GB2064151A

GB2064151A - Photopolymerisable composition

Info

Publication number: GB2064151A
Application number: GB8036790A
Authority: GB
Original assignee: Uniroyal Inc
Current assignee: Uniroyal Inc
Priority date: 1979-11-21
Filing date: 1980-11-17
Publication date: 1981-06-10
Also published as: IT8068790A0; DE3043910A1; JPS5682801A

Abstract

A photopolymer elastomeric article designed to be used primarily as a flexible printing plate comprising three constituents: a chlorinated polyethylene polymer matrix, a photopolymerizable, ethylenically unsaturated crosslinking agent, as typified by esters of acrylic or methacrylic acid and a photoinitiator, which is a compound that when irradiated, will initiate polymerization and/or crosslinking. The chlorinated polyethylene component may comprise a blend of chlorinated polyethylene with 5-50% of an elastomer e.g. butadiene-styrene, butadiene-acrylonitrile, styrene- butadiene-styrene block copolymer, styrene-isoprene styrene block copolymer, 1,4 polybutadiene, chlorosulphonated polyethylene and natural rubber.

Description

SPECIFICATION Photopolymerizable composition containing chlorinated polyethylene BACKGROUND OF THE INVENTION 1. Field of the invention The present invention relates to a photopolymer elastomeric article which is designed to be used primarily as a flexible printing plate. Flexographic printing is a method of rotary letterpress printing which uses flexible elastomeric printing plates. In this system the ink is transferred directly from the raised surfaces of the elastomeric plate to the printing stock.

Flexible photopolymer printing plates of the present invention comprise three elements: 1. An elastomeric polymer matrix, 2. A photopolymerizable, ethylenically unsaturated cross-linking agent, as typified by esters of acrylic or methacrylic acid and 3. A photoinitiator, which is a compound that when irradiated, will initiate polymerization and/or crosslinking.

The flexographic printer has to print many different substrates which vary widely in their composition and surface uniformity including metal foils, plastic films, kraft, corrugated board and laminated boards. To be able to print on this variety of materials, the printer must use a variety of ink types and printing press conditions. The ideal printing plate should withstand all common ink solvents and must also show no tendency to crack when flexed, must be soft enough to conform to irregular surfaces yet firm and durable enough to withstand the forces of printing. The ideal flexographic plate must also not crack or degrade when exposed to ozone in ambient air.

2. Description of the Prior Art U.S. Patents 2,791,504 and 2,760,863 disclose a photosensitive element and a process for the preparation of printing reliefs containing an addition polymerizable monomer and a photopolymerization initiator. These all use rigid polymeric binders and produce rigid relief images which are not useful for flexographic printing.

U.S. Patent 3,024,180 and Canadian 614,181 disclose relief plates based on 1,3-butadiene and halogen-substituted 1,3-butadiene, and mentions copolymers thereof, with aryl olefins, e.g., styrene, acrylic esters and nitriles.

U.S. Patent 3,674,486 discloses printing plate compositions based on block copolymers of styrene and polybutadiene (SBS) or polyisoprene (SIS).

None of these references disclose a flexible photopolymer plate in which the polymer matrix is chlorinated polyethylene.

SUMMARY OF THE INVENTION The present invention is a flexible photopolymer printing plate comprising: (1) a chlorinated polyethylene elastomer, or blends thereof, (2) a photopolymerizable, ethylenically unsaturated crosslinking agent, and (3) an addition polymerization initiator activatable by actinic radiation.

The article of the present invention, comprising a layer of the aforesaid photopolymerizable composition optionally adhered to a support layer, is exposed to actinic radiation through an imagebearing transparency, consisting of substantially clear and opaque areas, placed in intimate contact with the photopolymerizable layer. Irradiation causes addition photopoiymerization to occur in the area of the layer corresponding to the clear area of the process transparency, but substantially no polymerization takes place in the areas corresponding to the opaque areas of the process transparency.Removal of the non-polymerized area (hereinafter referred to as unexposed areas) by treatment with a suitable solvent, which dissolves the non-polymerized areas, but which has substantially no effect on the photopolymerized areas (hereinafter referred to as exposed areas) yields a relief printing plate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION Flexible photopolymer printing plates comprise three major elements: (1) An elastomeric polymer matrix; (2) A photopolymerizable ethylenically unsaturated crosslinking agent as typified by esters of acrylic or methacrylic acid; (3) A photoinitiator, which is a compound, that when irradiated, will initiate polymerization and/or crosslinking. In the present invention the elastomeric polymer matrix is a chlorinated polyethylene, or a blend thereof. The chlorinated polyethylenes operable in this invention are described in Rubber World 1979 Blue Book, Materials, Compounding Ingredients, and Machinery for Rubber, page 342.They are designated CM and have specific gravities of 1.16, 1.16 and 1.25 and Mooney viscosities, ML-4, at 250OF, of 70-80, 100-110 and 75-85 respectively. These synthetic rubbers are available under the tradename DOW Registered Trade Mark CM and the code numbers 0136, 0236 and 0342. These polymers contain from about 36% to 42% chlorine based upon the weight of the polymer.

All of the photopolymer flexographic printing plates mentioned in the prior art have limited utility to the printer because they cannot be used with all the common ink solvents. The plates disclosed in the prior art also tend to crack when exposed to ambient ozone. The present invention, which uses chlorinated polyethylene as the elastomeric polymer matrix, is particularly useful because it has all of the desirable properties and can be used with all common ink solvents. The printing plates of the present invention also resist ozone attack better than prior art printing plates. Further, the chlorinated polyethylene elastomer shows extraordinarily compatibility with a wide variety of other polymers which allows for easy adjustment of the printing plate properties.

In the prior art, all of the elastomer polymers used for making flexible photopolymer printing plates have unsaturation, either in the polymer backbone or in unsaturated pendant groups. It is generally believed that unsaturation in an elastomeric polymer is a necessary requirement for good cures in free radical initiated systems. Thus, it is totally unexpected that a photopolymer composition containing a fully saturated polymer such as chlorinated polyethylene would photocure to produce a plate superior to the aforementioned elastomers.

The chlorinated polyethylene elastomers are uniquely suited for flexible printing plates because they offer resistance to all common ink solvents including hydrocarbons, alcohols, water and esters. On the other hand, commercially available SBS and SIS polymers cannot be used with hydrocarbon inks because the ink solvents swells the printing plate severely and thus the resolution and durability are lost.

The commercially available photopolymer plates based on nitrile elastomers are considered to offer the best balance of ink solvent resistance. The printing plates of this invention are superior to the nitrile elastomer plates in resistance to alcohol and ester inks and comparable in resistance to hydrocarbon inks. Flexible printing plates of this invention, unlike those of the prior art, resist cracking and degradation caused by ozone attack of the elastomer backbone unsaturation since the chlorinated polyethylene is a completely saturated elastomer. The chlorinated polyethylene based plates also have other properties important to the printer such as durability, no tendency to crack on flexing, good image quality and fast processing times.

Chlorinated polyethylene is also unique among elastomers because it is compatible with a variety of other polymers such as SBS and SIS block copolymers, nitrile rubbers and polybutadiene polymers.

This permits for easy modification of the properties of the printing plate. Printing plates of the present invention can also be modified with oligomers such as chlorinated waxes, processing aids such as Strucktol WB212 and 60NS and by fillers such as silica.

The polymers which may be blended with CM include natural and synthetic dienoid elastomers and chlorosulfonated hydrocarbon elastomers. Synthetic elastomers include homopolymers of conjugated diolefin hydrocarbons and copolymers of such diolefins with mono-olefinic compounds copolymerizable therewith. Such monoolefins include styrene, alpha-methylstyrene, p-methylstyrene, alpha-p-dimethylstyrene, acrylic and methacrylic nitriles, amides, acids and esters, vinylpyridines, fumaric esters, methylenemalonic esters, vinylidene chloride, methyl vinyl ketone and methyl isopropenyl ketone. Mixtures of such mono-olefinic compounds can also be polymerized with the diolefin Of particular usefulness in blends with the CM are butadiene-acrylonitrile copolymers as prepared by the method described in U.S.Patent No. 1,937,000, butadiene-styrene copolymers and block copolymers of butadiene or isoprene with styrene of the SBS and SIS types as prepared by the method described in U.S. Patent No. 3,265,765, and chlorosulfonated polyethylene.

In order to maintain elastomeric properties, these copolymers should comprise at least 50% by weight of the diene. In the case of nitrile rubber, the copolymer contains 2 to 50% by weight of the copolymerized acrylonitrile (preferably 20 to 40% by weight) the balance comprising 1,3-butadiene.

In a styrene-butadiene copolymer rubber, the copolymer contains 21 to 45% of the copolymerized styrene. In a styrene-butadiene or styrene-isoprene block copolymer, the copolymer contains 10 to 40% copolymerized styrene.

Photopolymerizable ethylenically unsaturated crosslinking agents suitable for use in the invention include unsaturated esters of polyols, particularly such esters with alpha-methylene carboxylic acids, e.g. ethylene glycol diacrylate, diethylene glycol diacrylate, triethylene-glycol diacrylate, tetraethylene glycol diacrylate, glycerol diacrylate, glycerol triacrylate, mannitol polyacrylate, sorbitol polyacrylate, ethylene glycol dimethacrylate, 1,3-propanediol dimethacrylate, 1 ,2,4-butanetriol trimethacrylate, 1,4- butanediol diacrylate and dimethacrylate 1,4-cyclohexanediol diacrylate,1,4-benzenediol dimethacrylate, isosorbidene diacrylate, pentaerythritol di-, tri-, and tetra-methacrylate, dipentaerythritol polyacrylate, pentaerythritol di-, tri-, and tetraacrylates, 1 ,3-propanediol diacrylate, 1,6 hexanediol diacrylate and dimethacrylate, 1,5 pentanediol dimethacrylate, trimethylolpropane triacrylate, the bisacrylates and methacrylates of polyethylene glycols of molecular weight 200 to 4000; unsaturated amides, particularly those of the alpha-methylene carboxylic acids, and especially those of alpha, omega-diamines and oxygen-interrupted omega-diamines such as methylene bisacrylamide, methylene bis-methacrylamide, ethylene bis-methacrylamide, 1 ,6-hexamethylene bis-acrylamide, diethylene triamine tris-methacrylamide, 1 ,2-di(gamma-methacrylamidopropoxy)ethane, betamethacrylamidoethyl methacrylate, N-(beta-hydrnxyethyl)-2-(methacrylamido)-ethyl acrylate and N,N bis-(beta-methacrylyIoxyethyl)#acryIamide; ; vinyl esters such as divinyl succinate, divinyl adipate, divinyl phthalate, divinyl terephthalate, divinyl benzene-1 ,3-disulfonate, divinyl butane-1 ,4-disulfonate and unsaturated aldehydes, such as sorbaldehyde (hexadienal). An outstanding class of those preferred addition polymerizable crosslinking agents are esters and amides of alpha-methylene carboxylic acids and substituted carboxylic acids with polyols and polyamines wherein the molecular chain between the hydroxyls and amino groups is solely carbon or oxygen-interrupted carbon.

The useful addition polymerization and/or crosslinking initiators are those activatable by actinic light and thermally inactive at and below 1 850C. These include aromatic ketones such as benzophenone and substituted benzophenones, the substituted or unsubstituted polynuclear quinones which are compounds having two intracyclic carbonyl groups attached to intra-cyclic carbon atoms in a conjugated six-member carbocyclic ring, there being at least one aromatic carbocyclic ring fused to the ring containing the carbonyl groups.Suitable such initiators include 9,1 0-anthraquinone, 1 - chloroanthraquinone, 2-chloroanthraquinone, 2-methylanthraquinone, 2-tert-butylanthraquinone, octamethylanthraquinone, 1 ,4-naphthoquinone, 9,1 0-phenanthrenequinone, 1 ,2-benzanthraquinone, 2,3-benzanthraquinone, 2-methyl-1 ,4-naphthoquinone, 2,3-dichloro-naphthoquinone, 1 4- dimethylanthraquinone, 2,3-dimethylanthraquinone, 2-phenylanthraquinone, 2,3- diphenylanthraquinone, sodium salt of anthraquinone, alpha-sulfonic acid, 3-chloro-2 methylanthraquinone, retenequinone, 7,8,9,1 0-tetrahydronaphthacenequinone, and 1,2,3,4tetrahydrobenz (a) -anthracene -7,12-diene. Other photoinitiators which are also useful are described in U.S.Patent No. 2,760,863 and include vicinal ketaldonyl compounds, such as diacetyl, benzil, etc.; a ketaldonyl alcohols, such as benzoin, pivaloin etc., acyloin ethers, e.g. benzoin methyl and ethyl ethers, etc., a hydrocarbon substituted aromatic acyloins, including a-methylbenzoin, a-allylbenzoin and a- phenylbenzoin and 2,2 dialkoxy-2-phenylacetophenone.

The amount of elastomeric polymer blended with the CM may range from 5 to 50% of the total elastomer content, with 10 to 45% being the preferred range. The amount of photopolymerizable ethylenically unsaturated crosslinking agent may range from 5 to 50 parts per hundred of the total elastomer content, with 10 to 25 being preferred. Similarly, the range for the photosensitizer would be 0.1 to 10 parts per hundred of elastomer, with 1 to 4 being preferred.

The method of mixing the photosensitive compositions of the present invention is not critical.

Preferably, the photoinitiator is dissolved in the liquid monorrWer and the solution in turn is added to the chlorinated polyethylene elastomer by dry blending. The blend is then transferred to a two roll mill or an internal mixer and mixed for several minutes. Following mixing, the composition is then compression molded or extruded to the desired thickness in sheet form. The sheet is exposed through a photographic transparency with a UV source for a time sufficient to make the exposed areas insoluble in the developing solvent. The plate is then developed by brushing in the development fluid to remove the unexposed areas. The resulting relief plate is then dried in a forced air oven.

The supporting substrate for the photosensitive compound should be dimensionally stable andr solvent resistant. Commonly used supports are sheet metals, such as aluminium, and plastics, such as polyester and polyamide.

The following examples illustrate the invention in greater detail.

EXAMPLE 1 One and a half parts of benzophenone is dissolved in 20 parts of trimethylol propane triacrylate (Sartomer, SR-35 1) with heat. The solution is then hand-mixed with 100 parts of chlorinated polyethylene elastomer CM 0342. The dry blend is mixed on a two roll mill for 5 minutes at 230OF then compression molded onto a 5 mil thick polyester substrate to form a 6 x 6 x .062" plate. The plate is given 1 minute back exposure and 12 minutes face exposure through a lithographic negative using a "black-light" U.V. exposure unit. The unexposed areas arekemoved iri a rotary brushing unit with perchloroethylene solvent. The plate is dried in a forced air oven at 1 500F for 20 minutes. The developed plate had a good image, .032" relief, and a Shore A hardness of 75.

EXAMPLE 2 The same procedure was used as disclosed in Example 1, except that 100 parts of chlorinated polyethylene (CM 0136) is used as the binder. This plate was given T minute back exposure and 4 minutes face exposure and brushed in perchloroethylene solvent for 16 minutes to give 0.022" relief and a Shore A hardness of 80.

EXAMPLE 3 The same procedure was used as disclosed in Example 1, except 60 parts of chlorinated polyethylene (CM 0342) plus 40 parts of nitrile rubber containing 29.5% acrylonitrile and a specific gravity of 0.97 was used as a binder. This plate was given T minute back exposure and 6 minutes face exposure and developed for 20 minutes in perchloroethylene solvent to give a plate with a Shore A hardness of 69 and 0.022" relief.

EXAMPLE 4 About 1.6 grams of benzophenone is dissolved with 9.4 parts of tetraethylene glycol diacrylate and 9.4 parts of trimethylolpropane triacrylate. This is dry blended with 59 parts of CM 0342. The dryblend is then mill mixed at 2300F for 5 minutes with 41 parts of a nitrile rubber containing 29.5% acrylonitrile and a specific gravity of 0.97. The mix is compression molded onto a 0.005" thick polyester substrate to form a 6 x 6 x .062" plate. The plate was back exposed 8 minutes and then face exposed through a lithographic negative for 10 minutes. The plate was developed by brushing in perchloroethylene solvent for 20 minutes. The dried plate had good image quality, a Shore A hardness of 52, a relief depth of 0.028" and did not crack when flexed 1800.

EXAMPLE 5 The same formulation and procedure as in Example 4 was used except that 50 parts of CM 0136 replaced 50 parts of CM 0342. The resultant plate had good resolution, a Shore A hardness of 49, a relief depth of 0.048" and did not crack when flexed 1800.

EXAMPLE 6 Using the procedure in Example 4, 41 parts of a block copolymer of styrene and isoprene containing about 14% styrene was mill mixed with 59 parts CM 0136, 9.4 parts tetraethylene glycol diacrylate, 9.4 parts of trimethylolpropane triacrylate and 1.6 parts of benzophenone. The material was compression molded into a 6 x 6 x .062" plate. The plate was back exposed 8 minutes then face exposed 8 minutes through a lithographic negative. The exposed plate was developed in perchloroethylene solvent with rotary brushing. The dried plate has a Shore A hardness of 66 and a relief depth of 0.038".

EXAMPLE 7 The following materials are mill mixed and molded by the procedure in Example 4: 60 parts of CM 0136: 40 parts of nitrile elastomer containing 29.5% acrylonitrile and a specific gravity of 0.97; 22 parts of a chlorinated paraffin wax containing 70% chlorine by weight, a viscosity of 20,000 poises at 250C and a density of 1.55; 25 parts trimethylolpropane trimethacrylate and 2.2 parts benzophenone.

The resultant plate was given 14 minutes face exposure and 1 minute back exposure and developed in perchloroethylene solvent with rotary brushing for 14 minutes. The dried relief plate had excellent image quality, a Shore A hardness of 63,0.030" relief and did not crack on repeated flexing.

EXAMPLE 8 Using the milling and molding procedure given in Example 4, 50 parts of CM 0136 is mixed with 50 parts of chlorosulfonated polyethylene containing 35% chlorine and about 1% sulfur by weight, 10 parts 1,6-hexanediol diacrylate, 5.0 parts 1,6-hexanediol dimethacrylate, 0.2 parts ionol antioxidant and 2.0 parts Vicure 10 photoinitiator which is a mixture of benzoin alkyl ethers sold by Stauffer Chemical Company. The plate was face exposed 10 minutes, back exposed 1 minute and developed in perchloroethylene solvent for 10 minutes. The dried printing plate had 0.018" relief depth and good image quality.

EXAMPLE 9 This example shows the superior ink solvent resistance of flexible printing plates made from chlorinated polyethylene.

Percent Volume Swell After 4 Hrs. In Plate Composition Hexane Alcohol Ethyl Acetate Commercial "A" SIS Black Copolymer 230 -2 99 Commercial "B" Nitrile Rubber -2 25 157 Example 1 CM 0342 7 3 107 Example 2 CM 0136 8 -2 88 Example 3 CM 0342/Nitrile 5 4 127 This table shows that commercial plate A has poor hydrocarbon resistance, but fair ester resistance. Commercial plate B has good hydrocarbon resistance but marginal ester resistance.

Examples 1,2 and 3 have good resistance to all solvents.

Claims

1. An elastomeric article suitable for printing comprising (A) chlorinated polyethylene (B) between about 5 and 50 phr of a photopolymerized cross-linking agent derived from a photopolymerizable ethylenically unsaturated compound and (C) between about 0.1 and 5 phr of a photoinitiator activatable by actinic radiation.

2. An article according to claim 1, wherein the chlorinated polyethylene contains between about 36 and 42% of chlorine, said percentage being based upon the weight of said chlorinated polyethylene.

3. An article according to claim 1 or claim 2, wherein said chlorinated polyethylene has an ML-4 between about 70 and 110.

4. An article according to any of the preceding claims, wherein (A) is a blend of chlorinated polyethylene with 5~50% of an elastomer selected from butadiene-acrylonitrile, butadiene-styrene, styrene-butadiene styrene block copolymer, styrene-isoprene-styrene block copolymer, 1 ,4- polybutadiene, 1 ,2-polybutadiene, chlorosulfonated polyethylene and natural rubber.

5. An article according to claim 4, wherein (A) is a blend of chlorinated polyethylene and a butadiene-acrylonitrile copolymer containing 2~50% by weight of acrylonitrile.

6. An article according to claim 4, wherein (A) is a blend of chlorinated polyethylene and a butadiene-acrylonitrile copolymer containing 20~40% by weight of acrylonitrile.

7. An article according to claim 4, wherein (A) is a butadiene-styrene copolymer containing 21~45% by weight of copolymerized styrene.

8. An article according to claim 4, wherein (A) is a blend of chlorinated polyethylene and a styrenebutadiene-styrene block copolymer containing 10~40% by weight of copolymerized styrene.

9. An article according to claim 4, wherein (A) is a blend of chlorinated polyethylene and a styreneisoprene-styrene block copolymer containing 10~40% by weight of copolymerized styrene.

1 0. An article according to any of the preceding claims, wherein (B) is derived from an ester of acrylic acid or methacrylic acid.

11. An article according to any of claims 1 to 9, wherein (B) is derived from ethylene glycol diacrylate, ethylene glycol dimethacrylate, tetraethylene glycol diacrylate, tetraethylene glycol dimethacrylate, 1 ,4-butanediol diacrylate, 1 ,4-butanediol dimethacrylate, 1 ,6-hexanediol diacrylate, 1 ,6-hexanediol dimethacrylate, trimethylolpropane triacrylate or trimethylolpropane trimethacrylate, or a blend thereof.

1 2. An article according to any of the preceding claims, wherein (C) is benzophenone, a C1-C15 alkyl ether of benzoin, benzil, a C1-C15 alkyl monoketal of benzil, 2,2-dimethoxy-2phenylacetophenone.

13. An article according to any of claims 1 to 3, wherein (A) is chlorinated polyethylene, (B) is 10-20 phr trimethylolpropane triacrylate and (C) is 0.1-5 phr benzophenone.

14. An article according to any of claims 1 to 3, wherein (A) is chlorinated polyethylene, (B) is 10-20 phr tetraethylene glycol diacrylate or trimethylolpropane triacrylate or mixtures thereof, and (C) is 0.

1-5 phr benzophenone.

1 5. An article according to any of claims 1 to 3, wherein (A) is a blend of chlorinated polyethylene and chlorosulfonated polyethylene, (B) is 10-20 phr trimethylolpropane trimethacrylate and (C) is 0.1-5 phr benzophenone.

1 6. An article according to any of claims 1 to 3, wherein (A) is a blend of chlorinated polyethylene and a natural rubber, (B) is 10-20 phr trimethylolpropane trimethacrylate and (C) is 0.1-5 phr benzophenone.

1 7. An article according to any of claims 1 to 3, wherein (A) is a blend of chlorinated polyethylene and a block copolymer of styrene-isoprene-styrene, (B) is 10-20 phr tetraethylene glycol diacrylate or trimethylolpropane triacrylate or mixtures thereof and (C) is 0.1-5 phr benzophenone.

18. An article according to any of claims 1 to 3, wherein (A) is a blend of chlorinated polyethylene and a block copolymer of styrene-butadiene-styrene, (B) is 10-20 phr 1,6-hexanediol diacrylate and (C) is 0.1-5 phr 2,2-dimethoxy-2-phenylacetophenone.

19. An article according to any of claims 1 to 3, wherein (A) is a blend of chlorinated polyethylene and a butadiene-acrylonitrile rubber, (B) is 10~20 phr trimethylolpropane triacrylate or trimethacrylate and (C) is 0.175 phr benzophenone.

20. An article according to any of claims 1 to 3, wherein (A) is a blend of chlorinated polyethylene and polybutadiene rubber, (B) is 10-20 phr trimethylolpropane trimethacrylate and (C) is 0.1-5 phr benzophenone.

21; An article according to claim 1 and substantially as herein described.

22. An elastomeric article suitable for printing and substantially as described in any of the specific Examples.