Process for preparing dispersions of tio2 in the form of nanoparticles, and dispersions obtainable with this process and fuctionalization of surfaces by application of tio2 dispersions

Description

Field of the Invention

The present invention relates to the field of methods for the preparation of compounds in the form of nanometric particles, and more particularly to a process for the preparation of a TiO dispersion.₂in the form of nanoparticles.

State of the art

[0002] Titanium dioxide is used as a white pigment with good hiding power, especially in paints and in the production of paper and synthetic rubber. Newer applications of titanium dioxide include those that take advantage of its photocatalytic activity, i.e. its ability to generate, when exposed to ultraviolet light, radical forms capable of catalysing the oxidative degradation of harmful or toxic substances such as benzene, dioxane and other organic pollutants, as well as unpleasant and infectious substances such as molds and bacteria. Such applications range from anti-pollution in the field of environmental protection to the field of cleaning and sterilization.

[0003] In such applications, titanium dioxide is used as a coating on the treated surfaces so as to maximize the photocatalytic effect. The crystalline form of titanium dioxide, specifically anatase, is advantageous in this type of application as, in addition to being chemically stable and readily available, it also exhibits greater photocatalytic activity than the other two crystal forms, rutile and brukite.

[0004] On the other hand, the overlap of the absorption spectrum of titanium dioxide with the solar spectrum is not very great even in its anatase form, indicating a low photocatalytic efficiency. Accordingly, various attempts have been made to modify TiO₂for example by doping it with other metals or by making the compound of interest in the form of nanoparticles; in this way the surface area is significantly increased and thus the photocatalytic efficiency.

[0005] Various methods are known for producing TiO₂ in the form of anatase, even in the form of nanoparticles, but as far as the applicant is aware, all such methods lead to the production of TiO₂ in the form of a powder.

[0006] The method of producing a suspension of nanoparticles in a high-boiling alcohol is the polyol process described, for example, in C. Feldmann Polyol mediated synthesis of nanoscale functional materials, which makes it possible to obtain suspensions very stable for a long time, but contrary to the currently claimed method, it is used therein mineral acid as a polycondensation inhibitor (see also WO 99/62822 in this aspect). C. Feldmann, Adv. Funct. Mater. 2003, 13 (2), 101-107 describes suspensions of nanometric TiO particles₂ in glycol under acidic conditions.

[0007] EP 770 579 describes a modified titanium oxide sol by treating the titanium oxide sol with a compound active in phase transfer processes. In order for this powdered material to be useful in photocatalytic coatings, it must be dispersed in a suitable solvent and optionally formulated with additives that improve the adhesion of the coating. However, this causes the titanium dioxide particles to coagulate, making it impossible to maintain the photocatalytic activity and performance of the particulate material. In addition, TiO particles₂ in such dispersions, they tend to settle over time at the bottom of the containers in which they are stored, which gives rise to storage stability problems.

[0008] Accordingly, a need is felt to provide a method that allows the preparation of a nanoparticle dispersion of titanium dioxide in the form of anatase.

Summary of the invention

[0009] The Applicant has now developed a method for obtaining TiO nanoparticles₂ in anatase form already dispersed in suitable solvents, directly suitable for the preparation of photocatalytic coatings. The dispersions prepared by the process of the invention do not exhibit any coagulation phenomena even after long-term storage, which makes it possible to produce coatings which maintain the photocatalytic activity of the particulate material due to the uniformity of the dispersion.

[0010] Accordingly, the present invention provides a process for dispersing TiO nanoparticles₂as anatase in a mixture of water and a suitable complexing solvent, comprising the following steps:

i) reacting the titanium alkoxide with a suitable complexing solvent;

ii) distilling the solution coming from step i);

iii) adding water to the solution coming from step ii) together with said complexing solvent and one or more polycondensation inhibitors, and then refluxing the reaction mixture to obtain the desired nanoparticle dispersion.

[0011] Another method of producing titanium dioxide nanoparticle suspensions, TiO₂is the aqueous hydrolysis of titanium alkoxides such as methoxide, ethoxide, normal propanolate, isopropoxide, normal butanol, and titanium isobutoxide. Titanium isopropoxide is preferred in some of the respects previously described.

[0012] Titanium isopropoxide is added to a hot aqueous solution containing a mineral acid (such as hydrochloric or nitric acid) and a non-ionic surfactant (such as Triton Χ-100). The process is heated under reflux for 24 hours.

[0013] The invention also provides dispersions of TiO nanoparticles₂ in the form of anatase in a mixture of water and a suitable complexing solvent, prepared by the above-described process, and their use for the preparation of photocatalytic topcoats for antibacterial action, photocatalytic decontamination of gases and liquids, and for the production of cosmetic preparations to protect the skin from sunlight.

[0014] The characteristics and advantages of the invention will be illustrated in detail in the following description.

SHORT DESCRIPTION OF THE FIGURES

[0015]

Figure 1 shows the diffractogram obtained from the XRD analysis of the product obtained in Example 1 after drying at 200 ° C for 12 hours.

Figure 2 shows a TEM image of TiO nanoparticles₂ (90000x).

Figure 3 shows the diffractogram obtained from the XRD analysis of the product obtained in example 8.

Detailed description of the invention

[0016] According to the process of the invention, the formation of TiO₂ in anatase form takes place directly in the water / complexing solvent mixture used in step i) to form a dispersion of TiO particles at the end of the process₂ with a size between 3 and 20 nm. Particle size measurements were made by various techniques known to those skilled in the art, such as XRD (X-ray diffraction), FEG-SEM (scanning electron microscopy with half emission gun), TEM (transmission electron microscopy), and DLS (dynamic light scattering). Such dispersions, unlike those prepared by dispersing nanometric powders in solvent mixtures, show no agglomeration, no coagulation and precipitation phenomena, even after prolonged storage of the dispersion.

[0017] The advantages of such dispersions are obvious and relate to the uniformity and photocatalytic efficiency of the coatings obtainable therefrom. The polydispersity index of the dispersions according to the invention, measured by DLS (dynamic light scattering) technique, is below 0.3, which distinguishes the dispersions according to the invention from those produced by the traditional method of producing nanoparticle powder and then dispersing them in a solvent. A typical TEM pattern of a nanoparticle dispersion according to the invention is shown in Fig. 2. The titanium alcoholate used as a starting material in the process of the invention may for example be selected from the group consisting of methoxide, ethoxide, normal propoxide, isopropoxide, normal butoxide and titanium isobutoxide.

[0018] Of these compounds, titanium isopropoxide is the preferred starting material for the process according to the invention for various reasons. Of the titanium compounds that can be used, it is the cheapest and the one that shows the best reactivity under the conditions of the process according to the invention; moreover, its use leads to isopropyl alcohol obtained as a by-product in step ii), a product easily recovered from the process according to the invention and valuable due to its wide application in the detergent industry.

[0019] Complexing solvents commonly used in the process of the invention include ethylene glycol, diethylene glycol and polyethylene glycols with a molecular weight for example between 200 and 600. Longer chain polyethylene glycols with a molecular weight of up to 10,000 may also be used. end of process and after cooling instead of TiO dispersion₂ in the liquid, nanoparticles of TiO are obtained₂ dispersed in a solid matrix. The final product retains the nanometric dimensions of TiO₂ and the low polydispersity index observed with liquid dispersions. A preferred complexing solvent is diethylene glycol.

[0020] Excellent results have been obtained by carrying out the reaction step i) using titanium isopropoxide and diethylene glycol in a molar ratio of 1: 3.

[0021] Within the scope of the present invention, the term polycondensation inhibitor usually denotes a mixture containing at least one mineral acid and one organic acid, the mineral acid may be selected, for example, from the group consisting of hydrochloric acid, nitric acid, sulfuric acid, perchloric acid, hydrobromic acid and hydroiodic acid and the organic acid is preferably acetic acid.

[0022] According to a particularly preferred embodiment of the present process, the polycondensation inhibitor is a mixture of hydrochloric acid and acetic acid.

[0023] The amount of the polycondensation inhibitor added is such that the amount of mineral acid is between 0.1 and 10% by volume with respect to the total volume of the reaction mixture, and the amount of organic acid is between 1 and 20% by volume with respect to the total volume of the reaction mixture.

[0024] The water / complexing solvent mixture used in the invention also allows the dispersion to be used directly in the preparation of photocatalytic coatings in virtually any type of application, even for use in the field of cosmetics or fabrics for skin contact coating products.

[0025] When used to form coatings, the dispersions of the present invention may optionally be formulated with additives and diluents commonly used in the field of topcoats such as adhesion promoters or solvents such as water or ethanol to achieve the required dilution.

[0026] Instead, when used to decontaminate liquid or gaseous products, the dispersions of the present invention are suitably adsorbed onto a silica gel support or other suitable inorganic support with good adhesion characteristics, such as glass, ceramics, porous ceramics, fibers, fabrics. or the like which are then immersed in the liquid or placed as such or after dilution in containers through which the gas is bubbled, to be cleansed.

[0027] The carriers to which the topcoats made from the dispersions of the present invention can be applied can vary widely, ranging from textiles, roll or stacked, to ceramic products, glass, metal or specular substrates and the like.

[0028] The photocatalytic activity of the topcoats of the invention is manifested when the coating itself is exposed to light of a suitable wavelength, typically below 388 nm, to provide a surface with antimicrobial, bacteriostatic and superhydrophilic properties when exposed to UV light. TiO coated substrates₂ show a complete lack of hydrophobicity, known as superhydrophilicity, which is provided to TiO-treated surfaces₂ self-cleaning properties.

[0029] Moreover, due to the very small particle size of TiO₂, the dispersions of the present invention are nearly transparent, thus leaving the appearance of the surface to which they have been applied unchanged. Such transparency also makes them useful in the field of cosmetics for the production of sunscreens with a high degree of protection against UV.

[0030] A further advantage of the dispersions of the present invention is their behavior at high temperatures. In this aspect, the application of topcoats to ceramic substrates requires a high temperature treatment of the substrate to which the dispersion has been applied, the dispersions of the present invention retaining the appearance, anatase crystalline form and the nature of the nanoparticles as in the coating before heating.

[0031] According to a particular embodiment of the present process, the doping of Ti with a metal selected from the group of transition metals, in particular Ag, Cu or Ce, may be achieved by adding one of their salts in step i) or alternatively in step iii) of the process of the invention. In this way, the process will achieve the formation of a TiO dispersion₂ doped with Ag, Cu or Ce, which can exhibit catalytic activity even without being irradiated with UV light.

[0032] Some illustrative and non-limiting examples of the invention are given below.

EXAMPLE 1

Production of TiO nanoparticle dispersion₂ as anatase in diethylene glycol water starting from Ti isopropoxide

[0033] 5.53 liters of diethylene glycol are introduced into a 20 liter flask to which is added 5.54 liters of titanium isopropoxide. The reaction mixture is kept under stirring for 5 minutes and then heated to 120 ° C while the isopropyl alcohol formed is distilled off until a small volume is obtained. 11.1 liters of diethylene glycol, 125 ml of 32-33 wt.% Are added. hydrochloric acid, 2.07 liters of glacial acetic acid and 125 ml of deionized water. The temperature is brought to 180 ° C and the mixture is refluxed for 2 hours.

[0034] The product thus obtained was characterized as follows.

[0035] First, the concentration of TiO was measured₂ in the end product using inductively coupled plasma atomic emission spectroscopy (ICP) according to standard methodology. Based on this analysis, the amount of TiO₂ in the dispersion is 5.7 wt.% based on the total weight of the dispersion.

[0036] A sample of the dispersion obtained as described above was dried in an oven at 200 ° C for 12 hours until the solvent was completely evaporated. The thus obtained powder was analyzed by XRD using a Philips X'Pert PRO diffractometer to determine its crystal structure: as can be seen in figure 1, the position and intensity of the peak in the diffraction pattern are typical of anatase.

[0037] From the diffractogram of figure 1, and in particular from the width of the main peak, the average particle size of TiO was calculated.₂ using the Sherrer formula, and the mean diameter was found to be 4.5 nm.

[0038] This value was also confirmed by transmission electron microscopy observations of the dispersion sample obtained as described above after diluting 1: 100 with ethanol.

EXAMPLE 2

Production of TiO nanoparticle dispersion₂ as anatase in water / diethylene glycol starting from Ti ethoxide

[0039] 5.53 liters of diethylene glycol are introduced into a 20 liter flask to which is added 3.76 liters of titanium ethoxide. The reaction mixture is kept under stirring for 5 minutes and then heated to 130 ° C while the ethanol formed is distilled off. 11.1 liters of diethylene glycol, 125 ml of 32-33 wt.% Are added. hydrochloric acid, 2.07 liters of glacial acetic acid and 125 ml of deionized water. The temperature is brought to 180 ° C and the mixture is refluxed for 2 hours.

[0040] This product was characterized in the same way as in Example 1 as having the same crystalline phase and particles of similar size. In addition, the obtained product was used to carry out the same tests as those described above in Examples 2, 3 and 4 with results similar to those obtained with the product produced as in Example 1.

EXAMPLE 3

Production of TiO nanoparticle dispersion₂ as anatase in water starting from Ti isopropoxide

18.720 kg of an aqueous solution obtained by mixing water with 100 g of hydrochloric acid and 80 g of 1 wt. Triton Χ-100 solution in water is added to a 20 L flask. The reaction mixture is heated to 50 ° C. 1.280 kg of titanium isopropoxide are added. The reaction mixture is refluxed at 50 ° C for 24 hours. The product obtained in this way was characterized as follows.

[0042] First, the concentration of TiO was measured₂ in the end product using inductively coupled plasma atomic emission spectroscopy (ICP) according to standard methodology. Based on this analysis, the amount of TiO₂ % in the dispersion is 1.8 wt.% based on the total weight of the dispersion.

[0043] A sample of the dispersion obtained as described above was dried in an oven at 100 ° C for 12 hours until the solvent was completely evaporated. The powder obtained in this way was analyzed by XRD method using a Philips X'Pert PRO diffractometer in order to determine its crystal structure.

EXAMPLE 4

Application of TiO nanoparticle dispersion₂ in water / diethylene glycol on the fabric

[0044] 25 ml of deionized water was added to 75 ml of the dispersion prepared as previously described in Example 1 and the dispersion thus diluted was placed in the bowl. A 20 cm x 60 cm strip of cotton fabric was immersed in the bowl for 10 seconds, then removed and passed between two rollers of silicone material to remove excess solvents. The fabric was then oven dried, machine washed, dried again and the UV protection index (UPF) provided by the coated fabric was measured by standard spectrophotometric methods for this type of measurement and found to be 35.40 UPF.

EXAMPLE 5

Application of TiO nanoparticle dispersion? in e / ethylene glycol water on wool [0045] 25 ml of deionized water was added to 75 ml of the dispersion prepared as previously described in Example 1 and the dispersion thus diluted was placed in a bowl. A 20 cm x 60 cm strip of wool fabric was immersed in the bowl for 10 seconds, then removed and passed between two rollers of silicone material to remove excess solvents. The fabric was then oven dried, machine washed and dried again. The antimicrobial properties of this fabric were visually examined according to the AATCC ™ 100: 99 rule. The test results are shown in the table below.

<td></td><td colspan="3">Percentage of microbial strain kill</td>

<td>A sample</td><td>Staphylococcus aureus</td><td>Bacillus subtilis</td><td>Aspergillus niger</td>

<td>Pure wool</td><td> 0</td><td> 0</td><td> 77,60</td>

<td>Wool treated with titanium dioxide</td><td> &gt;99,94</td><td> 99,60</td><td> 99,47</td>

EXAMPLE 6

Application of TiO nanoparticle dispersion₂ in water / diethylene glycol on cotton thread [0046] 25 ml of deionized water was added to 75 ml of the dispersion prepared as described above in Example 1 and the dispersion thus diluted was placed in a bowl.

[0047] Cotton thread was dipped in the bowl, oven dried, and wound on a spool. This thread was knitted into a knitted fabric which was tested to determine the UV Protection Index (UPF) provided by the coated fabric. These properties were measured by standard spectrophotometric methods and the UPF was found to be 30.20.

EXAMPLE 7

Application of TiO nanoparticle dispersion₂ in water / diethylene glycol on ceramic surfaces - test of adhesion and resistance to high temperatures

[0048] The nanoparticle dispersion obtained as described above in Example 1 was used to form a photocatalytic coating on an unglazed stoneware substrate by adding 5% by weight of low melting frit to aid adhesion of the titanium dioxide to the substrate. A frit with a relatively low hemispherical temperature of 700 ° C and the following chemical composition was used:

<td>SiO₂</td><td> 48,32 %</td><td>CaO</td><td> 6,95 %</td>

<td>A1₂ABOUT₃</td><td> 2,22 %</td><td>MgO X</td><td> 6,95 %</td>

<td>K.₂ABOUT</td><td> 0,049 %</td><td>Li₂ABOUT</td><td> 13,9 %</td>

<td>On₂ABOUT</td><td> 0,06 %</td><td>ZnO</td><td> 4,05 %</td>

<td>B₂ABOUT₃</td><td> 22,55 %</td><td></td><td></td>

[0049] The dispersion of example 1 was applied by dip coating to a substrate which was subjected to thermal cycling at both 700 ° C and 600 ° C. After the baking treatment, the carrier retained its original appearance and showed good adhesion between the coating and the substrate.

[0050] The high temperature behavior of the coating of the present invention was investigated by high temperature powder diffraction (XRD-HT). In this way, it was observed that the phase conversion from anatase to rutile only started at about 800 ° C and was completed at about 900 ° C. Using the Sherrer formula, the dimensions of the nanocrystal at different temperatures were also calculated.

[0051] Table 1 below shows the angle 2Θ at which the measurement was made, the peak width at half height (FHWD), which, when inserted into the Sherrer formula, is used to calculate crystallite dimensions, crystallite dimensions and temperature, relative to the previous dimensions.

Table 1

<td>Crystalline phase</td><td>2Θ angle</td><td>FHWD</td><td>dimensions (nm)</td><td>T (° C)</td>

<td>anatase</td><td> 25,04</td><td> 1,3354</td><td> 60,9</td><td> 300</td>

<td>anatase</td><td> 25,11</td><td> 1,2553</td><td> 64,8</td><td> 400</td>

<td>anatase</td><td> 25,10</td><td> 1,1532</td><td> 70,6</td><td> 500</td>

<td>anatase</td><td> 25,05</td><td> 0,9405</td><td> 86,5</td><td> 600</td>

<td>anatase</td><td> 25,05</td><td> 0,4045</td><td> 201,2</td><td> 700</td>

<td>anatase</td><td> 25,03</td><td> 0,2614</td><td> 311,3</td><td> 800</td>

<td>anatase</td><td> 25,02</td><td> 0,1935</td><td> 420,5</td><td> 900</td>

<td>rutile</td><td> 27,10</td><td> 0,1401</td><td> 583,3</td><td> 900</td>

<td>rutile</td><td> 27,08</td><td> 0,137</td><td> 596,4</td><td> 1000</td>

[0052] The same method was used to evaluate the increase in crystallite size while keeping the frit firing temperature constant but at different times, and in this case good coating adhesion was found, even with extended firing times, with the crystallite size increasing over time but within an acceptable range. to a degree, but without reducing the photocatalytic efficiency of the coating.

[0053] To confirm the adhesion of the coating to the substrate, the entire sample was subjected to cycles of ultrasound in ethanol and acetone for various times (5 and 60 minutes) and repeated washing with fabrics with different abrasive properties (sponge washing). XRD analysis was performed after each ultrasound cycle to determine any reduction in the amount of anatase present in the coating, concluding that the treatment did not affect the TiO crystal form.₂ or adhesion of the coating to the substrate. EXAMPLE 8

Application of TiO nanoparticle dispersion₂ in water / diethylene glycol on ceramic surfaces - photocatalytic effect

[0054] Two samples of the same porcelain stoneware with white glaze were soiled with the same amount of a solution containing 10 ppm methylene blue. Only one of the two samples was precoated with the dispersion according to the invention as described in Example 4.

[0055] The two samples were then exposed to light from the UV lamp for different times: 10, 30, 60, 90 and 120 minutes. While no change in methylene blue soil was observed on the untreated sample, the gradual disappearance of the blue soil was observed for the sample coated with the dispersions of the invention. The same experiment was repeated with the marker pen-indelible soil, observing the soil disappearance only on the coated sample after 45 minutes exposure to UV light.

[0056] The above two experiments were repeated under sunlight instead of light from a UV lamp, and the same results were obtained.

EXAMPLE 9

Application of TiO nanoparticle dispersion? in water / diethylene glycol onto the glass.

[0057] The dispersion of example 1 was applied by dip or spray coating to a substrate which was subjected to thermal cycling for 30 minutes at 200 ° C for 30 minutes at 500 ° C. After the baking treatment, the support retained its original appearance and showed good adhesion between the coating and the substrate.

[0058] This sample was soiled with a solution containing 10 ppm methylene blue. The sample was then exposed to the light of a UV lamp and a gradual disappearance of the blue soil was observed. This experiment was repeated under sunlight instead of a UV lamp and the same result was obtained.

EXAMPLE 10

Application of TiO nanoparticle dispersion? in water / di-ethylene glycol on glass-ceramic surfaces,

[0059] The dispersion of Example 1 was applied by dip or spray coating to a substrate which was subjected to thermal cycling for 30 minutes at 200 ° C and 30 minutes at 700 ° C. After the baking treatment, the support retained its original appearance and showed good adhesion between the coating and the substrate.

[0060] This sample was soiled with a solution containing 10 ppm methylene blue. The sample was then exposed to the light of a UV lamp and a gradual disappearance of the blue soil was observed. This experiment was repeated under sunlight instead of a UV lamp and the same result was obtained.

EXAMPLE 11

Application of TiO nanoparticle dispersion? in water / diethylene glycol on various surfaces (glass, glass-ceramics, glaze, solid stoneware).

[0061] 0.01 to 10% of a surfactant was added to the dispersion of example 1, such as, for example, a nonionic surfactant (such as Triton X100) to improve surface distribution. This solution was applied by dip or spray coating to a substrate that was subjected to thermal cycling for 30 minutes at 200 ° C and 30 minutes at 500 ° C for glass or 700 ° C for glass-ceramics, glaze and bulk porcelain. . After the baking treatment, the support retained its original appearance and showed good adhesion between the coating and the substrate.

[0062] This sample was soiled with a solution containing 10 ppm methylene blue. The sample was then exposed to the light of a UV lamp and a gradual disappearance of the blue soil was observed. This experiment was repeated under sunlight instead of a UV lamp and the same result was obtained.

EXAMPLE 12

Application of TiO nanoparticle dispersion? in water on a ceramic composite made of an inorganic material and a polyester resin.

[0063] 50 ml of deionized water was added to 50 ml of the dispersion obtained as described in example 1 bis above and the dispersion thus diluted was placed in the spray gun. A sample was sprayed onto the surface of the composite material which was then held at 100 ° C for 1 hour.

[0064] This sample was soiled with a solution containing 10 ppm methylene blue.

The sample was then exposed to the light of a UV lamp and a gradual disappearance of the blue soil was observed. This experiment was repeated under sunlight instead of a UV lamp and the same result was obtained.