Polar migration behavior of phosphonate groups in phosphonate esterified acrylic grafted epoxy ester composites and their role in substrate protection

2.1 Materials

Epoxy resin (E20) was obtained from Jining Huakai Resin Co., Ltd (Jining, China). Linoleic acid (LOFA) was purchased from Anhui Rifende Oil Deep Processing Co., Ltd (Langxi, China). HPA, methyl methacrylate (MMA), methacrylic acid (MAA), butyl acrylate (BA), and styrene (St) were purchased from Tianjin Hedong Hongyan Reagent Plant (Tianjin, China). The above monomers are chemically pure. 2-Hydroxyethyl methacrylate phosphonate (PM-2) was supplied by Guangzhou Weber Technology Co., Ltd (Guangzhou, China). dibenzoyl peroxide (BPO), tert-butyl peroxybenzoate (TBPO), and tetrabutylammonium bromide (TBAB) were purchased from Kingchemical Co., Ltd (Shanghai, China). Antifoaming agent (TEGO 810) and dispersing agent (TEGO 760 W) were purchased from Evonik Specialty Chemicals (Shanghai) Co., Ltd.

2.2 Synthesis of waterborne phosphorylated acrylic resin (AR-P) composite modified epoxy esters (EL) composite emulsion (EL@AR-P)

The EL was prepared by using E20 and LOFA. E20 (0.1 mol) was taken in a three-necked round bottom flask, TBAB was added as an initiator, and LOFA (0.2 mol) was added into the reaction flask with continuous stirring at 130°C for 4 h. Then, EL and HPA (0.1 mol) etherified graft (EL@ HPA) at 90°C, followed by the addition of MMA, HPA, MAA, BA, St, and PM-2 for graft copolymerization, and BPO was added as an initiator; the reaction time was 3 h at 80°C, and then the temperature was raised to 110°C, TBPO was added, and the reaction was stirred for 2 h. The modified materials (EL@AR-P) were neutralized to 0.8 in DMEA and emulsified with deionized water to produce an emulsion with a solid content of 40% for the preparation of latex films and coatings. The use of PM-2was changed to 0, 1%, 2%, 3%, and 4% of the total mass of the monomer while keeping the other conditions constant, and the prepared modified emulsions were labeled as EL@AR-P₀, EL@AR-P₁, EL@AR-P₂, EL@AR-P₃, and EL@AR-P₄. The reaction process and reaction mechanism are shown in Figure 1.

Figure 1

The reaction process and reaction equation.

2.3 Preparation of latex film

The latex film was prepared as follows: 20 g of the composite emulsion was accurately weighed and poured into the PTFE template, left to stand for 7 days at room temperature, then allowed to dry naturally, and finally put into a vacuum drying oven at 60°C for 2 h, and the film was placed in a drier for characterization.

2.4 Fabrication of composite coatings

The general procedure for the preparation of composite coatings is as follows. The EL@AR-P emulsion was poured into a beaker and DI-water was added, dispersant, pigment filler (BaSO₄, carbon black), defoamer, stirred with dispersing machine, and then the composite emulsion was mixed evenly, and finally coated on tinplate according to the existing Chinese std (GB/T 1727-1992) (27).

2.5 Characterization

The structural properties of EL, EL@AR, and EL@AR-P were characterized through Fourier-transform infrared spectra (FT-IR, VECTOR-22, Bruker) between 500 and 4,000 cm⁻¹. The stability of the emulsion was tested by Stability Analyzer (Turbiscan Lab). The particle size of the composite emulsion was measured by laser particle scattering (Panalytical, Malvern). The test temperature was 25°C, the laser angle is 90°, and the test laser wavelength was 633 nm. The stability of the emulsion was measured by a stability tester, and the samples were tested using near-infrared light (λ = 880 nm) for 2 h.

WVTR measures of coatings were carried out through a water vapor permeability tester (WVTR-W6, Pubtester). All WVTR measures were performed at 25°C and 90% RH. The OTR of the coatings was obtained via an OTR tester (OTR-O1, Pubtester). All OTR tests were conducted at 25°C and 50% RH in 1 atm oxygen partial pressure condition. Atomic force microscopy (AFM, SPA-400, SEIKO) was applied to investigate the morphology of the coatings.

Electrochemical tests were conducted in a PARSTAT MC (Princeton) electrochemical workstation with a three-electrode system. A platinum foil was used as a counter electrode, and a saturated calomel electrode was set as a reference electrode, and all test samples were used as working electrodes with an exposed area of 1 cm⁻² and the electrolyte was 3.5% NaCl solution. EIS measurement was conducted at a set frequency from 1,00,000 to 0.01 Hz, and the sinusoidal voltage signal amplitude was 10 mV. EIS results were fitted with ZsimpWin software to record corresponding corrosion parameters. Polarization curves were performed under the set potential range from −1.000 to +2.000 V with a scan rate of 0.01 V/s (28,29,30,31).

3.1 Spectral analysis of acrylate-epoxy emulsion

To investigate the chemical structure of the composite emulsion, we made a latex film of the composite emulsion and analyzed its structure using FTIR Spectrometer. The results are shown in Figure 2.

Figure 2

Infrared spectrum of EL, EL@HPA, EL@AR, EL@AR-P latex film.

From Figure 2, the stretching vibration peaks of the hydroxyl and carboxyl groups in EL@AR-P are at 3,720 and 3,618 cm⁻¹, respectively; the tensile vibration peaks of –CH₃ and –CH₂ are at 2,927 and 2,856 cm⁻¹, respectively; and the tensile vibration peak of the carbonyl group (C═O) is at 1,738 cm⁻¹. The skeleton vibration peaks of the benzene ring are at 1,507 and 1,455 cm⁻¹, respectively, the bending vibration peak of –CH₃ is at 1,376 cm⁻¹, and the tensile vibration peak of the aromatic ether is at 1,236 cm⁻¹. The characteristic absorption peaks P═O and C–O–P appear at 1,184 and 1,107 cm⁻¹, respectively, and the unsaturated double bond peak is at 1,602 cm⁻¹. Comparing the EL and EL@HPA IR spectra of Figure 2, EL@HPA has an unsaturated double-bonded peak of HPA at 1,602 cm⁻¹. Comparing the EL@HPA and EL@HPA IR spectra of Figure 2, peaks of EL@AR at 3,720 and 3,618 cm⁻¹ are the stretching vibration peaks for the hydroxyl and carboxyl groups, respectively, and a weakly unsaturated double-bonded peak of LOFA at 1,602 cm⁻¹. Demonstration of completed polymerization of acrylic monomers. Comparing the EL@AR and EL@AR-P spectra, EL@AR-P shows the characteristic absorption peaks P═O and C–O–P at 1,184 and 1,107 cm⁻¹, respectively. This is a demonstration of the complete polymerization of phosphonate ester groups in acrylic resin.

3.2 Particle size and stability analysis

Figure 3 shows the particle size distribution of EL@AR-P emulsion, and Figure 4 shows the stability curve of EL@AR-P emulsion. As can be seen from Figure 3, with an increase in the use of PM-2, the average particle size of EL@AR-P₀, EL@AR-P₁, EL@AR-P₂, EL@AR-P₃, and EL@AR-P₄ emulsions is 123.2, 115.3, 139.8, 163.7, and 190.3 nm, respectively, and the particle size distribution of the composite emulsions becomes wider and the particle size increases from 115.3 to 190.3 nm. When the PM-2 content reached 3%, the average particle size of the emulsion increased significantly, which was caused by the high phosphonate activity and the high-density crosslinking between the polymer chain segments. In the process of film formation, the smaller the particle size of the pellet, larger the capillary force and total surface area of the film, which is conducive to the interpenetration of polymer chain segments to promote film formation and improve the density of the film to prevent the penetration of water and small molecules and thus improve anti-corrosion properties (32).

Figure 3

Particle size distribution of EL@AR-P emulsions.

Figure 4

Stability of EL@AR-P emulsions.

The emulsion dispersion stability index TSI curves can be used to characterize the effect of aging time on emulsion stability. The higher the number of TSI index in a certain time, the lower the emulsion stability (33). Figure 4 shows that as the test time increases, the TSI curves of all samples shown a tendency to sharp increase at first and then flatten out, indicating that the emulsion tends to stabilize after a certain aging time. During the 120 min test, the TSI index of the emulsion was EL@AR-P₀ < EL@AR-P₁ < EL@AR-P₂ < EL@AR-P₃ < EL@AR-P₄; with increasing PM-2 content, there is a slight decrease in instability in emulsions. The TSI indices were all less than 1. EL@AR-P₃ and EL@AR-P₄ emulsions showed a significant increase over EL@AR-P₀, EL@AR-P₁, and EL@AR-P₂ emulsions, which are consistent with the particle size test.

3.3 Effect of PM-2 content on the impermeability of EL@AR-P coatings

The impermeability of the coating plays an important role in its excellent corrosion resistance. Generally, higher resistance to permeation leads to better corrosion resistance of the metal (34,35). WVTR, OTR, and water-uptake rate are critical for anticorrosive coatings.

The water vapor, oxygen permeability, and water-uptake rates of EL@AR-P coatings with different PM-2 contents were shown in Figure 5. With an increase in the PM-2 content, the WVTR, OTR, and water-uptake rate of EL@AR-P₂ coating showed a tendency to decrease at first and then increased. For EL@AR-P₀ coating to EL@AR-P₂ coating, the gradual increase in the denseness of the coating reduced the water and oxygen permeability, improved the coating’s resistance to penetration. With an increase in the PM-2 content, EL@AR-P₃ coating and EL@AR-P₄ coating resulted in excessive coating cross-linking, local defects of the curve, and the increase in water vapor, oxygen permeability, and water-uptake.

Figure 5

Oxygen (a) and water vapor (b) transmission rates of the EL@AR-P coatings, water-uptake rate of the EL@AR-P coatings (c).

3.4 EIS analysis

Figure 6a shows the Bode plot of EL@AR-P₀, EL@AR-P₁, EL@AR-P₂, EL@AR-P₃, and EL@AR-P₄ coatings in 3.5 wt% NaCl solution. The Bode plots derived from the EIS measurements with frequency ranging from 10⁵ to 10⁻² Hz were used to investigate the role of composite coatings on the protection for the tinplate (31,36). Usually, the peak appearing in the high-frequency region (10⁵–10³ Hz) of the Bode plot could be applied as an indicator of coatings responses, and the peak located at medium (10³–10² Hz) and low frequency (10²–10⁻² Hz) corresponds to the responses of corrosion, reflecting the coating failure (37,38). Besides, the impedance module at low frequencies (|Z|0.01 Hz) is usually applied as a semiquantitative indicator of the protective properties for coatings (39). With the increasing usage of PM-2, there is an increasing trend of |Z|0.01 Hz (Figure 6a), indicating the enhanced effect of PM-2 on corrosion prevention. When the amount of PM-2 is 2%, |Z|0.01 Hz of EL@AR-P₂ coating (3.17 × 10⁷ Ω cm²) is EL@AR-P₀ coating 3 times (1.05 × 10⁷ Ω cm²) (Figure 6a), means the corrosion resistance strongly improved. Bode phase (Figure 6b) shows the phase angle (PA) changes for EL@AR-P samples immersed in 3.5 wt% NaCl (aq) as a function of frequency. Generally, an ideal PA value for a polymer coating is close to −90°, which indicates that the coating belongs to a pure nonconductive capacitor (31,40). This means that a PA value approaching −90° reveals the highly capacitive behavior of the coating. As can be observed from Figure 6b that the EL@AR-P₀ coating (−82.56°) showed the lowest PA, the EL@AR-P₂ coating (−88.53°) exhibited the highest capacitive response as well as the best corrosion resistance, while the other EL@AR-P coating samples displayed an intermediate PA.

Figure 6

Bode modulus plots (a), Bode phase plots (b).

After the previous EIS test, EL@AR-P₀ coating and EL@AR-P₂ coating were selected to test the changes in EIS from immersion in 3.5 wt% NaCl (aq). Figure 7 presents the Bode plots and the phase angle change of EL@AR-P₀ coating and EL@AR-P₂ coating at different exposure times in 3.5 wt% NaCl (aq). |Z|0.01 Hz for EL@AR-P₀ decreases from 1.05 × 10⁷ to 4.77 × 10⁴ Ω cm² (Figure 7a) and EL@AR-P₂ (|Z|0.01 Hz) decreases from 3.17 × 10⁷ to 1.02 × 10⁶ Ω cm² (Figure 7b) after 16 days of immersion, respectively. Figure 7a and b plots show that EL@AR-P₂ coating decreased much more slowly than EL@AR-P₀ coating with the extension of time. Especially after 8 days of immersing in 3.5 wt% NaCl (aq), EL@AR-P₂ coating did not appear a second time constant until 16 days of immersion (Figure 7d), whereas EL@AR-P₀ coating began to appear a second time constant as early as 8 days of immersion (Figure 7c). The results showed that the addition of PM-2 effectively improved the impermeability and corrosion resistance of the coating.

Figure 7

Bode modulus plots (a), Bode phase plots (c) of EL@AR-P₀, Bode modulus plots (b), Bode phase plots (d) of EL@AR-P₂.

3.5 Polarization curve (Tafel) analysis

The potentiodynamic polarization plots of the tinplate coated with composite coatings in 3.5 wt% NaCl (aq) as a function of immersion time are shown in Figure 8. The corrosion potential (E_corr) and corrosion current density (i_corr) were fitted by electrochemical software. The inhibition efficiency (IE) was calculated through Eq. 1 (41):

where i_tin and i_corr stand for the corrosion current density of bare tinplate and coated tinplate, respectively. The fitting electrochemical parameters are recorded in Table 1. i_corr of EL@AR-P₀ coating (2.34 × 10⁻⁷ A cm⁻²) is less than bare tinplate (9.86 × 10⁻⁶ A cm⁻²). The i_corr further decreased from 2.34 × 10⁻⁷ to 1.51 × 10⁻⁸ A cm⁻² as PM-2 increased from 0% to 2%. i_corr increased to 4.02 × 10⁻⁸ A cm⁻² when PM-2 is over 3%. The significant reduction in i_corr demonstrated more effective corrosion protection and better resistance toward diffusive ions (41,42). Besides, the current densities of EL@AR-P₂ coating are lower than the EL@AR-P₀ coating, indicating the efficient suppression effect of EL@AR-P₂ coating (43). Further, E_corr of EL@AR-P coatings shift to the positive direction, and the EL@AR-P₂ coating displays the most positive value (−0.512 V), suggesting better anticorrosive property than other samples, which are consistent with EIS results.

Figure 8

Potentiodynamic polarization curves.

3.6 Morphological analysis of corrosion area

After the electrochemical measurements, EL@AR-P₀ coating and EL@AR-P₂ coating were peeled off from the tinplate surfaces. SEM observations for the tinplate surfaces were performed to confirm the electrochemical measurements. The surface chemical composition of the tinplate sample was also detected using EDS. The SEM morphologies of the tinplate are shown in Figure 9. The surface of the tinplate protected by the EL@AR-P₀ coating suffered more serious corrosion, with a large and deep area of corrosion. While the tinplate protected by the EL@AR-P₂ coating had a smoother surface, with a small and shallow area of corrosion, which indicates that the EL@AR-P₂ coating has better corrosion resistance than EL@AR-P₀ coating.

Figure 9

SEM images on coating-exfoliated steel surfaces after 16 days of immersion in 3.5 wt% NaCl solution, (a–c) EL@AR-P₀, (d–f) EL@AR-P₂, (a and d) magnification level of 2,000×, (b and e) magnification level of 10,000×, (c and f) magnification level of 20,000×.

The EDS results are listed in Table 2. During immersion, corrosion products were mainly generated through the electrochemical reaction of tinplate with water molecules as well as oxygen in the electrolyte. The O and Fe are regarded as the two primary elements for chemical composition analysis on tinplate surfaces, whereas the C element was ignored (31). From Table 2, 53.13 wt% of the O element and 20.06 wt% of the Fe element and 44.30 wt% of the O element, and 29.95 wt% of the Fe element were detected on the tinplate surfaces coated by EL@AR-P₀ coating and EL@AR-P₂ coating. Therefore, it is easily concluded that the EL@AR-P₂ coating possesses better corrosion resistance than EL@AR-P₀ coating, which is consistent with the EIS results.

3.7 NSS test results analysis

Salt spray test is the use of a salt spray tester to simulate environmental conditions to evaluate the corrosion resistance of the coating. The EL@AR-P₀ coating and EL@AR-P₂ coating were tested using a salt spray tester for 48, 96, 144, 192, 240, 288, 336, and 384 h NSS experiments. The results of the experiment are shown in Figures 10 and 11.

Figure 10

Salt spray test diagram of EL@AR-P₀ film: 48 h (a), 96 h (b), 144 h (c), 192 h (d), 240 h (e), 288 h (f), 336 h (g), 384 h (h).

Figure 11

Salt spray test diagram of EL@AR-P₂ film: 48 h (a), 96 h (b), 144 h (c), 192 h (d), 240 h (e), 288 h (f), 336 h (g), 384 h (h).

From Figures 10 and 11, EL@AR-P₀ coating and EL@AR-P₂ coating showed almost the same corrosion in 96 h. However, EL@AR-P₂ coating showed significantly less corrosion. At 144–288 h, EL@AR-P₀ coating (Figure 10c–f) corrosion area had significant spread and more corrosion products appeared. There is a small amount of bubbling at 192 h (Figure 10d). While in the same period, there is no significant expansion in the corrosion area and fewer corrosion products for EL@AR-P₂ coating. And there is a small number of bubbles that appeared at 288 h (Figure 11f). At 288–384 h, the corrosion area of EL@AR-P₀ coating (Figure 10g and h) continues to expand, while the corrosion area of the EL@AR-P₂ coating (Figure 11g and h) did not expand significantly. Salt spray test results showed that EL@AR-P₂ coating has better corrosion resistance than EL@AR-P₀ coating. NSS result was consistent with EIS and SEM results.

3.8 Characterization of corrosive products

For a further study of the effect of composite coatings on the tinplate beneath them, XRD was employed to evaluate the chemical composition of the tinplate surface for 16 days of immersion in 3.5 wt% NaCl (aq). The tinplate coated by EL@AR-P₀ coating was applied as a control to contrast its corrosion condition with EL@AR-P₂ coating. The EL@AR-P₂ coating was selected due to its better corrosion resistance than other composite coatings and the worst protection performance among EL@AR-P₀ coating according to the electrochemical results. The XRD patterns of EL@AR-P₀ coating and EL@AR-P₂ coating are shown in Figure 12.

Figure 12

XRD patterns of surface of tinplate samples coated by EL@AR-P₀ (a) and EL@AR-P₂ (b) during immersion time.

Permeating through the coating, corrosive electrolytes were produced by FeOOH, Fe₂O_3, and Fe₃O₄ composed of a corrosion layer (28,29,31). During immersion, the corrosion products consisted of FeOOH, Fe₂O₃, and Fe₃O₄. The corrosion products of EL@AR-P₀ coating was mainly FeOOH, while those of the EL@AR-P₂ coating were mainly FeOOH, Fe₂O₃, and Fe₃O₄. Fe₂O₃ and Fe₃O₄ are caused by the lack of oxygen on the surface of the tinplate. These results indicated that EL@AR-P₂ formed a passive film consisting of Fe₂O₃ and Fe₃O₄ on the tinplate surface and assisted in prevent further corrosion of the substrate.

3.9 EDS and XPS analysis of cross-section

To further investigate the polar migration behavior of the phosphonate groups. The cross-section of EL@AR-P₂ emulsion film formation on PTFE and the cross-section of EL@AR-P₂ coating was carried out to EDC measurements that were taken to observe the distribution of P element. Figure 13a is the cross-section of coating and substrate, which shows that the distribution of P elements is more concentrated in the contact measurement part of coating and substrate, while Figure 13b is the cross-section of PTFE film formation, where the distribution of P elements is more uniform, which also shows that the phosphonate groups of EL@AR-P₂ are distributed on the surface of the substrate during the process of film formation. To determine whether the phosphonate groups exhibit polar migration behavior and whether the phosphonate ester chelates with the substrate surface, XPS was used to characterize the cross-section of the EL@AR-P₂ coating. Figure 14a proves that the P element is indeed present in the cross-section where the coating is in contact with the substrate. After treatment, the peak of the P element can be divided into three peaks, located at 131.7, 132.2, and 132.6 eV, respectively. The peaks at 131.7 and 132.2 eV belong to PO₄³⁻. The peak at 132.6 eV belongs to P–O–Fe (44,45) which is formed due to the chelation of the phosphonate group with the substrate surface. To confirm the above conclusion, an XPS fit analysis of the iron element was performed. As shown in Figure 14b, the peak was divided into six peaks. The peaks at 701.2 and 709.6 eV are attributed to Fe and FeO, respectively. 711.2 eV is the characteristic peak of Fe₂O₃. 723.6 eV is the characteristic peak of FeOOH (46). The remaining two peaks, 713.2 and 720.3 eV, are characteristic peaks of iron chelate phosphonate. EDS and XPS results demonstrated the existence of polar migration of phosphonate groups in the film-forming process.

Figure 13

(a) Coating cross-section, (b) EDS of coating cross-section, (c) cross-section of PTFE film formation, (d) EDS of film cross-section, (a and c) magnification level of 200×.

Figure 14

XPS spectra of cross-section of the EL@AR-P₂ coating: (a) P, (b) Fe.

3.10 Anticorrosion mechanism of EL@AR-P coatings

The anticorrosion mechanism of the modified EL@AR-P coatings can be explained by the variations of electrochemical and chemical reactions occurring at the coating interface. For electrochemical reactions, anodic half-reaction Eq. 2 and cathodic half-reaction Eq. 3 are the two dominant half-reactions accounting for the corrosion at the interface. The final corrosion reaction can be expressed by Eq. 4, while the presence of chloride ions as a catalyst will accelerate the corrosion reactions Eq. 5 and 6. Based on the above analysis, it can be concluded that the existence of the H₂O, O₂, as well as Cl⁻ in the electrolyte was the critical factor impacting the entire corrosion process of a polymer coating system. The chemical reaction that occurs at the EL@AR-P coating interface is as follows Eq. 7.

For the EL@AR-P coating, it is usually regarded as an organic barrier layer to prevent corrosive electrolytes from permeating the coatings along with the defects. The corrosion reactions near the interface of the coating/tinplate will result in the pitting and delamination of the EL@AR-P coating from the tinplate substrate. Finally, the tinplate loses protection from the EL@AR-P coating (46). The EL@AR-P₂ coating slows down the O₂, H₂O, and Cl⁻ invasion when compared with the EL@AR-P₀ coating. The EL/AR-P₂ coating improves the film formation of the composite emulsion due to the introduction of the phosphonate group and the coating is also polarized due to the polar migration. Improves the ability of the coating to bond tightly to the substrate surface, reducing possible defects in the coating and increasing the impermeability of the coating, thereby slowing down the corrosion rate of the metal substrate. Moreover, the phosphonate group can chelate with Fe²⁺ to reduce further corrosion reactions, which is reduced in Eq. 4, 5, and 7.