Study of nanofibre reinforced epoxy composites: curing...

Study of nanofibre reinforced epoxy composites: curing behaviour and thermo-mechanical properties.

Sam van der Heijden

Promotor: prof. dr. ir. Karen De Clerck Tutor: ir. Bert De Schoenmaker

Thesis submitted to obtain the degree of

Master of Textile Engineering

Department of Textiles Chairman: prof. dr. Paul Kiekens Faculty of Engineering Academic Year 2011-2012

Acknowledgment

I would like to thank all the people who contributed to the accomplishment of my master thesis.

First of all I would like to say thanks to my tutor, Bert De Schoenmaker. Not only for the uncountable hourshe spent on guiding me through the research and correcting all my writings, but also for all the fun momentswe had together. I truly enjoyed working with you.

I would like to show my gratefulness and utmost respect for my promoter, Karen De Clerck. Albeit her busyschedule, she was always there for advice and feedback.

Furthermore, my thanks go to the people from the research unit of physical chemistry and polymer sciencefrom the VUB. Thank you Guy Van Assche, Hubert Rahier and Nick Watzeels for your knowledge, help,time and for allowing me in the laboratory.

I would like to express my appreciation to Lieve Van Landuyt for helping me with my DVS and DSCexperiments and to Klaas Allaer for manufacturing all composite samples used in this thesis.

Also thanks to all those who were kind and patient enough to read and review my writings. Thanks toSusann Hermansson, Daan de Keyzer and my parents.

Finally, I would like to thank my parents for their support and encouragement during all these years.

Sam van der HeijdenJune 2012

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Copyright notice

The author gives permission to make this master dissertation available for consultation and to copy parts ofthis master dissertation for personal use. In the case of any other use, the limitations of the copyright haveto be respected, in particular with regard to the obligation to state expressly the source when quoting resultsfrom this master dissertation.

Sam van der HeijdenJune 2012

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Study of nanofibre reinforced epoxy composites:curing behaviour and thermo-mechanical properties.

by

Sam van der Heijden

Promoter: prof. dr. ir. Karen De ClerckTutor: ir. Bert De Schoenmaker

Thesis submitted to obtain the academic degree of Master in Textile Engineering

Department of TextilesChairman: prof. dr. Paul Kiekens

Faculty of EngineeringGhent University

Academic Year 2011-2012

In this thesis, polyamide 6 nanofibre composites are studied. First the effect of these nanofibres on the curingbehaviour of epoxy amine resins is investigated using mainly but not exclusively modulated temperaturedeferential scanning calorimetrie. Thereafter the effect of polyamide 6 nanofibres on the thermo mechanicalproperties of composites is investigated.

The first chapter includes an explanation of the different concepts combined with a literature review. Chapterone concludes with the aim of the thesis. In the second chapter a description of the used materials andmethods is given.

In a third chapter, the reproducibility of epoxy amine curing experiments is investigated, as well as theinfluence of the nanofibres on this reproducibility. Furthermore the effect of the nanofibres on the curingbehaviour is investigated and compared with the results found in literature.

The fourth chapter focuses on finding the actual cause of the effects of nanofibres on the curing behaviourof epoxy amine resins observed in chapter 3. The main focus of this chapter is to provide a detailed unders-tanding of the effect of the nanofibre moisture content on this curing behaviour.

Chapter five studies the effect of polyamide 6 nanofibres on the thermo mechanical properties of compo-sites.First nanofibre composites consisting of epoxy amine resin and polyamide 6 nanofibres are investigatedand compared to the properties of neat epoxy resin. In the second part of chapter five the effect of polyamide6 nanofibres on [+45,−45]2S GF-epoxy composites is investigated.

Finally a general conclusion and proposals for future work can be found in the last chapter.

Keywords: composites, polyamide nanofibres, epoxy, curing, MTDSC, DMA

Study of nanofibre reinforced epoxy composites: curingbehaviour and thermo-mechanical properties.

Sam van der Heijden

Supervisor(s): Karen De Clerk, Bert De Schoenmaker

Abstract— This research investigates the potential of polyamide 6(PA6) nanofibres as a secondary reinforcement in epoxy compositematerials. First focus is given to the effect of PA6 nanofibres onthe curing behaviour of epoxy amine resins, further the thermomechanical properties of PA6 nanofibre composite are studied.

Keywords—nanofibres, composites, toughening, curing, MTDSC, DMA

I. INTRODUCTION

Electrospun nanofibres are extraordinary materials with wide appli-cation potentials, one such application is the reinforcement of com-posite materials. Recent research shows nanofibres can increase thestiffness and toughness of epoxy resins [1]. Further a preliminary cu-ring study on a DGEBA-MDA epoxy resin showed that PA6 nanofibreshave a catalytic effect on the curing reaction, as well as a small plasti-cizing effect [2]. The aim of this thesis is to investigate the potential ofPA6 nanofibres for the reinforcement of the epoxy matrix.

II. EFFECT OF PA6 NANOFIBRES ON THE EPOXY RESIN CURINGBEHAVIOR

A. Materials and methods

The effect of electrospun PA6 nanofibres on the curing behaviour ofthe DGEBA-MDA as well as the RIM135-RIM137 resin hardener sys-tem was studied using (mainly) (quasi)-isothermal MTDSC and TAMexperiments. The moisture content of the nanofibres, conventional PA6microfibres as well as the resins was analysed using DVS. Compositesamples produced from the RIM135-RIM137 epoxy resin where testedusing DMA.

B. Catalysing effect

In previous research it was suggested that the nanofibre moisturecontent might have a significant influence on the curing behaviour ofthe the epoxy matrix. This hypotheses was thoroughly investigated inthis research. The most important result is shown in Figure 1 whichshows the initial heat flow signals (which are proportional to the initialreaction speed) in function of the nanofibre moisture content.

Fig. 1. Initial reaction speed of nanofibre (♦) and microfibre (�) containing samples infunction of fibre moisture content

A linear increase of the initial reaction speed in function of the nano-fibre moisture content can be noted, implying first order kinetics inmoisture content. This clearly indicates that the nanofibre moisturecontent is the most important cause of the catalysing effect observedwhen adding PA6 nanofibres to an epoxy resin. However it should be

noted that the 0% moisture sample still shows a slight increase in initialreaction speed over the pure resin sample. This last result indicatesthat there are also a limited amount of catalysing groups present on thenanofibres, independent of the moisture content.

Figure 1 also shows the effect of conventional PA6 microfibres onthe initial reaction speed. Although the moisture content in these mi-crofibre is approximately the same as the one in the nanofibres, cleardifferences in initial reaction speed between nanofibre samples (♦) andconventional PA6 microfibre (�) samples can be noted. The reactionspeed of the conventional polyamide samples follows a linear trend un-til a moisture adsorption of around 2,7%, after which the reaction speedof the nanofibre containing samples keeps increasing while those of theconventional microfibers stagnates.

This difference can be explained by higher diameter (60 times) andlower specific surface area of the conventional PA6 microfibres. Re-sulting in a much lower contact area between resin and fibre. In thisresearch it was shown that the diffusion of moisture into the DGEBA-MDA resin is a very slow process, so although the total amount ofmoisture in the PA6 microfibers is approximately the same as in thenanofibres, there is not enough time for this moisture to diffuse into theresin and cause an acceleration of the initial reaction speed.

In addition the curing study conducted on the DGEBA-MDA re-sin hardener system, research was done on the commercially availableREACH proof RIM135-RIM137 resin. It was shown that the nanofibremoisture content has a similar effect on this resin as well.

C. Plasticizing effect

To study the plasticizing effect, the final glass transition tempera-ture (Tg∞) was measured. Figure 2 shows that the Tg∞ of the nano-fibre containing samples is lower than that of the neat resin for all fibremoisture contents. As far as the relation ship between moisture contentand Tg∞ is concerned, statistical analysis of variance showed that themeans of the Tg∞ at the different moisture contents are approximatelyequal. Further when adding a drop of water to the resin (not contai-ning nanofibres) the Tg∞ did not decrease. This suggests that nanofibremoisture content is not the cause of the plasticizing effect of nanofibreson the DGEBA-MDA resin.

Fig. 2. Tg∞ of nanofibre (♦) and microfibre (�) containing samples in function of fibremoisture content

More likely this plasticizing effect is caused by preferential migra-tion (or adsorption) of one of the resin components to (or on) the po-lyamide nanofibres. This migration would disturb the stoichiometricepoxy-amine ratio and thereby lower the Tg∞. This hypothesis is sup-ported by previous research indicating the influence of the epoxy amine

ratio on the Tg∞ [3]. If this hypothesis is true one would expect conven-tional PA6 microfibre to have a less significant effect on the Tg∞ due tothere smaller surface area. This is also confirmed by the data, in Figure2 one can see that the Tg∞ of the conventional microfibre containingsamples is higher than that of the nanofibre samples for nearly all mois-ture contents.

III. EFFECT OF PA6 NANOFIBRES ON THE DYNAMIC MECHANICALPROPERTIES OF COMPOSITES

A. Effect on the stiffness

Fig. 3. Storage modulus of neat epoxy resin (· · · ) and nanofibre composite (—)

Figure 3 shows the storage modulus in function of temperature forboth a neat epoxy resin and for a nanofibre-epoxy composite sample.When comparing the two storage moduli it was found that at a tempe-rature below 21,69±7 ◦C the nanofibres increase the stiffness of theepoxy resin, this was also confirmed by tensile experiments carried outduring this research. Above this temperature the stiffness of the nano-fibre composites is lower than that of the epoxy resin samples. Thisdecreased stiffness is similar to what is observed when thermoplasticparticles are added to the epoxy resin [1], [4], [5]. The point where thestiffness of the nanofibre composite starts to become lower than that ofthe neat epoxy resin was found to be close to the onset point of the Tg

of the nanofibre web.

B. Effect on energy adsorption and toughness

Fig. 4. tan(δ) of neat epoxy resin (· · · ) and nanofibre composite (—)

In this research it was shown that the loss modulus which is a mea-sure for the energy dissipation via heat of a material showed an increaseof over 56% for the nanofibre containg samples. The damping factorshows similar behavior as the loss modulus with an increase of 54 % ascan be seen in Figure 4. Often a correlation between damping factorand fracture toughness as well as impact strength exists [6]. Around20 ◦C there is a zone were the storage modulus, the damping factor and

the loss modulus are higher for the nanofibre composite as compared tothe neat resin, suggesting that in this zone an increased stiffness com-bined with an increased toughness might be achievable. It can also benoted that the (Tg∞) (derived from the peak in dampingfactor) of thenanofibre composite is slightly lower than that of the neat resin. Thisis also what could be expected from the curing study on the DGEBA-MDA resin.

IV. CREEP BEHAVIOUR

A TTS creep study was conducted on [+45◦,−45◦]2S glass fibre-PA6 nanofibre-epoxy composite. These creep experiments suggestedthat the nanofibres increase the creep resistance of these composites.However the spread on these measurements was rather high,thereforfuture research is needed to verify whether the improved creep resis-tance was really due to the presence of the nanofibres.

Fig. 5. Average masters curves obtained from three TTS creep experiments conducted on[+45◦,−45◦]2S glass fibre-PA6 nanofibre-epoxy composite samples (—) as well asreference master curves obtained from three creep experiments conducted on normal[+45◦,−45◦]2S glass fibre-epoxy composite without nanofibres (· · · ).

V. CONCLUSION

It has been shown that the nanofibre moisture content an importantcause for the catalysing effect of PA6 nanofibres on the DGEBA-MDAresin hardener system. The observed plasticizing effect is most likelycaused by the adsorption of one of the resins components onto thenanofibre surface. PA6 nanofibres have the potential to increase thestiffness of epoxy at temperatures below 20 ◦C, at higher temperaturesthe energy dissipation via heat as well as the damping factor of thenanofibre composites is higher than that of neat resin. This increaseddamping factor might be correlated with and increased toughness andimpact resistance of the nanofibre composites.

REFERENCES

[1] X. Wang J. Zhang, Z. Lin, ,” Composites Science And Technology, vol. 70, pp. 1660–1666, 2010.[2] S. Moorkens, Innovative Polyamide Nanofibre Composites: Effect Of The Nanofibres On The Curing

Characteristics Of The Dgeba Matrix, Master thesis, 2011.[3] B. Van Mele S. Swier, G. Assche, ,” Journal Of Applied Polymer Science, vol. 91, pp. 2814–2833,

2003.[4] Z. Chen Y. Yunhua L. Haiyang Z. Shen J. Xiaolong Y. Xiaoping X. Zhongmin R. Seungkon L. Gang,

L. Peng, ,” Composites Science And Technology, vol. 68, pp. 987–994, 2008.[5] P. Terry Mcgrail T. Peijs P.J. Hogg D.W.Y. Wonga, L. Lin, ,” Composites: Part A, vol. 41, pp.

759–767, 2010.[6] M. Misra L.T. Drzal H. Miyagawa, A.K. Mohanty, ,” Macromolecular Materials And Engineering,

vol. 289, pp. 636–641, 2004.

Contents

Acknowledgment i

Copyright notice ii

Summary iii

Extended abstract vi

Contents vii

Utilized abbreviations viii

1 Introduction 11.1 Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1.1 What are composites? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1.2 Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1.3 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.1.4 Fillers and secondary reinforcement . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2 Nano reinforcements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.2.1 Inorganic nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.2.2 Thermoplastic (nano)-particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.2.3 Nanofibres as secondary reinforcement . . . . . . . . . . . . . . . . . . . . . . . . 3

1.3 Epoxy matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.3.1 Curing of a thermoset matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.3.2 Influence of nanotechnology on curing kinetics of epoxy . . . . . . . . . . . . . . . 7

1.4 Thermal analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81.4.1 Differential scanning calorimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . 81.4.2 Dynamic mechanical analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1.5 Objective of the thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2 Materials and methods 182.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.1.1 Fibrous materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182.1.2 Epoxy resins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182.1.3 Composite samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.2 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

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CONTENTS vii

2.2.1 Moisture absorption determination . . . . . . . . . . . . . . . . . . . . . . . . . . . 192.2.2 Modulated temperature differential scanning calorimetry . . . . . . . . . . . . . . . 192.2.3 Thermal activity monitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202.2.4 Dynamic mechanical analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202.2.5 Tensile experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222.2.6 Scanning Electron Microscopy (SEM) . . . . . . . . . . . . . . . . . . . . . . . . . 22

3 Reproducibility of curing experiments 233.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233.2 DGEBA-MDA system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.2.1 Neat resin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233.2.2 Resin combined with nanofibres . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

3.3 RIM135-RIM137 system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

4 Effect of nanofibre moisture content on curing kinetics 294.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294.2 Evaluation of moisture adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294.3 Catalysing effect of the nanofibre moisture content on the DGEBA-MDA curing reaction . . 324.4 Comparison between effect of the PA6 nanofibres and PA6 microfibres on the curing reaction 374.5 Plasticizing effect of PA6 nanofibres on the DGEBA-MDA resin . . . . . . . . . . . . . . . 394.6 Comparison between effect of the PA6 nanofibres and PA6 microfibres on the Tg of the fully

cured resin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414.7 RIM135-RIM137 resin hardener system . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

5 Thermo mechanical analysis of nanofibre composites 445.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 445.2 Dynamic mechanical properties of nanofibre epoxy composites . . . . . . . . . . . . . . . . 44

5.2.1 Reproducibility of oscillating temperature ramp experiments . . . . . . . . . . . . . 445.2.2 Stiffness of nanofibre composites . . . . . . . . . . . . . . . . . . . . . . . . . . . 465.2.3 Energy dissipation and toughness of nanofibre composites . . . . . . . . . . . . . . 495.2.4 Morphology of PA nanofibre composites . . . . . . . . . . . . . . . . . . . . . . . 50

5.3 Dynamic mechanical properties of glassfibre epoxy composites secondary reinforced withnanofibres . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 525.3.1 Effect of PA6 nanofibres on storage modulus and damping factor . . . . . . . . . . . 525.3.2 Effect of PA6 nanofibres on creep behaviour . . . . . . . . . . . . . . . . . . . . . . 54

6 Conclusion and future work 59

Bibliography 61

List of Tables 66

List of Figures 69

Utilized abbreviations

α ConversionANOV A Analysis of varianceAs Specific surface areaaT Shift factor used in time temperature superpositionCNF Carbon nanofibresCNT Carbon nano tubesdf Degrees of freedomdfibre Average fibre diameterδ Phase angle between stress and strain in dynamic mechanical analy-

sis∆Cp Change in heat capacity during vitrification∆Hisothermal Reaction enthalpy released during isothermal curing∆Hresidual Residual reaction enthalpy∆Htotal Total reaction enthalpy∆tvitrification Width of the vitrification region expressed in units of timeDGEBA Diglycidyl ether of bisphenol ADMA Dynamic mechanical analysisDVS Dynamic vapour sorptionE´ Storage modulusE´´ Loss modulusFav Average functionality used in carothers equationFc Clamping factor used to correct for clamping errors in dynamic me-

chanical analysisFRP Fibre reinforced polymerHF Heat flowI Moment of inertiaKs Stiffness measured by dynamic mechanical analysisL Length between the clamps (dynamic mechanical analysis)(MT)DSC (Modulated temperature) differential scanning calorimetryMDA MethylenedianilineMw Molecular weightMWNT Multiwalled carbon nanotubes

viii

UTILIZED ABBREVIATIONS ix

NF NanofibresPA PolyamidePAAM PolyacrilamidePANI PolyanilinePBI PolybenzimidazoleQ Flow rateR Universal gas constant 8, 3144621 J/(molK)

REACH Registration, Evaluation, Authorisation and Restriction of Chemicalsubstances

RH Relative humidityρ Mass densityσ StressSEM Scanning Electron MicroscopyS Creep complianceTAM Thermal activity monitorTcure Curing temperatureTg Glass transition temperatureTg0 Initial glass transition temperature of uncured resinTg∞ Final glass transition temperature of fully cured resintmaxHF Time needed to reach the maximum value of the heat flowTTS Time temperature superpositiontvitrification Time it takes until system starts to vitrify during isothermal curingtr Reduced time used in time temperature superpositionν Poisson’s ratiowt% Weight percentXn Number average molecular weight

Chapter 1

Introduction

1.1 Composites

1.1.1 What are composites?

In the most general case a composite material can be defined as an engineered or naturally occurring materialmade from two or more constituent materials with significantly different physical or chemical propertieswhich remain separate and distinct at the macroscopic or microscopic scale within the finished structure.Most composites consist of a matrix and reinforcement. The matrix is a continuous phase that binds thereinforcement together. It transfers the external loads to the reinforcement and gives the composite productits shape and surface appearance. Furthermore, it protects the reinforcement from the environment. Thereinforcement, on the other hand, gives the product its macroscopic strength and stiffness. Meanwhile itcarries the structural load in the composite material [1, 2, 3].

In the more specific case of fibre reinforced polymers (FRP) the reinforcement consist of fibres, these couldbe inorganic like e.g. glass or carbon fibres or polymeric e.g. aramide fibres. The matrix materials is usuallypolymeric, it could either be a thermoplastic material e.g. polypropylene, or a thermosetting material e.g.epoxy, vinyl ester or polyester. Most FRP used in industry today use glass or carbon fibres as a reinforcementand have a thermosetting matrix material. Thermoplastic matrix materials currently only have a limited usedue to the need for high temperatures and pressures during the production process. Although they dohave some advantages over thermosets, like a better toughness which is one of the main weaknesses ofthermosetting resins.

1.1.2 Properties

As mentioned before, FRP combine the properties of two or more physically different materials. Dependingon the combined materials and the configuration of the reinforcement (form, fibre length and diameter,orientation, distribution and volume fraction), the properties of the composite will be different. The resultingproperties should be superior and unique compared to the properties of the individual components.

Composites offer lots of advantages of classical materials like metals and ceramics, one of the main advan-tages is their high strength and stiffness to weight ratio, which makes a significant weight reduction possible.Moreover, the strength or stiffness can be tailored in the load direction. This is hardly possible with the clas-sical materials like steel and alumina. Since composites are more resistant to corrosion, they have a longer

1

CHAPTER 1. INTRODUCTION 2

lifetime compared to metals. Other advantages of composite materials are their good dimensional stabilityand good vibration damping. They give a high freedom for design and it is possible to make small seriesof highly individualized products as well as doing mass production. However, composites also suffer fromsome disadvantages; from an environmental point of view composites are hard to reuse or recycle. Apartfrom that, the analysis of the behaviour of composites is more difficult than that of classical materials and thecost of the raw material and fabrication can be relatively high. As far as mechanical properties are concernedthe usually low toughness of the matrix is one of the major disadvantages of composites [1, 4, 5, 6].

1.1.3 Applications

Composite materials are widely used in industry and have a wide range of applications. The CompositesInstitute from the Society of the Plastics Industry (SPI) divided all the applications in the composite marketinto nine major commercial segments. This is shown in figure 1.1 [5].

Polymer composites

Corrosion-Resistant Equipment (chemical-resistant service)

•pipe and fittings,

•Tanks and containers

•Pumps

•Filtration equipment, water/wastewater treatment equipment

•Oil fields sucker rods, grating and related products for the oil and gas industry

Electrical/Electronic

•Rods, tubes

•Electronic microwave antennas

•Injection molded boards

•Polyester-based panel boards, housings and circuit breaker boxes

Appliance/Business equipment

•Refrigerators and freezers

• Microwave ovens and microwave cookware

•Small household appliance

•Calculators, computers, copiers

Marine (commercial, pleasure and naval boats and ships)

•Moter covers

•Moorings

•Marine docks

•Buoys, floats

•Canoes, kayaks, personal watercraft and car-top boats

Construction (building trads)

•Swimming pools, whirlpools, bathtubs, shower stalls, lavatories

•Concrete pouring forms

•Paneling for greenhouses

•Bridge decks and components

•Highway signs

Aircraft, Aerospace, Defense(commercial, pleasure or military aircraft)

•Flight surface

•Cabin interiors and accessories

•Heat shields

•Components and rocket motor casings for aerospace and related applications

•Military helmets

•Rocket launchers

Consumer Products (sports and recreational)

•Fishing rods

•Skis and snowboards

•Bicycles

•Exercise equipment

•Serving trays

•Boxes and containers

•Furniture

Transportation

•(trailer) body panels, grill opening panels, instrument panels

•Truck cabs

•Wind deflectors

•Subway car seats and components

•Scooters

•Tractor parts

Other

Figure 1.1: Examples of composite applications divided in nine major commercial segments [3].

1.1.4 Fillers and secondary reinforcement

Apart from the matrix and the main reinforcing fibres, additional fillers can be used to improve the mecha-nical properties of composites. Such filler can be very different in nature from the matrix and main reinfor-cement. Examples of such fillers are rubber particles, thermoplastic particles, nano-clay particles,Al2O3,


CaCO3, carbon nanotubes and so called secondary fibrous reinforcements. Rubber and thermoplastic par-ticles can be added to improve the toughness of thermosetting matrix composites. Nano-clay particles,Al2O3, CaCO3 and carbon nanotubes mainly aim to improve the stiffness and the fibre matrix inter facialproperties. Secondary fibrous reinforcements are a special type of filler, they consist of fibrous structure witha substantially lower fibre diameter compared to the primary fibre to improve the mechanical properties ofcomposite materials [7].

1.2 Nano reinforcements

1.2.1 Inorganic nanoparticles

A large amount of research has been done to improve the mechanical properties of the thermosetting matrixby adding inorganic nano-particles, like nanoclays, vapour grown carbon nanofibres, and carbon nanotubes(CNT) to the matrix. Some of the principles used in the development of nanoparticle fillers might be appli-cable to nanofibres as well. However a full review of all papers published on the topic of nano-compositesis beyond the scope of this thesis, since the focus is given to electrospun nanofibres. Therefore, the researchcited in this section is only a fraction of the hundreds of articles that are available in literature. In CNT andvapour growth carbon nanofibre nanocomposites, one aims to improve the resin and fibre/resin interface pro-perties, by mixing in these nanoparticles with a theoretically exceptionally high stiffness and strength. Thisleads to improvements of the epoxy matrix stiffness, strength and toughness [8, 9, 10]. However one shouldnote that the overall improvement in mechanical properties is only moderate in most cases [11, 12, 13]. Oneof the latest developments in CNT and vapour grown carbon nanofibre nanocomposites, is the functionali-zation of these nanoparticles to allow covalent bonds between the reinforcing particle and the matrix. Thisfunctionalization allows for a significant improvement in fracture toughness and modulus as compared tothe non functionalized CNT [9, 14, 15, 16, 17]. The same can be said about nano-clay particles.

Although lots of positive results were obtained with using nanoparticles to improve the mechanical pro-perties of the epoxy matrix, at present, the use of nanoparticles still faces problems. An important issuein nanoparticle composites is the need for special mixing methods to obtain a homogeneous dispersion ofnanoparticles in the resin. Also the high production cost of CNT and more importantly the potential healthhazard involved with the use of nanoparticles needs to be tackled [18].

1.2.2 Thermoplastic (nano)-particles

Another method to improve the toughness of the matrix is incorporating rubber particles or embeddingthermoplastic inclusions. These thermoplastic materials can be added to the resin as nanoparticles butalso thermoplastic sheets, membranes and fibres are used. Although the toughness can be significantlyimproved by adding thermoplastic materials to the resin, the stiffness of the resins is usually compromised.Furthermore, obtaining a homogeneously dispersed mixture of resin and thermoplastic material is found tobe very difficult [19, 20, 21].

1.2.3 Nanofibres as secondary reinforcement

Polymer nanofibres might be especially interesting for improving the toughness of composite materials.As for conventional composites, the mechanical reinforcement requires an efficient transfer of mechanical


loads from the matrix to the (nano-)fibres. Due to the very small diameter of nanofibres, the nanofibres havea high specific surface area, roughly 100 times larger than for conventional fibres, ensuring a large contactarea between the reinforcement and the matrix, and strengthening their overall interaction.Furthermore nanofibrous webs can be readily embedded in the resin, they have the large benefit of theirinherent nanoscale distribution which may improve the traditional limitations in (nano)particle dispersion.Owing to their macro scale length, no health hazards are involved in the production and use of electrospunnanofibres.

Figure 1.2: scanning electro microscope (SEM) image of electrospun nanofibres compared to a human hair.

Nanofibres can be produced by different processing techniques. Electrospinning is proven to be an efficienttechnique which can also be used for mass production. Other processing techniques which are for exampledrawing, template synthesis, phase separation and self-assembly, but these are not suitable for mass produc-tion [22, 23]. Using electrospinning, nanofibres with a diameter far below 500 nm can be obtained. Thescanning electron microscope (SEM) image in Figure 1.2 illustrates the size of a electrospun nanofibre bycomparing it to a human hair.

For this thesis elctrospun polyamide 6 (PA6) nanofibres are used. Polyamide is a macromolecule in whichstructural units are interlinked with the amide groups (NHCO) linkage. There are several different poly-amides from which PA 6.6 and PA 6 are the two with the highest importance for commercial fibre pro-duction. Generally, polyamides have excellent mechanical properties and abrasion and chemical resistance.However, while they are tough under many impact conditions they tend to be notch sensitive and brittle atlow temperatures. To improve this, modification with various additives is needed [24]. Until now there hasonly been a limited amount of research available on the mechanical properties of nanofibres specifically. Arecent article produced by Woong-Ryeol et al. [25] showed that the mechanical properties of PA6 nanofibresare superior to those of PA6 microfibres and suggested its potential use in composite applications.

Until now the research published on the use of elctrospun nanofibres as secondary reinforcement has beenlimited to only a few articles which are summarized below.

Elif Ozden et al. [26] created nanofibre webs from polystyrene-co-glycidyl methacrylate P(St-co-GMA)functionlized by spraying over the ethylenediamine (EDA). An increased storage modulus was observedeven though the epoxy only contained 2 wt % of nanofibres. In the begining of 2012 Elif Ozden et. alpublished an other article in wich they combined elecrospun P(St-co-GMA) nanofibres with CNT. It wasfound that the stiffness of the material increased over 20 wt% as compared to the pure resin reference


sample.

Kim and Reneker [27] observed the influence of polybenzimidazole (PBI) on epoxy. They reported that thePBI nanofibers give a higher fracture toughness and modulus compared to the pure epoxy reference.

Some of the polymer composite applications reinforced with electrospun nanofibers have been developedmainly for providing some outstanding physical (e.g. optical and electrical) and chemical properties whilekeeping their appropriate mechanical performance [28]. An example of such a case (Berghoefs et al.) isthe addition of electrospun PA 4.6 nanofibers (30− 200 nm) in an epoxy composite. Apart from impro-ving the stiffness and strength of the composite compared to a reference matrix film, this composite has acharacteristic transparency due to the fiber sizes smaller than the wavelength of visible light [29].

Research by Gang Li et al. [19] focused on the use of electrospun nanofibres as an alternative to thermo-platic particles and sheets to evenly distribute the thermoplastic material into the resin. In that research thenanofibres were first embedded into the resin, in a later stage they were melted inside the composite. Andthe obtained composite showed an increased toughness but a decrease in stiffness.

UGent-Tex and UGent-MMS underpined the potential of nanofibres for composite applications [30]. Du-ring a thesis by Langendries et al. Glasfibre/epoxy composites secondary reinforced with nanofibres werestudied. From tension (shear) tests on [+45/ − 45]2s composites it was found that the tested glass fibrereinforced epoxy composites showed an increased shear modulus on the addition of PA6 nanofibres from3,8 GPa to 4,9 GPa. Also the work of rupture increased. Furthermore an increase in matrix breaking pointfrom 65 MPa, for pure glass fibre composites, to 75 MPa was observed.

1.3 Epoxy matrix

The matrix is an important component of the composite, it transfers the external loads to the reinforcementand gives the composite product its shape and surface appearance. Furthermore, it protects the reinforce-ment from the environment. Epoxy is the resin material of choice for composites which are used for highend applications like airplane fuselage, wind turbine blades, and other high temperature aerospace appli-cations. These critical applications can justify the use of this superior performing, but higher cost, resinsystem (compared to vinyl ester or polyester resin). Epoxies can be manufactured with most composite ma-nufacturing processes, particularly vacuum-bag molding, compression molding, filament winding and handlay-up are suited [5, 31].

The structure of an epoxy resin can be engineered. This results in a broad variation of commercially avai-lable epoxies. Due to these engineered epoxy structures, it is possible to have a number of different productswith varying levels of performance and a broad range of properties. In general, epoxies are known for theirexcellent adhesion, chemical and heat resistance, relatively good mechanical properties and very good elec-trical insulating properties. The main disadvantages of epoxy resins are a relatively high thermal coefficientof expansion, high degree of smoke liberation during combustion, slow curing and low toughness [5, 6].

1.3.1 Curing of a thermoset matrix

The solidification of a thermoset matrix is called curing. It is an exothermic reaction that includes polyme-rization and cross-linking. During the cure of a resin, the system changes from a viscous liquid to a highly


crosslinked network. The rate of the curing reaction is dependent on two main aspects: the activity and themobility of the functional groups. The first stages of the curing reaction are controlled by the activity of thefunctional groups or in other words, these stages are chemically controlled. The second stage of the reactionis controlled by the mobility of the functional groups or in other words, this stage is diffusion controlled.

The proceeding of the curing reaction results in an increase in chain branching. The molecular mass of thesystem grows gradually and will become infinite at a critical conversion. This phenomenon is known asthe gelation of the system. After the gelation, there is no increase in molecular weight, only an increase incrosslink density and a decrease in free chain segment length.

During the curing reaction the glass transition temperature (Tg) of the system increases from the initial Tgof the resin/hardener mixture (Tg0) to a final, higher Tg of the fully cured resin (Tg∞). If during this processthe Tg of the system becomes higher than the curing temperature one can observe a phenomenon calledvitrification. Vitrification is a totally different transition in the network formation than gelation. Duringvitrification, the thermoset passes from a rubbery state into a glassy state with other words at the vitrificationpoint the (Tg) of the resin becomes equal to the curing temperature. It can be achieved for isothermal aswell as non-isothermal curing and before or after gelation [32, 33]. When the system vitrifies the mobilityof the system decreases drastically, therefore a vitrified system will always be diffusion controlled [34].

Amines are the most commonly used curing agents for epoxy resins. The reactions occurring in an epoxy-amine system are step-growth polymerization reactions and more specific addition reactions. The mainparameters that control the polymer structure are the number of reactive sites per monomer (functionality)and the molar ratio between co-reactive sites. To obtain a crosslinked polymer, at least one of the monomersneeds to have a functionality higher than two [34, 35].

During the curing of an epoxy-amine system, several reactions take place at the same time. When thereis no catalyst involved, there are two basic reactions occurring as shown in figure 1.3. The first one is thereaction between a primary amine and an epoxide ring to form a secondary amine. The second one is thereaction of the formed secondary amine with an epoxide to form a tertiary amine. Due to the opening of theepoxy ring, both reactions will also produce a hydroxyl group. These hydroxyl groups can act as a catalystto accelerate the reaction at the early stages and so they give the typical course of an autocatalysed reaction.When the amine is present in less than stoichiometric concentrations, ether links and new hydroxyl groupscan be produced by the reaction between the (remaining) epoxy groups and the hydroxyl groups. Thisetherification is shown in Figure 1.4. When the epoxy and amine are mixed in stoichiometric quantities orwhen there is an excess of amine, the etherification reaction is negligible [36, 37, 38].The detailed cure mechanism of epoxy resins is very complex because many reactions take place at the sametime. Furthermore the reactions are affected by phenomena as vitrification, gelation and the change fromchemically controlled to diffusion controlled curing. Incorporation of nanoparticles or nanofibres makes thecuring process even more complicated [31].


Figure 1.3: Curing of epoxy using amines: two basic reactions [36].

Figure 1.4: Etherification of epoxy [36].

1.3.2 Influence of nanotechnology on curing kinetics of epoxy

Nano-particles

A limited number of results have been published concerning the influence of nano-sized components on thecure kinetics of epoxy systems. Cai et al. [31] and Xie et al.[39] reported an acceleration of the curingreactions by adding carbon nanofibres and multiwalled carbon nanotubes, respectively, as a result of thepresence of hydroxyl groups. Hydroxyl groups tend to react with the epoxide group and open the ringsof epoxide. Then, the etherification of opened epoxide rings with diamine can occur more easily than thedirect reaction of epoxide with diamine. An interesting remark by Xie et al. was that curing at a highertemperature than the vitrification temperature, hinders the reaction due to the decrease in segment mobility.Cai et al. also studied the effect of polyaniline (PANI) modified carbon nanofibres on the cure kinetics ofepoxy. They reported that this surface modification accelerates the cure reaction even more, compared toonly CNF. Additionally, they observed a significant increase in (∆Htot). They explained this effect due tothe presence of secondary amine groups in PANI. Earlier it was already shown by Palaniappan et al. [40] thatpolyaniline can be used as a curing agent for epoxy resins. Mijovic and Wang [41] reported that conventionalgraphite fibres itself only have a very small effect on the curing kinetics of epoxy. The influence of Al2O3

filler on epoxy matrices is less clear. Sanctuary et al. [42] and Ji et al. [43] described an acceleration of the


curing reactions, whereas Akatsuka et al. [43] reported a delay. Zhang et al. [44] and Le Pluart et al. foundthat silica grafted with polyacrylamide and montmorillontie clay treated with alkylammonium, respectively,promote the curing of epoxy. Whereas adding unmodified silica (SiO2) nanoparticles to an epoxy resin hasa decelerating effect on the curing reaction. The clay does not affect the final glass transition temperatureand network structure.

Effect of electrospun nanofibres

The influence of electrospun nanofibres on the curing behaviour or resins specifically has been limited to onemaster thesis by Moorkens et al. [3]. In this research the effect of nanofibres on the curing characteristicsof a DGEBA-MDA matrix was investigated by using quasi-isothermal modulated temperature differentialscanning calorimetrie (MTDSC) measurements. It was found that addition of polyamide nanofibres into theresin resulted in a catalytic effect on the DGEBA-MDA cure. Both the effect of PA6 nanofibres as wellas conventional fibres was investigated, as the difference in catalytic behaviour between conventional andnanofibres of PA6 was very small, the amide groups at the fibre surface were not thought to be responsiblefor the acceleration. It was suggested that moisture present in the (nano) fibrous structure might be the causeof this catalysing effect. In addition, the influence of the polyamide nanofibres on the curing characteristicsand kinetics was compared to the influence of PVAL nanofibres, which have hydroxyl groups, known tocatalyse the epoxy cure. Although the PVAL nanofibres have a lower moisture content, the acceleratingeffect by adding PVAL nanofibres was higher compared to the PA nanofibres, indicating that in this case thesurface hydroxyls are markedly catalysing the cure reaction.

1.4 Thermal analysis

1.4.1 Differential scanning calorimetry

In this section the basic working principle of differential scanning calorimetry (DSC) and modulated tem-perature differential scanning calorimetric (MTDSC) will be explained. The emphasis will be on the use ofMTDSC to study curing behaviour. A lot of valuable information on the curing kinetics of a thermoset canbe obtained from of thermal analysis because the degree of cure can be related to the heat of reaction [45].

DSC and MTDSC basic principal

Figure 1.5 shows a schematic setup of a DSC. Essentially a DSC measures the temperature differencebetween the pan containing the sample and the reference. From this temperature difference a heat flowsignal is calculated. The reference is an empty pan of identical making as the pan containing the sample. Itis important to note that the actual temperature of the sample is never measured. The temperature sensorsare build into the sample platforms as shown in Figure 1.5. In the best case the pan temperature is calculatedbased upon the pan material and shape.

The most basic conventional DSC devices use the heat flow equation shown in Equation 1.2. This equationassumes that the calorimeter thermal resistances are identical, the temperature of the furnace at the sampleand the reference calorimeters are equal and does not include other known heat flows. Modern DSC´s usemore complex equations and correct for several imbalances like differences in thermal resistance between


Figure 1.5: DSC basic compomentents

sample and reference, differences in heat capacity between sample and reference, and differences in heatingrate.

q =Tr − Ts

R(1.1)

In a DSC one can conduct isothermal experiments as well as temperature ramp experiments. The DSC cangive information about, glass transitions, melting and boiling points, crystallisation time and temperature,heats of fusion and reactions, specific heat capacity, oxidative and thermal stability, rate and degree of cure,reaction kinetics, purity,....

In MTDSC the heat flow is obtained in the same way as for DSC, the only difference between DSC andMTDSC is that in the case of MTDSC a modulation is applied on the temperature. This modulation ispresent in a temperature ramp experiment but also in an isothermal experiment. That is why for MTDSCthe term quasi-isothermal experiment is preferred since the temperature is never truly constant.

Generally heat flow can be divided in two part (as shown in Equation 1.2). A heat capacity componentCp.dTdt

and so called kinetic component, which is a function of time and temperature. Applying a modulation onthe temperature allows to divide the heat flow signal in two parts, one that does and one that not respond toa changing heating rate. The so called reversing and non-revesing part. In most cases, only heat capacityand melting respond to the changing heating rate, whereas kinetic components of the total heat flow can befound in the non-reversing part [35].

q = Cp.dT

dt+ f(T, t) (1.2)

The advantages of MDSC over DSC are for example the higher resolution and sensitivity, and the ability toseparate overlapping thermal transitions. The use of the MDSC is particularly useful in analysing heavily


filled composites, where differing small transitions can be a difficult task [46]. Another obvious benefitof MTDSC is that it is possible to measure the heat capacity during an isothermal experiment, which isimpossible in a normal DSC experiment.

Use in curing studies

As mentioned before MTDSC is a powerful tool for analysing the curing behaviour of resins, since the heatflow is proportional to the reaction speed. Figure 1.6 shows the heat flow signal from a quasi-isothermalMTDSC experiment.

Figure 1.6: Figure showing heat flow (A) and rev Cp signal obtained from isothermal MTDSC scan onepoxy resin

The reaction enthalpy can be obtained by integrating the heat flow signal. Form this same quasi-isothermalcuring experiment one can also obtain the heat capacity in function of time (rev. Cp). This heat capacity isa measure of the mobility of the system and can also be used to track down the vitrification point (if thereis any). Figure 1.6 (B) shows the Rev. Cp signal, the sudden downward step of this signal is associatedwith vitrification, the Tg of the reactive mixture becomes equal to the curing temperature and therefore themobility of the system decreases drastically.


Figure 1.7: Figure showing the heat flow signal obtained from a non-isothermal DSC scan on an epoxyamine resin, as well as the corresponding total reaction enthalpy

Figure 1.8: Illustration of different methods to obtain the Tg from an MTDSC measurement, A via the (rev)Cp , B via the (rev) heat flow, C using the derivative of the (rev) Cp, D using the derivative of the (rev) heatflow


So from a simple isothermal MTDSC scan one can already obtain two valuable parameters: the evaluationof the reaction speed in function of time and the reaction enthalpy and information about the system mobilityvia the rev. Cp signal.

By conducting further non-isothermal scans one can obtain even more information about curing reaction.The total reaction enthalpy of a curing reaction can be obtained by preforming a DSC scan over the fulltemperature range and integrating the obtained heat flow peak. It should be noted that the total reactionenthalpy is not necessarily the same as the reaction enthalpy obtained during a quasi isothermal MTDSCscan because some of the curing reactions might require a higher activation energy (higher temperature)than the isothermal curing temperature. Figure 1.7 shows a non-isothermal DSC scan on an epoxy amineresin as well as the corresponding total reaction enthalpy of this curing reaction.

When performing a non-isothermal MTDSC scan on a (fully) cured resin one can find the Tg of this resin.The Tg can be spotted as a downward step in the (rev.) heat flow signal or an upward step in the (rev.) Cp

signal (mobility of the system increases). In some cases the derivative of these signals is used so the stepsbecome peaks.

Figure 1.8 shows how the value of the Tg is obtained for each of these methods. One can notice that thesevalues are not exactly the same, however non of these results is more correct than the others.

One should not forget that the glass transition of a material does not occur at a single temperature but in a(sometimes wide) temperature region. To facilitate the comparison between different samples one associatesa single temperature with this region the Tg . This Tg depends on how it is defined, different definitions cangive rise to different numerical values of the Tg. In general the Tg obtained from a rev. signal heat flows/Cp

signal is always slightly higher than the one obtained from the normal heat flow/Cp signal. Further more thehigher the modulation frequency the higher the Tg one obtains. Therefore it is important to mention howthe the reported Tg was defined.

1.4.2 Dynamic mechanical analysis

Dynamic mechanical analyses basic principal

In this section the basic working principal of dynamic mechanical analyses (DMA) will be described, itis beyond the scope of this thesis to give a full explanation of all testing modes and abilities of DMA,rather than doing that the focus will be on those aspects relevant for the interpretation of DMA experimentsconducted in this thesis.

DMA is a thermal analysis technique that measures the properties of materials as they are deformed un-der periodic stress or load. Specifically, in DMA a variable sinusoidal stress is applied, and the resultantsinusoidal strain is measured or vice versa as can be seen in Figure 1.9 [47].

The phase difference between stress and strain together with the amplitude of the stress and strain wavesare the core parameters obtained from a DMA experiment, from these core parameters a variety of funda-mental material parameters like storage and loss modulus, damping factor, complex and dynamic viscosity,storage and loss compliance, transition temperatures, rate and degree of cure, sound absorption and impactresistance,... can be obtained. If the tested material is purely elastic, the phase difference between the stress


Figure 1.9: Figure illustrating the sinusoidal stress and strain as measured by DMA as well as the phasedifference between them

and strain sine waves is 0° (they are in phase). On the other hand, if the material is purely viscous, thephase difference is 90°. However, most real-world materials including polymers are viscoelastic and exhibita phase difference between those extremes.

Next to these so called oscillating experiments, the accurate force control and displacement measurement ofa DMA can also be used to carry out different types or non-oscillating experiments like creep and relaxationtests or small scale tensile tests. In the following subsections three types of experiment will be describedthat can be carried out on most commercially available DMA devices including the TA instrument Q800which is used for the experiments conducted in this thesis.

Oscillating experiments and Temperature ramps

One of the common DMA experiments found in literature is a temperature ramp, in such an experiment thetemperature is increased at a fixed rate usually between 1 to 10 ( C/min) and the oscillation frequency is keptconstant. The experiment can either be stress or strain controlled which means that either the amplitude ofthe sinusoidal stress or the amplitude of the sinusoidal strain wave is set as a constant input parameter for theexperiment. During the experiment the strain should always remain within the linear viscoelastic region ofthe examined material, this implies that the stress strain ratio is constant. Due to this reason, strain controlis mostly preferred over stress control definitely when the sample is examined over a broad temperaturerange. This is because the stiffness of most polymer materials varies a lot over a wide temperature range,at low temperatures the material is usually stiff and at high temperature it might become rubbery. In astress controlled experiment, the constant stress amplitude might cause displacements far above the linearviscoelastic region at high temperatures and displacement below the resolution of the displacement sensorat low temperatures. In a strain controlled experiment one will remain in the linear viscoelastic region sincethe maximum displacement is set as a constant over the full temperature range.

Figure 1.10 shows a typical result obtained from a temperature ramp, usually one is interested in threefundamental signals: the storage modulus (E´), the loss modulus (E´´) and the damping factor (tan δ).Figure 1.11 describes the mathematical relationship between these signals.


Figure 1.10: Typical signals obtained from DMA time-temperature ramp experiment. One can see the sto-rage modulus, loss modulus and damping factor in function of temperature for an epoxy glassfibre compositeheating rate 2,5◦C/min oscillation frequency 1Hz

The loss and storage modulus are often described as the ability to lose energy as heat and the ability torecover from deformation (elasticity) respectively [48]. For most clamping types the loss modulus is equalto the Young’s modulus, however, direct comparison between the loss modulus obtained using DMA and theclassic Young’s modulus obtained by tensile testing is not always possible, because the Young’s modulus,commonly thought of as a constant material property, is in general dependent upon time, temperature andthe direction of measurement. The modulus obtained by classic tensile tests depends upon the force rampapplied, while the modulus obtained via DMA depends upon the frequency at which it is measured.

Figure 1.11: Mathematical relation between storage modulus (E´), the loss modulus (E´´) and the dampingfactor (tan δ)

Recently some research has been done to find empirical correlations between the Young’s modulus obtainedby classic tensile and bending experiments and the storage modulus obtained by DMA, S. Denga et. alconcluded that a simple shift of the storage modulus already gives reasonably good results [49] as illustratedin Figure 1.12.

Besides the fact that the moduli and damping factor are interesting material properties by them self, theevolution of these signals as a function of temperature can be used to track down transitions in the material,one of the most obvious transitions that can be seen in case of polymer materials is the glass transition. The


Figure 1.12: Storage moduli measured by DMA compared with flexure testing results of Araldite-F resinmeasured at different temperatures [49]

glass transition can be spotted by a sudden drop in storage modulus and a peak in the loss modulus anddamping factor. When looking at Figure 1.10 one will notice that just like with the MTDSC measurementsdepending on which signal is used a different numerical value of the glass transition temperature will beobtained. These glass transition temperatures can be more than 15 °C apart. The reason for this spread inTg is that the Tg of a polymer is in fact never a sharply defined temperature but rather a region in which thematerial properties start to change drastically due to increased mobility in polymer chains. That is why, for agood comparison between experimental data, one should always mention how the reported Tg is mentioned.

Creep testing and time temperature superposition

The viscoelastic creep behaviour of polymers is a study field on its own, it has been shown that in some casesthe addition of nanoparticles to the composite matrix can improve the materials resistance to creep [50, 51].From this perspective it is worth investigating the possible effect of nanofibres on the creep behaviour ofepoxy amine based composites. To do this it is useful to provide a theoretical basis for such a study, thus thissection explains some basic models and principles useful for examining the creep behaviour of compositeswith DMA.

Creep is the time-dependent deformation of a material under constant load. While all materials exhibit aninitial elastic strain when loaded. As mentioned before, polymer materials (as well as other viscoelasticmaterials) show a combination of viscous and elastic responses to external forces, the material is consideredviscoelastic, or time-dependent. The strain of a viscoelastic material will be a function of both stress andtime [52].


ε = f(σ)g(t) (1.3)

In the case of a linear viscoelastic material, the stress dependency of the strain f(σ) is a linear function.To allow for a comparison between creep experiments at different stress levels, the creep compliance (S) isused. the creep compliance is defined as the strain (ε) divided by the applied (constant) stress (σ). For linearviscoelastic material the creep compliance is independent from the applied stress [53].

S(t, σ) = ε.(σ, t)

σ(1.4)

For most polymers, as well as for epoxy amine composite material, the evolution of strain in time canbe described by the curve in figure 1.13. One can divide this curve into four zones, zone i is the initialdeformation caused by the applied (constant) stress at time=0, zone ii is a transition zone, zone iii is calledthe equilibrium zone where the strain continues to grow in time, however, the rate of growth decreases.Finally, there is the recovery zone, at the onset of zone iv the force is removed and the material strain startsto decrease, even at infinite time the material might not recover to its original dimensions, in such a caseplastic deformation occurred inside the material.

Figure 1.13: Typical regions in creep recovery curve

Since the desired lifetime of composite materials is often measured in tens of years, it is impractical inmost cases to conduct long-term creep testing for the entire design lifetime of the material. Thus, muchresearch has been conducted and published on accelerated characterization of creep in composite materials[53, 54]. The time temperature superposition (TTS) principle is widely used in creep testing of compositesto determine the effect of temperature on the creep of FRPs. This theory was originally developed for usewith solid polymers, but has been expanded for use with fiber-reinforced composites [54]. By the principleof TTS, the effect of elevated temperature is assumed to be equivalent to stretching the real-time of the creepresponse by a certain shift factor. Through this method, short-term creep tests at a range of temperaturescan be used to generate a transient creep long-term compliance master curve [54]. The length of time of


the master curve is in most cases significantly longer than the short-term curves. With this method, theshort-term creep curves at each isotherm are plotted on a log scale. A reference temperature is chosen andthe other curves are shifted on a log scale by a shift factor, log aT . The shift factor is determined graphicallyby manually lining up the curves or by using a computer program. It was found that these shift factors oftenfollow the Arrhenius equation below the Tg of the material, above the Tg they often follow the Williams-Landel-Ferry equation.

1.5 Objective of the thesis

The main objective of this thesis is to study the effect of thermoplastic nanofibres on the epoxy matrix. Firstthe effect of polyamide 6 nanofibres on the curing behaviour of epoxy amine resins is investigated usingmainly but not exclusively modulated temperature deferential scanning caloremetrie. Thereafter the effectof polyamide 6 nanofibres on the thermo mechanical properties of composites is investigated.

Therefore chapter 3 contains: the reproducibility of epoxy amine curing experiments using MTDSC andTAM, as well as the influence of the nanofibres on this reproducibility. Furthermore the effect of the na-nofibres on the curing behaviour is investigated and compared with the results found in literature. Chapter4 focuses on finding the actual cause of the effects of nanofibres on the curing behaviour of epoxy amineresins observed in chapter 3. The main focus of this chapter is to provide a detailed understanding of theeffect of the nanofibre moisture content on the curing behaviour of epoxy amine resins. The moisture ad-sorption of epoxy resins as well as nanofibres and conventional fibres is investigated using DVS, the effect ofthis absorbed moisture is than studied using MTDSC, and TAM. Chapter 5 studies the effect of polyamide6 nanofibres on the thermo mechanical properties of composites using DMA. First nanofibre compositesconsisting of epoxy amine resin and polyamide 6 nanofibres are investigated and compared to the propertiesof neat epoxy resin. In the second part of chapter 5 the effect of polyamide 6 nanofibres on [+45,−45]2SGF-epoxy composites is investigated.

Chapter 2

Materials and methods

2.1 Materials

2.1.1 Fibrous materials

All polyamide 6 (PA6) nanofibres used in this thesis were produced at Ghent University department oftextiles by electrospinning these nanofibres had an average fibre diameter of 179, 2± 20, 3 nm [3]. Theconventional PA6 microfibres were obtained from Concordia Textiles and had an average fibre diameter of10, 43± 0, 14µm.

2.1.2 Epoxy resins

In this thesis, two different epoxy resin-curing agent systems are investigated. The first system uses theepoxy resin EPIKOTE™ RESIN 828 LVEL obtained from Hexion Specialty Chemicals. It is based on di-glycidyl ether of bisphenol A (DGEBA) and has an epoxy equivalent weight of 182− 187 g. As hardenerthe tetrafunctional amine methylenedianiline (MDA) was used, this is a solid hardener with an amine equi-valent weight of 49, 5 g and was purchased from Sigma-Aldrich. The resin and hardener were mixed instoichiometric quantities (epoxy/amine ratio = 1 ± 0.01) at elevated temperature. The 828 LVEL resin wasfirst heated at 160 ◦C for 15 min followed by addition of the grinded hardener and a quick stire. Immediatelyafter mixing, the hot mixture was cooled in liquid nitrogen (to stop the curing reaction).

The second system consists of EPIKOTE™ Resin MGS® RIM 135 with an epoxy equivalent weight of166− 185 g combined with a liquid diamine hardener, EPIKURE™ Curing Agent MGS® RIM H 137, withan average amine equivalent weight of 52 g/equivalent. Both, the resin and curing agent, are also obtainedfrom Hexion Specialty Chemicals. These the liquid resin and curing agent where mixed in stoichiometricquantities at room temperature (RT).

2.1.3 Composite samples

All composite samples were produced at the UGent department of Materials Science and Engineering, byresin transfer moulding based on the RIM135-RIM137 epoxy resin.

The nanofibre-epoxy composites contained an average of 18 weight percent nanofibres and had a total thi-ckness of 1 mm, the pure resin sample had the same thickness. The [+45,−45]2S GF-epoxy composites

18

CHAPTER 2. MATERIALS AND METHODS 19

secondary reinforced with PA6 nanofibres all had a thickness of 3 mm. The nanofibres were directly spunupon the glass fibre mats. A classic glassfibre-composite without nanofibres was used as a reference mate-rial. All these glass fibre containing samples where post cured at 110 ° for 500 min prior to testing.

2.2 Methods

2.2.1 Moisture absorption determination

The moisture adsorption of PA6 nanofibres and conventional PA6 microfibres as well as both epoxy resinswas evaluated using the TA instruments Q5000 dynamic vapour sorption (DVS) analyzer. All samples wereanalysed in TA instruments DVS quartz pans, at 23± 1 ◦C.

Both for nanofibres as well as for conventional PA6 fibres a full sorption and desorption isotherm wasmeasured. The method started with drying the fibres at 0 % relative humidity (RH) and 23 ◦C until theweight change of the sample was less than 0,01% for 15 min. In the following steps the moisture adsorptionof the fibres was determined at different relative humidity levels going from 5% to 95% relative humidity insteps of 10%. Once the moisture adsorption at 95% was determined, the moisture desorption was measuredby lowering the relative humidity from 95% to 5% in steps of 10%. For all adsorption and desrption stepsthe sample was allowed to equilibrate until the weight change was less that 0,01% during the last 15 min ofmeasurement.

For of the RIM135-RIM137 and DGEBA-MDA resin-hardener systems the maximum moisture adsorption(at 95 % RH) of the resin hardener mixture was determined, as an idication of the moisture adsption of theuncured resin (see Chapter 4.2). The drying step was not nessesary since the resins were heated to 160 ◦C

during preparation.

2.2.2 Modulated temperature differential scanning calorimetry

The Modulated Temperature Differential Scanning Calorimetry (MTDSC) measurements were performedusing a Q2000 from TA instruments. The DSC was calibrated in the range in which the measurementswere done, using an indium and tin standard. The MTDSC was calibrated in the right range, using asapphire standard. All measurements were conducted under a constant nitrogen flow of 50 mL/min. Thesamples were analyzed in Tzero aluminum hermetic pans which were filled with 3.50± 0.05 mg fibres and10± 0.5 mg resin. The reference was an empty pan of identical type as the sample pan. The pans wereclosed with a Tzero® DSC Sample Encapsulation Press from TA instruments.

The quasi-isothermal MTDSC-measurements started with loading the sample into the DSC at 0 ◦C followedby applying the modulation while keeping it five minutes on this temperature. The modulation amplitude ischosen on 0.5 ◦C with a period of 60 s, these values are based on previous findings by B. Van Mele et al[35]. When the modulation was applied, the cell was heated with 30 ◦C/min until the curing temperature of80 ◦C was reached. The system remained at this temperature for 250 min. For the determination of the glasstransition temperature Tg a non isothermal heating was preformed after the isothermal cure, the system wascooled with 30 ◦C/min until 0 ◦C and followed by a non isothermal heating at 2.5 ◦C/min from 0 ◦C until195 ◦C so that the PA6 fibres would not melt. The cell was cooled again until 0 ◦C with 30 ◦C/min coolingrate and the non-isothermal heating action was repeated a second time to determine the glass transitiontemperature of the fully cured resin.


The fully non-isothermal MTDSC-measurement was preformed on both the DGEBA-MDA and RIM135-RIM137 resin to evaluate the total reaction enthalpy, the MTDSC-measurements started with cooling thecell until 0 ◦C followed by applying the modulation while keeping it five minutes on this temperature. Themodulation amplitude is chosen on 0.5 ◦C with a period of 60 s. When the modulation was applied, the cellwas heated with 2, 5 ◦C/min until 230 ◦C

The measured values were analysed using TA Instruments’ Universal Analysis software package and statis-tical analysis was done using SPSS.

MTDSC Sample preparation

For this thesis DSC pans were prepared at different relative humidity (RH) conditions to obtain nanofibreswith different moisture contents. For samples prepared at (23±2 ◦C, 50±5%RH) a large conditioned roomwas available in which the samples could be prepared, all mixing and measuring actions were preformedinside the climate room. The samples with 0 % moisturecontent where prepared by drying the nanofibres at120C in an open DSC pan for two 2 h. This ensured that all moisture inside the nanfibre web was vapourised.The dryed nanofibres where cooled in liquide nitrogen prior to adding the resin.For all other relative humidity condition a WEISS WK340 climate chamber was used. The sample prepa-ration was carried out trough a small opening of approximately 20 cm diameter. During this procedure carewas taken that the relative humidity inside the climate chamber did not vary with more than 5%.

The nanofibres were weight and inserted in the pans after which the pans were stored in the climate room orclimate chamber 24 hours prior to adding the resin. Moreover also the resin and hardener were stirred underthe same conditions. After mixing the resin and hardener, a drop of the mixture was added into the pans (forthis action, an injection needle was used) after which the DSC pan was sealed. The mass of the added resinwas determined out of the total mass of pan and sample.

For accurate kinetic analysis, no or very little reaction should occur before doing the MTDSC experiment.For this reason the pans are stored in liquid nitrogen (−196 ◦C) [55] immediately after preparation to beable to make more pans at the same time.

2.2.3 Thermal activity monitor

The Thermal Activity Monitor (TAM) measurements were preformed using a TAMIII from TA instruments,an oil bath was used to keep the temperature constant within ± 0, 0001 ◦C during the experiment. TheTAM samples were prepared in the lab at room temperature, the relative humidity at the time of samplepreparation was 35± 5%. The samples contained 0, 135± 0.001 g of nanofibres and 0, 62± 0.01 g of resinthe reference samples had 0, 52± 0.01 g of resin. The sample preparation took about 10 min, in adtion tothis sample preparation time another hour of equilibration time, inside the machine was needed before theactual measurement began. After this equilibration an isothermal heat flow measurement at 25 ◦C for 90hours was carried out.

2.2.4 Dynamic mechanical analysis

The DMA-measurements were performed using a Q800 from TA instruments. Before the start of the DMAexperiments a complete calibration was carried out, this includes position calibration, electronic calibration,


force calibration, dynamic calibration, clamp and temperature calibration. The temperature calibration wascarried out using an indium standard.

The PA6 nanofibre webs were analysed using the TA instruments thin film clamp, a clamping force of4 lb/in was used. A static force of 0, 1 N was applied to the sample, the measurement was carried out usinga frequency of 1 Hz and and amplitude of 15µm. All samples had a width of 5, 2± 0, 1 mm the length of thesample was around 2 cm, the actual length was measured with an accuracy of 0, 001 mm by the DMA priorthe start of the experiment (after application of the static force). The experiment started with a soaktime of10 min at 0 ◦C followed by a temperature ramp at 2, 5 ◦C/min to 110 ◦C.

All experiments on composite samples were carried out with single cantilever clamps using a clamping forceof 10 lb/in. The single cantilever clamp was chosen due to the high modulus of the samples for GF reinfor-ced composites these was around 8 GP (see Table 2.1 for practical guidelines on the use of the appropriateclamp type, sample dimensions and heating rate as described in the handbook Dynamic Mechanical analysisa practical introduction by Kevin P. Menard [48]). All DMA samples were cut to 10± 0, 6 mm wide and17, 5± 0, 2 mm free length the thickness of the samples was 3± 0, 3 mm for all samples containing glassfibres and 1 mm for all non glass fibre containing samples. All samples dimensions were measured withan accuracy of 0,01 mm prior to measurement. The temperature ramp experiments started with cooling theDMA to−10 ◦C, the furnace was kept at this temperature for 15 min after which the temperature was raisedat 2, 5 ◦C/min until 120 ◦C. During these experiments the frequency was kept constant at 1 Hz and thestrain amplitude was set to 20µm , this value is within the viscoelastic region of all tested material. A TimeTemperature Superposition (TTS) creep test was preformed on the glass fibre-nanofibre-composite samplesand conventional glass fibre composite samples. To apply the TTS principal the composite should be fullycured, therefore all samples were subjected to a 500 min post cure at 110 ◦C. Creep tests were preformedstarting at the temperature of 25 ◦C until 90 ◦C in steps of 5 ◦C, for every creep test the applied stress was5 MPa, the samples were allowed to creep for 180 min at constant temperature. After this creep time thetemperature was raised and the sample could recover for another 180 min, were after the next cycle started.


Table 2.1: Practical guidelines for sample dimensions and clamp type and heathing rate. [48]

2.2.5 Tensile experiments

Tensile experiments where conducted using an Instron 3369 equipped with a 2000 N load cell. The gaugelength was set to 200 mm and the distance between the flat clamps was 85 mm. The tensile tests wherecarried out with a speed of 2 mm/min in controlled climate of (20±2 ◦C, 65±5%RH). All samples where1 mm tick and 22 mm wide.

2.2.6 Scanning Electron Microscopy (SEM)

A scanning electron microscope (Jeol Quanta 200 F FE-SEM) was used to examine the morphology epoxy-nanofibre composites. The acceleration voltage of the SEM was set on 20 kV. Before the SEM analysis, thesamples were coated with a gold/platinum alloy using a sputter coater (Balzers Union SKD 030).

Chapter 3

Reproducibility of curing experiments

3.1 Introduction

This research started with the evaluation of the reproducibility of the DGEBA-MDA and RIM135-RIM137resin harder systems, for all experiments mentioned in this chapter the nanofibres were aclimatized at23±2 °C and 50± 5 % RH. The resin DGEBA-MDA was chosen due to its successful use in previous curingstudies [3, 56]. The RIM135-RIM137 resin harder system is investegated due to its frequent use in comer-cial composite production [57]. Futhermore, this RIM135-RIM137 system is apporved by REACH whichis not the case for the DGEBA-MDA system. For a good fundamental study of the effect of nanofibres onthe curing behavior of epoxy amine resins, it is highly important to use a characterization methode and resinharder system for which the neat resin samples give very reproducible results.

3.2 DGEBA-MDA system

3.2.1 Neat resin

In Figure 3.1 the heat flow data obtained from several isothermal MTDSC curing experiments expressed inWatt per gram of resin is shown. Every curve represents the heat flow in function of time, obtained froma different series. For each series a new resin-hardener mixture was prepared. This heat flow signal isproportional to the reaction speed. To compare different heat flow signals with each other, it is necessary tohave a good definition of time 0, a first option would be to select the time at which the sample is loaded intothe DSC as time 0. However, in this case a difference in temperature equilibration time would cause a shiftof the heath flow signal which could give the false impression that one reaction is faster or slower than theother. A better approach is to look at how the temperature controller of the DSC, pushes the temperaturetowards the steady state quasi-isothermal curing temperature and to define the time corresponding to acertain value of the temperature signal as time 0. From Figure 3.2 one can see that the temperature increasedeclines rapidly from around 79 ◦C, the temperature 79 ◦C was chosen to define time 0 rather than the finalsteady state temperature of 80 ◦C, since it might take several more minutes before the exact value of 80 ◦C

is reached.

23

CHAPTER 3. REPRODUCIBILITY OF CURING EXPERIMENTS 24

Figure 3.1: Heat flow signal of neat resin samples obtained from a quasi-isothermal MTDSC scan at 80 ◦C,comparison between experimental results from this research sample 1-3 and Moorkens et al. [3] reference

For the evaluation of the data, several parameters are to be looked at. The first parameter is the initial reactionspeed defined as the value of the heat flow signal 5 min after time 0. This is the most useful parameter toevaluate the effect of a catalyst on a curing system, since at this stage of the reaction the system still mainlyexists of a mixture of neat resin and hardener, which means that the influence of possible side reactions orby-products is neglectable at this point. Furthermore, the viscosity of the system is relatively low at thisstage of the reaction and the reaction is still chemically controlled [56], which means that a difference inreaction speed at this point will be mainly due to catalysis.

Two other parameters, useful to evaluate possible catalytic effects, are the maximum reaction speed and thetime to reach the maximum reaction speed. The maximum reaction speed can be defined as the maximumvalue of the heat flow signal and the time to reach the maximum reaction speed is the time at which the heatflow signal reaches its maximum value (tmaxHF ). These parameters give an indication of the effects of thecatalyst in the further stages of the reaction. Apart from the reaction speed related data the conversion ofthe reaction can also be found from the heat flow signal, since the surface underneath the heat flow signalis equivalent to the enthalpy of the reaction (∆H ). By dividing this value by the total reaction enthalpy of406 J/g (obtained from a non-isothermal MTDSC measurement), the result will be the conversion (αiso)obtained during this isothermal heat flow experiment. As a last parameter, Table 3.1 includes the glasstransition temperature, as opposed to the other parameters mentioned, this parameter is not obtained fromthe isothermal heat flow measurement but from the reversed heat capacity signal obtained from a non-isothermal MTDSC scan. Apart from the fact that the glass transition temperature is a useful property toknow from a mechanical point of view, the final glass transition temperature (Tg∞) is very sensitive to theepoxy amine mixing ratio [56] which means that it is a good indicator to check for possible errors in this


Figure 3.2: Typical response of TA instruments Q2000 temperature controller to step input of 80 ◦C

mixing ratio.

When looking at Table 3.1 and Figure 3.1 one can notice that first of all, the results obtained during thisresearch are very reproducible as can be seen from the close overlap of all the black curves in Figure 3.1. Aswell as from the small spread in the measurements that can be observed by the small error interval mentionedin Table 3.1. Furthermore, all results are in excellent agreement with the research done by Moorkens et al.[3] as one can noticed by comparing the data from this research with the reference data in Table 3.1 andFigure 3.1. In conclusion, these curing experiments confirm that the DGEBA-MDA resin hardener systemgives rise to very reproducible results and is, therefore, an excellent epoxy amine resin to use in the study ofthe effect of nanofibres on the curing behaviour of these resins.

During further curing experiments there was always at least one neat resin pan in a series which was usedas a reference to check for any inconsistencies in resin preparation.

Table 3.1: Parameters of the curing behaviour of neat DGEBA-DMA resin, comparison between the resultsobtained in this research and those obtained by Moorkens et al. [3].

Initial reaction tmaxHF max reaction ∆H Tg

speed(W/g) (min) speed(W/g) J/g °CRef. values [3] 0, 022± 0, 01 72, 25± 1 0, 068± 0, 001 341, 33± 9 175, 5± 4

Values obtained 0, 017± 0, 03 74, 70± 2 0, 072± 0, 003 325, 79± 15 174, 8± 4


3.2.2 Resin combined with nanofibres

When nanofibres are added to the DGEBA-MDA system, one notices a slight reduction in reproducibility.Table 3.2 shows the average values of initial reaction speed, peak time, maximum reaction speed and reac-tion enthalpy as well as the intervals in which all measurements are found. A possible explanation for thesevariations are small differences in fibre moisture adsorption. It has already been shown by J. Baselga et al.[58] that water has a catalytic effect on the curing reaction of epoxy amine resins. In the next chapter we willstudy the effect of nanofibre moisture adsorption on the curing behaviour of epoxy amine resins in detail.Although some spread on the measurements exist, from Table 3.2 and Figure 3.3 it can be noticed that theerror interval is in accordance with the error interval obtained by Moorkens et al. [3] and therefore it is mostlikely due to a fundamental property of the nanofibres (noted as small variations in moisture adsorption) andnot due too flaws in the method used or application of this method as such.

Figure 3.3: Heat flow signal of PA 6 NF-resin samples obtained from an isothermal MTDSC scan at 80 ◦C,comparison between experimental results from this research (sample 1-4) and results from Moorkens et al.[3] (reference 1 and 2)

A comparison between Table 3.1 and 3.2 leads to the conclusion that the curing of the resin accelerateswhen PA 6 nanofibres are added to the resin system. The initial reaction rate is about twice as high whenPA6 nanofibres are present, indicating the presence of a more effective catalyst. This could be due to aminegroups, the presence of water in the nanofibres or a combination of both. The effect of water adsorbed onthe polyamide nanofibre surface will be studied further in Chapter 4. Next to the increase in initial reactionspeed it took approximately 75 minutes for the neat resin system to reach the maximum exothermic heatflow, and so the maximum rate of conversion. This was only 47 minutes when the PA6 nanofibres wereadded, which means the reaction up to the maximum was on average 40 % faster.


The glass transition temperature of a fully cured (Tg∞) neat resin is slightly higher then when the resinwas impregnated with nanofibres. The lower (Tg∞) of the fully cured system when nanofibres are present,suggests that inserting the fibres in the resin has either a small plasticizing effect on the cured DGEBA-MDAsystem or the network formation is somewhat disturbed by the presence of the nanofibres. The latter mightbe the result of a plasticizing effect arising from the water absorbed by the nanofibres or due to preferentialmigration (or adsorption) of one of the resin components to (or on) the polyamide nanofibres, disturbing thestoichiometric epoxy-amine ratio, and reducing the glass transition temperatures attained at high conversion.This could also explain why the resulting reaction enthalpy released during quasi-isothermal curing is a littlebit smaller for the neat resin compared to the impregnated sample. Whether or not this phenomenon of wateradsorption is the main cause of this plasticizing effect will be further investigated in Chapter 4.

Table 3.2: Kinetic parameters of the curing behaviour of DGEBA-DMA resin containing PA 6 NF, compa-rison between the results obtained in this research and those obtained by Moorkens et al. [3].

Initial reaction tmaxHF max reaction ∆H Tg

speed(W/g) (min) speed(W/g) J/g °CRef. values [3] 0, 038± 0, 03 44, 78± 5 0, 079± 0, 005 353± 14 167, 7

Values obtained 0, 034± 0, 04 47, 42± 5 0, 072± 0, 009 333± 20 171, 8± 4

3.3 RIM135-RIM137 system

Although the RIM135-RIM137 and DGEBA-MDA resin hardener systems are both epoxy amine basedresin, there are some fundamental differences between them. The DGEBA-DMA is a high temperatureresin while the RIM135-RIM137 system is designed for curing at room temperature. Another importantdifference is that the DGEBA-MDA system exists of almost neat DGEBA and MDA while the RIM135-RIM137 is a commercial product of which the composition is not available. That is why in this thesis thecuring studies will mainly be focused on the DGEBA-MDA system, however some experiments will alsobe carried out on RIM135-RIM137 to evaluate if the fundamental results obtained from the study of theDGEBA-MDA system can be applied to the RIM135-RIM137 system as well.

Since the RIM135-RIM137 resin hardener system is designed for low temperature curing it is advisable tostudy the effect of nanofibres on this resin under these conditions. In theory one could study the RIM135-RIM137 using MTDSC just like the DGEBA-MDA system. However, due to the fact that the curing tempe-rature should be around 25 ◦C, the heat flow signals are to low to be evaluated accurately with the MTDSC.In previous studies, attempts were made to study the influence of nanofibres on the RIM135-RIM137 withMTDSC at elevated curing temperatures, these attempts were unsuccessful because the elevated curing tem-perature resulted in high and irreproducible initial reaction speeds [3]. That is why in this thesis TAMexperiments at 25 ◦C were carried out on the RIM135-RIM137 resin hardener system, Figure 3.4 shows theresult of such a curing experiment, the heat flow signal obtained from a TAM experiment can be interpretedin the same manner as the heat flow signal obtained from a DSC experiment. One can notice that these roomtemperature curing experiments are very reproducible.


Figure 3.4: Heat flow signal of RIM135-RIM137 neat resin samples obtained from a isothermal TAMexperiments at 25 ◦C

Chapter 4

Effect of nanofibre moisture content oncuring kinetics

4.1 Introduction

In Chapter 3.2.2 it was suggested that water absorption by nanofibres might be the cause of, or at leastcontribute to the catalysing effect of the nanofibres on the DGEBA-MDA resin system. Furthermore, itmight also have an influence on the plasticizing effect caused by adding nanofibres to the resin. In thischapter the effect of the nanofibre moisture content on the curing behaviour of the DGEBA-MDA resin isstudied in detail. The moisture absorption of PA6 nanofibres, conventional PA6 microfibres fibres and theepoxy resin was studied using DVS. The effect of this absorbed moisture on the curing behaviour and theglass transition temperature of the DGEBA-MDA resin was evaluated using MTDSC and TAM. In addition,a comparison was made between the effect of PA6 nanofibres and conventional PA6 microfibres on thiscuring behaviour. In the last part of this chapter the RIM135-RIM137 resin harder system was studied tovalidate if the fundamental results obtained from the DGEBA-MDA curing experiments are representativefor other epoxy amine resins as well.

4.2 Evaluation of moisture adsorption

Figure 4.1 shows the adsorption and desorption isotherms of PA6 nanofibres (4.1 A) and conventional PA6microfibres (4.1 B) respectively. These measurements are very reproducible, as can be seen from the smallstandard deviation (indicated by the error bars).

It can be noticed that both the nanofibres and microfibres show only a small amount of hysteresis. Thisis in favour for the reproducibility of the curing experiments, because for these experiments the sampleswhere acclimatized in either adsorption or desorption mode, depending on whether the relative humidity inthe climate chamber was higher or lower than the relative humidity in the lab during the preparation of thesample.

When comparing the nanofibres to the microfibres we notice that their moisture adsorption is nearly the sameuntil about 65% relative humidity, after which the nanofibres start to show a higher moisture adsorption. Thelarge porosity of the nanofibre web as compaired to the conventional fibres could be a possbile explanation

29

CHAPTER 4. EFFECT OF NANOFIBRE MOISTURE CONTENT ON CURING KINETICS 30

Figure 4.1: Absorption (—) and desorption (· · · ) of (A) PA6 nanofibres and (B) conventional PA6 micro-fibres at 23 ◦C.

Figure 4.2: Moisture absorption of uncured resins at 95% RH A DGEBA-MDA B RIM135-RIM137

for this higher moisture adsorption. It can be assumed that from around 65% RH these pores start to becomefilled with moisture.

For the RIM135-RIM137 and DGEBA-MDA resin the maximum moisture adsorption of the uncured resin(at 95% relative humidity) was determined. As shown in Figure 4.2 A, for the DGEBA-MDA resin anequilibrium value of 0, 6 % was obtained. In case of the RIM135-RIM137 resin this was 2, 9 % (Figure4.2 B). In both cases the moisture adsorption of the uncured resin is siginifcantly lower than the moistureadsorption of the PA6 fibres. It should be pointed out that it is the moisture content of the uncured resin thatis of intrest and not that of the cured resin. Since this moisture content is representable for the amount ofmoisture availble in the resin to participate in the curing reaction.

From Figure 4.2 it can also be noted that it takes quite a long time (more than 12 hours) before equilibrium isreached. This time might have a significant influence on the measurement because the curing reaction startsfrom the moment resin and hardener are mixed together. This means that the maximum moisture adsorptiondetermined, might not be representable for the moisture adsorption of the uncured resin, since part of theresins is already cured before the end of the measurement.


Figure 4.3: Heat flow signal of DGEBA-MDA (· · · ) and RIM135-RIM137 (—) epoxy amine resins from aisothermal TAM curing experiment at 25 ◦C.

It has to be verified if the duration of the experiment is still short enough to assume that the resins are mainlyconsisting of monomers during the DVS experiment. To do this, TAM curing experiments were carried outat room temperature. From the heat flow measurements obtained from these TAM experiments (Figure 4.3)one can estimate the conversion of the curing reaction during the moisture adsorption experiment. Thisconversion will give an indication on how many chemical bonds are formed between the resin and hardenermolecules. The conversion can be calculated by integrating the TAM heat flow signal Hf(t) over the durationof the DVS measurement and then dividing this result by the total reaction enthalpy ∆Htotal as shows inFormula 4.1. ∆Htotal was obtained for a non isothermal DSC scan. In case of the DGEBA-MDA resinhardener system the conversion was found to be 3 % while in case of the RIM135-RIM137 resin hardenersystem this was 12 %.

Fav =

∫Hf(t)

∆Htotal(4.1)

For a stepwise polymerization, the average polymer chain length Xn can be related to the conversion αusing the Carothes equation 4.2 [59]. In this equation Fav represents the average functionality. In case ofthe DGEBA-MDA resin system the functionality of DGEBA is two and that of MDA is four. From equation4.3 it can be found that Fav equals 2,66 because since the epoxy amine mixing ratio was 1, NMDA is twotimes NDGEBA. Combined with the conversion obtained from the TAM curing experiment the numberaverage molecular weight of the resin components can be calculated. For the DGEBA-DMA resin one findsthat the average chain length is only 1,33 monomer units. For the RIM135-RIM137 resin hardener systemthis was 1,14. The results of these calculations are summarized in Table 4.1. It can be concluded that in


both cases the reactive mixture consisted mainly of monomers during the DVS experiment. Therefore, theobtained moisture adsorption is a good estimation of the moisture adsorption of the uncured resin.

Xn =2

2− Fav.α(4.2)

Fav =

∑Ni.fiNi

(4.3)

Table 4.1: Maximum resin moisture adsorption and conversion data showing that the reactive resin hardenermixture mainly consists of monomers during the DVS experiments.

Adsorption DVS time to reach ∆Htotal α Xn

[wt %] equilibrium [min] [J/g] [%]DGEBA-MDA 1, 88 1200 402± 5 3 1, 33

RIM135-RIM137 4, 45 110 365± 5 12 1, 14

4.3 Catalysing effect of the nanofibre moisture content on the DGEBA-MDAcuring reaction

To analyse the assumed catalysing effect of the nanofibre moisture content on the DGEBA-MDA resin har-dener system, curing experiments where carried out with samples acclimatized at different relative humiditylevels. This relative humidity was then linked to the moisture content in the fibres using the adsorptionisotherms obtained from the DVS experiments.

When comparing the heat flow per gram of the different pure resin samples (Figure 4.4) one notices that allthe curves nearly fall on top of each other. This should come as no surprise since the DGEBA-MDA resin isheated at 160 for 15 min to ensure good mixing of resin and hardener, from the DVS data it is found that themaximum moisture absorption is less than 0,6% so its very likely that all moisture inside the resin vaporisedduring this heating period. Further more, the small amount of moisture that could be absorbed by the resinduring the short exposure of the resin to the air before sealing the DSC pan, clearly has no significant effecton the curing behaviour of the DGEBA-MDA resin.


Figure 4.4: Heat flow signal obtained from a quasi-isothermal MTDSC scan at 80 ◦C illustrating that therelative humidity during sample preparation has no significant effect on th curing behaviour of the DGEBA-MDA resin

Figure 4.5: Heat flow signal obtained from a quasi-isothermal MTDSC scan at 80 ◦C illustrating the effectof the nanofibre moisture content on the curing behaviour of the DGEBA-MDA resin


Figure 4.5 shows the heat flow per gram of resin in function of the reaction time but in this case nanofibreswere added to the resin. First of all one notices that these heath flow signals do not fall on top of oneanother. This indicates that the moisture content in the fibres does have a significant influence on the curingbehaviour of the DGEBA-MDA resin. Moreover, some clear trends in these heat flow signals can be noted.When the moisture content in the fibres increases the initial reaction speed and the maximum reaction speedalso increase. Furthermore, there is a clear shift of the heat flow signals to the left and the time at which themaximum reaction speed occurs decreases with increasing nanofibre moisture content.

Until now only the heat flow signal was mentioned, however from an isothermal MTDSC scan one can alsoobtain another usefull signal namely the rev. Cp signal. As mentioned in Chapter 1.4 MTDSC allows tocalculate a heat capacity during a quasi isothermal experiment. This heat capacity can be related to themobility of the system, a system with higher mobility has a higher rev. Cp. From Figure 4.6 it can benoted that the rev. Cp. of the pure resin samples is significantly higher than that of the nanofibre containingsamples. This indicates that the nanofibre containing samples lost some of there mobility due to the presenceof nanofibres.

Figure 4.6: Rev. Cp signal obtained from a quasi-isothermal MTDSC scan at 80 ◦C illustrating the effectof the nanofibre moisture content on the curing behaviour of the DGEBA-MDA resin.

The rev. Cp values of some of the nanofibre samples appears to be higher than others, however there is nocorrelation between the moisture content and the rev. Cp step height. Furthermore the data in Figure 4.6 isbased on single curves only, from repeating the measurement at least 5 times it was found that the averagestep height is the same for all nanofibre moisture contents and equals 0, 024 J/(g C). Therefore it wasconcluded that the moisture content has no significant influence on the mobility of the system. The moisturecontent does however have an influence on the time of vitrification, this vitrification time is associated with


Figure 4.7: Initial reaction speed (A), tmaxHF (B), maximum reaction speed (C) and Reaction enthalpy(D) obtained from a quasi isothermal MTDSC scan at 80 ◦C of pure resin (4) and of nanofibre containingsamples (♦)

the step in Figure 4.6. One notices that the vitrification occurs sooner in the nanofibre containing samplesas compared to the pure resin sample. Furthermore, the higher the moisture content in the nanofibres, thesooner the vitrification occurs. So also this parameter indicates that there is an increasing catalysing effecton the MDA-DGEBA curing reaction with increasing nanofibre moisture content.

To gain a better understanding of this catalysing effect, a large amount of curing experiments where conduc-ted during this thesis. In Figure 4.7 the average values of initial reaction speed, tmaxHF , max reaction speedand tvitrification. tvitrification is defined as the time needed to reach half the step height of the vitrificationstep in the rev. Cp signal. The triangles in Figure 4.7 show the values of these parameters for the pure dryresin without any nanofibres. The error bars indicate the standard deviation on the different measurements.

When looking at the initial reaction speed in function of the fibre moisture content, a near linear trend canbe observed, implying first order kinetics in moisture content. The tmaxHF , maximum reaction speed andtvitrification also shows a near linear correlation, one notices however that the correlation is stronger for theinitial reaction speed as can be seen from the higher R-squared value. This is probably due to the influenceof side reactions, it was already mentioned in Chapter 3.2.1 that the initial reaction speed is most suitableto evaluate a catalytic effect since at this stage of the reaction the system mainly contains a mixture of pureresin and hardener. This means that the influence of possible side reactions or by-products is neglectable.

Although fibre moisture content is definitely the main cause of the catalysing effect, it should be noted thatthe 0 % moisture sample still shows a slight increase in initial reaction speed over the pure resin sample.From this result it might be suspected that there are also a limited amount of catalysing groups present on


the nanofibres, independent of the moisture content. When looking at the maximum reaction speed onenotices that in this case the 0 % moisture sample does collide with pure resin sample, indicating that at thisstage of the reaction the effect of such additional catalysing groups is neglectable.

Figure 4.8: Reaction enthalpies obtained after a quasi isothermal curing experiment at 80 C° for 250 min infunction of the nanofibre moisture content

The reaction enthalpy in function of the nanofibre moisture content is shown in Figure 4.8 it can be notedthat the reaction enthalpies of the nanofibre containing samples is slightly higher than that of the pure resinsamples. It is, however, hard to conclude if this increase in reaction enthalpy is due to the moisture in thenanofibres just by looking at Figure 4.8, because the standard deviation on the reaction reaction enthalpiesis quite high compared to the differences between the samples prepared at different relative humidity´s.To get a better understanding of the influence of the nanofibre moisture content on reaction enthalpy, astatistical analysis of variance (ANOVA) was preformed on this data using SPSS statistics software. Fromsuch an ANOVA test one can conclude if the means of the different datasets are significantly different fromeach other. However before one can use ANOVA analysis, one should verify if the preconditions for thistest are met. In ANOVA analysis one assumes, equality of variance between the test groups and a normaldistribution of the test result with one group.

In our case there are 5 data sets, one for every moisture content. Each dataset contains the reaction enthalpiesof all the experiments conducted at this specific moisture content. These 5 datasets where first tested forequality of variance. This test resulted in a significance level of 0,082>0,05 meaning that the differencein variance between the different data sets is small enough to preform ANOVA analyses. After testing theequality of variance the data was tested for normality using the Shapiro-Wilk test, the results of this test areshown in Table 4.2. It can be noted that the significance level is higher than 0,05 for all datasets, so we canassume that the data comes from a normally distributed population and therefore it is allowed to performANOVA analysis on these datasets.

Now one can test if the mean reaction enthalpies obtained for different nanofibre moisture contents aresignificantly different from each other. The result of the ANOVA analyses are shown in Table 4.3, a high


Table 4.2: Results of the Shapiro-Wilk test for normality

moisture content of group significance levelGroup 1 0 wt % 0, 413

Group 2 0,76 wt % 0, 883

Group 3 2,70 wt % 0, 185

Group 4 3,72 wt % 0, 055

Group 5 6,30 wt % 0, 570

significant level of 0,78 is obtained. Practically this mains that if all means would be equal, there is a 78 %chance that a relationship as strong as the one observed in the data would be present. This result indicatesthat there is no statistically significant difference between these different reaction enthalpies. Therefore onecan conclude that the moisturecontent most likely has no influence on the reaction enthaply.

Table 4.3: ANOVA analysis showing that there is no significant difference between the reaction enthalpyobtained at different nanofibre moisture contents

Sum Of Squares df Mean Square F sig.Between groups 366,908 4 3 91,727 0,78Withing groups 6072,973 29 209,413Total 6439,88 33

4.4 Comparison between effect of the PA6 nanofibres and PA6 microfibreson the curing reaction

Clear differences in initial reaction speed between nanofibre samples (♦) and conventional PA6 microfibre(�) samples can be noted (Figure 4.9). The reaction speed of the conventional polyamide samples followsa linear trend until a moisture adsorption of around 2, 7 %, after which the reaction speed of the nanofibrecontaining samples keeps increasing while those of the conventional microfibers stagnates.

This observation can be explained by looking at the difference in the microscopic structure between na-nofibre webs and microfibres. As shown in Table 4.4 the diameter of the conventional fibres is almost 60times higher than the diameter of the nanofibres. Consequently, the specific surface area (As) of the usedconventional fibres is almost 60 times smaller than the As of the used nanofibres as can be easily deducedout of equation 4.4 where ρ represents the density of the used material which is identical for both fibre types.

As =4

ρ · dfibre(4.4)

Thus in a first step moisture needs to diffuse over a larger distance in the fibre to reach the fibre/resininterface, moreover this interphase is smaller in conventional fibres due to the smaller specific surface area


Figure 4.9: Comparison between initial reaction speed of the DGEBA-MDA curing reaction at differentfibres moisture contents for pure resin (4), PA6 nanofibres (♦) and conventional PA6 microfibres (�)

Table 4.4: Characteristics of PA6 NF and PA6 CF.

dfibre As

[nm] [106 m2/kg]PA6 NF 179.2± 20.3 22.3/ρPA6 CF 10 428.1± 141.8 0.4/ρ

of these fibres. In in a second step, and most important, the moisture has to defuse over a larger distance inthe resin as well. From the DVS experiments (see Figure 4.10) it can be noted that the diffusion of moisturein the resin is a lot slower than in the fibre. This implies that the last step may be rate determining. Inconclusion, although the total amount of moisture in the PA6 microfibres is approximately the same as inthe nanofibres, there is not enough time for this moisture to diffuse into the resin and cause an accelerationof the initial reaction speed.


Figure 4.10: Evolution of moisture content in time at 95% RH of A uncured DGEBA-MDA resin B PA6nanofibres

4.5 Plasticizing effect of PA6 nanofibres on the DGEBA-MDA resin

In Chapter 3.2.2 it was mentioned that the introduction of PA6 nanofibres into the DGEBA-MDA resincauses a slight reduction of the glass transition temperature of the fully cured resin. In this section we willinvestigate if this plasticizing effect can be attributed to the moisture present in the nanofibres. Figure 4.11shows the glass transition temperature of pure DGEBA-MDA resin (represented by the triangle) as well asthe Tg obtained when nanofibres with different moisture contents were added to this resin.

It can be noted that the Tg is lower for all nanofibre containing samples, compared to the pure resin samples.An interesting fact to note is that the Tg of the dry resin-nanofibre sample is also lower than that of the pureresin sample. If the nanofibre moisture content would be the cause of this plasticizing effect, one wouldexpect the dry nanofibre sample to have the same Tg as the dry resin, since it also does not contain anymoisture. However, the data shows that this is not the case, the Tg of the dry nanofibre sample was onaverage 6, 9 ◦C lower than the dry resin sample. This drop in Tg is higher than for any other of the nanofibresamples that did contain moisture.

To evaluate if there is any significant difference between the glass transition temperatures of the sampleswith different nanofibre moisture contents, statistical analysis of variance (ANOVA) was performed usingSPSS statistics software. Just like in case of the ANOVA test performed in the previous section equality ofvariance between the datasets and normality of the datasets was tested.

In our case the test has 5 datasets containing the glass transition temperatures at the different nanofibremoisture contents. These 5 groups where first tested for the homogenitiy of variance. The test resulted ina significance level of 0,05. This is the border value so it is not advised to preform an ANOVA test onthese 5 groups. However if the 0 % moisture content group was removed a sigificance level 0,13>0,05 wasobtained, therefore one can assume equality of variance between these 4 groups. The removal of the 0 %can be justified by pointing out that these samples where prepared using a slightly different method (seeChapter 2.2.2) wich resulted in a smaller standard deviation. The 4 remaining groups were subsequentlytested for normality using a Shapiro-Wilk tests. The results of these tests can be found in Table 4.5. One


Figure 4.11: Tg of fully cured DGEBA-MDA resin containing nanofibres (♦) with different moisturecontents and dry resin (4) without nanofibres

notices that the significance level is higher than 0,05 for all groups therefore one can assume that the groupshave a normal distribution.

Table 4.5: Results of the Shapiro-Wilk test for normality

moisture content of group significance levelGroup 1 0,76 wt % 0, 916

Group 2 2,70 wt % 0, 494

Group 3 3,72 wt % 0, 154

Group 4 6,30 wt % 0, 585

The results of the ANOVA test are shown in Table 4.6, a significance level of 0,623>0,05 was obtained.In conclusion, there is no statistically significant difference between the glass transition temperatures atdifferent moisture contents.

Table 4.6: Results of ANOVA test to evaluate the significance between the difference of the means of theTg´s obtained at different nanofibre moisture contents.

Sum Of Squares df Mean Square F sig.Between groups 36,224 4 3 0,6 0,623Withing groups 382,299 19 20,121Total 418,523 22

To further underpin the the conclusion that the nanofibre moisture content is not causing the observed


plasticizing effect, measurements were preformed on samples to which an access of water was added. If thenanofibre moisture content would be the cause of this plasticizing effect, one would also expect a decreasein Tg when only water was added to DGEBA-MDA resin hardener system prior to curing. However, this isnot the case, the average Tg of a fully cured water-resin mixture turned out to be 175, 5 ◦C with a standarddeviation of 1, 5 ◦C, while the average Tg of the dry resin was 173, 5 ◦C with a standard deviation of 3 ◦C.

All previously mentioned results in this Chapter lead to the conclusion that nanofibre moisture content is notthe cause of the plasticizing effect of nanofibres on the DGEBA-MDA resin. Which implies that the causeof this plasticizing effect should be searched elsewhere. It is well known that the epoxy amine ration has ahuge influence on the Tg of epoxy resins. The highest Tg is achieved when the epoxy and amine groups arepresent in stoichiometric amounts [56]. Only in this case the maximum Tg can be achieved. Based on theseobservations, it is suggested that the cause of the plasticizing effect might be due to preferential migration(or adsorption) of one of the resin components to (or on) the polyamide nanofibres. This migration woulddisturb the stoichiometric epoxy-amine ratio and thereby lower the Tg

4.6 Comparison between effect of the PA6 nanofibres and PA6 microfibreson the Tg of the fully cured resin

Figure 4.12: Tg of fully cured DGEBA-MDA resin containing PA6 nanofibres (♦) or conventional PA6microfibres (�) with different moisture contents and dry resin (4) without fibres

In the previous section it was suggested that the plasticizing effect on nanofibres on the DGEBA-MDA resinwas caused by preferential migration (or adsorption) of one of the resin components to (or on) the poly-amide nanofibres. A comparison between the Tg of samples cured with conventional fibres and nanofibrecontaining samples allows for a verification of this hypothesis. Since nanofibres have a much higher spe-cific surface area than conventional PA6 fibres, there are a lot more adsorption spots available for a resincomponent to absorb upon on the nanofibre surface as compared to the conventional fibres. For this reasonone would expect the final Tg of the nanofibre containing sample to be lower than the Tg of the conventional


PA6 fibre samples.

The data in Figure 4.12 shows that this is indeed the case, the Tg of the conventional PA6 samples wasapproximately equal to the Tg of the pure resin samples while the Tg of the nanofibre containing sampleswas significantly lower. Further more it can be noted that the moisture content of the convention PA6samples has no significant influence on the Tg of resin, just like this was the case for the nanofibre containingsamples. This observation confirms once again that it is the adsorption of one of the resin components to, oron the polyamide nanofibres that is causing the plasticizing effect of nanofibres on the DGEBA-MDA resinand not the moisture content in the fibres.

4.7 RIM135-RIM137 resin hardener system

In this section we will look at the effect of moisture adsorption on the curing kinetics of the RIM135-RIM137 resin hardener system, to evaluate if the fundamental knowledge obtained from the DGEBA-MDAsystem can be transferred to other epoxy amine resins like e.g. the commercially available RIM135-RIM137resin.

Figure 4.13 shows the result of an isothermal TAM curing experiment conducted on both pure resin samplesand nanofibre embedded samples. There are many similarities between these curing experiments and thoseconducted on the DGEBA-MDA resin hardener system. Figure 4.13 shows 28 % increase in initial reactionspeed from 447, 4± 1E− 5 W/g to 574± 1E− 5 W/g as well as an increase in maximum reaction speedfrom 552± 1E− 5 W/g to 636± 1E− 5 W/g. Furthermore, tmaxHF decreased from 282± 1 min for pureresin to 178± 1 min for the nanofibre containing samples. All these parameters strongly suggest that PA6nanofibres have a catalytic effect on the curing reaction of the RIM135-RIM137 epoxy amine resin just likeon the DGEBA-MDA resin.

Due to the fact that no climate chamber was available at the facility where the TAM experiments were car-ried out, its was impossible to reproduce all the curing experiments (different nanofibre moisture contents)carried out on the DGEBA-MDA epoxy amine resin. However, Figure 4.13 already suggests that we canexpect similar result. In addition, an experiment was carried out to verify this hypothesis. A piece of na-nofibre web was impregnated with a few drops of water, after which the moisture content of the fabric wasmeasured. This moisture content was found to be 76%, we should note that this value is a lot higher than the9% that is absorbed at 95% RH. Therefore, the effect on the curing behaviour is also alot more extreme, ascan be seen in Figure 4.14. There is a drastic increase in initial reaction speed, the high amount of moisturepresent in the nanofibres even resulted in the fact that the peak in maximum reaction speed occurred beforethe start of the measurement. Although this experiment is not completely equivalent to the detailed studycarried out on the DGEBA-MDA system, this experiment does show that moisture present in the nanofibrestructure causes and acceleration of the curing reaction. It can be concluded that the presence of PA6 nano-fibres has a catalytic effect on the RIM135-RIM137 epoxy amine curing reaction of a similar nature as thecatalytic effect observed for the DGEBA-MDA system.


Figure 4.13: Comparison between heat flow signals obtained from a isothermal TAM curing experiment at25 ◦C of nanofibres (—) embedded in resin and pure RIM135-RIM137 (· · · ) epoxy amine resin.

Figure 4.14: Heat flow signal of nanofibres embedded in RIM135-RIM137 epoxy amine resin samplesobtained from a isothermal TAM curing experiment at 25 ◦C, comparison between 2 % (—) and 76 % (· · · )nanofibre moisture content samples.

Chapter 5

Thermo mechanical analysis of nanofibrecomposites

5.1 Introduction

In Chapter 3 and 4 the effect of PA6 nanofibres on the curing behaviour was studied. This curing behaviouralso has a direct influence on the thermo mechanical properties of the composites produced. These thermomechanical properties are studied in this Chapter. In the first section, dynamic mechanical analysis waspreformed on RIM135-RIM137 resin samples, electrospun PA6 nanofibre webs and nanofibre-resin com-posites. The effect of the PA6 nanofibres on the storage modulus, loss modulus and damping factor areevaluated in detail.

In the second part of this chapter, glass fibre composites with a secondary reinforcement of PA6 nanofibresare investigated. The main focus will be on the study of long term creep modelling using the time tempe-rature superposition principle to predict the creep behaviour of these composites over a period of over 50years.

5.2 Dynamic mechanical properties of nanofibre epoxy composites

5.2.1 Reproducibility of oscillating temperature ramp experiments

Figure 5.1 shows the storage and loss modulus from three neat epoxy resin samples. It can be noted that thesemeasurements were very reproducible, the small differences between the measurements are most likely thecause of errors in sample dimensions such as uneven thickness or width. The effect of dimensional errorscan be estimated from equation 5.1, this is the equation used by the DMA software to calculate the storagemodulus when single cantilever clamps are used based upon the measured stiffness Ks the length betweenthe DMA clamps L, the sample moment of inertia I, the poison ratio ν, the sample thickness t and theclamping factor Fc (given by Equation 5.2). The moment of inertia is calculated based on the assumptionthat the sample is a rectangular prism using Equation 5.3. It can be noticed that this moment of inertia isproportional to the width w and to t3, this implies that an error on the thickness has a greater effect on themeasurement than an error on the width. Figure 5.2 (A) shows a simulation of a 1 % (dashed lines) and 2 %(dotted lines) error in thickness, Figure 5.2 (B) shows the effect of such erros in width. When comparingFigure 5.1 (A) to Figure 5.2 (A) one notices that the spread on these experiments are of similar magnitude as

44

CHAPTER 5. THERMO MECHANICAL ANALYSIS OF NANOFIBRE COMPOSITES 45

Figure 5.1: Illustration of the reproducibility of the DMA experiments carried out in this thesis showing(A) storage modulus and (B) loss modulus of three epoxy resin samples

Figure 5.2: Figure illustrating the effect of a 1 % (- - -) and 2 % (....) dimensional error in width (A) orthickness (B) on the storage modulus


a 1 % error in thickness which, in this case, is an error of 0,01 mm. Therefore, it is reasonable to assume thatall deviation on these DMA measurements is due to errors in the sample dimensions and that the samplesthemselves have very reproducible and uniform properties.

E`=Ks

Fc.L3

12.I.[1 +

12

5.(1 + ν).(

t

L)2] (5.1)

Fc = 0, 7616− 0, 02713.

√L

t+ 0, 10831. ln(

L

t) (5.2)

I =w.t3

12(5.3)

5.2.2 Stiffness of nanofibre composites

Figure 5.3 shows the storage modulus which is a measure of the stiffness of a material in function of tempe-rature for both a neat epoxy resin (· · · ) and for a nanofibre-epoxy composite (—) sample. When comparingthese two storage moduli it can be noted that the thermoplastic PA6 nanofibres clearly effect the stiffness ofthe resin.

Figure 5.3: Storage modulus of PA6 nanofibre composite(—) and neat resin (· · · ) (A) full measurementrange, (B) zoom on modulus shift

At a temperature below 21,69±7 °C the nanofibres increase the stiffness of the epoxy resin. Above thistemperature the stiffness of the nanofibre composites is lower than that of the epoxy resin samples. Toexplain why this shift occurs, the thermo mechanical properties of the nanofibre web should be observed,Figure 5.4 shows the damping factor and storage modulus of a nanofibre web. One notices that the point


where the stiffness of the nanofibre composite starts to become lower than that of the neat epoxy resin,is close to the onset point of the glass transition temperature evaluated from the damping factor of thePA6 nanofibre web. This leads to the hypothesis that the shift in storage modulus is related to the glasstransition temperature of the PA6 nanofibres. When the nanofibres are below the onset of the Tg they areprobably stiffer than the resin and this enables them to increase the stiffness of the nanofibre composite,similar to the increase in modulus observed when adding nanoclay or carbon nanotubes to the resin. Athigher temperatures, the PA6 nanofibres show a high toughing potential(also see 5.2.3) with a lower storagemodulus. Therefore, they decrease the stiffness of the nanofibre composite. This last effect is similar towhat is observed when thermoplastic particles are added to the epoxy resin, these also decrease the stiffnessof the matrix [19, 20, 21].

Figure 5.4: (A) dampingfactor and (B) storage modulus of PA6 nanofibre web

An important remark should be made about the storage modules of the nanofibre web, this storage modulusappears to be more than 10 times smaller than the storage modulus of the resin. Therefore one mightassume that the stiffness of the nanofibres is inferior to that of the resins. However, such a conclusioncan not be made based on this measurement because the storage modulus is calculated not only from thedisplacement and force but also from the sample dimensions as shown in Equation 5.4 (for the thin filmclamp). This equation uses the cross section (A) of the nanofibre web to calculate the storage modulus. Thisis an overestimation of the combined cross section area of all nanofibres in a nanofibre web. This is due tothe fact that over 80 % of the volume of the nanofibre web is air, so the cross section of the sample used inthe calculation of the storage (and loss) modulus is far bigger than the sum of the cross scetional area of allnanofibres. Therefore, the values of the storage modulus shown in Figure 5.4 are a lot lower that the storagemodulus of the actual nanofibres. As an indication, the storage modulus of a conventional polyamide yarnis around 2900 MPa at 20 ° [60]. However, the data in Figure 5.4 can still be used to track down the Tg ofthe nanofibres and to evaluate how this storage molulus varies with temprature.

E`= Ks.L

A(5.4)

When looking at 5.3 it can be noted that the storage modulus of the neat epoxy resin sample shows a jumparound 100 ◦C, similar disturbances can also be noted in the tan(δ) and loss modulus curves. From a careful


visual examination of the samples it was concluded that this disturbance corresponds to a sudden plasticdeformation of the sample. This also implies that all data collected after this point of plastic deformation isno longer valid, since DMA experiments should always be conducted within the linear viscoelastic regionof the examined material. The nanofibre composite shown in Figure 5.3 did not show this deformation.Although, some of the nanofibre composite samples did show a similar failure, it was always at significantlyhigher temperatures of around 130 ◦C. This result suggests that the nanofibre composites might have ahigher resistance to plastic deformation above the Tg of the resin.

In addition to the storage modulus, the Young´s modulus of the nanofibre composites was also measured.For this purpose tensile test were carried out in an acclimatized lab at 20 ± 2 ◦C on nanofibre compositesamples. The results of these tensile tests are shown in Figure 5.5.

Figure 5.5: Tensile test at 20 ± 2 ◦C showing an increased Young´s modulus for nanofibre composite (—)compared to a neat resin reference sample (· · · )

The modulus of the nanofibre composite samples was found to be 1895± 95MPa which is slightly higherthan the 1821 MPa obtained for the neat resin reference sample. This result is in line with results obtainedfrom the DMA experiments, since at 20 °C the nanofibre composite samples also showed a higher storagemodulus than the neat resin.


5.2.3 Energy dissipation and toughness of nanofibre composites

In literature a higher damping factor and loss modulus are related to a higher fracture toughness and impactresistance, [61, 62, 63, 64]. Figure 5.6 shows a comparison between the loss modulus of neat epoxy resinand PA6 nanofibre composite, the loss modulus is also a measure for the energy dissipation via heat of amaterial. From a temperature of around 10 °C the loss modules of the nanofibre composite starts to riseuntil it reaches a value of around 73 MPa at a temperature of 32, 5 ◦C this is over 56 % higher than theloss modulus of the neat resin. The damping factor shows similar behaviour as the loss modulus with anincrease of 54 % as can be seen in Figure 5.7. Thus the PA6 nanfobrs seem to increase the fracture touhnessand impact strenght of the epoxy resin. To futher underpin this, futher research may be planned to test theimpact resistance and toughness of nanofibre reinforced epoxy resin on a larger scale. However this was notwithin the scope of the present thesis.

Figure 5.6: Loss modulus of PA6 nanofibre composite(—) and neat resin (· · · )

it can be noted that an increase in stiffness (storage modulus) is accompanied with a decrease in toughnessand vice versa. This result is in line with the results of previous research on the use of thermoplasticparticles, sheets and membranes which indicated that indeed the stiffness decreased with and increasedtoughness [19, 20, 21]. It should however be noted that the stiffness of the epoxy resin is always inferior tothe stiffness of the main reinforcement (eg. glass fibres or carbon fibres). Therefore, it is also the stiffness ofthe main reinforcing fibres that will determine the stiffness of the composite. Implying that, an increase instiffness of the matrix will probably have less effect on the overall mechanical properties of the composite,than an increase in toughness. Although, an increase in stiffness combined with an increases in toughnessis often hard to achieve, one can notice that there is also a small zone around 20 °C where the storagemodulus, the damping factor and the loss modulus are higher for the nanofibre composite as compared to


the neat resin, suggesting that in this zone an increased stiffness combined with an increased toughnessmight be achievable.

Figure 5.7: Damping factor of PA6 nanofibre composite(—) and neat resin (· · · )

Both the loss modules and the damping factor also provides information about the transition of the materials,the peak in loss modulus is often used to define the glass transition temperature of a polymer. The averagepeak in loss modulus for the neat epoxy resin samples was 79, 8± 0, 7 ◦C while for the PA6 nanofibrecomposite this was 75, 1± 0, 9 ◦C. Indicating that the nanofibres have a small plasticizing effect on theepoxy resin, this result is completely in line with what one would predict based upon the results from thecuring study of the DGEBA-MDA epoxy resin system.

5.2.4 Morphology of PA nanofibre composites

The tan(δ) peak also contains information about the network structure of the epoxy, the peak factor, τ , isdefined as the full width at half maximum of the tan(δ) peak divided by its height. The peak factor canbe qualitatively used to assess the homogeneity of epoxy network [65, 66]. The neat epoxy was observedto have a low peak factor of approximately 9 °C that indicated the crosslinked density and homogeneity ofthe epoxy network were high. For the with incorporated nanofibres epoxy, the peak factor was found to be36,5 °C. The higher peak factor for the epoxy with incorporated nanofibres is indicative of lower crosslinkdensity and greater heterogeneity, which suggests intercalation of nanofibres into the epoxy network.

Figure 5.8 shows a magnified image of the cross section of a PA6 nanofibre composite. It can be notedthat the nanofibres are homogeneously embedded into the matrix. Furthermore, nanofibres did not melt andthe nanofibrous structure of the PA6 nanofibres is maintained during the composite production process, as


Figure 5.8: SEM images of the cross section of a PA6 nanofibre composite

opposed to some of the previous researches were, the thermoplastic nanofibres melted during the productionprocess [19, 20].


5.3 Dynamic mechanical properties of glassfibre epoxy composites secon-dary reinforced with nanofibres

5.3.1 Effect of PA6 nanofibres on storage modulus and damping factor

In previous research by Langendries et. al [30] it was found that [+45,−45]2S glass fibre-epoxy com-posites secondary reinforced with nanofibres showed an increased modulus. Furthermore, also the matrixbreakpoint increased. The nanofibres used in these composites were PA6 nanofibres directly spun uponthe glass fibre mats before impregnation into the resin. During this thesis identical [+45,−45]2S glassfibre-nanofibre-epoxy composite samples were analysed using DMA.

Figure 5.9 shows the average storage modulus obtained from several temperature ramp experiments. It wasobtained by calculating the average in every data point from 4 temperature ramp experiments. One noticesthat the standard deviation on these measurements is relatively high compared to the accurate measurementsobtained for the nanofibre composite samples (previous section). The spread on these measurements is mostlikely not due to flaws in the DMA testing procedure, because in that case one would expect the experimentsconducted on the nanofibre composite samples to show larger deviations as well. More likely the spread isdue to imperfections in composite production. The DMA samples are only 10 mm wide and 20 mm longthis means that local imperfections in either the glass fibre mats, resin distribution, nanofibre content and ordistribution or sample thickness can lead to relatively high variations in thermo mechanical properties. Aninteresting detail is that the nanofibre containing samples show a higher deviation than the glass fibre-epoxycomposites.

Although some spread on the measurements exists, the nanofibre containing samples clearly show a higherstorage modulus. This result is in line with what was to be expected from the research by Langendries et.al [30]. The reason for this increased storage modulus can be subscribed to two causes, as indicated in theprevious section, the nanofibres will strengthen the matrix, also in places where the larger glass fibers cannot. Furthermore the nanofibres might adhere to the glass fibres, whilst also adhering well to the matrix. Inthis way, the adhesion between the reinforcement and the matrix is improved. The improved fibre matrixadhesion also explains why the modulus of nanofibre containing samples increased at all temperatures andnot only below the onset of the nanofibre glass transition.

The effect on the Tg can also be verified, Figure 5.10 shows the damping factor in function of temperature.It can be noted that the peak maximum as well as the width of the peak are approximately the same for boththe nanofibre containing sample (—) and the normal glass fibre-epoxy reference (· · · ). So in this case thepresence of the nanofibres does not significantly influence the Tg of the resin. In the case of the nanofibre-epoxy composites (previous section) a small decrease in Tg could be noted. The reason for this differentbehaviour is probably due to the fact that the nanofibre concentration is a lot lower for the glass fibre-nanofibre-epoxy composites. Therefore, the effect of a possible migration of one of the resin components tothe nanofibres which could in turn lower the Tg will be smaller as well.


Figure 5.9: Effect on the stiffness of adding PA6 nanofibres (—) to [+45,−45]2S glass fibre-epoxy compo-site (· · · )

Figure 5.10: Effect on the damping factor of adding PA6 nanofibres (—) to [+45,−45]2S glass fibre-epoxycomposite (· · · )

Due to the slightly lower peak height, the peak factor τ , is also a bit higher for the nanofibre containingsamples. The peak factor of the nanofibre containing samples was found to be 39,3 °C, whereas the peakfactor of the glass fibre-epoxy reference was 32 °C. The higher peak factor for the nanofibre containing


epoxy is indicative of lower crosslink density and greater heterogeneity, which suggests intercalation ofnanofibres into the epoxy network. It should be noted that both the peak factor of the reference and of thenanofibre containing samples are significantly higher than the peak factor of 9 °C obtained from the neatresin sample (as mentioned in previous section). This indicates that the presence of the glass fibres alreadygives rise to a lower crosslink density and greater heterogeneity of the epoxy network. The addition of thenanofibres will increase this heterogeneity even more.

5.3.2 Effect of PA6 nanofibres on creep behaviour

The creep behaviour of these glass fibres epoxy composites secondary reinforced with nanofibres was eva-luated and compared with a normal glass fibre epoxy composite. The creep evaluation was done by conduc-ting creep experiment at different temperatures after which a master curves were created using the TTSprinciple. However, to be able to apply the TTS principal, it should be ensured that the composite sampleis fully cured. If this is not the case the mechanical properties of the composites (for example the stiffness)would change during the TTS experiment due to post curing. A fully cured sample should have a stable Tg,this implies that when repeatedly determining the Tg, the Tg should stay the same whereas for a sample thatis not fully cured the Tg will go up. This is due to the fact that to determine the Tg with DMA, a temperatureramp experiment has to be preformed, this temperature ramp also acts as a non-isothermal post cure. If thesample is fully cured this non-isothermal post cure has no influence on the Tg.

Figure 5.11: Effect of post cure at 110 °C for 500 min on Tg , before post cure (—) after post cure (- - -) andverification (· · · )

Figure 5.11 shows the result of three temperature ramp experiments, the first experiment (indicated by the—) determined the Tg prior to posture. In a next step, the sample was post cured at 110 °C for 500 min.


After this post cure a second temperature ramp was carried out (- - -). One can notice that the Tg increasedfrom 90,88 °C to 98,03 °C. Finally, a third temperature ramp (· · · ) was carried out to verify if this post curewas sufficient to fully cure the sample. It can be noted that the Tg obtained from the third temperature rampequals 98,05 °C which is nearly identical to that of the one obtained after the second temperature ramp.Therefore, it can be concluded that an isothermal post cure at 110 °C for 500 min was sufficient to fully curethe sample.

Figure 5.12: Raw data from TTS creep experiment on [+45,−45]2S glass fibre-PA6 nanofibre-epoxy com-posite samples, showing the creep compliance obtained for several creep experiments at different tempera-tures ranging from 30 °C to 90 °C in steps of 5 °C.

On these post cured samples, creep test were preformed at different temperatures starting at 30 °C until90 °C. In between these creep test the samples were allowed to recover. Figure 5.12 shows the result of suchcreep test for a [+45,−45]2S glass fibre-epoxy composite sample secondary reinforced with nanofibres.These creep compliance curves were subsequently plotted on a log(time) scale and an appropriate shiftfactor aT was applied for each temperature. These shift factors are defined by Equation 5.5 in which trrepresents the reduced time which is the expanded time scale for the creep master curve at an isothermalreference temperature. This reduced time is also unknown, one way to find the these shift factors and thecorresponding reduced time is by visually overlapping the curves. In this way a master curve which predictsthe creep behaviour at 30 °C over an extended amount of time was constructed, it can be seen in Figure 5.13.

log aT = logt

tr(5.5)

TTS is an empirical method that is commonly used in literature [67, 68]. It gives a first indication of athe long therm creep behaviour of a material, without having to carry out extremely long creep tests. Toverify the validity of the TTS principle one should ideally carry out a creep test of several months andcompare the results from this experiment to the one found using the TTS principal. Such a verification wasnot done in this thesis because it is very time consuming, however, there are some other more theoretical


Figure 5.13: TTS master curve predicting the creep compliance for [+45,−45]2S glass fibre-PA6 nanofibre-epoxy composite samples at 30 °C.

principles to verify if the TTS principle can be applied in this situation. One way to check the validityof the TTS principal is by looking at the shift factors, previous research has shown that these shift factorsshould follow an Arrhenius relationship below the Tg of the tested material [67, 68]. The general Arrheniusequation is shown in Equation 5.6. This equation can now be transformed to Equation 5.7, which shows alinear relationship between log aT and 1

T . Figure 5.14 displays a plot of the logarithm of the shift factorslog aT (determined by visual overlap of the isothermal creep compliance curves) in function of 1000

T . Onecan notice that the shift factors follow the Arrhenius equation fairly well until a temperature of 75 °C, thistemperature is very close to the onset of the tan(δ) peak (start of the glass transition) of the resin as canbe seen in Figure 5.11. This is a good indication that the TTS can be applied to these composite samples.Around and above the Tg of the resin the Arrhenius relationship is no longer valid, this observation is inagreement with previous research [67, 68, 69].

aT = A. exp−Ea

R4 T(5.6)

log aT =logA

R.(

1

T− 1

Tref). log e (5.7)

Another factor that should be evaluated is the reproducibility of these TTS experiments. Figure 5.15 showsthe master curves for three [+45,−45]2S glass fibre-nanofibre-epoxy composite samples, it can be notedthat the reproducibility of these experiments is not optimal. The spread larges with higher prediction times(higher temperatures). The exact reason for this irreproducibility remains unclear. One possible explanationis that the TTS principle is not valid above a certain temperature. Another possible explanation could be


Figure 5.14: Logarithm of the shift factor in function of 1000/T, showing good fit with Arrhenius equationuntil the shift factor at 75 °C

Figure 5.15: Reproducibility of TTS creep experiments, the figure shows three master curves, predictingthe creep compliance for [+45,−45]2S glass fibre-PA6 nanofibre-epoxy composite samples at 30 °C


that the samples themselves show variations in mechanical properties. In the previous section it was alreadymentioned that the [+45,−45]2S glass fibre-PA6 nanofibre-epoxy composite samples showed quite somevariation in stiffness, the same might apply for the creep behaviour.

The spread on these TTS creep experiments makes it difficult to evaluate if the PA6 nanofibres have asignificant effect on the creep behaviour of these glass fibre-epoxy composites. Figure 5.16 shows themasters curves of three creep test conducted on [+45,−45]2S glass fibre-PA6 nanofibre-epoxy compositesamples (—) as well as three reference master curves obtained from creep experiments conducted on nor-mal [+45,−45]2S glass fibre-epoxy composite without nanofibres (· · · ). It appears to be that the creepcompliance of the nanofibre containing samples is on average lower than for the the standard glass fibre-epoxy samples. However, future research is needed to verify whether or not the improved creep resistancewas really due to the presence of the nanofibres.

Figure 5.16: Average masters curves obtained from three TTS creep experiments conducted on[+45,−45]2S glass fibre-PA6 nanofibre-epoxy composite samples (—) as well as reference master curvesobtained from three creep experiments conducted on normal [+45,−45]2S glass fibre-epoxy composite wi-thout nanofibres (· · · ).

Chapter 6

Conclusion and future work

In this thesis the effect of polyamide nanofibres on the curing characteristics of an epoxy amine resins aswell as on the thermo mechanical properties of composite materials was investigated.

The DGEBA-MDA resin hardener system was found to be very suitable for the study of the effect of nano-fibres on the curing behaviour of epoxy amine resins. MTDSC was shown to be a very suitable techniqueto analyse this system and very reproducible heat flow curves where obtained. The addition of PA6 nano-fibres to this resin harder system resulted in a catalytic effect. Furthermore a small plasticizing effect wasobserved. Both observations where in line with the data found in literature. Next to the DGEBA-MDA resinthe commercially available RIM135-RIM137 resin hardener system was also investigated. Due to the factthat the RIM135-RIM137 resin hardener system was designed for low temperature curing, TAM was a moresuitable method to study this resin. These TAM experiment also gave rise to very reproducible results.

The effect of the nanofibre moisture content on the curing behaviour of the DGEBA-MDA resin was subse-quently investigated. Therefore, the moisture adsorption of PA6 nanofibres, conventional PA6 microfibresand epoxy resin was determined using DVS. The curing experiments were carried out at a wide range ofnanofibre moisture contents. These curing experiments showed a correlation between the nanofibre mois-ture content and the catalytic effect of nanofibres on the DGEBA-MDA curing reaction. More specificallya linear relationship between the initial reaction speed of the DGEBA-MDA curing reaction and the nano-fibre moisture content was found. In addition there are probably also a limited amount of catalysing groupspresent on the nanofibres, independent of the moisture content.

In addition, the effect of nanofibres was compared with the effect of conventional PA6 microfibres. Althoughthe moisture adsorption of both fibres were very similar, it were found that the initial reaction speed of thesamples containing PA6 microfibres stopped increasing from a moisture content of around 2,7 % whereasthat of the nanofibre containg samples kept increasing. This difference could be explained by the highersurface area of the nanofibres as opposed to the conventional fibres. The contact area between the nanofibresand the resin is a lot larger than for conventional fibres. Therefore, the diffusion distance in the bulk of theresin (in-between the fibres) is larger in comparison of the conventional fibres. The diffusion of moisture inthe DGEBA-MDA resin was shown to be a very slow process. Thus, in the case of the conventional PA6fibres, there is not enough time for the moisture to diffuse into the resin and cause a catalytic effect.

Moreover also the effect of the moisture content on the Tg of the resin was investigated. Statistical analysis of

59

CHAPTER 6. CONCLUSION AND FUTURE WORK 60

the obtained data showed that there was no correlation between the nanofibre moisture content and the finalTg of the DGEBA-MDA resin. The reason for the plasticizing effect of nanofibres was thought to be causedby preferential migration (or adsorption) of one of the resin components to (and/or upon) the polyamidenanofibres. This migration would disturb the stoichiometric epoxy-amine ratio and thereby lowering theTg. This assumption is line with the observation that the drop in Tg for the conventional PA6 microfibresamples was significantly lower than that of the nanofibre containing samples. Because these conventionalPA6 microfibres have a lower amount of active surface, upon which resin components can absorb.

The study of the curing behaviour was extended to the RIM135-RIM137 resin hardener system as well.It was shown that, also for this epoxy amine resin, nanofibres give rise to a catalytic effect. Furthermore,it was shown that an increased moisture content also increases this catalytic effect similar to the case forthe DGEBA-MDA resin. Therefore, it was suggested that the fundamental principles obtained from theDGEBA-MDA curring study can be applied to other resin hardener systems as well.

Chapter 5 focused on the study of thermo mechanical properties of composites contain PA6 nanofibres. Firstthe PA6 nanofibres composite were studied and their properties were compared with those of the neat resin.The nanofibre composites showed an increased stiffness until a temperature of around 21,5 °C. At highertemperatures, the stiffness of the nanofibre composite was lower than that of the resin. The reason for thisshift in modulus was thought to be caused by the glass transition of the nanofibre web. The nanofibres alsoshowed and increased storage modulus and damping factor from a temperature of about 19 °C, indicating ahigher energy dissipation by heat. The increased storage modulus and damping factor might also be corre-lated with an increased fracture toughness and impact resistance of the nanofibre composite, as compared tothe neat resin sample. The glass transition temperature of the nanocomposite was found to be slightly lowerthan that of the neat resin sample, this observation was in agreement with the data obtained from the curingstudy in the previous chapters.

In the last part of chapter 5, the effect of PA6 nanofibres on [+45,−45]2S GF-epoxy composites was inves-tigated. It was found that the presence of nanofibres gave rise to a slightly higher stiffness of the composite.Although, the nanofibre containing samples showed on average a better creep resistance, care should betaken before making such conclusion since the time temperature superposition experiments showed quite ahigh amount of variation.

In conclusion, this thesis provides interesting insights into the curing behaviour and thermo mechanicalproperties of PA6 nanofibre composites, It is however also clear that further research is useful and advisable.Further research could focus on a more detailed study of epoxy-nanofibre composites investigating differentprocessing parameters, for example the nanofibre concentration. Additionally, a wide range of mechanicaltest could be used to further characterize the nanofibre composites, so far all testing was mainly limited totensile tests and dynamic mechanical analysis. The toughness and impact resistance of nanofibre compositesturn out to be interesting properties for further research.

The effect of PA6 nanofibres on the curing behaviour of epoxy resins has been the focus of the presentthesis, however, future research should not be limited to the use of unmodified PA6 nanofibres. The useof modified polyamides with different functional groups can be investigated, definitely functionalities thatallow covalent bonds between fibre and matrix could prove to be particularly interesting.

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List of Tables

2.1 Practical guidelines for sample dimensions and clamp type and heathing rate. [48] . . . . . . 22

3.1 Parameters of the curing behaviour of neat DGEBA-DMA resin, comparison between theresults obtained in this research and those obtained by Moorkens et al. [3]. . . . . . . . . . . 25

3.2 Kinetic parameters of the curing behaviour of DGEBA-DMA resin containing PA 6 NF,comparison between the results obtained in this research and those obtained by Moorkenset al. [3]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

4.1 Maximum resin moisture adsorption and conversion data showing that the reactive resinhardener mixture mainly consists of monomers during the DVS experiments. . . . . . . . . 32

4.2 Results of the Shapiro-Wilk test for normality . . . . . . . . . . . . . . . . . . . . . . . . . 374.3 ANOVA analysis showing that there is no significant difference between the reaction en-

thalpy obtained at different nanofibre moisture contents . . . . . . . . . . . . . . . . . . . . 374.4 Characteristics of PA6 NF and PA6 CF. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 384.5 Results of the Shapiro-Wilk test for normality . . . . . . . . . . . . . . . . . . . . . . . . . 404.6 Results of ANOVA test to evaluate the significance between the difference of the means of

the Tg´s obtained at different nanofibre moisture contents. . . . . . . . . . . . . . . . . . . . 40

66

List of Figures

1.1 Examples of composite applications divided in nine major commercial segments [3]. . . . . 21.2 scanning electro microscope (SEM) image of electrospun nanofibres compared to a human

hair. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.3 Curing of epoxy using amines: two basic reactions [36]. . . . . . . . . . . . . . . . . . . . 71.4 Etherification of epoxy [36]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.5 DSC basic compomentents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91.6 Figure showing heat flow (A) and rev Cp signal obtained from isothermal MTDSC scan on

epoxy resin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101.7 Figure showing the heat flow signal obtained from a non-isothermal DSC scan on an epoxy

amine resin, as well as the corresponding total reaction enthalpy . . . . . . . . . . . . . . . 111.8 Illustration of different methods to obtain the Tg from an MTDSC measurement, A via the

(rev) Cp , B via the (rev) heat flow, C using the derivative of the (rev) Cp, D using thederivative of the (rev) heat flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

1.9 Figure illustrating the sinusoidal stress and strain as measured by DMA as well as the phasedifference between them . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

1.10 Typical signals obtained from DMA time-temperature ramp experiment. One can see thestorage modulus, loss modulus and damping factor in function of temperature for an epoxyglassfibre composite heating rate 2,5◦C/min oscillation frequency 1Hz . . . . . . . . . . . 14

1.11 Mathematical relation between storage modulus (E´), the loss modulus (E´´) and the dam-ping factor (tan δ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

1.12 Storage moduli measured by DMA compared with flexure testing results of Araldite-F resinmeasured at different temperatures [49] . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

1.13 Typical regions in creep recovery curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.1 Heat flow signal of neat resin samples obtained from a quasi-isothermal MTDSC scan at80 ◦C, comparison between experimental results from this research sample 1-3 and Moor-kens et al. [3] reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3.2 Typical response of TA instruments Q2000 temperature controller to step input of 80 ◦C . . 253.3 Heat flow signal of PA 6 NF-resin samples obtained from an isothermal MTDSC scan at

80 ◦C, comparison between experimental results from this research (sample 1-4) and resultsfrom Moorkens et al. [3] (reference 1 and 2) . . . . . . . . . . . . . . . . . . . . . . . . . . 26

3.4 Heat flow signal of RIM135-RIM137 neat resin samples obtained from a isothermal TAMexperiments at 25 ◦C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

67

LIST OF FIGURES 68

4.1 Absorption (—) and desorption (· · · ) of (A) PA6 nanofibres and (B) conventional PA6 mi-crofibres at 23 ◦C. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

4.2 Moisture absorption of uncured resins at 95% RH A DGEBA-MDA B RIM135-RIM137 . . 304.3 Heat flow signal of DGEBA-MDA (—) and RIM135-RIM137 (· · · ) epoxy amine resins

from a isothermal TAM curing experiment at 25 ◦C. . . . . . . . . . . . . . . . . . . . . . . 314.4 Heat flow signal obtained from a quasi-isothermal MTDSC scan at 80 ◦C illustrating that the

relative humidity during sample preparation has no significant effect on th curing behaviourof the DGEBA-MDA resin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

4.5 Heat flow signal obtained from a quasi-isothermal MTDSC scan at 80 ◦C illustrating theeffect of the nanofibre moisture content on the curing behaviour of the DGEBA-MDA resin . 33

4.6 Rev. Cp signal obtained from a quasi-isothermal MTDSC scan at 80 ◦C illustrating theeffect of the nanofibre moisture content on the curing behaviour of the DGEBA-MDA resin. 34

4.7 Initial reaction speed (A), tmaxHF (B), maximum reaction speed (C) and Reaction enthalpy(D) obtained from a quasi isothermal MTDSC scan at 80 ◦C of pure resin (4) and of nano-fibre containing samples (♦) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

4.8 Reaction enthalpies obtained after a quasi isothermal curing experiment at 80 C° for 250 minin function of the nanofibre moisture content . . . . . . . . . . . . . . . . . . . . . . . . . . 36

4.9 Comparison between initial reaction speed of the DGEBA-MDA curing reaction at differentfibres moisture contents for pure resin (4), PA6 nanofibres (♦) and conventional PA6 mi-crofibres (�) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

4.10 Evolution of moisture content in time at 95% RH of A uncured DGEBA-MDA resin B PA6nanofibres . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

4.11 Tg of fully cured DGEBA-MDA resin containing nanofibres (♦) with different moisturecontents and dry resin (4) without nanofibres . . . . . . . . . . . . . . . . . . . . . . . . . 40

4.12 Tg of fully cured DGEBA-MDA resin containing PA6 nanofibres (♦) or conventional PA6microfibres (�) with different moisture contents and dry resin (4) without fibres . . . . . . 41

4.13 Comparison between heat flow signals obtained from a isothermal TAM curing experimentat 25 ◦C of nanofibres (—) embedded in resin and pure RIM135-RIM137 (· · · ) epoxy amineresin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

4.14 Heat flow signal of nanofibres embedded in RIM135-RIM137 epoxy amine resin samplesobtained from a isothermal TAM curing experiment at 25 ◦C, comparison between 2 % (—)and 76 % (· · · ) nanofibre moisture content samples. . . . . . . . . . . . . . . . . . . . . . . 43

5.1 Illustration of the reproducibility of the DMA experiments carried out in this thesis showing(A) storage modulus and (B) loss modulus of three epoxy resin samples . . . . . . . . . . . 45

5.2 Figure illustrating the effect of a 1 % (- - -) and 2 % (....) dimensional error in width (A) orthickness (B) on the storage modulus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

5.3 Storage modulus of PA6 nanofibre composite(—) and neat resin (· · · ) (A) full measurementrange, (B) zoom on modulus shift . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

5.4 (A) dampingfactor and (B) storage modulus of PA6 nanofibre web . . . . . . . . . . . . . . 475.5 Tensile test at 20 ± 2 ◦C showing an increased Young´s modulus for nanofibre composite

(—) compared to a neat resin reference sample (· · · ) . . . . . . . . . . . . . . . . . . . . . 485.6 Loss modulus of PA6 nanofibre composite(—) and neat resin (· · · ) . . . . . . . . . . . . . . 495.7 Damping factor of PA6 nanofibre composite(—) and neat resin (· · · ) . . . . . . . . . . . . . 50

LIST OF FIGURES 69

5.8 SEM images of the cross section of a PA6 nanofibre composite . . . . . . . . . . . . . . . . 515.9 Effect on the stiffness of adding PA6 nanofibres (—) to [+45,−45]2S glass fibre-epoxy

composite (· · · ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 535.10 Effect on the damping factor of adding PA6 nanofibres (—) to [+45,−45]2S glass fibre-

epoxy composite (· · · ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 535.11 Effect of post cure at 110 °C for 500 min on Tg, before post cure (—) after post cure (- - -)

and verification (· · · ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 545.12 Raw data from TTS creep experiment on [+45,−45]2S glass fibre-PA6 nanofibre-epoxy

composite samples, showing the creep compliance obtained for several creep experimentsat different temperatures ranging from 30 °C to 90 °C in steps of 5 °C. . . . . . . . . . . . . 55

5.13 TTS master curve predicting the creep compliance for [+45,−45]2S glass fibre-PA6 nanofibre-epoxy composite samples at 30 °C. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

5.14 Logarithm of the shift factor in function of 1000/T, showing good fit with Arrhenius equationuntil the shift factor at 75 °C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

5.15 Reproducibility of TTS creep experiments, the figure shows three master curves, predictingthe creep compliance for [+45,−45]2S glass fibre-PA6 nanofibre-epoxy composite samplesat 30 °C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

5.16 Average masters curves obtained from three TTS creep experiments conducted on [+45,−45]2Sglass fibre-PA6 nanofibre-epoxy composite samples (—) as well as reference master curvesobtained from three creep experiments conducted on normal [+45,−45]2S glass fibre-epoxycomposite without nanofibres (· · · ). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

Study of nanofibre reinforced epoxy composites: curing...

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