The present invention relates to a thiol-cured epoxy composition that is particularly useful as a sealer for a joint between two substrates.
Joints often are coated with a sealer or sealant to protect the joint against the penetration of water, oil, salt spray or other potentially harmful liquids. A specific type of joint that preferably is sealed is the joint between two structural members formed by welding or bonding with a structural adhesive. One example is in the joining of lightweight metal and plastic materials in the fabrication, repair and reconstruction of automotive and truck vehicle bodies and component panels and parts such as doors and hoods.
A typical method for bonding such substrates in the automotive industry is described in U.S. Pat. Nos. 5,487,803 and 5,470,416, both incorporated herein by reference. According to this method, an inner panel is positioned within an outer panel with the edge of the outer panel extending beyond the edge of the inner panel. The edge of the outer panel is then folded or crimped over the edge of the inner panel in a process known as hemming and the resulting structure is known as a hem flange. In order to permanently secure the inner panel with respect to the outer panel of the hem flange a structural adhesive is applied between the surfaces of the panel or the two panels are welded together to form a permanent hem flange assembly. A sealer is applied to cover and protect the adhesive or weld joint after the joint has been formed.
A particular problem with sealers used in connection with adhesive joints concerns cure temperature. (Meth)acrylic-based structural adhesives are becoming increasingly popular to form hem flange joints. (Meth)acrylic-based structural adhesives typically are cured at temperatures less than 121.degree. C. (250.degree. F.). Various plastisols that only cure at temperatures greater than 149.degree. C. (300.degree. F.) are currently used as hem flange sealers. However, if the previously applied (meth)acrylic-based adhesive has not itself completely cured before it is exposed during curing of the sealer to temperatures greater than 121.degree. C. (250.degree. F.), the (meth)acrylic-based adhesive tends to volatilize, thus losing its integrity. In addition, induction curing is also becoming increasing popular. Metal substrates can warp when subjected to induction curing at the temperatures required for the cure of plastisol sealants.
Sealers also must be paintable, chemically compatible with the adhesive, exhibit good flow characteristics, adhere to the substrate and joint surface and exhibit flexibility to absorb stresses caused by joint movement. Adhesion to metallic substrates can be especially problematic due to the presence of stamping, milling and drawing oils on the surface that are the result of the joint manufacturing process.
A low temperature curable sealer that exhibits good adhesion to metallic substrates thus would be very desirable.