The variety of adhesives allows them to be classified according to the following criteria:
- By field of application: for bonding metals, glass, rubber, heat-shielding coatings, paper and cardboard, leather and footwear, for the woodworking and furniture industry, construction, for the transport industry, for household consumption, for installation, etc.
- By physical state: liquid, solid, pasty, film.
- By curing temperature: cold or hot curing.
- By the main chemical component: phenol-formaldehyde, epoxy, siloxane, acrylate, and others.
- By the number of components: one-component, two-component and multicomponent.
- By operating temperature: cryogenic resistant; adhesives used at moderate temperatures; heat resistant.
- By the method of curing: anaerobic curing, moisture curing, thermal curing (insoluble and infusible; thermoplastic); cured by activation, polymerization.
- By origin: natural (natural), semi-synthetic and synthetic.
Natural adhesives are classified into the following groups:
- animals (proteinaceous, actually animals, casein, gelatinous, wax);
- vegetable (resinous – gum arabic, rosin, tragacanth, Canadian balsam; proteinaceous – from soybean protein; carbohydrate – starch, dextrins);
- elastomeric (from natural rubber – vinyl chloride, hydrochlorination, cyclized rubber, and its derivatives);
- mineral (from inorganic materials – silicates, phosphates, magnesium oxide, lead litharge, sulfur, etc.; from mineral waxes – paraffin wax; from mineral resins – copal, amber; bituminous, including asphalt).
Adhesives are widely used for commercial and personal purposes all around the globe. The advantages of the gluing method, in comparison with other methods, are as follows:
- Connection of dissimilar materials differing in physical and mechanical properties.
- Possibility of gluing thin-walled parts.
- Uniform distribution of stresses during gluing. Absence of large residual stresses, as in welding, and stress concentrators (holes for fasteners). Many sealants are elastic, which allows them to absorb and redistribute internal stresses.
- Fast and economical assembly method.
- A wide range of adhesive materials allows them to be used in many technological processes.
- The high strength of adhesive joints.
- Low cost and simplicity of technology; no need to use additional equipment.
- Possibility of reducing metal consumption and weight of the structure due to the use of glue instead of fasteners.
- Possibility of joining materials that are sensitive to heat or break down during welding or brazing.
- Good sealing, sound, heat, and electrical insulating properties.
- Resistance of adhesives to moisture and chemicals.
- The versatility of many adhesives allows a limited range of adhesives to be used in production.
- Reduction of electrolytic corrosion when bonding different metals.
Synthetic adhesives are of the following types:
- thermoplastic (from cellulose derivatives – acetates, acetobutyrates, caproic acid esters, nitrates, hydroxyethyl cellulose, ethylcellulose; from vinyl polymers – polyvinyl acetate, polyvinyl chloride, polyvinyl alcohol, and ethers; polystyrene and polyamide; polyester);
- thermosetting (from aminoplast; epoxy; epoxy-polyamide, epoxy-bitumen, epoxy-polysulfide; from phenolic resins and their modifications).
With thermosetting adhesives, curing occurs by heating (hot curing) or by the action of hardeners (cold-curing). When choosing an adhesive, one must, first of all, take into account the type of load acting on the adhesive joint, the magnitude and duration of the load, the operating temperature, and the type of working environment. For example, epoxy glue does not work well under vibration loads, and polyurethane is inferior in strength but withstands shock loads and intense vibration.