QUESTION: I am somewhat confused by the terms thermoplastic and thermoset as they concern adhesives. Can you explain the difference?ANSWER: Adhesives can be classified in several ways, including their material origin (natural or synthetic) or their type of cure (physical or chemical). The terms thermoset, thermoplastic, structural or non-structural are also used in the industry.
The term "thermoset" historically meant adhesives that cured, or "set," on heating. The oldest types of this adhesive are the so-called PF, UF and MF resins (phenol formaldehyde, urea formaldehyde and melamine formaldehyde, respectively), which are commonly used to make plywood and are cured using heat and pressure. However, many thermoset adhesives cure at room temperature (e.g., two-part epoxies, moisture-curing polyurethanes, anaerobics and reactive acrylics). What thermosetting really means today is that the polymer chains are chemically crosslinked and do not soften on heating after curing. This makes them very suitable for structural applications where they have to support heavy loads. In contrast, thermoplastic adhesives like polyvinyl acetate (white glue), cyanoacrylates or hot melts also tend to "creep" over time when loaded. In general, thermoset adhesives have better thermal, fluid and environmental resistance than thermoplastics.