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Dissertations in Forestry and Natural Sciences

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TANELI VÄISÄNEN

EFFECTS OF THERMALLY EXTRACTED WOOD DISTILLATESON THE CHARACTERISTICS OF WOOD-PLASTIC COMPOSITES


Wood-plastic composites (WPCs) represent an ecological alternative to conventional petroleum-derived materials. The wood

distillates studied in this thesis displayed good potential as bio-based additives for WPCs as they improved the water resistance and mechanical properties. It was also shown

that proton-transfer-reaction time-of-flight mass-spectrometry (PTR-TOF-MS) can be

applied to study the release of volatile organic compounds (VOCs) from WPCs.

TANELI VÄISÄNEN

TANELI VÄISÄNEN

Effects of ThermallyExtracted Wood Distillates

on the Characteristics ofWood-Plastic Composites

Publications of the University of Eastern FinlandDissertations in Forestry and Natural Sciences

Number 222

Academic DissertationTo be presented by permission of the Faculty of Science and Forestry for public

examination in the Auditorium SN201 in Snellmania Building at the University ofEastern Finland, Kuopio, on June, 10, 2016, at 12 o’clock noon.

Department of Applied Physics

Grano OyJyväskylä, 2016

Editors: Prof. Pertti Pasanen,Prof. Jukka Tuomela, Prof. Pekka Toivanen, Prof. Matti Vornanen

Distribution:Eastern Finland University Library / Sales of publications

P.O. Box 107, FI-80101 Joensuu, Finlandtel. +358-50-3058396

http://www.uef.fi/kirjasto

ISBN: 978-952-61-2123-9 (printed)ISBN: 978-952-61-2124-6 (PDF)

ISSNL: 1798-5668ISSN: 1798-5668

ISSN: 1798-5676 (PDF)

Author’s address: University of Eastern FinlandDepartment of Applied PhysicsP.O. Box 162770211 KUOPIOFINLANDemail: [email protected]

Supervisors: Professor Reijo Lappalainen, Ph.D.University of Eastern FinlandDepartment of Applied PhysicsP.O. Box 162770211 KUOPIOFINLANDemail: [email protected]

Laura Tomppo, Ph.D.University of Eastern FinlandDepartment of Applied PhysicsP.O. Box 162770211 KUOPIOFINLANDemail: [email protected]

Reviewers: Professor Raimo Alén, Dr.Tech.University of JyväskyläDepartment of ChemistryP.O. Box 3540014 JYVÄSKYLÄFINLANDemail: [email protected]

Professor Rupert Wimmer, Ph.D.University of Natural Resources and Life Sciences, ViennaSustainable Biomaterials Group Institute for Natural Materials TechnologyKonrad Lorenz Strasse 203430 TULLNAUSTRIAemail: [email protected]

Opponent: Professor Jyrki Vuorinen, Dr.Tech.Tampere University of TechnologyDepartment of Materials ScienceP.O. Box 58933101 TAMPEREFINLANDemail: [email protected]

ABSTRACT

The use of raw materials derived from renewable sources is increasingdue to the finiteness of crude oil reserves. In wood-plastic composites(WPCs), the plastic in a material is partially replaced by wood, whichis an abundantly available and inexpensive raw material. WPCs arematerials that encompass a wide range of performance levels suchthat they have diverse applications, e.g., in fencing and decking aswell as in the manufacture of automobiles. The use of WPCs in indoorapplications is also becoming increasingly popular. Despite theincreasing popularity of WPCs, certain inherent limitations mean thatthese materials are unsuitable for some applications. Examples of thelimitations associated with WPCs are their insufficient mechanicalstrength and their susceptibility to excess water absorption.Furthermore, the VOC (volatile organic compound) characteristics ofWPCs have not been widely studied and therefore, a betterunderstanding of these properties of WPCs would be of greatimportance. The properties of WPCs and their constituents can bealtered by incorporating additives. However, some additives arerather expensive and their incorporation into WPCs is notstraightforward. There is a clear need for novel, affordable andeffective filler materials, especially those that would minimize the useof expensive constituents.

Wood distillates are products originating from thermal processeswhere the components of wood are partly or completely decomposedinto charcoal, condensable vapors, and non-condensable gases.Although the liquid components of wood have many potentialapplications, large volumes of liquids are still being discarded and notexploited in industrial applications. Thus, the incorporation of moreof wood distillates into WPCs would enhance the use of raw materialsand secondary products from the wood-processing industries. Thiswould be both economically valuable and environmentally friendlysince it would represent sustainable development by makingcommercial use of a potentially hazardous waste product.

The main aim of this thesis was to investigate whether wooddistillates could be used as WPC components. Another aim was toassess the possibility to improve the mechanical properties and water

resistance of the WPCs with wood distillates. Furthermore, theapplicability of proton-transfer-reaction time-of-flight mass-spectrometry (PTR-TOF-MS) in determining the VOC emissioncharacteristics of WPCs was studied. The effects of incorporatinghardwood and softwood distillates into WPCs were examined bycharacterizing the mechanical properties, water resistance and VOCemissions of these WPCs modified with the distillates. The distillatecontent varied from 1 wt% to 20 wt%. The suitability of PTR-TOF-MSfor analyzing VOC emissions from WPCs was assessed by measuringVOC emissions from a WPC deck during a 41-day trial and comparingVOC emission rates between seven different WPC decks.

Both hardwood and softwood distillates exerted positive effects onthe water resistance of the WPC; the addition of hardwood distillatedecreased the water absorption of the WPC by over 25% whereas atleast a 16% decrease was observed for the WPC with the softwooddistillate. Moreover, a 1 wt% addition of hardwood distillate into theWPC led to a highly significant increase (11.5%, p < 0.01) in the tensilemodulus as well as achieving minor enhancements in some othermechanical properties. Similarly, when 2 wt% of softwood was addedto the WPC, a highly significant increase in the tensile strength (5.0%,p < 0.01) was observed. Even though the addition of the distillatesincreased the total release of VOCs, the emission rates of harmfulcompounds, such as benzene, remained low. Nonetheless, the resultsfrom the VOC analyses indicated that some of the compoundsinvestigated in this thesis may be smelled from the WPCs becausetheir odor thresholds were exceeded.

Wood distillates displayed good potential as natural additives inWPCs as they improved the mechanical properties and waterresistance. The results of this thesis provide a basis for the furtherdevelopment of wood distillates as bio-based additives in WPCs.

Universal Decimal Classification: 66.092, 662.712, 674.048, 674.816

Library of Congress Subject Headings: Composite materials; Wood distillation;Pyrolysis; Hardwoods; Softwood; Volatile organic compounds; Wood—Chemistry;Wood—Mechanical properties

Yleinen suomalainen asiasanasto: komposiitit; puu; muovi; kuivatislaus; fysikaalisetominaisuudet; vetolujuus; kosteudenkestävyys; haihtuvat orgaaniset yhdisteet;puukemia; puuteknologia

Acknowledgements

This thesis summarizes the studies performed in the Department ofApplied Physics at the University of Eastern Finland during the years2014 and 2015. The studies were financially supported by EuropeanRegional Development Fund (ERDF, granted by the Finnish FundingAgency for Technology and Innovation, project 70049/2011), Centrefor Economic Development, Transport and the Environment (NorthSavo, project S12261), the Academy of Finland (decision no. 252908)and Teollisuusneuvos Heikki Väänänen’s Fund.

First, it is my pleasure to thank my supervisors for their supportand guidance during the thesis project. I express my thanks to my firstsupervisor Prof. Reijo Lappalainen, Ph.D., for his trust, advice andencouragement during this process. I owe my deepest gratitude toLaura Tomppo, Ph.D., for her friendship, patience and promptassistance whenever it was needed. I also want to thank all my co-authors for their contributions, especially Jorma Heikkinen for hisexpertise in the preparation of the composite granules, and Pasi Yli-Pirilä, M.Sc., for the advice in PTR-TOF-MS analyses.

I am very grateful to the pre-examiners of this thesis, Prof. RaimoAlén, Dr.Tech., and Prof. Rupert Wimmer, Ph.D., for their commentsand constructive feedback. Furthermore, I am grateful to Prof. JyrkiVuorinen for accepting the role of my opponent at the defense of thisthesis and thus being part in one of the most important events of myacademic career. I also want to thank Ewen MacDonald, Pharm.D., forhis linguistic advice.

I express my special thanks to the co-workers in Panos buildingand in the new premises. The good humor, inspiration, and supportreally helped me to do my best and gave me extra motivation. Thesupport from my friends is also highly acknowledged.

I am thankful to my family, Raija, Matti, Jouni, Arja, Emma, Lauri,and Juuso, for the support throughout my life. Thank you forbelieving in me and for encouraging me to always follow my heart.

Finally, I am grateful to my wife Anne. Your support, love, humor,and care always make my day and give me strength to carry on.

Kuopio, June 2016 Taneli Väisänen

LIST OF ABBREVIATIONS

ASTM American Society for Testing and MaterialsCIS Charpy’s impact strengthDMTA Dynamic mechanical thermal analysisEBS Ethylene-bis-stearamideEDS Energy-dispersive x-ray spectroscopyFTIR Fourier transform infrared spectroscopyGC/MS Gas chromatography-mass spectrometryHDPE High-density polyethyleneHWD Hardwood distillateISO International Organization for StandardizationL/D Barrel-length-to-diameterLDPE Low-density polyethyleneLG LunaGrainMAPE Maleated polyethyleneMAPP Maleated polypropyleneMFA Microfibril angleMOE Modulus of elasticityPAH Polycyclic aromatic hydrocarbonPE PolyethylenePEEK Polyether ether ketonePLA PolylactidePP Polypropyleneppb Parts per billionppmv Parts per million by volumePS PolystyrenePTFE Polytetrafluoro-ethylenePTR-MS Proton-transfer-reaction mass-spectrometryPTR-TOF-MS Proton-transfer-reaction time-of-flight mass-

spectrometryPVC Polyvinyl chlorideSEM Scanning electron microscopySWD Softwood distillateRH Relative humidityRMT Reinforced matrix theory

TD-GC-FID/MS Thermal desorption/gas chromatography withflame ionization detector and mass spectrometry

TOF Time-of-flightUF UPM ForMiVOC Volatile organic compoundWPC Wood-plastic compositeXPS X-ray photoelectron spectroscopy

LIST OF SYMBOLS

A Cross-sectional areaAsample Area of a sampleb WidthB Bendingc Water absorptionCreal room Real room air concentration of a VOCCvoc Concentration of a VOCds Thicknessd DeflectionE Modulus of elasticityEvoc Emission rate of a VOC� StrainFvoc Flow rateF Load/forceFS Flexural strengthh HeightI�, I� Cellulose polymorphsk[r] Rate coefficientL0 Initial gauge lengthL Final length of the gaugeLs Length of spanLp Product loading factorMvoc Molar mass of a VOCm1 Mass of a dried specimenm2 Mass of a specimen after water immersionn Air exchange rateS1, S2, S3 Layers of secondary wallS Maximum surface stress� StressT TemperatureTM Tensile modulusTS Tensile strength�t Reaction time

LIST OF ORIGINAL PUBLICATIONS

This thesis is based on data presented in the following articles,referred to by the Roman numerals I–IV.

I Väisänen T, Tomppo L, Selenius M and Lappalainen R. Effects ofslow pyrolysis derived birch distillate on the properties of wood-plastic composites. Eur J Wood Prod. 74 (1), pp. 131-133, 2016.

II Väisänen T, Laitinen K, Yli-Pirilä P, Tomppo L, Joutsensaari J,Raatikainen O and Lappalainen R. Rapid technique formonitoring VOC emissions from wood-plastic composites.Submitted for publication.

III Väisänen T, Heikkinen J, Tomppo L and Lappalainen R.Improving the properties of wood-plastic composite throughaddition of hardwood pyrolysis liquid. J Thermoplast ComposMater. DOI: 10.1177/0892705716632862. 2016. In press.

IV Väisänen T, Heikkinen J, Tomppo L and Lappalainen R.Softwood distillate as a bio-based additive in wood-plasticcomposites. J Wood Chem Tech. 36 (4), pp. 278-287, 2016.

The original articles have been reproduced with permission of thecopyright holders.

AUTHOR’S CONTRIBUTION

This dissertation is based on four publications that examined thetreatment and testing of WPCs modified with two types of distillates,and the characterization of VOCs from various types of WPCs. PapersI, III, and IV were concerned with the characterization of WPCstreated with hardwood and softwood distillates. Paper II focused onthe evaluation of the applicability of PTR-TOF-MS for monitoringVOC emissions from WPCs.

The original idea for the utilization of wood distillates in WPCswas presented by Prof. Reijo Lappalainen, who also treated thegranules with the hardwood distillate in paper I. The author wasmainly responsible for the sample preparation. Moreover, the samplecharacterization and result analyses were conducted by the author.The author was also the main writer for the paper, with contributionsfrom other authors.

In paper II, the comparative measurements of seven different WPCdecks were conducted by the author with the kind help from Pasi Yli-Pirilä, M.Sc. Kimmo Laitinen, M.Sc., conducted the measurements forthe 41-day trial and wrote a part of the materials and methods -sectionfor the paper. The samples for the study were acquired by Dr. LauraTomppo and Prof. Reijo Lappalainen. The author analyzed the resultsand wrote the majority of the research paper. Pasi Yli-Pirilä, M.Sc., Dr.Laura Tomppo, Doc. Jorma Joutsensaari, Doc. Olavi Raatikainen, andProf. Reijo Lappalainen gave constructive comments and suggestionsfor the paper.

The ideas for papers III and IV were devised by the author and Dr.Laura Tomppo. Jorma Heikkinen processed the distillates andimpregnated the granules. The author was mainly responsible for thepreparation of the samples. In addition, the sample characterizationand sample analyses were undertaken by the author. The papers werewritten by the author with contributions from Dr. Laura Tomppo andProf. Reijo Lappalainen.

Dissertations in Forestry and Natural Sciences No 222 17

Contents

1 Introduction .................................................................................. 19

2 Wood-plastic composites ........................................................... 232.1 Raw materials ....................................................................... 24

2.1.1 Wood ............................................................................ 252.1.2 Polymers ....................................................................... 282.1.3 Additives ...................................................................... 31

2.2 Properties .............................................................................. 332.2.1 Mechanical properties .................................................. 342.2.2 Water absorption .......................................................... 382.2.3 VOC emissions ............................................................. 40

2.3 Manufacturing technologies ............................................... 422.3.1 Extrusion ...................................................................... 422.3.2 Injection molding ......................................................... 442.3.3 Compression molding ................................................... 452.3.4 Choosing appropriate manufacturing method ............. 46

3 Characterization of wood-plastic composites ........................ 493.1 Mechanical properties ......................................................... 49

3.1.1 Tensile strength ............................................................ 503.1.2 Flexural strength and modulus .................................... 523.1.3 Impact strength ............................................................ 53

3.2 Water absorption .................................................................. 553.3 VOC emissions ..................................................................... 56

3.3.1 TD-GC-FID/MS .......................................................... 563.3.2 PTR-MS ....................................................................... 57

4 Thermal processing of wood ..................................................... 614.1 ThermoWood® process ........................................................ 634.2 Slow pyrolysis of wood ....................................................... 654.3 Products obtained from the processes .............................. 67

4.3.1 Charcoal........................................................................ 67

Taneli Väisänen: Effects of Thermally Extracted Wood Distillates onthe Characteristics of Wood-Plastic Composites

18 Dissertations in Forestry and Natural Sciences No 222

4.3.2 Condensable vapors ...................................................... 684.3.3 Non-condensable gases ................................................. 70

5 Aims and significance ................................................................ 71

6 Materials and methods ............................................................... 736.1 Sample preparation .............................................................. 73

6.1.1 Distillates ..................................................................... 746.1.2 Impregnation of WPC granules ................................... 766.1.3 Injection molding ......................................................... 77

6.2 Mechanical properties ......................................................... 786.2.1 Tensile and flexural properties ..................................... 796.2.2 Charpy’s impact strength ............................................. 80

6.3 Water absorption .................................................................. 816.4 VOC emissions ..................................................................... 816.5 Statistical analyses ................................................................ 85

7 Results ........................................................................................... 877.1 Mechanical properties ......................................................... 887.2 Water absorption .................................................................. 897.3 VOC emissions ..................................................................... 91

8 Discussion .................................................................................... 978.1 Impregnation of WPC granules with wood distillates ... 978.2 Mechanical properties ......................................................... 988.3 Water absorption ................................................................ 1018.4 VOC emissions ................................................................... 1038.5 Limitations and future prospects ..................................... 109

9 Summary and conclusions ...................................................... 113

Bibliography ................................................................................. 115


1 Introduction

The finiteness of crude oil reserves is globally recognized, andtherefore, new raw material alternatives are being sought fromrenewable sources (Najafi et al. 2010). Wood is an inexpensiveand abundantly available material that possesses suitablecharacteristics for multiple applications, such as in theconstruction industry (Clemons 2008). On the other hand, thecombination of wood with the commodity plastics, adhesives,and other substances provide unique properties that cannot beachieved with either wood or plastic products on their own.Thus, wood-plastic composites (WPCs) are ecological, durable,and recyclable materials (Kim and Pal 2010). In WPCs, the woodfibers are surrounded by a continuous polymer matrix, and thecompatibility between these two constituents is typicallyenhanced by incorporating coupling agents and other additivesinto the composite. WPCs can be created with a wide range ofperformance levels, and therefore, they have many applicationsnot only in decking, and fencing, but also in more sophisticatedmanufacturing, e.g., in the car-making industry (Klyosov 2007,Faruk et al. 2012).

Even though the use of WPCs is becoming more and morecommon, at present, these materials cannot be used inapplications where high mechanical strength is required. This ismainly due to the weak bonding between the hydrophilic woodfibers and the hydrophobic polymer matrix (Gao et al. 2008,Yuan et al. 2008, Butylina et al. 2011). Moreover, wood fiberscontain a large amount of hydroxyl groups that can formhydrogen bonds with water molecules. Hence, WPCs aresusceptible to water absorption that induces thickness swellingand the creation of microcracks in the material (Li et al. 2014),which increases the risk of mold growth.

Many different approaches have been examined to eliminate,or at least reduce, these limitations in the present generation of



WPCs. There are several ways to modify wood fibers, e.g., heattreatment (Ayrilmis et al. 2011), the extraction of hemicelluloses(Hosseinaei et al. 2012) and the treatment with coupling agents(Müller et al. 2012); these modifications can considerablyincrease the water resistance of the WPCs. On the other hand, insome instances, the mechanical properties of WPCs can beenhanced by using recycled polymers instead of virgin material(Adhikary et al. 2008a). Moreover, the mechanical durability ofWPCs can be improved by incorporating additives, such asmaleated polypropylene or polyethylene (MAPP or MAPE)(Pérez et al. 2012, Ndiaye et al. 2013), waste charcoal (Li et al.2014, Das et al. 2015a), nanoclay (Abdolvahaba et al. 2014), orinorganic fillers (Gwon et al. 2012), into the composite.

WPCs are also increasingly used in indoor applications, suchas window frames and furniture. However, the impact of WPCson the quality of the indoor air has not been studied widely.Volatile organic compounds (VOCs) are chemicals that have ahigh vapor pressure at room temperature, allowing a greatnumber of molecules to evaporate from the material and enterthe surrounding air. VOCs include chemical compounds thatoccur in nature, and compounds that originate from humanactivity. Some VOCs exert harmful effects on human health andthe environment, and therefore, their release and maximumconcentrations in indoor air are regulated.

Several studies have examined the effects of organic wastesand residues on WPCs (Ashori and Nourbakhsh 2010, Hamzehet al. 2011, Madhoushi et al. 2014, Das et al. 2015b). Whenorganic waste is added to WPCs, typically there is anemphasized need for coupling agents to improve the bondingbetween the fibers and the polymer matrix. The choice,acquisition and application of the right coupling agent forWPCs is neither straightforward nor inexpensive, and thewhole process requires time. Therefore, there is a desire tominimize the use of coupling agents and other additives withsimilar challenges, especially if they can be substituted withmore affordable bio-based filler materials that can confer similarbenefits.

Introduction


A new and environmentally friendly approach to improvethe properties of WPCs is to add the thermal degradationproducts of wood into the composites (Das et al. 2015b). Woodcan be converted into charcoal, liquids, and non-condensablegases in pyrolytic processes (Klass 1998). The yields of theseproducts vary depending on the process type (Nachenius et al.2013). Since it is the primary product which is sought, thesecondary products of the processes are commonly consideredsimply as waste. The potential of biochar as an additive in WPCshas been investigated previously, and the positive effects ofincorporation of biochar into WPCs were evident (Li et al. 2014,Das et al. 2015a). Even though the liquid components of woodhave multiple applications, e.g., as biocides, pesticides, materialcoating, and medicine (Bridgwater 1996, Fagernäs et al. 2012a),their effects on the characteristics of WPCs have not beenstudied earlier. Nonetheless, since wood distillates have a ratherversatile nature, one could speculate that they could be utilizedin WPCs as ecological additives, coupling agents, lubricants,biocides or stabilizers after proper processing.

The present thesis project investigated the effects of liquidcomponents of wood on the properties of WPCs. It washypothesized that the utilization of liquid components of woodin WPCs could provide many advantages. First, the content ofrather expensive and petroleum-derived polymers in WPCscould be reduced. Second, the raw materials would be exploitedmore effectively as these liquids would otherwise be treated aswaste and therefore, not be further utilized. The focus of thisthesis was also on the determination of the VOC characteristicsof WPCs. Proton-transfer-reaction time-of-flight mass-spectrometry (PTR-TOF-MS) was used to analyze the levels ofVOCs emerging from WPCs. The effects of hardwood andsoftwood distillates were evaluated in mechanical tests, waterabsorption studies, and VOC analyses. The working hypothesiswas that WPC granules could be effectively impregnated withhardwood or softwood distillates to increase the waterresistance of WPCs and to strengthen the materials. The VOC



emission rates were expected to increase due to incorporationof these types of distillates.


2 Wood-plastic composites

By definition, composite materials are formed by combiningtwo or more constituent materials that have substantiallydifferent chemical or physical properties (Callister 2005). As aresult, the individual components remain distinct within thefinished material, and thus composites can possess propertiesthat cannot be achieved with the individual constituentmaterials. Composites can be classified into particle-reinforced,fiber-reinforced, and structural composites. There are manywell known composite materials, e.g., metal and ceramiccomposites, cements, concrete, and reinforced plastics.

In materials science, a fiber is commonly defined as asubstance that has been drawn into a long and thin filament, i.e.,the aspect ratio, which is defined as the ratio of fiber length todiameter, is at least 100 (Callister 2005). However, the term fibermay also refer to the spindle-shaped cells within the woodmaterial (Clemons 2008), and in the case of natural fiber-polymer composites, fiber can be defined as an object with anaspect ratio greater than one (e.g. Stokke et al. 2014). Thus, fromthe viewpoint of materials science, some WPCs can be classifiedas particle-reinforced composites although they are commonlyreferred to as fiber-reinforced composites. In this thesis, thedefinition of a fiber is adopted from the terminology used fornatural fiber-polymer composites.

WPCs are fiber-reinforced composites produced by mixingwood components and molten thermoplastics. In a WPC, apolymer forms a continuous matrix that surrounds thereinforcing wood components. The low price and high stiffnessof wood makes it an attractive reinforcement for the commodityplastics. Since the processability of WPCs is similar to theplastic, there are several appropriate manufacturingtechnologies available for WPCs. Although the majority of WPCproducts are extruded, injection and compression molding are



other major technologies used in WPC production. (Godavarti2005)

2.1 RAW MATERIALS

The properties of WPCs are mainly determined by thecharacteristics of their two main constituents. Even though bothare polymer-based materials, wood and plastic exhibitdistinctive properties and have different origins (Clemons 2008).Matrix polymers are high-molecular-mass materials created bythe polymerization of small repeating subunits, monomers.Polymers can be either natural or synthetic and furthermore,virgin material or recycled based on their origin (Adhikary et al.2008a, Adhikary et al. 2008b). Several polymers are used as thematrix material in WPCs, e.g., polyethene (PE), polypropene(PP), polyvinyl chloride (PVC), polystyrene (PS), andpolylactide (PLA). Due to the high molecular mass relative tothe small molecule compounds, polymers possess uniquephysical properties, such as viscoelasticity and toughness.

Wood is a natural composite consisting primarily of threepolymeric components: cellulose, hemicelluloses and lignin(Pettersen 1984). Cellulose constitutes 40–45%, hemicelluloses25–35%, and lignin makes up much of the remaining 20–30% ofwood. Wood is an attractive material to be incorporated inpolymer composites not only because it is abundant but alsodue to its light weight in relation to its mechanical properties.

In WPCs, the wood components are surrounded by thecontinuous polymer matrix. In general, the development of highquality WPCs is limited by two physical factors (Godavarti2005): the difference between the surface energy of the polymermatrix and wood components, and the upper temperature atwhich wood can be processed. There are several ways to offsetor minimize these limitations and to improve the generalperformance of the WPC. The most common approach involvesthe incorporation of different types of additives. Examples of

Wood-plastic composites


additives used in WPCs are coupling agents, lubricants,stabilizers, inorganic fillers, biocides, and flame retardants.

2.1.1 WoodWood has unique and useful properties – it is a recyclable,biodegradable, renewable, bendable, and relatively stablematerial. In addition, wood has an important role in carbonsequestration; growing trees take up and store considerableamounts of atmospheric carbon dioxide (CO2) (Hill 2006a).

The reinforced matrix theory (RMT) is a concept which canhelp to understand the cell wall structure of wood fibers, andultimately the properties of wood. In short, the RMT describesthe cell wall structure as follows: the cell wall of a plant consistsof the thermoplastic matrix (lignin) reinforced by the hightensile strength fibers (cellulose) and the hygroscopic material(hemicellulose). (Stokke et al. 2014)

Wood can be anatomically divided into two classes(Wiedenhoeft 2010, Wiedenhoeft 2012): softwoods(gymnosperms) and hardwoods (angiosperms). Whenexamined in the microscope, wood can be observed to be acomposite of many cell types. It is a complex biological structurewhose parts act together to fulfill the needs of a living plant: toconduct water from the roots to the leaves, to providemechanical support for the plant’s body, and to store andsynthesize essential biochemicals. Both softwoods andhardwoods consist mainly of tracheids – these are elongatedand hollow cells arranged in parallel to each other along thetrunk of the tree. In general, softwoods have a simpler structurethan hardwoods because softwoods have only two cell typesand less variation in the structure within the cell types(Pettersen 1984, Godavarti 2005, Wiedenhoeft 2012). The mostdistinctive difference in the structure between hardwood andsoftwood is the presence of vessel elements in hardwoods; theseelements are absent in softwoods. Generally, softwoods havelonger (3–8 mm) wood fibers than hardwoods (0.2–1.2 mm), butthe length of wood fibers varies between wood species(Wiedenhoeft 2010, Clemons et al. 2013).



The layered structure of wood fibers also explains the uniqueproperties of wood. As shown in Figure 1, the cell wall of awood fiber consists of two main parts: the primary andsecondary wall. The secondary wall consists of three separatelayers designated as S1, S2, and S3.

Figure 1. The layered structure of the cell wall of wood. The lines in the primary andsecondary cell wall layers describe the orientation of microfibrils.

The middle lamella is a lignin-rich region that binds the fiberstogether. The primary cell wall is made up of a loose and thin(0.1 µm) network of randomly oriented cellulose microfibrils. Italso consists of hemicelluloses, proteins, and pectin. The firstlayer of the secondary cell wall, S1, is approximately 0.2 µmthick with a relatively high microfibril angle (MFA). S2 is thethickest layer of the cell wall (up to 20 µm thick), and itprimarily defines the mechanical properties of the fiber. S2

consists mainly of cellulose and hemicelluloses. S3 is a thin layer(0.1 µm) of cellulose microfibrils. (Pettersen 1984, Stokke et al.2014)

The chemical composition of wood also varies from speciesto species. In general, dry wood has an elemental compositionof approximately 50% carbon, 6% hydrogen, and 44% oxygen.In addition, wood contains trace amounts of other elementssuch as calcium, potassium, sodium, magnesium, iron,manganese, sulfur, and phosphorous. (Rowell et al. 2013)



Cellulose is a linear and highly crystalline polymer of D-glucopyranose units linked together by �-(1�4)-glucosidicbonds (Pettersen 1984, Li 2011). The repeating unit in celluloseis a two-sugar unit, cellobiose. When randomly orientedcellulose molecules form intra- and intermolecular hydrogenbonds, the packing density of cellulose increases, leading to theformation of crystalline regions. For example, wood-derivedcellulose may contain as much as 65% of crystalline regions thatconfer the strength and structural stability to the wood (Rowellet al. 2013, Stokke et al. 2014). There are several differentcrystalline structures of cellulose. Cellulose I is the form ofcellulose found in nature (Thomas et al. 2011). It has structuresI� and I�, of which I� is enriched in the cellulose produced byalgae and bacteria, and I� in higher plants (Stokke et al. 2014).

Hemicelluloses are heteropolymers that includearabinoxylans, glucomannans, xyloglucans, glucuronoxylansand xylans (Rowell et al. 2013). In addition to glucose,hemicelluloses can be made of other sugar monomers, such asxylose, mannose, and galactose. They are present in plant cellwalls along with cellulose and lignin. In contrast to the linearand crystalline structure of cellulose, hemicelluloses arebranched and amorphous polymers with little strength.Whereas cellulose consists of approximately 10 000 glucosemolecules per polymer, hemicelluloses have shorter chains ofabout 2 000 sugar units (Pettersen 1984, Clemons 2008). In thecell walls of plants, hemicelluloses form a network of cross-linked fibers, thus endowing flexibility to the plant.

Lignin is a complex, amorphous and cross-linked polymer,consisting of aromatic alcohols known as monolignols(Pettersen 1984, Li 2011, Stokke et al. 2014). There are threemonolignol monomers incorporated into lignin during itsbiosynthesis in the form of phenylpropanoids: p-coumarylalcohol, coniferyl alcohol, and sinapyl alcohol. The chemicalcomposition of lignin varies in the different wood species. Forexample, lignin in the softwoods consists almost entirely ofguaiacyl moieties. In cell walls, lignin can be considered as achemical adhesive that fills the gaps between hemicelluloses



and cellulose. Lignin is covalently linked to hemicellulosemolecules, increasing the mechanical strength of the cell wall.Lignin is a non-polar hydrophobic polymer whereas celluloseand hemicelluloses are hydrophilic (Thomas et al. 2011). In thepulp industry, lignin is normally removed from the pulp(chemical pulp) when manufacturing bleached writing paper,because lignin is responsible for the yellowing of paper with age(Ek et al. 2009). Since lignin yields a considerable amount ofenergy when burned, it is considered as a potential alternativeto fuels derived from non-renewable sources. In addition, thepyrolysis of lignin yields chemical compounds that are thoughtto be potentially useful in many fields of applications (Lora andGlasser 2002). For instance, guaiacol, which is a thermaldegradation product of lignin, has smoky sensory notes and itcan be used as a flavorant (Goldstein 2002, Dorfner et al. 2003).

In addition to lignocellulose, wood contains small amounts(3–10%) of other organic components (Pettersen 1984, Rowell etal. 2013, Stokke et al. 2014). Wood extractives include simplesugars, fats, waxes, resins, proteins, terpenes, and gums. Theextractive compounds are crucial components of the defensesystem of the tree, but they also act as energy reserves andsupport tree metabolism (Clemons 2008). There are also traceamounts (about 1%) of inorganic ash in wood (Rowell et al.2013).

2.1.2 PolymersA variety of thermoplastic or thermosetting polymers can beused as the matrix material in WPCs (Clemons 2008). However,the low thermal stability of wood limits the polymers to thosethat have adequately low processing temperatures. The thermaldegradation of wood components begins at approximately120 °C, and major changes take place at over 200 °C. Thus,polymers which have a processing temperature lower than200 °C need to be used in WPCs (Godavarti 2005). The mostcommon polymers include PE, PP, and PVC that arethermoplastics.



The properties of a polymer are primarily defined by itsmolecular structure. Homopolymers contain only one type ofmonomer whereas copolymers or terpolymers consist of severalkinds of monomers. The branching of polymer chains hasmultiple effects on the polymer. For example, the highlybranched low-density polyethylene (LDPE) is softer and has alower density and poorer tensile strength than the more linearhigh-density polyethylene (HDPE). (Clemons et al. 2013)

The properties of polymers also depend on its tacticity – thearrangement of monomers along the polymer backbone.Polymer tacticity can be divided into three classes: an isotacticpolymer has all of its substituents on the same side of thebackbone, and polymers with alternating placements ofsubstituents along the backbone are called syndiotactic. Atacticpolymers lack any consistent arrangement in their substituents.(Clemons et al. 2013)

The monomers of a copolymer can also be organized in avariety of ways (Clemons 2008). An alternating copolymerconsists of two different monomers arranged in an alternatingsequence within the chain of the molecule (ABABAB…). Theorganization of monomers in random copolymers is not defined(ABAABBBA…). Statistical copolymers have monomersarranged according to a known statistical rule. Blockcopolymers are made up of polymerized monomer blocks. If acopolymer contains side chains that have a differentcomposition compared with the main chain, the polymer istermed as a graft copolymer.

The crystallinity of the polymer affects its thermal andphysical properties because the crystalline regions inside thepolymer structure increase the interactions between thepolymer chains. When the structure of the polymer is highlyordered, there are fewer possibilities for the polymer chains tomove relative to one another. Thus, more energy is required totransform the polymer into an unordered fluid state, meaningthat polymers with high crystallinity have higher melting pointsin comparison with their more amorphous counterparts. (Beylerand Hirschler 2001) High crystallinity also means that the



polymer will be strong but brittle, which accounts for the highmodulus and low impact resistance (Galeski 2003).Semicrystalline polymers have both crystalline and amorphousregions, i.e., these polymers combine the high strength ofcrystalline polymers with the flexibility of the amorphous types(Callister 2005). In composite materials, semicrystallinepolymers are typically more efficiently reinforced by fibers thanamorphous ones because the fibers act as nucleation sites for thecrystallization process with the fiber becoming surrounded bya finely divided microcrystalline structure, which improves themodulus, especially the flexural modulus (Quan et al. 2005).

At the moment, PEs are the most commonly used plasticsbecause they are easy to produce and modify. PE (Figure 2) is asemicrystalline polymer. The polymer chains in PE can branchin a different manner, resulting in polymers with differentproperties. HDPE has a variety of applications because of itsexcellent barrier properties and resistance to different solvents.LDPE is commonly used in containers, bottles, films, and plasticbags because it is a flexible and tough polymer with a goodresistance to chemicals. In addition, LDPE has good electricalproperties. (Klyosov 2007, Kim and Pal 2010)

Figure 2. The chemical structure of PE.

The properties of PP resemble those of PE. PP is asemicrystalline polymer with a methyl group (CH3) attached tothe polymer backbone (Figure 3), meaning that PP can be eitherisotactic, atactic or syndiotactic. However, over 90% ofproduced PP is isotactic. Like PE, PP finds its applications inpackaging, containers and films. Furthermore, PP is used in theautomotive industry and can be found in laboratory equipmentand textiles. (Kim and Pal 2010)



Figure 3. The chemical structure of isotactic PP.

PVC (Figure 4) is commonly used in construction, packaging,and insulation because it has an excellent weather resistanceand electrical properties. Additionally, PVC has goodmechanical properties. However, the processing of PVC isproblematic because it releases a toxic compound, hydrochloricacid (HCl), when burned or melted. Furthermore, the thermalstability of PVC is very poor but can be improved by addingheat stabilizers during processing. The presence of a chlorinegroup in PVC means that the polymer may have differenttacticity. Unlike PP, PVC has mainly an atactic stereochemistry.(Klyosov 2007, Kim and Pal 2010)

Figure 4. The chemical structure of atactic PVC.

2.1.3 AdditivesAdditives are introduced into a polymer to alter itsprocessability or performance (Clemons 2008). When apolymeric material contains additives, it is usually referred toas a “plastic”. Examples of additives are plasticizers, pigments,biocides, UV stabilizers, and antioxidants. The additive contentis usually low because these materials are often rather expensive.Moreover, an excessive amount of the additive may deterioratethe properties of the material (Clemons et al. 2013). In WPCs,additives are used to improve the processability of thecomposites and especially to enhance the coupling betweenchemically different wood fibers and plastics. Furthermore,



additives provide WPCs with a better surface appearance andlong-term durability (Sherman 2004).

Coupling agents are additives that improve the adhesionbetween wood and plastics, and their content in WPCs istypically less than 5%. Coupling agents can be classified into thesurface-active agents and functional modifiers. Surface-activeagents do not form covalent bonds with either the polymermatrix or the wood fiber; instead, they increase the interfacialadhesion of these constituents by acting as a solidsurfactant. MAPP is one of the most commonlyused coupling agents; its anhydride part forms ester bonds withwood’s hydroxyl groups and the long hydrophobic polymerincorporates into the polymer network. MAPP is therefore afunctional modifier. Consequently, the wood fibers and thepolymer matrix become bonded together, resulting in enhancedmechanical properties and reduced moisture absorption.Organosilanes, acrylic-modified polytetrafluoro-ethylene(PTFE), epoxides, isocyanates, organic acids, inorganic agents,and titanates are some other examples of the coupling agentsused in WPCs (Lu et al. 2000, Godavarti 2005).

Mineral additives are another major group of additives usedin WPCs. They include talc (Mg3Si4O10(OH)2), calcium carbonate(CaCO3), kaolin clay (Al2Si2O5(OH)4), and silica sand (SiO2). Inparticular, talc and calcium carbonate are commonly utilized inWPCs because they are abundantly available, inexpensive, andthey clearly enhance mechanical properties of the composite. Inaddition, talc has a natural affinity to oil, making it a good fillerand lubricant for the mineral oil derived plastics (Klyosov 2007).Due to its hydrophobicity and ability to close the pathways forwater in the composite, the addition of talc into WPCs results inreduced moisture absorption and less swelling liability.(Huuhilo et al. 2010)

The processability and surface appearance of WPCs can beimproved using lubricants, such as zinc stearate, paraffin waxes,oxidized PE, and ethylene-bis-stearamide (EBS). However, theuse of metal stearates together with maleated coupling agentscan nullify the effects of both additives. Typically, the amount



of lubricants in WPCs is less than 5%, but it is also dependenton the type of the polymer matrix. For example, the lubricantcontent level for a HDPE-wood composite (wood content50–60%) is usually 4–5% whereas a similar composite composedof PP as a polymer matrix instead of HDPE is typicallymanufactured with 1–2% of lubricant. (Sherman 2004)

Light stabilizers and colorants (pigments) are also added toWPCs to improve the resistance against color fade and UVdegradation, and to provide the desired appearance (Sherman2004, Clemons et al. 2013). The amount of pigments in WPCsmust be 1–3% or even higher to avoid color staining from thewood. Biocides, such as zinc borate, protect composites againstfungal and microbial attacks and maintain their surfaceappearance. They also reduce the moisture absorption. Flameretardants suppress the production of flames and therefore,prevent the spread of fire.

2.2 PROPERTIES

There are two common reasons to add wood to polymers: 1) tolower the price of the final product and 2) to reduce thedependency on mineral oil based products (Kim and Pal 2010).This, however, means a compromise as the properties of woodand plastics are altered, i.e., WPCs possess rather differentcharacteristics. For example, WPCs absorb less water than woodbut have higher tensile strength than plastics. WPCs havetherefore found use in multiple applications. The low densityand good processability of WPCs are favored in automobileindustry, for instance. On the other hand, WPCs are widelyused in building products, such as siding and decking becauseof their low water absorption and good creep performance.However, the properties of WPCs are highly dependent on theproduct formulation, manufacturing, and the quality of the rawmaterials.



2.2.1 Mechanical propertiesWood fibers are added to polymers to increase their stiffnessand strength (Wolcott and Englund 1999). The presence of woodfibers in the polymer matrix typically increases the strength andmodulus of the composite (Bhaskar et al. 2012, Li et al. 2014).However, both the polymer matrix and the fiber reinforcementare responsible for the mechanical performance of thecomposite. Tensile strength is more sensitive to the propertiesof the polymer matrix whereas the modulus of elasticity of thecomposite is primarily dependent on the properties of the fiber.In order to increase tensile strength, a strong fiber-matrixinterface, oriented fibers, and low stress concentration arerequired whereas the maximization of the tensile modulusrequires fiber wetting in the matrix phase, a high fiberconcentration and fibers with a high aspect ratio. (Saheb and Jog1999)

The fiber must have a certain minimum length, i.e., thecritical fiber length, in order to achieve the fully stressedproperties to the fiber in the polymer matrix (Stark andRowlands 2003, Sain and Pervaiz 2008). The critical lengthdepends on the fiber characteristics and shear strength of thefiber-matrix bond. The fiber-matrix interface is likely to fail dueto the debonding at lower stresses if the length of the fiber is lessthan its critical strength (Stark and Rowlands 2003, Bourmaudand Baley 2007). By contrast, exceeding the critical fiber lengthmay reduce the strength of the composite because the effectivestress transfer may be impaired due to fiber curling and fiberbending (Sreekumar et al. 2007).

Interphase and interface are two important concepts in fiber-reinforced polymer composites. The interface is a two-dimensional surface between the fiber and the matrix whereasthe interphase is the three-dimensional intermediate betweenthe matrix phase and the fiber phase (Pilato and Michno 1994,Oksman Niska and Sanadi 2008, Jesson and Watts 2012). Theinterface in any fiber-polymer composite system is responsiblefor transmitting stresses from the matrix to the fibers, and thecontribution of surfaces to stress transfer depends on both the



roughness and the surface chemistry of the constituents. Thestress in WPCs is transferred not only by shear along the lengthof the fiber, but also by tension at the fiber-matrix interface. Thestress transfer is limited by the fiber strength, the shear yieldstrength, and the tensile yield strength of the plastic matrixpolymer. (Sretenovic et al. 2006, Sain and Pervaiz 2008) Acomposite failure can occur through several scenarios, and theuneven nature of the surfaces makes the process even morecomplex. However, in the simplest case, an adhesive failure canoccur in the fiber-interphase interface or in the interphase-matrix interface. A cohesive failure of the interphase is alsopossible. The typical techniques to evaluate interfacialinteractions and adhesion between the main constituentsinclude surface analysis methods, such as X-ray photoelectronspectroscopy (XPS) and Fourier transform infraredspectroscopy (FTIR), microscopy, single fiber-pullout andmicrobond tests, and dynamic mechanical thermal analysis(DMTA). (Sretenovic et al. 2006, Oksman Niska and Sanadi 2008)

The mechanical properties of WPCs have been extensivelyinvestigated. Changes in the amount of the wood componentexert multiple effects on the characteristics of WPCs. When thewood fiber content is increased, the tensile and flexural modulitend to increase because wood, especially cellulose, is a highlycrystalline material compared to PE, PP, and PVC (Bhaskar et al.2012). However, the moduli of WPCs are highly dependent onthe fiber type and source (Bouafif et al. 2009, Butylina et al. 2011,Ashori et al. 2011, Adhikari et al. 2012, Migneault et al. 2015).Although an increase in the wood fiber content may also lead tothe higher hardness, it tends to reduce impact and tensilestrength (Bledzki et al. 2002, La Mantia et al. 2005, Ndiaye et al.2013). In addition, the tensile strain at break decreasesconsiderably. Huang and Zhang (2009) and Ashori et al. (2011)concluded that higher loadings of wood flour in WPCs inducedthe agglomeration of wood particles, which may impair themechanical durability of WPCs.

Interestingly, Bledzki et al. (2002) showed that WPCsconsisting of hardwood fibers had a higher elongation at break



and better impact strengths compared with WPCs containingsoftwood fibers. Their findings can be explained by thecompositional differences between hardwoods and softwoods;hardwoods contain more cellulose and hemicelluloses thansoftwoods. On the other hand, the higher lignin content insoftwoods could explain the better stiffness of those WPCsconsisting of softwood fibers. (Sain and Pervaiz 2008, Lai 2012)

The durability of WPCs can be considerably modified byaltering the characteristics of the wood fiber surface, whichchanges the compatibility between wood fibers and couplingagents. For example, if WPCs are manufactured with woodfibers from bark, then the esterification reactions betweenreinforcing fibers and the coupling agent are inadequate andthese WPCs are mechanically weaker. Conversely, themanufacture of WPCs with pure cellulose fibers leads tostronger WPCs because cellulose fibers and polymer matrix canbe more extensively coupled through coupling agents. Theunderlying reason for this phenomenon is the differencebetween the surfaces of fibers; the surface of pure cellulose fiberis more polar than the surface of bark because cellulose containsmore polar hydroxyl groups. In contrast, bark consists mainlyof lignin and extractives that are chemically non-polar.Furthermore, the coupling between wood fibers and polymerscan be altered by treating wood fibers with coupling agents,acids or alkalis. (Balasuriya et al. 2002, Bouafif et al. 2009, Farsi2010, Müller et al. 2012, Zhang et al. 2013, Migneault et al. 2015)

Thermal treatment of wood fibers is another way to modifythe properties of WPCs (Ayrilmis et al. 2011). In general, WPCsreinforced with thermally treated wood fibers are mechanicallyweaker than those reinforced with non-treated fibers. However,the thermal treatment of the wood fibers significantly increasesthe dimensional stability and water resistance of WPCs. Thethermal degradation of hemicelluloses begins already at 120 °C.As mentioned in section 2.1.1, hemicelluloses act as theconnective bridges between cellulose fibers and lignin, leadingto the stiffer wood material. The degradation of hemicelluloses,therefore, results in weakened mechanical properties.



Hosseinaei et al. (2012) showed that the extraction ofhemicelluloses from wood fibers significantly improved thetensile properties of WPCs because the resulting wood fiberswere more hydrophobic and less polar, enhancing thecompatibility between wood fibers and thermoplastics.

Bouafif et al. (2009) demonstrated that wood fiber size alsoaffected the mechanical properties of WPCs; increasing the fibersize improves the modulus of elasticity (MOE) and maximumstrength in both flexural and tensile tests; the results have beenconfirmed by Kociszewski et al. (2012). Migneault et al. (2008)demonstrated that increasing fiber length and maintainingconstant fiber diameter exerted beneficial effects on the tensileand flexural moduli and toughness of WPC.

The possibility of modifying the polymer matrix also resultsin the preparation of WPCs with distinctive characteristics. Forexample, using recycled polymers instead of virgin materialsmay improve mechanical properties of WPCs (Adhikary et al.2008a). However, the use of recycled polymers in WPCs can bechallenging since the post-consumer plastics waste may containseveral grades, colors, and contaminants, leading to varyingoutcomes when the plastics are combined with wood fibers(Najafi 2013). Sobczak et al. (2013) showed that the flexural andimpact strength of WPCs increase along with the mass-averagemolecular mass of the polymer. The polymer matrix of WPCsdoes not necessarily consist of one type of polymer; Gao et al.(2008) used a PE/PP-blend as a polymer matrix. Clemons (2010)investigated WPCs with varying HDPE:PP ratios and observedthat if the ratio was changed from 75:25 to 25:75, the tensile yieldstress of the WPC increased considerably whereas the oppositeeffect was observed for impact energies and yield strain.

There are other ways to optimize further the mechanicalproperties of WPCs, e.g., the incorporation of additives can helpto overcome an incompatibility between the wood and thepolymers. The use of MAPP or MAPE is a well-establishedapproach to improve the durability of WPCs. Several studieshave confirmed the effectiveness of MAPP and MAPE(Nourbakhsh and Ashori 2009, Pérez et al. 2012, Bhaskar et al.



2012, Ndiaye et al. 2013). In addition, new types of additiveshave been recently introduced. For example, Li et al. (2014) andDas et al. (2015a) used waste charcoal as an additive in WPCsand noted improvements in the tensile and flexural properties.Abdolvahaba et al. (2014) improved the durability of WPCs byusing nanoclay as a filler. Gwon et al. (2012) added threedifferent types of inorganic fillers (kaolin, talc, and zinc-borate)to WPCs and observed that those WPCs containing kaolin ortalc had higher mechanical strengths than WPCs with zinc-borate. The mechanical performance of WPCs with kaolin fillerwas the highest because kaolin has a staked plate shape, smallparticle size and a surface with highly hydrophiliccharacteristics.

To summarize, the mechanical properties of WPCs are highlydependent on the product formulation. The incorporation ofadditives, such as coupling agents, is usually required toproduce WPCs with adequate mechanical properties. Thus,new potential additives providing higher mechanical strengthare constantly being discovered and developed.

2.2.2 Water absorptionA well-known disadvantage resulting from the addition ofwood fibers in plastics is the consequent susceptibility to waterabsorption (Adhikary et al. 2008b). Moisture penetrates into thecomposite materials by three different mechanisms. The firstand the most common process is the diffusion of watermolecules inside the microgaps between the polymer chains.The second mechanism is capillary transport into the gaps andflaws at the interfaces between the fibers and polymers.Moisture transport by microcracks formed during theprocessing is another mechanism. In general, water absorptionon natural fiber reinforced composites follows the kinetics of aFickian diffusion process. (Espert et al. 2004)

Wang et al. (2006) studied moisture absorption in naturalfiber-plastic composites. They proposed that moistureabsorption occurred via two mechanisms depending on thefiber content of the composite. At higher fiber loadings, when



the accessible fiber ratio was high and the material morehomogeneous, the diffusion process was the dominantmechanism. At low fiber loading, percolation is the dominantmechanism; the fiber loading threshold for percolation is about50 wt%. Percolation was applicable for nonhomogeneousmaterials and it takes into account the randomness of thecomposite structure.

As WPCs absorb water, not only do they become morevulnerable to the dimensional changes and microbial attack, butthey also become mechanically weaker (Espert et al. 2004,Sombatsompop and Chaochanchaikul 2004, Tamrakar andLopez-Anido 2011). Several efforts have been made to improvethe water resistance and dimensional stability of WPCs. Somemanufacturers have attempted to reduce water absorption ofWPCs by the addition of zinc borate, which also improves thefungal resistance. On the other hand, along with theimprovements in mechanical properties, the addition of MAPPor MAPE also reduces moisture absorption (Adhikary et al.2008a, Najafi et al. 2010). Li et al. (2014) improved the waterresistance of WPCs by incorporating biochar into the composite,but other additives have also been proven to decrease waterabsorption of WPCs (Lee and Kim 2009, Huuhilo et al. 2010,Turku et al. 2014).

An increase in wood fiber content or fiber size leads to ahigher water absorption but like the mechanical durability, thisproperty is also highly dependent on the fiber type and source(Yang et al. 2006, Migneault et al. 2008, Bouafif et al. 2009,Migneault et al. 2009, Ayrilmis et al. 2011, Butylina et al. 2011).In addition, the characteristics of the polymer matrix exert aconsiderable impact on water absorption (Adhikary et al. 2008a,Najafi et al. 2010, Sobczak et al. 2013); in general, waterabsorption of WPCs with PP as the polymer matrix is higherthan of those with PE (Najafi et al. 2007). However, the waterabsorption of WPCs is also dependent on the temperature of thewater, i.e., by increasing the temperature, then one alsoincreases the amount of water absorbed (Najafi et al. 2007).



Water absorption of WPCs can be reduced by modifying thewood fibers. For instance, Dányádi et al. (2010) showed that thebenzylation of wood fibers resulted in decreased waterabsorption. The wood modifications conducted by Müller et al.(2012) had similar effects. Hosseinaei et al. (2012) extractedhemicelluloses from wood fibers, which also resulted in lowerwater absorption of their WPCs. Wei et al. (2013) modifiedpoplar wood fibers chemically by esterification and noted thatthe esterified fibers were more hydrophobic than theunmodified fibers. Consequently, the compatibility betweenwood fibers and the plastic matrix increased, leading to lowerwater absorption. Thermal modification of wood also results ina considerably lower water absorption of WPCs (Ayrilmis et al.2011, Butylina et al. 2011).

2.2.3 VOC emissionsVOCs have a low boiling point and therefore, a high vaporpressure at room temperature, leading to the evaporation of alarge number of molecules into the surrounding air. VOCs areabundantly present in nature, for example, they play animportant role in the communication between plants (Ueda etal. 2012). However, some VOCs exert adverse effects on humanhealth and may cause harm to the environment. Therefore,legislative efforts have been made to diminish the release ofharmful VOCs from commercial products. The regulation ofindoor VOC emissions aims to limit VOC emissions fromcommercial products into indoor air where concentrations arethe highest. The major difficulty in the research of VOCs andtheir effects is that their concentrations are usually low and thesymptoms and illnesses they evoke tend to develop very slowly.

The VOC emission characteristics of WPCs have not beenextensively studied because these materials are generally usedoutdoors. Nevertheless, WPCs are increasingly being usedindoors (Kim and Pal 2010). Therefore, their effects on indoorair quality are becoming more relevant. As WPCs consist mainlyof wood and plastics, it is often presumed that their VOCemissions are dominated by these two major constituents.



However, WPCs have usually been processed at hightemperatures, which may change the VOC emissioncharacteristics of the material. Furthermore, the incorporationof additives exert effects on the VOCs.

Schwarzinger et al. (2008) conducted an elemental analysisof different WPCs by two-stage pyrolysis-GC/MS (gaschromatography-mass spectrometry). In addition to theidentification of marker compounds for different wood types,they identified pyrolysis products from polymers (PE, PP, andPVC), which is important with respect to VOCs. Furthermore,their analysis of WPCs with various lignocelluloses providedfurther insights into the fundamental differences betweenWPCs with different reinforcements.

Félix et al. (2013) examined the release of VOCs from WPCsmade from landfill-derived plastic and sawdust. Their findingswere in accordance with those of Schwarzinger et al. (2008); thekey markers for WPCs were phenols and aldehydes. In general,the profile of VOCs displayed alkanes, alkenes, phenols,aldehydes, aromatic hydrocarbons, terpenes, carboxylic acids,esters, nitrogen compounds, ketones, and alcohols. The mostabundant VOCs in WPCs were furfural, �-pinene, 2-ethyl-1-hexanol, 2-methoxyphenol, N-methylphthalimide, butylatedhydroxytoluene, 2,4-di-tert-butylphenol, and diethylphthalate.In addition, Félix et al. (2013) demonstrated that theincorporation of additives increased the release of certain VOCs.Another important finding was that WPCs have the potential toemit off-odor compounds that cannot be completely masked byodorizing agents. The identified off-odor compounds in WPCsincluded acetylfuran, hexanal, 4-vinylguaiacol, acetic acid, and2-methoxyphenol.

One way to control VOC emissions from WPCs is toincorporate odorants into the composites to mask at least partof the off-odor compounds (Félix et al. 2013). Moreover, theaddition of other types of additives capable of affecting the odorcharacteristics along with the other properties of WPCs couldprovide a simple solution. Another approach, proposed by Yehet al. (2009), is to cover the composite with a thin layer of virgin



polymer in a co-extrusion process. This layer should be able todelay the release of VOCs and therefore, alter the odor profilesof WPCs.

2.3 MANUFACTURING TECHNOLOGIES

There are several alternative methods for manufacturing WPCs.Compounding is a process in which filler and additives areadded to the molten polymer. The compounded material can beformed into pellets or granules prior to future processing, orthey can be immediately shaped into the final product (in-lineprocessing) (Clemons et al. 2013). The processability of WPCs issimilar to plastics, which is an advantage since WPCs are cantypically be processed with the same machinery. Extruders arethe most commonly used systems for WPC compounding. Hot-cold mixers are also used but mainly for the processing of PVC-based WPCs (Schwendemann 2008).

The product manufacturing technologies for WPCs includesheet or profile extrusion, injection molding, and compressionmolding (Stokke et al. 2014). Profile extrusion is the mostcommonly used manufacturing method for a WPC, and it isused to produce composites with a continuous profile of thedesired shape (Gonçalves et al. 2014). WPC panels can beproduced by sheet extrusion. Injection and compressionmolding produce non-continuous pieces with a morecomplicated shape.

2.3.1 ExtrusionExtrusion produces continuous linear profiles by forcing amelted WPC through a die. Different types of extruders andprocessing strategies have been used to produce WPCs. Forexample, some processors manufacture WPCs in one step, usingtwin-screw extruders whereas some prefer to adopt severalextruders in tandem to compound and finally form the desiredprofile of the WPC. (Clemons et al. 2013)



A typical screw extruder consists of feeders, modular barrels,screws, a gearbox, a heating, and cooling unit (Figure 5), and acentralized control unit to adjust the extrusion speed, feedingrate, temperature, and other process parameters(Schwendemann 2008). The extruding screw system, consistingof screws and barrels, mixes, devolatilizes, and performs thereactions for multiple applications.

Figure 5. The basic structure of an extruder.

The screws mix the components in order to produce ahomogeneous blending fluid in the barrel. The screws areusually made up of three zones: the feeding, melting, and meltpumping zone. In the feeding zone, the raw materials for WPCare usually solid, but when they move to the melting zone, mostpolymers have melted while fillers and additives remain in asolid state. The melt pumping zone forms a continuous fiber-polymer blend, which is finally pumped to the pelletizer aftercooling or extruded through the die. (Stokke et al. 2014)

A typical barrel-length-to-diameter (L/D) ratio of a single-screw extruder varies from 20 to 30. The screw builds up highpressure in the composite melt so that it can be extrudedthrough the die. Even though the single-screw extruders are lessexpensive than those with twin-screw systems, they suffer froma limited mixing and self-cleaning ability as well as from theselective material intake. The twin-screw extruders, whether co-rotating or counter-rotating according to the screw rotation



directions, are used for compounding, mixing or reactivepolymer materials. L/D ratios for the twin-screw extruders varyfrom 39 to 48. The advantages of the twin-screw extrudersinclude the self-cleaning and high mixing ability. However,unlike the single-screw extruders, these systems cannot developa up high pressure in the melt pumping zone. In addition, thetwin-screw extruders are more expensive. (Schwendemann2008, Stokke et al. 2014)

The barrels are divided into the sections heated with theindividual control units (Stokke et al. 2014). The temperature ofthe barrel gradually increases from the rear to the front whichallows the material to melt gradually and to prevent thermaldegradation or overheating. Sometimes the friction and highpressure in the barrel provide the required heat for the system,and the heaters can be turned off.

Multi-layered WPC structures are produced by coextrusion.This process utilizes multiple extruders (single or twin-screw)to melt and deliver different types of materials to a singleextrusion die that will extrude the materials in the desired form(Stokke et al. 2014). In addition to the reduced material andproduction costs, coextrusion makes the properties of finalproducts highly controllable, which is a significant advantageover other production technologies.

2.3.2 Injection moldingInjection molding is used for producing large quantities of WPCpieces with complex geometries (Migneault et al. 2009). Themolding of WPCs begins by inserting the pelletized rawmaterial into the hopper, which feeds the material into theheated barrel with a reciprocating screw (Figure 6). The majorityof the injection molding machines are equipped with singlescrews. The increased thermal energy reduces the viscosity ofthe material, allowing the screw to push the material forward.The simultaneous mixing and homogenizing increase thefriction and heat within the barrel. The material is collected atthe front of the screw and then injected at high pressure andvelocity into the mold. The volume of the material that is used



to fill the mold is known as a shot (Stokke et al. 2014). The highpacking pressure completes the mold filling and compensatesfor thermal shrinkage. Once the cavity entrance solidifies, nomore material can enter the cavity. Consequently, the screwreciprocates and receives the new material for the next cycle.Meanwhile, the material inside the mold is cooled to the presettemperature and ejected from the mold. After the preparedpiece is demolded by an array of pins, the mold closes and theprocess is repeated.

Figure 6. The structure of an injection molding apparatus.

Most injection-molded WPCs are produced from pelletized rawmaterials. However, in-line compounding is also possible. It isa combination of a two-stage injection unit with a co-rotatingtwin screw. Once the raw materials are fed into the co-rotatingtwin screw, they are compounded and transferred to a shootingpot. The shooting pot pushes the material via the machinenozzle and hot runner into the mold. (Schwendemann 2008)

2.3.3 Compression moldingCompression molding of WPCs is utilized especially in theautomotive industry due to its capability to produce large andcomplex parts. Moreover, it wastes relatively little raw material,and therefore, it is one of the least expensive molding methods.However, the product quality is not always consistent and it can



be problematic to control for leakage between the two surfacesof the mold (flashing). (Clemons et al. 2013, Stokke et al. 2014)

The compression molding process (Figure 7) starts byplacing the WPC pellets or granules into the preheated mold(Stokke et al. 2014). To shorten the molding cycle time, thecharge is also usually preheated. The material is softened by theheat and as the upper half of the mold moves downward, thecharge is forced to conform to the shape of the mold. After themold is opened, the part is removed by the ejector pin.

Figure 7. A schematic drawing of compression molding process.

2.3.4 Choosing appropriate manufacturing methodProcessing methods have significant effects on the properties ofWPCs. For instance, Migneault et al. (2009) observed thatinjection-molded WPCs have better physical and mechanicalproperties and reduced water absorption than extruded WPCs.However, the extruded WPCs had higher densities. Theseinvestigators also discovered that wood fibers in injection-molded WPCs were aligned in the main flow direction whereasthe fibers in extruded samples were more randomly oriented.This resulted in a better stress transfer between wood fibers andthe polymer matrix in the injection-molded samples.

Bledzki et al. (2005a) compared three different compoundingprocesses (two-roll mill, high-speed mixer, and twin-screwextruder) and claimed that the composites compounded byextrusion had the best mechanical strength and lowest water



absorption. Yeh and Gupta (2008) observed that a change in theextrusion parameters did not exert any significant effect on themechanical properties of WPCs, but it did influence the waterabsorption behavior. Namely, the longer residence time andhigher screw speed resulted in a lower rate of water absorptionas well as lower density. They speculated that the lower densityand therefore, the lower rate of water absorption was caused bythe loss of hydrophilic compounds in WPCs. Goncalves et al.(2014) revealed that the design of the extrusion die had aconsiderable effect on the properties of a WPC deck. They useda computer system to design a die and then simulatedexperimental conditions, and the comparison betweennumerical and experimental results achieved good qualitativeagreement.

Even though there may be considerable differences in theproperties of WPCs produced with different manufacturingmethods, it is also important to assess the strengths andlimitations of the manufacturing processes in a productionpoint of view. Table 1 presents a simple comparison betweenextrusion, injection molding and compression molding.

Table 1. A comparison between extrusion, injection molding, and compressionmolding.

Parameter ExtrusionInjectionmolding

Compressionmolding

Setup costs Moderate High Low

Production costs Low Low Moderate

Production speed Moderate High Low

Product consistency High High Low

Product geometryLimited to parts of afixed cross section

ComplexLimited to flat or

curved parts


3 Characterization of wood-plastic composites

Material testing is usually the last step in the manufacturingprocess, and its purpose is to ensure that the material meets therequirements of its applications. WPCs are commonly used inapplications that require adequate mechanical strength andresistance to water absorption. The physical and mechanicalproperties of WPC products are typically evaluated withstandard laboratory tests. In general, WPCs and plastics aretested with similar procedures because they are typicallymanufactured with the same technologies.

The British standard BS EN 15534-1 was validated as astandard in the EU in 2014. This standard specifies theprocedures for the determination of physical and mechanicalproperties of WPCs. In addition, the test methods for durability,such as weathering and natural ageing, and thermal propertiesare defined.

The characterization of WPCs can also be carried outaccording to ISO (International Organization forStandardization) and ASTM (America Society for Testing andMaterials) standards. ASTM standards are widely applied in theUS whereas ISO standards are typically used in Europe.

3.1 MECHANICAL PROPERTIES

The mechanical properties of WPCs are characterized accordingto the standards originally developed for plastics since in mostcases, these standards are suitable for WPCs. However, it ispossible that the testing of WPCs according to these standardsdoes not provide valid results. For example, the effect of the sizeof wood fibers may be overemphasized at the sample



dimensions specified in the ISO and ASTM standards. In otherwords, it is possible that the stress required to fracture the WPCsample is relatively lower at smaller sample dimensions than itwould be at the dimensions of the final applications becausewood fibers are larger in relation to the cross-sectional area ofthe sample and thus undergo fractures more easily.

According to BS EN 15534-1, WPC samples should beconditioned in a standard atmosphere at 23 ± 2 °C and relativehumidity (RH) of 50 ± 10% before mechanical testing. Anatmosphere of 20 °C and 65% RH may also be used but thoseconditions should be declared.

When determining flexural, tensile, and impact strength ofWPCs, the preferred test specimen according to ISO standardsshould be 80 ± 2 mm in length, 10.0 ± 0.2 mm in width, and4.0 ± 0.2 mm in thickness. The corresponding specimendimensions for WPCs in ASTM standards are 125 mm × 12.7 mm× 3.2 mm. In total, a set of 10 specimens should be tested unlessthe coefficient of variation has a value less than 5%. In that case,a minimum number of five specimens may be sufficient.

3.1.1 Tensile strengthTensile strength is the measure of the maximum amount oftensile stress that a material can withstand while being stretchedbefore breaking. It is defined as a stress and expressed as forceper unit area. The most important parameters for tensile testinginclude testing speed, force capacity, precision, and accuracy.The testing speed is expressed as mm/min, and according to ISO527-1, it can vary between 0.125–500 mm/min depending on thesample type.

At the start of the measurement, the machine slowly extendsthe sample until it breaks. The elongation of the sample ismeasured against the applied force. With the measuredelongation, it is possible to calculate the strain, �, using thefollowing equation:

� = L-L0L0

, (3.1)

Characterization of wood-plastic composites


where L is the final length of the gauge and L0 is the initial gaugelength. The applied force is used to calculate the stress, �, usingthe following equation:

� = FA

, (3.2)

where F is the tensile force (N) and A is the cross-sectional areaof the sample. The relationship between the stress and strain ofa material can be displayed on a stress-strain curve (Figure 8).The curve also provides the fracture strength of a material,which is the final recorded point.

Figure 8. A typical stress-strain curve for WPCs.

Tensile properties of WPCs can be determined according to ISO527-1 and ASTM D638-14 standards. Although the testingprocedures presented in these standards are rather similar,there are some differences that can significantly influence theresults obtained. In ISO 527-1, it is stated that the specimensmust not be pre-stressed considerably prior to testing. Pre-



stresses can be induced when the specimen is centered in thegrips or when the clamping pressure is applied. The maximumallowable pre-stress must be less than 1% of the measured stressresults. The induced strain value must be less than 0.05%,accordingly. ASTM D638-14 does not contain thesespecifications, and therefore, it lacks the defined status of thestress after placing the specimen in the grips. Thus, thecorrected strain zero point is defined as the point where thelinear slope of the stress-strain curve crosses the strain axis. Thiscorrection can exert a significant effect on the measured tensilemodulus. In ISO 527-1, the point where the tensile modulus ismeasured is precisely defined whereas the definition of tensilemodulus in ASTM D638-14 is based on the corrected strain zeropoint. If the material lacks the linear region in the stress-straincurve, the modulus is determined from a secant modulus that isdetermined between the corrected strain zero point and a freelyselected point on the curve. Consequently, this may result insignificant variations between the results carried out accordingto ISO or ASTM standard.

Another difference between these standards is related to thetest speed used in determining tensile modulus. In ISO 527-1,the tensile modulus is measured with the lower test speeds thantensile strength, but the same test speeds are allowed to be usedthroughout the test in ASTM D638-14. There are also differencesin the requirements for extensometers; ISO 527-1 allows thelower measurement uncertainty than ASTM D638-14.

3.1.2 Flexural strength and modulusThe material’s ability to resist deformation under load is definedas the flexural strength, which is typically measured using athree- or four-point flexural test technique. During the test, thesample experiences many kinds of stresses throughout its depth.At the outside of the bend, the stress is tensile in its naturewhereas at the load-bearing side, the sample experiencescompressive stress (Sain and Pervaiz 2008). Usually most of thematerials fail under tensile stress rather than compressive stress,meaning that the maximum tensile stress value that the material



can withstand before breaking is its flexural strength.Mathematically, the formula to calculate the maximum surfacestress, S, for a rectangular sample in the three-point bending testis expressed as:

S = 3FLs2bds

2 , (3.3)

where F is the bending load (N) at the given point, Ls is thelength of span (mm), b is the width of the sample (mm), and ds

is the thickness of the sample (mm). In ISO 178, Ls isrecommended to be 64 mm.

Flexural modulus, E, is the ratio of stress to strain within theelastic region. It is computed from the slope of a stress-straincurve (Figure 8) obtained from the flexural strength test. Theflexural modulus for the three-point test of a rectangular samplecan be expressed as:

E = Ls3F

4bh3d , (3.4)

where h is the height of the sample (mm) and d is the deflection(mm).

The test parameters for flexural testing are defineddifferently in ASTM D790-15 and ISO 178 since the dimensionsof the specimen are also different. In addition, the point wherethe test is stopped is not the same. In ASTM D790-15, the test isstopped when a 5% deflection is reached or if the specimenbreaks this value. In ISO 178, the test continues until thespecimen breaks. If the specimen does not break, the stress at3.5% strain is reported. Consequently, these standards providedifferent results if the specimen strain is higher than 3.5%.

3.1.3 Impact strengthImpact strength can be determined using either the Charpy orIzod impact test. Although the principle of these tests is similar,there are some differences in the testing procedures. In Charpyimpact test, a specimen with dimensions of 4.1 mm × 10.1 mm ×55 mm is positioned horizontally in the middle of two supports.



A pendulum strikes the middle of the specimen that may beunnotched or have a U-notch or V-notch. In the Izod impact test,the dimensions of the specimen are 4.1 mm × 10.1 mm × 75 mm.The specimen is placed vertically on the support, and it can beunnotched or have only a V-notch. In the Izod test, the notch isfacing the pendulum whereas in the Charpy test the notch sideof the specimen faces away from the pendulum.

The Charpy impact test determines the amount of energyabsorbed by a material during the fracture. The measurementapparatus consists of a pendulum of a known mass and length.The pendulum is dropped from a known height so that it strikesthe specimen. The amount of energy absorbed by the materialat the impact can be determined by comparing the heights of thehammer before and after the fracture. The energy absorbed inthe breaking is expressed as impact energy (J/m). It is calculatedby dividing the energy by the thickness of the sample. In ISO179-1, the Charpy’s impact strength is reported in kJ/m2, whichis derived by dividing the impact energy by the area under thenotch. In ASTM D6110-10, the results are reported as J/m.

Notching has a considerable effect on the results of theimpact test. Thus, the exact geometries and dimensions ofnotches have been determined in ISO 179-1. In addition, the sizeof the sample can affect the results.

The differences between ISO and ASTM impact tests arerelated to the type of pendulum hammer used in the tests. InISO 179-1, the pendulum hammer may be used in the rangefrom 10 to 80% of its nominal potential energy whereas themaximum value in ASTM D6110-10 is 85%. In addition,according to ISO 179-1, the largest possible hammer must beused because the speed loss at the impact must be kept as lowas possible. ASTM D6110-10 defines that the standardpendulum hammer has a rated initial potential energy of 2.7 J,and the hammer size is increased by doubling its dimensions.However, unlike ISO 179-1, the smallest hammer in the rangehas to be used in the tests.



3.2 WATER ABSORPTION

Water absorption of WPCs is typically evaluated using thestandard methods developed for plastics, wood, and WPCs.However, the time to reach the moisture equilibrium is longerfor WPCs than for plastics or wood (Defoirdt et al. 2010); woodreaches an equilibrium in hours or weeks, plastics in weeks andWPCs in months. Therefore, the testing methods used for WPCsmay not allow the material to reach the moisture equilibriumand this may affect the results obtained. This, however, does notmean that the test methods described in these standards couldnot be used to investigate the differences between various WPCtypes; considerable differences in the water resistance can beobserved even with only two hours of water immersion.

Guidelines for the determination of water absorption ofWPCs are given in BS EN 15534-1. ISO 62 can also be usedbecause it applies to the reinforced plastics, to which WPCsbelong. Similarly, ASTM D570-98(2010)E1 can also be used todetermine the moisture absorption of WPCs. According to thestandards, the water absorption of the material can bedetermined by completely immersing the samples in water at23 °C or in boiling water. Before the immersions, the samplesmust be dried in an oven at 50 ± 2 °C for at least 24 h and thencooled to room temperature in a desiccator before weighing.After the immersion, water absorption, c (%), of the materialscan be determined by using the following formula:

c = m2-m1m1

� 100% , (3.5)

where m1 is the mass of the test specimen after initial drying andbefore the immersion, and m2 is the mass of the test specimenafter the immersion.

There are some differences in these standards. In BS EN15534-1, the immersion period in the water bath (23 °C) is 28 ± 1days whereas the immersion period in ISO 62 should be at least24 hours. In addition, the immersion time in boiling test is5 h ± 10 min in BS EN 15534-1, but in ISO 62 the immersion



should last for at least 30 ± 2 min. ASTM D1037-12 specifies twomethods to determine water absorption of WPCs. In method A,the WPCs are first immersed for two hours in fresh water(20 ± 1 °C) and then weighed. After weighing, the sample issubmerged in water for an additional 22 hours and thenweighed again. Method B is similar to the 24-hour immersionprocedure described in ISO 62.

3.3 VOC EMISSIONS

There are several methods available for the determination ofVOC emissions from solid products. If the product is primarilyintended for indoors use, the emissions are commonlydetermined according to ISO 16000-6. In this standard, theemissions are analyzed by thermal desorption/gaschromatography with flame ionization detector and massspectrometry (TD-GC-FID/MS) using a Tenax TA® absorbenttube as a collector for VOCs.

Proton-transfer-reaction mass-spectroscopy (PTR-MS) isanother way to determine and compare VOC emissionsbetween different samples. This is an online monitoringtechnique using gas phase hydronium ions as the ion sourcereagents. This technique is used in food science, medicine, andbiological and environmental research. (Schripp et al. 2014)

3.3.1 TD-GC-FID/MSThe guidelines for determination of VOC emissions frombuilding products using emissions test chamber system aregiven in ISO 16000-9. In this procedure, the air flow transfers theemitted compounds from the chamber to a Tenax TA® absorbenttube. After reaching the tube, the compounds of interest areadsorbed onto the surface of the material. When the compoundsare being analyzed, the tube is heated to a temperature over250–300 °C and the adsorbed compounds are released into theflow of carrier gas that transfers the compounds to the GC-MS.



The measurements are conducted under controlledconditions as defined in the standard. This states that theproducts should be tested at a temperature of 23 ± 2 °C and RHof 50 ± 5%, with an air velocity in the range 0.1–0.3 m/s. Inaddition, the temperature should not vary by more than ± 1.0 °Cduring the measurements, and RH and air flow rate canfluctuate by only ± 3%. Before the measurements, the testchamber must be cleaned with alkaline detergents and thenrinsed twice with distilled water. In addition, cleaning bythermal desorption is also allowed. To eliminate the possibleeffects of background emissions, an air sample of the emptyemission chamber is taken before the actual measurements.When the sample is placed in the chamber, it should bepositioned in the center of the chamber to ensure that the airflow is evenly distributed over the emitting surface. Themeasurements should be carried out at predefined samplingtimes that depend on the objective of the test. However,duplicate air samples should be taken at least at 72 ± 2 h and28 ± 2 days after the start of the test.

For TD-GC-FID/MS, the analysis of VOCs is optimal for therange of VOCs eluting between and including n-hexane andn-hexadecane (Woolfenden 2009). However, when the tube isheated, the adsorbed compounds are released slowly from thetube. This may lead to low sensitivity and widechromatographic peaks. In addition, this system is not capableof measuring the emissions of certain VOCs, such methane andformaldehyde. Modern TD-GC-MS systems avoid wide peaksby using cold traps to focus the samples before they reach thecolumn, but the properties of column also affect the wideness ofthe peaks. Overall, the sensitivity of TD-GC-MS is highlydependent on the absorbent material, system parameters andthe amount of the sample.

3.3.2 PTR-MSPTR-MS is an easy-to-use online VOC monitoring system withhigh sensitivity and rapid time response. In PTR-MS, the VOCtrace gases in the sampled air are ionized in proton-transfer



reactions using hydronium (H3O+) as the primary reagent ion(Lindinger et al. 1998, Lindinger et al. 2001). The concentrationsof the product ion and the reagent are then measured in a massspectrometer. The system consists of an ion source (hollowcathode), a drift tube reactor (reaction chamber) and a massspectrometer. In the ion source, a hollow cathode discharge inwater vapor produces H3O+ ions (de Gouw and Warneke 2007).These ions are then injected into the drift tube where VOCs areionized with H3O+ ions according to the following reaction:

H3O+ + R � RH+ + H2O (3.6)

Next, a homogeneous electric field in the drift tube transportsthe reagent and the product ions into the second intermediatechamber. Most of the air is pumped away and a small fractionof ions is extracted for analysis in the mass spectrometer. Theconcentration of trace gas [RH+] is computed from the followingequation:

[RH+] = [H3O+]0 (1 – e-k[r]�t) � [H3O+]k[R]�t (3.7)

The approximation of the equation is made assuming that [R] issmall and, therefore, [H3O+] is equal to [H3O+]0. The ratecoefficient k for the proton-transfer-reaction and reaction time tare predefined parameters, and the fraction of [RH+] and [H3O+]is obtained from the mass spectrometry.

The sensitivity of PTR-MS can be further improved bycombining the PTR ion source with a time-of-flight massspectrometer (TOF-MS). In this arrangement, the system canseparate most atmospherically relevant protonated isobaricVOCs and identify their corresponding empirical formulas(Müller et al. 2010, Schripp et al. 2014). However, the maximummeasurable concentration of PTR-MS is limited toapproximately 10 ppmv (parts per million by volume). If thetotal concentration of VOCs is too high, the concentrationcalculation will be incorrect. In addition, the identification ofcompounds is based on their mass, which is not always unique.



As PTR-MS is based on the proton-transfer-reaction with H3O+

as the primary reagent, the system detects only molecules thathave a higher proton affinity than water (Schripp et al. 2010).Moreover, the fragmentation of ions can influence the resultsobtained (Aprea et al. 2007).


4 Thermal processing ofwood

Thermal modification of wood has been conducted on anindustrial scale since the beginning of the 20th century toimprove the dimensional stability and decay resistance of wood.Nowadays, there are several commercial thermal modificationprocesses, such as ThermoWood® (Finland), Plato® (Holland),Perdure process, and Retification® (France). In general, thetemperatures of the thermal modification processes varybetween 180 °C and 260 °C. (Hill 2006b, Esteves and Pereira 2008,Navi and Sandberg 2011)

Thermal modification exerts multiple effects on the physicaland biological properties of wood (Hill 2006b); these are mainlyinduced by the chemical changes in the macromolecularconstituents. In addition to the improved dimensional stabilityand decay resistance, thermally modified wood absorbs lesswater but has a tendency to form cracks and splits and hasreduced impact toughness, modulus of rupture, and work tofracture. The process variables, such as time and temperature ofthe treatment, wood species, sample dimensions, and the use ofcatalysis, have considerable effects on the resulting changes.

When the temperature increases to over 300 °C, thedegradation behavior of wood changes, and it becomes severelydegraded (Hill 2006b). In slow pyrolysis, wood is slowly heatedin the absence of oxygen up to a final temperature of 400–500 °C.The heating rate of the process is typically in the range5–10 °C/min (Klass 1998, Mohan et al. 2006, Dahmen et al. 2010,Li and Suzuki 2010). The primary product of this process ischarcoal, but it also produces condensable vapors and non-condensable gases. Charcoal can be utilized as a smokeless solidfuel or as pure carbon in chemistry. On the other hand, thecharcoal-rich biochar could be exploited to boost agricultural



yields and control pollution (Cernansky 2015). Condensablevapors can be condensed into separate fractions according to theprocessing temperature, and the heat from the non-condensablegases can be reused in the process to diminish the need forexternal energy (Nachenius et al. 2013).

The degradation of wood constituents begins at 130–200 °Cwhen hemicelluloses start to decompose (Figure 9). However,most of the hemicellulose decomposition occurs above 180 °C,resulting in the formation of gases and relatively small amountsof liquids and charcoal (Finnish Thermowood Association 2003,Bhaskar et al. 2011). The decomposition of cellulose occurs inmultiple phases. First, very short-lived active cellulose isformed. It is then dehydrated, decarboxylated, and carbonizedat a temperature under 300 °C to produce charcoal and non-condensable gases. At over 300 °C, cellulose becomesdepolymerized and cleaved into condensable gases and vapors,including tar. In the final stage, cellulose undergoes secondaryreactions, cracking the vapors into secondary charcoal, tar andgases. Lignin decomposition produces primarily charcoal (40%),but liquid components (35%), and gases (10%) are also formed.The decomposition of lignin occurs over a wide temperaturerange; the process can begin at 200 °C, continuing to 450–500 °C.As wood contains many kinds of extractives, the decompositiontemperatures of these compounds vary extensively; thedecomposition begins at 150 °C and continues to approximately400 °C. Extractives usually evaporate or are cracked intosecondary products. (Mohan et al. 2006, Basu 2013, Nacheniuset al. 2013)

Thermal processing of wood


Figure 9. Thermal decomposition temperatures of wood constituents. The line at thebottom indicates the typical temperature ranges for the processes.

4.1 THERMOWOOD® PROCESS

ThermoWood® is an industrial scale heat treatment process forwood (Finnish Thermowood Association 2003). During theprocess, wood is heated to at least 180 °C while it is protectedwith steam. In addition, the presence of steam affects thecompositional changes taking place in the wood.

The process consists of three phases. In the first phase, thetemperature of the kiln is rapidly raised to approximately 100 °C.Then the wood is dried until it has a moisture content of nearly0% by elevating the temperature slowly up to 130 °C. In thesecond phase, the temperature of the kiln is increased to185–215 °C. Depending on the final application, the temperaturecan remain unchanged for 2–3 hours. In the third and the finalphase, the temperature is lowered back to below 100 °C usingwater spray systems. At 80–90 °C, the wood moisture contentincreases to 4–7% as re-moisturizing takes place. The totalduration of the process is typically about 36 hours, dependingon the raw material and desired outcome. (Navi and Sandberg2011)

ThermoWood® has two treatment classes: Thermo-S(stability) and Thermo-D (durability). In Thermo-S, the



maximum treatment temperature varies between 185–190 °Cdepending on the wood type. The maximum treatmenttemperature of Thermo-D ranges between 200–215 °C.

The ThermoWood® process has multiple effects on wood. Asshown in Figure 9, extractives are the first components tothermally degrade or evaporate from wood. The extractivesinclude terpenes, waxes and phenols that are easily evaporatedduring the treatment. In addition, changes occur incarbohydrates (cellulose and hemicelluloses). However, themajority of the changes take place in the hemicelluloses. Whenwood is heated to the treatment temperatures, acetic acid isformed from acetylated hemicelluloses by hydrolysis. Theformed acetic acid formed serves as a catalyst such thathemicelluloses are hydrolyzed to soluble sugars, and it alsodepolymerizes amorphous cellulose microfibrils into shorterchains. (Finnish Thermowood Association 2003)

Consequently, the hemicellulose content in wood is reducedand the degree of crystallinity in cellulose increases. Thesechanges result in the improved resistance to fungal decay, betterdimensional stability, and decreased water absorption. Thus,heat-treated wood is very suitable for outdoor applications.Heat treatment also results in minor changes in lignin, eventhough it is thermally the most stable component of wood.During heating, the bonds between phenylpropane units arepartly broken. When the temperature exceeds 200 °C, �-arylether bonds start to break. At high temperatures, the methoxycontent in lignin decreases and some non-condensed units aretransformed into diphenylmethane-type units. These reactionsexert considerable effects on the properties of lignin. Sincehardwoods contain more hemicelluloses than softwoods,hardwoods are more susceptible to thermal degradation thansoftwoods. (Finnish Thermowood Association 2003)

In addition to the chemical changes, the ThermoWood®

process results in multiple physical changes in wood. Loss ofmass and the formation of microcracks during the treatmentdecrease the density of treated wood. Consequently, heattreated wood has generally a lower strength than its untreated



counterpart. However, the thermal conductivity of wood afterheat treatment is reduced by over 20%. Visually, heat treatedwood has a darker color than untreated wood (Navi andSandberg 2011).

In total, heat-treated wood has lower VOC emissions.However, the release of acetic acid, furfural, and hexanalincreases significantly after heat treatment. Acetic acid is aharmful VOC that causes irritation of the respiratory system.Furfural has a smoky odor and it is thought to be partlyresponsible for the characteristic odor of heat treated wood. It istherefore uncertain whether heat-treated wood has a betterVOC profile than untreated wood (Manninen et al. 2002,Hyttinen et al. 2010). Despite the increased release of someharmful VOCs, heat treated wood can be regarded a safeconstruction material for indoor air quality (Manninen et al.2002).

4.2 SLOW PYROLYSIS OF WOOD

Slow pyrolysis refers to the thermal decomposition of wood orother types of biomass in the absence of oxygen. In this process,wood is slowly heated (5–10 °C/min) to 400–500 °C so that theheat energy breaks down the long chains of carbon, hydrogen,and oxygen into smaller compounds. The products of thisprocess include charcoal (35–40%), tar and liquids (30–45%),and gases (25–35%). The yields of the products are dependenton the process variables, such as the heating rate, finaltemperature and pressure, and on the type and size of the woodpieces. When the final temperature is low (~400 °C) and theheating rate is slow (< 5 °C/min), pyrolysis will yield mainlycharcoal. With a rapid or moderate heating rate (> 5 °C/min) anda high final temperature (~500 °C), mainly non-condensablegases are formed. If one wishes to optimize the yield of bio-oil,then an intermediate temperature and relatively high heatingrate must be used. Torrefaction is a mild or incomplete form ofslow pyrolysis, and thus, only a partial thermochemical



conversion and devolatilization take place; the maximumtemperature of the process typically ranges between 200 °C and300 °C. (Williams and Besler 1996, Klass 1998, Nachenius et al.2013)

Pyrolysis and charcoal production have a long history.Traditionally, charcoal has been produced in kilns, but moderncharcoal production utilizes large retorts with capacities of100 m3 or even more. In addition, these systems are typicallycombined with refining facilities to capture the volatile products.The most common types of processes used in the industry arethe Reichert retort process, the SIFIC, and the Lambiotte process.A Reichert facility typically consists of multiple retorts (up tosix), and therefore, has a high rate of production. For example,the Reichert production facility operated by proFagus GmbH inBodenfelde, Germany, has been designed to produce30 000 tons of charcoal, 5 200 tons of acetic acid, 1 800 tons ofpyroligneous spirit, and 12 000 tons of bio-oil per year. Thecharcoal production capacity in a traditional Lambiotte retort is12 000 tons per year. (Dahmen et al. 2010)

The decomposition of cellulose and hemicelluloses producesprimarily condensable vapors and non-condensable gases.Lignin decomposes into liquids, gases, and charcoal. Thedecomposition or volatilization of extractives produces liquidor gas products. Minerals and ash remain solid in charcoal, andthey exert a catalytic effect on the pyrolysis reactions; thesecompounds increase the yield of charcoal. (Williams and Besler1996, Nachenius et al. 2013)

The pyrolytic production processes require only smallamounts of external energy. When the temperature is below200 °C, the process is endothermic and it produces primarilywater, formic acid, and acetic acid. At 200–270 °C, the processbecomes partly exothermic. At this stage, the main products arecarbon dioxide, formic acid, and acetic acid. When thetemperature reaches 350–400 °C, the process reactions arehighly exothermic, producing a great number of products, suchas methanol, formaldehyde, and tar compounds. When 400 °Cis exceeded, the reaction heat becomes weakly endothermic,



and the process produces non-condensable gases. The energyefficiency of pyrolysis can be further optimized by recycling theheat energy present in the non-condensable gases. The moisturecontent of the raw material is an important factor affecting theenergy efficiency of the process; if the moisture content exceeds30%, the process will be endothermic. (Dahmen et al. 2010,Nachenius et al. 2013)

4.3 PRODUCTS OBTAINED FROM THE PROCESSES

Even though the most important and commercially relevantproduct in the ThermoWood® process is the thermally modifiedwood itself, other products are also formed during the process.The maximum temperature used in the ThermoWood® processis 215 °C, and the reactions are carried out in the presence ofsteam. Hemicelluloses and extractives are decomposed attemperatures below 215 °C (Figure 9), and some minor changesoccur in lignin. Therefore, the products formed in this processare primarily liquids and gases formed from these constituents.The formation of solid products, such as charcoal, is minor.

The maximum temperature in slow pyrolysis, in turn,exceeds the decomposition temperatures of all woodconstituents (Figure 9), and the products of slow pyrolysis canbe found in the gaseous, liquid and solid phases. Gaseousproducts include hydrogen, methane, carbon monoxide, andcarbon dioxide. The liquids include tars and oils that remain inthe liquid form at room temperature. Solid products are mainlycomposed of charcoal, but other inert materials are also present.(Dahmen et al. 2010, Bhaskar et al. 2011, Basu 2013)

4.3.1 CharcoalCharcoal is the main product from slow pyrolysis, consistingmainly of carbon (60–90%), hydrogen, and oxygen. Charcoalproduction requires slow heating rate for a long duration but ata relatively low temperature (400 °C). Charcoal is used as anenergy source especially in developing countries, but it has



recently attracted substantial interest because of its highpotential in other applications. For example, charcoal can beused as biochar in agriculture to improve the water holdingcapacity of soil and to retain nutrients more efficiently, andtherefore, improve yields. On the other hand, biochar is alsoable to combat pollution by binding the heavy metals in soilsand liquids, and by reducing the nitrous oxide emissionsbecause of its bulk surface area, pore size distribution, particlesize distribution, density, and packing. In this context, charcoalis referred to as activated carbon. However, the properties ofbiochars vary according to the processing method and rawmaterials used in their manufacture. The pyrolysis temperatureis the most important parameter affecting the properties ofbiochar. (Downie and Van Zwieten 2012, Nachenius et al. 2013,Cernansky 2015)

Charcoal is also used as an ore reductant in the metallurgicalindustry because of its low mercury and sulfur content. Its highcarbon content makes it a desirable material in chemistry. Otherapplications of charcoal include water and air (and gas)purification. The addition of charcoal has also beneficial effectson the properties of WPCs. (Li et al. 2014, Das et al. 2015a)

4.3.2 Condensable vaporsThe decomposition of hemicelluloses, cellulose, and ligninproduces vapors that can be condensed into several fractions(Fagernäs et al. 2012a, Fagernäs et al. 2015). Tars originateprimarily from lignin and they contain carbohydrates, phenols,and aldehydes. Hemicelluloses and cellulose decompose intoseveral liquid fractions that consist of acetic acid, formic acid,acetone, phenol, and water. The formation of liquid distillatesbegins at approximately 150 °C when the bound water in woodstarts to evaporate. Methanol, acetic acid, and furfural becomeevaporated at 180–210 °C, and tars are formed at approximately330 °C. At 400 °C, the majority of the compounds have beendistilled.

The detailed chemical composition of liquid products fromslow pyrolysis has been determined by Fagernäs et al. (2012a)



and Miettinen et al. (2015). Fagernäs et al. (2012a) analyzed slowpyrolysis products from birch and found that the aqueousphases were composed mainly of acetic acid (60% of thecompounds), methanol (9%), hydroxypropanol (5–6%), furfural(3–4%), and acetone (2–5%). The compositions of tars weresimilar to the aqueous phases, but they consisted also of ligninmonomers phenols, guaiacol, and syringols. The amount ofpolycyclic aromatic hydrocarbons (PAHs) in tars ranged from0.1 wt% to 0.4 wt%. PAHs have harmful effects on human healthand the environment, and therefore, Fagernäs et al. (2012b)conducted a further study of the PAHs present in pyrolysisproducts. PAHs are mostly concentrated in the heavy tars whichcollect at to the bottom of the retort, but low PAH contents arealso found in the tar-free aqueous phases as well as in the non-condensable gases. Miettinen et al. (2015) showed that the slowpyrolysis oil from unbarked pine was composed mainly ofvarious wood extractives and lignin degradation productswhereas the aqueous phase contained saturated fatty acids,degradation products of lignin, anhydrosugars, and otheroxygen-rich compounds.

Tars and liquids derived from the thermal processing ofwood have been stated to possess a huge potential in a vastnumber of applications (Fagernäs et al. 2015). So far, hundredsof chemicals have been identified from tars and liquids, andnew ways to separate valuable chemicals and chemical familiesfrom these fractions are being developed (Brown and Brown2014). Their potential applications include their use as biocides,repellents, pesticides, material coating and medicines. However,the presence of PAHs may limit the widespread applications oftars (Fagernäs et al. 2012a).

Examples of chemicals with a high commercial potential arelevoglucosan, furfural, glycolaldehydes, and phenoliccompounds, such as guaiacol and catechol (Abou-Zaid andScott 2012). Furfural, glycolaldehydes, guaiacol, and catecholcan be utilized in resin production whereas levoglucosan can beused as a pesticide and in the production of antibiotics andpolymers. Other rather valuable compounds identified in wood



distillates are vanillin, propionic acid, and syringol, all of whichcan be used in the food processing industry. However, many ofthe compounds listed above are present in rather lowconcentrations and their fractionation and subsequentindividual identification and separation would be commerciallyimpractical. Therefore, the synthesis of chemical compoundsfrom fossil resources is currently favored (Brown and Brown2014). Nevertheless, there are still many unidentifiedcompounds in a wide variety of wood distillates, some of whichmay prove to have very high values and even be easy toseparate.

4.3.3 Non-condensable gasesBy definition, non-condensable gases are the gases that remainonce they have passed the condensation stage in the pyrolysisprocess (Nachenius et al. 2013). Cellulose decompositionproduces a great amount of gases. In some applications, the heatenergy from non-condensable gases is utilized to drive thepyrolysis process or to dry the biomass feed. It is also possibleto release these gases to the atmosphere or to burn them and usetheir energy for other purposes.

The composition of non-condensable gases is largelydetermined by the pyrolysis temperature and the temperatureat which the condensable vapors become condensed(Nachenius et al. 2013). At lower temperatures, the non-condensable gases consist primarily of carbon monoxide andcarbon dioxide. High reaction temperatures result in increasedhydrogen and methane contents. As mentioned in section 4.2,the maximum yield of non-condensable gases can be achievedat high reaction temperatures. Gasification is an example of aprocess that primarily aims to convert biomass into non-condensable gases (Decker et al. 2007).


5 Aims and significance

Despite the fact that WPCs are recognized as potentialsubstitutes for the mineral oil based plastics and conventionalbuilding products, they still lack certain desirable properties.For example, one of the major limiting factors of theapplicability of WPCs is their excess water absorption, and thiscan cause their swelling and increase their susceptibility tomicrobial attack. In addition, as WPCs are increasingly beingused indoors, their impact on indoor air quality has not beenassessed. To overcome the limitations associated with WPCs, awide variety of additives has been developed (Sherman 2004).Even though the additives, such as coupling agents, usuallyprovide at least a partial solution to the problems, additives areusually relatively expensive and developed via synthetic routes.Thus, the WPC industry needs novel, bio-based andinexpensive solutions to replace these materials.

On the other hand, the efficient utilization of raw materials,including industrial by-products, is becoming more relevant.For example, considerable amounts of liquid waste aregenerated in charcoal and ThermoWood® production. The useof these liquids in WPCs could provide benefits for bothindustries, as material previously considered as waste wouldbecome a valuable additive in WPCs. In order to explore thispossibility, the following aims were set for this thesis:

1. To explore if WPC granules could be effectively impregnated withdifferent types of wood distillates. The impregnation of WPCgranules with the distillates was tested in studies I, III andIV using two types of commercial WPC granules and twotypes of wood distillates.

2. To study whether PTR-TOF-MS is an applicable technique fordetermining VOC emissions from WPCs. The suitability ofPTR-TOF-MS to determine VOC emissions from WPCs was



assessed in study II where the emissions of seven differentcommercial WPC decks were determined and compared. Ingeneral, VOCs are related to the safety of the materials andthey define the odor profile of the material. PTR-MS hasbeen previously used to determine material emissionsignatures from nine common building materials (Han et al.2010), but there are no studies where PTR-TOF-MS has beenused to monitor VOC emissions from WPCs over a moreprolonged period of time.

3. To determine the effects of hardwood distillate on the properties ofWPC. The impact of hardwood distillate on thecharacteristics of WPC was determined in studies I and III.In study I, 4.2–4.8 wt% of hardwood distillate was added tothe WPCs whereas 1–8 wt% of a similar distillate wasadded to the WPC in study III. The effects of the distillateaddition were evaluated in mechanical tests and waterimmersion assays.

4. To determine the effects of softwood distillate on the properties ofa WPC. The impact of softwood distillate on thecharacteristics of a WPC was determined in study IV where1–20 wt% of softwood distillate was added to the WPC. Theeffects of the distillate addition were evaluated byconducting mechanical tests and water immersion assays.

As far as the author is aware, this thesis is a pioneering workexamining the incorporation of wood distillates into WPCs. Thisstudy can serve as a basis for further studies with similarobjectives and it is anticipated that it will represent a milestonein reaching the ultimate goal of replacing synthetic additiveswith inexpensive, bio-based alternatives.


6 Materials and methods

Two types of commercial WPC granules were used in this thesis;UPM ForMi (UPM Biocomposites, Lahti, Finland), hereafterabbreviated as UF, was composed of cellulose fibers andadditives in a PP matrix whereas LunaGrain (LunaComp Ltd,Iisalmi, Finland), hereafter abbreviated as LG, consisted ofthermally modified sawdust (Scots pine, 50 wt%), PP andadditives. UF was available with four different cellulose fibercontents: UF20 contains 20 wt%, UF30 30 wt%, UF40 40 wt%,and UF50 50 wt% cellulose fibers. After the granules weretreated with the distillates and dried, the samples formechanical testing and water absorption tests (studies I, III, andIV) were prepared by injection molding.

Seven different commercial WPC decks were used in studyII to evaluate their VOC characteristics: two decks wereproduced by UPM (UPM ProFi), three by LunaComp, and twoby unknown Chinese manufacturers. The decks were eitherprovided by the manufacturers or obtained from a local retailer.Table 2 presents a summary of the materials and methods usedin this thesis.

6.1 SAMPLE PREPARATION

The WPC granules were prepared for distillate impregnationsand injection molding by drying them in a force-convectionoven. The temperature in the oven was set to 105 ± 2 °C and thegranules were dried in the oven for at least four hours. At thebeginning of the trial, the granules were weighed before andafter the drying for reference. Once dried, the granules weretightly packed in plastic bags to avoid humidity. The packedgranules were stored under laboratory conditions (T = 22 ± 2 °Cand RH = 50 ± 10%) before further processing.



Table 2. An overview of the materials and methods used in the thesis.

Study Materials*Distillate type and

contentStudy methods

I

Injection-moldedspecimens from LG andUF20, UF30, UF40, and

UF50

Hardwood distillate4.1–4.8 w%

Mechanical propertiesWater absorption

IISeven WPC decks from

four manufacturers- VOC emissions

IIIInjection-molded

specimens from LGHardwood distillate

1.0–8.0 w%

Mechanical propertiesWater absorptionVOC emissions

IVInjection-molded

specimens from LGSoftwood distillate

1.0–20.0 w%

Mechanical propertiesWater absorptionVOC emissions

* LG refers to LunaGrain and UF to UPM ForMi

The WPC decks used in study II were sawn into smaller piecesto fit into the glass vessels used in the VOC emissionmeasurements. The cut surfaces were covered with Kapton®

tape immediately after sawing to avoid any VOC emissionsfrom the newly-cut surfaces. The samples were then storedunder laboratory conditions (T = 22 ± 2 °C and RH = 50 ± 10%)and characterized shortly after sawing.

6.1.1 DistillatesTwo types of distillates were used in this thesis. In studies I andIII, the WPC granules were treated with hardwood distillatewhereas softwood distillate was used in study IV.

The conversion of birch into charcoal, liquids, and non-condensable gases was conducted using a two-part slowpyrolysis retort with an approximate capacity of 10 m3. First,5–15 cm thick blocks of birch were put inside the inner part ofthe retort. The temperature of the retort was slowly (1 °C/min)elevated to about 350 °C in the absence of oxygen. The processwas endothermic until the temperature reached 270 °C.Subsequently, the process became exothermic and theformation of non-condensable gases started to increase. The

Materials and methods


heat energy from these gases was recycled to maintain theprocess conditions.

The vapors were condensed into collectable liquids bytransporting them through cooled pipes. The evaporation ofmoisture from the raw material began below 100 °C. Asexpected, the liquids formed from these vapors consistedprimarily of water. At 100–300 °C, the formation of condensablevapors continued, but the composition of liquids changed; asthe temperature increased, the share of water decreased andthat of acetic acid and other organic acids increased. When thetemperature exceeded 300 °C, the formation of tars began andcontinued until the final temperature (350 °C) was reached. Thisfraction was used for the treatments.

The softwood distillate originated from industrialThermoWood® process conducted by LunaWood Ltd (Iisalmi,Finland). In the process, primarily Scots pine planks werethermally treated as described in section 4.1. During the process,the evaporation of condensable compounds resulted in theformation of liquids, a part of which was collected into a liquidcontainer (V = 1 m3) and further processed. Due to thedifferences in the compositions and densities, there were twodistinguishable phases in the container; the lighter phaseconsisted primarily of water and water-soluble organiccompounds, such as acetic acid, and the heavier phase, whichphysically resembled tar, was a mixture of acetic acid, methanol,phenols, long chain fatty acids and their esters, squalene, cyclichydrocarbons and PAHs. This heavier phase was separated,processed and used for the treatments.

Both distillates, hardwood and softwood, were processedsimilarly before they were used for the treatments. First, thedistillates were rinsed three times with water (T � 40 °C) toextract water-soluble compounds, such as simple carbohydrates,from the distillate. In general, simple carbohydrates have lowmelting points (< 200 °C) and therefore, they could start to burnduring injection molding. When the distillates were rinsed,water was added to the distillates in a ratio of 1:1, and theresulting blend was carefully mixed for 15 minutes. After the



mixture, the blend was stabilized and the lighter phasecontaining water and water-soluble compounds was removed.Next, the distillates were heated to approximately 105 °C toevaporate those compounds with relatively low boiling pointsbecause the evaporation of these compounds during injectionmolding could affect the product quality. Finally, the distillateswere filtered to remove the solid particles.

6.1.2 Impregnation of WPC granulesThe impregnation of the WPC granules with hardwood orsoftwood distillate was carried out in a similar manner.However, the softwood distillate was more viscous than thehardwood distillate, and therefore, the softwood distillate hadto be heated to approximately 80–90 °C in order to achieve anefficient impregnation. It was also found that the UF granuleswere not suitable for this type of impregnation because theywere too dense to be thoroughly treated with the distillate.Especially the granules with relatively low cellulose fibercontent (UF20 and UF30) were problematic because theyresembled plastic granules with a low porosity. Therefore, onlythe LG granules were treated with the distillate and they werethen mixed with the UF granules.

The impregnation began by mixing the WPC granules andthe distillate in a dish until the granules were thoroughlycovered with the distillate. The excess distillate was thenremoved from the blend by pouring the mixture onto a steelsieve, allowing the extra distillate to drip. Once the dripping ofthe distillate ended, the granules were placed into a force-convection oven in aluminum trays at 120–130 °C. Thetemperature in the oven was then raised to 170 °C for 30 minutesto polymerize the distillate. The mixture was then cooled toroom temperature and tightly packed in plastic bags. The finaldistillate content of the treated granules was determined byweighing the dried granules before and after the impregnationwith the distillate.

The distillate content in the WPC materials used for injectionmolding was controlled by mixing the impregnated granules



with the untreated granules and changing the mixing ratioaccording to the desired distillate content. Consequently, thedistillate content varied between 1–8 wt% for the WPCscontaining hardwood distillate, and 1–20 wt% for the WPCscontaining softwood distillate.

6.1.3 Injection moldingSpecimens with dimensions of 4.1 mm × 10.1 mm × 170 mmwere prepared by injection molding using a Haitian Mars MA1600/600 apparatus (Ningbo Haitian Huayuan Machinery Co.,Ltd, Ningbo, China) as presented in Figure 10. The screwdiameter was 50 mm with an L/D ratio of 18. The mostimportant parameters used in the injection molding are listed inTable 3.

Figure 10. Haitian Mars MA 1600/600.



Table 3. Some of the parameters used in injection molding.Parameter Value

Heater temperature (°C)Heater 1 Heater 2 Heater 3 Heater 4 Nozzle

100–150 140–165 165–175 180 185–195

Mold temperature (°C)Fixed Movable

50–75 50–75

Cooling time (s) 20–35

Holding pressure (MPa) 350–800

Holding time (s) 15.0

Injection pressure (MPa) 80–100

In comparison with the specimens prepared from LG, the UFspecimens were injection molded at higher heater and moldtemperatures but at lower holding pressures. In general, theWPCs containing distillates required lower holding andinjection pressures for injection molding. However, the additionof distillates resulted in smoother surfaces of the specimens, andthus, the specimens started to stick to the mold. Therefore,longer cooling times were used for the WPCs treated withdistillates. At least 50 pairs of specimens were prepared fromeach material type, and the specimens were stored in plasticbags under laboratory conditions.


The mechanical properties of the WPCs were determined usingthe injection-molded specimens. The measurements werecarried out within two weeks after the sample preparation, andthe samples were conditioned and stored in the laboratorybetween the measurements.

In studies I, III and IV, the tensile and flexural properties ofthe WPCs were characterized. Moreover, Charpy’s impactstrengths were determined in studies III and IV. A total of tenspecimens of each material type were examined in each test.



6.2.1 Tensile and flexural propertiesTensile strength, modulus, and strain of the WPCs weredetermined as specified in ISO 527-1. The measurements wereconducted using an Instron 8874 dynamic mechanical tester(Instron Industrial Products, Grove City, Pennsylvania, UnitedStates) under laboratory conditions (Figure 11). The test speedwas set to 5.0 mm/min. The system recorded strain, tensilemodulus, and the force needed to break the sample. Thicknessand width for each specimen were measured, and the cross-sectional area was used to calculate the tensile strength asdescribed in section 3.1.1.

The flexural properties of the WPC samples were determinedaccording to ISO 178. The measurements were carried out usingan Instron 8874 dynamic mechanical tester (Figure 11). Ls was64 mm and the speed of the loading edge was set to 2.0 mm/min.The system was tested with reference samples to verify thecorrect calibration prior the actual measurements.

The system was set to stop the loading and the measurementwhen the specimen broke. During the measurements, thesystem recorded the force and the corresponding deflection ofthe specimen and provided a complete stress-strain curve.

Figure 11. Instron 8874 dynamic mechanical tester used in studies I, III, and IV.



6.2.2 Charpy’s impact strengthIn studies III and IV, Charpy’s impact strengths (unnotched)were determined according to ISO 179-1. The measurementswere carried out using a Ray-ran advanced universal pendulumsystem (JD Instruments Inc., Houston, Texas, United States) aspresented in Figure 12. The injection-molded samples were cutinto pieces with dimensions of 4.1 mm × 10.1 mm × 80 mm.

Before the measurements, the test machine was calibrated todetermine the frictional losses and to correct for any absorbedenergy. In addition, the dimensions of one specimen from eachmaterial type were measured to ensure that the dimensionscorresponded to the guidelines given in ISO 179-1. The sampleswere then placed edgewise between two supports (Ls = 62 mm).The pendulum (m = 0.952 kg) was lifted up its prescribed heightto achieve a pendulum velocity of 2.9 m/s. The pendulum wasreleased and the result was recorded. For all material types, theimpact resulted in a complete fracture of the specimen.

Figure 12. Ray-ran advanced universal pendulum system used in studies III andIV.




In studies I, III, and IV, the water absorption of the WPCs wasdetermined from the injection-molded samples as outlined inISO 62. Sprues and runners were removed from the specimensto ensure that the specimens were similar in shape. A total ofthree samples of each material type was tested.

The specimens were first dried in a force-convection oven at50 ± 2 °C for 24 hours. The specimens were then allowed to coolto room temperature in a desiccator before they were weighedusing a Mettler Toledo AX205 –scale (Mettler-Toledo, LCC,Columbus (Ohio), United States). Immediately thereafter, thespecimens were immersed in distilled water (T = 21.0 °C) for 24and 48 hours. After the immersion, any excess water wasremoved from the surfaces of the specimens and they werereweighed within 1 minute of their removal from water. Thespecimens were weighed at least three times at each stage, withthe results being reported as the means of the separateweighings.

6.4 VOC EMISSIONS

The VOC emissions from the WPC samples were measuredusing a high-resolution PTR-TOF-MS (PTR-TOF 8000, IoniconAnalytik, Innsbruck, Austria) as presented in Figure 13. ThePTR-TOF-MS was operated under controlled conditions:2.3 mbar drift tube pressure, 600 V drift tube voltage and 60 °Ctemperature.



Figure 13. PTR-TOF 8000 used in studies II, III, and IV.

In study II, the VOC emissions from seven different WPC deckboards were determined. One of the decks (LunaComp) wasobtained from the manufacturer immediately after itsproduction to investigate the changes in the VOC emissionsduring the first 41 days. Starting from the first day after themanufacture, the VOC emissions for this sample werecharacterized 11 times over a 41-day period to monitor thechanges in the VOC emission rates. The sample was storedunder laboratory conditions between the measurements. Theages of the six remaining decks were unknown, but theyrepresented products that a consumer would use. As the WPCdeck samples were prepared from different products, theirshapes were irregular and dissimilar. Therefore, the VOCemission rates of these samples were determined with respectto their masses.

In studies III and IV, the VOC emission measurements werecarried out using the injection-molded specimens as samples.Five specimens of each material type were cut so that they haddimensions of 4.1 mm × 10.1 mm × 80 mm. The areas of thesamples were determined so that it was possible to convert theobtained emissions into area-specific emissions rates.



The emissions were determined using 1.5 L glass vessels asthe chambers. The glass vessels were prepared for themeasurements by cleaning the inner surfaces with detergentand then rinsing them with distilled water. Furthermore, toensure the purity of the chambers, the vessels were thermallycleaned by heating them in a force-convection oven at 120 °C forat least one hour. The tubes that introduce the air to the chamberand transport it to the PTR-TOF-MS system were installed onthe metal lid that covered the glass vessels. The air (RH < 5%)that was introduced into the chamber with a flow rate of 0.3L/min had been filtered with an active carbon/Purafil®/HEPAfilter. The air containing VOCs was then transported into thePTR drift tube via a polyether ether ketone (PEEK) tube at a totalflow rate of 0.1–0.3 L/min.

Before the samples were analyzed, the emissions from emptychambers were measured and this background was subtractedfrom the data during the analysis. Once the samples wereplaced in the chamber, the system immediately started to collectthe data. As the VOC emissions could be observed online, eachmeasurement was continued until the emissions of thecompounds of interest stabilized. This took approximately15 minutes for each material. After each measurement, thechamber was carefully flushed with purified air.

The obtained data were analyzed using PTR-MS Viewer3.1.0.27 software (Ionicon Analytik, Innsbruck, Austria). Theconcentrations were calculated by the program using a standardreaction rate constant of 2 × 10-9 cm3 s-1 molecule-1. The tracegases were detected from the spectral peaks assuming that themolecules consisted only of hydrogen, carbon, and oxygen. Thedetection of gases was further supported by the literature dataon the VOC emissions of wood materials and the matching ofthe isotope patterns, especially in cases where the detection wasambiguous. Table 4 lists the VOCs studied in each study.



Table 4. The VOCs studied in studies II, III and IV.VOCs Studies

Formaldehyde (m/z 31.018 CH3O+)Methanol (m/z 33.034 CH5O+)

Acetaldehyde (m/z 45.034 C2H5O+)Acetic acid (m/z 61.029 C2H5O2+)

Cyclohexene (m/z 83.0706 C6H11+)Furan (m/z 69.034 C4H5O+)

Benzene (m/z 79.0548 C6H7+)Furfural (m/z 97.1000 C5H5O2+)

Guaiacol (m/z 125.1233 C7H9O2+)Monoterpenes (m/z 137.1330 C10H17+ and 81.0704 C6H9+)

IIIII

II and IVII

II, III, and IVII

III and IVII, III, and IVII, III, and IVII, III, and IV

The VOC emission values calculated by the software wereexpressed as parts per billion (ppb). In studies III and IV, theseemission values were converted into emission rates (µg/m2h)according to the following equation:

Evoc = 0.0409 CvocFvocMvocAsample

, (6.1)

where Cvoc is the concentration of the VOC (ppb), Fvoc is the flowrate (m3/h), and Mvoc is the molar mass of the individual VOCmolecule (g/mol). The unitless constant 0.0409 was obtainedfrom the conversion of units using the ideal gas law at 1 atmpressure and 25 °C, and Asample is the area of the WPC sample. Instudy II, the emission rates were calculated with respect to themass of the samples; the emission rates were expressed asµg/kgh, therefore.

The emission rates were further converted into real room airconcentrations (µg/m3) using product loading factor (Lp), whichis equal to the sample surface area divided by the chambervolume. By applying this conversion, it was possible to comparethe VOC emissions of the material with the odor thresholds ofVOCs, and thus one could estimate whether a certain VOCcould be smelled. The real room air concentration is expressedas follows:

Creal room= EvocLp

n , (6.2)



where Evoc is the emission rate of VOC (µg/m2h), Lp is the loadingfactor of the sample (m2/m3) and n is the air exchange rate (1/h)in the chamber. In studies III and IV, Lp was calculated withrespect to the area of the samples, but in study II, sample masswas used to determine Lp. In study II, it was also assumed thatthe samples had similar compositions and shapes.

6.5 STATISTICAL ANALYSES

In studies I, III, and IV, the statistical analyses were performedusing Matlab R2013b (Mathworks, Natick, MA, US) and IBMSPSS Statistics 21 software (IBM Corp., Armonk, NY, US). Bothparametric and non-parametric statistical tests were considered.Mann-Whitney U test was found to be suitable for evaluatingthe statistical significance of the differences observed betweenthe material types when the number of specimens was morethan five. The limit for statistical significance was set at p � 0.05,and p < 0.01 was designated as high statistical significance.


7 Results

The most important findings from studies I–IV are summarizedin Table 5. The results will be presented in more detail in thefollowing chapters.

Table 5. A summary of the main findings in this thesis.

Study subject Studies The main findings

Impregnation of WPCgranules with the

distillatesI, III, and IV

The LG granules were successfullyimpregnated with the distillates.

Hardwood and softwood distillatesenhanced the processability of the WPC

granules.

Mechanical properties I, III, and IV

A small (1 wt%) addition of hardwooddistillate significantly increased the

tensile modulus of the WPC. A higherdistillate content (2–8 wt%) reduced

the mechanical properties of the WPC.

A minor (2 wt%) addition of softwooddistillate significantly increased the

tensile strength of the WPC. Strain andbending increased significantly with ahigh distillate content (over 4 wt%)whereas the strength of the WPC

declined.

Water absorption I, III, and IVThe WPCs containing wood distillates

absorbed less water than those withoutdistillates.

VOCs II, III, and IV

PTR-TOF-MS is an applicable method fordetermining VOCs from WPCs.

The VOC emissions from the WPCschange considerably as a function of

time.




The effects of wood distillates on the mechanical properties ofthe WPCs were determined in studies I, III, and IV. In study I,the addition of hardwood distillate did not improve themechanical properties of the WPCs studied. UF40 had thehighest flexural and tensile strength whereas the highestflexural modulus was detected for UF50 and the highest tensilemodulus for UF50 + LG50.

In studies III and IV, a minor addition (1–2 wt%) of wooddistillates significantly improved the mechanical properties ofthe WPC studied. In study III, the LG granules wereimpregnated with a similar hardwood distillate as used in studyI. The distillate content ranged from 1 to 8 wt%. In study IV,1–20 wt% of softwood distillate was added to the WPC. Theresults from studies III and IV are summarized in Table 6.

Tensile modulus increased highly significantly with 1 wt% ofhardwood distillate. Similar trends, although not statisticallysignificant, were observed for tensile and flexural strength andmodulus of elasticity.

The addition of softwood distillate had advantageous effectson the mechanical properties of the WPC in study IV. With2 wt% of softwood distillate, a highly significant increase wasobserved in the tensile strength. Another finding emerging fromstudy IV was that when the softwood distillate contentexceeded 4 wt%, statistically significant or highly significantincreases were observed in strain and bending.

Results


Table 6. The mechanical properties of the WPCs in studies III and IV (mean ±standard deviation). The underlined values indicate at least significant (p � 0.05)

difference in comparison with the other WPCs in the same study.

Property LG LG + HWD1 LG + HWD2 LG + HWD4 LG + HWD8

TS (MPa) 22.41 ± 0.82 22.85 ± 0.31 22.26 ± 0.30 22.05 ± 0.44 19.67 ± 0.52**

TM (GPa) 2.09 ± 0.20 2.33 ± 0.09** 2.24 ± 0.06 2.16 ± 0.06 1.86 ± 0.11**

� (mm) 1.86 ± 0.26 1.81 ± 0.12 1.83 ± 0.15 1.92 ± 0.17 1.85 ± 0.13

FS (MPa) 44.09 ± 2.00 45.27 ± 1.01 43.09 ± 0.83 42.86 ± 1.17 39.36 ± 0.63**

MOE (GPa) 3.05 ± 0.16 3.21 ± 0.08 3.11 ± 0.12 2.89 ± 0.09* 2.73 ± 0.09**

B (mm) 4.54 ± 0.28 4.42 ± 0.28 4.37 ± 0.30 4.72 ± 0.29 4.67 ± 0.27

CIS (kJ/m2) 11.57 ± 2.02 10.61 ± 1.27 11.17 ± 0.81 10.89 ± 1.13 9.42 ± 0.69*

Property LG + SWD1 LG + SWD2 LG + SWD4 LG + SWD8 LG + SWD20

TS (MPa) 21.13 ± 1.03*23.54 ± 0.66** 22.45 ± 0.81 19.51 ± 0.79** 15.46 ± 1.56**

TM (GPa) 1.98 ± 0.18 2.16 ± 0.06 1.96 ± 0.10 1.60 ± 0.10** 1.07 ± 0.23**

� (mm) 1.94 ± 0.12 2.07 ± 0.15 2.13 ± 0.12* 2.18 ± 0.14** 2.60 ± 0.39**

FS (MPa) 43.04 ± 2.48 45.47 ± 1.40 40.81 ± 4.08 39.39 ± 1.41** 32.80 ± 2.38**

MOE (GPa) 2.91 ± 0.28 3.10 ± 0.07 2.58 ± 0.33** 2.34 ± 0.15** 1.70 ± 0.29**

B (mm) 4.71 ± 0.21 4.73 ± 0.27 5.21 ± 0.35** 5.59 ± 0.30** 6.65 ± 0.71**

CIS (kJ/m2) 10.40 ± 1.52 12.08 ± 2.12 12.22 ± 1.65 11.26 ± 1.41 10.63 ± 1.14* p � 0.05 (a significant difference compared with the unmodified WPC).** p < 0.01 (a highly significant difference compared with the unmodified WPC).LG = LunaGrain, HWD = Hardwood distillate, SWD = Softwood distillate,TS = tensile strength, TM = tensile modulus, � = strain, FS = flexural strength,MOE = modulus of elasticity, B = bending, CIS = Charpy’s impact strength.


Water absorption of WPCs was determined in studies I, III, andIV. In study I, the addition of hardwood distillate did not haveany consistent effects on the amount of water absorbed by theUFs; for example, when distillate-treated LG was added to UF20,a minor increase was observed in water absorption, but theopposite effect was detected for UF30 and UF50. However,when 4.2 wt% of distillate was added to LG, water absorptionreduced by over 30%.

In study III, the addition of hardwood distillate considerablydecreased water absorption of the WPCs (Figure 14). The majordifferences occurred during the first 24 hours of immersion asthe differences between the material types remained more or



less the same after 48 hours even though there was an increasein the values for all the materials. After 48 hours of immersion,LG containing 8 wt% of hardwood distillate had absorbedapproximately 20% less water than unmodified LG.Accordingly, the difference between the unmodified LG and LGwith 1 wt% of hardwood distillate was about 10%.

The addition of softwood distillate did not decrease waterabsorption of the WPCs to the same extent as hardwooddistillate even though water absorption decreased as a functionof the distillate content (Figure 14). The difference betweenwater absorption of LG and LG + SWD20 was approximately16% whereas no difference was observed between LG and LGwith 1 wt% of distillate.

Study IV also shows that the difference in the waterabsorption values between LG and distillate-treated LGsdecreased after 48 hours of immersion. The WPCs containingsoftwood distillate absorbed more water between 24 and 48hours of immersion than those containing hardwood distillate.

Figure 14. Moisture content of the WPCs after 24 and 48 hours of waterimmersion in studies III and IV. HWD = hardwood distillate, and SWD = softwood

distillate.

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Results


7.3 VOC EMISSIONS

The VOC emission rates of a WPC deck determined in study IIare presented in Figures 15 and 16. The compounds of interestwere formaldehyde, acetaldehyde, acetic acid, cyclohexene,furan, furfural, guaiacol, and monoterpenes.

Decreasing trends of the emission rates were observed foracetaldehyde, furfural, and monoterpenes. Additionally, aminor decline was observed in the guaiacol emission rates.Formaldehyde and furan emission rates remained relativelystable during the experiment, but acetic acid emission ratesfluctuated, especially at the beginning of the trial. During thefirst 13 days of the experiment, cyclohexene emission ratesremained relatively stable. However, starting from the day 16,the emission rates of cyclohexene nearly tripled compared withthe values observed during the first 13 days.

The VOC emission rates of the seven different WPC decksdetermined in study II are presented in Figures 17 and 18. Oneof the decks (LunaComp 3) was also used in the 41-day trial.

Figure 15. The emission rates of formaldehyde, acetaldehyde, acetic acid, andcyclohexene from a WPC deck during a 41-day period.

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Figure 16. The emission rates of furan, furfural, guaiacol, and monoterpenesfrom a WPC deck during a 41-day period.

Figure 17. The emission rates of formaldehyde, acetaldehyde, acetic acid andcyclohexene from seven different WPC decks.

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Manufacturer 1 Manufacturer 2 UPM ProFi 1 UPM ProFi 2

LunaComp 1 LunaComp 2 LunaComp 3

Results


Figure 18. The emission rates of furan, furfural, guaiacol, and monoterpenesfrom seven different WPC decks.

An overview of the results reveals that the deck fromManufacturer 2 had the highest emission rates for cyclohexene,furan, furfural, and guaiacol. The highest formaldehydeemission rates were observed for the deck from Manufacturer 1.UPM ProFi had the highest acetic acid and acetaldehydeemission rates. Monoterpenes were most abundantly releasedfrom UPM ProFi 2. In general, LunaComp decks had the lowestVOC emission rates when compared with the othermanufacturers.

The emission rates obtained from the comparative studywere further converted into real room concentrations. Thisconversion revealed that acetaldehyde and guaiacol exceededtheir odor thresholds and therefore, it was likely that they couldbe smelled from the decks. The values for other VOCs were lowwith respect to their odor thresholds.

In study III, the effects of hardwood distillate addition on theVOC characteristics of the WPCs were assessed by determiningthe emission rates of cyclohexene, furfural, guaiacol,monoterpenes, methanol, and benzene (Figures 19 and 20).

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LunaComp 1 LunaComp 2 LunaComp 3



Figure 19. The emission rates of cyclohexene, furfural, and guaiacol from theWPCs treated with hardwood distillate.

Figure 20. The emission rates of monoterpenes, methanol, and benzene from theWPCs treated with hardwood distillate.

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Results


The addition of hardwood distillate increased the emission ratesof the studied compounds, especially for cyclohexene, furfural,monoterpenes, and methanol. Monoterpenes were mostabundantly emitted whereas only trace amounts of guaiacolwere detected. The emission rates of benzene were also low. Theconversion of the emission rates into real room concentrationsindicated that the odor threshold of guaiacol would be exceededfor all of these materials. The odor threshold for monoterpeneswas also exceeded when 8 wt% of hardwood distillate wasadded to the WPC.

Similar evaluations were done in study IV where WPCs weremodified with softwood distillate. The emission rates ofcyclohexene, furfural, guaiacol, monoterpenes, acetaldehyde,and benzene were determined (Figures 21 and 22).

The addition of softwood distillate clearly increased theemission rates of cyclohexene, furfural, and monoterpenes. Theemission rates of benzene and guaiacol, in turn, remained ratherlow although the odor threshold of guaiacol was exceeded in allof the materials. Acetaldehyde emission rates decreased belowthe odor threshold when softwood distillate content wasincreased from 1 wt% to 8 wt%. An increase in the distillatecontent from 8 wt% to 20 wt% resulted in a considerably higheremission rate of acetaldehyde, and as a consequence, the odorthreshold of acetaldehyde was exceeded.



Figure 21. The emission rates of cyclohexene, furfural, and guaiacol from theWPCs treated with softwood distillate.

Figure 22. The emission rates of monoterpenes, acetaldehyde, and benzene fromthe WPCs treated with softwood distillate.

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LG + SWD4 LG + SWD8 LG + SWD20


8 Discussion

In the present thesis, the effects of hardwood and softwooddistillates on the characteristics of commercial WPCs werestudied. The first objective of this thesis was to assess thesuitability of the impregnation method used to incorporate thedistillates into the WPC granules. The suitability of wooddistillates as additives in WPCs was determined by conductingmechanical tests, water absorption studies, and via acharacterization of VOC emission rates. In addition, theapplicability of PTR-TOF-MS for determining the VOCemissions from WPCs was assessed.

8.1 IMPREGNATION OF WPC GRANULES WITH WOODDISTILLATES

The impregnation of the WPC granules with the distillates wassuccessfully performed for the LG granules but not for the UFs.Commercially available granules were used in this thesisbecause the impregnation of the WPC granules with thedistillates was both straightforward and quick and there wereno suitable facilities with which to produce WPC granules fromraw materials. Commercialized WPC granules contain multipleadditives, and theoretically, these could have interfered withthe modifications investigated here. Nonetheless, the presentresults clearly demonstrated that wood distillates can be addedto certain types of WPC granules and this can improve theirprocessability during injection molding.

In future studies, the incorporation of distillates into WPCscould be carried out at an earlier stage of the WPC production.The impregnation of the agglomerate or wood particles with thedistillates could eliminate at least some of the limitationsassociated with the impregnation of WPC granules. For



example, the distillates could be directly incorporated onto thesurfaces of the WPC constituents. Moreover, this would permita more precise control of the amounts of other additives used inthe WPCs. It would also demonstrate whether wood distillatescould replace some of the additives currently used in WPCs.


The results from study I indicated that an increase in cellulosefiber content in UFs enhanced the modulus of elasticity andtensile modulus, a property attributable to the higher stiffnessand crystallinity of cellulose fibers compared to those of PP. Asimilar phenomenon, in accordance with the rule of mixtures,has also been observed in other studies (Balasuriya et al. 2002,Bouafif et al. 2009, Ashori et al. 2011). However, the maximumvalues of flexural and tensile strength for UFs were achievedwith cellulose fiber content of 40 wt%. A similar finding wasalso reported by Ndiaye et al. (2013), Bhaskar et al. (2012) andYuan et al. (2008). The decrease in flexural and tensile strengthwas probably due to the limited bonding between the cellulosefibers and the PP matrix. On the other hand, Huang and Zhang(2009) also postulated that when the fiber content exceeded 40wt%, WPCs became more susceptible to the formation of fiberagglomerates that decrease the mechanical strength of thecomposites.

When the distillate-treated LGs were added to UF20, UF30,and UF40, the resulting composites were mechanically weakerthan the untreated composites. However, the addition of LGand distillate-treated LG in UF50 exerted different effects; forexample, UF 50 containing LG or distillate-modified LGpossessed significantly higher flexural and tensile strengthsthan the unmodified material. These findings suggested that thedistillate improves the mechanical durability of WPCs onlywhen there is a higher fiber content (over 40%). On the otherhand, these results could also be attributed to the chemicalnature of the wood fibers in LG; as LG has thermally modified

Discussion


sawdust as its fiber material, i.e., the fibers are morehydrophobic, resulting in stronger interactions between thefillers and the polymer matrix – especially if there is a high fibercontent.

The LG composites had lower flexural and tensile strengththan the UFs. In addition, only UF20 had a lower modulus ofelasticity and tensile modulus than LG. As mentioned in section4.1, the degradation of wood components during the thermaltreatment process results in the formation of organic acids, suchas acetic acid, and these can catalyze the degradation of woodfibers. Therefore, composites having thermally modified wooddust as their filler have lower mechanical strengths thancomposites with unmodified wood fibers. Wood fiber size canalso have a significant effect on the strength of WPCs asdiscussed in chapter 3.1 and demonstrated by Bouafif et al.(2009) and Kociszewski et al. (2012). Optical microscopicanalyses (unpublished) for the samples in study I revealed thatLG had considerably larger fibers than UFs, which can alsopartly explain the differences in the results. Moreover, nodistillate agglomeration was observed. The energy required tobreak a specimen is known to be lower for those materialscontaining large fibers or fiber agglomerates because the crackstend to travel through the large particles (Griffith 1921, Weibull1951, Bouafif et al. 2009).

Incorporation of 4.2 wt% of hardwood distillate did notimprove the mechanical properties for LG. Since the distillateacts like a coupling agent or any other additive that enhancesthe bonding between wood fibers and polymer matrix, it ispossible that only a small amount of distillate would berequired to enhance the properties of WPCs. The optimumamount of coupling agents in WPCs has been reported to beapproximately 2 wt% (Balasuriya et al. 2002), and as mentionedin section 2.1.3, the typical amount of additives in WPCs is lessthan 5%. This theory was tested in study III, and the resultsrevealed that the mechanically strongest composite wasachieved when the hardwood distillate content was 1 wt%. Anincrease in tensile modulus is evidence that the composite had



become stiffer after the distillate addition. Mathematically, thetensile modulus is the slope of the stress-strain curve. Anincrease in the slope means that more force is required to strainthe material. The higher stiffness, which was also indicated bythe decreased strain and bending with the higher distillatecontent, could be attributed to the capability of hardwooddistillate to fill the gaps and pores in WPCs. This leads torestricted movement of the polymer chains within the material.The restricted movement of the polymer chains and the reducedductility also explains the reduced Charpy’s impact strength.On the other hand, as the distillate fills the pores in WPC, thismay increase the interfacial bonding between the polymer andwood fibers. Other studies have also revealed a correlationbetween material porosity and elastic modulus; as the materialbecomes less porous, elastic modulus increases (Patterson 2001,Bledzki et al. 2005b).

The possible inclusion of distillates into the polymer-fiberinterphase could also explain the improvements to themechanical properties of the WPCs. The distillates arechemically hydrophobic but they may be able to bind tohydrophilic wood fibers through mechanical interlocking. Onthe other hand, the chemical similarity between the polymermatrix and the distillates could create a rather strong polymer-distillate interface. The molecular entanglement between thepolymerized molecules in the distillates and the polymer chainsmay also account for the improved mechanical durability of theWPCs. Thus, the compositional differences between hardwoodand softwood distillate may affect the compatibilitycharacteristics and the interactions in the fiber-polymerinterphase. When the distillate content was higher than 4 wt%,the excess distillate may have agglomerated or otherwisedisrupted the interactions between the additives and otherWPC constituents. Thus, there was a reduction of the strengthof the WPCs. For more homogeneous distribution, the distillatesmay have to be added to the WPCs at the earlier stage of theproduction, which could also enhance the positive effect on themechanical properties of the material.

Discussion


Softwood distillate had distinct effects in comparison withhardwood distillate on the WPCs. The tensile strength of theWPC containing 2 wt% of softwood distillate was enhanced byover 5% compared to the unmodified WPC, and improvementsin some other properties were also observed. One of the mostclear distinctions between the WPCs with hardwood andsoftwood distillates was that the addition of hardwood distillatestiffened the composite whereas opposite effect was observedfor the WPCs supplemented with softwood distillate. Theimprovement in mechanical strength at 2 wt% distillate contentand a major increase in bending and strain at higher distillatecontents (over 4 wt%) indicated that softwood distillate mayimprove the interfacial properties of the WPC constituents.However, it was possible that unlike the hardwood distillate,softwood distillate did not restrict the movement of polymerchains within the composite.

The differences between the distillates can be attributed totheir compositional differences; the softwood distillate wasformed at lower temperatures (maximum temperature 215 °C)in ThermoWood® process whereas hardwood distillate wasobtained from a slow pyrolysis process where the maximumtemperature was approximately 350 °C. The softwood distillateprimarily consisted of hemicellulose degradation productswhereas the hardwood distillate was a mixture of cellulose,hemicellulose, extractive and lignin degradation products.Moreover, the composition of the distillates was affected by thefeedstock: softwood distillate was obtained primarily from pinewhereas hardwood distillate was acquired from the slowpyrolysis of birch. In general, hardwoods contain more cellulosethan softwoods, and the lignin content in softwoods is higherthan in hardwoods.


The positive effects of hardwood and softwood distillates onwater absorption of the WPCs were evident in studies III and IV.



However, no conclusions can be drawn on the effects ofhardwood distillate on water absorption of UFs because thegranules could not be successfully impregnated with thedistillate. As expected, and as shown previously by Ndiaye etal. (2013), water absorption of the composites increased as afunction of fiber content. However, water absorption did notconsistently decrease after the addition of distillate-treated LGs.An incompatibility of the raw materials may explain theinconsistent results as the wood-based fillers used in thecomposites are chemically different. Moreover, the distillatemay be more effective with WPCs that contain thermallymodified wood fibers as the reinforcing material.

The results from study I indicated that the extent of waterabsorption of LG was considerably lower than that of UF50 eventhough they possessed the same fiber content. This finding is inaccordance with other studies, and it can be attributed to the factthat thermally modified wood fibers are less hydrophilic thanpure cellulose fibers because of the degradation ofhemicelluloses and other changes occurring in the structure ofcellulose during the thermal modification process (Balasuriya etal. 2002, Ayrilmis et al. 2011). Water absorption of UF50 wasapproximately 0.63%, which was in agreement with the valuesfound in the literature; typical water absorption (24 h) for WPCswith a wood fiber content of 50–65 wt% has been reported to bein the range 0.7–2.0% (Klyosov 2007). According to themanufacturer (UPM), water absorption (ISO 62, 24 h) of UF40 is0.66% whereas water absorption of UF50 is 0.90%. Theconsiderably lower water absorption of the UFs used in thisthesis may be due to the differences in the manufacture of thesample. Moreover, it was unclear whether UPM used similarsamples in their tests.

Water absorption of LG can be further decreased by addinghardwood or softwood distillates; this was examined in studiesI, III, and IV. As discussed in the previous section, the distillatesmay fill the pores and voids in the WPC, and affect the fiber-polymer interface or interphase. Therefore, the penetration ofwater molecules into the hydrophilic wood fibers may be

Discussion


hindered (Oksman Niska and Sanadi 2008). This theory is alsoin accordance with the percolation theory presented by Wang etal. (2006). On the other hand, the hydrophobic nature of thedistillates could also explain the findings because bothdistillates were obtained from the water-insoluble fractions.Thus, the addition of distillates resulted in more hydrophobicWPCs that absorbed less water. In order to minimize waterabsorption of WPCs, high amounts of wood distillates shouldbe used, but this would result in poorer mechanical properties.

Hardwood distillate (study III) decreased the waterabsorption of LGs more efficiently than softwood distillate(study IV). This might be due to compositional differences of thedistillates but the different densities of the composites mightalso affect the water absorption. Namely, the composites treatedwith hardwood distillate had higher densities than the onestreated with softwood distillate. A higher density of the WPCindicates lower porosity (Klyosov 2007). Thus, WPCs with ahigh density are less prone to the water absorption than the low-density WPCs. In addition, the water absorption as well as thephysical and mechanical properties of WPCs are alsoprominently affected by the manufacturing technology and theprocess parameters (Yeh and Gupta 2008, Migneault et al. 2009).

8.4 VOC EMISSIONS

The emission rates of a WPC deck in the 41-day trial showedthat acetaldehyde, furfural, and monoterpenes were thecompounds most abundantly emitted. However, a trendtowards decreasing emissions could be identified for thesecompounds. A similar decrease, but not to the same extent, wasalso observed for guaiacol. Furan and formaldehyde wereemitted at a rather stable rate throughout the trial, but theemission rates of acetic acid and cyclohexene changedinconsistently. The results, therefore, indicate that the emissionsof WPCs fluctuate extensively after their manufacture and



furthermore they can change considerably during the storagedepending on the conditions.

The emissions of acetic acid and aldehydes from the WPCwere not surprising as these compounds are formed during thethermal modification and extrusion processes (Peters et al. 2008)and they originate primarily from the degradation ofhemicelluloses. However, the emissions of these compoundsneed to be carefully monitored because they have harmfuleffects on indoor air quality and they can damage human health.Acetic acid is an irritant compound with a rather low odorthreshold (Akakabe et al. 2006). Formaldehyde andacetaldehyde, in turn, are carcinogenic compounds withmutagenic and irritant properties (Silla et al. 2001). The emissionrates of formaldehyde remained low during the trial; those ofacetaldehyde decreased substantially after 41 days, but no signsof any trend-like behavior could be identified for acetic acid.Low formaldehyde emission levels have also been reported forwood (Roffael 2006). The proton affinity of formaldehyde isonly slightly greater than that of water, which may lead toreverse proton transfer reactions between the protonatedformaldehyde and water molecules (Hansel et al. 1995, Schrippet al. 2010). For this reason, it is possible that not all of theformaldehyde was detected by PTR-TOF-MS. On the otherhand, the TD-GC-FID/MS-system has similar limitations asdescribed in section 3.3.1.

Hyttinen et al. (2010) obtained similar results when theyevaluated the emission rates of acetic acid in their comparisonof VOC emissions between air-dried and heat-treated woodspecies. They postulated that the fluctuations in acetic acidemissions could be caused by the diffusion of the compoundfrom the inside of the material to the surface if it was not evenlydistributed within the sample. A similar explanation could bealso the case for WPCs. Yrieix et al. (2010) analyzed VOCs fromwood-based panels and observed that acetaldehyde emissionsdecreased by over 62% during their 25 day sampling period. Inaddition, they did not detect any major changes informaldehyde emissions during the trial. Even though their

Discussion


findings mirror the results obtained in study II, the materialthey used is not fully comparable to WPCs. Particleboards andwood-based panels sometimes contain formaldehyde-basedresins that can dramatically alter the emission characteristics.This could also be observed in their results; the emissions offormaldehyde were over ten times higher than those ofacetaldehyde after 28 days. Contrasting results were obtained instudy II: the emissions of acetaldehyde were approximately 3times higher than those of formaldehyde after 41 days.

Cyclohexene is an irritant to humans and it is considered tobe at least partly responsible for the unpleasant odor of WPCs.Cyclohexene and its derivatives are present in the essential oilsof multiple wood species (Amoore and Hautala 1983,Chowdhury et al. 2008, Peters et al. 2008). The emissions ofcyclohexene remained stable during the first 13 days of the trial.However, the detected emissions nearly tripled on day 16 andremained at that level for the rest of the trial. The emissions ofcyclohexene from WPCs have not been studied previously, sothere is no supporting information for this finding. There areseveral reasons to account for the unexpected change on day 16.For example, the unique release kinetics of cyclohexene canexplain the result. In addition, due to the extremely highsensitivity of PTR-TOF-MS, one cannot completely rule out thepossibility that some minor loosening or movements of theKapton® tape may have been responsible for the change in theemissions. However, this is unlikely because one would haveexpected to detect similar changes in the emission rates of otherVOCs.

Furan and guaiacol have similar odor characteristics andthey are formed in similar processes. They have odorsreminiscent of burning wood, and both are formed during thethermal treatment. (Goldstein 2002, Greenberg et al. 2006,Aigner et al. 2009) Furan has been classified as a possiblecarcinogen with a high odor threshold (Bakhiya and Appel 2010)whereas guaiacol has a very low odor threshold with no directhealth effects. The detected emission rates for furan were lowthroughout the trial, and there were no major changes observed



in the emission rates. Therefore, the results suggested that furandoes not contribute any major effects to the VOC characteristicsof WPCs. Guaiacol was also emitted to a low extent, but it ispossible that it can be smelled from WPCs due to its low odorthreshold.

Furfural was emitted most abundantly in the beginning ofthe trial. However, as the trial continued, the emission rates offurfural decreased continuously. Previously, the emissions offurfural from heat-treated wood species have been investigatedby Hyttinen et al. (2010). They demonstrated that heat treatmentcaused the formation and higher emissions of furfural. Incontrast to the present findings, the emission rates of furfuraldid not decline during the test period in their study – in contrast,the emission behavior of furfural resembled that of acetic acid.Furfural is formed from the degradation of hemicelluloses andcellulose, and therefore, it is possible that the degradationprocess had continued in the wood samples during themeasurements. The inconsistent emissions rates may be causedby the uneven diffusion of furfural. The plastic matrix in WPCsmay decelerate the diffusion of furfural from the inside of thematerial to the surface, and therefore, the detected emissionrates constantly declined.

Monoterpenes, such as �- and �-pinene, are abundant inwood (Roffael et al. 2015). They have a pleasant resinous aromawith pine-like odors, are highly repellant to insects and possessantioxidative properties (Goldstein 2002, Nerio et al. 2010, Silvaet al. 2012). Monoterpenes do not cause major changes in therespiratory tract or lung function, but they have evoked chronicreactions in the airways (Eriksson and Levin 1990, Falk et al.1990, Eriksson et al. 1997). The presence of monoterpenes istherefore permissible only to a certain extent. Monoterpeneemission rates decreased during the trial. Yrieix et al. (2010) alsoobserved a decrease in the monoterpene emissions from theirwood-based panels. Similarly, Roffael (2006) reporteddecreasing emission rates for monoterpenes from wood.

LunaComp decks, that have thermally modified sawdust asa reinforcement material, had the lowest VOC emission rates in

Discussion


the comparative study of the seven different WPC decks. Ingeneral, thermally modified wood has lower VOC emissionsthan unmodified wood, although there is greater release ofsome irritating compounds, such as acetic acid (Manninen et al.2002). This could not be observed in the present experiments,but it was probably due to the different ages of the decks.Moreover, possible variations in surface quality of the samples,the sample contamination and changes in the production canexplain the differences. This comparison, however, providedsome interesting aspects. When the emission rates wereconverted into real room concentrations, the results suggestedthat acetaldehyde and guaiacol, which have low odorthresholds, could be smelled from the decks.

In study III, methanol was monitored because it is a toxichydrocarbon that is abundantly emitted from various plantspecies (Rinne et al. 2007). It is found especially in woodpyrolysis liquids (Fagernäs et al. 2012a). As expected, theaddition of hardwood distillate lead to the increased overallVOC emissions. The composition of distillate could explain thefindings; most of the compounds monitored in study III areformed through the thermal degradation of the woodcomponents. The slow pyrolysis oil from hardwood has beenextensively analyzed by Fagernäs et al. (2012a), and theirfindings support the present results. The emission rates ofbenzene and guaiacol remained low, even when there was ahigh (8 wt%) distillate content, indicating that these compoundswere not abundantly present in the hardwood distillatepresumably due to relatively low process temperature. It is alsopossible that the plastic matrix in WPCs reduces the diffusion ofthese compounds within the material. It is desirable that thereare low emission rates of these compounds as benzene is acarcinogenic compound and guaiacol has an unpleasant odor.The odor of monoterpenes (�-pinene) was detectable with8 wt% of distillate.

The effects of softwood distillate on the VOC characteristicsof the WPC were different when compared with the hardwooddistillate, which could be explained by the compositional



differences between the raw materials (Schwarzinger et al. 2008).A comparison of the VOCs studied showed that monoterpeneswere the compounds being emitted most from the hardwooddistillate modified WPCs whereas cyclohexene was emittedmost abundantly from the softwood distillate modified WPCs.Surprisingly, monoterpene emissions were higher from thehardwood distillate modified WPCs than from the WPCscontaining the softwood distillate. In general, hardwoodsmainly contain sterols, triterpenoids, and higher terpeneswhereas softwood terpenes possess mono-, sesqui-, andditerpenes together with sterols (Björklund Jansson andNilvebrant 2009). It was, therefore, assumed that the WPCstreated with softwood distillate would have highermonoterpene emissions than those modified with hardwooddistillate. As terpenes are constructed from units of isoprene, itis possible that the higher terpenes in hardwood had thermallydecomposed during the slow pyrolysis process, which resultedin the formation of lower terpenes, such as monoterpenes.Yadav et al. (2014) demonstrated that isoprene is one of theproducts of the thermal decomposition of terpenes.

Both hardwood and softwood distillates contained onlysmall amounts of guaiacol and benzene even though guaiacolcould be smelled from the distillate modified WPCs.Interestingly, when the softwood distillate content wasincreased from 1 wt% to 8 wt%, the emissions of acetaldehydedeclined. This also affected the odor characteristics of the WPCsas acetaldehyde could not be smelled from the WPCs with2–8 wt% of distillate. This finding suggested that the distillatecontains chemicals that react with acetaldehyde to form othercompounds.

There are no statutory limits for the material VOC emissionsdetermined with PTR-MS. Thus, no conclusions could be madeabout whether the emission levels of a certain VOC exceed itssafety limits. Due to the extremely high sensitivity of PTR-TOF-MS, the emission rates presented in this thesis may beconsiderably higher than the levels that would be estimatedwith the standardized chamber test method. However, the

Discussion


comparison of VOC emissions between different material typesusing PTR-TOF-MS is feasible and sometimes more practicalthan the traditional chamber test method.

8.5 LIMITATIONS AND FUTURE PROSPECTS

In this thesis, the effects of hardwood (birch) and softwood(primarily pine) distillates on the properties of WPCs weredetermined. Similar studies have not been previouslyconducted, even though the potential of other types of organicwaste and residues as additives for WPCs has been extensivelyassessed (Hamzeh et al. 2011, Li et al. 2014, Madhoushi et al.2014, Das et al. 2015a). This thesis focused on the determinationof the effects of different wood distillates on the mechanicalproperties, water resistance and VOC emissions of WPCs.

The positive effects of both distillates on the mechanicalproperties of the WPCs were evident, and the underlying reasonfor the enhancement was anticipated to be the improvement inthe interfacial bonding between the polymer matrix and woodfibers. Further investigations of WPCs with scanning electronmicroscopy (SEM) and spectroscopic techniques would providemore detailed information on the interactions occurringbetween WPC constituents before and after the additions of thedistillates.

The considerably lower water absorption values for theWPCs treated with distillates were attributed to thecomposition of the distillates and to the ability of the distillatesto fill the gaps within the WPCs. It was assumed that thedistillates were hydrophobic as they were obtained from the tarphases and all the water-soluble compounds had been extractedfrom the distillates. However, the exact compositions of thedistillates were not characterized. The chemical composition ofbirch distillates has been determined by Fagernäs et al. (2012a),and it was presumed, based on our other studies, that thedistillate used in this thesis would be composed of similarchemical compounds. The composition of softwood distillates



was known only roughly and the analyses conducted in otherpublications were also used as supporting material (Vitasari etal. 2011, Miettinen et al. 2015, Özbay et al. 2015). Nevertheless,more information on the exact chemical composition of thedistillates could provide further insights into the chemicalinteractions between distillates and WPC constituents andexplain the phenomena observed in this thesis more rigorously.

In this thesis, water absorption of the WPCs was studied for24 and 48 hours. Considerable differences between differentmaterial types were observed even at these time intervals, but itwould be interesting to examine the differences after multipleweeks or months. It would also be interesting to determine theimpact of cyclic weather conditions or microbial exposure onthe properties of WPCs treated with wood distillates.Nonetheless, the results of this thesis suggest that the distillateshad improved the water resistance of WPCs, especially at highdistillate contents. The decreased water absorption of WPCsalso suggests that the addition of wood distillates couldimprove the fungal resistance of WPCs, and this propertyshould be evaluated in future studies.

PTR-TOF-MS was tested to assess its suitability fordetermining the VOC emissions from WPCs. The applicabilityof PTR-TOF-MS for monitoring the concentrations of VOCsemitting from WPCs was confirmed but further analyses wouldbe appropriate. In this thesis, the number of monitored VOCswas limited to ten compounds. It is apparent that furtheranalyses of VOCs are needed. For example, the monitoring ofhazardous VOCs, such as styrene, toluene, and naphthalene,could provide valuable information on the impact of WPCs onindoor air quality.

In study I, two kinds of WPCs were used whereas only onekind of WPC was evaluated in studies III and IV. In the future,examinations of the effects of wood distillates on thecharacteristics of other types of WPCs are appropriate. Wooddistillates may act differently if the polymer matrix or woodfiber type is changed. Furthermore, changes in the wood fiber

Discussion


content may lead to considerable changes in the properties ofWPCs modified with wood distillates.


9 Summary andconclusions

In the present thesis, the effects of thermally extracted wooddistillates on the properties of WPCs were successfullydetermined. The effects of the distillates were investigatedthrough the mechanical tests, water absorption studies, andVOC emissions analyses. The main conclusions with respect tothe aims are:

1. The impregnation of the WPC granules with the wooddistillate, originated from either hardwood or softwood, ispossible with the presented method. However, othermethods, such as impregnation of the agglomerate beforethe granulation process, could well be more suitable. Theaddition of distillates improved the processability of theWPC granules.

2. PTR-TOF-MS can be applied to determine and compare therelease of VOCs from WPCs. The advantages of PTR-TOF-MS include its rapid time response, high sensitivity, andease of use. In addition, there is no need for special samplepreparation. However, there are no regulatory limits forVOC emissions measured with PTR-MS.

3. A small (1 wt%) addition of hardwood distillatesignificantly increased the tensile modulus of the WPC.Higher distillate contents (2–8 wt%) reduced themechanical properties of the WPC. In addition, the abilityof the material to absorb water was considerably decreasedin those containing the hardwood distillate. The VOCemissions increased for the WPCs containing hardwooddistillate.

4. A minor (2 wt%) addition of softwood distillatesignificantly increased the tensile strength of the WPC.



Strain and bending increased significantly when there wasa high distillate content (over 4 wt%) whereas the strengthof the WPC declined. The water absorption of the WPC canbe reduced with distillate content higher than 2 wt%. Theaddition of softwood distillate increased the release ofVOCs studied.

To conclude, the studies presented in this thesis emphasize thatthe inclusion of wood distillates originating from industrialwood processes into WPCs can enhance the properties of thesematerials. In addition, it was shown that PTR-TOF-MS is afeasible technique for analyzing VOCs being emitted fromWPCs. The further developments in wood distillates and thediscovery of new ways to incorporate the distillates into WPCscould provide novel and ecological materials based more on therenewable resources.


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ISBN 978-952-61-2123-9ISSN 1798-5668


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TANELI VÄISÄNEN

EFFECTS OF THERMALLY EXTRACTED WOOD DISTILLATESON THE CHARACTERISTICS OF WOOD-PLASTIC COMPOSITES


Wood-plastic composites (WPCs) represent an ecological alternative to conventional petroleum-derived materials. The wood

distillates studied in this thesis displayed good potential as bio-based additives for WPCs as they improved the water resistance and mechanical properties. It was also shown

that proton-transfer-reaction time-of-flight mass-spectrometry (PTR-TOF-MS) can be

applied to study the release of volatile organic compounds (VOCs) from WPCs.

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Dissertations in Forestry and Natural Sciences · strength and their susceptibility to excess water...

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