Hot melt adhesive (HMA), also known as hot glue, is a kind of thermoplastic adhesive that is commonly sold as solid cylindrical sticks of various diameters designed to be applied 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 1 minute. Hot melt adhesives can 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 usually can be discarded without special precautions. Some of the disadvantages involve thermal load in the substrate, limiting use to substrates not sensitive to higher temperatures, and loss in bond strength at higher temperatures, approximately complete melting from 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 possibly is cured by ultraviolet radiation. Some HMAs might not be resistant to chemical attacks and weathering. HMAs do not lose thickness during solidifying; solvent-based adhesives may lose approximately 50-70% of layer thickness during drying.
Hot melt glues usually include one base material with various additives. The composition is normally formulated to possess a glass transition temperature (start of brittleness) underneath the lowest service temperature as well as 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 achieve the properties of semicrystalline polymers, amorphous polymers would require molecular weights too high and, therefore, unreasonably high melt viscosity; the use 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 of 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 just one common system, EVA is utilized since the main polymer, with terpene-phenol resin (TPR) since the tackifier. The two components display acid-base interactions in between the carbonyl sets 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 of the substrate is essential for forming a satisfying bond in between the Beam cutting machine and 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; also a relatively weak nonpolar-nonpolar surface interaction can form a fairly 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 more rigid and have higher cohesive strength compared to the corresponding amorphous ones, but also transfer more strain to the adhesive-substrate interface. Higher molecular weight from 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 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; 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 service temperature may be accomplished by formation of cross-links inside the polymer after solidification. This could be achieved by making use of polymers undergoing curing with residual moisture (e.g., reactive polyurethanes, silicones), exposure to ultraviolet radiation, electron irradiation, or by other methods.
Resistance to water and solvents is critical in a few applications. For example, in Printing Machine, effectiveness against 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 deficiency of odors is important for food packaging.