Hot melt adhesive (HMA), also referred to as hot glue, is a kind of thermoplastic adhesive which is commonly sold as solid cylindrical sticks of numerous diameters created to be used utilizing a hot glue gun. The gun uses a continuous-duty heating element to melt the plastic glue, that 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 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, as well as the drying or curing step is eliminated. Hot melt adhesives have long shelf life and often can be disposed of without special precautions. A number 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 is often reduced by making use of Hot melt adhesive laminating machine 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 will not be resistant against 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 normally formulated to get a glass transition temperature (beginning of brittleness) below the lowest service temperature as well as a suitably high melt temperature too. The level of crystallization needs to be as high as possible but within limits of allowed shrinkage. The melt viscosity and also the crystallization rate (and corresponding open time) could be tailored for your application. Faster crystallization rate usually implies higher bond strength. To arrive at the properties of semicrystalline polymers, amorphous polymers would require molecular weights excessive and, therefore, unreasonably high melt viscosity; using 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 used to increase tackiness (called tackifiers) influence the character of mutual molecular interaction and interaction with the substrate. In a single common system, EVA is utilized since the main polymer, with terpene-phenol resin (TPR) as the tackifier. The 2 components display acid-base interactions between the carbonyl teams 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 generally have better adhesion because of the 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; even a relatively weak nonpolar-nonpolar surface interaction can form a relatively 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 also have higher cohesive strength compared to corresponding amorphous ones, but additionally transfer more strain to the adhesive-substrate interface. Higher molecular weight of the polymer chains provides higher tensile strength and heat resistance. Presence of unsaturated bonds makes pqrpif adhesive 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 are also 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; when a water-clear appearance 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 inside the polymer after solidification. This is often achieved by utilizing polymers undergoing curing with residual moisture (e.g., reactive polyurethanes, silicones), contact with ultraviolet radiation, electron irradiation, or by other methods.
Resistance to water and solvents is critical in some applications. For example, in Hot Foil Stamping Machine For Leather/Fabric, potential to deal with 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 absence of odors is important for food packaging.