Effect of Polyolic Plasticizers on Rheological and Thermal Properties of Zein Resins

One of the zein film forming methods is thermomoulding by hot press. Pure zein film is very brittle and plasticizers can improve its mechanical and film making properties. In this research, polyols (glycerol, sorbitol, and manitol) at three levels (0.5, 0.7 and 1 g/g of zein resin) were used separately as a plasticizer and for determining their plasticization effectiveness the rheological and thermal properties of zein resin samples were studied. Sorbitol and glycerol had good plasticizing effects and could decrease viscoelastic modulus of zein resin considerably, but manitol was not as effective as them. Resins containing manitol remained rigid and had unfavorable chalky appearance. The effects of plasticizers on thermal properties of resin were investigated by DSC at -100 to 150°C. No crystallization and melting peaks related to zein resin and plasticizers were observed. Thermograms showed that polyolic plasticizers and zein resin remained a homogeneous material throughout the cooling and heating cycles. As well as, at glass transition temperature, there is not any significant difference among zein resins containing various plasticizers. The Tg of different resins were at 60 -70°C. This was probably due to similar molecular weight of the selected polyols. In addition, after initial decreasing of Tg of resin, subsequent increase of plasticizer levels did not cause considerable reduction in it.