WO2017017182A1

WO2017017182A1 - Method for preparing a natural rubber

Info

Publication number: WO2017017182A1
Application number: PCT/EP2016/067996
Authority: WO
Inventors: Katia BARAN; Céline LEDOUX; Marjorie GUINEL
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: FR3039552A1; FR3039552B1

Abstract

The present invention relates to a method for preparing a natural rubber, comprising the addition of a coagulant to a stabilised field latex, at latest 6 after the day of bleeding from a hevea. The method according to the invention enables preparation of a natural rubber which has the property, when it is used in a rubber composition containing a reinforcing filler, of improving the dispersion of the reinforcing filler in the composition and strongly reducing the hysteresis thereof.

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 (bottom of cup), the second on coagulation of the field latex using a chemical agent, preceded or not by centrifugation of the generally stabilized latex (latex route). 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 chosen preparation process can very strongly impact 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 specific method for preparing a natural rubber makes it possible to improve the dispersion of a reinforcing filler in a composition containing the natural rubber and to greatly reduce its hysteresis.

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

- (a) the addition, not later than 6 days after the day of the bleeding of a rubber tree, of a

coagulating to a stabilized field latex.

The invention also relates to a method for preparing a rubber composition based on a natural rubber and a reinforcing filler, characterized in that it comprises the steps of the process for preparing a natural rubber according to the invention .

The 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 component, 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" is meant 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, less in part, during the various 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.

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.

According to the present invention, the addition of the coagulant to a stabilized field latex takes place at the latest 6 days after the day of the bleeding of the rubber tree.

The field latex is stabilized by adding a stabilizer to a fresh field 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 stabilizer is selected from the group of stabilizers typically used in field latex stabilization.

The stabilization of natural rubber field latex is widely practiced by those skilled in the art in the manufacture of natural rubber from field latex, to avoid spontaneous (or so-called natural) coagulation of the latex and thus maintain the state of the art. latex. As known to those skilled in the art, the stabilization of the field latex is to maintain the pH of the latex at a value greater than the isoelectric point of the field latex. Typically, the stabilized latex has a pH of at least 7, preferably around 9. One skilled in the art uses a base, as a stabilizer, which he adds to the field 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.

Preferably, the stabilizer is added to the field latex no later than the day of bleeding.

Even better, a container for collecting the bleeding latex contains the stabilizer from the moment the latex flows into the container or alternatively before the latex flows.

The coagulant is added to the latex field once it is stabilized.

According to a preferred embodiment of the invention, the addition of the coagulant to the stabilized field latex is carried out in a period which extends from the second day after the bleeding (namely 2 days after the day of bleeding) until at 6 days after the day of bleeding. If the coagulant is added before the second day after bleeding, the coagulation yield is lower. If, on the contrary, the coagulant is added beyond 6 days after the day of bleeding, the use of the natural rubber thus coagulated leads to a poor dispersion of the reinforcing filler, in particular carbon black, in the natural rubber. as well as a degradation of the implementation of the rubber composition containing the natural rubber and the reinforcing filler.

Suitable coagulants are those known to coagulate a natural rubber field latex, whether stabilized or unstabilized. The coagulant acts on the so-called weak bonds that exist in the field latex. These bonds are described as weak because of their relative low binding energy compared to covalent bonds. These weak bonds are, for example, hydrogen bonds or ionic bonds.

The coagulant is preferably chosen from alcohols, acids and alkaline earth metal salts.

Carboxylic acids and alkaline earth metal salts are particularly preferred as coagulants. By way of alcohol, it is possible to use alcohols such as methanol, ethanol and isopropanol.

As acid, whether weak or strong, mention may be made of organic acids such as carboxylic acids and mineral acids. Acetic acid and formic acid are particularly preferred, also for their availability and cost. As salts of alkaline earth metals, the calcium and magnesium salts, in particular the calcium and magnesium chlorides or the magnesium sulphate, are suitable.

The amount of coagulant is adjusted by the skilled person to the case to cause the destabilization of the latex and its coagulation. Indeed, the amount of coagulant will depend for example on the chemical nature of the coagulant, the concentration of the latex, the pH of the latex.

The form in which the coagulant is added can vary and depends on the case. Indeed, if the coagulant is solid, it is preferably dispersed or in solution, preferably aqueous. If the coagulant is liquid, it can also be added in the form of a dispersion or a solution, preferably aqueous. The use of a dispersion or solution of the coagulant allows a better contact of the coagulant with the latex and facilitates the handling of the coagulant, and ultimately increases the efficiency of the process from the point of view of the amount of coagulant added. The concentration of the coagulant in the dispersion or the solution may be chosen by those skilled in the art so as to optimize the volume of dispersion or solution to be added to the latex relative to the volume of latex to be treated, taking into account in particular the viscosity of dispersion or solution. The method may further comprise at least one centrifugation step of the stabilized latex prior to the addition of the coagulant. In other words, the centrifugation takes place before step a).

The centrifugation step is generally followed by a new latex stabilization step which proceeds in the same manner as that carried out to stabilize the fresh field latex with the difference that it is conducted on a centrifuged latex.

Centrifugation is a well known operation in the traditional manufacturing process of natural rubber from a field latex and is widely used especially in the manufacture of concentrated latexes, particularly intended for use in the glue, glove, condoms, balloons. It makes it possible to eliminate a part of the proteins present in the field latex and to concentrate the latex in dry matter.

The centrifugation can be repeated several times on the stabilized field latex: each new centrifugation step is conducted on a stabilized latex. When the latex is centrifuged several times, the latex is generally diluted after each centrifugation before being centrifuged again. The dilution makes it possible to avoid a setting in mass during the centrifugation and to optimize the efficiency of the centrifugation, the setting in mass being caused by a concentration of the latex too high in dry matter. Dilution, generally carried out using an aqueous ammonia solution, makes it possible to readjust the latex concentration in dry matter, which is typically at most 30%. According to one variant, the process has no centrifugation step before step a). According to another variant, the process comprises from 1 to 6 centrifugation steps before step a), preferably from 1 to 4 centrifugation steps. These variants apply to any of the embodiments of the invention.

The product resulting from step a) is a coagulum composed of natural rubber and water. Hereinafter, it is referred to as coagulum. Following step a), the coagulum is usually soaked with water. The water comes from the field latex and optionally the addition of the stabilizer and that of the coagulant. The coagulum can be dried to remove water, especially before incorporation of a filler into the natural rubber. The removal of water is an operation widely practiced by those skilled in the art of natural rubber manufacture, especially for use in a tire rubber composition. This is why the water present in the coagulum is preferably removed.

According to any of the embodiments of the invention, the step of removing the water can be done by drying under an air or an inert gas, such as nitrogen, or under vacuum . The vacuum drying can be carried out under a stream of an inert gas, such as nitrogen or 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.

According to a particular embodiment of the invention, the drying of the coagulum impregnated with water is conducted on a coagulum granulated and drained. The granulation of the coagulum soaked with water is done by several passages of the coagulum on crêpe and shredder machines. This embodiment is entirely suitable for a process on an industrial scale.

Prior to the water removal step, the natural rubber may be antioxidized by adding an antioxidant after 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. Alternatively, to avoid oxidation of the polyisoprene chains of the natural rubber during its preparation process, for example during drying, the antioxidant may be added to the fresh field latex prior to step a).

The antioxidant can be any antioxidant, especially those conventionally used to protect the chains of polyisoprene, including thermo-oxidation. 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 d icyclopentadiene and para-cresol, especially the antioxidant "Struktol LA229" from Schill & Seilacher. Whether added to a latex or a coagulum, the antioxidant is preferably used in the form of a dispersion or solution, preferably aqueous. The use of an antioxidant dispersion or solution allows a better contact of the antioxidant with the latex or coagulum of the natural rubber and facilitates the handling of the antioxidant, and ultimately increases the efficiency of the process. view of the amount of antioxidant added. The concentration of antioxidant in the dispersion or antioxidant solution is not a critical element in the process. It may be chosen by those skilled in the art so as to optimize the dispersion volume or antioxidant solution to be added relative to the amount of natural rubber to be treated, taking into account in particular the viscosity of the dispersion or solution. These dispersions or solutions 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 of natural rubber. The maximum amount of antioxidant that can be used is usually limited for economic reasons. This is why the rate of antioxidant generally does not exceed 3 g per 100 g of natural rubber. Thus, the level of antioxidant added varies in a range from 0.001 g to 3 g per 100 g of natural rubber.

The macromolecular chains of the polyisoprene of natural rubber 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, for example by saponification, by enzymatic treatment,

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 CONHNH ₂ ,

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

Steps i, ii, iii or iv) can be carried out at any stage of the process, preferably before step a), depending on whether they are carried out in the latex phase or in bulk according to methods that are well known to those skilled in the art. art.

The natural rubber resulting from the process according to the invention is 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 targeted: 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 in a sustained manner, for example a motorcycle tire, a tire for passenger vehicle or for commercial vehicles such as Heavy Duty. 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 provide 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 - S _x - 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 inorganic filler. Its level is preferably between 0.5 and 12 phr, more preferably in a range from 3 to 10 phr. This rate 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 less than 120 ° C, for example between 40 ° C and 100 ° C, finishing phase during which is incorporated a crosslinking system.

By way of example, the first (non-productive) phase is carried out in a single thermomechanical step in the course of which is introduced into a suitable mixer such as a conventional internal mixer, all components of the rubber composition with the 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 with the elastomer, in this case natural rubber and, if appropriate, other diene elastomers, 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. of tire.

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 Natural rubbers NR1 to NR8 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), the same day of bleeding, and have an ammonia content of 0.7%.

The latices, when they are centrifuged, are 1 or 4 times according to a traditional industrial process, the ammonia content being kept constant after each centrifugation by the addition of the ammoniacal solution, the rate of dry matter being also readjusted at 30.degree. % after each centrifugation by dilution with water.

The latex are coagulated by adding an aqueous solution of MgCl ₂ to 0.1 mol / 1 (lane MgCl _2), or by adding an aqueous solution of acetic acid to 0.15 mol / 1 (acid method) at a 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. (nitrogen route (N 2)) to remove the water.

The rubbers NR1, NR8 and NR6 are prepared by a process not according to the invention because of the absence of coagulation for the first two (NR1 and NR8) and the addition of the coagulant 7 days after the day of bleeding. (NR6). The natural rubbers NR2, NR3, NR4, NR5 and NR7 are prepared by a process according to the invention, since the coagulant is added to the stabilized field latex at the latest 6 days after the day of bleeding. In fact, the addition of the coagulant to the stabilized latex is carried out 2 days after the day of bleeding for NR2, NR3, NR4, NR5 and NR7. The NR2 and NR7 natural rubbers differ from one another in that NR7 natural rubber is stored for 3 weeks at room temperature (air and ambient temperature) before being characterized and used in a rubber composition. Such storage conditions make it possible to evaluate the resistance of natural rubber to hardening during storage.

11-2. Characterization of natural rubbers:

Table II shows the Mooney plasticity of natural rubbers.

Plasticity Moonev 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 composition in the green state (ie, before firing) is molded in a cylindrical chamber heated to 100 ° C. After a minute of preheating, the rotor (of small size) 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 (MS 1 + 4) is expressed in "Mooney unit" (UM, with 1 MU = 0.83 Newton meter).

For reference, Table II also shows the characteristics of a TSR20 grade.

11-3. Preparation of rubber compositions

C0 to C8 rubber compositions are prepared according to the method described below with the content of the constituents expressed in phr in Table III:

In an internal mixer (final filling rate: about 70% by volume), the initial temperature of the tank is approximately 80 ° C., successively the natural rubber elastomer, the reinforcing filler, and the various other ingredients. 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 sulfur is incorporated and the accelerator is blended on a mixer (homo-finisher) at 70 ° C., mixing the whole (productive phase) for a suitable time (for example 3 to 4 hours). 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.

The rubber compositions differ in the nature of the natural rubber used in the rubber composition. The rubber compositions C1 to C8 respectively contain the natural rubbers NR1 to NR8.

The rubber composition C0 is a reference composition, it contains as natural rubber a TSR20 grade, derived from the bottom of the cup and traditionally used as an elastomer in rubber compositions based on natural rubber.

The rubber compositions C2, C3, C4, C5 and C7 are prepared by a method of manufacturing a rubber composition according to the invention, the natural rubber being prepared according to step a). The rubber compositions Cl, C6 and C8 are prepared by a rubber composition manufacturing method not in accordance with the invention, the natural rubber not being prepared by a process comprising step a). 11-4. Results:

The results are shown in Table IV.

For each of the vulcanized compositions at 150 ° C., the properties at break as well as the dynamic properties are measured under the test conditions described below. For each composition, the state of dispersion of the filler, in this case carbon black, is also determined in the natural rubber.

The reference composition C0 is taken as a control. The values measured in each of the tests are expressed in base 100 relative to the values of the reference composition. A value greater than 100 indicates a value greater than that of the control.

Dispersion of the charge:

In a known manner, the charge dispersion in an elastomer matrix can be represented by a note, note Z, which is measured, after crosslinking, according to the method described by S. Otto and Al in Kautschuk Gummi Kunststoffe, 58 Jahrgang, NR 7-8 / 2005, in accordance with ISO 11345.

The calculation of the Z score is based on the percentage of area in which the charge is not dispersed ("% undispersed area"), as measured by the "disperGRADER +" apparatus supplied with its operating mode and software. 'disperDATA' exploitation by Dynisco according to the equation:

Z = 100 - (% undispersed area) /0.35

The percentage of undispersed surface is measured by a camera observing the surface of the sample under 30 ° incident light. The bright spots are associated with charge and agglomerates, while the dark spots are associated with the elastomeric matrix; digital processing transforms the image into a black and white image, and allows the determination of the percentage of undispersed surface, as described by S. Oto in the aforementioned document.

A dispersion of good is qualified when this score is greater than 50. Otherwise it is qualified as bad.

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 tensile strengths (in MPa) and 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 natural rubbers NR2, NR3, NR4, NR5 and NR7 prepared according to the process according to the invention lead to the production of rubber compositions which have the best dispersions of carbon black in natural rubber in comparison with rubber compositions which contain a natural rubber also prepared by the latex route, but according to a method not in accordance with the invention. Moreover, it is noted that the improvement of these properties is obtained without being to the detriment of the limiting properties of the rubber composition.

It is also found that NR2, NR3, NR4, NR5 and NR7 natural rubbers make it possible to prepare less hysteretic rubber compositions than those containing a TSR20 grade.

It is also observed that NR2, NR4, NR5 and NR7 natural rubbers have the lowest plasticity values. These natural rubbers were prepared according to the process which combines steps a) and b) with one or more centrifugations: this particular embodiment of the invention makes it possible to further improve the rheology of natural rubber. With the improvement of the rheology of natural rubber, the mechanical work of natural rubber in mixers, including those used in tire manufacturing plants, is facilitated, which implies a productivity gain in such plants . Finally, natural rubber NR7 does not see its plasticity change despite storage at ambient temperature, which reflects a good stability of its viscosity storage. This result is achieved without the need to add a known compound, such as a hydroxylamine salt or a hydrazide, to stabilize the viscosity of the natural rubber on storage. In summary, the method for preparing natural rubber according to the invention makes it possible to prepare natural rubbers by the latex route which have the best properties among the natural rubbers resulting from the latex route and which are also more advantageous than the grade TSR 20 from the bottom of the cup.

Table I

Natural rubber Preparation method of natural rubber

N 1 (non-compliant) - stabilized field cool latex

- centrifuged 4 times

- dried (route N2)

NR2 (compliant) - stabilized field cool latex

- centrifuged 4 times

- coagulated (MgCl ₂ channel, 2 days after bleeding)

- granulated

- dried (route N2)

NR3 (compliant) - stabilized field cool latex

- coagulated (MgCl ₂ channel, 2 days after bleeding)

- granulated

- dried (route N2)

NR4 (compliant) - stabilized field cool latex

- centrifuged once

- coagulated (MgCl ₂ channel, 2 days after bleeding)

- granulated

- dried (route N2)

NR5 (compliant) - stabilized field cool latex

- centrifuged 4 times

- coagulated (acid route, 2 days after bleeding)

- granulated

- dried (route N2)

NR6 (non-compliant) - stabilized field cool latex

- centrifuged once

- coagulated (MgCl ₂ route, 7 days after bleeding)

- granulated

- dried (route N2)

NR7 (compliant) - stabilized field cool latex

- centrifuged once

- coagulated (MgCl ₂ channel, 2 days after bleeding)

- granulated

- dried (route N2)

- stored at room temperature

NR8 (non-compliant) - stabilized field cool latex

- centrifuged once

- dried (route N2)

Table II

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

not measured due to poor dispersion of the reinforcing filler.

Claims

A process for the preparation of a natural rubber which comprises the following step a): a) the addition, at the latest 6 days after the day of the bleeding of a rubber tree, from a coagulant to a latex of field stabilized.

2. The method of claim 1 wherein the field latex is stabilized by the addition of a stabilizer at the latest the day of bleeding.

3. A method according to any one of claims 1 to 2 wherein a container for collecting the fresh field latex flowing at the bleed contains a stabilizer from the moment the latex flows or alternatively before the latex does not flow.

4. Method according to any one of claims 1 to 3 wherein the addition of coagulant stabilized field latex is carried out in a period extending from the second day after bleeding up to 6 days after the day of bleeding .

5. Process according to any one of claims 1 to 4 wherein the stabilizer is a solution of ammonia in water.

The process according to any of claims 1 to 5 wherein the coagulant is a compound selected from the group consisting of alcohols, acids and alkaline earth metal salts.

7. Process according to any one of claims 1 to 6 wherein the coagulant is a calcium salt or a magnesium salt, preferably a calcium or magnesium chloride.

8. Process according to any one of claims 1 to 6 wherein the coagulant is an acid.

The process of claim 8 wherein the acid is acetic acid or formic acid.

The process of any one of claims 1 to 9 which comprises at least one centrifugation step of the stabilized latex prior to addition of the coagulant.

11. The method of any one of claims 1 to 9, which process is devoid of centrifugation step stabilized latex before the addition of the coagulant.

The method of any one of claims 1 to 11 which comprises a step of removing water from the coagulum resulting from step a).

13. Process for preparing a rubber composition based on at least one natural rubber and a reinforcing filler, characterized in that it comprises the steps of the process defined according to any one of Claims 1 to 12.

14. Use of a natural rubber in a rubber composition which comprises a reinforcing filler, which natural rubber is prepared according to the process defined according to any one of claims 1 to 12.

15. 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 12.