FIELD OF THE INVENTION
[0001] The present invention relates to a heat-mode method for preparing lithographic printing
plates.
[0002] More specifically the invention is related to a method for making a lithographic
printing plate by a negative working non-ablative process.
BACKGROUND OF THE INVENTION
[0003] Rotary printing presses use a so-called master such as a printing plate which is
mounted on a cylinder of the printing press. The master carries an image which is
defined by the ink accepting areas of the printing surface and a print is obtained
by applying ink to said surface and then transferring the ink from the master onto
a substrate, which is typically a paper substrate. In conventional lithographic printing,
ink as well as an aqueous fountain solution are fed to the printing surface of the
master, which is referred to herein as lithographic surface and 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.
[0004] Printing masters are generally obtained by the so-called computer-to-film method
wherein various pre-press steps such as typeface selection, scanning, colour separation,
screening, trapping, layout and imposition are accomplished digitally and each colour
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.
[0005] In recent years the so-called computer-to-plate method has gained a lot of interest.
This method, also called direct-to-plate method, bypasses the creation of film because
the digital document is transferred directly to a plate precursor by means of a so-called
plate-setter. In the field of such computer-to-plate methods the following improvements
are being studied presently :
(i) On-press imaging. A special type of a computer-to-plate process, involves the
exposure of a plate precursor while being mounted on a plate cylinder of a printing
press by means of an image-setter that is integrated in the press. This method may
be called 'computer-to-press' and printing presses with an integrated image-setter
are sometimes called digital presses. A review of digital presses is given in the
Proceedings of the Imaging Science & Technology's 1997 International Conference on
Digital Printing Technologies (Non-Impact Printing 13). Computer-to-press methods
have been described in e.g. EP-A 770 495, EP-A 770 496, WO 94001280, EP-A 580 394 and EP-A 774 364. The best known imaging methods are based on ablation. A problem associated with
ablative plates is the generation of debris which is difficult to remove and may disturb
the printing process or may contaminate the exposure optics of the integrated image-setter.
Other methods require processing with chemicals which may damage the electronics and
other devices of the press.
(ii) On-press coating. Whereas a plate precursor normally consists of a sheet-like
support and one or more functional coatings, computer-to-press methods have been described
wherein a composition, which is capable to form a lithographic surface upon image-wise
exposure and optional processing, is provided directly on the surface of a plate cylinder
of the press. EP-A- 101 266 describes the coating of a hydrophobic layer directly on the hydrophilic surface
of a plate cylinder. After removal of the non-printing areas by ablation, a master
is obtained. However, ablation should be avoided in computer-to-press methods, as
discussed above. US-P 5,713,287 describes a computer-to-press method wherein a so-called switchable polymer such
as tetrahydro-pyranyl methylmethacrylate is applied directly on the surface of a plate
cylinder. The switchable polymer is converted from a first water-sensitive property
to an opposite water-sensitive property by image-wise exposure. The latter method
requires a curing step and the polymers are quite expensive because they are thermally
unstable and therefore difficult to synthesise. EP-A- 802 457 describes a hybrid method wherein a functional coating is provided on a plate support
that is mounted on a cylinder of a printing press. This method also needs processing.
A major problem associated with known on-press coating methods is the need for a wet-coating
device which needs to be integrated in the press.
(iii) Elimination of chemical processing. The development of functional coatings which
require no chemical processing or may be processed with plain water is another major
trend in plate making. WO- 90002044, WO- 91008108 and EP-A- 580 394 disclose such plates, which are, however, all ablative plates. In addition, these
methods require typically multi-layer materials, which makes them less suitable for
on-press coating. A non-ablative plate which can be processed with plain water is
described in e.g. EP-A- 770 497 and EP-A- 773 112. Such plates also allow on-press processing, either by wiping the exposed plate with
water while being mounted on the press or by the fountain solution during the first
runs of the printing job.
(iv) Thermal imaging. Most of the computer-to-press methods referred to above use
so-called thermal materials, i.e. plate precursors or on-press coatable compositions
which comprise a compound that converts absorbed light into heat. The heat which is
generated on image-wise exposure triggers a (physico-)chemical process, such as ablation,
polymerisation, insolubilisation by cross-linking of a polymer, decomposition, or
particle coagulation of a thermoplastic polymer latex. This heat-mode process then
results in a lithographic surface consisting of ink accepting and ink repelling areas.
[0006] EP-A- 786 337 discloses a process for imaging a printing plate, wherein the printing plate is charged
over the whole surface and wherein the whole surface is covered with fluid toner particles,
which are charged oppositely. Thereon is the layer, formed by the particles imagewise
fixed or imagewise ablated by infrared exposure on the surface of the printing plate.
Thereafter the parts which are not fixed are removed and optionally the non-ablated
areas are fixed by heating over the whole surface of the plate. This process requires
a cumbersome development.
OBJECTS OF THE INVENTION
[0007] It is an object of the present invention to provide a method for making lithographic
printing plates having excellent printing properties, which is suitable for on-press
coating and on-press thermal imaging and which does not require chemical processing.
[0008] It is still a further object of the invention to provide a heat sensitive imaging
material for making lithographic printing plates which can be used in computer to
plate application.
[0009] Further objects of the present invention will become clear from the description hereinafter.
SUMMARY OF THE INVENTION
[0010] According to the present invention there is provided a method for making a lithographic
printing plate comprising the steps of
- applying a first magnetic field to a dry, light absorbing powder, which comprises
a magnetic material and a hydrophobic thermoplastic binder, thereby coating said powder
on a surface of a metal support;
- image-wise exposing to light the powder in contact with the surface of the metal support,
thereby increasing the adhesion of the powder to the surface of the metal support,
without substantially ablating the powder; and
- removing the non-exposed magnetic powder from the surface of the metal support under
action of a second magnetic field with a polarity substantially opposite to the first
magnetic field.
DETAILED DESCRIPTION OF THE INVENTION
[0011] The metal support is preferably pure aluminum or an aluminum alloy, the aluminum
content of which is at least 95%. The thickness of the support usually ranges from
about 0.13 to about 0.50 mm.
[0012] The preparation of aluminum or aluminum alloy foils for lithographic offset printing
comprises the following steps : graining, anodizing, and optionally sealing of the
foil.
[0013] Graining and anodization of the foil are necessary to obtain a lithographic printing
plate that allows to produce high-quality prints in accordance with the present invention.
Sealing is not necessary but may still improve the printing results. Preferably the
aluminum foil has a roughness with a CLA value between 0.2 and 1,5 µm, an anodization
layer with a thickness between 0.4 and 2.0 µm and is posttreated.
[0014] The roughening of the aluminum foil can be performed according to the methods well
known in the prior art. The surface of the aluminum substrate can be roughened either
by mechanical, chemical or electrochemical graining or by a combination of these to
obtain a satisfactory adhesion of a layer to the aluminum support and to provide a
good water retention property to the areas that will form the non-printing areas on
the plate surface.
[0015] The electrochemical graining process is preferred because it can form a uniform surface
roughness having a large average surface area with a very fine and even grain which
is commonly desired when used for lithographic printing plates.
[0016] The roughening is preferably preceded by a degreasing treatment mainly for removing
greasy substances from the surface of the aluminum foil, e.g. by applying a surfactant
and/or an aqueous alkaline solution.
[0017] Preferably roughening is followed by a chemical etching step using an aqueous solution
containing an acid. The chemical etching is preferably carried out at a temperature
of at least 30°C more preferably at least 40°C and most preferably at least 50°C.
[0018] After roughening and optional chemical etching the aluminum foil is anodized which
may be carried out as follows.
[0019] An electric current is passed through the grained aluminum foil immersed as an anode
in a solution containing an acid. An electrolyte concentration from 1 to 70 % by weight
can be used within a temperature range from 0-70°C. The anodic current density may
vary from 1-50 A/dm
2 and a voltage within the range 1-100 V to obtain an anodized film weight of 1-8 g/m2
Al
2O
3.H
2O. The anodized aluminum foil may subsequently be rihsed with demineralised water
within a temperature range of 10-80°C.
[0020] The 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 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, 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. 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.
[0021] The magnetic powder comprises a hydrophobic thermoplastic binder, a magnetic material
and preferably a release agent. The binder resin used in the present invention may
for example include hydrophobic thermoplastic vinyl resins, polyester resins and epoxy
resins, Among these, vinyl resins and polyester resins are preferred in view of fixability.
[0022] Examples of vinyl monomers to be used for providing a vinyl polymer constituting
the binder resin of the present invention may include: Styrene; styrene derivatives,
such as o-methylstyrene, m-methylstyrene, p-methylstyrene, p-methoxystyrene, p-phenylstyrene,
p-chlorostyrene, 3,4-dichlorostyrene, p-ethylstyrene, 2,4-dimethylstyrene, p-n-butylstyrene,
p-tert.-butylstyrene, p-n-hexylstyrene, p-n-octylstyrene, p-n-nonylstyrene, p-n-decylstyrene,
and p-n-dodecylstyrene; ethylenically unsaturated olefins, such as ethylene, propylene,
butylene and isobutylene; unsaturated polyenes, such as butadiene; halogeneted vinyls,
such as vinyl chloride, vinylidene chloride, vinyl bromide and vinyl fluoride; vinyl
esters, such as vinyl acetate, vinyl propionate and vinyl benzoate, methacrylates
such as methyl methacrylate, ethyl methacrylate, propyl methacrylate, n-butyl methacrylate,
isobutyl methacrylate, n-octyl methacrylate, dodecyl methacrylate, 2-ethylhexyl methacrylate,
stearyl methacrylate, phenyl methacrylate, dimethylamonoethyl methacrylate and diethylaminoethyl
methacrylate; acrylates, such as methyl acrylate, ethyl acrylate, n-butyl acrylate,
isobutyl acrylate, propyl acrylate, n-octyl acrylate, dodecyl acrylate, 2-ethylhexyl
acrylate, stearyl acrylate, 2-chloroethyl acrylate and phenyl acrylate; vinyl ethers
such as vinyl ethyl ether and vinyl isobutyl ether; vinyl ketones, such as vinyl hexyl
ketone and methyl isopropenyl ketone; vinyl naphthalenes. These vinyl monomers may
be used singly or in combination of two or more species.
[0023] Among these, a combination of monomers providing styrene-type copolymers and styrene-acrylic
type copolymers may be particularly preferred.
[0024] A suitable polyester resin for use in the present invention may preferably have a
composition that comprises 45-55 mole % of alcohol component and 55-45 mole % of acid
component
[0025] Examples of the alcohol component may include: diols, such as ethylene glycol, propylene
glycol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, diethylene glycol, triethylene
glycol, 1,5-pentanediol, 1,6-hexanediol, neopentylglycol, 2-ethyl-1,3-hexanediol,
hydrogenated bisphenol A.
[0026] Examples of the acid constituting at least 50 mole % of the total acid may include
benzenedicarboxylic acids, such as phthalic acid, terephthalic acid and isophthalic
acid, and their anhydrides; alkyldicarboxylic acids, such as succinic acid, adipinic
acid, sebacic acid and azelaic acid, and their anhydrides; C
6-C
18 alkyl or alkenyl-substituted succinic acids, and their anhydrides; and unsaturated
dicarboxylic acids, such as fumaric acid, maleic acid, citraconic acid and itaconic
acid, and their anhydrides.
[0027] Examples of polybasic carboxylic acids having three or more functional groups may
include;trimellitic acid, pyromellitic acid, benzophenonetetracarboxylic acid, and
their anhydrides.
[0028] A preferred polyester for use in the present invention may preferably have a glass
transition temperature of 50°-75°C, particularly 55°-65°C, a number-average molecular
weight (Mn) of 1,500-50,000, particularly 2,000-20,000, and a weight-average molecular
weight of 6,000-100,000, particularly 10,000-90,000 and a diameter between 0.50 and
10 µm.
[0029] Examples of the magnetic material contained in the magnetic powder used in the present
invention may include: iron oxides such as magnetite, hematite, and ferrite; iron
oxides containing another metal oxide; metals, such as Fe, Co and Ni, and alloys of
these metals with other metals , such as Al, Co, Cu, Pb, Mg, Ni, Sn, Zn, Sb, Be, Bi,
Cd, Ca, Mn, Se, Ti, W and V; and mixtures of the above.
[0030] Specific examples of the magnetic material may include: triiron tetroxide (Fe
3O
4), diiron trioxide (γ-Fe
2O
3), zinc iron oxide (ZnFe
2O
4), ytrium iron oxide (Y
3Fe
5O
12), cadmium iron oxide (CdFe
2O
4), gadolinium iron oxide (Gd
3Fe
5O
12), copper iron oxide (CuFe
2O
4), Lead iron oxide (PbFe
12O
19), nickel iron oxide (NiFe
2O
4), neodymium iron oxide (NdFe
2O
3), barium iron oxide (BaFe
12O
19), magnesium iron oxide (MgFe
2O
4), manganese iron oxide (MnFe
2O
4), lanthanium iron oxide (LaFeO
3), powdery iron (Fe), powdery cobalt (Co), and powdery nickel (Ni). The above magnetic
materials may be used singly or in mixture of two or more species. Particularly suitable
magnetic material for the present invention is fine powder of triiron tetroxide or
γ-diiron trioxide.
[0031] The magnetic material preferably shows the following magnetic properties when measured
by 10 kilo-Oersted, inclusive of: a coercive force of 20-150 Oersted, a saturation
magnetization of 50-200 emu/g, particularly 50-100 emu/g, and a residual magnetization
of 2-20 emu/g.
[0032] Preferred compounds capable of converting light into heat are dyes, preferably infrared
dyes and pigments, preferably infrared pigments. The powder of the present invention
preferably also contains a compound capable of converting light into heat. Alternatively,
the magnetic material itself may be used as light absorbing compound.
[0033] In the present invention, it is also possible to incorporate one or more species
of release agent, as desired within a magnetic powder.
[0034] Examples of the release agent may include : aliphatic hydrocarbon waxes, such as
low-molecular weight polyethylene, low-molecular weight polypropylene, microcrystaline
wax, and paraffin wax, oxidation products of aliphatic hydrocarbon waxes, such as
oxidized polyethylene wax, and block copolymers of these; waxes containing aliphatic
esters as principal constituent, such as carnabau wax, sasol wax, montanic acid ester
wax, and partially or totally deacidified aliphatic esters, such as deacidified carnabau
wax. Further examples of the release agent may include: saturated linear aliphatic
acids, such as palmitic acid, stearic acid, and montanic acid; unsaturated aliphatic
acids, such as brassidic acid, eleostearic acid and palmitiric acid; saturated alcohols,
such as stearyl alcohol, arachidic alcohol, behenyl alcohol, carnaubyl alcohol, ceryl
alcohol, and melissyl alcohol; aliphatic acid amides, such as linoleyl amide, oleylamide
and laurylamide, saturated aliphatic acid bisamides, such as methylene-bisstearylamide,
ethylene-biscaprylamide, and ethylene-biscaprylamide; unsaturated aliphatic acid amides,
such as ethylene-bisoleylamide, hexamethylene-bisoleylamide, N, N'-dioleyladipoylamide,
and N,N'-dioleylsebacoylamide; aromatic bisamides, such as m-xylylene-bisstearoylamide,
and N,N'- distearylisophthalylamide; grafted waxes obtained by grafting aliphatic
hydrocarbon waxes with vinyl monomers, such as styrene and acrylic acid; partially
esterified products between aliphatic acids and polyhydric alcohols, such as behenic
acid monoglyceride; and methyl ester compounds having hydroxyl group as obtained by
hydrogenating vegetable fat and oil.
[0035] The release agent may preferably be used in an amount of 0.20 wt. parts, particularly
0.5-10 wt. parts, per 100 wt. parts of the binder resin.
[0036] The release agent may be uniformly dispersed in the binder resin by a method of mixing
the release agent in a solution of the resin at an elevated temperature under stirring
or melt-kneading the binder resin together with the release agent.
[0037] The above described metal support was placed in a first magnetic field with the hydrophilic
surface located in the direction of the magnetic powder. The polarity of the magnetic
field is so that the magnetic powder is attracted to the hydrophilic surface of the
support. After covering the hydrophilic surface of the support by the magnetic powder,
said imaging element is image-wise or information-wise exposed.
[0038] 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.
[0039] After said image-wise exposure, the imaging element was freed of magnetic powder
on the non-exposed areas by applying a magnetic field with the substantially opposite
polarity of the first field to said imaging element.
[0040] According to the present invention the plate is then ready for printing without an
additional development and can be mounted on the printing press.
[0041] Alternatively, some or all of the above steps can be performed on-press, i.e. after
mounting the support on the press.
[0042] The following example illustrates the present invention without limiting it thereto.
All parts and percentages are by weight unless otherwise specified.
EXAMPLES
[0043] An aluminum support was electrochemically grained using hydrochloric acid, anodized
in sulphuric acid and subsequently treated with polyvinylphosphonic acid.
[0044] The above described aluminum support was placed together with a dry magnetic powder
in a first magnetic field in such a way that the magnetic powder is attracted to the
hydrophylic surface of the aluminum support.
[0045] Two types of magnetic powder were used
1. Canon(type CB 743) contains magnetite, a binder with a release agent.
2. Agfa (Type T19076) contains 48.6 % polyester, 16.2 % copolymer of styrene/acrylate,
34 % magnetite and 1.2 % release agent (wax).
[0046] Subsequently the aluminum support covered with one of the magnetic powders was exposed
with an 830 nm diode laser (Isomet-3600 dpi-spot size 11µm-at a speed of 3.2 m/s;
i.e. pixel dwell time of 3.4 µs; the image plane power was varied: 80mW-190 mW-292
mW were used). The same aluminum supports were also exposed with an 1060 nm NdYLF
laser (Isomet-spot size 18 µm-speed 3.2 m/s; the power was varied between 250 mW and
750 mW).
[0047] On the non exposed parts, the magnetic powder was removed by applying a second magnetic
field with an opposite polarity as the first magnetic field. The thus obtained printing
plates were mounted on a conventional offset printing machine equipped with a conventional
ink and fountain solution.
Printing was started without any further treatment, and resulted in good prints with
good image quality.