WO1994017243A1

WO1994017243A1 - Retention aids for mechanical pulps

Info

Publication number: WO1994017243A1
Application number: PCT/CA1994/000021
Authority: WO
Inventors: Robert Pelton; Archie E. Hamielec; Huining Xiao
Original assignee: Dorset Industrial Chemicals Ltd.
Priority date: 1993-01-19
Filing date: 1994-01-18
Publication date: 1994-08-04
Also published as: EP0680532A1; FI953468A0; FI953468A; GB9300939D0; CA2153997A1; JPH08504900A

Abstract

High molecular weight copolymers of molecular weight at least about 100,000 and comprising a water-soluble polymer backbone, generally polyacrylamide, and pendant poly(alkylene oxide) groups, generally poly(ethylene oxide) groups in an amount of about 0.15 to about 3 mole %, are used as retention aids in the formation of wood pulp sheet, particularly mechanical pulp, along with conventional phenolic compounds, in place of very high molecurar weight poly(ethylene oxide) polymers. The copolymers are much less sensitive to degradation and loss of effectiveness than the poly(ethylene oxide) polymers. The retention aid copolymers may be prepared by copolymerization of monomers or by graft polymerization of poly(ethylene oxide) polymer onto the polyacrylamide backbone, using radiation.

Description

TITLE OF INVENTION RETENTION AIDS FOR MECHANICAL PULPS

FIELD OF INVENTION The present invention relates to the provision of certain novel retention aids for mechanical pulps. BACKGROUND TO THE INVENTION In the papermaking process, wood pulp comprising a highly dilute dispersion of wood fibers is placed onto a forming wire on which liquid is drained from the fibers to form a fibrous mat which then is processed further to form a paper sheet. During the drainage of the liquid from the pulp fibers, useful particulate components of the pulp, such as fines, fillers and pigments, tend to pass out of the sheet and to be lost from the sheet into the filtrate.

Retention problems in the manufacture of fine papers have been alleviated, in part, by the use of polymeric flocculants, in the form of cationic polymers. However, such retention aids are ineffective in achieving fines retentioit in newsprint. To alleviate this problem, it has been proposed in U.S. Patent No. 4,313,790 to use water-soluble very high molecular weight polyoxy- ethylenes, preferably of molecular weights approaching 10 million, as retention aids. Such materials are used in conjunction with phenolic compounds, generally phenolic resins.

While such polymers provide improved retention in newsprint, nevertheless they suffer from several drawbacks. Such very high molecular weight polymers are expensive chemicals and because of their high molecular weight are difficult to dissolve in the pulp suspension. The polymers tend to be fragile chemicals, both mechani-cally and chemically, tending to breakdown into smaller molecules under the operating conditions of pulp formation, thereby losing their effectiveness. The sensitivity of the chemicals to time, shear and metal

SUBSTITUTE SHEET ions dictates that solutions be freshly-prepared on site at the pulp mill.

SUMMARY OF INVENTION The present invention provides a novel chemical retention aid for mechanical pulps which both is effective in achieving retention of fines and yet does not suffer the drawbacks of the prior art. The compounds utilized herein can achieve at least the same efficiency of fines retention in mechanical pulps as conventional high molecular weight polyoxyethylenes, but are stable and do not degrade as the prior art molecules do. Such retention aid comprises a water soluble copolymer having a molecular weight of at least about 100,000 and comprising a high molecular weight water-soluble polymer backbone having pendant poly(oxyalkylene) groups.

Accordingly, in one aspect of the present invention, there is provided a retention aid composition for use in the formation of a paper sheet from wood pulp, particularly mechanical pulp, comprising: (a) a water soluble copolymer having a molecular weight of at least about 100,000 and comprising a high molecular water-soluble polymer backbone having pendant poly(oxyalkylene) groups, and (b) a phenolic compound. In another aspect of the present invention, there is provided an improvement in a method of forming a paper sheet wherein a slurry of cellulosic fibrous material, particularly mechanical wood fibers, is dewatered and a retention aid composition is added to the slurry to improve the retention of fines, fibers and pigments in the paper sheet during the dewatering step. The improvement comprises utilizing, as a component of the retention aid composition, a water-soluble copolymer having a molecular weight of at least about 100,000 and comprising a high molecular weight water-soluble polymer backbone having pendant poly(oxyalkylene) groups. The copolymer employed herein may be formed from polymeric materials by grafting, preferably using radiation. Accordingly, in a further aspect of the invention, there is provided a method of forming a copolymer which comprises grafting pendant groups of poly(oxyalkylene) onto a high molecular weight polymer backbone, preferably by irradiation.

BRIEF DESCRIPTION OF DRAWINGS Figures 1 and 2 contain a graphical representation of the effect of the quantity of retention aid on retention ability, determined by flocculation ability as described in the Examples below;

Figure 3 contains a graphical representation of the effect of copolymer composition on retention ability, determined by flocculation ability as described in the Examples below;

Figure 4 contains a graphical representation of the effect of polyethylene oxide chain length on retention ability, determined by flocculation ability as described in the Examples below;

Figure 5 contains a graphical representation of the effect of aging on retention ability, determined by flocculation ability as described in the Examples below; Figure 6 contains a graphical representation of the effect of the composition of copolymer on retention ability, as determined by DDJ as described in the Examples below;

Figure 7 contains a graphical representation of the effect of PEO side chain length on retention ability, as determined by DDJ as described in the Examples below;

Figure 8 contains a graphical representation of the effect of shear on retention ability, as determined by DDJ as described in the Examples below; and

Figure 9 contains a graphical representation of the retention ability of copolymers prepared by polymer-to- polymer grafting, as determined by flocculation ability, as described i .the Examples below. GENERAL DESCRIPTION OF INVENTION The chemical retention aid provided herein employs a backbone polymer chain which is water soluble and of very high molecular weight onto which are provided many side chains of low molecular weight polyoxyethylene or other low molecular weight poly(oxyalkylene) so as to assist adhesion of the fines particles to the fibers during drainage of the pulp mat in sheet formation.

The backbone polymer chain has a molecular weight of at least about 100,000, generally in excess of about 500,000 and preferably at least about 3 million. In general, higher molecular weights lead to a higher efficiency of fines retention. The upper limit of molecular weight is largely determined by the ability to dissolve the polymer in the water.

The backbone polymer chain preferably is provided by polyacrylamide. However, any other water soluble, high molecular weight polymeric material may be used to provide the backbone polymer, such as water-soluble cellulose derivatives, polyvinyl alcohol, starch, poly(vinyl pyrrolidone) and poly(vinyl sulfate) . The invention is described hereinafter particularly with reference to polyacrylamide as the backbone.

The polyacrylamide backbone polymer chain is inexpensive to produce, is readily made and is much less sensitive to shear and metal ions than the high molecular weight polyethylene oxides conventionally employed, as may be seen in the experiments reported in the Examples below. The polyacrylamide has a high molecular weight, generally at least about 100,000, as noted above. The polyacrylamide polymer chain may be a homopolymer of acrylamide or lower alkylacrylamide, such as ethacrylamide or isopropylacrylamide, or a copolymer of two or more of such acrylamides. The polymer chain generally is provided as a linear chain, but may comprise a branched chain or may be cross-linked. The pendant groups generally comprise polyoxyethylene groups, although they may comprise polyoxypropylene groups, as well as copolymer mixtures or blocks of polyoxyethylene and polyoxypropylene. The polyoxyalkyl groups may be provided pendant from the backbone polymer chain in any convenient manner.

One such procedure involves the copolymerization of acrylamide or other backbone monomer with an unsaturated macromonomer, usually a poly(oxyalkyl) substituted acrylate or methacrylate. Such unsaturated macromonomer may have the formula:

R

CH₂=C

0=C - [CH₂CH₂0]_n-R where R and R' are independently -H or -CH₃ and n is the number of oxyethylene groups in each of the pendant chains, generally at least about 5. Such polymerization may be effected by free-radical polymerization employing any convenient initiator.

The pendant low molecular weight polyethylene oxide side chains also may be provided by graft polymerization of poly(alkylene oxide) polymer onto the polyacrylamide backbone or backbone of other high molecular weight polymer. For this purpose, a mixture of the low molecular weight polyethylene oxide, generally one having the formula:

wherein R, R' and R" are independently CH₃ or H and z is the number of pendant groups and is at least about 5, preferably at least about 10, and high molecular weight polyacrylamide, generally having a molecular weight of at least about 100,000, preferably at least about 500,000, more preferably at least about 1,000,000 and most preferably at least about 3,000,000, may be subjected to irradiation, such as by gamma radiation, to effect graft polymerization of the polyethylene oxide onto the backbone at random locations. Alternatively, chemical grafting agents, such as eerie ions or peroxides, may be employed with the mixture of polyethylene oxide and polyacrylamide.

The overall quantity of polyoxyethylene in the pendant side chains generally comprise at least about 0.1 mole % of the overall molecule and may vary up to a large value. However, it is usually preferred for the overall quantity of polyethylene to be about 0.15 to about 3 mole %.

Although the pendant poly(ethylene oxide) side chains provided in the copolymers utilized herein may still exhibit some sensitivity, the presence of a large number of much smaller chains decreases significantly the adverse effect of decomposition of polyoxyethylene in comparison to the conventional high molecular weight polyethylene material. This leads to the significant improvement in stability established by the copolymers used herein in comparison to the prior art high molecular weight polyethylene oxides, while achieving comparable efficiencies of fines retention.

A polymeric retention aid in accordance with the present invention may have a structure depicted as follows:

wherein R, R' and R" are independently H or CH₃, x is the number of acrylamide monomer units in the length of the backbone chain, which is at least about 1400 (corresponding to a molecular weight of at least about 100,000), preferably at least corresponding to a molecular weight of about 3 x 10⁶, Y is the number of pendant polyoxyethylene and/or polyoxypropylene side chains, as determined by the relationship f = X x + y where f is the mole fraction of y, which may be at least about 0.001 (0.1 mole %) , z is the number of oxyethylene and/or oxypropylene groups in the side chain, wherein z is at least about 5, preferably about 5 to about 40.

The novel retention aids are employed in conjunction with phenolic compounds, generally phenol-formaldehyde resins, as in the case of the conventionally employed high molecular weight polyoxyethylene polymers, in retaining fines in all forms of mechanical wood pulp.

EXAMPLES Example 1

This Example describes the preparation of graft copolymers by copolymerization of monomers.

A series of experiments was carried out in which copolymers comprising a long polyacrylamide backbone and short pendant polyethylene oxide groups were tested for efficiency as retention aid composition components.

The copolymers were synthesized by copolymerization of acrylamide with a variety of polyoxyethylene macromonomers of the structure:

R

OH₂=C 0=C - [0 CH₂ CH₂]_n - 0CH₃ where n = 5 to 40 and R = H or CH₃, carried out in aqueous solution at 25"C or 40°C in a batch reactor in the presence of potassium persulfate (K^O_g) initiator. The lower temperature of polymerization led to a higher molecular weight material. The compositions of the graft copolymers were measured by NMR and HPLC. Molecular weight of the graft copolymer was measured by GPC, light scattering and intrinsic viscosity. Example 2

This Example describes the experimental protocol for testing flocculation of polystyrene resin particles.

The graft copolymers prepared as described in Example 1, were used to test for the ability to cause monodisperse polystyrene latex particle to flocculate in the presence of wood fibers and phenol-formaldehyde resin, as a measure of the ability of the graft polymers to cause fines retention in wood pulps.

The polystyrene latex particles used in such tests were synthesized using emulsifier-free polymerization, employing 73 g styrene, 0.53 g of potassium sulfate in 670 g of distilled and deionized water. Polymerization was carried out at 70 ° C for 12 hours. The particle size of the polystyrene latex was measured using a BI-DCP particle sizer (Brookhaven Instrument Company) and is presented in the following Table I:

Table I

Flocculation of the latex was induced by the sequential addition of small quantities (1.0 to 4.0 mg/L) of graft copolymer or high molecular weight polyethylene oxide (Polyox 301, MWt. 4 million, Polyox 309, MWt. 8 million, Union Carbide Corporation, materials commercially used as retention aids) for comparison, and phenolic resin Cascophen C27, Borden) (40% aqueous solution) , in the presence of wood fiber.

For a flocculation experiment, 38 mL of distilled water, 10 mL of wood fiber of 0.5 wt.% consistency and 1 mL of 0.25 wt.% polystyrene latex were added to a 50 mL test tube. The pH of the solution was adjusted to about 5 by adding about 0.1 mL 0.02 N HC1. To maintain the electrolyte at a concentration of 1.0 x 10^"3 M, 0.05 mL of 1M NaCI were added. After addition of 0.4 mL of 0.25 wt.% phenolic resin dispersion (MWt. 30,000), the transmittance for the control was measured by using a UV Spectrometer at a wavelength of 500 nm.

The aqueous solution of graft copolymer or polyethylene oxide then was added at a concentration of 0.025 to 0.05 wt.% to a total volume of 50 mL. The samples were hand-shaken for 3 to 5 seconds and the suspended solids were allowed to settle for one hour at room temperature. The supernatant then was taken from the tube and filtered through a 200 mesh screen to remove suspended fiber fragments and the turbidity was measured. Example 3

This Example provides the results for experiments in which the polymer concentration was varied. In a series of relative turbidity measurements utilizing the procedure outlined in the previous Example, the amounts of copolymer and Polyox 301 and 309 were varied at a constant amount of phenol resin of about 2.0 mg/L. The results obtained are plotted graphically as Figures 1 and 2. Figure 1 shows the results for the Polyox 301 and 309 compounds while Figure 2 shows the results for the graft copolymer.

As may be seen from these results, both Polyox 301 and 309 have an optimum addition of 2 mg/L and hence an optimum weight ratio of polyethylene oxide to phenolic resin of 1:1. With the copolymers, the flocculation efficiency initially increased with increasing amounts of copolymer, the copolymer concentrations yielding minimum relative turbidity values ranged between 2 and 4 mg/L. In general, higher copolymer concentrations (> 4 mg/L) caused flocculation to deteriorate. While the polyethylene oxides alone did not flocculate the latex to any significant degree, the copolymers alone were able to achieve about 25% latex removal, corresponding to a relative turbidity of 0.75. No flocculation was observed using polyacrylamide and phenol-formaldehyde resin.

Example 4

This Example illustrates the effect of the composition of the copolymer on flocculation. Two series of copolymers from the copolymerization of MA-23 (i.e., the ethoxy polyethylene glycol monomethacrylate in which n = 23) and acrylamide at different ratios of monomers, one copoly erized at 25"C, another copolymerized at 40^*C, were tested for formulation performance following the procedure of Example 2. The results obtained are shown graphically in Figure 3.

As may be seen therein, for the copolymer produced at 40"C, maximum flocculation was achieved at about 0.8% (mol) of MA-23 in the copolymer, while for the copolymer produced at 25°C, flocculation increased with increasing MA-23 content, with no significant improvement being attained beyond 1% (mol) . In general, a lesser amount of macromonomer was required for copolymer synthesized at a lower temperature to attain the same degree of flocculation. Example 5

This Example illustrates the effect of polyethylene oxide chain length in the copolymer on flocculation. A series of copolymers from the copolymerization of acrylamide with similar mole feed ratios (about 1/100) of polyethylene oxide macromonomer having chain lengths of from 5 to 40 moles, prepared at 25°C and 40°C, were tested for flocculation performance following the procedure of Example 2. The results obtained are set forth in Figure 4. The copolymers in both cases contained about 0.7 to 0.8 % (mol) of macromonomer but had different chain lengths.

As may be seen from this data, a minimum pendant chain length was required to achieve significant flocculation, namely 5 EOS for polymerization carried out at 25°C (higher molecular weight polymer) and 9 EOS for polymerization carried out at 40"C (lower molecular weight polymer) . The efficiency of flocculation increased with length of polyethylene oxide pendant chain up to a chain length of about 10 units, beyond which no significant improvement was observed up to a chain length of 40 EO units.

Example 6

This Example illustrates the effect of molecular weight of copolymer on flocculation.

A series of copolymers of differing molecular weight were tested for flocculation performance following the procedure of Example 2. The results obtained are reproduced in the following Table II:

Table II

(2) A-40 = acrylate with n = 40

(3) A- 10 = acrylate with n = 10

As may be seen in the above Table II, improved flocculation is achieved at higher copolymer molecular weights. Example 7 This Example illustrates the effect of aging polymers in aqueous solution on flocculation.

A series of flocculation experiments was carried out using copolymers and Polyol 301 and 309, in aqueous solutions freshly prepared and stored at a polymer concentration of 0.025 wt.% at room temperature, using the procedure described in Example 2. The results obtained are reproduced in Figure 5.

As may be seen from this Figure, the copolymer provided herein maintained its flocculation performance over various periods of aging, while the performance of both Polyol 301 and Polyol 309 declined steadily with an increasing storage period. After 20 days of storage, the Polyols had almost completely lost their ability to flocculate, while the copolymer sample retained its performance over the same period. This copolymer had a molecular weight of 3.7 x 10° and contained 0.65 mole % of MA-23 macromonomer.

A sample of the same copolymer was stored in a refrigerator at approximately l^βC for six months at a concentration of 0.29 wt.%. At the end of this period, the copolymer exhibited the same flocculation performance as prior to storage. Example 8

This Example describes evaluation of the copolymers by Dynamic Drainage Jar (DDJ) .

The copolymers prepared as described in Example 1 were also tested for their fines retention ability using DDJ, as described by Pelton et al, Pulp & Paper Can. 81(1): T9-15 (1980). Retention performance was characterized by First Pass Retention (FPR) , determined from the relationship: FPR = Cs - Cw 100

Cs where Cs is the initial stock consistency and Cw is the white-warer consistency. The Dynamic Drainage Jar (Paper Research Materials Inc.) was fitted with a stainless steel screen with 0.6 mm diameter opening. The temperature was maintained at 50°C for the experiment and the drainage time ranged from 115 to 125 s/100 L. The propeller speed was controlled at 250 to 1000 rpm. The addition of copolymers was at 0.10 wt.% based on oven-dried (o.d.) pulp. Phenol-formaldehyde resin was added to the pulp before addition of the polymer, in an amount corresponding to a 1:1 weight ratio of phenol- formaldehyde resin to copolymer. The pH of the pulp was adjusted to about 5.2 by adding 0.02 N HCl solution.

Table III below illustrates the effect of molecular weight of copolymer on FPR of newsprint pulp:

Table III

* - Propeller speed at 250 rpm and Temperature at 50* C As may be seen from the results of Table III, more than eighty percent retention was observed for those copolymers of molecular weight higher than 3.5 million, which indicates a significantly improved retention of pulp fines, with copolymers of a molecular weight of about one million showing a notable increase in FPR.

FPR for PEO homopolymers also showed a strong dependence on molecular weight. FPR increased to 83.7% for Polyox 309 with molecular weight of 8 million from 70% for Polyox 301 with a molecular weight of 4 million.

Figure 6 shows the influence of composition on amount of polyethylene oxide pendant chains on FPR measurements for copolymers prepared at 25'C and 40"C.

(The polymers prepared at 25°C had a higher molecular weight). As little as 1.0 mole % macromonomers incorporated into the polymer achieved a significant enhancement of FPR. Further, increasing amounts of macromonomer did not significantly improve FPR performance. Figure 7 shows the influence of PEO pendant chain length in the copolymers. No significant dependence was observed for a pendant chain length of 10 to 40. The better results achieved for the higher molecular weight copolymer illustrated in Figures 6 and 7 again show the beneficial influence of higher molecular weight on FPR.

There are several variables affecting the FPR measurement, such as propeller speeds, screen hole size, pulp consistency and temperature. The details of the relationship between these factors and FPR results have been fully elucidated by Pelton et al, Pulp & Paper Can. 80(12): T425-29 (1979).

Figure 8 shows the effect of propeller speed of FPR of newsprint pulp, for copolymers provided herein and Polyox 309. The results depicted in Figure 8 show the improved resistance to shear exhibited by the copolymers provided herein in comparison to Polyox 309. A common trend was that FPR decreased as the propeller speed increased, but the decrease was more significant in the case of Polyox 309. Example 9

This Example illustrates the preparation of graft copolymers by grafting polymers.

High molecular weight graft copolymers were prepared by gamma-ray irradiation of mixture of low molecular weight polyethylene oxides (PEO) and high molecular weight polyacrylamide (PAM) (Mw = 5 x 10⁶) . Experiments were carried out using poly (ethylene oxide) (Mw = 300,000), poly(ethylene glycol) (Mw = 8000, 2000), poly(ethylene glycol methyl ether (Mw = 5000, 750) , and pluronics F127, F208, F88 and F68 (collectively termed PEO) . The required amounts of PAM and PEO were dissolved in distilled water and the resulting aqueous solution exposed to gamma-radiation from a ⁶⁰Co source at a dose rate of 34 krad h^'1 at room temperature. After irradiation, the product was precipitated by the addition of acetone, to leave PEO homopolymer in solution. The copolymer was dissolved in water, precipitated by acetone and dried at 60'C.

The properties of the copolymers prepared by this procedure are set forth in the following Tables IV and V:

Table IV - Summary of radiation grafting polymerization of PAM (M^ = 5 xlO*) and PEO; PAM: M_w=5xl0⁶, concentration: 1.16 wt%; dose rate: 34 krad /h; room temperature. * not measured

Table V - Effect of PEO molecular weight on the physical state of graft copolymer; PAM: ^=5 10*, concentration 1.16 wt%; dose rate: 34 krad/h; room temperature

Example 10

This Example illustrates the flocculation ability of the graft copolymers prepared as described in Example 9. The flocculation ability of the copolymers prepared in the preceding Example 9 was determined following the procedure described above in Example 2. The results obtained in comparison to PEO-309 are summarized in Figure 9.

As a result of the flocculation studies, it was found that the purified C-18-12 and B-18-12 were the most effective flocculants. The relative turbidity reduced to 0.37 to C-18-12 at a copolymer concentration of 4 mg/L. As may be seen from Figure 9, these results are comparable to those obtained for PEO-309. Further increasing the copolymer concentration caused a small increase in relative turbidity.

The cross-linked polymer gels with low swelling (hard gel) gave.no effective flocculation in the polymer concentration range studied (0 to 60 mg/L) . All copolymers in which the PEO content was below 13 wt.% gave weak flocculation. The flocculation was enhanced by removal of ungrafted PEO. SUMMARY OF DISCLOSURE

In summary of this disclosure, the present invention provides a novel chemical retention aid for mechanical pulps which is inexpensive to manufacture, easy to make and exhibit chemical and physical stability not evident in prior art retention aids. Modifications are possible within the scope of this invention.

Claims

CLAIMS What we claim is:

1. A retention aid composition for use in the formation of a paper sheet from wood pulp, comprising: a water soluble copolymer having a molecular weight of at least about 100,000 and comprising a high molecular weight water-soluble polymer backbone having pendant poly(alkylene oxide) groups, and a phenolic compound.

2. The composition of claim 1 wherein said water- soluble polymer backbone comprises polyacrylamide.

3. The composition of claim 2 wherein said water- soluble copolymer has the formula:

in which R, R' and R" are independently H or CH₃, x is at least about 1400, being the number of acrylamide monomer units in the length of the backbone chain, and Y is the number of pendant polyalkoxy side chains, as determined by the relationship: f = Y x + y wherein f is the mole function of y and is at least about 0.1 mole %, and z is the number of oxyethylene and/or oxypropylene groups in the side chain and is at least about 5.

4. The composition of claim 3, wherein x has a value sufficient to provide a polyacrylamide backbone having a molecular weight of at least about 500,000.

5. The composition of claim 4, wherein said molecular weight is at least about 1 million.

6. The composition of claim 4, wherein said molecular weight is at least about 3 million.

7. The composition of claim 4, wherein z is at least about 10.

8. The composition of claim 7 wherein f is about 0.15 to about 3 mole %.

9. The method claimed in any one of claims 1, 2, 3, 4, 5, 6, 7 or 8 wherein said phenolic compound is a phenol- formaldehyde resin.

10. In a method of forming a paper sheet wherein a slurry of cellulosic fibrous materials is dewatered and a retention aid composition is added to said slurry to improve the retention of fines, fillers and pigments in said paper sheet during said dewatering step, the improvement which comprises, utilizing as a component of said retention aid composition, a water soluble copolymer having a molecular weight of at least about 100,000 and comprising a high molecular weight water-soluble polymer backbone having pending poly(alkylene oxide) groups.

11. The method of claim 10 wherein said retention aid composition further comprises a phenolic compound.

12. The method of claim 11 wherein said phenolic compound is a phenol-formaldehyde resin.

13. The method of claim 10, 11 or 12 wherein said water- soluble copolymer is as claimed in any one of claims 2, 3, 4, 5, 6, 7 or 8.

14. A method of forming a copolymer, which comprises: grafting pendant groups of poly(alkylene oxide) onto a high molecular weight water-soluble polymer backbone.

15. The method of claim 14 wherein said grafting is effected in the presence of irradiation.

16. The method of claim 15 wherein said irradiation is gamma irradiation.

17. The method of claim 14 wherein said high molecular weight water-soluble polymer backbone comprises a polyacrylamide having a molecular weight of at least about 100,000.

18. The method of claim 18 wherein said polyacrylamide has a molecular weight of at least about 500,000.

19. The method of claim 18 wherein said polyacrylamide has a molecular weight of at least about 1,000,000.

20. The method of claim 19 wherein said polyacrylamide has a molecular weight of at least about 3,000,000.

21. The method of claim 17 wherein said poly(ethylene oxide) has the formula:

wherein R, R' and R" independently are H or CH₃ and z is the number of oxyethylene and/or oxypropylene groups in the poly(ethylene oxide) pendant chains and is at least about 5.

22. The method of claim 18 wherein z is at least about 10.

23. The method of claim 21 wherein the polymerization is effected to provide a mole fraction of grafted pendant poly(ethylene oxide) groups of at least about 0.1 mole %.

24. The method of claim 23 wherein said mole fraction is about 0.15 to about 3 mole %.