Should we be concerned about shrinkage of our adhesives on curing?
Some adhesives, such as solvent-based materials and latex adhesives, always shrink as the carrier evaporates or is absorbed by the substrates. However, even 100% solid adhesives can have considerable shrinkage on curing. Shrinkage occurs because the density of the polymer is higher than that of the monomers (e.g., epoxies or acrylics). Different adhesive types have different levels of shrinkage; for example, epoxies typically shrink in the 2-5% range, whereas shrinkage of unmodified acrylics can be much higher. A worst-case scenario is adhesives and sealants with very high levels of multifunctional acrylate or methacrylate monomers, where shrinkage can be over 10%.
Why should you be concerned about shrinkage? Shrinkage induces considerable stresses in the cured adhesive bond, which can cause debonding, deformation, or even cracking of some materials. I have personally seen optical technicians actually measure the bending of glass from the curing of an epoxy adhesive. To minimize the effect of shrinkage, you can use adhesion promoters to prevent debonding or add fillers to reduce the amount of monomeric material. Much research has been done on this for acrylic systems in the dental adhesives field.
In addition, adding flexibilizers such as plasticizers or elastomers will help to absorb the stresses. Acrylic adhesives for bonding optical components use flexible additives such as urethane methacrylates, which can reduce the shrinkage to less than 1%. ASI
Dr. Dave is a former vice president and director of Loctite Corp. and has spent many years in troubleshooting adhesive and sealant problems in the adhesives, sealants, specialty rubbers and plastics fields. Questions for publication should be directed to him at 242 Trails End, Aurora OH 44202; phone (440) 477-5164; email DrDave242@windstream.net; or visit www.fldenterprises.com.
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.