Rubber vulcanizates having improved hysteresis behavior

Abstract

The rubber vulcanizates according to the invention, which are produced using certain carboxyalkyl oligosulfides, are distinguished by improved hysteresis behavior; their mechanical properties remain substantially unaffected.

Description

This invention relates to rubber vulcanizates having improved hysteresis properties and a process for the production of the rubber vulcanizates.

In rubber technology, hysteresis losses are the losses of energy irreversebly converted into heat when the elastomer is subjected to dynamic stressing. Hysteresis losses are measured as the tan .delta. which is defined as the ratio of loss modulus to storage modulus, cf. for example DIN 53 513 and DIN 53 535. Any reduction in the tan .delta. in the applicationally important temperature/frequency or amplitude range leads, for example, to reduced heat buildup in elastomers. Tires may of rubber having a reduced hysteresis loss are distinguished by reduced rolling resistance and, hence, by lower fuel consumption of the vehicles fitted with them.

U.S. Pat. No. 4,690,695, for example, describes carbon-black-filled rubber vulcanizates which, by virtue of their content of nitrosoanilines, show a reduced heat buildup, i.e. a reduced hysteresis loss. EP 253 365 describes certain nitroamines which lead to comparable effects. However, on account of the danger of carcinogenic nitrosamines being formed by renitrosation, there is a need for rubber auxiliaries which are free from nitro and nitroso groups.

U.S. Pat. No. 4,761,446 describes rubber vulcanizates modified with certain benzimidazolines. U.S. Pat. No. 4,762,870 describes rubber vulcanizates modified with hydroxybenzimidazole oxides. U.S. Pat. No. 4,822,845 describes rubber vulcanizates modified with heterocyclic di-N-oxides. Although all three described rubber vulcanizates show improved hysteresis properties, such as lower heat buildup and a lower tan .delta., the additive leads to considerable reductions in elongation at break and tensile strength in all three cases. For this reason, these modifiers have never been successfully adopted in practice. In addition, the last three modifiers mentioned are only effective in SBR (and, even then, only in solution SBR). EP 366 952 describes a process for the production of rubber vulcanizates having reduced hysteresis losses. However, the described hysteresis improvers cannot be used in natural rubber, which is so important for tires, because they drastically reduce the molecular weight of natural rubber (cf. U.S. Pat. No. 2,470,948).

It has now surprisingly been found that filled rubber vulcanizates to which certain carboxyalkyl oligosulfides have been added show reduced hysteresis losses, better elasticity and hardly any reductions in strength and elongation.

The present invention relates to rubber vulcanizates produced from at least one rubber, a crosslinking agent, a filler, optionally other rubber auxiliaries and at least one carboxyalkyl oligosulfide corresponding to the following formula

R.sup.1 --(S).sub.r --R.sup.2 (I) in which R.sup.1 and R.sup.2 may be the same or different and represent a C.sub.1-18 alkyl radical or C.sub.5-18 cycloalkyl radical substituted 1 to 4 times by a carboxyl group or by a carboxylate group; in the case of a carboxylate group, the cation is an ammonium group, a C.sub.1-18 mono-, di- or trialkyl ammonium or C.sub.6-18 mono-, di- or triaryl ammonium or C.sub.7-21 mono-, di- or trialkylaryl ammonium group or a mono- to tetravalent metal atom and r is an integer of 3 to 6, in quantities of 0.1 to 10% by weight, based on rubber.

Preferred carboxyalkyl oligosulfides are compounds corresponding to formula (I) in which R.sup.1 and R.sup.2 have the same meanings and represent an aliphatic radical containing 2 to 6 carbon atoms. These compounds may be substituted 1 to 4 times by a carboxyl or carboxylate group. They are preferably substituted once or twice. The following carboxyalkyl oligosulfides are mentioned as examples: ##STR1##

In addition to the carboxyl compounds mentioned, the corresponding sodium, potassium, cobalt, iron, nickel, ammonium, magnesium, calcium, tin and/or zinc salts, preferably sodium, potassium, calcium, zinc, ammonium salts, may of course be used.

The following compounds of formula (I) ##STR2## are particularly preferred.

The carboxyalkyl oligosulfides corresponding to formula (I) may be used both individually and also in admixture with one another.

The compounds corresponding to formula (I) may be produced by methods known per se, for example by reaction of carboxyalkyl halides with sodium polysulfides in accordance with reaction scheme A:

HO.sub.2 C-Alkyl-Hal+Na.sub.2 S.fwdarw.(I) (with r=2-7)+Na-HalA or by reaction of corresponding .omega.-mercaptoalkyl carboxylic acids with SCl.sub.2 or S.sub.2 Cl.sub.2 in accordance with reaction schemes B and C: HO.sub.2 C-Alkyl-SH+SCl.sub.2 .fwdarw.(I) (with r=3)+HCl B HO.sub.2 C-Alkyl-SH+S.sub.2 Cl.sub.2 .fwdarw.(I) (with r=4)+HClC or, for example, by reaction of diorganodisulfides with SCl.sub.2 in accordance with reaction scheme HO.sub.2 C-Alkyl-S-S-H+SCl.sub.2 .fwdarw.(I) (with r=5)+HClD

The reaction of sodium polysulfides with alkyl halides in accordance with scheme A gives mixtures of different bis-(carboxyalkyl)-polysulfides because the sodium polysulfides are mixtures of different sulfides having the composition Na.sub.2 S.sub.x (for carrying out the reactions in accordance with schemes A-D, see Houben-Weyl, Methoden der organischen Chemie, Vol. 9, pages 88 et seq (1955) and Vol. E 11 (1985), Thieme Verlag, Stuttgart).

Both natural rubber and synthetic rubbers may be used as rubbers for the rubber vulcanizates according to the invention, the synthetic rubbers containing at least 5% by weight copolymerized units emanating from a C.sub.4-12 diene. Preferred synthetic rubbers are described, for example, in W. Hofmann, Kautschuktechnologie, Gentner-Verlag, Stuttgart, 1980, and include inter alia

BR--polybutadiene ABR--butadiene/C.sub.1-4 alkyl acrylate copolymers with acrylate contents of 5 to 60% by weight and preferably 5 to 50% by weight CR--polychloroprene IR--polyisoprene SBR--styrene/butadiene copolymers having styrene contents of 1 to 60% by weight and preferably 10 to 50% by weight NBR--butadiene/acrylonitrile copolymers having acrylonitrile contents of 5 to 60% by weight and preferably 10 to 50% by weight and also mixtures of these rubbers. The rubbers preferably used have glass transition temperatures below 20.C and, more preferably below 0.degree. C., as determined by the torsion pendulum test according to DIN 53 445. Particularly preferred rubbers are natural rubber, polybutadiene and styrene/butadiene copolymers.

The present invention also relates to a process for the production of rubber vulcanizates, characterized in that

(i) the unvulcanized rubber, (i) 10 to 120% by weight and preferably 30 to 80% by weight, based on rubber (i), of carbon black and/or silica and (iii) 0.1 to 10% by weight and preferably 0.1 to 3% by weight, based on rubber (i), of carboxyalkyl oligosulfides corresponding to formula (I) R.sup. --(S).sub.r --R.sup.2 (I) in which R.sup.1 and R.sup.2 may be the same or different and represent a C.sub.1-18 alkyl radical or C.sub.5-18 cycloalkyl radical substituted 1 to 4 times by a carboxyl group or by a carboxylate group; in the case of a carboxylate group, the cation is an ammonium group, a C.sub.1-18 mono-, di- or trialkyl ammonium or C.sub.6-18 mono-, di- or triaryl ammonium or C.sub.7-21 mono-, di-or tri- alkylaryl ammonium group or a mono- to tetravalent metal atom and r is an integer of 3 to 6, are mixed at melt temperatures of 120.degree. to 200.degree. C. and at shear rates of 1 to 1000 sec.sup.-1, other vulcanization chemicals (crosslinking agents) and optionally other rubber auxiliaries are subsequently incorporated at temperatures below 120.degree. C. and preferably at temperatures of 20.degree. to 110.degree. C. and the mixture is subsequently vulcanized at typical temperatures.

Any reinforcing carbon blacks may be used for the production of the rubber vulcanizates according to the invention having improved hysteresis properties. Preferred surfaces are from 35 to 200 m.sup.2 /g (CTAB determination). SAF, HAF, FEF, ISAF and SRF carbon blacks are mentioned in particular. Mixtures of two or more different carbon blacks or mixtures of carbon blacks with silicas (with and without filler activators) may readily be used.

In addition, any reinforcing "silicate fillers" may be used for the production of the rubber vulcanizates according to the invention having improved hysteresis properties.

Highly disperse silicas (silicon dioxide) having specific BET surfaces of 5 to 1,000 m.sup.2 /g and primary particle sizes of approximately 10 to 400 nm may be used as the "silicate fillers", as may synthetic and natural silicates and also glass fiber and glass fiber products (cf. DE 2 255 577). As already mentioned, the "silicate fillers" and carbon blacks may be used in admixture with one another and may be mixed in any ratios, depending in particular upon the particular application envisaged for the rubber vulcanizates.

For the process according to the invention, other rubber auxiliaries, such as plasticizers, resins, factices and stabilizers, may be added in typical quantities to the green compounds in order to obtain certain green compound and vulcanizate properties.

The crosslinking systems known from rubber technology, such as sulfur, peroxides, polyisocyanates, metal oxides, phenolic resins and combinations thereof, may be used for vulcanization. The crosslinking system used for vulcanization will preferably be adapted to the type of rubbers used. Sulfur crosslinking systems are particularly preferred.

Other additives, such as accelerators, retarders, etc., may be added in order to obtain optimal crosslinking kinetics.

The crosslinking systems (vulcanization chemicals) and rubber auxiliaries are incorporated at temperatures below 120.degree. C. and preferably at temperatures of 20.degree. to 110.degree. C. Vulcanization may be carried out at temperatures of 100.degree. to 200.degree. C. and preferably at temperatures of 130.degree. to 180.degree. C., optionally under a pressure of 10 to 200 bar.

The rubber, the fillers and the carboxyalkyl oligosulfides of formula (I) according to the invention may be mixed in typical mixing units, for example on mixing rolls and in internal mixers and mixing extruders which generally operate with shear rates of 1 to 1,000 sec.sup.-1 and preferably 1 to 200 sec.sup.-1.

The rubber vulcanizates according to the invention may be processed to articles intended to withstand severe dynamic stressing, such as vehicle tires, conveyor belts, drive belts and air suspension linings.

EXAMPLE 1 ##STR3##

92 g (1.0 mol) thiodiglycolic acid are dissolved in 500 ml toluene and 67.5 g (0.5 mol) disulfur dichloride are added to the resulting solution at 5.degree. C. while nitrogen is passed through. After heating to room temperature, the mixture is stirred for 8 hours. Filtration and drying leave approximately 110 g colorless crystals melting at 94.degree. C.

EXAMPLE 2 ##STR4##

67.5 g (0.5 mol) disulfur dichloride are added at 5.degree. C. to 106 g (1.0 mol) 3-mercaptopropionic acid in 500 ml toluene, followed by stirring for 8 hours at room temperature while nitrogen is passed through. Filtration and drying leave 133.5 g colorless crystals melting at 130.degree.-133.degree. C.

Elemental analysis: C: 26.3 (26.3), H: 3.5 (3.65), S: 46.2 (46.7).

Acid value: 407.

EXAMPLE 3 ##STR5##

The procedure is as in Examples 1 and 2 using 67 g (0.5 mol) 5-mercaptopentanoic acid and 33.6 g (0.25 mol) disulfur dichloride. Colorless crystals melting at 86.degree. C. are obtained in a yield of 79 g.

EXAMPLE 4 ##STR6##

51 g (0.5 mol) sulfur dichloride are added at 5.degree. C. to 106 g (1.0 mol) 3-mercaptopropionic acid in 500 ml toluene while nitrogen is passed through, followed by stirring for 8 hours at room temperature. Filtration and drying leave 124 g colorless crystals melting at 150.degree. to 152.degree. C.

EXAMPLE 5

Preparation of O.sub.2 C-CH.sub.2 -S-S-S-S-CH.sub.2 -CO.sub.2 Zn.sup.2+

246 g (I.0 mol) of the compound of Example are dispersed in 1 toluene and 40.5 g (0.5 mol) zinc oxide are added to the resulting dispersion at room temperature, followed by stirring for 3 hours at 50.degree. C. Filtration and drying leave 270 g colorless crystals melting at 110.degree. C.

EXAMPLES 6-9

Production of the Vulcanizates

Rubber mixtures are prepared in a kneader at an internal temperature of 170.degree. C. (rotational speed approx. 50 r.p.m.). The mixing time is 5 minutes.

______________________________________Mixture components:______________________________________Natural rubber SMR 5/Defo 700 100 parts by weightCarbon black N 339 (Degussa) 55 parts by weightAromatic plasticizer.sup.1) 3.0 parts by weightZinc oxide 5.0 parts by weightAntiozonant wax.sup.2) 1.0 part by weightN-isopropyl-N'-phenyl-p-phenylene- 1.0 part by weightdiamine.sup.3)Additive according to Examples 1-5 1.5 parts by weightThe mixtures are then mixed on a roll at 50.degree. C. withSulfur 1.5 parts by weightN-tert. butyl mercaptobenzthiazole 1.2 parts by weightsulfenamide.sup.4)and vulcanized for 15 minutes at 140.degree. C.______________________________________ .sup.1) .RTM. Ingralen 450 .sup.2) Ozonschutzwachs 111 (Rheinchemie) .sup.3) Vulkanox 4010 NA (Bayer) .sup.4) Vulkacit NZ (Bayer) TABLE 1__________________________________________________________________________ Example 6 Example 7 Example 8 Example 9 Comparison With comp. With comp. With comp. With comp. No additive of Ex. 1 of Ex. 2 of Ex. 3 of Ex. 6__________________________________________________________________________Modulus 300 18.8 19.5 21.0 19.5 19.0[MPa]Strength 23.5 23.3 23.5 23.5 23.2[MPa]Elongation 400 390 380 400 380[%]Tear propagation 360 365 320 360 400resistance[n/mm]Hardness 72 72 74 75 70[Shore A]Elasticity 43 49 48 47 49[%]Tan .delta. 20.degree. C. 0.146 0.120 0.117 0.125 0.114DIN 53 535__________________________________________________________________________

EXAMPLES 10-11

Rubber mixtures were prepared in a kneader at an internal temperature of 170.degree. C. (rotational speed approx. 50 r.p.m.). The mixing time is 5 minutes.

______________________________________Mixture components:______________________________________Solution SBR (Buna SL 750) 100 parts by weight(oil-extended,oil content 27.3%)Carbon black N 339 (Degussa) 85 parts by weightStearic acid 1.5 parts by weightAromatic plasticizer.sup.1) 5.0 parts by weightN-isopropyl-N'-phenyl-p-phenylene- 1.5 part by weightdiamine.sup.3)Zinc oxide 3.0 parts by weightAdditive of Examples 1-5 1.5 parts by weightThe mixtures are then mixed on a roll at 50.degree. C. withSulfur 1.2 parts by weightN-tert. butyl mercaptobenzthiazole 1.4 parts by weightsulfenamide.sup.4)and vulcanized for 15 minutes at 150.degree. C.______________________________________ TABLE 2__________________________________________________________________________ Example 10 Example 11 Comparison Comparison With comp. With comp. 1.0 phrb No additive of Ex. 2 of Ex. 6 US 4,822,845, Ex. H 65__________________________________________________________________________Modulus 300 5.6 8.0 11.0 Cannot be measured[MPa]Strength 19.8 19.0 17.8 15.0[MPa]Elongation 500 420 470 280[%]Tear propagation 215 180 205 135resistance[n/mm]Hardness 65 67 65 67[Shore A]Elasticity 36 40 38 39[%]Tan .delta. 60.degree. C. 0.193 0.157 0.169 0.153DIN 53 535__________________________________________________________________________

EXAMPLE 12

Rubber mixtures are prepared in a kneader at an internal temperature of 170.degree. C. (rotational speed approx. 50 r.p.m.). Mixing time 5 minutes.

______________________________________Mixture components:______________________________________SBR (Buna Huls 1712) 137.5 parts by weightCarbon black N 339 (Degussa) 85 parts by weightZinc oxide 3.0 parts by weightAromatic plasticizer.sup.1) 5.0 parts by weightN-isopropyl-N'-phenyl-p-phenylene- 1.5 part by weightdiamine.sup.3)Stearic acid 1.5 parts by weightAdditive of Examples 1-5 1.5 parts by weightThe mixtures are then mixed on a roll at 50.degree. C. withSulfur 1.8 parts by weightN-tert. butyl mercaptobenzthiazole 1.2 parts by weightsulfenamide.sup.4)and then vulcanized at 150.degree. C. (15 minutes).______________________________________ TABLE 3______________________________________ Comparison Example 12 No additive With comp. of Ex. 2______________________________________Modulus 300 11.2 15.9[MPa]Strength 21.3 20.7[MPa]Elongation 560 405[%]Tear propagation 205 180resistance[n/mm]Hardness 64 68[Shore A]Elasticity 29 32[%]Tan .delta. 100.degree. C. 0.138 0.109DIN 53 535______________________________________

Claims

1. Rubber vulcanizates produced from at least one rubber, a crosslinking agent, a filler, optionally other rubber auxiliaries and at least one carboxyalkyl oligosulfide corresponding to the following formula

R.sup.1 -(S).sub.r -R.sup.2 (I)

in which

R.sup.1 and R.sup.2 may be the same or different and represent a C.sub.1-18 alkyl radical or C.sub.5-18 cycloalkyl radical substituted 1 to 4 times by a carboxyl group or by a carboxylate group; in the case of a carboxylate group, the cation is an ammonium group, a C.sub.1-18 mono-, di- or trialkyl ammonium or C.sub.6-18 mono-, di- or triaryl ammonium or C.sub.7-21 mono-, di- or trialkylaryl ammonium group or a mono- to tetravalent metal atom and

r is an integer of 3 to 6,

in quantities of 0.1 to 10% by weight, based on rubber.

2. A process for the production of rubber vulcanizates, characterized in that

(i) the unvulcanized rubber,

(ii) 10 to 120% by weight, based on rubber (i), of carbon black and/or silica and

(iii) 0.1 to 10% by weight, based on rubber (i), of carboxyalkyl oligosulfides corresponding to formula (I)

R.sup.1 -(S).sub.r -R.sub.2 (I)

in which

R.sup.1 and R.sup.2 may be the same or different and represent a C.sub.1-18 alkyl radical or C.sub.5-18 cycloalkyl radical substituted 1 to 4 times by a carboxyl group or by a carboxylate group; in the case of a carboxylate group, the cation is an ammonium group, a C.sub.1-18 mono-, di- or trialkyl ammonium or C.sub.6-18 mono-, di- or triaryl ammonium or C.sub.7-21 mono-, di-or trialkylaryl ammonium group or a mono- to tetravalent metal atom and

r is an integer of 3 to 6,

are mixed at melt temperatures of 120.degree. to 200.degree. C. and at shear rates of 1 to 1000 sec.sup.-1, other vulcanization chemicals (crosslinking agents) and optionally other rubber auxiliaries are subsequently incorporated at temperatures below 120.degree. C. and the mixture is subsequently vulcanized at typical temperatures.

3. Moldings of the vulcanizates claimed in claim 1.