Epoxy Adhesives, Part 1

Posted on 7/10/2013 2:16:51 PM By Sandy Niks

Epoxy adhesives are frequently used for structural applications bonding rigid substrates together, such as metal to metal, fiber-reinforced plastic (FRP) to FRP, and FRP to metal. They are found in most, if not all, of the disciplines in the transportation segment – automotive, aerospace, marine, etc., although adhesive formulations will vary to accommodate the differing requirements. Some of this has been described in previous blogs, along with joint design and surface preparation.

Epoxy adhesives come in many forms – thin enough to be sprayed, thickened to a paste to be pumped as either a room temperature curing two-part, or a one-part adhesive requiring elevated cure. They can also be in the form of a supported or unsupported film, which would also require an elevated cure. When cured they form a thermoset polymer which may become rubbery at elevated temperatures, but cannot melt as a thermoplastic material would.  Epoxy adhesives can be formulated to be conductive; they can also be insulating when used as potting compounds. Formulating to meet these application requirements provides epoxy adhesives with a wide range of properties that includes good chemical resistance, excellent adhesion, good mechanical properties, and little shrinkage.

Formulating is determining what materials, and how much of each, need to be used to make the desired end product, in this case, an epoxy adhesive. Optimum cure and other processing conditions are also defined as part of this process.

Epoxy adhesives are named for the epoxy resin component, which is a relatively low molecular weight polymer containing at least two of the reactive “epoxide” groups, represented in its simplest form as:

epoxy adhesives, adhesives, transportation, adhesives in transportation

Many of the epoxy resins are variations of diglycidyl ether of bisphenol A (DGEBA). There are a range of properties available within the epoxy resins. Select the one that is within range of basic strength and viscosity requirements. For example, an epoxy resin that is solid at room temperature will need to be heated until fluid enough for mixing, compounded with the curing agent(s), fillers, and other additives to fine tune the desired final properties, then formed into the film, cooled to prevent further polymerization and curing,  and stored, often under refrigeration. Thermosetting hot melt adhesives could similarly start with one of the solid epoxy resins.

Curing epoxy resins is needed to achieve ultimate adhesion and mechanical properties. This is accomplished by extending the polymer chain and cross-linking between chains, with the cross-linking making the resultant polymer a thermoset. The reaction is often exothermic, releasing heat and furthering the cure. They are 100% reactive, meaning no by-products, so potential off-gassing issues are avoided. One chemical group used as a curing agent is amines, aliphatic amines for room temperature cures, aromatic amines for elevated cure. Polyamides may also be used, again curing at room temperature but imparting additional flexibility in the polymer chain. One of the latent hardeners is dicyandiamide (Dicy), often used in one-part formulations, curing only with heat. Another catalyst is the imidazole group, reacting similar to Dicy, but with increased elevated temperature resistance. Note that two-part adhesives would be formulated with the epoxy resin (or base) component separate from the curing agent (or hardener) component. The user would then mix the two together before applying to the bond area. One of the challenges in formulating for a two-part system is to make mix ratio as close to 1:1 as possible, as well as matching viscosities, to optimize even, consistent mixing.

Next time we will continue with a discussion of the diluents, accelerators, fillers, and other additives used in completing the formulation of epoxy adhesives.

Note: references will be included at the conclusion of this series on epoxy adhesives.

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