The Build-up of Adhesion

In general, maximum adhesion is not achieved as soon as the part is moulded, rather it increases after storage of the material for some time. At room temperature this time 

The storage period can be shortened significantly, if it is carried out at an elevated temperature. Approximately 0.5 to 1 hour at 80 °C to 130 °C will provide sufficient adhesion development. The storage or post cure temperature is determined by the properties of the substrate, as, for example in thermoplastics, warping might be the result of too high a temperature. 

It is also essential, that demoulding should be stress free, so that a disruption of the contact surface between silicone rubber and its substrate cannot take place. Once this interface is broken, it is unlikely that proper adhesion will be possible later. 

Adhesion depends strongly on the surface properties. It is built up faster on smooth surfaces than on rough, sand blasted or electrically eroded substrates. Among other reasons this can be explained by the fact that smooth surfaces are intimately covered to 100% before the cure of the rubber. Uneven surfaces can be interpreted as surfaces with very tiny undercuts, and hence much less than 100% will be covered by uncured rubber, as curing usually happens before these micro-undercuts are filled. As mentioned in Section 11.5.1, undercuts that are not entirely filled might lead to loss of anchorage. 

When working with standard silicone rubbers stronger adhesion to smooth surfaces is observed. However, cohesive failure is never found when separating the rubber from a substrate which has not previously been primered. 

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This chapter first reviews the general structures and properties of silicone polymers. It goes on to describe the crosslinking chemistry and the properties of the crosslinked networks. The promotion of both adhesive and cohesive strength is then discussed. The build up of adhesion and the loss of adhesive strength are explained in the light of the fundamental theories of adhesion. The final section of the chapter illustrates the use of silicones in various adhesion applications and leads to the design of specific adhesive and sealant products. 

The adhesive must adhere to a flush-mounted axial component for 50 percent of the component length and 25 percent of its diameter on one side. The build-up of adhesive should... 

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