EP0693029B1 - Pressure-sensitive recorder paper - Google Patents

Abstract

The invention concerns single-layer pressure-sensitive SC (self-contained) recorder paper in which the binder contains modified starch and/or a starch derivative plus a synthetic latex.

Description

The present invention relates to an SC-type pressure-sensitive recording paper in which microcapsules, a color developer for the color former solution contained in the microcapsules, spacers and a binder are contained in one layer.

Pressure sensitive recording papers of the SC type (self contained) are known. A distinction is made between recording sheets in which microcapsules and color acceptors are arranged in one layer or in layers one on top of the other.

DE-C-10 10 822 describes a binder-free SC paper which is produced in a coating process, the color acceptor consisting of attapulgite being added and mixed to the capsule composition suspended in water, and the paper coating composition formed being applied to a paper and dried. The capsule walls are made of hydrophilic colloid material.

Although the binder-free procedure described here has the advantage that the color former and color acceptor can come into contact without hindrance, there is the disadvantage that an unwanted premature color reaction is not prevented with sufficient certainty.

DE-A-16 71 544 relates to an SC paper in which, in order to avoid an unwanted color reaction of the coating composition containing the capsules, a water-soluble alkyd resin, a water-dispersible metal soap, a water-dispersible paraffin or an aqueous emulsion of a paraffin wax and natural resin or a mixture of these materials is attached. The color acceptor can optionally be added to the capsule mass or applied to the paper sheet either before or after application of the capsule mass. Gelatine and gum arabic are used to make the capsules.

DE-A-24 46 313 relates to color formers based on fluoran compounds and their use both in pressure-sensitive and in heat-sensitive recording materials. Both CB sheets and so-called SC sheets are described as examples. Pig skin gelatin and gum arabic are given as capsule wall material. Insofar as the production of SC sheets from a composition containing the microcapsules and the color developer is described, this composition contains polyvinyl alcohol.

A copying paper with microcapsules, the capsule walls of which are formed by polyaddition from an isocyanate and a tetra- or pentamine, is disclosed in DE-A-27 53 767. Among the requirements specified in this document, which are made of microcapsules in general, reference should be made in particular to the requirement for the capsules to be impermeable to the color former and the solvent used. In relation to the prior art, the arrangement of the microcapsule layer or color acceptor layer on top of one another on one and the same paper surface is also specified in this document.

Another SC paper according to DE-A-29 30 408 is known, the microcapsule walls consisting of polyurea formed by reacting a polyisocyanate with a polyamine and water and the color developer a polysilicate formed in situ during the reaction of the polyisocyanate and polyamine and / or aluminum oxide hydrate, which is formed by precipitation from silica sol and / or aluminum hydrate. The type of binder used is not specified.

In order to avoid unwanted color reactions in SC papers, both color formers have already been proposed according to DE-A-28 20 462, DE-A-29 43 792, EP-A-0 011 367 and EP-A-0 120 972 also include organic color acceptors in microcapsules.

According to DE-A-29 43 792 mentioned above, it is provided that in addition to the encapsulated color former there is also powdered color former in the recording layer. In addition to water-soluble natural and synthetic binders, in which u. a. oxidized starch and starch esters are also called latices, e.g. B. based on styrene-butadiene. A mixture of water-soluble natural binders with synthetic latex is not mentioned in connection with a single-layer SC paper.

The aforementioned EP-A-0 120 972 provides for avoiding undesirable discoloration, e.g. B. when folding, the incorporation of a wax-like material in a single-layer SC paper in which both the color former and the color developer are microencapsulated. In addition to hydroxyethyl cellulose, polyvinyl alcohol and modified starch, latex is also mentioned as a binder, a mixture of latex with starch or modified starch not being mentioned.

In its general embodiment, DE-A-28 20 462 mentioned above also provides for the unencapsulated use of either color formers or color developers, but modified starches and SBR latices are not disclosed among a large number of binders, but their joint use is not disclosed.

From the Patents Abstracts of Japan M-1279 of July 14, 1992, vol. 16 / no. 321 in accordance with JP 4-90376 A, a pressure-sensitive recording paper of the SC type is known in which, in addition to dialdehyde starch, a water-soluble polymer is added to the color development layer as a binder. The color developer, also encapsulated, is organic in nature.

In order to achieve better storage stability of carbonless copying papers in which the capsule walls consist of a product formed by polyaddition of a diisocyanate and a diamine, DE-A-29 09 950 has already proposed a solvent for the color former, a mixture of alkylnaphthalene and to use isoparaffin and to anneal the microcapsules after their formation. The manufacture of SC-type pressure sensitive papers has not been addressed.

Finally, an SC-type pressure-sensitive recording paper is known from GB-B-15 24 742. To avoid a premature color reaction, the color developer and / or the microcapsules are marked with a protective substance, e.g. B. encased a urea resin or a melamine-formaldehyde polymer.

The known proposals still have certain disadvantages, in particular that the microcapsules produced are not always sufficiently leak-tight. In economic terms, additional procedural measures and the use of additional materials are to be mentioned as a disadvantage if, for. B. additionally encapsulated the color developer or microcapsules and color developer are additionally coated with a protective substance. Microcapsules made of polyurea have meanwhile found widespread use. Depending on the type of solvent or solvent mixture used to dissolve the color formers, the capsule walls may not be sufficiently dense. This disadvantage naturally has an effect especially when the microcapsules are to be used in SC papers.

Based on the known disadvantages, the invention has for its object to provide a pressure-sensitive recording paper of the SC type, in which the microcapsules have an increased tightness. Even after prolonged storage, there should be no discoloration that would impair readability. The cumbersome measures of a special coating of the microcapsules and / or the color developer should be dispensed with and the production in one be carried out in a single operation, ie the application of the color reaction layer from a coating slip containing both microcapsules and color developers may be possible. The task also includes a specific problem that arises in the manufacture of coating slips made using microcapsules with polyurea walls and with mineral color developers. The problem is that between the mineral color developers made of pigment, e.g. B. acid clay and free amino groups of the capsule wall material consisting of polyurea, an interaction occurs which leads in individual cases to a viscosity shock of the aqueous coating slip, but at least to a sharp increase in viscosity. Strong stirring of the highly viscous mixture can damage the capsule wall or cause leaks in the microcapsules.

Another object of the invention is to achieve the desired improvement in capsule tightness without other valuable properties such as. B. decrease sufficient color intensity of the generated image or sufficient printability of the color reaction layer in offset printing.

Finally, the invention wants to provide a pressure-sensitive recording paper of the SC type which, on the one hand, has a higher resistance to mechanical stress, for example in the case of Further processing, but on the other hand has no poorer copy performance.

To achieve the object, the invention provides for a pressure-sensitive recording paper of the SC type, in which microcapsules, a color developer for the color former solution contained in the microcapsules, spacers and a binder are contained in a layer that the binder contains modified starch and / or a Starch derivative and a synthetic latex contains and the color developer is an acidic clay.

As experiments have shown, the addition of modified starch and / or a starch derivative considerably reduces the permeability of microcapsules to the color former solution contained in the microcapsules, especially if their walls consist of polyurea. Even with prolonged storage, there is therefore far less premature release of the color former solution and the associated unwanted color reaction with the color developer.

The binder preferably contains a starch ester or starch ether. A latex based on butadiene-styrene has preferably proven to be suitable as the synthetic latex.

Modified starch and / or starch derivative is preferably present in the layer in an amount of 5 to 25% by weight, very particularly preferably at least 8% by weight, based on the mass of the microcapsules, calculated as parts by dry weight. The amount of binder is preferably 5 to 20 parts by weight, in particular not more than 15 parts by weight, based on the basis weight of the layer, in each case calculated in parts by dry weight. As has been shown, the invention enables the layer containing the microcapsules and color developers to be produced without problems from a coating slip which contains all the components, even if the capsule wall material consists of polyurea and unfavorable solvents are used for the color formers.

Since tests have shown that particularly good results could be achieved if the walls of the microcapsule consist of polyurea, such an embodiment is preferred. However, the invention is not limited to this and can also be applied to other capsule wall materials, e.g. for microcapsules made of melamine formaldehyde.

According to a further embodiment, the modified starches and / or starch derivatives used in the binder are not film-forming. This has the particular advantage that there is no masking, ie a coating of the color developer, so that if the color former contained in the microcapsules is released, an uninhibited reaction between the color reactants can take place. There is therefore no loss of intensity.

According to the invention, acid clays, activated clays, preferably clays whose main constituent is montmorillonite, are used as color developers. The invention is not restricted to the color formers to be used and the solvents used to dissolve them, all color donors which donate electrons and react with mineral color developers (electron acceptors) can be used.

Undissolved starch granules or cellulose fibers are preferably used as spacers.

In addition to the color-reactive clays, inert pigments, such as kaolin or calcium carbonate, can also be present in the layer or the coating slip used for its production.

The layer is preferably applied with a mass per unit area of not less than 6 and not more than 11 g / m ² .

The following examples serve to explain the invention.

Comparative Example 1:

An aqueous coating slip with the following composition is produced: Microcapsule 31.6 parts by weight Starch grains as spacers 12.6 parts by weight Butadiene styrene latex 11.6 parts by weight sour clay 35.4 parts by weight kaolin 8.8 parts by weight $\overline{\text{100.0 parts by weight}}$

The solids content of the coating slip was 35% by weight. The microcapsule used is a polyurea capsule with a color former solution resulting in a black typeface.

Example 1:

A coating slip was produced in which the procedure was as in the comparative example, but as a binder, a mixture of 6.1 parts by weight of a modified starch (medium-depolymerized) - Sobex 222 from SüdStar and 5.1 parts by weight of the butadiene styrene latex used in the comparative example.

Example 2:

The procedure was as in Example 1, but a carboxymethyl starch ether - Quicksolan CMS from Avebe was used as the starch derivative.

With the help of a wire doctor, the coating slips were applied to a sized coating base paper, in the comparative example 10 g / m ² ; in Examples 1 and 2 each 9 g / m ² .

Vergleichsbeispiel 1:

82

Beispiel 1:

84

Beispiel 2:

82

A measurement immediately after the production of the papers showed the following brightness values in%:

Comparative Example 1:

82

Example 1:

84

Example 2:

82

Vergleichsbeispiel 1:

50 % Punkte

Beispiel 1:

25 % Punkte

Beispiel 2:

30 % Punkte

A measurement after 18 days of aging in a climate box at 70 ^o C and 75% RH showed the following drop in brightness:

Comparative Example 1:

50% points

Example 1:

25% points

Example 2:

30% points

Example 3:

An aqueous coating slip with the following composition is produced: Microcapsule 33.6 parts by weight Starch grains as spacers 12.6 parts by weight Butadiene styrene latex 14.0 parts by weight sour clay 37.5 parts by weight Starch ether as in Example 2 2.3 parts by weight $\overline{\text{100.0 parts by weight}}$

Comparative Example 2:

The composition of the microcapsule dispersion corresponds to that in Example 3, but no starch is used and instead the proportion of butadiene styrene latex is increased to 16.3 parts by weight.

With the aid of a wire knife, the microcapsule dispersions according to Example 3 and Comparative Example 2, which had been adjusted to a solids content of 35% by weight, were applied to a sized coating base paper with the application weights shown in FIG. 1.

The results of the roll test shown in FIG. 1 show the inventive recording papers produced according to Example 3 a lower sensitivity to mechanical stress, while it can be seen from FIG. 2 that the write-through capacity of the recording paper according to the invention has not deteriorated in comparison with the recording paper according to Comparative Example 2 containing no modified starch.

Description of the role test:

This test is carried out with the test pressure device from Prüfbau, Dr.-Ing. Dürner, Peißenberg near Munich. A 4.5 cm wide and 24 cm long test strip of the paper to be examined, which is covered with a protective paper of 40 g / m2 and fixed on a slide, is placed between two rolls at a speed of 2 m / sec. carried out, which are pressed against each other with a defined pressure, the roller acting on the protective paper being a 4 cm wide aluminum roller. After 24 hours of dark storage, the discolouration of the test strip is measured on a data color device as a difference (intensity) from the unprinted area.

Description of the printer test:

In this test method developed by the applicant, a test piece covered with 40 g / m 2 of protective paper is printed on a full-size 4 x 3.5 cm needle printer. The capsules are completely destroyed. The measurement is carried out as stated above for the role test.

Claims (9)

1. Pressure-sensitive recording paper of the SC type, in which there are contained in one layer:

   - microcapsules,

   - spacers,

   - an acidic clay as colour developer for the colour-forming solution contained in the microcapsules, and

   - a binder that contains modified starch and/or a starch derivative and also a synthetic latex.
2. Pressure-sensitive recording paper according to claim 1, characterised in that the starch derivative is a starch ester.
3. Pressure-sensitive recording paper according to claim 1, characterised in that the starch derivative is a starch ether.
4. Pressure-sensitive recording paper according to any one of claims 1 to 3, characterised in that the synthetic latex consists of a butadiene/styrene polymerisate.
5. Pressure-sensitive recording paper according to any one of claims 1 to 4, characterised in that the layer contains modified starch and/or starch derivatives in an amount of from 5 to 25 % by weight, based on the mass of the microcapsules, calculated as dry weight proportions.
6. Pressure-sensitive recording paper according to claim 5, characterised in that the layer contains modified starch and/or starch derivatives in an amount of at least 8 % by weight, based on the mass of the microcapsules, calculated as dry weight proportions.
7. Pressure-sensitive recording paper according to any one of claims 1 to 6, characterised in that the binder is present in an amount of from 5 to 20 % by weight, based on the mass per unit area of the layer.
8. Pressure-sensitive recording paper according to claim 7, characterised in that the binder is present in an amount of up to 15 % by weight, based on the mass per unit area of the layer.
9. Pressure-sensitive recording paper according to any one of claims 1 to 6, characterised in that the walls of the microcapsules consist of polyurea.

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