Know your Adhesives

Adhesives can be classified endlessly depending on various parameters such their chemical composition, component type (single or double component), physical state (liquid, paste), load bearing capacities, origin or many other properties and factors. We are touching here a few popular classifications for our readers which could be handy to know.

Classification by the number of components

Adhesives can be classified as single component or double component adhesives. Some adhesives such as double component adhesives will require two or more cans to be mixed together (In manufacturer’s specified ratio) for adhesive to work.

Single component adhesive come are one pack adhesive’s and offer benefits such as ease of use and control. Single component adhesive physically come as a single tube/tub but they use moisture, heat or light to cure. These external factors work as a second component with single component adhesives and help them cure. Common examples for single component adhesive are moisture curing adhesives, cyanoacrylates (super glue), silicon adhesives, modified silanes etc.

Two component adhesives are two pot mix adhesives which cure through a chemical reaction hence generally external factors such as heat, moisture or light are not required by them. Common examples are two component polyurethanes & two component epoxy adhesives.

Two component adhesive cure through a chemical reaction and because of this reason they generally have a higher shelf life as compared to single component adhesives. Single component adhesives have to be protected from external factors such as heat, moisture or light for a longer shelf life as once they are exposed they start curing and get hard. When it comes to two component adhesives the most common standard is mixing them in 1:1 ration hence these types of adhesives are pre-packaged widely by manufacturers in  a dual cartridge and a mixing nozzle may be included with packaging.

Classification by Origin

When classification is advised on the basis of origin we can say adhesives can be classified as natural or synthetic adhesives.

Natural adhesives are derived from natural or organic sources such as plant, starch, vegetable matter, natural resin and animals. Natural rubber is an example of natural adhesive.

Synthetic adhesives are man made and developed in a laboratory. Synthetic adhesives are derived from chemical materials such as elastomers, thermoplastic material etc. PVA (Polyvinyl Acetate) and cyanoacrylate glues are most popular types of synthetic glues we have at our homes.

They are the most widely used adhesives and have far better mechanical, physical and chemical properties when compared with natural adhesives.

Classification by the structure

Thermoplastic Adhesives

Thermoplastic adhesives also known as holt melt adhesives are basically kind of polymers which soften easily when heated which helps them to be applied. On cooling they start to harden up creating strong bonds between wide variety of materials.

Common example of thermoplastic adhesive are PVA, polyacrylate and cyanoacrylates.

Elastomeric Adhesive

Elastomeric adhesive has high elasticity and also one kind of a polymer. Curing of elastomer adhesives happens through solvent evaporation of thermal curing which results in cross linking of molecules. It may be noted that usually such type of adhesive does not melt and their physical state changes to gaseous before they can be melted. They are generally insoluble, flexible, economic and very elastic in nature.

Natural rubber, SBR, butyl rubber and nitrile rubber are common forms of elastomeric adhesives.

Thermosetting Adhesive

Molecular structure of thermosetting adhesive is different as in their structure one giant molecule is formed by strong covalent intermolecular bonding of crosslinked molecules. Bonding in these adhesives can be achieved by heating, evaporation of solvents or using chemical agents but these adhesives cannot be re-melt and cross linking is irreversible process. They are expensive, infusible and insoluble in nature and are used in severe service conditions such as extreme heat, cold or radiation such as structural and load bearing applications.

 Epoxies, polyamides are common examples of such kind of adhesives where epoxy resins are most widely used adhesives around us.

Classification by curing type

Physical curing adhesive

These types of adhesives require some type of physical force such as temperature or pressure to create bonding and they are always based on a polymer. They are different from chemically curing adhesives as no chain of chemical reaction is required to create a bond with them. Instead the polymers (binding agents) which are dissolved in solvents start losing solvents once an adhesive is applied. Binding agent molecules get attracted to each other forming a film as the solvent is fully reduced forming a bond between the substrates. This type of adhesive requires one of the substrates to be porous to enable curing process as solvent needs to escape.

They are highly resistant to temperatures (between 100 deg C to 190 deg C). Common examples are pressure sensitive adhesives, contact adhesives and hotmelt adhesives.

Chemical curing adhesives

Chemcial curing adhesive cure through a chemical reaction between two or more components to reach solid state from liquid or thermoplastic state. Generally, these adhesives provide high bond strength resistant to temperature and humidity. They offer very high resistance to temperatures up to 180 deg C and silicon based adhesives can even provide resistance up to 300 deg C . They have high elasticity properties before fracture sometimes even up to 600% in PU or silicone-based adhesives.

Common type of examples are polyurethane, epoxy and silicon adhesives.

Classification on mechanical properties

Elastomeric Adhesives

These types of adhesives as the name suggest have elastic properties when exposed to a pressure. Examples: silicone adhesives, silane modified adhesives, single component moisture cure PU.

Rigid Adhesives

These types of adhesive usually have low elastomeric properties but high impact resistance. Common examples are epoxy adhesives, anaerobic adhesives or single component heat cured PU.