Single-layer plastic films often cannot fully meet the requirements of protecting goods, beautifying them and adapting to processing. Therefore, there is a more ideal packaging material which is compounded with two or more layers of the same or different types of packaging materials. This composite material overcomes the shortcomings of a single material and obtains excellent properties that a single material does not have. At present, composite materials have dominated the food packaging bag
s. The basic structure of the composite material: Generally speaking, the outer layer material of the composite packaging material should be a material with a high melting point, good heat resistance, not easy to be scratched, sanded, good in printing performance and good in optical performance. Paper is often used. , aluminum foil, cellophane, polycarbonate, nylon, polyester, polypropylene, etc. The inner layer material should have heat-sealability, good adhesion, odorless, oil resistance, water resistance, chemical resistance and other properties, such as polypropylene, polyethylene, polyvinylidene chloride and other heat-resistant plastic materials. The middle layer of the composite material with more than three layers is usually made of materials with good performance and high mechanical strength, such as aluminum foil, polyvinylidene chloride, cellophane, paper, polyethylene, etc. Adhesives are applied between the layers for bonding. Generally, a solvent-based thermosetting polyurethane adhesive is used between the outer layer and the middle layer, and a modified polypropylene emulsion adhesive or a special modified carbonyl-containing propylene resin is used between the inner layer and the middle layer. The adhesive for the outer layer requires high bonding strength, simple process and low cost; the adhesive for the inner layer requires high temperature resistance, high peel strength, odorless, does not affect the nutritional content of the food, and can well maintain the color and aroma of the food. In recent years, foreign countries have developed a uniform coating of a coating agent on the surface of the substrate film to improve the air tightness, oil resistance and chemical stability of the packaging material, making it suitable for a variety of food packaging. Commonly used coating agents include polyvinylidene chloride resin and polypropylene-polyvinylidene chloride resin. Coating methods include single-sided coating and double-sided coating. Currently commonly used food packaging composite materials: double-layer composite such as cellophane/polyethylene, paper/aluminum foil, paper/polyethylene, polyester/polyethylene, polyvinyl chloride/polyethylene, nylon/polyvinylidene chloride, polyethylene/polyethylene Polyvinylidene chloride, polypropylene/polyvinylidene chloride, etc. Three-layer composite such as stretched polypropylene/polyethylene/unstretched polypropylene, polyester/polyvinylidene chloride/polyethylene, polyester/cellophane/polyethylene, cellophane/aluminum foil/polyethylene, wax/paper/polyethylene Ethylene etc. Four-layer composite such as cellophane/polyethylene/stretched polypropylene/polyethylene, polyvinylidene chloride/cellophane/polyvinylidene chloride/polyethylene, paper/aluminum foil/paper/polyethylene, etc. Five-layer composite such as polyvinylidene chloride/cellophane/polyethylene/aluminum foil/polyethylene, etc. Six layers composite such as polyethylene/paper/polyethylene/aluminum foil/polyethylene/polyethylene etc.
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