EP0086636A1

EP0086636A1 - Pressure sensitive record systems

Info

Publication number: EP0086636A1
Application number: EP83300687A
Authority: EP
Inventors: Hirokazu Tsukahara; Takahiro Torii
Original assignee: Appleton Papers Inc
Current assignee: Oldapco Inc
Priority date: 1982-02-13
Filing date: 1983-02-11
Publication date: 1983-08-24
Also published as: JPS58138689A; DE3360745D1; EP0086636B1

Abstract

A pressure-sensitive record system comprises a donor composition and an acceptor composition which react under pressure to generate a coloured image or print, in which the donor composition comprises microcapsules containing a solution in a high-boiling solvent (preferably an aromatic hydrocarbon solvent) of an electron-donating colourless dye and the acceptor composition comprises activated clay and an aryl or aralkyl ester of p-hydroxybenzoic acid. Record systems produced using this combination of specific donor and acceptor compositions have a high rate of colour development, a high stability of the coloured images formed, and stability of unprinted background portions, particularly on exposure to sunlight and atmospheric oxidizing gases, and consequently a long shelf life. The record system is preferably of the transfer type in the form of a manifold containing a CB-sheet coated with the donor composition and a CF-sheet coated with the acceptor composition and possibly one or more interposed CFB-sheets coated on opposite sides with the two compositions.

Description

This invention relates to pressure-sensitive record systems comprising a donor composition and an acceptor composition, said compositions reacting on pressure contact to develop a coloured image or print, and more particularly to such systems in which improved properties have been obtained by the use of combinations of specific donor and acceptor compositions.
A pressure-sensitive record system is an image- or print-forming system which develops a coloured image on reactive contact between a donor composition and an acceptor composition. The record system may be a so-called separate or transfer system having separate donor and acceptor sides. The donor side is usually formed by a so-called CB-sheet, obtained by coating microcapsules enclosing a chromogenic solution in the form of an electron-donating colourless dye in a high-boiling point solvent, on a sheet-like substrate, while the acceptor composition is generally in the form of a so-called CF-sheet obtained by coating an electron-accepting solid acid material (which is often referred to as a "colour developer") toqether with other auxiliary materials also onto a sheet-like substrate.
When both a CB- and a CF-sheet are brought into contiguous relationship at their respective coated surfaces and pressure is applied to the manifold set so produced by means of a pen, typewriter, impact printer or like means, in order to generate a coloured image or print, the microcapsules which are provided at the donor side are ruptured at each area to which pressure has been.applied, thereby releasing the solution of the electron-donating colourless dye. The solution so released is then transferred to the acceptor side, where it comes into reactive contact with the colour developer of the acceptor composition. This causes the colourless dye either to be adsorbed onto the surface of the solid colour developer and there ionized, or the high-boiling point solvent to take up the colour developer, thereby changing the pH of the solution of the colourless dye to the acid side and ionizing the colourless dye to form an image or print with the colour so generated.
If further copies are required, more than two record sheets may be combined to form a manifold set by interposing between a CB- and a CF-sheet, one or more so-called CFB-sheets, each obtained by coating a donor composition onto one surface of a sheet and an acceptor composition onto the other surface thereof, and the sheets being arranged in the manifold set so that each donor composition coating is in contiguous relationship with an acceptor composition coating.
Besides such separate- or transfer-type record systems, self-contained pressure-sensitive record systems are known in which both donor and acceptor compositions are coated in a laminated state or as a single layer on one surface of a sheet. Alternatively, the donor and acceptor compositions may be dispersed within the sheet itself. The colour-generating mechanism of such a self-contained record system is the same as that of separate record systems. Such self-contained record systems are also included in the scope of this invention.
Examples of electron-donating colourless dyes which can be used as chromogenic materials in the donor compositions of pressure sensitive record systems include dyes of the triphenylmethane lactone type typified by 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide (so-called Crystal Violet Lactone); fluoran-type dyes typified by 3-diethylamino-6-methyl-7-anilinofluoran; dyes of the acylated leucothiazine type; dyes of the acylated leucooxazine type; lactams and spiropyrans. All of these dyes may be employed in the present invention.
Examples of high-boiling point solvents which may be used in the donor compositions include aromatic hydrocarbons, such as alkyl-naphthalenes, typified by diisopropyl-haphthalene, diarylalkanes, typified by 1-phenyl-l-xylylethane; alkyl biphenyls typified by isopropyl biphenyl; triaryldimethanes; alkylbenzenes; benzylnaphthalenes; diarylalkylenes; arylindanes; carboxylic acid esters typified by dibutyl phthalate and dioctyl maleate; phosphoric acid esters typified by tricresyl phosphate; naturally-occurring animal and vegetable oils and fats, such as castor oil, soybean oil, and cotton oil; and natural high-boiling point fractions, consisting of aliphatic hydrocarbons, such as mineral oil. An aromatic hydrocarbon solvent is, however, preferably used in the present invention and any of the aromatic hydrocarbons listed above may be so used.
Many electron-accepting solid acid materials have been previously proposed for use as colour developers. These include naturally-occurring minerals, such as acid clay, attapulgite, clay and the like, and activated clay obtained by treating an acid clay (a clay mineral of the montmorrilonite series) with a mineral acid, as inorganic colour developers; and a variety of phenolic compounds, novolak phenolic resins, multivalent metal salts of aromatic carboxylic acids and the like as colour developers of the organic acid type. However, the above-mentioned inorganic colour developers have serious disadvantages in that the developed colour images or prints lack stability (i.e., are susceptible to fading of the colour on exposure to light and may vanish completely if exposed to water or steam) and their colour developing capability is reduced, due to the adsorption of substances contained in the air thereon, in other words, they have a reduced storage life. On the other hand, the colour developers of the organic acid type also have disadvantages in that the rate of colour development is slow and developed colour images or prints may disappear when brought into contact with plasticizers for plastics materials or with oils, and unprinted background areas of the sheet material undergo discolouration when exposed to sunlight or to oxidizing gases present in the atmosphere, such as nitrogen oxides. With a view to overcoming these disadvantages, attempts have been made to use one or more inorganic colour developers and one or more colour developersof the organic acid type together as a mixture. Such attempts have in some cases resulted in some improvement in certain aspects. However, no fully satisfactory results have as yet been obtained, because, for example, some of such attempts frequently led to discolouration of the unprinted background of CF-sheets.
It is an object of the present invention to provide a pressure-sensitive record system having significantly improved properties as compared with previously-proposed systems.
According to the invention, there is provided a pressure-sensitive record system which comprises a donor composition and an acceptor composition which react under pressure contact to develop a colour image or print, characterised in that said donor composition comprises microcapsules containing a solution of an electron-donating colourless dye in a high-boiling point solvent, and said acceptor composition comprises an activated clay in combination with a para-hydroxybenzoic acid ester of the formula (1) :-
where R is an aryl or aralkyl group.
The electron-donating colourless dye used as a chromogenic material may be any of the examples listed above, or a mixture thereof. The high-boiling point solvent used, which preferably has a boiling point greater than 200°C, is preferably an aromatic hydrocarbon, and may be any of the aromatic hydrocarbon type solvents listed above. (The use of the word "colourless" herein with reference to the electron-donating dye is not intended to exclude the possibility that the dye may itself be coloured, provided that the colour developed in contact with the colour developer is in strong contrast to the original colour of the dye).
The pressure-sensitive record systems of the invention (which may be either separate transfer record systems or self-contained systems) have been found to have significantly improved properties as compared with known record systems, as regards the speed of colour development, fastness of the coloured images or prints produced, particularly as regards stability to exposure to light, water or moisture, and to plasticizers and oxidizing gases contained in the atmosphere, and stability of the unprinted background areas of the sheet, i.e. resistance to discolouration of these areas on exposure to sunlight or oxidizing gases.
The activated clay, suitable for use in the present invention, may be obtained by subjecting acid clay (i.e. a clay mineral of the montmorrilonite series) to a mild to medium treatment with a mineral acid, so as to extract and remove therefrom acid-soluble cations, such as iron, magnesium, calcium and aluminium, to a certain extent. Thus, the activated clay is a silicic acid.anhydride still containing such cations, at substantial concentration levels. The activated clay has acid values pKa<-3.0, -3.0<pKa<+0.8, and +0.8<pKa<+4.8. (These acid values are associated with the presence of cations of mixed metals). Thus, transfer of electrons take place when an electron-donating colourless dye, such as Crystal Violet Lactone or Benzoyl Leuco Methylene Blue is adsorbed on activated clay, thereby ionizing the colourless dye and producing a colour print or image. As a typical example, reference may be made to activated clay produced from a domestic acid clay as starting material, e.g. that produced and sold under the Trade Mark "Shilton" by Mizusawa Chemical Industries Ltd.
Preferred examples of para-hydroxybenzoic acid esters which may be used together with the activated clay, include the following compounds: Benzyl p-hydroxybenzoate
o-Methylbenzyl p-hydroxybenzoate
p-Methylbenzyl p-hydroxybenzoate
p-Chlorobenzyl p-hydroxybenzoate
Phenyl p-hydroxybenzoate
p-Methylphenyl p-hydroxybenzoate
These para-hydroxybenzoates may be synthesized by esterification reactions between p-hydroxybenzoic acid and the corresponding alcohols and are available commercially from various sources.
The microcapsules provided at the donor side may be prepared by any known micro-encapsulation technique. It is possible to use, for example, the coacervation method relying on a combination of gelatin and high molecular anions; the in-situ polymerization method making use of the polycondensation reaction between urea and formaldehyde or melamine and formaldehyde; or the interfacial polymerization method making use of the interfacial polymerization reaction between an activated hydrogen compound and an isocyanate compound. The present invention is not, of course, limited to the use of any specific micro-encapsulation method.
Incidentally, Japanese Patent Specification No. 16969/1975 as published, discloses, as a colour-developing paper for pressure-sensitive copying (CF-paper), a paper sheet coated with an alkyl p- or o-hydroxybenzoate and activated clay. In this publication, as the matching CB-sheet, there is used a CB-sheet obtained by dissolving Crystal Violet Lactone and Benzoyl Leuco Methylene Blue in a partially-hydrogenated triphenyl (which is believed to be a partially-hydrogenated triphenyl, the aromatic nature of which has been lost to a considerable extent), forming the thus-obtained solution into microcapsules and then coating the microcapsules onto a paper sheet.
The present inventors, have found that in general alkyl o-hydroxybenzoates are in the liquid state at room temperature and give off a strong and characteristic odour. The proposal to use these compounds is therefore not considered to be a practical one. The use of an ester of p-hydroxybenzoic acid and an aromatic alcohol (together with activated clay) as required in the present invention has been found to provide an excellent colour developing capability, particularly for a CB-sheet provided with microcapsules containing an aromatic high b.p. solvent and an electron-donating colourless dye, as compared with using an alkyl p-hydroxybenzoate, together with activated clay.
Japanese Patent Application No. 33209/1973 as published, discloses the use of acid clay and p-phenylphenol or p,p'-isopropylidene diphenol (usually referred to as bisphenol A) in combination as a colour developer. The present inventors, however, have found that a CF-sheet even using bisphenol A, let alone p-phenylphenol, in combination with activated clay developed discolouration of unprinted background areas on exposure to light (e.g. yellowing of white background areas) and, especially, severe discolouration of such background areas on exposure to oxidizing gases generally present in the atmosphere (confirmed by the results of an NO gas exposure test). Such a CF-sheet is thus impractical and unsuitable for actual use.
The most important constituent elements in the pressure-sensitive record system according to this invention will be listed as follows:

(1) an electron-donating colourless dye;
(2) a high-boiling point, preferably aromatic hydrocarbon, solvent;
(3) microcapsules containing the components (1) and (2);
(4) activated clay; and
(5) a p-hydroxybenzoic acid ester of the formula (I) above.

A preferred embodiment of the invention is illustrated in the following Example, in which all parts and percentages are by weight.

Example:

The following components were ground for 2 days in a ball mill:
One hundred parts of activated clay produced and sold by Mizusawa Chemical Industries Ltd. (under the Trade Name: "Shilton M-140") and 50 parts of the wet-ground benzyl p-hydroxybenzoate dispersion prepared as above were dispersed in 200 parts of water containing 1 part of sodium pyrophosphate dissolved therein, followed by the addition of 50 parts of 10%-oxidized starch and 50 parts of 48% SBR-latex to form a coating composition. The coating composition so obtained was applied to a base paper sheet of 40 g/m ² to give a dry coating weight of 7 g/m , to form a CF-sheet (Sample B).
For comparison tests, three other types of CF-sheet were also prepared in the same manner, except that in one sheet (Sample A) benzyl p-hydroxybenzoate was not used at all (i.e. activated clay was used as a sole colour developer), in another sheet (Sample C),methyl p-hydroxybenzoate was used in place of benzyl p-hydroxybenzoate, and in a third sheet (Sample D), bisphenol A was also used in the necessary amount.
The corresponding CB-sheets were each prepared by adding 25 parts of wheat starch and 150 parts of a 10% aqueous solution of oxidized starch to 100 parts (in terms of dry weight) of microcapsules having a melamine resin base which had been obtained by micro-encapsulating, in accordance with the teachings of U.S. Patent Specification No. 4,233,178, a solution of an electron-donating colourless dye consisting of:
in a high-boiling point solvent of the aromatic hydrocarbon group, and then coating the composition so obtained onto a base paper sheet of 40 g/m ² to give a dry coating weight of 5 g/m².
On each of the four different CF-sheet samples, a respective CB-sheet was superposed so that the coated surfaces of the sheets faced each other and pressure was applied to the combined sheets by means of medium-pressure calender rolls (the nip pressure of the rolls being within the range of writing pressures). The density of the colour produced on the CF-sheet was measured 1 minute later, 1 hour later and 24 hours later, the results being given in Table 1 below. (By the term "density" as used herein, is meant optical reflection density). From the results, it will be seen that Sample B, which used the acceptor composition required for the invention, gave the highest density, the difference in density between the samples being particularly great after the lapse of 1 minute. This means that it can provide a darker copy promptly when written and is thus very much to be preferred from the practical viewpoint.
Next, in order to obtain colour images or prints of a saturated colour density, to the combination of a respective CB-sheet and each of the four CF-sheetspressure was applied by means of high-pressure calender rolls (i.e., super-calender rolls) and, after the passage of 1 day or longer, the coloured CF-sheet was subjected to tests in which it was exposed to each of sunlight, humidified air and an oxidizing gas normally present in the atmosphere (NO gas was used). The test results are shown in Table 2. From the results, it will be seen that Sample B, which was the combination used in the present invention, showed a small density drop, particularly on exposure to humidified air. Its resistance to fading on exposure to sunlight or to NO gas was superior to that of the sample using activated clay only (SampleA) and was comparable with those of the remaining comparative examples.
Exposure tests to sunlight and nitrogen oxide gas were then carried out to determine the resistance of each of the CF-sheets to deterioration as shown by discolouration of the background (i.e., its stability). The results are given in Table 3.
From Table 3, it will be seen that the CF-sheet (B) used in the present invention developed only an extremely small discolouration on exposure to sunlight or oxidizing gases present in the atmosphere (this means that the shelf life of the product is long) while use of activated clay and bisphenol A in combination in the comparative example (Sample D) showed severe discolouration on exposure not only to sunlight but also to NO gas. Moreover, Sample A, which used activated clay as its sole colour developer, almost completely lost its colour developing capability in the exposure tests.
Although the matter will not be discussed in detail, colour images produced respectively by the above-described Samples A, B, C and D, were all stable to plasticizers for plastics materials and did not disappear even when kept sandwiched between PVC films containing about 30% of dioctyl phthalate. However, in the case of a CF-sheet prepared using an organic colour developer only, its colour images disappeared in a similar test.
The density of each background was determined by measuring its reflection density using a blue filter. Note: ^* Concentration of nitrogen oxides: 300 ppm
As has been described above, the combination of specific donor and acceptor compositions according to this invention has been found to provide an excellent pressure-sensitive record system which features a fast rate of colour development, high stability to fading of the colour images produced, substantially no discolouration of the CF-sheets (e.g. yellowing of white sheets) and a long shelf life.
As previously stated, apart from their use in separate or transfer record systems as particularly described above, the combination of donor and acceptor compositions of the present invention may equally be applied to self-contained record systems.

Claims

1. A pressure-sensitive record system which comprises a donor composition and an acceptor composition which react under pressure contact to develop a colour image or print, characterised in that said donor composition comprises microcapsules containing a solution of an electron-donating colourless dye in a high-boiling point solvent,and said acceptor composition comprises an activated clay in'combination with a para-hydroxybenzoic acid ester of the formula (I)

where R is an aryl or aralkyl group.

2. A pressure-sensitive record system as claimed in Claim 1, characterised in that said high-boiling point solvent boils at above 200°C.

3. A pressure-sensitive record system as claimed in Claim 1 or Claim 2, characterised in that said high-boiling point solvent is an aromatic hydrocarbon solvent.

4. A pressure-sensitive record system as claimed in Claim 3, characterised in that said aromatic hydrocarbon high-boiling point solvent is selected from alkyl naphthalenes; diarylalkanes, alkyl biphenyls, triaryldimethanes, alkylbenzenes, benzylnaphthalenes, diarylalkylenes, arylindanes and mixtures thereof.

5. A pressure-sensitive record system as claimed in any one of the preceding Claims, characterised in that said electron-accepting colourless dye is selected from triphenylmethane lactone type dyes, fluoran-type dyes, acylated leucothiazine type dyes, acylated leucooxazine type dyes, lactams, spiropyrans, and mixtures of two or more such dyes.

6. A pressure-sensitive record system as claimed in any one of the preceding Claims, characterised in that said para-hydroxybenzoic acid ester is selected from the benzyl, o-methylbenzyl, p-methylbenzyl, p-chlorobenzyl, phenyl and p-methylphenyl esters of p-hydroxybenzoic acid.

7. A pressure-sensitive record system as claimed in any one of the preceding Claims, characterised in that said activated clay is an activated clay obtained by subjecting an acidic clay of the montmorrilonite group to mild to medium treatment with a mineral acid so as partially to remove acid-soluble cations therefrom.

8. A transfer type pressure-sensitive record system as claimed in any one of the preceding Claims, characterised in that a first substrate sheet (CB-sheet) coated on one surface with said donor composition and a second substrate sheet (CF-sheet) coated on one surface with said acceptor composition are combined to form a manifold with the coated surfaces of said sheets in contiguous relationship.

9. A transfer type pressure-sensitive record system as claimed in any one of Claims 1 to 7, characterised in that a first substrate sheet (CB-sheet) coated on one surface with said donor composition, a second substrate sheet (CF-sheet) coated on one surface with said acceptor composition, and one or more further substrate sheets (CFB-sheets) coated on one surface with said donor composition and on the other surface with said acceptor composition, are combined to form a manifold, said further CFB-sheet or sheets being interposed between said first CB-sheet and said second CF-sheet in such manner that at each adjacent pair of sheets a surface coated with the donor composition is in contiguous relationship with a surface coated with the acceptor composition.

10. A self-contained pressure-sensitive record system according to any one of Claims 1 to 7, characterised by the use as record material of a substrate sheet coated with or containing a mixture of said donor composition and said acceptor composition.