Hot melt adhesive (HMA), also called hot glue, is a type of thermoplastic adhesive that is commonly sold as solid cylindrical sticks of various diameters made to be applied utilizing a hot glue gun. The gun uses a continuous-duty heating element to melt the plastic glue, that 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 one minute. Hot melt adhesives can 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 often could be discarded without special precautions. A number of the disadvantages involve thermal load from the substrate, limiting use to substrates not responsive to higher temperatures, and loss in bond strength at higher temperatures, as much as complete melting in the adhesive. This can be reduced by making use of TPU film 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 may not be immune to chemical attacks and weathering. HMAs tend not to 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 various additives. The composition is generally formulated to have a glass transition temperature (start of brittleness) beneath the lowest service temperature and a suitably high melt temperature as well. The amount of crystallization needs to be up to possible but within limits of allowed shrinkage. The melt viscosity and the crystallization rate (and corresponding open time) may be tailored for the application. Faster crystallization rate usually implies higher bond strength. To achieve 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 usually 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 and the additives utilized to increase tackiness (called tackifiers) influence the character of mutual molecular interaction and interaction using the substrate. In one common system, EVA can be used as the main polymer, with terpene-phenol resin (TPR) since the tackifier. The two components display acid-base interactions between the carbonyl sets 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 essential for forming a satisfying bond between the Hydraulic die cutting machine and also the substrate. More polar compositions usually have better adhesion because of the higher surface energy. Amorphous adhesives deform easily, tending to dissipate most of mechanical strain inside their structure, passing only small loads on the adhesive-substrate interface; 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 level of crystallinity influences the width of melting temperature range. Polymers with crystalline nature are certainly more rigid and also have higher cohesive strength than the corresponding amorphous ones, but also 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 vunerable to autoxidation and UV degradation and necessitates usage of antioxidants and stabilizers.
The adhesives are generally clear or translucent, colorless, straw-colored, tan, or amber. Pigmented versions can also be made and also 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 repair temperature can be accomplished 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.
Potential to deal with water and solvents is critical in some applications. For example, in Printing Machine, potential to deal with dry cleaning solvents is usually necessary. Permeability to gases and water vapor might or might not be desirable. Non-toxicity of both the base materials and additives and absence of odors is essential for food packaging.