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DIFFUSION THEORY :

The fundamental concept of the diffusion theory is that that adhesion arises through

The inter diffusion of molecules from one material to another across the interface

The diffusion theory is primarily applicable when both the adhesive and adherend

Are polymeric , having compatible long – chain molecules capable of movement

The phenomenon is a two – stage process . wetting is followed by inter diffusion

Of chain segments across the interface to establish an entangled network of molecules

 FIGUR     inter diffusion  of  polymer molecules across an interface.

For diffusion to occur ,the adhesive and the adherend must be chemically compatible

In terms of miscibility . his means that the materials must be mutually soluble .

And the diffusing molecules must be mobile . the mobility requirement generally

Necessitates that the materials be in melt form of dissolved  in a solvent solution

Dissimilar polymers are usually incompatible  because of the their long chain

Nature and their low diffusion coefficients .

Situations in which the adherend and adhesive are soluble in one another are

relatively rare . therefore . the  diffusion theory  of adhesion can be applied in only

a limited number of cases .the theory is applicable primarily to the solvent or heat welding of thermoplastic substrates however the diffusion theory also explains  the self

adhesion (autohesion) of certain elastomers . poly isobutylene and silicone  elastomers

of a low molecular weight will bond to themselves under slight pressure

the diffusion theory also explains why certain monomers that are used in adhesive

and primers provide high bond strength on untreated low energy surfaces such as

polypropylene .the adhesive or molecules diffuse readily into the substrates and create

molecular interlocking .these products offer the method of obtaining high bond strength

to low energy substrates without a secondary surface treating process   Chemical bonding theory:

in certain applications the formation of covalent chemical bonds occurs across the interface . chemical bonding requires that there be mutually reactive chemical groups

tightly bound on the substrate and in the adhesive . these strong (150 to 900 kj\mol)

and durable bonds are generally the result of close contact or adsorption of the adhesive

on the surface followed by a chemical reaction .thus the term "chemisorption "

is often used to describe this mechanism.

This theory is demonstrated by one of the oldest commercial bonding applications:the

rubber-brass adhesion process during vulcanization of  the rubber . polysulfur bonds

appear between the copper surface atoms of the brass and the elastomer vulcanized

with sulfur.The grafting of a reactive polymeric molecule on a substrate to improve

bond strength to an adhesive is another modern-day example of chemical bonding.

Adhesives containing reactive functional groups such as hydroxyl or carbonyl,tend to

adhere more tenaciously to substrates containing similar groups.hydroxyl bonding is

one of the reasons why epoxy and polyurethane base polymer are often used in struc-

tural adhesive formulations.The relation between the adhesive strength of epoxy resins

and their hydroxyl content is illustrated in the following Fig.

perhaps the most widely employed example of the chemical bonding theory is with adhesion promoters or coupling agents . multifunctional chemical provide a "mole-

cular bridge " between the substrate and the adhesive .one end of the adhesion

promoter molecular has functionally that will react will the adhesive and the other

end has a functionally that will react with the substrate a strong and durable bond forms as the adhesive cures .

organosilanes are the most widely used adhesion promoter they are employed as primers on glass fibers to promote adhesion between the resin matrix and the glass

fiber glass reinforced plastics . they are also used as primers and as additives to promote adhesion of adhesives and sealants