Strategies to Produce Thermoplastic Starch–Zein Blends: Effect on Compatibilization

Abstract

Different strategies to produce thermoplastic materials using starch and zein were studied, aiming to investigate their effect on the compatibility of starch and zein. The strategies comprised the use of two different plasticizers for starch, two different compatibilizing agents, and two blending procedures. The plasticizers were mixtures of sorbitol and glycerol (SG) or urea and formamide (UF). The UF and maleated starch (MS) were used as compatibilizing agents. The blending procedures included: (1) thermoextruding starch and zein as premixed powder materials (TP[Mix]) and (2) coextruding the biopolymers previously thermoplasticized with suitable plasticizers (Mix[TP]). As observed by the tensile tests, scanning electronic microscopy, and dynamic mechanical analysis, the segregation of phases occurred at different extents in all the starch–zein blends. The materials made with MS through the TP[Mix] procedure presented the most severe phases segregation, while the materials made with UF showed higher compatibility between starch and zein. Fourier transform infrared spectroscopy (FTIR) suggests that increased zein content leads to a lower molecular order, which was ascribed to diminished molecular entanglement. Thermogravimetric analysis and FTIR analysis showed that the chemical interaction between starch and zein occurred more extensively in slabs made with UF than those made with MS. In addition, foamability was evaluated for the selected materials using supercritical CO₂. Neat thermoplasticized starch plasticized with UF and themoplasticized zein with polyethylene-glycol 400 showed good suitability to be foamed, producing foams with porosities above 85%. Starch plasticized with SG and starch–zein blends yielded compact structures with minor porosities after foaming processing.