CA1324707C - Retention and drainage aid for papermaking - Google Patents

Abstract

RETENTION AND DRAINAGE AID FOR PAPERMAKING
ABSTRACT OF THE DISCLOSURE
A papermaking stock comprising cellulose fibers in an aqueous medium at a concentration of preferably about 50% by weight of the total solids in the stock including a retention and dewatering aid comprising a two component combination of an anionic polyacrylamide and a cationic colloidal silicia sol.
The stock exhibits enhanced resistance to shear forces during the papermaking process. A papermaking process is also de-scribed.

Description

2471~7 ~ ~
RETENTION AND DRAINAGE AID FOR PAPERMAKING
, TECHNICAL FIELD
This invention is directed to an aid for use in enhancing the resistance to shear and the retention of fibrous -~ fines and/~r particul~te filler~ ln a paper web ~ormed by vacuum felting of a stock on a wire or the like, and enhancing ' the dewatering of the web in the course of its formation.
,! BACKGROUND ART
Various aids have been proposed heretofore which ` enhance the retention and/or dewatering characteristics of a 10 paper web. Specifically, U. S. Patent Nos. 4,578,150 and 4,385,961 disclose the use of a two-component binder system comprising a cationic starch and an anionic colloidal silicic I acid sol as a retention aid when combined with cellulose fibers 3 in a stock from which is formed a paper web by vacuum felting on a wire or the like. Finnish Published Specifications Nos.
67,735 and 67,736 refer to cationic polymeric retention agent compounds including cationic starch and polyacrylamide as useful in combination with an anionic silicon compound to improve the reception of a sizing. In Specification No.
20 67,735, the sizing agent is added in the furnish, whereas in Specification No. 67,736, the sizing is applied after the paper ~ web is formed. These documents do not propose nor suggest - enhanced resistance of the stock to shear or dewatering en-hancement.
Many other prior publications have suggested differ-ent combinations of cationic and anionic substances as useful in papermaking. Most frequently, such combinations are specif-ic as regards their relative proportions as in U.S. 4,578,150, or as regards their sequence of addition to the pulp slurry as 30 in U.S. 4,385,961. They further often are limited, as regards .

13247 07 ;
j their effectiveness, to specific pulps, e.g. chemical, mechan-... .... . .
ical, thermomechanical, etc.
In International Publication No. W086/05826 there is disclosed the use of anionic colloidal silica sol together with cationic polyacrylamide as a retention aid in a papermaking stock. This disclosure is diametrically oppo~ite to the combi-nation of the present invention.
The basic mechanism by which the cationic and anionic component aids function is often stated in terms of the compo-nents forming agglomerates, either alone or in combination withthe cellulose fibers, that result in retention of fiber fines and/or mineral fillers~ It is well recognized in the papermak-ing art that a pulp slurry, i.e. stock, undergoes severe shear stress at various stages in the papermaking process. After digestion, the stock may be beaten or refined in any of the ; . ~ . .. ..
several ways well known in the papermaking industry or it may , be subjected to other similar treatments prior to the deposi- -!i~ tion of the stock onto a papermaking wire or the like for dewatering and web formation. For example, in a typical paper-- 20 making process, after digestion (and possibly bleaching), and even after beating and refining steps, the stock is subjected `
.. . .
to shear forces associated with mixing and particularly to ~ ;
hydrodynamic shear associated with flow of the stock through such equipment as distribution devices, some of which divide the pulp stream and then recombine the streams at high veloci-ties and in a manner that promotes mixing by means of high . .
turbulence prior to the stock entering the headbox. Each time the stock is caused to flow from one location to another, it `
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encounters shear, as when flowing through a conduit. Such ~
30 shear is exarcebated by the high flow velocities encountered in ~ -the more modern mills where the paper web is formed at speeds in excess of 4000 feet per minute, thereby requiring larger -- 132~707 : , ~- volumes of stock flow which often translates into greater flow velocities and greater hydrodynamic shear. All of these sources of shear tend to diminish or destroy the flocs or agglomerates developed by the added aids.
Shear stress continues to be experienced by the stock, and in fact is more severe in many instances, as it leaves the headbox, flows onto the wire, and is dewatered.
Specifically, as the stock is discharged from the headbox -through a manifold, thence a slice, onto the moving wire, there are very strong shear forces exerted upon both the liquid and the ~olids content of the stock. For example, in those paper-{ making mechanisms which employ slice ~ets, there is boundary shear between the stream flowing through each jet and the jet -walls. The slice lips can be considered as flat plates held parallel to the main direction of flow as the fluid travels farther along the plate, the shearing forces, due to the region of viscous action, accomplish the retardation of a continually expanding portion of the flow. As the velocity gradient at the boundary surface i~ reduced, the growth in boundary layer thickness along the plate i6 paralleled by a steady increase in boundary shear. -.;~ . . .
~, The stock on the wire is subjected to still further . hydrodynamic, including shear, forces. Paper sheet forming is predominantly a hydrodynamic process which affects all the components of the stock including fibers, fines, and filler.
The fibers may exist as relatively mobile individuals or they may be connected to others as part of a network, agglomerate or mat. The motions of the individual fibers follow the fluid motions closely because the inertial force on a single fiber is small compared with the viscous drag on it. However, the response of the fibers to fluid drag may be drastically modi~

fied when they are consolidated in a network or fiber mat.

:"` 132~7~7 1, ,, - Chemical and colloidal forces are recognized to play a signif-icant part in determining whether the fibers assume a network or mat geometry, such being particularly true with respect to fines and fillers. In commercial systems, heretofore, it has ' 5 been generally conceded that the hydrodynamic forces exert a ,l significant influence upon the sheet formation and that the ~ ~ :
degree of this influence is in proportion to the geometry of ~, the fibers, fines and fillers in the stock as the stock reaches ` the wire and the degree to which this geometry is maintained 10 during the sheet forming stage. Examples of the shear forces experienced by a stock during sheet forming include oriented ,, - : .
`~ shear due to velocity differences between the flow of stock and the speed of the wire at the instant the stock contacts the wire. Other shear forces arise as a consequence of the several 15 water removal devices associated with the sheet forming includ- ~
~1 ing the application of vacuum at table rolls, drainage foils, ; -i etc. -' These shear forces encountered by the stock tend ~
toward deflocculation or deagglomeration of the fiber-fines-20 fillers-aids complexes whose intended function is to maintain -~
~-~ their identity in order to obtain the desired intended results s of filler and fines retention, good dewatering during web x~ formation, etc. with improved, or no substantial loss of ~-strength and like properties in the paper product. In the 25 prior art it is not known precisely what mec~anism6 takQ place as respects the complexing o~ cellulose fibers, fillers and cationic and anionic aids, but in any event, the present inven-tor has found that the deleterious effects of shear upon the complexes is reduced or substantially eliminated through the -30 use of the aid and process disclosed herein.
It is therefore an object of the present invention to provide a papermaking stock having improved resistance ~o ~ ~' .- 4 ~ ~

shear forces that arise in the course of the papermaking proc-ess.
.
~i DISCLOSURE OF INVENTION :
--~ It is another object of the invention to provide an 5 improved combination of additives for a papermaking stock. -y It is another object of the present invention to provide a papermaking stock having improved drainage and reten-- tion properties.
It is another object of the present invention to 10 provlde a papermaklng ~tock which exhlbits improved re~lstance to shear forces and improved retention and drainage properties ~l over a substantial range of pH values.
Y It is another object to provide an improved papermak-ing process.
Other objects and advantages will be apparent from the disclosure provided herein.
j In accordance with the present invention, a papermak-j ing stock comprising cellulose fibers in an aqueous medium at a concentration of preferably at least about 50 percent by weight -l 21) of the total solids in the stock is provided with a retention 3~ and dewatering aid comprising a two-component combination of an ', anionic polyacrylamide and a cationic colloidal silica sol in advance of the deposition of the stock onto a papermaking wire.
, The stock so combined has been found to exhibit good dewatering during formation of the paper web on the wire and desirably high retention of fiber fines and fillers in the paper web products under conditions of high shear stress imposed upon the stock. ~ -; The present invention has been found to be effective 30 with pulps of both hardwoods or softwoods or combinations ;
thereof. Pulps of the chemical, mechanical (stoneground), semichemical, or thermomechanical types are suitable for treat~
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ment in accordance with th~ present process. In particular, the present invention has been found to provide shear-resistant complexed stocks where there is present in the stock substan-tial lignosulfates or abietic acid as might be encountered ,5 especially in unbleached mechanical pulps or in other pulps due to accumulation of these substances in recirculated white water.
Inorganic fillers such as clays, calcium carbonate, ,titanium oxide, and/or recycled broke or other cellulosic waste may suitably be incorporated in stocks processed in accordance ;with the present invention.
The cationic component supplied to the stock is of a colloidal silica sol type such as colloidal silicic acid sol and preferably such a sol which has at least one layer of ¦15 aluminum atoms on the surface of the siliceous component. A
~Isuitable sol is prepared according to the methods such as ;~described in U. S. Patent No. 3,007,878; 3,620,978; 3,719,607 and 3,956,171.
,Such methods involve the addition of an aqueous colloi-i20 dal silica sol to an aqueous solution of a basic aluminum salt such that the silica surface is coated with a positive aluminum Ispecies rendering the sol cationic. This sol is unstable under normal conditions of storage and, therefore, is preferably stabilized with an agent such as phosphate, carbonate, borate, ~¦25 magnesium ion or ~he like as is known in the art. Surface aluminum to silicon mol ratios in the sol may range from be-tween about 1:2 to about 2:1, and preferably 1:1.25 to 1.25:1 ~and most preferable 1:1, the latter being desirably more sta-- ble.
Particle size of the sol particulates appears to exhibit a lesser effect in determining the e~ficacy of the sol .
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;- 13241~07 as used in the present process than certain other properties such as aluminum/silicon mol ratio, etc. Particle sizes of between about 3 and 30 nm can be employed. The smaller size ranges are preferred because of their generally superior per-formance.
The anionic component of the present invention com-` prises a polyacrylamide having a molecular weight in excess of 100,000, and preferably between about 5,000,000 and 15,000,000.
The anionicity (degree of carboxyl fraction present) of the polyacrylamide may range between about 1 to about 40 percent,but polyacrylamides having an anionicity of less than about 10 ~ percent, when used with the cationic colloidal silica sols, i have been found to give the best all- around balance between freeness, dewatering, fines retention, good paper formation and strength, and resistance to shear.
Suitable anionic polyacrylamides may be obtained either by hydrolysis of a preformed polyacrylamide or by coplymerization of acrylamide with acrylic acid. Anionic polyacrylamides and anionic copolymers derived from the copoly-~ 20 merization of acrylamide with methacrylamide also may be em-3 ployed in the present invention. The polymer products of either of these methods of production appear to be suitable in the practice of the present invention. As noted hereinabove, the lesser degrees of anionicity are preferred for all-around j~ 25 benefits but optimum shear resistance with acceptable accompa-~¦ nying retention and dewatering properties has been found to occur with those polyacrylamides having an anionicity of be-tween about 1 and lO percent. Suitable anionic polyacrylamides are commercially available from Hi- Tek Polymers, Inc., Louis-30 ville, Xentucky, ~Polyhall brand), from Hyperchem, Inc., Tampa, ~-`

Florida (Hyperfloc brand), or Hercules, Inc., Nilmington, Delaware (Reton brand) as indicated in the following Table A:
~ * Trademarks 7 ., :
f ~3247 07 TABLE A

Polymer Average % Carboxyl Molecular Weight Ranqe (MM) 1 5 Polyhall*650 10 5 i Polyhall 540 1o 15-20 3, Polyhall*2J 10-15 2 ; Polyhall*7J 10-15 7 Polyhall 21J 10-1~ 21 Polyhall*33J 10-15 33 Polyhall*40J 10-15 40 Polyhall*CFN020 5 5 ~Polyhall CFN031 10 12 qHyperfloc*AF302 10-15 2-5 15 Reten*521 15 10 Reten*523 15 30 Of these polymers, the Polyhall 650 provides a combination of good dewatering retention, and shear resistance, while minimiz-ing floc size, and therefore is a preferred polymer for use in the present invention. For addition to the stock, the anionic polymer is prepared as a relatively dilute solution containing about 0.15 percent by weight or less.
MODES FOR CARRYING OUT THE INVENTION
.
In the papermaking process, the cationic colloidal silica æol and the anionic polyacrylamide are added sequential-ly directly to the stock at or briefly before the stock reaches the headbox. Little difference in fines retention or shear resistance is noted when the order of component introduction is alternated between cationic component first or anionic compo-nent first although it is generally preferred to add the ca-tionic component first. As noted above, in the practice of the invention, the sol and polymer preferably are preformed as relatively dilute aqueous solutions and added to the dilute stock at or slightly ahead of the headbox in a manner that promotes good distribution, i.e. mixing, of the additive with the stock.

.~* Trademarlcs ~ 8 Acceptable dewatering, retention and shear resistance properties of the stock are obtained when the cationic and anionic components are added to the stock in amounts represent-ing between about 0.01 and about 2.0 weight percent for each 5 component, based on the solids content of the treated stock.
Preferably, the concentration of each component is between ~ about 0.2 to about 0.5 weight percent.
¦ In the following Examples, which illustrate various aspects of the invention, the cationic component was a cationic 10 colloidal silica sol prepared according to the teachings of U.S. 3,956,171. Specifically, in the production of the sol, conditions are selected to provide a surface aluminum/silicon mol ratio of from about 1:2 to 2:1, preferably about 1:1.25 to 1.25:1. It has been found that a sol having a surface alumi-15 num/silicon mol ratio of 1:1 is most stable under those condi-3 tions existing in papermaking, so that sols with the 1:1 mol ;~
;l ratio are most suitable. - -. The anionic component used in the Examples comprised ¦ various anionic polyacrylamides, each o~ which is commercially available and identifie~ hereinabove. For additlon to the papermaking stock, the anionic polyacrylamides were prepared as dilute solutions of 0.15 weight percent or less as noted.
Whereas the pH of the stock in the several Examples was chosen ~-to be pH 4 and pH 8, it is to be recognized that the present ;-invention i~ u~eful with stock~ having a pH in the r~nge Or about pH 4 to pH 9.

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DEWATERING OF GROUNDWOOD PULP

Groundwood pulp is characterized by having a high percentage of fines and low dewatering (freene~s). For these '' 5 tests a 0.3 wt. % stock was prepared from 100% stoneground wood (40% poplar, 60% black spruce). To the stock was added l.Sg/l of sodium sulfate decahydrate to provide a specific conductivity of 115mS/cm similar to that of a typical papermak-ing process. The pH of the stock was adjusted to either pH 4 or pH 8 by means of dilute sodium hydroxide and sulfuric acid solutions and Canadian Standard Freeness Tests were then run to determine drainage in the presence of various amounts of polya-crylamide and cationic sol.
The polyacrylamide used was Polyh~ll 650 and was added in amounts up to 1.0 wt. % (20 lbs./ton) based on the pulp content of the stock. The cationic sol used is described ~i above and was used in amounts up to 1.5 wt. % of the pulp.
In conducting the tests, one liter of stock was first measured into a Britt Dynamic Drainage Jar as described by K.
Britt and J. P. Unbehend in Research Report 75, 1/10, 1981, :; , .. . .
published by Empire State Paper Research Institute (ESPRI), Syracuse, NY 13210. The bottom of the jar had been blocked off ~
to prevent drainage but to maintain mixing conditions similar :-A,, .
to those used in subsequent retention and shear force tests -' 25 described in later examples. The stock was agitated at 800 rpm for 15 seconds and excellent agitatLon obtained by means of this and the vanes on the side of the jar. The cationic silica ~ -sol was next added as dilute solution with 15 seconds allowed ~:
for mixing followed by addition of the dilute polyacrylamide ~

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solution. After a further 15 seconds of mixing the contents of '~ the jar were transferred to the hold cup of a Canadian Standard ~1~ 10 ,.,,,,, ~

- 132~707 ' Freeness Tester and the freeness measured.
The results of these tests are presented in Table 1 where it may be seen that the polyacrylamide by itself showed no beneficial effect in increasing the drainage of the stock :~.
'' 5 either at pH 4 or pH 8 (Tests 1-3). Addition of papermakers alum to the system produced no beneicial effect at pH 4. At pH 8, lower loadings of alum increased drainage but this bene- ~
fit was lost as alum loading was increased (Tests 4-7). In ~.
contrast to this, use of the cationic sol in increasing amounts 10 produced a steady increase in drainage both at pH 4 and pH 8 ;:
(tests 8-12). Significant improvements in drainage were main-tained at both pH levels as the polyacrylamide loading was .; `~
reduced (Tests 13-15).
In Tests 16-20, the polyacrylamide and the cationic ; :
¦ 15 sol were increased to very high loadings to demonstrate ~hat further gains in drainage could be obtained and that the system ~ : .
. has a broad range of operability. ..

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DRAINAGE AS A FUNCTION OF SOL AND POLYMER LOADING

100% Stoneqround Wood (40% poplar, 60% Black Spruce) Polyhall 650 Polyacrylamide .~ .
I Test % Polymer % Cationic Sol % Alum Freeness. ml :~ No. Loadinq Loadinq Loadina PH 4 PH 8 2 0.1 - - 68 53 ~ ::

3 0.2 - - 58 38 ~ :
~. 4 0.2 - 0.5 80 150 , 3 5 0.2 - 1.0 75 163 :~
;~ 6 0.2 - 2.0 68 84 :~ 7 0.2 - 5.0 66 82 ~j 8 0.2 0.25 - 74 80 ``
; 9 0.2 0.5 - 106 116 0.2 0.6 - 130 134 ~.
11 0.2 0.75 - 190 180 `:~
12 0.2 1.0 - 200 246 . 13 0.1 1.0 - 192 205 .
14 0.05 1.0 - 160 156 '.
0.025 1.0 - 144 130 --~ :
. 16 0.4 1.0 - 205 265 17 0.6 1.0 - 220 310 18 0.8 1.0 - 235 320 19 1.0 1.0 - 240 330 1.0 1.5 - 335 376 , : .
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132~707 DRAINAGE AS A FUNCTION OF POLYMER ANIONICITY ~ ~.

In this series of tests, the freeness resulting from ~-the use of a variety of anionic polyacrylamides together with 5 cationic sol was examined in a similar manner to that described -~
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in Example 1. The stock was again 100% stoneground wood (40% : ~
~,poplar, 60% black spruce). It may be seen from the results in ;: .
Table 2 that all of the cationic sol/polymer combinations show improved drainage but that the changes in anionicity only show 10 significant variations under alkaline conditions. ::
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~324707 < TA~LE 2 DRAINAGE AS A ~UNCTION OF POLYMER ANIONICITY
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100~ Stoneground Wood (40~ poplar, 60~ Black Spruco) Varlous Polvh~ll PolY~cr~lamldes ~ . . .
',Te~t Polyhal 1 Wt. ~ Polymer Wt. ~ Catlon~c Freene~s~ ml o. PolYmer U~ed Loading Sol Loading . 2 2J 0.1 0.3 - 72 3 7J 0.1 0.3 - 72 2lJ 0.1 0.3 110 ~:I 5 33J 0.1 0.3 - 160 j 6 ~OJ 0.1 ~.3 - 1~0 7 540 0.1 O.S - 124 ; 8 2J 0.1 0.5 - 100 .~ 9 7J 0.1 O.S - 118 ~0 21J 0.1 O.S - 210 ~ 11 33J 0.1 0.5 - 245 j 12 ~OJ 0.1 O.S - 165 .
. 13 2J 0.1 1.0 - 320 : 14 7J 0.1 ~.0 - 350 lS 21J 0.1 1.0 - 355 16 33J 0.1 1.0 - 355 17 ~OJ 0.1 1.0 - 320 18 33J 0.05 1.0 - ~58 ~ 19 33J 0.10 1.0 - 355 :~ 20 33J 0.15 1.0 - ~15 . 21 33J 0.20 1.0 - 410 22 33J 0.30 1.0 - 360 23 S~0 0.2 1~0 207 24 2J 0.2 1.0 192 . 25 7J 0.2 1.0 213 , 26 21J 0.2. 1.0 218 --I 27 33J 0.2 1.0 182 -! 20 ~OJ 0.2 1.0 207 ~!y' ~

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1 3 2 4 7 ~ 7 DRAINAGE OF CHEMICAL PULP ~
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In this example a series of tests was conducted using a bleached chemical pulp comprised of 70% hardwood and 30% ;-softwood. A 0.3 wt. % stock was prepared and 1.5 g/l of sodium -sulfate decahydrate was again added to provide a specific conductivity similar to that of a typical white water. Drain- -age tests were conducted using various amounts of Polyhall ÇSO
anionic polyacrylamide, cationic sol and alum at both pH 4 and 1 10 pH 8.
It may be seen from the results in Table 3 that at pH
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4 the combination of the anionic polyacrylamide with the ca-tionic sol is far more effective in increasing drainage (free- ;
ness) than the combination of the polyacrylamidc with papermak~
ers alum (of Tests 4-7 with Tests 8-13). At p~ 8 the differ- --ences are not as large but higher freeness is still obtainable with the cationic sol. Tests 17-21 show that very high free-ness can be obtained by using larger quantities of the anionic ~
polyacrylamide and the cationic sol. ~ ~:
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DRAINAGE OF THERMOMECHANICAL PULP

In this example a 0.3 wt. % stock from a thermome-chanical pulp of 100~ Aspen origin was prepared. 1.5 g/l of sodium sulfate decahydrate was added to simulate electrolytes.
The Canadian Standard Freeness Tests listed in Table 4 show -that with this stock, improved drainage at both pH 4 and pH 8 was obtained using Polyhall 7J anionic polyacrylamide with j cationic sol versus the use of the same polyacrylamide with 10 alum. ;~

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DRAINAGE/RETENTION OF CHEMICAL THERMOMECHANICAL PULP

n this example, the freeness of a chemical thermome-chanical pulp was examined. In addition, to obtain a measure 5 of fines retention, turbidity measurements were made on the `
white water drainage from the freeness tests. The furnish was of 0.3 wt. % consistency with 1.5 g/l sodium sulfate decahy- -:1 -:, ... .
drate as electrolyte. The combination of anionic polyacryla-mide with cationic sol at pH 4 showed a greater re8ponse to 10 both improved freeness and improved retention (lower turbidity) ~
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than did the polyacrylamide combined with alum. At pH 8, the freeness of both combinations remained at comparable values although the cationic sol system showed better retention. The results are given in Table 5.
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FINES RETENTION AND DRAINAGE OF FILLED PULP -.. .:, :
For these tests a 0.5 wt. % filled pulp stock com- ;

prising 70% chemical pulp (70% hardwood, 30% softwood), 29%
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', 5 Klondyke clay and 1% calcium carbonate was prepared. 1.5 g~1 -i sodium sulfate decahydrate was added as electrolyte. ;
j Britt Jar Tests for fines retention were then con-ducted using various loadings of Polyhall 650 anionic polya-crylamide with either alum or cationic sol. A constant stirrer speed of 800 rpm was used and tests were made at both pH 4 and pH 8. Table 6 lists the results.
It may be seen that at Polyhall 650 anionic polya-crylamide loadings of 0.1 wt. %, use of the cationic sol gives superior retentions to the use of reference alum at both pH 4 and pH 8 (cf Tests 9-12 with Tests 3-5). At higher Polyhall 650 loadings of 0.2 wt. % superiority of the cationic sol over , alum is maintained at pH 4. At pH 8 the differences are no -~
longer marked. -: . .: .
Also included in Table 6 are some freeness values for the same pulp system (diluted to 0.3 wt. % consistency) at additive loadings corresponding to high fines retention levels. -A clear superiority in drainage for the use of cationic sol -versus alum is demonstrated.
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Freeness and white water turbidity measurements were made on a istock similar to that described in Example 6. Two commercial anionic polyacrylamide retention aids were used. Table 7 shows a significant enhancement in both freeness and fines retentlon .~ . .
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13247 ~7 RESISTANCE OF FINES RETENTION TO TURBULENCE
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The improved resistance of pulp fines flocs formedfrom the co-use of anionic polyacrylamide with cationic sol to ~ 5 the effects of machine shear forces was demonstrated by further .
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Further tests were conducted to demonstrate the retention, under conditions of increased shear, of the present invention versus a commercial prior art system employing col-, loidal silica. In these tests, the stock used was a fine paper stock comprising 70% pulp (70% hardwood and 30%
~, softwood), 29% clay and 1% calcium carbonate. The pH of the stock was adjusted to 4.5. In these tests, the loadings of the anionic polyacrylamide was selected at the equivalent of 3 lb/ton (0.15 wt. %) and the cationic sol at 12 lb/ton (0.6 wt.
%). Britt Jar tests were conducted at different agitation speeds to simulate different magnitudes of shear. The order of S addition of the cationic and anionic components were reversed in certain o~ the tests to illustrate the effect of order of component addition. The results of these tests are given in lS Table 9. Further tests were conducted in like manner except that lO0 ppm of lignin sulfonate, a representative anionic impurity, was added to the stock. The Table 10 shows the results of these tests and shows the superiority of the present invention. The "prior art" referred to in Tables 9 and lO
comprised anionic colloidal silica sol plus cationic starch marketed under the tradename Compozil*by Procomp of Marietta, Georgia. The loadings employed in all tests were of 8 lb/ton (0.4 wt. ~) of anionic colloidal silica plus 20 lb/ton (l.0 wt.
%) of cationic starch. The loadings stated for each system had been established as giving nearly optimum values in fines retention for that system.
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Claims (10)

1. In a papermaking stock including cellulose fibers in a concentration of at least about 50% by weight of such fibers in an aqueous medium the improvement characterized in that the stock includes:
a cationic component comprising a colloidal silica sol compound selected from the group consisting of colloidal silicic acid sol, and colloidal silicic acid sol modified with at least one surface layer of aluminum atoms, an anionic component selected from the group consist-ing of polyacrylamide prepared by the hydrolysis of polyacryla-mide, polyacrylamide prepared by the copolymerization of acryl-ic acid with acrylamide, and polyacrylamide derived from the copolymerization of acrylamide with methacrylamide, said cationic component being present in the stock in a concentration between about 0.01 to about 2.0 weight percent based on the solids content of the stock, said anionic component being present in said stock at a concentration from about 0.01 to about 1.0 weight percent based on the solids content of the stock, whereby said stock is rendered effectively resistant to destruction of its retention and dewatering properties by shear forces incurred by said stock in the course of forming of the stock into a paper web.

2. The papermaking stock of Claim 1 characterized in that said cationic component and said anionic components are present in a ratio of between about 1:100 and 100:1.

3. The papermaking stock of Claim 2 characterized in that said cationic component and said anionic components are present in a ratio of between about 1:10 and 10:1.

4. The papermaking stock of Claim 1 characterized in that the pH of said stock is between about 4 and about 9.

5. The papermaking stock of Claim 1 characterized in that said anionic component exhibits an anionicity of between about 1 and about 40 percent.

6. The papermaking stock of Claim 5 characterized in that said anionic component exhibits an anionicity of less than about 10 percent.

7. The papermaking stock of Claim 1 characterized in that said anionic component has a molecular weight of between about 100,000 and about 15,000,000.

8. The papermaking stock of Claim 7 characterized in that said anionic component has a molecular weight between about 5,000,000 and 15,000,000.

9. The papermaking stock of Claim 1 characterized in that said cationic component has a particle size of between about 3 and 30 nanometers.

10. A papermaking process employing a stock compris-ing at least about 50% by weight of cellulose fibers in an aqueous medium having a pH between about 3 and about 9, intro-duced from a headbox containing said stock onto a moving paper-making wire and vacuum felted thereon characterized in that there is introduced to said stock prior to its removal from said headbox onto said wire, a cationic colloidal silica sol component, and an anionic polycrylamide component, such components being introduced separately from one another and with a time lapse between their times of introduction that is sufficient to permit good mixing, said cationic and said anionic components being present in a ratio of between about 1:10 an 10:1 based on weight and each component representing between about 0.01 and 1.0 weight percent of said stock based on total soldis of said stock.