GB2246373A

GB2246373A - Nonwoven fabric

Info

Publication number: GB2246373A
Application number: GB9015349A
Authority: GB
Inventors: Roger Gavin Dingley
Original assignee: Arco Chemical Technology LP
Current assignee: Lyondell Chemical Technology LP
Priority date: 1990-07-12
Filing date: 1990-07-12
Publication date: 1992-01-29
Also published as: GB9015349D0

Abstract

A non-woven fibrous web capable of absorbing many times its weight of aqueous liquid without losing its integrity, but which on contact with excess aqueous medium substantially disintegrates, comprises a blend of superabsorbent fibres and fibres of at least one thermoplastic polymer said web having been treated to fuse the fibres preferably in discrete zones throughout the web. The thermoplastic fibres may be bicomponent polypropylene and fused by heated calender rolls.

Description

ABSORBENT PRODUCTS The present invention relates to non-woven fabrics having water absorbing properties and to absorbent products made therefrom. In particular, the invention concerns non-woven fabrics comprising fibres of a superabsorbent material blended with fibres of at least one thermoplastic material capable of being thermally bonded to itself and/or said superabsorbent material.

Superabsorbent materials in fibre form have been developed. These are crosslinked copolymers having pendant carboxy groups some of which are carboxylic acid groups and some of which are carboxylate salt groups.

Examples of these copolymers are crosslinked copolymers of maleic anhydride and isobutylene or styrene which are partially neutralised with for example sodium hydroxide, such as those described in EP 0361842 and EP 0272074.

These superabsorbent materials and fibres made therefrom are characterised by their ability to absorb large amounts of water or saline liquid, for example 5 grams of such a fibre can absorb up to 1 litre of water. When a large amount of liquid is absorbed by the fibres a gel-like consistency results. In general the amount of saline liquid which can be absorbed by the fibres is less than the amount of water.

These superabsorbent fibres are intended for use in disposable absorbent products having high strength and integrity when wet such as baby diapers, incontinence ware and feminine hygiene products. Such products presently available suffer from the disadvantage that they do not readily disintegrate in water and on absorbing a large amount of water may swell, causing blockages in domestic sewage systems. Also these products tend to be bulky if they are required to absorb large amounts of liquid.

According to the present invention there is provided a non-woven fibrous web comprising a blend of superabsorbent fibres and fibres of at least one thermoplastic polymer, said web having been treated to fuse the fibres, the relative proportions of the superabsorbent fibres and the fibres of at least one thermoplastic polymer and the manner of fusion being such that the web is capable of absorbing many times its weight of aqueous liquid without losing its integrity but being capable of substantial disintegration on contact with excess aqueous medium.

The whole of the web may be subjected to the treatment required for fusion, or the treatment may be confined to discrete zones of the web.

One suitable treatment comprises heating the web while applying pressure to compress it.

Without wishing to be bound by the following theory it is believed that when the web is subjected to heat treatment while compressing it, superabsorbent fibres become trapped between the fused thermoplastic fibres. On exposure to excess aqueous medium the resultant gellation of these superabsorbent fibres causes zones of weakness to form in the web allowing the web to disintegrate into discrete clusters of fused thermoplastic fibres which are easily flushable.

Very good results are obtained when only discrete zones of the web are subjected to the treatment.

Thus by means of the invention there is provided an absorbent product which is readily disposable by flushing and which reduces the risk of causing blockages in domestic sewage systems.

The preferred superabsorbent material is Fibers orb (Registered Trade Mark). Fibersorb fibres are formed from a polymer containing as a monomeric constituent an a,5-unsaturated compound having in its molecule one or two carboxyl groups or one or two groups convertible to carboxyl groups, which polymer is then reacted with a basic substance to partially neutralize the polymer. The partially neutralized polymer is then blended with either, (a) a non-reactive compound containing at least two hydroxyl groups; or (b) a reactive compound containing at least one hydroxyl group and at least one amine group. The resultant partially neutralized polymer syrup (blend or reaction product) is then attenuated to form fibers and heated to cure and render the fibers water-absorbing.

The thermoplastic fibre which together with the superabsorbent fibre forms the non-woven fibrous web must be capable of being thermally bonded and in general should have a softening point of less than 3000C, since heating above this temperature may cause the properties of the superabsorbent fibres to deteriorate. Examples of such thermoplastic fibres are polypropylene, polyester or acrylics.

Blends of thermoplastic fibres as well as blends of thermoplastic and natural fibres may also be used. Suitable natural fibres are cotton, wool, and especially wood pulp, and mixtures thereof.

The preferred thermoplastic fibres useful in the present invention are bicor fibres. These fibres have an outer sheath of a material having a lower softening point than the core material. Bicor fibres based on a polypropylene core with a polyethylene outer sheath such as "Cheso" or "Danaklon Es" are preferred, although bicor fibres based on polyesters having core and sheath polymers of different softening points are also useful in the invention.

The length of the thermoplastic fibre should suitably be less than 50mm in order that the fibres may be readily disposable by flushing when the web disintegrates in excess aqueous medium. The preferred length of the thermoplastic fibre is in the range of from about 10 to 20mm.

The composition of the fibrous web may vary according to the absorbancy and also the strength of the fibrous web required. In general the web will comprise from 10 to 75% by weight of superabsorbent fibre and from 90 to 25% by weight of other fibres of which at least a portion should be thermoplastic fibres. In general the preferred composition is in the region of 55-65 by weight of superabsorbent fibre and 35-45% by weight of the other fibres.

The production of the fibrous web may be carried out by blending the fibres and forming them into a non-woven web using any of the conventional fibre processing techniques available. Such techniques include carding and air laying.

Bonding of the fibrous web in order to produce a coherent but disintegratable product may be achieved by any technique in which a combination of heat and compression causes fusion of the thermoplastic fibres in the web. Depending on the nature of the final product and its intended use, fusion of the fibres may take place throughout the whole of the web or in discrete zones throughout the web; that is the zones where the thermoplastic fibres are fused are isolated from each other. When fusion occurs in discrete zones of the web the zones should also be sufficiently small in volume that the fused thermoplastic fibres form easily flushable clusters when exposed to excess aqueous medium.

One method of achieving fusion is to pass the preformed web through heated calender rolls. Embossed calender rolls may be used where fusion in discrete zones throughout the web is required. An embossed calender having a design which will result in a total embossed area of 10 to 20 percent is preferred.

The bonded composite web thus obtained typically has a thickness of 0.5 to 3 mm. The thickness of the web in the fused zone may be less than the thickness of the superabsorbent fibre. This indicates that the superabsorbent fibres have been compressed in the fused zones.

In the non-woven web, after passage through the heated calender rolls the thermoplastic fibres are fused either to other thermoplastic fibres or to the superabsorbent fibres. In the fused web, superabsorbent fibres are trapped between thermoplastic fibres which would otherwise have fused together. When the web is exposed to excess aqueous liquid, gellation of the superabsorbent fibres occurs, causing the web to disintegrate.

By means of the procedure described above it is possible to obtain a fused web which has high strength and integrity when dry and which is capable of absorbing many times its own weight of aqueous medium whilst still maintaining its strength and integrity.

The ability of the resultant non-woven web to disintegrate when placed in excess aqueous medium is due to the superabsorbent fibres becoming gel like in the excess aqueous medium and thereby forming zones of weakness in the web whereby it is then readily broken down by light mechanical agitation. The small discrete fused clusters of thermoplastic fibres which result are easily dispersed in the aqueous medium and are readily disposed of in conventional sewerage systems by flushing.

The invention is illustrated by the following Example.

A non-woven web of approximately 250g/sq.m was prepared from a blend of 60% by wt of superabsorbent fibre (Fibersorb SA 7000) with 40% by wt of bicor polypropylene (Daneklon ES) using a Rando Weber machine. This web was passed through an embossed roll calender with roll temperature 1320C, speed 3 metres/ minute and nip pressure of 1000 N/cm. The rolls were embossed with a square pattern of 36 points per square cm giving an embossed area of 14% The resultant fabric had a strength of 119 pounds per square inch average cross section when dry and absorbed 30.3 grams of 0.9% saline solution per gram of composite when tested by the standard "tea bag" absorption method. When placed in excess solution, the fabric disintegrated into its constituent fibres and could easily be disposed of in the normal sewage systems.

Figures 1 to 3 are photomicrographs of the fibrous web obtained in the above example. Figure 4 is a schematic diagram of the cross section of the web shown on Figure 3.

Figure 1 shows a top view of the web after calendering. The magnification is lOx.

Figures 2 and 3 show a cross section through the web after calendering. The magnification is 15x and 50x respectively.

In Figure 2 the circles indicate the areas of the web which have not been fused and the narrow areas connecting these circles are the fused zones of the web.

In Figure 3 which is a magnified cross section of a single fused zone of the web it can be seen that the thicker fibres which are the superabsorbent fibre may also be crushed in the fused zone. The superabsorbent fibres are randomly distributed between the fused thermoplastic fibres.

Figure 4 indicates schematically the relationship of the superabsorbent fibres to the thermoplastic fibres in the fused zone of Figure 3. From this Figure it can be seen how when the web is placed in excess aqueous medium so that the superabsorbent fibres absorb liquid and become gel-like, disintegration of the web will occur.

Claims

i. A non-woven fibrous web comprising a blend of superabsorbent fibres and fibres of at least one thermoplastic polymer, said web having been treated to fuse the fibres, the relative proportions of the superabsorbent fibres and the fibres of at least one thermoplastic polymer and the manner of fusion being such that the web is capable of absorbing many times its weight of aqueous liquid without losing its integrity, but which on contact with excess aqueous medium substantially disintegrates.
2. A non-woven fibrous web as claimed in claim 1 in which fusion is effected by a combination of heat and compression.
3. A non-woven fibrous web as claimed in claim 1 or claim 2 in which discrete zones of the web have been treated to fuse the fibres.
4. A non-woven fibrous web as claimed in claim 1, claim 2 or claim 3 which has been prepared by forming a fibrous web comprising said superabsorbent fibres and said thermoplastic fibres and calendering said web using an embossed calender.
5. A non-woven fibrous web as claimed in any preceding claim comprising from 10-75% by weight of said superabsorbent fibres and 90-25% by weight of said thermoplastic fibres.
6. A non-woven fibrous web as claimed in any preceding claim which further includes natural fibres.
7. A non-woven fibrous web as claimed in any preceding claim in which the superabsorbent fibre is a crosslinked copolymer of maleic anhydride and isobutylene or styrene which has been partially neutralised.
8. A non-woven fibrous web as claimed in any preceding claim in which the thermoplastic fibre is bicor polypropylene.
9. A method of forming a fibrous web which is capable of absorbing many times its weight of aqueous liquid without losing its integrity, but which on contact with excess aqueous medium substantially disintegrates, said method comprising forming a non-woven fibrous web comprising a blend of superabsorbent fibres and fibres of at least one thermoplastic polymer, and subjecting said web to a conbination of heat and compression whereby to thermally fuse fibres of the web.
10. A method as claimed in claim 9 in which the fibres are fused in discrete zones of the web.
11. A method as claimed in Claim 9 or claim 10 in which the fusion is effected by passing the web through heated embossed calender rolls.