Hot melt adhesive (HMA), also known as hot glue, is a form of Double Sided Fusible Interfacing that is commonly sold as solid cylindrical sticks of various diameters created to be applied using a hot glue gun. The gun utilizes 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 matter of moments 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 can be discarded without special precautions. A number of the disadvantages involve thermal load of the substrate, limiting use to substrates not responsive to higher temperatures, and loss in bond strength at higher temperatures, up to complete melting in the adhesive. This could be reduced by using a reactive adhesive that after solidifying undergoes further curing e.g., by moisture (e.g., reactive urethanes and silicones), or perhaps 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 consist of one base material with some other additives. The composition is usually formulated to possess a glass transition temperature (onset of brittleness) underneath the lowest service temperature and a suitably high melt temperature as well. The level 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) could 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 much and, therefore, unreasonably high melt viscosity; using 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 the additives utilized to increase tackiness (called tackifiers) influence the character of mutual molecular interaction and interaction using the substrate. In one common system, Hot Melt Adhesive Film for Textile Fabric is used because the main polymer, with terpene-phenol resin (TPR) since the tackifier. The two components display acid-base interactions involving the carbonyl groups 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 from the substrate is vital for forming a satisfying bond involving the adhesive as well as the substrate. More polar compositions usually 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; also a relatively weak nonpolar-nonpolar surface interaction can form a reasonably strong bond prone primarily to your 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 also have higher cohesive strength compared to the corresponding amorphous ones, but additionally transfer more strain towards the adhesive-substrate interface. Higher molecular weight from the polymer chains provides higher tensile strength and also heat resistance. Presence of unsaturated bonds helps make the Pellon SF101 Substitute more susceptible 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 can 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; when 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 achieved by formation of cross-links in the polymer after solidification. This could be achieved by making use of polymers undergoing curing with residual moisture (e.g., reactive polyurethanes, silicones), contact with ultraviolet radiation, electron irradiation, or by other methods.
Resistance to water and solvents is crucial in a few applications. As an example, in textile industry, resistance to dry cleaning solvents may be needed. Permeability to gases and water vapor might or might not be desirable. Non-toxicity of the base materials and additives and lack of odors is very important for food packaging.