The plastics used in food packaging bags are made of multi-layer plastic films, including:

1. Polyethylene (PE for short)

It is a thermoplastic resin prepared by polymerization of ethylene. In industry, it also includes ethylene and small amounts α- Copolymers of olefins. Polyethylene is odorless, non-toxic and feels like wax. It has excellent low temperature resistance (the minimum service temperature can reach - 100 ~ - 70 ° C), good chemical stability and can resist the erosion of most acid and alkali (not resistant to acid with oxidizing properties)“ Polyethylene is soft and tough, lighter than water, non-toxic and has superior dielectric properties. Easy to burn and continue to burn after leaving the fire. The permeability is low, but the permeability to organic vapor is large. The transparency of polyethylene decreases with the increase of crystallinity. At a certain crystallinity, the transparency increases with the increase of molecular weight. The melting point range of high-density polyethylene is 132-135 ℃, and the melting point of low-density polyethylene is low (112 ℃) and wide. It is insoluble in general solvents at room temperature, with small water absorption and excellent electrical insulation. Above 70 ℃, a small amount can be dissolved in toluene, amyl acetate, trichloroethylene and other solvents.

Processing aspect

Polyethylene can be processed by blow molding, extrusion, injection molding and other methods. It is widely used in the manufacture of films, hollow products, fibers and daily sundries.

Make film

It can be widely used as packaging materials for various food, clothing, medicine, chemical fertilizer, industrial products and agricultural films. It can also be processed into composite film by extrusion method for packaging heavy objects. Since 1975, high density polyethylene film has also been developed. It has high strength, low temperature resistance, moisture resistance, and good printability and processability. The biggest use of linear low-density polyethylene is also to make film. Its strength and toughness are better than low-density polyethylene, and its puncture resistance and rigidity are also better. Although its transparency is poor, it is still slightly better than high-density polyethylene. PE film has strong chemical stability and certain acid and alkali resistance at room temperature, and is insoluble in any solvent. It is often the first choice for the inner material of food packaging bags.

2. Polyester film (PET film for short)

It is characterized by high temperature resistance, good printing, easy processing, good voltage resistance, good insulation, fire resistance, UL certification, 94vtm-2 certification, environmental protection, SGS and Rosh certification. It is a thin film material made of polyethylene terephthalate by extrusion and biaxial stretching. It is a colorless, transparent and glossy film with excellent mechanical properties, high rigidity, hardness and toughness, puncture resistance, friction resistance, high and low temperature resistance, chemical resistance, oil resistance, air tightness and fragrance retention. It is one of the commonly used barrier composite film substrates. However, the price of polyester film is high, with a general thickness of 0.12mm. It is commonly used as the outer material of cooking packaging, with good printability.

3. Polyamide (PA)

The main chain of a series of molecules is a polymer containing many repeated amide groups. This kind of polymer is commonly known as nylon. Compared with ordinary plastics, polyamide has the advantages of wear resistance, toughness, drug resistance, heat resistance, cold resistance, easy molding, self lubrication, non toxicity and easy dyeing. The biggest feature of the film is low oxygen transmittance. As a packaging container, it is conducive to the long-term storage of products without air leakage and moisture deterioration. It is one of the necessary basic materials in vacuum packaging. Among all varieties of polyamide, biaxially oriented nylon film is mainly used for food packaging; The utility model has the advantages of high puncture strength, impact strength, friction strength and bending strength, and has good gas barrier. The disadvantage is that it has poor heat sealing performance. When used, it is often combined with the substrate film with good heat sealing performance,

For frozen food packaging, it mainly uses the characteristics of high film strength, good puncture resistance and excellent cold resistance.

It is used for seasoning packaging of soup noodle liquid, which mainly uses its high strength, good stab resistance, especially oil resistance.

It is used for cooking resistant food packaging, mainly using its characteristics of oil resistance, heat resistance, good gas barrier and puncture resistance.

4. Biaxially oriented polypropylene film (BOPP)

Its production is that the melt of polymer polypropylene is first made into sheet or thick film through long and narrow machine head, and then stretched in two vertical directions (longitudinal and transverse) simultaneously or step by step in a special stretching machine at a certain temperature and set speed, and then made into film after appropriate cooling or heat treatment or special processing (such as corona, coating, etc.).

BOPP has high transparency and gloss, excellent ink and coating adhesion, heat sealing strength, water vapor and grease barrier performance, ultraviolet barrier performance, high-grade appearance of pearl luster, high barrier and low electrostatic performance. It is often used as the basic material for bag making and printing composite of packaging bags.

The materials of packaging bags and containers are closely related to chemical properties. The above is a part of the basic material film of common food packaging bags. For example, kraft paper, aluminum foil, CPP and yopp are also one of the composite materials of food packaging bags.