FI121433B - Process for the production of coated paper or board - Google Patents

Description

Process for the production of coated paper or board

The present invention relates to a process for the production of coated paper or board which is pre-calendered before coating and dried after coating.

There are a number of different types of paper and board, and they can be divided into two grades by basis weight: single-ply paper with a basis weight of 25-300 g / m2 and board made with 10 multi-layered techniques and 150-600 g / m2. As noted, the boundary between paper and board is sliding, since the lightest board is lighter than the heaviest paper. Usually paper is used for printing and cardboard for packaging. The paper and board may be coated or uncoated.

15

The following descriptions are examples of currently used values for coated fibrous webs and may exhibit considerable variations in the values given. The descriptions are mainly based on Papermaking Science and Technology, part Papermaking Part 3, Finishing, 20 eds. Jokio, M., published by Fapet Oy, Jyväskylä 1999, 361 pages.

LWC = 40-60% mechanical pulp, 25-40% bleached softwood pulp and 20-35% fillers and coatings. Generic values for LWC paper can be as follows: basis weight 40-70 g / m2, Hunter gloss 50-65%, PPS S10 roughness 0.8-1.5 pm (offset) and 0.6-1.0 pm (roto), density 1100-1250 kg / m3, brightness 70-75% and opacity 89-94%.

Generic values for machine finished coated MFC paper can be as follows: 30 basis weight 50 - 70 g / m2, Hunter gloss 25 - 70%, PPS S10 roughness 2 2.2 - 2.8 μηι, density 900 - 950 kg / m3 , brightness 70-75% and opacity 91-95%.

Generic values for FCO (film coated offset) paper are: 5 basis weight 40-70 g / m2, Hunter gloss 45-55%, PPS S10 roughness 1.5-2.0 pm, density 1000-1050 kg / m3, lightness 70 75% and opacity 91-95%.

Common values for MWC (medium weight coated) paper are: 70-90 g / m2 basis weight, Hunter gloss 65-75%, PPS S10 roughness 10 0.6-1.0 pm, density 1150-1250 kg / m3, lightness 70 75% and opacity 89-94%.

The HWC (heavy weight coated) has a basis weight of 100-135 g / m2 and can be coated more than twice.

15 Coated pulp base wood-free printing papers, or WFCs, have varying levels of coating depending on requirements and applications. The following are typical values for single and double coated pulp-based printing paper: once coated ne-20 with a specific gravity of 90 g / m 2, Hunter gloss 65-80%, PPS slO roughness 0.75-2.2 pm, brightness 80-88% and opacity 91-94 % and double-coated 130 g / m 2, Hunter gloss 70-80%, PPS S10 roughness 0.65-0.95 µm, brightness 83-90% and opacity 95-97%. 1 2 3 4 5 6

Cartons form a rather heterogeneous group, including high-density species up to 500 g / m 2 and low-weight species having a basis weight of about 120 g / m 2, ranging from primary fiber to up to 100% recycled fiber based and coated. 5 multi-coated. Coated board is as follows: 6 - Primary fiber based (FBB = folding boxboard), SBS (solid bleached board), Liquid packing board (LPB), coated white top liner, carrier board - based on recycled fiber (white lined chipboard (WLC), coated recycled paperboard).

5 The structure of the coating and the printing properties of the coated fibrous web can be affected by the drying conditions. Fluctuations in the amount of coating due to the unevenness of the bottom of the coated fiber web cause problems with the drying of the coating. Too much drying power in the paste solidification zone 10 causes, for example, mottling in the print. The situation is particularly emphasized with paperboard, where coating is typically done at blade stations. When the overlay coating is designed with a blade, the variation in coating quantity may be several grams per square meter. The pre-coating may not completely cover the base board, which may result in absorption differences between the top coat. An attempt is made to control the drying of the coating 15 in a so-called. a "quality fluid", i.e. a fluidized bed dryer with a limited evaporation capacity (kg / m2 / h) to provide a controlled coating structure. The evaporation efficiency is limited by limiting the blowing speed and / or temperature. Excessive evaporation efficiencies result in uneven absorption of water-soluble binders and hence distribution in the coating layer and thus uneven absorption of 20 inks in the printed product. This is reflected by the spotty distribution of the ink. The success of the drying depends on how well the different amounts of coatings hit the "melting point" at the freezing point. In addition to the quality fluid, two other driers are typically required for drying, typically one before and one after "quality fluid", in some cases up to three other driers. Many dryers extend the coating station and raise the price.

The drying capacity calculations for the coating processes typically do not exceed the evaporation capacity by more than 40 kg / m2 / h in the solidification range of the coating paste. The hardening point 30 is attempted to hit a fluidized bed dryer with a maximum running value of 250 ° C and 40 m / s.

4

Figure 1 schematically illustrates a prior art coating apparatus arrangement. The arrangement is acting as a precalender kovanippika-calender 1, followed by a precoating head 2, with the first side 5 of a precoating head 2a, followed by infra red dry in 6a, with limited vaporization air dryer "quality air dryer" 7a and the air dryer 8a, from which the web is transferred to another aspect of precoating Ile 2b, followed by an infrared dryer 6b, a fluidized bed "quality float" 7b having a limited evaporation capacity, and a fluidized bed dryer 8b. The coating stations 2a, 2b are substantially blade-like. After the pre-coating station n 2, the fibrous web 10 is passed through the drying cylinders 20 to the actual coating station 3, which includes similar blade coating stations 3a, 3b and drying devices 9a, 10a, 11a; 9b, 10b, 11b as in the pre-coating step. The fibrous web is then passed through a second set of drying cylinders 21 to a soft calender 4 which acts as a final calender and from there through a drying rollers 22 to a reel 15 15.

It is an object of the present invention to provide a solution whereby the space requirement of a device for producing a coated fibrous web can be substantially reduced and thus cost savings can be achieved.

To achieve this object, the method according to the invention is characterized in that the method uses a precalender and / or an intermediate calender selected from the group consisting of a metal belt calender and a long nip calender; that the process comprises effective drying at an evaporation rate of more than 40 kg / m 2 / h after said at least one coating step; the pre-calendering is performed such that the surface to be coated has a Bendtsen roughness (SCAN-P21: 67) in the range of 50 to 1000 ml / min, preferably in the range of 100 to 300 ml / min; and that the residence time of the fibrous web in the calender nip or region of the precalender is greater than 10 ms.

30 5

The invention will now be described in more detail with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a prior art coating apparatus arrangement, Figure 2 shows experimental results of printing mottling values using the method of the invention compared to the prior art.

In order to carry out the process according to the invention, the following changes to the prior art coating device arrangement shown in Figure 1 are made: hard nip calender 1 is replaced by metal belt or longip piezo calender, The other half of the precoating head 2b of the dryers 6b and 7b are replaced by corresponding air dryers and dryer 8b is left out. Corresponding changes are made to the actual coating station 3, whereby in this 20 examples a total of four dryers 8a, 8b, 11a and 11b, marked with a cross in Figure 1, can be reduced.

Table 1 shows an exemplary calculation of drying using the prior art method and the method of the invention, wherein the final moisture in each case is substantially in the same order.

6

table 1

PRIOR ART The process of the invention

Travel speed 800 m / min 800 m / min 5 Base weight 260 g / m2 250 g / m2

Input Humidity for Station 7% 7% Coating Amount and KAP 12 g / m2 / 63% 12 g / m * / 63% \ Dryer Gas Infrared 3 Rows 100% PDPus 450 ° C 160 m / s 2nd Dryer Length 2.7 m 250 ° C / 40 ~ m / s 3rd Dryer Length 2.7 m 350 eC / 55 Length 2.7 m 350 eC / 55 m / sm / s

Cylinders 1 bar to 1 bar

Final Humidity 6.65% 6.75%

Evaporation at 25 kg / m2 / h at 90 kg / m2 / h at the pour point 15

The method according to the invention allows one or more dryers to be omitted after the blade coating station, which results in considerable cost savings. The overall savings are achieved by eliminating dehumidifiers and shortening equipment, which also reduces the need for hall space 20, which results in significant additional savings.

Figure 2 illustrates how the pre-calendering method and the drying conditions of the surface coating affect the mottling values of the printed surface. Figure 2 shows that when metal belt pre-calendering is used, the mottling values do not change substantially even though the infrared drying is replaced by efficient fluidized drying. By contrast, when using a Yankee cylinder and a hard nip calender for pretreatment of the bottom web, the mottling values are clearly elevated as the transition from infra crab drying to efficient fluidized bed drying. 1

In the method of the invention, the pre-calendering is performed with a long nip calender or a metal belt calender such that the fiber web has a residence time in the nip or calendering area of more than 10 ms and the fiber web has a Bendtsen roughness of 50-1000 ml / min. Preferably, the coating is carried out as a blade coating, e.g. 1-3 coats per side.

5

In prior art solutions, the power of the first dryer following the coating station should not be so high that the freezing point of the coating paste would hit it. Excessive evaporation efficiencies result in uneven absorption of water-soluble binders and hence distribution in the overcoat layer 10 and thus uneven absorption of ink in the printed product. Therefore, in the prior art, the pour point is attempted to hit a quality fluid having a maximum running value of 250 ° C and 40 m / s.

In the method of the invention, the pre-calendered base is made smoother by using a long nip calender or a metal belt pre-calender, whereby the coating provided by the blade coating is more uniform, allowing more efficient drying, whereby the first dryer can be freely dimensioned In the solution according to the invention, effective drying is carried out at an evaporation rate of more than 40 kg / m 2 / h at a freezing point, preferably at an evaporation rate of more than 60 kg / m 2 / h and can preferably be greater than 90 kg / m 2 / h. Such an effective dryer may be located after one or more coating stations or e.g. after each coating station. In addition, the method may use a long nip calender or a metal strap calender as an intermediate calender between the top stations.

Claims (5)

A process for the production of coated paper or board comprising the step of introducing the fibrous web to be coated into a pre-calendering apparatus prior to coating, and the process comprising coating the paper or board in one or more steps and drying after at least one coating step. an intermediate calender selected from the group consisting of a metal belt calender and a longip quick calender; that the process comprises effective drying at an evaporation rate of 10 over 40 kg / m 2 / h after said at least one coating step; wherein the pre-calendering is performed such that the Bendtsen roughness (SCAN-P21: 67) of the surface to be coated is in the range of 50 to 1000 ml / min, preferably in the range of 100 to 300 ml / min; and that the residence time of the fibrous web in the calender nip or region of the precalender is greater than 10 ms. 15 Process according to Claim 1, characterized in that the drying step after said at least one coating step has an evaporation efficiency of more than 60 kg / m2 / h. A process according to claim 1, characterized in that said drying step after the at least one coating step has an evaporation efficiency of more than 90 kg / m2 / h. Process according to one of Claims 1 to 3, characterized in that said effective drying is carried out in a multi-stage coating process after two or more coating stages. Method according to one of Claims 1 to 3, characterized in that said effective drying is carried out in the case of a multi-stage coating after each coating step. 6. A method according to any one of the preceding claims, characterized in that blade coating is used in the coating. 5