US 3919042 A
Produces paper and like products of modified surface properties by depositing electrostatically charged particles of dry starch on a water-wet web which may still be on the wire of a Fourdrinier machine. The starch particles are deposited as uniformly distributed segregated particles which remain on the surface of the paper to be swollen with the water of the web as the web is dried to bind the surface fibers to the body of the paper. The adhesive function of the starch does not require the particles to be gelatinized to the point of rupture and the paper, especially newsprint, retains its porosity. The lack of penetration of the starch into the body of the paper and its presence in the form of separated particles emphasizes the surface binding action of a small amount of starch.
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Description (OCR text may contain errors)
United States Patent [191 Spiller METHOD AND APPARATUS FOR APPLYING DRY STARCH PARTICLES T0 WATER WET CELLULOSIC WEBS USING ELECTROSTATIC ATTRACTION  Inventor: Lester L. Spiller, lndianapolis, Ind.
 Assignee: Ransburg Electro-Coating Corporation, Indianapolis. Ind.
22 Filed: Jan. 5, 1967 211 Appl. No: 607,418
 References Cited UNlTED STATES PATENTS 2.030.483 2/1936 Uong 162/184 3.210.240 10/1965 Read et a1 162/175 3.248.253 4/1966 Barford et a1 117/17 3.461.032 8/1969 Lichtenberger et a1 162/192 X FOREIGN PATENTS OR APPLICATIONS 704.036 Canada 162/186 Nov. 11, 1975 OTHER PUBLICATIONS Reif, An Electrostatic Process for Applying Dry Coatings on Paper," Tappi, Oct. 951. Vol. 38, No. 10, pp. 607-609.
Casey. Pulp and Paper, 1960, 2nd ed, lnterscience Publishers, New York, V01. 2, p. 951.
Primary Exmniner-S. Leon Bashore Assistant Eraminer-William F. Smith Auumey, Agent. or Firm-Merri1l N. Johnson; David H. Badger 1 5 7 1 ABSTRACT Produces paper and like products of modified surface properties by depositing electrostatically charged particles of dry starch on a water-wet web which may still be on the wire of a Fourdrinier machine. The starch particles are deposited as uniformly distributed segregated particles which remain on the surface of the paper to be swollen with the water of the web as the web is dried to bind the surface fibers to the body of the paper. The adhesive function of the starch does not require the particles to be gelatinized to the point of rupture and the paper, especially newsprint, retains its porosity. The lack of penetration of the starch into the body of the paper and its presence in the form of separated particles emphasizes the surface binding action of a small amount of starch.
26 Claims, 2 Drawing Figures U.S. Patent Nov. 11, 1975 FIGJ METHOD AND APPARATUS FOR APPLYING DRY STARCH PARTICLES TO WATER WET CELLULOSIC WEBS USING ELECTROSTATIC ATTRACTION This invention relates to the manufacture of cellulosic sheet material such as paper and paperboard to produce products in which the surface properties are m selectively altered by the electrostatic application of dry particulate starch to the water-wet fibrous cellulosic sheet material, as for example, as it exists on the Fourdrinier machine. The invention includes the new products which can be produced.
In the manufacture of paper and paperboard products, it is important to adapt the physical characteristics of the product to the intended use. For this reason, a variety of processes and techniques have been devised to improve or control the various physical characteristics of various paper and paperboard sheets. Among the properties that are important are: strength, rigidity, smoothness, porosity, and the tendency to absorb or to resist the penetration of various fluids such as water, oil, wax and ink.
In the case of printing papers, such as newsprint, one of the most important characteristics is the so-called pick resistance. It is a measure of the ease or difiiculty with which the surface fibers can be removed from the remainder of the sheet and is, therefore, a measure of surface strength. In the printing operation, ink is applied to the surface of the sheet by means of a printing plate or, in the case of offset printing, by a rubber transfer blanket or roll. When the sheet is removed from the plate or blanket, there is a tendency for the free or loose or inadequately bonded surface fibers to pick off of the surface of the sheet and remain on the plate or blanket. Increasing the rate at which the sheet is removed from the plate or blanket greatly accentuates the tendency of the surface fibers to pick ofi of the sheet. Similarly, the use of inks having a higher tack, as is necessary in multicolor printing, also greatly accentuates this fiber picking tendency. In extreme cases, portions of the surface of the sheet are actually removed by the printing plate leaving an abvious irregulanty 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 sheet surface and then collect on the printing plate or blanket. When enough of these have collected they interfere with the ink distribution and a mottled, uneven printed area results. It then becomes necessary to shut down and clean up the press, which operation is both time consuming and expensive.
Where the paper is to be used with printing inks that dry by absorption, the paper maker is presented with a difficult dilemma. For example, newsprint is almost always printed with inks that dry by absorption. Its surface must, therefore, not only be strong but must also be porous so that the vehicle in the ink will absorb and thus dry. While in the case of paperboard, internal strength and rigidity are of prime importance, if the board is to be printed, the surface properties are also important.
In the usual methods of starch application, starch is applied either in the interior or as a continuous film on the surface of the sheet. This can be done by a wide variety of known procedures, namely:
l. Incorporation in the dilute suspension of fibers,
. commonly known as the paper furnish, prior to its admission to the paper machine.
2. Application to the paper web during the process of formation on the paper machine wire by means of water sprays, or by a similar application to the dandy roll which then transfers it to the paper surface.
3. Application to the underside of the forming web by means of water sprays directed up through the forming wire to the bottom of the sheet.
4. Application by or between the press rolls by means of suitable distributing devices.
5. Application at the size press between the dryer rolls in the drying section of the paper machine.
6. Application on the paper machine calender stacks.
The merits of each method of applying starch to the paper sheet are dependent on numerous factors which include the conditions prevailing in the paper machine, the nature of the base sheet, the properties desired in the finished sheet and the characteristics of the starch product being used. For example, by appropriate chemical treatment a furnish addition starch may have a cationic charge which makes it substantive to the paper fibers and causes it to be retained in the sheet or web rather than to remain in the water which is removed from the web in the dewatering operation. Similarly, for the so-called surface applications, the rheological properties of the starch are usually changed and controlled to enable the starch to better perform the desired function.
However, all of the aforementioned methods of starch application have deficiencies which relate to lack of uniformity, inefficient use of starch, lack of economy due to expensive necessary equipment, requirement for premium grades of highly modified starches, and the necessity to cook the starch prior to use. These deficiencies are particularly acute for the surface sizing of paper or paperboard which is to be printed, especially with absorption type inks. If the starch is applied as a surface coating in an amount suffrcient to increase surface strength (pick resistance) the surface may not be as absorbent as desired.
In contrast, the present invention is particularly directed to themodification of the surface properties of a fibrous absorbent cellulosic sheet and it is especially concerned with the surface sizing of paper, such as newsprint. In this way, the surface strength, e.g. the pick resistance, can be appreciably increased without significantly reducing the capacity of the paper to absorb the inks which are used. A feature of the invention is the capacity to markedly alter the surface properties of the fibrous cellulosic sheet with a minimum proportion of starch.
In accordance with the invention, a water-wet sheet of fibrous cellulosic material is advanced past a particle deposition zone and electrostatically charged particles of dry starch are supplied to this zone, these charged particles repelling one another to become uniformly distributed while they are attracted to the wet paper web which is effectively grounded. The starch particles are in an electrostatic field which extends to the web and are deposited as individual segregated particles and they tend to remain associated with surface fibers so that the surface characteristics of the paper or paperboard being treated can be selectively modified. The wet sheet with the starch particles deposited on the surface thereof is then heated to dry the same and the deposited starch particles become adhesively related to the surface fibers contact thereby. This method has been found to be of especial value in the manufacture of paper, especially newsprint paper, in which the starch particles which are deposited appear to selectively contact and adhesively anchor the most prominent upstanding surface fibers which are those which are most easily picked off during a subsequent printing operation. In any event, an unusually small weight proportion of starch confers remarkably increased strength to the surface of the paper without significantly modifying the natural absorbency of the paper which is modified.
The invention is illustrated in typical embodiments in the accompanying drawing in which:
FIG. 1 is a perspective view of illustrating a conven tional papermaking machine including the means for the deposition of electrostatically charged particles on the paper web; and
FIG. 2 is a partial diagrammatic view illustrating some alternatives with respect to the electrostatic application of the starch particles to the paper.
Typical embodiments of the process or method of the present invention can be better appreciated by a consideration of the accompanying drawing. Referring first particularly to FIG. 1, the numeral generically identifies a Fourdrinier machine comprising a head box 1 l, a slice 12, a breast roll 13, table rolls 14, and a couch around which the wire 16 travels. The conventional dandy roll may be present if desired, but this roll is not shown. The conventional suction boxes are identified by the numeral 17. When the freshly formed wet paper web leaves the wire 16, it is moved to the presses, the press rolls being identified by the numeral 18 and the felts which transport the paper being identified by the numerals 19, 20 and 21.
The paper P as originally deposited from the head box 1 1 onto the wire in the vicinity of the breast roll 13 is mostly water and the water content is reduced progressively by drainage and then by suction to a level of roughly 80% in the vicinity of the couch roll 15. After the paper leaves the couch roll, it is passed to a press section where the press rolls 18 co-act to further express water from the paper web to still further reduce the water content of the paper and to further increase the internal strength of the sheet. Lastly, the partially dewatered web is passed to the dryers D which reduce the sheet moisture further to the level desired in the finished product.
FIG. 1 diagrammatically illustrates an embodiment of the invention in which the electrostatically charged starch particles are deposited upon the upper or felt side of the paper P before the paper web is advanced between the first pair of press rolls 18. As can be seen, the starch particles are supplied as indicated by the arrow 22 through a header 23 and then, via tubes 24 to distributing heads 25 which are positioned above the surface of the paper P. A metered amount of finely divided starch is blown in, scattered by the distributing heads 25 and these particles are electrostatically charged so as to form a uniform cloud of starch particles within the chamber 26 which are propelled into association with the paper P by virtue of their electrostatic charge as has been described herein before.
The starch application assembly may be positioned at various points along the travel of Fourdrinier wire. It may be positioned between or in place of one of the sets of press rolls or it may be located between the last press rolls and the dryer section. Similarly, it may be inverted and used to electrostatically apply the starch particles to the under surface or wire side of the paper as indicated by the arrow 32. The invention is not limited to operation on a Fourdrinier-type machine but is equally advantageous on cylinder or other type machines.
There is illustrated in FIG. 2 the application of starch particles to both sides of the wet paper web P while the same is moved through either a vertical or horizontal path as may occur as it travels between adjacent pairs of press rolls 18. For sake of illustration, on one side of the vertically moving wet paper web is a fluid bed 33 of electrostatically charged starch particles, the numeral 34 indicating a cloud of starch particles being electro statically transported out of the fluid bed 33 and into engagement with the wire side of the wet paper. Another structure for applying the starch particles electrostatically to the vertically moving paper web is shown for dusting felt side of the paper web. Here the structure is diagrammatically indicated in the form of an inclined platform 35 which is agitated as indicated by the double ended arrow 36 so that a metered amount of particles is uniformly dropped into the vicinity of the paper web from the lower end of the platform 35 where the electrostatic charge on the particles forms a cloud 37 in which the starch particles are electrostatically transported into engagement with the wet paper.
As illustrated, the electrostatic application of starch particles or granules in metered amounts from a position above or below the wet web can be achieved in various ways, the utilization of an inclined plate which is agitated to feed the starch and charged to charge the starch is feasible, such structures being shown in US. Pat. No. 2,748,018. The use of guns to project or spray metered proportions of electrostatically charged particles is preferred, such instruments being available in commerce and being described in a pamphlet entitled Ransburg Electrostatic Powder Coating, published in 1966. These structures can also be utilized to apply the starch to the paper as it moves through a vertical traverse thereof, in which case the supply of uncharged particles might well fail to deposit anything on the paper. When the charge is applied, the electrostatic transporting forces again control the flow rate of material as well as the deposit of the particles on the paper. In the preferred situation, the particles which are projected toward the paper or dropped upon it are charged, but uncharged particles can also be used and charged on the way to the paper, which can be accomplished by passing them through an electrostatic field.
In supplying starch in a metered amount, it is convenient to fluidize the dry starch particles with air and the dispersion so-formed is blown to the distributing heads, but it is also permissible to use other supply transport means, e.g. a screw conveyor, or a gravity fed venturi pump.
The application of starch particles in accordance with the invention may be made on one or both sides of the paper. As indicated previously, the preferred practice of the invention involves application of the starch to the upper or felt side of the paper web. It is to be observed that the wire side of the paper web is normally much stronger than the felt side so that, and especially in the newsprint industry, it is sufficient to modify and strengthen only the felt side of the paper in order to provide a satisfactory product. On the other hand, and in the production of finer grades of paper, it may well be desirable to apply the starch particles to both sides of the paper web and this can be done.
The degree of association between adjacent molecules in the native starch granule is sufficient to cause the granule to be relatively insoluble in and insensitive to water. Under the influence of heat these intermolecular bonds become relaxed and replaced, to a degree, by the association of the starch molecules with the surrounding water. The originally dense, hard, starch granules imbibe water and become swollen to many times their original size. The swelling increases as the heating is increased or continued and eventually the granules rupture or fragment and cause the formation of a uniform dispersion of hydrated starch particles commonly known as a paste: To function as an adhesive in cellulose structures, it isnot'essential that the starch granules be fully gelatinized with complete granule rupture. When starch is partially swollen it is able to associate with the surfaces of cellulose fibers. The starch used in this invention should be moisture-reactive to the extent that it either hydrates readily on contact with water at normal room temperatures, or is capable of becoming hydrated or at least partially swollen under the conditions of moisture and temperature that prevail in the sheet as it passesthrough the dryer section of the paper machine.
The starch powderfor electrostatic application may be ungelatinized granule starch, p're-gelatinized starch, such as drum-dried starch, on a mixture'of granule starch and pre-gelatinized starch. However, for use with a continuous paper machine particularly good results have been obtained when the starch powder is composed essentially of ungelatinized starch granules. With granule starch, which is dry when applied (except for normal internal moisture), any tendency of the applied starch to causesticking on the rolls of the paper machine is reduced to a minimum, while at the same time sufficient adhesive characteristics are imparted to the starch granules by-the drying operation to increase surface strength and'pick' resistance. While the degree of gelatinization of the granule 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 readily correlated so that the starch is only partially gelatinized during the-heat drying operation, the starch granules being brought into adhesive relation with the surface fibers whileretaining their granule structure. When using pre-gelatinized starch, the applied starch will absorb water and will also be brought into adhesive relation with the surface fibers during the drying operation and the use of pre-gelatinized starch makes the process independent of drying temperature and is conducive tea more extensive solubilization of the starch particles so that a more continuous surface treatment may be obtained.
The gelatinization temperature'referred to in the previous paragraph is measured by graphing the viscosity of a slurry of ungelatinized starch in water against temperature, the graph showing an increased viscosity as soon as a temperature is reached at which the starch particles imbibe moisture and swell. As will be evident, the starch used in the invention should have a gelatinization temperature which is sufiiciently low in relation to the temperatures which are encountered in the formation of the paper or other cellulosic web'such that the deposited starch will at least partially gelatinize to become adherently associated with the fibers of the web. In the preferred practice of the invention, the starch particles are applied ungelatinized and the gelatinization temperature of the starch is above the temperature of the water on the web. In this way and as the temperature is increased in the drying section of the paper-making process, the starch particles are partially gelatinized and are not ruptured so that swollen adhesive particles are produced which retain their particulate form in the final product whereby the fibers contacted by the swollen starch particles can be adhesively related to the body of the sheet without having the starch material become dispersed to reduce the absorbency of the sheet.
in the practice of this invention the moisture content of the paper web at the point of deposition of the starch particles is relatively less important to the operation of the invention than is the moisture-temperature balance in the dryers as this moisture is removed. As illustrated in the examples below, the web moisture at the time of starch deposition may be varied over wide limits while still obtaining satisfactory sheet properties. More particularly, a moisture content of from 25% to 95% by weight of water or more may be used, but it is preferred that the web moisture content at the time of starch deposition should be at least about 45% by weight, preferably above 60% by weight. This is because at moisture contents of 45% or more, there is a greater opportunity for the starch particles which are deposited to imbibe moisture and swell to provide the adhesive characteristics which are desired.
The electrostatic deposition of starch on the surface of a paper web is accompanied by unique advantages over the methods known to the prior art, and, conversely, many of the disadvantages of the prior art methods are noticeably absent in the process of the present invention.
In prior art processes involving the addition of starch to the paper furnish prior to web formation, the starch in the resulting sheet is distributed throughout the thickness of the paper web and much, if not most of it, does not, consequently, have an appreciable effect as regards improving the surface properties of the sheet.
Similarly, and when the starch is cooked to form a paste, which is the usual practice, the starch material is dispersed throughout the body of the web where it is poorly situated from the standpoint of surface characteristics. Moreover, in this dispersed condition, it significantly reduces the absorbency of the web. This is the situation in so-called surface sizing applications in which a considerable portion of the applied starch migrates down through the interstices between the fibers into the body of the sheet.
In applications involving a roll application of starch to a paper surface, the rheology of the starch material is usually such that a mottled or non-uniform size film is applied and irregular surface characteristics result.
In the use of water sprays to apply starch to the sheet surface, the use of an appreciable quantity of water is necessary in the sprayed material to improve distribution and to maintain the viscosity at the necessary low level. Consequently, an undesirably large volume of water must be used and this water must be removed during subsequent operations imposing an increased load on the presses and/or dryers. Also, and especially when the web is weak, the force of the water spray may disrupt the web, and this is especially the case when newsprint is being produced. Further, the spray droplets tend to deposit the starch particles in non-uniform piles. Interestingly, and especially where ungelatinized starch particles are deposited, these are very small and penetrate the interstitial voids between the paper fibers so that many of the deposited particles are unable to directly contribute to surface properties. Those particles left in piles at the surface tend to coalesce with one another when heat is applied instead of directly functioning to bind the surface fibers to the body of the paper web.
The application of a dry powder to a wet paper web by mechanical distribution has been proposed but has not developed commercially. Such a process would be of doubtful value because of the great air turbulence which exists at the surface of a fast moving web. This could create a serious dust problem which could not be controlled by practical means, and thereby result in appreciable product loss. Additionally, the dispersion characteristics of powders sprayed mechanically are such that uniform application is virtually impossible. If such a method were applied to starch, the dusting would create a contamination and explosion hazard. Further, the tendency of the starch powder to deposit in piles would be expected to give a non-uniform and mottled effect and, again, there is nothing to stop tiny particles of ungelatinized starch from penetrating the web, as will be easily understood when it is appreciated that the ungelatinized starch particle is generally no larger and frequently smaller than the diameter of the paper fibers which form the web.
Indeed, it has recently been reported that the application of aqueous starch dispersions to a paper sheet containing only -20% of web moisture caused the sheet to be completely penetrated. In contrast, the invention applies the starch powder to sheets containing far more web moisture without any perceptible penetration of the web.
The electrostatic deposition utilized in this invention for applying starch on the surface of a paper web is not noticeably affected by the turbulence of the air near the wire. The operation is free from dust and substantially all of the starch is deposited on the web. Additionally, the charged particles in transit between the distributing equipment and the paper web repel each other so that there is no tendency for them to form agglomerates or clumps and the resulting cloud of material is exceptionally uniform and deposits on the sheet surface in a very uniform manner.
The electrostatically charged starch particles are attracted to the wet paper surface and this attraction is able to effectively overcome the turbulence created by the air which moves along with the rapidly advancing web. The deposited starch particles remain at the surface of the paper and it is thought that there is a selecu've deposition of starch on those fibers protruding above the surface of the sheet. These are the fibers which have the greatest susceptibility to picking and removal by the inked surface during the printing operation. It is believed that this phenomenon of selective deposition on critically important portions of the web contributes to the unexpected, markedly improved sheet properties resulting from the use of very low levels of electrostatically deposited starch. It is also unusual and unexpected to obtain an improvement in the sheet properties of the magnitude observed with the degree of starch swelling as moderate as observed in the sheets treated according to this invention. Again, this may be due to the fact that the starch remains at the surface so particle rupture is not needed to bring the starch material into adhesive relation with the surface fibers. A microscopic examination of iodine-stained sheets made by the process of this invention indicated that the starch particles remain at the surface separated from one another and are swollen into adhesive association with the surface paper fibers to perform a discontinuous bonding which increases the strength at the surface of the sheet without forming a continuous film which would reduce the absorbency of the sheet. It appears that starch utilization is considerably improved since the bonding is at critically important points. Uniformity of distribution can be noted microscopically by comparing the number of starch particles per unit area over different portions of the paper surface; of course, on the microscopic level the starch particles are not arranged in a regular pattern. This uniformity can also be observed visually, for any agglomeration of starch on the paper surface would result in defects observable by the trained eye. Microscopic examination of the iodinestained products are conveniently made at 100 magnifications.
In practicing the present invention, the amount of starch applied can be varied considerably while still achieving some of the benefits of the invention. However, the results are maximized with minimum cost by applying a relatively small proportion of starch, such as 0.02 to 0.1 lbs. of starch per 1,000 sq. ft. of treated surface. More generally, the method of this invention can be used to apply from 0.02 to 0.5 lbs. of starch per 1,000 sq. ft. of surface. The results of the invention appear to be maximized at minimum cost in the range from about 0.04 to 0.2 lbs. starch per 1,000 sq. ft. surface.
Broadly, the particle size can vary within the framework of from 40-300 mesh, and even smaller particles may desirably be used. Preferably, the particle size is 80 mesh or smaller. Indeed, particles having an average diameter of one-fitth to one-tenth the average width of the paper fibers may be easily handled in the invention.
The invention is further illustrated in the following examples:
EXAMPLE I An experimental papermaking machine of conventional construction (similar to the one illustrated in FIG. 1) was modified to include a starch deposition zone utilizing equipment as indicated by numerals 22, 23, 24, 25 and 26 shown in FIG. 1 to transport and electrostatically deposit a finely divided starch on the wet paper web. A single electrostatic distributing head 25 was used with the distributing head spaced about ID to 12 inches above the web, but in commerce wider webs should be produced and more distributing heads would be used as shown in the drawing to apply the starch across the full width of such wider webs. The furnish used approximated a conventional newsprint groundwood 30% sulphite) and the final sheet weighed 32 lbs. per ream (24 inches X 36 inches 500 sheets) (3,000 sq. fi.).
The paperrnaking machine is operated ata speed of feet per minute and an uncooked powdered cornstarch which has been hydroxy ethylated to lower its initial gelatinization temperature to a temperature in the range of from l40- 145F. is applied to the felt side of the paper utilizing an electrical charge of 65-75 kilovolts on the heads, said charge being applied by a negative power pack. The starch particles are screened to remove particles larger than mesh. The starch application equipment is located so that the starch is applied to the upper surface of the sheet at a location between the last press rolls and the first dryer roll, the sheet moisture in this region being about 65%. Following the starch application, the sheet is continuously dried using dryer rolls heated with 30 p.s.i.g. steam and the dried sheet is calendered. The rate of application of the starch is such that the amount deposited is equivalent to 1% of the weight of the paper fiber, about 0.l lbs. per 1000 sq. ft. surface. The finished sheets are then conditioned and tested by the standard procedures of the Technical Association of the Pulp & Paper industry as follows: T459M-48 (Wax Test for Surface Strength of Paper); T499su-64 (Surface Strength of Paper lGT Tester); RC-l9 (Resistance of Paper or Paperboard to Printing lnk K & N lnk); T403ts-63 (Bursting Strength of Paper). In each instance the IGT tests are made using a No. 3 tack ink and a 35 -B spring drive. The values recorded are those obtained with and against the machine direction of the paper. The ink absorption is expressed as the brightness loss on inking as measured by a Photovolt Brightness Tester (Model 610 Photovolt Corporation, N.Y.C.).
Run Starch Wax Pick IGT Test K & N ink A None 6-7 70/60 29.0 .1 lbs./ l2 220/340 27.5
1000 sq. ft.
EXAMPLE 2 The procedure of Example 1 is repeated using Penford Gum 380 which is a commercially available illustration of an uncooked powdered cornstarch which is modified by hydroxy ethylation to lower its initial gelatinization temperature. This cornstarch is also screened to remove particles larger than 80 mesh and the powder is applied, successively, at rates which deposited l%, 2%, and 3% starch on the basis of the fiber weight, or about 0.1, 0.2, and 0.3 lbs. per 1000 sq. ft. The finished sheets are tested for sheet strength, surface pick, and ink adsorption as above with the following results:
EXAMPLE 3 The procedure of EXAMPLE 1 was repeated while applying the starch of Example 2 at the 1% level or about 0.1 lbs. per l000 sq. ft. For different portions of this run, the electrostatic application equipment was positioned successively at (l) the same position as in EXAMPLE 1, namely, between the last press roll and the first dryer roll where the sheet moisture was about (2) immediately ahead of the couch roll as illustrated in FIG. 1 where the sheet contains about 83% moisture, (3) further back over the paper machine wire where the sheet moisture was approximately 94%. In each case, the machine operation was completely satisfactory and the sheet tests were comparable regardless of the location of the application equipment.
EXAMPLE 4 The procedure of EXAlvfPLE l was again repeated but this time the starch of Example 2 was electrostatically deposited on the top of the sheet by a unit positioned over the sheet between the last press roll and the first dryer roll, and also on the bottom of the sheet by a unit positioned under the sheet at this same location. No operational problems were encountered and the improvement in the surface pick resistance tests previously noted was observed on both the top and bottom of the sheets.
In the foregoing Examples, a small amount of tricresyl phosphate, e.g., 1% by weight, can be used to help to maintain the starch particles in free flowing condition.
In the offset printing of newsprint the ink dries mainly by absorption so it is important that the sheet surface remains in such condition that it will readily absorb ink. With the heretofore most effective method of improving surface pick resistance, namely, surface sizing with a cooked starch, the absorption properties of the sheet are usually drastically altered and the application of such methods may be impractical if the strength of the sheet is inadequate as is the case with newsprint. ln contrast, and as shown by these examples, starch applied by the electrostatic deposition process of this invention forms a strong and porous surface and the ink absorption properties are only negligibly changed by the treatment.
As previously indicated, the starch which is used, while preferably ungelatinized, may be partially gelatinized, or pre-gelatinized. When minor reduction in the absorbency of the paper is desired, then the starch should be selected so that, under the conditions of application which are used, the starch particles swell and The data indicates that the mullen and ink absorption were relatively unchanged while again the surface pick test was significantly improved on the addition of starch.
become adhesive but are not significantly disrupted. On the other hand, there are instances in which it is desirable to minimize the water absorbency of the paper and, in such instances, the use of pre-gelatinized starch or partially gelatinized starch is conductive to the rupture of the starch particle and the consequent disper- 1 1 sion of the starch material throughout the surface areas of the sheet.
Accordingly, and while different starches may be preferred in order to better achieve different objectives or to accommodate the different conditions of paper manufacture which are employed, such as machine speed, point of starch application, dryer roll temperature and the like, it will be understood that various starches can be used alone or in mixtures including unmodified cornstarch, acid thinned starch, oxidized starch, derivatized starches, tapioca starch, potato starch, rice starch, pregelatinized starches, etc.
ln connection with the foregoing, unmodified com starch has a gelatinization temperature of about l58-l60F., and it is preferable to employ cornstarch which has been modified to decrease its gelatinization temperature in order to facilitate gelatinization thereof during the papermaking process and the development of desired adhesive attributes. In this connection, the modification of the cornstarch or other starch of high gelatinization temperature is desirably conducted to lower the gelatinization temperature to an extent of from l50F., preferably from l540F. This can be desirably accomplished by hydroxy alkylation of the starch, especially by hydroxy ethylation, such modifications of starch being, per se, well known.
It has also been found that different starches respond differently to the imposition of an electrostatic charge thereon, both from the standpoint of accepting and holding the charge to enhance deposition efficiency and also from the standpoint of retaining a charge after the imposed charge is dissipated by grounding. In this respect, it is desirable to treat the surface of the starch particle which is used in order to increase the surface conductivity thereof which provides a surface region of increased conductivity surrounding a more resistive or insulating core. Of course, the entire surface of the starch particle does not have to have an increased conductivity. The utilization of starch particles which are structured to include a surface portion of increased conductivity and a more insulating portion is preferred since it enhances the capacity of the particle to retain a charge after deposition. It is particularly preferred to employ 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 association of the deposited particles with the paper fibers at the surface of the paper web. It is here noted that hydroxy alkylation, e.g., hydroxy ethylation, functions to increase conductivity and, in many instances, to provide a starch particle which retains a positive charge when it is charged with a negative power pack and then grounded.
As the wet paper web passes through the particle deposition zone it is exposed therein to an electrostatic field from the charged distributing heads. These heads are charged to establish a field having an average potential gradient of at least 5 and preferably from 5 to kilovolts per inch of distance extending between the distributing heads and the surface of the effectively grounded web. It should be noted that the paper web itself is subjected to the electrostatic field which may induce conditions of field concentration in the vicinity of the protruding surface fibers or the field may induce other related effect on the wet paper web which can contribute to the observed phenomenon in which the starch particles remain at the surface in association with the paper fibers instead of grossly penetrating the and particles of polymers compatible with the starch selected such as blends of starch with polyvinyl alcohol, polyvinyl acetate and copolymers of styrene and butadiene.
The invention is not to be construed by any abstract of disclosure, but its features are instead characterized in the description given hereinbefore and is defined in the claims which follow.
1. A method of manufacturing a fibrous, absorbent, cellulosic sheet material, the steps comprising forming a water-wet web containing at least 25% by weight of water of fibrous cellulosic sheet material, depositing dry particles of starch upon said wet web by advancing said wet web past a particle deposition zone and supplying to said particle deposition zone said dry particles of starch electrostatically charged for mutual repulsion whereby said starch particles will be electrostatically attracted to and uniformly deposited upon said wet web in the form of separated particles and in amounts suffrcient to be capable of causing selective modification of surface properties of the sheet material, and then dewatering said web.
2. A method as recited in claim 1 in which said starch particles are ungelatinized and said wet web is heated to dry the same whereby said starch particles become at least partially gelatinized with the water of said web.
3. A method as recited in claim 2 in which said particles of starch are deposited in an amount of from 0.02 to 0.5 pounds of starch per 1000 square feet of surface.
4. A method as recited in claim 1 in which said particles of starch are formed to include a conductive surface and a portion of reduced conductivity.
5. A method as recited in claim 1 in which said starch is composed essentially of ungelatinized granule starch.
6. A method as recited in claim 5 in which said starch is cornstarch treated to lower the gelatinization temperature at the surface thereof from l050F.
7. A method as recited in claim 6 in which said comstarch in hydroxy ethylated at the surface.
8. A method as recited in claim 1 in which said sheet material is selected from the group consisting of paper and paperboard.
9. A method as recited in claim 1 in which said sheet material is paper.
10. A method as recited in claim 1 in which said sheet material is newsprint paper.
1 1. A method for the manufacture of paper, the steps comprising forming a waterwet web of paper containing at least 25% by weight of water, depositing dry particles of starch upon said water-wet web by advancing said wet web past a particle deposition zone and supplying to said particle deposition zone said dry particles of starch in an amount of from 0.02 to 0.5 pounds of starch per 1000 square feet of surface, said particles of starch being electrostatically charged for mutual repulsion whereby said starch particles will be electrostatically attracted to and uniformly deposited upon said 13 wet web in the form of separated particles, and then heating said web to dewater the same whereby said starch particles become at least partially gelatinized with the water of said web and adhesively related with the surface fibers of the paper.
12. A method as recited in claim 11 in which said paper in newsprint and the starch particles supplied to said particle deposition zone are ungelatinized and sup plied in an amount of from about 0.04 to about 0.2 pounds of starch per 1000 square feet of surface.
13. A method as recited in claim 11 in which said particles of starch are formed to include a conductive surface and a portion of reduced conductivity and are electrostatically charged to cause said particles to retain a positive charge after deposition.
14. A method as recited in claim 11 in which the web moisture content at the point of starch deposition is at least 45% by weight.
15. A method as recited in claim 11 in which said starch particles are ungelatinized at the point of deposition and are deposited upon a water-wet web containing at least 60% by weight of water.
16. A method as recited in claim 11 in which said particles are metered to said particle deposition zone and deposited upon said wet paper web from above the same.
17. A method as recited in claim 11 in which said electrostatically charged particles of starch are deposited upon the free upper surface of the paper as it is carried by a Foudrinier wire.
18. The process of selectively altering the surface properties of fibrous, absorbent, cellulosic sheet material as it moves in the form of a water-wet sheet containing at least 25% by weight of water, comprising the steps of passing said water-wet sheet into an electrostatic field deposition zone, projecting a stream of dry starch powder toward said sheet, the particles of starch in said stream being electrostatically charged for mutual repulsion while being attracted to said sheet to deposit substantially uniformly distributed individual starch particles on said surface in amounts sufficient to be capable of causing selective modification of surface properties of the sheet material, and subjecting said sheet to heat drying, where by said deposited starch particles become adhesively related to the surface fibers contacted thereby.
19. The process of claim 18 wherein the starch powder in said projected stream is composed essentially of ungelatinized starch granules.
20. The process of claim 18 wherein said electrostatic field deposition zone has an average potential gradient of at least kilovolts per inch.
21. A method of manufacturing a fibrous, absorbent, cellulosic sheet material selected from the group consisting of paper and paper board, wherein the sheet material is formed as a continuous water-wet web containing at least 25% by weight of water and having upper and lower surfaces, and said web is subjected to heat drying, the method of treating at least one surface of said sheet material consisting essentially of:
a. passing said wet web prior to heat drying through an electrostatic deposition zone,
14 b. directing a spray of dry starch powder onto at least one surface of said web substantially across the width thereof, the particles of starch forming said spray being electrostatically charged for mutual repulsion while being attracted to said web so as to deposit thereon substantially uniformly distributed individual starch particles in amounts sufficient to be capable of causing selective modification of the surface properties of the sheet material, the individual deposited particles of starch being substantially separated on said surface and contacting wet surface fibers thereof; and
c. thereafter subjecting said wet web with said separated starch particles on said surface to said heat said starch particles being brought into an adhesive relation with the surface fibers contacted thereby during said heat drying.
22. The method of claim 21 wherein said sheet material is newsprint paper and said starch powder is sprayed onto a wet newsprint web.
23. The method of claim 21 wherein the starch powder in said projected stream is composed essentially of ungelatinized starch granules, said granules being at least partially gelatinized during said heat drying and said starch powder is sprayed onto the upper surface of said web at the rate of substantially 0.04 to 0.2 pounds of starch per 1000 square feet of surface.
24. The process of improving the pick resistance of newsprint without significantly reducing the absorbency thereof comprising the steps of passing a waterwet web of newsprint containing at least 25% by weight of water into an electrostatic field deposition zone, projecting a stream of dry starch powder toward the upper surface of web at the rate of substantially 0.04 to 0.2 pounds of starch per 1000 square feet of surface, the particles of starch in said stream being electrostatically charged for mutual repulsion while being attracted to said web, to deposit depositing substantially unifonnly distributed segregated starch particles on the surface of said web, said starch particles becoming electrostatically adhered to the surface fibers of said web, and subjecting said web to heat drying to swell said starch particles with the water of said web while substantially retaining the granule structure of said starch particles whereby said starch particles remain segregated from one another and are substantially confined to the surface of said newsprint to adhesively bond the surface fibers of said newsprint to the body thereof.
25. In a paper-making machine the improvement which comprises means for advancing a wet paper web past a particle deposition zone, means for supplying to said particle deposition zone electrostatically charged particles of starch whereby such charged particles will be attracted to said paper web in the form of separate particles and unifomily deposited thereon.
26. Apparatus as recited in claim 25 in which said particle deposition zone is positioned above the free upper surface of the paper as it is carried by said advancing means constituted by a Foudrinier wire.
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