INFLUENCE OF THE DEGRADATION DURING THE PROCESSING ON THERMAL, RHEOLOGICAL AND TENSILE PROPERTIES OF POLY(LACTIC ACID) COMPOSITES

Abstract

Poly (lactic acid) (PLA) composites with corn starch were prepared in a corotating twin-screw extruder. The influence of incorporating different  proportions of  filler  was  studied in  terms  of  intrinsic  viscosities,  oscillatory  shear  flow,  thermogravimetric  analysis  (TGA), scanning electron microscopy and tensile properties. Results showed that the kinetics of the thermo-oxidative reaction of the PLA and also of the PLA phase in the composites with corn starch under isothermal conditions in the dynamic rheometer was mainly a random chain  splitting  process.  The  thermal  and  hydrolytic  degradation  of  the  PLA  phase  in  the  composites  with  corn  starch  during  their processing  were  fitted  in  an  autocatalyzed  carboxylic  group  degradation  model.  Also,  several  models  were  employed  to  predict  the viscosity  ratio  at  constant  frequency  and  temperature,  the  tensile  strength  and  elongation  at  break  ratios  and  it  was  found  that  the experimental values in the composites lie close to those of the Maron-Pierce and Thomas, Nicolais and Narkis, and Nielsen models, respectively.  Additionally,  results  showed  that  the  composites  of  PLA  with  corn  starch  have  higher  susceptibility  to  thermal  and hydrolytic degradation during its processing than the PLA processed only by injection molding due to higher water absorption of the corn starch while in the extruder feeding. However, the enhanced degradation of composites with corn starch increased the crystallinity degree  due  to  a  higher  molecular  mobility  which  produced  similar  tensile  strength  and  modulus values  to  those  of  injection-molded PLA. Nonetheless, the complex viscosity and storage modulus at low frequencies, the elongation at break and thermal and stability both in inert atmosphere (TGA) and in air (in the rheometer) under isothermal conditions are lower.