X-ray Reflectivity Study of Mixed Bis-aminosilane-Vinyl Triacetoxysilane Coatings

Yimin Wang, Jan Ilavsky* and Dale W. Schaefer

Department of Chemical and Materials Engineering,

University of Cincinnati, Cincinnati, OH 45221-0012

*UNICAT, Advanced Photon Source, Argonne National Laboratory, 9700 South Cass Avenue, Building 438 E, Sector 33, Argonne, IL 60439

Abstract

            Silane surface treatment of metals has emerged in recently years as a promising alternative for chromate pretreatment in metal-finishing industries. Silanes are hybrid organic¨Cinorganic chemicals with the general structure of (XO)₃Si(CH₂)_nY are used as coupling agents for adhesion between organic and inorganic materials. In recent years van Ooij et al [1,2] found that the bis-silanes with the general formula of (RO)₃Si(CH₂)₃-R'-(CH₂)₃Si(OR)₃ display better corrosion protection than the above monosilane coupling agents. In order to understand the water-barrier properties of the bis-silanes, Pan et al [3,4] investigated the water response of solvent-based bis-amino silane, bis-sulphur silane and their mixture by neutron reflectivity.

     Despite of the good corrosion performance of above solvent-based silanes, water-based silanes are of more interest because of low VOC emission, ease of preparation, and paint compatibility.  A good example of a water-based silane is the mixture of bis-aminosilane (A) and vinyl triacetoxysilane (V) reported by van Ooij [5]. The addition of V makes the system water soluble and inhibits the gelation of A in water.  Since the water-response of this system has not documented we study the influence of V, pH and hydrolysis time on the water-barrier properties and morphology of the film.

            A-V mixtures of different molar ratios (A/V =2, 3, 3.41, 5 and 6) were prepared by spin coating as a function of precursor hydrolysis time and pH.  X-ray reflectivity was used to investigate the film thickness and morphology before and after H₂O conditioning.

            From the SLD profiles we conclude that the water-barrier properties of the A-V mixtures depend on A/V ratio, pH and hydrolysis time.  The highest swelling is observed at A/V = 6 indicating that A deteriorates the water-barrier properties of the coatings.  For the fixed A/V ratio, the optimum water-barrier properties are observed at 10-hour hydrolysis time.  The pH behavior was studied by the addition of acetic acid. Phase separation was observed at low acetic acid concentration but disappeared at higher concentration.  

References:

[1] Danqing Zhu, Wim J. van Ooij, Electrochimica Acta 2004, 49 1113-1125

[2] W.  J. van Ooij, Danqing Zhu etc., Surface Engineering 2000 16 (5) 386-396

[3] Pan, P.;  Yim, H.;  Kent, M.;  Majewski, J.; Schaefer, D. W. J. Adhesion Sci. Technol. 2003, 17, 2175-2189.

[4] Pan, G.; Yim, H.; Kent, M.;  Majewski, J.; Schaefer, D. W. In Silane and Other Coupling Agents; VSP: Orlando, FL, 2004; 3, 1-11.

[5] Danqing Zhu, Corrosion protection of metals by silane surface treatment, PhD dissertation, 2002, University of Cincinnati.