WO2000016816A1

WO2000016816A1 - Water-absorbing compositions and processes for their manufacture

Info

Publication number: WO2000016816A1
Application number: PCT/GB1999/002947
Authority: WO
Inventors: Peter Ralph Skidmore
Original assignee: INDUSTRIAL ZEOLITE (UK) Ltd
Current assignee: INDUSTRIAL ZEOLITE (UK) Ltd
Priority date: 1998-09-22
Filing date: 1999-09-22
Publication date: 2000-03-30
Anticipated expiration: 2001-03-22
Also published as: GB9820573D0; GB2341866A; GB2341866B; AU6098599A; GB9922379D0

Abstract

Water-absorbing compositions in granular form, useful in personal hygiene products such as cateminials and diapers and personal care products such as incontinence padding, as well as also in horticultural or agricultural contexts, comprise both super-absorbent water-insoluble polymeric material capable of absorbing water to form a gel without dissolving therein and at least one other water-insoluble but non-polymeric material, the granular composition substantially consisting of a co-granulate between all the water-insoluble materials present wherein each granule contains all said materials in substantially the same relative proportions as those wherein they are present in the composition as a whole. Processes for making such compositions use especially zeolite slurry and/or silica gel with residual sodium sulphate as the non-polymeric material.

Description

WATER-ABSORBING COMPOSITIONS AND PROCESSES FOR THEIR MANUFACTURE

The present invention is concerned with water-absorbing compositions (particularly those based on hydrogel-forming super-absorbers) and processes*fgr their manufacture. Such compositions may find diverse uses, but most commonly in the preparation of items for personal hygiene, such as cateminials and diapers, and for personal care, such as incontinence padding - and this invention will be described below primarily in that context. Economics permitting, the compositions are however also highly effective for various horticultural and agricultural purposes, as will be mentioned hereinafter.

Water-insoluble, hydrogel-forming polymers are already well known for personal hygiene, and perhaps to a more limited extent [because of economic considerations] for horticultural and agricultural use, in view of their ability to take-up, and subsequently retain water ... swelling with the water-uptake but without dissolving therein. They are commonly known as "super-absorbers", or "super-absorbent polymers" (sometimes SAPs) and those terms will be used herein as a convenient abbreviation for such water-insoluble polymeric materials. Super-absorbers are usually employed in the form of granules, the optimum size of the granule depending on the intended use. Often, though not necessarily always, the super-absorbers used are based on part- neutralised polyacrylate polymers. Such polymers can be made by methods such as that disclosed in EP 0,530,438, and before use they are conventionally blended into intimate mixtures with other materials, for instance so as to modify their surface properties, as described in for example GB 2,162,525.

It is however by no means easy to obtain a suitable granular material that incorporates a variety of desirable but different components and yet has sufficient homogeneity overall. Apart from anything else, each of the intended components must be brought into the right physical form before blending, otherwise the mixture cannot be expected to be homogeneous. Thus, for example, the super-absorber polymer must be made, ground, sieved, cross-linked as necessary and granulated. And other components, such as zeolites and colloidal silica, while normally available in finely- powdered form, will usually need to be agglomerated to a suitable size prior to incorporation in the blend. Preliminary processing steps of this kind all add to the cost of preparing the desired final product. Even so, unless highly efficient blending methods are used, there is always a risk of some non- homogeneity in the resulting product when first made - and subsequently there is a danger of segregation of the various components leading to non- homogeneity especially if some of the granules are rather frangible and therefore liable to break up again into finer particles.

The present invention seeks to provide improved water-absorbing compositions and processes for their preparation. We have found that the homogeneity of the end-product can be ensured and retained by forming a co-granulate between the super-absorbent water-insoluble polymer(s) and other water-insoluble but non-polymeric materials, and furthermore that contrary to expectation their incorporation into such a co-granulate does not noticeably impair the ability of the super-absorbers to take up water. We have moreover also found that such co-granulates, at least those of the preferred nature and proportions hereinafter disclosed, also display a wholly- unexpected and highly-useful reduction in frangibility.

According to one aspect of this invention there are provided water- absorbing compositions in granular form comprising both super-absorbent water-insoluble polymeric material capable of absorbing water to form a gel without dissolving therein and at least one other water-insoluble but non- polymeric material, said granular composition substantially consisting of a co- granulate between all the water-insoluble materials present wherein each granule contains all said materials in substantially the same relative proportions as those wherein they are present in the composition as a whole. The super-absorbent polymeric material employed may be any such material that will absorb water without dissolving therein. The term is used by experts in this field, and therefore understood by them, so it should need no further definition. Merely in case there could be a need to distinguish them from ordinarily absorbent material, that could be done on the basis that super-absorbent materials will absorb and retain more than their own weight of water - but those habitually employed for this purpose should in fact be capable of absorbing and retaining several times as much as that. The already-used such water-absorbing resins employed in personal hygiene products include the hydrolyzate of a starch-acrylonitrile graft polymer, the neutralisation product of a starch-acrylic acid graft polymer, the saponification product of a vinyl acetate-acrylic ester copolymer, the hydrolysate of an acrylonitrile copolymer and the hydrolysate of an acrylamide copolymer.

The currently-preferred super-absorbent polymeric material for use in personal hygiene products is a part-neutralised water-insoluble hydrogel- forming resin derived from cross-linked polyacrylic acid.

Such super-absorbent polymeric materials may be prepared by bulk or precipitation polymerisation, for example by the method described in EP 0,530,438. For example, cross-linked polyacrylates may be prepared by mixing acrylic acid, or a derivative, such as acrylamide, methacrylic acid or methyl methacrylate, singly or in admixture, with cross-linkers and other desired additives, such as water-soluble modified cellulose, and polymerising the mixture in bulk, in film form (for example up to about 5cm depth) or dispersed as droplets in a non-polar solvent such as hexane.

The water-insoluble non-polymeric material(s) selected in any particular case will depend primarily on the intended use for the water- absorbing composition. Subject to circumstances, including availability and cost, a wide range of such materials may find employment. When the end-product is intended for horticultural or agricultural use, the water-insoluble non-polymeric material(s) may include absorbent earths (preferably a diatomaceous earth) or expanded clay minerals (e.g. bentonite, dicalite, kaolin and most advantageously perlite) as well as organic fibres (preferabty^cellulose-based fibres such as wood pulp and cotton-wool).

When the end-product is intended for use in personal care products (e.g. incontinence pads) where aesthetic considerations are not of paramount importance, the water-insoluble non-polymeric material(s) may include those suggested for other purposes, but also carbon (for example activated carbon or carbon black) which is highly effective but can impart an unsightly "greyness" to the end-product.

When the end-product is intended for use in personal hygiene products, we currently prefer that the other water-insoluble non-polymeric material employed should consist of or include an aluminosilicate, which may be amorphous or crystalline, for example a zeolite such as zeolite A, P, X or Y.

These forms of zeolite differ in their crystal structure and Al.Si ratios. The aluminosilicates may be in sodium or potassium form, or ion-exchanged with other cations such as barium, calcium, magnesium and zinc. Although all these forms of zeolite are about equally effective, for reasons of availability and cost we prefer to use zeolite A.

Alternatively or in addition, the water-insoluble non-polymeric material may also advantageously consist of or include a silicate, for example colloidal silica or silica gel. If silicate is used, it is preferably incorporated in the form of silica gel, suitably obtained from sodium silicate by treatment with acid, e.g. sulphuric acid. We have found that there is no need to remove the residual sodium salt thus produced (e.g. Na₂S0₄) which would normally be filtered off to provide a commercially-acceptable grade of silica gel. It has been found that the residual sodium sulphate or other salt does little or no harm even if serving merely as a filler (while it may arguably serve an useful purpose in combating bodily odours of aminic nature) and even if it were marginally unwanted any such disadvantage would be outweighed by the reduction in processing costs achieved by its non-removal.

It is in fact a preferred feature of the compositions of this invention, especially_;-when they are intended for a personal hygiene end-use, that the other water-insoluble non-polymeric materials should include both zeolite and silicate. This is thought to enhance the ability of the compositions to combat bodily odours. Zeolite is basic in nature, and therefore can be expected to react with and neutralize acidic odour-producing substances such as phenols and thiols, whereas silica is acidic in nature and therefore can be expected to react with and neutralize basic odour-producing substances such as amines.

When both zeolite and silica are present, since both are advantageous the benefits of their incorporation can be secured over a wide range of relative proportions. There is however some evidence that currently persuades us that in this case the optimum relative proportions of the basic ingredients is about

SAP : Zeolite : Silica = 1 : 0.5 : 0.7 where "SAP" is used as an abbreviation for super-absorber polymer.

Besides the basic ingredients specified above, the compositions of this invention may and often will additionally include one or more performance- enhancers) and/or filler(s).

In the case of compositions especially intended for end-use in personal hygiene or personal care products, the performance-enhancer can advantageously be a deodorizer, notably one or more source(s) of sulphur- reactive metal cations and/or insoluble compounds of such metals. There are a number of such metals capable of performing this function, but at the practical level of availability and cost the choice falls on soluble zinc salt(s) and/or zinc carbonate and/or zinc oxide.

In the case of compositions especially intended for end-use in an horticultural or agricultural context, the performance-enhancer can advantageously include one or more of the following, viz. plant-nutrients and/or trace elements and/or slow-release and/or systemic insecticide(s) and/or hormonal rooting stimulants and/or degradation retardant(s).

For horticultural or agricultural use a preferred composition comprises, in parts by^_, weight, from 90% to 99.5% of the super-absorbent polymeric material and from 10% to 0.5% of the water-insoluble non-polymeric material. The water-absorbing composition for use in personal hygiene products will preferably contain, in parts by weight, from 20% to 80% of the super- absorbent polymeric material, and from 80% to 20% of the other water- insoluble but non-polymeric material, the balance (if any) of the composition being water and/or binder(s) and/or enhancer(s).

It may also be useful in certain circumstances to apply a surface coating of an inert powder such as talc or zeolite (where the zeolite is not otherwise a component of the agglomerate), the surface coating acting as a flow aid, reducing the tendency to become sticky on storage.

According to another aspect of this invention there are also provided processes for preparing water-absorbing compositions in which a super- absorbent water-insoluble polymeric material in particulate form is intimately mixed with at least one other water-insoluble but non-polymeric material also in particulate form and the resulting mixture is granulated, together with a liquid granulating agent, so as to form a co-granulate wherein each granule contains all said materials in substantially the same relative proportions as those wherein they are present in the composition as a whole, and the resultant co-granulate is dried. The granulating agent may most conveniently and economically be water, which however can if desired include a suitable binding agent. Suitable binding agents include, but are not restricted to, modified starches and celluloses, gums and vinyl polymers.

Preferably, the super-absorbent water-insoluble polymeric material and the other water-insoluble but non-polymeric material may be supplied in finely divided form (thus without preliminary agglomeration) prior to co- granulation.

In the preparation of the composition:

- the super absorber polymeric material is preferably incorporated in ground forrri, and may be cross-linked;

- if it employs zeolite, this is preferably incorporated either as a powder or still more advantageously as a slurry in water; and:

- if it employs silicate, this is preferably incorporated as undried silica gel, which moreover can be premixed with metal compounds and complexes such as oxides, hydroxides, carbonates, molybdates and phosphates, which in the case of sources of sulphur-reactive metal cations (such as zinc ions and/or insoluble compounds of such metals, e.g. zinc carbonate and/or zinc oxide) will serve as deodorizers in personal hygiene products, and in other cases will serve as plant nutrients and/or trace elements (such as molybdenum and/or copper) in horticultural-type products.

It will be appreciated that the water present in the aqueous zeolite slurry and/or in the silica gel will serve as the granulating liquid or part of it.

The granulation procedures to be adopted will be chosen dependent on the size of granules to be obtained. Methods and procedures for size enlargement, such as granulation and agglomeration, are well known in the art and are described in standard text books such as the Chemical Engineers' Handbook (Perry R. H. & Chilton C. H., McGraw-Hill) as well as the literature available from manufacturers.

The drying conditions are standard, and the temperatures employed will normally be in the range of from 50° to 650°C, preferably between 100° and 150°C.

The granulation is suitably effected to give a granule particle size believed to be appropriate to the intended use. For the cases currently envisaged, the granule particle size will generally be within the broad range of from 45μm to 5mm. Within that broad range, there are however narrower ranges usually adopted for particular purposes. Thus for example in horticulture or agriculture the range of from 45 to 150μm is usually adopted when the end- use of the product is to be in seed-beds, while the range of from 500 to 850μm rτ ay be preferable when the end-use envisaged is to be in plant- containers such as pots or flower-baskets, and the range of from 2mm to 5mm is probably best, when the end-use contemplated is for packing round the root-ball of transplanted shrubs or small trees.

On the other hand, for end-use in personal care products and especially in personal-hygiene products a granule particle size in the range of from 100 to 500μm will generally be preferred.

We have found that the process in accordance with the invention enables one to employ readily available starting materials of moderate cost without the need for elaborate processing steps prior to formulation of the composition. Thus, for example there is a considerable saving on processing cost if one can use silica gel without the requirement first to filter off sodium sulphate therefrom and/or if one can employ zeolite still in its slurry form.

The resulting product has been found to exhibit excellent homogeneity and to present a comparatively large surface area of super-absorbent for use in taking-up water. In addition, mateπal segregation problems are reduced, because of the homogeneity of the product. Material handling properties are also improved because of the higher mechanical strength (low frangibility) of the co-granulates as compared with the granulated materials, such as Zeolite A normally used in the blended mixtures. Unwanted granule particle size reduction and dust formation are thus reduced.

The invention of course also extends to water-absorbing compositions when prepared by the processes as herein described.

In order that the invention shall be well understood it will now be further described, though only by way of illustration, in the following examples and with reference (where indicated) to the accompanying drawings. Example 1 : Water-Absorbing Composition containing Zeolite

5kg of finely divided cross-linked polyacrylate super-absorbent polymer (predominantly of particle size below about 150μm) and 5 kg of Zeolite 4A were blended in a ploughshare mixer fitted with internal chopper blades and. mixed until a visually homogeneous dry mix was produced. Mixing was continued and water sprayed onto the mix until a friable crumb was produced. The damp mix was then dried to produce hard granules which were then milled and graded through standard mesh sieves and the fine (<150μm) and coarse (>850μm) fractions removed for reprocessing.

Example 2: Water-Absorbing Composition containing Silica Gel

10kg of a 3.3:1 ratio (Si0₂:Na₂0) silicate solution containing 40% total solids was reacted with dilute (25%) sulphuric acid and the resultant gel chopped in the mixer of Example 1. 3kg of super-absorbent polymer were then blended into the mass, after which the mass was extruded, dried, milled and graded as in Example 1. Example 3: Comparative Frangibility Tests

For the purposes of this Example, frangibility is defined as the percentage reduction in median particle size (d50, log-probability) of a powder when milled in a standardised way. This means that the smaller the frangibility of a material the more resistant the material is to "work damage" and the smaller the amount of dust produced during processing.

For this Example there was used a stainless steel ball mill of 1.05 litre capacity with a polished interior surface, internal diameter 115mm, containing 8 polished stainless steel balls each weighing 28.0g.

The median particle size of a 50.0g sample of each material was determined, the sample was then milled for 5.00 minutes at 100 rpm, the median size particle size of the milled sample was again determined, and the frangibility of the granulate was calculated by comparing the "before" and "after" median particle sizes. The results thus obtained are set out in tabular form below:

TABLE I

Example 4: Various Water-absorbing Compositions containing Zeolite in different proportions and comparative Frangibility Tests thereon

A series of co-granulate compositions were prepared, by a procedure generally similar to that described in Example 1 above, but with differing relative proportions between the SAP and the zeolite. Each such composition was then subjected to the frangibility test described in Example 3 above.

The details of the compositions as thus prepared and tested, as well as the frangibility test results thus obtained, were as follows: Example 4(i): 33.3% SAP / 66.7% zeolite co-granulate showed a percentage frangibility of 13.23%

Example 4(H): 40% SAP / 60% zeolite co-granulate showed a percentage frangibility of 9.44% Example 4(iii): A 50/50 composition of the two chemicals showed a percentage frangibility of 15.78% Example 4(iv): A 60% SAP / 40% zeolite co-granulate showed a percentage frangibility of 15.78% Example 4(v): A 66.7% SAP / 33.3% zeolite co-granulate showed a percentage frangibility of 31.20% These results are depicted graphically in Figure 1 of the accompanying drawings, from which it appears that the optimum ratio is at or around 50% SAP / 50% zeolite.

Example 5: Various Water-absorbing Compositions containing Carbon

** Black in different proportions and comparative Frangibility Tests thereon

A series of co-granulate compositions were prepared, by a procedure akin to that described in Example 1 above, but using carbon black in place of zeolite and with differing relative proportions between the SAP and the carbon black. Each such composition was then subjected to the frangibility test described in Example 3 above.

The details of the compositions as thus prepared and tested, as well as the frangibility test results thus obtained, were as follows:

SAP% Carbon Black% Frangibility

33.3 66.7 17.59

50 50 14.05

66.7 33.3 26.1

Example 5(i): A 33.3% SAP / 66.7% carbon black co-granulate showed a frangibility of 17.59%

Example 5(ii) A 50/50 co-granulate showed a percentage frangibility of 14.05%

Example 5(iii ): A 66.7% SAP / 33.3% carbon black co-granulate showed a percentage frangibility of 26.10% These results are depicted graphically in Figure 2 of the accompanying drawings, from which it appears that the optimum ratio is in the region of 50% SAP / 50% carbon black. Example 6: Various Water-absorbing Compositions containing both Zeolite and Silica in different proportions and comparative Frangibility Tests thereon

A series of co-granulate compositions were prepared, by a procedure similar to that described in Example 1 above, but containing both zeolite and silica gel and with different relative proportions between the various ingredients. Each such co-granulate was then subjected to the frangibility test described in Example 3 above. The details of the compositions as thus prepared and tested, as well as the frangibility test results thus obtained, were as follows:

SAP % Zeolite % Silica % Frangibility %

Example 6(i): 29.4 29.4 41.2 32.94

Example 6(ii): 37 37 26 31.82

Example 6(iii): 40 35 25 25.26

Example 6(iv): 41.7 29.2 29.2 34.21

Example 6(v): 45.4 22.7 31.8 34.7

Example 6(vi): 50 15 35 40.9

These results are depicted graphically in Figure 3 of the accompanying drawings, from which it appears that the best ratio tested was 40% SAP / 35% zeolite / 25% silica gel - but an optimum value was not determined in this series of tests.

Example 7: Various Water-absorbing Compositions containing both Silicate and residual Sodium Sulphate (Na₂SO ) in different proportions and comparative Frangibility Tests thereon

A series of co-granulate compositions were prepared, by a procedure similar to that described in Example 1 above, containing silicate incorporated as silica gel still containing residual Na₂SO₄ with different relative proportions between the various ingredients. Each such composition was then subjected to the frangibility test described in Example 3 above.

^5> SAP % Silica % Na?SQ₄ % Frangibility %

Example 7(i): 35.87 35.87 28.26 18.36

Example 7(ii): 44.41 31.09 24.50 10.71

Example 7(iii): 52.8 26.4 20.8 9.24

These results are depicted graphically in Figure 4 of the accompanying drawings, from which it appears that the best ratio tested was 52.8% SAP / 26.4% silica / 9.24% Na₂SO₄, but an optimum value was not determined.

Claims

1. Water-absorbing compositions in granular form comprising both super-absorbent water-insoluble polymeric material capable of absorbing water to form a gel without dissolving therein and at least one other water-insoluble but non-polymeric material, said granular composition substantially consisting of a co-granulate between all the water-insoluble materials present wherein each granule contains all said materials in substantially the same relative proportions as those wherein they are present in the composition as a whole.

2. Compositions as claimed in claim 1 , especially suitable for use in an horticultural or agricultural context or in personal-care products, in which the other water-insoluble but non-polymeric material is or includes absorbent-earth(s) and/or expanded clay mineral(s) and/or organic fibre(s) and/or carbon.

3. Compositions as claimed in claim 1 , especially suitable for use in personal hygiene products, in which the other water-insoluble but non- polymeric material is or includes one or more aiuminosilicate(s) including zeolite.

4. Compositions as claimed in claim 1 or claim 3, in which the other water-insoluble but non-polymeric material is or includes one or more silicate(s) incorporated in the form of silica gel and optionally including sodium salt(s) formed during the production thereof.

5. Compositions as claimed in claims 3 and 4, in which the other water-insoluble non-polymeric material includes both zeolite A and silicate, preferably in relative proportions substantially as follows:

SAP : Zeolite : Silica = 1 : 0.5 : 0.7 where "SAP" stands for super-absorbent polymer.

6. Compositions as claimed in any of the preceding claims, especially for end-use in personal hygiene or personal care products that additionally include performance-enhancer(s) in the form of one or more source(s) of sulphur-reactive metal cations and/or insoluble compounds of such metals, notably soluble zinc salt(s) and/or zinc carbonate and/or zinc oxide.

7. Compositions as claimed in any of claims 1 to 5, especially for end-use in an horticultural or agricultural context, that additionally

in the form of one or more plant- nutrients and/or trace elements and/or slow-release and/or systemic insecticide(s) and/or hormonal rooting stimulant(s) and/or degradation retardant(s).

8. Processes for preparing water-absorbing compositions in which a super-absorbent water-insoluble polymeric mateπal in particulate form is intimately mixed with at least one other water-insoluble but non-polymeric material also in particulate form, and the resulting mixture is granulated, together with a liquid granulating agent, so as to form a co-granulate wherein each granule contains all said materials in substantially the same relative proportions as those wherein they are present in the composition as a whole, and the resultant co-granulate is dried.

9. Processes as claimed in claim 8, in which the liquid granulating agent is or includes water, with or without a binder.

10. Processes as claimed in claim 8 or claim 9, in which the super- absorbent polymeric material is incorporated into the co-granulate in finley-ground form, optionally cross-linked.

11. Processes as claimed in any of claims 8 to 10, in which the other water-insoluble but non-polymeric material is or includes a zeolite and in which the latter is incorporated into the co-granulate in the form of an aqueous slurry of finely-divided particulate-form zeolite, the water in said aqueous slurry serving as the granulating agent or part of it.

12. Processes as claimed in any of claims 8 to 11 , in which the other water-insoluble but non-polymeric material is or includes silicate and in which the latter is incorporated into the co-granulate in the form of undried silica gel, optionally pre-mixed with any desired metal compound(s) and/or complex(es), the water in said silica gel serving as the granulating agent or part of it.

13. Processes as claimed in any of claims 8 to 12, in which granulation is effected by conventional granulation techniques to yield a granulate particle size in the range of from 45μm to 5mm.

14. Processes as claimed in claim 13, intended to yield an end- product especially useful in personal care or personal hygiene products, in which granulation is effected to yield a granulate particle size in the range of from 100 to 500μm.

15. Use in the manufacture of personal hygiene products such as cateminials and diapers or personal care products such as incontinence padding of a water-absorbing composition as claimed in any of claims 1 to 7 and/or as prepared by the process of any of claims 8 to 14.