The effect of interfacial tension on the adhesion of cathodic E-coat to aluminum alloys

Adhesion of a cathodically electrodeposited paint (E-coat) to aluminum allo ys, Alclad 2024-T3, AA 2024-T3 and AA 7075-T6, was investigated to examine the influence of interfacial tension at the paint/metal interface. The surf ace energy of an aluminum plate was modified by depositing a plasma polymer of a mixture of trimethyl silane (TMS) and one of three diatomic gases (O- 2, N-2, and H-2) by cathodic plasma polymerization. The contact angle (thet a) of water on a modified surface changes as a function of the mole fractio n of the diatomic gas. The plot of cos theta(PP) Of a plasma polymer as a f unction of the mole fraction of the gas crosses the plot of cos theta(EC) o f the E-coat. The difference, Delta Cos theta = cos theta(PP) - cos theta(E C), is a parameter which indicates the level of interfacial tension at the paint/metal interface, Delta Cos theta = 0 represents the minimum interfaci al tension. The adhesion of a cured E-coat on a panel was evaluated by the N-methyl pyrrolidinone (NMP) paint delamination time test. The maximum peak of adhesion test values plotted as a function of Delta Cos theta occurred around the zero point, Delta Cos theta = 0, indicating that maximum adhesio n is obtained with minimum interfacial tension. Mixtures of TMS and N-2 On all three aluminum alloys studied consistently displayed longer delaminatio n times in the NMP test than mixtures of TMS and O-2 or H-2.