IE48477B1 - Phosphine activated photosensitive compositions and photopolymer printing plates made therefrom - Google Patents

Abstract

The compsn. comprises (A) a photopolymerisable, ethylenically unsatd. monofunctional, water-soluble monomer opt. with an ethylenically unsatd. polyfunctional monomer; (B) a water soluble polyvinyl acetate contg. both acetyl and OH gps., having a polymerisation degree of 300-2000 and a saponification degree of 65-99 moles %, (C) a photopolymerisation promoter activatable by actinic light and (D) an activator comprising a phosphine deriv. of formula P (X)(Y)(Z) where X, Y and Z are H, halogen, alkyl, alkoxy, aryl, vinyl or allyl, at most one of X, Y and Z being H and pref. one being aryl. Unlike previous prods. the above does not require prelimination or bump exposure. The exposed prod. is developed by simply washing with water to remove the unexposed zones. Used for prodn. of printing plates.

Description

The present invention relates to water-developable photopolymerizable compositions, and photosensitive elements and photopolymer printing plates incorporating such compositions.

Many photopolymerizable compositions and photosensitive elements useful in making printing plates are known in the art. One, if not the most, commercially successful of all such prior art oompositions is described in U.S. Patent 3,801,328 (Takimoto). Such composit10 ions, although they have met with enormous commercial success, nevertheless require some form of pre-exposure conditioning before imagewise exposure in order to produce commercially satisfactory relief images on printing plates that employ such compositions.

Two techniques for pre-exposure conditioning have evolved in the art. The first, commonly known as C02 conditioning, requires the maintenance of the photosensitive element or plate in a C02 atmosphere until just prior to the imagewise exposure of the element or plate. The ’ second known technique, which has supplanted the C02 conditioning technique and is commonly known as bump exposure, requires that the photosensitive element or plate be subjected to a brief (typically 2 to 8 seconds) light exposure immediately prior to the longer duration imagewise exposure, in the presence of a negative. - 3 It has been recognized, of course, that both preexposure conditioning techniques require special, costly and time consuming additional handling of the photosensitive element or plate which, from a plate processing standpoint should desirably be eliminated. Moreover, both techniques make the design and operation of automatic plate exposure and processing equipment unduly complicated and expensive. Perhaps even more importantly, both techniques, because of the necessity that imagewise exposure be carried out within a designated time period after pre-exposure conditioning occurs, introduce time controls in plate processing that are often difficult, if not impossible, to maintain in the large quantity, commercial (newspaper) applications for such plates.

We have found that a printing plate which has been bump exposed should desirably be imagewise exposed within approximately 30 seconds after completion of bump exposure. Variations in the period between completion of bump exposure and the initiation of imagewise exposure can produce developed plates having very different printing quality characteristics. Thus, two plates utilizing the same photopolymer composition and same negative can have different printing characteristics simply because of differences in the period between bump and imagewise exposure.

Moreover, photosensitive elements and plates, which are subjected to bump exposure, must typically be exposed under high intensity point sources of actinic light, such as carbon arc lamps or high pressure mercury vapour lamps. Such lamps not only have extremely high current demands and short lives, but in normal operation produce unwanted heat, which can cause or contribute to the degradation of the photopolymerizable compositions or cause negatives to stick to the plate photopolymer surface.

As a result of the many inconveniences, the added expenses and variations in product quality inherent in the - 4 use of plates requiring some form of pre-conditioning, either through C02 conditioning or bump exposure, a need has developed for water-developable photopolymerizable compositions (and resultant elements and plates) that entirely eliminate the necessity of pre-exposure conditioning, yet at the same time provide all of the advantageous properties of the commercially desirable compositions of the type described in the above-mentioned O.S. Patent 3,801,328. One such effort to solve the pre10 exposure conditioning problem is described in U.S. Patent 4,042,386, which describes a photopolymerisable composition comprising at least one water-soluble ethylenically unsaturated monomer, partially saponified polyvinyl acetate having defined properties, and a photoinitiator. In this photopolymerisable composition, the selection of the specific ingredients in specific amounts with specific properties is intended to eliminate the need for pre-exposure conditioning. This approach, however, suffers from the disadvantage that only carefully selec20 ted components, ratios and amounts of the various ingredients can be used, and from the additional disadvantage that only costly high intensity light sources (which have shortened lamp lives and generate excessive amounts of undesired heat) can be used. Moreover, we have found that the photopolymerizable compositions disclosed in the above-mentioned U.S. Patent 4,042,386 work best with lower reflectivity substrates (e.g., a high degree of antihalation) in order to provide developed photopolymerizable elements having the desired dot depth charac30 teristics in highlight, middle tone and shadow image areas.

We have now devised improved photopolymerizable compositions which can be used without any pre-exposure or CO2 conditioning, and can therefore be directly image35 wise exposed and then developed with water without costly or time-consuming pre-exposure conditioning.

According to the present invention, there is provided a water-developable photopolymerisable composition which comprises: (i) a photopolymerisation initiator; (ii) at least one ethylenically unsaturated monomer which is photopolymerisable in the presence of (i), said at least one monomer comprising at least one watersoluble monoethylenically unsaturated monomer and, optionally, at least one polyethylenically unsaturated monomer; and (iii) at least one partially saponified polyvinyl acetate having an average polymerisation degree of 300 to 2000 and an average saponification degree of 65 to 99 mole %.

The improvement according to the Invention is that the composition contains a phosphine of the formula in which X is an aryl group optionally substituted by alkoxy, halogen or alkyl and Y and Z are each independently alkoxy, hydrogen, halogen, any group that X may be, or an alkyl group optionally substituted by alkoxy or halogen.

It is preferred that not more than one of Y and Z is hydrogen; it is further preferred that X is optionally substituted phenyl, as is at least one of Y and Z. - 6 As mentioned above, the phosphine must contain at least one optionally substituted aryl group (X) since phosphines without such aryl groups (such as trin-butyl phosphine) have been found to be unsatisfactory.

Preferred phosphines include triphenyl phosphine, o-tolyl diphenyl phosphine, di-(o-tolyl) phenyl phosphine, triio-tolyl) phosphine, o-methoxyphenyl diphenyl phosphine, o-ethylphenyl diphenyl phosphine, and o-chlorophenyl diphenyl phosphine; triphenyl phosphine is most preferred because of its improved sensitivity, cost and air stability.

Photopolymerisable compositions containing a photopolymerisation iniator, one or more ethylenically unsaturated monomers and a phosphine falling within the above definition have been previously proposed, for example, in U.S. Patents 3832188 and 3933682 (although U.S. Patent 3832188 is concerned with a polymerisable system very different to that employed in the present invention).

U.S. Patent 3933682 suggests that organic compounds of nitrogen, phosphorus, arsenic and bismuth are all generally equivalent as activators for photopolymerisation initiators and there is no hint or suggestion that phosphines having at least one aryl or substituted aryl groups are much better in water-developable compositions. In fact, neither U.S. Patent 3832188 nor U.S. Patent 3933682 is concerned with water-developable compositions and neither has any disclosure of the inclusion of partially hydrolysed polyvinyl acetate as specified above in a photopolymerisable composition containing a photopolymerisation initiator and a phosphine.

The present invention also comprises a photosensitive element comprising a substrate having thereon a layer of photopolymerisable composition according to the invention; the present invention further comprises a process of producing a printing plate, which comprises - 7 exposing the layer of photopolymerisable composition of an element as just defined to light of a wavelength less than 400 nm, removing the photopolymerisable composition from the unexposed areas by washing with water, and drying the element.

The photosensitive elements according to the invention can incorporate a wide range of monomers and polymers, require shortened exposure times and less antihalation conditioning of the substrate, can be exposed with less intense light, and yet still provide consistently improved printing characteristics; e.g., improved dot depth in highlight, middle tone and shadow image areas, and improved minimum line width and shapes in reverse areas. Furthermore, the photopolymerizable compositions of the present invention can be applied to ultra-shallow relief plates, such as 0.1 to 0.2 mm in relief thickness, because such compositions are not adversely affected by oxygen exposure.

The initiator (i) is preferably an acetophenone, benzophenone or benzoin. Benzoin derivatives are generally soluble in the monomer component (ii) and in water, do not decompose thermally below about 100°C, and do not harden or become insoluble with heating during the preparation of the photopolymerizable composition. Specific examples of benzoin derivatives for use in oompositions according to the invention are the methyl, ethyl, isopropyl, isobutyl, octyl, vinyl, aryl and allyl ethers of benzoin. The preferred benzophenone is benzophenone itself; other suitable benzophenones include 2-methoxybenzophenone, 2-methylbenzophenone and 2,2-dimethoxybenzophenone . The most preferred initiators, are acetophenone derivatives, such as 2,2-dimethoxy-2-phenyl acetophenone, 2,2-diethoxy-2-phenyl acetophenone, 2,2diisopropoxy-2-phsnyl acetophenone, and 2,2-di-n-butoxy2-phenyl acetophenone, because of their unexpectedly high solubility in the monomer component (ii) and their - 8 excellent photoinitiation properties.

Although ordinarily photopolymerization requires a photoiniator such as an anthraquinone compound in addition to the polymerization initiators discussed above, a high photosensitivity can be attained in the present invention by using initiators such as acetophenones or benzoins alone.

The monomer component (ii) preferably comprises an acrylic or methacrylic ester, such as a monoethylenic10 ally unsaturated acrylic or methacrylic ester of a lower alkanol having one or more hydroxy groups, optionally together with a polyethylenically unsaturated monomer.

The latter is preferably an acrylic or methacrylic ester of polyethylene glycol /50 (CHjCE^O) wherein n is desirably from 1 to 23, or at least one of pentaerythritol dimethacrylate, trimethylolpropane trimethacrylate, tetramethylolmethane tetramethacrylate and trimethylolethane trimethacrylate.

Examples of suitable monoethylenically unsaturated monomers include beta-hydroxyethyl acrylate, beta-hydroxy ethyl nethacrylate and beta-hydroxypropyl acrylate.

As mentioned above, the monomer component (ii) may be a mixture of both mono- and polyethylenically unsaturated monomers, in which case the monoethylenically unsat urated monomer solubilizes (i), and the polyethylenically unsaturated monomer enhances the adhesive characteristics of the resultant photopolymerizable composition; this enables a balance of water-washability and adhesion to be obtained.

The polymer component (iii) is a partially saponified polyvinyl acetate having an average degree of polymerization of 300 to 2,000 and a saponification degree of 65 to 99 mole percent. If a suitable partially saponified polyvinyl acetate cannot be obtained by saponifying homopolymer polyvinyl acetate having a low saponification - 9 degree, a copolymer obtained, for example, by copolymerizing vinyl acetate with maleic anhydride can be partially saponified to give the desired polymer. Saponification is used herein in its usual sense to mean the conversion of ester groups into alcohol groups and the saponification degree therefore represents the extent to which ester groups have been converted to hydroxy groups.

The hardness of the obtained printing plate as well as the speed of washing out in development depends directly on the degree of saponification. There are some cases, for example, wherein a certain degree of saponification is required for compatibility with certain monomer components. Thus, the required degree of saponification may be obtained by mixing two or more partially saponified polyvinyl acetate polymers having different degrees of saponification provided that the arithmetic average of the different degrees of saponification is in the range specified above. Mixtures of two or more polymers can be used, therefore, each having a different saponification degree, to obtain the desired average saponification degree for any given monomer component.

This feature of the present invention adds significant flexibility to the formulation process and contributes significantly to the balance of properties that are desired for the photopolymerizable composition.

The amounts of the various components of the photopolymerizable composition of the present invention are preferably as follows: a) 0.1 to 3.0 parts by weight of (ii); b) 0.1 to 3.0 parts by weight of (iii); c) 0.001 to 0.3 parts by weight of (i); and d) 0.001 to 0.3 parts by weight of the phosphine (although proportions outside these ranges can be used if desired). When the concentration of the phosphine is outside the range specified, however, the photopolymerizable composition is so light sensitive that it is very difficult to handle the photosensitive element prior to imagewise exposure. Also, too - 10 much phosphine causes the photopolymerizable composition to become heat sensitive which further interferes with the practical use of the photosensitive elements and reduces their useful shelf life.

When, on the other hand, the concentration of the phosphine used is less than that specified in the broad concentration range above, the sensitivity of the photosensitive element may be too low to be practically used without the undesired pre-exposure conditioning.

It is desirable in some instances to add to the photopolymerizable composition of this invention 0.01 to 0.3 percent, based upon the amount of (ii), of a thermal polymerization inhibitor; examples of suitable such inhibitors include 2,6-di-t-butyl-p-cresol, hydroquinone, and p-methoxyphenol. The preferred inhibitor is 2,6-dit-butyl-p-cresol because of its compatibility with the other components.

Small amounts of dyestuffs may also be added to the photopolymerizable composition (to provide antihalat20 ion properties). Generally, the dye is added in an amount just below that at which the composition becomes hazy (for example, 20 to 150 ppm based on the amount of the photopolymerizable composition). Examples of suitable dyestuffs include one or more of rose bengal, eosine, methylene blue and malachite green.

The photosensitive element according to the invention comprises a substrate having the photopolymerisable composition according to the invention thereon, as mentioned above. The substrate may be of plastics or metal and is preferably chemically or physically roughened prior to application of the photopolymerisable composition in order to ensure strong adhesion. This roughening or grating of the substrate surface eliminates the need for a separate adhesive layer although such may be used, where desired, in certain instances. - 11 The preferred substrate is a metallic plate, more preferably an aluminium or tin plate. As described in U.S. Patent 3,877,939, the use of a dispersed antihalation material can be eliminated by treating the substrate with an aqueous solution of a chromic compound.

Thus, for example, an aluminium plate may be abraded and then chemically treated to impart the desired antihalation properties. The preferred chemical treatment for obtaining these antihalation properties is a chemical solution of a chromic compound which includes, for example, chromic acid, water soluble salts of chromic acid and ferricyanic or ferrocyanic acid and water soluble salts thereof.

Generally, a treating composition for such a purpose contains either chromic acid or sodium bichromate, and mixtures of potassium ferricyanide, barium fluosilicate, sodium ferrocyanide, sodium fluoborate, potassium fluozirconate and similar compositions. The presently preferred treating composition is 2 percent aqueous solution of Alodine 1200 manufactured by Amchem Products.

Inc., Ambler, Pennsylvania. The treating composition is typically first placed in a bath and then the aluminium or tin plate is immersed in the bath for approximately 40 seconds. After the plate has been treated in this fashion, it generally changes its colour from metallic to orange, whereby the plate provides antihalation activity.

As discussed above, the degree of antihalation required in the photopolymer plates made from photosensitive elements according to the present invention is reduced, since the presence of the phosphine in the composition minimizes the need for lower reflectivity substrates, and thus reduces the manufacturing cost of the photopolymerizable compositions.

Moreover, the exposure of the photosensitive element involves the step of subjecting the element to a single imagewise exposure of actinic light. Although Λ847 7 - 12 prior photosensitive elements required the use of high intensity point sources such as carbon arc lamps and/or high pressure mercury lamps, exposure of the photopolymerizable composition of the present invention may be carried out under diffusion light sources such as black light tubes, otherwise known as chemical lamps. These chemical lamps are far less expensive than the prior high intensity point sources, use considerably less energy, and generate less heat. Such chemical lamps give off actinic light with wavelengths in the desired range of 300-400 nm, and provide the further advantage of generating better shoulders in the final relief image which results in improved resolution and highlight reproduction.

By eliminating the previously required pre-exposure conditioning, the photopolymerizable compositions of the present invention enable the plate processing time to be shortened and enable the photosensitive elements to be readily used in automatic developing equipment. It is not necessary to engage in the time consuming task of first bump exposing a plate, and then placing a negative on the plate for imagewise exposure. Instead, a negative can be immediately placed on the plate (without bump exposure) and then exposed for a shorter period of time with less intense light, without sacrificing printing plate quality in terms of dot depth or minimum line size.

The photosensitive element according to the invention can have a wide range of thickness for the photopolymerizable composition coating. For example, the element may be a relief plate having photopolymer thicknesses greater than 0.5 mm and shallow relief plates having photopolymer layers between 0.1 and 0.5 mm thick. The preferred thickness is in the range 0.1 to 0.89 mm.

For shallow relief plates, an interlayer is preferably interposed between the substrate and photopolymer layer; a preferred interlayer contains dispersed within it particles which are present in a size and concentration - 13 sufficient to create a plurality of protuberances in background areas of the photosensitive element after its exposure and development, to create a relief image.

The interlayer preferably contains, as binder, a polyester, a polyurethane, an ethylene-butadiene copolymer, a polyvinyl acetate derivative, a polyamide, an epoxy resin, a styrene-butadiene copolymer (preferred), mixtures of such copolymers and partially hydrolyzed polyvinyl acetate. The polyester may be an unsaturated polyester made, for example, from diethylene glycol, maleic anhydride and phthalic anhydride, the polyester may be mixed with partially hydrolyzed polyvinyl acetate, or with a mixture of glyoxal and partially hydrolyzed polyvinyl acetate.

The dispersed particles may be, for example, of glass, polytetrafluroethylene (for example, available under the Trade Mark Teflon), and alumina beads. The particle size and concentration will, of course, be chosen depending on the printing parameters such as the thickness of the relief image, printing pressures and number of repetitions.

In order that the present invention may be more fully understood, the following Examples (in which all percentages and parts are by weight unless otherwise indicated) are given by way of illustration only.

EXAMPLE 1 Substrates for the photosensitive elements used below are prepared as follows: Method A: An aluminium plate is immersed in a surface treating agent comprising sodium bichromate (1.0 part), concentrated sulphuric acid (10 parts) and water (30 parts) at 70 to 80°C for about 20 minutes. Then, the plate is washed with water and dried. - 14 Method B: A tin plate is immersed in a detergent comprising Ridoline (1.0 part; manufactured by Amchem Products, Inc.) and water (30 parts) at 65°C for about 20 minutes. Then, the plate is washed with water and dried.

Method C: The solution (60 parts) which consists of 14 parts of partially saponified polyvinyl acetate (average polymerization degree, 500; saponification degree, 7.0 mole?) and 86 parts of water, and the wellground pigment paste (10 parts) which consists of 14 parts of the above-mentioned polyvinyl acetate, 56 parts of water and 30 parts of red iron oxide pigment (R0-5097) manufactured by Pfizer) are mixed together. A latex of carboxylated styrene/butadiene copolymer (20 parts) XD655 manufactured by Dow Chemical Co.) is added to the resulting solution gradually under stirring. The solution is cast on the plate which is described in Method B and dried for 2 minutes at 190°C to form the layer 15 pm in thickness.

Method D: The solution (60 parts) which consists of 14 parts of partialkly saponified polyvinyl acetate (average polymerization degree, 500; saponification degree, 78.0 mole%) and 86 parts of water, and the wellground pigment paste (15 parts) which consists of 14 parts of above-mentioned polyvinyl acetate, 56 parts of water and 30 parts of red iron oxide pigment (R0-5097) manufactured by Pfizer) are mixed together. The latex of carboxylated styrene/butadiene copolymer (20 parts) (XD-655 manufactured by Dow Chemical Co.) is poured into the resulting solution gradually under stirring, and 5 parts of Teflon powder (TL-115 by LNP Co.) is added into the solution. The solution is cast on a plate as described in Method B and dried for 2 minutes at 190°C to form the layer 20 pm in thickness. - 15 EXAMPLE 2 A mixture of partially saponified polyvinyl acetate (average polymerization degree, 500; saponification degree, 82.0 mole%)(35 parts), water (30 parts) and Rose bengal (30 ppm of all components by weight) is kneaded in a kneader at 80 to 90°C for 30 minutes. Then, this mixture is cooled to 60°C and a mixture of diethylene glycol dimethacrylate (5 parts), beta-hydroxyethyl methacrylate (28 parts), hydroquinone (0.1% of total monomer by weight), benzoin iso-propyl ether (0.5 part) and triphenyl phosphine (1.5 parts) is added and stirred for 30 minutes. The resulting photopolymerizable composition is cast on a plate as described in Example 1, Method C.

A polyester sheet is placed thereon and the resulting piled product is passed between two rolls. After cooling, the polyester sheet is peeled off and the plate is dried in a dryer at 75°C for 40 minutes to form a photosensitive layer 0.5 mm thick.

EXAMPLE 3 A mixture of partially saponified polyvinyl acetate (average polymerization degree, 500; saponification degree 77.0 mole%)(5 parts), partially saponified polyvinyl acetate (average polymerization degree, 500; saponification degree 82.0 mole%)(30 parts), water (30 parts) and Eosin (30 ppm of all components by weight) is kneaded in a kneader at 80 to 90°C for 30 minutes. Then, this mixture is cooled to 60°C and a mixture of ethylene glycol dimethacrylate (8 parts), beta-hydroxypropyl methacrylate (25 parts), hydro-quinone (0.1% of total monomer by weight), 2,2-dimethoxy-2-phenyl acetophenone (1.0 part) and triphenyl phosphine (1.0 part) is added and stirred for 30 minutes. The resulting photopolymerizable composition is cast on a plate as described in Example 1, Method C. The photosensitive plate is obtained by the same method as described in Example 2, and the photosensitive layer is 0.5 mm thick. - 48477 - 16 EXAMPLE 4 A mixture of partially saponified polyvinyl acetate (average polymerization degree 500; saponification degree 82.0 mole%)(35 parts), water (30 parts) and Rose bengal (30 ppm of all components by weight) is kneaded in a kneader at 80° to 90°C for 30 minutes. Then, this mixture is cooled to 60°C and a mixture of ethylene glycol dimethacrylate (5 parts), beta-hydroxypropyl methacrylate (28 parts), hydroquinone (0.1¾ of total monomer by weight), benzoin iso-propyl ether (1.0 parts) and O-tolyl diphenyl phosphine (1.0 part) is added and stirred for 30 minutes. The resulting photopolymerizable composition is cast on the plate which is described in Example 1, Method A. The photosensitive plate is obtained by the same method as described in Example 2, and the photosensitive layer is 0.5 mm thick.

EXAMPLE 5 The photosensitive resin which is obtained by the method in Example 3, is cast on a plate as described in Example 1, Method D, and the photosensitive layer is 0.125 mm thick after drying.

EXAMPLE 6 (comparative example) A mixture of partially saponified polyvinyl acetate (average polymerization degree, 500; saponification degree, 82.0 mole%)(35 parts), water (30 parts) and Rose bengal (50 ppm of all components by weight) is kneaded in a kneader at 80 to 90°C for 30 minutes. Then, this mixture is cooled to 60°C and a mixture of ethylene glycol dimethacrylate (2 parts), beta-hydroxyethyl meth30 acrylate (32 parts), hydroquinone (0.1% of total monomer by weight) and benzoin isopropyl ether (1.0 part) is added and stirred for 30 minutes. The resulting photopolymerizable composition is cast on a plate as described in Example 1, Method A. The photosensitive plate is obtained by the same method as described in Example 2, 4847 7 - 17 and the photosensitive layer is 0.5 mm thick.

EXAMPLE 7 (comparative example) The photosensitive resin which is obtained by the method in Example 6, is cast on a plate which is as described in Example 1, Method C, and the photosensitive layer is 0.5 mm thick after drying.

EXAMPLE 8 The photopolymer plate made according to Example 3 is placed in a vacuum frame and the photopolymerizable surface is brought into contact with a line negative or a half tone negative. It is exposed to a 3,000 watt high pressure mercury arc for 50 seconds from a distance of 50.8 cm. After exposure, the negative is stripped from the plate and the unexposed polymer is removed by spray washing with water (temperature, 49°C) under the pressure 2 of 0.28 N/mm for 3 minutes. The printing plate is dried at 110°C for 3 minutes. The printing plates thus prepared show excellent image quality and long press life when used for direct printing and also are utilizable as origi20 nal plates for a paper mache.

Table I demonstrates the comparison in quality between the photosensitive plates with (Examples 3 and 4) and without (Examples 6 and 7) phosphine derivatives. - 18 TABLE I Example 3 Example 4 Example 6 Example 7 Exposure Time (sec) 50 45 180 195 Dot Depth (pm) 10% Hi-light 270 260 160 175 45% Middle tone 125 120 50 60 90% Shadow 55 50 Plug Dot Plug Dot Width of Minimum Remaining Line (pm) 40 40 140 140 Gray Step 17 17 13 13 As shown in Table I above, photopolymer plates made with the photopolymerizable compositions of the ' present invention have improved dot depth characteristics in hi-light, middle tone and shadow areas. Dot depths that are less than 200 pm in hi-light areas, 80 pm in middle tone areas and 35 pm in shadow areas (as is the case in Examples 6 and 7) provide dark, unacceptable half tone areas.

Msreover, line widths above 50 to 60 pm are generally regarded as unacceptable in newspaper printing applications.

The significance of component selection in the present invention can be demonstrated through a series . of tests in which (1) derivatives employing Group V ele25 ments other than phosphorus are substituted for the phosphines used according to the present invention, (2) phosphines with no phenyl groups (not according to the invention) are used instead of phosphines having 1, 2 or 3 phenyl groups, and (3) halogenated compounds such as pentachlorobenzene are used as an additional component in - 19 the present invention. Table II sets forth on the following page shows the results of such tests: TABLE II in in 30 I 30 1_ 30 ppm CO in CM 0.1% of monomer i—f »—J fh Ch EH 0.5 f •sr in 30 30 a a o (n co 25 »W M 0 0 M ϋΡ 0 Η β • 0 o s I-1 pH CM. Eh o cn in 30 30 30 ppm co 25 j 0.1% of monomer (-1 pH Ch fh a o CM in 30 30 30 ppm co 25 0.1% of monomer rd BDP 1 o l—'1 in 30 30 30 ppm co 25 0.1% of j monomer rH pH ih Ch Eh o PVA (Saponification degree 77.0 mol %) PVA (Saponification degree 82.0 Mol %) Water Eosin Ethyleneglycol dimethacrylate Beta-Hydroxypropyl methacrylate Hydroquinone . Ch X 0 0 fi fi 0) -P o 0 nJ Η H ? & CM fl) ·* xS CM A * Activator Q) β Q) N β Q) Λ 0 0 rH Λ ϋ (ϋ fi β Q) Ch uo-rq-PTnuttoj TABLE II (continued) tn O in 270 120 in in o Xf rt Accept- able xp O d rt o in rt in in Plug Dot 140 rt rt Not Accept- able ro m in 10 d O d rt in in o in «X rt Accept- able d 00 tn in l· d O d rt in in O in KO rt Accept- able t—1 O tn 270 in d rt in in O XP t*· rt Accept- able Exposure Time (sec.) •P Xi Cn rt rt 1 rt K CUP O rt 45% Middle Tone 90% Shadow s B 0 n a β -π rt rt s cn ΜΗ β ° 3 Gray Step Overall Quality (“rt) qqdea qoa Width Remai AqiienQ equid Φ £ rt Λ 4J >i I fi tn rt Λ Φ rt rt β rt 4J >1 rt £· fi β g 9 nJ Φ Jo rt ,c rt 0 >1 ft >1 β fi rt β (0 V Φ Λ XJ +) b 4J cu fi ft Φ rt n rt rt M rt P M Ei Q H EH co cu z cu cu cu kJ Eh cu rt rt Φ fi •rl xj rt Cu tn CU >1 tn fi ο Φ •β ft ft !>< »ϋ fi rt Φ Λ CU rt U rt ft M H L Ή Ή Μ H Eh Eh ft ft < ft CQ ft m O Eh Ei 8 4 7 7 - 22 TABLE II (continued) στ m 3030 e a a o cn 00 25 0.1% of monomer H H Z a o 00 m 30 30 β a a o m oo -, 25 1 0.1% of monomer t—! 1-t _ a a CQ o Γ- m 30 o ω 30 ppm oo 25 0.1% of monomer r-l !”f CQ a o kO in30 i 30 β a a o ro co 25 0.1% of monomer , i-1 «—! Λ e< o PVA (Saponification degree 77.0 mol %) PVA (Saponification degree 82.0 mol %) Φ P rtj £ Eos in Ethyleneglycol dimethacrylate Beta-Hydroxypropyl methacrylate Hydroquinone 2,2-Dimethoxy-2phenyl acetophenone A 0 8 > •rl +> 0 rtj * Pentachlorobenzene uoxq.'Bxnuiaoj - 23 TABLE II (continued) cn in KO rd 130 in M· Plug Dot 130 CO r-d Not Accept- able co in in rri 140 o in Plug Dot o co ’S' rd Not Accept- able P- 135 155 in m Plug Dot O tn in rd Not Acceptable io 150 o ’T rd O in Plug Dot O tn in r-t Not Acceptable Exposure Time (sec.) 10% Hi-light 45% Middle Tone 90% Shadow Width of Minimum Remaining Line (pm) Gray Step Overall Quality (uirl) qqdaa qoa Aq-ptenO egeid 0) >1 a 0 •ri Λ +» β 6 03 •ri Φ s rQ φ β •ri »—1 β •ri +» >1 •κ c ri β φ 1 rd rd λ rd 0 a >1 β β β ri 11 φ X! £ +) X! a β a φ *ri •ri id ri •ri ri ri Ω Η Η ω Αι 2ί -ϋ Ai Αι Αι W Ed (0 h-i t-i 1) a -I Λ a tn Ο Λ a rd a v x Pi id ri Ed •Η β X! •ri a ti Φ tn α β 0 η •ri Λ 0 ti a Λ Pi rd Λ in rd 0 β >1 Λ Φ β Pi ti Φ rd Ρι Λ >1 •ri Ρι +> Ό 0 β rd ri Λ 0 1 +) rd β β Xj 1 Ο •ri 1 •ri ri β α Η Αι Ai Ai a u Ai w u (0 ri ri r—i E? CD ti Λ id ri E4 Ai < A) Ai H E« - 24 TABLE II (continued) CM H 30 o fi cu cu o co CO tn CM 0.1% of monomer rd rd CU ffl 0.5 1—1 rd m o m 30 30 ppm CO tn CM 0.1% of monomer r—1 o O o rd m O o 30 ppm co in CM 0.1% of monomer rd rd w Ed o PVA (Saponification degree 77.0 mol %) PVA (Saponification degree 82.0 mol %) Water 1 Eosin r-l ¢) 0 +> □ (0 H Η rd tn rt φ o rt rt Φ Λ Η -Ρ >t 0 Λ fi -Ρ ‘id Beta-Hydroxypropyl methacrylate Hydroquinone 2,2-Dimethoxy-2phenyl acetophenone rt 0 rt 5 4J ϋ Pentachlorobenzene uoigpinuiioj - 25 TABLE II (continued) CN rd o CN rd in rd in in Plug Dot 140 rd Not Acceptable r—! rd 160 O co rd O in Plug Dot 140 cn rd Not Acceptable o rd 180 120 in •ci· Plug Dot o in rd rd rd Not Acceptable | Exposure Time (sec.) | 10% Hi-light 45% Middle Tone 90% Shadow of Minimum .ing Line (pm) Gray Step Overall Quality (uirl) qgdsa gog Width Remain Xq-TinnO sqnid 0) ti id & 0 x: co id Φ s fi Φ β rd ti id •P o, id (3 ti (0 c Φ § rd rd h rd 0 >1 a >1 G ti rd ti (0 Φ >» Φ fi Λ +J Λ •P CU ti cu Φ id Λ4 id id P id P id ti Q H H ω PU SS ri! PU PU fi. ω CQ Eh J3 cu n id fi CU at β 1) 0 (0 •fd β Xi 0 XJ id a X! a fi rd a w 41 a rd u β m β x; id 0 Φ ti CU 10 ,fi XJ Φ rd P a cu Λ >1 Rj id a •P rd >1 •ti 0 ti >1 ti rd P 14 ti (!) 0 1 Φ Λ •P rd β Xi a 3 X! a ω 1 o id id p •rd P M Ε-» ti Q Ε-» En fU PU PU CU CU Q PU cn cu EH P3 u tl Eh 8 4 7 7 - 26 Examples 1, 2 and 3 demonstrate that the use of phosphine derivatives having 3 (triphenyl phosphine), (n-butyldiphenyl phosphine) and 1 (dichlorophenyl phosphine) phenyl groups in the phosphine activator com5 ponent all produce printing plates with satisfactory properties. Surprisingly, however, the degree of acceptance decreases as the number of phenyl groups decreases from 3 to 1, e.g., exposure time increases, dot depth decreases, and gray step values decrease, but all within acceptable limits.

Example 4 demonstrates that the use of a phosphine with no phenyl groups (tri-n-butyl phosphine), with all other parameters remaining the same, does not result in a printing plate having satisfactory conditions for news15 paper printing applications.

Likewise when derivatives of Group V elements such as nitrogen, arsenic, antimony and bismuth are used in place of a phosphine, the resultant printing plate has unsatisfactory properties (Examples 6-10). Surprisingly, this is the case even when three phenyl groups are used (triphenylarsine, triphenylantimony and triphenylamine), or when two phenyl groups are used (dibutylphenylbismuthine).

Finally, the presence of a halogenated compound such as pentachlorobenzene neither improves the properties of the resultant plate (compare Examples 1 and 5) nor renders the resultant plate satisfactory in the absence of a phosphine derivative with at least one phenyl group (compare Examples 4 and 12).

Claims (16)

1. CLAIM S:1. A water-developable photopolymerisable composition comprising: (i) a photopolymerisation initiator; (ii) at least one ethylenically unsaturated monomer which is photopolymerisable in the presence of said initiator, said at least one monomer comprising at least one water-soluble monoethylenically unsaturated monomer and, optionally, at least one polyethylenically unsaturated monomer; and (iii) at least one partially saponified polyvinyl acetate having an average polymerisation degree of 300 to 2000 and an average saponification degree of 65 to 99 mole%, characterised in that the composition contains a phosphine of the formula in which X is an aryl group optionally substituted by alkoxy, halogen or alkyl, and Y and Z are each independently alkoxy, hydrogen, halogen, any group that X may be, or an alkyl group optionally substituted by halogen or an alkoxy group.

2. A composition according to claim 1, characterised in that the phosphine is triphenyl phosphine, otolyl diphenyl phosphine, di-(o-tolyl)phenyl phosphine, tri-(o-tolyl)phosphine, o-methoxyphenyl diphenyl phosphine, - 28 o-ethylphenyl diphenyl phosphine or o-chlorophenyl diphenyl phosphine.

3. A composition according to claim 1 or 2, characterised in that the photopolymerisation initiator 5 is a benzoin, acetophenone or benzophenone.

4. A composition according to any of claims 1 to 3, characterised in that the water-soluble monomer is an acrylic or methacrylic ester.

5. A composition according to claim 4, charac10 terised in that the water-soluble monomer is betahydroxy ethyl acrylate or methacrylate.

6. A composition according to any of claims 1 to 5, characterised in that the polyethylenically unsaturated monomer is an ester of acrylic or methacrylic acid 15 and a polyethylene glycol having 1 to 23 ethylene oxide units, or at least one of pentaerythritol dimethacrylate, tetramethylolmethane tetramethacrylate and trimethylolethane trimethacrylate.

7. A composition according to any of claims 1 20 to 6, characterised in that it comprises 0.1 to 3.0 parts by weight of said at least one monomer, 0.1 to 3.0 parts by weight of said partially saponified polyvinyl acetate, 0.001 to 0.3 parts by weight of said initiator and 0.001 to 0.3 parts by weight of said phosphine. 25

8. A composition according to any of claims 1 to 7, characterised in that it contains a thermal polymerisation inhibitor in an amount of 0.01 to 0.3%, based on the weight of the monomer.

9. A composition according to any of claims 1 to 30 8, characterised in that it contains a minor amount of dyestuff. - 29

10. A photosensitive element comprising a substrate having thereon a layer of photopolymerisable composition, characterised in that the composition is as claimed in any of claims 1 to 9.

11. An element according to claim 10, characterised in that the substrate is a chemically or physically roughened metal or plastics plate.

12. An element according to claim 10 or 11, characterised in that the layer of photopolymerisable composition has a thickness of 0.1 to 0.89 mm.

13. An element according to any of claims 10 to 12, characterised in that an interlayer comprising a binder having particulate material dispersed therein is present between the layer of photopolymerisable composition and the substrate, the particulate material having a particle size and concentration such as to create a plurality of protuberances in background areas of the element after exposure and development thereof to produce a relief image.

14. An element according to claim 13, characterised in that the particulate material is glass beads, alumina, or polytetrafluoroethylene powder.

15. An element according to claim 13 or 14, characterised in that the binder in the interlayer is a styrene-butadiene copolymer or a polyester.

16. A process of producing a printing plate, which comprises exposing the layer of photopolymerisable composition of an element according to any of claims 10 to 15 to light of a wavelength less than 400 nm, removing the photopolymerisable composition from the unexposed areas by washing with water, and drying the element.