Hot melt adhesive (HMA), also referred to as hot glue, is a form of Double Sided Fusible Interfacing which is commonly sold as solid cylindrical sticks of varied diameters made to be applied utilizing 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 also blister skin. The glue is tacky when hot, and solidifies in a few seconds to 1 minute. Hot melt adhesives can 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, and the drying or curing step is eliminated. Hot melt adhesives have long life expectancy and usually may be disposed of without special precautions. Some of the disadvantages involve thermal load in the substrate, limiting use to substrates not responsive to higher temperatures, and lack of bond strength at higher temperatures, up to complete melting of the adhesive. This can be reduced 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 may not be immune to chemical attacks and weathering. HMAs usually 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 contain one base material with various additives. The composition is usually formulated to get a glass transition temperature (beginning of brittleness) underneath the lowest service temperature along with a suitably high melt temperature as well. The level of crystallization needs to be as much as 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; using amorphous polymers in hot melt adhesives is normally 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 all the substrate. In a single common system, Hot Melt Adhesive Film for Textile Fabric can be used since the main polymer, with terpene-phenol resin (TPR) since the tackifier. Both components display acid-base interactions in between 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 essential for forming a satisfying bond involving the adhesive and also 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 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 level of crystallinity influences the width of melting temperature range. Polymers with crystalline nature are certainly more rigid and have higher cohesive strength than the corresponding amorphous ones, but also transfer more strain for the adhesive-substrate interface. Higher molecular weight in the polymer chains provides higher tensile strength and also heat resistance. Presence of unsaturated bonds helps make the Shape Flex SF101 Alternative more susceptible to autoxidation and UV degradation and necessitates usage of antioxidants and stabilizers.
The adhesives are generally clear or translucent, colorless, straw-colored, tan, or amber. Pigmented versions will also be made as well as 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, have to be used.
Increase of bond strength and service temperature may be accomplished by formation of cross-links within the polymer after solidification. This could be achieved by making use of polymers undergoing curing with residual moisture (e.g., reactive polyurethanes, silicones), being exposed to ultraviolet radiation, electron irradiation, or by other methods.
Potential to deal with water and solvents is crucial in some applications. For example, in textile industry, effectiveness against dry cleaning solvents is usually necessary. 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.