Description

Vulcanizable rubber mixture and its use for rubber products

The invention relates to a vulcanizable rubber mixture and to its use for rubber products which need good grip in wet conditions or in icy conditions, in particular vehicle tyres and vehicle tyre treads, but also technical rubber products like belts on conveyor systems which can have exposure to weathering, technical hoses, or vibration- and impact-dampers.

The rubber products mentioned must in practice meet various requirements, and it is often difficult or impossible to meet these simultaneously. For example, rubber products and in particular vehicle tyres are required to have good elasticity, low abrasion, and good adhesion on various substrates, and are required to maintain the properties at high and low temperatures.

The service properties of vulcanizates or rubbers used for vehicle tyres are specifically adjusted inter alia via the selection of the rubber polymers used, via functionalization of the same, via chemical and/or adsorptive/physical binding to the filler, and via the selection of the fillers and additives. However, a conflict frequently arises because optimization of a desired property impairs another property that is likewise desired.

Tyre rubber compounds, in particular for treads, often comprise olefinic polymers having, or not having, aryl side chains. Material very often used in tyre tread mixtures are copolymer rubbers composed of conjugated dienes and of aromatic vinyl compounds. SBR rubber (styrene-butadiene rubber) is the most important member of this group.

Alongside the random SBR copolymers, block copolymers are also known, and block formation here appears to have a decisive effect on some service properties.

EP 396780 uses particular block constitutions in order to improve the service properties of a thermoplastic block copolymer which is intended to raise weathering resistance and impact resistance in rubbers and resins, a particular intention being to increase heat resistance. This is achieved via blockwise differences affecting the types of butadiene linkage and affecting vinyl contents.

EP 1110998 discloses a diene polymer modified terminally by a silanol group, and having, or not having, a polysiloxane spacer. The diene polymer can be an SBR rubber.

Therefore it was an object of this invention to provide a rubber mixture which can be used for rubber products and in particular vehicle tyres and treads of vehicle tyres and whose service properties have been improved in relation to grip in icy conditions and in wet conditions.

The object is achieved by a vulcanizable rubber mixture which comprises at least one block copolymer functionalized at least at one chain end, i.e. terminally, for binding to fillers, at least one filler and, if appropriate, additives, a feature of the mixture being that it comprises from 10 to 100 phr of a randomly (statistically) or microsequentially polymerized copolymer comprising a conjugated diene and an aromatic vinyl compound, and which has, polymerized onto at least one end of said main chain polymer in form of the statistically or microsequentially polymerized copolymer, a terminally functionalized block of different structure to the main chain, which structure is homopolymeric or copolymeric, from 5 to about 250 monomer units, more preferably between 20 and 180 monomer units long, and composed of at least one of the mers of the main chain copolymer, with the provisio that the glass transition temperatures on the one hand of the main chain and on the other hand of the terminal block or block polymer are different.

The term phr (parts per hundred parts of rubber by weight) used here is the usual term for amounts for mixing formulations in the rubber industry. The amount added in parts by weight of the individual substances is always based here on 100 parts by weight of entire weight of all of the rubbers present in the mixture.

Although, as described above, it was known that block polymers can be functionalized, it was not known that the properties of random and microsequential polymers of the SBR class, and the grip properties, in wet conditions and in icy conditions, of tyres and of other rubber products obtained using these rubber mixtures can be improved via addition of a block to the end of the main chain of at least on of the SBR-type copolymers, where the block is of 5 to about 250 units long and composed of at least one of the main chain mers. The terminal block bears a functional group for interaction with the filler and has a different structure compared to the main chain copolymer. In accordance with the different structure of the two polymeric regions of the copolymer, the different structures or different regions, i.e. the SBR-type copolymer main chain as cited above and the terminal block bearing the at least one terminal functional group, have different glass transition temperatures (Tg). The two regions of the copolymer can be characterized by these different glass transition termperatures.

With the help of the structurally different regions of the new copolymer of the mixture, characterized by different glass transition temperatures, the conflict arising when several performance properties shall be optimized at the same time can better be resolved.

It was found that this effect may even be enhanced when the terminal block is an internally homopolymeric block, which might be attributed to the fact that a uniform structure is introduced into the "glassy layer", i.e. the layer around the filler with which the functional group interacts within the vulcanizable mixture or within the rubber. Thus, preferably the terminal block is an internally homopolymeric block.

An advantageous process for the preparation of the functionalized copolymer used in the inventive mixture consists in using firstly a conjugated diolefin, such as preferably butadiene, and secondly an aromatic vinyl compound, such as preferably styrene, and polymerizing these randomly or microsequentially, i.e. under conditions where at least for a time both comonomers are present in the mixture, and with suitable reaction conditions (i.e. as in the prior art, for example

disclosed in EP 396 780 or WO 2004/099278), and specifically polymerizing, onto the living polymer, a short block of from about 5 to 250 monomer units, and functionalizing this "spacer block" terminally, as likewise known per se in the prior art.

The functionalization is usually introduced by way of a functionalized terminator. As an alternative, the active polymer chain end can, on termination of the polymerization, be reacted with a compound which comprises the desired functional group to be added to the end and also comprises a suitable leaving group. By way of example, this can be achieved via reaction of the reactive polymer ends with compounds such as ethylene oxide, benzophenone, carbon dioxide, dialkylaminobenzaldehyde, carbon disulphide, alkoxysilanes, alkylphenoxysilanes, phenoxysilanes, etc., as is previously known from the prior art to the person skilled in the art in this field. By way of example, EP 396 780 or EP 849 333 cites suitable processes and groups.

The functionalization is introduced in order to improve the cohesion or binding force between rubber and filler, i.e. carbon black or silicon-oxide-based (in this case preferably generally white) fillers. Bifunctional coupling agents, particularly preferably silanes and amines, can also be used as additives for the rubber mixtures.

Furthermore coupling agents like SnCl ₄ and SiCl ₄ can be added.

A wide variety of functionalization is in principle possible.

It is preferable that identical or different functional groups, as cited in more detail below, are introduced at at least one chain end or at both chain ends of the block copolymer, the degree of functionalization here, based on the block polymer, being from 0% to 100%, preferably from 50% to 95%, particularly from 75% to 95%.

One functional group can be introduced with the initiator during initiation of the chain, and the other can be introduced with the terminator at the end of the chain. These groups can be identical or different. It is preferable that a functional group is

introduced only at the chain end in direct contact with the internally homopolymeric block, rather than on the randomly or microsequentially polymerized chain, since this group is intended to interact or react with the filler, and the intention is that the location of the homopolymeric block be in the vicinity of the filler.

The initiator used can, by way of example, also comprise a bifunctional initiator, as described in WO 02/020623, the entire content of which is incorporated into this disclosure by way of reference. This method can be used to functionalize both polymer ends with the internally homopolymeric block and with the functionalization.

The terminal functional groups are preferably -OH, -COOH, -COX, where X = halogen, -SH, -CSSH, -NCO, amino, epoxy, silyl, silanol or siloxane groups inclusive of, in each case attached to the polymer chain with or without a spacer, polysiloxane groups, and siloxane and polysiloxane groups comprising amino groups. The spacer is a group indicated below by A.

In particular, the formulae of individual groups can be represented as follows: the amino groups by the formulae: -A-N(Ri) ₂ , -A-NHRi, -A-NH ₂ , the silyl, silanol and siloxane groups can be represented by the following formulae:

-A-SiH ₂ (OH), -A-Si(Ri) ₂ (OH), -A-SiH(OH) ₂ , -A-SiRi(OH) ₂ , -A-Si(OH) ₃ , -A- Si(ORi) ₃ , -A-(SiRiR ₂ O) _x -R ₃ , -A-Si(R ₃ ) ₃ , -A-Si(R ₃ /X) ₃ , where X = halogen, and the siloxane groups comprising amino groups can be represented via the following formula: -A ¹ -Si(A ² -N(H/Ri) ₂ ) _n (ORi) _m (R ₃ ) ₃ . _(n+m) in each case where Ri and R ₂ are identical or different, being namely alkoxy or alkyl, branched or straight-chain, cycloalkyl, aryl, alkylaryl, aralkyl, or vinyl, in each case having from 1 to 20 carbon atoms, and mononuclear aryl, x = a whole number from 1 to 1500, n = 0 - 3, m = 0 - 3, with n+m < 3 R ₃ is -H, or -alkyl, branched or straight-chain, or cycloalkyl, in each case having from 1 to 20 carbon atoms, or mononuclear aryl, and A ¹ and A ² are each a Co-Ci ₂ organic spacer chain, branched or unbranched, preferably Co-Ci ₂ -alkyl, -allyl or -vinyl, (Co meaning here the absence of the spacer chain).

Siloxane groups that contain amino groups, are represented best by the formula - Al-Si-A2-N((H)k(Ri)2-k))y(ORi) _z (R3)3-(y+z), where: k can vary from 0 to 2, y can vary from 1 to 3, and z can vary from 0 to 2, 0 < y+z < 3, provided that Ri and R ₂ are identical or different, and can be alkyl, linear or branched, cycloalkyl, aryl, alkylaryl, aralkyl or vinyl groups, in each case having 1 to 20 carbon atoms, and mononuclear aryl groups, R ₃ is H or alkyl, linear or branched, in each case having 1 to 20 carbon atoms, or a mononuclear aryl group, and Al and A2 are chains of up to 12 carbon atoms, linear or branched, preferably alkyl, allyl or vinyl.

For the terminal functional groups intended to interact with the filler, siloxane and silanol groups are preferred.

For the elastomers of this invention, most preferably siloxanes are used as terminal functional groups of the polymeric chains, in the form or structures that can be represented by the general formula -[— Si(RiR2)-O-] _n — Si(RiR ₂ VOH, where n is an integer up to 500, and Ri and R ₂ are identical or different, and can be alkoxy or alkyl, linear or branched, cycloalkyl, aryl, alkylaryl, aralkyl or vinyl groups, in each case having 1 to 20 carbon atoms, and n represents the number of units of the siloxane functional group before a silanol terminal group.

To obtain the functionalization in the extremities of the polymeric chains, the preferred reagent is hexamethylcyclotrisiloxane (D3), which allows the incorporation of continuous sequences of the siloxane function group -[-Si(CHs) ₂ - O-] — , in various lengths, and a silanol terminal group - Si(CHs) ₂ -OH.

The terminally functionalized particular block polymer of the invention, which forms entirely or to some extent the rubber content of the vulcanizable rubber mixture, is a randomly or microsequentially polymerized, generally solution- polymerized, copolymer which is composed of conjugated diene or diolefm and aromatic vinyl compound, and which has, polymerized onto at least one end, a terminally functionalized block, homopolymeric or copolymeric, but preferably homopolymeric, and composed of at least one of the mers of the copolymer, where "mers" means the type of mers, i.e. one conjugated diene and/or one aromatic

vinyl compound. The block is from 5 to about 250 monomer units long and because of its different structure characterized by a different glass transition temperature compared to the main chain to which it is attached.

The diolefm here can generally be a diene monomer having from 4 to about 10 carbon atoms, particular examples being 1,3 -butadiene, 2-alkyl-l,3-butadiene, 2,3- dialkyl-l,3-butadiene, 2-alkyl-3-alkyl-l,3-butadiene, 1,3-pentadiene, 1,3- hexadiene, 2,4-hexadiene, etc.

The polyisoprene can be either 1 ,4-polyisoprene or 3,4-polyisoprene. The polybutadiene can be either 1,4- or vinyl-polybutadiene.

The diene of the copolymer and/or of the polymerized-on block is preferably a butadiene, an isoprene, or, in the random or microsequential polymer, a mixture of the two. Both in the case of polybutadiene and in the case of polyisoprene, the length of the internally homopolymeric block at the chain end is preferably from 10 to 200 monomer units, more preferably from 20 to 180 monomer units, and polyisoprene can comprise either 1,4 or 3,4, and butadiene can comprise either 1,4 or 1,2-units. In total, from 8 to 80% of 1,2 and, respectively, 3,4.

The aromatic vinyl compound is preferably styrene, both within the random or microsequential copolymer and also within the polymerized-on block. However, in principle it is also possible to use other aromatic vinyl compounds, which, however, are not important industrially. Among these are in particular ortho, meta and/or para-methylstyrene, vinyltoluene, p-tert-butylstyrene, methoxystyrenes, vinylmesitylene and divinylbenzene.

According to one preferred embodiment, the internally homopolymeric block at the chain end is a polystyrene block whose length is preferably from 5 to 250 monomer units, more preferably from 10 to 200 or from 20 to 180 monomer units.

The random or microsequential copolymer can have any desired cis or trans structure. It is generally obtained via solution polymerization. "Microsequential" here means that one or both comonomers can be present at least partially in the

form of microblocks. The microblocks here preferably encompass from 2 to 10, more preferably from 3 to 10, and in particular from 3 to 6, connected units of a comonomer.

The diolefm content, which is preferably the butadiene content of the SBR-type polymer can comprise either 1,4-cis, 1,4-trans or 1,2-vinyl structures. The vinyl content is adjustable in a known manner by way of the polymerization conditions. The vinyl content for inventive functionalized copolymers is preferably, within the BR content or IR content, from 8 to 80%, preferably from 10 to 70%.

More specifically, the elastomers of this invention present a percentage composition in weight of their main chain, which can vary from 5 % to 50 %, for the aromatic vinyl monomer (e.g. styrene), and from 50 to 95 % for the conjugated diene (e.g.: 1,3 -butadiene). Preferably, these elastomers present a composition from 15 % to 40 % for the % w/w of the monomer with an aromatic vinyl structure, and from 60 % to 85 % for the % w/w of the conjugated diene incorporated in the copolymer.

The elastomers of the invention have a Mooney Viscosity (ML+4 at 100 ⁰ C) in a range from 30 to 90, and an average molecular weight in the range from Mw = 80,000 to 700,000, with a polydispersion in the range from 1,05 to 4,0, when analyzed by Size Exclusion Chromatography (SEC), based on polystyrene standards.

The elastomers present glass transition temperatures, Tg, in the range from -92 ⁰ C to +1 ⁰ C, in particular from -50 to 0 ⁰ C, depending on the chemical content of the aromatic vinyl monomer of the copolymer and the microstructure of the conjugated diene incorporated in the copolymer.

The rubber content of the mixture can comprise not only the SBR-class block copolymer used according to the invention here but also preferably the following blend constituents: natural rubber (NR), polybutadiene (BR), polyisoprene (IR), polychloroprene (CR), styrene-isoprene-butadiene terpolymer and/or ethylene-

propylene-diene rubber (EPDM), IBR, SBR. The rubber mixture preferably comprises 30 to 90 phr of the block copolymer having random or microsequential content and from 10 to 70 phr of residual rubber content preferably composed of at least one elastomer rubber from the group of natural rubber (NR), synthetic polyisoprene (IR), polybutadiene (BR) or polychloroprene (CR), styrene butadiene rubber (SBR), styrene isoprene butadiene rubber (SIBR), isoprene butadiene rubber (IBR).

The mixture can moreover comprise conventional additives, such as plasticizers, antioxidants, UV stabilizers, mould-release agents, polymerization accelerators and polymerization retarders, activators, e.g. zinc oxide and fatty acids (e.g. stearic acid), waxes and mastication aids in conventional parts by weight.

The filler present in the mixture can preferably be what is known as a "white filler" or else carbon black. In one preferred embodiment, the filler comprises at least one silicon-oxide-based filler (silica, silicic acid) or comprises another "white" filler individually or in a mixture and can also comprise carbon black in the mixture.

The filler preferably comprises a mixture composed of at least one silicon-oxide- based filler and of at least one grade of carbon black. Suitable silicon-based fillers and carbon blacks for technical rubber mouldings and tyre mixtures are known in the prior art and can be used here. The fillers and rubbers of the mixture can likewise have been modified in the manner known in principle from the prior art for rubber mixtures. The white filler can moreover preferably be an oxidic and/or hydroxidic inorganic or mineral filler, or comprise any desired combination of these substances. The white filler has preferably been selected from the group of silica (SiO ₂ ), phyllosilicates, chalk, starch, oxides and/or hydroxides of aluminium, magnesium, titanium or zinc.

In one preferred embodiment, the content present of the filler in the mixture can be from 20 to 200 phr, preferably from 30 to 150 phr, more preferably from 30 to 130 phr, in particular for tread mixtures for cars and light-truck tyres from 60 to 130 phr or preferably from 80 to 130 phr, of filler.

The rubber mixture can, if desired, also comprise a coupling reagent. Suitable coupling reagents are in principle likewise known to the person skilled in the art in this field.

The invention likewise encompasses the use of the inventive rubber mixture for the production of rubber products, especially technical rubber products which means products made of rubber with a technical field of application. In particular the rubber products of the invention encompass vehicle tyres, in particular vehicle tyre treads, hoses, belts, technical mouldings or especially vibration- or impact- dampers, and also comprises the associated tread and vehicle tyre. The vehicle tyre is very generally a pneumatic vehicle tyre.

For production of the rubber, the inventive unvulcanized rubber mixtures are further processed in a known manner, i.e. vulcanized with the aid of vulcanizing agents present in the mixture or added subsequently. The term "vulcanization" here is generally intended to mean the crosslinking of the substantially linear, not three- dimensional, rubber to give an elastomer rubber, irrespective of whether the traditional method using sulphur or sulphur-liberating reagents is used or other crosslinking agents suitable for natural rubbers and synthetic rubbers are used, these methods being known to the person skilled in the art.

The inventive rubber mixture is generally produced in a conventional manner, by first, in one or more stages of mixing, generally producing a parent mixture comprising all of the constituents except for the vulcanization system, and then producing the finished mixture by adding the vulcanization system. The mixture is then further processed, e.g. via an extrusion procedure, and converted to the appropriate form. The mixture is preferably converted to the form of a tread and, as is known, applied during production of the pneumatic vehicle tyre parison. However, the tread can also be wound in the form of a narrow strip of rubber mixture onto a green tyre which by this stage comprises all of the tyre parts except for the tread. It is of no importance for the tyres whether the entire tread has been produced from a single mixture or has, for example, a cap and base structure, as

long as at least the area coming into contact with the road has been produced from the inventive rubber mixture.

Inventive examples are used below for further illustration of the invention, but these are intended solely for illustration and to improve understanding of the invention and not to restrict the invention.

The mixture is produced in two stages, as described above, in a laboratory tangential mixer. Test specimens were produced from all of the mixtures via vulcanization at an elevated pressure and temperature and these test specimens were used to determine typical rubber-industry properties of the materials, and these can be utilized as indicators for particular behaviour of the materials, i.e. particular handling properties. The following test methods were used when testing the specimens:

- Shore A hardness at room temperature and 70 ⁰ C to DIN 53 505

- rebound at room temperature and 70 ⁰ C to DIN 53 512

- tensile strength at room temperature to DIN 53 504

- elongation at break at room temperature to DIN 53 504 - stress values at 100 and 300% static elongation at room temperature to DIN

53 504

- Tan delta at 0 ⁰ C to DIN 53 513 from measurement with variation of temperature using a dynamic deformation amplitude of 0.2% with 10% predeformation and 10 Hz. - average dynamic storage modulus E' at from -25 to -5°C to DIN 53 513 from measurement with variation of temperature using a dynamic deformation amplitude of 0.2% with 10% predeformation and 10 Hz.

EXAMPLES

Example group 1: Solution-polymerized SSBR with polymerized-on polystyrene block and terminal siloxane functionalization

Main polymer (block copolymer used according to the invention)

Table 1.1

O

Amino:

Nagata et al, Rubber chemistry and technology, 60, 1987, 837

Siloxane functionalization as described in EPl 10998 (hexamethyltrisiloxanol)

Mixing formulations:

Table 1.2

6PPD = N-(l,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine

TMQ = 2,2,4-trimethyl-l,2-dihydroquinoline

DPG = N,N-diphenylguanidine

CBS = benzothiazyl-2-cyclohexylsulphenamide

Oil/plasticizer = TDAE

Silane/functionalization reagent = SILQUEST A- 1589 SILANE, General Electric

Speciality, USA Silica = VN3, Degussa AG, Germany, nitrogen surface area: 175 m ² /g, CTAB surface area 160 m ² /g;

Test results:

Table 1.3

The results show that braking in wet conditions can be improved as desired while handling performance remains the same. Rebound at room temperature and tan d (0 ⁰ C) are conventional indicators for braking in wet conditions. Braking performance in wet conditions improves as the values for rebound at room temperature decrease and as the values for tan d (0 ⁰ C) increase. Braking performance in wet conditions is also better for all of the blends with the block copolymer of the examples.

The combination of the properties of rebound at 70 ⁰ C and MlOO is used to represent handling. The rebound at 70 ⁰ C here serves as an indicator for handling- manoeuvre grip (improving as the values decrease), and the stress value for 100%

elongation serves for stiffness during handling (improving as the value increases). The examples of polymers having a polystyrene block also exhibit advantages during handling, although at the same time there is an improvement in grip under wet conditions.

Example group 2: Solution-polymerized SSBR having polymerized-on butadiene block and terminal siloxane functionalization

Main polymer (block copolymer used according to the invention)

Table 2.1

Siloxane functionalization as described in EPl 10998 (hexamethyltrisiloxanol), the disclosure of which with respect to this siloxane functionalization is hereby incorporated into the content of this application.

Mixing formulations:

Table 2.2

Silica: VN3, Degussa AG, Germany, nitrogen surface area: 175 m ² /g, CTAB surface area 160 m ² /g;

Silane functionalization reagent: Silquest A 1589, General Electric Specialty, USA

6PPD = N-(l,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine

TMQ = 2,2,4-trimethyl-l,2-dihydroquinoline

DPG = N,N-diphenylguanidine

CBS = benzothiazyl-2-cyclohexylsulphenamide

Oil/ plasticizer = TDAE

Test results:

Table 2.3

The storage modulus E' at from -25°C to -5°C can be considered to be an indicator for the improvement in grip in icy conditions, a reduced storage modulus being regarded as equivalent to the improvement in grip in icy conditions. The results show that in the mixtures which comprise the block copolymer used according to the invention having a BR block, E' has been reduced, i.e. there is an improvement in grip in icy conditions, while wet grip is retained. The rebound at room temperature is used here as an indicator of wet grip (smaller values indicating improved wet grip).