Preparation of Palygorskite hybrid iron oxide red pigment and its application in waterborne polyurethane composite coatings and ceramics

Palygorskite (Pal) with the main composition of CaO (1%), Al₂O₃ (8%), Na₂O (2%), MgO (13%), SiO₂ (59%), K₂O (1%), and Fe₂O₃ (5%) was a gift from Jiuchuan Clay Technology Co. (Jiangsu, China). Red iron oxide (α-Fe₂O₃, 190) was provided by Jiangsu Yuxing Industry and Trade Co. Ltd (Jiangsu, China). The color light of 190 was L* = 37.56, a* = 25.60, and b* = 13.73. Meanwhile, the tinting strength of 190 was L* = 54.78, a* = 24.06, and b* = 3.80. Sodium hexametaphosphate ([NaPO₃]₆) was purchased from China National Medicines Co. Ltd (Shanghai, China). The dispersant of EDAPLAN^®494 for red iron oxide was obtained from MÜNZING (Germany). The deionized water was used throughout the experiments.

Before the synthesis of the Pal/α-Fe₂O₃ hybrid pigments, 1 g Pal was mixed with 0.01 g sodium hexametaphosphate. Then, 1, 2, 4, 8, 12, and 15 g red iron oxide dispersed in 0.01, 0.02, 0.04, 0.08, 0.12, and 0.15 g EDAPLAN^®494, respectively, were added to the above Pal mixture (The compositions of mixed pigments were referred to table S1 (available online at stacks.iop.org/MRX/9/065202/mmedia)). The resulting hybrid was grounded with ZrO₂ microsphere at 1350–1650 r min⁻¹ for 90 min, washed with deionized water three times, filtered with 200 mesh screens, and dried in an oven at 80 °C. After that, the as-prepared pigments were ground with agate mortar and screened with 140 mesh screens to obtain Pal/α-Fe₂O₃ hybrid pigments named cp1, cp2, cp4, cp8, cp12, and cp15, respectively.

According to the Commission Internationale de L'Eclairage 1976 color space (L*a*b*), the color of each Pal/α-Fe₂O₃ hybrid pigment was analyzed three times by a Datacolor 500 spectrophotometer (Datacolor, USA). In L*a*b* color space, each color is represented by three numbers L*, a*, and b*, while L* represents brightness, a* represents the green to the red component, and b* represents the blue to the yellow component. The larger L*, the higher the brightness. When L* is 0, it represents black, and when L* is 100, it represents white. When a* and b* are 0, both represent gray. a* and b* change from a negative number to a positive number, and the corresponding color changes from green to red and blue to yellow, respectively. The range of color channels a* and b* are −100 ∼ 100 and −128 ∼ 127. The − presented as ΔE = (ΔL² + Δa² + Δb²)^1/2, where ΔL = L_cp − L₁₉₀ is the lightness difference, Δa = a_cp−a₁₉₀ is the red/green difference, Δb = b_cp−b₁₉₀ is the yellow/blue difference. C = [(a)² + (b)²]^1/2 is the saturation of the color.

The tinting strength test was performed by diluting the pigment with a white base material (such as ZnO and TiO₂) and then measuring the color performance with a Datacolor 500 spectrophotometer. In this case, the sample was prepared by mixing the pigment and TiO₂ in a ratio of 1:20. The color performance was compared with the diluted 190 red iron oxide standard sample.

The XRD patterns were collected by the X-ray powder diffractometer (Ultima IV, Rigaku) at a measuring 2θ range of 5 ∼ 80°, with Cu-Kα ration (40 mA, 40KV), 0.025° (2θ) step size, and 10° (2θ) scan rate per minute. The powder sample was ground for 15 min and then laid flat on a glass sample plate with a thickness of about 0.5 mm. The Fourier Infrared Spectrum was conducted by Nicolet IS20 (Thermo Scientific, USA), in a range of 4000–400 cm⁻¹. UV–vis reflectance spectra were collected by UV-2600 (Shimadzu, Japan), in a range of 350–750 nm. The Zeta potential and particle size distribution were determined by NanoBrook Omni (Brookhaven, USA). The morphology of the pigments was observed by a field emission scanning electron microscope (Nova Nano SEM450, FEI, USA). The sample was ultrasonically dispersed in absolute ethanol for about 30 min and then dropped onto the silicon wafer. The thermal gravity (TG) curves were recorded by TG209 F3 thermogravimetric analyzer (NETZSCH, GER) in the heating range from room temperature to 800 °C, and the heating rate of 10 °C min⁻¹. For the high-temperature calcination test, the composite pigments were respectively treated with a muffle furnace (carbonite cwf1100, GER) at 700, 950, and 1100 °C for 30 min under a heating rate of 10 °C min⁻¹. After naturally cooling to room temperature, XRD and ultraviolet diffuse reflection were performed to evaluate the high-temperature resistance. The chemical composition of hybrid pigments was measured by ARL X4200 X-ray fluorescence spectrometer (Thermo Fisher Scientific, USA).

The waterborne polyurethane composite coating was synthesized by adding 7.5 g waterborne polyurethane, 0.5 g defoamer, and 0.2 g dispersant into a 25 ml beaker, and magnetically stirring for 30 min at the rotation speed of 900–1200 r min⁻¹. Then 1 g of Pal/α-Fe₂O₃ hybrid pigment and an appropriate amount of deionized water was added to adjust the viscosity. The mixture was kept under magnetic stirring at 1500–1800 r min⁻¹ for 4 h, followed by standing for 12 h for later use.

The ceramics containing 190 red iron oxide or composite pigment cp8 was synthesized by adding 8 g ZnO, 4 g MgO, 21 g B₂O₃, 12 g SiO₂, 10 g Al₂O₃, and 4 g cp8 into a ball mill tank and mixing them initially. After adjusting the viscosity with a certain amount of water, the mixture was grounded with an F-P2000 (FOCUCY, CHN) high-performance planetary ball mill for 2 h. Then, the mixture was dried in an oven at 100 °C followed by sieving with a 40-mesh screen to obtain the ceramic powder. The resultant powder was pressed into a 1.5 cm diameter disc with a tablet press machine and burned in a muffle furnace at 800 °C for 3 h to ceramic discs. After that, the color performances of the obtained ceramics were analyzed according to CIE 1976 l*a*b*standard colorimetric method.

The color light and tinting strength played key roles in the property and application of a red pigment (Jose et al 2019). The color light and tinting strength properties of each composite pigment were shown in figure 1 and table 1. After adding Pal to red iron oxide, the color light property of the composite pigment decreased slightly with the increase of Pal content. However, when the Pal/α-Fe₂O₃ mass ratio exceeded 1:12, the color light performance of the pigment decreased significantly and was close to that of pure red iron oxide. On the other hand, with the increase of Pal content, the tinting strength of the composite pigments increased largely and reached its largest value at the Pal/α-Fe₂O₃ mass ratio between 1:4 and 1:8. Thus, the optimum ratio of Pal/α-Fe₂O₃ for the preparation of composite pigments was between 1:4–1:8, which displayed the best color light and tinting strength properties. The composite pigments cp1 and cp2 had the highest color light redness values, but their tinting strength redness values were low (table 1). Meanwhile, the tinting strength of cp12 and cp15 was relatively improved, but their color light performance was poor. On the other hand, cp4 and cp8 had the most balanced color light and coloring intensity, the coloring intensity was higher than other composite pigments, and their color light redness value and tinting strength redness value were about 10% higher than those of 190 iron oxide red, showing the best color performances of Pal/α-Fe₂O₃ at the optimum ratio of 1:4–1:8. From the result of XRF (table S2), with the increase of Fe content in the hybrid pigment, the shading performance of the hybrid pigment will continue to improve, but when the Fe content reaches more than 92%, the increase in shading will tend to 0. At the same time, the tinting strength of the hybrid pigment will increase with the content of Fe first, reaching a peak when the Fe content is 70%–85%, and then the tinting strength will continue to decrease with the increase of the Fe content.

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The color of Pal/α-Fe₂O₃ was also analyzed by reflectance spectroscopy (Baryshev et al 2004, Wang et al 2018). The level of reflectivity (R%) at the same wavelength meant the saturation and lightness of the pigment color. The higher the reflectivity, the higher the color saturation and lightness. As shown in figure 2(a), the reflectance of composite pigments cp1, cp2, and cp8 was significantly higher than that of other pigments, indicating their excellent color saturation and lightness. Figure 2(b) was the absorption edge of 190 and the composite pigments. The absorption edge (λ) of the composite pigment was derived from the Kubelka–Munk function of F(R) = (1 − R)²/2 R (Wang et al 2020). The redness of red iron oxide pigment was closely related to the bandgap (Guan et al 2016). The bandgap (E_g ) of the composite pigments was calculated by the formula of E_g = 1240/λ. The absorption edge and bandgap of each composite pigment were displayed in table 1. The absorption edges of all composite pigments were blue-shifted, while the largest and the smallest bandgap were of cp1 and cp15, respectively. The bandgap decreased with the iron oxide content increasing, which was due to the change in the redness value (a) of the composite pigment. The changing trend of the chromatic redness value (a) of the composite pigment was basically consistent with the changing trend of the bandgap (table 1). The increase in the bandgap and redness value of the composite pigment was due to the quantum confinement effect induced by the small size region (Liu et al 2015).

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Figure 3 showed the SEM and TEM morphologies of the red iron oxide pigment (190), Pal, and Pal/α-Fe₂O₃ hybrid pigment. The red iron oxide pigment displayed an irregular spherical morphology with a size of 100–150 nm (figures 3(a) and (d)). The Pal showed a typical rod-like structure with rods length of 0.5–5 μm, and sizes of 30–60 nm (figures 3(b) and (e)). For Pal/α-Fe₂O₃ hybrid pigment, red iron oxide can be evenly coated on the Pal rod (figures 3(c) and (f)), which increased the distance between pigment particles and thereby improves the dispersion of pigment (Lu et al 2019). We reasoned that Pal plays an important role in controlling the structure and color performances of composite pigments, which was the major factor to improve the color performances of red iron oxide composite pigments (Wheeler et al 2012).

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There are four common types of iron oxide (Fe₂O₃), namely α-Fe₂O₃, β-Fe₂O₃, γ-Fe₂O_3, and ε-Fe₂O₃. α-Fe₂O₃ is the most common and stable type of iron oxide, with a uniform hexagonal crystal structure, and could be used as the red pigment (Song et al 2014). Figure 4 showed the x-ray diffraction patterns of 190, Pal, and composite pigments. The characteristic peaks of α-Fe₂O₃ crystal were observed at 24.14° (012 planes), 33.15° (104 planes), 35.61° (110 planes), 40.85° (113 planes), 49.48° (024 planes), 54.09° (116 planes), 57.59° (018 planes), 62.45° (214 planes) and 63.99° (300 planes), showing a high purity of α-Fe₂O₃ in red iron oxide pigment 190 (Liu et al 2013). Pal displayed the characteristic peaks at 8.41° (110 planes) and 13.86° (200 planes) (Giustetto and Wahyudi 2011, Boudriche et al 2014). After being hybridized with Pal, the characteristic peaks of α-Fe₂O₃ crystal in the composite pigment were maintained unchanged. However, as the mass ratio of Pal in complex pigments decreased, its characteristic peak gradually decreased. When the ratio of α-Fe₂O₃ in the composite pigment was increased to 1:4, the crystal structure of the composite pigment has become consistent with that of α-Fe₂O₃. This was possibly due to the decrease of Pal in composite pigments and the damage of Pal rod structure caused by mixing and grinding during the preparation of composite pigments (Bruni et al 1999, Ruiz-Hitzky et al 2013).

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FT-IR spectra showed two characteristic bands at 515 cm⁻¹ and 425 cm⁻¹ for red iron oxide pigment 190, which were attributed to Fe-O-Fe stretching vibration and Fe-O-Fe bending vibration, respectively (figure 5(a)) (Frost et al 2001). These two characteristic bands could also be observed from cp4 and cp8. Similarly, the characteristic bands of Pal at 3550 cm⁻¹ (stretching vibration of (Fe/Mg)OH or (Al/Mg)OH), 1660 cm⁻¹, and 1200 cm⁻¹ (stretching vibration of Si-O-Si groups connected two) also appeared in cp4 and cp8 (Yan et al 2012). The slight shift of 1660 cm⁻¹ indicated the strong interaction between Pal and α-Fe₂O₃ (Frini-Srasra and Srasra 2008).

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TG measurements showed that the red iron oxide pigment 190 has almost no mass loss (1.5%) even when heated to 750 °C (figure 5(b)). TG curve of Pal showed three stages of mass loss. The mass loss from room temperature to 100 °C came from the physically adsorbed water and a small part of zeolite water in Pal; the mass loss from 100 °C to 400 °C was related to the coordination water and most of the zeolite water in Pal; the loss of quality from 400 °C to 540 °C came from the release of structural water in Pal (Boudriche et al 2012). When the temperature rose to 540 °C, the mass loss reached 15%. At high temperatures, some hydroxyl groups in Pal were dehydroxylated, resulting in a slight mass loss (Gueli et al 2018). As the content of 190 in the composite pigment increased, the quality loss gradually decreased and mainly came from Pal. Therefore, the thermal stability of composite pigments was somewhat lower than pure α-Fe₂O₃.

As a colorant, the most important performance index of red iron oxide pigment was tinting power and shade, especially the tinting power reflecting the efficiency and ability of the pigment (Jose et al 2019). The dispersity and particle size of pigment were the main factors affecting these two properties. In general, the tinting strength of pigment increased with the decrease in particle size and the increase in uniformity and dispersion. As a white powder, Pal will reduce the concentration and tinting strength of the red iron oxide pigment after it is added to the red iron oxide pigment. However, the tinting strength of composite pigments is improved instead of undiminished by adding Pal. We added an experiment by only ball-milling 190 red iron oxide, without adding Pal. Then the tinting strength of the 190 iron oxide red pigment was monitored after ball milling. ΔL, Δa, and Δb were determined to be 0.57, 0.31, and 2.04, respectively. Although the a* of iron oxide red pigment 190 has been improved, its tinting strength was slightly less than that of cp8 Pal/α-Fe₂O₃ composite pigment. Therefore, we believe that Pal plays a key role in the performance of pigment tinting strength. Pal was negatively charged due to the abundant hydroxyl groups on its surface, while α-Fe₂O₃ is positively charged due to the presence of oxygen vacancies on its surface (Galan 1996). Therefore, α-Fe₂O₃ particles could coat to Pal surface through electrostatic interaction. This could be verified through the zeta potential test (figure 5(c)). 190 showed positive potential, while Pal and all composite pigments were negative potentials, indicating that α-Fe₂O₃ is successfully dispersed on Pal's surface.

In addition, Pal not only acted as the adhesion base of α-Fe₂O₃ in composite pigments, but also could increase the dispersion of α-Fe₂O₃ particles through steric hindrance, reduce the average particle size, and make the particle size distribution more uniform. The average particle size of composite pigments was determined by DLS measurements (figure 5(d)). The average particle size of 190 was about 350 nm, while cp1 and cp12 had particle sizes of 300 nm, cp4 and cp8 had particle sizes of 230 nm. At the same time, the particle size distribution of compound pigment was much narrower than that of 190, indicating that the particle size distribution of compound pigment became more uniform. This increased the tinting strength and tinting power of pigment.

Iron oxide red was a commonly used pigment, especially for building exterior walls, antirust coatings, and ceramic products. In this work, cp8 was used as a raw material to prepare water-based paint that can be coated on the surface of metal and glass. Figure 6(a) was the actual sample picture of the coating, showing that the coating exhibits a bright red color. The color of the paint only slightly changes in acid, alkali, and organic solvents, indicating that the pigment had good corrosion resistance and stability (table S3). The tape test method was used to measure the adhesion of the coating on the glass sheet and the iron sheet. According to the American Society of Testing Materials standard, the adhesion of the coating on the two substrates has reached the 4B standard. Under the same conditions, the paint with cp8 as the raw material was completely suspended for six months, and there was no change compared with the original paint, while the paint made of 190 iron oxide red had obvious delamination, which indicated that the water-based coating with the composite pigment cp8 had better storage stability (figure 6(b)).

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In addition, the waterborne coating with cp8 as the raw material could maintain a relatively stable color and light performance in different solvents or sodium chloride solution (table S3), indicating that Palygorskite could improve the color performance at the same time, ensure the resistance of the composite pigment, and would not reduce the viscosity and stability of the paint (figure 6(b)). Afterward, the effect of pH on the stability of the coatings was evaluated by measuring the Colorimetric coordinates of the coatings in solutions of different pH. The results (table S4) demonstrated that the changes of L*a*b* for the coating were slightly between different pHs. This showed that the coating could maintain good stability at different pHs.

Then we added cp8 to the ceramics during the preparation process to give the ceramic a red color and made the color not fade after high-temperature calcination. The XRD characteristic pattern of the composite pigment did not change significantly at all three calcination temperatures, and the characteristic peaks belonging to α-Fe₂O₃ did not disappear (figure S1). This showed that the crystal form and structure of the red iron oxide crystals in the composite pigment did not change significantly after high-temperature calcination, indicating that the composite pigment had good high-temperature resistance. From the digital picture of the ceramic sheet, the ceramic color after high-temperature firing was bright and smooth (figure 6(C)). The redness value a* of cp8 ceramic was 30.2, which was much higher than the redness value of 25.4 for 190 ceramic. Moreover, the color, hardness, and smoothness of the ceramic chip made of cp8 are better.