Hot melt adhesive (HMA), also referred to as hot glue, is a type of thermoplastic adhesive which is commonly sold as solid cylindrical sticks of numerous diameters made to be used utilizing a hot glue gun. The gun uses 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 couple of seconds to 1 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 often could be discarded without special precautions. A few of the disadvantages involve thermal load of 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 could be reduced by utilizing Fabric with film laminating machine that after solidifying undergoes further curing e.g., by moisture (e.g., reactive urethanes and silicones), or possibly 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 up to 50-70% of layer thickness during drying.
Hot melt glues usually contain one base material with some other additives. The composition is normally formulated to have a glass transition temperature (beginning of brittleness) below the lowest service temperature along with a suitably high melt temperature as well. The amount of crystallization should be as much as possible but within limits of allowed shrinkage. The melt viscosity as well as the crystallization rate (and corresponding open time) can be tailored for that application. Faster crystallization rate usually implies higher bond strength. To reach 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 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 and also the additives employed to increase tackiness (called tackifiers) influence the type of mutual molecular interaction and interaction with all the substrate. In one common system, EVA can be used as the main polymer, with terpene-phenol resin (TPR) since the tackifier. The 2 components display acid-base interactions between the carbonyl sets 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 of the substrate is vital for forming a satisfying bond involving the Hydraulic die cutting machine as well as the substrate. More polar compositions usually have better adhesion because of the higher surface energy. Amorphous adhesives deform easily, tending to dissipate most of mechanical strain inside their structure, passing only small loads on the adhesive-substrate interface; 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 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 also transfer more strain for the adhesive-substrate interface. Higher molecular weight of the polymer chains provides higher tensile strength as well as heat resistance. Presence of unsaturated bonds makes pqrpif adhesive more prone to autoxidation and UV degradation and necessitates use of antioxidants and stabilizers.
The adhesives are often clear or translucent, colorless, straw-colored, tan, or amber. Pigmented versions can also be made and even versions with glittery sparkles. Materials containing polar groups, aromatic systems, and double and triple bonds have a tendency to appear darker than non-polar fully saturated substances; each time a water-clear appearance is desired, suitable polymers and additives, e.g. hydrogenated tackifying resins, must be used.
Increase of bond strength and service temperature can be achieved by formation of cross-links inside the polymer after solidification. This can be achieved by utilizing 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 essential in certain applications. For example, in Sofa Fabric Bronzing Machine, resistance to dry cleaning solvents may be required. Permeability to gases and water vapor might or might not be desirable. Non-toxicity of both base materials and additives and deficiency of odors is essential for food packaging.