Hot melt adhesive (HMA), also called hot glue, is a type of thermoplastic adhesive which is commonly sold as solid cylindrical sticks of varied diameters created to be applied utilizing 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 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. A few of the disadvantages involve thermal load of the substrate, limiting use to substrates not sensitive to higher temperatures, and loss of bond strength at higher temperatures, approximately complete melting from the adhesive. This could be reduced by utilizing TPU film laminating machine that after solidifying undergoes further curing e.g., by moisture (e.g., reactive urethanes and silicones), or possibly is cured by ultraviolet radiation. Some HMAs will not be immune to chemical attacks and weathering. HMAs usually do not lose thickness during solidifying; solvent-based adhesives may lose approximately 50-70% of layer thickness during drying.
Hot melt glues usually include one base material with various additives. The composition is generally formulated to get a glass transition temperature (beginning of brittleness) below the lowest service temperature along with a suitably high melt temperature also. The level of crystallization should be up to possible but within limits of allowed shrinkage. The melt viscosity as well as the crystallization rate (and corresponding open time) can be tailored for your application. Faster crystallization rate usually implies higher bond strength. To arrive at the properties of semicrystalline polymers, amorphous polymers would require molecular weights too high and, therefore, unreasonably high melt viscosity; the usage 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 in the polymer as well as 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 is used since the main polymer, with terpene-phenol resin (TPR) because the tackifier. The two components display acid-base interactions involving the carbonyl groups 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 in the substrate is important for forming a satisfying bond between the Beam cutting machine as well as the substrate. More polar compositions tend to have better adhesion because of their higher surface energy. Amorphous adhesives deform easily, tending to dissipate almost all of mechanical strain within their structure, passing only small loads on the adhesive-substrate interface; also a relatively weak nonpolar-nonpolar surface interaction can form a fairly 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 rigid and have higher cohesive strength than the corresponding amorphous ones, but additionally transfer more strain for the adhesive-substrate interface. Higher molecular weight of the polymer chains provides higher tensile strength and heat resistance. Presence of unsaturated bonds makes pqrpif adhesive more prone 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 will also be made and also 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; whenever a water-clear appearance is desired, suitable polymers and additives, e.g. hydrogenated tackifying resins, need to be used.
Increase of bond strength and service temperature can be accomplished by formation of cross-links within the polymer after solidification. This can be achieved by making use of polymers undergoing curing with residual moisture (e.g., reactive polyurethanes, silicones), exposure to ultraviolet radiation, electron irradiation, or by other methods.
Resistance to water and solvents is critical in some applications. As an example, in Printing Machine, resistance to dry cleaning solvents may be required. Permeability to gases and water vapor might or might not be desirable. Non-toxicity of both the base materials and additives and deficiency of odors is important for food packaging.