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). Hot glue sticks are manufactured by combining polymers and additives. The molten mixture is then shaped, cooled and cut to size.
Hot melt adhesivesare 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. Hot melts are also used in the printing industry for joining book spines, in the textile industry for joining appliques and in the shoemaking industry for joining, for example, shoe soles. To avoid this problem, formulators have developed hot melts with a lower application temperature, sometimes referred to as “hot melts”.
Stabilizers delay oxidation, tackifiers improve bond strength, waxes reduce viscosity and alter surface characteristics, and various fillers increase viscosity, melting point and bond strength. These thermoplastic resins are considered the personality of a hot melt formula; they can be manipulated to affect many processing and performance attributes. They are compatible with many structural polymers and have a high softening point along with a low melt viscosity. 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.
To achieve the properties of semicrystalline polymers, amorphous polymers would require too high molecular weights and therefore an unreasonably high melt viscosity; the use of amorphous polymers in hot melt adhesives is normally only as modifiers. Melt viscosity and crystallization rate (and corresponding open time) can be adapted to the application. Hot melts are sold only with the manufacturer's name number or designation, without generic identification, as is common with most other adhesives. The hot melt adhesive in film or web form is heated by IR heaters and fed into the nip between the two substrates, as shown in Fig.
The molecular weight distribution and the degree of crystallinity influence the breadth of the melting temperature range. The most commonly used polymers in hot glue sticks include ethylene vinyl acetate (EVA), polyesters, polyethylene and ethylene methyl acrylate (EMA). The main advantages of hot melts are that they are generally quick-setting, allowing high production speeds, and have good void-filling properties, useful for rough surfaces. Hot melt adhesive (HMA), also known as hot glue, is a form of thermoplastic adhesive commonly sold as solid cylindrical sticks of various diameters designed to be applied with a hot glue gun.
Hot melt adhesives are thermoplastic adhesive materials applied as melts that achieve a solid state and the resulting strength upon cooling. . .