Process for coating a substrate

Abstract: A substrate is coated with a visible light sensitive composition comprising a free radical addition polymerizable or cross linkable material and a photobleachable ionic dye-counter ion compound capable of absorbing visible light and generating free radicals. The composition is exposed to light to harden the composition, and to bleach the composition to an essentially colourless state. Also described are photohardenable compositions containing photobleachable ionic dye-counter ion compounds as photoinitiators useful in the aforesaid process.

Description: This invention relates to processes for coating a substrate, and in particular for forming coatings without tint on a substrate. By coatings without tint we mean colourless, clear or transparent coatings and white coatings. European Patent Specification EP-A-0223587 is concerned with full colour imaging systems and with photohardenable compositions and photosensitive materials employing them, which are designed to be used in such systems, in which an encapsulated (usually microencapsulated) radiation sensitive composition associated with an image-forming agent (for example a colour precursor) is image-wise exposed to radiation which usually hardens the composition, and then subjected to a uniform rupturing force to release the colour precursor in an image-wise pattern to form a colour by reaction with a developer. The photocurable compositions described in EP-A-0223587 contain ionic dye complexes as photoinitiators. These complexes directly or indirectly yield free radicals upon exposure to actinic radiation and particularly visible light. By nature, compositions such as these which are sensitive to visible light appear coloured to the human eye. A coloured appearance in a cured composition is a drawback in certain applications where such a system might otherwise be advantageous. For example, in laminating glass, if the composition is coloured it imparts a tint to the glass which is generally undesirable. Similarly, in coating optical fibres a coloured appearance is a drawback. We have now appreciated that photobleachable ionic dye complexes would be particularly useful in such applications. We have observed that some of the dyes disclosed in the aforementioned EP-A-0223587 are photobleachable. The present invention may thus be seen at least in part as a selection from what is described in the said European Application applied to a novel use. The present invention provides a process for coating a substrate with a coating which is essentially without tint comprising the steps of: coating said substrate with a visible light sensitive composition comprising a free radical addition polymerizable or crosslinkable material and a photobleachable ionic dye-counter ion compound capable of absorbing visible light and generating free radicals, said ionic dye counter ion compound being a cationic dye-borate anion compound represented by the formulas (I): where D is a cationic dye moiety; and R¹, R², R³ and R⁴ are the same or different and selected from alkyl, aryl, aralkyl, alkaryl, alkenyl, alkynyl, alicyclic, heterocyclic, and allyl groups; exposing said composition to light to harden said composition; and bleaching said ionic dye to an essentially colourless state. Our process is useful in applications in which an essentially colourless, clear or transparent coating is desired. Typical examples of such applications are in laminating glass or coating optical fibres. Our process is also useful in coating using compositions containing white pigments such as titanium dioxide, where no tint is desired. Ionic dye complexes useful in the practice of the present invention are included within the general description of EP-A-0223587. The ionic dye complexes useful in the present invention are characterised in that they are photobleachable. The polymerizable or crosslinkable materials described in the aforementioned applications are useful herein. The photoinitiator systems described in the aforementioned applications are also useful herein with the requirement that the ionic dye-counter ion compounds be photobleachable. Of the dye complexes disclosed in EP-A-0223587, all of the cyanine dye complexes are photobleachable with varying efficiency and speed. Some of the more efficient photobleachable complexes are: The azine dye complex of Methylene Blue and triphenylbutylborate is one of the most efficient complexes in terms of photobleachability. Its structure is For the purpose of the present invention, the photobleachable dye complexes may bleach upon exposure to visible light upon curing the compositions, or they may be bleached upon post-exposure to a bleaching radiation such as ultraviolet radiation.

Claims: A process for coating a substrate with a coating which is essentially without tint comprising the steps of: coating said substrate with a visible light sensitive composition comprising a free radical addition polymerizable or crosslinkable material and a photobleachable ionic dye-counter ion compound capable of absorbing visible light and generating free radicals, said ionic dye counter ion compound being a cationic dye-borate anion compound represented by the formula (I): where D is a cationic dye moiety; and R¹, R², R³ and R⁴ are the same or different and selected from alkyl, aryl, aralkyl, alkaryl, alkenyl, alkynyl, alicyclic, heterocyclic, and allyl groups; exposing said composition to light to harden said composition; and bleaching said ionic dye to an essentially colourless state. A process according to Claim 1, further characterised in that said bleaching step takes place in the course of said exposure to light step to harden the composition. A process according to Claim 1, further characterised in that said bleaching step comprises exposure to a bleaching radiation such as ultra-violet radiation post exposure to said visible light. A process according to any preceding claim, further characterised in that, prior to exposure, said photobleachable ionic dye-counter ion compound is a stable, non-transient compound. A process according to any preceding claim, further characterised in that said photobleachable ionic dye-counter ion compound is soluble in said free radical addition polymerizable or crosslinkable compound. A process according to any preceding claim, further characterised in that, following exposure of said ionic dye-counter ion compound to light, an electron is transferred from said dye to said counter ion or from said counter ion to said dye, the rate of said electron transfer being greater than a diffusion controlled rate. A process according to any preceding claim, further characterised in that said cationic dye is selected from cationic methine, polymethine, triarylmethane, indoline, azine, thiazine, xanthene, oxazine, acridine, cationic cyanine, carbocyanine, hemicyanine, rhodamine, and azamethine dyes. A process according to Claim 7, further characterised in that at least one of R¹, R², R³ and R⁴ is an alkyl group. A process according to Claim 8, further characterised in that R¹, R², and R³ are aryl groups and R⁴ is an alkyl group. A process according to Claim 7, further characterised in that said dye is Methylene Blue. A process according to Claim 7, further characterised in that said cationic dye is selected from and A process according to any preceding claim, further characterised in that said polymerizable or crosslinkable material contains an ethylenically unsaturated compound. A process according to any preceding claim, further characterised in that said substrate is glass. A process according to any of Claims 1-12, further characterised in that said substrate is an optical fibre.

Inventors: GOTTSCHALK PETER, NECKERS DOUGLAS C, SCHUSTER GARY B, GOTTSCHALK, PETER, NECKERS, DOUGLAS C., SCHUSTER, GARY B.
Applicants: MEAD CORP, THE MEAD CORPORATION
Application year: 1988
Main classification: G03F7
IPC codes: G03C1, G03F7