DE69523789T2 - Device for applying a coating on sheets - Google Patents

Description

Technical area

The present invention relates to a device and a method for applying coating material to the two main surfaces facing away from each other of several individual sheets, e.g. sheets of paper.

Background of the invention

In certain fields it is necessary to apply coating material to paper and in certain cases it is necessary to apply coating material to the two opposite major surfaces of the paper. For example, in the manufacture of so-called repositionable note sheets it is necessary to apply a primer material to one side of the paper from which the repositionable note sheets are to be cut and to apply a low adhesion back finish or tack-repellent material to the other side of the paper. The repositionable adhesive is applied to the paper on top of the primer material. Traditionally, to produce repositionable note sheets the various coatings are applied to a paper web from a roll. The coating materials are dissolved in solvents and applied directly to the paper. The web is dried between coating operations and then rewound and the coated roll is subsequently cut into webs which are used to make the note sheets. If the paper web is coated with materials dissolved in an organic solvent and is under tension when the coatings are applied, the tendency of the paper web to curl or wrinkle can be substantially eliminated, whereas otherwise great care is required to compensate for the expansion of the paper web during coating.

WO-A-87/05315 describes a process for producing repositionable note sheets in which an anti-tack material and a primer material are applied sequentially to opposite sides of a paper web. In some cases it is desirable to apply a coating material to cut webs rather than to a paper web. When producing repositionable note sheets, it is desirable to have the option of using a stack of pre-printed sheets as a feed source rather than a simple paper web in order to increase the flexibility of the manufacturing process. Furthermore, for environmental reasons, there is a desire to abandon the use of organic solvent materials in coating processes in favor of water-based materials. In addition, many ink materials are soluble in organic solvents but insoluble in water. Problems can arise when applying coating materials (and particularly water-based coating materials) to individual sheets, as the risk of buckling or wrinkling of the sheets is considerably greater. Any buckling or wrinkling not only spoils the external appearance of the sheets, but can also make it difficult to apply further coating materials.

Copending U.S. Patent Application Serial No. 08/196,490, filed February 15, 1994, entitled "Method and Apparatus for Applying a Coating Material to a Sheet," describes apparatus and a method for forming pads of repositionable note sheets from a stack of uncoated individual paper sheets. The sheets are fed in an overlapped state from the stack to a coating station where a continuous layer of water-based primer material is applied to a major surface of the web-like overlapping sheets and a continuous layer of water-based Low Adhesion Backcoat (LAB) material is applied to the other major surface. The overlapping sheets are then dried and fed to a second coating station where strips of repositionable adhesive are applied to the web-like overlapping sheets, covering the area to which the primer was applied in the first coating station. The sheets are then adhered to one another in a stack and trimmed to form pads of repositionable note sheets. In this process, buckling or wrinkling of the sheets is avoided by applying the primer and LAB coating to both sides of the sheets at the same time and then drying both coatings simultaneously.

However, existing systems for applying coating materials to sheets, including that of the above-referenced U.S. Patent Application Serial No. 08/196,490, while having their own respective benefits, have proven not to be as effective or efficient as desired in coating some materials onto certain sheet materials. Therefore, there is a need for an improved apparatus for applying a coating material to sheets.

The present invention provides an apparatus for applying water-based coating material to both sides of a plurality of sheet members as defined in claim 1.

Further advantageous embodiments of the invention are set out in the dependent claims 2-11.

Short description of the drawings

In the following, embodiments of the invention are described by way of example with reference to the accompanying drawings, in which similar structures are designated by the same reference numerals throughout the several views.

Fig. 1 shows a schematic side view of the device designed according to the invention, which has a coating station;

Fig. 2 shows a schematic plan view of the device according to Fig. 1;

Fig. 3 shows a view of a coating station which is part of the device according to Fig. 1;

Fig. 4 shows a more detailed view similar to Fig. 3 of a part of the coating station;

Fig. 5 shows a coating material supply system for the coating station according to Figs. 3 and 4;

Fig. 6 shows a schematic side view of a part of the adhesive coating station arranged at the downstream end of the device according to Fig. 1 with an alternative coating section;

Fig. 7 shows a schematic side view of a modified version of the device designed according to Fig. 1 with an alternative guide section; and

Fig. 8 shows an enlarged partial view of a portion of Fig. 4.

Detailed description of the invention

The apparatus shown schematically in Figures 1 and 2 is specifically for use in making repositionable note sheets from webs of any suitable substrate material, e.g., paper, polymeric films or foils, such as metal foils, and particularly for applying primer material, low adhesion backing (LAB) material and a repositionable adhesive to individual sheets such that the sheets can subsequently be used to form repositionable note sheets. In the following description, it is assumed that the sheets (which may be pre-printed) are made of paper. The paper may be any suitable paper, e.g., the paper used to make the Post-it® brand repositionable note sheets sold by Minnesota Mining and Manufacturing Company ("3M") of St. Paul, Minnesota. In the following description, unless otherwise stated, it is assumed that the sheets (which may be pre-printed) are made of paper.

The apparatus includes a paper path inlet 1 which receives a succession of paper sheets (not shown) from any suitable source, e.g., a stack. From the paper path inlet 1, the sheets move in the direction shown by arrow 2 through a double coating station 3, a sheet overlap station 4, a dryer 5, and a sheet guide section 6. The apparatus may conveniently be used in conjunction with an adhesive coating station 7. The coating station 7 may be any suitable coating station, however, the present invention is most conveniently used in conjunction with the coating station described in copending U.S. patent application entitled "Method and Apparatus for Applying a Coating Material to Sheets," filed on even date and assigned to the assignee of the present invention, the contents of which copending application are hereby incorporated by reference into the present application. Alternatively, According to the present invention, the output material can be stored, e.g. in stacked form, and coated independently and sequentially by means of a separate coating station. The control and synchronization of the drives of the various stations can be carried out by a central electronic control unit (not shown), e.g. the Siemens PLC 135 unit.

Individual sheets arrive one at a time at the paper path inlet 1 of the apparatus and are fed by a nip roller pair 10 into the dual coating station 3 (shown in more detail in Figs. 3 and 4). In comparison with other arrangements, e.g. that according to the above-referenced copending U.S. application Serial No. 08,196,490, filed February 15, 1994, entitled "Method and Apparatus for Applying a Coating Material to Sheets", one of the advantages of the present invention is that the sheets are transported to the coating station in a non-overlapping condition. This exposes all or a substantial portion of the two major surfaces of the webs to coating with the LAB and the primer. Furthermore, handling of the sheets is easier because the "pseudo-web" has a uniform thickness.

The sheets are preferably all the same size, for example A2, and the same weight, for example 70 gsm. The paper may be of any weight, including, but not limited to, papers weighing between 45 grams per square meter (gsm) and 90 gsm. The double coating station 3 comprises an upper coating station 11 and a lower coating station 12, which are arranged above and below the paper path, respectively. As the sheets pass through the double coating station, the upper station 11 applies a coating of primer material to one side of each sheet, and the lower station 12 simultaneously applies a coating of LAB material to the other side of each sheet. According to one embodiment of the present invention, the primer material and the LAB material are substantially applied to both major surfaces of the leaves. Typically, mechanical devices are used to grip and advance the leaves through the apparatus, preventing the primer and LAB from being applied to the entire major surfaces of the leaves.

Paper is preferably formed by accumulating paper fibers on a wire network or grid and compressing the accumulated fibers between the grid and a "felt" or fabric layer opposite the grid layer. This produces paper with a "wire" side and a "felt" side. It has also proven advantageous to transport the sheets S through the device according to the invention in the state in which the "wire" side is presented for coating with the anti-tack material and the "felt" side is presented for coating with the primer and finally for coating with the adhesive.

The primer may, for example, be a water-based solution of an organic binder and a split mineral pigment. In particular, the primer material may be formed by mixing about 3-7% by weight of the binder MOWIOL (trademark), available from Hoechst AG, Frankfurt/Main, Germany, and about 3-8% by weight of the pigment AEROSIL (trademark), available from Degussa AG, Frankfurt/Main, Germany, with about 90% by weight of water.

The LAB material may be any suitable material including, but not limited to, acrylate copolymers, silicone materials, urethanes, and fluoropolymers. For example, the LAB may be a water-based solution of the material described in copending U.S. Patent Application Serial No. 08/040,876, filed March 31, 1994, assigned to the assignee of the present invention, the contents of which are hereby incorporated by reference into the present application. The solution typically comprises from about 5% to about 10% solids Other LAB materials that may be used with the present invention include those described in U.S. Patent Nos. 5,202,190 and 5,032,460.

The upper coating station 11 has a metering roller 13 and a coating roller 14 arranged above the path of the paper sheet through the double coating station. The coating roller 14 cooperates with a coating roller 16 of the lower coating station 12 which also has a metering roller 17 and a transfer roller 18 all arranged below the paper path through the double coating station. The coating roller 16 has a cut-out area 9 (shown in more detail in Fig. 8) of rectangular cross-section which has a conventional sheet gripping element 9A (shown closed in Fig. 8) for gripping sheets from the feed nip 10. Furthermore, the roller is covered by a cover 20 (not shown in Fig. 8) around less than half its circumference so that, as will be described, the coating roller 14 and the coating roller 16 only form a coating gap when the cover is located directly on the coating roller 14. As will be described, the upper coating station 11 essentially performs a full-surface roller coating with primer material on the upper surface of the sheets and the lower coating station 12 essentially performs a full-surface roller coating with LAB material on the lower surface of the sheets. In any event, the areas engaged by the gripping elements (as described here) are not coated.

In the upper coating station, a trough 15 for the printing material is formed by the surfaces of the metering and coating rollers 13, 14 at the gap between the rollers and at their upper side together with two opposite end walls (not shown) which engage in grooves (not shown) at the ends of the rollers. When the rollers 13, 14 rotate, the primer material forms a film on the coating roller 14 and is transferred to a sheet passing beneath the roller.

The thickness of the primer film on the coating roll 14 and hence the amount of primer applied to the sheet depends on the viscosity of the primer and on the pressure between the metering and coating rolls 13, 14 and can be adjusted for a given primer by moving the metering roll towards or away from the coating roll, thereby adjusting the pressure between the rolls. Primer is supplied to the trough 15 through nozzles 60 (see also Fig. 5), the nozzles receiving the primer from a tank 61 by means of a pump 62. The trough 15 is further provided with overflow outlets 63 through which excess primer is returned to the tank.

In the lower coating station 12, a trough 19 for LAB material is similarly formed between the metering roller 17 and the transfer roller 18. As with the upper station, the coating material on the coating roller forms a film whose thickness can be adjusted by moving the metering roller 17 toward or away from the transfer roller 18, thereby controlling the amount of LAB material transferred from the transfer roller to the cover pad 20 on the coating roller 16 (but not to the remainder of the roller which the transfer roller 18 does not contact).

In a similar manner to the trough 15 in the upper coating station 11, the trough 19 is fed with LAB material through respective nozzles 60A (see also Fig. 5) which receive the LAB material by means of a pump 62A from a respective tank 61A. The trough 19 is provided with overflow outlets 63A through which excess primer is returned to the tank 61A. While the roller 16, which is partially covered by the cover, moves on the coating roller 14 of the upper station, a sheet fed from the feed nip 10 of the double coating station 3 is fed by gripped by the gripping member 9A on the drum and moved through the coating gap between the roller 14 and the cover 20 on the drum 16, and as the sheet passes through the gap it is coated on one side with the LAB material. Sheet stripping members (not shown) are arranged on the downstream sides of the coating roller and the coating roll to ensure that the sheets do not wrap around the roller or the roll but are fed to the overlap station 4. The next sheet from the feed roll gap 10 is gripped and transported between the coating roller and the coating roll as the surrounding cover in turn moves around the roller 14.

It will be appreciated that the coating in the dual coating station 3 is discontinuous since it only occurs when the overlying cover 20 of the coating roll 16 is on the coating roller 14 (i.e. when a sheet is moving through the coating nip). A typical coating weight for the LAB material on the sheets is from about 0.5 gsm to about 12.0 gsm and the coating weight for the primer material is adjusted so that the coated sheets remain reliably flat. Since the primer and LAB material are applied simultaneously to the paper sheets in the coating station and are preferably selected to have appropriately designed properties such as viscosity, percent solids and coating weights, the risk of curling or wrinkling of the webs is substantially eliminated.

The shielding cover 20 on the coating roll 16 may be of any suitable type, for example, it may be the DuPont "CYRELL" type polyurethane cover sold by E.I. DuPont Demours of Wilmington, Delaware.

At the exit side of the coating nip 14,16 in the sheet overlap station 4, a gripping unit 27 is positioned to grip the sheets as they exit the coating nip and deposit them on a conveyor 30 (not shown in Fig. 4). The gripping unit 27, which may be of conventional design, comprises sheet gripping elements 28 carried by an endless chain 29, the movement of which is synchronized with the sheet feed such that a gripping element 28 is positioned in a manner suitable for gripping each sheet as it exits the coating nip. A fan 31, located under the paper path at the exit side of the coating nip, creates an air cushion to support the sheets as they are moved by the sheet gripping elements 28. The blower 31 contains a heater (not shown) which serves to dry the LAB coating on the sheet to some extent to prevent the sheets from sticking to the conveyor. The conveyor 30 is moved at a speed slower than that of the gripper unit chain 29 so that each sheet is placed on the conveyor with the leading edge of the sheet resting on the trailing edge of the previous sheet, thereby forming a pseudo-path of sheets. Typically, the amount of overlap is from about 1 centimeter (cm) to about 2 cm. The conveyor 30 is a vacuum conveyor connected to a vacuum source 32 so that the sheets are positively held on the conveyor and the overlap relationship between them is maintained.

At the output end of the conveyor 30, the sheets pass over an air knife 33 which is arranged to reverse the direction in which the sheets are brought into overlap. The air knife is positioned to deliver a stream of air to the sheets in an orientation which causes the trailing edge of each sheet to be moved from being under the leading edge of the succeeding sheet and instead to be placed on top of that leading sheet. Instead of using an air knife 33 to change the direction of overlap of the sheets, an equivalent mechanical arrangement may be used which may, for example, be similar to that described in GB-A-2-166 177.

The double coating station 3 together with the sheet overlapping station 4 can be designed on the basis of the coating device "GUILA SPEED GS GS 8000" available from Billhöfer Maschinenfabrik GmbH in Nuremberg, Germany.

Referring again to Fig. 1, the pseudo-web of sheets now moves out of the sheet overlap station 4 and into the dryer 5 where the moisture is removed from the primer and LAB coatings. The dryer 5 is preferably a high frequency dryer, e.g. a specially adapted version of model no. SP 890 GF"C"-AG manufactured by Proctor Strayfield Ltd. of Berkshire, England. The dryer 5 is provided with a control unit (not shown) which adjusts the speed of the dryer according to the line speed of the apparatus. The control unit may be, for example, a Siemens PLC 55 95U device connected to the central electronic control unit of the entire apparatus. The overlapping sheets move continuously through the dryer 5 on an endless belt 40 and are dried to reduce the tendency of the sheets to curl, but the drying ensures that the sheets emerge substantially dry. The use of a high frequency dryer is not essential and the overlapping sheets could instead be dried using infrared heating or hot air. Alternatively, the endless belt 40 can be heated to dry the sheets. However, high frequency drying is preferred due to its simplicity and lower energy consumption.

Downstream of the dryer 5, the overlapping sheets move through the guide section 6 where they are laterally brought into register alignment and aligned with each other to prepare for advancement to further processing stations, such as the adhesive coating station 7, which has already been mentioned as the subject of the copending U.S. patent application entitled "Method and Apparatus for Applying a Coating Material to Sheets" filed on the same date and assigned to the assignee of the present invention. As described in the copending application cited above, the overlapping sheets move through a transfer nip 50 (see also FIG. 6) where they contact an endless transfer belt 51 to which an adhesive coating has previously been applied in the form of a plurality of stripes running lengthwise along the belt. The adhesive may be a pressure-sensitive adhesive and is preferably a repositionable microsphere pressure-sensitive adhesive, e.g., as described in U.S. Patent No. 4,495,318 to Howard or U.S. Patent No. 3,691,140 to Silver. Adhesives useful in the present invention also include film-forming materials known in the art, including those containing organic solvents.

At the transfer nip 50, the adhesive is transferred in continuous stripes to the pseudo-web of overlapping sheets, to the surface of which the primer has been applied in the double coating station 3. The adhesive coated webs are then pulled away from the transfer belt 51 on a vacuum roller 52 (shown in Fig. 6) whose interior is connected to a vacuum source. Removal of the sheets from the transfer belt 51 is facilitated by the direction in which the webs overlap each other (i.e. by the fact that the air knife 33 has caused the leading edge of each sheet to underlap the trailing edge of the preceding sheet). Following removal from the transfer belt, the sheets are held on a vacuum belt 53 which runs around the roller 52 and moves the (still overlapping) sheets out of the adhesive coating station 7 to the sheet outlet 8 of the apparatus. The sheets can then be stacked and trimmed to form blocks of repositionable note sheets, such as those available under the trademark "Post-it"® and manufactured by Minnesota Mining and Manufacturing Company ("3M") of St. Paul, Minnesota.

Although the above description refers to sheets made of paper, the sheets (as already mentioned) may also be formed of other materials, e.g., polymer films or metal foils as previously described. When the sheets are made of paper, they are preferably fed through the apparatus such that the "machine direction" of the paper is aligned with the machine processing direction; in this way, the tendency of the paper to warp or wrinkle may be further mitigated.

If desired, papers of different weights and textures can be used. For example, although the above description refers to an A2 sheet size, the device described is capable of handling A4 size sheets. Furthermore, although the above description refers to sheets of 70 gsm, the device can also handle higher weight sheets (e.g. 90 gsm) and lower weight sheets (e.g. 70 gsm and possibly even as low as 45 gsm).

If the sheets fed to the double coating station are pre-printed, the process described above results in the LAB coating material being applied to the print on the sheets. The LAB coating then serves to protect the print, in particular against the print being removed by the adhesive when the sheets are subsequently stacked and cut to form pads of repositionable note sheets. The protection offered by the LAB coating allows consideration of using stronger adhesives on printed note sheets. Of course, the sheets can be coated behind the double coating station 3 by any conventional printing operation. printed, regardless of whether they are pre-printed or not.

Although it is advantageous if the primer and LAB coatings are applied simultaneously in the dual coating station 3 as described above, this is not essential. However, simultaneous application of the coatings is preferred, particularly when the coating materials are water-based, as this helps to maintain the sheets in a flat condition.

In Fig. 7, an apparatus is shown in which the primer and LAB coatings are not applied simultaneously. In this apparatus, the primer coating station 11 is located before the LAB coating station 12 and, consequently, the coating roller 14 of the upper station 11 and the coating roller of the lower station 12 both require a respective counterpressure roller 14A,16A. At each coating station, the arrangement for feeding coating material to the coating roller/drum differs from the arrangement shown in Fig. 1 in that in each case the metering roller 13,17 dips into a respective tank 13A,17A of coating material and applies the coating material to a respective transfer roller which in turn applies the coating material to the coating roller/drum 14,16. The remainder of the apparatus according to Fig. 7 (including the overlap station 4, the dryer 5, the guide section 6 and the adhesive transfer station 7) is effectively designed as described above with reference to Fig. 1.

The present invention has been described above in connection with various embodiments thereof. It will be apparent to those skilled in the art that numerous changes can be made to the described embodiments without departing from the scope of the invention. Thus, the scope of the present invention is not limited to the structures and processes described here, but only by the wording of the claims.

Claims (11)

1. Device for applying a water-based coating material to both sides of several sheet parts, with: (a) a conveyor for sequentially conveying the sheet-like members along a sheet path; (b) a dual coating station positioned to sequentially receive the sheet-like members from the conveyor, the dual coating station having first and second coating stations located on opposite sides of the sheet path passing through the dual coating station, each of the first and second coating stations having a respective source of water-based coating material and a respective coating member positioned to contact the sheet-like members at a point along the sheet path to apply coating material from the respective source to the respective side of each sheet-like member; (c) an overlap conveyor adapted to receive sheet-like parts from the dual coating station and movable at a speed such that the leading edge region of each sheet-like part leaving the dual coating station is deposited on the trailing edge region of the preceding sheet-like part on the conveyor, thereby forming a pseudo-sheet of overlapping sheet-like parts; (d) an air knife positioned to direct a stream of air onto the overlapping blades to adjust their relative positions in the pseudo-leaf so that the trailing edge region of each leaf-like part overlies the leading edge region of the succeeding leaf-like part; and (e) a dryer disposed in the sheet path for removing water from the coating material applied to the sheet members in the double coating station. 2. The device of claim 1, further comprising: (f) a further coating station having a source of water-based coating material, and a coating member positioned to contact the pseudo-sheet of overlapping sheet-like members at a point along the sheet path to apply coating material from the respective source to one side of the pseudo-sheet of overlapping sheet-like members. 3. Apparatus according to claim 1, wherein the coating members in the first and second coating stations of the dual coating station comprise coating rolls arranged in opposed relation to each other on opposite sides of the sheet path to form a coating nip, whereby coating material is applied simultaneously to both sides of a sheet-like member passing through the coating nip. 4. Apparatus according to claim 3, comprising a cover part over a part of the circumference of one of the coating rollers, whereby the coating gap is only formed when that part of the roller is arranged on the other coating roller. 5. Apparatus according to claim 4, wherein said one of said coating rollers is provided with a sheet gripping member for pulling each sheet-like member through the coating gap. 6. Apparatus according to claim 5, with a further sheet gripping element arranged downstream of the double coating station for removing each sheet-like part from the double coating station and depositing the sheet-like parts on the overlap conveyor. 7. Apparatus according to claim 1, comprising a blower positioned to create an air cushion for supporting the sheet-like parts during their removal from the double coating station. 8. Apparatus according to claim 7, wherein the fan is provided with a heating device to partially dry the sheets on the side subsequently deposited on the overlap conveyor. 9. The apparatus of claim 1, wherein the dryer includes a conveyor that is non-reactive to radio frequency radiation and is arranged to convey the overlapping sheet-like member along a portion of the sheet path, and a source of radio frequency radiation directed at the water-based coating material on the sheet-like members. 10. Apparatus according to claim 2, further comprising an alignment station arranged in the sheet path for aligning the overlapping sheet-like parts before they enter the further coating station. 11. Device according to claim 2, wherein the coating part arranged in the further coating station has a transfer belt with a transfer surface which is positioned such that it overlapping leaf-like parts contact each other at a point along the leaf path.