The curing principle of adhesives

The purpose of bonding is to have certain mechanical properties and to be able to transfer stress. The mechanical properties mainly depend on the mechanical properties of the cured adhesive.

Most adhesives take polymer as the main raw material, and the mechanical properties of solidified adhesives are those of solid polymer.

The surface free energy of the liquid polymer is relatively low, the surface of the adhesive is easy to wet, and it can be changed from liquid to solid through solvent volatilization, melt cooling, and polymer reaction.

The curing principle is generally as follows:

• The dispersion of solvent or dispersant, such as solvent-based glue, water solution glue, emulsion glue, etc
• The melt cools like a hot melt gel
• In situ polymerization
• Such as the mixed curing reaction of two-component epoxy glue, two-component polyurethane glue, the second generation of acrylic glue
• Moisture absorption curing such as room temperature curing silicone rubber, one component wet curing polyurethane glue, cyanoacrylate glue
• Anaerobic curing such as anaerobic glue
• Radiation curing such as ultraviolet (UV) curing glue, electron beam (EB) curing glue, etc
• Heat reaction curing such as one-component epoxy glue

In addition, there is non-curing glue such as pressure-sensitive glue. In recent years, multiple curing methods have also appeared in one, such as hot melt + reactive solvent + reactive hot melt + pressure sensitive solvent + pressure sensitive hot melt + photocuring + pressure sensitive photocuring + anaerobic curing.

The curing principle of solvent-based adhesives Solvent-based adhesives is prepared by adding linear polymer solution to other additives. The curing rate depends on the evaporation rate of the solvent. The solvent volatilization rate of adhesive is different from that of pure solvent.

As the solvent volatilized, the viscosity and Tg of the system increased, and finally, the polymer chain polymerized and formed a solid adhesive film. In this process, due to the extremely high viscosity and small diffusion coefficient, the volatilization rate is very slow, and sometimes it takes several years to volatilize completely. This is the source of the biggest pollution existing in solvent adhesives at present, but it is also the basis of contact adhesives.
The curing principle of latex type adhesive latex-type adhesive is made of solid adhesive into very small particles, dispersed in water to form like milk is milky white. Its curing is similar to solvent-based adhesive, through the dispersion medium (generally refers to liquid water) volatilization to make the latex particles coalesce, that is, the latex self-adhesive bond into an adhesive layer.

Before curing, light refracts and reflects through multiple-phase interfaces between latex particles and water,
The white light is diffused back, so it is milky white. When cured, the latex particles coalesce and the phase interface disappears or becomes thinner, so the latex becomes transparent. When the cured transparent adhesive layer is in contact with water, these hydrophilic groups can also absorb water, and sometimes there will be a whitening phenomenon, which is determined by the principle of latex synthesis and curing mechanism.

Hot melt adhesive refers to the thermoplastic polymer as the matrix resin, without the use of solvent or dispersant, through heating melting and low-temperature solidification, to achieve a class of adhesive. Because the adhesive molecules are still linear after curing, or the crosslinking density is very low, it can be repeatedly melted, softened, or dissolved in a solvent when heated. Hot melt adhesive belongs to the amorphous structure, there is no clear melting point, and the quantitative melting temperature of hot melt adhesive is often expressed by softening point.

The curing principle of reactive adhesive whether solvent-based or hot melt type, many properties of molecular weight of matrix resin requirements are contradictory, only chemical reactive adhesive can solve the process and high performance of molecular weight contradictory requirements.

Reactive adhesives can use relatively small molecular weight oligomers, monomers, or resins because the molecular weight is relatively small, and do not need.

The solvent can be used to wet the surface of the adhesive at room temperature, form effective molecular contact, form enough adhesion force, and then through the polymerization or crosslinking of the adhesive wetted in the gully on the surface of the adhesive, form a polymer with relatively large molecular weight or three-dimensional network, so as to achieve high adhesion performance. This polymerization at the bonding site is called in situ polymerization and also called in situ curing. Reactive adhesives must contain reactive groups, such as double bonds, epoxides, isocyanate groups, silyl hydroxyl, silhydrogen and silyl vinyl, sulfhydryl, hydroxymethyl, cyanate groups, etc. The focus of reactive adhesives is how to solve the contradiction between stable storage and fast curing.