Thermoplastic starch/polyvinyl alcohol blends modification by citric acid-glycerol polyesters.

Thermoplastic starch/polyvinyl alcohol (TPS/PVA) films have limitations for being used in long-term applications due to starch retrogradation. This leads to plasticizer migration, especially when low molecular weight plasticizers such as glycerol, are used. In this work, we employed mixtures of oligomers based on glycerol citrates with higher molecular weight than glycerol as plasticizers for potato-based TPS/PVA blends obtained by melt-mixing. This constitutes an alternative to reduce plasticizer migration while keeping high swelling degree, and to provide high mechanical performance. The novelty lies in the usage of these oligomers by melt-mixing technique, aspect not deeply explored previously and that represents the first step towards industrial scalability. Prior to the blending process, oligomers mixtures were prepared with different molar ratios of citric acid (0-40 mol%) and added them. This minimizes the undesirable hydrolysis effect of free carboxylic groups on starch chains. The results demonstrated that the migration of plasticizers in TPS/PVA blends decreased by up to 70 % when the citric acid content increased. This reduction was attributed to the higher molecular weight (the majority in the range 764-2060 Da) and the 3D structure of the oligomers compared to using raw glycerol. Furthermore, the films exhibited a 150 % increase in Young's modulus and tensile strength without a reduction in elongation at break, while maintaining a high gel content, due to a moderate crosslinking.

Study of the Plasticization Effect of 1-Ethyl-3-methylimidazolium Acetate in TPS/PVA Biodegradable Blends Produced by Melt-Mixing.

The first step towards the production and marketing of bioplastics based on renewable and sustainable materials is to know their behavior at a semi-industrial scale. For this reason, in this work, the properties of thermoplastic starch (TPS)/polyvinyl alcohol (PVA) films plasticized by a green solvent, as the 1-ethyl-3-methylimidazolium acetate ([Emim+][Ac-]) ionic liquid, produced by melt-mixing were studied. These blends were prepared with a different content of [Emim+][Ac-] (27.5-42.5 %wt.) as a unique plasticizer. According to the results, this ionic liquid is an excellent plasticizer due to the transformation of the crystalline structure of the starch to an amorphous state, the increase in flexibility, and the drop in Tg, as the [Emim+][Ac-] amount increases. These findings show that the properties of these biomaterials could be modified in the function of [Emim+][Ac-] content in the formulations of TPS, depending on their final use, thus becoming a functional alternative to conventional polymers.

Exploring the effect of humidity on thermoplastic starch films using the quartz crystal microbalance.

The quartz crystal microbalance (QCM) is used as a non-destructive and efficient characterization tool for thin thermoplastic starch (TPS) films. Thin TPS films (1-2 µm) were prepared with 30% (w/w starch) plasticizers using either glycerol or an ionic liquid, 1-ethyl-3-methylimidiazolium acetate ([emim+][Ac-]), as the plasticizer. The differences in the mechanical properties and environmental effects on the plasticized TPS films were explored. The modulus of starch-glycerol films was higher than starch-[emim+][Ac-], consistent with literature data and bulk AFM measurements, likely due to superior plasticization by the ionic liquid. The starch-[emim+][Ac-] films were shown to have relative stable properties at low humidity that may be due to some antiplasticization effects at low water content despite absorbing more water than starch-glycerol films at higher humidity.

Influence of Starch Composition and Molecular Weight on Physicochemical Properties of Biodegradable Films.

Thermoplastic starch (TPS) films are considered one of the most promising alternatives for replacing synthetic polymers in the packaging field due to the starch biodegradability, low cost, and abundant availability. However, starch granule composition, expressed in terms of amylose content and phosphate monoesters, and molecular weight of starch clearly affects some film properties. In this contribution, biodegradable TPS films made from potato, corn, wheat, and rice starch were prepared using the casting technique. The effect of the grain structure of each starch on microstructure, transparency, hydration properties, crystallinity, and mechanical properties of the films, was evaluated. Potato starch films were the most transparent and corn starch films the most opaque. All the films had homogeneous internal structures-highly amorphous and with no pores, both of which point to a good starch gelatinization process. The maximum tensile strength (4.48-8.14 MPa), elongation at break (35.41-100.34%), and Young's modulus (116.42-294.98 MPa) of the TPS films were clearly influenced by the amylose content, molecular weight, and crystallinity of the film. In this respect, wheat and corn starch films, are the most resistant and least stretchable, while rice starch films are the most extensible but least resistant. These findings show that all the studied starches can be considered suitable for manufacturing resistant and flexible films with similar properties to those of synthetic low-density polyethylene (LDPE), by a simple and environmentally-friendly process.