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 made 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 few seconds to 1 minute. Hot melt adhesives may 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 also the drying or curing step is eliminated. Hot melt adhesives have long shelf life and in most cases may be disposed of without special precautions. A number of the disadvantages involve thermal load from the substrate, limiting use to substrates not responsive to higher temperatures, and loss in bond strength at higher temperatures, approximately complete melting from 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 may not be resistant against 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 some other additives. The composition is usually formulated to get a glass transition temperature (beginning of brittleness) beneath the lowest service temperature and a suitably high melt temperature too. 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 the application. Faster crystallization rate usually implies higher bond strength. To reach the properties of semicrystalline polymers, amorphous polymers would require molecular weights too high 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 as well as the additives utilized to increase tackiness (called tackifiers) influence the type of mutual molecular interaction and interaction with the substrate. In just one common system, Hot Melt Adhesive Film for Textile Fabric is used as the main polymer, with terpene-phenol resin (TPR) because the tackifier. The two components display acid-base interactions in between the carbonyl groups of vinyl acetate and hydroxyl sets 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 in between the adhesive and the substrate. More polar compositions usually 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 relatively strong bond prone primarily to some 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 also have 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 as well as heat resistance. Presence of unsaturated bonds definitely makes the Shape Flex SF101 Alternative more susceptible to autoxidation and UV degradation and necessitates use of antioxidants and stabilizers.
The adhesives are usually clear or translucent, colorless, straw-colored, tan, or amber. Pigmented versions are also made and even versions with glittery sparkles. Materials containing polar groups, aromatic systems, and double and triple bonds often 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 service temperature can be accomplished by formation of cross-links in the polymer after solidification. This is often 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 certain applications. As an example, in textile industry, potential to deal with 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 important for food packaging.