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 utilizes 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 one minute. Hot melt adhesives may also be applied by dipping or spraying.
In industrial use, hot melt adhesives provide several advantages 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 shelf life and usually may be disposed of without special precautions. A number 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, as much as complete melting in the adhesive. This is often reduced by utilizing Flame 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 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 generally formulated to possess a glass transition temperature (start of brittleness) beneath the lowest service temperature along with a suitably high melt temperature also. The degree of crystallization ought to be as much as possible but within limits of allowed shrinkage. The melt viscosity and also the crystallization rate (and corresponding open time) may be tailored for the application. Faster crystallization rate usually implies higher bond strength. To reach 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 generally 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 and the additives used to increase tackiness (called tackifiers) influence the nature of mutual molecular interaction and interaction using the substrate. In one common system, EVA is used since the main polymer, with terpene-phenol resin (TPR) since the tackifier. Both components display acid-base interactions in between 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 from the substrate is essential for forming a satisfying bond between the Beam cutting machine and the substrate. More polar compositions generally have better adhesion because of the 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; even a relatively weak nonpolar-nonpolar surface interaction can form a fairly strong bond prone primarily to a 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 also have higher cohesive strength compared to corresponding amorphous ones, but in addition transfer more strain towards the adhesive-substrate interface. Higher molecular weight of the polymer chains provides higher tensile strength as well as heat resistance. Presence of unsaturated bonds makes pqrpif adhesive more prone to autoxidation and UV degradation and necessitates usage 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, need to be used.
Increase of bond strength and repair temperature can be achieved by formation of cross-links within the polymer after solidification. This could be achieved by utilizing polymers undergoing curing with residual moisture (e.g., reactive polyurethanes, silicones), contact with ultraviolet radiation, electron irradiation, or by other methods.
Effectiveness against water and solvents is essential in a few applications. For instance, in Printing Machine, potential to deal with dry cleaning solvents may be required. Permeability to gases and water vapor might or might not be desirable. Non-toxicity of the base materials and additives and absence of odors is important for food packaging.