Article pubs.acs.org/IECR
Modification of C.I. Pigment Red 21 with Sepiolite and Lithopone in Its Preparation Process Lingyun Cao,† Xuening Fei,*,†,‡ Tianyong Zhang,† Lu Yu,‡ Yingchun Gu,‡ and Baolian Zhang‡ †
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Colleges of Science, Tianjin Chengjian University, Tianjin 300384, China
‡
ABSTRACT: A novel method for modifying C.I. Pigment Red 21 with sepiolite and lithopone was applied during its preparation process. The morphology and structure of the modified pigment were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Fourier transform infrared (FT-IR) spectroscopy. The results showed that sepiolite was encapsulated by the organic pigment and lithopone was deposited on the surface of the organic pigment. The particle size of the modified pigment was comparable to that of the original pigment, but the modified pigment had a narrower scatter of diameter and a higher homogeneity than the original pigment. The properties of the modified pigment including tinctorial strength and flowability were slightly improved, and its heat resistance and light fastness were enhanced by 20 °C and one grade, respectively. Thermal and UV analyses showed that the thermo- and photostabilities of the modified pigment were significantly improved. its dispersion ability in waterborne systems.10 Some researchers have expanded these technologies in the modification of dyes to improve their application characterization. Zhao et al.11 increased the fluorescence intensity, nonleachability in polar solvents, and photostability of C.I. Basic Violet 10 (Rhodamine B) by encapsulating it within a silica layer using a sol−gel process. Modification with inorganic materials can obviously improve the properties of organic pigments, especially their thermal and photostabilities. However, most modifications are carried out after the preparation process, which prolongs the process and increases the costs. Organic polymers can provide active sites for modification, the improvement of hydrophilic/hydrophobic properties, and the formation of microcapsules, so they are widely used in the encapsulation of organic pigments12 and dyes.13 There are three major methods for encapsulating organic pigments with polymers, including polymer adsorption,14 polymer grafting,15,16 and miniemulsion polymerization.17,18 Polymer adsorption forms a coating layer through a simple physical adsorption process, but the coating layer is not strong enough for field applications. Polymer grafting improves the stability of the coating layer on the surface of the organic pigment through chemical bonding;19 however, this method requires the creation of specific functional groups on the organic pigment surface. Miniemulsion polymerization can fabricate sufficiently small polymer particles and is thus predominantly carried out within preexisting miniemulsion droplets, which is attractive for organic surface coating.18,20 The encapsulation of polymers can improve the dispersion ability in aqueous solution or organic solvent by the hydrophilic and hydrophobic groups on the polymers, but because of the inherent limitations this
1. INTRODUCTION Organic pigments have bright colors, light tones, and various species, so they are widely used in materials such as printing inks, plastics, and industrial coatings. However, the inherent structures of organic pigments lead to some drawbacks including low thermal and UV stabilities, low weather durability, and low migration resistance.1 To improve the application properties of organic pigments, many methods have been developed, such as the preparation of hybrid pigments and the encapsulation of organic pigments with inorganic materials or polymer. Adsorption of organic dyes onto inorganic materials is a common method to prepare hybrid-pigment.2−6 Jesionowski et al.2 prepared a series of hybrid-pigment by adsorption basic dyes and acidic dyes on the surface of silica supports modified with silane coupling agent. Tang et al.3,4 fabricated yellow pigments with excellent thermo- and photostability by the intercalation Mordant yellow 3 (MY3) anions and C.I. Mordant Yellow 10 (HSAB) to Zn−Al Layered Double Hydroxides (LDHs). Grafting and dry milling process can also produce hybrid pigments with core−shell structure. Fei et al.5 grafted the 808 scarlet (known as 808 Scarlet in China) onto inorganic silica core to enhance heat resistance, color strength and dispersing stability of the organic pigment. Hayashi et al.6,7 produced a nanosized core−shell pigment by dry milling of organic pigments together with nanosized and surface-modified silica powder. The core−shell organic pigment particles had a uniformed spherical shape and a narrow size distribution that controlled by the size of silica. Encapsulation with inorganic materials can improve the properties of organic pigments through the inherent advantages of inorganic materials. Yuan et al.8,9 encapsulated nanosilica onto the surface of a yellow organic pigment using a layer-bylayer assembly technique to improve the thermal stability, wettability, acid and alkali resistance, and weatherability of Pigment Yellow 109. Additionally, they covered Pigment Yellow 109 with titania using a sol−gel process to improve © 2013 American Chemical Society
Received: Revised: Accepted: Published: 31
July 10, 2013 November 28, 2013 December 4, 2013 December 4, 2013 dx.doi.org/10.1021/ie4021914 | Ind. Eng. Chem. Res. 2014, 53, 31−37
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Article
Scheme 1. Preparation of the Modified Pigment
Figure 1. SEM images of (a) the original pigment, (b) sepiolite, (c) the modified pigment, and (d) the structure of the modified pigment.
agitation for 1 h. Then, the mixture was filtered, washed to neutral, and dried at 120 °C for 12 h. Next, the naphthol AS solution and diazonium salt were prepared. A mixture of sulfonated castor oil (50%, 1.16 g), NaOH (2.16 g), Nekal BX (0.16 g), and water (45 mL) was heated to 65−70 °C with continuous magnetic stirring, and then naphthol AS (6.5 g) was added to obtain the complete solution. Activated sepiolite (1.5 g) was added to the complete solution to adsorb naphthol AS for 3−5 min, which then formed a turbid liquid. Then, the turbid liquid was poured into a four-neck round-bottom flask and mixed by mechanical agitation at 35−40 °C. Aniline (2.3 mL, 0.024 mol) was dissolved in a solution of water (35 mL) and HCl (32%, 7.2 mL), and then the mixture was cooled and kept at 0−5 °C. A NaNO2 (1.75 g, 0.025 mol) aqueous solution with a concentration of 30% (w/w) was dropwise added to the mixture, and this process was completed within 10−12 min. Finally, the modified pigment was prepared. The diazonium salt solution was poured into a pressure-equalizing funnel and added dropwise to couple with the naphthol AS solution; the coupling process was completed within 40 min. After the coupling reaction was completed, the pH of the solution was
methods cannot make obvious improvement in the thermal and photostability of organic pigments. Herein, we describe a novel method for modifying C.I. Pigment Red 21 with sepiolite and lithopone during its preparation process to simplify its modification process and reduce the cost. The properties of the modified pigment were determined and compared with those of commercial products. The morphology and crystalline forms of the hybrid pigments were analyzed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD).
2. EXPERIMENTAL SECTION 2.1. Materials. Sulfonated castor oil, Nekal BX, and naphthol AS were obtained commercially, as were the original pigment (C.I. Pigment Red 21), sepiolite, and lithopone B311. The other reagents NaOH, HCl, and NaNO2 were analytical reagents. 2.2. Methods. 2.2.1. Preparation of the Modified Pigment. First, sepiolite was activated by the following procedure: A mixture of sepiolite (80 g) and HCl (10%, 320 mL) was heated to 60−70 °C with continuous mechanical 32
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Figure 2. TEM analysis of the modified pigment.
heat resistance of the pigments was analyzed by differential thermal analysis (DTA) combined with thermogravimetric analysis (TGA), and the UV−vis spectra of the organic pigments were obtained with a Lambda 35 spectrometer in diffuse-reflectance mode in the powder state.
adjusted to 1−2 with HCl (30%, 2 mL), and the mixture was allowed to react for 30 min. Then, lithopone B311 (1.7 g) was added and the mixture was reacted for another 3−5 min. The mixture was heated and kept at 95−100 °C for 1−1.5 h. The filtered product was washed to neutral, dried at 75−80 °C and grinded to obtain modified pigment. Based on the preceding description, the process for preparing the modified pigment is depicted in Scheme 1. 2.2.2. Characterization and Property Determination of Modified Pigments. The tinctorial strength, flowability, and light fastness of the pigments were determined according to Chinese National Standards GB/T 1708-79, 1719-79 and 17102008, and the heat resistance was tested by a chemical industrial standard method (Determination of the Heat Resistance Property of Dry Powder Dyestuff, HG/T 3853-2006). To determine their particle size distributions, the pigment samples were pretreated by sonication in water for 2 h and then analyzed with a nanoparticle analyzer (DelsaNano C, Beckman Coulter, Fullerton, CA). The morphologies of the pigment samples were determined by scanning electron microscopy (SEM, S-4800, Hitachi Corporation, Tokyo, Japan) and transmission electron microscopy (TEM, JEM-2100, NEC Corporation, Tokyo, Japan). The crystalline forms of the pigments were measured by X-ray diffraction (XRD, BDX330, Peking University Instrument Factory, Beijing, China). The
3. RESULTS AND DISCUSSION 3.1. Characterization of the Modified Pigment. 3.1.1. SEM Analysis. The SEM images of the original pigment, sepiolite, the modified pigment, and the structure of the modified pigment are shown in Figure 1. As shown in Figure 1a,c, the particle size of the modified pigment was comparable to that of the original pigment, but the modified pigment had a more uniform particle size distribution than the original pigment. Comparison of panels b−d of Figure 1 indicates that most of the bar-shaped sepiolite was covered by organic pigment, as expected. 3.1.2. TEM Analysis. TEM images of the modified pigment are presented in Figure 2. As shown in Figure 2a, the barshaped sepiolite was fully encapsulated by the organic pigment, which is consistent with the SEM analysis results shown in Figure 1d. To observe the structure of the modified pigment, further analysis was carried out, and the results are shown in Figure 2b. It can be seen that the modified pigment had a clear core−shell structure with a sepiolite core and an organic 33
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disappeared because of the reaction between its composition ZnS and HCl in the preparation process. The reaction resulted in the release of H2S and then left holes on the surface of lithopone, which could provide binding sites for its assembly with organic pigments. Comparison of the curve of the modified pigment with that of sepiolite shows that the primary diffraction peaks at 25.53°, 17.74°, 15.08°, and 11.38° in the curve of sepiolite disappeared in the curve of the modified pigment. As reported in the literature,5 it was concluded that the sepiolite was fully encapsulated by the organic pigment. 3.1.4. Fourier Transform Infrared (FT-IR) Analysis. The results of infrared spectroscopy analysis of the original and modified pigments are shown in Figure 4. Figure 4a shows that the infrared spectrum of the modified pigment was similar to that of the original pigment. To investigate the interaction between sepiolite and the organic pigment, the infrared spectrum between the wavenumbers of 450 and 2000 cm−1 was observed on a large scale, and the results are shown in Figure 4b. In the curve of sepiolite, the characteristic peaks at 449, 940, and 1100 cm−1 correspond to the Si−O−Si stretching vibration absorption peak, the Si−O−Si rocking vibration absorption peak, and the Si−O asymmetric absorption peak. These characteristic peaks disappeared in the curve of the modified pigment, suggesting that the sepiolite was encapsulated by the organic pigment. Sepiolite is a natural nanoinorganic material with a porous structure and good adsorption capability,21 so it is widely used in wastewater treatment22 and the preparation of hybrid pigments.23 The sepiolite activated in the naphthol AS solution preadsorbed naphthol AS on its surface and in its porous channels, and a core−shell hybrid pigment was fabricated when diazonium salt was added. 3.2. Properties of the Modified Pigment. 3.2.1. Thermal Stability. The thermal stabilities of the original and modified pigments were investigated and compared, and the results are shown in Figure 5. Figure 5b shows that there were three weight loss stages for original pigment: one in the range of 150−340 °C, one in the range of 350−440 °C, and the last in the range of 450−600 °C. The modified pigment had a similar onset decomposition temperature as the original pigment at 245 °C, illustrating that the modification did not postpone the onset decomposition temperature. However, the modified pigment had a higher exothermic peak and lower and wider loss peaks than the original pigment, suggesting that the modification by sepiolite and lithopone depressed the decomposition rate of the organic
pigment shell. To determine the structure between lithopone and organic pigment, we also observed a single particle of the modified pigment in energy-dispersive spectroscopy (EDS) mode, and the results are shown in Figure 2c,d. As can be seen in Figure 2c, in EDS mode, the organic pigment was decomposed and left holes in the images. In addition, we found that the bar-shaped modified pigment had smooth edges, some holes near the edges, and an opaque middle core, illustrating that the modified pigment had a structure of inorganic edge−organic pigment middle−inorganic core. In Figure 2d, it can be seen that the edge and core parts had crystal lattices with different lattice dimensions, suggesting that the modified pigment had a sepiolite core and a lithopone edge. As stated above, we concluded that the modified pigment had a structure of sepiolite core−organic pigment middle−lithopone shell. 3.1.3. XRD Analysis. The XRD analyses of the original pigment, sepiolite, lithopone, and modified pigment are presented in Figure 3. The 2θ diffraction curve of the modified
Figure 3. XRD analyses of the samples.
pigment was nearly the same as that of the original pigment, but some new diffraction peaks at 42.73° and 28.83° were observed. As shown in the curve of lithopone, there are diffraction peaks at the 2θ angles where the new diffraction peaks were observed in the curve of the modified pigment. Additionally, the diffraction peaks at 9.54° and 31.00°
Figure 4. FT-IR analyses of the original and modified pigments. 34
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Figure 5. (a) Weight loss and DTA curves and (b) DTG curves of the original and modified pigments.
pigment. Additionally, the decomposition at 365 °C shown in the DTA curve of the original pigment was not observed in the curve of the modified pigment. All of these results were similar to those reported by Yuan et al.,8 showing that the modification obviously improved the thermal stability of C.I. Pigment Red 21. In addition, as reported in the literature,5,8 the improvement in thermal stability was due to the effects of the sepiolite core modification and the lithopone surface modification. 3.2.2. Particle Size Distribution. The particle size distribution plays a key role in the optical and rheological properties of organic pigments.24 In this work, the particle size distributions of the original and modified pigments were determined and compared using a nanoparticle analyzer, and the results are shown in Figure 6 and Table 1. The small, medium, and large
volume-based diameters D10, D50, and D90, respectively, and the average particle diameter of the modified pigment were approximately the same as those of the original pigment, suggesting that the particle size of the modified pigment was comparable to that of the original pigment. However, the particle size distribution of the modified pigment was obviously different from that of the original pigment. As shown in Table 1, the polydispersity indexes for the original and modified pigments was 0.787 and 0.0625, respectively. The modified pigment had a narrower scatter of diameter and a higher homogeneity than the original pigment, which would make the modified pigment a valuable material for use in coloring plastics.25 3.2.3. Properties of the Modified Pigment. The properties of Pigment Red 21 were obviously improved by the modification with sepiolite and lithopone. The essential properties of the modified pigments including tinctorial strength, flowability, heat resistance, and light fastness were tested and compared with those of the original pigment, and the results are reported in Table 2. As shown in Table 2, the Table 2. Properties of the Modified Pigment
Table 1. Particle Distributions of the Original and Modified Pigments original pigment
modified pigment
254 286.5 327.9 293.2 0.787
278 304.6 336.5 310.2 0.0625
original pigment
modified pigment
100 32.5 140 6
102 33.7 160 7
tinctorial strength of the modified pigment was improved by 2%, and the flowability increased from 32.5 to 33.7 mm. In addition, the light fastness was enhanced by one grade, and the heat resistance was improved by 20 °C. 3.2.4. UV−Vis Spectra. The UV−vis diffuse-reflectance spectra of the original and modified pigments are presented in Figure 7. Because the spectra were determined in the powder state and diffuse-reflectance mode, they helped to determine the absorbing and scattering properties of the pigments. The lower the diffuse-reflectance absorbance, the higher the scattering property of the pigment.10 As shown in Figure 7, the intensity of the reflectance peaks in the wavelength range of 200−500 nm for the modified pigment was significantly enhanced, indicating that the modification could obviously scatter the UV rays and the modified pigment had a better photostability. As mentioned earlier, the component ZnS of the lithopone would react with HCl to release H2S in the
Figure 6. Particle size distributions of the original and modified pigments.
D10 (nm) D50 (nm) D90 (nm) average (nm) polydispersity index
property tinctorial strength (%) flowability (mm) heat resistance (°C) light fastness (grade)
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preparation process, leaving BaSO4 as a component on the surface of the modified pigment. BaSO4 is a photostabilizer and provides high reflection of UV light between 200 and 400 nm, which was responsible for the improvement in the photostability of the modified pigment.
4. CONCLUSIONS The modification of C.I. Pigment Red 21 by sepiolite and lithopone was carried out during its preparation process. SEM, XRD, and FT-IR analysis results showed that, in the modified material, sepiolite was encapsulated by the organic pigment and formed a core−shell structure. TEM showed that the barshaped sepiolite was encapsulated by an organic pigment interlayer and a lithopone outer layer, forming a core−shell− shell structure. The particle size distribution results showed that the particle size of the modified pigment was comparable to that of the original pigment, but the modified pigment had a narrower diameter scatter and a higher homogeneity than the original pigment. The modification improved the properties of C.I. Pigment Red 21 including tinctorial strength, flowability, heat resistance, light fastness, and photostability. The heat resistance of the modified pigment was improved by 20 °C, and the light fastness was improved by one grade. In addition, the optical analysis results showed that the modified pigment had better UV stability. AUTHOR INFORMATION
Corresponding Author
*Tel.: +86-22-23773042. Fax: +86-22-23085032. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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Figure 7. Diffuse-reflectance UV−vis spectra of the original and modified pigments.
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ACKNOWLEDGMENTS
We are grateful for support from the key project (Grant 2011005) of the Education Ministry of China and Technology Development Foundation Plan Project of Tianjin Colleges (20120506). 36
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