On processing of thermoplastic starch
Starch is a biodegradable material produced in many different crop plants. Since it is biodegradable, inexpensive, generally has good barrier properties and comes from a renewable source it would be an ideal material to use in e.g. packaging applications. A major drawback is that starch is hygroscopic and that the mechanical and barrier properties are strongly affected by the water content. To decrease brittleness of starch materials, plasticisers, such as glycerol, can be used. By using starch from crops giving a desired chain constitution with regard to molecular weight, amount of branches and lengths of branches, problems associated with the hydrophobicity might be reduced. Starch consists of two polymers, amylose and amylopectin, the former is regarded as linear and the latter as branched.
In this work, potato starches with two different amylose contents have been melt-processed with glycerol as plasticiser. The processing techniques used included extrusion and compression moulding. Of special interest was here the processability of the starches and the properties of final processed material. The characterisation techniques used were mainly capillary viscometry, mechanical testing and dynamic-mechanical analysis. Changes in the crystallinity were evaluated with x-ray diffraction.
Processing of high amylose (HAP) materials was in general more difficult than materials with lower amylose contents. High moisture and glycerol contents combined with high shear rates, were required to melt the HAP-material sufficiently and turn it into thermoplastic sheets. The problems encountered included insufficient melt tenacity, pressure and flow fluctuations and clogging of the extruder die. The melt viscosities of HAP were higher than that of normal potato starch (NPS) at similar glycerol and moisture contents. By increasing the moisture content, the viscosities could however be reduced significantly. On the positive side, HAP-materials exhibited higher mechanical strength and stiffness. This was partly explained by a higher transition (softening) temperature observed for the HAP-material using dynamic mechanical analysis. Extruded and compression moulded samples appeared transparent and x-ray diffraction measurements indicated no crystallinity of the processsed sheets.