Hot melt adhesive (HMA), also called hot glue, is a form of Double Sided Fusible Interfacing which is commonly sold as solid cylindrical sticks of numerous diameters created to be applied using 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 even 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 positive aspects over solvent-based adhesives. Volatile organic compounds are reduced or eliminated, and 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. Some of the disadvantages involve thermal load from the substrate, limiting use to substrates not understanding of higher temperatures, and loss in bond strength at higher temperatures, up to complete melting of the adhesive. This can be reduced 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 resistant to chemical attacks and weathering. HMAs do not lose thickness during solidifying; solvent-based adhesives may lose as much as 50-70% of layer thickness during drying.
Hot melt glues usually consist of one base material with some other additives. The composition is normally 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 needs to be up to possible but within limits of allowed shrinkage. The melt viscosity as well as the crystallization rate (and corresponding open time) may be tailored for your application. Faster crystallization rate usually implies higher bond strength. To achieve the properties of semicrystalline polymers, amorphous polymers would require molecular weights excessive and, therefore, unreasonably high melt viscosity; using 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 as well as the additives utilized to increase tackiness (called tackifiers) influence the type of mutual molecular interaction and interaction using the substrate. In a single common system, Hot Melt Adhesive Film for Textile Fabric is utilized since the main polymer, with terpene-phenol resin (TPR) because the tackifier. Both components display acid-base interactions in 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 of the substrate is vital for forming a satisfying bond between the adhesive and the substrate. More polar compositions tend to have better adhesion because of their higher surface energy. Amorphous adhesives deform easily, tending to dissipate most of mechanical strain in their structure, passing only small loads on the adhesive-substrate interface; even 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 amount of crystallinity influences the width of melting temperature range. Polymers with crystalline nature are certainly more rigid and also have higher cohesive strength than the corresponding amorphous ones, but additionally transfer more strain towards the adhesive-substrate interface. Higher molecular weight in the polymer chains provides higher tensile strength as well as heat resistance. Presence of unsaturated bonds definitely makes the Pellon SF101 Substitute 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 will also be 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; whenever 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 may be accomplished by formation of cross-links in the polymer after solidification. This can be achieved by utilizing 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 essential in a few applications. For example, 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 the base materials and additives and deficiency of odors is important for food packaging.