CS272793B2 - Method of rubber vulcanization - Google Patents

Abstract

Rubber vulcanisates having low reversion can be obtained by means of a sulphur vulcanisation system containing a) sulphur or a sulphur donor, b) conventional sulphenamide accelerators, c) o-nitrophenylsulphenamide and optionally d) conventional vulcanisation aids.

Description

The invention relates to a process for vulcanizing rubbers by means of a vulcanization system comprising a sulfur or sulfur-providing substance, a nitro-free sulfenamide accelerator and an o-nitrophenylsulfenamide, optionally in the presence of conventional vulcanizing aids.

The vulcanization process according to the invention allows the production of reduced-reverse vulcanisates.

Reverse refers to a decrease in the degree of crosslinking and mechanical values of vulcanisates (such as tensile strength, tear resistance, elasticity) when the vulcanization exceeds the optimum. Reverse occurs in the vulcanization of pulp rubbers, especially natural rubber and synthetic polyisoprene at high vulcanization temperatures, or at the long vulcanization times required to produce bulky products (such as truck tires). Since it has not yet been possible to effectively limit this reverse, it has in many cases been necessary to abandon the high vulcanization temperatures which are desirable from an economic point of view.

So far, a reduction in reverse has only been possible on a sub-scale:

For example, attempts have been made to influence reverse by using sulfur-reduced vulcanization systems. While normal sulfur vulcanization uses sulfur in an amount of from about 1.8 to 2.5 parts by weight, based on 100 parts by weight of rubber, medium-active sulfur systems operate in an amount of from about 1 to 1.8 parts by weight, based on 100 parts by weight of rubber and effective sulfur systems in an amount below 1 part by weight based on 100 parts by weight of rubber. However, compared to normal sulfur vulcanization, there are drawbacks in terms of crack resistance, tear resistance, abrasion resistance and cord adhesion to vulcanisate when using less sulfur. vulcanization systems with reduced sulfur only for certain rubber products. Thus, for example, it is not possible to reduce the amount of sulfur in the tire production from about 1.5 parts by weight to 100 parts by weight of rubber. *

Furthermore, it is known that the type of vulcanization accelerator used has an influence on the possible reversion of the vulcanisate. While ultra-accelerators, such as dithiocarbamates, thiurans, and basic accelerators, such as diphenylguanidine, cause a greater reversal, mercaptobenzothiazole-based accelerators, dibenzothiazyldisulfide, and zinc salt such as the 2-mercaptobenzothiazole accelerator, cyclohexylsulphenamide, benzothiazyl-2-tert-butylsulphenamide, N-morpholinothio-2-benzothiazole and benzothiazyldicyclohexylsulphenamide, more preferably for the reversion of vulcanisate.

For pulp rubbers with reduced reversal, vulcanization systems of the following composition were often used in practice:

(a) sulfur in an amount of from about 0.8 to 1.8 parts by weight per 100 parts by weight of rubber; and

b) a sulfenamide accelerator in an amount of from about 1 to 2 parts by weight per 100 parts by weight of rubber.

However, such vulcanization systems provide reversal at an undesirable scale at high vulcanization temperatures (e.g., temperatures above 160 ° C) or long heating times.

It is further known that the above-described vulcanization systems with the addition of dithiophosphates, such as zinc dibutyldithiophosphate, reduce the reversion of the vulcanisates (cf. H. Ehrend: Gummi,

Asbest, Kunststoffe, 3c (1977), pp. 58-71). These dithiophosphate accelerators, however, undesirably reduce the time required to vulcanize the article. This is a disadvantage, since sufficient processing reliability is needed, especially when used in areas where reverse reduction is of particular importance, especially in the manufacture of truck tires and generally in the production of bulky products.

Some primary nitrophenylsulfenamides are known (cf. U.S. Pat. No. 2,382,813). However, as the only vulcanization accelerators, they are poorly effective and do not provide a sufficient degree of crosslinking, resulting in poor mechanical values of the vulcanisate (cf. Example 1).

Common sulfenaraid-based accelerators alone, such as benzothiazyl-2-cyclohexylsulfenamide (CBS), benzothiazyl-2-tert-butylsulfenamide (TBBS),

N-morpholinothio-2-benzothiazole (MBS) and benzothiazyldicyclohexylsulphenamide (DCBS) provide too high reversal volcanisates with normal amounts of sulfur.

It was therefore an object of the present invention to provide a vulcanization system which would reduce the reversal of the rubber vulcanisates while not reducing the vulcanization vulcanization time.

Surprisingly, it has been found that the use of sulfur-based vulcanization systems in combination with conventional sulfenamide accelerators free of nitro groups on the one hand and substituted o-nitrophenylsulfenamides on the other hand results in vulcanisates with a particularly reduced reverse. The low reverse thus achievable at a practical cross-linking degree is much lower than that achieved by the individual sulfenamides.

SUMMARY OF THE INVENTION The present invention provides a method for vulcanizing rubber using a sulfur-based vulcanization system comprising:

(a) 1 to 3 parts by weight of sulfur or 2 to 6 parts by weight of sulfur-producing substance,

b) 0.3 to 4 parts by weight of a nitro-free sulfenamide accelerator selected from the group consisting of benzothiazyl-2-cyclohexylsulfenamide (CBS) benzothiazyl-2-tert-butylsulfenamide (TBBS)

N-morpholinothio-2-benzothiazole (MBS) benzothiazyl-2-diisopropylsulfenamide (DIBS) benzothiazyl-2-t-amylsulfenamide (AMZ) benzothiazyl-dicyclohexylsulfenamide (DCBS) morpholinothiocarbamoylsulfenmorpholide (OT)

c) 0.5 to 4 parts by weight of o-nitrophenylsulfenamide from the group selected from compounds of formulas I, II and III

(II)

(III) _of which, independently of one another, is hydrogen or nitro,

R is a straight or branched (C 1 -C 6) alkyl group, optionally a C 1 -C 4 alkyl group, a phenyl group, a C 1 -C 6 trihaloalkyl group, preferably a trichloromethyl group, and a trifluoroethyl group; furthermore represents a C 5 -C 10 cycloalkyl group optionally substituted by a C 1 -C 4 alkyl group or a trihalomethyl group, for example a trifluoromethyl group, in

R is C 1 -C 8 alkylene, C 5 -C 6 cycloalkylene, C 6 -C 18 arylene optionally substituted with C 1 -C 4 alkyl or trihalomethyl; always refer to 100 parts by weight of rubber.

Sulfur donors referred to in (a) are compounds which degrade sulfur under vulcanization conditions.

Preferred sulfur scavengers include dimorpholyldisulfide (DTDM), tetramethylthiuram disulfide (TMTD), 2- (4-morpholinodithio) benzothiazole (MBSS), and caprolactam disulfide.

Preferred conventional sulfenamide accelerators mentioned in b) are benzothiazyl-2-cyclohexylsulfenamide, benzothiazyl-2-tert-butylsulfenamide and N-morpholinothio-2-benzothiazole.

Preferred o-nitrophenylsulfenamides of formula I include compounds with the following substituents:

C. R ¹ R ² 1 H methyl 2 H ethyl 3 H n-propyl 4 H isopropyl 5 H n-butyl 6 H tert.butyl 7 H n-amyl 8 H isoamyl 9 H tert.amyl 10 H n-hexyl 11 no ₂ tert.butyl 12 H phenyl 13 H o-methylphenyl 14 H 3-trifluoromethylphenyl 15 Dec H cyclohexyl 16 H 2-methylcyclohexyl 17 H 4-methylcyclohexyl 18 H 4-trifluoromethylcyclohexyl

Preferred o-nitrophenylsulfenamides of the formula: II include compounds of the following C. '1 2 R R 19 Dec H tert.butyl 20 May H cyclohexyl

Preferred o-nitrophenylsulfenamides of formula III include compounds with the following stituents: R ² C. R ¹ 21 H 1,2-ethylene 22nd H 1,4-tetramethylene 23 H 1,6-hexamethylene 24 H 1,2-propylene 25 H 2,4-toluylene 26 H 1,4-cyclohexylene

Preferably compound 6, i.e. o-nitrophenyl-tert-butylsulfenamide, is used.

The substituted o-nitrophenylsulfenamides used as component c) are for the most part known compounds. These compounds can be prepared analogously, according to the method of the Methoden der Organischen Chemie (Houben-Weyl), Vol. Georg Thieme Verlag

Stuttgart-New York 1985, pp. 110-116. The substituted o-nitrophenylsulfenamides which are used as component c in the process of the invention also include the novel compounds.

These novel o-nitrophenylsulfenamides can be characterized by the general formula IV €

αν) ι in which

R ² represents a tert-butyl group, a tert-amyl group, a 1,1,1-trifluoroethyl group or a 4-trifluormethylcyklohexylovou group.

A particularly economical process for the preparation of the o-nitrophenylsulfenamides of formula XV as well as the other o-nitrophenylsulfenamides used as component (σ) in the process of the invention is the reaction of o-nitrophenylsulfenyl chloride with the corresponding amine in a two-phase system consisting of an aqueous base and an inert organic solvents which are not miscible with water.

Thus, the novel o-nitrophenylsulfenamides used as component o) in the process of the invention are prepared from the o-nitrophenylsulfenyl chloride of formula V

in which

R ⁷ · is as defined above,

3 2 3 and an amine of formula R ₂ -NH or -NH-diamine -WHy -R in which R and R have the abovementioned meanings, a process which consists in reacting a solution of o-nitrofenylsulfenylchloridu in an inert organic solvent not miscible with water, in the presence of "2 (I) 1 to 1.1 moles of amine of formula R -NH- or" 2 (XI) 0.5 to 0.55 moles of amine of R -NH ₂ or (III) 0.5 up to 0.55 mol of NH ₂ -R ⁸ -NH ₂ diamine, based on 1 mol of sulfenyl alcohol of the formula V, and an aqueous solution of 1 to 10 equivalents of alkali metal or alkaline earth metal hydroxide, carbonate or bicarbonate per mole of hydrogen chloride leaving, temperatures from -10 to 40 ° C, preferably from 10 ° C to 40 ° C

-10 to 10 ° C, whereupon the organic phase is separated and sulfenamide as component c) is isolated from this organic phase.

The aqueous solution may be present or may be added simultaneously with the sulfenyl chloride solution to the present amine. It is also possible to add an amine and a sulfenyl chloride solution to the present aqueous solution.

Preferred inert organic solvents include, for example, chlorobenzene, dichlorobenzene, chloroform, carbon tetrachloride and dichlorodifluoromethane.

o nitrofenylsulfenamidy used as component c), however, can be prepared, 2, vat also sulfenyl chloride of formula V, an amine of formula R -NH ,, or diamine ^of the formula

NH 3 -R -NHg using tertiary amines as acid binding agents in homogeneous solution.

In addition, they may contain conventional vulcanizing agents such as accelerator activators and retarders.

Rubbers suitable for the vulcanization process of the invention are natural rubbers, synthetic rubbers, preferably halogen-free diene rubbers, and mixtures thereof.

Suitable synthetic rubbers are, for example: polyisoprene, polybutadiene, styrene-butadiene rubber, nitrile rubber and mixtures thereof.

These rubbers may contain conventional additives such as fillers, especially carbon black, mineral oils, emollients, tackifiers, accelerator activators, especially stearic acid, waxes, anti-aging agents, ozone protection agents, blowing agents, dyes, pigments.

Fillers such as the various types of carbon black used in the rubber industry, furthermore silicic acid, in particular finely dispersed, gas-phase silicic acid, as well as hydrophobic silicic acid and finely dispersed metal oxides, including mixed oxides and mixtures of oxides, are valuable components of the mixture.

The individual components of the vulcanization system used according to the invention may be added to the rubbers either separately or in the form of a mixture or in the form of a rubber-accelerator premix (cf. Ullmann's Encyclopedia der technischen Chemie, 3rd edition, Urban und Schwarzenberg, Miinchen-Berlin 1957, vol 9, p 364).

The vulcanization of rubbers is generally carried out at temperatures between 100 ° C and 300 ° C, preferably at temperatures between 120 ° C and 240 ° C. For this purpose, all conventional vulcanization processes such as compression heating, hot steam heating, hot air heating, saline can be used. bath, fluidized bed, high frequency heating and steam pipe.

The vulcanization systems used according to the invention have advantages in terms of reverse after a longer vulcanization time and / or after high vulcanization temperatures (e.g. 45 minutes at 180 ° C). The mechanical properties of the vulcanisate, such as stress magnitude, tensile strength, ductility, hardness, flexibility, tear resistance, fatigue resistance, heat build up, dynamic creep and abrasion, exhibit more favorable values than conventional vulcanization systems. .

The following examples illustrate the invention but do not limit it in any way.

Examples:

N-tert-butyl-2-nitrophenylsulfenamide

770 g of 2,2 ^1- dinitrodiphenyldisulphide are reacted in 2 liters of chlorobenzene at a temperature of 30 to 40 ° C with 180 g of chlorine. The reaction mixture was further stirred for 1 hour and then excess chlorine was removed under reduced pressure. Any solution of the reaction product of 2-nitrosulfenyl chloride was then added dropwise to 375 g of tert-butylamine in 1200 ml of chlorobenzene at -5 to 10 ° C over 180 minutes. Following the dropwise addition of 1/2 sulfenyl chloride, a solution of 275 g of sodium carbonate in 800 ml of water is added dropwise over 90 to 100 minutes. After stirring for 14 hours, the reaction mixture is diluted with 1000 ml of water, filtered and the chlorobenzene phase is separated. This phase was washed with water and evaporated under reduced pressure. First, the liquid product is poured onto a sheet for cooling. The product crystallizes during cooling. 1047 g of sulfenamide are obtained, m.p. 44-46 ° C. Recrystallization from petroleum ether / isopropyl alcohol 1: 1 melts at 49 ° C.

Examples illustrating vulcanization:

List of test methods:

Bayer product, heating for 1 minute measuring range 3/20 mV t (30) / 130 * C (min) Vulcanization time from the measurement at 130 c or s at 160 ° C, time interval until the elastic modulus curve rise in shear 30 mm above minimum; at t _g (15) 15 mm above minimum t _gQ (min) gradual vulcanization Μ _30θ (ΜΡα)

P (MPa)

D (%)

H (Shore A)

E (%) reverse (%) heating time, time to 90% of maximum shear modulus (Vulcameter, Bayer product) mm flap, press vulcanization stress value at 300% elongation, DIN 53 504 tensile strength, DIN 53 504

R 1 elongation, DIN 53 504, Rl Shore A hardness, DIN 53 504, R L reflection elasticity, DIN 53 512, flap 4 mm from gradual vulcanization:

^M 300 ^p min / 180 ° C

-maximum At 150 ° C x 100

Example 1

A rubber premix is prepared from the following components in a kneading machine:

parts by weight natural rubber, carbon black N 330 100,0 55,0 aromatic mineral oil 3.0 stearic acid 2.5 protective wax against the action of ozone of 1.0 ^N-phenyl-N-r -isopropy1 pf enylendiarain polymerized 2,5 2,2 4-trimethyldihydroquinoline 1,5 zinc oxide 5,0 Note: parts by weight refer to 100 parts by weight of rubber

The following components are admixed to the premix on a laboratory twin cylinder (cf. Table 1):

parts by weight System 1: sulfur 1.42 N-morpholinothio-2-benzothiazole 1,2 System 2: sulfur 1.42 N-morpholinothio-2-benzothiazole 4.0 System 3: Sulfur 1.42 o-nitrophenyl-tert-butylsulfenamide 4.0 System 4: Sulfur 1.42 N-morpholinothio-2-benzothiazole 2,0 o-nitrophenyl-tert-butylsulfenamide 2,0

Note: parts by weight refer to 100 parts by weight of rubber.

System 1 is represented by a useful medium-effective vulcanization system of sulfur and N-morpholinothio-2-benzothiazole. Renders after re-vulcanization at 180 ° C / 45 min. a relatively high reverse (cf. Table 1).

System 2 also consists only of sulfur and N-morpholinothio-2-benzothiazole at a higher dose. A dose of 4 parts by weight of N-morpholinothio-2-benzothiazole, based on 100 parts by weight of rubber, is too high for practical purposes and leads to too much crosslinking. However, it is intended to allow comparison with system 4 and therefore contains the same amount of accelerator as the sum of the number of accelerators in system 4.

System 2 provides a lower reverse than System 1.

System 3 contains, in addition to sulfur, only o-nitrophenyl-tert-butylsulfenamide, also in an amount of 4 parts by weight per 100 parts of rubber. There is no reversal here, since this accelerator still causes a gradual additional crosslinking even at 180 ° C; however, the degree of crosslinking thus obtained is insufficient and is significantly lower than, for example, when using 1.2 parts by weight of N-morpholinothio-2-benzothiazole (cf. system 1).

System 4 provides a significantly lower reverse than systems 1, 2 and 3 at a degree of crosslinking that is favorable to practical requirements (approximately equivalent to system 1 that is suitable for practical use) and a cure time that is also practically the same as system 1. Furthermore, it can be seen from Table 1 that the system 4 according to the invention has a better vulcanization time t ₉ ρ at 150 ° C than systems 1, 2 and 3 and also a shorter reaction time tgQ-bg than all other systems. The voltage value at 180 c / 45 min is significantly higher than the other systems.

Example 2

This example illustrates the superiority of system 2 of the invention over system 1, which contains zinc dibutyldithiophosphate to reduce reverse. The rubber premix is prepared in the same manner as in Example 1 and the following ingredients are incorporated into the mixture on a laboratory twin-cylinder:

parts by weight

System 1: Sulfur 1.42

Zinc dibutyldithiophosphate 1.20 N-morpholinothio-2-benzothiazole 1.20

System 2: Sir 1.42

N-morpholinothio-2-benzothiazole 1.50 o-nitrophenyl-tert-butylsulfenamide 2.0

Note: parts by weight are based on 100 parts by weight of rubber.

System 1 System 2 reverse (%) after 45 min / 180 * C 28 4.5 cure time t at 130 * C (min) 10.7 23.7

The system 2 according to the invention offers not only a substantially lower reversion, but also a significantly improved vulcanization resistance.

Example 3

This example illustrates a comparison between system 1 which contains, in addition to sulfur, only benzothiazyl-2-tert-butylsulfenamide and system 2 according to the invention, which consists of sulfur, benzothiazyl-2-tert-butylsulfenamide and o-nitrophenyl-tert-butylsulfenamide. Systems 1 and 2 are adjusted to approximately the same value of stress and hardness.

parts by weight

System 1: Sulfur 1,425 Benzothiazyl-2-tert-butylsulfenamide 1,2

System 2: sulfur 1,425 benzothiazyl-2-tert-butylsulphenamide 1,7 o-nitrophenyl-tert-butylsulphenamide 2,0 Note: parts by weight refer to 100 parts by weight of rubber.

reverse (%) after 45 min / 180 'c curing resistance t _g / 130 curing time tgθ / 150 * C (min)

F after 30 min / 150 ’C

D ^M 300

H

E

F for 45 min / 180 ° C

D ^M 300

H

E

System I System 2

34.5 3

C (min) 14.8 14.0

8,0 7,5

24,8 23,3

475 445

16,2 15,9

68

40

17,7 18,3

475 370

10.6 15,6

69

35

System 2 exhibits a distinctly lower reverse with nearly the same vulcanization resistance as System 1. Further, after re-vulcanization at 180 ° C, the system 2 has advantages in terms of tensile strength, stress value and hardness.

Example 4

This example illustrates the preponderance of three combinations according to the invention which consist of o-nitrophenyl-tert-butylsulfenamide and sulfenamide accelerators:

benzothiazyl-2-cyclohexylsulfenamide (system 2), benzothiazyl-2-tert-butylsulfenamide (system 3), and

N-morpholinothio-2-benzothiazole (system 4) χο over a conventional moderately active benzothiazyl-2-cyclohexylsulfenamide system alone. The same rubber masterbatch is used as in Example 1. In all systems, the amount of sulfur is always 1.42 parts by weight per 100 parts by weight of rubber.

system 1 2 3 4 Benzothiazyl-2-cyclohexylsulfene- • j x / amide 1,2 1.7 benzothiazyl-2-tert-butylx) sulfenamide 1.7 X / N-morpholinothio-2-benzothiazole - - - 1.7 o-nitrophenyl-tert-butylsulfene- amid - 2,0 2,0 2,0 reverse after 45 min / 180 * C (¾) 29.8 0 O 2.7 cure time tg / 130 * C (min) 11.7 10.5 11.3 13.2 voltage value ^M 300 P ° 30 min / 150 C (MPa) 13.7 13.4 15.3 14.7 voltage value M _3qo after 45 min / 180 * C (MPa) 9.9 14.1 15.8 14.8

x / parts by weight per 100 parts by weight of rubber

Example 5

This example illustrates the% reversal after re-vulcanization at 180 c / 45 min achieved with the same rubber composition as in Example 1. The compounds listed under 5, 6, 15, 17, 19, 20, 21, 24 are used. , 25 and 26 on pages 10 to 12 (each in two parts by weight per 100 parts by weight of the rubber composition) in combination with 1.2 parts by weight of N-morpholinothio-2-benzothiazole, based on 100 parts by weight of rubber. The amount of sulfur is 1.42 parts by weight per 100 parts by weight of rubber. For comparison, a conventional moderately active system consisting of sulfur and N-morpholinothio-2-benzothiazole (1.2 parts by weight per 100 parts by weight of rubber) is used.

reverse (%) after 45 min / 180 ° C

N-morpholinothio-2-benzothiazole 40.5

17

8

14

17

11

10

7

9

23.5

23

T a b uJL k a 1 syathia. aysta 2 systda 3 systda 4

Sulfur (parts by weight) 1.42 1.42 1.42 1.42 N-aorfolinothio-2-benzothiazole (parts by weight) 1,2 4.0 -  2,0 o-nitrophenyl-tert-butylsulfentaid (parts weight) - - 4.0 2,0 Total Amount of Accelerator (parts weight) 1.2 4.0 4.0 4.0 reverse (35) to 45 ain / 180 ° 0 40.4 14.7 5.9 vulcanization time t _ft / 130 ° C (ain) 22.1 17 45 19.9 watering time tgg / 150 ° C (ain) 11.4 10.4 23.1 10.3 reaction time tg _Q- t _B / 150 ° C (ain) 6.1 5.7 21.1 5.6 and a voltage value of 150 ° C at 150 ° C (MPa) 15.1 19.1 6.8 15.3

tensile strength P for 30 nin / 150 ° C. (MPa) 21.7 18.0 11.9 22.2 elongation D to 30 ain / 150 ° C (MPa) 435 310 485 455 hardness H to 30 nin / 150 ° C (MPa) 70 77 57 71 elasticity B to 30 oin / 150 ° C (MPa) 39 35 32 37

comparative example.

Claims (2)

CLAIMS 1. A method for vulcanizing rubber using a sulfur-based vulcanization system, characterized in that the vulcanization is carried out in the presence of a sulfur-based vulcanization system comprising 1 to 3 parts by weight of sulfur or 2 to 6 parts by weight of a sulfur-providing substance; 0.3 to 4 parts by weight of a nitro-free sulfenamide accelerator selected from the group consisting of benzothiazyl-2-cyclohexylsulfenamide, benzothiazyl-2-tert-butylsulfenamide, N-morpholinothio-2-benzothiazole, benzothiazyl-2-diisopropylsulfenamide, benzothiazylsulfenamide , benzothiazyldicyclohexylsulphenamide and morpholinothiocarbonylsulphenmorpholide and 0.5 to 4 parts by weight of o-nitrophenylsulphenamide from the group of compounds of formulas I, II and III (I) (II) (III) in which, independently of one another, are hydrogen or nitro; R is C 1 -C 6 alkyl, phenyl optionally substituted with C 1 -C 4 alkyl, trihaloC 1 -C 6 alkyl, C 5 -C 10 cycloalkyl optionally substituted with alkyl (C 1 -C 4) group or trihalomethyl group, R is C 1 -C 8 alkylene, C 5 -C 10 cycloalkylene, C 6 -C 18 arylene optionally substituted with C 1 -C 4 alkyl or trihalomethyl, the moieties by weight reported above, each refers to 100 parts by weight of rubber. 2. The process of claim 1, wherein the vulcanization is carried out in the presence of a sulfur-based vulcanization system containing 1 to 3 parts by weight of sulfur or 2 to 6 parts by weight of the sulfur-providing substance; 0.3 to 4 parts by weight of a nitro-free sulfenamide accelerator selected from the group consisting of benzothiazyl-2-cyclohexylsulfenamide, benzothiazyl-2-tert-butylsulfenamide, N-morpholinothio-2-benzothiazole, benzothiazyl-2-diisopropylsulfenamide 2-benzothiazylsulfenamide , benzothiazylcyclohexylsulfehamide and morpholinothiocarbonylsulfenmorpholide and 0.5 to 4 parts by weight of o-nitrophenylsulfenamide from the group of compounds of formulas I, II and III (III) in which, independently of each other R ¹ represents a hydrogen atom or a nitro group, R is C 1 -C 6 alkyl, phenyl optionally substituted with C 1 -C 4 alkyl; trihalomethyl; (C 5 -C 10) cycloalkyl optionally substituted with (C 1 -C 4) alkyl or trihalomethyl; R is C 1 -C 8 alkylene, C 5 -C 10 cycloalkylene, C 6 -C 18 arylene optionally substituted with C 1 -C 4 alkyl or trihalomethyl, the moieties indicated above by weight always refer to 100 parts by weight of rubber.