What groups can we divide adhesives into? Introduction.

We can divide adhesives according to various factors. The basic ones are the origin, the method of curing and the strength of the adhesive. Certainly, more classifications could be distinguished, such as the number of ingredients, but in our opinion these are enough to show the main differences between the given groups of adhesives.

Distribution of adhesives according to the method of curing

There are two basic mechanisms of curing adhesives:

• chemically hardening 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 properties (pressure, solvent evaporation, temperature)

A full classification of adhesives curing methods is presented in the graphic below:

Chemosetting adhesives

Chemo-curing adhesives are divided into cold-curing and thermosetting adhesives (which need temperature to start the cross-linking process).

Within this group, we can distinguish 3 types of hardening / chemical reaction:

Polymerization

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

Polyaddition

Polyaddition can be described as a type of chemical reaction, a polymerization in which there are no by-products. It is gradual and not chain-based. This is how polyurethane and epoxy adhesives, for example, harden

Polycondensation

In this type of reaction, polymerization is gradual, during which a low-molecular by-product is released. In this way, for example, silicone hardens, which is the reason for problems with painting surfaces that were close to the curing silicone

Adhesives with a physical curing mechanism

From this group of adhesives, we can additionally distinguish 4 methods of curing:

Dispersion

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

Solvent adhesives

These types of products harden by evaporating the solvent (e.g. alcohol, organic solvents). They consist of a liquid carrier, base and modifiers. After the carrier evaporates, the glue thickens and turns into a dry, uniform joint that permanently connects the elements. A very popular product of this type is butaprene, this group also includes e.g. a>. These types of adhesives are very often used for gluing large-size rubber, e.g. rubber mats.

Hot melt adhesives

A group of adhesives that hardens under the influence of temperature, i.e. melts and gains the ability to wet the surface - connect with it. We can distinguish 3 additional types of adhesives in this category:

• EVA adhesives (based on ethylene-vinyl acetate) - usually yellow in color, have a specific smell, commonly used for the production of cardboard packaging
• PO adhesives (based on polyolefins) - have better properties than traditional EVA adhesives, such as higher temperature resistance, better adhesion, increased flexibility at low temperatures
• PUR adhesives (based on polyurethane) - adhesives from this group react with moisture contained in the material or in the environment, e.g. in the air. This means that two effects take place simultaneously - hardening of the liquid adhesive and a chemical reaction, which additionally positively affects the adhesive properties. These are reactive adhesives (after cross-linking, exposure to high temperature does not change the concentration, i.e. the adhesive melts). They are the best in terms of water and temperature resistance. They are used in the production of furniture, in the automotive and textile industries
• PSA (Pressure Sensitive Adhesives) adhesives are so-called ever-living adhesives, which are characterized by permanent tack. Bonding with the material is achieved by means of pressure - the greater the force, the stronger the bond. Depending on the tack strength of the bond, the bonding process to various substrates can occur instantaneously or over time, which is why these adhesives are often used for pre-coating independently of the bonding process.
• animal glues - glue that is created by prolonged boiling of animal connective tissue. Most often mixed with water and heatedup to a certain temperature, but not higher than 70°C. At higher temperatures, the adhesive degrades. Used in bookbinding shops.

Division of adhesives by origin

In terms of origin, we can distinguish two groups of adhesives:

• glues of natural origin - based on starch, cellulose and animal
• synthetic adhesives - constitute the vast majority of currently used products

Classification of adhesives according to the activating (curing) factor

• chemically hardening adhesives - curing takes place with the use of a hardener, usually at room temperature
• thermosetting adhesives - hardening by high temperature,
• thermoplastic adhesives - they become liquid when heated, 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. Most often, they can withstand shear loads from 7 MPa at room temperature. The maximum strength of structural adhesives is about 80 MPa/mm2. The average strength is about 25 - 30 MPa per mm2. In load tests of adhesive joints when gluing plastics or composites, the results show that the force generated in the adhesive joint is equal to the glued native material.

Non-structural adhesives

Non-structural adhesives are products that have varying degrees of strength (from repositionable adhesives to high-strength products). In this group you can put:

• EVA, PA, PSA hot melt adhesives),
• contact adhesives based on neoprene, 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 adhesives, products containing the word hybrid in the name have often appeared on the market recently, but they do not meet the conditions to be called in this way. 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 product. Thanks to the formula based on two chemical bases, these adhesives achieve excellent properties, i.e. high strength, fast curing, good chemical and temperature resistance and the ability to fill gaps. This makes them very versatile products.

The article uses the following materials:

- www.ekologia.pl
- "Basics of adhesive applications" dr. inż Mariusz Tryznowski
- "Connections of assembly units"  dr inż. Marcin Słoma
- graphic design by Maciej Klus