Packaging bag heat seal layer material knowledge, a must-read

As a heat-sealing layer for composite packaging materials, because it is in direct contact with the packaged goods, its hygiene must first be considered. Other factors to consider include transparency, permeability resistance, good heat-sealability, and openability. Below, Shunxingyuan Packaging, a professional flexible packaging manufacturer, details the five basic film properties and application areas.

1. Polyethylene Film ( PE )

Characteristics

The polyethylene used for composite heat-sealing layers is mainly low-density polyethylene and linear low-density polyethylene (some products also use medium-density and high-density polyethylene films as heat-sealing layers), and generally, two types of polyethylene are blended in a certain proportion. Depending on the processing method, it can be divided into blown film, cast film, and extrusion coating film. The thickness of the polyethylene film used in composite materials is selected according to the requirements of the packaged goods; thin films can be around 20μ, while thicker films can be greater than 100μ. Due to the many additives in the resin used to produce the film, especially lubricants, attention must be paid to additive migration during production.

Application Areas

Metallocene polyethylene can be considered a revolution in polyethylene resins, resulting in films made from metallocene polyethylene having good heat-seal strength, heat-sealing strength, and certain anti-seal contamination properties. Currently, films produced by blending metallocene polyethylene with low-density polyethylene are widely used in oil packaging, shampoo packaging, and detergent packaging. Currently, the heat-sealing layer of most composite packaging uses polyethylene, such as instant noodles, biscuits, pickles, and preserved vegetables. Blown films and cast films must undergo corona treatment during processing to ensure the film surface tension is greater than 38 dynes/cm.

2. Polypropylene Film ( PP )

Polypropylene films can also be divided into blown films (downward blowing water cooling method), cast films, and extrusion coating films. Among polypropylene films, cast polypropylene films (i.e., CPP films) are the most widely used. They are mainly used in packaging requiring high-temperature sterilization and high transparency. After aluminum plating is applied to CPP films to create aluminum-plated CPP (VmCPP), they are used in the packaging of puffed foods and instant noodles.

The raw materials for high-temperature sterilization CPP and aluminum-plated CPP are different from those of ordinary CPP and have special requirements. In addition to general binary copolymers, block copolymers are used for high-temperature sterilization, and their high-temperature resistance should be the best; the raw materials for aluminum-plated CPP are preferably ternary copolymers. To save costs, some companies use polypropylene blown films for the inner layer of products with lower requirements.

3. Sealing Films

Most of the jelly, cold drinks, yogurt, and fruit milk on the market are packaged in plastic cups and bottles, and the cups and bottles are sealed with a film. Some require opening, while others do not. Heat-sealing layers that do not require opening are generally used in the packaging of milk and fruit milk, and the cup or bottle bodies are mostly made from high-density polyethylene (HDPE) resin. The inner layer material only needs to be made of the same material as the cup (bottle) body. On the one hand, it must have good sealing strength, and on the other hand, it must have peelable properties. For such special requirements, the selection of the heat-sealing layer is very important.

Modified EVA is a material that possesses both functions. EVA - ethylene-vinyl acetate copolymer, has good adhesion to polypropylene, polystyrene, etc., but its molecular structure is different from polypropylene and polystyrene. During heat sealing and bonding, the two resins cannot fully diffuse, penetrate, and melt together. Only bonding strength with a clear interface can be formed. Therefore, using modified EVA as the heat-sealing layer of the sealing film provides sufficient heat-sealing strength and easy peelability.

4. Special Heat-Sealing Materials

In some special packaging applications, such as coffee packaging, meat packaging, and form-fitting packaging, special heat-sealing materials are used, 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, and Japan, and were rarely used in China before. However, with the rapid development of China's national economy, the requirements for packaging materials are also getting higher and higher. The application of these special heat-sealing materials is becoming increasingly widespread.

5. Co-extruded Films

With the development of the packaging industry and fierce market competition, the numerous shortcomings of single-layer films have gradually become apparent, restricting the promotion and application of resins. Co-extruded films produced using co-extrusion blowing or co-extrusion casting processes have the following advantages:

1. Reduce Production Costs

To improve the heat-sealing performance of films, many manufacturers now use a blend of LDPE and mLLDPE to produce films. The increase in material cost per ton when using single-layer equipment is higher than when using multi-layer co-extrusion equipment.

2. Improve the Performance of the Heat-Sealing Layer

Barrier properties: When producing co-extruded films, using high-barrier materials such as NY and EVOH can significantly improve the film's barrier properties against gases.

Heat-sealing properties: Selecting suitable mLLDPE can improve the film's heat-sealing strength, heat-adhesive strength, sealing temperature, and heat-sealing temperature range.

Physical properties: Selecting suitable resins can improve the film's tensile strength, tear strength, impact strength, and elongation at break. In addition, according to different customer requirements, different resins and different resin grades can be selected to improve the film's stiffness, optical properties, and surface friction coefficient. In short, due to the excellent performance and reasonable price of co-extruded films, they have developed rapidly. Currently, co-extruded films have developed from two layers, three layers to five layers, seven layers, nine layers, and even more.

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