FR2867481A1 - Procedure of treatment of vegetable fibers, useful to include as reinforcement in a composite material containing base polymer, comprises grafting on a nitrile or amide group of vegetable fibers - Google Patents

Abstract

Procedure of treatment of vegetable fibers (I) to include as a reinforcement in a composite material containing base polymer comprises grafting on a nitrile or amide group of (I). An independent claim is also included for: (1) manufacture of a composite material containing polyamides or polyacrylonitrile bases including fibers treated by the procedure, comprising introduction of the fibers at a rate of 10-70% in the polyamide matrix; and (2) manufacture of a composite material to acrylonitrile butadiene styrene bases including fibers treated by the procedure, comprising introduction of the fibers at a rate of 10-70% in the arylonitrile butadiene styrene base matrix.

Description

2867481 1

  The present invention relates to a method for treating plant fibers to allow them to be included as reinforcement in a matrix of thermoplastic or thermosetting materials.

  The invention also covers the fibers obtained as well as the composite materials including them.

  The composite materials in fact comprise a matrix, in this case composed of resins and in the embodiment of the resins of the polyamide and polyacrylonitrile type, and a reinforcement consisting of a woven fabric of long woven or non-woven fibers, more particularly in the case of thermosetting materials or short fibers for thermoplastics.

  The best known reinforcements are glass fibers and other organic fibers. The constraints of these materials are either their weight in the case of fiberglass, or their price in the case of synthetic organic fibers especially when the mechanical strength performance is sought.

  Such composite materials are now introduced in large numbers in motor vehicle parts for example.

  Another constraint is then the recycling of parts comprising fibers which, they are not recyclable as in the case of glass fibers.

  It was therefore imagined to use plant-derived fibers obtained from annual plants such as flax or hemp or even wood fibers.

  2867481 2 These fibers have technical and especially mechanical characteristics that are entirely compatible with the needs of industries, as reinforcing fibers for composite materials.

  The modulus of elasticity is of the same order of magnitude as that of the 70 GPa glass fiber.

  The tensile strength is lower since the plant fibers have a strength of 300-400 MPa.

  Fiberglass has superior strength since it is in the range of 1000-1500 MPa but the strength / weight ratio is quite comparable.

  On the other hand, the use of vegetable fibers in composite materials poses problems at the fiber / resin interface. Indeed, to achieve the desired performance and get closer to the theoretical performance requires the binding of the resin matrix and fibers is adapted.

  Also, the incorporation of plant fibers into polymers requires a chemical sizing functionalisation. Such an operation makes it possible to optimize the energies at the fiber / resin interface. The size is used to fight against the phenomena of abrasion of the fibers against each other, against their fragility, against the presence of electrostatic charges or even against sensitivity to moisture. However, the known solutions involve the surface hydroxyl groups of the lignocellulosic fibers of the plants allowing the grafting of chlorides of acids, acid anhydrides or even acids which are often used for polymers of convenience such as polyolefins by grafting maleated polyethylene or polypropylene.

  The aim of the present invention is to propose a method for the specific treatment of plant fibers which allows the integration of these plant fibers, in particular those derived from wood, hemp, sisal, jute and / or flax in matrices with base of polyacrylonitriles or polyamides.

  The process is now described in detail according to two variants of the wood fiber treatment process, in this case spruce fibers, or by grafting nitrile groups for integration of these fibers into a matrix. polyamide either by grafting amide groups for integration into a polyacrylonitrile matrix.

  Figures 1 and 2 showing curves illustrating the results obtained and tables of values 3 and 4 are associated with the present description. This process consists first of all in a plant fiber treatment process to obtain plant fibers on which are grafted nitrile groups and two grafting means achieve this result.

  1 / A first grafting means consists of an ammoxidation reaction. This reaction is carried out in the gaseous phase in an oxygen / ammonia mixture in a ratio of between 0.1 and 4.0. The reaction is carried out at a temperature of between 180 and 250 ° C. in the presence of a metal catalyst such as Fe, Co, Ni, Cu, Mo or Cr, introduced in the form of a salt, in this case a molybdate solution of ammonium whose fibers are impregnated beforehand by the so-called dry technique and which consists of immersing the fiber in an aqueous solution of the catalyst salt of a volume at most equal to the water retention capacity of the fiber in question.

  The reaction that occurs is as follows Vegetable fiber C OH + NH3 + 02 -> plant fiber C = N + 3H20

OH

  It can be seen from the graphs of FIG. 1 that the first graph A, corresponding to native untreated spruce fiber, does not include any peak in the 2200 cm-1 band in the infrared and shows peaks corresponding to the groups surface hydroxyl.

  The graph B of this same figure, corresponding to the ammoxidized spruce fiber, has a very intense and very precise peak at 2200 cm -1 identifying nitrile groups while the peaks of the surface hydroxyls have disappeared.

  The degree of coverage in nitrile groups can be determined by a nitrogen analysis with variations induced by the conditions of implementation but reproducible for identical conditions under control.

  2 / A second means for grafting nitrile groups onto these spruce fibers is cyanoethylation. This reaction is obtained in the liquid phase in the presence of a mixture of sodium hydroxide and urea which is supplemented with acrylonitrile. The temperature is between 20 and 80 C.

  After filtration and washing until neutralization, the fibers are dried and are thus functionalized with nitrile groups. The reaction that occurs is as follows: Hz I2 Fiber OH + H2C C CN -> Fiber O C C C N vegetable 1 vegetable

H

  Figure 2 shows the two graphs A and B of untreated and cyanoethylated treated spruce fibers. It contains the presence of nitrile groups with their characteristic peak at 2200 cm-1 in the infrared.

  These functionalized and native fibers are now tested in a comparative manner as reinforcement in a polymer matrix in order to materialize the interest of such functionalization. The polymer retained is polyamide PAU.

  The table is indicated in FIG. 3.

  As can be seen, the presence of native fibers at low levels decreases the mechanical properties of maximum tensile strengths of the composite material. The fibers act as a filler but the fiber-polymer bonding energies being very limited, the resistance is lower than that of a 100% polyamide matrix. This result highlights the lack of compatibility between plant fibers and polyamide.

  The results clearly show the great interest of the addition of fibers treated by the process according to the present invention.

  On the other hand, the results show the very great advantage of the addition of fibers treated by the process according to the present invention.

  In fact, it is found that the maximum tensile stress is maintained when 50% of fibers treated by ammoxidation are introduced and the Young's modulus is substantially doubled. Taking into account the price of the polymer compared to that of the fibers, this proportion of 50% of fibers is extremely interesting in terms of the saving made in polymer necessary for the same piece to be made, in terms of improving the Young's modulus and also in terms of the recycling capacity of the end-of-life product.

  It is also found that the same improvement in mechanical properties is obtained with the fibers treated by cyanoethylation or even more important because in this case, with 50% fiber integration, an improvement in the maximum stress at break of 32% is obtained. and a Young's modulus three times higher than that of the virgin polymer.

  The supposed phenomenon leading to this result comes from the strong interaction between the polyamide and the plant fiber grafted with nitrile groups of the fact.

  dipole / dipole interactions of the nitrogen atoms of the polyamide and the nitrogen atoms of the nitrile groups, and the dipole / dipole interactions of the oxygen atoms of the polyamide and the carbon atoms of the nitrile groups.

  Another test may be conducted with a polymer-based matrix comprising nitrile groups. Compounds of this family of compounds include polyacrylonitrile homopolymer or acrylonitrile: butadiene-acrylonitrile, styrene-acrylonitrile, acrylonitrilebutadiene-styrene, vinyl chloride-acrylonitrile, vinyl acetate-acrylonitrile copolymers.

  In this case, in order to find the same dipole / dipole interactions, it is necessary to functionalize the fiber with amide groups. This is a symmetrical process from the previous one.

  For this purpose, the plant fibers functionalized with nitrile groups either by the ammoxidation reaction or by the cyanoethylation reaction are, for example, hydrolysed in a basic medium in the presence of hydrogen peroxide. The fixing of a molecule of water by nitrile group corresponds to the formation of amide according to the known mechanism: Vegetable fiber C> Plant fiber H2O / H2O2

OH-

  In parallel, it is possible to act with isocyanate grafting, for example an arylisocyanate, on the hydroxyl groups of the cellulose or on the lignin leads to arylurethanes. This reaction is obtained in the liquid phase in the presence of a mixture of ether and sodium ethanoate which is supplemented with aryl isocyanate.

  The temperature is between 20 and 80 C.

  After filtration and washing until neutralization, the fibers, functionalized with urethane groups are dried. The mechanism is as follows:

O

  Plant fiber = OH + RI N = C O ---- Vegetable fiber O C NH R1 The RI group is an aromatic ring in the case of arylisocyanate. We find a spatial geometry of the bond identical to that of the amide.

  The urethanes obtained can also lead to the dipole / dipole interactions previously demonstrated with acrylonitrile-based polymers or copolymers.

  Thus, the functionalization of vegetable fibers in surface amide is obtained by grafting an isocyanate.

  The table in FIG. 4 shows that, compared to the virgin polymer, the amide functions introduced on the plant fiber make it possible to increase by 45% the maximum breaking stress and by 89% the tensile modulus when the fibers are associated with Acrylonitrile Butadiene Styrene (ABS).

  The composite material contains fibers introduced from 10 to 70% in the matrix based on Acrylonitrile Butadiene Styrene.

Claims (13)

  1. A method of treating plant fibers to include as reinforcement in a polymer-based composite material, characterized in that it comprises a grafting step on said plant fibers of nitrile groups.   2. Process for treating plant fibers according to claim 1, characterized in that the polymer base intended to be associated with the grafted plant fibers is chosen from polyamides.   3. Process for treating plant fibers to include as a reinforcement in a polymer-based composite material, characterized in that it comprises a grafting step on said plant fibers of amide groups.   4. Process for treating plant fibers according to claim 3, characterized in that the polymer base intended to be associated with the grafted plant fibers is chosen from polyacrylonitriles.   5. Process for treating plant fibers according to any one of the preceding claims, characterized in that the grafting is carried out by an ammoxidation reaction.   6. Process for the treatment of plant fibers according to any one of the preceding claims, characterized in that the ammoxidation reaction is obtained in an oxygen / ammonia gas medium with an O 2 / NH 3 ratio of 0.1 to 4.0 and a temperature of between 180 ° C. and 250 ° C., in the presence of a catalyst chosen from cobalt, iron, nickel, chromium, or copper and more particularly molybdenum.   7. Process for the treatment of plant fibers according to claim 6, characterized in that the molybdenum is introduced in the form of ammonium molybdate whose fibers are impregnated dry.   8. Process for the treatment of plant fibers according to claims 1 to 4, characterized in that the grafting is carried out by a cyanoethylation reaction.   9. Process for treating plant fibers according to claim 8, characterized in that the reaction is carried out in the presence of a mixture of sodium hydroxide and urea which is additivated with acrylonitrile.   10. Process for the treatment of plant fibers according to claim 7 or 8, characterized in that the fibers are treated by hydrolysis to obtain graftings of amide groups.   11. A method of manufacturing a composite material based on polyamides or polyacrylonitriles including fibers treated by the process according to claims 1 to 10, characterized in that fibers are introduced in a proportion of 10 to 70% in the matrix of polyamide.   12. A method of treating plant fibers, characterized in that the functionalization of plant fibers in surface amide is obtained by grafting an isocyanate.   13. A process for manufacturing a composite material based on acrylonitrile butadiene styrene including fibers treated by the process according to claim 12, characterized in that fibers are introduced at a rate of 10 to 70% in the d-based matrix. Acrylonitrile Butadiene Styrene.