Polyurethane Laminate Performance and Application

2. Dry compound adhesive

Dry compounding refers to the use of an adhesive to bond two kinds of film substrates together. After the adhesive is applied to the substrate film, it is dried by heating to remove the solvent and undergo a chemical reaction and compounded (pressed). Therefore, the dry compound is called Process. In wet compounding, the adhesive is applied to a substrate film. Before the adhesive is dried, the other substrate film is affixed and then the cured film is dried or dried. This process is called Wet compound. There are at least one material with good moisture permeability, such as paper, wood, and woven fabric, among the wet film base films. If the composite substrate has poor moisture permeability, such as plastic film and aluminum foil, the solvent will always vaporize, and defects such as air bubbles, peeling, etc. will occur when the substrate is not penetrated. Therefore, the wet compounding is limited by the substrate film, and only relatively low-grade composite packaging materials can be manufactured, such as cigarette packaging paper in which paper and aluminum foil are compounded.

1. Advantages and disadvantages of dry compounding process

(1) Advantages The substrate film has a wide range of choices, and can be made into two-layer, three-layer, four-layer and multi-layer composite films, which have high composite fastness and can achieve peel strength of up to 9.8N/15mm. Therefore, a high-strength, high-performance composite product such as a high-temperature cooking bag can be produced. Compounding speed is relatively fast, generally 150-180m/min, up to 250m/min. Composite films can be printed on the surface or reversed. In addition, the composite film produced has high dimensional accuracy and excellent heat sealing properties.

(2) Disadvantages Adhesives are used, so there are residual organic solvent residues and contamination of the ring diameter. The base film having a low softening point tends to contract when passing through the hot air passage, and is easily deformed. The thickness of the substrate film requires higher precision.

2. Adhesives for composite packaging films have properties

In the dry compound process, the adhesive is a key factor affecting the film quality performance, so the adhesive for food packaging composite film should have the following properties.

(1) Adhesive Composite packaging films are made of a variety of different materials and are bonded together with an adhesive. Therefore, the adhesive must have good adhesion to various films. If only one of them sticks firmly, and the other film does not, it cannot be operated. Substrates used in composite packaging films include plastics, aluminum foils, papers, and the like, while plastics come in various types and their surface characteristics are different. Facing so many complex materials, adhesives must have the ability to bond two different materials at the same time.

(2) The flexibility of composite packaging materials has been widely welcomed, and light and flexible is very important. Nowadays, plastic-based composite materials are called soft packaging materials. In addition to being soft and collapsible, the adhesive itself must have this property. If the film is hard, brittle and unfoldable, it loses the meaning of packaging.

(3) Heat Resistance Many food packagings are subjected to high temperatures (180-220°C) during manufacturing operations. For example, when heat sealing a bag, the temperature of the hot knife sometimes reaches 220°C to make the composite film into a bag. In addition, after many food composite films are packaged, they must be subjected to high-temperature sterilization and sterilization. For example, steamed foods are subjected to high-temperature cooking and sterilization at 121 to 135°C/30 minutes, and yoghurts and jams are also subjected to high temperatures of 80 to 100°C. . This not only requires that a variety of substrate films withstand the test of high temperatures, the adhesive used must also withstand the test of high temperatures. If the adhesive can only be used at room temperature, but not high temperature, after the high temperature treatment, the original composite substrate film to peel off, it is not a composite packaging film. This must be carefully considered when choosing adhesives.

(4) Cold resistance Many foods are cold-stored or frozen after being packaged, which requires the packaging film itself to withstand low temperatures. If the adhesive hardens at a low temperature, brittle, delamination, peeling, degumming and other phenomena are not acceptable. Sometimes it is possible to require heat-resistant and cold-resistant packaging at the same time. The type of adhesive must meet the requirements.

(5) The anti-media food itself is a very complex composition of substances, water, oil, salt, wine, as well as spicy, spices, and even acetic acid, citric acid, lactic acid, sugar, sulfides, oxides and so on. In the face of these complex components, after being packaged, they are subjected to high-low temperature treatment and long-term storage tests. To maintain the integrity of the packaging film, in addition to the excellent resistance to media erosion of the substrate film itself, the stability of the adhesive is also important. To be able to teach the erosion of various media, otherwise it will cause layered peeling of the composite film, losing the packaging effect.

(6) Safe and hygienic food packaging Composite film protects and the life of the package is people's direct population. In order to be responsible for the health and safety of consumers, not only the substrate film is tasteless, odorless, and non-toxic, but also used. Adhesives must also have the same performance, and safety and sanitation are major issues that countries around the world are paying attention to. China has already developed a hygienic standard for complex packaging materials (GB 9683-88).

In addition, there are certain requirements for the appearance, transparency, flowability, cold resistance (non-freezing), initial tack, and sizing operations of the adhesive. (to be continued)