The compound aluminum foil is pasted on the pasting machine. The pasting machine is equipped with an unwinding shaft for aluminum foil and paper (or plastic film) and a rewinding shaft for the composite aluminum foil, a tray for holding the adhesive, and a roller system for applying the adhesive. Including guide rollers, flattening rollers, nip rollers, heating rollers, cooling rollers and temperature and tension adjustment devices.
There are three methods for pasting composite aluminum foil: wet method, dry method and thermosetting method.
Wet bonding is a method of using water solvent or water-dispersible adhesive to paste aluminum foil and composite layer together in a wet state, and then drying to evaporate water. Therefore, the composite layer is required to have certain permeability and porosity .
Wet bonding is commonly used for mounting paper. Binders should be soluble in water or can be diluted with water. They can be protein-based binders such as glue, animal glue and tyrosin, or carbohydrate-based binders such as starch, dextrin and cellulose. ; You can also use organic or inorganic binders such as water glass, acrylic acid, modified vinyl acetate and ethylene-vinyl acetate copolymer. The adhesive is usually neutral, but it can also be slightly alkaline, slightly corroding the aluminum foil and sticking firmly.
The most commonly used binders are water glass (sodium silicate, foaming alkali) and vinyl acetate. Water glass is cheap, but it becomes brittle after drying and turns yellow after long-term storage; vinyl acetate has strong adhesion, softness and water resistance, but the price is higher.
Dry bonding is used for bonding airtight plastic films. There are three types of binders: solvent types such as vinyl acetate and vinyl chloride; fiber types such as nitrocellulose, acetaldehyde cellulose, and formaldehyde cellulose; copolymers such as synthetic rubber and propylene resin.
Dry bonding first dissolves the adhesive in volatile solvents, such as acetone and ethanol, etc., coats the aluminum foil, dries, and then presses the film together with a certain pressure. Considering that the drying temperature (about 240-250°C) exceeds the plastic film’s ability to withstand the temperature and the physicochemical effect of the solvent on the plastic film, the adhesive is usually coated on aluminum foil. In order to improve the sticking fastness, the plastic film must be corona treated before sticking.
In wet and dry bonding, the type, concentration, viscosity, solid content (kg/m3), operating temperature and coating amount of the adhesive all affect the bonding fastness and operating speed. Characteristics to determine.
Thermosetting bonding is divided into two types: wax bonding and polyethylene bonding.
Wax-coated bonding uses molten paraffin as an adhesive to attach paper to aluminum foil. According to the melting temperature of wax during processing, it can be divided into low melting point (66-99 ℃), medium melting point (100-149 ℃) and high melting point (150-230 ℃) adhesive. In order to prevent surface agglomeration, about 5% of polyethylene can be added to the paraffin wax, and the use temperature at this time is 8°C higher than its cloud point. Paraffin foil aluminum foil has good elasticity and flexibility, and can be used to package cooked food such as candy, snacks, cream, bread, etc., and can also package small metal products, meters, instruments, and photographic equipment.
When pasting with polyethylene, the melting point of polyethylene is higher than that of wax. Roll coating can no longer be used. Extrusion coating is used: granular polyethylene is continuously added to the screw extruder and heated to soften. Extruded from the gap of the mold, coated between paper and aluminum foil, and paste them together. Polyethylene is divided into 3 grades: low, medium and high according to density. Low-density polyethylene should be used for bonding. The melting temperature is 260-280°C, the preheating temperature is 310°C, and the temperature of the sticking part is 205°C. The temperature should be controlled when sticking. The adhesion fastness is related to the coating thickness, generally 0.01-0.07mm, and the thicker one has greater adhesion fastness. In addition, this method preferably uses degreased soft aluminum foil. Due to the high pasting temperature, it can also be directly pasted with hard aluminum foil that has not been degreased. At this time, the temperature and speed should be mastered. In addition to ensuring firm pasting, the productivity should also be higher.