Langmuir 1992,8, 43-47
43
Suspension Interaction of Rutile and Anatase Titanium Dioxide Pigments in Organic Liquidsf K. V. S. N. Raju and M. Yaseen* Organic Coatings and Polymers, Indian Institute of Chemical Technology, Hydera bad -500 007, India Received September 4, 1990. I n Final Form: August 14, 1991 The physical interaction between Ti02 pigment and an organic liquid has been studied by suspending its finely ground particles in 62 individual liquids. The suspension interaction of the pigment in some liquids is observed to be such that its particles remain suspended in them for hours to days. To effect such an interaction, the liquid should get adsorbed on the pigment surface. Applying the principle that materials which have cohesive energy density in close range attract each other, the data of Hansen's cohesion parameters (based on semiempirical equations and theoretical subdivision of the Hildebrand parameter, 6) of organic liquids were fed into a computer program. The output data obtained from the three-dimensionalplots of spherical volumes of a suspension of the pigment have been used to characterize its surface in terms of Hansen's cohesion parameters of organic liquids in which the pigment particles remain suspended for a certain period of time. This type of characterization is applicable only to the surface and not to the bulk of the pigment.
Introduction In a good dispersion, it is necessary that the dispersed particles should interact favorably with the dispersing medium. Usually, in an organic coating formulation, pigments are premixed with a resin and organic liquids and then ground to the desired extent of fineness. The dispersed pigment particles remain suspended in the dispersion because of specific interactions among the components. In order to have a favorably stabilized dispersion of pigment in the resin solution, the solvents are to be chosen by taking into account the acid-base class1 of resins and pigments so that interaction between resin and solvent is greater than that between pigment and solvents. The adsorption of resin on the pigment surface is influenced by factors such as chemical groups, steric interaction, pigment modification, and nature of the organic liquids. Burrel12studied the adsorption of styrene copolymers on a variety of pigments in the presence of xylene and found the adsorption was influenced by the chemical groups present in the copolymer. Porowska and Hippe3 made a gravimetric study of the adsorption of polyesters on rutile Ti02 and found that polyesters prepared from the unsaturated acids adsorbed - 5 times more than those prepared from saturated acids. Abramov et al.4 studied the effect of hydroxyl number on the adsorption of polyesters on rutile TiO2. The thickness of the adsorbed resin layer can be measured by ellip~ometry,~,~ determination of sedimentation rate of dispersed p a r t i ~ l e sa, ~rheological method,8p9 turbidimetry,1° or infrared spectroscopy." Janardhan et
IICT Communication 2669. (1) Sorensen, P. J. Paint Technol. 1975,47, 31. (2) Burrell, H. Abstracts of Papers, 170th National Meeting of the American Chemical Society, Chicago, IL, 1975; American Chemical Society: Washington DC, 1975; ORPL 4. (3) Porowska, E.; Hippe, Z. Rapra Abstr. 1970, 6(4), abstr. 3425. (4) Abramov, Y. I.; Chupeev, M. A.; Eltekov, Yu. A. Lakokras. Mater. Ikh. Primen. 1972, No. 3, 9. (5) Staromberg, R.; Tutes, D. J.; Pessaglia, E. J. J. Phys. Chem. 1965, 69, 3955. (6) Killmann, E.; Eisenlauer, J.;Korn, M. J.Polym. Sci.,Polym. Symp. 1977, 61, 413. (7) Fontana, B. J.; Thomas, J. R. J. Phys. Chem. 1961,65, 480. (8) Ohm, 0. E. J. Polym. Sci. 1955, 17, 137. t
a1.12studied the adsorption of alkyd resin on an anatase Ti02 surface by using gel permeation chromatography and concluded that the amount adsorbed depends on the dielectric constant of the solvent and also on the solventresin interaction. They also studied13 the adsorption of polyurethane on the surface of iron oxide in methyl ethyl ketone and in a mixture of methyl ethyl ketone and nheptane. Schiesser14observed that the level of adsorption of polyamide resin on carbon black and on rutile Ti02 was optimum in a 1-butanol-water mixture. This was due to a progressive change in the interaction of pigment compared to that in 1-butanol or in a 1-butanol-decane mixture. Romo's15calculations based on the theories of Hamaker and Overbeck-Verwey led to the conclusion that the stability of Ti02 suspension in 1-butanol and in l-butylamine was due to electrostatic attraction. Witekowe and Kaminski16found that the relatively higher adsorption of methylene blue on activated carbon was due to an increase in the degree of fineness and a change in the electrochemical properties of charcoal. The Toronto Society17 studied the suspension behavior of pigments and classified Ti02 into alcohol and ether groups, red iron oxide in ketone groups, and phthalocyanines in ketone and ester groups on the basis of their interaction with organic liquids. In a suspension study of five a-phthalocyanine blue pigments by electrophoreticmobility techniques, the pigments were found to get charged in some organic liquids and remained su~pended.'~J~ (9) Doroszkowski, A., Lambourne, R. J. Colloid Interface Sci. 1968, 26, 214. (10) Jenekel, E.; Rumback, B. 2.Electrochem. 1951,55,612. (11) Sakai, H.; Imanura, Y. Bull. Chem. SOC.Jpn. 1980,53,1749. (12) Janardhan,R.; Gedam,P. H.; Sampathkumaran,P. S.;Narasimha, R.; Sajid Husain J.Appl. Polym. Sci. 1988, 36, 1781. (13) Janardhan, R.; Gedam, P. H.; Sampathkumaran, P. S. Langmuir 1989, 5, 1031. (14) Schiesser, R. H.; Schaeffer, W. D.; Zettlemoyer, A. C. J. Oil Colour Chem. Assoc. 1967,50, 865. (15) Romo, L. A. J. Phys. Chem. 1963, 67, 382. (16) Witekowa, S.; Kaminski, W. Zesz. Nauk Politech. Lodz., Chem. 1962, 12, 55. (17) Toronto Soc. Off. Dig. 1963, 35, 1211. (18) Petersen, A. Farg Och Lack 1966, 12(1), 4. (19) Vinther, A. Fiirg Och Lack 1965,11(20), 317.
0743-7463192124Q8-QQ43$03.0Q/Q0 1992 American Chemical Society
44 Langmuir, Vol. 8, No. 1, 1992
Pigment-Liquid Interaction in Terms of Hansen’s Cohesion Parameters of Dispersing Medium The Hildebrand parameter, 6, the square root of cohesive energy density (CED),a function of intermolecular forces, is usually expressed as20 6 = (AE/?fy
Here, V is molar volume and AE is the energy of vaporization, which includes energies arising from the modes of interactions, i.e., dispersion forces h E d , polar forces AE,, and hydrogen-bonding forces U h . h E f v = A E d / v + h E , f v + hEh/v
This expression is an empirical one based on semiempirical equations2’ and has been used for theoretical subdivision of the Hildebrand parameter. The symbols ad, 6,, and 6h represent Hansen’s cohesion parameters due to dispersion, polar, and hydrogen-bonding forces. Data on physical properties such as dipole moment, dielectric constant, and internal pressure are used for calculating these p a r a m e t e r ~ . ~ l - ~ ~ H a n ~ e nstudied ~ ~ the pigment-solvent interaction by suspending the pigment in different organic liquids and suggested an empirical method for characterizing the surface of a pigment in terms of Hansen’s cohesion parameters. Eissler et al.25 used multiple regression analysis for estimating the effect of surface treatment on solvent-pigment interactions. Cheever and Ulciny26used the solid-liquid contact angle for the estimation of energy of interaction between pigment and dispersing medium. Shareef et aLZ7estimated Hansen’s cohesion parameters of pigments by using their sphericalvolumes of suspension. The objective of this study is to estimate the interaction of Ti02 pigments in organic liquids and characterize the pigment surface in terms of Hansen’s cohesion parameters of liquids in which the pigment is found to have a certain degree of interaction.
Experimental Section Anatase (TravancoreTitanium Products Ltd.) of 3.84 specific gravity and more than 99% pure pigment and rutile (Woodall Duckham Chemicals Ltd.) of 4.12 specific gravity and almost 100% pure pigment, passing through 300 mesh, were used. Sixty-two organic liquids having Hildebrand parameters ranging from 14.40to 33.34 MPa1/2were selected for the pigment suspension study. Procedure. A 0.25-g sample of a pigment was taken in a 50-mLconicalflask and dried at 140OC for 8 h. A 25-mL aliquot of an organic liquid was added to the flask immediately after it was taken from the oven, and the flask was carefully closed. The pigment was suspended individually in 62 liquids following the above procedure. The flasks were shaken for 16 h and then the contents were transferred to 25-mL stoppered measuring cyl(20) Hildebrand, J.; Scott, R. L. The Solubility of Nonelectrolytes, 3rd ed.; Reinhold New York, 1949. (21) Barton, A. F. M. Handbook of Solubility Parameters and Other Cohesion Parameters; CRC Press Inc.: Boca Raton, FL, 1983. (22) Gardon, J. L.; Teas, J. P. Solubility Parameters. In Treatise on Coatings; Marcel Decker, Inc.: New York, 1976; Vol. 2, Part 11,Chapter 8.
(23) Begley, E. B. Solubility Parameters, Their Origin and Use. Paint Industry short course, Kent State University, Kent, OH, 1972. (24) Hansen, C. M. J. Paint Technol. 1967,39, 505. (25) Eissler, R. L.; Zgol, R.; Stolp, J. A. J . Paint Technol. 1970,42,483. (26) Cheever, G. D.; Ulciny, J. C. J. Coat. Technol. 1970,55(697), 53. (27) Shareef, K. M. A.; Yaseen, M.; Mohamood Ali, M.; Reddy, P. J. J . Coat. Technol. 1986, 58(773), 35.
Raju and Yaseen inders. The suspension behavior of each pigment sample was observed visually at different intervals (5and 30 min, 1,2,6,8, and 24 h, and every day for 7 days). Classification of Suspension. The suspension of the pigment in an organic liquid was classified into the following four classesby taking into consideration the periods duringwhich a certain amount of pigment remained suspended: class I, the pigment settled down within 30 min and left a clear supernatant liquid; class 11, a part of the pigment remained suspended for up to 8 h; class 111,a major part of the pigment remained suspended for up to 24 h; class IV, the pigment remained suspended for a period of more than 24 h. Computer Program for Characterization of Pigment Surface in Terms of Hansen’s Cohesion Parameters. The suspension behavior of Ti02 pigment was used to express ita interaction with organicliquids. A computer programn was used for plotting the sphericalvolume of suspension in terms of Hansen’scohesion parameters (&, d,, ah) of liquids, in which pigments has certain degree of interaction.
Results It was found that the size of the finely ground pigment particles as well as the viscosity of the liquids do not influence the suspension behavior of pigments.27 The factors responsible for keeping a pigment suspended in an organic liquid are the characteristics of the pigment surface and the forces of liquid-solid interaction. Data of Hansen’s cohesion parameters of organic liquids (Table I) in which the pigment was suspended individually were fed into the computer program. The output data from the computer program were used in characterizing the pigment surface in terms of its Hildebrand and Hansen’s cohesion parameters and radius of sphere, CR.
Discussion
In practice, the principle of Stokes’law is used to describe the suspension behavior of solid particles in a dispersing medium. The inorganic pigments are usually heavy and inert. Because of their higher densities, they usually settle in low-density media like water and organic liquids and their comparative rate of settling could be determined by applying Stokes’ law. However, Stokes’ law may not be applicable so effectively to systems in which particles remain suspended for a certain period of time,28because of their possible physical interaction with the dispersing medium. This in turn may influence the rate of sedimentation of the particles. Some w o r k e r have ~ ~ ~projected ~ ~ ~ a concept which takes into account the suspension behavior of untreated/ modified pigments in pure organic liquids for characterizing their surfaces. The basic principle of this concept is based on physical interaction between pigment particles and liquid molecules. The surface energy of an inorganic pigment is likely to lie far from that of organic liquids. Even then, in some cases the suspension interaction between pigment particles and liquid molecules is so strong that the finely divided pigment particles remain suspended for hours or days. The surface of untreated rutile and anatase Ti02 pigments has been characterized by taking into account their suspension interactions. The term cohesion parameter does not adequately represent the predictive possibilities of the system, particularly where a solid surface is involved. Terminology to be used in such a situation should be adopted with the term attraction leading to physical interaction. Applying the principles that materials with similar Hansen’s co~~
~
(28) Hansen, M. C. Three-Dimensional Solubility Parameter and Soluent Diffusion Coefficient;Danish Technical Press: Copenhagen, 1967.
Langmuir, Vol. 8, No. 1, 1992 45
Suspension Interaction of Ti02 Pigment in Organic Solvents
Table I. Hildebrand and Hansen's Cohesion Parameters 6 (MPa112)of Organic Liquids and Classification of Their Interaction with Ti02 Pigments ~
~~
solvent
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. a
Hansen's cohesion parameters Hildebrand parameter 6
8d
14.40 15.18 15.30 15.61 16.00 16.28 16.30 16.41 16.45 16.73 17.00 17.00 17.20 17.31 17.41 17.59 17.69 17.80 17.90 18.00 18.00 18.00 18.23 18.61 18.72 18.84 19.00 19.00 19.47 19.60 20.11 20.21 20.29 20.50 20.50 20.80 20.80 21.70 21.70 21.72 21.81 22.05 22.09 22.40 22.58 22.71 22.71 23.09 23.50 24.30 24.40 24.51 24.51 24.67 25.51 25.90 26.14 26.43 28.92 29.60 31.50 33.34
13.71 15.18 15.30 15.61 16.00 14.93 16.30 13.0 15.10 16.73 16.85 15.30 12.9 15.69 16.41 13.75 17.69 13.0 15.61 17.80 17.69 17.69 18.04 15.22 18.31 17.69 15.89 18.00 16.82 18.90 13.30 17.69 17.80 15.00 19.00 15.79 16.00 16.00 18.92 17.59 19.60 18.59 15.20 17.41 19.49 15.20 15.10 16.00 15.79 16.06 15.30 16.00 18.49 16.16 15.30 13.50 18.94 15.81 16.57 15.10 17.20 16.88
diisopropyl ether heptane octane decane white spirita diethylamine dibutyl ether allyl chloride isobutyl butyrate cyclohexane white spirit + xylene (1:l) methyl isobutyl ketone ethyl methacrylate 1-butyl acetate 1-butyl chloride ethyl propionate carbon tetrachloride methyl methacrylate 1-propyl acetate ethylbenzene xylene m-xylene toluene ethyl acetate benzene chloroform methyl ethyl ketone trichloroethylene tetrahydrofuran chlorobenzene ethyl acetoacetate cyclohexanone dichloromethane 1-hexanol 1,4-dioxane diacetone alcohol butyl cellosolve 1-pentanol pyridine nitrobenzene acetophenone dimethyl phthalate tert-butyl alcohol cyclohexanol aniline isooctyl alcohol isobutyl alcohol 1-butanol 2-propanol ethyl cellosolve acetonitrile 1-propanol allyl alcohol methyl cellosolve propionic acid acrylic acid butyrolactone benzyl alcohol 1,3-butanediol methanol ethanolamine ethylene glycol
6,
2.11 0.00
0.00 0.00 0.00 2.25 0.00 7.7 2.86 0.00 0.51 6.10 8.5 3.68 5.50 8.0 0.00 9.1 4.50 0.61 1.02 1.02 1.43 5.32 1.02 3.07 9.00 2.91 5.73 4.30 11.11 8.39 6.30 8.51 1.80 8.20 5.09 4.50 8.80 12.27 8.59 16.94 5.09 4.09 5.12 9.31 9.71 5.71 6.10 9.21 18.00 6.79 4.91 9.21 11.21 12.81 16.57 8.80 10.02 12.29 15.61 11.05
6h
3.91 0.00 0.00 0.00 0.00 6.14 0.00 6.6 5.93 0.00 1.53 4.09 7.8 6.34 2.11 7.1 0.00 8.0 7.59 1.43 3.07 3.07 2.05 9.21 2.05 5.73 5.11 5.30 7.98 2.11 10.21 5.12 6.10 13.71 7.41 16.94 12.29 13.91 5.93 4.09 3.70 4.89 14.89 13.50 10.23 14.09 16.00 15.79 16.41 14.32 6.10 17.41 13.91 16.36 17.10 17.90 7.36 19.43 21.48 22.30 21.29 25.98
classification of suspension rutile
I I I I I I1
I I I11 I I I11
I I I11
I I I I I I I1
I I I I I I I I I11 I1
I IV IV IV IV IV IV I1 IV IV IV I1 IV IV IV IV I1 IV I1 I1 I1
IV IV IV IV IV I11 IV
anatase
I I I I I I I I I I I I11 I I1 I I I I I
I I1 I1
I I I I1
I I I I1
I I11
I IV I1 I11 I11
I I11 I11 IV
IV
I IV I I1
I I1 I11 I1
IV I11 IV IV 111 IV
Distillation temperature, 157-167 "C.
hesion parameters attract each other, one can conclude that volumes of interaction of pigment and the liquid overlap. Consequently, the liquid should be adsorbed on the pigment surface to effect the interaction. On this basis, the concept of Hansen's cohesion parameters can be applied to the characterization of the pigment surface. Suspension of Ti02 Pigments in Organic Liquids. Rutile pigment particles are found to have good interaction with 32 liquids out of which they remained suspended in
20 for more than 24 h. On the other hand, anatase TiOz pigment was found to exhibit good interaction with 24 organic liquids. Since these liquids contain a hydroxyl, carboxylic, or amine group, the pigment surface may have acquired charge distribution in them and developed a strong physical interaction, which, in turn, kept the pigment particles in a suspended state for a longer period of time. As most of the liquids in which rutile Ti02 particles have a good interaction are amphoteric in nature,
46 Langmuir, Vol. 8, No. 1, 1992
Raju and Yaseen
Cohesion Parameters, 61 (MPai’2 derived from SVS alqorithm
6d
16.70
dp =
8.25
6,
= 14.05
5
= 23.33
CR = 12.27
b,
Figure 1. Spherical volume of suspension for rutile titanium dioxide. Key: (0) class I, (0)class 11, (A)class 111, (+) class IV.
r
r
r
Cohesion Parameters, d /(MPa) 112 derived from SVS algorithm
6~
16.15
6, =
8.55
6h = 15.05
6 CR
L
0
= 23.67 1k26
0 I
I
I
Figure 2. Spherical volume of suspension of anatase titanium dioxide. Key: (0) class I, (0)class 11, (A)class 111, (+) class IV.
on the basis of the acid-base concept, these pigments could be classified to be amphoteric. This type of character-
ization is applicable only to the surface but not to the bulk of the pigment.
Suspension Interaction of Ti02 Pigment in Organic Solueniis In studies on the charge of pigments in various organic liquids, earlier worker^^^^^^ reported that a pigment which was found to be positive in alcohol and in liquids near the hydrogen-bonding axis developed a negative charge in ketones and in liquids closer to the polar axis. Data on the classification of suspension interaction of Ti02 pigments in organic liquids are given in Table I. They were fed into the computer program to obtain the threedimensional plots with respect to rutile and anatase Ti02 pigments. The three plots in Figure 1 represent the spherical volume of suspension of rutile Ti02 in the 62 organic liquids in which the pigment was suspended. The surface of the pigment is characterized in terms of Hansen’s cohesion parameters of liquids, which have a certain amount of interaction (classes 11-IV). The values of the parameters derived from the output of the computer program for rutile Ti02 are given (MPa1l2): 6d = 16.70,6, = 8.25, 6h = 14.05, 6 = 23.33, and CR = 12.27. CR is the radius of the sphere which covers all the points representing the interaction of organic liquids with pigment in a threedimensional diagram. The data on the suspension of anatase with respect to the 62 organic liquids were also fed into a computer program and the output is illustrated by the threedimensional plots in Figure 2. The values of the cohesion parameters of anatase Ti02 derived from the plots are (29) Brintzinger, H.; Haug, R.; Sachs, G. Farbe Lack 1954,60, 15. (30) Hamann, K.; Florus, G. Farbe Lack 1956,62, 260.
Langmuir, Vol. 8, No. 1, 1992 47 given (MPa1/2):6d = 16.15,6, = 8.55, 6h = 15.05,6 = 23.67, and CR = 11.26. The technique of characterization of pigment surface, in terms of Hansen’scohesion parameters of organicliquids in which pigment particles show interaction, is an indirect method. It is based on the fact that liquids in which the pigment remains in a suspended state for at least 8 h or more must have a sufficient degree of interaction with the pigment. The critical analysis of suspension data of rutile and anatase pigments in various organic liquids (Table I) indicates that rutile has very good interaction (class IV) with 20, whereas anatase has the same in only 8. This feature of suspension behavior can be used in making a distinction between the anatase and rutile forms of TiOz. Registry No. 1,108-20-3; 2,142-82-5; 3,111-65-9; 4,124-18-5; 6,109-89-7; 7,142-96-1; 8,107-05-1; 9,539-90-2; 10,110-82-7; 12, 108-10-1; 13,97-63-2; 14,123-86-4; 15,109-69-3; 16,105-37-3; 17, 56-23-5;18,80-62-6; 19,109-60-4; 20,100-41-4; 21,1330-20-7; 22, 108-38-3;5, 13463-67-7; 23, 108-88-3;24, 141-78-6;25,71-43-2; 26,67-66-3; 27,78-93-3; 28,79-01-6; 29,109-99-9; 30,108-90-7; 31, 141-97-9;32,108-94-1; 33,75-09-2; 34,111-27-3; 35,123-91-1; 36, 123-42-2;37,111-76-2; 38,71-41-0; 39,110-86-1; 40,98-95-3; 41, 98-86-2;42, 131-11-3;43,75-65-0;44, 108-93-0;45, 62-53-3;46, 26952-21-6;47,7843-1;48,71-36-3; 49,67-63-0; 50,110-80-5; 51, 75-05-8;62,71-23-8;53,107-18-6;54, 109-86-4; 55, 79-09-4;66, 79-10-7;57,96-48-0;58, 100-51-6;59, 107-88-0;60,67-56-1;61, 141-43-5;62, 107-21-1.
