(819e) Development of Biodegradable Foams Through Processing and Material Property Enhancement

DEVELOPMENT OF BIODEGRADABLE POLY(HYDROXYALKANOATE) FOAMS THROUGH PROCESSING AND MATERIAL PROPERTY ENHANCEMENT

A. Tsui and C. Frank

Department of Chemical Engineering, Stanford University,

381 North-South Mall, Stanford, CA USA 94305

atsui@stanford.edu, curt.frank@stanford.edu

Abstract

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) is a member of the bacterial polyester family of poly(3-hydroxyalkanoates) (PHAs) known for its ability for enzymatic degradation and good mechanical properties.  The use of biodegradable polymers for producing sustainable materials has received increasing attention, and PHBV is an excellent candidate, shown to have similar properties to polypropylene (PP), known to be a tough and robust material.  Specifically, PHBV could be a useful material for construction materials such as insulation foam.  Current insulation foams are made primarily of polyurethane, polystyrene and polyethylene which are derived from non-renewable (petroleum) feedstock.  Conventionally, foam structures in nonaqueous systems, such as polymers are stabilized simply by solidification due to vitrification, cross-linking, or crystallization¹.  This requires that the polymer matrix have a high enough melt strength to retain cell structures until solidification occurs as well as sufficient gas solubility of the blowing agent during extrusion.  However, processing of the PHBV into foam is limited by its low melt elongational viscosity and narrow thermal processing window.  Low melt elongational viscosity contributes to cell coalescence and collapse, resulting in undesirable high density foam.  The poor foamability of PHBV can be addressed via understanding of gas solubility to inform processing conditions and the addition of long chain branching to the polymer to increase extensional viscosity.

Foaming of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) with 5% hydroxyvalerate content (Tianan Biologic Materials Company, China) was performed using  an activated azodicarbonamide (AZ) (Actafoam 765A, Chemtura) chemical blowing agent that decomposed at 152-160°C.  After blending and drying the PHBV and blowing agent, materials were extruded through a ¾-inch single-screw extruder (L/D ratio 25:1, compression ratio 3:1, C.W. Brabender) equipped with a 2-inch horizontal flex-lip ribbon die. The extruder temperature profile was chosen based on the temperature requirements for melting the PHBV and decomposing the blowing agent, while minimizing PHBV decomposition^2–4. Foaming occurred upon exiting the die.  SEM images obtained on cryo-fractured surfaces of extruded foam samples showed varying cellular morphologies with changes in AZ content. 

Gas solubility of nitrogen in PHBV could not be determined experimentally due to thermal degradation of the polymer during measurement.  Instead, a method of predicting solubility was validated and applied.  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 determined for polypropylene and poly(lactic acid) which have similar properties to PHBV and more experimental data available in literature. PVT properties of PP were found to be similar to experimental values in literature and the predicted solubility had a 10-14% error for PP and 20-24% error for PLA.  Foaming studies on PHBV were performed to evaluate the impact of 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 the pressure threshold, respectively.  As a result of the gas solubility predictions, it was also shown that CO₂ is more soluble in PHBV by a factor of approximately 3.5.  Therefore, we are also investigating various blowing agents that release CO₂.

In order to further improve the extensional rheology of PHBV, a reactive extrusion procedure was performed using dicumyl peroxide (DCP) as an initiator to produce long chain branching of PHBV.  This method has been shown to work previously in PHB⁵. We were able to significantly increase the viscosity of extruded PHBV using higher residence time and higher DCP content.  Further studies are being performed to determine appropriate processing schemes for PHBV branching and foaming using extrusion as well as optimal blend ratios of branched and linear PHBV.  Use of branched chain structure has been shown to improve foams of polypropylene⁶.

The biodegradable polyester PHBV is a promising material for replacing conventional petroleum-based polymers.  However, the melt strength and thermal processing window of PHBV must be improved in order to develop an alternative insulation foam.  A combination of maximizing gas solubility and reactive extrusion to add long chain branches to PHBV could be a method for yielding such improvement.    

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