1. Field of the invention.
[0001] The present invention relates to a heat sensitive material for making a lithographic
printing plate. The present invention further relates to a method for preparing a
printing plate from said heat sensitive material.
2. Background of the invention.
[0002] Lithography is the process of printing from specially prepared surfaces, some areas
of which are capable of accepting lithographic ink, whereas other areas, when moistened
with water, will not accept the ink. The areas which accept ink form the printing
image areas and the ink-rejecting areas form the background areas.
[0003] In the art of photolithography, a photographic material is made imagewise receptive
to oily or greasy ink in the photo-exposed (negative working) or in the non-exposed
areas (positive working) on a hydrophilic background.
[0004] In the production of common lithographic plates, also called surface litho plates
or planographic printing plates, a support that has affinity to water or obtains such
affinity by chemical treatment is coated with a thin layer of a photosensitive composition.
Coatings for that purpose include light-sensitive polymer layers containing diazo
compounds, dichromate-sensitized hydrophilic colloids and a large variety of synthetic
photopolymers. Particularly diazo-sensitized systems are widely used.
[0005] Upon imagewise exposure of the light-sensitive layer the exposed image areas become
insoluble and the unexposed areas remain soluble. The plate is then developed with
a suitable liquid to remove the diazonium salt or diazo resin in the unexposed areas.
[0006] On the other hand, methods are known for making printing plates involving the use
of imaging elements that are heat sensitive rather than photosensitive. A particular
disadvantage of photosensitive. imaging elements such as described above for making
a printing plate is that they have to be shielded from the light. Furthermore they
have a problem of sensitivity in view of the storage stability and they show a lower
resolution. The trend towards heat sensitive printing plate precursors is clearly
seen on the market.
[0007] For example, Research Disclosure no. 33303 of January 1992 discloses a heat sensitive
imaging element comprising on a support a cross-linked hydrophilic layer containing
thermoplastic polymer particles and an infrared absorbing pigment such as e.g. carbon
black. By image-wise exposure to an infrared laser, the thermoplastic polymer particles
are image-wise coagulated thereby rendering the surface of the imaging element at
these areas ink acceptant without any further development. A disadvantage of this
method is that the printing plate obtained is easily damaged since the non-printing
areas may become ink accepting when some pressure is applied thereto. Moreover, under
critical conditions, the lithographic performance of such a printing plate may be
poor and accordingly such printing plate has little lithographic printing latitude.
[0008] EP-A-514145 discloses a heat sensitive imaging element including a coating comprising
core-shell particles having a water insoluble heat softenable core component and a
shell component which is soluble or swellable in aqueous alkaline medium. Red or infrared
laser light directed image-wise at said imaging element causes selected particles
to coalesce, at least partially, to form an image and the non-coalesced particles
are then selectively removed by means of an aqueous alkaline developer. Afterwards
a baking step is performed. However the printing endurance of a so obtained printing
plate is low.
[0009] EP-A-599510 discloses a heat sensitive imaging element which comprises a substrate
coated with (i) a layer which comprises (1) a disperse phase comprising a water-insoluble
heat softenable component A and (2) a binder or continuous phase consisting of a component
B which is soluble or swellable in aqueous, preferably aqueous alkaline medium, at
least one of components A and B including a reactive group or precursor therefor,
such that insolubilisation of the layer occurs at elevated temperature and/or on exposure
to actinic radiation, and (ii) a substance capable of strongly absorbing radiation
and transferring the energy thus obtained as heat to the disperse phase so that at
least partial coalescence of the coating occurs. After image-wise irradiation of the
imaging element and developing the image-wise irradiated plate, said plate is heated
and/or subjected to actinic irradiation to effect insolubilisation. However the printing
endurance of a so obtained printing plate is low.
[0010] EP-A-625728 discloses an imaging element comprising a layer which is sensitive to
UV- and IR-irradiation and which can be positive or negative working. This layer comprises
a resole resin,a novolac resin, a latent Bronsted acid and an IR-absorbing substance.
The printing results of a lithographic plate obtained by irradiating and developing
said imaging element are poor.
[0011] US-P-5,340,699 is almost identical with EP-A-625728 but discloses the method for
obtaining a negative working IR-laser recording imaging element. The IR-sensitive
layer comprises a resole resin,a novolac resin, a latent Bronsted acid and an IR-absorbing
substance. The printing results of a lithographic plate obtained by irradiating and
developing said imaging element are poor.
[0012] US-P-4,708,925 discloses a positive working imaging element including a photosensitive
composition comprising an alkali-soluble novolac resin and an onium-salt. This composition
can optionally contain an IR-sensitizer. After image-wise exposing said imaging element
to UV - visible - or eventually IR-radiation followed by a development step with an
aqueous alkali liquid there is obtained a positive working printing plate. The printing
results of a lithographic plate obtained by irradiating and developing said imaging
element are poor.
[0013] EP-A-770494 and EP-A 770495 discloses a method for making a lithographic printing
plate using an imaging element comprising (i) on a hydrophilic surface of a lithographic
base an image forming layer comprising hydrophobic thermoplastic polymer particles
dispersed in a hydrophilic binder and (ii) a compound capable of converting light
to heat , said compound being comprised in said image forming layer or a layer adjacent
thereto. Both applications disclose that there can be one or more layers intermediate
layers provided between the lithographic base and the image forming layer. Said applications
do not disclose that said intermediate layer should be soluble in an aqueous solution
with a pH of at least 5.
[0014] GB 1,160,221 discloses a method of recording information, wherein a recording material
is used comprising a water-permeable recording layer which incorporates hydrophobic
thermoplastic polymeric material in the form of particles solid at room temperature
and which can be rendered water-impermeable or substantially less water-permeable
by the action of heat, said recording material also incorporating, in heat-conductive
relationship to said polymer particles, a substance or substances which is or are
distributed over the whole area of such material and is or are capable of being heated
by exposing the material to intense electro-magnetic radiation which is absorbed by
such substance or substances. Said substance or substances can be incorporated in
an intermediate layer. However the disclosure is silent about the fact that said layers
should be removable in an aqueous solution with a pH of at least 5. The examples even
disclose hardened gelatine layers, which are surely not removable.
[0015] EP-A-800 928 (prior art according to Article 54 (3) and (4) EPC) discloses a heat
sensitive imaging element comprising on a hydrophilic surface of a lithographic base
an image forming layer comprising hydrophobic thermoplastic polymer particles dispersed
in a water insoluble alkali soluble or swellable resin and a compound capable of converting
light into heat, said compound being present in said image forming layer or a layer
adjacent thereto, wherein said alkali swellable or soluble resin comprises phenolic
hydroxy groups and/or carboxyl groups. However the printing plates obtained from said
heat-sensitive imaging element gives prints with scumming.
[0016] All the disclosed systems either require a treatment after the development step and/or
or yield lithographic plates with poor printing properties. So, there is still a need
for a heat sensitive imaging element that is easy to process and yields a lithographic
plate with good or excellent printing properties.
3. Summary of the invention.
[0017] It is an object of the present invention to provide a heat sensitive imaging element
for making in a convenient way a lithographic printing plate having excellent printing
properties.
[0018] It is another object of the present invention to provide a method for obtaining in
a convenient way a negative working lithographic printing plate of a high quality
using said imaging element.
[0019] It is still another object of the present invention to provide a method for obtaining
in a convenient way a negative working lithographic printing plate which gives prints
without scumming using said imaging element.
[0020] Further objects of the present invention will become clear from the description hereinafter.
[0021] According to the present invention there is provided a heat sensitive imaging element
comprising a lithographic base with a hydrophilic surface, an image forming layer
including a hydrophobic thermoplastic polymer latex and a compound capable of converting
light into heat being present in said image forming layer or a layer adjacent thereto,
and a barrier layer between the lithographic base having a hydrophilic surface and
the image forming layer, said barrier layer and said image forming layer being removable
in an aqueous solution with a pH of at least 5 and the barriers layer containing a
binder as defined in present claim 1.
[0022] According to the present invention there is also provided a method for obtaining
a lithographic printing plate comprising the steps of:
(a) image-wise or information-wise exposing to light or heat an imaging element as
described above
(b) developing said exposed imaging element with an aqueous developing solution having
a pH of at least 5 in order to remove the unexposed areas and thereby form a lithographic
printing plate.
4. Detailed description of the invention.
[0023] It has been found that lithographic printing plates of high quality, giving prints
without scumming can be obtained according to the method of the present invention
using an imaging element as described above. More precisely it has been found that
said printing plates are of high quality and are provided in a convenient way, thereby
offering economical and ecological advantages.
[0024] An imaging element for use in accordance with the present invention comprises on
a hydrophilic surface of a lithographic base in the order given a barrier layer according
to claim 1 and thereby soluble in an aqueous medium of at least pH 5, preferably at
room temperature and an image forming layer comprising a hydrophobic thermoplastic
polymer latex, removable on the areas where the barrier layer is dissolved.
[0025] The barrier layer is preferably soluble in an aqueous solution having a pH of at
least 6, more preferably having a pH of at least 7. The barrier layer has preferably
a dry thickness ranging from 0.01 to 1 g/m
2, more preferably from 0.05 to 0.5 g/m
2.
[0026] In one embodiment the barrier layer is only soluble in an aqueous solution having
a pH of at least 10. Said alkali-soluble barrier layer comprises an alkali soluble
binder. Suitable alkali soluble binders for use in an image forming layer in connection
with this embodiment are for example synthetic novolac resins such as ALNOVOL, a registered
trade mark of Reichold Hoechst and DUREZ, a registered trade mark of OxyChem and synthetic
polyvinylfenols such as MARUKA LYNCUR M, a registered trade mark of Dyna Cyanamid.
[0027] The alkali soluble binder used in connection with the present embodiment is preferably
not cross-linked or only slightly cross-linked.
[0028] In another embodiment the barrier layer is already soluble in an aqueous solution
having a pH of at least 5. Said aqueous soluble barrier layer comprises a binder soluble
in an aqueous solution with a pH of at least 5. Suitable aqueous soluble binders for
use in an image forming layer in connection with this embodiment are for example polymers
containing an acid group, preferably a carboxyl group. More preferably said aqueous
soluble polymer is a (co)polymer containing hydroxy groups which have at least partially
reacted with a compound comprising at least two carboxyl groups. Most preferably said
alkali soluble polymer containing hydroxy groups also contains hydrophobic groups
such as acetal groups. Preferably the molecular weight of said alkali soluble polymer
ranges from 10,000 to 1,000,000, more preferably from 20,000 to 300,000.
[0029] Very preferred polymers for use in the barrier layer according to the invention have
a structure as represented by formula I, wherein n ranges from 50 to 78 %
m ranges from 21 to 49%
p ranges from 1 to 5 %
q ranges from 0 to 28 %
[0030] The alkali soluble binder used in connection with the present embodiment is preferably
not cross-linked or only slightly cross-linked.
[0031] According to one embodiment of the present invention, the lithographic base having
a hydrophilic surface can be an anodised aluminum. A particularly preferred lithographic
base having a hydrophilic surface is an electrochemically grained and anodised aluminum
support. According to the present invention, an anodised 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 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 can 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,
polyvinylsulphonic acid, polyvinylbenzenesulphonic acid, sulphuric acid esters of
polyvinyl alcohol, and acetals of polyvinyl alcohols formed by reaction with a sulphonated
aliphatic aldehyde. It is further evident that one or more of these post treatments
may be carried out alone or in combination.
[0032] According to another embodiment in connection with the present invention, the lithographic
base having a hydrophilic surface comprises a flexible support, such as e.g. paper
or plastic film, provided with a cross-linked hydrophilic layer. A particularly suitable
cross-linked hydrophilic layer may be obtained from a hydrophilic binder cross-linked
with a cross-linking agent such as formaldehyde, glyoxal, polyisocyanate or a hydrolysed
tetra-alkylorthosilicate. The latter is particularly preferred.
[0033] As hydrophilic binder there may be used hydrophilic (co)polymers such as for example,
homopolymers and copolymers of vinyl alcohol, acrylamide, methylol acrylamide, methylol
methacrylamide, acrylic acid, methacrylic 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 percent
by weight, preferably 80 percent by weight.
[0034] The amount of crosslinking agent, in particular of tetraalkyl orthosilicate, is preferably
at least 0.2 parts by weight per part by weight of hydrophilic binder, preferably
between 0.5 and 5 parts by weight, more preferably between 1.0 parts by weight and
3 parts by weight.
[0035] A cross-linked hydrophilic layer in a lithographic base used in accordance with the
present embodiment preferably also contains 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 can 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 cross-linked hydrophilic
layer is given a uniform rough texture consisting of microscopic hills and valleys,
which serve as storage places for water in background areas.
[0036] The thickness of a cross-linked hydrophilic layer in a lithographic base in accordance
with this embodiment may vary in the range of 0.2 to 25 µm and is preferably 1 to
10 µm.
[0037] Particular examples of suitable cross-linked hydrophilic layers for use in accordance
with the present invention are disclosed in EP-A 601240, GB-P-1419512, FR-P-2300354,
US-P-3971660, US-P-4284705 and EP-A 514490.
[0038] As flexible support of a lithographic base in connection with the present embodiment
it is particularly preferred to use a plastic film e.g. substrated polyethylene terephthalate
film, cellulose acetate film, polystyrene film, polycarbonate film etc... The plastic
film support may be opaque or transparent.
[0039] It is particularly preferred to use a polyester film support to which an adhesion
improving 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 619524, EP-A 620502 and EP-A 619525. Preferably,
the amount of silica in the adhesion improving layer is between 200 mg per m
2 and 750 mg per 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 per gram, more preferably at least 500 m
2 per gram.
[0040] The hydrophobic thermoplastic polymer latex can be dispersed in a hydrophilic binder.
[0041] The image forming layer comprising a hydrophilic binder used in connection with the
present invention is preferably not crosslinked or only slightly crosslinked. Suitable
hydrophilic binders for use in an image-forming layer in connection with this invention
are water soluble (co)polymers for example synthetic homo- or copolymers such as polyvinylalcohol,
a poly(meth)acrylic acid, a poly(meth)acrylamide, a polyhydroxyethyl(meth)acrylate,
a polyvinylmethylether, a copolymer according to formula I or natural binders such
as gelatin, a polysaccharide such as e.g. dextran, pullulan, cellulose, arabic gum,
alginic acid.
[0042] The hydrophilic binder can also be a water insoluble, alkali soluble or swellable
resin having phenolic hydroxy groups and/or carboxyl groups.
[0043] Preferably the water insoluble, alkali soluble or swellable resin used in connection
with the present invention comprises phenolic hydroxy groups. Suitable water insoluble,
alkali soluble or swellable resins for use in an image-forming layer in connection
with this invention are for example synthetic novolac resins such as ALNOVOL, a registered
trade mark of Reichold Hoechst and DUREZ, a registered trade mark of OxyChem and synthetic
polyvinylfenols such as MARUKA LYNCUR M, a registered trade mark of Dyno Cyanamid.
[0044] The hydrophobic thermoplastic polymer latex can also be dispersed in an aqueous medium
without a binder.
[0045] The hydrophobic thermoplastic polymer latices used in connection with the present
invention preferably have a coagulation temperature above 50°C and more preferably
above 70°C. Coagulation may result from softening or melting of the thermoplastic
polymer latices under the influence of heat. There is no specific upper limit to the
coagulation temperature of the thermoplastic hydrophobic polymer latices, however
the temperature should be sufficiently below the decomposition temperature of the
polymer latices. Preferably the coagulation temperature is at least 10°C below the
temperature at which the decomposition of the polymer latices occurs. When said polymer
latices are subjected to a temperature above the coagulation temperature they coagulate
to form a hydrophobic agglomerate so that at these parts the hydrophobic latices become
insoluble in plain water or an aqueous liquid.
[0046] Specific examples of hydrophobic. thermoplastic polymer latices for use in connection
with the present invention with a Tg above 80°C are preferably polyvinyl chloride,
polyvinylidene chloride, polyacrylonitrile, polyvinyl carbazole etc., copolymers or
mixtures thereof. Most preferably used are polystyrene, polymethylmethacrylate or
copolymers thereof.
[0047] When the hydrophobic thermoplastic polymer latex is dispersed in an aqueous medium
without a binder said hydrophobic thermoplastic polymer latex preferably contains
a water dispersing functional group such as an acid function. Preferred hydrophobic
thermoplastic polymer dispersed latices in such embodiment are polymers of therephthalic
acid or isophthalic acid with ethylene diglycol or copolymers of therephthalic acid
and isophthalic acid with ethylene diglycol, said polymers or copolymers comprising
sulphoisophthalic acid in an amount between 0.5 and 5%.
[0048] The weight average molecular weight of the hydrophobic thermoplastic polymer may
range from 5,000 to 1,000,000g/mol.
[0049] The hydrophobic thermoplastic polymer latex may have a particle size from 0.01 µm
to 50 µm, more preferably between 0.05 µm and 10 µm and most preferably between 0.05
µm and 2 µm.
[0050] The hydrophobic thermoplastic polymer latex is present as a dispersion in the aqueous
coating liquid of the image forming layer and may be prepared by the methods disclosed
in US-P-3,476,937. Another method especially suitable for preparing an aqueous dispersion
of the thermoplastic polymer latex comprises:
- dissolving the hydrophobic thermoplastic polymer in an organic water immiscible solvent,
- dispersing the thus obtained solution in water or in an aqueous medium and
- removing the organic solvent by evaporation.
[0051] The amount of hydrophobic thermoplastic polymer latex contained in the image forming
layer when said layer contains a hydrophilic binder is preferably between 20% by weight
and 65% by weight and more preferably between 25% by weight and 55% by weight and
most preferably between 30% by weight and 45% by weight.
[0052] The image forming layer if containing a hydrophylic binder can also comprise crosslinking
agents although this is not necessary. Preferred crosslinking agents are low molecular
weight substances comprising a methylol group such as for example melamine-formaldehyde
resins, glycoluril-formaldehyde resins, thioureaformaldehyde resins, guanamine-formaldehyde
resins, benzoguanamineformaldehyde resins. A number of said melamine-formaldehyde
resins and glycoluril-formaldehyde resins are commercially available under the trade
names of CYMEL (Dyno Cyanamid Co., Ltd.) and NIKALAC (Sanwa Chemical Co., Ltd.)
[0053] The imaging element further includes a compound capable of converting light to heat.
Suitable compounds capable of converting light into heat are preferably infrared absorbing
components although the wavelength of absorption is not of particular importance as
long as the absorption of the compound used is in the wavelength range of the light
source used for image-wise exposure. Particularly useful compounds are for example
dyes and in particular infrared dyes, carbon black, metal carbides, borides, nitrides,
carbonitrides, bronze-structured oxides and oxides structurally related to the bronze
family but lacking the A component e.g. WO
2.9. It is also possible to use conductive polymer dispersion such as polypyrrole or
polyaniline-based conductive polymer dispersions. The lithographic performance and
in particular the print endurance obtained depends on the heat-sensitivity of the
imaging element. In this respect it has been found that carbon black yields very good
and favorable results.
[0054] A light to heat converting compound in connection with the present invention is most
preferably added to the image forming layer but at least part of the light to heat
converting compound may also be comprised in a neighbouring layer. Such layer can
be for example the cross-linked hydrophilic layer of the lithographic base according
to the second embodiment of lithographic bases explained above.
[0055] In accordance with a method of the present invention for obtaining a printing plate,
the imaging element is image-wise exposed and subsequently developed with an aqueous
solution having a pH of at least 5.
[0056] Image-wise exposure in connection with the present invention is preferably an image-wise
scanning exposure involving the use of a laser or L.E.D.. It is highly preferred in
connection with the present invention to use a laser emitting in the infrared (IR)
and/or near-infrared, i.e. emitting in the wavelength range 700-1500nm. Particularly
preferred for use in connection with the present invention are laser diodes emitting
in the near-infrared.
[0057] After the development of an image-wise exposed imaging element with an aqueous solution
having a pH of at least 5 and drying the obtained plate can be used as a printing
plate as such. However, it is still possible to bake said plate at a temperature between
100°C and 230°C for a period of 40 minutes to 5 minutes. For example the exposed and
developed plates can be baked at a temperature of 230°C for 5 minutes, at a temperature
of 150°C for 10 minutes or at a temperature of 120°C for 30 minutes.
[0058] The following examples illustrate the present invention without limiting it thereto.
All parts are by weight unless otherwise specified.
EXAMPLE 1
Preparation of the lithographic base
[0059] A 0.20 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.
[0060] 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.
[0061] 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, posttreated with a solution containing polyvinylphosphonic
acid( 2.2 g/m
2).
Preparation of the imaging element
[0062] An imaging element according to the invention was prepared by first coating on the
lithographic base a 2 % solution in methylethylketone of a compound according to formula
I wherein n is 70%, p is 3%, m + q is 27% in a wet thickness of 20 µm (dry weight
of 0.3 g/m
2). Thereon was coated an aqueous dispersion of carbon black (0.06 g/m
2) and of a copolymer consisting of terephthalic acid (58 mol%), isophthalic acid (40
mol%) and sulphoisophthalic acid with ethylene glycol, said dispersed particles having
a particle size of 67 nm (0.54 g/m
2).
This layer was coated from aqueous medium at pH = 6.3. In a comparative example a
material was prepared, without first 'layer; the toplayer being directly coated onto
the lithographic base.
[0063] Both materials were imaged with an external drum IR-laser imaging apparatus (diode
laser 830 nm, drumspeed 1 m/s, adressability 5000 dpi, power level in image plane
80-120 mW), and develped in an aqueous developing solution (EN144 negative developer
commercially available from Agfa) having a pH = 8.3.
[0064] With the material having no first layer no selective clean-out could be obtained
:
- Optical density (macbetch RD918-SB/Black filter)
- imaged parts : 0.46
- non-imaged parts : 0.49
[0065] With the material having a first layer a selective clean-out could be obtained, with
full clean-out in the non-imaged parts :
- Optical density (Macbeth RD918-SB/Black filter)
- imaged parts : 0.42
- non-imaged parts : 0.00
EXAMPLE 2
[0066] An imaging element was prepared according to the invention as described in example
1 with the exception that the pH of the aqueous dispersion (the top layer) was 8.5.
[0067] This material was imaged with an external drum IR-laser imaging apparatus (diode
laser 830 nm, drumspeed 1 m/s, at addressabilities 5000 dpi and 200 dpi, power level
in image plane 60-120 mW), and developed in an aqueous developing solution (EN144
negative developer commercially available from Agfa) having a pH = 8.3, additionally
a gum solution was applied to the developed plate and it was subsequently baked for
2 minutes at 200 °C.
[0068] The material was selectively cleaned-out with full clean-out in the non-imaged parts;
optical density (Macbeth RD918-SB/Black filter) :
- imaged parts : 0.37
- non-imaged parts : 0.00
At 5000 dpi images were obtained using 60 mW power or more in imageplane.
At 200 dpi images were obtained using 70 mW power or more in imageplane.
[0069] This plate was used for printing on an Heidelberg GTO printing machine with a conventional
ink (AB.Dick 1020) and fountain solution (Rotamatic), resulting in good prints, i.e.
no scumming in non-imaged parts and good ink-uptake in imaged parts.
EXAMPLE 3
[0070] An imaging element was prepared according to the invention as described in example
1 with the exception that the pH of the aqueous dispersion (the top layer) was 10.
This material was imaged with an external drum IR-laser imaging apparatus (diode laser
830 nm, drumspeed 1 m/s, at addressabilities 5000 dpi and 200 dpi, power level in
image plane 60-120 mW), and developed in an aqueous developing solution (EN144 negative
developer commercially available from Agfa) having a pH = 8.3, additionally a gum
solution (Polychrome PC804 gum) was applied to the developed plate and it was subsequently
baked for 2 minutes at 200 °C.
[0071] The material was selectively cleaned-out with full clean-out in the non-imaged parts;
optical density (Macbeth RD918-SB/Cyan filter) :
- imaged parts : 0.31
- non-imaged parts : 0.00
At 5000 dpi images were obtained using 40 mW power or more in imageplane.
At 200 dpi images were obtained using 70 mW power or more in imageplane.
[0072] This plate was used for printing on an Heidelberg GTO printing machine with a conventional
ink (AB.Dick 1020) and fountain solution (Rotamatic), resulting in good prints, i.e.
no scumming in non-imaged parts and good ink-uptake in imaged parts.
EXAMPLE 4
[0073] An imaging element according to the invention was prepared by first coating on a
lithographic base as described in example 1 a 1.25% solution in methylethylketone
of Alnovol PN 249 binder ( 91%) and trihydroxybenzophenone (9%) in a dry weight of
0.1 g/m
2). Thereon was coated an 2% aqueous dispersion of carbon black (10%), polystyrene
latex (75%) and of a compound according to formula I wherein n is 70%, p is 3%, m
+ q is 27% in a dry weight of 0.6 g/m
2).
This layer was coated from aqueous medium at pH = 7.0.
[0074] The material was imaged with an internal drum IR-laser imaging apparatus (NdYAG laser
1060 nm, drumspeed 367 m/s, adressability 2400 dpi, power level in image plane 6 W),
and develped in an aqueous developing solution (mixture of 4 parts EN144 negative
developer and 1 part EP 351B positive developer, both solutions commercially available
from Agfa) with 3 parts of water, said solution having a pH = 13.
[0075] With this material a selective clean-out could be obtained, with full clean-out in
the non-imaged parts : This plate was used for printing on an Heidelberg GTO printing
machine with a conventional ink (AB.Dick 1020) and fountain solution (Rotamatic),
resulting in good prints, i.e. no scumming in non-imaged parts and good ink-uptake
in imaged parts.
EXAMPLE 5
Preparation of the lithographic base: as described in example 1
Preparation of the imaging element
[0076] An imaging element according to the invention was prepared by first coating on the
lithographic base a 1.25 % solution in methylethylketone of a compound according to
formula I wherein n is 70%, p is 3%, m + q is 27% in a wet thickness of 30 µm (dry
weight of 0.3 g/m
2). Thereon was coated an aqueous dispersion of carbon black (0.06 g/m
2) and of a copolymer consisting of terephthalic acid (58 mol%), isophthalic acid (40
mol%) and sulphoisophthalic acid with ethylene glycol, said dispersed particles having
a particle size of 67 nm (0.54 g/m
2).
This layer was coated from aqueous medium at pH = 5.8.
[0077] The material was imaged with an external drum IR-laser imaging apparatus (Nd laser
1064 nm, drumspeed 1 m/s, adressability 200 and 5000 dpi, power level in image plane
150-400 mW), and develped in an aqueous developing solution (EN144 negative developer
commercially available from Agfa, pH adjudsted with HCl to 6.6).
[0078] With these material a selective clean-out could be obtained, with full clean-out
in the non-imaged parts :
- Optical density (Macbeth RD918-SB/Black filter)
- imaged parts : 0.33
- non-imaged parts : 0.00
[0079] At both 200 and 5000 dpi images were obtained using 150 mW power in the image plane.
[0080] This plate was used for printing on an Heidelberg GTO printing machine with a conventional
ink (AB.Dick 1020) and fountain solution (Rotamatic), resulting in good prints, i.e.
no scumming in non-imaged parts and good ink-uptake in imaged parts.
1. Ein wärmeempfindliches bilderzeugendes Element, das
- eine lithografische Unterlage mit einer hydrophilen Oberfläche,
- eine bilderzeugende Schicht mit einem hydrophoben thermoplastischen polymeren Latex
und in der bilderzeugenden Schicht oder einer daran grenzenden Schicht eine Verbindung,
die Licht in Wärme umzuwandeln vermag, und
- zwischen der lithografischen Unterlage mit einer hydrophilen Oberfläche und der
bilderzeugenden Schicht eine Sperrschicht enthält,
dadurch gekennzeichnet, daß
- die Sperrschicht ein Bindemittel aus der Gruppe bestehend aus einem Polymeren mit
phenolischen oder Carboxylgruppen oder phenolischen und Carboxylgruppen und einem
(Co)polymeren mit Hydroxylgruppen, die zumindest teilweise mit einer zumindest zwei
Carboxylgruppen enthaltenden Verbindung reagiert haben, enthält, und
- die Sperrschicht und die bilderzeugende Schicht in einer wäßrigen Lösung mit einem
pH von zumindest 5 entfernbar sind.
2. Ein wärmeempfindliches bilderzeugendes Element nach Anspruch 1, dadurch gekennzeichnet, daß die Sperrschicht ein Novolakharz enthält.
3. Ein wärmeempfindliches bilderzeugendes Element nach Anspruch 1, dadurch gekennzeichnet, daß das Hydroxylgruppen enthaltende (Co)polymere ebenfalls hydrophobe Gruppen enthält.
4. Ein wärmeempfindliches bilderzeugendes Element nach irgendeinem der Ansprüche 1 bis
3, dadurch gekennzeichnet, daß die Verbindung, die Licht in Wärme umzuwandeln vermag, aus der Gruppe bestehend aus
einem infrarotabsorbierenden Farbstoff, Gasruß, einem Metallborid, einem Metallcarbid,
einem Metallnitrid, einem Metallcarbonitrid und einer leitfähigen Polymerdispersion
gewählt wird.
5. Ein wärmeempfindliches bilderzeugendes Element nach irgendeinem der Ansprüche 1 bis
4, dadurch gekennzeichnet, daß die lithografische Unterlage mit einer hydrophilen Oberfläche ein eloxierter Aluminiumträger
ist oder einen biegsamen Träger mit einer darüber vergossenen vernetzten hydrophilen
Schicht umfaßt.
6. Ein wärmeempfindliches bilderzeugendes Element nach irgendeinem der Ansprüche 1 bis
5, dadurch gekennzeichnet, daß die bilderzeugende Schicht ein hydrophiles Bindemittel enthält.
7. Ein wärmeempfindliches bilderzeugendes Element nach irgendeinem der Ansprüche 1. bis
5, dadurch gekennzeichnet, daß die bilderzeugende Schicht kein Bindemittel enthält.
8. Ein wärmeempfindliches bilderzeugendes Element nach irgendeinem der Ansprüche 1 bis
7, dadurch gekennzeichnet, daß der hydrophobe thermoplastische polymere Latex eine wasserdispergierende funktionelle
Gruppe enthält.
9. Ein durch die nachstehenden Schritte
gekennzeichnetes Verfahren zum Erhalt einer lithografischen Druckplatte :
(a) die bildmäßige oder informationsmäßige Beaufschlagung mit Licht oder Wärme eines
nach irgendeinem der Ansprüche 1 bis 8 definierten bilderzeugenden Elements,
(b) die Entwicklung des belichteten bilderzeugenden Elements mit einer wäßrigen Entwicklerlösung
mit einem pH von zumindest 5, um die unbelichteten Bereiche zu entfernen und dadurch
eine lithografische Druckplatte zu erhalten.