The Fundamentals of Hot Melt Adhesives
Hot melt adhesive, sometimes known as hot glue, is a form of thermoplastic adhesive designed as solid cylindrical sticks and applied using a hot glue gun. The gun uses a continuous heating element to melt the plastic glue, which is then pushed through the gun with a mechanical trigger mechanism or with direct finger pressure. When first squeezed out of the nozzle, the glue is hot enough to damage skin. It is sticky when hot, but solidifies in a few seconds to one minute. Hot melt adhesive can also be applied by dipping or spraying.
In addition to being popular with hobbyists, it has many industrial uses where it provides advantages over solvent-based adhesives. For example, volatile organic compounds are reduced or entirely eliminated, and hot melt adhesives eliminate the drying/curing step. They also have long shelf lives and can typically be disposed of without special precautions. However, hot melt adhesive is not without its disadvantages. The main disadvantage comes from the limited thermal loads of a substrate, meaning hot melt adhesives are limited to use on substrates that can withstand high temperatures. Another disadvantage is that the adhesive’s bond strength can diminish at higher temperatures, sometimes melting completely.
Properties of Hot Melt Adhesive
There are four main properties of hot melt adhesive: melt viscosity, melt flow index, pot life stability, and bond-formation temperatures. Melt viscosity is perhaps the most noticeable property. It affects the spreading of the adhesive as well as the wetting of the surfaces. The viscosity of hot melt adhesives is dependent on temperature - viscosity lowers as heat increases. Melt flow index is a value inversely proportional to the molecular weight of the base polymer. Adhesives with high melt flow indexes are easy to apply but have poor mechanical properties, while low melt flow indexes have better properties but are more difficult to apply.
Pot life stability refers to the adhesives stability in a molten state and its tendency to decompose and burn. This is an important property for industrial processing, where the adhesive is molten for long periods of time before being applied. The last important property is bond-formation temperature. This is the minimum temperature below which sufficient wetting of the substrate does not occur.
Hot melt adhesives are thermoplastic, 100% solid, and contain no solvents or water. They consist of three main elements: polymers, resins, and plasticizers. Polymers add strength and flexibility, providing the primary mechanical properties. Resins offer wetting and adhesion properties, and plasticizer controls the viscosity and allows the adhesive to be easily applied.
Application Considerations for Hot Melt Adhesives
The most important things to consider when applying hot melt adhesives are temperature, add-on level, compression, open time, and substrate. The temperature must be hot enough that the adhesive’s viscosity is low enough to sufficiently wet the substrate and will not cool before the materials are fully assembled. The add-on level refers to the amount of adhesive used in a given application. It affects the wetting of the substrate and the rate of cooling. Compression refers to the amount of pressure used to laminate the substrate, mainly affecting the bond to the secondary substrate. The open time is the time, usually in minutes, after application of an adhesive in which it will still bond materials together. Finally, the substrate texture and chemistry both affect bonding. Highly textured substrates allow for more mechanical adhesion, while high surface energy substrates promote wetting.
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