METHODS FOR FORMING AND USES OF TITANIA-COATED INORGANIC PARTICLES

Abstract

A method of forming a titania-coated inorganic particle comprising the steps of: (a) agitating a mixture of inorganic particle and organic solvent; (b) adding titania precursor dropwise into the mixture of step (a) under agitation; and (c) adding catalyst to the mixture of step (b) thereby converting said titania precursor to titania which then forms a coating on said inorganic particle; wherein steps (a) to (c) are performed at neutral pH and ambient temperature.

Claims

1. A method of forming a titania-coated inorganic particle, the method comprising steps of: (a) agitating a mixture of inorganic particle and organic solvent; (b) adding titania precursor dropwise into the mixture of step (a) under agitation; and (c) adding catalyst to the mixture of step (b) thereby converting said titania precursor to titania which then forms a coating on said inorganic particle; wherein steps (a) to (c) are performed at neutral pH and ambient temperature.

2. The method according to claim 1, comprising performing step (c) two or more times at predetermined time intervals, wherein each time interval is at least 0.5 hours apart.

3. The method according to claim 1, wherein in step (a), a mass ratio of inorganic particle to organic solvent is in a range of 0.02:1 to 0.1:1.

4. The method according to claim 1, wherein in step (b), a mass ratio of titania precursor to organic solvent is in a range of 0.01:1 to 0.12:1.

5. (canceled)

6. The method according to claim 1, wherein in step (c), a mass ratio of total amount of catalyst added over predetermined time intervals to organic solvent is in a range of 0.006:1 to 0.1:1.

7. The method according to claim 1, wherein a ratio of titania precursor to inorganic particle is in a range of about 1:1 to 7:1 (mmol/g).

8. The method according to claim 1, wherein a molar ratio of catalyst to titania precursor is in a range of about 9:1 to about 16:1.

9. The method according to claim 1, wherein said titania precursor is a titanium alkoxide.

10. The method according to claim 1, wherein said inorganic particle is selected from the group consisting of hollow glass beads, silicate platelets, silica glass particles, borosilicate glass particles, aluminosilicate glass particles, and mixtures thereof.

11. The method according to claim 1, wherein said organic solvent is selected from the group consisting of alcohols, ketones, ethers, and mixtures thereof.

12. The method according to claim 1, wherein said catalyst is selected from the group consisting of tap water, distilled water, and deionized water.

13. The method according to claim 12, further comprising a step of: (d) annealing the particles at a temperature of 200° C. to 1000° C. for 2 hours at a heating rate of 1° C. min.sup.−1m, thereby obtaining titania-coated particles that are coated with crystalline titania.

14. The method according to claim 1, wherein a yield of titania is above 90%.

15. The method of according to claim 1, wherein the titania-coated particle is coated with a uniform titania coating.

16. The method of according to claim 1, wherein said titania-coated particle is coated with titania with a thickness in a range of 20 nm to 500 nm.

17. A titania-coated particle produced by the method according to claim 1.

18. Use of the titania-coated particle according to claim 17 in reflective coatings for thermal insulation applications, antimicrobial coatings, or UV-shielding for packing applications.

19. A reflective coating for thermal insulation, an antimicrobial coating, or UV shielding for packing applications comprising a titania-coated particle according to claim 17.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0072] The accompanying drawings illustrate a disclosed embodiment and serves to explain the principles of the disclosed embodiment. It is to be understood, however, that the drawings are designed for purposes of illustration only, and not as a definition of the limits of the invention.

[0073] FIG. 1 is a series of scanning electron microscopy (SEM) images of (a) Hollow glass beads (HGBs) and (b)-(d) TiO.sub.2-coated hollow glass beads: (b) HGBs@TiO.sub.2#1, (c) HGBs@TiO.sub.2#2 and (d) HGBs@TiO.sub.2#3 of Examples 1 to 3. Left and middle panels show samples at different magnifications. Right panel shows samples at the fractured interface.

[0074] FIG. 2 is a series of transmission electron microscopy (TEM) images of (a) TiO.sub.2-coated silicate platelets at a scale of 0.5 μm, (b) TiO.sub.2-coated silicate platelets at a scale of 0.2 μm and (c) TiO.sub.2-coated silicate platelets at a scale of 20 nm of Example 5.

[0075] FIG. 3 is a series of ultraviolet-visible-near-infrared (UV-VIS-NIR) diffuse reflectance spectra of (a) HGBs, (b) HGB@TiO.sub.2#1, (c) HGB@TiO.sub.2#2 and (d) HGB@TiO.sub.2#3 of Examples 1 to 3.

[0076] FIG. 4 is a series of UV-VIS-NIR diffuse reflectance spectra of (a) HGB@TiO.sub.2#3 not thermally annealed, (b) HGB@TiO.sub.2#3 thermally annealed at 200° C. and (c) HGB@TiO.sub.2#3 of Examples 1 to 3 thermally annealed at 600° C.

EXAMPLES

[0077] Non-limiting examples of the invention and a comparative example will be further described in greater detail by reference to specific Examples and a Comparative Example, which should not be construed as in any way limiting the scope of the invention.

Materials

[0078] K25 hollow glass beads, obtained from 3M company of Minnesota of the United States of America.

[0079] Tetrabutyl titanate, obtained from Sigma Aldrich of St. Louis of Missouri of the United States of America.

[0080] Silicate platelets (montmorillonite), obtained from Nanocor Inc of Arlington Heights of Illinois of United States of America.

Example 1—Preparation of TiO.SUB.2.-Coated HBGs@TiO.SUB.2.#1

[0081] 2.4 g of K25 hollow glass beads were added to 72 mL of ethanol and shaken at 200 rpm at ambient temperature (25° C.) and neutral pH (pH of 7). 2.78 g of tetrabutyl titanate was added dropwise (0.5 g/min) into the hollow glass bead suspension under shaking. After 2 hours of shaking, 0.285 mL of tap water was added (0.2 g/min) to the suspension. The process was repeated by adding 0.285 mL of tap water for another 3 times at a time interval of 3 hours. It was further shaken for 12 hours. The suspension was filtered and the obtained wet powder was dried at room temperature. The powder was labelled as HGBs@TiO.sub.2#1.

Example 2—Preparation of TiO.SUB.2.-Coated HGBs@TiO.SUB.2.#2

[0082] 3.6 g of K25 hollow glass beads were added to 72 mL of ethanol and shaken at 200 rpm at ambient temperature (25° C.) and neutral pH (pH of 7). 2.78 g of tetrabutyl titanate was added dropwise (0.5 g/min) into the hollow glass bead suspension under shaking. After 2 hours of shaking, 0.285 mL of tap water was added (0.2 g/min) to the suspension. The process was repeated by adding 0.285 mL of tap water for another 5 times at a time interval of 3 hours. It was further shaken for 12 hours. The suspension was filtered and the obtained wet powder was dried at room temperature. The powder was labelled as HBGs@TiO.sub.2#2.

Example 3—Preparation of TiO.SUB.2.-Coated HGBs@TiO.SUB.2.#3

[0083] 4.8 g of K25 hollow glass beads were added to 72 mL of ethanol and shaken at 200 rpm at ambient temperature (25° C.) and neutral pH (pH of 7). 2.78 g of tetrabutyl titanate was added dropwise (0.5 g/min) into the hollow glass bead suspension under shaking. After 2 hours of shaking, 0.285 mL of tap water was added (0.2 g/min) to the suspension. The process was repeated by adding 0.285 mL of tap water for another 7 times at a time interval of 3 hours. It was further shaken for 12 hours. The suspension was filtered and the obtained wet powder was dried at room temperature. The powder was labelled as HGBs@TiO.sub.2#3.

Example 4—Preparation of Crystalline TiO.SUB.2.-Coated HGBs

[0084] 4.8 g of K25 hollow glass beads were added to 72 mL of ethanol and shaken at 200 rpm at ambient temperature (25° C.) and neutral pH (pH of 7). 2.78 g of tetrabutyl titanate was added dropwise (0.5 g/min) into the hollow glass bead suspension under shaking. After 2 hours of shaking, 0.285 mL of tap water was added (0.2 g/min) to the suspension. The process was repeated by adding 0.285 mL of tap water for another 7 times at a time interval of 3 hours. It was further shaken for 12 hours. The suspension was filtered and the obtained wet powder was dried at room temperature. An annealing process was applied to the TiO.sub.2-coated hollow glass balls powder, typically at 200-1000° C. for 2 hours at a heating rate of 1° C. min.sup.−1, to adjust the crystalline phase and crystallinity of TiO.sub.2.

Example 5—Preparation of TiO.SUB.2.-Coated Silicate Platelets

[0085] 1.0 g of pristine silicate platelets (montmorillonite) was mixed with 25 mL of water and stirred overnight. To exchange water with ethanol, 120 mL of ethanol was added to the suspension. The suspension was homogenized using IKA T18 Basic Ultra Turrax homogenizer at 15,000 rpm for 5 minutes. Thereafter, the slurry precipitate was filtered with a Buchner funnel and washed with ethanol. The collected slurry precipitate was re-suspended into 120 mL of ethanol and homogenized for 5 minutes at 15,000 rpm, followed by filtration and washing. The process of re-suspension, homogenization, filtration and washing was repeated for another 2 times. The collected precipitate was re-suspended into 50 mL of ethanol and stirred under 300 rpm.

[0086] At ambient temperature (25° C.) and neutral pH (pH of 7), 2.33 g of tetrabutyl titanate was added dropwise (0.5 g/min) into the suspension under stirring. After 5 minutes of stirring, 0.285 mL of deionised water was added (0.2 g/min) to the suspension. The process was repeated by adding 0.285 mL of deionised water for another 3 times at a time interval of 1 hour. The suspension was filtered, washed with ethanol and dried at room temperature.

Example 6—Characterization and Performance Test of Sample

[0087] Scanning electron microscopy (SEM, JEOL, JSM-6700F) was used to measure the morphology of TiO.sub.2-coated hollow glass beads and the thickness of TiO.sub.2 shell. Transmission electron microscopy (TEM, JEOL JEM-2010F, 200 kV) was used to study the morphology of TiO.sub.2-coated silicate platelets. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) was used to measure the TiO.sub.2 content in TiO.sub.2-coated hollow glass beads. UV-VIS-NIR spectrophotometer UV-3600 (Shimadzu) with integrating sphere ISR 3100 was used to measure the diffuse solar reflectance.

[0088] FIG. 1 shows the SEM images of (a) hollow glass beads and (b)-(d) the corresponding hollow glass beads after TiO.sub.2 coating of Examples 1-3, at different magnification, including the cross-sectional view.

[0089] FIG. 2 shows the TEM images of TiO.sub.2-coated silicate platelets after TiO.sub.2 coating of Example 5 at different magnifications.

[0090] The surface curvature of substrate materials may affect the morphology of the metal oxide formed. For instance, the deposition of TiO.sub.2 onto hollow glass beads with curved surfaces to create core-shell structures (FIG. 1), and the deposition of TiO.sub.2 onto silicate platelets with flat surfaces to create platelet-like structures immobilized with metal oxide particles (FIG. 2).

[0091] FIG. 1 shows SEM images of the TiO.sub.2-coated hollow glass beads. It reveals good integrity of hollow glass beads in all products. A broken sphere provides further insight into the core-shell structure of the products (FIG. 1, right panel) where the hollow glass bead core is homogenously covered by a TiO.sub.2 shell with micro-crack structures. FIG. 1 also shows that using the method as disclosed herein for TiO.sub.2-coating results in a coating of TiO.sub.2 exclusively deposited onto the surface of hollow glass beads without the disadvantageous presence of freestanding TiO.sub.2 particles. This also contributes to high yield of TiO.sub.2.

[0092] FIG. 2 shows TEM images of TiO.sub.2-coated silicate platelets. FIG. 2 shows that TiO.sub.2 particles are well-dispersed as a ring-shaped coating around the surface of silicate particles. The TiO.sub.2-coated silicate platelets have a coating thickness of about 20 nm and a titania yield above 90%.

[0093] The right panel of FIG. 1 shows the cross-sectional SEM images of (a) hollow glass beads and (b)-(d) the corresponding hollow glass beads after TiO.sub.2 coating as mentioned in Examples 1-3. The thickness of the TiO.sub.2-coating layer is about 200 nm in FIG. 1b, about 150 nm in FIG. 1c and about 100 nm in FIG. 1d. The results also show that the thickness of the TiO.sub.2-coatings decreases from 200 nm to 100 nm as the ratio of titania precursor to hollow glass beads decreases from 3.36:1 to 1.68:1 (mmol/g).

[0094] FIG. 3 shows the UV-VIS-NIR diffuse reflectance spectra of (a) hollow glass beads and (b)-(d) the corresponding hollow glass beads after TiO.sub.2 coating of Examples 1-3. An obvious red shift of diffuse reflectance spectra of the TiO.sub.2-coated hollow glass beads is observed in the region of 200-400 nm, when compared with the spectra of the original hollow glass beads, which further confirms the presence of TiO.sub.2-coating.

[0095] Table 1 below shows the preparation conditions, thickness of TiO.sub.2 shell, TiO.sub.2 yield and diffuse solar reflectance of TiO.sub.2-coated hollow glass beads. The results show that the TiO.sub.2-coated hollow glass bead samples prepared with a lower ratio of TiO.sub.2 precursor to hollow glass beads at about 1.68:1 (mmol/g) has a reduced thickness in TiO.sub.2-coating but comparable diffuse solar reflectance compared to the samples prepared with a higher ratio of TiO.sub.2 precursor to hollow glass beads at about 2.24:1 or 3.36:1 (mmol/g). Therefore, a lower ratio of TiO.sub.2 precursor to hollow glass beads at about 1.68:1 (mmol/g) is favourable for preparation of TiO.sub.2-coated hollow glass bead considering the utilization of TiO.sub.2 precursor, production cost and the diffusive solar light reflectance performance.

[0096] The loading of metal oxides on the substrate materials can be controlled by adjusting the amount of the substrate materials in the surface reactions. The results in FIG. 1 and Table 1 show that when the mass ratio of hollow glass beads to organic solvent increases from 0.04:1 (HGB@TiO.sub.2#1) to 0.06:1 (HGB@TiO.sub.2#2) and then to 0.08:1 (HGB@TiO.sub.2#3), the thickness of the TiO.sub.2 shell decreases from 200 nm to 100 nm, while the samples still maintained comparable reflective performance.

[0097] FIG. 3 and Table 1 show that the diffuse solar light reflectance of HGB@TiO.sub.2#1-3 is comparable and is not affected by the reduction of the shell thickness. As a result, there should be an optimized level of the TiO.sub.2 content to hollow glass beads, above which the diffuse solar light reflectance will not be further improved. This is crucial for determining the suitable ratio between the metal oxides and the substrate materials in the surface reactions and maximizing the performance and cost-saving potential.

[0098] Table 1 shows that the TiO.sub.2 yield obtained for HGB@TiO.sub.2#1-3 is above 90%. This may be ascribed to two mechanisms. Firstly, a sufficiently high amount of hollow glass beads was added in the suspension to provide a large contact surface area with the other reactants. This allows the yield of the metal oxides to be maximized and result in a high metal oxides yield. Secondly, catalyst was added at predetermined time intervals. The controlled addition of water enables exclusive deposition of metal oxide on the substrate surface and further contributes to the high TiO.sub.2 yield.

TABLE-US-00001 TABLE 1 Preparation conditions, thickness of TiO.sub.2 shell, TiO.sub.2 yield and diffuse solar reflectance of TiO.sub.2-coated hollow glass beads. Ratio of Ratio of titania hollow precursor glass TiO.sub.2 TiO.sub.2 to hollow bead to yield yield Diffuse glass organic Thickness (by (by solar bead solvent of TiO.sub.2 mass) ICP) reflectance Sample (mmol/g) (g/g) shell (nm) (%) (%) (%) HGB − − − − 84.1 HGB @ 3.36:1 0.04 200 97.0 70.8 89.9 TiO2 #1 HGB @ 2.24:1 0.06 150 98.3 83.1 87.5 TiO2 #2 HGB @ 1.68:1 0.08 100 91.9 77.7 88.9 TiO2 #3

[0099] FIG. 4 shows the UV-VIS-NIR diffuse reflectance spectra of HGB@TiO.sub.2#3 thermally annealed at different temperatures, (a) HGB@TiO.sub.2#3 not thermally annealed, (b) HGB@TiO.sub.2#3 thermally annealed at 200° C. and (c) HGB@TiO.sub.2#3 thermally annealed at 600° C. as mentioned in Example 4. TiO.sub.2 has different crystalline phases. Rutile TiO.sub.2 has the highest refractive index (˜2.73), followed by brookite TiO.sub.2 (˜2.58), anatase TiO.sub.2 (˜2.55) and amorphous TiO.sub.2 (˜2.45). A high refractive index would improve the solar light reflectance. Due to the high refractive index of crystalline TiO.sub.2, the reflectance in both the NIR and the visible regions is obviously increased with temperature. However, the reflectance in the ultraviolet region decreases, which could be ascribed to the strong ultraviolet absorbance of crystalline TiO.sub.2. Crystalline TiO.sub.2 exhibits high refractive index and strong ultraviolet absorbance. Thermal annealing at elevated temperature increases the proportion of crystalline TiO.sub.2, which may lead to increasing reflectance in both the near-infrared radiation and the visible regions while decreasing the reflectance in the ultraviolet region. The total solar reflectance increases as a combination of these two effects. The results show an improvement in total solar reflectance for samples thermally annealed at 200° C. or 600° C.

[0100] Crystalline TiO.sub.2 is known to inactivate microorganisms and exhibit excellent photocatalytic antimicrobial activity. The crystalline TiO.sub.2-coated hollow glass beads can thus be applied in antimicrobial coatings with the use of natural sunlight as an excitation source. A mixture with different ratios of crystalline TiO.sub.2 phases and crystallinity can be easily produced through thermal annealing of the TiO.sub.2-coated hollow glass beads. This method may be used to prepare mixtures of crystalline phases that are more active than single crystalline phase alone and at the same time offers longer shelf-life by preventing the coating from decomposition. Such mixtures could also be produced through mixing thermally and non-thermally treated TiO.sub.2-coated hollow glass beads at specific ratios.

[0101] Table 2 below shows the calculated diffuse solar reflectance of HGB@TiO.sub.2#3 thermally annealed at different temperatures based on the data presented in FIG. 4. The results show that the TiO.sub.2-coated HGBs samples prepared with thermal annealing shows an improvement in diffuse solar reflectance over a broad spectrum including total, ultra-violet, visible and near-infrared radiation ranges. Table 2 also shows that samples that were thermally annealed at higher temperature of 600° C. results in a greater improvement in diffuse solar reflectance compared to samples that were thermally annealed at temperature of 200° C.

TABLE-US-00002 TABLE 2 Diffuse solar reflectance of HGB@TiO.sub.2#3 not thermally annealed, HGB@TiO.sub.2#3 thermally annealed at 200° C. and HGB@TiO.sub.2#3 thermally annealed at 600° C. Diffuse solar reflectance (%) Near- infrared Sample Total Ultraviolet Visible radiation HGB@TiO.sub.2#3 88.9 64.8 89.4 89.6 HGB@TiO.sub.2#3_200° C. 89.7 63.9 89.3 91.2 HGB@TiO.sub.2#3_600° C. 90.8 54.5 90.3 92.7

COMPARATIVE EXAMPLE

[0102] 0.6 g of K25 hollow glass balls were added to 72 mL of ethanol (technical grade) and shaken at 200 rpm. 2.78 g of tetrabutyl titanate was added dropwise into the hollow glass ball suspension under shaking. After 2 hours of shaking, 0.285 mL of tap water was added to the suspension. It was shaken for another 2 hours. The suspension was filtered and the obtained wet powder was dried at room temperature. The powder was labelled as Comparative Example 1.

[0103] The TiO.sub.2-coated hollow glass beads of Examples 1 to 3 were compared against Comparative Example 1. As discussed in Examples 1 to 3 and as shown below in Table 3, the present invention uses different ranges of i) ratio of titania precursor to hollow glass bead and ii) ratio of water to titania precursor, as compared to Comparative Example 1.

[0104] Comparative Example 1 uses a ratio of 13.45:1 of titania precursor to hollow glass bead, whereas the present invention uses a ratio in the range of 1:1 to 7:1 (mmol/g) which is lower than that of Comparative Example 1. The use of a lower ratio of titania precursor to inorganic particles as disclosed in the present examples offers an unexpected technical effect of improved TiO.sub.2 yield, as compared to Comparative Example 1. It was surprisingly found that the lower ratio of titania precursor to inorganic particles enables sufficient inorganic particle surface area to be exposed to the reactants and hence maximizing the yield of titania.

[0105] Comparative Example 1 uses a ratio of 2:1 of catalyst to titania precursor, whereas the present invention uses a ratio in the range of 9:1 to 16:1 which is higher than that of Comparative Example 1 and which advantageously leads to higher yield of titania (as shown in Table 3 below).

[0106] Comparative Example 1 only uses one addition of catalyst, whereas the present invention discloses the multiple additions of catalyst at predetermined time intervals which advantageously control the concentration of catalyst in the system and avoids the growth of free-standing titania.

TABLE-US-00003 TABLE 3 Comparison with Comparative Example 1 Ratio of titania precursor to Ratio of Thick- TiO.sub.2 TiO.sub.2 Diffuse hollow water to ness yield yield solar glass titania of TiO.sub.2 (by (by reflec- Sam- Exam- bead precursor shell mass) ICP) tance ple ples (mmol/g) (mol/mol) (nm) (%) (%) (%) Com- #Com- 13.45:1  2:1 450 61.9 37.0 88.8 par- par- ative ative Exam- Exam- ple 1 ple 1 Exam- #1  3.36:1  9:1 200 97.0 70.8 89.9 ples 1 #2  2.24:1 12:1 150 98.3 83.1 87.5 to 3 #3  1.68:1 16:1 100 91.9 77.7 88.9

INDUSTRIAL APPLICABILITY

[0107] The disclosed method may be able to form TiO.sub.2-coated inorganic particles that are used in reflective coatings for thermal insulation applications, antimicrobial coatings or UV-shielding for packing applications. The TiO.sub.2-coated inorganic particles may be used in coatings to impart desired properties such as high solar light reflectance at low production cost due to the uniformity in coating and maximized utilization of TiO.sub.2 precursors.

[0108] The disclosed method may circumvent the problem of non-uniform coating, low yield and high production cost. The disclosed method may avoid the problem of freestanding agglomerates of TiO.sub.2 particles. The disclosed method may control the thickness and yield of TiO.sub.2 coating formation by controlling the ratio between the TiO.sub.2 precursor, inorganic particles, catalyst and organic solvent. The disclosed method may not require the use of pH control or temperature control. The disclosed method may not require the use of surface modification, coupling agent, precipitator or surfactant.

[0109] Through controllable surface reactions, TiO.sub.2 particles can be uniformly dispersed on the surface of silicate platelets. There is no need to make any surface modification of silicate substrates. It overcomes the agglomeration problem of nanoscale TiO.sub.2 when dispersed in the matrix materials, where nanoscale TiO.sub.2 is typically prone to aggregate due to the large surface area and high surface energy of nanoparticles. The TiO.sub.2-coated silicate platelets can be further incorporated into polymer matrix to produce UV-shielding coatings for packaging UV sensitive food. This invention discloses controllable surface reactions for uniformly coating metal oxides (e.g. TiO.sub.2) onto the surface of substrate materials (e.g. hollow glass beads and silicate platelets), easily adjusting and optimizing the loading of metal oxides and maximizing utilization of the metal oxide precursors. It offers new opportunities for developing metal oxides with controlled growth and dispersion on the surface of the substrate materials which can be applied for reflective coating, antimicrobial coating and packaging applications.

[0110] It will be apparent that various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims.