By Dr. Jody Muelaner
Hot melt adhesive (HMA) is a thermoplastic that’s melted, typically using a glue gun, and applied in a molten state. As it cools it solidifies and forms a bond. HMA’s are used to bond paper and card in packaging and bookbinding, wooden furniture, and fabrics.
They are used to fix, insulate and protect electronic components. Hot melt adhesives are also able to bond rubbers, ceramics, metals, plastics and glass. HMA’s are usually supplied as a cylindrical rod known as a glue stick. Standard diameters include 12, 15, and 43mm.
These sticks are compatible with glue guns that:
- Combine a heat source to melt the adhesive
- Use a nozzle through which the molten adhesive is extruded
- Have a trigger mechanisms that pushes the glue stick through the heater and nozzle
Generally, the heat is supplied using a mains electric heating element. However, gas powered glue guns are also available for wireless use.
Within manufacturing, hot melt adhesive may be applied using other processes — such as dipping, rolling, and spraying. The advantages of HMA’s include rapid bonding, zero use of solvents, and the ability to join many different materials. Additionally, HMA’s produce little if any toxic fumes, which can be a major advantage for industrial processes.
The glue sticks can be stored for long periods with no special requirements. However, care must be taken not to damage the parts being bonded through excessive heating. Hot melt adhesive must be applied at the correct temperature. If the heat is not high enough then the molten adhesive will be unable to wet out the surfaces, which will prevent a proper bond formation. If the adhesive is too hot then it may damage the surfaces or it may require too long to solidify.
Rapid solidification also depends on the correct pressure to squeeze the molten adhesive into a thin layer. It’s important that sufficient pressure is maintained until solidification has completed or the bond will be severely weakened.
Most common hot melt adhesives are thermoplastics. They are not waterproof and are unsuitable for external use. These include EVA, acrylic, polyamide, and polyolefin. Reactive polyurethane hot melt adhesive undergoes secondary curing to form a thermoset plastic.
This makes it waterproof, suitable for external use and gives it good resistance to heat and chemicals.