Corrosion Science 75 (2013) 106–113

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Corrosion Science

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Water and corrosion resistance of –acrylic– waterborne coatings: Effects of molecular weight, polar group and hydrophobic segment ⇑ Min Liu a, Xuhui Mao a, Hua Zhu a, An Lin a, Dihua Wang a,b, a School of Resource and Environmental Science, Wuhan University, Wuhan 430072, PR China b State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, PR China article info abstract

Article history: A two-step esterification process is developed for the synthesis of epoxy–acrylic-grafted-copolymer Received 7 January 2013 waterborne . The effect of synthesis parameters on and corrosion resistance of the water- Accepted 24 May 2013 borne coatings is investigated. The results reveal that moderate increasing of the resin molecular weight Available online 5 June 2013 (<8000 Da) and carboxyl content (<27 wt.%) increased the crosslinking property, thereby improved the anticorrosion performance of the coatings. Longer epoxy-octanoic hydrophobic chains can provide stron- Keywords: ger shielding effect on the hydrophilic portion of the matrixes. The polar group content in a A. Organic coatings waterborne resin can be optimized for better anticorrosion performance, whereas the optimal value is B. EIS coating-specific. C. Polymer coatings Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction found that water existed in the polymer matrix in two distinct forms: free water filling the microcavities and bound water com- Increasing environmental pressures are forcing the coating bined with polar groups of the polymer network. Viktor et al. industry to minimize the release of volatile organic compounds [20–22] investigated that absorbed water mostly located in the (VOCs) and result in a continuous shift from -borne coating hydrophilic part of the polymer matrixes and caused irreversible to waterborne coating [1,2]. At present, waterborne organic coat- swelling of . Vanderwel et al. [23–26] found that the ings have been wildly employed as building coatings and wood water absorption caused by the hydrogen bonding between water paints, while their application as anticorrosive coatings for metal and polar groups was common in hydrophilic coatings, and the ab- is still limited [3–6]. As is well known, hydrophilic components sorbed water induced the swelling of the crosslinked polymers and with polar groups or ionic groups are necessary for water-soluble weakened the mechanical property of the coatings. or hydrosol resins. However, these polar groups are believed to Many efforts have been made to improve the anticorrosive form water penetration channels in the polymer matrixes of water- property of waterborne coatings. An optional method is to improve borne coatings [7,8]. The formation of a waterborne film is a com- the crosslinking property [1,27–31]. Chemical reactions between plicated process consisting of three stages [8–15]: particle packing; different polar groups not only facilitate crosslinks and thus particle deformation and compression; and particle coalescence. In enhance the physical and chemical integrity of the coalesced film, the particle coalescence stage, molecular rearrangement occurred but also reduce the number of polar groups and thereby lower the and polymer particles with the groups of similar polarity tended water sensitivity of the coatings [15,27,31]. Reactive polar groups to aggregate [7,16]. As a result, the polar groups in the particles for coating systems usually include epoxy groups, hydroxyl groups, form polar channels for water permeation, accelerating the water carboxyl groups, amino groups, N-methylolacrylamide groups, uptake in waterborne coatings and deteriorating the corrosion blocked isocyanates and acetoacetate groups, etc. [7,15,27–31]. resistance. Perez et al. [5] reported that the water apparent diffu- Crosslinking property of a coating is known to be impacted by sion coefficient (Dapp) for waterborne acrylic paint was 10 times the number density of these reactive polar groups. However, it re- higher than that of organic-solvent paints. Mikols et al. [17–19] mains controversial regarding the exact relation between them. Many studies suggested that more crosslinked units were needed in each polymer chain to obtain better crosslinking property ⇑ Corresponding author at: School of Resource and Environmental Sciences, Wuhan University, Wuhan 430072, PR China. Tel./fax: +86 27 68775799. [7,15]. Others reported that an increase in density of reactive polar E-mail addresses: [email protected] (M. Liu), [email protected] (X. Mao), groups resulted in poor crosslinking property and reduced tensile [email protected] (H. Zhu), [email protected] (A. Lin), [email protected] strength of the cured films [32–34]. High reactive group content (D. Wang).

0010-938X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2013.05.020 M. Liu et al. / Corrosion Science 75 (2013) 106–113 107 caused fast crosslinking reactions, thereby suppressed further 85 °C and maintained till to the end of the dropwise addition polymer interdiffusion and lowered the gel content of the film [35]. (90 min). Afterwards, the reaction system was kept at 85 °C for Besides, the molecular weight of the polymer particles has been additional 120 min. The second esterification step was the reaction considered as another important parameter for crosslinked films between the prepared EP and PA prepolymers. Based on solid con- [7,14,36–38]. It is well known that coatings with high resin molec- tents of the resultant solutions, the mass ratio of EP to PA was 3:7, ular weight are superior in barrier performance, because more and the reaction condition was the same as that of the first ester- interdiffusion of polymer chains occurred between large resin mol- ification. Finally, the obtained epoxy–acrylic graft copolymer (EA ecules and led to a better crosslinking property [7]. However, some resin) was prepared. studies also reported that higher resin molecular weight may exert For the preparation of an unpigmented epoxy–acrylic coating negative influences on the properties of the coatings. With high (EA coating), the EA resin was neutralized by DMEA and mixed molecular weight resins, the waterborne coatings showed lower with amino resin (curing agent). Then the mixture of the two resins dispersibility and compatibility, hence poor film smoothness and was diluted to 45 wt.% solid content with distilled water prior to protective property [14,36]. use. The addition amount of amino resin was determined by the In order to elucidate the effect of resin molecular weight and reactive group contents of the EA resin. polar group content on the barrier performance of waterborne coatings, a two-step esterification process is developed in this 2.3. Preparation of samples study to prepare various epoxy–acrylic-grafted-copolymer water- borne resins (EA resins), and the water and corrosion resistance Commercial aluminum foil and galvanized steel sheet, with the of their unpigmented waterborne coatings were evaluated. Unlike compositions given in Table 2, were used as substrates for EA coat- the previously reported methods, the two-step esterification pro- ings. Samples on aluminum foils (150 mm  150 mm  0.01 mm) cess was designed to accurately control the molecular weight were used for water absorption test. Galvanized steel sheets and carboxyl content of the EA resins. Hence the prepared coatings (100 mm  50 mm  2 mm) were used as the substrates for elec- allow us to evaluate the effect of polar groups on the water and trochemical impedance spectroscopy (EIS) test and neutral salt corrosion resistance. The scientific hypothesis of this study is that spray (NSS) test. The metal substrates were degreased using ace- optimum resin molecular weight and carboxyl content of the EA tone solvent and dried in air before the experiments. The coatings resins not only ensure good water dispersibility, but also promise were painted by roll coating method at ambient temperature favorable crosslinking property and anticorrosion performance (25 °C), and were cured at 150 °C temperature for 30 min. After for the coatings. the curing process, the samples were kept in desiccators for 30 days before measured. The thicknesses of the obtained coatings 2. Materials and experimental are 10 ± 2 lm, as measured by a coating thickness gauge (TT260, Beijing TIME Corp., China). 2.1. Chemical materials 2.4. Measurements Two types of bisphenol-A epoxy resins (E-12 and E-20), the typ- ical structure of which was depicted in Fig. 1, were purchased from Molecular weights of the PA and EA resins were determined by Sinopec company (China) and were used as received. E-12 and E-20 gel permeation chromatography (GPC) (2690D, Corp., USA), hold a number-average molecular weight (Mn)of2000 and using tetrahydrofuran (THF) as an eluent. Glass transition temper-

1000, respectively. The curing agent was methylated amino resin ature (Tg) of the cured films was measured by differential scanning at 80 wt.% solid content (Type SM5717, Sanmu Group Co., China). calorimetry (DSC) (Q20, TA instruments, USA). The measurements All the other chemical reagents were provided by Sinopharm were carried out under the protection of nitrogen atmosphere with Chemical Reagent Co. (China). a scanning rate of 10 °C minÀ1 in the temperature range from À50 °C to 300 °C. The whole measuring process was operated 2.2. Preparation of epoxy–acrylic waterborne coatings according to ASTM/D3418-82. Gel content, representing the insol- uble fraction of a cured film in a good solvent [7], was tested by EA resins were synthesized by a two-step esterification process, extracting the cured film with refluxing 2-butanone in a Soxhlet as depicted in Fig. 2. The first esterification is to decrease the func- extractor for 10 h. Water absorption of the films was evaluated tionality of the epoxy resin. Epoxy monomers reacted with equal following a standard method (HG2-1612-1985). Viscosity of the mole ratio n-octanoic acid at 105 °C for 100 min to produce the coatings was determined by a digital viscometer (DV-79, Shanghai epoxy-octanoic ester (EP), and a certain amount of N,N-dimethy- Nirun Intelligent Technology Co., Ltd., China), at a constant temper- lethanolamine (DMEA) was used as the catalyst. The mixed organic ature of 25 °C. Thermal decomposition behavior of the films was for the reaction include 50 wt.% methyl examined with a thermo-gravimetric analysis (TGA) (Diamond ether and 50 wt.% n-butyl alcohol. PA prepolymer was prepared TG/DTA, PerkinElmer instruments, USA) under nitrogen flow. The via a free radical polymerization of several acrylic monomers TGA spectra were acquired in the temperature range from 30 °C (see the recipe described in Table 1). At the beginning of reaction, to 650 °C at a heating rate of 5 °C minÀ1. NSS tests were conducted one-third of portion I was added into a three-neck flask equipped in a salt spray cabinet (Wuxi Tainuo Testing Equipments Co., Ltd., with a constant temperature magnetic blender. When the temper- China), with spraying NaCl solution (50 ± 5 g dmÀ3, pH = 6.5–7) at ature rose to 75 °C, portion I, portion II and the remaining two- 35 ± 0.5 °C, lasting for 600 h. third of portion III were instilled into the flask by a constant flow EIS was performed with an electrochemical workstation (PAR- pump. After 60 min of reaction, the temperature was elevated to STAT 2273, Princeton Applied Research, USA) at room temperature

Fig. 1. General molecular structure of bisphenol-A epoxy resin (n = 5.5–6 for the E-12, n = 2–2.5 for the E-20). 108 M. Liu et al. / Corrosion Science 75 (2013) 106–113

Fig. 2. The schematic diagram for the two-step esterification process.

Table 1 modified coating was set to be the working electrode with a circu- Recipe for the synthesis of the acrylic polymer (total mass weight: 150 g). lar tested area of 13 cm2; a saturated calomel electrode (SCE) was Portion number Feed name Weight (g) the reference; and a platinum plate electrode with a dimension of À2 I Methacrylic acid (MAA) 13 10 mm  10 mm was the counter. The frequency range was 10 to 5 Methymethacrylate (MMA) 15 10 Hz, and the amplitude of the sinusoidal voltage was 10 mV. n-Butyl acrylate (BA) 72 II Azoisobutyronitrile (AIBN) 1.25 n-Dodecyl mercaptan (NDM) 1.25 3. Results and discussion Ethyl acetate 2.5 III Butyl acetate 20 3.1. Preparation of the EA resins Propylene glycol methyl ether (PM) 13 n-Butyl alcohol 12 In the first esterification step, the epoxy equivalent and acid value of the reactants were monitored (see Fig. A1 in the Supple- mentary data): epoxy equivalent increased as a function of reac- Table 2 Chemical composition of metal substrates. tion time, while the acid value decreased gradually. This observation suggested the consumption of octanoic acid, and the Element Material progress of epoxide ring-opening reaction. At the end of the ester- Galvanized steel sheet (wt.%) Aluminum foil (wt.%) ification, the epoxy equivalent increased by twice of the original Zn 99.7 0.003 value, which meant that half of the epoxy rings were exactly Al 0.058 99.5 opened when the octanoic acid was exhausted. As described in Mn 0.065 0.013 the experimental section, the remaining epoxy groups in the resul- Si 0.031 0.108 Fe 0.015 0.350 tants are designed to react with acrylic polymers in the second C 0.060 – esterification. P 0.004 – The molecular weights of the prepared PA are listed in Table 3. S 0.008 – It can be observed that the polydispersity index (PDI) was less than Cu – 0.014 two for all PAs, while the number-average molecular weight (M ) Ti – 0.017 n of the PA changed with the amount of the initiator (AIBN) and chain transfer agent (NDM). As the additive amount decreased,

(25 °C). A three-electrode cell arrangement was used in the the Mn of the PA increased from 2844 to 7142 Da accordingly. experiments [10,38]: the galvanized steel sheet coated with The relative narrow molecular weight distributions of the PA M. Liu et al. / Corrosion Science 75 (2013) 106–113 109

Table 3 Molecular weights of the prepared acrylic polymers with 20 wt.% MAA content under different additions of initiator (AIBN) and chain transfer agent (NDM).

a a b Acrylic polymers AIBN (wt.%) NDM (wt.%) Mn (Da) PDI PA I 3 4 2844 1.89 PAII 2 2 4326 1.99 PAIII 1.5 1 5278 1.96 PAIV 1 1 5793 1.96 PAV 1 0.5 7142 1.83

a wt.% is based on the total weight of the ingradients in Table 1. b PDI represents the polydispersity index.

(PDI < 2) also suggest that the acrylic polymerization proceeded in a controlled manner [39]. Using different EP and PA prepolymers, a series of EA resins were synthesized in the second esterification step, as shown in Table 4. Measurements using GPC and DSC were conducted to see if the two polymers were successfully grafted. As presented Fig. 3. GPC traces of the reactants (EP and PA) and the product (EA) in the second esterification step for the synthesis of Coating EA-12-20-65. in Fig. 3, Mn of the product (the EA resin) is approximately the sum of the Mn of the reactants (the EP and the PA), indicating that the EP was expectedly grafted with the PA (Fig. 2). The comparison 3.2. Effect of molecular weight on water and corrosion resistance of the DSC curves of the films (Fig. 4) further convinced the successful preparation of EA resins. Four kinds of resins, EP, PA, ‘‘EA-12-20-XX’’ in Table 4 represents a group of unpigmented the prepared EA and the blend of the EP and PA, were all cured EA coatings prepared by the resins with different molecular by AR, but the resultant films exhibited different DSC curves. For weights. The water absorption of their cured films, as shown in the case of individual resin, the relaxation transition occurred at Table 4, constantly decreases from 0.779% to 0.597% when the re- 0 °C for PA, and 40 °C for EP. For the blended one (BEA), two sin molecular weight increases. However, the decrease of water relaxation transitions, at around 0 and 40 °C, appeared in the curve, absorption seems to be weak when the resin molecular weight suggesting the separated status of the EP and PA in the blend. In reached 8000 Da (from 0.601% to 0.597%). Fig. 5a shows the bode contrast, the curve of the prepared EA resin looks very different plots of galvanized steel samples coated with different EA coatings. from the other three curves, suggesting that a homogeneous The initial log|Z| value increased as the resin molecular weight in- copolymer was successfully prepared via the chemical grafting creased, although the increasing trend became not evident after process presented in Fig. 2. 8000 Da. This phenomenon is in accordance with the water The EA resins and the properties of their unpigmented coatings adsorption trend, suggesting that a higher molecular weight was were listed in Table 4. PA resins with different Mn, ranging from preferable for water and corrosion resistance. 2800 to 7100 Da (see Table A1 in the Supplementary data), were In Table 4, it is notable that the increase in the molecular weight selected for the synthesis of EA resins with different molecular of the EA resins basically results in higher gel content (from 89.55% weights. Also, MAA addition in the preparation of PA was adjusted to 98.62%), even though the increasing trend was not obvious to achieve different carboxyl contents for the produced EA resins, when the molecular weight increased to 8000 Da. This observation since other ingredients are void of strong polar groups like meant that the crosslinking property, which is reflected by the gel carboxyl. Thus, the MAA addition (the mass ratio of the MAA to content [7], was improved with higher resin molecular weight. Due the total mass of the acrylic monomers, 8–27 wt.% in Table 4)is to the improved crosslinking property, better water and corrosion a simple indicator that reflects the carboxyl content, and thus resistance of the samples was observed for the samples with higher the polarity of the EA coatings. In the following sections, the resin molecular weights. MAA addition, instead of carboxyl content, was used for In order to quantitatively evaluate the anticorrosion perfor- discussion. mance of the EA coatings, the evolution of the low-frequency

Table 4 EA coatings prepared by EA resins with different molecular weights, carboxyl contents and epoxy monomers.

Coating type Epoxy resin MAA Mn of PA in the resin Resin molecular Water Gel content (%) Viscosity (EA-XXa-XXb-XXc) addition (wt.%) weight absorption (%) (mPa s)

Mn (Da) PDI EA-12-20-50 E-12 20 2844 1.89 5000 0.779 89.55 364.1 EA-12-20-65 E-12 20 4326 1.99 6500 0.685 91.40 841.2 EA-12-20-73 E-12 20 5278 1.96 7300 0.620 98.33 2636.9 EA-12-20-80 E-12 20 5793 1.96 8000 0.601 98.31 5172.1 EA-12-20-92 E-12 20 7142 1.83 9200 0.597 98.62 6255.2 EA-12-8-73 E-12 8 5138 1.99 7300 1.144 89.53 – EA-12-13-75 E-12 13 5533 1.91 7500 0.772 92.35 EA-12-27-74 E-12 27 5362 1.90 7400 1.135 99.39 EA-20-8-83 E-20 8 5963 1.95 8300 1.002 87.95 EA-20-13-80 E-20 13 5490 1.93 8000 0.809 92.03 EA-20-20-82 E-20 20 5793 1.96 8200 1.075 97.47 EA-20-27-81 E-20 27 5682 1.83 8100 2.540 99.67

a Type number of epoxy resin (12 and 20 represent E-12 and E-20, respectively). b MAA addition (wt.%). c Abbreviation for the resin molecular weight. 110 M. Liu et al. / Corrosion Science 75 (2013) 106–113

Fig. 4. DSC curves of the films prepared by different polymers cured with amino resins. EP represents the epoxy-octanoic ester (EP-12); PA represents the acrylic polymer (Mn = 4326; MMA addition is 20 wt.%); EA represents the copolymer product of EP and PA (the resin for Coating EA-12-20-65). BEA represents the physical blend of the EP and the PA. All films were cured with amino resins.

impedance module (|Z|f=10mHz) of the coated samples during a long- term immersion was depicted in Fig. 5b [40]. As can be seen, because of the penetration of water and electrolyte [41], the impedance of the samples all presented a decreasing trend. In the case of the samples with resin molecular weights more than

8000 Da (EA-12-20-80 and EA-12-20-92), the |Z|f=10mHz experi- enced a fast decrease in the time duration of 0–250 h. After 500 h of immersion, the one that exhibited the best corrosion 7 2 resistance was EA-12-20-73 (|Z|f=10mHz >10 X cm ), followed by EA-12-20-80 and EA-12-20-92. Namely, there is an optimal resin molecular weight for the long-term corrosion resistance of the Fig. 5. (a) Bode plots of the galvanized steel samples with EA coatings. Samples coating, and too high molecular weight may deteriorate the anti- were immersed for 2 h prior to measurement. (b) Time dependence of the low- frequency impedance modules (|Z|f=10mHz) of the galvanized steel samples with EA corrosion performance. In Table 4, the viscosities of the coatings coatings. Testing was conducted in 3.5 wt.% NaCl solution. The coatings included prepared by ‘‘EA-12-20-XX’’ resins are listed. A dramatical increase EA-12-20-50 ( ), EA-12-20-65 ( ), EA-12-20-73 (), EA-12-20-80 ( ) and EA-12- in viscosity from around 300 to 6000 mPa s can be observed as the 20-92( ). resin molecular weight increased. High viscosity resulted in poor paintability of the coatings, causing defect points and inferior smoothness of the cured film [14,36]. For the EA-12-20-80 and EA-12-20-73 sample held the highest |Z| of 109.4 X cm2 EA-12-20-92 samples, we thought the excessive viscosity f=10mHz (Fig. 6a). At longer times of immersion, EA-12-20-73 sample al- accounted for their lower corrosion resistance in comparison with ways kept the highest impedance, showing the best barrier perfor- the optimal one (EA-12-20-73) in EA-12-20-XX coating series. As mance for metal substrates (Fig. 6b). However, the EA-12-27-74 discussed above, increasing the resin molecular weight within a sample demonstrated a fast decrease in the impedance values soon certain range seems to be a good way to improve the anticorrosion after the start of the immersion. The |Z| value became even performance of the coatings. Since the coatings with resin molecu- f=10mHz lower than that of EA-12-13-75 sample after nearly 70 h of immer- lar weights higher than 8000 Da was too viscous to provide good sion (Fig. 6b), indicating that the 27 wt.% MAA addition greatly paintability, and hence did not show the best anticorrosion perfor- weakened the anticorrosion performance of coating. The observa- mance, in the following sections, the EA resins with molecular tion of the 600 h NSS test (see Fig. A2 in the Supplementary data) weights of 7000–8000 Da were selected for further study. further confirmed the results obtained in the impedance measure- ments. Rust spots appeared on the samples coated by Coating 3.3. Effect of carboxyl content on water and corrosion resistance EA-12-8-73 (at 124 h), EA-12-27-74 (at 264 h), and EA-12-13-75 (at 360 h), while EA-12-20-73 sample did not exhibit any visible The EA coatings with similar resin molecular weights (7300– rust spots at the end of the exposure (at 600 h). 7500 Da) but different carboxyl contents were denoted by The gel contents listed in Table 4 demonstrate that the cross- ‘‘EA-12-XX-7X’’ in Table 4. The MAA contents include four levels, linking property of the films was improved by increasing the car- 8, 13, 20 and 27 wt.%. It is observed that the water absorption de- boxyl content, and the highest gel content of 99.39% was creases from 1.144% to 0.620% with the increased carboxyl content achieved when the MAA addition was 27 wt.% (Coating EA-12- (MAA addition from 8 to 20 wt.%). However, for the coatings with 27-74). Considering the one with 20 wt.% MAA addition (Coating the highest MAA addition of 27 wt.%, the water absorption returns EA-12-20-73) demonstrates the best water and corrosion resis- to 1.135%, being equal to that with 8 wt.% MAA addition. A similar tance, it can be concluded that the improvement on crosslinking trend is observed in the bode plots of their coated samples (Fig. 6). property does not always result in better anticorrosive property After 2 h of immersion, the samples with higher MAA addition of the coating. The variation of the relation was obviously associ- (20 wt.% and 27 wt.%) basically exhibited higher impedance, and ated with the excess carboxyl content at 27 wt.% MAA addition. M. Liu et al. / Corrosion Science 75 (2013) 106–113 111

Table 5 The weight loss of the EA films revealed by TG curves.

Coating type Dm (%) Weight loss (%)

100 °C 200 °C Part I Part II EA-12-13-75 1.15502 1.51030 1.15502 0.35528 EA-12-20-73 0.55031 0.96395 0.55031 0.41364 EA-12-27-74 0.92078 1.98352 0.92078 1.06274

Table 6

Tg values of the EA coatings with different carboxyl contents before and after immersion.

Coating type Tg (°C) DTg Before immersion After immersion for 250 h EA-12-13-75 50.26 47.82 2.44 EA-12-20-73 51.29 49.91 1.38 EA-12-27-74 51.88 47.52 4.36

on the other hand, higher carboxyl content also increases the affin- ity of the polymer matrix to water, thereby weakening the corro- sion resistance of coatings. In order to verify the role of the carboxyl content on the corro- sion property of an EA coating, TGA and DSC were employed to investigate the absorbed water in the unpigmented films. Fig. 7 shows the TGA curves of the EA films after 1000 h of immersion in distilled water. According to the TGA curves, the weight losses within the temperature regions of 0–100 °C (Part I in Table 5), and 100–200 °C (Part II in Table 5) can be calculated, respectively. As generally recognized [21], the weight loss in the temperature range of 0–100 °C is associated with free water in the capillaries and microvoids within the polymer matrix, while the weight loss Fig. 6. (a) Bode plots of the galvanized steel samples with EA coatings. Samples in the 100–200 °C range is related to the bound water combined were immersed for 2 h prior to measurement. (b) Time dependence of the low- with the hydrophilic groups of the polymer matrix [26]. It is ob- frequency impedance modules (|Z|f=10mHz) of the galvanized steel samples with EA served that Coating EA-12-13-75, which has the lowest gel content, coatings. Testing was conducted in 3.5 wt.% NaCl solution. The coatings included showed the largest weight loss between 0 and 100 °C (Part I), sug- EA-12-8-73 ( ), EA-12-13-75 ( ), EA-12-20-73 () and EA-12-27-74 ( ). gesting that inferior crosslinking property did lead to larger absorption of free water; Coating EA-12-27-74, which has the highest carboxyl content, showed the largest weight loss between 100 and 200 °C (Part II), indicating that excess carboxyl groups accelerated the absorption of bound water. For Coating EA-12- 20-73, the Part II weight loss is slightly larger than that of Coating EA-12-13-75, whereas its total absorbed water (sum of Part I and Part II weight loss) is evidently the lowest. Water uptake was thought to result in the degradation of the polymer matrix, and

thus the reduction of Tg and mechanical properties [3,4]. As shown in Table 6, three films all revealed evident reductions of Tg after 250 h of immersion in distilled water. The change extent in Tg reduction (DTg) basically reflected the sequence of their adsorbed water amounts. Based on all the evidences above, the role of carboxyl content on water resistance can be illustrated in Fig. 8. Too low carboxyl content results in lower crosslinking level, and bigger water pene- tration channels including capillaries and microvoids are left be- tween polymer particles (see Fig. 8a). Hence, the largest amount of the absorbed free water (Part I in Table 5) was observed for Coat- ing EA-12-13-75. Excess carboxyl content, even though it enables Fig. 7. TG curves of the films prepared by different EA coatings (different MAA better crosslinking property, induces the aggregation of water additions), after 1000 h of immersion in distilled water. due to the strong hydrogen bonding between water molecules and hydrophilic groups, resulting in swelling of polymer and water We thought the effect of carboxyl content on the anticorrosion per- permeation (see Fig. 8c). An optimal carboxyl content is the key to formance of the EA waterborne coatings seemed like a double- balance the two sides of a coin; namely, favorable matrixes with edged sword: increase in the carboxyl content can improve the tighter crosslinks and relative lower polarity can decrease the per- crosslinking property, thus enhancing the barrier performance; meation and absorption of both free and bound water (Fig. 8b). In 112 M. Liu et al. / Corrosion Science 75 (2013) 106–113

Fig. 8. The schematic diagram of water permeation in the EA coatings: (a) low MAA addition; (b) optimal MAA addition; and (c) high MAA addition. this study, 20 wt.% MAA addition is demonstrated to provide the on the type of the epoxy resin. Among the two series of coatings, 9.4 2 optimum carboxyl content for enhancing water and corrosion the initial |Z|f=10mHz of the EA-12-20-73 sample is 10 X cm , resistance. being higher than that of all other samples. Its |Z|f=10mHz value still remained above 107 X cm2 after 500 h of immersion, demonstrat- ing the most admirable performance for corrosion protection. 3.4. Effect of hydrophobic segments on water and corrosion resistance Furthermore, comparisons can be carried out between the coatings derived from different epoxy resins: EA-12-13-75 vs EA-20-13-80, Except for the molecular weight and carboxyl content, epoxy EA-12-20-73 vs EA-20-20-82, and EA-12-27-74 vs EA-20-27-81. monomer was also a concerned factor for the anticorrosion perfor- The coatings in each group have similar resin molecular weights, mance of coatings. Two bisphenol-A epoxy resins, E-12 and E-20, gel contents, and equivalent carboxyl content due to the identical which had different polymer chain lengths (Fig. 1), were used to MAA addition. As can be seen from Table 4, the EA coatings pre- prepare two series of EA coatings (denoted by ‘‘EA-12-XX-XX’’ pared with E-12 resin always showed lower water absorption. and ‘‘EA-20-XX-XX’’ in Table 4). For example, for the group with 20 wt.% MAA addition, the water Coating EA-12-20-73 demonstrated the best anticorrosion per- adsorption of Coating EA-20-20-82 increased by 1.7 times as that formance in the coating series of EA-12-XX-XX since it showed the of Coating EA-12-20-73. The evolution of impedance (Figs. 6b lowest water absorption (Table 4), highest bode impedance during and 9) also confirmed that the coatings prepared with E-12 long-term immersion (Figs. 5 and 6), and the best corrosion protec- (EA-12 series) were superior to the EA-20 series on anticorrosion tion on metal substrate in the salt spay test (Fig. A2 in Supplemen- performance. tary data). In the case of EA-20-XX-XX series, the minimum water Because epoxy monomers with different chain lengths (E-12 absorption of 0.809% (Table 4), as well as the highest impedance and E-20) were used for the preparation of EP resin, the resultant (initial and long times immersion, see Fig. 9), occurred at 13 wt.% EA resins finally possess different lengths of hydrophobic epoxy-al- MMA addition (EA-20-13-80), instead of at 20 wt.% MAA addition kyl chains. Since other factors like molecular weight and carboxyl (EA-20-20-80). This observation implied that the optimal MMA content were controlled at the same level, we thought the differ- addition, or optimal polar group content, is actually dependent ences of water and corrosion resistance between EA-12 and EA- 20 coatings were associated with the lengths of the hydrophobic epoxy-alkyl chains. Longer hydrophobic chains in the molecules of the EA-12 series resins may provide stronger shielding effect on the hydrophilic parts and lower the water sensitivity of the polymer particles: the aggregation of absorbed water around the polar groups can be hindered to some extent, and the water per- meation takes longer pathway due to the presence of hydrophobic alkyl chains. The shielding effect of alkyl chains also explains the different optimal carboxyl contents for EA-12 and EA-20 coatings: E-12 resin, which holds the alkyl chain nearly two times longer than that of E-20, can shield more hydrophilic groups in the EA molecule. Thus, the optimal MAA addition for EA-12 series coatings is 20 wt.%, being higher than the 13 wt.% for the EA-20 series coatings.

4. Conclusion

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