Synthesis and evaluation of novel polymer modifiers for biodegradable polymers

Abstract

The effects of ester plasticizers and copolymers on the mechanical properties of the natural biodegradable polymers, poly(3-hydroxybutyrate) [PHB] and poly(lactic acid) [PLA] have been studied after subjecting to melt processing conditions. Ester plasticizers were synthesized from citric, tartaric and maleic acids using various alcohols. A variety of PLA copolymers have also been prepared from poly(ethylene glycol) derivatives using stannous octanoate catalysed ring opening polymerisations of DL-lactide. A novel PLA star copolymer was also prepared from an ethoxylated pentaerythritol. The structures of these copolymers were determined by NMR spectroscopy.
The plasticizing effect of the synthesised additives at various concentrations was
determined. While certain additives were capable of improving the mechanical
properties of PLA, none were effective in PHB. Moreover, it was found that certain
combinations of additives exhibited synergistic effects. Possible mechanisms are
discussed.
Biotic and abiotic degradation studies showed that the plasticizers (esters and
copolymers) did not inhibit the biodegradability of PHB or PLA in compost at 60°C.
Simple toxicity tests carried out on compost extract and its ability to support the
growth of cress seeds was established.
PLA was found to be susceptible to limited thermal degradation under melt
processing conditions. Conventional phenolic antioxidants showed no significant
effect on this process, suggesting that degradation was not predominantly a free
radical process. PLA also underwent photo-oxidative degradation with UV light and the process could be accelerated in the presence of a photoactivator such as iron (III) diisononyl dithiocarbamate.
The mechanisms for the above processes are discussed.
Finally, selected compounds were prepared on a pilot plant scale. Extruded and
blown films were prepared containing these additives with conventional polymer
processing equipment. The mechanical properties were similar to those obtained
with laboratory produced compression moulded films.

Date of Award	May 2002
Original language	English
Supervisor	Allan J Amass (Supervisor) & Khirud B. Chakraborty (Supervisor)