With the development of China's packaging industry, China's packaging has grown from simple paper packaging to single-layer plastic film packaging to the widespread use of composite materials. The composite film can make the contents of the package have the characteristics of moisturization, fragrance preservation, beautiful appearance, freshness preservation, light protection, anti-permeation, and extended shelf life, etc., so it has developed rapidly.

Experts point out that composite materials are two or more materials, which are combined together after one or more composite processes to form a composite material with certain functions. Generally can be divided into base layer, functional layer and heat seal layer. The base layer mainly plays the role of aesthetics, printing and moisture resistance. Such as BOPP, BOPET, BOPA, MT, KOP, KPET, etc .; the functional layer mainly plays the role of blocking, avoiding light, etc., such as VMPET, AL, EVOH, PVDC, etc .; the heat sealing layer is in direct contact with the packaging items, which is adaptable and resistant to penetration Performance, good heat-sealability, and transparency, opening day and other functions, such as LDPE, LLDPE, MLLDPE, CPP, VMCPP, EVA, EAA, E-MAA, EMA, EBA, etc.
The following is a brief description of the development, development and current situation of the inner layer of composite flexible packaging materials.
1. LDPE, LLDPE resin and film
China's composite film started in the late 1970s. From the early to mid-1980s, China began to introduce some extruders, film blowing machines and printing machines to produce simple two-layer or book-layer composite materials. Such as extruded composite BOPP / PE, paper / PE, pp / PE; dry composite BOPP / PE, PET / PE, BOPP / AL / PE, PET / AL / PE, etc. Among them, LDPE resin and film are often blended with a certain proportion of LLDPE to enhance its strength and stiffness. Mainly used in the packaging of instant noodles, biscuits, mustard and other foods. Common coating-grade LDPE resins are: IC7A, L420, 19N430, 7500, etc .; blown film-grade LDPE resins are: Q200, Q281, F210-6, 0274, etc .; LLDPE resins are: 218w, 218F, FD21H, etc.
2. CPP film, CPE film
From the end of the 1980s to the beginning of the 1990s, with the introduction of a new generation of flexible packaging equipment and casting equipment, the scope of packaging contents was further expanded. Some puffed foods, cereals and other packaging bags have higher transparency requirements, while boiling, high temperature Sterilization products have been asked to the market one after another, and the requirements for packaging materials have also increased accordingly. The inner layer materials mainly based on LDPE and LLDPE can no longer meet the requirements of the above products. Produced by the casting method has good heat sealability. CPP with oil resistance, transparency, fragrance retention, special low-humidity heat-sealability and high-temperature cooking property is widely used in packaging. Aluminium-plated CPP developed on this basis is widely used because of its metallic luster, beauty, and barrier properties. And the CPE film produced by the casting method is being further used because of its unidirectional easy tearing, low temperature heat sealing, and good transparency.
3. MLLDPE resin
With the continuous development and changes of the packaging market, there are more and more special requirements for packaging. Dow Chemical Company of the United States produced metallocene polyethylene MLLDPE by metallocene catalytic polymerization. Such as APFINITY, POP1880, 1881, 1840, 1450 and other resins. Then, Exxon in the United States, Mitsui in Japan, and Philips in the United States also produced MLLDPE. Such as EX-CEED350D60, 350D65 of Exxon, E-VOLVESP0540, SP2520 of Mitsui Petrochemical, MPACTD143, D139 of Philips, etc. Because MLLDPE has good blendability and easy processability with LDPE and LLDPE, MLLDPE can be mixed in blown film or casting processing, and the mixing ratio can be from 20% to 70%. This type of film has good tensile strength, impact strength, good transparency, and good low-temperature heat sealability and pollution resistance. The composite material with its inner layer is widely used in frozen, refrigerated food, shampoo, Oil, vinegar, soy sauce, detergent, etc. It can solve the packaging speed, breakage, leakage and penetration of the above products during packaging production and transportation.
4. Inner material of cover film
The main materials of jelly, juice, yogurt, fruit milk, soup and other liquid packaging cups and bottles are HDPE, PP, PS, etc. The cover film of this package must consider both the shelf life and the heat seal strength between the cover film and the cup, as well as the convenience of consumers for use-easy to tear. To achieve this particularity, the inner layer material can only form an interface bonding strength with the cup, but cannot fully penetrate and fuse together. Generally, modified EVA resin is used. Such as DuPont APPEEL53007 of the United States. TOPCOL-3388 of Japan Toyo, LOTRYL20MA08 of France, WT231 of HIRO.DINE of Japan, etc. Its structure can be: PET / PE / HM, BOPA / PE / HM, PET / VMPET / PE / HM, PET / AL / PE / HM, paper / PE / AL / PE / HM. For the requirement that the cover film and the bottom cup are firmly closed and not torn apart, it is generally required that the material of the bottom cup is the same as the material of the inner layer of the cover film so that the two materials are completely fused when heat-sealed. Such as HDPE cup, the inner material of the cover film is: LDPE or EAA; PP cup, the inner layer of the cover film is CPP film; PET bottle, we have found a modified PET film as a heat seal layer, Pesticide PET bottles were sealed and satisfactory results were achieved.
4. Coextruded film
Films produced with a single layer of LDPE or LDPE blended with other resins have a single performance and cannot meet the packaging requirements of modern article development. Therefore, the overall performance of coextruded films produced by coextrusion blown film or coextrusion casting equipment is improved. . Such as the mechanical strength, heat sealing performance, heat sealing temperature, barrier property, opening property, anti-pollution property and other comprehensive properties of the film are improved, and its processing cost is reduced, and it is widely used. Such as H layer co-extruded blown film hot melt adhesive film: PE / HM, cable film PE / EAA, MLLDPE low temperature heat seal film, EVA cover film and antistatic film, slip film. H-layer co-extruded cast co-extruded CPP, unmodified PP / heat sealable PP, etc. Three-layer and five-layer nylon coextruded membranes, five-layer and seven-layer EVOH, and PVDC high-barrier membranes are also continuously developed and widely used.
The development of the inner film of composite flexible packaging materials, from LDPE, LLDPE, CPP, MLLDPE, to the current large-scale use of co-extruded films, basically realize the functionalization and personalization of packaging, and meet the requirements of packaging quality, processing performance, transportation ,Storage conditions. With the continuous introduction of new materials and the improvement of inner layer film production technology and equipment, the inner layer film of composite flexible packaging materials will surely develop rapidly and promote food.
5. Other
With the progress of society, human needs continue to grow, and various functional and environmentally friendly packaging films continue to appear. For example, in addition to being a single-layer packaging material, water-soluble polyvinyl alcohol films that are environmentally safe, thoroughly degraded, and have good heat-sealing performance are also being developed as inner-layer films.

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