GB1586547A - Cross-linkable block copolymers and their use in screen printing - Google Patents

Description

(54) CROSS-LINKABLE BLOCK COPOLYMERS AND THEIR USE IN SCREEN PRINTING (71) We, ARTHUR D. LITTLE, INC., a corporation organized and existing under the laws of the Commonwealth of Massachusetts, United States of America, 25, Acorn Park, Cambridge, Massachusetts 02140, United States of America, do hereby declare 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:- This invention relates to block copolymers for use in preparing screen printing stencils, to coating compositions containing the same and to improved screen printing stencils and methods of using them. More particularly, this invention relates to the use of certain ultraviolet-sensitive copolyacrylate/polyurethane block copolymers in preparing screen printing stencils.

The art of screen printing was developed around 1900. This art was originally referred to as "silk-screen printing", that term being derived from the fine mesh silk which was originally used as the screen. According to this method of printing, a fine-mesh fabric, such as silk, is stretched across a frame and an image is formed on the fabric. In its simplest form, the so-called "silk-screen stencil" is formed by gluing a hand cut paper stencil to the silk screen. The side of the silk-screen stencil to which the paper stencil is attached is then placed against the surface to be imprinted, and ink is forced through the screen onto the surface. When the ink is forced through the openings in the silk mesh, it imprints on those areas of the surface which are not covered by pieces of the paper. Thus the printing operation is based on the ability of the ink to flow through only that part of the screen mesh which is not blocked by the paper, i.e. through the "cut image".

A more advanced method for preparing a screen printing stencil is the "tusche and glue" method. In accordance with this method, a design or image is formed on the screen with tusche, a type of lithographic ink, by painting the tusche on the screen. After the tusche has dried, the entire screen is covered with a glue, and when the glue dries, the entire screen is washed with an organic solvent, such as kerosene or turpentine. The solvent washes out the glue covering the tusche, but the remainder of the glue is retained on the screen. Thus, the only mesh areas which remain open for ink transfer are those defining the original tusche design.

An even more advanced method of forming screen printing stencils involves the use of photosensitive compositions to form images in response to light transmitted through a master. This method is discussed in U.S. Patent No.

3,246,986. In accordance with this method, a screen is coated with a colloidal solution containing a light sensitizer. The coated screen is allowed to dry and then exposed to light through a master (such as a photographic negative). The light which passes through the master and onto the coated screen causes the colloid to be hardened in a pattern corresponding with that of the master. Those portions of the coated screen which are protected from the light by the nontransparent part of the master are unaffected by the light, and the coating thereon remains unhardened. Since the unhardened colloid material is soluble in water and the hardened colloid is insoluble, an image can be "developed" on the screen by washing it with water. When this is done, the unhardened colloid material is washed away, while the image-defining hardened material remains on the screen, thus forming a screen printing stencil. The screen printing stencil so prepared may then be used for printing the image on receptor surfaces, such as paper and fabric.

Various colloids and light sensitizers have been used to prepare the light-sensitive coatings used in this method. Typical colloids used in preparing screen printing stencils by this method are listed in U.S. Patent No. 3,246,986 and include polyvinyl alcohol, partially acrylated polyvinyl alcohol, partially hydrolyzed polyvinyl acetate, partially acetylated polyvinyl alcohol, polyacrylic acid, methyl cellulose and gelatin. Typical light-sensitizing compounds used are potassium dichromate, ammonium dichromate and condensation products of diazo diphenylamines with aldehydes.

Unfortunately, these methods of preparing screen printing stencils and the screen printing stencils prepared by such methods often have undesirable characteristics. For example, many of the screen printing stencils prepared by the prior art photochemical methods do not perform well with water-based inks, because the indicia-defining materials on the screen printing stencils prepared by these methods are generally sensitive to the water-based inks, which are usually alkaline in nature. Although screen printing stencils which are compatible with water-based inks may be prepared using certain lacquers to define indicia thereon, we are aware of no photochemical method for preparing screen printing stencils with such materials.

Even where water insensitivity is not required, the prior art photochemical methods of preparing screen printing stencils are not completely satisfactory. In this regard, the use of dichromate light sensitizers in the prior art methods is of particular concern, since they are generally environmentally unacceptable, poisonous compounds.

Therefore a need exists for a new method of preparing screen printing stencils, for new coating compositions and copolymers useful in preparing screen printing stencils, for new screen printing stencils and for new screen printing methods.

It is therefore a primary object of this invention to provide a new photochemical method of preparing screen printing stencils without the use of the prior art diazo or dichromate light sensitizing compounds. It is another object of this The present invention provides a block copolymer of a polyurethane which is derived from a polyether or polyester polyol and which is connected through terminal urethane groups to addition polymer blocks derived by the free radical copolymerisation of one or more hydroxyalkyl acrylates or methacrylates and one or more dibromoalkyl acrylates or methacrylates, with or without acrylic acid, methyl methacrylate or butyl methacrylate.

These block copolymers may be used, in accordance with a feature of the invention to provide an ultra-violet cross-linkable liquid coating composition which comprises, in addition to the block copolymer a solvent for the block copolymer, an ultra-violet initiator and a cross-linking promoter.

Screen printing stencils may be produced by affixing to a transparent supporting sheet a coating of the above ultra-violet cross-linkable liquid coating composition, drying the affixed coating to produce a coated support sheet, forming in the resulting coating a predetermined indicia pattern, transferring the resulting indicia pattern to a printing screen substrate and exposing the indicia pattern to ultra-violet radiation. Alternatively the screen printing stencils may be made by affixing to a printing screen substrate a coating of the above coating composition, drying the coating to produce a coated printing screen substrate, forming in the coating a predetermined indicia pattern and exposing the indicia pattern to ultraviolet radiation.

In order to carry out screen printing a screen printing stencil produced by the above method then has its indicia-bearing side held in contact with a receptor surface and ink is then forced through those areas of the stencil which are not covered by indicia and into the contact with the underlying surface of the receptor.

The invention will now be described with reference to the following detailed description taken in connection with the accompanying drawings in which: Figs. 1--11 illustrate the steps in one method embodiment of this invention Fig. 12 illustrates the use of a screen printing stencil of this invention in printing on a receptor surface; Figs. 13-17 illustrate the steps of another method embodiment of this invention; Figs. 18-20 illustrate the steps of yet another method embodiment of this invention and Fig. 21 illustrates the steps of still another method embodiment of this invention.

The preparation of screen printing stencils in accordance with the method of this invention is illustrated diagrammatically in Figs. 1--11 in which the same reference numerals are used to identify the same components.

Fig. 1 illustrates one type of master 10 which is used in the method of this invention. This master is a negative transparency having a nontransparent background 11 and transparent indicia pattern 12. It is also, of course, within the scope of this invention to use a positive transparency as a master which is the reverse of that shown in Fig. 1.

As shown in Fig. 2, the ultraviolet-sensitive coating composition 21 of this invention is applied to one side of a light transparent support sheet 20, such as a polyethylene terephthalate sheet. The coated support sheet 20 is then (Fig. 3) exposed to an ultraviolet radiation source 25 through master 10, having ultraviolettransparent indicia 12 and nontransparent background 11. The ultraviolet radiation passing through indicia 12 of master 10 causes the partial cross-linking 22 (Fig. 4) of a portion of the coating 21 in a pattern corresponding to that of indicia 12, while the remainder of coating 21, which is protected from irradiation by nontransparent background 11 of master 10, is unaffected. Indicia 22 are then "developed" on support sheet 20 by placing support sheet 20 in an etching bath containing a water/alcohol solution, such as 1/3 (by volume) water/ethyl alcohol, to dissolve away the unexposed coating composition 21 leaving on the support sheet only the exposed, partially cross-linked indicia-defining coating 22 as shown in Figs. 5 and 6.

After drying indicia-defining coating 22, the coated side of support sheet 20 is brought into contact with a suitable screen 23 so that indicia-defining coating 22 is between screen 23 and support sheet 20, thus forming an assembly having indiciadefining, partially cross-linked coating 22 interposed between support sheet 20 and screen 23 as shown in Fig. 7. The indicia-defining coating 22 extends up into the interstices of screen 23 to form a good bond. Screen 23 is then moistened with an appropriate solvent, such as the water/alcohol solution used in the development step, and allowed to stand for about 30 seconds to improve the adhesion of indiciadefining coating 22 to screen 23. Alternatively, coated support sheet 20 may be brought into contact with screen 23 while indicia-defining coating 22 is still wet from the development step, in which case it will not be necessary to moisten the screen. Excess solvent is then removed by blotting with a water-absorbent surface such as newsprint. The assembly shown in Fig. 7 is then air dried for about 15 minutes, after which support sheet 20 is removed (Fig. 8) to leave indicia-defining coating 22 on screen 23 to form screen printing stencil 24 as shown in Figs. 9 and 10. The coating-bearing side of screen printing stencil 24 is dried for about 10 minutes, and then coating 22 on screen printing stencil 24 is post-cured by exposing it to ultraviolet radiation (Fig. 11), which completes the cross-linking of the partially-cross-linked, indica, defining coating composition 22 and promotes its bonding to screen 23.

Fig. 12 illustrates the use of the screen printing stencil of Fig. 11 in printing indicia onto a suitable receptor surface 30. As illustrated, the indicia-bearing side of screen printing stencil 24 is contacted with receptor surface 30. Printing ink is applied to the opposite side of screen printing stencil 24 and forced through those areas 26 (Fig. 12) of screen printing stencil 24 which are not blocked by indiciadefining coating composition 22. The ink passing through the screen printing stencil and onto receptor surface 30 defines indicia on receptor surface 30 which thereby becomes a right reading copy of master 10.

Figs. 13-18 illustrate the preparation of screen printing stencils in accordance with another embodiment of the method of this invention. As shown in Fig. 13, transparent support sheet 20, which is coated with coating composition 21, is brought into contact with screen 23 in such a manner as to create an assembly having coating composition 21 interposed between support sheet 20 and screen 23.

Screen 23 is then moistened with an appropriate solvent, such as a water/alcohol solution or some of the original coating solution, to help bond screen 23 to coating 21 on support sheet 20. Excess solvent is removed by blotting with newsprint or the like. The assembly is then dried in a manner not to induce premature cross-linking after which it is exposed to ultraviolet radiation through indicia master 10, from the direction of the support sheet side (Fig. 14). The ultra-violet radiation passes through transparent indicia 12 of master 10, then through transparent support sheet 20 to coating 21. This causes the partial cross-linking of those areas of coating 21 which correspond to indicia 12 of master 10 (Fig. 15). The remainder of coating 21, being protected from irradiation by nontransparent area 11 of master 10, is unaffected.

Support sheet 20 is then removed (Fig. 16) leaving the coating (both cross linked 22 and noncross-linked 21) on screen 23 (fig. 17). Indicia are then "developed" from the coating on the screen by placing the screen in an etching bath containing a water/alcohol solution which dissolves away noncross-linked coating 21 leaving only partially cross-linked indicia-defining coating 22 on the screen as previously shown in Fig. 9. The resulting unfinished screen printing stencil 24, carrying partially cross-linked indicia-defining coating 22 adhered thereto, is then dried and exposed to ultraviolet radiation to complete the cross linking of indicia-defining coating 22 and promote its bonding to the screen, as previously shown in Fig. 11. This completes the screen printing stencil.

Figs. 18-20 illustrate the preparation of screen printing stencils in accordance with still another method embodiment of this invention. In accordance with this embodiment coating composition 21 is applied directly onto both sides of screen 23, as shown in Fig. 18. Then, as shown in Fig. 19, coated screen 23 is exposed to ultraviolet radiation source 25 through master 10. The ultraviolet radiation passing through transparent indicia 12 of master 10 causes partial cross-linking 22 (Fig. 20) of that part of coating composition 21 corresponding to the pattern of indicia 12, while the remainder of coating 21, being protected from the ultraviolet radiation by the nontransparent background of master 10, is unaffected. Indicia are then "developed" from the coating on the screen by placing the screen in an etching bath containing a water/alcohol solution which dissolves away the noncross-linked coating 21 leaving only the partially cross-linked indicia-defining coating 22 on the screen as previously described. The screen is then dried and exposed to ultraviolet radiation to complete the cross-linking of coating 22 and promote its bonding to the screen, as previously illustrated in Fig. 11. The screen printing stencil is thus completed.

Fig. 21 illustrates the preparation of screen printing stencils in accordance with yet another method embodiment of this invention. In accordance with this embodiment, coating composition 21 is applied to the surface of support sheet 20.

After drying, indicia pattern 40 is formed from coating 21 on support sheet 20 but cutting out a portion of the coating corresponding to the pattern and removing that portion 41 of the coating which is excess to the pattern. The indicia pattern is then transferred to screen 23 and the screen printing stencil completed as previously shown in Figs. 7-11.

This invention involves the use of the above defined block copolymers and of coating compositions made therefrom. The block copolymers of this invention are copolyacrylate/polyurethane and copolymethacrylate/polyurethane block copolymers in which the copolyacrylate and copolymethacrylate blocks are copolymers of at least one hydroxy-alkyl acrylate or methacrylates and at least one dibromoalkyl acrylate or methacrylate. The polyurethane blocks are polyether urethane or polyester urethane, although polyether urethane is more preferred.

The block copolymers of this invention are prepared by the same general method as that disclosed by Tobolsky in U.S. Patents Nos. 3,865,898, 3,291,859, and 3,257,476. In accordance with the general method disclosed in those patents, a polymer which is a polyether or polyester polyol, is reacted with a diisocyanate to form what is referred to as a "diisocyanate-capped" prepolymer. The diisocyanate capped prepolymer is then reacted with a hydroperoxide or a dihydroperoxide, such as 2,5-dimethyl-2,5-bis(hydroperoxy)hexane to form a peroxycarbamate, such as bis(2,5-dimethyl-2-dihydroperoxyhexane-5-peroxycarbamate) polyether. Chain extension of the polyether or polyester polymer constituent may be effected in this second reaction if desired by addition of a suitable chain extender, such as butane diol, to the reaction mixture. The peroxycarbamate is then reacted with one or more of the ethylenically-unsaturated monomers which can be used, the active sites in the peroxycarbamate serving to initiate the free radical polymerization of the monomer.

The reaction forming the peroxycarbamate is best conducted in a solvent, such as toluene, at about 50 per cent by weight solids. The product of this reaction (i.e., the peroxycarbamate solution) is then mixed with additional solvent and the acrylate or methacrylate monomers which form the copolyacrylate or copolymethacrylate blocks of the block copolymers of this invention. These components are then reacted by heating and stirring, to form the copolyacrylate or copolymethacrylate polyurethane block copolymers which may then be used directly as a block copolymer-containing syrup. Alternatively, the block copolymer may be precipitated from the product syrup and recovered, as will be understood by those skilled in the art.

The polymeric constituents which are reacted with diisocyanates to form diisocyanate-capped prepolymers in the practice of this invention are hydroxylterminated polyethers or hydroxyl-terminated polyesters. Preferred hydroxylterminated polyethers are hydroxyl-terminated polyoxypropylene and hydroxylterminated polyoxyethylene and preferred hydroxyl-terminated polyesters are those formed by reacting adipic acid with propylene glycol, ethylene glycol or mixtures thereof.

The diisocyanates which may be used in the practice of this invention include, but are not limited to toluene diisocyanate, bis(4-isocyanatocyclohexyl)methane, hexamethylene diisocyanate, 4,4'-diphenylmethane diisocyanate, isophorone diisocyanate and trimethylhexamethylene diisocyanate. Bis(4isocyanatocyclohexyl)methane is preferred.

The hydroperoxides and dihydroperoxides used in preparing peroxycarbamates in the practice of this invention are those having one or more terminal -OOH groups and include, but are not limited to, cumene hydroperoxide, t-butyl hydroperoxide, tetralin hydroperoxide, phenylcyclohexane hydroperoxide, diisopropylbenzene hydroperoxide, p-menthane hydroperoxide, pinane hydroperoxide and 2,5-dimethyl-2,5-bis(hydroperoxy)hexane. Of these, 2,5dimethyl-2,5-bis(hydroperoxy)hexane is preferred.

The hydroxyl alkyl acrylates and methacrylates present in the copolyacrylate blocks of the block copolymers of this invention include, but are not limited to, 2hydroxyethyl acrylate, hydroxypropyl acrylates, 2-hydroxyethyl methacrylate and hydroxypropyl methacrylates. Of these, 2-hydroxyethyl methacrylate is preferred.

The dibromoalkyl acrylates and methacrylates which are used in forming the copolyacrylate and copolymethacrylate blocks of the block copolymer of this invention include, but are not limited to dibromopropyl methacrylate and dibromopropyl acrylate. Of these, dibromopropyl methacrylate is preferred.

The preparation of the copolymers of this invention is illustrated in greater detail in the following nonlimiting examples.

Example 1 500 grams of dry polyoxyethylene having a molecular weight range of 3000 4000, 292 grams of toluene, 93.7 grams of bis(4-isocyanatocyclohexyl)methane and 0.5 gram of dibutyltin dilaurate were added to a 1.5 liter 3-necked reaction flask equipped with an agitator and a nitrogen purge and heated in a controlled temperature oil bath. The contents were purged with dry nitrogen and heated rapidly to 750C while being agitated. An isocyanate analysis (ASTM Test No.

D2572-67T) was made after 5 hours at 750C and, based upon the results of this analysis, 16.3 grams of 2,5-dimethyl-2,5-bis(hydroperoxy)hexane, 7.9 grams of butane diol, 2.6 grams of dibutyltin dilaurateand 189 grams of toluene were added to the flask, and the temperature maintained at 500C for a period of 4 to 5 hours. At that time isocyanate analysis indicated an isocyanate content of less than 0.05 per cent by weight, which was taken as an indication that the reaction was complete.

The clear, viscous peroxycarbamate solution was transferred to glass bottles and refrigerated. Upon cooling, the product became a soft, waxy, opaque solid. Total solids were determined to be 54 per cent by weight.

Example 2 54.1 grams of the peroxycarbamate prepared in Example 1, 71.8 grams of 2hydroxyethyl methacrylate and 250 ml of ethylene glycol monomethyl ether were added to a 1000 ml 3-necked flask, equipped with an agitator, a carbon dioxide purge, a thermometer and a pressure equalizing addition funnel, and heated by a controlled-temperature oil bath. The contents were mixed until all ingredients were dissolved. 43.1 grams of 2,3-dibromopropyl methacrylate and 94 ml of ethylene glycol monomethyl ether were then added to the addition funnel. Both the flask and the addition funnel were purged with carbon dioxide gas for 3 minutes, after which time a slow carbon dioxide purge was maintained by subsurface bubbling through the liquid in the flask, with the purge gas escaping through a fine bore stopcock on top of the addition funnel. The temperature of the flask contents was brought to 800C and the 2,3-dibromopropyl methacrylate of the addition funnel was uniformly added over a 5-hour period. The temperature was maintained at 80"C for an additional three hours after the last of the 2,3-dibromopropyl methacrylate had been added to assure completion of the reaction.

The resulting polymer syrup had a viscosity of 500 cps. at 240C and a total solids content of 29 per cent by weight.

Example 3 The same procedure was followed as that described in Example 2, except that 2,3-dibromopropyl acrylate on an equal basis was substituted for the 2,3dibromopropyi methacrylate of Example 2. The resulting polymer syrup had a viscosity of 738 cps. at 16 C and a total solids content of 28.1 per cent by weight.

As will be understood by those skilled in the art, the relative amounts of polyurethane to copolyacrylate or copolymethacrylate, as well as the relative amounts of the acrylates and methacrylates making up the copolyacrylate and copolymethacrylate blocks of the block copolymers of this invention, may be varied by varying the relative amounts of the corresponding reactants used in preparing them.

In the practice of this invention the relative amounts of the various components making up the block copolymers are balanced to achieve desired characteristics in coatings made with the block copolymers. Thus, the relative amount of polyurethane in the block copolymer must be adjusted to achieve a balance of properties in the coating films when used to prepare screen printing stencils. For example, if too much polyurethane is present in the block copolymer, the coating films will be flexible but will lack durability; if too little polyurethane is present, the coating films will exhibit durability, but will be too brittle. The relative amount of dibromoalkylacrylate and/or methacrylate to hydroxy-alkyl acrylate and/or methacrylate must also be balanced in order to control the amount of crosslinking which may be effected in the coating film. The extent of cross-linking of the coating film is determined by the number of cross-linking sites, such as the dibromoalkyl groups on the block copolymer, and therefore, on the amount of dibromoalkyl acrylate and/or methacrylate present in the block copolymer. The relative amount of hydroxy-alkyl acrylate and/or methacrylate affects the solubility of the noncross-linked copolymer in polar solvents such as water/alcohol mixtures.

This characteristic is of particular importance during the "development" step of the method of this invention for preparing screen printing stencils.

We have found that block copolymers having characteristics which are desirable for the practice of this invention preferably comprise from 60 to 90 per cent by block copolymer weight of copolyacrylate or copolymethacrylate and from 10 to 40 per cent by weight of polyurethane. The copolyacrylate or copolymethacrylate preferably comprises from 50 to 95 per cent by copolyacrylate or copolymethacrylate weight of hydroxy-alkyl acrylate or methacrylate and from 5 to 50 per cent by weight of acrylate or methacrylate substituted with bromine or iodine. A preferred block copolymer for making screen printing stencils according to this invention is one comprising substantially 80 per cent copolyacrylate or copolymethacrylate and substantially 20 per cent polyurethane by weight, the copolyacrylate or copolymethacrylate comprising substantially 60 per cent 2hydroxyethyl methacrylate and substantially 40 per cent 2,3-dibromopropyl methacrylate by weight.

In addition to flexibility and durability, it is also possible to introduce other specific characterics into the block copolymers by including in the copolyacrylate blocks certain other.acrylates or methacrylates, in amounts of up to 15 per cent of copolyacrylate or copolymethacrylate by weight. Thus, for example, the substitution of acrylic acid for some of the hydroxy-alkyl acrylate or methacrylate will impart to the total copolymer a sensitivity to basic solvents, such as ammonia and sodium hydroxide solutions. This may be an important characteristic where it is desired to recycle used printing screens by washing the coatings from them with basic solvents. This particular substitution should only be made, however, in block copolymers used for preparing screen printing stencils for use with nonwater-based inks. This is because the water sensitivity of coating films made from block copolymers having this substitution is increased, thereby rendering them less durable in the presence of water than they would otherwise be. In a similar manner, the substitution of methyl methacrylate for some of the hydroxy-alkyl acrylate or methacrylate increases the hardness and water resistance of coating films made from the block copolymers, whilst substitution of butyl methacrylate for some of the hydroxy-alkyl acrylate or methacrylate imparts improved flexibility to the coating films. Therefore, where such specific characteristics are desired, the block copolymers comprise from 60 to 90 per cent by block copolymer weight of copolyacrylate or copolymethacrylate and from 10 to 40 per cent by weight of polyurethane, the copolyacrylate or copolymethacrylate comprising from 50 to 95 per cent by copolyacrylate or copolymethacrylate weight of hydroxy-alkyl acrylate or methacrylate, from 5 to 50 per cent by weight of dibromoalkyl acrylate or methacrylate and up to 15 per cent by weight of acrylic acid, methyl methacrylate or butyl methacrylate.

Although polymer syrups, such as those prepared in Examples 2 and 3 may be used directly in preparing the coating compositions of this invention, it may be preferable in some circumstances to recover the block copolymer from the syrup and use it in a purified form. The block copolymer may be recovered from the syrup in which it is prepared by precipitating it in an excess of water, removing the water and vacuum drying the precipitate, as is well known in the art. When such purified copolymer is used instead of the syrup, some improvement in the quality of developed indicia may result; and an improved response to ultraviolet radiation may also be observed. It is, of course, necessary to weigh the benefits of these improvements against the additional processing step required to achieve them.

In practicing this invention, we prefer to apply the above described block copolymers to substrates as solutions in suitable solvents. It is also preferred to add to the solution at least one cross-linking promoter and at least one ultraviolet.

initiator. Thus, a feature of this invention is a printing screen coating composition comprising a solution of the block copolymer in a suitable solvent containing a cross-linking promoter and an ultraviolet initiator.

When the purified form of t compounds serve to extend the degree of cross-linking which may be achieved by providing bridges or links between reactive sites of different block copolymer molecules. The nature of the cross-linking promoter and the amount used influence the degree of cross-linking, and therefore, the hardness and durability of the final coating. The hardness of the final coating, in turn, influences the performance characteristics of screen printing stencils having indicia defined by such final coatings. For example, if an insufficient degree of cross-linking is achieved, the indicia-defining coating may be too soft, and may not wear well during printing operations. If, on the other hand, too much cross-linking is achieved, the indicia-defining coating may be too hard and brittle, and may crack during printing operations thereby resulting in defective prints. There are many cross-linking promoters known to those skilled in the art which may be advantageously used in formulating the coating compositions of this invention, and it is well within the expertise of those skilled in the art to determine the relative amounts to use in the formulation. Preferred cross-linking promoters for use in preparing the coating compositions of this invention include, but are not limited to, polyethylene glycol diacrylates, pentaerythritol triacrylate, trimethylolpropane triacrylate and mixtures thereof. These are preferably used in concentrations of up to 20 per cent by weight based on the total weight of copolyacrylate or copolymethacrylate polyurethane block copolymer used.

We have also found it helpful to add small amounts of pigmenting material to the coating compositions of this invention. The presence of such pigmenting material improves the visibility of the coating composition during the preparation of the screen printing stencils of this invention as well as that of the indicia on the final screen printing stencils. This can be an important factor in discovering errors or defects during the preparation of the screen printing stencils, and prior to their use in printing operations. Although there may be other pigments which are useful for the purposes of this invention, we prefer to use phthalocyanine blue in concentrations of from 0.1 to 3.0 per cent by block copolymer weight.

We have also observed that, for reasons not fully understood, the solvent resistance of coatings formed from coating compositions based on block copolymers in which the dibromoalkyl acrylate or methacrylate is dibromopropyl acrylate, is improved by the presence in the coating composition of small amounts (i.e., about 1 per cent by block copolymer weight) of dimethylaminoethyl acrylate.

The coating compositions of this invention may be formulated from their individual components using conventional techniques. Thus, for example, the block copolymer may be added to the solvent and stirred until it is all in solution; then the ultraviolet initiator and cross-linking promoter may be added. A conventional mixer, such as a high-speed mixer may be used, and the whole operation may be conducted at room temperature. Since the coating compositions of this invention are generally noncorrosive, no special materials of construction are required, and conventional processing equipment may be used; however, we prefer to use stainless steel or glass lined equipment to minimize the possibility of contamination. The preparation of the coating compositions of this invention is further illustrated by Example 4, it being understood that this example is illustrative only.

Example 4 The polymer syrup prepared in Example 2 was mixed with phenanthrenequinone (ultraviolet initiator), trimethylpropane triacrylate (crosslinking promoter) and tetraethylene glycol diacrylate (cross-linking promoter) and filtered through glass fiber in a pressure filter. Phthalocyanine blue pigment (as a dispersion in ethylene glycol monomethyl ether) was added to the filtered solution which was then thoroughly mixed with a high-speed mixer to assure adequate pigment dispersion.

Since the resulting coating composition was sensitive to light, the formulation was carried out in amber glass containers and the final product was stored in amber glass containers.

A second coating composition was prepared following the same procedure as described above except that the polymer- syrup prepared in Example 3 was used and a small amount of dimethylaminoethyl acrylate was added to the formulation.

The actual formulation amounts for these two coating compositions are shown under "A" and "B", respectively in Table I.

Each of the two coating compositions thus prepared was used to prepare screen printing stencils by the method of this invention. The resulting screen printing stencils were used to print with a standard laboratory printing press and produced satisfactory results.

A third coating composition was prepared by the same procedure as the first two and using a polymer syrup such as that prepared in Example 2, except that uranyl nitrate was used as the ultraviolet initiator instead of phenanthrenequinone.

This coating composition was then used to prepare a screen printing stencil. The actual formulation amounts for this coating composition are shown under "C" in Table I.

TABLE I % by Weight of Total Composition Composition A B C polymer syrup from Example 2 (29 /n solids) 91.8 - - polymer syrup from Example 3 (28.1 /n solids) - 93.4 polymer syrup (29% solids) 96.4 phenanthrenequinone 1.9 2.6 uranyl nitrate 0.1 trimethylolpropane triacrylate 2.1 0.6 0.6 triethylene glycol diacrylate 2.1 0.6 0.6 phthalocyanine blue solution* 2.1 2.1 2.3 dimethylaminoethyl acrylate - 0.7 7 *8.7% solids in ethylene glycol monomethyl ether It will be understood by those skilled in the art, that it is important that the temporary support sheet (e.g, 20 of Fig. 3) used in the practice of the invention be transparent to ultraviolet radiation. The temporary support sheet should also have favorable adhesion and release characteristics with respect to the coating compositions of this invention. The adhesion between the coating and the support sheet should be sufficient to enable the engraving of certain parts of the coating film without disturbing the remainder of the film. It should also be strong enough to hold the image-defining cross-linked coating material during the development process, when the support sheet is washed with a water/alcohol solution. On the other hand, the adhesion between the coating and the support sheet should not be so strong as to interfere with transfer of indicia from the support sheet to the printing screen, i.e., the coating should be released to the printing screen by the support sheet.

The temporary support sheet should be sufficiently flexible to enable good contact with the printing screen, and should also be compatible with solvents, such as water/ethanol mixtures used in the development step of the method of this invention, and with the solvents used in preparing coating solutions.

While there may be many materials which are suitable for use as temporary support sheets in the practice of this invention, we prefer to use thin sheets of polyethylene terephthalate coated with an appropriate adhesion/release agent. The polyethylene terephthalate sheets are generally from 2-7 mils thick and preferably, about 3 mils thick. The adhesion/release agents with which the sheets are coated include, but are not limited to, thin films of natural or synthetic rubber, which may be applied to the support sheets as dilute (0.5-3 per cent by weight) solutions in volatile mixed aliphatic/ketone solvents.

The printing screens used in the practice of this invention are highly porous carrier materials, especially screens or fabrics made of polyamides, polyesters, silk or metals such as copper, brass, bronze and stainless steel. The screens used generally have a mesh number of 60 to 450, or more (US. standard sieves). The mesh number refers to the openings per linear inch and is measured from the center of any given wire to a point one inch away therefrom. For example, number 80-square mesh cloth would have 80x80 openings per square inch or 6400 openings per square inch.

The coating compositions of this invention may be applied to the appropriate substrates by any suitable conventional means. For example, they may be applied by painting, knife-coating, rod coating or flow coating. Subsequent to the application of the coating composition to the substrate, it may be air dried, vacuum dried or oven dried.

In practicing this invention, we have found that satisfactory results may be achieved with dry coating thicknesses of from 0.3-2.0 mils, although this range is by no means critical. As will be apparent to those skilled in the art, the coating may be applied in several layers to obtain the desired thickness.

The amount of ultraviolet radiation used in forming the screen printing stencils of this invention will vary according to several parameters, such as coating thickness, amount of ultraviolet initiator used and degree of cross-linking desired.

In this regard, it should be noted that less radiation is required for some steps than for others. Thus, where the coated support sheet is irradiated through a master, as shown in Fig. 3, to define an indicia with partially cross-linked material it will suffice to irradiate it only to the extent required to render the affected part of the coating insoluble in the solvent used for the subsequent development step. Where, on the other hand, the irradiation is used to complete cross-linking and bond the indicia-defining coating to the printing screen, as shown in Fig. 8, somewhat more irradiation may be required. Thus, in practicing this invention, the coatings are irradiated with ultraviolet radiation of sufficient intensity and for sufficient time to achieve the desired degree of cross-linking. As will be recognized by those skilled in the art, there are many ultraviolet radiation sources which may be used in the practice of this invention. These include, but are not limited to, carbon-arc lamps, high-intensity tungsten filament lamps, metal-halide lamps and mercury-vapor lamps. While the radiation intensity and exposure time will vary for different materials and different radiation sources, we have found that when using a standard 12-ampere carbon-arc source in the practice of this invention, exposure at a distance of 18 inches for periods of from about 2 to 4 minutes are sufficient to partially polymerize the coating compositions and render them insoluble in the development solvent, and exposures at a distance of 12 inches for periods of from about 5 to 10 minutes are usually sufficient to post-cure the coatings on the printing screens.

While there are many solvents which may be used as developer solutions to wash away the noncross-linked coating composition in the practice of this invention, we prefer to use water/alcohol solutions such as water/ethanol or water/iso-propanol mixtures. The ratios of water/alcohol in these solutions is usually in the range of from 1/1 to 1/3.

The practice of this invention for preparing screen printing stencils is further illustrated by the folldwing non-limiting examples.

Example 5 An 8xl0-inch support sheet of 3-mil polyethylene terephthalate film was coated with a thin layer of an adhesion/release agent applied as a dilute (0.75 by weight solids) solution of a white transparent rubber cement in a volatile mixed aliphatic/ketone solvent using a No. 3 wirewound Meyer rod. After the adhesion/release agent coating was allowed to dry, a coating of the printing screen coating composition prepared in Example 4 (Formulation "A" of Table I) was applied using a coating knife having a l0-mil clearance. The resulting coated support sheet was then air dried for about 2 hours and oven dried at 600C for an additional 2 hours.

The coated support sheet was then exposed through a standard test positive (from the direction of the uncoated side) to a standard 12-ampere carbon-arc light at a distance of about 18 inches for a period of about 3 minutes. The exposed coated support sheet was then placed in an etching bath of 1/3 (volume) water/ethyl alcohol solution and the unexposed coating was dissolved away leaving a printed negative image of exposed, partially cross-linked coating composition on the support sheet. The image-bearing support sheet was then air dried in front of a fan for about 10 minutes. The image-defining coating was then transferred from the support sheet to a clean, frame-mounted printing screen as follows: The support sheet was placed, coated side up, on a flat, raised surface slightly smaller than the inside dimension of the printing screen frame. The frame-mounted printing screen was then lowered onto the support sheet with the ink or squeegee side up. The screen was swabbed with a 1/1 water/ethyl alcohol blend and allowed to stand for about 30 seconds to soften the surface of the coating. The screen was then covered with newsprint and gently rolled to press the surface of the screen against the coated support sheet while soaking up excess solvent with the newsprint at the same time. Care was taken not to exert excessive pressure which would distort the image.

The screen/support sheet assembly was then placed in front of a fan and air dried for about 15 minutes, after which the support sheet was peeled off of the printing screen leaving the coating (i.e., the design) on the printing screen. The coating-bearing side of the printing screen was then air dried in front of the fan for about another 10 minutes. The printing screen was then exposed to a standard 12ampere carbon arc ultraviolet source at a distance of about 12 inches for about 10 minutes on each side to complete the cross-linking of the image-defining coating and promote its bonding to the screen, thus completing the screen printing stencil.

The resulting screen printing stencil was used to print with a standard laboratory screen printing press and produced satisfactory prints.

Example 6 An 8x 10-inch support sheet of 3 mil clear polyethylene terephthalate film was coated with a thin layer of an adhesion/release coating as a dilute (0.75 per cent by weight solids) solution of a white, transparent rubber cement in a volatile mixed aliphatic/ketone solvent using a No. 3 wire-wound Meyer rod. After the adhesion/release coating was dried, two successive castings of the coating composition of Example 4 (Formulation "A" of Table I) were applied using a casting knife with a clearance of 6 mils. After each layer of the coating composition was applied, the coated support sheet was air dried for 2 hours and oven dried at 60"C for 2 additional hours.

A clean, frame-mounted printing screen was then placed on top of the coated side of the support sheet. A thin film of the printing screen coating composition of Example 4 was then cast on top of the screen as an adhesive to bond the screen to the coating on the support sheet, after which the entire assembly was air dried in front of a fan for about 10 minutes. The assembly was then exposed through a standard test negative to a standard 12-ampere carbon-arc ultraviolet source at a distance of about 18 inches for a period of about 3 minutes. The support sheet was then removed, leaving the coating on the printing screen. The printing screen was placed in an etching bath of 1/1 (by volume) water/ethyl alcohol solution, and the unexposed coating composition was washed away leaving a printed positive image of exposed, partially cross-linked coating composition on the printing screen. The developed printing screen was then air dried in front of a fan for about 10 minutes on each side and then exposed to a standard 12-ampere carbon-arc ultraviolet source at a distance of about 12 inches for about 10 minutes on each side. The resulting screen printing stencil was used to print with a standard laboratory screen printing press and produced a number of good prints.

Example 7 A coating of the printing screen coating composition of Example 4 (Formulation "A" of Table I) was applied to both sides of a clean, frame-mounted printing screen and allowed to dry. The coated screen was then exposed, developed, dried and post cured as in the previous example. The resulting screen printing stencil was used to print with a standard laboratory screen printing press and produced good prints.

Example 8 An 8x 10-inch support sheet of 3-mil clear polyethylene terephthalate film was coated with a thin layer of an adhesion/release coating applied as a 3 per cent by weight solution of a white transparent rubber cement in a solvent comprised of a heptane/hexane/methyl ethyl ketone mixture using a No. 7 wire-wound Meyer rod.

After the adhesion/release coating was dried, two successive castings of the coating composition of Example 4 (Formulation "A" of Table I) were applied using a casting knife having a clearance of 6 mils. The coated sheet was air dried for 2 hours and oven dried for 2 hours at 600C after each layer of the coating composition was applied.

Once the coatings were dry, a sharp stencil-cutting knife was used to cut a design in the coating material, taking care not to score the polyethylene terephthalate film itself. The unwanted portion of the coating material was peeled away from the support sheet and discarded.

The completed design was transferred to a printing screen by the technique described in Example 5, and then cured by exposure to ultraviolet irradiation as described in Example 5.

The resulting screen printing stencil was used to print 400 copies of the design using a standard laboratory screen printing press. Of the 400 copies, 200 were made with a commercially available water-based ink and, after cleaning the screen printing stencil, 200 more copies were printed using a commercially available ketone-based ink.

Visual inspection of the screen printing stencil after printing 400 copies with it showed no sign of wear, and the stencil appeared to be in substantially the same condition as it was prior to being used to print the 400 copies.

Although the coated support sheets and printing screens of the preceding examples were air dried for two hours and oven dried for an additional 2 hours following the application of the coating material to them, these drying times may be reduced so long as the solvents are essentially all removed.

The screen printing stencil of this invention is a screen printing stencil having indicia defined thereon in the form of a cross-linked block copolymer having outer blocks of copolyacrylate and an inner block of polyurethane, the outer blocks of copolyacrylate being a copolymer of at least one hydroxy-alkyl acrylate or methacrylate and at least one dibromoalkyl acrylate and/or methacrylate.

Typically, the surfaces upon which indicia are printed using the screen printing stencils of this invention are paper, fabric or plastic materials.

The screen printing stencils of this invention represent a substantial advance in the art in that they are compatible with both oil-based and water-based inks. More specifically, these screen printing stencils combine the precision of photochemically prepared screen printing stencils with the durability of alkaline and water-resistant coatings for use with such water-based inks. As will be understood by those skilled in the art, the use of water-based inks in screen printing processes, enabled by the screen printing stencils of this invention, is particularly advantageous with respect to the environmental consequences of the drying of the ink after printing. Thus, where water-based inks are used instead of oil-based inks, the environmental problems associated with evaporation of the ink solvent into the atmosphere are substantially lessened.

WHAT WE CLAIM IS: 1. A block copolymer consisting of a condensation polymer block of a polyurethane which is derived from a polyether or polyester polyol and which is connected through terminal urethane groups to addition polymer blocks derived by the free radical copolymerisation of one or more hydroxyalkyl acrylates or methacrylates and one or more dibromoalkyl acrylates or methacrylates, with or without acrylic acid, methyl methacrylate or butyl methacrylate.

2. A block copolymer as claimed in claim 1, in which said polyurethane constitutes 10 to 40 /" by weight of said block copolymer and said addition polymer blocks constitute 90 to 60% by weight thereof, said addition polymer blocks comprising 50 to 95% by weight of hydroxyalkyl acrylate or methacrylate and 50 to 5% by weight of said dibromoalkyl acrylate or methacrylate, with or without up to 15% by weight of acrylic acid, methyl methacrylate or butyl methacrylate.

3. A block copolymer as claimed in either of claims I or 2, in which said polyurethane is a polyoxyethylene polyurethane or a polyoxypropylene polyurethane, said hydroxyalkyl acrylate or methacrylate is 2-hydroxyethyl acrylate or methacrylate or a hydroxypropyl acrylate or methacrylate and said dibromoalkyl acrylate or methacrylate is a dibromopropyl acrylate or methacrylate.

4. A block copolymer according to either of claims 2 or 3, in which said polyurethane constitutes substantially 20% by weight of said copolymer and said addition polymer blocks formed by free radical copolymerisation substantially 80 /" by weight thereof, said addition polymer blocks together comprising substantially 60% by weight of 2-hydroxyethyl methacrylate and 40% by weight of 2,3dibromopropyl methacrylate or 2,3-dibromopropyl acrylate.

5. A block copolymer according to claim I and substantially as hereinbefore described.

6. An ultra-violet cross-linkable liquid coating composition comprising (a) a block copolymer according to any of claims I to 5, (b) a solvent for said block copolymer, (c) an ultra-violet initiator, and (d) a cross-linking promoter.

7. A liquid coating composition according to claim 6, in which said solvent is ethylene glycol monomethyl ether, ethyl alcohol, n-propyl alcohol or isopropyl alcohol.

8. A liquid coating composition according to claim 6 or 7, in which the concentration of block copolymer in solvent is from 20 to 40% by weight.

9. A liquid coating composition according to any of claims 6-8, in which said

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

Claims (24)

**WARNING** start of CLMS field may overlap end of DESC **. Visual inspection of the screen printing stencil after printing 400 copies with it showed no sign of wear, and the stencil appeared to be in substantially the same condition as it was prior to being used to print the 400 copies. Although the coated support sheets and printing screens of the preceding examples were air dried for two hours and oven dried for an additional 2 hours following the application of the coating material to them, these drying times may be reduced so long as the solvents are essentially all removed. The screen printing stencil of this invention is a screen printing stencil having indicia defined thereon in the form of a cross-linked block copolymer having outer blocks of copolyacrylate and an inner block of polyurethane, the outer blocks of copolyacrylate being a copolymer of at least one hydroxy-alkyl acrylate or methacrylate and at least one dibromoalkyl acrylate and/or methacrylate. Typically, the surfaces upon which indicia are printed using the screen printing stencils of this invention are paper, fabric or plastic materials. The screen printing stencils of this invention represent a substantial advance in the art in that they are compatible with both oil-based and water-based inks. More specifically, these screen printing stencils combine the precision of photochemically prepared screen printing stencils with the durability of alkaline and water-resistant coatings for use with such water-based inks. As will be understood by those skilled in the art, the use of water-based inks in screen printing processes, enabled by the screen printing stencils of this invention, is particularly advantageous with respect to the environmental consequences of the drying of the ink after printing. Thus, where water-based inks are used instead of oil-based inks, the environmental problems associated with evaporation of the ink solvent into the atmosphere are substantially lessened. WHAT WE CLAIM IS:

1. A block copolymer consisting of a condensation polymer block of a polyurethane which is derived from a polyether or polyester polyol and which is connected through terminal urethane groups to addition polymer blocks derived by the free radical copolymerisation of one or more hydroxyalkyl acrylates or methacrylates and one or more dibromoalkyl acrylates or methacrylates, with or without acrylic acid, methyl methacrylate or butyl methacrylate.

2. A block copolymer as claimed in claim 1, in which said polyurethane constitutes 10 to 40 /" by weight of said block copolymer and said addition polymer blocks constitute 90 to 60% by weight thereof, said addition polymer blocks comprising 50 to 95% by weight of hydroxyalkyl acrylate or methacrylate and 50 to 5% by weight of said dibromoalkyl acrylate or methacrylate, with or without up to 15% by weight of acrylic acid, methyl methacrylate or butyl methacrylate.

3. A block copolymer as claimed in either of claims I or 2, in which said polyurethane is a polyoxyethylene polyurethane or a polyoxypropylene polyurethane, said hydroxyalkyl acrylate or methacrylate is 2-hydroxyethyl acrylate or methacrylate or a hydroxypropyl acrylate or methacrylate and said dibromoalkyl acrylate or methacrylate is a dibromopropyl acrylate or methacrylate.

4. A block copolymer according to either of claims 2 or 3, in which said polyurethane constitutes substantially 20% by weight of said copolymer and said addition polymer blocks formed by free radical copolymerisation substantially 80 /" by weight thereof, said addition polymer blocks together comprising substantially 60% by weight of 2-hydroxyethyl methacrylate and 40% by weight of 2,3dibromopropyl methacrylate or 2,3-dibromopropyl acrylate.

5. A block copolymer according to claim I and substantially as hereinbefore described.

6. An ultra-violet cross-linkable liquid coating composition comprising (a) a block copolymer according to any of claims I to 5, (b) a solvent for said block copolymer, (c) an ultra-violet initiator, and (d) a cross-linking promoter.

7. A liquid coating composition according to claim 6, in which said solvent is ethylene glycol monomethyl ether, ethyl alcohol, n-propyl alcohol or isopropyl alcohol.

8. A liquid coating composition according to claim 6 or 7, in which the concentration of block copolymer in solvent is from 20 to 40% by weight.

9. A liquid coating composition according to any of claims 6-8, in which said

ultra-violet initiator is phenanthrenequinone or uranyl nitrate in a concentration of 0.01 to 5% of the weight of the block copolymer.

10. A liquid coating composition according to any of claims 6-9, in which said cross-linking promoter is polyethyleneglycol diacrylate, pentaerythritol triacrylate, trimethylolpropane triacrylate or a mixture thereof.

11. A liquid coating composition according to any of claims 6-10, which also includes 0.1 to 3% of the weight of block copolymer of a pigment.

12. A liquid coating composition according to claim 11 in which said pigment is phthalocyanine blue.

13. A liquid coating composition according to any of claims 6-12 which also includes dimethylaminoethyl acrylate.

14. A liquid coating composition according to claim 6 and substantially as hereinbefore described with reference to Example A and Table I.

15. A method of preparing a screen printing stencil which comprises affixing to a transparent supporting sheet a coating of a coating composition according to any of claims 6-14, drying said coating to produce a coated supporting sheet, forming in said coating a predetermined indicia pattern, transferring the resulting indicia pattern to a printing screen substrate and exposing the indicia pattern to ultraviolet radiation.

16. A method according to claim 15 in which said predetermined indicia pattern is formed by exposure through a master defining said indicia for a time at least sufficient to partially crosslink said block copolymer in the exposed areas and developing said pattern by removing that portion of said coating which has not been cross-linked by exposure.

17. A method according to claim 15 in which said predetermined indicia pattern is formed by cutting away a portion of said coating corresponding to said pattern from the supporting sheet.

18. A method of preparing a screen printing stencil which comprises affixing to a printing screen substrate a coating of a coating composition according to any of claims 6-14, drying said coating to produce a coated printing screen substrate, forming in said coating a predetermined indicia pattern and exposing the indicia pattern to ultra-violet radiation.

19. A method according to claim 18 in which said predetermined indicia pattern is formed by exposure through a master defining said indicia for a time at least sufficient to partially crosslink said block copolymer in the exposed areas and developing said pattern by removing that portion of said coating which has not been cross-linked by exposure.

20. A method of forming a screen printing stencil according to claim 15 or claim 18 and substantially as hereinbefore described with reference to any one of Examples 5-8.

21. A screen printing stencil which has been prepared by the method of any one of claims 15-20.

22. A screen printing stencil having a predetermined pattern of indicia thereon formed of a cross-linked form of a block copolymer according to any of claims 15.

23. A method of screen printing which comprises holding the indicia-bearing side of a screen printing stencil according to either of claims 21 or 22 in contact with a receptor surface and forcing ink through those areas of the stencil which are not covered by indicia and into contact with said receptor surface.

24. A method of screen printing according to claim 23 and substantially as hereinbefore described.