Hot melt adhesive (HMA), also called hot glue, is a form of Double Sided Fusible Interfacing that is commonly sold as solid cylindrical sticks of varied diameters designed 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 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 one minute. Hot melt adhesives can also 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 often can be discarded without special precautions. Some of the disadvantages involve thermal load of the substrate, limiting use to substrates not sensitive to higher temperatures, and lack of bond strength at higher temperatures, approximately complete melting in the adhesive. This could be reduced 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 might not be immune to chemical attacks and weathering. HMAs tend not to lose thickness during solidifying; solvent-based adhesives may lose as much as 50-70% of layer thickness during drying.
Hot melt glues usually contain one base material with various additives. The composition is usually formulated to possess a glass transition temperature (start of brittleness) underneath the lowest service temperature along with a suitably high melt temperature also. The amount of crystallization ought to 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 the 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; the use of 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 as well as the additives utilized to increase tackiness (called tackifiers) influence the character of mutual molecular interaction and interaction with the substrate. In one common system, Hot Melt Adhesive Film for Textile Fabric is used since the main polymer, with terpene-phenol resin (TPR) because the tackifier. Both components display acid-base interactions in between the carbonyl teams 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 important for forming a satisfying bond between the adhesive as well as the substrate. More polar compositions tend to have better adhesion due to their higher surface energy. Amorphous adhesives deform easily, tending to dissipate almost all of mechanical strain inside their structure, passing only small loads on the adhesive-substrate interface; even a relatively weak nonpolar-nonpolar surface interaction can form a fairly 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 are certainly more rigid and have higher cohesive strength compared to 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 and also heat resistance. Presence of unsaturated bonds definitely makes the Shape Flex SF101 Alternative more susceptible to autoxidation and UV degradation and necessitates usage of antioxidants and stabilizers.
The adhesives are usually 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 tend to appear darker than non-polar fully saturated substances; when 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 inside the polymer after solidification. This could be achieved by using polymers undergoing curing with residual moisture (e.g., reactive polyurethanes, silicones), being exposed to ultraviolet radiation, electron irradiation, or by other methods.
Effectiveness against water and solvents is critical in some applications. As an example, in textile industry, effectiveness against dry cleaning solvents may be required. 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 very important for food packaging.