GB2240851A

GB2240851A - Photochromic imaging process

Info

Publication number: GB2240851A
Application number: GB9001013A
Authority: GB
Inventors: Peter Dobson; Fiona Jane Holmes
Original assignee: Courtaulds PLC
Current assignee: Akzo Nobel UK PLC
Priority date: 1990-01-17
Filing date: 1990-01-17
Publication date: 1991-08-14
Also published as: AU7052491A; WO1991010571A1; GB9001013D0

Abstract

A process of imaging a film containing a photochromic compound, which is a fulgide or fulgimide that can reversibly be converted by UV light from a relatively colourless form to a coloured form, includes the step of exposing the film to UV light in one or more selected areas to convert the photochromic compound at least partially to the coloured form in those areas, the photochromic compound in the remaining areas being in the relatively colourless form. The process is characterised in that the film so exposed is heated, either simultaneously with its exposure to UV light or subsequently, to a temperature, preferably in the range 50-180 DEG C, at which the coloured form of the photochromic compound is degraded to a relatively colourless permanently non-photochromic compound but the relatively colourless form of the photochromic compound is not degraded. The image formed can be used as an identifying mark, particulary as a security marking for goods, packages, documents or identification cards.

Description

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

A photochromic compound is a compound that undergoes a colour change after irradiation with light of a certain wavelength, which colour change may be reversible or irreversible. In general, the compounds are coloured by irradiation with UV light and convert to a pale or colourless form in visible light. Examples of reversible photochromic compounds are spiropyrans, fulgides and fulgimides.

Photochromic compounds, particularly those which are colourless under white light, can be used for marking. The marking can be irradiated by UV light and an image previously invisible under white light can then be observed.

A photochromic image can, for example, be printed on a substrate using an ink containing the photochromic compound.

European Patent Application No. 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-photochromic 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. After the layer has subsequently been irradiated with UV light a colourless image formed by the non-photochromic compound can be seen on a background of the coloured photochromic compound.

Over-exposure in selected areas can be achieved by using a UV laser or 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 of 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.

According to one aspect of the present invention, a process of imaging a film containing a photochromic compound, said compound being a fulgide or fulgimide 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 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 cnaracterised in that the film so exposed is heated to a temperature at which the coloured form of the photochromic compound is degraded to a relatively colourless permanently non-photochromic compound but the relatively colourless form of the photochromic compound is not degraded.

The product of this process is a film suitable for use as a security marking. It is a relatively colourless film which contains the permanently non-ph9tochromic compound in the selected area or areas and the photochromic compound in the remaining area or areas. It has substantially no apparent image in visible light but has a latent image which is revealed on irradiation with UV light, when the area(s) containing non-photochromic compound remain colourless but the photochromic compound is converted to its coloured form in the remaining area(s).

Photochromic fulgides are described for example in UK Patents 1 442 628 and 1 464 603, UK Patent Application 2 170 202A and European Patent Application 279600 and in a paper by A. Kaneko et al in Bull. Chem. Soc. Japan 61, pages 3569-3573 (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 R1 and R2 and at least one of R3 and R4 is other than hydrogen. The fulgides derive their photochromic characteristics from their ability to undergo reversible ring closure.For example, where R2 is 2 the aromatic group, ring closure occurs between R2 and the carbon atom to which R3 and R4 are attached. The group R1 attached to the same carbon atom as the aromatic group R2 usually has to be other than hydrogen for the compound to be photochromic. The fulgides in the coloured form thus generally have the formula

where A is a partially unsaturated ring derived from the aromatic group R2.

Photochromic fulgimides have the formula

in the relative colourless form and

in the coloured form, where R1, R2, R3, R4 and A have the meanings given above and R5 is hydrogen or a monovalent hydrocarbon group, preferably an alkyl group having 1 to 6 carbon atoms or a phenyl group. The preparation of fulgimides is described in UK Patent 1 464 603 and UK Patent Application 2170202A.

Preferred fulgides and fulgimides for use in the invention are those in which the aromatic group R2 contains at least one unsubstituted carbon atom ortho to the position at which R2 is bonded to the fulgide or fulgimide molecule. The group R2 can for example be

where X is oxygen, sulphur or -NR5, where R5 is defined as above, and R6 and R7 are each a hydrogen atom, or an alkyl, cycloalkyl, aryl, aralkyl or heterocyclic group, or can be

where R6, R5 and X are defined as above. Preferably both of the said ortho-positions have unsubstituted carbon atoms and R2 is a heterocyclic ring having aromatic character, that is R2 is

Most preferably R2 is unsubstituted.

Examples of preferred photochromic fulgides are those of formula (I) in which R1, R3 and R4 are all CH3 and R2 is an unsubstituted 3-furyl or 3-thienyl or 3-pyrryl group.

The film is preferably self-supporting but can be a surface coating on an article provided that the article itself is not damaged by the heat treatment. The film used in the invention comprises a layer of film-forming material containing a photochromic fulgide or fulgimide, the filmforming 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 300 nm. The most preferred film-forming polymer is cellulose acetate.

Alternatives are other cellulose esters, polyesters, acrylic polymers, for example polymethyl methacrylate, polyurethanes, olefin polymers, for example polyethylene or ethylene vinyl acetate copolymers, vinyl polymers, for example polyvinyl acetate or polyvinyl chloride, polycarbonates and polyamides. The photochromic fulgides and fulgimides 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 the fulgides are reactive to some extent with lower alcohols such as methanol and ethanol. The solution 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.

As an alternative method of incorporating the photochromic 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) of the film. In addition the rate may be increased by including in the dye bath a plasticiser which swells the film.

In the process of the invention the photochromic compound 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. Colouration in selected areas can be achieved by exposure to a UV lamp through a mask, for example using a Philips Blacklight fluorescent UV lamp array or a 100 to 125 watt medium pressure art lamp at a distance of 5 cm. Using a Philips 'Blacklight lamp array, the colour generated by UV radiation of the film is generally apparent after a few seconds and typically reaches maximum intensity in 5 to 25 minutes.

The time of UV exposure through the mask is most preferably in the range 3 to 25 minutes, or so that the intensity of colour of the photochromic compound reaches at least 50% of its maximum intensity. (Using this type of lamp total degradation of the photochromic fulgide or fulgimide by UV to a colourless non-photochromic compound takes 3 to 4 hours' exposure.) The film so exposed is heated. Heat treatment is preferably carried out in the dark to minimise reversion of the photochromic fulgide or fulgimide in the exposed areas to its relatively colourless form. The film is preferably kept from exposure to daylight or other visible light between UV exposure and heating.The temperature of the heat treatment is preferably in the range 50-180 C, most preferably 60#1200C. The temperature should not be such as to damage the film (not above 1150C for cellulose acetate film). The time of heating required to render the exposed areas substantially indistinguishable from the background areas containing the relatively colourless form of the photochromic compound is generally in the range 30 minutes to 4 hours.

Heat treatment can be carried out simultaneously with UV exposure if desired, although it is generally preferred to remove the film from UV exposure and continue heating in the dark.

The invention is illustrated by the following Example.

Example 1.0% (calculated on weight of solids) alpha-3-thienyl ethylidene (isopropylidene) succinic anhydride was dissolved in a 15% dope of cellulose acetate in acetone. The dope was then cast at 50; dry film thickness.

A small sample of the film was placed in a cardboard ultraviolet sample holder. A UV-visible spectrum of this sample was run. The sample was then exposed to periods of UV light from a Philips "Blacklight" lamp array, set 5 cm above the film. After each exposure, another spectrum was run. The results obtained for the absorbance of the colour peak (at 520.9 nm) after various UV exposures are shown below in Table 1.

Table 1 Time of radiation (seconds) Absorbance (A) 0 0.0841 30 0.1212 60 0.1578 120 0.2390 240 0.3814 480 0.5390 960 0.6014 1920 0.4517 Film which had been irradiated for 1100 seconds as described above was removed from the imaging unit and placed, in a light-proof envelope, in an oven at 100or for 135 minutes. The coloured form of the photochromic compound disappeared so that no image was visible.

When the film was subsequently exposed to the UV lamp over all its area, the non-imaged areas became coloured at substantially the rate shown in Table 1, but the imaged areas remained colourless.

Claims

1. A process of imaging a film containing a photochromic compound, said compound being a fulgide or fulgimide 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 UV light in one or more selected areas to convert the photochromic compound to the coloured form in the said areas, the photochromic compound in the remaining areas being in the relatively colourless form, characterised in that the film so exposed is heated to a temperature at which the coloured form of the photochromic compound is degraded to a relatively colourless permanently non-photochromic compound but the relatively colourless form of the photochromic compound is not degraded.

2. A process according to claim 1, characterised in that the exposed film is heated at a temperature in the range 50#1800C.

3. A process according to claim 1 or claim 2, characterised in that the film is removed from UV exposure and subsequently heated in darkness.

4. A process according to claim 1 or claim 2, characterised in that the film is heated simultaneously with its exposure to UV light in one or more selected areas.

5. A process according to claim 4, characterised in that the photochromic compound is a fulgide in which R2 is an unsubstituted 3-thienyl or 3-furyl group and the groups R1, R3 and R4 are all methyl groups.

5. A process according to any of claims 1 to 4, characterised in that the photochromic compound has the formula

in which R2 is an aromatic group having an unsubstituted carbon atom in at least one of the positions ortho to the position at which R2 is bonded to the fulgide or fulgimide molecule, and R1, R3 and R4 are each hydrogen or a monovalent hydrocarbon group, provided that at least one of R3 and R4 is other than hydrogen, and Y represents an oxygen atom or an NR5 group where R5 is hydrogen or a monovalent hydrocarbon group.