Sulfur-linkable rubber compound and vehicle tires

Description

[0001] The invention relates to a sulfur-crosslinkable rubber coating mixture for metallic reinforcements, which contains at least one novolak resin and at least one sulfenimide accelerator. Furthermore, the invention relates to a pneumatic vehicle tire comprising at least one such sulfur-crosslinked rubber coating mixture.

[0002] For rubber lining compounds for metallic reinforcements, especially brass-plated steel cord, the use of methylene acceptor-methylene donor pairs as adhesion systems is well known. In the so-called direct adhesion process for brass-plated steel cord, the rubber lining compound contains, for example, cobalt salts and a resorcinol-formaldehyde-silica system, with the formaldehyde (methylene donor) typically coming from formaldehyde donors such as etherified melamine resins. Examples of etherified melamine resins include hexamethoxymethylmelamine (HMMM) and hexamethylenetetramine (HMT). Resorcinol and resorcinol equivalents or their precondensates, as well as other phenols, are used as methylene acceptors. During the vulcanization process, a resin is formed from the methylene donor and the methylene acceptor.In addition to the sulfur network, a second network based on methylene donor and methylene acceptor forms, which interacts adhesively with the surface of the reinforcement. Reinforcing resins are also used to further improve adhesion, and the mixtures should contain a high amount of sulfur and less accelerator to ensure sufficient mechanical interlocking with the steel cord surface.

[0003] In addition to resorcinol, special resins such as novolak resins are also known as methylene acceptors in rubber compounds.

[0004] US 6,120,911 discloses cobalt-containing rubber coating mixtures for metallic reinforcements containing a cashew nut-modified novolak resin and a sulfenimide accelerator, namely N-tert-butyl-2-benzothiazolesulfenimide (TBSI).

[0005] US 2010 / 0200141 A1 describes cobalt-containing rubber compounds for metallic reinforcements that contain N-tert-butyl-2-benzothiazole sulfenimide (TBSI) as a vulcanization accelerator.

[0006] EP 2 432 810 B1 discloses adhesion-enhancing rubber coating mixtures for rubber articles, which contain at least one novolak resin containing alkyl urethane units and produced by reacting a phenol compound, an aldehyde, and a carbamate resin, the carbamate resin being produced by reacting alkyl urethane with an aldehyde, and at least one etherified melamine resin. The mixtures are said to be characterized by good hardness, tensile strength, and adhesion, while avoiding resorcinol-based systems that are hazardous to health and the environment. The mixtures described in EP 2 432 810 B1 contain the sulfenamide accelerators N-tert-butyl-2-benzothiazolesulfenamide (TBBS) and N,N-dicyclohexyl-2-benzothiazolesulfenamide (DCBS) as vulcanization accelerators.

[0007] The invention is based on the object of providing a sulfur-curable rubber coating mixture for metallic reinforcements which is improved with regard to the adhesion properties after aging and thus leads to an improvement in the durability of the reinforced rubber products.

[0008] The object is achieved according to the invention in that the rubber mixture 0.25 to 5 phr (parts by weight based on 100 parts by weight of the total rubbers in the mixture) of at least one novolak resin having alkyl urethane units and prepared by reacting a phenol compound, an aldehyde and a carbamate resin, wherein the carbamate resin is prepared by reacting alkyl urethane with an aldehyde, and 0.5 to 3 phr of at least one sulfenimide accelerator.

[0009] The term phr (parts per hundred parts of rubber by weight) used in this document is the standard quantity used in the rubber industry for compound 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 rubbers present in the compound. The mass of all rubbers present in the compound adds up to 100.

[0010] Surprisingly, it has been shown that by combining a special novolak resin with a sulfenimide accelerator in the specified amounts, a rubber coating mixture can be obtained which has good adhesion to the reinforcement even after aging.

[0011] The use of a sulfenimide accelerator also eliminates the need for sulfenamide accelerators, some of which are considered hazardous to health.

[0012] The rubber coating mixture contains 0.25 to 5 phr, preferably 1 to 4 phr, of at least one novolak resin containing alkyl urethane units and produced by reacting a phenol compound, an aldehyde, and a carbamate resin, the carbamate resin being produced by reacting alkyl urethane with an aldehyde. Several such resins can also be used.

[0013] The novolak resin is produced by reacting a phenol compound with an aldehyde and a carbamate resin. The phenol compound can be selected from the group consisting of phenol, o-, m-, and p-cresol, and o-, m-, and p-monoalkylphenols with alkyl radicals of up to 18 carbon atoms. Preferably, the phenol compound is phenol. The aldehyde can be selected from the group consisting of formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, and isobutyraldehyde. Preferably, the aldehyde is formaldehyde.

[0014] The carbamate resin is produced by reacting an alkyl urethane with an aldehyde. The alkyl urethane can be selected from the group consisting of ethyl urethane, butyl urethane, 2-ethylhexyl urethane, and decyl urethane. Preferably, the alkyl urethane is butyl urethane.

[0015] The aldehyde for the carbamate resin can be selected from the group consisting of formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, and isobutyraldehyde. Preferably, the aldehyde is formaldehyde.

[0016] The aldehydes for the novolak resin and the carbamate resin can be the same or different aldehydes.

[0017] For the rubber coating mixture, a novolak resin is preferably used, which is made from phenol, formaldehyde and a carbamate resin made from butyl urethane and formaldehyde (butylcarbamate-functionalized phenol-formaldehyde resin).

[0018] The rubberizing mixture according to the invention contains 0.5 to 3 phr, preferably 1 to 2 phr, of at least one sulfenimide accelerator. Sulfenimide accelerators are vulcanization accelerators based on primary amines. In contrast, sulfenamide accelerators such as DCBS (N,N'-dicyclohexyl-2-benzothiazole sulfenamide) or MBS (N-oxydiethylene-2-benzothiazole sulfenamide) are based on secondary amines. One or more sulfenimide accelerators may be present in the mixture.

[0019] For particularly good aging resistance of the reinforcement-rubber adhesion even under oxygen, the sulfenimide accelerator is N-tert-butyl-2-benzothiazolesulfenimide (TBSI, IUPAC name: N,N-bis(1,3-benzothiazol-2-ylsulfanyl)-2-methylpropan-2-amine).

[0020] According to a preferred embodiment of the invention, the rubber coating mixture contains 1 to 8 phr of at least one etherified melamine resin, preferably hexamethoxymethylmelamine (HMMM) and / or hexamethylenetetramine (HMT). This can further improve the adhesion between the rubber mixture and the reinforcement. The etherified melamine resin hexamethoxymethylmelamine (HMMM) is particularly preferred. This is a common, commercially available melamine resin that forms a good resin network. HMMM is used, for example, as a technical product—often on an inert carrier—with a degree of methylation <6.

[0021] The rubberizing mixture according to the invention can contain 1 to 10 phr of resorcinol and / or resorcinol-based methylene acceptors. To avoid processing disadvantages and with regard to ecological aspects, it has proven advantageous for the rubberizing mixture to be essentially free of resorcinol. Resorcinol and resorcinol-based methylene acceptors have disadvantages with regard to occupational safety and environmental protection.

[0022] To further improve the adhesion between the reinforcement and the rubber coating, it has proven advantageous if the rubber coating mixture contains at least one organic cobalt salt. The organic cobalt salts are typically used in amounts of 0.2 to 2 phr. Examples of cobalt salts that can be used include cobalt stearate, borate, borate alkanoate, naphthenate, rhodium, octoate, adipate, etc.

[0023] The metallic reinforcements are preferably brass-plated steel cord. The brass surface, through the formation of copper sulfide dendrites, ensures good chemical and mechanical interlocking with the adjacent rubber compound.

[0024] The sulfur-curable rubber compound contains other components commonly used in the rubber industry, in particular at least one rubber.

[0025] Diene rubbers can be used as rubbers. Diene rubbers include all rubbers with an unsaturated carbon chain that are at least partially derived from conjugated dienes.

[0026] The rubberizing mixture can contain polyisoprene (IR, NR) as the diene rubber. This can be either cis-1,4-polyisoprene or 3,4-polyisoprene. However, the use of cis-1,4-polyisoprenes with a cis-1,4 content of > 90 wt.% is preferred. Such a polyisoprene can be obtained by stereospecific polymerization in solution with Ziegler-Natta catalysts or using finely divided lithium alkyls. Natural rubber (NR) is also such a cis-1,4-polyisoprene, and the cis-1,4 content in natural rubber is greater than 99 wt.%. Natural rubber is defined as rubber that can be obtained by harvesting sources such as rubber trees (Hevea brasiliensis) or non-rubber tree sources (such as guayule or dandelion (e.g. Taraxacum koksaghyz)).

[0027] If the rubber compound contains polybutadiene (BR) as the diene rubber, it can be cis-1,4-polybutadiene. The preferred material is cis-1,4-polybutadiene with a cis-1,4 content of greater than 90 wt.%, which can be produced, for example, by solution polymerization in the presence of rare earth catalysts.

[0028] Other diene rubbers that can be used include vinyl-polybutadienes and styrene-butadiene copolymers. Vinyl-polybutadienes and styrene-butadiene copolymers can be solution-polymerized (styrene)-butadiene copolymers (S-(S)BR) with a styrene content, based on the polymer, of approximately 0 to 45 wt.% and a vinyl content (content of 1,2-bonded butadiene, based on the total polymer) of 10 to 90 wt.%, which can be produced, for example, using lithium alkyls in an organic solvent. S-(S)BR can also be coupled and end-group modified. However, emulsion-polymerized styrene-butadiene copolymers (E-SBR) and blends of E-SBR and S-(S)BR can also be used. The styrene content of the E-SBR is approximately 15 to 50 wt.% and the types known from the prior art, which were obtained by copolymerization of styrene and 1,3-butadiene in aqueous emulsion, can be used.

[0029] The diene rubbers used in the mixture, especially the styrene-butadiene copolymers, can also be used in partially or fully functionalized form. The functionalization can be carried out with groups that can interact with the fillers used, especially with fillers bearing OH groups. This can, for example, involve functionalization with hydroxyl groups and / or epoxy groups and / or siloxane groups and / or amino groups and / or phthalocyanine groups and / or carboxyl groups and / or silane sulfide groups. The diene rubbers can also be coupled additionally or alternatively.

[0030] In addition to the diene rubbers mentioned, the mixture may also contain other types of rubber, such as styrene-isoprene-butadiene terpolymer, butyl rubber, halobutyl rubber or ethylene-propylene-diene rubber (EPDM).

[0031] Reclaim can also be added to the rubber compound as a processing aid and to reduce the cost of the compound.

[0032] The rubber coating mixture may contain various fillers, such as carbon black, silica, aluminosilicates, chalk, starch, magnesium oxide, titanium dioxide or rubber gels in usual quantities, whereby the fillers may be used in combination.

[0033] If carbon black is used in the rubber compound, it is preferable to use grades with a CTAB surface area (according to ASTM D 3765) of more than 30 m² / g. These are easy to mix in and ensure low heat buildup.

[0034] If silicas are included in the mixture, they can be those commonly used in tire rubber compounds. It is particularly preferred to use a finely dispersed, precipitated silica with a CTAB surface area (according to ASTM D 3765) of 30 to 350 m² / g, preferably 110 to 250 m² / g. Both conventional silicas such as type VN3 (trade name) from Evonik and highly dispersible silicas, so-called HD silicas (e.g., Ultrasil 7000 from Evonik), can be used.

[0035] If the rubber compound contains silica or other polar fillers, silane coupling agents can be added to the compound to improve processability and bond the polar filler to the rubber. The silane coupling agents react with the surface silanol groups of the silica or other polar groups during mixing of the rubber or rubber compound (in situ) or even before the filler is added to the rubber as a pretreatment (premodification). Silane coupling agents that are known to those skilled in the art for use in rubber compounds can be used.Such coupling agents known from the prior art are bifunctional organosilanes which have at least one alkoxy, cycloalkoxy or phenoxy group as a leaving group on the silicon atom and which have as another functionality a group which, if appropriate after cleavage, can enter into a chemical reaction with the double bonds of the polymer. The latter group can, for example, be the following chemical groups: -SCN, -SH, -NH 2 or -S x - (where x = 2-8). Thus, silane coupling agents which can be used include, for example, 3-mercaptopropyltriethoxysilane, 3-thiocyanatopropyltrimethoxysilane or 3,3'-bis(triethoxysilylpropyl)polysulfides having 2 to 8 sulfur atoms, such as ... B. 3,3'-bis(triethoxysilylpropyl)tetrasulfide (TESPT), the corresponding disulfide or mixtures of the sulfides with 1 to 8 sulfur atoms with different contents of the various sulfides can be used.The silane coupling agents can also be added as a mixture with industrial carbon black, such as TESPT on carbon black (trade name X50S from Evonik). Blocked mercaptosilanes, such as those known from WO 99 / 09036, can also be used as silane coupling agents. Silanes such as those described in WO 2008 / 083241 A1, WO 2008 / 083242 A1, WO 2008 / 083243 A1, and WO 2008 / 083244 A1 can also be used. Examples of suitable silanes include those marketed under the name NXT in various versions by Momentive, USA, or those marketed under the name VP Si 363 by Evonik Industries. So-called "silated core polysulfides" (SCP, polysulfides with a silylated core) can also be used, which are described, for example, in US 20080161477 A1 and EP 2 114 961 B1.

[0036] Furthermore, the rubber coating mixture according to the invention can contain conventional additives in conventional parts by weight. These additives include plasticizers, such as glycerides, factices, hydrocarbon resins, aromatic, naphthenic or paraffinic mineral oil plasticizers (e.g. MES (mild extraction solvate) or TDAE (treated distillate aromatic extract)), oils based on renewable raw materials (such as rapeseed oil, terpene oils (e.g. orange oils) or factices), so-called BTL oils (as disclosed in DE 10 2008 037714 A1) or liquid polymers (such as liquid polybutadiene)); ageing inhibitors, such as B. 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, as known for example from J. Schnetger, Lexikon der Kautschuktechnik, 2nd edition, Hüthig Buch Verlag, Heidelberg, 1991, pp. 42-48, activators, such as B.Zinc oxide and fatty acids (e.g. stearic acid), waxes, tackifying resins such as hydrocarbon resins and rosin, and masticating agents such as 2,2'-dibenzamidodiphenyl disulfide (DBD).

[0037] Vulcanization is carried out in the presence of sulfur and / or sulfur donors, some of which can also act as vulcanization accelerators. Sulfur or sulfur donors are added to the rubber compound in the final mixing step in amounts customary in the art (0.4 to 8 phr).

[0038] In addition to the sulfenimide accelerators, the rubberizing mixture may also contain other vulcanization-influencing substances, such as other vulcanization accelerators, vulcanization retarders, and vulcanization activators, in conventional amounts. Preferably, the rubberizing mixture contains less than 0.5 phr of other vulcanization accelerators in addition to the sulfenimide accelerators.

[0039] The rubber compound according to the invention is produced in a conventional manner, whereby a base compound containing all components except the vulcanization system (sulfur and vulcanization-influencing substances) is first prepared in one or more mixing stages. The final compound is then produced by adding the vulcanization system. The compound is then further processed.

[0040] The rubber compound can be used in a wide variety of rubber products that contain reinforcements. These rubber products can include drive belts, conveyor belts, hoses, rubberized fabrics, or air springs.

[0041] The rubber compound is preferably used in pneumatic vehicle tires.

[0042] The rubber compound can be used to rubberize a wide variety of tire components with metallic reinforcements, such as the bead core, bead caps, bead reinforcements, the belt, the carcass, or the belt bandages. It is also possible to coat several components within a tire with the compound according to the invention. The pneumatic vehicle tires according to the invention are manufactured using processes known to those skilled in the art.

[0043] Preferably, the rubber compound is used as a belt rubber compound, where the good adhesion values ​​between the reinforcement and the rubber compound, even under aging, lead to a long service life of the pneumatic vehicle tire.

[0044] The invention encompasses all advantageous embodiments, which are reflected, among other things, in the patent claims. In particular, the invention also encompasses embodiments that result from the combination of different features, for example, components of the rubber coating mixture, and varying degrees of preference for these features, so that a combination of a first feature designated as "preferred" or described within the context of an advantageous embodiment with another feature designated, for example, as "particularly preferred" is also encompassed by the invention.

[0045] The invention will now be explained in more detail using the table below.

[0046] Table 1 shows mixtures for rubberizing steel cord using different vulcanization accelerators (DCBS and TBSI) and different novolak resins (phenol-formaldehyde resins without and with carbamate functionalization according to the invention).

[0047] The mixture was prepared under standard conditions by producing a base mixture and then the final mixture in a laboratory tangential mixer.

[0048] Test specimens were produced from the mixtures by optimal vulcanization under pressure at 160 °C and, using these test specimens, material properties typical for the rubber industry were determined using the test methods specified below. Shore A hardness at room temperature according to DIN ISO 7619-1 Rebound resilience at room temperature according to DIN 53 512 Tensile stress at 300% elongation at room temperature according to DIN 53 504 Elongation at break at room temperature according to DIN 53504

[0049] Furthermore, adhesion tests were conducted with the compounds from Table 1 to brass-coated steel cord (1+5x0.4 HT) according to ASTM 2229 / D1871 without aging and after two days of aging in oxygen at 70 °C (vulcanization: 30 min, 150 °C, embedment length in the rubber compound: 10 mm, pull-out speed: 125 mm / min). The pull-out force and coverage were determined.

[0050] For all properties determined, the measured value of Mixture 1 was set as 100%; the values ​​of the other mixtures were related to Mixture 1. Table 1 Components Unit 1 2 3 4 5 6 Natural rubber phr 80 80 80 80 80 80 Polybutadien phr 20 20 20 20 20 20 Silica phr 60 60 60 60 60 60 Plasticizers, processing aids, silane coupling agents phr 12,65 12,65 12,65 12,65 12,65 12,65 zinc oxide phr 8 8 8 8 8 8 Anti-aging agents phr 4,6 4,6 4,6 4,6 4,6 4,6 Cobalt borate alkanoate phr 0,4 0,4 0,4 0,4 0,4 0,4 HMMM a)< phr 3,85 3,85 3,85 3,85 3,85 3,85 Resorcin phr 2,5 - - 2,5 - - Phenol-formaldehyde resin b)< phr - 2,5 - - 2,5 - butylcarbamate-functionalized phenol-formaldehyde resin c)< phr - - 2,5 - - 2,5 DCBS phr 1,5 1,5 1,5 - - - TBSI phr - - - 1,5 1,5 1,5 Sulfur (67%) phr 6,45 6,45 6,45 6,45 6,45 6,45 Characteristics Hardness at RT % 100 100 101 100 99 98 Rebound load at RT % 100 98 100 100 98 100 Tensile strength at RT % 100 98 99 107 92 91 Elongation at break % 100 119 97 93 99 101 Pull-out force (unaged) % 100 105 98 93 104 103 Pull-out force (oxygen-aged) % 100 106 97 99 104 110 Cover (unaged) % 100 96 102 101 99 100 Cover (oxygen-aged) % 100 99 100 100 100 100 a)< Hexamethoxymethylmelamine 65% on silica b)< Elaztobond ®< A250LP from SI Group, USA c)< Alnovol ®< PN 760 / Past from Allnex Netherlands BV

[0051] A significant improvement in adhesion properties was achieved with the steel cord rubber linings 6 according to the invention in Table 1, which is particularly improved after oxygen aging compared to comparative mixtures 1 to 5. This improvement only occurs when both the butylcarbamate-functionalized phenol-formaldehyde resin and the TBSI are present in the mixture. The individual measures according to mixtures 3 and 4 and the use of a different phenol-formaldehyde resin without butylcarbamate functionalization according to mixtures 2 and 5 do not lead to such an improvement.

[0052] The improvement in adhesion properties ultimately leads to better durability for rubber products with correspondingly rubberized metallic strength carriers.

Claims

1. Sulfur-crosslinkable rubberization mixture for metallic strength members containing - 0.25 to 5 phr (parts by weight, based on 100 parts by weight of the total rubbers in the mixture) of at least one novolac resin which comprises alkylurethane units and is produced by reaction of a phenolic compound, an aldehyde and a carbamate resin, wherein the carbamate resin is produced by reaction of alkylurethane with an aldehyde, and - 0.5 to 3 phr of at least one sulfenimide accelerator.

2. Sulfur-crosslinkable rubberization mixture according to Claim 1, characterized in that it contains - 1 to 4 phr of at least one novolac resin which comprises alkylurethane units and is produced by reaction of a phenolic compound, an aldehyde and a carbamate resin, wherein the carbamate resin is produced by reaction of alkylurethane with an aldehyde.

3. Sulfur-crosslinkable rubberization mixture according to Claim 1 or 2, characterized in that the phenolic compound is phenol.

4. Sulfur-crosslinkable rubberization mixture according to at least one of the preceding claims, characterized in that the alkylurethane is butylurethane.

5. Sulfur-crosslinkable rubberization mixture according to at least one of the preceding claims, characterized in that the aldehyde is formaldehyde.

6. Sulfur-crosslinkable rubberization mixture according to at least one of the preceding claims, characterized in that it contains 1 to 2 phr of at least one sulfenimide accelerator.

7. Sulfur-crosslinkable rubberization mixture according to at least one of the preceding claims, characterized in that the sulfenimide accelerator is N-tert-butyl-2-benzothiazolesulfenimide (TBSI).

8. Sulfur-crosslinkable rubberization mixture according to at least one of the preceding claims, characterized in that it contains 1 to 8 phr of at least one etherified melamine resin.

9. Sulfur-crosslinkable rubberization mixture according to at least one of the preceding claims, characterized in that it contains at least one organic cobalt salt.

10. Sulfur-crosslinkable rubberization mixture according to at least one of the preceding claims, characterized in that the metallic strength member is a brass-plated steel cord.

11. Pneumatic vehicle tyre comprising a sulfur-crosslinked rubberization mixture according to Claim 1.

12. Pneumatic vehicle tyre according to Claim 11, characterized in that it comprises a belt rubberization composed of the rubberization mixture.