CA1110900A - Polythiol effect, curable monoalkenyl aromatic-diene and ene composition - Google Patents

Abstract

A B S T R A C T
Curable composition containing (1) at least 20% by weight monoalkenyl aromatic-diene copolymer resin; (2) at least 2% by resin weight of at least one monomer substantially compatible with said resin and having at least one cross-linkable C-C-double bond; (3) 0.1 to 35% by resin weight of polythiol containing at least two-SH groups per molecule; and, (4) a curing agent. The composition is particularly useful for printing plate production and the composition may contain 0.01 to 10% by resin weight of photoinitiator.
The monomer is preferably an addition photopolymerizable polyethylenically unsaturated acrylic or methacrylic acid ester containing two or more acrylate or methacrylate groups per molecule or mixtures thereof.
Low molecular weight liquid rubber polymer may be included in the composition.

Description

o This invention relates to curable compositions and especially to those that can have controlled cure through the use of appropriate curing agents.
British Patant 1,395,822 discloses a composition comprising an A-s-A type block copolymer present in an amount of at least 40O/o by weight, at least 5% by weight of an addition photopolymerizable polyethylenically unsaturated acrylic or methacrylic acid ester containing two or more acrylate or methacrylate groups per molecule and 0.1 to 10% by weight of an addition polymerizable initiator activatable by actinic light. As a preferred embodiment this British Patent discloses a composition having 60-85 parts by weight of the block copolymer, 40-15 parts by weight of ethylene glycol diacrylate or methacrylate, trimethylolpropane triacrylate or methacrylate or pentaerythritol tetraacrylate or methacrylate and from 1.5 to 4.0 parts by weight of benzophenone. An optional identified ingredient is a liquid rubber polymer having a molecular weight between 750 and 2000, which, in a preferred embodiment, is 5-30 parts by weight of liquid polybutadiene.
Another patent showing an A-E~A block copolymer composition is United States Patent 3,674,486. This is represented by control Example XIII of the present application. Yet another A-B-A
block copolymer composition is disclosed in British Patent No. 1,366,769. A-B block copolymer compositions are also disclosed in German Patent Publications DT OS 24 56 439 and DT OS 26 10 206.
As is illustrated in Table 1 below the compositions of the above references are less rapid in curing than the compositions of the present invention.
This is also illustrated by Example I-VI as contrasted with Control Examples VIT-XII which show that the compositions of the present invention produce printing plates that are not tacky while printing plates produced from the compositions of the above references are tacky.
A number of patents show photocurable compositions that have a polythiol as a major chemically functioning constituent. Examples of such patents are United States Patents 4,008,341, 3,843,572, 3,832,421, 3,783,152, 3,666,461, 3,661,744, 3,627,529 and 3,615,450 and Canadian Patents 930,094, 926,183, 924,047 and 885,388.
In the present invention the polythiol constituent appears to act in some way to regulate and make more effective the polymerization of the other constituent o components of the composition. The curable compositions of the present invention, as a general rule, cure within minutes without the presence of the polythiol.
The present invention provides a composition comprising:
~ 1) at least about 2~/o~ more preferably at least 4Go/o, by weight of the composition of a monoalkenyl aromatic-diene copolymer resin,

(2) at least about 2%, more preferably at least 10%l by resin weight of at least one monomer substantia~y compatible with said resin and having at least one cross-linkab~e C-C-double bond, and

(3) about 0.1 to about 35%, more preferably 0.5 to l~/ol by resin weight of a polythiol containing at least two-SH groups per molecule. These compositions - cure rapidly to give cured compositions having improved solvent resistance and a non-tacky surface. They are especially useful for making printing plates.
The present invention also provides a method for forming a printing plate which involves exposing to actinic radiation projected through an image-bearing transparency selected portions of a radiation curable composition as aforesaid, which also comprises

(4) 0.01 to 10% by resin weight of a photoinitiator.
For better printing plate products the 9~0 monoalkenyl aromatic-diene copolymer resin is present in the composition in an amount of at least 40/O by weight. The preferred monoalkenyl aromatic-diene copolymer resin has the general formula: A-B-A wherein

5 the end blocks A are monoalkenyl aromatic polymer blocks which may be the same or different and which have an average molecular weight of from 2,000 to 100,000 and a glass transistion temperature above 25C, the total block A content bein~ 10-5~o by weight of the copolymer, and B is an elastomeric diene polymer block having an average molecular weight of from 25 t 000 to 1,000,000 and a glass transition temperature below 10 C .
These end blocks (A) are typically derived from monoalkenyl aromatic compounds having the general formula:

¦ A
C = CH2 ~q ~Y)n ~

wherein X represents hydrogen or an alkyl radical containing 1 or 2 carbon atoms (methyl or ethyl), Y
represents hydrogen or an alkyl radical containing from 1 to 4 carbon atoms (methyl, ethyl, n propyl, isopropyl, n-butyl, sec-butyl or tert-butyl), and n represents an g~0 integer from 1 to 5. Examples of the alkenyl ~romatic compounds which are within the scope of formula A are, styrene, alpha-methyl styrene, tert-butyl styrene, vinyl toluene, ortho-and para-methyl styrenes, ortho- and para-methyl-alpha-methyl styrenes, and ortho- and para-ethyl styrenes.
The elastomeric block (B) is preferably a diene polymer block derived from a conjugated diene hydrocarbon compound having the general formula:

CH2 = C - C = CH2 B
R

wherein each R individually represents hydrogen or an alkyl radical containing 1 or 2 carbon atoms (methyl or ethyl). Examples of such conjugated diene hydrocarbon compounds are l,3-butadiene and 2-methyl-~,3-butadiene.
Typical examples of the just described monoalkenyl aromatic-diene block copolymers, useful in the present invention, are described in United States Patent 3,265,765.
An alternative and in general less desirable monoalkenyl aromatic-diene copolymer resin, for use in the present invention, has the general formula:

O

A-B

wherein blocks A and B are the same as those designated by the same letters in the ~-~-A bloc~
copolymer. The A-B type copolymer resin can have a sharp or a blurred, i.e. a "flowing" transition between the two block segments of the copolymer formed solely from diene-hydrocarbons or styrene-monomers. The two-block-copolymers contain 5 to 70, and preferably 10 to 40% by weight in units of the last mentioned monomer of the styrene type, and consequently 30 to 95, and preferably 60 to 90O/o, by weight of the polymerized units of diene~hydrocarbon. The production of such two-block copolymers is known and can in particular take place according to the procedure described in United States Patent 3,149,182, see I.
guntz, J. Polymer Sci. 54 (1961), 569-586 and Y. U.
Spirin et al, J. Polymer Sci. 58 ~1962) 1181-1189. The copolymers can be produced continuously or discontinuously.
In the production of copolymers with sharply separated block segments through stepwise copolymerization, it is advantageous to start with the polymerization of the monomer which quantitatively preponderates.
Advantageously, the two-block copolymers are produced by solution-polymerization in a suitable solvent such as in particular hydrocarbon,s or mixtures thereof as well as polar solvents such as tetrahydrofuran~ The type of solvent influences the fine structure of the tw~-block copolymer, e.g. the confi~uration of the diene-polymer segments. Prefera~ly hy~rocar~ons are used as so~vents.
It is particularly advantageous to use the solutions of the block copolymers produced by solution-polymerization either directly or in concentrated state, after adding the other coating constituents for producing the photo-cross-linkable layer by casting.
Suitable two-block copolymers of the indicated type, have viscosity numbers measured as 0.5% by weight solutions in toluene, of approximately 60 to 350 ml/~ and in particular 90 to 250 ml/g, corresponding to a molecular weight range Mv of approximately 75,000 to 200,000.
It is also possible to use mixtures of the copolymer resins of the indicated types or for special applications to mix the two-block copolymers with small quantities of other compatible polymers and particularly elastomers.
The monomer that is substantially compatible with the resin and has at least one cross-linkable C-C-double bond is present in an amount of at least 10% by resin weight. Preferably the monomer is present in an amount of 15-30% by resin weight. The monomer is preferably ~n addition photopolymerizable polyethylenically unsaturated acrylic or methacrylic acid ester containing two or more acrylate or methacrylate groups per molecule or mixtures ~.

g/.~3 there~. Exampl es of such multifunctional acrylates are ethylene glycol diacrylate, ethylene glycol dimethacrylate, trimet~ylolpropane triacrylate, trimet~y~pr~pane trimethacrylate, ~entaerythritol tetra-acrylate or pentaerythritol-tetramethacrylate, hexane-di~1-1.6-dimethacrylate, and diethylene-glycol-dimethacrylate.
Also useful in some special instances are mono-acrylates such as n-butyl-acrylate, n-butyl-methacrylate, 2-ethylhexyl-acrylate, lauryl-acrylate, and 2-hydroxypropyl-acrylate. Small quantities of amides of (meth)acrylicacid such as ~-methylol-methacrylamide-butyl-ether are also suitable~ ~-vinyl-compounds such as N-vinyl-pyrrolidone, vinyl-esters of aliphatic monocarboxylic acids such as vinyl-oleate, vinyl-ethers of diols such as butane-diol-l, 4-divinyl-ether and allyl-ether and allyl-ester are also suitable. Isocyanate-free reaction products of organic polyisocyanates such as hexamethylene-diisocyanate, isophorone-diisocyanate or tolylene-diisocyanate with hydroxyl group-containing (meth)acrylates such as glycol-monoacrylate, hydroxypropylmethacrylate or ,, 1,4-butane-diol-monoacrylate, are also suitable as monomers provided they are adequately compatible with the resin. The same applies regarding the reaction products of di- or polyepoxides such as butane-diol-l, 4-diglycidyl-ether or bisphenol-A-diglycidyl-ether with (meth)acrylic acid. ~he characteristics of the photo-i _ g _ ;
.

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polymerizable layers can be modified for the specificpurpose by a suitable selection of monomers or mixtures thereof.
The polythiols used in the present invention are simple or complex organic compounds containing 2 or more pendant or terminally positioned-SH functional groups per average molecule. They usually have a viscosity range of slightly above 0 to about 20 million centipoises (cps~ at 70C, as measured by a Brookfield Viscometer. This viscosity may be attained by including in the polythiol a reactive plasticizer such as diallyl phthalate. Suitable polythiols for use in the invention usually have molecular weights in the range about 94 to about 20,000 or more, preferably about 100 to about 10,000.
The polythiols which can be used in the invention have, for example, the general formula:
R8 ~ (-SH)n where n is at least 2 and R8 is a polyvalent organic moiety free from "reactive" carbon to carbon unsaturation. Thus, R8 may contain cyclic groupings and minor amounts of hetero atoms such as ~,S,P or 0 but, primarily, contains carbon-hydrogen, carbon-oxygen or silicone-oxygen containing chain linkages free of any "reactive'` carbon to carbon unsaturation.
'~ 25 One class o~ polythiols operable in the , instant invention to obtain essentially o~ourless cured /

' ' ~

coatings and relief images are e9ters of thiol containing acids of the general formula: HS-R9-COOH
where Rg iS an organic moiety containing no "reactive"
carbon to carbon unsaturation with polyhydroxy compound of the general structure: Rlo - (~OH~n where Rlo is an organic moiety containing no "reactive" carbon to carbon unsaturation and n is 2 or greater. These components react under suitable conditions to give a polythiol having the general structure:
O

Rlo-(-oc-R9 SH)n where Rg, Rlo and n are as hereinbefore defined.
Certain polythiols such as the aliphatic monomeric polythiols (ethane dithiol, hexamethylene dithiol, decamethylene dithiol, and tolylene-2,4-dithiol) and some polymeric polythiols such as athiol~terminated ethylcyclohexyl dimercaptan polymer, and similar polythiols which are conveniently and ordinarily synthesized on a commercial basis, although havirgobnoxious odours; can be used in this invention.
Examples of the polythiol compounds preferred for this invention because of their relatively low odour level and fast curing rate include ester of thioglycollic acid v (HS-C~2-CO~H), ~-mercaptopropionic acld ~S-CH~CH3)-COOH
and ~-mercaptopropionic acid (HS-CH2CH2CO~H) with polyhydroxy compounds such as glycols, triols, tetraols, pentaols, and hexaols. Specific examples of the preferred polythiols include ethylene glycol bis(thioglycolate)~
ethylene glycol bis(~-mercaptopropionate), trimethylol-propane tris(thioglycolate), pentaerythritol tetrakis-(thiolglycolate) and the most preferred pentaerythritol tetrakis(~-mercaptopropionate) and trimethylolpropane tris(~-mercaptopropionate), and mixtures thereof, all of which are commercially available.
The preferred polythiol compounds have a low level of mercaptan-like odour initially, and after reaction, give essentially odourless cured end products which are commercially useful resins or elastomers for coating and imaging end uses~ For better printing plate products the amount of the polythiol in the composition is 0.5 to 10% by resin weight.
The composition of the present invention is generally cured through the use of a curing agent which facilitates a controlled, rapid and determinable or predictable cure performance. The curing agent generally acts as a free radical generating agent.
Preferably the free radical generating agent includes initiation by radiation, pre~erably either W or high y~
o ener~y ioslizi~g radiatlon~ Most preferably W radiation is used~ The composition particularly lends itself to advantage in W curing in ~orming relief images and especially for printing indicia production.
Any type of actinic light may be used to supply the W radlation. For the preferred photocurable composition, it is preferred that the light emanate from a point source or in the form of parallel rays but divergent beams are also operable as a source of actinic light. Practically any convenient source of high intensity light may be used. Such sources include carbon arcs, mercury arcs, fluorescent lamps with special ultraviolet light emitting phosphors, xenon arcs, sunlight, tungsten halide lamps, argon glow lamps, photographic flood lamps, and lasers.
When W radiation is used for curing, a photoinitiator is normally added to the composition to increase the reaction rate. Initiators or catalysts which are operative in the present invention are those which are capable of initiating photopolymerization under the influence of actinic light. The preferred catalyst or initiators are substantially soluble in the photopolymerizable composition, and are effective in promoting rapid polymerization, by which the composition is cured. Many suitable photopolymerization initiators or catalysts are listed in United States Patent 4,00~,341, o to which re~erence should be made. The pre~erred photoinitiators are benzophenone and 2,2-dimethoxy-2-phenyl acetophenone. For better printing plate products the photoinitiators are pre~erably added in an amount ranging from 0.2 to 5% by weight of the resin.
~ he composition of the instant invention can also be cured by high energy ionizing irradiation or bombardment as described in the aforesaid United States Patent 4,008,341.
In an alternative preferred form of the present invention a low molecular weight liquid rubber polymer may be incorporated in the previously described compositions in order to increase the softness of relief plates made therefrom. Suitable liquid rubber polymers have molecular weights of 750 to 3000 and comprise butadiene homopolymers and copolymers containing at least 30/O by copolymer weight, more preferably at least 60%
by copolymer weight, butadiene with the remainder being either styrene or acrylonitrile. Such liquid rubber polymers should not constitute more than 50/O by weight of the total composition and preferably should constitute from 0.1 - 50/OI more preferably 5-30/OI by weight of the total composition.
The composition of the present invention may in proper instances include such additives as antioxidants, :;

i3-~10 inhibitors, activators, fillers, pigments, dyes, antistatic agents, viscosity modifiers, and plasticizers~ The aforesaid additives may be present in quantities up to 500 parts or more per 100 parts radiation curable composition by weight and preferably 0.0005 to 300 parts on the same basis. The type and concentration of the additives must be selected with care so that the ~inal composition remains suitable for its use and preferably radiation-curable.
The curing period may be from less than 1 minute to 30 days or more. Conventional curing inhibitors or retarders which may be used in order to stabilize the components or curable compositions so as to prevent premature onset of curing include hydroquinone ~-tert-butyl catechol, 2,6-di-tert-buty~-~-methylphenol, phenothiazine, ~-phenyl-2-naphthylamine, phosphorous acid, and pyrogallol. Conventional reaction promoters which give faster curing reaction rates and broaden the range of utilizable light wavelengths include trimethylphosphite, triethylphosphite, triphenylphosphite, rose bengal and acetone.
The method of mixing the components of the photosensitive compositions of the present invention is not critical. Solutions and dispersions o~ the ` 25 photopolymerizable compositions can be made with such solvents as aromatic hydrocarbons, e.g. benzene, toluene ,.

;o and xylene, chlorinated hydrocarbons, e.g. chloro~orm, carbon tetrachloride, trichloroethylene and chlorotoluene, ketones, e.g. methyl ethyl ketone, diethyl ketone and methyl isobutyl ketone, and blends of such solvents.
Solid mixes may be prepared by mixing the components on a mill or in an internal mixer such as a Banbury. The resultant polymerizable compositions may be in the form of a liquid solution, liquid dispersion, or as a solid mix. The liquid solution or liquid dispersion may be cast directly upon a substrate, or first cast upon a suitable wheel or belt, stripped, and then affixed to a substrate surface. The solid mix may be e~truded or calendered directly upon the substrate or as a self-supporting sheet and then affixed to the surface of the substrate.
A convenient method of forming a printing plate, which is a preferred use of the invention, is to place an image-bearing, line or halftone, stencil or positive or negative transparency parallel to the surface of the photocurable composition which is secured to a support or to a light-absorptive layer on the support. The image-bearing transparency and the surface of the photocurable composition should preferably be in contact.
To make possible easy removal of the image-bearing transparency from the printing plate after photopolymerization, it is desirable to coat the printing plate with a "parting"
layer or slip coat. Suitable coatings include the o nv~r)~ionl~ ryli(- ]~tox slip coats, an~ silicon gr~asf-~s.
p}~tocur~bl( layer is exposed through the tr~rlsparency t~ a s~urc~-~ oI actinic liyht untjl the ph~tocurable layer is cured to an insoluble state in the exposed areas. The thickness of the ultimate relief in such a process may be controlled by varying the thickness of the layer of the photocurable composition, and/or ~he exposure time.
Development can be carried out by conventional means. In general after exposure of the plates, the unexposed areas are removed by suitable means such as a suitable solvent liquid which has good solvent action on the mono-alkenyl aromatic-diene copolymer and relatively little action on the insolubilized, photopolymerized image or upon the substrate, antihalation coating, or any anchor layer, in the period required to remove the non-polymerized portions. Suitable organic solvents include aliphatic hydrocarbons such as hexane, octane, mineral oil, and naphthas, aromatic solvents such as toluene and xylene, halogenated organic solvents such as methylene chloride, 20 trichloroethane, and that soldunder the Trade Mark ~ -"Freon D", and blends of such solvents. The best solvent to use depends on the exact composition of the photosensitive printing plate. In the develcpment step where the relief is formed the solvent may be applled in any convenient manner, a6 by pouring, i~ rs on, or spray.

' &~:) Brushing or agitation aid in the removal of the non-polymerized portion of the composition. The use of ultrasollic washing techniques is a convenient means of removing the non-polymerized areas of the relief printing plate~
The relief plate of the present invention is preferahly formed in a layer of the photopolymerizable composition, typically 2 to 200 mils in thickness, adherent to a flexible backing. Layers ranging from 3 to 60 mils thickness are used for the majority of the letterpress printing plates. Layers thicker than 50 to 60 mils can be used for the printing of designs and relatively large areas in letterpress printing plates.
In general, the relief height-forming stratum of the photopolymerizable layer should be essentially non-light scattering. It is important that the final photosensitive composition possess sufficient clarity so as to permit the passage of light in sufficient quantity to effect the addition-photopolymerization.
In making printing plates it is important that the exposure be sufficient to harden the photocurable compositions in the exposed image areas without causing significant curing in the non-image areas. Aside from exposure time and light intensity, the extent of the exposure is dependent on the thickness of the photocurable layer, the curlng temperature, the monomer having the :, ~; - 18 -, phot~-cross-linkable ~-C-double bond and polythiol employed, the photoinitiator, diluent, the presence of ~ight absorbing pigments or dyes in the photocurable composition, and the character of the image to be reproduced. In general, the thicker the layer to be cured, the longer the exposure time. Curing generally starts at the surface of the photocurable layer closest to the light source and proceeds downward to the support.
With insufficient exposure, the layer may have a hard cure at the surface but, through lack of a clear-through cure, the relief will be removed when the unexposed area is removed. Inasmuch as the curing rate usually increases at higher temperature, less exposure is required thereat than at room temperature. Thus, ultraviolet light sources that emit heat are more efficient than cold ultraviolet light sources. However, care must be - exercised that too high a temperature is not attained during the photocure, as this leads, in some cases, to thermal expansion of the photocurable composition which results in image distortion. ~ence, it is preferred that the photocuring be carried out at a temperature in the range about 20 to about 70C. Due to the number of variables which effect exposure time, optimum results are best determined by trial and error, e.g. stepped exposures with characterization after each exposure.

3~1~

A s~pportir~g ~ase materia-, i.e. the sub~trate, which acts as a suppo~t for th~ photosensitive composition in the preferred printing plate products may be practically any natural or synthetic product capable of existing in film or sheet form, which is generally dimensionally stable and flexible. It is a common practice to use sheet metals such as aluminium or steel, or plastics such as polyester of polyamide film, as substrates. The preferred substrate supporting base material is a polyester film. Sush materials are rendered non-reflective, where necessary, by coating with an antihalation layer. Suitable antihalation coatings can be made by dispersing in a solution or aqueous dispersion of a resin or polymer a finely divided dye or pigment which substantially absorbs actinic light. The antihalation layer may also be formulated such that it acts as a binder or adhesive layer between the substrate and photosensitive composition. Antihalation pigments include carbon black, manganese dioxide and dyes, e.g.
Acid Blue Black (C120470) and Acid Magenta 0 (CI 42685).
A dyed metal plate is also useful.
In those instances where rotary press plates are desired, the support material may be used to form flat relief plates which are then formed to the desired shape. Such rotary press plates may also be prepared by using cylindrically shaped support plates of the various types carrying the curable composition and exposing them directly to actinic light through a concentrically disposed image-bearing transparency.

:
: .

qi9l`~io The invention is further illustrated by the following Examples I to VI and XIV~ Examples ~II to XIII are Controls.
EXAMæLE I
A photosensitive composition consisting of the following mixture was prepared:
a) 100 parts of styrene-isoprene-styrene (SIS) block copolymer.
b) 15 parts of trimethylolpropane trimethacrylate.
c) 2 parts pentaerythritol tetrakis~-mercaptopropionate).
d) 2 parts 2, 2-dimethoxy-2-phenyl-acetophenone.
e) 2 parts hindered phenol (Ionol~ , a curing inhibitor)O
The particular SIS copolymer was Kraton ~ 1107, manufactured by the Shell Chemical Company. and was composed of approximately 14% by weight polystyrene equally distributed between the two end blocks, the remainder being polyisoprene. The relief plate was prepared by dissolving the composition in toluene in a stirred resin kettle. Ingredient a) was charged first and dissolved followed by b), d) and e) in rapid succession and then c) after the others had dissolved.
The solution was then cast to a thickness of approximately 80 mils on a polyester backing sheet.
The dried photosensitive layer was then coated T

., .
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~q}9~

with a standard commercial amide-based slip coat to less than 1/4 mil. A 10-inch by 12-inch piece of the resultant plate was placed in a vacuum frame, and the coated photopolymer surface was brought into contact with a line process negative. The element was then exposed to actinic radiation (5000 ~W/cm2) for 1.00 min.
After exposure, the negative was stripped from the element, and the element was subiected to an etch bath containing trichloroethane to remove the unexposed polymer. A relief image corresponding to the clear areas of the negative was obtained. The photopolymerized relief plate had excellent softness, resilience, sharpness of image and essentially a tack free surface.
The plate was placed on a printing cylinder, on a flexo press and excellent prints of the original image were obtained on polyester film. The relief plate also had excellent abrasion resistance, durability, - and resistance to printing ink solvents as determined by visual observation.
The composition of Example I had the following properties 100 parts (SIS) x 100 = 82% by weight of the 121 parts-total all ingredients mono-alkenyl aromatic-diene copolymer resin . ~ ' ., .
:. ~

., l5 parts trimethylolpropane 100 parts tSIS) x lO0 = 15% ~y resin weight of m~n~mer substantially compatible with said resin and having at least one photo-cross-linkable C-C-double bond 2 parts pentaerythritol tetrakis(~-mercapto-propionate) x 100 = 2% by resin weight of 10 lO0 parts (SIS) polythiol containing at least two-SH groups per molecule 2 parts 2.2-dimethoxy-ne x 100 = 2% by resin weight of photoinitiator EXAMPLE II
A pr-.nting plate was prepared essentially . as in Example I except the composition consisted of the following mixture:
a) lO0 parts of the same SIS copolymer.
b) 2Q parts trimethylolpropane trimethacrylate.
c) 2 parts pentaerythritol tetraXis(~-mercaptopropionate).
d) 2 parts 2,2-dimethoxy-2-phenyl-acetophenone.
e) 2 parts hindered phenol.
The results were essentially the same as those described in Example I including the printing test results.

. - 23 -~,:

.:, EXAMPLE III
A printing plate was prepared essentially as in Example I except the composition consistea of the following mixtures:
a) 100 parts of the same SIS copolymer.
b) 15 parts trimethylolpropane trirnethacrylate.
c) 2 parts pentaerythritol tetra~is(~-mercaptopro~ionate).
d) 4 parts benzophenone~
e) 2 parts hindered phenol.
The results were essentially the same as those described in Example I, including the printing test results.
EXPMPLE IV
A printing plate was prepared essentially as in Example I, except the composition consisted of the following mixture:
a) 100 parts of the same SIS copolymer.
b) 15 parts trimethylolpropane triacrylate.
c) 2 parts pentaerythritol tetrakis(~-mercaptopropionate).
d) 2 parts 2,2-dimethoxy-2-phenyl-acetophenone.
e) 2 parts hindered phenol.
The results were essentially the same as those described in Example I, including the printing test results.

EXAMPLE V
A printing plate was prepared essentially as in Example I except the composition consisted of the following mixture:
a) 100 parts of styrene-butadiene-styrene (SBS) block copolymer (Kraton ~ 1102. Shell Chemical Company).
b~ 15 parts trimetholypropane trimethacrylate.
c) 2 parts pentaerythritol tetra~is ( ~ -mercaptopropionate).
d) 2 parts 2,2-dimethoxy-2-phenyl-acetophenone.
e) 2 parts hindered phenol.
The ingredients were not dissolved in toluene but were instead blended in an internal shear mixer and subsequently calendered onto a polyester backing.
The results were essentially the same as those described in Example I. including the printing test results.
EXAMPLE VI
A printing plate was prepared essentially as in Example I except the composition consisted of the ' 20 following mixture:
a) 100 parts of the same SIS copolymer.
b) 15 parts trimethylolpropane trimethacrylate.
c) 2 parts pentaerythritol tetrakis(~-mercaptopropionate).
d) 2 parts 2,2-dimethoxy-2-phenyl-acetophenone.
e) 2 parts hindered phenol.
f) 15 parts liquid butadiene rubber (Arco's product sold under the Trade Mark "Poly bd CS-15").
The results were essentially the same as those described in Example l, including the prin~ing test results EXAMPLES VII-XII
Controls The procedures of Examples I-V were repeated respectively except ingredient c), the pentaerythritol tetrakis ~-mercaptopropionate), was omitted in each Example.
The plates possessed similar properties except they had tacky surfaces to touch as tested by touching the surface with the hand. In addition the plates exhibited slower activation as is illustrated quantitatively in Table l which follows Example XIV.

EXAMPLE XIII
Control A printing plate was prepared essentially as in Example I except the composition consisted of the following mixture:
a) lOO parts of the same SIS copolymer.
b) lO parts trimethylolpropane trimethacrylate.
c) None. `~
d) l part 2,2-dimethoxy-2-phenyl-acetophenone.
e) 2 parts hindered phenol.
These results were essentially the same as .

B

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those of the other controls described in Examples VII-XII.
EXAMPLE XIV
-A printing plate was prepared essentially as in Example I except the composition consisted of the following mixture:
a) 100 parts of the same SIS copolymer.
b) 10 parts trimethylolpropane trimethacrylate.
c) 2 parts pentaerythritol tetrakis(B-mercaptopropionate).d) 1 part 2.2-dimethoxy-2-phenyl-acetophenone.
e) 2 parts hindered phenol.
The results were essentially the same as those described in Example I including the printing test results.

EXAMPLE ELONGATIO~ TE~SILE RESILIENCE SHORE A ACTIVITY

The higher the activity number the longer is the period required ~or adequate cure. Thus, as a general proposition, the lower the "activity" number the better the printing plate formation performance of the composition. The acti~ity num~er is the minimum number of light units required to stabilize a 5%-150 ,~' 9~V

line halftone dot in a photosensitive layer to form a pri~ting plate. The photosensitive layer is exposed th~ough a test negative containing the inaicated dot and then the photosensitive layer îs etched as described in the Examples except with varient li~ht units. The units recorded are read *rom a light integrator which controls the number of light units used. Elongation and Tensile were determined by ASTM method D2707-72.
Resilience was determined by ASTM method D2632-74 and Shore A was determined by ASTM method D2240-75.
This invention provides a simple~ effective relief printing plate utilizing inexpensive materials and minimal labour requirements. The images obtained are sharp and show fidelity to the original transparency both in small details and overall dimensions. A
significant advantage arises from the fact that the softness and flexibility of the photopolymerized printing plates makes possible the use of "kiss impression"
~; printing techniques~ This printing technique is ~- 20 preferred especially by the book publishing industry as it allows use of co~rse "antique" papers, as well as higher press speeds. The abrasion resistance of the photopolymerized printing plates make the plate more durable than those presently available. An important commercial advantage is their lightness in weight. This , invention also provides a composition that eliminat`es the s, use of a de-tackifying treatment in the plate-making process.
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Claims (20)

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

1. The method of forming a printing plate which comprises exposing to actinic radiation projected through an image-bearing transparency selected portions of a radiation curable composition comprising (1) at least about 20% by weight of said composition of monoalkenyl aromatic-diene copolymer resin;
(2) at least about 2% by resin weight of at least one monomer substantially compatible with said resin and having at least one photo-cross-linkable C-C-double bond;
(3) about 0.1 to about 35% by resin weight of polythiol containing at least two-SH groups per molecule; and (4) about 0.01 to about 10% by resin weight of photoinitiator.

2. The method of claim 1 wherein said resin (1) is present in an amount of at least 40% by composition weight and is a block copolymer having the general formula A-B-A with blocks A being monoalkenyl aromatic blocks which may be the same or different and which have an average molecular weight of from 2,000 to 100,000 and a glass transition temperature above 25°C and having the general formula:

wherein X represents hydrogen or an alkyl radical containing 1 or 2 carbon atoms, Y represents hydrogen or an alkyl radical containing from 1 to 4 carbon atoms, and n represents an integer from 1 to 5 and with block B being derived from conjugated diene hydrocarbon having the general formula:

wherein each R individually represents hydrogen or an alkyl radical containing 1 or 2 carbon atoms.

3. The method of claim 2 wherein the said monomer (2) is an addition photopolymerizable poly-ethylenically unsaturated acrylic or methacrylic acid ester containing two or more acrylate or methacrylate groups per molecule or mixture thereof and is present in an amount of at least 10% by resin weight.

4. The method of claim 2 or 3, wherein said polythiol (3) is pentaerythritol tetrakis(.beta.-mercapto-propionate), trimethylolpropane tris(.beta.-mercaptopropionate), or a mixture thereof and is present in an amount of 0.5 to 10% by resin weight.

5. The method of claim 2 or 3, wherein from about 0.1 to about 50% by resin weight of a liquid polymer that is a homopolymer of butadiene or a copolymer of at least 30% by weight butadiene with the remainder being either styrene or acrylonitrile is included in said radiation curable composition.

6. The method of claim 3 wherein the said monomer (2) is present in an amount of 15-30% by resin weight, the said photoinitiator (4) is benzophenone, 2,2-dimethoxy-2-phenyl-acetophenone or a mixture thereof and is present in an amount of 0.2 to 5%, the base substrate is of polyester film, and the said radiation curable composition is applied to the substrate in a coating thickness of about 2 to about 200 mils.

7. The method of claim 1, wherein the said resin (1) is present in an amount of at least 40% by composition weight and is a two block copolymer of 30-95% by weight diene-hydrocarbon with 4 to 5 C-atoms and 5 to 70% by weight of a monomer of formula , wherein R = ? or CH3, and R' = phenyl or C1-C4 alkyl-substituted phenyl.

8. The method of claim 7, wherein the said monomer (2) is an addition photopolymerizable poly-ethylenically unsaturated acrylic or methacrylic acid ester containing two or more acrylate or methacrylate groups per molecule or a mixture thereof and is present in an amount of at least 10% by resin weight, the said polythiol (3) is pentaerythritol tetrakis(.beta.-mercapto-propionate), trimethylolpropane tris(.beta.-mercaptopropionate) or a mixture thereof and is present in an amount of 0.5 to 10%, the said photoinitiator (4) is benzophenone, 2.2-dimethoxy-2-phenyl-acetophenone, or a mixture thereof, the base substrate is of polyester film, and the said radiation curable composition is applied to the said substrate in a coating thickness of 2-200 mils.

9. A composition comprising (1) at least about 20% by weight of said composition of a monoalkenyl aromatic-diene copolymer resin;
(2) at least about 2% by resin weight of at least one monomer substantially compatible with said resin and having at least one cross-linkable C-C-double bond; and (3) about 0.1 to about 35% by resin weight of a polythiol containing at least two-SH groups per molecule.

10. The composition of claim 9 wherein said resin (1) is present in an amount of at least 40% by composition weight and is a block copolymer having the general formula A-B-A with blocks A being mono-alkenyl aromatic blocks which may be the same or different and which have an average molecular weight of from 2,000 to 100,000 and a glass transition temperature above 25°C and derived from a monoalkenyl aromatic compound having the general formula:

wherein X represents hydrogen or an alkyl radical containing 1 or 2 carbon atoms, Y represents hydrogen or an alkyl radical containing 1 to 4 carbon atoms, and n represents an integer from 1 to 5, and with block B being derived from a conjugated diene hydrocarbon having the general formula:

wherein each R individually represents hydrogen or an alkyl radical containing 1 or 2 carbon atoms.

11. The composition of claim 10, wherein the said monomer (2) is an addition photopolymerizable polyethylenically unsaturated acrylic or methacrylic acid ester containing two or more acrylate or methacrylate groups per molecule or a mixture thereof and is present in an amount of at least 10% by resin weight.

12. The composition of claim 10 wherein the said polythiol (3) is pentaerythritol tetrakis(.beta.-mercaptopropionate), trimethylolpropane tris(.beta.-mercapto-propionate). or a mixture thereof, and is present in an amount of 0.5 to 10% by resin weight.

13. The composition of claim 10 which also includes from about 0.1 to about 50% by resin weight of a liquid polymer that is a homopolymer of butadiene or a copolymer of at least 30% by weight butadiene with the remainder being either styrene or acrylonitrile.

14. The composition of claim 9 wherein said resin (1) is present in an amount of at least 40%
by composition weight and is a two block copolymer of 30-95% by weight diene-hydrocarbon with 4 to 5 C-atoms and 5 to 70% by weight of a monomer having the general formula , wherein R = H or CH3 and R'= phenyl or C1-C4 alkyl-substituted phenyl.

15. The composition of claim 14 wherein said monomer (2) is an addition photopolymerizable poly-ethylenically unsaturated acrylic or methacrylic acid ester containing two or more acrylate or methacrylate groups per molecule or a mixture thereof and is present in an amount of at least 10% by resin weight, said polythiol (3) is pentaerythritol tetrakis(.beta.-mercapto-propionate), trimethylolpropane tris(.beta.-mercaptopropionate), or a mixture thereof and is present in an amount of 0.5 to 10% by resin weight, and the composition also includes from about 0.1 to about 50% by resin weight of a liquid polymer that is a homopolymer of butadiene or a copolymer of at least 30% by weight butadiene with the remainder being either styrene or acrylonitirile.

16. The method of forming a coating on a substrate comprising applying to the surface a layer of a composition as claimed in claim 9, and thereafter exposing the layer to radiation and curing said composition.

17. The method of claim 16 wherein the composition is as claimed in claim 10.

18. The method of claim 16 wherein the composition is as claimed in claim 11.

19. The method of claim 16 wherein the composition is as claimed in claim 14.

20. The method of claim 16 wherein the said composition also includes from about 0.01 to about 10%
by resin weight of a photoinitiator and the said radiation is actinic radiation.