Biodegradable Thermoplastic Starch (TPS): Present and Future Applications

M.A. VILLAR

San Sebastian, Donostia

Simposio; XVII Simposio Latinoamericano de Polímeros, SLAP?22, XV Congreso Iberoamericano de Polímeros, CIP?22 y XVI Reunión del Grupo Especializado de Polímeros, GEP?22; 2022

GEP

Nowadays, several new bio-based materials are developed in order to partially substitute synthetic ones. Among others, starch is a good candidate because of its availability, biodegradability, and low cost. Native starch presents a semi-crystalline granular structure which can be destroyed to convert it into thermoplastic starch (TPS), when processed under conditions of pressure and temperature in the presence of plasticizers such as water and glycerol. On the other hand, thermoplastic starch can be processed using technologies used for synthetic polymers, such as extrusion and blow molding, compression and injection molding, among others. Given the wide range of botanical sources, native starches can meet the requirements for some applications. However, native starches are not always suitable for some industrial applications due to functional limitations, such as low resistance to shear stress, poor thermal properties, and a high tendency toward retrogradation. These limitations can be improved greatly by starch modification, which generally involves physical (heat-moisture treatment, annealing, pregelatinization, high-pressure treatment, radiation, and sonication), chemical (crosslinking, substitution, acid hydrolysis, and oxidation/bleaching), or enzymatic processes. However, the hydrophilic nature of starch conditions its use in many applications, since it gives the final materials poor mechanical properties and a low barrier to water vapor. Then, for different applications, it is improbable to use starch as raw material, even after some modifications. Hence, it is necessary to use starch blends and composites. One approach to overcome these drawbacks is the reinforcement of TPS matrix adding natural organic or mineral fillers. There are many works dealing with TPS based materials containing different clay minerals. Among mineral fillers, talc is considered as a good candidate in order to reinforce different kind of matrixes. Talc is a phyllosilicate with a layered structure which consists of octahedral brucite [Mg(OH)2] sheets sandwiched between two tetrahedral silica [Si2O5] sheets. Talc structure is composed of two kinds of surfaces: a weakly energetic “basal surface” with basic and hydrophobic Si-O-Si groups and a more energetic “edge surface” with acid and hydrophilic Si-OH groups and residual magnesium cations. Thus, both layered structure and the nanometric size thickness make talc particles suitable to be intercalated by polymer chains during composite processing. Composites final properties and performance depend on the starch botanical source, its physical or chemical modifications and also on the filler nature, composition, and size. On the other hand, processing is also relevant in order to obtain a proper filler dispersion and adhesion to the matrix in order to guarantee a significant improvement in composites final properties.Blending TPS with other polymers also represents an alternative to improve some of the aforementioned limitations of TPS. However, there are some research and industrial challenges in terms of producing starch-based blends with commercial utility, such as: miscibility problems and poor mechanical properties at high starch contents, and costs reduction, especially for biodegradable starch-polyester blends at low starch contents (< 30 wt%). The most common biodegradable materials are blends of TPS and aliphatic/aromatic polyesters, such as poly(lactic acid) (PLA), poly(butylene adipate terephthalate) (PBAT), poly(hydroxybutyrate) (PHB), and poly(vinyl) alcohol (PVA). In the last years, PLA and PHB have attracted attention because they are obtained from renewable resources.As it has been mentioned, mechanical, thermal, and barrier properties of TPS materials can be tailored to meet specific products’ requirements. For vegetables packaging, TPS materials with low to moderate barrier to moisture and gases should be adequate to maintain the freshness, thus prolonging the shelf life of vegetables.Considering the global market of polymer materials, starch-based plastics occupied a relevant position amongst the bio-based ones, with a relevant role in several applications and an increasing market among bio-based materials in the last few decades.