Method for impregnating natural fibres with a polymer in aqueous dispersion and use of said fibres in composite materials

Abstract

A method for impregnating strands or strips of natural fibers, using the following successive steps: i) the impregnation of the strands or strips by immersion in a bath containing a fine aqueous polymer dispersion; followed by ii) the drying of the strands or strips using a heating system, with the progressive elimination of the water and the gradual melting of the polymer, and the coating of the strands or strips and the molten polymer incorporated into the core of the strands or strips as a binder between the fibers; iii) optionally, the forming of the treated strands or strips into their final shape; and iv) the cooling of the treated strands or strips. The aqueous polymer dispersion comprises at least one semi-crystalline or amorphous polymer and, in the case of an amorphous polymer, has a Tg varying between 50 C. and 175 C.

Claims

1. A process for impregnating, on line, rovings or ribbons of natural fibers, wherein the process comprises at least the following successive steps: i) impregnating said rovings or said ribbons of said fibers by immersing in a bath containing a fine aqueous polymer dispersion or by spraying said aqueous dispersion, said impregnating being followed by ii) drying said rovings or said ribbons by means of a heating system, with, during said drying, the gradual elimination of the water and the melting of said polymer as said drying advances, the coating of said rovings or ribbons with said molten polymer incorporated into the core of the bundle of fibers of said rovings or of said ribbons binding said fibers to one another, iii) optionally, final forming of said rovings or ribbons thus treated, of unidirectional ribbons or of rovings with a cylindrical cross section or in the form of pre-impregnated fabrics or in the form of an assembly of ribbons, iv) cooling said rovings or said ribbons thus treated, with said aqueous polymer dispersion comprising at least one amorphous or semi-crystalline polymer and, in the case of an amorphous polymer, said polymer having a Tg ranging from 50 C. to 175 C., and, in the case of a semi-crystalline polymer, said polymer having a melting point ranging from 70 C. to less than 220 C., said dispersion comprising a content by weight of said polymer ranging from 5% to 50% with particles in dispersion having a number-average size of less than 10 000 nm, wherein said polymer is a polyurethane formed from a polyisocyanate prepolymer comprising an ionic group, dispersed in water with chain extension in an aqueous medium or a copolyamide.

2. The process of claim 1, wherein said polymer is dispersible (or dispersed) in powder form in an aqueous medium without surfactant.

3. The process of claim 1, wherein said copolyamide bears amine groups.

4. The process of claim 1, wherein said copolyamide bears carboxy groups which are neutralized in salt form by a base.

5. The process of claim 1, wherein said copolyamide is semi-crystalline with a melting point of less than or equal to 150 C.

6. The process of claim 1, wherein said copolyamide comprises at least one of the following units: 5.9, 5.10, 5.12, 5.13, 5.14, 5.16, 5.18, 5.36, 6, 6.9, 6.10, 6.12, 6.13, 6.14, 6.16, 6.18, 6.36, 9, 10.6, 10.9, 10.10, 10.12, 10.13, 10.14, 10.16, 10.18, 10.36, 11, 12, 12.6, 12.9, 12.10, 12.12, 12.13, 12.14, 12.16, 12.18, 12.36, 6.6/6, 11/10.10 and mixtures thereof.

7. The process of claim 1, wherein said polymer is a copolyamide chosen from: PA 6/6.6/12, PA 6/6.6/11/12, PA 6/12, PA 6.9/12, PA Pip.9/Pip.12/11, PA 6/1PD.6/12, PA IPD.9/12, PA6/MPMD.12/12, PA 6/6.12/12, PA 6/6.10/12, PA 6/Pip.12/12, PA 6/6.6/6.10/6.I, PA 6.10/Pip.10/Pip.12, PA 6/11/12, PA Pip.12/12, PA IPD.10/12, PA Pip.10/12, PA 6/11, PA Pip.10/11/Pip.9, PA 6/6.6/6.10, PA 6/6.10/6.12 and mixtures thereof.

8. The process of claim 1, wherein said polyamide is semi-aromatic amorphous and corresponds to the following formulae: 6.I, 8.I, 9.I, 10.I, 11.I, 12.I, 6.I/9.I, 9.I/10.I, 9.I/11.I, 9.I/12.I, 9/6.I, 10/6.I, 11/6.I, 12:6.I, 10/9.I, 10/10.I, 10/11.I, 10/12.I, 11/9.I, 11/10.I, 11/11.I, 11/12.I, 12/9.I, 12/10.I, 12/11.I, 12/12.I, 6.10/6.I, 6.12/6.I, 9.10/6.I, 9.12/6.I, 10.10/6.I, 10.12/6.I, 6.10/9.I, 6.12/9.I, 10.I/6.I, 10.10/9.I, 10.12/9.I, 6.10/10.I, 6.12/10.I, 9.10/10.I, 9.12/10.I, 10.10/10.I, 10.12/10.I, 6.10/12.I, 6.12/12.I, 9.10/12.I, 9.12/12.I, 10.10/12.I, 11/6.I/9.I, 11/6.I/10.I, 11/6.I/11.I, 11/6.I/12.I, 11/9.I/10.I, 11/9.I/11.I, 11/9.I/12.I, 11/10.I/11.I, 11/10.I/12.I, 11/11.I/12.I, 6.I/10.I, 6.I/11.I, 6.I/12.I, 10.I/11.I, 10.I/12.I, 11.I/12.I, 12/6.I/10.I, 12/6.I/11.I, 12/6.I/12.I, 12/9.I/10.I, 12/9.I/11.I, 12.9.I/12.I, 12/10.I/11.I, 12/10.I12.I, 12/11.I/12.I, 12/11.I/12.I, the preceding terpolymer polyamides with 12/ replaced with 9/, 10/, 6.10/, 6.12/, 10.6/, 10.10/, 10.12/, 9.10/ and 9.12/, all the preceding polyamides, with isophthalic (I) partially replaced up to 40 mol % with terephthalic (T), naphthalene-2,6-dicarboxylic and/or with 1,3- or 1,4-CHDA (cyclohexanedicarboxylic acid), with all or some of the linear aliphatic diamines possibly being replaced with branched aliphatic diamines, all the preceding polyamides, wherein isophthalic (I) is partially or totally replaced with a linear or branched C6 to C18 aliphatic diacid and at the same time with total or partial replacement of the aliphatic diamine with a cycloaliphatic diamine among BMACM, BACM and/or IPD.

9. The process of claim 1, wherein said polymer is semi-crystalline with a melting point Mp greater than 90 C., and wherein the particles of said dispersion have a number-average size ranging from 50 to 5000 nm.

10. The process of claim 1, wherein the content by dry weight of said polymer relative to the dry weight of said fibers ranges from 0.5% to less than 50%.

11. The process of claim 10, wherein said content by weight ranges from 0.5% to 10%, and wherein said impregnation is limited to the consolidation of said fibers to one another, in addition to sizing.

12. The process of claim 10, wherein said content is greater than 25% and less than 50%, and wherein said impregnation in addition to said consolidation results in a prepreg of said fibers that is used or can be used separately or successively in the manufacture of composite materials.

13. The process of claim 1, wherein the viscosity of said dispersion at 23 C. ranges from 10 to 1000 mPa.Math.s.

14. The process of claim 1, wherein said polymer is biobased.

15. The process of claim 1, wherein said fibers are long flax fibers with L/D>2000.

16. The process of claim 1, wherein said rovings or ribbons are based on flax fibers having a tex ranging from 10 to 10 000.

17. The process of claim 1, wherein the process comprises at least one additional series of steps of impregnating according to i) and of drying according to ii) after a first series of steps of impregnating according to i) and of drying according to ii), with a polymer dispersion different than that of the preceding series.

18. The process of claim 1, wherein the process comprises at least one additional series of steps of impregnating according to i) and of drying according to ii) after a first series of steps of impregnating according to i) and of drying according to ii), wherein said dispersion remains the same, passing in at least one loop in the same impregnation bath or in the same spraying device and in the same drying system of the first series of steps.

19. The process of claim 11, wherein the process further comprises an additional step of impregnating and of coating in the molten state, after said drying step ii), with a polymer in the molten state, said polymer being different than but compatible with said polymer of said aqueous dispersion and said polymer is deposited in the molten state by passing said rovings or said ribbons through an extruder die, with optionally, after cooling according to step iv), passing through a system for reducing the size of said fibers thus treated.

20. The process of claim 1, wherein said heating means of said drying step ii) are chosen from: infrared (IR) radiation, microwaves, induction or by an oven with water extraction, pulsed-air oven or by calendering on heating rolls.

21. The process of claim 1, wherein said process comprises a step of forming said fibers in the shape of a ribbon with a width of less than 2000 mm.

22. The process of claim 1, wherein the process is continuously integrated into a composite material manufacturing line.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1. Flax roving is glued (with Loctite 401 glue) onto a cardboard frame.

(2) The following examples are presented by way of illustration of the invention and of the performance levels thereof and in no way limit the scope thereof.

1EXAMPLES

(3) In order to carry out the impregnation treatment with an aqueous solution of copolyamide, 10 I of several solutions (aqueous dispersions of copolyamide) were produced in a laboratory reactor.

(4) The copolyamides used are the Arkema commercial products Platamid 2592 and Platamid 1657.

(5) Their essential characteristics are presented in Table I below.

(6) TABLE-US-00001 TABLE I COOH functionality Platamid Melting point (eq/g) 2592 102 220 1657 107 180

(7) These products were introduced in dry powder form into a solution of water with sodium hydroxide (1% relative to the Platamid). The final dry extract (DE) is 30%.

(8) The reagents are charged to the reactor, and then the medium is inerted with nitrogen. The reagents are heated so as to reach the material temperature of 150 C. This heating phase is carried out with stirring at 1000 rpm. Between 100 and 120 C., the medium becomes homogeneous, white and opaque. The medium is kept stirring at 1000 rpm for 30 min at 150 C., then cooled with stirring at 300 rpm. The dispersions obtained are fluid, white and opaque.

(9) The particle size (size of the particles) of the powders was measured by means of observations made by scanning electron microscopy and also using the laser diffraction particle size analysis method. The 2 types of measurements are in agreement. The data indicated in the table below were obtained with the laser diffraction particle size analysis method.

(10) The particle size, the viscosity and the dry extract of the dispersions used are presented in Table II below.

(11) TABLE-US-00002 TABLE II Platamid Number-average Brookfield* Dispersion Test of the diameter of the viscosity at dry extract REF dispersion particles (nm) 23 C. (mPa .Math. s) (%) EP-063 Platamid 80 30 30 2592 EP-064 Platamid 140 20 30 1657 *the measurement was carried out with a No. 1 spindle at 60 rpm.

(12) These aqueous dispersions thus prepared were then used on the flax fiber treatment line, in non-diluted or diluted form.

(13) Except for Example 7, in which the impregnation method is specified, in all the other cases (unless specifically specified for each example), said impregnation was carried out by on line (continuous) spraying using a spraying device (spray) with a residence time in the jet of said spray of approximately 1 s. The drying is carried out by heating with an IR device. The cooling is carried out in the open air.

(14) The 1.sup.st series of tests was carried out on a flax roving (of fibers) of high count or grammage of 2190 tex, with various treatments: Counter Example 1:non-impregnated roving of 2190 tex Example 1: EP-063 ND: impregnation with a non-diluted solution (DE: 30%) of Platamid 2592. Example 2: EP-063 D50: impregnation with a dispersion, diluted to 50%, of Platamid 2592 (DE: 15%). Example 3: EP-064 ND: impregnation with a non-diluted dispersion of Platamid 1657 (DE: 30%). Example 4: EP-064 D50: impregnation with a dispersion, diluted to 50%, of Platamid 1657 (DE: 15%).

(15) The 2.sup.nd series of tests was carried out on a flax roving of low count (1030 tex) having undergone a treatment with Platamid 1657, at various degrees of impregnation. Counter Example 2: non-impregnated roving of low count (0% polymer). Example 5: impregnation with a 4-fold diluted solution with DE: 7.5%. Example 6: impregnation twice successively (2 passes) with the dispersion of Example 5. Example 7: impregnation by immersion (dipping) in the aqueous dispersion with a longer residence time (10 s).

2TENSILE MECHANICAL PROPERTIES

(16) 2.1Test Conditions

(17) Non-conditioned samples Temperature: 23 C.

(18) The flax roving is glued (with Loctite 401 glue) onto a cardboard frame (according to FIG. 1). The reference length L0 was chosen at 14 mm, in the knowledge that the average length of a flax fiber is approximately 30 mm. The upper and lower edges of the cardboard are held tight between the jaws of the dynamometer (Zwick machine) while the side edges are cut. The roving is then subjected to a tensile force at a speed of 1 mm/min (displacement of the crosshead).

(19) 2.2Results

(20) In order to compare the rovings weakly impregnated with resin (up to 10% of polymer), the breaking force of the various samples tested is measured. The results are presented in Table III below.

(21) TABLE-US-00003 TABLE III Counter Exam- Exam- Exam- Exam- Reference Example 1 ple 1 ple 2 ple 3 ple 4 Impregnation Non- EP63- EP63- EP64- EP64- type impreg- ND D50 ND D50 nated Resin content 0 2-10% 2-10% 2-10% 2-10% (% by weight) Number of 5 8 5 5 5 samples Mean breaking 443 775 723 888 770 force (N) Standard 33 98 68 23 73 deviation

(22) It is shown in Table III that the impregnation with a copolyamide dispersion makes it possible to very significantly increase the breaking force of the flax fiber roving.

(23) The two dispersions give results that are similar and along the lines of a consolidation (Examples 2 and 3 as opposed to Counter Example 1), thereby demonstrating that the consolidation dispersion proposed functions, including for dilutions of 50% (which corresponds to a dry extract of 15%).

(24) Table IV below gives the result of the various impregnations with Platamid 1657 and a roving of low grammage or count (1030 tex). It is noted that, with a 4-fold dilution with a final dry extract of 7.5%, a breaking force is obtained that is equivalent to that obtained with the non-impregnated roving of high count (2190 tex) (comparison of Example 5 with Counter Example 1): it has thus been demonstrated that the use of a copolyamide dispersion as defined above makes it possible to use a flax fiber roving that has half the count of the initial roving (1030 tex compared with 2190 tex), while at the same time retaining approximately the same mechanical strength (approximately the same breaking force).

(25) It is also shown that, by performing several impregnation passes (by spraying) with the same dispersion, the copolyamide content in the final roving is increased (comparison between Examples 5 and 6).

(26) To finish, it is shown, by means of an immersion (dipping) impregnation test with a much longer spraying time (10 s), that it is possible to impregnate the roving with close to 40% (by weight) of copolyamide (Example 7). This content corresponds to a resin (polymer) content found in usual pre-impregnated reinforcements, making it possible to thus directly produce composite parts, for example by thermocompression, without an additional operation of impregnating the fibrous reinforcement.

(27) The possibility, with the process according to the invention, of directly producing a composite semi-product of prepreg type that is ready to use is thus demonstrated.

(28) TABLE-US-00004 TABLE IV Counter Exam- Exam- Exam- Reference Example 2 ple 5 ple 6 ple 7 Impregnation Non- Product Product Immersion type impreg- diluted diluted impregnation nated 4-fold 4-fold for 10 s (DE: 7.5%) (DE: 7.5%) with 2 passes Polymer content 0 2.7 5.3 39.8 (% by weight/ fiber + polymer) Number of 5 5 5 3 samples Mean breaking 202 449 549 948 force (N) Standard 15 49 65 49 deviation