Mechanical Properties of Injection-Moulded Jute/Glass Fibre Hybrid Composites

Transcription

1 Mechanical Properties of Injection-Moulded Jute/Glass Fibre Hybrid Composites Tomoko Ohta, Yoshihiro Takai, Yew Wei Leong, and Hiroyuki Hamada* Advanced Fibro Science, Kyoto Institute of Technology, Matsugasaki Sakyo-ku, Kyoto Japan Received: 18 November 2008, Accepted: 29 May 2009 SUMMARY In order to improve the mechanical performance of natural fibre composites, glass fibres (GF) and natural fibres were simultaneously incorporated into a single matrix system. Such a practice would of course decrease the degree of green, which measures the amount (in weight percentage) of natural materials in a composite system. Nevertheless, it was found that hybrid composites containing up to 20 wt.% jute fibres exhibited similar tensile properties to those of glass fibre composites. The distribution of glass and jute fibres was anisotropic, wherein the GF tend to accumulate at the skin regions of the mouldings, thus orientating towards the resin flow direction. Jute fibres, on the other hand, tend to remain at the core regions and were mostly oriented transverse to the flow direction. This anisotropic distribution and orientation are thought to be responsible for the weakening of the composites at high natural fibre loadings. INTRODUCTION Recent developments in natural fibre (or green) composites indicate the vast awareness of the need to reduce the usage of inorganic reinforcements as well as petroleum based polymers. There are many types of natural fibres and green polymers that are suitable for various applications 1-3, and they are considered to be very promising materials for the future. However, several problems are still being confronted regarding the processing of natural fibre composites. One of them is the hygroscopic nature of natural fibres, which causes numerous surface and internal defects to the final product 3-4. For example, hemp fibre mats can absorb 5% moisture within 5 minutes after complete drying, which causes blistering and delamination of composite structures during postcuring processes under elevated temperatures. In addition, the bulk volume of short natural fibre is very large, which complicates the compounding process, especially when high fibre volume fraction is desired in the composite. The density of natural fibres is lower than that of resins; the densities of bamboo fibres and polypropylene (PP) are g/cm 3 5 and 0.9 g/cm 3, respectively. The side feeder technique that is commonly used for incorporating glass fibres during the compounding of composites is not applicable to low density natural fibres as this would produce pellets with inconsistent fibre volume fractions. Moreover, if the natural fibres are dry-blended with resin pellets, especially at high fibre contents, the material flow from the hopper to the extruder barrel can be difficult, which again causes inconsistencies in the compound. As far as the authors are concerned, a truly effective surface treatment for natural fibres to improve composite properties is yet to be developed 3-4,6-8. Mechanical properties of natural fibre composites with or without fibre treatments are still comparatively low and highly scattered. Without overcoming these problems, natural fibre composites would still be considered as materials that could only sustain low loads and are incapable of being used as structural materials. In this paper we propose two technologies for producing natural fibre composites of highly consistent fibre volume fraction with superior Figure 1. Comparison between the volume of PP resin (90 wt.%) and bamboo fibres (10 wt.%; volume fraction = 6.3%) *Corresponding author address: Hhamada1@aol.com Smithers Rapra Technology,

2 Tomoko Ohta, Yoshihiro Takai, Yew Wei Leong, and Hiroyuki Hamada performance. Firstly, the long fibre pellet production technique was adopted to resolve the abovementioned compounding difficulties as well as to control precisely the fibre volume fraction in the pellets. Secondly, the hybrid fibre concept was introduced, whereby natural and glass fibres were simultaneously incorporated to enhance the mechanical properties of the composite. Although this concept is not fully environmentally friendly, the problem of natural fibre composites having low mechanical properties can be addressed. Here, the Degree of Green terminology is introduced, which is an estimation of the content of natural materials in a composite; for example the Degree of Green in GF/PP composites is zero while a 20 wt.% jute fibre-pp composite has a Degree of Green of 20%. Long jute fibre-pp (jute/pp) and long glass fibre-pp (GF/PP) pellets were prepared by using the long fibre pellet pultrusion technology, whereby the fibre bundle is continuously twisted while being impregnated by a resin through a heated die before being cut into pellets with pre-determined lengths after sufficient cooling. The pellet length is longer than normal short fibre pellets so that higher fibre aspect ratios can be retained, which is known to contribute towards better tensile and impact properties in composites In this study, the lengths of the pellets are 7 mm and 11 mm for the jute/pp and GF/PP composites, respectively. These pellets were dry-blended prior to being injection moulded into dumbbell specimens. EXPERIMENTAL Materials and Specimen Fabrication Jute and glass fibres were used as reinforcements while PP resin acted as the matrix. The long fibre pellets were produced by Calp Co. Ltd. with the use of a long fibre pellet pultrusion machine, as schematically represented in Figure 2. Continuous fibres were fed into the extruder from a roving stand. A twin screw extruder is used to supply the resin to the fibre bundles while they are pulled through a die at which pressure is applied to impregnate the resin into the fibres. Upon sufficient cooling, the extrudate was then cut into the desired lengths. The pellets were then dry blended to obtain the desired composition prior to being injection moulded into dumbbell specimens with a 100 ton injection moulding machine. Before moulding, the pellets were dried at 80 C for 3 hours. The barrel temperature was set at 200 C while injection and holding pressures were 85 MPa and 70 MPa, respectively. Specimen designations according to the composition of the composites are listed in Table 1. The content of jute fibres was varied from 5 to 30 wt.% while the glass fibre content was kept constant at 10 wt.%. Jute/PP composites without glass fibres were also moulded for comparison purposes. Tensile tests were performed by using an Instron universal testing machine with a grip distance of 115 mm and crosshead speed of 10 mm/min. An extension meter with a 50 mm gauge length was used to measure the tensile modulus. Izod impact specimens were Table 1. Specimen designation Sample ID GF content (wt.%) Jute content (wt.%) PP content (wt.%) G G10J G10J G10J G10J J J J J Figure 2. Schematic of long fibre pellet pultrusion machine 488

3 obtained from the parallel part of the dumbbell specimens and tested with a 5.5 J impact energy hammer. Figure 3. Effect of jute fibre content on tensile modulus of monotonous jute/pp and hybrid composites RESULTS Figure 3 shows the relationship between tensile modulus and jute fibre contents in both monotonic and hybrid materials. With increasing jute fibre content in the monotonic materials, the elastic modulus increased linearly to about 4 GPa at 30 wt.% jute content. As for the hybrid composites, however, a similar stiffness of 4 GPa could be achieved even at 5 wt.% jute fibre content. The stiffness further increased linearly to 6 GPa as the jute fibre loading increases to 30 wt.%. These values indicate that there was an almost two fold increase in stiffness with the incorporation of only 10 wt.% glass fibres. Without the presence of jute fibre, the GF/PP composites only recorded a tensile modulus of 2.8 GPa, which indicates that there is a synergistic contribution from both the jute and glass fibres in enhancing the mechanical properties of the hybrid composite. Figure 4 shows the relationship between tensile strength and jute fibre content. The tensile strength of the monotonic jute/pp materials experienced a linear increment with increasing fibre content. This tendency is similar to that of tensile modulus. However, in the case of hybrid composites, the strength remained consistent at 58 MPa despite the increase in jute fibre loading. Above 20 wt.% jute content, a deterioration of strength was recorded. This indicates that the overloading of jute fibres could have affected the distribution of glass fibres, thus creating stress concentration points at parts of the specimens that are either rich in resin or jute fibres. Figure 4. Effect of jute fibre content on tensile strength of monotonous jute/pp and hybrid composites Figure 5. Effect of jute fibre content on un-notched Izod impact strength of monotonous jute/pp and hybrid composites Figure 5 shows the effect of jute fibre content on the Izod impact strength. At 5 to 10 wt.% fibre contents in 489

4 Tomoko Ohta, Yoshihiro Takai, Yew Wei Leong, and Hiroyuki Hamada monotonic jute/pp composites, the impact strength remained constant; thereafter a slight increase in toughness was noted as fibre content increased. On the other hand, the impact strength of the hybrid composites steadily decreased with jute fibre content. Perhaps one of the most interesting results is the sudden decrease in tensile strength of the hybrid composite only when the jute fibre loading reached 30 wt.%. In order to provide an explanation for this observation, the fracture surfaces of G10J30 and G10J20 specimens were observed by scanning electron microscopy (SEM) and the photographs are depicted in Figures 6a and b, respectively. The low magnification provides an overall view of the fibre distribution as well as fracture characteristics. The glass fibres are easily distinguishable by its fine diameter and clean surface while the thicker fibres with rough surfaces are jute fibre bundles. Normally, glass fibres are able to disperse well in PP. However, with the presence of jute fibres especially at high weight fractions, an anisotropic distribution could be observed whereby jute fibres appear to have remained in the form of fibre bundles at the core regions while glass fibres were mostly found near the surface of the mouldings. Furthermore, the orientation of jute fibres was mostly random or perpendicular to flow direction (90 direction) while glass fibres at the skin region were oriented parallel to flow direction (0 direction). When the jute fibre content was low, such as in the case of G10J20 specimens, the concentration of jute fibres at the core could not be observed. behind by the debonding between jute fibres and matrix were also detected, indicating weak interfacial interaction. It is worth mentioning again that there were significantly fewer glass fibres in this region than jute fibres. Figure 7b depicts high magnification SEM photographs of skin region. It can be seen that most of the skin region is dominated by glass fibres that are oriented in the 0 direction while some jute fibres can also be found with similar orientation. Figure 8 highlights the interphase region between the matrix and fibres. Figure 6. Fracture surface of tensile tested specimens: (a) G10J30; and (b) G10J20 Figure 7. SEM photograph of G10J30 specimen at: (a) the core; and (b) the skin (a) Figure 8. High magnification SEM photographs depicting the fibre/matrix interface in G10J30: (a) glass fibre; (b) jute fibre (b) Figure 7a shows high magnification SEM photograph at the core region of a fractured G10J30 specimen. At this region, a large presence of jute fibres that are mostly orientated perpendicular to the direction of flow could be seen, thus these fibres might not be able to fully contribute as a reinforcing agent. Holes left 490

5 Large gaps between the matrix and glass as well as jute fibres can be found. Figure 9. Marking of fibres to indicate the state of orientation of each fibre. Lines represent fibres oriented in the transverse (90 ) direction while those oriented in the flow direction (0 ) are marked with circles: (a) before marking; and (b) after marking) DISCUSSION According to SEM observations, jute fibres are highly concentrated at the core region and are mostly oriented in the 90 direction while the fibrematrix interphase characteristics are also found to be weak. These are the main factors contributing to the low tensile strength of G10J30 specimens. In order to quantify the distribution and orientation state of both fibres from the fracture surface in the G10J30 specimens, the fibres were marked as shown in Figure 9. The fibre orientation was roughly classified into longitudinal (0 ) direction and transverse (90 ) direction. Fibres oriented in the longitudinal direction were marked with a circle while those oriented in the transverse direction were marked with a line. Figure 10 shows a low magnification SEM photo with all the markings indicating longitudinally oriented glass fibres. The distribution of the longitudinally oriented glass fibres was quite uniform at most parts of the specimen except for the core region. The content of glass fibres at the core was far less than that at the skin. Figure 11 shows the preferential distribution of transversely oriented glass fibres that are mostly located at the core region of the specimen. The orientation and distribution of longitudinally oriented jute fibres are shown in Figure 12 and the number of markings is visibly smaller than those of glass fibres in Figure 10. Jute fibres oriented in the transverse direction are marked with thicker lines since they appear in the form of fibre bundles, as shown in Figure 13. It is clear that most of these fibres are concentrated at the core region. The reason for the anisotropic distribution of the glass and jute fibres is probably due to the distinct difference in density between these fibres. Figure 10. Markings of glass fibres oriented in the flow direction (0 ) in G10J30: (a) markings on actual photographs; (b) carbon copy of the markings Figure 11. Markings of glass fibres oriented in the transverse direction (90 ) in G10J30: (a) markings on actual photographs; (b) carbon copy of the markings 491

6 Tomoko Ohta, Yoshihiro Takai, Yew Wei Leong, and Hiroyuki Hamada Figure 12. Markings of jute fibres oriented in the flow direction (0 ) in G10J30: (a) markings on actual photographs; (b) carbon copy of the markings Figure 13. Markings of jute fibres oriented in the transverse direction (90 ) in G10J30: (a) markings on actual photographs; (b) carbon copy of the markings Figure 14 shows a compound figure that was produced by overlapping Figures The most interesting observation is that most of the jute fibres are accumulated at the core region and are oriented 90 to the flow direction. On comparing the fibre distributions between G10J30 in Figure 14 and G10J00 (only incorporated with GF) in Figure 15, a much larger presence of GF may be observed in the core region of GF10J00. In order to quantify the microscopic observations of fibre orientation state, the ratio between 0 o and 90 o oriented fibres was obtained numerically by using image analysis software (COSMOS32 Library Corporation). The volume fractions of fibres in different orientation states can be obtained from the following equations: V 0 = A 0 A total (1) and V 90 = A 90 A total (2) whereby V 0 and V 90 is the volume fraction of 0 and 90 oriented fibres, respectively, while A 0 and A total are the cross-sectional areas of 0 and 90 oriented fibres, respectively. Figure 14. Superposition of all the markings indicating the distribution of jute fibres in different orientations The orientation ratios of 90 to 0 fibres for all specimens are tabulated in Table 2. The orientation ratio of glass fibres in the monotonic GF10J00 specimens was found to be around 2.2, while glass fibres in the hybrid GF10J30 specimens recorded a significantly lower orientation ratio of about 1.4. This indicates that more glass fibres in the hybrid composites are oriented in the direction of resin flow and this could be due to the preferential distribution of the glass fibres towards the skin regions, as observed from the SEM micrographs in Figure 7b. On the other hand, the jute fibre orientation ratio in the hybrid 492

7 Table 2. Fibre orientation ratio of glass and jute fibres in monotonic and hybrid composites Sample Fiber Orientation angle (degree) Total area of fiber (TAF) Total area of specimen (TAS) TAF/TAS Orientation ratio (90 /0 ) G10J100 Glass G10J30 Glass Jute Figure 15. Superposition of all the markings indicating the distribution of glass fibres in different orientations composite was found to be more than 5.0, which indicates that the jute fibres are highly oriented towards the transverse direction. This highly anisotropic fibre orientation in the hybrid composites could be the main reason for the lower tensile strength especially when high jute content is present. The weak jute-pp interfacial properties further compounds to the failure of the fibres to effectively act as reinforcements and stress dispersion agents. Therefore it is possible that crack initiation occurred at the core region during tensile testing, which is similar to a specimen that has been notched from the inside. CONCLUSION The hybridisation of natural fibre composites by the addition of glass fibres can be an effective solution to improve the mechanical properties as well as consistency of the material. It was found that the hybrid composites could attain similar tensile strength to that of GF/PP composites with the incorporation of up to 20 wt.% jute fibres. Furthermore, a significant improvement in toughness could be observed in the hybrid composites compared to monotonic jute/pp composites, although a gradual deterioration in impact resistance was imminent at high jute fibre contents. Microscopic observations revealed that the distribution and orientation of glass and jute fibres would be different with increasing jute fibre content. A preferential distribution of glass fibres towards the skin and jute fibres towards the core was found when the jute fibre content reached 30 wt.%. This distribution would thus affect fibre orientation as well, i.e. glass fibres would tend to orientate parallel to flow direction while jute fibres would exhibit a more random orientation. This could be the cause of the severe deterioration of strength in composites with high jute fibre loadings. REFERENCES Broz M.E., VanderHart D.L., and Washburn N.R., Biomaterials, 24 (2003) Sheth M., Ananda Kumar R., Dave V., Gross R.A., and McCarthy S.P., J. Appl. Polym. Sci., 66 (1997) George J., Sreekala M.S., and Thomas S., Polym. Eng. Sci., 41 (2001) Nabi Saheb D. and Jog J.P., Adv. Polym. Tech., 18 (1999) Jain S. and Kumar R., J. Mater. Sci., 27 (1992) Van de Weyenberg I., Ivens J., De Coster A., Kino B., Baetens E., and Verpoest I., Comp. Sci. Tech., 63 (2003) Gauthier R., Joly C., Coupas A.C., Gauthier H., and Escoubes M., Polym. Comp., 19 (1998) Gassan J. and Bledzki A.K., App. Comp. Mater., 7 (2000) Lafranche E., Krawczak P., Ciolczyk J.P., and Maugey J., Adv. Polym. Tech., 24 (2005) Denault J., Vu-Khanh T., and Foster B., Polym. Comp., 10 (2004) Bijsterbosch H. and Gaymans R.J., Polym. Comp., 16 (1995)