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-abhesive
(ink-repelling) areas and during driographic printing, only ink is supplied to the
master.
[0003] Printing masters are generally obtained by the so-called computer-to-film method
wherein various pre-press steps such as typeface selection, scanning, color separation,
screening, trapping, layout and imposition are accomplished digitally and each color
selection is transferred to graphic arts film using an image-setter. After processing,
the film can be used as a mask for the exposure of an imaging material called plate
precursor and after plate processing, a printing plate is obtained which can be used
as a master.
[0004] A typical printing plate precursor for computer-to-film methods comprise a hydrophilic
support and an image-recording layer of a photosensitive polymer layers which include
UV-sensitive diazo compounds, dichromate-sensitized hydrophilic colloids and a large
variety of synthetic photopolymers. Particularly diazo-sensitized systems are widely
used. Upon image-wise exposure, typically by means of a film mask in a UV contact
frame, the exposed image areas become insoluble and the unexposed areas remain soluble
in an aqueous alkaline developer. The plate is then processed with the developer to
remove the diazonium salt or diazo resin in the unexposed areas. So the exposed areas
define the image areas (printing areas) of the printing master, and such printing
plate precursors are therefore called 'negative-working'. Also positive-working materials,
wherein the exposed areas define the non-printing areas, are known, e.g. plates having
a novolac/naphtoquinone-diazide coating which dissolves in the developer only at exposed
areas.
[0005] In addition to the above photosensitive materials, 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, decomposition, or particle coagulation of a thermoplastic
polymer latex.
[0006] WO97/39894 and EP-A 823 327 describe positive-working heat-sensitive materials comprising
a hydrophilic support and a oleophilic coating provided thereon. The coating comprises
a phenolic resin and a dissolution inhibitor, i.e. a compound which reduces the solubility
of the phenolic resin in an aqueous alkaline developer. The interaction between the
inhibitor and the phenolic resin is disrupted by exposure to heat or infrared light
and, as a result, the exposed areas of the coating dissolve faster in the developer
than the non-exposed areas and a lithographic image consisting of hydrophilic (exposed)
and oleophilic (non-exposed) areas is obtained. In order to provide a larger solubility
differentiation between exposed and non-exposed areas, WO 99/21725, EP-A 864 420 and
EP-A 950 517 disclose the use of developer resistance means such as polysiloxane compounds
which are capable of preventing the aqueous alkaline developer from penetrating into
the phenolic resin layer. The increased developer resistance thus obtained can be
reduced by exposure to heat or infrared light and upon subsequent immersion in the
developer, a positive lithographic image is obtained.
[0007] The coating of the known printing plates contain a colorant, also called contrast
dye or indicator dye, in order to provide a visible image after image-wise exposure
and development. Such colorants remain in the coating at printing areas and are removed,
together with the coating, at non-printing areas. Most of such prior art materials
are characterized by a low sensitivity and therefore require a high power during exposure.
SUMMARY OF THE INVENTION
[0008] It is an aspect of the present invention to provide a highly sensitive thermal lithographic
printing plate precursor. This object is realized by the material of claim 1. Preferred
embodiments are defined in the dependent claims.
[0009] The colorants, that are used as indicator dyes in the prior art materials, are typically
organic molecules containing quaternary nitrogen atoms, or carbonyl (-CO-), sulfinyl
(-SO-) or sulfonyl (-SO
2-) groups. Such groups have a dissolution inhibiting effect, probably due to hydrogen
bridge formation with the binder(s) present in the coating such as phenolic resins.
Known examples of such inhibiting dyes are the amino-substituted tri- or diarylmethane
dyes, e.g. crystal violet, methyl violet, violet pure blue, auramine and malachite
green.
[0010] According to the present invention, it has been found that the colorants which are
used in the prior art materials can be replaced by alternatives that are non-inhibiting.
Such colorants provide a higher sensitivity, even if the absorption efficiency at
the wavelength of the image-wise exposure is not affected thereby.
DETAILED DESCRIPTION OF THE INVENTION
[0011] The lithographic printing plate precursor of the present invention contains a hydrophilic
support and a coating comprising an oleophilic layer provided thereon. The printing
plate precursor is positive-working, i.e. after exposure and development the exposed
areas of the oleophilic layer are removed from the support and define hydrophilic
(non-printing) areas, whereas the unexposed layer is not removed from the support
and defines an oleophilic (printing) area.
[0012] The support has a hydrophilic surface or is provided with a hydrophilic layer. The
support may be a sheet-like material such as a plate or it may be a cylindrical element
such as a sleeve which can be slid around a print cylinder of a printing press. Preferably,
the support is a metal support such as aluminum or stainless steel.
[0013] A particularly preferred lithographic support is an electrochemically grained and
anodized aluminum support. The anodized aluminum support may be 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°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 a citric acid or
citrate solution. This treatment may be carried out at room temperature or may be
carried out at a slightly elevated temperature of about 30 to 50°C. A further interesting
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
US-P- 4 458 005.
[0014] According to another embodiment, the support can also be a flexible support, which
is provided with a hydrophilic layer, hereinafter called 'base layer'. The flexible
support is e.g. paper, plastic film, thin aluminum or a laminate thereof. Preferred
examples of plastic film are polyethylene terephthalate film, polyethylene naphthalate
film, cellulose acetate film, polystyrene film, polycarbonate film, etc. The plastic
film support may be opaque or transparent.
[0015] The base layer is preferably a cross-linked hydrophilic layer obtained from a hydrophilic
binder cross-linked with a hardening agent such as formaldehyde, glyoxal, polyisocyanate
or a hydrolyzed tetra-alkylorthosilicate. The latter is particularly preferred. The
thickness of the hydrophilic base layer may vary in the range of 0.2 to 25 µm and
is preferably 1 to 10 µm.
[0016] The hydrophilic binder for use in the base layer is e.g. a hydrophilic (co)polymer
such as homopolymers and copolymers of vinyl alcohol, acrylamide, methylol acrylamide,
methylol methacrylamide, acrylate acid, methacrylate acid, hydroxyethyl acrylate,
hydroxyethyl methacrylate or maleic anhydride/vinylmethylether copolymers. The hydrophilicity
of the (co)polymer or (co)polymer mixture used is preferably the same as or higher
than the hydrophilicity of polyvinyl acetate hydrolyzed to at least an extent of 60%
by weight, preferably 80% by weight.
[0017] The amount of hardening agent, in particular tetraalkyl orthosilicate, is preferably
at least 0.2 parts per part by weight of hydrophilic binder, more preferably between
0.5 and 5 parts by weight, most preferably between 1 parts and 3 parts by weight.
[0018] The hydrophilic base layer may also contain substances that increase the mechanical
strength and the porosity of the layer. For this purpose colloidal silica may be used.
The colloidal silica employed may be in the form of any commercially available water
dispersion of colloidal silica for example having an average particle size up to 40
nm, e.g. 20 nm. In addition inert particles of larger size than the colloidal silica
may be added e.g. silica prepared according to Stöber as described in J. Colloid and
Interface Sci., Vol. 26, 1968, pages 62 to 69 or alumina particles or particles having
an average diameter of at least 100 nm which are particles of titanium dioxide or
other heavy metal oxides. By incorporating these particles the surface of the hydrophilic
base layer is given a uniform rough texture consisting of microscopic hills and valleys,
which serve as storage places for water in background areas.
[0019] Particular examples of suitable hydrophilic base layers for use in accordance with
the present invention are disclosed in
EP-A- 601 240, GB-P- 1 419 512, FR-P- 2 300 354, US-P- 3 971 660, and
US-P- 4 284 705.
[0020] It is particularly preferred to use a film support to which an adhesion improving
layer, also called support layer, has been provided. Particularly suitable adhesion
improving layers for use in accordance with the present invention comprise a hydrophilic
binder and colloidal silica as disclosed in
EP-A- 619 524,
EP-A- 620 502 and
EP-A- 619 525. Preferably, the amount of silica in the adhesion improving layer is between 200 mg/m
2 and 750 mg/m
2. Further, the ratio of silica to hydrophilic binder is preferably more than 1 and
the surface area of the colloidal silica is preferably at least 300 m
2/gram, more preferably at least 500 m
2/gram.
[0021] The oleophilic layer contains a polymer that is soluble in an aqueous alkaline developer.
Preferred polymers are phenolic resins which are soluble in an aqueous developer,
preferably having a pH between 7.5 and 14. Suitable polymers are 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.
[0022] According to the present invention, the oleophilic layer also contains an organic
dye which absorbs visible light so that a perceptible image is obtained upon image-wise
exposure and subsequent development. The term "organic dye" shall be understood as
excluding pigments and metal ion complexes. Preferably, the dye has an absorption
maximum in the visible wavelength region (380-750 nm). The dye may also absorb the
infrared light that can be used for the image-wise exposure. In an alternative embodiment,
the dye does not substantially absorb the light that may be used for the image-wise
exposure and then it is advantageous to add an additional sensitizer to the coating
that is capable of absorbing the light used for the image-wise exposure. The latter
sensitizer is discussed in more detail below. 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. In a preferred
daylight stable embodiment, the coating does not comprise photosensitive ingredients,
such as diazide or diazonium compounds, photoacids, photoinitiators, sensitisers etc.,
which absorb the near UV and/or visible light that is present in sun light or office
lighting and thereby render the coating more soluble in exposed areas.
[0023] The dyes in the materials of the present invention are non-inhibiting, i.e. they
do not reduce the solubility of the above polymer in an aqueous alkaline developer.
"Reducing the solubility of the polymer" shall be understood as reducing the dissolution
rate of the polymer in the developer, rather than reducing the concentration of the
dissolved polymer in equilibrium conditions. As explained above, a positive working
material shows a faster dissolution of the oleophilic layer at exposed areas than
at unexposed areas. Preferably, the exposed areas are completely dissolved in the
developer before the unexposed areas are attacked so that the latter are characterized
by sharp edges and high ink-acceptance. It may be concluded that the solubility differentiation
between exposed and unexposed areas of the coating is induced by a kinetic rather
than a thermodynamic effect.
[0024] The inhibiting capability of the dye can easily be tested by coating two samples
of the oleophilic layer on a support : the reference sample contains only the polymer
and another includes both the polymer (in equal amounts as the reference) as well
as the dye. 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, which is referred to herein as "dissolution time". When
the dissolution time of the sample containing the dye is longer than the dissolution
time of the sample without the dye, then the dye clearly acts as an inhibitor. When
the dissolution time of the sample containing the dye is not longer than the value
of the reference sample, then the dye is non-inhibiting and, as a result, does not
reduce the solubility of the oleophilic layer in the developer.
[0025] The dye preferably has a chemical structure, wherein a chromophoric group, which
absorbs visible light is substituted by one or more solubilizing groups, as shown
in the following formula :
D - [(L)
x - (G)
y]
n
wherein D is a chromophoric group, L is a divalent linking group, x is 0 or 1, y and
n are at least 1, and G is an anionic group or a group which can be rendered anionic
by immersion of the coating in an aqueous alkaline solution. G is preferably selected
from the group consisting of -COOH, -OH, -PO
3H
2, -O-PO
3H
2, -SO
3H, -O-SO
3H, -SO
2-NH
2, -SO
2-NH-R, -SO
2-NH-CO-R and salts thereof, R being an optionally substituted alkyl or optionally
substituted aryl group. L is e.g. a group which comprises -O-, -CO-O-, -O-CO-, -N=N-,
-NR'-, -CO-NR'-, -NR'-CO-, optionally substituted arylene or optionally substituted
alkylene, R' being hydrogen, optionally substituted alkyl or optionally substituted
aryl. When x=0, then G is directly bonded to the chromophoric group D. When x=1, then
y may be larger than 1, i.e. the same linking group may carry more than one anionic
group. Each L and G can be independently selected from the other L and G groups. Dyes
wherein one or more anionic groups G are directly bonded to D and wherein one or more
other anionic group G are coupled to D by means of a linking group L also belong to
the scope of the present invention.
[0026] Suitable dyes correspond e.g. to one of the following formula :

wherein
- i and j are independently 0 to 3;
- k, l and o are independently 0 to 4;
- m and n are independently 0 to 5;
- p is 0 to 3;
- R1, R1', R1", R2 and R2' are independently selected from the group consisting of optionally substituted alkyl,
optionally substituted aryl, -G, -L-G , -CN, a halogen, -NO2, -ORd, -CO-O-Ra, -O-CO-Ra, -CO-NRdRe, - NRdRe, -NRd-CO-Ra, -NRd-CO-O-Ra, -NRd-CO-NReRf, -SO2-O-Ra, -SO2-NRdRe or wherein two adjacent radicals R1, R1', R1", R2 or R2' together form a condensed carbocyclic or heterocyclic ring;
- R3, R4 and R5 are independently selected from the group consisting of hydrogen, optionally
substituted alkyl, optionally substituted aryl, -CO-Rb, -CO-O-Rb, -CO-NRfRg and -L-G;
with
- L and G as defined above.
- Ra and Rb being an optionally substituted alkyl or an optionally substituted aryl group;
- Rd, Re, Rf and Rg being hydrogen, an optionally substituted alkyl or an optionally substituted aryl
group.
[0028] The non-inhibiting dye is present in an amount sufficient to give the coating a visible
color. It is self evident that the required amount depends on the extinction coefficient
of the dye. The concentration of a typical non-inhibiting dye in the oleophilic layer
may vary e.g. between 0.25 and 10.0 wt.% relative to the oleophilic layer, more preferably
between 0.5 and 5.0 wt.%.
[0029] The oleophilic layer may further contain other ingredients, e.g. additional binders
to improve the run length of the plate, such as those described in EP-A 933 682. Preferably,
also development accelerators are included, i.e. compounds which act as dissolution
promoters because they are capable of reducing the dissolution time of the oleophilic
layer, which can be tested by the same procedure as describe above in relation to
the inhibiting capability of the dye. For example, cyclic acid anhydrides, phenols
or organic acids can be used in order to improve the aqueous developability. 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. Patent 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 image forming composition is preferably in
the range of 0.05 to 20% by weight.
[0030] In a preferred embodiment, the coating also contains developer resistance means,
i.e. one or more materials which prevent penetration of the aqueous alkaline developer
into the oleophilic layer at unexposed areas. Such developer resistance means can
be added to the oleophilic layer or in a barrier layer provided on top of the oleophilic
layer. In the latter embodiment, 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 864 420, EP-A 950 517 and WO99/21725.
Preferred examples of the developer resistance means include water-repellent polymers
such as a polymer comprising siloxane and/or perfluoroalkyl units. In one embodiment,
the barrier layer contains such a water-repellent polymer in an amount between 0.5
and 25 mg/m
2, preferably between 0.5 and 15 mg/m
2 and most preferably between 0.5 and 10 mg/m
2. When the water-repellent polymer is also ink-repelling, e.g. in the case of polysiloxanes,
higher amounts than 25 mg/m
2 can result in poor ink-acceptance of the non-exposed areas. An amount lower than
0.5 mg/m
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-repellant
polymer is a block-copolymer or a graft-copolymer of a poly(alkylene oxide) and a
polymer comprising siloxane and/or perfluoroalkyl units. A suitable copolymer comprises
about 15 to 25 siloxane units and 50 to 70 alkyleneoxide 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 applied as an ingredient of the
coating solution of the oleophilic layer. 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 oleophilic layer.
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
material.
[0031] 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 converted into heat by a light absorbing
compound. Near infrared light is preferred. Said light absorbing compound can be the
non-inhibiting dye discussed above. The coating preferably comprises, in addition
to the non-inhibiting dye, a sensitizer which is a dye or pigment having an absorption
maximum in the IR wavelength range. The concentration of the sensitizing dye or pigment
in the oleophilic layer is typically between 0.25 and 10.0 wt.%, more preferably between
0.5 and 7.5 wt.% relative to said layer.
[0032] 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 :

[0033] The sensitizing dye or pigment may be present in the oleophilic layer, in the barrier
layer discussed above or in an optional other layer. According to a highly preferred
embodiment, the dye or pigment is concentrated in or near the barrier layer, e.g.
in an intermediate layer between the oleophilic and the barrier layer. According to
that embodiment, said intermediate layer comprises the light absorbing compound in
an amount higher than the amount of light absorbing compound in the oleophilic or
in the barrier layer. In a preferred embodiment, the barrier layer consists essentially
of water-repellent polymer, i.e. comprises no effective amount of sensitizer or other
ingredients.
[0034] The printing plate precursor of the present invention can be exposed to heat or to
infrared light, e.g. by means of a thermal head, LEDs or a laser. Most preferably,
the light used for the exposure is 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
2 of maximum intensity : 10-25 µm), 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).
[0035] 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.
[0036] 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.
US 5,174,205 and US 5,163,368.
[0037] 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 development step may be followed by a rinsing step, a gumming
step, a drying step and/or a post-baking step.
[0038] 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 inks which are suitable for use in the method of the present invention
have been described in US 4,045,232; US 4,981,517 and US 6,140,392. In a most preferred
embodiment, the single-fluid ink comprises an ink phase, also called the hydrophobic
or oleophilic phase, and a polyol phase as described in WO 00/32705.
EXAMPLES
Preparation of the support
[0039] A 0.30 mm thick aluminum foil was degreased by immersing the foil in an aqueous solution
containing 5 g/l of sodium hydroxide at 50°C and rinsed with demineralized water.
The foil was then electrochemically grained using an alternating current in an aqueous
solution containing 4 g/l of hydrochloric acid, 4 g/l of hydroboric acid and 5 g/l
of aluminum ions at a temperature of 35°C and a current density of 1200 A/m
2 to form a surface topography with an average center-line roughness Ra of 0.5 µm.
[0040] After rinsing with demineralized water the aluminum foil was then etched with an
aqueous solution containing 300 g/l of sulfuric acid at 60°C for 180 seconds and rinsed
with demineralized water at 25°C for 30 seconds.
[0041] The foil was subsequently subjected to anodic oxidation in an aqueous solution containing
200 g/l of sulfuric acid at a temperature of 45°C, a voltage of about 10 V and a current
density of 150 A/m
2 for about 300 seconds to form an anodic oxidation film of 3.00 g/m
2 of Al
2O
3 then washed with demineralized water, post-treated with a solution containing polyvinylphosphonic
acid and subsequently with a solution containing aluminum trichloride, rinsed with
demineralized water at 20°C during 120 seconds and dried.
Test of inhibiting capability of dyes
[0042] A layer of novolac (Alnovol SPN452 from Clariant, a 40.5 wt.% solution in methoxypropanol)
and the dyes specified in Table 1 were coated on the above support. After drying during
2 min at 120°C, the samples contained 0.9 g/m
2 of novolac. The samples were then dipped in an Ozasol EP26 developer from Agfa at
20°C and the dissolution time was determined as described above. Examples 2-4 contained
a dye according to the invention and showed shorter dissolution time values than Reference
Example 1 without dye. Comparative Example 5 contained Resolin Rot F3BS, which is
an inhibiting dye, inducing a longer dissolution time than for the materials of the
present invention.
Table 1
Example no. |
Dye (mg/m2) |
Dissolution time (sec) |
1 (reference) |
- |
40 |
2 (invention) |
Dye 1 (20) |
20 |
3 (invention) |
Dye 2 (20) |
20 |
4 (invention) |
Dye 3 (20) |
20 |
5 (comparative) |
Resolin Rot F3BS (12.5) |
60 |
Table 2
Ingredients (g) |
Ex. 6 (inv) |
Ex. 7 (inv.) |
Ex. 8 (inv.) |
Ex. 9 (comp.) |
Tetrahydrofuran |
186.2 |
= |
= |
207.12 |
Alnovol SPN452 |
103.7 |
= |
= |
116.29 |
Methoxypropanol |
439.93 |
= |
= |
376.56 |
Methylethylketon |
236.93 |
= |
= |
263.55 |
IR-1 |
2.27 |
= |
= |
2.53 |
Dye 1 |
1.00 |
- |
- |
- |
Dye 2 |
- |
1.00 |
- |
- |
Dye 3 |
- |
- |
1.00 |
- |
Resolin Rot F3BS |
- |
- |
- |
1.75 |
Tego Glide 410 * |
0.25 |
= |
= |
= |
2,3,4-trihydroxybenzophenone |
7.26 |
= |
= |
8.08 |
IR sensitivity (mJ/cm2) |
≤79 |
= |
= |
157 |
(*) Surfactant commercially available from Tego Chemie, Essen, Germany |
Plate precursor materials
[0043] The solutions in Table 2 were coated on the above support at a wet coating thickness
of 22 µm and then dried during 2 min at 120°C. The materials were then imaged on a
Creo Trendsetter 3244 (830 nm) using the following series of energy density settings
(power at the image plane) : 79 mJ/cm
2, 99 mJ/cm
2, 125 mJ/cm
2, 157 mJ/cm
2, and 197 mJ/cm
2. The plates were then processed in an Agfa Autolith PN85 processor operating at a
speed of 0.84 m/min using Agfa Ozasol EP26 developer at 25°C and finally gummed with
Agfa Ozasol RC795. The IR-sensitivity was defined as the energy density that is required
to obtain a 50% light absorption, measured on the developed plate at the wavelength
maximum of the dye, in areas which have been exposed with a dot area of a 50% screen
(@200 lpi).
[0044] The results in Table 2 indicate that the non-inhibiting dyes Dye 1-3 provide a higher
sensitivity (given by the lower energy density) than the inhibiting dye Resolin Rot
F3BS.
1. A heat-sensitive lithographic printing plate precursor comprising (i) a support having
a hydrophilic surface or which is provided with a hydrophilic layer and (ii) a coating
provided thereon, the coating comprising an oleophilic layer which, upon image-wise
exposure to heat or infrared light and subsequent immersion in an aqueous alkaline
developer, dissolves in the developer at a higher dissolution rate in exposed areas
than in unexposed areas, wherein the oleophilic layer comprises a polymer that is
soluble in the developer and an organic dye in a amount sufficient to provide a visible
color to the coating, characterized in that said organic dye does not reduce the dissolution rate of the unexposed areas in the
developer.
2. A lithographic printing plate precursor according to claim 1 wherein the organic dye
has a chemical structure according to the following formula :
D - [(L)x - (G)y]n
wherein D is a chromophoric group, L is a divalent linking group, x is 0 or 1, y and
n are at least 1, and G is an anionic group or a group which can be rendered anionic
by immersion of the coating in the developer.
3. A lithographic printing plate precursor according to claim 2 wherein each G is independently
selected from the group consisting of -COOH, -OH, -PO3H2, -O-PO3H2, -SO3H, -O-SO3H, -SO2-NH2, -SO2-NH-R, -SO2-NH-CO-R and salts thereof, R being an optionally substituted alkyl or optionally
substituted aryl group.
4. A lithographic printing plate precursor according to claim 3 wherein the organic dye
has a chemical structure according to one of the following formula :

wherein
- i and j are independently 0 to 3;
- k, l and o are independently 0 to 4;
- m and n are independently 0 to 5;
- p is 0 to 3;
- R1, R1', R1", R2 and R2' are independently selected from the group consisting of optionally substituted alkyl,
optionally substituted aryl, -G, -L-G , -CN, a halogen, -NO2, -ORd, -CO-O-Ra, -O-CO-Ra, -CO-NRdRe, - NRdRe, -NRd-CO-Ra, -NRd-CO-O-Ra, -NRd-CO-NReRf, -SO2-O-Ra, -SO2-NRdRe or wherein two adjacent radicals R1, R1', R1", R2 or R2' together form a condensed carbocyclic or heterocyclic ring;
- R3, R4 and R5 are independently selected from the group consisting of hydrogen,
optionally substituted alkyl, optionally substituted aryl, -CO-Rb, -CO-O-Rb, -CO-NRfRg and -L-G;
with
- Ra and Rb being an optionally substituted alkyl or an optionally substituted aryl group;
- Rd, Re, Rf and Rg being hydrogen, an optionally substituted alkyl or an optionally substituted aryl
group.
5. A lithographic printing plate precursor according to any of the preceding claims wherein
the oleophilic layer further comprises a compound which increases the dissolution
rate of unexposed areas in the developer.
6. A lithographic printing plate precursor according to claim 5 wherein the compound,
which increases the dissolution rate of unexposed areas, is a cyclic acid anhydride,
a phenol or an organic acid.
7. A lithographic printing plate precursor according to any of the preceding claims wherein
the coating further comprises means for providing increased developer resistance of
the coating, and wherein the developer resistance of the coating is reduced upon exposure
to heat or infrared light.
8. A lithographic printing plate precursor according to claim 7 wherein the means for
providing increased developer resistance are present in a barrier layer provided on
the oleophilic layer, wherein the solubility of the barrier layer in the developer
or the penetrability of the barrier layer by the developer is reduced upon exposure
to heat or infrared light.
9. A lithographic printing plate precursor according to claim 7 or 8 wherein the means
for providing increased developer resistance comprise a water-repellent polymer.
10. A lithographic printing plate precursor according to 9 wherein the water-repellent
polymer is
- a polymer comprising siloxane and/or perfluoroalkyl units; or
- a block- or graft-copolymer of a poly(alkylene oxide) and a polymer comprising siloxane
and/or perfluoroalkyl units.