Inorganic blue pigments from cobalt doped magnesium having transition element oxides and a process for the preparing the same

Abstract

The present invention relates to a new Inorganic Blue pigments from Cobalt doped Magnesium having Transition Element Oxides and a process for the preparing the same. The present invention more particularly relates to the development of blue pigments, comprising oxides of alkaline earth, and transition metals of the general formula Mg.sub.1-xCo.sub.xWO.sub.4 (x=0.1 to 0.5), Mg.sub.1-xCo.sub.xN-bO.sub.6 (x=0.1 to 0.5), and Mg.sub.1-xCo.sub.xTiO.sub.3 (x=0.1 to 0.5) and well suited for coloring applications of a wide variety of substrates for example paints, varnishes, plastics, ceramics etc. Raw materials such as MgO, CoO and one of WO.sub.3, TiO.sub.2, Nb.sub.2O.sub.5 and are weighted in the stoichiometric ratio and calcined in the range 1100-1300 C. for 6-12 hrs duration in air atmosphere. The well ground calcined powders were used for characterization of the pigments. The phase purity and optical properties of the prepared pigments were investigated.

Claims

1. A Blue pigment having the general formula Mg.sub.1-xCo.sub.xWO.sub.4 (x=0.1 to 0.5).

2. The Blue pigment according to claim 1 of the formula, Mg.sub.1-xCo.sub.xWO.sub.4 (x=0.1 to 0.5) having chromaticity coordinates, determined as per the CIE 1976 colour scales are L*=39.01 to 46.28, a*=0.10 to 6.33, b*=32.88 to 46.97.

3. The Blue pigment according to claim 1 of the formula, Mg.sub.1-xCo.sub.xWO.sub.4 (x=0.1 to 0.5) having NIR reflectance of 42 to 56% and NIR solar reflectance of 21 to 28.6%.

4. A Blue pigment having the general formula Mg.sub.1-zCo.sub.xNb.sub.2O.sub.6 (x=0.1 to 0.5).

5. The Blue pigment according to claim 4 of the formula, Mg.sub.1-x Co.sub.xNb.sub.2O.sub.6 (x=0.1 to 0.5) having chromaticity coordinates, determined as per the CIE 1976 colour scales are L*=52.78 to 68.05, a*=0.97 to 2.55, b*=27.64 to 36.16.

6. The Blue pigment according to claim 4 of the formula, Mg.sub.1-x Co.sub.xNb.sub.2O.sub.6 (x=0.1 to 0.5) having NIR reflectance of 86 to 74% and NIR solar reflectance of 38 to 43%.

7. A Blue pigment having the general formula Mg.sub.1-xCo.sub.xTiO.sub.3 (x=0.2, 0.3, or 0.4.

8. The Blue pigment according to claim 7 of the formula, Mg.sub.1-xCo.sub.xTiO.sub.3 (x=0.2, 0.3, or 0.4) having chromaticity coordinates, determined as per the CIE 1976 colour scales are L*=36.62 to 54.13, a*=11.04 to 15.73, b*=11.66 to 25.61.

9. The Blue pigment according to claim 7 of the formula, Mg.sub.1-xCo.sub.xTiO.sub.3 (x=0.2, 0.3, or 0.4) having NIR reflectance of 46 to 73% and NIR solar reflectance of 23 to 37%.

10. A process for the preparation of a blue pigment comprising Cobalt doped Magnesium and one Transition Element Oxide selected from Tungsten, Niobium and Titanium, comprising the steps of: mixing thoroughly MgO (purity 99%), CoO (99.99%) with one of the Transition Element Oxides (purity 99.995%) in a stochiometric ratio in agate mortar with a pestle; ii) calcining the mixture at 1100-1300 C. in air atmosphere for 6-12 hrs duration; and iii) getting blue pigment in the form of powder having particle size 1-5 m.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) For a better understanding of the invention an exemplary embodiment is described below considered together with the figures in which:

(2) FIG. 1. Powder X-ray diffraction pattern of Mg.sub.1-xCo.sub.xWO.sub.4 (x=0.2) pigments.

(3) FIG. 2. Diffuse reflectance spectra of Mg.sub.1-xCo.sub.xWO.sub.4 (x=0.2) pigments.

(4) FIG. 3. TGA of Mg.sub.1-xCo.sub.xWO.sub.4 (x=0.2) pigments.

(5) FIG. 4. Solar irradiance spectra of Mg.sub.1-xCo.sub.xWO.sub.4 (x=0.2) pigments.

(6) FIG. 5. Powder X-ray diffraction patterns of Mg.sub.1-xCo.sub.xNb.sub.2O.sub.6 (x=0.5) pigments.

(7) FIG. 6. Diffuse reflectance spectra of Mg.sub.1-xCo.sub.xNb.sub.2O.sub.6 (x=0.5) pigments.

(8) FIG. 7. TGA of Mg.sub.1-xCo.sub.xNb.sub.2O.sub.6 (x=0.5) pigments.

(9) FIG. 8. Solar irradiance spectra of Mg.sub.1-xCo.sub.xNb.sub.2O.sub.6 (x=0.5) pigments.

(10) FIG. 9. Powder X-ray diffraction pattern of Mg.sub.1-xCo.sub.xTiO.sub.3 (x=0.1) pigments.

(11) FIG. 10. Diffuse Reflectance spectra of Mg.sub.1-xCo.sub.xTiO.sub.3 (x=0.1) pigments.

(12) FIG. 11. TGA of Mg.sub.1-xCo.sub.xTiO.sub.3 (x=0.1) pigments.

(13) FIG. 12. Solar irradiance spectra of Mg.sub.1-xCo.sub.xTiO.sub.3 (x=0.1) pigments.

(14) It is to be understood that the plots are only for the purpose of illustrating the examples without limiting the scope thereof.

SUMMARY OF THE INVENTION

(15) Blue pigment comprising Cobalt doped Magnesium and one Transition Element Oxides selected from Tungsten, Niobium and Titanium and a Process for preparing the same. The present invention particularly relates to blue pigments (i) Mg.sub.1-xCo.sub.xWO.sub.4 (ii) Mg.sub.1-xCo.sub.xNb.sub.2O.sub.6 and (iii) Mg.sub.1-xCo.sub.xTiO.sub.3 well suited for colouring applications of a wide variety of substrates for example paints, varnishes, plastics, ceramics etc.

DETAILED DESCRIPTION OF THE INVENTION

(16) The detailed description of these inventions was explained with following examples but these should not construe to limit the invention:

Example 1

(17) Preparation of Mg.sub.1-xCo.sub.xWO.sub.4 Blue Pigment

(18) This example relates to the preparation of Mg.sub.1-xCo.sub.xWO.sub.4 (x=0.1, 0.2, 0.3, 0.4 &0.5). MgO (purity 99%) WO.sub.3 (purity 99.995%) and CoO (99.99%) were thoroughly mixed in the stoichiometric ratio in agate mortar with a pestle. The mixture was calcined at 1100 C. for 12 h in air. The obtained powders were examined by means of X-ray powder diffraction (XRD) using Ni filtered CuK1 radiation with a Philips X'pert Pro diffractometer. MgWO.sub.4 crystallizes in a monoclinic structure isomorphic to wolframite, with a space group P21c and has C.sub.2h point-group symmetry. The structure consists of layers of alternating MgO.sub.6 and WO.sub.6 octahedral units that share edges forming a zigzag chain. FIG. 1 shows the XRD patterns of cobalt doped MgWO.sub.4. All the diffraction peaks can be indexed to the monoclinic structure with P2/c space group in agreement with the JCPDS file No (01-073-0562). Morphological analysis was performed by means of scanning electron microscope with a JEOL JSM-5600LV SEM. The particle size of the pigment varies in the range 1-2.5 am. Optical reflectance of the powders was measured with UV-Vis spectrophotometer (Shimadzu, UV-2450) using PTFE as a reference is shown FIG. 2. The chromaticity coordinates, determined by the CIE-LAB 1976 colour scales. The values a* (the axis red-green) and b* (the axis yellow-blue) indicate the colour hue. The value L* represents the lightness or darkness of the colour as related to a neutral grey (Table 1). The colouring performance of cobalt bearing pigments depends very much on the coordination of Co.sup.2+ ions. In order to understand the origin of blue colour of the Mg.sub.1-xCo.sub.xWO.sub.4 powders we take the UV-Visible NIR spectrum. The given spectrum contains mainly three bands attributed to the three spin allowed transitions .sup.4T.sub.1g(.sup.4F)-.sup.4T.sub.2g(.sub.1), .sup.4T.sub.1g(.sup.4F)-.sup.4A.sub.2g(.sub.2), .sup.4T.sub.1g(.sup.4F)-T.sub.1g(.sup.4P)(.sub.3) appear at 1500 nm, 730 nm and 580 nm respectively. These are the three spin allowed transitions of CoO.sub.6 chromophore.

(19) L*=46.28, a*=6.33, b*=46.97 (x=0.2) &L*=42.54, a*=4.46, b*=43.2 (x=0.3)

(20) For the Purpose of evaluating the chemical and thermal stability of the synthesized pigments, we treated it with acid and alkali (Table 2). For this a small amount of weighed sample is mixed with 2% NaOH and 2% HCl and immersed for 1 hour with constant stirring.

(21) Then the pigment was filtered, washed with distilled water, dried and finally weighed. Negligible weight lose was observed for the acid and alkali treated samples. The L*a*b*values are found to be L*=41.53, a*=3.7, b*=41.16 (x=0.3) and L*=43.04, a*=4.04, b*=42.43 (x=0.3) for HCl and NaOH respectively. The delta E values are found to be within the allowed limit (<5). From this data we can concluded that the synthesized samples are chemically stable. Thermo gravimetric analyses (TGA) were performed (Schimadzu, DTG-60) on all samples in the temperature range 30-200 C., under air atmosphere at a heating rate of 20 C./min shown in FIG. 3. There is an increasing demand to develop new NIR reflective pigments which can be used for cool roof applications. Replacing conventional pigments with cool pigments that absorb less NIR radiation can provide coatings similar in color to that of conventional roofing materials, but with higher solar reflectance. Thus we perceived the need to develop new blue coloured NIR reflecting inorganic pigment. From FIGS. 2 & 4 it can be see that corresponding NIR& NIR solar reflectance (R*) of the synthesized Mg.sub.0.8Co.sub.0.2WO.sub.4 pigment is found to be 56% and 28.6%. This observation indicates that synthesized pigment serve as a potential candidate for cool roof applications.

Example 2

(22) Preparation of Mg.sub.1-xCo.sub.xNb.sub.2O.sub.6 Blue Pigment

(23) This example relates to the preparation of Mg.sub.1-xCo.sub.xNb.sub.2O.sub.6 (x=0.1, 0.2, 0.3, 0.4 &0.5). MgO (purity 99%) Nb.sub.2O.sub.5 (purity 99.995%) and CoO (99.99%) were thoroughly mixed in the stoichiometric ratio in agate mortar with a pestle. The mixture was calcined at 1300 C. for 6 h in air. The obtained powders were examined by means of X-ray powder diffraction (XRD) using Ni filtered CuK1 radiation with a Philips X'pert Pro diffractometer. Most of the niobium oxides related to AB.sub.2O.sub.6 Structure have columbite structure with pbcn space group. XRD pattern of the compound depicted in FIG. 5 is in good agreement with the powder X-ray diffraction file: (01-088-0708). Cobalt doped MgNb.sub.2O.sub.6 crystallizes in orthorhombic structure with pbcn space group. Morphological analysis was performed by means of scanning electron microscope with a JEOL JSM-5600LV SEM. The particle size of the pigment varies in the range 1.5-2.5 m. Optical reflectance of the powders was measured with UV-Vis spectrophotometer (Shimadzu, UV-2450) using PTFE as a reference is shown FIG. 6. The chromaticity coordinates, determined by the CIE-LAB 1976 colour scales. The values a* (the axis red-green) and b* (the axis yellow-blue) indicate the colour hue. The value L* represents the lightness or darkness of the colour as related to a neutral grey (Table 1). Optical absorption spectra of Mg.sub.1-xCo.sub.xNb.sub.2O.sub.6 contains mainly three bands attributed to the three spin allowed transitions. .sup.4T.sub.1g(.sup.4F)-.sup.4T.sub.2g(.sub.1), .sup.4T.sub.1g (.sup.4F)-.sup.4A.sub.2g(.sub.2), .sup.4T.sub.1g(.sup.4F)-T.sub.1g(.sup.4P)(.sub.3). A single very wide band located in the near-IR region around 1500 nm is due to the .sub.i transition. The bands at 730 nm, 580 nm is due to the .sub.2 and .sub.3 transition.

(24) L*=52.78, a*=0.97, b*=36.16 (x=0.5)

(25) For the Purpose of evaluating the chemical and thermal stability of the synthesized pigments, we treated it with acid and alkali (Table 2). For this a small amount of weighed sample is mixed with 2% NaOH and 2% HCl and immersed for 1 hour with constant stirring. Then the pigment was filtered, washed with distilled water, dried and finally weighed. Negligible weight lose was observed for the acid and alkali treated samples. The L*a*b* values are found to be L*=50.62, a*=0.19, b*=36.19 (x=0.5) and L*=51.23, a*=0.18, b*=37.08 (x=0.5) for HCl and NaOH respectively. The delta E values are found to be within the allowed limit (<5). From this data we can concluded that the synthesized samples are chemically stable. Thermo gravimetric analyses (TGA) were performed (Schimadzu, DTG-60) on all samples in the temperature range 30-200 C., under air atmosphere at a heating rate of 20 C./min shown in FIG. 7. There is an increasing demand to develop new NIR reflective pigments which can be used for cool roof applications. Replacing conventional pigments with cool pigments that absorb less NIR radiation can provide coatings similar in color to that of conventional roofing materials, but with higher solar reflectance. Thus we perceived the need to develop new blue coloured NIR reflecting inorganic pigment. From FIGS. 6 & 8 it can be see that corresponding NIR& NIR solar reflectance (R*) of the synthesized Mg.sub.0.5Co.sub.0.5Nb.sub.2O.sub.6 pigment is found to be 74% and 38%. This observation indicates that synthesized pigment serve as a potential candidate for cool roof applications.

Example 3

(26) Preparation of Mg.sub.1-xCo.sub.xTiO.sub.3Blue Pigment

(27) This example relates to the preparation of Mg.sub.1-xCo.sub.xTiO.sub.3 (x=0.1, 0.2, 0.3, 0.4 &0.5). MgO (purity 99%), TiO.sub.2 (purity 99.995%) and CoO (99.99%) were thoroughly mixed in the stoichiometric ratio in agate mortar with a pestle. The mixture was calcined at 1200 C. for 6 h in air. The obtained powders were examined by means of X-ray powder diffraction (XRD) using Ni filtered CuK1 radiation with a Philips X'pert Pro diffractometer. Geikielite (MgTiO.sub.3) belongs to the ilmenite structure type (ATiO.sub.3, A=Mg, Mn, Fe, Zn) with a rhombohedral space group R-3 and 6 formula units per unit cell. FIG. 9 shows the XRD patterns of cobalt doped MgTiO.sub.3. All the reflections can be well indexed according to the Powder diffraction file 01-079-0831. The structure of MgTiO.sub.3 consists of MgO.sub.6 octahedron and TiO.sub.6 octahedron. Morphological analysis was performed by means of scanning electron microscope with a JEOL JSM-5600LV SEM. The particle size of the pigment varies in the range 2-4 m. Optical reflectance of the powders was measured with UV-Vis spectrophotometer (Shimadzu, UV-2450) using PTFE as a reference is shown FIG. 10. The chromaticity coordinates, determined by the CIE-LAB 1976 colour scales. The values a* (the axis red-green) and b* (the axis yellow-blue) indicate the colour hue. The value L* represents the lightness or darkness of the colour as related to a neutral grey (Table 1). The blue colour of the Mg.sub.1-xCo.sub.xTiO.sub.3 powders is evident even for very low values of X. The UV-Visible NIR spectrum of Co.sup.2+ doped MgTiO.sub.3 shows that the blue colour is due to the octahedral incorporation of the Co(II). The given spectrum contains mainly three bands attributed to the three spin allowed transitions .sup.4T.sub.1g (.sup.4F)-.sup.4T.sub.2g (.sub.1), .sup.4T.sub.1g (.sup.4F)-A.sub.2g (.sub.2), .sup.4T.sub.1g (.sup.4F)-T.sub.1g(.sup.4P) (.sub.3) appear at 1500 nm, 730 nm and 580 nm respectively.

(28) L*=54.13, a*=11.04, b*=25.61 (x=0.1).

(29) For the Purpose of evaluating the chemical and thermal stability of the synthesized pigments, we treated it with acid and alkali (Table 2). For this a small amount of weighed sample is mixed with 2% NaOH and 2% HCl and immersed for 1 hour with constant stirring. Then the pigment was filtered, washed with distilled water, dried and finally weighed. Negligible weight lose was observed for the acid and alkali treated samples. The L*a*b*values are found to be L*=52.89, a*=11.07, b*=25.01 (x=0.1) and L*=56, a*=11.14, b*=25.85 (x=0.1) for HCl and NaOH respectively. The delta E values are found to be within the allowed limit (<5). From this data we can concluded that the synthesized samples are chemically stable. Thermo gravimetric analyses (TGA) were performed (Schimadzu, DTG-60) on all samples in the temperature range 30-200 C., under air atmosphere at a heating rate of 20 C./min shown in FIG. 11. There is an increasing demand to develop new NIR reflective pigments which can be used for cool roof applications. Replacing conventional pigments with cool pigments that absorb less NIR radiation can provide coatings similar in color to that of conventional roofing materials, but with higher solar reflectance. Thus we perceived the need to develop new blue coloured NIR reflecting inorganic pigment. From FIGS. 10 & 12 it can be see that corresponding NIR& NIR solar reflectance (R*) of the synthesized Mg.sub.0.9Co.sub.0.1TiO.sub.3 pigment is found to be 73% and 37%. This observation indicates that synthesized pigment serve as a potential candidate for cool roof applications.

(30) Table 1 Explains Colour Co-Ordinates & NIR Reflectance of Typical Compositions

(31) TABLE-US-00001 TABLE 1 Colour Co-ordinates & NIR Reflectance of Typical Compositions NIR Solar reflectance Composition L* a* b* (%) Mg.sub.0.8Co.sub.0.2WO.sub.4 46.28 6.33 46.97 28.6%. Mg.sub.0.5Co.sub.0.5Nb.sub.2O.sub.6 52.78 0.97 36.16. 38% Mg.sub.0.9Co.sub.0.1TiO.sub.3 54.13 11.04 25.61 37% CoAl.sub.2O.sub.4 44.8 2.1 32.7 29% Commercial
Table 2 Explains Acid & Alkali Tests

(32) TABLE-US-00002 Acid Alkali E Composition L* a* b* L* a* b* Acid Alkali Mg.sub.0.8Co.sub.0.2WO.sub.4 41.53 3.7 41.16 43.04 4.04 42.43 2.3 1 Mg.sub.0.5Co.sub.0.5Nb.sub.2O.sub.6 50.62 0.19 36.19 51.23 0.18 37.08 2.2 1.5 Mg.sub.0.9Co.sub.0.1TiO.sub.3 52.89 11.07 25.01 56 11.14 25.85 1.3 1.8