Hot melt adhesive (HMA), also referred to as hot glue, is a type of thermoplastic adhesive which is commonly sold as solid cylindrical sticks of varied diameters made to be applied utilizing a hot glue gun. The gun uses a continuous-duty heating element to melt the plastic glue, that the user pushes through the gun either with a mechanical trigger mechanism on the gun, or with direct finger pressure. The glue squeezed out of the heated nozzle is initially hot enough to burn and even blister skin. The glue is tacky when hot, and solidifies in a matter of moments to one minute. Hot melt adhesives can be applied by dipping or spraying.
In industrial use, hot melt adhesives provide several positive aspects over solvent-based adhesives. Volatile organic compounds are reduced or eliminated, and the drying or curing step is eliminated. Hot melt adhesives have long shelf-life and in most cases may be disposed of without special precautions. Some of the disadvantages involve thermal load of the substrate, limiting use to substrates not understanding of higher temperatures, and loss in bond strength at higher temperatures, up to complete melting in the adhesive. This can be reduced by using Hot melt adhesive laminating machine that after solidifying undergoes further curing e.g., by moisture (e.g., reactive urethanes and silicones), or is cured by ultraviolet radiation. Some HMAs might not be resistant to chemical attacks and weathering. HMAs do not lose thickness during solidifying; solvent-based adhesives may lose approximately 50-70% of layer thickness during drying.
Hot melt glues usually contain one base material with some other additives. The composition is normally formulated to get a glass transition temperature (beginning of brittleness) beneath the lowest service temperature along with a suitably high melt temperature also. The amount of crystallization ought to be as much as possible but within limits of allowed shrinkage. The melt viscosity as well as the crystallization rate (and corresponding open time) can be tailored for that application. Faster crystallization rate usually implies higher bond strength. To reach the properties of semicrystalline polymers, amorphous polymers would require molecular weights too high and, therefore, unreasonably high melt viscosity; using amorphous polymers in hot melt adhesives is generally only as modifiers. Some polymers can form hydrogen bonds between their chains, forming pseudo-cross-links which strengthen the polymer.
The natures from the polymer and also the additives utilized to increase tackiness (called tackifiers) influence the nature of mutual molecular interaction and interaction with all the substrate. In one common system, EVA is used as the main polymer, with terpene-phenol resin (TPR) because the tackifier. The 2 components display acid-base interactions between the carbonyl teams of vinyl acetate and hydroxyl teams of TPR, complexes are formed between phenolic rings of TPR and hydroxyl groups on the surface of aluminium substrates, and interactions between carbonyl groups and silanol groups on surfaces of glass substrates are formed. Polar groups, hydroxyls and amine groups can form acid-base and hydrogen bonds with polar groups on substrates like paper or wood or natural fibers. Nonpolar polyolefin chains interact well with nonpolar substrates.
Good wetting of the substrate is essential for forming a satisfying bond between the Abrasive paper disc travel head cutting machine and also the substrate. More polar compositions tend to have better adhesion because of their higher surface energy. Amorphous adhesives deform easily, tending to dissipate the majority of mechanical strain inside their structure, passing only small loads on the adhesive-substrate interface; even a relatively weak nonpolar-nonpolar surface interaction can form a reasonably strong bond prone primarily to some cohesive failure. The distribution of molecular weights and amount of crystallinity influences the width of melting temperature range. Polymers with crystalline nature tend to be more rigid and also have higher cohesive strength than the corresponding amorphous ones, but also transfer more strain to the adhesive-substrate interface. Higher molecular weight from the polymer chains provides higher tensile strength and also heat resistance. Presence of unsaturated bonds makes pqrpif adhesive more susceptible to autoxidation and UV degradation and necessitates use of antioxidants and stabilizers.
The adhesives are often clear or translucent, colorless, straw-colored, tan, or amber. Pigmented versions are also made as well as versions with glittery sparkles. Materials containing polar groups, aromatic systems, and double and triple bonds tend to appear darker than non-polar fully saturated substances; when a water-clear appearance is desired, suitable polymers and additives, e.g. hydrogenated tackifying resins, must be used.
Increase of bond strength and repair temperature may be accomplished by formation of cross-links in the polymer after solidification. This can be achieved by making use of polymers undergoing curing with residual moisture (e.g., reactive polyurethanes, silicones), contact with ultraviolet radiation, electron irradiation, or by other methods.
Resistance to water and solvents is crucial in certain applications. For instance, in PU Leather/PVC Bronzing Machine, effectiveness against dry cleaning solvents may be needed. Permeability to gases and water vapor might or might not be desirable. Non-toxicity of both base materials and additives and deficiency of odors is very important for food packaging.