WO2021111082A1

WO2021111082A1 - Method for preparing natural rubber

Info

Publication number: WO2021111082A1
Application number: PCT/FR2020/052269
Authority: WO
Inventors: Alexandre ARTHEMISE; Jérôme DUSSILLOLS
Original assignee: Compagnie Generale Des Etablissements Michelin
Priority date: 2019-12-04
Filing date: 2020-12-03
Publication date: 2021-06-10
Also published as: FR3104156A1; FR3104156B1

Abstract

The invention relates to a method for treating a natural rubber which comprises the extrusion of a mixture of a wet natural rubber coagulum and a peptizer in an endless screw machine equipped with a barrel, in which the screw turns, and with a perforated die at the end of the screw, in which method a) the mixture is compressed at a temperature greater than or equal to 130°C and less than or equal to to 210°C in the barrel, b) an adiabatic flash expansion at a differential pressure of greater than or equal to 40 bar is carried out at the die outlet. Such a method makes it possible to prepare a natural rubber which has an improved compromise between its resistance to ageing by thermal oxidation and its viscosity.

Description

Process for preparing a natural rubber

The present invention relates to a process for preparing a natural rubber.

Natural rubber comes from the rubbery dry material of the latex harvested by bleeding the rubber tree and collected in a bucket attached to the trunk of the rubber tree. Two traditional methods exist for recovering the rubber material from the latex. According to one of the methods, the latex which is still liquid in the cup is decanted, filtered, optionally stabilized or centrifuged, then coagulated, for example using a chemical agent. According to the other method, the latex is not collected before its coagulation in the cup, also called cup: one then collects a wet coagulum at the bottom of the cup, also known under the name of "bottom of cup" (in English "Cup lump"). After removal of plant debris and mineral debris from the coagulum, the natural rubber is dried, traditionally in air-circulating tunnels at a temperature of about 90 to about 130 ° C.

As the properties of natural rubber depend in part on the coagulation process used, it is customary to designate rubber with a name which originates from the choice of the coagulation process. We are also talking about natural rubber at the bottom of the cup and natural rubber latex.

Natural rubber is also distinguished from other synthetic elastomers by many characteristics: one of them is its particularly high viscosity. Before its use in a rubber composition, natural rubber is generally subjected to a plasticization operation or to a peptization operation with a view to reducing its viscosity to viscosity values compatible with its industrial use, in particular for the preparation of compositions of rubber. The plasticization operation consists of a thermomechanical treatment in an internal mixer and requires a significant energy cost, of the order of 140 kWh / t. The peptization operation is a plasticization step in the presence of chemical agents, called peptizers.

Internal mixers, installations well known to those skilled in the art of rubber compositions, are also used to prepare rubber compositions by thermomechanical mixing of one or more elastomers such as natural rubber and various ingredients such as fillers such as a carbon black or a silica, plasticizers such as an oil, crosslinking agents such as sulfur, a vulcanization accelerator. It follows that the machine time of the internal mixers devoted to the preparation of the rubber compositions is to be shared with the plasticization or peptization operations of natural rubber, which has the consequence of reducing the productivity of a production unit of rubber. rubber compositions. A plasticization operation reduces productivity more than does a peptization operation, because plasticization requires more machine time than peptization which has the advantage of benefit from the accelerating action of the chemical agent on the viscosity reduction kinetics. Unfortunately, natural rubber at the bottom of the cup does not lend itself to peptization unlike natural latex rubber, because natural rubber at the bottom of the cup is very sensitive to the presence of peptizers: its resistance to aging by thermooxidation is highly reduced, its viscosity decreases sharply and may become unsuitable for use in a rubber composition. This high sensitivity of the natural rubber at the bottom of the cup to peptization can also lead to a lack of control of the viscosity of the natural rubber during the peptization operation and consequently to a difficulty in controlling the quality of the production. natural rubber.

The Applicant, pursuing its efforts, has found a new process which makes it possible to solve the problems mentioned.

Thus a first object of the invention is a process for treating a natural rubber which comprises the extrusion of a mixture of a wet coagulum of natural rubber and a peptizer in a worm screw machine equipped with a sleeve, in which the screw turns, and a die with holes at the end of the screw, process in which: a) the mixture is compressed at a temperature greater than or equal to 130 ° C and less than or equal to 210 ° C in the sheath, b) an adiabatic flash expansion at a differential pressure greater than or equal to 40 bars is carried out at the die outlet.

The invention also relates to a natural cup bottom rubber having a plasticity retention index greater than 70 and a Mooney ML (1 + 4) viscosity at 100 ° C less than 60. The natural rubber according to the invention which can being prepared according to the process in accordance with the invention has the advantage of having a viscosity which does not require a plasticization step before its use in a rubber composition, which also gives the advantage to natural rubber of having a good resistance to thermo-oxidation aging.

detailed description

Any interval of values designated by the expression "between a and b" represents the range of values going from more than a to less than b (that is to say limits a and b excluded) while any interval of values designated by the expression “from a to b” means the range of values going from a to b (that is to say including the strict limits a and b). Unless expressly indicated otherwise, all the percentages (%) indicated are% by mass.

The machine used in step a) is typically an extruder, a worm machine which comprises a material inlet called a hopper, a body formed of a cylinder (also called a sheath) in which a screw (one or more) turns. endless and a head which serves as a support for a die. This machine makes it possible to apply mechanical drying or thermo-mechanical drying to a product soaked in a liquid to be removed by drying. The mechanical drying allows the elimination of the liquid by purely mechanical forces (pressing, spinning, ...). It can be achieved by simple transfer of momentum and possibly without thermal transfer. Thermomechanical drying is carried out by heating communicated to the product to be dried by degradation of the mechanical energy. The water included in the product to be dried is in the liquid state under pressure and at high temperature. A release of the stresses previously exerted on the natural rubber in the sleeve takes place at the die outlet by the elimination of the compression, which allows adiabatic flash expansion at the die outlet. On leaving the die, the expansion produced also makes it possible to flash the humidity and, if necessary, depending on the viscosity of the product, to fragment the product.

The extruder useful for the needs of the invention can be an extruder available on the market, in particular those sold by the companies Anderson, FOM and Welding, such as for example the Expander from Anderson, the Extruder Dryer from FOM, the VCU of Welding. The extruder useful for the purposes of the invention for any one of the embodiments of the invention is preferably a single screw extruder.

Variants of extruders are preferred in that they allow, at the die outlet, to achieve higher coagulum flow rates or to promote adiabatic expansion. Such a preferred variant is an extruder, the sleeve of which has, in the feed zone of the extruder, one (one or more) means for discharging water (free water, in liquid form). As evacuation means, mention may be made of grooves in the thickness of the sheath which open onto the inner surface of the sheath, one or more openings in the feed zone of the extruder, which opening makes it possible to evacuate water. out of the scabbard. These openings can be in the form of a slot, a grid or a circular hole. The feed area is the area below the opening of the hopper.

The coagulum used in step a) is a coagulation product of natural rubber latex, either obtained by spontaneous or induced coagulation. Preferably, the coagulum is a cup bottom. In the present application, the term “natural rubber latex” is understood to mean the latex obtained from the bleeding of the rubber tree.

The coagulum is said to be wet because it is soaked in water which comes in particular from the washing water resulting from the washing operations of the coagulum such as decontamination operations, generally carried out in a swimming pool under water. The coagulum used in step a) is preferably a coagulum which has undergone washing operations. It preferably contains more than 5% by weight of water, in particular between 5 and 40% by weight of water, more preferably between 8 and 30%, even more preferably between 8 and 25% by weight. The percentage of water is calculated with respect to the total mass of wet coagulum.

The wet coagulum can be in the form of granules commonly called crumbs or pancakes. Preferably, the coagulum used in step a) is in the form of granules (in English "crumbs"), in particular to facilitate its introduction into the extruder through the hopper. More preferably, the coagulum used in step a) is in the form of granules.

Preferably, the coagulum used in step a) is typically a coagulum which has undergone, prior to step a), decontamination work which generally breaks down into two steps, primary decontamination and secondary decontamination. The coagulum collected after bleeding the rubber tree very often contains more or less large contaminants, such as leaves, twigs, sand and other debris which contaminate the coagulum during harvesting. To carry out the primary decontamination which aims to eliminate the largest objects, the coagulum is traditionally cut and washed in pools of water. In the secondary decontamination which allows the elimination of the finest contaminants, the coagulum is traditionally shredded, then washed with water in swimming pools, then sent for example in crepers and shredders. The decontamination can comprise a step of filtering the coagulum, in particular under pressure, for example in a particular device which comprises an extruder and a suitable filtration means installed at the outlet of the extruder. Reference may for example be made to the filtration process described in patent application WO 2016162645 A2 or to that described in patent application WO 2018224773 A1. Such a process makes it possible to remove contaminants of size greater than 1 mm, advantageously greater. at 500 µm (0.5 mm), more preferably greater than 100 µm (0.1 mm).

When the coagulum used in step a) is decontaminated, the conforming process preferably comprises a step of filtering the wet coagulum before step a).

Peptizers are compounds well known to those skilled in the art. They make it possible to reduce the molecular mass of the polyisoprene chains of natural rubber by breaking them by thermooxidation reaction of the chains. They generally act as a catalyst for the thermooxidation reaction. The peptizer can also be a mixture of several peptizers. Mention may be made of aromatic mercaptans, aromatic disulphides, arylamines, phenols, sulphonic acids and their derivatives such as their metal salts. Preferably, the peptizer is a phenol or a metal salt of a phenol or else a dithiobisbenzamide. Very particularly suitable are pentachlorothiophenol, its zinc salt, 2-dibenzamido diphenyl disulfide (in English 2,2′-dibenzamido diphenyl disulfide).

Inside the sheath, under the effect of the thermal energy provided in particular by the rotation of the screw, the mixture of wet coagulum of natural rubber and peptizer heats up. It is assumed that this heating triggers the reaction of the peptizer with the polyisoprene chains of natural rubber and therefore leads to the formation of a peptized natural rubber inside the sheath. The amount of peptizer in the wet coagulum is adjusted by those skilled in the art to obtain the desired viscosity of natural rubber. It can vary over a wide range depending on the chemical nature of the peptizer and the temperature applied to the mixture of wet coagulum of natural rubber and peptizer in the sheath. Typically, it can vary within a range of from 0.01 to 0.5 gram of peptizer per kilogram of dry natural rubber.

According to a particular embodiment of the invention, the worm machine is fed with the mixture of wet coagulum of natural rubber and peptizer by a hopper fitted to the worm machine. According to this embodiment, the mixture of wet coagulum of natural rubber and peptizer is typically prepared during a preliminary step of bringing a wet coagulum of natural rubber into contact with a peptizer. According to this embodiment, step a) is preferably preceded by a step of soaking a wet coagulum of natural rubber in an aqueous solution of peptizer or by a step of watering a wet coagulum of a natural rubber with an aqueous solution of peptizer to form the mixture of wet coagulum of natural rubber and peptizer. When step a) is preceded by a sprinkling step, the wet coagulum is preferably sprinkled with an aqueous solution of peptizer at the rate of 0.1 to 1 liter of solution per kilogram of dry natural rubber, knowing that the solution has a concentration ranging from 0.05 to 3 grams of peptizer per liter of solution.

In step a), the mixture of wet coagulum of natural rubber and peptizer is compressed in the sleeve of the extruder. This compression is useful for subsequently subjecting natural rubber to adiabatic expansion. The pressure at which the natural rubber is compressed must be sufficient to allow adiabatic expansion to a differential pressure of at least 40 bars. At the pressure useful for the needs of the invention to achieve compression, the natural rubber is brought in step a) to a temperature ranging from 130 to 210 ° C. In an endless screw machine such as an extruder, the mechanical work under high pressure is accompanied by heating of the rubbery material of the coagulum, which has the effect of increasing the temperature of the coagulum. Below 130 ° C, the process is not efficient enough to reduce the moisture content of natural rubber. Preferably, the natural rubber is compressed in step a) at a temperature between 170 ° C and 210 ° C. More preferably, the natural rubber is compressed in step a) at a temperature between 180 ° C and 210 ° C. These more preferential temperature conditions make it possible to increase the efficiency of the process to produce a natural rubber with a lower residual humidity. To reach the temperatures useful for the needs of the invention, calories can also be supplied by heating the inside of the screw machine such as the screw or the casing of the extruder by means of a heating system such as a double jacket, heating resistors. The adiabatic expansion carried out in step b) is characterized as flash expansion in that it allows the natural rubber to pass from a compressed state to an uncompressed state almost immediately, typically in a time less than one second. It is carried out at a differential pressure greater than or equal to 40 bars or at a differential pressure greater than 100 bars. According to a first variant, the differential pressure in step b) is at most 100 bars, in particular between 40 and 100 bars. According to a second variant, the differential pressure is greater than 100 bars, or even at least 120 bars or 150 bars. The second variant has the advantage of also reducing the nitrogen content in natural rubber.

The expansion being adiabatic, the expansion occurs at the temperature at which the compression was carried out. At the end of the expansion, the coagulum is generally at atmospheric pressure and its moisture content is reduced, in particular to a content of less than 5%, preferably to a content of less than 3%. At the outlet of the die, the natural rubber can be cut, then packaged or alternatively cut, dried further, then packaged.

The natural rubber can be cut at the outlet of the die by a means which is capable of cutting natural rubber and which is disposed downstream of the die. The means suitable for cutting natural rubber may be a knife or a granulator, preferably a granulator.

Preferably, the natural rubber recovered at the outlet of the die is dried by additional drying to further reduce its residual moisture level, in particular to a rate of less than 0.8%. Before the additional drying, the natural rubber is advantageously cut at the outlet of the die by a means which is capable of cutting the natural rubber and which is placed downstream of the die. The means suitable for cutting natural rubber may be a knife or a granulator, preferably a granulator. The divided state in which the natural rubber is found after having been cut makes the additional drying more efficient.

The drying time is adjusted by those skilled in the art as a function of the drying temperature and as a function of the residual water content in the natural rubber at the end of step b). It is preferable to apply the shortest possible drying time to preserve the structure of the polyisoprene chains of natural rubber and its properties. Therefore, a drying time of less than 10 minutes is recommended and preferred. To obtain a natural rubber containing less than 0.8% water and considered to be dry with such short drying times, the drying is preferably drying by convection. Any known means of drying by convection may be suitable. In particular, a fluidized bed is preferred, such as a hot air vibrating screen, a device known and conventionally used in synthetic rubber manufacturing processes. Advantageously, the natural rubber recovered at the outlet of the die is dried by additional drying, preferably by convection, preferably by means of by means of a fluidized bed, more preferably by means of a hot air vibrating screen. The drying by convection is preferably carried out in air. Drying by convection in air can be carried out at a temperature ranging from 90 ° C to 180 ° C, preferably at a temperature ranging from 90 ° C to 130 ° C, in particular from 110 ° C to 130 ° C.

According to a preferred embodiment of the invention, the method further comprises a step during which a viscosity stabilizer is added, in which case the natural rubber is said to be stabilized. The viscosity stabilizer can be added inside the sleeve according to a first variant or outside the sleeve after step b) according to a second variant.

Viscosity stabilizers for stabilizing the viscosity of natural rubber are well known to those skilled in the art of natural rubber. They reduce or eliminate the tendency of natural rubber to harden on storage. As a viscosity stabilizer useful for the purposes of the invention, any compound known to stabilize the viscosity of natural rubber may be suitable. Mention may be made, for example, of hydroxylamine, its salts, hydroxyalkylamines, their salts, semicarbazide, dimedone, compounds having a triazole function and compounds having a hydrazide function. Preferably, the viscosity stabilizer is dimedone or a compound derived from ammonia chosen from compounds of formula XNH2 and their salts, where X is a group chosen from hydroxyl and C1-C4 hydroxyalkyl groups or a mixture of these compounds. The salt can be a weak acid salt of compounds of formula XNH2 or a strong acid salt of compounds of formula XNH2 optionally neutralized with a strong base. For neutralization with a strong base, one can for example refer to the description of patent application WO 2017085109.

More preferably, the viscosity stabilizer is hydroxylamine sulfate or hydroxylamine sulfate neutralized with sodium hydroxide, very advantageously hydroxylamine sulfate. Preferably, the viscosity stabilizer is added at a rate ranging from 2.4 mmoles to 24 mmoles, more preferably from 6 mmoles to 24 mmoles, even more preferably from 8 mmoles to 18 mmoles equivalent of dimedone or equivalent of XNH2 per kilogram of natural rubber .

According to the first variant, the viscosity stabilizer is injected by means of an injection device which comprises one or more orifices opening into the sleeve.

To implement this variant, the sleeve preferably carries over all or part of its length fingers which extend radially inwardly of the sleeve relative to the axis of rotation of the screw. Such fingers are placed in a zone which is downstream of the supply zone dedicated, for example, to the introduction of the mixture of wet coagulum and of peptizer. When the sleeve carries fingers which extend radially inwardly of the sleeve relative to the axis of rotation of the screw, the thread of the screw which is helical and which extends radially from a central shaft of the screw is interrupted so as to form cylindrical annular spaces in which the fingers are placed. In a well-known manner, an extruder has a feed zone and a pressurization zone (compression zone) dedicated to the rise in temperature and pressure of the coagulum and located downstream of the feed zone. Typically, a viscosity stabilizer is continuously introduced into natural rubber by injecting the viscosity stabilizer into the barrel of the extruder. Preferably, the viscosity stabilizer is injected in the form of an aqueous solution. The viscosity stabilizer is distributed within the natural rubber under the effect of the mechanical forces exerted in the sleeve during the operation of the extruder. The assembly formed by the natural rubber and the viscosity stabilizer is homogenized within the sleeve by the mixing function also provided by the operation of the extruder. The mixing can be improved by the presence of fingers in the sleeve which extend radially towards the inside of the sleeve, the fingers being able to be carried by the sleeve. The rise in temperature generated by the mixing allows the reaction between the viscosity stabilizer and the natural rubber. To facilitate the reaction, it is also possible to raise the temperature of the coagulum inside the sheath by means of a double jacket or any other heating system such as heating resistors fitted to the sheath and / or by means of a heating system incorporated in the screw. The viscosity stabilizer is added to the natural rubber in the sleeve to stabilize the viscosity of the peptized natural rubber.

To allow the injection of the viscosity stabilizer into the sleeve, the extruder useful for the needs of the invention is equipped with an injection device which comprises one or more orifices opening into the sleeve. The orifices called injection points are preferably located downstream of the supply zone, preferably in a compression zone downstream of the supply zone. The injection downstream of the limit feed zone, or even eliminates the part of viscosity stabilizer which would not be incorporated in the natural rubber, which has the effect of increasing the efficiency of the process with respect to stabilization of natural rubber. The location of the injection points in the compression zone makes it possible to further increase the efficiency of the process by ensuring good incorporation of the viscosity stabilizer into the natural rubber and sufficient contact time between the viscosity stabilizer and the rubber. natural before adiabatic relaxation.

Preferably, the injection points are located at the radially inner end of the fingers which are carried by the sleeve and which extend radially towards the inside of the sleeve relative to the axis of rotation of the screw. In other words, each injection point is located at the radially inner end of a finger which is carried by the sleeve and which extends radially towards the inside of the sleeve relative to the axis of rotation of the sleeve. screw. This location of the injection points also makes it possible to ensure effective incorporation of the viscosity stabilizer into the heart of the natural rubber and contributes to good distribution of the viscosity stabilizer in the natural rubber. More preferably, the injection points are located in fingers carried by the sleeve and extending radially towards the inside of the sleeve relative to the axis of rotation of the screw, such fingers being arranged in the zone of compression. This localization of the injection points makes it possible to further increase the efficiency of the process by combining the benefits provided by the localization in the fingers and the localization in the compression zone.

According to one embodiment of the invention, the sleeve carries a single finger extending radially towards the inside of the sleeve relative to the axis of rotation of the screw and at the radially inner end of which a point is located. injection, knowing that the sleeve can carry other fingers extending radially towards the inside of the sleeve relative to the axis of rotation of the screw and at the end of which there is no injection point. These other fingers promote the mixing of the natural rubber in the barrel and help to mix the natural rubber and the viscosity stabilizer.

Typically, the pressure at the injection point, in particular at the radially inner end of the finger, is greater than 0 bar relative, which reflects the presence of material (rubber coagulum) at the injection point and makes it possible to ensure that the viscosity stabilizer is well injected into the natural rubber. Injection of the viscosity stabilizer into a space in the barrel not filled with natural rubber could result in a loss of process efficiency. In fact, injection into an empty space promotes the risk of entrainment of part of the viscosity stabilizer with the water extracted from the coagulum. It also promotes the risk of decomposition of part of the viscosity stabilizer under the effect of heat in the sheath even before it comes into contact with the natural rubber. To further improve the efficiency of the process, the pressure at the point of injection is preferably greater than the saturated vapor pressure of the water at the temperature of the point of injection.

The reaction between the viscosity stabilizer and the natural rubber being activated by the temperature, the temperature at the point of injection is preferably greater than or equal to 100 ° C, more preferably from 130 to 210 ° C, even more preferably from 150 to 210 ° C. ° C. A temperature below 100 ° C does not allow such effective stabilization, the yield of the reaction being lower at these temperatures compared to temperatures above 100 ° C. Above 210 ° C, the polyisoprene chains of natural rubber can degrade.

According to the second variant, the addition of the viscosity stabilizer to the natural rubber carried out after step b) is typically done by spraying the natural rubber, preferably dry, with the desired amount of viscosity stabilizer, the natural rubber preferably being in the form of granules. For example, the natural rubber recovered from the die and dried by convection drying is sprayed with a viscosity stabilizer. To do this, the viscosity stabilizer is generally dissolved in water in order to be able to sprinkle the natural rubber. The step of adding the stabilizer viscosity according to the second variant is preferably followed by mechanical work at a temperature of at least 100 ° C. The mechanical work which has the function of dispersing the viscosity stabilizer in the natural rubber can be carried out by means of a shredding and homogenizing device. Typically, it is implemented by means of a device called a “pre-breaker”. A pre-breaker is a shredding and homogenizing device well known to those skilled in the art of natural rubber, since it is traditionally used in factories for “remilling” natural rubber. Reference may for example be made to patent application WO 2015189365 which gives a detailed description of a prebreaker.

Natural rubber, another subject of the invention, is a natural rubber at the bottom of a cup having a plasticity retention index (PRI) greater than 70, and a Mooney ML (1 + 4) viscosity at 100 ° C less than 60 , preferably at most 55, in particular from 30 to 55. The PRI is the ratio expressed as a percentage of the plasticity of aged natural rubber to the plasticity of natural rubber before aging. Its determination is used to give an indication of the resistance to oxidation of natural rubber. The higher the value, the better the resistance to oxidative aging. As the natural rubber in accordance with the invention has a Mooney viscosity of less than 60, its use in a rubber composition does not require a prior plasticization step. Owing to its PRI, it exhibits good resistance to aging by thermooxidation, which has the consequence of also improving the properties of a rubber composition containing it.

Such a compromise is not observed on conventional natural cup bottom rubbers such as grade TSR20. Because of their high viscosity, these grades must be plasticized to reduce their viscosity: the thermomechanical work carried out on natural rubber during plasticization is accompanied by thermooxidation of the polyisoprene chains, which inevitably leads to a reduction in the PRI.

The use of a natural rubber in accordance with the invention in a rubber composition makes it possible to improve both the properties of the rubber compositions and the productivity of the factories manufacturing these rubber compositions.

The natural rubber according to the invention can be prepared according to the process according to the invention, preferably according to particular embodiments of the process according to the invention. These particular embodiments can include a step of stabilizing the viscosity of natural rubber, implemented by adding a viscosity stabilizer to the mixture of wet coagulum and peptizer in the sheath or in a pre-breaker, preferably used. works by adding a viscosity stabilizer to the mixture of wet coagulum and peptizer in the barrel. The natural rubber in accordance with the invention is preferably a stabilized natural rubber. When the wet coagulum of natural rubber used in step a) is decontaminated by a filtration process described according to patent application WO 2016162645 A2, the natural rubber in accordance with the invention is advantageously devoid of impurities of size greater than 0.5 mm, very advantageously free of impurities greater than 0.1 mm in size. The filtration process makes it possible to remove impurities of size greater than 1 mm, advantageously greater than 500 μm (0.5 mm), more advantageously greater than 100 μm (0.1 mm).

In the production of conventional cup bottom grades such as TSR20, the impurities present in the wet coagulum of natural cup bottom rubber are typically leaves, twigs, sand, soil and other debris. Their removal from the coagulum traditionally involves a process which comprises successive operations of shredding and washing in a swimming pool. Unfortunately, the effectiveness of this process proves to be ineffective in removing all the impurities. The presence of impurities larger than 0.5 mm in a natural rubber at the bottom of the cup can be the cause of the formation of a hole in a fine semi-finished product, in particular during the manufacture of semi-finished product containing strips of very thin rubber composition. The presence of these impurities in natural rubber can therefore prove to be problematic in the manufacture of semi-finished products. The absence of impurities of size greater than 0.5 mm, or even greater than 0.1 mm in the natural rubber in accordance with the invention gives the natural rubber a quality superior to the conventional grades of the bottom of the cup.

In summary, the invention is advantageously implemented according to any one of the following embodiments 1 to 36:

Mode 1: A method of treating a natural rubber which comprises the extrusion of a mixture of a wet coagulum of natural rubber and a peptizer in a worm machine equipped with a sheath, in which the screw, and a die with holes at the end of the screw, process in which: a) the mixture is compressed at a temperature greater than or equal to 130 ° C and less than or equal to 210 ° C in the sheath, b) an expansion adiabatic flash at a differential pressure greater than or equal to 40 bars is produced at the die outlet.

Mode 2: Method according to Mode 1 in which the worm machine is fed with the mixture of wet coagulum of natural rubber and peptizer by a hopper fitted to the worm machine.

Mode 3: Method according to mode 1 or 2 in which the natural rubber is a natural rubber at the bottom of the cup.

Mode 4: Process according to any one of modes 1 to 3 in which the wet coagulum contains more than 5% by mass of water. Mode 5: Process according to any one of modes 1 to 4 in which the wet coagulum contains between 5 and 40% by mass of water.

Mode 6: Process according to any one of modes 1 to 4 in which the wet coagulum contains between 8 and 30% by mass of water.

Mode 7: Process according to any one of modes 1 to 6 in which the wet coagulum contains between 8 and 25% by mass of water.

Mode 8: Process according to any one of modes 1 to 7 in which the peptizer is a phenol or a metal salt of a phenol or else a dithiobisbenzamide.

Mode 9: Process according to any one of modes 1 to 8 in which the peptizer is pentachlorothiophenol, its zinc salt or 2-dibenzamido diphenyl disulfide.

Mode 10: Process according to any one of modes 1 to 9 in which the amount of peptizer is 0.01 to 0.5 gram of peptizer per kilogram of dry natural rubber.

Mode 11: Process according to any one of modes 1 to 10 in which step a) is preceded by a step of soaking a wet coagulum of natural rubber in an aqueous solution of peptizer or by a step of sprinkling a wet coagulum of natural rubber with an aqueous solution of peptizer to form the mixture of wet coagulum of natural rubber and peptizer.

Mode 12: Process according to mode 11 in which the wet coagulum is sprayed with an aqueous solution of peptizer at the rate of 0.1 to 1 liter of solution per kilogram of dry natural rubber, knowing that the solution has a concentration ranging from 0.05 to 3 grams of peptizer per liter of solution.

Mode 13: Process according to any one of modes 1 to 12 in which the natural rubber recovered at the outlet of the die is dried by additional drying.

Mode 14: Method according to mode 13 in which the additional drying is convection drying.

Mode 15: Process according to mode 13 or 14 in which the additional drying is drying by convection in air at a temperature ranging from 90 ° C to 180 ° C.

Mode 16: Process according to any one of modes 13 to 15 in which the additional drying is drying by convection in air at a temperature ranging from 90 ° C to 130 ° C.

Mode 17: Process according to any one of modes 13 to 16 in which the additional drying is drying by convection in air at a temperature ranging from 110 ° C to Mode 18: Method according to any one of modes 13 to 17 in which the additional drying is drying by convection by means of a fluidized bed.

Mode 19: Method according to any one of modes 13 to 18 in which the additional drying is drying by convection by means of a vibrating sieve with hot air.

Mode 20: Process according to any one of modes 13 to 19 in which, before the additional drying, the natural rubber recovered at the outlet of the die is cut by a means which is capable of cutting the natural rubber and which is placed downstream of the die. the die, preferably a granulator.

Mode 21: Process according to any one of modes 1 to 20 in which the natural rubber is compressed in step a) at a temperature between 170 ° C and 210 ° C.

Mode 22: Process according to any one of modes 1 to 21 in which the natural rubber is compressed in step a) at a temperature between 180 ° C and 210 ° C.

Mode 23: Process according to any one of modes 1 to 22 in which the differential pressure in step b) is at most 100 bars.

Mode 24: Process according to any one of modes 1 to 23 in which the differential pressure in step b) is greater than 100 bars.

Mode 25: Process according to any one of modes 1 to 24 in which the differential pressure in step b) is at least 120 bars.

Mode 26: Process according to any one of modes 1 to 25 in which the differential pressure in step b) is at least 150 bars.

Mode 27: Process according to any one of modes 1 to 26, which process comprises a step during which a viscosity stabilizer is added.

Mode 28: Process according to mode 27 in which the viscosity stabilizer is hydroxylamine, its salts, hydroxyalkylamines, their salts, semicarbazide, dimedone, compounds having a triazole function and compounds having a hydrazide function.

Mode 29: Process according to any one of modes 27 to 28 in which the viscosity stabilizer is a compound derived from ammonia chosen from compounds of formula XNH ₂ and their salts, where X is a group chosen from hydroxyl groups and C ₁ -C ₄ hydroxyalkyl or a mixture of these compounds.

Mode 30: Process according to any one of modes 27 to 29 in which the viscosity stabilizer is added at a rate ranging from 2.4 mmoles to 24 mmoles, preferably from 6 mmoles to 24 mmoles, more preferably from 8 mmoles to 18 mmoles equivalent of dimedone or equivalent of XNH2 per kilogram of natural rubber Mode 31: Method according to any one of modes 1 to 30, which method comprises a step of filtering the wet coagulum before step a).

Mode 32: Natural cup bottom rubber with a plasticity retention index greater than 70 and a Mooney ML (1 + 4) viscosity at 100 ° C less than 60.

Mode 33: Natural cup bottom rubber according to mode 32, which natural rubber has a Mooney ML (1 + 4) viscosity at 100 ° C of not more than 55.

Mode 34: Natural rubber at the bottom of the cup according to mode 32 or 33, which natural rubber has a Mooney ML (1 + 4) viscosity at 100 ° C from 30 to 55.

Mode 35: Natural cup bottom rubber according to any one of modes 32 to 34, which natural rubber is stabilized.

Mode 36: Natural cup bottom rubber according to any one of modes 32 to 35, which natural rubber is free from impurities larger than 0.5 mm.

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

Example

To measure Mooney viscosity, an oscillating consistometer as described in ASTM D1646-2007 (Reapproved 2012) is used. Mooney viscosity is measured according to the following principle: natural rubber is molded in a cylindrical chamber heated to 100 ° C. After one minute of preheating, the rotor turns within the specimen at 2 revolutions / minute and the torque useful to maintain this movement is measured after 4 minutes of 8 rotations. The Mooney viscosity (ML 1 + 4) is expressed in "Mooney unit" (MU, with 1 MU = 0.83 Newton.meter).

The plasticity retention index (PRI) is measured according to the ASTM D 3194-04 standard.

The water content is determined with a Mettler Toledo HB43-S halogen desiccator. The desiccator is an automated device which incorporates a cup, a scale and a cover intended to close the cup. The cup is positioned on the scale. The cover comprises a means of heating by a halogen lamp, this heating means being triggered when the cover is folded back on the cup. In the cup, a sample of 10 grams of natural rubber is weighed exactly: the device registers the corresponding weight "a". The cover is closed to close the cup, which triggers the temperature rise to reach a set point of 160 ° C. When the device detects a decrease in weight of less than 0.001 g per minute, the device detects a weight "b". The water content in the sample is given as a mass percentage by the following equation:

Water content {%) = 100 * ((ab) / a) Preparation of natural cup bottom rubbers NR1 and NR2 according to a process not in accordance with the invention:

A cup bottom coagulum is fed to an extruder in the form of granules having a water content of 24%. The extruder is a single screw extruder, it is equipped with a die with holes at the end of the screw and a granulator placed at the die outlet. The extruder has a double casing, its sheath having water discharge means (grooves, slots, holes) in the supply zone. The speed of the screw is 150 revolutions / min, the pressure is 63 bars, the temperature of the coagulum is 174 ° C, the temperature and the pressure being measured by sensors positioned as close as possible to the die, between the die and the end of the screw closest to the die. Coming out of an extruder, the coagulum in the form of granules is dried on a vibrating sieve with hot air at a temperature of 119 ° C. for about 5 minutes. The natural rubber NR1 is recovered and its Mooney viscosity and its plasticity retention index reported in Table 1 are measured.

Instead of being dried according to the procedure described above, the same starting coagulum as that used to feed the extruder, also in the form of granules, was dried according to a traditional method conventionally used for the manufacture of the grade. TSR20, i.e. drying in a hot air tunnel at a temperature ranging from 108 ° C to 125 ° C for 4h30 minutes. The natural rubber NR2 is recovered and its Mooney viscosity and its plasticity retention index reported in Table 1 are measured.

The Mooney viscosity and the plasticity retention index of natural rubbers NR1 and NR2 are measured.

Preparation of natural cup bottom rubbers NR3 to NR7 according to a process in accordance with the invention:

The natural rubbers NR3 to NR7 are prepared according to a process which differs from the manufacturing process for NR1 in that the introduction of the wet coagulum into the extruder is preceded by a sprinkling step with a solution of peptizer depending on the conditions. following.

For NR3: 20 kg of wet granules are sprayed three times in a row with 10 liters of an aqueous solution of 2-dibenzamido diphenyl disulfide concentrated at 1.5 g / L of aqueous solution.

For NR4: 20 kg of wet granules are sprayed three times in succession with 10 liters of an aqueous solution of 2,6-di-tert.-butyl-p-cresol concentrated at 0.5 g / L of aqueous solution. For NR5: 20 kg of wet granules are sprayed three times in succession with 10 liters of an aqueous solution of 2,6-di-tert.-butyl-p-cresol concentrated at 1 g / L of aqueous solution.

For NR6: 20 kg of wet granules are sprayed three times in succession with 10 liters of an aqueous solution of 2,6-di-tert.-butyl-p-cresol concentrated at 1.5 g / L of aqueous solution. For NR7: 20 kg of wet granules are sprayed three times in succession with 10 liters of an aqueous solution of 2,6-di-tert.-butyl-p-cresol concentrated at 1 g / L of aqueous solution. After drying on the vibrating screen, the natural rubbers NR3 to NR6 are recovered and their Mooney viscosity and their plasticity retention index measured. The values are reported in Table 1.

NR7 natural rubber is prepared according to a particular embodiment of the process according to the invention, since a viscosity stabilizer is added to natural rubber under the conditions described below:

An aqueous solution of hydroxylamine sulfate at 150 g / L of solution is injected continuously into the barrel of the extruder supplied with the mixture of wet coagulum of natural rubber and peptizer, at a rate of 0.08 part by weight of sulfate d hydroxylamine per 100 parts by weight of dry natural rubber. The pressure at the point of injection is 11.8 bars and the temperature of the mixture of wet coagulum and peptizer at the point of injection is 166 degrees.

The Mooney viscosities and the plasticity retention indices of the natural rubbers NR1 to NR7 recovered are measured before their storage. The values are reported in Table 1.

The Mooney viscosity and the plasticity retention index of natural rubber NR7 also measured after storage in ambient air at 25 ° C for two months are shown in Table 2.

Table 1

Table 2

The results show that compared to the process for manufacturing the TSR20 grade natural cup bottom rubber, the process according to the invention leads to the production of natural rubbers which, before their storage, exhibit an improved compromise between their resistance to aging by thermooxidation. and their suitability for working in mixing or calendering tools. Indeed, the plasticity retention index of natural rubbers NR3 to NR7 before storage is higher than that of NR2, while their Mooney viscosity is much lower than that of NR2. The process not in accordance with the invention and used for the synthesis of NR1 leads to the synthesis of natural rubber with a very high retention index, but its Mooney viscosity, also very high, is unfavorable to the productivity of a composition production line. made from natural rubber. The particular embodiment of the process in accordance with the invention which results in the production of NR7 also has the advantage of retaining the improved compromise of properties even after storage.

Claims

1. A method of processing a natural rubber which comprises extruding a mixture of a wet coagulum of natural rubber and a peptizer in a worm machine equipped with a sheath, in which the screw turns. , and a die with holes at the end of the screw, process in which: a) the mixture is compressed at a temperature greater than or equal to 130 ° C and less than or equal to 210 ° C in the sheath, b) a flash expansion adiabatic at a differential pressure greater than or equal to 40 bars is produced at the die outlet.

2. The method of claim 1 wherein the worm machine is fed with the mixture of wet coagulum of natural rubber and peptizer by a hopper equipping the worm machine.

3. The method of claim 1 or 2 wherein the natural rubber is a natural cup bottom rubber.

4. Method according to any one of claims 1 to 3 wherein the wet coagulum contains more than 5% by weight of water, in particular between 5 and 40% by weight of water.

5. Method according to any one of claims 1 to 4 wherein the wet coagulum contains between 8 and 30% by weight of water, preferably between 8 and 25% by weight of water.

6. Process according to any one of claims 1 to 5, in which the peptizer is a phenol or a metal salt of a phenol or else a dithiobisbenzamide.

7. Method according to any one of claims 1 to 6 wherein step a) is preceded by a step of dipping a wet coagulum of natural rubber in an aqueous solution of peptizer or by a sprinkling step. of a wet coagulum of natural rubber with an aqueous solution of peptizer to form the mixture of wet coagulum of natural rubber and peptizer.

8. The method of claim 7 wherein the wet coagulum is sprinkled with an aqueous solution of peptizer at a rate of 0.1 to 1 liter of solution per kilogram of dry natural rubber, the solution having a concentration ranging from 0.05 to 3 grams of peptizer per liter. of solution.

9. A method according to any one of claims 1 to 8 wherein the natural rubber recovered at the outlet of the die is dried by additional drying, preferably by convection, preferably by means of a fluidized bed, more preferably. by means of a hot air vibrating sieve.

10. The method of claim 9 wherein the additional drying is drying by convection in air at a temperature ranging from 90 ° C to 180 ° C, preferably from 110 ° C to 130 ° C.

11. The method of claim 9 or 10 wherein, before the additional drying, the natural rubber recovered at the outlet of the die is cut by means suitable for cutting natural rubber and disposed downstream of the die, preferably a granulator.

12. Method according to any one of claims 1 to 11, which method comprises a step during which a viscosity stabilizer is added.

13. A method according to any one of claims 1 to 12, which method comprises a step of filtering the wet coagulum before step a).

14. Natural cup bottom rubber having a plasticity retention index greater than 70 and a Mooney ML (1 + 4) viscosity at 100 ° C less than 60, which natural rubber is preferably stabilized.

15. Natural rubber for the cup base according to claim 14, which natural rubber is free from impurities of a size greater than 0.5 mm.