Method And Apparatus For Forming A Non-woven Fibrous Web From A Foamed Fiber Furnish

Abstract

Improvements are afforded in method and apparatus for making non-woven fibrous webs including paper from a foamed aqueous furnish containing a surfactant in that the foam acquires and has imparted thereto physical characteristics as regards attainment of such a highly emulsified state that the bubbles are scarcely visible to the naked eye, as regards viscosity build-up of the foam to at least 22 seconds as measured with a Ford Cup Type B-4 and as regards the specific gravity being no greater than 0.35. Other improvements pertain to vortical mixing in the production of the foam, to combining the method and apparatus with conventional methods and apparatus and to handling the surfactant-containing water when in the foamed furnish and likewise as drained from the formed web.

Parent Case Text

This application is a continuation-in-part of application Ser. No. 854,246, filed Aug. 29, 1969, which is a continuation in part of application Ser. No. 640,411 filed May 22, 1967, both of which are now abandoned.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the forming of non-woven fibrous webs and in particular to the production of such webs utilizing a liquid suspending medium in the form of an aqueous foam in which the fibers are dispersed and suspended and from which they are formed into a layer on the wire of a paper-making machine, which layer is then collapsed and drained to form the web.

2. Description of the Prior Art

It is known to form paper and non-woven fibrous materials by depositing a suspension of fibers in a liquid, usually water, onto a foraminous support, called the wire, which allows the liquid to drain through while retaining most of the fibers in the form of a web, or felt, in which the fibers lie intermeshed, all substantially in the plane of the web. Due to the random nature of the process of deposition, and also because of the natural tendency of most fibers to form flocs, or clumps, the web is usually not uniform but contains areas which are particularly thin or light, or which are particularly thick or heavy. The degree of uniformity, or the lack of uniformity, may be controlled to some extent by the exercise of the operator's skill and by the design of the machine. In particular, the formation of acceptably uniform webs from fibers which have an excessive tendency to flocculate, or clump together, such as long synthetic fibers, or long, lightly beaten cotton or wood fibers, or other natural fibers, requires that the fibers be dispersed in very large volumes of water. The subsequent drainage of such large volumes of water cannot be accomplished on the usual paper-making machinery but requires costly modifications thereto, or the use of machinery specially adapted for the rapid drainage of large volumes of water.

A less common known method of producing relatively uniform dispersions of fibers in a liquid medium, and hence substantially uniform fibrous webs, is that by which the fibers are dispersed in a liquid medium of high viscosity, such as aqueous solutions of sugar, or of natural gums. With this method, however, it is found that when the dispersing action ceases the fibers very quickly cease their motion, and become immobile before becoming reflocculated and clumped together to an appreciable extent. Thus the suspension of fibers in such a liquid medium remains well dispersed until the liquid drains and the web is formed. However, due to the high viscosity of the suspending medium its drainage through the forming web and through the supporting wire is slow and difficult so that this method is not well suited to the large-scale continuous manufacture of fibrous webs.

It likewise has been proposed to add a surfactant to the water conventionally used in the water-laying of non-woven webs and by agitation produce a foam for the purpose of assisting in the formation of non-woven webs and, more especially, non-woven webs comprising fibers longer than those conventionally employed in the manufacture of such webs. However, so far as is known, such proposals have not provided the correct conditions that are critically essential and, accordingly, have not been such as to warrant commercial exploitation.

SUMMARY

According to this invention, fibers to be formed into a web are dispersed in foamed surfactant-containing water which has been brought to such a state of extreme emulsification of air therein that the inadequacies of prior proposals for the employment of foam in a web-forming operation have been successfully overcome. A thick, viscous emulsion is formed by dispersing air in an aqueous solution of an emulsifying agent in the form of bubbles, so small that even the largest is barely visible to the naked eye. The number average bubble diameter, as determined in the manner hereinafter described, should be no greater than about 0.2 mm. The foam should contain at least 65 percent of air or, in other words, the specific gravity of the foam should not be greater than 0.35. Moreover, the viscosity of the foam emulsion should not be less than 22 seconds as measured at 20.degree. C using Ford Cup Type B-4 in the manner called for by British Standard 1733.

Other features of this invention which are novel and useful per se but which preferably are employed in combination relate to vortical foam production so as to cause maximum emulsification as minute bubbles to occur at the base of a vortex while coarser bubbles tend to rise and accumulate at the surface in equilibrium with the vortical forces which tend to suck them down into the vortex with air so as to become further emulsified into minute bubbles. The vortical foaming action is continued until the desired emulsification of air is attained in the region adjacent the base of the vortex from which the foam is taken for deposit on the web-forming foraminous surface of the web-forming machine. During the formation of the foam the fibers to be formed into the web are dispersed in the foam so as to be present in the highly emulsified foam taken to the headbox of the web-forming machine.

Other features of the invention relate to the sources of fibers and of surfactant-containing water and their introduction, preferably concurrently from separate sources, into the foam-producing means. Still other features relate to the manner of reutilizing the surfactant-containing water that drains from the foam when the foam collapses to form a fibrous web on the wire or other foraminous support of the web-forming machine and its reintroduction directly into the foam-producing means, said reintroduction preferably being controlled responsive to the liquid level occurring in the foam-producing means. It is a further feature that surfactant-containing liquid drained from the web during its formation is returned to the foam-producing means partly substantially fiber-free and partly with fibers dispersed therein. Another feature of the invention relates to balancing introduction of fresh water in a thickened fiber dispersion in relation to the dryness of the web that is removed from the web-forming machine whereby maximum conservation of surfactant may be afforded.

Still other feature relate to apparatus. One such feature is concerned with apparatus which comprises vortex type foam-producing means so combined with web-forming means that foamed fiber furnish deposited on the wire of the web-forming machine is taken from adjacent the base of the vortex that occurs in vortical type foam-producing means. Other features relate to the apparatus that is employed in utilizing and pumping surfactant-containing water drained from the web.

Further features of the method and apparatus of this invention are concerned with the overall system whereby the employment of a foamed fibrous furnish may be carried out in the commercial production of non-woven webs and in affording means in combination with conventional paper-making equipment whereby responsive to simple selective manipulation a non-woven web may be produced either in the conventional way or when employing a foamed furnish.

DESCRIPTION OF THE PRACTICE OF THE INVENTION

It has been found that when fibers are dispersed in a liquid medium of the kind specified in the above summary the medium possesses thixotropic-pseudoplastic properties in that it combines the advantages of high viscosity under low stresses such as occur when the fibers cease the motion imparted thereby by the dispersing machinery, with low viscosity and an acceptably fast rate of drainage when subject to pressure against a supporting wire during the drainage and formation of a fibrous web on a usual paper-making machine. It is also found that fibers dispersed in such a liquid medium quickly become immobile when agitation ceases and remain well dispersed to form a substantially uniform web. Materials formed in this manner have the advantage of very uniform distribution of fibers over their area, as compared with those obtained by methods known heretofore, especially if the fibers are long or liable to clump and flocculate together. Another advantage is that the suspension of fibers in an emulsion of air can be obtained at a consistency, that is ratio of fiber to liquid, substantially higher than is practicable with the previously known methods. Yet another advantage is that the deposition and drainage of the suspensions in order to form a fibrous web may be accomplished using currently available papermaking machinery with only slight modifications. Alternatively, new machinery can be constructed to form fibrous webs from the suspension, which machinery is substantially more simple in design and operation than is conventional paper-making machinery.

When preparing the thixotropic-pseudoplastic liquid medium the chemical nature of the emulsifying agent used is found not to be critical provided that it has an emulsifying power sufficient to produce a thick, viscous emulsion of very small air bubbles. The emulsifying agent may be anionic, cationic, or non-ionic and it has been found that proprietary surfaceactive agents such as that sold under the name "ACE" liquid, this being an anionic substance, by Industrial Soaps Ltd., that sold as "TEXOFOR FN 15," a non-ionic substance, by Glover Chemicals Ltd., and that sold as "AMINE Fb 19," a cationic substance, by Float-Ore Ltd., are all suitable for making the required viscous emulsion. Another non-ionic emulsifying agent that has been used is octylphenoxypolyethoxy ethanol. One of the most useful emulsifying agents is commercial grade dodecyl benzene sulfonate. Soap flakes may also be used. It is, however, to be understood that the emulsifying agent used may depend on the properties which it may impart to a finished fibrous material. The emulsifying agent is mixed with water and the use of hot water may be an advantage in preparing the emulsion but hot water is not essential.

The emulsion may be prepared in any suitable kind of emulsifying apparatus subject to the proviso that the apparatus is capable of emulsifying air in the form of very small bubbles, the largest of which is barely visible to the naked eye, in the liquid and of maintaining this dispersion. In general, simply stirring the liquid does not produce a sufficiently vigorous emulsifying action. Thus, while it was possible to incorporate 65 percent or more of air in a dilute dispersion of fibers in surfactant-containing water when using a conventional blender type mixer, the resulting foam fell considerably short of attaining the aforesaid minute bubble size or a viscosity of at least 22 seconds or failed to attain either and the web formed from the foamed furnish was inferior to a web formed from a corresponding fiber dispersion when the aforesaid properties were attained. Machines which provide high rates of shear are required. It has been found that a modified froth flotation cell of the type made by Denver Equipment Co., Denver, Colorado, provides a suitable emulsifying action. Effective vortical mixing utilizing such foam-producing means is described more in detail hereinbelow.

It is essential that the concentration of the emulsifying agent be sufficiently high and that the action of the emulsifying apparatus be sufficiently vigorous to produce a finely dispersed emulsion of air in water. It will be understood that if the concentration of the emulsifying agent is too low only a thin fluid emulsion will be obtained which will soon separate into a liquid layer and a coarse froth, and that if the concentration is sufficiently high but the emulsifying action of the apparatus is insufficiently vigorous a thick, viscous emulsion may not form at all. It will further be understood that it is necessary to control the amount of air dispersed in the emulsion because if too much air is folded in the emulsion will turn into a dry, coarse froth with large bubbles and will not be suitable for making a uniform dispersion of fibers. A satisfactory emulsion is one which has a viscosity greater than 22 seconds, when measured by Ford Cup Type B-4 according to British Standard 1733, and the air content of the foam is not less than 65 percent by volume, that is, the foam has a specific gravity not greater than 0.35, provided that the largest bubble is barely visible to the naked eye. When reference is made herein and in the claims to the viscosity being at least 22 seconds (measured in the manner aforesaid) the reference is to the viscosity of foam produced in the absence of the fibers, but under conditions corresponding precisely with the conditions that prevail when producing the foamed furnish, the reason being that the presence of the fibers interferes with obtaining an accurate measurement of viscosity. The extent to which the largest bubble may be visible to the naked eye may be determined by examination of the foamed furnish. The number average diameter of the bubbles in a foam is determined by placing a drop of foam produced in the absence of fibers but produced under conditions precisely corresponding with those that prevail during the formation of the foamed furnish on a microscope slide. Photomicrographs are taken at 100 magnification at successive intervals of 1 minute. The number of bubbles visible in the field of view on each photomicrograph is counted. It is found that the number of visible bubbles appears to decrease exponentially with time to a constant value, that is:

log (N-X) = No - k T

where N = the number of bubbles visible at time T

x = a number to be determined by trial and error

No = the (supposed) number of bubbles visible at time T = 0, i.e., when the sample is taken

k = a constant

T = time elapsed from the time the sample was taken.

The value of No is determined graphically by plotting log (N-X) against T, value of X being adjusted until the graph forms the best possible straight line. Extrapolating the graph to T = 0, namely, the value of No at the intersection of the line with the log (N-X) axis, one may obtain No, the "supposed" number of bubbles at the time when the sample was taken. Dividing the area of the field of view by No, one obtains the number average area per bubble, and hence the number average bubble diameter.

It is possible to disperse fibers in the liquid medium concurrently with the formation of the emulsion, or alternatively to insert the fibers, either dry or as a suspension of fibers in water, into the prepared, i.e., foamed, liquid medium and effect dispersion in the same apparatus as is used to prepare the liquid medium. The preferred method of effecting dispersion of fibers in the liquid emulsion is to transfer the prepared liquid medium from the apparatus in which it is prepared to another apparatus and there to add to it and to disperse in it a suspension of fibers in a small quantity of water.

The prepared, substantially uniform, suspension of fibers flows or is pumped to and deposited as a layer on the moving wire of a Fourdriner type paper-making machine, or the surface of a cylinder type board machine, or the web-forming area of other suitable forms of apparatus known in the art of paper making. Upon the application of suction to the other side of the wire the layer of emulsion collapses and drains, leaving on the surface of the wire a web of fibers of a superior degree of uniformity. While spreading of the suspension of fibers on the wire and the collapsing and draining of the resulting layer of emulsion can be effected by any of the methods known in the art, the spreading is preferably effected by the use of an enclosed flow spreader of relatively low volume, for example, of the kind described in British Pat. No. 1,075,103, and is then collapsed by means of suction applied to the underside of the wire, after which the formed web is handled as in a usual paper-making process.

Surfactant-containing water drained from the foam during formation on the foraminous support of the web-forming machine is effectively reutilized and at least a part thereof is returned directly to the foam-producing means as, for example, so as to be introduced adjacent the mouth of the vortex of a vortical foam-producing unit, and in order to control the consistency of the foamed fibers furnish its introduction may be permitted by a sensing device that is responsive to liquid level in the foam-producing means. Fiber may be introduced separately either dry or in the form of a thickened slurry. When introduced in the form of a thickened slurry, the fresh water content of the slurry is controlled in balanced relation to the water content of the web as it is removed from the web-forming machine. Part of the drained water may be mixed with edge trim or other fibers and separately introduced into the foam-producing means. Erratic pumping action occasioned by the presence of foam in the surfactant-containing water is successfully overcome preferably by the employment of a positive displacement pump whose capacity preferably is greatly increased in a novel way by sucking additional liquid into a jet stream of the output of said pump.

An advantage of the process according to the invention is that multilayer sheets may be formed without difficulty. Several layers of fibrous suspension may be collapsed and drained consecutively upon an already formed mat, that is, by using several headboxes suspended above and along part of the length of the wire. The position of each headbox would coincide with a bank of suction boxes positioned on the underside of the wire. In the normal paper-making process, where the fibers are dispersed in water this would necessitate large quantities of water from the second and successive headboxes being drained through the previously formed mats, resulting in poor formation. In the process according to the invention little liquid has to be drained through the previously formed mats and collapsing and draining of the emulsion does not unduly disturb the formation. Because of the high viscosity of the foamed suspension its drainage may be accelerated at a more controlled rate, namely, on a larger number of suction boxes, without the disadvantage of the stock flocculating.

DETAILED DESCRIPTION OF THE INVENTION

Further objects, features and advantages of the invention will become apparent in connection with the following description for purposes of illustration and exemplification of typical practice of this invention in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic side elevation of one type of system embodying features of this invention wherein a foam emulsion of the character aforesaid is produced and utilized according to this invention in the production of a non-woven web;

FIG. 2 is a side elevation of a preferred foam-producing means that is employed according to this invention for producing a fibrous foam suspension, the foam-producing means being shown on a larger scale as compared with the showing in FIG. 1;

FIG. 3 is a plan view of the foam-producing means shown in FIG. 2;

FIG. 4 is a detail sectional elevation through a foam-producing means of the type shown in FIGS. 2 and 3;

FIG. 5 is a side elevation, largely schematic, of the web-forming means wherein the foamed fiber furnish is formed into a web on the wet end of a machine of the Fourdrinier type, this portion of the system being shown in greater detail than the showing in FIG. 1;

FIG. 6 is a detail view on a larger scale of the headbox end of the Fourdrinier machine shown in FIG. 5;

FIG. 7 is an enlarged view of the positive displacement type pump which preferably is used at different locations in the system for pumping surfactant-containing water; and

FIG. 8 is a block flow diagram showing an overall system which may be selectively operated in conventional formation of a non-woven web or utilizing aqueous foam.

In the embodiment illustrated in connection with the drawings the fiber to be formed into the non-woven web is first beaten with water and water-containing stock after thickening is mixed with water containing surfactant to form a dilute fiber suspension wherein the fibers are entrained in a foam emulsion having the properties and characteristics hereinabove mentioned. To this end, the fiber which is introduced into the system is beaten or otherwise refined in a conventional beater 10, such as a Hollander beater, to form a stock consistency at about 4 percent by weight based on bone dry fiber. The stock is dropped by the line 11 controlled by the valve 12 to the machine chest 13 where it is held so as to provide bulk storage for the system, the fiber being kept uniformly in suspension as by the use of a conventional agitator 14. From the chest 13 the stock is directed by the line 15 containing the pump 16 to the constant headbox 17 which maintains its desired head by an overflow over a weir back into the chest 13 via line 18. Preferably the stock in the headbox 17 is passed through a conventional refiner 19 and back into the headbox from which it finally flows at a constant head by line 20 into the thickener 21. The thickener serves to de-water the stock to an extent which facilitates reuse of surfactant-containing liquid by balancing the amount of liquid entering the system with that removed from the system whereby surfactant containing water drained from the web during its formation on the web-forming machine may be conserved and reused. The consistency of the stock as it leaves the thickener is usually between 12 and 20 percent by weight based on bone dry fiber, but should be sufficiently high to minimize the input of fresh water into the system, depending upon the dryness of the web that is removed from the web-forming step prior to drying. Water extracted from the stock in the thickener 21 is removed through the drain line 22 and may either go to drain or re-directed into mill back-water for reuse in the initial beating step.

The thickened stock is next combined with surfactant-containing water in the foam-producing means, which foam-producing means is indicated generally in FIG. 1 by the reference character 23 and which is shown in detail in FIGS. 2, 3 and 4. In preferred practice of this invention the foam-producing means comprises a plurality of vortical mixing units arranged for sequential foam production and withdrawal of foamed furnish in the manner to be described. In the embodiment shown the foam-producing means comprises a succession of open top compartments A, B, C and D. Each of compartments A, C and D contains two vortical foam-producing units. Two are shown in each compartment and, while there are four compartments, this particular arrangement is not essential in the practice of this invention. In the system as illustrated in FIG. 1, the thickened stock from thickener 21 and the supply of surfactant-containing water from sources to be described are shown as being introduced into compartment A so that the foaming and the fiber dispersion occur simultaneously in this compartment. Alternatively, the surfactant-containing water may be introduced into compartment A and the thickened stock may be introduced into compartment C whereby the foam is initially produced to a substantial extent in the absence of the fiber and the production of the foam is completed while being blended with the fiber stock.

The vortical foaming unit which is preferably employed in the practice of this invention is, as aforesaid, a modification of foaming equipment designed for use in froth-type mineral separation by the Denver Equipment Co. However, instead of producing bubbles for the objective of producing a coarse froth which carries mineral particles to the surface where they overflow into a so-called launder, the modified vortical unit which is preferably employed in the practice of this invention effects vortical mixing under conditions such that the coarse bubbles rise to the surface without overflowing and with reintroduction into the vortex along with such air as may be necessary to provide the desired specific gravity so as to be further emulsified at the foot of the vortex; and when foam is taken from the foam-producing means it is taken from a region adjacent the base of the vortex of one of the vortical foaming units immediately after the emulsifying action of the impeller means at the base of the vortex by which the air is caused to be emulsified in the form of minute bubbles.

Each of the vortical foaming units comprises a bladed impeller 24 which is rotated at a suitable speed such as from 300 to 500 r.p.m. by means of the driven shaft 25 which rotates in bearings mounted within the housing 26. The impeller or rotor 24 is interposed between the floor 27 of the foaming unit and the stator 28 and there is upstanding from the stator 28 a collar 29 on the first two units only. The distance between the impeller 24 and stator 28 is chosen so that the fluid passing therebetween is subjected to a high rate of shear. In practice this distance is in the range 0.010 to 0.125 inch. During foaming in an operation wherein the liquids and fiber stock are all supplied to compartment A the fiber to water consistency in ordinary practice is about 1 to 2 percent and the liquid level is substantially above the upper extremity of the collar 29. When the vortical foaming units are in operation the aqueous furnish is acted upon with powerful shearing action by the impeller 24 and is projected outwardly centrifugally at the base of the vortex and at the same time the furnish is vortically drawn into the collar 29. Because of the vortex action air is drawn into the dilute furnish and caused to be produced in the form of bubbles which are effectively emulsified by repeated vortical circulation. While the mixing is being carried on coarse bubbles tend to rise to the surface but tend to be sucked into the vortex for further emulsification so that the amount of coarse bubbles in the region of the mouth of the vortex becomes relatively stable during flow of the furnish through the foam-producing means.

In the embodiment shown the foam produced in compartment A flows over the top of weir 30 which controls the liquid level in compartment A into compartment B, the principal function of which is to drain off any unfoamed surfactant-containing water through line 31 and return it into the vortex of the vortical foaming unit at the inlet 32 of each unit in compartment A to be further acted upon to emulsify air therein.

The foamed furnish flows under baffle 33 into compartment C in which there are two vortical foaming units, indicated generally by the reference character 34, whose construction is the same as that of the units in compartment A except for the omission of the collar 29 and inlet 32. In compartment C the vortical units serve to further emulsify the air so as to reduce the bubble size and, again, such coarse bubbles as there may be tend to rise to the surface and become sucked into the vortex again for further emulsification. In flowing from compartment C to compartment D the foamed fiber furnish passes through apertures 35 which are disposed below the level maintained in compartment C, with the result that coarse bubbles in compartment C at the surface are not transferred to compartment D and only the more minutely emulsified bubbles pass into compartment D through the apertures 35.

In compartment D there is another pair of vortical foaming units, indicated generally by the reference character 36, which are identical in construction and operation as compared with the units contained in compartment C. In compartment D such coarse bubbles as there may be again rise to the surface where they are recycled into the vortex for further emulsification and the foam is taken from the foam-producing means by the outlet conduit 37. Baffles 38 assist in maximizing the emulsified characteristics of the foamed furnish as it is withdrawn through the outlet conduit 37.

Depending somewhat on the amount and type of emulsifying surfactant which is used, the fineness of the bubble size of the emulsified air may vary somewhat responsive to different rates of rotation of the impeller in the vortical foaming units. It is well, therefore, before instituting a commercial run to make adjustments in the rotational speed of the impellers in the respective vortical foaming units in order to determine the most effective rotational speed for the impeller conditions otherwise prevailing during the operation in question. As aforesaid, when reference is made to bubble size and viscosity the reference is to the bubble size and viscosity of foam produced in the absence of fiber in the solution of the surfactant and water inasmuch as the presence of the fiber interferes with examination as regards pore size and also prevents the making of a viscosity determination of the kind referred to hereinabove.

When the dilute fiber suspension leaves the foam-producing means through the outlet conduit 37, the foam is of the highly viscous character hereinabove mentioned wherein the individual bubbles are scarcely visible to the naked eye. In this condition the foam is taken by the conduit 37, controlled by the valve 39 to the headbox 40' of the Fourdrinier type web-forming machine, as shown in FIG. 1 and in greater detail in FIGS. 5 and 6. For feeding the foam onto the wire 91 the headbox 40 may be provided with a simple open type Vee box and may be provided with a slice of known design.

Some drainage of the liquid phase water occurs at the table rolls 41 immediately after the breast roll 42. During travel of the wire over the wet vacuum boxes 43, the foam partially collapses. Some drainage of liquid continues to occur during the travel of the wire over the larger area in the region of table rolls 44 and further collapse and further drainage occurs at the dry vacuum boxes 45. After passing over the vacuum couch roll 46 the web is then carried on through the conventional press sections of a Fourdrinier-type machine and from there is passed on to conventional dryers. The final consolidation of the web occurs when the web is taken to the first press-felt. When the web leaves the vacuum couch roll the water content will be less than in the normal paper-making process, typically 75 percent instead of 82 percent (that is 25 percent instead of 18 percent by weight based on bone dry fiber), corresponding with the water content of the thickened pulp which is introduced into the foam-producing means and commingled with the surfactant-containing water to make the foam. The thickened pulp introduced into the foam-producing means should be de-watered at least to such an extent that nearly all of the surfactant-containing backwater may be returned to be commingled therewith, and only very little of the said back-water needs to be disposed of. If the pulp is de-watered to a sufficient extent, no back-water needs to be disposed of, and the system is then balanced. To the extent that surfactant contained in surfactant-containing water retained in the web leaving the vacuum couch roll or in the small quantity of back-water which is disposed of is removed from the system, a corresponding amount is introduced into the system as by metering it into the foam-producing means.

The vacuum maintained in the vacuum boxes 43 and 45 is provided by a conventional vacuum pump 47. Surfactant-containing liquid from the wet vacuum box 43 disposed closest to the breast roll 42 is removed therefrom by the line 48 under the influence of the applied vacuum to a separator tank 49 wherein the liquid becomes separated from air and the air is taken into the vacuum pump 47 by the line 50 from which it is discharged into the atmosphere. Surfactant-containing water is directed from the bottom of the tank 49 to the pump 51 by which it is pumped by the line 52 to the back-water tank 53.

While specific web-forming means has been shown and described whereby a fibrous web is formed with concomitant drainage of surfactant liquid, it is to be understood that this has been done for purposes of illustration and that other web-forming means may be employed since the particular means whereby the fibers are formed into a web is not critical.

The pumping of water containing surfactant in a quantity adapted to produce a viscous foam presents a special problem inasmuch as a conventional centrifugal pump is not adapted to handle liquid that contains air. Accordingly, in order that the surfactant-containing water may be effectively pumped a special type of positive displacement pump is employed. A pump which is commercially known as type F80 Mono pump has been found to be suitable. The pump 51 is a pump of the positive displacement type wherein, as shown in detail in FIG. 7, the rotation of the rotatable sinusoidal part 54 effects positive displacement pumping to produce a continuous unfluctuating stream. It is one of the novel features of this invention that the exit of the pump 51 is provided with a jet 55 which creates in the surrounding chamber 55A a suction imposed on line 56 which communicates with the other two wet vacuum boxes 43. In this manner the relatively small capacity positive displacement pump is used so as to greatly increase its capacity for withdrawing surfactant-containing water from the wet boxes 43 and directing the water to the back-water tank 53.

The drained water which during travel of the wire drains therefrom in the region of the table rolls 41 and likewise in the region of the table rolls 44 is collected by the collector 57 from which it drains to pump 58 which pumps the surfactant-containing water by the line 59 to the back-water tank 53.

Surfactant-containing water is directed from tank 53 to compartment A of the foam-producing means by line 88, the amount being controlled by passage through the pump 89. A sensing device which is responsive to liquid level in compartment D of the foam-producing means 23 regulates the speed of operation of the pump 89 so as to maintain the level in compartment D substantially constant. When make-up surfactant is needed it is metered into compartment A of the foam-producing means 23.

Vacuum provided by the vacuum pump 47 also communicates with the separator tank 60 into which surfactant-containing water is taken from dry vacuum boxes 45 by the line 61. The surfactant-containing water from the bottom of the tank 60 is pumped by the pump 61, which is of the same type as the pump 51 which has previously been described. Part of the water is pumped directly through the pump 61 and thence to a jet in the jet region 62 which serves to suck the balance of the water from bank 60 into the stream being pumped. The surfactant-containing water so propelled is directed by the line 63 so as to go into the hogpit 64.

The hogpit also normally receives the surplus trim width of the web and during any wet end break any such web material also falls into the hogpit wherein it is slushed by means of conventional agitating means such as the agitator 65. The slushed fiber in the hogpit is maintained at a consistency which is desired for being pumped by pump 66 and line 67 into the foam-producing means 23. The pump 66 again is of the positive displacement type in order to effectively pump the fibers slushed with surfactant-containing water back into the foam-producing means. Since some surfactant-containing liquid may be retained on the wire after it leaves the couch roll 46, a vacuum box 68 preferably is installed to extract the moisture from the interstices of the wire and any such collected surfactant-containing water is directed by line 69, which communicates with the line 61 leading to the separator tank 60, and any such liquid eventually goes into the hogpit.

In typical practice approximately 80 percent of the total recovered surfactant-containing liquid is returned to the back-water tank 53 and about 20 percent to the hogpit 64. However, the relative amounts thereof are variable.

On the underside of the wire where it is returned to the breast roll water sprays may be used for washing the wire and, in order that fresh water may not be introduced into the system via the wire, a further vacuum box 70 is applied to the wire immediately in front of the breast roll and fresh water discharged therefrom is allowed to go to drain along with the wash water from water sprays 71 which is collected by the collector 72 and directed to the drain 73.

Examples of the manufacture of fibrous web materials using the method according to the invention are as follows:

EXAMPLE 1

A web of filter paper grade was produced as above described. The furnish consisted of 50 percent bleached kraft, 25 percent sulfite and 25 percent cotton linters. The fibers were beaten and stored and after thickening as aforesaid were supplied to the foam-producing means at a consistency level of about 1 percent. The viscosity of the foam was 23 seconds. Its specific gravity was 0.3 and the number average diameter of the bubbles was less than 0.2 mm. The take-off moisture for the web was 73 percent, or 27 percent dry. The suction maintained in the suction boxes was about 10 inches Hg and that in the suction couch roll about 14 inches Hg. The surfactant solution fed into the mixing means contained about 0.2 percent of octylphenoxypolyethoxy ethanol as the emulsifying surfactant. The weight of the finished paper was approximately 120 g/sq.m. and it was formed into a web about 84 inches wide at the rate of about 100 ft. per minute. The mean residence time in association with each pair of vortical forming units was about 60 seconds. The fiber formation was excellent.

While the same fiber stock was being run long fibers were fed into the foam-producing means in sufficient quantity to constitute about 10 percent by weight of the total fiber. The long fibers that were added consisted of cut, staple nylon fibers about 1/2 inch in length and 2 denier per fiber. Similar fibers, but 3/4 inch in length, were added in another run. In each run the fiber formation in the web was excellent with good lateral fiber disposition in all directions.

EXAMPLE 2

Paper-making stock was prepared by known methods by disintegrating cotton linter fibers in water and beating to 18.degree. Schopper-Riegler. The stock was treated in the normal way with a wet strength resin, that used being 2 percent melamine formaldehyde by weight of fiber.

An emulsion of air in water was prepared by continuously pumping a 0.2 percent by weight solution of "TEXOFOR FN 15" (a polyoxyalkylene condensate) in water through a modified Denver flotation cell of the kind hereinabove described at such a rate that the mean residence time in the cell was 15 seconds. The admission of air to the impeller of the cell was restricted so that little coarse froth formed at the surface of the emulsion and was not allowed to overflow into the launder of the cell in the manner used in the froth flotation process.

The emulsion and the prepared stock were taken pumped into another Denver cell at such rates of flow that the mean residence time was 45 seconds and the proportion of fiber to water in the suspension was 2 parts by weight of fiber to 100 parts of water. The admission of air to the cell was restricted so that little of the coarse top froth was formed and the suspension did not overflow into the launder of the cell. The specific gravity of the suspension was 0.3 and the viscosity and bubble size were substantially as in Example 1.

The suspension was pumped from the bottom of the second cell adjacent the base of the vortex maintained in the cell and spread and deposited in a layer on the wire of a Fourdrinier type paper-making machine through a V-shaped nozzle of the kind described in British Pat. No. 1,075,103, the thickness of the layer of suspension being approximately 11/2 cm. The V-shaped nozzle was suspended over the wire at a position such that the suspension was deposited on the wire directly above the suction boxes and on passage over the surfaces of the suction boxes the layer collapsed and drained to form a web of fiber of a substance of 100 g/sq. cm. The suction in the boxes was approximately 11/2 to 3 inches Hg and that in the suction couch was 12 to 14 inches Hg.

The fibrous web was then transferred from the wire to the succeeding sections of the paper-making machine in the normal manner. The liquid and froth collected from the suction boxes was largely separated from the air in a cyclone type separator of known kind. The air was exhausted by a wet vacuum pump and the separated liquid and froth were returned to the Denver cell.

The uniformity of the resulting material was much greater than that of the usual material made from similar fibrous stock with the resulting advantage of more uniform distribution of the size of open pores in the material, and a superior filtering performance.

EXAMPLE 3

The process followed was that of Example 2 but the stock consisted of 67 percent of free beaten sulfite pulp and 33 percent of an artificial latex such as is referred to in British Pat. specification No. 920.848. The latex was added to the suspension in the second Denver type cell. The web formed had a substance of 40 g/sq.m. and the proportion of latex retained in the web was 85 percent to 90 percent as compared with 70 percent retained when a web is made by the usual known paper-making processes. Thus it will be understood that by the use of a suspension prepared in accordance with the invention there can be obtained an increased retention of loadings and other additives within the fibrous web.

EXAMPLE 4

Cotton linter stock as in Example 2 had added to it in the beater a proportion of 10 percent Terylene fibers 1 inch long and 15 denier. The stock was then pumped continuously through a thickener into a modified hydrapulper dispersing machine having a vortical mixing action very similar to that hereinabove described as produced when using the modified Denver cell, the hydrapulper screen having been removed. In the hydrapulper the furnish was diluted to a consistency of 1 percent and it had a proportion of 0.2 percent by weight of liquid of TEXOFOR FN 15 added to it. The mean residence time in the hydrapulper was approximately 5 minutes which was found to produce a sufficiently viscous suspension of fibers in emulsified foam meeting the aforesaid requirements as regards specific gravity and bubble size. This suspension was pumped from adjacent the base of the vortex onto the wire of a Fourdrinier type machine and collapsed there as described in the preceding examples. The resulting product was a filter paper of superior uniformity and heat resistance.

A further feature of this invention resides in using in combination with a Fourdrinier type machine or other web-forming machine apparatus which may be selectively employed either for the production of paper or other non-woven web material in the conventional manner or for utilizing fibers which have been dispersed in foam that is fed to the headbox of the web-forming machine. Such apparatus combination is illustrated in the block flow diagram of FIG. 8. In this flow diagram the operating components have been indicated generally. Means for preparing a fiber stock suspension is indicated at 74 and prepared stock is transferred to the thick stock storage means 75. The wet end of a paper- or other web-forming machine is indicated at 76. When employing conventional equipment the thick stock from storage means 75 is directed through the mixing means 77 wherein the furnish of desired dilution is produced that is fed into the headbox of the web-forming machine. Transfer of the thick stock to the mixing stage is controlled by valve 78 and transfer of the furnish to the headbox of the web-forming machine is controlled by valve 79. Water drained from the web during its formation may be directed to the back-water tank 80 so that it may be fed back into the mixing stage 77. The flow of water into the back-water tank is controlled by the valve 81. Of course, the web formed on the wet end of the web-forming machine is removed therefrom and processed in the usual way. When the apparatus is being employed in the conventional manner the valves 78, 79 and 81 are open and the valves 82, 83 and 84 are closed.

When it is desired to form the web from a foamed fiber furnish the valves 82, 83 and 84 are opened and the valves 78, 79 and 81 are closed. When the valve 82 is open, thick stock from the storage means 75 is transferred to the stock thickener 85 which corresponds with the stock thickener 21 shown in FIG. 1. Thick stock from the stock thickener 85 is directed to the foam-producing means 86 which in the flow diagram corresponds with the foam-producing means 23 shown in FIG. 1. The foamed fiber furnish may then be directed through open valve 83 to the headbox of the web-forming machine. In FIG. 8 all water separated from the foam after it is deposited on the foraminous surface of the web-forming machine is shown generally as being directed into the container or back-water tank 87 from which it is fed, as desired, into the foam-producing means 86. No attempt in FIG. 8 has been made to illustrate the different means shown in connection with FIG. 1 for returning the surfactant-containing water to the foam-producing means. Obviously, however, the expedients shown and described in connection with FIG. 1 may be employed in the combination shown as a flow diagram in FIG. 8.

Claims

We claim:

1. Method of producing a non-woven fibrous web which comprises

a. forming by a vigorous shearing action an aqueous foam consisting essentially of air dispersed in the form of very small bubbles in an aqueous solution containing a surface active agent, the number average diameter of said bubbles being not more than substantially 0.2mm, said foam having a viscosity of at least 22 seconds as measured by Ford Cup Type B-4 at 20.degree. C and having an air content by volume of at least about 65 percent;

b. forming a dispersion of fibers in said aqueous foam to produce a mixture possessing thixotropic properties with low viscosity and rapid rate of release of said fibers when subjected to draining action; and

c. depositing said fiber dispersion on a foraminous support with concomitant collapse of the foam and formation of a non-woven web on said support accompanied by drainage of residual water from the web.

2. The method of claim 1 in which the dispersion of fibers in said aqueous foam is formed by subjecting surfactant-containing water in which the fibers are dispersed to the vortical foaming action of an impeller which creates a vortex of liquid drawn towards the impeller at the base of the vortex.

3. A method according to claim 2, wherein surfactant-containing water and fibers to be dispersed therein are continuously supplied so as to be subjected to said vortical mixing action and wherein foam having the defined properties and characteristics is continuously removed from the region adjacent the base of the vortex.

4. Method of producing a non-woven fibrous web which comprises

a. forming a foam of surfactant-containing water, said foam being formed by a vigorous shearing action and consisting essentially of air dispersed in the form of very small bubbles, the average diameter of said bubbles being not more than substantially 0.2 mm, said foam having a viscosity of at least 22 seconds is measured by Ford Cup Type B-4 at 20.degree. C and having an air content by volume of at least about 65 percent, by subjecting water containing an effective amount of a foam-producing surfactant to the vortical foaming action of an impeller which creates a vortex of liquid drawn towards the impeller at the base of the vortex together with some air that is emulsified by the action of the impeller with return flow of foam to the mouth of the vortex carrying with it coarse bubbles which at least in part are drawn into the vortex;

b. removing foam from the region adjacent the base of the vortex while it is in the condition produced in that region by the action of the impeller;

c. dispersing fibers in said removed foam; and

d. depositing the dispersion of fibers in the removed foam on a foraminous support with concomitant collapse of the foam and formation of a non-woven web on said support accompanied by drainage of residual water from the web.

5. A method according to claim 4, wherein the fibers to be formed into the web are dispersed in the foam as it is formed by said vortical foaming action.

6. A method according to claim 4, wherein fibers are dispersed in the foam after removal of the foam from adjacent the base of the vortex and the foam with the fibers contained therein is subjected to a second vortical foaming step from which foam containing fibers therein is removed at the region adjacent the base of the vortex.

7. A method according to claim 4, wherein surfactant-containing water having fibers dispersed therein is subjected to a succession of vortical foaming steps with effluent from one flowing into the next, foam of emulsified air with the fibers dispersed therein being removed from adjacent the base of the vortex of the last vortical mixing step and being directed for deposit on the foraminous support of the web-forming machine.

8. A method according to claim 7, wherein at a point between the first vortical mixing step and the second any unfoamed surfactant-containing water is separated and is returned to the vortex maintained during the first vortical foaming step.

9. The method of claim 4, in which there are concurrently introduced from separate sources fibers and surfactant-containing water into a foam-producing zone in which said foam is produced by a vigorous shearing action to form a fiber dispersion possessing thixotropic properties with low viscosity and a rapid rate of release of said fibers when subjected to draining action.

10. A method according to claim 9, wherein fibers are introduced while dry.

11. A method according to claim 9, wherein fibers are introduced in the form of a thick stock of fibers dispersed in water that is substantially free of surfactant.

12. A method according to claim 9, wherein surfactant-containing water is drained from the web during its formation on the foraminous support, and wherein said surfactant-containing water drained from the web is introduced into the foam-producing zone simultaneously with the introduction of fibers into said zone.

13. A method according to claim 9, wherein the foam having fibers dispersed therein is continuously removed from the foam-producing zone and the rate of introduction into the foam-producing zone of surfactant-containing water drained from the web during its formation is controlled responsive to the level of the liquid head in the foam-producing zone.

14. Method of producing a non-woven fibrous web which comprises

a. forming a suspension of fibers in water;

b. thickening the suspension of fibers to form a thickened fibrous stock;

c. mixing the thickened fibrous stock with surfactant-containing water to form a dilute fiber furnish of web-forming consistency;

d. foaming the dilute furnish by subjecting the furnish to vigorous shearing action to produce therein an aqueous foam consisting essentially of air in the form of very small bubbles in an aqueous solution containing a surfactant, the number average diameter of said bubbles being not more than substantially 0.2 mm, said foam having a viscosity of at least 22 seconds as measured by Ford Cup Type B-4 at 20.degree. C and having an air content by volume of at least about 65 percent;

e. depositing said foamed furnish on a foraminous support to form a web with concomitant drainage of surfactant-containing water so as to partially dry the web;

f. removing partially dried web;

g. returning surfactant-containing water to the mixing step (c); and

h. substantially balancing the water introduced into said mixing step with the thickened stock, with the amount of surfactant-containing water contained in the partially dried web removed from web-forming step (e).

15. The method of claim 14, in which a portion of the surfactant-containing water drained from the web during its formation in step (e) is added to the surfactant-containing water that is introduced substantially free of fibers into the foam producing step (d); another portion of surfactant-containing water drained from the web is admixed with waste portions of the formed web to form a dispersion of fibers in surfactant-containing water; and said fiber dispersion is introduced into the foam-producing step.

16. A method according to claim 14, wherein the fibers are introduced into the foam-producing zone at a substantially constant rate and the rate of introduction of said surfactant-containing water that is substantially free of fibers is controlled responsive to the level of liquid in the foam-producing zone.

17. Apparatus for making a non-woven fibrous web which comprises in combination:

a. fiber stock supply means;

b. web-forming means provided with a foraminous support on which an aqueous fiber furnish is deposited with formation of fibrous web and drainage of aqueous liquid therefrom;

c. foam-producing means including a casing having means for admission of air thereto, a blade impeller mounted within said casing so as to be rotatable about an axis at right angles to the upper surface of the casing, the upper edge of said impeller being situated closely adjacent to the upper inner surface of said casing so as to produce a vortex in the aqueous liquid supplied thereto, the distance between said upper edge of the impeller and the surface of the casing being in the range 0.010 inch to 0.125 inch and such as to form a foam wherein the number average diameter of the bubbles is not more than substantially 0.2 mm, by vigorous shearing action on said aqueous liquid;

d. means for mixing said fiber furnish with said foam;

e. means for directing fiber stock from said stock supply means selectively to said mixing means and to said foam-producing means;

f. means for directing fiber furnish to said web-forming means selectively from said foam-producing means or from said mixing means; and

g. means for directing aqueous liquid drained from said web-forming means selectively to said mixing means and to said foam-producing means.

18. Apparatus according to claim 17, wherein fiber stock selectively directed to said foam-producing means from said fiber stock supply means is directed from said fiber stock supply means to stock thickening means and from said stock thickening means to said foam-producing means.

19. Apparatus for producing a non-woven fibrous web from a foamed fibrous furnish which comprises in combination:

a. foam-producing means comprising a mixing compartment, a rotary impeller adjacent the bottom of said compartment, and means for rotating said impeller at high speed, said foam-producing means being capable of forming a foam by vigorous shearing action on aqueous liquid supplied thereto the distance between the adjacent edge of said impeller and the bottom of said compartment being in the range 0.010 inch and 0.125 inch;

b. a web-forming apparatus including a travelling foraminous support on which an aqueous fiber furnish is deposited with formation of a fibrous web; and

c. means for removing foam-containing dispersed fibers from adjacent said impeller near the bottom of said mixing compartment and directing them onto said travelling foraminous support.

20. Apparatus according to claim 19, which comprises a reservoir, means for directing surfactant-containing liquid drained from said web during its formation on said foraminous support into said reservoir, and means for admitting surfactant-containing water from said reservoir into said compartment responsive to the level of liquid in said compartment.

21. The apparatus of claim 17, which further includes a reservoir for supplying surfactant-containing water to said foam-producing means, means for directing surfactant-containing water from said reservoir into said foam-producing means, means for collecting surfactant-containing water drained from said web-forming means, means for directing surfactant-containing water from said collecting means into said reservoir, and means for depositing foam produced by said foam-producing means having fibers disposed therein onto said foraminous support.

22. The apparatus of claim 21 wherein said collecting means comprises a first collecting means for collecting a first portion of said surfactant-containing water drained from said web-forming means, and a second collecting means for collecting a second portion of surfactant-containing water from said web-forming means, and wherein said means for directing water from said collecting means into said reservoir comprises a mixing reservoir, means for directing surfactant-containing water into said mixing reservoir, means for introducing fibrous stock into said mixing reservoir, and means for directing fibrous stock admixed with surfactant-containing water from said mixing reservoir into said foam-producing means.