JPH10329441A - Heat sensitive image forming element and method for manufacturing lithographic printing plate using it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a heat sensitive image forming element for manufacturing a lithographic printing plate having excellent ink receptivity by a simple method and a method for obtaining a negative operable lithographic printing plate by a simple method by using the image forming element. SOLUTION: In the heat sensitive image forming element comprising a lithographic printing base having hydrophilic surface, image forming layer containing hydrophobic thermoplastic polymer latex, and compound existing in the image forming layer or its adjacent layer to convert light into heat, the image forming layer contains compound having at least two carboxyl groups, and alkali soluble copolymer containing at least partly reacted acetal group and hydroxy group.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[1. FIELD OF THE INVENTION This invention relates to heat-sensitive materials for making lithographic printing plates. The invention further relates to a method for making a printing plate from said heat-sensitive material.

[0002]

[2. BACKGROUND OF THE INVENTION Lithographic printing is a method of printing from a specially made surface where some areas are capable of receiving lithographic printing ink, while others are not receiving ink when wetted with water. is there. The areas that receive ink form the printed image areas and the ink-repellent areas form the background areas.

In the field of photolithographic printing, photographic materials are image-wise converted to oily or greasy inks on a hydrophilic background in exposed areas (negative working) or in unexposed areas (positive working). It is considered receptive.

[0004] Surface lithographic printing plate (surface lit)
ho plates or planographic printing plates (p
lanographic printing plate
In the preparation of conventional lithographic printing plates, also referred to as es), a thin layer of the photosensitive composition is coated on a support that has an affinity for water or obtains such an affinity by chemical treatment. Coatings for that purpose include photopolymer layers containing diazo compounds, dichromate-sensitized hydrophilic colloids and various synthetic photopolymers. In particular, diazo-sensitized systems are widely used.

When the photosensitive layer is image-wise exposed, the exposed image areas become insoluble and the unexposed areas remain soluble. The plate is then developed using a suitable solution to remove the diazonium salt or diazo resin in the unexposed areas.

[0006] On the other hand, methods are known for making printing plates that involve the use of imaging elements that are heat-sensitive, rather than light-sensitive. A particular disadvantage of the photosensitive imaging element as described above for the preparation of a printing plate is that it must be shielded from light. Furthermore, they have sensitivity issues in terms of storage stability, and they exhibit relatively low resolution. There is a clear trend in the market towards heat-sensitive printing plate precursors.

[0007] For example, Research in January 1992
h Disclosure no. No. 33303 discloses a thermosensitive imaging element comprising a support having thereon a crosslinked hydrophilic layer containing thermoplastic polymer particles and an infrared absorbing pigment such as carbon black. By image-wise exposure to an infrared laser, the thermoplastic polymer particles coagulate image-wise, thereby rendering the surface of the imaging element in these areas ink-receiving without further development. A disadvantage of this method is that the resulting printing plate is easily damaged, since the non-printed areas can become ink-receptive if some pressure is applied thereto. Furthermore, under marginal conditions, the lithographic performance of such printing plates may be poor, and thus such printing plates have little lithographic latitude.

[0008] EP-A-514145 discloses a heat-sensitive imaging element comprising a coating containing core-shell particles having a water-insoluble heat-softenable core component and a shell component soluble or swellable in an aqueous alkaline medium. Is disclosed. The red or infrared laser light that is image-directed to the imaging element causes the selected particles to at least partially agglomerate and form an image, and then the non-aggregated particles are selectively removed using an aqueous alkaline developer. Is removed. Thereafter, a baking step is performed. However, the printing durability of the printing plate thus obtained is low.

[0009] EP-A-599510 describes (i) (1)
A dispersed phase comprising a water-insoluble thermosoftening component A and (2) a binder or a continuous phase comprising component B which is soluble or swellable in an aqueous, preferably aqueous alkaline medium, wherein at least one of components A and B is A layer containing reactive groups or precursors therefor, which at elevated temperature and / or upon exposure to actinic radiation causes insolubilization of the layer, and (ii) can strongly absorb radiation, and thus A heat-sensitive imaging element is disclosed that includes a substrate coated with a material that transfers the energy gained as heat to the dispersed phase and causes at least partial coating agglomeration. After image-wise irradiating the imaging element and developing the image-irradiated plate, the plate is heated and / or subjected to actinic radiation to effect insolubilization. However, the printing durability of the printing plate thus obtained is low.

[0010] EP-A 625728 describes UV- and I-
Disclosed are imaging elements that include a layer that is sensitive to R-rays and can be positive or negative working. This layer consists of resole resin, novolak resin, latent Bronsted acid and IR
-Contains absorbent material. The printing results of the lithographic printing plate obtained by irradiation and development of the imaging element are inferior.

US-P-5,340,699 is EP-A
Almost the same as -625728, but discloses a method for obtaining a negative working IR-laser recording imaging element. The IR-sensitive layer contains a resole resin, a novolak resin, a potential Bronsted acid and an IR-absorbing material. The printing results of the lithographic printing plate obtained by irradiation and development of the imaging element are inferior.

US-P-4,708,925 discloses a positive-working imaging element comprising a photosensitive composition containing an alkali-soluble novolak resin and an onium-salt. The composition may optionally contain an IR-sensitizer. After the image-forming element has been image-wise exposed to UV-visible- or optionally IR-rays and a development step with an aqueous alkaline solution is followed, a positive working printing plate is obtained. The printing results of the lithographic printing plate obtained by irradiation and development of the imaging element are inferior.

[0013] EP-A-514145 discloses a method for forming an image by directly irradiating a radiation-sensitive plate with a red or infrared laser beam and modulating the radiation. Radiation-sensitive plates include a coating containing core-shell particles having a water-insoluble thermosoftenable core compound and a shell compound that is soluble or swellable in an aqueous alkaline medium. The shell compound can be a dicarboxylic acid half ester of a hydroxyl group-containing polymer.
The material does not contain a hydrophobic thermoplastic polymer latex.

EP-A-96200972.6 is present in an image-forming layer containing hydrophobic thermoplastic polymer particles dispersed in a water-insoluble alkali-soluble or swellable resin and in or adjacent to the image-forming layer. A heat-sensitive imaging element comprising a lithographic base-based hydrophilic surface comprising a compound capable of converting light to heat, wherein the alkali-swellable or soluble resin comprises phenolic hydroxy and / or carboxyl groups. ing. However, by exposing the thermosensitive imaging element using a short pixel time,
Ablation occurs on the exposed areas, resulting in poor ink receptivity.

All of the disclosed systems require lithographic printing plates that require processing after a development step and / or have poor printability. Therefore, there is still a need for a heat-sensitive imaging element that is simple to process and provides a lithographic printing plate with very good printability.

[0016]

[3. SUMMARY OF THE INVENTION It is an object of the present invention to provide a heat-sensitive imaging element for preparing a lithographic printing plate having excellent ink receptivity in a simple manner.

Another object of the present invention is to provide a method for obtaining a negative-working lithographic printing plate having excellent ink receptivity by using the image forming element in a simple manner.

Yet another object of the present invention is to provide a method for obtaining a negative-working lithographic printing plate which provides a print having excellent printability using the image-forming element in a simple manner.

Further objects of the present invention will become clear from the description hereinafter.

According to the present invention, there is provided a lithographic printing base having a hydrophilic surface, an image forming layer containing a hydrophobic thermoplastic polymer latex, and a compound capable of converting light present in the image forming layer or a layer adjacent thereto into heat. Wherein the imaging layer comprises a copolymer containing acetal groups and hydroxy groups at least partially reacted with a compound having at least two carboxyl groups. Elements are provided.

According to the invention, (a) exposing the imaging element described above to light or heat imagewise or informationally;
A method is also provided for obtaining a lithographic printing plate comprising developing the exposed imaging element with an aqueous developer to remove unexposed areas, thereby making a lithographic printing plate.

[0022]

[4. Detailed Description of the Invention It has been found that according to the method of the present invention using the above described imaging element, a high quality lithographic printing plate can be obtained which gives a print with excellent ink receptivity. More precisely, it has been found that the printing plates are of high quality and are provided in a simple manner, thereby providing economic and ecological benefits.

The copolymer containing acetal groups and hydroxy groups at least partially reacted with a compound having at least two carboxyl groups is preferably soluble in an aqueous solution having a pH of at least 6.

The molecular weight of the polymers used in connection with the present invention is preferably between 10,000 and 1,000,000,
More preferably, it is in the range of 20,000 to 300,000.

The copolymer used in connection with this embodiment is preferably not crosslinked or only slightly crosslinked.

Highly preferred polymers for use in accordance with the present invention are those of the formula I

[0027]

Embedded image

Wherein n is in the range of 50-78%;
m is in the range of 21-49%, p is in the range of 1-5%, and q is in the range of 0-28%].

According to one embodiment of the present invention, the lithographic base having a hydrophilic surface can be anodized aluminum. A particularly preferred lithographic base having a hydrophilic surface is electrochemically grained.
ed), anodized aluminum support. Most preferably, the aluminum support is grained in nitric acid to provide an imaging element with higher sensitivity. According to the present invention, the anodized aluminum support can be treated to improve the hydrophilicity of its surface. For example, an aluminum support can be silicated by treating its surface with a sodium silicate solution at an elevated temperature, for example, 95 ° C.
In another case, a phosphating can be applied, which comprises treating the aluminum oxide surface with a phosphating solution, which can further contain inorganic fluoride. Additionally, the aluminum oxide surface can be rinsed with a citric acid or citrate solution. This treatment can be performed at room temperature or at a slightly elevated temperature of about 30-50 ° C. A further interesting treatment involves rinsing the aluminum oxide surface with a bicarbonate solution. In addition, polyvinyl phosphonic acid, polyvinyl methyl phosphonic acid, polyvinyl alcohol, phosphate ester of polyvinyl alcohol, polyvinyl sulfonic acid, polyvinyl benzene sulfonic acid, polyvinyl alcohol formed by reaction with sulfate ester of sulfonated aliphatic alcohol and sulfonated aliphatic aldehyde. It can be processed using acetal. One of these post-processing
It is further clear that one or more can be performed alone or in combination.

According to another embodiment related to the present invention,
A lithographic printing base having a hydrophilic surface is a flexible support,
For example, it can include a paper or plastic film provided with a cross-linked hydrophilic layer. Particularly suitable cross-linked hydrophilic layers can be obtained from hydrophilic binders cross-linked with cross-linking agents such as formaldehyde, glyoxal, polyisocyanate or hydrolyzed tetra-alkyl orthosilicate. The latter is particularly preferred.

As the hydrophilic binder, hydrophilic (co) polymers such as vinyl alcohol, acrylamide, methylolacrylamide, methylolmethacrylamide, homopolymers and copolymers of acrylic acid, methacrylic acid, hydroxyethyl acrylate, hydroxyethyl methacrylate or maleic anhydride Acid / vinyl methyl ether copolymer can be used. The hydrophilicity of the (co) polymer or (co) polymer mixture used is
It is preferred that the hydrophilicity of the polyvinyl acetate hydrolyzed to at least 60 weight percent, preferably to the extent of 80 weight percent, be the same or higher.

The amount of crosslinking agent, especially tetraalkyl orthosilicate, is preferably at least 0.2 parts by weight, preferably 0.5 to 5 parts by weight, per part by weight of hydrophilic binder.
More preferably, the amount is 1.0 part by weight to 3 parts by weight.

The lithographic base crosslinked hydrophilic layer used in accordance with this embodiment preferably also contains substances which improve the mechanical strength and the porosity of the layer. Colloidal silica can be used for this purpose. The colloidal silica used is, for example, up to 40 nm, for example 20 n
It can be in the form of any commercially available water-dispersion of colloidal silica having an average particle size of m. In addition, inert particles of larger dimensions than colloidal silica, for example, Colloid and Interface
Sci. , Vol. 26, 1969, pages 6
Particles having an average diameter of at least 100 nm can be added which are silica or alumina particles prepared according to StOber as described in 2 to 69 or particles of titanium dioxide or other heavy metal oxides. The introduction of these particles gives the surface of the cross-linked hydrophilic layer a uniform rough texture consisting of microscopic hills and valleys, which serves as a storage for water in the background areas.

The thickness of the crosslinked hydrophilic layer of the lithographic printing base according to this embodiment can vary within the range of 0.2 to 25 μm, preferably 1 to 10 μm.

Specific examples of suitable crosslinked hydrophilic layers for use in accordance with the present invention include EP-A 601240,
GB-P-1419512, FR-P-230035
4, US-P-397660, US-P-42847
05 and EP-A 514 490.

It is particularly preferable to use a plastic film such as a substrate-based polyethylene terephthalate film, a cellulose acetate film, a polystyrene film, a polycarbonate film, etc. as the lithographic printing base flexible support in connection with the present embodiment. The plastic film support can be opaque or transparent.

It is particularly preferred to use a polyester film support provided with an adhesion promoting layer. Particularly suitable adhesion-promoting layers for use in accordance with the present invention are EP-A 619.
524, EP-A 620502 and EP-A 619
525, comprising a hydrophilic binder and colloidal silica. The amount of silica in the adhesion promoting layer is m ² per 200Mg～m ² per 750mg is preferred. Furthermore, the ratio of silica to hydrophilic binder is preferably higher than 1, and the surface area of the colloidal silica is preferably at least 300 m ² per gram and at least 50 m ² per gram.
0 m ² is more preferred.

[0038] The hydrophobic thermoplastic polymer latex used in connection with the present invention is preferably above 50 ° C.
More preferably, the coagulation temperature is higher than 70 ° C.
emperative). Coagulation can result from the thermoplastic polymer latex softening or melting under the influence of heat. There is no specific upper limit for the coagulation temperature of a thermoplastic hydrophobic polymer latex, but the temperature must be well below the decomposition temperature of the polymer latex. The coagulation temperature is at least 10 ° above the temperature at which the decomposition of the polymer latex occurs.
C is preferably lower. When the polymer latices are subjected to temperatures above the coagulation temperature, they coagulate to form hydrophobic agglomerates, at which point the hydrophobic latex becomes insoluble in fresh water or aqueous liquids.

Particular examples of hydrophobic thermoplastic polymer latexes for use in connection with the present invention having a Tg of greater than 80 ° C. include, but are not limited to, polyvinyl chloride, polyvinylidene chloride, polyacrylonitrile, polyvinyl carbazole, and the like. It is a copolymer or a mixture. More preferred are polymethyl-methacrylates or copolymers thereof. Most preferably used are polystyrene copolymers and especially polymers of polystyrene itself or substituted styrene.

The weight average molecular weight of the hydrophobic thermoplastic polymer can range from 5,000 to 1,000,000 g / mol.

The hydrophobic thermoplastic polymer latex contains 0.1%.
01 μm to 50 μm, more preferably 0.01 μm to 1
It can have a particle size of 0 μm, more preferably 0.01 μm to 1 μm and most preferably 0.02 μm to 0.10 μm.

The hydrophobic thermoplastic polymer latex exists as a dispersion in the aqueous coating solution of the image forming layer and can be prepared by the method disclosed in US Pat. No. 3,476,937. Other processes which are particularly suitable for the preparation of aqueous dispersions of thermoplastic polymer latex are: dissolving a hydrophobic thermoplastic polymer in a water-immiscible organic solvent, and dissolving the solution thus obtained in water or an aqueous medium. And removing the organic solvent by evaporation.

The amount of the hydrophobic thermoplastic polymer latex contained in the image forming layer is preferably from 20% by weight to 95% by weight.
%, More preferably from 40% to 90%, most preferably from 50% to 85% by weight.

The imaging layer can also include a crosslinking agent, but this is not required. Preferred crosslinking agents are low molecular weight substances containing methylol groups, such as melamine-formaldehyde resins, glycoluril-formaldehyde resins, thiourea-formaldehyde resins, guanamine-
Formaldehyde resin, benzoguanamine-formaldehyde resin. A plurality of the melamine-formaldehyde resins and glycoluril-formaldehyde resins are CYMEL (Dyno Cyanamid Co., L.)
td. ) And NIKALAC (Sanwa Chemi)
cal Co. , Ltd. ) Is commercially available under the trade name.

The imaging element further comprises a compound capable of converting light to heat. Suitable compounds that can convert light into heat are preferably infrared absorbing components, but the absorption wavelength is not particularly important if the absorption of the compound used is within the wavelength range of the light source used for image-wise exposure. Absent. Particularly useful compounds are, for example, dyes, especially infrared dyes, carbon black,
Metal carbides, borides, nitrides, carbonitrids
es), bronze-structured oxides and oxides structurally related to the bronze group but without the A component, for example WO _2.9 . Conductive polymer dispersions, such as those based on polypyrrole or polyaniline, can also be used. The resulting lithographic performance, especially printing durability, depends on the heat sensitivity of the imaging element. In this regard, it has been found that carbon black gives very good and favorable results.

Most preferably, the light-to-heat conversion compound associated with the present invention is added to the imaging layer, although at least a portion of the light-to-heat conversion compound may be included in an adjacent layer. Such a layer can be, for example, a lithographic base crosslinked hydrophilic layer according to the lithographic base second embodiment described above.

According to the method of the invention for obtaining a printing plate, the imaging element is image-wise exposed to heat or light and subsequently developed with an aqueous solution having a pH of at least 6.

Image-wise exposure in connection with the present invention may be performed by laser or L.W. E. FIG. D. Preferably, the image-wise scanning exposure involves the use of It is highly preferred in connection with the present invention to use a laser that emits in the infrared (IR) and / or near infrared, that is to say in the wavelength range from 700 to 1500 nm. Particularly preferred for use in connection with the present invention are laser diodes that emit in the near infrared.

After the image-forming element exposed imagewise is developed with an aqueous alkaline solution and dried, the resulting plate can be used as it is as a printing plate. However, it is preferred to treat the plate with a gumming solution. The rubberizing liquid is a water-soluble (co) polymer, such as a synthetic homo- or copolymer, such as polyvinyl alcohol, poly (meth) acrylic acid, poly (meth) acrylamide, polyhydroxyethyl (meth) acrylate, polyvinyl methyl ether or a natural binder. For example, gelatin, polysaccharides such as dextran,
Contains pullulan, cellulose, gum arabic, arginic acid, etc. However, the developed plate, which is rubberized or not rubberized, is heated at a temperature of 100 ° C. to 300 ° C. at 40 ° C.
It can be baked for minutes to 30 seconds. For example, exposing and developing a plate at 270 ° C. for 2 minutes at 230 ° C.
5 minutes at a temperature of 150 ° C. for 10 minutes or 120
Can be baked for 30 minutes at a temperature of ° C.

The following examples illustrate the invention but do not limit the invention thereto. All parts are by weight unless otherwise specified.

[0051]

【Example】

Example 1 (Comparative Example) Production of a lithographic printing base An aluminum foil having a thickness of 0.20 mm was immersed in an aqueous solution containing 5 g / l of sodium hydroxide at 50 ° C,
Degreased by rinsing with deionized water. The foil is then electrochemically grained in an aqueous solution containing 4 g / l hydrochloric acid, 4 g / l boric acid and 5 g / l aluminum ions using an alternating current at a temperature of 35 ° C. and a current density of 1200 A / m ^2. Then, a surface topology having an average center line roughness Ra of 0.5 μm was formed.

After rinsing with deionized water, then 300 g /
at 60 ° C. using an aqueous solution containing 1
The aluminum foil was etched for 0 seconds and rinsed with deionized water at 25 ° C. for 30 seconds.

The foil was then placed in an aqueous solution containing 200 g / l of sulfuric acid at a temperature of 45 ° C., a voltage of about 10 V and 150
Anodize at a current density of A / m ² for about 300 seconds to form an anodized film of 3.00 g / m ² of Al ₂ O ₃ , then wash with deionized water and remove the 5% citric acid solution. And used for 90 seconds at room temperature.

Preparation of the Coating Composition 0.4 g of the surfactant Host having a particle size of 70 nm
140 g of a 20% dispersion of polystyrene stabilized in deionized water with apal B (available from Hoechst, Germany), 250 g water containing 0.35 g wetting agent, to raise the pH to 8.1. Of ammonium hydroxide, a 15% dispersion of carbon black in water containing a wetting agent, 434 g of water, 98 mol%
Hydrolyzed polyvinyl acetate (Hoechst, Ge
Mowiol 56-98 available from Rmany)
400 g of a 2% solution in water were continuously added with stirring.

Preparation of the Imaging Element By preparing the coating composition described above, coating it on the lithographic base described above in an amount of 20 g / m ² (wet coating amount) and drying it at 50 ° C. An imaging element was manufactured.

Production of a printing plate and production of a copy thereof An internal drum recorder emits light at 1.06 μm,
The imaging element was exposed using a scanning infrared laser with a scan speed of 18 m / sec and a spot size of 10 μm with a pixel dwell time of 0.05 μsec. Various energy levels in the image plane were tested: 2.22 mJ / mm ² ,
1.70 mJ / mm ² and 1.50 mJ / mm ² .

After exposure, the plate was removed from Ozasol EN 14
4 (Agfa AG, negative developer pH 8.3 available from Germany). Using the obtained lithographic printing plate, printing was performed in the same manner on a normal offset printing press using a normal ink and a fountain solution. After the specified number of prints, the density of the printed image was measured. The exposed and developed plate produced unacceptable ink receptivity (even after 100 prints, it was still unacceptable at the highest energy levels, lower energy levels were even worse).

Example 2 (Example according to the invention) Preparation of the coating solution 2.3 g of the surfactant Host having a particle size of 70 nm
To 546 g of a 20.6% dispersion in deionized water of polystyrene stabilized with apal B (available from Hoechst, Germany), 2104 g water containing 5 g wetting agent, to raise the pH to 8.1 100 g of a 15% dispersion of carbon black in water containing ammonium hydroxide, humectant, n is 70%, p is 3%, m + q is 27% and has a molecular weight of 60,000 22 of a 1.5% solution of a compound of formula I in water
50 g were added continuously with stirring.

Preparation of the Imaging Element By preparing the coating composition described above, coating it on the lithographic base described above in an amount of 20 g / m ² (wet coating amount) and drying it at 50 ° C. An imaging element was manufactured.

Production of a printing plate and production of a copy thereof An internal drum recorder emits light at 1.06 μm,
The imaging element was exposed using a scanning infrared laser with a scanning speed of 67 m / s, a spot size of 10 μm, and a pixel dwell time of 0.032 μs and an energy of 0.55 mJ / mm ^{2 at} the image plane (resolution exposure). (Res
solution exposure).

After exposure, the plate was removed from Ozasol EN 14
4 (Agfa AG, negative developer pH 8.3 available from Germany). Using the resulting lithographic printing plate, printing was performed in the same manner as in Example 1 on a normal offset printing machine using a normal ink and a dampening solution. After the specified number of prints, the density of the printed image was measured. The exposed and developed plate produced very good ink receptivity (<10 prints).

Example 3 (Example according to the invention) Preparation of the coating liquid 2.5 g of the surfactant Host having a particle size of 70 nm
To 619 g of a 20.6% dispersion of polystyrene stabilized in deionized water of apal B (available from Hoechst, Germany), 3531 g of water containing 5 g of wetting agent, to raise the pH to 8.1 100 g of a 15% dispersion of carbon black in water containing ammonium hydroxide, humectant, n is 70%, p is 3%, m + q is 27% and has a molecular weight of 60,000 750 g of a 1% solution of a compound of formula I in water
Was added continuously with stirring.

Preparation of the Imaging Element By preparing the above coating composition, coating it on the above lithographic base in an amount of 20 g / m ² (wet coating amount) and drying it at 50 ° C. An imaging element was manufactured.

Production of a printing plate and production of a copy The internal drum recorder emits light at 1.06 μm,
The imaging element was exposed using a scanning infrared laser having a scanning speed of 67 m / s, a spot size of 10 μm, and a pixel dwell time of 0.032 μs and an energy of 0.45 mJ / mm ^{2 at} the image plane (resolution exposure). (Res
solution exposure)).

After exposure, the plate was removed from Ozasol EN 14
4 (Agfa AG, negative developer pH 8.3 available from Germany). Using the resulting lithographic printing plate, printing was performed in the same manner as in Example 1 on a normal offset printing machine using a normal ink and a dampening solution. After the specified number of prints, the density of the printed image was measured. The exposed and developed plate produced very good ink receptivity (<10 prints).
It can be seen that this plate having a higher percentage of polystyrene in the sensitive layer than the imaging element of Example 2 requires a lower (and thus higher sensitivity) energy level due to exposure.

Example 4 (Example According to the Invention) A lithographic base A was prepared as described in Example 1, except that the anodized foil was post-treated with a solution of polyvinylphosphonic acid. Lithographic base B was prepared in the same manner as lithographic base A, except that the aluminum foil was electrochemically grained in a solution of nitric acid. Both lithographic bases were coated, exposed and developed as in Example 3, except that the energy at the image plane was adjusted to obtain the resolution exposure. The imaging element with lithographic base B had higher sensitivity than the imaging element with lithographic base A. The ink receptivity of both plates was very good.

Claims (2)

[Claims] 1. A lithographic printing base having a hydrophilic surface, an image forming layer containing a hydrophobic thermoplastic polymer latex, and a heat sensitive material containing a compound capable of converting light present in the image forming layer or a layer adjacent thereto into heat. An image-forming element, wherein the image-forming layer comprises an alkali-soluble copolymer containing acetal and hydroxy groups at least partially reacted with a compound having at least two carboxyl groups. 2. The method according to claim 1, wherein said alkali-soluble copolymer is of the formula I Wherein n ranges from 50 to 78%, m ranges from 21 to 49%, p ranges from 1 to 5%, and q ranges from 0 to 28%. 2. The heat-sensitive imaging element of claim 1, wherein the element has a structure.