|Doane, William - RETIRED USDA-ARS SCIENT|
Submitted to: Polymer Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 8, 2002
Publication Date: April 8, 2003
Citation: WILLETT, J.L., DOANE, W. EFFECT OF MOISTURE CONTENT ON TENSILE PROPERTIES OF STARCH/POLY(HYDROXYESTER ETHER)COMPOSITE MATERIALS. POLYMER. 2003. v.43. p. 4413-4420. Interpretive Summary: Starch can be used as a low-cost component in biodegradable plastics for disposable applications. We have recently developed materials by com- bining corn starch with a biodegradable plastic known as PHEE. Both starch and PHEE can absorb moisture from the surrounding environment, to an extent determined by the relative humidity. Because water affects the properties and processability of starch/PHEE blends, it is important to measure its effects in a controlled manner. Blends of starch and PHEE were prepared at different moisture contents between 1 and 10 percent, and the properties were measured. It was shown that as the moisture content in the blends increased, their strength decreased and they became more flexible. A sharp drop in strength and increase in flexibility occurred when the moisture content exceeded about 7 percent. The presence of moisture levels exceeding about 7 percent during processing also altered the structure of the starch. Analysis of the strength data indicated that the effect of moisture content on properties was primarily due to changes in the PHEE's properties with moisture content. Analysis of changes in the starch/PHEE materials with increasing temperature allowed an estimation of the relative amounts of moisture in each component. Know- ledge of the moisture content in the starch/PHEE blends allows prediction of properties such as strength and flexibility. These results are useful to other researchers working with starch-based materials, or other materials sensitive to moisture content.
Technical Abstract: The effect of moisture content on the mechanical properties of starch/poly (hydroxyester ether) (PHEE) composite materials has been characterized. Cornstarch with either 1 percent or 10 percent moisture content was extruded with PHEE. Total moisture content (TMC) during subsequent injection molding ranged from 1.0 percent to 10 percent (total basis). Starch granule structure, determined by X-ray diffraction and scanning electron microscopy, was progressively disrupted but not completely destroyed (as in thermoplastic starch) as TMC during molding increased. Tensile strength and modulus values were not significantly impacted when the TMC was 6 percent or less. As TMC increased above 6 percent both properties decreased rapidly, while strain to break increased. Tensile strength increased when samples prepared at high total moisture content were equilibrated to a lower value. Molecular weight changes in the PHEE during processing were not significant as measured by gel permeation chromatography, even at the highest moisture contents. The effects of total moisture content on mechanical properties of starch/PHEE materials are shown to be due to changes in the viscoelastic response of the PHEE matrix, by comparison of starch/PHEE results to those of neat PHEE (St. Lawrence et al Polymer 42, 5643 (2001)).