Pressure-sensitive mark-recording systems and solutions for use in such systems

Abstract

 The initial intensity and durability of marks produced in pressure-sensitive mark-recording systems employing a chromogen, an aromatic hydrocarbon solvent for the chromogen and a chromogen-sensitizing agent, are improved by effecting contact between the chromogen and the sensitizing agent in the presence of a promoter which is a compound that is an alkylated, aralkylated or cycloalkylated phenol or which contains an alkylated, aralkylated or cycloalkylated hydroxyphenol radical in its molecule, the structure of the phenol or radical being such that at least one of the positions ortho to the hydroxyl group is occupied by an alkyl group of at least four carbon atoms or by an aralkyl or cycloalkyl group.

Description

[0001] This invention relates to solutions which are useful in the production of pressure-sensitive mark-recording systems and to systems thus produced.

[0002] The most familiar form of pressure-sensitive mark-recording system is the so-called carbonless copying paper which comprises a two-sheet system in which the under surface of the top sheet has a coating of microcapsules of a solution of a colourless chromogen, while the upper surface of the lower (receiver) sheet has an absorbent coating including a sensitizing agent for the chromogen. When a marking instrument is applied to the top sheet, the microcapsules are locally ruptured, thereby releasing the chromogen solution from the affected microcapsules to react with the underlying sensitizing agent and form coloured marks on the receiver sheet corresponding to the marks applied to the top sheet.

[0003] A successful carbonless copying paper system needs to meet a number of criteria. For example, the marks on the receiver sheet should develop rapidly to a legible intensity of colour, and a legible mark should persist for as long as the sheet is required to be kept. Whether the various criteria are met depends on a number of factors including the nature of the solvent, the sensitizing agent and the chromogen, and many different materials of each category have been proposed.

[0004] We have now found that improvements are achieved in respect of the initial intensity of colour of the mark and its durability by including in the system certain phenolic compounds.

[0005] A solution of the invention comprises a liquid solvent comprising an aromatic hydrocarbon in which at least half the carbon atoms are benzenoid carbon atoms, and, dissolved in the solvent, a chromogen and a compound (hereinafter referred to as a promoter) which is an alkylated, aralkylated or cycloalkylated phenol or which contains an alkylated, aralkylated or cycloalkylated hydroxyphenyl radical in its molecule, the structure of the phenol or radical being such that at least one of the positions ortho to the hydroxyl group is occupied by an alkyl group of at least four carbon atoms or by an aralkyl or cycloalkyl group, and the solvent having a low volatility consistent with its use as a chromogen solvent in a system of the invention.

[0006] A pressure-sensitive mark-recording system of the invention comprises (a) sheet material, (b) promoted mark-forming components supported by the sheet material and arranged in juxtaposition but in unreactive condition, the said components comprising (i) a chromogen (ii) a sensitizing agent for the chromogen which produces a colour from the chromogen when brought into contact with the chromogen in the presence of a solvent and (iii) a promoter as defined above, and (c) the said solvent, which is a liquid comprising an aromatic hydrocarbon in which at least half the carbon atoms are benzenoid carbon atoms, and in which solvent both the chromogen and the promoter are soluble, supported by the sheet material but separated from the sensitizing agent by a physical barrier which is rupturable on the application of a marking instrumentto the sheet material.

[0007] The chromogens, sensitizing agents and the aromatic hydrocarbons useful in the present invention are,in general, materials conventionally employed in the art, and are more fully described below.

[0008] In the promoter, the or each phenol nucleus may contain from one to three alkyl, aralkyl or cycloalkyl (including alkylcycloalkyl) groups, and preferably contains two or three such groups, each of such groups preferably having up to 12 carbon atoms, with the proviso indicated above regarding occupancy of at least one of the positions ortho to the hydroxy group. An alkyl group occupying such a position is preferably an alkyl group of from 4 to 12 carbon atoms, especially a tertiary alkyl group, for example a t-butyl-, t-amyl, 1,1,3,3-tetramethylbutyl or 1,1,3-trimethylhexyl group. Alkyl substituents in other positions can have straight or branched chains, and usually contain from 1 to 12 carbon atoms,examples being methyl, ethyl, isopropyl, n-hexyl, 2-ethylhexyl, nonyl, decyl, and the tertiary alkyl groups named above. An alkyl group in such other position is often preferably an alkyl group containing from one to three carbon atoms, e.g. methyl, ethyl, or isopropyl, for example where the alkyl group concerned is a third alkyl group in a di-tertiary alkyl phenol, in a diaralkylated phenol or in a dicycloalkyl phenol, or where the alkyl group is present in an ortho- tertiary alkylated, aralkylated or cycloalkylated hydroxyphenyl nucleus that is linked to other atoms in the molecule of the promoter.

[0009] An aralkyl substituent in a promoter molecule is preferably a benzyl, alpha-methylbenzyl or alpha, alphadimethylbenzyl group. The last two are especially preferred where the substituent occupies a position ortho to a hydroxyl group. A cycloalkyl group occupying such a position is preferably an alpha-alkyl cycloalkyl group, for example an alpha-methylcyclohexyl or alpha-ethylcyclohexyl group.

[0010] Preferred promoters are mononuclear phenols, substituted in the manner defined; compounds containing two hydroxyphenyl radicals, each substituted in the manner ⁴ defined, and linked, preferably at the 2- or 4- positions, through a sulphur atom or through an alkylene or alkylidene group, usually a C_1-4 alkylene or alkylidene group; and compounds having the general formula

where A represents a hydroxyphenyl radical substituted in the manner defined, preferably linked at the 2- or 4-position to the (CH₂)p radical, p is an integer of from 1 to 4, m is an integer of from 1 to 4 and R represents a radical derived by the loss of m hydrogen atoms from an alkane of the formula C_nH_2n+2 where n is an integer of from 1 to 24, preferably of from m to 24.

[0011] Specific examples of promoters that are alkylated, aralkylated or cycloalkylated mono-nuclear phenols are ortho (alpha-methylbenzyl)phenol, 2,4-di(alpha-methylbenzyl)phenol, 2,6-di(alpha-methylbenzylphenol) and 2,4,6-tri(alpha-methylbenzyl)phenol and mixtures of these mono, di and tri(alpha-methylbenzylated)phenols, 2(alpha-methylbenzyl)-4-methylphenol, 2,6-di-t-butyl-4-methylphenol, 2,5-di-t-amyl- hydroquinone, 4,6-di-nonyl-2-methylphenol, 2,4-dimethyl-6 (d-methylcyclohexyl)phenol and 2,6-di(o(-methylcyclohexyl)-4-methylphenol.

[0012] Examples of promoters containing two hydroxyphenyl radicals substituted and linked in the manner defined above are 4,4'-thiobis(-2-t-butyl-5-methylphenol), 4,4'-butylidene- bis(2-t-butyl-5-methylphenol), 2,2'-methylenebis(4-methyl-6-t-butylphenol), 2,2'-methylenebis(4-ethyl-6-t-butylphenol), and bis[2-hydroxy-3(α-methylcyclohexyl)-5-methylphenyl] methane.

[0013] Examples of compounds having the general formula

are octadecyl 3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate and tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionyloxymethyl]methane.

[0014] Such promoters are commercially available materials.

[0015] The amount of the promoter used in a solution of the invention can be, for example, from 0.05 to 10% by weight of the solvent, but is preferably from 0.1 to 3%. The optimum will vary with the particular solvent and promoter used, but is often in the range 0.5 to 2% by weight of the solvent.

[0016] In addition to improving the initial intensity and durability of the mark, the presence of the promoter has been found to increase the solubility of the chromogen in the solvent. This effect could be used to increase the concentration of the chromogen in the solvent and thus obtain darker marks, but is mainly of interest in that it helps to avoid precipitation of the chromogen when the latter is used at conventional concentrations and the solution or system is stored at low temperatures.

[0017] The aromatic hydrocarbons suitable for use in the present invention can be single compounds, but are often mixtures of individual compounds, the aromatic hydrocarbon being such that at least half the carbon atoms in the compound or mixture are benzenoid carbon atoms. Aromatic hydrocarbons which can be used as, or as components of, the solvent in this invention include partially hydrogenated terphenyls, for example hexahydroterphenyls and dodeca- hydroterphenyls. As normally produced, materials of this type are mixtures containing in various proportions, fully hydrogenated terphenyls, partially hydrogenated terphenyls, and terphenyl itself. The terphenyl which is partially hydrogenated in obtaining such mixtures is itself a mixture of isomers. Other hydrocarbons include benzylated and alpha-methylbenzylated alkylbenzenes, e.g. mono- and dibenzyl ethylbenzene and mono(alpha-methylbenzyl)toluene, alkylnaphthalencs, e.g. dipropylnaphthalene and mono-alpha or -beta(2-hexyl) naphthalene, alkylbiphenyls, e.g. mono- and diisopropyl- biphenyls.

[0018] The solvent can be an aromatic hydrocarbon (provided the hydrocarbon is a liquid at room temperature) or a mixture of aromatic hydrocarbons, or an aromatic hydrocarbon or mixture together with a diluent, a diluent being an inert liquid miscible with the aromatic hydrocarbon(s). Examples of suitable diluents include mineral and vegetable oils, such as kerosene, paraffin oil, castor oil, soybean oil, corn oil and (long-chain alkylated)benzenes, for example (C₇-C₁₆ alk^ylated)benzenes. The diluent in a solvent of the invention functions to alter physical properties of the solvent, for instance viscosity or vapour pressure, where such alteration may be desired for optimum handling or processing. The solvent preferably contains at least 50% by weight of aromatic hydrocarbon but in some instances, the aromatic hydrocarbon may be diluted with up to, for example, 3 times its own weight of diluent. For rapid development of print intensity, the solvent is preferably one having a viscosity in the range 5 to 15 centisokes at 38°C.

[0019] The chromogen used in the presentinvention is usually an aromatic compound containing a double bond system which is converted to a more highly polarized conjugated and coloured form when reacted with an acidic sensitizing material. A particularly preferred class of chromogens includes compounds of the phthalide type, for instance crystal violet lactone which is 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminonaphthalide, and malachite green lactone which is 3,3-bis(p-dimethylaminophenyl)phthalide. Other phthalide-derived chromogens include 3,3-bis(p-dipropylaminophenyl)phthalide, 3,3-bis(p-methylaminophenyl) phthalide, 3-(substituted-phenyl)-3-(indole-3-yl)-phthalides such as 3-(p-dimethylaminophenyl)-3-(l,2-dimethylindol-3-yl)phthalide, 3,3-bis(methylindol-3-yl)-phthalides such as 3,3-bis(1,2-dimethylindol-3-yl)phthalide, 3-(phenyl)-3-(heterocyclic-substituted) phthalides such as 3-(p-dimethylaminophenyl)-3-(1-methyl -pyrr-2-yl)-6-dimethylaminophthalide, indole and carbazole-substituted phthalides such as 3,3-bis(1,2-dimethylindol-3-yl)-5-dimethylaminophthalide and 3,3-bis (9-ethylcarbazol-3-yl)-5-dimethylaminophthalide, and substituted indole phthalides such as 3-(1,3-dimethyl- indol-3-yl)-3-(2-methylindol-3-yl)-phthalide.

[0020] Other chromogens which can be used in the present invention include 3-dialkylamino-6-alkyl-7-alkylamino- fluorans, 3-dialkylamino-7-dialkylaminofluorans and 3-methyl-2,2'-spirobi(benzo[f]chromene).

[0021] An auxiliary colouring agent can be employed with the above chromogens. For example, crystal violet lactone (CVL) is usually used in conjunction with benzoyl leuco methylene blue (BLMB). The colouring agent complements the chromogen because on contact with the sensitizing agent the CVL produces an immediate dark blue colouration, but the colour tends to fade over a period of time. The BLMB when in contact with the sensitizing agent produces a somewhat pale initial colour, but the colour gradually intensifies by oxidation of the dye.

[0022] The sensitizing agent used in a system of the present invention can be, for example, an acid clay, for instance an attapulgite or bentonite clay, or an acidic organic polymer, for example a phenol-aldehyde polymer or a partially or wholly hydrolysed styrene/maleic anhydride or ethylene/maleic anhydride polymer. Particularly good results are obtained in the present invention using an acid clay as the sensitizing agent.

[0023] A system of the present invention can be prepared according to well known conventional procedures which are adequately described in the literature.

[0024] Although a preferred embodiment of this invention comprises a two-sheet system wherein the sensitizing agent is carried by one sheet and a marking fluid comprising a solution of the invention is carried by a second sheet, the invention is not limited to such systems alone. The only essential requirement is that the chromogen and the sensitizing agent be maintained in a separate or unreactive condition until pressure is applied to the system, and that upon the application of pressure the chromogen and the sensitizing agent are brought into reactive contact in the presence of the solvent and of the promoter. Thus it is possible to have the chromogen and sensitizing agent present in a dry and unreactive state on a common carrier and to have a solution of-the promoter in the solvent carried on a separate sheet whereupon the application of pressure would release the solution into the chromogen/sensitizing agent mixture and promote localized reaction and colour development. Obviously, many other arrangements, configurations and relationships of the solvent and the promoted mark forming materials with respect to their encapsulation and location on the supporting sheet or webs can be envisaged, and such arrangements are within the scope of the present invention. For example, it is possible to coat a single paper or support member with all the components of this system to form a single self- contained unit which can be marked by the movement of a stylus or other pressure-imparting means upon the surface of the paper. Such papers are particularly useful for use in inkless recording instruments.

[0025] Solutions of the invention were evaluated by the following technique :

[0026] A solution of a blend of equal parts by weight of CVL and BLMB in the solvent was prepared. A portion of this solution was used as the control, and in other portions there were dissolved the promoters to be evaluated. To estimate print intensity, a plate engraved with a pattern of dots was coated with sufficient of the solution to give a coating 18 microns in thickness, using a doctor blade. A carriage-supported roller having a paper sheet coated with an acid clay sensitizing agent wrapped round the roller was then moved slowly across the solution-coated plate under constant pressure. Colour developed on the paper. The paper was removed from the roller, and the intensity of colour was measured using a Macbeth RD 514 reflectometer calibrated against a "perfect white" of 0.07 units of optical density and a "perfect black" of 1.78 units of optical density, using standard "perfect white" and "perfect black" plates supplied by the manufacturer. After the initial reading, the paper was transferred to a humidified oven, and was removed at intervals for further colour intensity measurements.

[0027] Tc obtain the results shown in the Tables below, the colour intensity was measured each time at 20 different points on the paper surface and the values were averaged. The figures in the column headed "Standardized" are values of the average colour intensity on a scale on which the average initial colour intensity of the control is 100.

[0028] The results given in Table 1 were obtained using a solution containing 2.94% by weight of the blend of CVL and BLMB and 1.9% by weight of the promoter in a solvent consisting of a mixture of 7 parts by weight of hydrogenated terphenyls obtained by the hydrogenation of terphenyl with approximately 40% of the amount of hydrogen required for complete hydrogenation, and 3 parts by weight of a (C₉-C₁₁ alkyl)benzene. The oven was held at 75°C. and 60% relative humidity. The promoters used were as follows:

A - Mixture of mono- and di(α-methylbenzylated)phenol

B - 4,6-dinonyl-2-methylphenol

C - Tetrakis[(3-(3,5-di-t-butyl-4-hydroxyphenyl) propionyl)oxymethyl]methane

D - Octadecyl 3-(3,5-di-t-butyl-4-hydroxyphenyl) propionate

[0029] The results given in Table II were obtained using a solution containing 2.97% by weight of the blend of CVL and BLMB and 0.96% by weight of the promoter in a solvent consisting of a mixture of 7 parts by meight of hydrogenated terphenyls obtained by the hydrogenation of terphenyl with approximately 40% of the amount of hydrogen required for complete hydrogenation, and 3 parts by weight of kerosene. The oven was held at 70°C. and 50% relative humidity.

[0030] The improvements obtained in the presence of the promoter, both in respect of initial colour intensity and persistence of colour during accelerated storage, are apparent from the above results.

[0031] The effects of varying the proportions of CVL and BLMB in the blend in the presence and absence of promoter were investigated, with the results shown in Table III. The solvent was the same mixture as that used to obtain the results of Table II, and the oven was maintained at 75°C. and 60% relative humidity.

[0032] These results demonstrate the improvement obtainable by the addition of the promoter, and that when using a solution containing a promoter, the amounts of chromogen and auxiliary colouring agent can be reduced while maintaining good print intensity and durability.

[0033] The concentration of the chromogen in a solution of the invention is usually 0.75 - 2% by weight of the solution. Where both a chromogen and an auxiliary colouring agent are used, their proportions by weight may be, for example, from 40.60 to 80;20, and are preferably within the range 50:50 to 75:25.

Claims

1. A solution for use in a pressure sensitive mark-recording system, the said solution comprising a chromogen dissolved in a liquid solvent comprising an aromatic hydrocarbon in which at least half the carbon atoms are benzenoid carbon atoms, characterised in that there is also dissolved in the solvent a promoter which is a compound that is an alkylated, aralkylated or cycloalkylated phenol or which contains an alkylated, aralkylated or cycloalkylated hydroxyphenol radical in its molecule, the structure of the phenol or radical being such that at least one of the positions ortho to the hydroxyl group is occupied by an alkyl group of at least four carbon atoms or by an aralkyl or cycloalkyl group.

2. A solution according to Claim 1 in which the promoter is a mononuclear phenol; a compound containing two hydroxyphenyl radicals linked at the 2- or 4-position through a sulphur atom or through an alkylene or alkylidene group; or a compound having the general formula

where A represents a hydroxyphenyl radical linked at the 2-or 4-position to the (CH₂)pradical, p is an integer of from 1 to 4, m is an integer of from 1 to 4 and R represents a radical derived by the loss of m hydrogen atoms from an alkane of the formula C_nH_{2n + 2} where n is an integer of from 1 to 24.

3. A solution accordingto either of Claims 1 and 2 in which the structure of the phenol or radical is such that at least one of the positions ortho to the hydroxyl group is occupied by an alkyl group of from 4 to 12 carbon atoms, an alpha-alkyl benzyl group or an alpha-alkyl cyclohexyl group.

4. A solution according to Claim 3, in which the promoter is alpha-methylbenzylated phenol containing a mixture of ortho(alpha-methylbenzyl)phenol and 2,4-di(alpha-methylbenzyl) phenol and/or 2,6-di(alpha-methylbenzyl)phenol.

5. A solution according to Claim 3 in which the promoter is 4,6-dinonyl-2-methyl phenol.

6. A solution according to Claim 3 wherein the phenol or radical contains two or three alkyl, aralkyl or cycloalkyl substituent groups and thestructure of the phenol or radical is such that at least one of the positions ortho to the hydroxyl group is occupied by a tertiary alkyl group of from 4 to 12 carbon atoms or by an alpha-methylcyclohexyl group and at least one other position in the nucleus of the phenol or radical is occupied by an alkyl group of from one to three carbon atoms.

7. A solution according to Claim 6 wherein the promoter is 2,6-di-t-butyl-4-methylphenol, 2,4-dimethyl-6(alpha-methylcyclohexyl)phenol, 2,6-di(alpha-methylcyclohexyl)-4-methylphenol, 4,4'-thiobis(2-t-butyl-5-methylphenol), 4,4'-butylidene-bis(2-t-butyl-5-methylphenol), 2,2'-methylenebis (4-methyl-6-t-butylphenol), 2,2'-methylenebis(4-ethyl-6-t-butylphenol), or bis[2-hydroxy-3(alpha-methylcyclohexyl)-5-methylphenyf] methane.

8. A solution according to Claim 3 wherein the promoter is a compound having the general formula

where A represents a hydroxyphenyl radical linked at the 4-position to the (CH₂)p radical and having a tertiary alkyl group of from 4 to 12 carbon atoms in at least one of the 3- and 5-positions, and R represents a radical derived by the loss of m hydrogen atoms from an alkane of the formula C_nH_{2n + 2} where n is an integer of from m to 24.

9. A solution according to Claim 8 wherein the promoter is octadecyl 3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate or tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionyloxymethyl]methane.

10. A solution according to any of Claims 1 to 9, in which the amount of the promoter is from 0.1 to 3% by weight of the solvent.

11. A solution according to any of Claims 1 to 10, in which the chromogen is crystal violet lactone (CVL), and the solution also contains benzoyl leuco methylene blue (BLMB) as an auxiliary colouring agent, the proportions of CVL and BLMB being from 40:60 to 75:25 by weight.

12. A pressure-sensitive mark-recording system comprising (a) sheet material, (b) mark-forming components supported by the sheet material and arranged in juxtaposition but in unreactive condition, the said components comprising (i) a chromogen, (ii) a sensitizing agent for the chromogen which produces a colour from the chromogen when brought into contact with the chromogen in the presence of a solvent, and (c) the said solvent, being a solvent comprising an aromatic hydrocarbon in which at least half the carbon atoms are benzenoid carbon atoms, supported by the sheet material but separated from the sensitizing agent by a physical barrier which is rupturable on the application of a marking instrument to the sheet material, characterised in that the sheet material also supports a promoter which is an alkylated, aralkylated or cycloalkylated phenol or a compound which contains an alkylated, aralkylated or cycloalkylated hydroxyphenyl radical in its molecule, the structure of the phenol or radical being such that at least one of the positions ortho to the hydroxyl group is occupied by an alkyl group of at least four carbon atoms or by an aralkyl or cycloalkyl group, the promoter being soluble in the solvent, and the location of the promoter in the system being such that it is present when the chromogen is brought into contact with the sensitizing agent.