How to Operate Correctly Based on the Properties of Waterborne Polyurethane Adhesives?

Waterborne polyurethane adhesives still hold a certain market share in the flexible packaging market, and their properties and operability are completely different from those of existing ester-solvent polyurethane adhesives. How should we correctly apply waterborne polyurethane adhesives?

Performance Characteristics of Waterborne Polyurethane Adhesives

Compared with solvent-based polyurethane adhesives, waterborne polyurethane adhesives, in addition to being non-flammable, free of solvent odor and free from environmental pollution, also have the following characteristics:

1. Solvent-based two-component adhesives make full use of the reaction of NCO groups to enhance bonding performance during curing; since waterborne polyurethane adhesives contain no NCO groups, adhesion mainly relies on the cohesive force generated by polar groups within the polymer. Waterborne polyurethane contains carboxyl, hydroxyl and other groups, which can also participate in reactions under suitable conditions, causing the adhesive to undergo crosslinking reactions.
2. Important factors affecting the viscosity of waterborne polyurethane include ionic charge, core-shell structure, emulsion particle size, etc. The more ions and counterions in the polymer molecules, the higher the viscosity. However, factors such as the molecular weight, solid content and crosslinking agent of the polyurethane resin have no obvious effect on the viscosity of waterborne polyurethane, which is conducive to increasing the molecular weight of polyurethane so as to increase the cohesive strength of the adhesive. For polymers with the same solid content, waterborne polyurethane adhesives have lower viscosity than solvent-based ones.
3. Viscosity is an important indicator determining the application performance of an adhesive. The viscosity of waterborne polyurethane is generally adjusted by waterborne thickeners or water, while that of solvent-based polyurethane adhesives can be adjusted by increasing solid content or molecular weight, or by selecting a suitable solvent.
4. Since the evaporation rate of water is lower than that of organic solvents, waterborne polyurethane adhesives dry more slowly. Because water has high surface tension and poor wetting ability on the hydrophobic surface of substrates, if most of the water has not yet evaporated from the adhesive into the air, or is still absorbed by the porous substrate, and the adhesive is suddenly heated and dried, it is difficult to obtain a continuous coating. Since the main solid component of waterborne polyurethane is hydrophilic, the dried adhesive film will have poor water resistance unless a certain degree of crosslinking occurs.
5. Waterborne polyurethane adhesives can be mixed with a variety of waterborne resins to improve performance or reduce costs. At this time, attention should be paid to the pH and other properties of the waterborne adhesive, otherwise coagulation may occur.
6. Waterborne polyurethane adhesives have low odor, are easy to operate, and residual adhesive is easy to clean. In contrast, solvent-based polyurethane adhesives consume a large amount of solvent during use, and cleaning is not as convenient as with waterborne adhesives.

The operation of waterborne polyurethane is basically similar to that of solvent-based adhesives, such as manual brushing, mechanical roller coating, spraying, etc., but some special requirements need attention, especially wet lamination.

1. Adhesive coating weight: The lamination strength is related to the coating weight of the adhesive, especially for porous substrates. If the coating weight is too small, most of the adhesive will be absorbed by the substrate and the bonding strength cannot be achieved. Therefore, it is recommended that the wet adhesive coating weight be controlled at about 10 g to 20 g per square meter, with appropriate adjustment depending on different substrates.
2. The temperature of the drying tunnel should be controlled at 70°C-85°C-90°C, and the air volume should be increased to the maximum possible. Wet lamination generally does not require curing, and water evaporation is very important for waterborne polyurethane adhesives, so the temperature must be raised as much as possible and the air volume increased to speed up evaporation.
3. The coated substrate must be ensured to be free of oil stains and other impurities, so as not to affect the bonding strength.
4. This product cannot be mixed with adhesives of other different properties.
5. Before lamination, the temperature of the drying oven must be raised to the set temperature before starting the machine.
6. After coating is completed, first clean the adhesive on the coating roller with tap water, then clean it thoroughly with ethyl acetate.