Complex interplay of thermal constraints, rheological performance, and process design on extrusion foaming of biodegradable polyhydroxyalkanoates [electronic resource]
Keywords: Chemical Engineering.

Today, there are two major issues associated with conventional plastic use. The first is that plastics are typically sourced from non-renewable fossil fuels, either petroleum or natural gas, and the second is that they persist in the environment causing environmental issues both in the oceans and on land. Polyhydroxyalkanoates (PHAs) are now a well-known family of polymers that can address both challenges. PHAs are produced by many types of bacteria for carbon and energy storage. The feedstock can be many natural carbon sources such as sugars, starches, alcohols and other similar waste products. Because PHAs are naturally produced, they are also readily biodegradable in aquatic and anaerobic environments. This provides advantages to PHAs over poly(lactic acid) (PLA), another commonly used biopolymer that requires the high temperature and control of industrial composting environments for decomposition to occur. However, the main drawbacks that prevent more widespread use are their high cost, low melt strength, and susceptibility to thermal degradation. PHAs, in particular, are highly vulnerable to thermal degradation near their melting temperatures, leading to an extremely narrow thermal processing window and further reductions in melt strength during processing. This is particularly challenging for foam applications where high melt strength and elongational viscosity are important for stabilizing cell growth and achieving uniform, high cell density microstructure and low foam density. Yet, the ability to foam PHAs could reduce costs by lowering the amount of polymer required and expand PHA applications to packaging (e.g., foam packing), construction (e.g., insulation), and consumer products (e.g., serving ware). Given that both material and processing conditions can affect foam characteristics concurrently, this work seeks to understand and improve the processing and material properties of PHAs in order to yield biodegradable foams. Firstly, various studies were performed to understand and determine the impacts of some of the material choices and melt processing protocols on both foam microstructure and characterization. This initial work supported and complemented subsequent development of biodegradable PHA foams. Next, 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 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 PLA, which have similar thermal and rheological properties to PHBV. To address the material issues, poly(3-hydroxybutyrate-¬co-3-hydroxyhexanoate) (PHBHHx), a PHA with higher melt strength, was utilized. However, it was found to be susceptible to cell coalescence at the higher processing temperatures required for low bulk density. PHBV and orotic acid (OA) were used as crystal nucleating agent blend components to increase the crystallization temperature of PHBHHx, and thus induce solidification earlier during cooling. Blends of PHBHHx/PHBV and PHBHHx/OA were produced and characterized. PHBHHx and PHBV appear to be fully miscible across all compositions used in this study. Furthermore, both PHBV and OA could be used to generate a range of crystallization temperatures. As a result of the additives, PHBHHx was able to achieve greater than twofold expansion, which has not been achievable by PHBV with the same blowing agent. However, PHBV was able to maintain higher cell density, likely due to a lower viscosity promoting nucleation. Additionally, preliminary and complementary work was performed using other additives (silk fibroin, monohydrated orotic acid, and dicumyl peroxide), PHAs (PHBHHx with 11 mol% HHx), and blowing agents (sodium bicarbonate). The results from this body of research contribute to the understanding of the limitations and identification of opportunities of this biodegradable polymer for the production of extruded foams.