Process for the preparation of printing plates from urethane polymers

Abstract: A urethane polymer having free carboxyl groups on the backbone of the polymer and terminal methacrylate groups is prepared by (1) reacting a mixture of at least two aliphatic diols and an aliphatic dihydroxy mono- or dicarboxylic acid with an excess of an aliphatic, cycloaliphatic or aromatic diisocyanate in the presence of a catalyst to produce an isocyanate-terminated urethane polymer and (2) reacting the terminal isocyanate groups of the product of step (1) with an aliphatic monohydroxy methacrylate. The polymer can be used in photopolymerizable compositions for making tack-free flexographic printing plates.

Description: This invention relates to a process for the preparation of printing plates from photocurable polyurethane compositions. The use of various photopolymer resin compositions to make printing plates is known. For example, U.S. Patent 4,358,354 describes a urethane photosensitive resinous composition comprising at least one urethane prepolymer having carboxyl groups and acrylic residues only at the terminals of the molecule, (b) an ethylenically unsaturated compound and (c) a photopolymerization initiator. The composition provides letterpress printing plates that are substantially free of surface tack and have good resistance to moisture and to abrasion and wear. However, there is still a need in the art for improved photopolymer resin compositions that eliminate surface tack and improve plate washout and suspension of the washed out polymer in the bath. JP-A-61-168610 discloses that urethane oligomers may be obtained by reacting a polyalcohol, e.g. polyethylene glycol, and 1-50% by weight, based on the total alcohol, of dimethylolpropionic acid, of the formula Me(HOCH 2 ) 2 CCO 2 H, with a polyisocyanate, e.g. tolylene diisocyanate, at an OH/NCO molar ratio of less than 1 to form an isocyanate-terminated oligomer. The latter is reacted with an OH-containing (meth)acrylate. The resultant urethane oligomer may be mixed with a reactive diluent, e.g. trimethylolpropane tri(meth)acrylate, and a photopolymerization initiator, e.g. methyl o-benzoylbenzoate, to form a photocurable resin composition useful for inks (or paints), adhesives and photoresist coatings. JP-A-63-54416 discloses a UV-curable urethane (meth)acrylate resin composition suitable for silk-screen printing inks, which resin composition is prepared by reacting a mixture of 5-70 mol% of a diol having a molecular weight of at least 1500, e.g. polytetramethylene glycol, or higher and 30-95 mol% of a diol having a molecular weight of 200 or less, e.g. polypropylene glycol, together with an organic diisocyanate and also a (meth)acrylate of specified general formula, e.g. 2-hydroxyethyl acrylate. The present invention provides a printing plate manufactured from a urethane polymer that has free carboxyl groups and terminal methacrylate groups and that is obtainable by a process that comprises (1) reacting a mixture of at least two aliphatic diols and an aliphatic dihydroxy mono- or dicarboxylic acid with an excess of an aliphatic, cyclo-aliphatic or aromatic diisocyanate in the presence of a catalyst to produce an isocyanate-terminated urethane polymer having free carboxyl groups on the polymer backbone, and (2) reacting the terminal isocyanate groups of the product of step (1) with an aliphatic monohydroxy methacrylate. The mixture of diols used in step (1) of the above process includes at least two aliphatic diols, preferably those having a weight average molecular weight of 1000 to 4000. Suitable aliphatic diols include polyethylene glycol; polypropylene glycol; a block copolymer made by reacting a propylene oxide polymer with ethylene oxide; polytetrahydrofuran diol; polypropylene adipate diol; neopentyl glycol adipate diol; 1,4-butanediol ethylene glycol adipate diol; 1,6-hexanediol phthalate adipate neopentyl glycol phthalate adipate diol and ethylene glycol adipate diol. An aliphatic dihydroxy mono- or dicarboxylic acid is also used in step (1). Suitable dihydroxy carboxylic acids include bis-hydroxymethylpropionic acid, 2,3-dihydroxybutanoic acid; 2,4-dihydroxy-3,3-dimethylbutanoic acid; 2,3-dihydroxyhexadecanoic acid; dihydroxybutenoic acid, tartaric acid and 2,3-dihydroxy-2-methylpropionic acid. Bis-hydroxymethylpropionic acid is preferred. The concentration of hydroxy-functional carboxylic acid can be varied from 0.2% to 30%. From 2% to 5% is preferred. In this specification, all percentages are by weight, based on the total weight of the components used to prepare the urethane polymer. The mixture of diols is reacted with an excess of an aliphatic, cycloaliphatic or aromatic diisocyanate. Suitable diisocyanates include, for example, 2,4- or 2,6-toluene diisocyanate; isophorone diisocyanate; 1,6-hexamethylene diisocyanate; 2,2,4-trimethylhexamethylene-1,6-diisocyanate; p,p'-methylene-bis-phenylisocyanate or mixtures thereof. The equivalent hydroxyl to isocyanate ratio for the reaction can vary from 1:1.1 to 1:2.0. A ratio of 1:1.4 is preferred. The reaction temperature is preferably from 40°C to 90°C, most preferably from 70°C to 80°C. The reaction is preferably carried out in the presence of 50 to 300 ppm of an organic tin compound or from 0.1 to 3% of an aliphatic amine compound as a catalyst. Suitable catalysts include, for example, dioctyltin bis(isooctylmercaptoacetate), dibutyltin dilaurate, dibutyltin diisooctylmaleate, triethylenediamine, triethanolamine and triethylamine. Dioctyltin bis(isooctylmercaptoacetate) is preferred. The urethane polymer that is formed is characterized by the presence of free carboxylic acid groups on the backbone of the polymer and isocyanate groups at the ends of the polymer chain. The presence of the free carboxyl groups in the final product improves plate washout and suspension of the washed out urethane polymer in the alkaline washout bath. In the second step of the reaction, the product from step (1) is reacted with an aliphatic monohydroxy methacrylate in an amount sufficient to react with the isocyanate groups at the ends of the polymer chain. From 0.2 to 10 equivalents of excess hydroxy-functional methacrylate are preferred. Suitable methacrylates include, for example, 2-hydroxypropyl methacrylate, butanediol monomethacrylate, polyethylene glycol (10-20 moles ethylene oxide) monomethacrylate and polypropylene glycol (5-20 moles propylene oxide) methacrylate. An organic tin compound or an aliphatic amine compound of the types and amounts specified above can be used as a catalyst for this reaction if desired. The resulting urethane polymer is a liquid, has methacrylate groups on the ends of the polymer chain, free carboxyl groups on the polymer backbone, and an acid number of 1 to 150. The urethane polymers can then be blended with polymerizable monomers, photoinitiators and stabilizers to produce a composition that is sensitive to actinic radiation and can be used to produce low tack or tack-free flexographic printing plates. To produce the plates the liquid composition is exposed to ultraviolet radiation through a negative, the unexposed portion is washed away with an aqueous alkaline detergent solution, and the plate is exposed to ultraviolet radiation in air. Production of 150 to 250 plates per hour can be achieved in this manner. Example 1 A urethane polymer is made from the following charge according to the process of this invention: Eq. Wt. Wt. % Equiv. Poly G 55-53 995 29.28 0.0294 Poly G 20-56 996 42.63 0.0428 Bis-hydroxymethyl-propionic acid 67 2.37 0.0353 Toluene diisocyanate 87.1 14.07 0.1615 2-Hydroxypropyl methacrylate 144 3.88 0.0269 99.99 The Poly G 55-53, Poly G 20-56, and bis-hydroxymethylpropionic acid are dried under a nitrogen purge for 12 hours at approximately 60°C. ("Poly G" may be a registered trade mark.) Poly G 20-56 is polypropylene glycol and Poly G 55-53 is a block copolymer made by reacting ethylene oxide with a propylene oxide polymer. Both have a weight average molecular weight of about 2000 and are available from Olin Chemical Co. Dioctyltin bis(isooctylmercaptoacetate) catalyst (0.015%) is added, followed by the toluene diisocyanate. The reaction mixture, still under nitrogen, is heated to 75° to 80°C. Samples are taken for isocyanate analysis. At 1.8% isocyanate, the 2-hydroxypropyl methacrylate containing 0.10% (by weight of the total composition) butylated hydroxytoluene as a stabilizer and 0.015% (by weight of the total composition) of the same tin catalyst are added under an air atmosphere. The final product has an acid number of 10.2. Example 2 A urethane polymer is prepared as follows. Poly G 55-53 (37.01 g), 53.88 g Poly G 20-56 and 2.00 g tartaric acid are dried under a nitrogen purge overnight at approximately 60°C. The mixture is cooled to approximately 45° to 50°C and 0.015 wt. % dioctyltin bis(isooctylmercaptoacetate) catalyst is added. Toluene diisocyanate (13.36 g) is added in 1/3 increments at 45° to 50°C. After 30 minutes the reaction mixture, still under nitrogen, is heated to 70°C. The OH/NCO equivalent ratio is 1:1.3. Samples are taken for isocyanate analysis. At 1.5 to 2.0% isocyanate, 7.65 g 2-hydroxypropyl methacrylate containing 0.015 wt. % (based on the total weight of the composition) of the same tin catalyst and 0.10 wt. % (based on the total weight of the composition) butylated hydroxytoluene as a stabilizer are added under an air atmosphere. The temperature is then raised to approximately 75°C until the % NCO is less than 0.1 to 0.3%. The final product has an acid number of 18. Example 3 A proprietary photopolymerizable resin formulation containing the urethane polymer prepared as described in Example 1 is processed through a MERIGRAPH® SR-Y plate maker (Hercules Incorporated) at 215 plates/hour. The plates have low tack and are processable through a semi-automatic trim and bender machine supplied by K&F Printing Systems International. About 35,000 impressions of a 64 page edition consisting of old newspaper headlines are printed successfully. No solids build-up or fill-in are observed. It will of course be understood that the present invention has been described above purely by way of example and that modifications of detail can be made within the scope of the invention.

Claims: A printing plate manufactured from a urethane polymer, characterized in that the urethane polymer has free carboxyl groups and terminal methacrylate groups and is obtainable by a process that comprises (1) reacting a mixture of at least two aliphatic diols and an aliphatic dihydroxy mono- or dicarboxylic acid with an excess of an aliphatic, cyclo-aliphatic or aromatic diisocyanate in the presence of a catalyst to produce an isocyanate-terminated urethane polymer having free carboxyl groups on the polymer backbone and (2) reacting the terminal isocyanate groups of the product of step (1) with an aliphatic monohydroxy methacrylate. The printing plate of claim 1, characterized in that the aliphatic diols are a mixture of polypropylene glycol and a block copolymer of polypropylene oxide and ethylene oxide. The printing plate of claim 1 or 2, characterized in that the dihydroxy monocarboxylic acid is bis-hydroxymethylpropionic acid. The printing plate of claim 1, 2 or 3, characterized in that the diisocyanate is toluene diisocyanate. The printing plate of any of the preceding claims, characterized in that the monohydroxy methacrylate is 2-hydroxypropyl methacrylate. The printing plate of any of the preceding claims, characterized in that the catalyst is an organic tin compound. The printing plate of claim 6, characterized in that the tin compound is dioctyltin bis(isooctylmercaptoacetate). The printing plate of claim 6 or 7, characterized in that the catalyst is employed in an amount of 50 to 300 ppm based on the total weight of the components used to prepare the urethane polymer. The printing plate of any of claims 1 to 5, characterized in that the catalyst is an aliphatic amine compound, The printing plate of claim 9, characterized in that the catalyst is employed in an amount of 0.1% to 3% by weight, based on the total charge of the components used to prepare the urethane polymer. The printing plate of any of the preceding claims, characterized in that the ratio of hydroxyl to isocyanate equivalents in step (1) is 1:1.1 to 1:2.0. The printing plate of claim 11, characterized in that the ratio of hydroxyl to isocyanate equivalents in step (1) is 1:1.4. The printing plate of any of claims 1 to 12, characterized in that said process further comprises (3) blending the urethane polymer with a polymerizable monomer, a photo-initiator and a stabilizer to produce a composition that is sensitive to actinic radiation. A process for producing a printing plate according to any of claims 1 to 13, which comprises exposing the urethane polymer or a composition comprising the urethane polymer to ultraviolet radiation through a negative, washing away the unexposed portion with an aqueous alkaline detergent solution, and exposing the plate to ultraviolet radiation in air.

Inventors: YOKOYAMA WAKO, YOKOYAMA, WAKO
Applicants: MACDERMID IMAGING TECHNOLOGY I, MACDERMID IMAGING TECHNOLOGY INC.
Application year: 1991
Main classification: C08G18
IPC codes: G03F7, C08F299, C08G18, B41N1