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A novel processing technique for thermoplastic manufacturing of unidirectional composites reinforced with jute yarns
Composites: Part A 37 (2006) 2274–2284 www.elsevier.com/locate/compositesa
A novel processing technique for thermoplastic manufacturing of unidirectional composites reinforced with jute yarns O.A. Khondker *, U.S. Ishiaku, A. Nakai, H. Hamada Department of Engineering, Advanced Fibro-Science, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan Received 27 December 2005; received in revised form 28 December 2005; accepted 29 December 2005
Abstract This paper primarily investigates the fabrication process of long-fibre reinforced unidirectional thermoplastic composites made using jute yarns (both untreated and treated). Tubular braiding technique was used to produce an intermediate material called ‘‘microbraid yarn’’ (MBY) with jute yarn as the straightly inserted axial reinforcement fibre and polymer matrix fibre being braided around the reinforcing jute yarns. Microbraid yarns were then wound in a parallel configuration onto a metallic frame and compression molded to fabricate unidirectional composite specimens. In this study, two types of polymeric materials (biodegradable poly(lactic) acid and non-biodegradable homo-polypropylene) were used as matrix fibres. Basic static mechanical properties were evaluated from tensile and 3 point bending tests. Test results were analyzed to investigate the effects of molding temperature and pressure on the mechanical and interfacial behaviour. For the unidirectional jute fibre/poly(lactic) acid (PLA) composites, the results indicated that the molding condition at 175 C and 2.7 MPa pressure was more suitable to obtain optimized properties. Improved wettability due to proper matrix fusion facilitated thorough impregnation, which contributed positively to the fibre/matrix interfacial interactions leading to effective stress transfer from matrix to fibre and improved reinforcing effects of jute yarns. For the jute/PP unidirectional composites, specimens with only 20% of jute fibre content have shown remarkable improvement in tensile and bending properties when compared to those of the virgin PP specimens. The improvements in the mechanical properties are broadly related to various factors, such as the wettability of resin melts into fibre bundles, interfacial adhesion, orientation and uniform distribution of matrix-fibres and the lack of fibre attrition and attenuation during tubular braiding process. 2006 Elsevier Ltd. All rights reserved. Keywords: A. Yarn; A. Polymer (textile) fibre; E. Compression moulding; Microbraiding
1. Introduction Advances in science and technology pose new challenges in relation to certain environmental issues, such as biodegradability, recyclability, eco-friendliness etc., that need to be addressed to help preserve and protect our environment. Composite materials from the annually renewable natural fibres and biodegradable matrices have been developed in the past decade in an attempt to find alternatives to the fossil fuel-based polymeric materials in the automotive *
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and packaging industries. One of the problems associated with biodegradable composites is the price, which limits investigation of these materials to a laboratory scale. With the new economic climate turning attention to larger volume industrial uses, a wide variety of agro-based resources for the manufacture of new composite materials including geotextiles, filters, sorbents, structural/non-structural composites, molded products and combinations of other materials make it possible to explore new application areas such as packaging, housing, and automotive products [1]. Demands for natural fibres in plastic composites is forecast to grow 15–20% annually with a growth rate of 15– 20% in automotive applications, and 50% or more in
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selected building applications. At the moment, three quarters of agricultural fibres used in composites is wood fibre with the remaining one quarter kenaf, jute, hemp and sisal etc. Wood fibres are progressively being replaced by sustainable long natural fibres, such as jute. Other emerging markets are industrial and consumer applications such as tiles, flower pots, furniture and marine piers. Another economic reason for the growing popularity of natural fibres is their cheap price, while weight reduction is also responsible for the increasing use of natural fibres. Thermoplastic materials that currently dominate as matrices for natural fibres are PP, PE and PVC while thermosets, such as phenolics and polyesters are common matrices. Biodegradable polymers are yet to become popular as ‘‘green’’ substitutes for synthetic polymers. Besides, the use of biodegradable plastics appear as an attractive alternative for enhancing sustainable and environmentfriendly agricultural activities, especially in mulching and low-tunnels cultivation [2]. The increasing global interest on the agro-based structural materials from the natural, annually renewable and biodegradable sources was also driven by the conservation of forest and forest resources through reducing their massive usage, and the need for cost reduction in raw materials, manufacture and maintenance of composites. With a view to replacing the wooden fittings, fixtures and furnitures, natural composites reinforced with jute, kenaf, sisal, coir, straw, hemp, banana, pineapple, rice husk, bamboo etc. can be used instead of the conventional polymer composites reinforced with man-made fibres such as glass. Development of new composite product(s) from the existing resources has a strong potential to deliver a novel biodegradable and/or readily recyclable material suitable for the automotive and packaging industry to replace non-renewable fossil fuel-based polymers/ plastics. Lignocellulosic (hydrophilic) bio-fibres, when rightly combined with the recyclable thermoplastic polymers, can possess an outstanding potential to produce environmentfriendly composite materials having specific properties favourably comparable with those of glass-based composites in most cases [3–7]. Natural fibres have distinct advantages over synthetic glass fibre in the applications not requiring very high load bearing capabilities [3]. Complete matrix fusion to facilitate thorough fibre impregnation, formation of strong fibre/matrix interfacial bonding and matrix-to-fibre stress transfer efficiency [3,8,9] are vital requirements for the manufacture of reliable, eco-friendly natural composites that can possess better mechanical properties and withstand environmental attacks. Mohanty et al. [5] carried out a detailed review work on jute fibre reinforced thermosets, thermoplastic, and rubber-based composites. Jute is one of the most versatile agro-based fibres, mostly grown in south asian countries like Bangladesh and India, that has enormous potential in composite manufacture due to its cost-effective, renewable, versatile, non-abrasive, viscoelastic, biodegradable, combustible, compostable nature and better insulation against noise
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and heat due to it’s hollow cellular [4] structure. Jute-based composites, an alternative to wood products [7,10], are not severely affected by mites, moisture, and are generally considered to be fire-retardant and chemical-proof. Jute-based green composites would be suitable for even primary structural applications as indoor elements in housing [11,12], temporary outdoor applications like low-cost housing for defence and rehabilitation [13,14] and transportation. Due to its insulating characteristics, jute may find areas of applications in automotive door/ceiling panels and panels separating the engine and passenger compartments [4,15]. Such panels made from jute fibres and PP or other bio-thermoplastic and hybrid composites are already in use [4,16,17]. The durability of jute fibre-based phenolic composites has been studied under various humidity, hydrothermal and weathering conditions to assess the suitability of jute composites in damp and dry environment [18]. Fibre type, textile architecture, interphase properties, fibre mechanical properties and content were found to strongly affect the fatigue behaviour of flax and jute fibre reinforced epoxy, polyester and PP composites [19,20]. Much of the fibre degradation during conventional shear mixing processing techniques—extrusion compounding and injection molding methods [15,21–24] can be avoided when an innovative fibre mat production method by sprinkling the fibres down a drop feed tower [25] was used to manufacture natural fibre reinforced polypropylene composites. Most researches concentrate on jute fibre/nondegradable polymer composites but research reports on jute/biodegradable polymer composites are rather limited [26]. With the increasing popularity of natural fibres in polymer composites, it is timely to carry out fundamental studies on jute fibre-based composites, especially on their fabrication processes. In this study, a preliminary investigation on the fabrication of long-fibre reinforced unidirectional jute-based composite is presented. A tubular braiding machine [27] was used to produce micro-braid yarn using continuous jute yarn as the straightly inserted axial fibre and polymer fibre as the matrix material braided around the reinforcing jute fibre. Micro-braiding technique offers minimum or no damage to the reinforcement fibre bundles, when compared to using commingled yarns. 2. Experimental 2.1. Materials, fabrication and mechanical tests Jute yarns produced from 100% BT Tossa B-grade finer jute fibres (Corchoras olitorious) were commercially supplied by Bangladesh Janata Jute Mills Ltd. The matrix materials used in this study were biodegradable PLA fibres from Mitsui Chemical Co. Ltd., Japan and homo-PP fibres (HX100G) from Sumitomo Chemical Co., Ltd., Tokyo, Japan. Despite problems associated with poor fibre wetting due to high melt viscosities, homo-polypropylene fibres have wide selectivity, excellent property range, low processing temperature and recyclability, which have led to the
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Fig. 1. Fabrication process of microbraid yarns.
development of a number of manufacturing techniques, such as commingled yarn, powder impregnated yarn, coweaving etc. This paper explored a different molding technique primarily to optimize the processing condition for thermoplastic manufacturing of unidirectional composites. Good quality composite specimens with enhanced mechanical characteristics were expected. An intermediate material called ‘‘micro-braid yarn’’ (MBY) that has a unique combination of reinforcement and matrix phases, was produced using a tubular braiding machine consisting of 16 spindles (Fig. 1). MBY can be treated as a single fibre bundle so as to fabricate varieties of textile processed goods. Continuous jute yarns were used as the straightly inserted axial fibres, and matrix fibres (PLA and PP fibres) were braided around the reinforcing jute yarns. A hot compression molder was used to fabricate continuous fibre reinforced thermoplastic composites. Matrix fibres were melted by heating at appropriate molding temperatures and become the matrices for the fibre reinforced plastics/composites that easily wet out the reinforcing jute fibre.
Both natural (untreated) and treated or coated jute yarns were used to fabricate jute/PLA and jute/PP unidirectional composites. Fabrication of continuous jute yarn reinforced PLA or PP thermoplastic composites involved a twofold process. The first step was performed by initially winding microbraid jute yarns 40 times in two stages in a parallel configuration onto a metallic frame (Fig. 2a) with a spring mechanism enabling it to adjust tension caused by thermal shrinkage. The second step involved placement of the metallic frame containing microbraid yarns in a preheated molding die (Fig. 2b) for consolidation by compression molding (Fig. 2c) to produce composite specimens. The heated die was let to cool naturally with the molding dies being closed. Consolidation process involved three stages. At first, under certain temperature, the solid fibrous matrix materials became softened and then the matrix in the liquid form soaked and infiltrated the reinforcing fibres and finally during cooling stage, the matrix turned into solid form to hold the fibres in a definite position. The second step was considered to be the most important, as this decided uniform distribution of fibres in the matrix. There
Fig. 2. Consolidation set-up for compression molding: (a) unidirectional arrangement onto a tension rig, (b) molding die and (c) consolidation by compression molding.
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were used for this study. Bending specimens were 20 mm in nominal width and the span length was 56 mm. The specimens for bending test were also cut along the fibre axis. Thicknesses of the specimens were 3.4–3.6 mm. Tensile specimens were clamped over an area of 40 mm · 20 mm at each end leaving a gauge length of 100 mm. For tensile tests, an open mesh emery cloth (Polynet sheet—240 grit) was used in the gripped areas instead of aluminium end tabs. Strain was measured using strain gauges, which were bonded onto the central surface of the specimens. Both tension and 3 point bending tests were conducted on an Instron Universal Testing Machine (Type 4206) under nominal test speeds of 5.0 and 2.0 mm/min, in accordance with ASTM D638 and D790, respectively. No fewer than five specimens were tested for each specimen and test types. Tensile and bending properties were determined from the tests. Pre- and post- failure cross-sectional observations were carried out on the selected test specimens with a focus on matrix fusion, impregnation property related to the fibre/matrix interfacial bonding, and to study its relationships to the test results. 3. Results and discussion 3.1. Jute/PLA unidirectional composites Tensile and 3 point bending properties were evaluated for jute/PLA unidirectional composites. Tensile specimens had three variations in fibre contents designated as Vf1 (38%), Vf2 (27.5%) and Vf3 (22.5%). Specimens were tested to failure, and stress–strain curves, as shown in Fig. 3, indicated that all three types of composites samples processed at 170 C did not fail abruptly. Composite behaved in a pseudo-plastic manner before attaining a maximum stress value and then decayed. Fractured specimens were shown in Fig. 4. There was no indication of normal tensile fracture, as the failure of the composites was rather dominated by shear due to poor interfacial bonding between the fibres and the matrix. Tested samples also suggested that matrix failure occurred first and then the fibre slippage became dominant.
100 Tensile Stress (MPa)
is certainly a scope for improvement in the quality of the molded specimens by introducing a rapid cooling system attached to the compression molder. For Jute/PLA composites, matrix–fibre contents were varied by changing the number of resin fibre bundles to microbraid around the reinforcing jute yarn. Only untreated (natural) jute yarns were used in this study. The effects of processing temperature (170 C and 175 C) and pressure (2.3 and 2.7 MPa) were investigated. Three types of jute/PLA composite specimens were fabricated and the fibre volume fractions (Vf) were calculated as approximately 22.5%, 27.5% and 38% for the test samples. Samples were cut in the transverse direction, embedded in epoxy resin and then polished, in order to study fibre impregnation and interfacial properties by scanning electron microscopy (SEM) analysis. Thermal analyses of the PLA and PP fibres used in this study were carried out using differential scanning calorimetry (DSC 7, Perkin-Elmer). The 10 mg samples were placed in the aluminium pans, sealed and then introduced into the heating cell of the calorimeter. Heat treatment or thermal scanning was performed at a programmed rate of 10 C/min from 20 C to 250 C under constant nitrogen flow. The samples were then cooled, and data were analyzed by a computer. The first thermal scan melts the samples and erase its thermal history. DSC traces were obtained from the second thermal cycle as it provides accurate result for the melting temperature (Tm), measured from the peak value of the endotherm. The melting temperatures of PLA and PP matrix fibres were obtained as 168 C and 169 C, respectively. The melting endotherms of PLA and PP matrix fibres were completed at 170 C and 175 C, respectively. Both untreated and coated jute yarns were used in jute/ PP composites. The coating agents used on the jute yarns were PVA (Poly vinyl alcohol) and PP (Polypropylene) each of which having 1.5 wt% on the basis of jute yarn content. Untreated jute yarns look naturally dull and feels loose and scattered as opposed to the coated one, which has a shiny, dimensionally stable and compact surface. Processing condition for reinforced PP composites using microbraid jute yarns was set at 180 C for 10 min under 2 MPa molding pressure. For jute/PP composites, the fibre volume fraction (Vf) achieved was approximately 21.2% and no other variations in Vf were aimed at this stage. Research focused mainly on the fabrication process of jute/PP unidirectional composites by adopting tubular braiding technique. Temperature and pressure during fabrication influence the impregnation behaviour. Each variety of agro-based fibres has a particular structure and composition. Even though most lignocellulosic fibres cannot withstand processing temperatures higher than 175 C for long duration, limiting their ability to be used with some thermoplastic resins [4], there were no signs of fibre damage due to temperature during molding process. Tensile specimens of 180 mm · 20 mm in nominal dimensions, with the fibre axis along the loading direction
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80 60 40
Vf1 = 38% Vf2 = 27.5% Vf3 = 22.5%
20 0 0.00
0.03
0.06 0.09 Tensile Strain
0.12
0.15
Fig. 3. Tensile stress–strain curves for jute/PLA composites processed at 170 C.
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Bending Stress (MPa)
10 0 80 60 40 Vm1=62% (2.3 MPa)/170˚C) Vm2=77.5% (2.3 MPa)/170˚C)
20
Vm3=77.5% (2.3 MPa)/175˚C) V Vm4=77.5% (2.7 MPa)/175˚C)
0
Fig. 4. Tensile tested jute/PLA composite specimens processed at 170 C.
9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0
78 Tensile Strength Tensile Modulus
76 74 72 70 68 Vf1 = 38%
Vf2 = 27.5%
Tensile Modulus (GPa)
Tensile Stress (MPa)
80
Vf3 = 22.5%
Fibre Volume Fraction, Vf Fig. 5. Tensile properties of jute/PLA composites processed at 170 C.
2.3
3.5 4.9 Deflection (mm)
6.5
Bending Stress (MPa)
Fig. 6. Bending stress–deflection curves for jute/PLA composites processed at 170 and 175 C.
86 84 82 80 78 76 74 72 70 68 66
8
Bending Strength Bending Modulus
7 6 5 4 3 2 1
62
72.5 Matrix Content,Vm (%)
77.5
Bending Modulus (GPa)
Maximum tensile stresses and moduli are graphically presented in Fig. 5. As expected, jute/PLA composite samples exhibited superior tensile properties with increasing fibre content. The trend of improvement in both strength and modulus was fairly consistent with respect to the increase in the reinforcing fibre content. Four specimen types underwent 3 point bending tests. Samples had variations with respect to the processing conditions and were designated as Vm1 (62%), Vm2 (77.5%), Vm3 (77.5%) and Vm4 (77.5%). Bending test yielded a different trend (as exemplified in Figs. 6 and 7) when compared to the tensile results. This different trend could be accounted for by the differences in failure mechanism and the integrity of the fibre/matrix interface. Figs. 6 and 7 suggested that there could be an optimum processing condition for which composite might perform better in flexure. Post-failure bending specimens are shown in Fig. 8. As indicated in Fig. 6, for sample Vm4, a higher molding or holding pressure (2.7 MPa) with a higher molding temperature (175 C) imparted the best performances in bending. A relatively sharper drop in the gradient after the maximum value for sample Vm4, was indicative of tensile fracture, whereas for all the other samples processed at 170 C, only compressive failure was evident (Fig. 8a) while tensile failure was absent and the samples rather looked pale. This could also suggest a poor interfacial adhesion and incomplete matrix fusion as indicated in Figs. 9 and 10. The voids
0.0
0
Fig. 7. Bending properties of jute/PLA composites processed at 170 C.
within the matrix indicated boundary areas where complete fusion was not attained. It is noteworthy that the hollow cross-sectional structure of jute fibres cannot be seen even at higher magnifications. The process of embedding samples in epoxy resin and subsequent polishing must have covered the lumen or central cavities of jute fibres. Figs. 9–12 showed representative SEM micrographs of the polished cross-sections for the composite samples processed at different molding conditions. The effects of molding conditions on the matrix fusion and impregnation behaviour have been studied. Both samples Vm1 and Vm2, shown in Figs. 9 and 10 were processed at 170 C with 2.3 MPa molding pressure. It is evident in these figures that matrices did not completely fuse or achieve complete molten state to adequately impregnate the reinforcement jute yarns. Voids as unimpregnated regions were readily visible in these figures. When comparing Fig. 10b with Fig. 9b, fibre/matrix interfacial interactions might have shown a little improvement due to an increased amount of matrix content in Vm2 samples. Figs. 9 and 10 contrasted sharply with Fig. 11. Sample Vm3 (Fig. 11) had an increased amount of matrix content (same as sample Vm2) and were processed at 175 C and 2.3 MPa molding pressure. With a higher molding temperature matrix fusion
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cease to be visible. Molding pressure was increased from 2.3 MPa to 2.7 MPa in sample Vm4. Matrix content and processing temperature for Vm4 were kept unaltered as sample Vm3. Sample Vm4 showed relatively better matrix fusion. Better resin impregnation into the reinforcing fibres and lack of microvoids between the fibre and the matrix were evident in these figures (Fig. 12) contributing positively towards a better fibre/matrix interfacial interaction. The effect of molding temperatures and pressures were graphically presented in Figs. 13 and 14. Fig. 13 indicated that samples processed at 175 C showed better bending properties while application of a higher pressure at the same temperature yielded further improvement in bending properties (Fig. 14). As mentioned earlier, at this optimized processing condition, matrix resins were adequately fused and the presence of voids between the fibres and the matrices was minimum. An attempt to attain further higher pressure to fabricate composites resulted in the rupture of jute yarns due to excessive pressure leading to resin blow-out at both ends of the molds, and hence, composites could not be obtained at higher molding pressure. 3.2. Jute/PP unidirectional composites Fig. 8. Bending tested jute/PLA composite specimens processed at 170 C: (a) compression side and (b) tension side.
and fibre impregnation were much better, although voids denoting regions of unimpregnation did not completely
Tensile and 3 point bending tests were also conducted with the samples made from jute/PP composites that had microbraid structure. Only one fibre volume fraction (Vf = 21.2%) was considered. Studies with varied Vf were
Fig. 9. SEM photomicrographs of the polished cross-sections—jute/PLA composite (Vm1 = 68%; 2.3 MPa; 170 C).
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Fig. 10. SEM photomicrographs of the polished cross-sections—jute/PLA composite (Vm2 = 77.5%; 2.3 MPa; 170 C).
Fig. 11. SEM photomicrographs of the polished cross-sections—jute/PLA composite (Vm3 = 77.5%; 2.3 MPa; 175 C).
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170 C
175 C
100 80 60 40 20 0
62
77.5
9
120
175 C 9 8 7 6 5 4 3 2 1 0
Matrix Content,V Content, Vm (%) Fig. 13. Effect of molding temperature on bending properties of jute/PLA composites.
beyond the scope of the investigation carried out in this paper. The present study aimed at the fabrication of jute/ PP unidirectional composite by applying microbraiding technique. Tensile and 3 point bending properties were evaluated and results were graphically presented in Fig. 15a and b. Both tensile and bending strengths and moduli of the reinforced PP composites were compared with those of the virgin homo-PP materials. Both tensile and bending properties significantly improved due to introduction of jute fibres and improvement in fabrication process of thermoplastic composites via tubular (micro-)
Bending Strength (MPa)
170 C
100
Bending Strength Bending Modulus
8 7 6
80
5 60
4 3
40
2
20 0
1 2.3MPa
2.7MPa
Bending Modulus (GPa)
120
Strength Modulus
Bending Modulus (GPa)
Bending Strength (MPa)
Fig. 12. SEM photomicrographs of the polished cross-sections—jute/PLA composite (Vm4 = 77.5%; 2.7 MPa; 175 C).
0
Holding Pressure Fig. 14. Effect of molding pressure on bending properties of jute/PLA composites molded at 175 C.
braiding technique. Both elastic and bending moduli have shown considerable improvements. Moduli were determined from the initial slope of the stress–strain curves, considering only a very small strain region (0.001–0.003). Moduli are thought to be predominantly controlled by factors, such as reinforcement fibre content and fibre directionality. It can be anticipated that the orientation of the PP-matrix fibre used for microbraiding around the unidirectionally aligned jute yarns must have contributed to the resultant directionality of the reinforcing fibres, and
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200
Tensile Strength (MPa)
Tensile Strength 160
Bending Strength
142
122
114
120
69
80 40
141
36
0
Virgin PP
MB_UJY
MB_CJY
(a) 20
Tensile Modulus (GPa)
Tensile Modulus
Bending Modulus
16
11.0
12
9.3
8.1
9.1
8 Fig. 17. Post-failure bending tested jute/PP composite specimens.
4 0
1.6 2.3 Virgin PP
MB_UJY
MB_CJY
(b) Fig. 15. Mechanical properties of Jute/PP unidirectional composites using microbraid jute yarns [MB: Microbraid; UJY: Uncoated jute yarns; CJY: Coated jute yarns].
thereby, enhancing the moduli. Some improvements in tensile (stiffness by 18.3%) and bending properties (strength by 5.3% and stiffness by 12.3%) were recorded due to coating treatment on the fibre surface, except for tensile strength, which was practically not influenced by fibre coating. Microbraid specimens have shown a different failure mode known as splitting fracture (Fig. 16b) during tensile tests, where the fibre loading efficiency of each unidirectionally aligned jute yarn was positively influenced by the
orientation of matrix–fibre that has contributed to the resultant directionality of the reinforcing jute yarns (Fig. 16a). Bending samples failed in a compressive manner. Signs of fibre breakage with a kink (Fig. 17a) were visible on the compression side, whereas the tension side had no evidences of fibre breakage (Fig. 17b). A more detailed investigation on the jute/PP microbraid composites is underway to study and examine the level and quality of impregnation by determining the void content and checking the uniformity in the matrix distribution around the reinforcement jute fibres. At this stage, no detailed information on the applications of polymer coating procedures was available due to product confidentiality from the supplier’s point of view. Application of coating on jute yarns might have improved the dimensional instability resulting from the frequent weaker regions caused by naturally developed microvoids contained in jute fibres. Coating
Fig. 16. Tensile failure in microbraid jute/PP composite specimen: (a) microbraid specimens under tensile test and (b) Tensile failure (splitting fracture in the microbraid specimens).
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may have formed a ductile interface, which has contributed to the effective stress transfer from matrix to fibre, and thus improving the reinforcing effect of jute fibres. At this point, it can only be concluded that coating of jute yarns by poly(vinylalcohol) and polypropylene does affect fibre/ matrix interfacial adhesion and interactions, as suggested by the optical photomicrographs shown in Fig. 18. Relatively better matrix blends and improved impregnation within and at the exterior of the jute yarns were evidenced in the micrographs for the specimens having PVA/PP coated jute yarns (Fig. 18b, d and f) as compared to the uncoated specimens (Fig. 18a, c and e).
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4. Conclusions Tubular braiding technique was applied to produce microbraided yarns using jute yarns as the core reinforcement fibres and PLA and PP fibres as the matrices microbraided around jute yarns. For jute/PLA microbraid composites, maximum tensile stress and modulus increased with increasing fibre volume fraction. Bending test result showed a tendency that is opposite to tensile results. Only compressive failure occurred due to poor interfacial adhesion resulting from improper matrix fusion and associated fibre impregnation. It was indicated that higher pressure
Fig. 18. Representative optical micrographs of polished cross-sections of jute/PP microbraid specimens using (a,c,e) uncoated (natural) and (b,d,f) PP/PVA coated jute yarns.
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(2.7 MPa) and temperature (175 C) were more suitable as the processing condition for the fabrication of jute/PLA microbraid composites. On the other hand, for jute/PP microbraid composites, tensile and bending properties of virgin homo-PP materials increased significantly by the introduction of jute fibre reinforcement. It is believed that composites using microbraid yarns have held enormous promise and possibilities in the optimization of mechanical performances in the resultant composites. Microbraiding technique certainly can improve matrix fusion, wettability of resin melts into fibre bundles, interfacial adhesion properties, and can avoid fibre attrition and associated strength losses resulting from other processing techniques, such as injection molding. The molten viscosity of the thermoplastic polypropylene is extremely high, which makes it difficult to impregnate reinforcing fibre bundles. Processing conditions for composite manufacture by microbraiding needed to be studied thoroughly to examine the impregnation behaviour of the matrix into yarn bundles in order to optimize fibre/matrix interfacial properties. Information on the thermal stability of jute fibres was not available at this moment and work will be undertaken to include this study at a later stage. References [1] Rowell RM. Composites from agri-based resources. Proceedings No. 7286 of the conference entitled ‘‘The Use of Recycled Wood and paper in Building Applications’’ sponsored by the USDA Forest Services and the Forest Products Society, Madison, Wisconsin, September 1996. p. 217–22. [2] Briassoulis D. An overview on the mechanical behaviour of biodegradable agricultural films. J Polym Environ 2004;12(2):65–81. [3] Wambua P, Ivens J, Verpoest I. Natural fibres: can they replace glass in fibre reinforced plastics? Compos Sci Technol 2003;63:1259–64. [4] Netravali AN, Chabba S. Composites get greener. Mater Today 2003;6(4):22–9. [5] Mohanty AK, Misra M, Hinrichsen G. Biofibres, biodegradable polymers and biocomposites: an overview. Macromol Mater Eng 2000;276/277:1–24. [6] Leao AL, Rowell R, Tavares N. Applications of natural fibres in automotive industry in Brazil-thermoforming process. In: Prasad PN, editor. Science and technology of polymers and advanced materials. New York: Plenum Press; 1998. p. 755–61. [7] Biswas S, Srikanth G, Nangia G. Development of natural fibre composites in India. In: Proceedings of annual convention & trade show, COMPOSITES 2001, Composites Fabricator’s Association at Tampa, Florida, USA, 3–6 October 2001. [8] Karnani R, Krishnan M, Narayan R. Biofibre-reinforced polypropylene composites. Polym Eng Sci 1997;37(2):476–83.
[9] de Albuquerque AC, Joseph K, de Carvalho LH, d‘Almeida JRM. Effect of wettability and ageing conditions on the physical and mechanical properties of uniaxially oriented jute-roving-reinforced polyester composites. Compos Sci Technol 2000;60:833–44. [10] Nangia S, Mittal A, Srikanth G, Biswas S. Towards faster trains— role of composites. Sci Tech Enterpreneur 2000;8(2):63–8. [11] Biswas S, Mittal A, Srikanth G. Composites—the future building material. J Indian Build Congress 2001;8(2):199–206. [12] Biswas S, Mittal A, Srikanth G. Composites: a vision for the future. In: Proceedings of the international conference & exhibition on reinforced plastics—ICERP, IIT, Madras, 7–9 February 2002. p. 25– 36. [13] Nangia S, Biswas S. Jute composites: technology business opportunities. In: Proceedings of international conference on advances in composites, IISc & HAL, Bangalore, 24–26 August 2000. p. 295–305. [14] Biswas S, Mittal A, Srikanth G. Gujarat earthquake—composite materials towards re-building & rehabilitation. J Indian Build Congress 2001;8(1):383–90. [15] Rowell RM, Sanadi AR, Caulfield DF, Jacobson RE. Utilization of natural fibres in plastic composites: problems and opportunities. In: Leao et al., editor. Lignocellulosic-plastic composite, 1997. p. 23–51. [16] Al-Qureshi HA. The application of jute fibre reinforced composites for the development of a car body. In: UMIST Conference, UK, 2001. [17] Karus M, Kaup M. Natural fibres in the European automotive industry. J Indus Hemp 2002;7(1):S119–31. [18] Singh B, Gupta M, Verma A. The durability of jute fibre-reinforced phenolic composites. Compos Sci Technol 2000;60:581–9. [19] Gassan J. A study of fibre and interface parameters affecting the fatigue behaviour of natural fibre composites. Composites Part A 2002;33:369–74. [20] Gassan J, Bledzki A. The influence of fibre-surface treatment on the mechanical properties of jute–polypropylene composites. Composites Part A 1997;28:1001–5. [21] Wollerdorfer M, Bader H. Influence of natural fibres on the mechanical properties of biodegradable polymers. Indus Crops Products 1998;8:105–12. [22] Li Y, Mai Y-W, Ye L. Sisal fibres and its composites: a review of recent developments. Compos Sci Technol 2000;60:2037–55. [23] Karmaker AC, Youngquist JA. Injection molding of polypropylene reinforced with short jute fibres. J Appl Polym Sci 1996;62: 1147–51. [24] Rana AK, Mandal A, Bandyopadhyay S. Short jute fibre reinforced polypropylene composites: effect of compatibiliser, impact modifier and fibre loading. Compos Sci Technol 2003;63:801–6. [25] Jayaraman K. Manufacturing sisal–polypropylene composites with minimum fibre degradation. Compos Sci Technol 2003;63:367–74. [26] Corden TJ, Jones IA, Rudd CD, Christian P, Downes S, McDougall KE. Physical and biocompatibility properties of poly-e-caprolactone produced using in situ polymerization: a novel manufacturing technique for long-fibre composite materials. Biomaterials 2000;21: 713–24. [27] Sakaguchi M, Nakai A, Hamada H, Takeda N. Mechanical properties of thermoplastic unidirectional composites using microbraiding technique. Compos Sci Technol 2000;60:717–22.
A novel processing technique for thermoplastic manufacturing of unidirectional composites reinforced with jute yarns O.A. Khondker *, U.S. Ishiaku, A. Nakai, H. Hamada Department of Engineering, Advanced Fibro-Science, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan Received 27 December 2005; received in revised form 28 December 2005; accepted 29 December 2005
Abstract This paper primarily investigates the fabrication process of long-fibre reinforced unidirectional thermoplastic composites made using jute yarns (both untreated and treated). Tubular braiding technique was used to produce an intermediate material called ‘‘microbraid yarn’’ (MBY) with jute yarn as the straightly inserted axial reinforcement fibre and polymer matrix fibre being braided around the reinforcing jute yarns. Microbraid yarns were then wound in a parallel configuration onto a metallic frame and compression molded to fabricate unidirectional composite specimens. In this study, two types of polymeric materials (biodegradable poly(lactic) acid and non-biodegradable homo-polypropylene) were used as matrix fibres. Basic static mechanical properties were evaluated from tensile and 3 point bending tests. Test results were analyzed to investigate the effects of molding temperature and pressure on the mechanical and interfacial behaviour. For the unidirectional jute fibre/poly(lactic) acid (PLA) composites, the results indicated that the molding condition at 175 C and 2.7 MPa pressure was more suitable to obtain optimized properties. Improved wettability due to proper matrix fusion facilitated thorough impregnation, which contributed positively to the fibre/matrix interfacial interactions leading to effective stress transfer from matrix to fibre and improved reinforcing effects of jute yarns. For the jute/PP unidirectional composites, specimens with only 20% of jute fibre content have shown remarkable improvement in tensile and bending properties when compared to those of the virgin PP specimens. The improvements in the mechanical properties are broadly related to various factors, such as the wettability of resin melts into fibre bundles, interfacial adhesion, orientation and uniform distribution of matrix-fibres and the lack of fibre attrition and attenuation during tubular braiding process. 2006 Elsevier Ltd. All rights reserved. Keywords: A. Yarn; A. Polymer (textile) fibre; E. Compression moulding; Microbraiding
1. Introduction Advances in science and technology pose new challenges in relation to certain environmental issues, such as biodegradability, recyclability, eco-friendliness etc., that need to be addressed to help preserve and protect our environment. Composite materials from the annually renewable natural fibres and biodegradable matrices have been developed in the past decade in an attempt to find alternatives to the fossil fuel-based polymeric materials in the automotive *
Corresponding author. Tel.: +88 75 724 7844; fax: +88 75 724 7800. E-mail address: [email protected] (O.A. Khondker).
1359-835X/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2005.12.030
and packaging industries. One of the problems associated with biodegradable composites is the price, which limits investigation of these materials to a laboratory scale. With the new economic climate turning attention to larger volume industrial uses, a wide variety of agro-based resources for the manufacture of new composite materials including geotextiles, filters, sorbents, structural/non-structural composites, molded products and combinations of other materials make it possible to explore new application areas such as packaging, housing, and automotive products [1]. Demands for natural fibres in plastic composites is forecast to grow 15–20% annually with a growth rate of 15– 20% in automotive applications, and 50% or more in
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selected building applications. At the moment, three quarters of agricultural fibres used in composites is wood fibre with the remaining one quarter kenaf, jute, hemp and sisal etc. Wood fibres are progressively being replaced by sustainable long natural fibres, such as jute. Other emerging markets are industrial and consumer applications such as tiles, flower pots, furniture and marine piers. Another economic reason for the growing popularity of natural fibres is their cheap price, while weight reduction is also responsible for the increasing use of natural fibres. Thermoplastic materials that currently dominate as matrices for natural fibres are PP, PE and PVC while thermosets, such as phenolics and polyesters are common matrices. Biodegradable polymers are yet to become popular as ‘‘green’’ substitutes for synthetic polymers. Besides, the use of biodegradable plastics appear as an attractive alternative for enhancing sustainable and environmentfriendly agricultural activities, especially in mulching and low-tunnels cultivation [2]. The increasing global interest on the agro-based structural materials from the natural, annually renewable and biodegradable sources was also driven by the conservation of forest and forest resources through reducing their massive usage, and the need for cost reduction in raw materials, manufacture and maintenance of composites. With a view to replacing the wooden fittings, fixtures and furnitures, natural composites reinforced with jute, kenaf, sisal, coir, straw, hemp, banana, pineapple, rice husk, bamboo etc. can be used instead of the conventional polymer composites reinforced with man-made fibres such as glass. Development of new composite product(s) from the existing resources has a strong potential to deliver a novel biodegradable and/or readily recyclable material suitable for the automotive and packaging industry to replace non-renewable fossil fuel-based polymers/ plastics. Lignocellulosic (hydrophilic) bio-fibres, when rightly combined with the recyclable thermoplastic polymers, can possess an outstanding potential to produce environmentfriendly composite materials having specific properties favourably comparable with those of glass-based composites in most cases [3–7]. Natural fibres have distinct advantages over synthetic glass fibre in the applications not requiring very high load bearing capabilities [3]. Complete matrix fusion to facilitate thorough fibre impregnation, formation of strong fibre/matrix interfacial bonding and matrix-to-fibre stress transfer efficiency [3,8,9] are vital requirements for the manufacture of reliable, eco-friendly natural composites that can possess better mechanical properties and withstand environmental attacks. Mohanty et al. [5] carried out a detailed review work on jute fibre reinforced thermosets, thermoplastic, and rubber-based composites. Jute is one of the most versatile agro-based fibres, mostly grown in south asian countries like Bangladesh and India, that has enormous potential in composite manufacture due to its cost-effective, renewable, versatile, non-abrasive, viscoelastic, biodegradable, combustible, compostable nature and better insulation against noise
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and heat due to it’s hollow cellular [4] structure. Jute-based composites, an alternative to wood products [7,10], are not severely affected by mites, moisture, and are generally considered to be fire-retardant and chemical-proof. Jute-based green composites would be suitable for even primary structural applications as indoor elements in housing [11,12], temporary outdoor applications like low-cost housing for defence and rehabilitation [13,14] and transportation. Due to its insulating characteristics, jute may find areas of applications in automotive door/ceiling panels and panels separating the engine and passenger compartments [4,15]. Such panels made from jute fibres and PP or other bio-thermoplastic and hybrid composites are already in use [4,16,17]. The durability of jute fibre-based phenolic composites has been studied under various humidity, hydrothermal and weathering conditions to assess the suitability of jute composites in damp and dry environment [18]. Fibre type, textile architecture, interphase properties, fibre mechanical properties and content were found to strongly affect the fatigue behaviour of flax and jute fibre reinforced epoxy, polyester and PP composites [19,20]. Much of the fibre degradation during conventional shear mixing processing techniques—extrusion compounding and injection molding methods [15,21–24] can be avoided when an innovative fibre mat production method by sprinkling the fibres down a drop feed tower [25] was used to manufacture natural fibre reinforced polypropylene composites. Most researches concentrate on jute fibre/nondegradable polymer composites but research reports on jute/biodegradable polymer composites are rather limited [26]. With the increasing popularity of natural fibres in polymer composites, it is timely to carry out fundamental studies on jute fibre-based composites, especially on their fabrication processes. In this study, a preliminary investigation on the fabrication of long-fibre reinforced unidirectional jute-based composite is presented. A tubular braiding machine [27] was used to produce micro-braid yarn using continuous jute yarn as the straightly inserted axial fibre and polymer fibre as the matrix material braided around the reinforcing jute fibre. Micro-braiding technique offers minimum or no damage to the reinforcement fibre bundles, when compared to using commingled yarns. 2. Experimental 2.1. Materials, fabrication and mechanical tests Jute yarns produced from 100% BT Tossa B-grade finer jute fibres (Corchoras olitorious) were commercially supplied by Bangladesh Janata Jute Mills Ltd. The matrix materials used in this study were biodegradable PLA fibres from Mitsui Chemical Co. Ltd., Japan and homo-PP fibres (HX100G) from Sumitomo Chemical Co., Ltd., Tokyo, Japan. Despite problems associated with poor fibre wetting due to high melt viscosities, homo-polypropylene fibres have wide selectivity, excellent property range, low processing temperature and recyclability, which have led to the
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Fig. 1. Fabrication process of microbraid yarns.
development of a number of manufacturing techniques, such as commingled yarn, powder impregnated yarn, coweaving etc. This paper explored a different molding technique primarily to optimize the processing condition for thermoplastic manufacturing of unidirectional composites. Good quality composite specimens with enhanced mechanical characteristics were expected. An intermediate material called ‘‘micro-braid yarn’’ (MBY) that has a unique combination of reinforcement and matrix phases, was produced using a tubular braiding machine consisting of 16 spindles (Fig. 1). MBY can be treated as a single fibre bundle so as to fabricate varieties of textile processed goods. Continuous jute yarns were used as the straightly inserted axial fibres, and matrix fibres (PLA and PP fibres) were braided around the reinforcing jute yarns. A hot compression molder was used to fabricate continuous fibre reinforced thermoplastic composites. Matrix fibres were melted by heating at appropriate molding temperatures and become the matrices for the fibre reinforced plastics/composites that easily wet out the reinforcing jute fibre.
Both natural (untreated) and treated or coated jute yarns were used to fabricate jute/PLA and jute/PP unidirectional composites. Fabrication of continuous jute yarn reinforced PLA or PP thermoplastic composites involved a twofold process. The first step was performed by initially winding microbraid jute yarns 40 times in two stages in a parallel configuration onto a metallic frame (Fig. 2a) with a spring mechanism enabling it to adjust tension caused by thermal shrinkage. The second step involved placement of the metallic frame containing microbraid yarns in a preheated molding die (Fig. 2b) for consolidation by compression molding (Fig. 2c) to produce composite specimens. The heated die was let to cool naturally with the molding dies being closed. Consolidation process involved three stages. At first, under certain temperature, the solid fibrous matrix materials became softened and then the matrix in the liquid form soaked and infiltrated the reinforcing fibres and finally during cooling stage, the matrix turned into solid form to hold the fibres in a definite position. The second step was considered to be the most important, as this decided uniform distribution of fibres in the matrix. There
Fig. 2. Consolidation set-up for compression molding: (a) unidirectional arrangement onto a tension rig, (b) molding die and (c) consolidation by compression molding.
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were used for this study. Bending specimens were 20 mm in nominal width and the span length was 56 mm. The specimens for bending test were also cut along the fibre axis. Thicknesses of the specimens were 3.4–3.6 mm. Tensile specimens were clamped over an area of 40 mm · 20 mm at each end leaving a gauge length of 100 mm. For tensile tests, an open mesh emery cloth (Polynet sheet—240 grit) was used in the gripped areas instead of aluminium end tabs. Strain was measured using strain gauges, which were bonded onto the central surface of the specimens. Both tension and 3 point bending tests were conducted on an Instron Universal Testing Machine (Type 4206) under nominal test speeds of 5.0 and 2.0 mm/min, in accordance with ASTM D638 and D790, respectively. No fewer than five specimens were tested for each specimen and test types. Tensile and bending properties were determined from the tests. Pre- and post- failure cross-sectional observations were carried out on the selected test specimens with a focus on matrix fusion, impregnation property related to the fibre/matrix interfacial bonding, and to study its relationships to the test results. 3. Results and discussion 3.1. Jute/PLA unidirectional composites Tensile and 3 point bending properties were evaluated for jute/PLA unidirectional composites. Tensile specimens had three variations in fibre contents designated as Vf1 (38%), Vf2 (27.5%) and Vf3 (22.5%). Specimens were tested to failure, and stress–strain curves, as shown in Fig. 3, indicated that all three types of composites samples processed at 170 C did not fail abruptly. Composite behaved in a pseudo-plastic manner before attaining a maximum stress value and then decayed. Fractured specimens were shown in Fig. 4. There was no indication of normal tensile fracture, as the failure of the composites was rather dominated by shear due to poor interfacial bonding between the fibres and the matrix. Tested samples also suggested that matrix failure occurred first and then the fibre slippage became dominant.
100 Tensile Stress (MPa)
is certainly a scope for improvement in the quality of the molded specimens by introducing a rapid cooling system attached to the compression molder. For Jute/PLA composites, matrix–fibre contents were varied by changing the number of resin fibre bundles to microbraid around the reinforcing jute yarn. Only untreated (natural) jute yarns were used in this study. The effects of processing temperature (170 C and 175 C) and pressure (2.3 and 2.7 MPa) were investigated. Three types of jute/PLA composite specimens were fabricated and the fibre volume fractions (Vf) were calculated as approximately 22.5%, 27.5% and 38% for the test samples. Samples were cut in the transverse direction, embedded in epoxy resin and then polished, in order to study fibre impregnation and interfacial properties by scanning electron microscopy (SEM) analysis. Thermal analyses of the PLA and PP fibres used in this study were carried out using differential scanning calorimetry (DSC 7, Perkin-Elmer). The 10 mg samples were placed in the aluminium pans, sealed and then introduced into the heating cell of the calorimeter. Heat treatment or thermal scanning was performed at a programmed rate of 10 C/min from 20 C to 250 C under constant nitrogen flow. The samples were then cooled, and data were analyzed by a computer. The first thermal scan melts the samples and erase its thermal history. DSC traces were obtained from the second thermal cycle as it provides accurate result for the melting temperature (Tm), measured from the peak value of the endotherm. The melting temperatures of PLA and PP matrix fibres were obtained as 168 C and 169 C, respectively. The melting endotherms of PLA and PP matrix fibres were completed at 170 C and 175 C, respectively. Both untreated and coated jute yarns were used in jute/ PP composites. The coating agents used on the jute yarns were PVA (Poly vinyl alcohol) and PP (Polypropylene) each of which having 1.5 wt% on the basis of jute yarn content. Untreated jute yarns look naturally dull and feels loose and scattered as opposed to the coated one, which has a shiny, dimensionally stable and compact surface. Processing condition for reinforced PP composites using microbraid jute yarns was set at 180 C for 10 min under 2 MPa molding pressure. For jute/PP composites, the fibre volume fraction (Vf) achieved was approximately 21.2% and no other variations in Vf were aimed at this stage. Research focused mainly on the fabrication process of jute/PP unidirectional composites by adopting tubular braiding technique. Temperature and pressure during fabrication influence the impregnation behaviour. Each variety of agro-based fibres has a particular structure and composition. Even though most lignocellulosic fibres cannot withstand processing temperatures higher than 175 C for long duration, limiting their ability to be used with some thermoplastic resins [4], there were no signs of fibre damage due to temperature during molding process. Tensile specimens of 180 mm · 20 mm in nominal dimensions, with the fibre axis along the loading direction
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Vf1 = 38% Vf2 = 27.5% Vf3 = 22.5%
20 0 0.00
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0.06 0.09 Tensile Strain
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Fig. 3. Tensile stress–strain curves for jute/PLA composites processed at 170 C.
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10 0 80 60 40 Vm1=62% (2.3 MPa)/170˚C) Vm2=77.5% (2.3 MPa)/170˚C)
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Vm3=77.5% (2.3 MPa)/175˚C) V Vm4=77.5% (2.7 MPa)/175˚C)
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Fig. 4. Tensile tested jute/PLA composite specimens processed at 170 C.
9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0
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Fibre Volume Fraction, Vf Fig. 5. Tensile properties of jute/PLA composites processed at 170 C.
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3.5 4.9 Deflection (mm)
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Fig. 6. Bending stress–deflection curves for jute/PLA composites processed at 170 and 175 C.
86 84 82 80 78 76 74 72 70 68 66
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Maximum tensile stresses and moduli are graphically presented in Fig. 5. As expected, jute/PLA composite samples exhibited superior tensile properties with increasing fibre content. The trend of improvement in both strength and modulus was fairly consistent with respect to the increase in the reinforcing fibre content. Four specimen types underwent 3 point bending tests. Samples had variations with respect to the processing conditions and were designated as Vm1 (62%), Vm2 (77.5%), Vm3 (77.5%) and Vm4 (77.5%). Bending test yielded a different trend (as exemplified in Figs. 6 and 7) when compared to the tensile results. This different trend could be accounted for by the differences in failure mechanism and the integrity of the fibre/matrix interface. Figs. 6 and 7 suggested that there could be an optimum processing condition for which composite might perform better in flexure. Post-failure bending specimens are shown in Fig. 8. As indicated in Fig. 6, for sample Vm4, a higher molding or holding pressure (2.7 MPa) with a higher molding temperature (175 C) imparted the best performances in bending. A relatively sharper drop in the gradient after the maximum value for sample Vm4, was indicative of tensile fracture, whereas for all the other samples processed at 170 C, only compressive failure was evident (Fig. 8a) while tensile failure was absent and the samples rather looked pale. This could also suggest a poor interfacial adhesion and incomplete matrix fusion as indicated in Figs. 9 and 10. The voids
0.0
0
Fig. 7. Bending properties of jute/PLA composites processed at 170 C.
within the matrix indicated boundary areas where complete fusion was not attained. It is noteworthy that the hollow cross-sectional structure of jute fibres cannot be seen even at higher magnifications. The process of embedding samples in epoxy resin and subsequent polishing must have covered the lumen or central cavities of jute fibres. Figs. 9–12 showed representative SEM micrographs of the polished cross-sections for the composite samples processed at different molding conditions. The effects of molding conditions on the matrix fusion and impregnation behaviour have been studied. Both samples Vm1 and Vm2, shown in Figs. 9 and 10 were processed at 170 C with 2.3 MPa molding pressure. It is evident in these figures that matrices did not completely fuse or achieve complete molten state to adequately impregnate the reinforcement jute yarns. Voids as unimpregnated regions were readily visible in these figures. When comparing Fig. 10b with Fig. 9b, fibre/matrix interfacial interactions might have shown a little improvement due to an increased amount of matrix content in Vm2 samples. Figs. 9 and 10 contrasted sharply with Fig. 11. Sample Vm3 (Fig. 11) had an increased amount of matrix content (same as sample Vm2) and were processed at 175 C and 2.3 MPa molding pressure. With a higher molding temperature matrix fusion
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cease to be visible. Molding pressure was increased from 2.3 MPa to 2.7 MPa in sample Vm4. Matrix content and processing temperature for Vm4 were kept unaltered as sample Vm3. Sample Vm4 showed relatively better matrix fusion. Better resin impregnation into the reinforcing fibres and lack of microvoids between the fibre and the matrix were evident in these figures (Fig. 12) contributing positively towards a better fibre/matrix interfacial interaction. The effect of molding temperatures and pressures were graphically presented in Figs. 13 and 14. Fig. 13 indicated that samples processed at 175 C showed better bending properties while application of a higher pressure at the same temperature yielded further improvement in bending properties (Fig. 14). As mentioned earlier, at this optimized processing condition, matrix resins were adequately fused and the presence of voids between the fibres and the matrices was minimum. An attempt to attain further higher pressure to fabricate composites resulted in the rupture of jute yarns due to excessive pressure leading to resin blow-out at both ends of the molds, and hence, composites could not be obtained at higher molding pressure. 3.2. Jute/PP unidirectional composites Fig. 8. Bending tested jute/PLA composite specimens processed at 170 C: (a) compression side and (b) tension side.
and fibre impregnation were much better, although voids denoting regions of unimpregnation did not completely
Tensile and 3 point bending tests were also conducted with the samples made from jute/PP composites that had microbraid structure. Only one fibre volume fraction (Vf = 21.2%) was considered. Studies with varied Vf were
Fig. 9. SEM photomicrographs of the polished cross-sections—jute/PLA composite (Vm1 = 68%; 2.3 MPa; 170 C).
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Fig. 10. SEM photomicrographs of the polished cross-sections—jute/PLA composite (Vm2 = 77.5%; 2.3 MPa; 170 C).
Fig. 11. SEM photomicrographs of the polished cross-sections—jute/PLA composite (Vm3 = 77.5%; 2.3 MPa; 175 C).
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170 C
175 C
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9
120
175 C 9 8 7 6 5 4 3 2 1 0
Matrix Content,V Content, Vm (%) Fig. 13. Effect of molding temperature on bending properties of jute/PLA composites.
beyond the scope of the investigation carried out in this paper. The present study aimed at the fabrication of jute/ PP unidirectional composite by applying microbraiding technique. Tensile and 3 point bending properties were evaluated and results were graphically presented in Fig. 15a and b. Both tensile and bending strengths and moduli of the reinforced PP composites were compared with those of the virgin homo-PP materials. Both tensile and bending properties significantly improved due to introduction of jute fibres and improvement in fabrication process of thermoplastic composites via tubular (micro-)
Bending Strength (MPa)
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Fig. 12. SEM photomicrographs of the polished cross-sections—jute/PLA composite (Vm4 = 77.5%; 2.7 MPa; 175 C).
0
Holding Pressure Fig. 14. Effect of molding pressure on bending properties of jute/PLA composites molded at 175 C.
braiding technique. Both elastic and bending moduli have shown considerable improvements. Moduli were determined from the initial slope of the stress–strain curves, considering only a very small strain region (0.001–0.003). Moduli are thought to be predominantly controlled by factors, such as reinforcement fibre content and fibre directionality. It can be anticipated that the orientation of the PP-matrix fibre used for microbraiding around the unidirectionally aligned jute yarns must have contributed to the resultant directionality of the reinforcing fibres, and
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Virgin PP
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8 Fig. 17. Post-failure bending tested jute/PP composite specimens.
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(b) Fig. 15. Mechanical properties of Jute/PP unidirectional composites using microbraid jute yarns [MB: Microbraid; UJY: Uncoated jute yarns; CJY: Coated jute yarns].
thereby, enhancing the moduli. Some improvements in tensile (stiffness by 18.3%) and bending properties (strength by 5.3% and stiffness by 12.3%) were recorded due to coating treatment on the fibre surface, except for tensile strength, which was practically not influenced by fibre coating. Microbraid specimens have shown a different failure mode known as splitting fracture (Fig. 16b) during tensile tests, where the fibre loading efficiency of each unidirectionally aligned jute yarn was positively influenced by the
orientation of matrix–fibre that has contributed to the resultant directionality of the reinforcing jute yarns (Fig. 16a). Bending samples failed in a compressive manner. Signs of fibre breakage with a kink (Fig. 17a) were visible on the compression side, whereas the tension side had no evidences of fibre breakage (Fig. 17b). A more detailed investigation on the jute/PP microbraid composites is underway to study and examine the level and quality of impregnation by determining the void content and checking the uniformity in the matrix distribution around the reinforcement jute fibres. At this stage, no detailed information on the applications of polymer coating procedures was available due to product confidentiality from the supplier’s point of view. Application of coating on jute yarns might have improved the dimensional instability resulting from the frequent weaker regions caused by naturally developed microvoids contained in jute fibres. Coating
Fig. 16. Tensile failure in microbraid jute/PP composite specimen: (a) microbraid specimens under tensile test and (b) Tensile failure (splitting fracture in the microbraid specimens).
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may have formed a ductile interface, which has contributed to the effective stress transfer from matrix to fibre, and thus improving the reinforcing effect of jute fibres. At this point, it can only be concluded that coating of jute yarns by poly(vinylalcohol) and polypropylene does affect fibre/ matrix interfacial adhesion and interactions, as suggested by the optical photomicrographs shown in Fig. 18. Relatively better matrix blends and improved impregnation within and at the exterior of the jute yarns were evidenced in the micrographs for the specimens having PVA/PP coated jute yarns (Fig. 18b, d and f) as compared to the uncoated specimens (Fig. 18a, c and e).
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4. Conclusions Tubular braiding technique was applied to produce microbraided yarns using jute yarns as the core reinforcement fibres and PLA and PP fibres as the matrices microbraided around jute yarns. For jute/PLA microbraid composites, maximum tensile stress and modulus increased with increasing fibre volume fraction. Bending test result showed a tendency that is opposite to tensile results. Only compressive failure occurred due to poor interfacial adhesion resulting from improper matrix fusion and associated fibre impregnation. It was indicated that higher pressure
Fig. 18. Representative optical micrographs of polished cross-sections of jute/PP microbraid specimens using (a,c,e) uncoated (natural) and (b,d,f) PP/PVA coated jute yarns.
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(2.7 MPa) and temperature (175 C) were more suitable as the processing condition for the fabrication of jute/PLA microbraid composites. On the other hand, for jute/PP microbraid composites, tensile and bending properties of virgin homo-PP materials increased significantly by the introduction of jute fibre reinforcement. It is believed that composites using microbraid yarns have held enormous promise and possibilities in the optimization of mechanical performances in the resultant composites. Microbraiding technique certainly can improve matrix fusion, wettability of resin melts into fibre bundles, interfacial adhesion properties, and can avoid fibre attrition and associated strength losses resulting from other processing techniques, such as injection molding. The molten viscosity of the thermoplastic polypropylene is extremely high, which makes it difficult to impregnate reinforcing fibre bundles. Processing conditions for composite manufacture by microbraiding needed to be studied thoroughly to examine the impregnation behaviour of the matrix into yarn bundles in order to optimize fibre/matrix interfacial properties. Information on the thermal stability of jute fibres was not available at this moment and work will be undertaken to include this study at a later stage. References [1] Rowell RM. Composites from agri-based resources. Proceedings No. 7286 of the conference entitled ‘‘The Use of Recycled Wood and paper in Building Applications’’ sponsored by the USDA Forest Services and the Forest Products Society, Madison, Wisconsin, September 1996. p. 217–22. [2] Briassoulis D. An overview on the mechanical behaviour of biodegradable agricultural films. J Polym Environ 2004;12(2):65–81. [3] Wambua P, Ivens J, Verpoest I. Natural fibres: can they replace glass in fibre reinforced plastics? Compos Sci Technol 2003;63:1259–64. [4] Netravali AN, Chabba S. Composites get greener. Mater Today 2003;6(4):22–9. [5] Mohanty AK, Misra M, Hinrichsen G. Biofibres, biodegradable polymers and biocomposites: an overview. Macromol Mater Eng 2000;276/277:1–24. [6] Leao AL, Rowell R, Tavares N. Applications of natural fibres in automotive industry in Brazil-thermoforming process. In: Prasad PN, editor. Science and technology of polymers and advanced materials. New York: Plenum Press; 1998. p. 755–61. [7] Biswas S, Srikanth G, Nangia G. Development of natural fibre composites in India. In: Proceedings of annual convention & trade show, COMPOSITES 2001, Composites Fabricator’s Association at Tampa, Florida, USA, 3–6 October 2001. [8] Karnani R, Krishnan M, Narayan R. Biofibre-reinforced polypropylene composites. Polym Eng Sci 1997;37(2):476–83.
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