(433d) Biodegradable Poly(hydroxyalkanoate) Foams

Authors: 
Liao, Q., Stanford University


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 crystallization (1).  This requires that the polymer matrix have a high enough melt strength to retain cell structures until solidification occurs.  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 the addition of a nucleating agent, orotic acid.

Orotic acid (OA), a biodegradable and nontoxic chemical, was shown to be a very effective nucleating agent (NA) for crystallization in poly(3-hydroxybutyrate) (PHB) (2,3).  Nucleating agents can also be used to promote bubble formation (4).  With NAs, more nucleation sites are created for gas to diffuse into so bubbles are limited to a smaller, more uniform cell size (5), while also increasing cell density (6).  Additional benefit to using nucleating agents is that they often act as plasticizers which reduce the processing temperature needed for melt extrusion (4).  As NAs also promote crystallization, the addition of NAs may also reduce the time required for stabilization of polymeric foam structure.

To first evaluate the nucleating efficiency of OA in PHBV, films of PHBV and OA were made using the solvent casting method. The compositions ranged from 0 to 10 wt% OA.  Polarized light optical microscopy (PLOM) showed that the OA particles were well-dispersed throughout the films, which is optimal for homogeneity of nucleation sites.  Differential scanning calorimetry (DSC) indicated that 1 or 2 wt% OA resulted in the highest percent crystallinity.  It was also seen that presence of OA resulted in more uniform crystal size, as indicated by the narrower full-width-half-maximum (FWHM) values of the melting endotherm.  Further studies are being performed to evaluate the effect of OA content on crystallization kinetics. 

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 780, Chemtura) chemical blowing agent that decomposed at 140-150°C, generating approximately 150 cc of nitrogen per gram of solid.  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 preventing PHBV decomposition (7-9). Foaming occurred upon leaving the die.  SEM images obtained on cryo-fractured surfaces of extruded foam samples showed varying cellular morphologies with changes in AZ content.  Cell coalescence and collapse occurred when the amount of blowing agent AZ exceeded 1.5 parts per hundred (phr), suggesting that the PHBV melt strength does not withstand more increase in pressure posed by gas generation and expansion (10).  Using the same procedure as for the PHBV foams, results from the addition of 1 or 2 wt% OA will be shown.

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.  Adding orotic acid as a nucleating agent for both crystallization and bubble formation  could be a promising method for making such enhancements to the foamability of PHBV. 

References

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5.      Mills N Polymer Foams Handbook 2007, Chapter 3, 40

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10.  Q. Liao 2010 Ph.D. Thesis, Stanford University, Stanford, CA.

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