Hot glue
Overview
 
Hot melt adhesive also known as hot glue, is a form of thermoplastic
Thermoplastic
Thermoplastic, also known as a thermosoftening plastic, is a polymer that turns to a liquid when heated and freezes to a very glassy state when cooled sufficiently...

 adhesive
Adhesive
An adhesive, or glue, is a mixture in a liquid or semi-liquid state that adheres or bonds items together. Adhesives may come from either natural or synthetic sources. The types of materials that can be bonded are vast but they are especially useful for bonding thin materials...

 that is commonly supplied in solid cylindrical sticks of various diameters, designed to be melted in an electric hot glue gun. The gun uses a continuous-duty heating element
Heating element
A heating element converts electricity into heat through the process of Joule heating. Electric current through the element encounters resistance, resulting in heating of the element....

 to melt the plastic glue, which may be pushed through the gun by a mechanical trigger mechanism, or directly by the user. The glue squeezed out of the heated nozzle is initially hot enough to burn and blister skin.
Encyclopedia
Hot melt adhesive also known as hot glue, is a form of thermoplastic
Thermoplastic
Thermoplastic, also known as a thermosoftening plastic, is a polymer that turns to a liquid when heated and freezes to a very glassy state when cooled sufficiently...

 adhesive
Adhesive
An adhesive, or glue, is a mixture in a liquid or semi-liquid state that adheres or bonds items together. Adhesives may come from either natural or synthetic sources. The types of materials that can be bonded are vast but they are especially useful for bonding thin materials...

 that is commonly supplied in solid cylindrical sticks of various diameters, designed to be melted in an electric hot glue gun. The gun uses a continuous-duty heating element
Heating element
A heating element converts electricity into heat through the process of Joule heating. Electric current through the element encounters resistance, resulting in heating of the element....

 to melt the plastic glue, which may be pushed through the gun by a mechanical trigger mechanism, or directly by the user. The glue squeezed out of the heated nozzle is initially hot enough to burn and blister skin. The glue is tacky when hot, and solidifies in a few seconds to one minute. Hot melt adhesives can also be applied by dipping or spraying.

In industrial use, hot melt adhesives provide several advantages over solvent-based adhesives. Volatile organic compound
Volatile organic compound
Volatile organic compounds are organic chemicals that have a high vapor pressure at ordinary, room-temperature conditions. Their high vapor pressure results from a low boiling point, which causes large numbers of molecules to evaporate or sublimate from the liquid or solid form of the compound and...

s are reduced or eliminated, and the drying or curing step is eliminated. Hot melt adhesives have 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. This can be reduced by using a reactive adhesive that after solidifying undergoes further curing
Curing (chemistry)
Curing is a term in polymer chemistry and process engineering that refers to the toughening or hardening of a polymer material by cross-linking of polymer chains, brought about by chemical additives, ultraviolet radiation, electron beam or heat...

 e.g. by moisture (e.g. reactive urethanes and silicones), or is cured by ultraviolet radiation. Some HMAs may 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.

Glue sticks

Glue sticks are manufactured in several diameters for different glue guns. The most-used size has a diameter of 11 millimetre (0.433070866141732 in). Sticks are available in various lengths, from about 10 centimetres (3.9 in) up, although guns will accept sticks of any length. Thinner 7 mm (0.275590551181102 in) sticks are also used. Hot-melt and low temperature glue sticks are available for different types of guns, and some dual-use sticks melt at low temperatures but can be used at high temperatures without degradation. Hot melt adhesives are also available as granules, powder, slats, blocks, foils, ribbons, and solutions.

For household use, only a few types of stick are available, and they are more or less interchangeable. For industrial use, many types of sticks are available for special purposes, with the most common diameters being 12 mm (0.47244094488189 in), 15 mm (0.590551181102362 in), and 45 mm (1.8 in). Compositions resistant to plasticizer
Plasticizer
Plasticizers or dispersants are additives that increase the plasticity or fluidity of the material to which they are added; these include plastics, cement, concrete, wallboard, and clay. Although the same compounds are often used for both plastics and concretes the desired effects and results are...

s, usually based on polyamides, are needed for gluing PVC
Polyvinyl chloride
Polyvinyl chloride, commonly abbreviated PVC, is a thermoplastic polymer. It is a vinyl polymer constructed of repeating vinyl groups having one hydrogen replaced by chloride. Polyvinyl chloride is the third most widely produced plastic, after polyethylene and polypropylene. PVC is widely used in...

. Sticks have different open time and set time, varying from a few seconds to several minutes. The times depend on glue temperature, substrate, rate of heat loss from the glue mass, etc. A range of values of viscosity and heat resistance of the bond are available.

Important properties

The hot-melt adhesives have a number of important properties, making them suitable for different applications:
  • 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
  • Bond-formation temperature, minimum temperature below which sufficient wetting of substrates do not occur
  • Melt viscosity
    Viscosity
    Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...

    , one of the most important properties. Influences the spread of applied adhesive, and the wetting of the surfaces. Temperature-dependent, higher temperature lowers viscosity.
  • Melt flow index, a value 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 short polymer chains. Low melt flow index adhesives have better properties but are more difficult to apply.
  • Surface energy
    Surface energy
    Surface energy quantifies the disruption of intermolecular bonds that occur when a surface is created. In the physics of solids, surfaces must be intrinsically less energetically favorable than the bulk of a material, otherwise there would be a driving force for surfaces to be created, removing...

    , which influences wetting
    Wetting
    Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting is determined by a force balance between adhesive and cohesive forces.Wetting is important in the bonding or adherence of...

     of different kinds of surfaces.
  • Pot life stability, the degree of stability in molten state, the tendency to decompose and char. Important for industrial processing where the adhesives are molten for prolonged periods before deposition.
  • Tack, the degree of surface stickiness of the adhesive; influences the strength of the bond between wetted surfaces.

Materials used

Hot glue sticks are usually based on one or more base material, with various additives. The composition is usually formulated to have glass transition temperature (onset of brittleness) below the lowest service temperature and a suitably high melt temperature. 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 too high molecular weights and therefore unreasonably high melt viscosity; the use of amorphous polymers in hot melt adhesives is therefore usually only as modifiers and additives. Some polymers can form hydrogen bond
Hydrogen bond
A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond...

s between the chains, forming pseudo-cross-link
Cross-link
Cross-links are bonds that link one polymer chain to another. They can be covalent bonds or ionic bonds. "Polymer chains" can refer to synthetic polymers or natural polymers . When the term "cross-linking" is used in the synthetic polymer science field, it usually refers to the use of...

s strengthening the polymer.

The nature of the polymer and the tackifier additive influences the nature of mutual molecular interaction and interaction with the substrate; e.g. the EVA
Ethylene-vinyl acetate
Ethylene vinyl acetate is the copolymer of ethylene and vinyl acetate. The weight percent vinyl acetate usually varies from 10 to 40%, with the remainder being ethylene....

 together with terpene-phenol resin (TPR) tackifiers display acid-base interactions between the carbonyl
Carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups....

 groups of vinyl acetate and hydroxyl
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...

 groups of TPR, complexes are formed between phenolic rings of TPR and hydroxyl
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...

 groups on the surface of aluminium substrates, and interactions between carbonyl groups and silanol
Silanol
Silanol, also known as silyl alcohol, is a chemical with formula SiH3OH. It is the simplest silicon alcohol, and is a heavy, volatile, colorless, flammable liquid. At room temperature it is a polar liquid...

 groups on surfaces of glass substrates are formed. Polar groups, hydroxyls and amine groups can form acid-base and hydrogen bond
Hydrogen bond
A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond...

s with polar groups on substrates like paper or wood or natural fibers. Nonpolar polyolefin chains interact well with nonpolar substrates. Good wetting
Wetting
Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting is determined by a force balance between adhesive and cohesive forces.Wetting is important in the bonding or adherence of...

 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
Surface energy
Surface energy quantifies the disruption of intermolecular bonds that occur when a surface is created. In the physics of solids, surfaces must be intrinsically less energetically favorable than the bulk of a material, otherwise there would be a driving force for surfaces to be created, removing...

. Amorphous adhesives deform easily, tend 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 then 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
Autoxidation
Autoxidation is any oxidation that occurs in open air or in presence of oxygen and/or UV radiation and forms peroxides and hydroperoxides. A classic example of autoxidation is that of simple ethers like diethyl ether, whose peroxides can be dangerously explosive. It can be considered to be a slow,...

 and UV degradation
UV degradation
Many natural and synthetic polymers are attacked by ultra-violet radiation and products made using these materials may crack or disintegrate . The problem is known as UV degradation, and is a common problem in products exposed to sunlight...

 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. 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.

Increase of bond strength and service temperature can be achieved by formation of cross-link
Cross-link
Cross-links are bonds that link one polymer chain to another. They can be covalent bonds or ionic bonds. "Polymer chains" can refer to synthetic polymers or natural polymers . When the term "cross-linking" is used in the synthetic polymer science field, it usually refers to the use of...

s in the polymer after solidification. This can be achieved by using polymers undergoing curing with residual moisture (e.g. reactive polyurethanes, silicones), exposition to ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 radiation, electron irradiation, or by other methods.

Resistance to water and solvents is critical in some applications. E.g. in textile industry, resistance to dry cleaning
Dry cleaning
Dry cleaning is any cleaning process for clothing and textiles using a chemical solvent other than water. The solvent used is typically tetrachloroethylene , abbreviated "perc" in the industry and "dry-cleaning fluid" by the public...

 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 articles, e.g. diaper
Diaper
A nappy or a diaper is a kind of pant that allows one to defecate or urinate on oneself discreetly. When diapers become soiled, they require changing; this process is often performed by a second person such as a parent or caregiver...

s and hygienic products, necessitate development of biodegradable HMAs. Research is being performed on e.g. lactic acid
Lactic acid
Lactic acid, also known as milk acid, is a chemical compound that plays a role in various biochemical processes and was first isolated in 1780 by the Swedish chemist Carl Wilhelm Scheele. Lactic acid is a carboxylic acid with the chemical formula C3H6O3...

 polyesters, polycaprolactone
Polycaprolactone
Polycaprolactone is a biodegradable polyester with a low melting point of around 60°C and a glass transition temperature of about −60°C. PCL is prepared by ring opening polymerization of ε-caprolactone using a catalyst such as stannous octoate. Recently a wide range of catalysts for the ring...

 with soy protein
Soy protein
Soy protein is a protein that is isolated from soybean. It is made from dehulled, defatted soybean meal. Dehulled and defatted soybeans are processed into three kinds of high protein commercial products : soy flour, concentrates, and isolates. Soy protein isolate has been used since 1959 in foods ...

, etc.

Some of the possible base materials are:
  • Ethylene-vinyl acetate
    Ethylene-vinyl acetate
    Ethylene vinyl acetate is the copolymer of ethylene and vinyl acetate. The weight percent vinyl acetate usually varies from 10 to 40%, with the remainder being ethylene....

     (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 30-50 °C but are limited below 60-80°C and have low creep
    Creep (deformation)
    In materials science, creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses. It occurs as a result of long term exposure to high levels of stress that are below the yield strength of the material....

     resistance under load. 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% of stabilizers. Fillers
    Filler (materials)
    Fillers are particles added to material to lower the consumption of more expensive binder material or to better some properties of the mixtured material...

     can be added for special applications. Can be formulated for service temperatures ranging from -40°C to +80°C, 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
    Pressure sensitive adhesive
    Pressure sensitive adhesive is adhesive which forms a bond when pressure is applied to marry the adhesive with the adherend...

     (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, e.g. polyethylene, while increased content of vinyl acetate promotes adhesion to polar substrates, e.g. 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. 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. EVAs can be compounded with aromatic hydrocarbon resins. Grafting butadiene to EVA improves its adhesion. Its dielectric properties are poor due to high content of polar groups, the dielectric loss
    Loss tangent
    The loss tangent is a parameter of a dielectric material that quantifies its inherent dissipation of electromagnetic energy. The term refers to the tangent of the angle in a complex plane between the resistive component of an electromagnetic field and its reactive component.-Electromagnetic...

     is moderately high. Polypropylene HMAs are a better choice for high-frequency electronics. EVAs are optically clearer and more gas and vapor permeable than polyolefins. Nearly half of EVA HMAs is used in packaging applications. Cryogenic grinding
    Cryogenic grinding
    Cryogenic grinding, also known as freezer milling, freezer grinding, and cryomilling, is the act of cooling or chilling a material and then reducing it into a small particle size. For example, thermoplastics are difficult to grind to small particle sizes at ambient temperatures because they soften,...

     of EVAs can provide small, water-dispersible particles for heat-seal applications. EVA can degrade primarily by loss of acetic acid
    Acetic acid
    Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...

     and formation of a double bond in the chain, and by oxidative degradation. EVA can be compounded into a wide range of HMAs, from soft pressure-sensitive adhesives to rigid structural adhesives for furniture construction.
    • Ethylene
      Ethylene
      Ethylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...

      -acrylate
      Acrylate
      The acrylate ion is the ion of acrylic acid.Acrylates are the salts and esters of acrylic acid. They are also known as propenoates ....

       copolymers have lower glass transition temperature and higher adhesion even to difficult substrates than EVA. Better thermal resistance, increased adhesion to metals and glass. Suitable for low temperature use. Ethylene-vinylacetate-maleic anhydride
      Maleic anhydride
      Maleic anhydride is an organic compound with the formula C2H22O. It is the acid anhydride of maleic acid and in its pure state it is a colourless or white solid with an acrid odour....

       and ethylene-acrylate-maleic anhydride terpolymers offer very high performance. Examples are ethylene n-butyl acrylate (EnBA), ethylene-acrylic acid (EAA) and ethylene-ethyl acetate (EEA).
  • Polyolefin
    Polyolefin
    A polyolefin is a polymer produced from a simple olefin as a monomer. For example, polyethylene is the polyolefin produced by polymerizing the olefin ethylene. An equivalent term is polyalkene; this is a more modern term, although polyolefin is still used in the petrochemical industry...

    s (PO) (polyethylene
    Polyethylene
    Polyethylene or polythene is the most widely used plastic, with an annual production of approximately 80 million metric tons...

     (usually LDPE but also HDPE; HDPE has higher melting point and better temperature resistance), atactic polypropylene
    Polypropylene
    Polypropylene , also known as polypropene, is a thermoplastic polymer used in a wide variety of applications including packaging, textiles , stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers, automotive components, and polymer banknotes...

     (PP or APP), polybutene-1, oxidized polyethylene, etc.), low-performance, for difficult-to-bond plastics. Very good adhesion to polypropylene, good moisture barrier, chemical resistance against polar solvents and solutions of acids, bases, and alcohols. Longer open time in comparison with EVA and polyamides. Polyolefins have low surface energy
    Surface energy
    Surface energy quantifies the disruption of intermolecular bonds that occur when a surface is created. In the physics of solids, surfaces must be intrinsically less energetically favorable than the bulk of a material, otherwise there would be a driving force for surfaces to be created, removing...

     and provide good wetting of most metals and polymers. Polyolefins made by metallocene
    Metallocene
    A metallocene is a compound typically consisting of two cyclopentadienyl anions bound to a metal center in the oxidation state II, with the resulting general formula 2M. Closely related to the metallocenes are the metallocene derivatives, e.g. titanocene dichloride, vanadocene dichloride...

     catalyzed synthesis have narrow distribution of molecular weight and correspondingly narrow melting temperature range. Due to the relatively high crystallinity, polyethylene-based glues tend to be opaque and, depending on additives, white or yellowish. Polyethylene hot melts have high pot life stability, are not prone to charring, and are suitable for moderate temperature ranges and on porous non-flexible substrates. Nitrogen or carbon dioxide can be introduced into the melt, forming a foam
    Foam
    -Definition:A foam is a substance that is formed by trapping gas in a liquid or solid in a divided form, i.e. by forming gas regions inside liquid regions, leading to different kinds of dispersed media...

     which increases spreading and open time and decreases transfer of heat to the substrate, allowing use of more heat-sensitive substrates; polyethylene-based HMAs are usually used. Foamable HMAs are available on the market since 1981. Amorphous polypropylene HMAs have good dielectric properties, making them suitable for use at high frequencies. PE and APP are usually used on their own or with just a small amount of tackifiers (usually hydrocarbons) and waxes (usually paraffins or microcrystalline waxes, for lower cost, improved anti-blocking, and altered open time and softening temperature). The molecular weight of the polymer is usually lower. Lower molecular weights provide better low-temperature performance and higher flexibility, higher molecular weights increase the seal strength, hot tack, and melt viscosity.
    • Polybutene-1 and its copolymers are soft and flexible, tough, partially crystalline, and slowly crystallizing with long open times. The low temperature of recrystallization allows for stress release during formation of the bond. Good bonding to nonpolar surfaces, worse bonding to polar ones. Good for rubber
      Rubber
      Natural rubber, also called India rubber or caoutchouc, is an elastomer that was originally derived from latex, a milky colloid produced by some plants. The plants would be ‘tapped’, that is, an incision made into the bark of the tree and the sticky, milk colored latex sap collected and refined...

       substrates. Can be formulated as pressure-sensitive.
    • Amorphous polyolefin (APO/APAO
      APAO
      Amorphous Poly-alpha-olefins are produced by polymerisation of α-olefins, e.g. propylene or 1-butene with Ziegler-Natta catalysts. The polymers have an amorphous structure which makes them useful for the production of hot melt adhesives....

      ) polymers are compatible with many solvents, tackifiers, waxes, and polymers; they find wide use in many adhesive applications. APO hot melts have good fuel and acid resistance, moderate heat resistance, are tacky, soft and flexible, have good adhesion and longer open times than crystalline polyolefins. APOs tend to have lower melt viscosity, better adhesion, longer open times and slow set times than comparable EVAs. Some APOs can be used alone, but often they are compounded with tackifiers, waxes, and plasticizers (e.g. mineral oil
      Mineral oil
      A mineral oil is any of various colorless, odorless, light mixtures of alkanes in the C15 to C40 range from a non-vegetable source, particularly a distillate of petroleum....

      , poly-butene oil). Examples of APOs are e.g. amorphous (atactic) propylene (APP), amorphous propylene/ethylene (APE), amorphous propylene/butene (APB), amorphous propylene/hexene (APH), amorphous propylene/ethylene/butene. APP is harder than APE, which is harder than APB, which is harder than APH, in accordance with decreasing crystallinity. APOs show relatively low cohesion
      Cohesion
      Cohesion may refer to:* Cohesion , the intermolecular attraction between like-molecules* Cohesion , a measure of how well the lines of source code within a module work together...

      , the entangled polymer chains have fairly high degree of freedom of movement. Under mechanical load, most of the strain is dissipated by elongation and disentanglement of polymer chains, and only a small fraction reaches the adhesive-substrate interface. Cohesive failure is therefore a more common failure mode of APOs.
  • Polyamides and polyesters, high-performance
    • Polyamide
      Polyamide
      A polyamide is a polymer containing monomers of amides joined by peptide bonds. They can occur both naturally and artificially, examples being proteins, such as wool and silk, and can be made artificially through step-growth polymerization or solid-phase synthesis, examples being nylons, aramids,...

      s (PA), high-performance, for severe environments; high-temperature glues; typically applied at over 200 °C, but can degrade and char during processing. In molten state can somewhat degrade by atmospheric oxygen. High application temperature. High range of service temperatures, generally showing adequate bonding from -40 to 70°C; some compositions allow operation to 185°C if they do not have to carry load. Resistant to plasticizer
      Plasticizer
      Plasticizers or dispersants are additives that increase the plasticity or fluidity of the material to which they are added; these include plastics, cement, concrete, wallboard, and clay. Although the same compounds are often used for both plastics and concretes the desired effects and results are...

      s, therefore suitable for gluing polyvinyl chloride
      Polyvinyl chloride
      Polyvinyl chloride, commonly abbreviated PVC, is a thermoplastic polymer. It is a vinyl polymer constructed of repeating vinyl groups having one hydrogen replaced by chloride. Polyvinyl chloride is the third most widely produced plastic, after polyethylene and polypropylene. PVC is widely used in...

      ; only polyamides derived from secondary diamines however provide a satisfying bond. Resistant to oils and gasoline. Good adhesion to many substrates, e.g. metal, wood, vinyl, ABS, and treated polyethylene and polypropylene. Some formulations are UL-approved for electrical applications requiring reduced flammability. Three groups are employed, with low, intermediate, and high molecular weight; the low MW ones are low-temperature melting and easy to apply, but have lower tensile strength, lower tensile-shear strength, and lower elongation than the high-MW ones. The high-MW ones require sophisticated extruders and are used as high-performance structural adhesives. The presence of hydrogen bond
      Hydrogen bond
      A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond...

      s between the polymer chains gives polyamides a high strength at even low molecular weights, in comparison with other polymers. Hydrogen bonds also provide retention of most of the adhesive strength up almost to the melting point; however they also make the material more susceptible to permeation of moisture in comparison with polyesters. Can be formulated as soft and tacky or as hard and rigid. Niche applications, together with polyesters taking less than 10% of total volume of hot melt adhesives market. Absorption of moisture may lead to foaming during application as water evaporates during melting, leaving voids in the adhesive layer which degrade mechanical strength. Polyamide HMAs are usually composed of a dimer acid
      Dimer acid
      Dimer acids, or dimerized fatty acids, are dicarboxylic acids prepared by dimerizing unsaturated fatty acids obtained from tall oil, usually on clay catalysts. The CAS number of the material is [61788-89-4]. Dimer acids are used primarily for synthesis of polyamide resins and polyamide hot melt...

       with often two or more different diamines. The dimer acid usually presents 60-80% of the total polyamide mass, and provides amorphous nonpolar character. Linear aliphatic amines, e.g. ethylene diamine
      Ethylene diamine
      Ethylenediamine is the organic compound with the formula C2H42. This colorless liquid with an ammonia-like odor is a strongly basic amine. The liquid fumes upon contact with humid air...

       and hexamethylene diamine, provide hardness and strength. Longer chain amines, e.g. dimer amine, reduce the amount of hydrogen bonds per volume of material, resulting in lower stiffness. Polyether diamines provide good low-temperature flexibility. Piperazine
      Piperazine
      Piperazine is an organic compound that consists of a six-membered ring containing two opposing nitrogen atoms. Piperazine exists as small alkaline deliquescent crystals with a saline taste....

       and similar diamines also reduce the number of hydrogen bonds. Only polyamides based on piperazine and similar secondary amines form satisfactory bond with polyvinyl chloride
      Polyvinyl chloride
      Polyvinyl chloride, commonly abbreviated PVC, is a thermoplastic polymer. It is a vinyl polymer constructed of repeating vinyl groups having one hydrogen replaced by chloride. Polyvinyl chloride is the third most widely produced plastic, after polyethylene and polypropylene. PVC is widely used in...

      ; primary amines form stronger hydrogen bonds within the adhesive, secondary amines can act only as proton acceptors, don't form hydrogen bonds within the polyamide, and are therefore free to form weaker bonds with vinyl, probably with the hydrogen atom adjacent to the chlorine.
    • Polyester
      Polyester
      Polyester is a category of polymers which contain the ester functional group in their main chain. Although there are many polyesters, the term "polyester" as a specific material most commonly refers to polyethylene terephthalate...

      s, similar to the ones used for synthetic fiber
      Synthetic fiber
      Synthetic fibers are the result of extensive research by scientists to improve on naturally occurring animal and plant fibers. In general, synthetic fibers are created by forcing, usually through extrusion, fiber forming materials through holes into the air, forming a thread...

      s. High application temperature. Synthetized from a diol
      Diol
      A diol or glycol is a chemical compound containing two hydroxyl groups A geminal diol has two hydroxyl groups bonded to the same atom...

       and a dicarboxylic acid
      Dicarboxylic acid
      Dicarboxylic acids are organic compounds that contain two carboxylic acid functional groups. In molecular formulae for dicarboxylic acids, these groups are often written as HOOC-R-COOH, where R may be an alkyl, alkenyl, alkynyl, or aryl group...

      . The length of the diol chain has major influence to the material's properties; with increasing diol chain length the melting point increases, the crystallization rate increases, and the degree of crystallization decreases. Both the diol and acid influence the melting point. In comparison with similar polyamides, due to absence of hydrogen bonds, polyesters have lower strength and melting point, but are much more resistant to moisture, though still susceptible. In other parameters, and in applications where these factors do not play a role, polyesters and polyamides are very similar. Polyesters are often used for bonding fabrics. They can be used on their own, or blended with large amounts of additives. They are used where high tensile strength and high temperature resistance are needed. Most polyester hot melt adhesives have high degree of crystallinity. Niche applications, together with polyamides taking less than 10% of total volume of hot melt adhesives market. Water-dispersible amorphous polymers, modified by addition of sodium sulfonate
      Sulfonate
      A sulfonate is a salt or ester of a sulfonic acid. It contains the functional group R-SO2O-.- Sulfonate salts:Anions with the general formula RSO2O− are called sulfonates. They are the conjugate bases of sulfonic acids with formula RSO2OH. As sulfonic acids tend to be strong acids, the...

       groups for dispersability, were however developed for repulpable adhesives. Polyesters are often highly crystalline, leading to narrow melting temperature range, which is advantageous for high-speed bonding.
  • Polyurethanes
    • Thermoplastic polyurethane
      Polyurethane
      A polyurethane is any polymer composed of a chain of organic units joined by carbamate links. Polyurethane polymers are formed through step-growth polymerization, by reacting a monomer with another monomer in the presence of a catalyst.Polyurethanes are...

       (TPU) offer good adhesion to different surfaces due to presence of polar
      Chemical polarity
      In chemistry, polarity refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment. Polar molecules interact through dipole–dipole intermolecular forces and hydrogen bonds. Molecular polarity is dependent on the difference in...

       groups. Their low glass transition temperature provides flexibility at low temperatures. They are highly elastic and soft, with wide possible crystallization and melting point ranges. Polyurethanes consist of long linear chains with flexible, soft segments (diisocyanate-coupled low-melting polyester
      Polyester
      Polyester is a category of polymers which contain the ester functional group in their main chain. Although there are many polyesters, the term "polyester" as a specific material most commonly refers to polyethylene terephthalate...

       or polyether chains) alternating with rigid segments (diurethane bridges resulting from diisocyanate reacting with a small-molecule glycol chain extender). The rigid segments form hydrogen bonds with rigid segments of other molecules. Higher ratio of soft to hard segments provides better flexibility, elongation, and low-temperature performance, but also lower hardness, modulus, and abrasion resistance. The bonding temperature is lower than with most other HMAs, only about 50-70 °C, when the adhesive behaves as a soft rubber acting as a pressure-sensitive adhesive. The surface wetting in this amorphous state is good, and on cooling the polymer crystallizes, forming a strong flexible bond with high cohesion. Choice of a proper diisocyanate and polyol
      Polyol
      A polyol is an alcohol containing multiple hydroxyl groups. In two technological disciplines the term "polyol" has a special meaning: food science and polymer chemistry.- Polyols in food science :...

       combination allows tailoring the polyuretane properties; they can be used on their own or blended with a plasticizer. Polyurethanes are compatible with most common plasticizers, and many resins.
    • Polyurethanes (PUR), or reactive urethanes, for high temperatures and high flexibility. New type of hot melt thermosetting adhesives, introduced in early 1990s. Solidification can be rapid or extended in range of several minutes; secondary curing with atmospheric or substrate moisture then continues for several hours, forming cross-link
      Cross-link
      Cross-links are bonds that link one polymer chain to another. They can be covalent bonds or ionic bonds. "Polymer chains" can refer to synthetic polymers or natural polymers . When the term "cross-linking" is used in the synthetic polymer science field, it usually refers to the use of...

      s in the polymer. Excellent resistance to solvents and chemicals. Low application temperature, suitable for heat-sensitive substrates. Heat-resistant after curing, with service temperatures generally from -30°C to +150°C. Ink-solvent resistant. Often used in bookbinding
      Bookbinding
      Bookbinding is the process of physically assembling a book from a number of folded or unfolded sheets of paper or other material. It usually involves attaching covers to the resulting text-block.-Origins of the book:...

      , automotive, aerospace, filter and plastic bag applications. Susceptible to UV degradation
      UV degradation
      Many natural and synthetic polymers are attacked by ultra-violet radiation and products made using these materials may crack or disintegrate . The problem is known as UV degradation, and is a common problem in products exposed to sunlight...

       causing discoloring and degradation of mechanical properties, requires blending with UV stabilizers and antioxidants. Usually based on prepolymers made of polyol
      Polyol
      A polyol is an alcohol containing multiple hydroxyl groups. In two technological disciplines the term "polyol" has a special meaning: food science and polymer chemistry.- Polyols in food science :...

      s and methylene diphenyl diisocyanate
      Methylene diphenyl diisocyanate
      Methylene diphenyl diisocyanate, most often abbreviated as MDI, is an aromatic diisocyanate. It exists in three isomers, 2,2'-MDI, 2,4'-MDI, and 4,4'-MDI, but the 4,4' isomer is most widely used. This isomer is also known as Pure MDI. MDI reacts with polyols in the manufacture of polyurethane...

       (MDI) or other diisocyanate, with small amount of free isocyanate groups; these groups when subjected to moisture react and cross-link. The uncured solidified "green" strength tends to be low than non-reactive HMAs, mechanical strength develops with curing. Green strength can be improved by blending the prepolymer with other polymers.
  • Styrene block copolymers (SBC), also called styrene copolymer adhesives and rubber-based adhesives, have good low-temperature flexibility, high elongation, and high heat resistance. Frequently used in pressure sensitive adhesive
    Pressure sensitive adhesive
    Pressure sensitive adhesive is adhesive which forms a bond when pressure is applied to marry the adhesive with the adherend...

     applications, where the composition retains tack even when solidified; however non-pressure-sensitive formulations are also used. High heat resistance, good low-temperature flexibility. Lower strength than polyesters. They usually have A-B-A structure, with an elastic rubber segment between two rigid plastic endblocks. High-strength film formers as standalone, increase cohesion and viscosity as an additive. Water-resistant, soluble in some organic solvents; cross-linking improves solvent resistance. Resins associating with endblocks (cumarone-indene, α-methyl styrene, vinyl toluene, aromatic hydrocarbons, etc.) improve adhesion and alter viscosity. Resins associating to the midblocks (aliphatic olefins, rosin esters, polyterpenes, terpene phenolics) improve adhesion, processing and pressure-sensitive properties. Addition of plasticizers reduces cost, improves pressure-sensitive tack, decrease melt viscosity, decrease hardness, and improve low-temperature flexibility. The A-B-A structure promotes a phase separation of the polymer, binding together the endblocks, with the central elastic parts acting as cross-links; SBCs do not require additional cross-linking.
    • Styrene-butadiene-styrene (SBS), used in high-strength PSA applications
    • Styrene-isoprene
      Isoprene
      Isoprene , or 2-methyl-1,3-butadiene, is a common organic compound with the formula CH2=CCH=CH2. Under standard conditions it is a colorless liquid...

      -styrene (SIS), used in low-viscosity high-tack PSA applications
    • Styrene-ethylene/butylene
      Butylene
      In chemistry, butylene may be an alternate name for the hydrocarbon butene, . It is also a divalent functional group with formula 2• that can be seen as the result of removing two hydrogen atoms from a butane molecule, leaving two free bonds....

      -styrene (SEBS), used in low self-adhering non-woven applications
    • Styrene-ethylene/propylene (SEP)
  • Polycaprolactone
    Polycaprolactone
    Polycaprolactone is a biodegradable polyester with a low melting point of around 60°C and a glass transition temperature of about −60°C. PCL is prepared by ring opening polymerization of ε-caprolactone using a catalyst such as stannous octoate. Recently a wide range of catalysts for the ring...

     with soy protein
    Soy protein
    Soy protein is a protein that is isolated from soybean. It is made from dehulled, defatted soybean meal. Dehulled and defatted soybeans are processed into three kinds of high protein commercial products : soy flour, concentrates, and isolates. Soy protein isolate has been used since 1959 in foods ...

    , using coconut oil
    Coconut oil
    Coconut oil is an edible oil extracted from the kernel or meat of matured coconuts harvested from the coconut palm . Throughout the tropical world, it has provided the primary source of fat in the diets of millions of people for generations. It has various applications in food, medicine, and industry...

     as plasticizer, a biodegradable hot-melt adhesive investigated at Korea University
    Korea University
    Korea University is a prestigious nonsectarian, private research university located primarily in Seoul, South Korea, and one of the SKY universities, a historical acronym used in South Korea to refer to Seoul National University, Korea University, and Yonsei University. Founded by Lee Yong-ik in...

  • Polycarbonate
    Polycarbonate
    PolycarbonatePhysical PropertiesDensity 1.20–1.22 g/cm3Abbe number 34.0Refractive index 1.584–1.586FlammabilityV0-V2Limiting oxygen index25–27%Water absorption – Equilibrium0.16–0.35%Water absorption – over 24 hours0.1%...

    s
  • Fluoropolymer
    Fluoropolymer
    A fluoropolymer is a fluorocarbon based polymer with multiple strong carbon–fluorine bonds. It is characterized by a high resistance to solvents, acids, and bases.-History:Fluoropolymers were accidentally discovered in 1938 by Dr. Roy J...

    s, with tackifiers and ethylene copolymer with polar groups
  • Silicone rubber
    Silicone rubber
    Silicone rubber is an elastomer composed of silicone—itself a polymer—containing silicon together with carbon, hydrogen, and oxygen. Silicone rubbers are widely used in industry, and there are multiple formulations...

    s, undergo cross-linking after solidification, form durable flexible UV and weather resistant silicone sealant
  • Thermoplastic elastomer
    Elastomer
    An elastomer is a polymer with the property of viscoelasticity , generally having notably low Young's modulus and high yield strain compared with other materials. The term, which is derived from elastic polymer, is often used interchangeably with the term rubber, although the latter is preferred...

    s
  • Polypyrrole
    Polypyrrole
    Polypyrrole is a chemical compound formed from a number of connected pyrrole ring structures. For example a tetrapyrrole is a compound with four pyrrole rings connected. Methine-bridged cyclic tetrapyrroles are called porphyrins. Polypyrroles are conducting polymers of the rigid-rod polymer host...

     (PPY), a conductive polymer
    Conductive polymer
    Conductive polymers or, more precisely, intrinsically conducting polymers are organic polymers that conduct electricity. Such compounds may have metallic conductivity or can be semiconductors. The biggest advantage of conductive polymers is their processability, mainly by dispersion. Conductive...

    , for intrinsically conducting hot melt adhesives (ICHMAs), used for EMI
    Electromagnetic interference
    Electromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...

     shielding
    Electromagnetic shielding
    Electromagnetic shielding is the process of reducing the electromagnetic field in a space by blocking the field with barriers made of conductive and/or magnetic materials. Shielding is typically applied to enclosures to isolate electrical devices from the 'outside world' and to cables to isolate...

    . EVA compounded with 0.1-0.5 wt.% PPY are strongly absorbing in near infrared, allowing use as near-infrared activated adhesives.
  • various other copolymers


The usual additives are:
  • tackifying
    Tackifier
    Tackifiers are chemical compounds used in formulating adhesives to increase the tack, the stickiness of the surface of the adhesive. They are usually low-molecular weight compounds with high glass transition temperature...

     resin
    Resin
    Resin in the most specific use of the term is a hydrocarbon secretion of many plants, particularly coniferous trees. Resins are valued for their chemical properties and associated uses, such as the production of varnishes, adhesives, and food glazing agents; as an important source of raw materials...

    s (e.g. rosin
    Rosin
    .Rosin, also called colophony or Greek pitch , is a solid form of resin obtained from pines and some other plants, mostly conifers, produced by heating fresh liquid resin to vaporize the volatile liquid terpene components. It is semi-transparent and varies in color from yellow to black...

    s and their derivates, terpene
    Terpene
    Terpenes are a large and diverse class of organic compounds, produced by a variety of plants, particularly conifers, though also by some insects such as termites or swallowtail butterflies, which emit terpenes from their osmeterium. They are often strong smelling and thus may have had a protective...

    s and modified terpenes, aliphatic, cycloaliphatic and aromatic resins (C5 aliphatic resins, C9 aromatic resins, and C5/C9 aliphatic/aromatic resins), hydrogenated hydrocarbon resins, and their mixtures, terpene-phenol resins (TPR, used often with EVAs)), up to about 40% Tackifiers tend to have low molecular weight, and glass transition and softening temperature above room temperature, providing them with suitable viscoelastic properties. Tackifiers frequently present most of both weight percentage and cost of the hot-melt adhesive.
  • wax
    Wax
    thumb|right|[[Cetyl palmitate]], a typical wax ester.Wax refers to a class of chemical compounds that are plastic near ambient temperatures. Characteristically, they melt above 45 °C to give a low viscosity liquid. Waxes are insoluble in water but soluble in organic, nonpolar solvents...

    es, e.g. microcrystalline wax
    Microcrystalline wax
    Microcrystalline waxes are a type of wax produced by de-oiling petrolatum, as part of the petroleum refining process. In contrast to the more familiar paraffin wax which contains mostly unbranched alkanes, microcrystalline wax contains a higher percentage of isoparaffinic hydrocarbons and...

    es, fatty amide waxes or oxidized Fischer-Tropsch waxes; increase the setting rate. One of the key components of formulations. Waxes lower the melt viscosity and can improve bond strength and temperature resistance.
  • plasticizer
    Plasticizer
    Plasticizers or dispersants are additives that increase the plasticity or fluidity of the material to which they are added; these include plastics, cement, concrete, wallboard, and clay. Although the same compounds are often used for both plastics and concretes the desired effects and results are...

    s (e.g. benzoates, e.g. 1,4-cyclohexane dimethanol dibenzoate, glyceryl tribenzoate, or pentaerythritol tetrabenzoate, phthalates, paraffin oils, polyisobutylene, chlorinated paraffins, etc.)
  • antioxidant
    Antioxidant
    An antioxidant is a molecule capable of inhibiting the oxidation of other molecules. Oxidation is a chemical reaction that transfers electrons or hydrogen from a substance to an oxidizing agent. Oxidation reactions can produce free radicals. In turn, these radicals can start chain reactions. When...

    s and stabilizers (e.g. hindered phenols, BHT
    Butylated hydroxytoluene
    Butylated hydroxytoluene , also known as butylhydroxytoluene, is a lipophilic organic compound that is primarily used as an antioxidant food additive as well as an antioxidant additive in cosmetics, pharmaceuticals, jet fuels, rubber, petroleum products, electrical transformer oil, and embalming...

    , phosphite
    Phosphite
    A phosphite is a salt of phosphorous acid. The phosphite ion is a polyatomic ion with a phosphorus central atom where phosphorus has an oxidation state of +3...

    s, phosphates, hindered aromatic amines); added in small amounts (<1%), not influencing physical properties. These compounds protect the material from degradation both during service life, compounding and in molten state during application. Stabilizers based on functionalized silicones have improved resistance to extraction and outgassing.
  • UV stabilizers
    UV Stabilizers in plastics
    UV light stabilizers are used frequently in plastics, including cosmetics and films. The primary function is to protect the substance from the long-term degradation effects from light, most frequently ultraviolet light...

    , protecting the material against degradation by ultraviolet radiation
  • pigment
    Pigment
    A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...

    s and dye
    Dye
    A dye is a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and requires a mordant to improve the fastness of the dye on the fiber....

    s, glitter
    Glitter
    Glitter describes an assortment of very small pieces of copolymer plastics, aluminum foil, titanium dioxide, iron oxides, bismuth oxychloride or other materials painted in metallic, neon and iridescent colors to reflect light in a sparkling spectrum...

  • biocide
    Biocide
    A biocide is a chemical substance or microorganism which can deter, render harmless, or exert a controlling effect on any harmful organism by chemical or biological means. Biocides are commonly used in medicine, agriculture, forestry, and industry...

    s where hindering bacterial growth is desired
  • flame retardant
    Flame retardant
    Flame retardants are chemicals used in thermoplastics, thermosets, textiles and coatings that inhibit or resist the spread of fire. These can be separated into several different classes of chemicals:...

    s
  • antistatic agent
    Antistatic agent
    An antistatic agent is a compound used for treatment of materials or their surfaces in order to reduce or eliminate buildup of static electricity generally caused by the triboelectric effect. Its role is to make the surface or the material itself slightly conductive, either by being conductive...

    s
  • fillers
    Filler (materials)
    Fillers are particles added to material to lower the consumption of more expensive binder material or to better some properties of the mixtured material...

    , for reducing cost, adding bulk, improving cohesive strength (forming an aggregate-matrix composite material
    Composite material
    Composite materials, often shortened to composites or called composition materials, are engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct at the macroscopic or...

    ) and altering properties; e.g. calcium carbonate
    Calcium carbonate
    Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...

    , barium sulfate
    Barium sulfate
    Barium sulfate is the inorganic compound with the chemical formula BaSO4. It is a white crystalline solid that is odorless and insoluble in water. It occurs as the mineral barite, which is the main commercial source of barium and materials prepared from it...

    , talc
    Talc
    Talc is a mineral composed of hydrated magnesium silicate with the chemical formula H2Mg34 or Mg3Si4O102. In loose form, it is the widely-used substance known as talcum powder. It occurs as foliated to fibrous masses, its crystals being so rare as to be almost unknown...

    , silica, carbon black
    Carbon black
    Carbon black is a material produced by the incomplete combustion of heavy petroleum products such as FCC tar, coal tar, ethylene cracking tar, and a small amount from vegetable oil. Carbon black is a form of amorphous carbon that has a high surface-area-to-volume ratio, although its...

    , clay
    Clay
    Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...

    s (e.g. kaolin).


Fugitive glue
Fugitive glue
Fugitive glue, also called credit card glue, E-z-release glue, or booger glue, is a low-tack adhesive that produces a removable, non-permanent joint...

s and pressure sensitive adhesive
Pressure sensitive adhesive
Pressure sensitive adhesive is adhesive which forms a bond when pressure is applied to marry the adhesive with the adherend...

s can also be available in hot-melt form.

Additives and polymers containing unsaturated bonds are highly prone to autoxidation
Autoxidation
Autoxidation is any oxidation that occurs in open air or in presence of oxygen and/or UV radiation and forms peroxides and hydroperoxides. A classic example of autoxidation is that of simple ethers like diethyl ether, whose peroxides can be dangerously explosive. It can be considered to be a slow,...

. Examples include rosin-based additives. antioxidants can be used for suppressing this aging mechanism.

Addition of ferromagnetic particles, hygroscopic water-retaining materials, or other materials can yield a hot melt adhesive which can be activated by microwave heating.

Addition of electrically conductive
Electrical conductor
In physics and electrical engineering, a conductor is a material which contains movable electric charges. In metallic conductors such as copper or aluminum, the movable charged particles are electrons...

 particles can yield conductive hot-melt formulations.

Glue gun specifications and usage

Glue guns come in low-temperature and high-temperature (hot-melt) versions. Low-temperature glue guns operate at approximately 250 °F (121.1 °C) and are well suited when high temperatures are undesirable, such as gluing lace
Lace
Lace is an openwork fabric, patterned with open holes in the work, made by machine or by hand. The holes can be formed via removal of threads or cloth from a previously woven fabric, but more often open spaces are created as part of the lace fabric. Lace-making is an ancient craft. True lace was...

 and cloth. High-temperature guns operate at approximately 380 °F (193.3 °C) and produce a stronger bond. Dual guns have a switch for both low- and high-temperature use.

In addition to bonding surfaces together, hot-melt glue can be used to fill gaps, but the properties that allow gap-filling (high viscosity, high toughness, and so on) keep it from forming an adhesive film as thin and smooth as is possible with other adhesives. (For example, a wood joint properly made with hide glue may be invisible, marked only by a difference in grain at the seam line.) Bonds must be made quickly before the glue has time to harden. Usually it must be applied accurately with the glue gun, as it can not easily be spread, but it is always possible at any time to melt and spread the glue with a heat gun
Heat gun
A heat gun is a device used to emit a stream of hot air. They are superficially similar in shape and construction to a hair dryer, though they run at much higher temperatures...

 or a household clothes iron
Iron (appliance)
A clothes iron, also referred to as simply an iron, is a small appliance used in ironing to remove wrinkles from fabric.Ironing works by loosening the ties between the long chains of molecules that exist in polymer fiber materials. With the heat and the weight of the ironing plate, the fibers are...

, which helps when bonding larger areas.

Surprisingly, hot-melt glue can be used to assemble and repair foam models as an alternative to foam-safe Cyanoacrylate
Cyanoacrylate
Cyanoacrylate is the generic name for cyanoacrylate based fast-acting adhesives such as methyl 2-cyanoacrylate, ethyl-2-cyanoacrylate , and n-butyl cyanoacrylate...

 or UHU POR adhesive. Due to the insulating properties of the foam the hot-melt glue remains sticky for much longer than when used on wood, metal or plastics.

Another development of glue gun technology allows the user to 'spray' an area with a semi-pressure sensitive hot melt adhesive. Using compressed air to force the adhesive at a controlled rate through a specially developed glue gun nozzle, a spiral pattern of adhesive is applied. The tackiness of the adhesive / open time is limited to just a few minutes, after which the adhesive will lose its tack, so coverage is limited.

Applications

  • Hot melt adhesives are used to close corrugated fiberboard boxes and paperboard
    Paperboard
    Paperboard is a thick paper based material. While there is no rigid differentiation between paper and paperboard, paperboard is generally thicker than paper. According to ISO standards, paperboard is a paper with a basis weight above 224 g/m2, but there are exceptions. Paperboard can be single...

     carton
    Carton
    Carton is the name of certain types of containers typically made from paperboard which is also sometimes known as cardboard. Many types of cartons are used in packaging. Sometimes a carton is also called a box.-Folding cartons:...

    s.
  • Crafts in the home
  • Assembly of parts in manufacturing
  • Heat sometimes allows disassembly: Points of modern arrow
    Arrow
    An arrow is a shafted projectile that is shot with a bow. It predates recorded history and is common to most cultures.An arrow usually consists of a shaft with an arrowhead attached to the front end, with fletchings and a nock at the other.- History:...

    s, hockey sticks, etc.
  • Assembly and repair of foam model aircraft
    Model aircraft
    Model aircraft are flying or non-flying models of existing or imaginary aircraft using a variety of materials including plastic, diecast metal, polystyrene, balsa wood, foam and fibreglass...

    and toys.
  • Hot melt adhesive is used for disposable diaper construction where it is used to bond the nonwoven material with the backsheet and the elastics.
  • Hot glue is also used to affix parts or wires in electronic devices.
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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