EP0060113A1

EP0060113A1 - Method of coating a support sheet

Info

Publication number: EP0060113A1
Application number: EP82301148A
Authority: EP
Inventors: Nicholas J.B. Jones; Hugh Higgins; Thomas Galloway Green
Original assignee: DRG UK Ltd
Current assignee: DRG UK Ltd
Priority date: 1981-03-11
Filing date: 1982-03-05
Publication date: 1982-09-15
Also published as: FI820830L; EP0060114A1; FI820830A7; FI820831L; DE3268618D1; ATE17599T1; EP0060114B1; FI820831A7

Abstract

A method of coating both sides of a support sheet such as paper, one of the coatings containing microcapsules, by applying the coatings in liquid medium to opposite sides (10a, 10b) of the sheet simultaneously or sequentially, removing excess coating material preferably between an opposed pair of flexible blades (12) and subjecting the wet coated sheet to a single drying operation in a drying chamber (16) for both coated faces.

Description

This invention relates to a method of coating a support sheet especially but not exclusively for coating paper.
It is well known to coat paper with microcapsules contain- inga colour former so that on application of pressure the microcapsules rupture and release the colour former. When the coated face of the paper is in contact with a colour developer, which may be coated on a second sheet of paper, the released colour former reacts with the developer to produce a visible copy of the area of pressure. This is the principle of "carbonless copy paper", and if several copies are to be taken simultaneously the intermediate sheets of paper may have microcapsules coated on one face and developer on the other face.
To prepare such intermediate sheets having a coating on each face has hitherto involved the application of a coating to one face, drying of that coating, application of a coating to the other face and further drying. This has had several inherent disadvantages, for example the process has been slow due to the need to pass the sheet sequentially through two coating stages. Further, with the need for two drying stages, with the first coating being dried prior to application of the second to maintain the quality of the first coating and the strength of the paper, a very large amount of heat energy is required. This problem has on occasion been added to by the application of water to the face of the paper opposite to that which receives one of the coatings, to prevent the paper from curling when the coatings are dry. Curl is a problem which has beset the paper industry over the years, and its solution by wetting one of the faces of the paper has increased energy costs at the drying stage. With rapidly-increasing energy costs such a solution is becoming unacceptable.
It has previously been proposed to coat paper on both faces simultaneously with the same starch solution in order to produce a magazine-quality paper, and thereafter calendering the coated paper to produce a smooth finish. However, this technique has not hitherto been performed with different coatings on each face, nor has it been considered to be possible when the coating material has included microcapsules. Microcapsules have been applied to the paper by an air-knife technique whereby a dispersion of the microcapsules in water is applied in excess to the paper and the excess is removed by a controlled air blast. This technique has proved adequate for coating one face of the paper but the dispersion has of necessity included a high proportion of water as the air blast is insufficiently strong to remove dispersion having a high solids content. This again has added to the energy costs in drying the coating.
The air-knife technique produces a relatively rough surface finish for the coated paper and, as calendering cannot be performed on microcapsule coatings, this has not been entirely satisfactory.
In order to ensure that only a desired amount of coating material remains on the sheet, excess solution or dispersion is preferably removed prior to drying. This can be achieved in a variety of ways, for example by air-knife or by coating the sheet by roller, but the most preferable manner of removing the excess liquid from a face of the coated sheet is by feeding the sheet past a fixed blade so that the coating depth on that face is controlled by contact with the blade. The blade is preferably flexible and may be biassed towards the sheet, as in this way it not only controls the depth of coating but also spreads the coating evenly and smoothly across the sheet's face. Once dried the coated sheet then has a smoother finish than with an air-knife technique.
The blade may act in conjunction with a roller so that the coating depth is controlled on both faces simultaneously, the coated sheet passing through the nip between the blade and roller, or a pair of blades may be provided between whch the coated sheet passes. An especially effective method is where coating heads apply the solution or dispersion to each face of the sheet as it passes upwardly past the heads, and the coated sheet then continues upwardly between a pair of flexible blades whose edges are biassed towards one another so that excess liquid is removed from the coated sheet and falls downwards for collection and recycling to the coating heads.
Blades can be used on faces of the sheet which are coated without microcapsules, or with microcapsules whose walls are strong enough to withstand the mechanical forces imposed by the blades. Preferably the microcapsules have walls of hydrophobic material, for example synthetic polymers, rather than hydrophilic material such as gelatin. Hydrophobic materials retain their inherent strength when exposed to water whereas gelatin and the like materials swell with resulting weakening of the microcapsule walls and rupture on contact with the blades.
Suitable examples of polymers for use in manufacture of the microcapsule walls are polyurea, polyamide, urea-formaldehyde, melamine-formaldehyde, polyurethane, polyester, epoxy resin and polyvinyl acetate.
Use of blades is of considerable value also in allowing a much higher solids content of the microcapsule dispersion coated on the support sheet than is possible with the conventional air-knife technique. Thus while the single drying stage for the coatings on each face of the sheet in the method of this invention allows considerable savings in energy, this can be enhanced by the use of smaller quantities of water in the blade coating technique. Additionally or alternatively the feed rate of the support sheet can be increased and the amount of drying equipment can be reduced.
Thus the present invention makes it possible for increased efficiency, lower capital outlay and lower running costs to be obtained.
According to the present invention there is provided a method of coating a support sheet, comprising feeding the support sheet past coating means to apply a solution of dispersion of coating material to opposite faces of the sheet, the coating material applied to at least one of the faces of the sheets comprising microcapsules, and passing the coated sheet through drying means thereby concurrently to deposit the coating material from solution or dispersion onto both faces of the sheet.
A major advantage accruing from applying a wet coating to both faces simultaneously is that "curl" is eliminated without adding to the amount of water which eventually must be driven off.
To reduce further the space occupied by coating machinery it is of benefit to use driers which operate by subjecting the wet substrate to hot air.blast from jets in the form of nozzles or slots arranged along both sides of the coated sheet. The sheet passes between the jets without contacting the walls, and drying can thus be effected very rapidly and without disturbing the evenness of the coatings by solid contact. Any changes in direction of the wet coated sheet can be achieved by passing the sheet round an "air roll" whose surface has a cushion of air on which the sheet runs, so that again the wet coating is not disturbed.
The method of this invention is of particular benefit when the materials being coated on each face of the substrate are not merely different but also reactive with one another, as this method allows the materials to be simultaneously applied without reaction.
The apparatus may also be used with only one of the coating heads in operation so as to prepare a substrate having a coating on only one face; alternatively water or other non-reactive liquid can be applied to the other face to eliminate paper curl in this situation, in conventional manner.
An embodiment of the present invention will now be described by way of example with reference to the accompanying drawings, of which Fig. 1 illustrates schematically a paper making machine in combination with apparatus for use in applying to one face of the paper a coating of microcapsules containing colour former and to the other face a coating of clay containing colour developer, to produce an intermediate sheet of carbonless copy paper by the method of this invention, and Fig. 2 is a schematic detail view of the coating area of the apparatus of Fig. I.
Referring to the drawings, a slurry comprising 99.6% of water and 0.4% of wood fibre is led along a conveyor 1 having a surface formed of wire cloth. As the slurry moves along the conveyor water filters through the cloth and drops into a collecting area below. The slurry is then passed between a pair of rollers 2 which squeeze out more of the water, and around a set of rollers 3 which are steam heated. These rollers are at about 150°C and further dry the fibrous material to an extent that on leaving the rollers 3 the paper contains 15% water.
The paper, which is now a continuous strip from 2 to 7 metres wide, passes round guide rollers 4 and through a size press 5 and thence round further heated rollers 6, after which the moisture content is 4-6%.
The paper is then led between calendering rolls 7 which impart a smooth surface to the paper, and round a guide roller 8 which passes the paper upwardly into coating apparatus 9 which is shown in detail in Fig. 2. The paper enters the apparatus 9 between a pair of applicators in the form of jets lla, llb through one of which a dispersion of microcapsules in water is fed onto one face lOa of the paper and through the other of which a dispersion of acid activated bentonite or other colour developer in water is fed to the opposite face lOb. The microcapsules have hydrophobic synthetic polymer walls and contain a colourless colour former such as crystal violet lactone reactive with the colour developer. The microcapsule dispersion also contains starch particles whose particle size is greater than that of the microcapsules. The dispersions are applied across most of the width of the paper, but strips at each edge are left uncoated so that the dispersions on each face lOa, lOb of the paper do not come into contact.
The dispersions are fed to the paper in excess, and as the paper continues to move upwardly it passes between an opposed pair of flexible blades 12 each of which is about 80mm in width, of similar length to the paper width and 0.3 to 0.5 mm thick. These blades are biassed towards one another so that as the paper carrying its excess of dispersions travels between them the surplus of each dispersion is wiped off the surface lOa, lOb of the paper, while the residual coated surface is smoothed by the blades 12. The blades while flexible are rigidly held and this imparts stability to the moving web of paper and "fluttering" is prevented. The surplus dispersion removed by the blades 12 runs down the blades and blade holders 13 into a reservoir 14 from which it is pumped to the applicators lla, llb for recirculation.
As the microcapsules pass the blade they are protected to a considerable extent from mechanical damage by the starch particles which, being of greater size, contact the blade 12 and allow the microcapsules to pass in dispersion below the blade. The microcapsules are thus, at least in part, separated from the blade 12 by the starch particles.
A double-blade coating apparatus 9 as shown can be obtained as a Twinblade apparatus from Inventing AB of Sweden.
From the apparatus 9 the paper with its wet coatings of microcapsules on one face 10a and clay on the other lOb emerges vertically and is guided round an air roll 15 through 90° so as to move horizontally. The air roll 15 has a cushion of air continuously applied to its surface so that the paper rests on the air cushion, thus protecting the smooth surface of the coatings from damage.
The paper then enters a drying chamber 16 in which it passes between banks of air jets 17 which blow hot air against it, thus driving off the wa:er from the dispersions and depositing the solid material on the paper. On emerging from the drying chamber 16 the paper is hot and the coatings dry, and the paper then passes round further conventional guide rollers to a centre-driven rewind with a flying splice facility 18, where it is wound for storage and collection.
The embodiment of the invention described above allows an intermediate sheet for pressure-sensitive carbonless copy paper to be produced having microcapsules containing colour former on one face and colour developer reactive with the colour former on the other face. The sheet can be produced at speeds of up to 1000 metres per minute and with a much reduced amount of drying compared with conventional methods.
The combination of the paper-making machine with the coating apparatus allows considerable capital and running cost savings.
The arrangement in this embodiment of the invention allows either or both of the applicators lla, llb to be used, so that paper can be coated on one or both sides. This adaptability allows the quantity production of each type of paper to be maximised.
The apparatus of this embodiment is very compact as the drying chambers are relatively small and the coatings are applied simultaneously. The use of the twin blades also allows much higher solids content to be used in the dispersions with the resulting saving in the amount of drying required. Moreover, the coated paper has po tendency to curl as it is wetted on both faces simultaneously.
The following Examples illustrate the composition of dispersions which may typically be used for application to a paper support sheet in the manufacture of pressure-sensitive recording paper by the method of this invention.

EXAMPLE 1

Microcapsule dispersion (60% solids content) in water:

100 parts microcapsules having polyurethane walls and containing crystal violet lactone colour former
25 parts starch particles
10 parts polyvinyl alcohol binder

Colour developer dispersion (40% solids content) in water:

100 parts acid activated bentonite clay
15 parts modified potato starch binder
5 parts caustic soda

EXAMPLE 2

Microcapsule dispersion (60% solids content) in water:

100 parts microcapsules having polyamide walls and containing crystal violet lactone colour former
15 parts glass beads
20 parts acrylic latex binder

Colour developer dispersion (45% solids content) in water:

100 parts montmorillonite clay
25 parts acrylic latex binder
5 parts caustic soda

EXAMPLE 3

Microcapsule dispersion (60% solids content) in water:

100 parts urea-formaldehyde-walled microcapsules containing crystal violet lactone
40 parts starch particles
25 parts polyvinyl alcohol

Colour developer dispersion (50% solids content) in water:

50 parts acid activated montmorillonite clay
50 parts kaolin
5 parts caustic soda
10 parts modified potato starch
1 part carboxymethyl cellulose
10 parts styrene butadiene rubber latex

The dispersions of each of the above Examples were applied to the paper in the apparatus illustrated in Figs. 1 and 2.
Modifications and improvements may be made without departing from the scope of the invention.

Claims

1. A method of coating a support sheet, comprising feeding the support sheet past coating means to apply a solution or dispersion of coating material to opposite faces of the sheet, the coating material applied to at least one of the faces of the sheet comprising microcapsules, and passing the coated sheet through drying means whereby concurrently to deposit the coating material from solution or dispersion onto both faces of the sheet.

2. A method according to Claim 1, wherein the microcapsules have hydrophobic walls and excess dispersion of microcapsules is removed from the coated sheet by passage of the sheet past a flexible blade which contacts the dispersion of microcapsules prior to passage of the sheet through the drying means.

3. A method according to Claim 2, wherein the excess dispersion is removed by passage of the sheet through the nip formed between a flexible blade and a roller.

4. A method according to Claim 1, 2 or 3, wherein the microcapsules contain colourless colour former and the coating material applied to the face of the sheet opposite to the microcapsules comprises a colour developer reactive with the colour former.

5. A method according to any one of Claims 1 to 4, wherein the coating means applies solution or dispersion simultaneously to both faces of the sheet.

6. A method according to any one of Claims 1 to 5, wherein excess solution or dispersion is removed from both faces of the coated sheet simultaneously prior to passage of the sheet through the drying means.

7. A method according to Claim 6, wherein the excess is removed by passing the coated sheet between a pair of blades which are biassed together.

8. A method according to any one of Claims 1 to 7, wherein the drying means comprises opposed banks of jets between which the coated sheet passes, the jets being arranged to blow heated air against the coated faces of the sheet.

9. A method according to any one of Claims 1 to 8, wherein prior to passage through the drying means the coated sheet is led round a roller having a cushion of air applied to its surface thereby to minimise disturbance of the solution or dispersion on the sheet.

10. A method according to any one of Claims 1 to 9, wherein the support sheet is paper and the paper is fed continuously to the coating means from a machine on which the paper is manufactured from fibrous raw material.