Hot melt adhesive (HMA), also known as hot glue, is a type of Double Sided Fusible Interfacing which is commonly sold as solid cylindrical sticks of varied diameters designed to be applied using a hot glue gun. The gun utilizes a continuous-duty heating element to melt the plastic glue, that 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 matter of moments to one minute. Hot melt adhesives can 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 in most cases could be discarded without special precautions. A few of the disadvantages involve thermal load from the substrate, limiting use to substrates not understanding of higher temperatures, and loss of 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 possibly is cured by ultraviolet radiation. Some HMAs will not be resistant against 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 consist of one base material with some other additives. The composition is usually formulated to have a glass transition temperature (onset of brittleness) beneath the lowest service temperature and a suitably high melt temperature also. The degree 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) can be tailored for your 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 from the polymer and also the additives used to increase tackiness (called tackifiers) influence the character of mutual molecular interaction and interaction using the substrate. In a single common system, Hot Melt Adhesive Film for Textile Fabric is used since the main polymer, with terpene-phenol resin (TPR) because the tackifier. The two components display acid-base interactions 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 and also the substrate. More polar compositions generally have better adhesion because of the higher surface energy. Amorphous adhesives deform easily, tending to dissipate almost all of mechanical strain in their structure, passing only small loads on the adhesive-substrate interface; also a relatively weak nonpolar-nonpolar surface interaction can form a reasonably 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 have higher cohesive strength compared to the corresponding amorphous ones, but in addition transfer more strain to the adhesive-substrate interface. Higher molecular weight in the polymer chains provides higher tensile strength as well as heat resistance. Presence of unsaturated bonds definitely makes the shape flex sf101 alternative more vunerable to autoxidation and UV degradation and necessitates use of antioxidants and stabilizers.
The adhesives are generally 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 tend to appear darker than non-polar fully saturated substances; each time 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 achieved by formation of cross-links within the polymer after solidification. This is often 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 certain applications. For instance, in textile industry, effectiveness against dry cleaning solvents may be needed. Permeability to gases and water vapor may or may not be desirable. Non-toxicity of both base materials and additives and lack of odors is very important for food packaging.