Color-stable nitrile rubbers
11248078 · 2022-02-15
Assignee
Inventors
Cpc classification
International classification
Abstract
What are provided are colour-stable, low-salt nitrile rubbers and a process for production thereof under specific conditions for the coagulation and/or washing of the nitrile rubber. Additionally obtainable are the corresponding vulcanizable mixtures based on this nitrile rubber, a process for producing vulcanizates from these mixtures and the resultant vulcanizates having excellent processing properties.
Claims
1. A nitrile rubber comprising repeat units of at least one α,β-unsaturated nitrile monomer and of at least one conjugated diene monomer, wherein the nitrile rubber has an aluminium index E according to the general formula (I) of 7 ppm/MU to not more than 35.0 ppm/MU, based on the nitrile rubber,
2. The nitrile rubber according to claim 1, additionally comprising repeat units of one or more further copolymerizable monomers.
3. The nitrile rubber according to claim 1, wherein the nitrile rubber is a copolymer of acrylonitrile and 1,3-butadiene.
4. The nitrile rubber according claim 1, wherein the proportions of, or the sum total of the conjugated dienes is 20% to 95% by weight, based on the nitrile rubber, and the proportion of or the sum total of the α,β-unsaturated nitriles is in the range from 5% to 80% by weight, based on the nitrile rubber, where the proportions of all the monomers add up to 100% by weight in each case.
5. A process for producing the nitrile rubber according to claim 1, by emulsion polymerization of at least one α,β-unsaturated nitrile monomer and at least one conjugated diene monomer, the process comprising: (i) after the polymerization, coagulating the latex which contains the nitrile rubber using at least one aluminium salt, wherein the coagulating is effected at a pH of 1 to 4.5, wherein the at least one aluminium salt is used in the form of an aqueous solution which has a concentration of 40%-70% by weight, and is used in such an amount that, based on 100 parts by weight of nitrile rubber, 0.5-50 parts by weight of aluminium salt are used based on 100 parts by weight of nitrile rubber, and (ii) subsequently washing the nitrile rubber at a pH of 6 to 12 with an amount of wash water that is 0.5 to 20 parts by weight, based on 100 parts by weight of rubber.
6. The process according to claim 5, wherein the at least one aluminium salt is an aluminium salt of a mono- or divalent anion.
7. The process according to claim 5, wherein the at least one aluminium salt is aluminium chloride, aluminium sulphate, potassium aluminium sulphate (potassium alum), sodium aluminium sulphate (sodium alum) or any mixtures thereof.
8. The process according to claim 5, wherein the at least one salt additionally comprises one or more other, non-aluminium-based precipitation salts in an amount of not more than 10% by weight, based on the total amount of all the salts used for coagulation.
9. A vulcanizable mixture comprising the nitrile rubber according to claim 1 and at least one crosslinker.
10. A process for producing the vulcanizable mixture according to claim 9, the process comprising mixing the nitrile rubber with at least one crosslinker.
11. A process for producing vulcanizates, the process comprising vulcanizing the vulcanizable mixture according to claim 9.
12. Vulcanizates obtained by the process according to claim 11.
13. The vulcanizates according to claim 12, wherein the vulcanizates are mouldings comprising seals, caps, hoses and membranes.
14. The nitrile rubber according to claim 1, wherein: the aluminium index E is 7 ppm/MU to less than 35 ppm/MU, based on the nitrile rubber; the proportion of, or the sum total of the conjugated dienes is 40% to 90% by weight, based on the nitrile rubber, and the proportion of, or the sum total of the α,β-unsaturated nitriles is 10% to 60% by weight.
15. The nitrile rubber according to claim 1, wherein: the proportion of, or the sum total of the conjugated dienes is 60% to 85% by weight, based on the nitrile rubber, and the proportion of, or the sum total of the α,β-unsaturated nitriles is 15% to 40% by weight.
Description
EXAMPLES
(1) I Analysis
(2) The nitrogen content for determination of the acrylonitrile content (ACN content) in the nitrile rubbers is determined to DIN 53 625 according to Kjeldahl. Due to the content of polar comonomers, the nitrile rubbers are typically >85% by weight soluble in methyl ethyl ketone at 20° C.
(3) The values for the Mooney viscosity (ML 1+4@100° C.) are determined in each case by means of a shearing disc viscometer in accordance with DIN 53523/3 or ASTM D 1646 at 100° C. Uncalendered (“unmassed”) samples are used.
(4) The dried NBR rubbers were characterized via the Mooney viscosity before and after hot-air storage at 100° C. for 48 hours; in other words, the Mooney viscosity was determined once directly after the drying (i.e. before the hot-air storage) and subsequently after hot-air ageing at 100° C. for 48 hours.
(5) For further characterization, the colour of the aged rubber was assessed. The colour assessment of the aged rubber samples was made on a scale of 1-10, 1 corresponding to the colour of the unaged rubber and 10 to complete blackening.
(6) The determination of the aluminium content was made by digesting 0.5 g of the nitrile rubbers by dry ashing at 550° C. in a platinum crucible with subsequent dissolution of the ash in hydrochloric acid. After suitable dilution of the digestion solution with deionized water, the aluminium content was determined by ICP-OES (inductively coupled plasma-optical emission spectrometry) at a wavelength of 309.271 nm against calibration solutions adjusted with an acid matrix. According to the concentration of the elements in the digestion solution and/or sensitivity of the measuring instrument used, the concentrations of the sample solutions for each of the wavelengths used were fitted to the linear range of the calibration (B. Welz “Atomic Absorption Spectrometry”, 2nd Ed., Verlag Chemie, Weinheim 1965).
(7) The aluminium index E, reported in [ppm/MU], is calculated from the aluminium content in ppm per Mooney unit, based on the nitrile rubber.
(8) Scorch stability can be measured via the scorch characteristics and the scorch time t.sub.5 thereof to DIN53523. In this context, the scorch time t.sub.5 describes the time in minutes from the commencement of measurement until the Mooney viscosity rises by 5 Mooney units above the viscosity minimum at the given temperature.
(9) II Production of the Nitrile Rubber Latices for Examples A1-A18
(10) The polymerization was effected batchwise in an autoclave with a stirrer system. In each of the autoclave batches, the monomer mixture specified in Table 1, water and EDTA in an equimolar amount based on the Fe(II) were used. A portion of the amount of water was initially charged together with the emulsifier in the autoclave and purged with a nitrogen stream. Thereafter, the destabilized monomers and the amounts of the molecular weight regulator specified in Table 1 were added and the reactor was closed. After the reactor contents had been brought to temperature, the polymerizations were started by the addition of aqueous solutions of iron(II) salts (in the form of premix solutions) and of para-menthane hydroperoxide (Trigonox® NT50).
(11) The premix solution contained 0.986 g of Fe(II)SO.sub.4*7 H.sub.2O and 2.0 g of Rongalit® C in 400 g of water.
(12) The course of the polymerization was monitored by gravimetric determinations of conversion. On attainment of the conversions reported in Table 1, the polymerization was short-stopped by adding an aqueous solution of diethylhydroxylamine. Unconverted monomers were removed by means of steam distillation.
(13) TABLE-US-00001 TABLE 1 Recipe for the emulsion polymerization for preparation of the NBR latices A1-A18 Latex for Latex for Latex for Latex for examples examples examples examples Nitrile rubber A1-A12 A13-A14 A15-A16 A17-A18 Butadiene 44.2 73.9 37.0 67.5 Acrylonitrile 55.8 26.1 63.0 32.5 Total amount of water 220 170 200 200 Fatty acid 2.0 Oleic acid 1.4 1.4 1.4 Resin acid .sup.1) 0.6 0.6 0.6 AOS .sup.2) 0.4 0.4 0.4 Tert-TDM.sup.3) 0.6 0.25 0.61 0.46 Polymerization 13.0 ± 3 13.0 ± 3 13.0 ± 3 13.0 ± 3 temperature [° C.] Polymerization 73 75 68 75 conversion [%] Polymerization time 7 17 8.3 8.3 [h] .sup.1) sodium salt of the disproportionated resin acid, CAS 61790-51-0 .sup.2) AOS: sodium α olefinsulphonate .sup.3)tert-TDM: tert-dodecyl mercaptan
III Aluminium Precipitation of NBR Rubbers, Example A1-A12
(14) Prior to the coagulation of each NBR latex, a 50% dispersion of Vulkanox® BKF (0.3% by weight of Vulkanox® BKF based on NBR solids) was added to each. A short-stopped aqueous NBR latex dispersion is used, having an NBR solids content of 18% by weight, and where the polymer component in the NBR latex has a content of 44% by weight based on acrylonitrile monomer, and 56% by weight based on the butadiene monomer. The NBR dispersion is added gradually to an aluminium sulphate solution (T=70° C.) at the pH specified in the table and stirred vigorously. The concentration of the aluminium sulphate solutions is chosen such that homogeneous crumbs of the NBR rubber are obtained. The amounts of aluminium sulphate solutions based on the NBR rubber are 9+/−1 phr and are specified in Table 2 or 3. The resultant raw rubber is washed with water (T=60° C.) at the pH specified in Table 2 or 3and dried in a vacuum oven at 60° C. for 6 hours, such that the residual moisture content is <0.6%.
(15) TABLE-US-00002 TABLE 2 Aluminium precipitation of nitrile rubber at various pH values (Examples A1 to A6; inventive examples are identified by ″*″) Example A1 A2* A3* A4 A5 A6 Coagulation Temperature ° C. 70 70 70 70 70 70 pH 3.0 3.0 3.0 5.0 5.0 5.0 Precipitant salt Al.sub.2(SO.sub.4).sub.3 Amount of phr 9 9 9 9 9 9 precipitant salt Wash Temperature ° C. 60 60 60 60 60 60 pH 5.1 6.9 10.0 4.8 6.8 9.9 Testing Ion content Al ppm 1550 1640 1440 14550 14750 13250 Molecular weights GPC Mn — 70922 — — 71239 — Mw — 189329 — — 187693 — Polydispersity index PDI — 2.7 — — 2.6 — (PDI = Mw/Mn) Mooney viscosity MU 44 47 42 64 64 67 Aluminium index E ppm/MU 35.2 34.9 34.3 227.3 230.5 197.8 Testing after ageing 48 h/110° C. Mooney viscosity MU 47 49 46 63 63 65 Colour assessment 6 4 3 2 2 3
(16) TABLE-US-00003 TABLE 3 Aluminium precipitation of nitrile rubber at various pH values (Examples A7 to A12; all comparative examples) Example A7 A8 A9 A10 A11 A12 Coagulation Temperature ° C. 70 70 70 70 70 70 pH 7.0 7.0 7.0 10.0 10.0 10.0 Precipitant salt Al.sub.2(SO.sub.4).sub.3 Amount of phr 9 9 9 11 11 11 precipitant salt Wash Temperature ° C. 60 60 60 60 60 60 pH 5.1 6.9 9.9 5.2 7.0 10.0 Testing Ion content Al ppm 15100 14900 10800 10100 6300 5700 Molecular weights GPC Mn — 67906 — — 73191 — Mw — 189077 — — 190062 — Polydispersity index PDI — 2.8 — — 2.6 — (PDI = Mw/Mn) Mooney viscosity Mu 63 66 67 59 59 58 Aluminium index E ppm/Mu 239.7 225.8 161.2 171.2 106.8 98.3 Ageing 48 h/110° C. Mooney viscosity Mu 62 64 62 60 56 57 Colour assessment 2 2 3 7 9 10
(17) As expected, all the aluminium-precipitated samples show the same molecular weight in the GPC analysis. In the case of the Mooney viscosities, however, values between inventive examples and comparative examples are measured, which differ by up to 20 MU. This must presumably be attributable to the washing and non-removal of the aluminium salts which remain in the rubber in large amounts. The Mooney viscosity measured thus shows a combination of the desired viscous proportion of the polymers and a proportion of the undesirable salts, the latter being disproportionately high in the case of the comparative examples. The best aluminium index E, being the lowest salt burden per unit Mooney viscosity, is achieved only through controlled coagulation at an acidic pH in the range from 1 to 4.5 and a subsequent wash within a range from 6 to 12. It is apparent from Comparative Example I that a small deviation in the pH of the wash of 5.1, outside the range from 6 to 12 of relevance in accordance with the invention, leads to a substantial deterioration in colour assessment for the aged samples. It is also apparent that even a higher amount of aluminium sulphate solutions (phr) in the precipitation (Examples A10-A12 as compared with A7-A9) does not necessarily increase the amount of aluminium ions remaining in the rubber.
(18) After hot-air ageing of the rubbers, great colour differences are observed between the aged samples. The smallest colour difference after ageing of the rubbers is present in Examples A3-A9, but this is misleading in that this apparently positive result in these comparative examples is caused by the excessively high aluminium salt content. It is apparent that the rubber after the ageing test becomes ever lighter in colour with increasing salt content, since the aluminium salts lighten the rubber colour, but with simultaneous substantial losses in the other properties. The best colour effect with simultaneously minimum salt burden is surprisingly achieved only by coagulation at a pH in the range from 1 to 4.5 and a subsequent wash within a range from 6 to 12.
(19) III Aluminum Precipitation of NBR Rubbers. Examples A13-A18
(20) Prior to the coagulation of each NBR latex, a 50% dispersion of Vulkanox® BKF (0.4% by weight of Vulkanox® BKF based on NBR solids) was added to each. A short-stopped aqueous NBR latex dispersion is used, having an NBR solids content of 18.4% by weight, and where the polymer component in the NBR latex has the Mooney viscosity reported in Table 4 and the contents based on the acrylonitrile and butadiene that are likewise mentioned in Table 4. The dispersion is added gradually to an aluminium sulphate solution at the pH and temperature specified in Table 4 and stirred vigorously. The concentration of the aluminium sulphate solutions is chosen such that homogeneous crumbs of the NBR rubber are obtained. The amounts of aluminium sulphate solutions based on the rubber are reported in Table 4. The resultant raw rubber is washed with water at the pH and temperature specified in Table 4 and dried in a vacuum oven at 60° C. for 16 hours, such that the residual moisture content is <0.6%.
(21) TABLE-US-00004 TABLE 4 Aluminium precipitation of nitrile rubber in the case of coagulation at acidic pH and washing at basic pH (Examples A13 to A18; all inventive examples) Example A13* A14* A15* A16* A17* A18* Coagulation Temperature ° C. 60 60 65 65 60 60 pH 3.0 Precipitant salt Al.sub.2(SO.sub.4).sub.3 Amount of phr 7 7 7 7 6 6 precipitant salt Wash Temperature ° C. 60 60 65 65 60 60 pH 7.1 10.3 7.1 9.9 7.0 10.0 Testing Ion content Al ppm 1096 880 1626 1380 1216 1137 ACN content % 28 28 49 49 28.5 28.5 Mooney viscosity Mu 95 99 61 66 51 53 Aluminium index E ppm/Mu 11.5 8.9 26.7 20.9 23.8 21.5 Ageing 48 h/110° C. Mooney viscosity Mu 109 97 78 79 81 70 Colour assessment 4 4 4 3 4 4
(22) Table 4 shows that aluminium precipitation at an acidic pH of 3 with a subsequent basic wash in the pH range of 6 to 12 which is of relevance in accordance with the invention affords rubbers having only a low salt content, additionally having a low aluminium index E and also exhibiting good colour stability after ageing. It is advantageous that this is true both of nitrile rubbers having a very high acrylonitrile content and of those having a very low acrylonitrile content, since the latter are particularly sensitive.
(23) IV Production of Vulcanizates of the Nitrile Rubbers V1-V18
(24) Nitrile rubbers A1 to A18 were used as described below to produce vulcanizates V1 to V18. The constituents of the vulcanizable mixtures are based on 100 parts of rubber and are reported in Table 5. The vulcanizate number in Table 5 corresponds in each case to the nitrile rubber having the same number. Nitrile rubber A1 is used to produce vulcanizate V1, nitrile rubber A2 to produce vulcanizate V2, etc.
(25) The mixtures were produced in a Banbury mixer to ISO4658, section 5.2.3.3. For this purpose, the rubber and all the additives specified in Table 4 were mixed in each case at a maximum temperature of up to 60° C. for a total of 5 minutes.
(26) TABLE-US-00005 TABLE 5 Composition of the vulcanizable mixtures for vulcanizates V1-V18 Vulcanizable mixture for V1-V12 V13-V14 V15-V16 V17-V18 Polymer A1-A18* 100 100 100 100 IRB 7 .sup.1 40 40 40 40 EDENOR C 18 98-100 .sup.2 1 1 1 1 SULFUR SPIDER .sup.3 1.54 1.54 1.54 1.54 VULKACIT NZ/EGC .sup.4 0.7 0.7 0.7 0.7 ZINKOXYD AKTIV .sup.5 3 3 3 3 Total phr 146.24 146.24 146.24 146.24 Density g/ccm 1.167 1.129 1.176 1.139 .sup.1 IRB7 carbon black from Sid Richardson Carbon Co. .sup.2 Edenor C 1898-100 stearic acid from Caldic .sup.3 sulfur Spider sulphur (S8) from Krahn Chemie GmbH .sup.4 Vulkacit ® NZ/EG-C N-Cyclohexyl-2-benzothiazolesulphenamide (CBS) from Lanxess Deutschland GmbH .sup.5 Zinkoxid Aktiv zinc oxide from Lanxess Deutschland GmbH
(27) The resultant vulcanizates had the scorch time t.sub.5 to DIN 53523 reported in Tables 6 to 8 and the Mooney scorch at 140° C. (MS t.sub.5/140° C.) reported. It is apparent that the polymers having an aluminium index E of not more than 35 ppm/MU have a longer scorch time MS t.sub.3/140° C. than non-inventive samples; in other words, the former have very good processing reliability, as expected for aluminium-precipitated types. Aluminium-precipitated types which are not acid-precipitated within the range of values specified and washed at a basic pH within the range of values specified have very short scorch times and additionally high aluminium contents. (“nd” in Table 7 stands for “not determined”)
(28) TABLE-US-00006 TABLE 6 Mooney scorch of aluminium-precipitated nitrile rubbers Example V1 V2* V3* V4 V5 V6 Aluminium ppm/ 35.2 34.9 34.3 227.3 230.5 197.8 index E MU Mooney MS min 17.1 17.8 17.2 16.6 16.3 13.9 scorch t.sub.5/140° C.
(29) TABLE-US-00007 TABLE 7 Mooney scorch of aluminium-precipitated nitrile rubbers Example V7 V8 V9 V10 V11 V12 Aluminium ppm/ 239.7 225.8 161.2 171.2 106.8 98.3 index E MU Mooney MS min 16.0 15.2 12.1 11.4 nd 8.4 scorch t.sub.5/140° C.
(30) TABLE-US-00008 TABLE 8 Mooney scorch of aluminium-precipitated nitrile rubbers Example V13* V14* V15* V16* V17* V18* Aluminium ppm/ 11.5 8.9 26.7 20.9 23.8 21.5 index E MU Mooney MS min 9.6 9.7 13.6 14.6 13.0 12.7 scorch t.sub.5/140° C.