Heat mode recording element based on a thin metal layer

An improved heat mode recording element based on a thin metal layer, preferably bismuth, is disclosed, characterized in that it contains hypophosphorous acid, or phosphorous acid, or a mixture of both.

FIELD OF THE INVENTION
 The present invention relates to an heat mode recording element based on a
 thin metal layer with improved sensitivity for image formation by laser
 light.
 BACKGROUND OF THE INVENTION
 Conventional photographic materials based on silver halide are used for a
 large variety of applications. As is generally known silver halide
 materials have the advantage of high potential intrinsic sensitivity and
 excellent image quality. On the other hand they show the drawback of
 requiring several wet processing steps employing chemical ingredients
 which are suspect from an ecological point of view.
 In the past several proposals have been made for obtaining an imaging
 element that can be developed using only dry development steps without the
 need of processing liquids as it is the case with silver halide
 photographic materials.
 A dry imaging system known since quite a while is 3M's dry silver
 technology. It is a catalytic process which couples the light-capturing
 capability of silver halide to the image-forming capability of organic
 silver salts.
 Another type of non-conventional materials as alternative for silver halide
 is based on photopolymerisation. The use of photopolymerizable
 compositions for the production of images by information-wise exposure
 thereof to actinic radiation is known since quite a while. These methods
 are based on the principle of introducing a differentiation in properties
 between the exposed and non-exposed parts of the photopolymerizable
 composition e.g. a difference in adhesion, conductivity, refractive index,
 tackiness, permeability, diffusibility of incorporated substances e.g.
 dyes etc. The thus produced differences may be subsequently employed in a
 dry treatment step to produce a visible image and/or master for printing
 e.g. a lithographic or electrostatic printing master.
 As a further alternative for silver halide chemistry dry imaging elements
 are known that can be image-wise exposed using an image-wise distribution
 of heat. When this heat pattern is applied directly by means of a thermal
 head such elements are called thermographic materials. When the heat
 pattern is applied by the transformation of intense laser light into heat
 these elements are called heat mode materials or thermal imaging media.
 They offer the additional advantage compared to most photo mode systems
 that they do not need to be handled in a dark room nor that any other
 protection from ambient light is needed.
 In a particular type of heat mode elements, e.g. as disclosed in EP 0 674
 217, density is generated by image-wise chemical reduction of organic
 metal salts, preferably silver salts such as silver behenate, without the
 presence of catalytic amounts of exposed silver halide such it is the case
 in the dry silver system.
 Another important category of heat mode recording materials is based on
 change of adhesion, e.g. as disclosed in U.S. Pat. No. 4,123,309, U.S.
 Pat. No. 4,123,578, U.S. Pat. No. 4,157,412, U.S. Pat. No. 4,547,456 and
 PCT publ. Nos. WO 88/04237, WO 93/03928, and WO 95/00342.
 In still another particular type of thermal recording or heat mode
 recording materials information is recorded by creating differences in
 reflection and/or in transmission on the recording layer. The recording
 layer has high optical density and absorbs radiation beams which impinge
 thereon. The conversion of radiation into heat brings about a local
 temperature rise, causing a thermal change such as evaporation or ablation
 to take place in the recording layer. As a result, the irradiated parts of
 the recording layer are totally or partially removed, and a difference in
 optical density is formed between the irradiated parts and the
 unirradiated parts (cf. U.S. Pat. Nos. 4,216,501, 4,233,626, 4,188,214 and
 4,291,119 and British Pat. No. 2,026,346)
 The recording layer of such heat mode recording materials is usually made
 of metals, dyes, or polymers. Recording materials like this are described
 in `Electron, Ion and Laser Beam Technology", by M. L. Levene et al.; The
 Proceedings of the Eleventh Symposium (1969); "Electronics" (Mar. 18,
 1968), P. 50; "The Bell System Technical Journal", by D. Maydan, Vol. 50
 (1971), P. 1761; and "Science", by C. O. Carlson, Vol. 154 (1966), P.
 1550.
 Recording on such thermal recording materials is usually accomplished by
 converting the information to be recorded into electrical time series
 signals and scanning the recording material with a laser beam which is
 modulated in accordance with the signals. This method is advantageous in
 that recording images can be obtained on real time (i.e. instantaneously).
 Recording materials of this type are called "direct read after write"
 (DRAW) materials. DRAW recording materials can be used as a medium for
 recording an imagewise modulated laser beam to produce a human readable or
 machine readable record. Human readable records are e.g. micro-images that
 can be read on enlargement and projection. An example of a machine
 readable DRAW recording material is the optical disc. To date for the
 production of optical discs tellurium and its alloys have been used most
 widely to form highly reflective thin metal films wherein heating with
 laser beam locally reduces reflectivity by pit formation (ref. e.g. the
 periodical `Physik in unserer Zeit`, 15. Jahrg. 1984/Nr. 5, 129-130 the
 article "Optische Datenspeicher" by Jochen Fricke). Tellurium is toxic and
 has poor archival properties because of its sensitivity to oxygen and
 humidity. Other metals suited for use in DRAW heat-mode recording are
 given in U.S. Pat. No. 4,499,178 and U.S. Pat. No. 4,388,400. To avoid the
 toxicity problem other relatively low melting metals such as bismuth have
 been introduced in the production of a heat-mode recording layer. By
 exposing such a recording element very shortly by pulses of a high-power
 laser the writing spot ablates or melts a small amount of the bismuth
 layer. On melting the layer contracts on the heated spot by surface
 tension thus forming small cavitations or holes. As a result light can
 pass through these cavitations and the density is lowered to a certain
 Dmin value depending on the laser energy irradiated.
 According to EP 0 384 041 a process is provided for the production of a
 heat mode recording material having "direct read after write" (DRAW)
 possibilities wherein a web support is provided with a heat mode recording
 thin metal layer, preferably a bismuth layer, characterized in that in the
 same vacuum environment a protective organic resin layer in web form is
 laminated to said supported recording layer by means of an adhesive layer.
 A commercially available material manufactured according to the principles
 of cited EP 0 384 041 is MASTERTOOL MT8, registered trade name, marketed
 by Agfa-Gevaert N. V.
 A drawback of the method of preparation of a thin bismuth recording layer
 by vacuum deposition is the fact that this is a complicated, cumbersome
 and expensive process. Therefore, in pending European patent application
 appl. No. 98201117 an alternative process for applying a thin metal layer
 is described comprising the following steps:
 (1) preparing an aqueous medium containing ions of a metal,
 (2) reducing said metal ions by a reducing agent thus forming metal
 particles,
 (3) coating said aqueous medium containing said metal particles on said
 transparent support.
 In a preferred embodiment the metal layer is again a bismuth layer. However
 such bismuth layers coated from an aqueous medium suffer in their turn
 from another drawback. Compared to bismuth layers prepared by vacuum
 deposition their sensitivity to laser light is lower. This is due to the
 presence of a higher degree of oxidized bismuth, and to the presence of
 ballast compounds in the layer such as a binder and additives improving
 stability, which to a certain degree hamper the formation of microspheres
 by the action of laser radiation.
 The present invention extends the teachings on heat mode recording elements
 based on thin metal layers applied by coating from an aqueous medium.
 OBJECTS OF THE INVENTION
 It is an object of the present invention to provide a heat mode recording
 element based on a coated thin metal layer with improved sensitivity and
 Dmin.
 It is a further object of the present invention to provide a heat mode
 element less susceptible to bubble formation during laser recording.
 It is still another object of the present invention to provide such an
 improved heat mode recording element wherein the thin metal layer is
 coatable from an aqueous medium.
 SUMMARY OF THE INVENTION
 The above mentioned objects are realised by providing a heat mode recording
 element comprising, in order,
 (1) a transparent support optionally carrying a subbing layer,
 (2) a thin metal layer,
 (3) a protective layer or layer pack,
 characterized in that said heat mode recording element contains
 hypophosphorous acid, or phosphorous acid, or a mixture of both.
 In a preferred embodiment the thin metal layer is a bismuth layer. The
 hypophosphorous acid, or phosphorous acid, or mixture of both may be
 present in the metal layer, in the first protective layer of the
 protective layer pack, or in both.
 DETAILED DESCRIPTION OF THE INVENTION
 The different elements constituting the heat mode recording material
 obtained by the process according to the present invention will now be
 explained in more detail.
 Useful transparent organic resin supports include e.g. cellulose nitrate
 film, cellulose acetate film, polyvinylacetal film, polystyrene film,
 polyethylene terephthalate film, polycarbonate film, polyvinylchloride
 film or poly-.alpha.-olefin films such as polyethylene or polypropylene
 film. The thickness of such organic resin film is preferably comprised
 between 0.05 and 0.35 mm. In a most preferred embodiment of the present
 invention the support is a polyethylene terephthalate layer provided with
 a subbing layer. This subbing layer can be applied before or after
 stretching of the polyester film support. The polyester film support is
 preferably biaxially stretched at an elevated temperature of e.g.
 70-120.degree. C., reducing its thickness by about 1/2 to 1/9 or more and
 increasing its area 2 to 9 times. The stretching may be accomplished in
 two stages, transversal and longitudinal in either order or
 simultaneously. The subbing layer, when present, is preferably applied by
 aqueous coating between the longitudinal and transversal stretch, in a
 thickness of 0.1 to 5 mm. In case of a bismuth recording layer the subbing
 layer preferably contains, as described in European Patent Application EP
 0 464 906, a homopolymer or copolymer of a monomer comprising covalently
 bound chlorine. Examples of said homopolymers or copolymers suitable for
 use in the subbing layer are e.g. polyvinyl chloride; polyvinylidene
 chloride; a copolymer of vinylidene chloride, an acrylic ester and
 itaconic acid; a copolymer of vinyl chloride and vinylidene chloride; a
 copolymer of vinyl chloride and vinyl acetate; a copolymer of
 butylacrylate, vinyl acetate and vinyl chloride or vinylidene chloride; a
 copolymer of vinyl chloride, vinylidene chloride and itaconic acid; a
 copolymer of vinyl chloride, vinyl acetate and vinyl alcohol etc. Polymers
 that are water dispersable are preferred since they allow aqueous coating
 of the subbing layer which is ecologically advantageous.
 The process for preparing the thin metal layer on the transparent support
 will now be explained on the hand of the preferred embodiment wherein the
 metal is bismuth.
 In a first step an aqueous solution of bismuth ions is prepared. As most
 suitable bismuth salt bismuth nitrate is chosen. Almost all bismuth salts
 are poorly soluble in water. In order to maintain a sufficient amount of
 bismuth ions in solution, it is necessary to add a complexing agent. A
 preferred complexant is simply the well-known ethylenediaminetetraacetic
 acid (EDTA) or a homologous compound or a salt thereof. Another preferred
 one is citrate, e.g. triammonium citrate. Other suitable complexants
 include diethylenetriamine-pentaacetic acid (DTPA),
 trans-1,2-diaminocyclohexane-N,N,N',N'-tetraacetic acid (CDTA),
 ethyleneglycol-O,O'-bis(2-aminoethyl)-N,N,N',N'-tetraacetic acid (EGTA),
 N-(2-hydroxyethyl)ethylenediamine-N,N,N'-triacetic acid (HEDTA), etc.
 In a following step the bismuth ions in the solution are reduced to highly
 dispersed bismuth particles by means of the addition of a reducing agent.
 A preferred reducing agent is sodium hyposulphite. Another suitable
 reducing agent is KBH.sub.4. Others include glucose, formaldehyde,
 tin(II)chloride. The reducing agent can be added to the original bismuth
 salt solution as a solid powder. On the other hand the reducing agent can
 be dissolved separately in a second aqueous medium and added to the
 bismuth-tin salt solution according to a single jet or a double jet
 procedure. Preferably, according to the double jet principle, the aqueous
 medium containing the different metal ions and the second solution
 containing the reducing agent are added together to a third aqueous
 medium.
 In order to keep the bismuth particles formed by reduction in colloidal
 dispersion a protective binder is preferably added to one or more of the
 three aqueous solution involved. Preferably, this protective binder is
 added to the third aqueous medium wherein both others are jetted. A
 particularly preferred protective binder is carboxymethylcellulose (CMC),
 preferably of the high viscosity type. Other possible binders include
 gelatin, arabic gum, poly(acrylic acid), cellulose derivatives and other
 polysaccharides.
 When the reduction is substantially completed the aqueous medium can
 directly be coated on a support but more preferably the superfluous salts
 are first removed from the aqueous medium by a washing process, preferably
 involving optionally ultrafugation, ultrafiltration and/or diafiltration.
 In any of the solution involved in the preparation a so-called dispersing
 aid can be present. In a preferred embodiment this compound is added to
 the diafiltration liquid at the last stage of the preparation. Suitable
 dispersing aids in the case of bismuth are pyrophosphates, more
 particularly a hexametaphosphate such as sodium hexametaphosphate.
 Probably, the hexametaphosphate adsorbs to the surface of the bismuth
 particles so that they become negatively charged. By electrostatic
 repulsion they are kept in dispersion. So in a preferred embodiment the
 bismuth particles are ultrafiltrated e.g. through a Fresenius F60
 cartridge and subsequently diafiltrated against a solution of sodium
 hexametaphosphate in water/ethanol (98.5/1.5).
 In the final aqueous medium preferable an anti-oxidant, added at any stage
 of the preparation, such as ascorbic acid or a derivative thereof is
 present in order to avoid oxidation to bismuth oxide which would lead to
 an unacceptable density loss during drying after coating or during
 conservation of the unprotected bismuth layer. Finally, after the addition
 of one or more coating agents the obtained aqueous medium is coated on the
 transparent substrate by means of a conventional coating technique, such
 as slide hopper, curtain coating and air-knife coating.
 Suitable coating agents include non-ionic agents such as saponins, alkylene
 oxides e.g. polyethylene glycol, polyethylene glycol/polypropylen glycol
 condensation products, polyethylene glycol alkyl esters or polyethylene
 glycol alkylaryl esters, polyethylene glycol esters, polyethylene glycol
 sorbitan esters, polyalkylene glycol alkylamines or alkylamides,
 silicone-polyethylene oxide adducts, glycidol derivaties, fatty acid
 esters of polyhydric alcohols and alkyl esters of saccharides; anionic
 agenst comprising an acid group such as a carboxy, sulpho, phospho,
 sulphuric or phosphorous ester group; ampholytic agents such as
 aminoacids, aminoalkyl sulphonic acids, aminoalkyl sulphates or
 phosphates, alkyl betaines, and amine-N-oxides; and cationic agents such
 as aklylamine salts, aliphatic, aromatic, or heterocyclic quaternary
 ammonium salts, aliphatic or heterocyclic ring-containing phosphonium or
 sulphonium salts. Other suitable surfactants include perfluorinated
 compounds.
 The particle size of the reduced metalic bismuth is preferably comprised
 between 5 and 300 nm, most preferably 10 and 200 nm. The thickness of this
 Bi layer is preferably comprised between 0.1 and 1.5 .mu.m. When this
 thickness is too low the recorded images do not have sufficient density.
 When on the other hand the thickness is too high the sensitivity tends to
 decrease and the minimal density, i.e. the density after laser recording
 on the exposed areas tends to be higher.
 The formation of the thin metal recording layer has been described on the
 hand of the preferred embodiment wherein the metal is bismuth. However,
 the scope of the present invention is not limited to bismuth, but extends
 to other metals that can form thin metal recording layers by a similar
 procedure. Possible other metals for the recording layer in this invention
 include Mg, Mn, Cu, Ag, Au, Zn, Cd, Al, Ga, In, Ge, Sn, As, Sb, Se, Te,
 Sr, La, Pb, Nd, Ba, Be, Ca, and Ce.
 It will be readily understood that for each particular metal the choice of
 the metal ions, the complexant if any, the binder and dispersing aid, the
 reducing agent, etc., must be optimized and that the preferred embodiments
 will in most cases deviate from the preferred embodiments when the metal
 is bismuth.
 Preferably the hypophosphorous acid or the phosphorous acid or the mixture
 of both is incorporated, totally or partially, in the metal layer itself.
 The acid is preferably added after ultrafiltration to the final aqueous
 coating solution for the metal layer right before coating. Without willing
 to be bound by theory a possible explanation for the action of the
 (hypo)phosphorous acid in the case of a bismuth layer is the following. In
 the layer the acid reacts with unwanted residual trivalent bismuth thereby
 inducing the formation of hydrogen gas. This hydrogen gas, adsorbed to the
 metallic bismuth, will probably react in an exothermic way during the
 laser recording with one or more compounds present in the layer such as
 O.sub.2, CO.sub.2, carboxymethylcellulose and hexametaphosphate. Thanks to
 this extra thermal push a lower amount of laser energy is needed for the
 formation of micropheres which means that the material has become more
 sensitive. This hypothesis is sustained by ESCA measurements which show
 that the amount of trivalent bismuth is considerably less than in layers
 containing no (hypo)phosphorous acid.
 The layer may extra be stabilized by the addition of sulphite or
 hydroquinone or another anti-oxidant.
 Two additional advantages obtained by the addition of (hypo)phosphorous
 acid are worth mentioning. Firstly, a lower Dmin is obtained which can be
 attributed to the lower amount of bismuth oxide, and secondly, no bubble
 formation occurs any longer during laser recording. The bubble formation
 is probably due to the decarboxylation of the present bismutite at
 290.degree. C. Due to the lower amount of oxidized bismuth the amount of
 bismutite is also lowered.
 Although the (hypo)phosphorous acid is preferably present in the metal
 layer itself it can in principle be incorporated in the subbing layer
 covering the support or in one of the layers of the protective layer pack
 covering the thin metal layer. Such a protective layer or layer pack is
 highly wanted because the metal layer is very sensitive to mechanical
 damage.
 Three types of protective elements are preferred.
 In a first preferred particular embodiment this protective element
 comprises a transparent organic resin, acting as outermost cover sheet,
 and an adhesive layer. In this case the (hypo)phosphorous acid may be
 incorporated in the adhesive layer. The adhesive layer can be of the
 pressure-adhesive type or of the thermoadhesive type. Examples of
 pressure-adhesive resins are described in U.S. Pat. No. 4,033,770 for use
 in the production of adhesive transfers (decalcomanias) by the silver
 complex diffusion transfer process, in the Canadian Patent 728,607 and in
 the U.S. Pat. No. 3,131,106. When the adhesive layer is of the
 heat-activatable, also called thermoadhesive type, the adhesive layer is
 preferably applied on top of the metal layer by lamination together with
 the resin foil to which it is preliminary applied by coating. The exterior
 resin foil can be chosen from the group of polymeric resins usable for the
 support of the heat mode element. In a preferred embodiment the cover
 sheet is also polyethylene terephthalate but preferably substantially
 thinner (about 10 .mu.m) than the polyethylene terephthalate of the
 support.
 A survey of pressure and/or heat-sensitive adhesives is given by J. Shields
 in "Adhesives Handbook", 3rd. ed. (1984), Butterworths-London, Boston, and
 by Ernest W. Flick in "Handbook of Adhesive Raw Materials" (1982), Noyens
 Publications, Park Ridge, N.J.--USA.
 In a second preferred type of protective layer pack two layers are coated
 on top of the metal layer, a soft polymeric layer and an outermost hard
 polymeric layer. In this case the (hypo)phosphorous acid may be
 incorporated in the soft polymeric layer while still being effective.
 Combinations of useful compositions for the soft and the hard polymeric
 layers are described in Europen patent application appl. No. 98201117
 cited above.
 A third type of protective element consists of just one layer which due to
 the presence of a reactive monomer is radiation-curable, preferably
 UV-curable. Protective elements of this type are disclosed in pending
 European patent application appl. No. 97203857. In this case the
 (hypo)phosphorous acid may be incorporated in that curable protective
 layer.
 Since the (hypo)phosphorous acid can cause some instability in the coating
 solution containing the reduced metal dispersion it can be divided between
 this metal coating solution and the coating solution for a protective
 layer or for the subbing layer. In this way the stability of the coating
 layer can be assured. In this particular embodiment the acid is present in
 several different layers of the final heat mode recording element.
 For the formation of a heat mode image using the element of the present
 invention any laser can be used which provides enough energy needed for
 the production of sufficient heat for this particular process of image
 formation. In a preferred embodiment a powerful infra-red laser is used,
 most preferably a Nd-YLF laser or diode laser.
 The present invention will now be illustrated by the following examples
 without however being limited thereto.

EXAMPLES
 Example 1
 This example shows the improvements in sensitivity that can be obtained by
 adding hypophosphorous acid to the heat mode laser recordable Bi-layer.
 The following solutions were prepared:

Solution 1
 Water 400 ml
 Bi(NO.sub.3).sub.3.5H.sub.2 O 449 g
 Triammonium citrate (50% in water) 1200 ml
 NH.sub.3 (26% in water) (pH = 12) 300 ml
 Water to 2330 ml
 Solution 2
 Na.sub.2 S.sub.2 O.sub.4 (16.7% in water) 1238 ml
 Solution 3
 Water 1136 ml
 Carboxymethylcellulose (3% in water) 104 ml
 The samples, according to following table 1, were prepared as follows:
 To solution 3, held at 40.degree. C. and stirred at 450 rpm, solution 1 at
 a flow rate of 200 ml/min was simultaneously added with solution 2 at 117
 ml/min. After the reduction, the bismuth dispersion was ultrafiltrated
 through a Fresenius F60 cartridge and diafiltrated with a 0.2% solution of
 sodium hexametaphosphate in water/ethanol (98.5/1.5).
 The dispersion was stirred and 10 ml of a 12.5% solution of Saponine
 Quillaya (Schmittmann) in water/ethanol (80/20) was added.
 The dispersion was analysed for its particle size distribution (weight
 average d.sub.wa) with the Disc Centrifuge Photosedimentometer BROOKHAVEN
 BI-DCP. A d.sub.wa of 65 nm (s.sub.wa =6) was obtained.
 The dispersion was divided in small portions and to each of these portions,
 except for the control sample, a certain amount of a 50% solution of
 H.sub.3 PO.sub.2 (Merck) was added according to table 1.
 Subsequently these dispersions were coated on a substrated PET foil so that
 a density of 3.5 (Macbeth optical densitometer) was obtained. Thereupon a
 protective laminate comprising a 8 .mu.m thick pressure-adhesive layer,
 type DURO-TAK 380-2954, National Starch and Chemical Co., and a 12 .mu.m
 thick PET foil was laminated by using CODOR LAMIKER LPP650.
 The exposure was performed by a NdYLF laser emitting at 1064 nm. The image
 plane power was set between 200 and 450 mW maximum with intervals of 50
 mW. A spot size of 16 .mu.m was used together with a pitch of 8 .mu.m at a
 scan speed of 2 m/s. The sensitivity is defined as the energy necessary to
 obtain a linewith of 8 .mu.m in the image (microscopic evaluation) and is
 expressed in J/m.sup.2 (E.sub.SS =E.sub.single scan). The smaller this
 number, the more sensitive the film is.
 Table 1 lists the obtained results.
 TABLE 1
 ml 50% H.sub.3 PO.sub.2
 per liter E.sub.ss
 Sample dispersion (J/m.sup.2)
 1 (control) 0 5000
 2 (invention) 4 4750
 3 (invention) 8 3800
 4 (invention) 12 3600
 5 (invention) 22 3300
 As can be concluded from the results of table 1, adding hypophosphorous
 acid improves the sensitivity. No bubble formation cured during laser
 recording.
 Example 2
 This example shows the improvements in sensitivity and D.sub.min that can
 be obtained by adding hypophosphorous acid to a pressure-adhesive layer of
 the laminate in stead of adding the hypophosphorous acid to the heat mode
 laser recordable Bi-layer.
 The bismuth dispersion was prepared as was described in example 1. This
 dispersion was coated on a substrated PET foil so that a density of 3.5
 (Macbeth optical densitometer) was obtained.
 Pressure adhesive layers were prepared according to table II and coated
 with a barcoater of 40 .mu.m on a Melinex S (ICI) support of 23 .mu.m.
 This coatings were laminated against the Bi-layer by using the CODOR
 LAMIKER LPP650 at room temperature.
 Laser exposures of these samples were performed as described in example 1
 and the results are listed in table II.
 TABLE II