FIBROUS MATERIAL IMPREGNATED WITH THERMOPLASTIC POLYMER OF OPTIMUM MOLECULAR MASS AND VISCOSITY AND METHOD FOR THE PRODUCTION THEREOF

Abstract

The present invention concerns an impregnated fibrous material comprising at least one continuous-fiber fibrous material in the form of a roving or a plurality of parallel rovings and at least one thermoplastic polymer matrix, characterized in that said at least one thermoplastic polymer is an amorphous or semi-crystalline polymer having a glass transition temperature such that Tg≥40° C., especially Tg≥100° C., in particular ≥120° C., the fiber content of said impregnated fibrous material being from 45 to 65% by volume, preferably from 50 to 60% by volume, especially from 54 to 60% by volume, the number-average molecular mass Mn of said thermoplastic polymer being from 11,000 to 25,000 g/mol, the melt viscosity of said thermoplastic polymer being from 80 to 1500 Pa.Math.s, as measured by plane-plane rheology at 1 Hz and 2% deformation, at a temperature of Tg+220° C.

Claims

1. An impregnated fibrous material comprising at least one continuous-fiber fibrous material in the form of a roving or several parallel rovings and at least one thermoplastic polymer matrix, wherein a thermoplastic polymer of said at least one thermoplastic polymer matrix is an amorphous or semi-crystalline polymer having a glass transition temperature such that Tg≥40° C., as measured using a differential scanning calorimeter (DSC), after a second heating pass, according to standard ISO 11357-2:2013, with a heating and cooling speed of 20° C./min, the fiber content in said impregnated fibrous material being from 45 to 65% by volume, the number-average molecular mass Mn of said thermoplastic polymer being from 11,000 to 25,000 g/mol, the melt viscosity of said thermoplastic polymer being from 80 to 1500 Pa.Math.s, as measured by plane/plane rheology at 1 Hz and 2% deformation, at a temperature of Tg+220° C.

2. The impregnated fibrous material according to claim 1, wherein the polymolecularity index Ip of said thermoplastic polymer is from 2 to 6.

3. The impregnated fibrous material according to claim 1, wherein said at least thermoplastic polymer is chosen from: polyaryl ether ketones (PAEK); polyaryl ether ketone ketone (PAEKK); aromatic polyether imides (PEI); polyaryl sulfones; polyarylsulfides; polyamides (PA); PEBAs; polyolefins, polylactic acid (PLA), polyvinyl alcohol (PVA), and fluorinated polymers; and mixtures thereof.

4. The impregnated fibrous material according to claim 1, wherein the number-average molecular mass Mn of said thermoplastic polymer is from 14,000 to 25,000, and the molten viscosity of said thermoplastic polymer is from 150 to 1500 Pa.Math.s at a temperature of Tg+220° C., and said thermoplastic polymer is a polymer with Tg≥130° C.

5. The impregnated fibrous material according to claim 4, wherein said at least thermoplastic polymer is a polyamide.

6. The impregnated fibrous material according to claim 5, wherein said polyamide is chosen from semi-aromatic polyamides.

7. The fibrous material according to claim 6, wherein said semi-aromatic polyamide is optionally modified with urea units and chosen from a semi-aromatic polyamide of formula X/YAr; X.T denotes a unit obtained from the polycondensation of a Cx diamine and terephthalic acid, with x representing the number of carbon atoms of the Cx diamine, x being between 6 and 36.

8. The impregnated fibrous material according to claim 6, wherein said semi-aromatic polyamide is chosen from a PA MXDT/6T, a PA MPMDT/6T, a PA 11/BACT, a PA 11/6T/10T, a PA MXDT/10T, a PA MPMDT/10T, a PA BACT/10T, a PA BACT/6T, PA BACT/10T/6T, a PA 11/BACT/6T, a PA 11/MPMDT/6T, PA 11/BACT/10T, a PA 11/MXDT/10T, a PA 11/MXDT/6T.

9. The impregnated fibrous material according to claim 1, wherein the number-average molecular mass Mn of said thermoplastic polymer is from 11,000 to 20,000 g/mol, the molten viscosity of said thermoplastic polymer is from 80 to 650 Pa.Math.s at a temperature of Tg+220° C., and said thermoplastic polymer is a polyamide with Tg<130° C.

10. The impregnated fibrous material according to claim 9, wherein said polyamide with Tg<130° C. is chosen from aliphatic polyamides, cycloaliphatic polyamides and semi-aromatic polyamides with Tg less than 130° C.

11. The impregnated fibrous material according to claim 10, wherein said aliphatic polyamide is chosen among polyamide 6 (PA-6), polyamide 11 (PA-11), polyamide 12 (PA-12), polyamide 66 (PA-66), polyamide 46 (PA-46), polyamide 610 (PA-610), polyamide 612 (PA-612), polyamide 1010 (PA-1010), polyamide 1012 (PA-1012), polyamide 11/1010, polyamide 12/1010, or a mixture thereof or a copolyamide thereof, and block copolymers, and the semi-aromatic polyamides are chosen among MXD10, MXD6, PA 6/6T, a PA 66/6T, a PA 6I/6T, MPMDT/10T, 11/MPMDT/10T, MPMDT/6T, a PA BACT/10T, a PA 11/BACT/10T, a MXDT/10 and a PA 11/MXDT/10T.

12. The impregnated fibrous material according to claim 1, wherein the number of fibers in said fibrous material for carbon fibers is greater than or equal to is greater than or equal to 3K, or the grammage for the glass fiber is greater than or equal to 1,200 Tex.

13. The impregnated fibrous material according to claim 1, wherein the fibers of the fibrous material are non-sized.

14. The impregnated fibrous material according to claim 1, wherein the content of fibers by volume is constant in at least 70% of the volume of the impregnated fibrous material.

15. The impregnated fibrous material according to claim 1, wherein the material has a porosity level, that is the closed porosity level determined by electron microscopy or relative deviation between theoretical density and experimental density, in said impregnated fibrous material of less than 10%.

16. The impregnated fibrous material according to claim 1, wherein said impregnated fibrous material is single layer.

17. The impregnated fibrous material according to claim 1, wherein said fibrous material comprises continuous fibers selected from carbon, glass, silicon carbide, basalt, silica, natural fibers or amorphous thermoplastic fibers with a glass transition temperature Tg higher than the Tg of said polymer or said polymer mixture when the latter is amorphous or higher than the Tm of said polymer or said polymer mixture when the latter is semi-crystalline, or the semi-crystalline thermoplastic fibers with a melting temperature Tm higher than the Tg of said polymer or said polymer mixture when the latter is amorphous or higher than the Tm of said polymer or said polymer mixture when the latter is semi-crystalline, or a mixture of two or several of said fibers.

18. The impregnated fibrous material according to claim 1, wherein said thermoplastic polymer further comprises carbonaceous fillers.

19. The impregnated fibrous material according to claim 1, wherein said thermoplastic pre-polymer further comprises liquid crystal polymers or cyclic poly(butylene terephthalate), or mixtures containing said liquid crystal polymers or said cyclic poly(butylene terephthalate) as additives.

20. The impregnated fibrous material according to claim 1, wherein said impregnated fibrous material does not exhibit drapability.

21. A method for preparing an impregnated fibrous material as defined in claim 1, wherein it comprises a step of pre-impregnating or a step of impregnating said fibrous material with at least thermoplastic polymer being an amorphous or semi-crystalline polymer having a glass transition temperature such that Tg≥40° C., the fiber content in said impregnated fibrous material being from 45 to 65% by volume, the number-average molecular mass Mn of said thermoplastic polymer being from 11,000 to 25,000 g/mol, the melt viscosity of said thermoplastic polymer being from 80 to 1500 Pa.Math.s, as measured by plane/plane rheology at 1 Hz and 2% deformation, at a temperature of Tg+220° C.

22. The method for preparing an impregnated fibrous material according to claim 21, wherein it comprises a step of impregnating said fibrous material with at least thermoplastic polymer, the number-average molecular mass Mn of said thermoplastic polymer being from 11,000 to 20,000 g/mol, the molten viscosity of said thermoplastic polymer is from 80 to 650 Pa.Math.s at a temperature of Tg+220° C., and said thermoplastic polymer being a polyamide with Tg<130° C.

23. The method according to claim 22, wherein said impregnation step is carried out by molten route, at a speed >1m/min.

24. The method according to claim 22, wherein it comprises the following steps: i) impregnating a fibrous material with at least one nonreactive thermoplastic polymer by molten route, by crosshead-die extrusion of molten polymer, in order to obtain an impregnated fibrous material, said pre-impregnation step being carried out by at least thermoplastic polymer, the number-average molecular mass Mn of said thermoplastic polymer being from 11,000 to 20,000 g/mol, the molten viscosity of said thermoplastic polymer is from 80 to 650 Pa.Math.s at a temperature of Tg+220° C., and said thermoplastic polymer being a polymer with Tg<130° C. i) optionally a step of shaping and calibrating said impregnated fibrous material to obtain an impregnated fibrous material consisting of a ribbon in the form of a thin band having a thickness from 0.2 to 5 mm.

25. The method for preparing an impregnated fibrous material according to claim 21, wherein it comprises a step of pre-impregnating said fibrous material assuming the form of a roving or of several parallel rovings with at least one nonreactive thermoplastic polymer, the number-average molecular mass Mn of said thermoplastic polymer is from 14,000 to 25,000 and the molten viscosity of said thermoplastic polymer is from 150 to 1500 Pa.Math.s at a temperature of Tg+220° C., and said thermoplastic polymer is a polymer with Tg 130° C.

26. The method according to claim 25, wherein said pre-impregnation is carried out with a system chosen among a fluidized bed, spraying by spray gun, by continuous passage of the fibers in an aqueous dispersion of powder of said nonreactive thermoplastic polymer or aqueous dispersion of particles of said thermoplastic polymer or emulsion or aqueous suspension of said nonreactive thermoplastic polymer.

27. The method according to claim 25, wherein it comprises at least one heating step without supporter of said pre-impregnated fibrous material.

28. The method according to claim 25, wherein it comprises at least one heating step carried out by means of at least a supporting part (E) and at least one heating system, said roving(s) being in contact with part or all of the surface of said at least one supporting part (E) and scrolling partially or wholly on the surface of said at least one supporting part (E) at the heating system.

29. The method according to claim 27, wherein the heating system is chosen from an infrared lamp, a UV lamp, a convection heating, a microwave heating, a laser heating and a high frequency (HF) heating.

30. The method according to claim 25, wherein it comprises the following steps: i) Pre-impregnating a fibrous material with at least one nonreactive thermoplastic polymer by fluidized bed in a tank, equipped or not with at least one supporting part (E′), by spraying by nozzle or spray gun by dry route in a tank, equipped or not with at least one supporting part (E′) to obtain a pre-impregnated fibrous material, said pre-impregnation step being carried out by at least one nonreactive thermoplastic polymer, the number-average molecular mass Mn of said thermoplastic polymer is from 14,000 to 25,000, and the molten viscosity of said thermoplastic polymer is from 150 to 1500 Pa.Math.s at a temperature of Tg+220° C. and said thermoplastic polymer is a polymer with Tg≥130° C., ii) heating step without support of said pre-impregnated fibrous material to obtain a pre-impregnated fibrous material, iii) heating step carried out using at least one supporting part (E) and at least one heating system, as defined in claim 28 or 29, to obtain an impregnated fibrous material, iv) optionally, step of shaping and calibrating the roving or said parallel rovings of said impregnated fibrous material to obtain an impregnated fibrous material consisting of a ribbon in the form of a thin band.

31. The method according to claim 25, wherein it comprises the following steps: i) pre-impregnating a fibrous material with at least one nonreactive thermoplastic polymer, said pre-impregnation step being carried out by at least thermoplastic polymer and said thermoplastic polymer is a polymer with Tg≥130° C., said nonreactive thermoplastic polymer having a number-average molecular mass Mn of said thermoplastic polymer from 14,000 to 25,000 and the molten viscosity of said thermoplastic polymer is from 150 to 1500 Pa.Math.s, as measured in plane/plane rheology at 1 Hz and 2% deformation, at a temperature of Tg+220° C., ii) heating step without support of said pre-impregnated fibrous material to obtain an impregnated fibrous material, iii) optionally, heating step carried out using at least one supporting part (E) and at least one heating system to obtain an impregnated fibrous material, iv) optionally, step of shaping and calibrating the roving or said parallel rovings of said impregnated fibrous material to obtain an impregnated fibrous material consisting of a ribbon in the form of a thin band.

32. The method according to claim 21, wherein one or more supporter(s) (E″) is (are) present upstream from said system.

33. The method according to claim 25, wherein it is carried out for the dry powder route at a speed from 5 to 30 m/min and for the aqueous dispersion route at a speed of at least 15 m/min.

34. A use of an impregnated fibrous material, as defined in claim 1, for the preparation of ribbons suitable for the manufacture of three-dimensional composite parts, by the automatic laying of said ribbons by means of a robot.

35. The use of an impregnated fibrous material, as defined in claim 1, for the preparation of thermoformable sheets.

36. The use according to claim 35, wherein the impregnated fibrous material is pre-cut into pieces, said pieces being randomly associated or oriented for the preparation of the thermoformable sheet.

37. A use of at least one nonreactive thermoplastic polymer, said at least one nonreactive thermoplastic polymer being an amorphous or semi-crystalline thermoplastic polymer having a glass transition temperature such that Tg≥40° C., the fiber content in said impregnated fibrous material being from 45 to 65% by volume, the number-average molecular mass Mn of said thermoplastic polymer being from 11,000 to 25,000 g/mol, the molten viscosity of said thermoplastic polymer being from 80 to 1500 Pa.Math.s, as measured by plane/plane rheology at 1 Hz and 2% deformation, at a temperature of Tg+220° C., as defined in claim 1, for impregnating a fibrous material.

38. An impregnated fibrous material comprising at least one continuous-fiber fibrous material in the form of a roving or several parallel rovings and at least one thermoplastic polymer matrix, wherein a thermoplastic polymer of said at least one thermoplastic polymer matrix is an amorphous or semi-crystalline polymer having a glass transition temperature such that Tg≥40° C., as measured using a differential scanning calorimeter (DSC), after a second heating pass, according to standard ISO 11357-2:2013, with a heating and cooling speed of 20° C./min, the fiber content in said impregnated fibrous material being from 45 to 65% by volume, the number-average molecular mass Mn of said thermoplastic polymer being from 11,000 to 25,000 g/mol, the melt viscosity of said thermoplastic polymer being from 80 to 1500 Pa.Math.s, as measured by plane/plane rheology at 1 Hz and 2% deformation, at a temperature of Tg+220° C.; and wherein said thermoplastic polymer is an aliphatic polyamide chosen from: polyamide 6 (PA-6), polyamide 11 (PA-11), polyamide 12 (PA-12), polyamide 66 (PA-66), polyamide 46 (PA-46), polyamide 610 (PA-610), polyamide 612 (PA-612), polyamide 1010 (PA-1010), polyamide 1012 (PA-1012), polyamide 1 1/1010 and polyamide 12/1010, or a mixture thereof or a copolyamide thereof, and block copolymers, in particular polyamide/polyether (PEBA).

Description

BRIEF DESCRIPTION OF THE FIGURES

[0306] FIG. 1 shows the morphology of a composite plate which is microcracked but perfectly impregnated, obtained from a 11/10T/6T polymer of excessively low molecular mass Mn=10,500 g/mol, with a molten viscosity (plane/plane rheology at 1 Hz and 2% deformation) of 70 Pa.Math.s at 330° C.

[0307] The scale bar represents 50 μm.

[0308] FIG. 2 shows the morphology of the same microcracked composite plate as FIG. 1, but with a greater magnification.

[0309] The scale bar represents 20 μm.

[0310] FIG. 3 shows the morphology of a composite plate having areas of dry fibers obtained from a 11/10T/6T polymer of excessively high molecular mass Mn=27,200 g/mol and with a molten viscosity (plane-plane rheology at 1 Hz and 2% deformation) of 1790 Pa.Math.s at 330° C.

[0311] The scale bar represents 100 μm.

[0312] FIG. 4 shows the morphology of the same composite plate with dry areas as FIG. 3, but with a greater magnification.

[0313] The scale bar represents 20 μm.

[0314] FIG. 5 shows the morphology of a composite plate not having any microcracks or dry areas obtained from a 11/10T/6T polymer with molecular mass Mn=13,700 g/mol and with a molten viscosity (plane-plane rheology at 1 Hz and 2% deformation at Tg+220° C.) of 185 Pa.Math.s at 330° C., attesting that the molecular mass of the polymer is optimal.

[0315] The scale bar represents 200 μm.

[0316] FIG. 6 shows the morphology of the same composite plate with no microcracks or dry areas like FIG. 5, but with a greater magnification.

[0317] The scale bar represents 20 μm.

EXAMPLES

[0318] The following examples provide a non-limiting illustration of the scope of the invention.

Example 1 (Comparative Example)

[0319] Impregnation of a fibrous material with a PA of type 11/10T/6T, with Tg 110° C. with mass Mn=10,500 g/mol, with molten viscosity (plane/plane rheology at 1 Hz and 2% deformation) of 70 Pa.Math.s at 330° C.

[0320] This polymer is ground into powder with an average diameter D50=110 μm, then the powder is deposited by gravity on the surface of an Advantex SE4535 glass fiber reinforcement of 3B, woven in the form of a UDT (UD tape) of 400 g/m.sup.2: this type of reinforcement is a quasi-UD (unidirectional fibers), since 90% of the fibers are in the weft direction.

[0321] The assembly is heated by infrared heating to fix the powder.

[0322] The powder level is 30% by weight (or 50% by volume).

[0323] The prepreg thus obtained is cut into 300*200 mm formats, and 4 of these formats are superimposed to make up a preform which will next be consolidated by CARVER press at 330° C. for 15 min, then cooled and removed from the mold at 100° C.

[0324] Bending test specimens are taken in the minority direction of the fibers (90° orientation) tested according to standard 14125 (1998).

[0325] The results are provided in Table I below. The rupture stress value of 40 MPa in bending obtained with the mass of 10,500 is considered to be insufficient to protect against the presence of a premature microcrack of the composite at the interface of the fibers and resin, in the presence of a mechanical or thermal load.

[0326] This result is obtained despite a perfect impregnation of the fibers, but the presence of microcracks can be observed in the plates (see FIG. 1), which appear upon cooling under the effect of thermal stresses and affect the strength of the obtained composite: in this case, it is concluded that the polymer used has an insufficient molecular mass.

Example 2 (Comparative Example)

[0327] Impregnation of a fibrous material with a PA of type 11/10T/6T, with Tg 110° C. with mass Mn=27,200 g/mol, with molten viscosity (plane/plane rheology at 1 Hz and 2% deformation) of 1790 Pa.Math.s at 330° C. according to Example 1.

[0328] The nature of the fibrous reinforcement, the method for manufacturing the prepreg, the composite plate and the mechanical test protocol are identical to those of Example 1.

[0329] The result obtained in transverse bending (90°) according to ISO 14125 (1998) is shown in Table I: it is close to that obtained with the polymer of low mass of Example 1; this attests this time that the mass of the polymer is too high and prevents good impregnation of the fibers, which is observed (see FIG. 2) through the presence of dry zones devoid of resin in the consolidated plate.

[0330] Since the Mn of the polymer is too high, it is therefore too viscous and its impregnation therefore leads to dry zones.

Example 3

[0331] Impregnation of a fibrous material with a PA of type 11/10T/6T, with Tg 110° C. with mass Mn=13,700 g/mol, with molten viscosity (plane/plane rheology at 1 Hz and 2% deformation at Tg+220° C.) is 185 Pa.Math.s at 330° C.

[0332] The nature of the fibrous reinforcement, the method for manufacturing the prepreg, the composite plate and the mechanical test protocol are identical to those of Example 1.

[0333] A clear improvement of the transverse mechanical properties (90°) is observed according to ISO 14125 (1998) when the molecular mass of the resin goes from 10,500 to 13,700.

[0334] The morphology of the plates (see FIG. 3) shows that unlike comparative examples no. 1 and 2, no microcrack or dry zone are present in the plate, attesting that the molecular mass of the polymer is optimal.

Example 4

[0335] Impregnation of a fibrous material with a PA of type MPMDT/10T (67/33 mol %) with D50=115 μm, with Tg 125° C. Its mass is 14,000 g/mol (measurement by NMR) and its molten viscosity (plane/plane rheology at 1 Hz and 2% deformation at Tg+220° C.) is 214 Pa.Math.s at 345° C.

[0336] This polymer is ground into powder with an average diameter D50=115 μm. The powder is supplemented with dry blend with a heat stabilizing agent, then deposited according to Example 1 on the surface of an Advantex glass reinforcing fiber of 3B, SE4535 woven in the form of a UDT (90% of the fibers are in the weft direction and 10% and the warp direction) of 400 g/m.sup.2. The powder level is 30% by weight (or 50% by volume).

[0337] The prepreg thus obtained is cut into 300*200 mm formats, and 4 of these formats are superimposed to make up a preform which will next be consolidated by CARVER press at 345° C. for 15 min, then cooled and removed from the mold at 100° C.

[0338] Bending test specimens are taken in the majority direction of the fibers (weft direction) tested according to standard ISO 14125 (1998).

[0339] The results are provided in Table I below.

[0340] It is observed that the molecular mass molten viscosity compromise is satisfactory.

Example 5

[0341] Impregnation of a fibrous material with a PA of type BACT/10T with Tg 140° C. Its mass is 19,100 g/mol (measurement by NMR) and its molten viscosity (plane-plane rheology at 1 Hz and 2% deformation at Tg+220° C.) is 502 Pa.Math.s at 360° C.

[0342] This polymer is ground into powder with an average diameter DN50=110 μm. The powder is supplemented with dry blend with a heat stabilizing agent, then deposited according to Example 1 on the surface of an Advantex glass reinforcing fiber of 3B, SE4535 woven in the form of a UDT (90% of the fibers are in the weft direction and 10% and the warp direction) of 400 g/m.sup.2. The powder level is 30% by weight (or 50% by volume).

[0343] The prepreg thus obtained is cut into 300*200 mm formats, and 4 of these formats are superimposed to make up a preform which will next be consolidated by CARVER press at 360° C. for 15 min, then cooled and removed from the mold at 100° C.

[0344] Bending test specimens are taken in the majority direction of the fibers (weft direction) tested according to standard ISO 14125 (1998).

[0345] The results are provided in Table I below.

[0346] It is observed that the molecular mass molten viscosity compromise is satisfactory.

Example 6

[0347] The polymer is a MXD10 with mass 15,000 g/mol. Its Tg is 70° C. and its viscosity at 290° C. is 110 Pa.Math.s.

[0348] The method for manufacturing the composite plate is a pultrusion method with impregnation by molten route with a crosshead die. The polymer is introduced in the form of granules, compounded beforehand with a heat stabilizing agent, in an extruder which supplies the crosshead die.

[0349] The temperature at which the fibers have been impregnated was 290° C.

[0350] The line speed was 1.1 m/min pb/claimed speed range.

[0351] The fiber used is the Hypertex glass fiber of 3B SE4535.

[0352] The fiber level was 60% by volume.

[0353] Excellent mechanical properties are obtained, measured in bending according to standard ISO 14125 (1998). They are summarized in Table I below:

In this example, unlike in the other examples, no break is observed in transverse bending, up to 10% deformation, past which value the test is interrupted because it then goes beyond the conditions recommended by standard ISO 14125. In this case, the excellence of the transverse mechanical properties is judged by the ductility of the obtained composite (that is to say, the value of the deformation achieved perpendicular to the fibers (90° direction)), which attests to a good compromise between molten viscosity and molecular mass.

TABLE-US-00001 TABLE 1 Molten viscosity Modulus Stress Example Tg (Pa .Math. s) at 90° 90° no. Mn (° C.) Tg + 220° C. (GPa) (MPa) 1 10,500 110 70 7.7 41 2 27,200 110 1790 3.6 46 3 13,700 110 185 10.2 79 4 14,000 125 214 9.2 75 5 19,100 140 502 10.3 82 6 15,000 70 110 5.8  47** **no break observed in transverse bending up to 10% deformation.

[0354] All of Examples 1 to 5 led to breaking.

[0355] Example 6 therefore corresponds to a material which deforms greatly without breaking.

Example 7: Determination of the Porosity Level the Relative Deviation Between Theoretical Density and Experimental Density (General Method)

[0356] a) The required data are: [0357] The density of the thermoplastic matrix [0358] The density of the fibers [0359] The grammage of the reinforcement:
linear mass (g/m) for example for a ¼ inch band (coming from a single roving) surface density (g/m.sup.2) for example for a wider band or a fabric
b) Measurements to be carried out:

[0360] The number of samples must be at least 30 in order for the result to be representative of the studied material.

[0361] The measurements to be carried out are: [0362] The size of the samples taken:
Length (if linear mass is known).
Length and width (if surface density is known). [0363] The experimental density of the samples taken:
Mass measurements in the air and in water. [0364] The fiber level is measured according to ISO 1172:1999 or by thermogravimetric analysis (TGA) as determined for example in the document B. Benzler, Applikationslabor, Mettler Toledo, Giesen, UserCom 1/2001.

[0365] The measurement of the carbon fiber level can be determined according to ISO 14127:2008.

[0366] Determination of the theoretical mass fiber level:

a) Determination of the Theoretical Mass Fiber Level:

[0367] [00001]$\%{\mathrm{Mf}}_{\mathrm{th}}=\frac{m_l.Math.L}{{\mathrm{Me}}_{\mathrm{air}}}$

With

[0368] m.sub.i the linear mass of the tape,
L the length of the sample, and
Me.sub.air the mass of the sample measured in the air.

[0369] The variation of the mass fiber level is presumed to be directly related to a variation of the matrix level without taking into account the variation of the quantity of fibers in the reinforcement.

b) Determination of the Theoretical Density:

[0370] [00002]$d_{\mathrm{th}}=\frac{1}{\frac{1-\%{\mathrm{Mf}}_{\mathrm{th}}}{d_m}+\frac{\%{\mathrm{Mf}}_{\mathrm{th}}}{d_f}}$

With d.sub.m and d.sub.f the respective densities of the matrix and the fibers.

[0371] The theoretical density thus calculated is the accessible density if there is no porosity in the samples.

c) Evaluation of the Porosity:

[0372] The porosity then is the relative deviation between theoretical density and experimental density.