GB2242993A

GB2242993A - Imaging process

Info

Publication number: GB2242993A
Application number: GB9003014A
Authority: GB
Inventors: Colin Peter Sambrook-Smith; Michael Hawkins
Original assignee: Courtaulds PLC
Current assignee: Akzo Nobel UK PLC
Priority date: 1990-02-09
Filing date: 1990-02-09
Publication date: 1991-10-16
Also published as: WO1991012139A1; AU7337591A; GB9003014D0

Abstract

A process for imaging a film containing a reversible organic photochromic compound comprises exposing the film to UV light in one or more selected areas followed by treating it with a chemical reagent which reacts irreversibly with only the relatively colourless form of the photochromic compound. An alternative process comprises exposing the film to UV light and the chemical reagent followed by exposing it to high-frequency light in one or more selected areas to cause irreversible reaction. Preferably the photochromic compound contains a cyclic carboxylic acid anhydride ring and the reagent is ammonia or an amine. The imaged films are useful as security markings.

Description

IMAGING PROCESS This invention relates to the formation cf an image using a photochromic compound. The image formed can be used as an identifying mark, particularly a security marking, for goods, packages, identification cards or other documents.

A photochromic compound is a compound that undergoes a colour change when irradiated with light of â certain wavelength, which colour change may be reversible or irreversible. In general, the compounds are coloured when irradiated with UV light and convert to a pale or colourless form in visible light. Examples of reversible photcchromc compounds are fulgdes and 3,4-diarylfuran-2,5-diones Photochromic compounds, particularly those which are colourless under white light, can be used for marking. The marking can be illuminated by UV light and an image previously invisible under white light can then b seen.

Unlike a fluorescent image, the image remains after e > .posure to UV has ceased, so that anyone observed3 the image need not be exposed to UV. A photochromic image can, for example, be printed on a substrate using an ink containing the photochromic compound.

European Patent Application 279600 describes a marking comprising a layer, preferably of film-forming material, which contains a photochromic compound. The photochromic compound is capable of changing colour when exposed to UV light, but can be converted to a permanently non-photcchromic compound, preferably by over-exposure to UV light.

An image is formed in the layer by converting the photochromic compound to a permanently non- photochromic compound in one or more selected areas. When the layer is subsequently viewed under UV light a colourless image of non-photochromic compound can be seen on a background of coloured photochromic compound. Over-exposure in selected areas is preferably achieved by using a UV laser. Alternatively, over-exposure can be achieved by prolonged exposure to light from a UV lamp through a mask.

It is an object of the present invention to provide an imaging process which does not require the use cf a UV laser and is more rapid than the process of European Patent Application 279600 which uses prolonged exposure to light from a UV lamp through a mask.

U.S. Patent 3441411 describes a photographic method comprising exposing an imaging layer comprising an organic photochromic material to actinic electroma3netic radiation in image configuration of sufficient energy to convert at least a portion of said material from one photochromic state to another, contacting said photcchromic material with a reagent which is reactive with only one form of said photochromic material, and supplying sufficient heat energy to said photochromic material to cause a reaction between said reagent and one form of said photschromic material thereby forming a permanent differentially ascertainable reaction product in said layer.Although U.S. Patent 3441411 lists a wide variety of photochromi compounds,the only reagent which is described is an acid anhydride, which will react with photochromic compounds such as spiropyrans which have phenol functionality in the coloured photochromic form. The chemically reacted material is visible as a yellow image.

The method of imaging disclosed in U.S. 3441411 has the disadvantage that the chemical reagent used to develop the image remains in the article after imaging. The undeveloped area of the image is therefore prone to undergo the development reaction whenever the article is exposed to electromagnetic radiation to reveal the image, so reducing the contrast between foreground and background. It is an object of the present invention to provide a means of avoiding such a disadvantage.

According to a first aspect of the present inventicn.

a process of imaging a film containing a reversible photochromic compound which can be converted by UV light from G relatively colourless form to a coloured form, in which process the film is exposed to UV light in one or more selected areas to convert the photochromic compound at least partially to its coloured form in those areas, the photochromic compound in the remaining areas being in the relatively colourless form, is characterized in that the film so exposed is treated with a chemical reagent which reacts with the relatively colourless form of the photochromic compound to form a relatively colourless substantially permanently non-photochromic compound or a precurscr thereof but does not react with, or reacts reversibly with, the coloured form of the photochromic compound.

When the process is used for forming security markings, any reversible reaction product between the chemical reagent and the coloured form of the photcchromic compound in the chemically treated film is allowed to revert to the coloured form of the photochromic compound in the said selected areas. The film is then exposed to visible light to convert the coloured form of the photochromic compound into the relatively colourless form of the photcchromic compound, thereby forming a relatively colourless film which has no apparent image but which has a latent image of relatively colourless photochromic compound against a background of relatively colourless substantially permanently non-photochromic compound, which image can be seen clearly after exposure to UV light.

An alternative process according to the invention of imaging a film containing a reversible photochromic compound which can be converted by UV light from a relatively colourless form to a coloured form is characterized in that the film containing the photochromic compound in its coloured form is treated with a chemical reagent which reacts with the photochromic compound to form a temporarlly non-photochromic compound, which reaction is reversible for the coloured form of the photochromic compound, and the film is then exposed to high frequencY liqht in one or -

- more selected areas, + tne temporari iy non-pnoWocnromic compound is rapidly converted to a relatively colourless substantially permanently non-photochromic compound n the selected areas but remains re-convertable into the photochromic compound in areas which are not exposed to the high frequency light. By "high frequency light" we mean light of frequency within the visible or UV range and above a minimum value depending on the particular photochromic compound.

To form a film suitable for security marking, after exposure of the film to high frequency light the film is kept in conditions whereby the temporarily non-photochromic compound is allowed to revert to the coloured form of the photochromic compound in the non-exposed areas. The film is subsequently exposed to white light to ccnvert the coloured form of the photochromic compound into the relatively colourless form of the photochromic compound, thereby forming a relatively colourless film which has no apparent image but which has a latent image of relatively colourless substantially permanently non-photochromic compound against a background of relatively colourless photochromic compound ,which image can be seen clearly after exposure to UV light.

It is an advantage of the present invention as compared with the procedure disclosed in US Patent 3441411 that the chemical reagent used to develop the image is removed from the article once the image has been developed in it, thus allowing repeated or extended examination of the developed latent image without chemical degrEdation of this type. It is a further advantage of the present invention that it is not required, as it 5 in the invention disclosed in US 3441411, that the chemical reagent used for development of the image react with only one of the forms of the photochromic compounds.

The photochromic compound preferably contains a cyclic carboxylic acid anhydride group in both its coloured and relatively colourless photochromic forms. Examples of such compounds are fulgides and 3,4-diarylfuran-2,5-diones. The chemical reagent used to treat the photochromic film is preferably a reagent which will effect ring opening of the anhydride ring, for example ammonia or an amine, especially a primary or secondary amine.

The chemical and photochemical reactions involved in a preferred imaging process are summarised diagrammatically below:

where A represents the photochromic compound in its relatively colourless form; B represents the photochromic compound in its coloured form; R represents the chemical reagent used to treat the film; AR represents the reaction product of A with R; BR represents the reaction product of B with R; hv represents visible light; UV represents ultraviolet light; hv' represents high frequency light, that is light whose frequency is above a minimum value which depends on the particular photochromic system A/B; and X represents a physical process, for example heating or evaporation, by which R is removed.

The first described process according to the invention, together with the subsequent steps necessary to produce a substantially colourless security marked film, comprises the reaction scheme:

The alternative process according to the invention, together with the subsequent steps necessary to produce a substantially colourless security marked film comprises the reaction scheme:

The preferred photochromic compounds are fulgides, as described for example in UK Patents 1,442,628 and 1,464,603, published UK Patent Application 2,170,202A and European Patent Application 279600 and in a pape by A.

Kaneko et al in Bull. Chem. Soc. Japan 61, pages 3F69-'-573 (1988). The photochromic fulgides generally have the formula (in the relatively colourless form)

in which at least one of the substituents R1, R2, R3, and R4 is an aromatic group (which term includes heterocyclic aromatic groups), the other substituents being hydrogen or monovalent hydrocarbon groups, which can be substituted, provided that at least one of Rl and R2 and at least one of R3 and R4 is other than hydrogen. Preferably all the substituents are other than hydrogen. The fulgides derive their photochromic characteristics from their ability to undergo reversible ring closure.For example, where R is the aromatic group, ring closure occurs between R and the carbon atom to which RS and R4 are attached. The fulgides in the coloured form thus generally have the formula

where A is a partially unsaturated ring derived from the aromatic group R2 and R7 is hydrogen or a monovalent group present as a substituent in the aromatic group R.

Examples of preferred photochromic fulgides are those of formula (I) in which R1, R3 and R4 are all CH3 and R is an alkyl-substituted 3-furyl or 3-thienyl or 3-pyrryl group. Particularly preferred fulgides are those described in European Patent Application 279600 at page 4, lines 3 to 23 and the fulgides identified as 6, 7 and 9 in the above paper by Kaneko et al.

An alternative type of photochromic compound containing an anhydride ring is a 3,4-diarylfuran-2,5-dione having the formula (in its relatively colourless form)

where R8 and R9, which may be the same or differentare each aromatic groups and are preferably heterocyclic groups having an aromatic character. This type of compound has the formula

in its coloured form, where A' and A'' are partially unsaturated rings derived from R8 and R9 respectively. The groups R8 and R9 are preferably each substituted by at least one alkyl, for example methyl, group, particularly in the ring-closing position (that is, the position adjacent to that at which the group R8 or R9 is attached to the anhydride ring).Examples of preferred compounds of this type have the formula:

where X represents an oxygen or sulphur atom or an NR10 group, where R10 represents a monovalent hydrocarDon group such as C14 alkyl or phenyl. The thienyl compound where X is sulphur is particularly preferred. Such compounds in the coloured form have the formula:

The film used in the invention comprises a layer of film-forming material containing a photochromic compound, the film-forming material being substantially transparent to UV and visible light at the wavelengths that activate the photochromic compound. The photochromic compound is preferably incorporated in the film-forming material by dissolving or dispersing it in a solution of a film-forming polymer transparent to UV light of wavelength above oOOnm.

The most preferred film-forming polymer is cellulose acetate. Alternatives are other cellulose esters, polyesters, for example polyethylene terephthalate, acrylic polymers, for example polymethyl methacrylate, polyurethanes, olefin polymers, for example polyethylene or polypropylene or ethylene-vinyl acetate copolymers, vinyl polymers, for example polyvinyl acetate or polyvinyl chloride, polycarbonates and polyamides. The photochromic compound is preferably dissolved in the solution so that it is more uniformly dispersed in the film formed. The photochromic fulgides for instance are soluble in a wide range of organic solvents, for example ketones such as acetone or methyl ethyl ketone, esters such as ethyl acetate, aromatic hydrocarbons such as toluene, chlorinated hydrocarbons such as chloroform or methylene chloride, or ethers.They are not very soluble in water or aliphatic hydrocarbons and are reactive to some extent with lower alcohols such as methanol and ethanol. The soluticn can be cast or coated on a substrate to form a film. The photochromic fulgides for example can readily be incorporated in cellulose acetate film cast from acetone solution. The concentration of the photochromic compound is generally 0.03 to 10% by weight based on the film-forming material, preferably 0.1 to 5%, and most preferably 0.2 to 2%. The film is preferably colourless apart from the photochromic compound but alternatively can be lightly pigmented or dyed with a pigment or dye which is not degraded in UV light.

When the film-forming material is a melt-extruded polymer, for example polyethylene, polypropylene or copolymers thereof, or an ethylene-vinyl acetate copolymer, the photochromic compound can be dispersed in the polymer melt prior to extrusion, but care must be tahen not to thermally damage the photochromic compound during extrusion. Useful photochromic compounds in this instance generally are stable to temperatures up to 10000 or even 18000.

As an alternative method of incorporating the pcto- chromic compound, a film which is substantially transparent to UV and visible light at the wavelengths that activate the photochromic compound may be 'dyed' with a solution of the photochromic compound. Any of the above-mentioned film-forming materials may be used to form the film, although this dyeing method is particularly suitable for materials into which the photochromic compound cannot be readily incorporated because, for example, it is insoluble in the casting solvent or the extrusion temperature would damage the compound. Examples of such materials are certain polyesters and regenerated cellulosics. This photochromic dyeing can be achieved by immersing the film in a dye bath containing the photochromic compound dissolved in a solvent which is a non-solvent for the film.

The rate of dye uptake can, in general, be increased by increasing the temperature of the dye bath, especially by increasing it to a temperature above the glass transition point (but below the melting point) cf the film. In addition,the rate may be increased by including in the dye bath a plasticiser which swells the film.

The chemical reagent used to effect ring opening of the cyclic carboxylic acid anhydride ring of the photochromic molecule is preferably ammonia or a primary or secondary amine, for example of the formula HNR5R6,where R5 is hydrogen, alkyl of 1 to 4 carbon atoms, hydroxyalkyl of 1 to 4 carbon atoms or aminoalkyl of 1 to 4 carbon atoms and R6 is hydrogen, alkyl of 1 to 6 carbon atoms, hydroxyalkyl of 1 to 6 carbon atoms, aminoalkyl of 1 to 6 carbon atoms, aryl of 6 to 10 carbon atoms, aralkyl of 7 to 10 carbon atoms or a heterocyclic group. Ammonia is particularly preferred. Examples of amines which can be used are methylamine, ethylamine, isopropylamine, dimethylamine, diethylamine, ethylenediamine, ethanolamine, N-methylethanolamine or 2-(2-aminoethylamino)ethanol.

In the first process of the invention the photochromic compound is first exposed to UV light in one or more selected areas to convert the photochromic compound at least partially to its coloured form in those areas.

Colouration in selected areas can be achieved by exposure to a UV lamp through a mask, for example a 100 to 125 watt medium-pressure arc lamp. The colour characteristic of UV radiation is generally apparent in the selected areas in a second or two and typically reaches maximum intensity in 60 to 100 seconds. The time of UV exposure through the mask is most preferably 5 to 60 seconds.

The film so exposed is then treated with the chemical reagent. The film is preferably immersed in a bath of the reagent, which may be a solution of the reagent or undiluted liquid reagent. For a gaseous reagent such as ammonia a chamber containing the gaseous reagent can alternatively be used. Any solvent used should preferably be inert to the photochromic compound and should not dissolve or substantially swell the film so that the film is intact and undistorted after the chemical treatment.

Water is a preferred solvent for use with most film materials; for example ammonia and amines can be applied from aqueous solution. Water is a particularly good solvent for use with cellulose acetate film since cellulose acetate is permeable to water but is not dissolved or swollen by water. Ammonia can for example be applied from the commercial solution sold as "0.880 ammonia" or from a more dilute solution, for example having a concentration of above 10% by weight. Amines are preferably applied from solutions of concentration 10 to 70 > 0 by weight. The time of treatment is preferably in the range 1 second to 30 minutes; when treating a film containing a fulgide with ammonia or an amine times of 20 seconds to 20 minutes are generally used.

The reaction product of the photochromic compound containing an anhydride ring and ammonia or an amine is an amic acid:

For example, the coloured form of the fulgide is believed to react to form a mixture of the isomers:

This reaction product generally has a yellow colcur.

The reaction is reversible and the characteristic colour of the coloured form of the fulgide used will gradually reappear on standing, particularly under conditions conducive to the removal of the ammonia or amine. Preferably the film is subjected to conditions under which the ammonia or amine is evaporated from the film. The film can for example be heated at a temperature which is below the glass transition temperature of the film-forming material. The film can be heated by contact with a heated roller or in an oven.In general, more than 90%, for example 95%,of the colour of the photochromic compound before chemical treatment is regained after 5 minutes at 500 C. If ammonia or a gaseous or highly volatile amine such as methylamine or ethylamine is used, application of heat to the film is not necessary, although it is generally preferred to apply heat to give a shorter treatment time. When ammonia is used heating times of for example 1 to 5 minutes can be used.

The relatively colourless form of the fulgide is also believed to react with the ammonia or amine to form a mixture of isomers:

This amic acid reaction product is also relatively colourless. Unlike the amic acid derived from the coloured form of the photochromic fulgide, it will not undergo the reverse reaction to reform the anhydride ring of the fulgide to any significant extent. It is a relatively colourless substantially permanently non-photochromic compound. It may re-form the anhydride ring to some extent after a period of more than 3 months, but this is generally not significant in practical use as a security marking.

The chemically treated film is preferably kept in the dark during the re-formation of the coloured photochromic compound, since light may cause degradation of some of the amic acid before it has reverted to the photochromic anhydride compound. Light of too great a frequency will cause ring opening of the amic acids (VII) and (VIII) to amic acids (IX) and (X) corresponding to the relatively colourless form of the photochromic compound, that is to the same compounds as are present in the background areas of the film. The critical frequency for this is different for amic acids derived from different photochromic compounds.The amic acid derived from the coloured form of 2-(1-(2,5-dimethyl-3-thienyl)ethylidene)-0-isopropylidene- succinic anhydride, for example, will mainly revert to the coloured form of the fulgide on standing in daylight but will mainly be converted to amic acids (IX) and (X) under UV light or strong direct sunlight. The amic acid derived from the coloured form of 2-(1-(5-cyano-1,2-dimethyl-3- pyrryl)ethylidene)-3-isopropylidenesuccinic anhydride will mainly be converted to amic acids (IX) and (X) even in indirect daylight.

The image of the coloured form cf the photochromic compound in the selected areas against a background of relatively colourless substantially permanently non-photochromic compound can readily be used as a security marking by subjecting it to white light to convert the coloured form of the photochromic compound to the relatively colourless form. The image of the relatively colourless form of the photochromic compound is then substantially indistinguishable from the background of the relatively colourless substantially permanently non-photochromic compound (amic acids (IX) and (X)). When the film is subsequently exposed to UV light, however, the photochromic compound is converted to its coloured form in the selected areas while the background remains relatively colourless; that is the image reappears.

The alternative process according to the present invention makes use of the phenomenon described above that light of too great a frequency will convert the amic acids (VII) and (VIII) corresponding to the coloured form of the photochromic compound to the relatively colourless substantially permanently non-photochromic amic acids (IX) and (X). By "high frequency light" we mean light of sufficient frequency to cause this reaction. The minimum frequency will vary for different photochromic compounds. In general, UV light, for example light from a mercury arc lamp, will be of sufficiently high frequency for use with any photochromic fulgide.For certain fulgides, for example 2-(1-(5-cyano-1,2-dimethyl-3-pyrryl)ethylidene)-3-iso- propylidenesuccinic anhydride, white light, for example from a daylight lamp, will be of sufficiently high frequen cy.

In this alternative process the photochromic compound is incorporated into a film as described above. The film is subjected to UV light, for example by exposure to a mercury arc lamp for 5 to 100 seconds, to convert the photochromic compound to its coloured form. The film is then treated with the chemical reagent, for example ammonia. The photochromic compound is thereby converted to amic acids (VII) and (VIII-) over the whole area of the film. The film appears uniformly yellow. The film is then subjected to high frequency light, for example UV light, through a mask to form an image in selected areas. Alter natively, the film can be imaged by a UV laser. Although a laser is not required to achieve a short exposure time, it may be preferred if security films are to be marked with different images, for example serial numbers.In these areas the photochromic compound is converted to a relatively colourless permanently non-photochromic compound, that is the amic acids (VII) and (VIII) are converted to amic acids (IX) and (X). The film thus has an image of the relatively colourless substantially permanently non-photochromic compounds in the selected areas against a back ground of the amic acids (VII) and (VIII). The image can generally be seen at this stage,although not very distinctly, as a colourless image against a yellow backgrcund.

To form a film suitable for security marking from the film which has been exposed to high frequency light in selected areas, the film is kept in conditions whereby the amic acids (VII) and (VIII) are allowed to convert to the coloured form of the photochromic compound in areas other than the exposed selected areas. The preferred conditions are those conducive to the removal of the ammonia or amine used as chemical reagent. The film is preferably kept in the dark and is preferably heated at a temperature below the glass transition temperature of the film-forming material.

In the alternative process the imaging and heating steps can be combined if desired. In such a combined process the chemically treated film is covered with a black mask. The masked film is exposed to a floodlight which emits heat as well as light. For most photochromic compounds a UV floodlight is required, although for certain compounds such as cyano- or amido-substituted pyrryl fulgides a white light floodlight can be used. In the exposed areas the amic acid is converted to a substantially permanently non-photochromic compound. In the masked areas the heat and light emitted by the floodlight is absorbed by the black mask and transmitted as heat to the film, so that in the areas of the film beneath the mask the amic acid is converted by heat into the coloured form of the photochromic compound.

When the coloured form of the photochromic compound has reappeared the film is subsequently exposed to white light to convert the coloured form of the photochromic compound into the relatively colourless form of the photochromic compound. In the resulting film the selected areas of the film contain a relatively colourless substantially permanently non-photochromic compound while the background areas of the film contain the photochromic compound in its relatively colourless form. The film thus appears as a relatively colourless film with no apparent image. It has a latent image which can be seen after exposure to UV light as a colourless image against a background of (0 coloured form of the photochromic compound.

The coloured form of a photochromic 3,4-diarylfuran2,5-dione will react with ammonia or an amine of the formula HNR5R6 to form:

which may be the same compound or isomers depending on whether R8 and R9 are the same or different. This reaction is reversible on heating or on standing in the darh. The relatively colourless form of the 3,4-diarylfuran-2,5-dione will react to form:

The amic acid reaction product (XII) may re-form the anhydride ring to a significant extent on standing or heating, so that it is not substantially permanently nonphotochromic, unlike the amic acid reaction product (IX) and (X) derived from a fulgide.It canhowever,undergo cis-trans isomerisation when irradiated by UV light, y particularlyXhard UV of a wavelength of 250 nm or less.

The isomerisation product:

is a substantially permanently non-photochromic compound.

The amic acid reaction product (XII) can therefore be regarded as a precursor of a substantially permanently nonphotochromic compound.

When a 3,4-diarylfuran-2,5-dione is used in the first process according to the invention, an extra step cf UV irradiation is required after the chemical treatment with ammonia or an amine. This irradiation should take place soon after chemical treatment, and before any heating step has taken place, to minimise any reformation of the anhydride ring by (XII).

A 3,4-diarylfuran-2,5-dione can be used in the alternative process of the invention without the need for an additional process step, provided that the high frequency light to which the film is exposed imagewise is of sufficiently high frequency not only to cause ring opening of the amic acid reaction product (XI) to the amic acid (XII) corresponding to the relatively colourless form of the photochromic compound but also of sufficiently high frequency to isomerise the amic acids (XII) to the substantially non-photochromic compound (XIII).

A 3,4-diarylfuran-2,5-dione (III) can be used in a further imaging process according to the present invention.

In this process, a film containing the photochromic compound (III) in its relatively colourless form is treated with a chemical reagent such as ammonia or an amine to effect opening of the anhydride ring, forming amic acid reaction product (XII). The product of chemical reaction is then exposed to UV radiation, preferably hard UV radiation of wavelength 250 nm or below, in one or more selected areas. In the selected areas the amic acid reaction product (XII) is isomerised to (XIII), which is a substantially permanently non-photochromic compound. The film is then subjected to conditions conducive to removal of the ammonia or amine. Preferably the film is heated to cause evaporation of ammonia or amine from the film.In the background areas of the film which have not been exposed to UV radiation the anhydride ring will gradually re-form, so that the background areas of the film contain the relatively colourless form of the photochromic compound (III). The resulting film is useful as a security marking. It has no apparent image, but on subsequent irradiation with UV light the background areas will be converted to the coloured form (IV) of the photochromic compound. The exposed areas will be seen as a colourless image against a coloured background.

The invention is illustrated by the following Exsamples.

Example 1 2-( 1-(2,5-dimethyl-3-thienyl )ethyl idene)-3-isopropyl- idenesuccinic anhydride was incorporated in cellulose acetate film at a concentration of 1% by weight. A piece of the film was exposed to 5 minutes UV irradiation from a Philips "Blacklight" fluorescent UV lamp array over the whole film. The film was coloured to the magenta colour characteristic of this photochromic fulgide in its coloured form. The film was placed in 0.880 ammonia solution for 5 minutes, removed, washed with water and dried. The appearance of the film was now light yellow. A mask (a zinc sheet with 3 mm diameter holes punched in it) was placed over the film which was then subjected to irradiation from the same UV lamps for 5 minutes. At the end of this period the lamps were switched off and the mask removed.The film was observed to have an image of clear colourless circles on a yellow background. After 24 hours' storage in the dark the film was observed to be composed of clear circles on a background of magenta. Bleaching the above film with white light for 10 seconds gave a colourless clear film useful as a security label.

When the film was exposed to UV strip lamps, an image of clear slightly magenta coloured circles on a strong magenta coloured background appeared within seconds.

ExamDle 2 A piece of cellulose acetate film containing 1 by weight 2-(1-(2,5-dimethyl-3-thienyl )ethylidene)-3-iso- propylidenesuccinic anhydride was placed on a flat surface under the "Blacklight" UV lamp array. On top of the fulgide film was placed a mask of the type described in Example 1.

The UV array was then switched on for 5 minutes to form a pattern of magenta circles. At the end of this period the UV strip lamps were switched off and the whole film submerged in a solution of 0.880 ammonia for 5 minutes. The film was removed from the solution, washed with water to remove excess ammonia solution and dried.

The previously magenta areas were now coloured yellow. The remaining areas were clear and colourless. The sample was then stored in the dark at room temperature for 1 day. At the end of this period the yellow areas had recoloured to give a magenta colouration similar to that originally produced. The other areas remained clear and colourless.

Bleaching of the film with white light for 10 seconds gave a film which was clear and colourless and useful as a security label. Exposure of the film to UV strip lamps produced an image of strongly coloured magenta circles with a very pale magenta background within seconds.

Example 3 2-(1-(5-aminocarbonyl-1,5-dimethyl-3-pyrryl)ethylidene)3-isopropylidenesuccinic anhydride (XIV) was incorporated in cellulose acetate film at a concentration of 0.5% by weight. A piece of the film was placed on a flat surface under a 'Blacklight lamp array and irradiated for 10 minutes. The resulting blue film was placed in a solution of 0.880 ammonia for 5 minutes. At the end of this period the film was washed in water and dried. The appearance of the film was now yellow. The yellow film was placed between a glass slide and a mask (a piece of zinc sheet with 3 mm diameter holes punched in it). The slide, film and mask were held together in close contact. The masked sample was now held by hand in a beam of wavelength above 420 nm for 30 seconds. At the end of this period the film sample was removed from the mask and glass slide.The film was observed to be composed of clear circles on a yellow background. After 24 hours' storage in the dark the film was composed of clear colourless circles on a blue background. Bleaching of the film with white light for 10 seconds gave colourless clear film free from any swelling.

When the film was subsequently exposed to UV light, an image of very slightly blue circles on a strong blue background appeared within seconds.

Example 4 Example 3 was repeated using a cellulose acetate film containing 0.5% 2-(1-(5-cyano-1,2-dimethyl-3-pyrryl)ethyl- idene)-3-isopropylidene succinic anhydride and 0.5 (XIV).

The film was treated as was the sample in Example 3 above.

Bleaching of the imaged film with white light gave a clear colourless film. Subsequent exposure to UV light revealed an image of very light blue circles on a blue background.

Examples 5 and 6 The photochromic cellulose acetate films produced in Examples 3 and 4 were treated as in Example 2, except that a 10 minute exposure to the UV lamps was used.

This produced films when bleached with white light which were clear and colourless and free from swelling.

Exposure of each film to UV strip lamps revealed an image of blue circles on a pale blue background.

Claims

1. A process of imaging a film containing a reversible photochromic compound which can be converted by UV light from a relatively colourless form to a coloured form, in which process the film is exposed to UV light in one or more selected areas to convert the photochromic compound at least partially to its coloured form in those areas, the photochromic compound in the remaining area(s) being in the relatively colourless form, characterised in that the film so exposed is treated with a chemical reagent which reacts with the relatively colourless form of the photochromic compound to form a relatively colourless substantially permanently non-photochromic compound or a precursor thereof but does not react with, or reacts reversibly with, the coloured form of the photochromic compound.

2. A process according to claim 1 for forming a film suitable for security marking, characterised in that any reversible reaction product between the chemical reagent and the coloured form of the photochromic compound in the chemically treated film is allowed to revert to the coloured form of the photochromic compound in the said selected areas and the film is then exposed te visible light to convert the coloured form of the photochromic compound, thereby forming a relatively colourless film which has no apparent image but which has a latent image of relatively colourless photochromic compound against a background of relatively colourless substantially permanently non-photochromic compound, which image can be seen after exposure to UV light.

3. A process according to claim 1, characterised in that the relatively colourless form of the photochromic compound reacts with the chemical reagent to form a precursor which can be converted by UV radiation to a relatively colourless substantially permanently non-photochromic com pound, and the film is UV irradiated after treatment with the chemical reagent, any reaction product formed between the coloured form of a photochromic compound and the chemical reagent being substantially unaffected by the UV irradiation.

4. A process of imaging a film containing a reversible photochromic compound which can reversibly be converted by UV light from a relatively colourless form to a coloured form, in which process the film is exposed to high frequency light in one or more selected areas, characterised in that prior to the exposure to the high frequency light the film containing the photochromic compound in its coloured form is treated with a chemical reagent which reacts with the photochromic compound to form a temporarily non-photochromic compound which is rapidly converted to a relatively colourless substantially permanently non-photochromic compound in the selected areas when the film is subsequently exposed to high frequency light but which can be reconverted into the photochromic compound in areas which are not subsequently exposed to high frequency light.

5. A process according to claim 4 for forming a film suitable for security marking, characterised in that after exposure of the film to high frequency light the film is kept in conditions whereby the temporarily non-photochromic compound is allowed to revert to coloured form of the photochromic compound in areas other than the said selected areas and the film is subsequently exposed to white light to convert the coloured form of the photochromic compound into the relatively colourless form of the photochromic compound, thereby forming a relatively colourless film which has no apparent image but which has a latent image of relatively colourless substantially permanently non-photochromic compound against a background of relatively colourless photochromic compound, which image can be seen after exposure to UV light.

6. A film containing a photochromic compound which is reversibly convertable between a coloured form and a relatively colourless form and which in both forms has a cyclic carboxylic acid anhydride ring in its molecule, characterised in that in at least part of the area of the film the anhydride ring of the photochromic molecule has been ring opened to an amic acid or salt thereof.