PROCEDURE AND MEANS FOR COATING A PAPER WEB IN AT LEAST TWO STEPS

Abstract: A procedure for coating a paper web in at least two steps, the second step being implemented immediately after the first step so that the coating applied and scraped in the first step does not have time to dry before implementation of the second step.

Description: PROCEDURE AND MEANS FOR COATING A PAPER WEB IN AT LEAST TWO STEPS The present invention concerns a procedure for coating a paper web with an aqueous dispersion in at least two steps, employing in both steps a blade coat¬ ing method, in both steps coating material in excess being deposited on the surface of the paper, the coat¬ ing weight being controlled and the surplus being re- moved by scraping (with a blade) . The invention further concerns a means for coating a paper web in at least two steps, said means comprising a first coating means which has been provid¬ ed with an applicator and a doctor blade and arranged to apply aqueous dispersion on the paper, to adjust the coating weight and to remove the surplus coating mate¬ rial by scraping with the doctor blade against a moving base in the first coating step, and a second coating means which has been provided with an applicator and a doctor blade and arranged to apply aqueous dispersion on the paper, to adjust the coating weight and to remove the surplus coating material by scraping with the doctor blade against a moving base in the second coating step. Multiple-step coating without intermediate drying has previously been used in producing functional special coatings. The coating material is most often a solution or a polymer melt, sometimes also a dispersion (e.g. DE .3 235 151, GB 1.159,208, GB 1,159,598, GB 1,207,133 and US 4,537,801). The purpose with the coat- ing is to influence, for instance, the permeability to gases and aroma, release properties, photosensitivity, etc. It is important in all these instances that the coating forms a layer of uniform thickness, totally covering the surface. For this reason apparatus of the nozzle type is frequently employed to do the coating, with the aid of which on the surface of the paper is deposited a pre-adjusted quantity of the coating mate- rial, in a layer of uniform thickness. The invention concerns the coating of paper or cardboard intended for use as a printing substrate, by a blade, or doctor, coating method, the principal ob- ject being to smooth out the unevenness of the surface. Strict requirements are imposed on printing paper, that is, paper to be used as a printing sub¬ strate. The purpose with the coating is to level out all unevenness of the surface. This implies that the thickness of the coating is variable. The coating is accomplished in that on the surface of the paper is deposited a great excess of coating material and the major part thereof is scraped off. Coating material will then mainly remain in the depressions of the base paper, and the roughness of the paper will decrease. Sufficient smoothness is not achieved with one single coating run in the most exacting applications. It is therefore common practice to use two, or even three, consecutive coating processes, the coating being dried after each individual coating. This implies major space requirements and high apparatus investments. Another problem is associated with the use of small coating material quantities. When the coating colour is applied on the surface of the precoating, water is most rapidly absorbed therefrom into the base. As a result, its dry matter content may increase to such extent that serious difficulties are encountered in the scraping process, such as streak formation for instance. The object of the invention is to eliminate the problems mentioned. In addition, it is an ^object of the invention to provide a means for implementing the procedure and for attaining the aims mentioned in the foregoing. As regards the circumstances characterizing the procedure of the invention, reference is made to Claim 1. The invention is based on the fundamental idea that the second coating step is implemented immediately after the first coating step so that the surface of the coating spread on the paper, and smoothed out by scrap¬ ing, in the first step is wet when the second step coating is applied, that is, the coating has not time to settle before implementation of the second step. This is understood to mean that the second coating step is implemented before the surface of the first step coating has time to dry, i.e., before the water has time to be drained from the surface of the coating, prior to the second coating step. The coating applied on the surface of the paper in the first step is thus wet, and the second coating step takes place in that the coating is applied upon the wet coating. In the procedure constituting the object of the invention, two consecutive blade coatings are implemented, advantageously, with one and the same backing roll, without drying taking place in the mean¬ time. 'In the novel procedure'two coating events are effected with one and the same unit, whereby two sepa¬ rate coating units and an intervening drying process are obviated. In practice, the second coating step is imple- mented, as to location, substantially in conjunction with the first step, i.e., as seen in the direction in which the web travels, immediately after the first step. When a surface coating is applied on a wet substrate, those problems are solved at the same time which are caused by absorption of water into the base prior to scraping. It is thus easy to use small quan¬ tities of coating material, and also a high dry matter concentration of the paste. The first coating step may be implemented using any blade coating method, which implies that application of the coating colour, that is of the dis- persion, may be accomplished using e.g. roll applica¬ tion, nozzle or spraying application. In the first coating step, and similarly po¬ tentially in the second and/or additional coating step, advantageously at least the main part, e-g- more than 50%, of the coating applied on the surface of the paper is-removed by scraping. After the second step one may also spread coating material on the paper in extra steps, e.g. in three, four or several extra steps, using for instance the same quantities as in the first and/or second step. Furthermore, the procedure of the invention may be employed to coat paper on one and/or both sides. In that case either side of the paper may be coated in the manner just described in two or several steps. In accordance with what has been presented above, the coating of paper is carried out substantial¬ ly in a single run coating operation, that is, using two consecutive doctor blades substantially in one uni- tary coating apparatus. The first doctor spreads out the coating colour that has been applied on the surface of the paper and eliminates part of the surface rough¬ ness, said coating colour being advantageously an inex¬ pensive coating colour, with conventional coat weight or e.g. at 2-10 g/m 2 , advantageously 3-8 g/m 2 , depending on the roughness of the base web. In the second step one may in the surface of the paper apply the coating, if desired, in a very thin layer, for instance 0.1-12 g/m 2 , advantageously 0.5-10 g/m* 2 . Thanks to the invention, the coating of paper can be carried out as an economical, tailor-made job, considering the customer's special requirements. The properties of the paper can be regulated within wider limits than before. Thanks to the invention it becomes possible to use coating material of high-standard qual¬ ity, e.g. gloss-enhancing plastic pigments, precipitat¬ ed pigments improving the printer's ink absorption and the gloss, and optical brighteners boosting the brightness, and titanium dioxide, all of them with good economy. The procedure may also be used for surface sizing with which thin pigmentation is combined. Thanks to the small amount, if such is desired, of the coating applied in the second step. i.e., to its low coat weight, the procedure and means of the invention enable expensive special pigments and other raw materials to be used economically, that is in small quantities, In the present application, paper is under¬ stood to mean cellulose or mechanical pulp paper and/or cardboard types manufactured on a paper or cardboard machine. The invention is described in the following in detail with the aid of embodiment examples, referring to the attached drawings, wherein:- Fig. 1 presents the implementation of the procedure of the invention when a certain means according to the invention is being used, Fig. 2 presents the implementation of a procedure ac¬ cording to another embodiment of the invention when an¬ other means according to the invention is being used, Fig. 3 presents the implementation of a procedure ac¬ cording to a third embodiment of the invention when a third means according to the invention is being used, and Fig. 4 presents a fourth embodiment of the invention. In Fig. 1 is seen a means according to the invention, for coating the paper web l in two steps. The means comprises a first coating means 2; in the figure the coating means has been schematically depict¬ ed, in order to illustrate its principle design and function.- The coating means 2 comprises a conventional applicator roll 7, a coating material feeding means 8, and a doctor blade 3. The applicator roll has been dis¬ posed to spread coating material on the paper web moving in the direction of the arrow d , against a moving base, such as a backing roll 4, and the doctor blade 3 levels the coating. The means further comprises a second coating means 5, comprising a spray applicator 9 and, likewise, provided with a doctor blade 6 and arranged to apply coating material on the paper and to level the coating. The second coating means 5 has been placed, in the direction of travel d of the web 1, immediately after the first coating means 2 so that the second coating step is implemented immediately after the first coating step, so that the surface of the coating spread and scraped in the first step is wet, that is, the coating has not time to settle before the second step. The applicators 7,9 and doctor blades 3,6 have been disposed against one and the same backing roll 4. Hereby, the first coating means 2 and the sec¬ ond coating means 5 constitute a functional entity, that is one single unit in which the coating of the paper takes place in two steps, the latter step follow¬ ing immediately after the preceding step. In Fig. 1, the second coating means 5 is based on the spray application mode known in itself in the art, this means comprising a coating material spray applicator 9 for applying coating material on the sur¬ face of the paper, and a doctor blade 6. The applicator may be any kind of device known in the art. The quan¬ tity of the coating material that remains on the sur¬ face of the web is regulated by controlling the pres¬ sure of the doctor blade 6 against the web. The blade 6 may be positioned e.g. at an oblique angle against the web, e.g. 3-80°, such as 5-60°, sloping in the direc¬ tion of travel of the web. The feed 20 of coating colour to the lifting roll applicator 7, and similarly the feed 21 of coating paste to the spray applicator (as well as the design of the applicators, and the design of the respective doctor blades 3,6) is in itself known in the art, and no closer description of them is given in this context. The way in which the surplus pastes 23,24 are returned to the feed of the applicator is likewise known in the art and will not be more closely described. In Fig. 1 in the first coating step, that is in the applicator 2, lifting roll application is em¬ ployed, and in the second coating step, that is in the applicator 5, nozzle or spray application is used. Other alternatives are: lifting roll application in the first step and lifting roll application in the second; nozzle or spray application in the first step and noz¬ zle or spray application in the second; and nozzle or spray application in the first step and lifting roll application in the second step. In Fig. 2 is seen an embodiment of the means of the invention which comprises a first coating means 2 and a second coating means 5. The first coating means 2 comprises a nozzle applicator 10 and therewith con¬ nected a doctor blade 3 (so-called SDTA apparatus) , disposed to apply the coating paste on the web 1. The doctor blade has been arranged to scrape and level the coating, the surplus 23 (more than half, possibly more than 80%) to be conducted back to the coating material circulation. Furthermore in the embodiment here presented the second coating means 5 has been placed immediately after the first coating means, in the direction of travel of the web. The second coating means comprises a spray applicator 11 with feed 21, and a doctor blade 6 with return 24 to the feed 21. The coating means 5 has been arranged to feed the coating of the second step onto the web 1 after the doctor blade 3 of the first coating means 2. Coating of the web takes place on a relative short distance, i.e., the coating means have been placed immediately adjacent to each other, e.g. on a distance of 1-15 cm, such as 1-10 cm, possibly 1-5 cm. In Fig. 3 is depicted an embodiment comprising a first coating means 2, a second coating means 5 and a third coating means 12, with doctor blades 3, 6 and 13, respectively, disposed in this order in the direction of travel of the web 1 , immediately one after the other. The respective applicators are seen in the fig¬ ure: a lifting roll applicator 7, spray applicators 13 and 14, respectively, with feeds 29, 21 and 22, respec¬ tively. In Fig. 3 the coating means 2 corresponds to the first step applicator 2 in Fig. 1, and the appliσa- tors 5 and 12 correspond mainly to the coating means 5 in Fig. 2. The coating rate can be regulated in each step by controlling the pressure of the respective blades against the web. The surplus coating materials 23, 24 and 25 are returned to the respective feeds of the applicators. In Fig. 4 is seen an embodiment of the proce¬ dure and means of the invention in which the paper web 1 is coated on both sides, at first against the backing roll 5 with the aid of a first coating means 2, a sec- ond coating means 5 and a third coating means 12, all of the types shown in Fig. 2, and thereafter against the backing roll 4 s *, with the aid of a first coating means 2 2 , a second coating means 5 2 and a third coating means 12=, all likewise of the types shown in Fig. 2. The coating means 2, 5 and 12 then constitute a unitary entity for coating the web on one side in three consec¬ utive steps and as taught by the invention. The coating means 2 s *, 5 2 and 12= likewise constitute a unitary entity for coating the web on the opposite side in three consecutive steps and as taught by the invention. Example 1 In the test, the means for coating a paper web in two steps, shown in Fig. 2 was used. In the first step, precoat was applied on the surface of the web with the aid of the coating means 2 and in the second step, face coating with the aid of the coating means 5. The following characteristics of the coated paper were determined: basis weight, density, and gloss. The results are stated in Table 1. Table 1 Example 2 In the test, the means of Fig. 2 was used to coat LVC base paper; the base paper's basis weight was about 43 g/m 2 , density about 618 kg/m 3 , roughness about 360 ml/min., opacity about 87%. In the first step (l) , precoat was applied on the surface of the web with the aid of the coating means and in the second step (2) , face coating with the aid of the second coating means. The coating material used in step (1) was 100% carbo¬ nate; the coating material used in step (2) contained 10% plastic pigment (Rhodopas) and 90% SPS kaolin. The following characteristics of the coated paper were de¬ termined: basis weight, density, roughness and gloss. The doctor blades 1 and 2 were pressed against the web, steps I and II. The results are stated in Table 2. Table 2 C oat weight Basis Density Roughness Gloss Coat. Coat. weight Example 3 In the test, coating of paper was carried out in two steps, similarly as in Example 2. In step (1) , 1 0 0% carbonate was applied on the web and in step (2) , coating material containing 30% plastic pigment and 70% SPS kaolin.- The results are stated in Table 3. Table 3 C oat weight Basis Density Roughness Gloss Coat. Coat, weight 1 g/m 2 2 g/m 2 g/m 2 kg/m 3 6.75 2.25 52.0 1002 6.75 4.0 53.9 1005 6.75 7.25 56.8 1057 6.75 8.75 58.1 1074 11.25 3.75 58.1 1060 11.25 2.25 56.4 1019 ll 25 53.6 992 11.25 51.5 965 Example 4 In the test, coating of paper was carried out in two steps as in Example 2. In step (1) , 100% carbo¬ nate was applied on the web and in step (2) , coating material containing 50% plastic pigment and 50% SPS kaolin. The results are stated in Table 4. Table 4 Coat weight Basis Density Roughness Gloss Coat. Coat, weight 1 g/m 2 2 g/m 2 g/m 2 kg/m*- ml/min 75 . % 21 58.9 19 61.7 14 75, 15 77, 14 73, 17 68, 28 55, Example 5 In the test, coating of paper was carried out in two steps as in Example 2. In steps (l) and (2), coating material containing 100% SPS kaolin was applied on the web. The results are stated in Table 5. Table 5 Coat weight Basis Density Roughness Gloss Coat. Coat, weight 1 g/m 2 2 g/m 2 g/m 2 kg/m : ml/min 75 / o 10.5 2.0 56.4 10.5 1.0 54.1 10.5 - 52.1 Example 6 In the test, coating of paper was carried out in two steps as in Example 2. In step (l) , coating mate¬ rial containing 10% starch was applied on the web and in step (2) , coating material containing 100% SPS kaolin. The results are stated in Table 6. Table 6 Coat weight Basis Density Roughness Gloss Coat. Coat, weight 1 g/m 2 2 g/m 2 g/m 2 kg/m 3 ml/min 75",% The embodiment examples are meant merely to illustrate the invention, and embodiments of the inven¬ tion may vary within the scope of the claims following below.

Claims: CLAIMS 1. A procedure for coating a paper web with aqueous dispersion in at least two steps, employing in both steps a blade coating method, in both steps coat- ing material being deposited in excess on the surface of the paper, the coating weight being controlled and the surplus being removed by scraping, c h a r a c t¬ e r i z e d in that the second step is implemented im¬ mediately after the first step so that the surface of the coating applied and scraped in the first step is wet when the second step coating is applied. 2. Procedure according to claim 1, c h a r¬ a c t e r i z e d in that the second step is implemen¬ ted, as to location, substantially in conjunction with the first step, and in both steps the scraping is ef¬ fected against the same backing roll. 3. Procedure according to claim 1 or 2, c h a r a c t e r i z e d in that in the first step coating material is applied on the surface of the paper about 2-10 ' g/m 2 , advantageously 3-8 g/m 2 , and in the second step 0.1-12 g/m 2 , advantageously 0.5-10 g/m 2 . 4. Procedure according to any one of claims 1-3, c h a r a c t e r i z e d in that in the first step is used an application method selected from the group: roll application, nozzle application, and spray application. 5. Procedure according to any one of claims 1-4, c h a r a c t e r i z e d in that in the second step is used an application method selected from the group: roll application, nozzle application, and spray application. 6. Procedure according to any one of claims 1-5, c h a r a c t e r i z e d in that after the second coating step coating material is applied on the surface of the paper in at least one additional step, said coating is levelled by scraping and said additional step is implemented immediately after the preceding scraped in the preceding step is wet when the coating of the additional step is being applied. 7. Procedure according to any one of claims 1-6, c h a r a c t e r i z e d in that the paper is coated on both sides by a procedure according to any one of claims 1-6. 8. A means for coating a paper web (1) in at least two steps, comprising a first coating means (2) which has been provided with an applicator (7) and a doctor blade (3) and arranged to apply aqueous disper¬ sion on the paper, to control the coating rate and to remove the surplus coating material (23) by scraping with the doctor blade against a moving base (4) in the first coating step, and a second coating means (5) which has been provided with an applicator (9) and a doctor blade (6) and arranged to apply aqueous disper¬ sion on the paper, to control the coating rate and to remove the surplus coating material (24) by scraping with the doctor blade against a moving base (4) in the second coating step, c h a r a c t e r i z e d in that the second coating means (5) is placed, in the direction of travel (d) of the web (1) , immediately after the first coating means (2) so that the second coating step can be implemented immediately after the first coating step so that the surface of the coating applied and scraped in the first step is wet when the second step coating is applied. 9. Means according to claim 8, c h a r a c t¬ e r i z e d in that the first coating means (2) is placed in conjunction with . a backing roll (4) so that the doctor blade (3) has been arranged to be pressed against the web (1) and the backing roll, and that the second coating means (5) is placed in conjunction with the first coating means (2) so that the doctor blade (6) has been arranged to be pressed against the web and the same backing roll. 10. Means according to claim 8 or 9, c h a r- a c t e r i z e d in that the first applicator (7) is selected from the group: roll applicator (7) , nozzle applicator (11) , and spray applicator (9) . 11. Means according to any one of claims 8-10, c h a r a c t e r i z e d in that the first applicator (7) is selected from the group: roll applicator (7) , nozzle applicator (11) , and spray applicator (9) . 12. Means according to any one of claims 8-11, c h a r a c t e r i z e d in that the means comprises at least one additional coating means (12) which has been provided with an applicator (14) and a doctor blade (13) and placed, in the direction of travel of the web (1) , immediately after the preceding coating means so that the additional coating step can be implemented immediately after the preceding coating step so that the surface of the coating applied and scraped in the preceding step is wet when the coating of the additional step is being applied. 13. Means according to any one of claims 8-11, c h a ra c t e r i z e d in that on both sides of the paper web (1) has been placed at least one coating means (2,5; 2 2 ,5 2 ; 12,12 2 ) according to any one of claims 9- 12.

Inventors: NIEMI MARKKU, NIEMI, MARKKU
Applicants: KESKUSLABORATORIO, OY KESKUSLABORATORIO CENTRALLABORATORIUM AB
Application year: 1989
Main classification: B05C11
IPC codes: B05C1, B05C5, B05C9, B05C11, B05D1, B05D3, B05D7, D21H23