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Pigments on amorphous silica carriers
Powder Technology 132 (2003) 190 – 195 www.elsevier.com/locate/powtec
Pigments on amorphous silica carriers Andrzej Krysztafkiewicz, Slawomir Binkowski *, Iwona Wysocka Institute of Chemical Technology and Engineering, Poznan´ University of Technology, Pl. M. Sklodowskiej-Curie 2, 60-965 Poznan´, Poland A
Received 10 April 2001; received in revised form 12 February 2002; accepted 27 March 2003
Abstract Amorphous precipitated silica was applied as a carrier for pigments. The silica surface was modified with silane coupling agents, such as 3aminopropyltriethoxysilane,N-2-(aminoethyl)-3-aminopropyltrimethoxysilane and 3-ureidopropyltrimethoxysilane. Pigments were obtained by attaching organic dyes, C.I. Acid Red 18 and C.I. Acid Violet 1, to a modified silica surface. The adsorption process was conducted in an aqueous suspension of the modified silica in the presence of the dye. Various amounts of the modifying compounds were used in order to determine their effect on the dye adsorption process. The structural and microscopic properties, zeta potential and agglomerate size distribution were examined for the modified silicas and the obtained pigments. Colours of the obtained pigments were measured. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Nanosize pigments; Silica; Silane coupling agents; Adsorption of dyes
1. Introduction Silicas are extensively modifiable materials [1,2]. The modification results in products which carry on their surface new functional groups, capable of interacting with various organic compounds [3]. For example, application of aminosilane for the modification produces functional amino groups which may react with carbonyl groups of aldehydes, ketones or esters [4 –6]. The aminosilane-modified silicas find multiple applications in industry, serving, inter alia, as polymer fillers. Recently, they have been used with increasing frequency as coupling agents in pigment or organic dye systems. Until recently, studies on organic pigments involved modification of basic properties, such as stability or intensity of colours. In multiple cases, however, pigments must satisfy additional requirements, such as, e.g. well defined particle diameters. Pigments with silica cores can satisfy such requirements. The pigments mentioned above may be used in many applications. In earlier studies [7,8], azo pigments were described, suitable for application in electrophotographic toners. Those consist of a silica core of particle diameter below 10 Am with a mono- or polyazo dye coating, chemically bound to the silica surface by the silane. In other * Corresponding author. Tel.: +48-61-665-36-26; fax: +48-61-665-3649.
papers [9,10], the process of preparation of colourful silicas in which an organic pigment is coupled to the surface of modified silica by a covalent bond was described in detail. Such pigments are applied in the ink for ink jet printers. Tentorino et al. [11], Aiken et al. [12], Giesche and Matijevic´ [13], Hsu et al. [14], Wu et al. [15] and Matijevic´ [16] described preparation of such pigments on various types of inorganic carriers of strictly defined particle diameters by adsorption or incorporation of the dye in the course of carrier preparation. In this way, pigments can be obtained which are resistant to elution by an organic solvent. In the present study, we compared properties of pigments obtained using as carrier the precipitated silica (Syloid 244), as related to the type and amount of the silane coupling agent used for the modification and to the type of solvent which was used to deposit the silane on the silica surface.
2. Experimental 2.1. Materials Commercially available silica, namely, the precipitated silica, Syloid 244 (Grace Davison), was used. Silane coupling agents used for modification of the silica included:
E-mail address: [email protected] (S. Binkowski). 0032-5910/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0032-5910(03)00073-1
3-aminopropyltriethoxysilane [U-13, H 2 N(CH 2 ) 3 Si (OC2H5)3],
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N-2-(aminoethyl)-3-aminopropyltrimethoxysilane [U-15, H2N(CH2)2NH(CH2)3Si(OCH3)3], 3-ureidopropyltrimethoxysilane [U-17, H2NC(O)NH (CH3)3Si(OCH3)3], produced by UniSil.
sette) and passed through a sieve. The same procedure was repeated for the unmodified silicas. In the filtrate, the amount of unadsorbed dye was estimated by measuring absorbance in a SECOMAM S 750 spectrophotometer.
For preparation of colourful silicas, organic dyes which were not prepurified were used, namely, C.I. Acid Red 18 and C.I. Acid Violet 1 (Boruta-Kolor) of the following structures:
2.4. Evaluation of the pigments obtained
C.I. Acid Red 18
C.I. Acid Violet 1
2.2. Modification of the silica The silica surface was modified with silane coupling agents, such as U-13, U-15 and U-17. Solutions were prepared containing one, three and five parts by weight of the modifiers in water, methanol or methanol/water mixture (4:1) per 100 parts by weight of the modified silica [17]. The amount of the modifier was always so adjusted that exclusively uniform wetting of the silica surface took place in the mixer. Appropriate solutions of the modifier were gradually added dropwise to a round bottomed flask containing the silica. The solutions were prepared directly before the modification to prevent their aging. The modification was performed in a mixer for 1 h at room temperature. Following the cycle of mixing, the silica was dried at 105 jC and passed through a sieve of 0.2 mm mesh. 2.3. Colouration of the silica A total of 50 cm3 aqueous solution of an organic dye [18] of various concentrations and 1 cm3 HCl were introduced to a conical flask, containing each time 2 g of the modified silica, Syloid 244. The contents of the flask were magnetically mixed for 4 h at room temperature. Subsequently, the content was filtered under vacuum. The sediment was dried at 105 jC, ground in an electric mortar 02 (Fritsch Pulveri-
In order to determine the morphology of the pigment surface, selected samples were examined in a scanning electron microscope to observe the rough surface of the solids, such as fracture plane, surface structure and pigment agglomerates. Since silicas do not conduct electric current, irradiation of their surface with an electron beam of the scanning electron microscope results in an accumulation of an electric charge which deforms the pattern of surface topography. In order to avoid accumulation of the surface electric charge, silica preparations were made in tertiary butyl alcohol. In these studies, the scanning electron microscope Philips SEM 515 was used. Laser Doppler electrophoretic light scattering determinations were performed with a ZetaPlus instrument, purchased from the Brookhaven Instruments, in the reference beam mode at the wavelength of the laser light source of 635 nm, sampling time 256 As, modular frequency 250 Hz and scattering angle 15j. The standard error of the zeta potentials (f), converted from the experimentally determined electrophoretic mobilities according to the Smoluchowski limit of the Henry equation, was typically < 1.5% and the percent error < 5%. The zeta potential distributions were obtained by averaging three to five runs. Particle size distribution was also examined using a ZetaPlus apparatus. The particle size was measured using the dynamic light scattering (DLS) technique. The technique involved weighing out an appropriate sample, placing it in a small amount of water (0.1 g in 50 cm3 H2O) and stabilizing using an ultrasonic bath (50 kHz). The prepared sample was placed in a cuvette, and the size distribution of silica particles was then measured. The colorimetric data of the obtained pigments were measured using an instrumented colorimeter (Spectro Module 4000, JETI Technische Instrumente), which was calibrated using a white colour standard tile with tristimulus values: X = 78.7, Y = 83.1 and Z = 86.5 (standard no. 0085). Daylight (D65) was used as a standardized light source. A fixed amount of pigment sample was poured into the measurement cup. The instrument provides the colour of the samples in terms of the L, a, b colour space system. In this colour space, L represents the lightness, and a and b are colour coordinates: where + a is the red direction, a is the green direction, + b is the yellow direction and b is the blue direction.
3. Results and discussion In order to examine the uniformity of the silicas and of the pigments prepared from them, microscope studies were
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performed and the particle size distribution was examined. For the unmodified precipitated silica, Syloid 244, the representative electron micrograph is presented in Fig. 1A while the particle size distribution is presented in Fig. 1B. The silica did not exhibit a significant homogeneity, as proven by the intense band corresponding to agglomerate formation. The band was situated within the range of 1300– 1750 nm. The silica structure comprised also the intense band associated with aggregate presence and positioned within the range of 430 –570 nm (the maximum intensity of 100 corresponded to aggregates of diameter of 508.6 nm). The electron micrograph confirmed the presence of the agglomerates. Following modification with five parts by weight of U13 aminosilane (in a water system), the Syloid 244 silica
Fig. 2. (A) Scanning electron micrograph (SEM) of silica Syloid 244 modified with five parts by weight of U-13 silane (in water). (B) Particle size distribution of silica Syloid 244 modified with five parts by weight of U-13 silane (in water).
Fig. 1. (A) Scanning electron micrograph (SEM) of unmodified silica Syloid 244. (B) Particle size distribution of unmodified silica Syloid 244.
became less uniform (Fig. 2A). However, in the curve of particle size distribution (Fig. 2B), a low-intensity band could be observed, corresponding to the presence of agglomerates of 2500– 4100 nm in diameter (maximum intensity of 5 corresponded to agglomerates of diameter 2731.0 nm). On the other hand, the very intense band reflecting the presence of aggregates was situated in the range of 320 – 550 nm. The effective diameter of the silica agglomerates was 455.1 nm, and maximum intensity of 100 corresponded to an agglomerate diameter of 399.6 nm. The electron micrograph (Fig. 3A) clearly proved a highly uniform character of the sample and low tendency to form agglomerates after adsorption of C.I. Acid Red 18 dye on the so modified silica. Only an insignificant increase
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dispersed pigments. The modification of the silica surface resulted in an augmented value of zeta potential. Adsorption of the dye on the surface of the modified silica further augmented the zeta potential on the surfaces. Syloid 244 modified with aminosilane and following adsorption of C.I. Acid Red 18 dye reached on the surface a highly positive zeta potential ( + 48 mV). As documented by the data of Tables 1 and 2, the silica surface modification with silanes exerted a significant effect on the adsorption extent of C.I. Acid Red 18 and C.I. Acid Violet 1 on the silica surface. The least adsorption extent of C.I. Acid Red 18 was obtained for the unmodified surface (9.9%) and for surfaces modified with 1 part by weight of U13 silane (27.7 – 62.9%). The silica surface modification resulted in an clearly increased extent of adsorption of C.I. Acid Red 18 dye on the surfaces. In the case of C.I. Acid Violet 1 dye, no dye was adsorbed on unmodified silica surfaces. For both dyes, a particularly high extent of adsorption was obtained for Syloid 244 silica, modified with three or five parts by weight of U-13, U-15 or U-17 silanes. The silicas modified with three or five parts by weight exhibited the optimum dye adsorption and, thus, application of even higher concentrations of silanes would be pointless. It could also be noted that multiple adsorption of the same dye (C.I. Acid Violet 1) on Syloid 244 silica modified with five parts by weight of U-15 silane in methanol resulted in a decrease
Table 1 Adsorption extent of C.I. Acid Red 18 dye (adsorption time: 4 h) Amount of silane (weight parts)
Fig. 3. (A) Scanning electron micrograph (SEM) of silica Syloid 244 modified with five parts by weight of U-13 silane (in water) after adsorption of C.I. Acid Red 18. (B) Particle size distribution of silica Syloid 244 modified with five parts by weight of U-13 silane (in water) after adsorption of C.I. Acid Red 18.
in effective agglomerate diameter to 544.4 nm was observed (Fig. 3B). A low-intensity agglomerate band was formed, with an agglomerate diameter lower than that noted for the silica modified with the silane alone. The band was positioned within the range of 1200 – 1600 nm (maximum intensity of 36 corresponded to agglomerates diameter of 1337.6 nm). An intense band, corresponding to aggregates, was positioned within the range of 390– 550 nm (maximum intensity of 100 corresponded to aggregates diameter of 433.0 nm). Following modification and the surface coating with the organic dyes, the syloid 244 silica manifested most advantageous properties, which should characterize highly
Modification medium
Dyes concentration before adsorption (mg/cm3)
Dyes concentration after adsorption (mg/cm3)
Syloid 244 0 –
0.20
0.18030
9.9
U-13 3 5 1 3 5 1 3 5
water water methanol methanol methanol methanol/water methanol/water methanol/water
0.32 0.32 0.32 0.32 0.32 0.32 0.32 0.32
0.07918 0.00728 0.13030 0.04360 0.01436 0.11860 0.02357 0.02023
75.3 97.8 59.3 86.4 95.6 62.9 92.6 93.7
U-15 1 3 5 1 3 5
water water water methanol/water methanol/water methanol/water
0.32 0.32 0.32 0.32 0.32 0.32
0.06493 0.01778 0.01094 0.03111 0.03027 0.00332
79.7 94.4 96.6 90.3 90.6 98.9
U-17 1 3 5
methanol/water methanol/water methanol/water
0.32 0.32 0.32
0.07190 0.11070 0.01465
77.5 65.4 95.4
Disposal extent (%)
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Table 2 Adsorption extent of C.I. Acid Violet 1 dye (adsorption time: 4 h) Amount of silane (weight parts)
Dyes concentration before adsorption (mg/cm3)
Dyes concentration after adsorption (mg/cm3)
Syloid 244 0 –
0.4
0.4000
0
U-15 1 3 5 5 5 1 3 5
0.4 0.4 0.4a 0.4b 0.4c 0.4 0.4 0.4
0.37210 0.06556 0.03560 0.20400 0.38580 0.16930 0.10540 0.00237
7.0 83.7 91.1 49.0 3.6 57.7 73.7 99.4
U-17 1 3 5 1 3 5
Modification medium
methanol methanol methanol methanol methanol methanol/water methanol/water methanol/water
methanol methanol methanol methanol/water methanol/water methanol/water
0.4 0.4 0.4 0.4 0.4 0.4
0.28950 0.08350 0.02363 0.26900 0.05906 0.05007
Disposal extent (%)
27.7 79.2 94.1 32.8 85.3 87.5
a
First adsorption process onto the same sample. Second adsorption process onto the same sample. c Third adsorption process onto the same sample. b
of extent of the adsorption from 91.1% in the first adsorption to 3.6% in the third adsorption (Table 2). Colour of the obtained pigments was measured using the colorimeter described earlier. The data were presented in the form of table (Table 3) and as graphs in a tristimulus system, L*a*b* (Fig. 4). The L* value, brightness of the pigments, was most pronounced for pigments obtained by adsorption of C.I. Acid Red 18 on the unmodified silica Syloid 244 (Fig. Table 3 Colorimetric data for obtained pigments Sample
Colorimetric data C*
h*
Syloid 244 Syloid 244 + AR18 Syloid 244 + 1(m/w) U-17 + AR18 Syloid 244 + 3(m/w) U-17 + AR18 Syloid 244 + 5(m/w) U-17 + AR18 Syloid 244 + AV1 Syloid 244 + 1(m) U-15 + AV1 Syloid 244 + 3(m) U-15 + AV1 C.I. Acid Red 18 C.I. Acid Violet 1
100.00 82.31 75.49
0.56 14.07 29.64
2.64 13.52 20.65
2.73 19.44 36.09
259.11 43.81 34.83
75.05
28.36
20.81
35.01
36.31
70.85
34.90
24.58
42.80
35.12
85.89 61.68
6.76 11.43
6.84 16.90
9.47 20.50
314.40 304.12
59.34
10.69
15.95
19.20
304.01
33.10 26.38
24.65 0.64
23.10 6.75
33.85 6.82
43.56 85.43
L*
a*
b*
Fig. 4. Colorimetric data for pigments obtained from C.I. Acid Red 18 dye on the silica Syloid 244 modified with U-17 silane (in methanol/water mixture).
4). However, increase in the amount of U-17 silane used for modification of the silica in a mixture of methanol and water was associated with a decrease in brightness of the pigments obtained. The data corresponded to the obtained efficiencies of the adsorption (Table 1). The obtained brightness estimates (L*) might indicate that an increase in the applied silane amounts (independently of its type) was linked to an increase in dye adsorption. One can hypothesise that an increase in the applied silane concentrations has been paralleled by an increase in the number of adsorbing sites. The values a* and b* for the obtained pigments represent a qualitative index of their colour. For pigments obtained on the Syloid 244 silica, a* ranged between + 14.07 for the unmodified silica to + 34.90 for the silica modified with five parts by weight of U-17 silane in a methanol/water mixture, while the value b* ranged, respectively, from 13.52 to + 24.58. As mentioned earlier, + a corresponds to red, a to green, + b to yellow and b to blue colours. Summing up the above results, one may conclude that increase in the amount of the applied modifier results in increasing colour of the pigments (increasing values of a* and b*).
4. Conclusions
Example: SYLOID 244 + 3(m/w)U-13 + AR-silica SYLOID 244 modified with three parts by weight of U-13 silane (in methanol/water) after adsorption of C.I. Acid Red 18 dye.
Electron microscopic studies using the scanning technique have demonstrated that adsorption of dyes on modified silica carrier enables a uniform organic pigment to be obtained. Dye adsorption on a modified silica has resulted in clearly improved uniformity of the pigments obtained, based on Syloid 244 carrier, while ranges of the aggregate and agglomerate bands have become shifted toward smaller particles. The extents of the dye adsorption have been greatly affected by silica surface modification with silane coupling agents.
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As shown by colorimetric studies, the brightness of colours in the pigments obtained has been affected by the amount and type of the applied silane used for modification of the silica surface.
Acknowledgements The authors are indebted to ‘‘Boruta-Kolor’’ Sp. z.o.o. for a gift of the dyes used in the studies, and ‘‘Medson’’ S.C. for enabling us to do colorimetric measurements. This work was supported by grant no. DS 32/115/2003.
References [1] A.P. Legrand, The Surface Properties of Silicas, Wiley, New York, 1998. [2] E.F. Vansant, P. Van Der Voort, K.C. Vrancken, Characterization and Chemical Modification of the Silica Surface, Elsevier, Amsterdam, 1995.
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[3] K.I. Mittal, Silane and Other Coupling Agents, VSP, Utrecht, 1992. [4] N.R.E.N. Impens, P. Van Der Voort, E.F. Vansant, Microporous and Mesoporous Materials 28 (1999) 217. [5] J. Gulin´ski, H. Maciejewski, Chemik 5 (1998) 119. [6] H. Maciejewski, J. Gulin´ski, B. Marciniec, Polimery 43 (1998) 219. [7] US Patent 4 566 908 (1986). [8] US Patent 4 576 888 (1986). [9] US Patent 4 877 451 (1989). [10] F.M. Winnik, B. Keoshkerian, J.R. Fuller, P.G. Hofstra, Dyes and Pigments 14 (1990) 101. [11] A. Tentorino, E. Matijevic´, J.P. Kratohvil, Journal of Colloid and Interface Science 77 (1980) 418. [12] B. Aiken, W.P. Hsu, E. Matijevic´, Journal of American Ceramic Society 71 (1988) 845. [13] H. Giesche, E. Matijevic´, Dyes and Pigments 17 (1991) 323. [14] W.P. Hsu, R. Yu, E. Matijevic´, Dyes and Pigments 19 (1992) 222. [15] G. Wu, A. Koliadima, Y.-S. Her, E. Matijevic´, Journal of Colloid and Interface Science 195 (1997) 222. [16] E. Matijevic´, Journal of the European Ceramic Society 18 (1998) 1357. [17] A. Krysztafkiewicz, T. Jesionowski, S. Binkowski, Colloids and Surfaces A 173 (2000) 73. [18] S. Binkowski, T. Jesionowski, A. Krysztafkiewicz, Dyes and Pigments 47 (2000) 247.
Pigments on amorphous silica carriers Andrzej Krysztafkiewicz, Slawomir Binkowski *, Iwona Wysocka Institute of Chemical Technology and Engineering, Poznan´ University of Technology, Pl. M. Sklodowskiej-Curie 2, 60-965 Poznan´, Poland A
Received 10 April 2001; received in revised form 12 February 2002; accepted 27 March 2003
Abstract Amorphous precipitated silica was applied as a carrier for pigments. The silica surface was modified with silane coupling agents, such as 3aminopropyltriethoxysilane,N-2-(aminoethyl)-3-aminopropyltrimethoxysilane and 3-ureidopropyltrimethoxysilane. Pigments were obtained by attaching organic dyes, C.I. Acid Red 18 and C.I. Acid Violet 1, to a modified silica surface. The adsorption process was conducted in an aqueous suspension of the modified silica in the presence of the dye. Various amounts of the modifying compounds were used in order to determine their effect on the dye adsorption process. The structural and microscopic properties, zeta potential and agglomerate size distribution were examined for the modified silicas and the obtained pigments. Colours of the obtained pigments were measured. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Nanosize pigments; Silica; Silane coupling agents; Adsorption of dyes
1. Introduction Silicas are extensively modifiable materials [1,2]. The modification results in products which carry on their surface new functional groups, capable of interacting with various organic compounds [3]. For example, application of aminosilane for the modification produces functional amino groups which may react with carbonyl groups of aldehydes, ketones or esters [4 –6]. The aminosilane-modified silicas find multiple applications in industry, serving, inter alia, as polymer fillers. Recently, they have been used with increasing frequency as coupling agents in pigment or organic dye systems. Until recently, studies on organic pigments involved modification of basic properties, such as stability or intensity of colours. In multiple cases, however, pigments must satisfy additional requirements, such as, e.g. well defined particle diameters. Pigments with silica cores can satisfy such requirements. The pigments mentioned above may be used in many applications. In earlier studies [7,8], azo pigments were described, suitable for application in electrophotographic toners. Those consist of a silica core of particle diameter below 10 Am with a mono- or polyazo dye coating, chemically bound to the silica surface by the silane. In other * Corresponding author. Tel.: +48-61-665-36-26; fax: +48-61-665-3649.
papers [9,10], the process of preparation of colourful silicas in which an organic pigment is coupled to the surface of modified silica by a covalent bond was described in detail. Such pigments are applied in the ink for ink jet printers. Tentorino et al. [11], Aiken et al. [12], Giesche and Matijevic´ [13], Hsu et al. [14], Wu et al. [15] and Matijevic´ [16] described preparation of such pigments on various types of inorganic carriers of strictly defined particle diameters by adsorption or incorporation of the dye in the course of carrier preparation. In this way, pigments can be obtained which are resistant to elution by an organic solvent. In the present study, we compared properties of pigments obtained using as carrier the precipitated silica (Syloid 244), as related to the type and amount of the silane coupling agent used for the modification and to the type of solvent which was used to deposit the silane on the silica surface.
2. Experimental 2.1. Materials Commercially available silica, namely, the precipitated silica, Syloid 244 (Grace Davison), was used. Silane coupling agents used for modification of the silica included:
E-mail address: [email protected] (S. Binkowski). 0032-5910/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0032-5910(03)00073-1
3-aminopropyltriethoxysilane [U-13, H 2 N(CH 2 ) 3 Si (OC2H5)3],
A. Krysztafkiewicz et al. / Powder Technology 132 (2003) 190–195
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N-2-(aminoethyl)-3-aminopropyltrimethoxysilane [U-15, H2N(CH2)2NH(CH2)3Si(OCH3)3], 3-ureidopropyltrimethoxysilane [U-17, H2NC(O)NH (CH3)3Si(OCH3)3], produced by UniSil.
sette) and passed through a sieve. The same procedure was repeated for the unmodified silicas. In the filtrate, the amount of unadsorbed dye was estimated by measuring absorbance in a SECOMAM S 750 spectrophotometer.
For preparation of colourful silicas, organic dyes which were not prepurified were used, namely, C.I. Acid Red 18 and C.I. Acid Violet 1 (Boruta-Kolor) of the following structures:
2.4. Evaluation of the pigments obtained
C.I. Acid Red 18
C.I. Acid Violet 1
2.2. Modification of the silica The silica surface was modified with silane coupling agents, such as U-13, U-15 and U-17. Solutions were prepared containing one, three and five parts by weight of the modifiers in water, methanol or methanol/water mixture (4:1) per 100 parts by weight of the modified silica [17]. The amount of the modifier was always so adjusted that exclusively uniform wetting of the silica surface took place in the mixer. Appropriate solutions of the modifier were gradually added dropwise to a round bottomed flask containing the silica. The solutions were prepared directly before the modification to prevent their aging. The modification was performed in a mixer for 1 h at room temperature. Following the cycle of mixing, the silica was dried at 105 jC and passed through a sieve of 0.2 mm mesh. 2.3. Colouration of the silica A total of 50 cm3 aqueous solution of an organic dye [18] of various concentrations and 1 cm3 HCl were introduced to a conical flask, containing each time 2 g of the modified silica, Syloid 244. The contents of the flask were magnetically mixed for 4 h at room temperature. Subsequently, the content was filtered under vacuum. The sediment was dried at 105 jC, ground in an electric mortar 02 (Fritsch Pulveri-
In order to determine the morphology of the pigment surface, selected samples were examined in a scanning electron microscope to observe the rough surface of the solids, such as fracture plane, surface structure and pigment agglomerates. Since silicas do not conduct electric current, irradiation of their surface with an electron beam of the scanning electron microscope results in an accumulation of an electric charge which deforms the pattern of surface topography. In order to avoid accumulation of the surface electric charge, silica preparations were made in tertiary butyl alcohol. In these studies, the scanning electron microscope Philips SEM 515 was used. Laser Doppler electrophoretic light scattering determinations were performed with a ZetaPlus instrument, purchased from the Brookhaven Instruments, in the reference beam mode at the wavelength of the laser light source of 635 nm, sampling time 256 As, modular frequency 250 Hz and scattering angle 15j. The standard error of the zeta potentials (f), converted from the experimentally determined electrophoretic mobilities according to the Smoluchowski limit of the Henry equation, was typically < 1.5% and the percent error < 5%. The zeta potential distributions were obtained by averaging three to five runs. Particle size distribution was also examined using a ZetaPlus apparatus. The particle size was measured using the dynamic light scattering (DLS) technique. The technique involved weighing out an appropriate sample, placing it in a small amount of water (0.1 g in 50 cm3 H2O) and stabilizing using an ultrasonic bath (50 kHz). The prepared sample was placed in a cuvette, and the size distribution of silica particles was then measured. The colorimetric data of the obtained pigments were measured using an instrumented colorimeter (Spectro Module 4000, JETI Technische Instrumente), which was calibrated using a white colour standard tile with tristimulus values: X = 78.7, Y = 83.1 and Z = 86.5 (standard no. 0085). Daylight (D65) was used as a standardized light source. A fixed amount of pigment sample was poured into the measurement cup. The instrument provides the colour of the samples in terms of the L, a, b colour space system. In this colour space, L represents the lightness, and a and b are colour coordinates: where + a is the red direction, a is the green direction, + b is the yellow direction and b is the blue direction.
3. Results and discussion In order to examine the uniformity of the silicas and of the pigments prepared from them, microscope studies were
192
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performed and the particle size distribution was examined. For the unmodified precipitated silica, Syloid 244, the representative electron micrograph is presented in Fig. 1A while the particle size distribution is presented in Fig. 1B. The silica did not exhibit a significant homogeneity, as proven by the intense band corresponding to agglomerate formation. The band was situated within the range of 1300– 1750 nm. The silica structure comprised also the intense band associated with aggregate presence and positioned within the range of 430 –570 nm (the maximum intensity of 100 corresponded to aggregates of diameter of 508.6 nm). The electron micrograph confirmed the presence of the agglomerates. Following modification with five parts by weight of U13 aminosilane (in a water system), the Syloid 244 silica
Fig. 2. (A) Scanning electron micrograph (SEM) of silica Syloid 244 modified with five parts by weight of U-13 silane (in water). (B) Particle size distribution of silica Syloid 244 modified with five parts by weight of U-13 silane (in water).
Fig. 1. (A) Scanning electron micrograph (SEM) of unmodified silica Syloid 244. (B) Particle size distribution of unmodified silica Syloid 244.
became less uniform (Fig. 2A). However, in the curve of particle size distribution (Fig. 2B), a low-intensity band could be observed, corresponding to the presence of agglomerates of 2500– 4100 nm in diameter (maximum intensity of 5 corresponded to agglomerates of diameter 2731.0 nm). On the other hand, the very intense band reflecting the presence of aggregates was situated in the range of 320 – 550 nm. The effective diameter of the silica agglomerates was 455.1 nm, and maximum intensity of 100 corresponded to an agglomerate diameter of 399.6 nm. The electron micrograph (Fig. 3A) clearly proved a highly uniform character of the sample and low tendency to form agglomerates after adsorption of C.I. Acid Red 18 dye on the so modified silica. Only an insignificant increase
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dispersed pigments. The modification of the silica surface resulted in an augmented value of zeta potential. Adsorption of the dye on the surface of the modified silica further augmented the zeta potential on the surfaces. Syloid 244 modified with aminosilane and following adsorption of C.I. Acid Red 18 dye reached on the surface a highly positive zeta potential ( + 48 mV). As documented by the data of Tables 1 and 2, the silica surface modification with silanes exerted a significant effect on the adsorption extent of C.I. Acid Red 18 and C.I. Acid Violet 1 on the silica surface. The least adsorption extent of C.I. Acid Red 18 was obtained for the unmodified surface (9.9%) and for surfaces modified with 1 part by weight of U13 silane (27.7 – 62.9%). The silica surface modification resulted in an clearly increased extent of adsorption of C.I. Acid Red 18 dye on the surfaces. In the case of C.I. Acid Violet 1 dye, no dye was adsorbed on unmodified silica surfaces. For both dyes, a particularly high extent of adsorption was obtained for Syloid 244 silica, modified with three or five parts by weight of U-13, U-15 or U-17 silanes. The silicas modified with three or five parts by weight exhibited the optimum dye adsorption and, thus, application of even higher concentrations of silanes would be pointless. It could also be noted that multiple adsorption of the same dye (C.I. Acid Violet 1) on Syloid 244 silica modified with five parts by weight of U-15 silane in methanol resulted in a decrease
Table 1 Adsorption extent of C.I. Acid Red 18 dye (adsorption time: 4 h) Amount of silane (weight parts)
Fig. 3. (A) Scanning electron micrograph (SEM) of silica Syloid 244 modified with five parts by weight of U-13 silane (in water) after adsorption of C.I. Acid Red 18. (B) Particle size distribution of silica Syloid 244 modified with five parts by weight of U-13 silane (in water) after adsorption of C.I. Acid Red 18.
in effective agglomerate diameter to 544.4 nm was observed (Fig. 3B). A low-intensity agglomerate band was formed, with an agglomerate diameter lower than that noted for the silica modified with the silane alone. The band was positioned within the range of 1200 – 1600 nm (maximum intensity of 36 corresponded to agglomerates diameter of 1337.6 nm). An intense band, corresponding to aggregates, was positioned within the range of 390– 550 nm (maximum intensity of 100 corresponded to aggregates diameter of 433.0 nm). Following modification and the surface coating with the organic dyes, the syloid 244 silica manifested most advantageous properties, which should characterize highly
Modification medium
Dyes concentration before adsorption (mg/cm3)
Dyes concentration after adsorption (mg/cm3)
Syloid 244 0 –
0.20
0.18030
9.9
U-13 3 5 1 3 5 1 3 5
water water methanol methanol methanol methanol/water methanol/water methanol/water
0.32 0.32 0.32 0.32 0.32 0.32 0.32 0.32
0.07918 0.00728 0.13030 0.04360 0.01436 0.11860 0.02357 0.02023
75.3 97.8 59.3 86.4 95.6 62.9 92.6 93.7
U-15 1 3 5 1 3 5
water water water methanol/water methanol/water methanol/water
0.32 0.32 0.32 0.32 0.32 0.32
0.06493 0.01778 0.01094 0.03111 0.03027 0.00332
79.7 94.4 96.6 90.3 90.6 98.9
U-17 1 3 5
methanol/water methanol/water methanol/water
0.32 0.32 0.32
0.07190 0.11070 0.01465
77.5 65.4 95.4
Disposal extent (%)
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Table 2 Adsorption extent of C.I. Acid Violet 1 dye (adsorption time: 4 h) Amount of silane (weight parts)
Dyes concentration before adsorption (mg/cm3)
Dyes concentration after adsorption (mg/cm3)
Syloid 244 0 –
0.4
0.4000
0
U-15 1 3 5 5 5 1 3 5
0.4 0.4 0.4a 0.4b 0.4c 0.4 0.4 0.4
0.37210 0.06556 0.03560 0.20400 0.38580 0.16930 0.10540 0.00237
7.0 83.7 91.1 49.0 3.6 57.7 73.7 99.4
U-17 1 3 5 1 3 5
Modification medium
methanol methanol methanol methanol methanol methanol/water methanol/water methanol/water
methanol methanol methanol methanol/water methanol/water methanol/water
0.4 0.4 0.4 0.4 0.4 0.4
0.28950 0.08350 0.02363 0.26900 0.05906 0.05007
Disposal extent (%)
27.7 79.2 94.1 32.8 85.3 87.5
a
First adsorption process onto the same sample. Second adsorption process onto the same sample. c Third adsorption process onto the same sample. b
of extent of the adsorption from 91.1% in the first adsorption to 3.6% in the third adsorption (Table 2). Colour of the obtained pigments was measured using the colorimeter described earlier. The data were presented in the form of table (Table 3) and as graphs in a tristimulus system, L*a*b* (Fig. 4). The L* value, brightness of the pigments, was most pronounced for pigments obtained by adsorption of C.I. Acid Red 18 on the unmodified silica Syloid 244 (Fig. Table 3 Colorimetric data for obtained pigments Sample
Colorimetric data C*
h*
Syloid 244 Syloid 244 + AR18 Syloid 244 + 1(m/w) U-17 + AR18 Syloid 244 + 3(m/w) U-17 + AR18 Syloid 244 + 5(m/w) U-17 + AR18 Syloid 244 + AV1 Syloid 244 + 1(m) U-15 + AV1 Syloid 244 + 3(m) U-15 + AV1 C.I. Acid Red 18 C.I. Acid Violet 1
100.00 82.31 75.49
0.56 14.07 29.64
2.64 13.52 20.65
2.73 19.44 36.09
259.11 43.81 34.83
75.05
28.36
20.81
35.01
36.31
70.85
34.90
24.58
42.80
35.12
85.89 61.68
6.76 11.43
6.84 16.90
9.47 20.50
314.40 304.12
59.34
10.69
15.95
19.20
304.01
33.10 26.38
24.65 0.64
23.10 6.75
33.85 6.82
43.56 85.43
L*
a*
b*
Fig. 4. Colorimetric data for pigments obtained from C.I. Acid Red 18 dye on the silica Syloid 244 modified with U-17 silane (in methanol/water mixture).
4). However, increase in the amount of U-17 silane used for modification of the silica in a mixture of methanol and water was associated with a decrease in brightness of the pigments obtained. The data corresponded to the obtained efficiencies of the adsorption (Table 1). The obtained brightness estimates (L*) might indicate that an increase in the applied silane amounts (independently of its type) was linked to an increase in dye adsorption. One can hypothesise that an increase in the applied silane concentrations has been paralleled by an increase in the number of adsorbing sites. The values a* and b* for the obtained pigments represent a qualitative index of their colour. For pigments obtained on the Syloid 244 silica, a* ranged between + 14.07 for the unmodified silica to + 34.90 for the silica modified with five parts by weight of U-17 silane in a methanol/water mixture, while the value b* ranged, respectively, from 13.52 to + 24.58. As mentioned earlier, + a corresponds to red, a to green, + b to yellow and b to blue colours. Summing up the above results, one may conclude that increase in the amount of the applied modifier results in increasing colour of the pigments (increasing values of a* and b*).
4. Conclusions
Example: SYLOID 244 + 3(m/w)U-13 + AR-silica SYLOID 244 modified with three parts by weight of U-13 silane (in methanol/water) after adsorption of C.I. Acid Red 18 dye.
Electron microscopic studies using the scanning technique have demonstrated that adsorption of dyes on modified silica carrier enables a uniform organic pigment to be obtained. Dye adsorption on a modified silica has resulted in clearly improved uniformity of the pigments obtained, based on Syloid 244 carrier, while ranges of the aggregate and agglomerate bands have become shifted toward smaller particles. The extents of the dye adsorption have been greatly affected by silica surface modification with silane coupling agents.
A. Krysztafkiewicz et al. / Powder Technology 132 (2003) 190–195
As shown by colorimetric studies, the brightness of colours in the pigments obtained has been affected by the amount and type of the applied silane used for modification of the silica surface.
Acknowledgements The authors are indebted to ‘‘Boruta-Kolor’’ Sp. z.o.o. for a gift of the dyes used in the studies, and ‘‘Medson’’ S.C. for enabling us to do colorimetric measurements. This work was supported by grant no. DS 32/115/2003.
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