Recent advances in thermoplastic starch biodegradable nanocomposites

Abstract

The use of polymers capable of being degraded by the action of microorganisms and/or enzymes without causing harmful effects is a strategy in the management of waste and environmental care. Agro-polymers have begun to play a significant role among researchers and industry, since it has been found that these materials are Biodegradable and eco-friendly. Starch is a polymer belonging to the group of polysaccharides, which is produced by almost all plants using it as energy storage. Depending on the botanical origin of the plant, starch granules can have different shapes (spheres, platelets, polygonal) and size (from 0.5 to 175 μm). Its chemical composition consists of two components: amylose, composed of 1,4-α-D bonds of glucose in straight chains, and amylopectin, in which the glucose chains are highly branched. Starch is a naturally renewable carbohydrate polymer, abundant, and inexpensive, so it is mostly used as raw material in the production of Biodegradable polymers. However, since its thermal degradation and melting are overlapping processes, the structure of native starch must be physically modified by disrupting the crystalline structure of the granule, either by mechanical stress, pressure, or temperature, in the presence of aplasticizer. This process is called ?gelatinization? and the resulting product is known as ?Thermoplastic starch (TPS)?. This name is deduced by its processability characteristics similar to those of conventional thermoplastic polymers. The amount of plasticizer and its chemical nature exert a strong influence on the physical properties of starch in two aspects: (i) controlling its destructuring and depolymerization minimizing degradation during Processing; (ii) affectingthe final properties of the TPS, such as the glass transition temperature and mechanical properties. Starch has poor mechanical and barrier properties and is susceptible to changes in properties as a function of ambient humidity. The mechanical properties of Thermoplastic starch change as a function of time after gelatinization due to molecular reorganization, which depends on theProcessing method and storage conditions. When samples are stored below the Tg, they can suffer physical aging with densification of material. When T>Tg, samples develop retrogradation, increasing their crystallinity. Physical aging is observed for materials with plasticizer content less than 25% by weight. This phenomenon induces an increase in the strength of the material and a decrease in the deformation at break. Same strategies can be evaluated to reduce the disadvantages described above. Starch can be chemically modified producing the reaction of native starch with chemical reagents that introduce new functional groups, depending on the properties to be improved. Also, the incorporation of nanoclays to the polymer blends produces enhancements in the mechanical and barrier properties, driving to materials with high performance/cost ratio.The aim of this chapter is to evidence the advantages and disadvantages of the use of Thermoplastic starch as a replacement for conventional polymers, the strategies to improve its performance and also the use of nanoclays as fillers to improve the final properties of the material.