Ask Dr. Dave / Columns

Ask Dr. Dave

June 2007

Question: Can you give me some tips on formulating a high-temperature epoxy adhesive?

Answer: There are several things you can do to improve the heat resistance of such an adhesive. When cured at room temperature, simple two-part epoxies usually have a glass-transition temperature (Tg) only 20 or 30 degrees above room temperature, thus limiting their ability to bear loads at moderately elevated temperatures. More complete curing and higher Tgs can often be achieved by heat-curing at 100-130°C. Other ways of increasing heat resistance are to blend epoxy novolac resins with conventional bisphenol A-based resins and to use special types of hardeners. For example, anhydride hardeners can produce very high Tg resins, but require curing at high temperatures for extended periods.

Question: We are thinking of getting into manufacturing polyurethane sealants for some construction applications, but have heard that there are some modified urethanes that don’t involve the handling of isocyanates. Do you have any knowledge of these?

Answer: You’re thinking of two types of sealants. The first are based on polyether polyol backbones that are end-capped with alkoxy silane groups; the second are true polyurethane polymers, again with the alkoxy silane end groups. These polymers can react with moisture, liberate small amounts of alcohol and crosslink just like one-component RTV silicones. These sealants are sometimes called “modified silicones,” “modified urethanes” or “hybrid” sealants. Because they don’t contain isocyanates, they do not have some of the negatives associated with these, such as gas bubble formation or the irritancy of isocyanates. These sealants are said to be particularly useful in applications where their non-staining characteristics and excellent adhesion provide long-term use without significant changes in properties. They are also noted for low viscosities at low temperatures, which aids in application.


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