Hot-melt adhesive (HMA), also known as hot glue, is a form of thermoplastic adhesive that is commonly sold as solid cylindrical sticks of various diameters designed to be applied using 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 sticky when hot, and solidifies in a few seconds to one minute. Hot-melt adhesives can also be applied by dipping or spraying, and are popular with hobbyists and crafters both for affixing and as an inexpensive alternative to resin casting.
In industrial use, hot-melt adhesives provide several advantages over solvent-based adhesives. Volatile organic compounds are reduced or eliminated, and the drying or curing step is eliminated. Hot-melt adhesives have a long shelf life and usually can be disposed of without special precautions. Some of the disadvantages involve thermal load of the substrate, limiting use to substrates not sensitive to higher temperatures, and loss of bond strength at higher temperatures, up to complete melting of the adhesive. Loss of bond strength can be reduced by using a reactive adhesive that after solidifying undergoes further curing, whether by moisture (e.g., reactive urethanes and silicones), or ultraviolet radiation.
Some HMAs are more resistant to chemical attack and weathering than others; specifically, low-cost HMAs, such as those typically used in craft-oriented glue guns, possess less resistance to weathering and to certain chemicals than high-performance, specialized HMA formulations, which are engineered for demanding environments.
HMAs do not lose thickness during solidifying, whereas solvent-based adhesives may lose up to 50–70% of layer thickness during drying.
Properties
Melt viscosityOne of the most noticeable properties. Influences the spread of applied adhesive, and the wetting of the surfaces. Temperature-dependent, higher temperature lowers viscosity. Melt flow indexA value roughly inversely proportional to the molecular weight of the base polymer. High melt flow index adhesives are easy to apply but have poor mechanical properties due to shorter polymer chains. Low melt flow index adhesives have better properties but are more difficult to apply. Pot life stabilityThe degree of stability in molten state, the tendency to decompose and char. Important for industrial processing where the adhesive is molten for prolonged periods before deposition. Bond-formation temperatureMinimum temperature below which sufficient wetting of substrates does not occur.
General terms
Open time The working time to make a bond, where the surface still retains sufficient tack, can range from seconds for fast-setting HMAs to infinity for pressure-sensitive adhesives. Set time Time to form a bond of acceptable strength. Tack The degree of surface stickiness of the adhesive; influences the strength of the bond between wetted surfaces. Surface energy Influences wetting of different kind of surfaces.
Materials used
Hot-melt glues usually consist of one base material with various additives. The composition is usually formulated to have a glass transition temperature (onset of brittleness) below the lowest service temperature and a suitably high melt temperature as well. The degree 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) can 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; 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.Synthetically Designed Hot Melt Adhesives – Polyamides and Polyesters - Article . Specialchem4adhesives.com (2007-10-10). Retrieved on 2010-02-08.
The natures of the polymer and the additives used to increase tackiness (called tackifiers) influence the nature of mutual molecular interaction and interaction with the substrate. In one common system, EVA is used as the main polymer, with terpene-phenol resin (TPR) as the tackifier. The two components display acid-base interactions 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.M. Nardin et al.Effects of the composition of hot-melt adhesives on their bulk and interfacial properties, Journal de Physique IV, Volume 3, 1993, p. 1505 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 essential for forming a satisfying bond between the adhesive and the substrate. More polar compositions tend to have better adhesion due to 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; even a relatively weak nonpolar-nonpolar surface interaction can form a fairly 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 tend to be more rigid and have higher cohesive strength than the corresponding amorphous ones, but also 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 the adhesive 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 tend 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.Color and Clarity of Hot Melt Adhesives. Woodweb.com. Retrieved on 2010-02-08.
Increase of bond strength and service temperature can be achieved by formation of cross-links in the polymer after solidification. This can be achieved by using polymers undergoing curing with residual moisture (e.g., reactive polyurethanes, silicones), exposure to ultraviolet radiation, electron irradiation, or by other methods.
Resistance to water and solvents is critical in some applications. For example, in textile industry, resistance to dry cleaning solvents may be required. Permeability to gases and water vapor may or may not be desirable. Non-toxicity of both the base materials and additives and absence of odors is important for food packaging.
Mass-consumption disposable products such as diapers necessitate development of biodegradable HMAs. Research is being performed on e.g., lactic acid polyesters,Biodegradable/compostable hot melt adhesives comprising polyester of lactic acid polycaprolactone with soy protein,95-5 Development of biodegradable hot-melt adhesive based on poly-e-caprolactone and soy protein isolate for food packaging system. Ift.confex.com. Archived from the original on 2011-09-27. Retrieved on 2010-02-08. etc.
Some of the possible base materials of hot-melt adhesives include the following: Ethylene-vinyl acetate (EVA) copolymers, low-performance, the low-cost and most common material for the glue sticks (e.g., the light amber colored Thermogrip GS51, GS52, and GS53). They provide sufficient strength between and but are limited to use below to and have low creep resistance under load. Typical melting point is about . The vinyl acetate monomer content is about 18–29 percent by weight of the polymer. High amounts of tackifiers and waxes are often used; an example composition is 30–40% of EVA copolymer (provides strength and toughness), 30–40% of tackifier resin (improves wetting and tack), 20–30% of wax (usually paraffin-based; reduces viscosity, alters setting speed, reduces cost), and 0.5–1.0% of stabilizers.HMA - EVA based - UV/Light Stabilizers Center . SpecialChem4Adhesives. Retrieved on 2010-02-08. Fillers can be added for special applications. Can be formulated for service temperatures ranging from to , and for both short and long open times and a wide range of melt viscosities. High stability at elevated temperatures and resistance to ultraviolet radiation, which can be further enhanced with suitable stabilizers. High vinylacetate content can serve for formulating a hot-melt pressure-sensitive adhesive (HMPSA). EVA formulations are compatible with paraffin. EVA was the base for the original hot melt composition. The composition of the copolymer influences its properties; increased content of ethylene promotes adhesion to nonpolar substrates such as polyethylene, while increased content of vinyl acetate promotes adhesion to polar substrates such as paper. Higher ethylene content also increases mechanical strength, block resistance, and paraffin solubility. Higher vinyl acetate content provides higher flexibility, adhesion, hot tack, and better low-temperature performance. Adhesive grade EVA usually contains 14–35% vinyl acetate. Lower molecular weight chains provide lower melt viscosity, better wetting, and better adhesion to porous surfaces. Higher molecular weights provide better cohesion at elevated temperatures and better low-temperature behavior.Ethylene Vinyl Acetate (EVA) Copolymers(>50% Ethylene)Market Study Report - European Adhesives Industry . Chemquest.com. Retrieved on 2010-02-08. Increased ratio of vinyl acetate lowers the crystallinity of the material, improves optical clarity, flexibility and toughness, and worsens resistance to solvents. EVA can be crosslinked by, e.g., peroxides, yielding a thermosetting material.Ethylene vinyl acetate copolymers (EVA) . Plastiquarian.com. Retrieved on 2010-02-08. EVAs can be compounded with aromatic hydrocarbon resins.Young-Jun Park and Hyun-Joong Kim, "Hot-melt adhesive properties of EVA/aromatic hydrocarbon resin blend", International Journal of Adhesion and Adhesives, Volume 23, Issue 5, 2003, Page 383 Grafting butadiene to EVA improves its adhesion.Butadiene grafted ethylene-vinyl acetate hot melt adhesive Its dielectric properties are poor due to high content of polar groups, the dielectric loss is moderately high. Polypropylene HMAs are a better choice for high-frequency electronics.Hot melt glues (Barry L. Ornitz). Yarchive.net. Retrieved on 2010-02-08. EVAs are optically clearer and more gas and vapor permeable than polyolefins. Nearly half of EVA HMAs is used in packaging applications. Cryogenic grinding of EVAs can provide small, water-dispersible particles for heat-seal applications. EVA can degrade primarily by loss of acetic acid and formation of a double bond in the chain, and by oxidative degradation.John Moalli Plastics failure: analysis and prevention, William Andrew, 2001 p. 8 EVA can be compounded into a wide range of HMAs, from soft pressure-sensitive adhesives to rigid structural adhesives for furniture construction.