Green-High-Performance PMMA–Silica–Li Barrier Coatings

Abstract

Organic-inorganic coatings based on polymethyl methacrylate (PMMA)–silica–lithium are an efficient alternative to protect metals against corrosion. Although the preparation methodology is established and the thin coatings (~10 µm) are highly protective, the use of an environmentally friendly solvent has not yet been addressed. In this work, PMMA–silica coatings were synthesized using 2-propanol as a solvent and deposited on aluminum alloy AA7075, widely used in the aeronautical industry. Different concentrations of lithium carbonate (0–4000 ppm) were incorporated into the hybrid matrix to study the structural and inhibitive effects of Li+ in terms of barrier efficiency of the coatings in contact with saline solution (3.5% NaCl). Structural and morphological characterization by low-angle X-ray scattering, X-ray photoelectron spectroscopy, atomic force microscopy, thermogravimetric analysis, thickness, and adhesion measurements, showed for intermediate lithium content (500–2000 ppm) the formation of a highly polymerized PMMA phase covalently cross-linked by silica nodes, which provide strong adhesion to the aluminum substrate (15 MPa). Electrochemical impedance spectroscopy (EIS) results revealed an excellent barrier property in the GΩ cm2 range and durability of more than two years in a 3.5% NaCl solution. This performance can be attributed to the formation of a highly reticulated phase in the presence of Li, which hinders the permeation of water and ions. Additionally, the self-healing ability of scratched samples was evidenced by EIS assays showing a fast Li-induced formation of insoluble products in damaged areas; thus, constituting an excellent eco-friendly solution for corrosion protection of aerospace components.

High anti-corrosion barrier of poly (methyl methacrylate)-silica coatings explained: A thousand-days study

“…Small angle X-ray scattering (SAXS) pro les show no signi cant changes after exposure and are similar to those reported for PMMA-silica hybrids in previous studies [1,2,28,32] (Figure S2). The presence of a broad correlation peak in the high q-range indicates the presence of a concentrated set of nanometric silica scatterers dispersed homogeneously in the PMMA phase, con rming the reticulated structure of the hybrids.…”

“…A previous study showed that a higher BPO/MMA ratio increases the polymerization degree by shortening the polymeric chain segments 3 , thus resulting in an improved barrier performance. The molar proportions of other precursors and solvents in the synthesis were kept constant, as reported in previous studies [1][2][3]32].…”

“…Infrared spectra in Fig. 6 show the presence of vibrational bands characteristic for the precursor functional groups con rming for both samples (B001 and B005) the formation of the PMMA-silica nanocomposite [2,32]. The PMMA phase is evidenced by the stretching bands of the carbonyl group (C O) of ester at 1720 cm -1 [2,29,32,43], bending vibrations of the CH 2 and CH 3 groups at 1386 and 1445 cm -1 , and bands at about 2950 cm -1 , related to symmetric and asymmetric C─H stretching of CH 2 and CH 3 groups of MPTS and PMMA [2,32,43].…”

“…6 show the presence of vibrational bands characteristic for the precursor functional groups con rming for both samples (B001 and B005) the formation of the PMMA-silica nanocomposite [2,32]. The PMMA phase is evidenced by the stretching bands of the carbonyl group (C O) of ester at 1720 cm -1 [2,29,32,43], bending vibrations of the CH 2 and CH 3 groups at 1386 and 1445 cm -1 , and bands at about 2950 cm -1 , related to symmetric and asymmetric C─H stretching of CH 2 and CH 3 groups of MPTS and PMMA [2,32,43]. The presence of the silica nanodomains was identi ed by the absorption bands of Si─CH stretching vibrations at 750 cm -1 of the MPTS [2,29,32] precursor, asymmetric Si─OC 2 H 5 stretching vibrations at 983 cm -1 of the unhydrolyzed TEOS groups, and intense bands at 1055 and 1140 cm -1 associated with asymmetric Si─O─Si stretching vibrations [2,29,32,44].…”

“…The weak absorption band between 3100 and 3750 cm -1 is characteristic of the vibration of O─H groups of residual silanol groups and absorbed water [2,32,44,45].…”