Selective self-adhesive LSR (Liquid Silicone Rubber) offers good adhesion to thermoplastics, but does not adhere to the metal surfaces of molds, so it enables us to operate insert molding without any primers or to make a co-injection molding. This selective self-adhesive mechanism can be explained as follows: The adhesion promoter containing of functional groups X and Y in a molecule is insoluble in silicone polymer. The group X has similar structure to thermoplastic polymer while the group Y can be cross-linked with LSR. When the adhesion promoters added to the LSR migrate to the interface, group X tends to adjust themselves to make an interaction with thermoplastics, which results in strong bonding between LSR and thermoplastics. On the other hands there is a weak interaction between the adhesion promoters and metallic mold owing to the low reactivities of X and Y with metal surfaces.
The selective self-adhesive LSR is mainly applied for keypads of mobile phone, for example, 1) plastic keypads of frame type, and 2) film keypads of filed with silicone rubber. For the frame type, this LSR is usually used through the insert molding to polycarbonate parts, and film keypads are made of polyurethane films with this LSR by the compression molding after dispensation. In both cases, there is a good reason to choose this selective self-adhesive LSR, not only skipping priming process but also reducing labor-costs by eliminating time-consuming handling process.