CA1063348A - Heat-sensitive recording material - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A thermographic recording process in which a dye is produced by bringing image-wise into reactive contact with the aid of heat an acid-reacting compound with a dye precursor compound corresponding to the following general formula :

X - Ar - CH = N - R1 wherein :
Ar represents a bivalent aromatic nucleus, R1 represents an aryl group including a substituted aryl group, and X represents a

Description

1~)63348 :~ , ~ he present invention relates to heat-sensitive mate-rials suited for the recording and/or reproduction of infor-mation and to recording processes wherein such materials are used.
In common thermography a heat-sensitive sheet is brought into face to face contact wi-th a graphic original that carries an, image formed of infrared radiation absorbing material. ~heu the original is exposed to infrared radia-tion, the i~age portions thereof are heated selectively and cause development in the adjacent heat-sensitive sheet of a colour pattern corresponding to the original.
~ ransfer by heat of reactant materials to a receptor sheet has been desoribed,e.g.,in the United Kingdom Patent Specification 973,965 filed Saptember 29, 1960 by Minnesota Mining Manufacturing ~nd in the United ~tes Patents 2,770,534 .
of Walter S.Marx, Jr. issued ~ovember 13, 1956 and 3,476,578 of Erio Maria Brinckman issued November 4, 1969 Heat-sensitive copy sheets that change colour when heated and in whioh the d~e-~orming reac~io~ is based on the reaction 20 of an acid-reacting compound with a dye precursor compound, have been described in the Canadian Patent No. 787,326 filed }~y Allied .
Chem. Corp.
From the United States Patent 3,594,208 of Joseph A.Wiese, Jr. and Donald J.Williams issued July 20, 1971 it is further known to prevent premature reaction between a dye precursor and a proton-producing compound by applying the dye precursor GV.767C _ 1 ~ ~ ;
, - .

10~i334~3 ~
compound in a binder layer different ~rom the binder layer containing an acid. In practice, the acid-ca~taining coating is applied as an outermost layer from a solution in a volatile liquid vehicle, which is a non-solvent ~or the vinyl chloride polymer acting as binder for the dye precursor, the layer com-prising the dye precur~or being applied as the first coating to the support.
The dye precursorslu~ed in ~uch integral copy sheet having a dual coating are N-bis(p-dialkylaminoary~methane -~
derivat~es. Ma~y of these derivatives are not completely co-lOUrleS9 80 that images having a slightly coloured image back-ground are obtained therewith.
An importa~t dema~d for copying materials in~ended for projectio~ purposes is the production of highly fade-resistant dye images, i.e. dye images, the dyes of which withstand con-ti~uous exposure to light as applied e.g. ln a~ overhead pro-jector.
In the pro~ection of multicolour images there is a special ~eed for light resistant yella~d~es.
~here has now been found a thermographic recording pro-cess in which essentially yellow azom~hine d~e salt~ o~ good fade resistance are produ¢ed by bri~gin~ image-wise into reaotive co~taot with the aid of heat a~ aoid-reacting compound with a dye precur~or oompound oorre~ponding bo the ~ollowing gener~l formula : -~ - ~r - CE . N -GV.767a - 2 -;.,.- : ., . ... . ., . , ~. .. -, .; - ,. .: . . . . . . ... ..

~06334~3 wherein :
Ar represent~ a bivalent aromatic ~ucleus e.g. phenylene, R1 represènbs an aryl group including a substituted aryl group e.g. a phenyl, naphthyl or biphenyl group, substitu-ents of the aryl group being e.g. an alkyl group, an alkoxy group, an alkoxycarbonyl substituted alkoxy group, a car-box~l substituted alkoæy group, an aryl sulphonyl substituted alkoxy group, a phenyl carbamoyl substi~uted alkoxy group, an alkyl mercapto group, an alkylamido group, or halogen e.g. bromine, and X represents an a ~fR2 group wherein each of R2 and R3 (same or different) represents an alkyI group e.g. a C1-C5 alkyl group, a cycloalXyl group, an aralkyl group, or an aryl group including said groups in substituted form or R2 and R3 together represen~ the necessary atoms to close a nitrogen-co~nLng heterocyclic nucleus e.g. a piperidine, pyrrolidine~ or morpholine nucleus.
Representatives of particularly suitable d~e precursor compounds are given in the following table.
able _ _ ~ _ ~ _ ~_ __ ____ ~lti ~o. Dye precursor compound point - ~ - OC~3 - ~ i H ~ C~ o-aH2-a~2-o- ~ 152 ~ ~ l Cl-aH2-aH22~ ~ ~ -~H=~- ~ _o-~cH2)1o-cooc2H566

2 _________________________ _________ (,V.767C ~ ~ - 3 -.. . , : , .
~, , .

~63348 _ ~ . , ___ _ _ ___

3 ~3- ~-N=HC- ~-~CH3 208

4 H3CC~- ~ -CH=l 1- ~ -S- ( CH2) 1 5_CH3 99 . ~ ;

XE3Cc~ -CH=~- ~ -O- ( CH2) 1 5-CX3 98 . ' '~' ' '' ~CH~ ~CH3 112 : . .
~ : ~
! OCE3 :

7 E3C~~ ~-CH=l~- ~-OCH3 145 . , .~

8 ~-so2-cH2-cx2-o- ~-~-CH~ CH3 154 9 Br-g~-~=CE-~-N~c~33 160 10 H3CO- ~-N-CE- ~-~CcHH3 ,140 ~ E7= -e~-~O~E3 ¦ 122 _ ,X5C2 . .

______ ___ ________________________________________ ,_______ ~V ~ 767 C - 4 ' i 1C~63348 ___________ ___ ___ _ _____ __ __ __ .. ._ _ __ _ 12 ~ -~H-aO-CH2~0- ~ -~=CH~ C~ . 205 13 H3C- ~ -~=HC- ~ -N~CH33 120 CH~

4 H33C ~- ~ -CX=~ CH3 102 . . .

CX3 : .

15 ~ _~=XC~ o ~ ~7 . .;
16 CCl aCHH ~ 2C ~- ~ -CX=N- ~ -SCH3 114 : 17 HH3CC ~_ ~ -CH=~- ~ -0-(cH2)1o-cooH 140 . . .
: 20 18 ~ -N-XC~ a~2-coocE3 112 ~aE ~ C~ ~2 . :~
'19 ~_~,~C- e~-~CH22_cH22_cN

L20 ~ _~aEa~ ~ -~CH33 ___________ L . . ~ .
______---- , . .
GV.767C - 5 - :
:,. ~

~ 6334~3 ~

___ ________ _____ _______ ________ _ _ __ _ _ ~_ __ __ ~ : .

~21 01-0~22-~20~ a~=~- ~ oa~ ¦ 83 ~
. ''',,'' ~ ' . 22 ~,CH- ~ -~`CE2~CH3 98 ~ ;.-23 ~=CH~ CH22-CE22-0~ 148 24 H3CO- ~ -CH=~ 'C2HH5 9o .
. . ., 25 ~C-aH2-cE2~ OH-N~ a~5 150 ~ ~ " . ' "

~ ~20 26 HH30~ ~ -GE=~ a ~5 140 . . . .

27 H3C-OC-H~ HC- ~ -N~CCH3 215 . .

28 X30S- ~ _~=EC- ~ -~H33 i 150 ~

__ ___________________~ ______________________ ________ ____ GV.767a -- 6 -: . :'. .
,!

~`
;3348 r~ -------29~=CH- ~ ~ ~ CH22-CH22-Cl 104 ~ .

3~- ~ -C~ C~3 1 1C7 1C ~ 312 ~_ ~ -CE=~- ~ -cc~3 ¦ 14C

-C~=~- ~ ~ - =C~ CX33 1 2 ~

~he preparation of the dye precur~or compounds is illustra-ted by the following reaction scheme :
ethanol X-Ar-CH0 + H2~-R1 ~ X-Ar-c~ R

and preparation receipt.
1 mole of the aldehyde a~d 1 ~le of the amine are dissol-ved whilst heabing i~ 1200 ml to 2800 ml of etha~ol. 1 mole of acetic a¢id is added dropwi~e to the obtained mixture o~er a period o~ 10 min. ~he rea¢tion mi~ture i5 then allowed to `~
boil with reflux for 20 to 60 min. ~ter cooling the precipi-tate is sucked off and washed with cold ethanol. Wh~lnecessary the obtaine ~ roduct is recrystallized frcmmetha~ol or a higher GV.767C - 7 -1~633~ ~
al cohol .
~ or the preparation ~ co~p~unds 24 and 26 the acetic acid has been replaced by 3 moles of triethylami~e.
~he reaction of acid with the above dye precursor com- -pounds results in the production of protonated azomethine dyes with light-absorptlon maxima that are more bathochromic than in the non-protonated structures.
~ he above colour precursors are suited for use in a thermographic two-sheet system using a transfer a~a receptor sheet or for use in an integral copy-sheet co~taining on a same support the proto~ donor or acid-supplying ~actant out of direct chemical contact ~rom the dye precursor at room temperature (20-30C) but i~ such a condition that reactive contact can be effected through heati~ig at a temperature above 60C.
Preferably acid reactants are used that evolve a volatile acid or melt at the temperature applied in the thermogra~ic proce~s. In this respect the following reactants are pre-ferred : benzoic acid, succinic acid, citric acid, cyanoacetic 2C acid, gallic acid, salic~lic acid, 5-bromosaliaylic acid, a sulfamic acid (i.e~ an organi¢ a¢id of bhe typc (R1.R2)-~-S020H
in which R'1 a~d R'2 are organi¢ groups), malei¢ a¢id~
2~4-dichloromaleio aoid~ pht~li¢ a¢id~ and th~ a~hydrides of ~hese a¢id~i. Further use can be made of the half-osters of ~ bivalent carboxylic acids. ~hese half esters may be formed j in situ in the coating composition by dissolving the correspo~d-GV.767a - 8 -, .. .. .

106334~

ing anhydrides in an alcohol, e.g. ethanol. ~xamples of such half-esters are the monomethyl-, monoethyl- or monoisopropyl esters of tetrachloroph-thalic acid.
Particularly suited are mono-esters of aromatic ortho-carboxylic acids described i~ the Canadic~n Patent 1,034,379 issued July 11, 1978 to Agfa-Gevaer-t N.V. corresponding to the following general formula :

1~ .
~ C - OH
Z ~L C - OR

wherein :
Z represents the ~ecessary atoms -to close an aromatic nucleus or ring system including such nucleus or ring system in substituted form, e.g. a benzene nucleus and a halogen- `
substituted benzene nucleus, and R represents an aliphati~c or cycloaliphatic group of at least 4 carbon atoms or an aliphatic group substituted with hydroxyl, an~eth-rified hydroxyl or a~ acyloxy group.
;Acids having a pKa value between 2 and 5 are generally e~fect-ive.
Examples of salts that are reacting as an acid are mono-sodium citrate, potassium aluminium sul~ate, aluminium sulfate, potassium hydrogen tartrate, sodium hydro~en phosphate, ammo ' nium gallate, ammonium benzoate and dichlorobenzidinedihydro-chloride.

i ` ` GV. 767C _ g ~ he anh~dride co~pound~ of succinic acid an~ o~ maleic acid and some o~ t~ ammonium salt~ show little proton acti-vity at room temperature~ but obtai~ i~creased activity at elevated temperatures. Examples ~ solid prototropic compounds that are useful for the~thermographic dye format'ion of the present invention are saccharine, barbituric acid, and uric acid. The~e co~pounds have the property of not bei~g a~
active proton-donor at room temperature, 80 that premature colour formatio~ therewith i~ avoided. In thermographic -heating conditions above 60C the~'become actively proton-donating.
According to o~e embodiment of a two-sheet thermographic recordin~ system the dye'precursor compound is applied in such a condition to or into a receptor sheet that an æid transferred from a contacting image-wise heated transfer sheet can reach this compound and react therewith to form the desired dye. ~he dye precur~or compound i9 preferably applied to a support in a binder ¢oating, to which the acid, when heated, can be tran~iferred from the transfer sheet.
Suitable binde~s for that purpose are vinyl chloride homopolymer and copolym~rs e.g. vinyl chloride copolymer including from 75 to 95 % of vin~l chloride~ Copolymers of vin~l ohloride~m~omer and of vinyl acetate monomer are pre-ferred copolymer~.
Gther copolymers of vinyl chloride e.g. with acrylonitrile ar~ useful llkewise. Polymers and copolymers~ which as a re-' .

~ GV.767C - 10 - ;
'1 ~ ~ ' , . . . . .

~ 0 6 33 4 ~
sult of their molecular weight or composition become sticky on heatin6 have to be avoided ~ince they prevent the easy separation of the transfer sheet from the receptor sheet.
~he composition of the receptor sheet coating will usually consist exclusively of non-acidic vinyl chloride polymer or copol~mer and tke dye precursor co~pound, though this is not absolutely necessary. Indeed, the receptor coating or an adjacent coating may contain pig~e~ts that give an overall colour to the receptor ~heet e.g. for obtain- ;
.
~ ing more ima~e contrast.
~or example, white pigments or coloured pi~menbs con-trasting in colour with the dye image produced may be incor-porated too i~ the receptor sheet~ Suitable pigments for that purpose are e.g~ tita~ium dioxide particles. ~he receptor coating may contain dif~erent kinds o~ fillers or grainy material such as silica particles that e.g. improve the capa-i bility of being written on with pe~cil.
~urther it may contain gloss-improving substance a~d ~ anti-sticking agents, e.g. metal soaps, aluminium stearate 3 20 being an example th~reo~.
In the two ~heet sysbem good results have been obtalned with an amou~t of d~e pracursor compou~d in a ratio o~ 1 part by wei~lt with re~pect to 1 to 20 parts by wei~ht o~ binder.
~he support of the re¢eptor!sheet is preferabl~ ~lexible.
Any kind o~ paper or resin support may be used. However~
i~ the adharence o~ the receptor coating is too low, a sui-"I . , .
table subbing layer or layèrs ma~ be applied to the suppo~.

GV.767C - 11 -, I .

~ 633~8 ~he support has to be tran~p~rent ~or visible lightg when the ~opie~i obtai~ed with the recordin~ material have to be used for projection e.g. in an overhead projector.
In the mono-sheet sy~tem different techniques of keeping the acid ~a¢tant and the d~e procursor compound out of reactive chemical contact beow 60C may be applied. ~or example, the reactants are kept out of direct chemical con-tact by e~veloping at least oi~e of the reactants i~ a capsule or droplet that contains a shell or en~elope of a materlal, ~ normally a polymeric material or wax that preven~s the direct ~j ., .
contact with the other reactant. ~he capsule shell or droplet en~elope is ruptured or softened by,heating, as a result of whioh t~ reac~ants come into reactive contact.
~ he capsules or droplets containing a first reactant may be dispersed in the paper mabs of a paper sheet or in a ~inder or binder system containing the second reacta~t in dispersed or dissolved form.
~ he inner part of the capsule may be of organic non-water-miscible ~ature and the shell or envelope may contain or oonsist o~ a hydrophilic material e.g. hydrophilic polymer or colloid that is hardened optio~ally. aapsules of this type have ~en de~cribed e.g. in the U~ited Kin~dom Patent Specifi-oatio~s 1~281,492 ~iled April 19, 1971 bg l~at.Ca~h Re~ister, 1,276,598 îiled ~ugust 3, 197~ by ~u~i Photo ~ilm and 1,034,437 flled ~ebruary 20, 1963 by Gevaert Photo-Producten N .V .
GV . 767C _ 12 . . .
! ~
'`'I ~ .

~ 633~L~
Accordi~g to another embodiment the conte~ts of the capsule are hydrophilic. ~or example the capsule co~tai æ water and a first reàctant dissolved or dispersed therein. lhe capsule shell has a hydrophobic nature. he preparation of the latter ,.
type of capsules has been described in the United Eingdom Patents 1,048,696~ 1,048,697 both filed July 10, 1963 by Ge~aert Photo Producten ~.V. and 1,29i~,194 filed ~ovember 20, 1968 by Gevaert-Ag~a ~.V. and in the Belgian Patent 792,550 ~iled December 11, 1972 by Agfa-Gevaert ~.V.
Pre~erred integral copy sheets applied in the mono-sheet system co~-tain the dye preourso~ compound i~nd acid reacti~nt `, out of chemical reactive co~tact at least below 6~oa in apart ;I binder layers, thetop layer ha~ing been applied ~rom a solu-tion in a volatile liquid, which i~ a non-solverLt for the binder of the subjaoe~t other la~er. Premature reaotio~ is ~¦ avoided e~feotively when in a first layer on the ~upport o~ the reoording material a vinyl ohloride homopol~mer or oopolymer bi~der co~taining the dye precursor compound and bel~g i~isoluble or poorl~ ~oluble in ethi~nol or methanol is coated a~d the acid reactant a~d a pol~mer that is highly ~?l soluble in etha~ol e.g. oellulo~e ~itri~te or pol~vi~yl-acetate axe i~oorporated i~ a layer bonded to the ~ixist layer. aellulo~q nitrate conb~ g a small amou~t e~g.
2 % by weight o~ the oopolymer o~ methyl methaorylate and methacrylic aoid is a preferred bi~der oompositio~ for the la~er oo~ininS the a¢id rea¢ta~t. ~he methaor~lic açid content I of the aopolymer is pre~erably ~rom 10 to 60 % by weight.
:: 1 .
GV.767C - 13 - ` `
, i ~ .
b ~ i33~ /

.;

~ .~ I . . ' .
~ rystallization of the acid reactant in said~ethanol-solubIe binders may be avoided by incorporation therein of a suitable amount of plasticizer e.g. as described i~ the United States Patent Specification 3,~9~,208 as mentioned above.
If coated on a removable carrierj the very vinyl polymer film containing the dye precursor compound may serve as the backing but preferably it is permanently supported on a separate heat-resistant`film e.g. a polyester resin film, preferably a polyethylene terephthalate film. ~he ratio of vi~yl polymer to d~e precursor compound in the sin~le sheet system material may be in the range of about 20 to 3 parts by weight of polymer to 1 part by weight of dye precursor compound . i -A'preferred acid ~aotant is phthallc anhydride.
If plasticizers are used in the lay~ containing the acid reactant preference is given to those that do not opacify the recording material, in other words those that are compatible with the bi~der e.g. cellulose nitrate. ~he plasticizer should there~ore be 801uble in phe same solvent as the binder. It should be esse~ially non-volatile in normal storage conditions. Suitable plasticizers for cellulose nitrate are "Butvar B--76" ~rade Mark) a polyvinyl butyral, poly-alkylene ~lycol, ~nd camphor.
~ he following examples illustrate the present inventionwithout, however, limiting it thereto. The percentages and ratios are by weight, unless otherwise indicated.
GV.767~ - 14 -,:j, IFA~ `:

10633~8 :
Exam~l e . 1 A polyethylene terephthalate suppo~t o~ a thicknes~ of 0.10 ~m wa~ coated at a ratio of 33 ml per ~q.m with the ~ollowing compositio~ :
7 % solutio~ in methyl ethyl keto~o of copolymer of vinyl ¢hloride and vi~yl acetate (85/15) 700 ml 2 % solution i~ methyl eth~l ketone of the dye preour~or compound 1 of the 300 ml ~fter dr~i~g a seco~d layer was applied at a ratio of 70 ml per sq.m from the following compo~ition :

5 % soluffDn in ethanol of polyvinyl acetate 500 ml 10 % ~olution in ethanol of phthalic ~nhydride (actually the ethyl half ester af ortho- 50~1 phthalic acid is formed in ~i~u) ethanol 450 ml ~fter drying of the second layer at 50G the re~ulting ~ansparent recording material was exposed relecbographically -~
to infrared radiation? the second layer being held in direct contact with the infrared absorbing image markings of a printed tex~ paper origlnal. I~ accordance with the infrared absorbing lm~ge markings a yelbw dye h~s been formed in the recording material.
~he resulting copy was particularly ~uited ~or projection with an overhead projector.
Example 2 .
A polyethylene tereph~halate support having a thickne~
of 0.10 mm wa~ coated at a ratio of 30 ~ per ~q.m from the GV.767C ~ - 15-.~ ~;''.
...

~633~
followi~g compositio~:

5 a/o sol~on in methyl eth~l keto~e of copolymer of vinyl chloride and vinyl acet~æ (85/15) 400 ml 2.5 % solution~nmethyl ethyl keto~e of dye precursor compound Nr. 7 100 ml After dr~i~g at 70C a second coating was applied at a ratio of 20 ml per sq.m from the followi~g compositio~ :
5 ~ solution ln methanol of cellulose400 ml 10 % solution in ~ethanol of polyvi~yl butyral 20 ml 10 % solution i~ ethanol o~ salicylic acid~2 ml ~ `
~ethanol ~ 118 ml ~ he recording material was dried and exposed re~lecto-graphicall~ as de~cribed i~ Example 1.
A yellow image correspo~ding to the image marki~Bs of the origi~al was formed.
~ he image was particularly suited for use i~ the projec-tlo~ with overhead projector.

.

G~ . 767a - 1.6

Claims (22)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A thermographic recording process in which a dye is produced by bringing image-wise into reactive contact with the aid of heat at above 60°C an acid-reacting compound, exclusive of acid-reacting compounds having the general formula wherein:
Z represents the necessary atoms to close an unsubstituted benzene nucleus, a halogen-substituted benzene nucleus or a nitro-substituted benzene nucleus, and R represents an unsubstituted aliphatic group containing at least 4 carbon atoms, a cycloaliphatic group or an ali-phatic group substituted with hydroxyl, with an etherified hydroxyl group or with an acyloxy group, with a dye precursor compound corresponding to the following general formula:

X - Ar - CH = N - R1 wherein:
Ar represents a bivalent aromatic nucleus, R1 represents an aryl group including a substituted aryl group, and X represents group wherein each of R2 and R3 which may be the same or different represent a member selected from the group consisting of an alkyl group, a cycloalkyl group, an aralkyl group, and an aryl group including said groups in substituted form or R2 and R3 together represent the necessary atoms to close a nitrogen-containing hetero-cyclic nucleus.

2. A thermographic recording process according to claim 1, wherein the dye precursor compound and acid-reacting compound are applied on separate support sheets and one of them is transferred by heat from its support into reactive contact with the compound on the other support sheet.

3. A thermographic recording process according to claim 1, wherein the dye precursor compound and acid-reacting compound are used in an integral copy-sheet containing on a same support the acid-reacting compound out of direct chemical contact from the dye precursor compound but in such condition that reactive contact can be effected through heating at a temperature above 60°C.

4. A thermographic recording process according to claim 2, wherein the dye precursor compound and acid-reacting compound are applied on separate support sheets and the acid-reacting compound being volatile or meltable at the tempera-ture reached in the image-wise heating is transferred from a transfer sheet to a receptor sheet which contains the dye precursor compound to form therewith a dye.

5. A thermographic recording process according to claim 4, wherein the dye precursor compound is applied in a coating containing a non-acidic vinyl chloride polymer or copolymer.

6. A thermographic recording process according to any of claims 4 and 5, wherein the dye precursor compound is contained in a binder coating in a ratio of 1 part by weight with respect to 1 to 20 parts by weight of binder.

7. A thermographic recording process according to claim 3, wherein at least one of the reactants being the acid-reacting compound and dye precursor compound are kept out of direct chemical contact by enveloping at least one of the reactants in capsules or droplets from which the reactant is set free by image-wise heating the copy-sheet.

8. A thermographic recording process according to claim 7, wherein the capsules or droplets are dispersed in the paper mass of a paper sheet or in a binder or binder system containing the second reactant in dispersed or dis-solved form.

9. A thermographic recording process according to claim 3, wherein the dye precursor compound and acid reactant are kept out of chemical reactive contact at least below 60°C

in apart binder layers.

10. A thermographic recording process according to claim 9, wherein the top layer of the binder layers has been applied from a solution in a volatile liquid, which is a non-solvent for the binder of the subjacent other layer.

11. A thermographic recording process according to claim 10, wherein the dye precursor compound has been incor-porated in a first Layer on the support of the copy-sheet in a vinyl chloride homopolymer or copolymer binder and the acid-reacting compound has been applied on top of said first layer from a solution of a polymer in ethanol or methanol.

12. A thermographic recording process according to claim 11, wherein the integral copy sheet is a clear trans-parent heat-sensitive sheet material useful in the prepara-tion of a colour projection transparency by thermographic copying procedures and includes a first layer containing the dye precursor compound in a vinyl chloride polymer binder selected from homopolymer and/or copolymer vinyl chloride binder and a second coating bonded to said first layer con-taining the compound reacting as an acid in a binder mainly containing cellulose nitrate.

13. A mono-sheet thermosensitive recording material containing a dye precursor compound and a compound reacting as an acid in which material the compound reacting as an acid and the dye precursor compound at a temperature below 60°C

are kept out of direct chemical contact and the dye precursor compound corresponds to the following general formula:

X - Ar - CH = N - R1 wherein:
Ar represents a bivalent aromatic nucleus, R1 represents an aryl group including a substituted aryl group, and X represents group wherein each of R2 and R3 which may be the same or different represents a member selected from the group consisting of an alkyl group, a cycloalkyl group, an aralkyl group, and an aryl group including said groups in substituted form or R2 and R3 together represent the necessary atoms to close a nitrogen-containing hetero-cyclic nucleus, and the compound reacting as an acid is exclusive of those having the general formula wherein:
Z represents the necessary atoms to close an unsubstituted benzene nucleus, a halogen-substituted benzene nucleus or a nitro-substituted benzene nucleus, and R represents an unsubstituted aliphatic group containing at least 4 carbon atoms, a cycloaliphatic group or an ali-phatic group substituted with hydroxyl, with an etherified hydroxyl group or with an acyloxy group.

14. A mono-sheet thermosensitive recording material according to claim 13 wherein the dye precursor compound and compound reacting as an acid are kept out of chemical reactive contact below 60°C in apart binder layers.

15. A mono-sheet thermosensitive recording material according to claim 14, wherein one of the binder layers is a top layer that has been applied from a solution in a volatile liquid, which is a non-solvent for the binder of the subjacent other layer.

16. A mono-sheet thermosensitive recording material according to claim 13, wherein the recording material is a clear transparent heat-sensitive sheet material useful in the preparation of a colour projection transparency by thermographic copying procedures and includes a first layer containing the dye precursor compound in a vinyl chloride polymer binder selected from homopolymer and/or copolymer vinyl chloride binders and a second coating bonded to said first layer containing the compound reacting as an acid in a binder mainly containing cellulose nitrate.

17. A mono-sheet thermosensitive recording material according to claim 16, wherein the second layer contains as sole binding agent cellulose nitrate and a plasticizer for cellulose nitrate.

18. A mono-sheet thermosensitive recording material according to claim 17, wherein the plasticizer is polyvinyl butyral, a polyalkylene glycol or camphor.

19. A mono-sheet thermosensitive recording material according to claim 17, wherein 5 to 50% by weight of plasti-cizer are used with respect to the cellulose nitrate.

20. A mono-sheet thermosensitive recording material according to claim 16, wherein the second layer contains cellulose nitrate and up to 2% by weight with respect to the cellulose nitrate of a copolymer of methylmethacrylate and methacrylic acid in which copolymer the methacrylic acid content is from 10 to 60% by weight.

21. A mono-sheet thermosensitive recording material according to claim 16, wherein the dye precursor compound in its binder layer is used in the range of 1 part by weight with respect to about 20 to 3 parts by weight of binder.

22. A mono-sheet thermosensitive recording material according to claim 16, wherein the compound reacting as an acid is present in its binder layer in an amount of l part by weight with respect to about 20 to 3 parts by weight of binder.