WO2009127775A1

WO2009127775A1 - Method and assembly for controlling the curling of a paper/board web for a heat sensitive product

Info

Publication number: WO2009127775A1
Application number: PCT/FI2009/050074
Authority: WO
Inventors: Jouni Haavisto; Simo Rahkonen
Original assignee: Metso Paper, Inc.
Priority date: 2008-04-17
Filing date: 2009-01-30
Publication date: 2009-10-22
Also published as: AT11922U1; DE202008005359U1; JP3168446U

Abstract

The invention relates to a method (1) for controlling the curling of a paper/board web (W) for a heat sensitive product (2), said method comprising the use of an applicator device (5) for applying at least one coating agent or the like substance, containing one or more temperature sensitive components, onto at least one side (12) of the web (W), and said method further using equipment (100) for controlling the web (W) in terms of its curling, including first devices (10) for bringing at least one appropriate liquid agent to the web (W) onto its side (11) opposite to the side (12) to be applied and second devices (80) disposed in the web's (W) running direction downstream of the first devices (10) for drying the web (W). In the method, said first devices (10) and second devices (80) for controlling the web (W) in terms of its curling are disposed in the web's (W) running direction upstream of said application (5).

Description

Method and assembly for controlling the curling of a paper/board web for a heat sensitive product

The invention relates to a method for controlling the curling of a paper/board web for a heat sensitive product, said method comprising the use of an applicator device for applying at least one coating agent or the like substance, containing one or more temperature sensitive components, onto at least one side of the web, and said method further comprising the use of equipment for controlling the web in terms of its curling, including first devices for bringing at least one appropriate liquid agent to the web onto its opposite side and second devices disposed in the web's running direction downstream of the first devices for drying the web. The invention relates also to an assembly capable of implementing the method.

Temperature-sensitive products, especially thermal papers, are used e.g. in stickers, receipts, tickets, labels and the like items. The product consists of either one or most often several layers existing e.g. in a web of paper or board, one or more of said layers being provided with components which are highly responsive to temperature. When temperature rises to a sufficient height, these components produce chemical reactions and, as a result, the product develops marks optically detectable on its surface or alterations in the color or darkness thereof.

A typical way of providing layers containing such components is to apply onto the surface of a fiber web a coating layer or coating layers with such an appropriate temperature-responsive composition, i.e. to perform so-called thermocoating. Like- wise, adhesive layers or the like can be applied onto a web surface. This moist layer, diluted with an appropriate volatile liquid, for example water, must be dried prior to reeling the fiber web with a winder or the like or before finishing.

Paper and board grades come in a multitude of varieties and can be categorized in terms of grammage for two classes: papers with a single ply and a grammage of 25-300 g/m² and boards made in multiply technology and with a grammage of 100- 600 g/m². As noted, the dividing line between paper and board is fluctuating as the lowest-grammage boards are lighter than the heaviest papers. As a rule, paper is used for printing and board for packaging. Papers and boards can be coated or un- coated. The subsequent descriptions are examples of currently valid values for coatable fiber webs and may include considerable discrepancies from given values. The descriptions are principally based on the source publication Papermaking Science and Technology, volume Papermaking Part 3, Finishing, edited by Jokio, M., published by Fapet Oy, Jyvaskyla 1999, 361 pages.

Coated magazine paper (LWC=Light weight coated) contains mechanical pulp 40- 60%, bleached softwood pulp 25-40%, and fillers and coaters 20-35%. General values for LWC paper can be regarded as follows: grammage 40-70 g/m², Hunter gloss 50-65%, PPS SlO roughness 0.8-1.5 μm (offset) and 0.6-1.0 μm (roto), density 1100-1250 kg/m³, brightness 70-75%, and opacity 89-94%.

General values for MFC paper (machine finished coated) can be regarded as follows: grammage 50-70 g/m², Hunter gloss 25-70%, PPS SlO roughness 2.2-2.8 μm, den- sity 900-950 kg/m³, brightness 70-75%, and opacity 91-95%.

General values for FCO paper (film coated offset) can be regarded as follows: grammage 40-70 g/m², Hunter gloss 45-55%, PPS SlO roughness 1.5-2.0 μm, density 1000-1050 kg/m³, brightness 70-75%, and opacity 91-95%.

General values for MWC paper (medium weight coated) can be regarded as follows: grammage 70-90 g/m², Hunter gloss 65-75%, PPS SlO roughness 0.6-1.0 μm, density 1150-1250 kg/m³, brightness 70-75%, and opacity 89-94%.

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

In coated chemical pulp-based woodfree printing papers or fine grade papers (WFC), the amounts of coating fluctuate a great deal according to requirements and intended use. The following are typical values for once- and twice-coated chemical pulp-based printing paper: once-coated grammage 90 g/m², Hunter gloss 65-80%, PPS SlO roughness 0.75-2.2 μm, brightness 80-88%, and opacity 91-94%, and for twice-coated grammage 130 g/m², Hunter gloss 70-80%, PPS SlO roughness 0.65- 0.95 μm, brightness 83-90%, and opacity 95-97%. Boards make up quite a hetero- geneous group, including high grammage grades with a grammage possibly as high as 500 g/m² and low grammage grades with a grammage around 120 g/m², the grades ranging from those based on primary fiber to those based on fiber recycled by up to 100% and from uncoated to multi-coated grades. Coated boards include the following: - primary fiber-based (folding board (FBB=folding boxboard), bleached chemical pulp board (SBS=solid bleached board), LPB=liquid packaging board, coated white top liner, carrier board - recycled fiber-based (WLC=white lined chipboard, coated recyclable board).

Heat sensitive products can be manufactured basically for all above-mentioned paper and board grades. In the process of coating a web with coating agents, adhe- sives, pretreating agents, as well as with other substantially liquid treating agents, one detrimental occurring effect is a curling tendency of the web. In an effort to compensate for this effect, a prior known approach has been such that, after the application of treating agents and the web drying subsequent thereto, the opposite web side has been subjected to a further separate treatment. This has been performed by moistening with water or by similar application of a treating agent and by subsequent drying of a web, whereby the web curl has been neutralized to a desired extent by regulating these counter-measures. Thus, the objective is to expose the opposite web side to a compensatory curling effect of the matching order.

The after-treatment conducted in an effort to compensate for curling naturally requires that a web be subsequently dried again for clearing the web of volatile components applied to the web surface during the after-treatment process and for re- winding the web or conveying it to finishing. When the above-described coating process is conducted on a heat sensitive product as described above, it is further necessary to monitor vigilantly throughout the process that the web temperature does not inadvertently rise at any time over a maximum value set therefor. Otherwise, the product is likely to sustain irreparable damage. An important aspect in the process of manufacturing heat sensitive products is to control the temperature of a product to be dried, especially during the course of drying.

Because the application of water or other treating agent used for the curl control of a web is carried out on a web coated with a heat sensitive product, it is necessary that drying the same from the web be conducted by observing the same tempera- ture limitations as those employed in the thermal drying of the coating itself. Typically, these temperature limitations lie within the range of +55-6O⁰C. A result of such a limited temperature is that the drying performance remains low and requires plenty of time. At the same time, this implies limited running speeds as well as a large-scale dimensioning of the dryer equipment for providing a sufficiently long dwell time and running course for the web.

In addition, the water-heated cylinders used for drying have other shortfalls, e.g. when compared to steam cylinders. Among other things, there is always a risk in water-heated cylinders with regard to attaining a consistent CD-profile, because of a rather awkward condensate removal. There is no such risk in a steam cylinder. In a water-heated cylinder, the liquid water layer exceeds in terms of its thickness the amount of condensed water in a steam-heated cylinder. Because the condensate/water removal relies typically on siphons present at the ends, there is typically a water layer of more thickness in the middle of a cylinder. The distribution of heating water evenly over the entire cylinder jacket is also more difficult than that of steam.

An objective of the present invention is to eliminate the foregoing problems. It en- ables alleviating restrictions imposed on the operation of an entire line and its running speed by actions aimed at the compensation of curling, as well as to eliminate temperature-related risk factors regarding the product quality.

This objective is achieved by a method of the invention, which is characterized by what is set forth in the characterizing clause of the first claim. The features characteristic of an assembly applying the method according to the invention are in turn set forth in the characterizing clause of claim 5. A few preferred embodiments of the invention are additionally presented in dependent claims.

A central idea in the solution of the invention is the disposition of equipment for performing the actions applied in an effort to compensate for curling to lie in the running direction of a web upstream of a unit used for applying heat sensitive components. Above all, the invention enables omitting a temperature limitation from the drying process following the curl control. By reshuffling, according to the invention, the process working sequence, the drying can be intensified considerably with re- spect to its current performance while avoiding the risks of damaging the product. At the same time, the entire process becomes considerably simpler and the size of equipment more compact.

The invention will now be described in more detail with reference to the accompanying drawing, in which

Fig. 1 shows an assembly of the prior art for coating a web, as well as a curl control downstream thereof,

Fig. 2 shows an assembly of the invention for controlling the curling of a web.

In an assembly 1 of fig. 1, there is adapted to occur, according to the prior art, the coating of a web W, as well as the control of curling. The web W is delivered from an unwinder 15 and conveyed over guide rolls 3 to a coating or treating station 5, wherein at least one side 12 of the web is subjected to the application of a desired treating agent or coating. It should be noted that the procedure here does not necessarily have to be literally a coating process, or at least a coating process alone, but, instead, the question can be about some other application of an agent with an effect on the web's W properties on the web surface or about delivering it deeper in its layers. For the sake of simplicity, reference is hereinafter made to just coating.

The supply of agents onto a web can be performed by means of any prior art application method, such as for example blade coating,, film transfer coating, or curtain coating. The question is about heat sensitive products, i.e. at least one of the agents applied at the coating stage 5 contains, among others, such components which impart to the final product desired types of temperature-dependent response properties.

This if followed by drying the coated web in a drying process 20. As a result of temperature limitations, the drying is typically conducted by means of fluidized bed dryers arranged in series with each other, in which the web is subjected to modest temperatures. Due to a low drying performance, it is typically necessary to employ several successive drying units for a desired drying result, such as three successive units 2a, 2b, 2c in the example of fig. 1. The drying process 20 is followed by the adjustment of a curl developed in the web. A web surface 11, which is opposite to the web surface 12 treated in the coating process 5, is supplied in a moistening process 10 with an appropriate substance, for example water. A typical objective is to achieve a total compensation for the curling of a web, resulting in a web as straight and flat as possible in terms of its cross- section. The process 10 is followed by another web drying process 80. Because of temperature limitations, this drying is generally performed by using water-heated cylinders 18 having a temperature of about 50-60⁰C, the web, while pressed against the outer surface of said cylinders, making its way through the process and drying at the same time. A low drying performance necessitates the use of several such cylinders 18 and the dryer section 80 becomes bulky in size and expensive in costs. In most cases, it has been necessary to design the drying cylinders independently adjustable with the top and bottom cylinders having separate water circulations. The drying performance has also its limiting influence on the line's running speed. The cylinders must not have a temperature higher than +50-60° at the surfaces in contact with thermal paper. Consequently, the temperature of water in the cylinder must be less than 65°C.

Rg. 2 shows an assembly applying a method of the invention, wherein the entire process sequence is essentially different. Here, as early as immediately after an unwinding step 15, the web is anticipatorily subjected to a curl adjustment by carrying out moistening 10 and drying 80 of an opposite web surface 11 prior to an actual coating process 5. The moistening step can be performed as described in the prior art, such as for example by roll moistening, spray moistening, or by using a sizer or a blade station.

What is most essential is that the drying step 80, along with drying cylinders 28 therein, can now be conducted without temperature limitations like those encountered in the prior art solution. As opposed to needing typically from seven to eight water-heated drying cylinders 18 for the drying process, the operation can now be managed by using as few as two drying cylinders 28, which are steam heated and typically operated by normal back-pressure steam (3 bar, circa 14O⁰C). The drying is now distinctly more effective than before as temperature levels applied to a web can be increased from prior values of less than 6O⁰C to what are clearly above +100⁰C. The equipment can now be substantially downsized and thereby the proc- ess sequence and the considerably simplified hardware configuration of the invention are capable of providing a perfectly equal final result, and what is more, without jeopardizing the final product's quality.

And, even if the presently proposed changes in the process sequence were to increase the service demand of liquid intended for a curl control, it would still be sufficient, for example in the case of a double amount of liquid, to employ four, maximally 5-6 steam-operated drying cylinders 28, i.e. fewer than the currently employed number of cylinders, possibly just half of that. Likewise, since the drying process, conducted downstream of moistening, is no longer subject to limitations like the ones before, the moistening step can also be allowed to include measures more intense than before. It is possible, for example, to use water more than before with the certainty that the dewatering thereof is in any case achievable all the way to a desired dryness degree.

The elimination of a temperature limitation in drying also opens up a possibility of conducting the drying process subsequent to the moistening 10 by means of other known drying methods, such as for example infrared drying or fluidized bed drying with enhanced operating parameters, or by using combinations of various drying methods. This enables achieving an even further reduced hardware size and savings in terms of required space.

Hardware savings are also obtained regarding material requirements. With respect to water- and steam-heated cylinders, it is the case that the water-heated cylinders 18 typically necessitate the use of special materials in cylinder elements because of corrosion resistance. In the steam cylinder 28, on the other hand, the requirements are less stringent in this respect.

Another benefit offered by the solution is an excellent opportunity of applying the solution on presently existing machine lines. The assembly fits in considerably less space, is less expensive than before, better in terms of its production reliability and adjustability, more secure from the standpoint of product quality. It should also be pointed out that the coating and curl control of a heat sensitive product has been earlier conducted by means of an off-line type coater. Having managed to discard the drying-related limitations as far as finishing is concerned and having managed to make the hardware simpler and distinctly smaller than before, it is now also conceivable to link an assembly of the invention directly as an on-line system downstream of a papermaking machine.

The solution according to the invention is also better than before in terms of allowing the addition of supplementary features relevant to controlling the curl of a web. Downstream of the drying step 80 prior to the coating 5 can be further provided means for an appropriate pretreatment 29 of the web, for example. It is possible, for example, to conduct the temperature and/or moisture adjustment of a web, as well as a possible pretreatment with an appropriate agent, prior to the actual coating process.

Regarding temperatures, it is also beneficial that, while currently the web temperature upon arriving in the coating step 5 is typically about +30°, the web tempera- ture can now be set, by means of the extra devices 29 or solely by means of the hot steam cylinders 28, before the coating step 5 easily in the category of >+50°C. This enables a remarkable enhancement of the drying process 20 which follows the coating step 5. It is even possible to completely omit one fluidized bed drying unit.

Claims

1. A method (1) for controlling the curling of a paper/board web (W) for a heat sensitive product (2), said method comprising the use of an applicator device (5) for applying at least one coating agent or the like substance, containing one or more temperature sensitive components, onto at least one side (12) of the web (W), and said method further comprising the use of equipment (100) for controlling the web (W) in terms of its curling, including first devices (10) for applying at least one appropriate liquid agent to the web (W) onto its side (11) opposite to the side (12) to be applied and second devices (80) disposed in the web's (W) running direction downstream of the first devices (10) for drying the web (W), characterized in that said first devices (10) and second devices (80) for controlling the web (W) in terms of its curling are disposed in the web's (W) running direction upstream of said application (5).

2. A method as set forth in claim 1, characterized in that said application (10) involves moistening with water or other liquid agent or with a solution.

3. A method as set forth in claim 1 or 2, characterized in that the drying process (80) involves the use of one or more steam cylinders, infrared dryers, fluidized bed dryers or any combination thereof.

4. A method as set forth in any of claims 1-3, characterized in that downstream of the drying process (80) prior to the application (5) of a coating are further provided means (29) for adjusting the web's moisture and/or temperature and/or for pre- treating the web with desired agents.

5. An assembly (1) for controlling the curling of a paper/board web (W) for a heat sensitive product (2), said assembly comprising applicator devices (5) for applying at least one coating agent or the like substance, containing one or more temperature sensitive components, onto at least one side (12) of the web (W), and said assembly further comprising equipment (100) for controlling the web (W) in terms of its curling, including first devices (10) for applying at least one appropriate liquid agent to the web (W) onto its side (11) opposite to the side (12) to be applied and second devices (80) disposed in the web's (W) running direction downstream of the first devices (10) for drying the web (W), characterized in that said first devices (10) and second devices (80) for controlling the web (W) in terms of its curling are disposed in the web's (W) running direction upstream of said application (5).

6. An assembly as set forth in claim 5, characterized in that said application (10) comprises moistening with water or other liquid agent or with a solution.

7. An assembly as set forth in claim 5 or 6, characterized in that the drying process (80) involves the use of one or more steam cylinders, infrared dryers, fluidized bed dryers or any combination thereof.

8. An assembly as set forth in any of claims 5-7, characterized in that downstream of the drying process (80) prior to the application (5) of a coating are further provided means (29) for adjusting the web's moisture and/or temperature and/or for pretreating the web with desired agents.