Why research on pressure-sensitive adhesives for use in wet environments?

Adhesives are widely used in People’s Daily life, providing a variety of conveniences for people’s life. Pressure Sensitive Adhesive (PSA) is a very important adhesive. Psa is very sensitive to pressure and can be strongly bonded to the adhesive by applying mild pressure without the need for solvent, heat, or other means. Since the last century, the research and application of pressure-sensitive adhesive have been rapidly developed and has been widely used in all walks of life. However, most of the available pressure-sensitive adhesives will fail in humid environments, which greatly limits their application.

Therefore, the development of pressure-sensitive adhesives that can be used in wet environments has very important significance. Bonding is a complex physical and chemical process. So far, there are mainly several theories to explain the bonding mechanism, including adsorption theory, mechanical interlocking theory, diffusion theory, and electrostatic theory.

The bonding mechanism of pressure-sensitive adhesive is as follows: the pressure-sensitive adhesive will have viscous flow after being subjected to external pressure and can fully contact the substrate surface. When the pressure-sensitive adhesive has sufficient wettability to the substrate surface, the molecules between the interface can contact closely and generate intermolecular force to achieve adsorption equilibrium and generate strong adhesion. Therefore, the pressure-sensitive adhesive must have good viscoelasticity and good wettability to the substrate in order to produce sufficient adhesion.

In a humid environment, there is water on the surface of the adhesive. When the pressure-sensitive adhesive is in contact with the substrate, the surface of the two will be deposited in a layer of the water film. The water film will cause the pressure-sensitive adhesive to be unable to fully contact the surface of the substrate, which will lead to the failure of intermolecular force, resulting in bonding failure. In recent years, some progress has been made in the research of adhesives that can be used in a humid environment. Most of these adhesives are based on the dopamine system. However, the price of dopamine is relatively expensive, and it is easy to oxidize, and the bonding effect decreases with the increase of oxidation and bonding times.

Therefore, it is very important to develop a new type of pressure-sensitive adhesive which can be used in humid environments. In addition, after the preparation of pressure-sensitive adhesive is completed, there are few simulation tests and evaluations on the bonding ability of pressure-sensitive adhesive in wet environments and water in existing technologies. Therefore, how to evaluate the bonding ability of pressure-sensitive adhesive in a wet environment and water is also an urgent problem to be solved at present.