Synthesis of Ultrathin Poly(methyl methacrylate) Model Coatings Bound via Organosilanes to Zinc and Investigation of Their Delamination Kinetics

Abstract

Polymer coatings are widely used to protect metals from corrosion. Coating adhesion to the base material is critical for good protection, but coatings may fail because of cathodic delamination. Most of the experimental studies on cathodic delamination use polymers to study the corrosion behavior under conditions where the interfacial chemistry at the metal(oxide)/polymer interface is not well-defined. Here, ultrathin linear and cross-linked poly(methyl methacrylate) [PMMA] coatings that are covalently bound to oxide-covered zinc via a silane linker have been prepared. For preparation, zinc was functionalized with vinyltrimethoxysilane (VTS), yielding a vinyl monomer-covered surface. These samples were subjected to thermally initiated free radical polymerization in the presence of methyl methacrylate (MMA) to yield surface-bound ultrathin PMMA films of 10-20 nm thickness, bound to the surface via Zn-O-Si bonds. A similar preparation was also carried out in the presence of different amounts of the cross-linkers ethylene glycol diacrylate and hexanediol diacrylate. Functionalized and polymer-coated zinc samples were characterized by infrared (IR) spectroscopy, secondary ion mass spectrometry (SIMS), ellipsometry, and X-ray photoelectron spectroscopy (XPS). Coating stability toward cathodic delamination has been evaluated by scanning Kelvin probe (SKP) experiments. In all cases, the covalently linked coatings show lower delamination rates of 0.02-0.2 mm h(-1) than coatings attached to the surface without covalent bonds (rates ∼10 mm h(-1)). Samples with a higher fraction of cross-linker delaminate slower, with rates down to 0.03-0.04 mm h(-1), compared to ∼0.3 mm h(-1) without cross-linker. Samples with longer hydrophobic alkyl chains also delaminate slower, with the lowest observed delamination rate of 0.028 mm h(-1) using hexanediol diacrylate. For the coatings studied here, delamination kinetics is not diffusion limited, but the rate is controlled by a chemical reaction. Several possibilities for the nature of this reaction are discussed; radical side reactions of the oxygen reduction are the most likely path of deadhesion.

Waterborne chitosan–epoxysilane hybrid pretreatments for corrosion protection of zinc

“…For the reference system, the delamination rate found here is comparable to the rate found in our lab in previous works. 42 Chitosan-epoxysilane films showed a delamination rate of 0.24 ± 0.02 mm h −1 , therefore representing the highest inhibition of delamination found in this work. The delamination rate of chitosan-epoxysilane hybrid coatings is comparable to those of covalently bound coatings.…”

“…63 A time dependence of d ∝ t 1 (α = 1) is expected either if a first-order reaction is rate-determining or for short-distance ionic migration. 42,63 During the first stage of the delamination of the chitosan-epoxysilane coating, a linear time dependence was observed.…”

“…42,44,63 On the other hand, the α corresponding to unmodified chitosan and PVB are 0.70±0.03 and 0.81±0.04, respectively. The value for PVB is lowered by edge effects in the last data point, which was close to full delamination of the sample.…”

“…[38][39][40][41] Silanes covalently bound to the interface can significantly slow down delamination. 42 Ion migration to balance the electronic current through the solid is an important part of the mechanism.…”

Cyclodextrins as Carriers for Organic Corrosion Inhibitors in Organic Coatings

“…18,19 A critical point is the bonding between the metal and the polymer, which can significantly reduce the delamination rate. 12,20,21 An approach for further suppression of cathodic delamination is the addition of active components into the organic coating. 22,23 The most effective inhibitors are based on Cr VI .…”