Structural biocomposites from flax – Part II: The use of PEG and PVA as interfacial compatibilising agents

Composites: Part A 38 (2007) 1403–1413 www.elsevier.com/locate/compositesa

Structural biocomposites from flax – Part II: The use of PEG and PVA as interfacial compatibilising agents Q. Liu a, T. Stuart b, M. Hughes

c,* ,

H.S.S. Sharma

b,d

, G. Lyons

b,d

a The BioComposites Centre, University of Wales, Bangor, Gwynedd, LL57 2UW, United Kingdom Department of Applied Plant Science, School of Agriculture and Food Science, Queens University Belfast, New Forge Lane, Belfast, BT9 5PX, United Kingdom Laboratory of Wood Technology, Department of Forest Products Technology, Helsinki University of Technology, P.O. Box 6400, FI-02015 TKK, Finland d Applied Plant Science Division, Department of Agriculture for Northern Ireland, New Forge Lane, Belfast, BT9 5PX, United Kingdom b

c

Received 2 February 2006; received in revised form 10 July 2006; accepted 23 August 2006

Abstract Flax fibre, pre-treated in a 2-step process with a chelating agent for calcium followed by a commercial pectinolytic enzyme preparation, was modified with either PVA (poly(vinyl acetate)) or PEG (poly(ethylene glycol)). After treatment the fibres were found to have undergone surface and bulk chemical changes, identified through near infra-red spectroscopy (NIR) and differential thermo gravimetry (DTG). Changes to the linear density of the fibre were also found to have occurred. Modification with PVA and PEG did not result in any change in the fibre Young’s modulus, but did result in a loss in tensile strength of about 15%, accompanied by an increase in the coefficient of variation from around 10% to 25%, indicating structural change to the fibre. When used as reinforcement in an epoxy matrix composite, an increase in the composite’s Young’s modulus from 4.5 GPa to 5.5 GPa was observed, accompanied by a reduction in tensile strength, strain to failure and work-of-fracture. It is believed that the PVA and PEG modify the interfacial behaviour in these systems, improving fibre and matrix adhesion.  2006 Published by Elsevier Ltd. Keywords: A. Polymer–matrix composites (PMCs); B. Interface/interphase; B. Fibre/matrix bond

1. Introduction In recent years there has been an increase in the use of raw materials of a biological origin in the manufacture of composites destined for a variety of applications [1]. Latest reports, for example, have shown that in 2003 around 43 000 tonnes of natural fibre were being used by the European automotive industry as composite reinforcement [2]. In the search to replace traditional materials with ecologically sourced alternatives, developmental work has recently concentrated on the substitution of glass fibres with natural fibres such as flax and hemp in the reinforcement of thermosetting and thermoplastic composite materials [3]. With

*

Corresponding author. Tel.: +358 9 451 4266; fax: +358 9 451 4259. E-mail address: mark.hughes@tkk.fi (M. Hughes).

1359-835X/$ - see front matter  2006 Published by Elsevier Ltd. doi:10.1016/j.compositesa.2006.08.009

the notable exception of the automotive sector, however, there has been little or no commercial application of biocomposites [4]. In part, this has been due to the hitherto limited performance of biocomposite materials rendering them unsuitable for structural applications. Reported fibre Young’s modulus values of up to 100 GPa [5,6] and tensile strengths of 1500 MPa [7] would, however, suggest that structural composites reinforced with these fibres, should be possible. In the first part of this study it was shown that the application of the latest processing and characterisation techniques for flax fibres, used in the textile industry, can be employed to prepare bespoke fibre for composite reinforcement [8]. Furthermore, this improved fibre can lead to significant improvements in the performance of the resultant composites. It was observed that flax fibres, pre-treated to separate the fibre bundles (and hence increase the number

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of ultimate fibres) and also to clean the fibre surface of adhering non-cellulosic material, produced the best performing composites. In that study, however, no attempt was made to ‘‘engineer’’ the interface between the fibre and the encapsulating matrix; rather, the composite performance improvements were attributed to the substantially increased fibre surface area available for bonding with the matrix and the elimination of weak inter-fibre interfaces [8]. The work undertaken in this part of the study focused on the potential to improve, still further, the properties of biocomposites by applying a coupling agent to pre-treated, separated fibres, so as to enhance fibre–matrix bonding. In choosing suitable coupling agents, PVA and PEG have already proven to be beneficial in both textile and composite applications. PEG is widely used as a plasticizing agent whilst PVA is recognized for its adhesive properties. In a study of the curing of a 50% PVA solution onto flax and hemp fibres prior to embedding in phenolic and epoxy resins, both the tensile strength and stiffness of the resultant composites were observed to increase [9]. Further work on PVA, used to press together fragments of vegetable parenchyma material, produced composites with a strength of 70 MPa and modulus of 5.4 GPa [10]. Recently published work has shown that flax fibres with PEG (Mw = 350 and 750 g/mol) chemically grafted onto their surface, produced composites of the highest tensile strength within the study, when embedded in a PLA (poly(lactic acid)) matrix [11]. In textile applications, both PEG and PVA have been observed to improve crease (stiffness) and abrasion resistance in cotton fabric with good retention of tensile strength [12]. The abundance of hydroxyl groups present in PVA and low molecular weight PEG provide for a high degree of hydrogen bonding with a cellulosic fibre. An increase in the functional groups present on the surface of the fibre can be attained through its partial acid hydrolysis. Reducing and non-reducing end groups are freed up in the process through scission of polysaccharide chains within accessible material [13]. The hydrolysis proceeds via carbocation formation followed by hydration. From an initially high rate, the process slows considerably when the more accessible material has been digested, and the highly crystalline cellulose forms the remainder of the substrate. The digestion of the accessible material does lead to a significant loss in tensile properties [14]. The presence of the PVA and PEG compatiblising agents during the hydrolysis may also encourage other types of interaction with the fibre. PVA, with the aid of an acid catalyst, reacts with aldehydes to form acetals in aqueous conditions [15]. Hydrolysis of cellulose will liberate reducing aldehyde groups capable of undergoing acetal formation. A suitable lewis acid for the hydrolysis reaction is aluminium sulphate, a substance implicated in the acid degradation and ageing of paper [16]. Kinetic studies on aluminium doped paper in atmospheres of varying moisture content have revealed that aluminium sulphate dramatically increases the oxidative and hydrolytic degradation of paper at high

(>100 C) temperatures; hydrolysis was favoured in moist conditions. Aluminium salts are known to form complexes with pectin and are used to precipitate aluminium pectate in the fruit industry. Studies have highlighted that aluminium preferably complexes with citric acid under acid conditions in the presence of polygalacturonates [17]. The sequestration of the aluminium should thus be inhibited by the presence of citric acid. The aim of the investigation reported in this paper was to explore the possible benefit of utilising PEG and PVA as compatibilising agents to increase interfacial bonding between flax fibres and a model epoxy resin matrix after the flax fibres had first been separated and cleaned. 2. Materials and methods 2.1. Materials Flax was obtained from Terre Du Lin, France and had the following basic characteristics: a yellow/grey uneven colour; linear density of 68.5 DTex; and the presence of adhering non-fibrous material. Ethylene diamine tetraacetic acid (EDTA) and citric acid were analar grade and were obtained from BDH Laboratory Supplies, Poole, England. The PVA (80% hydrolysed, average Mw = 9000–10 000 g/mol), the PEG (average Mw = 400 g/mol), and the aluminium sulphate were sourced from Sigma–Aldrich Co. Ltd., Gillingham, UK. The non-ionic surfactant, Triton X100, was obtained from Amersham Biosciences UK, Ltd., Buckinghamshire, UK. Pectinex AR, a pectinase manufactured by Novo Nordisk, Denmark for use in the fruit juice industry, was assayed for xylanase, cellulase and pectinase activity (at 50 C for 5 min at pH 7.0 for cellulase and pH 5.0 for xylanase and polygalacturonase). The activities were xylanase, 747 U mL1; polygalacturonase 1170 U mL1; cellulase 91 U mL1. Epoxy resin (Ampreg 20, SP Systems, Isle of Wight, UK), for the matrix, was cured by the addition of hardener (Ampreg 20, ‘‘slow’’ hardener) in the ratio 4:1 (v/v) resin: hardener. Care was taken during mixing to minimise the inclusion of air bubbles and any extraneous matter. Prior to use, the catalysed resin was degassed under vacuum for 5 min. 2.2. Fibre treatment 2.2.1. Fibre pre-treatment The fibre surface was first cleaned and fibre separation accomplished in a two stage pre-treatment process using pectinase and EDTA as described fully in [8]; pectins that bind the adhering material to the fibre, as well as fibres to each other within fibre bundles, are removed through this procedure. The pre-treatment was carried out on a pilot plant scale (1.5 kg) at liquor (volume) to fibre (weight) ratio of 11:1 in a rove treating pilot plant (Ugolini, Italy). In the first stage, fibre was treated with EDTA (5 g/L), at a

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pH adjusted to 11.0 with NaOH, at 60 C for 3 h. In the second stage, fibre was processed in a liquor containing Pectinex AR (5 mL/L), adjusted to a pH of 4.5 with acetic acid, at 40 C for 2 h. The fibres were rinsed three times with tap water after each stage. 2.2.2. Fibre modification A liquor solution was prepared containing either PEG or PVA dissolved in distilled water at a concentration of 100 g/L, together with aluminium sulphate at 10 g/L, citric acid at 10 g/L and Triton X100 detergent at 1 g/L. Samples containing 30 g of flax fibre, pre-treated with the two stage process to remove pectins as described above, were immersed in the liquor at an 11:1 liquor (volume) to fibre (weight) ratio (v/w), then heated from ambient to 121 C in an autoclave over a period of 15 min. When the target temperature was reached, it was held there for 5, 10, or 20 min before being cooled to room temperature and washed for 1 h in running water. A control sample was immersed in liquor and soaked, at room temperature, for the equivalent time taken for the 20 min experiment, before being washed. Details of the relative proportions of the constituents and sample codes are presented in Table 1. 2.3. Composite fabrication Modified and unmodified fibres were chopped to 2 mm and formed into reinforcement mats using a semi-automatic handsheet former (British pulp evaluation apparatus, Mavis Engineering Ltd., England) as used in the production of laboratory paper specimens. Fibre mats were produced to a nominal weight of 0.65 g. Composite laminates were formed by infusing a stack of three reinforcement mats with catalysed resin. The infused mats were placed between glass sheets, cured for 24 h at room temperature and post-cured for 16 h at 50 C. 2.4. Testing 2.4.1. Fibre fineness measurement (airflow) Fibre fineness was measured on 3 g samples by the airflow method (WIRA fineness meter, Reynolds and Branson Ltd., Leeds, UK). Three sub-samples, prepared from 8 cm lengths of conditioned fibres were teased apart and

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formed into a ball of random orientation. Air permeability was measured for each sub-sample 10 times and the averaged permeability value converted into fibre fineness with a conversion table created from standard samples. 2.4.2. Differential thermogravimetry (DTG) Samples were cut finely with serrated scissors to give chopped fibres of a length between 0.5–1 mm. Three subsamples of each replicate were weighed out accurately with an amount between 3.000 and 3.200 mg. The samples were pyrolysed in a microbalance and furnace unit (SDTA 851e with TSO 80 RO sample robot, Mettler Toledo, Leicester, UK) over a temperature range from 30 C to 600 C at a heating rate of 20 C/min in an air atmosphere with flush rate of 20 mL/min. Weight loss over the temperature range and the first derivative of weight loss were obtained through the Mettler STAR software. The continuous weight loss data (1704 data points) of the replicate samples were averaged. 2.4.3. Near infra-red spectroscopy (NIR) Combined visible and near infrared spectra were obtained on a scanning monochromator (FOSS NIR systems, Model 6500, Silver Spring, USA) in reflectance mode over a wavelength range of 400–2498 nm at 2 nm intervals. The band pass value was 10 nm and wavelength accuracy 0.5 nm. The machine was calibrated regularly in accordance with manufacturer’s recommendations. All measurements were carried out in a standard conditioned atmosphere for textile testing (21 ± 1 C and 65 ± 2% r.h.). The samples (7.5 g) were first teased apart to separate single fibres from dryback (electrostatic bonding of fibres after drying) or the bonding effect of the coupling agent. They were then packed into a rectangular sample cup with dimensions 21 cm · 5 cm · 4 cm. Ten scans were made per sample with each scan being an average of 64 sweeps. Principle components for the NIR spectra were then calculated with multivariate analysis software (Unscrambler software package 7.6, CAMO, Trondheim, Norway). 2.4.4. Fibre tensile measurements Fibre (PVA10 and PEG10) tensile properties were measured using a MINIMAT miniature mechanical testing machine (Polymer Laboratories Ltd., UK) equipped with

Table 1 Details of treatments with abbreviated names applied to flax samples for altering fibre characteristics Sample code

Description

Two stage PEG0 (Control sample) PEG5 PEG10 PEG20 PVA0 (Control sample) PVA5 PVA10 PVA20

EDTA (5g/L), pH 11.0 with NaOH at 60 C for 3 h. Then Pectinex AR (5 mL/L), pH 4.5 at 40 C for 2 h Two stage pretreatment. Then 100 g/L PEG, 10 g/L Al2(SO4)3, 10 g/L citric acid, 1 g/L Triton X-100 at 121 C Two stage pretreatment. Then 100 g/L PEG, 10 g/L Al2(SO4)3, 10 g/L citric acid, 1 g/L Triton X-100 at 121 C Two stage pretreatment. Then 100 g/L PEG, 10 g/L Al2(SO4)3, 10 g/L citric acid, 1 g/L Triton X-100 at 121 C Two stage pretreatment. Then 100 g/L PEG, 10 g/L Al2(SO4)3, 10 g/L citric acid, 1 g/L Triton X-100 at 121 C Two stage pretreatment. Then 100 g/L PVA, 10 g/L Al2(SO4)3, 10 g/L citric acid, 1 g/L Triton X-100 at 121 C Two stage pretreatment. Then 100 g/L PVA, 10 g/L Al2(SO4)3, 10 g/L citric acid, 1 g/L Triton X-100 at 121 C Two stage pretreatment. Then 100 g/L PVA, 10 g/L Al2(SO4)3, 10 g/L citric acid, 1 g/L Triton X-100 at 121 C Two stage pretreatment. Then 100 g/L PVA, 10 g/L Al2(SO4)3, 10 g/L citric acid, 1 g/L Triton X-100 at 121 C

for for for for for for for for

0 min 5 min 10 min 20 min 0 min 5 min 10 min 20 min

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a 20 N load beam. Testing was conducted at a cross-head speed of 0.1 mm/min. Individual fibres were gripped in specially fabricated jaws, using paper in the jaws to prevent the fibre breaking near the grips. The gauge length was 30 mm. For the determination of cross-sectional area, a circular cross-section was assumed and the transverse dimensions of the fibre were measured by optical microscopy (HM Lux, Leitz, Germany). At least 20 fibre specimens for each modification type were tested. 2.4.5. Composite testing Tensile testing of the composite specimens was conducted on an Instron universal testing machine model 1195 at a crosshead speed of 1 mm/min. Strain was measured by an extensometer attached to the specimen, over a gauge length of 25 mm. Specimen dimensions were nominally 10 mm · 100 mm · 0.4 mm, with at least 10 replicates of every composition being tested. 2.5. Scanning electron microscopy (SEM) Samples of flax fibre were mounted on SEM sample supports (stubs) and sputter-coated with platinum before assessment with a FEI QUANTA-200 scanning electron microscope (FEI, Eindhoven, Holland). The fracture surfaces of composite specimens were examined using a Hitachi S-520 scanning electron microscope. Samples were first mounted on aluminium stubs and gold coated using a Polaron SEM coating unit E5000. 3. Results and discussion 3.1. Initial screening The first stage screening involved the use of a combination of airflow, NIR spectroscopy and DTG in order to

ascertain the effect of chemical modification with PVA and PEG on the flax fibre. 3.1.1. Fibre fineness The linear density of the samples, seen in Fig. 1, is measured as mass per unit length (Dtex) and is used as a measure of fibre fineness. The fineness (or reduction in fibre diameter) increases as linear density decreases. Fibre fineness increased after the two stage pilot scale cleaning and refining process, although the improvements were not as great as those previously achieved in laboratory scale experiments on flax from the same sourced batch [8]. Inefficient removal of pectic material at pilot scale explained this discrepancy. Increased inter-fibre adhesion due to the presence of the compatibilising agents caused small but significant decreases in fineness. This trend disappears with increasing treatment times through the hydrolysis of the acetylated portion of the PVA decreasing adhesion [18] or changes in hydrogen bonding through displacement of moisture [19]. Release of partially solubilised pectic material still present in the fibres, observed in previous studies, may also affect the fineness [8,20]. 3.1.2. NIR analysis The first derivative (Savitsky–Golay) of the spectra for all samples were described by four principle components (PC) and the scores for each sample projected onto the axes of the first and second principle components using Unscrambler 7.51 software (not shown). As differences between the uncoated (untreated and two stage) and coated (PEG and PVA treated) samples were the primary sources of variation and the benefits of the two stage process over the untreated material have already been previously established [8], it was decided to concentrate on modelling the variance between the coated samples. The loading plots of the two PCs in relation to the original spectral wavelengths are illustrated in Fig. 2. The pro-

75 0.20

0.15

0.10

x-loading

65

PVA20

PVA10

PVA5

PVA0

PEG20

PEG5

55

PEG10

60

PEG0

Fibre fineness, DTex

st

1 principle component nd 2 principle component

70

0.05

0.00

-0.05

-0.10

50 Untreated 2 stage

PEG

PVA

Treatment Fig. 1. Linear density of untreated, two stage pre-treated and fibre modified with PVA/PEG treated for various times (0–20 min). (Note that as linear density increases, fibre fineness decreases.)

-0.15

400

800

1200

1600

2000

2400

Wavelength, nm Fig. 2. Loading plots for first and second principle components with hydroxyl fingerprints circled.

Q. Liu et al. / Composites: Part A 38 (2007) 1403–1413

PVA10

0.0005

0.0000

PVA5

PEG5

PVA0

-0.0005

-0.0010

PEG PVA

PEG20 PEG10

-0.0015 -0.004 -0.003 -0.002 -0.001

0.000

0.001

0.002

0.003

0.004

1st principle component

Fig. 3. Principal components extracted from NIR spectra for treated samples.

jection of the first derivative spectra from the coated sample set onto the first two principle components can be seen in Fig. 3. The circled areas in the loading plot relate to the fingerprint regions for water which include the stretch-bend combination (1800–2500 nm, kmax = 1940 nm), first overtone (1350–1800 nm, kmax = 1450 nm) and a further, less intense band (1150–1350 nm, kmax = 1190 nm), in the near infrared region [21]. The first principle component (PC1) is heavily weighted in these regions, which leads to the conclusion that PC1 is strongly influenced by moisture present in the samples. As PC1 describes most of the variability between the samples (73% of variability in the sample set), the measured changes to the surface properties are strongly linked to alterations by moisture uptake in the samples. Moisture content decreases with increasing treatment duration due to the decreased hygroscopicity of PVA with increasing degree of hydrolysis [18]. There is also a tendency for natural fibres to become more hydrophobic after chemical modification, changing the physical properties of the fibres (see Section 3.2). PEG, being highly hygroscopic in its polymerised form, shows a smaller variation along PC1 due to hydrolysis. The second principle component (PC2, 11% of variability in the sample set) has the majority of its weighting in the combination and first overtone band and virtually none in the lower overtone bands (sub 1700 nm region of the NIR excluding the visible region). PC2 thus represents information from a much weaker signal within the spectra. The samples follow the sequence of their treatment duration along the PC1 axis except for PVA10, which is placed at a slightly more positive value than PVA5 sample. This deviation is also detected in the differential thermogravimetric measurements (see Section 3.1.3). PEG and PVA samples traverse in opposite directions along PC2. The increase or decrease in the scores follows treatment duration apart from one exception, the PEG20 sample. PC2 may be linked to discolouration of the fibre during processing as shown by its high loading in the visible region.

O

Temperature, C 250 0.000

-1

PVA20 0.0010

3.1.3. DTG Thermogravimetry is a technique that has been previously used to infer fibre quality from its bulk combustion characteristics [22]. The thermograms exhibited two sets of decomposition bands that are referred to as the primary peak and the secondary peak. The primary peak occurs between 320 C and 360 C and has a single maximum, whereas the secondary peak, possibly containing multiple maxima, is found between 400 C and 550 C. The primary peak is related to the pyrolysis of cellulose and non-cellulosic polysaccharides while the secondary peak represents combustion of the char remaining from the primary combustion. Studies of the pyrolysis of char from cellulosic material in argon have revealed the evolution of aromatic compounds over the temperature range of the secondary peak [23] and although the combustion was accomplished in the absence of oxygen, the evolved compounds contained oxygen obtained from the cellulosic materials themselves. The secondary peak is believed to be influenced by structurally significant material within the flax fibre [24]. Differential thermograms for PEG and PVA treated materials along with the baseline two stage material can be seen in Figs. 4 and 5 respectively. Examination of the

Diff. mass loss, mgmin

PEG0

2

nd

principle component

0.0015

1407

300

350

400

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550

-0.005 -0.010

SP -0.015

PP -0.020 -0.025 -0.030 -0.035

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500

520

530

PEG 10

-0.005

PEG 0

-0.010

PEG 5 PEG 20

2-stage

-0.015

510

2-stage PEG 0 PEG 5 PEG 10 PEG 20

Fig. 4. DTG graphs for PEG treated samples with primary peak (PP) and secondary peak (SP) illustrated.

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Temperature, C 250 0.000

300

350

400

450

500

550

Diff. mass loss, mgmin

-1

-0.005

material during the secondary combustion. The flattening of the peak is linked to an increase in different microstates and hence a decrease in the uniformity within the fibre with increasing duration of treatment.

-0.010

3.2. Fibre properties

SP

-0.015 -0.020

PP

-0.025 -0.030 -0.035

430 0.000

440

450

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490

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510

520

530

PVA 20 -0.005

PVA 0

PVA 5 PVA 10

-0.010

2-stage

-0.015

2-stage PVA 0 PVA 5 PVA 10 PVA 20

Fig. 5. DTG graphs for PVA treated samples with primary peak (PP) and secondary peak (SP) illustrated.

temperature and form of the primary peak reveals no clear pattern for either PEG or PVA treated samples. In contrast, the secondary peak for both sample sets clearly shifted to a higher temperature, with a distinct flattening of the peak as treatment duration increased. The exception to this trend, as with the scores of the first principle component, is the PVA10 sample which has a slightly lower secondary peak temperature. The correlation is significant (r = 0.88) between the peak temperature and the first principle component scores for all samples and can also be seen in the relation between secondary peak shape and PC1 scores. Both measurements showed that the PEG samples fell into two mini clusters based on duration. The PVA samples also showed similarities between the scores and peak shape. DTG measurements are indicative of bulk properties, while NIR relates to the surface properties of the fibre. The correlation between the two sets of values suggests a uniform distribution of the compatibilising agents throughout the fibre. The effect of the agents on the secondary peak of the thermograms, rather than the primary peak, signifies that the agents influence the formation of char rather than primary combustion. The shape of the secondary peak can also be related to the variety of microstates within the

3.2.1. Axial tensile properties A summary of the tensile mechanical properties of untreated, pre-processed and PVA/PEG treated fibres is shown in Table 2. As may be seen a slight, statistically insignificant, drop in Young’s modulus was observed following the two stage pre-treatment process. Treatment with either the PVA or PEG did not lead to any further significant change in the stiffness of the fibre. This result is not unexpected since although the PVA and PEG may penetrate the fibre, as evidenced by the results from the DTG analyses (Section 3.1.3), the tensile modulus will be dominated by the stiffness and orientation of microfibrils in the cell wall [25,26]. In bast fibres the microfibrils are orientated at small angles to the fibre axis. In hemp, for instance, this angle has been reported to be in the region of 2–3 to the axis of the fibre [27] and with a Young’s modulus estimated to be in the region of 134 GPa [28], the influence of the much lower modulus PVA or PEG would be negligible. There was, however, a distinct drop in the tensile strength of the fibres following pre-treatment (see Table 2). Previously, it has been speculated that this drop is most likely to be caused by the removal of interfibre pectins, brought about by the treatment, and a consequent weakening of the fibre [8]. This contention is supported by the observation that, with an average fibre length of 25 mm [29] and gauge length of 30 mm, it is likely that no single fibre ultimate would have spanned the distance between the jaws of the tensile testing instrument and thus the failure process was probably associated with interfibre rupture, rather than direct fibre fracture. Furthermore, distinct ‘pop-ins’ in the load–deformation history of some of the fibres tested were noted (Fig. 6), indicative of progressive failure. What is particularly interesting is the effect that the PVA and PEG treatments have upon the coefficients of variation (CV) of the means of both tensile strength and Young’s modulus. A high CV is to be expected in the measurement of the tensile properties of natural fibres; in the measurement of the tensile properties of wood fibres for example, Groom et al. [30] found CVs of between 20% and 25% for Loblolly pine latewood fibres. A high CV might thereTable 2 Axial fibre tensile properties

Untreated Two stage PVA PEG

N

Young modulus (GPa)

CV (%)

Tensile strength (MPa)

CV (%)

20 26 22 20

92.1 89.1 82.1 87.9

11.2 10.4 16.6 23.1

1000 650 552 543

10.0 11.1 34.6 33.9

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fore be expected, but the increase in the CV of the mean tensile strength from around 10% for the two stage treated fibre to in excess of 30% for the PVA/PEG treated fibres is indicative of some other structural change to the fibre. This aspect nevertheless requires further investigation.

900 800

Tensile stress, MPa

700

PVA 10 2 stage A

600

1409

3.2.2. Scanning electron microscopy Scanning electron micrographs of untreated, two stage, PVA and PEG treated fibres are shown in Fig. 7. As may be clearly seen, following pre-treatment (Fig. 7(b)), the fibre ultimates were well separated compared to the untreated fibre (Fig. 7(a)). Furthermore, the pre-treated fibres are seen to be free from debris adhering to the fibre surface. After treatment with PEG and PVA (Fig. 7(c) and (d), respectively), globular deposits were visible on the fibre surfaces. This would indicate that the PVA and PEG agglomerate on the fibre surfaces, suggesting that the treatments formed a layer on the surface of the fibre.

500

3.3. Composite axial tensile properties

400 300 200 100 0 -100

-0.5

0.0

0.5

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Extension,mm

Fig. 6. Typical tensile stress–extension curves for two stage pre-treated fibre and PVA10 modified fibre, showing distinct ‘pop-ins’ in the traces at A.

Representative stress–strain curves for the different laminate types are shown in Fig. 8. The manufacturing process used in the preparation of these composite laminates ensured that not only was the architecture of the reinforcement extremely regular, but also the volume fraction (Vf) of fibre was very reproducible, at around 20%. With homogeneous fibre architecture and minimal variation in Vf, it was to be expected that any variation in the properties of the composite would be attributable to the treatments applied to the fibres. In previous work [8], it has been

Fig. 7. SEM photographs of flax fibres that are untreated (a), pre-treated in a two stage (b), PEG10 (c) and PVA10 (d).

Q. Liu et al. / Composites: Part A 38 (2007) 1403–1413

20 10 0 0.0

0.5

1.0

1.5

2.0

Strain, % Fig. 8. Representative tensile stress–strain curves for laminates.

demonstrated that this technique is particularly sensitive and can be used to detect changes in properties arising from treatment to the fibre. Tensile strength, MPa

3.3.1. Tensile modulus As may be observed from Fig. 9, there is little difference between the tensile stiffness of the composite material reinforced with unmodified flax fibre and that reinforced with fibre subjected to the two stage pre-treatment process. This finding is somewhat unexpected given that although the same total volume fraction of fibrous material was available for reinforcing purposes and its orientation and packing arrangement were equivalent, morphological changes to the fibres themselves had taken place. In particular an increase in fineness was seen following pre-treatment, indicating a reduction in the effective cross-sectional area of the fibre; an increase in the extent of fibre separation was also noted from the microscopical evidence (see Fig. 7), corroborating the fineness data. This decrease in effective cross-

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PVA10

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PVA0

Tensile stress, MPa

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PEG10

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60

sectional area might, therefore, be expected to yield fibres of generally higher aspect ratio, s, and thus having greater reinforcing efficacy, leading to improve composite stiffness. Since no improvement in composite stiffness was seen then presumably, either s prior to pre-treatment was sufficiently high that any further increase in the aspect ratio did not lead to any greater reinforcement or, as has been postulated elsewhere [31], the fibres act as a series of short ‘‘segments’’ bounded by microcompressive defects and thus stiffness is independent of the overall fibre aspect ratio, but rather, it is dominated by the aspect ratio of the fibre ‘‘segments’’. What is of interest and again unexpected is the increase in the Young’s modulus of the laminates following modification of the two stage pre-treated fibre with both PVA and PEG. An increase in tensile stiffness from approximately 4.5 GPa for the two stage pre-treated fibre reinforced-

PEG5

70

PEG0

1410

0

Untreated

2 stage

PVA

PEG

Treatment Fig. 10. Tensile strength of composite laminates reinforced with untreated fibre, two stage pre-treated fibre, PEG and PVA treated fibre.

7

2.4

1.2

0

Untreated

2 stage

PEG

PVA

Treatment Fig. 9. Young’s modulus of composite laminates reinforced with untreated fibre, two stage pre-treated fibre, PEG and PVA treated fibre.

PVA10

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PEG

PVA

Treatment Fig. 11. Tensile strain to failure of composite laminates reinforced with untreated fibre, two stage pre-treated fibre, PEG and PVA treated fibre.

Q. Liu et al. / Composites: Part A 38 (2007) 1403–1413 45

35 30 25 20

0

Untreated 2 stage

PEG

PVA10

PVA5

5

PEG5

PEG0

10

PVA0

15

PEG10

Work of fracture, kJ/m

2

40

PVA

Treatment

Fig. 12. Work of fracture of composite laminates reinforced with untreated fibre, two stage pre-treated fibre, PEG and PVA treated fibre.

epoxy composites, to 5.5 GPa for the PVA0 and PEG0 fibre reinforced materials was seen. For both treatment types a slight, though statistically insignificant, drop in laminate stiffness with increasing fibre treatment times (0 ! 10 min) was also noted. It would seem that the

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increase in the Young’s modulus of the laminates cannot be directly attributed to any increase in the axial stiffness of the fibre following treatment, since there was no evidence to suggest this from the fibre property measurements. Change in the transverse or shear moduli of the fibres following treatment cannot be ruled out, since it would seem that their bulk properties were altered following modification. Nevertheless, for the same reasons outlined in Section 3.2.1, it seems unlikely that either the fibre transverse modulus or shear modulus would have been altered significantly; thus, any change in composite stiffness would, at best, be slight. A, perhaps, more likely explanation for the observed increase in composite stiffness is alteration to the behaviour of the interface, resulting from surface modification of the fibre following treatment. Further work is, however, required to verify or otherwise this hypothesis.

3.3.2. Tensile strength, strain to failure and work of fracture The tensile strength of laminates reinforced with untreated, pre-treated as well as PEG and PVA modified fibres are shown in Fig. 10. As has been reported earlier, pre-treatment by the two stage process of chelating agent followed by pectinolytic enzymes, has the effect of

Fig. 13. SEM photomicrographs of the fracture surfaces of modified and unmodified flax fibre reinforced epoxy composites, tested to destruction under tensile loading.

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improving the tensile strength of the resultant composites [8]. The rationale for subsequently treating fibre modified in this manner with PEG or PVA was to attempt to modify the interface in such a way that fibre–matrix bonding might be improved. Furthermore, it was expected that by engineering an interphase region, the worst effects of residual fibre defects and irregularities might be overcome, thereby improving the properties of the composite still further. Interestingly, however, the use of PVA and PEG has the opposite effect, reducing tensile strength to some extent (Fig. 10), whilst strain to failure and work of fracture (Figs. 11 and 12) also follow the same pattern. Treatment time appears to affect not only tensile strength, but also failure strain and work of fracture. The results of the fractographic examination (Fig. 13) are inconclusive as far as suggesting whether treatment with PVA/PEG resulted in any improvement in interfacial bonding. The mechanical property data would seem to indicate that improvements were made, however. In those composites with a strong interfacial bond, fibre–matrix debonding would tend to be suppressed and this would have the effect of inhibiting the ability of fibres to pullout from the matrix thereby giving rise to low strains to failure and to low works of fracture. 4. Conclusions Modification of flax with PVA and PEG would seem to affect the bulk and surface chemical properties of the fibre, as evidenced by the results of the DTG and NIR analyses. Modification also leads to a decrease in fibre fineness, most probably arising from the fibres tending to adhere together, again indicating modification to the surface. Modification would also seem to influence the mechanical properties of the fibre, for whilst no change in Young’s modulus was observed, tensile strength was diminished. When used as reinforcement in an epoxy matrix, PVA and PEG modified fibre resulted in composite materials with superior Young’s modulus. This was, however, accompanied by a decrease in tensile strength, strain to failure and work of fracture. These changes to the material behaviour would seem to indicate that modification of the interface between fibre and matrix takes place following treatment with PVA and PEG. However, the extent of this modification has not been quantified and further work should be undertaken. In summary, a two stage pre-treatment of flax fibre followed by modification with PVA and PEG has the potential to improve the performance of composites reinforced with this fibre. Stiffness improvements of more than 20% can be achieved through the use of this technique, albeit at the expense of some loss in strength and work of fracture. Acknowledgements The work reported in this paper formed part of the ‘‘NOVCOMPS’’ (Low Environmental Impact Polymer

Matrix Composites) project, funded under the LINK Competitive Industrial Materials from Non Food Crops programme by the Engineering and Physical Sciences Research Council (EPSRC). The authors gratefully acknowledge the financial support received for this work from the EPSRC under the following grant references GR/R88748 (Bangor) and GR/R88731 (Belfast). References [1] Van de Velde K, Kiekens P. Thermoplastic polymers: Overview of several properties and their consequences in flax fibre reinforced composites. Polym Test 2001;20:885–93. [2] Karus M. Market and economy of natural and wood fibre reinforced plastics. In: Proc of SusCompNet 9: Sustainability and biocomposites – A one day seminar, 2005, Risø National Laboratory, Denmark. Available from: http://www.risoe.dk/afm/suscompnet/Proceedings.htm, 28 June 2006. [3] Van de Velde K, Kiekens P. Biopolymers: Overview of several properties and consequences on their applications. Polym Test 2001; 21:433–42. [4] Hughes M, Hill CAS, Se`be G, Hague J, Spear M, Mott L. An investigation into the effects of microcompressive defects on interphase behaviour in hemp-epoxy composites using half fringe photoelasticity. Compos Interfaces 2000;7(1):13–29. [5] Bledzki AK, Reihmane S, Gassan J. Properties and modification methods for vegetable fibers for natural fiber composites. J App Polym Sci 1996;59:1329–36. [6] McMullen P. Fibre/resin composites for aircraft primary structures: A short history, 1936–1984. Composites 1984;15(3):222–30. [7] Bos HL, Van den Oever MJA, Peters OCJJ. In: Proc of the 4th int conf on deformation and fracture of composites. Manchester, UK; March, 1997. pp. 499–504. [8] Stuart T, Liu Q, Hughes M, McCall RD, Sharma S, Norton A. Structural biocomposites from flax – Part I: Effect of bio-technical fibre modification on composite properties. Compos Part A 2006; 37(3):393–404. [9] Hepworth DG, Bruce DM, Vincent JFV, Jeronimidis G. The manufacture and mechanical testing of thermosetting natural fibre composites. J Mat Sci 2000;35(2):293–8. [10] Hepworth DG, Bruce DM. The mechanical properties of a composite manufactured from non-fibrous vegetable tissue and PVA. Compos A 2000;31:283–5. [11] Zini E, Baiardo M, Armelao L, Scandola M. Biodegradable polyesters reinforced with surface-modified vegetable fibers. Macromol Biosci 2004;4(3):286–95. [12] Ghosh P, Das D. Modification of cotton by some low molecular weight glycols and a polyol. J Polym Mat 2002;19:209–20. [13] Gilbert SM, Smith BF. A degradation study of some formaldehydemodified celluloses. Text Res J 1970;40:720–7. [14] Buschle-Diller G, Zeronian SH. Enzymatic hydrolysis of cotton, linen, ramie and viscose rayon fabrics. Text Res J 1994;64(5):270–9. [15] Toyoshima K. In: Finch CA, editor. Acetalisation of Polyvinyl Alcohol in Polyvinyl Alcohol. Properties and Applications. London: John Wiley and Sons Ltd; 1973. p. 391–411. [16] Lojewska J, Lubanska A, Lojewski T, Miskowiec P, Proniewicz LM. Kinetic Approach to degradation of paper. In situ FTIR transmission studies on hydrolysis and oxidation. e-PS, www.e-PreservationScience.org, ISSN: 1581-9280, 2005; 2. pp. 1–12. [17] Ostatekboczynski Z, Kerven GL, Blamey FPC. Aluminum Reactions with polygalacturonate and related organic-ligands. Plant Soil 1995; 171(1):41–5. [18] Tsunemitsu K, Kishimoto H. In: Finch CA, editor. Use of polyvinyl alcohol in warp sizing and processing of textile fibres in polyvinyl alcohol. Properties and applications. London: John Wiley and Sons Ltd; 1973. p. 233–76.

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