Mechanical Properties of Hemp-Fibre-Reinforced ...

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Mechanical Properties of Hemp-Fibre-Reinforced Euphorbia Composites Leonard Y. Mwaikambo,* Nick Tucker, Andrew J. Clark

A composite material consisting of hydroxide-modified hemp fibres and euphorbia resin was produced. The composites were tested in tension, short-beam interlaminar shear stress and in impact. There was an increase in the tensile strength and modulus for resins with highhydroxyl-group based composites. Similar results were obtained for interlaminar shear stress while low-hydroxyl group euphorbia resin based composites exhibited high impact strength. The euphorbia resin with high hydroxyl content yielded composites with high stiffness. The use of euphorbia-based resins in composite manufacture increases the value of the euphorbia oil as well as creating a new route of composite manufacturing.

The application of plant-based materials is becoming increasingly important, leading industries, manufacturers and the public at large to seek to replace the ever dwindling fossil-based feedstock with agricultural-based materials. The search for materials from renewable resources supports global sustainability and comes at a time when there is, in certain quarters of the world, excess

L. Y. Mwaikambo Department of Engineering Materials, University of Dar Es salaam, P. O. Box 35131, Dar Es salaam, Tanzania Fax: þ255 22 241 0114; E-mail: [email protected] N. Tucker Crop and Food Research Centre, Christchurch, New Zealand A. J. Clark Department of Chemistry, University of Warwick, Coventry CV4 7AL, UK Macromol. Mater. Eng. 2007, 292, 000–000 ß 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

capacity in the agricultural industry. Therefore, the diversification into non-food uses addresses both an important global environmental issue and lends stability for an essential sector of our economy. Research has shown that plant fibres and plant-based oils can be mixed to produce composites that exhibit mechanical properties similar to those of fossil-based polymers reinforced with plant fibres such as polyester reinforced composites. For instance the tensile strength and Young’s modulus of untreated jute fibre polyester reinforced composites with a fibre volume fraction of 45% were found to be 60 MPa and 7 GPa, respectively, while the Charpy impact tests showed values of 29 kJ m2 at the same fibre volume fraction.[1] The average interlaminar shear strength (ILSS) was 10 MPa.[1] However, it has been observed that using natural fibres as reinforcement in composite materials results in several problems at the interface due to imperfect bonding.[2] Therefore, modification of the fibres by

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DOI: 10.1002/mame.200700092

Early View Publication; these are NOT the final page numbers, use DOI for citation !!

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Introduction

L. Y. Mwaikambo, N. Tucker, A. J. Clark

chemical treatment such as alkalisation reveals the microfibrils consisting hydroxyl groups and gives a rough surface topography to the fibre.[3] Sodium hydroxide is the most commonly used chemical for the cleaning of the surface of plant fibres. It also changes the fine structure of the native cellulose I to cellulose II.[4,5] Hydroxyl groups are responsible for the cross-linking with polar matrices while the rough surface results in fibre-matrix interlocking. These two fibre-matrix phenomenon results in increased composite mechanical properties compared to mechanical properties of composites reinforced with untreated plant fibres. Hemp fibre is one of the plant fibres that have recently attracted the attention of researchers and industry, particularly in the manufacture of automotive components in the form of composites. Hemp fibres belong to the group of plant fibres known as ‘bast fibres’. The dimensions of single cell (ultimates) hemp fibres have been reported to be between 8.3–14.1 mm in length and 17.0–22.8 mm in diameter.[6] Bast fibres are naturally arranged in bundles comprising several ultimate fibres. Figure 1 shows a cross sectional view of untreated and alkali treated hemp fibre bundles viewed using the scanning electron microscope (SEM). The ultimate fibres consisting a lumen (arrow L) at the centre are glued together by lignin (arrow G), a naturally occurring phenolic polymer, which constitute part of the plant cell wall.[4–8] Similarly, a new kind of thermosetting polymer has been developed from the plant Euphorbia Lagascae, grown mainly in temperate countries. The plant produces oil seeds. The oil can be chemically modified to graft the reactive hydroxyl groups, which are in turn cross-linked to produce thermosetting resins.[9] The euphorbia resin can be mixed with fibrous materials such as hemp fibres for the manufacture of thermosetting composites with high mechanical properties. Earlier research work indicates possible composite tensile strength and stiffness of 35 MPa and 4 GPa, respectively, for a fibre volume fraction of about 35%.[10] It is worth noting that the tensile strength and stiffness are basic mechanical properties of materials. The property of a composite is dependent on the interrelation between the fibres and the matrix and the fibre arrangements. In this research euphorbia resin was used as a binder for hemp fibre bundles in a mat form. The tensile strength, Young’s modulus, interlaminar shear strength (ILSS) and impact strength were measured. Theoretical models of modulus were used to predict the randomness value of the fibres, the maximum stress that can be applied to the fibre in the composite and the critical fibre length. The results of this work have been compared with literature values and end use of the new composites has been proposed. Macromol. Mater. Eng. 2007, 292, 000–000 ß 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 1. SEM micrographs of cross sectional view of: (a) untreated; and (b) alkali-treated hemp fibre bundles.

Randomness Factor Model The determination of the empirical value KS, which is a measure of randomness, was carried out on the basis that: (a) the shear strength of the composite (shown in Table 1 and Figure 5) represents the shear strength (stress), which is more or less the same as the yield stress usually closely related to the shear stress of the matrix used in this work; and, (b) to calculate KS the tensile strength and porosity of hemp fibre bundles of untreated fibres are 594 MPa and 2.46%, respectively, while the tensile strength and porosity of 0.24% NaOH treated hemp fibre bundles are 1 074 MPa and 3.49%, respectively.[11] Based on assumption (a) and

DOI: 10.1002/mame.200700092


Mechanical Properties of Hemp-Fibre-Reinforced Euphorbia Composites

Table 1. Mechanical properties of untreated and alkali treated hemp mat reinforced composites.

Vf

s

E

Impact

ILSS

%

MPa

GPa

kJ mS2

MPa

HEURE-low-OH

21.05

22.91 (1.06)

2.31 (0.53)

18.81 (2.17)

3.12 (0.54)

HEURE-high-OH

18.89

26.56 (1.85)

2.78 (0.92)

7.03 (1.13)

3.49 (0.45)

AHEURE-high-OH

20.39

34.69 (3.76)

3.13 (1.05)

9.15 (1.64)

4.73 (0.76)

the values for the tensile strength and porosity of untreated and alkali treated hemp fibre provided in (b), KS was determined using the model Equation (1), which is a variant of equal strain condition that applies to composite strength.[12,13] KS ¼

s c Vm s m Vf s f

(1)

where sC, sf, sm, Vf, Vm, and KS are the modulus of the composite, stress of the fibre, stress of the matrix, fibre volume fraction, volume fraction of the matrix and the randomness of factor of the staple fibres, respectively.

Critical Fibre Length Model Since the work involved the use of short fibre bundles, efforts were made to determine the fibre critical length. It is assumed that the stress applied on the staple fibre, in the composite, increases until it becomes equal to that on a continuous fibre. This consideration leads to the definition of the so-called critical fibre length, lC. It is also believed that for a particular length of the fibre (x) greater than or equal to the critical fibre length, the stress transmitted to the fibre becomes a maximum 2.[13] It is reported that for polymeric matrices the fibre shear stress (tx) is approximately equal to the matrix shear stress (tm).[13] The maximum stress that can be applied to a fibre is defined as its ultimate strength and can be expressed as shown in Equation (2):[13] s f ¼ ðEf "f Þmax

(2)

Having obtained the maximum stress that can be obtained from the fibre in the composite, the equation for the critical length under the critical load is obtained using Equation (3),[14,15] lc ¼

sf d 2t m

(3)

where d is the diameter of the fibre. Using the model Equation (1)–(3) the randomness factor and maximum Macromol. Mater. Eng. 2007, 292, 000–000 ß 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

stress that can be applied to a fibre and critical fibre length were obtained

Experimental Part Materials Samples of modified rapeseed oil (monomer) were supplied by the Department of Chemistry, University of Warwick as part of the products from a DEFRA funded project. Hemcore Limited supplied the needle punched non-woven hemp mat, which had an area density of about 450 g m2. The strength of the hemp mat was not determined. Methylene diisocyanate (MDI) and toluene were purchased from Aldrich chemical company UK. A 300 300 4 mm compression mould available at the Advanced Technology Centre, Warwick Manufacturing Group, University of Warwick was used to manufacture the composites. Chemical Release Company Limited, Harrogate United Kingdom, supplied a mould release agent PAT 607/PCM.

Alkalisation of the Hemp Mat Hemp mats were soaked in basins containing a solution of 0.16 wt.-% sodium hydroxide (caustic soda) concentration controlled at room temperature for 48 h. The hemp mats were then removed from the solution, washed with distilled water and then thoroughly rinsed with distilled water. The hemp mats were then dried to remove free water and used as reinforcements.

Composite Manufacture Compression moulding produced composites. Three types of composites were prepared: (i) untreated hemp fibre mixed with euphorbia resin (EURE) containing low hydroxyl groups (HEURElow-OH); (ii) untreated hemp fibre mixed with euphorbia resin with high content of hydroxyl groups (HEURE-high-OH); and, (iii) caustic soda treated hemp fibre mixed with euphorbia with high content of hydroxyl groups (AHEURE-high-OH). Three layers were prepared, weighed and hand-laid in the mould. The resin was then poured on the hemp mat layers except the sides facing the mould. The mould was then closed and compressed at 0.6 MPa. The process was carried out at room temperature and the composites were post cured at 60 8C for 10 h.

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Composite type


Tensile Properties

Impact Properties

The tensile testing of rectangular laminated composites was carried out according to ASTM D 3039-82 using an Instron tensile testing machine Model 4505 with installed computer programme. The machine was run at a crosshead speed of 5 mm min1. Hemp mat-EURE reinforced composites were tested in tension. 12 test pieces were used.

The impact strength was determined using the pendulum impacttesting machine provided with different operating hammer weights. The choice of the operating weight depends on the expected impact strength of the composites. ASTM D 256-92 applied for simple beams Charpy-type test was used. This is a test standard for plastic materials with or without fibre reinforcement. The specimens were un-notched. Impact loads were applied at a point perpendicular to the direction of the fabric plane. 12 test pieces were used.

Shear Properties (Short Beam) The short beam three-point bend test was performed using an Instron model 4505. The width and thickness of the specimen were measured and recorded. The specimen was placed on two lower rollers with a span depth ratio of 5:1. This ratio generates a high level of in-plane shear stress. Samples were tested at a crosshead speed of 1 mm min1. The tests were carried out in accordance with ASTM D 2344. The inter-laminar shear strength is calculated using Equation (4) and taken as the mean of 12 specimens for each set of samples:

t¼

3P 4bt

(4)

Scanning Electron Microscopy (SEM) Analysis Scanning electron microscopy was used to study the fractured surfaces of the impact tested composite samples. Prior to the analysis the samples were coated with Au/Pd alloy by means of a Polaron Sputtering apparatus.

Results and Discussion Tensile Properties

where P is the applied force and b and t are the width and thickness of the beam, respectively. Implicit in the equation is that the maximum bending moment is Pl/4, where l is the distance between the outer supports (support span). The dimensions of the test-piece are so specified as to induce inter-laminar shear failure along the neutral axis before failure in the tension surface of the beam. The inter-laminar shear strength, t, is taken to be the maximum shear stress predicted by classical theory on the neutral axis of an isotropic material.

Figure 2 shows representative stress-strain curves for untreated hemp/EURE-low-OH and hemp/EURE-high-OH, and alkalitreated hemp/EURE-high-OH composites. An initial linear trace of the curve is observed for the three composites. The inflection exhibited by the hemp/EURE-high-OH composite is due to machine slippage during testing. A higher failure strain for the untreated-hempreinforced EURE-low-OH composite is observed followed

Figure 2. Stress-strain curves of untreated and alkali treated hemp/EURE composites. Macromol. Mater. Eng. 2007, 292, 000–000 ß 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Macromol. Mater. Eng. 2007, 292, 000–000 ß 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 3. Tensile strength of untreated and alkali-treated hemp/ EURE reinforced composites.

fibre volume fraction, implying that the high content of the –OH groups on the EURE contribute to a greater extent on the increase in the tensile properties and that alkalisation of hemp fibre has a lesser contribution. Figure 3 and 4 shows the tensile strength and Young’s modulus, respectively, of untreated hemp/EURE-low-OH and -EURE-high-OH composites and alkalised hemp/ EURE-high-OH composites. Both figures exhibit an increasing trend due firstly to an increase in hydroxyl groups on the resin and secondly due to alkalisation of the hemp fibre mat. An increase in –OH groups on EURE increases the binding characteristics of the resin while alkalisation creates rough surfaces on hemp fibre thus providing mechanical interlocking particularly where the resin is non-polar and it also exposes the hydroxyl groups on the surface of hemp fibre, which are polar and would react with the highly polar EURE.

Figure 4. Young’s modulus of untreated and alkali-treated hemp/ EURE reinforced composites.

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by the alkali-treated hemp/reinforced EURE-high-OH composite and untreated hemp/EURE-high-OH composites increasing in that order. The high strain exhibited by the untreated hemp/EURE-low-OH composites is due to the unfolding of the amorphous regions and plasticisation of the hemp cell wall. The plasticisation of the hemp cell wall is caused by the presence of moisture this is common with plant fibre cell as reported in literature.[5,7,16] The high strain at failure for the alkali-treated hemp/EURE-high-OH composites is caused by the alkali treatment which tends to increase the amorphous regions following alkalisation.[5] This observation has been reported in earlier work.[11] The non-linearity of the curves is due to plastic deformation of the matrix and is more significant with untreated than treated hemp fibre bundles. Table 1 shows fibre volume fractions of untreated hemp/EURE consisting high content of hydroxyl groups on EURE structure decreased by 10% compared with fibre volume fraction of untreated hemp/EURE consisting of low content of hydroxyl groups on EURE. However, the HEURE- high-OH exhibit higher tensile strength and Young’s modulus by 16 and 20%, respectively, compared with untreated HEURE-low-OH composite. The increase in the tensile properties is attributed to the higher content of hydroxyl groups on the HEURE-high-OH. An increase in the –OH groups on the resin structure increases the fibrematrix interface strength. The tensile properties obtained are quite reasonable for the amount of fibre content exhibited. Wool et al.[10] obtained similar ranges of mechanical properties for vegetable fibre reinforced plantbased resin-matrices for about the same fibre volume fractions. The lower tensile properties exhibited by untreated hemp/EURE-low-OH compared with untreated hemp/ EURE-high-OH composites is due to the low level of hydroxylation on EURE-low-OH. This implies that the presence of more –OH groups or high-level of hydroxylation increases fibre-matrix contacts at the fibre-matrix interface. Alkalising the hemp mat shows an increase of the fibre volume fraction of the AHEURE-high-OH composite by 8% compared with the untreated HEURE-high-OH composite resulting in an increase in the tensile strength and Young’s modulus of 31 and 13%, respectively. Also, it can be deduced from Table 1 that high hydroxylation increased the tensile strength and Young’s modulus by 16 and 20%, respectively. Similarly, alkalisation of the hemp mat combined with hydroxylation of EURE shows an increase in the tensile strength and Young’s modulus of the composites by 51 and 36%, respectively, compared with the untreated HEURElow-OH composite. The increase in the tensile strength and Young’s modulus is exhibited despite the decrease in the


Figure 6. Shear strength of untreated and alkali-treated hemp/ low-OH, high-OH EURE composites. Figure 5. Impact strength of untreated and alkali-treated woven hemp/EURE-high-OH composites.

Impact Properties Figure 5 and Table 1 shows the impact energy of the untreated hemp/reinforced EURE-low-OH, untreated hemp/ EURE-high-OH and alkalised hemp/reinforced EURE-high-OH composites. The untreated hemp/reinforced EURE-low-OH composite exhibited the highest impact strength followed by alkalised hemp/reinforced EURE-high-OH composite. The high impact strength exhibited by the hemp/reinforced EURE-low-OH composite is due to possible plasticization of the resin, thus absorbing more energy, whereas that of the alkalised hemp/reinforced high-OH composite is due to the plasticization of the hemp fibre cell wall caused by the alkalisation process. Alkalisation has been found to plasticize the cell wall due to the presence of water used for the preparation of caustic soda.[11]

Inter-Laminar Shear Strength (ILSS) by Short Beam Flexure Results for short beam interlaminar shear tests are shown in Table 1 and Figure 6 for untreated hemp/reinforced EURE-low-OH, untreated hemp/reinforced EURE-high-OH and alkali treated hemp/reinforced EURE-high-OH composites. The short beam ILSS is ideally a measure of the resistance of the resin to stress. The results show that alkalised hemp/reinforced EURE-high-OH exhibits the highest ILSS followed by hemp/reinforced EURE-high-OH and hemp/reinforced EURE low-OH composites, respectively. The high ILSS of the alkalised hemp/reinforced EURE-high-OH composites is due to increase in the reactive –OH groups. The increase in the –OH groups has occurred on both the EURE as a result of the modification process of the euphorbia oil and due to the removal of the surface impurities on the hemp fibres following alkalisation. Since shear strength is closely a measure for the strength of the matrix plus the interface, it is therefore, logical to Macromol. Mater. Eng. 2007, 292, 000–000 ß 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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imply that the values for the shear strength (stress) represent more or less the shear stress of the neat matrices used in this work.[13,14]

Randomness Factor (Kc) Model Table 2 shows that the randomness factor, KS, which is an indication of the disorder[8] of fibres in the composite is highest for the HEURE-high-OH composite followed by that of the HEURE-low-OH and the AHEURE-high-OH composites, respectively. The high randomness factor of the HEURE-high-OH composite is attributed to the increased amount of reactive sites compared to the HEURElow-OH composite while the decrease in the randomness factor of the AHEURE-high-OH composite is due to the alkalisation of the hemp fibre bundles. Alkalisation will also reduce the diameter of the hemp fibre bundles thus increasing the chances for orderliness, which would result in lower randomness factor. Courteny[10] suggests that the randomness factor for short fibres should be not more than 2, this implies that the HEURE-high-OH composite, which exhibit a randomness factor (although insignificant) higher than 2, requires more attention during the manufacturing process. It is observed from Table 2 that the reduction in the maximum stress of hemp fibre bundles in the composite compared to the actual values is due to the fact that the load is shared amongst the components surrounding the fibre, which are the fibre bundle itself, resin-matrix and porosity plus the interface.

Critical Fibre Length Model The diameter of untreated and alkali treated hemp fibre bundle were 67.84 and 38.35 mm, respectively.[17] The critical fibre length was determined using the actual hemp

DOI: 10.1002/mame.200700092



Table 2. The mechanical properties, randomness factor and critical length of hemp fibre bundles, where lCf, rc, sc, s f , sAf and KS are the fibre bundle critical length, composite density, composite stress, theoretical maximum stress of hemp fibre bundle in the composite, tested (actual) stress of hemp fibre bundle and fibre randomness factor, respectively.

Type of composite

a)

sc

sHfa)

s f

r

MPa

MPa

MPa

%

KS

lC mm

HEURE-low-OH

22.91

593.72

68.29

2.46

0.16

6.45

HEURE-high-OH

26.56

593.72

68.29

2.46

0.21

6.45

AHEURE-high-OH

34.69

1073.72

150.62

3.49

0.14

6.60

See Mwaikambo and Ansell.[11]

fibre bundle stress rather than the theoretical stress of hemp fibre bundle and they are shown in Table 2. The critical fibre length of alkali treated hemp fibre bundle was found to be approximately 2% higher than that of the untreated hemp fibre bundle. The increase in the critical fibre length is attributed to the alkalisation of the hemp fibre bundle.

Maximum Stress of Fibre in the Composite Using Equation (5) the theoretical maximum stress that can be applied to a fibre in the composite (s f ) was found to be about 12% the actual stress of the untreated hemp fibre bundle and 14%. of the 0.24% NaOH treated hemp fibre bundle (Table 2). This implies that alkalisation has contributed by 2% the theoretical stress of the hemp fibre bundle.

Fracture Surface of the Neat Resins and Composites Figure 7(a), (b) and (c) show impact-fractured surface of untreated hemp/EURE (high-OH EURE), untreated hemp/ EURE (low-OH EURE) and alkali treated hemp/EURE

(high-OH EURE) composites, respectively. The untreated hemp fibre reinforced composite (Figure 7(a)) exhibit fracture at fibre/matrix interface showing debonding (arrow d) attributed to accumulation of moisture at the interface mainly from hemp fibre and partial debonding (arrow pd) indicating the presence of reactive hydroxyl groups on both the hemp fibre surface and EURE particularly considering that this is the EURE with high content of hydroxyl groups. Hydroxyl groups are grafted on the tryglyceride structure (euphorbia oil) to facilitate reactivity with polar materials in this case the cellulosic hemp. Apparently Figure 7(b) shows a coherent fibrematrix interface (arrow ch) indicating an efficient interaction between the fibre and the EURE-low-OH. Figure 7(c) shows debonding due to fracture as well as coherent fibre/ matrix interface. The debonding observed in Figure 7(c) is attributed to the presence of moisture at interface originating from the hemp fibre, which is the result of alkalisation. Both Figure 7(a) and (b) show a less clean and less rough surface of hemp fibre compared to hemp fibre shown in Figure 7(c). The removal of surface impurities on cellulosic fibres reveals the reactive hydroxyl groups while the rough surface facilitates mechanical interaction between the fibre and the matrix.

Figure 7. SEM micrographs of impact fractured composites: (a) untreated hemp/EURE-high-OH; (b) untreated hemp/EURE-low-OH; and, (c) alkali-treated hemp/EURE-high-OH.

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Untreated hemp/resin composites (Figure 7(a)) show a more brittle failure of the EURE matrix while Figure 7(b) show a bit of ductile fracture which can be attributed to the fewer –OH groups on the EURE structure. The ductile fracture profile observed in Figure 7(c) is caused by the increased amount of moisture due to alkalisation particularly that the resin preparation was carried out in situ with the composite moulding. The fibre bundle texture of alkalised hemp mat (Figure 7(c)) is friable and weak compared with the untreated hemp mat in Figure 7(a) and (b). In plant fibres fracture of the fibrils occurs at stress concentrations usually at the weakest points of the microfibrils. The fracture of micro-fibrils in plant fibre reflects the composite nature of the fibres and fibre bundles. The resin at the fibre/matrix interface of alkali treated hemp fibre bundle composites (Figure 7(c)) shows matrix shear hackles where the matrix transfers stress to the fibre bundles. The shear hackle indicates more plastic flow of the matrix implying that plasticization has occurred due to the presence of the aliphatic nature of the tryglycerides. Sanadi et al.,[18] made similar observations on sunn hemp reinforced polyester composites. This shows that resin curing continues after composite manufacture.

Conclusion The hemp fibre reinforced euphorbia exhibit similar tensile properties to other plant fibres reinforced composites, particularly the alkali treated hemp mat. The untreated hemp mat/EURE with low hydroxyl content showed better impact resistance than other composites tested in this research, which is due to the plasticization of the resin and the composite by low-hydroxyl groups present on the resin. The composite with high content of hydroxyl groups exhibited the least impact strength followed by composites with alkalised hemp fibre bundles, respectively, indicating increased density of hydroxyl groups at fibre/matrix interface compared with the untreated hemp mat composite material. The results also imply that high –OH based EURE is useful in applications where high strength and high stiffness composites are required. The lower randomness factor of the alkalised hemp-EURE high-OH content is attributed to the rough surface of hemp fibre caused by caustic soda treatment. It


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has therefore been shown that EURE is a viable alternative resin-matrix sourced from a renewable plant, with the potential of replacing the fossil-based resins. Received: March 24, 2007; Revised: June 5, 2007; Accepted: June 6, 2007; DOI: 10.1002/mame.200700092 Keywords: composites; fibers; interfaces; mechanical properties; resins

[1] T. M. Gowda, A. C. B. Naidu, R. Chhaya, Comp. Part A: App. Sc. Manuf. 1999, 30, 277. [2] B. Singh, M. Gupta, A. Verma, Polym. Comp. 1996, 17, 910. [3] L. Y. Mwaikambo, M. P. Ansell, Angew. Makrom. Chem. 1999, 272, 108. [4] S. G. Shenouda, ‘‘The structure of cotton cellulose’’, in: Applied Fibre Science, Vol. 3, F. Happey, Ed., Academic Press, London 1979, p. 275. [5] W. E. Morton, J. W. S. Hearle, ‘‘Swelling, Physical properties of textile fibres’’, Heinemann, London 1975, p. 223. [6] D. R. Perry, ‘‘Identification of textile materials’’, The Textile Institute, Manchester 1985, p. 243. [7] R. W. Moncrieff, ‘‘Viscose rayon, Man made fibres’’, Newnes-Butterworth, London 1975, p. 152. [8] B. A. Jayne, ‘‘Comprehensive view of the structure of paper, theory and design of wood and fibre composite materials’’, Syracruse University Press, New York 1972, p. 157. [9] L. Y. Mwaikambo, N. Tucker, A. J. Clark, in: Proceedings of the ICCP2007-International Conference on Polymer Processing 2007, p. 261. [10] R. P. Wool, S. H. Khot, J. J. La Scala, G. I. Williams, S. P. Bunker, S. S. Morye, Proceedings of ACUN-2 2000, p. 619. [11] L. Y. Mwaikambo, M. P. Ansell, J. Mater. Sci. 2006, 41, 2483. [12] T. H. Courteny, ‘‘Composite materials, mechanical behaviour of materials’’, 2nd Edition, McGraw-Hill Higher Education, Singapore 2000, p. 244. [13] A. A. Berlin, S. A. Volfson, N. S. Enikolopian, S. S. Negmatov, ‘‘Principles of polymer reinforcement with fillers: principles of polymer composites’’, Springer-Verlag, New York 1988, p. 5. [14] Z. D. Jastrzebski, ‘‘The nature and properties of engineering materials’’, Wiley, New York 1987, p. 522. [15] J. W. Martin, ‘‘Materials for engineering’’, The Institute of Materials, Oxford 1996, p. 179. [16] R. M. Rowell, ‘‘Property enhancement of wood composites, Composite applications-the role of matrix, fibre and interface’’, VCH Publishers, New York 1992, p. 365. [17] L. Y. Mwaikambo, PhD Thesis, University of Bath 2002. [18] A. R. Sanadi, S. V. Prasad, P. K. Rohatgi, J. Mater. Sci. 1986, 21, 4299.

DOI: 10.1002/mame.200700092
