The disclosure generally relates to epoxy adhesives and more particularly to a toughened epoxy adhesive with enhanced adhesion to sheet molding compound (SMC).
Sheet molding compound (SMC), for example, is defined (ASTM) as a molding compound in integral sheet form comprising a thermosetting resin, fibrous reinforcement, and additives required for processing or product performance, e.g., resin, catalyst, thickener, mold release agent, particulate filler, pigment, shrink control agent, etc. These materials and others generally are known as fiber-reinforced composites, reinforced composites, or simply composites. Due to their brittleness, traditional SMC's may micro-crack due to handling, flexing, etc., during and after molding. When those parts are painted, solvents used in the paint process can wick into the cracks and volatilize during the paint oven curing process. This volatilization pushes the paint off the substrate resulting in paint pops.
Recent developments in SMC technology for the automotive industry have resulted in improvements to the toughness of the SMC composites, resulting in reduced micro-cracking and, therefore, reduced paint pops and higher quality. Toughness refers to the resistance to cracking including higher elongation and increased area under the stress/strain curves. Past SMC composites have used brittle resins with high crosslink density to achieve the performance attributes needed for automotive applications. New SMC materials have been developed by increasing the elongation of the unsaturated polyester resins through a decrease in the crosslink density of the matrix resin. In addition, there can also be added improvements resulting from the inclusion of low Tg elastomeric toughening additives. This addition typically results in a 2-phase morphology that improves toughness through energy dissipation and crack stoppage mechanisms. These modification designed to increase toughness may result in a more difficult to bond to surface.
A common class of structural adhesives useful in adhering metal parts to the same and to different substrates (e.g. composites) is epoxy adhesives. Epoxy adhesive compositions most often contain a polyfunctional epoxy resin and are cured by addition of a curative, which typically is provided in a separate package, often referred to as Part A and Part B. The rate of cure and product characteristics are influenced by the choice of curing agent, which itself is influenced by the make-up of the adhesive composition, as dictated by the final properties desired by the user. Metal may include, but not be limited to, hot dipped galvanized steel, electro galvanized steel, e-coat steel, cold rolled steel, bare aluminum, anodized aluminum, etched aluminum, magnesium, etc.
Acceleration of two-part epoxy adhesives has taken many paths over the years, such as, for example, the use of acrylic esters, mercaptans, and hydroxyl groups. The acrylic esters produce a rapid reaction, but typically result in brittle-glassy polymers with low adhesion to SMC. The mercaptans also react vigorously, but result in brittle polymers with low adhesion and have a strong odor. The catalytic affect of hydroxyl groups was first reported by Shechter, et al., Industrial Engineering and Chemistry, Vol. 48, No. 1, pp. 94-97 (1956) where a termolecular mechanism was proposed. Phenol was cited as being far more efficient than aliphatic alcohols due to its relative acidity. U.S. Pat. No. 4,129,670 claims the use of dihydric phenols, such as catechol or resorcinol, as epoxy chain extenders that “may reduce the cure time and/or temperature”. U.S. Pat. No. 5,385,990 claims the use of substituted aromatic phenols, where the substitution is an electron withdrawing group that causes an increase in the pKa value. This latter patent cites the previous work of U.S. Pat. No. 4,129,670 and, thus, excludes use of non-substituted aromatic phenols. Their preferred additive is p-chlorophenol, which is quite effective; however, it is plagued by an objectionable strong odor and recently chlorinated compounds, as a class, have been under scrutiny for being hazardous to the environment by such companies as Ford and Volvo.
U.S. Pat. No. 6,645,341 B1 claims a two-part epoxy that contains a polyester or polyether polyol at 10%-40% with a monomeric acrylic toughener and a curative consisting of polyoxyalkyleneamine, polyamide, reactive liquid rubber, amino functional silane, and tris(dimethylaminomethyl)phenol. The disclosed adhesive composition utilizes a polyol but at concentrations less than about 9.9% and does not use polyoxyalkyleneamine, acrylic monomer, or silane.
Additional representative epoxy structural adhesive compositions can be found in, for example, U.S. Pat. Nos. 2,977,332, 5,385,990, 6,645,341, 6,451,876, 5,001,193, 4,740,539, 6,486,256, and 6,572,971 and published application US2004/0197563A1, the disclosures of which are expressly incorporated herein by reference. Various combinations of epoxy resins, rubber modifiers, amine curing agents, amide curing agents, mercaptan curing agents, etc. have been proposed for formulating high strength adhesive compositions.