Absorption Pigment Cores for Pearlescent Pigments

Abstract

A lustrous appearance and interference-based colors make pearlescent pigments attractive for use in applications such as automotive paints, plastics, consumer electronics, and cosmetics. A combination of interference and absorption in the visible light spectrum improves significantly the hiding power as well as the color strength of pearlescent pigments while potentially extending their color range. The aim of the present study was to introduce synthetic fluorohectorites, having an appreciable diameter (~20 μm) and aspect ratio (~1000), as promising colored cores for pearlescent pigments. Fluorohectorites can adopt a variety of colors by ion-exchange reaction with cationic organic dyes of high absorption coefficient. Unlike related dye-exchanged natural montmorillonite clays, which undergo acid activation accompanied by release of dye at low pH, as is required for subsequent coating with TiO₂ in an environment with low pH and elevated temperature, no leaching was observed with dye-exchanged synthetic fluorohectorites ([Na_0.5]^int.[Mg_2.5Li_0.5]^oct.[Si₄]^tet.O₁₀F₂). Due to its significantly greater layer charge, more organic dye molecules were adsorbed per volume of the fluorohectorite than for montmorillonite. Consequently, the free volume available in the interlayer space for H₃O⁺ diffusion was less for synthetic fluorohectorite than for montmorillonite. Acid attack via interlayer space was, therefore, retarded significantly for fluorohectorite. Acid attack from the external edges of synthetic fluorohectorites was in the range of conventionally applied mica pigment core (fluorophlogopite, ([K]^int.[Mg₃]^oct.[AlSi₃]^tet.O₁₀(F,OH)₂) because of the comparable large diameter of the platelets. Montmorillonite, however, occurs with particle diameters typically <200 nm and the much increased relative contribution of edges to the total surface area also makes them more prone to acid attack and concomitant leaching. Aside from leaching stability, the confinement of organic dyes in the interlayer space restricts rotational and vibrational motions, which in turn stabilizes the dyes typically by ~100°C against thermal decomposition as compared to chloride salts of the dyes.

Introduction

Due to the appealing interplay of color and gloss, pearlescent pigments have become popular in automotive paints, plastics, consumer electronics, and cosmetics (Horiishi et al. 2005; Maile et al. 2005). The color is obtained by interference of visible light on thin films of high and low refractive index. For luster and brilliance effects, a platy morphology of the pigment core and also the pigment coating are important. Applying layered materials, like mica, with an appreciable aspect ratio (ratio of diameter to thickness) as the pigment core, inevitably leads to a parallel alignment with respect to the applied surface giving the desired lustrous effect. By combining interference with absorption of visible light, the accessible color range can be extended by, for instance, applying light-absorbing coatings such as iron oxides (Pfaff and Weitzel 2004; Horiishi et al. 2005).

The coating environment imposes severe restrictions on the choice of core material. For instance, in the state-of-the-art commercial coating of mica, TiCl₄ is applied as the affordable TiO₂ source. The hydrolysis of TiCl₄ results in a low pH of ~3 which, in combination with typical processing temperatures of up to 100°C, represents a corrosive environment for oxides or even silicates (Pfaff 2002; Komadel and Madejová 2006).

A commonly applied mica, fluorophlogopite (Fig. 1a,b), is a 2:1 layered silicate with a high charge density of the silicate layer. The strong electrostatic attraction with K⁺ renders the interlayer space inaccessible to ion-exchange, hydration of K⁺, and even acid attack. Acid attack can consequently only occur through the edge surface, which is comparatively small due to the platy morphology and the large particle diameter (5–200 μm). Therefore, fluorophlogopite shows only negligible dissolution during the coating process.

Fig. 1. a Schematic structure of fluorophlogopite, b SEM images of fluorophlogopite, c montmorillonite, and d fluorohectorite platelets

For low-charge, so-called ‘swelling’ 2:1 layered silicates, such as natural montmorillonite, the interlayer space is available for ion-exchange and the cation exchange capacity (CEC) has been utilized to incorporate cationic organic dyes (Baranyaiová and Bujdák 2018; Boháč and Bujdák 2018). For this class of inorganic/organic hybrid pigments, infinite color variations and higher coloring strength are at hand due to high absorption coefficients of organic dyes in the visible region. They also show surprising thermal stability, allowing these montmorillonite-based pigments to be used for coloring polymers in melt compounding (Koyama et al. 2009; Smitha et al. 2011; Choudhary and Sengwa 2014).

Such montmorillonite-based hybrids are inappropriate as core materials for pearlescent pigments for two reasons. Firstly, their diameter is too small (<200 nm, Fig. 1c). Secondly, as will be shown below, montmorillonites exchanged with organic cations are not stable in acidic environments and the structure dissolves by the attack of octahedral cations releasing small amounts of the dye into solution.

Synthetic fluorohectorite is another swelling 2:1 layered silicate (Stöter et al. 2013) with accessible interlayers but which comes in diameters comparable to typical mica platelets (Fig. 1d), and is a promising new candidate for pearlescent pigment core platelets. Because the commercial coating process with TiO₂ takes place in an acidic environment at elevated temperature, the main objective of this study was to compare the stabilities of montmorillonite and fluorohectorite hybrids under given conditions.

EXPERIMENTAL SECTION

Synthesis of Fluorohectorite

Fluorohectorite (Hec) was synthesized from a melt as described in detail previously (Kalo et al. 2010). To improve intracrystalline reactivity, the pristine hectorite was heated in an autoclave for 72 h at 340°C.

Purification of SWy-1 Montmorillonite

SWy-1 montmorillonite, obtained from the Source Clays Repository of The Clay Minerals Society, was purified prior to use in order to remove residual carbonates and amorphous ferric (oxyhydr)oxides (Mehra and Jackson 1958). After purification, montmorillonite (Mnt) was dialyzed for two days until the conductivity of the water remained below 1 μS/cm.

Dyes

3,7-Bis(dimethylamino)phenothiazin-5-ium chloride (MB, Sigma-Aldrich, St. Louis, Missouri, USA), 4-N,N-dimethyl-2,5-cyclohexadien-1-iminium chloride (MAL, Molekula, Darlington, UK), and 3,7-Diamino-2,8-dimethyl-5-phenylphenazin-5-ium (SAF, Alfa Aesar, Haverhill, Massachusetts, USA) were used as received.

Ion-exchange

Solutions of dyes with a concentration of ~0.02 M and 0.2 wt.% clay dispersions were prepared with Milli-Q water. The interlayer Na⁺ ions were exchanged upon addition of an aliquot volume of dye solution, in the amount of 100% CEC. Afterward, the samples were shaken in an overhead shaker (IKA, Staufen im Breisgau, Germany) for 8 h at room temperature.

Acid Stability Test

40 mL of clay dispersion corresponding to a constant clay weight of 80 mg was adjusted to pH 1 with 1 M HCl. During the acid treatment, the sample was stirred at 75°C for 6 h. After the acid treatment, the sample was centrifuged at 14,090 × g and washed with Milli-Q water three times. The collected supernatant was filtered using a CHROMAFIL Xtra PET-20/25 membrane filter with 0.20 μm pore size and analyzed for dissolved Mg²⁺ by atomic absorption spectroscopy using a Perkin-Elmer 2380 spectrophotometer (PerkinElmer, Waltham, Massachusetts, USA).

Adsorption Capacities

In order to determine the adsorption capacities of the clays, the ion-exchanged samples were centrifuged at 14,090 × g for 15 min and the supernatant was filtered using a CHROMAFIL Xtra PET-20/25 membrane filter with 0.20 μm pore size. The amount of non-adsorbed dye in the filtered supernatant was determined colorimetrically with a Varian Cary 300 double beam spectrophotometer (Varian, Palo Alto, California, USA).

Powder X-ray Diffraction

Textured films were prepared by suspending a few drops of the dispersion onto microscope slides (Menzel glass). Prior to measurement, samples were dried at room temperature for 24 h. Powder X-ray diffraction (PXRD) patterns were recorded in Bragg-Brentano geometry using Ni-filtered CuKα radiation (λ = 0.1541 nm) using a PANalytical Empyrean diffractometer equipped with a PIXcel1D detector (Malvern Panalytical, Kassel, Germany). All patterns were analyzed using Panalytical’s Highscore Plus software.

Thermogravimetric Analysis

To speed up the kinetics of the thermal decomposition, the sample surface was increased by lyophilization using a Christ Alpha 1-4 freeze-dryer (Martin Christ, Osterode am Harz, Germany). The thermal stability of samples was then examined using a thermogravimetric analyzer NETZSCH STA 449 C (Netzsch, Selb, Germany) through the temperature range 25–900°C at a heating rate of 10°C/min under synthetic air atmosphere containing 20.5 vol.% O₂ and 79.5 vol.% N₂.

Infrared Spectroscopy

Infrared transmittance spectra were recorded using a Jasco FT/IR 6100 (Jasco, Hachioji City, Japan) spectrophotometer equipped with a diamond ATR (Pike technologies, Madison, Wisconsin, USA). Transmittance spectra were subjected to background subtraction and normalized.

Reflectance Spectroscopy and Chromaticity Coordinates

Diffuse reflectance spectra of solid samples were measured with a Varian Cary 300 double beam spectrophotometer (Varian, Palo Alto, California, USA) equipped with a DRA-CA-30I diffuse reflectance accessory. The measured spectrum was used to calculate the tristimulus values by multiplying the reflectance values by the values of the color-matching functions. Afterwards, the chromaticity coordinates of the CIE 1931 color space (x, y) were calculated by normalizing previously obtained tristimulus values (Torrent and Barrón 2008).

Scanning Electron Microscopy

For electron microscopy characterization, samples dispersed in water were dropped onto a silicon wafer and dried at room temperature for 24 h. Prior to the sample deposition, silicon wafers were cleaned in piranha solution (3:1 v/v H₂SO₄:H₂O₂) for 24 h and rinsed several times with Milli-Q water. To ensure a clean surface, the silicon wafer was also cleaned using the CO₂ snow jet procedure (Sherman et al. 1994). After cleaning, the surfaces of silicon wafers were hydrophilized by treating them with plasma. In order to visualize clay-hybrids, the samples were coated with a very thin (2 nm) layer of platinum using a Cressington Platin-Sputter Coater 208HR (Cressington Scientific Instruments UK, Watford, UK). Prepared samples were examined with a field emission scanning electron microscope Zeiss Ultra plus (Carl Zeiss AG, Oberkochen, Germany) under a vacuum of ~10^–5 mbar.

Calculations

The van der Waals volumes of the organic cations were calculated using chemicalize.com (Swain 2012).

RESULTS AND DISCUSSION

Coloring Strength

Colored clay hybrid materials were obtained by cation exchanging Na⁺ in Mnt ([Na_0.30][Al_1.50Fe_0.20Mn_0.01Mg_0.27 Ti_0.01][Si_3.99Al_0.01]O₁₀(OH)₂) (Hall and Astill 1989; Collins et al. 1992) and Hec by cationic dyes. The negative layer charge of these clay minerals is a consequence of an isomorphous substitution of aluminum mainly with magnesium and iron in Mnt, and substitution of magnesium by lithium within the octahedral sheet in Hec. The CEC as determined by the [Cu(trien)]²⁺ method (Ammann et al. 2005) was 0.71 mEq/g for Mnt and 0.90 mEq/g for Hec. By dispersing in deionized water, the clay minerals swelled osmotically (Stöter et al. 2013). After the ion-exchange with organic cations, the interlayer space became hydrophobic and collapsed yielding stacks of several silicate layers (Kunz et al. 2015).

Organic dyes MB, MAL, and SAF (Fig. 2) were chosen because they are soluble in water and possess a large absorption coefficient in the visible region. Consequently, even traces of these dyes remaining in solution after ion-exchange can be determined precisely by conventional UV-Vis spectroscopy. Choosing water as a polar solvent shifted the partition function towards hydrophobic dyes being intercalated (Stöter et al. 2014). When applying dyes in amounts corresponding to 100% of the CEC, all dyes are almost completely intercalated (Table 1) supporting the preference of organic dyes for the interlayer space.

Fig. 2. Structures of organic dyes and photographs of corresponding Hec-hybrids

Table 1. Amount of dye intercalated and calculated van der Waals volumes

Completeness of ion exchange was corroborated by PXRD diffraction patterns. Upon intercalation, the d spacings increased from 1.24 nm for the Na-Mnt to 1.56 nm, 1.76 nm, and 1.80 nm for MB-Mnt, SAF-Mnt, and MAL-Mnt samples, respectively, and from 1.24 nm for the Na-Hec to 1.81 nm, 1.97 nm, and 1.98 nm for MB-Hec, SAF-Hec, and MAL-Hec samples, respectively. For MB-Hec only, a shoulder at 1.24 nm might indicate some Na-interlayers remaining un-exchanged (Fig. 3).

Fig. 3. PXRD patterns of (dye-intercalated) Mnt and Hec with d ₀₀₁ spacings

By comparing absorption spectra of the dye solution at low concentrations, where a purely monomeric form of dye was present, with the absorption spectra of clay-hybrids obtained by transformation of reflectance spectra with the Kubelka-Munk function (data not shown) determining the type of the dye species responsible for the resulting clay-hybrid color impression (Fig. 4) was possible. Following this procedure, MB formed in the interlayer space of MB-clay hybrids H-dimers (Bujdák 2006) and monomers, while MAL and SAF were present in their monomeric forms.

Fig. 4. a Chromaticity diagram showing x and y coordinates of MB-clay hybrids, b MAL-clay hybrids, c SAF-clay hybrids before (open symbols) and after (full symbols) acid treatment in the CIE 1931 color space. Mnt-hybrids are represented by circles and Hec-hybrids by rectangles

Acid Stability

Pristine fluorohectorite was shown (Komadel and Madejová 2006) to be much less stable in inorganic acids than Al-rich montmorillonite due to the faster dissolution kinetics of octahedrally coordinated Mg²⁺ ions than of octahedrally coordinated Al³⁺ ions (Christidis et al. 1997; Gates et al. 2002). Contrary to expectations based on this result, the opposite trend in acid stability for the dye-intercalated hybrid materials was observed. Recently, Lackovičová et al. (2019) showed that not only is the dye capable of stabilizing the clay structure, but also the confinement of dye in the interlayer space helps to stabilize it chemically and to prevent reduction to its leuco-form. After intercalation of Hec and Mnt with organic dyes, Hec samples were much more resistant to acid attack than the Mnt samples (Fig. 5). Because of that, for dye-intercalated Hec no detectable amounts of dye were leached upon acid treatment, but the release of octahedral Mg²⁺ into the supernatant was determined as a measure for acid-attack kinetics. Because the Mg contents of the two clays differ, the relative amounts were compared. One possible reason for this unexpected acid stability of Hec-hybrids is the greater charge density. The charge density of Mnt (0.63 e/nm²) is significantly smaller than of Hec (1.04 e/nm²). Because the ab-dimensions of Mnt and Hec are comparable, fewer dye molecules being intercalated per formula unit in Mnt will result in a less dense packing of any given dye molecule in Mnt than in Hec (Table 2). The volume available to one dye molecule was estimated by multiplying the ab-dimensions of Mnt and Hec (0.230 nm² for Mnt, 0.237 nm² for Hec) with the height of the interlayer space (d spacing minus the thickness of the silicate layer which is 0.96 nm) divided by the number of interlayer cations per formula unit. As expected from CECs, these volumes were found to be considerably larger for Mnt samples than for Hec samples, corroborating a significantly greater packing density of dyes in the interlayer space of Hec than in Mnt samples. Multiplying the number of dye molecules per formula unit (p.f.u.; half unit cell corresponding to four tetrahedral cations) by the van der Waals volume and dividing it with available volumes allows estimation of a packing density of the interlayer space (Table 2). The packing density of Hec-hybrids was consistently >0.64 for all three dyes while for Mnt samples it remained at <0.51. This suggests that larger voids exist in the interlayer space of Mnt-hybrid materials that allow H₃O⁺ to diffuse more easily into the interlayer space. This allowed acid attack via the interlayer space on top of the attack via the edges. The extent of the voids in Hec samples was less, and presumably more isolated; therefore, the acid attack was restricted largely to edges. Due to its larger particle size, the edge:volume ratio of Hec is small (Table 3) and is negligible compared to that of Mnt. The Hec ratio is in the range commonly found in fluorophlogopite cores.

Fig. 5. Amount of dissolved Mg in the supernatant after 6 h of acid treatment with HCl at pH 1 and 75°C of Mnt (red bars) and Hec (black bars) hybrid samples

Table 2. Packing density per formula unit

Table 3. Calculated edge:volume ratio

The overcrowded situation with Hec-hybrids might result in a displacement of dye molecules residing at the edges, outside the interlayer space, similar to what was suggested by Lagaly (1982) for alkylammonium intercalates of vermiculite. The displaced parts of the dye molecules hanging over the edges will render them more hydrophobic and, thus, might even slow down acid attack via the edges.

The statements above are also supported by the scanning electron microscopy images of clay-hybrids before and after acid treatment (Fig. 6). Close examination revealed that the edges and the surfaces of Hec-hybrids remained unaffected, while after acid treatment, due to the leaching of octahedral cations, the Mnt-hybrids demonstrated the formation of bulges, probably consisting of amorphous silica. A more exact indicator of changes in the octahedral sheet was provided by the vibrations of groups of atoms including octahedral cations; the leaching process can also be confirmed, therefore, by IR spectroscopy in the mid-infrared region (Fig. 7). The characteristic Al–Al–OH, Al–Fe–OH, and Al–Mg–OH bands are located in Mnt-hybrid at 918, 883, and 836 cm^–1, respectively (Bishop and Murad 2004). After acid treatment, the intensities of these bands decreased slightly, indicating partial release of the structural octahedral cations.

Fig. 6. SEM images (scale bar: 1 μm) of MB-Hec sample a before and b after acid treatment, and of MB-Mnt sample c before and d after acid treatment

Fig. 7. IR transmittance spectra of MB-Mnt sample before (black line) and after (red line) acid treatment. The positions of vibrations of Al-Al-OH at 918 cm^–1, Al-Fe-OH at 883 cm^–1, and Al-Mg-OH at 836 cm^–1 are indicated by vertical lines

At this point, also important to know is to what extent the acidic environment affects the colorfastness. For this reason, the reflectance spectra of the samples before and after acid treatment were measured. The reflectance spectra obtained were subsequently transformed into CIE 1931 x, y coordinates, and are shown in a CIE 1931 chromaticity diagram (Fig. 4). One would expect that the dye molecules in the Mnt interlayer space were due to the presence of larger voids protonated during acid treatment, giving rise to the formation of species with different spectral characteristics which, in turn, would result in a notable color change. Reflectance measurements and the corresponding representations in the CIE 1931 chromaticity diagram showed, however, that the changes in color were insignificant, assuming little or no protonation under given conditions, and thus favoring the occurrence of acidic dissolution of octahedrally coordinated cations.

Thermal stability

As a first indication of the environmental stability of the newly developed absorption pigment cores, their thermal stability was tested. Pristine Hec is thermally stable up to 800°C (Tsurko et al. 2017). The thermal stability will, therefore, be limited by the decomposition of intercalated dyes. Fortunately, the thermal stability of organic ions has long been known to be improved by intercalation into the galleries of layered materials (Laguna et al. 2007; Tang et al. 2008). This general observation is confirmed by comparing the thermal stability of pristine organic dye salts (chlorides) and Hec-hybrids, where formally the chloride is replaced by the layered silicate polyanion. In the presence of synthetic air, the thermogravimetric curves show multistep decomposition of MB-Cl, MAL-Cl, and SAF-Cl. After the loss of physisorbed water in the temperature range 50–150°C, the onset of oxidative decomposition accompanied by exothermic peaks was observed at 185°C, 307°C, and 190°C for MB-Cl, MAL-Cl, and SAF-Cl, respectively (Figs 8a–c). These onset temperatures, as defined by the intersection of tangents, would cause doubt about the integrity of MB and SAF under certain processing conditions. The onset temperatures of decomposition of all hybrid materials were shifted considerably to higher temperatures: 281, 413, and 282°C for MB-Hec, MAL-Hec, and SAF-Hec, respectively. Presumably, the confinement of organic cations in the interlayer space restricts their rotational and vibrational motions and, therefore, the thermal decomposition of organic cations results in stabilization by ~100°C.

Fig. 8. Thermogravimetric analysis of a MB-Cl, MB-Hec, b MAL-Cl, MAL-Hec, and c SAF-Cl, SAF-Hec samples. Mass loss curves of dye-Cl (black full line) and dye-Hec (red full line), and DSC curves of dye-Cl (black dashed line) and dye-Hec (red dashed line) in a synthetic air (gas mixture consisting of 79.5 vol.% N2 and 20.5 vol.% O2, with possible impurities such as H2O, CO2, CO, CH4, NOx, SO2, or H2S) atmosphere with a heating rate of 10°C/min. Vertical full lines for dye-Cl (black) and dye-Hec (red) are used to show the onset temperatures

Conclusions

The coloring strength of clay-hybrids is affected directly by the amount of intercalated organic cations, and is due to the greater layer charge being more pronounced in Hec-hybrid samples. Surprisingly, the acid stability of Hec-hybrid samples was better than that of Mnt-hybrid samples because of larger packing density and smaller edge:volume ratios. As a consequence of organic dye confinement in the interlayer space, the rotational and vibrational motions of these organic cations are restricted, resulting in stabilization of their thermal decomposition by ~100°C. Consequently, all these properties make colored Hec hybrids promising in terms of the preparation of new, intensely colored pearlescent pigments. The next step will be the formulation of coatings with Hec-based pigment cores followed by weathering tests.