Ask Dr. Dave
Question: We are using a room-temperature-curing epoxy adhesive. Will heat affect the Tg of the adhesive? What difference does it make whether we cure at room temperature, 60°C or above?
Answer: The Tg, or glass transition, is the temperature where the polymer goes from a hard, glasslike state to a rubber-like state. The ultimate Tg is determined by the chemical structure of the epoxy resin you are using, the type of hardener and the degree of cure. In general, the highest Tg systems come from using multifunctional resins (e.g., trifunctional or tetrafunctional) and by curing at high temperatures for long periods; these curing conditions ensure maximum molecular mobility and complete curing. It is not possible to get very high Tg systems by room-temperature curing. In order to cure at room temperature, you have to use relatively low-molecular-weight difunctional epoxy resins (and often diluents) to make the systems sufficiently fluid - plus, only a few hardeners, such as polymercaptans and amidoamines, will cure the systems sufficiently quickly at room temperature. Normally, you won't get a Tg more than 20 or 30°C above the cure temperature. You will usually notice a dramatic loss in adhesive strength above about 50°C in a room-temperature-cured epoxy. With your adhesive, you should certainly see an increase in Tg by curing at higher temperatures, but the ultimate value will be determined by your particular composition.
Question: We are interested in using MMA adhesives for bonding several plastics, but we don't like the strong smell and flammability. Do you have any suggestions?
Answer: These adhesives are generally known as reactive acrylic adhesives. The monomer used in many of these adhesives, MMA (methyl methacrylate), does have a very objectionable odor and is quite flammable. It has been used historically for two reasons: It gives good adhesion on many low polarity surfaces and it is a low-cost monomer compared to most other acrylic monomers. So-called "low volatile" versions of these adhesives have been available for many years, though not many companies offer them. They are based on higher molecular weight monomers, such as tetrahydrofurfural methacrylate, cyclohexyl methacrylate and others. These monomers have some odor (but not as bad as MMA) and lower flammability. The systems they provide are usually not quite as strong as MMA systems, but they often cure faster.