EP0017386B1

EP0017386B1 - Self-contained pressure sensitive recording paper

Info

Publication number: EP0017386B1
Application number: EP19800300838
Authority: EP
Inventors: Mitsutu Fuchigami; Isao Nagayasu
Original assignee: Mitsubishi Paper Mills Ltd
Current assignee: Mitsubishi Paper Mills Ltd
Priority date: 1979-03-28
Filing date: 1980-03-19
Publication date: 1984-12-12
Also published as: DE3069782D1; JPS55128492A; JPS5833116B2; EP0017386A1

Description

This invention relates to a self-contained pressure sensitive recording paper.
A known pressure sensitive recording paper system generally comprises a combination of a top sheet coated with microcapsules enwrapping a colourless due as colour former and an undersheet coated on one side with a colour developer which is an acidic substance such as, for example, activated acid clay, a phenolic resin, or a salicyclic acid derivative or its metal salt. When both sheets are placed so that the coated sides may face each other, and localized pressure is applied by means of a typewriter or a writing instrument, the intended duplicate record is obtained on the side of the sheet coated with the colour developer. Such a recording paper system is used widely.
Another known pressure sensitive recording paper is "self-contained" and comprises a supporting sheet and, coated on one side, two consecutive coatings of a colour former-containing microcapsule layer and a colour developer layer. This recording paper can be printed with letters by means of a typewriter without using an inked ribbon, or can provide a copy by writing or printing on a superposed ordinary paper sheet. Such specific uses have recently given rise to increasing demand for the paper. JP-A-6,207/72 has disclosed an improved coater for applying two coating layers in manufacturing such a self-contained pressure sensitive recording paper.
It may be easily understood that a self-contained pressure sensitive recording paper of the single coating type having one of the reactants encapsulated is most preferable in view of not only production cost but also product quality, particularly the initial speed of image colour formation. However, commercialization of such a type of recording paper has been hindered by lack of proper microencapsulation techniques. For instance, if microcapsules having the wall phase of gelatin as major component produced by complex coacervation are used, coloration of the recording paper will occur all over the coated side. In the coacervation technique described in Research Disclosure No. 16645, February, 1978, a gelatin membrane wall is strengthened by cross-linking with a melamine-formaldehyde precondensate, which may be used as the sole hardening agent but is more preferably used as a secondary hardening agent together with glutaraldehyde or formaldehyde as primary hardening agent.
It is also known to form microcapsules having a wall phase of urea-formaldehyde resin, as described in 9,079/76. However, we find that when such a technique is used, coloured specks appear on the paper surface which is white in colour, and so makes the recording paper commercially of no value. GB-A-1 507739 described a microencapsulation technique in which the membrane wall is formed by cross-linking a water soluble polymer with a urea-formaldehyde precondensate, the wall being strengthened by additionally cross-linking with a melamine formaldehyde precondensate.
EP-A-001 1367, published on 28th May, 1980, and having a priority of 10/10/1978, discloses compositions for a self contained pressure sensitive recording paper, said compositions containing solid particles of a colour developer which are microencapsulated, preferably by in situ polymerization of melamine formaldehyde, microcapsules of a chromogenic material and about 25% by weight of the total weight of the capsules of colour developer and chromogenic material, of uncooked wheat starch.
DE-A-2.820.462 discloses a self-contained pressure sensitive recording paper of the single coating layer type comprising a supporting paper sheet and coated thereon a single layer of a mixture including (a) a colour former, (b) a colour developer, which colour developer is in a liquid medium encapsulated by a wall membrane of a urea-formaldehyde resin, the encapsulation being effected by in-situ polymerization using an ethylene-maleic-anhydride copolymer as emulsifier, the colour developer being a phenolic resin or salicyclic acid or a derivative thereof or a metal salt of salicyclic acid or of said derivative and (c) starch particles, the starch particles being present in an amount of at least 150% by weight of the total dry weight of the microcapsules.
A self-contained pressure sensitive recording paper of the single layer type applied with a single coating composition containing the colour former and the colour developer has been disclosed in JP-A-16,096/72. Although this type of recording paper provides an improvement in reducing spontaneous background staining (self-staining), we found that this recording paper has the disadvantage of marked yellowing and insufficient density of the developed image colour, this being because a hydrophilic film-forming polymeric substance impregnated with a phenol-aldehyde resin is used as the capsule wall material. JP-A-6,413/75 has proposed the addition of amines to alleviate the self-staining problem. Although effective to some degree, the addition of an amine in an amount required for the complete prevention of self-staining brings about an interference with the colour-forming reaction and also a tendency to produce yellowing of the sheet surface.
A method is described in JP-B-1 0,780/72, in which the coating composition is incorporated into the paper web during web formation. In such a recording paper, unlike the coated paper, the colour forming reaction takes place within the paper sheet, resulting in insufficient density of the image colour.
Further, as disclosed in JP-A-37,710/74, attempts have been made to prepare a self-contained pressure sensitive recording paper improved with regard to reduction of self-staining by improving the encapsulation method using the reaction of a polyvalent isocyanate, a polyhydroxy compound and a polyamine compound. Although self-staining is reduced by the use of a wall material and an encapsulation procedure as disclosed in that patent application, it is necessary, for the complete elimination of self-staining, to increase the capsule wall thickness, leading to deterioration of the colour forming performance of the coating layer. The disclosed method, moreover, is not suitable for encapsulating oil-soluble acidic colour developers such as phenolic resins, salicylic acid derivatives or metal salts thereof, this being because active hydrogen atoms of these colour developers react with the isocyanate compounds used in forming the capsule membrane.
This invention provides a self-contained pressure sensitive recording paper of the single coating layer type which is resistant to spontaneous background staining and has an excellent colour forming ability.
A self-contained pressure sensitive recording paper in accordance with the invention is of the single coating layer type and comprises a supporting paper sheet and coated thereon a single layer of a mixture including a colour former, a colour developer and starch particles.
The colour developer is a phenolic resin or a salicyclic acid or a derivative thereof or a metal salt of salicyclic acid or of the derivative.
At least the colour developer is encapsulated by a wall membrane of a melamine-formaldehyde resin, the encapsulation being effected by in-situ polymerization using a styrene-maleic anhydride copolymer as emulsifier.
The starch particles are present in the mixture in an amount of at least 150% by weight of the total dry weight of the microcapsules. This "dry" weight includes the weight of any liquid in the core but not that of moisture on the capsule wall.
We find that the abovementioned emulsifier gives improved results in the microencapsulation process and an improved pressure sensitive recording paper as compared with the microencapsulation technique described in previously mentioned DE-A-2820462.
At least the colour developer (and preferably the colour former also) is in a liquid medium and we find that, by encapsulating at least the colour developer (and preferably the colour former also) with the melamine-formaldehyde wall membrane material, self-staining can be prevented and colour forming quality improved.
A coating layer of a recording paper embodying the invention may be formed by the application of a mixture of the following five components onto a substrate sheet: (1) a colourless dye as the colour former, (2) a phenolic resin, salicylic acid or a salicyclic acid derivative or a metal salt of the said acid or derivative as the colour developer, (3) starch particles, (4) a pigment, and (5) a binder.
In encapsulating the colour former (1) or the colour developer (2), it is used generally in the form of dispersion or solution in an oily substance, except for the case of a phenolic resin (colour developer) if it is liquid at ambient temperatures. The colour former used may be a known electron donating leuco dye such as typically crystal violet lactone or a fluorane derivative. The colour developer used is typically a substituted phenol-formaldehyde resin described in JP-B-20,144/67 or a polyvalent metal salt of a salicylic acid derivative described in JP-B-25,174/76. Examples of desirable colour developers include p-phenylphenol-formaldehyde resin, 3,5-di-tert-butylsalicylic acid or zinc salt thereof, and 3,5-di(dimethylbenzyl) salicylic acid or zinc salt thereof.
The high-boiling oily substance for use in combination with a colour former or colour developer may, for example, be "SAS" (a commercially available diarylethane type oil produced by Nippon Petrochemical Co.) or "KMC" ( a commercially available dialkylnaphthalene type oil produced by Kureha Chemical Industry Co.), both of which have been used in conventional carbonless copy paper sheets. If necessary, these oils can be admixed with known oils of the ester type or aliphatic type.
The properties of capsule membrane are important particularly when a self-contained pressure sensitive recording paper is to be prepared with a single coating composition. As mentioned above, it is necessary to encapsulate at least one of the colour former and colour developer with a wall material comprising substantially a melamine-formaldehyde resin. A satisfactory self-contained pressure sensitive recording paper is not obtained if encapsulation is performed by conventionally used or proposed coacervation of gelatin or interfacial polymerization of a polyamide, presumably because of unsatisfactory properties of the capsule wall. Although the reason why the melamine-formaldehyde resin alone yields a satisfactory capsule membrane is yet to be elucidated, it seems that owing to dense and water-resistant capsule walls formed from said resin, the capsules are able to retain their contents firmly throughout the coating operation. The encapsulation is performed by an in-situ polymerization process such as that disclosed, for example, in P-A-49,984/79, but, as mentioned above, using a styrene-maleic anhydride as the emulsifier. Capsules excellent in uniformity of the particle size and heat resistance of the capsule wall may then be obtained.
Since in a recording paper of this invention a large amount of starch particles is employed in the coating layer (as mentioned above the amount is at least 1 50% by weight of the total dry weight of the microcapsules), improved properties as compared with those of conventional recording papers can be achieved.
Although the use of granular starch in carbonless copy sheets has been known (EP-A-0011367 and JP-B-1, 178/72 and JP-B-33,204/73), it is used, according to the description, in an amount of at most 50% of the weight of capsules. In the systems disclosed in these Japanese Patent Publications, the capsule-containing coatings coat both a top sheet and an intermediate sheet. In the case of a self-contained copy sheet of this invention, if starch is used in such a small amount, the resulting copy sheet is, we find, still susceptible to self-staining or causes smudging on other paper sheets.
A suitable size of starch particles in a coating of a recording paper embodying the invention is in the range of from about 10 to about 30 µ.
As for the pigments, use is made of those well known in the art such as clays, kaolin, calcined clays, calcium carbonate, titanium oxide, zinc oxide, and plastic pigments. The pigment may be used to improve the brightness of paper, printability and colour forming performance. The amount to be used is preferably in the range of from about 10 to about 100 parts for 100 parts of capsules, depending on the type of pigment used.
The binders used in recording paper embodying this invention are the same as those generally used in usual copying paper, such as hydroxyethylcellulose, modified starches, carboxymethylcellulose, acrylic latices, SBR latices, and polyvinyl alcohol.
As described before, most of the conventional self-contained pressure sensitive recording paper systems have been of the superposed coating layer type prepared by applying onto one side of a supporting sheet (paper, film or the like) two consecutive layers; a coating of colour former-containing microcapsules and a coating of colour developer. Quite recently, with technical progress in microencapsulation, it has become possible to obtain microcapsules less susceptible to leakage of the contained substance. Therefore, we have attempted to prepare a self-contained pressure sensitive copying paper with single layer of a coating composition containing a mixture of colour former and colour developer, either or each of which is entrapped within microcapsules. According to our experimental results, a self-contained pressure sensitive recording paper which is practically white in background colour, and which therefore is the most preferred embodiment of the invention, is obtained when the colour former and the colour developer were each microencapsulated and, in addition, microencapsulation of the colour developer with wall material of melamine-formaldehyde resin was carried out by the abovementioned in situ process.
We find that the microcapsules with the wall phase of a melamine-formaldehyde resin are most suitable for entrapping the colour developer for use in the above-said self-contained pressure sensitive recording paper of the single coating type having each of the colour-forming reactants encapsulated, in preparing a self-contained pressure sensitive recording paper of the single coating type having both of the reactants encapsulated. We find that this may be achieved without encountering the difficulties of known pressure sensitive recording papers previously discussed.
In preparing a recording paper embodying the invention, it is possible to use, as usual, other known additives, for example, ultraviolet absorbers, antioxidants, dispersants, water-proofing agents, lubricants and defoamers. The coating composition may be applied by known methods such as air-knife coating, roll coating or blade coating.
The formation of recording papers embodying the invention will now be illustrated in more detail below with reference to the following Examples, in which "parts" are by weight.

Example 1

Encapsulation of colour developer.
In 60 parts of "Hisol SAS" N-296 (a commercially available diarylethane oil produced by Nippon Petrochemical Co.), was dissolved with heating 40 parts of p-phenylphenol (PR 26142 produced by Sumitomo Durez Co.; m.p. 86°C) to prepare 100 parts of an internal phase oil. This internal phase oil was emulsified in 200 parts of a 5-% aqueous styrene-maleic anhydride copolymer-solution adjusted to pH 5.3. A melamine-formaldehyde early-stage condensate was prepared by heating at 80°C a mixture of 20 parts of melamine and 45 parts of 37-% formalin, said mixture having been adjusted to pH 8.5 with sodium hydroxide. The early-stage condensate was added to the above emulsion and the resulting mixture was allowed to react by heating at 70°C for one hour to obtain microencapsulated colour developer having an average particle size of 5.2 _fL.

Example 2

Encapsulation of colour developer.
Microencapsulation of a colour developer was carried out in a manner similar to that in Example 1 using an internal phase oil comprising 20 parts of zinc 3,5-di-tert-butylsalicylate, 40 parts of KMC-113 (an oil produced by Kureha Chemical Industry Co.) and 40 parts of a high-boiling aliphatic hydrocarbon oil (No. 3 ink solvent). The resulting microcapsules had an average particle size of 3.5,u.

Example 3

Encapsulation of a colour former.
Microencapsulation of a colour former was carried out in a manner similar to that in Example 1 using an internal phase oil comprising 5 parts of crystal violet lactone (CVL) and 95 parts of KMC-113 to obtain microencapsulated colour former having an average particle size of 4 µ.

Example 4

A uniform fluid composition was prepared by thorough mixing of the following components:
The above composition was applied by means of a metering bar onto a paper sheet to prepare a self-contained pressure sensitive sheet of single coating layer type having one of the colour-forming reactants microencapsulated. The application rate was 6 g/m². The above 20-% CVL dispersion was an aqueous dispersion of finely ground CVL cystals, said CVL being in solid state and not encapsulated. There was obtained a self-contained recording sheet of excellent brightness and exhibiting no self-staining tendency. Upon impression by means of a typewriter, a coloured sharp image was obtained.

Example 5

A coating composition of the following formulation was prepared.
The above coating composition was applied by means of an air-knife coater onto a sheet of fine paper at an application rate of 7.5 g/m² in terms of solids to prepare a self-contained recording sheet of the single coating layer type having colour-forming reactants each encapsulated. This white self-contained pressure sensitive recording sheet showed no self-staining tendency. Upon impression by means of a typewriter, there was instantly formed a sharp image, bright deep blue in colour.
The experimental results described in the above Examples demonstate the novel fact that by the encapsulation with a melamine-formaldehyde resin as wall material, it is possible to prepare a self-contained pressure sensitive recording paper of the single coating layer type having either or each of the colour-forming reactants encapsulated.

Claims

1. A self-contained pressure sensitive recording paper of the single coating layer type comprising a supporting paper sheet and coated thereon a single layer of a mixture including (a) a colour former, (b) a colour developer, which colour developer is in a liquid medium encapsulated by a wall membrane of an aminoplast resin, the encapsulation being effected by in-situ polymerization using a maleic-anhydride-containing copolymer as emulsifier, the colour developer being a phenolic resin or salicyclic acid or a derivative thereof or a metal salt of salicyclic acid or of said derivative and (c) starch particles, the starch particles being present in an amount of at least 150% by weight of the total dry weight of the microcapsules characterized in that the said wall membrane is of a melamine-formaldehyde resin and the said copolymer as emulsifier is a styrene-maleic anhydride copolymer.

2. A self-contained pressure sensitive recording paper according to Claim 1, wherein both of the colour former and the colour developer are each encapsulated by a melamine-formaldehyde resin using a styrene-maleic anhydride copolymer as emulsifier.