Mixture of organosilanepolysulphanes and a process for the production of rubber compounds containing these mixtures

This invention relates to mixtures of organosilane-polysulphanes of the general formula EQU (RO).sub.3 Si(CH.sub.2).sub.x S--S.sub.z --S(CH.sub.2).sub.x Si(OR).sub.3 (I) in which PA1 R means alkyl, linear or branched, having 1-8 C atoms, in particular 1-3 C atoms PA1 x means an integer from 1-8 PA1 z means 0 to 6, wherein sum of the proportions of polysulphanes in which z=0 and z=1 amounts to .gtoreq.80% by weight, providing that the proportion of compounds in which z=0 remains below 80% and the proportion of organosilanepolysulphanes in which z means an integer from 2 to 6 does not exceed a proportion of 20 wt. % in the mixtures, and to the rubber compounds produced using these mixtures, in particular for tire treads.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 This invention relates to mixtures of organosilane-polysulphanes having an
 elevated proportion of disulphanes and to a process for the production of
 rubber compounds containing these compounds.
 2. Prior Art
 Due to increasing environmental awareness, economies in fuel consumption
 and a reduction in pollutant emissions are today becoming a greater
 priority [Auto 91/92, Verband der Autombobilindustrie e.V., Frankfurt,
 ADAC-Motorwelt 11791, 50 (1991]. The challenge to tire manufacturers is to
 develop tires which are distinguished by very low rolling resistance
 combined with excellent wet skid resistance and good abrasion resistance.
 Proposals have been made in numerous publications and patents with regard
 to reducing tire rolling resistance and thus fuel consumption. Proposals
 which may be mentioned include reducing the carbon black content in the
 compound and using special carbon blacks (U.S. Pat. No. 4,866,131, U.S.
 Pat. No. 4,894,420). However, none of these proposed solutions has
 resulted in a satisfactory balance between the desired low rolling
 resistance and other important tire characteristics such as wet skid
 resistance and abrasion resistance.
 The use of highly active silica fillers in combination with organosilane
 bis(triethoxysilylpropyl)tetrasulphane (TESPT) to replace the carbon black
 normally used in rubber compounds is known to allow production of a tire
 having rolling resistance substantially reduced in comparison with
 standard tires, while simultaneously retaining or even improving the other
 two above-stated tire characteristics [EP 0 501 227, U.S. Pat. No.
 5,227,425; G. Agostini, J. Berg, Th. Materne: New Compound Technology,
 October 1994, Akron, Ohio, USA; S. Wolff, U. Gorl, M. J. Wang, W. Wolff:
 Silica based on tread compounds--background & performance, paper presented
 at TYRE TECH '93, October 1993, Basel, Switzerland; Ph. Cochet, L. B.
 Barriquand: Precipitated silica in tire tread, paper presented at ACS
 Meeting of the Rubber Division, October 1995, Cleveland, Ohio, USA].
 At the 1986 ACS meeting in New York S. Wolff [S. Wolff: The influence of
 fillers on rolling resistance, presented at the 129.sup.th meeting of the
 Rubber Division, American Chemical Society, Apr. 8-11, 1986, New York]
 presented a paper showing that it is possible to reduce rolling resistance
 in comparison with a carbon black filled standard compound while largely
 retaining wet skid resistance by using silica in combination with TESPT
 both in a passenger vehicle tire tread based on an emulsion
 styrene/butadiene rubber (E-SBR) and in a truck tire tread based on
 natural rubber.
 This system was further optimized with regard to all three characteristics
 by using specific styrene/butadiene polymers produced using a solution
 polymerization process (EP 0 447 066 A1), sometimes blended with other
 polymers, in particular polybutadiene and additionally using novel grades
 of silica (U.S. Pat. No. 5,227,425) and polymer blends specifically
 tailored to this application (EP 0 620 250 A1) sometimes with three to
 four different starting polymers [G. W. Marwede, U. G. Eisele, A. J. M.
 Sumner: paper presented to the ACS Meeting of the Rubber Division, October
 1995, Cleveland, Ohio, USA.].
 It is stated in all the publications and patents that, in order to achieve
 a lower rolling resistance while retaining or improving wet skid
 resistance and abrasion resistance, it is necessary to replace a large
 proportion of or the entire content of the normally used carbon black
 filler with a highly active silica [S. Wolff: The influence of fillers on
 rolling resistance, presented at the 129.sup.th meeting of the Rubber
 Division, America Chemical Society, Apr. 8-11, 1986, New York; U.
 LeMaitre: Tire rolling resistance, AFCEP/DKG Meeting, 1993, Mulhouse,
 France]. However, this replacement results in the desired objective only
 if the organosilane bis(triethoxysilylpropyl)tetrasulphane (TESTP) is used
 as a "coupling" agent between the silica and the polymer.
 It is known [S. Wolff: The role of rubber-to-silica bonds in reinforcement,
 presented at the First Franco-German Rubber Symposium, Nov. 14-16, 1985,
 Obernai, France; S. Wolff: Silanes in tire compounding after ten
 years--review, Third Annual Meeting & Conference on Tire Science &
 Technology, The Tire Society, Mar. 28-29, 1984, Akron, Ohio, USA] that the
 properties which may be achieved by using organosilanes in rubber
 compounds are dependent upon two independent reactions. Firstly, during
 production of the compound, preferably during the first compounding stage,
 a reaction occurs at elevated temperature between the silanol groups of
 the silica and the trialkoxysilyl groups of the silane with elimination of
 alcohol (hydrophobing or modification reaction). A complete reaction is of
 decisive significance to subsequent properties.
 Like all chemical reactions, this reaction proceeds faster at elevated
 temperatures [U. Gorl, A. Hunsche: Advanced investigations into the
 silica/silane reaction system, paper presented at ACS Meeting, Rubber
 Division, October 1996, Louisville, Ky., USA], such that the rubber
 compounder, desiring short compounding times, prefers to use the highest
 possible compounding temperature. The use of such high compounding
 temperatures is, however, limited by the fact that the second, so-called
 rubber-reactive group of TESPT consists of a group which is on average a
 tetrasulphane group having a significant proportion of longer sulphane
 chains (S.sub.5 -S.sub.8) [S. Wolff: Silanes in tire compounding after ten
 years--review, Third Annual Meeting & Conference on Tire Science &
 Technology, The Tire Society, Mar. 28-29, 1984, Akron, Ohio, USA].
 This rubber-reactive group is generally considered to give rise to a
 so-called filler/rubber bond, which determines the technical rubber
 properties of the finished article (for example tires). This reaction,
 which is desired during vulcanization, is influenced by the thermal
 lability of the tetrasulphane group and higher sulphane units. Practical
 experience has, however, shown that the reaction causes serious problems
 if it occurs during production of the unvulcanised compound, during which
 only the reaction between the filler and the silane should normally occur.
 If sulphur is eliminated from the long-chain sulphane units, it is
 incorporated into the polymer chain. This then brings about "scorching"
 and stiffening of the sheeted compound, which can even render the
 unvulcanized compound unprocessable. Scorching can be measured by
 determining the viscosity of the compound. EP-A1-0 732 362, which is not a
 prior publication, describes the use of organosilanedisulphides in rubber
 compounds.
 However, these sulphur compounds must be very pure or have a disulphide
 content of at least 80%.
 SUMMARY OF THE INVENTION
 The object of the invention is to provide mixtures of
 organosilanepolysulphanes which do not give rise to scorching at the
 elevated temperatures which may occur during production of unvulcanized
 rubber compounds, i.e. vulcanizable rubber compounds, which still do not
 yet contain the sulphur and accelerator(s) necessary for vulcanization.
 The present invention provides mixtures of polysulphanes which achieve this
 object.
 The mixtures of the invention comprise organosilanepolysuphanes of the
 general formula
EQU (RO).sub.3 Si(CH.sub.2).sub.x S--S.sub.z --S(CH.sub.2).sub.x Si(OR).sub.3
 (I)
 in which
 R is alkyl, linear or branched, having 1-8 carbon atoms, preferably 1-3
 carbon atoms,
 x is an integer from 1-8, and
 z is 0 to 6,
 wherein the sum of the proportions of organosilanepolysulphanes in which
 z=0 and z=1 amounts to .gtoreq.80% by weight, providing that the
 proportion of compounds in which z=0 remains below 80% and the proportion
 of organosilanepolysulphanes in which z is an integer from 2 to 6 does not
 exceed 20 wt. % of the mixtures.
 The latter should be stated as a content of .ltoreq.20% by weight and is a
 vital characterizing feature. Polysulphane fractions where z=7 or 8 are
 not generally found in the mixtures according to the invention. These
 polysulphane fractions can be present at contents of &lt;1%, for example as
 impurities, which have no effect on the use of the mixtures according to
 the invention.
 The sum of the constituents must, of course, always be 100%, if necessary
 taking account compounds where z is 7 or 8.
 Particularly suitable mixtures are those in which the proportions of the
 organosilanepolysulphanes have the following values:
 z=0 approx. 58 to &lt;80 wt %
 z=1&gt;0 to approx. 32 wt %, wherein the sum of these compounds is .gtoreq.80
 wt %, and
 z=2 to 6.ltoreq.20 wt %, preferably&lt;11 wt %.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 FIG. 1 graphically illustrates the scorching behavior of the unvulcanized
 compounds of Example 1. The rheometer data is obtained at 165.degree. C.
 FIGS. 2-5 graphically illustrate a comparison in vulcanized properties
 between a control (Si 69) and a product of the invention (Si 266 mod) as
 used in the specific examples.
 FIG. 2 shows that Si 266 mod exhibits distinctly improved scorching
 behavior at all compounding temperatures.
 FIG. 3 shows that Si 266 mod exhibits a distinctly more favorable
 vulcanization time in comparison with Si 69.
 FIG. 4 shows that even at elevated compounding temperatures, Si 266 mod
 exhibits no scorching and thus has distinctly better processing behavior.
 FIG. 5 shows that Si 266 mod exhibits distinct advantages in injection
 speed.
 SPECIFIC EXAMPLES
 The following examples are provided to further illustrate applicants
 invention.
 Test Standards Used in the Examples: