GB1583492A

GB1583492A - Method of dyeing or printing hydrogel resin

Info

Publication number: GB1583492A
Application number: GB25955/77A
Authority: GB
Original assignee: Toppan Printing Co Ltd
Current assignee: Toppan Inc
Priority date: 1976-06-24
Filing date: 1977-06-21
Publication date: 1981-01-28
Also published as: DE2728613A1; DE2728613B2; DE2728613C3; CA1093763A; FR2355642A1; FR2355642B1

Description

(54) METHOD OF DYEING OR PRINTING HYDROGEL RESIN (71) We, TOPPAN PRINTING COM PANY LIMITED, a Japanese corporation, of 5-1, l-chome, Taito, Taito-ku, Tokyo, Japan, do hereby dedare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention is concerned with dyeing or printing of hydrogel resins, such as mouldings, sheets or coatings of such resins.

Hydrogel resins generally contain hydroxyl groups and are partially cross-linked; they are generally highly hygroscopic and waterexpansible. Typical hydrogel resins are crosslinked polymers (including copolymers) of hydroxyalkylmethacrylate and derivatives thereof, polyacrylamide and derivatives thereof, graft polymers of polysiloxanes with hydrophilic monomers, copolymers of N-vinyl pyrrolidone with methacrylates, polyion complexes, cellulose derivatives (such as hydroxyethyl cellulose) and cellulose graft copolymers.

After fabricating such hydrogel resins into films, membranes or other shaped articles by conventional methods, it is often desired to colour the hydrogel resin by dyeing or printing (for example, by printing letters, pictures or other designs). Since the hydrogel resins are generally swollen by water, it is often extremely difficult to colour them using an oily ink, and while the resins can be coloured using an aqueous ink or dye solution, the aqueous colouring material tends to be dissolved out in time.

Regardless of whether an aqueous ink or dye solution (containing a water-soluble dyestuff) or an oily ink (containing the usual dyes or pigments and binder resins) is used, a distinct colour can be formed initially, but on prolonged contact with water, the colour tends to fade.

In recent years, soft contact lenses made of hydrogel resins of hydroxyalkylmethacrylate polymers have become more popular, because, when swollen in water, this type of hydrogel resin becomes soft and hydrophilic and does not cause discomfort to the wearer. The contact lens varies in curvature or degree with the user's eyesight and it is desirable to display marks and numerals near the periphery of contact lenses to distinguish between their types. Such marks or numerals should not fade in use or when the contact lens is disinfected by boiling in hot water or cleaned in a solution of protein-decomposing enzyme. Moreover, for hygienic reasons, the contact lens should not have a protrusion in the printed portion, which would result if a dye solution or ink contained a binder.

The curvature or degree of the soft contact lens may be measured, the result of the measurement being impressed on the contact lens, and thereafter the lens being stored in water, in the swollen form; or the curvature or degree of the swollen contact lens may be measured, the result of the measurement being impressed on the soft contact lens, and thereafter the impressed contact lens being stored in water.

We have now developed a method of dyeing or printing a hydrogel resin, which may be in the form of a contact lens, whereby in some embodiments at least, the dyed or printed resin can be disinfected by boiling without substantial fading.

According to the invention, there is provided a method of dyeing or printing a hydrogel resin, which comprises impregnating the hydrogel resin with a water-soluble vat dye or tetrazolium salt, and then converting the dye or salt in the resin to a water-insoluble coloured material.

As is well known, the water-soluble vat dye is a reduced form of a dye and is in the form of a colourless leuco compound. The vat dye may be soluble because of sulphonate groups therein. The colour is produced by oxidation, which may be effected using a developer comprising an aqueous solution of an oxidizing agent or by photo oxidation.

When an oxidizing agent is used, it may be, for example, an aqueous solution of sulphuric or nitrous acid or of sodium perborate, potassium bichromate, a pesulphate, sodium chlorite or hydrogen peroxide.

Some water-soluble vat dyes are photosensitive and can be photo-oxidized, such that, on exposure to light they are rendered waterinsoluble. Example of such photosensitive vat dyes are yellow grades 3, 4, 7 and 8, red grades 1, 2 and 10, blue grades 2, 5 and 8 and black grades 1, 4 and 6.

The water-soluble tetrazolium salts (which have great affinity for hydrogel resins) are generally colourless; they are converted to a coloured material by reduction, for example, by dipping in an aqueous solution of a reducing agent (developing bath), the waterinsoluble coloured material being a formazan.

Suitable tetrazolium salts include, for example, 2,3,5-triphenyl-2H-tetrazolium chloride, Terazol Blue, Tetrazolium Violet, Tetrazolium Purple and Nitro Blue Tetrazolium. Some Tetrazolium salts are photosensitive and can be converted to the coloured formazan by exposure to light or ultraviolet rays (instead of using a developing bath as described above).

When the vat dye of tetrazolium salt is photosensitive, as described above, an image may be projected on the resin through a transparent negative to form a dyed impression on the hydrogel resin. The resulting coloured hydrogel resin my then be boiled to remove any remaining water-soluble tetrazolium salt or vat dye in the unexposed portions.

The conversion of the vat dye or tetrazolium salt to a water-insoluble coloured material takes place in situ, in the molecular structure of the hydrogel resin. Accordingly, the colour is stable even when the hydrogel resin is swollen with water.

The hydrogel resin may be impreganated with the vat dye or tetrazolium salt by several methods, such as: (i) simultaneously swelling the hydrogel resin in water and impregnating with the vat dye or tetrazolium salt; (ii) first impregnating the hydrogel resin with the vat dye or tetrazolium salt and then swelling the resin; or (iii) first swelling the hydrogel resin and then impregnating the swollen resin with the vat dye or tetrazolium salt. These three methods will now be described in more detail.

(i) Unswollen hydrogel resin is dipped in a solution or in containing the vat dye or tetrazolium salt, followed by converting the dye or salt to a water-insoluble material.

(ii) Unswollen hydrogel resin is dipped in a solution or ink containing the water-soluble vat dye or tetrazolium salt, or such a solution or ink is used to print the resin, followed by exposing the impregnated resin to an atmosphere having a high temperature of 30 to 100"C and a high humidity of 60 to 90O/o to swell the hydrogel resin, thereby causing the vat dye or tetrazolium salt to permeate the hydrogel resin. The dye or salt is then rendered water-insoluble and coloured.

(iii) The unswollen hydrogel resin is first swollen in water; then dipped in a solution or ink containing the water-soluble vat dye or tetrazolium salt, or such a solution or ink is used to print the resin, followed by converting the dye or salt to a water-insoluble coloured material.

In each case, the solution or ink can be mixed, if necessary, with a stabilizer and other additives for elevating printability. Where the dye or salt is sensitive to air oxidation, then a small amount of an antioxidant or a small amount of sodium carbonate for alkalyzing the dye solution can be added as a stabilizer.

Additives for improving the printability of the dyestuff include defoaming agents (for suppressing the evolution of air bubbles on stirring) and glycerine and/or glycol (for controlling the dryability and viscosity of the solution).

It is difficult to print a water-swollen hydrogel resin using an ordinary printing device (such as a rubber stamp) since the ink used is repelled by surface water and consequently spreads over the surface of the hydrogel resin.

This is particularly disadvantageous where the resin is to be used as a soft contact lens, since if the ink contained a binder, any binder spread over the surface of the lens would cause surface irregularities and difficulties in en suring that the surface remains hygienic. It is therefore preferred, when the hydrogel resin is to be used as a contact lens, that the ink be free of binder and contain only the watersoluble vat dye or tetrazolium salt and a watersoluble solvent such as water, glycerine, glycol, alcohol, ketone or ester or a mixture thereof.

Such an ink for a soft contact lens has a lower viscosity and tack than customary printing ink. Therefore, the use of a nonporous rubber stamp of the letterpress type in printing a soft contact lens is accompanied with the difficulties that when the rubber stamp is pressed on the soft contact lens until the dye solution fully penetrates the contact lens, then the dye solution unnecessarily spreads out, providing an excessively broad marginal zone, resulting in insufficient or irregular dye concentration. Therefore, the present invention provides a method of easily making a firm impression on a soft contact lens by impregnating the fine cells of a porous rubber stamp manufactured by, for example, Japan NCR Company or Syachihata Kogyo Company with a printing ink containing the previously described vat dye or tetrazolium salt.

Other rubbery elastic materials which may be used as a porous stamp in the method according to the invention include, for example, porous materials generally referred to as sponge, materials prepared by compactly compressing, for example, fibrous cloth or paper, foamed silicone rubber sponge, or plastics material provided with dense fine pores to have a spongelike property for absorption of a liquid.

The above-mentioned porous stamp has the advantages that some of the water drops on the surface of the hydrogel resin are easily absorbed in the fine cells of the porous stamp when it is pressed on the surface, preventing the dyestuff present in the porous stamp from being unduly spread around the resulting impression and causing the dyestuff immediately to penetrate the swollen hydrogel resin; even printing ink having a low viscosity is prevented from being forced out to undesired portions of the surface, such that when the porous stamp is released, a distinct impression is provided.

Of course, such a porous stamp can also be used to print a hydrogel resin in the method according to the invention when the hydrogel resin is in the unswollen state (i.e., the abovementioned method (ii)). The above-mentioned method (iii) is very effective to make an impression on hydrogel resin having a relatively high water expansibility. However, it has been found that where an impression is made on hydrogel resin having a relatively low water expansibility according to the application intended, then method (iii) has the drawbacks that the vat dye or tetrazolium salt fails to fully penetrate the hydrogel resin, resulting in insufficient or irregular dye concentratioL; ard where the impressed resin is contacted with water for a long time, the colour of the dyestuff gradually fades.

A further method which alleviates the abovementioned difficulties is as follows: (iv) dipping the hydrogel resin in a solvent which has a greater power of swelling the hydrogel resin than water and which is highly compatible with water to fully swell the surface or the whole of the hydrogel resin, followed by treating the prescribed portion of the hydrogel resin with the water-soluble vat dye or tetrazolium salt.

This method (iv) has the very advantageous effect of causing the chemical structure of the hydrogel resin to be sufficiently broadened to admit of the easy entry of dye molecules.

Suitable solvents for use in this method (iv) include, for example, low alcohols, low ketones, low glycols and low glycol esters.

Specific examples thereof are methyl alcohol, ethyl alcohol, propyl alcohol, butyl alcohol, diacetone alcohol, acetone, methylethyl ketone, ethylene glycol, propylene glycol, diethylene glycol, 2-methoxy ethanol, 2-ethoxy ethanol, 2-butoxy ethanol, diethylene-glycol monoethyl ether and glycerine.

With respect to the hydrogel resin, for example, poly-2-hydroxyethyl methacrylate homopolymer (cross-linked form), comparison was made between the resin-swelling power of water and that of some of the above-listed solvents, showing that methanol, ethanol, 2ethoxyethanol and acetone had resin-swelling rates of 38%, 31%, 46% and 23% respectively as against 16% of water.

A close relationship exists between the rate at which solvents swell a hydrogel resin and the radius of the dye molecule capable of permeating the cross-linked structure of the hydrogel resin. The radius of the pore of a hydrogel resin which permits the passage of such a molecule is calculated to be 4.0s for 2-hydroxyethyl methacrylate and 6.0A for polyacrylamide. The molecule of a vat dye or tetrazolium salt used in the method according to the invention generally has a radius of at least 7 or SA, and consequently may have difficulty in permeating the crosslinked structure of the hydrogel resin. It is for this reason that swelling of hydrogel resin by a solvent having a greater swelling power than water is preferred, because this increases the pore radius of the resin and enables the molecules of the vat dye or tetrazolium salt to permeate the cross-linked structure of the resin.

According to the above-described method (iv), the resulting water-insoluble coloured material is firmly fixed in the reticular structure of the hydrogel resin, providing a distinct image having a dye concentration. The coloured image has good water-proofness, even when kept for a long time in contact with water, particularly boiling water.

In order that the present invention may be more fully understood, the following Examples are given by way of illustration only.

Example 1.

Transparent film formed of cross linked 2-hydroxyethyl methacrylate resin was preliminarily dipped for 10 minutes in an aqueous solution of a solubilized vat dye (manufactured by Mitsui Chemical Company under the trademark "Mikethren Soluble Blue O") of concentration 15 g/l. The dipped film was taken out to remove the solution deposited on the surface of the film. Later when dipped in a developing bath, i.e. an aqueous solution containing sulphuric acid of 66" Be (of concentration about 10 cc/l) and sodium nitrite (of concentration about 0.5 g/l), then the film was dyed uniformly blue. Thereafter, the film was washed to remove unreacted dyestuff. The film thus coloured was placed for several hours in boiling water to determine dyeability (dye fixation test), proving that the dyestuff firmly adhered to the film and the colour did not fade.

Another example of the above-mentioned film was dyed by a different solubilized vat dye (manufactured by Farbwerke Hoechst A.G. under the trademark "Anthrasol 04G") through the same process as described above, obtaining substantially the same result.

Example 2.

A picture image was projected by means of a 2-kilowatt mercury lamp through a transparent negative on a transparent film preliminarily dipped in an aqueous solution of the latter dyestuff used in Example 1, presenting a distinct blue picture. Later, the film thus coloured was fully washed with warm water to dissolve out dyestuff attached to the non exposed portions of the film. The coloured picture thus formed fully withstood the same dye fixation test as applied in Example 1.

Example 3.

A film whose triacetate base was uniformly coated with cross-linked 2-hydroxyethyl methacrylate resin was preliminarily dipped for about 15 minutes in a 20% aqueous solution of 2,3 ,5-triphenyl-2H-tetrazolium chloride. After the dipped film was taken out, the solution deposited on the surface of the film was removed. When again dipped about 10 minutes in a 10% aqueous solution of sodium boron hydride, the film was uni formly dyed red. This dye sufficiently with stood the same dye fixation test as used in Example 1. Application of another form of tetrazolium salt gave the same result as ob served in Example 1.

Example 4.

A picture image was projected 5 minutes by means of a 2-kilowatt mercury lamp through a transparent negative on a transparent film preliminarily dipped in the same manner as in Example 3, presenting a distinct red image. Thereafter, the film was dipped in warm water to remove the 2,3,5-triphenyl 2H-tetrazolium chloride remaining on the nonexposed portions of the film. The dye of the picture well withstood the same dye fixation test as applied in Example 1.

Example 5.

A soft contact lens fabricated from crosslinked 2-hydroxyethyl mathacrylate resin was swollen by being dipped in a physiological saline solution. After the soft contact lens was taken out, the water remaining on all the peripheral surface of the lens was removed. A letter was impressed at the prescribed posi tipn near the periphery of the lens by a porous sponge rubber stamp impregnated with ink formed of 3 parts by weight of solubilized vat dye (manufactured by Farbwerke Hoechst A.G. under the trademark "Anthrasol Pink 13B"), 13 parts by weight of glycerine and 14 parts of water. Later when the contact lens was dipped in the developing bath used in Example 1, then a distinct red letter appeared.

The dyestuff of this letter fully endured the same dye fixation test as applied in Example 1.

Example 6.

A soft contact lens impressed with a letter in the same manner as in Example 5 was exposed for about one minute to black light (2 mW/cm2). Then the letter was dyed dis-.

tinctly red. The dyestuff of the letter satisfactorily endured the same dye fixation test as conducted in Example 1.

Example 7.

An ink having the same composition as used in Example 5 was applied to the prescribed spot near the periphery of a soft contact lens fabricated by the same process as used in Example 5. A letter image was projected for about 2 minutes by means of a 2-kilowatt mercury lamp through a transparent negative on the ink-impressed spot of the lens.

Then the letter was dyed distinctly red.

Later, the lens was dipped in warm water to remove the dyestuff remaining on the nondyed portions of the lens. The dyestuff of the letter thus formed fully withstood the same dye fixation test as used in Example 1.

With respect to Examples 5, 6 and 7, application of another form of solubilized vat dye gave the same result as in Example 1.

Example 8.

A porous rubber stamp measuring 4 X 4 X 4 mm (a letter engraved therein having a measurement of 2 X 1 mm) manufactured by Japan NCR Company was impregnated with 0.03 cc of printing ink formed of 10 parts by weight of solubilized vat dye (manufactured by Farb werke Hoechst A.G. under the trademark "Brilliant Pink 13B"), 20 parts by weight of water and 10 parts by weight of glycerine. A water-expansible soft contact lens was dipped for 10 minutes in water. Less than 30 seconds after the lens was taken out of the water, the rubber stamp was pressed for 5 seconds to impress a letter on the lens. The soft contact lens thus impressed with a letter was exposed 30 seconds to a black light (2 mW/cm2). Then the letter was dyed distinctly purple. For comparison, a letter was impressed on the soft contact lens by an ordinary rubber stamp using the above-mentioned printing ink, under the same condition in which the soft contact lens was exposed for about 30 seconds to black light (2 mW/cm2). In this case, however, the printing ink was displaced from the rubber stamp, only providing an indistinct blurred letter image.

A letter image provided by the method of this Example 8 was firmly fixed in the resin constituting the soft contact lens. The surface of the soft contact lens presented no scars.

When the marked soft contact lens was subjected to a long boiling test required to examine the practicability as a soft contact lens, then the letter impressed on said soft contact lens did not fade at all.

Example 9.

A porous rubber plate impregnated with a 20% aqueous solution of 2,3,5-triphenyl-2H- tetrazolium chloride made a picture impression on a transparent film formed of unswollen cross-linked 2-hydroxyethyl methacrylate resin.

The film thus impressed was left for about 5 minutes in an atmosphere having a temperature of 60"C and humidity of 85%. Thereafter, the film was exposed to light about 20 seconds at a point spaced about 15 cm from a 2-kilowatt mercury lamp, resulting in a distinct red water-insoluble letter impression.

This impression was found to be highly wearresistant and waterproof.

Example 10.

A porous rubber stamp impregnated with a printing ink formed of 4 parts by weight of solubilized vat dye (manufactured by Farbwerke Hoechst A.G. under the trademark "Anthrasol Pink 13B"), 4 parts by weight of glycerine and 20 parts by weight of water made an impression on a soft contact lens formed of the same unswollen resin as used in Example 9. The soft contact lens thus impressed was left for 10 minutes in an atmosphere having a temperature of 70"C and a humidity of 85% and was exposed for about 90 minutes in this atmosphere to black light (2 mW/cm2). Then the soft contact lens had a distinct reddish purple letter image.

This letter impression had the advantages that it did not give rise to any convex portion on the soft contact lens, endured a long boiling test, was not affected by a cleaning solution (formed of protein-decomposing enzyme), and proved quite satisfactory for use with a soft contact lens.

Example 11.

A letter impression was made for about 3 seconds with the same kind of printing ink and the same kind of rubber stamp as used in Example 10 on a soft contact lens fabricated by swelling the same type of resin as used in Example 9 in water within 30 seconds after said contact lens was taken out of the water. The soft contact lens thus impressed was left about 10 minutes in an atmosphere having a temperature of 40"C and a humidity of 90%. And thereafter when dipped the lens about 15 minutes in a dye bath formed of an aqueous solution of sulphuric acid of 60 Be (of concentration about 10 cc/l) and sodium.

nitrite (of concentration about 0.5 g/l), then the soft contact lens had a letter impression of concentrated reddish purple colour. This letter impression indicated a more concentrated colour than that which was formed without being left in an atmosphere of high temperature and humidity, and moreover was elevated about four fold in resistance to boiling due to the deeper penetration of the dyestuff into the soft contact lens.

Example 12.

A porous rubber plate impregnated with a 20% aqueous solution of 2,3,5-triphenyl-2Htetrazolium chLoride made a letter impression on a transparent film formed of a cross-linked copolymer of polymethyl methacrylate and N-vinylpyrrolidone which had been dipped about 20 minutes in methanol for swelling.

The film thus impressed was exposed for about 30 seconds to a light at a spot spaced about 15 cm from a 2-kilowatt mercury lamp, providing a distinct red water-insoluble letter impression. The letter impression proved to be highly wear-resistant and waterproof.

Where the same type of film as described above was swollen by being dipped in water alone, the resultant letter impression had a very faint colour.

Example 13.

A moulding of cross-linked copolymer of 2-hydroxyethyl methacrylate and methyl methacrylate was dipped for about 2 minutes in a bath of 2-ethoxyethanol to swell the surface of the moulding. The moulding was preliminarily dipped for 10 minutes in an aqueous solution of solubilized vat dye (manufactured by Farbwerke Hoechst A.G. under the trademark "Anthrasol 04G") of concentration 30 g/l. Later, the solution deposited on the surface of the moulding was washed off. The moulding was again dipped 5 minutes in an aqueous solution of sulphuric acid of 65" Be (of concentration about 10 cc/l) and sodium nitrite (of concentration about 0.5 g/A), this being a developing bath. Then the surface of the moulding was uniformly dyed blue. Later, the moulding was water washed to remove unreacted dyestuff. To examine dyeability, the moulding was placed scores of hours in boiling water (dye fixation test), but was not substantially decolourised. Application of another form of the above-mentioned vat dye to dye the moulding gave approximately the same results.

Example 14.

A picture image was projected by means of a 5-kilowatt mercury lamp on the same kind of resin moulding as used in Example 13 which had been preliminarily dipped in 2-ethoxyethanol for swelling through a transparent negative, providing a distinct blue impression.

Later, the impressed moulding was thoroughly washed with boiling water to remove the dyestuff remaining on the non-exposed portion of the moulding. A picture impression thus dyed fully endured the same dye fixation test as applied in Example 13.

Example 15.

A soft contact lens fabricated only from cross-linked 2-hydroxyethyl methacrylate was dipped about 15 minutes for swelling in an ethanol-water bath (whose first and second components were mixed in the ratio of 3 : 7).

A letter was impressed at a spot near the periphery of the swollen soft contact lens by a porous rubber stamp impregnated with a printing ink formed of 15 parts by weight of solubilized vat dye (manufactured by Farbwerke Hoechst A.G. under the trademark "Anthrasol Pink 13B"), 10 parts by weight of glycerine and 75 parts by weight of water.

When exposed for about one minute to nearu!traviolet rays radiated by a black light (2 mW/cm'), the soft contact lens thus impressed displayed a distinct red letter. When the soft contact lens was placed in boiling water for one hundred and scores of hours, the letter impression fully endured this continuous boiling test. Even when the soft contact lens was not dipped in an ethanol bath for swelling, a letter impressed thereon was well dyed, but with a far lower resistance to boiling.

A soft contact lens prepared from copolymer of 2-hydroxyethyl methacrylate and N-vinylpyrrolidone did pot have a very wide difference between the immersion of the lens in ethanol and the nonimmersion thereof due to the copolymer itself having a very high water-expansibility. In contrast, a soft contact lens prepared from a copolymer of 2-hydroxyethyl methacrylate and methylmethacrylate presented a noticeable difference between the immersion of the lens in ethanol and the nonimmersion therof. Thus when a letter was impressed on a soft contact lens formed of the latter copolymer but not dipped in the ethanol, the dye of the letter only slightly penetrated the texture of the copolymer. Where, therefore, the soft contact lens was washed with water, the dyed letter faded. Conversely, where the soft contact lens was previously dipped in ethanol for swelling, then a letter impressed thereon was distinctly dyed with high resistance to boiling.

Example 16.

The soft contact lenses prepared from each of the three forms of resin used in Example 15 were dipped about 15 minutes in the same developing bath as used in Example 13 instead of being exposed to ultraviolet rays to display a letter impressed on the soft contact lenses. In this case, the same good results were obtained as in Example 15.

Example 17.

A soft contact lens was dipped for about 2 minutes in a bath of the same printing ink as used in Example 15 without impressing a letter by a porous rubber stamp. A letter was projected on the soft contact lens through a transparent negative by means of a 5-kilowatt mercury lamp. The letter was distinctly displayed. The lens was boiled to remove the printing ink in those portions of the soft contact lens which were not exposed to the light.

The dyed letter indicated the same good resistance to boiling as in Example 15.

Example 18.

An impression was made on the surface of a moulding of a cross-linked high molecular copolymer of 2-hydroxyethyl methacrylate and methylmethacrylate by a rubber stamp impregnated with a printing ink formed of 18 parts by weight of solubilized vat dye (manufactured by Farbwerke Hoechst A.G. under the trademark "Anthrasol 04G"), 20 parts by weight of 2-ethoxyethanol and 68 parts by weight of water. The impressed moulding was left for about 5 minutes in an atmosphere having a temperature of 60"C and a humidity of 85%. Later, the moulding was exposed to a light about 30 seconds at a point spaced about 5 cm from a 2-kilowatt mercury lamp.

Then a dyestuff deeply penetrating the texture of the moulding had a water-insoluble distinct bluish green impression which proved highly wear-resitant and waterproof.

Example 19.

A moulding of a cross-linked copolymer of a 2-hydroxymethacrylate and N-vinylpyrrolidone was swollen by being di 13 parts by weight of glycerine and 13 parts by weight of water. When dipped in a developing bath formed of an aqueous solution of sodium perborate (of concentration 1 g/l), then the moulding had a distinct red image.

When the moulding was placed for several hours in boiling water (dye fixation test), the image appearing thereon did not fade in colour, proving that the dyestuff was firmly fixed.

WHAT WE CLAIM IS:- 1. A method of dyeing or printing a hydrogel resin, which comprises impregnating the hydrogel resin with a water-soluble vat dye or tetrazolium salt, and thereafter converting said dye or salt in the hydrogel resin to a water-insoluble, coloured material.

2. A method according to claim 1, in which the impregnation is carried out by contacting the hydrogel resin with a solution or printing ink containing the vat dye or tetrazolium salt, and thereafter swelling the hydrogel resin by exposure to an atmosphere of high temperature and humidity.

3. A method according to claim 1, in which the impregnation is carried out by swelling the hydrogel resin in water, and then contacting the swollen resin with a solution or printing ink containing the vat dye or tetrazolium salt.

4. A method according to claim 1, in which the impregnation is carried out by swelling the hydrogel resin in a solvent which has a greater resin-swelling power than water and is compatible with water, or a mixture of such solvent with water, and then contacting the swollen resin with a solution or printing ink containing the vat dye or tetrazolium salt.

5. A method according to any of claims 1 to 4, in which the hydrogel resin is contacted with said printing ink by means of a porous printing stamp soaked with the printing ink.

6. A method according to claim 1, in which the impregnation is carried out by dipping the hydrogel resin in a solution or printing ink containing the vat dye or tetrazolium salt.

7. A method according to any of claims 1 to 6, in which the water-soluble vat dye in the hydrogel resin is converted to the waterinsoluble, coloured material by oxidation.

8. A method according to any of claims 1 to 6, in which the water-soluble vat dye in the hydrogel resin is photosensitive, and is converted to the water-insoluble, coloured material by exposure to light.

9. A method according to any of claims 1 to 6, in which the tetrazolium salt in the hydrogel resin is converted to the waterinsoluble, coloured material by reduction.

10. A method according to any of claims 1 to 6, in which the tetrazolium salt in the hydrogel resin is photosensitive, and is converted to the water-insoluble, coloured material by exposure to light.

11. A method according to claim 8 or 10, in which the exposure to light is imagewise.

12. A method according to any of claims 1 to 11, in which the hydrogel resin is a crosslinked polymer of hydroxyethylmethacrylate.

13. A method according to claim 12, in which the hydrogel resin is a cross-linked copolymer of hydroxyethylmethacrylate and N-vinylpyrrolidone.

14. A method according to claim 12, in which the hydrogel resin is a cross-linked copolymer of hydroxyethylmethacrylate and methylmethacrylate.

15. A method according to any of claims 1 to 14, in which the hydrogel resin is in the form of a soft contact lens.

16. A method of dyeing or printing hydrogel resin, substantially as herein described in any of the Examples.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

13 parts by weight of glycerine and 13 parts by weight of water. When dipped in a developing bath formed of an aqueous solution of sodium perborate (of concentration 1 g/l), then the moulding had a distinct red image.

When the moulding was placed for several hours in boiling water (dye fixation test), the image appearing thereon did not fade in colour, proving that the dyestuff was firmly fixed.

WHAT WE CLAIM IS:- 1. A method of dyeing or printing a hydrogel resin, which comprises impregnating the hydrogel resin with a water-soluble vat dye or tetrazolium salt, and thereafter converting said dye or salt in the hydrogel resin to a water-insoluble, coloured material.
2. A method according to claim 1, in which the impregnation is carried out by contacting the hydrogel resin with a solution or printing ink containing the vat dye or tetrazolium salt, and thereafter swelling the hydrogel resin by exposure to an atmosphere of high temperature and humidity.
3. A method according to claim 1, in which the impregnation is carried out by swelling the hydrogel resin in water, and then contacting the swollen resin with a solution or printing ink containing the vat dye or tetrazolium salt.
4. A method according to claim 1, in which the impregnation is carried out by swelling the hydrogel resin in a solvent which has a greater resin-swelling power than water and is compatible with water, or a mixture of such solvent with water, and then contacting the swollen resin with a solution or printing ink containing the vat dye or tetrazolium salt.
5. A method according to any of claims 1 to 4, in which the hydrogel resin is contacted with said printing ink by means of a porous printing stamp soaked with the printing ink.
6. A method according to claim 1, in which the impregnation is carried out by dipping the hydrogel resin in a solution or printing ink containing the vat dye or tetrazolium salt.
7. A method according to any of claims 1 to 6, in which the water-soluble vat dye in the hydrogel resin is converted to the waterinsoluble, coloured material by oxidation.
8. A method according to any of claims 1 to 6, in which the water-soluble vat dye in the hydrogel resin is photosensitive, and is converted to the water-insoluble, coloured material by exposure to light.
9. A method according to any of claims 1 to 6, in which the tetrazolium salt in the hydrogel resin is converted to the waterinsoluble, coloured material by reduction.
10. A method according to any of claims 1 to 6, in which the tetrazolium salt in the hydrogel resin is photosensitive, and is converted to the water-insoluble, coloured material by exposure to light.
11. A method according to claim 8 or 10, in which the exposure to light is imagewise.
12. A method according to any of claims 1 to 11, in which the hydrogel resin is a crosslinked polymer of hydroxyethylmethacrylate.
13. A method according to claim 12, in which the hydrogel resin is a cross-linked copolymer of hydroxyethylmethacrylate and N-vinylpyrrolidone.
14. A method according to claim 12, in which the hydrogel resin is a cross-linked copolymer of hydroxyethylmethacrylate and methylmethacrylate.
15. A method according to any of claims 1 to 14, in which the hydrogel resin is in the form of a soft contact lens.
16. A method of dyeing or printing hydrogel resin, substantially as herein described in any of the Examples.