Question: We have several industrial components to bond and seal. Some of the bonds have quite large gaps, while others are tight-fitting. How do I select the right adhesive or sealant?
Answer: This is a very interesting question. Many adhesives are used in what is called a “zero gap” situation, where the adhesive is applied between two closely fitting surfaces, which are then clamped together. In fact, this is usually the only situation where adhesives that cure by initiation from surfaces can be used, and at large gaps the center of the adhesive will fail to cure. Cyanoacrylate and anaerobic adhesives, while conveniently one-component, have the distinct disadvantage that their polymerizations are initiated from the surface being bonded, and the lack of molecular diffusion as polymerization proceeds prevents curing through large gaps. They are limited to gaps of about 20 mils (0.5 mm), although this can be improved slightly using surface activators.