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Hot-melt adhesive (HMA), also known as hot glue, is a form of
thermoplastic
A thermoplastic, or thermosoft plastic, is any plastic polymer material that becomes pliable or moldable at a certain elevated temperature and solidifies upon cooling.
Most thermoplastics have a high molecular weight. The polymer chains associate ...
adhesive
Adhesive, also known as glue, cement, mucilage, or paste, is any non-metallic substance applied to one or both surfaces of two separate items that binds them together and resists their separation.
The use of adhesives offers certain advant ...
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
A heating element converts electrical energy into heat through the process of Joule heating. Electric current through the element encounters resistance, resulting in heating of the element. Unlike the Peltier effect, this process is indepen ...
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
Resin casting is a method of plastic casting where a mold is filled with a liquid synthetic resin, which then hardens. It is primarily used for small-scale production like industrial prototypes and dentistry. It can be done by amateur hobbyists wit ...
.
In industrial use, hot-melt adhesives provide several advantages over solvent-based adhesives.
Volatile organic compound
Volatile organic compounds (VOCs) are organic compounds that have a high vapour pressure at room temperature. High vapor pressure correlates with a low boiling point, which relates to the number of the sample's molecules in the surrounding air, a ...
s 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 may not be resistant to chemical attacks and weathering. HMAs do not lose thickness during solidifying, whereas solvent-based adhesives may lose up to 50–70% of layer thickness during drying.
Hot melt specific-properties
; Melt
viscosity
The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water.
Viscosity quantifies the inte ...
: One 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 index
The Melt Flow Index (MFI) is a measure of the ease of flow of the melt of a thermoplastic polymer. It is defined as the mass of polymer, in grams, flowing in ten minutes through a capillary of a specific diameter and length by a pressure applied ...
: A 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 stability: The 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 temperature: Minimum 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
In surface science, surface free energy (also interfacial free energy or surface energy) quantifies the disruption of intermolecular bonds that occurs when a surface is created. In solid-state physics, surfaces must be intrinsically less energe ...
: Influences
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. This happens in presence of a gaseous phase or another liquid phase not miscible with th ...
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
The glass–liquid transition, or glass transition, is the gradual and reversible transition in amorphous materials (or in amorphous regions within semicrystalline materials) from a hard and relatively brittle "glassy" state into a viscous or rubb ...
(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 bond
In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a ...
s between their chains, forming pseudo-
cross-link
In chemistry and biology a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers can be either synthetic polymers or natural ...
s which strengthen the polymer.
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
Eva or EVA may refer to:
* Eva (name), a feminine given name
Arts, entertainment, and media Fictional characters
* Eva (Dynamite Entertainment), a comic book character by Dynamite Entertainment
* Eva (''Devil May Cry''), Dante's mother in t ...
is used as the main polymer, with terpene-phenol resin (TPR) as the tackifier. The two components display acid-base interactions between the
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. A compound containing a ...
groups of vinyl acetate and
hydroxyl
In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...
groups of TPR, complexes are formed between phenolic rings of TPR and
hydroxyl
In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...
groups on the surface of aluminium substrates, and interactions between carbonyl groups and
silanol
A silanol is a functional group in silicon chemistry with the connectivity Si–O–H. It is related to the hydroxy functional group (C–O–H) found in all alcohols. Silanols are often invoked as intermediates in organosilicon c ...
groups on surfaces of glass substrates are formed. Polar groups, hydroxyls and amine groups can form acid-base and
hydrogen bond
In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a ...
s with polar groups on substrates like paper or wood or natural fibers. Nonpolar polyolefin chains interact well with nonpolar substrates. Good
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. This happens in presence of a gaseous phase or another liquid phase not miscible with th ...
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
In surface science, surface free energy (also interfacial free energy or surface energy) quantifies the disruption of intermolecular bonds that occurs when a surface is created. In solid-state physics, surfaces must be intrinsically less energe ...
. 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
Autoxidation (sometimes auto-oxidation) refers to oxidations brought about by reactions with oxygen at normal temperatures, without the intervention of flame or electric spark. The term is usually used to describe the gradual degradation of organi ...
and
UV degradation
In polymer chemistry photo-oxidation (sometimes: oxidative photodegradation) is the degradation of a polymer surface due to the combined action of light and oxygen. It is the most significant factor in the weathering of plastics. Photo-oxidatio ...
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.
Increase of bond strength and service temperature can be achieved by formation of
cross-link
In chemistry and biology a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers can be either synthetic polymers or natural ...
s 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
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
radiation,
electron irradiation
Electron-beam processing or electron irradiation (EBI) is a process that involves using electrons, usually of high energy, to treat an object for a variety of purposes. This may take place under elevated temperatures and nitrogen atmosphere. Poss ...
, or by other methods.
Resistance to water and solvents is critical in some applications. For example, in textile industry, resistance to
dry cleaning
Dry cleaning is any cleaning process for clothing and textiles using a solvent other than water.
Dry cleaning still involves liquid, but clothes are instead soaked in a water-free liquid solvent. Tetrachloroethylene (perchloroethylene), known in ...
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
diaper
A diaper /ˈdaɪpə(r)/ (American and Canadian English) or a nappy ( Australian English, British English, and Hiberno-English) is a type of underwear that allows the wearer to urinate or defecate
Defecation (or defaecation) follows dig ...
s necessitate development of
biodegradable HMAs. Research is being performed on e.g.,
lactic acid
Lactic acid is an organic acid. It has a molecular formula . It is white in the solid state and it is miscible with water. When in the dissolved state, it forms a colorless solution. Production includes both artificial synthesis as well as natu ...
polyesters,
polycaprolactone
Polycaprolactone (PCL) is a biodegradable polyester with a low melting point of around 60 °C and a glass transition temperature of about −60 °C. The most common use of polycaprolactone is in the production of speciality polyure ...
with
soy protein
Soy protein is a protein that is isolated from soybean. It is made from soybean meal that has been dehulled and defatted. Dehulled and defatted soybeans are processed into three kinds of high protein commercial products: soy flour, concentrates, ...
,
[95-5 Development of biodegradable hot-melt adhesive based on poly-e-caprolactone and soy protein isolate for food packaging system](_blank)
Ift.confex.com. Archived fro
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 30 and 50 °C but are limited to use below 60–80 °C and have low
creep 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 0.5–1.0% of stabilizers.
Fillers
In processed animal foods, a filler is an ingredient added to provide dietary fiber, bulk or some other non-nutritive purpose.
Products like corncobs, feathers, soy, cottonseed hulls, peanut hulls, citrus pulp, screening, weeds, straw, and cere ...
can be added for special applications. Can be formulated for service temperatures ranging from −40 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
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...
, which can be further enhanced with suitable stabilizers. High vinylacetate content can serve for formulating a hot-melt
pressure-sensitive adhesive
Pressure-sensitive adhesive (PSA, self-adhesive, self-stick adhesive) is a type of nonreactive adhesive which forms a bond when pressure is applied to bond the adhesive with a surface. No solvent, water, or heat is needed to activate the adhesiv ...
(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. 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
1,3-Butadiene () is the organic compound with the formula (CH2=CH)2. It is a colorless gas that is easily condensed to a liquid. It is important industrially as a precursor to synthetic rubber. The molecule can be viewed as the union of two vi ...
to EVA improves its adhesion. Its dielectric properties are poor due to high content of polar groups, the
dielectric loss
Dielectric loss quantifies a dielectric material's inherent dissipation of electromagnetic energy (e.g. heat). It can be parameterized in terms of either the loss angle ''δ'' or the corresponding loss tangent tan ''δ''. Both refer to the ...
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, 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 parti ...
of EVAs can provide small, water-dispersible particles for heat-seal applications. EVA can degrade primarily by loss of
acetic acid
Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main component ...
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 (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon-carbon double bonds).
Ethylene i ...
-
acrylate
Acrylates (IUPAC: prop-2-enoates) are the salts, esters, and conjugate bases of acrylic acid. The acrylate ion is the anion C H2=CHC OO−. Often, acrylate refers to esters of acrylic acid, the most common member being methyl acrylate. These acr ...
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 is an organic compound with the formula C2H2(CO)2O. It is the acid anhydride of maleic acid. It is a colorless or white solid with an acrid odor. It is produced industrially on a large scale for applications in coatings and pol ...
and ethylene-acrylate-maleic anhydride
terpolymer
In polymer chemistry, a copolymer is a polymer derived from more than one species of monomer. The polymerization of monomers into copolymers is called copolymerization. Copolymers obtained from the copolymerization of two monomer species are some ...
s offer very high performance. Examples are ethylene ''n''-butyl acrylate (EnBA), ethylene-acrylic acid (EAA) and ethylene-ethyl acetate (EEA).
*
Polyolefin
A polyolefin is a type of polymer with the general formula (CH2CHR)n where R is an alkyl group. They are usually derived from a small set of simple olefins (alkenes). Dominant in a commercial sense are polyethylene and polypropylene. More speciali ...
s (PO) (
polyethylene
Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic. It is a polymer, primarily used for packaging ( plastic bags, plastic films, geomembranes and containers including bo ...
(usually
LDPE
Low-density polyethylene (LDPE) is a thermoplastic made from the monomer ethylene. It was the first grade of polyethylene, produced in 1933 by Imperial Chemical Industries (ICI) using a high pressure process via free radical polymerization. Its ...
but also
HDPE
High-density polyethylene (HDPE) or polyethylene high-density (PEHD) is a thermoplastic polymer produced from the monomer ethylene. It is sometimes called "alkathene" or "polythene" when used for HDPE pipes. With a high strength-to-density ratio, ...
, which has a higher melting point and better temperature resistance),
atactic
Tacticity (from el, τακτικός, taktikos, "relating to arrangement or order") is the relative stereochemistry of adjacent chiral centers within a macromolecule. The practical significance of tacticity rests on the effects on the physical p ...
polypropylene
Polypropylene (PP), also known as polypropene, is a thermoplastic polymer used in a wide variety of applications. It is produced via chain-growth polymerization from the monomer propylene.
Polypropylene
belongs to the group of polyolefins and ...
(PP or APP),
polybutene-1,
oxidized polyethylene, etc.), low-performance, for difficult-to-bond plastics. Very good adhesion to polypropylene, good
moisture barrier
A vapor barrier (or vapour barrier) is any material used for damp proofing, typically a plastic or foil sheet, that resists diffusion of moisture through the wall, floor, ceiling, or roof assemblies of buildings and of packaging to prevent inter ...
, chemical resistance against
polar solvent
A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for p ...
s and solutions of acids, bases, and alcohols. Longer open time in comparison with EVA and polyamides. Polyolefins have low
surface energy
In surface science, surface free energy (also interfacial free energy or surface energy) quantifies the disruption of intermolecular bonds that occurs when a surface is created. In solid-state physics, surfaces must be intrinsically less energe ...
and provide good wetting of most metals and polymers.
Metallocene
A metallocene is a compound typically consisting of two cyclopentadienyl anions (, abbreviated Cp) bound to a metal center (M) in the oxidation state II, with the resulting general formula Closely related to the metallocenes are the metallocene d ...
-catalyst-synthesised polyolefins have a 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
Foams are materials formed by trapping pockets of gas in a liquid or solid.
A bath sponge and the head on a glass of beer are examples of foams. In most foams, the volume of gas is large, with thin films of liquid or solid separating the reg ...
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, also called India rubber, latex, Amazonian rubber, ''caucho'', or ''caoutchouc'', as initially produced, consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds. Thailand, Malaysia, and ...
substrates. Can be formulated as pressure-sensitive.
**
Amorphous
In condensed matter physics and materials science, an amorphous solid (or non-crystalline solid, glassy solid) is a solid that lacks the long-range order that is characteristic of a crystal.
Etymology
The term comes from the Greek ''a'' ("wi ...
polyolefin (APO/
APAO
Amorphous poly alpha olefin (APAO; also known as atactic poly alpha olefin) is a commodity chemical used in multiple applications.
History
In the mid-to-late-1950s, atactic polypropylene (APP) was a by-product of the synthesis of isotactic poly ...
) 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 is any of various colorless, odorless, light mixtures of higher alkanes from a mineral source, particularly a distillate of petroleum, as distinct from usually edible vegetable oils.
The name 'mineral oil' by itself is imprecise, ...
, poly-butene oil). Examples of APOs include 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, 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
A polyamide is a polymer with repeating units linked by amide bonds.
Polyamides occur both naturally and artificially. Examples of naturally occurring polyamides are proteins, such as wool and silk. Artificially made polyamides can be made through ...
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
A plasticizer ( UK: plasticiser) is a substance that is added to a material to make it softer and more flexible, to increase its plasticity, to decrease its viscosity, and/or to decrease friction during its handling in manufacture.
Plasticiz ...
s, therefore suitable for gluing
polyvinyl chloride; only polyamides derived from secondary diamines however provide a satisfying bond.
[Specific adhesion model for bonding hot-melt polyamides to vinyl](_blank)
(PDF) . Retrieved on 2010-02-08. Resistant to oils and gasoline. Good adhesion to many substrates such as 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
In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a ...
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 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 such as
ethylene diamine
Ethylenediamine (abbreviated as en when a ligand) is the organic compound with the formula C2H4(NH2)2. This colorless liquid with an ammonia-like odor is a basic amine. It is a widely used building block in chemical synthesis, with approximately ...
and
hexamethylene diamine
Hexamethylenediamine is the organic compound with the formula H2N(CH2)6NH2. The molecule is a diamine, consisting of a hexamethylene hydrocarbon chain terminated with amine functional groups. The colorless solid (yellowish for some commercial sam ...
, provide hardness and strength. Longer chain amines such as dimer amine, reduce the amount of hydrogen bonds per volume of material, resulting in lower stiffness.
Polyether diamine
In organic chemistry, ethers are a class of compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula , where R and R′ represent the alkyl or aryl groups. Ethers can again be c ...
s provide good low-temperature flexibility.
Piperazine
Piperazine () is an organic compound that consists of a six-membered ring containing two nitrogen atoms at opposite positions in the ring. Piperazine exists as small alkaline deliquescent crystals with a saline taste.
The piperazines are a broad ...
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; 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 is a category of polymers that contain the ester functional group in every repeat unit of their main chain. As a specific material, it most commonly refers to a type called polyethylene terephthalate (PET). Polyesters include natural ...
s, similar to the ones used for
synthetic fiber
Synthetic fibers or synthetic fibres (in British English; see spelling differences) are fibers made by humans through chemical synthesis, as opposed to natural fibers that are directly derived from living organisms, such as plants (like cotton) ...
s. High application temperature. Synthetized from a
diol and a
dicarboxylic acid. 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 a 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
In organosulfur chemistry, a sulfonate is a salt or ester of a sulfonic acid. It contains the functional group , where R is an organic group. Sulfonates are the conjugate bases of sulfonic acids. Sulfonates are generally stable in water, non-o ...
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 (; often abbreviated PUR and PU) refers to a class of polymers composed of organic chemistry, organic units joined by carbamate (urethane) links. In contrast to other common polymers such as polyethylene and polystyrene, polyurethan ...
(TPU) offer good adhesion to different surfaces due to presence of
polar
Polar may refer to:
Geography
Polar may refer to:
* Geographical pole, either of two fixed points on the surface of a rotating body or planet, at 90 degrees from the equator, based on the axis around which a body rotates
* Polar climate, the c ...
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
In organic chemistry, isocyanate is the functional group with the formula . Organic compounds that contain an isocyanate group are referred to as isocyanates. An organic compound with two isocyanate groups is known as a diisocyanate. Diisocyan ...
-coupled low-melting
polyester
Polyester is a category of polymers that contain the ester functional group in every repeat unit of their main chain. As a specific material, it most commonly refers to a type called polyethylene terephthalate (PET). Polyesters include natural ...
or
polyether
In organic chemistry, ethers are a class of compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula , where R and R′ represent the alkyl or aryl groups. Ethers can again be c ...
chains) alternating with rigid segments (diurethane bridges resulting from diisocyanate reacting with a small-molecule
glycol
A diol is a chemical compound containing two hydroxyl groups ( groups). An aliphatic diol is also called a glycol. This pairing of functional groups is pervasive, and many subcategories have been identified.
The most common industrial diol is e ...
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
In organic chemistry, a polyol is an organic compound containing multiple hydroxyl groups (). The term "polyol" can have slightly different meanings depending on whether it is used in food science or polymer chemistry. Polyols containing two, thr ...
combination allows tailoring the polyurethane 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
In materials science, a thermosetting polymer, often called a thermoset, is a polymer that is obtained by irreversibly hardening (" curing") a soft solid or viscous liquid prepolymer (resin). Curing is induced by heat or suitable radiation and ...
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
In chemistry and biology a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers can be either synthetic polymers or natural ...
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 is the process of physically assembling a book of codex format from an ordered stack of ''signatures'', sheets of paper folded together into sections that are bound, along one edge, with a thick needle and strong thread. Cheaper, b ...
, automotive, aerospace, filter and plastic bag applications. Susceptible to
UV degradation
In polymer chemistry photo-oxidation (sometimes: oxidative photodegradation) is the degradation of a polymer surface due to the combined action of light and oxygen. It is the most significant factor in the weathering of plastics. Photo-oxidatio ...
causing discoloring and degradation of mechanical properties, requires blending with UV stabilizers and antioxidants. Usually based on prepolymers made of
polyol
In organic chemistry, a polyol is an organic compound containing multiple hydroxyl groups (). The term "polyol" can have slightly different meanings depending on whether it is used in food science or polymer chemistry. Polyols containing two, thr ...
s and
methylene diphenyl diisocyanate
Methylene diphenyl diisocyanate (MDI) is an aromatic diisocyanate. Three isomers are common, varying by the positions of the isocyanate groups around the rings: 2,2′-MDI, 2,4′-MDI, and 4,4′-MDI. The 4,4′ isomer is most widely used, and i ...
(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. Although hot melt adhesives have been around for decades, advancements in PUR development have made it popular for applications like bookbinding, woodworking, and packaging starting in the 1950s. Since it is highly flexible and has a broad thermal setting range, PUR is perfect for bonding difficult substrates.
*
Styrene
Styrene () is an organic compound with the chemical formula C6H5CH=CH2. This derivative of benzene is a colorless oily liquid, although aged samples can appear yellowish. The compound evaporates easily and has a sweet smell, although high concen ...
block copolymer
In polymer chemistry, a copolymer is a polymer derived from more than one species of monomer. The polymerization of monomers into copolymers is called copolymerization. Copolymers obtained from the copolymerization of two monomer species are ...
s (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 (PSA, self-adhesive, self-stick adhesive) is a type of nonreactive adhesive which forms a bond when pressure is applied to bond the adhesive with a surface. No solvent, water, or heat is needed to activate the adhesiv ...
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 olefin
In organic chemistry, hydrocarbons (chemical compound, compounds composed solely of carbon and hydrogen) are divided into two classes: aromatic compounds and aliphatic compounds (; G. ''aleiphar'', fat, oil). Aliphatic compounds can be Saturate ...
s,
rosin
Rosin (), also called colophony or Greek pitch ( la, links=no, pix graeca), 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 comp ...
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
1,3-Butadiene () is the organic compound with the formula (CH2=CH)2. It is a colorless gas that is easily condensed to a liquid. It is important industrially as a precursor to synthetic rubber. The molecule can be viewed as the union of two vi ...
-styrene (SBS), used in high-strength PSA applications.
** Styrene-
isoprene
Isoprene, or 2-methyl-1,3-butadiene, is a common volatile organic compound with the formula CH2=C(CH3)−CH=CH2. In its pure form it is a colorless volatile liquid. Isoprene is an unsaturated hydrocarbon. It is produced by many plants and animals ...
-styrene (SIS), used in low-viscosity high-tack PSA applications.
** Styrene-ethylene/
butylene
Butene, also known as butylene, is an alkene with the formula . The word ''butene'' may refer to any of the individual compounds. They are colourless gases that are present in crude oil as a minor constituent in quantities that are too small for ...
-styrene (SEBS), used in low self-adhering non-woven applications.
** Styrene-ethylene/propylene (SEP)
*
Polycaprolactone
Polycaprolactone (PCL) is a biodegradable polyester with a low melting point of around 60 °C and a glass transition temperature of about −60 °C. The most common use of polycaprolactone is in the production of speciality polyure ...
with
soy protein
Soy protein is a protein that is isolated from soybean. It is made from soybean meal that has been dehulled and defatted. Dehulled and defatted soybeans are processed into three kinds of high protein commercial products: soy flour, concentrates, ...
, using
coconut oil
frameless , right , alt = A cracked coconut and a bottle of coconut oil
Coconut oil (or coconut butter) is an edible oil derived from the wick, meat, and milk of the coconut palm fruit. Coconut oil is a white solid fat; in warmer climates duri ...
as plasticizer, a
biodegradable hot-melt adhesive investigated at
Korea University.
*
Polycarbonate
Polycarbonates (PC) are a group of thermoplastic polymers containing carbonate groups in their chemical structures. Polycarbonates used in engineering are strong, tough materials, and some grades are optically transparent. They are easily work ...
s
*
Fluoropolymer
A fluoropolymer is a fluorocarbon-based polymer with multiple carbon–fluorine bonds. It is characterized by a high resistance to solvents, acids, and bases. The best known fluoropolymer is polytetrafluoroethylene under the brand name "Teflon ...
s, with tackifiers and ethylene copolymer with polar groups
*
Silicone rubber Silicone rubber is an elastomer (rubber-like material) 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
An elastomer is a polymer with viscoelasticity (i.e. both viscosity and elasticity) and with weak intermolecular forces, generally low Young's modulus and high failure strain compared with other materials. The term, a portmanteau of ''elastic p ...
s
*
Polypyrrole
Polypyrrole (PPy) is an organic polymer obtained by oxidative polymerization of pyrrole. It is a solid with the formula H(C4H2NH)nH. It is an intrinsically conducting polymer, used in electronics, optical, biological and medical fields.
History ...
(PPY), a
conductive polymer
Conductive polymers or, more precisely, intrinsically conducting polymers (ICPs) are organic polymers that conduct electricity. Such compounds may have metallic conductivity or can be semiconductors. The biggest advantage of conductive polymers ...
, for intrinsically conducting hot-melt adhesives (ICHMAs), used for
EMI
EMI Group Limited (originally an initialism for Electric and Musical Industries, also referred to as EMI Records Ltd. or simply EMI) was a British Transnational corporation, transnational Conglomerate (company), conglomerate founded in March 1 ...
shielding. EVA compounded with 0.1–0.5wt.% PPY are strongly absorbing in
near infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from arou ...
, allowing use as near-infrared activated adhesives.
* various other
copolymer
In polymer chemistry, a copolymer is a polymer derived from more than one species of monomer. The polymerization of monomers into copolymers is called copolymerization. Copolymers obtained from the copolymerization of two monomer species are some ...
s
The usual additives include the following:
*
tackifying resin
In polymer chemistry and materials science, resin is a solid or highly viscous substance of plant or synthetic origin that is typically convertible into polymers. Resins are usually mixtures of organic compounds. This article focuses on natu ...
s (e.g., rosins and their derivates,
terpene
Terpenes () are a class of natural products consisting of compounds with the formula (C5H8)n for n > 1. Comprising more than 30,000 compounds, these unsaturated hydrocarbons are produced predominantly by plants, particularly conifers. Terpenes ar ...
s and modified terpenes,
aliphatic
In organic chemistry, hydrocarbons ( compounds composed solely of carbon and hydrogen) are divided into two classes: aromatic compounds and aliphatic compounds (; G. ''aleiphar'', fat, oil). Aliphatic compounds can be saturated, like hexane, ...
, 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
In materials science and continuum mechanics, viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. Viscous materials, like water, resist shear flow and strain linearly wi ...
properties. Tackifiers frequently present most of both weight percentage and cost of the hot-melt adhesive.
*
wax
Waxes are a diverse class of organic compounds that are lipophilic, malleable solids near ambient temperatures. They include higher alkanes and lipids, typically with melting points above about 40 °C (104 °F), melting to giv ...
es, e.g.,
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 p ...
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
A plasticizer ( UK: plasticiser) is a substance that is added to a material to make it softer and more flexible, to increase its plasticity, to decrease its viscosity, and/or to decrease friction during its handling in manufacture.
Plasticiz ...
s (e.g.,
benzoate
Benzoic acid is a white (or colorless) solid organic compound with the formula , whose structure consists of a benzene ring () with a carboxyl () substituent. It is the simplest aromatic carboxylic acid. The name is derived from gum benzoin, wh ...
s such as
1,4-cyclohexane dimethanol dibenzoate,
glyceryl tribenzoate, or
pentaerythritol tetrabenzoate,
phthalate
Phthalates (, ), or phthalate esters, are esters of phthalic acid. They are mainly used as plasticizers, i.e., substances added to plastics to increase their flexibility, transparency, durability, and longevity. They are used primarily to soften ...
s,
paraffin oils,
polyisobutylene
Polyisobutene (polyisobutylene) is a class of organic polymers prepared by polymerization of isobutene. The polymers often have the formula Me3C H2CMe2sub>nX (Me = CH3, X = H, F). They are typically colorless gummy solids.
Polymerization is typ ...
,
chlorinated paraffins, etc.)
*
antioxidant
Antioxidants are compounds that inhibit oxidation, a chemical reaction that can produce free radicals. This can lead to polymerization and other chain reactions. They are frequently added to industrial products, such as fuels and lubricant ...
s and stabilizers (e.g., hindered phenols,
BHT,
phosphite
The general structure of a phosphite ester showing the lone pairs on the P
In organic chemistry, a phosphite ester or organophosphite usually refers to an organophosphorous compound with the formula P(OR)3. They can be considered as esters of a ...
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 protect the material against degradation by
ultraviolet radiation
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...
*
pigment
A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compounds whereas pigments are often inorganic compo ...
s and
dyes,
glitter
Glitter is an assortment of small, reflective particles that come in a variety of shapes, sizes, and colors. Glitter particles reflect light at different angles, causing the surface to sparkle or shimmer. Glitter is similar to confetti, sparkle ...
*
biocide
A biocide is defined in the European legislation as a chemical substance or microorganism intended to destroy, deter, render harmless, or exert a controlling effect on any harmful organism. The US Environmental Protection Agency (EPA) uses a slig ...
s for hindering bacterial growth
*
flame retardant
The term flame retardants subsumes a diverse group of chemicals that are added to manufactured materials, such as plastics and textiles, and surface finishes and coatings. Flame retardants are activated by the presence of an ignition source a ...
s
*
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. Static charge may be generated by the triboelectric effect or by a non-contact process using a high ...
s
*
fillers
In processed animal foods, a filler is an ingredient added to provide dietary fiber, bulk or some other non-nutritive purpose.
Products like corncobs, feathers, soy, cottonseed hulls, peanut hulls, citrus pulp, screening, weeds, straw, and cere ...
, for reducing cost, adding bulk, improving cohesive strength (forming an aggregate-matrix
composite material
A composite material (also called a composition material or shortened to composite, which is the common name) is a material which is produced from two or more constituent materials. These constituent materials have notably dissimilar chemical or ...
) and altering properties; e.g.,
calcium carbonate,
barium sulfate
Barium sulfate (or sulphate) is the inorganic compound with the chemical formula Ba SO4. 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 an ...
,
talc
Talc, or talcum, is a Clay minerals, clay mineral, composed of hydrated magnesium silicate with the chemical formula Mg3Si4O10(OH)2. Talc in powdered form, often combined with corn starch, is used as baby powder. This mineral is used as a thi ...
,
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
,
carbon black
Carbon black (subtypes are acetylene black, channel black, furnace black, lamp black and thermal black) is a material produced by the incomplete combustion of coal and coal tar, vegetable matter, or petroleum products, including fuel oil, fluid ...
,
clay
Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4).
Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...
s (e.g.,
kaolin
Kaolinite ( ) is a clay mineral, with the chemical composition Al2 Si2 O5( OH)4. It is an important industrial mineral. It is a layered silicate mineral, with one tetrahedral sheet of silica () linked through oxygen atoms to one octahedral ...
).
Fugitive glues and
pressure-sensitive adhesive
Pressure-sensitive adhesive (PSA, self-adhesive, self-stick adhesive) is a type of nonreactive adhesive which forms a bond when pressure is applied to bond the adhesive with a surface. No solvent, water, or heat is needed to activate the adhesiv ...
s are available in hot-melt form. With a tack-like consistency, PSA are bonded through the application of pressure at room temperature.
Additives and polymers containing
unsaturated bonds are highly prone to
autoxidation
Autoxidation (sometimes auto-oxidation) refers to oxidations brought about by reactions with oxygen at normal temperatures, without the intervention of flame or electric spark. The term is usually used to describe the gradual degradation of organi ...
. Examples include
rosin
Rosin (), also called colophony or Greek pitch ( la, links=no, pix graeca), 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 comp ...
-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
Dielectric heating, also known as electronic heating, radio frequency heating, and high-frequency heating, is the process in which a radio frequency (RF) alternating electric field, or radio wave or microwave electromagnetic radiation heats a diel ...
.
Addition of
electrically conductive
Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allow ...
particles can yield conductive hot-melt formulations.
Electrically conductive hot-melt silicone adhesive composition - Patent 6433055
Freepatentsonline.com. Retrieved on 2010-02-08.
Applications
Hot-melt adhesives are as numerous as they are versatile. In general, hot melts are applied by extruding, rolling or spraying, and the high melt viscosity makes them ideal for porous and permeable substrates. HMA are capable of bonding an array of different substrates including: rubbers, ceramics, metals, plastics, glass and wood.
Today, HMA (hot-melt adhesives) are available in a variety of different types, allowing for use in a wide range of applications across several industries. For use with hobby or craft projects such as the assembly or repair of remote-control foam model aircraft
A model aircraft is a small unmanned aircraft. Many are replicas of real aircraft. Model aircraft are divided into two basic groups: flying and non-flying. Non-flying models are also termed static, display, or shelf models.
Aircraft manufactur ...
, and artificial floral arrangements, hot-melt sticks and hot-melt glue guns are used in the application of the adhesive. For use in industrial processes, adhesive is supplied in larger sticks and glue guns with higher melting rates. Aside from hot-melt sticks, HMA can be delivered in other formats such as granular or power hot-melt blocks for bulk melt processors. Larger applications of HMA traditionally use pneumatic systems to supply adhesive.
Examples of industries where HMA is used includes:
* Closing the flaps of corrugated boxes and paperboard cartons in the packaging
Packaging is the science, art and technology of enclosing or protecting products for distribution, storage, sale, and use. Packaging also refers to the process of designing, evaluating, and producing packages. Packaging can be described as a co ...
industry.
* Spine gluing in the bookbinding
Bookbinding is the process of physically assembling a book of codex format from an ordered stack of ''signatures'', sheets of paper folded together into sections that are bound, along one edge, with a thick needle and strong thread. Cheaper, b ...
industry[
* Profile-wrapping, product assembly and laminating applications in the woodworking industry][
* Disposable diapers are constructed through the use of HMA, bonding the non-woven material to both the backsheet and the elastics.
* Many electronic device manufacturers may also use an HMA to affix parts and wires, or to secure, insulate, and protect the device's components.
]
Hot-melt adhesives
can also be used to create temporary tattoos or portraits.
Format
Hot-melt adhesives are often sold in sticks or cartridges suited to the intended glue gun. Bulk pellets are also used: these are dumped or transported to an adhesive reservoir for subsequent application. Large open-head drums are also used for high volume application. Hot-melt drum pumps have a heated platen which melts the adhesive for pumping through heated hoses.
References
{{Power tools
Adhesives
Packaging materials