WO1993022048A1

WO1993022048A1 - Absorbing material

Info

Publication number: WO1993022048A1
Application number: PCT/SE1993/000379
Authority: WO
Inventors: Berne F. Ellers; Curt H. Appelgren
Original assignee: Ellers Berne F; Appelgren Curt H
Priority date: 1992-04-29
Filing date: 1993-04-29
Publication date: 1993-11-11
Also published as: SE9201349D0; AU4275593A

Abstract

The present invention relates to a process for preparing an absorbing material comprising a polymer material which swells and forms a gel while absorbing a liquid, whereby the absorbing material has been provided with a meltable, adhesive structure forming composition.

Description

ABSORBING MATERIAL.

DESCRIPTION Technical field

The present invention relates to a process for manufacturing a water absorbent article in which particles of a water swellable polymer are fixed to a substrate absorbing material.

The object of the present invention is to obtain absorbing pro¬ ducts, preferably water-absorbing products, which can be applied to a fibrous structure in a safe way.

Background of the invention.

Absorbing products, and in particular so called super absorbing products , are products and compounds that absorb typically 40 to 60 times its weight with respect to a saline solution and 100 to 200 times its weight with respect to pure water. The products are used within different sectors , but have preferably been used within the food industry sector, hygienic products sector, and medical technical sector. In the first mentioned case for the absorption of liquid from prepacked foodstuffs such as meat and fish, for example to absorb blood juice or melt water from iced fish, or in the second case with regard to hygienic articles of manufacture as diapers and sanitary napkins for the absorption of urine. These super absorbents are generally present in powd- ered form which means that when being introduced into a substra¬ te, fibrous structure of cellulose or synthetic fibres or mix¬ tures thereof, which is quiet common in hygienic products, these absorbents will be readily released from the structure during transport, and will fall out with an accompanying reduced ab- sorbing effect as a consequence. One has tried to eliminate this by compressing the structure maximally in order to bind the the absorbent but has not succeeded completely. Other ways of solv¬ ing the problem is to manufacture laminates where the absorbent is distributed between two layers of materials which are bound together using a thermoplastic powder. The absorbent can also be manufactured in the form of a polyacrylate fibre being coated with a high absorbing coating, but this will increase costs of the product considerably. As one, in the hygienic products of today, such as in a diaper, adds the absorbent intermittently in islands in an attempt to optimize the efficiency thereof, this will create a problem in that so called gel blocking easily oc¬ curs which means that all particles of the absorbent can not be reached by a liquid but remain unused. The technology of today further includes a limitation in that different qualities of ab¬ sorbents and/or sizes of particles of absorbent can not be used together in a simple way.

A further problem using the absorbents of today in the form they are used is that only certain particle sizes can be used and then preferably the larger particle sizes. The smallest particl¬ es can not, due to a the off-separation mentioned above be used which means increased costs.

At the formation of the final products generally dry methods are used as the presence of moisture means that the absorbents are being activated. Such dry methods are used at the manufacture of diapers, sanitary napkins , non-woven material for hygienic and medical use, filter materials, package materials, whereby soft paper, so called fluffy pulp of cellulose fibres and staple fib¬ res of polyethylene, polypropylene, polyesters, polyamides, or other poly olefinic fibres, is used. The selection of fibre de- pends on the different demands which are requested from the fi¬ nal product. The differences in the types of fibres are primar¬ ily in the specific area they provides. The advantage of so called poly olefinic fibre is that it has a large specific area,

2 8 to 15 m /g thus encapsulates a powder better in the capillary structure but as the same fibre requests high temperatures for being activated the cellulose fibre will easily become disco¬ loured and may even be ignited. Staple fibre, specific area 2 to

2 3 m /g has a very poor capability of binding the absorbent par¬ ticles in a non-woven material.

Fluffy pulp is most often formated on a tissue of cellulose in order to prevent losses of short cellulose fibres. In diapers and sanitary napkins a surface layer of non-woven material, such as of polypropylene, is most often used. In order to maintain the absorbent the request for density of these materials has been increased which in turn render passage of liquid of the surface layer more difficult.

As evident from above great problems exist in the off-separation of absorbent from the material.

There are thus a number of problems which look for their solu¬ tions.

Description of the present invention it has now surprisingly been shown possible to be able to solve these problems by means of the present invention which is cha¬ racterized in that the absorbing material is provided with a structure of adhesive material and in particular the process of the invention is characterized in that a) applying to the surface of water swellable particles a struc- ture of a meltable adhesive composition to form secondary parti¬ cles, the meltable, adhesive composition being applied in mol¬ ten, non-solvent containing form to said particles, b) bringing the secondary particles into contact with said sub¬ strate and c) applying heat to melt the meltable, adhesive composition of the said secondary particles to cause said secondary particles to adhere to said substrate.

Further characteristics are evident from the accompanying claims.

By means of the present invention there is provided (a) that the absorbent can be bound to a substrate, either completely or in fibre form by means of the meltable, adhesive composition, (b) that the absorbent can be bound to a fibre web while being formed in dry condition, (c) that the absorbent maintains its absorbing ability and (d) that different particle sizes of the absorbent as well as different absorbent qualities can be bound and used together. Further advantages are evident from the fol¬ lowing.

Absorbing material hereby intends to mean polyaer lates, car- boxymethyl cellulose, modified cross-linked starch, and other polysaccharides. Such absorbents are sold under e.g. the trade names SALSORB 84, and SALSORB DD (polysodiumacr late) , AQUA KEEP 10 SH-P, AQUA KEEP X 50, AQUA KEEP X 65, LANSEAL-F (polyacrylate fibre), DRYTECH 510 (cross-linked aerylate copolymer), GELOK 40 (cross-linked starch grafted polyacrylate polymer) , AKUCELL S ( cross-linked sodium carboxymethyl cellulose).

Suitable structure forming materials to provide the meltable, adhesive composition are polyols , polyethylene glycol, emulsi- fiers in the form of esters (Dimodan, Dimodan TH; distilled mo¬ noglycerides, Acidan; citric acid esters of monoglycerides, Ce- todan; acetic acid esters of monoglycerides, Lactodan; lactic acid esters of monoglycerides , Panodan; diacetyl tartaric acid ester of monoglycerides, Promodan; propylenglycol esters of fat¬ ty acids, Triodan; polyglycerol esters of fatty acids), fatty acid alcohols (cetanol, stearyl alcohol, palmitol) , fatty acids (stearic acid, palmitinic acid), hardened fats, waxes (bees wax, bamboo leaf wax, carnauba wax, Japan wax, shellac wax, cotton wax, micro-crystalline wax), paraffin wax, melt glue (Lunatack based on colophonium resins), INSTANT-LOK, either used alone or in different combinations thereof.

The structure forming material may be provided with up to 30 % by weight of a plasticiser in the form of glycerin, diethyl- phtalate, Citroflex esters of citric acid), triacetin, propylene glycol, or sorbitane fatty acid esters, such as sorbitane raono- laurate, sorbitane monostearate, sorbitan monopalmitate, and sorbitane sesqui oleate, sorbitan monooleate, as well.

Different structure forming materials can be combined, as well as different plasticisers. Hereby a slower softening temperature

Δ _ is achieved, which is positive contrary to a very rapid, exact melting point.

The term structure forming material used means that the material does not form a continous coating onto the waterswellable par¬ ticles, but form either a net structure, or a spotwise appearing structure which is able to form a mechanical bond between the absorbent and the substrate used as a carrier to said absorbent.

The structure of meltable material should have a melting tempe¬ rature exceeding the temperature of use with about 10 C, i.e. in case of use in diapers and sanitary napkins, a melting tempera¬ ture of at least 50 C.

To obtain an improved water penetration at the use of water in¬ soluble structure forming materials water soluble crystals can be incorporated into the structure.

Other substances that may be incorporated into the structure are odour eliminating substances in the form of calcium hydrogen phosphate having 2 crystals of water, flavour providing com¬ pounds (deodorants), bacteriostatic or bacteriocidal compounds such as iodine, sorbic acid, bensoic acid, hexachlorophen, lac¬ tic acid.

Further examples of substances that might be incorporated are fertilizers.

The amount of structure forming material onto the absorbent should amount to 5 % by weight.

The absorbent having a particle size of from <5 _χum to up to 1 mm, typically 90 to 8000 ,um, can be provided with the structure forming material in a coating pan (pan-coating), drum coating, roto-coater, intense mixer, so called batch mixer, fluid bed coater, or using a melt application technology disclosed in SE- A-8500487-7. At the application in an intense mixer the absorbent and the structure forming materials are brought and treated together during heat addition. At the application in a fluid bed device the absorbent is introduced into the device and is fluidized us¬ ing air. The structure forming material can then be added as a solid product and applied to the absorbent while heated, or be added as a melt through nozzles provided above or underneath the fluidized bed.

The application of the solid product with itself can be done in an apparatus described in SE-C-7903053-2 and in such a way as described in SE-A-8500487-7.

The apparatus according to SE-C-7903053-2 comprises a mixing house provided with a cover. Within the housing a disc is ar¬ ranged which disc consists of an upper disc and a lower disc, which disc is rotatably mounted around a shaft by means of a bearing. Between the upper and the lower discs there is a hollow space which is connected to the upper side of the upper disc via an annular slot consisting of two parts which both take the shape of a circumferential surface of a cut cone with its point directed downwardly. The upper disc has blades cut on its upper side and optionally cut, or mounted blades on its side under- neath. Between the blades spaces exist which are intended to re¬ ceive particles to be coated while said space optionally being divided into compartments by means of said blades is intended to receive the melt which is to be used for coating. The lower disc is provided with an edge extending around it close to the open- ing of the slot. At its periphery the lower disc is provided with blades for throwing out the final product via a diffusor and an outlet. The cover is provided with a compartment placed above the centre of the disc which compartment is arranged to receive the particulate solid material for a further transport thereof to said above mentioned spaces between the blades . The particulate solid material is fed to the compartment of the co¬ ver by means of a transporting device, e.g. a feeding screw. The liquid phase is being fed to the space via a tube.

At the application operation according to SE-A-8700487-7 the discs are rotated within the housing around the shaft with a re- volving speed of between 1000 to 500 rpm. A suitable peripheral speed using a disc diameter of 300 mm is 1500 to 5000 m per min¬ ute whereby the lower speed is used for embedment/agglomeration and the higher for coating. The coating melt is thereby fed to the apparatus via the tube to the hollow space between the upper and the lower discs. The tube is thereby provided with heating means in order to keep the material in melted, liquid form. By means of the centripetal force and the blades the melt is thrown out-wardly through the slot and further through the outer slot. The melt hereby takes the form of a film which extends outwardly all the time simultaneously as it becomes thinner. When the melt leaves the outer slot the film is torn up into very small drop¬ lets when it leaves the edge , whereby a mist curtain of droplets having a microscopic size are formed. Simultaneously with the addition of melt through the tube a particulate material is add- ed by means of the transporting device to the compartments and further on to the spaces on the upper side of the upper disc. From there the particulate material is thrown, by means of the centripetal force, and the blades, outwardly towards the peri¬ phery of the upper disc, simultaneously as it is deagglomerated into primary particles which meet with and pass the mist curtain at the edge. The particles hereby obtain a surface coating of the melt and are further thrown outwardly to the blades arranged at the periphery of the lower disc, which blades throw the pro¬ duct out off the apparatus via the diffusor and the outlet. De- pending on the number of blades their height and the throughput, which can be varied using the outlet area, one can get an agglo¬ meration of the particles , or get the particles out as separate sole, coated particles. The granulate obtained is directly chilled with air during the throwing out thereof and is further chilled at a following separation in a cyclone or the like.

The invention will be further disclosed in the following with reference to some embodying examples .

Example 1

A 50:50 mixture of a slow absorbent. Aqua Keep X65, and a rapid absorbent. Aqua Keep 10 SH-P (acrylic acid polymer) was provided with 23 % calculated on the final weight, with a structure form¬ ing material consisting of 25 % of Dimodan TH, 50 % of poly¬ ethylene glycol (PEG 6000/PEG 400 90:10), and 25 % of Lunatack (melting glue) according to the process according to SE-A- 8500487-7.

Example 2

A 50:50 mixture according to Example 1, was provided with 20 % by weight of the final weight, with a structure forming material consisting of 75% of Dimodan (an emulsifier) , and 25% of Cetodan (an emuldsifier) according to the same process as in Example 1.

Example 3

A 50:50 mixture according to Example 1, was provided with 20 % by weight of the final weight, with a structure forming material consisting of 75% of^" Dimodan TH, and 25% of Cetodan according to the same process as in Example 1.

Example 4

A 50:50 mixture according to Example 1, was provided with 20 % by weight of the final weight, with a structure forming material consisting of polyethylene glycol (PEG 6000/PEG 400 90:10) ac¬ cording to the same process as in Example 1.

The products thus obtained were tested with regard to adhesive¬ ness to a dry formulated web having a layer of staple fibre sur¬ rounded on each side by a layer of tissue fibre (non-woven, cel¬ lulose fibre). The granular grains provided with a meltable structure forming material and having a size of 500 to 1000 .urn were adhered using heat radiation for 20 sec. At shaking no off- separation could be observed. The products were tested with regard to absorbent activity as well, in a 0.9 % saline solution. Hereby 1 g of the respective granular powder was dissolved in 70 ml saline solution for 5 min whereupon the gel obtained was allowed to drip off placed on a screen for 2 in., whereupon the volumes obtained were measured. Then the gel was placed between two pieces of filter paper the weights of which were determined prior to and after 60 sec. The ability of the filter paper to absorb liquid from the gel is a measure of the so called rewetting degree of the gel. The fol- lowing data were obtained, Table 1.

1 g of powder contains 0.8 g of absorbent. The amount of liquid absorbed is direct proportional to the weight of absorbent. Re¬ vised values with regard to weight are thus given. It is evident from these that complete absorption is obtained in spite of the applied structure forming material. In the same way the slow ab- sorbent is thus influenced that the combination becomes faster in Ex. 2 and Ex. 3 than can be expected.

Besides the above indicated areas of use one can utilize the present invention for the binding of grains/seeds in a non-woven structure. Thus it is possible to bind grass seeds in a simple way together with an absorbent and then roll it out for provid¬ ing of a lawn.

Claims

- CLAIMS.

1. A process for manufacturing a water absorbent article in which particles of of a water swellable polymer are fixed to a substrate, which process comprises: a) applying to the surface of water swellable particles a struc¬ ture of a meltable adhesive composition to form secondary par¬ ticles, the meltable, adhesive composition being applied in mol¬ ten, non-solvent containing form to said particles, b) bringing the secondary particles into contact with said sub- strate and c) applying heat to melt the meltable, adhesive composition of the said secondary particles to cause said secondary particles

^'to adhere to said substrate.

2. Process according to claim 1, characterized in that the melt¬ able, adhesive composition is at least 5 % by weight of the fi¬ nal product.

3. Process according to claim 1, characterized in that the ab- sorbing material is polyacrylates, carboxymethyl cellulose, mod¬ ified cross-linked starch and other pol saccharides.

4. Process according to claim 1, characterized in that the melt¬ able, adhesive structure forming material is a polyol, poly- ethylene glycol, emulsifiers in the form of esters, fatty alco¬ hols, fatty acids, waxes, paraffin waxes, melting glue, and com¬ binations thereof

5. Process according to one or more of the preceding claims, characterized in that the structure formingmaterial further com¬ prises a plasticiser in the form of glycerine, diethylphtalate, Citroflex, triacetin, propylen glycol, and combinations thereof.

6. Process according to one or more of the preceding claims, characterized in that the structure forming material further comprises odour eliminating compounds, flavour providing sub¬ stances (deodorants), bacteriostatic or bacteriocidal compounds, and combinations thereof .

7 . The use of an absorbing material prepared in accordance with claim 1 to 6 , in a product intended for food-stuffs .

8. The use of an absorbing material prepared in accordance with claim 1 to 6, in a product intended for hygienic purposes.

9. The use of an absorbing material prepared in accordance with claim 1 to 6, in medical technical product.

10. The use of an absorbing material prepared in accordance with claim 1 to 6, in technical horticultural product.