Waterborne polyurethane adhesives still hold a certain market share in the flexible packaging market. Their performance and operability are completely different from the existing ester-soluble polyurethane adhesives. How should we correctly apply waterborne polyurethane adhesives?
The performance characteristics of waterborne polyurethane adhesives
Compared with solvent-based polyurethane adhesives, waterborne polyurethane adhesives, in addition to having the advantages of being non-flammable, having no solvent odor, and not polluting the environment, also have the following characteristics:
1.Solvent-based two-component adhesives fully utilize the reaction of NCO groups to enhance bonding performance during the curing process. Since waterborne polyurethane adhesives do not contain NCO groups, the adhesive force mainly relies on the cohesion generated by the polar groups within the polymer. Waterborne polyurethane contains carboxyl, hydroxyl and other groups, which can also participate in the reaction under suitable conditions, causing the adhesive to undergo cross-linking reactions.
2.The important factors influencing the viscosity of waterborne polyurethane include ionic charge, core-shell structure, and emulsion particle size, etc. The more ions and counterions there are in the polymer molecules, the greater the viscosity. However, factors such as the molecular weight, solid content, and crosslinking agent of
Polyurethane Resin have no significant impact on the viscosity of waterborne polyurethane. This is conducive to increasing the molecular weight of polyurethane to enhance the cohesive strength of the adhesive. For polymers with the same solid content, the viscosity of waterborne polyurethane adhesives is lower than that of solvent-based ones.
3.Viscosity is an important indicator that determines the performance of adhesives in use. According to Coating Online, the viscosity of waterborne polyurethane is generally adjusted by waterborne thickeners or water, while solvent-based polyurethane adhesives can be adjusted by increasing solid content, molecular weight or choosing appropriate solvents.
4. Due to the lower evaporation rate of water compared to organic solvents, the drying of waterborne polyurethane adhesives is relatively slow. Due to the high surface tension of water, its wetting ability on the hydrophobic surface of the substrate is poor. If most of the water has not yet evaporated from the adhesive into the air or has been absorbed by the porous substrate before it is suddenly heated and dried, it is difficult to obtain a continuous coating. As the main solid component of waterborne polyurethane is hydrophilic, if the dried adhesive film does not undergo a certain degree of cross-linking, its water resistance will be poor.
Operating conditions
The operation of waterborne polyurethane is basically similar to that of solvent-based adhesives, such as manual brushing, mechanical roller coating, and spraying, but some special requirements need to be noted, especially wet lamination.
1. Adhesive application amount: The composite strength is related to the amount of adhesive applied, especially on porous substrates. If the amount of adhesive applied is too small, most of the adhesive will be absorbed by the substrate and the bonding strength will not be achieved. Therefore, Coating Online suggests that the amount of wet adhesive applied should be controlled at about 10 to 20 grams per square meter, and adjusted appropriately according to different substrates.
2. The temperature of the drying tunnel should be controlled at 70℃-85℃-90℃, and the air volume should be increased to the maximum as much as possible. Wet lamination generally does not require curing, but water evaporation is very important for waterborne polyurethane adhesives. Therefore, it is necessary to increase the temperature and air volume as much as possible to accelerate the evaporation rate.
3. The coated substrate must be free of oil stains and other impurities to avoid affecting the bonding strength.
Precautions
1.This product must not be mixed with other adhesives of different properties.
2. Before compounding, the temperature of the drying oven must be raised to the set temperature before starting the machine.
3.After the gluing is completed, first clean the glue on the gluing roller with tap water, and then thoroughly clean it with ethyl acetate.
