GB2179360A - Aqueous developable photopolymer compositions - Google Patents

Abstract

An aqueous developable photopolymerizable composition for use in the manufacture of flexographic printing plates which comprises an admixture of: (A) carboxylated poly(butadiene-acrylonitrile) elastomer; (B) chlorosulfonated polyethylene; (C) ethylenically unsaturated crosslinking agent; and (D) photopolymerization initiator activatable by actinic radiation. Generally, the (A)/(B) weight ratio is 75/25-40/60; the concentration of (C) is 2-40 parts per 100 parts of (A) plus (B); and the concentration of (D) is 0/01-10 parts per 100 parts of (A) plus (B); all by weight.

Description

SPECIFICATION Aqueous developable photopolymeric compositions containing chlorosulfonated polyethylene Background of the invention Flexographic printing plate compositions prepared from photopolymerizable compositions are well known.

However, the compositions heretofore used have suffered from many problems often difficu It to resolve. A major dilemma posed in the desire to have a printing plate which is readily developable after exposure (that is, a non-exposed composition which can be easily and rapidly separated from the exposed composition) and a residual polymer (that is, the polymer formed upon exposure) which meets the demanding physical and chemical requirements of a finished printing plate.

U.S.Patent No.4,265,986 (Allen et al) discloses a flexographic printing plate composition of chlorosulfonated polyethylene blended with natural and synthetic dienoid elastomers and chlorinated hydrocarbon elastomers and specifically exemplifies the use of a blend of chlorosulfonated polyethylene and noncarboxylated butadiene-acrylonitrile copolymer. This composition has a limited tendency to crack and good copy resolution. However, to develop such composition, organic solvents must be used with the attendant prob lems oftoxicity and flammability, environmental pollution, and high cost.

Because ofthe desirability of an aqueous-developable system, considerable research and development work has been expended in this area. In fact, many aqueous-developable systems have been reported, as shown in U.S.Patents 3,416,922; 4,023,973; 4,233,391; 4,239,849; 4,177,074; 4,272,608; and 4,273,857. Some of these systems have even reached commercial acceptability. Unfortunately, however, such systems do not produce flexographic printing plates of the desired quality, especially ones which can be readilyfabricated economically.

For example, in U.S.Patents '074 and '608, high molecularweight butadiene-acrylonitrile copolymers have been blended with low molecular weight butadiene-acrylonitrile softeners wherein the total blend must have at least 2% by weight carboxyl groups. Unfortunately, these compositions cure relatively slowly.

A major problem in this area has been to find compositions of compatible polymers which obviate or at least minimize the aforedescribed deficiencies.

Summary of the invention In accordance with the invention, it has now been discovered that an aqueous developable photopolymer comprising a blend of chlorosulfonated polyethylene and a carboxylated poly(butadiene-acrylonitrile) rubber compatible therewith can be prepared which has an improved cure rate, and excellent relief and detail characteristics after cure. In addition, the printing plates made with the composition of the invention have good resistance to water- and oil-based inks alike.

The elastomeric photosensitive composition comprises: (A) carboxylated poly(butadiene-acrylon itrile) rubber; (B) chlorosulfonated polyethylene; (C) ethylenically unsaturated crosslinking agent; and (D) photopolymerization initiator activatable by actinic radiation; wherein the (A)/(B) ratio is 75/25 - 40/60, the concentration of(C) is 2 -40 parts per 100 parts of (A) plus (B), and the concentration of (D) is 0.01 - 10 parts per 100 parts of (A) plus (B), all byweight.

The compositions of this invention have utility especially in raised image printing processes such as letterpress, letterset and flexographic printing; however, the compositions may also be used in other applications where image formation is utilized such as photoresists, planographic plates, "silk screen" printing and stencils.

Detailed description of the invention The photopolymerizable compositions used to make the flexographic printing plates of the invention are composedofthefollowingfouressentialcomponents: (A) carboxylated poly(butadiene-acrylonitrile elastomer; (B) chlorsulfonated polyethylene; (C) ethylenically unsaturated crosslinking agent; and (D) photopolymerization initiator activatable by actinic radiation.

Generally, the (A)/(B) weight ratio is 75/25 - 40/60, preferably60/40 - 45/55, and most preferably 55/45 - 45/55.

The concentration of (C) may be 2 - 40 parts, preferably 5 - 25 parts, usually 7.5 - 15 parts per 100 parts of (A) plus (B); andtheconcentration of (D) generally is 0.01 - 10 parts, preferably 0.1 -8 parts, most preferably 0.2 -5 parts per 100 parts of (A) plus (B); all byweight.

Suitable carboxylated elastomers usually have a butadiene-acrylonitrileweight ratio of 95/5 - 50/50, prefer ably 90/10 - 60/40, most preferably 75/25 - 65/35, and a carboxyl content of 1 - 15%, usually 2 - 10% by weight.

The number average molecular weight of such carboxylated nitrile rubber may rangefrom 20,000to 120,000 preferably from 25,000 to 75,000 and usually from 30,000 to 50,000.

The chlorosulfonated polyethylene elastomers (CSM) employed for the purpose ofthis invention generally have a chlorine content of 15-50%, usually 20-45% by weight, with the sulfur content being about 0.5-2% usually 1 - 1.5% by weight. The Mooney viscosity (ML-4 at 100 C) may vary from 20 to 100, preferably from 25to 95.

That the aforesaid photosensitive composition can be of use to prepareflexographic printing plate compositions having the long sought balance of properties is particularly surprising. One skilled in the artwould not normally expect that a composition containing 50% of chlorosulfonated polyethylene (an aqueous-insoluble, high molecularweightelastomer) could be utilized in an aqueous or semi-aqueous developable system.

Similarly, priorflexographic printing plates based on chlorosulfonated polyethylene polymer matrices which have been described in the aforementioned U.S.Patent 4,265,986, are not aqueous-developable.

Ethylenically unsaturated crosslinking agents to be employed in the compositions of this invention include unsaturated esters of polyols, particularly such esters with alpha-methylene carboxylic acids, e.g., ethylene glycol diacrylate, diethylene glycol diacrylate, glycerol diacrylate, glycerol triacrylate, mannitol polyacrylate, sorbitol poiyacrylate, ethylene glycol dimethacrylate, 1 ,3-propanediol dimethacrylate, 1 ,3,4-butanetriol trimethacrylate, 1 ,4-butanediol diacrylate or dimethacrylate, 1 4-cyclohexanediol diacrylate, 1 ,4-benzenediol dimethacrylate, pentaerythritol di-,tri- and tetramethacrylate, pentaerythritol di-, tri- and tetracrylate, 1,3propanediol diacrylate,1,6-hexanediol diacrylate or dimethacrylate, 1 5-pentanediol dimethacrylate, trimethylolpropane triacrylate, bisacryiates and methacrylates of polyethylene glycols of molecularweight 200to4000, unsaturated amides of alpha-methylene carboxylic acids, such as of alpha, omega-diamines and oxygen-interrupted omega-diamines, such as methylenebis(acrylamide), methylenebis(methacrylamide), ethylenebis(methacrylamide), 1 ,6-hexamethylenebis(acrylamide), diethylenetriamine,trimethacrylamide, 1 ,2-di(gamma-methacrylamidopropoxy)ethane, beta-methacrylaminoethyl methacrylate, N-(betahydroxyethyl)-2-(methacrylamido)-ethyl acrylate and N,N-bis(beta-methacryloxyethyl)acrylamide; vinyl esters such as divinyl succinate, divinyl adipate, divinyl phthalate, divinyl terephthalate and unsaturated aldehydes, such as sorbaldehyde (hexadienai). Preferred additional polymerizable crosslinking agents are the esters and amides of alpha-methylene carboxylic acids and substituted carboxylic acids with polyols and polyamineswherein the molecular chain between the hydroxyls and amino groups is solely carbon or oxygen-interrupted carbon. The most preferred unsaturated crosslinking agents are 1,4-hexanediol diacry late, 1 ,4-hexanediol dimethylacrylate, trimethyloipropane triacrylate and pentaerythritol triacrylate.

The crosslinking initiators useful for this invention are activatable by actinic radiation and are essentially thermally inactive at or below about 185 C. Such initiators include aromatic ketones such as benzophenone and substituted benzophenones, substituted or unsubstituted polynuclear quinones, e.g., 9,10- anthraquinone, 1 -chloroanthraquinone, 2-chloroanthraquinone, 2-methylanthraquinone, 24ert- butylanthraquinone, octamethylanthraquinone, 1 ,4-naphthoquinone, 9,1 0-phenanthrenequinone, 1,2 benzanthraquinone, 2,3-benzanthraquinone, 2-methyl-1,4-naphthoquinone, 2,3-dichloronaphthoquinone, 1 ,4-dimethylanthraquinone, 2,3-dimethylanthraquinone, 2-phenylanthraquinone, 2,3diphenylanthraquinone, sodium salt of anthraquinone alpha-sulfonic acid, 3-chloro-2-methylanthraquinone, 7,8,9,1 O4etrahydronaphthalenequinone, and 1 ,2,3,4-tetrahydrobenz-alpha-anthracene-7,1 2-dione. Other useful photoinitiators are vicinal ketaldonyl compounds such as diactyl benzil, etc., alpha-hydrocarbon substituted aromatic acyloins, including alpha-methylbenzoin, alpha-allylbenzoin and alpha-phenylbenzoin and 2,2-dialkoxy-2-phenylacetophenone.

The photosensitive compositions are prepared by thoroughly mixing the components, i.e., (A) carboxylated butadiene-acrylonitrile copolymer, (B) chlorosulfonated polyethylene, (C) ethylenically unsaturated crosslinking agent, and (D) photopolymerization initiator, either by solvent blending in suitable solvents such as chlorinated hydrocarbons, e.g., trichloroethylene, trichloroethane and chlorotoluene; ketones, e.g., methyl ethyl ketone, diethyl ketone, and methyl isobutyl ketone; or aromatic hydrocarbons, e.g., toluene, xylene or by open milling or in an internal mixerfor a time long enough to blend the ingredients essentially homogeneously.

The non-light-scattering compositions are formed into sheets in any desired thickness from 0.0005 to .250 inch (0.0013 - 0.64 cm). Such forming methods are, for example, solvent casting, hot pressing, calendering or extrusion at room temperature or at elevated temperature. The sheet may be supported during forming by a suitable adhesive coated supporting substrate or the sheet may be applied to a supporting substrate after forming. The supporting substrate can be a translucent natural or synthetic material as a flexible or rigid film or sheet. Commonly used supports for this composition in flexographic printing include metal sheets such as steel ortin coated steel and plastic films made, for example, from polyester or polyamide.The preferred supporting substrates are dimensionally stable and resistant to the washout solutions.

As these are photo-activated systems, it is advisable to protectthe above mentioned sheets from surface contamination bydirtand dust during storage before being exposed and washed into afinished relief-imaged element. This is accomplished by application of a flexible protective cover sheet to the surface of the element opposite that of the substrate. Since these elements are usually tacky, it is also desirable to apply a releasefilm to the surface of the element before application ofthe protective cover sheet. This release film may be about an 0.0002 inch (0.0051 mm) thickflexible polymeric film such as made from polyamide polymer orvinyl acetate-type copolymer. Similarly, release agents known to the art may also be used forthis purpose, e.g., silicones, fluorocarbons and waxes as long as they are transparent and do not interfere with exposure to actinic radiation. These release agents orfilms promote good intimate contact between the surface ofthe element opposite to the substrate and an image-bearing negative or transparency applied to the element.

Intimate contact is essential for accurate reproduction ofthe image on the element.

Relief printing plates are produced from the composition ofthis invention from the above described sheets.

The construction usually comprises a polyester substrate from .001 to .020 inch (0.025-0.51 mm) and prefer- ably 0.004 to .006 inch (0.010-0.015cm), adhered to a sheet of the composition from .01 to .250 inch (0.025-0.64 cm) thick coated with a ca. 0.0002 inch (0.005 m thick polyamide release film on the side opposite of the substrate. This polyamide film is covered and adhered to a second polyester film (ca. 0.005 inch; 0.013 cm) which is a protective cover sheet. This construction is then exposed to actinic radiation through the substrate, if necessary, depending on thickness, in order to crosslinkvia additional polymerization the non-relief backing area of the plate.This exposure will control the depth of relief produced in the plate (thin plates do not require this step).The plate is then turned over, and the protective cover sheet removed.

Photographic negatives ortransparencies or other image-bearing media, with transparent and opaque areas used to produce the image, are laid in intimate contact with the polyamide release coated surface of the sheet of the composition of the invention. Good contact between the negative and release surface can be provided by a vacuum system that presses the negative against the sheet surface. This structure is then exposed to actinic radiation, e.g., ultraviolet radiation at a wavelength between 2500A and soooA, with preferred being 3600 . On exposure, the transparent areas of the negative permit addition polymerization or crosslinking to take place.Crosslinking will occur only in the exposed areas of the sheet (transparent areas of the negative) with no significant crosslinking occurring in the non-image unexposed areas ofthe sheet (opaque areas ofthe negative).

Actinic radiation may be provided from any source, such as carbon arcs or high pressure mercury lamps.

Preferred sources are so-called blacklight fluorescent types. Exposure times will vary from a few seconds to several minutes, depending on the output ofthe lamps, distance from the lamp, the relief depth desired and the thickness ofthe plate.

On completion of the exposure, the negative is removed, and the exposed sheet is ready for development of the relief image by washing with aqueousorsemi-aqueous basic solution or solvent wash. These wash systems remove the unexposed, uncrosslinked composition from the exposed sheet and do not adversely affect the exposed, crosslinked composition that forms the raised relief image. The sem i-aq ueous basic wash method is preferred.

Solvent wash may be accomplished as in the prior art with organic solvents such as 2-butanone, benzene, toluene, xylene, trichioroethylene, tetrachloroethylene, methylchloroform, or solvent mixtures such astet- rachloroethylenewith n-butanol.

Suitable semi-aqueous washout mixtures include sodium hydroxide/isopropyl alcohol/water, sodium carbonate/2-butoxyethanol/water, sodium borate/2-butoxyethanol/water, sodium silicate/2-butoxyethanol/ glycerol/water, sodium carbonate/2-(2-butoxyethoxy)ethanol/water, sodium hydroxide/2-(2 butoxyethoxy)ethanol/water and sodium hydroxide/i ,2-ethanediol/water. Normal use ofthese mixtures is at elevated temperature.

The following examples illustrate the invention.

Example 1 Photosensitive elastomeric printing plates were prepared by mixing all ingredients listed below on a two-roll mill (preheated to about 60 C) until a seemingly homogeneous composition was obtained. Subsequently, a laminate printing plate structure was made by placing into a platen press a translucent polyester support sheet (0.005 inch); followed by the photosensitive compositions and then a release-coated polyester cover sheet (0.005 in.). The thickness of the composition layer was controlled by a template, and the plate was formed by exerting 1,000 psi pressure at 1 200C for 5 minutes.After cooling to room temperature, the laminate was removed from the press and each plate was exposed through the support sheet for a certain period oftime using Greig Viking [trademark] #3022 lamps. The distance from lamp to composition was 2 inches. Thereafter, the cover sheet was removed, and on the front side was placed a negative which was held in close contactwith the plate by vacuum. The plate was then exposedthroughthe negativefora certain duration using above conditions.

After exposure, the plates were thoroughly brushed in the presence of a mixture of 1800 ml water, 210 ml butyl Carbitol [trademark] and 16 g sodium carbonate. After brush-out, the plates were rinsed with water and dried for about 15 minutes at 65"C. Relief depth was then determined. The plates were then post-exposed on the image side for5 minutes, and hardness and quality of image reproduction were evaluated.

Recipes (parts) Run No. 1 2 3 4 XNBR-1 50 50 50 XNBR-2 - - - 50 CSM-1 50 - CSM-2 - 50 50 50 TMPTA 10 - - HDDA - 9.0 6.0 6.0 HDDM - 6.0 4.0 4.0 DMPA 1.5 1.5 1.5 1.5 Stabilizer-1 1.5 1.5 1.5 1.5 Stabilizer-2 0.75 0.75 0.75 0.75 Processing aid - 10 10 10 Ingredients used XNBR-1: Carboxylated NBR; ACN content = 32% carboxycontent = 4.5%; ML-4 at 100 C = 55 XNBR-2: Carboxylated NBR; ACN contentca. 27% carboy content = 3.4%; M L-4 at 1 000C = 32 CSM-1: Chlorosulfonated polyethylene; Cl = 29%, S = 1.4%; ML-4 at 100 C = 28 CSM-2:Chiorosulfonated polyethylene; Cl = 35%,S = 1.09% ML-4 at 1 000C = 56 TMPTA: Trimethylolpropane triacrylate; crosslinking agent HDDA: Hexanediol diacrylate; crosslinking agent HDDM: Hexanediol dimethacrylate; crosslinking agent DMPA: 2,2-dimethoxy-2-phenylacetophenone; initiator Stabilizer-1: Nonylatedtriphenylphosphite Stabilizer-2: 2,6-di-t-butyl-p-cresol Processing aid: Depolymerized polyisoprene; Viscosity = 40,000 cps at 100 F.

Results Run No. 1 2 3 4 Platethickness, inch 0.067 0.071 0.071 0.069 Exposure; back, sec 5 20 20 45 front,min. 8 15 15 15 Brush-out at 55 C,min. 30 30 30 30 Relief, mil 20 30 25 32 Hardness,ShoreA 55 55 49 51 Reliefquality good excellent excellent very good The data clearly indicate that aqueous processable flexographic printing plates can be prepared from the compositions ofthis invention having good hardness and excellent detail quality.

Example 2 Following essentially the procedure of Example 1, additional compositions were prepared. Run Nos. Sand 6 showing the use of each ofthe polymers alone. Run No.7 using the blend, illustrates the composition ofthe invention.

Recipes Run No. 5 6 7 XNBR-1 100 - 50 CMS-2 - 100 50 HDDA 6 6 6 HDDM 4 4 4 DMPA 1.5 1.5 1.5 Stabilizer-1 1.5 1.5 1.5 Stabilizer-2 0.75 0.75 0.75 Processing aid 10 10 10 Results Run No. 5 6 7 Platethickness,inch 0.071 0.067 0.070 Exposure: back, sec. 45 5 45 front, min 15 10 15 Brush-outat55"C,Min. 25 30 30 Relief, mil 33 0 31 Relief, quality: 0.005 very - slightly inch lines wavy wavy The results show that Run No.7, the composition of this invention, is superiorto Run No.5, the composition using solely the carboxylated NBR as the elastomer base, since it has a faster cure rate as exemplified by the significantly better line definition. Run No.6, using only chlorosulfonated polyethylene as base polymer, does not provide any relief after using the development system of Example 1.

Claims (12)

1. An elastomeric photosensitive composition comprising: (A) carboxylated poly(butadiene-acrylonitrile) rubber; (B) chlorosulfonated polyethylene; (C) ethylenically unsaturated crosslinking agent; and (D) photopolymerization initiator activatable by actinic radiation; wherein the (A)/(B) ratio is from 75/25 to 40/60, the concentration of (C) is 2 - 40 parts per 100 parts of (A) plus (B), and the concentration of (D) is 0.01 - 10 parts per 100 parts of (A) plus (B), all byweight.

2. A composition according to claim 1 wherein said (A) has a carboxyl content of 1-15 percent byweight, and said (B) has a chlorine content of 15-50 percent by weight and a sulfur content of 0.5-2 percent by weight.

3. A composition according to claim 1 or claim 2 wherein said (A)/(8) ratio is 60/40 - 45/55 byweight.

4. A composition according to any of the preceding claims wherein said (B) has a Mooneyviscosity(ML-4 at 100 C) of from 20 to 100.

5. An elastomeric photosensitive composition comprising: (A) carboxylated poly(butadiene-acrylonitrile rubber; (B) chlorosulfonated polyethylene; (C) ethylenically unsaturated crosslinking agent; and (D) photopolymerization initiator activatable by actinic radiation; wherein the (A)/(B) ratio is from 55/45 to 45/55, the concentration of (C) is from 7.5 to 15 parts per 100 parts of (A) plus (B), and the concentration of (D) is from 0.2 to 5 parts per 100 parts of (A) plus (B), all byweight.

6. A photosensitive flexographic plate which comprises a compositions as claimed in any ofthe preceding claims.

7. Aflexographic plate prepared by selectively exposing the plate of claim 6 and thereafter developing the exposed plate.

8. A process for preparing a flexographic plate which comprises forming a sheet of the photosensitive composition of any of claims 1 to 5, selectively exposing said composition to actinic radiation thereby crosslinking the exposed portion of said composition, and thereafter developing said composition so asto remove the unexposed portion of said composition.

9. A composition according to claim 1 and substantially as herein described.

10. An elastomeric photosensitive composition substantially as described in the specific Examples.

11. A process according to claim 8 and substantially as described herein.

12. A process for preparing a flexographic plate substantially as described in the specific Examples.