SUSCEPTIBILITY OF STARCH-FILLED AND STARCH-BASED LDPE TO OXYGEN IN WATER AND AIR

The susceptibility of starch-filled and starch-based polyethylenes to oxygen in water and air was analyzed and compared. LDPE containing 7.7 % starch and a prooxidant formulation in the form of masterbatch (LDPE -MB) was compared to pure LDPE, LDPE with 7.7% starch (LDPE-starch), a nd a blend with 70% starch, and 30% ethylene maleic anhydride (starch- EMA). Thermal ageing at 80 degrees C in air and water was followed by monitoring the molecular weight changes, the formation of carbonyl gro ups, and degradation products by SEC, FTIR, and GC-MS. It was demonstr ated that LDPE-MB was the most susceptible material to degradation in both environments, although the degradation was faster in air than in water. The slower degradation in water is explained by a deactivation or leaching out of the pro-oxidant during the aging. The degradation o f pure LDPE and starch-EMA is faster in water than in air. LDPE-starch was the only material that did not degrade during 11 weeks in water a t 80 degrees C. The addition of starch to LDPE made this material even more stable than pure LDPE to aging in water. The molecular weight di stribution of LDPE-MB narrowed during aging in air. In water, on the o ther hand, the MWD of LDPE-MB, LDPE, and LDPE-starch broadened. The lo wer oxygen concentration in water increases the probability for molecu lar enlargement reactions in comparison to the case in air. Mono- and dicarboxylic acids were the major products identified in both environm ents. Ketoacids were formed in both air and water, but ketones and hyd rocarbons were only identified after aging in air. Either these produc ts are not formed or they remain in the polymer matrix rather than mig rate out into the water. Lactic acid and 2-furancarboxaldehyde were on ly identified in the starch-EMA material degraded in water at 80 degre es C. LDPE, LDPE-starch, and starch-EMA did not form any degradation p roducts during 11 weeks at 80 degrees C in air in agreement with the n eglible molecular weight changes observed. (C) 1997 John Wiley & Sons, Inc.