WO2005061126A1

WO2005061126A1 - Control of air boundary layer

Info

Publication number: WO2005061126A1
Application number: PCT/FI2004/050190
Authority: WO
Inventors: Johan Grön; Reima Kerttula
Original assignee: Metso Paper, Inc
Priority date: 2003-12-22
Filing date: 2004-12-21
Publication date: 2005-07-07
Also published as: FI115547B; FI20035247A0

Abstract

The present invention relates to a method for coating a surface of a continuous web with a coating powder comprising steps of : Allowing the web to move between electrodes, which are in different potentials; Applying the coating powder comprising inorganic material and polymeric binder material on the surface of the web by utilizing the difference in the electric potential; And finishing the coated surface of the web. Before the web is allowed to move between the electrodes the air boundary layer is removed at least partially by an obstacle.

Description

Control of air boundary layer

The present invention relates to a method for coating a surface of a continuous web with a coating powder comprising steps of: - allowing the web to move between electrodes, which are in different potentials, - applying the coating powder comprising inorganic material and polymeric binder material on the surface of the web by utilizing the difference in the electric potential, and - finishing the coated surface of the web.

A problem with the above-mentioned method is that an air boundary layer, which forms on the surface of the web, prevents the coating powder to reach the surface of the web. Consequences of such phenomenon are for example an uneven coated surface and dusting in the process. To cross the air boundary layer, more energy is needed when speeds in the coating process increase. The roughness of the web has also an effect on the air boundary layer.

The method of the invention is characterized in that that before the web is allowed to move between the electrodes the air boundary layer is removed at least partially by an obstacle.

When the air boundary layer is removed at least partially the coating powder places itself easily on the web, transfer to the web is more efficient, and dusting diminishes remarkably. Depending on the effectiveness of the obstacle, it is possible that the air boundary layer is removed totally.

The dry surface treatment process of paper or board substrates, in which the method of the invention is mainly applied, comprises principally the following steps: Allowing the web to move between electrodes, which are in different potentials, applying the coating powder comprising inorganic material and polymeric binder material on the surface of the web by utilizing the difference in the electric potential, and finishing the coated surface of the web. The coating powder is applied at the powder application unit. The coated surface of the web is finished for example by a thermomechanical treatment. The fibrous portion of the continuous web to be treated usually consist of papermaking fibres but also other substrates can be used. For example, the dry surface treatment process can be used to coat plastic or metallic films.

The obstacle of the invention is placed just before the powder application unit. The obstacle can be a mechanical obstacle, or it can be based on gaseous flows, such as air streams. The mechanical obstacle is put in contact with the web, or close to the web in such a manner that it is not in a direct contact with the web but close enough to carry out the desired act. When the mechanical obstacle is in contact with the web it may be necessary to have a counter surface on the other side of the web to support the obstacle. Suitable mechanical obstacles include for example doctoring devices, such as blades, but other mechanical obstacles are as well usable. The other obstacles can be for example bars or plates. Naturally, the obstacle extends over the whole web in the cross direction of the web.

When the obstacle is based on gaseous flows both suction and blowing are included. The suction or blowing means are directed in such a manner that they do not disturb the powder application act, preferably they are directed to suck or blow in the opposite direction compared to the running direction of the web. By using suction a further advantage is achieved: Before the powder application any foreign particles can be removed from the web in a very tidy manner. As with mechanical obstacles, the obstacles based on gaseous flows also extend over the whole width of the web. The suction or blowing means can comprise for example a row of ducts, or a long slit through which gas, usually air, is blown or sucked.

The distance between the obstacle and the powder application unit in the length direction of the web is also important. Normally, the distance is at the most 100 mm. According to tests made by the applicant, the vanishing of the air boundary layer is only momentary and it reappears at some distance from the obstacle.

The dry surface treatment process in which the obstacle of the invention is placed is explained next in more details. The application of the coating powder utilises an electric field to transfer the coating particles to the paper surface and to enable an electrostatic adhesion prior to the finishing step. Both the final adhesion and the surface smoothening of the coating layer are executed simultaneously through thermomechanical treatment or another suitable treatment.

The coating powder comprises either separate inorganic material particles and polymeric binder material particles or particles including both inorganic material and polymeric binder material (so-called hybrid particles). The average diameter of the material particles is usually 0.1 - 500 μm, preferably 1 - 15 μm. A particle size close to 10 μm is in most cases preferable in respect to charging properties. The coating powder material shall be taken into consideration because the components of the coating powder can have varying electrical properties, such as particle surface charging and discharging rate. The coating powder may comprise up to 99.5 wt-% (dry weight) of inorganic material and the rest is preferably polymeric binder material. The coating powder is in a substantially dry form (moisture content under 10 wt.-%), and it comprises air and the material particles whose portion in the air/particle mixture is above 1 vol.-%.

In dry surface treatment process, the powder is sprayed through an area of strong electric field and high free-ion concentration to the surface of the substrate. The coating powder is put into the coating feeder chamber and transferred to the powder deposition unit with compressed air. The coating powder is charged in the powder deposition unit. The dry powder is supplied to the powder application unit with compressed air or another transport medium that promotes particle charge. Voltage and current are varied with the required distance between the charging and the grounding electrodes, the material properties (e.g. dielectric constants) of the electrodes, the powder composition (organic-inorganic ratio, dielectric constants of the powder etc.), the powder amount, the supply medium moisture content, and pressure. The voltage varies from 5 kV to 1000 kV and the current from 30 μA to 1000 A. The powder properties and the application concept guides set-up of the charging electrodes. The charging electrodes are however either positive or negative.

In practice, the grounded electrode may be a static grounding plate or a moving grounding device. The moving device can be a rotating device, for example a grounding roll, an endless conductive wire, or belt. The grounding roll may form a nip with a hot roll, which at least partially melts the binder of the coating powder. The finishing can be finalised in the next nip formed by the hot calender roll and a resilient roll. The grounding roll, the hot roll and the resilient roll can form a calender stack. The web in contact with the grounding roll is earthed down to the nip formed by the grounding roll and the hot roll. It is possible that there are also other nips through which the web travels. The finishing can also be finalised by using chemicals, or a suitable radiation, for example UV radiation, to fix the coating powder to the web.

The application of the coating powder may also be done by using a belt or a like. The belt is charged by a corona charging electrode to have an even charge all through the surface of the belt. The belt shall have sufficiently high resistivity because the belt shall maintain its charge. The charged belt catch the particles of the coating powder and convey them over the web to be coated. The particles are released from the belt by using a corona charging electrode having an opposite polarity compared to the polarity of the corona charging electrode used for charging the belt. In this case, the obstacles may be required both at the belt and the web.

One possibility to charge the coating powder instead of using corona is to transfer the particles of the coating powder by using a static electric field between a high voltage electrode and an earthed duct supplying the coating powder particles. The substrate is not charged by the field because there are no free ions and there is no need to ground the substrate. The voltage used is preferably 60 - 80 kV. Instead of a grounded duct can be used a grounded heavy-duty grinder. The large agglomerates are ground to fine particles and it is possible to add some auxiliary substances to the grinder.

Some auxiliary substances can also be sprayed simultaneously with the coating powder onto the web. They are preferably in a liquid form but also solids are used. The auxiliary substance is charged to have a similar charge as the coating powder and it is blown among the coating powder. The auxiliary substance may be for example water, lime water, cationic starch, polyvinylalcohol in a granular form or carboxymethylcellulose.

In the dry surface treatment process, it is also possible to coat the both sides of the web simultaneously. To coat the both sides of the web at the same time, a grounding electrode can be replaced by an electrode having an opposite polarity compared to the first electrode. The web is between the two electrodes and hence the particles drawn by the electric field having an 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 is positive and vice versa. When the first corona charge electrode is negative the particles of the coating powder charged by negative electrons of the negative corona charge electrode move towards the positive corona charge electrode which is located on the other side of the web. The difference in potentials of the two electric fields is remarkable, and thus those two electrodes strengthen the function of each other. In this embodiment, the obstacles are required at both sides of the web.

The dry coated substrate may also comprise more than one coating layer on the same side of the substrate. The layers can be different from each other. The charges, which are formed for the application of the coating powder, can be eliminated or changed to have a different sign after fixing the coating powder with heat and pressure. When a first application is done by a negative charge a second application can be made by a positive charge and hence the layers are adhered to each other properly due to the electric attraction. The obstacles shall be placed sequentially.

A considerable reduction of the polymer binder content in the dry powder has been achieved due to further optimised fixing conditions (e.g. surface moisturizing, moisture content of the web, dwell-time, surface temperature and linear load). The polymeric binder concentration and its thermal deformability during thermomechanical treatment determine paper properties such as a coating layer density, openness, smoothness, strength, and optical properties. A binder content of less than 10 wt.-% is in some cases enough to give a sufficient surface strength. The glass transition temperature (T_g) of the binding polymers have ranged from 20 to over 100 °C, where the lowest glass transition temperature (T_g) has been restricted by the required drying and refining conditions. Usage of other binders, such as starch, has given certain desirable paper properties in combination with higher base substrate moisture content or moisturising prior to the thermomechanical fixation. The moisture may dissolve the starch granule and allow it to work as a binder under certain process conditions, but less effective than the copolymer latex binder. Starch can be produced dry as a granule through grinding, but preferable dissolved in liquid to gain its binding properties.

The preferred ranges for the thermomechanical treatment are: The temperature of 80-350°C, the linear load of 25-450 kN/m and the dwell time of 0.1-100 ms (speed 150-2500 m/min; nip length 3-1000 mm). The fixation can be reinforced in different ways to achieve desired paper properties. In this novel process solution, the polymer also creates physical adhesion of the coating layer to the paper surface, which replaces the lack of a penetration effect and mechanical interlocking present in a conventional process. The thermomechanical treatment can be made by various calendering methods or calendering-like methods. The methods utilize nips formed between rolls, or substantially long nips formed between two counter surfaces. Examples of such nips are hard-nip, soft-nip, long-nip (e.g. shoe-press or belt calender), Condebelt-type calender and super-calender.

One of the most essential parts in the thermomechanical fixing is the non-adhesive property of the roll surfaces to avoid blocking, sticking, or other build-up of polymer based deposits. When powders with the polymer content less than 20 wt.-% are used, hard roll cover materials such as hard chrome or wolfram-carbide based are suitable. When powder with a high polymer contents are used, the roll cover must have better non-sticking properties, e.g. usage of Teflon based cover materials. Another way to avoid the above mentioned problem is to use a calender comprising a nip formed between a hard hot roll and a resilient roll. The web is conveyed to the nip so that the coating layer touch the resilient roll. The heat acts through the web melting the binder, especially the lower part of the coating layer thus enhancing the adherence of the coating powder.

An alternative to the heated roll is to use a suitable solvent to dissolve the binder, or a suitable radiation to melt the binder. The wave length of the radiation is chosen so that the radiation does not absorb into the web but into the coating powder. After the radiation unit there can be a calender to give a sufficiently strong pressure treatment. The roll in contact with the coating layer is a resilient roll.

Increased surface moisture content of the base paper may improve the powder deposition and fixing to the substrate surface. An incoming substrate moisture content (e.g. paper bulk moisture) can be maximised or adjusted to optimise the layer strength and other paper properties. For example, starch requires a higher moisture content than copolymer latex binders to reach equivalent surface strengths of the surface treated paper or board. This can be explained by the need to solubilise the starch to give binding properties and then an excess energy is required for the water evaporation. The surface moisture can also be adjusted through nozzle application onto the substrate surface. Then only a water amount evaporating in the fixing process is applied and the moisture balance over the fixing stage remains constant. The nozzle application can be done before the powder application or the thermomechanical fixing.

In the following, the invention will be described by means of figures in which

Figure 1 shows a schematic view about a process in which a two- sided coating is accomplished,

Figure 2 shows a schematic view about a process in which two coating layers are formed on top of each other, and

Figure 3 shows a schematic view about a process in which a two- sided coating is accomplished at the same time to the both sides.

In figure 1 , on both sides of a web W there is an obstacle 1 , such as a blade, and a powder application unit 2. The air boundary layer is removed at least partially by the obstacle before the web W is coated with dry coating powder obtained from the application unit 2. Opposite to the application unit there is an electrode 3, which in a different potential compared to a potential of at least one electrode in the application unit 2. The electrode 3 may be grounded. The first coating layer is fixed in a nip N1, and the second coating layer is fixed in a nip N2. The fixing capability of the nips N1 and N2 is based on heat and pressure.

In figure 2, the principle is the same except the both coating layers are formed on the same side of the web W. Thus, sequential obstacles 1 are required.

In figure 3, the both sides of the web W are coated simultaneously. Electrodes in the application units 2 have different potentials at the opposite sides of the web. In this embodiment, only one nip is required for the fixing step. The obstacle 1 has to be before the application unit 2 on both sides of the web.

The above-mentioned embodiments of the invention do not restrict the scope of the claims. The main idea of the invention is that the air boundary layer can be removed so that the dry powder can be applied to the web more easily.

Claims

Claims:

1. A method for coating a surface of a continuous web with a coating powder comprising steps of: - allowing the web to move between electrodes, which are in different potentials, - applying the coating powder comprising inorganic material and polymeric binder material on the surface of the web by utilizing the difference in the electric potential, and - finishing the coated surface of the web, characterized in that before the web is allowed to move between the electrodes the air boundary layer is removed at least partially by an obstacle.

2. The method according to claim 1, characterized in that the obstacle is a mechanical obstacle.

3. The method according to claim 2, characterized in that the obstacle is a doctoring device.

4. The method according to claim 1 , characterized in that the obstacle is formed of at least one gaseous flow.