Hot glue, frequently sold as solid sticks for use in a heating gun, is a form of plastic formally known as a hot-melt adhesive (HMA). Its classification as a plastic comes from its fundamental chemical structure as a synthetic polymer. HMA is designed to be 100% solid at room temperature, containing no water or solvents. The ability of hot glue to transition repeatedly between a solid and a liquid state simply by changing its temperature defines its category of plastic.
The Chemistry: Identifying Hot Glue as a Thermoplastic
Hot glue belongs to the family of plastics called thermoplastics. These polymers become soft and pliable when heated and solidify upon cooling. This physical change is entirely reversible and can be repeated many times without chemical degradation. The process relies on the long polymer chains moving more freely when thermal energy is applied, allowing the material to flow like a liquid.
This characteristic contrasts with thermoset plastics, which undergo an irreversible chemical reaction, or curing, when first heated. Once a thermoset material has cured, reheating it causes it to burn or decompose, not melt back into a liquid state. Because hot-melt adhesive relies only on temperature change to form a bond, it sets very quickly as it cools down. The bond forms purely through physical solidification, making the application process fast and efficient.
Key Components and Additives
The base material for most common, general-purpose hot glue is the synthetic polymer Ethylene Vinyl Acetate (EVA). EVA is a copolymer made of two different monomer units—ethylene and vinyl acetate—linked into long chains. The ratio of these components determines the final properties, with higher vinyl acetate content leading to greater flexibility and adhesion.
The glue stick is an engineered compound containing several other components besides EVA. Tackifying resins are included to increase the wetting ability of the molten glue, helping it adhere better to different surfaces. Waxes, such as paraffin wax, are also mixed in to control the melt viscosity and setting speed. These additives allow the glue to flow smoothly out of the gun nozzle and quickly solidify once applied.
Safe Handling and Disposal
The primary safety concern when handling hot glue is the physical hazard of the application temperature, which can range from 120°C to over 200°C. The molten adhesive can cause serious skin burns upon contact, which is the most common injury associated with its use. Once the adhesive has cooled and solidified, the EVA-based polymer is considered inert and poses a very low chemical toxicity risk.
Hot glue is typically not accepted in standard municipal curbside recycling programs. Although the base polymer is plastic, the final product is a mixture of polymer, resins, and waxes, making it a complex, mixed material. This multi-component nature can contaminate batches of higher-value plastics during the recycling process.
Furthermore, in paper and cardboard recycling, the adhesive residue is difficult to separate from the paper fibers during pulping. If the glue is not fully removed, it creates sticky residue that negatively impacts the quality of the new paper product. Consequently, hot glue sticks and the cooled adhesive should be disposed of with regular household waste.