FIELD OF THE INVENTION
[0001] The present invention relates to a heat-sensitive material for preparing lithographic
printing plates.
[0002] More specifically the invention is related to a processless heat-sensitive material
which yields lithographic printing plates with a high endurance.
BACKGROUND OF THE INVENTION
[0003] Lithographic printing is the process of printing from specially prepared surfaces,
some areas of which are capable of accepting ink, whereas other areas will not accept
ink.
[0004] 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 ink-repelling background.
[0005] 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.
[0006] Upon imagewise exposure of such 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.
[0007] 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 daylight. Furthermore they
have a problem of unstable sensitivity with regard to the storage time and they show
a lower resolution. The trend towards heat-sensitive printing plate precursors is
clearly seen on the market.
[0008] 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 accepting 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.
[0009] Furthermore
EP-A- 770 494, 770 495, 770 496 and
770 497 disclose a method for making a lithographic printing plate comprising the steps of
(1) image-wise exposing to light a heat-sensitive 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; (2) and developing a thus obtained image-wise exposed
element by rinsing it with plain water.
[0010] The above mentioned heat-sensitive imaging elements for making lithographic printing
plates are not optimal regarding staining and scratch resistance.
OBJECTS OF THE INVENTION
[0011] It is an object of the present invention to provide a processless heat-sensitive
imaging material for making lithographic printing plates having excellent printing
properties.
[0012] It is a further object of the invention to provide a heat sensitive imaging material
for making lithographic printing plates with an improved scratch resistance.
[0013] Further objects of the present invention will become clear from the description hereinafter.
SUMMARY OF THE INVENTION
[0014] According to the present invention there is provided a heat-sensitive material for
making lithographic printing plates comprising on a lithographic support an image-forming
layer comprising a hydrophilic binder, a cross-linking agent for said hydrophilic
binder, metal oxide particles with a mean diameter of at least 100 nm and dispersed
hydrophobic thermoplastic polymer particles, characterized in that said image-forming
layer has a ratio of specific surface (in m
2 per g) over mean roughness( in µm) of more than 0.75 and that the mean pore width
is less than 15 nm.
DETAILED DESCRIPTION OF THE INVENTION
[0015] The specific surface of the coating (in m
2 per g) is measured by a Micromeritics ASAP2400-apparatus. Therefore the material,
including the support, is cut in small pieces and introduced into the apparatus, then
a sorption/desorption isotherm of the material is measured with nitrogen-gas as adsorbate.
[0016] From the obtained sorption/desorption isotherms, the specific surface is calculated,
following the sorption/desorption approximation corresponding with BET. Also the mean
pore diameter is calculated by the method of Barett, Joyner and Hallender.
[0017] The average surface roughness of the plate (in µm) is measured with a perthometer
MAHR PERTHEN S6P containing a measuring head RTK50 (tradename of Feinprüf Perthen
GmbH, Goettingen, Germany) equipped with a diamond stylus with a diameter of 50µm
under a pressure of 1.0 mN according to techniques well known in the art. The sampling
length Ls which is the reference length for roughness evaluation measures 0.25 mm.
The evaluation length Lm, being that part of the travelling length Lt which is evaluated
for acquiring the roughness profile R contains standard 5 consecutive sampling lengths.
The traversing length Lt is the overall length travelled by the tracing system when
acquiring the roughness profile. The average roughness Ra is the measured roughness
averaged over the evaluation length Lm.
[0018] Preferably the ratio of specific surface over mean roughness is more than 0.75, more
preferably more than 0.85. The mean pore width is preferably less than 10 nm, more
preferably less than 7 nm.
[0019] According to the present invention to improve sensitivity and throughput and to avoid
scumming an imaging element is provided comprising preferably hydrophobic thermoplastic
polymer particles with an average particle size between 40nm and 150nm. More preferably
the hydrophilic thermoplastic polymer particles are used with an average particle
size of 40nm to 80nm. Furthermore the hydrophobic thermoplastic polymer particles
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 particles under the influence of heat. There
is no specific upper limit to the coagulation temperature of the thermoplastic hydrophobic
polymer particles, however the temperature should be sufficiently below the decomposition
temperature of the polymer particles. Preferably the coagulation temperature is at
least 10°C below the temperature at which the decomposition of the polymer particles
occurs. When said polymer particles are subjected to a temperature above the coagulation
temperature they coagulate to form a hydrophobic agglomerate in the hydrophilic layer
so that at these parts the hydrophilic layer becomes hydrophobic and oleophilic.
[0020] Specific examples of hydrophobic polymer particles for use in connection with the
present invention have a Tg above 80°C. Preferably the polymer particles are selected
from the group consisting of polyvinyl chloride, polyvinylidene chloride, polyacrylonitrile,
polyvinyl carbazole etc., copolymers or mixtures thereof. Most preferably used are
polystyrene, polymethylmethacrylate or copolymers thereof.
[0021] The weight average molecular weight of the polymers may range from 5,000 to 5,000,000g/mol.
[0022] The polymer particles are 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 particles 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.
[0023] The amount of hydrophobic thermoplastic polymer particles contained in the image-forming
layer is preferably at least 10% by weight and more preferably at least 15% by weight
and most preferably at least 20% by weight of the total weight of said layer.
[0024] 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 or natural binders such as gelatin, a polysaccharide such as
e.g. dextran, pullulan, cellulose, arabic gum, alginic acid, inuline or chemically
modified inuline.
[0025] A cross-linked hydrophilic binder in the heat-sensitive layer used in accordance
with the present embodiment also contains substances that increase the mechanical
strength and the porosity of the layer e.g. metal oxide particles having an average
diameter of at least 100 nm which are particles of titanium dioxide or other metal
oxides. Incorporation of these particles gives the surface of the cross-linked hydrophilic
layer a uniform rough texture consisting of microscopic hills and valleys. Particularly
preferable is titanium dioxide, used in 50 to 95 % by weight of the heat-sensitive
layer, more preferably in 60 to 90% by weight of the heat-sensitive layer.
[0026] The image-forming layer also comprises crosslinking agents. such as formaldehyde,
glyoxal, polyisocyanate or a hydrolysed tetraalkylorthosilicate. The latter is particularly
preferred.
[0027] The imaging element can further include 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 and pigments and in particular infrared pigments
such as 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 i.a.on the heat-sensitivity of
the imaging element. In this respect it has been found that carbon black yields very
good and favorable results.
[0028] 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.
[0029] The imaging layer preferably contains surfactants which can be anionic, cationic,
non-ionic or amphoteric. Perfluoro surfactants are preferred. Particularly preferred
are non-ionic perfluoro surfactants. Said surfactants can be used alone or preferably
in combination.
[0030] The weight of the imaging layer ranges preferably from 1 to 12 g/m
2, more preferably from 3 to 9 g/m
2.
[0031] The lithographic base according to the present invention can be aluminum e.g. electrochemically
and/or mechanically grained and anodised aluminum.
[0032] Furthermore in connection with the present invention, the lithographic base can be
a flexible support. As flexible support in connection with the present embodiment
it is particularly preferred to use a plastic film e.g. substrated polyethylene terephthalate
film, polyethylene naphthalate film, cellulose acetate film, polystyrene film, polycarbonate
film, polyethylene film, polypropylene film, polyvinyl chloride film, polyether sulphone
film. The plastic film support may be opaque or transparent.
[0033] Still further also paper or glass of a thickness of not more than 1.2 mm can also
be used.
[0034] In accordance with the present invention the imaging element is image-wise exposed.
During said exposure, the exposed areas are converted to hydrophobic and oleophilic
areas while the unexposed areas remain hydrophilic.
[0035] Said image-forming can be realized by direct thermal recording wherein the thermal
transfer is effected by heat radiation, heat conductivity or inductive heat transport.
On the heated areas the hydrophobic polymer particles coagulate and forms a hydrophobic
area while on the non-heated areas the hydrophobic polymer particles remain unchangedand
said area remains hydrophilic.re
[0036] Said image-forming can also effected by irradiation with high intensity light. The
heat-sensitive material should then comprise a compound capable of converting light
into heat.
[0037] 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. Preferably used are lasers
that operate in the infrared or near-infrared, i.e. wavelength range of 700-1500 nm.
Most preferred are laser diodes emitting in the near-infrared.
[0038] According to the present invention the plate is then ready for printing without an
additional development and can be mounted on the printing press.
[0039] According to a further method, the imaging element is first mounted on the printing
cylinder of the printing press and then image-wise exposed directly on the press.
Subsequent to exposure, the imaging element is ready for printing.
[0040] The printing plate of the present invention can also be used in the printing process
as a seamless sleeve printing plate. In this option the printing plate is soldered
in a cylindrical form by means of a laser. This cylindrical printing plate which has
as diameter the diameter of the print cylinder is slid on the print cylinder instead
of mounting a conventional printing plate. More details on sleeves are given in "Grafisch
Nieuws" , 15, 1995, page 4 to 6.
[0041] The following examples illustrate the present invention without limiting it thereto.
All parts and percentages are by weight unless otherwise specified.
EXAMPLES
Comparative example
[0042] To 446 g of an aqueous dispersion comprising 25% by weight of TiO
2 with average particle size between 0.3 and 0.5 µm and 2.5% by weight of polyvinylalcohol
(hydrolysed polyvinylacetate, marketed by Wacker Chemie GmbH, F.R. Germany, under
the trademark POLYVIOL WX), 218 g of an aqueous dispersion of hydrolysed tetramethoxysilane
(22% by weight of hydrolysed tetramethoxysilane) was added. To this mixture 10g of
a 4.1% by weight solution of AKYPO OP80 ™ was added. Akypo OP80 is a commercial available
surfactant from Chemy. Also 2 g of a 5% by weight solution of a fluorosurfactant,
N-polyoxyethyleneethyl-perfluorooctane acid amide was added.
[0043] The volume was adjusted to 1000 ml with distilled water. The pH was adjusted to 4.0
with NaOH.
[0044] The solution was applied to a heat-set, biaxially oriented polyethylene terephtalate
film with a thickness of 175µm, so that a total thickness of 6.83 g/m
2 of the coating was present. The coating was applied at a wet thickness of 50 µm and
the film was dried under impingement drying with air from 50°C and a moisture content
of 4 g/m
3.
Example 1
[0045] To 348 g of an aqueous dispersion comprising 25% by weight of TiO2 with an average
particle size between 0.3 and 0.5 µm and 2.5% by weight of polyvinylalcohol (hydrolysed
polyvinylacetate, marketed by Wacker Chemie GmbH, F.R. Germany, under the trademark
POLYVIOL WX), 170 g of an aqueous dispersion of hydrolysed tetramethoxysilane (22%
by weight of hydrolysed tetramethoxysilane) was added. To this mixture 10g of a 4.1%
by weight solution of AKYPO OP80 ™ was added. Akypo OP80 is a commercial available
surfactant from Chemy. Also 2 g of a 5% by weight solution of a fluorosurfactant,
N-polyoxyethyleneethyl-perfluorooctane acid amide was added.
[0046] Then 242.5 g of a polystyrene emulsion was added. This emulsion was 12.05% by weight
and non-ionically stabilised.
[0047] The volume was adjusted to 1000 ml with distilled water. The pH was adjusted to 4.0
with NaOH.
[0048] The solution was applied to a heat-set, biaxially oriented polyethylene terephtalate
film with a thickness of 175µm, so that a total thickness of 6.83 g/m
2 of the coating was present. The coating was applied at a wet thickness of 50 µm and
the film was dried under impingement drying with air from 50°C and a moisture content
of 4 g/m
3.
Example 2
[0049] To 312 g of an aqueous dispersion comprising 25% by weight of TiO
2 with an average particle size between 0.3 and 0.5 µm and 2.5% by weight of polyvinylalcohol
(hydrolysed polyvinylacetate, marketed by Wacker Chemie GmbH, F.R. Germany, under
the trademark POLYVIOL WX), 152 g of an aqueous dispersion of hydrolysed tetramethoxysilane
(22% by weight of hydrolysed tetramethoxysilane) was added. To this mixture 10g of
a 4.1% by weight solution of AKYPO OP80 ™ was added. Akypo OP80 is a commercial available
surfactant from Chemy. Also 2 g of a 5% by weight solution of a fluorosurfactant,
N-polyoxyethyleneethyl-perfluorooctane acid amide was added.
[0050] Then 330.6 g of a polystyrene emulsion was added. This emulsion was 12.05% by weight
and non-ionically stabilised. Also 2 g of an IR-dye (structure I) was added. This
compound was premixed in 18 g of ethanol.
[0051] The volume was adjusted to 1000 ml with distilled water. The pH was adjusted to 4.0
with NaOH.
[0052] The solution was applied to a heat-set, biaxially oriented polyethylene terephtalate
film with a thickness of 175µm, so that a total thickness of 6.83 g/m
2 of the coating was present. The coating was applied with a wet thickness of 50 µm
and the film was dried under impingement drying with air from 50°C and a moisture
content of 4 g/m
3.
Example 3
[0053] To 332 g of an aqueous dispersion comprising 25% by weight of TiO
2 with an average particle size between 0.3 and 0.5 µm and 2.5% by weight of polyvinylalcohol
(hydrolysed polyvinylacetate, marketed by Wacker Chemie GmbH, F.R. Germany, under
the trademark POLYVIOL WX), 79.1 g of an aqueous dispersion of hydrolysed tetramethoxysilane
(22% by weight of hydrolysed tetramethoxysilane) was added. To this mixture 10g of
a 4.1% by weight solution of AKYPO OP80 ™ was added. Akypo OP80 is a commercial available
surfactant from Chemy. Also 2 g of a 5% by weight solution of a fluorosurfactant,
N-polyoxyethyleneethyl-perfluorooctane acid amide was added.
[0054] Then 331 g of a polystyrene emulsion was added. This emulsion was 12.05% by weight
and non-ionically stabilised.
[0055] The volume was adjusted to 1000 ml with distilled water. The pH was adjusted to 4.0
with NaOH.
[0056] The solution was applied to a heat-set, biaxially oriented polyethylene terephtalate
film with a thickness of 175µm, so that a total thickness of 6.83 g/m
2 of the coating was present. The coating was applied at a wet thickness of 50 µm and
the film was dried under impingement drying with air from 50°C and a moisture content
of 4 g/m
3.
Example 4
[0057] To 308 g of an aqueous dispersion comprising 25% by weight of TiO
2 with average particle size between 0.3 and 0.5 µm and 2.5% by weight of polyvinylalcohol
(hydrolysed polyvinylacetate, marketed by Wacker Chemie GmbH, F. R. Germany, under
the trademark POLYVIOL WX), 73.5 g of an aqueous dispersion of hydrolysed tetramethoxysilane
(22% by weight of hydrolysed tetramethoxysilane) was added. Also 175 g of a 5% by
weight of a polyvinylalcohol solution was added. The used polyvinylalcohol is POLYVIOL
WX 48/20, commercially available from Wacker, Burghausen, Germany. To this mixture
10g of a 4.1% by weight solution of AKYPO OP80 ™ was added. Akypo OP80 is a commercially
available surfactant from Chemy. Also 2 g of a 5% by weight solution of a fluorosurfactant,
N-polyoxyethyleneethyl-perfluorooctane acid amide was added.
[0058] Then 331 g of a polystyrene emulsion was added. This emulsion was 12.05% by weight
and non-ionically stabilised.
[0059] The volume was adjusted to 1000 ml with distilled water. The pH was adjusted to 4.0
with NaOH.
[0060] The solution was applied to a heat-set, biaxially oriented polyethylene terephtalate
film with a thickness of 175µm, so that a total thickness of 6.83 g/m
2 of the coating was present. The coating was applied with a wet thickness of 50 µm
and the film was dried under impingement drying with air from 50°C and a moisture
content of 4 g/m
3.
Example 5
[0061] To 314 g of an aqueous dispersion comprising 25% by weight of TiO
2 with average particle size between 0.3 and 0.5 µm and 2.5% by weight of polyvinylalcohol
(hydrolysed polyvinylacetate, marketed by Wacker Chemie GmbH, F.R. Germany, under
the trademark POLYVIOL WX), 74.6 g of an aqueous dispersion of hydrolysed tetramethoxysilane
(22% by weight of hydrolysed tetramethoxysilane) and 7.4 g of glycerol was added.
To this mixture 10g of a 4.1% by weight solution of AKYPO OP80 ™ was added. Akypo
OP80 is a commercial available surfactant from Chemy. Also 2 g of a 5% by weight solution
of a fluorosurfactant, N-polyoxyethyleneethyl-perfluorooctane acid amide was added.
[0062] Then 331 g of a polystyrene emulsion was added. This emulsion was 12.05% by weight
and non-ionically stabilised.
[0063] The volume was adjusted to 1000 ml with distilled water. The pH was adjusted to 4.0
with NaOH.
[0064] The solution was applied to a heat-set, biaxially oriented polyethylene terephtalate
film with a thickness of 175µm, so that a total thickness of 6.83 g/m
2 of the coating was present. The coating was applied with a wet thickness of 50 µm
and the film was dried under impingement drying with air from 50°C and a moisture
content of 4 g/m
3.
Nanostructure of Heat-sensitive Imaging element
[0065] The specific surface and the pore diameter of the coating was measured by a Micromeritics
ASAP2400-apparatus.
[0066] The average surface roughness of the plate is measured with a perthometer MAHR PERTHEN
S6P containing a measuring head RTK50 (tradename of Feinprüf Perthen GmbH, Goettingen,
Germany) equipped with a diamond stylus with a diameter of 5µm under a pressure of
1.0 mN.
Lithographic properties:
Sensitivity to staining:
[0067] The lithographic properties of the thermal imaging element was tested on a Heidelberg
GTO 52 with a Van Son Rubberbase RB2329 ink and Rotamatic fountain. Before testing
the lithographic properties, the press was ran during 3000 prints to obtain 'equilibrium'-conditions.
Then the test plates were mounted on the press without wetting. The press was then
rotated 10 times with contact from the plates with the Dahlgren dampening system.
Then contact was made with the ink rollers and after 5 rotations, contact was made
with paper. Staining on the printed papers was given a visual quotation.
Thermal sensitivity:
[0068] The above mentioned materials were imaged in an OYO GS618 thermal printer, with a
resolution of 400 dpi, printed under standard conditions at 0.2 inch/s.
[0069] After imaging, the plates were tested on an AB Dick 360 press, using Van Son Rubber
Base ink and Tame 2% fountain solution. On the printed papers, image quality was visually
evaluated. The run length was 250.
Physical properties:
[0070] The physical properties of the imaging element were evaluated by measuring the scratch
resistance. In this test, the mechanical properties and the adhesion are quantified.
Sratching the heat-sensitive imaging element
[0071] The above mentioned materials were scratched in the test 'Linisoft'. This test simulates
the mechanical strain in the printing process. First, the imaging element was swollen
in distilled water untill equilibrium occured . For safety, a time of 2 minutes was
applied.
[0072] In this test scratches are formed by displacing needles at a speed of 96 cm/min,
under well defined loads. The needles are of ruby type with a radius of 1.5 mm. 15
scratches are formed under following loads: 57 - 85 - 114 - 142 - 170 - 113 - 169
- 225 - 282 - 338 - 400 - 600 - 800 - 1000 en 1200 mN.
Evaluation of the scratch resistance
[0073] After drying the image element, the 15 scratches were controlled on width of damage
and given a corresponding quotation as indicated in table 1.
[0074] When the depth of the scratch was down to the support, this means the total layer
was removed, then an extra value was added. This phenomenon was visible by a discoloration
to transparency in the scratch region. This value was 3 when the discoloration was
local. When the entire scratch was transparant, a value of 5 was added.
Table 1
Quotation |
Width of scratch |
0 |
no scratch visible |
1 |
scratch smaller than 50µm |
2 |
width between 50 and 100 µm |
3 |
width between 100 and 150 µm |
4 |
width between 150 and 200 µm |
5 |
width greater than 200 µm |
+3 |
when scratch is broken white line |
+5 |
when scratch is fully white |
[0075] The sum of all given quotations resulted in the scratch resistance of te material.
The lower the value, the better the scratch resistance.
Results:
[0076]
|
Ratio |
MPW |
Linisoft |
Staining |
Image quality/print length |
Comparative |
0.339 |
3.9 |
0 |
very good |
not thermally sensitive |
Ex 1 |
0.892 |
1.6 |
0 |
very good |
very good |
Ex 2 |
0.775 |
4.1 |
0 |
good |
good |
Ex 3 |
0.720 |
19 |
59 |
bad |
bad |
Ex 4 |
0.591 |
4.0 |
29 |
bad |
bad |
Ex 5 |
1.256 |
17 |
38 |
bad |
bad |
Ratio: Ratio of specific surface on mean roughness (g/m2.µm)
MPW: mean pore width (nm). |
[0077] From the results it is clear that and the ratio of specific surface over mean roughness
should be higher than 0.65 and the mean pore width should be less than 15 nm of an
heat-sensitive material in order to obtain a lithographic plate with a good scratch
resistance and no or almost no staining.