CA2066812A1 - Method and apparatus for the production of multiply cellulosic board and product obtained thereby - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

METHOD AND APPARATUS FOR THE PRODUCTION
OF MULTIPLY CELLULOSIC BOARD
AND PRODUCT OBTAINED THEREBY

A method and apparatus for the production of multiply cellulosic fiber board wherein first and second streams of cellulo5ic pulp are deposited on a wire, partly dewatered, mechanically integrated and conditioned to form a bilayered web, followed by thc deposition of a third stream of cellulosic pulp onto the top of the bilaycred web and further dewatering in a flow direction opposite the di.rec-tion of flow of the dewatering of the bilayered web to hydraulically integrate and form a trilayered web. Prefera-bly, the total quantity of fibers contAincd in the seconc1 (inner) layer is greater than the quantity of fibers in either the first or third outer layer, thereby developing a board product that exhibits an appareIlt bulk at least equal to the apparent bulk of a single layer board formed from the same quantity of fibers, but containing between about 9% and 11% fewer fibers than such single layered board. The novel product exhibits substantially improved physical and other properties,especially stiffness. Coating of the board with a polymer and formation of the coated board into liquid containers is disclosed.

U.S. APPLICATION
SERIAL NO. 071690,518

Description

~ 20~6~12 MF.T~IOD 7~ND ~PPI~ TU_FOR Tlll~ L'l~ODUCTION
OF MULTIPLY C~LT ULOSIC E30ARD
7~NI) I'RODUCT 013T7~INED 'I'IIE:REBY

This invention relates to multiply cellulosic, e.g. paper, board which is particularly suitable for use in the manufacture of containers for liquid food products, and more particularly for disposable mil]c cartons, and to meth-ods and appara~us ~or the manuFacture of such board.
Disposable containers for liquid food productshave long been manufactured from celluIosic board that is formed using conventional fourdrinier papcrmakin~ machines.
Such board is most usually sinqle ply ancl of a basls wcight in excess of about 150 lb./~000 ft2. 01 recent there has been eonsiderable e~fort exerted toward produciny multip]y board for use ln such disposab]e containers in ~n cf~ort to reduce the overall eost of the board, while maintaining those board properties that arc essential for its successful use in disposabIe liquid food containers. Systems such as those shown in U.S. Pakent Nos. 3,6~1,193, 3,~91,501, 4,00~,968 and ~,~72,2~ have been su~ested for use in makinq multiply paper board. In these patents the apparatus and methods disclosed for the manufacture of multiply board, i.e. three or more plies, require complica~ed and expensive equipment and in most, there is duplication of equipment for developing each of the plies. It is desired therefore that there be a method for producing multiply board of at least three~layers whieh requires less extensive, hence less expensive, modification of existin~ papermaking equipment and which provides a multiply board havin~J cl lessor quantity of fibers in the board, but which provides properties equal or superior to sin~lc ply boarcl.

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206~812 In accordance witll the metllod of tlle present invention, ~irst and second streams of cellulosic pulps are deposited substantial].y si.mu].taneously onto a ~orwardly moving foraminous papermaking fabric, e.g. a fourdrinier wire, to develop Ei.rst and second layers of a mul-tiply web.
These overlaid layers are dewatered to a consistency of between about 1.~% and about 3.5%, by weight, and thereupon are mechanically int~grat:ed ~t their interface and thcir formatlon enhanced. Such mechanical intec3ration ~urther conditions the upper sur~ace oE the second layer ~or the receipt of a third layer of pulp. Such thircl layer is developed by depos.iting a stream of ccllulosic pulp onto the upper surface o~ the second layer at a location just down-stream of the wet line o~ the bilayered web on the wire.
].5 Substantlally immecliately followiny tllC dePOSitiOn Of SUCh third layer, the three layers are captured bctwccn the initial forming fabric and a further form~minous ~orming fabric that is ovcrlaid on-to the top surfacc of tlle third .
layer. Thereafter, the multilayered web is dewatered up-20~ wardly through the several layers of the web to hydraul.ical-ly integrate the second and thlrd layers alld enhancc the integration of the first and second layers. Thereafter, the ~:
web is further dewatered, dried and collected. In a pre-erred embodiment, the web is dried and fed through a size press prior to final drying to develop a surface size on opposite surfaces of the web, and then calendered. Still ~: .
further, in the preferred embodi.ment, the composition of the second layer o~ fibers includes less expensive fibrous matter, such as a larger percentage of hardwood fibers, and the total quant.ity of fibers dcposited as the secolld layer preferably is between ahout 0% and about 300% greater than 20g6812 -the quantity oE tl~e ~ibers IcE)osited in ~ormincJ eithcr tl~e ~irst or third out~r layers. Ill tili~ m~nncr, thc apparell~
bulk of th~ secorld (inner) ]aycr prc~er~hly is ~Jrc~t~r tll.ln th~t of either o~ t~le first or tl~.ird layel.~, but t~ ov~r~ll S caliper of the board product is m~intained at about th~ s~me caliper as single ply boar-l made ~rom t11e same total wcight of fi~ers. The multiply bo.lrd of the pres~nt illVCl-tiOn exhibits pertinent: propertie~ t:h~t ar e equal to or s~lperior to the same properties of sincJle ply ~o~rd. Especially, the present board exhibits the modu1~s, stiffness, bulqe resist-ancc, and other propcrties oi a s.inr~lc pl.y boarcl, ~nd docs SO Witll t~le pres~rl~ board collt~.inill~ betwcen about ~ and 11% ]css ~ih~r COIltCIlt, I:)y wei-~lt.
~urther objcctivos alld advallt~cJes, ~s well as understandinc3 o~ the prcsent invelltionr will bc proviclccl from the followin~ description, incl~l-lill(J thc fi~ures, i Wll i. C ll:
Fic~ure 1 is a sc~lem~lt.ic r~prcscntati.oll o~ onc embocliment o~ an apparatus ~or us~ in carryi:llc~ out the method oE the pres~nt invention;
Fic~ure 2 :is a schcmc~tic r~r?r~s~nt~ion oE ~ m~lti-ply board in accord~ncc with thc prcsent inv~ntion a~
:
d~pictin~3 variDus ~eatures tl~er~of, and Figure 3 shows a turned-up corner portion of a web roduced in accordance with an embodiment o~ the ~resent invention.
With speci~ic re~crerlce to l~i-Jure 1, ther~ is depicted a preferred embodi.ment oE apr?aratus ~or carryillq out the met}~od of the present invention anc~ compri.ses a ; continuous lool1 ~ourdrinicr wire lO which is traincd ~bout a breast roll 12, a couch ro]l 1~, nnd one or more idler rolls 16 and 1~. 'rhe wire inclucles an upper run 20 whicll i5 supported as by c~ plurality of suction devices 22 ~ncl/or foi]s 2~, all as are well knowll in tlle art. The wire is moved in a ~orwarcl clir~ction, by dr;ve meall3 not SllC)WIl, as ~l 2~812 indicated by arrow 26. ~djacent the breast roll 12, there is provided a headbox 2~ which in the preferxed embodiment comprises two flow channels 30 and 32, cach of whicll is in fluid communication with its respective source of cellulosic pulp (not shown). Pulp streams from the respective channels 30 and 32 are maintained as separate streams until substan-tially the moment of their discharge from a dual slice 34.
These two streams are deposited substantlally simultaneously as separate layers of pulp onto the wire 12 as it i5 moving forwardly to form a bilayered web 35 on the wire, such web comprising first and second layers, 31 and 33, respectively.
In FIGU~E l, the thickness of the layers Oll the wirc 10 arc exaggerated for purposes of illustra-tion. Onc suitable headbox is a Strataflo unit manufactured by Beloit Corpora-tion of Beloit, Wisconsin. As the bilayered web on the wireis move forwardly, it is partially dewatered as by the suction devices 22 arld the foils 2~. ~t that point aloncJ
the length o~ the upper run 20 o~ the wire 12 at whic~l tlle consistency of the fibers in thc web has reached a value of between about 1.~% and about 3.5%, the bilayered weh is contacted by a dandy roll 36. Such roll 36 prefcrably comprises an open mesh formed into a cylindrical cJeometry and positioned with its len-Jtll transverscly of the clirection o~ forward movement of the web. The roll 36 is preferably driven at a tan~ential speed that is subs-tantially cquiva-lent to the forward lineal speed of the wire, e.~., 100~ ~
5%. ~urther, the roll 36 i5 mounted so that it can be forced into pressurized contact with the upper surface of the web 35, such that between about 2 to 4 inches of the circumfer-ential dimension of the roll is in contact with the wcb asthe ~eb moves forwardly. This 2 to ~ inch "footprint" of ..... ~

2~6812 the roll 36 extends across the full width of the web ~5.
The open mesh character Oe tl~e roll 36 serves to mechanical-ly enga~e the fibers of the web and enhance the integration Oe the first and sccond layers Oe the web at their int~rface as well as enhancing the overall formation of the web.
Further, the open mesh smooths and conditions the top sur-face of the second layer 33 for receivin~ a further layer of pulp thereon. One suitable dandy roll is formed of phospllor bronæe wire having a mesh count of 15 x 13 cm, an opell area of about 39.5%, a warp yarn diameter Oe 0.2G mm, and a weft yarn diameter of 0.25 mm.
Followin~ inte~ration of the first and second layers of the web, and at a location substant1ally immedi-ately downstream of the wet line of the bilayered web on the wîre, a ~urther, i.e. third, layer oL pulp is deposited onto the upper surface Oe tlle web as ~rom a second~ry he~d-box ~3. This headbox may be of conventional sin~le-slice desi~n. The pulp deposited onto the we~ erOm the secondary headbox preferably is subst.antially equivalcnt in composi-tion and quantity as the pulp deposited onto the wire from ~; the channel 32 of the headbox 2~, thereby causing the first and third layers of the web to be substantially identical in a preferred embodiment. Substantially immediately after the third layer of pulp has been deposited onto the web 35, the trilayered web 1s captured between a further foraminouspapermaking fabric ~0 which is trainecl about a plurality of rolls 42, 4~, 46 and 48. In a preferred embodiment, such fabric 40 is a part of a device known in the art as a Bel Bond unit, manufactured by 13eloit Corporation of Beloit, Wisconsin. The Bel Bond unit includes one or more suction devices 50 disposed on that sicle of the wire ~0 oppositc the web 35 and adapted to withclraw water erOm the web in an . ,.. ,_,~, .
.. . . . . . .

2~6~12 ~pward direction. lrhis action serves to hyd~a~llically inte~rate the second ancl third layers of the web, as well as to further dewater the web.
The partially dewatered web is withdrawn from tlle wire 12 at the couch roll 1~ and directed tl-rou~ll a wct press 62. In the depicted wet press section G2, the web 35 is first cc)ntacted by a suction pic~c up roll 66 about wilicl there is trained a fir~t feit 6~. Tlle web is next capturcd between the first felt 6~ and a second felt 70 and clirccted throuc3h a first press nip 72 betwcen a groovcd roll 7~ and a suction roll 76. Thereafter, the web, while still on the first felt 6~ and trained about the suction roll 76, is passed through a second press nip 78 developed between a suction roll 76 and a hard-sur~aced roll ~0. ~ollowinc3 the s~cond press nip 7a, the wcb is arJaill c~L>tured b~twccn th~
irst felt 63 and a third felt ~2 and conveyecl throur3ll a third press nip 8~ established between a further c3rooved roll ~6 and a smooth roll ~. Pressurc loads in thc press nlps of 200, 300, and 600 p.l.i., respectively, have been found suitable. Other wet press desic3ns known in tlle art would also suffice.
The web exiting tlle wet press section is conveyed through a dryer section 6~ within which the web is passed~
over a series of heated rolls 90 and dried. After the lnitial drying, a water solution or slurry of sizinc3 material may be depositecl on the surEace of the shect in a size press 60. Surrace siæing further strengthens the shect sur~ace layer and can include materials that promote a hydrophobic nature of the sheet surfacc. In thc depicted size press 60, the web 35 is ~ccl ovcr a roll G1, tllc through the nip 71 betwcen a pair oE rolls 63 and 65.

. 2~66gl2 Siziny solution is fe~ into ~le nip 71 from one or ~Ot~l o~
sources 67 and 69 of sizincJ solution, dcpendin~ upon whether one or hoth surfaces oE the weh are to receive sizin~. From thc nip 71, the size~ wch i5 fed thro-lcJ}l a second drycr 6~' which includes heated rolls ~0'. The dried web may be passed through one or more nips (calendered) to i1nprove surPace smoothness. The dry web is collected in a roll 92.
/ ~ turned-up corner portion 9~ of a web 35 produced in accordance witll the presellt method is depic~ed in Fi~ure 3. The depicted w~b compriscs a first (~ottom) layer 31, a second ~inner) layer 32 and a third ~top) layer 33. In the depicted web portion, the several layers are delineated for purposes of illustration, but it is to be rccoqni7.ed that the inter~aces between laycrs are not so pronounced in the actual web.
Thus, ~hc pre~errccl cmbo~liment o the me~ho(l of the present invention comprises the stcps of preparill~
first, second, ancl thi].d .slurries oF ce].lulosic fibers i.n an aqueous medium, dcpositin~ a stream oE the first slurry onto a forwardly movin~ papermaking Pabric at a irst veloc-ity sufficient to form a Pirst layer of fibers on said fabric, substantially simultaneously depositing a stream of the second slurry onto the upper surface of the first layer of fibers at a velocity sufficicnt to dcposit onto said 25 first laycr betwc¢n about 0% and 300% ~reater quantity oP
fibers from the second slurry than the quantity of fibers deposited from the first slurry, commencing dewaterin~ of the bilayered web and when it has achieved a consistency of between about l.B% and about 3.5~, mechanically integrating the first and socond layers at their intcrf~ce, depositin[J a stream o~ the third slurry onto the upper surface of the inteyrated bilayered weh at a location .immediately down-U

2 ~ 2 stream of the wet line of the web on the formin~ fabric,substantially .immediately after depos.ition of the third layer, capturing -the ~eb be~ween the Eirst form.in~ fabric and a further foraminous fabric, and withdrawing water from the trilayered web throu~h the further fabric to hydrauli~
cally integrate the second and third layers of the web. ~s desired a surface size may be deposited on the opposite flat surfaces of the web, and the web therea~ter dried and/or calendered.
The pulp slurries employed in the present inven-tion are selected to develop ~irst and tllird outer layers of the present board that c~pture therebetwcen a secolld, i.e.
inner, layer which exhib.its an apparent bullc that is sub-stantially qreater than the apparent bulk of the outer layers. In th.is mann~r, the overall caliper of the boar~l is developed with less fibrous content of the board tllan for sinyle ply board forme~ from li.ke ~ibers. In the preferred embod.iment, the pulp used for thc firs~ ancl third layers is of the same composition, namely about 75% sol`twood ancl 25%
hc~rdwood flbers, at a consistency o~ about 0.8~ by wei~ht, based on oven dried f1bers and a C.S.F. of about 500. The :~ preferred composition of the inner layer is about ~.5% soft-; wood and 75% hardwood fibers, at a consistency of about 0.~%
: :
~ : : and a C.S.~. of about G10. The greater percentage o~ soft-~:25 wood fibers in the pulp for the outer layers provides ~or the development oE stren~Jth in these layers, ~ood surface .~smoothness of the board product, and other properties. ~s noted above, the quanti.ty of fibers for the inner layer deposited on the wire is between about 0% and 300% greater than the quantity of fihers c~eposited in the formation of each of the outer layers. l3y this means, the inner layer `:~

2~66~12 develops an apparent hulki.ness which aids in il~partinc3 to the boarcl product a final caliper that i.5 equiv~lent to the caliper o a si.ngle ply boarcl, but whose total fiber con-tent is about 9% to 1.1% less than the fiber content of A
single ply board. In this manner, the present invention provides the means for produciny more board product with less fibers, and doincJ so without loss of -the desired properties of the board. ~s the relative volume o the pulp for forming the inner ply varies bclow about 0~ or above about 300% there is a nc~ti.ceable dccrease in the desircd properties of the board.
~ key proper-ty for judgincJ strenc3th of three-ply versus si.n~le ply board is stiffness. In tlle present dis-closure, stiffness refers to the ~eometric mean value of 1.5 stiffness (square root of thc product of machine dircction [M. D. ] and cross direction [C.D.] sti~ne~;s). ~ti~fness .is related to basis weight by the equation:

stiffness = stifness constant x ~q. l (caliper)l 6 x : ~ 20 basi.s weicJht or : stiffness = (stiffness constant x ~q. 2 : : basis weight2 6)/apparent density ::

;~:25 Yield improvement ~is calculated also using these equations, ;i.e. percent reduction in basis weight that ~ives equal stiffness. The followincJ Table I shows the avera~e apparent density, average stiffncss constant anc~ calculated yield improvement of various boards madc in accordance with the pr~sent method:

. 2a66s12 rr~r3r,li` I

~vera~e ~vera~e Improved 13Oard ~pparent Stiffness Yield Samples Density constant %
~s-made single ply 9.63 0.00~8 1~.9 three ply 9.~7 0.0065 Surface sized/
uncalendered single plylO.lO 0.0066 9.0 three ply 9.~ O.OOnl Surface sized/
calendered sinyle plyll.23 0.0061 ~.7 three ply lO.93 0.007~

Vario~s of the softwoods and/or hardwoods may be employed ln the pulps employed in the present inve1ltion.
International Pine softwood fibers and ~0-2 llardwood pulps hclve been found most suitable, are re-1dily available and simi].ar to pulps produced in khe souther1l United Stat~s. In the formation of the pulps, there may be added thereto the usual wet-end chcmicals to improve dry strcn~th, improve wet stren~th, improve retentio1l, alter p11, etc., such as Kymene, Acco-stren~th ~6, c~ustic for pll adjustment, etc., as de-sired. Further, tests have shown that the additio1l of lO~
or more of brolce to the pulp has no detectable deleterious effect upon the desired properties of the board product.
~ Whereas the consistency of the pulp may be the same for cach i~ layer, pre~erably from about 0.5 to about 0.~, the consist-ency of pu1p for each layer may be selected to be of a specific value for that layer. The average consistency of the pulps used for formation of the first and second layers (total amount of so1ids/total flow from both the channels 30 and 32 of the headbox 2~) may ran~Je from between about 0.6~
to about l.1%, dependinq upon the desired basis wei~ht of ' . 2~6812 the board produet. The pulp consistellcy employed to obtain a particular basis weight of product is also a function of the wire speed. Table II presents the data from a series of ~sts ~mployiny the present method to produce bo~rd ~roduct of various basis weights.

T~BLE II

~asisWire Speed Primary Primary Wei~ht (fpm) ~low 1Consistency2 (lb~3000 (l/min.) (%~
t~) _ 160 129~ ~9~9 .~o ~ o .92 200 1030 ~9~9 .76 220 93~ ~7~ 3 250 ~o~ ~765 .75 2~2 725 ~552 .~
2~2 705 ~91 .~7 2~2 77~ ~30~ .9~
282 B53 ~552 1.02 1 qlotal flow from lleadbox 2~
:
2 Total amount of solids/primary flow Board product use~ul in the manufacture of con-tainers for liquid food products preferably contain a starch size on the opposite outer surfaces o the board. ~ccord-incJly, it is preferred in the present method to pass theformed web throu~h a size press containin3 a conventional starch size to thereby deposit between about 1 and nbout 3 lb (based on 3000 ft2) o sizinc~ onto each of the opposite i, ~:~ :
surfaces of the web. In a typical mill run, about 35 lb of ~starch per ton of fibers, produces a suitable sizing of the ~ web. Other sizes, combinations of sizes, and/or quantities ;~ of slzes may be employed to obtain specifie results.

;~ Tlle sized web may be cal~ndered as desirecl.
i : :
. .
, , ..

2~68~2 EX~MPLF I
Multiple test rulls us:in~ the present method werc made to produce both sin~le ply and multiply boarcl suitable for use in disposable containers for liquid food products.
In the several runs, the composition of the pulps employed for the several layers of the multiply product and the wire speed were selected to procluce different wei~hts of board.
All runs were made Oll apparatus as shown in the Fi~ures and described herein, except that the dandy roll ancl secondary headbox were eliminated when making the single ply board.
The pulp employed in thc sin~le ply board was a 50/50 pine to hardwood blend at 610 C.S.F. and 0.~% consistency. Other variables were sct as noted in the tables presented hcrein-after and in Table II above.

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206f~2 Paper board manufactured in accordance with the present invention was converted to quart-size and half-qallon-size milk cartons and tcsted. Prior to its conver-sion, boards havinc3 a basls wei~ht oE l~0, 200 and 220 lb/3000 ft2 were passed through an extruder and coated wi-th about 16.7 lb of matte po]yetllylcne on the outer sur~ace of the third layer of the boards and about 10.7 lb. of gloss polyethylene on the outer surEace oE the first (wire) layer of the boards. Boards havinq a basis weicJht oE 250 and 2~0 lb/3000 ft2 were coated with 18.9 lb. of matte PE on their third layer surfaces and 11.1 lb of ~loss ~E on their wire sides. The results of teStincJ of converted milk cartons are given in Tables VII and VIII. These clata show that the cartons made from the multiply board of the present inven-lS tion compared favorably to li~e cartons macle from sinc~le ply board.

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Claims (9)

1. Claim 1. A method for the manufacture of a board suitable for use in the fabrication of a container for liquid food products comprising the steps of:
   forming first and second slurries of cellulosic fibers in a flowable medium each slurry having a consistency of between about 0.6% and about 1.12%, by weight, directing said first slurry onto a foraminous forwardly moving papermaking forming fabric to develop a first layer of fibers on said fabric, substantially simultaneously directing said second slurry onto the exposed surface of said first layer of fibers on said fabric to develop a second layer of fibers on said first layer of fibers on said fabric, the quantity of fibers deposited from said second slurry onto said fabric being between about 0% and about 300% greater than the quantity of fibers deposited on said fabric from said first slurry, partially dewatering said first and second layers on said fabric to a consistency of between about 1.8% and about 3.5% by weight to form a bilayered web on said fabric, and thereupon mechanically integrating said first and second layers of said bilayered web and conditioning the upper surface of said second layer for receiving a third layer of fibers, substantially immediately downstream of the wet line of said bilayered web on said fabric, directing a further slurry of fibers onto the exposed surface of said second layer to develop a third layer of fibers on said fabric to form a trilayered web on said fabric, substantially immediately downstream of the depo-sition of said further slurry of fibers, capturing said trilayered web on said fabric between said fabric and a further foraminous fabric, and withdrawing liquid through said further fabric to partially dry said web and hydraulically integrate said second and third layers at their interface.
2. Claim 2. The method of Claim 1 wherein said first and further slurries of fibers are substantially identical in composition.
3. Claim 3. The method of Claim 1 and including the step of applying a surface size to said web.
4. Claim 4. The method of Claim 1 and including the further step of applying a coating of polymeric material to the exposed surfaces of said web.
5. Claim 5 The method of Claim 1 wherein the board product has a stiffness ratio of at least about 1.80 and a mean stiffness of at least about 110.
6. Claim 6. Apparatus for the manufacture of a cellulosic board suitable for use in the fabrication of containers for liquid food products comprising a first foraminous forming fabric, means mounting said forming fabric and moving the same in a forward direction and defining a run thereof, a source of a first slurry of cellulosic fibers disposed in a flowable medium, means depositing a stream of said first slurry of fibers onto said run of said fabric to develop a first layer of fibers on said fabric, a source of a sccond slurry of cellulosic ibers disposed in a flowable medium, means depositing a stream of said second slurry of fibers onto said first layer of fibers substantially simul-taneously with the deposition of said first layer of fibers, and developing a second layer of fibers on said first layer of fibers, and including means controlling the quantity of said second slurry deposited onto said first layer such that there is deposited onto said first layer a quantity of fibers of between about 0% and about 300% greater than the quantity of fibers deposited by said first slurry onto said fabric, means for withdrawing liquid from said layer of fibers on said fabric through said forming fabric to form said first slurry of fibers into a web on said forming fabric, whereby there is developed a bilayered web on said fabric, means for mechanically integrating said first and second layers of fibers on said fabric and conditioning said second layer of fibers for receiving a third layer of fibers thereon, said means being located downstream of said means for depositing said fibers onto said fabric a distance sufficient to permit said liquid withdrawal to proceed to the extent that the combined consistency of said first and second layers of fibers is between about 1.8% and 3.5% by weight, a source of a third slurry of cellulosic fibers, means depositing a stream of said third slurry onto the exposed surface of said second layer of fibers on said fabric to develop a third layer of fibers on said first fabric, said means being located substantially immediately downstream of the wet line of the bilayered web on said first fabric, further foraminous fabric means including a run disposed in substantially parallel relationship to said first fabric and in contact with the exposed surface of said third layer of fibers on said first fabric, means disposed on that side of said further fabric opposite said first fabric for withdrawing liquid from said fibrous layers on said first fabric and hydraulically inte-grating said second and third layers of fibers to establish a trilayered web on said first fabric.
7. Claim 7. A paper board useful in the fabrication of containers for liquid food products manufactured in accordance with the method of any of Claims 1 through 6 .
8. Claim 8. A container for liquid food products comprising a cellulosic fiber board including at least three layers integrally bonded one to another to the extent that their interbond strength equals or exceeds the internal bond strength of either of the individual layers of the board and the board exhibits a caliper and overall strength equal to or exceeding the caliper and overall strength of a single ply board containing between 9% and 11% more fibrous content than said three-layered board.
9. Claim 9. A planar sheet of base stock for use in production of a disposable container for liquid food products and particularly for milk and milk-based products, comprising a first layer of cellulosic fibers formed by the deposition of a slurry of said fibers in a flowable medium at a consistency of between about 0.6% and about 1.12% onto a papermaking forming fabric, said fibers comprising between about 70% and 80% hardwood fibers and between about 20% and 30% softwood fibers, by weight.
   a second layer of cellulosic fibers formed by the substantially simultaneous deposition of a slurry of said fibers in a flowable medium at a consistency of between about 0.6% and 1.12% onto said first layer of fibers on said forming fabric, said fibers comprising between about 20% and 30% hardwood fibers and between about 70% and 80% softwood fibers, by weight, said first and second layer of fibers being mechanically integrated at least at their layer inter-face after their respective fiber consistencies have been increased to between about 2% and 3.5%, by weight, a third layer of cellulosic fibers of substantial identity as the fibers of said first layer formed by the deposition of a slurry of said fibers onto said second layer after said combined first and second layers have passed the wet line of said papermaking forming fabric, said third layer of fibers being hydraulically integrated with said fibers of said second layer at their layer interface, said layers thereafter being further dewatered and dried, said layers being surface sized with a coating pickup of between about 2.3 to about 3.9 lb./3000 ft2, and thereafter calendered, and a layer of polymeric material bonded to the oppo-site flat surfaces of said sheet, wherein said sheet exhibits a basis weight between about 160 and about 210 lb./3000 ft2, a caliper of between about 0.014 and about 0.025 inch, a stiffness ratio of not less than about 1.80, a mean stiffness of at least about 110,- an interlayer bonding strength that exceeds the inter-nal bonding strength of said layers, a Sheffield porosity of between about 100 and about 250 units/in2, a tensile strength of between about 55 and about 100 lb/inch width, and an MIT fold of between about 350 and about 1250 double folds.