In his October 2008 column, Dr. Dave discusses the proper formulation of two-part epoxies and the use of new adhesives in the assembly of materials that do not require special surface treatment.

Question: We need to formulate some fast-curing two-part epoxies for both assembly and repair applications. What would you recommend?

Answer: There are a few choices for this type of adhesive. Using a standard bisphenol-A epoxy resin, certain aliphatic or cycloaliphatic amines will give fast cures. Tertiary amines will usually act as accelerators with these hardeners, the most common one being 2,4,6-tris (dimethylaminomethyl)phenol. The addition of multifunctional acrylic monomers to epoxies can also give rapid cures and lower the viscosity of the overall system. One type of hardener that seems to be making something of a comeback is mercaptans. Although these sulfur-based compounds have a distinctive odor, this disappears on curing. Mercaptans have the advantages of very fast, controllable curing, a very low toxicity profile and a convenient 1:1 mix ratio that is relatively insensitive to “off-mix” ratios. Increased competition in the supply of these hardeners has made them more cost-effective for a range of applications.

Question: We assemble many items using polyethylene and polypropylene, and we hear that there are some new adhesives that can be used for these materials that do not require special surface treatment. Can you please elaborate on this?

Answer: For non-structural applications, hot-melt adhesives will frequently work well, particularly those based on polyolefins as opposed to traditional EVA (ethylene-vinyl acetate). However, for high-performance applications, it has been necessary for many years to use special treatments on these so-called low-energy surfaces in order to bond them. Most of these treatments involved some sort of oxidation of the surfaces, using either chemicals or a plasma method. In recent years, there have been two major developments: the creation of special primers for use with cyanoacrylate adhesives, which work well for relatively small assembly jobs but are not always cost-effective for large-scale applications, and the development of acrylic adhesives based on organoborane free radical initiators, which are mixed two-component adhesives that work particularly well on plastics such as filled polypropylene, making them very suitable for many industrial and automotive applications.