GB2351742A

GB2351742A - Absorbent fibre

Info

Publication number: GB2351742A
Application number: GB9915836A
Authority: GB
Inventors: Jason Yorke; William Brunskill
Original assignee: Acordis Fibres Holdings Ltd; Technical Absorbents Ltd
Current assignee: Technical Absorbents Ltd; Lenzing Fibers Ltd
Priority date: 1999-07-06
Filing date: 1999-07-06
Publication date: 2001-01-10
Anticipated expiration: 2019-07-06
Also published as: GB2351742B; GB9915836D0

Abstract

Superabsorbent fibre, which can be prepared by extruding filaments of a crosslinkable water-soluble polymer, cutting the filaments to form staple and crosslinking the polymer to a water-insoluble water-absorbent polymer, is characterised in that the fibre is coated with a hydrophobic finish (such as a silicone), which decreases its rate of uptake of moisture vapour and/or aqueous liquid. The hydrophobic finish can be applied to the filaments or to the fibres. The coated fibres may form the strike zone of an absorbent pad.

Description

2351742 Absorbent Fibre This invention relates to superabsorbent f ibre,

that is to say fibre having a waterabsorbency of at least 20 grams 0. 9% by weight saline solution per gram of fibre as measured 5 by the free swell method described in US-A-5582786, to a process for its production and to absorbent products containing the superabsorbent fibre.

The production of superabsorbent fibre by extruding filaments of a crosslinkable water-soluble polymer and 10 crosslinking the polymer to a water-insoluble water absorbent polymer is described in US-A-5582786, USA3954721, US-A-3983095, EP-A-268498, EP-A-269393, EP-A- fibre 342919, EP-A-397410 and FR-A-2355929. Superabsorbent can alternatively be produced by chemical treatment of a preformed fibre to increase its water absorption, for example carboxymethylation of a cellulosic fibre as described in WO-A-93/12275, US-A-4044766, GB-A-2094802 and GB-A-2220881.

The only one of the above-mentioned patent documents which discusses fibre finishes is GB-A-2220881, which applies a lubricating- gent from a solvent which does not le JD% r 1Dr swell the fibre. The pr ferred lubricating agent contains more than 10% by weight nonionic polyethylene oxide adduct which does not impair the water absorption of the fibre, for example a mixture of oleyl alcohol ethylene oxide adduct with sodium lauryl phosphate or with vegetable fat and polypropylene glycol ether.

Superabsorbent fibre according to the present invention is characterised in that the fibre is coated with a hydrophobic finish which improves processability of the fibre and decreases its rate of uptake of moisture vapour and/or aqueous liquid.

Superabsorbent fibre coated with a hydrophobic f inish according to the present invention has improved processability. Uncoated superabsorbent fibre, cr superabsorbent fibre coated with a conventional hydrophilic finish, absorbs moisture from the atmosphere very quickly, becomes plastic and can cause problems during air laying at high ambient humidity and/or high temperature. The fibre of the present invention can be air laid without problems in hot humid atmospheres.

Superabsorbent fibre coated with a hydrophobic finish according to the present invention generally has a reduced rate of uptake of aqueous liquids and this leads to improved performance of a web containing the superabsorbent fibres in absorbent products in terms of increased liquid spread and reduced rewetting as discussed in more detail below.

The invention also includes a process for the preparation of a superabsorbent staple fibre by extruding filaments of a crosslinkable water-soluble polymer, cutting the filaments and crosslinking the polymer to a water- insoluble water-abso.rbent polymer, characterised in that a hydrophobic finish is applied to the filaments or fibre.

The fluid uptake and rate of fluid uptake can be measured by the absorbency (AUL) test in which 0.16g absorbent staple fibre is placed in a test cell of 2.54 cm diameter having an open perforated plate base and compressed under a retaining load of 0.02 KPa, and the material in the test cell is then contacted through the perforated plate with the aqueous fluid, for example 0.9% by weight saline solution. In this test, superabsorbent fibres free from finish typically absorb 20g saline solution per g fibre within 1.5 to 5 minutes. A finish may be regarded as hydrophobic if a superabsorbent fibre, which in unfinished state absorbs 20g/g saline in 2 to 5 minutes, takes at least 10 minutes to absorb 20 g/g saline when coated with 0.2% by weight of the finish. Preferably the finish is sufficiently hydrophobic that such a superabsorbent fibre takes at least 20 minutes, most preferably at least 40 minutes, to absorb 20 g/g saline when coated with 0.2% by weight of the finish.

The moisture vapour uptake can be measured by placing S the fibre in an air flow of 0.5 m/sec at 300C and 70% relative humidity. This simulates the conditions to which the fibre might be subjected during air laying in a hot humid environment. In this test, superabsorbent fibres free from finish typically have a moisture regain of more than 2; by weight per minute and reach a moisture regain of 35?6 after 15 minutes. Fibre which has gained more than 35% moisture begins to spoil and to become too sticky for successful air laying. A finish which reduces the rate of moisture regain to less than 2% per minute may be regarded as hydrophobic. Preferably the fibre is rendered sufficiently hydrophobic to reduce the rate of moisture regain to less than 1.516 per minute.

The crosslinkable water-soluble polymer is preferably a polymer containing carboxylic acid or anhydride groups and hydroxyl, epoxide or amine groups. For example, the watersoluble polymer is a copolymer of 50 to 95% by weight ethylenically unsaturated carboxylic monomer, e.g. acrylic or methacrylic acid,'and 5 to 50 by weight copolymerisable ethylenically unsaturated monomer.

The extruded filaments can in general be dry spun, wet spun or melt spun filaments consisting at least partly of a hydroxy-functional or epoxidefunctional comonomer so that ester crosslinks are formed by reaction between carboxylic acid groups derived from the carboxylic monomer and hydroxyl or epoxide groups derived from the comonomer during the crosslinking step. US-A- 3954721, US-A-3983095 and EP-A272074 disclose copolymers of maleic anhydride and at least one vinyl comonomer in fibrous form, the fibrous copolymers being rendered hydrophilic and water-swellable by reaction with ammonia or an alkali metal hydroxide. Substantially linear copolymer can be crosslinked by a diol or diamine as described in US-A-3810468 or with glycerol diglycidyl ether as described in US-A- 3980663. The comonomers preferably also include a plasticising monomer such as methyl acrylate, for 5 example at a level of 5 to 40't; by weight of total. The crosslinking step is usually carried out by heating at a temperature in the range 125 to 2500C, although crosslinking by application of chemicals is possible. As an alternative to use of a crosslinking comonomer the fibre can be treated with a crosslinking agent to convert it from the watersoluble form to water-insoluble water-swellable superabsorbent fibre. Such a crosslinking agent can be applied in liquid or vapour form.

Cutting the filaments into staple fibre before is crosslinking, as described in US-A-5582786, is generally preferred, since the crosslinked fibres are relatively brittle and generate more fly on cutting. Cutting of the non-crosslinked filaments can, however, cause problems of smearing of polymer on the cutter, as discussed in WO-A- 99/04069, and even blockage of the cutter at high throughput. The hydrophobic finish is preferably applied to the filaments before they are cut. Application of a hydrophobic finish to the filaments according to the invention reduces problems of smearing and blockage of the cutter.

The hydrophobic finish preferably comprises a silicone and more preferably contains at least 50%, most preferably at least 75%, by weight silicone based on non-volatile matter in the finish. Most preferably, the finish consists essentially of a silicone or mixture of silicones. Silicone finishes are reviewed by S. Cray and G. Budden in "Textile Month", March 1996, at page 33. A preferred silicone is a polydiorganosiloxane containing reactive groups. The reactive groups can for example be Si-H groups, or can alternatively be amine, hydroxyl, Si-bonded alkoxy or Sibonded acyloxy reactive groups. The silicone finish can - 5 alternatively or additionally contain an unsubstituted polydiorganosiloxane. One preferred silicone finish comprises a silicone which contains reactive groups and an unsubstituted polysiloxane which is of lower viscosity than the silicone containing reactive groups and which acts as a diluent for the reactive silicone. The unsubstituted polydiorganosiloxane can for example be low-viscosity polydimethylsiloxane such as that sold under the Trade Mark DC-200 or a higher viscosity polydimethylsiloxane such as that sold under the Trade Mark Thredol or a more hydrophobic methyl phenyl polysiloxane such as that sold under the Trade Mark DC-710. We have found that low-viscosity polydimethylsiloxane such as DC-200 used alone as a fibre finish may not give all the benefits of a hydrophobic.

1S finish. Silicone finishes containing DC-200 preferably also contain at least 20%- by weight of a reactive silicone; mixtures containing DC-200 and at least 50?6 by weight of a reactive silicone are for practical purposes as hydrophobic as the reactive silicone alone. Other unsubstituted polydiorganosiloxanes and reactive silicones can in general be used in mixtures in any proportions, for example from 5:95 to 95:5 by weight.

Silicone finishes have additional advantages over other hydrophobic finishes in that they are extremely stable to heat and on storage, and they do not form any tar when heated during the fibre crosslinking step.

Alternative hydrophobic finishes which can be used in the present invention are fluorocarbons such as fluorinated long-chain hydrocarbons, fluorinated polyethers or low molecular weight fluorinated aliphatic polyesters or acrylic polymers, or hydrocarbon waxes.

The amount of hydrophobic finish applied is generally at least 0.005 or 001k by weight based on the filament or fibre, preferably at least 0.02%, and can be up to 2"6, preferably up to 1%, by weight. The finish can be applied at 100% solids (particularly if a low-viscosity silicone forms part of the finish) or can be applied from solution in an organic solvent which does not substantially swell the filament or fibre. Such a solution preferably contains at least 50% by weight of the hydrophobic finish. The finish can contain a minor amount, preferably no more than 25% and most preferably no more than 10%, by weight based on the hydrophobic material of the finish, of a hydrophilic material, for example an alkoxylated alcohol or amine such as polyethoxylated sorbitan ester.

The finish is preferably applied to the filaments before they are cut, and a hydrophobic finish which is reactive with the fibre is preferably applied before the crosslinking step, particularly if heat treatment is needed for the finish to react with the fibre, but in general the finish can be applied after cutting and crosslinking if desired, for example by spraying onto the fibres.

The superabsorbent fibre of the present invention can be used, for example, in absorbent personal products such as disposable diapers, sanitary napkins, incontinence pads or tampons. The superabsorbent fibre is preferably used in combination with one or more other fibrous materials, for example fluffed pulp, such as wood pulp or synthetic fibre pulp, for example polyolefin pulp or cellulosic fibre such as cotton or regenerated cellulose fibre, including multilimbed cellulose fibre as described in EP-A-301874, or polypropylene or polyester fibre. The superabsorbent fibre can be intimately mixed with the other fibrous material, for example by air-laying the fibres together to form a web of mixed fibres. The proportion of superabsorbent fibre in a blend with fluffed pulp or cellulosic fibre for absorbent products can for example be at least 5% and up to 95% by weight, preferably at least 10% and up to 50% by weight. Alternatively, the superabsorbent fibre can be formed, for example air laid as a layer, consisting wholly or partly of superabsorbent fibre which can be located at one surface of an absorbent pad or sandwiched between layers of other fibrous material of the type described above. Such a layer preferably consists mainly of superabsorbent fibre but may contain up to 5001, preferably up to 2026, by weight of other 5 fibres.

Absorbent products containing the superabsorbent fibre of the present invention, for example absorbent pads to be used as the core of a diaper or incontinence device, can have any of the configurations known for such pads, for example as described in US-A-4935022, US-A-5439548 or EP-A565606. The absorbent pad can have channels cut therein or can be in the form of a C- fold which naturally has a channel between the folded layers.

The superabsorbent fibre coated with a hydrophobic finish according to the present invention generally has a reduced rate of uptake of aqueous liquids. One consequence of this is that when aqueous liquid impinges on a web comprising the superabsorbent fibre, the liquid spreads further over the area of the web. Liquid striking a web of uncoated superabsorbent fibre, or superabsorbent fibre coated with a hydrophilic finish, is absorbed rapidly in the strike zone where the liquid first contacts the web until the superabsorbent fibre in the strike zone is swollen to a saturated gel. If a second liquid insult impinges on tile same strike zone, it cannot be absorbed in the strike zone which is perceived as a wet area. This lack of absorption in the strike zone on rewetting is a particular disadvantage in diapers, incontinence pads and feminine hygiene pads which are required to cope with liquid insults in the form of fairly large amounts of fluid passed intermittently. Liquid striking a web of the superabsorbent fibre of the present invention, on the other hand, spreads much further over the area of the web so that the superabsorbent fibre in the strike zone is much less likely to become saturated and th e strike zone is not perceived as a wet patch when it receives a second liquid insult. This wetness can be measured by gently blotting the web with an absorbent material such as filter paper, after it has received each liquid insult, and measuring the amount of liquid absorbed by the filter paper. The wetness after the first liquid insult is very low for any superabsorbent web designed to cope with that amount of liquid; it is usually even lower for the superabsorbent fibre which is uncoated or coated with a hydrophilic finish than for superabsorbent fibre coated with hydrophobic finish. The wetness after the second or third liquid insult, however, is much higher for superabsorbent fibre which is uncoated or coated with hydrophilic finish than for superabsorbent fibre coated with hydrophobic finish. Superabsorbent fibre coated with a hydrophobic silicone finish gives particularly good results in this rewetting 1S test.

The invention thus includes an absorbent pad comprising superabsorbent fibres, characterised in that the superabsorbent fibres in the strike zone of the absorbent pad are coated with a hydrophobic finish. usually it is more convenient if all the superabsorbent fibres in the absorbent pad are fibres coated with a hydrophobic finish so that the whole pad is made of a single web of fibres, but if the pad is being constructed from different component parts it may be only the superabsorbent fibres in the strike zone which are coated with a hydrophobic finish according to the present invention.

The superabsorbent fibre of the present invention can be used in many other applications of the types described in Research Disclosure, January 1992 at pages 60-61, for - disposable wipes, mats, shoe insoles or example in filters, bed sheets, swellable gaskets or seals, moisture- retention mats in horticulture, moisture-retaining packaging or swellable self-sealing stitching threads, or as a yarn or fabric tape used to wrap cable or laid longitudinally in the cable to prevent ingress of water in underground cables.

9 - The invention is illustrated by the following Examples, in which parts and percentages are by weight unless otherwise stated:

Example 1

A 38% aqueous solution of a copolymer of 78 % acrylic -e acid (75% neutralised as sodium salt), 20 P6 methyl acrylat and 2 % hexapropylene glycol monomethacrylate was spun into fibre through a spinneret into a cell where water was evaporated from the fibre. The temperature of the solution at the spinneret was between 90 and 10011C. The cell was heated by tube wall heaters at 1500C and was of the type described in US-A-5582786. Fibre emerged from the spinning cell through a small hole as a tow of continuous filaments. A silicone finish was applied to the tow as it emerged from the hole. The fibre had a moisture content of 16% based on dry fibre and a weight per filament of approximately 15 dtex and was collected as a tow at approximately 200 m/min.

The silicone finish was a liquid polyorganosiloxane containing reactive Si-H groups sold by Dow Corning tinder the Trade Mark DC-1107. It was applied to the tow at 0.2;.

The treated tow was passed to a staple cutter where it was cut to a f ibre length of 6 mm using a rotary cutter. There was no smearing of polymer from the fibres onto the cutter blade surface.

Thecut fibre was dried in a forced air oven at 700C to a moisture content of 7% based on dry fibre. The dried j'-'ibre was then crosslinked in an oven at 2000C for 10 minutes to form a water-absorbent water-insoluble fibre.

The fibre produced had a free swell absorbency of 50 g/g with a retention under load of 35 g/g in the absorbency tests described in US-A-5582786.

The f ibre was placed in an air f low of 0. 5 m/sec at 300C and 70% relative humidity. The moisture regain rate of the fibre was measured and was found to be 14-0o after 10 minutes and 24% after 20 minutes and to have reached 350-.

after 34 minutes. By comparison, similar fibre finished with "Atlas G1096" (Trade Mark), a conventional hydrophilic ethoxylated sorbitan ester finish, had a moisture regain of 26-0. after 10 minutes and 35% after 17 minutes.

In the AUL test described above, the AUL of the fibres of Example I was 29 g/g, although this was only reached after 90 minutes contact. By comparison, fibre finished with "Atlas G109611 had AUL 35 g/g reached after 20 minutes contact.

% of the superabsorbent fibre was blended with 50% fluffed pulp and 10,16 "Thermo Bond" (Trade Mark) thermoplastic polyolefin fibre to form 200 g/M2 sheets. The sheets were bonded and densified to a 0.14g/cm3 web by a heated calendar. The bonded webs were folded into C-folds to form absorbent pads.

The absorbent pads were tested in a rewetting test in which pads were wetted with 5 ml 0.9% aqueous saline solution, allowed to absorb the liquid for 5 minutes, then blotted with 10 x 5.5 cm filter papers at 0.5 psi for 15 seconds each. The total absorbed by the filter papers, (11rewet") was weighed. The pads were then wetted with a further 5ml saline and the rewet measured again. In another similar test, pads were subjected to 2 10ml saline insults. The results are shown in Table 1 below:- Table 1

Example Fibre Rewet Rewet Total Rewet Rewet Total Finish (A) - (B) of (C) (D) of after after (A) after after (C) First second and first second and SM1 SM1 (B) 10M1 10M1 (D) saline saline saline saline DC-1107 0.35g 0.7g 1.05g 0.5g 2.4g 2.9g Compara A t 1 a s 0.059 1.65g 1.7g 1.1g 2.8g 3.9g tive G-1096 The hydrophobic silicone-finished fibre produced a pad with lower rewet, particularly after the second liquid insult.

ExgMles 2 to 7 Fibre was produced according to Example 1, but using 15 the following finishes in place of DC-1107.

Example 2 - low viscosity polydimethylsiloxane silicone fluid sold by Dow Corning under the Trade Mark IIDC-20011 Example 3 - A reactive ethoxylated polydimethylsiloxane sold by Dow Corning under the Trade Mark "4100-NW".

Example 4 - An 80:20 mixture of DC-200 and DC-1107.

Example 5 - A 60:40 mixture of DC-200 and DC-1107.

Example 6 - a 50:SO mixture of DC-200 and DC-1107.

Example 7 - Polydimethylsiloxane fluid sold under the Trade Mark "Thredol".

The fibres of Examples 2 to 6 were tested for AUL with the following results:- Example No. AUL Time to reach maximum AUL 2 36 g/g 19 minutes 3 30 g/g 26 minutes 4 35 g/g 36 minutes 5 17 g/g 56 minutes 6 23 g/g 90 minutes The moisture regain at 300C and 70% relative humidity was measured for the fibre of Examples 2, 3 and 7 with the following results:- Example No. Moisture Regain Time to 35% after 10 minutes moisture regain 2 19% More than 20 minutes 3 26% 18 minutes 7 12% More than 35 minutes

Claims

1. Superabsorbent f ibre, characterised in that the f ibre is coated with a hydrophobic finish which decreases its rate of uptake of moisture vapour and/or aqueous liquid.

2. Fibre according to claim 1, characterised in that the hydrophobic finish comprises a silicone.

3. Fibre according to claim 2, characterised in that the silicone is a polydiorganosiloxane containing reactive Si-H groups.

4. Fibre according to claim 2, characterised in that the silicone is a polydiorganosiloxane containing amine, hydroxyl, Si-bonded alkoxy or Si-bonded acyloxy reactive groups.

5. Fibre according to claim 3 or claim 4, characterised in that the silicone finish additionally contains an unsubstituted polydiorganosiloxane.

6. Fibre according to any of claims 1 to 5, characterised in that the fibre is coated with 0.02-1.0% by weight of the hydrophobic finish.

7. A process for the preparation of a superabsorbent staple fibre by extruding filaments of a crosslinkable water-soluble polymer, cutting the filaments and crosslinking the polymer to a water-insoluble water absorbent polymer, characterised in that a hydrophobic finish is applied to the filaments or fibres.

8. A process according to claim 7, characterised in that the finish is applied to the filaments before they are cut.

14

9. An absorbent pad comprising superabsorbent fibres, characterised in that the superabsorbent fibres in the strike zone of the absorbent pad are coated with a hydrophobic finish.