Printing plate protectant

A composition useful for the protection of actinic light-exposed presensitized lithographic printing plates. The composition contains an effective amount of the di-potassium salt of hydroxyethylidene diphosphonic acid dissolved in water. The composition is applied to the surface of the revealed substrate of the plates after they have been developed but before treatment to heat, to prevent surface contamination from the heat treatment.

FIELD OF THE INVENTION 
This invention relates to a printing plate protectant composition 
comprising an effective amount of the di-potassium salt of 
hydroxyethylidene diphosphonic acid dissolved in water. The composition is 
useful for protecting the surface of actinic light-exposed presensitized 
lithographic printing plates, when applied to the surface of the revealed 
surface of the plates after they have been developed but before treatment 
to heat, thereby preventing surface contamination resulting from the heat 
treatment. 
BACKGROUND OF THE INVENTION 
Radiation-sensitive plates suitable for lithographic printing are well 
known. Such plates typically consist of a substrate such as aluminum that 
may be grained and/or anodized, or of zinc, magnesium, copper or steel or 
a combination such as a bimetal or laminate, and a layer of 
radiation-sensitive material deposited on the substrate. 
Image-wise exposing the radiation-sensitive layer to actinic radiation 
through a transparency causes the solubility of the radiation-exposed 
areas to change relative to that of the unexposed areas. Upon treatment of 
the exposed plate with a suitable developer, the more soluble areas can be 
readily removed to reveal the underlying substrate and leave an image on 
the substrate constituted by the less soluble areas. The areas of the 
substrate revealed upon development constitute the non-image areas. 
A type of radiation-sensitive materials known as photopolymers become less 
soluble after exposure to radiation and therefore a negative transparency 
is used in the exposure. In this case, it is the non-radiation-exposed 
areas that are removed upon development and the radiation-exposed areas 
that remain on the substrate form the image. Examples of suitable 
photopolymers include diazo resins, chromium-sensitized colloids, 
diazonium or azide-sensitized resins or polymers bearing such groups. 
Plates having a radiation-sensitive layer based on such materials are 
known as negative-working plates. 
Radiation-sensitive materials such as those based on orthoquinone diazides 
become more soluble after exposure to radiation and therefore a positive 
transparency is used in the exposure. In this case, it is the 
radiation-exposed areas that are removed by development and the 
non-radiation-exposed areas that remain on the substrate form the image. 
Plates having such radiation-sensitive layers are known as 
positive-working plates. 
The life, in terms of the number of copies it can produce, of a printing 
plate can often be increased by ("burning-in") the image areas, provided, 
of course, that the material of the image areas is suitable. "Burning-in" 
is a well-established practice in the art of producing lithographic 
printing plates from radiation-sensitive plates. The "burning-in" causes 
extensive crosslinking to occur in the polymeric structure of the material 
comprising the image area. The limiting temperature and time of the 
"burning-in" is that at which the aluminum anneals, resulting in a loss of 
strength required for a printing plate. 
U.K. Patent 669,412 discloses the burning-in of images based on 
naphthoquinone diazides. In accordance with the teachings of this patent, 
a radiation-sensitive plate including a layer of the diazide is image-wise 
exposed, developed with an alkaline solution to remove selectively those 
areas of the layer exposed to radiation, and then placed in an oven to 
heat the image constituted by those areas of the layer that were not 
exposed to radiation. Thereafter, it is necessary to treat the plate with 
an alkaline solution again in order to remove contaminating residues from 
the plate and make the plate ready for printing. 
In many cases, the image areas to be heated may be reinforced by 
incorporating reinforcing material in the radiation sensitive layer and/or 
by applying the reinforcing material in the form of a reinforcing lacquer 
to the image areas after development. Novolak resins and/or resol resins 
are examples of commonly used reinforcing materials. However, as disclosed 
in U.K. Patent 1,154,749, heating at a temperature sufficient to harden 
resin-reinforced image areas causes those areas of the substrate revealed 
on development to be at least partially covered with a contaminating layer 
which is ink accepting and which would therefore cause scumming and yield 
a soiled background during printing. This layer must therefore be removed 
before printing is initiated and this is achieved in accordance with the 
teachings of the patent by treating the plate with aqueous alkaline 
solution. 
U.S. Pat. No. 4,294,910 discloses the use of various aqueous compositions 
known as "gumming" or "pre-bake" solutions to avoid problems resulting 
from the burning-in process. Such solutions contain materials such as 
sodium dodecyl phenoxy benzene disulphonate, alkylated naphthalene 
sulphonic acid, sulphonated alkyl diphenyl oxide, methylene dinaphthalene 
sulphonic acid, etc. 
U.S. Pat. No. 4,786,581 discloses the use of "gumming" solutions for 
protecting plates during the burning-in process; these aqueous solutions 
contain a hydrophilic polymer component and an organic acid component. The 
organic acid component (or water-soluble salt thereof) contains di- or 
greater acid functionality and encompasses the benzene carboxylic acids, 
sulphonic acids and phosphonic acids including alkane phosphonic acids. In 
contradistinction to the materials recited in the '581 patent, the present 
invention does not require the use of a hydrophilic polymer. Furthermore, 
the diphosphonic acid (present as the di-potassium salt) employed in the 
present invention provides superior results in protecting the surface of 
aluminum plates to be burned-in. Such superior protectant properties would 
be related to the ability of the di-potassium salt of the 
hydroxyethylidene diphosphonic acid to complex with the aluminum oxide 
surface of the printing plate, whereas the alkane phosphonic acids 
disclosed in the '581 patent do not possess such a property. 
The contaminating layer produced as the result of the burning-in process is 
not, as a rule, discernible to the naked eye and it is difficult to ensure 
that all the contamination has been removed. Moreover, in the case of 
those substrate surfaces that are porous, as is the case of an anodized 
aluminum plate, contamination may be present in the pores. Such 
contamination is likely to cause scumming during long printing runs as the 
substrate surface is gradually worn away. The alternative of redeveloping 
the plates after burning-in in order to remove the contaminating layer is 
costly and inconvenient since the plates have to be returned to the plate 
fabrication facility after they have been removed from the oven. 
In view of the difficulties associated with the removal of the 
contamination which is produced by the burning-in procedure, it is 
desirable to prevent such contamination from occurring in the first 
instance. It has been found that the contamination apparently arises as a 
result of some component of the image material subliming from the image 
areas during heating and subsequently being redeposited on the areas of 
the substrate revealed on development. Even plates that contain no 
substances that could generate contamination during heating nevertheless 
become contaminated by deposition of contaminating material previously 
deposited on the internal surfaces of the burning-in oven as a result of 
prior usage. 
DETAILS OF THE PRESENT INVENTION 
The present invention involves the use of an aqueous composition which is 
applied to the surface of the developed plate prior to burning-in. The 
composition acts as a protectant in preventing contamination from 
occurring during burning-in and thereby obviates the necessity for 
redeveloping the plate. The potassium salt form of the hydroxyethylidene 
diphosphonic acid is utilized since other cationic forms of this acid 
(e.g. the sodium salt form) are not sufficiently soluble to provide the 
desired concentration in water. Generally, the di-potassium salt of 
hydroxyethylidene diphosphonic acid will be present in the solution in a 
concentration of 1 to 20 wt. %, preferably 2 to 10 wt. %. 
The pH of the aqueous composition containing the di-potassium salt is 
generally in the range of 4 to 7, preferably 5 to 6. At a pH of about 5.5, 
the tri-potassium salt form of the acid commences being formed in the 
solution, and the concentration of the tri-potassium salt form increases 
to a maximum as the pH is raised. For the purposes of this invention, the 
tri-potassium salt form may be present in an amount of 0 to 15 wt. %, 
preferably 0 to 5 wt. %. 
Preferably, the composition also includes a salt which is selected from the 
group consisting of stannous salts, zinc salts and mixtures thereof. The 
preferred salts are the halides such as the chlorides and mixtures of the 
stannous and zinc salts are particularly preferred. The content of these 
salts may be 1 to 7 wt. %, preferably 1 to 3 wt. %. When a mixture of the 
salts is employed, the ratio of stannous salt to zinc salt may be in the 
range of 12:1 to 0.2:1, preferably 3:1 to 1:1, parts by weight. 
The composition desirably also includes an anionic surfactant that contains 
functionalities of a salt of a strong organic acid and a salt of a weak 
organic acid. The surfactant may be present in an amount of 3 to 11 wt. %, 
preferably 5 to 9 wt. %. Examples of suitable surfactants of anionic and 
other types include tetrasodium N-(1,2-dicarboxyethyl)-N- octadecyl 
sulfosuccinamate (which is preferred), the tetrapotassium salt form of the 
foregoing, disodium cocoamphocarboxypropionate, sodium 
cocoamphopropylsulfonate, sodium cocoamphoglycinate, alkyl (50% C.sub.14, 
40% C.sub.12, 10% C.sub.16) dimethylbenzyl-ammonium chloride, sodium 
N-methyl-N-cocoyltaurate, modified soya dimethylethylammonium ethosulfate, 
cocamidopropylhydroxysultaine, disodium dicarboxyethyl 
phosphoethylimidazoline, and the like. 
It is also desirable to include a salt of citric acid, e.g. di-potassium 
citrate and/or tri-potassium citrate in the aqueous composition in an 
amount of 1 to 9 wt. %, preferably 2 to 6 wt. %. 
The aqueous composition desirably also contains one or more polyhydroxy 
compounds containing 3 to 10 carbon compounds; preferred examples of the 
polyhydroxy compounds include sorbitol and glycerol and mixtures thereof. 
Sorbitol may be present in amounts of 1 to 15 wt. %, preferably 3 to 7 wt. 
%, while glycerol may be present in amounts of up to about 3 wt. %, 
preferably 0.1 to 1.0 wt. %; where sorbitol and glycerol are both present, 
the ratio of sorbitol to glycerol will be in the range of 0.3:1 to 150:1, 
preferably 3.5:1 to 70.1. The use of the polyhydroxy compound permits the 
quantity of the surfactant to be drastically reduced, i.e. to 0.04 to 8.0 
wt. %, preferably 0.08 to 0.8 wt. %. 
The components of the aqueous composition should be such that they do not 
volatilize at the burning-in temperature, and are selected according to 
the nature of the substrate, radiation-sensitive layer, and the purpose 
for which the plate is to be used. For example, in the case where a 
lithographic printing plate is being produced, the selected component must 
not only be an effective physical barrier, but it must also be such that 
it does not deleteriously affect the image (e.g. by dissolving it or by 
rendering it non-ink receptive). Further, it must be readily removable 
after the burning-in process without the image and non-image areas being 
adversely affected. 
The foregoing examples of components are particularly useful in protecting 
lithographic printing plates from contamination occurring as a result of 
the burning-in process. The compositions of the present invention are 
easily applied to the surface of the developed plate by conventional 
methods, e.g. spraying, dipping, brushing, etc. and provide the desired 
protective shield. After burning-in has been completed, the shield can be 
readily removed from the plate merely by wiping the plate with water. This 
removal step can be effected while the plate is actually mounted on the 
printing press, and thus the plate need not be returned to the 
plate-making facility after completion of the burning-in procedure. 
Rinsing with water to remove the shield and desensitizing with, e.g. gum 
arabic, is all that is then required to make the plate ready for printing. 
The aqueous compositions of the present invention are also quite versatile 
in that they may also be employed as preparation and storage gums where 
the burning-in process is not desired. When used as a preparation gum, the 
composition is coated onto the plate after development; the composition 
acts to improve the hydrophilic properties of the aluminum oxide-coated 
base. When used as a storage gum, the composition is coated onto the 
developed plate, and the thus-coated plate may be stored for extended 
periods of time, since the adsorbent aluminum oxide surface is protected 
from contamination. Even after prolonged storage, the plates may be taken 
directly to the press and they produce good copies without any further 
preparation. 
The following examples serve to illustrate the invention (unless otherwise 
indicated, all percentages are by weight):

EXAMPLE 1 
Using agitation, 527 g 87.1% potassium hydroxide were dissolved in 3 kg 
deionized water. After cooling, 1.301 kg of a 60% solution of 
1-hydroxyethylidene-1,1-diphosphonic acid were added and rinsed in with 
192 g deionized water. Thereafter 76 g anhydrous stannous chloride were 
added and, after dissolution, 45 g anhydrous zinc chloride were added. The 
solution was cooled and thereafter a solution of 95 g 87.1% potassium 
hydroxide in 250 g deionized water was added and rinsed in with 25 g 
deionized water. To the resultant mixture were added 44 g 30% hydrogen 
peroxide and rinsed in with 25 g deionized water. Subsequently, a solution 
of 239 g 87.1% potassium hydroxide in 775 g deionized water was added and, 
with cooling, 310 g citric acid monohydrate. Afterwards, 2.233 kg of a 35% 
solution of tetrasodium N-(1,2-dicarboxyethyl)-N-octadecyl succinamate 
were added and rinsed in with 500 g deionized water. Finally, the 
concentration was adjusted by the addition of 2 kg deionized water and the 
solution was filtered. The resultant solution had a solids content of 
22.2% and had a pH of 5.9. 
EXAMPLE 2 
Using agitation, 527 g 87.1% potassium hydroxide were dissolved in 3 kg of 
deionized water. After cooling, 1.301 kg 
1-hydroxyethylidene-1,1-diphosphonic acid (60% solution) were added and 
rinsed in with 192 g deionized water. To this solution, 76 g anhydrous 
stannous chloride were added and, after dissolution. 45 g anhydrous zinc 
chloride. The solution was cooled and thereafter a solution of 95 g 87.1% 
potassium hydroxide in 250 g deionized water were added and rinsed in with 
25 g deionized water. To the solution were added 44 g 30% hydrogen 
peroxide, rinsing in with 25 g deionized water. Subsequently, 1.485 kg of 
the succinamate of example 1 were added and rinsed in with 245 g deionized 
water. The concentration was adjusted with 1.430 kg deionized water, and 
the solution was filtered, thereby resulting in a final solution 
containing 21.4% solids and having a pH of 5.8. 
EXAMPLE 3 
Using agitation, 4.047 kg 87.1% aqueous potassium hydroxide were dissolved 
in 30 kg deionized water. After cooling, 9.949 kg of a 60% solution of 
1-hydroxy-ethylidene-1,1-diphosphonic acid were added and rinsed in with 
1.0 kg deionized water. Thereafter, 1.159 kg anhydrous stannous chloride 
were added, and after complete dissolution, 821 g 87.1% potassium 
hydroxide in 2.753 kg deionized water added and rinsed in with 50 g 
deionized water. To this solution were added 5.973 kg 70% sorbitol and 500 
g of deionized water as a rinse. In addition, 335 g 99% glycerol in 500 g 
deionized water were rinsed in with 500 g deionized water. Subsequently, 
70 g of an 80% solution of C.sub.14 -C.sub.16 alkyl dimethylbenzyl 
ammonium chloride dissolved in 300 g deionized water were added and rinsed 
in with 200 g deionized water. Finally, the concentration was adjusted 
with 1.24 kg deionized water, resulting in a solution containing 21.3% 
solids and having a pH of 5.6. 
EXAMPLE 4 
Using agitation, 628 g 87.1% aqueous potassium hydroxide were dissolved in 
5 kg deionized water. To this solution were added 1.558 kg 
1-hydroxyethylidene-1,1-diphosphonic acid (60% solution) and an additional 
160 g deionized water for rinse purposes. The solution was cooled and 532 
g 70% sorbitol were added, followed by a solution of 56 g glycerol in 58 g 
deionized water, using a total of 200 g deionized water to rinse in the 
components. Thereafter, 11 g of a solution of 80% of C.sub.14 -C.sub.16 
alkyl dimethylbenzyl ammonium chloride in 100 g deionized water was added 
and rinsed in with 90 g deionized water. Finally, the solution was diluted 
with 355 g deionized water to result in a solution containing 20.3% solids 
and having a pH of 5.4. 
EXAMPLE 5 
Positive working plates were hand developed in the usual manner. Subsequent 
to development, and while still damp, the plates were treated with the 
solutions of each of Examples 1-4, except for the control plate which was 
not treated, by applying a few milliliters of solution to the plate 
surface, followed by rubbing-in with a cloth. The wet plates were then 
buffed to near dryness and then air dried. Based upon the weight gain, it 
was determined that the weight of solution that was applied to the plate 
surface was 4.0.+-.0.5 g/m.sup.2. The plates were then baked in an oven 
(Wisconsin: type SPC-II30) having a conveyor belt, convection heat and 
forced air, at the temperatures and for the periods of time indicated in 
Table I below. The baked plates were then washed down with water and then 
examined. This first examination was for the purpose of determining 
whether there was any burning-through or penetration to the base of the 
plate to produce pinholes, the plate surface was also examined to 
determine whether there was any residue thereon. The plates were then 
inked by rubbing their surfaces with a cloth containing ink and water and 
again examined. This second examination was conducted to determine whether 
there was any ink take-up by the base in the exposed areas of the plate, 
the preciseness of inking of the image. (i.e. no ink build-up or loss of 
fine detail) and correctness of inking of the unexposed areas of the test 
pattern (i.e. the oleophilicity of the plate surface should not have been 
affected by the baking step). 
TABLE I 
__________________________________________________________________________ 
232.degree. C. 
246.degree. C. 
260.degree. C. 
260.degree. C. 
274.degree. C. 
274.degree. C. 
288.degree. C. 
288.degree. C. 
Solution 
3.5 min. 
3.5 min. 
2.5 min. 
3.5 min. 
1.5 min. 
3.5 min. 
1.5 min. 
3.0 min. 
__________________________________________________________________________ 
Ex 1 Pass Pass Pass Pass Pass Pass Pass Pass 
Ex 2 Pass Pass Pass Pass Pass Pass Pass Pass 
Ex 3 Pass Pass Pass Pass Pass Pass Pass Pass 
Ex 4 Pass Pass Pass Pass Fail Fail Fail Fail 
Control 
Fail Fail Fail Fail Fail Fail Fail Fail 
__________________________________________________________________________ 
The results set forth in Table I above show that the solutions of Examples 
1 to 4 provide excellent protection against the effects of baking. In 
particular, it is noted that the control (i.e. the plates that were not 
treated prior to baking) failed the second examination after the plates 
were baked at the indicated temperatures and periods of time. In contrast 
thereto, the plates treated with the solutions of Examples 1 to 4 passed 
both examinations after baking at all the indicated temperatures and 
periods of time. The plates treated with the solution of Example 4 failed 
only at the higher temperatures, thus indicating the desirability of 
incorporating a stannous salt (Example 3) or a mixture of a stannous salt 
and a zinc salt (Examples 1 and 2).