Amylose content and chemical modification effects on the extrusion of thermoplastic starch from maize

Abstract

The effects of starch structural properties and starch modification on extruder operation were monitored via die pressure, motor torque, mean residence time and specific mechanical energy (SME). The structural properties studied involved variations in the ratios of amylose and amylopectin as well as the effect of a hydroxypropylated starch on the fore mentioned extruder properties. A full factorial design of experiments (DOE) was used to then determine the influence of starch type (unmodified starches with 0%, 28%, 50% and 80% amylose; 80% amylose hydroxypropylated starch) and screw speed (250, 300 and 350 rpm) on these processing parameters. The effects of starch type and screw speed on extrusion operation that were systematically investigated using the DOE and have provided valuable insight into the relationships between starch structure and processing. The design of experiments showed that starch type for both unmodified and modified maize had a statistically significant effect on parameters such as torque, die pressure and specific mechanical energy and that screw speed also significantly effected specific mechanical energy. Residence time distributions differed according to starch type (amylose content, hydroxypropylation) and screw speed. The additional study of residence time distribution also gave an indication of the degree of mixing in the extruder. Starch type variations were apparent at low screw speed however at higher screw speed the influence of starch type decreased significantly.

Introduction

The development of a fully biodegradable, natural, renewable thermoplastic is of increasing interest due to its low environmental impact when compared to current petroleum based products that contribute to litter and landfill problems (Swift, 1996). In response to these problems, research into the synthesis of materials from natural sources such as starch and cellulose is being undertaken with the aim of replacing their non-biodegradable counterparts.

Starch is a polymeric material that is biodegradable, renewable and also available worldwide at low cost, which makes it attractive as a substitute for petroleum based plastics (Trommsdorff & Tomka, 1995). However, simple extruded starches with water products are brittle and highly sensitive to water (Ollett et al., 1991, Slade and Levine, 1993). As a result, development of practical thermoplastic starch resins includes the addition of processing aids and plasticisers to aid gelatinisation during processing thus producing suitable mechanical properties in the finished product (Doane, 1992, Shogren et al., 1992). Recently, commercially biodegradable packaging has been developed to overcome these problems (Halley, Mcglashan, & Gralton, 2006 Patent No. 7094817), giving the opportunity to manufacture products from starch-based thermoplastic resins (Plantic Technologies Ltd YeBiodegradable Lethal Ovitrapar, 2007).

Prior to thermoplastic processing such as injection moulding, starch is extruded and gelatinised to form a thermoplastic material that can be subsequently processed into viable products (Wiedmann & Strobel, 1991). Different extrusion processing conditions will alter the transformation of the starch during the preparation of the thermoplastic starch resin (Wiedmann & Strobel, 1991), which ultimately affects the mechanical properties of the finished product (Van Soest, De Wit, & Vliegenthart, 1996). Screw speed is a particularly useful processing variable, since it is readily altered during extrusion operation, controls the amount of work done on the material during processing, affects the extent of degradation of starch and alters the rheology of starch melts (Tolstoguzov, 1993, Van Soest et al., 1996).

Just as there is a wide variety of synthetic thermoplastic polymers available which differ in their monomers, their structure (molecular weight of chains, extent of branching), particular processing characteristics, and desired products physical properties (e.g. mechanical properties, barrier properties, appearance), similarly, with thermoplastic starches, the type of starch, chemical modification of the starch and tailored processing conditions has been shown to be able to be optimised for particular of the finished product (Halley, et al. 2006).

When starch type and screw speed are considered together, they have been shown to have a major effect on extrusion and finished product properties. In a study of (Van Soest, De Wit, et al., 1996) it was shown that increasing the screw speed, increased single helical type crystallinity for higher amylose content starches, thus affecting the final product mechanical properties.

The objective of our research was to study the effects of varying amylose/amylopectin ratios of maize starches when processed using a twin screw extruder. Hydroxypropylated starch was used for two reasons. Firstly, starch modification is carried out to improve the functional and physicochemical parameters in various industries since native starch itself may not give optimal performance (Lawal, 2004) and it is high amylose hydroxypropylated starch that is commercially available in Australia to produce thermoplastic materials. The effects of starch type (0–80% amylose maize starch; hydroxypropylation of 80% amylose maize starch) and mechanical processing conditions (screw speed) on extruder operation (motor torque, SME, die pressure, mean residence time) were analysed.