How are the characteristics of adhesives classified?

(1) Classified by origin: Adhesives can be categorized into natural adhesives and synthetic adhesives.
Natural adhesives are those primarily composed of raw materials derived from natural sources, such as shellac, animal glue, starch, dextrin, and natural rubber.
Synthetic adhesives are those formulated primarily from synthetic resins or synthetic rubbers, such as epoxy resins, phenolic resins, neoprene (polychloroprene), and nitrile rubber.
(2) Classified by application: Adhesives can be categorized into general-purpose adhesives and specialty (application-specific) adhesives. Specialty adhesives are further subdivided based on the materials they are designed to bond, such as those for metals, wood, glass, ceramics, rubber, or polyethylene foam.
(3) Classified by bonding strength: Adhesives can be categorized into structural adhesives and non-structural adhesives.
Structural adhesives are characterized by their ability to withstand significant stress, regardless of the specific bonding location. Under static loading, these adhesives can achieve a shear strength of 7 MPa, while also exhibiting high resistance to non-uniform (peel/cleavage) forces and fatigue. Non-structural adhesives are not designed to bear heavy loads; in principle, they are used for bonding smaller components or for temporary fixation during assembly processes.
(4) Classified by curing temperature: Adhesives can be categorized into room-temperature-curing, medium-temperature-curing, and high-temperature-curing adhesives.Room-temperature-curing adhesives are those that can cure at room temperature—typically below 30°C.
Medium-temperature-curing adhesives are those that can cure between 30°C and 99°C.
High-temperature-curing adhesives are those that can cure at temperatures above 100°C.
(5) Classified by the properties of the adhesive layer after curing: Adhesives can be categorized as thermoplastic or thermosetting.
Thermoplastic adhesives possess a linear molecular structure and typically cure through solvent evaporation, the cooling of a melt, or the coagulation of an emulsion. The resulting adhesive layer softens when heated and dissolves in solvents; it exhibits relatively low cohesive strength and poor heat resistance.
Thermosetting adhesives possess a three-dimensional network structure. They do not soften when heated and are insoluble in solvents. They offer high cohesive strength, as well as resistance to heat, chemical/medium corrosion, and creep. Their drawbacks include low impact strength and low peel strength.
(6) Classified by the base material: Adhesives can be categorized as resin-based, rubber-based, inorganic, or natural adhesives.
(7) Classified by other special properties: Adhesives can be categorized as electrically conductive, magnetically conductive, or spot-welding-compatible adhesives, among others. To ensure effective wetting of the substrate surfaces, an adhesive must be in a liquid state (or rendered into a liquid state) prior to bonding; it only develops structural strength once it transitions into a solid state after bonding. The process of transforming the adhesive layer from a liquid to a solid state—achieved through appropriate methods—is known as "curing." Different types of adhesives typically require different curing processes.