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 varied diameters made to be used 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 from the heated nozzle is initially hot enough to burn and even blister skin. The glue is tacky when hot, and solidifies in a matter of moments to 1 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, and also the drying or curing step is eliminated. Hot melt adhesives have long life expectancy and often could be disposed of without special precautions. Some of the disadvantages involve thermal load from the substrate, limiting use to substrates not sensitive to higher temperatures, and loss of bond strength at higher temperatures, up to complete melting in the adhesive. This can 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 is cured by ultraviolet radiation. Some HMAs may not be immune to chemical attacks and weathering. HMAs do not lose thickness during solidifying; solvent-based adhesives may lose as much as 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 possess a glass transition temperature (beginning of brittleness) below the lowest service temperature as well as a suitably high melt temperature too. The level of crystallization ought to be up to possible but within limits of allowed shrinkage. The melt viscosity and also the crystallization rate (and corresponding open time) could 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 high 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 of the polymer and the additives employed to increase tackiness (called tackifiers) influence the type of mutual molecular interaction and interaction using the substrate. In just one common system, EVA is utilized as the main polymer, with terpene-phenol resin (TPR) as the tackifier. The 2 components display acid-base interactions between the carbonyl teams of vinyl acetate and hydroxyl groups 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 Automatic conveyor belt 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 almost all of mechanical strain in 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 your 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 possess higher cohesive strength compared to the 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 and heat resistance. Presence of unsaturated bonds makes pqrpif adhesive more vunerable to autoxidation and UV degradation and necessitates use of antioxidants and stabilizers.
The adhesives are generally clear or translucent, colorless, straw-colored, tan, or amber. Pigmented versions are also made and also 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, have to be used.
Increase of bond strength and repair temperature may be accomplished by formation of cross-links within 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 critical in a few applications. For example, in Hot Foil Stamping Machine For Leather/Fabric, resistance to dry cleaning solvents may be needed. Permeability to gases and water vapor may or may not be desirable. Non-toxicity of both base materials and additives and absence of odors is essential for food packaging.