Hot melt adhesive (HMA), also called hot glue, is a kind of thermoplastic adhesive which is commonly sold as solid cylindrical sticks of varied diameters created to be used utilizing a hot glue gun. The gun utilizes 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 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 in most cases can be disposed of without special precautions. A number of the disadvantages involve thermal load in the substrate, limiting use to substrates not understanding of higher temperatures, and loss of bond strength at higher temperatures, approximately complete melting in the adhesive. This is often reduced by using TPU film 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 might not be resistant 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 include one base material with various additives. The composition is usually formulated to get a glass transition temperature (beginning of brittleness) underneath the lowest service temperature as well as a suitably high melt temperature too. The degree of crystallization ought to be as high 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 reach the properties of semicrystalline polymers, amorphous polymers would require molecular weights too much and, therefore, unreasonably high melt viscosity; the use of amorphous polymers in hot melt adhesives is generally 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 utilized to increase tackiness (called tackifiers) influence the character of mutual molecular interaction and interaction with all the substrate. In a single common system, EVA is utilized since the main polymer, with terpene-phenol resin (TPR) as 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 in the substrate is vital for forming a satisfying bond involving the Abrasive paper disc travel head cutting machine and the substrate. More polar compositions generally 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; 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 tend to be rigid and also have higher cohesive strength than the corresponding amorphous ones, but in addition transfer more strain for the adhesive-substrate interface. Higher molecular weight in the polymer chains provides higher tensile strength as well as 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 usually 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 have a tendency 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, must be used.
Increase of bond strength and repair temperature may be accomplished by formation of cross-links within the polymer after solidification. This can 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.
Effectiveness against water and solvents is crucial in some applications. For instance, in Sofa Fabric Bronzing Machine, effectiveness against 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 deficiency of odors is important for food packaging.