ADHESION MECHANISMS OF SILICA LAYERS ON PLASMA-TREATED POLYMERS .2. POLYPROPYLENE

We have investigated the influence of plasma pretreatment in Various g ases (Ar, NH3, N-2) on the adhesion of plasma-deposited silica layers to polypropylene (PP). UV-visible ellipsometry measurements enabled us to detect crosslinking at the polymer surface. All treatments, and es pecially the Ar-diluted N-2 treatment, induced crosslinking in the sur face layer. The adhesion of silica to PP, as measured by the micro-scr atch test, was improved by every treatment. In the case of Ar, the adh esion improvement is attributed to crosslinking, and possibly to surfa ce activation by radical formation. In addition, surface functionaliza tion was studied by in situ IR ellipsometry, X-ray photoelectron spect roscopy, and contact angle measurements. N-2 plasma treatments induced a significant nitrogen grafting (17.5 at.% concentration after dilute N-2 treatment, in the form of amines and imines), leading to an incre ase in both the electron-acceptor and the electron-donor components of the surface free energy. In contrast, dilute NH3 plasma treatment see med to result in weak bonding of the nitrogen atoms to the surface, as deduced from a comparison of measurements performed in situ after thi s treatment, after aging, and after an additional SiH4 plasma treatmen t. The dilute NH3 treatment, performed after Ar treatment, did not fur ther improve the adhesion of silica to PP, while an additional improve ment was obtained with dilute and pure N-2 treatments. This may be att ributed to C-N-Si bond formation or to acid-base interactions between the nitrogenated polymer surface and silica.