DE102009059207A1 - Zinc oxide-free, sulfur-crosslinkable rubber mixture, useful for making treads of tires, comprises a synthetic diene rubber and a dithiocarbamate accelerator - Google Patents

Abstract

Zinc oxide-free, sulfur-crosslinkable rubber mixture (I) comprises 80-100 phr (parts by weight based on 100 parts by weight of rubber) of at least one synthetic diene rubber and 0.1-5 phr of at least one dithiocarbamate accelerator. An independent claim is included for tire, where at least a part of tread of tire is made of (I) that is vulcanized with sulfur.

Description

The invention relates to a sulfur-crosslinkable rubber mixture, in particular for treads of tires, comprising at least one synthetic diene rubber and at least one dithiocarbamate accelerator.

The invention further relates to a tire whose tread consists, at least in part, of the sulfur vulcanized rubber composition.

Dithiocarbamate accelerators have long been known in rubber technology for accelerating the sulfur crosslinking of diene rubbers and are referred to as ultra-accelerators because of their high vulcanization rate. For sufficient processing safety they are often used in combination with other, slower heating accelerators. When processing sulfur-crosslinkable rubber mixtures, it is also customary and, in order to avoid adverse effects on the vulcanizate properties, it is necessary to use zinc oxide as the essential activator for the dithiocarbamate accelerators accelerated with dithiocarbamate accelerators (cf. W. Hofmann, H. Gupta, Handbook of Rubber Technology, dr. Gupta Verlag, Ratingen, 2001, Ch. 7, p. 28 ff ). In the presence of zinc, complexes form with the accelerator which activate the sulfur for the crosslinking of the rubber.

However, zinc compounds in rubber compounds have come into the discussion, since zinc is a heavy metal, which also has potential for damaging the aquatic environment. It is feared that especially by abrasion of tires larger amounts of zinc could get into the environment. On the other hand, high levels of zinc in the mixture pose a problem in tire manufacturing because the vulcanization of the tires produces zinc sulfide, which deposits on the vent valves of the tire molds and can result in blocked vent valves. The blocked vent valves then lead to so-called shrinkage points in the tire by trapped air bubbles.

Due to the aforementioned problems, efforts are being made to reduce the proportion of zinc compounds in rubber compounds for tires. A complete abandonment of zinc in sulfur-crosslinkable diene rubber mixtures is currently generally not possible, since it then comes to low crosslinking strengths and speeds and significantly lower stiffnesses in the vulcanizates and thus unwanted vulcanizate properties. Typically, sulfur-crosslinkable rubber compounds for tire treads today use from 2.5 to 5 phr of zinc oxide.

The invention is based on the object of reducing the zinc content in sulfur-crosslinkable diene rubber mixtures without having to accept losses in the vulcanization behavior and in the tire-relevant vulcanizate properties.

The object is achieved according to the invention characterized in that the rubber mixture contains 80 to 100 phr of at least one synthetic diene rubber and 0.1 to 5 phr at least one dithiocarbamate accelerator and at the same time is free of zinc oxide.

The term phr (parts per hundred parts of rubber by weight) used in this document is the quantity used in the rubber industry for mixture formulations. The dosage of the parts by weight of the individual substances is always based on 100 parts by weight of the total mass of all the rubbers present in the mixture.

By using a dithiocarbamate accelerator in a rubber mixture, which is based essentially on synthetic diene rubbers, it was surprisingly possible to increase the vulcanization rate by simultaneously omitting the previously considered essential zinc oxide with constant processability through constant scorch time. This leads to the improvement of productivity by Heizzeitverkürzung. The resulting vulcanizates are also characterized in hardness adjustment by an increased dynamic storage modulus E 'at higher temperatures and increased damping, resulting in more grip and better handling when used as a tire tread strip. These properties are particularly desirable in so-called ultra high performance tires (UHP tires, high speed tires). The mixture contains significantly less zinc ions if the accelerator used is a zinc dithiocarbamate (higher molecular weight than zinc oxide) and at the same time no zinc oxide. If a zinc-free dithiocarbamate accelerator is used, the mixture is zinc-free.

The reduced level of zinc in the mixture provides environmental benefits and tire production is significantly less hindered by sulphide deposits in the vent valves of the tire molds.

The sulfur crosslinkable rubber mixture contains 80 to 100 phr of at least one synthetic diene rubber. The diene rubbers can be used in the blend. The diene rubbers include all rubbers having an unsaturated carbon chain derived at least in part from conjugated dienes. Particularly preferred is when the diene rubber is a styrene-butadiene copolymer (SBR). This offers special advantages in terms of grip and handling.

The styrene-butadiene copolymer can be a solution-polymerized styrene-butadiene copolymer (S-SBR) having a styrene content, based on the polymer, of about 10 to 45% by weight and a vinyl content (content of 1.2 bound butadiene, based on the total polymer) of from 10 to 70% by weight, which can be prepared, for example, using lithium alkyls in organic solvent. The S-SBR can also be coupled and end-group modified. But it can also emulsion-polymerized styrene-butadiene copolymer (E-SBR). and blends of E-SBR and S-SBR. The styrene content of E-SBR is about 15 to 50% by weight and the types known in the art obtained by copolymerization of styrene and 1,3-butadiene in aqueous emulsion can be used.

Other synthetic diene rubbers which can be used are, for example, synthetic polyisoprene (IR), polybutadiene (BR), styrene-isoprene-butadiene terpolymer, butyl rubber, halobutyl rubber, ethylene-propylene-diene rubber (EPDM) or modified diene rubbers. The modified diene rubbers may be based on solution or emulsion polymerized styrene-butadiene rubber. The modification may be those having hydroxyl groups and / or epoxy groups and / or siloxane groups and / or amino groups and / or aminosiloxane and / or carboxyl groups and / or phthalocyanine groups.

According to a preferred embodiment of the invention, the rubber mixture contains 5 to 10 phr of natural rubber (NR), which leads to improved abrasion behavior when used in tire treads.

The rubber mixture according to the invention contains 0.1 to 5 phr of at least one dithiocarbamate accelerator. The dithiocarbamate accelerators include ammonium, zinc and other metal dithiocarbamates such as lead, cadmium, copper, bismuth and nickel dithiocarbamates, which may also be used as a mixture.

For the highest possible vulcanization rate with at the same time still good processing safety by not too fast vulcanization, it has proved to be advantageous if the dithiocarbamate accelerator is a zinc dithiocarbamate. Thus, for example, zinc dimethyldithiocarbamate (ZDMC), zinc diethyldithiocarbamate (ZDEC), zinc dibutyldithiocarbamate (ZDBC), zinc pentamethylenedithiocarbamate (Z5MC), zinc ethylphenyldithiocarbamate (ZEPC) and zinc dibenzyldithiocarbamate (ZBEC) or mixtures of these can be used ,

According to a preferred development of the invention, the dithiocarbamate accelerator used is zinc dibenzyldithiocarbamate (ZBEC). This has the advantage of a particularly high level of processing reliability, since the scorching time is relatively high.

The mixtures according to the invention without zinc oxide, are softer without any other adjustments of the mixing system, show a lower Shore hardness. To counteract this softening, it has proved to be advantageous if the proportion of plasticizer in the mixture is reduced and amounts to a maximum of 40 phr. So you can get mixtures that are characterized by the same hardness by a high attenuation at high temperatures. The rubber composition may include plasticizers such as aromatic, naphthenic or paraffinic mineral oil plasticizers, MES (mild extraction solvates) or TDAE (treated distillate aromatic extract) or liquid polymers (eg liquid polybutadiene).

The rubber mixture may contain as fillers carbon black and / or silica in conventional amounts. It may also contain other fillers such as aluminosilicates, chalk, starch, magnesia, titania or rubber gels.

The usable carbon blacks preferably have the following characteristics: DBP number (according to ASTM D2414 ) 90 to 200 mL / 100 g, CTAB count (according to ASTM D 3765 ) 80 to 170 m ² / g and iodine adsorption number ( according to ASTM D 1510 ) 10 to 250 g / kg. The silicas may be the silicas known to those skilled in the art which are suitable as a filler for tire rubber mixtures. However, it is particularly preferred if a finely divided, precipitated silica is used, which has a nitrogen surface (BET surface area) (according to DIN 66131 and 66132 ) from 35 to 350 m ² / g, preferably from 145 to 270 m ² / g and a CTAB surface (according to ASTM D 3765) of 30 to 350 m ² / g, preferably from 120 to 285 m ² / g , Such silicas lead z. B. in rubber mixtures for tire tread to particularly good physical properties of the vulcanizates. In addition, advantages in compounding processing can result from a reduction in mixing time with consistent product properties leading to improved productivity. As silicas can thus z. B. both those of the type VN3 (trade name) from Degussa and highly dispersed silicas, so-called HD silicas (eg Ultrasil 7000 from Degussa) are used.

To improve the processability and to attach the silica and other optional polar fillers to the diene rubber silane coupling agents can be used in rubber mixtures. The silane coupling agents react with the superficial silanol groups of the silica or other polar groups during the mixing of the rubber or the rubber mixture (in situ) or even before the addition of the filler to the rubber in the sense of a pretreatment (pre-modification). All silane coupling agents known to the person skilled in the art for use in rubber mixtures can be used as silane coupling agents. Such known from the prior art coupling agents are bifunctional organosilanes having on the silicon atom at least one alkoxy, cycloalkoxy or phenoxy group as a leaving group and have as other functionality a group which may optionally undergo a chemical reaction with the double bonds of the polymer after cleavage , In the latter group may be z. These may be, for example, the following chemical groups: -SCN, -SH, -NH ₂ or -S _x - (where x = 2-8). Thus, as silane coupling agents z. For example, 3-mercaptopropyltriethoxysilane, 3-thiocyanato-propyltrimethoxysilane or 3,3'-bis (triethoxysilylpropyl) polysulfides having 2 to 8 sulfur atoms, such as. For example, 3,3'-bis (triethoxysilylpropyl) tetrasulfide (TESPT), the corresponding disulfide or mixtures of the sulfides having 1 to 8 sulfur atoms with different contents of the various sulfides, are used. TESPT can be added, for example, as a mixture with carbon black (trade name X50S from Degussa). Also blocked mercaptosilanes, as z. B. from the WO 99/09036 are known, can be used as a silane coupling agent.

The silane coupling agents are used in amounts of from 0.2 to 30 parts by weight, preferably from 1 to 15 parts by weight, based on 100 parts by weight of filler, in particular silica, since optimum bonding of the filler to the rubber (s) can then take place.

Furthermore, the rubber mixture according to the invention may contain customary additives in customary parts by weight. These additives include anti-aging agents, such as. N-phenyl-N '- (1,3-dimethylbutyl) -p-phenylenediamine (6PPD), N-isopropyl-N'-phenyl-p-phenylenediamine (IPPD), 2,2,4-trimethyl-1, 2-dihydroquinoline (TMQ) and other substances, such as those from J. Schnetger, Encyclopedia of Rubber Technology, 2nd edition, Hüthig Book Verlag, Heidelberg, 1991, pp. 42-48 are known activators, such. As fatty acids (eg., Stearic acid), waxes, resins and Mastikationshilfsmittel, such as. B. 2,2'-Dibenzamidodiphenyldisulfid (DBD).

The vulcanization is carried out in the presence of sulfur or sulfur donors, with some sulfur donors can also act as a vulcanization accelerator. Sulfur or sulfur donors are added in the last mixing step in the amounts customary by the person skilled in the art (0.4 to 4 phr, sulfur, preferably in amounts of 1.5 to 2.5 phr) of the rubber mixture.

Furthermore, the rubber mixture may contain vulcanization-influencing substances such as further vulcanization accelerators, vulcanization retarders and vulcanization activators in conventional amounts in order to control the required time and / or temperature of the vulcanization even better and to further improve the vulcanizate properties. The further vulcanization accelerators can be selected, for example, from the following groups of accelerators: thiazole accelerators, such as, for example, B. 2-mercaptobenzothiazole, sulfenamide accelerators such as. B. benzothiazyl-2-cyclohexylsulfenamid (CBS), thiuram accelerators such as tetrabenzylthiuram disulfide (TBzTD), guanidine accelerators such as. N, N'-diphenylguanidine (DPG) and disulfides. The accelerators can also be used in combination with each other, which can result in synergistic effects.

The preparation of the rubber mixture according to the invention is carried out in a conventional manner, wherein initially, as a rule, a base mixture containing all constituents except the Vulkanisationssystems (sulfur and vulcanization-influencing substances), is prepared in one or more mixing stages and is subsequently produced by adding the vulcanization system, the finished mixture. Subsequently, the mixture is further processed, for. B. brought by an extrusion process, and in the appropriate form. Preferably, the mixture is brought into the form of a tread. A tread blending blank produced in this way is laid in the production of the green tire, in particular the pneumatic vehicle tire blank, as known. However, the tread may also be wound onto a green tire, which already contains all the tire parts except the tread, in the form of a narrow rubber compound strip. The tires produced in this way with the mixture according to the invention have a low zinc content and are characterized by good grip and good handling.

The invention will now be explained in more detail with reference to comparative and exemplary embodiments, which are summarized in Table 1.

The comparison mixtures are with V, the mixture according to the invention are marked with E. Blend 5 (E) was adjusted to a hardness similar to Blend 1 (V) by less plasticizer.

• • Shore-A-Härte bei Raumtemperatur und 70°C gemäß DIN 53 505
• • Rückprallelastizität bei 70°C gemäß DIN 53 512
• • dynamische Speichermodul E' bei 55 und 100°C aus dynamisch-mechanischer Messung gemäß DIN 53 513 bei dynamischer Verformungsamplitude von 0,2–12% bei 0,2% Vorverformung und 10 Hz
• • Verlustfaktor tan δ bei 55 und 100° aus dynamisch-mechanischer Messung gemäß DIN 53 513 bei dynamischer Verformungsamplitude von 0,2–12% bei 0,2% Vorverformung und 10 Hz

Tabelle 1 Bestandteile Einheit 1(V) 2(V) 3(V) 4(E) 5(E) Naturkautschuk phr 5 5 5 5 5 E-SBR(ölverstreckt)^a phr 130,625 130,625 130,625 130,625 130,625 Ruß^b phr 85 85 85 85 85 Weichmacheröl phr 45 45 45 45 33 Alterungsschutzmittel phr 3,5 3,5 3,5 3,5 3,5 Ozonschutzwachs phr 0,5 0,5 0,5 0,5 0,5 Zinkoxid phr 4 - 4 - - Stearinsäure phr 0,5 0,5 0,5 0,5 0,5 ZBEC phr - - 1,0 1,0 1,0 CBS phr 2,8 2,8 2,8 2,8 2,8 Schwefel phr 1,7 1,7 1,7 1,7 1,7 Eigenschaften t₅ min 2,0 1,8 1,9 1,7 1,9 t₉₀ min 7,2 11,9 4,9 3,7 4,1 Shore-A-Härte RT Shore A 50 49 52 42 48 Shore-A-Härte 70°C Shore A 41 40 46 34 40 Rückprallelast. 70°C % 37 36 42 32 31 tan δ bei 55°C - 0,289 0,276 0,255 0,310 0,317 tan δ bei 100°C - 0,205 0,208 0,184 0,245 0,254 E' bei 55°C MPa 7,9 8,2 7,7 6,9 8,6 E' bei 100°C MPa 5,6 5,2 5,7 5,0 6,2 ^a ESBR 1739, verstreckt mit 30,625 Teilen Öl auf 100 Teile ESBR, The Dow Chemical Company, Deutschland
^b CB 2115, Columbian Chemicals Company, USAMixture preparation was carried out under standard conditions in two stages in a laboratory tangential mixer. Turnover times were reached until the relative crosslinking levels of 5 and 90% (t ₅ , t ₉₀ ) were reached via a rotorless vulcameter (MDR = Moving Disc Rheometer) according to DIN 53 529 determined. From all mixtures test specimens were prepared by optimal vulcanization under pressure at 160 ° C and determined with these specimens for the rubber industry typical material properties with the test methods given below.

• • Shore A hardness at room temperature and 70 ° C according to DIN 53 505
• • Resiliency at 70 ° C according to DIN 53 512
• • dynamic storage modulus E 'at 55 and 100 ° C from dynamic-mechanical measurement according to DIN 53 513 at dynamic deformation amplitude of 0.2-12% at 0.2% pre-deformation and 10 Hz
• • Loss factor tan δ at 55 and 100 ° from dynamic mechanical measurement according to DIN 53 513 with dynamic deformation amplitude of 0.2-12% at 0.2% pre-deformation and 10 Hz

Table 1 ingredients unit 1 (V) 2 (V) 3 (V) 4 (E) 5 (E) natural rubber phr 5 5 5 5 5 E-SBR (oil stretched) ^a phr 130.625 130.625 130.625 130.625 130.625 Carbon black ^b phr 85 85 85 85 85 processing oil phr 45 45 45 45 33 Anti-aging agents phr 3.5 3.5 3.5 3.5 3.5 Ozone protection wax phr 0.5 0.5 0.5 0.5 0.5 zinc oxide phr 4 - 4 - - stearic acid phr 0.5 0.5 0.5 0.5 0.5 ZBEC phr - - 1.0 1.0 1.0 CBS phr 2.8 2.8 2.8 2.8 2.8 sulfur phr 1.7 1.7 1.7 1.7 1.7 properties t ₅ min 2.0 1.8 1.9 1.7 1.9 t ₉₀ min 7.2 11.9 4.9 3.7 4.1 Shore A hardness RT Shore A 50 49 52 42 48 Shore A hardness 70 ° C Shore A 41 40 46 34 40 Rebounds load. 70 ° C % 37 36 42 32 31 tan δ at 55 ° C - 0,289 0.276 0,255 0,310 0.317 tan δ at 100 ° C - 0,205 0.208 0.184 0.245 0,254 E 'at 55 ° C MPa 7.9 8.2 7.7 6.9 8.6 E 'at 100 ° C MPa 5.6 5.2 5.7 5.0 6.2 ^a ESBR 1739 stretched with 30.625 parts oil to 100 parts ESBR, The Dow Chemical Company, Germany
^b CB 2115, Columbian Chemicals Company, USA

From Table 1 it can be seen that despite reduced zinc content in the mixtures 4 (E) and 5 (E) (only zinc from ZBEC in the mixture, attention should be paid to the different molecular weights of zinc oxide and ZBEC and the percentage of zinc at these) a high vulcanization rate with acceptable scorch time with good dynamic mixing properties is present. Furthermore, the mixture 5 (E), which has been set to comparable hardness, results in an increased dynamic storage modulus E 'and a significantly increased damping resulting from a low resiliency at 70 ° C. and increased tan δ values at 55 and 100 ° C read off. Especially at 100 ° C, the increase in attenuation is extremely high.

When using such a mixture 5 (E) as a tread of pneumatic vehicle tires, this means that more grip can be expected with a tendency for higher dynamic stiffness. This is advantageous both in handling and in behavior at high speeds, such as on racetracks.

Finally, the rubber composition according to the invention enables the improved production of almost zinc-free tire tread compounds which cause a high level of grip in the tire.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• WO 99/09036 [0021]

Cited non-patent literature

• W. Hofmann, H. Gupta, Handbook of Rubber Technology, dr. Gupta Verlag, Ratingen, 2001, Ch. 7, p. 28 et seq. [0003]
• ASTM D2414 [0020]
• ASTM D 3765 [0020]
• ASTM D 1510 [0020]
• DIN 66131 and 66132 [0020]
• J. Schnetger, Encyclopedia of Rubber Technology, 2nd edition, Hüthig Buch Verlag, Heidelberg, 1991, pp. 42-48 [0023]
• DIN 53 529 [0029]
• DIN 53 505 [0029]
• DIN 53 512 [0029]
• DIN 53 513 [0029]

Claims (7)

Zinc oxide-free sulfur-crosslinkable rubber compound, in particular for treads of tires, containing 80 to 100 phr of at least one synthetic diene rubber and 0.1 to 5 phr of at least one dithiocarbamate accelerator. Rubber mixture according to claim 1, characterized in that the synthetic diene rubber is a styrene-butadiene copolymer (SBR). Rubber mixture according to claim 1 or 2, characterized in that it contains as further rubber 5 to 10 phr of natural rubber. Rubber mixture according to at least one of the preceding claims, characterized in that the dithiocarbamate accelerator is a zinc dithiocarbamate. Rubber mixture according to claim 4, characterized in that the dithiocarbamate accelerator is zinc dibenzyldithiocarbamate. Rubber mixture according to at least one of the preceding claims, characterized in that it contains a maximum of 40 phr plasticizer. A tire whose tread consists, at least in part, of a sulfur vulcanized rubber composition according to at least one of claims 1 to 6.