Adhesive curing process

Adhesive curing process

Curing method :

The curing of adhesives is done by physical methods, such as evaporation of solvents, emulsion coagulation and melt cooling, and chemical methods.

• (1) Hot melt adhesive: The polymer melt solidifies by cooling after it wets the surface to be bonded.

(2) Solution adhesive: As the solvent evaporates and the solution concentration increases, it gradually solidifies and has a certain strength.

(3) Emulsion adhesive: As the water in the emulsion gradually penetrates into the porous adherend and evaporates, the emulsion concentration continues to increase, and finally due to the effect of surface tension, the polymer colloidal particles condense. When the ambient temperature is high, the emulsion condenses into a continuous adhesive film, while when the ambient temperature is low and the minimum film forming temperature (MFT), a white discontinuous adhesive film is formed. Emulsion adhesives are mainly polyvinyl acetate and its copolymers and acrylic acid copolymers.

(4) Thermosetting adhesives The multifunctional monomers or prepolymers of thermosetting resins undergo polymerization reactions. As the molecular weight increases, the molecular chains change and cross-link, forming an insoluble gel or basic solidification. Prolonging the curing time and increasing the curing temperature within a certain range are not equivalent, and lowering the curing temperature is difficult to compensate by prolonging the time. Because a certain chemical reaction needs to occur between the adhesive and the surface of the adherend, this requires a sufficiently high temperature to proceed.

Curing pressure:

It is beneficial for the adhesive to fully infiltrate the surface; it is beneficial for removing low molecular volatiles produced by the adhesive curing reaction; it is beneficial for removing volatile solvents remaining in the adhesive layer; it is beneficial for controlling the thickness of the adhesive layer; adhesives with high viscosity often have thicker adhesive layers, and the adjustment of curing pressure controls the thickness range of the adhesive layer.

After applying the glue, let it sit for a while, which is called pre-curing. When the viscosity of the glue increases, apply pressure to ensure the uniformity of the thickness of the glue layer.

Curing temperature:

If the curing temperature is too low, the cross-linking density of the adhesive layer is too low, and the curing reaction is incomplete; if the curing temperature is too high, it is easy to cause the glue to lose or make the adhesive layer brittle, resulting in a decrease in bonding strength. Heating is conducive to the diffusion of molecules between the adhesive and the bonded parts, which can help form chemical bonds.

(1) Direct heating method in an oven: Use an air blowing device to ensure uniform heat transfer.

(2) External heating method: The heat is quickly transferred to the inside of the adhesive layer, greatly shortening the curing time. Ultrasonic heating method: It is suitable for heat curing of viscoelastic adhesives and solvent-free adhesives, but not suitable for thermosetting rigid adhesives.