Hot-melt adhesive

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"Glue Gun" redirects here. For the band, see Glue Gun (band).
A hot glue gun loaded with a glue stick

Hot-melt adhesive (HMA), also known as hot glue, is a form of thermoplastic adhesive that is commonly sold as solid cylindrical sticks of various diameters designed to be applied using a hot glue gun. The gun uses a continuous-duty heating element to melt the plastic glue, which the user pushes through the gun either with a mechanical trigger mechanism on the gun, or with direct finger pressure. The glue squeezed out of the heated nozzle is initially hot enough to burn and even blister skin. The glue is sticky when hot, and solidifies in a few seconds to one minute. Hot-melt adhesives can also be applied by dipping or spraying, and are popular with hobbyists and crafters both for affixing and as an inexpensive alternative to resin casting.

In industrial use, hot-melt adhesives provide several advantages over solvent-based adhesives. Volatile organic compounds are reduced or eliminated, and the drying or curing step is eliminated. Hot-melt adhesives have a long shelf life and usually can be disposed of without special precautions. Some of the disadvantages involve thermal load of the substrate, limiting use to substrates not sensitive to higher temperatures, and loss of bond strength at higher temperatures, up to complete melting of the adhesive. Loss of bond strength can be reduced by using a reactive adhesive that after solidifying undergoes further curing, whether by moisture (e.g., reactive urethanes and silicones), or ultraviolet radiation. Some HMAs may not be resistant to chemical attacks and weathering.[citation needed] HMAs do not lose thickness during solidifying, whereas solvent-based adhesives may lose up to 50–70% of layer thickness during drying.[1]

Properties

Melt viscosity
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
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.[2]

General terms

Open time
The working time to make a bond, where the surface still retains sufficient tack, can range from seconds for fast-setting HMAs to infinity for pressure-sensitive adhesives.
Set time
Time to form a bond of acceptable strength.
Tack
The degree of surface stickiness of the adhesive; influences the strength of the bond between wetted surfaces.
Surface energy
Influences wetting of different kind of surfaces.

Materials used

Hot-melt glues usually consist of one base material with various additives. The composition is usually formulated to have a

shrinkage. The melt viscosity and the crystallization rate (and corresponding open time) can be tailored for the application. Faster crystallization rate usually implies higher bond strength. To reach the properties of semicrystalline polymers, amorphous polymers would require molecular weights too high and, therefore, unreasonably high melt viscosity; the use of amorphous polymers in hot-melt adhesives is usually only as modifiers. Some polymers can form hydrogen bonds between their chains, forming pseudo-cross-links which strengthen the polymer.[3]

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,

UV degradation
and necessitates use of antioxidants and stabilizers.

The adhesives are usually clear or translucent, colorless, straw-colored, tan, or amber. Pigmented versions are also made and even versions with glittery sparkles.[5] 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.[6]

Increase of bond strength and service temperature can be achieved by formation of

electron irradiation
, or by other methods.

Resistance to water and solvents is critical in some applications. For example, in textile industry, resistance to dry cleaning solvents may be required. Permeability to gases and water vapor may or may not be desirable. Non-toxicity of both the base materials and additives and absence of odors is important for food packaging.

Mass-consumption

biodegradable HMAs. Research is being performed on e.g., lactic acid polyesters,[7] polycaprolactone with soy protein,[8]
etc.

Some of the possible base materials of hot-melt adhesives include the following:

  • dielectric loss is moderately high. Polypropylene HMAs are a better choice for high-frequency electronics.[15] EVAs are optically clearer and more gas and vapor permeable than polyolefins. Nearly half of EVA HMAs is used in packaging applications. Cryogenic grinding of EVAs can provide small, water-dispersible particles for heat-seal applications. EVA can degrade primarily by loss of acetic acid and formation of a double bond in the chain, and by oxidative degradation.[16]
    EVA can be compounded into a wide range of HMAs, from soft pressure-sensitive adhesives to rigid structural adhesives for furniture construction.
  • polar solvents and solutions of acids, bases, and alcohols. Longer open time in comparison with EVA and polyamides.[18] Polyolefins have low surface energy and provide good wetting of most metals and polymers. Metallocene-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 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.[19]
    • rubber substrates. Can be formulated as pressure-sensitive.[20]
    • APAO) 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, 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.[21]
  • Polyamides and polyesters, high-performance
    • hexamethylene diamine, 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 diamines provide good low-temperature flexibility. Piperazine 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, do not 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.[22]
    • Polyesters, similar to the ones used for synthetic fibers. High application temperature. Synthesized 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 groups for dispersability, were however developed for repulpable adhesives.[23] Polyesters are often highly crystalline, leading to narrow melting temperature range, which is advantageous for high-speed bonding.
  • Polyurethanes
    • Thermoplastic
      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 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.[24]
    • Polyurethanes (PUR), or reactive urethanes, for high temperatures and high flexibility. New type of hot-melt
      UV degradation causing discoloring and degradation of mechanical properties, requires blending with UV stabilizers and antioxidants.[25] Usually based on prepolymers made of polyols and methylene diphenyl diisocyanate (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.[26]
      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.[27]
  • 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 applications, where the composition retains tack even when solidified; however non-pressure-sensitive formulations are also used. High heat resistance, good low-temperature flexibility.[28] 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.[29]
    • Styrene-butadiene-styrene (SBS), used in high-strength PSA applications.
    • Styrene-isoprene-styrene (SIS), used in low-viscosity high-tack PSA applications.
    • Styrene-ethylene/
      butylene
      -styrene (SEBS), used in low self-adhering non-woven applications.
    • Styrene-ethylene/propylene (SEP)
  • biodegradable hot-melt adhesive investigated at Korea University.[8]
  • Polycarbonates[30]
  • Fluoropolymers, with tackifiers and ethylene copolymer with polar groups[31]
  • Silicone rubbers, undergo cross-linking after solidification, form durable flexible UV and weather resistant silicone sealant[32]
  • Thermoplastic elastomers
  • near infrared, allowing use as near-infrared activated adhesives.[34]
  • various other copolymers[35]

The usual additives include the following:

  • viscoelastic
    properties. Tackifiers frequently present most of both weight percentage and cost of the hot-melt adhesive.
  • 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.[37]
  • chlorinated paraffins
    , etc.)
  • phosphites, 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.[38]
  • ultraviolet radiation
  • pigments and dyes, glitter
  • biocides for hindering bacterial growth
  • flame retardants
  • antistatic agents
  • kaolin).[39]

Fugitive glues and pressure-sensitive adhesives are available in hot-melt form. With a tack-like consistency, PSA are bonded through the application of pressure at room temperature.[40]

Additives and polymers containing

unsaturated bonds are highly prone to autoxidation. 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.[41]

Addition of electrically conductive particles can yield conductive hot-melt formulations.[42]

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.[43] HMA are capable of bonding an array of different substrates including: rubbers, ceramics, metals, plastics, glass and wood.[40]

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, 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.[43]

Examples of industries where HMA is used includes:

  • Closing the flaps of corrugated boxes and paperboard cartons in the
    packaging industry.[44]
  • Spine gluing in the bookbinding industry[44]
  • Profile-wrapping, product assembly and laminating applications in the woodworking industry[44]
  • 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.

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

  1. ^ "Hot Melt Adhesives｜Technical Issues". pprc.org. Pacific Northwest Pollution Prevention Resource Ctr. Archived from the original on 4 May 2010. Retrieved 4 June 2020.
  2. ^ Gierenz, Gerhard; Karmann, Werner (2001). Adhesives and Adhesive Tapes. John Wiley & Sons.
  3. ^ Synthetically Designed Hot Melt Adhesives – Polyamides and Polyesters - Article. Specialchem4adhesives.com (2007-10-10). Retrieved on 2010-02-08.
  4. doi:10.1051/jp4:19937235
  5. ^ "AKORD Glitter Hot Melt GLUE Adhesive Sticks 100x7mm for Heating GLUE Gun, Multi-colour, Set of 36 Piece : Amazon.co.uk: DIY & Tools". Amazon UK.
  6. ^ Color and Clarity of Hot Melt Adhesives. Woodweb.com. Retrieved on 2010-02-08.
  7. ^ Biodegradable/compostable hot melt adhesives comprising polyester of lactic acid U.S. patent 6,365,680
  8. ^ a b 95-5 Development of biodegradable hot-melt adhesive based on poly-e-caprolactone and soy protein isolate for food packaging system. Ift.confex.com. Archived from the original on 2011-09-27. Retrieved on 2010-02-08.
  9. ^ MSDS – Detailed View
  10. ^ HMA - EVA based - UV/Light Stabilizers Center. SpecialChem4Adhesives. Retrieved on 2010-02-08.
  11. ^ Ethylene Vinyl Acetate (EVA) Copolymers(>50% Ethylene)Market Study Report - European Adhesives Industry Archived 2008-11-18 at the Wayback Machine. Chemquest.com. Retrieved on 2010-02-08.
  12. ^ Ethylene vinyl acetate copolymers (EVA) Archived 2009-07-15 at the Wayback Machine. Plastiquarian.com. Retrieved on 2010-02-08.
  13. doi:10.1016/S0143-7496(03)00069-1
  14. ^ Butadiene grafted ethylene-vinyl acetate hot melt adhesive U.S. patent 3,959,410
  15. ^ Hot melt glues (Barry L. Ornitz). Yarchive.net. Retrieved on 2010-02-08.
  16. ISBN 1-884207-92-8
    p. 8
  17. ^ Hot melt applications - Ethylene Copolymers Center. SpecialChem4Adhesives. Retrieved on 2010-02-08.
  18. ^ Polyolefins - Antioxidants Center. SpecialChem4Adhesives. Retrieved on 2010-02-08.
  19. ^ Solvent-free adhesives By T.E. Rolando, iSmithers Rapra Publishing, 1998
    ISBN 1-85957-133-6
    p. 17
  20. ISBN 3-527-30110-0
    , p. 22
  21. ^ Amorphous Poly-Olefin (APO/APAO)-based Hot Melt Adhesives Archived 2008-07-24 at the Wayback Machine. Hot Melt News (2006-07-18). Retrieved on 2010-02-08.
  22. ^ a b Specific adhesion model for bonding hot-melt polyamides to vinyl. (PDF) . Retrieved on 2010-02-08.
  23. ^ Odorless, Water-Dispersible Sulfopolyester for Recyclable Hot Melt Adhesives - Article. Specialchem4adhesives.com (2002-05-22). Retrieved on 2010-02-08.
  24. ISBN 0-07-147916-3
  25. ^ Reactive Hot-melts - UV/Light Stabilizers Center. SpecialChem4Adhesives. Retrieved on 2010-02-08.
  26. ISBN 0-444-51140-7
    , p. 785
  27. ^ [1]. LD Davis Glues & Gelatins. Retrieved on 2019-01-19.
  28. ^ Applications. Hbfuller.com. Retrieved on 2010-02-08.
  29. ISBN 0-387-31235-8
    , p. 484
  30. ^ Reactive hot melt composition - Patent 4996283. Freepatentsonline.com (1991-02-26). Retrieved on 2010-02-08.
  31. ^ U.S. patent 4,252,858
  32. ^ Hot Melt Assembly Sealant. Dow Corning. Retrieved on 2010-02-08.
  33. doi:10.1016/S0379-6779(99)00061-2
  34. doi:10.1039/b708707a
  35. ^ High Performance Industrial Hot Melts. (PDF) . Retrieved on 2010-02-08.
  36. ^ Additives, Polymers and Tackifiers Database - The Online Experts on Polymer Additives & Colors Archived 2014-08-05 at the Wayback Machine. Specialchem4adhesives.com. Retrieved on 2010-02-08.
  37. ^ Using Waxes in Hot Melt Adhesives - Article. Specialchem4adhesives.com (2009-12-16). Retrieved on 2010-02-08.
  38. ^ Olga I. Kuvshinnikova and Robert E. Lee Silicon-based antioxidants for hot melt adhesives TAPPI JOURNAL, October 1998, Vol.81(10) pp. 214–218
  39. ^ Polyamide adhesives having improved bookbinding characteristics - Patent 5989385. Freepatentsonline.com. Retrieved on 2010-02-08.
  40. ^ a b Davis, Joseph R (1992). ASM Materials Engineering Dictionary. ASM International. p. 215.
  41. ^ Hot melt adhesive for microwave heating Archived 2010-04-23 at the Wayback Machine. Freshpatents.com. Retrieved on 2010-02-08.
  42. ^ Electrically conductive hot-melt silicone adhesive composition - Patent 6433055. Freepatentsonline.com. Retrieved on 2010-02-08.
  43. ^ a b "Adhesives and Sealants 101: Hot Melts". Adhesives & Sealants Industry. October 1, 2008. Retrieved 11 November 2015.
  44. ^ a b c von Byern, Janek; Grunwald, Ingo (2010). Biological adhesion systems : from nature to technical and medical application (1st ed.). Wien: Springer Science & Business Media. pp. 198–199.
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