Preparation And Corrosion Resistance Of Coating Materials Based On Silane-Functional Polybenzoxazine

Metallic corrosion has a huge economic and environmental impact on virtually all facets of the world’s infrastructure. In addition to causing severe damage and threats to public safety, corrosion disrupts operations and requires extensive repair and replacement of failed assets. Organic coatings have been employed to protect steel surfaces against versatile corrosion environments for a long time by introducing a barrier to prevent ionic transport and electrical conduction. Polybenzoxazines have the potential to apply as corrosion protective coating due to their unique properties such as low water absorption, low surface free energy, near-zero shrinkage, and excellent dielectric properties, which are superior to those of epoxy resins and conventional phenolics. Therefore, a novel silane-functional polybenzoxazine (PB-TMOS) with hydrophobicity in nature was applied as a corrosion protection coating material with superior adhesion at first. A series of novel composite material based on PB-TMOS were prepared and investigated in this dissertation.Firstly, PB-TMOS was prepared on mild steel (MS) through a dip-coating and thermal curing method. The coating properties were investigated by Fourier transform infrared spectroscopy (FTIR), solid-state nuclear magnetic resonance spectroscopy (29Si NMR), static contact angle measurement, electrochemical tests, and thermogravimetric analysis. Good adhesion was obtained between substrate and PB-TMOS coating due to the silane structure in PB-TMOS. The corrosion protection performance of PB-TMOS coated specimens was considerably enhanced by the high hydrophobicity and water resistance resulted from the dual cross-linking network of polybenzoxazine and Si-O-Si in PB-TMOS matrix. For the PB-TMOS coated sample, a reduction of5times on corrosion rate (CR)(6.80x10-3mm year"’) were observed comparing to that of the bare MS sample (3.43×10"2mm year-1) under the same conditions.Secondly, in order to improve the corrosion resistance of PB-TMOS, a series of nanocomposite coatings consisting of PB-TMOS and SiO2nanoparticles were developed for corrosion protection of MS. The influence of silica content on corrosion resistance of PB-TMOS/S1O2coatings was investigated by electrochemical measurements. The surface chemistry of nanoparticles and its effect on morphology of the PBS coating was also studied utilizing FTIR,29Si NMR and scanning electron microscopy (SEM). The results indicate that the presence of the covalent bond between nanoparticles and PB-TMOS, greatly improves the interfacial interactions at the polymer/filler interfaces resulting in a better corrosion performance. Role of nanoparticles in enhancing the corrosion resistance of PBS coating should be attributed to the synergistic effect of barrier property induced from SiCb nanoparticles in its matrix and anodic inhibition induced from passive Fe-silicate compounds. The synergistic effects made the corrosion resistance of PBS coatings was improved with the increase of the SiO2nanoparticles content at the range of1-5wt%. The CR value (3.43X10-4mm year"1) reduced respectively5times and2orders of magnitude for the sample with5wt%SiO2nanoparticles comparing to that of the PB-TMOS coated sample and bare MS.Thirdly, two types of nanoclays, Nanocor’s Nanomer I.44P and I.30P, at various concentrations were tested as nanofillers in PB-TMOS coatings applied on MS. Coated steel panels were characterized in terms of microstructure and corrosion performance in3.5wt%NaCl solution. Dimethyl dioctadearyl quaternary ammoniums (in I.44P) promoted an intercalated structure for composites, and the d-spacing value of nanoclay was increased by compositing with PB-TMOS. An improvement on corrosion protection abilities of PB-TMOS was also observed at all studied amounts, evidently due to a favorable aspect ratio and orientation of the clay I.44P. Addition of clay I.30P with octadearyl quaternary ammonium as pillaring agent yielded coatings where the nanoclays were relatively randomly oriented which introduced an exfoliated morphologies. The barrier and corrosion protection properties were clearly compromised for clay I.30P. For the sample with3wt%I.44P, corrosion rate (4.03×lO-6mm year-1) reduced3and4orders of magnitude compared to PB-TMOS and bare MS, respectively.Finally, additional cross-linking points were introduced into the composites by the reaction between the phenolic-OH groups of the ring-opened benzoxazine and epoxide groups. Epoxy (E-51) was copolymerized with PB-TMOS to improve corrosion performance without compromising the hydrophobic properties. Information on the structure of the hybrid systems (PBE) was received from FTIR analyses, which confirmed the presence of possible polymerization reactions. The results showed that hydrophobicity and corrosion protection properties of PBE absolutely depended on the PB-TMOS/E-51ratio (100/0-20/80), which influenced the hydrogen bonding network and hybrid coating compactness significantly. The ratio of PBE sample with best corrosion performance was observed at30/70of PB-TMOE/E-51,1and2orders of magnitude reduction of corrosion rate (1.46×10-4mm year-1) were obtained compare with PB-TMOS and bare MS, respectively.