Ask Dr. Dave
Moisture-Sensitive Urethane Adhesives and the Pros and Cons of Epoxies
Dr. Dave tackles two questions this month, one related to package stability for moisture-sensitive urethane adhesives, and the other regarding epoxy adhesives.
Question: My colleague and I are working on a unique challenge of trying to find a solution for manufacturing urethane-based adhesives that are quite sensitive to moisture exposure. We typically package in large quantities (5-gal pails and 55-gal drums), which poses some issues when trying to eliminate all points of potential exposure to moisture. If exposed to too much moisture, rather than foaming like most urethanes, the material will “crust” and form a barrier on the material that is undesirable and could ultimately lead to customer rejections.
Answer: Package stability is usually ensured by adding small amounts of 2-3% of PTSI (p-toluenesulfonylisocyanate) or oxazoladines to mop up moisture in your system. Blanketing the pail or drum with dry nitrogen before sealing is also a standard technique.
Question: What are the pros and cons of using epoxy adhesives?
Answer: Epoxies are quite versatile, high-strength adhesives with excellent performance on metals, ceramics, and glass. They will fill very large gaps, which is a major advantage over alternative systems like reactive acrylics and cyanoacrylates, and they have excellent thermal and fluid resistant properties.
Disadvantages include their two-component (or heat cure) form, inherent lack of toughness, and limited performance on plastics (where reactive acrylics, cyanoacrylates, and polyurethanes are usually superior). Also, because the base resins are relatively high-molecular-weight species, it is not possible to formulate very low-viscosity adhesives unless reactive diluents are added, with a corresponding drop in performance.
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