CA1206302A - Desensitizing gum for lithographic printing plate - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A desensitizing gum for lithographic printing plates is disclosed. The gum is comprised of 5 to 35 wt%, based on the total weight of the gum, of a film-forming organic high molecular compound substantially soluble in cold water. The compound is selected from the group consisting of dextrine, soluble starch, water-soluble cellulose ether, water-soluble vinyl polymers, plurune and its derivatives, sterabic, stractan and alginates. The gum further includes 0.02 to 2 wt%, based on the total weight of the gum, of an alkali metal hexametaphosphate or ammonium hexametaphosphate. The desensitizing gum is used in connection with lithographic printing plates and aids in maintaining the ink-repelling properties of the non-image areas of the plate. Further-more, the gum does not spoil the ink receptive properties of the image areas and possesses good plate-protecting ability while providing good coating surface properties.

Description

DESEMSITIZING GUM
FOR LITHOGRAPHIC PRINTIWG PLATE

FIELD OF THE INVENTION

The present invention relates to a desensitizing.gum for lithographic printing plates.
BACKGROUND OF THE INVENTION ..:
S - In making. lithographic-. printing plates,-a..step of,~
coating a desensitizing gum, called. a. gumming~up.- step,.,is~
provided as a final..step.. -The purposes:of-applying.the ...-desensitizing..gum are to protect- the plate-.surface from .

stains causing print stains during: the period after plate,' =-:-:making and before mounting the plate on a press,and to pre- :

vent the plate. surface from being damaged when stored:in ~
: piles or when in contact with foreign matter. It is most important to prevent the plate surface frbm being directly : ~ exposed to the airO The exposu~e to the air causes oxida- .,.
lS tion of the plate which .deteriorates the ink-repelling. .

properties of non~image areas. In addition, application of the desensitizing gum serves to increase ink-repelling pro- - -,-perties of non-image areas, thus being i,ndispensable~

The desensitizing gum which has been used .is about...
15 to about 3~ wt% aqueous solution of gum arabic...This gum. ..:.

aabic is extremely excellent.as.:a~ desensitizing .gum with .::
respect. to its desensitizing. action, filming'propertiest :.

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solubility, etc. However, due to its strong desensitizing action, when used as a desensitizing gum, it often decreases ink receptivity of image areas so much that many spoiled copies must be printed beEore the image fully accepts ink.
One known procedure involves coating the image areas with fats and oils before the gumming-up step for the pur-pose of removing the above-described defect. However, this makes the plate-making step complicated and deteriorates workability and, in. addition, it.is not preferable:-.due to the pollution and health hazard created Accordingly, it -has been attempted.-to use other-water-soluble organic high molecular. compounds ..than gum arabi as desensitizing .gum. For example, Japanese Ratent-: ~
Application (OPI) Nos. 56603/77 and GB 2,010,298,.We~st German Patent 2,504,594, and Soviet Patent 623,755 disclose dextrin, plurune and its derivatives, carboxy-containing:
polyacrylamide .derivatives~ :methyl acrylate- or methyl- .:.
methacrylate-grafted polyacrylamide copolymer, etc. as sub-stitutes for.-gum arabi. However, all of these compounds show weaker desensitizing action on non-image areas than gum arabi does. . In addition, they are. inferior în coating performance, .and, particularly. when applied as aqueous solutions to an exposed and developed lithographic printing:
plate using a.gum coater, they-are- difficult to coat uni-f.ormly, making c.oating. uneven.... .;.

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SUMMARY OF TE~E INVENTION
It is, therefore, an object of the present invention to provide a desensi~izing gum which ~eeps ink-repelling properties of non-image areas and does not spoil ink-recep-tive properties of image areas and which possesses good :.plate-protecting ability while providing good -coating sur- -face properties. --As a -result of intensive investigations to attain~
the above-described objects, the inventor has found that a..-. -.
desensitizing gum wkich shows a.desensitizing action.:without.... :
reducing ink-receptive properties of image areas and which .:.
provides excellent coating surface properties can-be.obtain ed by adding to a film-forming,. water-soluble organic high ..-molecular..compound substantially soluble in cold water an .alkali metal or ammonium hexametaphospha~e as a stain-pre-venting agent and a coating surface--improving agentO
That is, the present invention is directed to a desensitizing gum for lithographic printing plates which :.
contains:
(a) 5 to 35 wt%, preferably 10 to 20 wt%, of one or more members of film-forming organic high molecular compounds.-substantially soluble in cold water and selected. from the group consisting of dextrin, soluble starch~.water-soluble .-cellulose..ether, water-soluble vinyl polymers 21urune tr-mer of:.glucose.and. - .
.its derivatives, sterabic, stractan (arabinogalacton), and .-

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alginates; and (b) 0.02 to 2 wt%, preferably 0.05 to 0.5 wt~, of an alkali metal or ammonium hexametaphosphate~
The remainder other than the film-forming organic high mole- -cular compounds and the alkali metal or ammonium hexameta-phosphate comprises water.
DETAILED DESCRIPTION OF THE INVENTION
The alkali metal or ammonium hexametaphosphate to be used in the present invention includes sodium hexametaphos-~
phate, potassium .-hexametaphosphate, ammonium.hexame.taphos~ s --phate, etc L
: These alkali metal.and ammonium hexametaphosphates are added in amounts of 0.02 to 2 wt~, prefera~ly Ø0S to :
.
0.5 wt%, based on the total weight o.E the desensitizing.gum composition.
. Of the component a) above described, dextrin is most preferable. The dextrin to be used in the. present invention is that obtained by hydrolyzing starch. Dextrins.different . in properties are obtained depending upon the hydrolyzing process and the degree of hydrolysis, including those which are only soluble in warm water and those~ which are almost soluble in cold water. Dextrins usable. in the present in--vention are those which contain at least 60 -wt~, preferably more than 70 wt%, soluble components in-water a~ 20C. ..
.: .... . - .
25 . In the practice of the present invention, it.is........ .;

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suitable to adjust the whole water solubility by mixing a dextrin having a lower content of water solubles and a dextrin having a higher content of water solubles. The use of a dextrin having a lower content of water solubles can reduce the ink-receptive properties of image areas and solidify in the aqueous solution due to poor storage .:
stability.
Similarly with the dextrin, other soluble starch,~
water-soluble cellulose ethers, water-soluble. vinyl poly~
mers, plurune and its derivatives, sterabic,. stractan:-..:..
(arabinogalacton), and alginates to be used contain at least .
60 wt~ preferably more than 70 wtg, of ingredients soluble .:: .
in 20C waterO In addition to the above-described:compo~
nents, the desensitizing gum of the present- invention...can -:.-~contain an acid, an antiseptic, a wetting agent, and otherinorganic salts.
The wetting agent prevents ~he desensitizing- gum on the plate from being dried upon drying, which serves to facilitate removal of-the desensitizing gum upon subsequent -washing with water or upon the plate surface coming into , contact with dampening rbllers on a press. Rapid removal ofthe desensitizing gum favors the ink receptivity of image :-:
areas. -Useful wetting agents include polyethylene glycol, polypropylene glycol, and low molecular polyols such as. .

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ethylene glycol, propylene glycol and glycerin. Other useful wetting agents are anionic surfactants and nonionic surfactants The anionic surfactants include alkylbenzene-sulfonates, alkylnaphthalenesulfonates, naphthalenesulfonate condensates, dialkylsulfosuccinates, . alkylsulfonic ac.id salts containing 8 or more carbon atoms, R ~ O ~ (R:

- S03Na S03Na alkyl group or hydrogen atom), alkylphenol. type:.phosphates, alkyl type phosphates, fatty acid salt soaps, .rhodinic.acid soaps, castor oil sulfuric~.ester salts, sulfuric ester salts o alkyl.alcohols containing 8 or more carbon atoms, alk~
aryl ether sulfuric ester.salts, polyoxyethylene aIkyl-ethèr sulfuric ester salts, etc. 0f-these, aliphatic or aromatic~
sulfonic acid salts are excellent due to their low-foaming properties. The nonionic surfactants include ethoxylated .aliphatic fluorocarbons, ethoxylated vegetable oils, ethoxy-:
lated aliphatic alcohols, polyoxyethylene aliphatic alco-hols, ethoxylated alkylphenols, tetramethyldecynediol, sor-bitol and sorbitan derivatives. Of these nonionic surfac-tants, those with an ~LB of 10 to 18 are. most usefulO
Generally, the wetting agent is added in an amount o 0.05 to 5 wt%, desirably 0.1 to 2 wt%, based on-the total weight of the desensitizing gum composition.-Addition.of acids.or other inorganic salts serves to -- 6 -- ..

`2 adjust p~ and raise the action of rendering the base plate hydrophilic. Such acids include phosphoric acid, citric acid, oxalic acid, ~artaric acid, boric acid, aliphatic or aromatic sulfonic acids, organophosphonic acids, etc. These inorganic salts include sodium nitrate, sodium sulfa-te, cal-cium nitrate, magnesium nitrate, magnesium sulfate, sodium phosphate, ammonium phosphate, ammonium nitrate, ammonium sulfate, so~ium acetate, sodium tartarate, sodium phosphate, sodium dihydrogenphosphate, disodium .hydrogenphosphate, am-monium - phosphate, ammonium dihydrogenphosphate, diammonium hydroqenphosphate, etc. These acids and inorganic salts are~
added in amounts which do not cause corroding of the plate.
In general, they are added in amounts such that the pH of the resulting composi.tion ~ecomes aboaut 2.~5..~to about : 15 9.5. Phosphor.ic acid.and its salts have been found to be the most useful.
The desensitizing gum of the present invention can be applied to various lithographic .printing plates, parti-cularly preferably to lithographic printing plates obtained by imagewise exposing and developing presensitized litho-graphic printing plates (printing plates having a previously given light-sensitivity which are called PS plates) com-prising a support of an aluminum plate having provided-thereon a llght-s~nsitive layer. Preferable examples of ~5 such PS plates include: PS plates comprising an aluminum 12~63C~

plate support having provided thereon a light-sensitive layer composed of a mixture of diazo resin (salt of a con-densate between p-diazodiphenylamine and paraformaldehyde) and shellac as described in British Patent 1,350,521, negative-working PS plates comprising an aluminum support . ..
having provided thereon a light-sensitive layer composed oE . :
a mixture of diazo resin and a polymer having hydroxyethyl . .
methacrylate units or hydroxyethyl acrylate units as major ..
repeating unitst as described in British Patents 1,460,978 - .
and 1,505,739; and positive-working PS plates comprising an.. ~
aluminum plate having -provided thereon a light-sensitive . ~--layer composed of a mixture of an o-quinonediazide light~
sensitive compound- and a novola~ type phenol resin, as..:. ..
described in. U.S. Patent 4,123,279. ~urther~ PS--plates-~.--~-comprising an aluminum plate having provided thereon-a 1ight-sensitlve layer of photo-cross-linkable photopolymer specifically described in U.S. Patent 3,860,426, PS plates .
comprising an aluminum support having provided thereon a.
: light-sensitive layer of. photopolymerizable photopolymer . .
composition as described in U.S. Patents 4,072,528 and

4,072,527, and PS plates comprising an aluminum plate having .~ .
provided thereon a light-sensitive layer composed of a mix- --ture of.an azide and a water-soluble polymer as described in .
British. Patents 1,235,281 and 1,495,861 are also prefer- .. -able. :.. -. .

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One embodiment of applying the desensitizing gum of the present invention to a PS plate is descri.bed below.
A PS plate is first imagewise exposed, then devel-oped to prepare a lithographic printing plate. This lithographic printing plate is washed with water and, after squeezing away the water on the plate surface, a suitable amount of the desensitizing gum of the present invention is applied to .:the plate surface, followed by rubbing with a sponge so as to spread the gum all over the plate surface..
Thus, non-image areas of the printing plate a're-protected, and the resulting lithographic printing plate can be stored.
In order to starting..printing, the gum on the plate surface is washed away,'and subsequent procedures are conducted in a usual manner to print copies. ' -The use.of the desensitizing gum of the present in-vention eliminates the necessity of using an oily protective ink which has been conventionally used for retaining- ink~
receptive properties 'o~ image a.eas, thus being extremely, . ~ advantageous with respect to workability and the,prevention of pollution. Upon printing, sufficiently sharp and clear copies can be obtained immediately without producing many spoiled copies as has been encountered in the prior art.
The present invention will now be described in more, detail by reference to non-limiting examples. '~-.

_ g _ `` ~ZO~i3~2 140 Parts by weight of dextrin conta.ining 95 wt% or more water solubles (cream dextrin made by Matsutani Chemi-cal Industry Co.~ Ltd.) and 60 parts by weight ,of dextrin containing about 90 wt% water solubles (white dextrin made by Matsutani Chemical Industry Co., Ltd.) were dissolved in 8Q0 parts by weight of water warmed to 40C. AEter cooling .
to room temperature, 5 parts by weight of a 33~ aqueous solution of sodium isobutylnaphthalenesulfonate, 2 parts. by weight of sodium hexametaphosphate, and 2 parts by weight of,. '' ''ammonium dihydrogenphosphate were dissolved therein-,to.pr:e--. c.-pare a desensitizing gum of the present invention.
Separately~ l part. by weight of a naphthoquinone~
l,2 diazido-5-sulfonic .ester - of polyhydroxyphenyl ob~tained , by polycondensation of acetone and pyrogallol. described.in~
U.S. Patent 3,635,709 and 2 parts by weight of a novolak., type cresol-formaldehyde resin were dissolved in 40 parts by weight of methyl cellosolve to prepare a light-sensitive solution.
A 0.2-mm thick, grained aluminum plate was well . -, washed and dried, and the above-described light-sensitive solution was coated on the aluminum-plate using a r,otary coater to prepare - a positive-positive type presensitized, printing plate having a.light-sensitive layer o a thickness .-.
of about 2.0 g/m2. This plate was exposed using a half ton~e .'positive transparency, developed w.i.th a 3% sodium.silicate.: 'n '~

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aqueous solution, washed with water, and dried.
The resultiny printing plate was cut into three pieces. One piece of the plate was coated with a 14Bé gum arabic (about 27% aqueous solution of gum arabic) having been conventionally used as a desensitizing gùm, followed by wiping off excess gum with a cloth to prepare sample A.
A second piece was coated with the above~prepared desensitizing gum o~ the present invention, ~ollowed by similarly wiping off excess gum with a cloth to prepare sample B.:
The third piece was coated with a desensitizing.g.um prepared .by excluding sodium hexametaphosphate -from the~
desensitizing gum of the present .inventionj foll.owed by .wiping off excess .gum~with a cloth to prepare.sample.C.
These three samples A, B and C were placed in a constant temperature and moisture both at 60C for 3 days.
Then, the desensitizing gum was removed in a usual manner, and each of the sample plates was mounted on an offset press . ~ to print in a usual manner.
With sample A, 35 spoiled copies had to be printed before sharp and clear copies were printed and, with samples B and C, S spoiled copies. had to be printed. With respect to stain during printing, samples A and B suffered no stains, whereas sample C .was extrèmely easily stained.
Thus, sample B.using the desensitizing gum of .the present 63~3~

invention was found to be excellent in ink-receptive pro-perties o~ image areas and ink-repelling properties of non-image areas. As to coating surface properties upon coating the desensitizing gum, samples A and B showed satisfactor~
properties, whereas sample C showed coating streaks after wiping with a cloth.

60 Parts by weight of dextrin containing 95 wt% or more water solubles, 160 parts by weight of soluble starch containing 95 wt% or more water solubles, Amycol l~ (made by Nichiden Chemical Co., Ltd.), 4 parts by weight of sodium hexametaphosphate, 1 part by weight of phosphoric acid, and

5 parts by weight of a 20% aqueous solution of sodium di-octylsulfosuccinate were dissolved in 770 parts by weight of water to prepare a desensitizing gum. This was applied to the same printing plate as described in Example l~
As a result of printing by mounting the plate on an offset press, only 5 spoiled copies were printed beEore sharp and clear copies were printed, and no stains occurred.
~0 Thus, the desensitizing gum showed satisfactory performance as a desensitizing gum.

30 Parts by weight of dextrin containing 95 wt% or more water solubles, 80 parts by weight of stractan, 2.5 parts by weight of a 50% aqueous solution of l-hydroxy-*Trade ~ark - 12 -

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ethane~ diphosphonic acid, 5 parts by weight of sodium isopropylnaphthalenesulfonate, 1 part by weight of ammonium dihydrogenphosphate, 1 part by weight of sodium hexameta-phosphate, and 0..05 part by weight of an aqueous emulsion solution of silicone oil (containi.ng 40 wt~ solids) were dissolved in 800 parts by weight of water to prepare a desensitizing gum.
Separately, a negative-working PS plate comprising diazo resin described in Example 7 of Japanese.Patent Publi-cation No. 34929/80 was exposed and developed in the same manner as described in Example 7 of the publicationj washed with water, dried, and cut into three pieces. One of the pieces was coated with a 14Bé gum arabic using a gum coater (G800-II, made by Fuji Photo Film Co.,-Ltd.) to prepare ~15 sample A. A second piece was coated with the above-prepared desensitizing gum using the gum coater to prepare sample B.
The tnird piece was coated with a desensitizing gum prepared by excluding sodium hexametaphosphate from the formerly prepared desensitizing gum using the gum coater to prepare sample C. Samples A, ~ and-C were placed in a thermo-hygro-stat at 45C and 85~ humidity for 7 days, then processed in .a conventional manner, followed by printing using .an offset press.
With sample A, 250 spoiled copies had to he printed before sharp and clear copies were printed.and, with samples *.Trade Mark - 13 -~1~206~ )2 B and C, 18 and 20 spoiled copies had to be prin~ed, respectively. Sample C was susceptible to stains. On the other hand, sample B was extremely satisfactory.
As to coating surface properties upon coating each of the desensitizing gum samples using a gum coater, sample A and sample B were satisfactory, whereas sample C showed serious coating unevenness.

30 Parts by weight of dextrin containing 95 wt% or more water solubles and 80 parts by weight o~ soluble starch, Amycol lB (made by Nichiden Chemical Ca. t Ltd.), were dissolved in 800 parts by weight of water at 40C and, after cooling, l part by weight of sodium laurylbenzenesul-onate, 2 parts by weight of sodium hexametaphosphate, 2 parts by weight of citric acid, 5 parts by weight of meth-anol, and 0.1 part by weight of ethyl p-hydroxybenzoate were dissolved therein to prepare a desensitizing gum. When this gum was applied to the same printing plate as used in Example 3 in the same manner as in Example 3, only 20 spoil~d copies had to be printed before sharp and clear copies were printed even after storing the plate for 7 days at a temperature of 45C and a humidity of 85%, and no stains occurred. Thus, the desensitizing gum of the present invention showed extremely satisfactory performance as a desensitizing gum.

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While the invention has been described in detail andwith reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.

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Claims (9)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1, A gumming process comprising applying a desentizing gum to an imagewise exposed and developed presensitized lithographic plate, wherein said desensitizing gum comprises:
5 to 35 wt%, based on the total weight of the gum, of a film-forming organic high molecular compound substan-tially soluble in cold water, selected from the group consisting of dextrine, soluble starch, water-soluble cellulose ether, water-soluble vinyl polymers, plurune and its derivatives, sterabic, stractan, and alignates;
0.02 to 2 wt%, based on the total weight of the gum, of an alkali metal hexametaphosphate or ammonium hexametaphosphate.

2 . A gumming process as claimed in Claim 1, wherein the alkali metal hexamethaphosphate is selected from the group consisting of sodium hexametaphosphate and potassium hexametaphosphate.

3. A gumming process as claimed in Claim 1, wherein the alkali metal hexametaphosphate or ammonium hexametaphosphate is present in an amount in the range of 0.05 to 0.5 wt% based on the total weight of the gum.

4. A gumming process as claimed in Claim 1, wherein dextrine is present in an amount in the range of 5 to 35 wt% based on the total weight of the gum and further wherein the dextrine contains at least 60 wt% of components soluble in water at 20°C.

5. A gumming process as claimed in Claim 4, wherein the dextrine contains at least 70 wt% of components soluble in water at 20°C.

6. A gumming process as claimed in Claim 1, wherein the gum includes a wetting agent selected from the group consisting of aliphatic and aromatic sulfonic acid salts present in an amount in the range of 0.05 to 5 wt% based on the total weight of the gum.

7. A gumming process as claimed in Claim 6, wherein the wetting agent is present in an amount in the range of 0.1 to 2 wt% based on the total weight of the gum.

8. A gumming process as claimed in Claim 1, wherein the gum includes a pH adjusting agent selected from the group consisting of acids and inorganic salts, the agent being present in an amount such that the pH of the resulting gum becomes about 2.5 to about 9.5.

9. A gumming process as claimed in Claim 8, wherein the agent is phosphoric acid or its salts.