US3489592A - Method and device for coating or covering paper or sheet material with surface layers - Google Patents

Description

Jan. 13, 1970 H. I. WALLSTEN 3,489,592

METHOD AND DEVICE FOR COATING OR COVERING PAPER OR SHEET MATERIAL WITH SURFACE LAYERS Filed April 26, 1966 2 Sheets-Sheet 1 HAN-S /VAR WALLS TEN 1 N VENTOR.

ATTORNEY.

Jan. 13, 1970 H. I; WALLSTEN METHOD AND DEVICE FOR COATING OR COVERING PAPER OR SHEET MATERIAL WITH SURFACE LAYERS 2 Sheets-Sheet 2 Filed April 26. 1966 HANS /VAR IA ALLS TEN INVENTOR.

ATTORNEY United States Patent 3,489,592 METHOD AND DEVICE FOR COATING 0R COVERING PAPER OR SHEET MATERIAL WITH SURFACE LAYERS Hans Ivar Wallstu Fengersfors, Sweden, assignor to AB Billingsfors-Langed, Billingsfors, Sweden Filed Apr. 26, 1966, Ser. No. 545,383 Claims priority, application Sweden, Apr. 27, 1965,

,507/ 65 Int. Cl. D21h 1/10 US. Cl. 11768 5 Claims ABSTRACT OF THE DISCLOSURE A method and apparatus for coating sheet material simultaneously on both sides by passing a sheet through a press nip immediately after the coating material is applied. One side of the press nip is a flexible blade to apply pressure along a straight line of contact with the sheet. Film splitting is avoided by deflecting the sheet at a small angle after it passes through the press nip.

Known coating methods for applying a surface layer to both sides of paper by means of a coating knife, necessitate the use of two coating stations, viz one for coating either side of the paper. Even though known blade-coating methods possess great advantages in comparison with many other coating methods, primarily due to the smoothening action of a coating blade on the unevennesses of the surface layer of the base paper the use of two coating stations involves considerable capital and out-lay with respect to the costly blade-coating stations. In comparison with many other coating methods coating by means of a blade or knife has a further essential advantage, viz that this method leaves no so called film splitting.

When coating by means of rollers, e.g. in a sizing press or similar method, the paper web is caused to pass between two rollers, so-called coating rollers, which are urged towards each other. In the manufacture of doublesided coated paper the coating composition is usually applied on both sides of the paper web. In such a roll-coating method the surface layer of the base paper is not smoothed out to the same degree of smoothness as that achieved by means of a coating blade or knife.

Another disadvantage which prevails in such stations is namely the so called film splitting, which means that the coating, immediately after the formed press nip, easily presents a non-desired so-called orange peel pattern, i.e. a surface unevenness which in the majority of printing methods gives an impaired printing result when the paper is printed, and furthermore results in a bad appearance.

It can be mentioned in this context that the problem of film splitting is substantially due to the properties of the coating composition.

When manufacturing ofl-set paper, for instance, using the roll-coating method with high binder content in the coating material, the tendency to film splitting is often increased.

It is also known to blade-coat paper by passing it around a so called breast roll, a steel doctor knife of suitable thickness being urged against the paper web using the breast roll as a support. As a rule the breast roll is driven, and serves partly as a support against the doctor knife and the paper web, and partly to feed the paper web. However, in this way only one side of the paper will be covered, resulting in the above disclosed disadvantages.

The present invention intends to exploit the favourable effects of the blade coating method, i.e. optimal levelling ice of the unevenness of the surface of the base paper, while avoiding the disadvantages of the expensive coating stations associated with this method, by coating simultaneously the two sides of the paper web by means of a relatively simple arrangement, without the occurrence of film splitting. The method can suitably be employed in coating directly in a paper machine as well as at a subsequent separate coating station.

According to the invention the coating composition is applied on the two sides of the paper or sheet material, immediately in front of (viewed in the advancing direction of the sheet material) a press nip adapted to distribute and to apply the coating composition on both sides of the sheet material, said sheet material being passed through the press nip and between two pressing members of which at least one comprises a thin flexible blade shaped coating member. Substantially immediately after having passed through the press nip the sheet material, thus coated on both sides, is suitably deflected at an angle to its travelling path through the press nip. It has been proved that in this way it is possible to avoid the above mentioned film-splitting eifect.

Several examples of a device for carrying out the invention are shown diagrammatically on the accompanying drawings, in which:

FIG. 1 relates to an embodiment having a blade-like coating member and a support member mounted opposite to said blade member on the opposite side of the sheet material, said support having a substantially flat support surface abutting the sheet material;

FIG. 2 shows details of FIG. 1 in a large scale;

FIG. 3 shows a similar embodiment in which the sheet material after having passed through the press nip is deflected in the opposite direction to FIG. 2;

FIG. 4 illutrates an embodiment in which one blade member is replaced by a rotating cylinder, whereas FIG. 5 illustrates a particular problem which can arise in conjunction with the invention.

In FIG. 1 the reference numeral 9 indicates a blade or a steel doctor knife of suitable thickness, the edge of which is pressed towards one side of the paper web 11, whilst the other side, opposite the blade 9, is supported against a substantially plain surface of a holding-on surface 10, which can consist of another blade but of larger thickness than the blade knife. The paper web 11 is fed through the press nip defined by contact lines between the blades 9 and 10. A coating composition 12 is passed on both sides of the paper web 11. By using a relatively thin coating blade 9 which urges the paper web 11 against the relatively thick coating blade 10 the difficulties of allowing two thin coating blades to meet along a clearly defined contact line are overcome. When the coated paper web has passed through the press nip it is suitably deflected, as shown at 11a, in order to stabilize the paper web on its passage through the coating apparatus.

FIG. 2 shows details of FIG. 1 on a larger scale, and it can be seen how contact between the two blades 9 and 10 is eflected. Due to the relatively large thickness of the blade 10 with respect to the blade 9, a clearly defined contact line between the two blades can be obtained without difficulty, the paper web 11 being passed along said contact line. The blade 10' has such a thickness that it is very rigid, even in the case with relatively high tensions in the paper web.

According to FIG. 2 the paper web 11 is suitably deflected over the two edge lines 13 and 14, but even other means for guiding the paper web can be conceived. Thus, the paper web can be curved against the edge 15 of the thinner blade 9, as shown in FIG. 3, or also passed straight on.

Another embodiment of the invention is shown in FIG.

4. Similarly to the previous embodiments a thin flexible blade 9 is urged against the paper web 11 which is supported by a holding-on surface at the opposite side of said paper web at the same time as coating composition 12 is applied to both sides of said paper web 11. In this embodiment, however, the holding-on surface is not constituted of a blade but of a rotating cylinder or roller 16, and when the paper web 11a leaves the nip between the blade 9 and the holding-on roll 16 the web 11a is deflected against the side at which the blade 9 is located, or in other words the paper web 11 is deflected so that an angle a occurs between the tangent to the holding'on roller 16, through the point of contact, and the downward passing portion of the paper web 11.

It is clearly seen from FIG. 4 that the thin flexible blade 9, having a thickness of say about 0.30 mm., will form a dam on both sides of the paper web so that the coating composition or paste will be dammed up on each side of the paper web across the whole width thereof, said dams being defined on one side by the paper Web and the blade 9 and on the other side of the roller shell and the paper web. Laterally the dams are limited by suitable sealing means. In this manner the paper web will be coated simultaneously on both sides.

Certain advantages are obtained with this embodiment as should be apparent from FIG. 5. If certain coating compositions are used in the embodiments according to FIGS. 2 and 3 there is a risk that during passage of the coating composition through the nip impurities in the form of solid particles contained in said composition might lodge between the edge of the blade 9 and the paper Web 11. Such a particle is shown at 17 in FIG. 5. With paper of low surface weight and insignficant sizing the mechanical strength of the paper web becomes relatively low when the mixture 12 wets the same. This implies the risk that such a particle 17 might tear the paper web as a result of being pressed in between the two blades 9, 10. In such a case the paper web 11 will be perforated, and due to the movement of said web, will split or slit up, resulting in a break in the web. Under certain conditions the embodiment according to FIG. 4 as aforesaid, possesses certain advantages. Because the holding-on roller 16 is constituted of a rotating roll, particles which possibly accompany the coating mixture are carried down to the press nip between the blade 9 and the paper web 11 when perforation of said paper web occurs, and the particle is carried by the paper web so as to prevent it from being scored. It should be pointed out that the number of revolutions of the roll 16 need not necessarily correspond to the speed of the paper web. In connection with papers of high weights per unit of area it may even be suitable to have the roller 16 rotating at a lower speed than or at the same speed as the paper web whereas experience shows that for small weights per unit of area it is more favourable to have the counter roller 16 rotating at the same speed as or even faster than the paper web. In the last mentioned case the friction between the counter roller and the applied coating composition or paste and the surface of the paper web facing the roller will contribute to pull or draw the paper web, so that it will be stretched in an improved manner during the coat ing process.

Some data concerning the application of the invention will now be mentioned by way of example. Thus, coating may successfully be elfected according to the invention in a paper machine in which the paper is pre-dried to a moisture content of between 20%. A 0.30 mm. steel blade of the same quality as is usually used for blade-coating purposes may be used as a doctor blade, and a hardrubber roll having a hardness of approximately 30 P&] (that is, measured on the scale of the standard Perse and Jones testing method) may be used as a holding-on or counter pressure roll. The thin blade is clamped in a blade holder and its free width measured from the edge of the doctor knife to the free edge of the blade is approximately 20 mm.

Paper of gram weights varying from 50 grams up to g./m. has been coated. The coating compositions are based on aqueous China clay dispersions with synthetic and/ or natural binders, dispersing agents and other agents known within the coating technology. A different coating mixture can possibly be used for different sides of the paper.

When performing coating according to the invention it has been proved possible to obtain a rather considerable increase in production, particularly in such paper machines in which the coating unit is mounted so as to allow the use of a preliminary drying portion of a relatively high capacity as compared with that of the post drying portion. This increase in production is attributed firstly to the blade coating technique as such, according to which it is possible to Work with coating compositions of relatively high dry content and secondly to the fact that the drying portion can be better utilized in that the method permits a relatively high dry content of the paper web before it is coated.

As is generally known, the sides of a base paper to be coated, in similarity with the majority of other types of paper, are dissimilar due to the fact that the side of the paper facing the Wire in the paper machine, the so called wire side, often has a lower surface uniformity than the opposite side, the so called upper side, inter alia due to markings obtained from the wire and the felts. This is also applicable to crude paper of relatively high China clay content, the China clay tending to concentrate towards the upper side and thus further contributing towards said dissimilarity. Due to the new method, particularly in the embodiment according to FIG. 4 for example, it is possible to cause the blade 9 to coat the wire side of the paper, which contributes towards smoothing out the irregularities. A particularly even surface on the side of the crude paper 11 facing the blade 9 during the coating process is obtained in the embodiment according to FIG. 4.

It has also been found to be very important in the embodiment according to FIG. 4 to deflect the paper web from the roller and in practice it has been found that the angle a should not be lower than 5 to 10. It should be noted that the risk for film splitting is eliminated by deflecting the paper web against the blade by the angle a. This effect occurs both at low and high rotational speeds of the breast roller 16.

What I claim is:

1. A method of coating paper or other sheet material with surface layers by applying a coating composition to both sides of a travelling sheet by feeding the sheet between two opposed members defining a press nip immediately after applying the coating composition to distribute the coating composition evenly on both sides of the sheet, maintaining pressure in a thin line against both sides of the sheet as it passes through the press nip by using a resilient thin blade having a sharp edge engaging the sheet as one member of the press nip, the other member of the press nip being relatively rigid and supporting the sheet against pressure imposed by said one member.

2. The method of claim 1 including deflecting the sheet at an angle to its direction of travel through the press nip substantially immediately after the sheet passes through the press nip.

3. In an apparatus for coating paper or sheet material with surface layers by applying a coating composition to both sides of a travelling sheet, a press nip having two opposed members on opposite sides of sheet material movable through the press nip after the application of coating material to both sides of the sheet material for distributing the coating material evenly on both sides of the sheet material, one member of the press nip being a resilient thin blade forming a thin sharp edge engaging the sheet material for maintaining line pressure against the sheet material, the other member of the press nip being more rigid than said one member and comprising a mainly flat supporting surface engaging one side of the sheet material under pressure imposed by said one member, and means for deflecting the sheet material at an angle to its direction of travel through the press nip substantially immediately after the sheet passes through the press nip to prevent film splitting.

4. Apparatus according to claim 3 wherein the deflecting means is adapted to deflect said sheet material in a direction away from the plane of said flat surface.

5. In an apparatus for coating paper or sheet material with surface layers by applying a coating composition to both sides of a travelling sheet, a press nip having two opposed members adapted to press against both sides of the sheet material only along a single line of contact with said material immediately after the application of coating material to both sides of the sheet material for distributing the coating material evenly on both sides of the sheet material, one member of the press nip being a thin blade forming a thin sharp edge for engaging the sheet material to maintain line pressure against the sheet material, the other member being a rotary roller having a cylindrical surface adapted to contact the sheet material only along a line, and means for feeding sheet material References Cited UNITED STATES PATENTS 1,484,705 2/1924 Frothingham et al. 118-405 X 2,051,403 8/1936 Boyer et al.

2,855,895 10/1958 Burns et al. 1l8407 FOREIGN PATENTS 1,308,186 9/1962 France.

ALFRED L. LEAVITT, Primary Examiner CHARLES R. WILSON, Assistant Examiner US. Cl. X.R.

"H059 UNITED STATES PATENT OFFICE CERTIFICATL OF CORRECTlON Patent NO- Q 4RQ RQ7 I Dated January 13- 1970 Inventor( HANS IVAR WALLSTEN It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Page 1, column 1, line 6 should include the statement:

The assignment was not of record at the date of issue of this patent Signed and sealed this 6th day of June 1972.

(SEAL) Attest:

EDWARD M.FLETGHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents

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