doi: 10.1007/978-3-319-32101-1 106€¦ · alkoxysilanes, polysiloxanes, colloidal silica, or...

Sol-Gel Wood Preservation 97Thomas H€ubert and Muhammad Shabir Mahr

ContentsIntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2796Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2798

Wood and Chemicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2798Wood Impregnation Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2801Determination of Structure and Properties of Impregnated Wood . . . . . . . . . . . . . . . . . . . . . . . . . 2803

Structure of Sol-Gel Impregnated Wood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2803Properties of Sol-Gel Modified Wood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2804

Mechanical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2804Weathering Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2805Sol-Gel Based Wood Preservation for Improved Biodurability . . . . . . . . . . . . . . . . . . . . . . . . . . . 2811Sol-Gel-Based Wood Preservation for Improved Fire Retardancy . . . . . . . . . . . . . . . . . . . . . . . . 2820

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2833References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2835

AbstractThe sol-gel-based modification of wood introduces chemical substances intowood in order to improve its characteristics and impart new properties. It stabi-lizes dimensions of wood (timber) components, increases its strength and resis-tance to water, and reduces cracking. Many sol-gel-based impregnations aim toprotect against wood rot and fire. In most cases, the treatments are performed withalkoxysilanes, polysiloxanes, colloidal silica, or organically modified silica. Inaddition further substances such as titania, copper, and boron compounds havebeen applied on different types of wood. The precursor solutions were introduced

T. H€ubert (*) · M. Shabir MahrFederal Institute for Materials Research and Testing (BAM), Berlin, Germanye-mail: [email protected]

# Springer International Publishing AG, part of Springer Nature 2018L. Klein et al. (eds.), Handbook of Sol-Gel Science and Technology,https://doi.org/10.1007/978-3-319-32101-1_106

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by immersion, painting, or spray coating of wood followed by a drying and heattreatment process. The structure of the impregnated wood was investigated bySEM, EDX, TEM, FT-IR, NMR, and XRD. Frequently, test procedures accordingto standards were applied in order to assess the improvement in properties.Investigations demonstrate that silica and titania impregnations change propertiesfirst of all with increasing the amount of absorbed inorganic material (WPG),which is determined by the concentration of precursors, amount and size ofparticles in the sol, and the impregnation method. Sol-gel impregnation can beconsidered as an environmentally friendly approach of wood preservation. Var-ious improvements in wood properties can be achieved such as reduced wateruptake and volume swelling, improved weather stability, resistance against bio-degradation, and fire retardancy.

Introduction

Wood is a porous and fibrous tissue of plants and is composed mostly of thebiopolymers, cellulose, hemicellulose, and lignin. Additionally it contains so-calledextractives, such as resins, fatty acids, waxes, and terpenes. One can distinguishsapwood as the younger, living outer wood part of a growing tree from heartwoodwhich has formed as a result of naturally occurring transformation and consists in thefinal form of dead cells. Early wood is formed earlier in the growth season and hasthin-walled cells with large cell cavities (so-called lumen), whereas latewood hasthick-walled cells with very small cavities and it is denser, harder, smoother, anddarker than early wood. These differences in morphology and widths of latewoodhave been shown to affect wood properties, weathering, and bio-decay. Furthermore,wood properties are anisotropic in relation to the main growth direction of the treetrunk and are axial (or longitudinal), radial, and tangential in 3D space. Theanisotropy is due to the orientation of the wood cell, preferred in axial direction.

Wood has been used by mankind for thousands of years due to its numerousfavorable properties for building homes, bridges, fences, ships, and many otherstructures. Wood is available in many species, sizes, and shapes. Most people arefamiliar with its aesthetic inherent beauty. It is an easy to work and handle material.When dry, wood has good insulating properties against heat, cold, sound, andelectricity. It is low cost, sustainable, environmentally friendly, and renewable.

Unfortunately, many types of wood have some drawbacks in their application,such as instability against weathering due to moisture, rain, and ultraviolet radiation.Wood is combustible when provided with adequate heat and oxygen. In fact, it is themost widely used fuel in many parts of the world. Wood has a porous structure, andthe biopolymers can be served as foodstuff for fungi, insects, bacteria, and marineborers resulting in biodegradation and loss of its functionality. Even though sometypes of wood are quite resistant against biodegradation due to their inherentcomponents, usually, wood preservation is needed. The aim of contemporarywood preservation is to protect wood from weathering, photodegradation (graying),fire, and wood pests. This can be performed in a constructive way, e.g., by avoiding

2796 T. H€ubert and M. Shabir Mahr

contact with water. However, this cannot always be excluded. Therefore, wood canbe classified in respect to its uses into five classes:

1. Under cover, not exposed to the weather and wetting.2. Under cover and not exposed to the weather (particularly rain and driven rain),

but not persistent, wetting can occur.3. Above ground and exposed to the weather (particularly rain).4. Indirect contact with ground and/or fresh water.5. Seawater contact (EN 335: 2013).

These different application classes imply consequences for the needed protectionof wood (EN599: 1996).

Chemical wood preservation has been known since long term. Since ancienttimes, olive oil and tar were used for wood preservation. In the nineteenth century,railroad ties and telegraph poles were immersed in creosote, a product of coal tardistillation. Since then, various substances and their mixtures have been suggested,investigated, and commercialized. However, the use of wood preservatives isambivalent, because poisoning substances are used for protection of wood whichmay leach out to the environment and have impact on animals and plants. Therefore,some substances are nowadays in part prohibited to use, such as chlorinatedorganics, arsenic, or chromium(VI) compounds. In order to protect wood fromfire, inorganic substances such as aluminum hydroxide and organic compoundscontaining phosphor or halogens were used. Protection against graying can beachieved by the application of UV-absorbing substances, such as benzophenone orzinc oxide.

This chapter focuses on wood protection by no or scant colored impregnationswhich preserve the original wood surface instead of hiding it under a thick opaque orin-transparent pigmented layer. Paints based on a polymeric matrix which cancontain sol-gel-derived pigments and fillers are not considered in the following.

Today’s impregnations for increasing stability toward photodegradation,bio-decay, and flammability are limited in durability and environmental compatibil-ity. Therefore, further research and development has been carried out. Contemporarywood protection should pursue a multiple approach to improve the propertiesaltogether in relation to the intended application of this material (see Fig. 1). Apromising approach of a multifunctional protection and improvement of woodproperties is sol-gel technology based on compounds which contain in particularnontoxic elements such as silicon or titanium. However, a benchmark for innovationis the approved commercial wood preservations.

First attempts to improve fire retardancy of wood by sol-gel technology startedalready in the nineteenth century by application of potassium water glass (Fuchs1825). Systematic investigations in this area were performed by Furuno in the 1980sof the last century who investigated the silica mineralization of wood and performedtreatments with aqueous silicate solutions (Furuno et al. 1986, 1991). Schneider andSaka impregnated wood by several alkoxysilanes (Schneider and Brebner 1985;Saka et al. 1992). This approach is inspired by a natural process of the formation of

97 Sol-Gel Wood Preservation 2797

silicified wood. Silicified wood develops over millions of years through the infiltra-tion of silicic acid into the wood tissue. A silica gel is formed by polycondensation,which further reacts to form quartz (chalcedony) and opal (wood opal).

Several researchers have applied sol-gel technology to impregnate wood withinorganic (mostly oxide) or hybrid (organic/inorganic) materials. On this topic, morethan 70 papers were published between 2010 and 2016. An overview of sol-gel-derived approaches for wood modification is given in Table 1 (see page 2799).However, their technical use and commercialization are still pending. This is becauseof a long-term process for commercial approval. In many cases, these sol-gel-basedimpregnations were called wood composites because of the presence of relativelyhigh amount of new ingredients in wood and the resulting change in properties.However, it should be considered that these impregnations are not always chemicallymodifying wood. A strong chemical bonding between the dispersant and the matrixis not likely; mostly the wood structure is preserved.

Experimental

Wood and Chemicals

Several types of wood (mostly sapwood) are used for sol-gel-based impregnations,often in respect to the sphere of activity of the investors. Examples are pine, spruce,larch, birch, beech, aspen (poplar), robinia, oak, and exotic wood species such asJapanese cedar and cypress, Chinese fir, teak, rubberwood, bamboo, white lauan,yemane, eucalyptus wood, and Brazilian pine.

Fig. 1 Targets of contemporary wood preservation


Table 1 Precursors for wood impregnation

No Precursor

Hydrolysis/

decomposition/

solvent

Application/

target property Wood Reference (selection)

1 Silicates (water glass)

Lithium, sodium, or potassium

silicate solutions

Additionally: Al2(SO4)3, CaCl2,

BaCl2·2H2O, B2O3, H3BO3,

Na2B4O7·10H2O, K2B4O7·4H2O,

(NH4)2O·5B2O3·8H2O

Basic,

water, ethanol

Biocide

(fungi and termites)

Fire retardant

Japanese cedar (Cryptomeria japonica)

Monterey pine

Brazilian pine (Araucaria angustifolia)

Furuno et al. 1991

Furuno et al. 1992

Furuno and Imamura 1998

Marney et al. 2008

Pereyra and Giudice 2009

Lithium silicates + dibutyl amine

phosphate + ZnSO4

Water Fire retardant Monterey pine (Pinus radiata) Canosa et al. 2011

Metyltrimethoxysilane (MTMOS),

Sodium methoxide,

Sodium acetate

Acidic (HAc) or

alkaline

Fire retardant Western hemlock (Tsuga heterophylla) Miyafuji and Saka 2001

2 Pure silica sol, silica emulsion

Silica sols, silicic acid Basic,

water

Biocide Japanese cedar Yamaguchi 1994a

Yamaguchi 1994b

Yamaguchi 2002

Tetraethoxysilane (TEOS) Acidic

Basic

Water uptake

Dimensional stability

Fire retardant

Hardness

Biocide

Poplar (Populus ssp.)

Scots pine (Pinus sylvestris)

Monterey pine, Castanospermum australe

Saka et al. 1992;

Cookson et al. 2007

Mahltig et al. 2008

Unger et al. 2013

3 Silica + salts or oxides

Colloidal silicic acid + H3BO3 Water,

ethanol

Biocide (termites)

Fire retardant

Japanese black pine (Pinus thunbergii )

Western hemlock

Pine

Yamaguchi 2003

Böttcher et al. 1999

Silica + H3BO3

or + CuCl2, ZnCl2, FeCl2, CoCl2

Water Biocide (brown-rot

fungi)

Japanese cedar Yamaguchi 2002

SiO2 + ammonium salt (TMSAC) Acidic (HAc),

ethanol

Biocide Hinoki (Chamaecyparis obtusa) Tanno et al. 1998

Silica + aluminum oxychloride Water Water uptake

Biocide (blue strain)

Scots pine,

beech

Pries and Mai 2013b

4 Organo-functional silane

Decyltrimethoxysilane,

Hexadecyltriethoxysilane

n-heptane Water repellent

Fire retardant

Saka and Ueno 1997;

Miyafuji et al. 1998

HDTMOS + MTMOS Acidic (TFA) Water repellant

Weathering

Loblolly pine (Pinus taeda L.) Tshabalala and Gangstad

2003

Metyltriethoxysilane (MTES)

Propyltriethoxysilane (PTEO)

Tetrapropoxysilane (TPOS)/

Propyltriethoxysilane (PTEO)

Acidic (HCl),

Ethanol,

Water

Water uptake,

Anti-swelling

Scots pine,

Beech (Fagus sylvatica)

Donath et al. 2004

Methyltrimethoxysilane (MTM)

Octyltriethoxysilane (OTES)

Triethoxysilane (TES) +

polydimethylsiloxane (PDMS)

Methoxy-terminated

dimethylphenylsiloxane (DMS) +

N-octyltriethoxysilane (n-OTES)

Water,

Solvent

Biocide

Discoloration

Pine,

Beech

DeVetter et al. 2009b

PDMS/(n-O)TES,

Methyltrimethoxysilane (MTM)

Water,

Solvent

Water repellent,

Anti-swelling,

Biocide (fungi)

Pine

Beech (Fagus sylvatica)

De Vetter et al. 2010

Pries et al. 2013

HDTMOS + TEOS Basic (NH4OH), Water repellant Chinese fir (Cunninghamia lanceolata) Chang et al. 2015

Methylmethoxydisiloxane +

methyltrimethoxysilane +

trimethylborate (TMB)/H3BO3 +

trimethyphosphite (TEP)/H3PO4

trimethylphosphite (TMP)

Water Fire retardant Western hemlock Miyafuji et al. 1998

Vinyl- or (3-mercaptopropyl)-

trimethoxysilane and an organically

modified zirconium-oxocluster

Acidic,

THF

Biocide (brown rot)

Fire retardant,

Discoloration

Pine,

Larch

Maggini et al. 2012

Girardia et al. 2014

TEOS+MTMOS, 2-heptadeca-

fluorooctyl-ethyltrimethoxysilane

(HFOETMOS)

Acidic (HAc)

Ethanol,

Methanol

Water repellant,

Water uptake

Western hemlock Miyafuji and Saka 1999

TEOS + 2-heptadecafluoro-

octylethyltrimethoxysilane

(HFOETMOS) +

propyldimethyloctadecyl

ammonium chloride (TMSAC)

Acidic (HAc),

Ethanol

Methanol

Water repellent

Biocide

Hinoki Tanno et al. 1998

Silica + perfluoroalkyl methacrylic Water Water repellent Pine (Pinus taiwanensis) Hsieh et al. 2011

copolymer

TEOS +

dimethyldiethoxysilane,

perfluoorooctyltriethoxysilane

(PFOS)

Phenyltriethoxysilane (PHTES)

Acidic (HCl)

Ethanol, water

Water repellent Larch (Larix decidua),

Pine

Cappelletto et al. 2013

Chlorotrimethylsilane (CTMS)

octadecyltrichlorosilane (OTS)

dichlorodiphenylsilane (DPS)

dichlorodimethylsilane (DDS)

Acidic (TFA)

n-heptane

Water repellent

Water uptake

Apple (Malus sylvestris),

Eucalyptus,

Spruce (Picea abies)

Mohammed -Ziegler et al.

2008

TEOS + octadecyltrichlorosilane

(OTS) + polyvinyl acetate (PVA)

Basic (NH4OH)

Ethanol

Water repellent Poplar Liu et al. 2013

TEOS +

3-aminopropyltriethoxysilane

(APTES), + CuCl2/+ H3BO3

Aminopropylmethyldiethoxisilane

Ethanol

Basic (NH4OH)

Bio attack

Fire retardant

Scots pine

Brazilian pine (Araucaria angustifolia)

Palanti 2012b

Giudice et al. 2013a

Alkoxisilane +

Amino/alkyl functionalized

siloxane

Acidic (HCl),

Ethanol,

Water

Water uptake Scots pine Donath 2006

(continued)


For sol-gel-based investigations, commercial colloidal sols prepared from hydro-lysis of alkoxides or originated on water glass were available. Alternatively, theinvestigators prepared their precursor solutions by themselves from a broad varietyof available alkoxides, siloxanes, and further components1.

Frequently used solvents are water, ethanol, or isopropanol. Hydrolysis wasperformed in acidic or alkaline media using, for example, acetic and hydrochloricacid, or sodium hydroxide or ammonia.

Precursor compositions for silica-based sols have an alkoxide/water molar ratiosof <1–4 and alkoxide/solvent molar ratios of 1–120. The resulting particulatematerial content in the sols is 1–20%. However, precursor preparation can be carriedout in steps: firstly, silica particles were synthesized via a Stöber process in basicmedium and then organo-functional silanes were added and subsequently hydro-lyzed under acidic conditions for surface modification of nanoparticles and their

Table 1 (continued)

Alkoxysilane +

trimethylborate (TMB)

trimethylphosphite (TMP)

Fire retardant Miyafuji and Saka 1996

TEOS + APTES +

copper sulfate (CuSO4) or H3BO3

TEOS/silicic acid + H3BO3

Ethanol Biocide (termites) Scots pine Feci et al. 2009

MTMOS + HDTMOS + Aluminum

isopropoxide (AIP)

Acidic (TFA)

Isopropanol

Photodegradation,

UV resistant

Tshabalala 2007

TEOS+

2,2’,4’-trihydroxy-4- [2-hydroxy-3-

(3-trimethoxysilylpropoxy)

propoxy]

benzophenone (BP)

Acidic (HAc) UV stability Western hemlock Miyafuji 2004

TEOS + 3-(trimethoxysilyl) propyl

octadecyl ammonium chloride

(TMSAC)

Acidic (HAc)

THF,

n-butylacetate

Biocide Hinoki Tanno et al. 1998

Polydimethylsiloxanes bearing

amino and quaternary ammonium

groups

Isopropanol Biocide (white, brown

rot, blue stain)

Scots pine,

Beech

Gosh et al. 2012a

Methacryloxypropyltrimethoxy- Methanol Biocide, Eastern white pine (Pinus strobus), Schneider and Brebner

silane (TMPS)

Vinyltrimethoxysilane (VTMS)

Anti-swelling White birch (Betula papyrifera),

Trembling aspen (Populus tremuloides)

Corsican pine (Pinus nigra)

1985

Hill et al. 2004

3-trimethylsilylpropanoic anhydride,

2-trimethylsilylmethylglutaric

anhydride, trimethylsilylethenone ,

bis(trimethylsiloxy)methylsilane,

poly-dimethylsiloxane,

3-isocyanatepropyl triethoxysilane

(IPTEOS)

3-glycidoxypropyltrimethoxysilane

(GPTMS),

n-propyltrimethoxysilane (PTMS)

Acidic (HAc)

DMF,

Pyridine

Water repellent,

Anti-swelling

Maritime pine (Pinus pinaster) Sèbe and De Jeso 2000

Sebe and Brook 2001

Sebe et al. 2004

Silica epoxysilane surface modified Acidic,

Water

Water repellant

Biocide

Scots pine Liu et al. 2015

Pries and Mai 2013

5 Non-silicon compounds

TiO2 nano particles Water Biocide (mould) Paulownia Chen at al. 2009

TiO2 dispersion + UV radiation Water Biocide Scots pine, silver fir, walnut, chestnut,

wild cherry, sessile oak, beech, ash

De Filpo et al. 2013

TiO2 + (heptadecafluoro-1,1,2,2-

tetradecyl trimethoxosilane

Acidic Water repellant

Tetrabutyl orthotitanate (TBOT),

tetraisopropyl titanate (TPT)

+ Zinc nitrate

Basic,

Ethanol

Water resistance

Water resistance

Fire retardant

Poplar (Populus ussienis)

Poplar

Gao et al. 2015

Sun et al. 2010

Sun et al. 2012

Titanium(IV) n-butoxide (TBT)

titanium isopropoxide (TIP)

Acidic (HAc,

HCl), 2-

Methoxyethanol,

Ethanol

Biocide Scots pine

Poplar

Chinese fir

Hübert et al. 2010;

Qin and Zhang 2012;

Wang et al. 2012

Zn acetate, Zn nitrate, Basic (NaOH),

Ethanol, water

Water repellant Polar (Populus euramericana) Fu et al. 2012

Trimethyl borate (TMB) Acidic (HAc),

Ethanol

Leachablility Poplar Zhang 2015

1The chemical labeling of the abbreviations is given in Table 1.


interconnection. The chosen pH values of the medium and the organic solventsshould avoid conditions that would break chemical bonding in wood or dissolvewood-stabilizing compounds.

Wood Impregnation Methods

The practical procedures of wood impregnation can be classified as diffusive,capillary, or hydrostatic (pressure treatment), depending on the physical phenomenainvolved in the process. In industry, impregnation of wood is primarily done in avacuum pressure process. Dried wood is placed in a vessel, evacuated, and thenimpregnated under pressure of up to 1.4 MPa.

The sol-gel-based impregnation of wood samples is mostly performed in alaboratory. A preceding wood treatment, e.g., in a Soxhlet extractor in a dilutedaqueous sodium hydroxide solution followed by purging with pure water, can beperformed to activate cellulose for improved interaction with the impregnating agent(Giudice et al. 2013b).

In general, two different strategies have been applied. Either a colloidal solution,a sol of oxide, or organic modified particles (e.g., silicic acid, silica) from a priorhydrolysis is used for treatment or precursors of non- and partly hydrolyzed mono-mers of alkoxides which then polymerizes and condensates inside into the wood.The latter promotes a better infiltration into the cell walls, a mechanical fixation, anda chemical interaction with the biopolymers.

The impregnation can be performed by brush painting or spraying (Chu andSeeger 2015). Much deeper infiltration can be achieved by dipping wood into thesol, by steeping, and by application of vacuum and/or pressure (Tshabalala andGangstad 2003; Donath et al. 2006). However, the vacuum impregnation is rela-tively cumbersome and time consuming. An example of vacuum-enhanced solimpregnation is given in Fig. 2.

The achievement of impregnation depends strongly on viscosity, wettability, andparticle size of the precursor solutions and of the wood and its moisture content. Theuptake of solutions is better in axial (longitudinal) direction than in radial direction,because of the orientation of the wood cells. Additionally, sapwood is easier toimpregnate then heartwood.

A measure for the uptake of impregnates is the so-called weight percent gain(WPG) which has been determined by using the equation

WPG ¼ mt � moð Þ=mo½ � � 100, in %ð Þ (1)

where mo and mt are the masses of the untreated and impregnated wood specimens,respectively, after their oven drying up to constant weight.

The WPG varies from 1% to 40% and can increase up to 60% depending on thewood species and sol-gel technology. High WPG can be achieved not only from highsolid content and low particle size in the sol, but also a repeated impregnationprocedure of kiln-dried wood is conducive. An increased WPG can be achieved by


use of ultrasonic-assisted impregnation (Ogiso and Saka 1993; Wang et al. 2014). Itshould be considered that, the WPG is not always equal to the content of oxides in thewood. After impregnation and drying, solvents (water, alcohol) or other organics suchas alkoxides may be still present, whereas extractives (resin) may be leached out.

Corresponding to the mass gain due to uptake of a sol, or a solution, an increase ofvolume can be observed. A measure for this so-called bulking effect (B) is thevolumetric swelling coefficients that can be determined as follows:

B ¼ Vt � Voð Þ=Vo½ � � 100, in %ð Þ (2)

where Vt is the volume of the sol-gel impregnated wood and Vo is that of theuntreated wood sample, respectively.

Both parameters, weight percent gain (WPG) and bulking (B), are decisive fortailoring a number of physical properties of the resultant impregnated wood. Figure 3illustrates the increase of WPG and B in dependence of the amount of particulatematter in silica and titania sols. Almost a linear dependency can be observed, and aWPG of 40% (even up to 55%) can be obtained for titania- and silica-basedimpregnations (Unger et al. 2013). Bulking of impregnated wood reaches valuesup to 4% which are comparable with results reported, e.g., from Saka and Miyafuji(Saka et al. 1992; Miyafuji and Saka 1997; B€ucker et al. 2003; Donath et al. 2004),however, larger values up to 12% were also reported.

Fig. 2 Schematic outline of a vacuum-assisted sol-gel impregnation procedure


Determination of Structure and Properties of Impregnated Wood

The structure of the obtained inorganic wood composites was examined by scanningelectron microscopy (SEM, environmental SEM), energy dispersive X-ray analysis(EDX), transmission electron microscopy (TEM), Fourier transform infrared spectros-copy (FTIR), nuclear magnetic resonance spectroscopy (NMR), and X-ray diffraction(XRD). Further methods for testing specific wood properties are described together withthe obtained results in section “Properties of Sol-Gel Modified Wood.” In many cases,these methods for determination of mechanical properties, weathering, biological sta-bility, and fire retardancy are stated in common accepted standards of ISO, CEN,or ASTM.

Structure of Sol-Gel Impregnated Wood

The main bulk of wood consists of elongated cells (e.g. tracheids in softwoods aswell as libriform fibres and vessel elements in hardwoods) oriented in an axialsystem, aligned with the vertical orientation of a standing tree. These cells areseparated by cell walls which enclose a free intracellular volume, called lumen.These cell lumina are interconnected by pits in the cell wall, that allow transport ofliquids between these cells in the sapwood. The sol penetrates in particular throughthe lumen (see Fig. 4).

Structural investigations indicate that the impregnation can result in an incorpora-tion of inorganic or hybrid materials more than 4 mm deep into the poplar or pinewood structure. However, the depth and amount of uptake depend on the grainorientation and growth direction (Mahltig et al. 2008; Weigenand et al. 2007). The

40

30

20

10

0

0 5

mixedsilicatitania

10

WP

G /

%

solid content / %15 20

0

1

2

3

4

5

B /

%

6

7

8

9

10

Fig. 3 Correlation between particular material (solid content) in the sol, the weight percentage gain(WPG), and bulking (B) of the impregnated pine sapwood (Shabir Mahr 2013)


precipitates are in most cases localized in the lumen and can also close the pores (seeFigs. 4 and 5). More material is soaked into sapwood in comparison to the more densehardwood. The pores to the wood cells are only 4 nm in diameter (Hill andPapadopoulos 2001). Therefore, the solid particles in the colloidal solution need tohave an adequate diameter in order to be infiltrated into the wood cells. Unambiguousexamples of sol-gel infiltration into the wood cells are rare (Saka et al. 1992; Donathet al. 2004). In many cases, the solid is precipitated above all only in the lumen.

Also the evidence of chemical bonding of silica or other metal ions to thebiopolymers is under discussion. Indications for stable Si–O–C bonds in woodwere obtained by 29Si and 13C NMR and by thermal analysis, in an observed shiftin the fire behavior of 100 K along with a change in pyrolysis behavior (Magginiet al. 2012; Cappelletto et al. 2013; B€ucker et al. 2014; Unger et al. 2013).

Properties of Sol-Gel Modified Wood

Mechanical Properties

Sol-gel-based impregnations of wood can enhance mechanical properties, albeit dra-matic changes are not observed (Lathela et al. 2014). Thus, the Brinell hardness ofpoplar and pine woods increased from 13 to 20 MPa for poplar and from 18 to 21 MPafor pine wood through impregnation by silica or titania sols (Mahltig et al. 2008).Similar results were obtained with an increase of Brinell hardness from about 7 to

Fig. 4 Cells and lumen of silica-treated pine sapwood

Fig. 5 SEM pattern (a), silicon (b), and titanium (c) element distribution in a silica/titania sol-gel(MC1) impregnated pine sapwood (Shabir Mahr et al. 2012b)


12MPa and the compressive strength from 33 to 55MPa through the ultrasonic-assistedimpregnation of Chinese fir by silica (Lu et al. 2014). Brinell hardness of spruce and oakwood was increased by treatment with silica sol from alkaline hydrolysis of TEOS from50 and 80 MPa to 100 and 135 MPa respectively (Götze et al. 2008).

Also an improved abrasion and scratch resistance of silica and titania sol-treatedwood was observed which depends on the thickness of the applied nano-sol coating.Protective layers were prepared from silicon dioxide (SiO2) nanoparticles andMTMS or HDTMS and PDMS (Kanokwijitsilp et al. 2016; Chang et al. 2015b).The SiO2 nanocomposite coating showed an improvement in mechanical robustnessin abrasion resistance property as tested with Taber abrasion and has a lower weightloss than an uncoated counterpart. The nanocomposite film also showed a higherwear resistance than the commercial polyurethane coating. Also silica mixed withpolyvinyl alcohol and a chlorosilane (OTS) gives mechanical robust coatings with-out significant reduction in hydrophobicity after the abrasion test (Liu et al. 2013).

Wang (Wang et al. 2012) investigated mechanical properties of titania wood com-posites. From the stress–strain curves, higher Young’s modulus was obtained for treatedsamples. It was concluded that titania densifies the cell walls by filling the voids withinorganic particles which causes it to stiffen. Additionally, a reduced moisture uptake(reduced hygroscopicity) may contribute to improve mechanical properties.

The bending strength of the silica- and titania-modified wood was increased up to40%, when compared to unmodified wood. These improvements are attributedmainly to gel depositions in the wood structure that physically interact with thewood matrix consequently reinforcing strengths of the fiber.

The decrease of modulus of elasticity (MOE) after exposure in ground contact for42 months was substantially reduced for Scots pine samples only if treated withMTES or PTES combined with boric acid (Panov and Terziev 2015).

Mechanical properties are related to processing properties and workability ofmaterials. However, the consequences of sol-gel treatment on these characteristicshave not been investigated yet.

Weathering Stability

Water RepellencyThe exposure to water, heat, wind, and light radiation changes wood properties,leaches out wood constituents (lignin, resin), and results among other things in lesswater repellency. The surface becomes more hydrophilic that enhances the wateruptake. The effect of weathering depends on the wood morphology; latewood withthicker cell walls being denser and harder degrades slower than earlywood.

Besides others, sol-gel-based impregnations aim to improve wood surface hydro-phobicity. Organo-functional silanes in combination with alkoxysilanes have beendescribed to increase the water repellency and reduce moisture uptake and leachingof chemicals from wood. Because of their high weathering stability, the applicationof these formulations on wood exposed to conditions of use class 3 (outside exposurewithout soil contact) was recommended.


Ameasure for water repellency is the water surface tension or the contact angle ofa water droplet which is typically between 20� and 80� of untreated wood. Waterrepellant coatings resulted in water contact angles up to 150� by impregnations usingsols based on TEOS and HDTMOS (Chang et al. 2015). Other alkoxysilane-basedformulations give so-called super-hydrophobicity with contact angles of more than150�, such as silicon dioxide (SiO2) nanoparticles and MTMS (Kanokwijitsilpet al. 2016) or silica and polyvinyl alcohol (PVA) (Liu et al. 2013). An ultrasonic-assisted sol-gel method for the preparation of SiO2-wood composites has signifi-cantly reduced the hygroscopicity of wood and consequently improved the mechan-ical performance of modified wood (Lu et al. 2014). An option to increase thesurface hydrophobicity is by the silanization with alkyltrichloro-/alkyltrialkoxysilanes, such as OTS, CTMS that gives contact angles above 100�

(Mohammed-Ziegler et al. 2008). The impregnation with commercial silicananoparticles with an average size of 20 nm mixed with perfluoroalkyl methacryliccopolymer yields contact angles for water on coated wood of more than 160� (Hsiehet al. 2011). High water-repellant coatings on wood surfaces were also prepared byzinc oxide doped with stearic acid (Wang et al. 2011) or based on ZnO nanorods thatwere grown hydrothermally (Fu et al. 2012). However, a crucial point is the long-term effectiveness of coatings. During storage over days, a decline of the highcontact angle has been observed.

Water Absorption and SwellingDue to its hydroxyl groups, wood is hydrophilic and has great affinity to interact withmoisture. The water absorption and desorption of wood results in an unwantedswelling and shrinking.

Not only the dimensional instability but also a possible crack formation which arepreferred areas for fungal attack should be avoided. Thus, controlling moisturecontent of wood by various treatments is a promising strategy to enhance its lifeduring service. The moisture and water uptake of wood can be reduced by sol-gel-based impregnations, whereas the quantitative effect depends on the kind of wood.

Silica-based impregnations of pine sapwood can reduce the water uptake in moistair from 25% to 5–10%, while relevant volume is diminished from 14% to 6%(Unger et al. 2013).

The water uptake of beech wood reduced from 40% to 13–20% by modifica-tions with various organo-functional silanes and polysiloxanes (Mahltiget al. 2008). The water uptake of treated pine sapwood was considerably dimin-ished from 35–50% to 10–20%, especially after treatment with multifunctionalwater-borne silane systems containing both hydrophilic and hydrophobic groups(Donath et al. 2006).

It was demonstrated that an impregnation with a fluorosilane (HFOETMAS)containing silica sol can reduce four times the water uptake of hinoki cypress andwestern hemlock (Pinaceae) (Saka and Ueno 1997; Miyafuji and Saka 1999).

Titania impregnated pine sapwood samples have a minimum of 4%mass gain dueto moisture sorption and a 2% relevant volume increase instead of 24% and 13%,respectively, for the untreated wood samples (H€ubert et al. 2010).


For illustration, results of water dip test for silica and titania sol-gel impregnatedwood are shown in Fig. 6a. Compared to untreated controls, relative water uptakewas reduced significantly for all samples tested implying that wood becomes morewater resistant as a result of sol-gel treatment. After immersion in water for 6 h, allimpregnated samples showed 47–52% lower relative water uptake compared tountreated pine sapwood controls. This absorption trend remained steady for testedsamples at the end of dipping (96 h) showing around one half of the water absorbedby untreated wood.

It is noted that the degree of reduction is independent of the sol-gel materialsdeposited (titania, silica, or mixed) because all the tested composites of similarweight gains (WPG) showed no visible difference in their water absorption. It isassumed that introduction of material reduces free cavities, closes pores, and hindersflow of water within the cell walls leading to different water absorption behavior forimpregnated specimens than controls.

The absorption of water from moisture was tested under moist environment of95% relative humidity for more than 75 days. The resultant moisture sorptionbehavior of tested samples is displayed in Fig. 6b. In the early phase of exposure(first 10 days), all the test variants gained their masses rapidly indicating that theirmoisture absorption rates were high in this period. After few days, certain saturationwas reached. Beyond this, no more moisture was absorbed and the moisture contentremained unchanged in tested variants for rest of the testing period. The maximalabsorption observed for untreated wood controls was around 21%, while impreg-nated samples took up only a fraction of this value ranging from 56% to 67%. Silicaimpregnated wood (SC) showed maximum reduction in moisture sorption (around44% in relation to untreated wood control). Titania impregnated wood(TC) exhibited fairly less performance (just 33% reduction in moisture sorptioncompared to controls).

Resulting improvement in moisture lowering was attributed to the reducedsorption sites as well as reducing the cell wall space for water by depositing thematerial into cell wall cavities (Papadopoulos and Hill 2003). There are some

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Fig. 6 Water absorption properties of impregnated pine sapwood wood as determined (a) in waterimmersion and storage in moist air with relative humidity 95% (b) (Shabir Mahr 2013)


indications of partly blocking the water active sites by the FT-IR measurements. Butthese changes are rather small and do not sustain due to lack of a real chemical bondbetween hydroxyl (OH) groups and sol-gel depositions. Therefore, reduction inmoisture sorption by OH blocking seems to be irrelevant for these materials.

Several models for water sorption have been proposed. One such model for fittingwater sorption isotherms to empirical data was introduced by Hailwood andHorrobin (Hailwood and Horrobin 1946). This model does not require any assump-tions to be made regarding the nature of the geometry of the micropores within thecell wall and has been applied to equilibrium moisture data, both for untreateddifferent wood species and for chemically modified wood (Mantanis andPapadopoulos 2010).

As a result of water uptake, a volume increase of wood occurs. A measure forassessment of a reduced swelling is given by the anti-swelling efficiency (ASE) thatis calculated according to

ASE ¼ B0 � Btð Þ=B0½ � � 100, in %ð Þ (3)

where B0, is the volumetric swelling coefficient of untreated and Bt the volumetricswelling coefficient of the treated wood, respectively.

In general, moisture and water sorption capacities were decreased by 43–50%,while anti-swelling efficiency (ASE) of the precursor-modified wood was increasedup to 34%, when compared to unmodified wood. Figure 7 shows that the anti-swelling efficiency (ASE) of impregnated wood was increased monotonically withthe WPG as well as cell wall bulking B (see Fig. 7a, b). It rises up to 30% for silicaimpregnated wood with WPG of 21% and with bulking of 3.9%. This result iscomparable to the literature (Donath et al. 2004), even some authors observed forlower WPG, higher ASE values up to 70% for impregnations with silica and waterglass (Saka et al. 1992; Furuno et al. 1992). This may depend on the type of woodand the degree of infiltration of cell walls (Schneider and Brebner 1985). For titaniaand mixed sol impregnations, ASE was increased up to 34% and 31% with WPG of39% and 23%, respectively.

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Fig. 7 Anti-swelling efficiency (ASE) for sol-gel impregnated wood (Shabir Mahr 2013)


The formation of TiO2 nanostructures on the surface of wood by the cosolvent-controlled hydrothermal method improved its water-repellent characteristics, i.e.,remarkably decreased water absorption and prevented dimensional swelling of wood(Sun et al. 2010).

These results proved that the TiO2 coating acted as a moisture barrier andimproved the water resistance, as well as dimensional stability of wood. However,this is significantly less improved in comparison to other wood modification treat-ments such as acetylation and furfurylation. Acetylated wood showed an improve-ment in ASE of 90% at 30%WPG (Hill and Jones 1999) and furfurylated wood up to70% at WPG of 47% (Lande et al. 2004).

The improvements by gel depositions in the wood structure are attributed mainly tophysically interact with the wood matrix consequently narrowing the main flowpassages there and reinforcing strengths of the fiber. The dimensional stability of testedsol-gel-derived silica and titania wood impregnations is mainly attributed to cell wallbulking because of the permanent cell wall swelling caused by the sol-gel treatment.

Organo-functional silanes also have reduced water uptake, and anti-swellingefficiency as well as increased water contact angle, and water repellency that wasobserved for treatment on pine and beech wood (Hager 1995; Lukowsky et al. 1997;Donath et al. 2004; Weigenand et al. 2007). However, this effect can diminish afteraging of the samples, and it was observed that equilibrium moisture content (EMC)and maximum swelling were not lowered (De Vetter et al. 2010; Donath et al. 2006).The organosilicons may have a positive effect on the wood–moisture relationshipreducing moisture-related damage like surface erosion, defibrillation, crack forma-tion, etc., in wood and thereby contributing to a longer service life of the treatedwood (De Vetter et al. 2009a). The main sites of reaction are the hydroxyl groups ofthe biopolymers. It was concluded that deposition of the modifying agents in the cellwall is crucial for successful modification. Therefore, the chemical has to be able topenetrate into the cell walls in order to react with the hydroxyl groups of the cell wallconstituents.

UV StabilitySurface color is a very important quality criterion in the utilization of wood.Unfortunately, light irradiation induces color changes. The effect of light on thecolor of wood surface is mainly to fade or to darken and to bring about changes intones, e.g., gray shading, pink-orange, and reddish-orange discoloration. Extensivestudies and observations have shown that mostly the rate of discoloration is usuallyrelated to the intensity of light. It is well known that different wood species reactdifferently to UV light exposure, and the results are influenced by experimentalparameters. These color changes are caused by severe chemical modifications ofthe structure of cellulose, hemicelluloses, and lignin. Lignin in wood absorbsultraviolet light. This results in a radical induced depolymerization of both ligninand cellulose on wood surface. A quantification of this effect can be obtained byaccelerated weathering tests (Oltean et al. 2008; Teaca et al. 2013). Changes ofcolor can be determined by colorimetry using CIE L*a*b* method (EN ISO11664–4:2011).


Several UV-absorbing substances can be used to protect wood from discolorationusing clear coatings. Organic UV absorbers such as benzophenone, benzoate, orheterocyclic organic nitrogen-containing compounds were deposited on wood sur-face, mostly as part of a finishing or paint (Kiguchi and Evans 1998). Some inorganicmaterials like titania, magnetite particles, or zinc oxide are also commercially used asUV absorbers (Aloui et al. 2007).

Whereas impregnation by water glass or an amino-alkyl functionalized siloxanecould not prevent a color change (Pfeffer et al. 2012). Silylation of wood bymethacryloxysilanes and epoxysilane provides improved photo stability (Petric2013; Pfeffer et al. 2012). Color changes are reduced due to coatings of vinyl-functionalized silicon zirconium oxoclusters (Girardia et al. 2014). Highphotostability was obtained by impregnations of silica with a benzophenone silane(BP) with a WPG of 15% (Miyafuji et al. 2004). Additionally, the weight loss ofwood due to an accelerated weathering test can be reduced by wood impregnationwith HDTMOS and MTES (Tshabalala and Gangstad 2003).

The stability of the sol-gel impregnated wood against UV light in dry, wet air,and in water was investigated up to 1,038 h (43 days). Pine sapwood samples wereimpregnated by Ag-TiO2 formed by the hydrolysis of TIP in isopropanol andAgNO3 (series 1 s) and SiO2 impregnation formed by acidic hydrolysis of TEOSin ethanol (series 2 s). As demonstrated in Fig. 8, due to this weathering experi-ment, decolorization and bleaching of the wood surface can be observed. Anaccelerated weathering test showed that coatings of hybrid silica with rutileTiO2 reduced the color changes in comparison to untreated wood (Zhenget al. 2015).

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Fig. 8 Discoloration of sol-gel impregnated pine sapwood due to an accelerated weathering test byexposure to UV radiation in dry air, wet air, and water (1 s: TiO2 precursor forms hydrolysis of TIPin isopropanol, 2 s: Ag-TiO2 precursor forms hydrolysis of TIP and AgNO3 in isopropanol (H€ubertand Wachtendorf 2008)


Sol-Gel Based Wood Preservation for Improved Biodurability

OverviewVarious organisms can colonize wood and metabolize their components. Thisdegrades wood properties ending in a complete destruction. The most commontype of wood decay is due to fungi which produces enzymes or chemicals (e.g.,hydrogen peroxide, oxalic acid) to break down wood components by differentmechanisms (Hill 2002; Ibach 2013). Three main types of biodegradation by fungiare reported according to the appearance of wood following degradation:

• Brown rot is most prevalent with regard to attack on conifer and is caused, e.g., byPoria placenta, Polyporus meliae, Serpula lacrymans (true dry rot), andConiophora puteana (cellar fungus). The fungi break down cellulose and hemi-cellulose that result in typically brown cubic-shaped fragments.

• White rot associated with hardwood decay and caused by basidiomycete fungi,e.g., Trametes versicolor L., Phanerochaete chrysosporium, and Coriolusversicolor (turkey tail), breaks down lignin selectively. The rotted wood becomessoft, spongy, or stringy with white or yellow appearance.

• Soft rot is caused by various fungi, e.g., Chaetomium globosum, Kretschmariadeusta, Stachybotrys chartarum. The fungi need high moisture content for theircolonization and to secrete an enzyme from their hyphen that breaks downcellulose and lignin.

Furthermore molds, staining fungi (blue stain), and black yeasts can colonizepreferentially sapwood resulting in unwanted discoloration, however without severewood degradation. Algae, lichens, and bacteria, such as Escherichia coli (Qin andZhang 2012) or cyanobacteria, can colonize wood surfaces as well. They can makewood more susceptiple for further bio attack, but they do not have a significantimpact wood properties.

Wood is exposed by attack of insects, especially by termites (Isoptera) and severalwood-boring beetles such as European house borer (Hylotrupes bajulus L.), com-mon furniture beetle (Anobium punctatum), or wharf borer (Narcerdes melanura), aswell as bees, ants, and wasps. In contact with water, wood can be attacked bymussels, e.g., boring teredinids (Teredo navalis) and crustaceans (Limnoria sp.,Sphaeroma, etc.).

Drying of wood can prevent the growth of fungi. The avoidance of water uptakealong with swelling and crack formation as starting points for biological attack cangive a certain protection. However, these measures are not sufficient for woodpreservation used in contact with environment (especially in contact with waterand soil). Therefore, several organic and inorganic substances as fungicides orinsecticides were used in wood preservation with specific applications. Most com-monly used organic wood preservatives are various alkyl ammonium compoundssuch as tertiary amine salts and quaternary ammonium compounds. Other organicwood preservatives contain substances such as carbamates, sulfamides, permethrin,


and azoles. Further, oil-borne preservatives, linseed and rapeseed oil, and wax wereused; formulations can contain pentachlorophenol (Schultz et al. 2008).

Many inorganic preservations contain copper, such as chromated copper arsenate(CCA), that is highly effective and protects also against UV light. However, it is verypoisonous and therefore most countries have banned its use because of its unaccept-able effects on humans, animals, and environment (98/8/EC: 1998). Therefore, woodpreservatives have undergone rapid and dramatic changes worldwide in the pastdecade owing to environmental concerns and governmental regulations. Neverthe-less, copper remains the primary biocide component still effective to protect woodused in ground contact or fully exposed to weather. It was found that copper is moreeffective in alkaline and neutral conditions than in acidic.

Preservation systems currently employed for residential applications are based inmany countries on copper (copper(II) ions) with an organic co-biocide added tocontrol copper-tolerant fungi (Freeman and McIntyre 2008). Boron-containing pre-servatives are boric acid, oxides, or salts, e.g., disodium octaborate tetrahydrate(DOT) (Caldeira 2010). These compounds are not very fixed in the wood and leachout which limits their application for wood protection (Lesar and Humar 2009).Another method of wood preservation by killing insects uses gases, such asbromomethane (CH3Br) or sulfuryl difluoride (SO2F2).

Various other types of compounds have been used for the modification ofwood in chemical reactions, such as acetylation with acetic anhydride(Accoy®), acidification with acid chloride or carboxylic acids, and chemicalreaction with organic polymers, e.g., isocyanates, epoxides, or enzymatic treat-ment to bound functional groups on the wood surface (Rowell 2006). Besideschemical modification (acetylated and furfurylated wood), thermal treatmentmethods at elevated temperatures (160–240 �C) were developed and commercial-ized (ThermoWood®), or plasma and microwave treatments were suggested(Denes et al. 1999; Petric 2013). However, an effective persistent preservationof wood for all use classes over long time has not been reached. For example,some brown-rot fungi, such as Serpula lacrymans or Tyromyces palustris, aretolerant against copper ions (Cervantes and Guiterrez-Corona 1994). Many woodpreservatives are poisoning for human and animals. Their release into the envi-ronment is problematic and has to be controlled (CEN 15083-1: 2006). Develop-ing effective, economically and environmentally friendly wood preservatives inareas with high or severe deterioration hazard, especially ground-contact appli-cations, will be difficult. Therefore, alternatives for long-term preservation are ofinterest, and sol-gel technology can be considered hereby as a promisingapproach.

Test ProceduresTo access the efficiency of wood preservatives, commonly standardized tests areused for aboveground applications, and the mass loss caused by bio-decay isdetermined as a characteristic parameter (Aloui et al. 2007; EN117: 2005;ASTM1037: 2012). Alternatively, the degree of wood colonialization by fungi canbe determined for decay assessment.


Wood mini block samples of sizes such as 50 � 25 � 15 mm3 or20 � 20 � 5 mm3 with the longitudinal faces parallel to the grain of the wood wereused in many cases. After sol-gel-based impregnation (see section “Wood ImpregnationMethods”), samples were stored over 2 weeks for fixation of chemicals. Other smallersizes of wood specimens were suggested in order to accelerate test procedures (Nicholasand Militz 2008). However, it is not always agreed that these smaller samples will giverepresentative results for mass-scale wood application.

To use class 4 applications, the test in soil at least 5-year duration requires woodbars of 500 � 50 � 25 mm3 size. The biodeterioration is accessed visually and byloss of elasticity modulus MOE (EN252: 2015; ENV807: 2001).

LeachingWood preservatives may be washed out from wood when expose to water that maylimit their application. A typical example is boron compounds (e.g., boric acid orzinc borate) which are soluble in water, and therefore a large amount of them areeasily washed out even if inserted via sol-gel process. In the case of wood applicationin soil or exposed to weathering (classes 3 and 5), the leaching of preservatives canbe tested by storage of impregnated wood samples in water over 14 days accordingto EN 84 (EN84: 1997 CEN15119: 2005). The resulting relative mass loss is calledleaching formulation (LF) and is determined according to

LF ¼ mt � moð Þ= m1 � moð Þ½ � � 100, in %ð Þ (4)

where mo, mt, and ml are the oven dry masses of untreated, sol-gel treated, and thenleached wood samples.

The leaching tests of poplar wood impregnated with hydrolyzed trimethyl borateshow high mass losses, and only 3–6% of the initial boron remains in the poplarsamples (Zhang 2015). However, the leaching of boron and copper can be reducedby silica or titania impregnations. Silica- and titania-based impregnations are utmostleach resistant after posttreatment because of the insolubility of gel depositions inwater as well as due to their fixture into the wood matrix. In addition to this, gelprecipitates can also reduce substantially the release of hazardous active agents (e.g.,CuCl2 or boron) of wood preservatives into the environment (Böttcher et al. 1999;Nami et al. 2009; Feci et al. 2009; Altun et al. 2010; Shabir Mahr et al. 2013a). Thisanti-leaching effect is supposed to be due to the reduced ion mobility of active agentsinside the wood by their encapsulations into the gel matrix, due to the lowering ofwater penetration into the wood depths, as well as due to the internal covering actionof the gel layer. Combining the organo-functional silanes and organic biocides in thewater-based and solvent-based treating solution does not change the protectiveeffectiveness of the treated wood, also they do not contribute to a reduced leachingof the biocides either (De Vetter et al. 2009a).

IncubationWood samples were preconditioned for 2 weeks at 20 �C and relative humidity of65% to achieve moisture equilibration. It was required to sterilize wood specimens


previously, e.g., in an autoclave or by γ-radiation (25 kGy). Always one treated andone untreated reference sample in size of mini blocks were placed in a Kolle flask orPetri dish where the fungus was previously grown on a malt-agar medium. Five ormore repetitions for each fungus were used and incubated under controlled condi-tions 25 � 5 �C and 60–70% relative humidity for 10, 12, as well as 16 weeks.Then, the specimens were removed from the culture flasks and sterilized again;mycelia were subsequently withdrawn. Finally, the specimens were placed in anoven at 100 � 3 �C and dried for 18 h up to constant mass. The mass loss (ML) wasdetermined for each specimen by using the following equation:

ML ¼ mo � mf

� �=mo

� �� 100, in %ð Þ (5)

where mo and mf are the masses of the dried specimen without and with exposure tofungi, respectively.

Alternatively, the heat-energy production of the fungal decay in wood can bemeasured in a micro calorimeter (Verma et al. 2008).

Sol-Gel Based Wood Preservation for Improved BiodurabilitySince the nineteenth century, aqueous sodium and potassium silicate solutions (waterglass) have been used as deterrent against insect attack. Combinations of water glasswith further inorganic components (e.g., boric acid) were investigated by Furunoet al. and indicated decay resistance against white and brown-rot fungi (Furunoet al. 1992; Furuno and Imamura 1998). However, these compounds are hygroscopicand are easily washed out by moisture.

Treatment with pure silicic acid or colloidal silica from TEOS alone did notsignificantly improve the anti-decay properties of wood (Yamaguchi and Östman1996). On the other hand, impregnation with hydrolyzed silanes (MTES and PTEO)is very effective and gave mass loss reduction from 38% for untreated beech wood to5–9% after 6-week exposure to T. versicolor. Siloxane-treated beech and pine woodafter 24-week soil test have no reduced loss of modulus of elasticity (MOE) incomparison to untreated controls. However, after longer exposure, this effect waslost (Donath et al. 2004). Sol-gel impregnation based on alkoxysilanes causes only adelay of fungal attack, while after longer exposure, heavy decay occurs (Donathet al. 2004).

Solvent-based impregnations with alkoxysilanes (MTM and OTES) and a water-based siloxane microemulsion (PDMS and TEOS) of Scots pine (Pinus sylvestris L.)and beech (Fagus sylvatica L.) could be partly protected against decay caused bybasidiomycetes (De Vetter et al. 2009a).

Treatment of wood with amino- and ammonium-functionalized silicones wasshown to inhibit colonization by staining and mold fungi (Ghosh et al. 2009).Blue-stain fungal attack by A. pullulans on pine sapwood can only be inhibited bytreatment with silica sols; however, amino-silicone emulsion resulted in strongresistance to staining (Pries and Mai 2013b; Ghosh et al. 2009).

Scots pine and European beech samples treated with quaternary (quat) andamino-silicone (QS and AS) solutions of different chain lengths were tested against


brown-rot and white-rot fungi as well as blue-stain colonization. The treatment withshort-chained silicones bearing quaternary ammonium or amino functional groupsreduced the mass loss (ML) caused by wood decay fungi at 15% treatment concen-tration (Ghosh et al. 2012b).

Hinoki cypress was impregnated by hybrid silica-based sols, and the mass losswas measured after exposure to white-rot and brown-rot fungi (Tanno et al. 1997.The mass loss of untreated wood of 10% was reduced to about 3% by impregnationof silica with addition of an amino silane (TMSAC) and to zero due to a fluorosilane(HFOETMOS).

Complex polymeric coating on wood based on vinyl- or (3-mercaptopropyl)-trimethoxysilane and an organically modified zirconium oxocluster showed resis-tance to brown-rot fungus C. puteana (Maggini et al. 2012).

Silicic acid and metal compounds such as copper iron, cobalt, or zinc wereeffective wood preservatives.

Treatments by commercial silica sols showed a reduction in mass loss comparedto the control samples after incubation with C. puteana for pine sapwood andT. versicolor for beech according to EN 113 and CEN/TS 15083-1, especially inthe case of modification with an acidic aluminum oxychloride (Pries and Mai2013b).

Because copper ions can be grafted by amino complexes, a combination ofsilanes or nano-silica emulsions and amino alkoxysilanes APTES) can fix copperions (CuCl2) by coordinative interactions and can thus reduce leaching (Palanti et al.2012a).

A silicic acid solution and boric acid, as well as addition of transition metal (Fe,Co, Cu, Zn) chlorides, exhibited high anti-decay properties against fungal attack(Tyromyces palustris) and hardly dissolved out from the treated wood (Yamaguchiand Östman 1996). It is assumed that silicon was substituted by boron, and a boronion was introduced into the network structure of silicic acid, preventing leaching ofboron from the wood. Investigating the mechanism of preservation, it was observedthat when the concentration of boric acid was high, depression of pH due to fungimetabolism is small, mycelial growth was inhibited completely, and no proteinproduction was detected. When the amount of boric acid was low, the enzymeactivities were lower than with control wood.

Sol-gel impregnations were tested as preservatives against insects. Sodium waterglass in combination with borates or alkaline earth chlorides had great efficiency in atermite test (Ogiso and Saka 1993; Furuno and Imamura 1998). Impregnations bysilica sols hydrolyzed from TEOS give no unambiguous results. A certain resistanceagainst larvae of wood-boring beetle (Hylotrupes bajulus) was observed (Reinschet al. 2002). A 3-year test showed that silica impregnated wood (Castanospermumaustrale) was not attacked by larvae of a wood boring beetle (Lyctus brunneus)(Cookson et al. 2007). Resistance in 6- or 7-year-long trials against marine borescould not be obtained by impregnation with pure silica, but through combination ofsilica and copper chromium arsenate (CCA) (Kl€uppel et al. 2015; Cooksonet al. 2007). Also termites (Coptotermes acinaciformis) readily consumed allTEOS-treated sapwood (Cookson et al. 2007). On the other hand, impregnation of


low molecular weight silicic acid and boric acid agents in a single step prevents over3-year vermin damage by termites (Yamaguchi 2003).

Wood treated with quaternary ammonium and amino-functionalized siliconesdemonstrated resistance against subterranean termites even at less than WPG of15%. Alkyl-functional silane emulsions and the water glass treatment also reducedthe damage by termites to a considerable extent (Ghosh et al. 2012b). It is assumedthat in addition to the fungicidal effect of the amino groups, the combined impact ofhydrophobazation, cell wall bulking, and change of the wood surface energy isresponsible for the successful performance of silicone compounds as woodpreservatives.

Wood modification with the solution of TEOS and APTES in ethanol coupledwith both copper and boron is effective against subterranean termites, although themode of action is very different. While silica and copper seem to act more like arepellent, boron has a real toxic action and termites die after ingestion of the activeingredient. The siloxane alone seems to act in another way, hypothetically with adelayed effect or with a chronic toxicity produced by a sublethal dose of silica,whose effects are evident a long time after the ingestion (Feci et al. 2009).

Titania-Based ImpregnationsThe fungal resistance of pine sapwood impregnated with titania precursors (based ontitanium(IV) isopropoxide in 2-propanol or ethanol with solid content of 5–16% andsubsequently dried under ambient and moist air was tested against brown-rot fungiConiophoria puteana and Poria placenta for an incubation time of 10 and 16 weeks.Figure 9 indicates a remarkable shrinking of the untreated wood sample, whereas thetitania-treated sample shows no change in size and color. Figure 10 presents ESEMmicrographs showing the wood structure of untreated and titanium alkoxide impreg-nated wood after incubation with brown-rot fungus (C. puteana). It was observedthat untreated wood was intensively colonized (Fig. 10a). All the cell walls of earlyand late wood were disintegrated and cells were even fully collapsed due to fungalinfestation. C. puteana forms a network of hyphae, indicating the intense growth ofthe fungus on the wood substrate. On the other hand, Fig. 10b shows that hardly anyhyphae of the fungi were visible within the wood matrix of the wood treated withtitanium alkoxides, revealing no indications of fungal attack. It was noted that

Fig. 9 Photographs ofuntreated wood (left) andtreated wood with titania-based precursor (T-prop withequivalent TiO2 solid contentof about 13% and WPG 18%),(right) aftergrowth of thebrown-rot fungus Coniophoraputeana


titania-coated cell walls and cell lumens were distinguishable and still intact,although some look disintegrated. This could be due to the cut preparation of thesample for ESEM because tested samples were quite frail after the fungal test.

Results of mass loss given in Fig. 11 indicate a large decrease in mass loss for alltreated samples compared to untreated wood (38% mass loss caused by C. puteana

Fig. 10 ESEM images of crosscut section aftergrowth of the brown-rot fungus Coniophoraputeana in a untreated wood and b treated wood with titania-based precursor T-prop with equivalentTiO2 solid content of about 13% and WPG 18% (Shabir Mahr 2013b)

05 12 16 5 12

solid content / %

control

10 weeks16 weeks

controlC. puteana P. placenta

16

123456

24

mas

s lo

ss /

% 30

36

42

48

54

60

Fig. 11 Mass losses of pine sapwood mini blocks treated with titania precursor solutions (TIP withdifferent concentrations) and subsequently dried under ambient air, after exposure to brown-rotfungi Coniophora puteana and Poria placenta, respectively (Shabir Mahr 2013b)


and 50% mass loss caused by P. placenta) after 10-week exposure. Even withprolongation of further 6 weeks, mass loss of the tested samples did not exceed5%. However, the virulence of P. placenta seems to be higher. Generally, biodete-rioration and mass loss caused by fungal attack depend on time necessary for fungalcolonization, growth, as well as the decay rate of mineralization of cell wallcomponents and are deviant for different fungi (Weigenand et al. 2008). The resultsdepicted in Fig. 11 should be therefore considered as a snapshot. Effectiveness ofwood protection by titania precursors depends on several factors. For example, theconditions of treatment after impregnation have impact. An increased water contentin the wood stimulates fungi growth, whereas residual organics, such as alkoxides oralcohols, have an inhibitory effect (Shabir Mahr et al. 2013b).

Almost full decay protection was achieved against wood-destroying brown-rotfungi (C. puteana and P. placenta) in 10-week laboratory trials only with lowloadings (WPG). The mass loss in prolonged test (16 weeks) was similar to normal,indicating that protection is permanent. It was noted that wood treated with a lowconcentration of precursor (equivalent solid content of 5%) showed higher efficacy,with a mass loss below 3%. Better protection at lower concentrations may be a resultof a better penetration due to smaller size as well as shielding of interior and exteriorsurfaces of the treated wood. To investigate this question, wood treated withprecursor of titanium(IV) isopropoxide with an equivalent TiO2 solid content of5% by mass (WPG 9%) and 16% (WPG 22%) was analyzed by ESEM. Thickerlayers of solids led to cracks and defects in the titania gel protective layers, whereasthinner layers led to a more uniform coating on the cell walls (see Fig. 12).Imperfections in the deposited layers allow relatively easy access to fungal hyphaeto penetrate into the wooden structure, resulting in slightly less efficacy againstdecay. Perhaps better shielding with lower concentrated precursor solution is not theonly reason for higher protection. Similar mass losses after 16 weeks of incubationprovide a hint that the biocidal mode of protection of titania-based precursor isworking as well. A combined action of biocidal activity and shielding effect oftitania gels is the probable antifungal protection mechanism of wood treated withtitanium alkoxide precursors.

0

mas

s lo

ss /

%

C. puteanaP. placenta

0123456

243036424854

a b

84 12

WPG / %

16 20 24

Fig. 12 Mass loss of wood samples impregnated by different titania content (left 5%, right 16%)after 16 week exposure to fungi


It can be demonstrated that titania precursor solution with equivalent TiO2

content of about 5% is sufficient to impart complete protection to pine sapwoodagainst brown rot. Moreover, higher loadings greater than this value do not providemore protection. Even the treatment with higher concentrated precursors (solidcontent 12% and 16%) decreases the decay resistance of treated wood.

These impregnations result in remarkable resistance against biodeterioration.Almost full decay protection was achieved against wood-destroying brown-rotfungi (C. puteana and P. placenta). In addition, composites imparted moderateresistance against soft rot fungi and very minor against surface blue stain. Thelowering in moisture content that discourages fungal colonization, gel layer thatprovides better shielding against fungal exposure, and un-hydrolyzed alkoxides(organics) that probably induce biocidal effects are the main reasons for protectionagainst biological deterioration of these materials. Furthermore, impregnations by anaqueous dispersion of nanoscaled titania exhibited an antifungal effect againstAspergillus niger in the presence of UV irradiation (Chen et al. 2009).

Role of Sol-Gel Process in Wood PreservationThe results of the empirical studies depend on the chosen substances, the effectivenessof impregnation, the kind of wood, and the activity of microbes, and even quantitativedata give only relative statements. Nevertheless, there are several strategies to preventbiodeterioration which can be used for sol-gel-based wood impregnation:

Passivation and hydrophobization of surfaces. The modified surface will not berecognized by microbes as wooden. The blocking of pores and micropores hindersthe penetration into the wood structure, e.g., by hyphens. The reduction of water(moisture) uptake avoids repeated swelling and shrinking which results in crackformation as starting areas for fungal growth. The reduced water content in the woodhinders fungi growth. However, the impregnation by modified silica sols has only aretardation effect and a limited resistance against bio-attack (Yamaguchi 2002;Yamaguchi and Östman 1996).

The increased hydrophobicity of the surface decreases the amount of the adsorbedwater as well as diminishing the adhesion of microbial cells.

Inhibition and Devitalization of the Fungi by Biocidal AgentsAlready acidic and alkaline conditions, a pH value of larger than 8 or smaller than2, inhibit mycelial growth; however, extreme values can destroy the wood structure.

Titania impregnations have an effect of decay protection against brown-rot fungi(Shabir Mahr et al. 2013b). However, the effect is not linear to the amount of soliduptake correlated. Figure 11 demonstrates that the increased solid content (WPG)results in the formation of cracked titania films and causes higher mass loss due tobio-decay. Also, there are very robust fungi which are persistent and can overcomesol-gel protections, and therefore, the life time of environmentally friendly woodpreservatives is limited in particular for use of wood in soil.

For long-term preservation especially in contact to water and soil, the applicationof biocidal substances, such as metal salts, fluorine, or ammonium compounds, isstill unavoidable (Tanno et al. 1997; Yamaguchi 2002). Copper- or boron-based


compounds act as fungistatics or fungicides and are able to inhibit or devitalize thefungi by different modes of action of fungicides such as inhibition of respirationprocesses, inactivation of enzymes, etc., within fungi. Cu(II) ions from coppercompounds are a typical inhibitor of respiration processes in fungal cells. Boron-based compounds usually form stable complexes with vitamins; enzymes and otherbiological molecules consequently suppress the activity of wood-degrading fungi byinhibiting their metabolic activity, enzymatic function, and growth (Eaton and Hale1993; Reinprecht et al. 2010).

Improvements of Efficiency of BiocidesSol-gel-based impregnations can tailor and retard the release of the biocidal compo-nents such as boric acid or copper ions and may prolong its effectiveness. Gels formedby the impregnated substances have the tendency to capture or encapsulate the activeingredients and hence retain them efficiently into wood even under extreme moistconditions. Such stable localizations into the gel within wood structure improveefficacy of the biocides. However, it is still a matter of further research to understandto which extent the performance of the biocides when applied with sol-gel impreg-nations is compatible with one when biocides are applied to wood alone.

Sol-gel-based wood impregnations can give a substantial contribution in improv-ing wood preservation; however, further developments seem to be needed forcommercialization.

Sol-Gel-Based Wood Preservation for Improved Fire Retardancy

OverviewFire is an exothermic oxidation reaction of a fuel, a combustion accompanied with aflame. Compartment fire development can be comprised of four stages: incipient,growth, fully developed, and decay characterized based on increasing, enduring, anddeclining heat release.

In case of severe heat entry, the thermal degradation of wood can hardlysuppressed; however, inorganic or organic substances can be used as fire retardants.Inorganic fire retardants are oxides and hydroxides, such as aluminum, magnesium,calcium hydroxide, and antimony oxide or zinc borate. Other relevant groups of fireretardants are bromine- or chlorine-containing organic compounds and organo-nitrogen and phosphor compounds. Often used chemicals for wood fire retardancyare mono- and di-ammonium phosphate (MAP, DAP) or ammonium sulfate(AS) (Marney et al. 2008; Terzi et al. 2011). These substances have differentmechanisms for fire retardancy. They cause a cooling due to water disposal andevaporation or in a radical interception (scavenger). A protection layer is formed asheat isolation or to prevent oxygen supply (Vignali et al. 2011). Recently ionicliquids, such as 1-ethyl-3-methylimidazolium bromide, 1-ethyl-3-methylimi-dazolium tetrafluoro borate, and 1-ethyl-3-methylimidazolium hexafluoro phosphatewere suggested for improvement of fire resistance of wood (Miyafuji and Fujiwara2013). However, some of these have the disadvantage of leaching fastness when in


contact with water. Other flame-retardant additives such as chlorinated or bromi-nated biphenyls and diphenyl ether carry high environmental and health risks. Analternative approach on fire protection is the use of sol-gel technology.

Systematic investigations on application of aqueous alkaline silicates for fireprotection were performed by Furuno (Furuno et al. 1991, 1992, 1993) and also inthe group of Saka which included alkoxysilanes and colloidal silica (Sakaet al. 1992; Saka and Tanno 1996; Saka and Ueno 1997; Miyafuji and Saka 2001).

In addition to the silicate- and silica-based precursors, the impact of furthercomponents such as boric acid was investigated to increase fire resistance (Furunoet al. 1991, 1992, 1993; Yamaguchi and Östman 1996). MulticomponentSiO2–P2O5–B2O3 wood impregnations were prepared via sol-gel (Miyafujiet al. 1998). Transparent ceramic coatings on wood prepared via sol-gel methodby siloxane and silica sol as the matrix, and organophosphates as the fire-retardantagent, increase the limit time of fire endurance (Liu et al. 2012). Hybrid organic–i-norganic zirconium- and silica-based coating (vinyl-functionalized zirconiumoxoclusters copolymerized with vinyltrimethoxysilane) on pine and larch woodresulted in a flame retarding effect (Girardia et al. 2014).

Titania (TiO2)-based impregnations were tested for improving fire retardancy(Miyafuji and Saka 1997; H€ubert et al. 2010; Wang et al. 2012). Also TiO2/ZnOnanoparticles prepared via a hydrothermal process could functionally act as aprotective material to change inflammable wood into nonflammable wood, protectwood to extend the combustion time, and be an especially strong shield that inhibitssmoke spreading (Sun et al. 2012; Fu et al. 2012).

The fire behavior of wood and the impact of impregnation are of wide complexdepending on different fire scenarios and conditions of ignition. It can be investi-gated by different methods.

Thermal AnalysisIn many cases, thermal analysis (TG, DTA, or DSC) was applied for investigationsof the thermo-oxidative decomposition of wood. This method gives fast results onpyrolysis and the protection effects of preservatives on wood. However, the resultsdepend on heating rate and surrounding media, especially on the oxygen content inthe gas atmosphere (Sandermann and Augustin 1963).

Thermogravimetric and differential thermal analyses showed that incorporationof silica and titania in wood enhances the thermal stability. The heat effects arereduced and shifted partially to higher temperatures. It demonstrates that the impreg-nation retards thermal decomposition and complete combustion of the wood matrix(Saka and Ueno 1997; Miyafuji and Saka 1997; Unger et al. 2013; Lu et al. 2014).

To study thermal decomposition, silica and titania impregnated wood samples incomparison with untreated wood were thermally analyzed with TG and DTA (Ungeret al. 2013; H€ubert et al. 2010; Shabir Mahr et al. 2012a). Representative TG curvesunder air of titania impregnated sapwood are displayed in Fig. 13a showing massloss as a function of temperature. For untreated wood as well as silica and titaniaimpregnated wood, similar TG curves were obtained, consisting of three mass lossregions (1, 2, and 3) in different temperature ranges.


In region 1, below 220 �C, only a slight mass loss was observed for treated anduntreated samples. This mass loss was related to moisture content liberated duringmeasurement as well as the release of volatile organic residues (un-hydrolyzedalkoxide) and partial decomposition of least stable wood component hemicellulosesthat began to degrade from 200 �C (Shen et al. 2010).

A considerable second and third mass loss (regions 2 and 3) occurred in the rangeof 220–350 �C and 350–500 �C, respectively. Mass loss in this range was related tooxidative decomposition of wood components (Kaur et al. 1986), while mass loss inregion 3 was attributed to char oxidation (Wegner 1989). After 500 �C, no furthermass loss implied that untreated wood was completely thermo-oxidized with verysmall residue (<1% by mass). In the range 2, a mass loss of around 40% wasapparent for both untreated wood and impregnated wood without showing pro-nounced mass loss difference in this region. However, in region 3 the sampleswere decomposed with different final mass loss caused by remaining silica or titaniaas stable residues (see Fig. 13a). In this region, all curves of impregnated woodsamples were hardly shifted to higher temperatures indicating any considerabledelay in the oxidation of char. The yielded residual masses depended on WPG ofthe tested impregnated wood samples. However, the WPG may contribute not onlythe formed oxide (silica or titania) in the wood but also less volatile organic residuals(un-hydrolyzed alkoxide as a result of incomplete hydrolysis and condensation).

DTA thermograms of untreated wood and titania impregnated wood are placed inFig. 13b. The obtained diagrams show in many cases a weak endothermic and twoexothermic effects. The endothermic peak is related to the decomposition of woodwhich already starts at temperatures below 100 �C.

Two distinct exothermic peaks appeared in temperature range of 240–360 �C and410–470 �C for untreated wood, in which the peak intensity and peak maximumtemperature are influenced by the heating rate. First exothermic narrow peak wasmainly ascribed to oxidative decomposition of wood cell wall components (hemi-celluloses and cellulose and lignin), while second was attributed to oxidation of char

0

–1.0

–0.5

0.0

3

TC6TC4TC3UN

untreated (UN)

TC1

TC2

TC3

TC4

TC5

TC6

2

rela

tive

mas

s Io

ss

1

200 400

temperature / ˚C

600 800 200

ba

End

o.E

xo.

400

temperature / ˚C

600 800

Fig. 13 Thermal behavior of titania wood composites in air. TG – relative mass loss (a) and DTA(b) of non-treated wood (UN) and samples impregnated with titania sols (TC1–TC6), heating rate2 K/min (Shabir Mahr et al. 2012a)


as similarly described above for respective TG curve. A peak due to lignin decom-position cannot be observed, although the lignin decomposition occurs between240 �C and 260 �C (Murugan et al. 2008). In comparison to softwoods, thedecomposition of hardwoods starts at higher temperatures, the hemicelluloses asso-ciated shoulder or peak is more intensive, and both the hemicellulose and cellulosereaction zones are narrower (Gronli et al. 2002).

Apparently, for all cases of impregnated wood, the intensity of the first peak isconsiderably reduced. The intensity of second peak was also reduced gradually withincreasing WPG and was slightly shifted toward higher temperatures revealing adelay in oxidation of char. These results are similar to previously reported results fortitania impregnated wood (titania wood-inorganic composites) (Miyafuji and Saka1997; H€ubert et al. 2010).

Undoubtedly, this TG/DTA method is quite efficient and quick to characterize thethermal behavior of materials and thus able to deliver some valuable hints for the firebehavior such as char yield and pyrolysis temperatures. Nevertheless, these data arenot sufficient to assess reasonably the fire properties of macroscopic specimens indifferent fire scenario. Thus, further studies under different fire scenarios employingmeaningful fire tests were performed to get deeper insight to the thermal and firebehavior.

FlammabilityFlammability is a complex behavior and can be characterized by various testssimulating different scenarios.

Limiting Oxygen Index (LOI)This test reflects the reaction to a small flame according to ISO 4589 (ISO4589:2006); alternatively ASTM D 2863 can be applied (ASTM2863: 2013). LOI is theminimum amount of oxygen in oxygen/nitrogen mixture required to sustain candle-like combustion in the specified setup. The higher the LOI value, the more flameretardant the material. It is calculated by

LIO ¼ O2½ �O2½ � þ N2½ � � 100, in %ð Þ (6)

For LOI investigations, wood specimens (pine sapwood, Pinus sylvestris L.) of asize of 80 � 10 � 4 mm3 were impregnated by sol of silica, titania, and mixturesof both.

LOI values of untreated pine sapwood (UN) and silica and titania impregnatedsamples are summarized in Table 2.

The measured LOI value for untreated pine wood was 23% by volume. Thisexperimental value is smaller than the reported value (29.4%) for pine wood (Baysal2002), demonstrating that this method delivers only relative values depending onorigin and dimensions of the specimens. A remarkable increase in LOI valuesranging from 26% to 41% by volume that are up to 70% was observed due to sol-


gel impregnation. Comparable values are obtained by Canosa et al. for phosphate-modified water glass impregnations (Canosa et al. 2011).

Increase in LOI correlates with the concentration of nanoparticles in the sol(titania or silica) that results in higher WPG. Also double impregnation can increasethe WPG as well as the LOI value. The LOI roughly depends linearly from the WPGas demonstrated in Fig. 14. Thus, it was concluded that the treatment of wood withmore concentrated precursor solutions as well as higher titania or silica loadings inthe derived wood samples remarkably enhances the flame retardancy of thesematerials with respect to reaction to a small flame such as LOI.

UL 94 TestThe small-scale flame test UL 94 is a method to describe and to classify theflammability of a material (UL94: 1996; IEC60965: 2003). Samples of125 � 13 � S mm3 (0.4 mm � S � 3 mm) were ignited by a 20 mm high flameand usually 50W power for 5–30 s. The test accesses the material’s property to eitherextinguish or spread the flame of the specimen that has been ignited. An HB flamerating indicates that the material was tested in a vertical position and self-extinguished within a specified time after the ignition source was removed. All thetested sol-gel impregnated samples have achieved horizontal burning HB in accor-dance to UL 94 classification (see Table 2). Most specimens show self-extinguishingin HB before reaching any testing marks in contrast to pure wood. Furthermore, theflame spread velocity for the few burning specimen was much slower than foruntreated wood. These results correspond well to the LOI results implying that

Table 2 Flammability (reaction to small flame) of single and double impregnated TiO2 and SiO2

pine sapwood

Code Basic alkoxide

WPG LOIa UL 94

�4% � 1% O2 Classification

UN Untreated – 23 HB

SC1 TEOS single coated 21 28

SC2 TEOS double coated 31 30

TC1 TIP single coated 3 26 HB

TC1-2 TIP double coated 4 29 HB


TC3 TIP single coated 12 29 –




TC5-2 TIP double coated 49 38 HB

MC1 TIP, TEOS single coated 23 29

MC2 TIP, TEOS double coated 40 35

TSC2 TIP+TEOS double coated 37 35

STC2 TEOS+TIP double coated 42 41aLOI determined with a device of Stanton Redcroft, East Grinstead, UK


titania-based wood impregnations are slow burning or flame-retardant materials(Weil et al. 1992). On the other hand, the materials did not achieve vertical burningrating in accordance to UL 94 showing that these systems were still flammablematerials in the more challenging vertical configuration with ignition from thebottom. The difference in horizontal and vertical burning UL 94 testing indicates astrong influence of upward and downward flame configurations on the flammabilityof the investigated materials as it is often observed for charring approaches. It wasreported that heat transfer and burning rates under downward flaming configurationwere greatly altered from the upward flaming configuration (vertical burning UL 94).Moreover, in both test geometries, the residual impact was changed that consider-ably influenced the flame retarding effect.

Small Flame TestA further variant of a flammability test in a laboratory-scale apparatus in accordancewith the Italian UNI standard 8457:2008, in which a small flame impinges directlyon a specimen to assess its ignitability, was tested (UNI8457: 2008).

Specimens cut in the radial direction to 70 � 40 � 1.4 mm3 in size were placedvertically in a U-shaped specimen holder, and a propane gas flame 20 mm in heightwas brought into contact with the specimen for 30 s at an angle of 45�. The time ofignition, flame propagation along the specimen, the duration of flaming and glowing,the dimensions of the area damaged, and whether flaming particles fell from theproduct and the sample were classified according to criteria based on theseobservations.

0 5

LOI /

%

10 15 20 25

WPG / %


UN20

25

30

35

40

30 35 40 45 50

Fig. 14 Correlation between weight per gain (WPG) of impregnated wood and limiting oxygenindex (LOI)


The flame tests showed that vinyl-functionalized zirconium oxoclusterscopolymerized with vinyltrimethoxysilane coatings on wood are slightly better interms of self-extinguishing after the small flame was removed, such that theyrevealed none of the post-combustion or postincandescence phenomena, usuallyseen on the uncoated samples. The damaged area was reduced in height by 55% andwidth by 30% (Girardia et al. 2014).

2-Foot Tunnel MethodThis test according to ASTM D 3806 determines the comparative surface-burningcharacteristics of a coating by evaluating the flame spread over the surface whenignited under controlled conditions in a small tunnel (ASTM3806: 2004). Coatedand uncoated sample panels of 605 � 100 � 6 mm3 in size are exposed to a flame ina tunnel and the flame advancement observed. In addition to the flame spread rate,the mass of sample consumed, time of after-flaming and afterglow, char dimensionand index, and height of intumescence can be determined in this test.

The fire-retardant performance of wood particleboards treated with 8, 12, and16% of boric acid was determined. Fire-retardant characteristics were measuredsubsequently according to ASTM D3806 by employing standard 2-foot tunnelmethod. It was noted that fire retardancy properties were enhanced with the concen-tration of boric acid. Particleboards treated with 16% boric acid showed the best fire-retardant properties, particularly in terms of weight loss, flame spread speed, andafter-flame time (Pedieu et al. 2012).

CombustionThe cone calorimeter is a key tool in fire research and can be used for ranking ofproducts based on fire performance (ASTM1354: 2015; ISO5660: 2015). It is aperformance-based bench-scale device for fire testing using forced flaming condi-tions and provides a comprehensive insight into fire risks and hazards determining aset of characteristics such as time to ignition (TTI), heat release rate (HRR), total heatrelease (THR), mass loss rate, combustion products (smoke and CO/CO2), and otherparameters (Babrauskas 2002; Schartel and Hull 2007). However, results stronglydepend on calorimeter parameters, also on type of wood, its density, heat capacity,and thermal conductivity, as well as sample size and grain orientation.

The wood specimens (pine sapwood, Pinus sylvestris L.) with a size of100 � 100 � 10 mm3 were impregnated by sols of silica, titania, and a mixtureof both and were investigated with a cone calorimeter (Fire Testing Technology,East Grinstead, UK) in the horizontal position at a constant irradiation of 50 kW/m2 in triplicate. The obtained values were averaged out and are compiled inTable 3.

Time till Ignition and Burning TimeThe time of ignition of different untreated wood varieties is in the range of 8–34 s forirradiance levels of 50 KW/m2 (Harada 2001). The investigation shows that sol-gel-based impregnations increases the time of ignition (TTI) and burning time (TFO),and a correlation to the amount of precipitated oxide (WPG) can be demonstrated as


Table

3Con

ecalorimeter

dataforsol-gelim

pregnatedwoo

d

Cod

eWPG

TTI/s

TFO/s

Residue/%

THR/M

J/m

21.PHRR/kW/m

21.TPHRR/s

2.PHRR/kW/m

22.TPHRR/s

TSRm

2/m

2COY/kg/kg

UN

–19

445

17.4

59.7

209

2726

132

932

10.00

62

UNa

–20

409

17.3

57.0

209

2926

530

729

80.00

52

SC1

2128

455

25.0

64.6

195

3322

435

619

20.00

44

SC2

3130

521

29.0

62.0

189

3119

136

618

40.00

43

TC1

323

471

20.0

52.9

228

3321

435

314

20.00

45

TC1-2

423

494

24.1

51.0

206

3219

635

9112

0.00

40

TC2

1023

515

24.9

55.6

194

3518

636

713

60.00

41

TC5

2733

503

28.8

56.6

178

4122

639

314

70.00

29

TC5-2

4932

571

33.9

67.6

206

6019

944

614

60.00

38

MC1

2329

521

25.5

61.5

182

4220

937

516

20.00

35

MC2

4035

552

33.8

55.6

166

3715

742

1114

0.00

35

STC2

4241

590

31.7

61.8

176

4017

642

190

0.00

38

TSC2

3735

539

35.0

60.0

187

4618

140

518

00.00

53

TTItim

eto

ignitio

n,TFO

timeof

flam

eout

(burning

time),residu

e(m

ass-totalmassloss)/mass,THRtotalheat

release,PHRRpeak

ofheat

release(firstand

second

),TPHRRtim

eof

PHRR(firstandsecond

),TSP

totalsm

okerelease,COYtotalCO

yield


shown in Fig. 15. Furthermore a correlation between LOI and TTI for impregnatedwood can also be observed (Weil et al. 1992).

Also results from literature show that for wood which was treated with mixturesof silicic acid or silanes along with further components such as boric acid, theignition time was delayed (Yamaguchi and Östman 1996). Larch wood treatedwith partially hydrolyzed TEOS in mixture with phenyl- and fluoro-modified sili-cones showed increased time of ignition from 13 to 18 s (Cappelletto et al. 2013). Itwas demonstrated also that impregnations of larch and pine by vinyl-functionalizedsilane and zirconium enlarge the time of ignition and time to flame out (Girardiaet al. 2014).

Heat ReleaseThe heat release rate (HRR) is one of the main parameter measured in the conecalorimeter and is related to flame spread and fire growth. The HRR profiles formeasured samples are illustrated in Fig. 16.

Always, the HRR representative curves are a two-peak profile typical for thickchar-forming materials like wood. Out of two peaks, first one is caused afterignition by the increasing amount of volatile compounds oxidizing within thegrowing flame due to the rapid increase in mass release rate before a char layeris formed limiting the pyrolysis. As the burning proceeds and a char layer is formedon the surface of wood, the HRR value decreases. The second peak corresponds tochar layer cracking and increased rate of volatile formation in the thin unburnedpart of specimen prior to the end of flame burning. The last part of the curve afterthe volatiles were expended characterizes the non-flame burning, i.e., glowing.Flame burning is of relevance in the growth and fully developed phases of the fireand corresponds to the intensity of the fire, while the glowing corresponds mostlyto the last phase of the fire concerns to the fire resistance (Parker and Tran 1992;Grexa and L€ubke 2001).

0

20

25

30

35

40

400

450

500

550

600

5

TT

I / s

TF

O /

s

10 15 20 25 30 35 40 45 50 0

UNUN



5

ba

10 15 20 25

WPG / %WPG / %

30 35 40 45 50

Fig. 15 Dependency of time of ignition (TTI) (a) and burning time (TFO (b) on weight percentgain (WPG) of silica and titania impregnated pine sapwood


In comparison to untreated wood, the intensity of the second peak of HRR (2.PHRR) of silica and titania impregnated wood was reduced in particular toward theend of burning (see Figs. 16 and 17a). Figure 17a reveals that impregnations of lowWPG from 3% to 10% (TC1, TC1-2, and TC2) showed already a reduction in the 2.PHRR compared to untreated wood indicating a reduced protection action of theresidues (see Table 3). Further increase in WPG from 27% to 42% (TC5, MC2) doesnot result in additional reduction in the intensity of second PHRR (2.PHRR), but,however it increases the time to second peak of HRR (2.TPHRR). This performancemay be attributed to degree of cracking occurred in the char formed of the testedsilica/titania or mixed impregnated wood samples (see Fig. 18). An improvement inthe protection action was observed when titania gels introduced in the secondimpregnation healed most of the cracks formed during first impregnation conse-quently reduced the second PHRR (compare TC1 and TC1-2 or TC5 and TC5-2).

0

HR

R /

MJm

–2

HR

R /

MJm

–2

200

a b

400

time / sec600

TCSMC1SC1UN

TC5-2MC2

SC2TSC2STC2

UN

800 1000 0

0

50

100

150

200

250

0

50

100

150

200

250

200 400

time / sec600 800 1000

Fig. 16 Heat release rate (HRR) profiles for sol-gel-derived single-layered (a) and (b) double TiO2

and SiO2 impregnated wood along with untreated pine sapwood obtained from cone calorimeter testperformed at an external heat flux of 50 kW/m2. The arrows indicate the changes induced (ShabirMahr 2012b)

2.P

HR

R /

kW/m

2

150

200

250

300

350

400

450

50

UN

a b

SC1

SC2

2.PHRR2.TPHRR

2.T

PH

RR

/ s

150

200

250

300

350

400

450

10 15 20 25

WPG / %

30 35 40 45 50 0

0.05

0.10

0.15

0.20

0.25

5

UN

FP

I / m

2 s/ k

W

10 15 20 25

WPG / %


30 35 40 45 50 55

Fig. 17 Dependencies of second peak of HRR, time to second PHRR (a), and fire performanceindex (FPI) (b) on WPG for TiO2 and SiO2 impregnated pine sapwood


On the basis of these results, it was concluded that not the residual content(indirectly WPG) as such controls the second PHRR of the tested impregnatedwood samples. Apparently, protection properties of the titania depositions are notonly dependent on the amount deposited into the wood (WPG) but on the crack-free effective fixation of material into the wooden structure that leads to a closedbarrier layer with greater capability to hinder the heat and mass transportation inthe condensed phase.

In contrast to untreated wood, the times to second PHRR (2.TPHRR) wereappreciably altered depending on the WPG values for tested impregnated pinesapwood probes (see Fig. 17a). Time related to second maximum of HRR alreadyfor low WPG (TC2) was delayed about 15%, while that of high WPG (TC5-2) about45%. As a consequence, burning times (time of flame-out) for all tested impregna-tions were greater than untreated wood increasing with WPG. That is, in contrast tothe second PHRR, the time to second PHRR increases linearly with the increasingfire residue. Thus, it is proposed that the HRR was mainly controlled by theprotection properties of the residue, whereas the time to PHRR goes along withthe heat capacity of the residue and its amount.

Fig. 18 Morphology of fire residues obtained after the cone calorimeter fire test for untreated pinesapwood (UN) and impregnated wood with SiO2 (SC1), TiO2 (TC1), and SiO2/TiO2 (MC1)mixed sols (Shabir Mahr 2012b)


It is noticed that HRR considerably decreased (but not tends to zero) after thesecond PHRR. This fire response is assigned to an after glowing of char throughthermo-oxidative decomposition. The reduction in heat release rate is insignificantfor the first maximum in HRR. Beyond first PHRR, HRR decreases because of theprotection of carbonaceous char. It is concluded that the protection performance wasimproved by the applied silica or titania deposition layers. It was reported before thatdegree of reduction, shape, width, and shift observed in the second PHRR dependson the protection properties of residues (Wu et al. 2012).

The fire performance index (FPI) which is defined as the ratio of TTI to 1.PHRRcan be used to characterize a fire risk of a material (Hirschler 1995). The FPI raisedwith increasing weight per gain of the impregnated wood samples (see Fig. 17b). Theincreased FPI are due to the increased TTI and the reduced intensity of the first HRRpeak. The results suggest a considerable reduction of fire risk for these treated samples.

These findings showed similarities to DTA curves presented in Fig. 11 wheresimilar effects were observed in term of intensity reduction and peak shifting incomparison to untreated wood. Of course, the experiments were performed underdifferent environments that led to different fire scenarios. It is well supported byresidual amount obtained for untreated wood of 17.3% by mass in the cone calo-rimeter compared with <1% in TG. Pyrolysis was mainly anaerobic in the conecalorimeter, whereas thermo-oxidative decomposition took place in TG under air.Therefore, the shifts and reduction observed in DTAwere not directly related to theshifts observed for the second PHRR in the cone calorimeter. Nevertheless, thesimilarities give a remarkable hint to a general protection mechanism active in theresidues of homogeneous sol-gel silica and titania impregnated wood.

Smoke Release and CO ProductionA remarkable reduction in the range of 50–62% in total smoke release (TSR) wasachieved for all impregnated wood samples compared to untreated indicating sig-nificant improvement in reducing fire hazards by sol-gel treatment to wood.

The CO production was decreased for most of the tested samples indicatingchanges in the volatile pyrolysis products and oxygen deficit in combustion.

ResiduesThe amount of residues increases lucidly with loading of incombustible oxides (seeTable 3, Column 5). Residues of tested samples collected after the end of test arevisualized in Fig. 18. It shows the residues of untreated wood and the testedimpregnated samples after the test in a cone calorimeter. Untreated wood wascombusted to light brownish–black residues. Cracking of the residue toward theend of burning and subsequent after glowing finally decomposed the material toblack ash containing larger pieces disintegrate to each other. The residuals of silicasol-treated wood samples (SC1) are black to gray colored and cracked in smallerpieces. Combustion of titania sol-gel-treated wood (TC1) derived from dilute pre-cursor solutions (WPG up to 10%) yields white residual masses with more compactand integrated structures. White appearance of residues is attributed to titania gelnetwork homogeneously distributed in whole wood matrix. Similar to TC1, even


more stable and compact residue containing fewer cracks was left for mixed sol-gelimpregnated wood (MC1) and well supports the greater reduction in its 2.PHRR.Larger cracks were found in the residual mass of silica impregnated pine sapwoodSC1. It is assumed that thicker SiO2 coatings in the interior of wood matrix tend to bemore frail and hence to crack when heated. As a result, more cracked and rigidresidual mass were obtained for SC1 accompanied by a least reduction in its secondmaximum of HRR compared to its counterparts MC1 and TC1 (see Table 3).

Nevertheless, titania-based sol-gel wood is valuable environmentally friendlymaterial that shows significant improvements in most of its properties includingfire and flame retardance at low titania loadings (WPG). In practice, 3–4% criticalloading of titania gels by single impregnation of titanium alkoxide precursors towood is sufficient to produce fire-retardant wood inorganic composites with fewerfire risks such as low CO and smoke release. An approach to use higher loadings byinfiltrating more concentrated titanium alkoxide solution only slightly improves thefire performance of impregnated wood. In comparison with typical boron-based fireretardants (boric acid, borates, etc.) under similar loadings (3–5%), similar fire/flameretardancy enhancements in terms of second peak HRR (2.PHRR) reduction andLOI increase in the small-scale flammability conditions were realized (Baysal 2002;Wang et al. 2004).

However, at higher loadings the latter performed better flame retardance (basedon their LOI values) than the underinvestigated TiO2 wood impregnations. However,due to better leach resistance, TiO2-based wooden materials may have edge overboron-based ones especially under moist conditions.

Discussion• For the investigation of fire retardancy, the use of methods and equipment stated

in common agreed standards, e.g., ISO 5660, is preferable because it can deliverresults of better comparability (ISO5660: 2015; Harada 2001). Wood type,structure, growth orientation, and moisture content as well as test apparatus andparameters such as heat irradiation have impact on the results.

• Silica and titania impregnations showed resistance against fire of different sce-narios in terms of reduced flame spread as well as suppressed fire growth. Insmall-scale fire, their flame was retarded markedly (up to 78% in optimum case)in comparison to wood controls as studied by oxygen index test (LOI). Conecalorimetric investigations revealed their better fire retardancy in terms of timeresolved heat release rates (HRR) in the developing fire scenario. A remarkablereduction of 40% in the second peak of HRR was achieved through sol-geltreatment. Furthermore, fire hazards such as CO and total smoke producti onwere considerably lowered as well for these materials. Besides these improve-ments, reductions in first peak HRR and in fire load (total heat evolved) were notworth considering in compliance to fire retardancy principles. The results alsoindicate that with weight gain increase, the performance of impregnants againstfire does not increase proportionally, and imply that the film quality has impact(Giudice et al. 2013b).


• Heat release rates and second maximum of HRR for these systems weredecreased, and corresponding burning times and time to second maximum ofHRR were increased moderately due to change in their residual properties underforce-flame fire conditions. No further improvement was realized in terms of fireload as total heat evolved was unchanged very similar to untreated wood. Fireretardancy of sol-gel impregnated wood in general is minor compared to thecommercial fire retardant tested (Shabir Mahr et al. 2012b; Pries and Mai 2013a).

• More efficiency may be obtained by addition of further components. Woodtreated with nitrogen- and phosphorous-based fire retardants showed almostthree times more flame resistance (increase in LOI) than TiO2 wood impregna-tions at the similar respective loadings of nitrogen and phosphorous in the testedspecimens (Gao et al. 2003, 2004, 2006).

When wood was treated with silicic acid and boric acid, the ignition time wasdelayed, the rate of heat release during 300 s after ignition per exposed area wasdecreased, the quantity of fumes produced at combustion was also decreased, andthe apparent flashover time was delayed as well as the after-flaming andafterglowing combustion time in comparison with those of untreated (Yamaguchiand Östman 1996; Yamaguchi 2003). Cone calorimetry investigations of woodtreated with partially hydrolyzed TEOS in mixture with phenyl- and fluoro-modified silanes showed increased time of ignition and reduced total heat releaserates and average mass loss (Cappelletto et al. 2013).

• The low-cost treatment of wood with alkaline silicates gives fire-retardant effects.The glassy layer of Na2O–SiO2 and intumescent structure formed over the cellwalls were thought to prevent oxidation and heat transfer from proceeding intothe inner portion of the wood cell walls (Miyafuji and Saka 2001). However,essential disadvantage of the treatments by inorganic silicates is the high alkalin-ity of the solutions that demands special care for their manipulation. An impreg-nation with aluminum oxychloride-modified silica sol increased fire resistance ofthe treated wood. Mass loss and velocity of mass loss as well as burning time werereduced; glowing of the formed charcoal was completely prevented. The effec-tiveness increased with increasing weight percent gain of the silica in the wood(Pries and Mai 2013a).

• Mechanism of fire protection by sol-gel impregnations could be based mainly onbarrier effects of formed char (Giudice and Pereyra 2010). It can be concludedthat crack-free gel layers (independent of material loading) inside the woodmarkedly retard the proceeding combustion processes (oxidation) after firstpyrolysis by improving the overall protection properties of the fire residue.

Conclusions

Sol-gel technique offers a broad variety in chemical wood modification and canimprove wood properties. Various wood properties such as mechanical and dimen-sional stability, water uptake and repellency, weathering, and bio- and fire resistancecould be improved in multiple approaches. For this purpose different substances and


mixtures, such as alkaline silicates (water glass), silicic acid, silica, organic-modifiedsilanes, and further non-silicon compounds, are used.

The mechanical and strength properties of wood can be modified by sol-gel-basedimpregnations and hardness increased and abrasion reduced. However, the impact onelasticity and mechanical workability (sawing, planes, and grinding) has to beinvestigated in detail.

Impregnations of wood with silica- or organic-modified silanes show improve-ments in lowering water uptake and repellency, dimensional stability, as well asstability against weathering. Titania exhibits similar effects and can provide anadditional benefit in UV protection.

Wood protection against biodegradation using silicate solutions (water glass) wassuggested. However, these impregnations can worse wood strength properties anddimensional stability. Its alkaline nature tends to destroy the polymeric structure ofwood because of hydrolysis. In addition, water glass-treated wood displayed highhygroscopicity, so that the resistance against basidiomycetes appeared to be due to avery high moisture content and pH.

Solutions of alkoxysilanes, monomeric and polymeric silicic acid, or organicmodified silanes slightly increased bioresistance. However, they have not replacedtraditional wood preservatives in class 3–5 uses yet. For this reason, these basicprecursors were combined with fungicides such as boron and quaternary ammoniumcompounds.

Fire retardancy of wood can be improved with silicate solutions, colloidal silica,or organic-modified silanes in particular when combined with boron or phosphorouscompounds. Titania is an inert environmentally friendly compound that can protectwood from weathering by absorption of UV radiation, as well as acting againstbio-decay and fire. It can be considered as an alternative to silica. However, theimpact of combinations of titania with other biocidal compounds is rarelyinvestigated.

Four strategies of further deployment of sol-gel technology for wood treatmentare identified:

• The application of sol-gel-based substances as a mild, alternative environmentallyfriendly solvent-free impregnation, but with limited efficiency.

• Because of low biocidity of silicic acid, silica, or titania, their combination withwell-known wood protecting compounds is suggested to improve and prolong theeffectiveness. Silica or titania can provide a matrix for inserted or grafted com-ponents as well as it may provide skeleton for biocidic functional groups.

• Avoiding or reducing leaching of classical wood preservations such as boron orcopper by embedding in a sol-gel matrix.

• Even though sol-gel impregnations do not have in many cases the performance oftraditional commercial substances for improving bioresistivity or fire retardancy,however, several sol-gel formulations can give a remarkable improvement ofwood parallel in many properties (multiple improvements) or present an alterna-tive to traditional agents.


In future, research should be considered on restraints such as an increased mass ofthe impregnated wood, in particular for WPG larger than 10%, to overcome addi-tional expenses for treatment and overhead for processing. Unfortunately, all thesesol-gel-based treatments are not yet feasible in practice, since these require a costlyadditional impregnation and heat treatment steps. In many cases the long-termstability over years of impregnations is unclear and difficult to predict. Investigationson wood composites, plywood, and veneers based on sol-gel impregnations arestill rare.

Acknowledgment We are highly grateful to our colleagues Brita Unger, Ina Stephan, Carlo Tiebe,Heidi Lorenz, Michael B€ucker, Bernd Schartel, Birgit Strauss, Martin Sabel, and Kerstin Klutznyfor their long-lasting cooperation in our research endeavors.

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