Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21F—PAPER-MAKING MACHINES; METHODS OF PRODUCING PAPER THEREON
  • D21F9/00—Complete machines for making continuous webs of paper
  • D21F9/003—Complete machines for making continuous webs of paper of the twin-wire type
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21F—PAPER-MAKING MACHINES; METHODS OF PRODUCING PAPER THEREON
  • D21F11/00—Processes for making continuous lengths of paper, or of cardboard, or of wet web for fibre board production, on paper-making machines
  • D21F11/002—Processes for making continuous lengths of paper, or of cardboard, or of wet web for fibre board production, on paper-making machines by using a foamed suspension

Definitions

• TECHN ICAL FI ELD This invention relates to the manufacture of nonwoven fibrous webs, such as paper, which are typically formed from an aqueous slurry of fibers.
• Paper and other nonwoven fibrous materials are typically manufactured by depositing fibers suspended in a liquid onto a foraminous support which allows the liquid to drain through while retaining most of the fibers in the form of a web .
• the fibers lie intertangled in the plane of the web and adhere to each other by papermaking bonds or by binder added to the web.
• the suspension of fibers in water commonly referred to as "the stock” is flowed onto a horizontal upper run of a continuously moving endless belt of wire cloth wrapped around a breast roll on one end and a couch roll on the other. Water progressively drains from the stock through the wire as the wire carries it from the breast roil towards the couch roll .
• Supporting the upper run of the wire between the breast roll and the couch roll are a number of rotating rolls, known as “table rolls", whose function is to accelerate the drainage of water from the stock.
• the wire runs over “foils” which support the wire and provide a more gentle drainage than the table rolls . Foils are more commonly used at higher machine speeds .
• suction boxes are employed to further assist drainage of water from the stock.
• the wire slides over the flat upper faces of the suction boxes, which are provided with openings in communication with vacuum pumps . The vacuum is necessary to draw additional water from the web after all of the "easy” water is removed by table rolls or foils .
• the wet web of fibers is removed from the wire after passing partially about a perforated "suction couch roll" provided with an internal suction box connected to a vacuum pump.
• the wet web is then passed through a press section where additional water is squeezed out of the web by press rolls and is absorbed by a felt traveling along with the web.
• Final drying of the web takes place in the dryer section or on a yankee dryer.
• a typical dryer section consists of a series of small heated drums about which the web passes .
• the stock is usually flowed onto the wire at a "consistency" of one part of fibers to about two hundred parts of water, by weight. This is described as "0.5 9 o consistency", meaning that the fiber weight is 0.5% of the stock weight.
• the stock loses water gradually ⁇ sively as the wire passes from the breast roil to the couch roll, and therefore, the consistency of the stock increases progressively as it travels away from the breast roll .
• a consistency of about 2.5 table rolls or foils are no longer effective in accelerating the drainage of water from the web, and relatively high vacuum (250mm. of Hg. for example) must be applied through the suction boxes for further drainage.
• the web typically will have a consistency of about 20% when it leaves the couch roll .
• the consistency of the web is further increased to about 40% in the press section, and the consistency of the web leaving the dryer section is greater than 90% and considered completely dry for all practical pu rposes .
• a disadvantage of the conventional method of forming a web is the enormous volume of water which must be handled because of the low consistency of the stock, conventionally kept in the neighbor ⁇ hood of 0.5% when flowed onto the wire in order to achieve accept- ably uniform dispersion of the fibers in the water. Unless this uniformity of dispersion is achieved, the "formation of the sheet" is unacceptable. This is due to the phenomenon of "floccing" of the fibers . That is, the fibers insist upon agglomerating unless dispersed in a dilute suspension .
• the amount of water removed from the web in this conventional papermaking process is enormous .
• to make one Kg. of paper requires about 200 Kg . of water to be deposited on the wire at the breast roll . From this, about 175 Kg . are removed by gravity or the slightly accelerated drainage at the table rolls or foils .
• Another 20 Kg . are removed at the suction boxes and couch roll and 2.5 Kg. in the press section . Final drying in the heated dryer section only takes out about 1 .5 Kg . of water.
• the point where fiber mobility has been sufficiently eliminated to assure maintenance of fiber formation is commonly referred to as the "dry line" , because the appearance of the stock suddenly changes from wet to dry. Typically, the dry line occurs at 3i% to 4 * 2% consistency . Prior to reaching the dry line, great care must be taken in dewatering the web. Fiber mobility in stock, on the other hand, has always been believed necessary in the early steps of dewatering to get the desired fiber orientation and uniformity of formation . That is why prior art processes which employ so-called "high consistency" stock use stock consistencies only up to 1% or 2%. These stocks are still well below the dry line consistency of about 4%. The nonwovens and papermaking art can be viewed as being divided into two distinct types of processes .
• the present invention permits the formation of paper with much less water than in conventional paper- making .
• the stock can be deposited on the wi re at a fiber consistency well beyond the dry line stage, and the web can be formed instanta ⁇ neously with good fiber formation .
• the amount of water needed in the process can be as little as from 8% down to 2% of that used conventionally.
• the commercial impact of the savings is staggering .
• the invention can provide products with improved proper- ties over conventional paper. Examples of such improved properties are greater bulk and better formation , as well as others . These sheets can be produced without loss of strength .
• the present invention is a method and apparatus for producing a nonwoven fibrous web in a very simple manner which provides good formation and uniform basis weight across the width of the papermaking machine.
• the present invention in one form is a method of producing paper with good formation which comprises the steps of:
• the fiber consistency is greater than 10%, greater than 15% or greater than 20%.
• the invention in another form is a method of producing a nonwoven fibrous web which comprises the steps of : A. forming a foamed dispersion of fibers and liquid, said foam being generated by intermixing a gas into the liquid, the fiber consistency in the dispersion being greater than 6% of the dispersion weight;
• the foam is preferably removed by breaking it through means of a pressure differential applied outwardly from the dispersion on at least one side and preferably both .
• the invention in another form is a method of producing a nonwoven fibrous web which comprises the steps of:
• the invention in another form is a method of producing a low density nonwoven fibrous web comprising the steps of:
• the invention is also the paper products and the nonwoven fibrous webs produced by the above described methods .
• the invention in another form is an apparatus for producing a nonwoven fibrous web comprising : A . means for providing a reservoir of foamed fibers and liquid dispersion ;
• reduced pressure means adjacent the reservoir for providing on the side of the carrier member opposite the reservoir a pressure which is lower than that of the foam dispersion for breaking the foam adjacent the carrier member and for forming a fibrous web on the carrier member;
• a preferred form of the apparatus further includes means for preventing unbroken foam dispersion from leaving the reservoir with the fibrous web on the carrier member.
• the carrier members are preferably guided by two rotatable vacuum rolls spaced apart at the point of closest convergence.
• the invention in another form is a method for producing a nonwoven fibrous web comprising the steps of :
• B depositing the foamed dispersion into a reservoir extending continuously across the width of the papermaking machine; C. continuously moving at least one foraminous carrier member in contact with the reservoir;
• FIG . 1 of the drawings illustrates schematically the preferred apparatus for carrying out the present invention .
• FIG . 2 is an enlarged view of the section of the apparatus where the web is formed adjacent to the reservoir.
• the stock preparation section 1 is illustrated by foaming appa ⁇ ratus 4 being fed stock at a high consistency through inlet 5 and being supplied air through inlet 6.
• a suitable apparatus which provides mechanical agitation of the stock in the presence of air is used as the foaming apparatus 4.
• An example of a satisfactory apparatus is a Micar Processor Reactor manufactured by Black Clawson, which consists of a shell and rotor fitted with vanes or bars arranged in a variety of angles to impart micro and macro mixing.
• the stock is fed into the foaming apparatus 4 in the form of a very high consistency slurry of papermaking fibers and water.
• the consistency of the stock should be as high as possible while still providing the desired product properties and will preferably vary between about 6% and about 30%. Consistencies lower than this range do not benefit from all of the advantages of the invention . Consistencies higher may not have enough free water to permit satisfactory foaming .
• su rfactant in water is added to the foaming apparatus 4 th rough inlet 7.
• the su rfactant can be provided by any conventional surface tension reducing material which will permit foaming of water and which is compatible with the properties sought in the paper being manufactured .
• satisfactory surfactants are sodium lau ryl sulfate, rosin size and an aqueous blend of anionic su rfactants and ethylene glycol monobutyl ether.
• the amount of surfactant can be varied to provide the desired foam stability, but will typically be on the order of 0.2% by weight of the stock.
• Air can be delivered along with the stock through inlet 5 or it can be delivered in part or wholly th rough air inlet 6 under sufficient pressure to enable it to enter the apparatus 4. Higher pressure is unnecessary, since foaming is accomplished through the mechanical agitation of the apparatus driven by motor 8.
• the surfactant, liquid and fibers are subjected to mechanical shearing action in the presence of the air to produce a foamed liquid and to uniformly disperse the fibers .
• the stock is sufficiently foamed to provide a volumetric increase wherein preferably at least 66% of the volume is air and more preferably from about 75% to about 95% of the volume is air.
• the minimum air volume ratio is generally needed to provide good formation . The higher the ratio, the better separation of the fibers .
• the air bubbles in the foam replace water in a conventional water dispersion and act to keep the fibers dispersed and separated from each other. Accordingly, the most uniform dispersion requires higher air volume ratios for higher consistency stock.
• the bubble size of the foam is not critical in the sense of having an exact size, but it must be small enough to provide a foam which remains stable until the web is formed. That is to say, the foam must hold the fibers in their dispersed state until they are about to take a permanent placement in the web . The smaller the bubbles the more stable the foam will be. Typically, the bubble size will be small enough to be barely visible to the naked eye.
• the foamed dispersion will be permanently stable. That is to say, the dispersion will remain intact indefinitely unless some special step is taken to destabilize and collapse the foam.
• steps are external mechanical pressure being applied to the foam or the foam being subjected to a pressure which is lower than the pressure in the foam.
• the latter is preferably accomplished by exposing the foam to a partial vacuum, such as by providing a suction through a screen which collects the fibers . It would also be possible to produce and maintain the foam at a higher than atmospheric pressure and then collapse the foam by exposing it to atmospheric pressure.
• the permanently stable foamed dispersion of the present invention when left to dry, either to the air or in a heater, without subjecting it to a destabilizing step will maintain the fiber dispersion even after drying.
• Subject only to a small amount of shrinkage from drying the high volume, low density structure of the foamed dispersion can be maintained even after drying .
• shrinkage will be less than 50% and preferably less than 20%.
• Useful fibrous products can be made with all or at least a majority of the fibers being papermaking fibers or all or at least a majority of the fibers being nonpapermaking fibers if an adhesive is added to the liquid before foaming .
• Such products can be useful for insulation or cushioning materials, among other things .
• the foamed stock is delivered from the foaming apparatus 4 through a conduit 9 to a stock reservoir 10 formed by two continu ⁇ ously moving foraminous or permeable wires 11 and 12 converging towards each other to form a nip .
• the wires can be provided by conventional fou rdrinier wires .
• the wi res are brought closely together as they move past vacuum foam breakers 13 and 14 spaced apart a short distance from each other.
• Each of the vacuum foam breakers 13 and 14 are preferably provided by suction rolls having a partial vacuum within .
• the vacuum is preferably greater than 75 mm. of Hg .
• the foamed stock emitted from conduit 9 piles up in the reservoir 10 to form a pond 15 (a reservoir or accumulation of material unenclosed or open to the atomosphere) of stock extending continuously across the width of the papermaking machine and from which a thin wide sheet-like flow of stock is continuously drawn into the nip.
• the suction from vacuum foam breakers 13 and 14 assists the moving wires in drawing stock into the nip.
• the suction creates a zone of lower pressure than in the foam, which collapses the foam and forms the web.
• An exceptionally valuable advantage obtained by this apparatus is that the web so formed will have uniform basis weight and uniform formation acrpss its width even with wide variations of height across the width of the pond or reservoir. There is no need for a complex flow spreader to deliver the stock.
• the conduit 9 can be provided by a series of circular pipes positioned across the width of the machine, the number of which depends upon the width of the wires .
• the suction from the foam breakers 13 and 14 simultaneously accomplishes in a single step destabilizing and breaking or collapsing of the foam, removal of at least a portion of the foam medium (air along with some water and surfactant) and formation of the web on the wires 11 and 12.
• Prior to collapsing the foam essentially no water will drain from the foam.
• a continuous web 20 is formed between the two wires 11 and 12, and it travels with the wires for a short distance until the two wires separate.
• a vacuum box 16 placed behind wire 11 is used to gently urge the web 20 to follow wire 11 when the wires separate.
• felt 17 is brought into contact with the web and the web is transferred to the felt with the assitance of vacuum box 18. The felt 17 .
• the web 20 then travel together t rough press rolls 19 where water leaves the web .
• the web 20 is removed from felt 17 and delivered to one or more additional press rolls (not shown) and then to a conventional drying section (not shown) , if desired .
• the web may be dried by through-air driers. Where a press section is employed, the sheet can enter it drier than a conventional sheet, thereby requiring less pressing and resulting in higher bulk.
• wi res 11 and 12 continue around a continuous loop driven by nip-forming drive rolls 21 and about carrying rolls 22 and guide rolls 23.
• felt 17 continues around carrying rolls 22 and tension roll 24.
• Located adjacent the path of the felt 17 is a shower 25 for washing the felt 17 and a suction box 26 for dewatering .
• reservoir 10 formed by pond 15 of foamed dispersion extends downwardly to the point of closest convergence between carrier wire 11 and carrier wire 12.
• a reservoir is defined herein as an accumulation of material in a stream of flow wherein the quantity of the material being removed from the reservoir can differ widely from the quantity entering the reservoir from moment to moment.
• the two wires converge together at this point to form a nip, the spacing of which is established by the spacing between vacuum foam breakers 13 and 14.
• One wire is needed to form the web against, and the other wire serves to prevent unbroken foam from leaving the reservoir.
• Each foam breaker shown here schematically in cross-section consists of outer rotatable cylinders 27 and 28 drilled or otherwise perforated to provide openings to vacuum chambers 29 and 30.
• the foam breakers may be provided by conventional rotatable suction rolls commonly used in papermaking .
• the vacuum chambers are provided by adjustable radially extending baffles which define the suction zone in each foam breaker.
• the lower baffle of each is preferably positioned at the nip or the point of closest convergence of the wires, which is also the boundary of the reservoir. From the point of the lowermost baffle downstream the foam in the dispersion has been collapsed and the fibrous web 20 formed between the wires 11 and 12.
• Each lower baffle could be positioned above (upstream of) the nip in some circumstances, but generally it would not be desi rable, since there would be no means of escape for ai r and/or water squeezed out as the web proceeded into the nip . Also, it generally would not be desi rable to place it below (downstream of) the nip because it would tend to draw the wire from its natural straight path after leaving the nip.
• Each upper baffle forming the vacuum chamber is positioned at another point adjacent the reservoi r to place the entire vacuum chamber in vacuum communication with the reservoir. Its preferred position will depend upon machine speed and vacuum level .
• An example of a satisfactory position is between about 2° and 15° ci rcumferentially from the center point of the nip. It may be desirable for the positions of the upper and lower baffles in the two foam breakers to be different in some circumstances.
• the vacuum levels in the two foam breakers 13 and 14 do not have to be the same. I n fact, it is preferable that the vacuum be slightly higher in foam breaker 13 to encourage the formed web to
• the spacing between the foam breakers will be related to the desired web thickness and vacuum level .
• the spacing between wires 11 and 12 will typically be between about 0.3 mm. and about 1 .5 mm. for 100 g./m. 2 basis weight paper.
• the foam breakers may have their positions fixed rigidly or, preferably, mounted for slight movement towards and away from each other and being resiliently urged towards each other by springs or fluid driven pistons .
• the fibers employed in the present invention are preferably papermaking fibers in the majority, and more preferably in total , which form papermaking bonds upon drying, nonpapermaking fibers, such as synthetic fibers, can also be employed, in which case it may be desirable for adhesives to be added for additional bonding.
• nonpapermaking fibers such as synthetic fibers
• foamed fiber papermaking stocks of softwood and hardwood fibers were prepared at 8%, 15% and 20% consistency (fiber weight to stock weight) .
• the surfactant was a mixture of the following with about 180 Kg . of water added.
• Mearlcel 3005 an aqueous blend of anionic . 19 Kg. surfactants and ethylene glycol monobutyl ether, The Mearl Corp. )
• the foam was formed in a Micar Processor apparatus and had an air content of about 87% by volume.
• Each of the stocks were further subdivided into th ree parts .
• One part was not subject to fu rther treatment, a second part included a starch binder and a third part included starch binder and clay pigment.
• Each of the slu rry samples were deposited onto a wi re cloth and formed into webs in a web forming section similar to that disclosed in the drawings .
• the webs were subjected to the following conventional paper tests to determine their usefulness as conventional products:
• Opacity This test measures the ability of paper to prevent light from passing through it. For a plain uncoated paper a unitless value of 70 is acceptable. Coated papers for printing, would be expected to have opacity values in the nineties .
• Gurley Density This porosity test measures in seconds the amount of time it takes for 100 cc. of air to pass th rough 6.45 cm . 2 of paper. Without any addition of filler or coating, a plain uncoated paper should exhibit a minimum porosity of about 1 or 2 sees . A minimum for coated printing base papers should be about 20 sees .
• Mullen This test measures in Kg. /cm. 2 the amount of pressure paper will withstand before it ruptures . A range of about 0.35 Kg . /cm. 2 for uncoated printing paper to 9.8 Kg. /cm. 2 for coated paper is normal .
• Scott Bond This test measures in cm. - Kg . , the amount of work necessary to split a sheet in the Z-direction .
• Coated papers may be as high as 460-580.
• Table I displays the results of testing Examples 1 -3, Table I I for Examples 4-6, and Table I I I for Examples 7-9.
• OMPI e.g. , clay
• Example 1 The conventional sample and Example 1 were further subjected to a calendering operation by passing the samples twice through a nip created by steel rolls at a nip pressure of 36 Kg . per linear cm. On visual inspection of the samples after calendering, it was apparent that the formation of the foamed fiber sheet was very good and the surface appearance was good, and both were as good as or better than for the conventional sample. Table II
• EXAMPLE 10 Dry lap pulp having a mixture of about 60% softwood fibers and 40% hardwood fibers was cut up and added to a Micar processor apparatus with simultaneous addition of water to provide a fiber consistency of about 25%.
• a Micar processor apparatus with simultaneous addition of water to provide a fiber consistency of about 25%.
• O PI sulfate (a surfactant manufactured by du Pont as Duponal C) was metered in as a 0.5% solution in water and mixed with the pulp. Generation of foam in this mixture was almost immediate as it progressed to the first mixing/processing disc. The processor dispersed the fibers in the foam and also broke up lumps .
• the mixture was passed through a second treating unit consisting of a 15 cm. diameter tube about 1 .3 m. long with a motor driven shaft extending through the center and having agitating fingers to further insure dispersion of the fibers in the foam and to increase the foam ratio.
• the foamed slurry had a fiber consistency of about 15% by weight and an air content of about 93% by volume.
• the foamed fiber slurry was delivered to a downwardly converging nip formed by two air permeable wires traveling at a speed of about 16 m./min . in an arrangement similar to the web formation section described in the drawings, except that the foam breakers were provided by non rotating slotted pipes .
• the slot for one pipe was 6 mm. high and the slot for the other was 3 mm. high . Both extended across the full width of the web to be formed.
• the vacuum applied was about 100 mm. of Hg. for one foam breaker and about 40 mm. of Hg. for the other foam breaker.
• the gap between the wires at the nip was 0.38 mm.
• the angle of the center point of the 6 mm. vacuum slot at the higher vacuum foam breaker was about 5° upwards from the nip.
• the angle of the center point of the 3 mm. wide vacuum slot at the other foam breaker was about 0.75° upwards from the nip.
• the web followed one wire and was transferred to a felt using a suction pick-up device. It was then passed through a double felted first press, transferred to the second felt by suction pick-up, removed from the second felt and then dried.
• the web prepared by this process had a basis weight of 118 g./m. 2 and when examined had better appearance and feel than most sheets formed by conventional means . I n general the formation was excellent, the look-through appearance having much more uniform translucency than conventional papers .
• the formation approached the parchment-like softness of good cellulose drafting paper normally made by heavily refining the fibers .
• the floe size distribution was greatly reduced compared to conventionally made paper, from a characteristic range of 1 to 20 mm. down to a range of about 1 to 3 mm.
• the floe clouds larger than 10 mm. or so common in many conventional papers were completely absent.
• the densest floes in the paper of the example had less than half the density of floes in conventional paper.
• EXAMPLE 11 A foam was created by mixing in a Waring blender under high speed 20 ml . of water and 45 cc. of sodium lauryl sulfate surfactant (2% solution in water) . With continuous high speed mixing, 47.6 g . of a 25% consistency slurry of papermaking fibers and 14 g . of titanium dioxide pigment were added, A uniform foam fiber dispersion of about 26% solids was obtained.
• the mixtu re was applied to a forming apparatus having a web formation section similar to that used for Example 10, except that one of the wires was covered with an impermeable plastic film to block any drainage or vacuum effects from that side. This film was also intentionally wrin kled slightly .
• the vacuum applied by the uncovered foam breaker was about 125 mm . of Hg . , and it collapsed the foam and formed a web.
• the paper sheets thus formed exhibited conventional wire pattern on the side formed on the unblocked wire, but the side formed against the film covered wire was smooth overall with ridges replicating the wrin kles of the plastic film.
• This example illustrates the ability to form a two-sided sheet with significant variations in the surface effects .

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• 1984
  • 1984-02-07 CA CA000446944A patent/CA1230995A/fr not_active Expired
  • 1984-02-08 EP EP19840901040 patent/EP0136329A4/fr not_active Withdrawn
  • 1984-02-08 JP JP50102384A patent/JPS60500628A/ja active Pending
  • 1984-02-08 WO PCT/US1984/000168 patent/WO1984003112A1/fr not_active Application Discontinuation
  • 1984-10-04 FI FI843904A patent/FI843904L/fi not_active Application Discontinuation

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