Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
  • D21H23/00—Processes or apparatus for adding material to the pulp or to the paper
  • D21H23/02—Processes or apparatus for adding material to the pulp or to the paper characterised by the manner in which substances are added
  • D21H23/22—Addition to the formed paper
  • D21H23/46—Pouring or allowing the fluid to flow in a continuous stream on to the surface, the entire stream being carried away by the paper
  • D21H23/48—Curtain coaters
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21G—CALENDERS; ACCESSORIES FOR PAPER-MAKING MACHINES
  • D21G1/00—Calenders; Smoothing apparatus
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
  • D21H19/00—Coated paper; Coating material
  • D21H19/80—Paper comprising more than one coating
  • D21H19/84—Paper comprising more than one coating on both sides of the substrate
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
  • D21H25/00—After-treatment of paper not provided for in groups D21H17/00 - D21H23/00
  • D21H25/08—Rearranging applied substances, e.g. metering, smoothing; Removing excess material
  • D21H25/12—Rearranging applied substances, e.g. metering, smoothing; Removing excess material with an essentially cylindrical body, e.g. roll or rod
  • D21H25/14—Rearranging applied substances, e.g. metering, smoothing; Removing excess material with an essentially cylindrical body, e.g. roll or rod the body being a casting drum, a heated roll or a calender

Definitions

• the invention relates to a method for coating a fibrous web with one or more layers of coating material.
• the invention relates also to an apparatus for performing the method.
• a so-called cast-coating technique it is conventional to effect coating of a fibrous web by means of a so-called cast-coating technique. It involves applying a coating material first in one or more operations onto the surface of a fibrous web. The application can be followed by performing an intermediate drying step or an intermediate moistening step or by performing a final drying process directly on the coating material, bonding it to the fibrous web, as well as glazing it in a compressive contact with a hot contact surface. This process is generally carried out by means of a Yankee cylinder.
• An essential feature in the cast-coating process is to establish a direct contact between a still wet coating and a hot, typically polished metal surface.
• the coating While in contact with a hot surface, the coating undergoes drying and its surface becomes glazed as it adheres to a surface of the fibrous web.
• the process enables providing a fibrous web surface with particularly good printing qualities.
• the surface has a high-quality smoothness and the surface layers also absorb printing ink quickly and drying of printing ink occurs quickly as well.
• a problem in this type of process has been caused e.g. by limitations on running speed. Higher running speeds have been denied both by application methods and limited operating characteristics of a Yankee cylinder generally employed in the contact process. Consequently, it has been generally necessary to use an off-line machine for a cast-coating type of coating process.
• Another obvious problem has been caused by the adherence of a wet coating material to a hot metal surface. Attempts have been made to alleviate the adherence problem by means of anti-adhesive agents. Such a release agent has been applied, for example, first directly to a hot contact surface and then, during the actual compressive contact, between the contact surface and the coating.
• prior technology comprises also a multiple coating process by means of a curtain coating technique.
• this technique enables the application at one or separate process stages of several layers of coating material or layers of some other functional material in precisely desired thicknesses onto the surface of a fibrous web.
• the actual layers of coating material can be topped by applying other additional coats without affecting the underlying material layers in any way.
• the curtain coating technique also facilitates higher running speeds with respect to many other coating techniques.
• one important objective of the present invention is to provide a method, which offers benefits equal to those of a cast-coating process in terms of the printability of a coated fibrous web and which at the same time enables avoiding the above type of problem involving the adherence of coating materials to contact surfaces and facilitates higher running speeds, the method thus becoming more applicable than before also in the context of on-line machines.
• the inventive method offers numerous benefits with respect to prior art solutions.
• An essential advantage gained by the inventive method is that the use of a curtain coating process enables a controlled application of release agents to a coating, for example either to a top liner and on top of the top liner as a separate coat. This makes it easier to avoid the problems involved in a cast-coating process as the coating adheres to a hot contact surface.
• the application of a release agent to the coating can be performed as early as concurrently with the actual application of the coating itself.
• This when considered in combination with a higher running speed of application generally achieved in curtain coating technique, enables a higher running speed in terms of the entire coating process. At the same time, this enables the coating process to be implemented as an on-line machine.
• curtain coating process and especially a multiple curtain coating process, enables also the forming of several separate layers of coating material and thereby a desired type of lamellar structure for providing layer- specific coating characteristics.
• Various layers of the coating can be provided with qualities as desired.
• the capabilities of regulating the characteristics of a coating within various layers of the coating have been decidedly more limited.
• the curtain coating process essentially immediately after the application of a coating as required by the invention, is adapted to include a compression contact with a hot contact surface, the coating surface of excellent smoothness will even be obtained. This has a major significance in terms of printability.
• one adverse factor in coating methods based on contactless application of a coating has been the fact that the coating develops a rough surface matching the paper's original roughness.
• a method of the invention is capable of providing the end product with a smooth surface, which is usually not possible for a contactless coating method. In spray and curtain coating, for example, the surface is usually quite rough, matching the paper's original roughness.
• the top liner surface of a coating becomes substantially smoother.
• a still further significant benefit offered by the inventive solution is a possibility of designing particularly compact and hence low-cost machine configurations, regarding especially the implementation of a multiple coating process and compression contact stages preceding and following it.
• Several available calendering processes consist of a plurality of discrete sub- processes, such as e.g. a two-nip soft calender and a multi-nip calender with a number of identical nips in succession.
• the invention offers a possibility of exploiting a calender of several treatment stages in a fibrous web coating process, such that a coating method is provided, which enables the use of an apparatus as compact as possible.
• a fibrous web to be coated is conveyed, upstream of a coating operation of the fibrous web, into one nip or treatment zone of a calender, for example for pre-calendering. This is followed by executing a coating operation of the fibrous web, and after the coating operation the fibrous web is conveyed into a second nip or treatment zone of the same calender for final calendering.
• Fig. 1 shows a prior known cast-coating method for coating a fibrous web
• fig. 2 shows one apparatus applying a method of the invention for coating a fibrous web, the compression contact being effected by means of a metal belt calender.
• Figs. 3-5 depict schematically a few metal belt calenders suitable for the execution of a method according to a further embodiment of the invention
• fig. 6 depicts schematically one two-nip soft calender suitable for the execution of a method according to a further embodiment of the invention.
• figs. 7-8 depict schematically a few multi-nip calenders suitable for the execution of a method according to a further embodiment of the invention.
• the cast-coating type fibrous web coating process shown in fig. 1 is executed with an off-line machine, as typical for the method.
• a fibrous web W is delivered from an unwinder 10 and to its surface is applied a coating.
• the application can be performed by using a number of prior known techniques, such as for example an air brush coating process 11 shown in fig. 1.
• the application is followed by an intermediate drying 12 as well as a remoistening 13 prior to leading the fibrous web to a hot compression contact.
• the drying of a coating is effected in a compressive contact with a hot, typically polished metal surface.
• the compression contact is in this case performed by means of a Yankee cylinder 20, the Yankee cylinder's surface thereby functioning as a hot contact surface.
• an anti-adhesive agent either as such or for example admixed with a coating material.
• a further coating with a puddle coating method 14 as well as the addition of an anti-adhesive agent just before treatment in the Yankee cylinder.
• the treatment is followed by subjecting the dried and still warm coated fibrous web to a cooling step 16 and possibly also to a remoistening step, for example in a moistening/steaming cabinet 17, and once more to another cooling step. Finally, this is followed by a possible calendering step 18 and other possible finishing procedures prior to winding for a roll 19.
• Fig. 2 illustrates a coating process of the invention, in which a fibrous web W to be coated is conveyed to the treatment either straight from a paper/board machine or is unrolled from an unwinder (not shown).
• the application of a coating onto the fibrous web's surface is presently effected by means of a curtain coater 21.
• the curtain coater can be either a slot-fed or slide-fed curtain coater. In the latter case, several layers of coating material can be preferably applied at the same time.
• the application of even a multiple coating can be performed in its entirety in a single process operation by means of a single apparatus.
• the coating can be formed of layers highly diverse both in terms of properties and thicknesses thereof. Each layer can be adjusted for its properties to comply precisely with those required.
• the underlayer including a bonding agent, can be formulated for a high adherence to the surface of a fibrous web, the middle layers can be formed of less expensive bulk layers.
• the upper layers can be made while keeping in mind the requirements set by printability and, particularly in the case of a top liner, the problems regarding adherence to a hot contact surface.
• the layer can either be supplemented with or composed solely of an anti-adhesive agent. What is essential with regard to the adherence problem is now the fact that the properties of top layers can be regulated as desired without affecting adversely the other material layers of a coating.
• the fibrous web is immediately ready for proceeding to a compression contact with a hot treatment surface. It is also plausible to effect a pre-drying 23 of the fibrous web W as well as to perform yet another moistening 24 prior to the treatment.
• a pre-drying 23 of the fibrous web W as well as to perform yet another moistening 24 prior to the treatment.
• a pre-drying 23 of the fibrous web W As well as to perform yet another moistening 24 prior to the treatment.
• a pre-drying 23 of the fibrous web W As well as to perform yet another moistening 24 prior to the treatment.
• a Condebelt type of apparatus such as a Yankee cylinder, as well as a variety of calenders with at least one heated metal surface, such as a machine, soft or shoe calender.
• the treatment of a fibrous web against a hot contact surface be performed by means of drying cylinders.
• the compression contact applied to a fibrous web in an arrangement of the invention is effected by means of a metal
• the coating applied to the surface of a fibrous web continues to be substantially moist upon entering the treatment, the dry content being typically less than 90%.
• the treatment is adapted to take place substantially immediately after the application of a coating, such that, without specific drying processes applied to the fibrous web, there is not enough time for the coating to undergo significant drying before the treatment.
• the fibrous web can be subjected both to a treatment time sufficiently long in terms of drying the coating and also to an adequately high pressure effect for providing a desired action on the structure of both the coating and the fibrous web. If necessary, the pressure effect can be increased by means of a press nip N established with a separate nip roll 26 in the metal belt calender's treatment zone.
• the coating can be provided with a surface of excellent smoothness and desired properties for printing.
• the length of a treatment zone can be adjusted, nor is the running speed a limiting factor as compared for example to a curtain coating process.
• the heating can also be provided on a metal belt calender's counter-element, which is usually a roll 25.
• the preferred temperature range for contact surfaces is from about +50 0 C to +400 0 C.
• the application of an entire coating is performed by means of curtain coating. It is naturally also conceivable that some of the coating be applied by other known methods. What is essential in an arrangement of the invention is that at least the uppermost material layers of the coating, as well as an anti-adhesive agent or agents, possibly either blended therewith or applied entirely in its own layer, be applied to the fibrous web's surface by a curtain coating method. This enables performing the application without substantially affecting the underlying layers of coating material.
• the treatment in a compression contact can be preceded by applying to the coating surface an anti-adhesive agent for example by a spray method.
• the agent can be applied as such in an appropriately thin liquid or, for example, blended in water or some other liquid.
• the coating process can also be effected on both sides of a fibrous web. This can be implemented either by performing the application with curtain coating first for one side and then for the other side or by performing the application on both sides of a fibrous web prior to treating the fibrous web in a hot compression contact.
• Rg. 3 in turn illustrates one arrangement for executing a method according to a preferred further embodiment of the invention, which is based on a metal belt calender.
• the arrangement comprises two separate metal belts 2a and 2b circling around guide rolls 3 for establishing, by means of a roll 5 present therebetween and functioning as a counter-element, two treatment zones N 1 and N2 for a fibrous web W 1 , W2. At least some of the guide rolls 3 within the belt circles are made displaceable for adjusting tension of the belts 2a and 2b and the running thereof as desired.
• the fibrous web is subjected to a desired pressured impulse and heat effect as a function of time.
• the counter-roll 5, as well as the nip roll 6, may or may not be a deflection-compensated roll, and it is chosen from a group, including: a flexible-surface roll, such as a polymer-coated roll, a rubber-coated roll or an elastomer-surface roll, a shoe roll, a thermal roll and a filled roll.
• a fibrous web W 1 is first conveyed, prior to a coating stage, through a treatment zone N 1 for pre-calendering the fibrous web. This is followed by conveying the fibrous web to a coating process 15 for coating the same with an appropriate coating method, especially a curtain coating method.
• the coating method be selected from a group, including dry coating, cast coating, blade coating, spray coating, microjet coating or film coating.
• the coating process is followed by conveying the coated fibrous web, represented by reference numeral W2, through a second treatment zone IM2 of the same calender unit for bonding the coating and final calendering the fibrous web.
• the term calendering is used in reference to both calendering and also generally to compression contact or hot pressing contact applied to a fibrous web.
• Fig. 4 illustrates a configuration 40, which also includes two belt circles 2a, 2b and therebetween a counter-roll 5 for establishing two treatment zones N 1 and N 2 as in fig. 1.
• the number of coating stages is optionally more than one, whereby e.g. a coating stage 15a can be used for coating one side of a web and a coating stage 15b for the other side of the web.
• a single coating stage is sufficient for implementing a concurrent application of several discrete layers of coating material.
• a fibrous web W 1 is conveyed first to a pre-calendering process N 1 and then to the coating stage 15a, followed by leading it by way of a deflection roll 7 to an optional second coating stage 15 and thence to a bonding/final calendering stage N 2 .
• a configuration 50 shown in fig. 5 comprises a first metal belt circle 2a and a first counter-roll 5a for establishing therebetween a first treatment zone N 1 , as well as a second metal belt circle 2b and a second counter-roll 5b for establishing therebetween a second treatment zone N 2 .
• the counter-rolls 5a, 5b are disposed for establishing with each other a bonding nip N, preferably a heated one, for the coating.
• the fibrous web W 1 is first conveyed for pre-calendering through the treatment zone N 1 and then through a coating stage 15a and by way of a deflection roll 7 further through a second coating stage 15b, from which it is delivered to the bonding nip N for bonding the coating material to the fibrous web and for effecting final calendering.
• This process can be followed by conveying a coated web W 2 out of the apparatus to a further processing, such as e.g. to a winding process.
• the coating stages 15a and 15b can be optional to each other or both of those can be used either for coating one side of a web with several layers or for coating opposite sides of a web.
• the web Downstream of the bonding nip N, the web can optionally have a third coating stage 15c, from which the web can be delivered by way of a deflection roll 7a to the second treatment zone N 2 for a bonding/final calendering process.
• the apparatus may even have a fourth coating stage 15d, which can function e.g. as a pre-coating stage upstream of the actual coating stage 15a and/or 15b and/or 15c.
• the coating stages 15a-15d can be optionally active one or more than one at a time for treating one fibrous web so as to provide a single or multiple coating on one or each side of the fibrous web.
• the traveling direction of a fibrous web can be reverse relative to what is described above, whereby e.g. the treatment zone N 2 or the bonding nip N can function as a pre-calender and the treatment zone N 1 as a final calender.
• a coating station 15 is disposed between two nips N 1 and N 2 of a soft calender, whereby one compact apparatus is capable of performing pre-calendering prior to a coating process and final calendering after the coating process.
• a coating stage 15 is positioned at a location between two nips of a multi-nip calender, and in a configuration 60 of fig. 6, a coating stage 15 is disposed at a location between two arrays of rolls.
• the fibrous web passes through a plurality of nips before and after a coating process for effective pre- calendering and final calendering.
• a supercalender can be divided for two or more sections, more specifically for two separate sets of rolls, between which is adapted to be performed an appropriate coating process.
• the coating process 15, 15a, 15b shown in the configuration of figs. 3-8 includes the application of a coating material, i.e. its transfer onto the surface of a web.
• the coating process may further include drying, heating and bonding, as necessary.
• the proposed method is suitable for a coating process effected with a coating material in dry form (dry coating).

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• Paper (AREA)
• Application Of Or Painting With Fluid Materials (AREA)
• Coating Apparatus (AREA)

Priority Applications (2)

Applications Claiming Priority (4)

Publications (1)

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Cited By (1)

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Citations (5)

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• 2005
  • 2005-07-01 WO PCT/FI2005/050267 patent/WO2006003263A1/en active Application Filing
  • 2005-07-01 DE DE112005001564T patent/DE112005001564T5/de not_active Withdrawn
  • 2005-07-01 JP JP2007517325A patent/JP2008503659A/ja active Pending

Patent Citations (5)

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