CA2292393A1 - Rubber mixtures which contain organosilanepolysulfanes - Google Patents

Abstract

The invention provides the in-situ desulfurization of organosilanepolysulfanes during the preparation of rubber mixtures by the addition of, in particular, phosphines or phosphites.

Description

Rubber Mixtures Which Contain Organosilanepolysulfanes The present invention provides rubber mixtures which contain organosilanepolysulfanes.
The use of organosilanepolysulfanes as coupling agents or reinforcing additives in oxide-filled rubber mixtures such as, for example, the treads and other parts of car tires has been disclosed (DE 2 141 159, DE 2 212 239, US 3 978 103, US 4 048 206). These types of organosilanepolysulfanes such as, for example, bis-(3-[triethoxysilyl]-propyl)tetrasulfane (TESPT), generally consist of a polysulfane mixture, wherein the length of the sulfane chains (SX) is generally in the range 2 to 10.
It is also known, when using these types of coupling agents in oxide-filled rubber mixtures, that processing temperatures higher than 130°C have to be maintained in order to enable the reaction between silica and the organosilane to take place. The plasticity of the mixture is then reduced. The reaction of organosilanes with silica and the emission of the alcohol which is released accelerates with increasing temperature of the mixture.
Furthermore, it is known that the organosilanepolysulfanes which are mainly used, such as bis-(3-[triethoxysilyl]-propyl)tetrasulfane, require particular attention when being incorporated into rubber in order to avoid pre-vulcanisation when mixing the components. In particular, reactive longer-chain polysulfanes with SX>4 tend to enter into unwanted cross-linking reactions with the rubber at temperatures higher than 140°C. This is made obvious, inter alia, by an increase in the plasticity of the mixture (Gorl, Munzenberg, ACS-Meeting Rubber Division, Anaheim, California/USA, May 1997, 38).
The use of organosilanes with shorter polysulfane chains has also been disclosed (WO-A 97/48264, D-A 197 02 046).

Known organosilanes with shorter polysulfane chains may be obtained by the reaction (desulfurization by nucleophilic reagents) of longer-chain organosiIanepolysulfanes with trivalent phosphorus compounds, sulfites or cyanides (D 195 41 404 and EP-A 845 472). However, the preparation of these organosilanepolysulfanes requires at least one additional process step. It is complicated and expensive.
The invention provides rubber mixtures which contain at least one organosilane and at least one desulfurizing l0 reagent from the class of compounds which contains trivalent phosphorus compounds.
According to the invention, it was found that, when preparing the mixture, the tendency to pre-vulcanisation when using longer-chain organosilanepolysulfanes in rubber mixtures can be largely avoided by the direct addition of trivalent phosphorus compounds, sulfites or cyanides which are capable of reducing the proportion of longer-chain polysulfanes via a desulfurization reaction.
Known organosilanepolysulfanes may be used as organosilanepolysulfanes. In particular organosilanepolysulfanes which correspond to formula I are used.
(R1 Rz R3 SiR4) zSx I
wherein R1, RZ , R3 may be identical or di f f erent and may be H, (C1-C4) alkyl, CZ-C4 alkoxy or halogen, wherein the halogen may be C1 or Br;
wherein, preferably R1 = RZ = R3 = methoxy or ethoxy.
R4 may be a (C1-C6) linear or branched alkylidene;
X = 2 to 10.

The following nucleophiles are particularly suitable for desulfurizing the organosilanpolysulfanes in the mixture:
phosphines with the general structure: P(R1)3 and P(NRZR3)3, where R1, Rz and R3, independently, represent H, alkyl or aryl; in particular R1 - phenyl;
phosphites with the general structure: P(OR4)3 and HOP(OR4)z where R4 - alkyl or aryl;
and dithiophosphites with the general structure:
,OR~ ,R60~
RSO Pv ~ ~P ORS n where RS and R6, independently, represent alkyl or aryl.
Rubber mixtures which contain a combination of an organosilanepolysulfane and a nucleophile according to the invention for desulfurization and the moulded articles resulting after a vulcanization step, in particular pneumatic tires or tire treads, in addition to increased scorch resistance, surprisingly also have a higher 300~/100~ modulus which points to the higher coupling effectiveness of the coupling agent. This is also reflected in a lower tan 8 (60°C) value which correlates with a lower rolling resistance.
In accordance with one embodiment of the invention, the rubber mixtures may contain an organosilanepolysulfane in amounts of 0.1 to 15 wt.~, in particular 5 to 10 wt.~, with respect to the amount of filler used, and at least one reagent for desulfurization which is capable of reducing the proportion of longer-chain polysulfanes via a desulfurization reaction in amounts of 5 to 80 wt.~, in particular 10 to 40 wt.~, with respect to the amount of organosilanepolysulfane used.

In a preferred embodiment of the invention, the mixtures may contain a synthetic rubber and a silica as filler. The rubber mixtures according to the invention may be prepared by blending the rubber, at least one filler, an organosilanepolysulfane and a desulfurization reagent with each other.
Addition of the organosilanes, the nucleophiles to desulfurize the same, and the addition of fillers, preferably takes place in a common procedure in a thermomechanical mixing step at bulk temperatures of 80 to 200°C, in particular 140 to 180°C.
The nucleophiles may preferably be added at the start of mixture preparation, in order to ensure the increased thermal stability of the mixture according to the invention at the earliest possible stage of mixture preparation.
Fillers which may be used for rubber mixtures according to the invention are:
- carbon blacks, which may be prepared by the flame, furnace or gas carbon black process and have BET
surface areas of 20 to 200 m2/g.
- highly dispersable silicas prepared, for example, by precipitation from solutions of silicates or by flame hydrolysis of silicon halides with specific surface areas of 5 to 1000, preferably 20 to 400 m2/g (BET
surface area) and with primary particle sizes of 10 to 400 nm. The silicas may optionally also be present as mixed oxides with other metal oxides such as A1, Mg, Ca, Ba, Zn and titanium oxides.
synthetic silicates such as aluminium silicate, alkaline earth silicates such as magnesium silicate or calcium silicate with BET surface areas of 20 to 400 mz/g and primary particle diameters of 10 to 400 nm.

- aluminium oxides with a proportion of -OH
functionalities.
- natural silicates such as kaolin and other naturally occurring silicas.

5 - glass fibres and glass fibre products (mats, ropes) or glass microbeads.
Carbon blacks with BET surface areas of 20 to 400 m2/g or highly dispersed silicas, prepared by precipitation from solutions of silicates, with BET surface areas of 20 to 400 mz/g may preferably be used in amounts of 5 to 150 parts by wt., each with respect to 100 parts of rubber.
The fillers mentioned above may be used individually or as a mixture. In a particularly preferred embodiment of the process, 10 to 150 parts by wt. of pale filler, optionally together with 0 to 100 parts by wt. of carbon black, and 0.1 to 15 parts by wt., preferably 5 to 10 parts by wt., of an organosilanepolysulfane, each with respect to 100 parts by wt. of the filler used, and at least one nucleophile which is capable of reducing the proportion of longer-chain polysulfanes via a desulfurizing reaction, may be used in amounts of 5 to 80 wt.~, in particular 10 to 40 wt.~, with respect to the amount of organosilanepolysulfane used, may be used to prepare the mixtures.
The organosilane may be a pure compound or may be combined with a support, preferably carbon black.
The nucleophile may be added directly to the mixture as such or else mixed with another constituent of the mixture, preferably the silane or the rubber auxiliary substances.
The nucleophile, the organosilane and/or the rubber auxiliary substances may be used as pure substances or mixed/combined with a support, preferably carbon black.

In addition to natural rubber, synthetic rubbers may also be used to prepare rubber mixtures according to the invention. Preferred synthetic rubbers are, for example, described in W. Hofmann, Kautschuktechnologie, Genter Verlag, Stuttgart 1980. They include, inter alia, - polybutadiene (BR) - polyisoprene (IR) - styrene/butadiene copolymers with styrene contents of 1 to 60, preferably 5 to 50 wt.~ (SBR) - isobutylene/isoprene copolymers (IIR) - butadiene/acrylonitrile copolymers with acrylonitrile contents of 5 to 60, preferably 10 to 50 wt.~ (NBR) - partly hydrogenated or fully hydrogenated NBR rubbers (~R) - ethylene/propylene/diene copolymers (EPDM) and mixtures of these rubbers. Anionic polymerised S-SBR
rubbers with a glass transition temperature above -50 °C and their mixtures with diene rubbers are used in particular for the production of vehicle tires.
Rubber vulcanisates according to the invention may contain further rubber auxiliary substances such as reaction accelerators, anti-ageing agents, heat stabilisers, light stabilisers, anti-ozonants, processing aids, plasticisers, tackifiers, blowing agents, colorants, waxes, extenders, organic acids, retarding agents, metal oxides and activators such as triethanolamine, polyethylene glycol or hexanetriol.
The rubber auxiliary agents may be used in conventional amounts which depend, inter alia, on the ultimate use.
Conventional amounts may be, for example, amounts of 0.1 to 50 wt.~, with respect to the rubber. The organosilanepolysulfanes may be used on their own as cross-linking agents. The addition of other cross-linking agents is generally recommended. Sulfur or peroxides may be used as other known cross-linking agents. In addition, rubber mixtures according to the invention may also contain vulcanization accelerators. Examples of suitable vulcanization accelerators are mercaptobenzthiazoles, sulfenamides, guanidines, thiurams, dithiocarbamates, l0 thiourea and thiocarbonate. The vulcanization accelerator and sulfur or peroxides are used in amounts of 0.1 to 10 wt.~, preferably 0.1 to 5 wt.~, with respect to the rubber.
Rubber mixtures according to the invention may be vulcanised at temperatures of 80 to 200°C, preferably 130 to 180°C, optionally under a pressure of 10 to 200 bar. Mixing the rubber with the filler, optional rubber auxiliary substances, the organosilanes and the nucleophiles according to the invention may be performed in conventional mixing equipment such as rollers, internal mixers and mixer-extruders. Rubber vulcanisates according to the invention are suitable for producing moulded articles, for instance for the production of pneumatic tires, tire treads, cable sheathing, hoses, drive belts, conveyer belts, roller coatings, tires, soles of shoes, sealing rings and damping elements.
Examples Examples 2 and 3 demonstrate the advantages of the use according to the invention of a combination of an organosilanepolysulfide and a nucleophile for desulfurizing, as compared with the prior axt (comparison example 1).

General method used in the examples The formulation used for the rubber mixtures is given in table 1. The unit phr means proportion by weight, with respect to 100 parts of the crude rubber used.
Table 1 Substance Amount [phr]

1st stage Buna VSL 5025-1T"' 96.0 Buna CB 2 4~' 3 0 . 0 Ultrasil VN3~'~' 80.0 Zn0 3.0 Stearic acid 2.0 Naftolene ZD 10.0 Vulkanox 4020" 1.5 Protector G35P~" 1.0 TESPT 6.4 Triphenylphosphine 0 to 4 2nd stage Batch stage 1 3rd stage Batch Stage 2 Vulkacit DT'' 2.0 Vulkacit CZT' 1.5 Sulfur 1.5 The polymer VSL 5025-1 is a solution polymerised SBR
copolymer from Bayer AG, with a styrene content of 25 wt.~
and a butadiene content of 75 wt.~. 73 ~ of the butadiene is 1,2 linked, 10 ~ is cis-1,4 linked and 17 ~ is trans-1,4 linked. The copolymer contains 37.5 phr of oil and has a Mooney viscosity (ML 1+4/100°C) of 50 ~ 4.
The polymer Buna CB 24 is a 1,4-cis polybutadiene (Neodyme type) from Bayer AG with a cis-1,4 content of 97 ~, a trans-1,4 content of 2 ~, a 1,2 content of 1 ~ and a Mooney viscosity of 44 ~ 5.
The silica VN3 from Degussa AG has a BET surface area of 175 m2/g.
Bis-(3-[triethoxysilyl]-propyl)tetrasulfane (TESPT) is sold by Degussa AG under the tradename Si 69 and has an average sulfane chain length of 4 and a polysulfane proportion S(x>4) > 25~.
Triphenylphosphine in accordance with examples 2 and 3 was purchased from the Merck Co.
Naftolen ZD from Chemetall is used as an aromatic oil.
Vulkanox 4020 is a PPD from Bayer AG. Protektor G35P is an anti-ozonant wax from HB-Fuller GmbH. Vulkacit D (DPG) and Vulkacit CZ (CBS) are commercial products from Bayer AG.
The rubber mixture is prepared in three stages in an internal mixer in accordance with table 2:

Table 2:
Stage 1 Settings Mixing unit Werner & Pfleiderer E-Typ Friction 1:1.11 Speed 70 min 1 Core pressure 5.5 bar Void volume 1.6 L

Filling extent 0.55 Thru'put temp. 80 C

Mixing process 0 to 1 min Buna VSL 5025-1 + Buna CB 24 1 to 3 min 1/2 Ultrasil VN3, ZnO, stearic acid, Naftolen ZD, silane, optional nucleophile 3 to 4 min 1/2 Ultrasil VN3, Vulkanox 4020, Protector G35P

4 min clean 4 to 5 min mix 5 min clean 5 to 6 min mix and discharge Batch temp. 140-150C

Storage 24 h at room temperature Stage 2 Settings Mixing unit same as stage 1 down to:

Speed 80 min 1 Filling extent 0.53 Thru'put temp. 80 C

Mixing process 0 to 2 min stage 1 batch broken up 2 to 6 min Batch temperature 150C by varying the speed 6 min discharge Batch temp. 150-155C

Storage 4 h at room temperature Stage 3 Settings Mixing unit same as stage 1 down to Speed 40 min 1 Filling extent 0.51 Thru'put temp. 50 C

Mixing process 0 to 2 min stage 2 batch + Vulkacit CZ + Vulkazit D

+ sulfur 2 min discharge and form a sheet on a laboratory mixing roller (diameter 200 mm, length 450 mm, throughput temperature 50C) Homogenise:

cut into and rotate 3* left, 3* right and then compress 8* with narrow roller gap (1 mm) and 3* with wide roller gap (3.5 mm) and then draw out as a sheet Batch-Temp. 85-95C

The general procedure for preparing rubber mixtures and their vulcanisates is described in the following book:
"Rubber Technology Handbook", W. Hofmann, Hanser Verlag 1994.
The vulcanisation time for the test specimen was 60 minutes at 165°C.
Rubber-engineering tests were performed in accordance with the test methods given in table 3.
Table 3 Physical tests Standard/
Conditions ML 1+4, 100C DIN 53523/3, ISO 667 Vulcameter test, 165C DIN 53529/3, ISO 6502 Tensile test on a ring, 23C DIN 53504, ISO 37 Tensile strength Modulus Elongation at break Shore A hardness, 23C DIN 53 505 Visco-elastic properties, 0 and DIN 53 513, ISO 2856 60C, 16 Hz, 50 N preliminary force and 25 N Amplitude force Complex modulus E*, Loss factor tan 8 DIN abrasion, 10 N force DIN 53 516 Dispersion ISO/DIS

Examples 1, 2 and 3: Triphenylphosphine as nucleophile Examples 1 (comparison example), 2 and 3 are performed in accordance with the general instructions given above, wherein no triphenylphosphine is added to the mixture in comparison example 1.
Differently from example 1, in the 1st mixing stage an addition 2 phr of triphenylphosphine is incorporated into the mixture in example 2 and an additional 4 phr of triphenylphosphine is incorporated into the mixture in example 3.
In figure 1, the changes in torque with time at 165°C in the 2nd mixing stage, for examples 1, 2,and 3, are plotted, wherein the increase in torque corresponds to the tendency to pre-vulcanisation at the stated temperature.
It can be seen, from figure 1, that the increase in torque for examples 2 and 3 which are in accordance with the invention is much less than for comparison example 1 in accordance with the prior art.
The rubber-engineering data for the crude mixture and the vulcanisate are given in table 4.

Table 4:
Crude mixture re;aults Feature: Units: - 1 - - 2 - 3 --ML(1+4) at 100C (3rd stage)[ME] 71 71 69 Vulcameter test 165C

Dmax-Dmin [dNm] 18.7 16.21 16.29 t 10~ [min] 1.41 1.49 1.41 t 90~ [min] 27.1 24.8 19.9 Vulcanisate resu:Lts Feature: Units: - 1 - - 2 - 3 --Tensile test Tensile strength [MPa] 16.1 15.5 16.6 Modulus 100 [MPa] 2.4 1.9 2.1 Modulus 300 [MPa] 10.9 9.3 10.4 Modulus 300~/100~ [ ] 4.5 4.9 5.0 Elongation at [~] 380 400 400 break Fracture energy [J] 84.6 80.1 85.9 Shore A hardness [SH] 67 61 60 DIN abrasion [mm'] 74 67 58 Visco-elastic Properties Complex modulus E* (0C) [MPa] 31.2 19.5 16.3 Complex modulus E* (60C) [MPa] 11.8 8.8 7.8 Loss factor tan b (0C) [-] 0.348 0.415 0.406 Loss factor tan 8 (60C) (-] 0.108 0.102 0.098 Dispersion [-] 6 6 6 It can be seen from table 4 that a generally balanced effective rubber-engineering set of values is produced for examples 2 and 3. In particular the modulus 300~/100~, which points to increased coupling effectiveness, and a low tan b (60°C) value, which correlates with a low rolling resistance, appear to be positive features.

Claims (11)

1. A rubber mixture, containing at least one organosilanepolysulfane and at least one desulfurization reagent from the class of compounds which includes trivalent phosphorus compounds.

2. The rubber mixture according to claim 1, containing organosilanepolysulfane in an amount of 0.1 to 15 wt.%, with respect to the amount of filler used, and desulfurization reagent in an amount of 5 to 80 wt.%, with respect to the amount of organosilanepolysulfane used.

3. The rubber mixture according to claim 1, containing organosilanepolysulfane in an amount of 5 to 10 wt.%, with respect to the amount of filler used, and the desulfurization reagent in an amount of 10 to 40 wt.%, with respect to the amount of organosilanepolysulfane used.

4. A rubber mixture according to claim 1, 2 or 3, including a synthetic rubber, and a silica as filler.

5. A process for preparing a rubber mixture according to any one of claims 1 to 4, comprising mixing together rubber, at least one filler, an organosilanepolysulfane and a desulfurization reagent.

6. The process for preparing rubber a mixture according to claim 5, wherein the organosilanepolysulfane and the desulfurization reagent are added during a thermomechanical mixing step at a temperature range of 80°C to 200°C.

7. An article moulded from a rubber mixture according to any one of claims 1 to 4.

8. The moulded article according to claim 7, wherein the article is a pneumatic tire.

9. The moulded article according to claim 7, wherein the article is a tire tread.

10. Use of a rubber mixture according to any one of claims 1 to 4 to produce moulded articles.

11. The use according to claim 10, wherein the moulded articles are pneumatic tires or tire treads.