KR101765730B1 - Coated light fabric, in particular for a flight sail - Google Patents

Abstract

The present invention relates to a lightweight fabric consisting of continuous warp and weft and coated on one or both surfaces with polyurethane, wherein the bare fabric has a coverage rate TC of 1.8 to 4, preferably 2.6 to 3.2, Wherein the polyurethane is made of a polyether and polycarbonate having a 100% elongation tensile stress of 5 MPa or less and has an impregnation rate of 10% by weight or more, preferably 12 to 30% by weight, Is from 15 to 25% by weight and the fabric comprises the coating and has a weight of from 25 to 40 g / m < 2 >. The present invention also relates to a method of making such a fabric. The invention can be used particularly for flight sails, especially for para-gliders.

Description

[0001] Coated lightweight fabrics, especially lightweight fabrics for flying sails,

The present invention relates to a lightweight fabric formed of continuous warp and weft and coated on one or both surfaces with polyurethane. Such lightweight fabrics are particularly applicable to the flying sail field. The present invention also relates to a method of making such a fabric.

In the field of flight sails, such as para-gliders, textile manufacturers have sought to combine lightweight, low porosity and durability for a considerable period of time. These equations are difficult to solve because the porosity reduction is generally very close to the weight gain. In practice, porosity depends on the density of the fabric, i. E., The number of warps and wefts per surface unit. The porosity also depends on the presence of a coating that somewhat closes the pores of the fabric. The coating is indispensable and occupies a non-negligible part of the weight of the coated fabric, i.

Such coatings must be durable, since they are essential to impart the required porosity. The concept of durability can cover various criteria, such as UV stability and hydrolytic stability, and more generally stability to atmospheric conditions and moisture. Hydrolytic stability should be considered as the most important factor for preservation of porosity over time.

The coating is also important for imparting adequate stiffness in the bias direction.

Currently, flying sails such as para-gliders use polyester-based polyurethane (PU) as the coating material. Such coatings have relatively good UV resistance, but have limited durability associated with hydrolytic stability. The weight of the lightest coated fabric produced in this way is close to 45 g / m 2.

By combining yarn density, the choice of coating material and the thickness and weight of the coating, a higher weight is obtained, which enables the production of fabrics having hydrolytic stability and acceptable levels of porosity. However, this is too heavy to be used for manufacturing high-flying sails.

FR 2 840 625 discloses a diagonally-stabilized fabric consisting of a ripstop-type weave comprising stitches and wefts interlaced with stabilizing strips and zones, . Such wovens are coated with an aqueous mixture of polyurethane and silicone. The physicochemical properties of the polyurethane are not specified and the weight of the fabric is 45 g / m 2.

EP 0 305 888 describes a textile material formed from a substrate impregnated with a water repellent agent and coated with a polysiloxane modified polyurethane resin layer. The modifying resin is prepared by reacting a siloxane polymer with a basic component of a polyurethane, isocyanate and a diol (polytetramethylene glycol, polyether diol, ring-opening lactone diol, polycarbonate diol). The weight of the fabric is greater than 40 g / m2.

EP 2 184 399 describes a textile material for para-gliders comprising a fabric coated with a urethane-silicone copolymer resin layer on the base of a polyester fiber. This material is said to have a weight of 21 to 100 g / m < 2 >. The examples illustrate a weight of 45 g / m 2 or much larger.

EP 0 552 374 describes a textile material which can be used in the manufacture of boats, para-gliders, or parachute sails. This material includes polyester fabrics that can be potentially coated with a resin that can be selected from these without any additional accuracy or any preference for the properties of polyurethane, silicone resin, and polyvinyl chloride. It is explicitly stated that this treatment is optional so that the textile material need not be treated with a resin, and the weight of the uncoated or coated fabric may be 20-100, preferably 30-50 g / m2. The examples illustrate 25, 48 or 85 g / m < 2 > of polyethylene terephthalate fabric coated with polyurethane resin.

JP 2000234272 has a polyamide warp and weft base and can be selected from among these without any additional accuracy for the physicochemical properties of acrylic resin, polyurethane resin, silicone resin, polyamide resin, polyester resin, polyimide resin Desc / Clms Page number 2 > describes a fabric for para-glider sails that can be coated with a resin.

The prior art fails to identify an intrinsic standard that allows for optimization of both the mechanical properties of the coated fabric to obtain the fabric to enable the production of improved flying sails as well as the weight and porosity of the coated fabric. Therefore, the conventional technique does not confirm the coating rate TC as an essential standard. Conventional techniques have also failed to confirm the importance of selecting the physicochemical properties of the coating resin. The prior art was unable to combine these criteria to obtain fabrics with a low weight, especially coatings of 40 g / m < 2 > or less. Conventional techniques have also failed to combine these different criteria to obtain an optimum level of air permeability for the reduced weight.

It is therefore an object of the present invention to provide a coated fabric combining light weight, porosity, and durability, especially moisture stability.

It is a further object of the present invention to provide a fabric having a suitable stiffness in the bias direction.

It is a further object of the present invention to provide a method of making such fabrics.

Other additional objects will appear by reading the specification of the present invention.

The present invention relates in particular to a polyether or polycarbonate based polyurethane comprising continuous warp and weft and having one surface or both surfaces having a 100% elongation tensile stress (measured according to standard DIN 53504) of 5 MPa or less To a lightweight fabric having a weight of 25 to 40 g / m2, preferably 31 to 36 g / m2 and a coverage TC of 1.8 to 4, preferably 2.6 to 3.2, inclusive of said coating.

According to one embodiment, the polyurethane has a 100% elongation tensile stress of 4 MPa or less, preferably 3 MPa or less. The tensile strength may therefore be from 1 to 5, 4 or 3 MPa.

The coverage TC is the coverage of the fabric obtained from the weaving process before any calendering or similar process. The TC is calculated as follows:

TC = (number of filaments / ㎝ × one diameter of the filament) + _slope (filament diameter × 1 number of one filament) _wefts,

The diameter of the filament is expressed in cm.

The TC value used in the present invention corresponds to a value giving a sufficiently closed structure to the fabric and is emphasized by calendering and on the one hand the impregnation rate of the coating material to obtain a low porosity suitable for the field of use of the fabric And consequently on the other hand to limit the final weight of the coated fabric.

According to one embodiment, the rate of dry impregnation of the coating material is at least 10% by weight, in particular from 10 to 30% by weight, preferably from 12 to 30% by weight, more preferably from 15 to 25% by weight. The dry impregnation rate is the weight ratio of the dry coating (especially PU and crosslinking agent) to the coated fabric and represents the dry / crosslink coating weight present in the final fabric.

In one embodiment, the fabric of the present invention is characterized by stiffness in the bias direction. The oblique direction bias is 45 DEG in the inclination reference direction. The weft bias is 45 ° in terms of weft. The percent elongation is measured under a load of 3 pounds (lbs, i.e., 1.36 kg) applied in the bias direction. These elongations characterize the fabric stiffness in the bias direction. The standard used is NF EN ISO 13934-1: test specimens are produced with a width of 50 mm and a length of 300 mm. The jaw of the dynamometer is moved 200 mm apart and the measurement is made at a speed of 100 mm / min.

In one embodiment, the fabric according to the present invention has an elongation of less than 10% with a warp and weft direction bias below 3 lbs. Thus, this elongation may be from 1 to 10%, preferably from 3 to 10%, more preferably from 5 to 10%.

According to one embodiment, the lightweight fabric has an air permeability of less than 20 L / m < 2 > / min under a pressure of 2000 Pa when measured according to standard NFG 07111 (100 cm2 surface measurement).

According to one embodiment, the lightweight fabric includes warp and / or weft yarns having a dtex of 11 to 44 dtex and a DPF (denier per filament) of 1 to 4.

According to one embodiment, the weight of the bare fabrics is 20 to 33 g / m2, preferably 24 to 28 g / m2.

According to one embodiment, the weight of the coated fabric is 25 to 40 g / m2, preferably 31 to 36 g / m2.

According to one embodiment, the fabric comprises 30 to 50 warp / cm and 30 to 50 weft / cm.

According to one embodiment, the fabric comprises a yarn having a DPF of 1 to 3 filaments (decitex per filament).

In one embodiment, the lightweight fabric of the present invention is obtained by coating with a polyurethane aqueous dispersion. The coating may have any one of the properties mentioned above.

The present invention also relates to a method of making a coated lightweight fabric, such as:

(a) a fabric having a coverage TC of from 1.8 to 4, preferably from 2.6 to 3.2, is used and TC is calculated as follows:

TC = (number of filaments / ㎝ × 1 size of one filament) + _slope (filament number / ㎝ × 1 size of one filament) _wefts,

(b) one surface or both surfaces of the fabric is coated with a polyether-based or polycarbonate-based polyurethane having a 100% elongation tensile stress (measured according to standard DIN 53504) of 5 MPa or less and a dry impregnation rate Is 10% by weight or more, preferably 12 to 30% by weight, more preferably 15 to 25% by weight,

(c) the fabric is heated until the coating is dried / crosslinked,

(d) the fabric is recovered and has a weight of 25 to 40 g / m < 2 >, including the coating.

According to one embodiment, the polyurethane has a 100% elongation tensile stress of 4 MPa or less, preferably 3 MPa or less. The tensile stress can therefore be from 1 to 5, 4 or 3 MPa.

According to one embodiment, the fabric comprises warp and / or weft yarns having a dtex of 11 to 44 dtex and a DPF (denier per filament) of 1 to 4.

According to one embodiment, the fabric is calendered before being coated in step (b). Calendering crushes the fabric and spreads the yarn as well as the filament components, which contributes to closing the pores of the fabric and reducing porosity.

According to one embodiment, the calendering is performed between a cal- liding tool, a cylinder, or a roller and a jack. The surface of the fabric through which the calendering tool passes is called calendering surface and is smoother than other surfaces.

According to one aspect, a coating is performed on the calendering surface. Pre-treatment of the surface on a smooth surface can improve the catching of the polymer. This may be a physical treatment or a chemical treatment. This may be, for example, a chemical treatment that imparts a functional group that reacts with the group of the polymer to form a chemical bond.

According to another aspect, the coating may be performed on another non-smooth surface. It is understood that the rate of impregnation varies with the surface of interest, and this rate of impregnation is higher at the non-smooth surface, so that one skilled in the art can manipulate the amount and weight of the coating. It is also possible to coat both surfaces.

According to another embodiment, the calendering is performed between two calendering tools, cylinders or rollers. Both surfaces of the fabric are smooth. One surface or both surfaces are then coated with or without the processing described above.

The calendering is preferably carried out at a temperature of 150 to 250 캜, preferably 180 and 210 캜. The calendering is preferably carried out at a pressure of 150 to 250 kg, preferably 180 to 230 kg. The rotation speed of the calender may be 1 to 30 m / min, preferably 10 to 20 m / min.

According to one embodiment, the weight of the fabric used in step (a) is 20 to 33 g / m 2, preferably 24 to 28 g / m 2.

According to one embodiment, the weight of the fabric obtained from step (c) is 25 to 40 g / m 2, preferably 31 to 36 g / m 2.

According to one embodiment, the fabric used comprises 30 to 50 warps / cm and 30 to 50 warps / cm.

According to one embodiment, the fabric used comprises a yarn having a DPF of 1 to 3 filaments (decitex per filament).

In one embodiment, the lightweight fabric of the present invention is obtained by coating with an aqueous dispersion of polyurethane.

In one embodiment, the lightweight fabric of the present invention is obtained by coating with a polyurethane dispersion in a solvent phase.

The polyurethane includes a rigid part (isocyanate) and a flexible part (polyol). The ratio between these two components makes it possible to obtain a somewhat stiff polymer. Polyether-type or polycarbonate-type polyols are hydrolytically stable and therefore durable.

The coating composition comprises a crosslinking agent, in particular isocyanate or melamine, or a mixture of both.

Isocyanates refer to both isocyanates and polyisocyanates, which may be either alone or in admixture with one or more other isocyanates and / or polyisocyanates. The term "isocyanate" should be understood to include the terms "isocyanate" and "polyisocyanate" herein.

According to one embodiment, the content of the dry crosslinking agent is 5% by weight or more than 5% by weight, in particular 10 to 30% by weight, based on the dry polyurethane.

According to a preferred embodiment, the polyurethane is polyether based.

In one embodiment, the polyether-based polyurethane is linear or branched and comprises a polyether-type polyol moiety and an isocyanate moiety. In certain embodiments, the isocyanate moiety is preferably aliphatic; Aromatic isocyanates have the disadvantage of yellowing with time.

According to another embodiment, the polyurethane is a polycarbonate based.

In one embodiment, the polycarbonate-based polyurethane is linear or branched and comprises a polycarbonate-type polyol moiety and an isocyanate moiety. In certain embodiments, the isocyanate moiety is preferably aliphatic; Aromatic isocyanates have the disadvantage of yellowing with time.

In one embodiment, the isocyanate crosslinker used is blocked (especially the isocyanate functionality is masked) so as to have a longer pot life product, as is originally known. In general, those skilled in the art are well aware of the polyurethane field and are able to propose coating compositions according to the present invention and are particularly capable of producing coatings having suitable elongation tensile stresses.

The coating step is carried out using techniques traditionally used for coating textiles, such as direct coatings.

Direct coating refers to a coating by direct deposition using a scraper or cylinder or air knife, such as pad coating, Meyer bar coating (or Champion method).

In one embodiment, the method comprises, after step c), one or more post-treatment step (s) imparting a stain-proofing and / or water repellency to the fabric.

The antifouling treatment refers to a treatment using an antistatic agent and / or an anti-tack agent. The water repellent treatment refers to a treatment using a fluorinated resin with or without an isocyanate extender. The water-repellent treatment is followed by a drying / crosslinking step.

In one embodiment, the post-treatment is applied by padding, coating, spraying, or plasma treatment, in particular using any method known to those skilled in the art.

According to one embodiment, the polyurethane used is flexible and durable to mechanical stresses. In particular it meets the following specifications: 1 to 5 MPa according to standard DIN 53504.

In the aqueous phase embodiment, the polymer is finely dispersed in water in the form of micrometric beads. This dispersion is stable due to the addition of surfactants. In the course of this process, the coating is deposited on the surface of the dry fabric (evaporation of water) and coalescence of the beads by the heat of the furnace. Thus, a microporous film is obtained. As the furnace temperature increases, the beads melt to form a continuous film.

The aqueous polyurethane has the inherent properties of the polymer and the physicochemical properties associated with its shaping. Thus, in one embodiment of the present invention, the aqueous polyurethane is characterized by a 100% elongation tensile stress of less than 5 MPa. The aqueous phase polyurethane is also characterized by a 30 to 60% dispersion concentration. Finally, the aqueous phase polyurethanes of the present invention can be characterized by a viscosity (standard DIN EN ISO / A3) of less than 10,000 Pa · s at 23 ° C.

In solvent phase embodiments, the polymer is dissolved in the medium to prevent coalescence. The film naturally forms during the evaporation of the solvent.

In one embodiment of the invention, the solvent is selected from the group consisting of aromatic solvents, alcohols, ketones, esters, dimethylformamide, and n-methylpyrrolidone.

In a particular embodiment, the solvent is selected from the group consisting of toluene, xylene, isopropanol, butanol, 1-methoxypropan-2-ol, methyl ethyl ketone, acetone, butanone, ethyl acetate, dimethylformamide, Money.

In one embodiment, the solvent-based polyurethane may be characterized by a concentration of 20 to 50%.

In one embodiment, the solvent-based polyurethane may be characterized by a viscosity (standard DIN EN ISO / A3) of less than 100,000 Pa · s at 23 ° C.

The aqueous polymer dispersion, which is a coating composition for fabrics of the present invention, may also comprise additives.

The additive may be any additive conventionally used in textile coating compositions. These are in particular selected from the group consisting of viscosity regulators, UV stabilizers, pigments, dispersants and surfactants. According to one embodiment, the coating comprises an anti-UV agent.

The fabric of the present invention may be any fabric known to those skilled in the art in which coatings are required. In particular, the fabrics of the present invention can be made of synthetic or synthetic yarns. This may in particular be polyamide, polyester or viscose.

The present invention has the advantage of using a fine yarn containing a plurality of filament components. This, in addition to imparting lightness to the fabric, can substantially reduce the porosity of the fabric prior to coating, particularly if the step of spreading the fibers by calendering is preceded by a coating step, so that the amount of polymer impregnated The relative weight of the coating can be reduced, and finally the final weight of the fabric can be reduced while providing superior porosity and durability of the fabric.

In one embodiment of the invention, the fabric is formed of warp and weft yarns having different mechanical properties.

The present invention also relates to a lightweight fabric which can be obtained by implementing the method according to the invention.

This type of fabrics can be advantageously used for flying sails, particularly paragliders, which require significant mechanical strength, resistance to weather conditions, and strength over time.

The fabric according to the invention advantageously has a high durability, in particular a high water stability. This stability can be evaluated by different accelerated aging methods as described in the Example section:

- Porosity after cleaning: preferably maintained below 70 L / m2 / min at 2000 Pa;

Porosity after mechanical stress: preferably less than 40, preferably 20 L / m < 2 > / min under 2000 Pa;

Porosity after hydrolysis and mechanical stress: preferably below 70 L / m2 / min at 2000 Pa.

The fabrics according to the invention are applicable in all fields where weight, durability, in particular moisture stability, and increased porosity are advantageous.

The invention thus relates to items such as para-glider sails comprising or made from a fabric according to the invention.

While the invention has been described using an embodiment corresponding to a preferred embodiment, it is provided for informational purposes only.

Example  One:

Slope: 22 dtex, 1.7 DPF

Weft: 33 dtex, 1.1 DPF

Seal material: polyamide, polyester or viscose

Weaving: 43.7 slope / cm and 47.6 weft / cm

Coverage rate 2.8

The calendering is carried out at a temperature of 180 to 210 DEG C and a pressure of 180 to 230 kg at a speed of 10 to 20 m / min.

Coating composition:

Aliphatic polyether-based PU: 70% (dry: 31.5%) having 100% elongation tensile stress of 2 MPa

Isocyanate-based curing agent: 15% (drying: 6.8%)

Additive: 10%

Thickener: 5%

(Unless otherwise indicated,% is by weight)

The coating was carried out with a scraper, followed by a drying step (90 to 120 ° C temperature) and a crosslinking step (140 to 210 ° C temperature). The speed was 10 to 30 m / min.

The obtained fabric was subjected to water repellency treatment:

Composition: Fluorinated resin 10%

      Water 89%

      Wetting agent 1%

The post-treatment was applied at a rate of 10-30 m / min with padding, drying (90-120 ° C temperature) followed by crosslinking (140-210 ° C temperature).

Example  2:

Slope: 22 dtex, 1.7 DPF

Weft: 33 dtex, 1.1 DPF

Thread material: Polyamide

Weaving: 43.7 slope / cm and 47.6 weft / cm

Fabric Weight: 27.7g / ㎡

Coverage rate 2.8

The calendering is carried out at a temperature of 180 to 210 DEG C and a pressure of 180 to 230 kg at a speed of 10 to 20 m / min.

Coating composition: (5.8 g / m 2)

Evotop U80ES (aliphatic polyether-based PU with 100% elongation tensile stress of 2 MPa): 70%

Rottal 444: 15% (isocyanate curing agent)

Additive: 10% (UV inhibitor, humectant, biocide)

Rotta 1227: 5% (binder)

The coating was carried out with a scraper, followed by a drying step (90 to 120 캜) and a crosslinking step (140 to 210 캜). The speed was 10 to 30 m / min.

The obtained fabric was subjected to water repellency treatment (0.5 g / m 2):

Composition: Thobotex SLF 10% (fluorinated resin)

      Water 89%

      Wetting agent 1%

The post treatment was applied at a rate of 10-30 m / min with padding followed by drying (90-120 ° C temperature) / crosslinking (140-210 ° C temperature).

The resulting fabric had a weight of 34 g / m 2 (coated fabric weight 27.7 g / m 2, the water repellent agent contribution to the final weight of the fabric was about 0.5 g / m 2).

The elongation was measured in% under a force of 3 pounds (lbs.) Applied in the bias direction. These elongations characterize the fabric stiffness in the bias direction. The standard used is NF EN ISO 13934-1. The test specimens were produced with a width of 50 mm and a length of 300 mm. The jaws of the dynamometer were moved 200 mm apart and measured at a speed of 100 mm / min.

Elongation to oblique bias below 3 lbs.: 6.0%

Elongation to weft bias at 3 lbs.: 6.3%

The aging of the fabric was also measured using different methods.

The porosity of the fabric was measured after four 1 hour wash cycles at 30 DEG C water. Ideally, the porosity after treatment should be less than 70 L / m2 / min.

Porosity after mechanical stress was also measured. To perform the porosimetry, the fabric was allowed to float (four blade assemblies, the fabric bound at the end of each blade) fixed to a mill-type assembly. Ideally, after 1.5 hours of treatment, the porosity should be less than 4 L / m 2 / min, preferably less than 20 L / m 2 / min.

Porosity after hydrolysis was also measured. To this end, the fabric was left in a pressure cooker for 4 hours with heated water and working pressure. The porosity of the fabric was measured by applying the suspension for 1 hour as above and the porosity after the treatment should be less than 70 L / ㎡ / min.

Result: A new porosity of 1 L / m 2 / min under 2000 Pa.

      After washing 4 times at 30 캜, the porosity was 52 L / m 2 / min under 2000 Pa.

      Porosity after hydrolysis and floatation: 9 L / m < 2 > / min under 2000 Pa.

      Porosity after 90 minutes flotation: 10 to 14 L / m < 2 > / min under 2000 Pa.

Claims (21)

A lightweight fabric formed of continuous warp and weft, characterized in that one or both surfaces are coated with polyurethane and characterized by a combination of the following characteristics:
- the bare fabric has a coverage rate TC of 1.8 to 4, said TC being calculated as follows:
TC = (diameter of filament number / ㎝ × one filament (cm)) + _slope (filament number / cm × diameter of the one filament (cm)) _{a weft,}
- the polyurethane has a polyether or polycarbonate base with a 100% elongation tensile stress (measured according to standard DIN 53504) of 5 MPa or less, a dry impregnation rate of at least 10% by weight,
The fabric has a weight of 25 to 40 g / m < 2 > including the coating. The lightweight fabric of claim 1, wherein the polyurethane has an elongation tensile stress of 3 MPa or less. The lightweight fabric of claim 1, having an air permeability of less than or equal to 20 L / m 2 / min under 2000 Pa when measured according to standard NFG 07111 for a measured surface area of 100 cm 2. The lightweight fabric of claim 1, having an elongation of less than 10% in said warp and weft direction bias below 3 lbs. In accordance with standard NF EN ISO 13934-1. The lightweight fabric of claim 4 having an elongation of 1 to 10% with said warp and weft direction bias below 3 lbs. The lightweight fabric of claim 1, wherein the warp and / or weft has a dtex of from 11 to 44 dtex and a DPF of from 1 to 4 (decitex per filament). The lightweight fabric of claim 1, wherein the weight of the bare fabric is 20 to 33 g / m 2. The lightweight fabric of claim 1, wherein the weight of the coated fabric is 25 to 36 / m2. The lightweight fabric of claim 1, wherein the fabric comprises 30 to 50 warps / cm and 30 to 50 weights / cm. The lightweight fabric of claim 1, wherein the polyurethane coating comprises an isocyanate and / or a melamine crosslinker, wherein the crosslinker content is 5 wt.% Or greater than 5 wt.% Relative to the polyurethane on a dry weight basis. The lightweight fabric of claim 1, comprising antifouling and / or water repellent. 12. A method of producing a lightweight fabric according to any one of claims 1 to 11,
(a) a fabric having a coverage TC of 1.8 to 4 is used, TC is calculated as follows:
TC = (number of filaments / ㎝ × one diameter (cm) of the filament _slope + (number of filaments / ㎝ × diameter (cm)) of one _weft filament,
(b) one surface or both surfaces of the fabric is coated with a polyether or polycarbonate-based polyurethane having a 100% elongation tensile stress (measured according to standard DIN 53504) of 5 MPa or less, Is not less than 10% by weight,
(c) the fabric is heated until the coating is dried and crosslinked,
(d) the fabric is restored to have a weight of 25 to 40 g / m < 2 >
A method of making a coated lightweight fabric. 13. A method according to claim 12, characterized in that the fabric is calendered before being coated in step (b). 14. The method of claim 13, wherein the coating is performed on a calendered surface of the fabric. 14. The method of claim 13, wherein a chemical treatment that imparts a functional group that reacts with the groups of polymers to form a chemical bond is applied on the surface of the fabric prior to coating the surface, and increasing the bonding. 14. The method of claim 13, wherein coating is performed on a fabric surface opposite the calendered surface. 14. The method according to claim 13, comprising a post-treatment step of imparting antifouling and / or water repellency to the fabric. The lightweight fabric of claim 1, wherein the bare fabric has a coverage rate TC of 2.6 to 3.2. The method according to claim 1,
Characterized in that the dry impregnation rate of said polyurethane is from 12 to 30% by weight. 20. The method of claim 19,
Characterized in that the dry impregnation rate of said polyurethane is from 15 to 25% by weight. 11. The method of claim 10,
Characterized in that the crosslinking agent content is from 10 to 30% by weight, based on dry weight, of the polyurethane.