XXVI International Mineral Processing Congress (IMPC 2012) Interaction Between TiO2 and Calcite Mixture During Grinding of Pigment in Water Based Paints F Karakaş, B V Hassas and M.S.Çelik Reference Number: (Include reference number provided by IMPC 2012 Conference Secretariat) Contact Author: (Use “Author Details” style) Fırat KARAKAŞ Designation: Dr. Organisation Name: Istanbul Technical University Address: Istanbul Technical University, Faculty of Mines, Mineral Processing Engineering Department, 34469, Maslak, Istanbul, TURKEY. Email:
[email protected] Phone: +90 212 2856123 Fax: +90 212 2856128 Mobile: +90 535 9369726 Email:
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Interaction Between TiO2 and Calcite Mixture During Grinding of Pigment in Water Based Paints F Karakaş1, B V Hassas2 and M.S.Çelik3 1. Istanbul Technical University, Faculty of Mines, Mineral Processing Engineering Department, 34469, Maslak, Istanbul, TURKEY. Email:
[email protected]
2. Istanbul Technical University, Faculty of Mines, Mineral Processing Engineering Department, 34469, Maslak, Istanbul, TURKEY. Email:
[email protected]
3. Istanbul Technical University, Faculty of Mines, Mineral Processing Engineering Department, 34469, Maslak, Istanbul, TURKEY. Email:
[email protected]
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ABSTRACT Water based paint formulations consist of some industrial minerals with quantities varying from 20 to 50 %. Quality of paint is directly related to these minerals used as pigment or filler in the production. Titanium dioxide is the most important and expensive pigment in paint formulations. Other minerals such as calcite and calcined kaolin are used as filler and mainly as a substitute for TiO 2. Consequently, particle-particle interactions including adsorption, coating and size distribution of pigment mixture directly affects the paint quality. In the present paper, a new type of pigment mixture for possible use in architectural water based paints was developed through grinding. The effect of different types of grinding methods on physical properties of TiO2 and calcite mixture and the quality of paint produced by these mixtures have been revealed. Three different types of mills; conventional ball mill, vibratory ball mill and high speed attritor were used for grinding of TiO2 and calcite. Viscosity, zeta potential and particle size distributions were used to characterize the pigment mixtures obtained upon grinding. The quality of paint was evaluated by standards based on both wet and dry paints such as viscosity, density, opacity and gloss. The paint produced with the pigment mixtures prepared in vibratory ball mill and high speed attritor showed improvement in physical properties particularly in the distribution and dispersion of mineral particles. Keywords: Grinding, pigment, paint,TiO2, calcite
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INTRODUCTION Various inorganic minerals with different dosages are used in water borne paints formulations. These minerals are used with the aim of decreasing the cost and improving some physical and mechanical properties of paints (Karakaş et al, 2009; McGonigle, 1996). Calcite is widely used due to its low cost. Interaction of these inorganic minerals with the rest of the paint ingredients is important for improving paint formulations. Understanding of the mechanism of these interactions will help produce paints with better quality and lower cost. Especially interaction of calcite and TiO2 before and during paint manufacture has to be revealed for effective use of these materials. It is already known that calcite particles with optimized size distribution contribute to the better distribution of TiO2 particles in paint medium (Karakaş and Çelik 2012; Werner, 1998). Calcite is positive and TiO2 negatively charged in paint medium. Heterocoagulation occurs under these conditions; calcite behaves like a spacer. Finer size and oppositely charged TiO2 make it easier to adsorb onto relatively larger calcite particles resulting in a better distribution of TiO 2 in the paint. This improvement leads to higher opacity and in turn compensates the poor technical properties of calcite. The same mechanism can be mimicked if calcite particles are mechanically coated with TiO2 before the paint production. Pre-Mixing and mechanical energy may contribute to the coating of calcite particles with TiO2 which create calcite particles that behave like TiO2 particles. TiO2 coated calcite and calcined kaolin are considered low-cost and environmentally friendly alternative for pure TiO2. There are some studies in the literature that show production of TiO 2 coated calcite, kaolin or calcined kaolin with almost the same technical properties as TiO 2. The similarity of TiO2+calcite with pure TiO2 was confirmed by X-ray diffraction (XRD) scanning electron microscope (SEM) and infrared spectra (IR) (Baikun et al, 2010; Lu et al, 2009). In this study, the effect of different types of milling action provided by conventional ball mill, attritor and vibratory ball mill on TiO2+calcite mixture was investigated. This mixture was then used in water born paints to investigate the effectiveness of pre-mixing using standard paint tests along with particle size, viscosity and zeta potential measurements of the same suspensions.
MATERIALS AND METHODS Typical properties of calcite and titanium dioxide used in the study are given in Table 1. Calcite was obtained from “Som Group” companies” of Turkey and titanium dioxide was received from Tronox of USA. Styrene acrylic copolymer used as binder was obtained from “Organic Chemical” of Turkey. Apart from pigments, fillers and binders used in paint formulation consists of various chemicals such as dispersants, thickeners and surfactants in order to improve the quality of the paint. These ingredients were obtained from “Ishakol Paint Company” in Turkey.
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Grinding of the mixture Three different types of mills, conventional ball mill, vibratory ball mill and high speed attritor were used for grinding of calcite and TiO2 mixture. Dry grinding was applied in all cases. Grinding time was kept the same at 3 hours for all the mills. Grinding parameters for each mill type were optimized before the experiments to identify the mechanism of coating and the effect of milling conditions on the properties of the mixture. The composition of mixture before grinding was 23.5 % TiO2 and 76.5% calcite by weight. Conventional ball mill used for producing a mixture of pigment was 110 mm in diameter with ceramic balls of 18 mm in diameter. In vibratory mill, the mixture is exposed to a vibration through the milling without revolving. The size reduction in this mill is relatively high; however, the interaction between particles of the mixture occurs through their surface energies and electrostatic forces which are high enough to agglomerate them without dispersant. The attritor used in this study was a high speed vertical attritor with zirconium oxide balls. This batch type machine was purchased from Union Process Company. Size reduction in this type of mill is the highest; however the size variation of the mixture is not much to affect the results. The effectiveness of different mill types in terms of particle-particle interactions was tested using viscosity and zeta potential measurements. Zeta meter 3.0+ and Brookfield DVII+ were used for zeta potential and viscosity measurements of suspensions, respectively. Viscosities of the samples were measured with low viscosity type spindles. Standard sample cell with temperature control unit was used for the measurements. Zeta-APS from Matec was used to analyze size distribution of the samples with solid content of 10 %wt. Measurement principles of this equipment is based on acoustic attenuation spectra.
Paint production and analysis TiO2 and calcite mixture produced in different mills was used in paint recipes to determine the quality of the paint according to the standard test procedure. Architectural water based paint formulation used as a base recipe in this study contains 8.6 % TiO2 and 28 % calcite. Paint production is explained elsewhere in detail (Karakas and Çelik, 2012). Paints were analysed in both wet and dry film form. The paint was characterized by standard analyses such as viscosity, gloss, opacity and density. A comparison among paints with pigment mixtures obtained from different mills was made based on these characterization tests. The viscosity and density of paints were determined by “Stormer Krebs” viscosimeter and liquid picnometer, respectively (ASTM D562-10, 2010). The contrast ratio measured by reflectometer was used to evaluate the hiding power of paint films (ASTM D 2805-11, 2011). Gloss value of the paint films were measured at three 0
0
0
different incident angles; 20 , 60 and 85 using a glossmeter (ISO 2813, 1994).
RESULT & DISCUSSION Grinding of the mixture The size distribution of TiO2, calcite and their mixtures obtained from different mills is given in Table 2. Particle sizes obtained from different mills are close to each other. Therefore, this parameter is not highly 5
effective on paint properties especially on opacity. Consequently, all the outward changes originate from the effect of particle-particle interactions and alteration of TiO2 and calcite surfaces though the milling processes. As shown in Table 2, particle size of mixtures is close that of individual TiO2; this indicates that there is more size reduction on calcite particles compared to TiO 2. Moreover, the particle size distribution of TiO2 and calcite mixtures obtained through attritor mill exhibits the narrowest one compared to conventional and vibratory ball mills. This distribution is much more effective for getting high quality paint. Conventional and vibratory ball mills yielded relatively larger particles probably due to long grinding time. Because of grinding mechanism, agglomeration without dispersant occurs after a certain period of time. Zeta potential of mixtures is given in Table 3. Evidently, the zeta potential of the mixture is similar to calcite but drifted towards TiO2 indicating that some tiny TiO2 particles have adhered to the surface of calcite particles. The standard mixture of the specimen without milling demonstrates two different zeta potentials as given in Table 3. The first measurement taken after 1 minute of conditioning yielded a zeta potential value of +24.5 mV which is close to zeta potential of calcite at pH 8.6. At this pH value, the zeta potential of calcite is around +17 mV. However, after 2 minutes particles started to move in the opposite direction. The zeta potential for the second measurement was -7.5 mV which shows the dominant effect of TiO2 on zeta potential. The zeta potential of TiO2 at pH 8.6 is around -21.1 mV. The probable mechanism for this finding is that relatively coarse calcite particles settle in the second set of measurement and thus report nearer to the zeta potential of fine TiO2 particles. It can be inferred that grinding of TiO2 and calcite result in a new particle that shows more homogenous and different surface properties than TiO2 and calcite alone. The viscosity and colloidal characteristics of the produced mixture at 10 % wt. was subjected to viscosity measurements with an ascending and descending shear rates. Results are shown in Figure 1. All the suspensions have thixotropic behaviour in terms of viscosity against shear rate; however, some alterations have been observed. Suspension of TiO2 and calcite mixtures obtained from both vibratory ball mill and attritor has smaller viscosity values than that obtained from conventional ball mill on all shear rates.
Paint production and analysis In this section the paint produced with milled mixture was compared with the standard paint using the same proportion of individual TiO2 and calcite. The quality of the paint was compared with each other based on both wet and dry standard paints properties such as viscosity, density, opacity and gloss. Results of the standard paint and paints that consist of pre-mixed TiO2 and calcite mixture are shown in Table 4. Table 4 interestingly shows a remarkable improvement in opacity upon milling. The rest of the paint properties such as density, viscosity and gloss are virtually similar. The standard paint by definition consists of TiO2 and calcite, which were not mixed before the paint production. Unlike the rest of the paints, it illustrates the lowest opacity value. Similarly the opacity of paint produced with TiO2 and calcite mixture obtained from conventional ball mill follows the standard paint. The opacity values of the standard and conventional ball mill exhibit a critical value as stated in the related standards i.e., paint with opacity value above 95 % and below 98 falls in the third class. On the other hand, the opacity of paints produced with the addition of TiO2 and calcite mixture obtained from both vibratory ball mill and attritor are close to the second
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class with their contrast ratios of 97.27% and 97.12%, respectively. There is almost a difference of 2 % in opacity values between the standard-conventional ball mill and vibratory ball mill-attritor. This is rather important for both environmental concerns and cost reduction as it utilizes less quantity of the material. This difference comes from the efficient use of TiO2 as TiO2 is the only pigment with its high refractive index that can contribute the opacity of paint unequalled to any other substance of paint formulation. The only way to improve the opacity of paint apart from increasing of TiO2 content, is to create a perfect dispersion of TiO2 and use each individual TiO2 particle as effective as possible in terms of light scattering. This phenomenon can be only accomplished through the use of proper type of dispersants, e.g. mostly polymers with low molecular weight (Farrokhpay 2009; Liufu et al, 2005; Chenet al, 2004; Filiatre et al, 2003; Tiarks et al, 2003). In addition, some inorganic particles with relatively coarse particle sizes and oppositely charged zeta potentials compared to TiO2 might be used to create homogenous dispersion of TiO2 particles with the aim of facilitating heterocoagulation (Bieleman et al, 2000). Smaller TiO2 particles are shown to adsorb onto larger calcite particles and lead to reduction in agglomeration of individual TiO2 particles. Moreover, a better distribution of pigment particles is achieved with the addition of these coarse inorganic particles. This phenomenon has been shown in another paper (Karakas and Çelik, 2012). On the other hand, the present study has shown that the paint properties of a mixture of TiO2 and calcite can be improved without altering the particle size of TiO2. Prior to introducing into the paint, TiO2 could be adhered on to calcite particles via electrostatic interaction with the help of mechanical forces. The type and magnitude of mechanical force appears to be crucial. For instance, impact force applied in a conventional ball mill seems to hinder the coating of TiO2 onto calcite leading to bad opacity in paint. On the other hand, it is envisaged that the applied shear force exerts a better energy level for coating of TiO2 on calcite. Attritor and vibratory ball mill can apply more shear than impact force compared to conventional ball mil. Therefore, better interaction of TiO2 and calcite is achieved in attritor and vibratory ball mill that is supported by opacity measurements in paint.
CONCLUSIONS In this study, pre-mixing of TiO2 and calcite by different modes of grinding before the paint production and its effects on paint quality were investigated. A strong interaction between TiO2 and calcite was found upon applying the mechanical energy on the mixture. This revealed more homogenous suspensions of TiO2 and calcite mixture as characterized by zeta potential and viscosity measurements. Attritor and vibratory mills were found to be more effective than the conventional ball mill since the latter apply more impact force than shear force. Paints with pre-mixed with TiO2+calcite using attritor and vibratory ball mil yielded 2 % more contrast ratio than the paint pre-mixed with TiO2+calcite using conventional ball mill and respective standard paint. This improvement in opacity is explained on the basis of more efficient interaction of TiO2 and calcite that create more stable pigment particles. Further studies and analyses such as XRD and SEM are in progress to reveal the interaction mechanism of TiO2 and calcite.
ACKNOWLEDGEMENTS We thank “Istanbul Technical University Research Fund” for supporting this work.
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REFERENCES (USE “HEADING 1” STYLE) ASTM D562-10, 2010, Standard Test Method for Consistency of Paints Measuring Krebs Unit (KU) Viscosity Using a Stormer-Type Viscometer. ASTM D 2805-11, 2011, Standard Test Method for Hiding Power of Paints by Reflectometry. Baikun W., Hao D.,Yanxi D., Characterization of Calcined Kaolin/TiO2 Composite Particle Material Prepared by Mechano-Chemical Method, Journal of Wuhan University of Technology-Mater. Sci, 2010. Bieleman, J., Heilen, W., Silber, S., Ortelt, M., Scholz, W., 2000, Surface Active Agents, Chapter 4 in Additives for Coatings, edited by Bieleman J., 67-99, Wiley VCH, Federal Republic of Germany. Chen, J., He, T., Wu, W., Cao, D., Yun, J., Tan, C.K., 2004, Adsorption of sodium salt of poly(acrylic) acid (PAANa) on nano-sized CaCO3 and dispersion of nano-sized CaCO3 in water, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 232, 163–168. Farrokhpay, S., 2009, A review of polymeric dispersant stabilisation of titania pigment, Advances in Colloid and Interface Science, 151, 24–32. Filiatre, C., Bertrand, G., Coddet, C., Foissy, A., 2003 Alumina slurry formulation intender for spray-dried powder production, Polymer International, 52, 586–589. ISO 2813, 1994, Paints and varnishes – determination of specular gloss of nonmetallic paint films at 20 degrees, 60 degrees and 85 degrees. Karakas F. and Çelik M.S., 2012, Effect of quantity and size distribution of calcite filler on the quality of water borne paints, Prog. Org. Coat. doi:10.1016/j.porgcoat.2012.02.002 Karakaş, F., Güldan, G., Ersever, G., Erkan, İ., Güven, O., Çelik, M.S., 2009, Endüstriyel hammaddelerin boya üretiminde kullanımı, 7. Uluslararası Endüstriyel Hammaddeler Sempozyumu, Kuşadası, 25-27 Şubat, 416-424. Liufu, S., Xiao, H., Li, Y., 2005, Adsorption of poly(acrylic acid) onto the surface of titanium dioxide and the colloidal stability of aqueous suspension, Journal of Colloid and Interface Science, 281, 155–163. Lu Z., Ren M., Yin H., Wang A., Ge C., Zhang Y.,Yu L., Jiang T., Preparation of nanosized anatase TiO2coated kaolin composites and their pigmentary properties, Powder Technology 196 pp 122–125, 2009. McGonigle F., Ciullo P.A., 1996, Paints & Coatings, Chapter 4 in Industrial Minerals and Their Uses, A Handbook & Formulary, edited by Ciullo P.A., Noyes Publication, Westwood New Jersey, 125-136. Tiarks, F., Frechen, T., Kirsch, S., Leuninger, J., Melan, M., Pfau, A., Richter, F., Schuler, B., Zhao C.L., 2003, Formulation effects on the distribution of pigment particles in paints, Progress in Organic Coatings, 48, 140–152.
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Werner, E., 1998, Step particle size distribution curves as a determining factor in the use of fine extenders in various coating systems, Omya Publication, FSCT Annual Meeting Technical Program (Revised Text), New Orleans, USA.
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FIGURE CAPTIONS Fig 1 - Viscosity against shear rate of TiO2 and calcite mixtures obtained from different milling conditions.
TABLE CAPTIONS Table 1 Specifications of TiO2 and calcite given by the manufacturer. Table 2 Size distributions of TiO2, calcite and their mixtures obtained from different mill types. Table 3 Zeta potential and corresponding pH values of TiO2, calcite and mixtures obtained from different types of mill Table 4 Standard analyses of the paints.
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FIGURES
500
without milling Conventional Ball Mill
Apparent Viscosity, cP
400
Vibratory Ball Mill Attritor
300
200
100
0 0
5
10
15
Shear Rate,
20
25
30
s-1
Fig 1 – Viscosity against shear rate of TiO2 and calcite mixtures obtained from different milling conditions.
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TABLES Table 1 Specifications of TiO2 and calcite given by the manufacturer. Property
TiO2
Calcite
Specific Gravity,(g/cm )
4
2.7
Suspension pH, 10 %wt.
6.5-7.5
9.3-9.4
Mean Particle Size, (µm)
0.21
0.9
Oil Absorption, (ml/100 g)
19-21
19
Refractivity Index
2.7
1.58
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Table 2 Size distributions of TiO2, calcite and their mixtures obtained from different mill types. %
Original Size, µm
Mean Size (d50) after milling, µm
TiO2
Calcite
Attritor Conventional Ball Mill
Vibratory Ball Mill
25
0.15
0.50
0.12
0.12
0.15
50
0.19
0.90
0.15
0.15
0.16
75
0.24
2.15
0.19
0.18
2.26
85
0.31
3.86
0.21
2.27
2.44
95
1.43
4.50
0.80
2.80
2.63
Mixtures
Table 3. Zeta potential and corresponding pH values of TiO2, calcite and mixtures obtained from different types of mill Material
Zeta Potential, mV
pH
Calcite
17
8.6
TiO2
-21.1
8.6
24.5, -7.5
8,6
without grinding Conventional Ball Mill Vibratory Ball Mill Attritor
6,86 15,4 17,73
8,6 8,6 8,6
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Table 4 Standard analyses of the paints. Mixture Type
Density, g/cm
3
Viscosity,Krebs
Gloss
Opacity, %
20
60
85
Without Milling
1.3
132.8
1.2
3.3
2.9
95.5
Conventional Ball Mill
1.3
130.5
1.3
3.5
3.3
95.4
Vibratory Ball Mill
1.3
121.3
1.4
4.3
2.13
97.3
Attritor
1.3
>140
1.4
4.1
2.8
97.1
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