Waterborne polyurethane adhesives (referred to as waterborne PU adhesives) refer to adhesives made from waterborne polyurethane as the base material. They are an important type of waterborne adhesives. Due to their excellent bonding properties, outstanding solvent resistance, acid and alkali resistance, impact resistance, wear resistance, and resistance to high and low temperatures, they have developed rapidly in recent years due to the strict environmental protection requirements of the state.
1.Most waterborne polyurethane adhesives do not contain -NCO groups, and thus curing mainly relies on the cohesion and adhesion generated by the polar groups within the molecule. Solvent-based or solvent-free one-component and two-component polyurethane adhesives can fully utilize the reaction of -NCO to enhance the bonding performance during the bonding and curing process. Waterborne polyurethane contains carboxyl, hydroxyl and other groups. Under suitable conditions, they can participate in the reaction to cause cross-linking of the adhesive.
2. Besides the added high-molecular thickeners, the important factors affecting the viscosity of waterborne polyurethane also include ionic charge, core-shell structure, and emulsion particle size, etc. The more ions and counterions (referring to free ions with opposite polarity to the ionic groups contained in the main chain and side chains of polyurethane in the solution) on the polymer molecule, the greater the viscosity. However, factors such as solid content (concentration), molecular weight of
Polyurethane Resin, and crosslinking agent have no obvious influence on the viscosity of waterborne polyurethane, which is conducive to the high-molecular quantification of polyurethane. To enhance the cohesive strength of the adhesive. In contrast, the main factors influencing the viscosity of solvent-based polyurethane adhesives include the molecular weight of polyurethane, the degree of branching, and the concentration of the adhesive, etc. For the same solid content, the viscosity of water-based adhesives is lower than that of solvent-based adhesives.
3. Viscosity is an important parameter of the performance of adhesives in use. The viscosity of waterborne polyurethane is generally adjusted by water-soluble thickeners and water. Solvent-based adhesives can be adjusted by increasing the solid content, the molecular weight of polyurethane or choosing an appropriate solvent.
4. Due to the poorer volatility of water compared to organic solvents, waterborne polyurethane adhesives dry more slowly. Moreover, because of the high surface tension of water, its wetting ability on hydrophobic substrates is poor. If most of the moisture has not yet evaporated from the bonding layer or coating into the air or been absorbed by the porous substrate before heating and drying, it is difficult to obtain a continuous adhesive layer.
5. Waterborne polyurethane adhesives can be mixed with a variety of waterborne resins to improve performance or reduce costs. At this point, attention should be paid to the ionic properties and acidity or alkalinity of ionic water-based adhesives; otherwise, coagulation may occur. Due to the influence of compatibility between polymers or solubility in certain solvents, solvent-based polyurethane adhesives can only be blended with a limited number of other resin adhesives.
6. Water-based polyurethane adhesives have a mild odor, are easy to operate, and the residual glue is easy to clean. In contrast, solvent-based polyurethane adhesives sometimes consume a large amount of solvents during use and are not as convenient to clean as water-based adhesives.
