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 various diameters made to be used using a hot glue gun. The gun works with 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, as well as the drying or curing step is eliminated. Hot melt adhesives have long shelf-life and usually can be disposed of without special precautions. A few of the disadvantages involve thermal load from the substrate, limiting use to substrates not sensitive to higher temperatures, and loss in bond strength at higher temperatures, up to complete melting in the adhesive. This can be reduced by using 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 approximately 50-70% of layer thickness during drying.
Hot melt glues usually contain one base material with some other additives. The composition is usually formulated to get a glass transition temperature (beginning of brittleness) beneath the lowest service temperature and a suitably high melt temperature too. The degree of crystallization should be as high as possible but within limits of allowed shrinkage. The melt viscosity and 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; using 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 as well as 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 since the main polymer, with terpene-phenol resin (TPR) as the tackifier. The two components display acid-base interactions in between the carbonyl groups 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 in the substrate is essential for forming a satisfying bond between the Abrasive paper disc travel head cutting machine and the substrate. More polar compositions usually have better adhesion because of their higher surface energy. Amorphous adhesives deform easily, tending to dissipate the majority of mechanical strain in their structure, passing only small loads on the adhesive-substrate interface; also a relatively weak nonpolar-nonpolar surface interaction can form a relatively strong bond prone primarily to some cohesive failure. The distribution of molecular weights and degree of crystallinity influences the width of melting temperature range. Polymers with crystalline nature are certainly more rigid and have higher cohesive strength compared to corresponding amorphous ones, but also transfer more strain to the adhesive-substrate interface. Higher molecular weight in the polymer chains provides higher tensile strength and also heat resistance. Presence of unsaturated bonds makes pqrpif adhesive more prone to autoxidation and UV degradation and necessitates utilization 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; 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 inside the polymer after solidification. This could be achieved by using polymers undergoing curing with residual moisture (e.g., reactive polyurethanes, silicones), exposure to ultraviolet radiation, electron irradiation, or by other methods.
Effectiveness against water and solvents is crucial in a few applications. For instance, in PU Leather/PVC Bronzing Machine, resistance to 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 deficiency of odors is important for food packaging.
