Method and installation for producing flat slivers of plant natural fibres

Abstract

The invention relates to a method and an installation for manufacturing flat wicks of natural plant fibers. According to the method of the invention, the following steps are carried out: a) a tape of plant fibers is supplied, b) a drawing operation is carried out on said tape of plant fibers, thereby forming a drawn tape of plant fibers (12), and c) a consolidation operation is carried out on the drawn tape of plant fibers, comprising the following substeps: c1) said drawn tape of plant fibers (12) is impregnated with a solution (112) containing a binder, thereby obtaining a drawn and impregnated tape of plant fibers (13), c2) said drawn and impregnated tape of plant fibers (13) is dried, thereby obtaining a flat wick of plant fibers (15).

Description

Technical field

[0001] The present invention relates to a method and an installation for manufacturing flat wicks of natural plant fibers.

[0002] These flat strands of natural plant fibers are used in particular for the manufacture of composite products, as reinforcing elements within a matrix. Natural plant fibers have many advantages over synthetic fibers, including their low environmental footprint, their biodegradable nature, high-performance mechanical characteristics and low density.

[0003] These flat strands of natural plant fibers are presented in the form of a continuous basic bundle containing parallel filaments assembled without intentional twisting. These filaments are in particular long, discontinuous natural plant fibers, such as flax or other plant fibers. In the world of composite materials, filament strands are often called "roving", which also applies to these flat strands of natural plant fibers.

[0004] These flat strands of natural plant fibers require treatment to give them properties compatible with their use in composite materials. Thus, despite the fact that they are made of discontinuous fibers and not continuous fibers, they must have a hold and cohesion allowing their handling and sufficient mechanical resistance for their use in a composite product.

[0005] Twisting such strands of natural plant fibers is avoided because, although this improves their mechanical strength in the strand state, it reduces certain mechanical performances of the composite materials made with these strands, such as the rigidity and breaking strength of the composite, because twisting the strands tends to misalign the natural fibers from the direction of stress exerted. Thus, for composite product reinforcement applications, unlike textile applications, we seek to form untwisted strands whose fibers have not been misaligned by the twisting step. Twisting also tends to make the strand round, not flat. The round section of the strand tends to create a strong smearing of fabric manufacturing with such a strand. Smearing also has the effect of misaligning the fibers with respect to the stresses exerted on the composite parts.Thus, the absence of twisting allows for a flat wick to be kept, with very little twisting, keeping the fibers in the plane of the stresses exerted.

[0006] It is therefore necessary to improve the cohesion of these natural fiber strands by means other than twisting the strand. State of the art

[0007] Document FR2839001A1 describes a process for impregnating reinforcing tape with natural fibers using a molten thermoplastic material as a binder. Since the tape is thus completely impregnated with a molten thermoplastic material, it will not be compatible with the use of thermosetting materials in the composite product, such as, for example, an epoxy resin (epoxy polymer) or an unsaturated polyester. In addition, due to the relatively high viscosity of molten thermoplastic materials, it is difficult to completely and homogeneously impregnate such a material into a natural fiber tape without damaging the latter.

[0008] Also known is the impregnation of a natural fiber ribbon with a thermoplastic or thermosetting fusible polymer by dusting a powder. This procedure also has the disadvantage of having to use for the impregnation of the natural fiber ribbon particles of a polymer specifically compatible with the polymer used in the final impregnation of the composite product. Typically, either a powder formed from thermoplastic polymer particles, such as polypropylene (PP), polylactic acid (PLA) or polyamide (PA) or from thermosetting polymer particles, such as very high viscosity non-crosslinked epoxy, is used. After dusting, the polymer is then melted in a dual belt press system heating the polymer until it melts, then cooling the polymer, while applying pressure between the belts.Alternatively, an infrared heating system or a heating calender system followed by cooling in a cold calender, or by air can be used.

[0009] According to another method, document FR3020819A1 provides for a distribution of a thermoplastic binder in a natural fiber reinforcing tape, by means of an aqueous dispersion of particles of a thermoplastic material. In this case, it is difficult to maintain a controlled and homogeneous concentration of the thermoplastic material in the bath. Here again, the use of thermoplastic which is melted in the impregnation step prevents good compatibility with the use of thermosetting matrices in the composite product using such impregnated natural fiber tapes.If such impregnated natural fiber ribbons can be placed as a reinforcing element within a thermosetting matrix, the thermoplastic binder of the ribbons is likely to prevent good impregnation with the thermosetting matrix, worsen the adhesion of the matrix to the fibers and negatively influence the thermomechanical properties of the resin, such as the glass transition temperature. Also, the use of a thermoplastic binder makes it difficult to bleach and dye the natural fiber reinforcements, as the thermoplastic binder prevents good access to the fibers. Brief summary of the invention

[0010] An aim of the present invention is to propose a method and an installation for manufacturing flat wicks of natural plant fibers free from the limitations of known solutions.

[0011] Another aim of the invention is to then enable the use of these strands of natural plant fibers in a composite product with a thermosetting matrix or in a composite product with a thermoplastic matrix.

[0012] A further object of the invention is to obtain a means by which the binder is distributed homogeneously in the natural plant fibers of the ribbon, with good penetration into the plant fiber ribbon.

[0013] Also, according to another aim of the invention, a way of operating is sought which allows easier cleaning of the binder on the reinforcements made with flat wicks of vegetable fibers.

[0014] Also, according to another aim of the invention, a way of operating is sought which makes it possible to bleach and dye the reinforcements made with flat wicks of vegetable fibers.

[0015] According to the invention, these aims are achieved in particular by means of a method for manufacturing flat wicks of natural plant fibers, in which the following steps are implemented: a) providing a ribbon of plant fibers, b) carrying out an operation of drawing said ribbon of plant fibers, whereby a drawn ribbon of plant fibers is formed, and c) carrying out an operation of consolidating the drawn ribbon of plant fibers, comprising the following sub-steps: c1) impregnating said drawn ribbon of plant fibers with a solution containing a binder, whereby a drawn and impregnated ribbon of plant fibers is obtained, c2) drying said drawn and impregnated ribbon of plant fibers, whereby a flat strand of plant fibers is obtained.

[0016] Thus, in particular by using impregnation using a solution containing a binder and not a dispersion containing a binder, the control and stability of the binder concentration in the liquid coming into contact with the natural plant fibers is improved. This also allows the binder to penetrate the fibers of the ribbon well, and therefore to have an effective binding action between the fibers, all along and in the heart of the ribbon. Finally, the binder can be easily cleaned using the solvent used in the solution that has been applied.

[0017] The use of a solution as a vector for impregnating the binder allows the binder, i.e. the material contained in the solution, to be applied to all the fibers of the ribbon, in their volume. This allows for good shape retention of the assembly of fibers constituting the ribbon, by effectively binding the fibers together. This good coverage and effective penetration of the fibers by the binder notably improve the cohesion, homogeneity and breaking strength of the resulting strand. This strand of natural plant fibers can then be used to effectively manufacture reinforcements for composite products, avoiding breakage of the strand during weaving or manufacturing of the reinforcement. If necessary, the binder can be cleaned once the reinforcement has been manufactured, as the strength of the strand itself is no longer essential at this stage.The binder allows partial impregnation of the natural plant fibers of the tape, this leaves room for the penetration of the resin, or any other material that will constitute the matrix, when impregnating the composite product with the resin / matrix material.

[0018] According to one embodiment, said solution is an aqueous solution containing a hydrophilic solute or a mixture of hydrophilic solutes. The hydrophilic solute or hydrophilic solutes thus participate(s) in the formation of the binder, which keeps the fibers in shape in the wick after drying of the water which forms the solvent. This is an advantageous arrangement from the point of view of implementation, and which avoids the use of more polluting solvents which require taking specific measures for example for the capture of their gaseous form during drying or during cleaning of installations or tools, or during recycling of the solution. Also, with a water-soluble binder, it can be removed from the wick when the latter is mounted on or in a base support such as a mat, a sheet, a fabric, the whole forming a composite product, for example during a dyeing step or application of any other treatment to the composite product.

[0019] According to one embodiment, the hydrophilic solute belongs to the group comprising polyvinyl alcohol or poly(vinyl alcohol) (PVAL), starch, modified starch, carboxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, cellulose acetate, chitosan, pectin, and an acrylic polymer modified by a polycarboxylic acid and a polyol (in particular the product known under the trade name Acrodur ®<, such as ACRODUR 950 L or ACRODUR DS 3530).

[0020] According to one embodiment, the drawing operation and the consolidation operation are carried out on independent installations. In this way, the manufacturing process is not carried out continuously on the same production line. This makes it possible in particular to adapt the working speed and advance of the ribbon, as well as the number of ribbons processed in parallel to the type of operation carried out. Indeed, for example, the consolidation operation can be carried out in parallel on a large number of ribbons already drawn (up to 100, or even 200), and this with a different advance speed of the ribbon, for example higher, than at the exit of the drawing operation.

[0021] According to one embodiment, the drawing operation is carried out by means of a drawing unit comprising a first pair of inlet rollers driving the plant fibers at a first speed and a second pair of outlet rollers driving the plant fibers at a second speed higher than the first speed, creating the drawing of the plant fiber ribbon. These two pairs of rollers form a drawing module. A single drawing module or several drawing modules which follow one another can be used to carry out a drawing in several stages of a plant fiber ribbon, or several plant fiber ribbons.

[0022] During the consolidation operation, the step of impregnating a stretched ribbon of plant fibers or several stretched ribbons of plant fibers is carried out. For this step of impregnation with the solution, one or other of the following impregnation possibilities can be used: by spraying, by watering, by dipping in a bath or by impregnation by contact with an applicator, such as one or more roller(s), brush(es), etc.

[0023] According to one embodiment, during the consolidation operation, between the impregnation step c1) and the drying step c2), said stretched and impregnated plant fiber ribbon is squeezed out in order to remove a portion of the solution from said stretched and impregnated plant fiber ribbon. Thus, before the drying step, a maximum of liquid (solution containing the binder) is squeezed out, for example by passing the stretched and impregnated plant fiber ribbon between one or more pairs of rollers pressing on the stretched and impregnated plant fiber ribbon. This makes it possible to reduce the drying time and energy. If the solution comprising the binder is supplied by an applicator roller, it is also possible to impregnate and squeeze out with a single pair of rollers, applying and squeezing out the excess solution.

[0024] According to one embodiment, the step of drying the stretched and impregnated plant fiber ribbon is carried out by convection, by infrared radiation, by microwave radiation, or by contact with a heating element.

[0025] Preferably, the plant fibers belong to the group comprising the following materials: linen, hemp, sisal, jute, abaca, kenaf, nettle, ramie, henequen, pineapple, banana, and palm. The process steps are in fact adapted to rather long fibers, and fibers with sufficient reinforcing properties are preferred depending on the intended application for the composite product that receives the wick comprising this fiber.

[0026] According to one embodiment, said stretched plant fiber ribbon, which is obtained according to the manufacturing process, is a linen fiber ribbon and has a weight of between 200 tex and 2,000 tex, preferably between 300 and 1,000 tex, and preferably between 300 and 500 tex. This weight corresponds to the stretched plant fiber ribbon that is recovered at the outlet of the installation allowing the implementation of this process, as soon as the drawing operation is finished. This weight remains close and of the same order of magnitude (generally to within about 1%, or even a few percentages, possibly up to 10%) as that of the final roving that comprises the stretched plant fiber ribbon and the binder.

[0027] The present invention also relates to an installation for the manufacture of flat wicks of natural plant fibers, characterized in that it comprises: a unit for drawing a ribbon of plant fibers, at the inlet of which is arranged a feeding module for drawing, comprising a ribbon of plant fibers and at the outlet of which is arranged a recovery module for drawing, capable of receiving a drawn ribbon of plant fibers, and a unit for consolidating the drawn ribbon of plant fibers, comprising from upstream to downstream: * at least one feeding module for consolidation, comprising a drawn ribbon of plant fibers, * a device for impregnating at least one ribbon of plant fibers stretched with a solution containing a binder, at the outlet of which is formed at least one ribbon of plant fibers stretched and impregnated, and * a device for drying said at least one ribbon of plant fibers stretched and impregnated, at the outlet of which is formed at least one flat wick of plant fibers,and * at least one recovery module for consolidation capable of receiving said at least one flat wick of plant fibers.

[0028] This solution has the particular advantage over the prior art of allowing a controlled and constant binder concentration to be maintained for the impregnation. Indeed, in the usual case of using a binder dispersion to impregnate the natural fiber ribbon, it is not only difficult to control the binder particle concentration over time, but there is also often a filtration effect of the dispersion during the impregnation, which is detrimental in terms of the quality and homogeneity of penetration of the binder into the fibers.

[0029] With a binder comprising compounds in solution, we maintain control over the quantity of binder present in the impregnation liquid, and therefore in the plant fibers, particularly flax fibers. In addition to the quantity, it is also the control of the consistency of this quantity of binder throughout the wick, which allows the latter to present homogeneous physical and mechanical qualities along its entire length.

[0030] The applicant company noted that the use of a solution comprising a binder for impregnation gives satisfactory results in terms of cohesion and strength of the wick of natural plant fibres obtained. Brief description of the figures

[0031] Examples of implementation of the invention are indicated in the description illustrated by the appended figures in which: [ Fig.1A] [Fig.1B] [Fig.1C] schematically illustrate different possible arrangement possibilities for a composite product in the form of a sheet, using flat strands of plant fibers obtained according to the manufacturing process of the invention, [ Fig.2 ] schematically represents a unit for stretching a ribbon of plant fibers, making it possible to carry out, according to the invention, a stretching operation by which a stretched ribbon of plant fibers is formed, stored on a recovery module, and [ Fig.3 ] schematically represents a consolidation unit of another stretched ribbon of plant fibers, this consolidation unit being not attached to the drawing unit of the Fig. 2, and making it possible to carry out, according to the invention, a consolidation operation by which a flat wick of plant fibers is formed, capable of being used, after possible other treatment, as a reinforcing element within a matrix or on a support, all of which allows the formation of a composite product. Example(s) of embodiment of the invention

[0032] THE Figures 1A, 1B, 1C represent several possible implementations of reinforcements for composite products, these reinforcements being formed from or comprising flat wicks 15 obtained according to the method of the invention. In this example, on the Figure 1A , it is a unidirectional reinforcement 20 formed of flat strands 15 obtained according to the method of the invention, parallel to each other and held together by a light weft thread 21. It is generally a thread of less than 30 tex, for example made of natural fibers, polyester or any other thermoplastic. On the Figure 1B, a multiaxial reinforcement 22 is shown, formed from several non-woven reinforcements. More precisely, it is an assembly of layers of unidirectional reinforcements 23, 24 whose flat strands 15 are sewn together, between two neighboring layers 23, 24 superimposed, by a sewing thread 25. Each layer of unidirectional reinforcement 23, 24 is oriented in a different direction, which makes it possible to produce multiaxial reinforcements 22 adapted to the desired mechanical resistance requirements. Then, in general, several layers of these multiaxial reinforcements 22 are used to manufacture parts. In this example, on the Figure 1C , it is a fabric 26 made with flat wicks 15, represented according to a twill weave. Such a fabric 26 can then be placed in a matrix, alone or with other superimposed fabrics.

[0033] THE figures 2 And 3schematically illustrate respectively the operations of treatment by drawing and consolidation of a ribbon of vegetable fibers, on an installation which comprises a drawing unit 100 ( figure 2 ) and a consolidation unit 200 ( figure 3 ).

[0034] In the illustrated example, the drawing unit 100 and the consolidation unit 200 are independent. This involves a drawing unit 100 and a consolidation unit 200 that are physically separate and operate independently of each other. Thus, each operation can be carried out on one (or more) different ribbon sections, at a speed adapted to the operation in question. In addition, if an incident occurs on one of the units 100 and 200, the other unit can continue to operate. This may be an incident related to the devices and modules making up each of these two units 100 and 200. It may also be a problem related to the fiber ribbon. In particular, in the case of a breakage of a fiber ribbon on the drawing unit 100, the latter requires a stoppage which does not affect the consolidation unit 200.Thus, more generally, any incident during the drawing operation does not impact the consolidation operation, which increases the overall efficiency of the process.

[0035] Thus, for example, as at the outlet 102 of the drawing unit 100, the last pair of rollers 112 has a given speed (for example of the order of 30 m / minutes), thanks to the separate nature of the two units 100 and 200, it is possible to dispense with the use of this speed for the continuation of the treatment of the drawn plant fiber ribbon 12, namely for the consolidation operation on the consolidation unit 200. To do this, the drawn plant fiber ribbon 12 is temporarily stored on a support such as a reel, and it is sufficient to subsequently take up this support / reel to carry out the consolidation operation with a feed speed of the drawn ribbon at the inlet 201 of the consolidation unit 200 different from the speed at the outlet 102 of the drawing unit 100 (for example of the order of 50 m / minutes).Also, this arrangement makes it easier to adapt the number of ribbons processed in parallel and at the same time, respectively on the drawing unit 100 and on the consolidation unit 200. Furthermore, in the event of a malfunction or need to adjust a parameter, it is easier to slow down or stop the advance of the fiber ribbon on the unit concerned among the drawing unit 100 and the consolidation unit 200, without impacting the speed of performance of the other operation on the other unit among the drawing unit 100 and the consolidation unit 200.

[0036] The present invention, however, also applies to an installation making it possible to continuously carry out the successive operations of treatment by drawing a ribbon of plant fibers and then of consolidation of the same ribbon of plant fibers. In the latter case, not shown, the drawing unit 100 is placed upstream of the consolidation unit 200, with the outlet 102 of the drawing unit 100 which continues directly to the inlet 201 of the consolidation unit 200, the outlet 102 of the drawing unit 100 being adjacent to and upstream of the inlet 201 of the consolidation unit 200, the two being connected to each other.

[0037] A ribbon of plant fibers 10 is generally provided at the inlet 101 of the drawing unit 100 (of the drawing operation) with a weight of at least 10,000 tex, preferably between 20,000 tex and 50,000 tex, and preferably between 30,000 tex and 40,000 tex.

[0038] As an example, in this detailed description, a ribbon of plant fibers 10 formed from flax fibers will be considered.

[0039] We refer to the figure 2 , illustrating a drawing unit 100 for drawing a ribbon of plant fibers 10 which arrives, at the inlet 101 of the drawing unit 100, at a first drawing module 110. In this example, the ribbon of plant fibers 10 comes from two ribbons 10a, 10b put together. Each individual ribbon 10a, 10b is supplied on a feed module 103 which allows the progressive advancement of the corresponding ribbon at the inlet 101 of the first drawing module 110 of the drawing unit 100. This feed module 103 is in particular a feed reel 103b (for the ribbon 10b on the figure 2 ) or a 103a power pot (for the 10a ribbon on the figure 2) or any other support for storing and then unwinding a ribbon of plant fibers. Alternatively, and not shown, the ribbon of plant fibers 10 which is going to be stretched may initially consist of a single ribbon and come from a single supply module 103, or else initially consist of more than two ribbons (for example three ribbons, four ribbons, five ribbons... or even more ribbons) and come from more than two supply modules 103.

[0040] Downstream of the inlet 101 of the drawing unit 100, the first drawing module 110 comprises a first pair of rollers 111 at the inlet, rotating in opposite directions to each other and thus driving, by their rotation and their close contact with it, the ribbon of plant fibers 10 passing between the two rollers of a pair, at a first speed V1, in the direction of advance A of the first drawing module 110 of the drawing unit 100 (from left to right on the figure 2, indicated by the arrow A). This first drawing module 110 also comprises, at a distance and downstream from the first pair of rollers 111, a second pair of rollers 112, arranged in a similar manner to the first pair of rollers 111, rotating at a second speed V2 higher than the first speed V1. This difference between the second speed V2 and the first speed V1 creates the drawing of the fiber ribbon 10, which thus lengthens with a decreasing linear title / weight. In the example illustrated, at the outlet of the first drawing module 110, a partially drawn plant fiber ribbon 11 is formed, which is flat, and which arrives at a first recovery module for drawing 114, in particular a reel 114b (see figure 2). This partially stretched ribbon of plant fibers 11 is thus either wound on reels 114b, or placed in large pots 114a awaiting the next step, namely another drawing step which is carried out separately on a second drawing module 120. It is then sufficient to take a full reel 114b or any other recovery module for drawing, such as a pot 114a, carrying a partially stretched ribbon of plant fibers 11 from the first drawing module 110, and to use it as a supply module for the second drawing module 120.

[0041] In the case of the figure 2, the drawing unit 100 comprises three drawing modules: a first drawing module 110 as previously described, a second drawing module 120 and a third drawing module 130 which operate in parallel separately to carry out a drawing in several stages (here three stages) of a ribbon of fibers. It is understood that the ribbon of fibers is drawn more on each drawing module over which it passes compared to the previous drawing module.

[0042] For this purpose, the drawing unit 100 also comprises a second drawing module 120 used after the first drawing module 110 for a given fiber ribbon, and a third drawing module 130 used after the second drawing module 120 for a given fiber ribbon, placed downstream of the second drawing module 120. The second drawing module 120 and the third drawing module 130 are similar to the first drawing module 110, with a positive rotational speed difference between the two pairs of rollers 111, 112 of each drawing module 120, 130.

[0043] The spacing between the pair of input rollers (first pair of rollers 111) and the pair of output rollers (second pair of rollers 112) is adjusted to the fiber length of the plant fiber ribbon 10. Between the pair of input rollers (first pair of rollers 111) and the pair of output rollers (second pair of rollers 112), the fiber ribbon 10 is transported by a conveyor module 113, for example a needle field (not shown) or a pair of belts (see figure 2 ).

[0044] Alternatively, and not shown, the stretching unit 100 comprises only a single stretching module formed by the first stretching module 110, or it is made up of more than three successive stretching modules 110, 120, 130 (for example four stretching modules, five stretching modules, six stretching modules, etc., or even more stretching modules). Thus, the operation of stretching the plant fiber ribbon 10 is carried out in as many stretching steps as there are stretching modules 110, 120, 130, etc., i.e. three stretching steps, four stretching steps, five stretching steps, six stretching steps, etc., or even more successive stretching steps.

[0045] Alternatively, and not shown, the stretching unit 100 comprises a second stretching module 120 placed directly downstream and continuously from the first stretching module 110 and a third stretching module 130 placed directly downstream and continuously from the second stretching module 120. The second stretching module 120 and the third stretching module 130 are similar to the first stretching module 110, with a positive difference in rotation speed between the two pairs of rollers of each stretching module 120, 130, and a rotation speed of these rollers which is, at the inlet of a module, substantially equal to the rotation speed of the pair of rollers located at the outlet of the previous module (located upstream). In this case, the fiber ribbon 10 undergoes the three (or more) drawing steps continuously, passing successively over each drawing module 110, 120, 130, without stopping between each step and without intermediate storage on a recovery module for drawing.

[0046] At the outlet of the drawing unit 100, therefore of the last (third or more) drawing module 130, a stretched ribbon of plant fibers 12 is formed, which is flat, and which arrives on a last recovery module for drawing 114, for example a reel. This stretched ribbon of plant fibers 12 is thus either wound on reels 114b, or deposited in large pots 114a awaiting the next step, namely the consolidation operation which is carried out separately and later in the embodiment example presented here.

[0047] In the illustrated example, the stretching operation is carried out in several stages on several (here three) successive machines each forming a stretching module to progressively reduce the weight of the ribbon of plant fibers. Also, in the illustrated example, the ribbons at the input of each stretching module 110, 120, 130 are doubled (ribbons 10a and 10b on the figure 2) to standardize the weight of the material: the defects of each ribbon are thus averaged). According to another example not shown, we can have four ribbons of 35 ktex at the input of a stretching module, we stretch everything by a factor of ten, and we obtain a single ribbon of 14 ktex at the output of the stretching module. Depending on the quality desired for the stretched ribbon formed at the output of the stretching module 110, 120, 130, we can use two, three, four, five, six, seven or eight ribbons of plant fibers put together at the input to form a superposition of ribbons stretched together, and a single ribbon comes out at the output of the stretching module 110, 120, 130.

[0048] Optionally, at the outlet of the drawing modules 110, 120 and 130, the partially drawn fiber ribbon 11 (the drawn fiber ribbon 12) may be slightly twisted (with a twist of less than 20 turns per meter or rpm) when it is wound onto the reel 114b serving as a recovery module, (for example by applying the spindle bench technique) to improve its hold. The partially drawn fiber ribbon 11 (the drawn fiber ribbon 12) may also be rubbed between two sleeves to improve its hold.

[0049] In the figures and in the preceding description, the drawing of a single fiber ribbon 10 (formed by the association of several individual ribbons 10a, 10b) has been mentioned / illustrated. In the context of an operation, in reality, it is usual for many plant fiber ribbons 10, placed side by side and separately from each other, to be drawn at the same time on the same drawing module 110 (or 120 or 130) by all passing between the two rollers of the first pair of rollers 111 and the second pair of rollers 112. As many drawn plant fiber ribbons 12 are obtained, which are stored separately and individually before the next consolidation step.This two-step process makes it possible to have different plant fiber ribbon feed speeds in the drawing and consolidation stages, a different number of plant fiber ribbons in the two drawing and consolidation stages, in order to best adapt to each stage, and to avoid stoppages of the entire production in the event of a stoppage due to a problem with one or other of the ribbons being processed or due to a malfunction in one or other of the drawing and consolidation stages, on one or other of the drawing 100 and consolidation 200 units.

[0050] The stretched plant fiber ribbon 12 is then taken from the last recovery module for stretching 114 (reel 114b or pot 114a) of the stretching unit 100 to be consolidated on the consolidation unit 200, to be typically usable for a weaving operation.

[0051] We now refer to the figure 3, illustrating a consolidation unit 200. At entrance 201 of consolidation unit 200 (on the left on the figure 2 ) is arranged a supply module for consolidation 203, such as a reel 203b (or a pot 203a), comprising a stretched ribbon of plant fibers 12. At the outlet 202 of the consolidation unit 200 (on the right on the figure 2 ) a recovery module for consolidation 204 is arranged, such as a reel, receiving the flat wick of plant fibers 15 ready for its subsequent use.

[0052] We now describe the modules of the consolidation unit 200, which intervene successively in the consolidation operation, in the direction of advance of the ribbon 12, from left to right on the figure 2 , indicated by arrow B.

[0053] Downstream of the consolidation feed module 203, the stretched plant fiber ribbon 12 arrives at the impregnation device 210. This impregnation device 210 comprises a reservoir 211 or any other means for feeding the solution 212 for impregnation, and means 213 for applying this solution 212. The step of impregnating the stretched plant fiber ribbon 12 with the solution 212 is carried out for example with an applicator roller. This impregnation step allows the stretched plant fiber ribbon 12 to receive a binder in solution 212, which easily penetrates into and between all the fibers, over their entire length, to form a stretched and impregnated plant fiber ribbon 13.

[0054] More generally, the impregnation device 210 comprises, as application means 213, spraying means, watering means, means of dipping in a bath or means of impregnation by contact with an applicator (roller, brush, etc.).

[0055] In the case of the embodiment envisaged as an example illustrated, the solution 212 of the impregnation device 210 is an aqueous solution containing a hydrophilic solute or a mixture of hydrophilic solutes. According to one possibility of this embodiment, the aqueous solution contains at least polyvinyl alcohol or poly(vinyl alcohol) and known by the acronyms PVA, PVAL or PVOH. This compound is often used as a sizing agent and allows good core impregnation of natural plant fibers, in particular flax fibers.

[0056] In this case, starch or modified starch is optionally added to this poly(vinyl alcohol) (PVA). The latter compound (modified starch) remains a starch because it is a polymer chain whose monomers are glucose molecules, linked together by glycosidic bonds of the α-(1 → 4) or α-(1 → 6) type, on which there are three hydroxyl (-OH) groups to which other chemical functions can be partially substituted (e.g. hydroxypropyl) in order to increase the solubility of the starch.

[0057] More broadly, and without being exhaustive, the hydrophilic solute or several hydrophilic solutes of the solution 212 used for the impregnation belong(s) to the group comprising polyvinyl alcohol or poly(vinyl alcohol) (PVAL), starch, modified starch, carboxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, cellulose acetate, chitosan, pectins, and an acrylic polymer modified by a polycarboxylic acid and by a polyol. Preferably, the quantity of the solute (or solutes) remaining in the wick 15 after evaporation of the solvent of the solution 212 is between 0.2-10% by weight, preferably between 0.5-5% by weight, and preferably between 0.5 and 2% by weight. With polyvinyl alcohol or poly(vinyl alcohol) (PVA) as solute, such percentages can be used, and good results are also obtained with between 1 and 2% by weight of solute in the wick 15.

[0058] Within the scope of the invention, solvents other than water are possible in the solution 212 used for impregnating the fibers of the stretched plant fiber ribbon 12. For example, the solvent of the impregnation solution 212 may be acetone. With acetone as the solvent, the solute or one of the solutes present in the solution 212 may be cellulose acetate.

[0059] In the case of the embodiment envisaged as an example illustrated, between the impregnation device 210 and a drying device 230, the consolidation unit 200 comprises a wringing device 220. This is used to express a portion of the solution 212 from the stretched and impregnated plant fiber ribbon 13. In the case of the illustrated embodiment, such a wringing device 220 carries out calendering with one (or more) pair(s) of rollers 221, 222. A recovery tank 223 placed under the pair(s) of rollers 221, 222 makes it possible to receive the liquid expressed from the stretched and impregnated plant fiber ribbon 13, and to recycle it by reintroducing it into the tank 211 of the impregnation device 210. This step of expressing the solution 212 from the fiber ribbon stretched and impregnated plant fibers 13 results, at the outlet of the impregnation device 210, in a stretched, impregnated and wrung out ribbon of plant fibers 14.This step of squeezing the solution 212 out of the stretched and impregnated plant fiber ribbon 13 makes it possible to reduce the drying time and energy in the drying device 230 placed downstream of the wringing device 220. Indeed, by removing a maximum of liquid, therefore of solution 212, during this squeezing step, efficient drying can be carried out with less energy effort.

[0060] For example, the squeezing step is carried out in order to obtain in the stretched, impregnated and wrung out plant fiber ribbon 14, a residual moisture content after "squeezing", i.e. a moisture content of less than 50%, preferably between 15 and 40%.

[0061] In the case of a variant not shown, the impregnation device 210 and the wringing device 220 form a single piece of equipment within the consolidation unit 200. For example, it is possible to use an applicator roller formed from a single pair of rollers, which carries out both the impregnation and the squeezing of the stretched ribbon of plant fibers 12. Such an arrangement makes it possible in particular to reduce the complexity, in particular for the recycling of the solution 212 squeezed out of the ribbon of plant fibers.

[0062] The drying device 230 for the stretched and impregnated plant fiber ribbon 13 (if there is no wringing device 220 in the consolidation unit 200) or for the stretched, impregnated and wrung plant fiber ribbon 14 (if there is a wringing device 220 in the consolidation unit 200) is now described. This drying device 230 is placed downstream of the drying device 230 or, in the absence of a wringing device 220 in the consolidation unit 200, downstream of the impregnation device 210. This drying device 230 comprises convection heating means, infrared radiation heating means 232 ( figure 3 ), means of heating by microwave radiation, or means of heating by contact with a heating element.

[0063] At the outlet of the drying device 230, one (or more) flat strand(s) of natural plant fibers 15 are obtained. These are (are) possibly further cooled by cooling means before their arrival at the outlet 202 of the consolidation unit 200, and their storage on a recovery module for consolidation 204.

[0064] A consolidated flat wick of plant fibers 15 is thus obtained on the recovery module for consolidation 204. In general, such a recovery module for consolidation 204 is a reel onto which the consolidated flat wick of plant fibers 15 has been wound in a precision cross-winding pattern keeping the wick flat with a controlled winding angle on the reel.

[0065] According to one embodiment, this consolidation operation is carried out simultaneously in parallel on several stretched ribbons of plant fibers. Indeed, since heating and possibly cooling means are used, it is advantageous from an energy point of view in particular to use the same means for several ribbons in parallel.

[0066] To do this, said consolidation unit 200 is arranged to simultaneously process in parallel several stretched plant fiber ribbons. Thus, it is possible to provide that groups of 2 to 200 stretched plant fiber ribbons 12, preferably 10 to 100 stretched plant fiber ribbons 12 are consolidated in parallel in the same consolidation unit 200. In this case, the strands 15 obtained after consolidation are then wound individually onto bobbins 204, with precision cross-winding keeping the continuous strand flat and controlling the winding angle on the bobbin.

[0067] In particular, flat wicks of consolidated plant fibers 15 are obtained having an RKM resistance greater than or equal to 5, preferably greater than or equal to 7, preferably greater than or equal to 9. As a reminder, this RKM resistance or kilometer resistance of a thread corresponds to the breaking length of a thread under its own weight, in kilometers.

[0068] These flat strands of consolidated plant fibers 15 can then be used to manufacture reinforcing elements for the composites industry. The small quantities of binding agent used during the impregnation of the consolidation operation are particularly suitable for impregnation with a thermosetting matrix (for example an epoxy resin, polyester, etc.). Among other results, the applicant company has found that polyvinyl alcohol or poly(vinyl alcohol) (PVA) as well as starch have shown good compatibility with these matrices. Thus, the use of an aqueous solution of PVA, or an aqueous solution of starch (or modified starch) or even an aqueous solution of a mixture of PVA and starch (or modified starch) gives good results.

[0069] It has been found that the implementation of this process is applied with better results on rather long natural plant fibers. The process is therefore well suited to natural plant fibers whose average length is greater than 120 mm, preferably natural plant fibers whose average length is greater than 150 mm and more preferably to natural plant fibers whose average length is greater than 200 mm.

[0070] The implementation of this process can be applied indifferently to raw natural vegetable fibers, or to bleached natural vegetable fibers, or even to bleached and dyed natural vegetable fibers. This process also applies to the previously mentioned natural vegetable fibers to which a finish or other treatment has been applied, provided that this finish or treatment does not prevent the penetration of the solution containing the binder.

[0071] According to an alternative embodiment, not illustrated, the geometry of the section of the wick is shaped to precisely control its width. For this purpose, at least one die, and preferably several dies, are used, through which the ribbon of natural plant fibers passes within the consolidation unit.

[0072] According to one possibility, one die is placed upstream of the impregnation device, and another die downstream of the impregnation device, therefore upstream of the drying device.

[0073] These dies may have a passage whose section has different geometries, including a circle shape, an oblong shape, an oval shape, a polygon shape, in particular a polygon with rounded vertices, such as a rectangle, a square, a trapezoid. Preferably, the section of the opening of the die has a ratio between its width and its height greater than or equal to 10.

[0074] At the exit of the last die, a wick is thus obtained whose section has a shape which may be close to one of the following shapes: a circle shape, an oblong shape, an elliptical shape, an oval shape, a polygon shape, in particular a polygon shape with rounded vertices, such as a rectangle, a square, a trapezoid. Preferably, the flat wick of plant fibers obtained has a section with a ratio between its width and its height greater than or equal to 10 which makes it appear flat. [Tables 1] Reference signs used in the figures A Direction of advance of the stretching operation B Direction of progress of the consolidation operation V1 First gear V2 Second gear 10 Plant fiber ribbon 11 Partially stretched plant fiber ribbon 12 Stretched plant fiber ribbon 13 Stretched and impregnated plant fiber ribbon 14 Stretched, impregnated and expressed plant fiber ribbon 15 Flat wick of vegetable fibers 20 One-way reinforcement 21 Weft thread 22 Multiaxial reinforcement 23 Unidirectional reinforcement layer 24 Unidirectional reinforcement layer 25 Sewing thread 26 Fabric 100 Stretching unit 110 First stretching module 101 Entrance to the stretching unit and the first stretching module 102 Release of the first stretching module 103 Feeding module for stretching 103a Feeding pot 103b Power supply coil 111 First pair of rollers 112 Second pair of rollers 113 Conveyor module 114 Recovery module for stretching 114a Recovery pot 114b Recovery reel 120 Second stretching module 130 Third stretching module 200 Consolidation unit 201 Consolidation Unit Entrance 202 Exit from the consolidation unit 203 Power module for consolidation 204 Recovery module for consolidation 210 Impregnation device 211 Solution feeder (reservoir) 212 Solution containing the binder 213 Ways of applying the solution 220 Wringing device 221 First spin roller 222 Second spin roller 223 Recovery tank

Claims

1. A method for manufacturing flat strands of natural plant fibers (15), in which the following steps are carried out: a) a ribbon of plant fibers (10) is provided, b) a drawing operation is carried out on said ribbon of plant fibers (10), whereby a drawn ribbon of plant fibers (12) is formed, and c) a consolidation operation is carried out on the drawn ribbon of plant fibers (12), comprising the following sub-steps: c1) impregnating said drawn ribbon of plant fibers (12) with a solution (212) containing a binder, whereby a drawn and impregnated ribbon of plant fibers (13) is obtained, c2) drying said drawn and impregnated ribbon of plant fibers (13), whereby a flat strand of plant fibers (15) is obtained.

2. The method of claim 1, wherein said solution (212) is an aqueous solution containing a hydrophilic solute or a mixture of hydrophilic solutes.

3. The method of claim 2, wherein the hydrophilic solute belongs to the group comprising polyvinyl alcohol or poly(vinyl alcohol) (PVAL), starch, modified starch, carboxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, cellulose acetate, chitosan, pectin, and an acrylic polymer modified by a polycarboxylic acid and by a polyol.

4. Method according to one of claims 2 and 3, in which said aqueous solution (212) contains at least polyvinyl alcohol or poly(vinyl alcohol) (PVAL).

5. Method according to one of claims 1 to 4, in which the stretching operation and the consolidation operation are carried out on independent installations (100, 200).

6. Method according to one of claims 1 to 5, in which the step of impregnation with the solution (212) is carried out by spraying, watering, soaking in a bath or impregnation by contact with an applicator (213).

7. Method according to one of claims 1 to 6, in which during the consolidation operation, between step c1) of impregnation and step c2) of drying, said stretched and impregnated plant fiber ribbon (13) is expressed in order to cause a part of the solution (212) to come out of said stretched and impregnated plant fiber ribbon (13).

8. Method according to one of claims 1 to 7 in which the plant fibers belong to the group comprising the following materials: linen, hemp, sisal, jute, abaca, kenaf, nettle, ramie, henequen, pineapple, banana, and palm.

9. Method according to one of claims 1 to 8, in which said stretched plant fiber ribbon (12) is made of flax fibers and has a weight of between 200 tex and 2,000 tex, preferably between 300 tex and 1,000 tex, and preferably between 300 and 500 tex.

10. Installation for the manufacture of flat wicks of natural plant fibers (15), characterized in thatit comprises: - a unit (100) for drawing a strip of plant fibers (10), at the inlet of which is arranged a drawing feed module (103), comprising a strip of plant fibers (10) and at the outlet of which is arranged a drawing recovery module (114), capable of receiving a drawn strip of plant fibers (12), and - a unit (200) for consolidating the drawn strip of plant fibers (12), comprising from upstream to downstream: * at least one consolidation feed module (203), comprising a drawn strip of plant fibers (12), * a device (210) for impregnating at least one drawn strip of plant fibers (12) with a solution (212) containing a binder, at the outlet of which is formed at least one drawn and impregnated strip of plant fibers (13), and * a drying device (230) of said at least one stretched and impregnated ribbon of plant fibers (13), at the outlet of which at least one flat wick of plant fibers (15) is formed,and * at least one recovery module for consolidation (204) capable of receiving said at least one flat wick of plant fibers (15)., 11. Installation according to claim 10, wherein said stretching unit (100) and said consolidation unit (200) are independent.

12. Installation according to one of claims 10 to 11, in which the solution (212) of the impregnation device is an aqueous solution containing a hydrophilic solute or a mixture of hydrophilic solutes.

13. Installation according to the preceding claim, in which the hydrophilic solute belongs to the group comprising polyvinyl alcohol or poly(vinyl alcohol) (PVAL), starch, modified starch, carboxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, cellulose acetate, chitosan, pectin, and an acrylic polymer modified by a polycarboxylic acid and by a polyol.

14. Installation according to claim 162 or 13, in which said aqueous solution (212) contains at least polyvinyl alcohol or poly(vinyl alcohol) (PVAL).

15. Installation according to one of claims 10 to 14, in which the plant fibers belong to the group comprising the following materials: linen, hemp, sisal, jute, abaca, kenaf, nettle, ramie, henequen, pineapple, banana, and palm.

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