WO2017017180A1

WO2017017180A1 - Method for preparing a natural rubber

Info

Publication number: WO2017017180A1
Application number: PCT/EP2016/067994
Authority: WO
Inventors: Marjorie GUINEL; Katia BARAN
Original assignee: Compagnie Generale Des Etablissements Michelin; Michelin Recherche Et Technique S.A.
Priority date: 2015-07-30
Filing date: 2016-07-28
Publication date: 2017-02-02
Also published as: FR3039551A1

Abstract

The present invention relates to a method for preparing a natural rubber which includes, after tapping, adding an antioxidant to a fresh field latex of natural rubber. The method according to the invention makes it possible to prepare a natural rubber which has the property, when used in a rubber composition containing a reinforcing filler, of reducing the hysteresis of the rubber composition, without degrading the other properties of the rubber composition and without modifying the implementation properties of the natural rubber.

Description

Process for preparing a natural rubber

The present invention relates to a process for preparing a natural rubber. Natural rubber is an elastomer very widely used in the field of pneumatics because of its remarkable properties. For example, it is used in rubber compositions for the manufacture of semi-finished for vehicles carrying heavy loads, because of the compromise of performance that it can bring to the tire. Indeed, the introduction of natural rubber into a rubber composition reinforced by a reinforcing filler such as a carbon black or a silica gives the rubber composition a very interesting compromise in terms of hysteresis and cohesion which results in terms of performance for the tire by a good compromise between rolling resistance, endurance and tire wear.

The natural rubber used as elastomer in the rubber compositions comes from the rubbery solids of the natural rubber latex, very often extracted from the rubber tree. It is generally recovered according to two major processes. The first is based on the coagulation of spontaneous natural rubber in the bottom of the cup from field latex, the second on coagulation of the field latex with the aid of a chemical agent, preceded or not by centrifugation of the latex generally. stabilized. The unitary steps which constitute each of the processes are far from having no effect on the final chemical composition of the natural rubber, in particular on the macromolecular structure of the polyisoprene. It is therefore known to those skilled in the art that the process of preparation chosen can have a very strong impact on the properties of natural rubber and therefore those of rubber compositions based on natural rubber.

Moreover, there are numerous published works dealing with natural rubber treatments, for example in its latex form, such as the elimination of proteins or lipids from natural rubber (respectively deproteinization or delipidation in English), the functionalization of polyisoprene chains. These treatments are also described to modify the properties of natural rubber, and hence the properties of rubber compositions based on natural rubber. However, it is still a constant concern to improve the properties of rubber compositions based on natural rubber in order to increase the performance of tires comprising such compositions.

Continuing their efforts, the Applicants have discovered that a process for preparing a natural rubber which comprises adding an antioxidant to a fresh field latex natural rubber makes it possible to prepare a natural rubber whose property, when used in a rubber composition containing a reinforcing filler, to reduce the hysteresis of the rubber composition without degrading the other properties of the composition of rubber, without changing the processing properties of natural rubber.

Thus, a first object of the invention is a method for preparing a natural rubber which comprises, after bleeding a rubber tree, the following step a):

a) the addition of an antioxidant to a fresh natural rubber field latex.

Another object of the invention is a method of manufacturing a rubber composition based on a reinforcing filler and a natural rubber, which process comprises the steps of the process for preparing a natural rubber according to the invention. invention.

The present invention also relates to the use of a natural rubber in a rubber composition which comprises a reinforcing filler, which natural rubber is prepared according to the process for preparing a natural rubber according to the invention.

The invention also relates to the use of a natural rubber in a tire rubber compound, which natural rubber is prepared according to the method of preparing a natural rubber according to the invention. I. DETAILED DESCRIPTION OF THE INVENTION

By the term "composition based on", it is understood to mean a composition comprising the mixture and / or the reaction product of the various constituents used, some of these basic constituents being capable of or intended to react with one another, at least in part, during the different phases of manufacture of the composition, in particular during its crosslinking or vulcanization.

By the term "part by weight per hundred parts by weight of elastomer" (or phr), is meant within the meaning of the present invention, the mass part per hundred parts of elastomer present in the rubber composition considered.

In the present description, unless expressly indicated otherwise, all the percentages (%) indicated are percentages (%) by mass. On the other hand, any range of values designated by the expression "between a and b" represents the range of values from more than a to less than b (i.e. terminals a and b excluded) while any designated range of values by the expression "from a to b" means the range from a to b (i.e., including the strict limits a and b).

The compounds mentioned in the description other than natural rubber and its latex, may be of fossil origin or biobased. In the latter case, they can be, partially or totally, derived from biomass or obtained from renewable raw materials derived from biomass. In particular, polymers, plasticizers, fillers, etc. are concerned. Natural rubber and its latex are of plant origin. In the present application, by natural rubber field latex is meant the latex resulting from the bleeding rubber tree. In the remainder of the description, the term field latex refers to natural rubber field latex. A field latex is an aqueous dispersion comprising several species that can be classified into two families: the natural rubber elastomer present in particle form and the non-rubbery compounds.

In the present application, natural rubber is understood to mean the elastomeric part of the natural rubber latex.

In the present application, by fresh field latex is meant a field latex which has not undergone the maturation phenomenon, also known as the fermentation of the latex. The maturation of the latex is a phenomenon well known to those skilled in the art and designates an evolution of the field latex which occurs after the bleeding of the rubber tree under the effect of bacteriological or enzymatic actions, which would be caused by latex contamination by microorganisms naturally present in the plantation environment.

The addition of the substance being carried out on a fresh field latex, the skilled person understands that this addition is done before the coagulation of the latex. Preferably, the fresh field latex is a latex which has been harvested at the latest the day after bleeding. In other words, the antioxidant is added to the field latex no later than 1 day after the day of bleeding.

More preferably, the fresh field latex is a latex which has been collected at the latest the day of bleeding. In other words, the addition of the antioxidant is carried out at the field latex no later than the day of bleeding.

Even more preferably, a container for collecting the bleeding latex contains the antioxidant from the moment the latex flows into the container or alternatively before the latex flows. The antioxidant can be any antioxidant, especially those conventionally used to protect the chains of polyisoprene, including thermooxidation. Of course, the antioxidant can be a mixture of several antioxidants. Among the antioxidants useful for the purposes of the invention, mention may be made of antioxidants belonging to the family of phenols, amines, quinones, tocopherols, tocotrienols, thiols. By way of example, vitamin E, derivatives of para-phenylenediamine, also known in the known para-phenylenediamine diamines, such as for example N, 3-dimethylbutyl-N'-phenyl-p-phenylenediamine ( more commonly known by the abbreviated term "6-PPD"), N-isopropyl-N'-phenyl-p-phenylenediamine (abbreviated "1-PPD"), phenyl-cyclohexyl-p-phenylenediamine, N, N'-di (1,4-dimethylpentyl) -p-phenylenediamine, N, N'-diaryl-p-phenylene diamine ("DTPD"), diaryl-p-phenylenediamine ("DAPD") ), 2,4,6-tris (N, 4-dimethylpentyl-p-phenylenediamino) -1,3,5-triazine, and mixtures of such diamines, quinoline derivatives ("TMQ.") such as that for example 1,2-dihydro-2,2,4-trimethylquinoline and 6-ethoxy-1,2-dihydro-2,2,4-trimethylquinoline, derivatives of phenols, especially those of cresol, such as 2,2'-methylenebis (6-tert-butyl-4-methylphenol) known as "A02246", copolymers of dic yclopentadiene and para-cresol, especially the antioxidant "Struktol LA229" from Schill & Seilacher.

According to a preferred embodiment of the invention, the antioxidant is used in the form of a dispersion, preferably aqueous. The use of an antioxidant dispersion allows a better contact of the antioxidant with the latex and facilitates the handling of the antioxidant, and ultimately increases the efficiency of the process in terms of the amount of antioxidant added . The antioxidant concentration in the aqueous antioxidant dispersion is not a critical element in the process. It may be chosen by those skilled in the art so as to optimize the volume of aqueous dispersion of antioxidant to be added to the latex relative to the volume of latex to be treated, taking into account in particular the viscosity of the dispersion. These dispersions may be commercially available, for example by Schill & Seilacher, especially under the name "Struktol LA", in particular "Struktol LA229", "Struktol LA190".

The amount of antioxidant used should be sufficient to act as an antioxidant, but it is well known that a small amount is sufficient. Typically, the added antioxidant level is at least 0.001 g per 100 g latex solids. The maximum amount of antioxidant that can be used is generally limited for economic reasons. This is why the antioxidant rate generally does not exceed 3 g per 100 g of latex solids. Thus, the level of antioxidant added to the latex varies in a range from 0.001 g to 3 g per 100 g of latex solids. In the present application, the terms "antioxidized natural rubber latex" or "antioxidated latex" are used interchangeably to denote the natural rubber latex resulting from step a). The antioxidized latex can then be treated according to different unit operations according to the intended application of the natural rubber, in particular according to the form in which the natural rubber will be used. Indeed, the natural rubber can be used in the form of latex, for example for applications in glues or in the solid state, for example in the form of a natural rubber ball.

The antioxidized latex resulting from step a) contains in particular water from the field latex and, if appropriate, from the addition of the antioxidant. As a result, the natural rubber can be dried before being used in a rubber composition, especially before incorporating a filler into the natural rubber.

Preferably, the water present in the mixture resulting from step a) is removed. The removal of water can be carried out from the antioxidated latex or from the coagulum resulting from the coagulation of the antioxidized latex. According to the first variant, the process comprises after step a) the following step b):

b) removing water from the antioxidized latex.

According to the second variant, the method comprises in this order step a), then step c) following, then step d) next

c) coagulation of the antioxidated latex to form a coagulum,

d) removal of water from the coagulum.

The use of step b) or d) makes it possible to recover the natural rubber in the solid state, containing variable volatile content levels according to the drying conditions applied. Preferably, the removal of water is conducted on the coagulum resulting from step c).

The coagulation of the antioxidated latex may be spontaneous, for example in the container for collecting the fresh field latex on bleeding. The spontaneous coagulation of the mixture proceeds from the same principle as the spontaneous (or so-called natural) coagulation of well-known field latexes to give the cup-cups (in English "cup lumps"), raw material of grades TSR10 and TSR20. The coagulation of the antioxidated latex may be caused by the addition of an additive, as for example this is traditionally done in the machining plants of natural rubber, in particular to produce grades TSR3L and RSS. The coagulation additives of natural rubber latexes are well known to those skilled in the art. By way of example, mention may be made of metal ions, carboxylic acids and alcohols.

The so-called non-spontaneous coagulation is generally carried out on a stabilized latex. Therefore, the method according to the invention may comprise after step a) and before step c) a step e) stabilizing the antioxidized latex.

Step e) is a unitary operation widely practiced by those skilled in the field of the manufacture of natural rubber, in particular grades TSR3L and RSS, to avoid spontaneous coagulation of the latex and thus maintain the state of the art. latex. According to the process according to the invention, step e) is carried out in a manner identical to that conventionally used in machining plants to stabilize the field latexes. Step e) therefore consists in maintaining the pH of the antioxidated latex at a value greater than the isoelectric point of the field latex. Typically, the latex thus stabilized has a pH of at least 7, preferably around 9. This stabilization step may consist in adding a base to the latex. Preferably, the base is added in the form of an aqueous solution. As base is suitable for example ammonia, sodium hydroxide or potassium hydroxide, preferably ammonia, more preferably a solution of ammonia in water. Step b) or d) removal of water can be done by drying, for example under vacuum, preferably under a stream of an inert gas, such as nitrogen. The vacuum drying can be carried out in the presence of air. Drying is preferably carried out at a moderate temperature, especially at a temperature of at most 80 ° C, for example from room temperature (23 ° C) to 80 ° C. The drying of the coagulum can be conducted under the drying conditions traditionally used in the machining plants (in English "remilling factory") of natural rubber, used in particular in the manufacture of grades TSR or RSS. In this respect, drying is possible under the sole action of the ambient air, with or without smoking (respectively "air drying" and "smoke drying"), drying in machines such as tunnel-type dryers.

Prior to the water removal step, the natural rubber whether in the form of coagulum or in the form of latex may be antioxidized by a further addition of an additional antioxidant after step a). The new addition of the additional antioxidant is to be distinguished from the addition of the antioxidant in step a) since it is not performed on a fresh field latex as defined in the present application. The additional antioxidant may be the same or different than the antioxidant used in step a). This particular embodiment is preferred when the drying is carried out in the presence of air, in particular at temperatures of at least 60 ° C.

The polyisoprene chains of the natural rubber resulting from step a) may be subjected to treatments known to those skilled in the art which are the following:

i. the functionalization of the polyisoprene chains of natural rubber,

ii. the elimination of proteins, lipids, phospholipids from the antioxidated latex, for example by saponification of the antioxidized latex, by enzymatic treatment of the antioxidized latex or by addition of an antioxidated latex surfactant,

iii. stabilizing the viscosity of the natural rubber with an additive such as, for example, hydroxylamine or one of its salts, with a compound having a hydrazide function CONHNH2,

iv. the dilution of the latex or, on the contrary, the concentration of the latex, in particular by centrifugation, by creaming (in English "creaming") or by evaporation.

Steps i, ii or iii may be carried out on the polyisoprene chains just after step a), or for some of them after step c), b) or d) according to methods well known in the art. skilled in the art.

The natural rubber resulting from the process according to the invention can be used in a rubber composition which comprises a reinforcing filler. The composition of the invention comprises any type of so-called reinforcing filler known for its ability to reinforce a rubber composition that can be used for the manufacture of tires, for example an organic filler such as carbon black, a reinforcing inorganic filler such as silica to which is associated in a known manner a coupling agent, or a mixture of these two types of filler. Such a reinforcing filler typically consists of nanoparticles whose average size (in mass) is less than one micrometer, generally less than 500 nm, most often between 20 and 200 nm, in particular and more preferably between 20 and 150 nm.

Preferably, the total reinforcing filler content (carbon black and / or reinforcing inorganic filler such as silica) is between 20 and 200 phr, more preferably between 30 and 160 phr, more preferably from 30 to 90 phr. the optimum being in a known manner and different according to the particular applications concerned: the level of reinforcement expected on a bicycle tire, for example, is of course less than that required on a tire capable of driving at high speed sustained, for example a motorcycle tire, a tire for a passenger vehicle or for a commercial vehicle such as a truck.

Suitable carbon blacks are all carbon blacks, especially blacks conventionally used in tires or their treads (so-called pneumatic grade blacks). Among these, the reinforcing carbon blacks of the 100, 200, 300 series or the 500, 600, 700 or 900 series blacks (ASTM grades), for example the blacks N115, N134, N234, N326, are particularly suitable. , N330, N339, N347, N375, N550, N683, N772). These carbon blacks can be used in the isolated state, as commercially available, or in any other form, for example as a carrier for some of the rubber additives used.

"Reinforcing inorganic filler" means any inorganic or mineral filler, irrespective of its color and origin (natural or synthetic), also called "white" filler, "clear" filler or even "non-black" filler. as opposed to carbon black, capable of reinforcing on its own, without any other means than an intermediate coupling agent, a rubber composition intended for the manufacture of pneumatic tires, in other words able to replace, in its function reinforcement, a conventional carbon black of pneumatic grade; such a filler is generally characterized, in known manner, by the presence of hydroxyl groups (-OH) on its surface.

Suitable reinforcing inorganic fillers are in particular mineral fillers of the siliceous type, preferentially silica (SiO ₂ ). The silica used may be any reinforcing silica known to those skilled in the art, especially any precipitated or fumed silica having a BET surface and a CTAB specific surface both less than 450 m ² / g, preferably between 60 and 300 m ² / g- As highly dispersible precipitated silicas (called "HDS"), there may be mentioned for example the silicas "Ultrasil" 7000 and "Ultrasil" 7005 from the company Degussa, the silicas "Zeosil" 1165MP, 1135MP and 1115MP of the Rhodia company, the "Hi-Sil" silica EZ150G from the company PPG, the "Zeopol" silicas 8715, 8745 and 8755 from the Huber Company, the high surface area silicas as described in the application WO 03/016387. In the present disclosure, the BET surface area is determined in a known manner by gas adsorption using the Brunauer-Emmett-Teller method described in "The Journal of the American Chemical Society" Vol. 60, page 309, February 1938, specifically according to the French standard NF ISO 9277 of December 1996 (multipoint volumetric method (5 points) - gas: nitrogen - degassing: time at 160 ° C - relative pressure range p / po: 0.05 at 0.17). The CTAB specific surface is the external surface determined according to the French standard NF T 45-007 of November 1987 (method B). In order to couple the reinforcing inorganic filler to the diene elastomer, an at least bifunctional coupling agent (or bonding agent) is used in a well-known manner to ensure a sufficient chemical and / or physical connection between the inorganic filler (surface of its particles) and the diene elastomer. In particular, organosilanes or at least bifunctional polyorganosiloxanes are used.

In particular, polysulfide silanes, called "symmetrical" or "asymmetrical" silanes according to their particular structure, are used, as described for example in the applications WO03 / 002648 (or US 2005/016651) and WO03 / 002649 (or US 2005/016650).

In particular, polysulphide silanes having the following general formula (I) are not suitable for the following definition:

(I) Z - A - Sx - A - Z, wherein:

x is an integer of 2 to 8 (preferably 2 to 5);

- the symbols A, which are identical or different, represent a divalent hydrocarbon group (preferably an alkylene Ci-Ci ₈ or an arylene group C ₆ -Ci2, particularly alkylene Ci-Ci _0, in particular Ci-C ₄ , especially propylene);

the symbols Z, identical or different, correspond to one of the three formulas below:

-

in which:

- the radicals R ^1, substituted or unsubstituted, identical or different, represent an alkyl group _Ci-8 cycloalkyl, C ₅ -C ₈ aryl or C ₆ -C ₈ (preferably alkyl groups -C ₆ , cyclohexyl or phenyl, especially C ₁ -C ₄ alkyl groups, more particularly methyl and / or ethyl).

- the radicals R ^2, substituted or unsubstituted, identical or different, represent an alkoxy group or Ci-Ci ₈ cycloalkoxy, C ₅ -C ₈ (preferably a group selected from alkoxyls and C ₈ cycloalkoxyls C ₅ -C ₈ , more preferably still a group selected from C ₁ -C ₄ alkoxyls, in particular methoxyl and ethoxyl).

The content of coupling agent is advantageously less than 20 phr, it being understood that it is generally desirable to use as little as possible. Typically the level of coupling agent is from 0.5% to 15% by weight relative to the amount of filler inorganic. Its level is preferably between 0.5 and 12 phr, more preferably in a range from 3 to 10 phr. This level is easily adjusted by those skilled in the art according to the level of inorganic filler used in the composition.

The rubber composition may furthermore comprise all or part of the usual additives normally used in elastomer compositions intended to constitute mixtures of finished articles of rubber such as tires, in particular treads, such as, for example, plasticizers. or extender oils of aromatic or nonaromatic nature, pigments, protective agents such as antiozone waxes, chemical antiozonants, anti-oxidants, anti-fatigue agents, reinforcing resins (such as resorcinol or bismaleimide), acceptors (for example phenolic novolac resin) or methylene donors (for example HMT or H3M) as described, for example, in application WO 02/10269, a crosslinking system based on either sulfur or donors sulfur and / or peroxide and / or bismaleimides, vulcanization accelerators or retarders, vulcanization activators.

The rubber composition may contain, in addition to the natural rubber resulting from the process according to the invention, another diene elastomer. By "diene" elastomer (or indistinctly rubber) is to be understood in known manner an elastomer consisting at least in part (ie, a homopolymer or a copolymer) of monomeric diene units (monomers carrying two carbon-carbon double bonds, conjugated or not). As an elastomer, mention may be made of those conventionally used in the tire, such as polybutadienes (BR), synthetic polyisoprenes (IR), butadiene copolymers, isoprene copolymers, and mixtures of these elastomers.

The rubber composition is typically prepared by incorporating the reinforcing filler into the natural rubber resulting from the process according to the invention, in particular by kneading, more particularly by thermomechanical kneading.

According to any of the embodiments of the invention, the rubber composition can be manufactured in mixers using two successive preparation phases well known to those skilled in the art: a first phase of work or thermomechanical mixing (phase so-called "non-productive") at high temperature, up to a maximum temperature of between 110 ° C and 200 ° C, preferably between 130 ° C and 185 ° C, followed by a second mechanical working phase (so-called "Productive") to a lower temperature, typically below 120 ° C, for example between 40 ° C and 100 ° C, finishing phase during which is incorporated the crosslinking system. By way of example, the first (non-productive) phase is carried out in a single thermomechanical step during which all the constituents of the rubber composition are introduced into a suitable mixer such as a conventional internal mixer. exception of the crosslinking system. The total mixing time, in this non-productive phase, is preferably between 2 and 10 min. After cooling the mixture thus obtained during the first non-productive phase, the low temperature crosslinking system is then incorporated, generally in an external mixer such as a roll mill; the whole is then mixed (productive phase) for a few minutes, for example between 5 and 15 min.

The first kneading step is generally carried out by incorporating the reinforcing filler to the elastomer in one or more times by thermomechanically kneading. In the case where the reinforcing filler, in particular carbon black, is already incorporated wholly or partly into the elastomer in the form of a masterbatch, it is the masterbatch which is directly kneaded and, if appropriate, incorporates other elastomers or reinforcing fillers present in the composition which are not in the form of masterbatch, as well as additives other than the crosslinking system.

The final composition thus obtained is then calendered, for example in the form of a sheet or a plate, in particular for a characterization in the laboratory, or else extruded in the form of a rubber profile that can be used, for example, as a tread. tire for passenger vehicle.

The rubber composition may be in the green state (before crosslinking) or in the fired state (after crosslinking).

The rubber composition may be used in a tire, especially as a composition of one of the constituent elements of the tire. These constituent elements may be a rubber component such as a tread, a sheet or a filling compound.

The aforementioned features of the present invention, as well as others, will be better understood on reading the following description of several embodiments of the invention, given by way of illustration and not limitation.

II. EXAMPLES OF CARRYING OUT THE INVENTION II-1. Preparation of natural rubbers The NR1, NR2, NR3 and NR4 natural rubbers are prepared according to the methods of preparation described in Table I.

All natural rubbers come from the same fresh field latex.

The latex, when stabilized, is with an ammonia solution (0.5% solution of ammonia in water), and have an ammonia content of 0.7%.

The latices, when centrifuged, are 4 times according to a traditional industrial process, the ammonia content being kept constant after each centrifugation by adding the ammoniacal solution, the dry matter content is also readjusted to 30% after each centrifugation by dilution with water.

The latices are coagulated by adding an aqueous solution of MgCl ₂ at 0.1 mol / l, at the rate of 3 volumes of solution for a volume of latex.

The coagulums are granulated according to a traditional industrial process by passing the coagula into a device comprising crepe and extruder or shredder.

The latexes are dried for 48 hours under nitrogen and under vacuum at 65 ° C. to remove the water.

The antioxidant used is the "Struktol LA229" product from Schill & Seilacher which is an aqueous dispersion of a butylated reaction product of p-cresol with dicyclopentadiene. The rate of antioxidant introduced into the cup is 0.2 phr.

In the preparation mode of NR2, the antioxidant is in the cup at the time of bleeding: the field latex is antioxidized while it is a fresh field latex. NR2 natural rubber is a natural rubber prepared according to the process according to the invention. In the mode of preparation of NR3, the antioxidant is added to the coagulum by spraying the dispersion of the antioxidant. In that of NR4, the antioxidant is added to a field latex that is not a fresh field latex. NR1 is obtained from a stabilized field latex, centrifuged, coagulated, granulated and then dried: it has not undergone any treatment with an antioxidant.

Only NR2 natural rubber is a natural rubber prepared according to the process according to the invention.

11-2. Characterization of natural rubbers:

Table II shows:

the rate of antioxidant measured on each of the natural rubbers NR1, NR2, NR3 and NR4, expressed in percent by mass,

the freezing rate of the natural rubbers, expressed in percent by weight,

the average molar mass in number and mass of natural rubbers (soluble phase),

their Mooney plasticity. Determination of the antioxidant level:

The level of antioxidant in natural rubbers, in this case the product "Struktol LA229", is determined by reverse-phase liquid chromatography coupled with ultraviolet detection. The mobile phase is a mixture of water and methanol, the stationary phase a phenyl-grafted silica.

The preparation of the sample consists of the solubilization of a test portion of between 10 mg and 1 g. A volume between 1 and 50 μί of this solution is then injected into the chromatographic column located in an oven at an isotherm between 25 and 60 ° C. The eluted antioxidant is separated from the natural rubber is detected according to the absorbance that generates ultraviolet between 150 and 350 nm. The survey of the area of the chromatographic peak thus recorded makes it possible to determine the antioxidant content with regard to a calibration curve previously established using reference solutions.

Determination of gel ratio and number average molecular weights (Mn) and mass (Mw) by flow-flow analysis FF / RI / MALS (Flow Field-Flow Fractionation / Refractive Index / Multi-Angle Light Scattering):

This technique makes it possible to evaluate the macrostructure of the soluble part of the natural rubber and to quantify the macrogel (insoluble part).

The measurement of the macrogel consists in solubilizing a sample of natural rubber (NR) at a concentration of between 0.5 and 10 g / l in an organic solvent for a period of between 1 and 14 days in a water bath with stirring at a temperature of temperature between 15 ° C and 45 ° C.

Once solubilization is performed, centrifugation is performed. The soluble fraction is then separated from the gel. Only this soluble fraction is analyzed in Flow-FFF / RI / MALS.

FFF is a technique that separates macromolecules (very high molar masses) according to their hydrodynamic volume. The principle is based on a laminar flow and a longitudinal flow through a channel. The separation is done using an orthogonal force which can be of different kinds (electric potential, magnetic field, liquid flow, ...).

Flow-FFF is the most common FFF technique. The orthogonal flow is ensured by the suction of the solvent through the bottom of the channel, through a semi-permeable membrane. This is called Asymmetrical Flow FFF (AF4). The longest elastomer chains or high molar masses are eluted last.

We first start by performing a focus step. It serves to position all the compounds in the same plane so as not to distort the elution times.

When this step is finished, comes the elution step during which the orthogonal flow also called "crossflow" decreases over time.

After separation, the macromolecules are detected using a refractometer (RI) and a light scattering device (MALS). These detectors allow you to have information on molar mass distributions or on the architecture of polymers.

Mooney plasticity:

An oscillating consistometer is used as described in the French standard NF T 43-005 (1991). The Mooney plasticity measurement is carried out according to the following principle: the raw composition (i.e., before firing) is molded in a cylindrical chamber heated to 100 ° C. After one minute of preheating, the rotor rotates within the test tube at 2 revolutions / minute and the useful torque is measured to maintain this movement after 4 minutes of rotation. Mooney plasticity (ML 1 + 4) is expressed in "Mooney unit" (UM, with 1 MU = 0.83 Newton meter).

It is observed that the process according to the invention leads to a natural rubber NR2 which has processing properties at least as good as the other natural rubbers NR1, NR3 and NR4. The addition of the antioxidant has no effect on the rate of freezing in the natural rubber. However, only the process according to the invention makes it possible to obtain the highest average mass molar masses with the same percentage of soluble fraction. 11-3. Preparation of rubber compositions

The rubber compositions C1, C2, C3 and C4 are manufactured from the respective natural rubbers NR1, NR2, NR3 and NR4 according to the process described below with the content of the constituents expressed in phr in Table III:

In an internal mixer (final filling rate: approximately 70% by volume), the initial tank temperature of which is approximately 100 ° C., the natural rubber elastomer, the reinforcing filler and the various other ingredients are introduced successively. with the exception of the vulcanization system. Thermomechanical work (non-productive phase) is then carried out in one step, which lasts a total of about 3 to 4 minutes, until a maximum temperature of "fall" of 165 ° C is reached. The mixture thus obtained is recovered, cooled and then sulfur is incorporated and the sulfenamide type accelerator on a mixer (homo-finisher) at 70 ° C., mixing the whole (productive phase) for a suitable time (for example 3 hours). at 4 minutes).

The compositions are then calendered in the form of a plate with a thickness ranging from 2 to 3 mm or thin rubber sheets for the measurement of their physical or mechanical properties.

11-4. Rubber results:

For each of the vulcanized compositions at 150 ° C., its breaking properties as well as its dynamic properties are measured under the test conditions described below. The composition Cl is taken as a control. The values measured in each of the tests are expressed in base 100 relative to the values of the control composition. A value greater than 100 indicates a value greater than that of the control. The values measured in base 100 are shown in Table IV. Mooney Plasticity ML:

It is measured according to the method described in paragraph 11-2.

Traction tests:

The tensile tests make it possible to determine the elastic stress and the properties at break. Unless otherwise indicated, they are carried out in accordance with the French standard NF T 46-002 of September 1988. Traction data processing also makes it possible to draw the modulus curve as a function of elongation. The nominal secant modulus calculated at the initial elongation of the test specimen (or apparent stress, in MPa) at 100% and 300% elongation noted MSA100 and MSA300 respectively is measured at first elongation. The breaking stresses (in MPa) and the elongations at break (in%) are measured at 60 ° C.

Dynamic properties:

The loss factor tan (δ) max is measured on a viscoanalyzer (Metravib VA4000) according to ASTM D 5992-96. The response of a sample of vulcanized composition (in the form of 2 test pieces 2 mm thick and 10 mm in diameter) is recorded, subjected to sinusoidal stress in alternating simple shear, at the frequency of 10 Hz, at 60 °. C according to ASTM D 1349-99. A strain amplitude sweep is performed from 0.1% to 50% (forward cycle) and then from 50% to 0.1% (return cycle). The result exploited is the loss factor tan (δ). For the return cycle, the maximum value of tan (δ) observed, denoted tan (δ) max, is indicated.

The dynamic shear complex module (G *) is also measured at 50% deformation.

The rubber composition C2 which contains as elastomer NR2 natural rubber prepared according to the process according to the invention, is the least hysteretic rubber composition. This result is obtained without being to the detriment of the other properties of the rubber composition, in particular the implementation and its properties in elongation (stress at break - elongation). The other compositions C1, C3 and C4 which respectively contain a natural rubber prepared according to a process not according to the invention, respectively NR1, NR3 and NR4, do not exhibit a compromise of such interesting properties. The addition of the antioxidant according to the process according to the invention is of interest as regards the overall compromise of the properties. Table I

Board

* : not determined Table III

(1) "Wingstay 100" (mixture of N, N '- (phenyl and benzene-1,4-diamines),

(2) N-1,3-dimethylbutyl N-phenylparaphenylenediamine

(3) N-cyclohexyl-2-benzothiazylsulfenamide

Table IV

Claims

1. A process for preparing a natural rubber which comprises, after bleeding a rubber tree, the following step a):

a) the addition of an antioxidant to a fresh natural rubber field latex.

2. The method of claim 1 wherein the addition of the antioxidant is carried out at the latest 1 day after the day of bleeding.

3. Method according to any one of claims 1 to 2 wherein the addition of the antioxidant is carried out at the latest the day of the bleeding.

A process according to any one of claims 1 to 3 wherein a container for collecting the fresh field latex flowing at the kerf contains the antioxidant from the moment the latex flows or alternatively before the latex does not flow.

5. Process according to any one of claims 1 to 4 wherein the antioxidant is in the form of a dispersion, preferably aqueous.

6. A method according to any one of claims 1 to 5 wherein the added antioxidant level is in a range from 0.001 g to 3 g per 100 g of latex solids.

7. Method according to any one of claims 1 to 6 which comprises after step a) a step of removing water present in the mixture resulting from step a).

The process of claim 7 wherein the step of removing water is the following step b):

b) removing water from the natural rubber latex resulting from step a).

9. The method of claim 7 wherein the step of removing water is the following step d), preceded by the following step c):

c) coagulation of the antioxidated latex to form a coagulum,

d) removal of water from the coagulum.

A process for producing a rubber composition based on a reinforcing filler and a natural rubber, which process comprises the method defined in any one of claims 1 to 9 for producing the natural rubber.

11. Use of a natural rubber in a rubber composition which comprises a reinforcing filler, which natural rubber is prepared according to the method defined according to any one of claims 1 to 9.

12. Use of a natural rubber in a tire rubber component, which natural rubber is prepared according to the method defined in any one of claims 1 to 9.