Hot melt adhesive (HMA), also referred to as hot glue, is a type of thermoplastic adhesive that is commonly sold as solid cylindrical sticks of varied diameters made to be used using 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 may also 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 shelf-life and in most cases can be discarded without special precautions. A number of the disadvantages involve thermal load from the substrate, limiting use to substrates not understanding of higher temperatures, and loss of bond strength at higher temperatures, as much as complete melting from the adhesive. This could be reduced by using Hot melt adhesive 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 resistant against chemical attacks and weathering. HMAs usually do not lose thickness during solidifying; solvent-based adhesives may lose up to 50-70% of layer thickness during drying.
Hot melt glues usually consist of one base material with some other additives. The composition is usually formulated to get a glass transition temperature (start of brittleness) beneath the lowest service temperature as well as a suitably high melt temperature also. The amount 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 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 also the additives used to increase tackiness (called tackifiers) influence the type of mutual molecular interaction and interaction with the substrate. In a single common system, EVA is used because the main polymer, with terpene-phenol resin (TPR) as the tackifier. Both components display acid-base interactions in between the carbonyl sets 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 vital for forming a satisfying bond in between the Hydraulic die cutting machine and 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 relatively strong bond prone primarily to your cohesive failure. The distribution of molecular weights and degree of crystallinity influences the width of melting temperature range. Polymers with crystalline nature tend to be rigid and 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 susceptible to autoxidation and UV degradation and necessitates use of antioxidants and stabilizers.
The adhesives are usually 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; 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 repair temperature can be achieved by formation of cross-links in the polymer after solidification. This is often achieved by using polymers undergoing curing with residual moisture (e.g., reactive polyurethanes, silicones), being exposed to ultraviolet radiation, electron irradiation, or by other methods.
Potential to deal with water and solvents is crucial in some applications. As an example, in Sofa Fabric Bronzing Machine, potential to deal with dry cleaning solvents may be required. Permeability to gases and water vapor may or may not be desirable. Non-toxicity of both the base materials and additives and absence of odors is important for food packaging.