Description of Related Art Paper and board webs are often coated with dispersions of pigment slurry and binding polymers and other additives. The most widely used pigments include kaolin, calcium carbonate and precipitated calcium carbonate. The purpose of coating is to improve surface smoothness, printability and ink picking, the opacity and surface gloss of the products and also often to improve barrier properties of the web.

The normal solids content of a coating suspension, in the following also called a "coating colour", is nowadays often near 70 % and the amount of binding polymer is between 3 and 15 % of the final dry weight of the coating layer. The viscosity of coating colours increases rapidly when the solids content is increased, whereby it becomes difficult or impossible to apply the coating colour to a web. On the other hand, if the water content is increased, more drying capacity is needed for drying the coated web. The lower the solids content, the easier is the coating process and the more expensive is the drying process. Drying also requires time because of the long path of the water molecules for diffusion to the outer surface of paper, where from the evaporation happens to the air. Because of the extended drying times, long drying sections are needed especially in case of thick coating layers.

Long drying sections are also necessary for drying coated cardboards, because water is absorbed deep into the base web and it is difficult to evaporate the water from within the web.

It is known in the art to dry paper and cardboard webs by using infrared dryers.

The very high energy impact imposed on the web during, e. g. high intensity infrared drying, may however cause water to boil within the coating layer and the dissolved gases give rise to porosity in the coating layer. In order to avoid porosity, diffusion of water from the pigment coating layer must be obtained promptly in order to prevent water from boiling. This becomes more difficult as the particle size of the pigments becomes smaller. The smaller the particle size, the closer the particles are to each other and the more difficult it is for water or steam to pass through the particle layer.

In order to increase productivity, web speeds of paper and paperboard making machines are rapidly increasing. The higher speed requirements of on-and off- line coating apparatuses lead to a need for higher drying rates. Since it is difficult to increase the rate of heat energy applied to the web within a prescribed time period without causing damage to the base web or to the coating, other means for improving water removal are necessary.

It is known in the art to remove water from different objects by ultrasonic treatment and also by so called electroacoustic effects. In these methods, an acoustic signal is directed to the object, which is subjected to water-removal, and the energy of the signal causes water molecules to move whereby they are separated from the object.

Drying paper webs by ultrasonic treatment has been carried out by sonicating the surface of a drained wet web with an ultrasound whistle and by treating the web with a pressure difference at the same time (cf. R. E. White, Tappi J. Vol 47. No 8 August 1964, and Tappi J. May 1986/US Patent 4, 561, 953). These tests showed that the studied system worked, but it is expensive.

Further developments have led to a new method called Electro-Acoustic Dewatering (EAD) comprising the simultaneous use of electrosmosis and sonic treatment for separating particles from liquid via filtration. To mention an example, in US Patent 4, 561, 953 ultrasound is used in combination with an electric field for removing water from sludges and slurries containing fine particles. In said method, water is removed through a permeable electrode, which makes the method suitable only for media which are in liquid phase or which can be pumped. Since it

is almost impossible to place an operating permeable electrode in contact with a moving web, these known methods cannot be used for removing water from paper webs and other moving webs.

In all the above-mentioned articles, the source of acoustic energy comprises sirens arranged at a suitable distance from the object to be dewatered. Even though up to 10 times higher drying rates have been found and the moisture has been reduced from 40 to 15 % with the aid of ultrasound, the known methods have never come to practical implementation. This is probably partly because the methods described in the above-mentioned TAPPI articles were designed for steady state sheet-by-sheet processes. Further, the known methods do not solve the problems of continuous processing ; dewatering by filtration is also problematic in continuous mode if applied to solid moving webs.

Summary of the Invention It is an object of the present invention to eliminate the problems of the prior art and to provide a novel method for dewatering a moving web.

It is another object of the invention to provide an apparatus for dewatering moving webs.

The present invention is based on the concept of subjecting a wet web to a plurality of successive ultrasonic signal bursts in order to achieve water removal from the web or coating. The expression"ultrasonic signal burst"signifies a distinct ultrasonic vibrational signal having a limited (temporal) duration. Typically, the duration of the burst is about 0. 0001 to 10 s, preferably about 0. 001 to 1 s.

When ultrasonic vibration is directed towards a water-containing matter, the energy of the water molecules is increased and a mist is formed. The formation of the mist is greatest at the beginning of the vibration period. Therefore, pulsed ultrasonic bursts according to the present invention are much more effective than regular steady wave fronts. When an ultrasonic burst starts, its influence on the formation of water droplets is extremely high compared to continuous operation of an ultrasonic emitter. One reason for this is probably the large energy consumption by potential counter-effects caused by the reattachment of the

formed droplets to the surface and by resonance in the treated matter.

The basic idea of the present invention can be implemented by arranging the web to run over a plurality of bars (having a limited extension in lateral direction). Using said bars mechanical high frequency ultrasonic vibration is applied to the running web or another moving object which is to be dried directly on contact with the running web or another moving object that is dried. The shorter the contact time is, the more efficient the action of the vibration is per length or per time unit. It is preferred to subject the web to instant vibrations of a pulse so that the web as such does not or does not essentially being to vibrate at all, only the water is expelled from the web.

Alternatively, if direct contact is not possible, the ultrasonic waves are focused through a resonance reflector onto the web, and the formed mist or water droplets are removed from the vicinity of the web.

An apparatus according to the present invention for implementing the above- described method of dewatering a moving web comprises at least one ultrasound emitter which extends over the width of the web. The emitter has a limited length in the travel direction of the web. It comprises an elongated and narrow bar or similar having a width of less than 10 %, preferably less than 5 %, and in particular about 0. 1 to 5 % of its length (the width, i. e. the contacting length, is, e. g. about 1 to 50 cm). Its central axis is placed essentially transversally to the running direction of the web. Further, the apparatus comprises means, such as rolls, for passing the web via the ultrasound emitter, and means, such as a fan or suctional means for removing the mist formed by the emitter from the vicinity of the web.

More specifically, the present invention is mainly characterized by what is stated in the characterizing part of claim 1.

The apparatus according to the present invention is characterized by what is stated in the characterizing part of claim 10.

Considerable advantages are obtained by the present invention. Thus, energy can be saved and the temperature gradient of the handled web is not a limiting factor in drying of paper and paper board web, as is the case with conventional drying,

when a decrease of the drying time or an increase of web speed is desired.

Furthermore, the coating layer can be effectively compacted to provide a barrier to water and water vapour. The present invention can be carried out in connection with modern drying methods such as Conderbelt drying.

Other objects and features of the invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are intended solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.

Brief Description of the Drawings Figure 1 shows diagrammatically a first embodiment of the invention, comprising web rollers and ultrasonic emitter bars placed in intimate contact with the web ; Figure 2 shows diagrammatically a second embodiment of the invention, comprising web rollers and focussing means ; Figure 3 shows diagrammatically a third embodiment of the invention, comprising focussing means placed on both sides of the web ; Figure 4 shows diagrammatically a fourth embodiment of the invention, wherein ultrasonic emitters are combined with wire mesh arrangements ; and Figure 5 shows diagrammatically as a side-view a rotating cylinder provided with a layer of piezocrystals on the surface.

Detailed Description of the Invention As already briefly discussed above, a major finding of the present invention is that ultrasonic energy can generate a great number of microdroplets on a wet surface.

These can be driven out as a liquid phase without direct evaporation to surrounding air or hot air. We have found that a separate, generally laminar aqueous phase will, at least on some occasions, be formed on the web or coating.

Subjected to ultrasonic vibration, water present in that aqueous phase will give rise to microwaving or surface waving. As a result, the aqueous phase will increase mass and heat transfer in the boundary layer between air and web and enhance dewatering of the web. The thickness of the aqueous layer subjected to microwaving depends on the web and on the ultrasonic energy applied, but it is

typically in the range of about 0. 1 to 100 um.

The prior art mentioned above fails to reveal any a method of the present kind, in which water is removed from webs and coatings covering the webs via air in the form of tiny droplets achieved by ultrasonic energy source which vibrates in contact underneath the object to be dried or is focused to form a non-contacting ultrasonic handling line on web to be treated.

The ultrasonic vibration according to the present invention is directed to the web with an elongate ultrasonic emitting bar or a focused emitter extending over the width of the web. The length of the area over which the effect of the energy is targeted is very narrow in the travelling direction of the web. When several successive emitters are used, this leads directly to pulsed delivery of the ultrasonic vibration. The distance between the ultrasonic emitters or source bars depends on the speed of the web. Preferably, the ultrasonic emitters are spaced apart at such a distance that the previous burst of microwaves accompanied by drops or droplets (or a fog) of water is removed before the next ultrasonic burst is directed towards the web. Typically the distance between the emitter bars is in the range of 0. 01 to 20 m, preferably about 0. 1 to 10 m. The time interval between two successive ultrasonic burst is, depending on the speed of the web, about 0. 01 to 10 s, preferably about 0. 1 to 1 s. The energy intensity of the ultrasonic vibration energy directed to the web is generally in the range of 3 to 1000 W/cm2 in particular 5 to 100 W/cm2. The frequency is 0. 01 to 10 Mhz, preferably about 1 to 1000 kHz.

The method according to the present invention can be carried out at ambient temperature, although an increased temperature will enhance water removal.

Thus, the temperature of the web is preferably kept over 60 °C, in particular at about 65 to 95 °C, during ultrasonic treatment,.

We have found that focused contact is almost as effective as direct contact. For that purpose, emitter bars or other emitter means (cf. Figure 5) can be replaced with ultrasonic emitters equipped with focussing means in order to focuse the vibration between 10 to 200 times with a convergent metal focuser which is vibrated in direct contact with the ultrasound source. Focusing will concentrate the ultrasonic vibration effect to a very narrow part of the treated web in the direction

of the travel of the web, of course depending on the type of the focusing used.

According to the invention the water droplets are removed either with an air stream only or with an air stream augmented with an electric field or a ion basting method.

As will appear from the examples below, according to a first apparatus embodiment several contacting bars emitting ultrasonic vibration or focused ultrasonic emitters can be arranged sequentially in the running direction of the web along the path of the web, each emitter producing a short pulse of ultrasonic energy on a length of the web travelling past the emitter. The bars or similar elongated means are mounted such that their central axes extend essentially transversally to the running direction of the web.

The ultrasonic emitter bars can also be arranged on opposing sides of the web in an alternating fashion. This makes it easier to adjust the flatness of papers. This embodiment also provides for a more regular orientation of the fillers, fibrils etc. contained in the web (one-way effect) because of regular dewatering of both sides.

Sectional application can employed for regulating the profile of the web.

The present invention can be applied to uncoated (base) papers and paperboards as well as, and preferably, to paper and paperboard webs which have been coated one time or several times. Regarding the treatment of coated webs it should be pointed out that the higher speed needed for on-and off-line coaters at paper making processes requires higher drying rates. Compacting the pigment of the coating layer in order to drive each particle as near to the other particles as possible is particularly advantageous. If the pigment layer can be compacted and water separated from it before evaporation, the drying will become much easier later on. At the same time it is desirable to coagulate and compact also the emulsified binding aid of the coating slurry. Coated paper or board webs also need an equal diffusion of binding polymer parts throughout the coating layer.

Taking out water of the web in the present way means coming close to an optimal distribution of binders and fillers before polymerization takes place. This might help to reduce the quantity of polymers.

All the above features can be achieved when the coating layer is handled with ultrasonic energy by direct contact to the wet web or with focused indirect contact.

It is known that ultrasonic waves create instabilities at phase boundaries, but the use of that phenomenon in web drying methods have not been found earlier.

Also a more dense coating can be achieved by subjecting a coating layer to an ultrasonic treatment than with a normal drying process. This improves the barrier properties of the coating layer as well as the whole product.

A further feature and benefit of the present invention comprises treating base webs containing fiber bunches. As known in the art, webs contain fibers that are not equally distributed throughout the web but form denser bunches and areas where the amount of fibers in volume unit is larger than elsewhere in the web.

These areas are stiffer than other parts of the web because they are thicker and more dense. The fiber bunches also dry more slowly than the surrounding areas.

This inhomogenity causes stresses in the web and leads to curling of the material.

However, it has now turned out that ultrasonic treatment and water removal of base webs of paper and paper board has an equalizing effect. When the web is subjected to ultrasonic treatment, the ultrasonic vibration homogenizes the moisture on these denser areas and prevents too early drying of the surface of these areas. No curling occurs because of stresses formed during drying. The effect of ultrasound appears to be more pronounced the denser the material is.

Therefore the denser parts of the material will absorb more energy than thinner areas. Strong ultrasound bursts may even partly break the denser parts of the web and, thus, equalize differences in density.

It is preferred, although by no means mandatory, to carry out the present ultrasound treatment of the web at the very beginning of the wet web handling. In many paper machine operations and similar drying operations, the wet web runs between two wire meshes and water is withdrawn in both directions. Normally this section is vertical so that web runs upwards. At this stage the ultrasound vibration by direct contact method is effected very easily.

In the appended drawings, the following reference numerals are used : 1, 43 web

2 film transfer coater 3, 13, 24 guide roll 4, 14, 24 guide roll 5 ultrasonic emitter bars 7, 17, 27 housing 8, 18, 28 air inlet 9, 19, 29 air outlet 16, 26 reflector means 31 guide rolls 32 wire mesh 33 suction means 34 emitter means 41 rotating cylinder 42 piezo crystal segment Figure 1 shows a water removal method according to the invention with a film transfer coater 2. The web 1 to be coated is lead to the applicator roll of the film transfer coater, wherein the coating mix spread on the applicator roll attaches on the web 1. Thereafter the web 1 is supported and guided by two guiding rolls 3, 4 arranged at a distance from each other in direction of the travel of the web. In this example the guide rolls 3, 4 are arranged on same level horizontal, but any other arrangement is also feasible. Ultrasonic emitter bars 5 that extend over the width of the web are arranged between the guide rolls. On the opposite side of the web there is arranged a housing or case 7 by which the mist emitted from the surface of the web is collecte. For this purpose, air is fed into the case through tube 8 and moist air is sucked from the case through tube 9.

The operation of this apparatus is very simple. The emitter bars 5 direct ultrasonic energy onto the web and water within the web forms droplets that exit from the surface of the web and are collecte in the air stream.

In the embodiment shown in the Figure 2 the emitter bars 5 of Figure 1 are replace with ultrasonic emitters equipped with focussing means 16.

As shown Figure 3, the emitter bars, or as the case is in Figure 3, the reflector/emitters 26 can also be arranged on opposing sides of the web in an alternating fashion. On the opposite side of each emitter or reflector means there is arranged a suction housing 27-29 for removing mist or droplets released by the web during ultrasonic treatment. This makes it easier to adjust the flatness of papers. This embodiment also provides for a more regular orientation of the fillers,

fibrils etc. contained in the web (one-way effect) because of regular dewatering of both sides.

Figure 4 illustrates the application of the present invention to drying of a wet web at the very beginning of the drying operation, i. e. when the web runs between two wire meshes 32 supported by guide rolls 31. The ultrasonic energy is directed to the web from emitter means 34 placed on each side of the web and the mist and droplets are withdrawn with suctional means 33. At this stage the ultrasound vibration by direct contact can be performed very easily. The direct contact device can be a bar or a roll that rotates with the same pyrophoric speed as the web and the wire mesh. It contains all around ultrasound generating units, driven by electricity and pressure air.

The ultrasound bar can also be a rotating cylinder running at the same speed as the web (Figure 5). That cylinder 41 is provided with peripheral piezo crystal segments 42.

In the following the invention is explained by referring to experiments conducted by the inventors.

Example Drying with ultrasound vibration was compared to drying with normal air flow.

The material was a PCC water suspension having an initial solids content of 69. 92 %. In the experiments the vibration source (emitter bar) was arranged under the sample and placed in intimate contact with the sample. The frequency of the ultrasound vibration was 40 kHz.

The ultrasound sample dried 69. 92 %------> 80. 30 % The comparable sample dried 69. 92 %------> 70. 56 % Time : 300 sec.

The evaporation rate was 16. 2 times higher under ultrasound than for a similar material without the use of ultrasound.

At all experiments it could visually be seen, that when the ultrasound device was turned on, evaporation and especially liberation of fine mist droplets began at once and after fractions of seconds the mist droplet formation had already been reduced. The mist formation can easily be measured with laser beam of beam length n->m as the side visibility of the bean on length of the beam indicates always a foggy area since light scatters from the droplets. If there is no mist, there is no side visibility. According to this, the drying of the web is preferably done so, that the web runs over multiple ultrasound emitter bars, whereby the non- resonance misting effect at the beginning of the operating area of each beam will be highest.

Another alternative is the non-contacting system where focusing of ultrasonic waves are needed. The focusing is easily made by curved plate having the centre of the radius on the level of the web. The focuser is advantageously made of aluminium and it is vibrated by a direct contact ultrasound source. The focusing should be 10 to 200 times, which means that the initial vibrating area and energy is focused on an area 10 to 200 times smaller.

The energy intensity of the ultrasonic vibration energy directed to the web amounts to 3 to 1000 W/cm2 in particular 5 to 100 W/cm2.

This same phenomenon can naturally be applied to drying of a coated web or a web having no coating at certain dry substance area of paper or a drying of a base web having no coating at all. This method can also be combined with any known drying method.

By contacting wet paper with the ultrasound vibrating surface at the same time when drying air was blown at a 45° angle to the wet paper, the following phenomenon was observed : With ultrasound and air dryer : 0. 39 g paper (RH 60%)-> wetted 0. 77 g-> dried 0. 41 g in 79 s.

Using only air dryer : 0. 40 g paper-> wetted 0. 88 g-> dried 0. 43 g in 138 s.

The grammage of the experimental paper was 80 g/m2, surrounding temperature

23°C, and air blower temperature at nozzle 85 C, and at paper level 37 °C.

Heavier writing boar sheet ; 127 g/m2, gave the following results : With ultrasound : 1. 27 g-> wetted 2. 25 g-> dried 1. 50 g in 90 s.

Using only air blower : 1. 29 g-> wetted 2. 16 g-> dried 1. 39 g in 168 s.

Heavy pulp sheet : 770 g/m2 With ultrasound : 3. 16 g-> wetted 7. 76 g-> dried 6. 67 g in 189 s.

Using only air blower : 3. 32 g-> wetted 7. 98 g-> dried 7. 03 in 234 s.

The results are very logical. That means, that thin paper, where the total compressibility at Z-axis direction is low because of low thickness, will transfer the ultrasound vibration more effectively to the droplet formation of water as compared to thicker sheets. On the other hand, the more dramatic difference at the drying of the surface pigment layer is explained with the high elasticity modulus of inorganic pigment material.

When pigment slurry were dried under ultrasound influence, the surface of the glass container collecte very hard closely packed layer of pigment, where the dry content was more that 85 %, when the starting slurry was 70 % w/w PCC in water.

This means that the ultrasound vibration will compact also the pigment particles of a surface coating layer near each other. This means better usage of binding polymer, better barrier properties of the layer and better printing characteristics.

When the frequency of ultrasounds is for instance 40, 000 Hz, this means that in water, where sound velocity is 1500 m/s, the wave length is about 37 mm and when the frequency is 1, 000 kHz the wave length is 1. 48 mm.

The particle displacement, that ultrasound can achieve at maximum with 1 MHz, A = v (max)/O =, where v (max) = vo x 1. 42 and O = p x 1000 kHz

A = wave length in water.

If the particle size is 0. 1 micron = 1000 A, the best frequency to move 0. 1 micron particles should be then 200 kHz.

As apparent from the above, drying can be achieved already at frequencies in the range of 20 to 40 kHz, but real particle compacting needs theoretically higher frequencies. The operable frequency is very large but a range of 0. 01 to 10 MHz is preferred and above mentioned values give other indications of preferred frequencies.

The viscosity and surface tension and density of water are dependent on temperature : viscosity surface tension density micro Pas dynes/cm kg/m3 20 °C 1005 73 998 100 °C 284 62 954 The droplet size and pressure inside the droplet depends : Pst=4 st/D, wherein st = surface tension, D = diameter, and Pst = pressure caused by surface tension We also know that Pi = Pst + Pd wherein Pd = external dynamic pressure and Pj = internal pressure

When Pi > Pex wherein PeX = external pressure, a drop forms, or can be broken down.

The droplet size will obey the following correlation : D (stm) = k x St(+0.6) Vis(0.2) dP(-0.4) wherein Vis = viscosity As we can see, the higher temperature will favour smaller droplets and also smaller surface energy needed for droplets to break down. Therefore it is beneficial if the temperature of the web is elevated, for example over 60°C.

When ultrasound has to change phase, quite a much energy and pressure potential will be lost as reflection.

The energy transport from phase to phase can be expressed as : e = 4 Z, x Z2/ [ (Zl+ Z2) 2], wherein Z, and Z2 are the wave resistances of different phases.

Z, in air is 45 g/m2s and Z2 in water is 15 E+4 g/m2s.

Z = density x sound velocity in this medium so e = 0. 12 %.

The normal efficiency from iron to water is about 13 % maximum and from aluminium to water about 30 %. This means that the focusing power must be about-100 to overcome the direct contact.

It is known from molten metal handling that ultrasonic treatment will degas the mixture and prevent this way the porosity of final metal. The same degassing of the coating suspension will happen also here.

When the ultrasonic vibration is concentrated on the web it is always found a frequency and specific power where the free water phase will vibrate at unstable stage and at this stage the surface will release fine droplets as form of mist from the surface. The instability means that vibration and resonance forces are at least locally bigger than surface tension at that point.

The instability point will depend on thickness, elasticity, water content, mineral filler content and quality and temperature and viscosity of free liquid phase as well as the boundary conditions of the area to be dried. Direct formula or mathematic conditions can not be easily given but experimentally can be seen by vibrating a layer of water on a plate the amount of evaporation is low until the layer is thin enough whereafter it starts to vibrate so that the ultrasound energy will push out high fume clouds from the surface of water.

The focused ultrasound emitter is best made from aluminium focuser and also a direct contact between water and metal surface is best carried out with aluminium.

Aluminium has the lowest sonic impedance, which means that the absorbency of sound energy is lowest in this metal. Of course, any other metal can also be used.

One embodiment of the invention is a method wherein a coating slurry is dewatered on a surface and attached after dewatering on a surface of a web to be coated.

Based on experimental data, we have found that by means of the present invention it is possible to increase the intensity of the drying with up to 100 % by only increasing the energy consumption with about 10 to 20 %.

Thus, while there have been shown and described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the method and apparatus may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and method steps which perform substantially the same results are within the scope of the invention. Substitutions of the elements from one described embodiment to another are also fully intended and contemplated. Thus, the principle of the invention is applicable to drying and

compacting of polymeric webs, coated metallic webs and combinations thereof. It is also to be understood that the drawings are not necessarily drawn to scale but they are merely conceptual in nature. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.