GB1574690A - Finishing process for flexographic printing plates using isocyanates - Google Patents

Description

(54) FINISHING PROCESS FOR FLEXOGRAPHIC PRINTING PLATES USING ISOCYANATES (71) We, E.I. DU PONT DE NEMOURS AND COMPANY, a Corporation organised under the laws of the State of Delaware, United States of America, of Wilmington, State of Delaware, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The invention relates to a process for the after-treatment of photopolymer printing plates for flexoprinting.

The printing of packaging materials, e.g., paper, cardboard or plastic films, is often accomplished by means of the flexoprinting process. Preponderantly, rubber plates are used as printing plates for this purpose due to their excellent elasticity. The preparation of rubber plates is, however, very cumbersome and time consuming, so that recently considerable efforts have been made to replace rubber plates by other materials. Thus e.g., printing plates with an elastic surface of photopolymerizable or photocrosslinkable light-sensitive mixtures have become known. These printing plates comprise an elastomer binder, a photopolymerizable or photocrosslinkable compound and an initiator or initiator system. A series of requirements must be placed on printing plates for flexoprinting, which are very difficult to combine with each other. On the one hand, these plates must be soft, flexible and elastic, while, on the other hand, the relief remaining after development or washing out must have a certain hardness and wear resistance without loss of elasticity.

Phdtopolymer printing plates, whose binders essentially consist of a synthetic rubber, have been especially proved useful for flexoprinting, because they are equivalent in elasticity with conventional rubber plates. Such flexoprinting plates are, for example, described in German Published Application 2,215,090 and German Preliminary Published Applications 2,138,582 and 2,223,808.

It is disadvantageous, that flexoprinting plates, which contain these types of binders, after development, have a tendency for surface tackiness and are also not completely resistant against solvents of flexoprinting inks, but rather will swell in these solvents. This leads to an impairment of the printing quality, e.g., by adhering paper dust, by abrasions, impaired dimensional stability and the like.

Presently, a series of measures are known to improve the mechanical surface features of these printing plates during the printing process.

Thus, it is known to add an aftertreatment step after the developing or washing out process, where the surface of the plate is chlorinated. Although the tackiness of the plate is hereby reduced, nevertheless, the surface simultaneously becomes brittle and loses elasticity. It is also disadvantageous that the chlorination process requires a considerable expenditure for apparatus and is very difficult to control.

Moreover, it is known to apply surface lacquer or membrane coatings onto the plate. These coatings, however, also bring about a loss of elasticity, and, in addition the printing quality of the plates is impaired by the coatings.

The present invention makes it possible to provide an aftertreatment process for photopolymer printing plates, for flexoprinting, substantially containing a synthetic rubber as binder, in which the resultant plates are non-tacky and possess improved resistance against solvents of printing inks without impairing the elasticity of the plate.

In accordance with this invention, there is provided a process for the aftertreatment of photopolymer printing plates for flexoprinting which contain a synthetic rubber as a binder, which process comprises treating the printing plate after the imagewise exposure and development with a solution comprising a di- or polyfunctional isocyanate in a solvent of formula: HO-(CH2COO)nR or I-no-(-cH,-cw,-s-),-IP, wherein R is C4-C10 alkyl, or cycloalkyl, for example cyclobutyl, cyclopentyl or cyclohexyl, R, is C1-C6 alkyl, or acyl of formula -CO-R2 where R2 is C1-C4 alkyl, n is O or 1, and m is 1 to 3 Suitable solvents are found among the group of higher alcohols, ethers and esters of glycol, as well as esters of glycolic acid.

Particularly mentioned are: decanol-(l), hexyl alcohol, cyclohexanol, ethylene glycol-monomethyl ether, ethylene glycol monoethyl ether, 2-phenoxy ethanol, hexyl glycol, diethylene glycol monomethyl ether, glycolic acid butyl ester and. ethylene glycol acetate.

The di- or polyfunctional isocyanates, which can be used to prepare the aftertreatment bath of the invention, are known in great numbers and are available as commercial products (see Ullmanns Encyclopedia of Industrial Chemistry, third edition, 1957, volume 9, pages 1--13).

Mentioned are: 2,4 - toluene diisocyanate, 2,5 - toluene diisocyanate, 2,6 - toluene diisocyanate, 1,5 - naphthylene diisocyanate, 4,4' - diphenyl methane diisocyanate, 4,4',4" - triphenyl methane triisocyanate (obtained as a methylene chloride solution), thiophosphoric acid-tris (p - isocyanate phenyl ester).

To prepare the aftertreatment baths, the isocyanate compound is dissolved in at least one of the above-mentioned solvents, whereby the concentration of the isocyanale compound can amount to approximately 2 to 50% by weight. The preferred concentration range lies between 20 and 40 /n by weight.

Depending on the type of isocyanate compound used and the solvent, however, other concentrations. are possible, whereby the respective optimal concentration can be easily determined by simple tests.

The described aftertreatment baths excel by having great stability and can be stored over longer periods of time, without deterioration.

The application of the aftertreatment solution onto the flexoprinting plate can be by manual means, e.g., by applying the solution to the whole plate surface using a cotton swab, sponge or brush or by dipping the plate into the solution. As a result of its excellent stability, the solutions can also be applied by mechanical means, e.g., by spraying or rinsing.

Aftertreatment takes place after the development of the imagewise exposed plate or, if necessary, subsequent to the afterexposure by applying the treatment solution to the whole plate surface at room temperature. The effect of the aftertreatment solution can be accelerated by a subsequent heat treatment. Generally, it suffices to heat the flexoprinting plate 5 to 30 minutes at 60 to 1000C.

The aftertreatment process of the invention in a simple manner leads to completely non-tacky flexoprinting plates which have improved resistance against solvents contained in the printing inks. The plates remaining non-tacky during the printing process without impairing the original flexibility of the plate. It is of further advantage that the excellent storage stability of the treatment solution, in addition to manual application, also makes possible a mechanical application.

The following Examples illustrate the invention. In all the Examples the percent data relates to percent by weight.

EXAMPLE 1 A flexoprinting plate prepared according to the data of German Published Application 2,215,090, exposed and developed, is afterexposed according to the data of Example 1. Then, a 20% solution of triphenyl methane triisocyanate in glycolic acid butyl ester is applied uniformly onto the plate surface using a sponge and is allowed to react in approximately ten minutes. Subsequently, the excess treatment solution is wiped off, and the plate is heated approximately 20 minutes to 6800C.

A completely non-tacky printing plate is obtained, which retains non-tackiness even during the printing process.

EXAMPLE 2 Onto the surface of an exposed and developed flexoprinting plate, as described in Example 1, is sprayed each one of the following solutions in known manner: a) a 45% solution of toluene diisocyanate in ethylene glycol monoethyl ether, b) a 30% solution of thiophosphoric acid tris - (p - isocyanate phenyl ester) in ethylene glycol monoacetate.

In both instances a non-tacky flexoprinting plate is obtained having high resistance against the solvent of the printing ink.

EXAMPLE 3 A flexoprinting plate as described in Example 1 is treated according to the data of Example I using a 25% solution of toluene diisocyanate in cyclohexanol.

The flexoprinting plate is completely nontacky after the treatment and during the printing process is resistant against the solvents of the printing ink.

The same results are obtained when using the following treatment baths: a) a 10% solution of triphenyl methane triisocyanate in decanol-(l), b) a 5% solution of triphenyl methane triisocyanate and diethylene glycol monomethyl ether.

WHAT WE CLAIM IS: 1. Process for the aftertreatment of a photopolymer printing plate for flexoprinting which contains a binder which is a synthetic rubber, which process comprises treating the printing plate after imagewise exposure and development with a solution comprising a di- or polyfunctional isocyanate in a solvent of formula: HO-(-CH2-COO-)n-R or HO(CH2CH2O)rnR1 in which: R is C4-C10 alkyl, or cycloalkyl, R1 is C1-C6 alkyl, or acyl of formula -CO-R2 where R2 is C1-C4 alkyl, n is 0 or 1, and m is 1 to 3.

2. A process according to claim 1, wherein the treatment solution contains the di- or polyfunctional isocyanate compound at a concentration of 2-50% by weight.

3. A process according to claims I and 2, wherein the treatment solution contains triphenyl methane triisocyanate and glycolic acid butyl ester.

4. A process according to any one of the preceding claims, wherein the printing plate is subjected to heat treatment following the aftertreatment.

5. A process according to claim 1, substantially as hereinbefore described in any one of Examples 1, 2 or 3.

6. A photopolymer printing plate for flexoprinting which contains a binder which is a synthetic rubber which has been subjected to an aftertreatment process as claimed in any one of the preceding claims.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. The flexoprinting plate is completely nontacky after the treatment and during the printing process is resistant against the solvents of the printing ink. The same results are obtained when using the following treatment baths: a) a 10% solution of triphenyl methane triisocyanate in decanol-(l), b) a 5% solution of triphenyl methane triisocyanate and diethylene glycol monomethyl ether. WHAT WE CLAIM IS:

1. Process for the aftertreatment of a photopolymer printing plate for flexoprinting which contains a binder which is a synthetic rubber, which process comprises treating the printing plate after imagewise exposure and development with a solution comprising a di- or polyfunctional isocyanate in a solvent of formula: HO-(-CH2-COO-)n-R or HO(CH2CH2O)rnR1 in which: R is C4-C10 alkyl, or cycloalkyl, R1 is C1-C6 alkyl, or acyl of formula -CO-R2 where R2 is C1-C4 alkyl, n is 0 or 1, and m is 1 to 3.

2. A process according to claim 1, wherein the treatment solution contains the di- or polyfunctional isocyanate compound at a concentration of 2-50% by weight.

3. A process according to claims I and 2, wherein the treatment solution contains triphenyl methane triisocyanate and glycolic acid butyl ester.

4. A process according to any one of the preceding claims, wherein the printing plate is subjected to heat treatment following the aftertreatment.

5. A process according to claim 1, substantially as hereinbefore described in any one of Examples 1, 2 or 3.

6. A photopolymer printing plate for flexoprinting which contains a binder which is a synthetic rubber which has been subjected to an aftertreatment process as claimed in any one of the preceding claims.