Effect of Reaction Conditions and Alkyl Chain Lengths on the Properties of Hydroxyalkyl Starch Ethers

Abstract

Starch applications in the plastics industry have been investigated intensively for many years. Unfortunately, native starch is unsuitable for most applications in polymers, because of its hydrophilic and brittle character. Modification of native starch can be a solution of this problem. In our research programme we developed an environmentally friendly process for the preparation of starch derivatives with long hydrophobic side chains based on natural products derived from vegetable fats and oils on the one hand and amylomaize starch on the other hand. In this process modification is carried out by the addition of longchain 1,2-epoxyalkanes (chain lengths varying between 6—18 C atoms) to starch in an alkaline aqueous medium. Reaction yields and molar degrees of substitution depended on chain length, reaction time, temperature and catalyst concentration. Under optimised reaction conditions molar degrees of substitution of hydroxyalkyl starch ethers for theoretical molar substitution (MS) = 3.0 decreased from MS=1.8 (C6 starch ether) to MS=0.2 (C18 starch ether), respectively. Hydrophobic properties of starch products were measured in order to reveal structural effects obtained at different reaction conditions. Water solubility and water absorption properties of hydroxyalkyl starch ethers decreased with increasing chain length and number of hydroxyalkyl substituents. Thermal and thermoplastic properties of starch ethers were investigated in order to determine their potential use as biodegradable polymers in the plastics industry. As expected, thermoplasticity of starch ethers increased with increasing MS and increasing chain length of the hydroxyalkyl side groups.