GB2052588A - Process for Producing Pressure- Sensitive Copying Paper - Google Patents

Abstract

A process for producing a pressure-sensitive copying paper in which a coating solution containing microcapsules and microspheres serving as a microcapsule protector and expandable upon heating (said microspheres having substantially the same particle size (before expanding as the microcapsules) is applied to a continuously travelling web by a blade-coating or bar-coating method to form a coating film on the web and then the thus coated web is heated to expand the microspheres. This method is capable of producing an excellent pressure- and abrasion- resistant pressure-sensitive copying paper at a high speed and at a low cost. <IMAGE>

Description

SPECIFICATION Process for Producing Pressure-sensitive Copying Paper This invention relates to a process for producing a pressure-sensitive copying paper, and more particularly, it relates to a method for coating a continuous flexible support (hereinafter referred to as "web") with a coating solution composed of microcapsules and a protector therefor by using a blade-coating or bar-coating method.

Heretofore, a pressure-sensitive copying paper has been manufactured by continuously applying micro-capsules containing a colour former and a coating solution containing a protector for said microcapsules and an adhesive to a web by an air-knife coating method. Incorporation of a protector into the coating solution in intended to prevent the microcapsules from being ruptured under external pressure or frictional force to which they are subjected during treatment other than the inherent use. The microcapsules, when ruptured, are reacted with the developer to cause colour development which taints the paper. Fine powder of cellulose, starch and the like have been generally used as such a protector.

As mentioned above, it is desirable that the pressure-sensitive copying paper has no probability of developing colour in handling other than the inherent use, but on the other hand, highly efficient colour formation is required in the inherent use such as printing. Thus, the pressuresensitive copying paper is required to have a property that it develops colour when a predetermined amount of pressure is applied to the paper but no colour development should take place under a pressure lower than a predetermined level.The conventional pressuresensitive copying paper was, however, unsatisfactory in this respect; it involved the possibility that, in handling other than the inherent use such as printing, the microcapsules be ruptured to develop colour, and it would also often happen in the inherent use such as printing that the microcapsules should fail to break, inducing no or, if any, insufficient colour formation.

Further, increasing demand for pressuresensitive copying paper in recent years makes necessary a method for manufacturing such a paper with a high productivity and a low drying energy cost. However, the currently used air-knife coating method can hardly meet such a demand because of involvement of some serious problems. For instance, in order to improve the productivity, a large amount of energy is required in the drying step because the coating solution is low in solid content, and also there is produced much noise and mist owing to the increased air pressure applied to the air-knife in high-speed operation.

For these reasons, there has recently been desired the production of pressure-sensitive paper by a blade-coating or bar-coating method enabling a coating solution with a high solid content to be coated. Unlike the air-knife coating method, according to the blade-or bar-coating methods, the coating solution is directly scraped down in the space between the blade or bar and the web, and therefore, an intensified classifying action is caused in that area, so that when particles of different sizes are contained in the coating solution, particles having greater sizes are scraped down preferentially to those having smaller sizes, resulting in a change in the coating solution composition with the lapse of time and simultaneously a change in the properties of the coated paper, making it impossible to obtain products of uniform quality.However, because of attractive merits of the blade-coating or barcoating method which has a high productivity, many and extensive studies are being made for solving the above-mentioned problems. For instance, Japanese Patent Application Kokai (Laid-Open) No. 15709/77 shows a method by which a coating solution comprising microcapsules and another coating solution comprising a protector are coated in multi-layer on a support, while Japanese Patent Publication No. 4284/79 discloses a method by which a coating solution consisting of micro-capsules and a protector is foamed so as to prevent classification of both materials by means of a combination of the foams with the blade-coating method.

It is an object of this invention to provide a method for producing a pressure-sensitive copying paper having excellent pressure-and abrasion-resistance, said copying paper being substantially free of one or more of the conventional defects mentioned above and the microcapsules on the paper being not ruptured in handling other than the inherent use.

Another object of this invention is to provide a method for producing a pressure-sensitive copying paper having excellent pressure-and abrasion-resistance at a high speed by utilizing the blade-coating or bar-coating method.

Still another object of this invention is to provide a method for producing a pressuresensitive copying paper having excellent pressure-and abrasion-resistance at a low cost by minimizing the amount of the protector added.

A further object of this invention is to provide a method for producing a pressure-sensitive copying paper which is capable of producing a high colour density in the inherent use.

According to this invention, there is provided a method for producing a pressure-sensitive copying paper which comprises coating a continuously travelling web with a coating solution containing microcapsules and microspheres as a protector for said microcapsules, said microspheres having substantially the same particle size as the microcapsules and expandable, and preferably foamable, upon heating by a blade-coating or bar-coating method, to form a coating film on the web, and then heating the thus treated web to expand the microspheres.

The term "microspheres expandable upon heating" referred to herein means thermoplastic microspheres which expand upon heating and in each of which a low-boiling volatile foaming agent such as isobutane, n-hexane or the like is enclosed in a high-molecular weight substance such as a vinylidene chloride-acrylic acid copolymer, a styrene-acrylonitrile copolymer, etc., the extent of expansion being variable by appropriately adjusting the thickness of the shell, the amount of expanding agent, and particle size.

Also, when the phrase "microspheres having substantially the same particle size as the microcapsules and expandable upon heating" is used herein, it refers to those microspheres which have substantially the same particle size as the microcapsules before expansion but, when heated, expand to 1.5 to 5 times the particle size before heating. The microspheres used in the method of this invention can be produced, for example, according to the method disclosed in U.S. Patent Specification No. 3,945,956.

The term "microcapsules" used herein means fine capsules of 2 to 20 ju in average particle size, in which capsules an oily solution having dissolved therein a basic colourless colour former is enclosed in a wall consisting of a high molecular weight material insoluble in both water and the oily solution. As the wall material, there may be used a combination of a polycation and a polyanion such as a gelatin-gum arabic combination, a condensation polymerization type composition such as urea-formaldehyde polymer, a condensation type composition such as polyisocyanate-polyamine condensate or combinations thereof.

The web used in this invention may be a commonly used type of paper or paper-like material including high quality paper, mediumquality paper, machine coated paper, art paper, cast coated paper, synthetic polymer paper, resin coated paper, plastic film, etc.

The blade-coating method referred to in this invention includes the coating techniques by means of a trailing blade-coater, a flexiblade coater, a flatted nip blade-coater, a fountain blade-coater, a billblade coater, a twin-blade coater, etc.

The bar-coating employed in this invention may be accomplished by a known coating method using a metalling bar-coater, "Champflex" coater or the like.

A method embodying this invention is described in further detail below with reference to the accompanying drawings, in which Fig. 1 is a schematic illustration of an example of apparatus for producing a pressure-sensitive copying paper, which apparatus is used in practicing the method; Fig. 2 is a schematic cross-sectional view showing the structure of the pressure-sensitive copying paper just after coating, and Fig. 3 is a schematic cross-sectional view showing the structure of the pressure-sensitive copying paper which has been heated and dried.

A web 3 is delivered out from a web feed roll 2 by a feeder 1 and sent to a coating means, while adjusting the tension of the web by dancer rolls 4.

In coating means, the web 3 is coated with an excess of a coating solution 7 containing microcapsules and microspheres which solution has been picked up by a coating roll 5 from a coating solution pan 6 containing the coating solution, and then the amount of the coating solution is adjusted to the desired coating amount by a blade 8. During this operation, there is no fear that the microspheres alone will be scraped down because the microspheres used as the protector have substantially the same particle size as the micro-capsules. Superfluous coating solution scraped down by the blade 8 is recovered into the coating solution pan 6.The web 3 is changed in its travelling direction by a back-up roll 9 and led to a drying zone 10 where the web is passed while being carried by a canvas 11 and hot air is blown onto the coating film to effect drying, whereby the microspheres are expanded to 2 to 3 times the original size. Leaving the drying zone 10, the web 3 is taken up on a takeup roll 13 by a take-up means 12.

This invention provides one or more of the following advantages: (1) Since the microcapsules are well protected by the expanded microspheres, there is little or no fear that the microcapsules will be ruptured to taint the paper in handling other than the inherent use.

(2) Since no classifying action is caused in the coating step, the coating rate can be increased to meet the request for increased paper output.

(3) Since the protector is composed of the expanded microspheres they are deformed or ruptured by a strong pressure such as printing by a typewriter. Therefore, a pressure-sensitive copying paper having a protector which does not impede the inherent use of the paper such as printing can be obtained.

In order to further clarify the effects of this invention, it is further described below referring to Examples. In the Examples, all "parts" and "%" are by weight unless otherwise specified, and all the coating amounts are indicated in dry weight.

Example 1 100 parts of a 10% aqueous solution of the hydrolyzate of an ethylene-maleic anhydride copolymer, 10 parts of urea, 1 part of resorcinol and 200 parts of distilled water were placed in a stirring vessel and mixed. The pH of the solution was adjusted to 3.5 with a 20% aqueous sodium hydroxide solution. Then, 200 parts of diisopropylnaphthalene oil having dissolved therein 3% of crystal violet lactone and 2% of benzoyl leucomethylene blue was emulsified in said solution to form oil droplets having a size of 4-8 microns. Then, 25 parts of a 37% aqueous formaldehyde solution was added and the system was heated to 550C with stirring. About two hours later, the system temperature was lowered to room temperature to complete the encapsulation, whereby a microcapsule dispersion having a solid concentration of 43% was obtained.

200 parts of this microcapsule dispersion, 40 parts of cake-like microspheres (solid content: 67%; particle size: 4-10 microns; expansion ratio: about twice in particle size; foaming agent n-hexane) serving as a microcapsule protector, 90 parts of a 10% aqueous oxidized starch solution serving as an adhesive and 75 parts of water were mixed to prepare a coating solution.

The thus obtained coating composition containing said microcapsules and microspheres was applied to a paper web with a basis weight of 40 g/m2 which was travelling at a rate of 100 m/min in a coating amount of 5 g/m2 by means of a trailing blade-coater, and then the web was passed through a hot air dryer at an internal temperature of 1 200C to effect drying, thereby obtaining a pressure-sensitive copying paper. The coating operation was continued for several hours, but the composition of the coating composition remained substantially unchanged throughout the operation.

Separately, 200 parts of activated clay was dispersed in 800 parts of water, and to this dispersion was added a 20% aqueous sodium hydroxide solution to adjust the pH of the dispersion to 10.0, after which 60 parts of a 50% styrene-butadiene copolymer latex and 60 parts of a 10% aqueous oxidized starch solution were added thereto to prepare a developer solution, and this developer solution was applied to a paper web with a basis weight of 40 g/m2 in a coating amount of 6 g/m2 by means of an air-knife coater to obtain a developer coated paper.

The thus obtained pressure-sensitive copying paper and developer-coated paper were put on each other so that the coated sides thereof faced each other, and this set of sheets was subjected to printing by a typewriter, whereby a good colour density was obtained.

In handlings other than printing, no colour formation took place and no trace of taint was seen.

Comparative Example 1 A coating composition was prepared by mixing 200 parts of the microcapsule dispersion obtained in Example 1, 27 parts of wheat starch particles (particle size: 20--30 microns, 3-5 times the capsule particle size) as a microcapsule protector, 90 parts of a 10% oxidized starch solution as an adhesive and 88 parts of water.

This coating composition was applied to a paper web with a basis weight of 40 g/m2 which was travelling at 100 m/min, in a coating amount of 5 g/m2 by means of a trailing blade-coater, and then the web was passed through a hot air dryer to effect drying, thereby obtaining a pressuresensitive copying paper. When the coating operation was continued for several hours, the coating composition was different from that at the beginning of the coating operation and the proportion of the wheat starch particles used as protector had increased excessively.

The thus obtained pressure-sensitive copying paper and the developer coated paper obtained in Example 1 were put on each other so that the coated sides faced each other, and this set of sheets was printed by a typewriter. As a result, although good colour formation was obtained, colour was also formed in handlings other than printing and a marked extent of taint was seen.

Example 2 5 parts of the adduct of 3 mole of tolylene diisocyanate and 1 mol of trimethylolpropane and 1 part of the adduct of polyolsorbitol and propylene oxide were dissolved in 30 parts of diisopropylnaphthalene having dissolved therein 3% of crystal violet lactone and 2% of benzoyl leucomethylene blue. This oily solution was gradually poured into 35 parts of a 10% aqueous polyvinyl alcohol solution at 200C under vigorous stirring to form oil droplets having a size of 5-10 microns. 50 parts of water of 40 C was added to this emulsion while continuing stirring, and then the system was gradually heated to 900C and maintained at this temperature for 20 minutes to complete encapsulation. There was thus obtained a microcapsule dispersion with a solid concentration of 33%.

100 parts of the thus obtained microcapsule dispersion, 1 5 parts of cake-like microspheres (solid content: 67%; particle size: 6-12 microns; expansion ratio: about twice in particle size; foaming agent: n-hexane) as a protector and 35 parts of a 10% aqueous solution of oxidized starch as an adhesive were mixed to prepare a coating composition. This coating composition was applied to a paper web with a basis weight of 40 g/m2, which was travelling at 100 m/min, in a coating amount of 4 g/m2 by means of "Champflex" coater, and the coated web was passed through a hot air dryer with an internal temperature of 1 200C to effect drying, thereby obtaining a pressure-sensitive copying paper. The microspheres had expanded to 10-25 microns.

The coating operation was continued for several hours, but the coating composition remained substantially unchanged throughout this period.

Separately, 100 parts of a 40% suspension of a p-phenylphenol-formaldehyde condensate, 1 60 parts of Kaolin and 80 parts of a 50% styrenebutadiene copolymer latex were put into 350 parts of water, and these materials were well stirred and mixed to prepare a developer solution.

This developer solution was applied to a paper web with a basis weight of 40 g/m2 in a coating amount of 6 g/m2 by means of an air-knife coater to obtain a developer-coated paper.

The thus obtained pressure-sensitive copying paper and developer-coated paper were put on each other so that the coated sides faced each other, and this set of sheet was printed by a typewriter. There was obtained excellent colour density.

No colour development nor any tainting took place in handling other than printing.

Claims (10)

Claims

1. A method for producing a pressure-sensitive copying paper, characterised in that a coating solution containing microcapsules and microspheres serving as a protector for said microcapsules, said microspheres having substantially the same particle size as said microcapsules and expandable upon heating, is applied to a continuously travelling web by a blade-coating or bar-coating method to form a coating film on the web and then said web is heated to expand the microspheres.

2. The method according to Claim 1, wherein the microspheres expand to 1.5 to 5 times the original particle size by heating.

3. The method according to Claim 1 or Claim 2, wherein the web is fine-quality paper, mediumquality paper, machine coated paper, art paper, cast coated paper, synthetic polymer paper, resin coated paper or plastic film.

4. The method according to any one of Claims 1-3, wherein the microcapsules contain an oily solution having dissolved therein a basic colourless colour former, said oily solution being encapsulated by a high-molecular weight material which is insoluble in both water and said oily solution.

5. The method according to any one of Claims 1-4, wherein the microcapsules have an average particle size of 2-20 ,u.

6. The method according to Claim 4, wherein the encapsulating material is a gelatin-gum arabic combination, urea-formaldehyde polymer or polyisocyanate-polyamine combination.

7. The method according to any one of Claims 1-6, wherein the microspheres are thermoplastic microspheres in which isobutane or n-hexane is enclosed in a shell consisting of a vinylidene chloride-acrylic acid copolymer or a styrene-acrylonitrile copolymer.

8. The method according to any one of Claims 1-7, wherein the blade-coating is accomplished by using a trailing blade-coater, a flexiblade coater, a flattened nip blade coater, a fountain blade-coater, a billblade coater or a twin-blade coated.

9. The method according to any one of Claims 1-7, wherein the bar-coating is accomplished by using a metalling bar-coater or Champflex coater.

10. A method according to Claim 1 substantially as herein described and exemplified.

1 A pressure-sensitive copying paper which has been produced by the method claimed in any preceding claim.