This is a good question for two reasons: first, adhesives and sealants often have to fill large irregular gaps and expansion on curing enables them to achieve this; and second, the products usually shrink on curing. Solvent-based or latex products shrink because of the loss of the carrier liquid on drying. Reactive adhesives that cure from a liquid to a solid can also shrink quite substantially. This shrinkage can be mitigated in highly filled or plasticized systems, but it can become a problem where unfilled systems such as ultraviolet (UV) adhesives or low-viscosity acrylic sealants are used. The shrinkage can cause very large stresses at the bond line, which can, in extreme cases, lead to bond failure or create leak paths in sealants.

Many foamable sealants, normally polyurethanes, are used in the building or DIY industry. Well-known one-component polyurethane consumer adhesives react with moisture and generate carbon dioxide, which causes them to expand on curing up to three or four times in volume. Be careful in using them to not over apply them, however, or they will often expand outside the bond line, which can necessitate removing the excess for cosmetic reasons.

Other, possibly less well-known systems are pre-applied threadlockers and sealants that use a chemical reaction to cause expansion, as well as adhesives and sealants that contain expandable microspheres. These microspheres contain a volatile liquid or a gas, which expands them when heated. Applications involve specialty adhesives or sealants and mastics for auto-body sealing.
 

Any views or opinions expressed in this column are those of the author and do not represent those of ASI, its staff, Editorial Advisory Board or BNP Media.