| Anticorrosive coatings, as the primary metal protection technique, have been attached great importance and get their rapid development recently. However, volatile organic compounds (VOCs) released from their production and application have been a major source of air pollution, causing adverse effects on the environment and human health. Increasing environmental pressures are forcing the anticorrosive coating industry to minimize the release of VOCs and result in a continuous shift from solvent-based coatings to waterborne coatings. As is well known, hydrophilic components with polar groups or ionic groups are necessary for water-soluble or hydrosol resins, in order to improve the water dispersibility and lower the VOCs of waterborne coatings. Nevertheless, these polar groups are believed to form polar channels for water/electrolyte penetration during the formation of waterborne films, accelerating the water uptake and deteriorating the corrosion resistance. Accordingly, the application of waterborne anticorrosive coatings is still limited. An optional method for the enhancement of the anticorrosive properties is to improve the crosslinking property of the waterborne coatings using crosslinkable polar groups. Chemical reactions between these polar groups not only facilitate crosslinks and thus enhance the physical and chemical integrity of the polymer matrixes, but also reduce the number of polar groups and thereby lower the water sensibility of the coatings, hindering the penetration and diffusion of water/electrolyte. Recently, few researches were conducted on the effects of polymer polarity on the coating performance and the protection mechanism for waterborne coatings. Clarifying these effects can provide theoretical basis for the formulation design and performance improvement of waterborne anticorrosive coatings, and deserves in-depth studies, In this work, the effects of resin molecular weight, polar group content and hydrophobic segments on the barrier performance of Epoxy-acrylic-grafted-copolymer (EA) waterborne coatings were studied in details. On the basis, the protection mechanism of the EA coatings was discussed. The proper additions of inorganic nanoparticles were investigated, as well as the anticorrosive properties of the EA resins/nanoparticles composite coatings. The main conclusions of the work are as follow:(1) A stable two-step esterification process was developed to control the chemical properties of the EA resins, consisting of the molecular weight, polar group content, and the hydrophobic segment. The process includes three stages:the first esterification of epoxy monomer and octanoic acid to produce epoxy-octanoic ester; free radical polymerization of acrylic monomers to produce acrylic polymer; and the second esterification between the epoxy-octanoic ester and the acrylic polymer to prepare the EA resin. The key technical point of the two-step process is to control the ring-opening extent of the epoxy monomer, in order to reserve one epoxy group unopened in the epoxy-octanoic ester that designed to react with acrylic polymers in the second esterification. Controlling the catalyst of0.2wt.%and the reaction temperature of105℃enables the implement of the technical point. The molecular weight (Mn) and glass transition temperature (Tg) of the reagents and product of the second esterification were investigated to confirm the preparation of homogeneous copolymer via this chemical grafting process.(2) Seventeen types of EA resins containing different molecular weights(2000～11000Da), Methacrylic acid (MAA) contents (8～27wt./%), and epoxy monomers (E-12and E-20) were synthesized by the two-step esterification process. The properties of the EA vanishes were determined. Accordingly, the effects of resin molecular weight, polar group content and hydrophobic segment on the coating performance were investigated. Increasing viscosity, increasing crosslinking degree and decreasing water absorption of the EA vanishes were found when the resin molecular weight increased. The corrosion resistance is also enhanced, as demonstrated by the better salt spray resistance and higher initial impedance. However, weaker anticorrosive properties were found in the EA coatings with the resin molecular weights higher than8000Da, due to their excessive viscosity. The proper molecular weight range of the EA resins was determined to be7000～8000Da. EA coatings with higher polar group contents demonstrated higher viscosity, crosslinking degree and adhesion strength. However, the coatings with excess polar groups (27wt.%MAA) showed poor anticorrosive properties demonstrating the highest water absorption, the rapid decreasing rate of impedance during the EIS test, and the appearance of more rust spots during the salt spray test. EA coatings with longer hydrophobic chains showed the lower viscosity, lower water absorption and better corrosion resistance. It was indicated that the anticorrosive performance was associated with the hydrophobic epoxy-alkyd chains. Longer epoxy-alkyd chains in the EA resins may provide stronger shielding effect on the polar groups, and enhance the water and corrosion resistance of the EA coatings. The optimal resins for the high-performance EA anticorrosive waterborne coatings have the molecular weight between7000Da and8000Da,20wt.%MAA content, and E-12epoxy monomer.(3) Thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC) were employed to investigate the absorbed water in the polymer matrixes and their extent of swelling of the EA vanishes with different polar group contents. Accordingly, the water preventing mechanism was analyzed. It was found that, the coating with low polar group content absorbed more free water than bound water; the "defects-caused water permeation channels" resulted in the water uptake. The coating with excess polar group content had high water absorption of both the free water and bound water; the water uptake was due to the "polar water permeation channels". The coating with optimal polar group content had much lower water absorption of both the free water and bound water, and resulted in degradation of the polymer matrix to a less extent. The water preventing mechanism is attributed to the favorable matrixes with tighter crosslinks provide by the optimal polar group content, and the stronger shielding effect provided by the longer hydrophobic chains. Therefore, the matrixes have less water permeation channels, both the defects-caused channels and the polar channels, and the water penetration and diffusion is hindered. The deterioration process of four EA coatings with different polymer polarity was investigated using electrochemical impedance spectroscopy (EIS). The evolution of Nyquist spectra was analyzed in details and the values of|Z|f=10mHz, Rc, Ret, Cc and fb as a function of immersion time were compared to obtained more characteristics of the degradation process. It was indicated that, EA resins with too low polarity resulted in low crosslinked polymer structures of the EA coatings and more defects-caused channels for electrolyte penetration accordingly; these channels were expanded during the immersion. Hence, the corrosion reactions occurred at early time produced more corrosion products accumulated on the metal surface. The weak adhesion strength permitted the diffusion of the corrosion products and caused the delamination and breakage of the coatings. EA resins with too high polarity resulted in less defects-caused channels but more polar channels formed in the EA coatings during the immersion. The corrosion reactions began later and produced less corrosion products.The stronger adhesion strength of the coatings limited the diffusion of the corrosion products and the delamination of the coatings did not happened accordingly.The protection mechanism of EA coatings with proper polymer polarity is attributed to the barrier performance of the polymer matrixes to the electrolyte penetration, and the shielding effect of the metal/coating interface to the charge transfer of the corrosion reactions. Tighter crosslinks and longer hydrophobic chains enable the barrier performance, and higher adhesion strength of the coatings benefits the shielding effect.(4) Ultrasound/alcoholizaiton dispersion process was emplyed to predisperse the TiO2and SiO2nanoparticles. The predispersed nanoparticles were added into the EA vanishes to prepare the composite coatings. The compatibility of the nanopartilces and the EA resins was investigated and the optimal additions of the nanoparticles were determined accordingly, as well as the protection mechanism of the composite coatings. It was found that, the protection property of the composite coatings was related to the EA resins/nanoparticles compatibility and the addition of nanoparticles. Coatings containing low contents of nanoparticles demonst- rated low adhesion strength, high water absorption and weak corrosion resistance. The poor anticorrosive performance was due to the insufficient addition of nanopartilces. Coatings with high contents of nanoparticles also showed weak anticorrosive properties during the long-term immersion. Particle agglomeration occurred in the coatings and experienced the position shift in the swollen polymer matrixes, resulting in the generations of defects channels for water/electrolyte permeation. Accordingly,"the second absorption process" of water uptake appeared. The optimal addition of nano-TiO2was3wt.%and that of nano-SiO2was3-4.5wt.%. Coatings containing the proper contents of nanoparticles demonstrated higher adhesion strength, better water and corrosion resistance. The protection mechanism is attributed to the enhanced water/electrolyte preventing mechanism and the improved adhesion strength provided by the nanoparticles. |
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