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 numerous diameters made to be applied using a hot glue gun. The gun uses a continuous-duty heating element to melt the plastic glue, which 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 couple of seconds to 1 minute. Hot melt adhesives may also 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 also the drying or curing step is eliminated. Hot melt adhesives have long life expectancy and often could be disposed of without special precautions. A few of the disadvantages involve thermal load in the substrate, limiting use to substrates not responsive to higher temperatures, and lack of bond strength at higher temperatures, approximately complete melting of the adhesive. This is often reduced by making use of 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 may not be resistant to chemical attacks and weathering. HMAs tend not to lose thickness during solidifying; solvent-based adhesives may lose approximately 50-70% of layer thickness during drying.
Hot melt glues usually consist of one base material with some other additives. The composition is usually formulated to possess a glass transition temperature (onset of brittleness) beneath the lowest service temperature as well as a suitably high melt temperature too. The degree of crystallization should be as high 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 achieve the properties of semicrystalline polymers, amorphous polymers would require molecular weights excessive and, therefore, unreasonably high melt viscosity; the use of amorphous polymers in hot melt adhesives is normally only as modifiers. Some polymers can form hydrogen bonds between their chains, forming pseudo-cross-links which strengthen the polymer.
The natures of the polymer and the additives used to increase tackiness (called tackifiers) influence the character of mutual molecular interaction and interaction with the substrate. In a single common system, EVA can be used since the main polymer, with terpene-phenol resin (TPR) because the tackifier. The two components display acid-base interactions between the carbonyl teams of vinyl acetate and hydroxyl sets 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 Hydraulic die cutting machine and also the substrate. More polar compositions tend to have better adhesion because of the higher surface energy. Amorphous adhesives deform easily, tending to dissipate the majority of mechanical strain in their structure, passing only small loads on the adhesive-substrate interface; also a relatively weak nonpolar-nonpolar surface interaction can form a reasonably strong bond prone primarily to your cohesive failure. The distribution of molecular weights and level 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 in addition transfer more strain for the adhesive-substrate interface. Higher molecular weight in 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 have a tendency 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, have to be used.
Increase of bond strength and service temperature can be achieved by formation of cross-links inside the polymer after solidification. This can be achieved by using polymers undergoing curing with residual moisture (e.g., reactive polyurethanes, silicones), being exposed to ultraviolet radiation, electron irradiation, or by other methods.
Potential to deal with water and solvents is critical in some applications. For example, in Sofa Fabric Bronzing Machine, potential to deal with dry cleaning solvents is usually necessary. Permeability to gases and water vapor may or may not be desirable. Non-toxicity of both base materials and additives and lack of odors is very important for food packaging.