CA1091998A

CA1091998A - Copy paper sheet and method of making the same

Info

Publication number: CA1091998A
Application number: CA238,450A
Authority: CA
Inventors: Guido Dessauer
Original assignee: Feldmuehle AG
Current assignee: Feldmuehle AG
Priority date: 1974-10-29
Filing date: 1975-10-28
Publication date: 1980-12-23
Also published as: FI61838B; AT344201B; IT1056303B; BE834984A; FR2289355B1; FR2289355A1; DK141928B; DK141928C; SE409968B; DE2451216A1; JPS5168306A; BR7507104A; DE2451216C3; SE7511780L; ATA824575A; FI61838C; CH603369A5; NL7512525A; FI752988A7; DK487575A

Abstract

ABSTRACT OF THE DISCLOSURE
A copy paper sheet capable of forming a colored image after contact with a master sheet carrying a pattern of a suit-able, weakly colored or colorless dye precursor is prepared on a paper machine from stuff which, in addition to web forming fibers predominantly consisting of cellulose, contains a coll-oidal dispersion of a hydrated clay, or by sizing a web of the fibers on the sizing press of the machine with an aqueous com-position containing the hydrated, colloidal clay. Upon dry-ing of the sheet, the fibers throughout the sheet thickness or at least adjacent the sized surface are substantially com-pletely enveloped by a hydrogel of the clay which develops the color of the dye by interaction with the precursor. For useful color strength, the clay must amount to at least 5% of the weight of the enveloped fibers.

Description

109~

This invention relates to duplicating papers, and parti-cularly to a copy paper sheet capable of forming a colored image after contact with a master sheet carrying a pattern of a suitable dye precursor, and to methods of preparing such a copy paper sheet.
It is known to produce images on copy paper sheets by contact with a master sheet carrying a pattern of a dye precursor, -when the copy paper sheet contains an acceptor for producing a dye with the precursor which is transferred to the ¢opy sheet by contact with or without the assistance of a solvent. A typical known dye precursor is leucoauramine which forms auramine by interaction with an acidic acceptor, whereas crystal violet and methylene blue are formed at high intensity from the respective colorless precursors by alkaline acceptors.
The known copy paper sheets carry the acceptor in a coat-ing applied to the surface o a basepaper after the dry paper has been removed from the paper machine. The coating compositions are relatively expensive, and the known process requires two separate operations, the manufacture of the base paper and the coating of the paper.

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Schot:L ~TAP~:I 5~ ) 7~-753, ~1ay 1971] has repo~ted -that sodium montmorillonite is~delaminated by contact with wa-ter into layers of approximately molecular dimensions. ~Ie has discovered that the hydrated, colloidal clay is adsorbed by cellulose fibers in a practically uniform envelopiny layer and is rPtained tenaciously. In amounts of less than 1~ of the fiber we~ght, the montmorillonite was found not to impair-the strength o paper hand sheets made from the clay-carrying fi-bers. However, such hand sheets, when contacted wlth master ~0 sheets carrying a dye precursor, do not d;evelop colored ima~es apparent to the unaided eye.
It has been found that fibers predominantly consistin~
of cellulose are capable of adso~bin~ a much greater amount of sodium montmorillonite and other hydrated, colloidal clay than was considered by Schott, and that ~opy ~aper sheets devel~

.
oping strongly colored images upon ,contact with suitable ~astQr sheets should contain at least 5 parts b~ weight clay hydrogel ~on a dry basis) per 100 parts o ~/eb forming Eibers. Visible color efects can be achieved with as little as 2~ clay under favorable conditions, but they are not sufficient for normal copying or duplicating purposes. When the clay content of the paper sheet exceeds 10~, the mechanical stren~th of the sheet - is impaired, but copy sheet papers useful for some purposes may contain as much as 25~ clay inthe form of its hydrogel.
The copy sheets according to the invention may be pre-pared in a one-step operation on a conventional paper machine of any type ~y colloidally dispersing the clay in an aqueous - suspension of the web forming fibers, and feeding the stuff so ~roduced to the ~per machine for conversion to a sheet in a conventional manner. It is also possible to form a web

- 2 -9~

from the fibers only, and to deposit on at least one surEace of the web a colloidal, aqueous dispersion of the clay which penetrates the web surface and forms an envelope of clay hydrogen at least on the fibers near the surface. Conveniently, the dispersion is appl-ied to the web on the sizing press of the paper machine. Both methods may be combined, particularly if a sized paper is required, and the sizing applied on the sizing press would dim the color effect produced on the fibers in the body of the paper sheet.
The present invention provides a copy paper sheet for forming a colored image consisting of a fibrous web, the fibers of which are uniformly coated with a thin coating of a clay hydrogel, formed from a clay which swells in water, is colloidal and can form thin films in the hydrogel form, the amount oE said clay being from 5 to 25~ of the weight of the coated fibers on a dry basis and a dye precursor which forms a colored image when contacted with the coated fibers.
In another aspect the present invention provides a combin-ation of a donor layer or master sheet comprising a color former and an acceptor sheet of paper or copy paper sheet of which at least the individual fibers of one surface thereof are substantially completely and uniformly enveloped by a color developer film deriv-ed from at least one hydrated film-forming colloidal clay, the color developer and color former being capable of reac-ting image-wise and the acceptor sheet having been formed by a process in which at least one hydrated film-forming colloidal clay is incorp-orated with paper stock containing fibers, the amount of said clay being 5% to 25% of the enveloped fibers a dry basis, and in which the paper stock is made into sheet form on paper-making apparatus and is de-watered and dried.
Preferably, the total weight of the hydrated film-forming colloidal clay (s) is in the range of from 8 to 15 percent, based on the total weight of the fibers prior to encapsulation.
' The hydrated film-forming colloidal clay may comprise a colloidal attapulgite or a colloidal montmorillonite of the sodium bentonite type. The hydrated film-forming colloidal clay may be hydrated colloidal sodium attapulgite or hydrated colloidal,sodium montmorillonite.
The acceptor sheet preferably includes a total of not more than 50,percent by weight, based on the total weight of the hydrated film-forming colloidal clay (s), of one or more non-colloidal color developers. In addition the acceptor sheet may comprise one or more metal compounds which may be d~valent metal salts selected from the metals of Group IIa and Group IIb of the Periodic Table and the transition metals. The metal compound may comprise a total of from 3 to 15 percent by weight, based on the hydrated colloidal clay (s).
The metal compound may be applied to the acceptor sheet in the form of a surface preparation, which may also contain a hydrated film-forming colloidal clay.
Preferably, either one or more binders compatible with the metal compound (s) are present'in the acceptor sheet or sub-stantially no binders are present.
The acceptor sheet is preferably made from a paper stock having a pH in the range of from 4 to 5. The donor layer may be -3a-9199~

carried on one side of the acceptor sheet itself. In this case, a barrier layer may be included between the paper of the acceptor sheet and the donor layer. The barrier layer must be capable of preventing penetration of the color former into the paper.
Alternatively, the barrier layer of this type may be included on the side of the acceptor sheet not carrying the donor layer.
Preferably the barrier layer comprises at least one polyethylene glycol wax and has a weight within the range of from 2 to 4 g/m2.
In another form, the acceptor sheet itself carries the donor layer on one side and has another donor layer on its other side.
The combination may also be arranged to include a second sheet of paper carrying the donor layer. In this instance, the second sheet may be a sheet of carbonising base t:issue and the donor layer is on the reverse side of the sheet and comprises a layer of wax or foamed plastics material containing the color former.
Alternatively, the acceptor sheet may have a reverse-side color former layer arranged for imagewise donor-acceptor re-action with a further acceptor sheet as specified earlier.
The combination may also comprise a plurality of acceptor sheets, as set forth earlier and having a reverse-side color former layer arranged for imagewise donor-acceptor reaction with the next acceptor sheet and a final acceptor sheet as described hereinbefore.
The acceptor sheet may have an obverse-side color former layer, with a first sheet of paper. In this instance the -3b-B.

~9~L998 combination will include a first sheet of paper and a plurality of acceptor sheets as defined earlier and with the color former layer on the obverse side. The combination may also be varied to include a first sheet of paper, a second sheet of donor-acceptor paper and a third sheet of acceptor paper.
The combination may also include a writing implement containing a colorless ink which comprises the color former and provides the donor layer. The writing implement may be a felt-or fiber-tipped pen.
Finally in another aspect the invention includes a process for the preparation of an image-carrying sheet of acceptor paper, which comprises effecting imagewise reaction in a duplicating pro-cess, in a combination as set forth above, between the color former o the donor layer and the color developer o the acceptor sheet.
The donor layer may comprise a colorless printing ink and is applied imagewise to a single acceptor sheet by a stencil printing process. It may alternatively be applied to the single acceptor sheet by an ink-jet recording process. The reaction may also be effected by heat in an electrophotographic process.
Obviously there can also be provided a hectographic dup-licating process in which, in a series of combinations, the donor layer comprises a colorless color former which has been transferred to a master from a donor sheet and is transferred from that master sheet, by means of a solvent, onto a run-off sheet comprising the acceptor paper.
All clays are sufficiently similar in their chemical composition that the ratio of aluminum oxide, silicon dioxide, and other components in the clay does not affect operativeness in the papers according to this invention. The sole critical r -3c-. ~ .

io9~9~

factor is the ability of the clay to swell or hydrate until the individual particles are no longer visible in an optical microscope and are colloidally dispersed when contacted with an aqueous medium whose pH may have an important bearing on the speed with which the colloidal dispersion is formed.
Attapulgite has been found to be superior to montmorillo-nite under many conditions, and preliminary tests indicate that vermiculite can be hydrated and colloidally dispersed for intro-duction in cellulose-based papers of the invention.
Clay particles of a size greater by several orders of magnitude than the colloidal particles of the hydrogels in the papers of the invention are known acceptors used in coated copy papers. However, the amounts of the pulverulent clays required to produce colored images in paper coatings are relatively great, and they cannot be incorporated in the body oE a paper sheet without reducing the paper strength to an unac--3d-ceptable value. They must be bond~d to the paper surface by adll~sives. The colloidal clay particles in hydrogels are bonded to the cellulose in the paper fibers and to each other by hy-drogen bonds and do not require carriers or adhesives.
Adequate color intensity can be had from copy paper oE
the inventi~n provided`with a clay-bearing sizing composition on the sizing press found on most paper machines. The sizing press deposits the composition on both faces of tlle paper, and a deposit of 2 to 4 y/m2 on each face is generally adequate if it contains approximately 20~ - 25% colloidal clay on a dry basis. When the sizing compos.ition contains 2 to 10% alkali me-tal orthosilicate (water glass) or other suitable electrolyte, the viscosity o the clay dispers.ion is lowered sufficiently ~or si2ing press application, and the clay hydrogel ultimately formed can penetrate the fiber web and uniformly envelop the cellulose bearing fibers at least adjacent the surface to which the composition was applied. Lighter coats of sizing may be applied if additional colloidal clay is incorporated in the body of the paper sheet as by dispersing the clay in the fiber suspension prior to paper making.
The reason for the much stronger colors de~elopsd under otherwise .identical.cond~-tions by the papers of the invention as compared to known copy papers carrying clay in a surface coating becomes apparent upon microscopic examination. In the papers of the invention, the fiber surfaces are practically completely covered with a colored, very thin layer of hydrogel.
The white or colorless surfaces of the fibers are completely obscured by the colored material. The available surface of ~ranular clay in a conventional copy paper sheet .is much

3~ ,smaller, and the color developed on the clay surEace is dilut~
ed by colorless or white, exposed coating ingredients.

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The depth of color achieved with the hydrogels of the invention is often strong enough to permit dilution of the colloidal clay with conventional and less effective acceptors such as convent-ional clay or acidic phenols.
As is known in itself, the several dye precursors are most effectively converted to image forming dyes at different pH values, and the paper sheets of the invention may be formulated accordingly.
Minor variations will be readily devised by those skilled in the art to adapt the copy sheets of the invention to the master sheets with which they are to be used.
The following Examples are further illustrative of this invention. All percentage values are by weight unless specifically stated otherwise.
EX~MPLE 1 Long-fibered cellulose produced from softwood was ground to 25 to 2~ SR (Schopper-Riegler), and 20% of the cellulose was mixed with 70% groundwood of 69 to 72 SR. The fibrous material was further mixed in a mixing tank with 10% waste paper containing a high percentage of wood fibers. The aqueous dispersion so obtain-ed contained 3.5% dry solids.
In a separate container, a 715% colloidal, aqueous dis-persion of hydrated sodium attapulgite was prepared, care being taken to ensure full swelling of the clay and separation of the clay particles into layers of practically monomolecular thickness, the completion of the swelling process being determined by inspection of the film formed upon drying of a dispersion sample.
Enough of the attapulgite dispersion was added to the fiber dispersion to provide 15% sodium attapulgite based on dry .

~091~98 fibrous material. 0.7% Rosin size was added and enough alum to adjust the pH value of the mixture to 4.5. It was then diluted with water to a fiber concentration of approximately 0.6%.
While the diluted fiber suspension was fed to the Fourdri-nier wire of a paper machine, its pH was further adjusted to 4 +
0.15 by automatic addition of dilute sulfuric acid, and paper was made in a conventional manner. The continuous web discharged from the dry section of the paper machine was cut to standard sheet size.
The copy paper so prepared showed a strongly colored image when brought into contact with a master sheet carrying a pattern of leucoauramine.
Rather surprisingly, the paper, which consisted of much more than 50% wood fibers, was successfully used in a xerographic copying machine.
EXAr~PLE 2 An aqueous fiber suspension containing about 3.5% fiber solids was prepared in a mixing tank from equal weights of bleached sulfate cellulose obtained from softwood and ground to 60 SR and bleached birch cellulose ground to 25 SR. There were added se-quentially 5% Silton clay and 10% attapulgite gel based on the fibèrweight, the attapulgite being added in the form of a colloidal 5%
solution, the percentage value of the attapulgite being calculated from the dry weight of the clay. The pH of the mixture was adjusted to 9.7 with sodium hydroxide.

The mixture was then diluted with water to a concentration suitable for paper making (about 0.6%), and fed to the Fourdrinier wire of a paper machine at a rate to produce a web having a dry weight of 78 g/m2. Easch side of the web was provided on the size press of the machine with 1 g/m of a composition containing, per 1~19~

liter, 20 g 50% styrene acrylate copolymer dispersion, 20 g of a commerGial surface sizing composition mainly consisting of a dis-persion of acrylate copolymers, 24 g hydrated, colloidal attapulgite.
The pH of the sizing composition was adjusted to 9.8 - 10 with sodium silicate.
The paper so obtained was sufficiently sized to permit writing thereon, and any dimming effect tha~ the sizing might have on the color intensity of a copy prepared by contact with a master sheet carrying a latent image of crystal violet lactone was com-pensated for by the colloidal clay content of the sizing composition.

A fiber suspension prepared from sulfate cellulose as in Example 2 was fed to the Fourdrinier wire of paper machine at a rate to produce a dry web of approximately 75 g/m2. On each face of the web, there was applied a composition in a dry amount of 3 g/m2 and prepared from 170 liters water, 0.5 kg sodium hydroxide flakes, 12 kg Silton clay, 30 kg (on a dry basis) of hydrated, colloidal attapulgite, 3.7 kg 50% styrene butadiene copolymer latex, by means of the sizing press of the paper machine.
Because of the low viscosity of the sizing composition, it penetrated into the web, and the fibers of the web were found to be almost uniformly coated with the clay hydrogel over much of the web thickness, as determined by microscopic examination of the material after staining of the clay with dye transferred from a master sheet in the form of the colorless precursor.

Using the apparatus and the fiber suspension described in Example 3, the web formed on the paper machine was covered on the sizing~press with a composition prepared from 120 liters water, 24 kg sodium orthosilicate (waterglass), 36 kg hydrated colloidal atta-pulgite, 3.5 kg 50% styrene butadiene latex in an amount sufficient to deposit a dry weight of 3.5 g/m on each side.
The properties of the sized paper were closely similar to those of the paper produced in Example 3.

A paper web of cellulose fibers prepared on a paper machine as in Example 3 was coated on the sizing press of the machine on each side with 3.5 g/m2 tdry weight) of a composition prepared from 120 liters water, 0.5 kg sodium hydroxide flaked, 12 kg China clay, 24 kg hydrated, colloidal sodium montmorillonitç of high speci~ic surEace area, and 3 kg 50% styrene butadiene copolymer latex, and sheets were prepared as described in Example 1.
The intensity of the color produced by contact with a master sheet in the gel of montmorillonite enveloping all flbers near the paper surface was so great as not to.be impaired signifi-cantly by the presence of the China clay, a filler not having signif-icant color developing properties of its own.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR PRIVILEGE
IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A copy paper sheet for forming a colored image consisting of a fibrous web, the fibers of which are uniformly coated with a thin coating of a clay hydrogel, formed from a clay which swells in water, is colloidal and can form thin films in the hydrogel form, the amount of said clay being from 5 to 25% of the weight of the coated fibers on a dry basis and a dye precursor which forms a colored image when contacted with the coated fibers.

2. A sheet as set forth in claim 1, wherein said clay is attapulgite.

3. A sheet as set forth in claim 1, wherein said amount of said clay is substantially uniform throughout the thickness of said sheet.

4. A sheet as set forth in claim 1, wherein said amount of said clay is not greater than 10%.

5. A sheet as set forth in claim 1, wherein said fibers predominantly consist of cellulose.

6. A sheet as set forth in claim 5, wherein at least 50% of said fibers are wood fibers.

7. A method of preparing the fibrous web of the copy paper sheet set forth in claim 1, which comprises suspending said fibers in an aqueous medium, depositing the fiber suspension so formed on a porous substrate permeable to the medium but retaining the fibers, whereby a web of fibers is formed, and depositing on at least one face of said web an aqueous, colloidal dispersion of said clay, the amount of said clay being 5% to 25% of the weight of the coated fibers on a dry basis, the viscosity of said dispersion being low enough to permit penetration of said web by said clay, and drying the web.

8. A combination of a donor layer comprising a color former and an acceptor sheet of paper of which at least the individual fibers of one surface thereof are substantially completely and uniformly coated by a color developer film derived from at least one hydrated film-forming colloidal clay, the color developer and color former being capable of reacting imagewise and the acceptor sheet having been formed by a process in which at least one hydrated film-forming colloidal clay is incorporated with paper stock containing fibers, the amount of said clay being 5% to 25% of the weight of the coated fibers on a dry basis, and in which the paper stock is made into sheet form on paper-making apparatus and is de-watered and dried.

9. A combination as claimed in claim 8, wherein the total weight of the hydrated film-forming colloidal clay(s) is in the range of from 8 to 15 percent, based on the total weight of the fibers prior to coating.

10. A combination as claimed in claim 8 or 9, wherein the or at least one hydrated film-forming colloidal clay comprises a colloidal attapulgite or a colloidal montmorillonite of the sodium bentonite type.

11. A combination as claimed in claim 8 or 9, wherein the or at least one hydrated film-forming colloidal clay is hydrated colloidal sodium attapulgite or hydrated colloidal sodium montmorillonite.

12. A combination as claimed in claim 8 or 9, wherein the acceptor sheet also includes a total of not more than 50 percent by weight, based on the total weight of the hydrated film-forming colloidal clay(s), of one or more non-colloidal color developers.

13. A combination as claimed in claim 8, wherein the acceptor sheet also comprises one or more metal compounds.

14. A combination as claimed in claim 13, wherein the or at least one metal compound is a salt of a divalent metal selected from the metals of Group IIa and Group IIb of the Periodic Table and the transition metals.

15. A combination as claimed in claim 13 or 14, wherein the acceptor sheet comprises a total of from 3 to 15 percent by weight, based on the hydrated colloidal clay(s), of the metal compound(s).

16. A combination as claimed in claim 13, wherein the or at least one metal compound has been applied to the acceptor sheet in the form of a surface preparation.

17. A combination as claimed in claim 16, wherein the surface preparation also contained a hydrated film-forming colloidal clay.

18. A combination as claimed in claim 13, wherein the acceptor sheet comprises one or more binders compatible with the metal compound(s).

19. A combination as claimed in claim 13, wherein the acceptor sheet is substantially free from binders.

20. A combination as claimed in claim 13, wherein the acceptor sheet has been made from a paper stock having a pH in the range of from 4 to 5.

21. A combination as claimed in claim 13, wherein the acceptor sheet itself carries the donor layer on one side.

22. A combination as claimed in claim 8, wherein the acceptor sheet itself carries the donor layer on one side and a barrier layer is included between the paper of the acceptor sheet and the donor layer, which barrier layer is capable of preventing penetration of the color former into the paper.

23. A combination as claimed in claim 8, wherein the acceptor sheet itself carries the donor layer on one side and a barrier layer, capable of preventing penetration of color former is included, on the side of the paper not carrying the donor layer.

24. A combination as claimed in claim 21 or 22, wherein the barrier layer comprises at least one polyethylene glycol wax.

25. A combination as claimed in claim 21 or 22, wherein the barrier layer has a weight within the range of from 2 to 4 g/m2.

26. A combination as claimed in claim 8, wherein the acceptor sheet itself carries the donor layer on one side and has another donor layer on its other side.

27. A combination as claimed in claim 8, which comprises a second sheet of paper carrying the donor layer.

28. A combination as claimed in claim 27, wherein the said second sheet comprises a sheet of carbonising base tissue and the donor layer is on the reverse side of the sheet and comprises a layer of wax or foamed plastics material containing the color former.

29. A combination as claimed in claim 27, wherein the acceptor sheet has a reverse-side color former layer arranged for imagewise donor-acceptor reaction with a further acceptor sheet as specified in claim 1.

30. A combination as claimed in claim 27, wherein there is a plurality of acceptor sheets, each being as specified in claim 1 and having a reverse-side color former layer arranged for imagewise donor-acceptor reaction with the next acceptor sheet, and a final acceptor sheet as specified in claim 1.

31. A combination as claimed in claim 21, in which the acceptor sheet has an obverse-side color former layer, with a first sheet of paper.

32. A combination as claimed in claim 31, comprising a first sheet of paper and a plurality of acceptor sheets each as specified in claim 21 or 23 and with the color former layer on the obverse side.

33. A combination comprising a first sheet of paper, a second sheet of donor-acceptor paper as specified in claim 27 and a third sheet of acceptor paper as specified in claim 8.

34. A combination as claimed in claim 8, which includes a writing implement containing a colorless ink which comprises the color former and provides the donor layer.

35. A combination as claimed in claim 34, wherein the writing implement is a felt-tipped or fiber-tipped pen.

36. A process for the preparation of an image-carrying sheet of acceptor paper, which comprises effecting imagewise reaction by the pressure of a writing implement, in combination as claimed in claim 8, between the color former of the donor layer and the color developer of the acceptor sheet.

37. A process for the preparation of an image-carrying sheet of acceptor paper, which comprises effecting imagewise reaction by a stencil printing process in a combination as claimed in claim 8, and wherein the donor layer comprises a colorless printing ink and is applied imagewise to a single acceptor sheet.

38. A process for the preparation of an image-carrying sheet of acceptor paper, which comprises effecting imagewise reaction by an ink-jet recording process in a combination as claimed in claim 8 and wherein the donor layer comprises a colorless printing ink and is applied to a single acceptor sheet.

39. A process for the preparation of an image-carrying sheet of acceptor paper which comprises effecting imagewise reaction in a hectographic duplicating process in which, in a series of combinations as claimed in claim 27, the donor layer comprises a colorless color former which has been transferred to a master from a donor sheet and is transferred from that master sheet, by means of a solvent, onto a run-off sheet comprising the acceptor paper.

40. A process for the preparation of an image-carrying sheet of acceptor paper which comprises effecting imagewise reaction by heat in an electrophotographic copying process in a combination as claimed in claim 8 between the color former of the donor layer and the color developer of the acceptor sheet.

41. A process for the preparation of an image-carrying sheet of acceptor paper which comprises effecting imagewise reaction in a duplicating process in a combination as claimed in claim 8 between the color former of the donor layer and the color developer of the acceptor sheet.