US3536580A

US3536580A - Paper making methods and apparatus involving electrostatic spray coating

Info

Publication number: US3536580A
Application number: US675159A
Authority: US
Inventors: Richard L La Fave
Original assignee: Ransburg Corp
Current assignee: Ransburg Corp
Priority date: 1967-10-13
Filing date: 1967-10-13
Publication date: 1970-10-27
Anticipated expiration: 1987-10-27
Also published as: DE1802693A1; FI51513B; FR1588333A; NO131274B; DE1802693B2; FI51513C; GB1257483A; SE357589B; NO131274C; CH479769A

Description

0a. 27, 1970 I R. LA FAVE 3,536,580

PAPER MAKING METHODS AND APPARATUS INVOLVING ELECTROSTATIC SPRAY COATING Filed Oct.

m R fi GNg UR P 6 3 3 4 7 2 8 4 3 3% 3 a 90 6M. 344 4 9 4 Fig 2 INVENTOR RICHARD L. LGFAVE United States Patent Ofice 3,536,580 Patented Oct. 27, 1970 3,536,580 PAPER MAKING METHODS AND APPARATUS INVOLVING ELECTROSTATIC SPRAY COATING Richard L. La Fave, Indianapolis, Ind., assignor to Ransburg Electra-Coating Corp., Indianapolis, Ind., a corporation of Indiana Filed Oct. 13, 1967, Ser. No. 675,159 Int. Cl. D21d 3/00; B05b 5/04 US. Cl. 162-186 5 Claims ABSTRACT OF THE DISCLOSURE This invention relates to the manufacture of cellulosic sheet material, such as paper and paperboard, in which a spray of materials is electrostatically applied to a water-wet fibrous cellulosic sheet material, as for example exists on the foraminous wire net of a Fourdrinier machine.

In the manufacture of cellulosic sheet products such as paper and paperboard, it is important to adapt the physical characteristics of the product to different uses. For this reason, it is frequently desirable to deposit materials onto the cellulosic sheet during its formation. For example, in the case of printing papers such as newsprint, one of the most important characteristics is socalled pick resistance. It is a measure of the ease or difficulty 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 rubber transfer blanket or roll. When a sheet is removed from the blanket or roll there is a tendency for free, or loose, or inadequately bonded surface fibers to pick off the surface of the sheet and to remain on the plate or blanket. When enough of these fibers have collected, they interfere with the ink distribution and a mottled uneven printing area results. To improve the pick resistance of the sheet it is desirable to apply starch sizing to the surface of the paper. The starch is applied to the surface of the web of water-wet paper pulp as it moves on the Fourdrinier wire. Other important properties which may be improved by the deposition of materials to a paper web are strength, rigidity, smoothness, porosity, brightness, opacity and a tendency to absorb or resist penetration of various fluids such as Water, oil, wax and ink.

In many coating operations it is desirable that the material be deposited upon the cellulosic sheet while the sheet is supported by the Fourdrinier wire. However, the water-wet cellulosic sheet material supported on the Wire is very fragile and may be easily disrupted or its surface marred by "the deposition of large droplets of liquid. Imperfections caused by the deposition of large droplets at the Fourdrinier wire may be apparent in the formed product and are thus undersirable.

In non-electrostatic methods of deposition of coating materials onto a cellulosic sheet, a spray of material is deposited on the cellulosic sheet material by either the force of gravity or mechanical forces imparting momentum in the direction of the sheet. With these methods of application the fineness of the spray which can be deposited is limited by movement of air adjacent the Fourdrinier wire. Spray droplets have small diameters have large surface-to-mass ratios and are easily moved by moving air streams. Where deposition is effected by gravity or by the momentum imparted to the spray, penetration of the moving air adjacent the web can be achieved only with relatively large spray droplets. However, fine spray particles can be deposited if they are electrically charged and directed into the vicinity of the cellulosic sheet while the cellulosic sheet material is maintained at a spray attracting potential by grounding the Fourdrinier wire. The spray particles, if highly charged, will be attracted through the moving air adjacent the sheet and deposited on the surface of the sheet. Apparatus and method to form and highly charge a spray of liquid material are shown in US. Pats. 3,169,882 and 3,169,883. Such apparatus can atomize and charge a liquid dispersion of finely divided solid material to be applied to the sheet, such as uncooked starch in water, or a material in liquid form. Materials in dry powder form may be applied to the cellulosic sheet by fiuidizing the powdered material, entraining the fluidized powder in a moving air stream by means of an injection pump immersed in the fluidized powder material and feeding the air-entrained powder material to a dispersing spray device carrying a spray charging electrode connected with a high potential source. It is frequently desirable to locate the electrostatic spraying apparatus over the Fourdrinier wire to achieve uniform distribution of materials across the sheet.

The ambient atmosphere adjacent a paper making machine is heavily laden with water vapor from the evaporation of water from the head box, the water-laden paper pulp, the water draining from the pulp, and the water vapor produced by the drying section of the paper making machine. The water vapor in the ambient atmosphere condenses on any cool surface, and the condensation will coalesce into droplets which rain from such a surface. Where electrostatic spray apparatus is located above the surface of the pulp web on a Fourdrinier wire, water condensed on the surfaces of the electrostatic spraying apparatus will drip on the pulp web, thus disrupting the web or producing craters in the sheet. In addition, the electrostatic spraying apparatus is generally mounted on a grounded support member. The electrostatic spraying apparatus includes insulating members which carry the spray charging electrode. The surfaces of these insulating members are electrically highly stressed by the high voltage applied to the spray charging electrode. Water formed on such surfaces provides a conductive path along the insulating surface so that excessive currents can flow between the spray charging electrode and the grounded support member of the spraying apparatus. Such a condition may ultimately damage the surface so that a permanent conductive path is formed. In addition, where the spraying apparatus includes a resistor having a high value of resistance in series with the spray charging electrode, the flow of current through the conductive path formed by collected moisture on the insulating surface may reduce the potential of the spray charging electrode to such a point that charging of spray particles will be impaired.

In accordance with my invention the electrostatic spraying apparatus is surrounded with a gas and the gas is diffused outwardly from the spray apparatus and into the ambient atmosphere to prevent the formation of water by condensation upon the electrostatic spraying apparatus. In apparatus to practice my invention a porous nonconductive housing encloses the electrostatic spraying apparatus and has an opening for the forward portion of the spray device and the spray charging electrode. The

housing is supplied with a gas under pressure and the interior of the housing is pressurized above the pressure of the ambient atmosphere. The gas escapes through the porous walls of the housing and through the opening surrounding the forward portion of the spray device and spray charging electrode, preventing the collection of condensed moisture on the housing and the exposed portions of the spraying apparatus.

The accompanying drawings are helpful in describing my invention and a typical embodiment of apparatus by which it is accomplished.

FIG. 1 is a perspective view illustrating the embodiment of my invention in a paper making machine including four electrostatic spraying devices.

FIG. 2 is a cross-sectional view of a portion of the paper making machine through the axis of one of the electrostatic spraying devices, showing the apparatus in greater detail.

Referring to FIG. 1 the numeral numerically identifies the Fourdrinier machine comprising a head box 11, sluice 12, and a breast roll 13, table rolls 14 and couch 15 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 numerals 17. The paper pulp P originally deposited from the head box onto the wire in the vicinity of the breast roll 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. After the paper leaves the Fourdrinier machine it is passed through a dryer section where the press rolls 18- and heated rolls D further remove water from the paper web transported by

felts

19, 20, and 21. The heated rolls D reduce the sheet moisture to the level desired in the finished product.

FIG. 1 also shows an electrostatic spraying apparatus 22 depositing a dry powdered sizing material upon the surface of the pulp web P. This apparatus comprises a plurality of electrostatic powder spray devices 23 which are spaced to uniformly distribute the powder on the web. Since the spray devices 23 are similar, the detailed description of operation will be limited to one device. The powdered coating material is entrained in air means of a special pump and delivered to a spray device 23 through a hose 24, and is discharged at the forward end of the device from a rotating distributor assembly. A high voltage D.C. power supply 25 is connected through cable 26 and a series resistor in each spray device with a conductive spray charging electrode portion of the rotating distributor assembly of that device, establishing an electrostatic field between the spray charging electrode and the pulp web being coated. The voltage applied is preferably negative. The powder particles acquire an electrical charge, are discharged into the electrostatic field and attracted to and deposited on the surface of the web.

The source of powder particles includes a fiuidizing bed 27 in which a quantity of the powder is maintained in a fluid state by the passage of air therethrough. A control assembly includes pressure regulator 28 which controls the flow of air from a source (not shown) through a pipe and a valve 29 to the fiuidizing bed. An injector pump (not shown) for each spray device 23 positioned in the fluidizing bed has associated with it a pressure regulating valve 31 and pressure indicator 32. The outlet of the pump is connected through hose 24 with the spray device. The pressure of the air to the injector pump controlled by valve 31, varies the rate of fiow of the powder particles. The air pressure, as indicated on gauge 32, provides a reference which can be used to conveniently and readily duplicate desired powder flow rates. Individual control of the spray devices is desirable in obtaining a uniform distribution of sizing on the web. The portion of electrostatic spraying apparatus 22 over the web P is surrounded by a housing of porous polyethylene of the type formed by a sintering process and sold under the registered trade name Vyon. The porous housing is pressurized with a gas, such as air, which diffuses outwardly through the porous housing and away from the electrostatic spray devices and their supports, thereby preventing the collection of condensed water on the surfaces of the electrostatic spraying apparatus.

The construction of the spray apparatus 22 and, more particularly, of each spray device 23 is shown in FIG. 2. Distributor assembly 33 is rotated by an air motor 34 to effect a distribution of the powder particles. Motor 34 is connected through a hose, a pressure regulating valve, and an on-off valve with a source of compressed air (all of which are not shown). Variation of the air pressure to motor 34 changes the speed of rotation of distributor assembly 33. To assure a suitable distribution of the powder particles, the speed of rotation should preferably be of the order of 400 revolutions per minute or more. A barrel 35' of insulating material extends forwardly from a conductive rear body portion 36. A tubular shaft 37 of insulating material is rotatably carried inside the barrel by a bearing in the rear body portion 36 and bearing surface at the forward end of barrel 35. A gear on shaft 37 is meshed with a gear driven by air motor 34 to rotate shaft 37. Spray charging electrode 38 comprises a resistive coating of the type disclosed in US. Pat. 3,021,077 upon the rear face of the distributor 33. The nonconductive barrel portion 35 and tubular shaft 37 provide an insulating support for the spray charging electrode 38 spaced from the conductive rear body portion 36. With this arrangement the conductive rear portion of the spray device 23 may be grounded while maintaining the spray charging electrode 38 at a high potential with respect to ground. The nonconductive barrel portion of the body also carries a resistor 39 having a high resistance of several hundred megohms. High voltage cable 26 enters the rear of the spray device 23 and is connected with the rear of resistor 39 within the insulating barrel 35. A brush member 40 is carried at the forward end of barrel 35 and provides the electrical connection between the front of the resistor and the spray charging electrode.

Hose 24 carrying the air entrained powder is connected with a passageway 41 in the rear body portion 36 of the spray device. Passageway 41 is sealed with the tubular shaft 37 by a rotary surface seal, and the air entrained powder flows through the spray device to the distributor assembly 33 which is mounted at the forward end of rotating tubular shaft 37 The distributor assembly 33 has a hub portion which telescopes over the end of tubular shaft 37. A passage through the distributor assembly is closed by cap 42 and thus terminates in a lateral outlet aperture 43 through which the powder particles flow in a direction generally at right angles to the axis of the distributor assembly. Cap 42 and distributor 33 are preferably of nonconductive material with the rear face of distributor 33 being provided with a conductive coating having a high resistivity to provide the spray charging electrode 38.

The distributor bell 33 has a face or forward surface which is pitched forward slightly from a plane at right angles to the axis of the distributor. The outlet aperture 43 is immediately adjacent the front concave face of the distributor near the rotational center thereof, so that the flow of coating material particles is outward along the forward surface. Rotation of the bell as the particles are discharged tends to effect a uniform distribution of the particles on the sheet. The forward surface of the cap 42 is preferably spaced slightly behind the forward edge of the distributor 33. The electrostatic field for charging and depositing the powder particles preferably extends from the edge of the distributor. The charge on the powder particles is greatest when the electrostatic field gradient at the edge of the distributor is greatest.

Spray devices 23i s supported from a bar 44 by cap screws 45 threaded into the conductive rear body portion 36. Bar 44 passes horizontally above the Fourdrinier wire to provide support for other spray devices. Bar 44 may be supported by any suitable structure.

Spray devices 23 is supported from a bar 44 by cap housing 46. The sides 47 and top 48 of the porous housing are fabricated by filter grade Vyon having a thickness of of an inch. The bottom 49 of the porous housing is A; inch thick grade Vyon to obtain preferential air flow around the distributor 38. The Vyon material can be heat formed into the required shapes to provide an enclosure and welded at the seams to prevent air flow through the seams. The opening between the rotating distributor 33 and the housing 46 is kept small, for example an annular opening with a width of approximately M of an inch has been found satisfactory. About 500 cubic feet per minute of gas at a pressure of 2 inches of water will supply the entire apparatus, including four housings 46 and housings for the support bar. The interior of housing 46 is at a sufficient pressure above the pressure of the ambient atmosphere that the gas diffuses through the porous surfaces of housing 46 outwardly away from the electrostatic spraying devices 23 and the supporting structure 44.

The porosity of the sides and top of such a housing is such that air 'will flow at 13 cubic feet per minute through each square foot of the housing surface if a diiferential pressure of 2 inches of water is maintained across the surface and the surface is 1 of an inch thick. correspondingly, air will flow at 24 cubic feet per minute through each square foot of such a surface if the interior of the housing is maintained at a pressure of 4 inches of water. The porosity of the bottom of the housing is such that 30 cubic feet of air will flow through each square foot of surface if a differential pressure of 2 inches of water is maintained across the surface and the surface is Ms of an inch thick. Although the specific porosity indicated above has been found to be effective in preventing the collection of condensed water vapor on the housing surface, other porosities may be effective so long as the ambient atmosphere laden with water vapor is prevented from reaching the exposed surfaces of the electrostatic spraying apparatus and its supporting structure over the Fourdrinier wire. More porous surfaces will permit larger air flow rates and will, of course, increase the cost of operation of the apparatus.

Heated air may also be used, and it has been found that air at temperatures on the order of 140 F. are also effective. The use of heated air is particularly effective when a portion of the outer surface of the housing for the electrostatic spraying apparatus cannot be of porous material. The flow of hot gas through the porous section of the housing ensures a continuous supply of hot gas to the interior of the housing and that the non-porous surface portion of the housing will be maintained at such a temperature that the water vapor will not condense upon it. Hot air for the dryer section of the paper making machine may be conveniently used to supply the housing.

EXAMPLE A paper making machine of conventional construction (similar to the one illustrated in FIG. 1) is modified to include a starch deposition zone utilizing equipment as indicated in FIG. 1 to electrostatically charge a finely divided starch powder. Four electrostatic powder guns are used with the guns spaced about 7 inches above a 54 inch Web and with the gun oriented downwardly normal to the web. Bell-like distributors of the type shown in FIG. 2 on each powder gun are rotated at approximately 700 r.p.m. The powder guns are spaced 14 inches apart and are staggered so that the starch is deposited uniformly across the Web. A porous housing encloses the powder guns and their supporting structure so that the electrostatic spray apparatus can be surrounded with air which is diffused outwardly away from the spray apparatus. Air at a pressure of about 2 inches of water is continuously supplied to the porous housing. The furnish used approximated a conventional newsprint (65% groundwood- 35% sulphite) and the final sheet weighed 37 /2 lbs. (Jet 0 English) per ream (25" x 38500 sheets) (3300 sq. ft.).

The paper making machine is operated at a speed of 1370 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 140-145 F. 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 entrained in air and are delivered to each powder gun at a rate of 6 pounds per hour. The starch spray application equipment is located so that the starch is deposited upon the upper surface of the sheet while it is on the Fourdrinier wire and nearly over the couch roll, the sheet moisture in this region being about Following the starch spray application, the sheet is continuously dried using dryer rolls heated with steam and the dried sheet is calendered. The rate of starch deposition is such that the amount of starch deposited is equivalent to /2 of the weight of the paper fiber, about .06 lb. per 1000 sq. ft. of surface. The finished sheets possess greatly improved pick resistance on the felt side despite little loss of absorbency, and show no imperfection due to the impact of water droplets.

Essentially the same results are obtained utilizing the commercial hydroxy ethylated cornstarch sold by Penrick and Ford, Limited, under the trade identification Penford Gum 300.

The invention is not to be construed by any abstract of disclosure, but its features are instead characterized in the preceding description and it is defined in the claims which follow.

I claim:

1. In a paper making apparatus including:

a source of paper pulp,

a Fourdrinier wire,

means for distributing paper pulp on the Fourdriner wire in the form of a web for the removal of water from the pulp,

an electrostatic spraying apparatus spaced from the Fourdrinier wire, said apparatus having a spray device, means connecting the spray device to a source of material to be sprayed, a spray charging electrode adjacent the spray device, means connecting said spray charging electrode to a high voltage source to establish an electrostatic field from the electrode for charging and deposition of the sprayed material,

a drier to dry the web,

the improvement which comprises:

a porous nonconductive housing enclosing within its interior the electrostatic spraying apparatus and including an opening through which the spray device and spray charging electrode project,

means for supplying the housing with gas under pressure, the interior of the housing being above the pressure of the ambient atmosphere whereby the gas escapes through the porous housing and the opening preventing the collection of condensed moisture on the housing, the spray device, and the spray charging electrode.

2. The apparatus as set forth in claim 1 wherein the housing has a surface adjacent the projecting spray device and spray charging electrode with greater porosity than the remainder of the housing.

3. The apparatus as set forth in claim 1 in which a portion of the surface of the housing is a sintered polyehylene.

4. In a method of making paper including the steps of:

uniformly distributing a paper pulp as a web on a foraminous wire net,

transporting the web horizontally while removing water therefrom and through a coating zone,

forming a spray of charged material over the web,

directing the charged spray into the coating zone while 7 maintaining the foraminous wire net at a spray attracting potential to deposit the material on the web, and removing the web from the foraminous wire net for further drying, the improvement which comprises during the steps of forming the spray of charged material and directing the charged spray into the coating zone,

substantially completely surrounding the spray forming means With gas, and diffusing the gas outwardly from the spray forming means over the web and into the ambient atmosphere to prevent water condensation.

5. The method as set forth in claim 4 including the 15 step of heating the gas prior to surrounding the spray forming means.

References Cited UNITED STATES PATENTS 2,680,079 6/1954 Huebner 11793.4 X 3,114,482 12/1963 Dunaway. 3,461,032 8/1969 Lichtenberger et a1. 162266 S. LEON BASHORE, Primary Examiner A. DANDREA, JR., Assistant Examiner US. Cl. X.R.