PHOTOCHROMIC MATERIALS

The present invention relates to photochromic materials and more particularly to photochromic clays, pigments, powder plastic (e.g. finely divided CR39 glass or polyester glass) in which a photochromic compound is incorporated in the clay, pigment, plastic powder or other material e.g. silica gel.

It is an object of the present invention to provide a photochromic clay, pigment, powder plastic etc which may be incorporated into such items as paper to produce a photochromic paper. Care must be taken that in any processing stages following the incorporation of the photochromic compound the temperatures are kept to within defined limits which do not destroy the photochromic properties of the original compound.

Accordingly the present invention provides a photochromic material for incorporation into an end product including a finely divided substance in which a photochromic compound is added to the finely divided substance prior to incorporation of the photochromic material into the end product.

Photochromic compound may for the purpose of the present invention include one or more compounds which are combined to produce a desired effect. An end

product may be a photochromic paper wherein the photochromic material is introduced either as a bulk material during manufacture of the paper or as a surface coating subsequent to manufacture of the paper. In a preferred embodiment the finely divided substance into which the photochromic compound is added is a clay. The clays exemplified in the subsequent description are usually white or near white fine powders. When photochromic compounds are mixed with clays they are surprisingly found to exhibit a colour change in that the frequency of maximum absorbance moves towards the red end of the spectrum and therefore the photochromic clays appear more blue than the photochromic material on filter paper or in solution. The invention therefore also provides a photochromic clay including a clay into whi-:h a photochromic compound has been mixed, in which the frequency of maximum absorbance for such a photochromic clay has been shifted towards the red end of the spectrum.

Such photochromic clays can be used as surface coatings on paper and the shift in frequency response may enable LED's to be used to change the colour of the photochromic material from one state to another thereby rendering possible the use of photochromic materials

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for photocopying and/or data storage. Such photochromic clays because they are white or pale yellow in their uncoloured state may also be incorporated into a plastic medium. The present invention will now be described, by way of example with reference to a number of examples of incorporation of photochromic materials into various clays, pigments and powder plastic.

In order to study the effect of different clays, pigments or powder plastic on the absorption maxima and photochromic stability of fulgides, a variety of clays pigments and powder plastics were boiled with a fulgide in a solvent such as refluxing toluene. The sample was boiled for 30 minutes to 3 hours. The clay, pigment or powder plastic samples were either dry powder or thin films. The Photochromic Compounds A to E are shown in the accompanying Figures 1 to 5. A Bentonite

0.5 g of A in toluene was heated for 1.5 hours with a thin flake of a bentonite and samples of the flake removed at 30 minute intervals washed with toluene and air dried then irradiated. Samples Effect of uv. Effect of white light

0 (0 mins) No colour change 1 (30 ins) Gives blue colour Regenerates original clay colour, Recolours with uv.

2 (60 mins) " "

3 (90 mins) " » ^EJTEX;

The period of time used for heating does not appear to effect the colour attained or the effects of white light reversal.

Texas ont. 0.5 g of A in toluene was heated for 1 hour with

1 g of Texas mont. The sample was filtered and washed with cold toluene, then air dried. On irradiation at

366nm, the sample, on filter paper, changed to red/blue.

On warming to further dry the clay, the colour changed to deep blue. Irradiation with white light caused some colouring of the previously uncoloured area but only slightly affected the coloured area after 30 minutes.

Fluoro Laponite

0.5 g of A was heated in toluene with 1 g of Fluoro Laponite. The product after 1 hour was filtered and washed to yield a yellow clay. Irradiation at

366nm gave a red/blue colour which, on warming the coloured clay to further dry it, became more blue.

Irradiation with white light gave the original clay colour after approximately 5 minutes. The sample could be recoloured.

Jap. Mont. Nwt 20

0,5 g A was heated in toluene with 1 g of Jap.Mont

Nwt 20 for 1 hour. The product was filtered, washed with toluene and air dried. Irradiation at 366nm

caused a change similar to that for F.Laponite. Irradiation with white light gave reversal after about 7 minutes. The sample could be recoloured. Jap Mont Nwt 19 0.5 g of A was heated in toluene with 1 g of Jap Mont Nwt 19 for 1 hour. The product was filtered washed with toluene and air dried. Irradiation at 366nm yielded a red/blue colour which rapidly became blue on heating the clay to further dry it. Irradiation with white light caused colouring of the previously uncoloured section and only slight reversal of the coloured section after 30 minutes.

The four samples, when compared visually together, yielded the following results:- on initial irradiation, Nwt 20 and Fluoro Laponite gave red/blue colours which became bluer on warming but still exhibited a large red component. Both colours reversed under white light F.Laponite being the faster. Nwt 19 and T.Mont both gave the red/blue colour on irradiation at 366nm but on heating became completely blue. irradiation with white light caused only a small change over 30 - 60 minutes. Saponite

0.5 g of A was heated in toluene with Ig Saponite for 30 minutes. The product was filtered, washed with toluene and air dried. Irradiation at 366nm caused a

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purple colour which was reversed on exposure to white light.

Laponite CP

0.5 g of A was heated in toluene with lg Laponite CP for 30 minutes. The sample was filtered, washed with toluene and dried. Irradiation at 366nm caused a purple colour which was reversed on exposure to white light.

Beidellite 0.5 g of A was heated in toluene with lg

Beidellite for 30 minutes. The sample was filtered, washed with toluene and air dried. Irradiation at

366nm gave a red colour which became blue when the irradiated clay was further dried by heating. The colour was reversed on exposure to white light for 1 hour.

Kaolin

0.5 g of A was heated in toluene with lg Kaolin for 30 minutes. The sample was filtered, washed with toluene and air dried. Irradiation at 366nm caused a red colour which showed a colour change towards the blue, when the irradiated clay was further dried by heating.

Barium titanite BaTiO 3 0.5 g of A was heated in toluene with 2g barium

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titanate for 30 minutes. The sample was filtered, washed with toluene, and air dried. Irradiation of the solid at 366nm gave a pink colour which was rapidly reversed on exposure to white light. Zirconium oxide ZrO _?

0.5 g of A was heated in toluene with 2 g zirconium oxide for 30 minutes. The sample was filtered, washed with toluene and air dried. Irradiation at 366nm of the solid gave a red colour which was reversed by white light, but more slowly than in the above example.

Photochromic material compound B as shown in Figure 2 will now be considered.

1. A Bentonite Compound B was heated in toluene with a flake of

Bentonite as used with compound A. After 2 hours the sample showed no colour change on exposure to uv radiation.

2. H & W Bentonite Compound B in toluene was heated with a flake of H & W Bentonite for 2 hours. The yellow flake, on irradiation give a blue green colour. This colour rapidly reversed with white light.

3. Hectorite 34 Compound B in toluene was heated with a flake of

Hectorite 34 for 2 hours. Irradiation gave results similar to those for H & W Bentonite.

4. Al. G.M.

Compound B in toluene was heated with a thick flake of Al G.M. for 2 hours. Irradiation gave results similar to those for H & W Bentonite.

The above samples 2,3 were thin flakes and yellow in colour where as 4 was thicker and dark grey but all samples gave the same colours on irradiation which may suggest that the green colour is not due to a blue photochrome over a yellow background. All samples when coloured remained so when kept in the dark for 2 days.

Compound B was heated in toluene with 1 g of T.Mont.

After 1 hour, the sample was filtered, washed with toluene and air dried. Irradiation with 366nm gave a green colour which rapidly reversed with white light and recoloured with uv light. Warming had no effect. The result is unusual considering the result with compound A. The fact that warming did not affect the colour of the sample after irradiation is possibly further evidence for the colour being other than blue over a yellow background.

Jap. Mont. Nwt 19

0.5 g of C in toluene was heated with lg of Jap. mont Nwt 19 for 30 minutes. The sample was filtered.

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washed with toluene and air dried. On irradiation at 366nm, the sample changed to blue. On drying the irradiated sample by warming, the colour changed to green. On prolonged exposure to white light, the green colour faded very slightly. Texas mont.

0.5 g of C in toluene was heated with lg of Texas mont for 30 minutes. The sample was filtered, washed with toluene and air dried. On irradiation at 366nm, the sample changed to blue. On drying the irradiated sample by warming, the colour changed to green. On prolonged exposure to white light, the green colour faded slightly. Texas mont. 0.5 g of D in toluene was heated with lg Texas mont. for 30 minutes. The sample was filtered, washed with toluene and air dried. On irradiation at 366nm, the clay showed a pale green colour. Reflectance spectroscopy indicated a strong absorption above 700nm with a maximum at ca. 750nm. Texas mont.

0.5 g of E in toluene was heated with lg Texas mont. for 30 minutes. The sample was filtered, washed with toluene, and air dried. On irradiation at 366nm, the colour changed to yellow. The colour was reversed

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on exposure of the coloured clay to white light. Silica gel Siθ2 (60-120 mesh B.D.H. Chemicals).

0.5 g of A was heated in toluene with 3 g silica gel for 30 minutes. The sample was filtered, washed with toluene, and air dried. Irradiation at 366nm caused silica gel to turn deep red. The colour was reversed on exposure to white light. Alumina Al ₂ 0 ₃ (type UG 200 mesh, Koch-Light).

0.5 g of A was heated in toluene with 3 g alumina for 30 minutes. The sample was filtered, washed with toluene, and air dried. Irradiation at 366nm caused the alumina to slowly turn orange. The colour was reversed on exposure to white light. Smectite clays Clay Ideal formula montmorilloniteζbentonite) ⁽ Al ₃ 5^0^. ^si ₈ ⁰ 20 ^{(OH }} 4 ^Na o.5 ^(H hectorite (+ laponite) (Mg _{5 5} i _Q>5 )Sig0 ₂₀ (OH,F)Li _{0 5} ⁽ H

beidellite ^A1 4 ^(si 7.5 ^Al 0.5 ^{) (OH)} 4 ^Ca 0.25 ^(H 2 ⁰⁾ saponite (V.gum) Mg ₆ ⁽ Si _{? 5} Al _{Q 5} ⁾⁽ 04 ₄ ⁾ ₄ ^Ca o.25 ^(H 2 ^o) ver iculite Mg ₆ (Si _? Al) (OH) ₄ Mg _Q _ ₅ (H ₂ 0) ₃ kaolinite Al ₄ Si ₄ 0 _1Q (OH) ₄ _

Texas Mont - Texas Montmorillonite 'cobbed hard helms' RL02064 Ex 1SB 816181. _ Fluoro Laponite - Laponite B. (synthetic Fluorohectorite. Jap Mont. - Japanese montmorillonite coded NWT 19, 20 Al G.M. — Aluminium exchanged clay. More acidic.

The following materials were found not to be suitable for mixing with a photochromic compound. Titanium Oxide Tiθ2 (P25 Degussa)

0.5 g of A was heated in toluene with 2g Ti0 ₂ - for 3 minutes. The sample was filtered, washed and air dried. The resulting powder was not photochromic and did not show a colour change on irradiation at 366nm. Amatase Ti0 ₂ (Laporte Tionag.)

0.5 g of A. Not photochromic, preparation of sample as above.

Fumed Silica, (Siθ2-, Fumed Titania (Ti0 ₂ ), Fumed Alumina (AI2O3) and Talc (CaC03) were also found not to be suitable even though a number of methods of introducing the photochromic were tried. It should be noted that all materials are white or near white prior to incorporation and that materials prior to irradiation are white. Incorporated materials prior to irradiation are white, near white or 'pale yellow. Colour changes are indicated. Photochromic materials could be recoloured.

The advantage of producing a photochromic material in which a finely divided substance is mixed with a photochromic compound is that the material may be added to for example the surface layer of a paper in predetermined quantities to produce a desired tint.

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The addition of two or more photochromic materials in various chosen quantities can produce a wide range of colours. By incorporating two or more of the photochromic materials into the surface layer of for example paper a finely spread mixture may be obtained which is evenly distributed on the surface. Each photochromic compound therefore receives the same illumination. This is in contrast to other incorporation methods such as spraying or imbibation where one photochromic material may be positioned on top of another and may be a change of colour to a more absorbent form affect the performance of the second photochromic compound.

In addition when the photochromic compound or compounds are incorporated into clays two unexpected effects are produced. Firstly as explained hereinbefore a shift in the wavelength of maximum absorbency is produced. This therefore extends the range of colours available. Secondly the absorption of a photochromic compound into a clay is observed to slow down the bleaching response of the compound. This feature can be used for copying applications when the photochromic clay is used as a surface layer on a paper or in a plastics material. Optical printing and laser beam writing become possible because of the wavelength

shift into the longer wavelengths of light emitting diodes (LED's). As an alternative the photochromic clay may be used in ink as a filler and the ink printed on a surface such that the printing is photochromic. It may be seen therefore that the introduction of photochromic compounds into clay produces two distinct effects. Firstly a colour change towards the blue is observed when the colour is compared with that of the photochromic in solution or in other vehicles. Secondly the fade rate of the photochromic from its coloured form is considerably reduced. This latter effect can enable paper with the photochromic clay used as a surface layer to be used for photocopying or non contact printing. The colour changes observed when the photochromic material was introduced into the clay were large. Changes in wavelength of maximum absorbance of over 70 nanometres can be achieved.

In addition to the photochromic compounds A to E other photochromic compounds may be used such as those described in British Patents No's 1,464,603 and 2002752 and U.S. Patent No 4,220,708.

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