WO2012067590A1

WO2012067590A1 - Coating of tio2 rutile nanoparticles in a suspension with hydrated sio2 and ai2o3

Info

Publication number: WO2012067590A1
Application number: PCT/SI2011/000064
Authority: WO
Inventors: Nika Veronovski; Dejan VERHOVŠEK; Aljaž SELIŠNIK
Original assignee: Cinkarna Metalursko Kemicna Industrija Celje Dd
Current assignee: Cinkarna Metalursko Kemicna Industrija Celje Dd
Priority date: 2010-11-19
Filing date: 2011-11-18
Publication date: 2012-05-24
Anticipated expiration: 2013-05-19
Also published as: SI23547A

Abstract

The invention refers to a rapid, adaptable, manageable and reproducible approach to a synthesis of coated Ti0₂ nanoparticles with hydrated silicon and aluminium oxides with the use of harmless chemicals. The proposed method is based on a preparation of a well dispersed system of Ti0₂ nanoparticles and a controlled hydrolysis of Si0₂ from an alkaline precursor Na₂Si0₃ in the presence of the mineral acid H₂S0₄ or a hydrolysis of AL₂0₃ from an alkaline precursor of NaAL0₂ in the presence of the mineral acid H₂S0₄. The analyses have proved that the longer the coating times, the higher the Si0₂quantity and consequently the thickness of the coating. Controlled synthesis conditions were used to coat individual Ti0₂ nanoparticles. The mechanism of inorganic coating of Ti0₂ nanoparticles can be categorised as precipitation of hydrated oxides in the form of a thin layer onto already present particles (heterogeneous nucleation). The advantage of the method is its simplicity and the fact that a stable suspension of Ti0₂ nanoparticles is present in the whole method from the beginning to the end, which provides for healthy work conditions of industrial workers and users and also prevents negative effects on the environment (emission of nanoparticles).

Description

COATING OF Ti0₂ RUTILE NANOP ARTICLES IN A SUSPENSION WITH HYDRATED Si0₂ AND A1₂0₃

SUBJECT OF INVENTION

The subject of the invention is coating of Ti0₂ rutile nanoparticles in a suspension with Si0₂ and A1₂0_3.

The present invention belongs to the field of colloidal chemistry and more precisely it comprises suspensions of Ti0₂ nanoparticles in a crystal rutile structure with the stress being on their surface treatment by using sodium silicate and sodium aluminate as reagents for the coating of nanoparticles with thin layers of hydrated silica or alumina (Si0₂ and A1₂0₃).

FIELD OF INVENTION

The invention provides for a controlled coating of nanoparticles in the form of Ti0₂ suspension with thin layers of hydrated alumina and silica (Si0₂ and Al₂0₃). A decision about a production of Ti0₂ nanoparticles, in which products are exclusively produced in the form of a suspension, is based on a care for healthy conditions of workers in production, use and prevention of negative effects on the environment (emission of nanoparticles). Coating is based on the principle of hydrolysis of sodium silicate (Na₂Si0₃) and sodium aluminate (NaA10₂). Ti0₂ nanoparticles are coated with inorganic oxides in order to limit formation of free radicals on the surface of Ti0₂ and in order to change electrokinetic properties. A surface treatment results in a change in a surface charge, which increases dispersibility in polar media. A surface treatment with inactive inorganic materials results in a preparation of suspensions without considerable agglomerates. This increases transparency level (of coatings containing coated Ti0₂ nanoparticles) in the visible part of the spectre, increases coverage of paints/lacquers/coatings and protects the basic substrate. Coated Ti0₂ nanoparticles preserve their technological and applicability properties, and what's more, they exhibit a higher application value, as inclusion of UV-absorbers, like Ti0₂, is an efficient way for the improvement of UV-resistance of materials. The coated Ti0₂ nanoparticles in a rutile crystal structure serve as UV-absorbers and they are applied in different fields: cosmetics, plastics, paints, lacquers, coatings, textiles, etc. The coating of the invention is budget-priced. Nano titanium dioxide (Ti0₂) is one of new modern materials. It has changed properties in comparison with pigment Ti0₂. Ti0₂ nanoparticles differ from pigment Ti0₂ in size and physical and chemical properties, which results in their application being different than that of pigment Ti0₂. Particle size, specific surface area, shape of particles and crystal structure are key parameters that have influence on the properties and applicability of the final product. As far as its crystal structure is concerned, Ti0₂ is a substance with several known polymorphs. Among all crystal structures of Ti0₂ only rutile and anatase are of any importance. The main properties of nano Ti0₂ are absorption of UV-light and photocatalytic activity. Pigment Ti0₂ is also photocatalytically active, but this property is even more distinctive in Ti0₂ nanoparticles. Ti0₂ nanoparticles are most often used as UV absorbers and photocatalysts. The Ti0₂ nanoparticles in the anatase crystal structure have a high photocatalytic activity and are therefore used as self-cleaning agents, bactericides in the cleaning of waste water and also as semi-conductors in the production of photocells. The Ti0₂ nanoparticles in the rutile crystal structure are excellent UV absorbers and are added to various paints and coatings in order to achieve better UV and weather resistance. The properties of Ti0₂ nanoparticles can be improved by being coated with other materials. Surface treatment of Ti0₂ or coating is achieved by a hydrolysis of oxides onto a surface of a Ti0₂ nanoparticle.

Water glass is sodium silicate (Na₂Si0₃) by its chemical structure. It is formed by dissolving quartz sand with soda. Na₂Si0₃ is soluble in water. It is stable in an alkaline medium at pH values exceeding pH 10. Below this value hydrolysis starts, Na₂Si0₃ hydrolyses and Si0₂ is obtained. In a surface treatment with Si0_2; nanoparticles of various characteristics can be formed based on the conditions of the oxide hydrolysis. A hydrolysis at acid or neutral pH leads to the formation of an amorphous Si0₂ coating composed of submicron particles bound in a gel structure. The coating thus obtained allows better dispersibility of (Ti0₂) particles in a medium and consequently improvement of optical properties. Neutral or poor acid conditions cause a formation of coatings having a lower shine level - matting. A slow hydrolysis at an alkaline pH leads to a formation of a thick, glass-like Si0₂ coating and results in improved particle persistence and duration.

Sodium aluminate (NaA10₂) is an important inorganic chemical. It works as an effective source of alumina for many industrial and technical applications. Anhydrous sodium aluminate is a white crystalline substance having a formula variously given NaA10₂, Na₂0 · A1₂0₃, or Na₂Al₂0 . Sodium aluminate is available both in a solution and in solid form. Sodium aluminate is manufactured by dissolution of aluminium hydroxide in a solution of NaOH. Both NaA10₂ and Na₂Si0₃ are soluble in water. An NaA10₂ aqueous solution is stable in an alkaline medium at pH values exceeding pH 10. Hydrolysis starts below this value, NaA10₂ hydrolyses and A1₂0₃ is formed. Surface treatment with hydrated alumina may be the most commonly used surface treatment of Ti0₂. Hydrated alumina can hydrolyse from sodium aluminate which reacts with an acid. The morphology of coating depends on coating conditions. Hydrated aluminium oxides which hydrolyse from sodium aluminate at acid conditions are of amorphous shape. Hydrated Α1₂0₃ particles on the surface of a Ti0₂ nanoparticle cause reduced attractive forces among the particles and improve dispersibility. In case of surface treatment with several layers, A1₂0₃ is the most often the last coating.

PRIOR ART

The properties of Ti0₂ nanoparticles can be improved by being coated with other materials. The most often used surface treatments are: treatment with Si0₂, A1₂0₃ and/or Zr0₂. Various approaches on the molecular level have been known for the formation of layers of oxides in controlled syntheses. A Ti0₂ synthesis can be performed by various methods, yet the most important is the sol-gel technique that allows a formation of a microstructure by selecting a type of precursor and process conditions. Despite all advantages offered by the sol-gel method the numerous existing industrial productions of Ti0₂-Si0₂ composites in the synthesis mode are most often hindered by factors like: use of expensive and dangerous chemicals in the sol-gel method; long synthesis times.

Two important publications in which the authors used sodium silicate (Na₂Si0₃) and sodium aluminate (NaA10₂) as a source of Si0₂ and A1₂0₃ for the coating of Ti0₂ nanoparticles are mentioned in the continuation.

S. R. Frerichs and W. H. Morrison searched a surface treatment of Ti0₂ nanoparticles with Si0₂ and A1₂0₃ in the presence of citric acid. The authors used powdered Ti0₂ nanoparticles to prepare a slurry. The surface treatment comprises: preparation of a slurry of Ti0₂ nanoparticles; addition of citric acid; addition of a source of metallic oxide selected from a group consisting of an A1₂0₃ source and Si0₂ source; final treatment of surface treated Ti0₂ nanoparticles. Hydrolysis of Si0₂ was performed at increased T (T=30-100 °C) at pH ~ 7. After the surface treatment with Na₂Si0₃ and NaA10₂, the particles were dried, crushed and sieved. The coated Ti0₂ nanoparticles contained ~ 4 % Si0₂ and ~ 6 % AI2O3. The particles coated by the described method exhibited high illumination stability and lower tendency towards agglomeration.

C. R. Bettler and M. P. Deibold searched a method for the preparation of a highly persistent Ti0₂ pigment that disperses more easily. The method comprises the following steps: heating of a slurry of Ti0₂ particles up to T=85-100 °C; addition of citric acid; adjustment of pH to pH>10; addition of an aqueous solution of Na₂Si0₃; neutralisation of the slurry with a mineral acid (nitric, hydrochloric or sulphuric acid); adjustment of T (T=55-90 °C); addition of an aqueous solution of NaA10₂; adjustment of pH to 5-9 with a mineral acid (nitric, hydrochloric or sulphuric acid).

Literature contains no data about a modification mode of non-powder Ti0₂ nanoparticles. There is further no explanation about the stability of Na₂Si0₃ and NaA10₂ precursors in various pH conditions by using a titration approach and explaining electrokinetic properties of Ti0₂ nanoparticles that have influence on the selection of process conditions and finally on the result of coating. Moreover, no data can be found about the influence of washing of surface treated Ti0₂ nanoparticles. The described mode of modification is different than the remaining coating processes in that the method does not include a drying phase and consequently powder particles that have a negative effect on industrial workers, users and on the environment. Coated Ti0₂ nanoparticles will be available for various applications in the form of a suspension not powder. The costs of heating in calcination and the agglomeration of coated nanoparticles that appears in calcination are thus avoided.

DESCRIPTION OF THE INVENTION

The invention is explained by the following figures:

Figure 1 : A TEM image of untreated Ti0₂ nanoparticles with a crystal structure of rutile. Figure 2: A TEM image of submicron Si0₂ particles formed after Embodiment A bound into a gel structure. A TEM image of coated Ti0₂ nanoparticles with a crystal structure of rutile, with an amorphous layer of hydrated Si0₂ obtained by a hydrolysis of Na₂Si0₃ according to Embodiment C, which is equally distributed over the surface of Ti0₂ nanoparticles.

A TEM image of coated Ti0₂ nanoparticles with a crystal structure of rutile, with an amorphous layer of hydrated Si0₂ and A1₂0₃ obtained according to Embodiment D, which are equally distributed over the surface of Ti0₂ nanoparticles.

A SEM image of unwashed coated Ti0₂ nanoparticles with a crystal structure of rutile, with an amorphous layer of hydrated Si0₂ obtained by a hydrolysis of Na₂Si0₃ according to Embodiment B, which is equally distributed over the surface of Ti0₂ nanoparticles and salt crystals formed in reactions during surface treatment.

A SEM image of washed coated Ti0₂ nanoparticles with a crystal structure of rutile, with an amorphous layer of hydrated Si0₂ obtained by a hydrolysis of Na₂Si0₃ according to Embodiment C, which is equally distributed over the surface of Ti0₂ nanoparticles.

A SEM image of washed coated Ti0₂ nanoparticles with a crystal structure of rutile, with an amorphous layer of hydrated Si0₂ and A1₂0₃ obtained according to Embodiment D, which are equally distributed over the surface of Ti0₂ nanoparticles.

The present invention describes coating of Ti0₂ nanoparticles with thin layers of hydrated amorphous alumina and silica (Si0₂ and A1₂0₃). The essence of coating of Ti0₂ nanoparticles of the invention is controlled conditions that lead to a formation of homogeneous layers of hydrated oxides on the surface of Ti0₂ nanoparticles, which is reflecte4d in changed electrokinetic properties of Ti0₂ nanoparticles. Further, such physical and chemical conditions have been determined and used that lead to desired morphology and property of coatings. In the case of controlled coating of Ti0₂ particles, homogeneous coatings have appeared on their surface, which entirely cover the surface of particles. An advantage of the described method is the absence of a final phase of calcination. A suspension of Ti0₂ nanoparticles with a mass concentration γ = 100 - 300 g/L is used for surface treatment. An average size of an individual Ti0₂ nanoparticle in a suspension is 80x20 nm. Specific surface area of Ti0₂ nanoparticles is 130 m /g. An analysis of electrokinetic properties determined the isoelectric point (IEP) of Ti0₂ nanoparticles, which is at pH value ~ 6.5. An increase in pH value of a suspension of Ti0₂ nanoparticles from a very acidic pH to a pH 10.5 is achieved by an addition of a base. As a base for adjusting the pH of the medium 10 - 60 wt. % of sodium hydroxide is used. Stabilisation of a suspension of Ti0₂ nanoparticles with a basic pH is carried out in a range of temperatures from 40 to 100 °C in a period of two hours or less. As a stabilising agent preventing agglomeration of Ti0₂ nanoparticles in the pH range close to the isoelectric point 0.5 - 2 wt. % of citric acid is added. Reaction with citric acid is carried out at a temperature range from 40 to 100 °C in a period of two hours or less under simultaneous stirring at 150 to 400 rev/min. The precursors used are Na₂Si0_3; 10 - 30 wt. % (Ysi02 = 220 g/L) and NaA10_2; 10 - 30 wt. % (γΑΐ203 = 280,3 g/L). A synthesis of the coating of Si0₂ nanoparticles is from sodium silicate with an addition of a mineral acid. As a mineral acid for the precipitation of Si0₂ nanoparticles 10 - 60 wt. % sulphuric acid is used. The Si0₂ nanoparticles participate at pH 7, at which the suspension is stirred for three hours or less. A diluted H₂S0₄ is used as hydrolysis and condensation initiator, since a concentrated acid could cause local or homogeneous nucleation of Si0₂, which is undesirable in our case. A synthesis of the coating of A1₂0₃ nanoparticles is carried out from sodium aluminate by a simultaneous adition of a mineral acid at a pH value between 6.5 and 7.5. The obtained suspension is mixed at a temperature range between 40 and 100 °C in a period of one hour or less. pH is adjusted with 10 -60 wt. % of a mineral sulphuric acid and 10 - 60 wt. % of NaOH.

Prior to the method of surface treatment, both precursors were analysed by using titrations, whereas Ti0₂ nanoparticles were determined electrokinetic properties or the isoelectric point (IEP). The pH value of the aqueous Na₂Si0₃ was determined (ysi02 = 220 g/L), which amounted for 1 wt. % of Na₂Si0₃ solution to 10.3, whereas the pH of undiluted aqueous Na₂Si0₃ is 1 1.7. The alkaline solution was titrated with CHCI ⁼ 0.1 mol/L. The Na₂Si0₃ solution was still stable at pH 10.5, whereas the solution is no longer stable at pH < 10, which is evident from the curve and the inflection point. pH of the diluted aqueous NaA10₂ is ~ 1 1.5, if acid is added or pH lowered, hydrolysis of the hydrated A1₂0₃ occurs. IEP of Ti0₂ nanoparticles was determined by use of an apparatus for determining the charge of particles (PCD). IEP lies at pH 6.5. The stability of the aqueous solution of Ti0₂ is the lowest at given pH, Ti0₂ nanoparticles have a high tendency to agglomerate. The surface charge of particles conditions the stability of dispersions, so the charge on the surface of particles is created by adjusting pH to 10.5. This was also the pH value, at which the stability of both precursors was determined, which provides optimal conditions, at which homogeneous thin layers of hydrated alumina and silica (Si0₂ and Al₂0₃) will occur that will entirely cover the surface of individual Ti0₂ nanoparticles and not agglomerates.

Once the surface treatment is completed, the particles are washed on a centrifuge to remove impurities, especially various salts that appeared in reactions of the surface treatment of Ti0₂ nanoparticles.

Embodiment A:

500 mL of distilled water was poured into a flask, pH was adjusted to pH 9.5 - 10.5 by an addition of 10 - 60 wt. % NaOH. Heating was simultaneously performed at a temperature of 40 -100 °C. Na₂Si0₃ at this pH is still stable. 10 - 30 wt. % of Na₂Si0₃ was slowly added (by drops). Na₂Si0₃ was the source of Si0₂. The suspension was stirred for 5 - 30 minutes, then 10 - 60 wt. % of H₂S0₄ was added until pH 6.5 - 7.5 was reached. The suspension having this pH value was then stirred for 0.5 - 4 hours for the entire Si0₂ to precipitate. Then followed maturing.

Embodiment B:

500 mL of a prepared suspension of rutile Ti0₂ nanoparticles with a mass concentration of γ = 100 - 300 g/L was poured into a flask. The suspension was earlier dispersed by using Ultra Turrax T25 dispersing instrument (IKA, Germany) at 8000 - 13500 rev/min. The suspension of rutile Ti0₂ nanoparticles is heavily acidic after the synthesis (pH ~ 0). To a suspension was added 0.5 - 2 wt. % of citric acid in the form of a solution under constant stirring with a propeller stirrer at 150 - 400 rev/min. While passing the isoelectric point during the process of surface treatment, the particles can get heavily agglomerated and are as such unsuitable for coating with Si0₂. When Ti0₂ nanoparticles are coated with Si0₂, agglomeration of particles needs to be avoided, which means that a stable suspension in water needs to be prepared. Agglomeration can be avoided by binding citric acid to the surface. The acid bound to the surface sterically prevents the nanoparticles from agglomerating and simultaneously creates a high charge on their surface, which contributes to electrostatic stabilisation of the suspension (electrosteric stabilisation). The mixture was heated to 40 - 100 °C and stirred at 150 - 400 rev/min for 0.5 - 2 hours. Citric acid thus chemically bound to the surface of nanoparticles. The pH value of the suspension was then adjusted to pH 10.5 by adding 10 - 60 wt. % NaOH. At such pH value the particles have a very high negative surface on the surface, which is evident as a high ζ-potential, which prevents their heavy agglomeration. The suspension was stirred for 0.5 - 2 hours in order to reach an equal distribution of the charge on the surface of Ti0₂ nanoparticles. While further heating and stirring 10 - 30 wt. % of Na₂Si0₃ was added. Na₂Si0₃ in this case was the source of Si0₂. The suspension was stirred for 5 - 30 minutes, then 10 - 60 wt. % of H₂S0₄ was added, until pH 7 was reached. At this pH value the suspension was stirred for 0.5 - 3 hours in order to separate the whole Si0₂. Then followed maturing and centrifugation.

Embodiment C:

The beginning of Embodiment C equals that of Embodiment B. After maturing followed centrifugation and washing. The process of washing of coated Ti0₂ nanoparticles is an important phase, since washing removes the salts formed during the reactions of hydrolysis of Si0₂ and other impurities that could have a negative effect on the applicative properties of coated Ti0₂ nanoparticles.

Embodiment D:

The beginning of Embodiment D equals that of Embodiment A. After the suspension was stirred for 0.5 - 3 hours at neutral pH and the entire Si0₂ precipitated, coating continued by adding 10 - 30 wt. % of NaA10₂ and simultaneously adding 10 - 60 wt. % of H₂S0₄, with which the pH value was maintained at 6.5 - 7.5. The suspension was then stirred and heated at constant temperature for 10 - 60 minutes. Then followed maturing and washing of surface treated Ti0₂ nanoparticles, wherein the salts formed during the reactions of precipitation of Si0₂ and A1₂0₃ and the remaining impurities are removed.

The coating of rutile nanoparticles of the invention is thus characterised in that

an acidic suspension of Ti0₂ nanoparticles is transformed into a stable suspension with a basic pH with an addition of a suitable quantity of citric acid in a dissolved form,

- reaction with citric acid is carried out in a temperature range between 40 and 100 °C in a period of two hours or less under simultaneous stirring at 150 to 400 rev/min, - the increase in the pH value from heavily acidic to pH 10.5 is achieved by an addition of a base,

- stabilisation of a suspension of Ti0₂ nanoparticles with basic pH is carried out in a temperature range between 40 and 100 °C in a period of two hours or less,

- the synthesis of coating with Si0₂ nanoparticles is performed from sodium silicate with an addition of a mineral acid,

- precipitation is achieved at pH value 7, at which the suspension is stirred for three hours or less,

- the synthesis of coating with A1₂0₃ nanoparticles is performed from sodium aluminate with a simultaneous addition of a mineral acid at a pH value between 6.5 and 7.5,

- the obtained suspension is stirred for one hour or less in a temperature range between 40 and 100 °C,

the coated Ti0₂ nanoparticles can be purified with centrifugation, wherein the centrifugation cycle can be repeated several times.

The suspension of Ti0₂ nanoparticles has a mass concentration of Ti0₂ between 100 and 300 g/L and has an acidic pH value. As stabiliser 0.5 to 2 wt. % of citric acid is used. The base used for the adjustment of the pH medium is 10 - 60 wt. % sodium hydroxide. The source of Si0₂ is 10 - 30 wt. % sodium silicate of mass concentration Si0₂ 220 g/L. 10 - 60 wt. % sulphuric acid is used as mineral acid for the precipitation of Si0₂ nanoparticles. The source of A1₂0₃ is 10-30 wt. % sodium aluminate with mass concentration of A1₂0₃ 280 g/L. 10 - 60 wt. % sulphuric acid is used as mineral acid for the precipitation of A1₂0₃ nanoparticles. By centrifuging the neutralised suspensions of coated Ti0₂ nanoparticles, the salts that formed during neutralisation with base and acid are removed.

Claims

1. Coating of rutile Ti0₂ nanoparticles in a suspension with Si0₂ and A1₂0_3; wherein the method starts from an acidic suspension of Ti0₂ nanoparticles in crystal structure, characterised in that

- the acidic suspension of Ti0₂ nanoparticles is transformed into a stable suspension with a basic pH with an addition of a suitable quantity of citric acid in a dissolved form,

- reaction with citric acid is carried out in a temperature range between 40 and 100 °C in a period of two hours or less under simultaneous stirring at 150 to 400 rev/min,

- the increase in the pH value from heavily acidic to pH 10.5 is achieved by an addition of a base,

- the coated Ti0₂ nanoparticles can be purified with centrifugation, wherein the centrifugation cycle can be repeated several times.

2. Coating as claimed in claim 1, characterised in that the suspension of Ti0₂ nanoparticles has a mass concentration of Ti0₂ between 100 and 300 g/L and has acidic pH value.

3. Coating as claimed in claim 1, characterised in that citric acid of 0.5 and 2 wt. % is used as stabiliser.

4. Coating as claimed in claim 1, characterised in that 10 - 60 wt. % sodium hydroxide is used as the base for the adjustment of the pH medium.

5. Coating as claimed in claim 1, characterised in that the Si0₂ source is 10 - 30 wt. % sodium silicate with a mass concentration of Si0₂ 220 g/L.

6. Coating as claimed in claim 1, characterised in that the mineral acid used for the precipitation of Si0₂ nanoparticles is 10 - 60 wt. % sulphuric acid.

7. Coating as claimed in claim 1, characterised in that the A1₂0₃ source is 10-30 wt. % sodium aluminate with a mass concentration of A1₂0₃ 280 g/L.

8. Coating as claimed in claim 1, characterised in that the mineral acid used for the precipitation of A1₂0₃ nanoparticles is 10 - 60 wt. % sulphuric acid.

9. Coating as claimed in claim 1, characterised in that by centrifuging the neutralised suspensions of coated Ti0₂ nanoparticles, the salts that formed during neutralisation with base and acid are removed.