Composition analysis and curing attention of polyurethane adhesive

Most polyurethane adhesives do not immediately produce high bonding strength when bonded and need to be cured. The so-called curing refers to the process of turning the liquid adhesive into a solid, and the curing process also includes post-curing, that is, the reactive group in the adhesive after initial curing further reacts or produces crystallization to obtain the ultimate curing strength. For polyurethane adhesives, the curing process is to make the NCO group reaction in the adhesive complete, or make the solvent volatilize completely, the polyurethane molecular chain crystallization, so that the adhesive and the substrate produce sufficient high adhesion process.

For two-component or multi-component polyurethane adhesive, it should be prepared according to the instructions when used. If the hydroxyl content and isocyanate group content of the components are known, the allocation ratio of each group can be determined by chemical calculation, and the isocyanate hydroxyl index (NCO/OH) is generally controlled in the range of 0.5-1.4. When the two-component solvent-based polyurethane adhesive is mixed, the ratio tolerance of the two components is larger than that of the non-solvent-based polyurethane adhesive. However, if there are too many NCO groups in the mixed component, the curing is incomplete, and the cured adhesive layer is high hardness and even brittle. If the hydroxyl component is too much, the adhesive layer is sticky, the cohesion is low, and the bonding strength is poor. The tolerance of the solvent-free two-component adhesive ratio is smaller than that of the solvent-based, which is due to the small initial molecular weight of each component, if one component is excessive, it will cause slow curing and easy to cure incomplete so that the adhesive layer appearance sticky strength is low.

The adhesive component contains a catalyst, or in order to speed up the curing speed when the catalyst is added to the adhesive, the application period of the adhesive after the adhesive is short, and the common curing catalyst is organotin compounds. In addition, the environmental temperature has a great influence on the application period, the application period is short in summer and long in winter, and the application period of two-component polyurethane adhesive diluted by polyurethane organic solvent can be extended. General solvent-based two-component adhesives, such as two-component polyurethane adhesives for soft plastic composite films, should be applied for more than 8 hours (that is, about one working day).

The adhesive component contains a catalyst, or in order to speed up the curing speed when the catalyst is added to the adhesive, the application period of the adhesive after the adhesive is short, and the common curing catalyst is organotin compounds. In addition, the environmental temperature has a great influence on the application period, the application period is short in summer and long in winter, and the application period of two-component polyurethane adhesive diluted by polyurethane organic solvent can be extended. General solvent-based two-component adhesives, such as two-component polyurethane adhesives for soft plastic composite films, should be applied for more than 8 hours (that is, about one working day).

The heating process should be gradually heated. In the process of drying, most of the solvent has evaporated, and the remaining solvent slowly diffuses through the adhesive layer, if the heating is too fast, the solvent vaporizes and bubbles in the softened adhesive layer, and bubbles are formed in the joint, which can seriously extruded most of the uncured fluid viscous adhesive from the joint, forming a bond internal vacancy that will affect the bonding strength. For the two-component solvo-free adhesive and the one-component moisture curing adhesive, the heating should not be too fast, otherwise the reaction between the NCO group and the moisture in the air on the glue or the surface of the substrate will be aggravated, and the CO2 gas generated will be too late to diffuse, and the viscosity of the adhesive layer will grow quickly, and the bubbles will remain in the adhesive layer, reducing the bonding emphasis.

One-component moisture curing polyurethane adhesive depends on the moisture curing in the air, so moderate air humidity should be maintained, and it is appropriate to slow curing at room temperature. If the air is dry, a small amount of water can be sprayed on the coated surface to promote curing. If the glue is sandwiched between the dry and hard adhesive, and the adhesive layer is thick, the interface and the outside water is not easy to penetrate into the glue, it is easy to cure incomplete, in this case, a very small amount of water can be injected into the glue.