WO2001016226A1

WO2001016226A1 - Hardness stabilization by mercaptopyridines

Info

Publication number: WO2001016226A1
Application number: PCT/EP2000/008490
Authority: WO
Inventors: Rabindra Nath Datta; Nicolaas Maria Huntink; Auke Gerardus Talma
Original assignee: Flexsys B.V.
Priority date: 1999-08-31
Filing date: 2000-08-29
Publication date: 2001-03-08
Also published as: AU7512400A

Abstract

The invention relates to a sulfur vulcanization process comprising (a) a non-productive stage of mixing of a rubber composition comprising styrene-butadiene rubber, (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 in the non-productive or productive stage of mixing there is added 0.1 to 1 phr of a hardness stabilizing coagent according to any one of formulae (I)-(IV) wherein R?1, R2, and R3¿ independently represent H or a C¿1?-C5 hydrocarbon group, optionally containing one or more oxygen, nitrogen or sulfur atoms, R?1 and R2¿, provided that R1 is in the 4-position, or R?2 and R3¿ 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¿1?-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 to 4. The invention also relates to an article of manufacture, such as a pneumatic tyre, comprising the rubber vulcanizate obtained by said process.

Description

HARDNESS STABILIZATION BY MERCAPTOPYRIDINES

The invention relates to a sulfur vulcanization process comprising (a) a non-productive stage of mixing of a rubber composition comprising styrene- butadiene rubber, (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, but such vulcanizates tend not to have good aging properties. A typical aging phenomenon is hardening, which is explained below.

Uncured as well as cured rubbers are prone to aging effects. The unsaturated groups in diene rubbers, e.g. styrene-butadiene rubber (SBR) or a blend of SBR with natural rubber, butadiene rubber or with both, make it possible to cure with sulfur, but at the same time they exhibit a sensitivity toward oxygen, ozone, and other reactive substances causing changes such as hardening in the rubber. Since soft rubbers based on diene rubbers contain free double bonds, they remain sensitive to the above reactive substances even after vulcanization. Higher temperatures make these effects more noticeable. Also, since unreacted double bonds are present in the rubber vulcanizate, there is the possibility of further reaction with sulfur causing hardening, i.e. additional crosslinking, of the vulcanizate. The use of antioxidants will retard the oxygen-induced aging of the vulcanizate, but will not affect the increase in hardness due to sulfur- induced crosslinking.

According to the present invention, it has been found that by adding low amounts of certain coagents to a rubber composition comprising styrene- butadiene rubber, vulcanizates, from which, e.g., pneumatic tyres can be made, having improved properties can be obtained. These coagents have the effect of stabilizing the hardness properties of the rubber vulcanizate, e.g., during the service life of a pneumatic tyre, without bringing the flex and dynamic properties of the rubber such as torque and modulus to undesired levels, and are hereinafter also referred to as hardness stabilizing coagents.

The process according to the present invention is characterized in that in the non-productive or productive stage of mixing there is added 0.1 to 1 phr of a hardness stabilizing coagent according to any one of formulae l-IV:

IV

wherein

R¹, R², and R³ independently represent H or a C^Cs hydrocarbon group, optionally containing one or more oxygen, nitrogen or sulfur atoms,

R¹ and R², provided that R is in the 4-position, or R² and R³ 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_rC₅ 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 to 4.

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 mercaptopyridines 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-pyridylthiooxide (further referred to in this application as zinc 2-mercaptopyridine N-oxide) is vulcanized. 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 sulfur-induced hardening which is the subject of the present invention and which is particularly relevant to rubber compositions comprising styrene-butadiene rubber.

L.H. Davis et al. in Rubber Chemistry and Technology, Vol. 60, 1987, 125- 139, disclose the use of 2,2'-dithiobispyridine-N-oxide and the zinc salt of pyridine-2-thiol-N-oxide (further referred to in this application as zinc 2- mercaptopyridine N-oxide) as a primary accelerator alone or in combination with a low amount of a benzothiazole-2-sulfenamide accelerator in the sulfur vulcanization of polyisoprene, e.g., natural, rubber compounds. This document only relates to polyisoprene rubbers which do not harden.

JP-A-04068042 discloses a vulcanizable rubber composition comprising 0.1 to 10, preferably 0.2 to 5 phr of a pyridine compound such as 2- mercaptopyridine, 2-mercaptopyridine-N-oxide, 2,2'-dithiodipyridine, and 2,2'-dithiobis(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. The rubber is selected from natural rubber, styrene-butadiene rubber, polyisoprene rubber, nitrile rubber, chloroprene rubber, butyl rubber, and ethylene-propylene terpolymer. In Examples 1-4 of this document, 1.5 phr of a pyridine compound is used in rubber compositions comprising natural rubber.

This document does neither disclose using mercaptopyridines in the amount claimed in accordance with the present invention process, nor does it relate to the problem of hardening which is the subject of the present application and which problem is related to unsaturated rubbers. Natural rubber-comprising vulcanizates do not suffer from hardening. This document also does not disclose compounds according to formula I and II.

The hardness stabilizing 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 hardness stabilizing coagent according to formula I or III, R¹, R², and R³ represent H. Preferably, for a hardness stabilizing coagent according to formula II or IV, R¹ represents a C_rC₅ hydrocarbon group and R² and R³ form a 5 or 6-membered ring. Preferably, the C_rC₅ hydrocarbon group is a C C₅ alkyl group, most preferably a methyl group. Preferably, the hydrocarbon group or ring does not contain an oxygen, nitrogen or sulfur atom. Preferably, n is 2. Preferably, M is Zn. Preferably, x is 1 or 2.

Typical examples of suitable hardness stabilizing coagents include 2,2'- dithiobispyridine (formula IV), 2,2'-dithiobis(pyhdine N-oxide) (Pyro-S2, formula III), zinc 2-mercaptopyridine (ZnDTPy, formula II), zinc 2- mercaptopyridine N-oxide (ZPNO, formula I), sodium 2-mercaptopyridine (formula II), sodium 2-mercaptopyridine N-oxide (formula I), 2,2'- thiobislepidine (MTIep, formula IV), 2,2'-dithiobislepidine (DTIep, formula IV), and zinc 2-mercaptolepidine (ZnDTIep, formula II). Preferably, the hardness stabilizing coagent in accordance with the present invention is a coagent according to formula IV, most preferably 2,2'- thiobislepidine or 2,2'-dithiobislepidine.

Either a single coagent of formula I, II, III or IV or a mixture of coagents may be used. Either styrene-butadiene rubber (SBR) or a blend of SBR with one or more other rubbers can be used in the invention process.

Typically, SBR, a blend of SBR with natural rubber (NR), a blend of SBR with polybutadiene rubber or butadiene rubber (BR), or a blend of SBR with NR and BR is used. Preferably, a blend of SBR and BR is used in the process of the present invention.

In the case of a blend of SBR with another rubber, hardening is significant when the rubber blend contains 20 parts by weight or more of SBR per 100 parts by weight of the total rubber blend.

Preferably, the amount of hardness stabilizing coagent employed in the process of the present invention is 0.1 to 0.9, more preferably 0.1 to 0.75, most preferably 0.25 to 0.5 phr.

It can be derived from the results of the Examples shown below, that amounts higher than 1 phr will not improve hardness stabilization much further, while torque (Delta S) and modulus (Mod200), which inter alia reflect the flex and dynamic properties of the rubber vulcanizate, will increase to undesirable levels.

In the process of the present 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 process of 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 present 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 (MBTS), benzothiazole-2-sulfenamide-based accelerators, such as N-cyclohexyl-benzothiazole-2-sulfenamide, N-tert- butyl-benzothiazole-2-sulfenamide (TBBS), N,N-dicyclohexyl- benzothiazole-2-sulfenamide, and 2-(morpholinothio)benzothiazole, thiophosphohc acid derivatives, thiurams, dithiOcarbamates, diphenylguanidine (DPG), diorthotolyl guanidine, dithiocarbamyl sulfenamides, xanthates, and mixtures of one or more of these accelerators. Preferably, the vulcanization accelerator comprises a benzothiazole-2-sulfenamide. Mixtures of one or more benzothiazole-2-sulfenamide-based vulcanization accelerators with diphenylguanidine are preferred, for example a combination of MBTS, TBBS, and DPG.

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.

A typical sulfur-vulcanizable rubber composition to be used in the process in accordance with the present invention comprises SBR, preferably a blend of SBR and BR, 0.1 to 10 phr of sulfur and/or a sulfur donor, 0.1 to 5 phr of a vulcanization accelerator, preferably comprising a benzothiazole-2- sulfenamide, and 0.1 to 1 phr of a hardness stabilizing coagent according to any one of formulae l-IV. Preferably, the composition comprises 0.1 to 5 phr of sulfur and/or sulfur donor, 0.3 to 3 phr of a vulcanization accelerator, and 0.1 to 0.9 phr of a hardness stabilizing coagent according to any one of formulae l-IV.

Conventional rubber additives may also be included in the sulfur- vulcanizable rubber composition in accordance with the present invention. Examples include reinforcing agents such as carbon black, silica, clay, whiting and other mineral fillers, processing oils, tackifiers, waxes, (phenolic) antioxidants, (phenylenediamine) antidegradants, antiozonants, pigments, e.g. titanium dioxide, resins, plasticizers, factices, and vulcanization activators, such as stearic 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 hexacarboxylic trianhydride, 4-methylphthalic anhydride, trimellitic anhydride, 4-chlorophthalic anhydride, salicylic acid, benzoic acid, maieic anhydride, citraconic anhydride, itaconic anhydride, and N- nitrosodiphenylamine 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 sulfur 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 SBR, carbon black, a vulcanization activator, and a processing oil, in an internal mixer such as a Banbury mixer or a Werner & Pfleiderer mixer, i.e., the non-productive stage of mixing, and subsequently adding a vulcanization system comprising sulfur and a vulcanization accelerator, and the hardness stabilizing coagent in accordance with the present invention 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. The hardness stabilizing coagent may be added in the non-productive or productive stage of mixing. Preferably, the coagent is added in the productive stage of mixing, certainly in the case of a hardness stabilizing coagent according to formula III and IV with x is 3 or 4.

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, and antidegradant was made in an internal mixer. The sulfur, accelerators, and hardness stabilizing coagent were mixed on a two-roll mill at approx. 50-70°C. Rubber compounds were vulcanized by compression moulding at 145°C for a period of time equal to 1.7xt_go. After cooling the vulcanized rubber sheets for 24 h, test pieces were cut and analyzed.

The rheological properties were determined on a Monsanto Rheometer MDR2000E, arc 0.5°, 145°C/60 min. Scorch time (t_s2) is the time to increase the torque 2 dNm above the minimum torque (M_L). Optimum vulcanization time (t₉₀) 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).

The rubber test pieces were aged in a hot air circulation oven for 3 days (72 h) at 100°C to simulate hardening during use, for example, as a tyre. The hardness stabilization characteristics were determined by calculating the so-called modulus stabilization (MS).

The modulus stabilization is the ratio of the modulus at elongation 200% (Mod200) of the aged and the unaged rubber test pieces and is expressed as a percentage by multiplying this ratio by 100%. The lower the ratio Mod200_aged/Mod200_unaged, the better the modulus retention or hardness stabilization. The Mod200 was obtained from tensile stress-strain tests which were performed in accordance with ISO 37-1994 (dumb-bell type 2).

Comparative Examples A, B, and C: control without coagent Examples 1 and 2: zinc 2-mercaptopyridine N-oxide (ZPNO) Examples 3 and 4: 2,2'-dithiobis(pyridine N-oxide) (Pyro-S2) Example 5: 2,2'-thiobislepidine (MTIep) Example 6: zinc 2-mercaptolepidine (ZnDTIep) Example 7: 2,2'-dithiobislepidine (DTIep) Example 8: zinc 2-mercaptopyridine (ZnDTPy)

The coagents were compounded into a rubber composition comprising SBR as shown in Tables 1 and 4. The rheological properties are shown in Tables 2 and 5, and the modulus stabilization, an indication of the hardness stabilizing effect of a coagent, is shown in Tables 3 and 6.

Table 1 : Rubber compositions

Table 3: Modulus stabilization or MS¹

¹MS = Mod200_aged/Mod200_unaged x 100% Table 4: Rubber compositions

Table 5: Rheological properties at 145°C/60min

Table 6: Modulus stabilization or MS¹

¹MS = Mod200_aged/Mod200_uπaged x 100% The above data, in particular in Tables 3 and 6, shows that the use of a low amount of a hardness stabilizing coagent in accordance with the present invention leads to the formation of rubber vulcanizates showing improved modulus stabilization (MS), i.e. hardness stabilization, as compared to the control. The results for Examples 1-4 indicate that a two-fold increase in the amount of the hardness stabilizing coagent does not improve modulus stabilization much further.

Claims

1. A sulfur vulcanization process comprising (a) a non-productive stage of mixing of a rubber composition comprising styrene-butadiene rubber, (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 in the non-productive or productive stage of mixing there is added 0.1 to 1 phr of a hardness stabilizing coagent according to any one of formulae l-IV:

IV

wherein

R¹, R², and R³ independently represent H or a C C₅ hydrocarbon group, optionally containing one or more oxygen, nitrogen or sulfur atoms, R¹ and R², provided that R¹ is in the 4-position, or R² and R³ 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 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 to 4.

2. A process according to claim 1 , characterized in that the amount of coagent is 0.1 to 0.9 phr.

3. A process according to claim 1 or 2, characterized in that for a coagent according to formula I or III, R¹, R², and R³ represent H and that for a coagent according to formula II or IV, R¹ represents a C.,-C₅ hydrocarbon group and R² and R³ form a 5 or 6-membered ring.

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 or 2.

7. A process according to any one of claims 1-6, characterized in that the coagent is a coagent according to formula IV, preferably 2,2'- thiobislepidine or 2,2'-dithiobislepidine.

8. A process according to any one of claims 1-7, characterized in that the rubber composition comprises styrene-butadiene rubber and butadiene rubber.

9. 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.