Physical and Chemical Investigations of Starch Based Bio-Plastics

Starch is one of the most common and easily obtained natural polymers, making it attractive as a potential bio-based alternative to synthetic polymers. The plasticisation of starch is complex due to the extensive hydrogen bonding between chains. This study shows that a simple quaternary ammonium salt combined with hydrogen bond donor (HBD) forms effective modifiers that produce flexible plastics with good mechanical properties that are comparable to some polyolefin plastics. Starch-based plastics can be formed by the same processes as current commercial plastics, giving similar mechanical strength to some polyolefin plastics. The processing conditions are shown to significantly affect the structure of the polymer which has a concomitant effect upon the mechanical and physical properties of the resulting plastic. Using a glycerol based modifier results in a totally sustainable and biodegradable material which can be formed by extrusion, pressing, vacuum forming and injection moulding. Most significantly, it is shown that these plastics are environmentally compatible, recyclable, bio-degradable and compostable. This study has demonstrated the optimisation of the parameters for a range of techniques that are currently used in processing this type of plastic. These include: temperature profiling for both compression moulding and extrusion, residence time in the extruder, drying time and pressing time. Glycerol/choline chloride is the plasticiser that was found to give optimal properties and has been used for most of this study. The optimum ratio is shown to be 1:3 by weight plasticiser: starch. In addition, five different systems of fillers have been added to the starch based plastics to study their effect on tensile strength and strain, glass transition temperature, viscoelasticity, crystallinity, morphology and rheology. The chemical changes have also been investigated after the addition of the five fillers which are eggshell, wood-flour, silk, zein and lignin. Furthermore, the fillers have an effect on stabilising water content in thermoplastic starch. Water uptake and water loss have been examined using thermogravimetric analysis, water absorption isotherms, water absorption capacity and contact angle. Finally, some applications have been presented for protecting these plastics from surrounding moisture.