Hot Melt Glues are usually used in industrial applications as the use of Volatile organic compounds are either eliminated or reduced and the curing/drying time considerably shortened. These have a very high shelf life and hence can be stored and used after long periods. These, however, have a few disadvantages, in that substrates that are sensitive to high temperature cannot be used. Hence the use of HMG is limited to only those substrates that are not sensitive to high temperatures. The advantages far outweigh the drawbacks as these are used to bond a wide variety of materials and are often called the perfect bond. The nature of the glue allows it to flow into gaps thereby making the joint near perfect. Further the HMG do not shrink in layer thickness unlike their solvent based adhesives. These adhesives are usually supplied in sticks of different sizes and are pushed through an electric gun. The electric gun heats the sticks and pushes out molten glue. The glue when released is very hot and can damage one's skin. The molten glue cures in a few seconds to a minute. The Hot melt glue can either be applied or used by dipping.
Hot melt glues are made from thermoplastic polymers. These polymers when heated melt and are tacky enabling easy application on surfaces to be bonded. Certain additives such as waxes are also added to the glue in order to reduce the viscosity. The use of additives also helps in regulating the speed at which the curing takes place. Hot melt glues are made of several chemicals such as Polyethylene, Ethylene vinyl acetate and amorphous polypropylene. Polyethylene based hot melt glues are used in sealing cases and bags while the amorphous polypropylene is used in bonding paper to paper as well for sealing tapes which are water resistant. These are, however, weak and hence cannot used in applications where the strength of the joint is of importance. The newer range of hot melt glues are made up of copolymers of Styrene and butadiene. These are used in pressure sensitive adhesives for tapes, labels and bottles, being water resistant.
Water based adhesives are those which use water as the solvent. These adhesives also use rubber compounds along with synthetic additives. Commonly used additives are hydrocarbon resins, pine sap, starch, raw flour, corn starch etc. These can also be used after hydrolysis where these are broken down into smaller units which facilitates combining with fillers such as Sodium Hydroxide, Borate salts and plasticizers. Water based adhesives may also contain Volatile Organic Compounds to help reduce viscosity and regulate the speed of drying.
Water based adhesives could either be water soluble or water dispersed. The former includes animal glue and the latter include natural rubber, synthetic rubber and post formed rubber. Natural Rubber is drawn directly from the tree while the synthetic rubber is obtained from polymers such as chloroprene, vinyl acetate and vinyl chloride. The process of bonding takes place by penetration of water into the paper and drying of the same. The bonds made by water based adhesives are stronger than the hot melt glue in the case of bonding paper to paper. Hot melt glues can, however, bond a wide range of materials unlike water based adhesives. Water based adhesives find several applications in corrugated cartons, labeling of cans, sealing of bags, bonding of fabrics, tiles, laminations etc.
