Except when starch is used as filler to produce reinforced plastics, its application as a bioplastic requires the transformation of the semicrystalline starch granules into a homogeneous, essentially amorphous matrix, in order to enhance the processability as compared to granular starch. The disruption of the molecular order within the granules can be accomplished by thermal or mechanical energy input associated with the addition of plasticizers such as water. “Destructurized starch” (DS) and “thermoplastic starch” are terms commonly employed to designate starch in this form.
Early patents (Stepto, Tomka, and Beat, 1988; Stepto and Beat, 1989) described destructurized starch as a material obtained by thermoplastic melt formation, by heating starch and water, with moisture content in the range of 5 to 40% (w/w), in a closed volume. The heating proceeds until all endothermic transitions have occurred, including the final narrow one, just prior to the endothermic change characteristic of oxidative and thermal degradation of starch. The starch is heated above the melting and glass transition temperatures of its components so that they undergo an endothermic transition. As a consequence, a melting and disordering of the molecular structure of the starch granule takes place, so that a destructurized starch is obtained. The applicability of DS is limited because of the degradation of starch due to water loss at elevated temperatures. Hence, the materials can only be processed by the addition of water, other plasticizers, or melt flow accelerators after the granular disruption step.
Thermoplastic starch (TPS)
In order to overcome these problems, some patents (Tomka, 1994; Jurgen, Winfred, and Harald, 2001) described a starch-based composition, made by mixing starch with an appropriated additive or plasticizing agent, such as glycerol and sorbitol, under conditions that yield a thermoplastic starch melt after thermomechanical transformations. The additive or plasticizer is a substance that, when mixed with starch, reduces its melting point, bringing it below the decomposition temperature. This homogeneous amorphous material was claimed as a thermoplastically processable starch or thermoplastic starch in the patents.
As intermediate forms of starch are difficult to distinguish as DS or TPS from the descriptions above, van Soest (1996) suggested an empirical distinction of these starch materials. Accordingly, DS is essentially an amorphous starch made from granular starch of which the native order and crystallinity were completely lost by thermomechanical processing irrespective of the type of additives or storage conditions encountered after processing. When DS is obtained by any conventional thermoplastic process, such as extrusion and injection molding into shaped forms without the addition of extra plasticizers or melt flow accelerators, it can also be designated TPS.
Recently, Avérous (2004) introduced other terms to designate starch products as a function of water content and destructuring level of granules. Most starch applications require water dispersion and partial or complete gelatinization. According to this classification, when the material is obtained with the combination of water (high content) and heat, it is called gelatinized starch. Conversely, thermoplastic starches (or plasticized starches) are described as starch materials that were obtained with both low water content (less than 20 wt.%) and a high level of destructuration, corroborating the concept suggested by van Soest (1996).