Thermographic Recording Material

Abstract

A photothermographic method of reproducing an original bearing an image of material absorbing visible or infrared radiation and converting the same into heat in which the original is arranged with its image material in heat conductive relation with a recording layer containing a normally solvent soluble material adapted to react with a cross-linking agent and would be thereby rendered less soluble, a compound decomposing when heated to yield a cross-linking agent for said cross-linkable material and uniformly distributed therethrough a finely divided substance absorbing visible or infrared radiation and converting the same to heat, and while the original and layer are thus arranged one of them is uniformly exposed for a time not in excess of 10.sup.-.sup. 2 seconds to light or infrared radiation sufficiently intense to decompose the compound in the areas of the layer corresponding to the nonimage areas of the original whereby such nonimage areas undergo a loss in solubility whereas the image areas remain relatively soluble. Reflectographic exposure with the radiation impinged upon the recording layer is preferred, with the layer containing a sufficient amount of the finely divided material to impart thereto an optical density of 0.2-0.8.

Description

This invention relates to a thermographic recording process based on insolubilization under the influence of heat, and to the production of resist images thereby.

A method has now been found for recording or reproducing information, which method comprises recordwise or informationwise exposing to electromagnetic radiation a recording material, which comprises at least one recording layer (which may be carried by a support or may be a self-supporting layer or sheet) incorporating a composition which under the influence of heat can be completely or relatively insolubilized with respect to a solvent or solvents wherein it was previously soluble, and which composition contains at least one compound that on heating yields an intermediate reactive for cross-linking, and wherein said compound stands in heat-conducting relationship with one or more substances, which absorb electromagnetic radiation in at least a part of the wavelength range of said radiation and convert at least a part of the thus-absorbed radiation into heat, the exposure being of such duration and intensity that an image or record is formed in or on said layer in terms of differential solubility.

It is preferred that the total amount of electromagnetic energy used in the method of the invention is provided by an exposure of 10.sup.-.sup.4 to 10.sup.-.sup.2 sec., preferably with an energy intensity at the recording material of at least 0.1 watt. sec./ sq. cm. In practice a maximum intensity of 1.5 watt. sec./sq. cm. at the recording material will suffice.

By recordwise or informationwise exposure to electromagnetic radiation is meant that the exposure may be progressive (in the sense that relative movement takes place between the source of the image or information and the recording material) or simultaneous, e.g. as is the case of reflectographic or transmission exposure respectively to or through an original, e.g. a printed text or silver image transparency, When assuming for example, that the information to be recorded is in the form of written or printed matter, the record is in terms of a differentiation in solubility in a solvent or mixture of solvents.

The exposure is applied and is preferably carried out by means of electromagnetic radiation mainly within a wavelength range above 390 nm.

The heat applied by irradiation of said substances and the concentration thereof in the recording member are chosen in a sufficient degree to produce a useful differentiation in insolubilization.

Having stated in general the concept of this invention, reference will now be made in more detail to the composition and structure of preferred heat-sensitive materials, and to suitable methods of carrying out imagewise or recordwise exposure.

Preferably, the heat-sensitive material used in the performance of the invention comprises at least one recording layer (which may be carried by a support or may be a self-supporting layer or sheet) preferably consisting for at least 80 percent by weight of a compound e.g. a polymer that on heating yields an intermediate reactive for cross-linking when heated between 50.degree. to 250.degree. C. The applied electromagnetic radiation is preferably mainly visible light (e.g. at least 70 percent. The exposure time influences image quality and for obtaining sharp images, e.g., of printed texts, it is preferable for the exposure time to be no longer than 10.sup.-.sup.1 second, and more preferably for a time not exceeding 10.sup.-.sup.2 second. Such a brief exposure is preferably achieved by the use of a flash lamp as the radiation source.

Polymers that on heating yield an intermediate reactive for cross-linking are, e.g., polymers containing aromatic azide groups and/or carboxylic acid and/or sulfonic acid azide groups. The azide system is already known in photoresist formation. Particulars about such a cross-linking system which is suited for use according to the present invention can be found in J. Appl. Polymer Sci., 7, 273-279 (1963), and in the Belgian Pat. specifications Nos. 650,367, 656,511, 665,427, 665,428 and 665,429.

As heat-sensitive monomeric cross-linking agents for a polymer or polymer composition containing either active hydrogen atoms such as present in a hydroxyl group, a thiol group, an amino group, or a phenyl or pyridine group are mentioned monomeric polyazide compounds, e.g. aromatic bisazides such as p,p'-diazido-o,o'-disulfonic acid stilbene, bis-carboxylic acid azides and bis-sulphonic acid azides. Further are mentioned heat-sensitive latent polyisocyanates, e.g. those described in the Belgian Pat. specification No. 612,896, which polyisocyanates are also reactive for polymers containing active hydrogen atoms.

Further are mentioned polyamine compounds which can be used as cross-linking agents for a polymer containing isocyanate groups. Compounds, which on heating split off a polyamine, are described in the Belgian Pat. specification No. 612,963.

Another polymer system wherein on heating reactive groups for cross-linking are formed but in the presence of a peroxide catalyst are polyallyl resins e.g. o-diallyl phthalate and m-diallylphthalate resins.

Allylic homopolymerization is extremely slow at room temperature and catalyzed compositions may be stored for a year or more at ambient temperatures without substantial curing. The curing rate increases rapidly with temperature, however, and fast curing is obtained at temperatures of about 150.degree. C. and above.

Allylic polymerization and curing occur by intermolecular and intramolecular additions. Unlike the condensation polymerization reactions of azide polymers no gases are evolved, which results in a low shrinkage of the cured composition. As suitable peroxide catalysts are mentioned t-butyl perbenzoate, dicumylperoxide, cumene hydroperoxide either alone or in combination with more reactive peroxides such as benzoylperoxide. As very effective catalysts are further mentioned bifunctional peroxide compounds such as 2,5-dimethyl-2,5-di(tert.-butylperoxy)-hexyne-3. Their effectiveness is explained by the fact that they decompose rapidly at temperatures at which the unsaturation in the resins becomes highly reactive (above 150.degree. C.). Furthermore, since they decompose into biradicals, they also may act as cross-linking agents.

If a catalyst is used for the cross-linking reaction it is preferably formed in the recording member itself during recording by the action of the imagewise applied heat.

According to a preferred embodiment of the present invention it is possible to use a recording material wherein the compound producing on heating the cross-linking agent, or catalyst and the cross-linkable polymer or composition are applied in separate layers. Such a composition then has the advantage of being more stable under storage conditions, particularly if both layers are separated by an interlayer, which, however, should not prevent mixing by diffusion of the cross-linking agent and/or catalyst with the cross-linkable polymer on heating.

The exposure of the heat-sensitive member containing said light-absorbing substance(s) converting that light into heat is preferably carried out with a radiation source producing to a major extent visible light of a high intensity during a very short exposure time, i.e. in a time not longer than 10.sup.-.sup.1 seconds. The intensity of the light incident upon the recording member is preferably at least 0.1 watt. sec./sq. cm. So-called flashlamps, which produce also infrared radiation and ultraviolet radiation but only to a minor extent i.e. normally less than 30 percent of the total irradiation energy are preferred.

According to the present invention good results can be obtained with a xenon gas discharge lamp, which can supply an energy of 200-2,000 watt. sec. in a period of 10.sup.-.sup.4 to 10.sup.-.sup.2 seconds.

According to a preferred arrangement the discharge lamp is in the form of a thin tube fitted in a hollow glass cylinder in order to make possible a uniform exposure of the recording material the latter being applied to the periphery of the cylinder. More details about such a gas discharge lamp can be found in Belgian Pat. specification No. 664,868 and published Dutch Pat. application No. 6606719. The intensity of the light emitted by such a gas discharge lamp is particularly high in the region of the visible spectrum.

It is possible to employ a number of flash tubes operating simultaneously, or to obtain a suitable image differentiation by flashing a single tube at suitable intervals. Reflectors and other optical components may be included to provide irradiation of maximum uniformity.

Evidently radiation sources with a much lower energy than those mentioned can be used if the light energy is focused onto a relatively small heat-sensitive area e.g. by using a laser beam or by carrying out the exposure progressively and/or intermittently. Thus, the heat-sensitive material containing the light-absorbing substance(s), which convert(s) incident electromagnetic energy into heat, may be scanningwise exposed, e.g. by means of an imagewise modulated high-intensity light spot, or may be progressively exposed through a slot wherein light, e.g. of a tubelike radiation source, is focused.

It is evident that the heat-sensitive material, before or during the imagewise heating, can be integrally heated to a certain temperature below the temperature at which the heat-sensitive recording element is substantially insolubilized.

The substance(s) for absorbing radiation and converting it into heat may be (a) finely divided solid substance(s), distributed nondifferentially in the heat-sensitive layer of the recording material. Finely divided substances suited for use in the recording member and which convert visible light and infrared light into heat are, e.g., carbon black, graphite, oxides or sulfides of heavy metals, particularly of those heavy metals themselves in finely divided state, such as silver, bismuth, lead, iron, cobalt, or nickel. Preference is given to carbon black as light-absorbing material in heat-conductive relationship with the insolubilizable composition.

The heat-sensitive layer may comprise from 0.01 to 1, preferably from 0.1 to 0.5 percent by weight of such finely divided substances or pigments calculated on the weight of the recording layer. Black or deep-black colored pigments are preferred. Particularly in the case of reflex exposure, the optical density of the light-absorbing recording layer should preferably be between 0.20 and 0.80 and may be above 1 for direct exposure.

The heat-sensitive layer may further comprise plasticizers for the applied polymers, such as glycerol, sorbitol, polyglycols, polyethylene glycols and esters thereof such as glyceryl monolaurate, polyethyleneglycol distearate and others.

The thickness of the recording layer depends on the use for which the recording material is intended. If a relief image is to be produced the thickness depends on the particular kind of master to be manufactured, e.g. a printing master, such as a planographic, intaglio or letterpress printing master. In general, the thickness of the layer will vary from about 0.001 mm. to about 7 mm. Layers ranging from about 0.001 mm. to about 0.70 mm. thickness will be used for halftone plates (screen images). Layers ranging from about 0.25 to 1.50 mm. thickness will be used for the majority of letterpress printing plates.

If the surface of the recording layer is somewhat sticky e.g. in the case the recording layer is formed by means of a cross-linkable high-viscous composition, the exposed recording layer can be developed by means of a powder adhering to the areas of the recording layer that have not been exposed to heat. If a sticky character of the recording layer is not desired the latter may contain an amount of filler or pigment, e.g. zinc oxide, which reduces the adhesive character.

The base or support material for the cross-linkable layer can be of any natural or synthetic product capable to be worked up in fabrics, film or sheet form. It can be flexible or rigid, reflective or nonreflective for the copying light. For the manufacture of printing materials metals are normally preferred as the base materials. However, e.g. where weight is critical, synthetic resin or polymer sheets are desirable base materials. Rotary pressplates can be prepared by using cylindrically shaped base plates carrying the curable composition and by exposing them directly through a concentrically disposed image-bearing transparency to electromagnetic radiation e.g. infrared radiation, which radiation is converted into heat in the recording element. The support can also be a screening material, which is coated with a heat-sensitive cross-linkable composition. As screening material Japan paper (Yoshino paper), nylon fabrics with a size of mesh of 0.2 to 0.08 mm. and woven bronze wire are particularly suited. The screening material impregnated or coated with the cross-linkable composition forms a screen- or stencil-printing master blank. By washing away the portions of the coated composition, which were not cross linked, a printing master ready for screen printing is obtained.

The recording member containing the cross-linkable substance or substances used in the present invention can be applied to a support or base from a solution or dispersion by coating techniques known in the art.

If the composition is sufficiently film forming, it can also be cast or extruded on a casting wheel or belt in the form of a self-supporting sheet. Later on the sheet can be affixed to the surface of a permanent support if necessary.

Exposure can be a direct exposure or a reflectographic exposure. During reflectographic exposure the recording material which for the purpose is sufficiently radiation transparent is placed between the radiation source and the original. The original may be a transparency or an opaque element, having an image formed by surface areas only some of which absorb the radiation to be employed, or which reflect such radiation but to different extents.

In order to obtain very sharp images preference is given to that type of exposure wherein the heat-sensitive recording layer stands in contact with the image areas or portions of the original. Even if image markings of an original are in heat-conductive relationship with the recording layer during the exposure, any radiation absorbed by such markings will not affect the recording member, providing the exposure is sufficiently short (preferably lower than 10.sup.-.sup.2 second) to prevent heat accumulating in such portions to such an extent that an effective amount of heat transfers by conduction from said portions to the recording member in the areas corresponding with these portions.

The effectiveness of the recording with electromagnetic radiation depends on the type of radiant energy, in other words the said energy has to be emitted in a wavelength range wherein the substances converting electromagnetic radiation into heat absorb.

The applied radiation energy incident upon the recording material is preferably not lower than 0.2 watt. sec./sq. cm. It is not necessary for the recording material to be in heat-conductive relationship with an original during an exposure. The recording material may comprise, for instance, a heat-insulating or poorly heat-conducting support sheet (such a resin sheet) for the recording layer, the recording material being located with the support sheet in contact with the original. Alternatively for excluding such a heat-conductive contact an auxiliary thin nonheat-conductive sheet e.g. resin sheet may be placed between the original and the recording layer.

The recording material containing after the exposure an imagewise differentiation in solubility can be used in various kinds of techniques wherein relief or resist images have to be produced. In these techniques the soluble portions of the imagewise exposed recording element are removed e.g. dissolved in an appropriate solvent, which should be selected with care, since it should have good solvent action on the unexposed areas, yet have little action on the exposed areas and also on the base material whereto the thermoinsolubilizable composition has been applied. In view of the uses aimed at, the solvent-treated material may serve as a resist image for the etching of a relief printing plate or for the formation of a printing plate carrying hydrophilic and hydrophobic portions and which is suitable for lithographic printing. More particularly are mentioned applications directed to the production of ornamental plaques, patterns for automatic engraving machines, cutting and stamping dies, relief maps for braille and as resists in the preparation of printed or etched circuits.

After washing away the portions that have not been exposed to heat, the polymer parts made insoluble by the imagewise heating may be supplementarily more thoroughly hardened by any known hardening technique applicable thereto. The purpose of this additional hardening is to strengthen the insolubilized polymer parts as much as possible. If, e.g., the remaining insolubilized polymer surface is to be used as a printing plate, such subsequent hardening is often desirable.

A differentiation in cohesion power of the recording element, more particularly in wet state, can be used in a transfer process wherein the uncross-linked parts of the recording layer are transferred to a receiving material forming thereon a print or relief image in correspondence with the areas which have not been exposed to heat.

A differentiation in liquid permeability, which is practically always the result of the created difference in solubility, can be used in techniques wherein substances, e.g. substances which can color or bleach the recording material, are selectively absorbed into the recording material. For such techniques reference is made, e.g., to out Belgian Pat. specification No. 656,713.

FIGS. 1 and 2 represent two possible methods of reflectographic exposure.

As schematically illustrated in FIG. 1, the element 22 represents a xenon gas discharge lamp, while 24 represents a line original carrying light-absorbing characters 27, on the two sides of a light-reflecting support 25. The heat-sensitive material 28 consists of a transparent support 29 and of a heat-sensitive layer 31 containing light-absorbing particles 32.

FIG. 2 represents another method of reflectographic exposure. Here the support 29 of the heat-sensitive material 28 is in direct contact with the characters 27 of the original 24.

Direct exposure through the original is illustrated in FIGS. 3 and 4. In FIG. 3 the light from the gas discharge lamp 22 passes first through the transparent support 26 of the original 23, before it strikes the heat-sensitive layer 31 of the heat-sensitive material 29. In this case the support 30 of this material can be opaque or transparent. The light cannot pass through the parts of the original 23 which bear the light-absorbing characters 27.

In FIG. 4, the original 23 has its side bearing the light-absorbing characters 27 turned towards the gas discharge lamp 22.

In addition to the exposure methods illustrated by these figures, several other exposure methods are possible. For a man skilled in the art, these methods are not difficult to find out.

The following examples illustrate the present invention.

EXAMPLE 1

The following ingredients are thoroughly mixed in a ball-mill for 6 h.:

reaction product of 1 mole of p-azido- benzoic acid chloride with the poly- ether, prepared by making to react 1 mole of 2,2-bis(4-hydroxyphenyl)- propane with 1.05 mole of epichloro- hydrin 10 g. carbon black 0.3 g. methylene chloride 100 cc.

The suspension obtained is applied to a support of polyethylene terephthalate in such a way, that the layer after being dried possesses a density of 0.2 measured by transmitted light.

As illustrated by FIG. 3, this heat-sensitive material obtained is irradiated for 8/1,000 sec. through a negative transparency by means of an electronic flash lamp with an energy output of 1.03 watt. sec./sq. cm. Then the exposed material is dipped into methylene chloride for some 30 sec. so that the exposed areas of the heat-sensitive layer are removed. A black positive image is obtained.

If the carbon black is omitted from the recording layer, the latter is completely dissolved by methylene chloride after an imagewise exposure.

EXAMPLE 2

A heat-sensitive recording material is prepared as in example 1, with the difference, however, that the reaction product mentioned therein is replaced by the same amount of the reaction product of the polyether, prepared by making to react 1 mole of m-azidosulphonylbenzoic acid chloride with the polyether prepared by reacting 1 mole of 2,2-bis-(4-hydroxyphenyl)-propane with 1.05 mole epichlorohydrin. Coating is executed in such a way that an optical density of 0.5 is obtained measured by transmitted light.

As illustrated in FIG. 2, the rear side of the heat-sensitive recording material is laid on an original to be copied, and the whole is irradiated reflectographically by means of an electronic flash lamp having an energy output of 0.90 watt. sec./sq. cm.

After soaking the exposed material in methylene chloride, a black negative image of the original is obtained.

EXAMPLE 3

The following coating suspension is ground for 6 h. in a ball mill:

poly(vinyl-n-butyral) containing 76.6 percent of acetal groups 3 g. carbon black 0.2 g. isophthalic acid diazide 0.4 g. methylene chloride 100 cc.

This suspension is coated on polyethylene terephthalate film in such a way that after drying this layer possesses a density of 0.28 measured by transmitted light. Exposure and rinsing are done as described in example 1. A sharp, black, positive image is obtained.

When the isophthalic acid diazide is omitted from the layer, an uncompletely washed and vague image is obtained.

EXAMPLE 4

A mixture of the following products is mixed in a ball mill for 3 h.:

40 percent solution in acetone of a branched polyester of adipic acid, phthalic acid, and glycerol, having still 8.5 to 9 percent of free hydroxyl groups 50 cc. zinc oxide 40 g. addition product of 2,4-toluene-diiso- cyanate and diethyl malonate 4 g. carbon black 0.2 g. acetone 50 cc.

This dispersion is applied to a support of polyethylene terephthalate in such a way that the dried layer has a thickness of 10 .mu.. The material obtained is exposed as described in example 1 and rinsed with acetone. A dark grey positive image is obtained.

EXAMPLE 5

The following ingredients are thoroughly mixed in a ball mill for 6 h.:

ethylcellulose 4 g. acetone 96 cc. carbon black 0.4 g. addition product of toluene diiso- cyanate and diethyl malonate 1 g.

The fine suspension obtained is coated onto a subbed polyethylene terephthalate film in such a way that after drying the applied layer possesses a density of 0.32 measured by transmittance. This heat-sensitive material with its sensitive side is laid onto an original to be copied and the whole is exposed reflectographically by means of an electronic flash lamp having an intensity of 1.03 watt. sec./sq. cm. Then the exposed material is pressed against a paper sheet, moistened with methanol. After peeling apart both materials, a greyish black print of the original is obtained on the latter. By pressing the same master again and again onto other paper sheets moistened with methanol, some 5 of such prints are obtained.

EXAMPLE 6

The following ingredients are mixed for 4 h. in a ball mill:

poly(diallyl o-phthalate) 25 g. methyl isobutyl ketone 175 cc. cumene hydroperoxide 7 g. carbon black 0.6 g.

The resulting fine suspension is coated onto a subbed polyethylene terephthalate support in such a way that after drying, the layer possesses a density of 0.26 measured by transmittance.

After exposure as in example 1, the material is rinsed with methyl isobutyl ketone for a short while. A black positive image is left on the film.

Claims

We claim:

1. A photothermographic method of reproducing an original carrying on at least one side thereof an image formed of material absorbing radiation in the visible to infrared region of the spectrum and converting such absorbed radiation into heat, which comprises the steps of arranging said original with the image material in direct face-to-face contacting heat-conductive relation with a recording layer consisting essentially of the combination of a polymer which is normally soluble in a solvent and is adapted to be rendered insoluble in said solvent by reaction with a cross-linking agent with a compound decomposing when heated to release a cross-linking agent reactive with said polymer and uniformly distributed through said layer finely divided particles of a substance absorbing radiation in said region of the spectrum and converting the same to heat, said combination being selected from a polymer containing active hydrogen groups with a monomeric aromatic polyazide, a monomeric polycarboxylic acid azide, or a monomeric polysulfonic acid azide, a polymer containing hydroxyl groups with a heat-sensitive latent polyisocyanate, a poly (diallyl-phthalate) with a peroxide catalyst, and polymers containing aromatic azide, carboxylic acid azide and sulfonic acid azide groups that on heating decompose into intermediates that are self-cross-linking, and said particles being present in sufficient amount to impart to the layer an optical density of about 0.20-0.80; and uniformly exposing the thus arranged original and said recording layer through said layer for a time not in excess of 10.sup.-.sup.2 seconds to radiation within said region of an intensity sufficient to initiate said cross-linking reaction in the areas of said recording layer corresponding to the nonimage areas of said original, whereby the nonimage areas of said layer become insoluble as a consequence of the cross-linking reaction occurring therein whereas the image areas of said layer remain soluble in said solvent.

2. A method according to claim 1, wherein the said substances absorbing visible light are finely divided black or dark colored substances.

3. A method according to claim 2, wherein the said substances are carbon particles.

4. A method according to claim 1, wherein the said layer is exposed with a flash lamp.

5. A method according to claim 1, wherein the said layer previously to the exposure is soluble in at least one organic solvent and after the exposure is contacted with said solvent.

6. A method according to claim 5, wherein the said layer after the said treatment is pressed against a receiving material, in order to transfer thereon by separation a stratum from the image areas thereof.