Adhesives can be classified in several ways, including their material origin (e.g., natural or synthetic) and their type of cure (e.g., physical curing such as drying or chemical curing). Terms such as 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 type 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 loads and be resistant to both heat and fluids.
In contrast, thermoplastic adhesives like polyvinyl acetate (white glue), cyanoacrylate, or hot melts soften on heating. Their assembled bonds tend to “creep” over time when loaded. In general, thermoset adhesives have better thermal, fluid, and environmental resistance than thermoplastics. There may be some concerns in the future that thermoset adhesives are not easily removable or recyclable, and it might be desirable to make higher performance thermoplastic systems.
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