CA2148505C

CA2148505C - Method and apparatus for coating paper or the like

Info

Publication number: CA2148505C
Application number: CA002148505A
Authority: CA
Inventors: Patrick Sundholm; Robert Donnelly; Martti Kangas
Original assignee: Individual
Current assignee: Individual
Priority date: 1992-11-06
Filing date: 1993-11-05
Publication date: 2005-03-08
Anticipated expiration: 2013-11-05
Also published as: EP0682571B1; DE69323438T2; BR9307399A; FI934890A; EP0682571A1; FI934890A0; AU5421794A; CA2148505A1; DE69323438D1; WO1994011116A1; FI96894C; FI96894B

Abstract

An application method and apparatus making able to apply a paper web or like in whatever stage of the process from the first dewatering step until the final drying step. The process comprises forming a fog from the coating slurry and directing the fog into a nozzle (24), which is not in physical contact with the web (11). The coating layer is smoothened after the application, using e.g. a doctor blade (32), or in surface sizing the web is fed between two rollers.

Description

VNO 94!11116 ~ ~ ~ PCTlFI93/00453 METHOD AND APPARATUS FOR COATING PAPER OR THE LIKE.
It is well known that paper, paper board, or other materi-als of this kind can be applied by various substances to change the surface texture, colour or the like. These coa-tings often contain clay (kaolin) or other relatively heavy materials as talc or chalk and binders as latex or modified starch, for filling up the somewhat porous surface of the paper or paper board to produce a smooth surface.
The coatings improve the quality of the paper, e.g. the shine, the brightness, the transparency, the smoothness, the surface strength, the absarption characteristics. One or more coating layers are applied onto the surface of the uncoated paper to fill up roughness of the paper surface and thus to render the said surface even, to give a more smooth and microporous surface. Depending on the quality of the paper, the paper web is applied with different amounts of coating in one or more steps. An appropriate surplus of wet coating is applied onto the paper web using a dosing device, through which the paper is conveyed. To give an even thickness to the coating on the surface of the paper web, a doctor or a so called doctor blade or knife, which extends over the whole width of the paper web, is installed

2~ spaced from the dosing device. Until 90 $ of the coating is scraped off. This wears down the blade and the maintenance, e.g. the changing of the doctor, is considerably time con-suming.
As a coating, e.g. only starch, can be employed, as is the case in manufacturing of copying or letter paper, then this is called surface sizing. In surface sizing after the sur-face coating, the paper web is led through a nip formed between two rolls. Ln among others the publications Tappi J. August 1992 pp 79-84 (Jarkko Tehomaa et al.: A compa-rison of different high speed surface sizing techniques for fine paper) and Tappi J. Dec. 1990 pp 69-75 (Charles P.

PcreF~93eoo4s~

Klass: Trends and developments in size press technology), examples on surface sizing are given.
It is well known in the field, that paper is usually manu factured in a continuous process, where the web first has a very high water content, whereafter the water content is gradually decreased, until essentially all water has been removed from the web. Because of the nature of the process and apparatus for applying the coatings, the choice of the process step, where the coatings are to be added is very limited. Especially clay or the like is applied on the sur-face and scraped to the desired thickness using a .doctor blade or a corresponding mechanical device, e.g. an air knife. Very many different types of doctors are known and even if in the present text for simplicity reference is always made to doctors, it is emphasized that the invention is not limited to the use of only doctors but that it can be applied to all such devices that are extending over es-sentially the whole width of the paper web and which are used after the paper coating has been applied.
Some efforts have been made to spray the coatings on paper like substrates, but these spray techniques have never led to successful application. In FI application 911390 a pro-25~cess and apparatus for coating a web is disclosed without mechanical contact, in which process the web is conveyed continuously along a path, the coating material is formed on the web in a slurry containing a solid particulate coa-ting, the slurry is broken up into small particles for pro-ducing a fog of coating material to be applied onto the web, said fog having solid particulate material as a gas suspension, a sufficient gas stream is produced to maintain the solid material in particle form in suspension, and the fog containing the solid-~particulate material is directed towards the web for adherence the solid particulate mate-rial onto the web.

WO 94/1111b PCT/FI93/O(1453

3 For many years higher paper web speeds have been desired for to increase production rates. Presently paper web speeds between 1000-1500 m/min are common and in"'the future even greater speeds will be possible.
For resolving the problems mentioned above, the present invention shows the characteristics disclosed in the cha-racterizing part of claim 1.
The objective of this invention is generally coating or sizing paper webs or like and concerns especially a process and an apparatus for coating a web. Another objecaive of the invention is to obtain a smooth paper surface. Yet another objective is to obtain a better paper quality, so I5 that the pager can be coated with a greater amount of coa-ting and/or the formerly attained coating amounts can be obtained using fewer coating steps. The pressure of the mechanical doctor can be decreased. The doctor blade pres-sure needed is considerably smaller than in existing devi-ces in industry. The wetting of the paper web can be dec-reased. The productional efficiency of the surface sizing is increased.
This invention comprises making a slurry to be used as the 25~coating material mixture or sizing mixture and producing a fog. The coating material mixture or sizing mixture, before adherence it onto the paper, is broken up into a fog using a gas under pressure. After spraying, the coated or surface sized iaeb is treated~by a doctor or it is pressed between two. rotating rolls or like, to smooth the coating or sizing layer and to making possible the chemicals, the coating and/or the sizing to penetrate into the paper.
In different embodiments the fog can be mixed with such fogs that contain other coatings if desired; then the final coatings are directed towards the web. In,one embodiment the fog and the raeb can contain static charges, which help ~:~~094/»»~ ~1~~505

4 the fog to adhere and to stay on the web, even if, the pro-cess is successful even without using static cha=ges. Furt-hermore, the fog can be generated in many different..saays e, by using normal spray nozzles. In the invention ultra j g.
sonic energy and ultra sonic nozzles well known in the be em toyed.
field can P
These and other features and advantages of the invention are disclosed in the following description, when read to-gether with the therewith attached drawing, where:
Figure 1 presents a cross sectional view of a schematic drawing of a coating nozzle.
Figure 2 presents a schematic flow diagram showing the do-sing device of the invention for coating paper-Figure 3 presents different alternatives for smoothing the paper web, which has been coated using a pager machine or a separate coating machine.
Figures 4 and 5 present structures of nozzles.
Referring now to the drawing and to the embodiments of this invention disclosed here, it should be mentioned, that many of the features disclosed in FI apglication 911390 can be incorporated to the apparatus.
In Figure 1 an example of an application nozzle.24 is g~-v-en, where air channels 25 are disposed around the capillary tube 26 of the slurry channel. The coating slurry is brought in the form of a slurry to the slurry channel 26.
The slurry channel and the a~.r channels narrow off towards the outlet. The air from the air channels is directed to break up the slurry into a fog and to further break up it, if the slurry already has been reduced into a fog in an earlier stage. The coating mixture hits the paper web and adheres to it. The distance of the application nozzle from the paper can be adjusted according to the need. The slurry is conducted to the slurry channel in form of a slurry it it is dilute wherein it is broken up just a moment before it hits the paper. The outlet end of the nozzle has e.g. a diameter of 1-5 mm or 3-5 mm. The nozzles can be aligned in a row, over the width of the web to be coated, either in the sane or a different plane, and the air and slurry chan-nels are arranged concentrically so that the fog broken up through them is applied evenly on the surface of the paper.
The nozzle according to Figure 1 can be formed into one uniform nozzle where the outlet of the slurry channel is formed into a slot extending over the whole width of the web and its dimensions e.g. 2-5 mm x 6 m. The air channels are then arranged on the both sides of the slot and spaced apart from each others. In the more advantageous situation a dosing device of the usual type can be replaced only by the nozzle 24 with the air and slurry channels. Figures 4 and 5 show further different embodiments of the nozzles.
Furthermore, the amount of the coating applied can be easi-ly adjusted. The penetration of and the adhered coating can be optimized by defining for each spray a suitable hit angle of the spray. The optimal value of the angle is not critical in all circumstances as the excess coating can always be collected or scraped off, but then the paper strain is added. A suitable pressure far the breaking up/-accelerating gas (air) is 0,5-25 bar. A great pressure usu-ally leads to a greater coating mixture velocity after the nozzle and this results that the coating penetrates deeper in the paper pores. The deepest penetration in the pager is obtained when the nozzles are slightly inclined against the direction of the incoming paper web. The optimal angle of the nozzles against the paper web line depends, as was sta-ted, on:
- the velocity of the paper web - the average droplet velocity of the broken up coating fog I
WO 94/11116 PCT/F193/00453 ' ~~.4~50~

produced by the nozzle (the velocity of the breaking up air, that breaks up drops and gives them velocity, which at the point of discharge carx approach the sonic speed,.depend-ing on the structure of the opening arland the slot/slots), - the distance of the nozzle from the web (0,02-2 m) - the diameter (0,02-10 mm) of the application opening or the width of the coating slot of the nozzle and the star-ting velocity given to the coatinglsizing by a pump or an inner ejector.
In displacing the fog the pressure can be adjusted as nee-ded to obtain an acceptable coating. The apparatus is natu-rally provided with an appropriate air-conditioning system, which is not disclosed herb.
After application the web is conveyed through the doctor 32. The doctor blade is pressed against the roll 33. It should be mentioned, that the knife pressure can be consi-derably lower than in coventional application techniques, as. .the .amounts of the fed coating can be easily dosed, wherein an excess amount of coating can be avoided. The main function of the doctor can be to meter or level. In a pilot-plant test a coating amount of 25 g/m~ was obtained in one coating step, whereas in conventional processes a coat-i,ng amount of 15-16 g/m2 in one step was obtained on, one side. A mechanical (or air) knife can function as brushing or removing slurry paste. The choice of the main function, between brushing or removing the paste, depends on the speed of the web to be coated, the type of paste and on the desired amounts of coating. A high blade pressure leads to a scraping phenomena and to an unwanted straining of the paper in removing the paste from the paper web. It is to be noted, that when paper is sized with a starch solution, the j characteristics of this solution are totally different from the usual mineral coating or slurry.
In the process according to the invention the surface si-v WO 94/ 11116 2148~~~

pC I"/Fi93/00453 zing and application speeds can be increased and it is es-timated that the velocity of 2000 m/min can be reached, which was the maximum speed of the test machine in"one of the pilot-plant tests. Thus, it can be said, that the limit speed of the application can be said to be dependant on the mechanically reachable speed of the machine as on the ap-plication process itself. The application cdn be Performed in one or more steps. The coated paper can be smoothened, doctored, instead of using a doctor or in combination with a doctor e.g. by passing it between two rolls. An air knife or air brush or any combination of these can also be employed.
Upon visual inspection using ultra violet light it was no-ticed, that, for both gravure paper and off set paper in experimental application tests using paper BPeeds from 1000 to 100 m/min, the process according to the invention gave a smoother surface to the paper than non-using doctoring and spraying, ,. , , .., .
In the embodiment according to Figure 2 the slurry is remo-ved from the tank 10 by a pump,l4, the outlet of which is connected to a line 15 for feeding a nozzle 16. The nozzle 16 breaks up the slurry from the tank 10 so that the slurry 25~becomes a spray having a very small particle size or a fog size. In this application.the material will be referred to as a fog and it should be understood that this term in-cludes range of forms from a very small particle aerosol to a relatively. small .'particle spray. ' The nozzle 16 may comprise many specific p~.eces of hardwa-re. It is possible, that by using, the pump 14 having suffi-cient pressure, the nozzle 16 may actually be a fluid type nozzle, wherein.the~nozzle will. break up the fluid that flows therethrough to produce a .fine particle spray, or a nozzle, where a gas under pressure breaks up the liquid and mixes therewith to form a fog. It is also possible to use .,-~. ~::WO 94/11116 ~ ~ 4 8 ~ D 5 p~/Fi93/00453 an ultrasonic nozzle, usually of the type disclosed in the US patent 4352459 of Berger et al. The ultrasonic nozzles are well known in the art, and those skilled in the art will understand without further explanation.
One further embodiment that the nozzle 16 may take, is a transducer located at the bottom of the tank 18. It will be noted, that the tank 18 is described as having some slurry in the bottom of it, and the fog in.the upper part of the tank 18. By placing-the transducer at the bottom of the tank 18, ultra sonic energy can break up the slurry into fine particles to produce the desired fog above the liquid, and the additional tog being generated is removed for use.
It will be noted that the fog from the tank 18 is directed to a mixing chamber 19, and further that there is a second tank 18A, which also has its output directed to the mixing chamber 19. If desired, one might have two or more tanks such as the tanks 18 and 18A, each of the tanks 18 and 18A
containing a different.slurry and.different fog so that two or more materials can be coated on a web 11 simultaneously.
Another means for providing two.different materials for coating the web 11 is to provide two or more of the tanks as.the tank 10. In the Fig. 2 a second tank l0A is disclo-sed, and a pump l4A moves material from the tank l0A and feeds it through the line 15A to a nozzle 16A in the tank 18. Thus, two different types of fogs are generated within the tank 18 by the nozzles 16 .az~d ~ 16A. The mixed fogs will then be either~directed to a mixing tank 19 or directly to the application nozzle 24.
This invention provides also an injection device 23, which injects air or other gas~to the~stream of fog. The injecti-on means 23 are placed adjacent to the walls of the appli-cation nozzle 24 and in this position a curtain of gas is placed along~.the walls, to prevent the attachment of the -.:WQ 94l1711b 21 ~ $ ~ ~ 5 PCTlFI93l00453 ,.

droplets on tl~e walls .
To assist in causing the fog to attach to the substY'ate 11, it is contemplated that a static electric charge will be utilized on the fog and on the web 11. Those skilled in the art will readily understand that the web 11 can be charged, and that the fag can be charged by means of grating or the like. For purposes of illustration a charge generator is indicated at 30, there being only one charge generator shown. Nevertheless, it will be understood, that one charge (e.g. a negative) can be generated on the substrate 11, while the opposite charge (e. g. a positive) can be placed on the fog. These opposite charges will cause the fog to be attracted to the substrate 11 and stick thereto.
Different alternatives for smoothing (doctoring) of the paper using a paper machine or separate application ma-chines are disclosed in Figure 3. Figure 3a discloses an embodiment of a doctor 32, an air brush or their combinati-on. Figure 3b discloses a pair of rollers 34, used for a surface sized paper web, through which rollers the paper web is conducted. An air knife presented by a broken line can be added after the roller pair, if desired. The pager web can move upwards or in a horizontal direction. A small amount of pigment can be added to the starch, and this is called pigmentation..
An important feature of this invention is the application of the fog containing coating material to a web 11 at low pressure and without mechanical manipulation or the like.
This allows the system of the invention to be utilized for coating paper anywhere along the paper production line, from the first de-watering stage until the paper has been completely dried. If desired, the paper can be manufactured and rolled up, and the rolls can be transported to another location, unrolled and then coated using the system accor-ding to the invention. .

:- <..yy0 94/11116 P~C'T/F193/00453 214~~05 to It will therefore be seen that this invention provides an extremely simple method and apparatus for coating a paper web or like. Since the slurry to be coated on the web is transformed into a fog and the fog is applied at very low

5 pressure, it will be understood, that the web will never be harmed, even when the web contains much water. Furthermore, it will be understood that any conventional drying techni-que is appropriate so that infrared lamps or the like can be utilized to dry the coating on paper or board.
The extra coating material that is accumulated to the doc-tor blade, can be collected by usual means for later ex-ploitation.
The invention has been described above referring to only one of its preferable embodiments, but it is clear, that the invention can be realized in other possible ways within the scope of the patent claims without changing the scope of the present invention. 2t is possible to use different coating techniques or machines. A remarkable improvement has been done in view of known art.

Claims

We Claim:

1. A process for coating a paper-like substrate, the process comprising the steps of applying a coating mixture to the substrate; and smoothening or doctoring the coated substrate using a smoothening device;
wherein said coating mixture is broken up into a fog, said fog being applied to the substrate at pressure through a nozzle.

2. A process according to claim 1, wherein the paper-like substrate, when coated by a starch fog, is smoothened between two rollers, said rollers being placed on either side of the substrate.

3. A process according to claim 2 for use in a surface sizing unit.

4. A process according to claim 1, wherein the coated paper-like substrate is smoothened using a smoothening device selected from the group consisting of a doctor blade, a pair of rotating rollers, an air knife, an air brush and any combination thereof.

5. A process for coating a paper-like substrate, wherein the substrate moves continuously along a path, the process comprising the steps of providing at least one slurry comprising the material to be coated on the substrate;
breaking up the at least one slurry into small particles to produce a fog containing said material to be coated on the substrate;
directing said fog towards the substrate for adherence to the substrate; and smoothening the coated substrate by smoothening means, wherein the step of directing said fog towards the substrate further comprises the steps of placing an application nozzle adjacent to the substrate and providing a positive force to urge said fog through said application nozzle and towards the substrate.

6. A process according to claim 5 wherein a plurality of slurries is provided and a plurality of fogs is produced prior to mixing said plurality of fogs and directing said resulting mixture towards said substrate.

12 An apparatus for coating a continuously moving paper-like substrate having a width, with a coating mixture, which apparatus comprises:
an application nozzle located at a distance from the substrate for forming the coating mixture into a fog and directing said fog towards the substrate; and a smoothening device for smoothening the coated substrate, wherein said fog is formed by treating the coating mixture with a gas under pressure.

An apparatus for coating a continuously moving paper-like substrate, which apparatus comprises:
at least one means for providing a slurry comprising a solid material to be coated onto the substrate;
at least one means for breaking up said slurry into fine particles for producing a fog containing said solid material;
application means for directing said fog towards the substrate, said application means being adjacent to and spaced from the substrate;
means for producing a current of gas for entraining and carrying said fog through said application means and towards the substrate; and a smoothening device for smoothening the coated substrate.

9. An apparatus according to claim 7 or 8, wherein the substrate is coated by a starch slurry fog and wherein the smoothening device is selected from the group consisting of a doctor blade and two rollers placed on both sides of the paper-like substrate.

10. An apparatus according to claim 8 wherein the apparatus is a surface sizing unit.

11. An apparatus according to claim 8, wherein the smoothening device is selected from the group consisting of a doctor blade, an air knife and an air brush.

12. An apparatus according to claim 7, wherein said application nozzle is provided with at least one slurry channel along which the coating mixture is directed from an entry end to an exit end, at least one air channel is located adjacent said at least one slurry channel and said fog is directed across the entire width of the substrate.

13. The apparatus of claim 12 wherein said application nozzle has one slurry channel and said exit end of said slurry channel extends across the width of the substrate.

14. The apparatus of claim 12 wherein said application nozzle has more than one slurry channel and said exit ends of said slurry channels collectively extend across the width of the substrate.