Extruded foams from microbial poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) and its blends with cellulose acetate butyrate

Abstract

Extrusion foaming of poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) (PHBV) and two blends of PHBV with cellulose acetate butyrate (CAB) were studied using an azodicarbonamide (AZ) blowing agent and a single‐screw extruder. The concentration of the blowing agent was systematically varied from 0 to 4.0 phr to achieve maximum density reduction reaching 41%, as well as to obtain information on the dependence of cell growth on blowing agent concentration. Extruded foams were characterized in terms of their bulk densities and cellular morphologies. Stereological and statistical methods permitted full characterization of the three‐dimensional cell size distributions, assessing the average cell diameters (ranging from 58 to 290 μm) and cell densities (ranging from 650 to 180,000 cm−3). The variation in cellular morphology among foams consisting of different polymer matrix or blowing agent concentration was compared. The results were analyzed by considering the influence of viscoelastic properties of the polymer matrix on the bubble growth during foaming. Significantly higher melt viscosity and elasticity and reduced gas solubility of the PHBV/CAB blends are believed to retard cell coalescence and collapse during foam expansion, resulting in more uniform cell size distribution and better homogeneity of cellular morphology. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers

Prediction of gas solubility in poly(3-hydroxybutyrate-co -3-hydroxyvalerate) melt to inform process design and resulting foam microstructure

“…Liao et al previously found that the bulk density of PHBV foamed with AZ began to increase after 1.5 phr and suggested this was due to the known poor melt strength. Figure compares the results by Liao et al with those achieved by using higher screw speeds to maintain higher pressures.…”

“…To reduce cost as well as expand to broader applications, it is attractive to foam PHAs and PLA. However, attempts at foaming PHAs have seen limited success and require additives or blend components for improvement . The reason has primarily been attributed to poor melt strength and poor elongational viscosity associated with the linear polymers .…”

“…However, attempts at foaming PHAs have seen limited success and require additives or blend components for improvement . The reason has primarily been attributed to poor melt strength and poor elongational viscosity associated with the linear polymers . PHAs, in particular, are highly susceptible to thermal degradation near their melting temperatures, leading to an extremely narrow processing window and further reductions in melt strength during processing.…”

“…Additionally, insufficient solubility of the blowing agent in the polymer has been suggested as another limitation to producing low density microcellular foams, but there has been little investigation into its impact on foam microstructure, particularly in biodegradable foams. Many foam studies simply aim for very high pressure conditions to ensure solubility without knowledge of actual values or use model polymers or other gases as reference points for the solubility . Xu et al indicated that large bubbles are observed in foams produced below the solubility pressure, though the cell density was not significantly impacted.…”

“…Given that both material and processing properties can affect foam characteristics concurrently, care should be taken to distinguish between them to make sufficient improvements. Liao et al observed that a minimum in bulk density was achieved at moderate amounts of blowing agent, but density would increase at higher content. This observation was attributed to low melt viscosity, leading to cell coalescence and collapse at high gas content.…”