Preparation and anticorrosion performance of graphene composited waterborne coatings

Waterborne coatings are safe and environmentally friendly and widely used in automobile, construction and other fields. The low hardness of films formed by water-based anticorrosion coatings and their poor anticorrosion effect can be resulted from the addition of hydrophilic substances, which make it easy for water penetration. In this experiment, glycidyl methacrylate (GMA), butyl acrylate (BA), methyl methacrylate (MMA), and acrylic acid (AA) were used as the polymerization monomers to enhance the hardness, adhesion, and film-forming properties of the coatings by introducing the epoxy bond of GMA in the synthesis. The appropriate formula ratios for the waterborne coatings were determined by conditioning the content of wetting agent, thickener, levelling agent, defoamer, and modified acrylic-based resin. Based on this formula, suitable additive ratios of graphene were determined by condition experiments. When the addition ratio of graphene was 0.6wt% - 0.8wt%, the obtained coatings exhibited satisfactory hardness, film-forming properties, strong adhesion and anticorrosive properties. The results of salt spray tests and potentiodynamic polarization experiments verified that graphene in a certain proportion can effectively block the penetration of water, so that the coating presented excellent properties.


Introduction
Currently, water-based coatings are mostly applied to interior and exterior wall coatings, metal coatings, and automotive coatings [1].In comparison to traditional solvent-based coatings, waterborne coatings are environmentally friendly [2].However, waterborne coatings have nonnegligible problems, e.g., poor dispersion stability and poor shear resistance [3], which influences their anticorrosion performance.
Graphene has good flexibility, electrical conductivity, and high strength [4].The atomic layer of graphene can be used as a protective layer to inhibit metal corrosion, which greatly improves the physical barrier effect of the coatings and prolongs the penetration paths of corrosive media [5].
Thereby, graphene exhibits wild application prospects in anticorrosion fields.In addition, cheap anticorrosion acrylic-based coating formulations need to be developed intensively.
In this paper, series acrylic components emulsion polymerization and resin emulsions were studied.Additives and graphene were added to formulate the coatings, and the preparation process was optimized.The resulting resin coatings and graphene composited coatings were examined for their basic properties.The optimal formulation was finally determined to achieve satisfactory anticorrosive performance.

Synthesis and formulation of waterborne acrylic coatings
Mix polyethylene glycol octyl phenyl ether (OP), sodium dodecyl sulfate (SDS), Tween-80 (ratio of 1:1:1) as a composite emulsifier, and take methyl glycidyl methacrylate (GMA), methacrylate (MMA), acrylic acid (AA) and butyl acrylate (BA) as composite monomers.Ammonium persulfate (APS) (1% of the total mass of the monomers) was dissolved in the appropriate amount of water as an initiator solution for polymerization, and finally the pH was adjusted to neutral to produce a waterborne acrylic emulsion.The typical waterborne acrylic emulsion was added different ratios of defoamer, thickener, wetting agent, and levelling agent to prepare coatings.The formed films were studied structural and physical properties e.g. the hardness and adhesion.The chosen acrylic-based coatings were investigated for the graphene addition condition experiments and a different amount of graphene results in different hardness, adhesion and anticorrosion properties.

Performance characterization
By testing the hardness and adhesion of four waterborne acrylic resin emulsions, suitable acrylic resin formula was found.The results indicated that the acrylic resin coating with the formula ratio of wetting agent, thickener, levelling agent and deformer at 2:4:2:1 present best hardness and adhesion performance among the pristine acrylic resin coatings; hence this formula ratio was selected for further comparison.Fig. 1 shows the formation film, hardness and adhesion performance of acrylic resin coatings with and without graphene.The pristine acrylic resin coating shows a flat film formation, while the content of the added graphene exhibits significant influence on the coatings' performance.As shown in Fig. 1b, graphene with a mass ratio of 0.6% performed strong hardness of H and high adhesion of 5B, which is much better than that of pristine acrylic resin coating (Fig. 1a).The corrosion resistance of the water-based coatings with different added mass ratios of graphene were measured by a salt spray corrosion testing machine and the tests were performed with 70 mm×150 mm sample plates (without scratches) for 0 h, 24 h, 48 h and 96 h, shown in Fig. 2. It plays a very good role in the anti-corrosion of the steel plate after adding graphene additives.It is seen that better anticorrosion effect is achieved when the mass ratio is 0.6-0.8wt%,and there is no large amount of corrosion phenomenon in the metal surface and its edges.The formula ratio of aqueous coatings greatly affects the performance of coatings, and the addition of graphene can effectively improve the anti-corrosion performance of waterborne acrylic coatings.
The corrosive environment was simulated with 3.5 wt% NaCl solution, and the corrosion resistance of various coating systems were further obtained by potentiodynamic polarization tests.The potentiodynamic polarization curves are shown in Fig. 3, and the corrosion potential (Ecorr) and corrosion current density (jcorr) of each sample were investigated by the Tafel extrapolation method.In general, high Ecorr and low jcorr values indicate that the sample exhibit low corrosion rate and is difficult to be corroded under the test condition [6].When the graphene mass ratio is 0.8%, the coating system has the smallest jcorr value of 2.78×10 -9 A/cm², indicating that the sample has the best corrosion resistance.As seen in Fig. 3, the graphene content has a significant effect on the corrosion resistance of the aqueous acrylic resin system.With the increase of graphene content, the corrosion parameters of the acrylic resin coatings show fluctuations, in which too high or too low graphene content will adversely affect the corrosion resistance of the waterborne coating system.That means it is related to the dispersion degree of graphene in the resin matrix.The above results imply that the Figure 2. Salt spray corrosion tests of water-based acrylic resin coatings with different graphene content of (1) 0.0 wt%, (2) 0.2 wt%, (3) 0.4 wt%, (4) 0.6 wt%, (5) 0.8 wt%, and (6) 1.0 wt% for respectively time of (A) 0 h, (B) 24 h, (C) 48 h and (D) 96 h.synergistic effect of graphene and aqueous acrylic resin can improve the coating performance and the addition of the appropriate amount of graphene effectively strengthens the corrosion resistance of the underlying metal substrate.