Method for making a lithographic base and a lithographic printing plate therewith

DESCRIPTION
1. Field of the Invention
The present invention relates to a lithographic base. More particularly the present invention relates to an improved adhesion of the hydrophilic lithographic layer to the support of the lithographic base.
2. Background of the Invention
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, generally hydrophobic areas, and the ink-rejecting areas form the background areas, generally hydrophilic areas.
In the art of photolithography, a photographic material is made imagewise receptive to oily or greasy inks in the photo-exposed (negative-working) or in the non-exposed areas (positive-working) on a hydrophilic background.
In the production of common lithographic printing plates, also called surface litho plates or planographic printing plates, a lithographic base that has affinity to water or obtains such affinity by chemical treatment is coated with a thin layer of a photosensitive composition. Compositions for that purpose include light-sensitive materials such as light-sensitive polymers, diazonium salts or resins, a photoconductive layer, a silver halide emulsion etc. These materials are then image-wise exposed to actinic radiation and processed in the appropriate manner so as to obtain a lithographic printing plate.
In another embodiment, a silver precipitating (nucleating) agent is located in or on top of the hydrophilic surface. An image is obtained on the precipitating layer according to the silver salt diffusion transfer process by contacting said precipitating layer with an exposed silver halide emulsion in the presence of a silver halide developing agent and a silver halide solvent.
Several types of supports can be used for the manufacturing of a lithographic imaging printing plate. Common supports are for example organic resin supports, e.g. polyesters, and paper bases, e.g. polyolefin coated paper. These supports, if not sufficient hydrophilic by themselves, are first coated with a hydrophilic layer forming the hydrophilic lithographic background of the printing plate.
It is known to use as hydrophilic layer in these systems a layer containing polyvinyl alcohol and hydrolyzed tetra(m)ethyl orthosilicate and preferably also silicium dioxide and/or titanium dioxide as described in e.g. GB-P-1419512, FR-P-2300354, U.S. Pat. Nos. 3,971,660 and 4,284,705, EP-A-405016 and 450199 and U.S Ser. No. 07/881,718.
In the use of such lithographic printing plates, an oleophilic (ink receptive) image is present on a hydrophilic background. In printing, the printing plate is continuously wetted with water and ink. The water is selectively taken up by the hydrophilic areas, the ink by the oleophilic areas of the printing surface. During the printing process, there occurs abrasion of the hydrophilic layer due to poor adhesion of this layer to the support either point by point or over the whole surface. The adhesion of the hydrophilic layer to the support is adversely influenced by a higher water adsorption and thus a less rigid hydrophilic layer. Due to said abrasion the hydrophobic support comes to the surface. This leads to ink acceptance in the non-printing areas, causing staining of the plate.
In order to assure the adhesion of the hydrophilic layer to the support the support is first coated with one or more subbing layers on which are coated in direct contact therewith the hydrophilic layer. However said subbing layer(s) contain(s) organic compounds, the irreproducibility of which leads to adhesion problems between the subbed support and the hydrophilic layer. Furthermore a decreased water absorption resulting in less emulgation with the ink and thereby an improved printing comfort and a sharper printed image is still wanted. Still further an improved dry start-up is also wanted.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for making a lithographic base having on a support a hydrophilic layer of which the adhesion to the support is improved, especially qua reproducibility.
It is a further object of the present invention to provide a method for making a lithographic base having on a support a hydrophilic layer of which lithographic base the water absorption is decreased.
It is still a further object of the present invention to provide a method for making a lithographic base having on a support a hydrophilic layer of which lithographic base the dry upstart is improved.
Further objects of the present invention will become clear from the description hereinafter.
According to the present invention there is provided a method for obtaining a lithographic base comprising on a hydrophobic support a hydrophilic layer contiguous to said support containing a non-gelatineous hydrophilic (co)polymer or (co)polymer mixture, characterized in that said hydrophobic support is treated with a plasma treatment with an applied power density during the plasma treatment of at least 70 W min/m.sup.2 before applying to said support said hydrophilic layer.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
In the discipline of physics, the term "plasma" describes a partially ionized gas composed of ions, electrons and neutral species. This state of matter may be produced by the action of either very high temperatures, strong electric or radio frequency (R.F.) electromagnetic fields. High temperature or "hot" plasmas are represented by celestial light bodies, nuclear explosions and electric arcs. These are not suitable for the modification of polymeric materials. Glow discharge plasmas are produced by free electrons which are energized by an imposed direct current (DC) or R.F. electric fields and then collide with neutral molecules. These neutral molecule collisions transfer energy to the molecules and form a variety of active species including metastables, free radicals and ions. These active species are chemically active and/or physically modify the surface of materials and may therefore serve as the basis of new chemical compounds and property modifications of existing compounds. A plasma is also called the fourth aggregation phase. A plasma can be obtained by adding to a gas enough energy. Plasmas which are used to functionalize the surface of a support are preferentially created by means of an electric field. By exposing a polymeric support to a plasma, there are introduced a variety on functional groups on the surface. The bulk of the polymer remains unchanged.
Surprisingly it has been found that a lithographic base prepared by applying on a hydrophobic support which is plasma treated under the conditions given above a non-gelatinous hydrophilic layer contiguous to said support has improved properties qua adhesion of the hydrophilic layer to the support, qua water absorption and qua dry upstart. The power during the plasma treatment is preferably at least 120 W min/m.sup.2, more preferably at least 175 W min/m.sup.2.
Power density is expressed by the following formula
Pd=Ps/WW.times.Ls
wherein Pd stands for power density (in Watt min/m.sup.2)
Ps stands for power supply (in Watt),
WW stands for web width (in m), and
Ls stands for line speed (in m/min).
Various hydrophobic supports may be used in the lithographic base in connection with the present invention. Examples of such supports are organic resin supports e.g. cellulose acetate films and polyolefin (e.g. polyethylene) coated paper. A preferred support is polyethylene naphthalenedicarboxylate, a more preferred support is polyethylene terephthalate film. Said support has preferably a thickness between 40 and 500 .mu.m, more preferably between 100 and 350 .mu.m.
As hydrophilic (co)polymers in the hydrophilic layer of the lithographic base in connection with the present invention a non-gelatinous, preferably a non-proteinic (co)polymer is used. One may use, 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 the same as or higher than the hydrophilicity of polyvinyl acetate hydrolyzed to at least an extent of 80 percent by weight, preferably 95 percent by weight. Most preferably polyvinyl alcohol is used in the hydrophilic layer in connection with the present invention.
The hydrophilic layer of the lithographic base is preferably hardened. Preferred hardening agents are those of the epoxide type, those of the ethylenimine type, those of the vinylsulfone type e.g. 1,3-vinylsulphonyl-2-propanol, aldehydes e.g. formaldehyde, glyoxal, and glutaraldehyde, N-methylol compounds e.g. dimethylolurea and methyloldimethylhydantoin, dioxan derivatives e.g. 2,3-dihydroxy-dioxan, active vinyl compounds e.g. 1,3,5-triacryloyl-hexahydro-s-triazine, active halogen compounds e.g. 2,4-dichloro-6-hydroxy-s-triazine, and mucohalogenic acids e.g. mucochloric acid and mucophenoxychloric acid. These hardeners can be used alone or in combination. The binders can also be hardened with fast-reacting hardeners such as carbamoylpyridinium salts of the type, described in U.S. Pat. No. 4,063,952.
Preferably used hardening agents are tetraalkyl orthosilicate crosslinking agents. Examples of tetraalkyl orthosilicate crosslinking agents are hydrolyzed tetraethyl orthosilicate and hydrolyzed tetramethyl orthosilicate. The amount of tetraalkyl orthosilicate crosslinking agent is at least 0.2 parts by weight per part by weight of hydrophilic (co)polymer, preferably between 0.5 and 5 parts by weight, most preferably between 1 and 3 parts by weight.
The lithographic base can comprise only one hydrophilic layer. However the lithographic base can also include more than one hydrophilic layer, the hydrophilic layer contiguous to the support then being a part of the hydrophilic element of the lithographic base. The hydrophilic layer or layers not contiguous to the hydrophobic support differs from the hydrophilic layer contiguous to the hydrophobic support either in the nature of the hydrophilic (co)polymer and/or the nature of the crosslinking agent and/or the ratio between the hydrophilic (co)polymer and the crosslinking agent.
The hydrophilic layer or layers of the lithographic base 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 Stober 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 layer is given a uniform rough texture consisting of microscopic hills and valleys, which serve as storage places for water in background areas.
More details about suitable hydrophilic layers for use in connection with the present invention can be found in e.g. GB-P-1419512, FR-P-2300354, U.S. Pat. No. 3,971,660, U.S. Pat. No. 4,284,705, EP-A-405016, EP-A-450199 and U.S. Ser. No. 07/881,718.
In a first method to obtain a lithographic plate the hydrophilic lithographic base in accordance with the present invention may be coated with a thin layer of a heat- or photosensitive composition. The heat- or photosensitive composition can also be present partially or completely in the hydrophilic layer used in accordance with the present invention. Compositions for that purpose include heat- or light-sensitive substances such as heat-or light-sensitive polymers, diazonium salts or resins, quinonediazides, photoconductive layers, silver halide emulsions etc. These materials are then imagewise exposed to actinic radiation and processed in the appropriate manner so as to obtain a lithographic printing plate.
According to one embodiment of the present invention an imaging element is prepared by applying a layer comprising a photopolymerizable composition and a silver halide emulsion layer to the lithographic base of the present invention. After imagewise exposure of the silver halide emulsion layer and subsequent development a silver image is obtained. The thus obtained silver image is subsequently employed as a mask for the photopolymerizable composition during an overall exposure of the imaging element. Finally the silver image and the non-exposed photopolymerizable composition are removed so that a lithographic printing plate is obtained.
According to another embodiment of the present invention an imaging element is prepared by applying a layer comprising a o-naphtoquinonediazide compound and an alkali soluble resin to the lithographic base of the present invention. After imagewise exposure of the photosensitive element and subsequent development a lithographic printing plate is obtained.
More details about suitable o-naphtoquinonediazide containing lithographic compositions for use in connection with the present invention can be found in e.g. EP-A-345016 and EP-A-508268.
According to still another embodiment of the present invention a lithographic printing plate is produced by the following steps: (i) uniformly electrostatically charging a photoconductive layer, such as a coating of zinc oxide photoconductive pigment dispersed in the hydrophilic layer of the present invention by means of a corona-discharge, (ii) image-wise discharging said photoconductive layer by exposing it to electromagnetic radiation to which it is sensitive, (iii) applying electrostatically charged oleophilic toner particles to develop the resulting electrostatic charge pattern and (iv) fixing the toner to the photoconductive layer. Fixing is usually accomplished by the use of heat which causes the toner resin powder to coalesce and adhere to the photoconductive layer.
More details about suitable electrophotographic lithographic compositions for use in connection with the present invention can be found in e.g. U.S. Pat. No. 2,993,787.
In a preferred embodiment there is provided a heat recording material comprising the hydrophilic lithographic base of the present invention containing in homogeneously distributed state throughout the entire hydrophilic layer hydrophobic thermoplastic polymer particles having a softening or melting temperature of more than 30.degree. C. and that are capable of coagulating when brought above their softening or melting temperature, forming a hydrophobic agglomerate in the hydrophilic layer so that at these parts the hydrophilic layer becomes sufficiently hydrophobic to accept a greasy ink in lithographic printing wherein a dampening liquid is used.
Specific examples of hydrophobic polymer particles for use in connection with the present invention are e.g. polyethylene, polyvinyl chloride, polymethyl(meth)acrylate, polyethyl (meth)acrylate, polyvinylidene chloride, polyacrylonitrile, polyvinyl carbazole, polystyrene etc. or copolymers thereof.
The molecular weight of the polymers may range from 5,000 to 1,000,000.
The hydrophobic particles may have a particle size from 0.01 .mu.m to 50 .mu.m, more preferably between 0.05 .mu.m and 10 .mu.m and most preferably between 0.05 .mu.m and 2 .mu.m. The larger the polymer particles are the less the resolving power of the heat recording material will be.
The polymer particles are present as a dispersion in the aqueous coating liquid and may be prepared by the methods disclosed in U.S. Pat. No. 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.
The amount of hydrophobic thermoplastic polymer particles contained in the hydrophilic layer is preferably between 20% by weight and 65% by weight and more prerably between 25% by weight and 55% by weight and most preferably between 30% by weight and 45% by weight. When too low amounts are used the hydrophobicity produced at the exposed areas may be too small and as a consequence ink acceptance will be poor in these areas whereas too large amounts of the hydrophobic thermoplastic polymer particles may result in ink acceptance in the non-image areas due to a too large overall hydrophobicity of the hydrophilic layer.
The above described heat recording material can be exposed by actinic radiation while in contact with an original that contains a pattern of areas that are capable of converting the radiation into heat at these areas so that the hydrophobic thermoplastic polymer particles in the hydrophilic layer are softened or melted and coagulate in the exposed areas thereby increasing the hydrobicity of the hydrophilic layer at these areas.
An especially suitable radiation is e.g. infrared or near infrared radiation. As an original there may be used e.g. an imaged silver halide photographic material.
More details in connection with this embodiment can be found in e.g. U.S. Pat. No. 3,476,937 and U.S. Pat. No. 3,971,660.
In a further preferred embodiment there is provided a light-sensitive material comprising a diazonium salt or resin, contained in homogeneously distributed state throughout or preferably coated from a hydrophilic solution over the hydrophilic lithographic base of the present invention.
Examples of low-molecular weight diazonium salt for use in the present invention include: benzidine tetrazoniumchloride, 3,3'-dimethylbenzidine tetrazoniumchloride, 3,3'-dimethoxybenzidine tetrazoniumchloride, 4,4'-diaminodiphenylamine tetrazoniumchloride, 3,3'-diethylbenzidine tetrazoniumsulfate, 4-aminodiphenylamine diazoniumsulfate, 4-aminodiphenylamine diazoniumchloride, 4-piperidino aniline diazoniumsulfate, 4-diethylamino aniline diazoniumsulfate and oligomeric condensation products of diazodiphenylamine and formaldehyde.
Examples of diazo resins useful in the present invention include condensation products of an aromatic diazonium salt as the light-sensitive substance. Such condensation products are known and are described, for example, in DE-P-1214086. They are in general prepared by condensation of a polynuclear aromatic diazonium compound, preferably of substituted or unsubstituted diphenylamine-4-diazonium salts, with active carbonyl compounds, preferably formaldehyde, in a strongly acid medium.
Said light-sensitive layer preferably also contains a binder e.g. polyvinyl alcohol and may be applied to the lithographic base in a thickness of 0.2 .mu.m to 5 .mu.m. Said presensitized imaging element advantageously contains water-soluble dyes such as rhodamines, sudan blue, methylen blue, eosin or triphenylmethane dyes such as crystal violet, victoria pure blue, malachite green, methylviolet and fuchsin or dye pigments which are essentially water insoluble. Said dyes and/or dye pigments may be present in any layer comprised on the support of said presensitized imaging element but are preferably present in said hydrophilic layer and/or light-sensitive layer.
Exposure of the presensitized imaging element advantageously proceeds with ultraviolet light optionally in combination with blue light in the wavelength range of 250 to 500 nm. Useful exposure sources are high or medium pressure halogen mercury vapour lamps, e.g. of 1000 W. Since most lithography is done by the offset process, the imaging element is exposed in such a way that the image obtained thereon is right reading. The exposure may be an exposure using optics or a contact exposure.
The diazo resin or diazonium salts are converted upon exposure from water soluble to water insoluble (due to the destruction of the diazonium groups) and additionally the photolysis products of the diazo may induce an advancement in the level of crosslinking of the polymeric binder or diazo resin, thereby selectively converting the surface, in an image pattern, from water soluble to water insoluble. The unexposed areas remain unchanged, i.e. water soluble.
When mounted on a printing press the printing plate is first washed with an aqueous fountain solution. To prevent this fountain solution from being contaminated by residual non-exposed diazo the unexposed diazo resin or diazonium salt should be removed from the printing plate before mounting it on a printing press. This removal can be achieved by rinsing or washing the imaging element with water or an aqueous solution.
More details in connection with this embodiment can be found in e.g. FR-P-2300354 , U.S. Pat. No. 4,284,705 and EP-A-92203835.1.
In another method to obtain a lithographic plate the hydrophilic lithographic base in accordance with the present invention is used as an image-receiving element for a heat- or photosensitive composition.
In one embodiment the lithographic base in accordance with the present invention may be used as a receiving element in a thermal transfer process where a hydrophobic substance or composition is information-wise transferred from a donor element to said lithographic base. Such a process is described in e.g. U.S. Pat. Nos. 3,060,024, 3,085,488, 3,649,268 and EP-A-502,562.
According to a preferred embodiment of the present invention a toner image may be transferred to the lithographic base of the present invention during an electrophotographic process as disclosed in e.g. U.S. Pat. No. 3,971,660 and EP-A-405,016.
According to the most preferred embodiment of the present invention a layer of physical development nuclei may be applied to the lithographic base of the present invention. Suitable physical development nuclei for use in accordance with the present invention are e.g. colloidal silver, heavy metal sulphides e.g. silver sulphide, nickel sulphide, palladium sulphide, cobalt sulphide, zinc sulphide, silver nickel sulphide etc. The layer of physical development nuclei may contain a hydrophilic binder but preferably does not contain a binder. The physical development nuclei contained in the image receiving layer can also be present partially or completely in the hydrophilic layer used in accordance with the present invention. A thus prepared element can be used as the image-receiving element in a DTR-process. According to this method an image-wise exposed photographic material comprising a silver halide emulsion layer is contacted with said image-receiving element and developed in the presence of a silver halide solvent e.g. thiosulphate or thiocyanate and one or more developing agents. Both elements are subsequently separated and a silver image is formed in the layer of physical development nuclei comprised on the image-receiving element. More details about this process for obtaining a silver image in said receiving layer can be found in e.g. U.S. Pat. No. 4,649,096 or EP-A-397926. Said silver image is oleophilic while the background of the image-receiving element is oleophobic so that a lithographic printing plate results. It may however be advantageous to improve the oleophilicity of the silver image by treating the silver image with so-called hydrophobizing agents. U.S. Pat. No. 3,776,728 describes the use of heterocyclic mercapto-compounds, e.g. a 2-mercapto-1,3,4-oxadiazole derivative as hydrophobizing agents, U.S. Pat. No. 4,563,410 discloses hydrophobizing liquids containing one or more mercaptotriazole or mercaptotetrazole derivatives.
The following examples illustrate the invention without however limiting it thereto. All parts are by weight unless otherwise specified.

EXAMPLE 1
Preparation of the hydrophilic layer
To 440 g of a dispersion containing 21.5% TiO.sub.2 (average particle size 0.3 to 0.5 um) and 2.5% polyvinyl alcohol in deionized water were subsequently added, while stirring, 250 g of a 5% polyvinyl alcohol solution in water, 105 g of a hydrolyzed 22% tetramethyl orthosilicate emulsion in water and 22 g of a 10% solution of a wetting agent. To this mixture was then added 183 g of deionized water and the pH was adjusted to pH=4.
Preparation of the lithographic base
To five samples of polyethylene terephthalate support,treated as described in table 2 was applied the above mentioned hydrophilic layer to a wet coating thickness of 50 g/m.sup.2, dried at 30.degree. C. and subsequently hardened by subjecting it to a temperature of 60.degree. C. for 1 week. This layer was applied on the subbed side of the support for sample 1, on a raw side of the support for sample 2 and on the plasma treated side for sample 3, 4 and 5. The influence of the pretreatment of the support on the adhesion, water absorption of the lithographic bases and on a dry start-up is shown in table 2.
The adhesion was determined as follows:
place the test strips in an apparatus containing five small balls, the strips are transported through the apparatus
first ball 400 gram
second ball 600 gram
third ball 800 gram
fourth ball 1000 gram
fifth ball 1200 gram
after the transport note the quantitative results as given in table 1.
TABLE 1______________________________________Evaluation Damage______________________________________0 none 0.5 very superficially 1 damage not till the support 2 locally damaged till the support 3 fine scratch till the support 4 broad scratch till the support (>1 mm) 5 very broad scratch till the support (>2 mm)______________________________________
move the sample 5 mm perpendicular on the moving direction and repeat the previous step with balls of the following weight:
first ball 113 gram
second ball 163 gram
third ball 225 gram
fourth ball 282 gram
fifth ball 338 gram
note again the results.
move the sample 5 mm perpendicular on the moving direction and repeat the previous step with balls of the following weight:
first ball 57 gram
second ball 85 gram
third ball 114 gram
fourth ball 142 gram
fifth ball 170 gram
note again the results.
Sum up the fifteen noted result for each sample. The maximum value is 75. The lower the value of a sample the better the adhesion between the support and the hydrophilic layer of the sample.
The amount of water absorption, expressed in g/m.sup.2 was determined as follows:
preserving the dry film for 15 minutes in a conditioning room at 20.degree. C. and 30% RH,
covering the backing topcoat layer of the dry film with a water impermeable tape,
weighing the dry film,
immersing the material in demineralized water of 24.degree. C. for 10 minutes,
sucking up the excessive amount of water present on top of the outermost layers and
immediately determining the weight of the wet film and
calculating the measured weight differences between the wet and the dry film per square meter.
The dry start-up is measured as follows:
On the samples of the lithographic printing plate is drawn a rectangular of 30 mm width with an ink accepting fluid.
the samples are brought under identical conditions on a printing press Heidelberg GTO52 with a Dahlgren 3-rol dampening unit and a compressible rubber blanket. The ink used was Van Son Rubberbase RB2329 and the fountain used was Rotamatic 100%, sold by Rotaprint.
Start the press and let the Dahlgren dampening unit make contact with the lithographic plate. After 10 revolutions, let the plate cylinder make contact with the inking cylinders for five revolutions and start then printing. Print till 100 copies. The number given is the first good copy, free of background staining.
TABLE 2______________________________________sample 1 2 3 4 5______________________________________Subbing layer yes no no no noplasma treatment 0 0 70 140 200 (W min/m.sup.2) adhesion 8 75 10 8 1 water absorption 2.2 -- 0.7 0.9 1 dry upstart >100 del.sup.a 100 40 25______________________________________
a) del: delamination
It is clear from the results that a lithographic base obtained from a support that has undergone plasma treatment has one or more of the following advantages when compared with a lithographic based obtained from a subbed support: adhesion, water absorption and or dry start-up.

Number	Name	Date
3971660	Staehle	Jul 1976
4036136	Takagi	Jul 1977
4451497	Dolezalek et al.	May 1984
4476153	Staehle	Oct 1984
5196288	Nakamura	Mar 1993

Method for making a lithographic base and a lithographic printing plate therewith

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