Radiation imagery chemistry: process – composition – or product th – Imaged product – Antifraud or antitampering
Patent
1990-01-11
1991-02-12
Michl, Paul R.
Radiation imagery chemistry: process, composition, or product th
Imaged product
Antifraud or antitampering
430 17, 430 19, 430345, 430962, G03C 300
Patent
active
049923471
DESCRIPTION:
BRIEF SUMMARY
This invention relates to the marking of items such as goods, packages, documents or identification cards, for example to provide security markings, using photochromic compounds.
A photochromic compound is a compound that undergoes a colour change when irradiated with light of a 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 photochromic compounds are spiropyrans and fulgides, the latter being described in UK Patent Nos. 1 442 628 and 1 464 603, and in published UK Patent Application No. 2 170 202A. Films containing a reversible photochromic compound have been suggested for the temporary recording of information, for example by a laser of visible light which converts the photochromic compound to its pale or colourless state, creating a recorded image which can be stored in the dark and erased by uv. Such a system is suggested by H. G. Heller in IEE Proceedings, Volume 130, part 1, no. 5, October 1983, and in British Patent No. 1 600 615.
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 be observed. A photochromic image can, for example, be printed on a substrate using an ink containing the photochromic compound. Security marking applied as an ink has disadvantages in that the presence of ink markings can usually be detected even if the ink is colourless and forging of the markings is possible by anyone having access to the photochromic ink. The present invention relates to more secure marking using photochromic compounds.
Accordingly the present invention provides a marking comprising a photochromic layer which contains or consists of a photochromic compound, which layer has an image formed therein by complete or partial conversion of the photochromic compound to a permanently non-photochromic compound in one or more selected areas. The invention also includes articles and materials having such a marking and a process for producing such a marking by forming the image in the layer.
The preferred method of conversion of the photochromic compound to a permanently non-photochromic compound (hereinafter referred to as degradation) is by over-exposure to uv light. The published literature on photochromics emphasises their reversibility but we have found surprisingly that over-exposure to uv light can completely degrade some photochromic compounds to a relatively colourless non-photochromic form. This non-photochromic form undergoes substantially no colour change under uv or visible light and cannot readily be distinguished by the eye against a background of the pale or colourless form of the photochromic compound under visible light, but after irradiation with uv light it is readily distinguished against a background of the more coloured form of the photochromic compound.
Advantageously the photochromic compound is reversible, preferably converting from pale or colourless to coloured under uv light and reverting to pale or colourless under visible light. Photochromic compounds which are irreversible or substantially irreversible can, however, be used for applications where the image is only to be viewed once or where it is not necessary that the image, once viewed, will revert to its invisible form. In general, it is preferred to employ photochromic compounds that are thermally stable in their coloured state, although photochromic compounds that thermally revert to their pale or colourless state may be used if desired. The thermal reversion may occur at room temperature or below but, where a thermally reversible photochromic compound is used, the compound is preferably stable at room temperature for long enough for the image to be clearly seen but is preferably also capable of relatively fast reversion to its pale or colourless state at a more elevated temperature, for example from
REFERENCES:
patent: 4220708 (1980-09-01), Heller
Glaze et al, J. Chem. Soc. Perkin Trans. I, 957-961 (1985).
A. E. J. Wilson, Molecular Electronics 1, Phys. Technol., 15:232-238 (1984).
Bowyer Arthur G.
Hawkins Michael
Chea Thorl
Courtaulds PLC
Michl Paul R.
LandOfFree
Marking does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Marking, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Marking will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-19987