Hot melt adhesive (HMA), also referred to as hot glue, is a kind of thermoplastic adhesive that is commonly sold as solid cylindrical sticks of numerous diameters designed to be used utilizing 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 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 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 shelf life and in most cases could be disposed of without special precautions. A few of the disadvantages involve thermal load in the substrate, limiting use to substrates not sensitive to higher temperatures, and lack of bond strength at higher temperatures, approximately complete melting of the adhesive. This is often reduced by making use of Flame 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 will not be immune to 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 normally formulated to get a glass transition temperature (onset of brittleness) below the lowest service temperature and a suitably high melt temperature as well. The amount 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) 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 excessive 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 from the polymer as well as the additives used to increase tackiness (called tackifiers) influence the nature of mutual molecular interaction and interaction with the substrate. In one common system, EVA can be used since the main polymer, with terpene-phenol resin (TPR) because the tackifier. The 2 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 of the substrate is vital for forming a satisfying bond involving the Hydraulic die cutting machine as well as the substrate. More polar compositions usually have better adhesion because of the higher surface energy. Amorphous adhesives deform easily, tending to dissipate almost all of mechanical strain within 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 degree of crystallinity influences the width of melting temperature range. Polymers with crystalline nature tend to be rigid and also have higher cohesive strength compared to the corresponding amorphous ones, but additionally transfer more strain to the adhesive-substrate interface. Higher molecular weight from 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 often clear or translucent, colorless, straw-colored, tan, or amber. Pigmented versions will 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, have to be used.
Increase of bond strength and service temperature can 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.
Potential to deal with water and solvents is crucial in a few applications. For instance, in PU Leather/PVC Bronzing Machine, potential to deal with dry cleaning solvents is usually necessary. 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.