FIBROUS LAYER HAVING HYDROPHILIC PROPERTIES AND A FABRIC COMPRISING SUCH LAYER

Abstract

A fibrous layer, wherein surface of the fibres has surface energy below 50 mN/m, characterised in that the calculated strike through time coefficient (cSTT) of the fibrous layer is below 20 and the fibrous layer is bonded in its entire volume at fibre to fibre contact bonding points, wherein the specific fibre surface is the surface area of the fibres in m.sup.2 per 1 m.sup.2 of the fibrous layer, basis weight is the weight of the layer in kg per 1 m.sup.2 of the fibrous layer, the specific void volume is the volume of empty spaces between the fibres in m.sup.3 per 1 m.sup.2 of the fibrous layer.

[00001]$\mathrm{cSTT}=\frac{{\left(\mathrm{specific}\mathrm{fibre}\mathrm{surface}\right)}^2\times \left(\mathrm{basis}\mathrm{weight}\right)}{\left(\mathrm{specific}\mathrm{void}\mathrm{volume}\right)\times {\left(\mathrm{surface}\mathrm{energy}\mathrm{of}\mathrm{fibre}\mathrm{surface}\right)}^3}\times 600$

Claims

1. A fibrous layer, wherein surface of the fibres has surface energy below 50 mN/m, characterised in that the calculated strike through time coefficient (cSTT) of the fibrous layer is below 20 and the fibrous layer is bonded in its entire volume at fibre to fibre contact bonding points, wherein $\mathrm{cSTT}=\frac{{\left(\mathrm{specific}\mathrm{fibre}\mathrm{surface}\right)}^2\times \left(\mathrm{basis}\mathrm{weight}\right)}{\left(\mathrm{specific}\mathrm{void}\mathrm{volume}\right)\times {\left(\mathrm{surface}\mathrm{energy}\mathrm{of}\mathrm{fibre}\mathrm{surface}\right)}^3}\times 600$ wherein the specific fibre surface is the surface area of the fibres in m.sup.2 per 1 m.sup.2 of the fibrous layer, basis weight is the weight of the layer in kg per 1 m.sup.2 of the fibrous layer, the specific void volume is the volume of empty spaces between the fibres in m.sup.3 per 1 m.sup.2 of the fibrous layer.

2. The fibrous layer according to claim 1, wherein the calculated strike through time coefficient (cSTT) of the fibrous layer is below 15, preferably below 10, more preferably below 7, most preferably below 5.

3. The fibrous layer according to claim 1, wherein the basis weight of the fibrous layer is within the range of 8 to 200 gsm, and more preferably the basis weight of the fibrous layer is more than 15 gsm, more preferably more than 20 gsm, most preferably more than 30 gsm, and/or the basis weight of the fibrous layer is less than 150 gsm, more preferably less than 100 gsm, more preferably less than 80 gsm, most preferably less than 60 gsm.

4. The fibrous layer according to claim 1, wherein all components of the fibres of the fibrous layer are arranged across the cross-section of the fibres in a non-crimpable configuration.

5. The fibrous layer according to claim 4, wherein the fibres comprise at least one polymeric material from a group consisting of polyesters, polyamides and their blends.

6. The fibrous layer according to claim 5, wherein the fibres comprise at least one polymeric material from a group consisting of PET, coPET, PLA, coPLA and their blends.

7. The fibrous layer according to claim 1, wherein all components of the fibres of the fibrous layer are arranged across the cross section of the fibres in a crimpable configuration.

8. The fibrous layer according to claim 7, wherein the fibres comprise at least one polymeric material from a group consisting of polyesters, polyamides and their blends.

9. The fibrous layer according to claim 8, wherein the fibres comprise at least one polymeric material from a group consisting of PET, coPET, PLA, coPLA, PP, PE, PP/PE copolymer, and their blends.

10. The fibrous layer according to claim 1, wherein the fibres are crosslinked cellulose fibres.

11. A fabric comprising the fibrous layer according to claim 1, wherein the fibrous layer forms a first fibrous layer (A) and the fabric comprises a second fibrous layer (B) arranged adjacent the first fibrous layer (A), wherein the difference between the calculated strike through time coefficient cSTT of the first fibrous layer (A) and of the second fibrous layer (B) is at least 0.5, preferably at least 1.0, more preferably at least 1.5, most preferably at least 2.0.

12. The fabric according to the claim 11, wherein the first fibrous layer (A) comprises crosslinked cellulose fibres.

13. A fibrous structure comprising at least two layers, one of them comprising cellulosic crosslinked, stiffened and curled fibres and another one comprising synthetic fibres, wherein the cellulosic fibres exhibit fibrils in their cross section and the synthetic fibres comprise homogeneous polymer or polymers in its cross section, and the cellulosic fibres have an average length of maximum 8 mm or less and the synthetic fibres have an average length larger than 80 mm and at least one of the layers contains bonding material.

14. The fibrous structure according to the claim 13, wherein the synthetic fibres are endless spunbond multicomponent filaments.

15. The fibrous structure according to claim 13, wherein the bonding material is comprised in the fibres or filaments of any layer, preferably the bonding material is present as a component of surface of these fibres or filaments.

16. The fibrous structure according to claim 13, wherein the bonding material is in the form of powder binder added into one or more fibrous layers or in between them.

17. A fabric comprising at least two fibrous layers (A, B), wherein the first layer (A) comprises hydrophilic spin finish at least partially soluble in water solution on at least a part of fibre surface and the cSTT for the first layer (A) is lower than cSTT for the second layer (B), preferably wherein the difference in cSTT for the layers (A, B) is at least 5.0, preferably at least 10.0, preferably at least 15.0, more preferably at least 20.0, wherein for each fibrous layer $\mathrm{cSTT}=\frac{{\left(\mathrm{specific}\mathrm{fibre}\mathrm{surface}\right)}^2\times \left(\mathrm{basis}\mathrm{weight}\right)}{\left(\mathrm{specific}\mathrm{void}\mathrm{volume}\right)\times {\left(\mathrm{surface}\mathrm{energy}\mathrm{of}\mathrm{fibre}\mathrm{surface}\right)}^3}\times 600$ wherein the specific fibre surface is the surface area of the fibres in m.sup.2 per 1 m.sup.2 of the fibrous layer, basis weight is the weight of the layer in kg per 1 m.sup.2 of the fibrous layer, the specific void volume is the volume of empty spaces between the fibres in m.sup.3 per 1 m.sup.2 of the fibrous layer.

18. An absorbent article, comprising topsheet, backsheet and at least one intermediate nonwoven fibrous layer arranged between the topsheet and the backsheet and comprising polymeric superabsorbent particles, wherein at least one of the topsheet, backsheet and the intermediate nonwoven fibrous layer is formed by a fibrous layer according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0117] FIG. 1 shows hydrophilic, hydrophobic and superhydrphobic behaviour,

[0118] FIGS. 2a to 2c show fibers exhibiting various degrees of swelling,

[0119] FIGS. 3a to 3c show various capillary effects based on philic properties of material,

[0120] FIG. 4a shows dry polyolefin fibers and FIG. 4b shows the same fibers in water.

EXAMPLE 1

[0121] A nonwoven fabric was produced using two subsequent bi-component REICOFIL spunbond beams with the same settings, with a round shape core-sheath type fibre. The core was produced from PET (Type 5520 resin from Invista) and the sheath from two different copolymers (type RT5032 from Trevira and type 701k from Invista). The process conditions and final fabric parameters for each of the Examples 1A to 1D are shown in Table 3 below.

TABLE-US-00003 TABLE 3 Example 1A 1B 1C 1D Basis weight set 80 75 75 60 [g/m.sup.2] Fibre composition PET/ PET/ PET/ PET/ coPET coPET coPET coPET coPET type RT5032 701k 701k RT5032 from from from from Trevira Invista Invista Trevira Cross section C/S C/S C/S C/S Mass ratio 77:23 70:30 70:30 77:23 Activation 140 140 140 140 temperature [° C.] Bonding 140 155 155 140 temperature [° C.] Bulk type controlled controlled controlled controlled shrinkage shrinkage shrinkage shrinkage Hydrophilic treatment no no no no Thickness [mm] 2.96 3.51 3.44 2.37 Apparent fibre 31.23 36.11 33.22 32.50 diameter [μm] Fibre surface energy 45.8 36.7 36.7 45.8 [mN/m] Basis weight measured 0.0802 0.0750 0.0756 0.0593 [kg/m.sup.2] Specific fibre 7.49 6.06 6.64 5.33 surface [m.sup.2/m.sup.2] Void space [m.sup.3/m.sup.2] 2.90*1e−3 3.46*1e−3 3.38*1e−3 2.33*1e−3 cSTT 9.68 9.67 11.97 4.52 Measured STT 8.52 9.03 10.77 3.83

EXAMPLE 2

[0122] A nonwoven fabric was produced using two subsequent bi-component REICOFIL spunbond beams with the same settings, with a round shape core-sheath type fibre. The process conditions and final fabric parameters for each of the Examples 2A to 2D are shown in Table 4 below.

TABLE-US-00004 TABLE 4 Example 2A 2B 2C 2D Basis weight set [g/m.sup.2] 25 40 60 80 Fibre composition PET/PE PET/PE PET/PE PET/PE Cross section C/S C/S C/S C/S Mass ratio 70:30 70:30 77:23 70:30 Core polymer PET PET PET PET Invista Invista Invista Invista 5520 5520 5520 5520 Sheath polymer PE Aspun PE Aspun PE Aspun PE Aspun 6834 6834 6834 6834 Activation temperature 140 140 140 140 [° C.] Bonding temperature 130 130 130 130 [° C.] Bulk type controlled controlled controlled controlled shrinkage shrinkage shrinkage shrinkage Hydrophilic treatment no no no no Thickness [mm] 0.85 1.39 1.21 2.33 Apparent fibre diameter 27.83 36.58 26.70 35.23 [μm] Fibre surface energy 32.7 32.7 32.7 32.7 [mN/m] Basis weight measured 26.0 40.2 61.5 78.1 [kg/m.sup.2] Specific fibre surface 3.01 3.55 7.25 7.14 [m.sup.2/m.sup.2] Void space [m.sup.3/m.sup.2] 0.83*1e−3 1.36*1e−3 1.16*1e−3 2.27*1e−3 cSTT 4.85 6.39 48.02 30.13 Measured STT 4.89 5.92 over 50 28.70

EXAMPLE 3

[0123] A nonwoven fabric was produced using two subsequent bi-component REICOFIL spunbond beams with the same settings, with a round shape side-by-side type fibre. The core was produced from PLA (type 6202 resin from Nature Works) and the sheath from PP (Tatren HT2511 from Slovnaft) polymer. The process conditions and final fabric parameters for each of the Examples 3A to 3D are shown in Table 5 below.

TABLE-US-00005 TABLE 5 Example 3A 3B 3C 3D Basis weight set [g/m.sup.2] 35 35 35 35 Fibre composition PLA/PP PLA/PP PLA/PP PLA/PP Cross section S/S S/S S/S S/S Mass ratio 50:50 70:30 70:30 70:30 Bulk type heat heat heat Heat activated activated activated activated self- self- self- self- crimping crimping crimping crimping Hydrophilic treatment no no no spin finish PHP10 Thickness [mm] 0.39 0.44 0.84 0.44 Apparent fibre diameter 13.76 17.45 31.70 17.45 [μm] Fibre surface energy 37.85 41.15 41.15 55.4 [mN/m] Basis weight measured 31.83 35.18 36.01 35.18 [kg/m.sup.2] Specific fibre surface 9.22 6.96 3.92 6.96 [m.sup.2/m.sup.2] Void space [m.sup.3/m.sup.2] 0.36*1e−3 0.41*1e−3 0.81*1e−3 0.40*1e−3 cSTT 91.74 35.57 5.89 14.68 Measured STT over 50 36.94 6.17 13.88

EXAMPLE 4

[0124] A nonwoven fabric was produced using two subsequent bi-component REICOFIL spunbond beams with the same settings, with a round shape side-by-side type fibre. The core was produced from PLA (type 6202 resin from Nature Works) and the sheath from PE (Bio-PE SHA 7260) polymer. The process conditions and final fabric parameters for each of the Examples 4A to 4D are shown in Table 6 below.

TABLE-US-00006 TABLE 6 Example 4A 4B 4C 4D Basis weight set [g/m.sup.2] 35 35 35 35 Fibre composition PLA/PE PLA/PE PLA/PE PLA/PE Cross section eC/S eC/S S/S S/S Mass ratio 70:30 80:20 70:30 60:40 Bulk type heat heat heat heat activated activated activated activated self- self- self- self- crimp crimp crimp crimp Hydrophilic treatment no no no no Thickness [mm] 0.33 0.48 0.37 0.51 Apparent fibre diameter 19.37 40.03 19.37 32.37 [μm] Fibre surface energy 34.3 34.3 41.8 41.8 [mN/m] Basis weight measured 35.13 35.29 34.63 34.09 [kg/m.sup.2] Specific fibre surface 6.27 3.13 6.18 3.74 [m.sup.2/m.sup.2] Void space [m.sup.3/m.sup.2] 0.30*1e−3 0.45*1e−3 0.34*1e−3 0.48*1e−3 cSTT 68.57 11.47 32.32 8.15 Measured STT over 50 12.01 34.12 6.99

EXAMPLE 5

[0125] The nonwoven fabric was produced using two subsequent bi-component REICOFIL spunbond beams with the same settings, with a round shape core-sheath type fibre. The core was produced from PET (Type 5520 resin from Invista). All samples were hydrophilised by means of a spin finish (PHP 90 from Schill and Seilacher) using the kiss roll. The process conditions and final fabric parameters for each of the Examples 5A to 5D are shown in Table 7 below.

TABLE-US-00007 TABLE 7 Example 5A 5B 5C 5D Basis weight set [g/m.sup.2] 30 60 40 80 Fibre composition PET/PE PET/PE PET/ PET/ coPET coPET Cross section c/s C/S C/S C/S Mass ratio 77:23 77:23 77:23 77:23 Sheath polymer PE PE coPET coPET Aspun Aspun Trevira Trevira 6834 6834 Bulk type con- con- con- con- trolled trolled trolled trolled shrink shrink shrink shrink Hydrophilic treatment spin spin spin spin finish finish finish finish PHP 90 PHP 90 PHP 10 PHP10 Thickness [mm] 0.78 1.34 1.23 2.86 Apparent fibre diameter 31.49 35.77 28.52 37.33 [μm] Fibre surface energy 52.7 52.7 54.3 54.3 [mN/m] Basis weight measured 30.1 60.5 40.6 83.2 [kg/m.sup.2] Specific fibre surface 3.0 5.3 4.2 6.5 [m.sup.2/m.sup.2] Void space [m.sup.3/m.sup.2] 0.75*1e−3 1.29*1e−3 1.20*1e−3 2.80*1e−3 cSTT 1.48 5.45 2.19 4.71 Measured STT 1.23 4.04 1.18 4.38

EXAMPLE 6

[0126] Two layers were combined in one composite. The layers and their specification are shown in the table 8 below:

TABLE-US-00008 TABLE 8 Example 6A 6B 6C 6D Layer A Example Example Example Example 1D 2A 2A 3B cSTT of layer A 4.52 4.85 4.85 5.89 Layer B Example Example Example Example 2B 2D 3C 3C cSTT of layer B 6.39 10.8 35.6 35.6 Total basis weight 100 60 60 70 [kg/m.sup.2] Total Specific fibre 8.87 7.88 9.97 10.88 surface [m.sup.2/m.sup.2] Total Void space 3.68*1e−3 1.45*1e−3 1.25*1e−3 1.22*1e−3 [m.sup.3/m.sup.2] Sum of A(cSTT) + 11 16 40 41 B(cSTT) Measured STT 4.9 8.2 12.8 14.1

[0127] These samples provided an excellent distribution of liquid within the layers.

[0128] In the following table two layers are combined together in one composite, layer A is a 40 gsm nonwoven fabric made from crosslinked, curled and stiffened fibres supplied by International Paper (former Weyerhaeuser). These cellulose fibres had an average thickness of 25.33 microns (fibre surface area of 4.16 m2/m2) with a surface energy of 46.4 mN/m and a thickness 2.2 mm (0.0022 m3/m2 void space), which provided the cSTT of 1.86 and also in reality the layer drew in the liquid very quickly with a dry surface after liquid absorption.

TABLE-US-00009 TABLE 9 Example 6E 6F 6G Layer A cellulose cellulose Example 2A cSTT of Layer A 1.86 1.86 1.86 Layer B Example Example Example 2B 2D 3C cSTT of Layer B 6.39 35.6 Total basis weight [kg/m.sup.2] 80 75 Total Specific fibre 7.71 11.1 surface [m.sup.2/m.sup.2] Total Void space [m.sup.3/m.sup.2] 3.57*1e−3 2.63*1e−3 Summed A(cSTT) + B(cSTT) 8 37 Measured STT 2.7 3.2

EXAMPLE 7

[0129] Two layers were combined in one composite. Both layers were 60 gsm PET/PE fabrics as described in examples 5B and 2C. The layer combination is shown in the table below:

TABLE-US-00010 Example 7A 7B 7C Layer A Example 2C Example 5B Example 5B cSTT of Layer A 48.02 5.45 5.45 Hydrophilic treatment of no yes yes Layer A Layer B Example 2C Example 2C Example 5B cSTT of Layer B 48.02 48.02 5.45 Hydrophilic treatment of no no yes Layer B Total basis weight [kg/m.sup.2] 120 120 120 Total Specific fibre surface 14.8 12.6 10.6 [m.sup.2/m.sup.2] Total Void space [m.sup.3/m.sup.2] 2.3*1e−3 2.45*1e−3 2.58*1e−3 Sum of A(cSTT) + 96 53 11 B(cSTT) Measured STT over 50 2.67 1.87

Testing Methodology

[0130] The “Basis weight” of a nonwoven web is measured according to the European standard test EN ISO 9073-1:1989 (conforms to WSP 130.1). There are 10 nonwoven web layers used for measurement, sample area size is 10×10 cm2.

[0131] The “Thickness” or “Calliper” of the nonwoven material is measured according to the European standard test EN ISO 9073-2:1995 (conforms to WSP 120.6) with the following modification:

1. The material shall be measured on a sample taken from production without being exposed to higher strength forces or spending more than a day under pressure (for example on a product roll), otherwise before measurement the material must lie freely on a surface for at least 24 hours.
2. The overall weight of the upper arm of the machine including added weight is 130 g. “Median fibre diameter” in a layer is expressed in SI units—micrometers (μm) or nanometers (nm). To determine the median, it is necessary to take a sample of the nonwoven fabric from at least three locations at least 5 cm away from each other. In each sample, it is necessary to measure the diameter of at least 50 individual fibres for each observed layer. It is possible to use, for example, an optical or electronic microscope (depending on the diameter of the measured fibres). In the event that the diameter of fibres in one sample varies significantly from the other two, it is necessary to discard the entire sample and to prepare a new one.

[0132] In the case of round fibres, the diameter is measured as a diameter of the cross-section of the fibres. In the event of any other shape of the fibre (e.g. hollow fibre or trilobal fibre), the cross-section surface shall be determined for each measured fibre and recalculated for a circle with same surface area. The diameter of this theoretical circle is the diameter of the fibre.

[0133] The measured values for each layer composed of all three samples are consolidated into a single set of values from which the median is subsequently determined. It applies that at least 50% of the fibres have a diameter less than or equal to the value of the median and at least 50% of the fibres have a diameter greater than or equal to the median. To identify the median of the given sample set of values, it is sufficient to arrange the values according to size and to take the value found in the middle of the list. In the event that the sample set has an even number of items, usually the median is determined as the arithmetic mean of the values in locations N/2 and N/2+1.

[0134] The “void volume” herein refers to the total amount of void space in a material relative to the bulk volume occupied by the material.

[0135] The bulk volume of the material is equal to the bulk volume of the nonwoven and can be calculated from fabric thickness (calliper) using the following equation:

bulk volume (m.sup.3)=(calliper (m))*1 (m)*1 (m)

The total amount of void space in a material can be calculated using the equation:

void space=bulk volume (m.sup.3)−mass volume (m.sup.3)

[0136] The total mass volume can be calculated using the equation:

mass volume (m.sup.3)=(weight in kilograms based on basis weight (kg))/mass density (kg/m.sup.3)

[0137] Where the mass density can be calculated from a known composition or measurement according to the norm ISO 1183-3:1999.

[0138] So the void volume can be calculated using the equation:

Void volume (%)=[1−(volume of filaments in 1 m.sup.2 nonwoven fabric layer/volume of 1 m.sup.2 nonwoven fabric layer)]*100%

[0139] Thus, for single component filaments:

Void volume (%)=[1−(basis weight (g/m.sup.2)/calliper (mm))/mass density (kg/m.sup.3)]*100%

[0140] Of course, when multi-component filaments are considered, wherein the components differ in density, the volume of filaments within a square meter of nonwoven fabric (NT) must be calculated accordingly.

[0141] The “recovery” of the bulkiness after the application of pressure herein refers to the ratio of the thickness of the fabric after it is freed from a load to the original thickness of the fabric. The thickness of the fabric is measured according to the EN ISO 9073-2:1995 using a preload force of 0.5 kPa). The recovery measurement procedure consists of following steps: [0142] 1. Prepare fabric samples measuring 10×10 cm [0143] 2. Measure the thickness of 1 piece of fabric [0144] 3. Measure the thickness of a pile of 5 pieces of fabric using a preload force of 0.5 kPa (Ts) [0145] 4. Load the pile of 5 fabric sheets on to a thickness meter (2.5 kPa) for 5 minutes [0146] 5. Release the weight and wait for 5 minutes [0147] 6. Measure the thickness of pile of the 5 fabrics using a preload force of 0.5 kPa (Tr) [0148] 7. Calculate the recovery according to the following equation:

Recovery=Tr/Ts(no unit) [0149] Ts=thickness of fresh sample [0150] Tr=thickness of recovered sample

[0151] The “compressibility” herein refers to the distance in mm by which the nonwoven is compressed by the load defined in the “resilience” measurement. It can be also be calculated as resilience (no unit)*thickness (mm). The “resilience” of a nonwoven is measured according to the European standard test EN ISO 964-1 with the following modification: [0152] 1. The thickness of one layer of the fabric is measured. [0153] 2. A pile of fabric samples is prepared so that the total thickness is at least 4 mm, optimally 5 mm in total. The pile of fabrics contains at least 1 piece of fabric. [0154] 3. The thickness of the pile of fabric samples is measured [0155] 4. A force of 5N with loading speed of 5 mm/min is applied to the pile of nonwoven samples [0156] 5. The distance of the clamp movement is measured [0157] 6. Resilience is calculated according to the equation:

R(no unit)=T1 (mm)/T0 (mm)

Or

R (%)=T1 (mm)/T0(mm)*100%

T1=distance of the clamp movement under the load 5N [mm]=how much was the pile of fabrics compressed
T0=thickness (acc. EN ISO 9073-2:1995 using the preload force of 1.06N) [mm] [0158] The “degree of crimping” is measured according to ASTM D-3937-82 with the following modification:

[0159] 1. the used unit of measurement is “crimps/cm” [0160] Setting the degree of crimping in a bonded layer is an issue since single fibres are bonded to each other and it is not possible to remove one of them from the composition (without the danger of affecting the original crimp level) and to measure the crimp value and the fibre length. For the purpose of this invention, the following estimation may be used: [0161] 1) A picture of the assessed layer is provided in such a magnification that the fibres can be well seen [0162] 2) One single fibre is chosen and its path through the picture or at least part of the picture is marked [0163] 3) The length of the marked fibre in the picture is measured [0164] 4) The number of crimps in the measured length is counted [0165] In contrast to the measurement of individual fibres, it is not possible to place the fibre in such a way that all the crimps can be seen clearly and then counted in a repeating sequence. In a bonded structure, some parts may be masked in the z direction, some parts may be masked by other fibres; some parts may be masked by bonding. Each fibre turn shall be counted as half a crimp. Also, a change from sharp to blurry on one fibre shall be counted as half a crimp [0166] 5) The number of crimps/cm is calculated

[0167] It has to be kept in mind that the value is calculated from a 2D picture of a 3D object and that the length of the fibre in the z direction is not covered. The real length of the fibre would probably be higher. Also, a 2D picture can mask certain crimps on the fibre, especially in the vicinity of a bonding point. Nevertheless, it is assumed that the described calculation can provide a relevant estimate of fibre crimping.

[0168] The “Bulk density” of a nonwoven material is calculated using the following equation:

ρ.sub.b=bulk density [kg/m.sup.3]=basis weight (g/m2)/fabric thickness (mm)

BW=basis weight (acc. EN ISO 9073-1:1989) [g/m.sup.2]
T=thickness (acc. EN ISO 9073-2:1995) [mm]

[0169] The bulk density of one layer in a composite: [0170] 1) Using an optical method, the thickness of a single layer in the cross-section of a nonwoven is measured. The number of samples is at least 10 and the number is set so that the corrected sample standard deviations shall be smaller than 30% of the average value (v is below 30%) [0171] 2) The basis weight is measured [0172] a. The production value is taken [0173] b. To obtain an approximate value, it is possible to do the following: [0174] i. A sample of a known surface area is taken [0175] ii. The layers are carefully separated from each other, or the fibres from the layers are separated out, [0176] iii. The weight of the separated layers and the fibres from them are measured. [0177] iv. The basis weight is calculated from the known surface area and the weight of layer. [0178] v. The number of samples is at least 10 and the number is set so that the corrected sample standard deviation s is less than 20% of the average value (v is below 20%)

[0179] The corrected sample standard deviation shall be calculated using following formula:

[00007]$s=\sqrt{\frac{1}{N-1}\underset{i=1}{\overset{N}{.Math.}}{\left(x_i-\stackrel{\_}{x}\right)}^2}.$$v=\frac{s}{x}.Math.100\left(\%\right)$

Where:

[0180] N—number of samples
xi—single measured value
x—average measured value