Abstract: The effect of glycerol on the mechanical, thermal and water absorption properties of thermoplastic tapioca starch reinforced rice straw fiber grinded in the range below 100 μm was investigated. Compound formulation was done on twin screw extrusion and the extrudates were pelletized. The compounded samples were processed using compression moulding to form sheet at 150°C for tensile and thermal gravimetric. SEM studies were investigated on fracture surface of composites. Tensile test was done in accordance to ASTM D638. Thermal analysis was used to determine the degradation temperature of composite. Increasing the amount of glycerol increased the tensile strength of composite up to 30 phr of glycerol, however, increasing to 40 and 50 phr glycerol was decrease the strength but elongation at break of composites was increased with increasing glycerol content. Thermal Gravimetric Analysis (TGA) result showed that increased amount of glycerol content lead to decrease in degradation temperature of the composite and SEM micrograph showed that good dispersion and adhesion between rice straw fiber and starch.
INTRODUCTION
Plastics materials have been used widely worldwide and very resistant to micro-organism and other natural degradation forces because of their chemical and physical nature which ensures longevity and long-lived properties. This will led to environmental problems and also in conjunction with the land shortage problems for solid waste management (Shah et al., 2008).
Starch is an inexpensive natural biopolymer which is totally biodegradable in a wide variety of environments. Different approaches have been adopted to use starch, in combination with other synthetic thermoplastic polymers, for the production of totally or partially biodegradable materials. Natural starch exhibits a pronounced macromolecular structure which is suitable for the production of bioplastics. However, prior to production of such materials, the structure of native starch should be suitably modified. This is necessary because starch is a multi hydroxyl polymer. There were vast intermolecular and intramolecular hydrogen bonds in starch that native starch is considered not a true thermoplastic. But in the presence of plasticizer at high temperatures (90-180°C) and under shear it readily melted and flowed, allowing for its use as an injection, extrusion or blow molding material, similar to most conventional synthetic thermoplastic polymers (Cordoba et al., 2008; Ma et al., 2007).
Thermoplastic Starch (TPS) is relatively new material for application as biodegradable plastic. It is used alone or compound, usually with polar synthetic polymers, in contents that usually exceed 50% (Shogren et al., 1993). However thermoplastic starch have two main disadvantages. When compare to most plastics currently in use which it mostly water soluble and has poor mechanical properties. The water resistance may be improved by mixing it with biodegradable polymers as studied by Averous et al. (2000) Adding crosslinking agent such as tri sodium, tri-meta phosphate also can improve water resistance of thermoplastic starch as reported by Demirgoz et al. (2000).
Another option is the used of natural fibers as reinforcement for thermoplastic starch. Ma et al. (2005) have proved that natural fiber not only improved water sensitivity of thermoplastic starch but also increase its strength. This fact has been attributes to the chemical similarity of polysaccharides and plant fibers, providing good compatibility between them.
Rice straw was usually disposed of by open field burning because it is a cheap disposal method and it is also helps to avoid propagating fungal stem rot disease caused by Sclerotinum Oryzae. Although open field burning is a convenient method but it produces visible smoke and most dangerously it produce silica emission which if not specifically monitored can pose health hazard (Kadam et al., 2000).
Therefore, in this study rice straw are being prepared together with tapioca starch and the glycerol effect onto this composite have been investigated.
MATERIALS AND METHODS
Materials: The starch used for preparation of composites was conventional tapioca starch. The rice straw fibers RSF used as reinforcing agent were grinded and sieved into size below 100 μm. The plasticizer (glycerol) was purchased from Fisher Chemical USA.
Mixing process: The starch, fiber and glycerol were first premixed using hand. After that the plasticization and melting of the blend were carried out in a twin screw extruder set temperature of 90°C and screw velocity 100 rpm. Four different formulations were prepared with varying glycerol content in each formulation.
Compression moulding: Test specimens for evaluation of mechanical were made by compression molding. The molding temperature used was 150°C and with 3 min preheat and 10 min compression time.
Tensile test: Tensile test was made according to technical standard method ASTM D638 using Llyod machine. Measurements were carried out at 25°C and crosshead rate 10 mm (min-1). For each sample 5 specimens were tested.
Thermogravimetric Analysis (TGA): Thermogravimetric Analysis were carried out using Perkin Elmer 7 model TGA instrument under nitrogen atmosphere at a heating rate of 20°C min-1. The range of scanning temperature was from room temperature to 800°C.
Scanning electron microscopy: The native starch and the fracture surfaces of extruded composites samples were examined with a JSM-6360 scanning electron microscope, operating at an acceleration voltage of 15 kV. The extruded composite were fractured in liquid nitrogen and stuck onto the aluminium stubs and then vacuum coated with gold for SEM.
RESULTS AND DISCUSSION
Mechanical properties: The mechanical properties of TPS/RS with different glycerol content were shown in Fig. 1 and 2. From the graph shown that with increasing the glycerol content form 20 to 30 phr, the tensile strength of the composite was increased.
| Fig. 1: | Tensile strength of TPS/Rice straw with different glycerol content |
| Fig. 2: | Elongation at break (%) of TPS/Rice straw at different glycerol content |
However, after addition of 40 phr glycerol and 50 glycerol the tensile strength tend to decreased. Plasticizer could form the hydrogen bond interaction with starch and weaken the interaction of starch molecules and slippage movement among starch molecules was facile, when TPS materials experience the tensile testing. Therefore, with increasing of glycerol content, tensile strength of composite decreased while the elongation at break improved. The other reason might be due to excess glycerol are being absorbed by rice straw thus make the rice straw soften and resulted poor mechanical properties of 40 and 50 phr glycerol content. However, when the glycerol is not enough to destructurized starch, thus resulting poor mechanical strength and elongation at break on 20 phr glycerol.
Thermogravimetric analysis: Figure 3 presented the TG experimental results. There is no significance different on the trend of the graph, however the there is slightly on different thermal behaviour of composites between 200 to 350°C. Between this temperature glycerol start to evaporate and resulted different percent weight loss to composites based on their glycerol content. In the graph the mass loss below 100°C was mainly ascribed to water loss. Glycerol start to evaporate on 200°C and fully evaporate on 300°C.
| Fig. 3: | TG curves of glycerol, rice straw, tapioca starch and TPS/Rice straw composites with different glycerol content |
At this temperature, it can be conclude that increasing of glycerol content was decreasing thermal decomposition of composite however the different is not significance unless the huge amount of glycerol being added into composites.
Scanning Electron Microscopy (SEM): The microscopic morphology of pure starch and starch granule are compared in Fig. 4. It was shown afew starch granule was not melted in 20 phr glycerol content in composites thus resulted poor mechanical properties. Plasticizer are known to distrupt intermolecular and intramolecular hydrogen bonds and makes the native starch plastic. In the present study, due to the high shear and temperature conditions with the action of glycerol, native tapioca starch granules were molten or physically broken up into small fragments, glycerol penetrated into the starch granules and formed hydrogen bonds with starch molecules which weaken the strong action of starch intermolecular and intramolecular hydrogen bonds (Bergo et al., 2008).
| Fig. 4: | SEM morphology or tapioca starch and TPS/Rice straw at different glycerol content (a) tapioca starch (b) 20 phr lycerol (c) 30 phr glycerol (d) 40 phr glycerol and (e) 50 phr glycerlol |
It can be seen in the Fig. 4a-e that all that all fibers were well disperse and no fibers pull out or debonding was observed even in low glycerol content (20 phr). This behaviour might be due good interaction between starch and fiber because both of them posses OH group in their molecular structure.
CONCLUSION
Homogenous extruded TPS/Rice straw pellets were prepared and successfully been extruded and compression moulded into sheets. The tensile strength of TPS/rice straw increased up to 30 phr glycerol but decreases by the addition of 40 and 50 phr of glycerol, however elongation at break of the composites increased with increasing of glycerol content. TGA results showed that increasing amount of glycerol of composites will decrease thermal stability, however the different was not significance. SEM studies showed that, starch was fully plasticized above 30 phr glycerol, however certain area of TPS in the composites on 20 phr glycerol was not plasticized which resulted poor mechanical properties in the composites.
ACKNOWLEDGMENTS
The authors thank Universiti Teknologi Malaysia and Polymer Engineering Department for the laboratory facilities and technical support given throughout the research work. We also like to express our gratitude to MOSTI for providing research grants VOT 79284 and Research Management Center-UTM (UTM-RMC) for managing the fund.