Influence of processing and processing additives on the degradation of aliphatic polyesters

Abstract: Poly(3-hydroxybutyrate-co-3-hydroxyvalerate), P(3HB-co-3HV),poly(?-caprolactone), PCL, and poly(lactic acid), PLA,have been subjected to film-blowing, melt-extrusion andmelt-pressing. The influence of processing conditions andprocessing additives (i.e. plasticizer, antiblocking and slipagents) on properties and degradation have been studied. Thepolyesters were exposed to pure fungal cultures(Aspergillus fumigatus,Phanerochaete chrysosporiumandPenicillium simplicissi-mum), composting and chemicalhydrolysis.The molecular weight of P(3HB-co-3HV) decreased withincreasing processing temperature and decreasing screw speedduring melt-extrusion. The mechanical properties were dependenton the molecular weight. The tensile strength decreased rapidlyat weight average molecular weights below 150 000 g/mol. Thedegree of crystallinity was largely unaffected by the extrusionconditions. An increase in the processing temperature increasedthe intensity of the (020) reflection and decreased theintensity of the (110) reflection. The changes in thediffraction patterns indicate changes in the positions of theoxygen atoms due to differences in the torsion angles in themain chain bonds. The rate of bioerosion in pure fungalcultures for P(3HB-co-3HV) was highest for the samplesexhibiting high peak ratio of the (020) and (110) reflectionsindicating that the changes in the positioning of the oxygenatoms affect the recognition capability of the extra-cellularenzymes. The erosion rate was not controlled by molecularweight or lamellar distribution, however, it decreased withincreasing degree of crystallinity.Film-blowing caused minor reductions in the molecular weightof PCL. Film blown from reprocessed PCL, and from reprocessedPCL containing additives exhibited lower degradation rates incompost than films from virgin PCL. The difference could beattributed to the reprocessing rather than the additives.Melt-pressed PCL film with SiO2(5.0 w%) exhibited higher erosion rates than purePCL during composting and exposure toA. fumigatus. Melt-pressed film with erucamide (1.0 w%)exhibited higher hydrophobicity but no reduction of thebiodegradation rate was observed. The rate of biodegradationfor PCL increasedwith decreasing film thickness. Thebiodegradation of P(3HB-co-3HV) and PCL was characterized bysignificant weight loss with minor reductions of the molecularweight.Film-blowing caused a 50 % decrease in the weight averagemolecular weight of PLA. CaCO3(2 w%) added to PLA reduced the chain scissionduring melt-processing. However, the rate of (bio)degradationwas also reduced. SiO2(5 w%) added to PLA did not affect the thermaldegradation or the rate of biodegradation. The (bio)degradationof PLA was characterized by significant reductions in molecularweight due to chemical hydrolysis. The onset of biodegradationoccurred when the molecular weight was sufficiently reduced.Extracellular enzymes fromA. fumigatuswere more effective in degrading PLA thanthose fromP. simplicissimum.