Hot melt adhesive (HMA), also called hot glue, is a kind of Double Sided Fusible Interfacing which is commonly sold as solid cylindrical sticks of various diameters made to be applied utilizing a hot glue gun. The gun works with a continuous-duty heating element to melt the plastic glue, which the user pushes from 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 also blister skin. The glue is tacky when hot, and solidifies in a few 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, as well as the drying or curing step is eliminated. Hot melt adhesives have long life expectancy and usually may be discarded without special precautions. A number 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 of the adhesive. This could be reduced simply by using a reactive adhesive that after solidifying undergoes further curing e.g., by moisture (e.g., reactive urethanes and silicones), or is cured by ultraviolet radiation. Some HMAs might 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 consist of one base material with various additives. The composition is generally formulated to possess a glass transition temperature (start of brittleness) below the lowest service temperature and a suitably high melt temperature as well. The degree of crystallization should be up to 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 excessive 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 in the polymer and also the additives employed to increase tackiness (called tackifiers) influence the nature of mutual molecular interaction and interaction with all the substrate. In just one common system, Hot Melt Adhesive Film for Textile Fabric can be used because the main polymer, with terpene-phenol resin (TPR) since the tackifier. The two components display acid-base interactions between the carbonyl groups 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 from the substrate is essential for forming a satisfying bond in between the adhesive and the substrate. More polar compositions generally have better adhesion due to their higher surface energy. Amorphous adhesives deform easily, tending to dissipate the majority of mechanical strain inside their structure, passing only small loads on the adhesive-substrate interface; a relatively weak nonpolar-nonpolar surface interaction can form a relatively strong bond prone primarily to a cohesive failure. The distribution of molecular weights and amount 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 corresponding amorphous ones, but also transfer more strain to the adhesive-substrate interface. Higher molecular weight of the polymer chains provides higher tensile strength and heat resistance. Presence of unsaturated bonds makes the Shape Flex SF101 Alternative 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 as well as 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 caarow 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 in 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.
Resistance to water and solvents is crucial in certain applications. For instance, in textile industry, resistance to dry cleaning solvents may be required. 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 essential for food packaging.