Application of Waterborne Polyurethane Adhesives in the Fabrication of Composite Films (I)

Foreword

Due to its function of shielding, printing and heat sealing, the composite film flexible packaging is widely used in packaging of food, medicine, cosmetics and office consumer goods, and partially replaces packaging materials such as glass, tinplate, paper and cardboard. Due to the presence of urethane groups (-NHCOO-) and or isocyanate groups (-NCO-) in the molecular chain, the performance of polyurethane adhesives is superior. At present, more than 90% of polyurethane adhesives are used in the dry composite manufacturing process of composite films. One of the areas in which the solvent-based polyurethane adhesive is used is the composite adhesive for food packaging composite films and decorative paper. However, the evaporation of a large amount of solvents not only causes environmental pollution, but also the residual solvents in the food bags can affect the quality and safety of the food. At present, developed countries in Europe and the United States have already established relevant food hygiene regulations, such as the US Food and Drug Administration FDA, the EU's EU90/128, and Germany's BgVV, all clearly stipulating the types of adhesives used for food and pharmaceutical packaging as long as they are regulated. Chemicals not mentioned in the list are prohibited from use. Therefore, the current use of solvent-based adhesive composite packaging materials in Europe and America has been reduced from 80% 10 years ago to the current 30%. At present, most of the thousands of compound packaging production lines in China still use solvent-based adhesives. China's exports to Japan, the European Union's tea, mushrooms, meat and other foods have been returned due to health and safety issues. Therefore, food hygiene and packaging not only threaten people's health, but also affect China's export economy. With the continuous enhancement of people's environmental awareness, food packaging materials will be green and environmentally friendly. Water-based polyurethane adhesive is a kind of polyurethane adhesive that uses water instead of organic solvents. It is an environmentally friendly adhesive. No harmful solvent is volatilized and no odor is produced in the production process of composite packaging. The residual solvent is fundamentally eliminated. The biggest advantage of this kind of water-based adhesive is that it can be directly used without modification and additional investment in the existing film production line of solvent packaging adhesives in food packaging plants, and the comprehensive cost of water-based adhesives is not higher than that of existing domestic solvents. Adhesives have been widely promoted in domestic "funded enterprises".

1 Composite film production process and its drawbacks

The production methods of composite films are mainly extrusion compounding and dry compounding, and the dry compounding technique is suitable for the compounding of various film substrates. At present, adhesives used in all dry methods in the world are polyurethane adhesives, and can produce flexible packaging composite films that meet the performance requirements of heat resistance, cold resistance, oil resistance, acid resistance, drug resistance, gas barrier, transparency, abrasion resistance, and puncture resistance. Dry compounding refers to the use of adhesives to bond two kinds of film substrates together. After the adhesive is coated on the substrate film, it is heated and dried to remove the solvent and undergo a chemical reaction for compounding (pressing). Therefore, the dry compound process is called. Also known as dry lamination. That is, the adhesive layer is heated and the tackiness-free adhesive layer is heat-melted to impart adhesiveness, and the resin is recrystallized after bonding to obtain a high initial adhesive strength immediately. This bonding method is expected to have a low thermal activation temperature in industrial operations. There are disadvantages in the dry compound process for solvent-based adhesives to prepare composite films. For example, the residual organic solvent in the adhesive layer may migrate into the packaged food. The volatilized solvent may cause serious environmental pollution, requiring a large amount of investment to recover the solvent and Establish protective facilities. In order to solve the problem of pollution caused by organic solvents, in 1974, German company Henbets first used a solvent-free polyurethane adhesive to make composite packaging materials. Although the solvent-free adhesive process can eliminate the above drawbacks, it requires low viscosity for the convenience of coating, and the corresponding molecular weight of the polyurethane is small, resulting in insufficient initial strength of the composite. A low-viscosity polyurethane composite adhesive with large cohesive force is being researched and developed. By modifying the properties of the waterborne polyurethane film, it has become possible to apply the waterborne polyurethane to the flexible packaging of the composite film. Many foreign companies have developed such adhesives. For example, WD-4003, WD-4006, and WD-4007 polyurethane dispersions developed by Fuller have excellent adhesion to a variety of flexible packaging materials, and the adhesive strength after cross-linking is higher than the film material itself. After compounding in a drying tunnel at 93°C for 24s, partially cross-linked, sufficient strength can be cut to size.

2 Characteristics of Waterborne Polyurethane Adhesives

Waterborne polyurethane adhesives are adhesives that are formed when polyurethanes are dissolved in water or dispersed in water. In practical applications, there are few aqueous solutions, which are mainly aqueous polyurethane dispersions or emulsions. They are water-based binary colloidal systems in which polyurethane particles are dispersed in a continuous aqueous phase. Waterborne polyurethane adhesives have the advantages of being non-toxic, non-flammable, non-polluting, energy saving, safe and reliable, not easy to damage the surface to be coated, suitable for being easily eroded by organic solvents, easy to operate and modified, etc. And wood adhesives and other fields have been widely used and are gradually replacing solvent-based polyurethanes. In addition to the above advantages, compared with solvent-based polyurethane adhesives, water-based polyurethane adhesives also have some of their own characteristics: (1) Most water-based polyurethane adhesives do not contain NCO groups and therefore rely mainly on intramolecular polar groups for cohesion and adhesion. Attachment, waterborne polyurethane contains carboxyl, hydroxyl and other groups, under appropriate conditions can participate in the reaction, so that the adhesive cross-linking. (2) High relative molecular mass, low viscosity, and emulsion viscosity have nothing to do with the relative molecular weight of the resin. (3) High solid content, low viscosity, low viscosity product can be obtained through control of emulsion particle size. (4) Since the volatility of water is inferior to that of the organic solvent, the water-based polyurethane adhesive dries slowly, and since the surface tension of water is large, the wettability of the surface-hydrophobic substrate is poor. If most of the water is not yet evaporated from the adhesive layer, coating, or air, or absorbed by the porous substrate, it will not be able to obtain a continuous adhesive layer. Since most of the water-borne polyurethanes are made of hydrophilic polyurethanes as the main solid component and sometimes contain water-soluble macromolecular thickeners, the water resistance is not good if the adhesive film does not form a certain degree of cross-linking after drying. (5) Waterborne polyurethane adhesives can be mixed with a variety of waterborne resins to improve performance or reduce costs. (6) The water-based polyurethane adhesive has a small odor, is easy to operate, and the residual rubber is easy to clean.

3 Insufficient Use of Waterborne Polyurethane Adhesives for Preparation of Composite Films and Improvement Methods for Drying

In the composite process, the adhesive is a key factor affecting the quality and performance of the composite film. Therefore, the adhesive for food packaging composite film should have the following properties: (1) Adhesive composite packaging film using plastic, aluminum foil, etc., and plastic In many cases, bonding these films with different surface properties requires that the adhesive must have the ability to simultaneously bond two different film materials. (2) Softness Composite materials based on plastics, also known as soft packaging materials, are popular for their lightness and flexibility. In addition to being soft and collapsible, the adhesive itself must have this property. If the film is hard, brittle and unfoldable, the meaning of the package is lost. (3) Heat resistance Many food packagings are subjected to high temperatures (180-220°C) during manufacturing operations. For example, heat-sealed bags, high temperature sterilization of packaged foods, and cooking of foods, etc. The substrate film is subjected to a high temperature test, and the adhesive used can withstand the test of high temperature. Otherwise, the film peeled off after high temperature treatment 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 is hardened, brittle, delaminated, peeled, and debonded at low temperatures, it does not 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, packaging must be subject to high and low temperature treatment and long-term storage. 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 resist the erosion of various media, otherwise it will cause the layered peeling of the composite film and lose the packaging effect. (6) Safe and hygienic food packaging The composite film protects the packaged material and is directly imported. In order to be responsible for the health and safety of consumers, not only the substrate film is odorless, odorless, and non-toxic, but also the adhesive used. Also have the same performance, safety and health is a major issue that countries around the world attach great importance to.