The present invention relates to a tungsten oxide compound, use thereof for shielding solar radiation and a process for the preparation thereof.

Tungsten oxides and tungstates are known as infrared shielding material. US 2007/0187653 discloses an infrared shielding nanoparticle dispersion comprising tungsten trioxide having reduced oxygen. In US2007/0187653, the starting materials are heat treated at a temperature 100-850 'Ό in an atmosphere of a reducing gas, and subsequently heat treated at a temperature of 550-1200 'Ό in an atmosphere of an inert gas.

WO 2009/059900 discloses tungsten oxide of the formula W0 ₃ - _x wherein W is tungsten, O is oxygen, and x is 0.1 -1 and tungstate of the formula MxWyOz wherein M is one or more element selected from NH ₄ , H, Li, Na, K, Rb, Cs, Ca, Ba, Sr, Fe, Sn, Mo, Nb, Ta, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Al, Ga, In, Tl; W is tungsten, O is oxygen, 0.001 <x/y<1 , and 2.0<z/y<3.0. They are used to increase the heat-input amount of near infrared radiation in the process of NIR curing of coatings and NIR drying of coatings. According to this publication, the target of the invention was to find a white or colourless IR absorber which can be an alternative for carbon black. The publication mentions that the tungsten oxide material of this publication was slightly bluish to grayish.

Although the process for making a tungsten oxide material known from the prior art is satisfactory for some applications, there is still a need in the industry for a new process which is more simple and have a higher flexibility. Further, there is also a need for a new tungsten oxide compound having different physical properties from known tungsten oxide compounds.

It is an object of the present invention to provide a new process for preparing a tungsten oxide compound which overcomes the above and/or other problems. It is a further object of the present invention to provide a tungsten oxide compound having different physical properties from known tungsten oxide compounds.

Accordingly, the present invention provides a process for making a tungsten oxide compound comprising the steps of:

a) providing an aqueous suspension comprising a tungstate and a phosphate compound;

b) heating the solution in an autoclave at a temperature of at least 200 ^ to obtain a reaction mixture and

c) cooling the reaction mixture to ambient temperature.

It was surprisingly found that the tungsten oxide compound obtained by this process shows good NIR and IR absorption properties while absorbing only a small portion of the visible light to a small degree. The compound according to the invention was also found to be stable, i.e. the absorption properties were retained for a reasonably long period of time. Generally, the compound according to the invention was found to be have a dark blue color.

During step a), the relevant components are mixed preferably while stirring, in order to make a homogeneous sample.

The tungsten of the tungstate to be heated in step b) has an oxidation state of +6. During the hydrothermal process of step b), it is believed that the oxidation state of the tungsten is lowered to less than +6. The bluish color is believed to be an indication of the lower oxidation state.

The temperature of at least 200 °C was found to be essential for the reduction of the tungsten during the hydrothermal process. Preferably, the solution is heated in step b) at a temperature of between 250 'Ό and 350 °C, preferably around 300 'Ό . This temperature range was found to give a very effective reduction of the tungsten. The heating is performed preferably for 30 minutes to 48 hours, preferably 45 minutes to 24 hours, more preferably 60 minutes to 2 hours. The increase of the temperature from room temperature to the desired temperature is preferably gradual. The increase of the temperature may be performed over at least 30 minutes, preferably at least 60 minutes. For efficient processing, the period for the increase of the temperature preferably does not exceed 24 hours. The heating of the reaction is preferably carried out at a pressure between 0.5 and 25 MPa, or between 1 and 15 MPa.

Preferably, the tungstate is selected from ammonium tungstate, and ammonium paratungstate, ammonium metatungstate. The tungstate may also be sodium tungstate.

The solution provided in step a) comprises a phosphate compound.

The phosphate compound was found to increase the stability of the obtained tungsten oxide compound. Particularly preferred is ammonium hydrogen phosphate. Ammonium hydrogen phosphate helps in stabilizing the tungsten oxide compound obtained and assists in particle formation. Step c) is preferably performed within a relatively short period of time. Step c) is preferably performed within 8 hours, more preferably within 4 hours. Generally, step c) is preferably performed within 0.5-1 hour.

The solution provided in step a) may be in an acidic condition or an alkaline condition, but an alkaline condition was found to give a more stable product. The solution provided in step a) may have a pH of 2-12, preferably 4-9. Further bases may be used to induce an alkaline condition. Accordingly, in some embodiments of the present invention, the solution provided in step a) comprises a base such as NaOH or Sn(OH) ₄ .

The solution provided in step a) may further comprise a reducing agent. This was found to greatly increase the stability of the obtained tungsten oxide compound. Examples of suitable reducing agents include thio compounds, oxalic acid and SnCI ₂ for its high effect of increasing the stability of the obtained tungsten oxide compound. Particularly preferred is (NH4) ₂ S0 ₃ , K ₂ S ₂ 0 ₃ , Cs ₂ S ₂ 0 ₃ and Na ₂ S ₂ 0 ₃ .

The aqueous solution may further comprise an ammonium-containing compound. Examples include tetramethyl ammonium hydroxide (TMAH). TMAH helps in solubilizing the tungstate.

The tungstate in the solution provided in step a) may be reduced by a reducing gas such as N2 and H2. Surprisingly, however, it was found that step b) of the process according to the present invention can be performed under air. Compared to conventional processes where a reducing gas is used, the process can be performed in a simpler manner.

According to a further aspect of the present invention, a tungsten oxide compound obtainable by the process according to the present invention is provided.

According to an yet further aspect of the present invention, a tungsten oxide compound is provided having the following structure N _w M _x W _y O _z .L _v , wherein N is nitrogen, M is one or more elements selected from H, He, alkali metals, alkaline-earth metals, rare-earth metals, Mg, Zr, Cr, Mn, Fe, Ru, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Al, Ga, In, Ti, Si, Ge, Sn, Pb, Sb, B, F, P, S, Se, Br, Te, Ti, Nb, V, Mo, Ta, Re, W is tungsten, O is oxygen, L is selected from phosphate, sulfate and acetate, w is between 0.001 and 1 and satisfying 0.001 <=x/y<=1 , 2.0<z/y<=3.0 and v <=0.1 .

The tungsten oxide compound obtainable by the process according to the present invention has favorable absorption properties. The absorption property of the compound is measured by ultraviolet-visible spectroscopy. Herein, the absorption of the compound is measured for a testing sample which is an aqueous

suspension/dispersion/solution containing 0.13 wt% of the solid. The solid content of the testing sample was calculated based on the amount of the solid of the raw material and the water used for the preparation of the suspension of the aqueous tungsten oxide compound in step a). The sample obtained after step c) was diluted with water so that the calculated solid content would be 0.13 wt%, assuming that no evaporation took place during the reaction process and that the content of the solid did not change between before and after the process.

The absorption property of the compound is characterized by an absorption factor (AF) defined by the following formula:

AF = A850 (t=0 min) / A500 (t=0 min)

where A850 (t=0) denotes the absorption at 850 nm measured immediately after (within 1 minute) after the compound is prepared and

A500 (t=0 min) denotes the absorption at 500 nm measured immediately after the compound is prepared. Preferably, AF is at least 1 .2, more preferably at least 1 .3, more preferably at least 1 .5.

The stability of the absorption property of the compound is

characterized by an absorption stability factor (SF) defined by the following formula:

SF = A850 (t=1 day) / A850 (t=0 min)

where A850 (t=1 day) denotes the absorption at 850 nm measured 24 hours after the compound is prepared and

A850 (t=0) denotes the absorption at 850 nm measured immediately after the compound is prepared.

Preferably, SF is at least 0.5, more preferably at least 0.6, more preferably at least 0.7, more preferably at least 0.8, more preferably at least 0.9, even more preferably at least 0.95.

The overall absorption performance of the compound may be characterized by absorption performance (AP) defined by the following formula:

AP = A850 (t=0 min) ^* AF ^* SF

Preferably, AP is at least 10%, more preferably at least 30%, more preferably at least 50%, even more preferably at least 100%.

The absorption property may also be characterized by AF ^* SF.

Preferably, AF ^* SF is at least 0.4, more preferably at least 0.7, more preferably at least 1 .0, more preferably at least 1 .1 , more preferably at least 1 .2, more preferably at least 1 .3, more preferably at least 1 .4.

According to a further aspect, the present invention provides a near infrared and/or infrared light shielding film comprising the compound according to the present invention dispersed in a thermoplastic polymeric matrix. The type of thermoplastic polymeric matrix is not critical and may be of any known polymer.

According to a further aspect, the present invention provides use of the compound according to the present invention for shielding a near infrared and/or infrared light.

It is noted that the invention relates to all possible combinations of features described herein, particularly features recited in the claims.

The present invention will now be explained in further detail referring to the following non-limiting examples.

Following materials were used.

- ammonium para tungstate (ApT): (NH ₄ ) oW ₂ 0 ₄ i .5aq purchased from Inframat Advanced Materials, Manchester, US

- sodium thiosulphate (STS): Na ₂ S ₂ 0 ₃ .5aq purchased from Merck, Ludwigshafen, Germany

ammonium hydrogen phosphate (AHP): purchased from Sigma Aldrich Chemie GmbH, Germany

tetramethyl ammonium hydroxide (TMAH): purchased from Honeywell, US - Na ₃ P0 _4: purchased from Merck, Ludwigshafen, Germany

Example 1

2.45 gr ApT was suspended in 17.15 gr demineralized water (d-aq). In order to reduce the tungstate, 0.79 gr STS was added. After mixing, 0.14 gr AHP was added.

Part of the gained suspension was gradually heated to 300 'Ό in 75 minutes in an autoclave and kept at 300 'Ό for 70 minutes. The autoclave was quickly cooled down to room temperature in 10 minutes.

A dark blue dispersion was removed from the autoclave. The dispersion was very stable, which maintained its color for more than a week under air atmosphere.

Example 2

2.45 gr ApT was suspended in 17.15 gr d-aq. In order to reduce the tungstate, 0.79 gr STS was added. After mixing 0.14 gr AHP was added.

Part of the suspension was heated in 60 minutes to 300 °C in an autoclave bomb. After being heated at this temperature for 60 minutes the autoclave was slowly (overnight) cooled down to room temperature.

A nice dark blue dispersion was removed from the autoclave bomb. The gained product contained micronsized needles which sedimented quickly from the suspension upon standing.

Example 3

2.53 gr ApT was suspended in 16.02 gr d-aq. In order to reduce the tungstate, 0.80 gr STS was added. After mixing 0.15 gr AHP was added to result in a pH of 8. With 0.71 gr acetic acid the pH was lowered to 5,5.

Part of the suspension was heated in 69 minutes to 300 °C in an autoclave bomb. After being heated at this temperature for 62 minutes the autoclave was quickly cooled down to room temperature in 10 minutes.

A nice dark blue dispersion was removed from the autoclave bomb.

Example 4

2.99 gr ApT was suspended 1 .00 gr NaOH(15W%). After thorough mixing 18.02 gr d-aq was added. Then 0.17 gr AHP was added. The pH of the suspension was 8.

Part of the suspension was heated in 64 minutes to 300 °C in an autoclave bomb. After being heated at this temperature for 60 minutes the autoclave was quickly cooled down to room temperature in 10 minutes.

A nice dark blue dispersion was removed from the autoclave bomb.

Example 5

2,51 gr ApT was suspended in 15.12 gr d-aq. As a base 2.48 gr of 12.5% TMAH was added. In order to reduce the tungstate, 0.76 gr STS was added. After mixing, 0.15 gr AHP was added.

Part of the gained suspension was heated to 300 'Ό in 60 minutes in an autoclave and kept at 300 °C for 40 minutes. The autoclave was quickly cooled down to room temperature in 10 minutes.

A dark blue dispersion was removed from the autoclave. The dispersion was stable.

Example 6

2.51 gr ApT was suspended in 15.98 gr d-aq. In order to reduce the tungstate, 0.84 gr STS was added. After mixing 0.14 gr AHP was added to result in a pH of 8. With 2.00 gr acetic acid the pH was lowered to 3.55.

Part of the suspension was heated in 61 minutes to 300 °C in an autoclave bomb. After being heated at this temperature for 63 minutes the autoclave was quickly cooled down to room temperature in 10 minutes.

A nice dark blue dispersion was removed from the autoclave bomb.

Example 7

2.60 gr ApT was suspended in 18.35 gr d-aq. After mixing, 0.15 gr AHP was added.

Part of the gained suspension was heated to 300 'Ό in 65 minutes in an autoclave and kept at 300 °C for 60 minutes. The autoclave was quickly cooled down to room temperature in 10 minutes.

A dark blue dispersion was removed from the autoclave.

Example 8

2.50 gr ApT was suspended in 17.98 gr d-aq. After mixing 0.15 gr Na ₃ P0 ₄ was added.

Part of the suspension was heated in 65 minutes to 300 °C in an autoclave bomb. After being heated at this temperature for 65 minutes the autoclave was quickly cooled down to room temperature in 10 minutes.

A nice dark blue dispersion was removed from the autoclave bomb.

Example 9

2.96 gr ApT was suspended in 17.92 gr d-aq. After mixing, 0.17 gr AHP was added. In order to reduce the tungstate, the sample was purged with N ₂ gas and the autoclave was closed under N ₂ atmosphere.

Part of the gained suspension was heated to 300 'Ό in 73 minutes in an autoclave bomb and kept at 300 'Ό for 65 minutes. The autoclave was quickly cooled down to room temperature in 10 minutes.

A dark blue dispersion was removed from the autoclave.

Example 10

2.46 gr ApT was suspended in 15.10 gr d-aq. As a base 2.50 gr TMAH was added. After mixing, 0.16 gr AHP was added.

Part of the gained suspension was heated to 300 'Ό in 70 minutes in an autoclave and kept at 300 °C for 70 minutes. The autoclave was quickly cooled down to room temperature in 10 minutes.

A dark blue dispersion was removed from the autoclave.

The absorption at 850 nm and 500 nm were measured by Ultraviolet- visible spectroscopy (Pye Unicam UV-4).The absorption of the compound was measured for a testing sample which is an aqueous suspension/dispersion/solution containing 0.13 wt% of the solid. The solid content of the testing sample was calculated based on the amount of the solid of the raw material and the water used for the preparation of the suspension of the aqueous tungsten oxide compound in step a). The sample obtained after step c) was diluted with water so that the calculated solid content would be 0.13 wt%, assuming that no evaporation took place during the reaction process and that the content of the solid did not change before and after the process. Results are summarized in Table 1 . The conditions and the results of these

experiments are summarized in Table 2.

Table 1

AP = A850 (t=0 min) ^* (A850 (t=0 min) / A500 (t=0 min))

* (A850 (t=1 day) / A850 (t=0 min))

Table 2

Example AHP Na ₃ P0 ₄ STS PH NaOH TMAH Temp Remark AP

1 y y 8 300 127,2

2 Y y 8 300 Cooled 1 19,2 down

overnight

3 y y 5.5 300 109,9

4 y 8 y 300 108,6

5 y y 8 Y 300 101 ,2 6 y y 3.5 300 100,4

5

7 y 8 300 75,23

8 y 8 300 67,91

9 y 8 300 Reduced 44,62 with N ₂

10 y 8 Y 300 7,23

Further experiments have been performed as follows.

Experiment A

6.76 gr ApT was suspended in 15.77 gr d-aq. As a base 0.60 gr TMAH was added.

Part of the gained suspension was heated in an autoclave at 180 °C for one night. The autoclave was quickly cooled down to room temperature in 10 minutes.

A white precipitated material was removed from the autoclave.

Experiment B

2.85 gr ApT was suspended in 3.02 gr of an aqueous solution of NaOH (15wt%). After thorough mixing, 14.77 g of d-aq was added.

Part of the gained suspension was gradually heated in an autoclave to 250 'Ό in 2 hours and kept at 250 'Ό for one night. The autoclave was quickly cooled down to room temperature in 10 minutes.

A slightly bluish suspension was removed from the autoclave. The gained product contained micron-sized needles which sedimented quickly from the suspension upon standing.

Experiment C

2.99 gr ApT was suspended in 0.99 gr NaOH (15wt%). After thorough mixing 18.01 gr d-aq was added.

Subsequently part of the gained suspension was gradually heated to

300 'Ό in 1 hr in an autoclave bomb. After being heated at this temperature for 65 minutes the autoclave was quickly cooled down to room temperature in 10 minutes.

A grey product with big needles was removed from the autoclave.

Experiment D

3.98 gr ApT was suspended in 24.96 gr d-aq. In order to reduce the tungstate, 1 .22 gr STS was added.

Part of the suspension was heated in 1 hr to 300 'Ό in an autoclave bomb. After being heated at this temperature for 61 minutes the autoclave was quickly cooled down to room temperature in 10 minutes.

A blue/green dispersion was removed from the autoclave bomb. The gained product contained micronsized needles which sedimented quickly from the suspension upon standing

Experiment E

2.78 gr ApT was dissolved in 6.88 gr of an aqueous solution of TMAH (12.5w%).

Part of the solution was gradually heated to 300 °C in 90 minutes in an autoclave and kept at 300 °C for 1 hour. The autoclave was quickly cooled down to room temperature in 10 minutes.

A dark blue dispersion was removed from the autoclave .

Experiment F

2.52 gr Apt was suspended in 17.99 gr d-aq.

Part of the solution was heated in 69 minutes to 300 °C in an autoclave bomb. After being heated at this temperature for 61 minutes the autoclave was quickly cooled down to room temperature in 10 minutes.

A grey blue dispersion was removed from the autoclave bomb.

Experiment G

1 .76 gr ApT was suspended in a mixture of 1 1 gr d-aq and 1 .2 g of acetic acid glacial (purchased from Panreac).

Part of the gained dispersion was gradually heated in an autoclave to 300 'Ό in 100 minutes and kept at 300 °C for 16 minutes. The autoclave was quickly cooled down to room temperature in 10 minutes.

A turbid greyish dispersion was removed from the autoclave .

Experiment H

61 .40 gr 12.5% TMAH and 30.04 gr ApT were subsequently added to 61 .82 gr Sn(OH) ₄ (Kriya Materials, Netherlands). After mixing thoroughly 90.37 gr d-aq was added to the mixture.

Part of the gained suspension was gradually heated to 300 'Ό in 75 minutes in an autoclave and kept at 300 'Ό for 2 hours. The autoclave was quickly cooled down to room temperature in 10 minutes.

A blue dispersion was obtained, which was rather instable. The conditions and results of these experiments are summarized in Table 3. The absorption properties were poor in all examples A-H.

Table 3

Experiments l-L

Experiments have been performed with tungstates in combination with lanthanide oxides and transition metal oxides. All experiments result in dark blue dispersions.

Example I

2.72 ammoniumparatungstate (ApT purchased from Inframat Adv. Materials) was dispersed in 20.96 gr demineralized water. Next, 0.15 gr

ammoniumhydrogenphosphate (purchased from Sigma Aldrich) and 0.59 gr Ce0 ₂ (from Philips NV, Netherlands) was added. After mixing thoroughly 3.01 gr was heated in a closed vessel. The temperature during heating was gradually increased in 80 min to 300 'Ό and maintained at this temperature for an extra 100 min. After cooling down the mixture a deep blue colored dispersion was removed from the vessel. Example J

3.05 ammoniumparatungstate was dispersed in 30.01 gr

demineralized water. Next, 0.18 gr ammoniumhydrogenphosphate and 1 .31 gr Eu ₂ 0 ₃ (from Philips, Netherlands) was added. After mixing thoroughly 2.71 gr was heated in a closed vessel. The temperature during heating was gradually increased in 60 min to 300 'Ό and maintained at this temperature for an extra 3:47 hr. After cooling down the mixture a greyish dispersion with needle shaped particles was removed from the vessel. Example K

2.81 ammoniumparatungstate was dispersed in 14.97 gr

demineralized water. Next, 0.56 gr ammoniumhydrogenphosphate and 5.49 gr Sn(OH) ₄ dispersion (from Philips, Netherlands, 12w%) was added. After mixing thoroughly 2.82 gr was heated in a closed vessel. The temperature during heating was gradually increased in 1 hr to 300 'Ό and maintained at this temperature for an extra 1 :14 hr. After cooling down the mixture a dark blue dispersion with some coarse particles was removed from the vessel.

Example L

2.44 ammoniumparatungstate was dispersed in 24.25 gr

demineralized water. Next, 0.15 gr ammoniumhydrogenphosphate and 0.62 gr Ce0 ₂ (from Philips NV, Netherlands) and 0.12 gr Sn (purchased from Merck) was added. After mixing thoroughly 2.82 gr was heated in a closed vessel. The temperature during heating was gradually increased in 1 hr to 300 'Ό and maintained at this temperature for an extra 1 :14 hr. After cooling down the mixture a dark blue dispersion with some coarse particles was removed from the vessel.