GB1595300A - Non woven fabrics - Google Patents

Description

(54) NON WOVEN FABRICS (71) We, CARL FREUDENBERG, a Germany Company, (a Kommanditgesellschaft the present personally responsible partners of which are Helmut Fabricius, Hans Erich Freudenberg, Otto Schildhauer, Hermann Freudenberg, Dieter Freudenberg and Reinhart Freudenberg) of 6940 Weinheim Bergstrasse, Hohnerweg 2, Germany do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a textile sheet structure formed hydrodynamically and to a process for the production thereof.

Hydrodynamically formed sheet structures, such as papers or non-woven fabrics, are frequently bonded with the use of secondary bonding agents which are generally introduced as a latex and are either deposited on the pulp or fibres or precipitated as isolated coagulate particles during the preparation of the suspension of material. The resultant products tend to have rather low strength, especially if they have an open structure or textile properties.

Another disadvantage is that their properties tend to be determined strongly by the bonding agent used and its concentration. For physiological reasons, for example, when the products are used in the fields of medicine and hygiene, limitations may have to be placed on the use of some (usually water soluble) bonding agents and so it may not be possible to use a bonding agent that would be preferred for strength reasons.

A different hydrodynamic process for forming non-wovens is described in Austrian Patent specification No. 266573. This process starts from the use of thermally softenable fibres and consolidation and bonding of the non woven is obtained on a Fourdrinier while the non woven is still in a moist state by subjecting it to a heat treatment sufficient to cause the fibres to stick together.

This bonding process described might appear to be based on the same principle as is used in bonding dry laid non wovens by means of thermoplastic bonding fibres. It is also already known from for example, Krcma, Handbuch der Textilverbundstoffe (Manual of Composite Textile Fabrics), 1970, to employ bonding fibres of different materials for the consolidation and bonding of dry laid non wovens.

In such dry laid products the bonding fibres should have thermoreversible melting and adhesion behaviour. Fibres of such materials may, if necessary, be thermally treated as often as desired over a lengthy space of time without their chemical or physical structure (and consequently their bonding capacity) changing in the process. Difficulties arise when bonding a dry laid non woven using bonding fibres of the kind in which irreversible changes which counteract the bonding capacity take place on heating. An example of an occurrence of this kind can be observed in the crystallisation of unstretched polyester fibres.

Unstretched polyester fibres have basically a degree of crystallisation of between 4.5 and 5.4%, i.e. the remainder of the molecular structure is present in an amorphous state below the vitreous transformation point. If such a fibre is subjected to heating, its adhesive properties decrease very rapidly on heating above the vitreous transformation temperature (70"C), in consequence of the increasing rate of crystallisation. The rate of decrease is so great that they tend to be unusable as bonding fibres in most processes. Consequently, unstretched polyester fibres can be used as bonding fibres in a non woven fabric only when it is ensured that their working-up takes place at room temperature at the most and when their activation is produced in the shortest possible time with the simultaneous application of pressure and heat, for example in a heating calender. However these conditions can only be produced in certain processes for the formation of dry laid non wovens. They cannot be, produced in processes for wet laid non wovens since completely different initial conditions exist in their formation.

Hydrodynamically formed non wovens are first formed by mechanical dewatering of a fibre suspension on an endless wire and are then transferred to a drier in which the residual water that remains is evaporated. The temperature applied and the duration of the drying step are related to one another and to the amount of water evaporated, and were always such that the heating was too slow and/or at too high a temperature to reproduce the conditions known to be necessary in dry lay processes to permit the use of polyester bonding fibres. Thus it appeared inevitable that the theoretically possible final crystallinity of conventional polyester filaments of 42% would be reached, this being synonymous with a complete loss of technically utilisable bonding power. Thus it appeared inevitable that subsequent heat calendering of a hydrodynamically formed non woven fabric of this kind would inevitably yield poor bonding, and would yield distinctly poor strength than can be obtained in some instances by heat calendering a dry laid sheet structure of the same composition.

A method according to the invention of making a non woven fabric comprises forming a web by dewatering a suspension containing unstretched polyester fibres as binder fibres, drying and prebonding the web by heating at a temperature below 1800C until the moisture content of the web has decreased to from 10 to 40% based on dry weight, and in a separate process completing the bonding by hot calendering the web at a temperature above 1800C and with a linear pressure of more than 30 kp/cm.

Thus, by the invention it is possible to combine the advantages that follow from the use of polyester fibres, especially their high chemical and heat resistance, with the advantages of hydrodynamic formation methods and eliminate the need for secondary bonding agents, while obtaining a strength at least equivalent to that obtainable using dry laid fabrics formed of substantially the same composition. The invention is based on the surprising discovery that despite the crystallisation which has been observed in dry lay processes, in the described wet lay process satisfactory prebonding and good final bonding can be obtained when unstretched polyester fibres are included in the web as the binder fibres.

The non woven fabric may be formed solely of unstretched polyester fibres or of a mixture of such fibres with other fibres. Also the fabric may contain unstretched polyester fibres, and optionally other fibres, with other fibrous particles, such as fibrids, or other particulate bodies having a particle size larger than the mesh size of the screen on which the wet lay fabric is made. Such fibres, fibrids or other particulate bodies may be of organic or inorganic material and generally the web will contain 0 to 90% of them and 10 to 100% of the unstretched polyester fibres. Particularly preferred fibres that may be included with the unstretched polyester are stretched polyester fibres and/or polyacrylamide fibres and/or polyamide imide fibres.

The inclusion of stretched polyester fibres is valuable because it results in the final product having particularly good and uniform properties. Thus the final product, if it consists solely of stretched and unstretched fibres, has very good temperature resistance, good resistance to acid, especially oxidative and hydrolytic effects, and good dielectric properties. However the inclusion of polyamide imide or aromatic polyamide fibres is also desirable in these and all other products of the invention since they can give improvement in properties, particularly at high temperatures.

The web may be formed by dewatering a suspension of the chosen fibres and other particles (if any) having a concentration typical of normal concentrations for dewatering webs, the dewatering preferably being conducted on a Fourdrinier machine. Preferably the machine has a steep screen. The web is then generally transferred to a drier in which the drying and prebonding is conducted. During the drying some sticking together of the fibres occurs. This prebonding should be sufficient to ensure that the web is bonded sufficiently strongly to permit handling during the subsequent processing operation. To achieve adequate prebonding it may be desirable to exert some pressure on the web so as to hold the fibres in contact with one another so as to increase the amount of prebonding and therefore the strength of the prebonded web. This consolidation is best brought about by applying only a negative pressure through the web, i.e. by sucking the web against a suitable surface. Thus, preferably the drying is conducted on a suction drier. The surface against which the web is sucked must not adhere to the web and so may carry a suitable non stick coating, such as a coating of a silicone or "Teflon" ("Teflon" is a registered Trade Mark).

Preferably the temperature during the drying is below 1500C and the drying is terminated when the residual moisture content of the web, based on the weight of dry fibre, is from 10 to 40% by weight.

The dried web is then subjected to hot calendering under the described conditions, this resulting in compression of the web and final bonding of the bonding fibres within the web.

The products of the invention have a wide variety of uses, and these are not limited, as are many prior art uses, by the choice of a secondary bonding agent.

The products of the invention have very high uniformity both of thickness and weight per unit area over their entire width. This makes them very suitable for use in the electrical engineering field. It also makes them very suitable for use as carriers for semipermeable membranes that may be used for, for example, reverse osmosis or ultra filtration. Thus a semi permeable membrane can be deposited on a bonded fabric according to the invention, e.g. by casting a cellulose acetate solution and then coagulating it to form a layer often 500 to 5000 angstroms thick, and then the resultant combination of membrane and carrier can be supported on a suitable supporting element such as a rod or tube having appropriate holes, channels or ducts in it for transferring liquid through or along it, the non woven fabric thus serving both as a drainage layer and as a carrier for the membrane.

The following are some Examples of the invention Example 1 A fibre mixture consisting of 35% by weight of unstretched polyester fibres, dtex 6.8/12 mm, and 65% by weight of stretched polyester fibres, dtex 1.3/12 mm, is mixed with water to form a 0.2% fibre suspension. In order to obtain optimum dispersion into single fibres in the process, 40 g of a non-ionic surfactant are then added per cubic metre. The suspension obtained is then fed to a steep-screen paper machine, the concentration of material being reduced to about one tenth of the original concentration by means of the circulating water.

After the water has been drawn off by suction, a laminar structure with a moisture content of 86% is formed on the steep screen.

This loose sheet structure, which is held together only by the moisture adhering to the fibres, is then transferred to a rotary suction drier and dried. The temperature during this process is 140"C, with a dwell time of the sheet structure of 11 seconds. The negative pressure is adjusted to 150 mm water column. After leaving the drier, the non-woven fabric is a slightly consolidated voluminous structure having a tearing strength of 14 N in the longitudinal direction and of 7 N in the transverse direction. Its dry weight is 80 g/m2. The strength values obtained are sufficient to be able to roll the material up and unroll it without any trouble. Its moisture content is between 10 and 40% based on its dry weight.

Following the drying and proconsolidation, the non-woven fabric obtained was consolidated in a heated two-roll calender having a steel roll and a cotton roll at a linear pressure of 70 kg/cm. The steel roll was at a temperature of 210 C and the cotton roll was at a temperature of 1600C. Tearing strengths between 390 and 400 N/5 cm strip width were then measured, which corresponded to about a 27-fold increase in the strength of the material in comparison with the uncalendered material.

Example 2 The almost dry non-woven fabric formed and preconsolidated in accordance with Example 1 was placed in boiling water in numerous partial steps commencing with .4 seconds up to a maximum of 180 seconds. On termination of the desired period of action, the sample was quenched in cold water, then squeezed out and dried at room temperature.

The non-wovens pre-treated in this way were than calendered under the conditions given in Example 1. In all cases there resulted tearing strengths which did not differ specially from those in Example 1.

Example 3 The almost dry non-woven fabric formed and preconsolidated in accordance with Example 1 was pressed uniformly by means of a screen against a steam-heated Teflon-coated cylinder for 4 seconds up to a maximum of 180 seconds. The temperatures at the surface of the cylinder were in stages 1200C, 1500C and 1800C.

The samples heat-treated in this way were then calendered under the conditions given in Example 1. In the final material no decrease worth mentioning in the tearing strengths could be found even in the samples which were treated for 180 seconds at 1200C in comparison with the materials of Examples 1 and 2. From 1500C, on the other hand, a distinct decrease in the tearing strength with increasing duration of action of the heat treatment could be observed within the interval of measurement. This tendency naturally appears even distinctly more markedly at 180 C.

The test data are compiled in the following Table. The tearing strengths given are average values of five measurements.

Table 1: Effect of pre-treatment on the tearing strength of the calendered non-woven fabric.

Tearing strength (N/Scm) at drier temperature Time (sec.) 120 C 150 C 1800C 0 395 378 357 4 397 363 343 8 424 362 345 15 390 367 303 30 385 339 283 60 393 317 273 90 361 318 292 180 379 309 186 Example 4: The almost dry non-woven fabric formed and preconsolidated in accordance with Example 1 is heat pre-treated at 1800C in accordance with Example 3 and then consolidated in the two-roll calender mentioned in Example 1, the temperature of the steel roll being 195"C. The textile roll is unheated and the speed is 4 m/min. with a linear pressure of 50 kg/cm. Table 2 shows the tensile strengths obtained in dependence upon the duration of the heat pre-treatment.

By reason of the gentler calendering conditions, the finished material shows distinctly poorer tearing strengths than the materials according to Examples 1 to 3. The crystallization of the unstretched polyester fibres taking place with the increasing action of heat is obviously covered only to a greatly reduced extent by the heat calendering.

Claims (1)

1. Table 2: Tearing strength N/5 cm Time (sec.) longitudinal transverse thickness (microns) 0 uncalendered 14 7 800 0 calendered 125 113 140

   4 109 98 140

   8 76 63 140

   16 54 . 37 150

   48 40 20 160

   80 34 18 160 180 36 17 170 In our copending application 50215/77 (serial No. 1595299) we describe and claim a permeable bonded non-woven fabric suitable for use as a carrier for a semi-permeable membrane, which has been prepared by wet-laying a mixture of fibres consisting substantially only of synthetic non-bonding fibres having a size of from 1 to 7 dtex and synthetic thermoplastic bonding fibres having a size of from 4 to 30 dtex, thereby obtaining a wet-laid non-woven fabric having the highest concentration of bonding fibres at one surface of the fabric, and heat-bonding the wet-laid fabric.

   WHAT WE CLAIM IS: 1. A method of making a non woven fabric comprising forming a web by dewatering a suspension containing unstretched polyester fibres as binder fibres, drying and prebonding the web by heating at a temperature below 1800C until the moisture content of the web has decreased to between 10 and 40%, based on dry weight, and in a separate process completing the bonding by hot calendering at a temperature above 1800C and a linear pressure of more than 30 kp/cm.

   2. A method according to claim 1 in which the drying and prebonding is conducted at a temperature below 1500C.

   3. A method according to claim 1 or claim 2 in which the drying and prebonding is conducted while applying only a negative pressure through the web.

   4. A method according to any of claims 1 to 3 in which the web contains 10 to 100% unstretched polyester fibres and 0-90% other fibres, fibrids or other particulate material.

   5. A method according to any of claims 1 to 3 in which the web is formed solely of unstretched polyester fibres or a mixture of unstretched polyester fibres with one or more of stretched polyester fibres, polyacrylamide fibres and polyamide imide fibres.

   6. A method according to claim 1 substantially as herein described with reference to Example 1.

   7. A product made by a method according to any of claims 1 to 6.

   8. A semi permeable membrane coated on a product according to claim 7.