Chemical and morphological characterisation of long-term properties of environmentally degradable polymers

Abstract: The environmental degradation of an aliphatic polyester,poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and a polyesteramide,poly(butylene adipate-co-amino caproate) (PEA) was studied to answer questionsregarding their life-time in natural environments,e.g.municipal solid waste compost. Characterisation oflong-term properties is done by a series of techniques. Inorder to predict long-term properties and lifetime ofdegradable polymers, it is necessary to compare morphologicalproperties,e.g.melting temperature and heat of fusion, withchemical properties, such as molecular weight, functional groupchanges and the evolution of degradation products.Pyrolysis-GC/GC-MS is also presented and discussed as a tool tostudy structural changes in polymers.PHBV was readily biodegradable and the major degradationpathway during composting was mediated by microorganisms. Theprimary degradation products formed during fungal degradationof PHBV were monomers and oligomers. These were furthermetabolised and converted into fatty acids, such as aceticacid, butanoic acid and pentanoic acid. The biodegradation wasaffected by the copolymer composition. Results indicated thatthe fungusAspergillus fumigatuspreferred 3-hydroxybutyrate unitsrather than 3-hydroxyvalerate units. Abiotic factors such asheat and moisture have less effect on the environmentaldegradation of PHBV in garden/household waste compost during aperiod of 50 days.PEA was less susceptible to biodegradation than PHBV. Noneof the fungiAspergillus fumigatus,Aspergillus nigerorPhanerochaete chrysosporiumwere able to grow on PEA ascarbon source. PEA was, however, sensitive to abiotic esterhydrolysis, especially at temperatures above 60°C, and wasreadily degraded through surface erosion. The fungaldegradation of PEA also occurred through surface erosion, butno preference for ester rather than amide bonds was observed.Degradation products formed during fungal degradation were1,4-butanediol, 1,6-hexanedioic acid, 6-aminocaproic acid and acomplex mixture of oligomers. Abiotic factors, such as heat incombination with moisture, have thus a large effect on thedecomposition of PEA in compost.Pyrolysis-GC/GC-MS is a promising tool to differentiatebetween degradation mechanisms of polymers, but needs furtherdevelopment for composition determination of biodegradablecopolymers.Keywords:polyester, poly(3-hydroxybutyrate-co-3-hydroxyvalerate), polyesteramide, poly(butyleneadipate-co-amino caproate), degradation, fungi, composting,pyrolysis.