FI122859B - Method and device for surface treatment of paper / board - Google Patents

Description

Method and apparatus for surface treatment of paper / board

The present invention relates to a method and apparatus for surface treatment of paper / board.

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High bending stiffness is desired for printing papers and board to achieve sufficient runnability and printability. The stiffness can be increased e.g. surface sizing and various gradient structures (multilayer structures and gradient calendering). A high sweater is sought to achieve a constant density of 10 with less fiber.

The purpose of surface sizing is to improve the strength properties of paper or paperboard, such as internal strength (peel strength) or surface strength (dust resistance). The chemicals used in surface sizing are water-soluble polymers-15, which mainly use starches because of their low cost. The starch raw material is plants such as corn, wheat, barley, potato, tapioca, etc., from which tubers, seeds, etc., starch is obtained. Starch (C6H10O5) consists of straight chain amylose and branched amylopectin. Other chemicals used in surface sizing include: various cellulose derivatives (CMC) and polyvinyl alcohol (PVA).

In calendering, the stiffness decreases as the web is compressed and the thickness ^ decreases, i.e., the sleeve decreases. The calculated bending stiffness increases with respect to the third power of thickness according to the following formulas: S 25 ti: OJ Si = E 1 ht 3 x S 2 E 2 h 12

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S 30 E2 h!

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tbh3! = hl2 hih32 h22 2 where

Si and S2 are the rigidities of calendered and non-calendered paper, and hi and hi 2 are the respective thicknesses of calendered and non-calendered paper, and

Ei and E2 are the corresponding Kimmo cards for calendered and non-calendered paper.

The following describes the types of paper and board generally, and the various calendering and coating methods used to make them.

Paper and board grades

There are a number of different grades of paper and board, and they can be divided into two classes, based on their basis weight: single-layered papers with a basis weight of 25-300 g / m2 and cartons which may be made using multilayer technology 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 following descriptions are examples of the current values of fibrous webs and may exhibit considerable variations in the values given. The descriptions are mainly based on the source paper Paper § 25 king Science and Technology, part Papermaking Part 3, Finishing, ed. Jokio, oj M., published by Fapet Oy, Jyväskylä 1999, 361 s and Paper and Board grades, Papermaking Science and Technology, eds. Paulapuro, H., published by Fapet Oy, Jygagskylä 2000, 134 pages.

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o ° 30 Mechanical pulp or wood-containing printing papers are newsprint, uncoated magazine paper and coated magazine paper.

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Newsprint can either consist entirely of mechanical pulp or may contain low levels of bleached softwood pulp (0-15%) and / or recycled fiber may be used to replace mechanical pulp. Generic values for newsprint 5 could be as follows: basis weight 40-48.8 g / m2, ash content (SCAN-P 5:63) 0-20%, PPS slO roughness (SCAN-P 76-95) 3.0-4.5 pm, Bendtsen roughness (SCAN-P21: 67) 100-200 ml / min, density 600-750 kg / m3, lightness (ISO 2470: 1999) 57-63% and opacity (ISO 2470: 1998) 90-96%.

Uncoated magazine paper (SC = supercalendered) generally has a mechanical pulp of 50-70%, bleached softwood pulp 10-25% and fillers 15-30%. Typical values for calendered SC paper (including, inter alia, SC-C, SC-B and SC-A / A +) are 40-60 g / m 2, Ash content (SCAN-P 5:63) 0-35%, Hunter gloss (ISO / DIS 8254/1) <20-50%, PPS slO roughness (SCAN-P 76:95) 1.0 - 2.5 pm, density 700 - 1250 kg / m3, brightness (ISO 2470: 1999) 62 - 70% and opacity (ISO 2470: 1998) 90 -95%.

Table 1 shows typical values for mechanical pulp coated papers. (MFC = machine finished coated, FCO = film coated offset, LWC = light weight coated, MWC = medium weight coated, ^ FIWC = heavy weight coated) δ

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MFC FCO LWC MWC HWC

basis weight, (g / m 2) 50-70 40-70 40-70 70-90 100-135

Hunter gloss (ISO / DIS 25-40 45-55 50-65 65-70 8254/1), (%) PPS-slO roughness, (μηι) 2.2-1.5-0.8-1.5 (off- 0.6-1.0 ( SCAN-P 76/95) 2.8 2.0 set) 0.6-1.0 (ro-to) density, (kg / m3) NIGHT1 1000-1100-1250 1150-950 1050 1250 Brightness (ISO 70-75 70-75 70-75 70 - 75 2470: 1999), (%) Opacity (ISO 91-95 91-95 89-94 89-94 2470: 1998), (%) Lightweight Coated Magazine Paper (LWC) contains mechanical pulp 40 - 60%, bleached softwood pulp 25-40% and filler and coating agents 20-35%. Fleavy weight coated (FIWC) can be applied up to ™ more than twice.

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w Pulp wood-free printing papers, or fine papers, are coated-

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£ 10 unpeeled and coated cellulose-based printing papers with a mechanical pulp content of less than 10%.

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m co o cv Uncoated pulp-based printing papers (WFU) contain bleached birch pulp 55-80%, bleached softwood pulp 0-30% and fillers 5-10-30%. The values for WFU range widely: 50 to 90 g / m2 (up to 240 g / m2), Bendtsen roughness 250 to 400 ml / min, brightness 86 to 92% and opacity 83 to 98%.

5 For coated cellulose-based printing papers (WFCs), the amount of coating varies greatly according to requirements and application. The following are typical values for single and double coated pulp-based printing paper: 90 g / m2 coated once weight, Hunter gloss 65 -80%, PPS slO roughness 0.75-2.2 pm, brightness 80-88% and opacity 91-104% twice coated basis weight 130 g / m 2, Hunter gloss 70-80%, PPS slO roughness 0.65-0.95 µm, brightness 83-90% and opacity 95-97%.

For release papers, the basis weight ranges from 25 to 150 g / m2.

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Pulp, mechanical pulp and / or recycled pulp are used for the production of paperboard. The cartons can be divided into e.g. to the following main categories according to their intended use.

20 Corrugated cardboard with a liner and fluting.

Boxes for making boxes, boxes. The cartons are e.g. liquid packaging board (FBB = folding boxboard, LPB = liquid packaging § board, WLC = white-lined chipboard, SBS = solid bleached sulfite, SUS = solid § 25 unbleached sulfite).

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I Graphic paperboard, of which cards, folders, folders, cd covers, covers, etc.

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o ^ 30 Wallpaper base.

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calendering

The surface properties and thickness profile of the various papers and board are adapted to the requirements of the printing process and further processing by calendering. Coated species are typically pre-calendered prior to coating and final calendered after coating.

Calendars are divided into machine calendars, soft calendars and multi roll calendars. The machine calender typically has 1-2 nips and both rolls 10 of the nip are hard rolls. The soft calender generally has 1-4 nips and at least one of the nip rolls is covered with a soft coating. The multi roll calender typically has 5-11 nips. The multi-roll calender track includes both heated rollers and soft-surface rollers.

15 Special calendars include: wet stack calender, breaker stack and long nip calendars.

The Wet stack calender is approximately identical to the multi-roll machine calender but the calendering process is completely different. The Wet Stack Calender 20 effectively utilizes a moisture gradient so that the moisture entering the calender web is only about 1-2%. The Wet Stack Calender has water sheets to form a film of water on the surface of the web before the nip which is pressed onto the surface of the web in the nip. In this case, the web is only wetted from the surface by calendering ^ more from the surface than from the overdried interior. The Wet Stack Calender is used as a § 25 calender for several board grades.

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| An intermediate calender is a machine calender located in the drying section of a paper machine.

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g Long nip calendars include a shoe calender with a soft sheath o £ 3 around 30 shoe rolls, typically with a nip length of 50-400 mm, and strap calendars. A conventional belt calender consists of a soft roll calender thermo roll, a belt roll and an inside roll roll counter roll, which may be a hard or a soft roll. The belt rotates through the counter roller and the guide / tension rollers. A special application of the belt calender is a metal belt calender, wherein the calender belt is a metal belt that rotates around the guide rolls and forms, together with a counter member, which is typically a roll, a long nip zone that can be up to 5000 mm. In addition, a compression member, e.g. a deflection compensated roll, may be provided inside the belt passage, by means of which a nip point of a higher compression pressure can be provided in the middle of the long nip zone.

10 Coating Technologies As paper types and coatings become more common, increasing demands are placed on coating processes and equipment. In coating, more specifically in pigment coating, a coating-coating layer is formed on the surface of the paper at a coating station, followed by drying of the water supplied with the coating composition. The formation of the coating composition layer can be divided into exporting the coating composition to the surface of the paper, i.e. application, and adjusting the final amount of coating. The most important pigment coating method is the so-called blade coating, in which the amount of coating is controlled by a so-called doctor blade. The most common types of blade coating stations are a blade roller with a brush roller and a blade coating with a nozzle application. In addition, the coating utilizes a so-called film transfer coating, which has become more common in recent years. Similarly, curtain coaters are becoming available as a new technology for No. 25 high-speed printing presses.

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| From a practical point of view, the most important difference between the various coating devices is the application of the coating and the penetration therein, i.e. the coating penetration into the paper.

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In the production of high-quality coated printing papers, more and more attention has been paid to productivity in recent times. Likewise, the quality of lightly coated papers, such as the bulk product, is enhanced. Almost all pavers are under pressure to improve quality, productivity and speed.

In the blade coating brush roll application, the application is done by lifting the coating mixture onto the lower surface of the paper supported by the counter roll by means of a rotating roll. The application rate is normally 200 to 250 g / m2. In the Sively-10 roller application, the coating composition effectively penetrates the base paper. In addition, the base paper fibers have time to swell prior to scraping, increasing the paper's roughness volume.

In nozzle application, the coating composition is in turn introduced directly onto the surface of the web by a nozzle. The advantage over the brush roll applicator is the lack of a rotating roller and thus a better suitability for high speeds. A further advantage is the lower application pressure pulse, which improves runnability. In nozzle application, the wetting event of the web is an intermediate form of the brush roll and the short stay. The application rate for nozzle application is typically 130 to 220 220 g / m 2.

In short-stay application, the coating composition is led to an applicator chamber just behind the doctor blade, one side of which is formed by a moving paper web supported by a counter roll. The moving paper web causes turbulence in the application chamber and the flow rate of the coating mixture on the surface of the paper web is the same as the paper web speed. For short stay application, the paper is wet low due to low application area pressure and short action distance, g The swelling of the paper fibers only partially occurs after the doctor blade, which

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g roughens the surface of the blade. The smoothness achieved by the short-stay coater is thus inferior to that of the roller and nozzle coaters.

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The amount of coating that remains on the surface of the paper is affected by many different variables. The properties of the base paper also increase the amount of coating as the roughness, porosity and water absorption increase. Likewise, as the solids content and viscosity of the coating composition increases, the amount of coating increases. On the other hand, the increase in the water retention of the coating composition reduces the amount of coating. In turn, as the load on the doctor blade increases, the blade angle and blade thickness increase, the amount of coating decreases. Among other factors, the increase in travel speed and the application pressure increase the amount of coating.

10 In addition to the blade coaters described above, other equipment is used for coating and surface treatment. The following are some of the most commonly used options. The adhesive press unit consists of two rotating rollers. Here, the mixture applied to the surface of the web is applied to the web in a pond between the web and the rolls. In addition to surface sizing, pigmentation can also be performed with an adhesive press. The amount of coating is about 1.0 to 2.0 g / m 2 / side. A problem with a normal size press has been the instability of the application pond at high running speeds (> 600 m / min).

To overcome this problem, film adhesive presses have been developed in which the desired coating or adhesive layer is first applied to the surface of the paper web 20 onto the surface of the press rolls, from which the layers transfer to the paper in the nip between the rolls. Units such as short-stay coatings are used as applicoin devices. Advantages of the apparatus include controlled application at high speeds and the possibility of pigmentation (2-6 g / m 2 / side). In addition, the dry solids content of the blend can be increased to § 25 compared to a normal size press. The coating of the film blends can be performed either on one side or on both sides. Film transfer coating drive- | Good quality compared to blade coating. With respect to the blade coating, the coating layer obtained by the film miniature transfer coating usually fits better

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g shape and is more opaque in that sense. However, film coating coatings cannot achieve high coatings (greater than 10 to 12 g / cm 2).

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In airbrush coating, the application of the coating composition is performed either by a single or multiple roller applicator or by means of a nozzle. In turn, the amount of coating is adjusted by means of an air jet. Airbrush coating is used almost exclusively in board coating 5 because of its good coverage. Weaknesses include the run speed limit of the process and the relatively low dry solids content of the coating composition. The coating layer formed by the air brush is uniform in thickness, conforming to the shape of the paper surface.

10 Thus, the basic solutions for the coating of paper web today are short end and seam roll heads, nozzle application devices and film squeezers. It has not yet been possible to develop a universal coating for all purposes.

15 Blade coating in various forms is and will continue to be the most common coating method. As drive speeds increase and application rate ranges expand, solutions based on nozzle applications will likely displace the paint roller almost completely.

20 The so-called curtain coating technology is also emerging as a new technology. Curtain coaters can be divided into slot-fed or slide-fed coatings. In a sheet-fed curtain coater, the coating is drained along an inclined plane and the curtain is formed as g coating drips from the edge of the plane. In slit-fed applicator beams g 25, the coating is pumped through a distribution chamber into a narrow vertical slot, g, on whose lip the curtain forms and drips onto the web. The coating may be applied in one or more layers.

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o LO Curtain coating exerts a much smaller force on the web of o g 30 compared to the blade coating and thus causes fewer web break breaks and thus improved runnability. Curtain coating 11 does not achieve the same smoothness as blade coating, but the coverage achieved by it is better than that of blade coating. The curtain wrapper is primarily thought of as a replacement for the airbrush.

Patent FI 115732 discloses a method and apparatus for improving the penetration of a surface sizing agent into a fibrous web by means of a vacuum device.

It is an object of the present invention to provide a solution which provides 10 further improvements in the stiffness and bulk of paper / board relative to the values obtained by normal surface sizing and subsequent calendering. To accomplish this object, the method according to the invention is characterized by the features set forth in the characterizing part of claim 1.

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The apparatus according to the invention, in turn, is characterized by the features set forth in the characterizing part of claim 4.

The invention will now be further described with reference to the accompanying drawings, in which:

Figure 1 is a schematic schematic view of an exemplary hardware solution according to the invention, and

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§ 25 Figure 2 illustrates one hardware solution for enhancing surface treatment.

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| The exemplary apparatus illustrated in Figure 1 includes, for example, a vacuum pressurized surface sizing apparatus 1 according to FI patent 115732 g and a metal belt calender 10.

S Surface sizing apparatus 1 includes a first applicator la to apply starch solution 30 to a first web Wi and a first vacuum device 3a wherein a prevailing underpressure causes a differential pressure over 12 W of web and causes the starch solution to migrate and penetrate. to the desired depth. The exemplary apparatus further comprises a second appliance device Ib arranged on the second side W2 of the web and a second vacuum device 3b arranged on the first side W1 of the web 5. This arrangement allows for the mutual surface treatment of the web W, whereupon the web W is guided, after the first vacuum device 3a, by means of guide and guide rolls 8 to the second applicator device Ib. After the surface treatment of the first page W1 of the web, it may be desirable to dry the web W at least to some extent prior to the surface treatment of the second page W2 thereof. This is illustrated by a non-contact dryer, denoted by reference numeral 9, for example an infrared dryer. Furthermore, in Fig. 1, the folding and guiding rolls, by which the direction of travel of the web W is rotated in the desired manner, are indicated by reference numerals 4a and 5a.

The metal belt calender 10 includes a metal calender belt 12 which wraps around the guide rollers 13. The calendering belt 12 passes around a roll 15 arranged outside it, whereby a calendering area is formed between the belt 12 and the roll 15. The material path W to be calendered passes through the calendering region N, whereby it is subjected to a desired pressure pulse and thermal effect as a function of time. Figure 1 illustrates the shape of the pressure effect when the calendering belt 12 is provided with a nip roll 14 acting as a press member which presses against the roll 15 to form a higher pressure inside the calendering region.

The dashed line 18, in turn, illustrates the shape of the pressure effect when the contact pressure acting on the calendering region 25 is formed solely by the tension of the belt oj 12, the nip roll 14 being released from the pressing contact of the belt | nail 12 (or when nip roll 14 is not mounted at all on the inside of belt 12).

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In accordance with the invention, it has been found that the flexural stiffness and / or bulk of the surface-glued paper / board can be improved by applying a vacuum application 13 to the surface-gluing followed by a treatment step to prevent or limit drying shrinkage, thereby increasing drying stresses. This phenomenon is based on the drying shrinkage effect of the starch, whereby the drying shrinkage after the addition of starch results in increased drying stresses. By using metal belt calendering in the post-sizing treatment step, the drying of the web is accomplished supported by a closed nip formed by the metal belt and roll, whereby the generation of drying tension is controlled. The surfaces of the fibrous web 10 between the hot surfaces of the metal belt calender dry and the moisture is transferred to the central portion, whereby increased humidity swells the central portions of the fibrous web, thereby increasing the sweat which increases the thickness. At the same time, drying tensions develop on the surfaces, whereby the modulus of elasticity of the surface parts is increased. As the fibrous web leaves the treatment area, the evaporated water in the central portion is discharged through the surface layers without condensation (which would result in the relaxation of the newly obtained high elastic modulus), thereby substantially retaining the structure achieved in metal belt calendering. This results in significant raw material savings as well as quality improvements.

Figure 2 shows another way to enhance the surface treatment. The processing device solution 30 of Figure 2 combines a film transfer coater and a metal belt calender. A surface treating agent, e.g.

i § 25 The film transfer nip is formed between the application roll 31 and the return roll 34. In addition, a metal belt 33 is passed through the nip i oj to provide a top of the fibrous web. knuckle and metal belt calendering in one nip. It is possible to use high temperatures in the metal belt n. The fibrous web is coming out of the nip

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g surface dry for contact drying, and depending on the running speed and temperatures ≤ 30, it is possible to reach sufficiently high dry matter concentrations without the need for separate drying. This combination solution provides 14 significant investment and operational cost savings while at the same time increasing the rigidity of the fiber web by preventing drying shrinkage in the metal belt nip.

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Claims (6)

15 A method for surface treatment of paper / board, characterized in that the method comprises treating the fibrous web with a drying shrinkable chemical such that the chemical penetrates substantially throughout the thickness of the fibrous web and thereafter the fibrous web is exposed to two heated metal surfaces (12, 15). ), whereby the surfaces of the fibrous web between the hot surfaces dries and moisture is transferred to the central portion, whereby the increased humidity swells the central portions of the fibrous web to prevent shrinkage of the surface portion 10 surface layers, resulting in an elevated surface elastic modulus and, at the same time, a low center density. 15 Method according to Claim 1, characterized in that the method uses a processing device (10) provided with a metal belt, which forms said processing area between the heated metal surfaces (12, 15). 20 Process according to claim 1 or 2, characterized in that the processing chemical is starch. CVI Apparatus for surface treatment of paper / board, characterized in that the apparatus includes an application apparatus (1) which allows the chemical to cause drying shrinkage black to be applied to the fibrous web so that the (M x medial) penetrates substantially throughout thickness of the fibrous web, and the CC fibrous web is arranged to pass through a treatment area (N) between two heated metal surfaces (12, CO 8 15) so that the surfaces of the fibrous web 30 dry between the hot surfaces and the moisture is transferred to the middle swells the middle portions of the fibrous web, preventing the surface portion from shrinking, and the surface portion is subjected to high drying tension, whereupon the fibrous web exits the treated area through the pores of the heated and dried surface rapidly without substantial condensation in the surface layers; density of the central part of the substance. 5 Apparatus according to Claim 4, characterized in that the apparatus comprises a processing device (10) provided with a metal belt, said treatment area (N) between the heated metal surfaces (12, 15). Apparatus according to Claim 4 or 5, characterized in that the application apparatus (1) comprises chemical applicator means (1a, Ib) arranged on one side of the fibrous web to apply the chemical to the surface (W1VV2) of the fibrous web to be treated and 3b) providing a differential pressure across the thickness 15 of the fibrous web (W) to cause the chemical to move from the side of the application means toward the opposite side. C \ J δ CM CO O CO CM X cc CL CD O CO LO CD O O CM 17