GB2100269A

GB2100269A - Preparation of particulate gels

Info

Publication number: GB2100269A
Application number: GB8216933A
Authority: GB
Original assignee: National Research Development Corp UK; National Research Development Corp of India
Current assignee: National Research Development Corp UK; National Research Development Corp of India
Priority date: 1981-06-12
Filing date: 1982-06-10
Publication date: 1982-12-22
Also published as: EP0068684B1; US4542176A; DK171937B1; JPS585331A; GB2100269B; DK260682A; DE3272562D1; JPH0334491B2; EP0068684A1

Description

1 GB 2 100 269 A 1

SPECIFICATION Preparation of particulate gels

This invention relates to the preparation of gels, especially hydrogels, in particulate form.

In many applications to which gels are put it is more convenient that they be in particulate form. Conventionally, such particulate gels are prepared by an emulsion or suspension polymerisation process 5 in which, however, there are separation and liquid handling problems. Bulk polymerisation processes, where such problems are either absent or diminished, result in the production of gels in more massive form.

According to the present invention, there is provided a process for the preparation of a particulate gel, which process comprises (i) contacting the gel with a swelling agent and (H) subjecting the swollen 10 gel to shear stress such that the swollen gel is comminuted to particles. By the term "gel" as used herein we mean a polymeric material which is swellable by, rather than soluble in, a given liquid.

Usually, but not necessarily, the polymeric material may be rendered swellable by being chemically cross-linked. It may instead, either in the case of high molecular weight material, be trapped in an entanglement network, or be formed with hydrophobic blocks to given an amphipathic material. 15 In accordance with a particularly preferred feature of the invention the gel is a hydrogel and the swelling agent comprises water, optionally in admixture with one or more other polar liquids, for example Cl to C4 alkanols such as ethanol. From the standpoint of cheapness and availability, however, it is preferred that the hydrogel is swollen with a swelling agent which consists essentially of water, except where the hydrogel is readily hydrolysable in aqueous media. It is to be noted, however, that 20 hydrogels can also be swollen by non-aqueous solvents, especially non- aqueous polar solvents: for example, chloroform.

Suitably, the hydrogel comprises a hydrophilic homo- or copolymer comprising residues derivable from at least one of the following monomer classes- (a) (meth)acrylic acid, (meth)acrylamide or an unsubstituted or hydroxy- substituted ethyl or 25 propyl (b) a substituted or unsubstituted cyclic mono or poly ether having from 3 to 6 ring atoms or cyclic imine having 3 ring atoms; or (c) a substituted or unsubstituted vinyl alcohol, aldehyde, ether, acetal, ketone, ester or substituted or unsubstituted N-vinyl heterocyclic compound.

By "(meth)acryl- we mean herein "methacryi" or---acryl- or a copolymer comprising both.

Monomers in class (b) include epoxides such as ethylene oxide, propylene oxide, 1,2-epoxybutane, 2,3-epoxybutane glycidyl ethers, N-(epoxy substituted) heterocyclic compounds such as N-(2,3epoxypropyi)-pyrrolidone; epihalohydrins, while not themselves cyclic ethers, do give homo and copolymers derivable therefrom: examples include epifluorohydrin, epichlorohydrin and epibromohydrin. 35 Other cyclic mono- or poly ethers include oxetane, tetrahydrofuran, dihydropyran, dioxolane and trioxane. Homo- and copolymers derivable therefrom include partially Cl to C, alkyl etherified celluloses and starches, homo- and co-poly(alkylene oxides) which may be cross-linked through reaction with isocyanate or unsaturated cyclic ether groups such as polyoxym ethylene, polyethylene glycols and polypropylene glycols, and polyesters thereof with dicarboxylate acids such as maleic acid.

Monomers in class (c) include methyl and ethyl vinyl ether, methyl vinyl ketone, methallyl alcohol, N-vinyl pyrrolidone, N-vinyl carbazole, Nvinyl pyridine, N-vinyl oxazole, N-Anyl methyloxazolidone, vinyl formal, vinyl butyral, vinyl methoxyacetal. Homo- and copolymers derivable therefrom include polyvinyl acetate and polyvinyl alcohol.

The process of this invention is particularly suited to homo- or copolymers which exhibit syneresis. 45 Examples include homo- and co(polyethylene oxide) and the process of the present invention is particularly suited to poly(ethylene oxide). By the term "syneresis" as used herein we mean the spontaneous exudation of at least some of the swelling agent from the swollen gel which results when the temperature of the swollen gel is varied. In the case of homo- and co(polyethylene oxide) swollen with aqueous solvents and some non-aqueous solvents (for example formamide and acetic acid) 50 syneresis occurs as the temperature is raised while for such polymers swollen with non-aqueous solvents syneresis in general occurs as the temperature is lowered.

It is found that the process of the present ivention provides particulate gels of average particle size typically from 0.05 mm to 7 mm, preferably from 0.1 mm to 5 mm, of narrow particle size distribution substantially free from "fines", and that a very precise degree of control over the average particle size 55 may be attained by appropriate regulation of one or more of the following parameters; (a) the composition of the gel; (b) the composition of the swelling agent; (c) the temperature at which the swollen gel is subjected to shear stress; (d) the rate of shear; (a) the shear power; (f) the nature and configuration of the shear means; and (g) the duration of shear.

With regard to (a), the gel composition is usually dictated by the application to which the gel is GB 2 100 269 A 2 put. However, a relative increase (in the case of a hydrogel copolymer) in the amount of a hydrophobic component or of chemical cross-linking, will reduce the amount of swelling caused by contact with an aqueous medium and, for a given set of parameters (b) to (g), will increase the average particle size of the sheared, swollen gel. The converse will also apply.

With regard to (b) the composition of the swelling agent will, for a given set of parameters (a) and (c) to (g), control the degree of swelling and the average particle size of the sheared, swollen gel. For example, a relative increase (in the case of a hydrogel polymer) in the amount of C3 or C4 alkanol in an aqueous swelling agent will decrease the amount of swelling and increase the average particle size of the sheared, swollen gel. However, by contrast an ethanol/water mixture approximating to 1:1 by weight composition will somewhat increase the amount of swelling, relative to water as the swelling 10 agent, and decrease the average particle size of the sheared, swollen gel. Where (a) is such that the gel is sensitive to hydrolysis or alcoholysis, for example by being hydrolysable in aqueous media, then it may be swollen by any suitable non-aqueous swelling agent, for example chloroform. The gel may also be swollen with any suitable non-aqueous swelling agent and then sheared in aqueous media. Nonaqueous swelling agents can often provide greater swelling (for example, chloroform provides a ten-fold 15 swelling relative to the dry weight, whereas water provides a four-fold swelling of poly (ethylene oxide) of equivalent weight about 3,500 at 371C) and thus a relatively smaller particle size after shearing and drying.

With regard to (c), where the swollen gel exhibits syneresis as the temperature is raised, in the case of homo- and co(polyethylene oxides) swollen in aqueous media, the regulation of the temperature 20 at which the swollen gel is subjected to shear stress to effect syneresis will result in a less swollen gel and, for a given set of parameters (a), (b), (d), (e) and (f), will increase the average dried particle size of the sheared, swollen gel. The converse will also apply. This is a particularly preferred feature of this invention.

The shear may suitably be provided by an industrial mincer or blender, such as a Waring Blender; it 25 is often convenient first to effect a coarse comminution followed by a fine comminution to specification.

Thereafter, the swollen particulate gel may be dried, optionally after impregnating with active substance which, where convenient may be incorporated in the swelling agent.

In the case of gels, notably hydrogels comprising homo- and co-poly (ethylene oxide) swollen in aqueous media, which exhibit syneresis at elevated temperature, an initial drying stage may comprise 30 contacting the swollen, sheared gel with hot water, for example at a temperature from 901 to 1 0011C.

The resulting syneresis will result in a reduction of the water content of the hydrogel the drying of which. if desired, can be completed after filtration by contact with a warm fluid current such as warm air.

In the case of gels, notably hydrogels comprising homo- and co-poly (ethylene oxide) having an equivalent weight greater than about 1,400 but swollen in non-aqueous media, which exhibit'syneresis at reduced temperatures, an initial drying stage may comprise cooling the swollen, sheared gel, for example to a temperature from -250C to +251C, preferably -200C to OIC whereupon the gel crystallises and the swelling agent is exuded.

The particulate gels, especially hydrogels, prepared by the process of this invention may be used in numerous applications. For example, they may be used as absorbents in diapers, tampons, incontinence 40 pads and wound dressings; as gelling agents for containing aqueous media: for example, in bedpans or incontinence bags where the aqueous medium is urine; as carriers, excipients and delivery mechanisms for active substances: for example, fertilisers, nutrients, herbicides, pesticides, flavourants and drugs, as described in our Specifications 2047093, 2047094 and EP 0000291 B 1 and, in particular, in encapsulated controlled release oral ly-admin istered dosage forms; as soil conditioners; as humectants:

for example in dried foods, tobacco, tea or coffee; and as extractive agents, as described in our Complete Specification 1573201.

In the specification accompanying our co-pending application 121959/121958/121640/2 we have described a hydrogel, suitable as a wound dressing agent and in the preparation of a controlled release composition comprising an active substance, which comprises polymerised moieties derivable 50 from 0) at least one polymerisable cyclic (thio) ether and from (ii) at least one hydrophilic homo- or copolymer.

The process of this invention is particularly useful in preparing such hydrogels for use in such inventions.

In particular, the process of this invention, when practiced on hydrogels comprising poly(ethylene 55 oxide) moieties cross-linked through reactions with unsaturated cyclic ethers, such as acrolein tetramer, produces fine powders (with a mean diameter from 0.1 mm to 0.2 mm) very effectively with a short period of comminution in a Waring Blender.

In the case of hydrogels comprising poly(ethylene oxide) moieties crosslinked through reaction with isocyanates, such as an aliphatic diisocyanate, the effectiveness of comminution may be increased 60 by foaming by the accidental or deliberate presence of a pneumatogen. This may comprise a small amount (typically less than 1 % of reactants) of 0) water (which reacts in known manner with an isocyanate group to form carbon dioxide, the effective blowing agent, and an amine group which latter reacts with further isocyanate to form a urea group); (ii) an inert low boiling liquid (such as a 65 halogenated, for exampe a chlorofluorinated hydrocarbon such as a freon); or (ill) a solid pneumatogen 65 1 A 3 GB 2 100 269 A -3 which decomposes on heating, for example a bicarbonate.

The process of the present invention is particualrly advantageous in that the energy investment of comminution is low. The product is also formed in a particularly clean state for medical applications. In comparative tests, colourless unswollen hydrogen was comminuted by cryogenic grinding. A powder of broad particle size distribution was formed of grey or brown colour caused by metallic or other 5 inclusions from the grinding mill. (Cryogenic grinding is also expensive. ) The product produced by the process of this invention is a clear white powder of narrow particle size distribution with concomitant reduction in waste from material which is out of specification, or in energy investment in a classification process. The dried product, while free-flowing, does not impart slip to surfaces onto which it is poured, unlike certain presently available powders, comprising spherical particles. Furthermore, the swelling 10 process enables free oligomeric and monomeric material to become extracted simultaneously.

On occasion, flocs of very fine (0.05 mm to 0.2 mm) particles can form on drying. This can be overcome by dry ball-milling.

The invention will now be further illustrated, by way of example, with reference to the accompanying drawings, in which: 1 Figure 1 represents a diagrammatic flow-chart of the complete process; and Figure 2 represents a diagrammatic detailed view of a drying embodiment.

In the drawing, a poly(ethylene oxide) gel is manufactured by bulk polymerisation in reactor 1. The gel is conveyed to a swelling tank 2 where it is contacted in massive form overnight with water or a non-aqueous swelling agent becoming swollen thereby. The swollen gel together with excess water or 20 nonaqueous swelling agent is then passed through a heat exchanger 3 in which it is brought to the particular temperature which will give the desired particle size after comminution in a coarse reduction stage 4 and a fine reduction stage 5. The comminuted slurry is next conveyed through a heat exchanger 6 in which it is heated to 1 001C or cooled to -251C to effect syneresis of water or non-aqueous swelling agent from the swollen gel. (The heat exchanger fluid exiting from 6 in the case where 6 is at 25 1 OOOC may be pumped in closed circuit to 3.) The less swollen gel leaving 6 is then dried to a free flowing powder in a continuous drier 7 and next passed through a screening station 8 to give a comminuted product of specified particle size distribution.

In the particular embodiment shown in Figure 2, the comminuted slurry leaving 5 is gravity fed by a guide 9 in 6 onto a moving endless belt 10. Water at 1 001C enters 6 at point 11 and cooler water 30 leaves at point 12. The less swollen gel leaving 6 is then gravity fed into 7 where it meets a vibrating screen 13 of specified mesh size. Screened gel passes through a static retaining mesh 14 while excess water is removed under tension through line 15. The screened gel is continually drained dry and next further dried by the passage of warm air through line 16. The dried gel is finally air veyed through line 17 to a fabricating station (not shown).

The following Examples illustrate the invention. In each case the gel was fully swollen before comminution. Gel particle sizes refer to the dry particles; mesh sizes refer to the mesh presented to the swollen polymer feedstock which is to be comminuted.

Gel particles were dried first by treating with boiling water for one minute on a vacuum filter on which they were then allowed to dry in air. Next they were fully dried using a through circulation air drier 40 at 1 OOOC.

Dried gel particles were sized using brass sieves in accordance with British Standard BS410/43.

EXAMPLE 1

A polyethylene oxide hydrogen cross-linked through urethane groups was first made by reacting 45 poly(ethylene oxide) (M- = 8307) with 1,2,6-hexanetriol and Hylene W (bis- (4'-isocyanatocyclohexyi) methane) in the molar ratio 43:8.5 essentially as described in Example 1 of our specification

2047093A.

A swelling curve (which gives an indication of the extent of swelling and the time taken to attain this) for the hydrogen was then obtained by plotting water ratio against time at a constant ambient. 50 temperature of 200C. (The water ratio is the ratio of the weight of water absorbed by the polymer at that temperature to the dry weight of polymer.) In the present case the equilibrium water ratio was found to be 3.5 and was attained after 6 hours (average of 3 samples).

Samples of the swollen hydrogel were next liquidised in a domestic liquidiser (Kenwood---Chef") for differing lengths of time but at constant liquidising shear (13,500 rpm) volume and weight ratio of polymer:ice:water (1:1:1). Groups of 5 samples were each liquidised for 1, 3, 5, 7 and 10 minutes making a total of 25 experimental runs: see Table 1. It was found that as the time of shear increased the percentage of sample weight remaining as coarse particles (that is, with a means diameter greater than 1 mm) decreased with concomitant increase in the amount of finer particles. It was also found that for shear times of 1 and 3 minutes the fraction which comprises the greatest percentage of sample weight was, in all cases, coarse; but that for a shear time of 5 and 7 minutes it was the fraction < 1 003ju but > 60 699p and for a shear time of 10 minutes it was the fraction <6991,t but > 500jj.

These results indicate the desirability of increasing shear time to obtain increase amounts of finer particles.

4 GB 2 100 269 A' EXAMPLE 2

Samples of swollen hydrogel, prepared as described in Example 1, were liquidised in the same liquidiser at different blade speeds but at constant liqudising times (1 min.), volume and weight ratio of polymer:ice:water (1:1:1). Groups of 5 samples were each liqudised at blade speeds of 4,000; 5,500; 8,000; 10,500 and 13,500 rpm making a total of 25 experimental runs: seeTable 2. It was found that as the blade speed increased the percentage of sample weight remaining as coarse particles (that is, in this instance, with a means diameter greater than 2.4 mm) decreased with a concomitant increase in the amount of finer particles. At the three highest blade speeds the fraction which comprised the greatest percentage (the percentage increasing with blade speed) of sample weight was, in each case, the fraction < 1405ju but > 1003A. At the highest blade speed this fraction comprised about 35% by weight of the sample and more than 50% by weight was under 2000A.

These results indicate the desirability of increasing shear to obtain increased amounts of finer particles.

EXAMPLE 3

Samples of swollen hydrogel, prepared as described in Example 1, was liquidised in the same 15 liquidiser at different weight ratios of polymer:ice: water but at constant liquidising times (10 minutes), shear (13,500 rpm) and volume. A sample was liquidised at each of the polymer:water:ice weight ratios 4:1:1, 3:1A, 2:1A and 1:1A making a total of 4 experimental runs: see Table 3. It was found that as the ratio increased an optimum value, at about 3:1A, was reached at which the percentage of sample weight converted to fine particles was greatest. The 3:1A sample gave approximately 60% by weight of the sample in the fraction < 600A but > 500p whereas the 4:1:1 samples gave approximately 60% by 20 weight of the sample in the fraction < 1 003p but >599,u. The other two samples gave a high percentage of sample >1 003A with a poor yield of small particles.

These results indicate the desirability of using a high polymer:water:ice ratio but not one so high that the viscosity of the resulting mix reduces the shear (as was believed to be the case at 4:1M.

EXAM P LE 4 Samples of swollen hydrogel of different volume, prepared as described in Example 1, were liquidised in the same liquidiser at constant liquidising times (10 minutes), shear (13,500 rpm) and 25 weight ratio of polymer:ice:water (2:11) to ascertain whether any---scale- up- effect existed. Samples which had the volume of an equilibrated weight of polymer of 363,487, 603 and 823 g, respectively, were each liquidised in a total of 4 experimental runs. It was found that as the volume of the sample increased the percentage of sample weight remaining as course particles (that is, with a mean diameter greater than 1 mm.) also increased. However, the corresponding decrease in the amount of finer particles at any given fraction was not great. For example, the fraction <600,u but >500A comprised about 38%,36%, 31 % and 27% of the sample weight for the 363, 487, 603 and 823g samples, respectively.

Accordingly, these results indicate that it may be more economical to use larger volume samples:

while the percentage of sample weight yielded as fine particles will be smaller the actual weight of fine 35 particles produced in unit time will be greater.

EXAM P LE 5 In this Example fractions from previous liquidising treatments, as feedstock, were extruded through a domestic mincer (Kenwood---Chef--- )modified by interposing a wire mesh between the supplied plate so as to obtain different mesh sizes. Resu its are shown in Table 5. These indicate that the 40 several meshes did function to reduce a major amount of each feedstock below the mesh size; however, at small mesh sizes it appears that flocculation may occur. It is also possible that syneresis of the polymer occurs, due to heating, not only making it more difficult to comminute but also, thereafter, less able to dry down to the expected size.

EXAMPLE 6

A polyethylene oxide hydrogel crosslinked through reaction with unsaturated cyclic ether groups was made by reacting 550g polyethylene glycol (5n = 7000) with 21.06g 1,2,6-hexane triol and 126.68g acrolein tetramer, 3,4-dihydro-2H pyran-2 methyi(3,4-dihydro-2H pyran-2 carboxylate). The reactants differed from the stoichiometric molar ratio of 12:4 by the use of 80% excess of acrolein tetramer. The reaction was initiated by the addition of 0.7g anhydrous ferric chloride. i.e. 0.1% by weight of the reactants.

The ferric chloride catalyst was dissolved in the triol at 901; the dihydropyran and the moisture free molten polyethylene glycol were then added and the reactants thoroughly mixed. The gel formed very rapidly and polymerisation was completed by curing for four hours at 901.

The fully swollen gel was mixed with its own weight of water and liquidised in a domestic 55 liquidiser (Kenwood Chef) for 10 minutes at 13,500 rpm. The gel particles were filtered off, treated with boiling water and then by through circulation warmed air.

The dried particles were sized by sieving to show a typical distribution of:

1 GB 2 100 269 A 5 Particle size (microns); <420; 420-500; 500-699; 699-1400 >1400 % by weight 19; 13; 12; 39; 17 The particles were aggregated and could readily be reduced by further processing, for example light manual grinding with a mortar and pestle of particles larger than 420 microns reduced their size below 5 this level.

The fine powder absorbs water very readily; 13.5 mi of water were absorbed by pouring from a measuring cylinder on to 3g of the dry hydrogel, a water to dry powder ratio of 4.5.

EXAMPLE 7

A hydrogen was prepared as described in Example 1 using 67.8g of polyethylene glycol (Mn 10 7000) 2.60g of 1,2,6 hexane trio] and 10.42g of acrolein tetramer, i.e. 20% excess of the dihydropyran.

The hydrogel tended to disintegrate on swelling water and readily comminuted with shearing. Thus, liquidising a 50,50 mixture of water and swollen gel for 20 minutes at maximum speed in a Kenwood Chef produced a frothy dispersion of very fine particles, estimated by optical microscopy to be in the 100 micron range. A slightly coarser dispersion made by liquidising at maximum speed for five minutes was filtered to give a filter cake of small discrete particles aggregated together to form a tough horn-like material.

a) TABLE 1

PARTICLE SIZES WEIGHT PERCENTAGES TIME OF LIQUIDISING MESH 1 MIN 3 MIN 5 MIN 7 MIN 10MIN (MINS) 1 3 5 7 10 NO SAMP. SAMP. SAMP. SAMP. SAMP.

JU weight of polymer/gms; 82.719 82.392 82.003 82.164 82.446 rough > 1003 76. 27 41.50 20.84 16.93 8.96 weight of ice/qms 81.857 81.858 82.039 82.284 81.027 16 mesh < 1003 but > 699 18.60 43.32 43.12 50.83 36.84 weight of water/gms 83.463 82.573 82.679 82.107 82.181 22 mesh <699 but > 500 4.30 12.85 26.70 25.13 38.02 weight of dried poiymer/gms 14.406 12.314 12.037 11.645 11.376 30 mesh <500 but >422 0.46 1.28 4.40 3.40 6.94 36 mesh <422 0.36 1.04 5.00 3.71 9.24 1 1 a) ca N) 0 N 0) (0 m 1, bl TABLE 2

PARTICLE SIZES WEIGHT PERCENTAGES 4000 5500 8000 10500 13500 SPEED OF BLADES MESH rev/ rev/ rev/ rev/ rev/ REWMIN 4000 5500 8000 10500.13500, NO min min min min min setting on control 1 3 5 7 MAX rough >2411 60.08 55.97 12.11 24.60 15.06 wt. of polymer/gms 82.104 81.740 82.172 82.774 82.013 7 mesh <241 1 but > 1676 29.30 29.84 21.99 22.02 12.92 wt. of ice/gms 82.938 81.740 82.706 82.113 82.218 10 mesh <1676 but >1405 5.72 6.99 23.30 18.49 19.69 wt. of water/gms 82.062 85.846 82.001 82.182 82.945 12 mesh <1405 but > 1003 3.79 5.87 30.72 25.42 34.54 wt. of wet polymer/gms 78.456 54.376 85.974 80.914 77.796 16mesh <'1003but>699 0.85 1.04 8.90 5.43 12.73 wt. of hot water dried/gms 63.597 45.266 50.640 65.214 65.292 22 mesh <699 0.26 0.28 2.98 4.02 5.06 % wt. loss due to H.W. 18.94 16.75 41.10 19.40 16.07 wt. of dry polymer/gms 13.100 12.304 11.886 1.384 12.665 G) W N 0 0 N a) W ---i 0G TABLE3

PARTICLE SIZES WEIGHT PERCENTAGES POLYMER:1CE: MESH WATER RATIO 1:1A 21:1 3:1A 4:1A NO A 1:1A 2:1:1 3:11 4:1A wt. of polymer/gms 59.433 119.413 179.693 239.964 rough >1003 23.19 15.38 0.52 0.55 wt. of ice/gms 59.436 59.098 60.451 60.838 16 mesh < 1003 but > 699 43.52 29.98 6.09 30.92 wt. of waterigms 59.350 59.573 60.124 60.898 22 mesh <699 but >599 15.47 21.85 30.66 31.09 wt. of wet polymer/gms 60.378 60.78 143.323 211.374 25 mesh <599 but >500 8.76 15.50 30.76 19.24 wt. of hot water dried/gms 39.475 51.78 121.131 207.097 30 mesh <500 but >422 3.93 7.10 17. 14 8.29 % wt. loss due to H.W. 34.62 14.81 15.51 2.02 36 mesh 022 but > 251 4.69 9.11 13.88 9.28 wt. of dry polymer/gms 9.038 18.027 26.419 38.073 60 mesh <2 51 but >21 1 0.22 0.57 0.49 0.33 72 mesh <211 but >178 0.17 0.35 0.35 0.22 mesh <178 0.05 0.16 0.11 0.09 1.

W co TABLE4

PARTICLES SIZES WEIGHT PERCENTAGES MESH 363 487 603 823 TOTALVOLUME/GMS 362.991 486.935 603.232 822.626 NO m gms gms gms gms wt. of polymer/gms 180.551 240.782 302.274 399.955 rough >1003 4.94 4.90 7.73 4.85 wt. of ice/gms 91.992 119.832 149.691 222.175 16 mesh < 1003 but >699 11. 43 17.84 30.30 43.22 wt. of water/gms 90.448 120.321 151.267 200.496 22 mesh <699 but >599 38. 38 35.54 31.29 27.39 wt. of wet polymer/gms 168.694 206,216 331.574 449.467 25 mesh <599 but > 500 21.57 21.06 16.48 13.21 wt. of H.W. drying/gms 116.464 143.158 263.163 374.417 30 mesh <500 but > 422 9.99 9.25 6.54 5.12 % wt. loss due to H.W. 30.96 30.58 20.51 16.70 36 mesh <422 but >251 12. 77 10.80 7.17 5.69 wt. of dry polymer/gms 27.332 34.214 51.434 63.408 60 mesh <251 but >21 1 0.56 0.41 0.23 0.25 72 mesh <21 1 0.36 0.34 0.27 0.25 G) m N CD CD N 0) co C.0 TABLE 5

PERCENTAGE (BEST) BY WEIGHT FEEDSTOCK NOT FEEDSTOCK PRODUCT THROUGHPUT EXCEEDING MODE PTCLE.

SAMPLE FRACTION MESH MODE PTCLE. SIZE (g min-1CM-2) SIZE 1 > 1 0001t 91 0p 600p 1.806 2 < 100011 but > 700p 91 Oiu 600A 3.671 50 3 <750p but >450p 910 600ju 3.970 4 > 1 000p 580t 422,u 0.883 < 1 000p but > 700p 5801t 422M 2.078 60 6 <750p. but >450p. 580p 422p 1.542 7 > 100011, 3201t 40011 0.155 8 < 1 000,u but > 700M 320A 4001,t 0.168 <50 9 <750p but >450p 320p, 400jj, 0.137 > 100011 180p 1241,t 0.011 11 < 1 000p but > 700A 180p, 124p 0.018 <50 12 <750p but >450p 180p 124p, 0.016 believed due to flocculation G) m 11 k GB 2 100 269 A

Claims

CLAIMS 1. A process for the preparation of a particulate gel, which

process comprises (i) contacting the gel with a swelling agent and (ii) subjecting the swollen gel to shear stress such that the swollen gel is comminuted to particles. 5
2. A process according to Claim 1 wherein the gel is a hydrogel and the swelling agent comprises 5 water.
3. A process according to Claim 2 wherein the swelling agent consists essentially of water.
4. A process according to Claim 2 or 3 wherein the hydrogen comprises a hydrophilic homo- or copolymer comprising residues derivable from at least one of the following monomer classes:

(a) (meth)acrylic acid, (meth)acrylamide or an unsubstituted or hydroxysubstituted methyl or 10 ethyl (meth)acrylate; (b) a substituted or unsubstituted cyclo mono or poly ether having from 3 to 6 ring atoms; or (c) a substituted or unsubstituted vinyl alcohol, aldehyde, ether, acetai, ketone, ester or substituted or unsubstituted N-vinyl heterocyclic compound.
5. A process according to any preceding claim wherein the gel exhibits syneresis.
6. A process according to any preceding claim wherein the hydrogel comprises a homo- or copoly(alkylene oxide).
7. A process according to Claim 6 wherein the poly(alkylene oxide) is cross-linked through reaction with isocyanate or unsaturated cyclic either groups.
8. A process according to Claim 6 or 7 wherein the hydrogel comprises a poly(ethylene oxide). 20
9. A process according to any preceding claim wherein the temperature at which the swollen gel is subjected to shear stress is regulated to provide a desired average particle size.
10. A process according to any preceding claim wherein an active substance is incorporated in the swelling agent.
11. A process according to any preceding claim wherein the swollen particulate gel is dried. 25
12. A particulate gel prepared in accordance with the process of any of the preceding claims.
13. A particulate gel according to Claim 12 which comprises an active substance.

Printed for. Her Majesty's Stationery Office by the Courier Press, Leamington Spa, 1982. Published by the Patent Office, Southampton Buildings, London, WC2A lAY, from which copies may be obtained