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A Processing, Characterization and Marine Biodegradation Study of Melt-Extruded Polyhydroxyalkanoate (PHA) Films

Thellen, Christopher, Coyne, Megan, Froio, Danielle, Auerbach, Margaret, Wirsen, Carl, Ratto, Jo Ann
Journal of polymers and the environment 2008 v.16 no.1 pp. 1-11
X-radiation, X-ray diffraction, biodegradability, biodegradation, chromatography, crystal structure, differential scanning calorimetry, extrusion, gels, glass transition, light microscopy, loss modulus, marine environment, mineralization, modulus of elasticity, molecular weight, oxygen, polarized light, polyhydroxybutyrate, resins, seawater, temperature, tensile strength, water vapor, weight loss
A series of polyhydroxyalkanoates (PHA), all containing 1% nucleating agent but varying in structure, were melt-processed into films through single screw extrusion techniques. This series consisted of three polyhydroxybutyrate (PHB) and three polyhydroxybutyrate-valerate (PHBV) resins with varying valerate content. Processing parameters of temperature in the barrel (165-173 °C) and chill rolls (60 °C) were optimized to obtain cast films. The gel-permeation chromatography (GPC) results showed a loss of 8-19% of the polymer's initial molecular weight due to extrusion processing. Modulated differential scanning calorimetry (MDSC) displayed glass transition temperatures of the films ranging from -4.6 to 6.7 °C depending on the amount of crystallinity in the film. DSC data were also used to calculate the percent crystallinity of each sample and slightly higher crystallinity was observed in the PHBV series of samples. X-ray diffraction patterns did not vary significantly for any of the samples and crystallinity was confirmed with X-ray data. Dynamic mechanical analysis (DMA) verified the glass transition trends for the films from DSC while loss modulus (E') reported at 20 °C showed that the PHBV (3,950-3,600 MPa) had the higher E' values than the PHB (3,500-2,698 MPa) samples. The Young's modulus values of the PHB and PHBV samples ranged from 700 to 900 MPa and 900 to 1,500 MPa, respectively. Polarized light microscopy images revealed gel particles in the films processed through single-screw extrusion, which may have caused diminished Young's modulus and tensile strength of these films. The PHBV film samples exhibited the greatest barrier properties to oxygen and water vapor when compared to the PHB film samples. The average oxygen transmission rate (OTR) and water vapor transmission rate (WVTR) for the PHBV samples was 247 (cc-mil/m²-day) and 118 (g-mil/m²-day), respectively; while the average OTR and WVTR for the PHB samples was 350 (cc-mil/m²-day) and 178 (g-mil/m²-day), respectively. Biodegradation data of the films in the marine environment demonstrated that all PHA film samples achieved a minimum of 70% mineralization in 40 days when run in accordance with ASTM 6691. For static and dynamic incubation experiments in seawater, microbial action resulting in weight loss as a function of time showed all samples to be highly biodegradable and correlated with the ASTM 6691 biodegradation data.