Quick Overview Of Adhesive Reactivity Mechanisms

Adhesives are typically characterised by the method of adhesion. These are then characterised into reactive and non-reactive adhesives, which refers to whether the adhesive chemically reacts in order to harden. Alternatively they can be organized by whether the raw stock is of natural, or synthetic origin, or by their starting physical phase. The reactivity of an adhesive refers to the method by which the adhesive solidifies and the adhesive bond sets. Depending on the adhesive to be used, there are different solidification methods including:

-> Chemical reaction
-> Loss of solvent
-> Cooling

Let's discuss some the adhesive reactivity mechanisms

Solidification by Solvent Loss
Solvent-based adhesives solidify due to the evaporation or diffusion of the solvent. Solvents serve as carrier materials within these types of adhesives, lowering their viscosities and allowing for their application onto the substrate. Once a solvent-based adhesive is applied onto a substrate, the solvent evaporates from the adhesive into the atmosphere or diffuses into the substrate, allowing viscosity to increase, the adhesive to dry, and eventually, the adhesive bond to set. This type of adhesives can be seen in many forms such as;

Contact adhesives: Applied through spray or roll coating, contact adhesives are adhesives of high cohesive strength which must be applied to both substrates to form an adhesive bond. After application to the substrates, the adhesive is subjected to a drying period in which a portion of its solvent is evaporated (in some cases, with the addition of heat) to develop the adhesive’s tackiness.

Pressure-sensitive adhesives: While pressure-sensitive adhesives are applied similarly to contact adhesives, unlike contact adhesives they exhibit permanent tackiness, even after complete solidification. Because of this quality, these types of adhesives can be pre-applied to a substrate and dispensed as needed.

Resinous solvent adhesives: They are also known as resin adhesives  are typically applied to porous substrates. The porosity allows these adhesives to flow into and around the cavities and protrusions of the substrates. As the solvent evaporates or diffuses into the substrate, the adhesive hardens and mechanically interlocks the two substrates together.

Reactivatable adhesives: They are  also known as solvent-activated adhesives which are pre-applied to a substrate and allowed to dry to a non-tacky state for storage and shipment purposes. As needed, these adhesives can be re-activated by the application of a solvent, which allows substrates to be bonded together given contact and pressure.

Solidification by Chemical Reaction
With regards to adhesives, chemical reaction refers to the adhesive’s reaction to a curing agent or another catalyst, such as heat, moisture, ultraviolet (UV) radiation, and lack of oxygen. Adhesives which solidify by chemical reaction are, by nature, thermosetting. Therefore, they generally express high bond strengths, offer significant temperature and solvent resistances, and are suitable for structural and non-structural applications. Additionally, they can be used for bonding substrates with larger surface areas. Chemically-reactive adhesives are available in several forms, including single-part and multipart solutions, as well as liquids, pastes, tapes, films, and powders.

Solidification by Cooling
Solidification by cooling occurs in solid adhesives (typically thermoplastic-based adhesives) which are subjected to melt conditions. Melting the adhesives allows them to soften, flow out across, and be applied to the substrate. After application, the melted adhesive cools and hardens, forming a bond between substrates.