Epoxy Adhesives

Epoxy adhesives are widely used because of their good wetting and superior adhesion characteristics. They are available in a wide array of formulations. The most common form is a room temperature cure, two component system. Single component systems requiring heat cure are also available. Although epoxies perform best when the adherents are properly prepared, they are more forgiving than most adhesives when it comes to cured strength, because they tolerate being applied in thick sections, helping overcome poor joint design.

Recent developments include the design of thermally removable epoxy adhesives that behave as cross-linked thermoset plastics below 140°F (60°C), but when heated above 194°F (90°C) they are no longer cross-linked. This behaviour allows simplified disassembly of applications requiring access after testing or at the time of recycling.

Mechanical properties of epoxy resins

It is inappropriate to attempt to classify epoxy resins in a generic sense because their properties vary widely depending on the formulation. An unmodified cured epoxy is inherently brittle and has low shrinkage and high creep resistance, which makes them excellent materials in nonimpact or static applications. Fortunately, epoxy resins can be cured with various chemical agents and modified with numerous materials, all of which affects the resulting cured material. For example, diluents can be added to reduce the viscosity of epoxy potting compounds. Conversely, fillers such as fibers can be added to produce a thixotropic compound; minerals, to reduce cost by acting as an extender or improve electrical or thermal conductivity; and rubbers, to improve impact and shock-resistance. Formulated hybrids extend the offering by modifying the epoxy by combining it with other compatible chemical families such as phenolics, nylons, polysulfides, and vinyls, all of which alter the formulation to provide the desired material properties (uncured and cured).

Bonding Issues

There are few materials that epoxies will not adhere to, which is why they are widely used. They form their strongest bonds with metals and ceramics because of the benefits of a thin bond line and lack of porosity. Other materials, including wood and paper-based products, plastics, and many plating and coatings also bond well with epoxy adhesives. Materials that promote limited adhesion include those with low surface energies such as PTFE, acetals, and specialized coatings, such as PTFE impregnated anodize.

Surface Preparation for epoxy adhesives

Epoxy adhesives do not typically require extensive surface preparation to provide an effective bond. Use of solvent-based and non solvent-based degreasers are usually sufficient, unless significant surface deterioration is present that necessitates mechanical or chemical processes to remove it.

Performance

Curing of epoxies is accomplished using many different agents which produce different material properties, both before and after curing. They affect cohesive strength, hardness, and environmental performance such as temperature and solvent-resistance.

Single-form epoxies generally cure in one of two ways. One, from the release of insoluble curing agents upon the application of heat and two, by chemically inhibiting the process at room temperature, requiring heat to unblock the reaction. Although greater processing time is often the penalty for using single part epoxy adhesives, the improved crosslinking and resulting higher quality bond necessitiates their use.

Specialized single-forms include B-staged and UV curable adhesives.  

B-staged epoxies are resins that are semi-cured and in a solid state, requiring only heat to complete the curing process. The advantage of using these forms is that there is reduced waste and better consistency, because the adhesive is in its final or near final form. Single and two component epoxies provide the ease of processing associated with acrylic adhesives, with the rigid, low shrinkage properties possible with single- or two-part epoxies at the expense of strength and some environmental resistance.

Two-component systems require mixing to initiate the curing process, which results in an exothermic reaction that produces no physical by-products. Thorough mixing in the correct proportions are necessary. Manufacturers provide syringe type systems with mixing nozzles that simplify the process while improving the quality and consistency of the bonding process. High temperature formulations that require heat curing can resist oils, moisture and many solvents while offering thermal resistance to 200°F (98°C). Temperatures reaching 400°F (204°C) can also be applied but with a significant reduction in bond strength. UV exposure can also affect two part epoxy adhesives.