Tensile and impact properties of sugarcane bagasse pith and rind filled poly(vinyl chloride) composites
ABSTRACT: Sugarcane bagasse is divided into two main components, pith and rind, with “pith” representing the inner part of the sugarcane bagasse and “rind” as the outer part. In this study, the tensile and impact properties of untreated pith unplasticised poly (vinyl chloride) composites were compared to that of untreated rind composites in the same matrix with variation of fibre content. The composites were produced by a compression moulding method, and the fibre contents were 10%, 20%, 30%, and 40% in weight. Tensile and impact tests were carried out to measure the mechanical properties of the resulting composites. It was observed that the tensile strength and modulus of rind/PVC composites are higher than the neat PVC at composite fibre contents of 30% and 40%. Additionally, the rind composites exhibited superior strength and stiffness in comparison with the pith composites.
Composites: polymer matrix (A), mechanical properties (E), thermal analysis (G).
Sugarcane bagasse (SB) is a residue of the sugarcane milling process. During this milling process, the sugarcane stalk is crushed to extract sucrose, and the process produces a large volume (32%) of bagasse. In Malaysia, over a million tonnes of sugarcane were produced in 2002, and hence, bagasse is easily obtained as a waste product . As a result, utilisation of sugarcane bagasse may contribute to environmental and economic development.
Poly (vinyl chloride) is one of the most well-known polymers and is commonly abbreviated as PVC. As one of the cheapest thermoplastics, PVC is used in a broad range of applications and its use has grown rapidly. This material is 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 adjusted by the addition of plasticiser and other additives for production of a rigid or flexible product [2, 3]
The stalk of the sugarcane plant includes an outer rind and inner pith. The rind is made up of a hard fibrous substance surrounding a central core of pith, which is softer due to a spongy structured component. Studies of sugarcane bagasse PVC composites have been reported by other researchers . However, studies focusing on the use of pith and rind components , especially in thermoplastic matrix, have not been widely reported. The only study of the use of pith and rind components in polymer composites with a thermoset used as the matrix .
In this study, the tensile properties of sugarcane bagasse pith and rind were evaluated as well as the tensile properties of pith and rind PVC composites with variation of the bagasse content. The effects of bagasse content and stalk components (pith/rind) on the tensile properties of the composites were examined.
2 Materials and methods
The matrix used in this study was unplasticised poly (vinyl chloride) compound (PVC) IR045A supplied by Polymer Resources Sdn. Bhd., Kelang, Selangor, Malaysia. This compound consists of medium molecular weight PVC, processing aids, lubricant, and certain additives. The studied sugarcane bagasse (Saccharum officinarum) is a residue of the sugarcane milling process gathered from sugarcane juice makers in Malaysia. For the purpose of single fibre tensile testing, single fibres of pith and rind were extracted manually. For the purpose of composite production, fibres were extracted and cut using a knife-ring flaker. The fibres were sieved and 40 mesh sizes of fibre were used in this study.
2.1 Preparation of Composites
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 SB. The total mixing time was 15 minutes. In this study, 10%, 20%, 30%, and 40% weight fractions of pith and rind fibre were prepared.
After the thermal mixing process, 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 dimensions 15 cm x 15 cm x 1 mm and 15 cm x 15 cm x 3 mm. The former was used for tensile testing, while the latter was used for impact testing.
2.2 Single fibre tensile testing
The single fibre tensile test specimens were prepared based on the ASTM D3379 procedure for single fibre tensile testing. The specimen was then tensile tested with a 5 kN load-cell equipped Instron 3365. The gauge length was 50 mm with a cross-head speed of 2 mm/min. A total of 30 specimens were tested for each component.
2.3 Tensile testing
Specimens for tensile testing were cut using a dog-bone dumbbell as per ASTM D638. The specimen was then tensile tested using the Instron 3365 machine with a crosshead-speed of 2 mm/min. Tensile strength and tensile modulus were calculated and recorded, and averages of five specimens were reported.
2.4 Impact testing
Hot pressed products with 3 mm thickness were cut using a band saw machine to form a rectangular shape with dimensions 5.5 cm x 12 cm x 3 mm. A notching cutter was used to prepare a v-notched impact specimen conforming to ASTM 256. Izod impact testing was then executed using a TMI Monitor/Impact tester with a maximum pendulum capacity of 1 Joule.
3 Results and discussion
3.1 Single fibre tensile testing
The tensile strength of SB was confirmed to be very non-uniform. The test results show that the tensile strength of pith and rind varied from 13 MPa to 76 MPa and 58 MPa to 430 MPa, respectively. The Weibull distribution was used to describe and analyse the strength of the fibres. From the two-parameter Weibull model, the cumulative failure probability is given by :
where s is the tensile stress, s0 is the Weibull scale parameter or the characteristic stress value, and m is a Weibull parameter that measures the variability of the fibre strength. A larger value of m means smaller scatter in strength value.
The cumulative failure probability, Pi, under a particular strength was approximated by :
where n is the number of fibres that failed at or below a certain value of stress and N is the total number of fibres measured.
A plot of ln [-ln(1-P)] versus ln (s) can be drawn for calculating the Weibull parameters, s and m, where
3.2 Tensile testing of composites
2 shows that the tensile strengths of both pith/PVC and rind/PVC composites at low fibre content (10%) were lower than that of neat PVC. However, the tensile strength of rind/PVC composites was observed to increase with increased fibre content. The tensile strengths of 30% and 40% rind/PVC composites were higher than that of neat PVC. In addition, the observation of decreased tensile strength of pith/PVC composites continued up to a fibre content of 30%, but the tensile strength of 40% pith/PVC composite was higher than that of 30%. However, this value is still significantly lower when compared to that of neat PVC. Despite the fact that the tensile strength of pith/PVC composites was higher than that of rind/PVC composites at 10% fibre content, the tensile strength of rind/PVC composites were superior at higher fibre content. This is due to the higher tensile strength of rind fibre compared to that of pith fibre.
The low of tensile strength of rind/PVC at low fibre content (10%) can be explained by the fact that there is a big gap between pulled out fibre and PVC in the composites as shown in 3(a), which is not found at the pith/PVC composites ( 3(b)). This gap indicates low interphase interaction between fibre and the matrix and is suspected as the origin of rupture or crack initiation . On the other hand, a fibre may also act as crack inhibitor . When a load is applied to the composite, a crack can be arrested by the fibre and prevent rupture from occurring. Hence, a higher load is needed to break up the composite at the increase of fibre content. Higher fibre content means higher density of crack inhibitors, and as a result, the tensile strength of the composite is increased.
At low fibre content, the effect of crack initiation was more dominant as compared to the effect of crack inhibition. As a result, the tensile strength of the composite was decreased. However, at high fibre content, the effect of crack inhibition was more dominant and a higher tensile strength of the composites was observed.
Another explanation is that the decrease of tensile strength, especially at low fibre content, may be caused by the reduction in area that participates in transfer of the loading stresses, called the effective cross-sectional area. If there is perfect adhesion between fibre and matrix, all of the loading stresses would be transferred to the fibre and no reduction of effective cross-sectional area would result. In the case of no adhesion, the interface layer between fibre and matrix are not able to transfer the stress . In the actual case, there are variable qualities of adhesion between matrix and fibre, which range from poor (almost no adhesion) to excellent (almost perfect adhesion). This variable quality of adhesion affects the efficiency of stress transfer. Better quality of adhesion results in higher efficiency of stress transfer.
The effect of fibre content on the tensile modulus of the composites is presented in 4. It seems that there is a similar trend observable between the tensile modulus and tensile strength, especially for pith/PVC composites. The effect of crack initiation and crack inhibition, however, is not adequate to explain the trend of tensile modulus. This is because tensile modulus is calculated as ratio of strength and strain at the elastic region, while the phenomena of rupture occurs beyond the elastic region of the tress-strain curve. The trend was mostly affected by the efficiency of stress transfer. There was adequate quality of adhesion bonding between the PVC and bagasse fibres to increase the tensile modulus when the fibre content was increased. However, the reduction of effective cross-section results in a lower tensile modulus as compared to that of neat PVC, especially at low fibre content.
3.3 Impact testing of composites
5 clearly shows that the impact strengths of both pith/PVC and rind/PVC composites are lower than that of neat PVC. This is not surprising, since both pith and rind are less ductile as compared to PVC. Like other mechanical properties, the impact strength of composites is highly affected by the fibre. The maximum tensile strain of PVC may reach more than 50%, while that of pith and rind are below 5%, as presented in Table 1. This low ductility results in low energy required to break the fibre. Because the tensile strain of pith fibre is higher than that of rind fibre, the impact strengths of pith/PVC materials are higher than that of rind/PVC composites.
The fibre content and fibre source (pith/rind) of sugarcane bagasse affects tensile and impact properties significantly. In general, the incorporation of low content pith and rind fibre decreases the tensile strength and modulus of PVC. However, the tensile strength and modulus increase with the fibre content. Rind/PVC composites exhibited superior strength and stiffness as compared to pith/PVC composites. At fibre contents of 30% and 40%, the tensile strength and modulus of rind/PVC composites are higher than the matrix material. In contrast, the impact strengths of both pith/PVC and rind/PVC were lower than that of PVC.
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