What are the types of adhesives? The division of adhesives due to the chemical type, strength and method of curing method

What types of adhesives are there? Classification by chemistry, strength and curing method

How can we divide adhesives into groups? Introduction

Adhesives can be divided using various criteria. The basic ones are origin, curing method and adhesive strength. Of course, more classifications could be distinguished, such as the number of components, but in our opinion these are sufficient to show the main differences between the main adhesive groups.

Key takeaways

✅ The most practical classification uses: origin, curing mechanism and strength level

✅ Two main curing mechanisms are: chemical curing and physical curing

✅ In real process selection, consider: substrate, process conditions, joint loads and service requirements

Classification by curing mechanism

There are two basic mechanisms of curing adhesives:

• chemically curing adhesives – the curing process (polymerization) is initiated by mixing component A with component B. These are mostly two-component adhesives.
• adhesives with a physical curing mechanism – curing occurs by providing the appropriate physical factor (pressure, solvent evaporation, temperature).

Adhesive curing methods

1) Chemically curing adhesives

• Cold curing: by polymerization / polyaddition / polycondensation

• Heat curing: by polymerization / polyaddition / polycondensation

2) Adhesives with a physical curing mechanism

• Dispersion

• Solvent-based

• Contact / pressure

• Hot-melt

Polymerization

This is a chemical reaction during which low-molecular-weight chemical compounds (monomers) or a mixture of such compounds combine with each other until the free functional groups are exhausted. This group includes, for example, anaerobic adhesives and cyanoacrylate adhesives.

Video: anaerobic adhesives as an example (polymerization)

Polyaddition

Polyaddition can be described as a type of chemical reaction, a polymerization in which there are no by-products. It is gradual rather than chain-based. Polyurethane and epoxy adhesives, for example, cure this way.

Polycondensation

In this type of reaction, polymerization proceeds gradually and a low-molecular by-product is released. Silicone cures this way, which is one reason for issues with painting surfaces that were close to curing silicone.

Adhesives with a physical curing mechanism

Within this group, we can additionally distinguish four curing mechanisms:

Dispersion

Dispersion adhesives are adhesives in which the adhesive is a suspension in another substance (usually water). Curing occurs by evaporation of water. They are commonly used in carpentry, the paper industry and in construction, e.g., for bonding floor coverings.

Solvent adhesives

These products cure by solvent evaporation (e.g., alcohol, organic solvents). They consist of a liquid carrier, base and modifiers. After the carrier evaporates, the adhesive thickens and forms a dry, uniform bond that permanently connects the parts. A popular example is butaprene; this group also includes, for example, TEROSON SB 2444. Such adhesives are often used for bonding large rubber parts (e.g., rubber mats).

Hot-melt adhesives

This group cures under the influence of temperature: the adhesive melts and gains the ability to wet the surface and bond to it. Within this category we can distinguish:

• EVA adhesives (ethylene-vinyl acetate based) – usually yellow, with a characteristic smell, commonly used for cardboard packaging production,
• PO adhesives (polyolefin based) – improved properties vs traditional EVA, e.g., higher temperature resistance, better adhesion, increased flexibility at low temperatures,
• PUR adhesives (polyurethane based) – react with moisture in the material or in the environment (air). Two effects occur simultaneously: hardening of the liquid adhesive and a chemical reaction that additionally affects bonding properties. These are reactive adhesives (after crosslinking, exposure to high temperature does not change the state, i.e., the adhesive does not melt). They are used in furniture manufacturing, automotive and textile industries,
• PSA (Pressure Sensitive Adhesives) – so-called “always tacky” adhesives with permanent tack. Bonding is achieved by pressure: the greater the force, the stronger the bond. Depending on tack, bonding can occur instantly or over time; they are often used for pre-coating independently of the main bonding process,
• animal glues – produced by prolonged boiling of animal connective tissue. Usually mixed with water and heated to a certain temperature, but not higher than 70 °C. At higher temperatures the adhesive degrades. Used in bookbinding.

Classification by origin

In terms of origin, two groups can be distinguished:

• natural-origin adhesives – based on starch, cellulose and animal-derived materials,
• synthetic adhesives – the vast majority of adhesives used today.

Classification by the activating (curing) factor

• chemically curing adhesives – curing occurs using a hardener, usually at room temperature,
• thermosetting adhesives – curing by high temperature,
• thermoplastic adhesives – become liquid when heated and solidify when cooled.

Structural and non-structural adhesives

Structural / construction adhesives

These are adhesives with very high mechanical (cohesive) strength and resistance to many aggressive external factors. They can often withstand shear loads from 7 MPa at room temperature. Maximum strength is about 80 MPa, and average strength about 25–30 MPa. In load tests of adhesive joints when bonding plastics or composites, results show that the strength generated in the adhesive joint can be comparable to the bonded base material.

Non-structural adhesives

Non-structural adhesives are products with varying strength levels (from repositionable adhesives to high-strength products). This group can include:

• hot-melt adhesives (EVA, PA, PSA),
• contact adhesives based on neoprene and nitrile,
• sealants based on polyurethane, butyl and nitrile rubbers in semi-liquid form,
• water dispersion adhesives.

Hybrid adhesives

Hybrid adhesives are a specific type of adhesive. Recently, many products have appeared with the word “hybrid” in the name, but they do not always meet the criteria to be called that. These are mostly polymer adhesives cured by polyaddition. In our opinion, the only true hybrid adhesives on the market today are LOCTITE hybrid adhesives, which combine a cyanoacrylate adhesive with an epoxy or acrylic adhesive depending on the product. Thanks to a formulation based on two chemical bases, these adhesives can offer high strength, fast curing, good chemical and temperature resistance and gap-filling capability, which makes them versatile.

Video: Melkib training videos

FAQ – adhesive classification

Why is classification “by chemistry” alone often not enough?

Because in practice selection also depends on curing mechanism, process conditions, joint loads and the service environment.

What should you watch out for with physically curing adhesives?

Process conditions (pressure, evaporation time, temperature) and whether the environment interferes with forming a stable bond line.

What’s the difference between structural and non-structural adhesives?

Structural adhesives are intended for higher loads, while non-structural adhesives cover a broader range of strength levels depending on the application.

Author

Marcin Filipczyk – long-time specialist in adhesive bonding.

Sources

Materials used in the article:

- www.ekologia.pl
- “Podstawy aplikacji klejowych” Dr Eng. Mariusz Tryznowski
- “Połączenia jednostek montażowych” Eng. Marcin Słoma
- graphic design: Maciej Klus