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A study of the retention of pigment during paper formation
A Study of the Retention of Pigment during Paper Formation 1 MICHAEL J. JAYCOCK 2 AND JOSEPH L. PEARSON 3 Department of Chemistry, University of Technology, Loughborough, Leicestershire, L E l l 3TU, Engla d Received February 26, 1975; accepted November 1l, 1975 Pigment retention measurements have been carried out using various inorganic materials with and without the use of a flocculant. Using DLVO theory for a spherical particle interacting with a flat plate, the observed variations in pigment retention have been explained in terms of heteroflocculation of the cellulose fibers and pigment particles. Because of the presence of fibrils on the surface of the fibers, the flat plate model is obviously an approximation, but nevertheless good correlation is obtained between the experimentally obtained and theoretically predicted heteroflocculation regions. It is found that in the absence of a sizing agent or flocculant the fiber fines flocculate with the filler particles which results in all the various pigments becoming electrokinetically similar. To obtain increased retention it would be necessary to produce a fines free beaten pulp, which would be impracticable on a commercial scale, although feasible for experimental research purposes. The flocculant employed is aluminum sulphate, which adsorbs either as positively charged aluminum species or in the form of aluminum rosinate onto both the cellulose fibers and pigment particles. This causes an increase in the width of the pH range in which heteroflocculation occurs.
In a recent series of papers we have been investigating the factors that influence the retention of inorganic pigments (fillers) (1), and, in particular, titanium dioxide (2) during the process of papermaking. We have postulated that the fillers are retained by means of a heteroflocculation process involving cellulose fibers and the filler particles. It is the purpose of this paper to look at the system more quantitatively and discuss the factors which affect this heteroflocculation process. A basic papermaking furnish consists of a suspension of cellulose fibers, a filler that is usually an inorganic pigment, a flocculant, or retention aid and a sizing agent. The purpose of the sizing agent is to avoid penetration of liquid through the sheet, and a flocculant to increase the retention of both size and filler. In this investigation a sizing agent will not be 1 Presented at the 49th National Colloid Symposium, Potsdam, New York, June 16-18, 1975. 2 To whom correspondence should be addressed. 3 Present address: I. C. I. Paints Division, Wexham Road, Slough, Berks SL2 5DS.
discussed in great detail, as it tends only to cloud the issue and make theoretical predictions impracticable. A retention aid that has been used for many years is aluminum sulphate, known as papermakers alum, and it is the effect of aluminum sulphate that will be primarily discussed in this paper. EXPERIMENTAL
Materials. The furnish was prepared from a bleached sulphate pulp, "Analar" aluminum sulphate and a series of inorgalaic fillers, listed in Table I. A list of the relevant physical constants are also shown in this table. All the water used was triply distilled from ~t quartz still and was stored in glass vessels to avoid contamination. The sulphuric acid and sodium hydroxide used to adjust the pH of the furnish were of Analar grade, the pH measurements being made using a PHM52 Radiometer pt~ meter. Preparation and characterization of thefurnisk; The basis of the papermaking furnish was
181 Copyright ~ 1976 by Academic Press, Inc. All lights of reproduction in any form reserved.
Journal of Colloid and InterJace Science, Vol. 55, No. 1, April 1976
182
JAYCOCK AND PEARSON TABLE I Physical Constants of Fillers Used Filler
Particle size (vm)
Ref.
H a m a k e r constant Alsl X 10 +13
Ref.
Anatase "Tioxide A-HR ''a Anatase "Tioxide A-DM TM Ruffle "Tioxide R-CR ''~ ~x-Alumina Silica Stannic oxide
0.15 0.14 0.25 0.3 0.15 0.18
(3) (3) (3) (3) (3) (3)
2.5 2.5 5.9 4.17 0.28 4.3
(4) (4) (4) (5) (6) (4)
" "Tioxide" is a registered trademark of Tioxide International Limited. prepared by beating a suspension of the bleached sulphate pulp in a single rotor blender at a consistency of 2%. The length of time the pulp is beaten determines the "freeness" of the suspension. A "free" pulp is described as being one that allows drainage of the water to occur quickly. A pulp that takes a long time to drain is termed "slow." The rate of drainage under standard conditions provides a convenient method of measuring the pulp's freeness. This measurement forms the basis of
l/WIRE ~,,.~.~.=.:~~RU BB ER t~
RIN6S
BUTI RSESS~ ~ ALU MN IEU IM JOINT '~ ~,, VAI A B FIG. 1. Glass handsheet machine. Journal of Colloid and Interface Science, Vol. 55, No. 1, April 1976
the Canadian Standard Freeness test. The pulps used were all of 450 ml CS. • The filler and aluminum sulphate were then added to the fiber suspension after it had been diluted to a consistency of 0.2% in the proportions 7.5 and 20"/0, respectively, by weight of fibers. Before and after the additions of the filler and aluminum sulphate the furnish was stirred for 5 min using only moderate agitation, so that further beating of the fibers was avoided. In some cases the filler could not be fully dispersed in water simply by stirring so dispersion was achieved by means of an ultrasonic probe. Retention measurements. Because of the unavailability of a commercial handsheet machine, a glass one was produced, which was based on a commercial design (7) (see Fig. 1). Comparison of results obtained on this machine with ones obtained using a large scale Fourdrinier machine and a commercial handsheet machine have been made, and satisfactory correlation has been obtained, as shown in Fig. 2. The glass machine consists of upper and lower compartments that are held together by means of buttress joints. Between the two compartments is situated the "wire," which consists of a disc of fine mesh placed on top of a stronger coarser mesh, the purpose of which was to give strength to the wire system. The wire is sealed in place by means of rubber rings. The purpose of the aluminum vane is to avoid swirling below the level of the wire, which causes a nonuniform sheet to be pro-
RETENTION OF PIGMENT duced. The lower chamber is attached to a Buchner Flask, and in the present work complete drainage of the sheet at 450 CS could be achieved at atmospheric pressure. To operate the machine, first water is passed into the lower compartment via tap A with tap B closed until the water level is just above the wire. After closing tap A the papermaking stock is placed in the upper compartment and stirred. On opening tap B the stock drains through the wire leaving the sheet deposited on it. After drying, the sheet is weighed and either it or the white water is analyzed for filler content. Unless otherwise stated the paper was made at a consistency of 0.20-/0 and a basis weight of 80 g m -2. Analysis of filler retained. The analysis of fillers present in paper is usually performed gravimetrically by ashing the paper and weighing the ash. This method, apart from being time consuming, needs fairly large amounts of paper before it is at all accurate and also the technique needs to be carefully carried out. Because of this it was decided to determine the amount of filler present in the "white" water, which is the water that drains through the wire while the sheet is being formed. This was achieved by measuring the turbidity of the white water using a spectrophotometer, after filtration to remove any fiber fines or large dirt particles. This technique was tested by measuring the turbidity of standard sols after they had been focculated and redispersed. In some cases, especially at low pH, the white water contained mostly filler flocs, which were redispersed using the ultrasonic probe, adjustments of p H being made where necessary. The only difficulty with this method would arise if the amount of aluminum sulphate present in the stock was high, when flocculation would be such as to prevent redispersion, however in the present work this is not the case, the molar concentration being only of the order of 7 X 10-~ M. Measurement of zeta-potentials. The zeta (~-) potentials of the fibers were initially determined using the streaming potential technique,
L
183
7.1xlO-SM
5o
AI2{S0/)3 z
=
3o
pH
Fio. 2. Comparison of anatase retention results on -O- a Fourdrinier machine, - A - a commercialhandsheet machine and -I-q- a glass handsheet machine.
the cell used being an all glass modification of the design of Goring and Mason (8). The electrodes were constructed from platinum mesh coated with platinum black. These were fused into glass plungers, which were perforated at the ends where they supported the electrodes. This support was to prevent electrode deformation during pad compression, while still allowing liquid to pass through the cell. The cell was contained in an air thermostat operating at 25 4- 0.1°C. The potentials were measured with a Keithley 610C electrometer fitted with digital output, and the pad conductivities with a Wayne Kerr B221 conductivity bridge. The ~'-potentials were calculated using the logarithmic function of Chang and Robertson (9). Recently it has been postulated by Strazdins (10) that the f-potentials of the fiber fines that originate from the fiber surface are representative of the ~'-potential of the surface as a whole, this having been substantiated by us in recent work (25). To measure the ~'-potentials of the fines, the microelectrophoretic technique was used, which was also used to measure the f-potentials of the filler particles. The electrophoretic cell used was rectangular in shape, and constructed from silica. It had a Journal of Colloid and InterJace Science, VoL 55, No. 1, April 1976
184
JAYCOCK AND PEARSON
height to depth ratio of 8 to 1, and was viewed horizontally along its central axis. It was fitted with ground silica sockets into which the electrode systems were fitted. Each of the electrodes was made of two platinum wires separated by a glass partition. Each of the wires was coated with platinum black to minimize the effects of polarization. This was augmented by employing the double electrode system in the following way. An electric potential was passed across the outer pair of platinum wires and measured across the inner pair by means of a high impedance digital voltmeter. The effective cell length was calculated by measuring the conductivity of the cell filled with 0.01 N KC1 solution, using a Wayne Kerr B221 conductivity bridge. The cell was immersed in a water bath mounted on the microscope which was supplied with water at 25°C from a thermostat bath. The cell was tested by measuring the velocity parabola for a 10-4 M AgI sol. The mobilities of the fiber fines were measured by allowing the fiber suspension to settle out until all the large fibers had sedimented to the bottom of the container. A sample of the top layer of liquid was then removed and transferred to the electrophoresis cell by means of a pipette. It should be pointed out that the beating action of the blender is such that some cutting of the fibers takes place, which gives rise to extremely small fibers that have the same radius as the original fibers. It is these rather than the colloidal material that are measured in the electrophoresis experiments. THEORETICAL
Electrophoresis and streaming potential. For large particles, whatever their shape or orientation, the mobility u, calculated from electrophoretic measurements can be converted to the t-potential using the equation of Smoluchowski (11)
to convert the mobilities to ~--potentials for the fibers because of their size. If it was the colloidal material that was measured, then this could not be used. This may account for the discrepancy some authors have found between results obtained from streaming potential and microelectrophoretic data (12, 13). The ~'-potentials of the fibers were also obtained from streaming potential data using the logarithmic equation of Chang and Robertson (9), which can be written in the form
EX~L/(P~) = A~e-Bc/(4r),
[-2-]
where E is the measured streaming potential observed when a pressure difference P is applied across a pad of length L, area A, concentration c and conductance ~. In this equation, B is a constant. Recently, this equation has been criticized by Ciriacks and Williams (14) using results on Dacron fibers. There is no experimental evidence on cellulose fibers to support this work and the results obtained from the Chang and Robertson equation were both realistic and in good agreement with the results of other workers (10, 15). For spherical particles the most comprehensive treatment has been given by Wiersema, Loeb, and Overbeek (16). For small values of ~" the equations of Overbeek (17) give similar results. For fairly simple electrolytes, the data of Wiersema et al. is tabulated; however, for more complicated electrolytes, i.e., A12(SO4)3, they are not, so in this case the equation of Overbeek has to be used. For an unsymmetrical electrolyte this has the form ~ { u = -fl(Ka) 6r,
et (z_ - z , ) - - f2(Ka) kT
(z+p+ + z_p_) kT
(e~/kT) 2f4(Ka) (z+ + z_)e 6~r~e
I
[-3.]
where Kis the reciprocal double layer thickness, a the radius of the particle, z. the valencies, u = e~'/(4r~), 1-17 p . the friction factors of the ions involved, where , is the dielectric constant and n is the k the Boltzmann constant, T the absolute viscosity of the medium surrounding the temperature, e the electronic charge, and f l to particle. This equation can be used, therefore, f4 are functions of ~a tabulated by Overbeek. Journal of Colloid and Interface Science, Vol. 55, No. 1, April 1976
RETENTION OF PIGMENT The comparison of the results of streaming potential measurements and electrophoretic measurements on fiber fragments is shown in Fig. 3. The results agree to within the limits of experimental error, and consequently, since the electrophoretic method yielded results more rapidly, the ~'-potentials reported elsewhere in this paper are derived by this method. Calculation of the potential energy of interaction. The total potential energy of interaction VT is calculated by summing the potential energies of attraction VA, which arises due to van der Waals type forces and of repulsion Vn, which results from the interaction of electrical double layers. Because the fibers are large compared with the filler particles the interaction was assumed to be one of a spherical particle and a flat plate. This is an approximation because of the presence of fibrils on the fibers. Because the potentials involved are small, the equation of Hogg, Healy, and Fuerstenau (18) can be used to calculate Vn. This equation can be written in the form ~a(f ~2 + ;2 '2)
VR--
:
(~1 + ~-2)
In [11 +-- exp-(--KH°)l exp (--~H0)-J
+ In[-1 -- exp (--2KHo)-]} ,
it
o Streoming Pvtentie[
-Ill w
3
5
pH
7
9
I
FIG. 3. Comparison of ~'-potential measurements of cellulose fibers by -O- streaming potential and -Omicroelectrophoreticmethods. distances and the other at longer distances of separation. These equations were used because they allowed the calculation of VA for all distances of separation. Once the potential energy diagram of VT versus H0 has been constructed the stability of the system can be defined in terms of a stability ratio W, which is given by
u
W = 2
fl
ds exp (VT/kT) s 2 ,
[6]
RESULTS AND DISCUSSION [-4]
where ~'z and ~'2 are the ~'-potentials of the sphere and plate, a is the radius of the sphere, and H0 is the distance of separation between the two particles. To calculate the potential energy of attraction VA, the equations of Vincent (19, 20) were used, these equations having form --12VA = E fn(A+)HJ,
~ - - -1C
where s = r/a (where r is the minimum distance of separation). The Hamaker constant for swollen cellulose was taken as 6.20 X 10-1~ erg.
4 × {2r1
185
[-5]
where fn(AOHj is a function of the Hamaker constants Ai, of the various materials, and the geometry H~, of the system. Two functions describing H are given, one for use at short
Eject of pH on the retention o] inorganic fillers. To determine the effect of pH on the retention of the various fillers, paper was made from cellulose fibers and filler only and the retentions determined. The results are shown in Figs. 4 and 5. Figure 4 shows the results obtained for the various TiO2 fillers used, and Fig. 5 for A1203, SiO2, and SnO~. It is found for all of the fillers that the retention is low until the pH is taken below a value of 4.0. The three TiO2 fillers gave very similar results, which are also similar to SnO~. The results for A1203 and SiO2 increase over the same range, but are not retained to such an extent as the other fillers. If the retention is governed by heteroflocculation of filler particles and fibers, then it will Journal of Colloid and Interface Science, Vol. 55, No. 1, April 1976
186
JAYCOCK AND PEARSON
3O
20
10
o
#
Fie. 4. Effect of pH on the retention of - O - Anatase "A-HR," -•- Anatase "A-DM," and -El- Ruffle "R-CR" in the absence of aluminum sulphate.
%
30
2D
t
L
i
i
i
2
J
~.
5
6
pH
Fro. 5. Effect of pH on the retention of - O - Alumina, - A - Silica and -IS1- SnO2 in the absence of aluminum sulphate. Journal of Colloid and Interface Science,
Vol. 55, No. I, April 1976
be controlled by the ~'-potentials of both of these components. The iso-electric points of the fibers and various fillers are shown in Table II. These are compared to literature values where possible. I t would seem from this data that increased retention should not occur over the same range of p H for each of the fillers. For instance, for the case of alumina, the fibers and filler particles are oppositely charged or have the same small charge until the furnish is made more alkaline than about 9.5, so increased retention should be observed over all this range. The results shown in Table II, however, do not take into account any effect that the fiber fines or other colloidal or ionic material that originates from the fibers may have on the ~'-potentials of the various fillers. To find out what this effect would be the variation of ~'-potential of the fillers was measured as a function of p H in the presence of the fiber fines, typical results being shown in Fig. 6. This figure shows results obtained for A1203 and A - H R , the other fillers giving results which are very similar to each other. I t can be seen that the iep is moved towards a value of about 2.7. Using these results along with those for the cellulose fibers, which are also shown on Fig. 6, the barriers to heteroflocculation can be calculated, and these converted to stability ratios, the values of which are shown as a function of p H for the heteroflocculation of cellulose fibers with A - H R by the dotted line on Fig. 6. Similar curves are obtained for the other fillers. I t is found that heteroflocculation of the fibers and filler particles is predicted below a p H of 3.8, which is the exact region over which increased filler retention is observed. I t is found, as shown by Fig. 4, that only about 40% of the filler is retained in the sheet. Electronmicrographs indicate that the relative coverage of the fibers with filler is low, and that a large amount of uncovered surface remains. This may be due to one of two reasons, or a combination of both. First, in describing a stability ratio W that determines the state of heteroflocculation of the system, we must take into account the two homoflocculation
RETENTION OF PIGMENT TABLE II Isoelectric Points of Fibers and Fillers Used Material
iep
Bleached sulphate pulp Anatase (A-HR) Anatase (A-DM) Ruffle (R-CR) a-Alumina Silica Stannic oxide
2.5 3.8 3.8 6.2 9.0 ~2.0 5.8
Literature value
Ref.
6.7 9.3 "~2.0 4.5~ 7.3
(21) (22) (22) (22)
processes that may also occur. The most important will obviously be the homoflocculation of filler particles producing aggregates with a smaller diffusion rate, and would have to rely on mechanical filtration as a retention mechanism. Previous work has indicated that mechanical filtration is an unlikely mechanism to explain retention due to the size of the pores in the paper compared with the size of the filler particles or flocs. Because of this any flocs of filler particles produced are unlikely to be retained, this is substantiated by electron micrographs, which show the filler to be present largely as single particles. The second argument why 100% of the filler is not retained is that the flocs produced from the fibers and filler particles may be "soft," i.e., it is easily dispersed by hydrodynamic shear, which occurs during the formation of the sheet. This hypothesis has been supported by work by Britt (23) who determined that the flocs could be dispersed by mechanical shear. There is also the problem that filler focculated with fiber fines is found in the white water and is not included as a part of the percentage retained, whereas it is in fact heterofloeculated. This is not thought to be a major factor. The relative amount by which each of the fillers is retained will probably be a function of the value of the stability ratio of heteroflocculation to the value for the homoflocculation of filler particles. This point is difficult to quantify at present because of the effect of fiber fines upon the f-potentials of the fillers and the resultant particle geometry. The
187
calculated stability ratio for a fines covered system would b'e essentially the same as for the fibers themselves. What would determine the ratio of homo- to heteroflocculation in systems of the type described in this paper will be the diffusion and flocculation rates of the components, together with the stability of the various flocs to hydrodynamic shear. These aspects are the basis of further study at present being undertaken. The relative stability ratios of fines coated anatase A - H R for homoflocculation and for the heteroflocculation calculated using the mean particle radius can be seen by comparing Figs. 6 and 7, and indicate that at pH values between approx 4.2 and 8 heteroflocculation will be the major factor. Similar calculations have not been attempted for the other pigments because their particle sizes are not known with the same precision as for anatase A - H R and the particle radius is the major parameter in the calculation.
The effect of aluminum sulphate on the retention of inorganic fillers. The effect of 2~o aluminum sulphate on the retention of in-
.20 ~
,,-,R''
.lO
'~
~I0 ~--i---'/-
-I0
'I00
-20 -30 -Z,O
y, l -60
i
2
3
k
5
5
pH
FIG. 6. Variation of the ~'-potential with p H for both - O - Anatase " A - H R , " - / k - Alumina in the presence of cellulose fibers, and - 0 - fibers alone. T h e dashed line shows the stability ratio for homoflocculation of anatase " A - H R . " Journal of Colloid and Interface Science, Vol. 55, No. 1, April 1976
188
JAYCOCK AND P E A R S O N LO
I
"Si 02" lO0
30 lO
2C
I0 l LD
I00
30 20
g -/.0
I00
20 l0
l
j~,~a~
L0
/
"A-HR"I00
30 2O
l0
___J __
t__ pH
FIG. 7. Effect of pH on the retention of (i) Silica, (ii) Ruffle " R - C R , " (iii) Alumina, and (iv) Anatase " A - H R " in the presence of 2% (7.1 X 10-5 M) aluminum sulphate. The dashed line shows the stability ratio for heteroflocculation.
organic in Fig. R-CR, similar
fillers is shown by the results presented 7, for the retention of A - H R , A1203, and SiO~. It is found in all cases that results are obtained. For convenience
sake the curves can be split into two halves. Below a p H of 4 the retention increases in a similar manner to the previous section, i.e., the effect of pH. Above a p H of 4 instead of falling to a low value the retention is increased reaching a maximum between p H values of 6 and 8. The retention is then found to drop to a low value around a p H of 8.0 to 8.5. Once again the remarkable fact is that all the fillers give results that are so very similar. The effect of multivalent ions on cellulose fibers is to adsorb on the surface and modify the electrokinetic potential, however, the extent to which modification takes place is not fully understood. For instance, Carolane (24) found that the ~'-potential remained negative for various kinds of pulps for alum concentrations up to 1 g/l, which is equivalent to about 10.3 M. This would substantiate our results, shown in Fig. 8. Using a 0.2% suspension, the effect of different aluminum sulphate concentrations on the f-potential versus p H relationship was found. The f-potential is not made positive at concentrations of aluminum sulphate of 10.4 M and less. On further addition precipitation takes place, which hinders measurements. It is found that the largest modification of the f-potential occurs over the p H range 4 to 9. As the concentration of aluminum sulphate is increased, a kink develops in the
.lO
_-l
-20
-30
-LO
pH
FIG. 8. Variation of the ~'-potential with p H of cellulose fibers in the presence of aluminum sulphate. - O - [-AI~(SO4)~] = 0; A - 1.015 X 10-4 M ; - v ] 8.46 X 10.6 M ; - 0 1.012 X 10-6 ~k/. Journal of Colloid and Interface Science, Vol. 55, N o . 1, A p r i l 1976
RETENTION OF PIGMENT curve between a p H of 5 and 6 that corresponds to the p H where maximum aluminum adsorption occurs, Fig. 9 (25). The adsorption of hydrolyzable ions onto the surface of metal oxides has undergone much work and a thorough study has recently been published by James and Healy (26). To see what effect aluminum sulphate has on the fillers in the presence of the fibers the ~'-potential of the filler particles was measured as a function of p H using the paper stock containing the 2% alum, the results being shown in Figs. 8 and 10. Once again, results are presented for A - H R , A1203, R - C R , and SiO2. I t is found for each of the four fillers that the curves are similar, and are also similar in shape to those obtained for the effect of aluminum sulphate on the ~'-potential of the cellulose fibers. Once again greatest modification occurs over the p H range 4 to 9 as in the case of the fibers. The amount of aluminum sulphate added, arrived at by the papermakers experimentally, is just the right concentration to keep the f-potential of the filler particles very nearly zero over a large p H range. I t is very difficult to measure the ~--potentials of the fillers in the presence of fiber fines
% ,-7
20
c2n E
<
8
7o
v-ta2 3
L,
5
6
pH FIo. 9. The adsorption isotherm for a 2% dispersion .of cellulose fibers from an initial 10-4 M aluminum sulphate solution.
189
"s20/2
0
QI
"R- CR"
-20I -L0t
g- 0L- _ ~
'~,~0f_ _
-2(1 -L0 0[ -- ---- "
A
-
H
-2Di -t,Oi 2
.3 . . C. . 5
fi
6
')' - - ~ -
9'
]'6
Fro. 10. Variation of the ~'-potential with pH of various fillers in the presence of 2% (7.1 X 10-5 M) aluminum sulphate and cellulose fiber fines. (i) Silica; (ii) Ruffle "R-CR;" (iii) Alumina; (iv) Anatase "A-HR."
in the furnish, but with extreme perseverance it can be accomplished, the results being shown in Fig. 10. It can be seen that the curves for each of the fillers are similar in shape, suggesting that the coating of filler particles with fiber fines is fairly complete. Using these results the potential energy barriers to flocculation can be found, and these once again converted to stability ratios, which are shown as a function of p H for each of the four fillers by the dotted curves in Fig. 7. The proportion of fiber fines retained has not been measured because of experimental difficulties. I t is of interest to note that when the aluminum sulphate is added to the furnish flocculation with or adsorption of the fines occurs first of all, followed by adsorption of the aluminum species. It can be seen from the dotted line in Fig. 7 that the region of predicted heteroftocculation coincides with the region of increased filler retention. Journal c)f['olloid and Interface Science,
Vol. 55, No. 1, April 1976
190
JAYCOCK AND PEARSON
T h e introduction of a sizing agent into the system results in a positively charged aluminum rosinate precipitate being produced t h a t acts similarly to the aluminum species discussed here, and gives the same end result, t h a t is it increases the retention of the fillers b y adsorbing onto the filler particles and cellulose fibers, followed b y heteroflocculation of the system (27). ACKNOWLEDGMENT Tioxide International Ltd. are thanked for the financial support which allowed this work to be carried out, and for the results on the commercial handsheet machine. Mr. R. Counter of the same Company is warmly thanked for invaluable discussion. REFERENCES I. JA¥COCK,M. J. ANDPEARSON,J. L., J. Appl. Chem. Biotech. to appear. 2. JAYCOCK,M. J. ANDPEARSON,J. L., Sven. Papperstidn. 78, 289 (1975). 3. TIOXIDEINTERNATIONALLTD., Private communicat.ion. 4. FOWK~S, F. M., in "Surfaces and Interfaces," Vol. I. (Burke, Ed.), p. 199. Syracuse Univ. Press, New York, 1967. 5. KRUPP, H., SCHNABEL,W., AND WALTER, G., J. Colloid Interface Sci. 39, 421 (1972). 6. WATILLON,A. ANDGI~RARD,PH., Proc. 4th I.C.S.A., Brussels, 2, 1261 (1964).
Journal of Colloid and Interface Science.Vol. 55. No. I. April 1976
7. I~ARSON, J. L., Ph.D. Thesis, Loughborough Univ., 1973. 8. GoRx~G,D. A. I. ANDMASON,S. G., Canad. J. Res. B28, 307 (1950). 9. CHANG,M. Y. ANDROBERTSON,A. A., Pulp Pap. Mag. Can. 66, 438 (1967). 10. STRAZDINS,E., Tappi 55, 1691 (1972). 11. S'~OLUCrIOWSKI,M. yON, Bull. Acad. Sci. CracoviC 182 (1903). 12. EK.LUND,D., Norsk. Skodind. 21, 140 (1967). 13. HINTON, A. J. AND QumN, N., Paper Teehnol. 5, 60 (t964). 14. CIRXACKS,J. A. AND WILLIAMS,D. G., J. Colloid Interface Sci. 26, 446 (1968). 15. HuKxI, R. T. ANDRINNE, R., Papper Tr~ 36, 129 (1954). 16. WIERSEMA,P. H., LOEB, A. L., AND 0VERBEEK,J. TH. G., J. Colloid Interface Sei. 22, 78 (1966). 17. LYKLEMA,J. AND OVERBEEK,J. TH. G., J. Colloid Sci. 16, 501 (1961). 18. HOGG,R., HEALY,T. W., ANDFUERSTENAU,D. W., Trans. Faraday Soe. 62, 1638 (1966). 19. VINCEI~T,B., J. Colloid Interface Sci. 42, 270 (1973). 20. OSMOND, D. W. J., VINCm~% B., AND WAITE, F. A. W., J. Colloid Interface Sci. 42, 262 (1973). 21. PURCELL,G. ANDSUI%S. C., Trans. A. L M. E. 226, 6 (1963). 22. PARKs, G. A., Chem. Roy. 65, 177 (1965). 23. BRir'r, K. W., Tappi 56, 83 (1973). 24. C~LROLAN~,K. C., Appita 15, 3 (1961). 25. JAYCOCK,M. J. ANDPEARSON,J. L., Sven. Papperstidn. 78, 167 (1975). 26. Jams, R. O. ANDHEALY,T. W. ; J. Colloid Interface Sci. 40; 42, 53, 65 (1972). 27. COUNTER,R., JAYCOCK,M. J., ANDPEARSON,J. L., Sven. Papperstidn. 78, 333 (1975).
In a recent series of papers we have been investigating the factors that influence the retention of inorganic pigments (fillers) (1), and, in particular, titanium dioxide (2) during the process of papermaking. We have postulated that the fillers are retained by means of a heteroflocculation process involving cellulose fibers and the filler particles. It is the purpose of this paper to look at the system more quantitatively and discuss the factors which affect this heteroflocculation process. A basic papermaking furnish consists of a suspension of cellulose fibers, a filler that is usually an inorganic pigment, a flocculant, or retention aid and a sizing agent. The purpose of the sizing agent is to avoid penetration of liquid through the sheet, and a flocculant to increase the retention of both size and filler. In this investigation a sizing agent will not be 1 Presented at the 49th National Colloid Symposium, Potsdam, New York, June 16-18, 1975. 2 To whom correspondence should be addressed. 3 Present address: I. C. I. Paints Division, Wexham Road, Slough, Berks SL2 5DS.
discussed in great detail, as it tends only to cloud the issue and make theoretical predictions impracticable. A retention aid that has been used for many years is aluminum sulphate, known as papermakers alum, and it is the effect of aluminum sulphate that will be primarily discussed in this paper. EXPERIMENTAL
Materials. The furnish was prepared from a bleached sulphate pulp, "Analar" aluminum sulphate and a series of inorgalaic fillers, listed in Table I. A list of the relevant physical constants are also shown in this table. All the water used was triply distilled from ~t quartz still and was stored in glass vessels to avoid contamination. The sulphuric acid and sodium hydroxide used to adjust the pH of the furnish were of Analar grade, the pH measurements being made using a PHM52 Radiometer pt~ meter. Preparation and characterization of thefurnisk; The basis of the papermaking furnish was
181 Copyright ~ 1976 by Academic Press, Inc. All lights of reproduction in any form reserved.
Journal of Colloid and InterJace Science, Vol. 55, No. 1, April 1976
182
JAYCOCK AND PEARSON TABLE I Physical Constants of Fillers Used Filler
Particle size (vm)
Ref.
H a m a k e r constant Alsl X 10 +13
Ref.
Anatase "Tioxide A-HR ''a Anatase "Tioxide A-DM TM Ruffle "Tioxide R-CR ''~ ~x-Alumina Silica Stannic oxide
0.15 0.14 0.25 0.3 0.15 0.18
(3) (3) (3) (3) (3) (3)
2.5 2.5 5.9 4.17 0.28 4.3
(4) (4) (4) (5) (6) (4)
" "Tioxide" is a registered trademark of Tioxide International Limited. prepared by beating a suspension of the bleached sulphate pulp in a single rotor blender at a consistency of 2%. The length of time the pulp is beaten determines the "freeness" of the suspension. A "free" pulp is described as being one that allows drainage of the water to occur quickly. A pulp that takes a long time to drain is termed "slow." The rate of drainage under standard conditions provides a convenient method of measuring the pulp's freeness. This measurement forms the basis of
l/WIRE ~,,.~.~.=.:~~RU BB ER t~
RIN6S
BUTI RSESS~ ~ ALU MN IEU IM JOINT '~ ~,, VAI A B FIG. 1. Glass handsheet machine. Journal of Colloid and Interface Science, Vol. 55, No. 1, April 1976
the Canadian Standard Freeness test. The pulps used were all of 450 ml CS. • The filler and aluminum sulphate were then added to the fiber suspension after it had been diluted to a consistency of 0.2% in the proportions 7.5 and 20"/0, respectively, by weight of fibers. Before and after the additions of the filler and aluminum sulphate the furnish was stirred for 5 min using only moderate agitation, so that further beating of the fibers was avoided. In some cases the filler could not be fully dispersed in water simply by stirring so dispersion was achieved by means of an ultrasonic probe. Retention measurements. Because of the unavailability of a commercial handsheet machine, a glass one was produced, which was based on a commercial design (7) (see Fig. 1). Comparison of results obtained on this machine with ones obtained using a large scale Fourdrinier machine and a commercial handsheet machine have been made, and satisfactory correlation has been obtained, as shown in Fig. 2. The glass machine consists of upper and lower compartments that are held together by means of buttress joints. Between the two compartments is situated the "wire," which consists of a disc of fine mesh placed on top of a stronger coarser mesh, the purpose of which was to give strength to the wire system. The wire is sealed in place by means of rubber rings. The purpose of the aluminum vane is to avoid swirling below the level of the wire, which causes a nonuniform sheet to be pro-
RETENTION OF PIGMENT duced. The lower chamber is attached to a Buchner Flask, and in the present work complete drainage of the sheet at 450 CS could be achieved at atmospheric pressure. To operate the machine, first water is passed into the lower compartment via tap A with tap B closed until the water level is just above the wire. After closing tap A the papermaking stock is placed in the upper compartment and stirred. On opening tap B the stock drains through the wire leaving the sheet deposited on it. After drying, the sheet is weighed and either it or the white water is analyzed for filler content. Unless otherwise stated the paper was made at a consistency of 0.20-/0 and a basis weight of 80 g m -2. Analysis of filler retained. The analysis of fillers present in paper is usually performed gravimetrically by ashing the paper and weighing the ash. This method, apart from being time consuming, needs fairly large amounts of paper before it is at all accurate and also the technique needs to be carefully carried out. Because of this it was decided to determine the amount of filler present in the "white" water, which is the water that drains through the wire while the sheet is being formed. This was achieved by measuring the turbidity of the white water using a spectrophotometer, after filtration to remove any fiber fines or large dirt particles. This technique was tested by measuring the turbidity of standard sols after they had been focculated and redispersed. In some cases, especially at low pH, the white water contained mostly filler flocs, which were redispersed using the ultrasonic probe, adjustments of p H being made where necessary. The only difficulty with this method would arise if the amount of aluminum sulphate present in the stock was high, when flocculation would be such as to prevent redispersion, however in the present work this is not the case, the molar concentration being only of the order of 7 X 10-~ M. Measurement of zeta-potentials. The zeta (~-) potentials of the fibers were initially determined using the streaming potential technique,
L
183
7.1xlO-SM
5o
AI2{S0/)3 z
=
3o
pH
Fio. 2. Comparison of anatase retention results on -O- a Fourdrinier machine, - A - a commercialhandsheet machine and -I-q- a glass handsheet machine.
the cell used being an all glass modification of the design of Goring and Mason (8). The electrodes were constructed from platinum mesh coated with platinum black. These were fused into glass plungers, which were perforated at the ends where they supported the electrodes. This support was to prevent electrode deformation during pad compression, while still allowing liquid to pass through the cell. The cell was contained in an air thermostat operating at 25 4- 0.1°C. The potentials were measured with a Keithley 610C electrometer fitted with digital output, and the pad conductivities with a Wayne Kerr B221 conductivity bridge. The ~'-potentials were calculated using the logarithmic function of Chang and Robertson (9). Recently it has been postulated by Strazdins (10) that the f-potentials of the fiber fines that originate from the fiber surface are representative of the ~'-potential of the surface as a whole, this having been substantiated by us in recent work (25). To measure the ~'-potentials of the fines, the microelectrophoretic technique was used, which was also used to measure the f-potentials of the filler particles. The electrophoretic cell used was rectangular in shape, and constructed from silica. It had a Journal of Colloid and InterJace Science, VoL 55, No. 1, April 1976
184
JAYCOCK AND PEARSON
height to depth ratio of 8 to 1, and was viewed horizontally along its central axis. It was fitted with ground silica sockets into which the electrode systems were fitted. Each of the electrodes was made of two platinum wires separated by a glass partition. Each of the wires was coated with platinum black to minimize the effects of polarization. This was augmented by employing the double electrode system in the following way. An electric potential was passed across the outer pair of platinum wires and measured across the inner pair by means of a high impedance digital voltmeter. The effective cell length was calculated by measuring the conductivity of the cell filled with 0.01 N KC1 solution, using a Wayne Kerr B221 conductivity bridge. The cell was immersed in a water bath mounted on the microscope which was supplied with water at 25°C from a thermostat bath. The cell was tested by measuring the velocity parabola for a 10-4 M AgI sol. The mobilities of the fiber fines were measured by allowing the fiber suspension to settle out until all the large fibers had sedimented to the bottom of the container. A sample of the top layer of liquid was then removed and transferred to the electrophoresis cell by means of a pipette. It should be pointed out that the beating action of the blender is such that some cutting of the fibers takes place, which gives rise to extremely small fibers that have the same radius as the original fibers. It is these rather than the colloidal material that are measured in the electrophoresis experiments. THEORETICAL
Electrophoresis and streaming potential. For large particles, whatever their shape or orientation, the mobility u, calculated from electrophoretic measurements can be converted to the t-potential using the equation of Smoluchowski (11)
to convert the mobilities to ~--potentials for the fibers because of their size. If it was the colloidal material that was measured, then this could not be used. This may account for the discrepancy some authors have found between results obtained from streaming potential and microelectrophoretic data (12, 13). The ~'-potentials of the fibers were also obtained from streaming potential data using the logarithmic equation of Chang and Robertson (9), which can be written in the form
EX~L/(P~) = A~e-Bc/(4r),
[-2-]
where E is the measured streaming potential observed when a pressure difference P is applied across a pad of length L, area A, concentration c and conductance ~. In this equation, B is a constant. Recently, this equation has been criticized by Ciriacks and Williams (14) using results on Dacron fibers. There is no experimental evidence on cellulose fibers to support this work and the results obtained from the Chang and Robertson equation were both realistic and in good agreement with the results of other workers (10, 15). For spherical particles the most comprehensive treatment has been given by Wiersema, Loeb, and Overbeek (16). For small values of ~" the equations of Overbeek (17) give similar results. For fairly simple electrolytes, the data of Wiersema et al. is tabulated; however, for more complicated electrolytes, i.e., A12(SO4)3, they are not, so in this case the equation of Overbeek has to be used. For an unsymmetrical electrolyte this has the form ~ { u = -fl(Ka) 6r,
et (z_ - z , ) - - f2(Ka) kT
(z+p+ + z_p_) kT
(e~/kT) 2f4(Ka) (z+ + z_)e 6~r~e
I
[-3.]
where Kis the reciprocal double layer thickness, a the radius of the particle, z. the valencies, u = e~'/(4r~), 1-17 p . the friction factors of the ions involved, where , is the dielectric constant and n is the k the Boltzmann constant, T the absolute viscosity of the medium surrounding the temperature, e the electronic charge, and f l to particle. This equation can be used, therefore, f4 are functions of ~a tabulated by Overbeek. Journal of Colloid and Interface Science, Vol. 55, No. 1, April 1976
RETENTION OF PIGMENT The comparison of the results of streaming potential measurements and electrophoretic measurements on fiber fragments is shown in Fig. 3. The results agree to within the limits of experimental error, and consequently, since the electrophoretic method yielded results more rapidly, the ~'-potentials reported elsewhere in this paper are derived by this method. Calculation of the potential energy of interaction. The total potential energy of interaction VT is calculated by summing the potential energies of attraction VA, which arises due to van der Waals type forces and of repulsion Vn, which results from the interaction of electrical double layers. Because the fibers are large compared with the filler particles the interaction was assumed to be one of a spherical particle and a flat plate. This is an approximation because of the presence of fibrils on the fibers. Because the potentials involved are small, the equation of Hogg, Healy, and Fuerstenau (18) can be used to calculate Vn. This equation can be written in the form ~a(f ~2 + ;2 '2)
VR--
:
(~1 + ~-2)
In [11 +-- exp-(--KH°)l exp (--~H0)-J
+ In[-1 -- exp (--2KHo)-]} ,
it
o Streoming Pvtentie[
-Ill w
3
5
pH
7
9
I
FIG. 3. Comparison of ~'-potential measurements of cellulose fibers by -O- streaming potential and -Omicroelectrophoreticmethods. distances and the other at longer distances of separation. These equations were used because they allowed the calculation of VA for all distances of separation. Once the potential energy diagram of VT versus H0 has been constructed the stability of the system can be defined in terms of a stability ratio W, which is given by
u
W = 2
fl
ds exp (VT/kT) s 2 ,
[6]
RESULTS AND DISCUSSION [-4]
where ~'z and ~'2 are the ~'-potentials of the sphere and plate, a is the radius of the sphere, and H0 is the distance of separation between the two particles. To calculate the potential energy of attraction VA, the equations of Vincent (19, 20) were used, these equations having form --12VA = E fn(A+)HJ,
~ - - -1C
where s = r/a (where r is the minimum distance of separation). The Hamaker constant for swollen cellulose was taken as 6.20 X 10-1~ erg.
4 × {2r1
185
[-5]
where fn(AOHj is a function of the Hamaker constants Ai, of the various materials, and the geometry H~, of the system. Two functions describing H are given, one for use at short
Eject of pH on the retention o] inorganic fillers. To determine the effect of pH on the retention of the various fillers, paper was made from cellulose fibers and filler only and the retentions determined. The results are shown in Figs. 4 and 5. Figure 4 shows the results obtained for the various TiO2 fillers used, and Fig. 5 for A1203, SiO2, and SnO~. It is found for all of the fillers that the retention is low until the pH is taken below a value of 4.0. The three TiO2 fillers gave very similar results, which are also similar to SnO~. The results for A1203 and SiO2 increase over the same range, but are not retained to such an extent as the other fillers. If the retention is governed by heteroflocculation of filler particles and fibers, then it will Journal of Colloid and Interface Science, Vol. 55, No. 1, April 1976
186
JAYCOCK AND PEARSON
3O
20
10
o
#
Fie. 4. Effect of pH on the retention of - O - Anatase "A-HR," -•- Anatase "A-DM," and -El- Ruffle "R-CR" in the absence of aluminum sulphate.
%
30
2D
t
L
i
i
i
2
J
~.
5
6
pH
Fro. 5. Effect of pH on the retention of - O - Alumina, - A - Silica and -IS1- SnO2 in the absence of aluminum sulphate. Journal of Colloid and Interface Science,
Vol. 55, No. I, April 1976
be controlled by the ~'-potentials of both of these components. The iso-electric points of the fibers and various fillers are shown in Table II. These are compared to literature values where possible. I t would seem from this data that increased retention should not occur over the same range of p H for each of the fillers. For instance, for the case of alumina, the fibers and filler particles are oppositely charged or have the same small charge until the furnish is made more alkaline than about 9.5, so increased retention should be observed over all this range. The results shown in Table II, however, do not take into account any effect that the fiber fines or other colloidal or ionic material that originates from the fibers may have on the ~'-potentials of the various fillers. To find out what this effect would be the variation of ~'-potential of the fillers was measured as a function of p H in the presence of the fiber fines, typical results being shown in Fig. 6. This figure shows results obtained for A1203 and A - H R , the other fillers giving results which are very similar to each other. I t can be seen that the iep is moved towards a value of about 2.7. Using these results along with those for the cellulose fibers, which are also shown on Fig. 6, the barriers to heteroflocculation can be calculated, and these converted to stability ratios, the values of which are shown as a function of p H for the heteroflocculation of cellulose fibers with A - H R by the dotted line on Fig. 6. Similar curves are obtained for the other fillers. I t is found that heteroflocculation of the fibers and filler particles is predicted below a p H of 3.8, which is the exact region over which increased filler retention is observed. I t is found, as shown by Fig. 4, that only about 40% of the filler is retained in the sheet. Electronmicrographs indicate that the relative coverage of the fibers with filler is low, and that a large amount of uncovered surface remains. This may be due to one of two reasons, or a combination of both. First, in describing a stability ratio W that determines the state of heteroflocculation of the system, we must take into account the two homoflocculation
RETENTION OF PIGMENT TABLE II Isoelectric Points of Fibers and Fillers Used Material
iep
Bleached sulphate pulp Anatase (A-HR) Anatase (A-DM) Ruffle (R-CR) a-Alumina Silica Stannic oxide
2.5 3.8 3.8 6.2 9.0 ~2.0 5.8
Literature value
Ref.
6.7 9.3 "~2.0 4.5~ 7.3
(21) (22) (22) (22)
processes that may also occur. The most important will obviously be the homoflocculation of filler particles producing aggregates with a smaller diffusion rate, and would have to rely on mechanical filtration as a retention mechanism. Previous work has indicated that mechanical filtration is an unlikely mechanism to explain retention due to the size of the pores in the paper compared with the size of the filler particles or flocs. Because of this any flocs of filler particles produced are unlikely to be retained, this is substantiated by electron micrographs, which show the filler to be present largely as single particles. The second argument why 100% of the filler is not retained is that the flocs produced from the fibers and filler particles may be "soft," i.e., it is easily dispersed by hydrodynamic shear, which occurs during the formation of the sheet. This hypothesis has been supported by work by Britt (23) who determined that the flocs could be dispersed by mechanical shear. There is also the problem that filler focculated with fiber fines is found in the white water and is not included as a part of the percentage retained, whereas it is in fact heterofloeculated. This is not thought to be a major factor. The relative amount by which each of the fillers is retained will probably be a function of the value of the stability ratio of heteroflocculation to the value for the homoflocculation of filler particles. This point is difficult to quantify at present because of the effect of fiber fines upon the f-potentials of the fillers and the resultant particle geometry. The
187
calculated stability ratio for a fines covered system would b'e essentially the same as for the fibers themselves. What would determine the ratio of homo- to heteroflocculation in systems of the type described in this paper will be the diffusion and flocculation rates of the components, together with the stability of the various flocs to hydrodynamic shear. These aspects are the basis of further study at present being undertaken. The relative stability ratios of fines coated anatase A - H R for homoflocculation and for the heteroflocculation calculated using the mean particle radius can be seen by comparing Figs. 6 and 7, and indicate that at pH values between approx 4.2 and 8 heteroflocculation will be the major factor. Similar calculations have not been attempted for the other pigments because their particle sizes are not known with the same precision as for anatase A - H R and the particle radius is the major parameter in the calculation.
The effect of aluminum sulphate on the retention of inorganic fillers. The effect of 2~o aluminum sulphate on the retention of in-
.20 ~
,,-,R''
.lO
'~
~I0 ~--i---'/-
-I0
'I00
-20 -30 -Z,O
y, l -60
i
2
3
k
5
5
pH
FIG. 6. Variation of the ~'-potential with p H for both - O - Anatase " A - H R , " - / k - Alumina in the presence of cellulose fibers, and - 0 - fibers alone. T h e dashed line shows the stability ratio for homoflocculation of anatase " A - H R . " Journal of Colloid and Interface Science, Vol. 55, No. 1, April 1976
188
JAYCOCK AND P E A R S O N LO
I
"Si 02" lO0
30 lO
2C
I0 l LD
I00
30 20
g -/.0
I00
20 l0
l
j~,~a~
L0
/
"A-HR"I00
30 2O
l0
___J __
t__ pH
FIG. 7. Effect of pH on the retention of (i) Silica, (ii) Ruffle " R - C R , " (iii) Alumina, and (iv) Anatase " A - H R " in the presence of 2% (7.1 X 10-5 M) aluminum sulphate. The dashed line shows the stability ratio for heteroflocculation.
organic in Fig. R-CR, similar
fillers is shown by the results presented 7, for the retention of A - H R , A1203, and SiO~. It is found in all cases that results are obtained. For convenience
sake the curves can be split into two halves. Below a p H of 4 the retention increases in a similar manner to the previous section, i.e., the effect of pH. Above a p H of 4 instead of falling to a low value the retention is increased reaching a maximum between p H values of 6 and 8. The retention is then found to drop to a low value around a p H of 8.0 to 8.5. Once again the remarkable fact is that all the fillers give results that are so very similar. The effect of multivalent ions on cellulose fibers is to adsorb on the surface and modify the electrokinetic potential, however, the extent to which modification takes place is not fully understood. For instance, Carolane (24) found that the ~'-potential remained negative for various kinds of pulps for alum concentrations up to 1 g/l, which is equivalent to about 10.3 M. This would substantiate our results, shown in Fig. 8. Using a 0.2% suspension, the effect of different aluminum sulphate concentrations on the f-potential versus p H relationship was found. The f-potential is not made positive at concentrations of aluminum sulphate of 10.4 M and less. On further addition precipitation takes place, which hinders measurements. It is found that the largest modification of the f-potential occurs over the p H range 4 to 9. As the concentration of aluminum sulphate is increased, a kink develops in the
.lO
_-l
-20
-30
-LO
pH
FIG. 8. Variation of the ~'-potential with p H of cellulose fibers in the presence of aluminum sulphate. - O - [-AI~(SO4)~] = 0; A - 1.015 X 10-4 M ; - v ] 8.46 X 10.6 M ; - 0 1.012 X 10-6 ~k/. Journal of Colloid and Interface Science, Vol. 55, N o . 1, A p r i l 1976
RETENTION OF PIGMENT curve between a p H of 5 and 6 that corresponds to the p H where maximum aluminum adsorption occurs, Fig. 9 (25). The adsorption of hydrolyzable ions onto the surface of metal oxides has undergone much work and a thorough study has recently been published by James and Healy (26). To see what effect aluminum sulphate has on the fillers in the presence of the fibers the ~'-potential of the filler particles was measured as a function of p H using the paper stock containing the 2% alum, the results being shown in Figs. 8 and 10. Once again, results are presented for A - H R , A1203, R - C R , and SiO2. I t is found for each of the four fillers that the curves are similar, and are also similar in shape to those obtained for the effect of aluminum sulphate on the ~'-potential of the cellulose fibers. Once again greatest modification occurs over the p H range 4 to 9 as in the case of the fibers. The amount of aluminum sulphate added, arrived at by the papermakers experimentally, is just the right concentration to keep the f-potential of the filler particles very nearly zero over a large p H range. I t is very difficult to measure the ~--potentials of the fillers in the presence of fiber fines
% ,-7
20
c2n E
<
8
7o
v-ta2 3
L,
5
6
pH FIo. 9. The adsorption isotherm for a 2% dispersion .of cellulose fibers from an initial 10-4 M aluminum sulphate solution.
189
"s20/2
0
QI
"R- CR"
-20I -L0t
g- 0L- _ ~
'~,~0f_ _
-2(1 -L0 0[ -- ---- "
A
-
H
-2Di -t,Oi 2
.3 . . C. . 5
fi
6
')' - - ~ -
9'
]'6
Fro. 10. Variation of the ~'-potential with pH of various fillers in the presence of 2% (7.1 X 10-5 M) aluminum sulphate and cellulose fiber fines. (i) Silica; (ii) Ruffle "R-CR;" (iii) Alumina; (iv) Anatase "A-HR."
in the furnish, but with extreme perseverance it can be accomplished, the results being shown in Fig. 10. It can be seen that the curves for each of the fillers are similar in shape, suggesting that the coating of filler particles with fiber fines is fairly complete. Using these results the potential energy barriers to flocculation can be found, and these once again converted to stability ratios, which are shown as a function of p H for each of the four fillers by the dotted curves in Fig. 7. The proportion of fiber fines retained has not been measured because of experimental difficulties. I t is of interest to note that when the aluminum sulphate is added to the furnish flocculation with or adsorption of the fines occurs first of all, followed by adsorption of the aluminum species. It can be seen from the dotted line in Fig. 7 that the region of predicted heteroftocculation coincides with the region of increased filler retention. Journal c)f['olloid and Interface Science,
Vol. 55, No. 1, April 1976
190
JAYCOCK AND PEARSON
T h e introduction of a sizing agent into the system results in a positively charged aluminum rosinate precipitate being produced t h a t acts similarly to the aluminum species discussed here, and gives the same end result, t h a t is it increases the retention of the fillers b y adsorbing onto the filler particles and cellulose fibers, followed b y heteroflocculation of the system (27). ACKNOWLEDGMENT Tioxide International Ltd. are thanked for the financial support which allowed this work to be carried out, and for the results on the commercial handsheet machine. Mr. R. Counter of the same Company is warmly thanked for invaluable discussion. REFERENCES I. JA¥COCK,M. J. ANDPEARSON,J. L., J. Appl. Chem. Biotech. to appear. 2. JAYCOCK,M. J. ANDPEARSON,J. L., Sven. Papperstidn. 78, 289 (1975). 3. TIOXIDEINTERNATIONALLTD., Private communicat.ion. 4. FOWK~S, F. M., in "Surfaces and Interfaces," Vol. I. (Burke, Ed.), p. 199. Syracuse Univ. Press, New York, 1967. 5. KRUPP, H., SCHNABEL,W., AND WALTER, G., J. Colloid Interface Sci. 39, 421 (1972). 6. WATILLON,A. ANDGI~RARD,PH., Proc. 4th I.C.S.A., Brussels, 2, 1261 (1964).
Journal of Colloid and Interface Science.Vol. 55. No. I. April 1976
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