Question: We have been considering organosilane adhesion promoters for use in several of our adhesives, and I am trying to understand how the organosilanes act chemically. Most literature shows them working primarily in adhesives for bonding glass. Will they work for other substrates (e.g., metals)?

Answer: Yes, organosilanes have been used for many years to enhance the durability of adhesive bonds, particularly to glass surfaces. In many cases, the organosilanes don’t necessarily improve the initial adhesion but prevent hydrolysis of the adhesive at the adhesive/glass interface. Organosilanes act as bridging molecules between the inorganic glass surface and the organic adhesive.

Most organosilanes consist of a silicon atom with three alkoxy groups, and a fourth group such as vinyl, methacryloxy, or glycidoxy. This group is chosen to be compatible with the adhesive and preferably to be able to copolymerize into the adhesive. In contact with surface or atmospheric moisture, the alkoxy groups hydrolyze and the silane forms Si-O-Si linkages with the glass surface. Note that this bonding is essentially the same as the chemical structure of glass, namely silicon dioxide.

The silanes will also often work in applications where glass is not involved, however. Bonding metals to metals with organosilane adhesion promoters has been well-documented over the years. I have personally used them very successfully to significantly improve the adhesion of acrylic adhesives when bonding aluminum to aluminum.

Question: We bond metal pieces to rubber. Solvent-based rubber cements seem to work well but cure too slowly for our required production rates. I would like to try a cyanoacrylate adhesive, but hear that they are very brittle when cured.

Answer: It is certainly true that most cyanoacrylates tended to be somewhat brittle in comparison to other adhesives, such as tough acrylics and polyurethanes, when used to bond rigid substrates, but more flexible versions are now available. Rubber-toughened cyanoacrylates are true impact-resistant structural adhesives. In your case, you can probably use a standard non-toughened grade because the rubber will absorb most of the stresses on the bond. The big advantage of cyanoacrylates is their extremely fast bonding speed. Your supplier can adjust this from a fixture speed of one or two seconds up to around 30 sec.