Biodegradable polymers based on starch were studied. Starch has been thought the most abundant and the lowest cost natural polymer on the market. Starch seems to be the easiest material for biodegradable polymers but it needs to be modified before blending with synthetic polymers. Since starch with three hydroxyl groups is hydrophilic while the synthetic polymers are usually hydrophobic. There had been many modification methods to give the compatibility to starch-low density polyethylene(LDPE) blends. Otey and co-workers(1987) reported a technique for blending gelatinized starch with poly(ethylene-co-acrylic acid) to produce flexible blown films that contain high levels of starch. The esterification of the hydroxyl group of starch with acetic anhydride was performed by Lemmerling and co-workers(1966). Other method was the copolymerization of acrylonitrile and styrene onto starch[Gugliemelli et al., 1969, 1977].
The esterification reaction of the hydroxyl group of starch with Surlyn was taken in this study. In treating starch by Surlyn, two reaction conditions were taken to optimize the esterification reaction. As the first condition, the esterification reaction was carried out in 110℃ xylene with oleic acid as a catalyst. In the second one the reaction carried out in the 90℃ mixture of water and xylene with ceric ammonium nitrate(CAN) as a initiator. The reaction temperature was above the gelatinization temperature of starch.
The blends of the modified starch with LDPE and poly(hydroxybutyrate)(PHB) were examined by tensile strength, percent elongation, Young' modulus, rheological properties and the morphology.
In this study, starch was successfully modified by Surlyn to have the good compatibility with LDPE. However, Surlyn-treated -starch did not show the good compatibility with PHB. The blend of LDPE and Surlyn-treated-starch must be the biodegradable polymer which retains the required properties sufficiently. PHB has the similar properties with polypropylene. The weakness of PHB is that it is very expensive. The PHB blended with Surlyn-treated -starch would be cheaper material than pure PHB. The PHB used in this study was appropriate for bottles and disposable containers because it was hard and brittle. It is expected to improve the properties of PHB by blending with commercial polymers in the future.
