FI115547B - Control of the air boundary layer - Google Patents

Description

115547 Control of the boundary layer

The present invention relates to a method of coating a continuous web surface with a coating powder comprising the steps of: - allowing the web to move between electrodes of different potentials, - applying a coating powder of an inorganic material and a polymeric binder material to the surface of the web. and - finishing the coated surface of the web.

A problem with the above method is that the air boundary layer formed on the surface of the web prevents the coating powder from reaching the surface of the web 15. The consequence of such a phenomenon is, for example, an unevenly coated surface and dusting in the process. As the speed of the coating process increases, more energy is required to cross the air boundary layer. Also the unevenness of the web affects the air boundary layer.

The process according to the invention is characterized in that, before the web is allowed to move between the electrodes, the barrier layer is at least partially removed by an obstacle.

*: When the air boundary layer is at least partially removed, the coating powder is readily deposited on the web, is more efficient in transferring to the web, and reduces dust deposition. Depending on the effectiveness of the barrier, it is possible that the air barrier layer will be completely removed.

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The process of dry surface treatment of paper or board trays, in which the process of the invention is mainly applied, comprises the main process; The following steps are carried out: The web is conveyed between the electrodes of different potentials:: ··: inorganic material is applied to the web surface ·! ·, and a coating powder comprising polymeric binder material is utilized, and the coated surface of the web is stamped. The coating powder is applied in a powder application unit. The coated surface of the 115547 nanoparticle is finished, for example, by thermomechanical treatment. The fiber portion of the continuous web to be treated is generally composed of papermaking fibers, but other basic materials may also be used. For example, the dry surface treatment method may be used to coat plastic or metal films.

The barrier according to the invention is placed just before the powder application unit. The barrier may be a mechanical barrier or may be based on gas flows such as air flows. The mechanical barrier is brought into contact with or close to the web so that it is not in direct contact with the web but close enough to perform the desired operation. When the mechanical barrier is in contact with the web, it may be necessary to have a counter surface on one side of the web to support the barrier. Suitable mechanical barriers include, for example, scrapers such as blades, but other mechanical barriers may be used. Other obstacles may be bars or plates, for example. Obviously, the barrier extends over the entire width of the web in the transverse direction of the web.

When the barrier is based on gas flows, both suction and blowing are $ 20. The suction or blowing means are oriented so as not to interfere with the powder application process; preferably, they are directed to suck or j * blow in the direction opposite to the web running direction.

: An additional advantage is obtained when using suction; • All foreign particles can be removed very cleanly from the fabric before applying the powder. Like mechanical barriers, barriers based on gas flows extend over the entire width of the web. The suction or discharge means may comprise, for example, a series of ducts or a long slot through which gas, usually air, is supplied. blown or sucked.

The distance between the barrier and the powder application unit in the longitudinal direction of the web is also important. Usually the distance is up to 100 mm. According to the applicant's ·: ··· experiments, the air boundary layer disappears only momentarily and,:, reappears at a certain distance from the obstacle.

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The dry surface treatment process in which the barrier according to the invention is placed will now be described in more detail. An electric field is used to apply the coating powder to transfer the coating particles to the surface of the paper and to allow electrostatic adhesion prior to the finishing step. Both final adhesion and leveling of the coating surface layer are simultaneously performed by thermomechanical treatment or other suitable treatment.

The coating powder comprises either separate inorganic material particles and polymeric binder particles or particles containing both inorganic material and polymeric binder material (so-called hybrid particles). The average diameter of the material particles is generally 0.1 to 500 µm, preferably 1 to 15 µm. A particle size close to 10 µm is in most cases the most suitable for charge characteristics. The coating powder material should be taken into consideration since the coating powder components may have different electrical properties, such as particle surface charge and discharge rates. The coating powder may comprise up to 99.5% by weight (dry weight) of an inorganic material, with the remainder preferably being 20 polymer binder materials. The coating powder is in substantially dry: '·· (moisture content less than 10% by weight) and comprises air and mate; j * particles in the air-particulate mixture of more than 11%.

• In the dry surface treatment process, the powder is sprayed with a high electrical: v. 25 through the field and a high free ion concentration area to the substrate surface. The coating powder is placed in the coating feeder.

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into the chamber and transferred to the powder application unit by means of compressed air. The coating powder is charged in the powder application unit.

* ·; · * 30 The dry powder is fed to the powder application unit by means of compressed air or any other particulate carrier. Tenderness: · *; power and current are varied by the required distance between charge and earth electrodes, material properties of electrodes (e.g., dielectric constants), powder composition (organic to inorganic '·· * 35 ingredient ratio, powder dielectric constants, etc.), powder volume, and 4115547 feed mass, under pressure. Voltage ranges from 5 kV to 1000 kV and current from 30 pA to 1000 A. The powder properties and application concept control the structure of the charge electrodes. However, the charge electrodes are either positive or negative.

In practice, a grounded electrode may be a static ground plate or a moving earthing device. The moving device may be a rotating device, for example an earthing roll or an endless conductive wire or belt. The ground roller may form a nip with the hot roller, which at least partially melts the binder of the coating powder. The finishing can be completed in the next nip formed by the hot calender and the soft surface roll. A ground roll, a hot roll, and a soft roll roll can form a calender stack. The web in contact with the ground roll is grounded from the shape of the ground roll and the hot roll to the nip of the 15 ground. The web may also pass through other nipples. The finishing can also be completed using chemicals or suitable radiation, such as UV radiation, for attaching the coating powder to the web.

The application of the coating powder may also be carried out using a belt or the like. The belt is charged with a corona charging electrode to provide a uniform charge on the entire belt surface. The strap must be · sufficiently resistive to maintain its charge. Busy belt; adheres to the coating powder particles and transports them over the web to be coated. The particles are released from the belt using a corona charge

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. ···. an electrode having a polarity opposite to the polarity of the corona charging electrode used to charge the belt. In this case, barriers may be needed on both the belt and the web.

One way to charge the coating powder instead of the corona is to transfer the V-j coating powder particles by means of a static electric field between the high voltage electrode and the grounded channel supplying the coating particles: ···: The base material is not prepared for the effect of the field because the base material has no free ions and no grounding of the base material is required. The voltage used is preferably 5 115547 60-80 kV. An earthed high power refiner may be used instead of a grounded channel. Large agglomerates are comminuted into fine particles and some additives may be added to the refiner.

Some additives may also be sprayed simultaneously with the coating powder. They are preferably in liquid form, but solids are also used. The additive is charged with a charge similar to that of the coating powder and blown among the coating powder. The additive may be, for example, water, limestone, cationic starch, polyvinyl alcohol in granular form or carboxymethyl cellulose.

In the dry surface treatment process, both sides of the web can also be coated simultaneously. Each side of the web at the same time, the ground electrode 15 for coating can be replaced with an electrode having a polarity opposite to the first electrode. The web is sandwiched between the two electrodes, whereby particles attracted by the electric field, which are of opposite sign, place them on the surface of the web. If the first electrode is negative, the second electrode on the opposite side of the web 20 is positive, and vice versa. When the first corona charge electrode is negative, the coating powder particles charged with the negative corona charge electrons move toward the positive corona charge electrode: located on the other side of the web. The potential difference between the two electric fields' · *: 25 is considerable, and thus the two electrodes reinforce each other. In this embodiment, barriers are required on both sides of the web.

The dry-coated base material may also comprise more than 30 coatings on one side of the base material. Layers ·. ·; can be different. Provisions created for the application of the coating powder may be eliminated or modified so that: ·! ·. they have a different mark after the coating powder has been applied with heat and pressure. When the first application is carried out with a negligible charge, the second application can be performed with a positive charge of 6 115547, whereby the layers are properly bonded to one another by the action of an electric pull. Obstacles must be placed one after the other.

Significant reduction in the amount of polymer binder in the dry powder 5 has been achieved by further optimized bonding conditions (such as surface wetting, web moisture content, dwell time, surface temperature and line load). The concentration of the polymer binder and its thermoplasticity during thermomechanical processing determine the properties of the paper, such as the density, transparency, smoothness, strength and optical properties of the coating layer. In some cases, a binder content of less than 10% by weight is sufficient to achieve sufficient surface strength. The bonding polymers have a glass transition temperature (Tg) ranging from 20 ° C to over 100 ° C, whereby the minimum glass transition temperature (Tg) is limited by the required drying and grinding conditions. The use of other binders, such as starch, has provided some desirable paper properties in combination with higher moisture content of the base material or wetting prior to thermomechanical fixing. Moisture can dissolve the starch granule and allow it to act as a binder under certain process conditions, but it is not as effective as copo-20 polymer latex binder. The starch may be prepared as a dry granulate by grinding, but is preferably dissolved in a liquid to achieve its binder properties.

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Preferred ranges for thermomechanical treatment are: Temperature 25 between 80 and 350 ° C, a line load of 25-450 kN / m and a residence time of 0.1-100 ms. downtime (speed 150-2500 m / min; nip length 3-1000 mm). The attachment can be reinforced in various ways to achieve the desired paper properties. In this novel process solution, the polymer also causes the coating layer to physically adhere to the surface of the paper, which replaces the lack of penetration effect and mechanical bonding in conventional processes. Thermo ·: ·: Mechanical treatment can be carried out by several calendering methods or by calendering-like methods. The methods utilize-L. nips formed between rollers, or substantially long nips formed between two counter surfaces 35. Such nipples include 7115547 hard nip, soft nip, long nip (e.g., shoe press or belt calender), Condebelt type calender and supercalender.

One of the most important things in thermomechanical bonding is the non-stick nature of the roll surfaces, which prevents the polymer-based layers from sticking together, sticking or otherwise piling up. When using powders with a polymer content of less than 20% by weight, hard roll coating materials such as hard chromium or tungsten carbide based materials are suitable. When using powders of high polymer content, the non-stick properties of the roll surface must be improved and, for example, Teflon-based materials may be used as coating material. Another way to avoid the above problem is to use a calender formed between a hard hot roll and a soft surface roll. The web is conveyed to the nip so that the coating layer contacts a soft surface roll. The heat acts through the web to melt the binder, especially the lower part of the coating layer, thereby promoting adhesion of the coating powder.

As an alternative to the heated roll, a suitable solvent may be used which dissolves the binder, or suitable radiation which melts the binder.

: “The wavelength of the radiation is chosen so that the radiation is not absorbed into the web .. Γ but in the coating powder. The radiation unit can be followed by a calender which * j performs a sufficiently strong compression treatment. The roll in contact with the coating layer jf is a flexible roll.

25; * ··. Increased moisture content of the base paper can improve the spread and adherence of the powder to the substrate surface. The incoming moisture content (e.g., paper pulp moisture) can be maximized or adjusted to optimize the layer strength and other properties of the paper. For example, starch requires a higher moisture content than V in the copolymer latex binders in order to achieve corresponding surface strengths in surface-treated paper or board. This is explained by the star, ···. need to dissolve the lysate to achieve the bonding properties, and more energy is needed to evaporate the water. The moisture of the surface 35 can also be adjusted by nozzle application to the surface of the base material 8 115547. In this case, only the volatile water is applied during the attachment process and the moisture balance of the attachment step remains stable. Nozzle application may be performed prior to application of the powder or thermomechanical attachment.

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The invention will now be described with reference to the drawings, in which Fig. 1 schematically depicts a process of double sided coating, 10 Fig. 2 schematically depicts a process of forming two overlays, and Fig. 3 illustrates a process in which duplexing is performed on each side.

In Figure 1, there is a barrier 1, such as a blade, on each side of web W and a powder spreading unit 2. The air boundary layer is at least partially removed by the barrier before the web W is coated with dry 20 coating powder. Opposite the application unit is an electrode 3 which is: · · different in potential from the application potential of at least one electrode of the application unit. The electrode 3 may be grounded. First coating layer: applied on nip N1, and second coating applied on nip • N2. The N1 and N2 nipples are fastened with heat and pressure.

! 25! ··. In Figure 2, the principle is the same except that both coating layers are formed on the same side of web W. Therefore, successive obstacles 1 are needed.

In Figure 3, both sides of the web W are simultaneously coated, y.i The electrodes of the spreader units 2 are at different potentials on opposite sides of the web. In this embodiment, only one nip is required for the fastening step. The barrier 1 must be on both sides of the web *; *, * before the application unit 2.

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The foregoing embodiments of the invention do not limit the scope of the claims. The basic idea of the invention is that the air boundary layer can be removed so that dry powder can be more easily applied to the web.

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Claims (4)

A method of coating the surface of a continuous web with a coating powder, which method comprises the following steps: 5. the web is allowed to move between electrodes which are at different potentials, one spreads on the surface of the web one of an inorganic material and a polymer binder. resistant coating powder by utilizing the difference of the electrical potential, and - the fine surface of the web is honed, characterized in that, until the web is allowed to move between the electrodes, the air interface layer is removed at least partially with an obstacle. Method according to claim 1, characterized in that the obstacle is a mechanical obstacle. Method according to claim 2, characterized in that the obstacle is a scraper. 20 Method according to claim 1, characterized in that the obstacle is formed by at least one gas flow. *. · T ->) I * • ·