In last week's SpecialChem4Adhesives article, the various means by which moisture can affect adhesive and sealant joints were examined. A number of consequences result from the ingress of moisture to an adhesive or sealant including plasticization of the system and disruption of the interfacial region between the substrate and the organic phase.
This week's article will focus on the several steps by which the formulator and the end-user can improve the moisture resistance of bonded joints. The major pathways to improve moisture resistance are via the bulk polymer, its fillers or additives, and the adhesive - adherend interface region.
Addressing the Bulk Adhesive or Sealant
All polymers absorb moisture to some extent and in doing so become plasticized by the water molecules. The bulk properties are changed: glass transition temperature, tensile strength, and modulus are lowered and elongation is increased. In sealants, swelling and deformation are also noted. These properties generally recover on drying unless hydrolysis takes place.