Process for the preparation of nitrile rubbers
09650502 ยท 2017-05-16
Assignee
Inventors
Cpc classification
International classification
B29C45/00
PERFORMING OPERATIONS; TRANSPORTING
Abstract
Process for the preparation of a nitrile rubber comprising: subjecting at least one ,-unsaturated nitrile, at least one conjugated diene and, optionally, one or more further copolymerizable monomers, to emulsion polymerization, in the presence of at least one alkyl thiol containing from 12 to 16 carbon atoms and at least three tertiary carbon atoms and having the sulfur bound to one of said tertiary carbon atoms, at a pH ranging from 7 to 10, preferably from 8 to 9, obtaining a latex; subjecting said latex to coagulation, in the presence of at least one metal sulfate selected from magnesium, sodium, potassium, preferably magnesium, at a temperature higher than or equal to 30 C., preferably ranging from 40 C. to 60 C., obtaining a coagulated nitrile rubber: subjecting said coagulated nitrile rubber to washing with water, in the presence of at least one inorganic base, selected from sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide, lithium hydroxide, preferably potassium hydroxide, at a pH higher than or equal to 10, preferably ranging from 11 to 12. The nitrile rubber thus obtained is capable of giving vulcanizable elastomeric compositions having a high vulcanization rate and a good vulcanization yield. Furthermore, said elastomeric compositions cause a low fouling of the molds and can consequently be advantageously used in injection molding processes.
Claims
1. A process for the preparation of a nitrile rubber comprising: subjecting at least one ,-unsaturated nitrile, at least one conjugated diene and, optionally, one or more further copolymerizable monomers, to emulsion polymerization, in the presence of at least one alkyl thiol containing from 12 to 16 carbon atoms and at least three tertiary carbon atoms and having the sulfur bound to one of said tertiary carbon atoms, at a pH ranging from 7 to 10, obtaining a latex; subjecting said latex to coagulation, in the presence of at least one metal sulfate selected from magnesium, sodium, and potassium, at a temperature higher than or equal to 30 C., obtaining a coagulated nitrile rubber; and subjecting said coagulated nitrile rubber to washing with water, wherein to said water is added at least one inorganic base, selected from sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide, and lithium hydroxide, at a pH higher than or equal to 10.
2. The process for the preparation of nitrile rubber according to claim 1, wherein said emulsion polymerization is carried out at a pH ranging from 8 to 9.
3. The process for the preparation of nitrile rubber according to claim 1, wherein said coagulation is carried out in the presence of magnesium sulfate.
4. The process for the preparation of nitrile rubber according to claim 1, wherein said coagulation is carried out at a temperature ranging from 40 C. to 60 C.
5. The process for the preparation of nitrile rubber according to claim 1, wherein said washing with water is carried out in the presence of potassium hydroxide.
6. The process for the preparation of nitrile rubber according to claim 1, wherein said washing with water is carried out at a pH ranging from 11 to 12.
7. The process for the preparation of nitrile rubber according to claim 1, wherein said ,-unsaturated nitrile is selected from ,-unsaturated nitriles having from 3 to 5 carbon atoms.
8. The process for the preparation of nitrile rubber according to claim 1, wherein said conjugated diene is selected from conjugated dienes having from 4 to 6 carbon atoms.
9. The process for the preparation of nitrile rubber according to claim 1, wherein said nitrile rubber is an acrylonitrile-butadiene (NBR) copolymer.
10. The process for the preparation of nitrile rubber according to claim 1, wherein said one or more copolymerizable monomers are selected from ,-unsaturated monocarboxylic or dicarboxylic acids, their esters or their amides, or mixtures thereof.
11. The process for the preparation of nitrile rubber according to claim 1, wherein said alkyl thiol is selected from: 2,2,4,6,6-pentamethylheptane-4-thiol, 2,4,4,6,6,8,8-pentamethyl-nonane-4-thiol, or mixtures thereof.
12. The process for the preparation of nitrile rubber according to claim 11, wherein said alkyl thiol is 2,2,4,6,6-pentamethylheptane-4-thiol.
13. The process for the preparation of nitrile rubber according to claim 1, wherein said alkyl thiol is a mixture comprising: 2,2,4,6,6-pentamethylheptane-4-thiol; 2,4,4,6,6-pentamethylheptane-2-thiol; 2,3,4,6,6-pentamethylheptane-2-thiol; and 2,3,4,6,6-pentamethylheptane-3-thiol.
14. The process for the preparation of nitrile rubber according to claim 1, wherein said alkyl thiol is used in an amount ranging from 0.05 parts by weight to 3 parts by weight per 100 parts by weight of the mixture of polymerization monomers.
15. The process for the preparation of nitrile rubber according to claim 1, wherein said metal sulfate is present in the latex in an amount ranging from 0.5 phr to 200 phr.
16. The process for the preparation of nitrile rubber according to claim 1, wherein the latex obtained from the polymerization has a concentration of solids ranging from 1% by weight to 40% by weight with respect to the total weight of the latex.
17. The process for the preparation of nitrile rubber according to claim 1, wherein said washing with water is carried out at a temperature ranging from 35 C. to 90 C.
18. The process for the preparation of nitrile rubber according to claim 1, wherein the amount of water used in said washing with water ranges from 0.5 phr to 20 phr.
19. A vulcanizable elastomeric composition comprising at least one nitrile rubber obtained according to claim 1 and at least one vulcanizing agent.
20. A method comprising: injection moulding a vulcanizable elastomeric composition according to claim 19.
Description
EXAMPLES
(1) The following characterization and analysis techniques were used.
(2) Properties of the Copolymer
(3) Determination of the Ion Content of the Nitrile Rubber
(4) The inductively coupled plasma-atomic emission spectrometry (ICP-AES) technique was used on a sample of nitrile rubber suitably mineralized according to the following process.
(5) 2 g of nitrile rubber were incinerated in a microwave muffle, at about 550 C., for 3 hours, in a porcelain crucible. 2 ml of concentrated nitric acid at 90% by weight were added to the residue obtained which was then heated on a plate for a few minutes. The solution obtained was transferred to a 50 ml flask and brought to volume with water. The solution was examined using an ICP-AES spectrometer at the following specific absorption wavelengths of each metal: Calcium: 317.93 nm; Magnesium: 285.21 nm; Potassium: 766.49 nm; Sodium: 589.59 nm.
(6) The calibration solutions of the ICP-AES spectrometer were obtained by dilution of standard concentrated and certified solutions. Three calibration points were normally used: 10 ppm, 5 ppm, 1 ppm, in a solution of nitric acid at 5% by weight.
(7) Determination of the pH of the Nitrile Rubber
(8) 3 g of nitrile rubber cut into small pieces were introduced, for the purpose, into a 150 ml cylinder containing 100 ml of chloroform and the whole mixture was subjected to stirring and under a nitrogen seal to prevent contact with carbon dioxide (CO.sub.2). The nitrile rubber was left to dissolve completely at room temperature (25 C.) and, after dissolution, 50 ml of bidistilled water adjusted to pH=7 were added with a calibrated pipette with an aqueous solution at 1% of sodium hydroxide, and the solution was left under vigorous stirring for 2 hours. The solution obtained was subsequently collected and subjected to centrifugation at 3,000 revs for 30 minutes. At the end of the centrifugation, the aqueous phase was removed and the pH was measured.
(9) Determination of the Content of Acrylonitrile Bound to the Nitrile Rubber
(10) The content of bound acrylonitrile was determined by measuring the total nitrogen in the nitrile rubber obtained using a LECO FP 528 analyzer, calculating the bound acrylonitrile from the total nitrogen content.
(11) Determination of the Mooney Viscosity of the Nitrile Rubber
(12) The measurement was carried out at 100 C., using 45 g of nitrile rubber, by means of a 200 E instrument of Alpha Technology according to the method ASTM D1646.
(13) Determination of the Ion Index of the Nitrile Rubber
(14) The ion index (II1) and the ion index (II2) of the nitrile rubber were determined according to the following formulae (1) and (2), respectively:
(15)
wherein c(Ca.sup.2+), c(Mg.sup.2+), c(Na.sup.+), and c(K.sup.+), indicate the concentrations of the calcium, magnesium, sodium and potassium ions in the nitrile rubber in ppm.
Properties of the Elastomeric Composition
(16) The quantities of the compounds used for the preparation of the elastomeric composition are indicated in Table 1. The elastomeric composition was prepared in an open mixer according to the method ASTM D3182.
(17) The vulcanization curves and the measurement of the relative parameters (MH, ML, t90, ts2) were carried out using a rheometer 100 S Monsanto, at a temperature of 160 C., according to the method ASTM D3187.
(18) TABLE-US-00001 TABLE 1 QUANTITY QUANTITY COMPONENTS (phr) (g) Nitrile rubber (NBR) 100 400 Zinc oxide 3 12 Sulfur 1.5 6 Stearic acid 1 4 HAF IRB6* 40 160 TBBS** 0.7 2.8 *HAF IRB6: carbon black (Greif Corp.); **TBBS: N-t-butyl-1,2-benzothiazolesulfenamide (Nist).
Mould Fouling Evaluation
(19) The elastomeric composition obtained as described above and indicated in Table 1, was injected through a 1 cm hole into a closed mould consisting of two plates with about 12 holes having a diameter of 10 mm, perfectly coinciding, having a sandwich configuration, with a thickness of the interspace thus obtained of about 2 mm. Once the mould had been filled, the elastomeric composition was vulcanized at about 220 C. for about 2 minutes, at a pressure of about 20 Kg/cm.sup.2.
(20) At the end of the vulcanization, the mould was opened to remove the vulcanized elastomeric composition and then re-closed to proceed with a subsequent injection of elastomeric composition again through the hole between the two metallic plates to proceed with a new vulcanization.
(21) Said procedure was repeated 50 times and the fouling evaluation was given according to a scale of values from 1 to 6 (the maximum value 6 indicates the worst fouling, the minimum value 1 indicates no fouling), observing the O-rings of elastomeric composition obtained after each opening of the mould and verifying the presence of anomalous deposits in both the O-rings obtained and in the mould.
Examples 1-3
(22) Three latexes A, B and C were produced using the components indicated in Table 2: the quantities of the various components are expressed in phr.
(23) TABLE-US-00002 TABLE 2 EXAMPLE 1 EXAMPLE 2 EXAMPLE 3 COMPONENTS (Latex A) (Latex B) (Latex C) Deionized water 180 180 180 Butadiene 72.75 66.42 72.75 Acrylonitrile 8.35 13.53 8.35 (initial charge) Acrylonitrile 18.9 23.1 18.9 (feeding) DIHP.sup.(1) 0.10 0.10 0.1 TDM.sup.(2) 0.312 0.45 0.45 (initial charge) TDM.sup.(2) 0.039-0.039 0-0.08 0-0.08 (booster1-booster2) Fatty soap.sup.(3) 0.48 0.48 0.48 DNMK.sup.(4) 3.30 3.30 3.30 Reducing agent.sup.(5) 0.12 0.12 0.12 Shortstop.sup.(6) 10.8 10.8 10.8 .sup.(1)di-isopropyl-benzene hydroperoxide (Sasol); .sup.(2)tertiary dodecyl mercaptan (Lanxess); .sup.(3)aqueous solution at 5.4% by weight of potassium stearate (Oleon); .sup.(4)aqueous solution at 45% by weight of potassium salt of naphthalenesulfonic acid condensed with formaldehyde (Dalton); .sup.(5)aqueous solution of sodium formaldehyde sulfoxylate (FSS) prepared as indicated in Table 3; .sup.(6)aqueous solution at 2% by weight of hydroxylamine sulfate (Basf).
(24) TABLE-US-00003 TABLE 3 COMPONENTS phr Water 9.9 EDTA.sup.(1) 0.01 FSS.sup.(2) 0.075 H.sub.2SO.sub.4 0.0008 FeSO.sub.4*7H.sub.2O 0.004 .sup.(1)bisodium salt of ethylenediamine tetracetic acid (Trilon B-Basf); .sup.(2)formic aldehyde sulfoxylate (rodite) (Buggolite C-Bruggeman).
(25) The three latexes A, B and C, were prepared in a 70 liter steel autoclave, equipped with a stirrer. Details relating to the preparation of the three latexes are provided hereunder.
(26) Preparation of Latex A
(27) The following components were charged in order into the autoclave: 9.92 kg of deionized water, 0.53 kg of aqueous solution at 5.4% by weight of potassium stearate and 0.440 kg of DNMK. The mixture obtained was stirred and its pH was corrected by adding an aqueous solution at 10% by weight of potassium hydroxide, up to a pH equal to 8.5. The autoclave was then closed and flushed with nitrogen. A vacuum was applied and the following components were charged in order: 500 g of acrylonitrile, 18.72 g of TDM and 4.37 kg of butadiene: the mixture obtained was subjected to stirring and subsequently cooled to 7 C. When this temperature had been reached, 6 g of DIHP were injected, immediately followed by 139 g of aqueous solution of sodium formaldehyde sulfoxylate (FSS) prepared as indicated in Table 3. The polymerization mixture was maintained at a temperature of 7 C. for the whole polymerization time. The remaining acrylonitrile (1.134 kg) was then fed in continuous, with a flow-rate equal to 0.162 kg/h.
(28) The conversion degree was followed during polymerization by collecting samples of latex from the bottom of the autoclave and determining the solid content percentage. At 37% of conversion, a first addition was carried out (booster 1) of 2.34 g of TDM. A subsequent addition (booster 2) of 2.34 g of TDM was carried out at a conversion of 59%.
(29) The polymerization was interrupted at 70% conversion (after 7 hours) by adding 515 g of aqueous solution at 2% by weight of hydroxylamine sulfate. The non-reacted monomers were removed by distillation in a stream of vapour (stripping).
(30) Preparation of Latex B
(31) The following components were charged in order into the autoclave: 9.92 kg of deionized water, 0.53 kg of aqueous solution at 5.4% by weight of potassium stearate and 0.440 kg of DNMK. The mixture obtained was stirred and its pH was corrected by adding an aqueous solution at 10% by weight of potassium hydroxide, up to a pH equal to 8.5. The autoclave was then closed and flushed with nitrogen. A vacuum was applied and the following components were charged in order: 815 g of acrylonitrile, 21.85 g of TDM and 4.0 kg of butadiene: the mixture obtained was subjected to stirring and subsequently cooled to 7 C. When this temperature had been reached, 6 g of DIHP were injected, immediately followed by 139 g of aqueous solution of sodium formaldehyde sulfoxylate (FSS) prepared as indicated in Table 3. The polymerization mixture was maintained at a temperature of 7 C. for the whole polymerization time. The remaining acrylonitrile (1.209 kg) was then fed in continuous, with a flow-rate equal to 0.173 kg/h.
(32) The conversion degree was followed during polymerization by collecting samples of latex from the bottom of the autoclave and determining the solid content percentage. At 59% of conversion, an addition was carried out (booster 2) of 5.45 g of TDM.
(33) The polymerization was interrupted at 70% conversion (after about 7 hours) by adding 515 g of aqueous solution at 2% by weight of hydroxylamine sulfate. The non-reacted monomers were removed by distillation in a stream of vapour (stripping).
(34) Preparation of Latex C
(35) The following components were charged in order into the autoclave: 9.92 kg of deionized water, 0.53 kg of aqueous solution at 5.4% by weight of potassium stearate and 0.440 kg of DNMK. The mixture obtained was stirred and its pH was corrected by adding an aqueous solution at 10% by weight of potassium hydroxide, up to a pH equal to 8.5. The autoclave was then closed and flushed with nitrogen. A vacuum was applied and the following components were charged in order: 500 g of acrylonitrile, 21.85 g of TDM and 4.37 kg of butadiene: the mixture obtained was subjected to stirring and subsequently cooled to 7 C. When this temperature had been reached, 6 g of DIHP were injected, immediately followed by 139 g of aqueous solution of sodium formaldehyde sulfoxylate (FSS) prepared as indicated in Table 3. The polymerization mixture was maintained at a temperature of 7 C. for the whole polymerization time. The remaining acrylonitrile (1.134 kg) was then fed, in continuous, with a flow-rate equal to 0.162 kg/h.
(36) The conversion degree was followed during polymerization by collecting samples of latex from the bottom of the autoclave and determining the solid content percentage. At 59% of conversion, an addition was carried out (booster 2) of 5.45 g of TDM.
(37) The polymerization was interrupted at 70% conversion (after about 7 hours) by adding 515 g of aqueous solution at 2% by weight of hydroxylamine sulfate. The non-reacted monomers were removed by distillation in a stream of vapour (stripping).
(38) The characteristics of the three latexes obtained at the end of the stripping are specified in Table 4.
(39) TABLE-US-00004 TABLE 4 Latex A Latex B Latex C Solid content 15.6 16.2 15.2 (%) pH 8.5 8.5 8.6 Content of bound 28.1 33.9 27.9 ACN (%) Mooney Viscosity 60.5 32.1 29.0
(40) Before being coagulated, a dispersion at 50% of antioxidant (2,6-di-t-butyl-p-cresol-BHT-Todini), was added to the above latexes, in a quantity equal to 1% by weight of active substance with respect to the total weight of the nitrile rubber. Said dispersion was prepared by means of an Ultraturex, by stirring 100 g of BHT in powder form, 8 g of DNMK and 100 g of deionized water.
(41) Coagulation of the Latexes
(42) The latexes A, B and C were subjected to coagulation in an open 70 liter recipient, under stirring, using a saturated aqueous solution of magnesium sulfate, operating under the following conditions.
(43) 43 kg of water and 4 kg of a saturated aqueous solution of magnesium sulfate were charged into a 70 liter recipient. The whole mixture was subjected to stirring and heated to 45 C. When this temperature had been reached, 10 kg of latex obtained as described above (Latex A, Latex B, Latex C) were gradually added, under continuous stirring, over a period of about 10 minutes. At the end of the addition of latex, the whole mixture was left under stirring for a further 10 minutes.
(44) Washing of Coagulated Nitrile Rubber
(45) The coagulated latexes were filtered to recover the crumbs of nitrile rubber which where then subjected to washing, operating under the following conditions.
(46) 40 kg of deionized water were charged into an open 70 liter recipient and brought to a pH of 11.5-11.8 (see Table 5 for the pH values) by the addition of 14 g of potassium hydroxide and subsequently heated to 45 C. When this temperature had been reached, 1.5 kg of crumbs of nitrile rubber obtained from the previous coagulation were introduced and the whole mixture was left, under stirring, for about 10 minutes, after which the nitrile rubber was filtered and dried in an air oven, at 100 C., for 24 hours (residual humidity: less than 1%).
(47) Table 5 indicates the following characteristics of the nitrile rubber (NBR) obtained: content of metals present (ppm), pH and ion indexes.
(48) Table 6, on the other hand, indicates the rheometric evaluations of the elastomeric compositions obtained as described above and indicated in Table 1, comprising the nitrile rubbers obtained according to the present invention and the classification obtained in the relative evaluation of the mould fouling.
(49) TABLE-US-00005 TABLE 5 pH pH II(1) II(2) Coagulation (washing (NBR Ca Mg K Na (ppmxg/ (ppmxg/ SAMPLES salt H.sub.2O) obtained) (ppm) (ppm) (ppm) (ppm) mole) mole) EXAMPLE 1 MgSO.sub.4 11.5 10.5 171 1068 110 120 4.8 138.3 (NBR from Latex A) EXAMPLE 2 MgSO.sub.4 11.8 11.0 98 1114 175 219 6.7 132.6 (NBR from Latex B) EXAMPLE 3 MgSO.sub.4 11.5 10.2 54 855 210 120 6.5 100.3 (NBR from Latex C)
(50) TABLE-US-00006 TABLE 6 MH ML MH ML T.sub.S2 T.sub.90 T.sub.90 T.sub.S2 Mould SAMPLES (dNm) (dNm) (dNm) (min) (min) (min) fouling EXAMPLE 1 37 7.75 29.25 5 13.5 8.5 1 (NBR from Latex A) EXAMPLE 2 35 4.5 30.5 4.5 18.5 14 1 (NBR from Latex B) EXAMPLE 3 34.5 5.5 29 5 14 9 1 (NBR from Latex C)
(51) From the data reported above, it can be deduced that the elastomeric compositions comprising the nitrile rubber obtained according to the present invention (Examples 1-3) have a high vulcanization rate (low T.sub.90-T.sub.S2 values), a good vulcanization yield (relatively high MH-ML values) and a low fouling of the moulds. In all three cases, in fact, the injection moulding could be repeated more than 50 times without observing any type of deposit or pitting attributable to fouling, either in the mould or, to an even lesser extent, in the O-rings.
Examples 4-6 (Comparative)
(52) For comparative purpose, the crumbs of nitrile rubber obtained as described above in Examples 1-3, were subjected to a different washing. For this purpose, the crumbs of nitrile rubber coming from the coagulation of the latex (Latex A, Latex B, Latex C), were subjected to washing with deionized water at neutral pH (see Table 7 for pH values).
(53) Table 7 indicates the following characteristics of the nitrile rubber (NBR) obtained: content of metals present (ppm), pH and ion indexes.
(54) Table 8, on the other hand, indicates the rheometric evaluations of the elastomeric compositions obtained as described above and indicated in Table 1, comprising the nitrile rubbers obtained according to Examples 4-6 (comparative), and the classification obtained in the relative evaluation of the mould fouling.
(55) TABLE-US-00007 TABLE 7 pH pH II(1) II(2) Coagulation (washing (NBR Ca Mg K Na (ppmxg/ (ppmxg/ SAMPLES salt H.sub.2O) obtained) (ppm) (ppm) (ppm) (ppm) mole) mole) EXAMPLE 4 MgSO.sub.4 8.0 8.5 37 466 120 8 0.65 57.6 (NBR from Latex A) EXAMPLE 5 MgSO.sub.4 7.2 8.0 163 501 166 22 7.01 69.63 (NBR from Latex B) EXAMPLE 6 MgSO.sub.4 7.5 7.5 84 476 66 8 4.25 63.75 (NBR from Latex C)
(56) TABLE-US-00008 TABLE 8 MH ML MH ML T.sub.S2 T.sub.90 T.sub.90 T.sub.S2 Mould SAMPLES (dNm) (dNm) (dNm) (min) (min) (min) fouling EXAMPLE 4 33 7.7 25.3 5 20 15 4 (NBR from Latex A) EXAMPLE 5 31 4.0 27 4.0 23 19 4 (NBR from Latex B) EXAMPLE 6 30 5.5 24.5 5.2 19 13.8 5 (NBR from Latex C)
(57) From the data reported above, it can be deduced that the elastomeric compositions comprising the nitrile rubber obtained after washing under neutral pH conditions (Examples 4-6 comparative) have a lower vulcanization rate (higher T.sub.90-T.sub.S2 values), a worst vulcanization yield (lower MH-ML values) and fouling of the moulds. In all three cases, in fact, at the end of the 50 injection moulding cycles numerous defects were observed (punctiform deposits) in both the mould and in the O-rings obtained.
Examples 7-9 (Comparative)
(58) For comparative purpose, the latexes obtained as described above in Examples 1-3 (Latex A, Latex B, Latex C), were coagulated using 4 kg of a saturated aqueous solution of calcium chloride. The Latex A1, Latex B1 and Latex C1, thus obtained, were subsequently subjected to washing with deionized water which was brought to pH 11.5-11.6 (see Table 9 for the pH values) by the addition of potassium hydroxide (same washing indicated in Examples 1-3).
(59) Table 9 indicates the following characteristics of the nitrile rubber (NBR) obtained: content of metals present (ppm), pH and ion indexes.
(60) Table 10, on the other hand, indicates the rheometric evaluations of the elastomeric compositions obtained as described above and indicated in Table 1, comprising the nitrile rubbers obtained according to Examples 7-9 (comparative), and the classification obtained in the relative evaluation of the mould fouling.
(61) TABLE-US-00009 TABLE 9 pH pH II(1) II(2) Coagulation (washing (NBR Ca Mg K Na (ppmxg/ (ppmxg/ SAMPLES salt H.sub.2O) obtained) (ppm) (ppm) (ppm) (ppm) mole) mole) EXAMPLE 7 CaCl.sub.2 11.5 10.2 1100 8 111 90 128.25 128.6 (NBR from Latex A1) EXAMPLE 8 CaCl.sub.2 11.5 10.6 990 5 88 107 127.34 127.97 (NBR from Latex B1) EXAMPLE 9 CaCl.sub.2 11.6 10.7 1200 17 125 115 135.8 137.93 (NBR from Latex C1)
(62) TABLE-US-00010 TABLE 10 MH ML MH ML T.sub.S2 T.sub.90 T.sub.90 T.sub.S2 Mould SAMPLES (dNm) (dNm) (dNm) (min) (min) (min) fouling EXAMPLE 7 30 7.75 22.25 5 25 20 6 (NBR from Latex A1) EXAMPLE 8 31 4.5 26.5 4.75 29.5 24.75 6 (NBR from Latex B1) EXAMPLE 9 31 7.5 23.5 5.0 25 20 6 (NBR from Latex C1)
(63) From the data reported above, it can be deduced that the elastomeric compositions comprising the nitrile rubber obtained by subjecting the latexes to coagulation with a saturated aqueous solution of calcium chloride (Examples 7-9 comparative) have a worst vulcanization rate (higher T.sub.90-T.sub.S2 value), a lower vulcanization yield (lower MH-ML values) and a high fouling of the moulds. In all three cases, in fact, already after only a few injection moulding cycles, numerous defects were observed (punctiform deposits) in both the mould and in the O-rings obtained: in no case, however, was it possible to exceed 50 moulding cycles.