WO1995034602A1

WO1995034602A1 - Secondary accelerators for the sulfur-vulcanization of rubber

Info

Publication number: WO1995034602A1
Application number: PCT/EP1995/002253
Authority: WO
Inventors: Rudolph Frank De Block; Rabindra Nath Datta; Hendrick Wulfert Hovius
Original assignee: Akzo Nobel N.V.
Priority date: 1994-06-10
Filing date: 1995-06-09
Publication date: 1995-12-21
Also published as: TW294692B

Abstract

Secondary accelerators of formula (I), wherein at least one of R, R1 and R2 is a dicarbamoyl radical of the formula (II), which are useful in combination with a primary accelerator selected from sulfenamides and thiazoles, in the sulfur-vulcanization of rubber compositions, are disclosed. Also disclosed are sulfur-vulcanized rubber compositions which are vulcanized with a primary accelerator and in the presence of one or more of the compounds of formula (I) and a sulfur-vulcanization process carried out in the presence of a primary accelerator and one or more of the compounds of formula (I). The present approach achieves a clear improvement in the vulcanization time as compared to a system without a secondary accelerator and, further, when compared with other secondary accelerators, the scorch time is improved by the present secondary accelerators.

Description

Secondary Accelerators for the Sulfur-Vulcanization of Rubber

The present invention relates to secondary accelerators which are used in combination with a primary accelerator selected from sulfenamides and thiazoles, in the sulfur-vulcanization of rubber compositions.

It is essential in the injection moulding and continuous vulcanization of rubber articles that the mouldable rubber remains processable (i.e. readily flowable) in both the extruder and the conveyor to the mould. In the mould, however, the rubber should vulcanize as rapidly as possible. Faster vulcanization permits a higher rate of production.

Since the viscosity of the mouldable rubber decreases with higher temperatures, it is desirable to maintain the temperature in the extruder and the conveyor to the mould as high as possible without risking premature crosslinking. In practice, the maximum processing temperature in the extruder/conveyor is at least 30° to 50°C below the temperature in the mould (vulcanization temperature). A small increase in the processing temperature and/or extension of the residence time in the extruder/conveyor can, above a certain critical level (or threshold-value), lead to premature crosslinking, which is characterized by wrinkled or "scorched" spots on the smooth surface of the rubber article produced. This phenomenon is commonly known as "scorch".

The tendency of a rubber to scorch (indicated by the t_s2) under commercial operating conditions may be measured by means of a moving die rheometer which procedure is described in International Standard ISO 6502. Mooney scorch is measured with a Mooney viscosimeter according to ISO 667.

Indicative of the scorch time of a rubber is the t_s2 value, which is the time to 2% of the delta torque above the minimum torque (ML). Delta torque or extent of crosslinking is the maximum torque (MH) minus the minimum torque (ML). Indicative of the residence time in the mould is the t₉₀ value (vulcanization time), which is the time to obtain 90% of the delta torque.

Until now, in the sulfur-vulcanization of rubber, a combination of a primary accelerator and a secondary accelerator have been employed. This combination has the distinct advantage that it gives a faster vulcanization (lower t₉₀ value) than when only a primary accelerator is employed, thereby increasing the production rate. However, for some applications such as injection moulding, these secondary accelerators suffer from the disadvantage that they also give a corresponding reduction in the scorch time (t_s2 value) thereby increasing the risk of scorch in such processes.

Accordingly, there is a need in the sulfur-vulcanization field for a secondary accelerator which reduces the vulcanization time without producing a correspondingly large decrease in the scorch time, thereby providing a shorter production time with improved process safety.

The present invention solves this problem through the use, in the sulfur-vulcanization of rubber with a primary accelerator selected from a sulfenamide and a. thiazole such as 2-mercaptobenzothiazole, zinc-2-mercaptobenzothiazole and dibenzothiazyl disulfide, of a sufficient amount of a secondary accelerator to improve the scorch ratio, characterized in that said secondary accelerator is a compound of the formula I:

wherein R, R₁ and R₂ are independently selected from hydrogen, C₁-C₁₀ alkyl, C₇-C₁₀ aralkyl, C₂-C₁₀ alkenyl, C₆-C₁₀ aryl and C₇-C₁₀ alkaryl, and one of R, R₁ and R₂ may be halogen, nitro, hydroxyl and at least one of R, R₁ and R₂ is a dicarbamoyl radical of the formula II:

wherein R₄ and R₅ are independently selected from C₁-C₁₀ alkyl, C₇-C₁₀ aralkyl, C₂-C₁₀ alkenyl, C₆-C₁₀ aryl , C₇-C₁₀ alkaryl, and R₄ and R₅ can combine to form a C₄-C₇ cycloalkyl group or a heterocycloalkyl group; R6 and R7 are independently selected from hydrogen, C₁-C₁₀ alkyl, C₇-C₁₀ aralkyl, C2-C₁₀ alkenyl, C₆-C₁₀ aryl and C₇-C₁₀ alkaryl, and R₅ and R₇ can combine to form a C₄-C₇ cycloalkyl group.

The present invention also relates to sulfur-vulcanized rubber compositions which are vulcanized with a primary accelerator and one or more of the compounds of the formula I and to a sulfur-vulcanization process carried out in the presence of a primary accelerator and one or more of the compounds of the formula I.

The present approach achieves a clear improvement in the vulcanization time (t₉₀) as compared to a system without a secondary accelerator and, further, when compared with other secondary accelerators the reduction in vulcanization time is of approximately the same order but with the approach of the present invention the scorch time is significantly improved thereby offering the important advantage that a relatively long scorch time can be combined with a short production time.

Some of the compounds of the formula I are known from U.S. patent 2,757,174 which also teaches that these compounds may be employed as accelerators in the vulcanization of natural and synthetic rubber. A similar disclosure is found in British Patent 722,870.

Some of the compounds of the formula I are also known to be vulcanization accelerators for the sulfur-vulcanization of rubber from Japanese published patent application no. 44245/1975. This patent application employs such compounds as accelerators for rubber compositions comprising two or more rubber components having significantly different vulcanization rates in order to render these compositions more compatible during vulcanization. The primary advantage taught in this patent application is that these compounds are readily soluble in each of the individual rubber components.

Thus, from these publications one might conclude that most of the compounds of the formula I are effective as vulcanization accelerators in the sulfur-vulcanization of rubber. However, British Patent 1,049,535 discloses that some compounds of the formula I can be employed as antioxidants in styrene/butadiene copolymers. Further, this patent states that these compounds are of special advantage since they have little effect on any vulcanization process to which the rubber may be subjected. Thus, this publication casts doubt on the general teachings of the earlier U.S. patent 2,757,174 and British Patent 722,870.

Other patents such as U.S. patent 3,117,947, European Patent Application 278 890 and U.S. patent 5,019,611 all teach that these types of compounds are effective as stabilizers such as antioxidants or antiozonants.

None of the foregoing patent publications teaches or suggests the use of the compounds of the formula I as secondary accelerators in the sulfur-vulcanization of rubber in combination with a primary accelerator. Further, the important advantage achieved by the compounds of the formula I, namely that they improve the scorch time in comparison to known, commercially employed secondary accelerators such as thiurams, thiocarbamoyl sulfenamides, xanthates, dithiophosphates, guanidines and mixtures thereof, is neither taught nor suggested by these publications.

More specific examples of such secondary accelerators include tetramethyl thiuram disulfide (TMTD), tetramethyl thiuram monosulfide (TMTM), tetraethyl thiuram disulfide (TETD), tetrabenzyl thiuram disulfide (TBzTD), tetrabutyl thiuram disulfide (TBTD), diisopropyl xanthate, zinc dialkyldithiophosphate, diphenylguanidine (DPG), di-o- toluylguanidine (DOTG), N-oxydiethylene

dithiocarbamoyl-N'-oxydiethylene sulfenamide (OTOS) and N-oxydiethylene thiocarbamoyl-N'-tert-butyl sulfenamide.

Accordingly, the present inventors have found that a significant improvement in the scorch (t_s2) can be achieved by replacing the known secondary accelerators with one or more of the compounds of the formula I. This is an important advantage for particular applications where a short production time is desirable but also a rather long scorch time is necessary, such as in injection moulding.

The present invention provides the ability to achieve, during vulcanization, a relatively long scorch time and a shorter vulcanization time without the crosslink density being adversely affected. Furthermore, the invention also provides improved rubber compositions which are characterized by better stress-strain properties.

The present invention is applicable to all natural and synthetic rubbers which contain unsaturated groups. Examples of such rubbers include, but are not limited to, natural rubber (NR), ethyl ene- propylene-dienemonomer terpolymers (EPDM), styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber (NBR), isoprene rubber (IR), butadiene rubber (BR), polychloroprene rubber (CR), halogenated isoprene-isobutylene rubber (BUR or CIIR), isoprene-isobutylene rubber (IIR), chloro polyethylene rubber (CM/CPE) and chlorosulfonyl polyethylene rubber (CSM), as well as combinations of two or more of these rubbers and combinations of one or more of these rubbers with other rubbers and/or thermoplastics.

Examples of sulfur which may be used in the present invention include various types of sulfur such as powdered sulfur, precipitated sulfur and insoluble sulfur. Also, sulfur donors may be used in place of, or in addition to sulfur in order to provide the required level of sulfur during the vulcanization process. Examples of such sulfur donors include, but are not limited to, tetramethylthiuram disulfide, tetraethylthiuram disulfide, tetrabutylthiuram disulfide, pentamethylene thiuram hexasulfide, pentamethylene thiuram tetrasulfide, dithiodimorpholine, caprolactam disulfide and mixtures thereof. In this text, references to sulfur shall include sulfur donors and mixtures of sulfur and sulfur donors. Further, references to the quantity of sulfur employed in the vulcanization, when applied to sulfur donors, refer to a quantity of sulfur donor which is required to provide the equivalent amount of sulfur that is specified.

The amount of sulfur to be compounded with the rubber is, based on 100 parts of rubber, usually 0.1 to 25 parts by weight, and more preferably 0.2 to 8 parts by weight. The amount of sulfur donor to be compounded with the rubber is an amount sufficient to provide an equivalent amount of sulfur which is the same as if sulfur itself were used.

Certain known primary vulcanization accelerators may be employed. The preferred vulcanization accelerators include thiazole and sulfenamide accelerators. Examples of thiazole accelerators are

2-mercaptobenzothiazole, zinc-2-mercaptobenzothiazole and

dibenzothiazyl disulfide. The sulfenamide accelerators may be N-cyclohexyl-2-benzothiazyl sulfenamide, N-t-butyl-2-benzothiazyl sulfenamide, N-t-amyl-2-benzothiazyl sulfenamide,

N-oxydiethylene-2-benzothiazyl sulfenamide and

N,N-dicyclohexyl-2-benzothiazyl sulfenamide.

The primary vulcanization accelerator is employed in quantities of from 0.1 to 8 parts by weight, based on 100 parts by weight of rubber composition. More preferably, the primary vulcanization accelerator comprises 0.3 to 6.0 parts by weight, based on 100 parts by weight of rubber.

The secondary vulcanization accelerator is selected from one or more compounds of the formula I. These compounds can be made by processes known in the art such as, for example, is described in U.S. Patent 3,658,743. The most preferred secondary accelerators for use in accordance with the present invention are the (dialkyl-hydroxybenzyl)-dialkyl dithiocarbamates, (dialkyl-hydroxybenzyl)-dibenzyl dithiocarbamates and the (dialkyl-hydroxybenzyl)-oxydiethylene dithiocarbamates. Especially preferred are the compounds (3,5-dimethyl-tert butyl-4-hydroxybenzyl)-dibenzyldithiocarbamate (HB-BEC, most preferred),

(3,5-dimethyl-tert butyl-4-hydroxybenzyl)-dimethyldithiocarbamate (HB-DMC), and (3,5-dimethyl-tert

butyl-4-hydroxybenzyl)-diethyldithiocarbamate (HB-DEC). An example of a heterocycloalkyl group which can be attached to the di thiocarbamoyl radical is a morpholino group.

Other conventional rubber additives may also be employed in their usual amounts. For example, reinforcing agents such as carbon black, silica, clay, whiting and other mineral fillers, as well as mixtures of fillers may be included in the rubber composition. Other additives such as process oils, tackifiers, waxes, antioxidants, antiozonants, pigments, resins, plasticizers, process aids, factice, compounding agents and activators such as stearic acid and zinc oxide may be included in conventional, known amounts. For a more complete listing of rubber additives which may be used in combination with the present invention see, W. Hofman, "Rubber Technology Handbook", Chapter 4 and "Rubber Chemicals and Additives", pp. 217-353, Hanser Publishers, Munich 1989.

Further, scorch retarders such as phthalic anhydride, pyromellitic anhydride, benzene hexacarboxylic trianhydride, 4-methyl phthalic anhydride, trimellitic anhydride, 4-chlorophthalic anhydride, N- cyclohexyl-thiophthalimide, salicylic acid, benzoic acid, maleic anhydride and N-nitrosodiphenylamine may also be included in the rubber composition in conventional, known amounts for special applications although, in general, the present invention obviates or greatly reduces the need for such scorch retarders. 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 present invention also relates to a vulcanization process which comprises the step of vulcanizing at least one natural or synthetic rubber in the presence of 0.1 to 25 parts by weight of sulfur, or a sufficient amount of a sulfur donor to provide the equivalent of 0.1-25 parts by weight of sulfur, per 100 parts by weight of rubber, 0.1 to 8.0 parts by weight of a primary vulcanization accelerator selected from thiazole and sulfenamide accelerators; and a sufficient amount of a secondary accelerator to provide a weight ratio of said primary accelerator to said secondary accelerator of from 10:90 to 90:10, said secondary accelerator being selected from compounds of the formula I given above.

The process is carried out at a temperature of 110-220°C over a period of up to 24 hours. More preferably, the process is carried out at a temperature of 120-190°C over a period of up to 8 hours in the presence of 0.05 to 5.0 parts by weight of the secondary accelerator. Even more preferable is the use of 0.1-3.0 parts by weight of secondary accelerator. All of the additives mentioned above with respect to the rubber composition may also be present during the vulcanization process of the invention.

In a more preferred embodiment of the vulcanization process, the vulcanization is carried out at a temperature of 120-190°C over a period of up to 8 hours.

Finally, the present invention also includes articles of manufacture, such as tires, which comprise sulfur-vulcanized rubber which is vulcanized in the presence of the secondary accelerators of the formula I. The invention is further illustrated by the following examples which are not to be construed as limiting the invention in any way. The scope of the invention is to be determined from the claims appended hereto.

Experimental Methods Used in the Examples

Mixing

First Step:

Type of Mixer : Werner & Pfleiderer (Volume 5.0 liters, 70% load factor)

Rotor Speed : 30 rpm

Start Temperature : 50°C

Mixing Time : 5 minutes

Mixing Procedure : 0 minutes - addition of rubber

1 minute - ½filler, zinc oxide & stearic acid

2½ minutes - ½ filler, oil & antioxidants

4 minutes - Sweep

5 minutes - Dump

Second Step:

Type of Mixer : Two-rol l mi l l

Rol l er Fri ction : 1/1.22

Start Temperature : 40-50°C

Mixi ng Procedure : ISO 2393 Vulcanization

Method : Compression Moulding

Temperature : 160°C unless otherwise indicated

Time :t₉₀ minutes

Test Methods Employed

Mooney Viscosity : ISO R-289 - ML 100°C

Mooney Scorch : ISO R-667 - MS 121°C

Rheology : Monsanto MDR 2000E - Vulcanization Temperature

160°C, vulcanization time 30 minutes, range 2.5

Nm, arc 0.5° - ISO 6502-91

Tensile Strength : ISO 37/2 - Dumb bell

Elongation at Break : ISO 37/2 - Dumb bell

Modulus : ISO 37/2 - Dumb bell

Hardness : ISO 48 (IRHD)

El asti city : ISO 4662

Hot Ai r Agei ng : ISO 188 - 3 days at 100°C

Density : ISO 2781

Example 1 and Comparative Examples A-C

The rubber compositions given in Table 1 were compounded and vulcanized according to the procedures given above. The rheological properties during vulcanization are given in Table 2. The mechanical properties of the resultant vulcanized rubber composition are given in

Table 3. Curing was performed at 150°C and at 170°C. The mechanical properties for the vulcanizates cured at 170°C are given in parentheses. From these examples it can be seen that when used in combination with a sulfenamide primary vulcanization accelerator, a compound of the present invention,

3,5-di-t-butyl-4-hydroxybenzyl-dibenzyldithiocarbamate (HB-BEC), gives a clear improvement in the vulcanization time (T₉₀) as compared to the control without a secondary accelerator (Comparative Example A). Further, when compared with two commercially available secondary accelerators in Comparative Examples B-C, it can be seen that the reduction in vulcanization time is of approximately the same order but with the compound of the present invention the scorch time is significantly improved compared to the scorch time obtained with commercially available secondary accelerators.

Table 1 Compound Composition C

NR SMR CV = Natural Rubber

CBS = N-cyclohexyl-2-benzothiazyl sulfenamide

TMTD = tetramethyl thiuram disulfide

TBzTD = tetrabenzyl thiuram disulfide

HB-BEC = 3,5-di-t-butyl-4-hydroxybenzyl-dibenzyldithiocarbamate Table 2 Rheological Properties

Table 3 Mechanical Properties of the vulcanizates cured at 150°C and 170°C.

Values between parenthesis are for curing at 170°C Comparative Examples D-F and Example 2

These Examples demonstrate that a secondary accelerator of the present invention, when used in combination with a sulfenamide primary vulcanization accelerator, gives even better scorch safety than a combination of the commercially available secondary accelerator with a known scorch retarding agent and the same sulfenamide primary vulcanization accelerator. Thus, the compounds of the present invention can be employed to replace a combination of the commercially available secondary accelerator with a scorch retarding agent and a further improvement is still realized over this combination, namely in the stress-strain properties of the rubber.

The ingredients of the rubber composition are given in Table 4. The cure characteristics and mechanical properties are given in Tables 5-8.

Table 4 Compound composition

TQ = 2,2 ,4-Trimethyl -1 ,2-di hydroqui nol i ne

6PPD = N-1 ,3-dimethyl butyl -N ' -phenyl -p-phenyl enedi ami ne

CTP - cycl ohexyl thi ophthal imi die Table 5 Cure data of the mixes obtained at 160°C

Table 6 Mooney scorch of the mixes obtained at 121°C

Table 7 Physico-mechanical properties of the vulcanizates cured at

160°C for t90 times

Table 8 Physico-mechanical properties of the aged (3d/100°C) vulcanizates cured at 160°C for t90 time

Example 3 and Comparative Examples G-H

In these examples, a secondary accelerator in accordance with the present invention is employed in combination with the primary accelerator 2-mercaptobenzothiazole (MBT) and compared to a combination of TMTM with MBT. This type of formulation is typical of compositions which are used in tyre carcasses. The vulcanization was carried out at 150°C and 170°C and the values of the mechanical properties are given in Table 11.

The formulations are given in Table 9, the cure properties in Table 10 and the mechanical properties are given in Table 11. The compound of the present invention gives three important advantages over TMTM and TBzTD, no nitrosamine problems, improved scorch and improvements in some mechanical properties, particularly stress-strain properties. Table 9 Compound Composition

Table 10 Cure data of the mixes obtained at 150°C & 170°C

Table 11 Physico-mechanical properties of the vulcanizates cured at 150°C/20 minutes

Examples 4-5 and Comparative Example I

In these examples it is shown that a secondary accelerator in accordance with the present invention can be used as a replacement for thiocarbamyl sulfenamides. The formulations are given in Table 12 and the cure properties in Table 13.

Table 12 Compound composition

OTOS = N-oxydiethylene-N'-oxydiethylene-thiocarbamyl sulfenamide Table 13 Cure data of the mixes at 170°C

The results show that HB-BEC can function as delayed action thiuram- thus the replacement of OTOS is possible. OTOS is based on morpholine which is carcinogenic and replacement of this ingredient is therefore highly desirable.

Examples 6-7 and Comparative Example J In these examples it is demonstrated that lower levels of a secondary accelerator according to the present invention improve the reversion resistance in conventionally cured compounds, The formulations are given in Table 14 and the cure properties in Table 15. Table 14 Compound composition

Table 15 Cure data of the mixes obtained at 150°C

The results show that small addition of HB-BEC into the compounds improves the reversion resistance with improvement in cure parameters.

Examples 8-10 and Comparative Example K

These examples demonstrate the use of some further secondary accelerators according to the present invention and their beneficial properties. Table 16 shows the compound compositions and Table 17 the rheolocical properties at 150°C.

Table 16 Coumpound composition

Table 17 Rheological data at 150°C

The foregoing examples were presented for the purpose of illustration and description only and are not to be construed as limiting the scope of the invention in any way. The scope of the invention is to be determined from the claims appended hereto.

Claims

What is claimed is:

1. Use in the sulfur-vulcanization of rubber with a primary accelerator selected from a sulfenamide and a thiazole, of a sufficient amount of a secondary accelerator to improve the scorch ratio, characterized in that said secondary accelerator is a compound of the formula I:

wherein R₄ and R₅ are independently selected from C₁-C₁₀ alkyl, C₇-C₁₀ aralkyl, C₂-C₁₀ alkenyl, C₆-C₁₀ aryl, C₇-C₁₀ alkaryl, and R₄ and R₅ can combine to form a C₄-C₇ cycloalkyl group or a heterocycloalkyl group; R₆ and R₇ are independently selected from hydrogen, C₁-C₁₀ alkyl, C₇-C₁₀ aralkyl, C₂-C₁₀ alkenyl, C₆-C₁₀ aryl and C₇-C₁₀ alkaryl, and R₆ and R₇ can combine to form a C₄-C₇ cycloalkyl group.

2. Use as claimed in claim 1 wherein said secondary accelerator is used in combination with a sulfenamide accelerator selected from N-cyclohexyl-2-benzothiazyl sulfenamide, N-t-butyl-2-benzothiazyl sulfenamide, N-t-amyl-2-benzothiazyl sulfenamide,

N-oxydiethylene-2-benzothiazyl sulfenamide and

N,N-dicyclohexyl-2-benzothiazyl sulfenamide.

3. Use as claimed in claim 1 wherein said secondary accelerator is used in combination with a thiazole accelerator selected from 2-mercaptobenzothiazole, zinc-2-mercaptobenzothiazole and dibenzothiazyl disulfide.

4. Use as claimed in any one of claims 1-3 wherein the weight ratio of primary accelerator to secondary accelerator is from 10:90 to 90:10.

5. A sulfur-vulcanized rubber composition which comprises the vulcanization reaction product of:

(A) 100 parts by weight of at least one natural or synthetic rubber;

(B) 0.1 to 25 parts by weight of sulfur and/or a sufficient amount of a sulfur donor to provide the equivalent of 0.1 to 25 parts by weight of sulfur;

(C) 0.1 to 8.0 parts by weight of a primary vulcanization accelerator selected from thiazole and sulfenamide accelerators; and (D) a sufficient amount of a secondary accelerator to provide a weight ratio of said primary accelerator to said secondary accelerator of from 10:90 to 90:10, said secondary accelerator being selected from compounds of the formula I:

wherein R₄ and R₅ are independently selected from C₁-C₁₀ alkyl, C₇-C₁₀ aralkyl, C₂-C₁₀ alkenyl, C₆-C₁₀ aryl, C₇-C₁₀ alkaryl, and R₄ and R₅ can combine to form a C₄-C₇ cycloalkyl group or a heterocycloalkyl group; R₅ and R₇ are independently selected from hydrogen, C₁-C₁₀ alkyl, C₇-C₁₀ aralkyl, C₂-C₁₀ alkenyl, C₆-C₁₀ aryl and C₇-C₁₀ alkaryl, and R₆ and R₇ can combine to form a C₄-C₇ cycloalkyl group.

6. A sulfur-vulcanized rubber composition as claimed in claim 5 wherein said sulfenamide accelerator is selected from N-cyclohexyl-2-benzothiazyl sulfenamide, N-t-butyl-2-benzothiazyl sulfenamide, N-t-amyl-2-benzothiazyl sulfenamide,

N-oxydiethylene-2-benzothiazyl sulfenamide and

N,N-dicyclohexyl-2-benzothiazyl sulfenamide.

7. A sulfur-vulcanized rubber composition as claimed in claim 5 wherein said thiazole accelerator is selected from 2-mercaptobenzothiazole, zinc-2-mercaptobenzothiazole and dibenzothiazyl disulfide.

8. A sulfur-vulcanized rubber composition as claimed in any one of claims 5-7 further comprising one or more additives selected from antiozonants, antioxidants, fϋlers, waxes, retarders and processing oils.

9. A process for the vulcanization, at a temperature of from 110 to 220°C for up to 24 hours, of a vulcanizable composition comprising at least one natural or synthetic rubber in the presence of 0.1 to 25 parts by weight of sulfur or a sufficient amount of a sulfur donor to provide the equivalent of 0.1 to 25 parts by weight of sulfur and a primary accelerator selected from sulfenamide and thiazole accelerators, characterized in that said process is carried out in the presence of a sufficient amount of a secondary accelerator to improve the process safety, said secondary accelerator being a compound of the formula I:

10. A process as claimed in claim 9 wherein said sulfenamide accelerator is selected from N-cyclohexyl-2-benzothiazyl sulfenamide, N-t-butyl-2-benzothiazyl sulfenamide, N-t-amyl-2-benzothiazyl sulfenamide,

N-oxydiethylene-2-benzothiazyl sulfenamide and

N,N-dicyclohexyl-2-benzothiazyl sulfenamide.

11. A process as claimed in claim 9 wherein said thiazole accelerator is selected from 2-mercaptobenzothiazole,

zinc-2-mercaptobenaothiazole and dibenzothiazyldisulfide.

12. A process as claimed in any one of claims 9-11 wherein the weight ratio of primary accelerator to secondary accelerator is from 10:90 to 90:10.

13. A process as claimed in any one of claims 9-12 wherein said secondary accelerator is selected from dialkyl-hydroxybenzyl dialkyldithiocarbamates, dialkyl-hydroxybenzyl-dibenzyl

dithiocarbamates and dialkyl hydroxybenzyl-N-oxydiethylene dithiocarbamates.

14. A process as claimed in claim 13 wherein said secondary accelerator is (3,5-dimethyl-tert

butyl-4-hydroxybenzyl)-dibenzyldithiocarbamate.

15. A process as claimed in claim 13 wherein said secondary accelerator is (3,5-dimethyl-tert

butyl-4-hydroxybenzyl)-dimethyldithiocarbamate or

(3,5-dimethyl-tert butyl-4-hydroxybenzyl)-diethyldithiocarbamate.