What are the basic structures of composite materials?

Generally speaking, the outer material of composite packaging material should be high melting point, good heat resistance, not easy to scratch and grind, good printing performance and good optical performance. Paper, aluminum foil, cellophane, polycarbonate, nylon, polyester, polypropylene and so on are commonly used.

The inner material should have heat sealing, good adhesion, odorless, non-toxic, oil-resistant, water-resistant, chemical-resistant properties, such as polypropylene, polyethylene, polyvinylidene chloride and other heat-resistant plastic materials.

The middle layer of three or more layers of composites is usually made of materials with high mechanical strength, such as aluminum foil, polyvinylidene chloride, cellophane, paper, polyethylene and so on.

Adhesives are coated between layers for bonding. Solvent based thermosetting polyurethane adhesives are used between general and intermediate materials. Modified polypropylene emulsion adhesive or special modified carbonyl propylene copolymer resin are used between the inner layer and the middle layer. The adhesive for outer layer requires high bonding strength, simple process and low cost, while the adhesive for inner layer requires high temperature resistance, high peeling strength, non-toxic and tasteless, which does not affect the nutritional composition of food and can maintain the color and fragrance of food well.

In recent years, a uniform coating agent has been developed on the surface of the base material film abroad to improve the air tightness, oil resistance and chemical stability of packaging materials and make them suitable for a variety of food packaging. The common coating agents are polyvinylidene chloride resin and polypropylene-polyvinylidene chloride resin. There are two ways of coating: one-sided coating and two-sided coating.