Determination of the mechanical properties of a biodegradable thermoplastic – polylactic acid and comparison with those of polyethylene

Abstract

Polyethylene (PE) bags have become the most widely used plastic commodity due to the fact that they are light and have good barrier properties against water and water – borne organisms. The energy requirements for their production as compared to other materials are much lower and the effluents emitted during manufacture of PE compared to paper bags (from trees) are much lower. They are however non-biodegradable and hence are one of the top environmental pollutants. Environment friendly biodegradable thermoplastics such as Polylactic acid (PLA), produced by microorganism synthesis through the fermentation of starch can replace PE. In this study, the dynamic mechanical properties of PLA and PE were compared. A4 PLA films of thickness 20 μm were obtained from Polyfilms limited. PE films of thickness 20 μm were sourced from a retail store. The Dynamic mechanical analyzer (DMA) model 2980 was used in the Multistrain mode at different amplitudes. The storage modulus of PLA was found to be 2220.000 MPa at 50˚C and amplitude of 10μm. This was higher than that of PE which was found to be 232.500 MPa at the same temperature and amplitude. The glass transition temperature (Tg) of PLA was found to be 65 ˚C against the documented Tg of PE of -78 ˚C. At 50˚C the loss tangent values of PLA was found to be 0.055 while that of PE was found to be 0.191 indicating higher dissipation of energy in PE. Analysis of the wet samples indicated a rapid loss of storage modulus of PLA while that of PE remained fairly constant. The modulus values were also found to be independent of the amplitude of the force. The densities of PE and PLA were 920 kgm-3 and 1249 kgm-3 respectively, which are close to those of PP and PS. This indicates that the properties of PE and PLA are similar, with PLA having the advantage of superior mechanical properties as well as being fully biodegradable. The replacement of PE with PLA will aid solid waste management by eliminating the need for recycling and reduce the landfill problem.