Crystallinity in starch bioplastics

Abstract

Thermoplastic starch (TPS) materials have been prepared by kneading, extrusion, compression moulding and injection moulding of several native starches with the addition of glycerol as a plasticizer. Two types of crystallinity can be distinguished in TPS directly after processing: (i) residual crystallinity: native A-, B- or C-type crystallinity caused by incomplete melting of starch during processing; (ii) processing-induced crystallinity: amylose V H−, V A− or E H−type crystallinity which is formed during thermomechanical processing The amount of residual crystallinity is related to processing conditions like processing temperature or applied shear stress. The composition of the mixture indirectly influences the amount of residual crystallinity. Lower amounts of glycerol cause a reduction in residual crystallinity. This effect is attributed to the increase in melt viscosity at decreasing plasticizer content, which causes an enhancement of shear stress on the melt. It has been concluded that composition and processing parameters are interrelated. Processing-induced crystallinity, also influenced by processing parameters, is caused by the fast recrystallization of amylose into single-helical structures. Increasing the screw speed during extrusion or increasing residence time during kneading causes an increase in single helical type crystallinity. The amount of crystallized amylose is proportional to the amount of amylose. The addition of complexing agents like calciumstearate or the presence of lysophospholipids cause the crystallization of amylose into these type of structures. In waxy starches, containing no amylose, obviously no V- or E-type crystallinity is formed.