Oxidative Degradation of Thermoplastic Corn Starch Induced by Ultraviolet Radiation

M.M. QUISPE; O.V. LÓPEZ; M.A. VILLAR

Rio de Janeiro

Conferencia; Fifth International Conference on Natural Polymers, Biopolymers and biomaterials: Applications from macro to nanoscale.; 2017

Universidade Federal de Rio de Janeiro

The interest on biopolymers is due mainly to the large accumulation of non-degradable materials in the environment. Among other biopolymers, starch is a good candidate because of its natural origin, biodegradable character, low cost, and availability. In this work, it was studied the changes induced on thermoplastic corn starch (TPS) by ultraviolet (UV) radiation, analyzing the material structural properties and mechanical behavior. TPS was obtained through the thermo-mechanical processing of native corn starch with water (45 %w/w) and glycerol (30 %w/w) as plasticizers. Films were obtained by thermo-compression and conditioned (ASTM D618-05) to reduce the material fragile nature, obtaining the specimens to be tested. UV photodegradation process was performed following the method described by ASTM D5208-91, exposing TPS to 96 h UV radiation. Specimen?s weight loss was determined gravimetrically. Chemical changes were studied by Fourier Transform Infrared Spectroscopy (FTIR) and morphological modifications were analyzed by Scanning Electron Microscopy (SEM). The average molecular weight was measured by Light-Scattering (LS). Changes in mechanical properties was studied from tensile tests (ASTM D638-98). After 96 h exposure, TPS specimens presented a weight reduction of 4?6 %, attributed mainly to loss of plasticizers. By FTIR, it was detected a decrease in the band intensity of ?OH after 72 h exposure, as consequence of oxidation processes. SEM observations showed that UV radiation induced morphological changes on TPS, evidenced by an increment of the specimens cracking. The weight average molar mass of the native starch was in the order of 107 g/mol. TPS exposure to UV radiation decreased significantly its molar mass, confirming molecular degradation. When TPS was exposed during 48 h, it was detected a considerable decrease in elongation at break values (~ 85 %), indicating that TPS flexibility was reduced. On the other hand, after 48 h exposure, TPS elastic modulus was 55 times higher than those of the unexposed specimens, evidencing an increase in material rigidity. TPS maximum tensile strength was also increased by UV light, registering an increment of ~ 400 % after 48 h exposure. Results revealed that starch based materials can be degraded by exposure to UV radiation, modifying their microstructure and mechanical performance.