CA1133170A - Method of preparing screen printing stencils using novel compounds and compositions, improved screen printing stencils and method of screen printing - Google Patents

Abstract

ABSTRACT
Screen printing stencils are prepared by affixing to a printing screen substrate an indicia-defining, ultraviolet-sensitive coating and cross-linking the coating by exposure to ultraviolet radiation. The coating compositions are comprised of copolyacrylate/polyurethane block copolymers in which the copolyacrylate blocks are copolymers of at least one hydroxy-containing acrylate and at least one acrylate which is partially substituted with bromine or iodine. Screen printing stencils are provided which are compatible with both water-based and oil-based inks. The disclosed screen printing stencils are used in improved screen-printing methods.

Description

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~ his invention relates to methods of preparing screen printing stencils, to coating compositions and copolymers for use in preparing screen printing stencils, 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 copolymeric compositions 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 origi-nally 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.

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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.
~ n 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 3,246,986. In accordance with this method, a screen is coated with a colloid solution containing a light sensitizer. The coated screen is allowed to dry and then expcsed 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 to 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 wash-ing it with water. When this is done, the unhardened colloidmaterial is washed away, while the image-defining hardened mate-rial remains on the screen, thus orming a screen printing stencil.
The screen printing stencil so prepared may then be used for printing the image on receptor surfaces, such as paper, fabric and the like. 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 3,246,986 and include polyvinyl alcohol, partially acrylated polyvinyl alcohol, par-tially hydrolyzed polyvinyl acetate, partially acetylated ~3;3~7~

polyvinyl alcohol, polyacrylic acid, methyl cellulose and gela-tin. Typical light-sensitizing compounds used are potassium dichromate, ammonium dichromate and condensation products of diazo diphenylamines with aldehydes.
Unfortunately, these methods of preparing screen print-ing stencils and the screen pxinting stencils prepared by such methods often have undesirable characteristics. For example, manv 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 w:ith such materials.
; Even where water insensitivity is not required, the ~` prior art photochemical methods of preparing screen printing stencils are not completely ~atisfactory. 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 co-polymers 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 print-ing stencils without the use of the prior art diazo or dichromate light sensitizing compounds. It is another object of this invention to provide new ultraviolet-cross-linkable liquid coating compositions for preparing screen printing stencils for printing with either water-based or oil-based inks. It is another primary object of this invention to provide new copolyacrylate/polyurethane block copolymers which, when cross-linked, form materials having physical characteristics and solvent resistance suitable for coa~ings on screen printing stencils for use with either water-based or oil-based inks.
It is yet another object of this invention to provide screen printing stencils and screen printing methods for printing with either water-based or oil-based inks. Other objects o~
the invention will in part be obvious and will in part be apparent hereinafter.
The invention accordingly comprises the several steps and the relation of one or more of such steps with res~
pect to each of the others, the composition possessing the features, properties and the relation of constituents and the article possessing the ~eatures, properties and the relation ; of elements, all as exemplified in the detailed disclosure hereinafter set forth, and the scope of the invention will be indicated in the claims.
In accordance with one aspect of this invention there is provided a method of preparing a screen printing stencil, characterized by the step of affixing to a printing screen substrate an indicia-defining coating, the coating comprising an ultraviolet-cross-linked block copolymer formed of blocks of copolyacrylate and polyurethane. The copolyacry-late is a copolymer of a hydroxy-containing acrylate and an acrylate which is partially substituted with bromine or iodine.
In preferred embodiments of the method of this invention, a predetermined indicia pattern is produced on a ~33~ D

substrate coated with an ultraviolet~cross-linkable coating by exposing the coated substrate to ultraviolet radiation through a master defining the indicia as a transparent pattern for a time sufficient to partially cross-link the coating in areas corresponding to the transparent pattern, and developing the indicia pattern by removing from the substrate that portion of the coating which is not cross-linked by the exposure to the ultraviolet radiation.
In another aspect of this invention there is prov-ided an ultraviolet-cross-linkable liquid coating composition comprising a block copolymer formed of blocks of a copoly-acrylate and a polyurethane, the copolyacrylate comprising a copolymer of a hydroxy-containing acrylate and an acrylate partiall~ substituted with bromine or iodine, a solvent for the block copolymer, an ultraviolet initiator and a cross-` linking promoter.
In accordance with another aspect of this inventionthere is provided a new block copolymer formed of blocks of copolyacrylate and polyurethane, the copolyacrylate comprising a copolymer of a hydroxy-containing acrylate and an acrylate which is partially substituted with bromine or iodine.
In accordance with yet another aspect of this invention there is provided a new screen printing stencil having indicia defined thereon by a cross-linked block co-polymer formed of blocks of a copolyacrylate and of a poly-urethane, the copolyacrylate comprising a copolymer of a hydroxy-containing acrylate and an acrylate partially substi-tuted with bromine or iodine.
In accordance with still another aspect of th;s invention there is provided an improved method of screen printing wherein indicia are printed onto a receptor surface by ' ~ - 5 -~33~

bringing the indicia-bearing side of a screen printing stencil having indicia defined thereon by an indicia-defining coating into contact with the receptor surface and forcing ink through those areas of the screen printing stencil not blocked by the indicia-defining coating, the improvement comprising preparing the screen printing stencil by the method of this -invention.
In accordance with a paritcular embodiment of the invention, a method of preparing a screen printing stencil -is characterized by the step of affixing to a printing screen substrate an indicia-defining coating, said coating comprising an ultraviolet cross-linked block copolymer formed of blocks of copolyacrylate and polyurethane, said block copolymer comprising from about 60 to about 90 percent by block copolymer weight of copolyacrylate and from about 10 to about 40 percent by weight of polyurethane, said copolyacrylate weight of a hydroxy-containing acrylate and from about 5 to about 50 percent by weight of an acrylate substituted with bromine or iodine.
In accordance with a further embodiment of the invention, a method of preparing a screen printing stencil comprises the steps of a. affixing to a transparent support sheet a coating composition comprising (1) a block copolymer formed of blocks of a copolyacrylate and a polyurethane said block copolymer comprising from about 60 to about 90 percent by block copolymer weight of copolyacrylate and from about 10 to about 40 percent by weight of polyurethane, said copoly-acrylate comprising from about 5O to about 95 percent by copolyacrylate weight of a hydroxy-containing acrylate and from about 5 to about 50 percent by weight of an acrylate substituted with bromine or iodine, (2) an ultraviolet initiator, and ~ .

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(3) a cross-linking promoter, (b) drying said coating composition to form an ultraviolet cross-linkable coating on said support sheet thereby to provide a coated support sheet;
(c~ producing in said coating a predetermined indicia pattern, (d) transferring said indicia pattern to a printing screen substrate, and (e) exposing said indicia pattern on said screen substrate to ultraviolet radiation thereby to form a printing screen stencil.
In accordance with a still further embodiment of -the invention, a method of preparing a screen printing stencil comprises the steps of (a) affixing to a printing screen sub-strate a coating composition comprising (1) a block copolymer formed of blocks of a copolyaerylate and a polyurethane said block eopolymer comprising from about 60 to about 90 pereent by bloek eopolymer weight of eopolyacrylate and from about 10 : to about ~0 pereent by welght of polyurethane, said copoly-acrylate eomprising from about 50 to about 95 percent by copoly-: acrylate weight of a hydroxy-containing acrylate and ~ro~ about 5 to a~out 50 percent by weight of an acrylate substituted with bromine or iodine, (2) an ultraviolet initiator, and (3) a cross-linking promoter' (b) drying said coating composition to form an ultraviolet cross-linkable coating on said printing screen substrate, (c) producing in said coating a predetermined indicia pattern; and (d) exposing said indicia pattern on said screen subs~rate to ultraviolet radiation thereby to form a screen printing stencil.
From a different aspect, and in accordance with the invention, there is ~rovided a block copolymer formed of blocks of copolyacrylate and polyurethane, said block copolymer comprising from about 60 to about 90 percent by block copolymer weight of copolyacrylate and from about 10 - 6a -~3~

to about 40 percent by weight of polyurethane, said copoly-acrylate comprising from about 50 to about 95 percent by copolyacrylate weight of a hydroxy-containing acrylate and from about 5 to about 50 percent by weight of an acrylate substituted with bromine or iodine.
In accordance with a further embodiment of the invention, a screen printing stencil has indicia defined thereon by a cross-linked block copolymer formed of blocks of a copolyacrylate and a polyurethane, said block copolymer com-prising from about 60 to about 90 percent by block copolymerweight of copolyacrylate and from about 10 to about 40 percent by weight of polyurethane, said copolyacrylate containing from : about 50 to about 95 percent by copolyacrylate weight of a hydroxy-containing acrylate and from abou-t 5 to about 50 percent by weight of an acrylate substituted with bromine or iodine.
In accordance with a still further embodiment of the invention, an ultraviolet cross-linkable liquid coating composition comprises (a) a block copolymer formed of blocks of a copolyacrylate and a polyurethane, said block copolymer comprising from about 60 to about 90 percent by block ~:
copolymer weight of copolyacrylate and from about 1~ to about 40 percent by weight of polyurethane, said copolyacrylate comprising from about 50 to about 95 percent by copolyacrylate weight of a hydroxy-containing acrylate and from about 5 to 50 percent by weight of an acrylate substituted with bromine or iodine (b) a solvent for said block copolymer, (c) an ultra violet initiator and (d) a cross-linking promoter~
For a fuller understanding of the nature and object of this invention, reference should be had to the following detailed description taken in connection with the accompany-6b -ing drawings in which Figs. 1-11 illustrate the steps of 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 inventionO
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 1~. It is also, of course, within the scope of this invention to use a positive trans-parency as a master which is the reverse of that shown in -~
Fig. 1.
As shown in Fig. 2, the ultraviolet-sensitive coat-ing composition 21 of this invention is applied to one side of a - 6c -~3~

light transparent support sheet 20, such as polyethylene tere-phthalate. The coated support sheet 20 is then (Fig. 3) exposed to an ultraviolet radiation source 25 through master 10, having ultraviolet-transparent 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 por-tion 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 hetween screen 23 and support sheet 20, thus forming an assembly having indicia-defining, partially cross-linked coating 22 interposed between support sheet 20 and screen 23 as shown in Fig. 7. The indicia-de~ining 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 develop-ment step, and allowed to stand for about 30 seconds to improvQ
the adhesion of indicia-defining coating 22 to screen 23. Alter-natively, 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 neces-sary to moisten the screen. Excess solvent is then removed by 33~

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, indicia-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 300 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 not blocked by indicia-defining coating com-position 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.
Figs1 13-18 illustrate the preparation of screen print-ing 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 mannex as to create an assembly having coating composition 21 interposed between support sheet 20 and screen 230 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 coat-ing 21 on support sheet 20~ Excess solvent is removed by blotting with newsprint or the like. The assembly is then dried in amanner not to induce premature cross-linking after which it is exposed to ultraviolet radiation through indicia master lO, from the direction of the support sheet side (Fig. 14). The ultra-violet radiation passes through transparent indicia 12 of master lO, 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 ll 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). lndicia 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. ll. This completes the screen printing stencil.
Figs. 18-20 illustrate the preparation of screen print-ing 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 FigO 18. Then, as shown in Fig. l9, 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 tFig.
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 nontxansparent background of master 10, is unaffected. Indicia are then "de-veloped" 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 FigO 11. The screen printing stencil is thus completed.
Fig. 21 illustrates the preparation of scxeen 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 by cutting out a portion of the coating corre-sponding to the pattern and removing that portion 41 of the coat-ing 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~
The objects of this invention are attained through the use of a unique class of block copolymers and coating com-positions made therefrom. The unique block copolymers of this invention are copolyacrylate/polyurethane block copolymers in which the copolyacrylate blocks are copolymers of at least one hydroxy-containing acrylate and at least one acrylate which ~L~L33~7~

is partially substituted with bromine or iodine. Preferably, the polyurethane blocks are polyether urethane or polyester ure-thane, although polyether urethane is more preferred.
The block copolymers of this invention are prepared by the same general method disclosed by Tobolsky in U.S. Patents 3,865,898, 3,291,859, and 3,257,476. In accordance with the general method disclosed in those patents, a polymeric constit-uent such as a polyether, for example, is reacted with a diiso-cyanate to form what is referred to as a "diisocyanate-capped"
10 prepolymer. The diisocyanate-capped prepolymer is then reacted with a hydroperoxide or a dihydroperoxide, such as 2 r 5-dimethyl-2l5-bis(hydroperoxy)hexane to form a peroxycarbamate, such as bist2,5-dimethyl-2-dihydroperoxyhexane-5-peroxycarbamate) poly-ether. Chain extension of the polyether polymer constituent may be effected in this second reaction if desired by addition ; of a suitable chain extender, such as butane diol, to the reac-tion mixture. The peroxycarbamate is then reacted with one or more ethylenically-unsaturated monomers, the active sites in the peroxycarbamate serving to initiate the free radical polym-erization of the monomer.
The reaction forming the peroxycarbamate is best con-ducted in a solvent, such as toluene, at about 50 per cent by weight solids. The product of this reaction (i.e., the peroxy-carbamate solution) is then mixed with additional solvent and the acrylate monomers which form the copolyacrylate blocks of the block copolymers of this invention. These components are then reacted by heating and stirring, to form the copolyacrylate/
polyurethane block copolymers which may then be used directly as a block copolymer-containing syrup. Alternatively, the block 3~.~

copolymer may be precipitated from the product syrup and recov-ered, as will ~e 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 hydroxy-terminated polyethers or hydroxy-terminated polyesters. Preferred hydroxy-terrninated poyethers are hydroxy-terminated polyoxypropylene and hydroxy-terminated polyoxyethylene and preferred hydroxy-terminated polyesters are those formed by reacting adipic acid with propyl- -ene 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 diiso-cyanate, bis(4-isocyanatocyclohexyl)methane, hexamethylene diiso-cyanate, 4,4'diphenylmethane diisocyanate, isophorone diisocyanate and trimethylhexamethylene diisocyanate. Bis(4-isocyanatocyclo-hexyl)methane is preferred.
The hydroperoxides and dihydroperoxides used in pre-paring 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, diiso-propyl benzene hydroperoxide~ p-menthane hydroperoxide, pinane hydroperoxide and 2,5-dimethyl-2,5-bis(hydroperoxy)hexane.
Of these, 2,5-dimethyl-2,5-bis(hydroperoxy)hexane is preferred.
The hydroxy-containing acrylates present in the copoly-acrylate blocks of the block copolymers of this invention include, but are not limited to, 2-hydroxyethyl acrylate, hydroxypropyl acrylates, 2-hydroxyethyl methacrylate and hydroxypropyl methacrylates. Of these, 2-hydroxyethyl methacrylate is preferred.

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The acrylates which are partially substituted with bromine or iodine and which are present in the copolyacrylate blocks of the block copolymer of this invention include, but are not limited to dibromopropyl methacrylate and dibxomopropyl acrylate. Of these, dibromopropyl methacrylate is preferred.
The preparation of the copolymers of this invention is illustxated in greater detail in the following nonlimiting examples.

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 gxam dibutyltin di-laurate were added to a 1.5 liter 3-necked reaction flask equipped with an agitator and a nitrogPn purge and heated in a controlled temperature oil bath. The contents were purged with dry nitrogen and heated rapidly to 75C while being agitated. An isocyanate anaylsis (ASTM Test No. D2572-67T) was made after 5 hours at 75C 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 dibutyltin dilaurate and 189 grams toluene were added to the flask, and the tempera~ure maintained a~ 50C for a period of 4 to 5 hours. At that time isocyanate analysis indi-cated 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 produc~ became a soft, waxy, opaque solidO Total solids were determined to be 54 per cent by weight.

Example 2 54.1 grams of the peroxycarbamate from Example 1, 71.8 grams of ~-hydroxyethyl methacrylate and 250 ml of ethylene ~33~7~

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 con-trolled-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 addi-tion 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 80C and the 2,3-dibromopropyl methacrylate of the addition funnel was uniformly added over a 5-hour period. The temperature was maintained at 80C 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 24C and a total solids content of 29 per cent by weight.
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The same procedure was followed as that of Example

2, except that 2,3-dibromopropyl acrylate on an equal weight basis was substituted for the 2,3-dibromopropyl methacrylate of Example 2. The resultinq polymer syrup had a viscosity of 738 cps. at 16C and a total solids content of 2801 per cent by weight.
As will be understood by those skilled in the artl the relative amounts of polyuxethane to copolyacrylate, as well as the relative amounts of the acrylates making up the copoly-acrylate blocks of the block copolymers of this invention, may ~L33~

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 poly-urethane 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 bromine- or iodine-substituted acrylate to hydroxy-containing acrylate must also be balanced in order to control the amount of cross-linking which may be effected in the coating film. The extent of cross-linking of the coating film is determined hy the number of cross-linking sites, such as bromine or iodine substituents on the ; block copolymer, and therefore, on the amount of bromine- or iodine-substituted acrylate present in the block copolymer.
The relative amount of hydroxy-substituted acrylate affects the solubility of the noncross-linked copolymer in polar solvents such as water/alcohol mixtures. This characteristic is of par-ticular importance during the "development" step of the method of this invention for preparing screen printing stencils.
We have found that block copolymers having charac~er-istics which are desirable for the practice of this invention preferably comprise from about 60 to about 90 per cent by block copolymer weight of copolyacrylate and from about 10 to about 40 per cent by weight of polyurethane. The copolyacrylate ~ ~ ~3~
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preferably comprises from about 50 to about 95 per cent by copoly acrylate weight of hydroxy~containing acrylate and frsm about 5 to 50 per cent by weight of acrylate substituted with bromine or iodine. A preferred block copolymer for making screen print-ing stencils according to this invention is one comprising about 80 per cent copolyacrylate and about 20 per cent polyurethane by weight, the copolyacrylate comprising about 60 per cent 2-hydroxyethyl methacrylate and about 40 per cent 2,3-dibromo-propyl methacrylate by weight.
In addition to flexibility and durability, it is also possible to introduce other specific characteristics into the block copolymers by including in the copolyacrylate blocks certain other acrylates, in amounts of up to about 15 per cent by copoly-acrylate weight. Thus, for example, the substitution of acrylic acid for some of the hydroxy-contain:ing acrylate will impart to the total copolymer a sensitivity to basic solvents, such as ammonia and sodium hydroxide solutions. This may be an impor-tant 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 dura-ble in the presence of water than they would otherwise be. In a similar manner, the substitution of methyl methacrylatQ for some of the hydroxy-containing acrylate increases the hardness and water resistance of coating films made from the block copoly~
mers, and substitution of butyl methacrylate for some of the hydroxy-containing acrylate imparts improved flexibili~y to the ~L33~

coating films. Therefore, where such specific characteristics are desired, the block copolymers comprise from about 60 to about 90 per cent by block copolymer weight of copolyacrylate and from about 10 to about 40 per cent by weight of polyurethane, the copolyacrylate comprising from about 50 to about 95 per cent by copolyacrylate weight of hydroxy-containing acrylate, from about 5 to about 50 per cent by weight of acrylate substituted with bromine or iodine and up to about 15 per cent by weight of another acrylate, such as 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 aforedescribed 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, another emodiment of this invention is a print-ing screen coating composition comprising a solution of the block copolymer in a suitable solvent containing a cross-linking ~L33~

promo~er and an ultraviolet initiatorO
When the purified form of the copolymer is used in preparing the coating compositions of this invention, the copoly-mer is dissolved in a solvent to prepare a coating solution.
When, however, the copolymer syrup is used directly, it is gener-ally not necessary to add any additional solvent, since the solids level of the syrup itself is usually in the range desired.
That is to say, the cross~linking promoter and ultraviolet initi-ator and any other desired additives may be added directly to the polymer syrup~
Although we have found it convenient to use coating compositions having solids contents of about 30 per cent by weightl the desired solids level and therefore viscosity may vary over a relatively wide range depending upon such factors as the technique used in applying the coating composition to the substrate, the nature of the solvent used, the amount of cross-linking promoter used, the nature of the substrate being coated, and the like. Therefore, although solids levels of from about 20 to about 40 per cent by weight of the coating composition are preferred, the practice of this invention is not limited to any particular range of solids levels or viscosities. The formulation of the coating compositions of this invention to achieve solids levels or viscosities appropriate to the particular use contemplated is well within the expertise of those skilled in the art and need not be further defined.
The solvents used in preparing coating compositions according to this invention may be important factors in the ulti-mate performance of the coating compositions. Thus, where a relatively nonvolatile solvent is used, the time required to dry the coating, once applied to the substrate, could be so long ~ ~33~7~ `

as to present a processing disadvantage. On the other hand, if a highly volatile solvent is used, it may evaporate so rapidly as to make the application of a uniform coating to the substrate being coated rather difficult. Therefore, we prefer to use sol-vents of intermediate volatility such as ethylene glycol mono-methyl ether, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, blends of ethylene glycol monomethyl ether with methylethyl ke-tone, ethyl alcohol, dimethyl formamide, isopropyl alcohol and the like. It is, of course, also within the scope of this inven-tion to choose other organic solvents.
In practicing this invention, we have found it conven-ient to add small amounts of ultraviolet initiators to the coating compositions. The ultraviolet initiators contribute to the rapid cross-linking or curing of the coatings in response to exposure to ultraviolet irradiation. When exposed to ultraviolet radiation, these initiators are thought to generate free radicals which initiate the cross-linking of the copolymer chains in the coating.
While there are many ultraviolet initiators known to those skilled in the art, persons so skilled also know that certain ultraviolet initiators may perform better in particular chemical systems than others. Thus, while there may be other ultraviolet initia-tors which may be used in the coating compositions of this inven-tion, we prefer to use phenanthrenequinone or uranyl nitrate~
While the actual amounts of ultraviolet initiator used in formu-lating the coating compositions of this invention may vary, depending on the particular formulation and needs of the formu-lator, we have found that the ultraviolet initiator concentration preferably ranges from about 0.01 to about 5.0 per cent by block copolymer weight. However, concentrations ranging from about 0.1 to about 3 per cent by weight are preferred.

--19~

~ ;33~

The coating compositions of this invention also gener-ally include small amounts of one or more cross-linking promoters in their formulation. These 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 mole-cules. 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 character-istics of screen printing stencils having indicia defined bysuch 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 exper~ise of those skilled in the art ~o determine the rela-tive amounts to use in the formulation. Preferred cross-linking promoters for use in preparing the coating composi~ions of this invention include, but are not limited to, polyethylene glycol diacrylates, pentaerythritol triacrylate, trimethylolpropane triacrylate and mixtures thereof. These are preferably used at concentrations of up to about 20 per cent by weight based on the total weight of copolyacrylate/polyurethane block copoly-mer used.
We have also found it helpful to add small amounts of pigmenting material to the coating compositions of this --~0--~33~7~

invention. The pre~ence of such piymenting material improves the visibility of the coating composition during the preparation of the screen printing stencils of this inven*ion 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 at concentrations of from about 0.1 to about 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 coat-ing compositions based on block copolymers in which the bromine-or iodine-substituted acrylate 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 for-mulated 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-gpeed 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 ~33~

4, it being understood that this example is illustrative only and in no way limits the scope of the invention.

The polymer syrup prepared in Example 2 was mixed with phenanthrenequinone (ultraviolet initiator~, trimethylpro-pane triacrylate (cross-linking promoter) and tetraethylene gly-col 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 light sensitive, the formulation was carried out in amber glass con-tainers and the final product was stored in amber glass containers.
A second coating composition was prepared using the same procedure as described above except that the polymer syrup prepared in Example 3 was used and a small amount of dimethylamino-ethyl acrylate was added to the formulation. The actual formula-tion 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 pro-duced 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 r ~3.

printing stencil. The actual formulation amounts for this coat-ing composition are shown under l'C'~ in Table I.

Table I

% by Weight of Total Composition Composition A B C

polymer syrup from Example 2 (29% solids) 91.8 polymer syrup from Example 3 (28.1% solids) - 93.4 pol~mer 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 *8.7% solids in ethylene glycol monomethyl ether As will be understood by those skilled in the art, it is important that the temporary support sheet te.g., 20 of Fig. 3) used in the practice of the invention be transparent to ultraviolet radiation. The ~emporary support sheet should also have favorable adhesion and release characteristics with respect to the coating compositions of this invention. The adhe-sion between the coating and the support sheet should be suffi-cient to enable the engraving of certain parts of the coating film without disturbing the remainder of the film. It ~hould 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 ~L33~L ,7~

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 lQ solutions.
While there may be many materials which are suitable for use as temporary support sheets in the practice of this in-vention, we prefer to use thin sheets of polyethylene terephthal-ate coated with an appropriate adhesion/release agent. The poly-ethylene 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 ap-plied 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 in-vention 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. 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 there-from. For example, number 80-square mesh cloth would have 80 x 80 openings per square inch or 6400 openings per square inch.
The coating compositions of this invention may be ap-pli~d to the appropriate substrates by any suitable conventional ~24-~L~33~6~

means. For example, they may be applied by painting, knife coating, rod coating or flow coatiny. Subsequent to the applica-tion 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 satis-factory 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 accord-ing to several parameters, such as coating thickness, amount of ultraviolet initia~or used, degree of cross-linking desired, etc. 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 mate-rial, it will suffice to irradiate it only to the extent required to render the affected part of the coating insoluble in the sol-vent used for the subsequent development step. Where1 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 for sufficient time to achieve the desired degree of cross-linking. As will be recognized by those skilled in the art, there are many ultra-violet 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 -~5-~13~17CD

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, expo-sure at a distance of 18 inches ~or 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 solu-tions is usually in the range of from 1/1 to l/30 The practice of this invention for preparing screen printing stencils is further illustrated by the following non-limiting examples.
Example 5 An 8 x 10-inch support sheet of 3-mil polyethylene terephthalate film was coated with a thin layer of an adhesion/
release agent applied as a dilute tO.75~ by weight solids) solu-tion 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 agent coating was allowed to dry, a coating of the printing screen coating composition prepared in Example 4 (Formulation "A" of Table l) was applied using a coating knife having a 10-mil clearance. The resulting coated support sheet was then air dried for about 2 hours and oven dried at 6GC for an additional 2 hours.

~33~;~

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 minutesO 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 sec-onds 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 soakin~
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 12-ampere carbon ~L~3;~

arc ultraviolet source at a distance of about 12 inches for about 10 minutes on e~ch 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.
Exam~le 6 An 8 x 10-inch support sheet of 3 mil clear polyethyl-ene terephthalate film was coated with a thin layer of an adhe-; sion/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 succes-sive castings of the coating composition of Example 4 (Formulation "A" of Table I) were applied using a casting knife with a clear-ance 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 60C 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 assemhly was air dried in front of a fan for about 10 minutes. The assem~
bly was then exposed through a standard test negative to a stand~
ard 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

3~

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 fxont 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.
Exam~le ?
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 8 x 10-inch support sheet of 3-mil clear polyethyl-ene 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 60C aft~r each layer of the coating composition was applied.

~3~ 7~

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 S, 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 print-ing 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 avail-able ketone-based ink.
Visual inspection of the screen printing stencil after printing 400 copies with it showed no sign of wear, and the sten-cil 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 preceeding 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 novel screen printing stencil of this invention is a screen printing stencil having indicia defined ~hereon in the form of a cross-linked block copolymer having alternating blocks of copolyacrylate and polyurethane, the copolyacrylate being a copolymer of at least one hydroxy-containing acrylate and at least one acrylate which is partially substituted with bromine or iodine. Typically, the surfaces upon which indicia are printed using the screen printing stencils of this invention are paper, fabric, plastic or the like.
~ he screen printing stencils of this invention repre-sent a substantial advancement of the art in that they are com-patible with both oil-based and water-based inksO More specifi-cally, these screen printing stencils combine the precision of photochemically prepared screen printing stencils with the dura-bility 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 pxo~
cesses, 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.
Although this invention has been described in terms of specific embodiments which are set forth in considerable detail, it should be understood that this is by way of illustra-tion only, in that the invention is not necessarily limitedthereto, since alternate embodiments and techniques will become apparent to those skilled in the art in view of the disclosure.
Accordingly, modifications are contemplated which can be made without departing from the spirit of the described inven~ion.

Claims (47)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A method of preparing a screen printing stencil, characterized by the step of affixing to a printing screen substrate an indicia-defining coating, said coating comprising an ultraviolet cross-linked block copolymer formed of blocks of copolyacrylate and polyurethane, said block copolymer com-prising from about 60 to about 90 percent by block copolymer --weight of copolyacrylate and from about 10 to about 40 per-cent by weight of polyurethane, said copolyacrylate comprising from about 50 to about 95 percent by weight of a hydroxy-containing acrylate and from about 5 to about 50 percent by weight of an acrylate substituted with bromine or iodine.

2. A method in accordance with claim 1 wherein said copolyacrylate includes up to about 15 percent by copolyacry-late weight of acrylic acid.

3. A method in accordance with claim 1 wherein said copolyacrylate includes up to about 15 percent by copolyacry-late weight of methyl methacrylate.

4. A method in accordance with claim 1 wherein said copolyacrylate includes up to about 15 percent by copolyacry-late weight of butyl methacrylate.

5. A method in accordance with claim 1 wherein said hydroxy-containing acrylate is 2-hydroxyethyl methacrylate and said acrylate partially substituted with bromine or iodine is a dibromopropyl methacrylate.

6. A method in accordance with claim 5 wherein said block copolymer comprises about 80 percent copolyacrylate and about 20 percent polyurethane by weight, said copolyacrylate comprising about 60 percent 2-hydroxyethyl methacrylate and about 40 percent 2,3-dibromopropyl methacrylate by weight.

7. A method in accordance with claim 1 wherein said coating includes a pigment.

8. A method of preparing a screen printing stencil, comprising the steps of a. affixing to a transparent support sheet a coating composition comprising (1) a block copolymer formed of blocks of a copolyacrylate and a polyurethane said block copolymer comprising from about 60 to about 90 percent by block copoly-mer weight of copolyacrylate and from about 10 to about 40 percent by weight of polyurethane, said copolyacrylate com-prising from about 50 to about 95 percent by weight of a hydroxy-containing acrylate and from about 5 to about 50 percent by weight of an acrylate substituted with bromine or iodine, (2) an ultraviolet initiator, and (3) a cross-linking promoter;
(b) drying said coating composition to form an ultraviolet cross-linkable coating on said support sheet thereby to provide a coated support sheet;
(c) producing in said coating a predetermined indicia pattern;
(d) transferring said indicia pattern to a print-ing screen substrate; and (e) exposing said indicia pattern on said screen substrate to ultraviolet radiation thereby to form a printing screen stencil.

9. A method in accordance with claim 8 wherein said producing said indicia pattern in said coating comprises the steps of (a) exposing said coated support sheet to ultra-violet radiation through a master defining said indicia as a transparent pattern for a time sufficient to partially cross-link said coating in areas corresponding to said transparent pattern; and (b) developing said indicia pattern by removing from said support sheet that portion of said coating which is not cross-linked by exposure to ultraviolet radiation.

10. A method in accordance with claim 9 wherein said step of transferring said indicia pattern to said printing screen substrate is carried out subsequently to said steps of exposing said coated support sheet and of developing said indicia pattern.

11. A method in accordance with claim 9 wherein said step of transferring said indicia pattern to said printing screen substrate is carried out simultaneously with said steps of exposing said coated support sheet and of developing said indicia pattern.

12. A method in accordance with claim 8 wherein said step of producing said indicia pattern in said coating com-prises cutting out a portion of said coating corresponding to said pattern from said support sheet.

13. A method in accordance with claim 8 wherein said hydroxy-containing acrylate is 2-hydroxyethyl acrylate and said acrylate partially substituted with bromine or iodine is a di-bromopropyl methacrylate.

14. A method in accordance with claim 13 wherein said block copolymer contains about 80 percent copolyacrylate and about 20 percent polyurethane by weight, said copolyacrylate being comprised of about 60 percent 2-hydroxyethyl methacry-late and about 40 percent 2,3-dibromopropyl methacrylate by weight.

15. A method in accordance with claim 8 wherein said ultraviolet initiator is phenanthrenequinone or uranyl nitrate at a concentration of from about 0.01 to about 5 percent by block copolymer weight.

16. A method in accordance with claim 8 wherein said cross-linking promoter is a polyethylene glycol diacrylate, pentaerythritol triacrylate, trimethylolpropane triacrylate or mixtures thereof at concentrations of up to about 20 percent, by block copolymer weight.

17. A method in accordance with claim 8 wherein said coating composition also contains from about 0.1 to about 3 percent by block copolymer weight of a pigment.

18. A method of preparing a scrren printing stencil, comprising the steps of (a) affixing to a printing screen substrate a coating composition comprising (1) a block copolymer formed of blocks of a copolyacrylate and a polyurethane said block copolymer com-prising from about 60 to about 90 percent by block copolymer weight of copolyacrylate and from about 10 to about 40 percent by weight of polyurethane, said copolyacrylate com-prising from about 50 to about 95 percent by copolyacrylate weight of a hydroxy-containing acrylate and from about 5 to about 50 percent by weight of an acrylate substituted with bromine or iodine.

(2) an ultraviolet initiator, and (3) a cross-linking promoter;
(b) drying said coating composition to form an ultraviolet cross-linkable coating on said printing screen substrate;
(c) producing in said coating a predetermined indicia pattern; and (d) exposing said indicia pattern on said screen substrate to ultraviolet radiation, thereby to form a screen printing stencil.

19. A method in accordance with claim 18 wherein said producing said indicia pattern in said coating comprises the steps of (a) exposing said coated printing screen sub-strate to ultraviolet radiation through a master defining said indicia as a transparent pattern for a time sufficient to partially cross-link said coating in areas corresponding to said indicia pattern; and (b) developing said indicia pattern by removing from said printing screen substrate that portion of said coating which is not cross-linked by exposure to ultraviolet radiation.

20. An ultraviolet cross-linkable liquid coating composition comprising (a) a block copolymer formed of blocks of a copoly-acrylate and a polyurethane, said block copolymer comprising from about 60 to about 90 percent by block copolymer weight of copolyacrylate and from about 10 to about 40 percent by weight of polyurethane, said copolyacrylate comprising from about 50 to about 95 percent by copolyacrylate weight of a hydroxy-containing acrylate and from about 5 to about 50 percent by weight of an acrylate substituted with bromine or iodine, (b) a solvent for said block copolymer;
(c) an ultraviolet initiator; and (d) a cross-linking promoter.

21. A coating composition in accordance with claim 20 wherein said copolyacrylate includes up to about 15 percent by copolyacrylate weight of acrylic acid.

22. A coating composition in accordance with claim 20 wherein said copolyacrylate includes up to about 15 percent by copolyacrylate weight of methyl methacrylate.

23. A coating composition in accordance with claim 20 wherein said copolyacrylate includes up to about 15 percent by copolyacrylate weight of butyl methacrylate.

24. A coating composition in accordance with claim 20 wherein said hydroxy-containing acrylate is 2-hydroxethyl acrylate, a hydroxpropyl acrylate, 2-hydroxyethyl methacrylate or a hydroxypropyl methacrylate.

25. A coating composition in accordance with claim 20 wherein said hydroxy-containing acrylate is 2-hydroxyethyl methacrylate or 2-hydroxyethyl acrylate.

26. A coating composition in accordance with claim 20 wherein said hydroxy-containing acrylate is 2-hydroxyethyl methacrylate.

27. A coating composition in accordance with claim 20 wherein said acrylate partially substituted with bromine or iodine is a dibromopropyl acrylate or a dibromopropyl meth-acrylate.

28. A coating composition in accordance with claim 20 wherein said acrylate partially substituted with bromine or iodine is 2,3-dibromopropyl methacrylate.

29. A coating composition in accordance with claim 20 wherein said polyurethane is a polyoxyethylene urethane or a polyoxypropylene urethane.

30. A coating composition in accordance with claim 20 wherein said solvent is ethylene glycol monomethyl ether, ethyl alcohol, n-propyl alcohol or isopropyl alcohol.

31. A coating composition in accordance with claim 20 wherein the concentration of said block copolymer in said solvent is from about 20 to about 40 percent by weight.

32. A coating composition in accordance with claim 20 wherein said ultraviolet initiator is phenantrenequinone or uranyl nitrate at a concentration of from about 0.01 to about 5 percent by block copolymer weight.

33. A coating composition in accordance with claim 20 wherein said cross-linking promoter is a polyethylene glycol diacrylate, pentaerythritol triacrylate, trimethylolpropane triacrylate or mixture thereof.

34. A coating composition in accordance with claim 20 including from about 0.1 to about 3 percent by block copoly-mer weight of a pigment.

35. A coating composition in accordance with claim 20 wherein said block copolymer contains about 80 percent copoly-acrylate and about 20 percent polyoxyethylene urethane by weight, said copolyacrylate being comprised of about 60 per-cent 2-hydroxyethyl methacrylate and about 40 percent 2,3-di-bromopropyl methacrylate by weight; said solvent for said block copolymer is ethylene glycol monomethyl ether, said ultra-violet initiator is phenanthrenequinone and said cross-linking promoter is a mixture of trimethylolpropane triacry-late and triethylene glycol diacrylate.

36. A coating composition in accordance with claim 35 further characterized as containing phthalocyanine blue as a pigment.

37. A coating composition in accordance with claim 20 wherein said block copolymer contains about 80 percent copoly-acrylate and about 20 percent polyoxyethylene urethane by weight, said copolyacrylate being comprised of about 60 per-cent 2-hydroxyethyl methacrylate and about 40 percent 2,3-di-bromopropyl acrylate by weight, said solvent for said block copolymer is ethylene glycol monomethyl ether, said ultra-violet initiator is phenanthrenequinone and said cross-link-ing promoter is a mixture of trimethylolpropane triacrylate and triethylene glycol diacrylate.

38. A coating composition in accordance with claim 37 further characterized as containing dimethylaminoethyl acry-late as a solvent resistance improver.

39. A coating composition in accordance with claim 38 further characterized as containing phthalocyanine blue as a pigment.

40. A block copolymer formed of blocks of copolyacrylate and polyurethane, said block copolymer comprising from about 60 to about 90 percent by block copolymer weight of copoly-acrylate and from about 10 to about 40 percent by weight of polyurethane, said copolyacrylate comprising from about 50 to about 95 percent by copolyacrylate weight of a hydroxy-containing acrylate and from about 5 to about 50 percent by weight of an acrylate substituted with bromine or iodine.

41. A block copolymer in accordance with claim 40 wherein said copolyacrylate includes up to about 15 percent by copolyacrylate weight of acrylic acid.

42. A block copolymer in accordance with claim 40 wherein said copolyacrylate includes up to about 15 percent by copolyacrylate weight of methyl methacrylate.

43. A block copolymer in accordance with claim 40 wherein said copolyacrylate includes up to about 15 percent by copolyacrylate weight of butyl methacrylate.

44. A block copolymer in accordance with claim 40 wherein said polyurethane is a polyoxyethylene urethane or a polyoxypropylene urethane, said hydroxy-containing acrylate is 2-hydroxyethyl acrylate, a hydroxypropyl acrylate, 2-hydroxyethyl methacrylate or a hydroxypropyl methacrylate and said acrylate which is partially substituted with bromine or iodine is a di-bromopropyl acrylate or a dibromopropyl methacry-late.

45. A block copolymer in accordance with claim 44 containing about 80 percent copolyacrylate and about 20 per-cent polyoxyethylene urethane by weight, said copolyacrylate being comprised of about 60 percent 2-hydroxyethyl metha-crylate and about 40 percent 2,3-dibromopropyl methacrylate or 2,3-dibromopropyl acrylate by weight.

46. A screen printing stencil having indicia defined thereon by a cross-linked block copolymer formed of blocks of a copolyacrylate and a polyurethane, said block copolymer comprising from about 60 to about 90 percent by block copoly-mer weight of copolyacrylate and from about 10 to about 40 percent by weight of polyurethane, said copolyacrylate con-taining from about 50 to about 95 percent by copolyacrylate weight of a hydroxy-containing acrylate and from about 5 to about 50 percent by weight of an acrylate substituted with bromine or iodine.

47. In a method of screen printing wherein indicia are printed onto a receptor surface by bringing the indicia-bearing side of a screen printing stencil having indicia defined thereon by an indicia-defining coating into contact with said receptor surface and forcing ink through those areas of the screen printing stencil not blocked by said indicia-defining coating, the improvement which comprises preparing said screen printing stencil by the step of affixing to a printing screen substrate an indicia-defining coating, said coating comprising an ultraviolet cross-linked block copolymer formed of blocks of copolyacrylate and polyurethane, said block copolymer comprising from about 60 to about 90 percent by block copolymer weight of copolyacrylate and from about 10 to about 40 percent by weight of polyurethane, said copolyacrylate weight of a hydroxy-containing acrylate and from about 5 to about 50 percent by weight of an acrylate substituted with bromine or iodine.