This project was carried out in conjunction with Plantic Technologies, which has developed and now manufactures a new low cost, starch-based biodegradable plastic. The aim of this project was to determine the effect of varying the number of extrusion stages on the degree of gelatinization of the extrudate to be used in injection moulded, starch-based plastic products. It was carried out in the Centre for High Performance Polymers (CHPP), as part of the UQ Plantic research project
The first step undertaken in the project was a review and critical analysis of relevant literature in order to develop a clear understanding of starch-based plastics and to determine the best techniques to use to ascertain the degree of gelatinization of extruded material. The literature revealed that the term gelatinization is defined as the transformation of granular starch into a turbid, viscoelastic paste in the presence of heat and water (Dalziel, 2004).
Three separate extrusion trials were then carried out. The first (trial 1) was the base material used in existing Plantic products, R1. This was used as a comparison to the other two trials. The second trial (trial 2) was the same as trial 1 with the addition of plasticizers to aid in processing and broaden the extrudate property range. The third extrusion trial (trial 3) used the R1 extrudate from trial 1 as feed and also incorporated the same plasticizers as in trial 2. These formulations were extruded to compare the degree of gelatinization between the onestage process (trial 2) and the two-stage process (trial 3). The degree of gelatinization of all three trials was determined using differential scanning calorimetry (DSC), microscopy, water absorption index (WAI) and water solubility index (WSI).
The DSC results indicated that the trial 1 starch granules were totally collapsed, indicating full gelatinization. Trial 2 extrudates were found to be between 47-81% gelatinized, with no real trend within the results except that they were all less gelatinized than trial 1 and 3. The DSC results for trial 3 indicated, unexpectedly, that it was only 82% gelatinized. The granular starch in this trial was expected to be completely transformed into a viscoelastic paste, because trial 1, which was fully gelatinized, was used as the feed material.
The results from the WAI and WSI analysis indicated that trial 1 was 44% gelatinized. The results indicated that trial 2 varied over a range of 19-31%, and a clear trend of increasing gelatinization with temperature was evident. Trial 3 was found to be 37% gelatinized. The WAI and WSI results were much lower than those obtained on the DSC. This difference may be due to the fact that pure starch was used in determination of the WAI and WSI calibration chart (Figure 10), but the extruded samples contain additional more soluble additives, glycerol and sorbitol.
All the samples analysed using optical microscopy showed evidence of the presence of birefringent starch granules. This means that the all the starch-based materials extruded were not fully transformed from granular starch into amorphous, disordered thermoplastic starch. The comparison between the one and two stage extrusion processes revealed that neither was totally gelatinized, but trial 3 (two-stage process) was more gelatinized than trial 2 (one-stage process). Trial 1 (existing formulation) was also not fully gelatinized, but more so than trials 2 and 3. This means the presence of plasticizers has an effect on the gelatinization of starch during extrusion and further work will be required to clarify these results.