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Hot melt adhesive (HMA), also referred to as hot glue, is a kind of thermoplastic adhesive which is commonly sold as solid cylindrical sticks of various diameters made to be applied using a hot glue gun. The gun works with 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 from the heated nozzle is initially hot enough to burn and even blister skin. The glue is tacky when hot, and solidifies in a few 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, as well as the drying or curing step is eliminated. Hot melt adhesives have long life expectancy and often can be discarded without special precautions. Some 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 in the adhesive. This could be 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 perhaps 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 up to 50-70% of layer thickness during drying.

Hot melt glues usually include one base material with some other additives. The composition is usually formulated to have a glass transition temperature (onset of brittleness) beneath the lowest service temperature as well as a suitably high melt temperature as well. The degree of crystallization should be as much as possible but within limits of allowed shrinkage. The melt viscosity and the crystallization rate (and corresponding open time) may 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 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 as well as the additives employed 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 as the main polymer, with terpene-phenol resin (TPR) as the tackifier. Both components display acid-base interactions involving 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 in the substrate is important for forming a satisfying bond between the Abrasive paper disc travel head cutting machine and also the substrate. More polar compositions generally have better adhesion because of their higher surface energy. Amorphous adhesives deform easily, tending to dissipate almost all of mechanical strain inside 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 some cohesive failure. The distribution of molecular weights and amount of crystallinity influences the width of melting temperature range. Polymers with crystalline nature are certainly more rigid and have higher cohesive strength compared to the corresponding amorphous ones, but also transfer more strain towards the adhesive-substrate interface. Higher molecular weight of 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 utilization of antioxidants and stabilizers.

The adhesives are generally clear or translucent, colorless, straw-colored, tan, or amber. Pigmented versions can also be made as well as versions with glittery sparkles. Materials containing polar groups, aromatic systems, and double and triple bonds often appear darker than non-polar fully saturated substances; each time 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 achieved 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 essential in some applications. As an example, in Printing Machine, effectiveness against dry cleaning solvents is usually necessary. Permeability to gases and water vapor might or might not be desirable. Non-toxicity of the base materials and additives and lack of odors is essential for food packaging.

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