No abstract

The influence of an accelerated hygrothermal ageing simulation test on a commercial PLA and its three subsequent mechanically-reprocessed materials was studied. The analysis was focused on the water diffusion kinetics and the physico-chemical changes induced by the hygrothermal degradation. Water diffusion proceeded faster than chain relaxation processes, as defined by a Case II absorption model. It was proved that the water diffusion rate decreased with subsequent reprocessing cycles and increased with higher hygrothermal ageing temperatures. Hydrolytic chain scission provoked significant molar mass decays and consequent general losses of thermal and mechanical performance. The rearrangement into crystalline fractions of shorter chains provoked by hygrothermal ageing was qualitatively and quantitatively followed by both Fourier-Transform Infrared Spectroscopy and Differential Scanning Calorimetry. The microstructural changes were monitored by the cold-crystallization temperature, the crystallinity degree X C and the absorbance intensity ratio I 921 /I 955. A Weibull model showed that the crystallites were formed faster at higher reprocessing cycles and at lower hygrothermal ageing temperatures. All these effects were particularly significant for PLA reprocessed more than one time.

The performance of biocompositesof polyhydroxybutyrate-co-valerate (PHBV) and sisal fibre subjected to hydrothermal tests at different temperatures above the glass transition of PHBV (TH= 26, 36 and 46 ºC) was evaluated in this study. The influences of both the fibre content and presence of coupling agent were focused. The water absorption capability and water diffusion rate were considered for a statistical factorial analysis. Afterwards, the physico-chemical properties of water-saturated biocomposites was assessed by Fourier-Transform Infrared Analysis, Size-Exclusion Chromatography, Differential Scanning Calorimetry and Scanning Electron Microscopy. It was found that the diffusion rate increased withboth temperature and percentage of fibre, whereas the amount of absorbed water was only influenced by fibre content. The use of coupling agent was only relevant at the initial stages of the hydrothermal test, giving an increase in the diffusion rate. Although the chemical structure and thermal properties of water-saturated biocomposites remained practically inherent to hydrolytical degradation, the physical performance was considerably affected, due to the swelling of fibres, which internally blow-up the PHBV matrix, provoking cracks and fibre detachment.