**Modifications of Thermoplastic Starch**

**1** 

*México* 

**Melt Blending with Thermoplastic Starch** 

Starch is a natural polymer synthesized by green plants as energy source. In comparison with low-cost synthetic polymers, starch is inexpensive, abundant and renewable raw material for the development of polymeric sustainable materials. It has be used in its native granular form as rigid filler or transformed in a thermoplastic material for melt blending with synthetic or natural polymers. Polymers filled with dry starch granules behave as typical composite materials where modulus increases and ductility decreases due to the stiffening effect of the starch granules (Willett, 1994, Kim et al, 1995, Chandra & Rustgi, 1997). An important disadvantage showed by polymeric composites filled with granular starch is the low starch content that can be added, especially for application where high ductility is required (Griffith, 1977). In contrast to the ordered structure of starch molecules in granular starch, thermoplastic starch (TPS) is an amorphous material that can flow and be deformed as any synthetic polymer (St.-Pierre et al., 1997). Crystallinity of starch granules is destroyed by the application of heat and shear in the presence of moisture during the gelatinization process. The addition of a good plasticizer, such as glycerol, allows TPS to be extruded at the processing temperatures of most commodity polymers (St.-Pierre et al., 1997). Mechanical performance of TPS material blended with synthetic polymers depends on a series of parameters including blend morphology (particle size and shape, and particle dispersion and distribution), interfacial adhesion and the intrinsic characteristics of TPS (Rodriguez-Gonzalez et al., 2003b). It has been reported that melt blending of TPS with synthetic polymer is an excellent alternative for the development of sustainable and more

environmentally friendly product (Rodriguez-Gonzalez et al., 2003b).

The gelatinization of starch is a process that permits the release of starch macromolecules from granules. It can be carried out by exposing starch granules to heat and shear in the presence of moisture. In the gelatinization of starch during extrusion, it is important to have strict control of the energy applied and the moisture content. The gelatinization process is depicted in Figure 1. The application of excessive heat and shear, such as that observed during extrusion processing of starch at low moisture content, leads to its thermo-mechanical degradation (Gomez & Aguilera, 1983, 1984, Lai & Kokini, 1991). Products of starch degradation are mainly dextrin, and in more extreme cases oligomer and sugar (Gomez & Aguilera, 1983, 1984). Once starch granules are disrupted, the resulting gelatinized starch (GS) can be mixed with a

**2. Thermoplastic starch preparation** 

**1. Introduction** 

Francisco J. Rodríguez-González *Centro de Investigación en Química Aplicada* 
