Double-Layer Coating Films Prepared from Water ... - ACS Publications

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Double-Layer Coating Films Prepared from Water-Borne Latexes of Acrylate−Vinylidene Chloride Copolymers: Investigating Their Heavy-Duty Anticorrosive Properties Ce Fu,† Tong-Xian Zhang,† Fa Cheng,† Wen-Zhu Cui,‡ and Yu Chen*,† †

Department of Chemistry, School of Sciences, Tianjin University, and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, P. R. China ‡ COSCO Kansai Paint & Chemicals Co., Ltd., No. 42, 5th Avenue TEDA, 300457 Tianjin, P. R. China S Supporting Information *

ABSTRACT: Aqueous latexes of copolymers of vinylidene chloride (VDC) with an acrylate, namely, methyl acrylate (MA), ethyl acrylate (EA), butyl acrylate (BA), or 2-ethylhexyl acrylate (EHA), were employed to form a double-layer coating film for the heavy-duty anticorrosion of metal. Measurements of the water-vapor transmission rate and oxygen-gas transmission rate and electrochemical impedance spectroscopy (EIS) demonstrated that the barrier properties of MA−VDC and EA−VDC films were better than those of BA−VDC and EHA−VDC films. Among the MA−VDC and EA−VDC coatings, EA−VDC85 showed better comprehensive properties and, thus, was selected as the top layer of the designed double-layer coating film. Adhesion tests demonstrated that the BA−VDC and EA−VDC coatings had better adhesion to tinplates than the MA−VDC and EA−VDC coatings and, thus, were suitable for use as the bottom layer of the designed double-layer coating film. The characterizations of the double-layer coating films by adhesion tests and EIS showed that BA−VDC75 was the optimal bottom layer. Under harsh salt-spray corrosion conditions, the optimal double-layer coating film (ca. 50-μm thickness) with EA−VDC85 as the top layer and BA−VDC75 as the bottom layer could protect tinplate well for at least 800 h, and the adhesion of the coating film to the tinplate was still excellent even after 1000 h of corrosion. Scanning electron microscopy, differential scanning calorimetry, and Fourier-transform infrared spectroscopy were used to evaluate the corroded double-layer coating films. very poor. Moreover, PVDC is unstable and poor in film formation.24 Consequently, PVDC itself cannot act as a heavyduty anticorrosive coating material. To date, studies of copolymer latexes of vinylidene chloride (VDC) with (meth)acrylate, acrylonitrile, or vinyl chloride have focused on improving film formation and the stability of the formed film.25,26 Such materials are targeted for applications as coatings for paper, fibers, or packaging.27−31 Certain patents32,33 have claimed that acrylate−VDC copolymer latexes can be used as the polymer resin to mix with other materials, such as pigment dispersions and corrosion inhibitors, to form water-borne paints. Such paints can be used to protect metals. In such acrylate−VDC copolymers, more than two types of acrylates are concurrently copolymerized with VDC, and the function of each acrylate unit has been never elucidated. In a previous work, to clarify the contribution of each acrylate unit to the anticorrosion performance of acrylate−VDC copolymers, we prepared a series of aqueous latexes of binary acrylate−VDC copolymers and systematically studied the influences of the content and type of acrylate unit in the copolymers on their anticorrosive performances.34 Unlike the patents,32,33 we evaluated the anticorrosion performances of only the pure copolymers, not the paints composed of the copolymers. Under

1. INTRODUCTION In high-humidity or underwater environments, heavy-duty anticorrosive coating materials are required for metal protection.1 Normally, these heavy-duty anticorrosive coating materials need at least two characteristics. One is good barrier properties against corrosive substances.2,3 The other is tight adhesion to metal substrates,4−6 because sufficient adhesion can protect the substrate more effectively, especially under humid conditions, where water can make the adhesion lose efficacy. Furthermore, occasional scratches and abrasions on the coating are normally unavoidable, and tight adhesion can prevent or retard peeling of the coating from the metal surface. Water-borne coating materials have attracted great interest in both academic and industrial research because of the increasing requirements for environmental protection.7−14 However, water-based coating materials usually exhibit poorer protection performances than corresponding solvent-based materials.15 Technological advances have produced water-borne epoxy resins with high performances, comparable to those of their solvent-borne counterparts.16 Aqueous latexes of poly(vinylidene chloride) (PVDC) or its copolymers can be synthesized by emulsion polymerization,17−20 and they are much less expensive than other waterborne coating materials applicable in heavy-duty anticorrosion, such as epoxide resins. PVDC films have excellent barrier properties,20−23 better than those of nylon, ethylene vinyl alcohol, poly(ethylene terephthalate), and low-density polyethylene.23 However, the adhesion of PVDC films to metals is © 2014 American Chemical Society

Received: Revised: Accepted: Published: 4534

October 10, 2013 February 23, 2014 February 24, 2014 February 24, 2014 dx.doi.org/10.1021/ie403396e | Ind. Eng. Chem. Res. 2014, 53, 4534−4543

Industrial & Engineering Chemistry Research

Article

a reflux condenser, and a thermometer. Aqueous SGN-30 (15.4%, 11.0 g) was added first, followed by half of an initiating system composed of 3.3 g of aqueous (NH4)2S2O8 (9.1%), 3.3 g of aqueous Na2S2O5 (9.1%), and 1.0 g of aqueous FeSO4· 7H2O (0.23%). The prepared emulsion solution was then added dropwise to the flask at a constant rate for 2.5 h at 30 °C. After all of the emulsion solution had been introduced, polymerization was performed at 30 °C for 4 h. The other half of the initiating system was then added, and polymerization was carried out at 40 °C for 2 h to complete the reaction. The solid content of the mixture was determined according to ASTM D1489.35 Without filtration, the emulsions were characterized directly. 2.4. Preparation of Monolayer Coating Film. BYK-348 (about 1%) was added to the latex solution under vigorous stirring for 30 min to improve substrate wetting. After being filtered with a 0.125-mm fiber mesh, the mixture was centrifuged at 3000 rpm for 10 min to remove bubbles. A wet film of the latex, with a thickness of 150 μm, was cast with a mold on a tinplate sheet abraded with 240-grit silicon carbide paper to eliminate mechanical interlocking effects. The periphery of the tinplate was coated with more latex. Because of the different minimum film-formation temperatures (MFFTs), the coating films were placed in an oven at 60 °C for 0.5 h and then dried at room temperature for 48 h. The films were held in the oven at 60 °C for 4 h before determination of their properties. Dry coating films with a thickness of 50 ± 2 μm were applied to tinplate sheets. For measurements of oxygen-gas transmission rate (OGTR), watervapor transmission rate (WVTR), and mechanical properties, dry coating films with a thickness of 150 ± 5 μm (wet films with a thickness of 500 μm) made on glass panels were utilized. 2.5. Preparation of Double-Layer Coating Film. A monolayer coating film was first prepared on a tinplate sheet abraded with 240-grit silicon carbide paper as described above. The coating film was placed in an oven at 60 °C for 0.5 h and then dried at room temperature for 24h. Subsequently, another layer was coated similarly. The dry double-layer coating films were 50 ± 2 μm in thickness. 2.6. Characterization. Fourier-transform infrared attenuated total reflectance (FTIR-ATR) spectroscopy (Shimadzu FTIR-8400S) was used to examine the dry films. Gel permeation chromatography (GPC) of the samples was carried out using a Viscotek GPC270 system. Freshly distilled tetrahydrofuran (THF) was filtered through a membrane with an average pore size of 0.22 μm and used as an eluent at 30 °C. The flow rate was set at 1.0 mL min−1. The latex particle diameter and size distribution were measured using a Malvern Mastersizer 2000 instrument. Scanning electron microscopy (SEM; Philips XL-30) was used to observe the film/tinplate interface. Differential scanning calorimetry (DSC) measurements were carried out using a Perkin-Elmer DSC8000 instrument in the temperature range from −40 to 180 °C at a heating rate of 20 °C/min. The glass-transition temperature (Tg) was determined as the median of the glass-transition range of the second heating process. Mechanical properties of the films were examined using a materials testing machine (EZ TEST, Shimadzu) with an elongation speed of 20 mm min−1 at 23.5 °C and a relative humidity of 56.5%. OGTRs of the films were measured at room temperature and 23% relative humidity, using a Systech Illinois model 8001 oxygen permeation analyzer. WVTRs of the films were measured at 37.8 °C and 90% relative humidity, using a Systech Illinois model 7002

harsh salt-spray corrosion conditions, the optimized binary acrylate−VDC copolymer film with a thickness of ca. 50 μm could protect tinplate from loss of metallic luster for at least 170 h, but the tinplate lost its metallic luster after 250 h. To improve the anticorrosive performance of the polymer coating film, an effective method is the addition of functional materials, such as corrosion inhibitors32 and conducting polymers.10,13 With the help of these functional materials, certain water-borne epoxides have been developed to satisfy the requirements of heavy-duty anticorrosion. For instance, an aqueous epoxy coating (thickness of 90 ± 10 μm) containing the conducting polyaniline/partially phosphorylated poly(vinyl alcohol) spherical nanoparticles was found to protect mild steel from corroding for 720 h under harsh salt-spray corrosion conditions.10 Bagherzadeh et al. reported that the surfaces of steel samples coated with a model water-based epoxy coating (thickness of 60 ± 5 μm) corroded seriously after 650 h of exposure to the salt-spray medium, whereas the addition of nanopolyaniline markedly improved its anticorrosion performance.13 Compared with the anticorrosive performances of the water-borne expoxides described above, that of water-borne binary acrylate−VDC copolymers is far worse. Therefore, the main target of this study was improving the anticorrosive performance of water-borne binary acrylate−VDC copolymers. In this work, we did not employ the traditional method of adding functional additives, but rather elucidated the anticorrosion functions of each type of acrylate unit in the binary acrylate−VDC copolymers and used the findings to prepare double-layer coating films. After the type and content of acrylate units in the top and bottom layers had been optimized, the obtained new coating film showed performance that was not only superior to that of the corresponding monolayer coating film formed by water-borne binary acrylate− VDC copolymers but also comparable to that of films based on water-borne epoxides with functional additives.

2. EXPERIMENTAL SECTION 2.1. Materials. Vinylidene chloride (VDC) was purchased from Tianjin Daguchem and distilled at 35 °C before use. Methyl acrylate (MA, 98%), ethyl acrylate (EA, 98%), butyl acrylate (BA, 98%), ammonium persulfate (AR), sodium metabisulfite (AR), and ferrous sulfate (AR) were purchased from Tianjin Kewei Chemical Company and used directly. 2Ethylhexyl acrylate (EHA, 98%) was purchased from Alfa Aesar and used directly. Acrylic acid (99.5%) was purchased from the Damao Chemical Reagent Factory and used directly. The commercially available emulsifiers SGN-20 and SGN-30, mixtures of anionic and nonionic surfactants, were purchased from Tianjin Hezhiyongtai Company. BYK-348, used as an additive, was purchased from Altana. 2.2. Nomenclature. Acrylate−VDC represents the latex of binary acrylate−VDC copolymer. Acrylate−VDCX represents the binary acrylate−VDC copolymer, where X is the weight percentage of VDC units. 2.3. Preparation of Latex by Emulsion Polymerization. A total of 86.7 g of a mixture of VDC, acrylate, and acrylic acid (3.3%) was added to 50.0 g of aqueous SGN-20 (3%) in a 250 mL round-bottom flask under vigorous stirring at 5 °C to form an emulsion. The mixture was stirred for 1 h in a closed system sealed with a glass stopper to prevent VDC from volatilizing. The polymerization was performed under a nitrogen atmosphere in a 250 mL four-necked flask equipped with a Teflon mechanical stirrer, a constant-pressure dropping funnel, 4535

dx.doi.org/10.1021/ie403396e | Ind. Eng. Chem. Res. 2014, 53, 4534−4543


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copolymers to be modulated in opposite ways. To satisfy the requirements of these two factors simultaneously, the acrylate content and type of the copolymer should be limited to a narrow range. Among these acrylate−VDC coatings, BA− VDC80 exhibited the best potential for use in heavy-duty anticorrosive applications. However, its performance was still inferior to that of the water-borne epoxides. Therefore, it is clear that it is difficult for such a monolayer coating film to achieve both excellent adhesion and barrier performance under harsh corrosion conditions. In this study, we designed a doublelayer coating film. In such a film, the bottom layer that directly contacts the metal is mainly responsible for the good adhesion to metal, whereas the top layer is responsible for the good barrier performance. The VDC contents of the acrylate−VDC copolymers utilized here were in the range of 65−90%. The type and content of acrylate units in the copolymers have a significant influence on their film-formation ability. For BA−VDC and EHA−VDC, good films can be prepared conveniently at room temperature because of their low MFFTs, which are similar to their Tg values (Table S2, Supporting Information). Moreover, BA− VDC and EHA−VDC films are flexible, as a result of their low Young’s moduli and high tensile strains (Table S2, Supporting Information). MA−VDC and EA−VDC are hard polymers, as indicated by their higher Tg values, high Young’s moduli, and low tensile strains, especially those with lower VDC contents (Table S2, Supporting Information). The MFFTs of EA−VDC copolymers are similar to their Tg values, whereas the MFFTs of MA−VDC copolymers are higher than their Tg values (Tables S2, Supporting Information). The MFFTs of MA− VDC and EA−VDC copolymers are normally higher than room temperature. Among the many acrylate−VDC copolymers examined in this work, MA−VDC65 was found to have the highest MFFT of 50 °C. For the effective comparison of different coating films, the film-formation temperatures for all of these acrylate−VDC copolymers were set at 60 °C. First, the acrylate−VDC coating films suitable for the top layers were explored. The barrier performances of the formed coating films against the main corrosive materials, H2O and O2, were measured. Because certain coating films adhered strongly to tinplate, it was difficult to tear them off the tinplate surface to obtain a piece of intact film for the measurement of WVTR and OGTR. Therefore, intact films for the measurements of WVTR and OGTR were made on glass panels. MA−VDC65, MA− VDC70, and all acrylate−VDC90 films suitable for WVTR and OGTR measurements could not be made because they were so brittle that no intact films could be obtained. The WVTR and OGTR data (Table S3, Supporting Information) of other acrylate−VDC films show that reducing the VDC content of the films generally led to poorer barrier performances against H2O and O2. Moreover, MA and EA were found to be better than BA and EHA in providing acrylate−VDC films with comprehensive barrier capacities against H2O and O2. For instance, the WVTR values of MA−VDC85, MA−VDC80, MA−VDC75, EA−VDC85, and EA−VDC80 films were all below 2 g m−2 day−1, and their OGTR values were all close to 0 mL m−2 day−1. In contrast, the WVTR values of all of the BA− VDC and EHA−VDC films were more than 2 g m−2 day−1, and their OGTR values were more than 20 mL m−2 day−1. According to the WVTR and OGTR data, the barrier ordering of the films with good barrier properties is MA−VDC85 > MA−VDC80 > EA−VDC85 > MA−VDC75 > EA−VDC80.

water vapor transmission analyzer. The experiments lasted for 24 h. The films were cut into 33 mm × 33 mm specimens before use. MFFTs were measured at room temperature and 58% relative humidity, using a model QMB instrument from Tianjin Yonglida Material and Testmart Company. Contact angles were measured on a model JC2000C1 optical contactangle analyzer from Shanghai Zhongchen Digital Technic Apparatus Company. Anticorrosion measurements were performed in a salt-spray testing chamber (Q-FOG Cyclic Corrosion Tester) with 5% aqueous NaCl at 35 °C, according to ASTM B117-07.36 The adhesion strength of the coating to the tinplate was measured using the cross-hatch tape test, according to ASTM D3359-08.37 The coating adhered to the tinplate was cut into 100 grids of about 0.01 cm2 in area. Transparent tape from 3M (catalog no. 600) with a width of about 25 mm was adhered to the grids. Then, the tape was removed to see the damaged grids in each sample. According to the evaluation indicators of ASTM D3359-08, the adhesion strength is divided into six grades denoted as 5B, 4B, 3B, 2B, 1B, and 0, which indicate that the peeling area is 0%, 65%, respectively. According to the evaluation indicators of ASTM D714-02,38 blister size is divided into five grades denoted as 10, 8, 6, 4, and 2, in which 10 represents no blisters, 8 represents the smallest-size blisters, and 2 represents the largest-size blisters. The blister frequency standard has four grades: D, dense; MD, medium dense; M, medium; and F, few. The rust creepage from scribe was investigated according to ASTM D1654-08.39 On the basis of corrosion expansion from the scribe, samples are divided into 11 grades, where 10 represents no progression of corrosion and grades of 9−0 represent progressively severe corrosion areas around the scribe. Electrochemical impedance spectroscopy (EIS) was used to evaluate the coatings’ barrier properties. Tinplates coated with the films and subjected to different exposure times in the salt-spray chamber were immersed in 3.0 wt % NaCl solution. A three-electrode system was used, in which a saturated calomel electrode acted as the reference electrode, a graphite rod acted as the counter electrode, and the coated sample was the working electrode. The exposed area of each sample was 1.0 cm2. EIS measurements were performed at the open-circuit potential in the frequency range from 0.01 Hz to 100 kHz using a sinusoidal signal of 20 mV. The measurements were conducted at room temperature using a Zennium workstation from Zahner-Elektrik.

3. RESULTS AND DISCUSSION According to our previous work,34 a series of aqueous latexes were synthesized by binary emulsion copolymerization of VDC with an acrylate, namely, MA, EA, BA, or EHA. The properties of these latex solutions were as follows (Table S1 in the Supporting Information): (1) solid contents were 56%−59%; (2) viscosities were less than 30 mPa s; (3) pH values were 2− 3; (4) average latex particle diameters were in the range 104− 130 nm, with particle size distributions of around 0.26; and (5) molecular weights of all the samples were more than 105. In our previous publication,34 such acidic latex solutions were directly deposited onto tinplate sheets to form dry-coated monolayer films with a thickness of 50 ± 2 μm. Barrier properties against corrosive substances and adhesions to metal substrates were two basic properties employed to evaluate their performances primarily under harsh salt-spray corrosion. However, it was found that good adhesion and good barrier performance required the content and type of acrylate units in the 4536



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Figure 1. Impedance plots of the tinplates coated with monolayer acrylate−VDC films (ca. 50-μm thickness) after 400 h of salt-spray corrosion.

The barrier properties of coating films (ca. 50-μm thickness) directly deposited onto tinplates were further measured by EIS. Generally, a Bode plot can be used to investigate the impedance nature of coatings. Higher impedance values at lower frequencies represent better anticorrosive coating films for metals. Before corrosion, the impedance values of all of the samples were higher than 1 × 1010 Ω, and their differences were minor. Figure 1 shows Bode plots of tinplates covered with acrylate−VDC coatings after 400 h of salt-spray corrosion. The impedance values of the tinplates covered with semicrystalline copolymers,34 including BA−VDC85, EHA−VDC85, EHA− VDC80, and all of the acrylate−VDC90 materials, were on the order of 102 Ω, similar to that of pure tinplate, indicating that these coatings completely lost their protection for the metal. For the other amorphous acrylate−VDC copolymers, their protection performances for the tinplates decreased with decreasing VDC content. Furthermore, after 400 h of saltspray corrosion, the impedance values of the tinplates covered with BA−VDC or EHA−VDC coatings were all below 107 Ω, whereas the impedance values of the tinplates covered with MA−VDC85, MA−VDC80, MA−VDC75, EA−VDC85, and EA−VDC80 coatings were still above 107 Ω. These results also demonstrate that MA−VDC and EA−VDC are better barrier coatings for protecting metals than BA−VDC and EHA−VDC. According to the EIS measurements, the barrier ordering of these five coatings is MA−VDC85 > EA−VDC85 > MA− VDC80 > EA−VDC80 > MA−VDC75. Why do the different acrylate units endow the corresponding acrylate−VDC coating films with such different barrier properties? A possible explanation is tentatively discussed as follows: It is known that acrylate units affect copolymer chains

in two ways. One is the polar interactions between VDC and acrylic ester units, which lead to stiffening of the polymer chains and, thus, to an increase in Tg.40 The other is the plasticizing effect, which enhances the mobility of the polymer chains and, thus, results in a decrease of Tg.41 Among these four acrylate monomers, the longer the hydrophobic ester side chain, the stronger the plasticizing effect. In the copolymers with acrylate units containing long ester side chains, such as EHA and BA, the plasticizing effect was pronounced, as reflected by low Tg values and flexible films with low Young’s moduli and high tensile strains. It is easier for the main corrosive molecules, O2 and H2O, to pass through more flexible films. Therefore, all of the BA−VDC and EHA−VDC copolymer coating films showed poor barrier properties. For the MA−VDC and EA− VDC copolymers, the ester side chains of the MA and EA units were very short, and their plasticizing effects were weak. The polar interactions between these acrylate units and VDC segments were therefore dominant, as reflected in high Tg values. The copolymer chains were stiffened upon the introduction of MA or EA units; thus, the formed films were hard and dense, as reflected in their high Young’s moduli and low tensile strains, which restricted the passage of O2 and H2O. It is known that PVDC films have excellent barrier properties;20−23 correspondingly, the MA−VDC and EA−VDC copolymers with higher VDC contents were found to exhibit better barrier properties than those with lower VDC content. The copolymers with the highest VDC content of 90% exhibited the poorest barrier properties, which can be ascribed to two possible reasons: (1) the latexes with VDC contents of 90% were not stable, leading to the inhomogeneity of the 4537



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Figure 2. Adhesion of acrylate−VDC coatings to tinplate (A) before and (B) after being corroded in a salt-spray chamber for 100 h (ASTM D335908; grades 5B, 4B, 3B, 2B, 1B, and 0 indicate that the peeling areas of the coating on the metal surface are 0, 65%, respectively).

formed films; , and/or (2) the formed films were very brittle and thus easily broken. Among the five acrylate−VDC coatings with good barrier properties, EA−VDC85 exhibited the best comprehensive properties. For instance, EA−VDC85 film was not as hard as the others, as reflected in its lower Young’s modulus and higher tensile strain (Table S2, Supporting Information). Moreover, EA−VDC85 showed a better film-forming capability because its Tg and MFFT were both below room temperature, whereas the Tg and MFFT values of the others were all above 25 °C (Table S2, Supporting Information). Therefore, in this work, EA− VDC85 was chosen as the top layer of the designed doublelayer coating films. Subsequently, the acrylate−VDC coatings suitable for the bottom layers by virtue of their good adhesion to metals were explored. The adhesion of acrylate−VDC coating film to tinplate was measured using the cross-hatch tape test, according to ASTM D3359-08.37 The higher the adhesion grade, the stronger the adhesion between the metal and the coating material. Figure 2A shows that all of the MA−VDC coatings had nearly no adhesion to tinplate. The initial adhesion of EA− VDC coatings to tinplate increased with decreasing VDC content, and the highest grade, 5B, was achieved by only EA− VDC65. In contrast, the BA−VDC and EHA−VDC coatings exhibited excellent adhesion to tinplate when the VDC content was within the wide range of 65−85%. Figure 2B shows the adhesion of these acrylate−VDC coatings to tinplate after the samples had been corroded by salt-spray for 100 h. It is clear that all of the MA−VDC and EA−VDC coatings adhered poorly to tinplate. The EHA−VDC coatings maintained their good adhesion only when the copolymers had a relatively low VDC content (65−75%). Compared with other coatings, the BA−VDC coatings maintained good adhesion over a wider range of VDC contents. At a VDC content of 85%, its adhesion grade decreased from 5B to 3B, unlike the others, whose adhesion grades fell to 0. In the VDC content range of 65− 80%, the adhesion grades of the BA−VDC coatings were still the highest, grade 5B. It can therefore be concluded that the incorporation of acrylate units bearing a relatively long and hydrophobic ester side chain into the PVDC main chains can effectively enhance the adhesion of coatings to metal when the content of acrylate units reaches a certain extent. Why do the different acrylate units endow the acrylate−VDC coating films

with such different adhesion properties? A possible explanation is tentatively provided as follows: Adhesion can be affected by many factors, such as the wetting ability of the copolymer latex on the surface of the metal, flash corrosion, and mechanical properties. First, consider the wetting abilities of all of the acrylate−VDC emulsions. The contact angles of all of the acrylate−VDC emulsions on tinplate were measured to be in the range of 53−55°, indicating that they had similar wetting abilities. Their similar wetting abilities might be ascribed to the same emulsifiers and functional units on the surface of the acrylate−VDC latexes. Then, the flash corrosion situations of the tinplates covered by the acrylate−VDC coatings were measured by optical microscope, and small bubbles could be observed in the dry coating films (Figure S1, Supporting Information), demonstrating the existence of flash corrosion. After the coating films had been peeled off the tinplates, all of the tinplate surfaces still exhibited metal luster, and no dark rust could be observed, indicating that no flash rust was formed after these acrylate−VDC coating films were cast onto the tinplates. After the different influences of wetting ability and flash corrosion on adhesion had been excluded, the different adhesions exhibited by different acrylate−VDC coatings could be correlated to their different acrylate units that endow different mechanical and thermal properties to the coating films. When the VDC content was no more than 85%, the MA−VDC and EA−VDC copolymers were amorphous; however, these MA−VDC and EA−VDC copolymer films were very brittle or hard due to the strong polar interactions among polymeric chains, which stiffened the polymer chains. Therefore, the MA−VDC and EA−VDC coating films showed high Young’s moduli, low tensile strains (Table S2, Supporting Information), and high Tg and MFFT values (Table S2, Supporting Information). The hard nature of these copolymers lowered their ductility and penetration ability into the unsmooth metal surface. Therefore, the contact of MA−VDC and EA−VDC coatings with the metal surface was not sufficient, and not enough polymer−metal interlocking joints formed, resulting in the poor adhesion of MA−VDC and EA− VDC coatings to tinplates. The acrylate units with longer hydrophobic side chains, such as BA and EHA, have good plasticizing effects, reflected in the low Young’s moduli, high tensile strains, and low Tg and MFFT values (Table S2, Supporting Information) of the corresponding acrylate−VDC 4538



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Figure 3. Adhesion to tinplate after different periods of harsh corrosion of coating films with EA−VDC85 as the top layer and (A) BA−VDC or (B) EHA−VDC as the bottom layer (ASTM D3359-08; grades 5B, 4B, 3B, 2B, 1B, and 0 indicate that the peeling areas of the coating on the metal surface are 0, 65%, respectively).

Figure 4. Impedance plots of the tinplates covered by double-layer coating films with EA−VDC85 as the top layer and (A) BA−VDC or (B) EHA− VDC as the bottom layer after 400 h of salt-spray corrosion (ca. 50-μm thickness).

copolymers. The flexible nature of BA−VDC and EHA−VDC allows them to spread efficiently on the unsmooth metal surface and form sufficient polymer−metal interlocking joints, resulting in the good adhesions of BA−VDC and EHA−VDC coatings to tinplates. After harsh corrosion, the adhesions of EHA− VDC80, EHA−VDC85, and BA−VDC85 became poor, which can be ascribed to their semicrystalline nature.34 It is known that the lowest VDC contents for crystallization of the copolymers are around 80% and 75% for BA−VDC and EHA−VDC, respectively,34 indicating that BA−VDC copolymers in a wider range of VDC contents are in the amorphous state or that the crystallinity of BA−VDC copolymers is less than that of EHA−VDC copolymers with the same VDC content. Therefore, BA−VDC copolymers can maintain good adhesion to the metal over a wider range of VDC contents. From the above experimental results, it can be concluded that BA−VDC and EHA−VDC copolymers with VDC contents of no more than 80% are suitable for use as the bottom layer of the designed double-layer composite film because of their good adhesion. With EA−VDC85 as the top layer and BA−VDC or EHA−VDC having a VDC content in the range of 65−80% as the bottom layer, double-layer coating films were formed on tinplate. As with the monolayer films, the total thickness of the dry double-layer films was also kept at ca. 50 μm. The thickness ratio of top/bottom layers was kept at

1:1. Figure 3 shows the adhesion of such double-layer coating films after different periods of harsh corrosion in the salt-spray chamber. The films with BA−VDC or EHA−VDC having VDC contents in the range of 65−75% as the bottom layers always showed good adhesion to tinplate even after 800 h of harsh saltspary corrosion, whereas the coating films with BA−VDC80 and EHA−VDC80 as the bottom laters lose their adhesions to tinplates after 400 and 100 h, respectively, of harsh corrosion. The barrier properties of double-layer coating films with EA−VDC85 as the top layer and BA−VDC or EHA−VDC as the bottom layer were measured by EIS (Figure 4). To obtain good adhesion, the VDC content of the bottom layer was controlled in the range of 65−75%. Before the harsh corrosion, the impedance values of all of the samples were higher than 1 × 1010 Ω. The differences between the different samples were very small. After 400 h of harsh salt-spray corrosion, their protection performances for tinplates decreased with reducing VDC content, just as for the monolayer coatings. Moreover, BA−VDC as the bottom layer was found to be better than EHA−VDC in favoring the barrier properties of the doublelayer coatings. For instance, among the many double-layer coatings, the coating film with BA−VDC75 as the bottom layer exhibited the highest impedance value of 6.8 × 107 Ω at lower frequencies after 400 h of harsh corrosion, whereas the impedance value was 3.2 × 107 Ω for the coating film with 4539



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EHA−VDC75 as the bottom layer. Therefore, BA−VDC75 was chosen as the optimal bottom layer of the designed doublelayer coating films. Figure 5 shows EIS plots of tinplates protected by the optimized double-layer coating film after different times of

Table 1. Corrosion Situation around the Scribe in DoubleLayer Coating Films after Different Periods of Salt-Spray Corrosion creepage from scribe

blister

corrosion time (h)

distance (mm)

rating numbera

size ratingb

frequencyc

adhesion graded

0 400 800 1000

0 0−0.5 0−0.5 0.5−1

10 9 9 8

10 10 10 10

none none none none

5B 5B 5B 5B

a

10 represents no progression of corrosion, and 9−0 represent progressively more severe corrosion areas around the scribe. bBlister size is divided into five grades, namely, 10, 8, 6, 4, and 2, in which 10 represents no blisters, 8 represents the smallest-size blisters, and 2 represents the largest-size blisters. cBlister frequency standard has four grades: D, dense; MD, medium dense; M, medium; and F, few. d Adhesion strength is divided into the six grades 5B, 4B, 3B, 2B, 1B, and 0, which indicate that the peeling area is 0%, 65%, respectively.

comparable to those of films based on water-borne epoxides with functional additives.10,13,32 SEM was used to visualize the interfaces of the double-layer coating films and tinplates before and after corrosion in saltspray for 800 h (Figure 7). As shown in Figure 7A, before being corroded, the surfaces of the tinplate and coating film make tight contact. After 800 h of harsh corrosion, obvious crevices in the interface zone appeared; however, many polymer joints still existed in the interface. This is why the adhesion of the coating film to the tinplate was still very good after 800 h of corrosion. The thermal properties of the corroded coating film with EA−VDC85 as the top layer and BA−VDC75 as the bottom layer were analyzed by DSC (Figure 8). The Tg values of the BA−VDC75 and EA−VDC85 copolymers were found to be 12.4 and 18.0 °C, respectively. The Tg value (15 °C) of the double-layer coating film falls between those of BA−VDC75 and EA−VDC85, and two glass-transition regions are not obvious in the DSC diagram, which might be due to the small Tg difference between BA−VDC75 and EA−VDC85. Compared with that obtained before corrosion, the DSC diagrams measured after 400 and 800 h of corrosion did not show any obvious differences. This implies that the double-layer coating film was relatively stable under harsh salt-spray corrosion. FTIR-ATR spectroscopy was used to monitor the changes in the air and tinplate sides of the double-layer coating film during corrosion (Figure 9). We mainly compared the changes in the peaks at around 3418 cm−1 (OH of COOH and water), 1720 cm−1 (ester carbonyl), 1640 cm−1 (COO− carbonyl), and 1062 and 1040 cm−1 (crystallinity of PVDC segments). After 800 h

Figure 5. Impedance plots of the tinplates covered by double-layer coating films with EA−VDC85 as the top layer and BA−VDC75 as the bottom layer after different periods of salt-spray corrosion (ca. 50-μm thickness).

exposure to salt-spray corrosion. The initial impedance value was above 1 × 1010 Ω. After 400, 800, and 1000 h under harsh corrosion, this value decreased to 6.8 × 107, 6.3 × 106, and 2.3 × 105 Ω at lower frequencies. The corrosion situation of tinplate covered by the optimized double-layer coating film was further observed. A scribe was made in the coating film before the corrosion test (Figure 6). Rating of corrosion at the scribe is reported according to ASTM D1654 (Table 1), and the blister size and frequency around the scribe are reported according to ASTM D714 (Table 1). After 800 h of harsh corrosion, the corrosion in the scribe progressed slightly, and the corrosion rating was 9, only a little less than the best value of 10. Moreover, no obvious blisters could be observed around the scribe. When the harsh corrosion time was prolonged to 1000 h, the corrosion around the scribe progressed further, and the rating decreased to 8. In addition, obvious blisters could not be observed around the scribe. The adhesion of this optimized double-layer composite film still maintained the best grade of 5B to tinplate even after 1000 h of harsh corrosion (Table 1). The above results demonstrate that the optimized doublelayer coating films showed performances that were not only superior to those of the previous monolayer coating films formed by water-borne acrylate−VDC copolymers, but also

Figure 6. Corrosion progress around scribes on the tinplate covered by double-layer coating films with EA−VDC85 as the top layer and BA− VDC75 as the bottom layer after different periods of salt-spray corrosion (ca. 50 μm thickness). 4540



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Figure 7. SEM images of lateral views of the contact interface of the tinplate surface and the coating film with EA−VDC85 as the top layer and BA− VDC75 as the bottom layer (A) before and (B) after corrosion in a salt-spray chamber for 800 h.

metal-catalyzed hydrolysis. Moreover, the peak at around 1042 cm−1 became a little stronger, whereas that at around 1066 cm−1 became weaker, which indicates that the crystallinity of the metal-side surface of the coating became higher after 800 h of corrosion.

4. CONCLUSIONS Double-layer coating films with excellent anticorrosive properties for metals were prepared from aqueous latexes of binary acrylate−VDC copolymers. Acrylates with short ester side chains, such as MA and EA, could endow the coating films better barrier properties and were thus suitable to be used as the top layer. Moreover, higher VDC contents, but less than 90%, in the acrylate−VDC copolymer were found to be preferrable for better barrier properties. The acrylates with long ester side chains, such as BA and EHA, could endow the coating films with better adhesion to metal and were thus suitable for use as the bottom layer. The overall adhesion performances of the films with EHA−VDC as the bottom layer were inferior to those of films with BA−VDC as the bottom layer. The bottom layers with higher VDC contents, but less than the amount leading to the partial crystallization of the copolymer, favored the better anticorrosion performance of the double-layer coating film. EA−VDC85 and BA−VDC75 were found to be the optimal top and bottom layers, respectively. Under harsh salt-spray corrosion conditions, the double-layer coating film (ca. 50-μm thickness) with EA−VDC85 as the top

Figure 8. DSC diagrams of coating films with EA−VDC85 as the top layer and BA−VDC75 as the bottom layer before and after corrosion in a salt-spray chamber for different times (second heating run).

of harsh corrosion, the signal at around 3418 cm−1 from the airside surface of the film increased slightly, whereas the other typical peaks were almost unchanged, indicating that a little hydrolysis occurred. With respect to the metal-side surface of the film, the signal at around 3418 cm−1 increased more after the 800 h of harsh corrosion, and the signal intensity at around 1640 cm−1 also increased slightly. This means that, after 800 h of harsh corrosion, the coating was still stable but with a little

Figure 9. FTIR-ATR spectra of coating films with EA−VDC85 as the top layer and BA−VDC75 as the bottom layer before and after corrosion in salt-spray chamber for different times: (A) air side and (B) tinplate side. 4541



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(11) Wang, J.; Tang, J.; He, Y. The Particle Size Effect of MALPBDEA Dispersions on Their Anticorrosion Performances as Waterborne Coatings on Galvanized Sheet. J. Coat. Technol. Res. 2010, 8, 11−17. (12) Bagherzadeh, M. R.; Mousavinejad, T. Preparation and Investigation of Anticorrosion Properties of the Water-Based EpoxyClay Nanocoating Modified by Na+-MMT and Cloisite 30B. Prog. Org. Coat. 2012, 74, 589−595. (13) Bagherzadeh, M. R.; Mahdavi, F.; Ghasemi, M.; Shariatpanahi, H.; Faridi, H. R. Using Nanoemeraldine Salt-Polyaniline for Preparation of a New Anticorrosive Water-Based Epoxy Coating. Prog. Org. Coat. 2010, 68, 319−322. (14) Bagherzadeh, M. R.; Daneshvar, A.; Shariatpanahi, H. Novel Water-Based Nanosiloxane Epoxy Coating for Corrosion Protection of Carbon Steel. Surf. Coat. Technol. 2011, 206, 2057−2063. (15) Caprari, J. J.; Sarli, A. R. D.; del Amo, B. Zinc Phosphate as Corrosion Inhibitive Pigment of Water-Borne Epoxy Paints Used for Steel Protection. Pigm. Resin Technol. 2000, 29, 16−22. (16) Jackson, M. A. Guidelines to Formulation of Water-Borne Epoxy Primers: An Evaluation of Anticorrosive Pigments. J. Prot. Coat. Linings 1990, 7, 54−64. (17) Lee, K. C.; El-Aasser, M. S.; Vanderhoff, J. W. Emulsion Copolymerization of Vinylidene Chloride and Butyl Methacrylate, 83:17 in mol %. J. Appl. Polym. Sci. 1991, 42, 3133−3145. (18) Ruckenstein, E.; Li, H. The Tensile Behaviour and Toughness of Poly(vinylidene chloride)/Poly(butyl methacrylate) Composites Prepared by the Concentrated Emulsion Approach. Polymer 1994, 35, 4343−4348. (19) Obi, B. E.; DeLassus, P. T.; Howell, B. A.; Dangel, B. Crystallization Kinetics for Semicrystalline Random Copolymers of Vinylidene Chloride (VDC) with Methyl Acrylate (MA), and the Effects on the Internal Morphology of the Resin Particles Formed during Synthesis. J. Polym. Sci. B: Polym. Phys. 1995, 33, 2019−2032. (20) Penzal, E.; Haberkorn, H.; Heilig, J. C. Some Properties of Copolymers of Vinylidene Chloride with Acrylates and Methacrylates, Part 1. Angew. Makromol. Chem. 1999, 273, 15−27. (21) Wessling, R. A.; Gibbs, D. S.; Delassus, P. T.; Obi, B. E.; Howell, B. A. Vinylidene Chloride Monomer and Polymers. In Kirk−Othmer Encyclopedia of Chemical Technology; John Wiley & Sons, Inc.: New York, 1997; Vol. 24. (22) Hsieh, T. H.; Ho, K. S. Effects of Thermal Stability on the Crystallization Behavior of Poly(vinylidene chloride). J. Polym. Sci. A: Polym. Chem. 1999, 37, 3269−3276. (23) Hwang, T.; Pu, L.; Kim, S. W.; Oh, Y. S.; Nam, J. D. Synthesis and Barrier Properties of Poly(vinylidene chloride-co-acrylonitrile)/ SiO2 Hybrid Composites by Sol−Gel Process. J. Membr. Sci. 2009, 345, 90−96. (24) Wessling, R. A. Polyvinylidene Chloride. In Polymer Monographs; Morawetz, H., Ed.; Gordon and Breach Science Publishers: New York, 1977; Vol. 5, p 706. (25) Kodani, T.; Sakai, H.; Okabe, T.; Nomura, M. Effect of Vinylidene Chloride Content on Film-Formation Property of Vinylidene Chloride-Methyl Acrylate Copolymer Latex. J. Appl. Polym. Sci. 1998, 69, 565−572. (26) Collins, S.; Yoda, K.; Anazawa, N.; Birkinshaw, C. The Thermal Stability of Some Vinylidene Chloride Copolymers. Polym. Degrad. Stab. 1999, 66, 87−94. (27) Schidknecht, C. E. Vinyl and Related Polymers; John Wiley and Sons: New York, 1952. (28) Wessling, R. A.; Gibbs, D. S.; Obi, B. E.; Beyer, D. E.; Delassus, P. T.; Howell, B. A. Vinylidene Chloride Polymers. In Encyclopedia of Polymer Science and Technology; John Wiley and Sons: New York, 1975; Vol. 14. (29) Strandburg, G.; Delassus, P. T.; Howell, B. A. Thermodynamics of Permeation of Flavors in Polymers: Prediction of Solubility Coefficients. In Food and Packaging Interactions II; Risch, S. J., Hotchkiss, J. H., Eds.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991; Vol. 473, Chapter 12, pp 133−148. (30) Delassus, P. T.; Brown, W. E.; Howell, B. A. Vinylidene chloride copolymers. In The Wiley Encyclopedia of Packaging Technology, 2nd

layer and BA−VDC75 as the bottom layer could protect tinplate well for at least 800 h, and the adhesion of the coating film to tinplate was still excellent even after 1000 h of corrosion. The optimized double-layer coating films showed performances that were not only superior to that of monolayer coating films formed by water-borne acrylate−VDC copolymers but also comparable to that of films based on water-borne epoxides with functional additives. Their thermal properties did not change after 800 h of corrosion, but a little hydrolysis of the film occurred, and the crystallinity of the metal-side surface of the coating became slightly higher.

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ASSOCIATED CONTENT

S Supporting Information *

Tables containing the physical properties of the obtained latex emulsions, the thermal and mechanical properties of the films, and WVTRs and OGTRs data of the films. Optical microscope image of acrylate−VDC coating film cast onto tinplate. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for financial support from the Program for New Century Excellent Talents in University and the National Natural Science Foundation of China (21074088).

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REFERENCES

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