1. Corona treatment of plastic film surface:
Before printing and compounding, the substrate PE, PP, or even aluminum foil must be corona treated. The purpose is to improve the firmness of the printing compound, to achieve the desired effect, and to ensure the critical surface tension during printing and compounding Greater than 38 mN / m; for composite films used for cooking, the thickness of the inner substrate should be greater than 70μ to ensure sufficient heat seal strength.
2. Compound process:
The composite film can improve various characteristics of a single film and can improve the protection of the contents, including moisture resistance, oil resistance, barrier properties, shading properties and air tightness. According to different substrates and different uses, the processing methods of composite films can be roughly divided into the following five categories:
A. Dry compound method
In this method, the adhesive is applied to the surface of the base material by a laminating machine, and is laminated with other films by heating rollers. This method can be applied to almost all substrates, and any number of layers can be compounded, which has been widely spread to various fields of packaging materials. The equipment cost is cheaper than the extruder. Recently, due to the more stringent requirements on packaging function for cooking treatment and boiling sterilization, adhesives are mostly made of polyurethane series with excellent adhesion. In addition, the residual solvent in the dry compound method causes hazards such as pollution, fire, and explosion. To solve these problems, a compound system of different organic solvents should be developed. It has the following advantages:
· No solvent, reduce environmental pollution.
· There is no danger of fire and explosion.
· No solvent drying process is needed to save energy and equipment area.
· No solvent erosion of printing ink.
· No investment in solvent recovery equipment is required.
· Cost reduction due to increased production speed.
B. Wet compound method
This method is mainly used for the combination of aluminum foil and porous materials such as paper, tissue paper, and ordinary cellophane. The water-soluble adhesive (gelatin, starch) and water-dispersible adhesive (vinyl acetate latex, etc.) are applied to the surface of the base material in a wet state and compounded with other materials, and then pressed and dried by roller. This method is simple to operate, and can be compounded with a small amount of adhesive. The extremely thin aluminum foil (5 to 6μ) can also be compounded. Compared with other compounding methods, it is fast and can be processed in large quantities, and the processing cost is very low. It does not use organic solvents, has good safety, no residual solvents, and the market centered on paper-based materials is relatively stable.
C. Extrusion compound method
This is the most commonly used method in compound processing. An extruder is used to extrude PP, PE, EVA (ethylene-vinyl acetate copolymer) ionic resin, etc. into a film form, which is coated with a processing agent (polyethyleneimine, Polyurethane resin, etc.) are compounded on various films, and then cooled and solidified. This method saves labor due to the use of adhesive resins (maleic anhydride-modified olefin resins, block-modified olefin resins such as maleic anhydride, etc.). Therefore, no adhesive is needed, and the process can be reduced to achieve the purpose of reducing costs.
D. Hot melting compound method
The method is to mix rosin, xylene resin, styrene resin, etc. with base resins such as EVA (ethylene-vinyl acetate copolymer), EEA (ethylene-ethyl acrylate copolymer), PE, butyl rubber, etc. Adhesives, paraffin wax and other bottom-reducing agent hot melts are coated on the surface of film, paper, and aluminum foil, and are immediately compounded on other substrates and cooled.
It is mainly used in the production of lid materials for various containers, and requires retort resistance. It is mixed with an adhesive agent in EVA and other substrate resins, and it is used as a substrate for extrusion coating to improve heat resistance.
E. Coextrusion compound method
This method is formed by two or three extruders at the same time to obtain more than two layers of film, there are T die method and blow molding method, can be a variety of compound, can produce ultra-thin film. Japan's annual output reaches nearly 10,000 tons, while more than a quarter of the US composite film is coextruded. Representative films are: PE / PVDC / PE, PE / PP / PE, PP / PE / PVA, PE / EVA, N / PE, etc.
Although this method has no problem in the bonding of the same resin, it can not be used to bond nylon or EVAL with polyolefin, and it can be prepared by using modified polyolefin with ionic resin-maleic anhydride palace energy group Good composite membrane. These coextruded films have absolute advantages in the field of food packaging, and the market share is very high.
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