EP0848767A1 - Cellulose fibres with improved elongation at break, and methods for producing same - Google Patents

Abstract

Fiber made of cellulose formate and fiber made of cellulose regenerated from cellulose formate. These fibers exhibit high tenacity and modulus properties, combined with improved values of elongation at break and of energy at break. Their elongation at break, in particular, is greater than 6%.Methods for producing these fibers. The fiber made of cellulose formate is obtained by spinning a liquid crystal solution of cellulose formate according to the so-called dry-jet-wet spinning method, the coagulation stage and the neutral washing stage which follow both being carried out in acetone. The fiber made of cellulose formate in a highly concentrated aqueous sodium hydroxide solution. The spinning and regeneration methods can be employed in line and continuously.Reinforcing assemblies based on such fibers. Articles reinforced by such fibers or assemblies, these reinforced articles being in particular tires.

Description

 CELLULOSIC FIBERS WITH IMPROVED RUPTURE ELONGATION

 AND METHODS FOR OBTAINING THEM.

The invention relates to fibers of cellulose derivatives and fibers of cellulose regenerated from these derivatives.

In known manner, the term “cellulose derivatives” is understood here to mean the compounds formed, as a result of chemical reactions, by substitution of the hydroxyl groups of the cellulose, these derivatives also being called substitution derivatives. The term "regenerated cellulose" means a cellulose obtained by a regeneration treatment carried out on a cellulose derivative.

The invention relates more particularly to fibers of cellulose formate and to fibers of cellulose regenerated from this formate. as well as the processes for obtaining such fibers.

Fibers of cellulose formate and fibers of cellulose regenerated from this formate have in particular been described in international patent application WO 85/05115 (PCT / CH85 / 00065), filed by the applicant, or in equivalent patents EP -B-179 822 and US-A-4 839 113. These documents describe the obtaining of spinning solutions based on cellulose formate, by reaction of the cellulose with formic acid and phosphoric acid. These solutions are optically anisotropic. that is, they have a liquid crystal state. These documents also describe the cellulose formate fibers obtained by spinning these solutions, according to the so-called "dry-jet-wet spinning" technique, as well as the cellulose fibers obtained after a regeneration treatment of these formate fibers.

Compared to conventional cellulosic fibers such as rayon or viscose fibers, or to other conventional non-cellulosic fibers such as nylon or polyester fibers for example, all spun from optically isotropic liquids, the cellulose fibers of the application WO 85/05115 are characterized by a much more ordered structure, due to the liquid-crystal nature of the spinning solutions from which they come. They thus have very high mechanical properties in extension, in particular very high values of toughness and modulus, but, on the other hand, are characterized by rather low elongation at break values, these values being on average between 3 % and 4%. and not exceeding 4.5%.

However, higher elongation at break values may be

desirable when such fibers are used in certain technical applications, in particular as reinforcing elements for a tire casing, in particular for a tire carcass reinforcement. The primary object of the invention is to provide fibers in cellulose formate as well as regenerated cellulose fibers which, compared to the fibers of application WO 85/05115, have a significantly improved elongation at break, as well as properties high energy at break.

The second object of the invention is to obtain the above improvements without reducing the tenacity of the fibers, which is a major advantage of the invention.

Another object of the invention is to obtain regenerated cellulose fibers from cellulose formate, the fatigue resistance of which, in particular in tires, is significantly improved compared to that of regenerated cellulose fibers on demand. WO 85/05115 cited above.

The cellulose formate fiber of the invention is characterized by the following relationships:

- Ds≥ 2;

 - Te> 45;

 - Mi> 800;

 - Ar> 6;

 - Er> 13.5,

Ds being the degree of substitution of the cellulose in formate groups (in%), Te being its tenacity in cN / tex, Mi being its initial modulus in cN / tex, Ar being its elongation at break in%, Er being its energy at break in J / g.

The cellulose fiber of the invention, regenerated from cellulose formate, is characterized by the following relationships:

- 0 <D _s <2;

- T _E >60;

- M _I >1000;

- A _R >6;

- E _R > 17.5,

Dg being the degree of substitution of the cellulose for formate groups (in%), T _E being its toughness in cN / tex, M _I being its initial modulus in cN / tex, A _R being its elongation at break in%. E _R being its energy at break in J / g.

The cellulose formate fiber and the regenerated cellulose fiber above are both obtained by novel and specific methods which constitute other objects of the invention.

The spinning process of the invention, for obtaining the cellulose formate fiber of the invention, consisting in spinning a solution of cellulose formate in a solvent based on phosphoric acid, according to the so-called "dry" spinning method -jet-wet spinning ", is characterized in that the coagulation step of the fiber and the neutral washing step of the coagulated fiber are both carried out in acetone.

The regeneration process of the invention, for obtaining the regenerated cellulose fiber of the invention, consisting in passing a cellulose formate fiber through a regenerating medium, washing it and then drying it, is characterized in that the regenerating medium is an aqueous sodium hydroxide solution (NaOH) whose sodium hydroxide concentration, denoted Cs, is greater than 16% (% by weight).

The invention further relates to the following products:

- the reinforcement assemblies each comprising at least one fiber according to the invention, for example cables, plies, fibers

 multifilamentary twisted on themselves, such reinforcement assemblies which can be for example hybrid, that is to say composite, comprising elements of different natures, possibly not in accordance with the invention;

the articles reinforced with at least one fiber and / or an assembly in accordance with the invention, these articles being for example articles made of rubber or plastics, for example plies, belts, pipes, tire casings, in particular tire carcass reinforcements.

The invention will be easily understood with the aid of the description and nonlimiting examples which follow.

I. MEASUREMENTS AND TESTS USED

I-1. Degree of polymerization

The degree of polymerization is noted DP. The DP of the cellulose is measured in a known manner, this cellulose being in the form of a powder, or previously transformed into a powder.

The inherent viscosity (IV) of the cellulose in solution is first determined, according to Swiss standard SNV 195 598 of 1970, but at different concentrations which vary between 0.5 and 0.05 g / dl. The inherent viscosity is defined by the equation:

IV = (1 / C _e ) × Ln (t ₁ / t ₀ ) in which C _e represents the concentration of dry cellulose, t ₁ represents the duration of flow of the dilute polymer solution, t ₀ represents the duration of flow of pure solvent. in an Ubbelhode viscometer, and Ln represents the natural logarithm. The measurements are carried out at 20 ° C.

The intrinsic viscosity [η] is then determined by extrapolation at zero concentration of the inherent viscosity IV.

The weight average molecular mass M _w is given by the Mark-Houwink relation:

[η] = K × M _w ^α where the constants K and α are respectively:

K = 5.31 × 10 ^-4 ; α = 0.78. these constants corresponding to the solvent system used for determining the inherent viscosity. These values are given by L.

Valtasaari in document Tappi 48, 627 (1965).

The DP is finally calculated according to the formula:

DP = (M _w ) / 162,

162 being the molecular weight of the elementary motif of cellulose.

When determining the DP of cellulose from cellulose formate in solution, we must first isolate this formate, then regenerate the cellulose.

We then proceed as follows:

the solution is first coagulated with water in a dispersing device. After filtration and washing with acetone, a powder is obtained which is then dried in a vacuum oven at 40 ° C for at least 30 minutes. After isolating the formate, the cellulose is regenerated by treating this formate at reflux with normal sodium hydroxide. The cellulose obtained is washed with water, dried and the DP is measured as described above.

I-2. Degree of substitution

The degree of substitution of cellulose for cellulose formate is also called the degree of formylation.

The degree of substitution determined by the method described here gives the percentage of alcohol functions of the cellulose which are esterified, that is to say transformed into formate groups. This means that a degree of substitution of 100% is obtained if the three alcohol functions of the cellulose unit are all esterified. or that a degree of substitution of 30%, for example, is obtained if 0.9 out of three alcohol functions, on average, is esterified. The degree of substitution is measured differently depending on whether one characterizes cellulose formate (formate in solution, or fibers in formate) or cellulose fibers regenerated from cellulose formate.

I-2.1. Degree of substitution on cellulose formate:

If the degree of substitution is measured on cellulose formate in solution, this formate is first isolated from the solution as indicated previously in paragraph I-1. If it is measured on formate fibers, these fibers are first cut into pieces 2 to 3 cm long.

200 mg of cellulose formate thus prepared is weighed with precision and placed in an Erlenmeyer flask. 40 ml of water and 2 ml of normal sodium hydroxide (1 N NaOH) are added. The mixture is heated at 90 ° C. at reflux for 15 minutes under nitrogen. The cellulose is thus regenerated by retransforming the formate groups into hydroxyl groups. After cooling, the excess soda is titrated back with a solution of decinormal hydrochloric acid (0.1 N HCl), and the degree of substitution is thus deduced therefrom.

In the present description, the degree of substitution is noted Ds when it is measured on cellulose formate fibers.

I-2.2. Degree of substitution on regenerated cellulose fibers:

About 400 mg of fiber are cut into pieces 2 to 3 cm long, then weighed with precision and introduced into a 100 ml Erlenmeyer flask containing 50 ml of water. 1 ml of normal sodium hydroxide (1N NaOH) is added. The whole is mixed at room temperature for 15 minutes. The cellulose is thus completely regenerated by transforming the last formate groups which had resisted the regeneration conducted, after spinning, directly into continuous fibers into hydroxyl groups. The excess soda is titrated with an acid solution

 hydrochloric decinormal (HCl 0, 1 N), and we thus deduce the degree of substitution.

In the present description, the degree of substitution is noted D _S when it is measured on regenerated cellulose fibers.

I-3. Optical properties of solutions

The optical isotropy or anisotropy of the solutions is determined by placing a drop of solution to be studied between crossed linear polarizers and analyzers with an optical polarization microscope, and then observing this solution at rest, i.e. by the absence of dynamic stress, at room temperature. In known manner, an optically anisotropic solution is a solution which depolarizes light, that is to say which exhibits, thus placed between crossed linear polarizer and analyzer, a transmission of light (colored texture). An optically isotropic solution is a solution which, under the same observation conditions, does not have the above depolarization property, the field of the microscope remaining black.

I-4. Mechanical properties of fibers

By "fibers" is meant here multifilament fibers (also called "spun"), constituted in a known manner of a large number of elementary filaments of small diameter (small titer). All the mechanical properties below are measured on fibers which have been subjected to prior conditioning. "Pre-conditioning" means storing the fibers for at least 24 hours, before measurement, in a standard atmosphere according to European standard DIN EN 20139 (temperature of 20 ± 2 ° C; hygrometry of 65 ± 2%).

For cellulosic fibers, such prior conditioning makes it possible, in a known manner, to stabilize their moisture content (residual water content) at a natural equilibrium level of less than 15% by weight of dry fiber (approximately 11 to 12%, in average).

The fiber titer is determined on at least three samples, each corresponding to a length of 50 m, by weighing this length of fiber. The title is given in tex (weight in grams of 1000 m of fiber).

The mechanical properties of the fibers (toughness, initial modulus, elongation and energy at break) are measured in known manner using a tensile machine

ZWICK GmbH & Co (Germany) type 1435 or type 1445. The fibers, after having received a slight preliminary protective twist (helix angle of approximately 6 °), undergo a traction over an initial length of 400 mm at a speed of 200 mm / min (or at a speed of 50 mm / min only when their elongation at break does not exceed 5%). All the results given are an average of 10 measurements.

The toughness (breaking force divided by the title) and the initial modulus are indicated in cN / tex (centinewton per tex - reminder: 1 cN / tex equal to approximately 0.11 g / den (gram per denier)). The initial modulus is defined as the slope of the linear part of the Force-Elongation curve, which occurs just after the standard pretension of 0.5 cN / tex. The elongation at break is indicated as a percentage. The energy at break is given in J / g (joule per gram), that is to say per unit mass of fiber. II. CONDITIONS FOR CARRYING OUT THE INVENTION

We first describe the production of spinning solutions, then the spinning of these solutions to obtain cellulose formate fibers. In a third paragraph, the step of regenerating the cellulose formate fibers is explained, to obtain the fibers of regenerated cellulose.

II- 1. Realization of spinning solutions

Cellulose formate solutions are made by mixing cellulose, formic acid, and phosphoric acid (or an acid-based liquid

phosphoric), as indicated for example in the aforementioned application WO 85/05115.

The cellulose can be in different forms, in particular in the form of a powder, prepared for example by spraying a plate of raw cellulose. Preferably, its initial water content is less than 10% by weight, and its DP between 500 and 1000.

Formic acid is esterification acid, phosphoric acid (or liquid based on phosphoric acid) being the solvent for cellulose formate, called "solvent" or "spinning solvent" in the description below. -after. In general, acid

phosphoric used is orthophosphoric acid (H ₃ PO ₄ ), but one can use other phosphoric acids, or a mixture of phosphoric acids. The phosphoric acid can, depending on the case, be used solid, in the liquid state, or dissolved in formic acid.

Preferably the water content of these two acids is less than 5% by weight; they can be used alone or optionally contain, in small proportions, other organic and / or mineral acids, such as acetic acid, sulfuric acid or hydrochloric acid for example.

In accordance with the description made in the aforementioned application WO 85/05115, the cellulose concentration of the solution, denoted "C" below, can vary to a large extent; concentrations C of between 10% and 30% (% by weight of cellulose - calculated on the basis of non-esterified cellulose - on the total weight of the solution) are for example possible, these concentrations being in particular a function of the degree of polymerization cellulose. The weight ratio (formic acid / acid

phosphoric) can also be adjusted within a wide range.

When making cellulose formate, the use of formic acid and phosphoric acid makes it possible to obtain both a high degree of substitution for cellulose formate, generally greater than 20%, without excessive reduction the initial degree of polymerization of the cellulose, as well as a homogeneous distribution of these formate groups, both in the amorphous zones and in the crystalline zones of the cellulose formate. The appropriate kneading means for obtaining a solution are known to those skilled in the art: they must be capable of kneading, kneading correctly, preferably at an adjustable speed, the cellulose and the acids until obtaining of the solution. By "solution" is meant here, in a known manner, a homogeneous liquid composition in which no solid particle is visible to the naked eye. The mixing can be carried out for example in a mixer comprising Z-shaped arms, or in a continuous screw mixer. These kneading means are preferably equipped with a vacuum evacuation device and a heating and cooling device making it possible to adjust the temperature of the mixer and its contents, in order to accelerate, for example, the dissolution operations. , or to control the temperature of the solution being formed.

By way of example, the following procedure can be used:

Cellulose powder (the humidity of which is in equilibrium with the ambient humidity of the air) is introduced into a double-jacket mixer, comprising Z-shaped arms and an extrusion screw. A mixture of orthophosphoric acid (99% crystalline) and formic acid is then added, containing for example three quarters of orthophosphoric acid per quarter of formic acid (parts by weight). The whole is mixed for a period of approximately 1 to 2 hours for example, the temperature of the mixture being maintained between 10 and 20 ° C., until a solution is obtained.

The spinning solutions thus obtained are ready to spin, they can be transferred directly, for example by means of an extrusion screw placed at the outlet of the mixer, to a spinning machine to be spun there, without further processing. prior to usual operations such as degassing or filtration steps for example.

The spinning solutions used for implementing the invention are optically anisotropic solutions. Preferably, these spinning solutions have at least one of the following characteristics:

- Their cellulose concentration is between 15% and 25% (% by weight), calculated on the basis of a non-esterified cellulose;

- their concentration of total formic acid (i.e. the part of formic acid consumed for esterification plus the part of free formic acid remaining in the final solution) is between 10% and 25% (% in weight) ;

- their concentration of phosphoric acid (or liquid based on phosphoric acid) is between 50% and 75% (% by weight); the degree of substitution of the cellulose for formate groups in the solution is between 25% and 50%, more preferably between 30% and

45%;

- The degree of polymerization of the cellulose, in solution, is between 350 and 600;

- they contain less than 10% water (% by weight). II-2. Spinning solutions

The spinning solutions are spun according to the so-called "dry-jet-wet-spinning" technique: this technique uses a non-coagulating fluid layer, generally air, placed at the outlet of the die, between the die and the means of coagulation.

At the outlet of the mixing and dissolving means, the spinning solution is transferred to the spinning block where it feeds a spinning pump. From this spinning pump, the solution is extruded through at least one die, preceded by a filter. It is during the journey to the die that the solution is gradually brought to the desired spinning temperature, generally between 35 ° C and 90 ° C, depending on the nature of the solutions, preferably between 40 ° C and 70 ° C. The term "spinning temperature" therefore means the temperature of the spinning solution at the time of its extrusion through the die.

Each die can include a variable number of extrusion capillaries, this number being able to vary for example from 50 to 1000. The capillaries are generally of cylindrical shape, their diameter being able to vary for example from 50 to 80 μm

(micrometers).

At the outlet of the die, a liquid extnidate is therefore obtained which consists of a variable number of elementary liquid veins. Each elementary liquid vein is stretched (see below spinning drawing factor) in a non-coagulating fluid layer, before entering the coagulation zone. This non-coagulating fluid layer is generally a layer of gas, preferably air, the thickness of which can vary from a few mm to several tens of mm (millimeters), for example from 5 mm to 100 mm, depending on the conditions. specific spinning; in known manner, the thickness of the non-coagulating layer is understood to mean the distance separating the underside of the die, arranged horizontally, and the entry to the coagulation zone (surface of the coagulating liquid).

After crossing the non-coagulating layer, all the liquid veins thus stretched enter the coagulation zone and come into contact with the coagulating medium. Under the action of the latter, they are transformed, by precipitation of the cellulose formate and extraction of the spinning solvent, into solid filaments of cellulose formate which thus form a fiber. The coagulating medium used is acetone.

The temperature of the coagulating medium, noted Tc, is not a critical parameter for the implementation of the invention. By way of example, for spinning solutions containing 22% by weight of cellulose, it has been observed that a variation in temperature Tc, in the entire temperature range from -30 ° C to 0 ° C, does not had virtually no effect on the mechanical properties of the fibers obtained.

We will preferably choose a negative temperature Tc, that is to say less than 0 ° C, and even more preferably less than -10 ° C.

Those skilled in the art will be able to adjust the temperature of the coagulating medium as a function of the characteristics of the spun solution, and of the targeted mechanical properties, by simple optimization tests. In general, the temperature Tc will be chosen the lower the lower the concentration C of the spinning solution.

The level of spinning solvent in the coagulating medium is preferably stabilized at a level of less than 15%, even more preferably less than 10% (% by weight of coagulating medium).

The coagulation means to be used are known devices, composed for example of baths, pipes and / or cabins, containing the coagulating medium and in which the fiber circulates during formation. It is preferable to use a coagulation bath placed under the die, at the outlet of the non-coagulating layer. This bath is generally extended at its base by a vertical cylindrical tube, called "spinning tube", through which the coagulated fiber passes and the coagulating medium circulates.

The depth of the coagulating medium in the coagulation bath, measured from the entry of the bath to the entry of the spinning tube, can vary from a few millimeters to a few centimeters for example, depending on the particular conditions of implementation of

the invention, in particular according to the spinning speeds used. The coagulation bath can be extended if necessary by additional coagulation devices, for example by other baths or cabins, placed at the outlet of the spinning tube, for example after a horizontal deflection point.

Preferably, the method of the invention is implemented so that at least one of the following characteristics is verified: a) the rate of residual solvent in the fiber, at the outlet of the coagulation means (denoted Rs) , is less than 100% by weight of dry formate fiber; b) the stress of tension undergone by the fiber, at the exit of the means of

coagulation (noted σ _c ), is less than 5 cN / tex, and, even more preferably, so that the two characteristics a) and b) above are simultaneously verified.

Thus, according to the above preferential conditions, the fiber is left in contact with the coagulating medium until a significant portion of spinning solvent is extracted from the fiber. On the other hand, during this coagulation phase, an effort is made to maintain the tensions undergone by the fiber at a moderate level: to control this, these tensions will be measured immediately at the output of the coagulation means, using tensiometers appropriate.

In general, if it is desired above all to favor the elongation properties at break of the formate fibers. the invention will preferably be implemented so that the following two relationships are verified:

Rs <50%; σ _c <2 cN / tex.

For the measurement of the residual solvent level Rs present in the coagulated formate fiber, the procedure is for example as follows: fiber is taken at the outlet of the coagulation means, with its coagulating medium; then it is wiped on the surface with absorbent paper, without pressure, so as to eliminate most of the coagulating medium (acetone) which is contained in the surface layer surrounding the fiber, and which itself contains a certain fraction of solvent of spinning (phosphoric acid or liquid based on phosphoric acid) already extracted from the fiber; the fiber is then washed completely with water, in a laboratory device, so as to completely extract the phosphoric acid which it contains, then this phosphoric acid is titrated in return with sodium hydroxide; for more precision, the measurement is repeated 5 times and the average is calculated.

At the outlet of the coagulation means, the fiber is taken up on a device

drive, for example on motorized cylinders. The speed of the spun product, on this drive device, is called "spinning speed" (or also calling or driving speed): it is the speed of travel of the fiber through the spinning installation. , once the fiber is formed. The ratio between the spinning speed and the speed of extrusion of the solution through the die, defines what is known, in known manner, the drawing stretch factor (abbreviated as FEF), which is for example between 2 and 10.

Once coagulated, the fiber must be washed until neutral. By "neutral washing" is meant any washing operation making it possible to extract all or almost all of the spinning solvent from the fiber.

Those skilled in the art have naturally hitherto been inclined to use water as a washing medium: in a well-known manner, water is indeed the "natural" swelling medium of the fibers of cellulose or of cellulose derivatives (see for example example US-A-4,501,886), and therefore the medium likely to offer, a priori, the best washing efficiency. By way of example, the patents or patent applications EP-B-220642, US-A-4,926,920, WO 94/17136, like the aforementioned application WO 85/05115 (page 72, examples II-1 and following) , describe the use of water, at the outlet of the coagulation means, for washing fibers of cellulose formate.

However, such a conventional step of washing with water does not make it possible to obtain fibers of cellulose formate in accordance with the invention.

Quite surprisingly, the Applicant has found that acetone used as a washing medium, despite a washing power which is, in known manner, significantly lower than that of water, leads to fibers which have a once completed (ie washed until neutral, then dried), very markedly improved properties, as regards firstly their elongation at break, when they are compared to the fibers described in application WO 85/05115.

For the implementation of the process of the invention, the step of coagulating the fiber and the step of neutral washing of the coagulated fiber must both be carried out in acetone.

The temperature of the washing acetone is not a critical parameter of the process. It goes without saying, however, that excessively low temperatures are avoided so as to favor the washing kinetics. Preferably, the temperature of the washing acetone, denoted T1, will be chosen positive (by this is meant a temperature equal to or greater than 0 ° C), and even more preferably greater than + 10 ° C. Advantageously, it is possible to use uncooled acetone, that is to say acetone at room temperature, the washing operation then preferably being carried out in a controlled atmosphere.

Known washing means can be used, consisting for example of baths containing washing acetone and in which the fiber to be washed flows. The washing times in acetone can vary, typically, from a few seconds to a few tens of seconds, depending on the particular conditions of implementation of the invention.

Of course, both the washing medium and the coagulating medium may contain constituents other than acetone, without the spirit of the invention being modified, provided that these other constituents are present only in minor proportion. ; the total proportion of these other constituents will preferably be less than 15%, more preferably less than 10% (% by total weight of coagulating medium or of washing medium). More particularly, if water is present in the coagulation or washing acetone, its content will preferably be less than 5%.

After washing, the cellulose formate fiber is dried by any suitable means, in order to remove the washing acetone. Preferably, the level of acetone at the outlet of the drying means is adjusted to a rate of less than 1% by weight of dry fiber. For the drying operation, one can operate for example by continuously scrolling the fiber on heating rollers, or use, as a main or

complementary, a previously heated nitrogen blowing technique. Preferably, a drying temperature at least equal to 60 ° C is used, more preferably between 60 ° C and 90 ° C.

The method of the invention can be implemented in a very wide range of spinning speeds, which can vary from several tens to several hundred meters per minute, for example to 400 m / min or 500 m / min, or even more . Advantageously, the spinning speed is at least equal to 100 m / min, more preferably at least equal to 200 m / min.

If it is desired to isolate the fiber in cellulose formate, that is to say not to regenerate it immediately, in particular to control its mechanical properties before the regeneration operations, the washing step will preferably be carried out so as to that the level of residual spinning solvent in the finished fiber, ie washed and dried, does not exceed 0.1% to 0.2% by weight, relative to the weight of dry fiber.

The cellulose formate fiber thus spun can also be sent directly to the regeneration means, online and continuously, in order to prepare a regenerated cellulose fiber.

II-3. Regeneration of formate fibers

In a known manner, a process for regenerating a fiber into a cellulose derivative consists in treating this fiber in a regenerating medium so as to eliminate almost all of the substituent groups (so-called saponification treatment), in washing the fiber thus regenerated, then to dry it, these three operations being in principle carried out continuously on the same treatment line called "regeneration line".

Concerning cellulose formate, the regenerating medium usually used is an aqueous solution of soda (sodium hydroxide NaOH) weakly concentrated, containing only a few% soda (% by weight), for example from 1 to 3% (see for example PCT / AU91 / 00151).

Low concentrated aqueous sodium hydroxide solutions, of sodium hydroxide concentration not exceeding 5% (% by weight), have also been described in patents or patent applications EP-B-220642, US-A-4,926,920, WO 94 / 17136 and WO 95/20629 for the regeneration of cellulose formate fibers. They have been used by the applicant for the regeneration of the cellulose formate fibers described in the aforementioned application WO 85/05115, as for the regeneration of the cellulose formate fibers of the present invention; these weakly concentrated solutions turn out to be entirely satisfactory for conducting the regeneration proper, that is to say for eliminating almost all of the substituent formate groups: they make it possible to obtain regenerated fibers without difficulty, the degree of substitution in formate groups is less than 2%. In an attempt to increase the sodium hydroxide concentrations beyond 5%, the Applicant has found that the filaments of the cellulose formate fibers (whether or not they conform to the invention) undergo partial, superficial dissolution, as soon as that the sodium hydroxide concentration reached and exceeded approximately 6% by weight, the regenerating medium then becoming a true solvent for cellulose formate. Such dissolution, even partial, is completely detrimental to the mechanical properties of the fiber: presence of bonded filaments, drop in resistance of the attacked filaments, difficulties in washing the fiber, etc.

Such problems of parasitic dissolution were moreover foreseeable, knowing for example that cellulosic fibers of the viscose type are partially or completely soluble in 10% sodium hydroxide (see PH Hermans, "Physics and Chemistry of Cellulose Fibers", lst part , Elsevier 1949), or that 5% of native cellulose dissolve in an aqueous solution of 8 to 10% of NaOH (see T. Yamashiki, Journal of Applied Polymer Science, vol. 44, 691-698, 1992).

Given the different elements above, the skilled person was therefore quite naturally inclined to use aqueous solutions of weakly concentrated sodium hydroxide, for the regeneration of fibers in cellulose formate.

However, by continuing to increase the sodium hydroxide concentration of the regenerating medium well beyond the above 5 to 6%, the Applicant has found, quite surprisingly, that beyond a certain concentration threshold, not only the parasitic dissolution phenomena disappeared, but still and above all that certain properties of the regenerated fiber were improved very appreciably, in particular the elongation at break and the energy at break.

In other words, if a conventional regenerating medium (ie weakly concentrated in sodium hydroxide) is certainly quite sufficient to regenerate fibers of cellulose formate, such a medium does not however make it possible to obtain the fibers of regenerated cellulose conforming to the invention.

The process of the invention, for obtaining a regenerated cellulose fiber according to the invention, by regeneration of a cellulose formate fiber, is characterized in that the regenerating medium is an aqueous solution of highly concentrated sodium hydroxide, the concentration of soda, noted Cs, is greater than 16% (% by weight).

Preferably, a concentration Cs greater than 18% is used, and even more preferably, a concentration of between 22% and 40%; it has in fact been found that such concentration ranges were, as a general rule, more particularly beneficial to the elongation at break of the regenerated fiber, the optimal concentration range being between 22% and 30%. For the implementation of the regeneration process of the invention, it is preferably started from a cellulose formate fiber according to the invention, having in particular an elongation at break Ar greater than 6%.

The regeneration line consists concretely, and conventionally, of regeneration means, followed by washing means, themselves followed by drying means. All these devices are not critical for the implementation of the invention, and those skilled in the art will be able to define them without difficulty. The regeneration and washing means may consist in particular of baths, pipes, tanks, cabins, in which the regenerating medium or the washing medium circulate. It is possible, for example, to use cabins each equipped with two motorized cylinders around which the fiber to be treated is wound, this fiber then being showered with the liquid medium used (regenerating or washing).

The residence times in the regeneration means will of course have to be adjusted so as to regenerate the formate fibers substantially, and thus to verify the following relationship on the final regenerated fiber:

0 <D _S <2.

Those skilled in the art will know how to adjust these residence times, which, depending on the particular conditions for implementing the invention, can vary for example from 1 to 2 seconds up to 1 to 2 tens of seconds.

The washing medium is preferably water. Indeed, after the operation of

regeneration above, the cellulose fiber can be washed with its natural swelling medium, that is to say with water, the latter having the best washing efficiency. Water is used at room temperature, or at a higher temperature, if necessary, to increase the washing kinetics. To this washing water can optionally be added a neutralizing agent for the soda not consumed, for example formic acid.

The drying means may consist, for example, of ventilated heating tunnels through which the washed fiber circulates, or alternatively in heating cylinders on which the fiber is wound. The drying temperature is not critical, and can vary over a wide range, in particular from 80 ° C. to 240 ° C. or more, depending on the particular conditions of implementation of the invention, in particular according to the speeds. of passage on the regeneration line. Preferably a temperature not exceeding 200 ° C is used.

At the outlet of the drying means, the fiber is taken from a take-up reel, and its residual moisture level is checked. Preferably, the drying conditions (temperature and duration) will be adjusted so that the residual humidity level is between 10% and 15%, even more preferably of the order of 12% to 13% by weight of dry fiber. Typically, the washing and drying times required vary from a few seconds to a few tens of seconds, depending on the means employed and the particular conditions for carrying out the invention.

During the passage through the regeneration line, of course excessive tension will be avoided so as not to damage the fiber on the one hand, and on the other hand not to lose a significant part of the elongation at break potential offered by the use of regenerating medium concentrated in soda. These voltages are generally difficult to access inside the various means used: they can be checked and measured at the input of these different means, using suitable tensiometers.

Thus, if it is desired to favor the elongation at break of the regenerated fiber, the tension stresses at the input of the regeneration means, washing means and drying means, will preferably be chosen lower. at 10 cN / tex. and even more preferably less than 5 cN / tex.

In real industrial regeneration conditions, and in particular for high regeneration rates, the lower limits of these tension constraints are generally around 0.1 to 0.5 cN / tex, lower values being unrealistic of an industrial point of view, and even undesirable. It was noted in particular that the mechanical properties of the regenerated fibers could be adjusted more or less by playing on these tension constraints.

The regeneration speed (denoted Vr), that is to say the speed of passage of the fiber through the regeneration line, can vary from several tens to several hundred meters per minute, for example up to 400 or 500 m / min or more; advantageously, this speed Vr is at least equal to 100 m / min, more preferably at least equal to 200 m / min.

Finally, the regeneration method of the invention is preferably implemented in line and continuously with the spinning method of the invention, in such a way that the entire production chain, from the extrusion of the solution through the until the regenerated fiber is dried, or uninterrupted.

III. EXAMPLES OF EMBODIMENT OF THE INVENTION

The tests described below can be either tests in accordance with the invention, or tests not in accordance with the invention.

III- 1. FIBERS IN THE FORM OF CELLULOSE

A) Fibers in accordance with the invention (Table 1):

A total of 14 spinning tests of cellulose formate fibers are carried out, according to the spinning process of the invention, and in particular conforming to the

 indications provided in paragraphs II-1 and II-2 above.

The coagulation step and the neutral washing step of the coagulated fiber are both carried out in acetone.

Table 1 gives both the specific conditions for carrying out the process of the invention, and the properties of the fibers obtained.

The abbreviations and the units used in this table 1 are as follows:

Test No .: test number (referenced from A-1 to A-14);

 N: number of filaments of the fiber;

 C: cellulose concentration in the spinning solution (% by weight);

 DP: degree of polymerization of the cellulose in the spinning solution;

 Vf: spinning speed (in m / min);

 Tc: temperature of the coagulating medium (in ° C);

 Rs: rate of residual solvent in the fiber at the outlet of the coagulation means (% by weight);

σ _c : tension stress undergone by the fiber at the exit of the coagulation means (in cN / tex);

 Ti: fiber title (in tex):

 Te: fiber tenacity (in cN / tex);

 Mi: initial fiber modulus (in cN / tex);

 Ar: elongation at break of the fiber (in%);

 Er: energy at break of the fiber (in J / g);

 Ds: degree of substitution of cellulose for formate groups. in fiber (in%);

In addition, the following special conditions are used to carry out these tests:

- all spinning solutions are prepared from powdered cellulose (initial water content equal to about 8% by weight, degree of polymerization between 500 and 600), formic acid and orthophosphoric acid (each containing about 2.5% by weight of water);

- these solutions contain (% by weight) from 16 to 22% of cellulose, from 60 to 65% of phosphoric acid, and from 18 to 19% of formic acid (total), the weight ratio (formic acid / phosphoric acid ) initial being approximately 0.30;

- These solutions are optically anisotropic, and contain in total less than 10% water (% by weight);

the degree of substitution of the cellulose in the solutions is between 40 and 45% for the solutions containing 16% by weight of cellulose, between 30 and 40% for the other more concentrated solutions;

- the channels include 500 or 1000 capillaries of cylindrical shape, with a diameter of 50 or 65 μm;

- the spinning temperatures are between 40 and 50 ° C;

- the FEF values are between 2 and 6 (between 2 and 4 for tests A-1, A-5 to A-9, A-14; between 4 and 6 for other tests);

the non-coagulating fluid layer consists of an air layer (thickness varying from 10 to 40 mm depending on the tests);

the level of phosphoric acid in the coagulating medium is stabilized at a level of less than 10% (% by weight of coagulating medium);

- the temperature of the washing acetone (Tl) is always positive, between 15 and 20 ° C;

- the drying of the fiber is carried out at 70 ° C, by passage over heating cylinders, with in addition a blowing of nitrogen heated to 80 ° C; the acetone level at the outlet of the drying means is less than 0.5% (% by weight of dry fiber);

- on the finished fiber, ie washed and dried. the residual phosphoric acid level is less than 0.1% (% by weight of dry fiber).

On reading Table 1, it is noted in particular that with the exception of test A-13, the temperature Tc of the coagulation acetone is always negative, lower than -10 ° C. in the majority of cases.

The DP of the cellulose, in the solution, is between 400 and 450, which shows in particular a low depolymerization after dissolution.

It is further noted that for all the tests in Table 1, at least one of the following preferential conditions is satisfied:

Rs <100%; σ _c <5 cN / tex, and that these two relationships are simultaneously verified in the majority of cases.

Even more preferably, the following two relationships are verified simultaneously:

Rs <50%; σ _c <2 cN / tex,

On the other hand, the spinning speeds are high, since most of them are equal to 150 m / min.

All the mechanical properties indicated in table 1 are mean values calculated over 10 measurements, with the exception of the titer (mean over 3 measurements), the standard deviation over the mean (in% of this mean) being

generally between 1 and 2.5%.

On reading Table 1, it can be seen that all of the fibers verify the following relationships:

- Ds≥ 2;

 - Te> 45;

 - Mi> 800;

 - Ar> 6;

 - Er> 13.5.

Preferably, for the cellulose formate fibers of the invention, the values of Ds are between 25 and 50%. It can be seen that in these examples, they are all between 30 and 45%: in practice, they are identical to the values of degrees of substitution measured on the spinning solutions.

corresponding.

Preferably, their elongation at break Ar is greater than 7% (examples A-4 to A-6), even more preferably greater than 8% (examples A-5 and A-6). In addition, these fibers in Table 1 mostly verify the following preferential relationships:

Te> 60; Mi> 1200; Er> 20.

Even more preferably, at least one of the following relationships is verified:

Te> 70; Mi> 1500; Er> 25.

For all the examples in Table 1, we also note that the following relation is verified:

Mid <1800.

However, particularly high initial modulus values, for example between 1800 and 2200 cN / tex, or even more, are still accessible on the formate fibers according to the invention, normally at the expense of elongation at break, in adapting the parameters of the spinning process according to the invention. This could be achieved in particular by increasing the tension stresses on the spinning line, for example at the outlet of the coagulation means, during washing or during the drying of the fiber; it was also observed that the use of relatively high concentrations C, in particular between 24 and 30%, were favorable for obtaining initial moduli and very high toughness. B) Fibers not in accordance with the invention (Table 2):

5 spinning tests are carried out (referenced from B-1 to B-5) of cellulose formate fibers, according to a spinning process not in accordance with the invention.

The general and specific conditions used for spinning are the same as those used for the fibers of table 1 above, with one exception: the neutral washing step of the coagulated fiber is carried out with water (as in the request WO 85/05115 cited above), and not with acetone. This washing water is industrial water, at a temperature close to 15 ° C. On the other hand the fibers contain 250 to 1000 filaments.

Table 2 gives both the specific conditions for carrying out the process of the invention, and the properties of the fibers obtained. The abbreviations and the units used in this table 2 are the same as for the previous table 1.

It is noted that these fibers of table 2, spun according to the method taught by the aforementioned application WO 85/05115, can have quite interesting characteristics of tenacity and initial modulus; in particular, after a conventional regeneration step according to the prior art (weakly concentrated aqueous NaOH solution), they can be transformed into regenerated fibers having very high toughness (110 to 120 cN / tex, or even more) combined with very high initial modulus values (3000 to 3500 cN / tex, or even more).

However, none of these fibers in Table 2 is in accordance with the invention, the following relationship not being verified:

Ar> 6.

III-2. REGENERATED CELLULOSE FIBERS

A) Fibers in accordance with the invention (Table 3):

A total of 23 regeneration tests of cellulose formate fibers are carried out, in accordance with the regeneration process of the invention, according to the indications provided in paragraph II-3 above.

All these regeneration tests are carried out online and continuously with the spinning operation, the latter being carried out in accordance with the spinning process of the invention: in particular, the coagulation step and the neutral washing step of the Coagulated fiber are both made in acetone.

The regenerating medium is an aqueous sodium hydroxide solution, the Cs concentration of which is in all cases greater than 16%.

Table 3 gives both specific conditions for carrying out the process of the invention, and the properties of the fibers obtained.

The abbreviations and the units used in this table 3 are as follows:

Test No .: test number (referenced from C-1 to C-23);

 N: number of filaments of the regenerated fiber;

 Cs: concentration of sodium hydroxide in the regenerating medium (% by weight);

 Vr: regeneration speed (in m / min);

T _I : title of the fiber (in tex);

T _E : tenacity of the fiber (in cN / tex);

M _I : initial modulus of the fiber (in cN / tex);

A _R : elongation at break of the fiber (in%);

E _R : energy at break of the fiber (in J / g); In addition, the following special conditions are used to carry out these tests:

- The starting cellulose formate fibers, a sample of which (a few hundred meters) was systematically taken at the outlet of the spinning means, to check their mechanical properties, all conform to the invention; in particular, they all have a

 elongation at break greater than 6%;

- the regenerating medium used is at room temperature (about 20 ° C);

- The regeneration, washing, and drying means consist of cabins equipped with motorized cylinders on which the fiber to be treated is wound;

- the regeneration being carried out in line and continuously with the spinning, the regeneration speed Vr indicated in table 3 (from 55 to 200 m / min) is therefore equal to the spinning speed Vf;

- washing is carried out with industrial water at a temperature of about 15 ° C;

the drying of the washed fiber is carried out on heating cylinders, at different temperatures varying from 80 ° C to 240 ° C, according to the specific scheme below: from 80 ° C to 120 ° C for tests C-2, C-3, C-5, C-10, C-17; at 240 ° C for test C-11; from 160 ° C to 190 ° C for the other tests;

- the stresses of tension measured at the entry of the means of regeneration, washing, and drying are always lower than 10 cN / tex, in the majority of the cases lower than 5 cN / tex, except for tests C-7, C-9, Cl 5 where a tension equal to or greater than 5 cN / tex has been measured at the input of at least one of the above means; these stresses of tension are lower than 2 cN / tex with each entry of the three means stated above

 (regeneration, washing and drying) for a large number of tests: C-2 to C-5, C-10 to C-11, C-13 to C-14, C-16 to C-23;

the residence times in the regeneration means are of the order of

15 s, as in the washing means, while they are of the order of 10 s in the drying means;

- At the outlet of the drying means, the fibers have a residual moisture content of the order of 12% to 13% (% by weight of dry fiber).

A measure of the degree of substitution, as indicated in paragraph I-2.2, has shown that all of the fibers in Table 3 have a Dg value between 0 and 2%, in the vast majority of cases between 0, 1 and 1%.

As for the previous results, all the mechanical properties indicated in table 3 are mean values calculated over 10 measurements, with the exception of the titer (mean over 3 measurements), the standard deviation over these different means (in% of the average) being generally between 1 and 2.5%.

It can be seen that the regenerated fibers of Table 3 verify all of the following relationships:

- T _E >60;

- M _I >1000;

- A _R >6;

- E _R > 17.5.

Preferably, their elongation at break A _R is greater than 7% (examples C-4 to C-11, C-13 to C-16, C-19 and C-20), even more preferably greater than 8% (example C-4).

The best elongation value at break (A _R = 8.4% for test C-4) was notably obtained by spinning and online regeneration of a solution containing 16% by weight of cellulose and whose DP was about 420.

The corresponding formate fiber sample, taken at the spinning outlet to measure the mechanical properties, showed the following properties:

Ds = 40; Te = 60; Mi = 1290; Ar = 8.4; Er = 25.3.

Furthermore, the vast majority of the fibers in Table 3 verify the following relationships:

T _E >80; M _I >1500; E _R > 25, a large number of them verifying at least one of the following relationships:

T _E >100; M _I >2000; E _R > 30.

Particularly high toughness is noted (equal to or greater than 100 cN / tex) in the case of tests C-1, C-7, C-18, C-21 and C-22, combined with high values of elongation and energy at break, or even at high initial modulus values, greater than 2400 cN / tex in the case of tests C-18, C-21 and C-22. For all the examples in Table 3, we also note that the following relation is verified:

M _I <2600.

However, particularly high initial modulus values, for example between 2600 and 3000 cN / tex, are still accessible on the regenerated fibers according to the invention, normally at the expense of elongation at break, by adapting the parameters of the regeneration process according to

 the invention. This can be achieved in particular by increasing the stress constraints on the regeneration line, or by selecting starting fibers (in cellulose formate) already having particularly high values of initial modulus, for example between 1800 and 2200 cN / tex .

If for the majority of the examples in Table 3, the filamentary title (title of the fiber T _I divided by the number N of filaments) is equal to approximately 1.8 dtex (decitex) (most common filamentary title for cellulosic fibers) , the latter can vary to a large extent, for example from 1.4 dtex to 4.0 dtex, or even more, by adjusting the spinning conditions in known manner. For example, the regenerated fibers of tests Cl 9 and C-20 have, respectively, a

2.9 dtex and 3.6 dtex filamentary. In general, an increase in the elongation at break A _R has been observed, combined with a decrease in the toughness T _E and of the initial modulus M _I , when the filamentary title increases. B) Fibers not in accordance with the invention (table 4):

A total of 9 regeneration tests of cellulose formate fibers (referenced from D-1 to D-9) are carried out, according to a regeneration process not in accordance with the invention.

The regeneration conditions are the same as those used for the fibers in accordance with the invention of table 3 above, with one exception: the regenerating medium is an aqueous sodium hydroxide solution whose Cs soda concentration is at most equal to 16% .

Table 4 gives both the specific conditions for carrying out the process of the invention, and the properties of the fibers obtained. The abbreviations and the units used in this table 4 are the same as for the previous table 3.

All the fibers obtained are well regenerated, insofar as, after control, the values of degree of substitution D _S are always less than 2%, more precisely between 0.1% and 1.0%.

These fibers of Table 4 may have particularly high characteristics of toughness and initial modulus (see in particular D-7 to D-9), but it can be seen that none of them is in accordance with the invention, the relationship following is not verified:

A _R > 6.

In Examples D-4 and D-5 (Cs = 6% and 12%), it was observed in particular a partial dissolution on the surface of the filaments, leading to the presence of married filaments, to a poor general state of the fiber causing very great difficulties in achieving a neutral wash. In example D-6, the same phenomena were encountered, but to a lesser degree: here we are at the limits of the process of the invention (Cs = 16%), and we note in particular a very long elongation at break close to 6%.

A comparison of Examples D-3 and C-12 (Table 3) turns out to be quite interesting, insofar as the regeneration operations were carried out on the same fiber in cellulose formate and, with the exception of the concentration of sodium hydroxide in the regenerating medium (3% for test D-3, 30% for test C-12), under strictly identical particular conditions.

In fact, it can be seen that, compared with a conventional regeneration with a weakly concentrated soda solution (test D-3), the process of the invention (test C-12) made it possible to very significantly improve the toughness values. (18% increase), elongation at break (33% increase), energy at break (55% increase), without any significant change in the initial modulus value.

All the fibers of Tables 1 to 4 above, in cellulose formate or in regenerated cellulose, whether or not they conform to the invention, have a structure and a morphology typical of products spun from a liquid crystal solution , as described in particular in basic application WO 85/05115.

In particular, when studying their filaments with an optical microscope or a scanning electron microscope, a morphology is observed such that each filament is constituted at least in part by layers nested one inside the other surrounding the axis of the filament; it is further noted that in each layer, in general, the optical direction and the direction of crystallization vary quasi-periodically along the axis of the filament. Such a structure or morphology is commonly described in the literature under the name of "band structure". C) Other properties of the regenerated cellulose fibers in accordance with the invention - Use in tires:

In addition to the improved mechanical properties described above, the regenerated cellulose fibers of the invention have many other advantages when compared with the fibers described in the aforementioned basic application WO 85/05115 on the one hand, with conventional fibers of the type radiates on the other hand.

C-1. Comparison with regenerated cellulose fibers according to WO 85/05115:

Compared with the fibers described in basic application WO 85/05115, the fibers of the invention notably have a very significantly improved fatigue resistance, both in laboratory test and in pneumatic rolling.

Compression endurance (laboratory test):

For technical fibers, intended in particular for reinforcing tire structures, the fatigue resistance can be analyzed by subjecting assemblies of these fibers to various known laboratory tests, in particular to the fatigue test known under the name of "Disc Fatigue Test "(see for example US 2,595,069, standard ASTM D885-591 revised 67T).

This test, which is well known to those skilled in the art (see for example US 4,902,774), essentially consists in incorporating plied fibers of the fibers to be tested, previously glued, in rubber blocks, then, after baking, to tire the gum test tubes. thus formed in compression, between two rotating discs, a very large number of cycles (for example between 100,000 and 1,000,000 cycles). After fatigue, the plies are extracted from the test pieces and their residual breaking strength is compared to the breaking strength of control plies extracted from non-tired test pieces.

The fibers of the invention, compared to the fibers of basic application WO 85/05115, systematically show clearly improved endurance on the "Disc Fatigue Test".

By way of example, fibers according to the invention having a preferred elongation at break greater than 7%, as well as fibers according to application WO 85/05115, all having an elongation at break less than 5%, have been assembled to form plied (type "A" and "B", respectively) having the same formula 180x2 (tex) 420/420 (t / m).

In known manner, such a formula means that each ply consists of two yarns (multifilament fibers), each having a count of 180 tex before twisting, which are first twisted individually at 420 rpm in one direction during a first step, then twisted the two together at 420 rpm in the opposite direction during a second step. For such a plied twist, the helix angle is approximately 27 ° and the coefficient of torsion (or torsional factor) K is approximately 215, with:

K = Twisted twist (in t / m) × [Twisted title (in tex) / 1520] ^1/2 .

(cellulose density: 1.52)

Several twists of type "A" (according to the invention) and of type "B" (according to WO 85/051 15) were subjected to the "Disc Fatigue Test" above (6 hours at 2700 cycles / min, with a maximum compression rate of the test piece of approximately 16% in each cycle); the following force-breaking lapses were recorded on the extracted twists (data in relative values, with a base 100 for the maximum lapsing recorded on a type “B” plied):

- plied type "A": 25 to 40;

 - plied type "B": 70 to 100.

The fatigue resistance of the regenerated fibers of the invention is therefore significantly improved - by a factor of two to three on average - compared to the regenerated fibers of the initial application WO 85/05115.

Tire endurance:

The capacity of technical fibers to reinforce tires can be analyzed, in a known manner, by reinforcing a rubber sheet with plies of the fibers to be tested, previously glued, by incorporating the fabric thus formed in a tire structure, for example in a carcass ply reinforcement, then subjecting the tire thus reinforced to a rolling test.

Such rolling tests are widely known to those skilled in the art, they can for example be implemented on automatic machines making it possible to vary a large number of parameters (pressure, load, temperature, etc.) during rolling. . After rolling, the plies are extracted from the tire tested, and their residual breaking strength is compared to that of control plies extracted from control tires which have not been rolled.

It has been found that the fibers of the invention, when used to reinforce a radial tire carcass, show an endurance which is significantly improved compared to the fibers according to WO 85/05115. In particular, it has been observed that where fibers according to the prior art did not resist (rupture of the plies of type "B" above), due to particularly severe driving conditions, the fibers of the invention

(twisted type "A" above) showed almost no lapse, even after several tens of thousands of kilometers.

C-2. Comparison with conventional rayon type fibers:

In addition to their significantly higher mechanical properties in extension, the regenerated fibers of the invention have other quite advantageous characteristics compared to conventional rayon fibers.

Moisture resistance:

The resistance to humidity of cellulosic fibers can be analyzed using various known tests, a simple test consisting for example of completely soaking the fibers in a water bath, for a determined time, then measuring the strength- rupture of the fibers in the wet state, by pulling them immediately out of the water bath after having simply drained them.

After 24 hours of storage in water at room temperature, it is found that the breaking strength in the wet state, for the fibers of the invention, represents 80 to 90%, depending on the case, of the nominal breaking strength (ie in the dry state, measured as indicated in paragraph I-4.). For rayon fibers, it only represents around 60% of the nominal breaking strength.

The fibers of the invention are therefore much less sensitive to moisture than conventional rayon fibers, they have better dimensional stability in a humid environment.

Mechanical properties on plied:

The fibers of the invention can be assembled, as described

 previously, to form reinforcement assemblies with high or very high mechanical properties, in particular plies, the

 construction can be adapted to a very large extent depending on the intended application. We know for example that an increase in torsion, i.e. in the helix angle, generally improves the endurance of the plied, increases its elongation at break, while being however

 detrimental to its toughness and its extension module.

Even for very high twists, corresponding for example to a helix angle of the order of 29-30 °, which give the twists excellent endurance properties, the fibers of the invention, in the twisted state , have a toughness which is even greater than the tenacity of untwisted rayon fibers. By way of example, the plies according to the invention, prepared according to known twisting methods from the fibers of the invention, exhibit, when the helix angle of the plies is varied from 20 to 30 degrees, a toughness which can vary from 75-80 cN / tex up to 45-50 cN / tex, for example a toughness of the order of 58-66 cN / tex for a helix angle of 23-24 ° (K = 180 approximately), or 53-57 cN / tex for a helix angle of 26-27 ° (K = 215 approximately), as well as an elongation at break which can reach values close to 10% , or even higher.

Thus, the tenacities of the plies according to the invention, with equivalent torsion (same helix angle), are generally much greater than the tenacities on plies which can be obtained from fibers of the rayon type whose tenacity hardly exceeds, so known, 45-50 cN / tex before twisting. They can therefore be used in a smaller quantity in articles usually reinforced with conventional rayon fibers.

Tire endurance:

For real running conditions, used on passenger vehicles fitted with tires of size 165/70 R 13, it was unexpectedly found that fibers of the invention (despite a clearly more rigid and more crystallized structure since '' they come from a liquid-crystal phase), revealed throughout the running tests (for example checks every 5,000 km, from 20,000 to 80,000 km) an endurance identical to that of a conventional rayon fiber , for an identical twisted construction.

Extension modules:

The fibers of the invention, the primary characteristic of which is a

improved elongation at break, have an initial modulus which remains quite high (for example 1500 to 2600 cN / tex approximately in table 3), in all cases very much higher than that of conventional rayon fibers (1000 cN / tex approximately, in a known manner).

This superiority of the fibers of the invention in terms of modulus, which is of course found on the reinforcement assemblies of these fibers, can be quite advantageous for articles usually reinforced with conventional rayon technical fibers, by offering such the possibility of improved dimensional stability: indeed, for the same variation Δ (F) of the load or force "F" acting on an assembly of each type, the assembly according to the invention will undergo a variation Δ (A) of length or elongation "A" much less. In conclusion, a comparison of the results of the invention with those described in application WO 85/05115, both for the fibers of cellulose formate and for the fibers of regenerated cellulose, shows that the invention has not only made it possible to increase very significantly the elongation at break values, which are more than doubled in some cases, but still maintain the toughness values at a very high level, or even improve them in many cases.

The advantage of such a result must be particularly emphasized.

The improvement brought by the invention does not consist in a simple displacement towards another optimum of a given combination [tenacity-elongation at break], with an energy at break remaining substantially the same (total surface under the curve of traction Force - Elongation remaining substantially constant); it consists in fact of a very appreciable improvement in any combination [toughness-elongation at break], making it possible to "extend" the force-elongation curves obtained for the fibers of the initial application WO 85/05115, and thus obtaining a very markedly improved energy at break (increased surface area under the Force-Elongation curve).

Of course, the invention is not limited to the examples previously described.

Thus, for example, different constituents can be optionally added to the basic constituents previously described (cellulose, formic acid, phosphoric acid, acetone, soda), without the spirit of the invention being modified.

Thus the term "cellulose formate" used in this document covers cases where the hydroxyl groups of the cellulose are substituted by groups other than the formate groups, in addition to the latter, for example ester groups, in particular acetate groups, the degree of substitution of cellulose with these other groups being preferably less than 10%.

The additional constituents, preferably chemically non-reactive with the basic constituents, may for example be plasticizers, sizes, dyes, polymers other than cellulose which may possibly be esterified during the production of the solution. It can also be various additives making it possible, for example, to improve the spinability of spinning solutions, the use properties of the fibers obtained, the adhesiveness of these fibers to a gum matrix.

The invention also covers the cases where a die is made up of one or more non-cylindrical capillaries, of various shapes, for example of a single capillary in the form of a slit, the term "fiber" used in the description and the claims before then be understood in a more general sense, which may include in particular the case of a cellulose formate film or of a regenerated cellulose film.

Claims

1. Fiber in cellulose formate, characterized by the following relationships:

- Ds≥ 2;

 - Te> 45;

 - Mi> 800;

 - Ar> 6;

 - Er> 13.5,

Ds being the degree of substitution of the cellulose for formate groups (in%), Te being its toughness in cN / tex. Mi being its initial modulus in cN / tex, Ar being its elongation at break in%. Er being its energy at break in J / g.

2. Fiber according to claim 1. characterized by the following relationship:

Ar> 7.

3. Fiber according to claim 2, characterized by the following relationship:

Ar> 8.

4. Fiber according to any one of claims 1 to 3, characterized by the following relationships:

Te> 60; Mi> 1200; Er> 20.

5. Fiber according to claim 4, characterized by at least one of the following relationships:

Te> 70; Mi> 1500; Er> 25.

6. Process for spinning a solution of cellulose formate in a solvent based on phosphoric acid, according to the spinning method called "dry-jet-wet spinning", to obtain a fiber in accordance with any one of the Claims 1 to 5, characterized in that the step of coagulating the fiber and the step of neutral washing of the coagulated fiber are both carried out in acetone.

7. Method according to claim 6, characterized in that the temperature of the coagulation acetone is negative, and in that the temperature of the washing acetone is positive.

8. Method according to claim 7. characterized in that one has the following relationships:

Tc <-10 ° C: T1> + 10 ° C, Tc being the temperature of the coagulation acetone and T1 being the temperature of the washing acetone.

9. Method according to any one of claims 6 to 8, characterized in that at least one of the following characteristics is verified: a) the rate of residual solvent in the fiber, at the outlet of the coagulation means (denoted Rs) , is less than 100% by weight of dry fiber; b) the stress of tension undergone by the fiber at the exit of the coagulation means (noted σ _c ) is lower than 5 cN / tex.

10. Method according to claim 9. characterized by the following relationships:

Rs <50%; σ _c <2 cN / tex.

11. Cellulose fiber regenerated from cellulose formate, characterized by the following relationships:

- 0 <D _s <2;

- T _E >60;

- M _I >1000;

- A _R >6;

- E _R > 17.5,

D _S being the degree of substitution of the cellulose for formate groups (in%), T _E being its toughness in cN / tex, M _I being its initial modulus in cN / tex, A _R being its elongation at break in%, E _R being its energy at break in J / g.

12. Fiber according to claim 1 1, characterized by the following relationship:

A _R > 7.

13. Fiber according to claim 12, characterized by the following relationship:

A _R > 8.

14. Fiber according to any one of claims 11 to 13, characterized by the following relationships:

T _E >80; M _I >1500; E _R > 25.

15. Fiber according to claim 14, characterized by at least one of the following relationships:

T _E >100; M _I >2000; E _R > 30.

16. Method for obtaining a regenerated cellulose fiber according to any one of claims 1 1 to 15, by passing a cellulose formate fiber through a regenerating medium, washing, then drying, characterized in that the regenerating medium is an aqueous sodium hydroxide solution whose sodium hydroxide concentration, noted Cs, is greater than 16% (% by weight).

17. Method according to claim 16, characterized in that there is the following relation:

Cs> 18%;

18. Method according to claim 17, characterized in that there is the following relation:

Cs> 22%;

19. Method according to any one of claims 16 to 18, characterized in that the tension stresses undergone by the fiber, at the input of the regeneration means, washing means and drying means, are less than 10 cN / tex.

20. Method according to claim 19, characterized in that the tension stresses are less than 5 cN / tex.

21. Method according to any one of claims 16 to 20, characterized in that one starts from a cellulose formate fiber according to any one of claims 1 to 5.

22. Method according to any one of claims 16 to 21, characterized in that it is implemented online and continuously with the method according to any one of claims 6 to 10.

23. Reinforcement assembly comprising at least one cellulose formate fiber according to any one of claims 1 to 5 and / or at least one regenerated cellulose fiber according to any one of claims 11 to 15.

24. Article reinforced with at least one cellulose formate fiber according to any one of claims 1 to 5 and / or at least one regenerated cellulose fiber according to any one of claims 11 to 15 and / or at least one assembly according to claim 23.

 25. Article according to claim 24, characterized in that it is a tire casing.