Graduated density felt

Abstract: An absorbent material having graduated density is manufactured by needling a web. Before needling but after tacking the web is of substantially constant density across its thickness, and needling is carried out so that the needles penetrate the web to the extent that only the surfaces of the web are needled. The absorbent material is useful in particular for medical and surgical applications, where it is desirable that the material should exhibit delayed fluid strike through.

Description: The present invention relates to a method for the manufacture of an absorbent material having graduated density, and to an absorbent material manufactured by this method. Absorbent materials of this type are particularly suitable for use as medical or hygienic absorbents, for the absorption of body fluids, for example in surgical dressings, sanitary products and incontinence products. It is a requirement of such absorbents that they should absorb the fluid away from the surface of the absorbent in contact with the source of fluid, but that fluid strike-through, that is the passage of fluid through the surface of the absorbent remote from the source of fluid, should be delayed as much as possible. In this way, the interval between the replacement of a used absorbent with a new absorbent can be extended. Attempts have been made to provide absorbent materials with delayed fluid strike-through, and it has been found that materials which have dense surfaces and a less dense centre in which the fluid can pool demonstrate this delayed fluid strike-through. One such material has been produced which comprises two stitch-bonded layers lightly tacked together by needling. This construction has the effect of sandwiching a layer of air between two relatively dense layers and results in a composite material with delayed fluid strike-through. This material has the disadvantage that it is relatively expensive to produce and, when used as a surgical dressing, provides limited cushioning for the wound. It is also known to produce a needled material having graduated density by laying down a plurality of layers of fibres superposed on one another, wherein the fibres of each layer are of a different decitex, and needling the thus superposed layers together, the finer fibres producing a region of greater density and the coarser fibres a region of lower density, the layers being so arranged that the density of the needled material is graduated across its thickness. The disadvantage of such materials is that they are expensive to produce. Such materials have not been used as surgical dressings, but as filtration materials and as synthetic leathers. It is an object of the present invention to provide an improved method for the manufacture of an absorbent material having graduated density in which the above disadvantages are reduced or substantially obviated. It is a further object of the invention to provide an improved absorbent material having a graduated density wherein the above disadvantages are reduced or substantially obviated. The present invention thus provides, in one of its several aspects, a method for the manufacture of an absorbent material having graduated density by needling a web, characterised in that the web, before needling but after tacking, is of substantially constant density across its thickness, and in that needling is carried out so that the needles penetrate to the extent that only surface regions of the web are needled. The web used in carrying out the method according to the invention may comprise a major proportion, up to 100%, of hydrophilic fibres, which may be blended with a minor proportion, preferably up to 30%, of hydrophobic fibres. The hydrophobic fibres, when incorporated, provide a "scaffolding" which resists compression and helps to maintain the low density structure of the centre of the material both during needling and in subsequent use, especially when the material becomes wet. The hydrophilic fibres used in the web are preferably rayon, acrylic or cotton fibres, or a mixture thereof, more preferably rayon fibres. The web for use in the method according to the invention may alternatively comprise 100% hydrophobic fibres. In this case, the absorbency of the web is due to the structure of the material, rather than any inherent absorbency of the fibres. The hydrophobic fibres used in the web are preferably polyester or polyolefin fibres, more preferably polyester fibres. In carrying out the method according to the present invention barbed needles are preferably used of the type which have a section without barbs close to the point of the needle and then a section which is provided with barbs. Suitable needles include those marketed by Foster Needle under the reference number 15 x 8 x 40 x 3.5CB F.20 9-18-3B and those marketed by Groz Beckaert under the reference numbers 15 x 18 x 43 x 3.5 C333 G1002, G92919. During the needling process, after tacking, the depth of penetration of the needles is controlled so that the first barb on the needle penetrates the web to a depth such that substantially only those fibres which are in the surface region thus penetrated are re-oriented and those in the centre region remain substantially in their original orientation. Needling may be carried out to a similar penetration at each surface of the web, so that a symmetrical absorbent material which can be used with either surface contacting the source fluid is obtained. Alternatively, needling may be carried out to a different penetration at the two surfaces of the web, to provide a material having surface regions of different densities. Such an arrangement of different surface densities has the effect of improving the fluid uptake while maintaining delayed strike-through. When, for example, a material having surface regions of different densities is used as a surgical dressing, the less dense surface is disposed adjacent the wound and the more dense surface remote from the wound. In use, the absorbent material is preferably provided with a surface covering which is non-adherent. The absorbent material may be wrapped, in known manner, in a non-adherent material or may be bonded, also in known manner, to such a material, to form a composite article. The composite articles manufactured in this way are particularly suitable for use as surgical dressings, sanitary towels or incontinence products. Other aspects of the present invention will be found set out in Claims 9 and 11 below. The invention will now be further described with reference to the following Examples. In these Examples, the effects achieved by varying the weight of the final product and/or the needle penetration can be observed. As a general rule, where the weight of the final product is reduced, the absorption is reduced and handling the fabric becomes more difficult. However, when the weight of the finished product is reduced, this can be advantageous in certain applications. It appears that as the weight of the material is increased the absorption may increase more than linearly therewith, apparently because as the weight increases there is a proportionately greater quantity of fibres in the absorbent central region. The weight of the fibre layer(s) introduced into the loom is determined according to the desired weight of the final material. In practice the weight of said layer(s) is determined empirically, the feed of the fibres being controlled according to the actual weight of the final product. Example 1 A batt made up of one or more layers was produced from a fibre blend of 80% 1.7 dTex 51mm staple viscose fibres and 20% 5.OdTex 40mm staple polyester fibres. The batt was then needled in a three-stage operation: In a first, tacking, stage the batt was passed between an up-stroking needle board and a down-stroking needle board, each fitted with 15 x 18 x 40 x 3 F222 G92919 needles marketed by Groz Beckaert. The penetration of both the top and bottom needles was 14mm. In a next following first needling stage, the thus tacked batt (or web) was passed under a single needle board fitted with a mixture of 67% 15 x 18 x 40 x 3.5 R333 G1909 needles and 33% Foster 15 x 18 x 40 x 3.5CB F20 9-18-3B needles and having a needle punch density of 83 needles/cm² . Needling was thus carried out using a downstroke only; needle penetration in this stage was 6.1mm. Thereafter in a second needling stage, the partially needled batt (or web) was passed between an up-stroking needle board and a down-stroking needle board, each fitted with a mixture of 67% 15 x 18 x 40 x 3.5 R333 G1909 needles and 33% Foster 15 x 18 x 40 x 3.5CB F20 9-18-3B needles and having a needle punch density of 329 needles/cm² and a head speed of 509 rpm. The top penetration in this case was 4.6mm and the bottom penetration 5.6mm. The final product weight was 300gsm. The product was satisfactory for use as a medical absorbent, with relatively dense surface regions and a less dense central region. Example 2 In this Example the same procedure, using the same needle boards, was followed as in Example 1 except that in the second needling stage the penetration by the needles of the down-stroking needle board was 0.2mm and that by the up-stroking needle board 5.6mm. Moreover in this case the needle punch density was 392 needles/cm² and the head speed 605 rpm. The final product weight was again 300gsm. The products demonstrated excellent properties for use as a medical absorbent, with dense surfaces and more open centre. Example 3 Again in this Example the same procedure was followed as in Example 1, except that the needle punch density in the first needling stage was 86 needles/cm², and further in the second needling stage the penetration by the needles of the down-stroking needle board was 4.7mm and that by the up-stroking needle board 5.6mm. Moreover, in this case the needle punch density was 337 needles/cm² and the head speed 501 rpm. The final product weight was 190gsm. The product was thus a material of lighter weight than the other two Examples. Such material would be suitable for a use in medical applications where the absorbency required was not so high and light weight was an advantage.

Claims: A method for the manufacture of an absorbent material having graduated density by needling a web, characterised in that the web before needling but after tacking is of substantially constant density across its thickness, and in that needling is carried out so that the needles penetrate the web to the extent that only surface regions of the web are needled. A method according to Claim 1 characterised in that needling is carried out to the same penetration at each surface region. A method according to Claim 1 characterised in that needling is carried out so that the penetration at one surface region is greater than the penetration at the other surface region. A method according to any one of Claims 1 to 3 characterised in that the web comprises from 50 to 100% of hydrophilic fibres and from 50 to 0% hydrophobic fibres. A method according to Claim 4 characterised in that the web comprises from 70 to 100% of hydrophilic fibres and from 30 to 0% of hydrophobic fibres. A method according to Claim 4 or Claim 5 characterised in that the hydrophilic fibres are rayon, cotton or acrylic fibres. A method according to any one of Claims 1 to 3 characterised in that the web comprises substantially 100% hydrophobic fibres. A method according to any of Claims 4 to 7 in which the hydrophobic fibres are polyester or polyolefin fibres. A non-woven absorbent material manufactured by a method according to any one of the preceding Claims and characterised by a denser surface region in which substantially the fibres are re-oriented from their original orientation (i.e. prior to needling), and by a less dense central region in which the fibres remain substantially in such original orientation. A material according to Claim 9 which is provided with a non-adherent surface covering. A surgical dressing, sanitary towel or incontinence product comprising a material as claimed in Claim 9 or Claim 10.

Inventors: BRASSINGTON NIGEL, DOWNIE MALCOLM, SUNLEY CLYDE, BRASSINGTON, NIGEL, DOWNIE, MALCOLM, SUNLEY, CLYDE
Applicants: BRITISH UNITED SHOE MACHINERY, DVSG HOLDING GMBH, BRITISH UNITED SHOE MACHINERY LIMITED
Application year: 1990
Main classification: D04H1
IPC codes: D04H1