Hot melt adhesive (HMA), also referred to as hot glue, is a kind of Double Sided Fusible Interfacing which is commonly sold as solid cylindrical sticks of numerous diameters designed to be applied using a hot glue gun. The gun works with a continuous-duty heating element to melt the plastic glue, that 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 may 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 life expectancy and usually could be discarded without special precautions. A number of the disadvantages involve thermal load from the substrate, limiting use to substrates not sensitive to higher temperatures, and lack of 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 is cured by ultraviolet radiation. Some HMAs might not be resistant to chemical attacks and weathering. HMAs usually 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 assorted additives. The composition is generally formulated to possess a glass transition temperature (beginning of brittleness) beneath the lowest service temperature as well as a suitably high melt temperature as well. The level of crystallization needs to be up to possible but within limits of allowed shrinkage. The melt viscosity and also the crystallization rate (and corresponding open time) could be tailored for the 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; the usage 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 from the polymer and also the additives utilized to increase tackiness (called tackifiers) influence the type of mutual molecular interaction and interaction with the substrate. In one common system, Hot Melt Adhesive Film for Textile Fabric can be used because the main polymer, with terpene-phenol resin (TPR) as the tackifier. The 2 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 from the substrate is essential for forming a satisfying bond between the adhesive as well as 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 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 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 rigid and possess higher cohesive strength than the corresponding amorphous ones, but also 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 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; whenever a water-clear caarow is desired, suitable polymers and additives, e.g. hydrogenated tackifying resins, need to be used.
Increase of bond strength and repair temperature may be accomplished by formation of cross-links in the polymer after solidification. This could 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.
Effectiveness against water and solvents is crucial in a few applications. For example, in textile industry, effectiveness against dry cleaning solvents may be needed. Permeability to gases and water vapor may or may not be desirable. Non-toxicity of the base materials and additives and lack of odors is essential for food packaging.