WO1993017066A1

WO1993017066A1 - Absorbent polymers and their production

Info

Publication number: WO1993017066A1
Application number: PCT/GB1993/000357
Authority: WO
Inventors: Ian Michael Johnson
Original assignee: Allied Colloids Limited
Priority date: 1992-02-20
Filing date: 1993-02-19
Publication date: 1993-09-02

Abstract

Absorbent particles consisting essentially of zones of polymeric material interconnected by linkages through an effective agglomerating amount of silica or silicate can be made by forming a gel mass by mixing particles of polymeric material with water and an effective agglomerating amount of a silica or silicate, and drying and comminuting the gel mass.

Description

Absorbent Polymers and their Production This invention relates to aggregates of water swellable polymeric material.

We describe in, for instance, EP-A-401044 processes of making water insoluble, water swellable, particulate polymeric material from dry particles of water swellable polymer derived from water soluble ethylenically unsaturated material, wherein the process comprises forming a gel mass by absorbing aqueous solution of cross linking agent into the dry particles, causing cross linking of the polymer in the gel mass by reaction between the cross linking agent and pendant groups in the polymer, and drying and comminuting the gel mass. We also describe water insoluble, water swellable, particulate polymeric materials formed from ethylenically unsaturated material wherein the polymeric material has a polymer structure that is formed of zones in which the polymer is substantially free of cross links between pendant groups in the polymer and the zones are interconnected by cross links through pendant groups in the polymer. We describe that a polyvalent metal compound such as a compound of aluminium can be suitable for causing the cross linking between the polymer particles, and this will lead to ionic cross linking. A suitable material for this purpose is aluminium sulphate or other polyvalent metal compound salt.

We also describe that the cross linking compound is preferably one that leads to covalent cross linking and various hydroxy-containing and silane compounds are described for this purpose. We describe that the technique is particularly valuable because it is capable of converting polymer fines into useful, coarser products, and that absorbent particles obtained in that invention, especially when the starting particles are cross linked sodium acrylate or similar particles, have particularly suitable properties when high gel capacity and high rate of absorption are required. Although these processes are very successful for many purposes, it is sometimes found that the resulting aggregates crack or disintegrate during transport or use. It would be desirable to find a way of conducting the process, and producing products, that have satisfactorily low friability and advantageous properties from the point of view of total absorption and rate of absorption, for instance in diapers.

In the invention, an absorbent product comprises water swellable, particulate aggregates which consist essentially of polymeric material derived from ethylenically unsaturated material and which each comprise zones of the polymeric material interconnected by linkages through an effective agglomerating amount of silica or silicate. A process according to the invention for making water swellable, particulate aggregates from particles of a water swellable polymeric material derived from water soluble ethylenically unsaturated polymer comprises forming a gel mass by mixing the particles of polymeric material with water and an effective agglomerating amount of a silica or a silicate, and drying and comminuting the gel mass. •

The mixing conditions must be such as to convert the mixture of polymer particles, water and silica or silicate to a gel mass, and if there is any tendency for the particles to remain separate it may be desirable to increase the force of mixing. For instance the gel mass may be formed in an extruder used for the comminution.

In the invention, a mixture is formed of the polymer particles (generally in dry form) , the silica or silicate, and water. The silica may be present as a suspension in water and the silicate may be present as a suspension or a solution in water,, in which event the mixing stage can be conducted merely by mixing the polymer particles with the resultant aqueous suspension or solution. Preferably, however, dry particulate silica or silicate is mixed with the polymer particles generally while they are still wholly or substantially dry, and additional water is then mixed with the mixture to form the gel mass.

The preferred technique comprises dry mixing the polymer particles with dry silica particles, and then mixing the resultant mixture with water to form the gel mass.

The water that is used for mixing with the polymer particles may be a carrier for the silica or silicate. When, as is preferred, the polymer contains pendant groups (e.g. carboxylic groups) the water may also contain cross linking agent that will cross link between such groups. Suitable cross linking agents are described in EP 401044, such as any of the hydroxy or epoxy or silane compounds mentioned therein as cross linking agents. The silica is preferably a fumed or precipitated silica and so will have very large surface area when dispersed in water, for instance 50 to 1000 g/m and usually 100 to 750 g/m². Preferably the surface area is above 100 or 150 g/m , preferably above 200 g/m . The particle size of the individual particles is generally below 30μm and typically in the range 0.1 to lOμm, often below 5μm, for instance 1 to 5μm. The silica is often supplied as aggregates, e.g. having a size of 10 to lOOμm.

Instead of using fumed silica or precipitated silica, insoluble silicates can be used although they are usually less effective. The silicate is preferably silicic acid or a swelling clay, colloquially known as bentonite, which in practice is normally a smectite clay. If it is particulate, it should have small particle size and large surface area, e.g. as described above.

Colloidal or true solutions of silicates or silicic acid may be used instead of suspensions of insoluble silicates, but again they are usually less effective than fumed or precipitated silica. Sufficient water must be applied to convert the starting polymer fines to an aqueous gel mass and the actual amount in any particular process will depend on the moisture content of the polymer fines and the nature of the silica or silicate. For instance when, as is preferred, the silica is capable of absorbing significant amounts of water it is necessary to include sufficient water to allow for this absorption and for conversion of the particles to a gel mass. The total amount of water typically ranges from 25 to 300% based on the dry weight of polymer particles, with the higher amounts of water (e.g., above 150%) being preferred when water absorbent silica is incorporated. As explained in EP 401044, the extent of cross linking of the polymer particles influences the amount of water that is optimum.

The amount of silica or silicate must be sufficient to provide sufficient linkages between adjacent starting polymer particles in each aggregate such that the dry aggregates do not disintegrate to any significant extent when they absorb urine or other liquid. If too much is used, it may act as a release or barrier coating and prevent linkages being formed. If too little is used, it may not permit adequate linkages to be formed. The optimum depends on the material but is typically from 1 to 20% by weight based on the dry weight of polymer particles, with best results generally being obtained with amounts in the range about 3 or 5 to about 15%. However higher amounts, ©•?• up to 25% or even more, may be appropriate in some instances especially when using a coarser or less effective material than fumed or precipitated silica.

The polymer particles that are aggregated in the invention can be water soluble polymer particles or, preferably, water swellable but water insoluble polymer particles. The aggregates are preferably water insoluble.

The dry particles that are used to form the gel mass preferably have a substantial proportion of fine particles in them. Generally at least 30%, and often at least 50%, may be fines, for instance below 200μm and usually below 150μm, preferably below lOOμm. Particles below 50μm can cause particularly serious dusting problems and so the invention is of particular value when applied to a fraction having at least 50%, and sometimes at least 90% by weight below 50μm.

The initial particles can be a fraction sieved from a product, in which event at least 90% of the particles will generally be below 200 and usually below 150μm, usually below lOOμm.

The invention is, however, also of value for upgrading the quality of a particulate product that does contain particles of a satisfactory size above 200μm but also contains an unacceptably large proportion of fines. Thus the dry particulate product that is formed into the gel mass can be a product which is 0 to 70% by weight, often 0 to 50% by weight, above a chosen particle size and 30 to 100%, often 50 to 100% by weight below that particle size, generally 200μm or 150μm, more preferably lOOμm.

The dry particles may have been made by reverse phase bead polymerisation, generally followed by sieving to produce a fines fraction for use in the invention, but more usually is made by a process comprising drying and comminution of larger particles. These larger particles may have been made by reverse phase bead polymerisation but, more usually, are made by comminution of gel made by bulk gel polymerisation. The polymer in these particles can be linear but is generally a cross linked polymer. Usually it is a polymer formed from water soluble ethylenically unsaturated material comprising polyethylenically unsaturated cross linking agent with the result that the polymer is cross linked in the backbone of the polymer. The water soluble ethylenically unsaturated material may be a single water soluble ethylenically unsaturated monomer or may be a water soluble blend of ethylenically unsaturated monomers.

The ethylenically unsaturated material can be non- ionic, anionic or cationic. If it is ionic, it can be a blend of non-ionic and ionic monomers, the amount of non- ionic monomer often being 0 to 90%, and usually 0 to 70%, by weight of the blend.

The preferred ethylenically unsaturated material is generally anionic and may be formed from anionic monomer alone or a blend of anionic and non-ionic monomers. Although sulphonic monomers (e.g., allyl sulphonate or 2- acrylamido-2-methyl propane sulphonate) can form part of the anionic monomer, it is generally preferred that the ethylenically unsaturated material should comprise carboxylic monomer, for instance methacrylic acid, maleic acid, itaconic acid, crotonic acid or, preferably acrylic acid.

Suitable non-ionic monomers include vinyl pyrollidone, methacrylamide or, preferably, acrylamide. Suitable cationic monomers include dialkylaminoalkyl (meth) -acrylamides and -acrylates, usually as acid addition or quaternary salts. Examples are dimethylaminoethyl (meth) acrylates and dimethylaminopropyl; (meth) acrylamides. Preferred polymers are formed from 10 to 100%, often 30 to 100%, acrylic acid with the balance being acrylamide. The acrylic acid can be entirely in the form of free acid but, provided this does not interfere with the cross linking reaction, at least 50% and usually at least 75% of the acid groups are present in the form of sodium or other alkali metal or other water soluble salt.

The dry particles will often have conventional dryness as a result of drying followed by exposure to the ambient atmosphere, for instance below 15% moisture, and so the process can be applied to the treatment of previously made product. However the process of the invention can also be conducted as part of the overall production process starting with polymerisation of the monomer or monomer blend and then the dry particles do not have to be dried fully before they are rewetted and can, instead, still be slightly moist, for instance having a moisture content up to 20 or 25%. The must however be sufficiently dry that they behave as fully dried particles rather than as sticky particles.

The aggregates are particulate and generally have a particle size of at least 90% by weight between 100 and lOOOμm. They generally have a gel capacity of at least 20

(and preferably at least 50) grams deionised water per gram aggregate.

The absorbent products of the invention can be used as a replacement for conventional absorbent polymeric particles, e.g. in dewatering of mineral slurries or in absorbent articles such as diapers or sanitary napkins.

Example l

Fines (generally having a size in the range 3 to 50μm) of cross linked sodium polyacrylate are dry blended with particulate fumed silica having a particle size below 4μm and the mixture is then sprayed with sufficient water to convert the mixture to a substantially homogeneous a d sticky gel. When the amount of silica is 5%, an appropriate amount of water is 35g per 50g fines, but when the amount of silica is 15% an appropriate amount of water is 105g.

In each instance, the resultant gel is extruded, comminuted and dried, for instance in a fluid bed dryer, in a manner that is conventional for drying polymer gel. The resultant product has very high absorbency and high gel strength both when dry and when swollen. For instance the gel strength in one example was 47,000 at a torque of 100,000 and an amplitude of 0.01A, compared to a gel strength of 36,000 when the silica was omitted and the water contained 0.1% ethylene glycol diglycidyl ether, as in EP 401044.

Example 2

The process of Example 1 is repeated except that 1% of that ether is incorporated in the water that is used for bonding the polymer-silica mixture. The gel strength increases from 47,000 to 75,000. Further increase is obtainable when the ether is replaced by the silane cross linking agent used in the examples in EP 401044. Example 3

In an alternative process, instead of dry mixing the silica with the polymer and then adding water, 47.5g polymer fines are mixed with a suspension of 2.5g silica in 25g water. The mix is comminuted and dried as in Example 1. This gives slightly lower gel strength and absorption capacity than when the silica is dry mixed, but the results are still satisfactory.

Claims

CLAIM8

1. A process for making water swellable particulate aggregates from particles of water swellable polymeric material derived from water soluble ethylenically unsaturated polymer comprising forming a gel mass by mixing the particles with water and drying and comminuting the gel mass, characterised in that the water includes an effective agglomerating amount of a silica or silicate.

2. A process according to claim 1 in which the water includes an effective agglomerating amount of a fumed or precipitated silica.

3. A process according to any preceding claim comprising forming a substantially dry mix of the polymer particles and the silica or silicate and then mixing the water with this mix.

4. A process according to any preceding claim comprising dry mixing dry fumed or precipitated silica with the polymer particles while substantially dry and then mixing the resultant mixture with water to form the gel mass.

5. A process according to any preceding claim in which the polymer includes pendant groups and the water contains cross linking agent that will cross link between the pendant groups.

6. A process according to any preceding claim in which the silica or silicate provides a surface area of above 150g/m , the amount of silica or silicate is from 1 to 20% by weight based on the dry weight of polymer and the amount of water that is added to form the gel mass is from 25 to 300% by weight based on the dry weight of polymer.

7. A process according to any preceding claim in which the aggregates are water insoluble water swellable aggregates and are formed from water insoluble water swellable polymer particles.

8. A process according to claim 7 in which the water swellable polymer particles are formed of a cross linked polymer derived from 10 to 100% acrylic acid and 90 to 0% acrylamide.

9. A process according to any preceding claim comprising blending cross linked sodium polyacrylate particles having a size range mainly 3 to 50μm with fumed or precipitated silica and the resultant dry mix is mixed with water to form a substantially homogenous and sticky gel, and this gel is comminuted and dried.

10. An absorbent product comprising water swellable, particulate aggregates which consist essentially of polymeric material derived from ethylenically unsaturated material and which each comprise zones of the polymeric material interconnected by linkages through an effective agglomerating amount of silica or silicate.