CARBON BLACK RUBBER COUPLING BY MERCAPTOPYRIDINES

Abstract: The invention relates to a sulfur vulcanization process comprising (a) a non-productive stage of mixing of a rubber composition comprising a polyisoprene rubber and carbon black, (b) a productive stage of mixing of a rubber composition further comprising sulfur and a vulcanization accelerator, and (c) vulcanization of said rubber composition and is characterized in that the non-productive stage of mixing is carried out in an internal mixer in the presence of 0.1 to 5 phr of a carbon black coupling coagent according to any one of formulae (I)-(IV) wherein R<1>, R<2>, and R<3> independently represent H or a C1-C5 hydrocarbon group, optionally containing one or more oxygen, nitrogen or sulfur atoms, R<1> and R<2>, provided R<1> is in the 4-position, or R<2> and R<3> together with the carbon atoms to which they are attached may form a 5 or 6-membered ring, said ring optionally being substituted with a C1-C5 hydrocarbon group, said group optionally containing one or more oxygen, nitrogen or sulfur atoms, n is 1 to 4, M is selected from Na, Zn, Mg, AL, Fe, and Te, and x is 1, 1.5 or 2. The invention also relates to rubber articles, such as pneumatic tyres, comprising the rubber vulcanizate obtained by said process.

Description: CARBON BLACK RUBBER COUPLING BY MERCAPTOPYRI DINES The invention relates to a sulfur vulcanization process comprising (a) a non-productive stage of mixing of a rubber composition comprising a polyisoprene rubber and carbon black, (b) a productive stage of mixing of a rubber composition further comprising sulfur and a vulcanization accelerator, and (c) vulcanization of said rubber composition. It further relates to rubber articles comprising the rubber vulcanizate obtained by said process. Vulcanizing rubber compositions by heating a sulfur-vulcanizable rubber composition with sulfur and/or a sulfur donor and a vulcanization accelerator has been known for many years. By this process vulcanizates having acceptable physical properties including tensile strength, resilience, and fatigue resistance can be obtained. Most rubber compositions typically contain a reinforcing filler such as carbon black or a combination of carbon black with a light-coloured filler such as silica. A known problem of rubber vulcanizates containing reinforcing fillers is the poor interaction between the filler and the rubber, which leads to deteriorated physical and dynamic properties of the vulcanizate, in particular increased hysteresis and increased heat build-up. This problem is particularly relevant to rubber compositions comprising a polyisoprene rubber such as natural rubber. In order to solve this problem it is customary in the industry nowadays that when silica is included as the reinforcing filler, in particular in so-called green tyres, to incorporate a silane coupling agent into the rubber composition before vulcanization. Although carbon black coupling agents are also known in the art, see for example T. Yamaguchi et al. in Kautschuk Gu mi Kunststoffe, Vol. 42, No. 5, 1989, pages 403-409, which describes a coagent called Sumifine ® (i.e. N,N'-bis(2-methyl-2-nitropropyl)-1 ,6-diaminohexane), and L. Gonzalez et al. in Rubber Chemistry and Technology, Vol. 69, 1996, pages 266-272, they are not used commercially, most likely due to the toxic/carcinogenic byproducts formed during decomposition. Up to now, carbon black has been the most widely used reinforcing filler in the rubber industry, and it is often used in combination with a smaller amount of silica. The problem of carbon black to rubber coupling has not been adequately solved so far, hence, there still is a need in the art for effective carbon black coupling coagents. We now have found carbon black coupling coagents which can be used advantageously in rubber compositions. The coagents in accordance with the present invention have the effect of improving the hysteresis and heat build-up properties of carbon black-reinforced polyisoprene rubber vulcanizates and showing a decreased Payne effect. The interaction between filler and rubber preferably is expressed in the art as the Payne effect and is described for example in the article of T. Yamaguchi et al. mentioned above and in B. Freund and W. Niedermeier in Kautschuk Gummi Kunststoffe, Vol. 51 , No. 6, 1998, pages 444-449. The process according to the present invention is characterized in that the non-productive stage of mixing is carried out in an internal mixer in the presence of 0.1 to 5 phr of a carbon black coupling coagent according to any one of formulae l-IV: IV wherein R 1 , R 2 , and R 3 independently represent H or a C,-C 5 hydrocarbon group, optionally containing one or more oxygen, nitrogen or sulfur atoms, R 1 and R 2 , provided that R is in the 4-position, or R 2 and R 3 together with the carbon atoms to which they are attached may form a 5 or 6-membered ring, said ring optionally being substituted with a C r C 5 hydrocarbon group, said group optionally containing one or more oxygen, nitrogen or sulfur atoms, n is 1 to 4, M is selected from Na, Zn, Mg, Al, Fe, and Te, and x is 1 , 1.5 or 2. In this application, the abbreviation "phr" means the number of parts by weight per 100 parts by weight of rubber. In the case of a rubber blend, it is based on 100 parts by weight of total rubber. Examples of mercaptopyπdines according to formulae l-IV are already known to the person skilled in the art of rubber vulcanization and for example have been described in the following documents SU-A-960203 discloses a sulfur vulcanization process wherein a composition comprising an unsaturated rubber such as cis-polybutadiene, 0 5-3 0 phr of zinc oxide, and 0 5-1 5 phr of zinc 2-pyrιdylthιooxιde (further referred to in this application as zinc 2-mercaptopyrιdιne N-oxide) is vulcanized The rubber composition is prepared on a roll mill It is mentioned that the rubber vulcanizates obtained by this process have an increased dynamic fatigue resistance This document does not relate to the problem of carbon black coupling, which is the subject of the present invention Moreover, carbon black coupling is less of a problem for rubber compositions comprising cis- polybutadiene or butadiene-nitπle rubber L H Davis et al in Rubber Chemistry and Technology, Vol 60, 1987, 125- 139, disclose the use of 2,2'-dιthιobιspyrιdιne-N-oxιde and the zinc salt of pyrιdιne-2-thιol-N-oxιde (further referred to in this application as zinc 2- mercaptopyπdine N-oxide) as a primary accelerator alone or in combination with a low amount of a benzothιazole-2-sulfenamιde accelerator in the sulfur vulcanization of polyisoprene, e g , natural, rubber compounds It is mentioned that the curatives which include the mercaptopyridine are added This document does not relate to the problem of carbon black coupling either, all the more so because the mercaptopyridine is added in the productive stage of mixing of the rubber composition, i e on the mill As is shown in the Examples below in Table 3, carbon black coupling is only observed when the coagent is added in the non-productive stage of mixing JP-A-04068042 discloses a vulcanizable rubber composition comprising 0.1 to 10 phr of a pyridine compound such as 2-mercaptopyridιne, 2- mercaptopyndine-N-oxide, 2,2'-dιthiodipyridine, and 2,2'-dithiobιs(pyridine- N-oxide) and 0.1-8 phr of sulfur It is mentioned in the description that the pyridine compound is used as a vulcanizer and that the rubber vulcanizates have anti-scorching properties and heat resistance This document also does not relate to the problem of carbon black coupling. Moreover, it is clear to a person skilled in this art that the pyridine compound is added in the productive stage of mixing of the rubber composition which, as described above, does not lead to carbon black coupling This document also does not disclose compounds according to formula I and II The carbon black coupling coagents according to formulae l-IV can be prepared by synthetic methods that are known to a person skilled in the art and using conventional equipment In the coagents according to formulae l-IV, the 5 or 6-membered ring may be saturated or unsaturated, preferably unsaturated Preferably, for a carbon black coupling coagent according to any one of formulae l-IV, R 1 , R 2 , and R 3 represent H Preferably, the C r C 5 hydrocarbon group is a C r C 5 alkyl group, most preferably a methyl group Preferably, the hydrocarbon group or ring is unsubstituted and does not contain an oxygen, nitrogen or sulfur atom. Preferably, n is 2 Preferably, M is Zn. Preferably, x is 1 Typical examples of suitable carbon black coupling coagents include 2,2'- dithiobispyndine (Pyr-S2, formula IV), 2,2'-dithιobιs(pyπdιne N-oxide) (Pyro- S2, formula III), zinc 2-mercaptopyrιdιne (formula II), zinc 2- mercaptopyridine N-oxide (ZPNO, formula I), sodium 2-mercaptopyrιdιne (formula II), and sodium 2-mercaptopyπdιne N-oxide (formula I) Preferably, a carbon black coupling coagent according to formula I or III is used in the invention process Most preferably, in the invention process use is made of 2,2'-dιthιobιs(pyrιdιne N-oxide) Either a single coagent of formula I, II, III or IV or a mixture of coagents may be used The polyisoprene rubber that is used in accordance with the invention process may be a natural rubber such as natural rubber (NR), a synthetic rubber such as isoprene rubber (IR) or a mixture thereof Preferably, the rubber composition comprises NR Blends of a polyisoprene rubber with one or more other rubbers such as polybutadiene rubber or butadiene rubber (BR), styrene-butadiene rubber (SBR), and a mixture of BR and SBR may also be used In the case of a blend of a polyisoprene rubber with another rubber, the rubber blend contains more than 80 parts by weight of the natural rubber per 100 parts by weight of the total rubber blend The carbon black reinforcing filler that is used in accordance with the present invention is well-known to the person skilled in the art Any carbon black or a combination of carbon black with any light-coloured filler such as silica may be used in accordance with the present invention The reader is referred to W Hofmann, Rubber Technology Handbook, Hanser Publishers, Munich 1989, in particular pages 277-294 A sulfur-vulcanizable rubber composition in accordance with the present invention typically contains 10 to 80, preferably 20 to 60, more preferably 40 to 60 phr of carbon black and 0 to 80, preferably 0 to 60, more preferably 0 to 40 phr of silica. Preferably, the amount of carbon black coupling coagent employed in the process of the present invention is 0.2 to 4, more preferably 0.25 to 2, most preferably 0.5 to 1 phr. In the process of the invention sulfur, a sulfur donor or a mixture thereof is employed. The amount of sulfur to be compounded with the rubber usually is 0.1 to 10, preferably 0.1 to 5, more preferably 0.5 to 3 phr. If a sulfur donor is used, the amount thereof should be calculated in terms of the amount of sulfur. Typical examples of sulfur donors that can be used in accordance with the present invention include dithiodimorpholine, caprolactam disulfide, tetramethylthiuram disulfide, and dipentamethylenethiuram tetrasulfide. The reader is referred to W. Hofmann, Rubber Technology Handbook, Hanser Publishers, Munich 1989, in particular pages 231-233. In the process of the invention either a single vulcanization accelerator or a mixture of accelerators can be employed. For vulcanization accelerators that can be used in accordance with the present invention the reader is referred to W. Hofmann, Rubber Technology Handbook, Hanser Publishers, Munich 1989. Typical vulcanization accelerators include thiazole- and benzothiazole- based accelerators, for example 2-mercaptobenzothiazole and bis(2- benzothiazolyl) disulfide, benzothiazole-2-sulfenamide-based accelerators, such as N-cyclohexyl-benzothiazole-2-sulfenamide (CBS), N-tert-butyl- benzothiazole-2-sulfenamide, N,N-dicyclohexyl-benzothiazole-2- sulfenamide, and 2-(morpholιnothιo)benzothιazole, thiophosphoπc acid derivatives, thiurams, dithiocarbamates, diphenylguanidine, diorthotolyl guanidine, dithiocarbamyl sulfenamides, xanthates, and mixtures of one or more of these accelerators Preferably, the vulcanization accelerator comprises a benzothιazole-2-sulfenamιde In the process of the present invention the vulcanization accelerator usually is employed in an amount of 0 1 to 5, preferably 0 3 to 3, most preferably 0 5 to 2 5 phr Preferably, the carbon black coupling coagent used in accordance with the present invention is present in the invention process in a lower amount than the amount of vulcanization accelerator A typical sulfur-vulcanizable rubber composition to be used in the process in accordance with the present invention comprises a polyisoprene rubber, preferably comprising NR, 0 1 to 10 phr of sulfur and/or a sulfur donor, 0 1 to 5 phr of a vulcanization accelerator, preferably comprising a benzothιazole-2-sulfenamιde, 10 to 80 phr of carbon black, 0 to 80 phr of silica, and 0 1 to 5 phr of a carbon black coupling coagent according to any one of formulae l-IV, preferably according to formula I or III Preferably, the composition comprises 0 1 to 5 phr of sulfur and/or sulfur donor, 0 3 to 3 phr of a vulcanization accelerator, 20 to 60 phr of carbon black, 0 to 60 phr of silica, and 0 2 to 4 phr of a carbon black coupling coagent Conventional rubber additives may also be included in the vulcanizable rubber composition in accordance with the present invention Examples include processing oils tackifiers, waxes, (phenolic) antioxidants, (phenylenediamine) antidegradants, antiozonants, pigments, e g titanium dioxide, resins, plasticizers, factices, and vulcanization activators, such as steaπc acid and zinc oxide These conventional rubber additives may be added in amounts known to the person skilled in the art of rubber compounding The reader is also referred to the examples that are described below Further, vulcanization inhibitors, i e scorch retarders, such as cyclohexyl- thiophthalimide, phthalic anhydride, pyromellitic anhydride, benzene hexacarboxy c tπanhydnde, 4-methylphthalιc anhydride, trimellitic anhydride, 4-chlorophthalιc anhydride, salicylic acid, benzoic acid, maleic anhydride, citraconic anhydride, itaconic anhydride, and N-nitrosodiphenyl- amine may be included in conventional, known amounts For further details on these typical rubber additives and vulcanization inhibitors, see W Hofmann, Rubber Technology Handbook, Hanser Publishers, Munich 1989 Finally, in specific applications it may also be desirable to include steel-cord adhesion promoters such as cobalt salts and dithiosulfates in conventional, known quantities The vulcanization process of the present invention can be carried out using means and equipment that are well-known to a person skilled in the art Suitable vulcanization procedures are described in W Hofmann, Rubber Technology Handbook, Hanser Publishers, Munich 1989 A typical method comprises preparing a masterbatch comprising a polyisoprene rubber, carbon black, optionally silica, a vulcanization activator, a processing oil, and the carbon black coupling coagent in accordance with the present invention in an internal mixer such as a Banbury mixer or a Werner & Pfieiderer mixer i e , the non-productive stage of mixing, and subseguently adding a vulcanization system comprising sulfur and a vulcanization accelerator to the masterbatch on a two-roll mill, i e the productive stage of mixing The uncured rubber composition is then vulcanized by heating, e g , by compression moulding In the case of a carbon black coupling agent according to formula III and IV with x is 1.5 or 2, it should be ensured that the temperature in the internal mixer does not become too high. The invention vulcanization process typically is carried out at a temperature of 110-200, preferably 120-190, more preferably 140-180°C for a period of time of up to 12, preferably up to 6, more preferably up to 3 hours. The present invention also pertains to articles of manufacture, such as pneumatic tyres, e.g., for passenger cars and trucks, and industrial rubber goods, which comprise the rubber vulcanizate obtained by the invention process. The invention is illustrated by the following examples. EXAMPLES A masterbatch of rubber, carbon black, stearic acid, zinc oxide, processing oil, antidegradant, and carbon black coupling coagent was made in an internal mixer (dump temperature 140-160°C). Subsequently, sulfur and an accelerator were mixed with the masterbatch on a two-roll mill at approx. 50-70°C. Rubber compounds were vulcanized by compression moulding at 150°C for a period of time equal to t 90 . After cooling the vulcanized rubber sheets for 24 h, test pieces were cut and their properties were determined. The rheological properties were determined on a Monsanto Rheometer MDR2000E, arc 0.5°, 150X/60 min. Scorch time (t s 2) is the time to increase the torque 2 dNm above the minimum torque (M L ). Optimum vulcanization time (t 90 ) is the time at 90% of the maximum torque (M H ). T end is the time at the rheometer and is set at 1 h. Delta torque (Delta S) is the difference between the minimum and the maximum torque. The slope of a rheogram between M L and M H is a measure of the cure rate (RH). Hysteresis is the percentage of energy lost per cycle of deformation. The ratio of loss modulus to storage modulus is defined as mechanical loss and this corresponds to tangent delta (tan d). Heat build-up (HBU) measurements, i.e. determination of the needle temperature, were carried out in accordance with ASTM D623/A (load: 10.8 kg; stroke: 4.45 mm; duration: 30 min; start temp.: 100°C). The dynamic mechanical properties were determined using a Metravib R.D.S. viscoanalyzer (deformation type: tension-compression; temp.: 60°C; frequency: 15 Hz; dynamic strain: strain sweep of 0.01 % to 10% strain, i.e. 9 intervals on a logarithmic scale). From the data the Payne effect is calculated, the difference between the modulus at 0.01 % strain and at 10% strain. The lower the number, the better the carbon black rubber coupling. Comparative Examples A and C: control without coagent Comparative Example B: Pyro-S2 mixed at 50-70°C on mill Examples 1 and 2: Pyro-S2 mixed at 140-160°C in internal mixer Pyro-S2 is 2,2'-dithiobis(pyridine N-oxide) Examples 3 and 4: Pyr-S2 mixed at 140-160°C in internal mixer Pyr-S2 is 2,2'-dithiobispyridine Example 5: ZPNO mixed at 140-160°C in internal mixer ZPNO is zinc 2-mercaptopyridine N-oxide The carbon black coupling coagent was compounded into compositions comprising NR as shown in Tables 1 and 4. The rheological properties of the rubber vulcanizates are shown in Tables 2 and 5, and the Payne effect is shown in Tables 3 and 6. Table 1 : Rubber compositions Addition of Pyro-S2 (Examples 1 and 2) resulted in an equal scorch time (t s 2), a large increase in tensile modulus (Delta S), a faster cure rate (RH), and less reversion (tan d). The use of Pyr-S2 reduced the scorch time as compared to the control. It was found that the needle temperature (HBU measurements, cure at 150°C/2xt 90 ) of the vulcanizate prepared in the presence of a coagent in accordance with the present invention was significantly lower (i.e. 147.2°C and 144.9°C for Examples 1 and 2, respectively, and 141.8°C and 144.8°C for Examples 3 and 4, respectively) than that of the control (i.e. 168.7°C for Comparative Example A). This effect was even more pronounced when the rubber composition was vulcanized at 180°C, i.e. needle temperatures of 149.8°C (Example 1 ') and 146.1°C (Example 2') were observed as compared to 186.3°C for the control. Hence, the rubber vulcanizate obtained by the process of the present invention showed a lower hysteresis, which is an indication of a lower rolling resistance, as compared to the control. The latter was confirmed, via measurements in a viscoanalyzer, cure at 150°C/1.5xt 90 , by the lower tangent delta (i.e. 0.122 and 0.116 for Examples 1 and 2, respectively, and 0.125 and 0.110 for Examples 3 and 4, respectively) at 60°C, 15 Hz, and 1 % strain for the vulcanizate in accordance with the present invention as compared to the control (i.e. 0.151 for Comparative Example A). This effect was even more pronounced in the case of vulcanization at 180°C (data not shown). The higher tensile modulus and reduced hysteresis are an indication of the chemical coupling of carbon black to natural rubber. Table 3: Payne effect 1 Vayne effect is E at 0.01 % strain - E at 10% strain The vulcanizate of Examples 1 and 2 showed a smaller difference between the modulus at low strain and at high strain (i.e. Payne effect) than the control A. The conclusion is that coagent Pyro-S2 is acting as a coupling agent between carbon black and natural rubber. Comparative Example B showed that when Pyro-S2 was added in the productive stage of mixing on the mill, the Payne effect increased relative to the control A. Table 4: Rubber compositions Table 5: Rheological properties at 150°C/60 min It was found that the needle temperature (HBU measurements, cure at 150°C/2xt 90 ) of the vulcanizate prepared in the presence of a coagent in accordance with the present invention was significantly lower (i.e. 45.9°C for Example 5) than that of the control (i.e. 78.5°C for Comparative Example C). Hence, the rubber vulcanizate obtained by the process of the present invention showed a lower hysteresis, which is an indication of a lower rolling resistance, as compared to the control. The latter was confirmed, via measurements in a viscoanalyzer, cure at 150°C/1.5xt 90 , by the lower tangent delta (i.e. 0.121 for Example 5) at 60°C, 15 Hz, and 1% strain for the vulcanizate in accordance with the present invention as compared to the control (i.e. 0.152 for Comparative Example C). The higher tensile modulus and reduced hysteresis are an indication of the chemical coupling of carbon black to natural rubber. Table 6: Payne effect 1 1 Payne effect is E at 0.01 % strain - E at 10% strain The vulcanizate of Example 5 showed a smaller difference between the modulus at low strain and at high strain (i.e. Payne effect) than the control C. The conclusion is that ZPNO is acting as a coupling agent between carbon black and natural rubber.

Claims: CLAIMS 1. A sulfur vulcanization process comprising (a) a non-productive stage of mixing of a rubber composition comprising a polyisoprene rubber and carbon black, (b) a productive stage of mixing of a rubber composition further comprising sulfur and a vulcanization accelerator, and (c) vulcanization of said rubber composition, characterized in that the nonproductive stage of mixing is carried out in an internal mixer in the presence of 0.1 to 5 phr of a carbon black coupling coagent according to any one of formulae l-IV: III IV wherein R 1 , R 2 , and R 3 independently represent H or a C r C 5 hydrocarbon group, optionally containing one or more oxygen, nitrogen or sulfur atoms, R 1 and R 2 , provided that R 1 is in the 4-position, or R 2 and R 3 together with the carbon atoms to which they are attached may form a 5 or 6- membered ring, said ring optionally being substituted with a C,-C 5 hydrocarbon group, said group optionally containing one or more oxygen, nitrogen or sulfur atoms, n is 1 to 4, M is selected from Na, Zn, Mg, Al, Fe, and Te, and x is 1 , 1.5 or 2. 2. A process according to claim 1 , characterized in that the amount of coagent is 0.2 to 4 phr. 3. A process according to claim 1 or 2, characterized in that for a coagent according to any one of formulae l-IV, R 1 , R 2 , and R 3 represent H. 4. A process according to any one of claims 1-3, characterized in that n is 2. 5. A process according to any one of claims 1-4, characterized in that M is Zn. 6. A process according to any one of claims 1-4, characterized in that x is 1. 7. A process according to any one of claims 1-6, characterized in that the coagent is a coagent according to formula I or III, preferably 2,2'- dithiobis(pyridine N-oxide). 8. A process according to any one of claims 1-7, characterized in that the rubber composition comprises natural rubber. . A process according to any one of claims 1-8, characterized in that the vulcanization accelerator comprises a benzothiazole-2-sulfenamide. 10. An article of manufacture, such as a pneumatic tyre, comprising the rubber vulcanizate obtained by the process according to any one of the preceding claims.

Inventors: DATTA RABINDRA NATH, HUNTINK NICOLAAS MARIA, NIEUWENHUIS PAULUS GERHARDUS J, TALMA AUKE GERARDUS, DATTA, RABINDRA, NATH, HUNTINK, NICOLAAS, MARIA, NIEUWENHUIS, PAULUS, GERHARDUS, JOHANNES, TALMA, AUKE, GERARDUS
Applicants: FLEXSYS B V, FLEXSYS B.V.
Application year: 2000
Main classification: C08K5
IPC codes: C08K5