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Mechanical theory :

At one time ,adhesion was thought to occur only by the adhesive flowing and filling pores

Holes, crevices ,and micro voids on substrate . when the adhesive then hardens ,the substrate Are held held  together mechanically .the surface of a solid material is never truly smooth

But consists of a maze of peaks and valleys . according to the mechanical theory of adhesion

In order to function properly the adhesive must penetrate the cavities on the surface displace

The trapped  air at the interface ,and lock on mechanically to the substrate .

One way the surface roughness aid in adhesion is by increasing the total contact area

Between the adhesive or sealantand the adherend  .if interfacial or intermolecular attraction

Is the basis for adhesion , increasing the actual area of contact will increase the total energy

Of surface interaction by a proportional amount .thus , the mechanical interfacing theory

Generally teaches that roughening of surface is beneficial because it (1)give "teeth"to the substrate and (2) increases the total effective area over which the forces of adhesion

Can develop.

The mechanical theory also teaches that joint designs that have large bonding areas are

Better than joint design having a smaller area . however ,there is a point where increasing

The joint area ---- for example , the overlap  distances of a lap joint ----has diminishing

Returns.

MAIN CATEGORIES OF SUBSTRATE SURFACE TREATMENT USED TO ALETER

                                     THE SURFACEENERGY

Surface roughness generally aids in adhesive bonding by providing a mechanical

Interlocking effect.With a surface preparation that roughens the substrate,the adhesive

Would have to pass through the adherend in order for separation to take place.What

Generally occurs is that the roughness or micro-overhangs causes the adhesive to

Plastically deform,which absorbs energy.Consequently,the strength of the adhesive

Joint increases.

Schematic showing (a) tortuous interface between two adhering material with rough sur

faces And (b)between two adherends with smooth surfaces note that in (a) the applied

forced can Not cleanly follow the path between  two adherends . as excursions are made ,

energy is Dissipated.

Another benefit of mechanical interlocking is that a rough surface will provide a crack

Propagation barrier.We notice in the previous fig.that as a wedge is driven into the edge

Of a smooth interface,little energy dissipation is required to separate the adherends,and

a clean separation is possible.In this case,the substrates may simply"unzip"However,if

there is surface roughness,then a tortuous interface between the adhering materials will

act as "path breaks"between the separating adherends.These excursions dissipate energy

and increasethe resulting strength of the joint.

Thus,there are many cases where the physical-chemical forces of adhesion and mechan-

Ical interlocking forces are working together in the same joint.In these case,the practical

work of adhesion is equal to the work developed by adhesion mechanisms(Van der Waals

forces)in addition to the work developed by mechanical mechanisms(elastic deformation).

Mechanical anchoring of the adhesive appears to be a prime factor in bonding many por-

ous Substrates . Adhesives also frequently bond  better to nonporous abraded surfaces

than to Natural surfaces.This beneficial effect of surface roughening may be due to:

v Mechanical interlocking.

v Formation of  a clean surface.

v Formation of a highly reactive surface.

v Formation of a larger surface.

The  electrostatic theory states that forces in the form of an electrical double layer are   produced at the adhesive adherend interface .these forces primarily dispersion forces andForces a rising from interaction of permanent dipoles . such interaction are only the sixth of seventh power of the distance of separation .

In the electrostatic theory . the joint can be treated as a plate capacitor as illustrate  in figure

The energy of adhesion of the system is equal to the energy of separation of the two capacitor  Plates :

                                   WA    =    ? h \ 2 ?    .

Where  ? is the surface charge density ; h ; the distance between the chargedPlates ; and , the ?    ,the dielectric constant of the medium .

Figure       : schematic diagram of the electrical double layer at a polymer metal interface

This theory gathered support from the fact that electrical discharges have been noticed

When adhesive is peeled  from substrate . it was not until recently that electrostatic

Forces were confirmed with out actually destroying the bond . it is believed that for

Conventional adhesive bond the electrostatic contribution to the total work of adhesion

Is quite small in comparison to van der waals forces

However the electrostatic theory is strongly evident in certain applications electrostatic

Adhesion theory is regarded as an accepted theory for biological cell adhesion  . a sim-

Ple from of adhesion can also a rise from direct contact electrification .  this has been

Demonstrated for thin films of metal sputtered onto polymeric surfaces , for attraction

Of dust particles to certain surfaces and for a new type of material called "electret"

Self – cling films or electrets can be fabricated by providing a polymeric film with

A semi permanent charge (generally by corona treatment ).these films will cling to

Themselves and many other substrates for substantial periods of time until the charge

Is lost .these products are used commercially to construct removable labels , signs ,and posters  . non woven electret textiles have also been found to be useful in manufacturing

Ultra filtration products where the charges is to attract and hold airborne particles