Adhesives can be classified in several different ways: by chemical family, curing or setting mechanism, origin, number of components, physical form and the role the bonded joint performs in the final assembly. No single classification is sufficient for every engineering decision.
For practical adhesive selection, the most useful approach is to combine chemistry, curing mechanism, substrates, joint geometry, process conditions and service requirements. Two adhesives from the same chemical family can behave very differently if they use different cure systems or are formulated for different applications.
Key takeaways
- Adhesives can be classified by chemistry, cure or setting mechanism, origin, component count and structural role.
- Chemically reactive adhesives do not have to be two-component systems. Anaerobic, cyanoacrylate, moisture-curing and light-curing products can be one-component.
- Physically setting systems include water- or solvent-borne adhesives and conventional thermoplastic hot melts.
- Pressure-sensitive adhesives (PSAs) do not cure simply because pressure is applied. Pressure improves contact and wetting while the adhesive remains permanently tacky.
- Reactive PUR hot melts combine fast physical setting on cooling with later chemical crosslinking triggered by moisture.
- There is no universal MPa threshold that alone defines whether an adhesive is structural. The intended load-bearing function and validated joint performance are more important.
How can adhesives be classified?
The same product can belong to several categories at once. For example, a two-component epoxy can be described as:
- synthetic by origin,
- reactive by curing mechanism,
- two-component by packaging and mixing method,
- thermosetting after cure,
- structural if it is designed and validated to transfer significant loads in an assembly.
This is why classifications should be treated as complementary rather than competing systems.
| Classification criterion | Typical groups | What it tells you |
|---|---|---|
| Chemical family | Epoxy, polyurethane, acrylic / methacrylate, cyanoacrylate, silicone, rubber-based, anaerobic and others | Gives a first indication of possible performance, cure chemistry and compatibility, but does not determine the final application by itself. |
| Curing or setting mechanism | Chemical reaction, water or solvent evaporation, cooling from a melt, pressure-sensitive bonding | Determines how the bond develops and what process conditions are required. |
| Number of components | 1K, 2K and multi-component | Determines mixing, metering, pot life and process-control requirements. |
| Origin | Natural and synthetic | Describes the source or base material, not necessarily the performance level. |
| Joint function | Structural, semi-structural, non-structural, sealing or temporary / repositionable | Relates the adhesive to the role of the joint in the final assembly. |
Chemically reactive adhesives
Reactive adhesives build their final polymer network through a chemical reaction. The reaction may be initiated in several ways, including:
- mixing resin and hardener,
- exposure to atmospheric moisture,
- exclusion of oxygen between metal surfaces,
- contact with surface moisture,
- UV or visible light,
- heat,
- an activator or initiator.
Therefore, it is incorrect to define chemically curing adhesives simply as “two-component products mixed A+B”. Two-component epoxies and polyurethanes are important examples, but many industrial reactive adhesives are one-component systems.
Polymerisation, polyaddition and polycondensation
Older bonding literature often divides chemical curing into polymerisation, polyaddition and polycondensation. This is useful as a teaching model, provided the terms are not treated as rigid product categories.
Chain-growth polymerisation
In chain-growth polymerisation, reactive monomers add to an active chain centre. Industrial bonding examples include cyanoacrylate adhesives and many anaerobic acrylic systems. Their initiation mechanisms are different, but both form polymer networks through polymerisation reactions.
Video: anaerobic adhesives as a practical polymerisation example
This Melkib playlist shows practical anaerobic applications such as threadlocking and sealing.
Polyaddition
Polyaddition is a step-growth reaction in which multifunctional molecules react without releasing a small-molecule by-product. Many polyurethane systems and many epoxy-hardener reactions can be described within this broader step-growth / addition framework.
The exact reaction depends on the formulation, so “epoxy” or “polyurethane” should not be used as a substitute for the product-specific curing mechanism in a technical process specification.
Polycondensation
Polycondensation is a step-growth process in which small molecules are released during network formation. Some moisture-curing silicone sealants and adhesives use condensation-cure chemistry.
However, not all silicones are condensation-curing. Addition-cure silicone systems are also widely used. WACKER and Dow both document industrial silicone families using condensation and addition cure mechanisms.
Physically setting adhesives
Some adhesives develop handling strength mainly through a physical change rather than through chemical crosslinking during the bonding step. Typical mechanisms include evaporation of water or solvent and cooling of a molten thermoplastic adhesive.
Water-dispersion adhesives
In dispersion adhesives, polymer particles are dispersed in a liquid phase, commonly water. As water is removed, the polymer particles come into closer contact and form the adhesive film. Such systems are widely used in woodworking, paper converting, packaging, flooring and other applications.
Drying conditions matter. A joint between two poorly permeable substrates can trap water and slow or prevent proper film formation if the product was not designed for that geometry.
Solvent-based and contact adhesives
Solvent-based adhesives contain polymer or resin dissolved or dispersed in a volatile carrier. Bond formation depends on controlled solvent evaporation and, for contact adhesives, joining the surfaces within the specified open time.
TEROSON SB 2444 is an example of a polychloroprene-based contact adhesive used for rubber profiles, upholstery and insulation applications.
In a production process, ventilation, solvent flash-off time, coating weight and pressure after assembly can be as important as the adhesive chemistry itself.
Hot-melt adhesives: thermoplastic and reactive systems
Conventional hot-melt adhesives are applied in the molten state, wet the substrate while hot and develop strength as they cool and solidify. This is a physical setting mechanism, not chemical curing.
Common thermoplastic hot-melt families include EVA, polyolefin and polyamide formulations, with different open times, temperature resistance and substrate compatibility.
Reactive PUR hot melts are different
Reactive polyurethane hot melts combine two mechanisms:
- physical setting: the molten adhesive cools and develops early handling strength,
- chemical curing: the polyurethane subsequently reacts with moisture and crosslinks.
Henkel describes its TECHNOMELT PUR products as moisture-curing reactive hot melts that chemically crosslink after the initial hot-melt application. After full cure, they behave differently from conventional remeltable thermoplastic hot melts.
Pressure-sensitive adhesives (PSA)
Pressure-sensitive adhesives are permanently tacky systems used in tapes, labels, films and pre-applied adhesive components. Pressure helps the adhesive wet and conform to the substrate, increasing real contact area.
A finished PSA does not normally undergo a liquid-to-solid cure simply because pressure is applied. 3M summarises the defining PSA characteristics as permanent tack, adhesion with light pressure and no liquid-to-solid phase change during bond formation.
The final bond depends on the balance between:
- tack – ability to make rapid initial contact,
- adhesion – interaction with the substrate,
- cohesion – internal strength of the adhesive.
Natural and synthetic adhesives
By origin, adhesives can be broadly divided into:
- natural-origin adhesives – for example starch-, dextrin-, cellulose-, protein- or animal-derived systems,
- synthetic adhesives – including the majority of modern industrial reactive, thermoplastic, dispersion and pressure-sensitive systems.
Origin alone does not indicate strength, durability or suitability for an industrial process.
Thermoplastic and thermosetting adhesives
This classification describes the behaviour of the polymer rather than a single cure trigger.
- Thermoplastic adhesives soften or melt when heated and solidify again on cooling. Conventional hot melts are a typical example.
- Thermosetting adhesive systems form a crosslinked network during cure. Once fully crosslinked, they do not simply melt back into the original liquid state when reheated.
Reactive PUR hot melts are a useful reminder that practical adhesive technology can cross classification boundaries: they are applied as hot melts but later form a chemically crosslinked network.
Structural and non-structural adhesives
A structural adhesive is intended to transfer significant loads as part of the engineering function of an assembly. Epoxies, acrylics / methacrylates, polyurethanes and some hybrid systems can all be formulated for structural bonding.
There is no universal adhesive-strength number such as 7 MPa, 25 MPa or 80 MPa that by itself determines whether a product is “structural”. The classification depends on the application, substrate, joint design, test method, temperature, environment, ageing and required safety factor.
Non-structural adhesives cover a wide range of tasks such as:
- temporary or repositionable attachment,
- labels and tapes,
- flooring and interior assembly,
- packaging,
- insulation and upholstery,
- applications in which the adhesive is not responsible for the primary structural load path.
What are hybrid adhesives?
The word “hybrid” is used for several types of products in the adhesive industry. In general, it describes a formulation or system that intentionally combines different polymer chemistries or performance mechanisms.
LOCTITE HY products are one industrial example: selected formulations combine cyanoacrylate chemistry with acrylic or epoxy chemistry to combine fast fixture with greater gap-filling ability and improved toughness compared with a conventional instant adhesive.
They are not the only products on the market that can legitimately be described as hybrid. The current Melkib Hybrid adhesives category itself includes products from more than one manufacturer.
Which adhesive family should you consider?
| Adhesive family | Typical process advantage | Typical limitation to check |
|---|---|---|
| Cyanoacrylate | Very fast fixture for small, close-fitting parts. | Gap size, peel / impact loading, moisture and substrate compatibility. |
| Anaerobic | Excellent for close-fitting metal assemblies such as threads, flanges and retaining joints. | Requires the correct metallic joint geometry and appropriate cure conditions. |
| Epoxy | High strength, good gap filling and broad formulation range. | Mixing, cure time, peel toughness, temperature and surface preparation. |
| Acrylic / methacrylate | Fast structural bonding and good compatibility with many industrial substrates. | Working time, odour / process controls, surface condition and formulation-specific compatibility. |
| Polyurethane | Useful where flexibility, toughness or bonding of dissimilar substrates is required. | Moisture, cure conditions, temperature resistance and product-specific safety requirements. |
| Silicone | Flexibility and performance over a broad temperature range in suitable formulations. | Cure chemistry, surface preparation, paintability and contamination sensitivity. |
| Hot melt | Very fast handling and easy automation. | Heat resistance, open time, application temperature and whether the system is thermoplastic or reactive. |
| PSA | Immediate attachment with pressure and no cure time at the assembly stage. | Surface energy, pressure, dwell time, shear loading, temperature and ageing. |
How should an adhesive be selected in practice?
- Identify the substrates. Determine the exact material, grade, coating and surface condition.
- Define the joint geometry. Check bond-line thickness, overlap, gap and whether the joint is loaded in shear, peel, cleavage or tension.
- Define the production process. Consider open time, fixture time, cure time, mixing, dosing, clamping and automation.
- Define service conditions. Include temperature, humidity, chemicals, UV, vibration, fatigue and thermal cycling.
- Check disassembly requirements. Permanent structural bonding and serviceable fastening are different design goals.
- Validate the actual joint. Test representative substrates, surface preparation, geometry and ageing conditions rather than relying on chemistry alone.
Video: Melkib adhesive training playlist
FAQ: adhesive classification
What is the most useful way to classify adhesives?
For engineering selection, combine chemical family with the curing mechanism, substrate, joint geometry, process requirements and service environment. Chemistry alone is rarely enough.
Are all chemically curing adhesives two-component?
No. Two-component epoxy, polyurethane and acrylic products are common, but many cyanoacrylate, anaerobic, moisture-curing and light-curing adhesives are one-component systems.
Does a PSA cure when pressure is applied?
No. Pressure primarily improves wetting and contact between the permanently tacky adhesive and the substrate. A finished PSA normally does not undergo a new liquid-to-solid cure during application.
Do all hot-melt adhesives work in the same way?
No. Conventional thermoplastic hot melts set mainly by cooling. Reactive PUR hot melts first set physically and then undergo moisture-driven chemical crosslinking.
Do all silicones cure by condensation?
No. Condensation-cure and addition-cure silicone systems both exist. Always check the actual product chemistry and technical data sheet.
What makes an adhesive structural?
Its intended load-bearing role and validated joint performance. A single strength value is not sufficient because real joints can be governed by peel, fatigue, impact, temperature, ageing or substrate failure.
Need to choose an adhesive family for a real application?
Start with the substrates, joint geometry, required production time, loads and service environment. These inputs make it possible to narrow the choice before comparing individual products.
Technical sources
- Melkib – current Polish counterpart of this article.
- Melkib – current English article page.
- 3M – Common Chemistry of PSA Tapes.
- Henkel – TECHNOMELT PUR reactive hot-melt technology example.
- WACKER – silicone bonding and condensation- / addition-cure systems.
- Melkib – TEROSON SB 2444, current English product page.
- Melkib – current English Hybrid adhesives category.
Legacy references retained from the original article: “Podstawy aplikacji klejowych” by Dr Eng. Mariusz Tryznowski; “Połączenia jednostek montażowych” by Dr Eng. Marcin Słoma; original graphic design credit: Maciej Klus.
Log in with Facebook
Log in with Google