NO127412B - - Google Patents

Description

Procedure for electrostatic coating

of a cellulosic fiber material, in particular paper,

with particles in powder form.

This invention relates generally to the manufacture of sheet material based on cellulose, such as paper and cardboard, to produce products whose surface properties are optionally modified by electrostatically applying dry particulate starch to the moist fibrous cellulose sheet material, such as the .ex. present in a Fourdrinier machine.

In the production of paper and cardboard products, it is important to adapt the physical properties of the product to the intended use. For this reason, many different processes and techniques have been developed to improve and regulate the different physical properties of different paper products. and cardboard or webs. Among the properties that are important can be mentioned: Strength, stiffness, smoothness, porosity and tendency to absorb or resist the penetration of various fluids, such as water, oil,

wax and ink or printing ink.

In the case of printing paper, e.g. newsprint, is one of

the most important properties the so-called surface firmness (pick resistance). This is a measure of how easily or difficult surface fibers can be removed from the rest of the sheet, and is therefore a measure of surface strength. During the printing operation, printing ink is deposited on the surface of the sheet by means of a printing plate or - in the case of offset printing - by a rubber transfer plate or roller. When the sheet is removed from the plate or the like, there is a tendency for the free or loose or insufficiently bonded surface fibers to tear out from the surface of the sheet and remain on the plate or roller. Increasing the rate at which the sheet is removed from the plate or roller greatly increases the tendency of the surface fibers to be torn from the sheet. in a similar way, the use of printing inks with greater adhesion or stickiness, which is necessary for multi-colour printing, will also greatly enhance this tendency to pull out fibres. in extreme cases, portions of the sheet's surface are actually removed by the printing plate, leaving a noticeable unevenness in the surface, which is extremely detrimental to the appearance of the printed area." In less severe cases, individual fibers may be removed from the surface of the sheet and then collect on the printing plate or the roller. When a sufficient amount of these fibers are collected, it will disturb the printing ink distribution, and the result is a stained or unevenly printed area. It then becomes necessary to stop operation and clean the press, and this operation is both time-consuming and expensive.

When the paper is to be used with printing inks that dry

in the case of absorption, the paper manufacturer faces a difficult problem. E.g. newsprint is almost always printed with printing inks that dry by absorption. The surface of such paper must therefore not only be strong, but must also be porous, so that the carrier or liquid in the printing ink is absorbed and thereby dries. While internal strength and stiffness are of primary importance when it comes to cardboard,

the surface properties are also important if the cardboard is to be printed on.

In the usual methods of applying starch, this is applied either in the interior or as a continuous film on the surface of the sheet. This can be done with the help of many.

various known methods, namely:

1. Addition to the diluted fiber suspension, usually

known as pulp suspension, prior to its introduction into the paper machine.

2. application to the paper tap during the forming process on the wire section of the paper machine by means of water sprays, or by a similar application to the dandy roll, which then transfers it to the paper surface.. 3. <p>application to the underside of the paper tap being formed, by help from. water sprays directed upwards through the forming wire towards the bottom or underside of the sheet. 4. Application of or between the gusk presses or press cylinders using suitable distribution devices. 5. <p>application by the glue press between the drying cylinders in the drying section of the paper machine.

6. Application to the paper machine's calender rollers.

The ability of each of these methods to apply starch to the paper sheet depends on many factors including conditions present in the paper machine, the nature of the output sheet, the properties desired in the finished sheet, and the properties of the starch product.

product used. With appropriate chemical treatment, e.g. a starch for addition to the pulp suspension•have a cationic charge-

causing it to be directly absorbed by the paper fibers and causing it to be retained on the sheet or paper web rather than remaining in the water removed from the paper web during the dewatering process. In the case of the so-called surface applications, in a similar way the rheological properties of the starch are usually changed and regulated to do so. possible for the starch to perform the desired function in a better way.

However, all the aforementioned methods of starch application have disadvantages related to non-uniformity, inefficient use of starch, unfavorable economic conditions due to expensive, necessary, equipment, need for first-class quality of highly modified starches and requirements for boiling the starch before use. These disadvantages are particularly prominent when surface gluing paper or cardboard to be printed on, especially when printing inks of the absorption type are to be used. If starch-

then is applied as a surface coating with a sufficient amount of

those to increase the surface strength (surface firmness), the surface will

may not be as absorbent as desired.

In contrast, the present invention is particularly directed to the modification of the surface properties of a fibrous absorbent cellulose sheet material, and it relates more particularly to the surface bonding of paper, such as newsprint. In this way, the surface strength, e.g. the surface firmness is increased significantly without appreciable reduction of the paper's ability to absorb the printing inks used. A feature of the invention is the ability to significantly change the surface properties of fibrous cellulose sheet material with a minimum of starch addition.

This invention relates more specifically to the electrostatic application of dry starch powder to the wet web of fibrous cellulose sheet material which is formed during the manufacture of paper. The starch interacts with the water in the wet web, so that it adheres to the surface fibers in the web, so that when the web is dewatered, the surface fibers will be bound to the main mass of paper. This improves the mentioned surface strength of printing paper. The use of electrostatic forces to apply dry starch particles has many advantages including the unusually moderate weight fraction of starch required to increase the surface strength of the paper compared to various other methods of introducing starch into paper.

In order to put this invention in its proper perspective, it may be helpful to compare it with methods that are already known in the papermaking industry. The conventional method consists of mixing the starch into pulp suspension. However, this method requires starch which is suitable to be dispersed in the pulp suspension, and such starch is expensive. Furthermore, the starch that is introduced will not necessarily be transferred to the surface of the paper where it is needed, and large amounts of starch

se is lost in the backwater. The starch that is lost represents an economic burden, and the starch in the waste water causes severe pollution of waterways.

In another common method, the paper is dried and then glue is applied. This method requires renewed wetting and repeated drying of the paper. It is also difficult to reduce the amount of starch used. The costs and the unnecessary thermal pollution arising from this method represent serious disadvantages for its applicability.

An attempt could be made to apply dry powdered starch

on wet paper, but the starch tends to be carried away from

the wet web due to the airflow accompanying the moving paper; Starch powder is very finely divided and light,

and it not only blows away easily, but it also forms an explosive mixture with air. If the application zone for starch particles is tightly encapsulated, the starch powder will circulate within

for the enclosure where there is high humidity due to water vapor from the wet web, which causes the starch particles to adhere to each other and to the enclosure. This method would undoubtedly result

tere in a random application of starch to the wet web in the form of large agglomerated lumps of starch which would destroy the surface of the web and which would cause uneven coverage of the surface of the web.

Another method involves distributing the starch in water,

so that a starch slurry appears. This slurry is sprayed onto the wet track. With this method, the starch is not evenly distributed on the web, and the water spray has a tendency to

break up the delicate path. Furthermore, large quantities of water are required to form the slurry, which results in an increased load on the presses and drying devices used to dewater the web.

In contrast to all the above-mentioned known methods

the present invention involves utilizing electrostatic forces

to separate the starch particles from each other and thereby prevent their agglomeration. Furthermore, the electrostatic forces cause the starch particles to penetrate the air flow at the moving wet web, to gently bring the starch particles towards the surface of the web without destroying it. introduce

gene of the starch particles in the surface of the web significantly reduces

equal to the amount of starch per surface unit of the web necessary to selectively modify the surface properties of the web to thereby achieve very significant financial savings.

Further details of the invention as well as the new and distinctive

ne features of this are taken up in the patent claims.

The invention is illustrated in the following under reference

ning to typical embodiments shown in the drawings, where:

Fig. 1 is a perspective view showing a conventional paper

machine, comprising a device for applying electrostatically charged particles to the paper web and

Fig. 2 is a partial schematic view showing some alternatives

with regard to the electrostatic application of starch- . the particles on the paper.

Typical embodiments of the process or method according to the present invention can be better understood by considering the drawings. First, special reference is made to fig. i, where the reference numeral 10 generally denotes a Fourdrinier machine, comprising an inlet box 11, a foam ruler 12, a breast roller 13, register rollers 14 and a gusset roller 15 around which the wire 16 is guided. A conventional dandy roller can be used if desired, but such a roller is not shown in the drawing. The conventional suction boxes are indicated by the reference number 17. When the newly formed, wet paper tap leaves the wire 16, it is led to the presses, where the press rollers or cylinders are designated by the number 18 and the felt that advances the batch is designated by the respective reference numbers 19, 20 and 21.

The paper P, originally discharged from the inlet box 11 to the wire near the breast roll 13, consists mostly of water, and the water content is gradually reduced by drainage and then by suction to a level of about 80% near the gusset roll 15. After the paper leaves the gus roller it is taken to a pressing section where the pressing rollers 18 cooperate to further squeeze out water from the paper web to further reduce the paper's water content and to further increase the internal strength of the sheet. Finally, the partially dewatered web is led to the drying cylinders D

which further reduces the moisture of the sheet to the level desired in the finished product.

Fig. 1 schematically shows an embodiment of the invention, in which the electrostatically charged starch particles are applied to the upper or felt side of the paper P before the paper web is passed between the first pair of press rollers 18. As can be seen from the figure, the starch particles are supplied as indicated by the arrow 22, through a distribution channel 2 3 and then through pipes 24 to distribution heads 25 which are placed above the surface of the paper P. A dosed amount of finely divided starch is blown in, dispersed by means of the distribution heads 25, and the particles are electrostatically charged to thereby form a uniform cloud of starch particles in the chamber 26, which starch particles deposit on the paper P as a result of their electrostatic charge, as described above.

The device for applying starch can be placed at various places along the course of the Fourdrinier wire. It can be placed between or instead of one of the sets of press rolls, or it can be placed between the last press roll and the drying section. Likewise, the device can be reversed and used for the electrostatic application of starch particles to the lower surface or wire side of the paper, as indicated by arrow 32. The invention is not limited to use on a Fourdrinier type paper machine, but is equally advantageous on cylinder type machines or other types.

In fig. 2 shows the application of starch particles on both sides of the wet paper web P, while this is moved through either a vertical or a horizontal course which can occur when the paper web moves between adjacent pairs of press rollers 18. As an illustrative example, on one side of the wet paper running in the vertical direction is shown a fluidized supply or table 33 with electrostatically charged starch particles, where the reference number 34 indicates a cloud of starch particles which are electrostatically transported out of the fluidized supply 33 and into contact with the wire side of the wet paper. Another construction for applying the starch particles electrostatically to the vertically guided paper is shown for application on the felt side of the paper web. Here, the construction is schematically indicated in the form of an inclined plane 35 which is vibrated as indicated by the double arrow 36, so that a dosed quantity of particles is brought to fall evenly into the vicinity of the paper web from the lower end of the inclined plane 35, where the electrostatic charge on the particles forms a cloud 37 in which the starch particles are brought electrostatically into contact with the wet paper.

As shown, electrostatic deposition or application can

of starch particles or grains in metered quantities from a position above or below the wet web of material can be achieved in various ways, the use of an inclined surface which is vibrated to feed the starch and charged to charge the starch is possible, which constructions are shown in US Patent 2,748,018. The use of spray guns for sending or spraying metered quantities of electrostatically charged particles is preferred, which apparatus is commercially available and is described in a pamphlet entitled "Ransburg Electrostatic Powder Coating", published in 1966. These structures can also be used to apply the starch to the paper as it is moved through a vertical course, in which case the supply of uncharged particles could very well result in nothing being deposited at all. of the paper When the charge is applied, the electrostatic movement or transfer

forces in turn control the flow rate and the amount of material as well as the deposition of the particles on the paper. In the preferred situation, the particles sent towards the paper or caused to fall against it are charged, but uncharged particles can also be used and charged on their way to the paper, which can be done by passing them through an electrostatic field.

When supplying starch in metered quantities, it is practical to fluidize the dry starch particles with air, and the dispersion thus formed is blown to the distribution heads,

but it is also possible to use other feeding or conveying means, e.g. a screw conveyor, or a gravity ma-

leaky venturi pump.

The application of starch particles according to this invention can be carried out on one or both sides of the paper.

As previously indicated, the preferred embodiment of the invention involves applying the starch to the upper or felt side of the paper web. It should be noted that the wire side of the paper web usually

vis is much stronger than the felt side, so that - and especially in the newsprint industry - it is sufficient to modify and reinforce only the felt side of the paper to produce a satisfactory product. On the other hand - and when producing paper of finer qualities - it may very well be desirable to apply the starch particles to both sides of the paper web, and this is possible to do.

The degree of connection or connection between the proximity

the molecules in the natural starch grain are sufficient to cause such a grain to be relatively insoluble and unaffected

of water" Under the influence of heat, these intermolecular

re bonds triggered and to a certain extent replaced by the starch moles in connection with the surrounding water. The originally dense, hard starch grains absorb water and swell to many times their original size. The swelling increases as the heating is increased or continued, and eventually the grains will break up or split, causing the formation of a uniform dispersion of hydrated starch particles which. usually referred to as a paste or a paste. In order to act as an adhesive in cellulosic structures, it is not of crucial importance that the starch grains are completely gelatinized with complete grain breakage. When starch is partially swollen, it is able to bind to the surfaces of cellulose fibers. The starch used in this invention,

should be sensitive or susceptible to moisture to such an extent that it is either readily hydrated by contact with water at normal room temperatures or is capable of being hydrated or at least partially swollen under the moisture and temperature conditions prevailing in the sheet when it passes through the drying part in the paper machine.

The starch powder for electrostatic application may be ungelatinized, granular starch, pregelatinized starch, such as drum-dried starch, or a mixture of granular starch and pregelatinized starch. For use in a continuous paper ma-

skin, however, particularly good results are obtained when the starch slurry consists mainly of ungelatinized starch grains. With granular starch that is dry when applied (apart from normal internal moisture), any tendency of the applied starch to cause adhesion to the rolls of the paper machine is reduced to a minimum, while at the same time imparting sufficient adhesive properties to the starch granules. during the drying operation to increase the surface strength and surface firmness. Although the degree of gelatinization of the granular starch will depend on the gelatinization temperature of the starch in relation to the temperature of the web in the drying section of the machine, the gelatinization and drying temperatures can be easily adjusted one after the other, so that the starch only is partially gelatinized during the heat-drying operation, where the starch granules are brought to adhesive conditions with surface fibers-

ne , while retaining their granular structure. When pregelatinized starch is used, the applied starch will absorb water and will also be brought into an adhesive relationship with the surface fibers during the drying operation and the use of pregelatinized starch makes the process independent of the drying temperature and contributes to a more extensive solubility of the starch particles, so that a more continuous surface treatment can be achieved.

The gelatinization temperature referred to in the preceding paragraph is measured by recording the viscosity of a slurry of ungelatinized starch in water in relation to the temperature, with the resulting curves showing an increased viscosity as soon as a temperature is reached at which the starch particles absorb moisture and swell. It will be obvious that the starch used according to the invention should have a gelatinization temperature that is sufficiently low in relation to the temperatures that occur during the formation of the paper or other cellulose webs, so that the applied starch at least partially will gelatinize, so that it acquires a .. adhesive relationship with the fibers in the web. In the preferred embodiment of the invention, the starch particles are applied ungelatinized,

and the gelatinization temperature for. starch late is higher than.tem<p>e-^' the raw ride of the water in the lane. In this way and when the temperature is increased in the drying part of the papermaking process, the starch particles are partially gelatinized and are not broken or divided, so that swollen adhesive particles are produced which retain their particle shape in the final product, whereby the fibers touched by the swollen starch particles, may be adherent to the bulk of the sheet without allowing the starch material to be dispersed to reduce the absorbency of the sheet.

Under the embodiment of this invention is the moisture content of the paper web. at the point of application of the starch particles relatively less important for the operation according to the invention than the balance between moisture and temperature in the drying section when this moisture is removed. As shown in the examples below, the moisture of the web at the time the starch is applied can be varied between wide limits, while. satisfactory properties are still obtained in the sheet. More particularly, a moisture content of 25-95 percent water by weight or more can be used, but it is preferred that the moisture content of the web at the time when the starch is applied,

is at least about 45% by weight, preferably over 60% by weight. percent. This is because moisture content of 45% or more provides greater opportunity for the applied starch particles to imbibe moisture and swell to produce the desired adhesive properties or conditions.: The electrostatic application of starch to the surface of a paper tap is accompanied by completely distinctive advantages, compared to previously known methods, and on the other hand, many of the disadvantages of the previously known methods are noticeably eliminated by the method according to the present invention...

In previous processes involving the addition of starch to the pulp suspension prior to the web forming stage, the starch in the resulting sheet is distributed throughout. the thickness of. the paper web,' and much if not most of the starch consequently has no appreciable effect in terms of improving the surface properties of the sheet. Similarly, when the starch is cooked to form a paste, which is common practice, the starch material is dispersed or distributed throughout the bulk or thickness of the web, so that it is poorly placed from the point of view of affecting the surface properties. this dispersed state considerably reduces the starch material's absorbency of the web. This is the situation with so-called surface adhesive applications, where a significant part of the applied starch migrates down through the spaces between the fibers into the main part or the interior of the sheet.

In such cases which include roller application of starch

late on a paper surface, the rheology of the starch material is usually such that a mottled or uneven adhesive film is applied, and uneven surface properties result.

When using water sprays for the application of starch

on the sheet surface, it is necessary to use a significant amount of water in the sprayed material to improve distribution and keep the viscosity at the necessary low level. Consequently, a must be used. an undesired large volume of water, and this water must be removed during subsequent operational steps which result in an increased load on the presses and/or drying cylinders. Furthermore - and especially when the web is weak - the force of the water spray can disrupt or damage the web, and this is especially the case when newsprint is produced. Furthermore, splash droplets have a tendency to deposit the starch particles as uneven accumulations or piles. It is interesting to note, especially when ungelatinized starch particles are deposited, that these are very small and penetrate through the intermediate openings or voids between the paper fibers, so that many of the deposited particles are unable to directly contribute to surface properties.

The particles that remain in piles on the surface tend to grow together with each other when heat is applied, instead of directly serving to bond the surface fibers to the main mass of the paper web.

Application of a dry powder to a wet paper tap by mechanical distribution has been proposed, but has not been the subject of commercial development. Such a process would be of tolerable value because of the strong air turbulence that exists at. the surface of a rapidly advanced trajectory. This could create a serious dust problem that could not be brought under control with the help of practical

ke agents and would thus result in a significant loss of product... In addition to this, the dispersion conditions of the powder that is sprayed mechanically are such that even application in reality is impossible. If such a method were applied to starch, dust formation would occur.

risk of contamination and explosion. Furthermore, the tendency of the starch powder to settle in accumulations or piles would presumably give an uneven* and spot-like effect, and here too there is nothing to prevent small particles of ungelatinized starch from

penetrate into the web, which is easy to understand when it is considered that the ungelatinized starch particles are usually not larger, but often smaller, than the diameter of the paper fibers that form the web.

The fact is that in recent times it has been found that application

of aqueous starch dispersions on a paper sheet containing only 15-20% moisture, caused the sheet to be completely permeated,

In contrast, according to the invention, starch powder is applied

right on sheets containing much more moisture without any noticeable penetration into the web.

The electrostatic application used according to this invention, for the deposition of starch on the surface of a paper

tap , is not appreciably affected by the turbulence in the air that is close to the wire. The process is free of dust, and approx.

met all the starch is deposited on the track. In addition to this, the charged particles which are in motion between the distribution equipment and the paper web will repel each other so that there is no tendency for these to form agglomerates or lumps, and the resulting

the cloud of material is extremely even and is deposited on the sheet surface

the surface evenly.

The electrostatically charged starch particles are attracted

ket to the wet paper surface, and this attraction is able to effectively overcome the turbulence produced by the air moving along the rapidly advancing path. The deposited starch particles remain on the surface of the paper, and it is believed to be a selective deposition of starch on the fibers that protrude above the surface.

ten of the sheet. These are the fibers that have the greatest tendency to be torn out and removed from the blackened surface during the printing operation. It is assumed that this phenomenon, selective deposition on the decision-

the important parts of the track, contribute to. the unexpected and noticeably improved sheet properties that result from the use of very low values or small amounts of electrostatically applied starch.

It is also quite unusual and unexpected to achieve an improvement in sheet properties of the magnitude observed with the degree of observed moderate swelling of starch in sheets treated according to this invention. Again, this may - be due to the fact that starch-

then remains on the surface, so that particle break-up is not

necessary to bring the starch material into adhesive relation to the surface fibers. Microscopic examination of iodinated sheets produced by the process of this invention indicated that the starch particles remain on the surface separated from each other and are swollen into adhesive attachment to the surface paper fibers to produce a discontinuous bond which increases the strength of the sheet surface without forming a continuous film which would reduce the absorbency of the sheet. It turns out that the utilization of the starch is significantly improved as the binding takes place at crucially important points. The evenness of the distribution can be found - microscopically by comparing the number of starch particles per area unit over different parts of the paper surface, as the starch particles are of course not arranged in a regular pattern when viewed microscopically. This evenness can also be observed visually, as any agglomeration of starch on the paper surface would result in defects that can be observed with a trained eye. Microscopic examination of iodine-applied products can be conveniently carried out at 100 times magnification.

When carrying out this invention, the amount of applied starch can be varied considerably, while still achieving some advantages according to the invention. However, the results are optimal with the least possible costs when applying a relatively small amount of starch, such as 0.1-0.5 g of starch per m 2 treated surface. More generally, the method according to this invention can be used for applying from 0.1 to 2.5 g of starch per m 2 of the surface. The results obtained with this invention seem to have an optimum with the least possible costs in the range from about 0.2 to 1.0 g of starch per m 2 of the o*surface.

Generally, the particle size can vary within the range from 40 to 300 mesh, and it may be desirable to use even smaller particles. Preferably, the particle size is 80 mesh or less. In fact, particles having an average diameter of 1/5 to 1/10 of the average width of the paper fibers can be easily processed in the practice of this invention.

The invention shall be further illustrated by means of the following examples: Example 1

An experimental paper machine of conventional construction (similar to that shown in Fig. 1) was modified to include a starch application zone using equipment indicated by reference numerals 22, 23, 24, 25 and 26 in Figs. 1,

to advance and electrostatically apply finely divided starch to

the wet paper path. A single electrostatic distribution head 25 was used with the distribution head placed at a distance of about 25-30 cm above the web, but in practical production wider webs would be produced and consequently more distribution heads used, as shown in the drawing, to apply the starch across the entire width of such wider lanes. The pulp suspension used was similar to a conventional pulp for newsprint (70% wood pulp and 30% sulphite)

and the finished sheet weighed 14.6 kg/rice (61 x 91 cm - 500 sheets or 280 m<2>).

The paper machine was operated at a speed of 23 m/min.

and an uncooked powdered corn starch which was hydroxyethylated to lower the initial gelatinization temperature to a temperature in the range of 60 to 63°C was applied to the felt side of the paper by applying an electrical charge of 65-75 kV to the heads, which charge is provided by using a power supply unit with negative polarity. The starch particles were sieved to remove

no particles larger than 80 mesh. The starch application equipment was positioned so that the starch was deposited on the upper surface of the ar-

ket in a place between the last press rollers and the first drying cylinder, the sheet's humidity in this area being around 65%. After the starch application, the sheet was dried continuously using drying cylinders heated with steam at a pressure of 2.1 kg/cm 2 , and the dried sheet was calendered. The rate or amount of application of the starch was such that the amount applied was equivalent to a percentage by weight of the paper fibers, namely about 0.5 g/m 2 of surface.

The finished sheets were then processed and tested using standardized procedures according to the Technical Association of the Pulp & Paper Industry as follows: T459M-48 (paper surface strength wax test); T499su^64 (surface strength of paper - IGT tester) ; RC-19' (the resistance of paper or cardboard to pressure

tea - K & N. Ink); T403ts-63 (breaking strength of paper)„ In each case the IGT tests were carried out using a pressure ink no.

3 and a spring drive device designated 35-B. The values obtained are respectively in the direction with and against the machine direction of the paper. The printing ink absorption is expressed as brightness loss of the ink, as measured by a "Photovolt Brightness Tester (Model 610 - Photovolt Corporation, N.Y.C.'!).

The resistance to surface stripping, as shown by both the wax stripping test and the IGT test, is markedly improved.' • The ink absorption, as indicated by the K & N ink test,

is reduced by a negligible value.

Substantially the same results are obtained using the commercial hydroxyethylated corn starch marketed by Penick and Ford, Limited under the trade name "Penford Gum 300".

Example 2

The process of Example 1 was repeated using "Penford Gum 380", which is a commercially available example of

an uncooked, powdered cereal starch that has been modified by hydroxyethylation to lower its original gelatinization temperature. This grain starch was also sieved to remove particles larger than 80 mesh, and the powder was successively applied in amounts depositing respectively 1%, 2% and 3% starch on a fiber weight basis, or about 0.5, 1, 0 and 1.5 g/m 2. The finished sheets were examined with regard to sheet strength, surface firmness and printing ink absorption as above with the following results:

The figures' indicate that the Mullen and printing ink absorption was relatively unchanged, while here, too, the surface firmness test was significantly improved by the addition of starch.

Example 3

The process in Example 1 was repeated, with the starch according to Example 2 being applied at the level of 1% or at about 0.5 g/m 2. During different parts of this experiment, the electrostatic application equipment was placed respectively at (1) the same position as in example 1, namely between the last press roller and the first drying cylinder where the moisture of the sheet was about 65%, (2) immediately in front of the gusset roller as shown in fig. 1, where the sheet contains about 83% moisture, (3) further back over the paper machine wire, where the sheet's moisture was about 94%. In each case the operation of the machine was completely satisfactory and the sheet samples were comparable regardless of the location of the application equipment.

Example 4

The process of Example 1 was again repeated, but this time the starch according to Example 2 was electrostatically deposited on the upper side of the sheet by means of a device placed above the sheet between the last press roller and the first drying cylinder, and likewise starch was applied to the underside of the sheet by using a device placed under the sheet in this same place. There were no problems with the operation, and the improvement of the surface tear-out resistance, as in the previous tests, was observed both on the upper and lower sides of the sheets.

In the preceding examples, a small amount of tricresyl phosphate, e.g. 1% by weight, can be used to help keep the starch particles in a free-flowing state.

In offset printing of newsprint, the printing ink dries mainly by absorption, so it is important that the surface of the sheet remains in such a state that it easily absorbs printing ink. With the hitherto most effective method of improving surface tear resistance, namely surface sizing with a boiled starch, the absorptive properties of the sheet are usually changed drastically, and the application of such methods may be impractical if the strength of the sheet is insufficient, as in the case of newsprint. In contrast to this and as shown by the examples, starch applied by means of the electrostatic deposition process according to this invention will form a strong and porous surface and the printing ink absorption properties are only negligibly changed by this treatment.

As previously stated, the starch used, although it is preferably ungelatinized, can be partially gelatinized or pregelatinized. When. a smaller reduction in the absorption of the paper is desired, the starch should be chosen so that under the application conditions used, the starch particles will swell and become adherent,

but is not broken up to any significant extent. On the other hand, there are cases where it is desirable to reduce the water absorption capacity of the paper, and in such cases the use of pregelatinized starch or partially gelatinized starch will contribute to breaking up or dividing the starch particles and consequently dispersion of the starch material over the entire surface area of the sheet.

Consequently - and although different starches may prefer -

kes to be able to better achieve different purposes or to take into account the different conditions used during paper production, such as machine speed, place of application of starch, drying cylinder temperature, etc. - it will be understood that it can be used

different starches alone or in mixtures including unmodified corn starch, acid diluted starch, oxidized starch

see, derivative starches, tapioca starch, potato starch, rice starch

see, pregelatinized starches etc.

In connection with the foregoing, it can be mentioned that unmodified grain starch has a gelatinization temperature of around 70-71°C, and it is preferable to use grain starch that has been modified to reduce the gelatinization temperature, so that gelatinization occurs more easily during paper production and similar development - the ling or the development of the desired adhesive properties.

In this connection, it should be noted that the modification of cereal starch or other starches with a high gelatinization temperature is preferably carried out to lower the gelatinization temperature to an extent corresponding to from 5.5 to 28°C, preferably from 8.3 to 22.3°C. This can be done in the desired way by hydroxyalkylation of the starch, especially by hydroxyethylation, which modification of starch is known in and of itself.

It has also been found that different starches are affected differently by the application of an electrostatic charge, both in terms of accepting and retaining the charge to improve deposition efficiency and also in retaining a charge after the applied or imparted charge has passed

lost as a result of the grounding. In this regard, it is desirable to treat the surface of the starch particle which is "used, in order to

increase surface conductivity, leading to a surface zone of increased conductivity surrounding a core that is more insulating or

has greater resistance. Of course, the entire surface of the starch particle must not have an elevated conductivity. The use of starch particles which have such a structure that they comprise a surface area with increased conductivity and a more insulating part is preferred as this improves the capacity of the particle, so that it retains a charge after deposition. It is particularly preferred to use starch particles which retain a positive charge after deposition, and it is possible that this positive retained charge contributes to the rapid and relatively complete attachment or connection between the applied particles and the paper fibers at the surface of the paper web. It should be noted here that hydroxyalkylation, e.g. hydroxyethylation, acts to increase conductivity and in many cases to produce a starch particle that retains a positive charge when charged with a negative voltage source and then grounded.

When the wet paper tap passes through the particle application zone, it is there exposed to an electrostatic field from the charged distribution heads. These heads are charged to create a field with an average potential gradient of at least 2 and preferably from 2 to 4 kilovolts per cm of the distance between the distribution heads and the surface of the effectively earthed track. It should be noted that the paper web itself is exposed to the electrostatic field which can cause field concentration conditions in the vicinity of protruding surface fibers, or the field can produce other similar effects on the wet paper in the web, which can contribute to the observed phenomenon that the starch particles remains on the surface in association with the paper fibers rather than essentially penetrating through the web as would be expected due to the high moisture content of the paper web at the point where the starch is applied and due to the very small particle size of the starch particles applied .

As will be understood, the application or deposition may

of starch particles according to this invention be accompanied by the simultaneous application of other particulate materials which can be mixed with the starch particles and applied together with them. These other particulate materials can e.g. be pigments, fillers and particles of polymers which are compatible or compatible with the chosen starch, e.g. mixtures of starch with polyvinyl alcohol, polyvinyl acetate and copolymers of styrene and butadiene.

Claims (5)

1. Method for the electrostatic coating of a cellulose fiber material, in particular paper, with particles in powder form, characterized in that a wet web of fibrous cellulose sheet material containing at least 25% water by weight is advanced past a particle application zone, to which zone dry particles of starch, preferably ungelatinized starch grains, which are electrostatically charged to give mutual repulsion, whereby the starch particles are electrostatically attracted to and deposited evenly on the wet web in the form of separate particles, after which dewatering and further paper formation takes place in a known manner, during which starch particles at least partially present in a swollen state, so that they bind the surface fibers together. 2. Method according to claim 1, characterized in that the starch particles which are known in and of themselves are applied in an amount of o from 0.1 to 2.5 g of starch per m 2 of the surface. 3. Method according to claim 1, characterized in that the starch particles are treated before application to provide a conductive surface and an inner part with unchanged conductivity. 4. Method according to claim 3, characterized in that the starch is grain starch, treated to lower the gelatinization temperature at the surface of the grains by from 5 to 28°C. 5. Method according to claim 4, characterized in that the grain starch is hydroxyethylated on the surface before application.