With the continuous development of the social economy, consumers have put forward higher requirements for the convenience, safety and aesthetics of food packaging, which makes the application of new packaging including easy-tear covers increasingly widespread.
As a new type of cover film packaging, the easy-tear cover has the advantages of safety and hygiene, beautiful appearance and easy opening. It is mainly used in food packaging and has now become a standard cover type of international food metal cans. In addition, it is notable for its environmental friendliness – the wafers left by the punching process of the base cover can be recycled and reused; the food-grade composite film on the cover is smaller in thickness and mass, saving natural resources, even if it is opened by consumers Discarded after use will also degrade naturally without causing environmental pollution.
As far as easy-tear lid heat-sealing materials are concerned, as a solution to overcome the problem of “easy rust in the incision” caused by the easy-tear lid sealing film, domestic easy-tear lid heat-sealing materials have achieved great results in product development and application exploration. Significant progress has been made, and it is expected to help industry users improve economic benefits through import substitution.
Easy-tear cover processing technology
Easy-tear cover is a new composite technology that is bonded by aluminum foil and tinplate or coated metal substrate. It has the characteristics of high sealing and certain pressure resistance. Its processing technology includes: using tinplate or aluminum as the basic material, after punching, stretching, and rounding, shaping treatment, so as to achieve the purpose of being safe and not hurting hands after opening; covered with metal composite film or ultra-high temperature resistant The sterilized composite plastic film is then processed by handle making and embossing for a beautiful appearance and easy opening.
Types and application characteristics of easy-tear lid sealing film
Most of the heat-sealed easy-tear covers use the aluminum-plastic lamination process, and the materials used to achieve the bonding properties are modified polyolefins or polyvinyl acetates. After heating, the film layer melts and is bonded to the substrate.
Polyolefin-based film Polyolefin-based film has excellent water resistance due to its low polarity, no hydrophilic group, and is often used in the easy-tear-cover packaging of yogurt, lunch boxes, hot pot takeaway and other products. But it is also because of the low polarity that modified polyolefins are usually pre-coated with film glue on the aluminum foil, and the film can be well adhered to the aluminum foil substrate through the glue.
Polyvinyl acetate film Polyvinyl acetate film has a high content of ester bonds in the molecular structure, and its water resistance is weakened, so it is often used in the easy-tear cover packaging of dried fruits, tea leaves and nuts.
After long-term research and development, the application of coated iron or coated aluminum easy-tear cover products has gradually matured. However, since the metal substrate at the incision is directly exposed after the lamination product is formed, it is easy to corrode, which affects the food safety and the adaptability of the product. Therefore, how to effectively avoid or reduce the corrosion of the incision has become the current lamination and easy-tear cover industry. a major problem.
Types and application characteristics of easy-tear cover heat sealant (roller hot melt adhesive)
At present, in addition to film-coated easy-tear cover products, there are also some easy-tear covers that use a roll-coating hot-melt adhesive process, and the film thickness is relatively thin (about 10 μm). Compared with the lamination products obtained by extrusion coating, cast film, blown film and other production methods, the use of roll coating hot melt adhesive technology can not only improve production efficiency, save material costs, but also obtain higher product quality stability. More importantly, due to the strong adhesion between the coating and the substrate, the degree of notch corrosion is reduced to a certain extent.
The hot melt adhesives involved in the roll coating process of the easy-tear cover are currently concentrated on materials such as modified saturated polyester, ethylene acrylate copolymer, and modified polyacrylate. , the market scale is gradually expanding, and the application prospect is broad.
Saturated polyester resins Saturated polyester resins are prepared from different polyacids and polyols. By controlling the structure of the monomers used, two different structures, Linear and Branched, can be obtained (Figure 4). If a linear structure of polyols and polyacids is used, the synthesized resin has a linear structure and is very flexible. If a polybasic acid and polyol containing a benzene ring are used, the synthesized resin contains a benzene ring structure. The rigidity of the benzene ring can give the resin hardness, and the stable structure of the benzene ring can give the resin chemical resistance. Because the main chain of the polyester resin contains more ester bonds, the molecular chain has good flexibility, and the terminal or side chain contains polar carboxyl or hydroxyl groups (that is, the acid value or hydroxyl value we often say), they can either As a heat sealant itself, it can also be cured into a film by cross-linking with isocyanate, amino resin and other resins to improve the chemical resistance of the adhesive layer.
Formula developers can screen resins according to product requirements, select appropriate curing agents for product design, and related easy-tear lid product applications include takeaway lunch boxes, dried fruit cans, milk powder cans, etc.
Ethylene acrylate copolymer Ethylene acrylate copolymer is a polymer based on acrylate monomers, by introducing ethylene or vinyl monomers for copolymerization, and using oxygen or peroxide as initiators through free radical polymerization. Its structure is shown in Figure 5(A). The introduction of vinyl monomers endows polymers with adhesiveness and film-forming properties; acrylate monomers provide polymers with good substrate adhesion properties. As the content of vinyl monomers increases, the properties of such polymers will gradually approach those of vinyl polymers—chemical resistance increases, but their stability in solution may decrease to a large extent, so it is necessary to adjust the two The proportion of class monomers to achieve the desired product performance. At present, this kind of polymer has oily and water-based systems, and the oil-based system is widely used. Because vinyl monomer is difficult to disperse in aqueous solution, its application is limited to a certain extent. At present, the vinyl content of water-based ethylene acrylate copolymer on the market is less than 20%. Milk cans, etc.
Modified polyacrylate polymer, modified polyacrylate polymer refers to the polymer obtained by repolymerization or graft modification of polyacrylate through chemical reaction between macromolecules after the completion of polymerization. Modified polyacrylate polymers include polyurethane-modified polyacrylates, polyester-modified polyacrylates, silicone-modified polyacrylates, etc., all of which have the advantages of both modified polymers and polyacrylates. Polyacrylate has the flexibility of the polymer chain, its glass transition temperature (Tg) is low, and the temperature varies with the number of carbon atoms in the ester group and its branching, and it will form a glossy paint film after coating. , strong adhesion, not easy to peel off, can be used in adhesives in different occasions, including easy-tear cover products.
Formula developers can screen resins according to product requirements and select suitable curing agents for product design. The application of related easy-tear lid products covers takeaway lunch boxes, dried fruit cans, milk powder cans, etc.
The localization of easy-tear cover hot melt adhesive will help users reduce costs
At present, most suppliers of easy-tear cover materials are foreign manufacturers, with high prices and relatively limited supply. Therefore, realizing the localization of easy-tear cover materials is a direction for material people.