GB2160882A - Flexographic printing methods and materials - Google Patents

Abstract

One edge of a solid flexographic printing plate 1 is butt joined to an adjacent edge of a solid flexographic printing plate by injecting liquid photopolymerisable resin 4 between the edges and then photocuring the resin. The or each plate initially carries a flexographic image in relief before the process or such an image is formed after injecting the resin. The photopolymerisable resin used in the invention comprises a diene type prepolymer having ethylenically insulated groups, together with polymerisable ethylenically unsaturated monomer and photo initiator. <IMAGE>

Description

SPECIFICATION Flexographic printing methods and materials One standard method of printing sheet material (that is material consisting of individual panels, or a single web) is by flexography, that is to say by contact with a flexographic printing plate that comprises a flexographic relief surface bonded to a substrate and that is carried by a printing cylinder.

Ink on the raised portion of the relief surface is transferred image-wise to the sheet material. The relief must be formed of flexible material. A flexographic printing plates can be made by engraving a sheet of flexible material or, more usually by image-wise photoexposure of material in contact with the substrate followed by removal of the non-image areas of material. The photopolymer may be a liquid which is cured by photoexposure or a solid which is rendered differentially soluble, e.g. in alkali or organic solvent, by photoexposure.

If the length of the pattern that is to be printed, and thus the length of the plate and the circumference of the printing cylinder, is relatively short, for instance uo to 30 cm, standard practice is to secure a flexographic printing plate to the printing cylinder by some adhesive system, for instance doublesided tape, and to butt-join the adjacent sides of the plate using solvent-based liquid adhesive, for example of the type sold under the trade name Superglue.

Such butt-joining systems suffer the disadvantage that solvents in the printing inks and in the solutions used to wash-down the printing cylinders may attack the joint so that the edges of the plates become unstuck from each other. The solvent can then attack the adhesive so that the plates may then be released from the cylinder. This is obviously undesirable. This potential problem of adhesion of the edges of the plates is especailly severe using plates formed solid photopolymer, which tend to shrink during exposure and/or subsequent treatment and to curl up at the edges. inks used to print paper bags and foil films are usually based on solvents that are particularly corrosive to the adhesive butt joints.

It is known to form printing plates that have a substantially continuous cylindrical surface and that can then be subjected to image-wise photoexposure to form cylindrical flexographic printing plates. For instance in GB 2,043,274 liquid photohardenable resin is cast between a cylinder and a transparent cylindrical rigid plate. Earlier methods of achieving a cylindrical photosensitive plate are described in Japanese Laid-open Patent Applications Nos.143404/78 and 29804/78. In 29804/78 a solid photosensitive printing plate is wrapped around a cylindrical substrate and liquid phtosensitive resin is flowed into the gap between the abutting edges of the plate. The amount of liquid resin that is applied is such that the height of the resin in the gap is at or above the level of the top surface of the printing plate.The liquid resin in the gap is eventually converted to solid by irradiation.

The cylindrical printing surface is, after application of the liquid resin in the gap, subjected to imagewise photoexposure. Thus it is intended that the photopolymer applied in the butt joint will become part of the image print surface.

We are concerned primarily with systems in which the plate has been exposed imagewise before butt jointing and with systems in which the butt joint is preferably in the non-image, or recessed, areas although it is also possible for the butt joint to be in image areas if the overall quality of the joint or the print makes this permissible. The disclosure in Japanese 29804/78 is therefore not related to the problem that the present invention seeks to solve.

We have noted that there is no suggestion in JP 29804/78 of the use of any particular liquid photosensitive resin for the butt jointing nor of any particular properties of the resins. In investigations subsequent to the making of this invention we have found that attempts at using conventional liquid photopolymers fur butt jointing in the manner described in JP 29804/78 is unsatisfactory during the printing uses with which we are concerned since during prolonged printing use the bond between the abutting edges breaks and the plate is liable to peel from the substrate.Thus even if the method of JP 29804/78 is used as a replacement for conventional butt jointing systems as described above, rather than as a way of making a cylindrical, imageable, print surface, the method has no advantage over the conventional and well-established methods using a solvent-based liquid adhesive for butt jointing in combination with doublesided tape. Both methods suffer equally from the fact that during conventional printing use there is a severe tendency for the butt joint to degrade and for the plate to lift off the cylindrical substrate.

In the invention one edge of a solid flexographic printing plate is butt jointed to an adjacent edge of a solid flexographic printing plate by injecting liquid photopolymerisable resin between the edges and then curing the resin by photoexposure and the solid flexographic printing plate or plates initially carries a flexographic image in relief or a flexographic image in relief is formed in the plate or plates after injecting the liquid resin composition, and the liquid composition comprises 100 parts by weight of a diene type prepolymer having ethylenically unsaturated groups, 5 to 100 parts by weight of a polymerisable ethylenically unsaturated monomeric compound and 0.001 to 10 parts by weight of a photopolymerisation initiator.

We have surprisingly found that this particular type of resin composition can be used successfully to form a butt joint which will resist degradation during use much more effectively than butt joints formed using either solvent-based liquid adhesives as discussed above or using the great majority of liquid photopolymerisable resins. It seems that the resins described for use in the invention have particular curing properties that result in the butt joint being very long lasting despite onerous use, for instance when being used for printing an aqueousbased or organic solvent-based ink. Thus the cured resin is resistnt to water-based inks and organic solvent-based inks.

The diene type prepolymer having ethylenically polymerisable unsaturated groups is a polymer prepared from a diene into which ethylenically polymerisable unsaturated groups are introduced, and has a molecular weight within the range of from 1,000 to 30,000, more preferably from 2,000 to 20,000. If the molecular weight is lower than 1,000, the photocured product may become too hard or lowered in tensile strength to give undesirably no sufficient bonding strength. On the contrary, a molecular weight over 30,000 will result in too high viscosity of the composition, whereby filling workability of the liquid photosensitive resin becomes undesirably very bad during joining of plates.

The above polymer prepared from diene as the starting material is a liquid polymer containing either of carboxyl groups, hydroxyl groups, mercapto groups, amino groups or epoxy groups as the terminal functional groups, which polymer is based on, for example, 1,4-polybutadiene, 1,2-polybutadiene, hydrogenated 1,2-polybutadiene, styrene-butadiene copolymer, acrylonitrile-butadiene copolymer, (meth)acrylic ester-butadiene copolymer, polyisoprene, 1,3- polypentadiene, etc., as such or a chain-extended polymer formed though urethane bonds, ester bonds or amide bonds, and (meth)acrylic acid residue, fumaric acid residue, itaconic acid residue, allyl group, etc., may be introduced into this polymer through utilisation of the above functional groups in a conventional manner to give the aforesaid prepolymer.Otherwise, it is also possible to use a polyene itself as the prepolymer of the present invention and, in this case, it is preferred to use it in combination with a polythiol.

Among the prepolymers as mentioned above, those excellent in solvent resistance and tear strength may include the following prepolymers A prepolymer having terminal ethylenically unsaturated groups and a number average molcecular weight of about 6,000 to 30,000 whose main segment is (i) a residue of a hydrogenated 1,2-polybutadiene having an average of about 1.2 to 2.0 terminal hydroxy groups per molecule and a degree of hydrogenation of pendant vinyl groups of the 1,2-addition units of about 50 to 100 percent; (ii) a residue of a chain-extended hydrogenated 1,2- polybutadiene having 2 to 20 urethane bonds obtained by reaction between the hydrogenated 1,2-polybutadiene (i) and a diisocyanate; (iii) a residue of a hydrogenated 1,2-polybutadiene having terminal carboxyl groups obtained by reaction between the hydrogenated 1,2-polybutadiene (i) and a dicarboxylic acid; or (iv) a residue of a chain-extended hydrogenated 1,2-polybutadiene having terminal carboxyl groups obtained by reaction between a chian-extended hydrogenated 1,2-polybutadiene having terminal hydroxy groups and 2 to 20 urethane bonds prepared by reaction between the hydrogenated 1,2-polybutadiene (i) and a diisocyanate, and a dicarboxylic acid.

Examples of suitable polymerizable, ethylenically unsaturated monomeric compounds include: (1) Compounds of formula1 wherein: Bi represents hydrogen or methyl R5 represents an alkyl group having 1 to 20 carbon atoms, cyclohexyl, an alkoxyalkyl group having at most 15 carbon atoms, a cyanoalkyl group having at most 8 carbon atoms, a tertiary amino alkyl group having at most 18 carbon atoms, a hydrogen atom, an oxyalkylene group of formula 2 or 3 wherein: R3 represents a hydrogen atom, methyl, monochloromethyl or monobromemethyl, p represents an integer of 1 to 20, the -(CH2)q-OH group, wherein q is an integer of 5 to 20, the -CH2C(CH3)2CH2OH group or the -CH2C(CH2Br)2CH2OH group; (2) Compounds of formula 4 wherein:: R' represents hydrogen or methyl, R6 and R7 each represent a hydrogen atom, an alkyl group having 1 to 12 carbon atoms, cyclohexyl or benzyl, or R6 is hydrogen and R7 is -RS-OH, where Rs is an alkylene group having 1 to 12 carbon atoms; (3) Compounds of formula wherein: Ri represents a hydrogen or methyl g is an integer of 2 or 4, R9 represents a radical of a polyol having g terminal hydroxy groups and a number average molecular weight of at most 1,000; (4) Compounds of formula 6 wherein: R1 and Ril each represent hydrogen or methyl, and Rio represents an alkylene group having 1 to 6 carbon atoms; (5) Compounds of formula 7 wherein:: R1 represents hydrogen or methyl, Rio represents hydrogen, methyl, monochloromethyl or monobromomethyl; (6) Aromatic compounds having at least one CH2=C group and one benzene nucleus; (7) Heterocyclic compounds having one ethylenically unsaturated group; and (8) Other ethylenically unsaturated monomeric compounds.

Exemplary compounds (1) include methyl acrylate or methacrylate, ethyl acrylate or methacrylate, isopropyl acrylate or methacrylate, n-butyl acrylate or methacrylate, n-pentyl acrylate or methacrylate, neopentryl acrylate or methacrylate, dibromonoeopentyl acrylate or methacrylate, n-hexyl acrylate or methacrylate, n-octyl acrylate or methacrylate, ndecyl acrylate or methacrylate, isodecyl acrylate or methacrylate, lauryl acrylate or methacrylate, stearyl acrylate or methacrylate, 2- ethylhexyl acrylate or methacrylate, cyclohexyl acrylate or methacrylate, methoxymethyl acrylate or methacrylate, ethoxybutyl acrylate or methacrylate, 2-methoxypropyl acrylate or methacrylate, n-butoxymethyl acrylate or methacrylate, cyanomethyl acrylate or methacrylate, cyanobutyl acrylate or methacrylate, (N,N-dimethylamino)-methyl acrylate or methacrylate, 2-(N,N-dimethylamino)-ethyl acrylate or methacrylate, 2-(N,N-diethylamino)-ethyl acrylate or methacrylate, 2-(N,N-dibenzylamino)-ethyl acrylate or methacrylate, 2-(N,N-diethylamino)-propyl acrylate or methacrylate, acrylic acid, methacrylic acid, and the same compounds (i) as described above.

Exemplary compounds (2) include acrylamide, methacrylamide, N,N- dimethyl acrylamide or methacrylamide, N,N-diethyl acrylamide or methacrylamide, N,N-diisorpopyl acrylamide or methacrylamide, N,N-dodecyl acrylamide or methacrylamide, N-isopropyl acrylamide or methacrylamide N-cyclohexyl acrylamide or methacrylamide, N- benzyl acrylamide or methacrylamide, N-methylol acrylamide or methacrylamide, 2-hydroxyethyl acrylamide or methacrylamide, 2-hydroxypropyl acrylamide or methacrylamide, 4hydroxy-n-butyl acrylamide or methacrylamide and the products obtained by ester-amide exhcnage reaction between methayl acrylate or methacrylate and an amino-alcohol having at most 12 carbon atoms.

Exemplary compounds (3) include ethyleneglycol di-acrylate or - methacrylate, diethyleneglycol diacrylate or -methacrylate, triethyleneglycol di-acrylate or -methacrylate, tetraethyleneglycol di-acrylate or -methacrylate, polyethyleneglycol (number average molcecular weight: 200 to 1,000) di-acrylate or -methacrylic, propyleneglycol di-acrylate or -methacrylate, dipropylene-glycol di-acrylate or methacrylate, polypropyleneglycol (number average molecular weight: 100 to 1,000) di-acrylate or methacrylate, butyleneglycol di-acrylate or -methacrylate, trimethylolethane tri-acrylate or -methacrylate, trimethylolpropane tri-acrylate or methacrylate, pentaerythritol tetra-acrylate or methacrylate, neopentryl glycol di-acrylate or methacrylate, dibromoneopentyl glycol di-acrylate or -methacrylate, 1,8-octanediol di-acrylate or methacrylate, 1,9-nonanedio di-acrylate or -meth- acrylate, 1,1 0-decanediol di-acrylate or -methacrylate, 1,12-dodecanediol di-acrylate or -methacrylate and 1,18-octadecanediol di-acrylate or -methacrylate, etc.

Exemplary compounds (4) include N,N'-methylenebisacrylamide, N,N'-methylenebis-methacrylamide, N,N'-trimethylenebisacrylamide, N,N'trimethylenebismethacrylamide, N,N'-hexamethylenebisacrylamide and N,N'-hexamethylenebismethacrylamide.

Exemplary compounds (5) include 2-acid phosphoxyethyl acrylate or methacrylate, 3-chloro-2acid phosphoxypropylacrylate or methacrylate and 3-bromo-2-acid phosphoxypropyl acrylate or methacrylate.

Exemplary compounds (6) include styrene, alphamethylstyrene, alphachlorostyrene, p-tertbutylstyrene, p-sec-butylstyrene, aminostyrene, methoxystyrene, vinyltoluene, vinylbenzoic acid, vinylphenol, allylbenzene, allyltoluene, monoallylphthalate, diallylphthalate and divinylbenzene.

Exemplary compounds (7) include N-vinylcarbazole, N-vinylpyrrolidone, 2-vinylpyridine, 4-vinylpyridine, vinylquinoline, 2-vinylimidazole, 4vinylimidazole, 5-vinylimidazole, 2-vinylbenzimidazole, N-vinyltetrazole, 2- vinylpyrazine, 2-vinylthiazole, N-vinyloxazolidone, 2-vinylbenzoxazole, 2vinylfuran, 2-isopropenylfuran and 2- vinylthiophene.

Exemplary compounds (8) include vinyl acetate, vinyl propionate, vinyl laurate, methylvinyl benzoate, beta-hydroxyethyl vinyl benzoate, vinyl suc ci nate, vinyl adipate, divinylphthalate, divinylterephthalate, diphenylpropane derivatives such as 2,2' -bis -(4 -methacryloydiethoxyphenyl) -propane, 2,2' - bis -(4*acryloxydiethoxyphenyl) -propane, 2,2' bis - (4 -methacryloxyethoxyphenyl) -propane and 2,2' -bis - (4 -acryloxyethoxyphenyl) -propane; acrylonitrile; oligoesters having terminal ethylenically unsturated groups and a number average molecular weight of at most about 1,000 obtained by recting ethylene glycol, propylene glycol, polyoxyethylene diol having a number average molecular weight of at most about 400 or polyoxypropylene diol having a number average molecular weight of at most about 400 or a mixture thereof with a decarboxylic aicd or its mixture as described in the preparation of the prepolymer (IV), and subsequently reacting the resulting oligoester with a carboxylic acid or dicarboxylic acid as described in the preparation of the prepolymer (I), and the same alcohols (ii) as described above.

Of these compounds, a compound of formula 8 or 9 wherein: t is an integer of 8 to 20 is especially effective for improving the solvent resistance of photopolymerized articles.

The amount and kind of the polymerizable ethylenically unsaturated monomeric compounds are suitably chosen depending upon the properties of photopolymerized articles in accordance with their use and the processability of the photosensitive compositions containing such monomeric compounds. The amount is in the range of from about 5 to 100 parts by weight, and preferably 20 to 70 parts by weight based on 100 parts by weight of the prepolymer.

It is necessary that the reaction of photosensitive compositions should be initiated only by the action of the appropriate light and that they should be thermally stable. Therefore, preferably, polymerization initiators are thermally inactive below 40"C and initiate photo polymerization upon irradiation with actinic light. The light is preferably ultra violet.

Exemplary phtopolymerization initiators include benzoins such as benzoin, benzoin methyl ether, benzoin ethyl ether, benzoin n- propyl ether, benzoin isopropyl ether, benzoin n-butyl ether, benzoin isobutyl ether, benzoin sec-butyl ether, benzoin namyl ether, benzoin isoamyl ether, alpha-methylbenzoin, alpha- ethylbenzoin, alphamethyl benzoin methyl ether, alpha-phenylbenzoin, alphaallylbenzoin; anthraquinones such as anthraquinone, chloroanthraquinone, methylanthraquinone, ethylanthraquinone, tertiary butyl-anthraquinone; diketones such as benzil, diacetyl; phenones such as acetophenone, benzophenone, omega-bromoacetophenone-dimethoxy phenyl acetophenone; 2naphthalene sulfonyl chloride; disulfides such as diphenyl disulfide, tetraethylthiuram disulfide; dyes such as Eosine G (C.l. 45380) and Thionine (C.l.

52025); and the like.

These photopolymerization initiators are used in an amount of from about 0.001 to 10 weight percent, and preferably from about 0.01 to 5 weight percent of the total weight of the prepolymer and the polymerizable, ethylenically unsaturated monomeric compound.

Known stabilizers (i.e., thermal polymerisation inhibitors) are employed for the purpose of maintaining storage stability (shelf life) of the photosensitive compositions. Such stabilizers may be added when the components of a photosensitive composition are admixed or may be added to each component separately prior to admixing of the components.

Exemplary stabilizers include hydroquinone, monotert-butyl hydroquinone, 2,5-di-tert-butyl hydroquinone, catechol, tert- butyl catechol, 2,6-ditert-butyl cresol, benzoquinone, 2,5- diphenyl-pbenzoquinone, p-methoxyphenol, picric acid and di- p-fluorophenylamine.

These stabilizers are added only for preventing thermal polymerization without the actinic radiation set forth above without restraining the photopolymerization reaction. Consequently, the amount of the stabilizers may be from about 0.001 to 2.0 percent by weight, and preferably from about 0.005 to 1.0 percent by weight of the total weight of the prepolymer and the polymerizable, ethylenically unsaturated monomeric compound.

Furthermore, various compounds may be incorporated into the photosensitive compositions in order to adjust the viscosity of the photosensitive composition, and the mechanical and other properties after photopolymerization in an amount of from about 0.01 to 100 parts by weight based on 100 parts by weight of the photosensitive composition. These compounds include, for example, dioctylphthalate, butylphthalyl butylene glycolate, polyester type plasticizers, epoxy type plasticizers, various phosphates and polyethers such as polypropylene glycol process oil, liquid paraffin, liquid rubber such as butyl rubber, butadiene rubber.

Particularly suitable liquid photosensitive resin compositions are described and exemplified in British patent specification no.1,575,584 and Japanese Laid-open Patent Publication No. 89101/75. A typical composition suitable for use in the invention is supplied by Asahi Chemical Industry (UK) Limited under the trade name APR Photopolymer K11.

The or each solid flexographic printing plate used in the invention generally carries a flexographic image in relief before the butt jointing process of the invention, in contrast to the process of JP 29804178 wherein the flexographic image must be formed by photoexposure after the injection of the liquid resin.

The butt joint is generally in a non-image area of the printing plate, that is say in one of the recessed areas and so conformity of the top surface of the cured butt joint with adjacent top parts of the resin plate is not essential. However the butt joint can be in raised, image, areas but this is generally preferred only if those image areas will be in unimportant parts of the image, for instance creases of a carton, since then it is unnecessary to achieve perfect registration between the surface of the butt joint and the surface of adjacent image areas.

The printing plate that is used in the invention has preferably been formed by imagewise photoexposure of a liquid photocurable resin that has been cured to form the relief printing surface and that is carried by a polyester or other sheet substrate. Often the entire plate, apart from the substrate, is formed of a single composition but the plate can be formed of two or more different compositions, for instance as described in British Patent 2,012,977. Suitable photocurable resins are described in that patent.

Instead of the plate having been formed by imagewise photoexposure of liquid material it may have been formed by imagewise exposure of a solid photopolymer material. This may be exposed either before or after being secured to the cylinder and before or after butt-joining. Usually a plate formed from a solid photopolymer is exposed imagewise before being secured on the printing cylinder and thus before butt-joining.

The photoexposure of a solid photopolymer plate will impart imagewise differences in the solubility, e.g., in alkali or organic solvent.

The plate may alternatively be a solid plate, of e.g. rubber, which is hand engraved either before or after securing to the cylinder, but the invention is most useful in methods of mounting photopolymer plates on printing cylinders.

If the plate is formed from a liquid photopolymer, then the liquid phtocurable resin may be the same as, or chemically similar to, that from which the printing plate is formed so as to promote bonding of the polymer to the edges of the plate.

The photosensitive plate compositions suitable for making the plate that is to be butt jointed may include solid compositions such as those disclosed in British patent no.1,577,706, 1,552,653, 2,033,598, 1,366,769, French patent 2,103,825, German patent (DE) 2,456,439, 3,137,416 and Japanese Laid-open no.143,557/1980, 110,289/1979 and 65,004/1981.

The liquid phtosensitive resin compositions suitable for making the plates used in the present invention may include those disclosed in United States Patent No.3783,152, 3,556,791, 3,858,510, 3,960,572, 4,266,007 and British patent no.1,575,584.

Although the invention can be used for butt jointing two separate printing plates, generally supported by a substrate, it is of particular value for butt jointing the opposite edges of a printing plate that is wrapped around a cylinder. The butt joint may be sufficient to give a permanent fit of the plate on to the cylinder or, if desired, adhesive may be provided between the plate and the cylinder, for instance double-sided adhesive tape. Since the butt joint is resistant to the solvents used in printing inks possible solvent sensitivity of the adhesive tape is of little or no importance.

The invention is illustrated by reference to the accompanying drawing which is a section through part of a printing cylinder during the process of the invention.

A previously imaged flexographic printing plate 1 (formed by imagewise photoexposure of a liquid resin) is wrapped around a cylinder 2 and liquid photocurable resin 4 of the type described above is injected into the gap between the adjacent and opposite edges of the plate generally using a high pressure injection gun in combination with a hypodermic needle. This is desirable in order that the liquid fills the gap completely, down to the cylinder, and excludes oxygen from the gap. Advantageously the liquid resin is covered by a sheet 3 of transparent material, for instance polyester, to minimise the contact of the resin with oxygen and to smooth the surface of the liquid.

The liquid resin is cured by irradiating with, generally, ultra- violet light from a source 5 and which passes through the transparent protecting sheet.

After curing the protective sheet is removed from the plate, and the resin may be dried using hot air and/or may be treated using conventional surface chemical treatment solutions, to eliminate any stickiness.

Joining the edges of a printing plate using the method of the invention produces a uniform plate on a cylinder, which will leave no mark or a negligible mark of discontinuity on the substrate which is to be printed.

Usually the plate is held onto the cylinder using double- sided adhesive tape between the cylinder and the plate.

The method of the invention is particularly suitable for continuous printing cylinders in which there is a very small gap between images, and particularly for applications requiring the use of small printing cylinders and high solvent inks. Examples of methods for which the inventon will be useful are for printing paper bags and foil films, for example crisp packets or the like.

Preferably cylinders used in methods according to the invention have a diameter of between 5 and 30 cm. The total thickness of plates used in the method according to the invention is usually between 0.75 and 10 mm, preferably from 3 to 7 mm.

The depth of the relief is generally between 0.5 and 5 mm. The gap that is filled is usually between 0.2 and 5 mm.

Claims (10)

1. A process in which one edge of a solid flexo graphic printing plate is butt jointed to an adjacent edge of a solid flexographic printing plate by injecting liquid photopolymerisable resin between the edges and then curing the resin by photoexposure and in which the or each solid printing plate initially carries a flexographic image in relief or a flexographic image in relief is formed in the or each plate after injecting the liquid composition, and in which the liquid composition comprises 100 parts by weight of a diene type prepolymer having ethylenically unsaturated groups, 5 to 100 parts by weight of a polymerisable ethylenically unsaturated monomeric compound and 0.001 to 10 parts by weight of a photopolymerisation initiator.

2. A process according to claim 1 in which the or each plate carries a flexographic image in relief before the butt jointing.

3. A process according to claim 2 in which the or each plate is formed from a liquid photopolymerisable resin by imagewise photoexposure of the resin.

4. A process according to any preceding claim in which the butt joint is in a non-image area.

5. A process according to any preceding claim in which the edges that are butt jointed are opposing edges of a printing plate that is wrapped around a cylinder.

6. A process according to claim 5 in which the printing plate is additionally secured to the cylinder by double-sided adhesive tape

7. A process according to any preceding claim in which the photosensitive resin is introduced into the gap by injection under pressure sufficient to exclude air from the joint.

8. A process according to any preceding claim in which the resin in the butt joint is covered with a transparent film before the photoexposure.

9. A process according to any preceidng claim in which the diene type prepolymer having ethylenically unsaturated groups is a prepolymer having terminal ethylenically unsaturated groups and a number average molecular weight of about 6,000 to 30,000 whose main segment is a residue of a chain-extended hydrogenated 1,2- polybutadiene having 2 to 20 urethane bonds obtained by reaction between the hydrogenated 1,2-polybutadiene having an average of about 1.2 to 2.0 terminal hydroxy groups per molecule and a degree of hydrogenation of pendant vinyl groups of the 1,2addition units of about 50 to 100 percent and a diisocyanate.

10. A process according to any preceding claim substantially as herein described.