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(11) | EP 0 150 777 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Method for the manufacture of a non-woven fibrous web |
| (57) Apparatus and method for the manufacture of a non-woven fibrous web such as paper
from a dispersion of fibers in a foamed liquid in which fiber fumish in a foamed liquid
comprising a solution of surfactant in water is discharged from a headbox into the
nip of a twin forming wire prior to its passage over a forming roll. The watersurfactant
solution is drained from the web and recycled as a foamed liquid containing about
65 % air in the form of bubbles of from about 20 to about 200 microns in diameter.
The foamed liquid is directed into a mix tank wherein a slurry containing fibers at
20 % to 55 % solids is added and mixed. The slurry is pumped to the headbox and into
the nip of the forming wires. |
[0001] This invention relates to imprcvements in paper and other non-woven fibrous webs. In one of its more specific aspects, this invention is directed to a method of making improved non-woven fibrous webs and to the resulting products of the process. By the process of this invention, non-woven fibrous webs, e.g. paper towels and facial or toilet tissue, having superior uniformity and controlled tensile strenght may be produced on high speed paper making machines.
[0002] High quality facial and toilet tissue are usually produced from refined high grade wood pulp on a fourdrinier type paper making machine wherein a dispersion or furnish of bleached wood pulp in an aqueous carrier is uniformly dispensed onto a moving belt to form continuous web. Conventionally the wood fibers are dispersed in water to form the furnish; recently foam forming processes have been developed in which the fibers are dispersed in a foamed aqueous solution of a surfactant which forms a relatively stable foam capable of supporting the fibers.
[0003] Among the prior art processes for producing fibrous webs by various foam-forming methods are those disclosed in U.S. patents Nos. 3,716,449; 3,938,782; 3,871,952; 3,837,999; 4,443,297; and 4,443,299 incorporated herein by reference. The method of this invention produces foamed liquid having the desired air content, viscosity, specific gravity, and related characteristics required for forming a fibrous web without the need for a special foam forming or turbulence generating device of the earlier patents and without the need for a foam storage silo.
[0004] Fig. 1 is a diagrammatic perspective view, with portions fragmented, of apparatus suitable for carrying out the process of this invention.
[0005] Fig. 2 is a sectional view with portions fragmented taken generally along the line of 2-2 of Fig. 1.
[0006] With reference to Fig. 1 of the drawings, a preferred form of apparatus of the twin-wire type for making a non-woven fibrous web, such as paper, is illustrated wherein reference numerals 11 and 12 designate first and second endless, woven, fluid permeable forming wires of substantially similar weave and type used in the forming of the non-woven webs. Forming wire 11 is supported in a conventional manner on rolls, including those designated generally by the numerals 13, 14, 15, and 16. Similarly, forming wire 12 is supported on rolls of conventional design, two of which are illustrated and designated by reference numerals 13 and 18a. The support rolls for forming wires 11 and 12 are so positioned as to cause the wires to converge to form a nip 17 just ahead of cylindrical forming roll 19 as illustrated in Fig. 2. The wires 11 and 12 are driven so that the wrapped portions on forming roll 19 move undirectionally, at the same speed, in the direction of rotation A of roll 19.
[0007] As illustrated in Fig. 2, wires 11 and 12 converge on forming roll 19, at slightly different angles, forming a wedge-shaped nip 17 therebetween into which a jet 20 of a foamed liquid-fiber dispersion is directed from a pressurized headbox 21 as illustrated. The surface of roll 19 is smooth and fluid impervious, and wires 11 and 12 are so tensioned that they are operative to squeeze the foamed liquid-fiber dispersion between them to force liquid 20a through the wire 11, hereinbelow also referred to as the outer wire. Liquid 20a forced through the outer wire 11 is directed through the open inlet port 23 of a saveall 22, and, with the aid of deflectors 22a, collected therein as seen at 20b. Wire 12, the inner wire on the forming roll, supports web W as it is carried away from the forming roll for drying and further conventional treatment.
[0008] Again with reference to Fig. 1, the foamed liquid and fiber furnish is supplied to headbox 21 through a conduit 24, and the residual liquid removed from the web W is withdrawn from saveall 22 through a conduit 25, to a pump 28 for recirculation through conduit 24 leading to headbox 21. A parallel liquid flow circuit comprises conduit 29 connected to conduit 25, a pump 30, a conduit 31 leading into the top of mix tank 32, and a conduit 33 leading from the bottom of mix tank 32 provided with a pump 34 which a dispersion of fibers in foamed liquid through conduit 35 to conduit 24. A water-surfactant solution is supplied to the mix tank 32 from a source 36 through conduit 40. Pulp comprising fibers of the type used in paper making is supplied to tank 32 through conduit 30 leading from a de-watering press 37 to which a pulp slurry is supplied from a suitable source (not illustrated). An agitator 38 is positioned in and operative to mix the contents of tank 32. The rate of pulp feed to the de-watering press is controlled to produce webs of the desired basis weight at the production speed of the machine. Typical basis weights are in a range of from about 8 lbs/ream (3000 ft2) to about 38 lbs/ream.
[0009] In a typical startup procedure, water from a suitable supply source 46 is introduced into mix tank 32 through supply conduits 45 and 45a. A concentrated aqueous solution of surfactant is added to tank 32 through conduit 40 in the amount necessary to provide a predetermined surfactant concentration in mix tank 32 required to produce a foamable liquid capable of producing a relatively stable foam which will support the fibers making up the foamed fiber furnish supplied to the headbox. The mix tank 32 is partially filled, e.g., to about one half to three fourths its capacity, with sufficient foam forming liquid to fill the pumps, conduits, headbox, and saveall when circulation is initiated and to provide a residual volume in the mix tank in the range of one fourth to one third of its capacity. An aqueous solution of a suitable anionic surfactant, e.g., an alpha olefin sulphonate has been used successfully at a preferred concentration of about 300 ppm by volume. A number of other surfactants are suitable as a water additive for purposes of the present invention, being generally classified as nonionic, anionic, cationic, or amphoteric.
[0010] Selection of a suitable class of surfactant is dependent upon chemical characteristics of such other commonly used additives which may be present in the manufacture of fibrous webs. These other additives include, singly or in homogeneous mixtures thereof, latexes, binders, de- bonding agents, dyes, corrosion inhibiting agents, pH controls, retention aids, creping aids, and other substances used in papermaking processes.
[0011] U.S. Patent Nos. 3,716,449 and 3,871,952 disclose specific nonionic, anionic, and cationic surfactants that have been found suitable in the art of forming fibrous webs from dispersions of fibers in foam. U.S. Patent No. 4,056,456 discloses additional surfactants, including some classified as amphoteric, that are suitable for practice of the present invention. The disclosures of these patents are included, by reference, in the present application for their teachings of surfactant materials.
[0012] The forming wires 11 and 12 are driven at a speed of about 2500 fpm (feet per minute). The pumps 28, 30, and 34 are energized to pump foamable liquid from saveall 22 and the suspension of
[0013] fibers in foam from mix tank 32 to headbox 21, from which jet 20 is directed into nip 17 at the juncture of the forming wires 11 and 12. The rotational speeds of pumps 30, 34 and 38 are regulated to establish fluid flow rates through the system which result in a preferred materials balance at typical flow rates, pump 28 handles about three fourths of the desired volume of flow to the headbox 21 while pump 34 handles the remainder. Pump 30 is regulated to maintain a substantially constant level in mix tank 32. The flow rate of foamed fiber furnish is regulated to achieve a jet velocity in the range of from about 90% to about 150% of the speed of the forming wires. Usually a jet velocity of about 110% of the speed of the wires is preferred. Forming wire speeds may be in the range of from about 1000 fpm to about 7000 fpm, or more, depending upon the operating conditions and the physical properties of the foamed fiber furnish and the forming wires.
[0014] When the foamed fiber furnish impinges on the forming wires 11 and 12, the furnish is uniformly distributed over the width of the wires. As the outer wire 11 converges with the inner wire 12, at nip 17, pressure is applied to the furnish which, combined with the force of liquid jet 20, causes the foamable liquid to flow through interstices of outer wire 11. The inner wire 12 has its interstices closed to fluid flow by the underlying solid surface of forming roll 19. As the expressed foamable liquid passes through the outer wire, air travelling with the wire as well as air in its interstices is entrained, thereby generating foam in the foamable liquid so that, once started, the foam generation (or regeneration) is a self sustaining operation.
[0015] Foam 20a is collected in saveall 22 and directed to the mix tank 32 via conduits 25, 29 and 31 and to headbox 21 by way of conduits 25, 27, and 24. The method of generating and regenerating foam is so effective that no other means of foam generation is required. In test operations, starting up without any fiber addition to mix tank 32, during an operating period of about 5 minutes, the air content of the foamable liquid increased from nearly zero to a preferred value of about 67 volume percent. Maximum bubble size of the foam during operation is, for example, in a range from about 20 microns to about 200 microns, which is less than the lengths of the suspended fibers. Optimum relationships of bubble dimension to fiber dimensions are dealt with in the referenced U.S. Patent Nos. 3,716,449 and 3,871,952, and are readily achieved by the apparatus and method of the present invention.
[0016] A pulp of papermaking fiber in water is prepared conventionally to a consistency about 1.0 to 4.0% fiber by weight. The conventionally prepared pulp is dewatered in a stock press 37 and then introduced into mix tank 32. Leaving the stock press 37 through line 37a, the pulp has a consistency sufficient to require the addition of make- up water and surfactant solution to the system via lines 40 and 40a respectively. The desired consistency of the pulp in line 37a can be calculated easily by material balance on the basis of limiting the loss of surfactant from the system to that amount inevitably carried away in the wet web 12. In general, the pulp consistency is between 8 and 50 weight percent fiber, preferably between 15 and 35 weight percent. Water removed by press 37 may be recycled. The dewatered high consistency pulp from line 37a is introduced to the mix tank 32 well below the liquid level therein at a rate dependent upon the material balance. About 4 to 22 pounds of surfactant per ton of dry fiber in web W is lost from the system and is made up through lines 40 and 40a.
[0017] Fiber is introduced from the dewatering press 37 to mix tank 32 at a rate corresponding to the desired web production rate. A slurry of about 3 weight percent fibers normally is fed to press 37, and a slurry of from about 25 to about 50 weight percent, preferably about 35 weight percent fibers, leaves the press 37 as feed to mix tank 32. If desired, dry fibers may be introduced directly to the foamed liquid in mix tank 32 in suitable proportions for achieving desired basis weights. With all pumps energized, the foam-fiber mixture is directed by pump 34 from mix tank 32 through conduits 33 and 35 into conduit 24, where it combines with foamed liquid from saveall 22, through conduits 25, 27 and 24, and the resulting foamed fiber furnish supplied to headbox 21 from which it is fed onto wires 11 and 12. Fibers and some of the liquid remain on the wires forming the product web. The major portion of the fcamable liquid passes through wire 11. Foam is regenerated by air from the wires as explained above. Control of air content of the foam is achieved by controlling the amount of surfactant added to the system in mixing tank 32.
[0018] In operation, a balance of air loss through foam degradation and air gain through regeneration is necessary to maintain proper foam air content and bubble size. The surfactant concentration is the primary factor determining the rate of foam degradation. The bubble size of the foam becomes the primary controlling factor on air gain through regeneration. The bubbles in the 20 to 200 micron size are significantly smaller than the openings in the weave of the forming wire thus passing through without the fluid film surrounding the bubble being broken into smaller bubbles. Bubbles of 20-200 micron size thus expell the air in the forming wire interstices without excessive foam generation.
[0019] Another naturally occurring phenomenon assists air content control. When the air content of the foam exceeds 67% air by volume, the foam becomes progressively more viscous with increasing air content. As the viscosity of the foam increases, it becomes more difficult to remove from the web. Thus more surfactant is lost from the ' system with the web as it is formed tending to restore the surfactant concentration balance.
[0020] The actual concentration of surfactant needed is a function of many variables and is best determined by trial. some of the variables are surfactant type, water hardness, water temperature, furnish ingredients and circulation time in the system.
[0021] A loss of foam occurs following the introduction of fiber and its deposition on the forming wires, since liquid is removed from the system with the fibrous web. The foamable liquid lost in this manner is continuously replenished, the water being replenished by water contained in the pressed pulp from press 37 supplemented by water supplied through conduit 45 and the surfactant solution replenished through supply conduit 40. The relative proportions of water and concentrated surfactant solution are suitably regulated in the range of 150 to 450 ppm (parts per million) surfactant by weight, to maintain air content of the foam in the desired range of from about 55 to about 75 percent. For example, in test runs, a concentration of about 340 ppm of an alpha olefin fulfonate (Arco A-OK) in water in the circulating foamable liquid was sufficient to maintain the air content in the foamed liquid at a preferred value of about 67% air by volume. It is well known in the art, as exemplified by the referenced U.S. Patent Nos. 3,716,449 and 3,871,952, that air contents below about 55% are conducive to fiber agglomeration, and air contents above about 75% are conducive to fiber bundling, both undesirable.
[0022] Control of air content is achieved by maintaining a predetermined concentration of surface active agent in the foamable liquid. The requisite concentration of surfactant depends on many factors including the particular choice of surfactant, the temperature of the system, the hold up time, i.e. time required to make one complete cycle of foam through the system and the speed of the wires, and is best determined for any given system by trial. By controlling the surfactant concentration, other factors remaining constant, the air content of the foam can be held substantially constant without the need for a foam generating device or for metering of air by separate means.
[0023] Foamed liquid from the saveall 22 is transferred by pump 30 through lines 25, 29 and 31 to mix tank 32. Pump 30 preferably is of the twin screw type capable of transferring low density liquids such as the foamed liquid. The volume of foamed liquid thus transferred is that amount necessary to obtain a mix tank consistency of between about 0.3 to about 4 percent fiber by weight, preferably between 1.5 to 4 percent. An agitator 38 provides the requisite energy to disperse the fibers in the foamed liquid. The foamed liquid furnish leaves the mix tank 32 by line 33, a twin screw pump 34 providing the motive energy therefor. The discharge from pump 34 through line 35 is passed through line 24 to headbcx 21. In a preferred embodiment, that is, where the mix tank consistency is between 1.5 to 4.0 percent fiber by weight, additional foamed liquid is pumped from the saveall 22 by twin screw pump 28 through line 24, and is combined with the mix tank discharge line 35, the combined streams flowing through line 24 to headbox 21. The flow rates in lines 24 and 35 are such that the furnish of line 24 is diluted to a final (headbox) consistency of between about 0.3 to about 1.2% by weight. Where the mix tank consistency is less than 1.2% fiber by weight, further dilution is not required.
[0024] In mix tank 32, the foamed liquid has substantially the same air content and bubble size quality as in the foam recovered in saveall 22 as the amount of water added with the untreated fibers through line 45 is minor in comparison to the water in recycled foamed liquid added through line 31. At the viscosity values of the foamed liquid in mix tank 32, the fibers from press 37 can be dispersed rapidly.
[0025] At a consistency above 1.5% in mix tank 32, several advantages are realized. First, the size of the mix tank and accompanying equipment is reduced, the ability to rapidly disperse the fibers enhanced, and mixing energy is reduced. The foamed liquid is subjected to shearing action in the mixer 38 which helps maintain fine foam structure while, at the same time, the fibers are subjected to less intensive shearing action than in a conventional water dispersion system so that less alteration of the fiber structure takes place. Consistency of the foamed liquid is ensured by blending the dispersion of fibers in foamed liquid from mix tank 32 with foamed liquid from line 24 to that in line 35 is in the range of from about 6:l to about 1:1 in the preferred process embodiment. Hence, when foam from line 27 is combined with the dispersion from mix tank 32, the foamed liquid in line 24 will have substantially the same quality as that in the saveall 22.
[0026] The final (headbox) furnish in line 24 is at a consistency of about between 0.3 to about 1.2% fiber by weight, and has a viscosity of about 10 cps to about 35 cps on a fiber free basis. Because of the head induced by pumps 38 and 34, the bubble size of the foamed liquid, which is a compressible fluid, is reduced to about 20 to about 200 microns, the averaging bubble size being in the range of about 50 to about 100 microns. The pressure drop through nozzle 22 is about 5 to 25 psi (pounds per square inch), preferably 10 to 20 psi. As the foam expands across the nozzle, the bubbles become larger and the density and viscosity of the foam decreases. The foamed fiber furnish is injected into nip 17 and uniformly across the wires 11 and 12. It has been found that the velocity of the jet from nozzle 22 relative to the speed of the forming wire is a critical factor in the production of a web having a high degree of uniformity of fiber distribution and the desired tensile or tear strength relationships.
[0027] In the method of this invention, twin forming wires are employed with forming wire speeds in the range of from about 1000 fpm to about 7000 fpm or more. It is essential that the pressure and flow rate of the furnish be regulated to achieve a jet velocity from nozzle 22 of from about 90% to 150% of the speed of the forming wires. The jet is directed into the nip 17 in a direction of travel of the forming wires.
[0028] As the foamable liquid impinges on the forming wires 11 and 12 within the confines of the nip 17, the kinetic energy of the jet is converted to potential energy building up sufficient pressure between the wires to lift wire 11 away from wire 12 and at the same time distribute the foam furnish uniformly along the wires without comb or drag relative to the headbox. The relationship of velocity of the jet relative to speed of the wires effectively prevents striation of the fibers in the web. The pressure created as the outer wire 11 moves onto the inner wire 12, combined with the force of liquid jet 20, causes the foamable liquid to flow through interstices of outer wire 11. Substantially no liquid flows through the inner wire 12 which has its interstices closed to fluid flow by the underlying solid surface of forming roll 19. Pressure resulting from closure of the nip between wires 11 and 12, together with centrifugal force resulting from movement of the wires about forming roll 19 and the forces resulting from impingement of liquid jet 20 on the wires, cooperate to produce combined compressive and shear forces on the liquid passing through the outer wire. These forces combine to deposit the dispersed fibers uniformly on the forming wires and permit control of relative tensile strengths in the machine direction and in the cross machine direction of the resulting web. At the same time air traveling with the wire and air in its interstices, simultaneously generates the desired foamed liquid for recirculation in the process.
[0029] By the process of this invention, the formation of the product web is greatly improved as compared with webs produced by conventional processes, that is, the uniformity of the distribution of individual fibers comprising the web is enhanced as observed by absence of flocs in the web upon visual inspection. A better formed web characteristically improves subsequent web processing operations inasmuch as the web is less likely to tear during drying, creping, embossing and the like on a high speed fourdrinier machine.
[0030] Formation of the web may be measured in a Thwing formation tester under Method No. 525 of the Institute of Paper Chemistry. In this procedure, the degree of uniformity of the web is ascertained by the degree of uniformity of light transmission through an area of the web. The Thwing Index (TI) is the ratio of localized variations in transparency to average transparency. Low basis weight products obtained by conventional web processing methods, e.g., tissue, towel, and napkin products having a basis weight between about 8 to 30 Ibs/ream (3000 sq ft), have a TI of between 5 and 15, which values are, of course, dependent upon process conditions and operations. At slower wire speeds, TI values are higher while at faster speeds, the formation is affected adversely. For webs prepared on a high speed pilot machine in accordance with the process of the present invention, TI values were measured at between about 20 to 25, significantly higher than comparative wet laid webs.
[0031] The tensile strengths in the machine direction of the product webs produced by the process of this invention are generally comparable to those produced by conventional wet pulp papermaking processes employing a water slurry of papermaking fibers and considerably greater in the cross machine direction than-those produced with conventional processes employing foamed liquid as the carrier liquid for the fiber furnish. Conventionally foamed fiber furnish processes produce a product having a low tensile strength in the cross machine direction. The relatively low tensile strength in the cross machine direction in conventional foamed liquid systems apparently occurs because the fibers in the web are combed or striated as they are laid on the carrier wire from the headbox by reason of the speed of the wire relative to the headbox and the thickness and relative stability of the foamed fiber furnish laid on the carrier wire. In our process, the foamed fiber furnish is jetted into the nip in a twin wire system at a velocity nearly equal to or in excess of the wire speeds avoiding combing or striation. Sufficient hydrogen bonding of the fibers is obtained to provide a product web of adequate strength in both the machine direction and the cross machine direction.
[0032] The geometric mean tensile strengths of webs produced by the process of the present invention are equivalent to those produced by conventional water-formed fourdrinier webs when the furnish, refining and basis weights are equivalent. In certain products, for example, low basis weight tissue and towel products, a selected ratio of machine direction (MD) to cross machine direction (CD) tensile strengths is advantageous. Machine direction tensile strength is important for processing webs . through operations,such as embossing, slitting, perforating and rewinding into consumer rolls, whereas in the end use by the consumer, equal tensile strengths in cross machine and machine direction may be desireable. The process of this invention allows the tensile ratio to be optimized to fill the needs of tensile ratio demands.
[0033] With seme fibers, a bonding agent may be necessary to provide adequate tensile strength as required by the ultimate end use. Suitable bcnding agents include cationic starch; polyvinyl alcohol; pearl starch; natural gums (tragacanth, karaya, guar); natural and synthetic latex, including polyacrylates, e.g., polyethylacryl- ate, and copolymers; vinyl acetate-acrylic acid copolymers; polyvinylacetates; polyvinyl chlorides; ethylenevinyl acetates; styrene-butadiene carbcxylates; polyacrylonitriles; and thermosetting cationic resins, e.g. , urea formaldehyde resins, melamine formaldehyde resins, glyoxal-acrylamide resins and polyamide-epichlorhydrin resins as disclosed in U.S. 3,819,470. Bonding materials are desirable where the fibers used in the web are not self-bonding, as in certain synthetic and chemically modified cellulosic fibers.
[0034] In the process of this invention, the fiber furnish may be made up of unrefined softwood fibers of a type which readily floc on water-forming papermaking equipment, and may comprise in addition to wood pulp, synthetic fibers, e.g. rayon fibers up to 6 denier in diameter and 3/4 inch long which do not yield satisfactory tissue products in conventional water laid papermaking processes.
[0035] The advantages of the present process and products produced thereby will be evident from the following examples.
Example 1
[0036] A run was made on a high speed twin wire paper machine at about 1000 fpm using repulped Ontario Softwood Kraft (OSWK) fibers that had been refined to 400 CSF. The fibers were added directly to the mix tank, the furnish therein being at 1.8% consistency. Headbcx consistency was adjusted to 0.45% by dilution with foamed liquid from the silo. Arco "Ultrawet A-OK" surfactant was used to generate the foam. The webs were wet pressed and subsequently dried and creped on a Yankee dryer, but were not calendered.
[0038] As above indicated, the ratio of the machine direction (MD) tensile strength to cross machine direction tensile strength was 3.1, i.e. , the tensile strength in this cross machine direction is about 32 percent of the tensile strength in the machine direction.
Example 2
[0039] Test runs were made on a high speed twin wire paper machine operation at 1500 fpm. In each run a pulp of 3.5% consistency was made comprising 50% OSWK and 50% OHWK (Ontario Hardwood Kraft) fibers. After pulping, the slush pulp was pressed to 28 weight consistency and added to the mix tank, and a foamed liquid furnish of about 0.6% percent fiber by weight delivered to the headbox. Air content ranged between 58 to 70%. The webs were wet pressed, dried and creped. In Runs 2 and 3 the webs were calendered.
Example 3
[0041] Three runs were made on the high speed machine operating at 1000 fpm using a foamed liquid furnish comprising 100% OSWK fibers refined to 480 CSF.
Example 4
[0042] Several test runs were made on a high speed twin wire papermaking machine at wire speeds of 2000 feet per minute.
[0043] In these runs, the furnish was repulped baled fiber without any refining consisting of 50% Ontario hardwood and 50% Ontario softwood bleached sulfate wood pulp. The . air content of the foam was maintained at 60% while wire tension was 30 pli. The product was creped and calendered and samples were removed from a rewound roll for testing. The product tensile strength at a theoretical efflux ratio of 1.09 tested 298 gm/3 inch machine direction and 300 gm/3 inch cross machine direction for a web of 9 pounds per ream basis weight.
1. A method for the production of non-woven fibrous web on a moving foraminous support
in which a foamed fiber furnish is formed by dispersing fibers in a foamable liquid
comprising water and a surface active agent and containing 55 to 75 percent air by
volume and supplying the foamed fiber furnish to a moving foraminous support web-forming
means without substantial further turbulence or agitaiton effecting formation of said
web and entraining air in foamable liquid passing through said foraminous support
characterized by collecting foamed liquid passing through said foraminous support
and recirculating said collected foamed liquid containing from 55 to 75 percent air
by volume directly to the foamed fiber furnish production step while maintaining the
desired air content of the foam solely by adjusting the concentration of surface active
agent in the foamable liquid.
2. A method according to Claim 1 wherein the foamed fiber furnish contains 0.3 to
1.2 weight percent fibers.
3. A method according to any of the preceding claims wherein said air in said foamed
liquid is in the form of bubbles having an average diameter in the range of from about
20 to 200 microns:
4. A method according to any of the preceding claims wherein said foamable Liquid
is brought into contact with said moving foraminous support as a jet of liquid having
a velocity in the range of 90 to about 150 percent of that of said moving foraminous
support.
5. A method according to claim 4 wherein the velocity of said jet is about 110 percent
of that of said moving foraminous support.
6. A method according to any of the preceding claims wherein the concentration of
surface active agent in water is sufficient to maintain an air concentration in said
foam within the range of 55 to 75 percent by volume.
7. A method according to any of the preceding claims wherein the aqueous foam containing
from about 55 to 75 percent air by volume is prepared by passing said foamable liquid
substantially free from fibers to and through the moving foraminous support web-forming
means producing a foamed liquid by entrainment of air therein and the foamed liquid
passing through said foraminous support is collected and recirculated to and through
the foraminous support until the foamed liquid contains from about 55 to 75 percent
air by volume in the foam of bubbles having an average diameter in the range of 20
to 200 microns.
8. An improved foam-formed non-woven fibrous web characterized by uniformity of fiber
distribution and tensile strength in the cross web direction at least equal to tensile
strength in the machine direction produced by the method of any of the preceding claims.