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Thursday, May 20, 2010

Joint Material Using Adhesives

Bonding agents are used on metals (metal adhesives) to complement or replace riveting, soldering, welding and folding. Outer surfaces of aircraft, bridges, roof and window structures and autoparts are bonded nowdays with adhesives. Other examples of adhesive bonding are; joining pipes, sticking on brake linings and bonding cutting tips to cutting tools.

Advantages and disadvantages of adhesive bonding
Adhesive bonding offers special advantages in joining together of similar materials (aluminium on steel or steel on glass). The positive result are smooth surfaces, gap free seams, uniform when the strength along the entire cross section, insulating effects and weight saving.

The distribution of stress is uniform when the adhesive joint is subjected to load, which results in a high long-term strength. The adhesive joint damps vibrations, and in gas, and water-tight. Neither stresses nor distortions take place in metal workpieces at the low temperatures required for adhesive bonding.

The disadvantages of adhesive joints are sensitivity to shock and impact and low deformation capacity. Two component adhesives must be mixed in the correct ratio to achieve the desired strength. The setting and hardening times limit applications in miss production.

Working Principle of Adhesives
Bonding agents function on the basis of adhesive effects between the agent and the workpiece and the cohesion within the agent itself.

Example of demonstration: one-glass plate is laid upon another and then lifted up again. The surfaces are then moistened with water. The plates now sticks to each other and can now only be separated by sliding. High tensile forces can be transmitted but only low shearing forces.

Adhesion can be improved by various forms of surface pre-treatment. Cohesion ensures that the material particles in a uniform material hold together. Among other things, cohesion depends upon the temperature and determines the state of aggregation of the material. Strong cohesion increased internal strength and weak cohesion leads to a weakening of the structure. This cohesion determines the toughness of a bonding material during processing as well as the strength after hardening.

Surface and Adhesion
The joining surfaces must be cleaned thoroughly, i.e. they must be free of dirt and grease, so that the molecules of the adhesives may get as close contact to the material as possible.

The viscosity of the adhesive must be suited to the roughness of the surface, i.e. the depressions in the surface must be filled with the adhesive and the layer of adhesive above must be as uniform as possible. Since the smoothing out is never very satisfactory, the efficacy of adhesion is greater the less rough the surface is.