About composite flexible packaging heat sealing layer materials

First, polyethylene film

Polyethylene used for composite heat seal layer, mainly low density polyethylene and linear low density polyethylene (some products use medium density and high density polyethylene film as heat seal layer), and generally use two kinds of polyethylene Use after a certain proportion of blending. According to different processing methods, it can be divided into blown film, cast film and extrusion coated film. The thickness of the composite polyethylene film is selected according to the requirements of the packaged material, and the thickness may be about 20 μ, and the thickness may be more than 100 μ. Since there are many additives in the resin for producing a film, particularly a slip agent, attention should be paid to the migration of the additive during production.

Metallocene polyethylene can be said to be a revolution of polyethylene resin, which makes the film made of metallocene polyethylene have good hot-adhesive strength, heat-sealing strength and certain resistance to sealing pollution. At present, the film produced by blending metallocene polyethylene and low-density polyethylene has been widely used in oil bags, shampoo bags, washing powder bags and the like. At present, the heat sealing layer of the composite package is mostly made of polyethylene, such as instant noodles, biscuits, mustard, and pickles. The blown plastic film and the cast film are subjected to an electric spark treatment in the middle of the process to make the film surface tension greater than 38 dynes/cm.

Second, polypropylene film

The polypropylene film can also be classified into a blown film (under water blowing method), a cast film, and an extrusion coated film. Among polypropylene films, cast polypropylene film (CPP film) is the most used one. It is mainly used in high temperature cooking packaging, high transparency packaging, and CPP film aluminum plating to make aluminum CPP. (VmCPP), used in the packaging of puffed food, packaging of instant noodles, etc.

The high temperature cooking CPP and the aluminum plating CPP raw materials are different from the ordinary CPP raw materials, and have special requirements. In addition to the general binary copolymer, the high temperature cooking is performed by using a block copolymer, and its high temperature resistance should be The best; and the raw material of the aluminized CPP is preferably a terpolymer. In order to save costs, some companies can use polypropylene blown film as the inner layer for products that are not demanding. The surface of the film is also subjected to an electric spark treatment, preferably with a surface tension of more than 40 dynes/cm.

Third, the cover film

Most of the foods such as jelly, cold drinks, yogurt, and fruit milk that are supplied on the market are packaged in plastic cups and plastic bottles, and the cups and bottles are sealed with a film. Some requirements can be uncovered, and some do not need to be uncovered. The heat-sealing layer that is not to be uncovered is generally used in packaging of milk, fruit milk, etc., and the cup body or the bottle body is basically made of high-density polyethylene (HDPE) resin. The inner layer material may be made of the same material as the cup (bottle). On the one hand, there must be better sealing strength, and the other side must have releasable properties. For this special requirement, the choice of heat sealing layer is very important.

The material that has both functions is a modified EVA. EVA-ethylene-vinyl acetate copolymer, which has good adhesion to polypropylene, polystyrene, etc., but it has different molecular structure from polypropylene and polystyrene. When heat-sealing and bonding, the two resins cannot be completely Diffusion, penetration, and melting together. Only a bond with a distinct interface can be formed. Therefore, the modified EVA is selected as the heat seal layer of the cover film, which has sufficient heat seal strength and easy peeling releasability.

Fourth, special heat sealing materials

In some special occasions, such as coffee packaging, meat packaging, body packaging, etc., some special heat sealing materials, such as ionic polymer, EAA (ethylene-acrylic acid copolymer), EEA ( Ethylene-ethyl acrylate), EMAA (ethylene-methacrylic acid), and the like. These special materials need to be imported from developed countries such as Europe, America, and Japan, and they are rarely used in China. However, with the rapid development of China's national economy, the requirements for packaging materials are getting higher and higher. The use of these special heat sealing materials is also becoming more widespread.

Five, co-extruded film

With the development of the packaging industry, the market competition is fierce, and more defects of single-layer film gradually appear, which restricts the promotion and application of resin. The use of co-extrusion blown film or co-extrusion casting process to produce a coextruded film has the following advantages:

1. Reduce production costs: In order to improve the heat sealing performance of the film, many manufacturers now use LDPE and mLLDPE blends to produce films. The increase in cost per ton of material when produced in single-layer equipment is higher than the cost per ton of material produced when using multi-layer co-extrusion equipment.

2. Improve the performance of the heat seal layer:

(1) Barrier performance. When producing co-extruded film, high barrier materials such as NY and EVOH can be used to greatly improve the barrier properties of the film to gas.

(2) Heat sealing performance, the selection of suitable mLLDPE can improve the heat sealing strength, hot tack strength, sealing temperature and heat sealing temperature range of the film.

(3) Physical properties, the selection of suitable resin can improve the tensile strength, tear strength, impact strength and elongation at break of the film. In addition, according to different requirements of customers, different resins and different grades of resin are selected to improve the stiffness, optical properties and surface friction coefficient of the film.

In short, due to the excellent performance and reasonable price of the coextruded film, it has developed rapidly. At present, coextruded films are developed from two layers, three layers to five layers, seven layers, nine layers or even more.