J. Coat. Technol. Res. DOI 10.1007/s11998-017-9923-8
A novel acrylate-PDMS composite latex with controlled phase compatibility prepared by emulsion polymerization Weili Li, Wenjun Shen, Wei Yao, Jijun Tang, Jie Xu, Lei Jin, Jide Zhang, Zexiao Xu
Ó American Coatings Association 2017 Abstract In this paper, a series of acrylate-polydimethylsiloxane (PDMS) composite latexes was prepared and studied systematically to find the factors that affect their performances. At first, the modified PDMS was synthesized to react with acrylate monomers and participate in free radical polymerization. Then, the modified PDMS was blended with acrylate monomers, and the acrylate-PDMS composite latexes with different formulas were obtained by emulsion polymerization. Because the blending monomers were constrained in the micelle, the two components were interconnected with each other by a covalent bond and the phase compatibility between the two components could be controlled well. Chemical constitution and the morphology of acrylate-PDMS composite latexes were confirmed by using FTIR spectroscopy, TEM, and SEM measurements, respectively. Thermophysics and heat resistance of the dried
coatings based on acrylate-PDMS composite latexes were studied using DSC and TGA tests, respectively. Anticorrosion properties of the cured coatings based on acrylate-PDMS composite latexes were confirmed by potentiodynamic polarization test. With low surface tension, good toughness, excellent weather-proof properties, and good high and low temperature stability, the modified PDMS component can improve the performance of the traditional acrylate latexbased waterborne resin effectively, and the prepared acrylate-PDMS composite latexes can be used in heavy-duty anticorrosion applications. Keywords Emulsion polymerization, AcrylatePDMS composite latex, Heat resistance, Hydrophobicity, Anticorrosion
Introduction
Weili Li and Wenjun Shen have contributed equally to this work. W. Li (&), W. Shen, W. Yao, J. Tang, J. Xu, L. Jin School of Material Science and Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, China e-mail:
[email protected] J. Zhang (&) Key Laboratory of Green Packaging and Biological Nanotechnology, Hunan University of Technology, Zhuzhou 412007, China e-mail:
[email protected] Z. Xu Suzhou Jiren Hi-Tech Material Co., Ltd, Suzhou 215143, China
With the rapid development of society, the cost and environmental consequences of corrosion problems have become a major challenge to engineers.1 Due to the airborne pollution from traditional solventthinned coatings, novel waterborne coating for the protection of metal substrate have attracted attention. Among various preparation processes, emulsion polymerization technology plays an important role due to its controllable preparation process, fast polymerization rate, and improved molecular weight of the synthesized polymers. The methods of emulsion polymerization can be subdivided into conventional emulsion polymerization,2–4 miniemulsion polymerization,5–7 microemulsion polymerization,8,9 soap-free emulsion polymerization,10,11 etc. The products are environmentally friendly alternatives to traditional solvent-based systems, and they can be applied in those areas, such as adhesives, paints, and additives for textiles.
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Acrylic resin has been extensively studied because of its good properties including gloss retention,12 color retention,13 heat resistance,14 corrosion resistance,15 and so forth. Thus, it is widely used in automotive coatings and architectural coatings. However, to make it applicable for a wider range, its weatherability and intrinsic chemical stability need to be further enhanced.16 Compared with acrylic resin, polydimethylsiloxane (PDMS) has good properties such as higher thermal stability, hydrophobicity, and chain flexibility, and it has been used in many fields.17–19 The enhanced thermal stability of PDMS is due to its higher bond energy, and the bond energy of Si–O bond (451 kJ/mol) is far greater than that of C–C bond (356 kJ/mol) and C–O bond (336 kJ/mol). Additionally, the chain of PDMS has a helical structure, and methyl group arranges outward and revolves around the Si–O chain. The molecule of PDMS is bulky, and its cohesive energy density is relatively low. In this regard, it has excellent properties of water repellency and dust resistance. Therefore, it is interesting to implant PDMS in other polymer systems to obtain composite materials with excellent properties. In the past few decades, composite polymers combined with PDMS have been widely studied.20–25 However, to obtain the composite polymer materials with high performance, it is very important and difficult to solve the problem of phase incompatibility between the two components, which will impact the performance of the final cured products. Bai et al.26 prepared an inorganic–organic trilayer core–shell PSQ/PA/PDMS hybrid latex. However, the Z-average particle size of the trilayer core–shell hybrid latex particles was 360.7 nm, and the reaction time was relatively long. Deng et al.27 prepared core/shell (PMMA/PDMS) particles via dispersion polymerization by using methanol as the solvent. The average size of the core/shell particles was 13.525 lm, and there were also a few polysiloxane homopolymers that could not be grafted onto the surface of the particles. Accordingly, it needed to be centrifuged and washed with methanol and water three times. So, it is necessary to find a simple experimental method to prepare acrylate-PDMS composite materials with good phase compatibility. To solve the problems mentioned above, a novel acrylate-PDMS composite latex was prepared by emulsion polymerization. First, PDMS was modified to make it react with acrylic monomers by radical polymerization and form an interconnection system; second, using an emulsion polymerization method, the reacting monomers with different polarity could be bounded in the micelle and react with each other. For the obtained acrylate-PDMS composite latexes with different formulas, phase separation of the two components was inhibited effectively. The factors affecting the performance of the dried coatings were studied systematically.
Experiment Materials Methyl methacrylate (MMA, C.P.), 2-ethylhexyl acrylate (2-EHA, C.P.), and methacrylic acid (MAA, C.P.) were purchased from Shanghai Wulian Chemical Factory Co., Ltd., China. Polydimethylsiloxane (PDMS, Mn = 1.3 9 105) was supplied by Jinan Beyond Co., Ltd., China. c-methacryloxy propyl trimethoxyl silane, which was used as silane coupling agent, was purchased from Aldrich. Dibutyltin dilaurate (DBTDL, National Medicine Group Shanghai Chemical Reagent) was added as a catalyst. Potassium persulfate (KPS, Shanghai Su Yi Chemical Reagent Co., Ltd., China) was used as the initiator. Sodium bicarbonate (NaHCO3, Changsun specialty products Co., Ltd., China) was used as buffering agent. NH3ÆH2O (Shanghai Su Yi Chemical Reagent Co., Ltd., China) was used as pH regulator. THF (Sinopharm Chemical Reagent Co., Ltd., China) was used as the solution, and deionized water was obtained in our laboratory. Sodium lauryl sulfate (SDS, Sinopharm Chemical Reagent Co., Ltd., China) and octylphenol polyoxyethylene ether-10 (OP-10, Changsun specialty products Co., Ltd., China) were used as anionic emulsifier and nonionic emulsifier, respectively. Preparation of modified PDMS At first, PDMS was blended with c-methacryloxy propyl trimethoxyl silane in a three-necked flask, and THF and dibutyltin dilaurate were added in as the solution and catalyst, respectively. Then, they were heated up to 60°C and reacted for about 5 h with continuous stirring. Lastly, the modified PDMS, which could participate in free radical reactions, was obtained through removing THF solution by rotoevaporation. Preparation of acrylate-PDMS composite latex The preparation process was carried out as follows. At first, the emulsifiers and deionized water were charged into the flask with gentle stirring. At the same time, monomers were mixed with each other according to the predetermined ratio. The initial reaction temperature was set at 70°C, and then a portion of KPS solution and a monomer mixture were added in via two separate feeding inlets. The reaction continued for 30 min at 75 ± 2°C with strong agitation. When the reacting emulsion turned bluish, the rest of the monomer mixture and KPS solution were added in through the two separate feeding inlets. The feeding rates should be adjusted to make the feeding process complete within 2.5–3 h at 75 ± 2°C. After completing the feeding process, the reaction temperature was further raised up to 80 ± 2°C and reacted for another 1 h. Before
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completing the reaction, products were cooled down to room temperature and a small amount of NH3ÆH2O was added to adjust pH value of the final latex in the range of 8–9. Detailed recipes of the samples are shown in Table 1. For clarity, the samples are named as SMA-X (X = 1–8). From SMA-1 to SMA-5, the content of modified PDMS is set at 3 wt%, by increasing Tg value of acrylate component from 0 to 40°C; while for the samples SMA-6, SMA-7, SMA-3, and SMA-8, Tg value of acrylate component is set at 20°C, and the content of modified PDMS increases from 0 to 5%. Coating procedure To study the properties of the dried coatings, the prepared acrylate-PDMS composite latexes were coated onto different substrates, such as steel panels, tin plate, and glass panel. Among them, steel panels were used to study the coatings’ corrosion resistance, mechanical properties, and hydrophobic nature; tin plate was used for the toughness test; glass panel was used for optical microscope observation. Before being coated, steel panel and tin plate were subjected to pretreatment according to the following steps: polishing by using metallographic sand paper, alkaline washing for removing grease, acid polishing, and drying. The wet coatings were applied by the Applicator Frame for a thickness of 0.5 mm. After being coated, the acrylate-PDMS composite latexes were allowed to dry for about one week at ambient temperature (20°C). Characterization FTIR spectroscopy of the different acrylate-PDMS composite latexes was carried out on BRUKER VECTOR-22 spectrometer (FT-5200, DIGILAB Company, USA) at room temperature. The spectra were collected over the range 400–4000 cm1 by averaging 128 scans at maximum resolution of 2 cm1.
TEM measurement (JEM 100CX II) was taken to observe the morphology of acrylate-PDMS composite latex (SMA-8 was selected as the example). In a typical sample preparation, the sample was stained with 2% phosphotungstic acid (PTA) solution after it was diluted to a certain concentration. The surface morphological aspects of the dried coatings based on acrylate-PDMS composite latexes were further characterized by using high resolution scanning electron microscopy (HR-SEM) technique (XL30-ESEM, PHILIPS). A field emission microscope model Quanta 3D provided by FEI Instruments was used, and the system operated at high vacuum mode. The samples were sputter coated with gold at accelerating voltage of 10 kV to prevent any charge-up effect. Particle sizes and the particle size distribution (PSD) of acrylate-PDMS composite latexes were tested by dynamic light scattering (DLS) measurement on a 90 Plus/B1-MAS Zetaplus Zeta Potential Analyzer (Brookhaven Instruments Corp). Gel fraction of the synthesized acrylate-PDMS composite latexes was obtained as follows. The coagula from the latex, the reactor, and the stirrer were collected and washed with distilled water. They were dried in an oven at 120°C to constant weight. Then, the gel rate was determined by the following equation (1): Gel rateð%Þ ¼
W0 100% W1
ð1Þ
where W0 is the constant weight of the coagula (g) and W1 is the weight of all monomers used (g). The hardness of the dried coatings based on acrylate-PDMS composite latexes was measured by using a Pencil Scratch Hardness Tester (QAQ film pencil scratch hardness tester, state-run weida test factory in TianJin, China) according to ASTM D 3363-05. The adhesion strength of the dried coatings based on acrylate-PDMS composite latexes to the metallic substrate was measured by using a Scotch tape test according to ASTM D 3359-2002. Typical procedure of
Table 1: Recipes of SMA varied with Tg value of acrylate component and the content of PDMS Recipes MMA (g) 2-EHA (g) MAA (g) Mod(PDMS) (g) SDS/OP-10 (g) KPS (g) NaHCO3 (g) Deionized water (g)
SMA-2 SMA-3 SMA-4 SMA-5 SMA-6 SMA-7 SMA-8 SMA-1 (Tg = 0°C) (Tg = 10°C) (Tg = 20°C) (Tg = 30°C) (Tg = 40°C) (PDMS = 0%) (PDMS = 1%) (PDMS = 5%) 20.01 17.39 0.40 1.20 0.8/0.5 0.16 0.15 60
23.21 15.19 0.40 1.20 0.8/0.5 0.16 0.15 60
25.26 13.14 0.40 1.20 0.8/0.5 0.16 0.15 60
27.17 11.23 0.40 1.20 0.8/0.5 0.16 0.15 60
28.97 9.43 0.40 1.20 0.8/0.5 0.16 0.15 60
26.06 13.54 0.40 0 0.8/0.5 0.16 0.15 60
25.80 13.40 0.40 0.4 0.8/0.5 0.16 0.15 60
24.76 12.86 0.38 2.0 0.8/0.5 0.16 0.15 60
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Scotch tape test was given as follows: At first, the surface of the coating was cut by a razor to obtain grid lines. The total test area was about 1 cm2 with each square grid dimension of 1 9 1 mm2. The Scotch tape (a width of 18.0 mm, fabricated by 3 M) was applied firmly to cover the grid area of the coating at room temperature. After around 1 min, the Scotch tape was stripped off by one quick peeling. The adhesion strength of the coatings was estimated by counting the number of squares peeled off compared to the total number of squares. The toughness of the dried coatings based on acrylate-PDMS composite latexes was carried on by Standard Test Method for Coating Flexibility of Prepainted Sheet according to ASTM D4145-2010. During the test, the dried coatings coated on tin plate were bent to 180° with stainless steel bars of different radii at room temperature. Each test was repeated at least three times. Water absorption of the dried coatings based on acrylate-PDMS composite latexes was examined by immersing the coatings (15 mm 9 30 mm; original weight designated as W0) in water for 24 h at room temperature (20°C). After the testing, coatings were taken out, residual water was wiped off from the surface of the coatings by using filter paper, the value weight (W1) was measured immediately, and the value of water absorption was calculated as follows (equation 2): Water absorptionð%Þ ¼
W1 W0 100% W0
ð2Þ
Water contact angles (WCA) of the dried coatings based on acrylate-PDMS composite latexes were measured by using a Drop Shape Analyzer (DSA) (JC2000D, Powereach Corporation, Shanghai, China). The volume of water droplet was around 4 lL, and the averages of 10 results were reported as the water contact angle. Thermal stability of the dried coatings based on acrylate-PDMS composite latexes was assessed by thermogravimetric analysis (TGA Q500, PE company, USA) from room temperature to 800°C under nitrogen atmosphere at heating rate of 15°C/min. During heating period, the change of weight loss with temperature was recorded as a function of temperature. Thermal dynamics of the dried coatings based on acrylate-PDMS composite latexes were determined by differential calorimetry (DSC Q100, TA Instruments, USA) with a temperature of range between 50 and 120°C at a heating rate of 10°C/min. All the thermograms were baseline corrected and calibrated by using Indium metal. All the experimental specimens (8– 10 mg) were dried at 60°C under vacuum for 24 h before being measured. During the testing procedure, the samples were first annealed at 120°C for 3 min, and then cooled to 50°C with liquid nitrogen and scanned for the measurement.
The anticorrosion behavior of the dried coatings based on acrylate-PDMS composite latexes was studied by potentiodynamic polarization test with a dual unit electrochemical work station (EG-GPARC M283) (EG&G Company, USA) connected to a corrosion analysis software program. During potentiodynamic polarization test, a three-electrode cell system which included the dried coating as the working electrode, saturated potassium sheet as the reference electrode, and platinum sheet as the counter electrode was employed, and each sample was immersed in 3.5 wt% NaCl solution and scanned at the rate of 5 mV/s.
Results and discussion FTIR measurement FTIR spectra of ordinary acrylate latex (SMA-6) and acrylate-PDMS composite latex (SMA-8) are shown in Fig. 1, and the two curves show similar tendency. For the former, the broad absorption at around 3444 cm1 is attributed to the stretching vibration of carboxyl group. The absorption peaks at 2948, 2866, and 1727 cm1 are ascribed to the stretching vibrations of –CH3, –CH2, and C=O, respectively. No absorption peak at around 1640 cm1 indicates that no C=C bond exists in the prepared polymer, and all the monomers have reacted during the reaction. According to the FTIR spectrum of SMA-8, two new peaks appear at 1062 and 807 cm1 which may be attributed to Si–O and Si–C of PDMS.
A B –OH
Si–CH3
A SMA–6
–CH
Si–O C=O
B SMA–8
4000
3500
3000
C–O 2500 2000 1500 Wavenumber/cm–1
1000
500
Fig. 1: FTIR spectra of ordinary acrylate latex and acrylatePDMS composite latex. (A) Acrylate latex (SMA-6), (B) Acrylate-PDMS composite latex (SMA-8)
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Morphology measurements SMA-8 was taken as the example, and the microstructure of acrylate-PDMS composite latex was observed by TEM measurement. According to Fig. 2, the synthesized composite latex particles have evenly spherical morphology, and the diameter of particles is about 100 nm. During the reaction, the modified PDMS and acrylic monomers were wrapped in the micelle formed by emulsifying agent and reacted with each other in the aqueous system. SEM images of the dried coatings based on pure acrylic latex and acrylate-PDMS composite latexes are shown in Fig. 3. As to the former, its surface appears smooth and uniform, and no cracks are found. However, for the latter, especially in the sample SMA-8, microscopic phase separation is found. Lower polarity of the modified PDMS phase tends to aggregate and presents as the island, while the major content of acrylic resin surrounds around like the sea. The wettability of the dried coating depends not only on its chemical composition, but also on its topography. According to Lotus effect, the microscopic phase separation structure of the dried coatings may be helpful to its hydrophobic property. The properties of dried coatings are under the influence of their morphologies in part. From the results of SEM, we find that there is phase separation in microscopic scale for the dried coatings when the content of modified PDMS exceeds a certain level. In the following, we will observe the dried coatings at macroscopic scale by a 3D measuring laser microscope. Due to good film-forming property, no obvious phase separation or cracks are observed from the two
Fig. 2: TEM image of acrylate-PDMS composite latex particles (SMA-8)
samples. For the sample SMA-6, its surface is smooth and uniform, as shown in Fig. 4. However, there are a lot of small particles evenly distributed in the dried coatings of SMA-8, which may be the excessively modified PDMS component. Study on the influence of acrylate component on the properties of acrylate-PDMS composite latexes Mechanical properties, which include hardness, flexibility and adhesion, etc., are under the influence of glass transition temperatures (Tg) of mainly polymers. At first, relative content of modified PDMS is set as a constant, Tg value of acrylate component is varied from 0 to 40°C to find the best value. Table 2 lists the mechanical properties of the dried coatings varied with it. The hardness of the dried acrylate-PDMS coatings increases with Tg value; however, their flexibility and the adhesion properties may be ruined by the rigid polymer chains. Considering all these factors comprehensively, SMA-3 presents the optimal performance. In the next, Tg value of acrylate component is set at 20°C, and acrylate-PDMS composite latexes are studied by varying the content of the modified PDMS. Study on the influence of the modified PDMS on acrylate-PDMS composite latexes At first, Tg value of acrylate component was set at 20°C, and the influence of modified PDMS on the particle size and PDI of acrylate-PDMS composite latexes was tested by DLS measurement. As is seen from Table 3, all the acrylate-PDMS composite latexes reveal a uniform distribution, and the low PDI values indicate monodisperse properties of the synthesized acrylate-PDMS latex. However, the size of the latex particles decreases linearly with the increasing content of modified PDMS. The ordinary latex particle is composed of hydrophobic core and hydrophilic layer. The modified PDMS polymer chains may shrink in the hydrophobic core, which will lead to a decrease in the size of latex particles (Fig. 5). With multifunctional group of modified PDMS, the synthesized acrylate-PDMS composite latexes may present a slightly crosslinked structure, which will lead to the emergence of sediment during the process of polymerization. According to Table 4, gel fraction of the synthesized acrylate-PDMS composite latexes increases linearly with the content of modified PDMS. However, its value can be accepted even when the content of modified PDMS reaches 5% (SMA-8). Figure 6 presents the thermodynamics properties of the dried coatings based on ordinary acrylate latex (SMA-6) and acrylate-PDMS composite latex (SMA8), respectively. Comparing these two samples, the obtained Tg value of SMA-8 presents a little higher than that of SMA-6 although the theoretical Tg values of the acrylate component of these two samples are the
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Fig. 3: SEM images of (a) SMA-6, (b) SMA-7, (c) SMA-3, (d) SMA-8
Fig. 4: Optical images of the surface of the coatings (a) SMA-6, (b) SMA-8, optical images of three-dimensional shape of the coatings (c) SMA-6, and (d) SMA-8
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Table 2: Mechanical performance of SMA varied with Tg value of acrylate component SMA
Pencil hardness
SMA-1 SMA-2 SMA-3 SMA-4 SMA-5
(Tg (Tg (Tg (Tg (Tg
= = = = =
0°C) 10°C) 20°C) 30°C) 40°C)
Flexibility
B B HB HB
Adhesion
1 mm 1 mm 1 mm 3 mm Cannot form a film
0 0 1 3
Table 3: Average particle size and its PDI values of the synthesized acrylate-PDMS composite latex varied with the content of PDMS Characteristic properties
Sample
Z-average (d nm) PDI
SMA-6
SMA-7
SMA-3
SMA-8
86.24 0.073
81.20 0.086
81.76 0.102
76.05 0.164
20
20
(a)
(b) 15 Intensity %
Intensity %
15
10
10
5
5
0
0 0
100
200 300 Size (d nm)
400
500
0
100
200 300 Size (d nm)
400
20
20
(c)
(d) 15
Intensity %
Intensity %
15
10
10
5
5
0
500
0 0
100
200 300 Size (d nm)
400
500
0
100
200 300 Size (d nm)
400
500
Fig. 5: Particle size and PDI of acrylate-PDMS composite latexes varied with the content of PDMS (a) SMA-6, (b) SMA-7, (c) SMA-3, and (d) SMA-8
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Table 4: Gel fraction of acrylate-PDMS composite latexes varied with the content of modified PDMS SMA-6
SMA-7
SMA-3
SMA-8
0.08
0.15
0.53
1.58
SMA–6 SMA–7 SMA–3 SMA–8
100
80
SMA–6 SMA–8
60 100
362°C
20
21.91°C
355°C
40 Weight /%
Weight /%
Gel fraction (%)
325°C
80
347°C
60
0
300
0
30.90°C
350
400
Temperature/°C
100
200
300 400 500 Temperature/°C
600
700
Fig. 7: TGA curves of the dried coatings based on acrylatePDMS composite latexes. (a) SMA 6, (b) SMA 7, (c) SMA 3, and (d) SMA 8
–40
–20
0
20 40 60 Temperature/°C
80
100
120
Fig. 6: DSC thermograms of SMA-6 and SMA-8
same. First, slightly crosslinked structure of the composite latexes may inhibit segmental motion of the polymer chains; second, a little higher molecular weight of modified PDMS component may also lead to the raise of glass transition temperature. Thermal stability of the dried coatings was studied by TGA measurement. Thermal decomposition temperature of the dried coating based on pure acrylate latex (SMA-6) is about 325°C, while for the dried coatings based on acrylate-PDMS composite latexes, their thermal decomposition temperatures raise up to 347–362°C. Due to higher chemical-bond energy of Si– O (452 kJ/mol), more energy is needed to destruct the dried coatings based on acrylate-PDMS composite latexes, which will be benefit to their thermotolerance property (Fig. 7). Hydrophobicity properties of the dried coatings were tested by WCA measurement. From Fig. 8, it can be seen that the contact angle of the dried coatings increases linearly with the content of modified PDMS. The dried coating based on ordinary acrylate latex (SMA-6) presents hydrophilic property (WCA = 66.86°), while for the sample SMA-8, when the content of modified PDMS is 5%, the dried coating presents hydrophobic property (WCA = 102.6°). The dried coating presents low polarity with relatively good hydrophobic property. First, the polymer chains of modified PDMS will lead to decreased polarity of the
acrylate-PDMS composite latexes; Second, the densities of acrylic resin and the modified PDMS are 1.18 and 1.00 g cm3, respectively. The differences in polarity and density will cause microscopic phase separation during the film formation process although the two components are linked by a covalent bond. When the acrylate-PDMS composite latex is coated onto the substrate, the modified PDMS component tends to float to the upper layer of dried coatings. According to the results of SEM and 3D measuring laser microscope, the microscopic phase separation morphologies of the dried coatings also support our conclusion. The improved hydrophobic property of the dried coating is one of the most important factors in its corrosion prevention ability. The ability for the dried coatings to absorb water is also an important parameter for the synthesized materials.28 According to Fig. 9, water absorption ratio of the dried coatings based on acrylate-PDMS composite latexes is greatly influenced by the copolymerized PDMS component, and the water absorption ratio of the dried coatings decreases linearly with the increased content of modified PDMS component. For the dried coating based on ordinary acrylic latex (SMA-6), water absorption ratio is 15.5%; however, for the sample SMA-8, water absorption ratio reduces to 5.11%. It may be caused by the hydrophobic properties of the modified PDMS component.29–31 Inhibition of the penetration of water effectively may enhance corrosion resistance of the dried coatings. Lastly, the tested steel panels, which were coated with different samples, were soaked into 3.5 wt% NaCl solution to study the anticorrosion behavior of the dried coatings by potentiodynamic polarization test. The thickness of the dried coatings was set around
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Fig. 8: Image of WCA for the dried coatings varied with the content of modified PDMS (a) SMA-6, (b) SMA-7, (c) SMA-3, and (d) SMA-8
20
16
0.5
SMA–6 SMA–7 SMA–3 SMA–8
0.0
12 E /V
Water absorption /%
Bare sample
SMA–6 SMA–7 SMA–3 SMA–8
–0.5
8 –1.0
4
–1.5
0 0
1
2
3
4 5 Time (day)
6
7
8
–10
–8
–6 Log I /A cm–2
–4
–2
Fig. 9: Effect of modified PDMS on water absorption of dried coatings
Fig. 10: Potentiodynamic polarization curves of samples covered with various coatings: Bare sample, SMA-6, SMA-7, SMA-3 and SMA-8
100 lm, and they were dried for 7 days before being tested. Figure 10 shows the result of the polarization curves of the tested samples, and the obtained electrochemical parameters are listed in Table 5. With the content of modified PDMS enhanced, Ecorr of the dried coatings increases from 0.677 V (SMA-6) to 0.454 V (SMA-8), while Icorr of the dried coatings decreases from 2.13 9 108 A cm2 (SMA-6) to 1.26 9 108 A cm2 (SMA-8). It indicates that the
corrosion resistance of the dried coatings increases with the content of modified PDMS. The reasons are listed as follows: firstly, low polarity and high molecular weight of PDMS polymer chains can prevent the penetration of corrosive medium; secondly, a slightly crosslinked structure of modified PDMS will also benefit to improve corrosion resistance of the dried coatings.
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Table 5: Electrochemical parameters obtained from potentiodynamic polarization curves Samples
Bare sample SMA-6 SMA-7 SMA-3 SMA-8
Coating properties Ecorr (V)
Icorr (A cm2)
0.919 0.677 0.548 0.513 0.454
5.26 2.13 1.50 1.33 1.26
Conclusion A series of acrylate-PDMS composite latexes was prepared by emulsion polymerization. With the effect of physisorption of emulsifying agent, the modified PDMS and acrylate monomers could disperse and copolymerize in the micelles of the aqueous phase. The latex particles presented evenly spherical morphology, and the modified PDMS component with low polarity was absorbed in the latex particles. The dried coatings based on acrylate-PDMS composite latexes presented Sea-Island Morphology, which would enhance the hydrophobic property (SMA-8, WCA = 102.6°) and corrosion resistance of the cured coatings. With the modified PDMS, the cured coatings also present good heat resistance, and the thermal decomposition temperature can increase to 347–362°C. All of these outstanding performances make them be applicable in the heavy-duty chemical industry. Acknowledgments This work is funded by the National Natural Science Foundation of China (Grant No. 51673088), 60th Postdoctoral Foundation of China (Grant No. 2016M601750), Postdoctoral Foundation of Jiangsu Province (Grant No. 1501091c). Author also thanks Jiangsu University of Science and Technology for its support to the Innovative Programs of Undergraduate Students.
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9 9 9 9 9
105 108 108 108 108
Corrosion rate (mm a1) 0.609 2.47 9 1.74 9 1.53 9 1.45 9
104 104 104 104
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