PROCESS FOR TWO-FACED COATING OF A TRAVELLING WEB

Abstract: A process for two-faced coating of a travelling web (7) by means of two juxtaposed transfer rollers (3, 5) while depositing a coating agent on both sides of the web (7), the transfer rollers (3, 5) being imparted mutually different peripheral speeds, at least one of said speeds deviating from the speed of the web (7), both transfer rollers (3, 5) and web (7) moving in the same direction.

Description: PROCESS FOR TWO-FACED COATING OF A TRAVELLING WEB The present invention relates to a process for two- faced coating of a travelling web, for example a paper web, while using two juxtaposed transfer roller between which the web travels with deposition of coating agent on both surfaces of the web. General techniques in coating and sizing of paper consists in metering the coating liquid in a roller nip formed between two rollers which are in engagement against each other. In the so called size press technique the coa¬ ting liquid is supplied directly into the nip, and this technique is commonly used mainly in the so called surface sizing of paper. According to other known variants of roller coaters the coating agent is transferred indirectly to the paper by being first metered onto a press roller, so called transfer roller. The pre-metered liquid film is then transferred to the paper web without the formation of excess of coating agent in the press nip. An example of this technique is the so called gate roller coater which comprises two juxtaposed pressure rollers between which the web travels for levelling out the coating agent. These two press rollers are flanked on either side of roller pairs by means of which coating agent is transferred onto the mantel surfaces of press rollers and from said sur¬ faces to the paper web. Other examples for metering coating liquid onto a roller are coaters using metering by means of blades or by means of rotary bars. However, the above described known techniques are subject to certain draw-backs, among which there may be mentioned uneven dosage, the formation of orange peel pattern in view of the so called film-split effect etc. Therefore, there is a need for new improved technique concerning the coating of travelling webs, particularly paper webs. The present invention therefore has for an object to provide an improved process for two-faced coating of a travelling web while using two juxtaposed transfer rollers between which the web travels with deposition of coating agent on both sides of the web. The invention is based on the principle that the transfer rollers are imparted mutu¬ ally different peripheral speeds, at least one of said pe¬ ripheral speeds deviating from the the speed by which the web is fed. In addition, the process according to the in- vention is characterized by the fact that both transfer rollers in connection with the position of engagement with the web move in the same direction and in the same direc¬ tion as the web. In a preferred embodiment of the process according to the invention both peripheral speeds of the transfer rol¬ lers deviate from the speed of the travelling web, and it is particularly preferred that the peripheral speed of one of the transfer rollers exceeds the speed of the web, whereas the peripheral speed of the other transfer roller is lower than the speed of the web. In this connection it is preferred that the peripheral speeds of the transfer rollers deviate from the speed of the web at about the same degree. Even if the degree of deviation from the speed of the web is not particularly critical it is preferred that it does not exceed about 5%, and a preferred range concerning deviation of peripheral speed is from about 0,5 to about 2,5%. In order to obtain optimum result while avoiding the draw-backs of the prior art the process according to the invention operates with a roller pressure in the nip be¬ tween the juxtaposed transfer rollers lower than that which is commonly used, and it is preferred that the transfer rollers are set at an engagement pressure in the nip of up to about 10 N/mm. It is also preferred to use rollers which are made of a soft material, for example rubber, and to use transfer rollers having a surface hardness of at most about 60 Shore A. For the purpose to obtain a certain feeding effect for the web it is suitable to bring same to an extended engagement against the transfer roller the peripheral speed of which exceeds that of the web. In summary it is particularly preferred that the transfer rollers are set at an engagement pressure in the nip of from about 1 to about 10 N/mm and that transfer rollers are used having a surface hardness of at most about 60 Shore A. The technique according to the present invention is particularly suitable for the coating of travelling paper webs. The invention will in the following be further de¬ scribed by a non-limiting example in conjunction with the appended drawing which in a side view diagramatically shows a coating device for applying process according to the invention. The coating device illustrated in the drawing and generally indicated 1 comprises two transfer rollers 3 and 5, both rotating in the direction indicated by arrows a). Between these two transfer rollers 3,5 a paper web 7 is fed in the direction indicated by arrow b). Transfer rollers 3,5 are supplied with coating composition by means of applicators 13 and 15, respectively, and excess of coating composition 21,23 is returned to containers 17,19. Metering rollers 9,11 are rotated in a direction indicated by arrows c) and they have a speed which is significantly lower than the rotational speed of transfer rollers 3,5. As is clear from the drawing paper web 7 is fed in such a manner as to come into engagement with the mantel surface of the upper roller over a sector segment cor¬ responding to about 30° of the circumference. The engage¬ ment against the lower roller is theoretically along a line but in view of the softness of the rollers somewhat larger than the theoretic engagement. In the described example there is used a relatively soft rubber in the material forming the surface of trans- fer rollers 3,5 and the hardness is about 50 Shore A. The nip pressure between the rollers is about 5N/mm. Further¬ more, the rollers are driven with peripheral speeds devi¬ ating from the speed of the web in that transfer roller 3 has a peripheral speed exceeding that of paper web 7 by about 1,5%, whereas the peripheral speed of transfer rol¬ ler 5 is somewhat lower than that of paper web 7 by about 1,5. In view of the extended engagement against transfer roller 3 a certain feeding effect of the paper web 7 will be obtained, whereas the differences in speed given advan- tageous levelling effect on both sides of paper web 7. The surprising improvement of the coating obtained by the process according to the present invention is above all related to the so called film-split effect which cau¬ ses orange peel pattern in the layer applied. The low en- gagement pressure enables differences in the peripheral speeds of transfer rollers 3,5, on the one hand in rela¬ tion to each other, on the other hand in relation to the travelling speed of paper web 7.

Claims: CLAIMS 1. A process for two-faced coating of a travelling web by means of two juxtaposed transfer rollers while de¬ positing a coating agent on both sides of the web, charac¬ terized in that the transfer rollers are imparted mutually different peripheral speeds, and that at least one of said speeds deviates from the web speed, both transfer rollers and web moving in the same direction. 2. A process according to claim 1, characterized in that both peripheral speeds of the transfer rollers devi¬ ate from the speed of the travelling web. 3. A process according to claim 2, characterized in that the peripheral speed of one of the transfer rollers exceeds that of the web, whereas the peripheral speed of the other transfer roller is lower than that of the web. 4. A process according to claim 3, characterized in that both peripheral speeds deviate from the speed of the web at about the same degree. 5. A process according to any of claims 2 to 4, cha¬ racterized in that the deviation of the peripheral speeds from the speed of the web is up to about 5%. 6. A process according to claim 5, characterized in that said deviation lies within the range about 0.5 to about 2.5%. 7. A process according to any preceding claim, cha¬ racterized in that the transfer rollers are subjected to an engagement pressure in the nip of up to about 10 N/mm. 8. A process according to any preceding claim, cha¬ racterized in that the transfer rollers have a surface hardness of up to about 60 Shore A. 9. A process according to any of claim 3 to 8, cha- racterized in that the web is brought to an extended engagement against the transfer roller the peripheral speed of which exceeds that of the web, whereby a certain feeding effect will be obtain. 10. A process according to claim 3, 4 or 5, charac- terzied in that the transfer rollers are set at an en¬ gagement pressure in the nip of from about 1 to about 10 N/mm, and that the transfer rollers have a surface hard¬ ness of not more than about 100 P&J. 11. A process according to any preceding claim, cha¬ racterized in that it is used for two-sided coating of a travelling paper web.

Inventors: ERIKSSON TORE, ERIKSSON, TORE
Applicants: BTG KAELLE INVENTING AB
Application year: 1991
Main classification: B05C1
IPC codes: B05C1, B05C9, B05D1, B05D7, D21H23