Effect of hydrolysis on the properties of the copolymers of ethylene-co-vinyl-acetate: DSC and permeation

Abstract

As soon as they are separated from their roots, fruits and vegetables exhibit multiple biochemical reactions consuming O₂ and producing H₂O and CO₂. To obtain a controlled atmosphere favorable for food conservation, packages must have high H₂O/CO₂ and O₂/CO₂ selectivities. It has been demonstrated that poly(ethylene-co-vinyl-acetate) films (EVA) of various vinyl acetate quantities are promising materials. By changing the content of vinyl-acetate (VA) in polyethylene (PE), it has also been observed that a composition of 70% w/w of VA in PE leads to a completely amorphous material. In this work, pseudo ≪ bi-layer ≫ membranes, based on poly(ethylene-co-vinyl acetate) (EVA) containing 70 wt% of vinyl acetate, have been prepared by a treatment of unilateral hydrolysis using solutions of sodium hydroxide dissolved in a mixture of water and methanol. The influence of this hydrolysis treatment on O₂ and H₂O permeation properties is investigated. The existence of structural changes as a function of the hydrolysis duration (0–16 h) is checked by means of DSC. Without hydrolysis, a glass transition at T_g = -20 °C with ΔC^p ≈ 0.56 J/g/K is obtained. By increasing the hydrolysis time, we find that T_g = constant, ΔC_p decreases and an endothermic melting peak (≈125 °C) appears. These results lead to the conclusion that a crystalline phase occurs through hydrolysis. On the other hand, the O₂ permeability decreases with the reaction time, while the H₂O permeability passes through a maximum for a hydrolysis lasting 30 min.

Keywords:

• Water & gases permeation
• DSC
• EVA
• EVOH
• Hydrolysis
• Plasticization
• Clustering