Thermoplastic starch crystallinity

In nature, starch is based on crystalline beads of about 15-100 microns in diameter. Crystalline starch beads in plastics can be used as fillers or can be transformed into thermoplastic starch, which can either be processed alone or in combination with specific synthetic polymers. To make starch thermoplastic, its crystalline structure has to be destroyed by pressure, heat, mechanical work or use of plasticisers. Three main families of starch polymer can be used pure starch, modified starch and fermented starch polymers. 

In the patent literature, the terminology destructurized starch [46-65] refers to a form of thermoplastic starch described as molecularly dispersed [56]. Destracturization of starch is defined as melting and disordering of the molecular structure of the starch granules as a molecular dispersion [55,56]. It means that, at the same time, the native crystallinity of starch... 

Starch, Main sources, Macromolecular composition. Polymer structure and crystallinity. Granule disruption, Thermoplastic starch. Plasticization, Moisture uptake. Degradation, Blends, Composites... 

Van Soest J.J.G., Hulleman S.H.D, de Wit D, Vhegenthart J.F.G. Changes in the mechanical properties of thermoplastic starch in relation with changes in B-type crystallinity, Carbohydr. Polym. 29(1996)225. 

Thermal properties of thermoplastic starch composites reinforced with pehuen husk showed the potential of this bioliber as an excellent reinforcement for composite materials. TPS composites showed a good interaction between the fibers and the plasticized starch matrix due to the natural affinity between husk and starch in the pehuen seed. TPS/PLA/PV A blend showed partial miscibility or co-continuous phase and TPS/PLA/PV A composites presented also discontinuities at the biofiber-polymeric matrix interface. The incorporation of biofiber improved the thermal stability of the composites, increasing the initial decomposition temperature. The biofiber hinders the out-dififusion of the volatile molecules (e.g., glycerol), retarding the decomposition process of starch composites. On the other hand, the degree of crystallinity of composites decreases when pehuen husk content increases (Castano et al. 2012). 

Intercalation of thermoplastic starch with clay nanocomposites has been shown to improve modulus strength, thermal resistance, and enhance hardness [98,99], while the migration of additives has been shown to be at the level acceptable for food contact [ 100]. Elasticity of the films was however reduced though barrier properties are reported to be improved, without major change in the crystallinity of the polymers [101]. 

In its natural form, starch is not meltable and therefore caimot be processed as a thermoplastic. However, starch granules can be thermoplasticized through a gelatinization process. In this process, the granules are disrupted and the ordered crystalline structure is lost under the influence of plasticizers (e.g., water and glycerol), heat, and shear. The resultant melt-processable starch is often termed thermoplastic starch (TPS). Since the advent of TPS,... 

Mihai M, Huneault MA, Favis BD, Li H. Extrusion foaming of semi-crystalline PLA and PLA/ thermoplastic starch blends. Macromol Biosci 2007 7 907-20. 

The original crystalline order of native starch is destroyed in the production of thermoplastic starch, but even below the Tg, chains exhibit enough mobility and re-order over time. This re-forming of ordered crystalline regions is called retrogradation and can be observed using Different plas-... 

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