Method for fiber loading a chemical compound

Abstract

The present invention relates to a method for loading a chemical compound within the fibers of a fibrous material and to the fibrous materials produced by the method. In the method, a fibrous cellulose material is provided which consists of a plurality of elongated fibers having a fiber wall surrounding a hollow interior. The fibrous material has a moisture content such that the level of water ranges from 40-95% of the weight of the fibrous material and the water is positioned substantially within the hollow interior of the fibers and within the fiber walls of the fibers. A chemical is added to the fibrous material in a manner such that the chemical is disposed in the water present in the fibrous material. The fibrous material is then contacted with a gas which is reactive with the chemical to form a water insoluble chemical compound. The method provides a fibrous material having a chemical compound loaded within the hollow interiors and within the fiber walls of the plurality of fibers.

Description

RELATED APPLICATION

This application is a continuation-in-part of Application Ser. No. 665,464, filed Mar. 6, 1991 entitled "A Method for Loading a Chemical Compound Within the Hollow Interior of Fibers" now abandoned.

1. Field of the Invention

The present invention relates generally to a method for loading a chemical compound within the hollow interior, cell walls and on the surfaces of the fibers of a fibrous material. More particularly, the present invention is directed to an improved process for the production of filler-containing paper pulp in which the filler is formed in situ while in proximity to the paper pulp and a substantial portion of the filler is disposed in the lumens and cell walls of the cellulose fibers of the paper pulp, to the paper pulp produced thereby and to papers produced from such pulp.

2. Background of the Invention

Paper is a material made from flexible cellulose fibers which, while very short (0.02-0.16 in. or 0.5-4 mm), are about 100 times as long as they are wide. These fibers have a strong attraction for water and for each other; when suspended in water they swell by absorption. When a suspension of a large number of such relatively insoluble in water (1.2 and 1.6 grams per liter, respectively) and there is no substantial free surface moisture on the fibers, the mechanism whereby the calcium oxide is drawn into the water located in the hollow fiber interior and the fiber walls is not completely understood. Calcium oxide, however, reacts vigorously with water in an exothermic reaction to produce calcium hydroxide, enough for 100 grams of quicklime to heat 200 grams of water from 0° F. to boiling. While not wishing to be bound by any theory, it is believed that the calcium oxide reacts with water at the surface openings of the fiber to form calcium hydroxide and that the calcium hydroxide is drawn into the cell walls and hollow interior of the cellulose fibers by hydrostatic forces. For this reason, the highly reactive forms of calcium oxide (quicklime) are preferably used in the process of the invention. The less reactive forms, such as dolomitic limestone and dead burned limestone are less suitable.

The calcium oxide or calcium hydroxide may be added at any desired level up to about 50%, based on the weight of the dry cellulosic material. The lower limit for addition of the calcium oxide may be as low as desired, but is preferably not less than about 0.1%. Most preferably, the calcium oxide or calcium hydroxide is present at a level of from about 10% to about 40%, based on the weight of the dry cellulosic material. The carbon dioxide is added at a level sufficient to cause complete reaction of the chemical with the gas to form the water insoluble chemical compound. Excess gas can be used since no further reaction takes place. Since there is no extraneous chemical material formed, such as would be the case with precipitating a water-insoluble chemical compound with two water soluble salts, there is no need to wash the cellulosic material after treatment with carbon dioxide in accordance with the invention to load the fibers with the precipitated calcium carbonate. In the case of paper pulp, the paper pulp can be immediately transferred to a papermaking operation where it is formed into a slurry, refined and placed onto a Fourdrinier machine or other suitable papermaking apparatus. Alternatively, the paper pulp having the chemical compound loaded therein may be further dried and shipped as an item of commerce to a papermaking facility for subsequent usage.

It has been determined that the precipitation of calcium carbonate in cellulosic fibers containing from about 40% to about 85% of moisture (15% to 60% of fiber) and loaded with from about 10% to about 40% of calcium oxide or calcium hydroxide is easily effected in a pressurized container with low shear mixing. The carbon dioxide pressure in the container is preferably from about 5 psig to about 60 psig and the low shear mixing is preferably continued for a period of from about 1 minute to about 60 minutes.

It has also been determined that for fibers containing from about 95% to about 85% of moisture (5% to 15%) of fiber) and the same calcium oxide loading, that high shear treatment during contact with the carbon dioxide is required to cause complete precipitation of calcium carbonate. In this connection, any suitable high shear mixing device can be used. Preferably, the high shear treatment is sufficient to impart from about 10 to about 70 watt hours of energy per kilo of fiber, dry weight basis.

It has been determined that a simple way to provide contact of the carbon dioxide with the paper pulp under high shear treatment is by means of a pressurized refiner. The pressurized refiner is a well known piece of apparatus utilized in the papermaking industry and consists of a cylindrical hopper into which the paper pulp is loaded. The cylindrical hopper is gas tight and can be pressurized with a gas. A rotating shaft containing beater arms is disposed within the hopper to keep the paper pulp from matting. An auger screw is located beneath the hopper for conveying the paper pulp into the interior space between a set of matched discs. One of the discs is stationary whereas the opposing disk disc is driven by means of a motor. The discs are spaced apart by a distance sufficient to shred the pulp crumbs as the pulp passes between the stationary disk disc and the revolving disk disc. The discs may be provided with refining surfaces The use of a "devil's tooth" plate, or fiberizing plate, has also been found to be suitable. Prior to forcing the pulp into contact with the rotating plate, the carbon dioxide is pumped into the sealed hopper to pressurized pressurize the hopper with carbon dioxide and remains in contact with the pulp while the paper pulp is stirred in the hopper and while the pulp is being transported by the auger through the refiner discs.

It has also been determined that it is not possible to effect the reaction between the calcium oxide or calcium hydroxide and the carbon dioxide by blowing the carbon dioxide through the mixture of dewatered crumb pulp and the calcium oxide or calcium hydroxide.

Through an investigation of handsheets prepared in accordance with the invention, it has been determined that about 50% of the precipitated calcium carbonate is retained by the pulp fibers. The remaining 50% is recovered as white water which can be used to fill paper on the papermaking machine in accordance with conventional surface filling processes. The retained calcium carbonate is distributed approximately equally in the lumen, within the cell walls of the cellulose fibers and on the surface of the cellulose fibers. A higher level of retention is attained by precipitation of calcium carbonate in a pressurized container with low shear than through use of the pressurized refiner. The quality of handsheets prepared from pulp wherein the precipitation is effected with the pressurized refiner is, however, superior.

The following example further illustrates various features of the invention, but is intended to in no way limit the scope of the invention as set forth in the appended claims.

Materials

Pulp--The pulps used were a softwood pulp mixture and a hardwood pulp mixture that were supplied by Consolidated Paper Company and refined further in a single disk refiner to pulp freenesses of 410 and 180 (CSF) for the softwood, and 395 and 290 (CSF) for the hardwood.

Calcium reactants--Calcium oxide used was a technical grade (Fisher Chemical Company) or a high reactivity Continental lime (Marblehead Lime Co.). Reagent grade calcium hydroxide (Aldrich Chemical) was also used. For the direct loading comparison, papermaker grade calcium carbonate (Pfizer) was used.

Equipment

Mixer--A bench-model 3-speed Hobart food mixer with a 20 quart stainless steel bowl and flat beater was used for mixing the calcium reactants with the pulp.

Refiner--A Sprout-Bauer pressurized disk disc refiner was used as both the reaction chamber and refiner for precipitating calcium carbonate and incorporating it into pulp fibers.

Filtering centrifuge--This 2-speed centrifuge is equipped with a perforated vessel lined with a canvas bag to filter a continuous flow of low consistency slurries.

Bauer-McNett Fiber Analyzer--An industry standard method for determining non-leachable filler retention.

Muffle furnace--A Thermodyne furnace was used for ashing samples.

Typical Refiner Run Procedure

Hobart--For each run, 1 kg pulp (based on dry weight of fiber) was blended in the Hobart mixer with varying amounts of calcium reactant and water required for a specific chemical load and consistency. The pulp was mixed for 15 minutes at low speed (approximately 110 rpm) to uniformly incorporate the calcium.

Refiner--The high consistency pulp was then loaded into the hopper of the refiner which was closed and sealed. Carbon dioxide was injected into the hopper to react with the calcium hydroxide. Carbon dioxide was held in the tank at 20 lbs. pressure for 15 minutes. During this interval, calcium carbonate was precipitated in the pulp fibers by the reaction of calcium oxide or calcium hydroxide with the carbon dioxide. The pulp is then refined in a carbon dioxide atmosphere at the desired plate gap and feed rate to provide intimate contact of the carbonate and fibers.

Direct loading--For comparisons, pulps were loaded directly with calcium carbonate without the aid of the pressurized refiner. Pulp for direct loading was fiberized in the British Disintegrator according to Tappi Standard T-205 for 60g/m2 m² handsheet preparation and poured into the doler tank. Varying amounts of calcium carbonate was added to the low consistency pulp slurry in the doler tank and stirred to assure uniform distribution prior to making handsheets.

Centrifuging--In order to avoid the high consistency mixing step using the Hobart mixer, pulps were sometimes loaded with calcium oxide or calcium hydroxide at low consistency and then dewatered. Pulp and the calcium reactant was stirred at 2% consistency with an air stirrer for 15 minutes. The pulp slurry was the then into the filtering centrifuge to dewater the pulp to approximately 30% consistency. The pulp was removed from the centrifuge bag, shredded and loaded into the pressurized refiner for reaction with carbon dioxide.

TEST MATERIALS

Scanning Electron Microscopy (SEM)--SEM observations and X-ray microanalysis was carried out on transverse sections of pulp fibers and handsheets. Sections were hand-cut with a razor blade. The dry pulps and strips of handsheets (1 cm×0.3 cm) were cemented to aluminum stubs and sputter-coated with gold. Samples were photographed in JEOL 840 SEM at an accelerating voltage of 20 kv.

SEM X-ray microanalysis--Samples were prepared as for SEM observation, but were adhered to carbon specimen stubs and coated with a conductive carbon layer. X-ray microanalysis was performed with a Tracor Northern T-2000/4000 energy-dispersive spectrometer in combination with the scanning electron microscope. The microanalysis spectra were recorded in an energy range of 15 keV.

The specimen preparation procedures for x-ray analysis make it necessary for controls to be employed if x-ray data are to be compared with any validity. The samples of pulp and handsheets were dried at the same time, under the same conditions. This eliminates variations arising from inconsistencies in procedures. Once a sample is dried, care was taken to keep it free of moisture. The samples were not exposed to room air and not stored in a desiccator with chemical desiccants for fear of elemental contamination. All x-ray data to be compared was obtained with the same specimen current for biological x-ray microanalysis.

Carbonate Test

Pulp and handsheet specimens were placed in 1% aqueous silver nitrate for 30 minutes, rinsed in §distilled water and placed in 5% aqueous sodium thiosulfate for 3 minutes and washed in tap water (Van Kossa's method for carbonates). Carbonate groups (calcium) stain black. Rapid spot tests were run on samples to confirm the presence of carbonates.

Pulp/Paper Tests

As each filled pulp sample was discharged from the refiner, a random sample was taken for the determination of freeness, pH and ash content. Ash content of the pulp was assessed by Tappi Method T-211. Handsheets (60g/m²) were prepared from the pulp by standard Tappi Method T-205. Again, the ash content was determined on the handsheet, and the percent retention is reported as the percent filler in the handsheet based on the percent filler in the pulp (and subtracting the small blank of the pulp's original ash content). Percent retention, therefore, represents the filler retention that stays with the pulp during standard handsheet formation. Another sample of pulp from the refiner discharge was subjected to a thorough washing (20 minutes) with tap water in a chamber of a Bauer-McNett fiber fractionator and collected on a 200 mesh screen. The ash content was determined on this Bauer-McNett washed pulp sample, and is identified in the data tables as B/M ash%.

The handsheets were used for evaluation of §burst index and for the evaluation of optical properties. Burst index, as determined by Tappi Method T-403, is a convenient measure of strength and an accepted measure of fiber bonding. Densities of the handsheets were measured according to Tappi Method T-220 and appeared to correlate meaningfully with both freeness and burst index. Optical properties of brightness, opacity and scattering coefficient were determined on a Technidyne photometer. Spread sheets of all the test data obtained on the pulp and handsheets are attached in the appendix.

SEM

Initial loading experiments using CaO indicated that rhombohedral calcite crystals in the 1 to 3 micron size were attained, as evidenced by electron microscopy. Scanning electron microscopy of the cross-sections of pulp and handsheet fibers showed that calcium carbonate was precipitated as discrete angular particles, i.e., crystals. Crystalline aggregates can be seen in the lumen and on the surface. The distinctive spectrum of calcium is found within the cell-wall as well as on the fiber surface and in the cell lumen. This latter information indicates that a portion of the calcium ions can diffuse into the fiber wall as well. Calcium carbonate was confirmed to be in the lumen and on the surface of pulp and handsheet fibers.

Table 1 is a comparison of the burst and optical properties (at the same initial freeness) of refiner-run handsheets. The two numbers in parentheses, such as (15,20), indicate the pulp consistency and the calcium reactant loading, respectively. Also for comparison, are the burst and optical properties of handsheets in which the filler loading was obtained by direct addition during handsheet formation of papermaker's grade carbonate (Pfizer). The results in Table 1 are also presented in the FIGS. 1-7. If scattering coefficient, opacity or brightness are plotted versus burst index, FIGS. 1-7 points from the fiber loaded handsheets lie approximately on the same curves as the points from the direct-loaded handsheets. These plots indicate the expected inverse relationship between optical properties and strength; that is, as burst strength increases, the desirable optical properties decreases decrease. The fact that both fiber loaded handsheets and direct loaded handsheets of the invention lie on the same curves means that for any given gain in optical properties, one would expect a comparable loss in strength properties regardless of how the filler is incorporated.

TABLE 1
__________________________________________________________________________
COMPARISON OF BURST AND OPTICAL PROPERTIES
BETWEEN FIBER LOADED & DIRECT LOADED HANDSHEETS
P.   Scatt.
Brightness
Opacity
Coeff.
Density
Burst Index
Paper Ash
B/M Ash
Type       (%)   (%)  (m2 /Kg)
(kg · m3)
(KPa · m2 /g)
(%)   (%)
__________________________________________________________________________
CTRL-Bl.HWC (395)
87.7  78.5 47.7 717.7
3.14   0.24  --
46% D.CaCO3
90.6  87.2 101.6
648.4
1.12   16.25 --
36% D.CaCO3
90.3  86.2 93.0 651.6
1.26   12.35 0.35
**27% D.CaCO3
89.6  84.6 79.6 671.7
1.65   8.80  0.35
16% D.CaCO3
88.5  81.5 60.4 676.2
2.03   4.10  --
12% D.CaCO3
88.1  81.5 58.2 687.2
2.23   3.02  --
10% D.CaCO3
88.6  81.5 60.3 679.2
2.12   3.83  --
5% D.CaCO3
87.8  79.5 53.5 696.0
2.57   1.74  --
Run #214 (21.20)
89.0  82.2 64.1 722.6
1.70   9.82  4.19
Run #233 (21.20)
88.8  82.5 63.9 750.8
1.92   10.48 5.34
Run #243 (21.20)
88.7  82.2 62.6 741.1
1.86   9.38  3.80
Run #245 (21.20)
88.7  82.4 64.0 738.5
1.81   9.51  3.30
Run #275 (21.20)
88.6  82.2 63.1 737.1
1.78   9.16  3.34
Run #265 (21.20)
88.7  83.0 66.7 727.2
1.71   10.17 3.77
Run #213 (18.20)
88.8  82.2 64.3 736.3
1.80   10.04 3.59
Run #217 (18.30)
90.0  84.5 78.9 719.2
1.27   15.39 5.22
Run #211 (15.20)
88.8  82.7 65.1 712.6
2.10   10.58 3.54
Run #218 (18.10)
87.8  79.8 53.2 720.7
2.34   5.11  2.69
__________________________________________________________________________

FIG. 4 is a plot of burst index versus ash content. The direct loaded handsheets lie on a smooth curve; again demonstrating that as the ash content increases, the burst strength decreases. The points from the fiber-loaded handsheets are plotted in the same figure and all of the fiber-loaded handsheets lie considerably above the direct-loaded curve. This means that at comparable ash contents, the fiber-loaded §handsheets of the invention are considerably stronger. The converse also holds true, as seen in FIGS. 5-7, when optical properties are plotted versus ash content. At equal ash content, the direct-loaded handsheets exhibit better optical properties than the fiber-loaded handsheets of the invention.

Conclusions

It has been demonstrated that fiber loading with calcium carbonate can be accomplished by an in situ reaction between calcium oxide (or hydroxide) and carbon dioxide in high consistency dewatered crumb pulps. A pressurized Sprout-Bauer disk disc refiner adequately serves as both reaction chamber and as a means for obtaining a good dispersion of filler and fiber. SEM examination has revealed the presence of calcium carbonate crystals on both external fiber surfaces and within the cell lumen; and x-ray microprobe analysis indicates the presence of calcium within the cell wall. Optimum conditions for fiber loading using the pressurized refiner occur at pulp consistency of 18% for softwood pulp and 21% for hardwood pulp.

In some respects, handsheet properties prepared from fiber-loaded pulp outperformed out performed direct loaded handsheets. When compared at equal filler content and equal freeness, the fiber-loaded handsheet exhibited greater bursting strength. This indicates that comparable burst strength can be obtained at higher ash content for handsheets made from fiber loaded pulp than handsheets made from direct loaded pulp. Also, at the same burst strengths, similar optical properties are obtained. This permits lower cost calcium carbonate to be substituted for higher cost fiber at no loss in burst or optical properties. This is a potential large saving in papermaking costs.

At equal ash contents, the poorer optical properties in comparison to the direct loaded sheet is partly understandable because the papermakers' carbonate was specifically designed in terms of crystal morphology and particle size to achieve maximum scattering power. In addition, filler in close contact with cell-wall material (as for example inside cell lumen) may inherently scatter less because the difference in refractive index between filler and cell-wall material is smaller than the difference in refractive index between filler and air.

Claims

1. A method for loading cellulosic fibers with calcium carbonate comprising:

(a) providing a cellulosic fibrous material comprising a plurality of elongated fibers having a fiber wall surrounding a hollow-interior, said fibrous material having moisture present at a level sufficient to provide said cellulosic fibrous material in the form of dewatered crumb pulp;

(b) adding a chemical selected from the group consisting of calcium oxide and calcium hydroxide to said pulp in a manner such that at least some of said chemical becomes associated with the water present in said pulp; and

(c) contacting said cellulosic fibrous material with carbon dioxide while subjecting said cellulosic fibrous material to higher high shear mixing so as to provide a cellulosic fibrous material having a substantial amount of calcium carbonate loaded within the hollow interior and within the fiber walls of the plurality of cellulosic fibers.

2. A method in accordance with claim 1 wherein the moisture content of said fibrous material is from about 40% to about 95% by weight.

3. A method in accordance with claim 1 wherein said chemical is added at a level of from about 0.1% to about 50% by weight based on the dry weight of said fibrous material.

4. A method in accordance with claim 1 wherein said chemical is added at a level of from about 5% to about 20% by weight based on the dry weight of said fibrous cellulose material.

5. A method in accordance with claim 1 wherein said contact with carbon dioxide is effected in a closed container pressurized with carbon dioxide gas.

6. A method in accordance with claim 5 wherein said carbon dioxide gas pressure is from about 5 psig to about 60 psig.

7. A method in accordance with claim 5 wherein said carbon dioxide is maintained in contact with said pulp for a period of from about 1 minute to about 60 minutes.

8. A method in accordance with claim 1 wherein said high shear mixing is sufficient to impart from about 10 to about 70 watt hours of energy per kilo of fiber, dry weight basis.

9. A method in accordance with claim 1 wherein said higher high shear mixing is effected by means of a pressurized paper refiner.

10. A method in accordance with claim 9 wherein said refiner is provided with devil's tooth refining blades.

11. A method for making a filled paper from cellulose fibers having tubular walls and lumens which contain precipitated calcium carbonate comprising:

(a) providing cellulose fibers containing water;

(b) adding a chemical selected from the group consisting of calcium hydroxide and calcium oxide to the cellulose fibers;

(c) contacting said fibers with carbon dioxide gas while subjecting said fibers to high shear mixing so that there is a reaction with the chemical to form precipitated calcium carbonate both in the interior of the fibers and in the fiber walls; and

(d) forming paper from said fibers.

12. A method in accordance with claim 11 wherein the water is present at a level of from about 40% to about 95% based on the dry weight of said cellulose fibers.

13. A method in accordance with claim 11 wherein said chemical is added at a level of from about 0.1% to about 50% by weight based on the dry weight of said cellulose fibers.

14. A method in accordance with claim 11 wherein said chemical is added at a level of from about 5% to about 20% by weight based on the dry weight of said cellulose fibers.

15. A method in accordance with claim 11 wherein said contact with carbon dioxide is effected in a closed container pressurized with carbon dioxide gas.

16. A method in accordance with claim 15 wherein said carbon dioxide gas pressure is from about 5 psig to about 60 psig.

17. A method in accordance with claim 15 wherein said carbon dioxide is maintained in contact with said pulp for a period of from about 10 minutes 1 minute to about 60 minutes.

18. A method in accordance with claim 11 wherein said high shear mixing is sufficient to impart from about 10 to about 70 watt hours of energy per kilo of fiber, dry weight basis.

19. A method in accordance with claim 11 wherein said high shear mixing is effected by means of a pressurized paper refiner.

20. A method in accordance with claim 19 wherein said refiner is provided with devil's tooth refining blades.

21. A method for loading cellulosic fibers with calcium carbonate comprising:

(a) Providing a cellulosic fibrous material comprising a plurality of elongated fibers having a fiber wall surrounding a hollow interior, said fibrous material containing from about 40% to about 85% moisture, and having the form of dewatered crumb pulp;

(b) adding a chemical selected from the group consisting of calcium oxide and calcium hydroxide to said pulp in a manner such that at least some of said chemical becomes associated with the water present in the hollow interior of said pulp said chemical being added at a level of from about 0.1% to about 50% by weight based on the dry weight of said fibrous material; and

(c) contacting said cellulosic fibrous material with carbon dioxide so as to provide a cellulosic fibrous material having a substantial amount of calcium carbonate loaded within the hollow interior and within the fiber walls of the plurality of cellulosic fibers, said contact with said carbon dioxide being effected in a closed container pressurized with carbon dioxide gas. 22. A method in accordance with claim 21 wherein said chemical is added at a level of from about 5% to about 20% by weight based on the dry weight of said fibrous cellulose material.

23. A method in accordance with claim 21 wherein said carbon dioxide gas pressure is from about 5 psig to about 60 psig. 24. A method in accordance with claim 21 wherein said carbon dioxide is maintained in contact with said pulp for a period of from about 1 minute to about 60 minutes. 25. A method in accordance with claim 21 wherein said contact with carbon dioxide is effected while subjecting said cellulosic material to low shear mixing. 26. A method in accordance with claim 21 wherein said cellulosic fibrous material is subjected to high shear mixing after contacting with carbon dioxide. 27. A method in accordance with claim 26 wherein said high sheer mixing is sufficient to impart from about 10 to about 70 watt hours of energy per kilo of fiber, dry weight basis. 28. A method in accordance with claims 26 wherein said high sheer mixing is effected by means of a pressurized paper refiner. 29. A method in accordance with claim 28 wherein said refiner is provided with devil's tooth refining blades. 30. A method for making a filler paper from cellulose fibers having tubular walls and lumens which contain precipitated calcium carbonate comprising:

(a) providing cellulose fibers containing from about 40% to about 85% moisture;

(b) adding a chemical selected from the group consisting of calcium hydroxide and calcium oxide to the cellulose fibers said chemical being added at a level of from about 0.1% to about 50% by weight based on the dry weight of said fibrous material;

(c) contacting said fibers with carbon dioxide gas so that there is a reaction with the chemical to form precipitated calcium carbonate in the interior of the fibers, in the fiber walls, and on the surface of the fibers, said contact with said carbon dioxide being effected in a closed container pressurized with carbon dioxide gas; and

(d) forming paper from said fibers. 31. A method in accordance with claim 30 wherein said chemical is added at a level of from about 5% to about 20% by weight based on the dry weight of said cellulose fibers. 32. A method in accordance with claim 30 wherein said carbon dioxide gas pressure is from about 5 psig to about 60 psig. 33. A method in accordance with claim 30 wherein said carbon dioxide is maintained in contact with said pulp for a period of

from about 1 minute to about 60 minutes. 34. A method in accordance with claim 30 wherein said contact with carbon dioxide is effected while subjecting said cellulosic material to low shear mixing. 35. A method in accordance with claim 30 wherein said cellulosic fibrous material is subjected to high shear mixing after contacting with carbon dioxide. 36. A method in accordance with claims 35 wherein said high shear mixing is sufficient to impart from about 10 to about 70 watt hours of energy per kilo of fiber, dry weight basis. 37. A method in accordance with claims 35 wherein said high shear mixing is effected by means of a pressurized paper refiner. 38. A method in accordance with claim 37 wherein said refiner is provided with devil's tooth refining blades.