US2599092A - Multiple layer paper containing pigmented pulp and method of making - Google Patents

Description

Patented June 3, 1952 vMULTIPLE LAYER PAPER CONTAINING PIG- MENTED PULP AND METHOD OF MAKING William Lutton Craig, Westport, Conn., assignor to R. T. Vanderbilt Company, Inc., New York, N. Y., a corporation of New York No Drawing. Application April 30, 1946,

Serial No. 666,199

Claims. (01. 92-3) paper or board is commonly carried out in cylinder machines having a number of cylinder vats and cylinder molds, each separately supplied with the desired stock, and each forming a layer, with the layers combined to form the final sheet of multi-ply paper or board, the number of lay-' ers depending upon the number of cylinder vats and molds in use. The outside layers of the composite or multi-ply paper or board are formed from stock from the first and last vats and molds and are called liners; and the intermediate layers are called fillers, and are commonly made of less expensive stock.

In such cylinder machines the stocks are supplied from the stuff chests to regulating boxes and from the regulating boxes to the cylinder vats, each containing a cylinder having a cylindrical screen on which a layer of partially formed Wet paper is produced, which is transferred to a felt. The successive layers of partially formed Wet paper from the cylinder molds form a composite or multi-ply series of layers, which are carried with the felt between rolls to squeeze out Water and to compact the layers, after which the sheet passes through press rolls to drying cylinders and to calenders where a size may be applied. This method of forming laminated or multi-layer paper or board, and one form of apparatus therefor, are described, for example, in vol. V of The Manufacture of Pulp and Paper (2nd Edn.) section 1, pages 251-264 (McGraw- Hill Book Company). Such composite multilayer paper or board can similarly be produced in multi-Fourdrinier and other machines.

The improved process of the present invention can advantageously be carried out in such cylinder machines to give an improved coated board or sheet without subsequent coating of the finished sheet or board in a coating machine.

According to the present invention, the composite sheet or board is provided on one side or on both sides with a surface coating layer or liner in the form of a specially prepared pigment-fiber layer or fibrous pigment of highly pigmented, modified cellulosic fibers, or a layer containing such highly pigmented modified cellulosic fibers admixed with untreated fibers, which layer or liner, when sized,'gives an improved coated board or sheet on a paper-making machine and without subsequent machine coating of the dried board or sheet.

The present invention is based upon the discovery that a portion of the cellulosic fiber stock (chemical, semi-chemical, or mechanical pulp) used in makingpaper, including cellulose fibers (chemical pulp) such as kraft, sulfite, soda pulp, etc, and particularly groundwood (ligno-cellulose) fibers (mechanical pulp) can be treated in the form of a suspension in water or pulp with calcium chloride in the form of a strong solution to modify the cellulose or the ligno-cellulose and that the so modified cellulosic fibers can then be treated with a solution of a soluble salt cellulose fibers such as chemical p'ulp (white pulp, sulfate pulp, soda pulp, etc.) as well as semi-chemical pulp and groundwood fibers (mechanical pulp), as well as mixtures of such fibers or pulps.

The cellulosic fibers are advantageously treated in the form of a relatively concentrated pulp or stock. Thus, stocks containing up to 4% or 5% or more of fibers (dry basis) are advantageously treated, and the treatment is also applicable to much more concentrated'stocks at a stock consistency as high as 13% or higher, where higher concentrations are available.

The stock treated may be bleached or unbleached stock and may be stock which has been beaten in abeater, particularly in the case of long fiber chemical pulp or unbeaten disintegrated stock.

The treatment of the fibers in suspension in strong calcium chloride solution of sufficient strength to modify the cellulose or ligno-cellu- I about a'profound change in the nature and apcalcium chloride combines witht hecellul'ose to form complex products, or it may.;be the-,caleiu n chloride exerts a modifying or hydrating efiect upon the cellulose molecules. Whatever the i; 1-

theoretical explanation of the action of thecalcium chloride on the cellulose maybe; it brings pearance of the cellulose fibers. Similarly,; when H semi-chemical pulp or groundwood fibers are similarly, treated with calcium chloridethe action. is tobring abouta profound changein the nature ds-appearancee -m sh s- After thi' .treatmntWith calcium chloride the stoclg-is treated with .a' solution of a. soluble salt. such assodiumcarbonatel(sodaiash) I or' sodium bor =1, t e e.lv g'., m 'th'elform l of. 'a 10%..sol1itioz1' Lin suflicient amount to react with the calcium..chloride-and precipitate an insoluble pigment, e; g.,. calcium carbonate; .or calcium, .bo'rate in and on thelflinodifiedjfibe'r'ai. The amount ofv added solu ble saltf is,.equaltoI-orsomewhat more; or less. than'tliearnbu whi'ch suificient to react with" r all lot =thelcalcium chloride used. iWitli cellulose. fibers 'thegfamountl; of. soluble salt isl'adv'antageouslyflsli'ghtly, excess .of that required to react wit1 .a11',. z t e calcium chloride; but with grouri godlfibers' .care [should ingeneral ibe' takenQto.ayoidianyexcessof the alkali salt which 7 would darken or otherwise 'afiect the stock; and. 40 with suc toicki 'thecamum chloride and. sodium ;7 carbonat' are; us'edflin .exajct' reacting proportions gsli'gh tl'xcess. .of 'calciuins'chl'oride to I avoid anyvexcessflgfisodium carbonate;

'i'hejqre ii ins'itr a ed nd ipi mi nted. sioc i 4 highly: hydrated and; has 1ittle str'er'igth,";or rela'- tivel'y', l m'uch 1essj.strength Y than the; untr d"? stock; but itfcarriesfthe precipitated. pigment; dist b ited. Within .thih rat d' fi slwfii e p merit is 'aIsQpreci'pitated' on the hydratedifib'ers dh t e-W t i .whic the fibers iare sus-f ,i--. .4'.. T V e 'spe i lly r atede d d fid1a dp s1 nt d stoc s hish1y;pie. en edI ..i.An'ian'1.ounit of piem ntq lb eemhine with thj' O k which I is c demb yii 'exq s's o "the wei ht of .t g stock treated (dry basis). Thus cellulose fibers" can bem d fied and .pi m n edt eivea' p p t on Q pi nent" isto fd y ba i uri'tgo aroundtwotopne 1r more; GI'OlllfldWOOd stock can be pigmentedii'tdia greater extent, up to." aroundf i parts offpigment 'to 1 of fiber (dry;

basis) or even higher.;

Theresulting stockfcontainingthe modified and p m ted fiber w thit e piement prec pie tated in and on jan'd" around the fibers, is used in the ,improyed process; of thepresentjjinven-p' tion, either alone or .admixed with, untreated; stock, informing the coating or liner layer. of thecompositemulti -layer paper or board. I The stock, after; treatment with; the1so1uble I 7 sodium salt, will contain e. g; sodium chloride in" solution; When 1 this" modified and j pigmented 1 stool; is used without'jadrnixture' of untreatedf stockjinmaging 'the coating ;or lin n layer. it

'at least 50 %2"- andadvantageously-nonsiderably I may not be necessary to remove the sodium chloride, as most of the sodium chloride will be removed in the cylinder machine in the white water, and most of the remainder will be squeezed out by the pressure of the squeezing and press rolls....,It..is sometimes advantage u h w ig remove the s'odium; chloride solution by washing the treated stock, or to remove the greater portion of the sodium chloride solution and replace it with fresh water, particularly where the stock isfii'niired witliuntr'eate'd stock and the mixed stock- -is-us'ed "for"forming the coating or liner layer. The washing. of the treated stock can be carried jout by dec'antati'on or by washing in pulp was, with. -replacement of the sodium chloride solution with iresh water, care being taken that theiwater drawn off is clear or relatively clea r water to avoid or minimize loss of finely suspendedpigment during the washing operation. I

The modified and pigmented stock before use.

' in thelast 'cylin'derwat of the cylindermachine;

may be [diluted to propelffconsistency; e.'. 'g. M around 1% of "cellulosic "fibers (dry basis) f or. f, somewhat less This 'stock,t in addition to its content of modified cellulose, 'will containv a highf' percentage of precipitatedlpigment. someofthe'jf pigment may escape with the white water but anf amount can be retained inth'e surface layer so 3O that thisllayer may containeg. from 40 to u of precipitated calciunfi' carbonate distributed inf and on and around the inodified fibersg The. m fie d. p m nted. w ihv t admixture of untreated fibers,' can;advantage ously; be used iniorming the coating or l iner' layer of the oompositemulti-layer pulp or larc'iarcig This treated and pigmented stock is highly hyi drated; and the liner layer'formed therefrom has; little strength or relatively muchless'streng'th .thanfa layer "formed'ir'o'm 'the untreated stock;-

or iiner' rbr:mmtamy pmxr and boardwhibh will have increasedfstrengtlr-andgwhich win also be' heavily pigmented;

VVHerethE' modi'fiEd and pigmented-stock :is-

thus-admixed withiintreated stock; varying proportions of the treated and untreated stock's can be used; For making coated board directly on the paper machine, the proportion of pigment inthe liner or coating -"layer-shoi 1ld,fin generalfbe above 5 0 I A highly pigmented coate'd "board can thus. be prodiicediwhich, aften s'izi t e calender-rolls; givsfa coated aboard v thepaper machine-which'tcanbereadily pi irite f It is sometimes-desirable 'to' rdrm mu1ti p1y 'board 'with use-p gment mati g. g' ar'oiiiij 3p to 40% pigrnm'it' -ifi' 'tli{ctiiatilig layer; a multi ply paper orboard wmemwhai 'si'Z'edIean' also'be readily=prifitedHf Forfhighly' igmented coated board;-i t"i s desire" 'ableand 'advant; eons; fwh'err untreated fibers are m xe w h h' i d fl i mented-st c i to usejon y'a e t fel si ell r porti n 1. 1 treatedistock e.fg.r 10% -or 2 0%j.,ofviohglJfib' percentages of untreated stock can be used, and

even when 50% of the stock is treated and pigmented and 50% is untreated, a coating layer or liner can be formed which will contain considerably in excess of 50% of pigment (as determined by ash) as compared with the weight of fiber (dry basis). Thus, where 50% of the stock in the form of groundwood is treated to form a modified pigmented stock containing 4 parts by weight of pigment to 1 part by weight of fibers (dry basis) and when this treated stock is admixed with 50% of untreated cellulose stock, the weight of the pigment in the resulting stock and in the coating layer or liner formed therefrom may approximate twice the weight of the fibers (dry basis). When 80% of the stock such as groundwood is treated and admixed with 20%.of untreated long fiber chemical pulp, the pigment content of the coating layer or liner may be around '75 to 80% of the weight of the coating layer or liner.

The process can be modified by adding organic binders, such as starch or casein, etc., to the stock before it is used on the cylinder machine, e. g. an amount equal to around more or less, based on the carbonate present.

In the carrying out of the process on a multiple cylinder machine, the new stock is used in the last cylinder vat, which produces the outer layer or liner of the composite sheet or board. Various stock can be used in the other cylinder vats, such as are commonly used in making multiple layer sheets and board, such as less expensive stock made from waste paper or reclaimed kraft, etc., for the intermediate or filler layers. Except for the use of the new pigmented and modified cellulosic stock for the outer layer or liner, the process can be carried out much the same as that now used in making multi-layer sheets or board, and with use of varying numbers of cyl; inder vats and cylinder molds to form composite sheets of from two to six or more layers, and with the outer layer or liner formed from the specially prepared and pigmented stock.

t is one advantage of the present invention that a particularly high proportion of pigment can be incorporated in the stock used for the outer layer or liner and that the proportion of cellulosic fibers can be relatively small. A single sheet of paper made from such stock would have little strength, but when used as an outer layer or liner for a multi-layer sheet or board, sufficient strength can be given by the other layers while the outer layer or liner is a highly pigmented layer which gives an improved pigmented finish to the sheet or board.

When the specially treated stock is formed into a sheet on the screen of the cylinder mold, the pigment tends to concentrate on the screen side of the layer, giving a smoother and more finished appearance to the sheet than would the outer surface of the layer. The outer surface with a more fibrous appearance is the surface picked up by the felt so that the pigmented surface formed on the screen is the outer surface of the composite sheet or board.

With highly pigmented stock containing a higher proportion of pigment than of fiber, and particularly with two or three or four times as much pigment as fiber, the dried composite sheet may contain so much pigment that it will tend to rub off unless protected. But by applying size to this surface of the sheet in the calenders, the size will hold the excess pigment and give a highly pigmented and sized surface with a uniform coating that completely covers all fibers.

and gives a pigment face with a high proportion of finely divided calcium carbonate pigment.

Finished sheets or boards can thus be produced with a white surface which can be printed, lacquered, etc.

The danger of removal of the surface pigment by dusting can also be reduced by adding binders to the stock before, it is applied to form the surface layer or liner; and a further improved result can be obtained by sizing the sheet in the calenders in accordance with the common practice of applying sizing to sheets during the calendering operation.

Where the treated and pigmented stock is admixed with untreated stock, the untreated stock may be stock which is prepared e. g. in a beater in the ordinary way and which may be sized and prepared for use before the treated stock is added. The stocks, are thoroughly admixed together before the composite stock goes to the cylinder vat, and the mixing is advantageously carried out at a point close to the cylinder vat.

Rosin soap and alum are commonly added to the untreated stock for sizing. Where untreated stock is admixed with the treated and pigmented stock, if the stocks after admixture are not sufficiently acid, a further amount of alum is advantageously added at the time of mixing or before the admixed stocks go to thecylinder vat or is added to the cylinder vat. The untreated stock may thus be ordinary stock which is prepared in the beater and which may be sized and prepared before the specially treated and pigmented stock is admixed therewith to form the composite pigmented stock for use in making the coating layer or liner of the multi-ply paper or board.

Where admixed untreated stock and treated pigmented stock are used, the portion. of stock treated may be either cellulose stock (chemical pulp) or semi-chemical pulp or groundwood stock. In many cases it is more advantageous to treat groundwood stock because of the increased amount of pigment which can be incorporated in itand retained by it, care being taken as above explained to avoid excess of alkali in the-treatment which would tend to darken or otherwise affect the groundwood stock treated. When untreated stock is admixed with the treated stock, the untreated stock is advantageously cellulose stock such as chemical pulp, and particularly long fiber chemical pulp which will give increased strength to the coating layer or liner.

The invention will be further illustrated by the following specific examples:

Example I.Sulfate kraft pulp from long-fibered wood is prepared in a beater with a content of around 4 to 5% of fibers (dry basis). The stock so prepared is placed in a separate tank provided with agitators, and solid calcium chloride is added in amount sufiicient to form a solution containing about 20% of calcium chloride. The stock is stirred to insure thorough distribution of the calcium chloride and to permit its action on the fibers, and this is continued for a period of 5 to 10 minutes, but may be continued for a longer period. After the calcium chloride has exerted its modifying action on the cellulose fibers, there is added a sufficient amount of 10% sodium carbonate (soda ash) solution to react with thecalcium chloride. This reaction takes place in a period of about 5 to 10 minutes with continued agitation, but the action can be permitted to take place for a longer time. If the sodium chloride, is to be'removed more-or less 1 7'1 completelyflbm th'e'astflckti' it iszmvasheds-or de -a cantedwith the -addition of; treshiwaterfito ei-i. place the salt solutioni 'Betoreuse in the cylinder machine; thes stock is diluted to proper consistency; Gaga; around1% i-5 of modified cellulose or a SOmGWhaULLIOWBI'iLpBZZ Ln' centage, and is-thenc suppliedato thesstufiiehest regulating box and cylinder avat forluse;in::mak+i. ing the-outer layer or liner of the compositeishetz'; or board v The -multii cylinderz machine 'may hex-:10. otherwise operated in the -usual way=to iformatheze multi layer sheet or board i-whichsiis -;.;pa'ssed ;a through the: squeeze and-pressure: ro'llsiandaoveren the dryers, and calenders, being givenz-c-aisizi-ngrs; treatmentby applying. a; starchisolutiomor a waf- L te'rprooi'size on the 'calenders E$ample: II.+-2400 parts of :sgrounded-tipuip on: 2.5% fibercontent equivalent to=-60?,"parts=-=of:d fibers are 'pumped to a treatingtanlneandtze lmz parts of calcium: chloride ;.74-%" CaCl2)' are added 120.2;

and-mixed thoroughly. and d-issolved and tlre -agi tation is continuedioraboutll) minute's to effect thorough treatment and gelling of the stock by the action of calciumchloride; A sodiunrearbonate solution is made by dissolving tiofl poundsot swa 5;:

layer or liner makes it desirable to use a size in ash in water to form a solution :ofabout: 10-7 This solution is pumped into the=caloium chloride t at p t b a ralmw djt mix-therewith, witlw continuediagitationg untilicomplete precipitation 1 has occurredozThe resulting treated stock-in =the form-bf:afibrous precipitate is'pumped to storage' tanks oand hllowedt to settlel' without i agitation.

Afters. settling the-=top liquor- -(-sodiu'm chloride solution) is drawn: off zand replaced with fresh water toxgive a treated:stoele Which;-afteimifllifibers The procedure. is otherwisethe sameas A tionsto propercconsistency; is used in the cylinde'r machine .for? formingetht: outer layeror linerof theacomposite sheet orboard "as-described'- in Example-IL 8Z2 thereto-1: talc-, size; alumi: andsstarchr. the" :proet portion-of 25:ipar ts oiitalc; 2.5? parts of resin size' (rosin -soap). 3.5 parts ofchighfree rosiriisiZer; (rosinsoap cont'aining about 46 Ifreevrosin)2 6 parts' -of:alum :and 6 parts of starchfor 240 parts oifimixed stock (dry basis). This treated-stock is-passedflfrom the beater through the. Jordan, through the machine chestand-head box inlthe screens to the: sluice leading to the cylinder vat, and th'e treated-stock is admixed with the um-- treated stoek in. the slui'ce, the mixed stocks being then pumpedto theLcylin'der' vat.

Tlfaproportions'of stocks admixedare 15% 01 1 thestochivzdry .basis) which has been "specially-' treated to imodify. and pigment it, 7 and 25% '-Jof th'e untreatedstock furnish; Alum is added-to keep the stock in the cylinder vat ac'id and: at a a. pH of-arQundBl-O 1106.2.

The. mixed-stock from the cylinder vatis used toiiform the-outer layerlor liner. of a multi-ply paper sheet or board in a regular multi cylin'der h machine, and the resulting composite paper or board is passed from the cylinder' vats-through the: usual equipment including calender rolls.

The: porous and absorptive nature of the outer sizing the board Which is a non-penetrating size such as a-low viscosity; high- 'solids starch ize containing addition agents which give a non- :penetr'ating size; V

ExampZcI-VJ-The sametreateotand untreated stocksare used as in Example IIL' but the -proportionszof treated and untreated stock are the same,-that-is,' 50% of the fibers are modified and pigmented fibers 'and- 50% are regular untreated that: of the preceding :example.- Although only half of -the stock-used in forming the outer layer or liner -is modified and pigmented stock, a high Examples =I anduII describe-the use---o the '40 iproportionof pigment is includedin the commodified andipigmentedstock withoutadmixture-1 of .untreatedtstockiormaking the coating la'yer or lin'er of thevmulti=ply paper or board;

Thefollowing examplesedescribthe also" of admixed i'itreated xanctspigmented stook -with eun- -45 treated stools. 1

Example: III."- '-The-stobk-duinishwa portion-off whiohxisi untreated anda Q0rtrinf0f' whioh istreated, is. made-sit 1 850 parts of "unbleached dong-5 fiber .sulfite i ipulpgs35ii 'parts offiyleaf shavings eo 3 loitparts; of ozhard white stock (long fiber bleached-j sulfite) Y and SLOQOiiIpaItSKJf deinked fstoele idry basis). This stoolofurnish is prepared in*abeate;- in? the regular-manner a-t fi 7% consistency;

A portion of this stock-*iuT-n-ish *amourrting'$065 3,000 pounds of mixed sto ck at-' 6-%"-consistency l-pounds 'of "fiber dry -'bas is)1 i s 'placedjn" a washer in which it -'-=kept in circulation andf somewhat diluted to permit'-;goodcirculation-. T" 1 this .1 diluted "stockis added1-,000- pounds of'7 5'%-}"fi0 calcium? chloride 4750 pounds CaGI2Fiwhich -isi; dissolved: andall'owed to-"* reac t fo'r 15minutes; with continued circulation 700 poun'ds'of1soda: ash is dissloved in :water to --form -afi 1(I-%" solu' tion, and after the-reaction of the-calcium phi ride onlthe stockis 'completed the soda' ash solu; tion is: pumpedin with continued ;circul-a, tio1 d; of the-Strick and precipitation of calcium: carbonate" 1 allowed to. take 1 place; this carbonate bmg pre V cipitatedswithinas Well as-=on and around' the fibers; i Afternthewprecipitatiorr ise completed; they stocks-is Washed lbyrdrawingoflthesodiunr-chlio ride;solution:and-meplaoingdt with-fresh water:

A separatez;portiomiofi the same. stock {Turn-15h pOSi-te stock, amounting to considerably more th'an=:50% of the weight of the liner layer While inv the multi-ply paper or boarcl of Example III; a much higher proportion of pigment is-containew in the:outer--layer or liner of the composite sheet ori'boardit EzcampZe- V-InsteaJd of using treated stock- 1 Which is a cellulose stoclnor largely a cellulose stock,= as= in Examples III and -I V, groundwood canbeitreatedto modify and: pigment it, as in Example-:II, and the resulting modified-and pigmented groundwoodstock-i admixed with 'untreated cellulose stock-and the resulting mixture used toi'form the outer layer or liner ofthe compositepaper or board, accordingto the procedure followed in Examples III-or IV.-

While the foregoing examples illustrate particular m'ethodsof treatment andparticular pros portions of admixedtreated and pigmented stock and' untrea-ted stockgand'the-use-of treated-stock alone-,' for-forming the outerlayeror liner ofthe multi ply paper or board, it willbe -evident thatvariations can be made -in' the amount of "pig ment -incorporated in the tre'ated an'd'pigmented stoek and also in the proportions of pigmented stock and untreated stock, where such mixtures are used.-Thus,- the outer-layer or "liner may be -made entirely- 0f the treated-and-pigmented stockg as illustrated in Examples I andII, and thepropo'rtion of pigment'to fiber-(dry. basis) in'the treated' stool; may be considerably in ex.- 7, cess of'-5 0%="andmay amount 'to as much as '4, times theaweight of thej'fiber or more:

I Where highlypigmcnted imodifieid". .fiberseare admixed with untreatedifibera'the 'proportionof' 1 untreated fibers is'for many'purposes advantageously only a small percentage of the total fibers, e. g.,-10% of long fiber chemical pulp admixed with 90% of pigmented cellulose pulp or groundwood stock; or the proportion of. untreated stock may be higher, e. g. 20% or 25% or moreof the total or in some cases as muchas 50%, as illustrated in Example IV.

It is one advantage of the process of the present invention that it can be carried out at room temperature throughout and with the use of existing equipment in a multi-cylinder machine, al though the tank or apparatus where the calcium chloride is added should be constructed of material which is not corroded by the calcium chloride solution.

It is another advantage of ,the invention that it enables a finished'carbonatewoated sheet to be produced directly on the. paper making machine without subsequent treatment of the sheet or board by subsequent machine coating.

While the invention has been described more particularly in connection with the use of a-multicylinder machine, the process can be carried out in a multi-Fourdrinier machine or in other machines for making laminated or'multi-ply paper where separate wet layers are formedand joined together to make the composite sheet or board.

The present invention enables a uniformly coated sheet to be cheaply produced with the coating formed as a part of the paper making operation and without subsequent coating.

The number of plies or layers of the multi-ply sheet or board can be varied. For example, in making a sheet or board weighing about 50 pounds per thousand square feet, layers or plies may be used, with the outer pigmented layer or liner forming about pounds of the total weight per thousand square feet, or somewhat more or less, and with the pigmented outer layer varying in calcium carbonate content from an amount equal to or somewhat less than the fiber content of this layer up to an amount 2 or 3 or 4 times as large as the fiber content or even higher. The finished board, weighing around 50 pounds per thousand square feet, may thus contain from 5 to 10 pounds of the coating layer per thousand square feet, with the highly pigmented white surface coating finally sized to give a white finished surface which can be printed, lacquered or otherwise finished when the board is used in making boxes, cartons, etc.

The new composite sheet or board is characterized and distinguished by its outer layer or liner of highly pigmented, modified cellulosic fibers containing pigment both in and on and around the fibers. Even with a large excess of calcium carbonate pigment in the surface layer or liner, the application of non-penetrating starch size in the calender will hold any otherwise loose particles of pigment firmly bound in the surface layer or liner.

Even where highly pigmented coated sheets or board are not desired, but a filled surface board of lower pigment content, such board can readily be produced by the use of admixed modified pigmented stock with untreated stock to give a surface layer or liner containing e. g. around 30 to 40% of pigment. Such a composite sheet, after sizing, gives directly on the paper machine a filled sheet which can be printed, etc.

With higher amounts of pigment, a coated sheet can be directly produced on the paper machine, so that it will have a hard surface which can be printed, etc., without subsequent coating treatment.

It will thus be seen that the present invention provides an improved process for making multiple layer sheets of paper and. board and gives a. resulting product which is an improved of increased strength and which will contain the untreated fibers together with the modified and pigmented fibers intimately admixed and combined therewith in the outer surface layer or liner. v

I claim:

1. The method of making multiple layer paper sheets and board whi'chcomprises subjecting an aqueous suspension of cellulosic fibers, at ordinary temperatures, to the action of a strong solution of calcium chloride until the calcium chloride brings about a gelatinization of the cellulosic fibers, adding a soluble salt which will react with the calcium chloride to precipitate a calcium compound pigment in and on the gelatinized fibers, the amount of calcium chloride in the solution being sufficient that the amount of precipitated calcium compound remaining in and on the fibers is in excess of the weight of the fibers, and applying the resulting stock to form the outer layer or liner in making a multiple layer sheet or board.

2. The method of making multiple layer paper sheets and board as set forth in claim 1 in which the cellulosic fibers treated are groundwood fibers.

3. The method of making multiple layer paper sheets and board as set forth in claim 1 in which the cellulosic fibers treated are chemicallytreated fibers.

4. The method of" making multiple layer paper sheets and board as set forth in claim 1 in which sodium carbonate is added to react with the calcium chloride to form a calcium carbonate pigment in and on the gelatinized fibers.

5. The method of making multiple layer paper sheets and board with a highly pigmented outer surface layer or coating which comprises applying, in making the outer layer or liner of the multiple layer sheet or board, a pulp of highly pigmented and gelatinized cellulosic fibers containing an amount of finely-divided precipitated calcium compound pigment in excess of the dry weight of the fibers, said pigmented fibers being produced from swollen, hydrated and gelatinized cellulosic fibers containing a soluble salt largely within the fibers by reaction of another soluble salt therewith to precipitate the calcium compound pigment largely within the fibers as well as on said fibers.

6. The method according to claim 5 in which the highly pigmented and gelatinized cellulosic fibers are groundwood and carry an amount of pigment more than twice the dry weight of the fibers.

'7. The method according to claim 5 in which the highly pigmented and gelatinized cellulosic fibers are chemically-treated fibers and carry an amount of pigment more than twice the dry weight of the fibers.

8. A composite multi-layer sheet of paper hav-

Claims (1)

1. 8. A COMPOSITE MULTI-LAYER SHEET OF PAPER HAVING AN OUTER OR SURFACE COATING LAYER FORMED OF HIGHLY PIGMENTED AND GELATINIZED CELLULOSIC FIBERS CONTAINING AN AMOUNT OF FINELY-DIVIDED PRECIPITATED CALCIUM COMPOUND PIGMENT IN EXCESS OF THE DRY WEIGHT OF THE FIBERS, SAID GELATINIZED AND PIGMENTED FIBERS HAVING BEEN PRODUCED FROM SWOLLEN, HYDRATED AND GALATINIZED CELLULOSIC FIBERS CONTAINING A SOLUBLE SALT LARGELY WITHIN THE FIBERS BY REACTION OF ANOTHER SOLUBLE SALT THEREWITH TO FORM THE PRECIPITATED CALCIUM COMPOUND PIGMENT LARGELY WITHIN THE FIBERS AS WELL AS ON THE FIBERS.