Submitted to: Polymer Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 8, 2002
Publication Date: N/A
Interpretive Summary: Disposable foam packaging, plates, etc. made from corn starch have the potential to replace those made from petroleum based plastics, but the strength and water resistance of the starch based products need to be improved. There have been many reports in the scientific literature that the addition of certain polymers to starch foams improve their properties, but there is little information available regarding the mechanism of these improvements. In this work, it was found that starch foam expansion and resistance to impact increased as polymer concentration on the starch foam surface increased. These results helped identify certain polymer characteristics such as size, branching and flexibility which may be needed to give improved starch foams. Such information should be useful to industrial, academic and government scientists trying to design improved starch based plastic foams.
Technical Abstract: Expanded foam peanuts made from blends of thermoplastic corn starch and various polymers were studied in order to better understand relationships between structure and properties in these systems. X-ray powder diffraction revealed that all foams had the V-type pattern indicative of crystalline starch/lipid complexes. Polymer crystallization was observed in foams containing polycaprolactone, polybutylene succinate/adipate, polyester-amide and polybutylene adipate/terephthalate but not in those containing polylactic acid (PLA), poly (hydroxyester-ether) (PHEE), polyvinyl alcohol (PVOH). Scanning electron micrographs of solvent extracted foams showed that most of the polymer occupied spherical to elongated domains 1-10 micro long. Domain sizes for PLA were much smaller (0.1 micro) suggesting better compatibility with starch than the other polymers. X-ray photoelectron spectroscopy results showed that polymers migrated to the foam surface, leading to a relative enrichment there, especially for PHEE and polyhydroxybutyrate-co-hydroxyvalerate (PHBV). Radial expansion ratios were directly correlated with polymer surface concentrations, but expansion for crystalline polymers fell well below the line for amorphous polymers. Foam friability decreased (impact resistance increased) as the polymer surface concentration increased. These data suggest that lower molecular weight, amorphous and tough polymers which can migrate to the foam surface give foams with the highest expansion and impact properties.