Elastic and viscoelastic properties of sugarcane bagasse filled poly (vinyl chloride) (PVC) were conducted by means of three point bending flexural test and dynamic mechanical analysis (DMA). Elastic modulus, storage modulus, loss modulus, and damping parameter of the composites at fibre content of 10%, 20%, 30%, and 40% in weight were determined as well as those of unfilled matrix. It was observed that there was a correlation in trend between elastic modulus and storage modulus of the composites. Moreover, the elastic and viscoelastic properties of the composites were highly influenced by fibre content.
Keywords: Viscoelastic properties, thermal properties, thermoplastics, composites.
In past few decades, there has been a growing attention in the development of new materials that involves natural resources as the raw material, especially in composite materials. Both properties and cost of various natural fibres and their composites were evaluated to determine their potential to replace glass fibre reinforced plastic composites in some applications. As compared to glass fibre, the advantages of natural fibres include their low cost, low energy consumption, zero CO2 emission, low abrasive in nature, low density, high resistance to crack propagation, non-toxic, and they are available continuously1-3.
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One of natural fibres with high availability is sugarcane bagasse, a residue of sugarcane milling process. In Malaysia, the annual production of sugarcane reaches a number of a million tonne, and it is less than 0.1% of the world annual production 4 . Nearly 30% of that number will turn into bagasse when it is crushed in a sugar factory. This procedure produces a large volume of bagasse wastes that may have an extremely harmful effect upon the environment if not suitably treated 5,6. Moreover, As the stock is abundant, the price of sugarcane bagasse is less expensive than that of other natural fibres 6
Matrix selected in this study was poly (vinyl chloride) (PVC). It is one of the most well-known and the least expensive thermoplastic polymer. This thermoplastic is used in a broad range of applications and its use has grown rapidly. Advantages of this material includes easy to fabricate, long lasting, exhibits good mechanical and chemical properties, and can be used in wide range of corrosive fluid environments. Furthermore, the properties of PVC can be customised by the addition of plasticiser and other additives for production of a rigid or flexible product 7,8. Many researchers have investigated the properties of PVC filled natural fiber composites. So far, at least there are nine variants of natural fibers that have been studied for filler/reinforced materials. They are wood, bamboo, pine, rice straw, sisal, oil palm, banana, coconut, and sugarcane bagasse9-21. However, the dynamic mechanical analysis (DMA) behavior of natural fibre filled PVC, especially sugarcane bagasse filled PVC, is barely discussed.
In this study, elastic and viscoelastic properties of sugarcane bagasse filled PVC composites were determined by means of three point bending flexural test and dynamic mechanical analysis (DMA). Static flexural and DMA behaviour of unfilled PVC as well as the sugarcane bagasse filled PVC were examined in various fibre loadings (10%, 20%, 30% and 40%).
Figure 1 Shows the flow chart of the experiment. In general, the experiment was divided into three steps of process. The steps are materials preparation, composite fabrication, and materials characterization.
The matrix used in this study was poly (vinyl chloride) compound PVC IR045A supplied by Polymer Resources Sdn. Bhd., Kelang, Selangor, Malaysia. This compound consists of a medium molecular weight PVC resin, PVC resin, processing aids, lubricant and some additives designed for general purpose application.
The sugarcane bagasse fibre studied was a residue of sugarcane milling process obtained from a traditional sugarcane juice maker in Malaysia. The stalk of the sugarcane plant was divided into two parts; outer rind and inner pith. The effect of different fibre sources (pith and rind) was observed as well as the effect of fibre content on the modulus of elasticity and viscoelastic behaviour of the composites.
Preparation of composites
Both pith and rind of sugarcane bagasse were sun-dried for 2 X 12 hours before they were fed into a ring knife flaker separately to obtain short fibres (below 3 cm in length). The fibres were then sieved to obtain more homogeneous dimension followed by oven-drying process at 80 oC for 24 hours. The 40 mesh of fibres were used in this study.
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A thermal mixing process was carried out using a Haake Polydrive R600 internal mixer at a temperature of 170oC and rotor speed of 50 rpm. PVC pellets were fed into the chamber and mixed for five minutes, followed by feeding of the fibres. The total mixing time was 15 minutes. In this study, 10%, 20%, 30%, and 40% weight fractions of pith and rind fibres were prepared.
The final stage of the composite preparation process in this study was hot-pressing. Hot pressing was carried out at a temperature of 170oC for 12.5 minutes, and the mixture was then cooled under pressure to room temperature. The final products were in the form of plates with dimension of 15 cm X 15 cm X 3 mm. The plates were then cut into rectangular shape with dimensions of 13 cm X 1.3 cm X 3 mm and 5.5 cm X 1.3cm X 3 mm for static flexural and DMA measurements, respectively.
A three point bending flexural test was conducted using Instron 3365 machine with span length of 10 cm at crosshead-speed of 2 mm/min. The modulus of elasticity was calculated from formula represented in eq. (1).
where L, b,d and m are the span length, the width of the test specimen, the depth of the span, and the slope of the tangent to the initial straight line portion of the load-displacement curve, respectively.
Meanwhile, DMA measurements were executed on a TA Instruments DMA Q800 equipment operating in a dual cantilever bending mode at 1 Hz in frequency with the span length of 5 cm. Each specimen was heated at the rate of 10 oC/min from 25 oC to 120 oC. Storage modulus (E'), loss modulus (E''), and tan ï¤ (E''/E') were measured with the function of temperature
Results and Discussion
The effects of filler on elastic modulus
The elastic modulus of rind/PVC was superior compared to pith/PVC composites at the same fibre content (see Figure 2). In addition, the elastic moduli of both pith/PVC and rind/PVC composites, in general, were observed to increase with the increased of fibre content. It is reported that the rind component consists of smaller size of hollow cavity and higher quantity of cellulose fibers as compared to those of pith 22. As a result, the rind offer better mechanical properties compared to that of pith.
The low value of elastic modulus at low fibre content (10% and 20% for pith/PVC and 10% for rind/PVC) can be explained by the fact that there was a reduction in effective cross-sectional area, which is an area that participates in transfer of the loading stresses. The loading stress can be transferred completely if only there is perfect adhesion between fibre and matrix. In the absence of adhesion, the interface layer between fibre and matrix are not able to transfer the stress 23. In the actual case, the qualities of adhesion between matrix and fibre are varied, which range from poor (almost no adhesion) to excellent (almost perfect adhesion). This quality of adhesion affects the effective cross-sectional area. Better quality of adhesion results in higher effective cross-sectional area.
The effects of filler on storage modulus
The storage modulus at elevated temperature of the pith/PVC and rind/PVC composites at various fibre contents is depicted in Figures 3 and 4. There was a correlation in trend between storage modulus at initial temperature (25 oC) and modulus of elasticity obtained from static bending test. Similar to modulus of elasticity, the storage modulus of the composites in low fibre content (10% and 20% for pith/PVC and 10% for rind/PVC) were found to be lower than that of unfilled PVC. The storage modulus of the composites, however, increased with the increase of fibre content. At the higher fibre content (30% and 40%), the storage moduli of the composites were higher than that of unfilled PVC. The correlation in trend between storage modulus and modulus of elasticity is an agreement with the result of static and dynamic moduli of jute/vinylester composites reported by Ray et al. 24. Hence, the trend of storage modulus at initial temperature was also representing the trend of the static modulus of elasticity of the composites.
It is observed that the storage modulus decreased at the elevated temperature. Moreover, a large fall in modulus was observed at a range of temperature between 70 and 90 oC, which indicates a glassy-rubbery transformation. Furthermore, it is also observed that curves of pith/PVC were shifted to the right as the fibre contents were increased. It shows that the incorporation of fibre increased the thermal stability of the PVC. On the other hand, the storage modulus of the filled systems, including the low fibre content, above Tg were higher compared to that of unfilled PVC. It indicates higher temperature stability of the composites.
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Different result was observed in rind/PVC composites. Figure 4 shows that the curves of rind/PVC composites were shifted to the left as the fibre content increased up to 30% and then bounced to the right at the fibre content of 40%. It indicates that the thermal stability of the composite decreased with the increase of fibre content up to 30% and then increased when the fibre content increased to 40%. Hence, among the four types of fibre content, 40% is the most effective fibre content to improve the thermal stability of rind/PVC composites.
In addition, the storage modulus of both in glassy and rubbery regions can be used for calculating C coefficient, which represents the effectiveness of fillers on the modulus of the composites 25.
where E'G and E'R are the values of storage modulus in the glassy and rubbery regions, respectively. Higher value of the constant C represents lower effectiveness of the filler. The measured E' values at 60 and 100 oC were employed as E'G and E'R, respectively. The value of C obtained for various fibre contents are represented in Figure 5. It can be seen that the coefficients of both pith/PVC and rind/PVC composites were decreased exponentially with the increase of fibre content. In other words, the effectiveness of the filler increased with the increase of fibre content until the fibre content of 40%. It indicates that the fibres embedded in the viscoelastic system successfully reduced the mobility and deformability of the matrix. Moreover, the coefficient of rind/PVC was higher than that of pith/PVC composites. It indicates that pith was more effective in reducing the mobility and deformability of PVC as compared to rind.
The effect of filler on loss modulus
The loss modulus, E", is defined as the ratio of stress to strain when the stress is 90o out phase with the strain and expressed in eq. 3:
where ï³0, ï¥0, and ï¤0 are maximum stress, strain at maximum stress, and the difference between applied stress and resultant strain, respectively. It measures the viscous response of material, which is the energy lost or dissipated when the material is deformed. 25. The viscous response of of pith/PVC and rind/PVC are represented in Figures 6 and 7, respectively. Both curves show that there is a loss modulus peak for each fibre content of the composites, which is attributed to the mobility of the matrix molecules 24. The maximum of loss modulus was reached as the storage modulus decreased because of the free movement in the polymeric chain at higher temperature 26. The peak of pith/PVC loss modulus, which is also considered as the glass transition temperature, Tg, shifted to the higher temperature when the fibre content increased. The polymer molecules near the surface of pith were immobilised due to various molecular interactions, which increased the Tg of the composites.
Different trend of results was performed by rind/PVC composites as compared to pith/PVC composites. When the Tg of pith/PVC composites increased with the increase of fibre content, the Tg of rind/PVC performed otherwise up to fibre content of 30%. The increase of Tg was only obtained at the fibre content of 40% , however the value was still lower compared to that of unfilled PVC. It seems that the addition of rind was less effective to immobilise the PVC molecules near the surface of rind at particular temperature due to the lack of molecular interaction between rind and PVC, especially in low fibre content. Surface interaction was observed to be better at fibre content of 40%. However, it was not good enough to increase the Tg of the system.
Another valuable object to observe is the width of the peak, that indicates the quantity of fibre-matrix interaction. When a fibre is incorporated to a polymer matrix, there will be an interaction between fibre and matrix that generates a difference in the physical state of matrix surrounding the fibres to the rest of matrix, called interface layer 25. The schematic diagram of fibre, matrix, and the interface layer is represented in Figure 8. The volume of the interface layer, which is not present in the unfilled matrix, may cause an additional transition resulting broader glass transition behaviour. Higher volume of the interface layer would result the higher width of loss modulus peak.
The increase of peak width at half height of loss modulus corresponding to the increasing of fibre content up to 40% is depicted in Figure 9. It indicates the increment of interface layer volume as the fibre content was increased. Moreover, it also indicates that the fibres were distributed well and optimum condition was not reached at fibre content of 40. When the fibre content is beyond the optimum condition, the peak width of the loss modulus would decrease as a consequence of fibre agglomeration and an increase in fibre to fibre contact 27. It seems that there is no significant difference between pith/PVC and rind/PVC composites in the trend of loss modulus width. It should be noted that the broadening width of peak in loss modulus curve was a result of quantity (volume) of interface layer instead of its quality, such as the strength of fibre-matrix adhesion.
The effects of filler on damping parameter
Damping parameter, tan ï¤, is the ratio of loss to storage modulus. It is dimensionless property that related to the ability of a material to absorb a vibrational energy. While storage and loss moduli indicate elastic and viscous responses, damping parameter is an indicator of viscoelastic response of a material. As well as the peak of loss modulus curve, the damping peak occurs in the region of glass transition where a material behaviour transforms from a rigid (glassy) state to a more elastic (rubbery) state and can be used to assign the Tg 26. The value, however, may be different 24. Hence, it is important to write which peak was used when reporting Tg..
Figures 10 and 11 represent the damping parameter curves of pith/PVC and rind/PVC composites, respectively. It can be clearly observed that the height of peak of both composites decreased with the increase of fibre content. In a composite system, the composition of composting materials highly affects the damping parameter. When the elastic fibre is incorporated, the system would be more elastic. Since damping parameter is the ratio of viscous and elastic responses, it would decrease with the increase of the content of elastic substance.
Moreover, it is observed that the trend of Tg based on tan ï¤, have an agreement with that based on loss modulus. The values, however, were higher than those obtained from loss modulus curve. The peaks of damping parameter were located at the temperature of more than 80 oC (Figure 10) while the peaks of loss modulus were located at the temperature less than 80 oC (Figure 6).
Elastic and viscoelastic properties of sugarcane bagasse filled PVC composites are highly affected by the fibre source and fibre content. Incorporation of low fibre content of pith or rind into PVC may decrease elastic properties, by means of modulus of elasticity and the storage modulus of PVC at initial temperature. However, the elastic properties of composites increases with the increase of fibre content. Increasing the fibre content may increase the effectiveness of the filler on the elastic and storage modulus increment. In addition, the rind/PVC composites offer superior elastic response compared to pith/PVC composites. The pith/PVC composites, however, offer better thermal stability and interfacial bonding compared to rind/PVC composites. In term of viscoelastic properties, it is observed that the incorporation of fibres may decrease the damping parameter of the composites.
The authors wish to thank Universiti Putra Malaysia for financial support of this study and fellowship funding for the main author through the Research University Grant Scheme (RUGS; Project no: 05/01/07/0190RU) and Graduate Research Fellowship (GRF), respectively. Part of this paper have been submitted to The International Conference on Plant Equipment and Reliability 2010 (ICPER2010), Kuala Lumpur, Malaysia.