Hot Melt Adhesives are thermoplastic solids at normal temperatures, which need heating and melting before application. Adhesive is applied, hot and liquid, on either one or both the substrates that are to be bonded together. Substrates are brought together immediately after adhesive application and as the adhesive cools and dries an adhesive bond between the substrates is achieved.

Base polymers are synthetic resins such as ethylene vinyl acetate copolymers, polyamides, polysters, and block polymers. The base polymer forms the backbone of the adhesive, providing strength and toughness.

These could be hydrocarbon resins, rosin esters, polyterpenes that act as modifying resins that contribute surface wetting and tack.

Hot melt adhesives do have a shelf life, while the physical appearance may not change appreciably with time, the material gets progressively brittle and lose specific adhesion. Shelf life begins from the date of manufacture and hence the product should be rotated i.e. the material should be used on the first come first consumed basis.

Heat history is the record of number of hours the adhesive has been subject to high temperatures during the manufacturing process. This is a critical parameter that governs the quality of the product manufactured.

Charring is the oxidation of hot melt adhesive at elevated temperatures due to prolonged and repeated heating. Charring is more pronounced at temperatures exceeding 200 Centrigrade. Charred material will not dissolve in any solvent nor can it be remelted. Charred product will coke hoses and nozzles and damage the equipment.

Charring is inherent and can only be reduced but not eliminated fully. Users are advised to remove from the machine, charred materials periodically before it accumulates and causes damage to the equipment.

Since charring is temperature related, a new class of adhesives called cold melts/warm melts are being developed. These adhesives work typically at application temperatures that are lower by about 25C-30C as compared to normal hot melt adhesives.Lower application temperatures lessen the rate of charring.

Antioxidants lower the tendency to charring but due to their high value increase the cost of formulated adhesive.

Being thermoplastic in nature, hot melt adhesives have poor heat resistance. Adhesive bond weakens at elevated temperatures.

The colour of the adhesive is due to the colour of the tackifier. Generally the better and stable grades of tackifiers have a lighter colour. The light colour of unpigmented adhesive would generally signify a better product.

Heat resistance of hot melt adhesive is given by Ring&Ball softening point. Higher the softening point , higher is the heat resistance. If the softening point is 'X C', the product is likely to work satisfactory upto a temperature of 'X-20 C'.