Polyurethane structural adhesives have been used for many years in industry to adhere parts fabricated from fiberglass reinforced polyester (FRP) materials. These materials are also commonly referred to as sheet molding compound (SMC).
The adhesive bonds must be strong and must maintain their strength both at elevated temperatures and when exposed to wet environments. Over the years substantial research effort has been devoted to improving the strength of these polyurethane structural adhesives. Two patents serve to describe what are, at the present time, typical commercially available polyurethane structural adhesives. U.S. Pat. No. 3,935,051, "Polyurethane Composition and Laminates Made Therewith", combines a prepolymer component and a curative to make an adhesive for FRP panels. The prepolymer component is the reaction product of polypropylene glycol and toluene diisocyanate. The curative contains polypropylene ether glycol and N, N, N', N'-tetrakis (2-hydroxypropyl) ethylene diamine. Bond strengths were reported to be greater than 60 pounds per square inch.
U.S. Pat. No. 4,552,934, "Two-Component Adhesive Sage Resistant At Essentially 1:1 Ratio", describes another two component polyurethane adhesive also having a prepolymer component and a curative component: This prepolymer is polypropylene ether glycol reacted with aromatic isocyanate and has 3 to 15 percent by weight free isocyanate. The curative is polypropylene ether glycol and polypropylene ether triol partially reacted with some polyarylmethane polyisocyanate before adding polyamine.
In addition to improvements in the formulations of the adhesive components themselves, a great deal of work has been directed to the development of primers to be used on FRP substrates to better prepare the substrate for application of the polyurethane adhesive. Primers generally are assumed to be absolutely essential elements for strongly bonded laminates.
Bond strength measurements are made both at room temperature and at elevated temperature, that is, at 82.degree. C. Only those adhesives having high bond strengths at 82.degree. C. are considered to be acceptable by the automotive industry. While adhesives used without primers may pass room temperature strength tests, at present only the use of primers provides adequate bond strengths at 82.degree. C.
For example, the General Motors Corporate Specifications for new plastic automotive substrates require the use of wash primers on test substrates prior to the application of adhesives to these candidate substrates.
A number of representative patents serve to illustrate the efforts made to improve bond strengths by developing better primers. U.S. Pat. No. 3,647,513, "Method for Improving the Adhesion of Polyester Compositions", teaches the need for a surface treating agent to enhance the bond strength between adhesives and fiberglass reinforced polyester substrates. The surface treating agent or primer recommended is a tertiary amine in low-boiling inert organic liquid solvent.
U.S. Pat. No. 3,945,875, "Method For Obtaining Polyester Laminates", teaches the use of a surface treating agent on FRP substrates prior to use of a polyester adhesive. The recommended surface treatment agent is an organic isocyanate containing ethylenic unsaturation. The isocyanate is used in a toluene or methyl ethyl ketone solution.
U.S. Pat. No. 4,004,050, "Two-Part Primer System for FRP Bonding", states that adhesion between a polyurethane adhesive and a polyester surface is improved by treating the surface with separate applications of an organic polyisocyanate and a tertiary amine, each applied in solvent solution to the polyester surface. Methylethyl ketone and the halogenated hydrocarbon, chlorinated naptha, are recommended solvents for the surface treatment agents. Practice of this invention requires three steps; that is, two surface treatments followed by application of the adhesive.
U.S. Pat. No. 4,500,606, "Sealer of Polyester and Method of Use to Obtain Laminates", teaches the need for surface treatment of FRP substrates with a sealer prior to the use of polyurethane adhesives. The recommended sealer is a mixture of phenol-formaldehyde resin, polyvinyl acetal film former and epoxy resin. This is a typical two step procedure requiring application of a sealer (or primer) followed by application of the adhesive.
Commercially available primers often contain chlorinated hydrocarbons such as methylene chloride, a compound under study by the Environmental Protection Agency as being hazardous in the work place. Manufacturers are actively studying ways to minimize or eliminate this and other harmful solvents from the work environment.
It would be a great improvement over the present technology if the cumbersome two-step process of first applying primer and then applying adhesive could be streamlined into a one step process in which the primer step was completely eliminated and the adhesive applied directly onto the FRP substrate. Elimination of the primer would be a significant advance not only in the saving of time, but also, in the elimination of a solvent-exposure opportunity, thus improving the environment for the workers.