Effect of the plasticizer on permeability, mechanical resistance and thermal behaviour of composite coating films

Thin layer deposit of a composite material on solid particle surfaces used in the food industry aims to ensure the protection of food powder against aggressive environments such as a moist atmosphere. The layer, having a thickness of a few fractions of millimetre, must have certain physico-chemical properties: it must be compatible with the product, it must be impermeable to water and oxygen, it must have good mechanical strength and good adhesion to the surface of the coated powder. Furthermore the layer must fulfil the regulatory requirements for food ingredients. Film properties like continuity, permeability, and mechanical resistance depend on the choice of the excipients included in the formulation and the operating conditions which can modify the constraints generated at the interface film-powder. As a consequence, the scientific issue consists of combining the local phenomena happening at a microscopic level on the surface of the particle with the processing technology and the process parameters. In a first step, the attention is focussed on the film and its formulation. For this step, films are prepared separately and they are dried under very smooth conditions. Test samples are taken from the formed composite films and contain hydroxypropyl methylcellulose as matrix (67% of dried material), micronised stearic add as hydrophobic filler (20% of dried material) and a plasticizer (13% of dried material). The film formation procedure and the test method are described in detail. The effect of the type of plasticizer (different grades of PEG) on mechanical, thermal and permeability properties of the coating film is studied. The results show that PEG with higher molecular rate provides a better plasticizing effect for the film but increases the water vapour permeability of the film. (C) 2012 Elsevier B.V. All rights reserved.