As the heat-sealing layer of the composite packaging material, since it is in direct contact with the packaged object, and the packaged object is mostly food, it is first necessary to consider its hygienic properties, and also its transparency, penetration resistance, Good heat sealability and opening properties. When designing packaging, the heat-sealing layer is an important part of composite packaging to meet the requirements of item packaging and must not be taken lightly. The following is a description of the materials used in the heat-sealing layer of the composite packaging material.
1. Polyethylene film
The polyethylene used for the composite heat-sealing layer is mainly low-density polyethylene and linear low-density polyethylene (some products also use medium-density and high-density polyethylene films as the heat-sealing layer), and generally use two types of polyethylene Use after blending in a certain proportion. According to different processing methods, it can be divided into blown film, cast film and extrusion coating film. The thickness of the composite polyethylene film can be selected according to the requirements of the packaged material. The thinner can be about 20μ, and the thicker can be greater than 100μ. Since there are many additives in the resin used to produce films, especially slip agents, the migration of additives should be paid attention to during production.
Metallocene polyethylene can be said to be a revolution of polyethylene resin, so that the film made of metallocene polyethylene has good thermal adhesive strength, heat sealing strength, and certain anti-sealing pollution performance. At present, the film produced by blending metallocene polyethylene and low-density polyethylene has been widely used in oil bags, shampoo bags, and washing powder bags. At present, most of the heat-sealing layer of composite packaging is made of polyethylene, such as instant noodles, biscuits, mustard, pickles, etc. The blown plastic film and cast film must be processed by electric spark in the middle of the processing, so that the surface tension of the film is greater than 38 dyne/cm.
2. Polypropylene film
Polypropylene film can also be divided into blown film (down-blowing water cooling method), cast film and extrusion coated film. Among polypropylene films, cast polypropylene film (ie CPP film) is the most used one. It is mainly used in high-temperature cooking packaging and packaging with high transparency requirements. Aluminized CPP is made of CPP film. (VmCPP), used in puffed food packaging, instant noodle packaging, etc.
The raw materials of high-temperature cooking CPP and CPP for aluminum plating are different from those of ordinary CPP and have special requirements. In addition to the general binary copolymers, block copolymers are used for high-temperature cooking. Its high temperature resistance should be The best; the raw material of aluminized CPP is preferably a terpolymer. In order to save costs, some companies use polypropylene blown film as the inner layer for products that are not demanding. The surface of the film should also be treated with electric spark when the film is produced. It is best to make the surface tension of the film greater than 40 dyne/cm.
3. Covering film
Most of the jelly, cold drinks, yogurt, fruit milk and other foods available on the market are packaged in plastic cups and plastic bottles, and the cups and bottles are sealed with film. Some requirements can be uncovered, and some do not need to be uncovered. The heat-sealing layer that does not need to be uncovered is generally used on the packaging of milk, fruit milk, etc. The cup or bottle is basically made of high-density polyethylene (HDPE) resin. The inner layer material only needs to be made of the same material as the cup body (bottle body). On the one hand, it must have better sealing strength, and on the other hand, it must have releasability. For this special requirement, the choice of heat-sealing layer is very important.
Four, special heat sealing material
In some special occasions packaging, such as coffee packaging, meat packaging, skin packaging, etc., some special heat-sealing materials, such as ionic polymers, EAA (ethylene-acrylic acid copolymer), EEA ( Ethylene-ethyl acrylate), EMAA (ethylene-methacrylic acid), etc. These special materials need to be imported from developed countries such as Europe, America, Japan, etc., and they are rarely used in China. However, with the rapid development of my country’s national economy, the requirements for packaging materials are getting higher and higher. The use of these special heat-sealing materials is becoming more and more extensive.
Five. Co-extruded film
With the development of the packaging industry and fierce market competition, many shortcomings of single-layer films have gradually emerged, restricting the popularization and application of resins. Using co-extrusion blown film or co-extrusion casting process to produce co-extruded film has the following advantages:
1. Reduce production costs: For example, in order to improve the heat-seal performance of the film, many manufacturers now use LDPE and mLLDPE blends to produce films. The increase in material cost per ton when produced with single-layer equipment is higher than that when produced with multi-layer co-extrusion equipment.
2. Improve the performance of the heat seal layer:
(1) Barrier performance. When producing co-extruded films, using high barrier materials such as NY and EVOH can greatly improve the film’s gas barrier performance.
(2) Heat-sealing performance. Choosing appropriate mLLDPE can improve the heat-sealing strength, heat-adhesive strength, unsealing temperature and heat-sealing temperature range of the film.
(3) Physical properties. Choosing appropriate resins can improve the tensile strength, tear strength, impact strength and elongation at break of the film. In addition, according to the different requirements of customers, different resins and different grades of resins are selected to improve film stiffness, optical properties, surface friction coefficient, etc. In short, due to the excellent performance and reasonable price of co-extruded films, rapid development has been achieved. At present, the co-extruded film has developed from two-layer, three-layer to five-layer, seven-layer, nine-layer and even more.