DE102018205700B4 - Sulfur-curable rubber mixture and its use - Google Patents

Abstract

Sulfur-crosslinkable rubber mixture, in particular for at least the part of a tread of a pneumatic vehicle tire that comes into contact with the road, containing at least one diene rubber selected from polyisoprene, polybutadiene and / or styrene-butadiene copolymer, containing 50 to 100 phr of natural polyisoprene as diene rubber , - at least one filler, - 0.1 to 7 phr sulfur, - 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine (TBD) as at least a first vulcanization accelerator, which is a bi- or polycyclic amine base with a melting point of less than 140 °C.

Description

The invention relates to a sulfur-crosslinkable rubber mixture, in particular for at least the part of a tread of a pneumatic vehicle tire that comes into contact with the road, containing at least one diene rubber selected from polyisoprene, polybutadiene and/or styrene-butadiene, at least one filler, sulfur and at least one first vulcanization accelerator . The invention further relates to the use of the sulfur-crosslinked rubber mixture for the part of the tread of a pneumatic vehicle tire that comes into contact with the road.

Sulfur-curable rubber mixtures are usually vulcanized using vulcanization accelerators, which lead to a higher vulcanization speed and/or better physical properties of the vulcanizate. It is also common to use several accelerators in combination in order to achieve synergistic effects with regard to the desired vulcanization and vulcanization times and the desired physical properties.

A well-known class of accelerators are the guanidine accelerators, of which diphenylguanidine (DPG) and di-o-tolylguanide (DOTG, N,N'-bis(2-methylphenyl)-guanidine) are most commonly used. Guanidine accelerators are often used as secondary or second accelerators because, as activating second accelerators, they often bring about a drastic increase in the vulcanization speed and the degree of crosslinking compared to the primary accelerators. For example, it is known to use DPG in combination with other accelerators, for example from the sulfenamide class, in rubber mixtures for tire treads, whereby the vulcanization temperature is usually higher than the melting temperature of the DPG, so that the accelerator can fulfill its full function.

For rubber compounds that are not crosslinked with sulfur, such as B. acrylate rubbers or (hydrogenated) nitrile rubbers, the use of bi- or polycyclic amine bases and di-o-tolylguanide (DOTG) as accelerators is known. For acrylate rubber, the use of di-o-tolylguanide (DOTG) as an accelerator is e.g. B. in the EP 2 292 687 A1 described. Di-o-tolylguanide (DOTG) is used as an accelerator for hydrogenated nitrile rubbers, e.g. B. in the EP 2 145 921 B1 .

From the EP 3 505 366 A1 are sulfur-crosslinkable rubber mixtures which contain at least one diene rubber, selected from polyisoprene, polybutadiene and/or styrene-butadiene copolymer, at least one filler, 0.1 to 7 phr of sulfur and as a bi- or polycyclic amine base 2,3,4,6, 7,8,9,10-Octahydro-pyrimido[1,2-a]azepine (DBU) is known.

In the case of sulfur-curable rubber mixtures, the use of DPG has proven to be disadvantageous if the mixtures are vulcanized at low temperatures (< 140 °C), as the DPG with a melting point of 144 °C may not have its full acceleration effect. The resulting vulcanizates are then often too soft and have too low a cross-linking density. There is also the possibility that DPG releases aniline during processing and vulcanization, which is classified as hazardous to health and the environment.

The present invention is based on the object of providing a sulfur-crosslinkable rubber mixture which has optimal vulcanization behavior with high crosslinking density even at vulcanization temperatures of less than 140 ° C.

• - zumindest einen Dienkautschuk, ausgewählt aus Polyisopren, Polybutadien und/oder Styrol-Butadien-Copolymer, wobei sie als Dienkautschuk 50 bis 100 phr natürliches Polyisopren enthält,
• - zumindest einen Füllstoff,
• - 0,1 bis 7 phr Schwefel und
• - 1,3,4,6,7,8-Hexahydro-2H-pyrimido[1,2-a]pyrimidin (TBD) als zumindest einen ersten Vulkanisationsbeschleuniger, der eine bi- oder polycyclische aminische Base mit einem Schmelzpunkt von weniger als 140 °C ist,

enthält.The task is solved by a sulfur-crosslinkable rubber mixture, of the type mentioned at the beginning

• - at least one diene rubber selected from polyisoprene, polybutadiene and/or styrene-butadiene copolymer, containing 50 to 100 phr of natural polyisoprene as diene rubber,
• - at least one filler,
• - 0.1 to 7 phr sulfur and
• - 1,3,4,6,7,8-Hexahydro-2H-pyrimido[1,2-a]pyrimidine (TBD) as at least a first vulcanization accelerator which is a bi- or polycyclic amine base with a melting point of less than 140 °C is,

contains.

It has been found that sulfur-vulcanizable rubber mixtures with aminic bases of this type can be crosslinked well even at vulcanization temperatures of less than 140 ° C, achieving a high crosslinking density. The vulcanizates obtained have the desired stiffness and can be vulcanized with suitable vulcanization and vulcanization times. This could be due to the fact that the vulcanization accelerators melt during vulcanization and can thus develop their optimal effect. They are then no longer present as discrete particles in the rubber mixture or adhered to the filler. According to the invention, the rubber mixture can contain one or more bi- or polycyclic amine bases as vulcanization accelerators.

1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine (TBD) is used as the bicyclic amine base. It has a melting point of approximately 128 °C and is therefore particularly suitable for vulcanization at temperatures of less than 140 °C. Good vulcanizates were obtained with this substance and it also has the advantage that it does not release aniline, which is harmful to the environment or health, when heated. The substances are easy to produce and show good acceleration behavior.

According to a preferred development of the invention, the at least one first vulcanization accelerator with a melting point of less than 140 ° C is contained in the rubber mixture in amounts of 0.1 to 5 phr, preferably 0.1 to 3 phr. With these amounts, particularly good results are achieved with regard to the vulcanization speed and the vulcanizate properties.

The specification phr (parts per hundred parts of rubber by weight) used in this document is the usual quantity specification for mixture recipes in the rubber industry. In this document, the dosage of the parts by weight of the individual substances is based on 100 parts by weight of the total mass of all high-molecular and therefore solid diene rubbers present in the mixture.

In order to positively influence the vulcanizate properties with regard to the physical properties of Shore hardness, rebound elasticity, tensile strength and elongation at break and at the same time to improve the vulcanization kinetics in such a way that good processability is ensured, the rubber mixture contains at least a second vulcanization accelerator, which is selected from the group of dithiocarbamates , the thiuram accelerator, the mercapto accelerator and/or the sulfenamide accelerator.

In order to achieve a particularly high level of processing reliability in the rubber mixture, the bi- or polycyclic amine base and the second vulcanization accelerator from the group of dithiocarbamates, the thiuram accelerators, the mercapto accelerators and/or the sulfenamide accelerators are in a molar ratio of 1:4 to 4: 1 in front.

The rubber mixture according to the invention contains at least one diene rubber selected from natural polyisoprene, polybutadiene and/or styrene-butadiene copolymer. Accordingly, several diene rubbers of this type can also be used in the blend.

The natural and/or synthetic polyisoprene can be both cis-1,4-polyisoprene and 3,4-polyisoprene. However, the use of cis-1,4-polyisoprene with a cis 1.4 proportion > 90% by weight is preferred. On the one hand, such a polyisoprene can be obtained by stereospecific polymerization in solution with Ziegler-Natta catalysts or using finely divided lithium alkyls. On the other hand, natural rubber (NR) is a cis-1,4 polyisoprene; the cis-1,4 content in natural rubber is greater than 99% by weight.

Furthermore, a mixture of one or more natural polyisoprenes with one or more synthetic polyisoprenes is also conceivable.

The styrene-butadiene rubber (styrene-butadiene copolymer) can be both solution-polymerized styrene-butadiene rubber (SSBR) and emulsion-polymerized styrene-butadiene rubber (ESBR), including a mixture of at least one SSBR and at least one ESBR can be used. The terms “styrene-butadiene rubber” and “styrene-butadiene copolymer” are used synonymously in the context of the present invention. In any case, preference is given to styrene-butadiene copolymers with an M _w of 250,000 to 600,000 g/mol (two hundred fifty thousand to six hundred thousand grams per mol).

The styrene-butadiene copolymer(s) used can be end group modified with modifications and functionalizations and/or functionalized along the polymer chains. At the Modification can be those with hydroxy groups and/or ethoxy groups and/or epoxy groups and/or siloxane groups and/or amino groups and/or aminosiloxane and/or carboxy groups and/or phthalocyanine groups and/ or silane sulfide groups. However, other modifications known to those skilled in the art, also referred to as functionalizations, are also possible. Metal atoms can be part of such functionalizations.

The butadiene rubber (BR, polybutadiene) can be any type known to those skilled in the art with an M _w of 250,000 to 5,000,000 g/mol. These include, among others, the so-called high-cis and low-cis types, with polybutadiene with a cis content greater than or equal to 90% by weight being the high-cis type and polybutadiene with a cis content smaller than 90% by weight. % is referred to as low-cis type. A low-cis polybutadiene is e.g. B. Li-BR (lithium-catalyzed butadiene rubber) with a cis content of 20 to 50% by weight. With a high-cis BR, particularly good abrasion properties and low hysteresis of the rubber mixture are achieved.

The polybutadiene used can also be end-group modified and/or functionalized along the polymer chains with the modifications and functionalizations mentioned above for the styrene-butadiene rubber. According to the invention, liquid polybutadienes do not fall under the term solid butadiene rubbers.

The rubber mixture contains 50 to 100 phr of natural polyisoprene as diene rubber. Such mixtures are often vulcanized at temperatures of less than 140 °C, so that the accelerating effect of the bi- or polycyclic amine base with a melting point of less than 140 °C is particularly effective here.

The sulfur-crosslinkable rubber mixture also contains at least one filler. Different fillers can be present individually or in combination in the mixture.

The filler can z. B. be soot. All types of soot known to those skilled in the art are conceivable. However, preference is given to using a carbon black which has an iodine adsorption number according to ASTM D 1510 of 30 to 180 g/kg, preferably 30 to 130 kg/g, and a DBP number according to ASTM D 2414 of 80 to 200 ml/100 g, preferably 100 up to 200 m1/100g, particularly preferably 100 to 180 m1/100g. This achieves particularly good rolling resistance indicators (rebound elasticity at 70 °C) with good other tire properties for use in vehicle tires.

Known polar and/or non-polar fillers such as silica, aluminosilicates, chalk, starch, magnesium oxide, titanium dioxide or rubber gels can be used as further fillers. Furthermore, carbon nanotubes (carbon nanotubes (CNT) including discrete CNTs, so-called hollow carbon fibers (HCF) and modified CNT containing one or more functional groups, such as hydroxy, carboxy and carbonyl groups) are conceivable.

Graphite and graphene as well as so-called “carbon-silica dual-phase fillers” are also conceivable as fillers.

Zinc oxide is not one of the fillers in the context of the present invention.

According to a preferred development of the invention, the rubber mixture contains 40 to 200 phr of at least one silica. Such mixtures can be vulcanized particularly well and easily with the special vulcanization accelerator. The low melting point probably prevents deposits on the silica and thus limited effectiveness.

According to the invention, a wide variety of silicas, such as “low surface area” or highly dispersible silica, can also be used in a mixture. Silicas with a CTAB surface area (according to ASTM D 3765) of 50 to 180 m ² /g and a nitrogen surface area (BET surface area) (according to DIN ISO 9277 and DIN 66132 ) from 60 to 190 m ² /g used.

• Es ist bekannt, in kieselsäurehaltigen Mischungen zur Verbesserung der Verarbeitbarkeit und zur Anbindung des polaren Füllstoffes an den Kautschuk Silan-Kupplungsagenzien zuzusetzen. Die Silan-Kupplungsagenzien reagieren mit den oberflächlichen Silanolgruppen der Kieselsäure oder anderen polaren Gruppen während des Mischens des Kautschuks bzw. der Kautschukmischung (in situ) oder bereits vor der Zugabe des Füllstoffes zum Kautschuk im Sinne einer Vorbehandlung (Vormodifizierung). Derartige Silan-Kupplungsagenzien sind bifunktionelle Organosilane, die am Siliciumatom mindestens eine Alkoxy-, Cycloalkoxy- oder Phenoxygruppe als Abgangsgruppe besitzen und die als andere Funktionalität eine Gruppe aufweisen, die gegebenenfalls nach Spaltung eine chemische Reaktion mit den Doppelbindungen des Polymers eingehen kann. Bei der letztgenannten Gruppe kann es sich z. B. um die folgenden chemischen Gruppen handeln: - SCN, -SH, -NH2 oder -Sx- (mit x = 2-8). So können als Silan-Kupplungsagenzien z. B. 3-Mercaptopropyltriethoxysilan, 3-Thiocyanato-propyltrimethoxysilan oder 3,3'-Bis(triethoxysilylpropyl)polysulfide mit 2 bis 8 Schwefelatomen, wie z. B. 3,3'-Bis(triethoxysilylpropyl)tetrasulfid (TESPT), das entsprechende Disulfid (TESPD) oder auch Gemische aus den Sulfiden mit 1 bis 8 Schwefelatomen mit unterschiedlichen Gehalten an den verschiedenen Sulfiden, verwendet werden. Die Silan-Kupplungsagenzien können dabei auch als Gemisch mit Industrieruß zugesetzt werden, wie z. B. TESPT auf Ruß (Handelsname X50S der Firma Evonik). Auch geblockte Mercaptosilane, wie sie z. B. aus der WO 99/09036 bekannt sind, können als Silan-Kupplungsagens eingesetzt werden. Auch Silane, wie sie in der WO 2008/083241 A1 , der WO 2008/083242 A1 , der WO 2008/083243 A1 und der WO 2008/083244 A1 beschrieben sind, können eingesetzt werden. Verwendbar sind z. B. Silane, die unter dem Namen NXT^® in verschiedenen Varianten von der Firma Momentive, USA, oder solche, die unter dem Namen VP Si 363 von der Firma Evonik Industries vertrieben werden. Falls die erfindungsgemäße Kautschukmischung ein Silan-Kupplungsagens enthält, handelt es sich vorzugsweise um ein geblocktes und/oder ungeblocktes Mercaptosilan, besonders bevorzugt um 3-Octanoylthio-1-propyltriethoxysilan.

In addition to the components essential to the invention that have already been explained, the rubber mixture can contain other components that are common in the rubber industry. Such components are set out below:

• It is known to add silane coupling agents to mixtures containing silica to improve processability and to bind the polar filler to the rubber. The Silane Kupp Lung agents react with the surface silanol groups of the silica or other polar groups during the mixing of the rubber or the rubber mixture (in situ) or before the filler is added to the rubber in the sense of a pre-treatment (pre-modification). Such silane coupling agents are bifunctional organosilanes which have at least one alkoxy, cycloalkoxy or phenoxy group on the silicon atom as a leaving group and which, as other functionality, have a group which, after cleavage, can optionally enter into a chemical reaction with the double bonds of the polymer. The latter group can be e.g. B. are the following chemical groups: - SCN, -SH, -NH2 or -Sx- (with x = 2-8). For example, silane coupling agents can be used: B. 3-mercaptopropyltriethoxysilane, 3-thiocyanato-propyltrimethoxysilane or 3,3'-bis (triethoxysilylpropyl) polysulfides with 2 to 8 sulfur atoms, such as. B. 3,3'-bis(triethoxysilylpropyl)tetrasulfide (TESPT), the corresponding disulfide (TESPD) or mixtures of the sulfides with 1 to 8 sulfur atoms with different contents of the different sulfides can be used. The silane coupling agents can also be added as a mixture with industrial carbon black, such as. B. TESPT on carbon black (trade name X50S from Evonik). Blocked mercaptosilanes, such as those found e.g. B. from the WO 99/09036 are known, can be used as a silane coupling agent. Also silanes, as found in the WO 2008/083241 A1 , the WO 2008/083242 A1 , the WO 2008/083243 A1 and the WO 2008/083244 A1 described can be used. Can be used, for example: B. Silanes, which are sold under the name ^NXT® in various versions by the company Momentive, USA, or those which are sold under the name VP Si 363 by the company Evonik Industries. If the rubber mixture according to the invention contains a silane coupling agent, it is preferably a blocked and/or unblocked mercaptosilane, particularly preferably 3-octanoylthio-1-propyltriethoxysilane.

Plasticizers such as aromatic, naphthenic or paraffinic mineral oil plasticizers, such as MES (mild extraction solvate) or RAE (residual aromatic extract) or TDAE (treated distillate aromatic extract), or phosphoric acid esters or rubber-to-liquid oils (RTL) can be used in the rubber mixture. or biomass-to-liquid oils (BTL) are preferably used with a polycyclic aromatic content of less than 3% by weight according to method IP 346 or rapeseed oil or facts or plasticizer resins.

1. a) Alterungsschutzmittel, wie z. B. N-Phenyl-N'-(1,3-dimethylbutyl)-p-phenylendiamin (6PPD), N,N'-Diphenyl-p-phenylendiamin (DPPD), N,N'-Ditolyl-p-phenylendiamin (DTPD), N-Isopropyl-N'-phenyl-p-phenylendiamin (IPPD), 2,2,4-Trimethyl-1,2-dihydrochinolin (TMQ),
2. b) Aktivatoren, wie z. B. Zinkoxid und Fettsäuren (z. B. Stearinsäure) oder Zinkkomplexe wie z. B. Zinkethylhexanoat,
3. c) Wachse,
4. d) Mastikationshilfsmittel, wie z. B. 2,2'-Dibenzamidodiphenyldisulfid (DBD) und
5. e) Verarbeitungshilfsmittel, wie z. B. Fettsäuresalze, wie z. B. Zinkseifen, und Fettsäureester und deren Derivate.

Furthermore, the rubber mixture can contain customary additives in customary parts by weight, which are preferably added in at least one basic mixing stage during its production. These additives include

1. a) Anti-aging agents, such as E.g. N-phenyl-N'-(1,3-dimethylbutyl)-p-phenylenediamine (6PPD), N,N'-diphenyl-p-phenylenediamine (DPPD), N,N'-ditolyl-p-phenylenediamine (DTPD ), N-isopropyl-N'-phenyl-p-phenylenediamine (IPPD), 2,2,4-trimethyl-1,2-dihydroquinoline (TMQ),
2. b) Activators, such as B. zinc oxide and fatty acids (e.g. stearic acid) or zinc complexes such as. B. zinc ethyl hexanoate,
3. c) waxes,
4. d) mastication aids, such as B. 2,2'-dibenzamidophenyl disulfide (DBD) and
5. e) processing aids, such as B. fatty acid salts, such as. B. zinc soaps, and fatty acid esters and their derivatives.

The vulcanization of the sulfur-crosslinkable rubber mixture according to the invention is carried out in the presence of sulfur and/or sulfur donors, whereby some vulcanization accelerators can also act as sulfur donors. The rubber mixture contains 0.1 to 7 phr of sulfur to achieve optimal crosslinking results.

The rubber mixture may also contain vulcanization retarders.

The sulfur-crosslinkable rubber mixture according to the invention is produced according to the process common in the rubber industry, in which a basic mixture with all components except the vulcanization system (sulfur and substances influencing vulcanization) is first produced in one or more mixing stages. The finished mixture is created by adding the vulcanization system in a final mixing stage. The ready mix is z. B. further processed by an extrusion process and brought into the appropriate shape. Further processing then takes place by vulcanization, with sulfur crosslinking taking place due to the vulcanization system added in the context of the present invention.

The rubber mixture can be used for a wide variety of rubber items, such as solid rubber tires, bellows, conveyor belts, air springs, belts, straps, hoses or shoe soles.

However, the rubber mixture is preferably used in pneumatic vehicle tires, with at least the part of the tread that comes into contact with the road having the rubber mixture crosslinked with sulfur. Such a pneumatic vehicle tire can be vulcanized easily and safely - even at lower temperatures.

For use as a tread in pneumatic vehicle tires, the mixture is brought into the form of a tread, preferably at least into the form of a tread cap, as a ready-made mixture before vulcanization and placed in the known manner during the production of the vehicle tire blank. The tread, preferably at least the tread cap, can also be wound onto a blank tire in the form of a narrow strip of rubber mixture.

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

The comparison mixture is marked with V, the mixtures according to the invention are marked with E. The comparison mixture contains diphenylguanidine (DPG) with a melting point of 144 °C as a vulcanization accelerator. The mixtures according to the invention contain 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine (TBD) with a melting point of 128 ° C as a vulcanization accelerator.

The mixture was produced according to the procedures common in the rubber industry under usual conditions in two stages in a laboratory mixer in which all components except the vulcanization system (sulfur and substances influencing vulcanization) were mixed in the first mixing stage (basic mixing stage). The finished mixture was produced by adding the vulcanization system in the second stage (finished mixing stage), with mixing at 90 to 120 °C.

The mixtures were vulcanized at 160 ° C for 20 minutes and the relative degree of crosslinking of 5% or 10% (t ₅ , t ₁₀ vulcanization time) and 90% (t ₉₀ , vulcanization time) was determined using a rotorless vulcanometer (MDR = Moving Disc Rheometer). DIN 53 529 determined. Table 1 components Unit 1(V) 2(E) 3(E) Natural rubber phr 100 100 100 Zinc oxide phr 1 1 1 Stearic acid phr 1 1 1 DPG ^a) phr 1.5 - - TBD ^b) phr - 1 1.5 sulfur phr 1.2 1.2 1.2 Characteristics _t5 min 0.59 0.76 0.48 _t10 min 0.75 1.09 0.60 _t90 min 16.15 18.79 11.44
^a) Diphenylguanidine, M _w = 211 g/mol, mp = 144 °C
^b) 1,3,4,6,7,8-Hexahydro-2H-pyrimido[1,2-a]pyrimidine, M _w = 139 g/mol, mp = 128 °C

Table 1 shows that vulcanization can be accelerated with TBD. Since the melting point is lower than 140 °C, TBD can also be used at low vulcanization temperatures, such as B. can be used in natural rubber mixtures. It is also advantageous that TBD does not release any environmentally hazardous aniline.

Claims (6)

Sulfur-crosslinkable rubber mixture, in particular for at least the part of a tread of a pneumatic vehicle tire that comes into contact with the road - at least one diene rubber selected from polyisoprene, polybutadiene and/or styrene-butadiene copolymer, containing 50 to 100 phr of natural polyisoprene as diene rubber, - at least one filler, - 0.1 to 7 phr sulfur, - 1,3,4,6,7,8-Hexahydro-2H-pyrimido[1,2-a]pyrimidine (TBD) as at least a first vulcanization accelerator which is a bi- or polycyclic amine base with a melting point of less than 140 °C is. Sulfur-curable rubber mixture Claim 1 , characterized in that the at least one first vulcanization accelerator with a melting point of less than 140 ° C is contained in the rubber mixture in amounts of 0.1 to 5 phr, preferably 0.1 to 3 phr. Sulfur-curable rubber mixture Claim 1 or 2 , characterized in that it contains at least one second vulcanization accelerator, which is selected from the group of dithiocarbamates, thiuram accelerators, mercapto accelerators and/or sulfenamide accelerators. Sulfur-crosslinkable rubber mixture according to at least one of the preceding claims, characterized in that the bi- or polycyclic amine base and the second vulcanization accelerator are selected from the group of dithiocarbamates, thiuram accelerators, mercapto accelerators and/or sulfenamide accelerators in a molar ratio of 1:4 to 4 :1 available. Sulfur-crosslinkable rubber mixture according to at least one of the preceding claims, characterized in that it contains 40 to 200 phr of at least one silica. Use of a sulfur-crosslinked rubber mixture according to one of the Claims 1 until 5 for the part of the tread of a pneumatic vehicle tire that comes into contact with the road.