Environmental concerns have led to the development of biorenewable polymers with the ambition to utilize them at an industrial scale. Poly(lactic acid) and poly(hydroxyalkanoates) are semicrystalline, biorenewable polymers that have been identified as the most promising alternatives to conventional plastics. However, both are inherently susceptible to brittleness and degradation during thermal processing; we discuss several approaches to overcome these problems to create a balance between durability and biodegradability. For example, copolymers and blends can increase ductility and the thermal-processing window. Furthermore, chain modifications (e.g., branching/crosslinking), processing techniques (fiber drawing/annealing), or additives (plasticizers/nucleating agents) can improve mechanical properties and prevent thermal degradation during processing. Finally, we examine the impacts of morphology on end-of-life degradation to complete the picture for the most common renewable polymers.

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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

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Foaming studies on poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) (PHBV) were performed to evaluate the potential impact of nitrogen gas solubility on foam microstructure and properties. It was found that there was a step change from low to high cell density below and above a specific pressure threshold. This pressure threshold, which is associated with nitrogen gas solubility in PHBV, was calculated in order to inform processing requirements for extrusion foaming of PHBV. In the process, the PVT properties of PHBV were also calculated to account for polymer swelling at high temperature and pressure. To validate the predictive method applied in this work, gas solubility of nitrogen was also determined for polypropylene (PP) and poly(lactic acid) (PLA), which have similar thermal and rheological properties to PHBV. Predicted PVT properties for PP were found to be similar to experimental values in the literature, with the predicted solubility having a 10–14% error for PP and 20–24% error for PLA. Application of the same error range to the solubility predicted for PHBV aligned well with experimental observations. POLYM. ENG. SCI., 54:2683–2695, 2014. © 2013 Society of Plastics Engineers