Hot melt adhesive (HMA), also known as hot glue, is a kind of thermoplastic adhesive that is commonly sold as solid cylindrical sticks of various diameters created to be used utilizing 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 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 few seconds to one minute. Hot melt adhesives can 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 could be discarded without special precautions. Some of the disadvantages involve thermal load in the substrate, limiting use to substrates not responsive to higher temperatures, and loss in bond strength at higher temperatures, up to complete melting in the adhesive. This is often reduced by using 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 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 various additives. The composition is usually formulated to have a glass transition temperature (start of brittleness) below the lowest service temperature along with a suitably high melt temperature as well. The level of crystallization needs to 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 arrive at the properties of semicrystalline polymers, amorphous polymers would require molecular weights too much 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 and the additives used to increase tackiness (called tackifiers) influence the nature of mutual molecular interaction and interaction using the substrate. In just one common system, EVA is used because the main polymer, with terpene-phenol resin (TPR) because the tackifier. Both components display acid-base interactions involving 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 from the substrate is important for forming a satisfying bond involving the Abrasive paper disc travel head 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 most 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 some 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 in addition transfer more strain for 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 use of antioxidants and stabilizers.
The adhesives are often clear or translucent, colorless, straw-colored, tan, or amber. Pigmented versions can also be made and even 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; each time 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), being exposed to ultraviolet radiation, electron irradiation, or by other methods.
Effectiveness against water and solvents is crucial in some applications. For example, in Hot Foil Stamping Machine For Leather/Fabric, effectiveness against dry cleaning solvents may be required. Permeability to gases and water vapor may or may not be desirable. Non-toxicity of the base materials and additives and lack of odors is very important for food packaging.