ADHESIVE BONDING

Adhesive bonding is most popular with aircraft and automobile industries where sheet metals. plastics and other components are joined by adhesives. The process is becoming popular in other manufacturing and construction industries also. In many applications, adhesive bonding has effectively replaced rivets, bolts, nails and other type of mechanical fasteners. Adhesive bonding has been a common method of joining and assembling in applications such as labelling, packaging, book binding, footwear, home furnishing, etc.

Adhesives are available in many forms such as liquid, paste, emulsion, solution, powder, tape or film. When applied, adhesives generally are about 0.1 mm thick. There is no mechanical bond in case of adhesives, but there are two types of adhesive action, 

(i) Specific adhesionresulting from the interatomic or intermolecular action (or attraction) between the adhesive and the assembly components, and 

(ii) Mechanical adhesion produced by the penetration of the adhesive into the surfaces of components and its hardening there resulting into the anchoring of the joint surfaces.

The adhesion which results from the interatomic or intermolecular action is caused by 

(i) van der Waals force of attraction(secondary bonding) due to the constant movement of positive and negative charges of molecules and 

(ii) Homo polar bonding between the adhesive and the oxide film on metal surface, the latter bonding force being many times more in magnitude than the van der Waals force (which gives a weak bond for metals but is effective for plastics, and other low melting point materials). Sufficient adhesive should be used to fill the voids and the irregularities of the surfaces being joined and to permit for shrinkage during solidification of the adhesive. Also, the strength of the adhesive bond joint increases with overlapped area (of joint) and reduces with the joint thickness.

The adhesives can be used to join practically all industrial materials. Elimination of rivets and other fasteners with the use of adhesives has resulted into smoother joint surfaces and considerable savings in weight and also the space. These features being important in the construction of airplanes, adhesives are used there with great advantage.

Adhesives also prevent fluid leakage and the biometallic corrosion at joints and give some degree of thermal and electrical insulation. Adhesive bonding is suited to mass production since there are now available many types of high-speed hand and mechanical applicators for adhesives including brushes, sprays, rollers. scrapers, pressure guns, tapes, etc.

Most adhesives have limited temperature range of service. Also, the surfaces to be bonded must be very clean. Most glues require clamping and heat for proper setting since the bond is not instantaneous. Machines are available which automatically apply correct bonding pressure and cause adhesion quickly with dielectric or radio frequency heating. Most adhesives are, however, suitable for only certain specific set of service conditions,

Properties of Adhesives To be suitable for a particular application, an adhesive should possess one or more of the following properties.

• Strength (both shear and peel)
• Resistance to chemicals and other fluids
• Resistance against environmental degradation (heat and moisture)
• Ability to wet the surfaces to be bonded
• Toughness

Nomenclature of Adhesives

The following definitions are applicable as nomenclature of adhesives.

Tack is the characteristic that causes one surface, coated with adhesive, to adhere to another on contact. It is regarded as the essential stickness of the adhesive. Wet strength is the bond strength realized immediately after adhesive-coated surfaces are joined, and before curing occurs.

Ultimate bond strength is the strength of the bond after the cure has been completed or substantially completed. Non-locking properties are opposed to tack, and indicate the freedom of the adhesive coated materials from sticking to unwanted materials such as the operator’s hands. Case stability is the length of time the adhesive can be held in storage without deterioration Pot life is the length of time the adhesive remains usable after being put into serviceable condition.

Specific service conditions are such items as colour, nonstaining, thermal range, resistance to solvent and clean-up.

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