glue

Glue is an adhesive substance used to join materials by forming a bond at the microscopic level. It typically consists of a liquid or semi-solid that hardens, cures, or sets through evaporation, chemical reaction, or cooling.

Glues can be natural or synthetic. Natural glues include animal-derived hide glue, casein glue, and starch-based pastes. Synthetic glues include epoxies, cyanoacrylates, polyurethanes, silicone, acrylics, and hot-melt adhesives. They may cure by evaporation of solvent or water, chemical reaction with hardeners, or cooling from a molten state. They are also categorized by base material: water-based, solvent-based, reactive, and hot-melt.

Common applications span woodworking, paper crafts, packaging, construction, automotive, electronics, and medical fields. Properties of glues—bond strength, curing time, temperature and moisture resistance, flexibility, and aging stability—determine suitable uses. Preparation of surfaces and clamping or pressing during curing are often required.

Safety and environmental considerations include exposure to solvents and fumes, skin or eye irritation, and proper ventilation. Recycling and disposal depend on the adhesive type. Some glues have low volatile organic compound content; non-toxic formulations are available for crafts and food-contact surfaces.

Historical note: Adhesives have been used since ancient times; hide glue and starch pastes were common before synthetic polymers emerged in the 20th century, expanding the range of bonding options.