FR2478653A1 - PROCESS FOR THE STABILIZATION OF THE VISCOSITY OF NATURAL RUBBER DURING STORAGE AND THE JOINT IMPROVEMENT OF RESISTANCE TO THERMO-OXIDATION OF RUBBERS CAUSED BY LATEX NATURAL COAGULATION AND ENHANCED NATURAL RUBBERS RESULTING FROM THIS PROCESS - Google Patents

Abstract

PROCESS FOR THE STABILIZATION OF THE VISCOSITY OF NATURAL RUBBER DURING STORAGE, WITH PRESERVATION OF ITS VULCANIZATION CHARACTERISTICS AND ITS MECHANICAL PROPERTIES, AND FOR THE JOINT IMPROVEMENT OF THE RESISTANCE TO THERMO-OXIDATION OF THE COOLING OF THE RUBBER LATEX. THIOSEMICARBAZIDE OR ITS SALTS FORMED BY ADDITION OF AN ACID, PREFERABLY IN THE FORM OF AN AQUEOUS SOLUTION POSSIBLY EMULSIONED IN A MINERAL OR VEGETABLE OIL, A PROPORTION OF THIOSEMICARBAZIDE FROM 0.1 TO 0.5 BY WEIGHT IS USED AS A STABILIZING AGENT. IN RELATION TO DRY RUBBER BEING SUFFICIENT AND THE STABILIZING AGENT BEING ADDED EITHER TO LATEX, OR TO COAGULATED AND PELLET RUBBER, BUT NOT YET DRY. THE PROCESS PROVIDES A RUBBER HAVING A LOWERED AND STABILIZED VISCOSITY ALLOWING ENERGY SAVING DURING MASTICATION AND MIXING, BETTER DAMPING OF THE MATERIAL, SAVINGS OF INVESTMENT.

Description

It has been known for a long time (MARTIN - Rubb Res Scheme

  of Ceylon, 2nd Quaterly Circular, 23 (1930); R.I. WOOD - J.Rubb.

  Res. Inst. Malaya 14, 20 (1952)) that the natural rubber

  latex of Hevea Brasiliensis sees its consistency Mooney increase

  when storing. This increase in consistency, which can

  exceed 30 Mooney points, implies the need for a premastic

  longer gum in the factory, where energy costs are

  and immobilization of equipment. On the other hand, this increase

  consistency is variable depending on the time and temperature of storage, the type of rubber and machining, the clone from which the dry rubber is derived, etc ... This results in a great variability of Mooney consistency, a batch to another, so technical complications at the time of setting

implemented.

  It is SEKHAR (BC SEEHAR, Rubb.Chem.Technol.31 (3), 425 (1958), BC SERMAR, J. Polym Sc, 48, 133 (1960), BC SEKHAR, Proc Nat Nat. Res., Conf., Kuala-Lumpur, 512 (1960), BCSEKHAR, Proc., 4th Rubb., Technol Conf., London, 460 (1962)) which gave

  the first an explanation for this phenomenon of hardening

  the consequence of crosslinking reactions (this term refers to

  In fact, as in the present text, a "rectification",

  that is to say a three-dimensional crosslinking)> between

  posed amines present in natural rubber and

  carboxyl or carbonyl moieties carried by the cis-1,4 chalne

polyisoprene itself.

  SUBRAMANIAN (A. Subramanian, Proc., Int., Rubb., Conf., Kuala-Lumpur, 4, 3 (1975); A SUBRAMANIAN, J. of the R.R.I.M., 25,

  part 2, 61 (1977)) studied more precisely the

  bonyle, while BURFIELD and GAN have focused on the identification of other oxygen groups, the epoxy groups

  (D.R. BURFIELD, Nature, 249, 29 (1974), D.R. BUTJRFIELD and S.N.

  GAN, J. Polym. Sci. Polym. Chem. Ed., 13, 2725 (1975); D.R.

  BURFIELD, L. C. CHEW and S. N. GAN, Polymer 17, 713 (1976)), whose role in storage hardening phenomena is not yet very clearly established. The pH of the latex and the nature

  Amino products also seem to be of decisive importance.

  terminating on these crosslinking processes (D.R. BURFIELD and S. N. GAN, J. Polym Chem.Ed. 15, 2721 (1977); M.J.GREGORY and

  A. S. FlAN, Proc. Int. Rubb. Conf., Kuala-Lumpur 4, 29 (1975)).

  In order to neutralize the hardening on storage, SEEHAR imagined blocking the carbonyl groups by reacting them with hydroxylamia sulfate or hydrochloride.

  This is how the CV rubbers (viscosity

  aunt) which are more and more requested by the manufacturers.

  The stabilization of the viscosity was mainly applied to granulated rubbers obtained in particular according to the hevacrum process (PS CHIN, J. of the RRIM, 22, (1), 56 (1969)), to deproteinized rubber (PI.AJ YAPA , QJ of RRI Sri Lanka 52, 1 (1975)), at Rubber Tire (S. SETHtJ, RRIM Short course on

  Natural Rubber Processing, 82 (1978)), at GP Rubber (R.R.I.M.

  Technical Bulletin of the SMR na 9 (1979)) and even at

  concentrated latex (R.R.I.N., British Patent No. 1,307,678 (1969)).

  On the other hand, the evolution of the PRI (retention index of

  plasticity) and the viscosity of CV rubbers during storage

  The long-term study has been studied (G. M. BRISTOW, NR Technol.5 (1), 1 (1974), G. M. BRISTOW, NR Technol 8 (2), 21 (1977)). Finally,

  a laboratory test known as the ASHT test (Accele-

  rated Storage Hardening Test = accelerated hardening test

  storage) has been developed (R.R.I.M., Information Bulletin

  technical assistance of SMR na 7, 24 (1970) (ASHT test)) in order to

  quickly characterize the stabilized viscosity rubbers.

  Rubbers which, after testing, have an increase in viscosity equal to or less than 8 Wallace units are said

  "stabilized viscosity", or constant viscosity (CV).

  Following the work of SEEHAR, a patent was taken in

  1961 (N.R.P.R.A., British Patent No. 965,757, 24.2.1961), which

  sells a method of stabilizing the viscosity of the rubber

  natural rubber with the aid of a compound of the general formula X - INH 2 where X is a group: - hydroxyl (example: hydroxylamine) - aromatic (example: naphthylamine)

  hydroxyalkyl (example: ethanolamine).

  Other patents claim: - either improvements in the process of incorporation of derivatives of hydroxyalamine (CEEW EAM CHOING, British Patent No. 1,259,717, 12.12.1969, R.R.I.M., British Patent No. 1,373,630,

31.01.1971);

  - the use of other products such as hydra-

  zines R - CO - NH - NH 2 (R.R.I.M., British Patent No. 1,472,064,

  8.6.1973) or the semicarbazide 1NH2 - CO - NH - NH2 introduced,

  others by soaking natural rubber granules

  (R.R.I.M., British Patent No. 1,484,252, 22.10.1973).

  Another product that stabilizes the viscosity of natural rubber is hydroxylamine - O - sulfonic acid (AHOS), which was published by the Institute for

  Rubber in 1977 (R. PAUTRAT, G. LOYEN and J. MARTEAU, Rev. Gen.

  Rubber-Dam. Plast. 54, 572, 47 (1977)).

  Compared with other qualities of natural rubber that harden on storage, stabilized viscosity rubbers

  offer the advantage of having, after machining, a lower viscosity

  not increasing substantially over time.

  However, the various viscosity stabilizers

  of rubber have the major drawbacks of

  the speed of vulcanisation and a decrease in

mechanical properties.

  The present invention aims to overcome these drawbacks.

vantages.

  For this purpose, it relates to a process for stabilizing

  the viscosity of the natural rubber during storage, with preservation of its vulcanization characteristics and its mechanical properties, and for the joint improvement of the resistance to thermooxidation of the rubbers produced by the

  natural coagulation of latex, characterized in that

  as a stabilizing agent for thiosemicarbazide or its salts

by addition of an acid.

  The acid used can be a hydrohalic acid or any

  other mineral acid, or an organic acid such as formic acid

that or acetic acid.

  It is advantageous to use thiosemicarbazide or its

  salts in the form of an aqueous solution, which may possibly

  be emulsified in mineral or vegetable oils.

  the. A concentrated solution of the stabilizing agent is generally used.

  The stabilization sought is sufficiently

  by the addition to the rubber of the stabilizing agent in the proportion of 0.1 to 0.5% by weight of thiosemicarbazide relative to the dry rubber. Said agent may be added either to the latex, before the coagulation thereof with the acid, or to the wet rubber granules, obtained after acid coagulation or natural coagulation of the latex, followed by machining of the latex.

  coagulum; in this second case, we operate by sprinkling or

  granules by means of a solution of the stabilizing agent, left in contact for a few minutes, then the granules are dried. The subject of the invention is also the improved natural rubber obtained, which is characterized in that it has a lowered and stabilized viscosity, with preservation and even improvement of

  its vulcanization characteristics and its mechanical properties.

  as will be seen below in the examples. In addition, the rubber resulting from the natural coagulation of the latex acquires, by the treatment according to the invention, a high resistance to thermooxidation and, consequently, a high MIC compared to that of most untreated dry rubbers of the same origin. The fact that the viscosity of the raw rubber is lowered

  stabilized, which avoids hardening when stored

  ge, allows the manufacturer to reduce the time of mastication and mixing cycles, thus increasing the performance of chewing and mixing machines, with all the economic consequences that result, including energy saving, material damping over a longer time, an investment saving. The Rubber Research Institute has therefore highlighted the very interesting properties of the thiosemicarbazide of formula

NH10 - - - NCHE

  X This product, because of the presence of the function N2 - -,

  is able to block the carbonyl groups of the natural rubber

  therefore stabilize its viscosity. In addition, it has the following advantages over hydroxylamine and its derivatives: its slightly accelerating character of vulcanization, da in the presence of the group CS-NHI2, makes it possible to obtain a

  kinetics of vulcanization close to that of natural rubbers

  current and high mechanical properties due to a good state of vulcanization;

  - its analogy with mercaptobenzimidazole and thiou-

  This gives it a deactivating effect, leading to better resistance to the thermal oxidation of raw rubber in the case of naturally coagulated latexes (as proved by PRI tests according to Standard IB0 2930), and some improvement in

  aging resistance of vulcanizates.

  In general, thiosemicarbazide is shown to be

  active at doses of 0.1 to 0.2% by weight relative to the rubber.

dry cowhide.

  This activity seems to find a technical and economic optimum

  nomic for a proportion of 0.3% by weight. It is possible to increase the dose further, but the gain in property becomes minimal while the expense, due to the price of the product, increases; therefore, in general, a proportion of 0.5% in

weight.

  The activity of thiosemicarbazide is independent of the time

  drying temperature of rubber, generally between

and 12000 C.

  The thiosemicarbazide used in aqueous solution must have a concentration less than or equal to 1.4% by weight,

  since its solubility limit is slightly

above this value.

  Thiosemicarbazide thus diluted can be added to the latex either indifferently or be used to treat by spraying or

  by dipping coagulated rubber granules.

  In the latter case, the activity of the product is effectively manifested for short contact times, for example 5 minutes. She hardly believes with longer soaking times. The following examples, which are purely indicative and in no way limitative, illustrate the action of thiosemicarbazide

in these different areas.

  Application Examples (All percentages shown

are by weight).

1st example

  - Thiosemicarbazide treatment of coagulated latex rubber

Dar acidification.

  One liter of natural latex of clone GT-1 is mixed with a

  Another liter of clone PR 107 latex.

  lablement slightly diluted with water to give them a

  dry rubber content of 28%, which is a common practice

te in planting plant.

  The two liters thus obtained are coagulated at pH 5.2 using acetic acid diluted to 5% and they will provide the rubber

witness referred to below (1).

  Two liters are added with a solution

  to 1.4% of hydroxylamine sulphate so as to have a

  0.15% of hydroxylamine sulfate relative to the

  dry cowhide. In this way, after a coagulation identical to the previous one, a stabilized viscosity rubber is obtained.

  of standard type, referenced hereinafter (2).

  Two other similar liters of latex are added with a solution of 1.4% thiosemicarbazide, so as to have a content of 0.3% of thiosemicarbazide relative to the dry rubber, to obtain, after coagulation identical to the previous, a improved rubber according to the present invention, referenced

hereinafter (3).

  After coagulation and overnight maturation, the

  three coagulae are machined separately by a granula-

  rotary cutter, preceded by a single rolling on the tight rolls of a creper, then they are placed in a rubber dryer under a hot air stream at 1050C for 2 hours 30 minutes; dry rubber granules are thus obtained. Each of the experimental rubbers was reprocessed and resubmitted to the tests nine times to provide results.

significant means.

  A. Properties of natural rubber, dry and raw (ie

  say without additives and unvulcanized).

  Table 1 below shows the average

  prepared on the natural rubbers, dry and

  (1): control rubber, unstabilised,

  (2): rubber stabilized with hydroxylamine sulphate (rubber)

strain C V according to the prior art),

  (3): rubber stabrilized with thiosemicarbazide, according to the pre-

this invention.

TABIEAU 1

  Test according to (1) (2) (3) ISO standard ISO (1) - (2) (3) Plasticity Wallace (Po) 2007 49 36 39

PRI 2930 86 87 87

Mooney Viscosity 269 - 77 57 61

Increase in plasticity

  quoted after ASHT test 18 2 4 Nitrogen content% 1656 0.49 0.48 0.52

  For the ASHT test, refer to the bibliographic reference

than mentioned.

  B. Vulcanization characteristics

  For each of the rubbers 1, 2 and 3, a mixture is preferably

  prepared according to the following formula: - Rubber 100 parts by weight - Zinc oxide '6' - Sulfur 3.5 '- Stearic acid 0.5' Mercaptobenzothiazole 0.5 'The vulcanisation characteristics of the three mixtures thus prepared (each mixture was repeated three times to obtain reliable average results) are then measured with a rheometer (according to ISO0 3417), according to the usual methods, with the following adjustment conditions: - Test temperature 1600C Angle d ' Rotation of the rotor 1 degree - No preheating - Selectivity The rheometric curves are then recorded and their analysis gives the following figures on two essential values (see Table 2):

TABIEAU 2

(1) (2) (3)

  Time to reach 90% of the maximum torque of 7.2 8.4 5.8 T90 vulcanization (min) Maximum torque (Nm) 2.20 2.09 2.25 Compared to the unstabilized rubber (1), respectively a decrease and an increase of the speed

  for vulcanising rubbers (2) and (3).

  C. Mechanical Properties of the Vulcanizates After vulcanization for 40 minutes at 1400 ° C. of the mixture described in B, the following mechanical properties were measured on the

  three types of rubber and are shown in Table 3.

TABLE 3

  We note that the rubber (3) treated with thiosemicarbazide

  has the highest mechanical properties.

  All the tests described above were repeated by drying the rubbers at 12000 instead of 1050C. The order of magnitude of the results obtained does not differ from the previous ones and

  leads to the same conclusions. The usual drying temperatures

  therefore, they have no influence apart from the fact that

  viscosity during the storage hardening test

  is lower for a drying temperature of 1200C compared with

port at 10500C.

  Conclusion of Example 1 The present invention relates to a novel method of

  preparation of the natural rubber which makes it possible to obtain

  derived from acidified latex, with lowered viscosity and

  (Table 1), without any reduction in the vulcanization rate (Table 2) nor any depression of the mechanical properties (Tables 2 and 3) of the vulcanizates, contrary to what is usually found with the so-called rubbers. at

  stabilized viscosity (CV) prepared especially with the use of hydro-

xylamine as stabilizer.

  1. Advanced rubbers are obtained by adding

  before coagulation of thiosemicarbazide in solution.

  at doses between 0.1 and 0.5% of the

(1) (2) (3)

  Breaking Strength (MPa) 21.6 19.2 22.1 Modulus at 100% elongation (MPa) 0.64 0.70 0.73 weight of dry rubber. These dry rubbers, obtained after usual machining, coagulation and drying, have a viscosity lowered and stabilized so that the phenomenon, called storage hardening, is practically eliminated for at least a year, like rubbers (CV ) stabilized in particular

  by adding hydroxylamine for example (Table 1).

  2. The acidified latex rubbers thus treated with

  addition of thiosemicarbazide have vulcaniza-

  sation, determined by measuring the rheometer t90 and the 100% elongation modulus under normalized conditions, which are at least equal, if not greater, than acidified latex rubbers

  witnesses, unlike those of rubber added with hy-

  droxylamine, which vulcanize more slowly (Tables 2 and 3).

  3. Vulcanizates of acidified latex rubbers

  thus treated by the addition of thiosemicarbazide possess

  mechanical characteristics at least equal to and even greater than

  those of acidified control acid-treated latex rubbers treated with

  droxylamine. Resistance to fracture, modulus at 100% elongation and maximum dynamic modulus values obtained at

  rheometer clearly show (Tables 2 and 3).

  It can be seen that the use of thiosemicarbazide under the conditions

  indicated, thus stabilizes the viscosity of the rubber.

  Acidified latex bunch by improving slightly but systematically

  the vulcanization speed of raw rubber and

  mechanical properties of vulcanizates, contrary to the use of hydroxylamine, which causes a slowing of the speed

  of vulcanization and some depression of the properties of

  -mechanical. It should be noted that the addition of the stabilizing agent in the form of an aqueous solution can take place on the rubber

  moist coagulated and granulated, by spraying or soaking thereof.

  In this case, identical results are obtained for tests A, B and C.

2nd example

  I. Treatment of Cumulative Lump Rubbers (Polybag) These rubber coagulated naturally by bagging sealed bags have plasticity retention index (MIC) somewhat lower on average than those of other latex rubbers. Their MICs are also irregular, especially when they come, at the end of the rainy season, from clones that are sensitive to natural coagulation, usually resulting in a low RRI. This is for example the case of clone GT-1. They also have an increasing viscosity during storage

  like all raw natural rubbers.

  800g of rubber from batches of rubber collected

  after eight cumulative bleeds, naturally coagulated in a polyethylene bag and derived from GT-1 latex, are rolled by ten creping passes, tight cylinders, before being introduced

  in a rotating knife granulator.

  Granules obtained are then soaked for 15 minutes.

  - in water containing no chemical products (reference rubber 4); in a 3% aqueous solution of hydroxylamine sulphate (rubber referenced 5);

  in a 1.4% aqueous solution of thiosemicarbazide (rubber

referenced rubber 6).

  After soaking, the granules are placed in a dryer

  rubber under hot air at 11000 for three hours.

  res. The values reported here are the average of nine

  repetitions of each test, which have allowed us to

  the low variability of the results obtained.

  A. Properties of raw rubber Table 4 below indicates the average properties

  compared with the three types of rubbers

  (4): unstabilized rubber, (5): hydroxylamine sulfate stabilized rubber (CV rubber according to the prior art), (6): thiosemicarbazide stabilized rubber, according to US Pat.

present invention.

TABLE 4

  The stabilizer is also effective with times

  dipping agents which are to be recommended as being better adapted

  to industrial constraints. A soaking time of 2 minutes in an aqueous solution of thiosemicarbazide at 1.4% already makes it possible to note the effectiveness of the product. She is slightly

  more intense for a time of 5 minutes.

  Granules, obtained according to the method described above, were treated under the same conditions reducing the duration

  dipping at 5 minutes and drying the rubber at a temperature of

  erase 120o0C; rubber referenced 7: control; rubber

  8: granules dipped in a 3% solution of sodium sulfate

  droxylamine; rubber referenced 9: granules impregnated in a

  1.4% solution of thiosemicarbazide. The results obtained are

  listed in Table 5. TABLE 5

BOARD

(7) (8) (9)

Plasticity Wallace Po 44 27 35

PRI 45 67 80

  Mooney Viscosity 81 54 66 Increase in plasticity after ASHT test il11 4 3

(4) (5) (6)

Plasticity Wallace Po 46 35 37.5

PRI 52 64 74

Mooney Viscosity 85 61 66

Increase in plasticity

  cited after ASHT test 9 1 1.5 Nitrogen content% 0.37 0.33 0.37

_. J ,,.

B. Characteristics of vulcanization From the rubbers 4, 5 and 6 mentioned in Table 4, mixtures are then prepared according to the formula indicated in

  paragraph B of example 1 above and are subject to

  res similar to those described in this paragraph for

  know their vulcanization characteristics.

  Their average rheometric values are then

lowing:

TABLE 6

_ ..

  C. Mechanical properties of vulcanizates

  After vulcanization for 40 minutes at 1400C of the premixed mixtures

  the following mechanical properties were measured on the three types of rubber and are shown in Table 7:

TABLE 7

  350 II. Treatment of cup bottom rubbers

  These rubbers, spontaneously coagulated like rubber

  cumulative bloodletting mentioned in paragraph I, possesses

  like them, but to a lesser degree, MICs are lower and less regular than those of a coagulated latex rubber; with acid. They also have an increasing viscosity during storage.

(4) (5) (6)

  Time to reach 90% o of maximum vulcanization torque t 90 (min) 5, 55 6.05 5.00 Maximum torque (N.m) 2.38 2.13 2.44

_, ..... _

(4) (5) (6)

  Breaking Strength (MPa) 21.7 20.6 23.6 Module at 100% elongation (MPa) - 0.83 0.75 0.8

  800g of rubber from cups of cups

  clones GT-1 and PR 107 are homogenized and rolled by 10 crepe passes, tight rolls, before being introduced

  in a rotating knife granulator.

  The granules obtained are then soaked for 15 minutes - in water containing no chemical products (control rubber referenced 10); in a 3% aqueous solution of hydroxylamine sulphate (rubber referenced 11); in a 1.4% aqueous solution of thiosemicarbazide

(rubber referenced 12).

  After soaking, the granules are placed in a dryer at

  rubber under hot air at 1100C for 3 hours.

  The values reported below are the average of six repetitions

  of each test, which has

  low variability of the results obtained.

A. Properties of the raw rubber.

  Table 8 below shows the average properties

  compared with the three types of raw rubbers.

TABLE 8

  B. Vulcanization characteristics All rubbers are then used for the preparation of

  mixtures according to the formula given in paragraph B of the

  ple 1 above and are subject to measures similar to those

(10) (11) (12)

Plasticity Wallace Po 46 33 39

PRI 53 67 76

  Mooney viscosity 79 60 71 Increased plasticity after ASHT test 7 1 2 Nitrogen content% 0.37 0.43 0.43 -04

  already described in that paragraph to know their characteristics

  vulcanization characteristics. The rheumatic values are then obtained

following average grades:

TABIAU 9

(10) (11) (12)

  5. _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ _ ,.

  Time to reach 90% of the maximum torque of vulcanis 6,20 5,45 5,07 canisation t 90 (mn) Maximum torque (N.m) 2,37 2,21 2,42 C. Mechanical properties of vulcanizates

  After vulcanization for 40 minutes at 1400C of the above mixtures

  teeth, the following mechanical properties were measured on

  the three types of rubber and are shown in Table 10.

TABLE 10

  Conclusion of the Second Example The present invention also relates to a new method for preparing natural rubber which makes it possible to obtain

  rubbers made from naturally coagulated, viscose latex

  lowered and stabilized tee with high resistance to thermooxygen

  dation, and this without any depression of the mechanical properties

  vulcanizates, contrary to what is usually

  with the so-called stabilized viscosity (CV) rubbers

  especially trimmed with use of hydroxylamine.

(10) (11) (12)

  Tensile Strength (MPa) 20.4 20.7 21.1 100% Alloy Modulus (MPa) 0.78 - 0.73 0.82 These rubbers are obtained by dipping wet natural coagula granules into thiosemicarbazide solution; 1. These dry rubbers, which are dried after the usual drying, have a lowered and stabilized viscosity in such a way that the so-called storage hardening phenomenon is practically eliminated for at least one year, like rubbers (CV).

  stabilized in particular by soaking in a hydroxylated solution

mine (Tables 4 and 5).

  2. These dry rubbers thus obtained have a resistance

  This is largely due to the rise in heat-oxidation, which is close to that of acidified latex rubbers as shown by the PRI plastic retention index values in Tables 4,5 and 8, compared with Table 1. obtain both a PRI level of such value

  related to such viscosity stabilizing characteristics ,. for naturally coagulated rubbers.

  3. These rubbers, enjoying a prior treatment of soaking in thiosemicarbazide solution, similar to

  one of those described above, have a velocity of

  tion, determined by the measurement of the t 9D obtained by the rheometer and by the value of the modulus of elongation at 100% in

  standardized conditions, which is higher than that of rubber

  sample bucks or soaked in hydroxylamine solution

(Tables 6, 7, 9 and 10).

  The vulcanizates of the rubbers thus treated with thio-

  Semicarbazide by dipping wet granules have

  mechanical characteristics superior to those of the control rubbers or soaked in a solution of hydroxylamine. The values of tensile strength, modulus at 100% elongation and

  maximum dynamic modulus obtained from the rheometer show it clearly

(Tables 6, 7, 9 and 10).

  It can be seen that the indicated method of soaking in thio-

  Semicarbazide thus makes it possible to stabilize the viscosity of the rubber.

  natural coagulation bunches by improving both

  their resistance to thermooxidation (PRI) and their characteristics

  mechanical ticks and vulcanization.

Claims (10)

- CLAIMS -   1.- Process for stabilizing the viscosity of natural rubber during storage, with preservation of its   characteristics of vulcanization and its mechanical properties   and for the joint improvement of resistance to thermooxidation of natural coagulation   latex, characterized in that the stabilizing agent   thiosemicarbazide or its salts formed by the addition of an acid. 2.Procédé according to claim 1, characterized in that the thiosemicarbazide or its salts are used in the form an aqueous solution.   3. A process according to claim 2, characterized   in that the said aqueous solution is used in the emulsion state.   in mineral or vegetable oils.   4. A process according to claim 2, characterized in that a 1.4% by weight aqueous solution of thiosemicarbazide is used.   5. Process according to any one of the claims   1 to 4, characterized in that the thiosemicarbazide or its salts is added to the rubber in the proportion of 0.1 to 0.5%   by weight of thiosemicarbazide relative to dry rubber.   6. A process according to claim 5, characterized   in that the thiosemicarbazide or its salts is added to the rubber   in the proportion of 0.3% by weight of thiosemicarbazide compared to dry rubber.   7. Process according to any one of the claims   1 to 6, applied to the stabilization of the viscosity of natural rubber, derived from Hevea Brasiliensis latex coagulated with acid, characterized in that the stabilizing agent is added to the latex, before the acid coagulation of that This is followed by   machining the coagulum transforming it into granules, which are   to provide dry and raw stabilized rubber.   8. Process according to any one of the claims   2 to 6, applied to the stabilization of the viscosity of natural rubber, derived from latex of Hevea Brasiliensis, coagulated with acid or coagulated naturally, characterized in that one   treated by spraying or dipping, with a thiosemide solution   carbazide or a salt thereof, the wet granules of   rubber obtained from the acid or naturally coagulated latex, with subsequent granulation of the coagulum, in that said granules and said solution are left in contact for a few minutes and then the granules are skunted to obtain the rubber dry and raw stabilized.   9. Improved natural rubber resulting from the process according to any one of claims 1 to 8, characterized   in that it has a lowered and stabilized viscosity, with   and even improvement of its vulcanization characteristics.   and its mechanical properties.   10.- Improved natural rubber, made from Hevea latex   Brasiliensis naturally coagulated and resulting from   claim 8, characterized in that it has a viscosity   lowered and stabilized and a high resistance to thermooxyda-   tion, with preservation and even improvement of its   vulcanizing ticks and its mechanical properties.