Phenolic-epoxy adhesives have been known for over 50 years and were one of the first commercial high temperature adhesives. ¹ However, despite their age, until recently these products have still faced a major problem due to their short shelf-lives. Now, researchers have discovered how to resolve this issue, without sacrificing the adhesives performance advantages or creating toxicity issues as some previous attempts have.

This problem is part of the same characteristic that is also considered a virtue of phenolic-epoxies, namely their low curing temperature requirements in comparison to other phenolic-based resins. Their reactivity at low temperatures limits long-term stability, meaning that they often need to be stored at temperatures as low as -18°C.

Once cured, phenolic-epoxy adhesives retain their adhesive properties over a large temperature range, have high shear strengths, and are resistant to weathering, oil, solvents, and moisture. Some studies have shown that phenolic-epoxies cured at 74-95°C can retain reasonably good strengths at 482°C. Higher cure temperatures, often around 165-175°C, can give these adhesives better structural properties, however.

The ability to resist high temperatures has long proved useful, in applications like aircraft engines. However, today the increasing amounts of heat that some electronic devices now produce also see them exploited in applications including insulating or anisotropically conductive adhesives in LCDs ², circuit boards, flip chip, and other semiconductor devices.

Phenolic-epoxies are available commercially as either one-part or two-part adhesives in several forms, including pastes, solutions in organic solvents or water, and supported films. Of the three forms, the adhesive film generally provides better adhesive strength.⁴ These take advantage of the three stages of curing often seen in these adhesives. In the first stage, the adhesive is applied and solvents are dried off of at up to 90°C, and here it can be mounted onto a carrier film. The second step involves baking at around 0.07-0.40 MPa pressure and at temperatures up to around 165°C. A final structural enhancement can then be attained by post curing at elevated temperatures if necessary.

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The right properties for manufacturing

Logically the one-part adhesive contains both phenolic and epoxy, meaning the consumer need not mix the components. However, this shortens the adhesives shelf-life at room temperature because the highly reactive phenolic immediately initiates the cure chemistry of the epoxy once it is added. This effect is even more pronounced above room temperature, where the usable lifetime of the adhesive can be reduced to minutes. Supported films can also have a shelf life short enough that they must be stored under refrigerated conditions before use. In addition, one-part adhesive manufacturers must also deal with the same problem potentially increasing solution viscosity and severely compromising processability.

In a two-part adhesive, the phenolic and epoxy are separated and supplied as two distinct packages, and therefore do not suffer from a reduced shelf-life. However, the adhesive does have a reduced green time and/or pot life, the time during which it remains workable after the phenolic and epoxy are mixed. In this case the solution viscosity problem is the consumers and not the manufacturers.

The most well-known phenolic-epoxy resins are novolac epoxies, using novolac resins formed by reacting formaldehyde with an excess of phenol. However, bis- and poly-phenols can also