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Composites of sugarcane bagasse in poly (vinyl chloride) (PVC) matrix were produced by a compression moulding method followed by various heat treatment processes, involving slow cooling (annealing), fast cooling (quenching), and re-heating to a temperature below Tg. The effects of the thermal history were examined by the measurement of tensile strength and elongation at break as well as by differential scanning calorimetry (DSC). It was observed that the heat treatments affect the elongation at break of unfilled PVC significantly with less significant effect on the tensile strengths. In contrast, various tensile strengths of sugarcane bagasse PVC composites were observed after various heat treatments with less significant effect to the elongations at break. In addition, recycling of the composites may erase the effect of thermal history. Uniform tensile properties of the composites were achieved after recycling process.
Keywords: Heat treatment, recyclability, bagasse, PVC, composite.
Mechanical properties of a polymer are affected by large number of variables. Beside the chemical composition and molecular weight, a number of thermally induced phenomena, such as crystallization and physical aging may contribute to the microstructure and thus influence physical as well as mechanical properties [1,2]. The structure of a polymer can vary depending on its thermal history, which is involving temperature and time and, as a result, its properties can vary as well . It is therefore necessary to study the effects of thermal history on the properties of a polymeric material.
Meanwhile, natural fibre reinforced polymer composites have attracted a great attentions and interests in recent years. One of the major reasons is the considerations of developing an environmentally friendly material. Moreover, natural fibre composites have advantages over currently used glass fibres in fibre reinforced composites, which are including their low cost, low energy consumption, zero CO2 emission, low abrasive nature, low density, non-toxic, and high as well as continuous availability [4-7].
Although there are a large number of research on various natural fibre composites [4,8-16], the effects of thermal history on the properties of natural fibre composite, especially sugarcane bagasse/PVC, has not been widely discussed. In this study, the tensile properties of heat treated PVC and its composites with sugarcane bagasse were evaluated to observe the effect of thermal history on the tensile properties of sugarcane bagasse/PVC composites.
Unplasticised poly (vinyl chloride) compound (PVC) IR045A supplied by Polymer Resources Sdn. Bhd., Kelang, Selangor, Malaysia was used in form of pellets. It consists of medium molecular weight PVC with some additives. The studied sugarcane bagasse (Saccharum officinarum) was a residue of the sugarcane milling process which is obtained from sugarcane juice makers in Malaysia. The bagasse was sun-dried and its fibres were extracted and chopped in a knife-ring flaker followed by sieving to obtain 40 mesh size of fibres.
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 [8,9]. Due to its mechanical characteristic, sugarcane bagasse rind (SBR) is used in this study.
Preparation of composites and samples
PVC and SBR were compounded in 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 SBR for the total mixing time of 15 minutes. In this study, the composites were prepared in 50% weight of the rind fibre.
Hot pressing was then carried out at a temperature of 170oC for 12.5 minutes, following by cooling the mixture under pressure to room temperature. The final products were in the form of plates with dimensions 15 cm x 15 cm x 1 mm.
Heat treatments were executed in four different processes, as described in figure 1. After hot-pressing, the products were cooled to room temperature in two different cooling rates. Fast cooling (quenching) was conducted by placing the products between two water circulated platens and pressed. Slow cooling (annealing) was performed by turning off the heating module of hot-press machine and let the composites cooling to room temperature over a period of 7-9 hrs. The quenched product was denoted by HP-Q, while the annealed was denoted by HP-A. Three plates of fast cooled and one plate of slow cooled composite were prepared.
Two plates of fast cooled composites were then re-heated to 60 oC and hold for 30 min. One of them was then quenched and the other was annealed. In this process, the quenched was denoted by T-Q and the annealed was denoted by T-A.
All of the heat-treated materials were crushed into small pieces. The pieces were then fed into thermal mixer at 170 oC for 10 minutes. The rest procedure was following the preparation of composites (hot-press and cutting).
Tensile test samples were cut using a dog-bone dumbbell as per ASTM D638. An Instron 3365 machine was used for tensile testing with a crosshead-speed of 2 mm/min. Tensile strength, modulus and strain at break were calculated and recorded. The report was based the average values of at least five measurements.
Differential scanning calorimetry (DSC)
DSC thermogram were recorded using a Mettler Toledo DSC apparatus with samples weighing 5-10 mg, under a nitrogen atmosphere with the heating rate of 10 oC/min.
RESULTS AND DISCUSSION
Effects of thermal history on tensile properties of PVC
Tensile test results of PVC composites after various heat treatments are presented in Figures 2 to 4. It is observed that there is no significant effect of thermal history on the tensile strength and modulus of PVC. Various heat treated PVC performed uniform values of tensile strength and modulus (see Figures 2 and 3). Various results, however, were observed on the strain at break of PVC after various heat treatments. The highest value of strain at break strength was performed by the HP-Q specimen followed by T-Q, T-A, and HP-A, respectively. It indicates that the thermal history influences more on tensile strain at break rather than on tensile strength and modulus.
The recorded DSC thermograms of PVC after various heat treatments are shown in Figure 5. The exothermal peak observed on the heated samples at a temperature above Tg (between 80-90 oC) is recognized as crystallisation peak which is considered to be caused by rapid crystallisation from the amorphous part and occur when PVC is heated above Tg [17,18]. Hence, higher area of the peak indicates lower crystallinity of the sample at ambient temperature. Figure 5 shows that the crystallisation peak was revealed on HP-Q, T-Q, and T-A samples. It is an evidence that there is no direct effect of heat treatment below Tg on the crystallisation. On the other hand, different result was performed by PVC that was directly slow cooled after hot press (HP-A). There is no crystallisation peak observed on the thermogram of HP-A sample, indicating higher crystallinity of HP-A sample as compared to the others. As a result, low tensile strain at break was obtained.
Another peak that is interesting to observe is the endothermal peak which was revealed at the temperature below crystallisation peak (between 60-80 oC). This peak is associated with free volume of the amorphous parts. Higher area of the endothermal peak indicates lower free volume formation of the sample at room temperature . Figure 5 shows that the endothermal peak was revealed in all specimens with various areas. It is observed that the endothermal peak area of fast cooled samples (HP-Q and T-Q) were lower than that of slow cooled samples (HP-A and T-A), indicating that the endothermal peak was influenced by cooling rate as well as low temperature heat treatment (below Tg). The fact that lower cooling rate results in higher area of endothermal peak indicates that a glassy polymer with a smaller free volume is formed on lower cooling rate, which is in agreement with the results of other researchers [1,17,18]. The small free volume was also resulted in low strain at break of the polymer.
Effects of thermal history on tensile properties of SBF/PVC composites
In contrast with tensile properties of heat treated PVC, The heat treated SBF/PVC composites perform various results in tensile strength and modulus (see Figure 6 and 7) with uniform results in the tensile strain at break (see Figure 8). It is interesting that there was a huge difference between the tensile strength of HP-Q composites with that of HP-A composites (Figure 6). Moreover, the tensile strength of HP-A composites was lower than that of PVC after the same heat treatment process (26.90 MPa and 39.63 MPa respectively). Poor tensile strength of a composite compared to the matrix indicates that the layer between fibre and matrix are not able to transfer the stress effectively, as a result of poor adhesion . This result indicates that cooling rate affected the fibre-matrix adhesion quality, which can be explained by the fact that there is a different rate of shrinkage between fibre and matrix. The different rate of shrinkage or contraction may develop an internal stress at the interface layer and, at worst, separating the two substances that can be observed as a crack that decreased the strength of the composite. In this case, crystalline cellulose that is containing SBF perform lower rate of shrinkage compared to the semi-crystalline PVC. Higher internal stress was achieved at low cooling rate due to complete shrinkage of the polymer. At high cooling rate, the material had been â€œfrozenâ€ before the shrinkage occurred completely. For that reason, fast cooling resulted better tensile strength of composites compared to slow cooling.
Similar trend was also found on the values of tensile modulus (Figure 7). Tensile modulus was calculated as ratio of strength and strain at the elastic region. Hence, it is also affected by the stress transfer efficiency. The decrease of tensile modulus of annealed composites, however, was not as large as the decrease of tensile strength. The value was comparable with that of PVC with the same thermal history. This is due to the fact that the tensile modulus is calculated at lower strength, before the crack propagation was occurred. Hence, the presence of cracks do not influence the tensile modulus as much as it affected the tensile strength.
The DSC thermograms of the composites are represented in Figure 9. It can be seen that the curves were different from the those performed by unfilled PVC. However, the effect of different cooling rate can be observed clearly. When endothermal peak was revealed on fast cooled samples, the slow cooled samples revealed the baseline shift due to glass transition. Hence, the thermal history was also influence the structural order of the matrix in SBF/PVC composites, such as crystallinity and free volume, which lead to variation in shrinkage rate of the matrix.
Effects of recycling
Figures 10 to 12 show that there is no difference among tensile properties of recycled composites. The effects of HP-A, T-Q, and T-A treatments that previously was conducted to the samples is no longer revealed. It indicates that recycling process may erase the effect of heat treatment. When the composites were recycled, the thermal history was â€œforgottenâ€, indicating the change of properties occurred without permanent modification of the structure of the material . Moreover, it can be observed that the tensile properties of recycled composites were lower than that of non-recycled HP-Q composites, indicating there was degradation of material during recycling process.
The incorporation of sugarcane bagasse rind into PVC has changed the effects of thermal history on mechanical properties of the material. In an absence of fibre, thermal history affects more on the strain at break rather than on other tensile properties, such as tensile strength and modulus. Contrary, in the presence of fibre, thermal history affects more on the tensile strength and modulus rather than on strain at break. The variation strain at break of PVC in various thermal histories was mainly caused by structural order of the polymer after heat treatment. Heat treatment above Tg caused change in the crystallinity, while heat treatment below Tg affected the free volume. On the other hand, the variation of tensile strengths an moduli of SBF/PVC composites was mainly influenced by the various internal stress at the fibre-matrix interface layer due to different rate of shrinkage.
Moreover, quenching process after hot pressing was the best processing method for both unfilled PVC and SBR/PVC composites. It offered the best tensile properties. Lower cooling rate would result lower strain at break of neat PVC and lower tensile strength and modulus of SBF/PVC composites.