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ACID – BASE THEORY OF ADHESION :

    A special type of interaction , the acid – base theory is a fairly recent discovery . it is basedon the chemical concept of lewis acid and base . the acid \ base definition was proposed separately by  J . N . bronsted  and  G . N LEWIS   restatement  of these definitions by

           LEWIS IN 1938  led to their popularity  and acceptance . the LEWIS definitions are :

     ( an acid is a substance which can accept an electron pair from abase ; an base is a substance which can donate an electron pair .)by this definition every cation is an acid .in addition to chemical compounds such as  BF3&  SIO2 conversely , anions & compounds like  NH3 , PH3 , & C6H5CH2NH2  are basesaccording to acid \ base theory , adhesion results from the polar attraction of  LEWIS acidand bases ( i e ,electron – poor & electron – rich elements ) at the interface . this theory is attributed to the work by Fowkes ct al Gutman &Bulger &Michael IN BF 3 the higher electronegativity of fluorine atoms preferentially displaces the shared electrons away from the boron atom . a bipolar molecules is thus created that has a positive on the boron side and is negative on the fluorine sides making it a  LEWIS acid on the other hand  NH3  has a negative nitrogen end that renders it a  LEWIS  base . the positive boron end of  BH3& negative nitrogen end of  NH3  interact A special case of acid – base interaction is hydrogen bonding , such as water molecules that exhibit both acidic & basic tendencies . while substantially less than ionic& covalent bond energies is one of the more significant secondary interactions the reader can refer to inorganic chemistry texts to learn about LEWIS acids & bases & their chemical reactions it is sufficient to say that the interactions between compounds capable of electron donation & acceptance from the foundation of the acid – base theory of adhesion

         WEAK BOUNDERY LAYER THEORY :

       this theory , first described by  BIKERMAN states the bond failure at the interface is caused by either a cohesive break or a weak boundary layer . weak boundary layers can originate from the adhesive the adehrend the environment , or a combination of any of the three weak boundary layers can occur in the adhesive or adherend if impurity  concentrated near the bonding surface & forms a weak attachment to the substrate . when failure take place at the adhesive – adherend interface polyethylene & metal oxides are examples of material that may inherently contain weak boundary layers  polyethylene has a weak low molecular weight constituent that is evenly distributed throughout the polymer this weak boundary layer is present at the interface & contributes to low – stress failing when polyethylene is used as an adhesive or an adherend some metal oxides are weakly attached to their base metals failure of adhesive joints made with these material occurs cohesively withen the oxides such as aluminum oxide are strong & do not significantly impair joint strength weak boundary layers such as those found in polyethylene & metal oxides ,