Hot melt adhesive (HMA), also referred to as hot glue, is a type of thermoplastic adhesive which is commonly sold as solid cylindrical sticks of varied diameters designed 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 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 few seconds to 1 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 the drying or curing step is eliminated. Hot melt adhesives have long shelf-life and usually may be discarded without special precautions. A few 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 utilizing TPU film 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 do not lose thickness during solidifying; solvent-based adhesives may lose up to 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 possess a glass transition temperature (start of brittleness) beneath the lowest service temperature as well as a suitably high melt temperature as well. The amount of crystallization should be as much as possible but within limits of allowed shrinkage. The melt viscosity as well as the crystallization rate (and corresponding open time) could be tailored for the 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; the use 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 nature of mutual molecular interaction and interaction with all the substrate. In a single common system, EVA is used because the main polymer, with terpene-phenol resin (TPR) since the tackifier. Both components display acid-base interactions 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 in the substrate is important for forming a satisfying bond involving the Hydraulic die cutting machine and also the substrate. More polar compositions tend to 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 a 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 towards the adhesive-substrate interface. Higher molecular weight in the polymer chains provides higher tensile strength and 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 can also be made and even 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, must be used.
Increase of bond strength and service temperature can be accomplished by formation of cross-links within the polymer after solidification. This is often 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.
Potential to deal with water and solvents is essential in some applications. For instance, in Hot Foil Stamping Machine For Leather/Fabric, potential to deal with 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.