Rubber feet will encounter adhesion problems in manufacturing or application: at the manufacturing stage, it is necessary to bond with the reinforcing material. In the application and assembly stages, it has to be bonded to other materials to form a whole. Whether it is bonded to itself (rubber) or other materials before or after vulcanization, it is bonded to a process that is widely used in the processing of rubber feet. There are many theories concerning adhesion, but they are mainly summarized in two aspects, namely “infiltration” and “source of adhesion”.

Infiltration of the bonding interface

(1) Surface infiltration

When rubber feet are bonded to other materials, firstly, an adhesive (glue) is applied to the interface, and the two adhesive surfaces are sufficiently wetted by means of an adhesive, and then the mutual interaction between the interface molecules Based on the action, the surface of the adherend is in close contact with the adhesive to produce an intermolecular force (van der Waals force) or a chemical bond force. Through these two types of interactions, the two surfaces are brought into close proximity and contact. This good wetting state is indispensable for rubber bonding, because any adhesive surface is uneven after microscopic enlargement. If the infiltration is not achieved, it will hinder the flow of the adhesive, and the weak link of bonding will occur. As a result, a strong bond cannot be achieved, and even bubbles are left at the bonding interface, which is a bane for future debonding.

When rubber and liquid adhesives are used as bonding intermediate materials in contact with solid adherends (rubber, metal, fiber, etc.), they must be sufficiently wetted, which is related to the surface tension of both sides. The surface tension of the adhesive, the surface tension of the solid adherend and the surface tension between the solid/liquid must satisfy the thermodynamic conditions shown in the following formula:
 
(3) Flow and diffusion of adhesive

As mentioned above, a certain infiltration condition creates conditions for the desired bond. However, it is necessary to have the aid of “flowing” and “diffusion” because the viscosity of the adhesive has a negative impact on this aspect. The greater the viscosity of the adhesive, the slower the flow. As the viscosity increases, the time required for infiltration can increase from a few seconds to a few hours. However, the effect of temperature is just the opposite. The higher the temperature, the shorter the required wetting time, because the increase in temperature will result in a decrease in viscosity.