|Lawton Jr, John|
|Doane, W. - FORMER ARS|
Submitted to: Journal of Applied Polymer Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 3, 2005
Publication Date: February 27, 2006
Citation: Lawton Jr, J.W., Doane, W.M., Willett, J.L. 2006. Aging and moisture effects on the tensile properties of starch/poly(hydroxyester ether) composites. Journal of Applied Polymer Science. v.100. p.3332-3339. Interpretive Summary: Starch can be used as a filler in poly(hydroxyester ether)(PHEE) to make plastic composites. Previous work has shown that the starch adheres to the PHEE very well, thus making for stronger plastic composites than other composites made of starch and synethic plastics. However, if wet starch is used to make the composite, the adhesion between the starch is diminished and the composite is not as strong. This study investigated the effects of different types of modified starch fillers and storing the composites in 50% relative humidity (RH) and dry. Composites containing starch that had surface modification had lower overall strength than did composites containing unmodified starch, probably because the modification interfered with the adhesion between the starch and PHEE. Composites stored at 50% RH loss strength after storage. These composites absorbed water during their storage and this water probably interfered with the adhesion between the starch and PHEE. Storing composites dry where they could not absorb water did not have the same loss of strength. Composites made with native starch, PHEE and also poly(lactic acid)(PLA) did not lose strength when stored at 50% RH, even though these composites absorbed the same amount of water. This research shows the starch/PHEE composites are susceptible to water in the environment, but this can be overcome with the addition of PLA. This work will allow starch filled PHEE to be more effective in real world applications.
Technical Abstract: The effect of starch and aging on the mechanical properties of starch/poly(hydroxyester ether)(PHEE) composite materials has been characterized. Native cornstarch or modified cornstarch was extruded with PHEE. Composites were aged for up to 20 months at either 23 degrees C and 50% relative humidity (RH) or over calcium sulfate to keep the composites dry. Tensile strength (TS) of the composites was affected by the type of starch filler they contained. Composites containing native or cross-linked starch had significantly greater TS than did composites containing ocenylsuccinated starch. It is thought that the ocenylsuccinated modification of the starch granule is affecting the adhesion between the starch and PHEE. Aging the composites at 50% RH had the greatest effect on the mechanical properties of the composites with TS and YM significantly decreasing and elongation to break (%E) significantly increasing after 20 months of storage. Composites stored at 50% RH absorbed water, which caused a change in mechanical properties in two ways. First, the absorbed water disrupted the adhesion between the starch and PHEE. Secondly, the absorbed water plasticized the PHEE. Composites containing polylactic acid (PLA) and PHEE in the composite did not experience large reduction in TS and YM when aged at 50% RH, even thought they also absorbed water.