Utilization of cassava and sweet potato starch to produce degradable polystyrene plastic for packaging

This experiment was made to study the feasibility to produce biodegradable packaging using polystyrene, a dominant packaging material which was incorporated with commonly available starch in Thailand, such as cassava and sweet potato starch. The starch content varied from 40, 60, 80 and 100 PHR (Part per Hundred Resin) with oil contents varying from 0 (zero), 5, 10, 15, 20, 25 and 30 parts. Mechanical properties, such as the tensile strength and the elongation at break, together with physical properties, such as specific gravity, density, hardness and melt flow index were used to determine the optimum proportion of starch to be incorporated with the polystyrene polymer. Starch incorporation at 60 PHR with 5 parts oil content was found to have the optimum proportion. For cassava starch, the tensile strength was 201.35 kgf/square cm, the percentage elongation at break was 2.747, the hardness value was 75.3 shore-D and the melt flow index (MFI) was 24.4 g/10 mins. For sweet potato starch, the tensile strength, the percentage elongation at break, the hardness value and the MFI was 165.70 kgf/square cm, 3.208, 75.10 shore-D and 27.19 g/10 mins, respectively. Three strains of Aspergillus spp. were used to study biodegradability characteristics, namely S. Artjariyasripong 1-2, a soil microorganism which is an amylase producer at 12.5 unit/ml, Niger spp. (ATCC) 9642, recommend for determining deterioration of plastic and J. Sukhumavasi No. 527, which has the highest activity for amylase production at 30.92 unit/ml. The comparison between the degradability effects on virgin polystyrene (control), polystyrene with additives (control with oil) and the starch/plastic mixture, during the degradable period, showed that microorganisms can not consume plastic since no change was observed on the control. The loss of the polarization cross on the starch granules indicated the biodegradability effects on starch/plastic mixtures, since the starch was consumed by the microorganisms. No differences in biodegradability was found between the microorganisms used. The factors effecting the rate of biodegradability were found to be the starch content and the quantity of oil incorporated in the mixture. An increase in these two factors, increased the rate of biodegradability.