This invention relates to a method of improving the adhesion between metal substrates and thermoset resins using a polymeric coupling agent. More particularly, this invention relates to a method of improving the adhesion between metal substrates (e.g. steel and copper) and thermoset resins (e.g. epoxy) utilizing a polymer composed of a strong, hydrophobic polymer backbone having functional groups attached thereto at spaced intervals that will chemically bond to both the metal and thermoset resin. This invention also relates to a preferred method of making the novel polymeric coupling agent.
It is well known in the adhesive bonding art that water is extremely detrimental to metal/thermoset adhesion systems. It is believed that diffusion of water into the interfacial region between the polymer and metal will reduce the strength of the metal/polymer bond for several reasons. For example, since water molecules are very strong hydrogen bonding agents, they can readily break non-covalent bonds between the metal and polymer and form new hydrogen bonds with the oxide surface of the metal. A weak water layer results, reducing the strength of the entire adhesion system. Water can also weaken the interfacial region by initiating corrosion and/or hydration reactions with the base metal, oxides or the polymer itself.
Epoxy resins are well known examples of thermosetting polymeric material whose bonds with metals are adversely affected by water. Thus, while an epoxy resin will normally exhibit a high strength and strong bond to metal surfaces such as steel, the integrity of the epoxy/metal bond is significantly reduced when exposed to high humidity or water.
Attempts have been made to improve the bonding between polymers and metals (particularly the adverse effect of water on the bond) by the use of intermediate compounds which are designed to form chemical bonds with both the metal substrate and the polymeric resin. In essence, these intermediate compounds chemically link or bridge the polymer to the metal substrates. These linking compositions have consisted of various low molecular weight coupling agents which have the ability to form chemical bonds across the metal/polymer interface. U.S. Pat. Nos. 4,448,847 and 4,428,987 to Bell et al, the contents of which are fully incorporated herein by reference, both disclose the use of such low molecular weight coupling agents. U.S. Pat. No. 4,448,847 describes a method of improving the adhesion of epoxy resins to steel substrates using either a beta-diketone or mercaptoester coupling agent. U.S. Pat. No. 4,428,987 describes a method of improving the adhesion of epoxy resins to copper substrates using benzotriazole, benzothiazole and related compounds as the coupling agent.
While the low molecular weight coupling agent of the type described hereinabove have improved the durability of polymer/metal bonds, these compositions nevertheless suffer from certain deficiencies and disadvantages. For example, in order to achieve the greatest effect, the prior art coupling agents should be coated onto the metal substrate in a single layer. However, single layer thicknesses are extremely difficult to produce. As a consequence, bond failure can result between layers of the coupling agent itself thereby dramatically reducing the bond strength of the overall polymer/metal bond. In addition, low molecular weight coupling agents are normally quite water permeable. Therefore, their presence can actually promote the infiltration of destructive water molecules into the interfacial region.
Low molecular weight coupling agents also provide little help in alleviating internal stresses in metal/thermoset adhesion systems. It will be appreciated that significant stresses can develop during post cure cooling of a thermoset such as an epoxy, primarily due to the thermal expansion coefficient mismatch between the metal and the polymer. This interfacial stress is believed to be the cause of failure in a number of adhesion systems.