Hot melt adhesives are thermoplastic, 100% solid and contain no solvents or water. Hot glue sticks are manufactured by combining polymers and additives. The molten mixture is then shaped, cooled and cut to size. Each additive used to make hot melt glue will give the adhesive a different property.
Hot melt adhesives are generally 100% solids formulations based on thermoplastic polymers. They are solid at room temperature and are activated by heating them above their softening point, where they are liquid and can therefore be processed. After application, they retain the ability to wet the substrate until they solidify. When they solidify, they return to a physical state that has structural integrity and can function as an adhesive.
The adhesive is applied by extrusion, lamination or spraying and bonding is carried out immediately after application or after reheating the solidified layer. The variety of polymers of this class is very wide and includes both natural and synthetic polymers. The high viscosity of the melt makes them particularly suitable for porous and permeable substrates that would otherwise be more difficult to bond with a solvent system. A feature of hot melts is that, when cooled, they quickly increase their internal strength, which allows quick assembly and subsequent processing.
Because they are based on thermoplastic polymers, hot melts can be repeatedly heated to melt and cooled to solidify. This property limits the temperature resistance of hot melt joints and they also have a tendency to deform when subjected to continuous stresses or high temperatures. On the plus side, these adhesives can be used to create bonded joints that are thermally removable and can also be reattached. Theoretically, any thermoplastic can be a hot melt adhesive, but the ten or more preferred materials are typically solids up to 79.4°C or more, then melt sharply to give a low viscosity fluid that is easily applied and capable of wetting the substrate to be bonded, followed by a rapid setting to the cool down.
The main polymers used in hot melts are ethylene-vinyl acetate (EVA), polyolefins, polyamides and polyesters, styrene block copolymers, polyethylene and ethylene-methyl acrylate (EMA) or ethylene n-butyl acrylate (ENBA). Modified rosins and modified terpenes, which have a molecular weight to softening point ratio of less than about 10, when used as tackifiers alone or in combination in a hot melt, provide adhesives that can be applied at a low temperature and have a high heat resistance and good cold resistance. The automotive industry employs hot melts for a variety of applications, including bonding insulating and damping materials, joining headlight covers to metal frames and wheel covers. Formulation options for hot melts are broad and dependent on the end use, e.g.
styrene block polymers can be used for pressure sensitive, and polyolefins such as polyethylene, ethylene vinyl acetate and polypropylene can be used for coating and laminating as well as for end uses of filling and filling indicated above. The key performance characteristics of hot melt packaging adhesives are their versatility in terms of adhesion; their longer shelf life (aging performance of hot melt adhesives); and their lower odor, cost and application temperature. Hot melt can also be applied in various ways including nozzles, extrusion, melt blowing, spiral spraying, screen printing and slot nozzle coating. Stabilizers delay oxidation, tackifiers improve bond strength, waxes reduce viscosity and alter surface characteristics, and various fillers increase viscosity, melting point and bond strength.
With a lower wax content, hot melt will have a higher viscosity and greater flexibility, and will bond more aggressively. Pressure-sensitive hot melts are beginning to replace solvent-based ones, partly because they offer better resistance to heat and plasticizers. The application of hot melt is a delicate matter and a range of tools are available to achieve this purpose, with the gun being the most widely used. The composition is normally formulated to have a glass transition temperature (onset of brittleness) below the lowest service temperature and also a suitably high melting temperature.
Hot melts are sold only with the manufacturer's name number or designation, without generic identification, as is common with most other adhesives. The main determinants of the softening point of the hot melt are the melting point of the wax used or the transition temperature of the base polymer (in the case of hot melt based on styrene block copolymer). . .
