U.S. patent application number 10/612211 was filed with the patent office on 2005-11-24 for positive-working lithographic printing plate precursor.
This patent application is currently assigned to AGFA-GEVAERT. Invention is credited to Van Damme, Marc, Vermeersch, Joan.
Application Number | 20050260934 10/612211 |
Document ID | / |
Family ID | 35375802 |
Filed Date | 2005-11-24 |
United States Patent
Application |
20050260934 |
Kind Code |
A1 |
Van Damme, Marc ; et
al. |
November 24, 2005 |
Positive-working lithographic printing plate precursor
Abstract
A positive-working lithographic printing plate precursor is
disclosed which comprises (i) a grained and anodized aluminum
support having a hydrophilic surface and (ii) a heat-sensitive
oleophilic coating provided on the hydrophilic surface, wherein
said coating is capable of dissolving in an aqueous alkaline
developer at a higher dissolution rate in areas of said coating
which are exposed to heat or infrared light than in unexposed
areas, characterized in that the hydrophilic surface has a surface
roughness, expressed as arithmetical mean center-line roughness Ra,
which is less than 0.40 .mu.m. The use of a low surface roughness
as defined above provides an improved shelf life of the
coating.
Inventors: |
Van Damme, Marc; (Bonheiden,
BE) ; Vermeersch, Joan; (Deinze, BE) |
Correspondence
Address: |
LEYDIG VOIT & MAYER, LTD
TWO PRUDENTIAL PLAZA, SUITE 4900
180 NORTH STETSON AVENUE
CHICAGO
IL
60601-6780
US
|
Assignee: |
AGFA-GEVAERT
Mortsel
BE
|
Family ID: |
35375802 |
Appl. No.: |
10/612211 |
Filed: |
July 2, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60395831 |
Jul 15, 2002 |
|
|
|
Current U.S.
Class: |
451/49 |
Current CPC
Class: |
B41N 3/034 20130101 |
Class at
Publication: |
451/049 |
International
Class: |
B24B 005/04 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 3, 2002 |
EP |
02100778.6 |
Claims
1. A positive-working lithographic printing plate precursor
comprising (i) a grained and anodized aluminum support having a
hydrophilic surface and (ii) a heat-sensitive oleophilic coating
provided on the hydrophilic surface, wherein said coating is
capable of dissolving in an aqueous alkaline developer at a higher
dissolution rate in areas of said coating which are exposed to heat
or infrared light than in unexposed areas, characterized in that
the hydrophilic surface has a surface roughness, expressed as
arithmetical mean center-line roughness Ra, which is less than 0.40
.mu.m and comprises more than 3.0 g/m.sup.2 of aluminum oxide.
2. A plate precursor according to claim 1 wherein the hydrophilic
surface has a surface roughness, expressed as arithmetical mean
center-line roughness Ra, which is less than 0.3 .mu.m.
3. A plate precursor according to claim 1 wherein the aluminum
support comprises more than 4.0 g/m.sup.2 of aluminum oxide at the
hydrophilic surface.
4. A plate precursor according to claim 1 wherein the coating
comprises (a) a hydrophobic polymer which is soluble in the
developer and (b) a dissolution inhibitor.
5. A plate precursor according to claim 4 wherein the dissolution
inhibitor is a water-repellent polymer.
6. A plate precursor according to claim 5 wherein the
water-repellent polymer is (a) a polymer comprising siloxane and/or
perfluoroalkyl units or (b) a block- or graft-copolymer of a
poly(alkylene oxide) block and a block comprising siloxane and/or
perfluoroalkyl units.
7. A plate precursor according to claim 4 wherein the dissolution
inhibitor is an organic compound comprising an aromatic group and a
hydrogen bonding site.
8. A plate precursor according to claim 1 wherein the coating
further comprises a dissolution accelerator.
9. The plate precursor according to claim 2 wherein the aluminum
support comprises more than 4.0 g/m.sup.2 of aluminum oxide at the
hydrophilic surface.
10. The plate precursor according to claim 2 wherein the coating
comprises (a) a hydrophobic polymer which is soluble in the
developer and (b) a dissolution inhibitor.
11. The plate precursor according to claim 4 wherein the coating
further comprises a dissolution accelerator.
12. The plate precursor according to claim 3 wherein the coating
comprises (a) a hydrophobic polymer which is soluble in the
developer and (b) a dissolution inhibitor.
13. The plate precursor according to claim 6 wherein the coating
further comprises a dissolution accelerator.
14. The plate precursor according to claim 7 wherein the coating
further comprises a dissolution accelerator.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a heat-sensitive
positive-working lithographic printing plate precursor that
requires aqueous alkaline processing.
BACKGROUND OF THE INVENTION
[0002] Lithographic printing presses use a so-called printing
master such as a printing plate which is mounted on a cylinder of
the printing press. The master carries a lithographic image on its
surface and a print is obtained by applying ink to said image and
then transferring the ink from the master onto a receiver material,
which is typically paper. In conventional lithographic printing,
ink as well as an aqueous fountain solution (also called dampening
liquid) are supplied to the lithographic image which consists of
oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling)
areas as well as hydrophilic (or oleophobic, i.e. water-accepting,
ink-repelling) areas. In so-called driographic printing, the
lithographic image consists of ink-accepting and ink-adhesive
(ink-repelling) areas and during driographic printing, only ink is
supplied to the master.
[0003] Printing masters are generally obtained by the image-wise
exposure and processing of an imaging material called plate
precursor. A typical positive-working plate precursor comprises a
hydrophilic support and an oleophilic coating which is not readily
soluble in an aqueous alkaline developer in the non-exposed state
and becomes soluble in the developer after exposure to radiation.
In addition to the well known photosensitive imaging materials
which are suitable for UV contact exposure through a film mask (the
so-called pre-sensitized plates), also heat-sensitive printing
plate precursors have become very popular. Such thermal materials
offer the advantage of daylight stability and are especially used
in the so-called computer-to-plate method wherein the plate
precursor is directly exposed, i.e. without the use of a film mask.
The material is exposed to heat or to infrared light and the
generated heat triggers a (physico-)chemical process, such as
ablation, polymerization, insolubilization by cross-linking of a
polymer, heat-induced solubilization, or particle coagulation of a
thermoplastic polymer latex. Examples of such thermal plates,
wherein also the role of surface roughness of the aluminum support
is discussed, are disclosed in U.S. Pat. No. 6,242,156 and EP-A 884
647.
[0004] A specific problem associated with thermal plate precursors
is the limited shelf life of the coating. Especially when stored at
elevated temperature, which is inevitable during transport by
truck, boat, etc. the plates may show significant toning
(ink-acceptance in the non-image areas).
SUMMARY OF THE INVENTION
[0005] It is an aspect of the present invention to provide a
positive-working thermal lithographic printing plate precursor with
improved shelf life. This object is realized by the material of
claim 1, having the characterizing feature that a low surface
roughness of the grained and anodized aluminum support unexpectedly
provides an improved shelf life of the material. Specific
embodiments of the invention are defined in the dependent
claims.
DETAILED DESCRIPTION OF THE INVENTION
[0006] The support of the plate precursor of the present invention
is a grained and anodized aluminum support having a hydrophilic
surface that is characterized by a low surface roughness, expressed
as arithmetical mean center-line roughness (Ra), sometimes also
referred to as CLA (center-line average). Ra as used herein is
defined in ISO 4287/1 (=DIN 4762) and references therein. Ra values
reported herein have been measured according to ISO 4288 and
references therein by a mechanical profile method using a contact
stylus with a very thin tip (also optical profile methods are
known; such optical methods systematically provide higher values
than the ISO method). The apparatus used for measuring Ra was a
Talysurf 10 from Taylor Hobson Ltd.
[0007] The Ra value of the hydrophilic surface of the grained and
anodized aluminum support used in the material of the present
invention is lower than 0.40 .mu.m, preferably lower than 0.3 .mu.m
and even more preferably lower than 0.25 .mu.m. A grained and
anodized s aluminum support having a hydrophilic surface
characterized by the mentioned low Ra values is briefly referred to
herein as a "smooth support". The lower limit of the Ra value may
be 0.05 .mu.m, preferably 0.1 .mu.M.
[0008] Graining and anodizing of aluminum lithographic supports is
well known. The grained aluminum support used in the material of
the present invention is preferably an electrochemically grained
support. The acid used for graining can be e.g. nitric acid. The
acid used for graining preferably comprises hydrogen chloride. Also
mixtures of e.g. hydrogen chloride and acetic acid can be used.
[0009] The relation between electrochemical graining and anodizing
parameters such as electrode voltage, nature and concentration of
the acid electrolyte or power consumption on the one hand and the
obtained lithographic quality in terms of Ra and anodic weight
(g/m.sup.2 of Al.sub.2O.sub.3 formed on the aluminum surface) on
the other hand is well known. More details about the relation
between various production parameters and Ra or anodic weight can
be found in e.g. the article "Management of Change in the Aluminium
Printing Industry" by F. R. Mayers, to be published in the ATB
Metallurgie Journal. So the skilled person is well aware of the
settings of the various parameters which are required for making a
smooth surface on a grained aluminum support or for making a given
anodic weight during aluminum anodization. According to the present
invention, further improvements can be obtained for a given
roughness Ra by forming more than 3.0 g/m.sup.2 of aluminum oxide
at the hydrophilic surface, a value above 4.0 g/m.sup.2 being even
more preferred.
[0010] The grained and anodized aluminum support may be
post-treated to improve the hydrophilic properties of its surface.
For example, the aluminum support may be silicated by treating its
surface with a sodium silicate solution at elevated temperature,
e.g. 95.degree. C. Alternatively, a phosphate treatment may be
applied which involves treating the aluminum oxide surface with a
phosphate solution that may further contain an inorganic fluoride.
Further, the aluminum oxide surface may be rinsed with an organic
acid and/or salt thereof, e.g. carboxylic acids, hydroxycarboxylic
acids, sulfonic acids or phosphonic acids, or their salts, e.g.
succinates, phosphates, phosphonates, sulfates, and sulfonates. A
citric acid or citrate solution is preferred. This treatment may be
carried out at room temperature or may be carried out at a slightly
elevated temperature of about 30 to 50.degree. C. A further
post-treatment involves rinsing the aluminum oxide surface with a
bicarbonate solution. Still further, the aluminum oxide, surface
may be treated with polyvinylphosphonic acid,
polyvinylmethylphosphonic acid, phosphoric acid esters of polyvinyl
alcohol, polyvinylsulfonic acid, polyvinylbenzenesulfonic acid,
sulfuric acid esters of polyvinyl alcohol, and acetals of polyvinyl
alcohols formed by reaction with a sulfonated aliphatic aldehyde.
It is further evident that one or more of these post-treatments may
be carried out alone or in combination. More detailed descriptions
of these treatments are given in GB-A-1 084 070, DE-A-4 423 140,
DE-A-4 417 907, EP-A-659 909, EP-A-537 633, DE-A-4 001 466,
EP-A-292 801, EP-A-291 760 and U.S. Pat. No. 4,458,005.
[0011] The coating provided on the support is heat-sensitive,
thereby providing a plate precursor which can be handled in normal
working lighting conditions (daylight, fluorescent light) for
several hours. The coating preferably does not contain UV-sensitive
compounds which have an absorption maximum in the wavelength range
of 200 nm to 400 nm such as diazo compounds, photoacids,
photoinitiators, quinone diazides, or sensitizers. Preferably the
coating neither contains compounds which have an absorption maximum
in the blue and green visible light wavelength range between 400
and 600 nm.
[0012] The coating may comprise one or more distinct layers.
Besides the layers discussed hereafter, the coating may further
comprise e.g. a "subbing" layer which improves the adhesion of the
coating to the support, a covering layer which protects the coating
against contamination or mechanical damage, and/or a light-to-heat
conversion layer which comprises an infrared light absorbing
compound.
[0013] The coating is positive-working and capable of heat-induced
solubilization, i.e. the coating is resistant to the developer and
ink-accepting in the non-exposed state and becomes soluble in the
developer upon exposure to heat or infrared light to such an extent
that the hydrophilic surface of the support is revealed
thereby.
[0014] Preferably, the coating comprises a hydrophobic polymer that
is soluble in an aqueous alkaline developer. Preferred polymers are
phenolic resins, e.g. novolac, resoles, polyvinyl phenols and
carboxy-substituted polymers. Typical examples of such polymers are
described in DE-A-4007428, DE-A-4027301 and DE-A-4445820. In
addition, the coating may comprise polymers which improve the run
length and/or the chemical resistance of the plate. Examples
thereof are polymers comprising sulfonamido (--SO.sub.2--NR--) or
imido (--CO--NR--CO--) pendant groups, wherein R is hydrogen,
optionally substituted alkyl or optionally substituted aryl, such
as the polymers described in EP-A 894622, 901902, 933682 and
WO99/63407.
[0015] In a preferred embodiment, the coating also contains one or
more dissolution inhibitors, i.e. one or more materials which
reduce the dissolution rate of the hydrophobic polymer in the
aqueous alkaline developer at the non-exposed areas of the coating.
The dissolution inhibiting capability of the inhibitor can easily
be tested by coating two samples on a support: a reference sample
containing only the hydrophobic polymer and another including both
the polymer (in equal amounts as the reference) as well as the
inhibitor. A series of unexposed samples is immersed in an aqueous
alkaline developer, each sample during a different time period.
After the immersion period, the sample is removed from the
developer, immediately rinsed with water, dried and then the
dissolution of the coating in the developer is measured by
comparing the weight of the sample before and after the
development. As soon as the coating is dissolved completely, no
more weight loss is measured upon longer immersion time periods,
i.e. a curve representing weight loss as a function of immersion
time reaches a plateau from the moment of complete dissolution of
the layer. A material has good inhibiting capability when the
coating of the sample without the inhibitor has dissolved
completely in the developer before the sample with the inhibitor is
attacked by the developer to such an extent that the ink-accepting
capability of the coating is affected.
[0016] The dissolution inhibitor(s) can be added to the layer which
comprises the alkali-soluble hydrophobic polymer discussed above.
In this embodiment, the dissolution rate of the non-exposed coating
in the developer is reduced by interaction between the hydrophobic
polymer and the inhibitor, due to e.g. hydrogen bonding between
these compounds. The dissolution inhibiting capability of the
inhibitor is preferably reduced or destroyed by the heat generated
during the exposure so that the coating readily dissolves in the
developer at exposed areas. Such inhibitors are preferably organic
compounds which comprise at least one aromatic group and a hydrogen
bonding site, e.g. a carbonyl group, a sulfonyl group, or a
nitrogen atom which may be quaternized and which may be part of a
heterocyclic ring or which may be part of an amino substituent of
said organic compound. Suitable dissolution inhibitors of this type
have been disclosed in e.g. EP-A 825927 and 823327. Some of the
compounds mentioned below, e.g. infrared dyes such as cyanines and
contrast dyes such as quaternized triarylmethane dyes can also act
as a dissolution inhibitor.
[0017] Water-repellent polymers represent a second type of suitable
dissolution inhibitors. Such polymers seem to increase the
developer resistance of the coating by repelling the aqueous
developer from the coating. The water-repellent polymers can be
added to the layer comprising the hydrophobic polymer and/or can be
present in a separate layer provided on top of the layer with the
hydrophobic polymer. In the latter embodiment, the water-repellent
polymer forms a barrier layer which shields the coating from the
developer and the solubility of the barrier layer in the developer
or the penetrability of the barrier layer by the developer can be
reduced by exposure to heat or infrared light, as described in e.g.
EP-A 864420, EP-A 950517 and WO99/21725. Preferred examples of the
water-repellent polymers are polymers comprising siloxane and/or
perfluoroalkyl units. In one embodiment, the coating contains such
a water-repellent polymer in an amount between 0.5 and 25
mg/m.sup.2, preferably between 0.5 and 15 mg/m.sup.2 and most
preferably between 0.5 and 10 mg/m.sup.2. When the water-repellent
polymer is also ink-repelling, e.g. in the case of polysiloxanes,
higher amounts than 25 mg/m.sup.2 can result in poor ink-acceptance
of the non-exposed areas. An amount lower than 0.5 mg/m.sup.2 on
the other hand may lead to an unsatisfactory development
resistance. The polysiloxane may be a linear, cyclic or complex
cross-linked polymer or copolymer. The term polysiloxane compound
shall include any compound which contains more than one siloxane
group --Si(R,R')--O--, wherein R and R' are optionally substituted
alkyl or aryl groups. Preferred siloxanes are phenylalkylsiloxanes
and dialkylsiloxanes. The number of siloxane groups in the
(co)polymer is at least 2, preferably at least 10, more preferably
at least 20. It may be less than 100, preferably less than 60. In
another embodiment, the water-repellent polymer is a
block-copolymer or a graft-copolymer of a poly(alkylene oxide)
block and a block of a polymer comprising siloxane and/or
perfluoroalkyl units. A suitable copolymer comprises about 15 to 25
siloxane units and 50 to 70 alkylene oxide groups. Preferred
examples include copolymers comprising phenylmethylsiloxane and/or
dimethylsiloxane as well as ethylene oxide and/or propylene oxide,
such as Tego Glide 410, Tego Wet 265, Tego Protect 5001 or
Silikophen P50/X, all commercially available from Tego Chemie,
Essen, Germany. Such a copolymer acts as a surfactant which upon
coating, due to its bifunctional structure, automatically positions
itself at the interface between the coating and air and thereby
forms a separate top layer even when the whole coating is applied
from a single coating solution. Simultaneously, such surfactants
act as a spreading agent which improves the coating quality.
Alternatively, the water-repellent polymer can be applied in a
second solution, coated on top of the layer comprising the
hydrophobic polymer. In that embodiment, it may be advantageous to
use a solvent in the second coating solution that is not capable of
dissolving the ingredients present in the first layer so that a
highly concentrated water-repellent phase is obtained at the top of
the coating.
[0018] Preferably, also one or more development accelerators are
included in the coating, i.e. compounds which act as dissolution
promoters because they are capable of increasing the dissolution
rate of the non-exposed coating in the developer, which can be
tested by the same procedure as described above in relation to
dissolution inhibitors. The simultaneous application of dissolution
inhibitors and accelerators allows a precise fine tuning of the
dissolution behavior of the coating. Suitable dissolution
accelerators are cyclic acid anhydrides, phenols or organic acids.
Examples of the cyclic acid anhydride include phthalic anhydride,
tetrahydrophthalic anhydride, hexahydrophthalic anhydride,
3,6-endoxy-4-tetrahydrophthalic anhydride, tetrachlorophthalic
anhydride, maleic anhydride, chloromaleic anhydride,
alpha-phenylmaleic anhydride, succinic anhydride, and pyromellitic
anhydride, as described in U.S. Pat. No. 4,115,128. Examples of the
phenols include bisphenol A, p-nitrophenol, p-ethoxyphenol,
2,4,4'-trihydroxybenzophenone, 2,3,4-trihydroxy-benzophenone,
4-hydroxybenzophenone, 4,4',4"-trihydroxy-triphenylmethane, and
4,4',3",4"-tetrahydroxy-3,5,3',5- '-tetramethyltriphenyl-methane,
and the like. Examples of the organic acids include sulfonic acids,
sulfinic acids, alkylsulfuric acids, phosphonic acids, phosphates,
and carboxylic acids, as described in, for example, JP-A Nos.
60-88,942 and 2-96,755. Specific examples of these organic acids
include p-toluenesulfonic acid, dodecylbenzenesulfonic acid,
p-toluenesulfinic acid, ethylsulfuric acid, phenylphosphonic acid,
phenylphosphinic acid, phenyl phosphate, diphenyl phosphate,
benzoic acid, isophthalic acid, adipic acid, p-toluic acid,
3,4-dimethoxybenzoic acid, phthalic acid, terephthalic acid,
4-cyclohexene-1,2-dicarboxylic acid, erucic acid, lauric acid,
n-undecanoic acid, and ascorbic acid. The amount of the cyclic acid
anhydride, phenol, or organic acid contained in the coating is
preferably in the range of 0.05 to 20% by weight, relative to the
coating as a whole.
[0019] The material can be image-wise exposed directly with heat,
e.g. by means of a thermal head, or indirectly by infrared light,
which is preferably converted into heat by an infrared light
absorbing compound, which may be a dye or pigment having an
absorption maximum in the infrared wavelength range. The
concentration of the sensitizing dye or pigment in the coating is
typically between 0.25 and 10.0 wt. %, more preferably between 0.5
and 7.5 wt. % relative to the coating as a whole. Preferred
IR-absorbing compounds are dyes such as cyanine or merocyanine dyes
or pigments such as carbon black. A suitable compound is the
following infrared dye: 1
[0020] The coating may further contain an organic dye which absorbs
visible light so that a perceptible image is obtained upon
image-wise exposure and subsequent development. Such a dye is often
called contrast dye or indicator dye. Preferably, the dye has a
blue color and an absorption maximum in the wavelength range
between 600 nm and 750 nm. Although the dye absorbs visible light,
it preferably does not sensitize the printing plate precursor, i.e.
the coating does not become more soluble in the developer upon
exposure to visible light. Suitable examples of such a contrast dye
are the quaternized triarylmethane dyes.
[0021] The infrared light absorbing compound and the contrast dye
may be present in the layer comprising the hydrophobic polymer,
and/or in the barrier layer discussed above and/or in an optional
other layer. According to a highly preferred embodiment, the
infrared light absorbing compound is concentrated in or near the
barrier layer, e.g. in an intermediate layer between the layer
comprising the hydrophobic polymer and the barrier layer.
[0022] The printing plate precursor of the present invention can be
exposed to infrared light with LEDs or a laser. Preferably, a laser
emitting near infrared light having a wavelength in the range from
about 750 to about 1500 nm is used, such as a semiconductor laser
diode, a Nd:YAG or a Nd:YLF laser. The required laser power depends
on the sensitivity of the image-recording layer, the pixel dwell
time of the laser beam, which is determined by the spot diameter
(typical value of modern plate-setters at 1/e.sup.2 of maximum
intensity: 10-25 .mu.m), the scan speed and the resolution of the
exposure apparatus (i.e. the number of addressable pixels per unit
of linear distance, often expressed in dots per inch or dpi;
typical value: 1000-4000 dpi).
[0023] Two types of laser-exposure apparatuses are commonly used:
internal (ITD) and external drum (XTD) plate-setters. ITD
plate-setters for thermal plates are typically characterized by a
very high scan speed up to 500 m/sec and may require a laser power
of several Watts. XTD plate-setters for thermal plates having a
typical laser power from about 200 mW to about 1 W operate at a
lower scan speed, e.g. from 0.1 to 10 m/sec.
[0024] The known plate-setters can be used as an off-press exposure
apparatus, which offers the benefit of reduced press down-time. XTD
plate-setter configurations can also be used for on-press exposure,
offering the benefit of immediate registration in a multi-color
press. More technical details of on-press exposure apparatuses are
described in e.g. U.S. Pat. No. 5,174,205 and U.S. Pat. No.
5,163,368.
[0025] In the development step, the non-image areas of the coating
are removed by immersion in an aqueous alkaline developer, which
may be combined with mechanical rubbing, e.g. by a rotating brush.
The developer preferably has a pH above 10, more preferably above
12. The development step may be followed by a rinsing step, a
gumming step, a drying step and/or a post-baking step.
[0026] The printing plate thus obtained can be used for
conventional, so-called wet offset printing, in which ink and an
aqueous dampening liquid is supplied to the plate. Another suitable
printing method uses so-called single-fluid ink without a dampening
liquid. Single-fluid ink consists of an ink phase, also called the
hydrophobic or oleophilic phase, and a polar phase which replaces
the aqueous dampening liquid that is used in conventional wet
offset printing. Suitable examples of single-fluid inks have been
described in U.S. Pat. No. 4,045,232; U.S. Pat. No. 4,981,517 and
U.S. Pat. No. 6,140,392. In a most preferred embodiment, the
single-fluid ink comprises an ink phase and a polyol phase as
described in WO 00/32705.
EXAMPLES
Preparation of Lithographic Support 1 (Comparative Example)
[0027] A 0.30 mm thick aluminum foil was degreased by immersing the
foil in an aqueous solution containing 40 g/l of sodium hydroxide
at 60.degree. C. for 8 seconds and rinsed with demineralized water
for 2 seconds. The foil was then electrochemically grained using an
alternating current in an aqueous solution containing 12 g/l of
hydrochloric acid and 38 g/l of aluminum sulphate (18 hydrate) at a
temperature of 33.degree. C. and a current density of 130
A/dm.sup.2 to form a surface topography with an average center-line
roughness Ra of 0.5 .mu.m. After rinsing with demineralized water
for 2 seconds, the aluminum foil was then etched with an aqueous
solution containing 155 g/l of sulfuric acid at 70.degree. C. for 4
seconds and rinsed with demineralized water at 25.degree. C. for 2
seconds. The foil was subsequently subjected to anodic oxidation in
an aqueous solution containing 155 g/l of sulfuric acid at a
temperature of 45.degree. C., at a current density of 22 A/dm.sup.2
to form an anodic oxidation film of 2.90 g/m.sup.2 of
Al.sub.2O.sub.3, then washed with demineralized water for 2 seconds
and posttreated for 10 seconds with a solution containing 4 g/l
polyvinylphosphonic acid at 40.degree. C., rinsed with
demineralized water at 20.degree. C. during 2 seconds and
dried.
Preparation of the Lithographic Support 2 (Invention)
[0028] The same procedure was used as described for lithographic
support 1 with the proviso that the current density during graining
and anodizing was 90 A/dm.sup.2 and 30 A/dm.sup.2 respectively. As
a result, the Ra value was 0.2 .mu.m and the anodic weight was 4.0
g/m.sup.2.
Preparation of Printing Plate Precursor 1 and 2
[0029] The comparative printing plate precursor 1 and the printing
plate precursor 2 according to the invention were produced by
coating the solution defined in Table 1 onto the above described
lithographic substrates 1 and 2. The coating solution was applied
at a wet coating thickness of 26 .mu.m on a coating line operating
at a speed of 10.8 m/min and then dried at 135.degree. C.
1TABLE 1 composition of the coating solution Parts (grams)
Tetrahydrofuran 209.20 Alnovol SPN452 (1) 102.02 Dowanol PM (2)
332.13 Methyl ethyl ketone 266.20 S0094 (3) 2.10 1 wt. % solution
of Basonyl Blue 640 (4) in Dowanol PM 53.00 1 wt. % solution of
TegoGlide 410 (5) in Dowanol PM 8.50 1 wt. % solution of TegoWet
265 (5) in Dowanol PM 21.55 3,4,5-trimethoxy cinnamic acid 5.30 (1)
Alnovol SPN452 is a 40.5 wt. % solution of novolac in Dowanol PM
(commercially available from Clariant). (2) 1-methoxy-2-propanol
from Dow Chemical Company. (3) S0094 is an IR absorbing cyanine dye
commercially available from FEW Chemicals. S0094 has the chemical
structure IR-1 shown above. (4) Basonyl Blue 640 is a quaternized
triarylmethane dye commercially available from BASF. (5) TegoWet
265 and TegoGlide 410 are both block-co-polysiloxane/poly(alkylene
oxide) surfactants commercially available from Tego Chemie Service
GmbH.
EVALUATION AND RESULTS
[0030] One part of each printing plate precursor was packaged in an
open paper bag and stored during 7 days at ambient temperature.
Another part of the printing plate precursor was packaged in an
open bag and stored for 7 days at 50.degree. C.
[0031] The printing plate precursors were then exposed on a
CreoScitex Trendsetter 3244 operating at a drum rotation speed of
150 rpm and the energy on the plates was varied from 80 mJ/cm.sup.2
up to 200 mJ/cm.sup.2 with steps of 20 mJ/cm.sup.2. The plates were
then processed in an Agfa Autolith T processor operating at a speed
of 0.96 m/min using Agfa TD5000 developer at 25.degree. C. and
RC795 as gum. Only the plates obtained with the optimum exposure
energy were used in the evaluation, using a Heidelberg GTO52
printing press with K+E 800 Skinnex Black (commercially available
from BASF) as ink and ROTA-MATIC (commercially available from
Unigraphica GmbH) as fountain. When stored at ambient temperature,
both the printing plate precursor 1 and 2 had a very good
lithographic behavior and printed without toning. The plate
obtained from precursor 1 stored at elevated temperature showed
severe toning in the non-image areas and was a useless printing
plate. The plate which was obtained from the precursor 2 that had
been stored at elevated temperature printed without toning.
* * * * *