Sherman & Shalloway
Support for liquid photosensitive resin
 A support for use in preparing a printing plate using a liquid photosensitive resin. The support includes a flexible self-supporting base plate and an adhesive layer thereon for applying a layer of a photosensitive resin. The adhesive layer is a layer of a cross-linked polyester-polyurethane resin formed by reacting (A) a linear polyester-polyurethane resin with (B) a polyfunctional isocyanate on the surface of the base plate. This invention relates to a flexible support for use in preparing a printing plate consisting of the support and a layer of a liquid photosensitive resin applied in situ to the support and solidified upon exposure, especially to a support including an adhesive layer for firmly bonding the solidified photosensitive resin layer to the support. A method has previously been known which comprises applying a layer of a solid or liquid photosensitive resin to the surface of an iron plate, aluminum plate or resin sheet through an adhesive layer and an antihalation layer, exposing the photosensitive resin layer to actinic light such as ultraviolet rays through an image-bearing film to solidify the image areas in relief, and washing out the uncured portion with a developer solution, thereby to prepare a relief printing plate having superior properties. Where the photopolymerizable photosensitive resin layer in the unexposed state is a solid which is completely devoid of flowability at room temperature, the manufacturers consume sufficient time to bond the unexposed photosensitive resin layer firmly to the base plate, and sell the resulting plate-making material as merchandise. Thus, it is not so difficult to obtain an adhesive layer which adheres sufficiently firmly to the base. However, when the photopolymerizable photosensitive resin layer is a liquid resin composition having flowability at room temperature, it is the practice to cast the resin liquid on a separately prepared base plate immediately before plate making, and then immediately exposing it to light to adhere reliefs of all shapes forming the image area firmly to the base plate. Thus, many problems to be solved exist as regards the adhesive layer when using liquid photosensitive resins. Above all, when a polyethylene terephthalate film having low adhesive strength is used, there is a problem of not only the adhesion between the photosensitive resin layer and the adhesive layer, but also the poor adhesion between the adhesive layer and the base plate. The adhesive layer must primarily satisfy all the following requirements. PA1 1. It should be moderately attacked at its surface by the liquid photosensitive resin in contact with it. PA1 2. It should have flexibility so that it can maintain adhesive strength sufficiently endurable to heavy mechanical stress during the handling of the printing plate. PA1 3. It should have sufficient resistance to chemicals such as the developer solution or ink washing liquids. PA1 4. It should be free from any unnecessary tackiness on the surface. In order to improve the chemical resistance of the adhesive layer, there has already been proposed a technique whereby a cross-linked partially hardened barrier layer is provided between the adhesive layer and the photosensitive resin layer (Japanese Pat. Nos. 23761/61 and 12104/65) the latter corresponding to U.S. Pat. No. 3,287,152 to Alles et al. This technique is intended to prevent the reduction of the adhesive strength of the adhesive layer which occurs as a result of the attacking of the contact surface between the photosensitive resin layer and image relief areas by a developing solution when washing out the uncured portion of the photosensitive resin. This method, however, is economically disadvantageous because control of the conditions for coating a barrier layer forming composition on the adhesive layer is difficult and the barrier layer must be partially hardened which is also a troublesome procedure. Furthermore, the polymerization reaction taking place during the formation of the barrier layer renders the thin base plate unsatisfactorily warped, and frequently causes marked disadvantages in the subsequent operations. Attempts have also been made to use various resins considered to be relatively well bonded to the surface of a base plate which permits difficult bonding, such as polyethylene terephthalate, for example, copolyester resins such as copolyesters derived from ethylene glycol, terephthalic acid and isophthalic acid, resorcinol-formaldehyde resins, and vinyl copolymers such as vinylidene chloride-type copolymers disclosed in U.S. Pat. No. 2,762,720 and Belgian Pat. No. 655,644. But none of these attempts have ever proved satisfactory, and suffer from one or more defects. For example, high temperatures and long periods of time for curing are required, or the adhesive layer has poor chemical resistance, or the adhesive layer has poor affinity for, and swellability with, the liquid photosensitive resin, or has poor flexibility. Because of these defects, the resulting adhesive layers are scarcely feasible. Accordingly, an object of this invention is to provide a support for a liquid photosensitive resin, which is free from the various defects of the prior techniques described above, is cheap, has superior properties, lends itself to easy handling, and can be used for making a printing plate in situ. Another object of this invention is to provide a support for a liquid photosensitive resin having an adhesive layer thereon for bonding a layer of the photosensitive resin firmly thereto. These objects of this invention can be achieved by a support for use in preparing a printing plate using a liquid photosensitive resin, comprising a flexible self-supporting base plate and an adhesive layer thereon for applying a layer of a photosensitive resin, said adhesive layer being a layer of a cross-linked polyester-polyurethane resin formed by reacting (A) a linear polyester-polyurethane resin with (B) a polyfunctional isocyanate on the surface of the above base plate. Preferably, the flexible base plate used in this invention is a base plate composed of a thin transparent substance. For example, a film of a polyester such as poly(ethylene 2,6-naphthalate) or polyethylene terephthalate, a polycarbonate film, a film of a cellulose, a film of a polyolefin such as polyethylene or polypropylene, a polyamide film, and a polyvinyl chloride film can all be used. Of these, the polyethylene terephthalate film which has dimensional stability and is tough against mechanical breaking power such as tear is especially preferred. In order to increase the adhesive strength between the base plate and the adhesive layer, the surface of the base plate can be subjected to various pre-treatments. For example, the surface may be sanded to form a mat surface. Or the surface can be activated with an oxidizing strong acid, or by corona discharge or ioninzing radiation. Furthermore, the surface may be coated with a thin layer of a titanic acid ester, or may be chemically roughened using a solvent. As the base plate, a film of the transparent resin as exemplified above is suitably used, but a non-transparent plate can also be used. For example, a thin sheet of a metal such as zinc, aluminum or steel, a rubber sheet, paper, or a non-woven fabric can be used. The adhesive layer is applied as a layer of a moderately cross-linked polyester-polyurethane resin. Specifically, a hydroxyl-terminated bifunctional polyester resin is reacted with a diisocyanate to form a high-molecular-weight linear polyester-polyurethane resin, and the polyester-polyurethane resin is reacted with a polyfunctional isocyanate on the surface of the base plate to cross-link it and simultaneously bond it firmly to the base plate. The linear polyester-polyurethane is obtained by reacting a linear polyester resin substantially having a hydroxyl group at both ends with a bifunctional isocyanate compound. Examples of preferred polyester resins are aliphatic linear polyesters, for example, polyesters derived from dibasic organic carboxylic acids as a dicarboxylic acid component, such as adipic acid, maleic acid or dimeric acid and glycols as a glycol component such as ethylene glycol, propylene glycol, trimethylene glycol, tetramethylene glycol, or diethylene glycol, and polycaprolactone. Linear polyesters containing an aromatic portion obtained by replacing a part of the dibasic organic carboxylic acid by phthalic acid can also be used. The polyester may include an amide group or ether group inert to the isocyanate group, in its chain. Preferably, the polyesters should have an average molecular weight of 500 to 6,000, especially 800 to 3,000. The polyester portion incorporated in the chain of the linear polyester-polyurethane chain forms a soft segment portion. The soft segment portion permits the moderate permeation of the liquid photosensitive resin, and plays the most important role in the present invention. If the molecular weight of the polyester is too low, there tends to be obtained a hard adhesive layer as a result of excessive intermolecular bonding. On the other hand, if it is too high, there tends to be obtained a tacky film having inferior durability. Examples of the bifunctional isocyanate compound are 2,4-tolylene diisocyanate (TDI for short), diphenylmethane diisocyanate (MDI for short), para-phenylene diisocyanate, 2-chloro-1,4-phenyl diisocyanate, 1,5-naphthalene diisocyanate, 3,3'-dimethyl-4,4'-diphenyl diisocyanate, 3,3'-dimethyl-4,4'-diphenylmethane diisocyanate, hexamethylene diisocyanate, and xylylene diisocyanate. Preferably, the linear polyester-polyurethane resin obtained by reacting the polyester with the bifunctional isocyanate usually has a number average molecular weight of 10,000 to 100,000, especially 20,000 to 50,000. The terminal groups of the linear polyester-polyurethane resin may be hydroxyl groups, isocyanate groups, or other organic groups inert to the isocyanate groups. Preferably, the polyester-polyurethane resin has a hydroxyl value of not more than 20, more preferably not more than 5. Polyester-polyurethane resins most suitably used are those having an average molecular weight of about 20,000 to 50,000 which are obtainable by chain-extending a polyester derived from a dicarboxylic acid component composed mainly of adipic acid and a glycol component composed mainly of 1,4-butanediol or ethylene glycol, with TDI or a mixture of TDI and MDI. Examples of suitable polyfunctional isocyanates that can be used in this invention are an adduct formed between trimethylol propane and TDI (for example, Desmodur L-75, the tradename for the product of Bayer AG), an adduct formed between trimethylol propane and xylylene diisocyanate, and condensates of diisocyanates such as Desmodur IL, Desmodure CT, a trimer of TDI, or Desmodur N. The bifunctional isocyanates useful for chain-extending the linear polyesters to form the polyester-polyurethane resins can also be used as the polyfunctional isocyanates. The amount of the polyfunctional isocyanate is usually 5 to 30% by weight, preferably 5 to 20% by weight, based on the total amounts of the linear polyester-polyurethane resin and the polyfunctional isocyanate, although the amount varies according to the structure of the polyfunctional isocyanate. According to this invention, the above-described linear polyester-polyurethane resin is reacted with the polyfunctional isocyanate on the surface of the base plate, thereby to form a cross-linked polyester-polyurethane resin layer on the surface. The above reaction can also be carried out in the presence of an additional component which is a non-gaseous compound having a photopolymerizable ethylenic double bond in the molecule. Examples of the compound having a photopolymerizable ethylenic double bond include acrylic or methacrylic acid esters such as 2-ethylhexyl acrylate, lauryl acrylate, cyclohexyl acrylate, 2-hydroxyethyl acrylate, 2-hydroxymethyl methacrylate, 2-hydroxypropyl methacrylate, diethylene glycol diacrylate, polyethylene glycol acrylate, diethylene glycol dimethacrylate, polyethylene glycol dimethacrylate, glycidyl methacrylate, tetrahydrofuryl methacrylate, glycerol monoacrylate, or 3-chloro-2-hydroxypropyl methacrylate; styrene and various styrene derivatives; vinyl esters such as vinyl acetate or vinyl butyrate; amide compounds such as acrylamide, methacrylamide, N-methylol acrylamide, diacetone acrylamide, or N,N'-dialkyl acrylamides; polyfunctional acrylic or methacrylic acid esters such as trimethylol propane triacrylate or methacrylate, trimethylol propane diacrylate or dimethacrylate, trimethylol propane triacrylate or trimethacrylate, pentaerythritol tetraacrylate or tetramethacrylate, or pentaerythritol triacrylate or trimethacrylate; allyl monomers such as diallyl phthalate (or its prepolymer), triallyl cyanurate or triallyl isocyanurate; and various high-molecular-weight photopolymerizable compounds having a plurality of vinyl groups at the ends of the molecule or at the side chains. Of these compounds, those having a boiling point of at least 100.degree. C. at atmospheric pressure are preferred, and those having at least 2 ethylenic carbon-to-carbon double bonds in the molecule are more preferred. Furthermore, the compounds having a photopolymerizable ethylenic double bond can more suitably be used when they contain active hydrogen capable of reacting with isocyanate groups. While we do not intend to limit the present invention by any theory, we assume that the compounds having active hydrogen capable of reacting with isocyanate groups especially give adhesive layers of improved bond strength because these compounds also react at the time of reacting the polyester-polyurethane resin with the polyfunctional isocyanate and are present in the adhesive layer in the chemically bound state. Of such compounds, polyfunctional vinyl compounds having a hydroxyl group in the molecule are especially preferred for their moderate reactivity. The amount of the compound having a photopolymerizable ethylenic double bond is usually 1 to 50% by weight, preferably 3 to 30% by weight, based on the polyester-polyurethane resin. Furthermore, a catalyst for promoting the cross-linking reaction, a stablizer, or a plasticizer such as benzylbutyl phthalate can also be added to the reaction components. The adhesive layer can be given an antihalation effect by adding a dye or pigment which absorbs actinic light. The reaction of the linear polyester-polyurethane and the polyfunctional isocyanate and if desired, the compound having a photopolymerizable ethylenic carbon-to-carbon double bond can be conveniently carried out by coating a solution of these components on the surface of the support to form the desired adhesive layer. The linear polyester-polyurethane resin is applied as a solution uniformly dissolved in a solvent inert to isocyanate groups, such as an acetic acid ester, toluene, benzene, xylene, dichloroethane, methyl ethyl ketone, or dimethyl formamide, and in this case, the polyfunctional isocyanate and possibly the compound having a photopolymerizable ethylenic double bond are simultaneously mixed, and applied. The adhesive layer coated on the base plate is usually dried by hot air to remove the solvent. The thickness of the layer is desirably at least one micron, especially suitably 5 to 20 microns. The hot air drying is performed immediately after coating, for 1 to 2 minutes at 100.degree. to 150.degree. C. The conditions are of course varied according to the solvent used. In any case, this heating step is to be understood as a step of merely removing the solvent. The reaction of isocyanate groups with active hydrogen atoms of the trunk polymer and possibly, the compound having active hydrogen in the molecule and a photopolymerizable ethylenic double bond, and the reaction of the isocyanate groups with the surface of the base plate proceed effectively and rapidly even at room temperature after drying, and upon complete disappearance of the isocyanate groups, a sufficiently cross-linked adhesive layer firmly adhered to the base plate is formed. Since the trunk polymer in the adhesive layer is polyurethane having a high degree of polymerization, no undesirable tackiness of the adhesive layer exists even when the crosslinking still does not proceed. Therefore, the adhesive layer does not suffer from a blocking phenomenon whereby the coated surface intimately adheres to other surfaces of the base sheet. As is clear from the above description, the method of forming the adhesive layer in accordance with this invention differs from the method of forming a coating of Desmophen-Desmodur (D--D lacquer) which has been widely accepted as a polyurethane lacquer. In the case of the D--D lacquer, a polyester resin itself containing a hydroxyl group is mixed with a polyfunctional isocyanate, and chain-extension is performed in situ and simultaneously, cross-linking is performed to a great extent. Thus, unless heat curing is performed for sufficiently long periods of time after coating, the adhesive strength between the coated film and the base plate is insufficient. Furthermore, the filmy base plate is frequently warped miserably as a result of shrinkage due to the polymerization of the adhesive layer, and because of the excessive cross-linkage, the adhesive strength between the adhesive layer and the photosensitive resin is low. It has also been found that when the polyester resin used is based on an aromatic dicarboxylic acid, the adhesive strength of the adhesive layer is extremely poor because of the crystallinity of the molecular chain and the high hardness of the coated film. The liquid photosensitive resin to be applied to the adhesive layer on the support of this invention to form a printing plate in situ is a mixture of a polymerization initiator capable of being easily activated by actinitic light and a composition consisting mainly of a compound having a polymerizable or cross-linking ethylenic carbon-to-carbon double bond. This photosensitive resin is generally applied to the adhesive layer as a uniform layer having a thickness of 0.2 to 10 mm. Examples of suitable liquid photosensitve resins are those consisting of polymers such as a vinyl chloride copolymer, an acrylic ester copolymer, a polyvinyl alcohol derivative, a cellulose derivative (e.g., cellulose ether), a polyamide, polyurethane, or a polyester, especially an unsaturated polyester having an unsaturated bond in the molecule, and a liquid photopolymerizable monomer consisting of acrylic acid, methacrylic acid, a derivative of any of these such as an ester or amide, styrene, a styrene derivative, or a vinyl ester, or a mixture of at least two of these. The support of this invention is especially suitable for application of a liquid photosensitive resin composed of a mixture of an unsaturated polyester having at least one carbon-to-carbon double bond inert to actinic light in the molecular chain, and a liquid or solid addition-polymerizable monomer of various kinds. The good affinity of such a photosensitive resin for the adhesive layer is combined with its moderate anchoring effect, and reliefs after exposure and development exhibit strong adhesion to the adhesive layer. Usually, the liquid photosensitive resin contains a photopolymerization initiator such as diacetyl, benzyl, benzoin or benzoin alkyl ethers, anthraquinone, Eosine, Thionine, or thiol. Sometimes, the liquid photosensitive resin is coated on the support after being given flowability using a solvent. However, if the solvent cannot be a constituent of image areas solidifed as a result of exposure of actinic light, the solvent cannot be used in a great quantity as a matter of course. Plate making is carried out by exposing the platemaking material imagewise to actinic light. Examples of the light sources for actinic light are a tungsten-filament lamp, a fluorescent lamp, a carbon arc lamp, a mercury lamp, or Xenon lamp. It is chosen according to the properties of the photosensitive resin. Usually, light sources emitting actinic lignt including ultraviolet rays are used. It is desirable that at the time of exposure, the plate-making material composed of the photosensitive resin layer, the adhesive layer and the support should be maintained at an elevated temperature of about 20.degree. C. to 70.degree. C. Immediately after exposure, the uncured portion is washed out with a developer solution. Under ordinary conditions, the adhesive layer formed in accordance with this invention is quite stable to the developer solution such as an alkaline aqueous solution. Accordingly, the reduction of the adhesive strength of the image areas after development is not observed at all. The photosensitive resin plate obtained by using the support of this invention can be produced at low cost and with good efficiency, and the printing plate produced from it exhibits sufficient resistance to external mechanical forces and can endure use over prolonged periods of time during printing or handling after printing because it includes the flexible base plate and the stretchable adhesive layer. The following Examples and Comparative Examples illustrate the present invention in greater detail. In these examples, all percents and parts are by weight. 
Teijin Limited
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Hikoichiro Yasuhisa
Yamada Kitanishi