BACKGROUND OF THE INVENTION

Field of the Invention

[0001] This invention relates generally to rubber compositions and more particularly, to those rubber compositions useful in tires and other articles made from rubber.

Description of the Related Art

[0002] The manufacturing of certain rubber articles, including rubber components that may be found in tires, require a rubber composition that has a high rigidity or a modulus MA at lower strain such as 10%, MA10. Tires and other articles that are made of rubber are manufactured from rubber compositions that include rubber, e.g., natural rubber, synthetic rubber or combinations thereof, as well as fillers, plasticizers, vulcanizing agents and other chemicals that improve the physical characteristics of the cured rubber composition.

[0003] Those skilled in the art understand that there are different ways of increasing a rubber composition's rigidity such as the addition of a reinforcing filler or a reinforcing resin. Reinforcing fillers are well known, examples of which include carbon black and silica. Reinforcing resins are also well known and typically include a methylene donor and a methylene acceptor.

[0004] In general, resins are typically (but not always) nonvolatile, solid organic substances that are produced naturally by plants or synthetically from petrochemicals or other sources of hydrocarbon materials. As used in rubber compositions, resins may be classified as either reinforcing resins or as plasticizing resins. Plasticizing resins are added to a rubber composition to improve the plasticity or workability of a rubber composition.

[0005] As noted, reinforcing resins are added to a rubber composition to increase the rigidity of the cured rubber composition. These reinforcing resins intermix with the rubber polymer chains and, when reacted with a linking agent or with each other, form a three- dimensional network that improves the physical characteristics of the cured rubber composition. Many of these resins are classified as being methylene acceptor/donor systems that react together to generate a three-dimensional reinforcing resin network by a condensation reaction.

SUMMARY OF THE INVENTION

[0006] Particular embodiments of the present invention include rubber compositions having a particular rigidity booster and articles made therefrom. Such embodiments include rubber compositions that are based upon cross -linkable rubber compositions, the cross- linkable rubber compositions comprising, in parts by weight per 100 parts by weight of rubber (phr) a highly unsaturated diene rubber having a content of units of diene origin that is greater than 50 mol% and a reinforcing filler.

[0007] In addition, such rubber compositions include a methylene acceptor and a methylene donor that is a methylene donor/methylene acceptor reinforcing resin system and the rigidity booster that has a structure as follows:

o

(R C O^ M ^+N where R is a saturated or unsaturated aliphatic moiety having a chain length of between CIO and C30, and M ⁺ⁿ is a metallic cation selected from K ⁺ , Na ⁺ , Zn ⁺⁺ , Ca ⁺⁺ , Mg ⁺⁺ or combinations thereof, and n is an integer 1 or 2. In particular embodiments, the rubber composition further includes no cobalt compounds. It should be noted that when n is greater than 1, R may be the same or different in each of the n-moieties bonded to the metallic cation.

[0008] The foregoing and other objects, features and advantages of the invention will be apparent from the following more detailed descriptions of particular embodiments of the invention.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

[0009] Particular embodiments of the present invention include rubber compositions and rubber articles manufactured from such rubber compositions. It has been found that adding a rigidity booster to the rubber compositions described below significantly enhances its rigidity when such rubber compositions further include a methylene acceptor/methylene donor resin system. The rubber compositions that include the rigidity booster are particularly useful for rubber articles that benefit from being formed from a high-rigidity rubber composition.

[0010] Examples of such articles include, inter alia, tire components, hoses, motor mounts and conveyor belts. Tire components may include, for example, those components found in the bead area, e.g., the apex or bead filler and toe guard and also in the tire tread, including retread rubber useful for retreading a tire. Particular embodiments may be limited to bead area tire components, namely the apex, the bead filler, the toe guard or may be limited to just one or more of these components.

[0011] Surprisingly the use of the rigidity booster in the rubber formulations provides an exceptional increase to the rigidity of the rubber composition without impacting the processability of the uncured rubber composition or the cured cohesiveness. Thus the rigidity booster has broken the compromise that typically occurs with an increase in a rubber composition's rigidity.

[0012] It is known by those skilled in the arts to use cobalt compounds, such as cobalt stearate, to increase the adhesion of rubber compositions to metallic components of a tire. The rubber compositions disclosed herein are not meant for use as coatings of metal cables, as are found in tire belts and cords. Therefore particular embodiments of the rubber compositions disclosed herein particularly exclude cobalt compounds such as cobalt stearate and other cobalt compounds from being included in such rubber compositions.

[0013] As used herein, "diene elastomer" and "rubber" are synonymous terms and may be used interchangeably.

[0014] As used herein, a "non-productive" mix includes many of the components of a rubber composition but includes no vulcanization agents or primary accelerators. A "productive" mix results after the vulcanization agents and any primary accelerators are added to the non-productive mix.

[0015] The quantities of components added to the rubber compositions disclosed herein are expressed in terms of parts by weight of the component per hundred parts by weight of the rubber in the rubber composition, which is commonly expressed as parts per hundred parts of rubber, phr. [0016] As used herein, "based upon" is a term recognizing that embodiments of the present invention are made of vulcanized or cured rubber compositions that were, at the time of their assembly, uncured. The cured rubber composition is therefore "based upon" the uncured rubber composition. In other words, the cross-linked rubber composition is based upon or comprises the constituents of the cross -linkable rubber composition.

[0017] As is generally known, a tire tread includes the road-contacting portion of a vehicle tire that extends circumferentially about the tire. It is designed to provide the handling characteristics required by the vehicle; e.g., traction, dry braking, wet braking, cornering and so forth - all preferably being provided with a minimum amount of generated noise and at low rolling resistance.

[0018] Treads of the type disclosed herein include tread elements, the structural features of the tread that contact the ground. Such structural features may be of any type or shape, examples of which include tread blocks and tread ribs. Tread blocks have a perimeter defined by one or more grooves that create an isolated structure in the tread while a rib runs substantially in the longitudinal (circumferential) direction and is not interrupted by grooves that run in the substantially lateral (axial) direction or any other grooves that are oblique thereto. The radial (depth) direction is perpendicular to the lateral direction.

[0019] It is recognized that treads may be formed from only one rubber composition or in two or more layers of differing rubber compositions, e.g., a cap and base construction. In a cap and base construction, the cap portion of the tread is made of one rubber composition that is designed for contact with the road. The cap is supported on the base portion of the tread, the base portion made of different rubber composition. In particular embodiments of the present invention the entire tread may be made from the rubber compositions disclosed herein while in other embodiments only the cap portion or at least a part of the cap portion of the tread may be made from such rubber compositions or only the base may be made from such rubber compositions.

[0020] The bead section of a tire includes the portion having the beads. The beads are typically nonextensible steel wire hoops that anchor the tire plies and also provide the structure for locking the tire onto the wheel assembly so that it doesn't slip or turn on the wheel rim. [0021] The other components in the bead section include the bead apex or the bead filler. The bead apex is typically of triangular shape and is adjacent to the bead so that it can provide a cushion between the bead and the other tire components such as the inner liner and the tire plies. The toe guard is also in the bead section of the tire and forms the radially innermost portion of the bead section, mounting against the wheel rim.

[0022] The rubber compositions disclosed herein include a highly unsaturated diene elastomer. Diene elastomers or rubber is understood to mean those elastomers resulting at least in part (i.e., a homopolymer or a copolymer) from diene monomers (monomers bearing two double carbon-carbon bonds, whether conjugated or not). Essentially unsaturated diene elastomers are understood to mean those diene elastomers that result at least in part from conjugated diene monomers, having a content of members or units of diene origin (conjugated dienes) that are greater than 15 mol.%.

[0023] Thus, for example, diene elastomers such as butyl rubbers, nitrile rubbers or copolymers of dienes and of alpha-olefins of the ethylene-propylene diene terpolymer (EPDM) type or the ethylene-vinyl acetate copolymer type, do not fall within the preceding definition and may in particular be described as "essentially saturated" diene elastomers (low or very low content of units of diene origin, i.e., less than 15 mol. %). Particular embodiments of the rubber compositions disclosed herein may include no essentially saturated diene elastomers.

[0024] Within the category of essentially unsaturated diene elastomers are the highly unsaturated diene elastomers, which are more particularly understood to mean diene elastomers having a content of units of diene origin (conjugated dienes) that is greater than 50 mol%. Particular embodiments of the rubber compositions disclosed herein may include not only no essentially saturated diene elastomers but also no essentially unsaturated diene elastomers that are not highly unsaturated.

[0025] Examples of highly unsaturated diene rubber components that may be used in particular embodiments of the rubber compositions disclosed herein include polybutadienes (BR), polyisoprenes (IR), natural rubber (NR), butadiene copolymers, isoprene copolymers and mixtures of these elastomers. The polyisoprenes include synthetic cis-\,A polyisoprene, which may be characterized as having greater than 90 mol% cis-1,4 bonds or alternatively greater than 98 mol% cis-1,4 bonds.

[0026] Other suitable examples include copolymers such as butadiene- styrene copolymers (SBR), butadiene-isoprene copolymers (BIR), isoprene-styrene copolymers (SIR) and isoprene-butadiene-styrene copolymers (SBIR) and mixtures thereof.

[0027] It should be noted that any of the highly unsaturated elastomers may be utilized in particular embodiments as a functionalized elastomer. Elastomers can be functionalized by reacting them with suitable functionalizing agents prior to or in lieu of terminating the elastomer. Exemplary functionalizing agents include, but are not limited to, metal halides, metalloid halides, alkoxysilanes, imine-containing compounds, esters, ester- carboxylate metal complexes, alkyl ester carboxylate metal complexes, aldehydes or ketones, amides, isocyanates, isothiocyanates, imines, and epoxides. These types of functionalized elastomers are known to those of ordinary skill in the art. While particular embodiments may include one or more of these functionalized elastomers solely as the rubber component, other embodiments may include one or more of these functionalized elastomers mixed with one or more of the non-functionalized highly unsaturated elastomers.

[0028] As noted above, the rubber compositions disclosed herein include a rigidity booster. Therefore in addition to the rubber components described above, the rubber compositions further include the rigidity booster that may be described as having the following structure:

where R is a saturated or unsaturated aliphatic moiety having a chain length of between CIO and C30 and M ⁺ⁿ is a metallic cation selected from K ⁺ , Na ⁺ , Zn ⁺⁺ , Ca ⁺⁺ , Mg ⁺⁺ or combinations thereof, and n is an integer 1 or 2. It should be noted that when n is greater than 1, R may be the same or different in each of the n-moieties bonded to the metallic cation.

[0029] These rigidity boosters may be described as being fatty acid salts (or soaps).

[0030] Fatty acid salts are well known and may be, for example, the result of a reaction between a long chain fatty acid and a metal hydroxide such as NaOH, KOH, Mg(OH) ₂ , Ca(OH) ₂ and Zn(OH) ₂ . Particular embodiments include fatty acid salts having the metallic cation selected from K ⁺ , Na ⁺ , Ca ^"1"1" , Mg ⁺⁺ and Zn** and combinations thereof or alternatively from K ⁺ and Na ⁺ and combinations thereof. Particular embodiments may be limited to just any one of these metallic cations singly or alternatively to any particular combination of two or more. Non-limiting examples of useful fatty acids for forming such salts include palmitic acid CH ₃ (CH ₂ )i ₄ COOH, myristic acid CH ₃ (CH ₂ )i ₂ COOH, lauric acid CH ₃ (CH ₂ )i ₀ COOH, stearic acid CH ₃ (CH ₂ )i ₆ COOH, oleic acid CH ₃ (CH ₂ ) ₇ CH=CH(CH ₂ ) ₇ COOH, linoleic acid CH ₃ (CH ₂ ) ₄ CH=CHCH ₂ CH=CH(CH ₂ ) ₇ C0 ₂ H, linolenic acid CH ₃ (CH ₂ CH=CH) ₃ (CH ₂ ) ₇ C0 ₂ H, ricinoleic acid CH ₃ (CH ₂ ) ₅ CH(OH)CH ₂ CH=CH(CH ₂ ) ₇ COOH, octanoic acid CH ₃ (CH ₂ ) ₆ COOH and undecylenic acid CH ₂ =CH(CH ₂ ) ₈ COOH.

[0031] Examples of such fatty acid salts include potassium oleate (CAS 143-18-0), sodium undecylenate (CAS 3398-33-2), potassium undecylenate (CAS 6159-41-7), potassium octanoate (CAS 764-71-6), sodium oleate (CAS 143-19-1), sodium palmitate (CAS 408-35-5), sodium laurate (CAS 629-25-4), sodium linoleate (CAS 822-17-3) and sodium stearate (CAS 822-16-2), zinc stearate (CAS 557-05-1), zinc oleate (CAS 557-07-3), zinc undecylenate (CAS 557-08-4), C16-C18 zinc salts (CAS 91051-01-3), C16-C18 unsaturated zinc salt (CAS 68188- 86-3), to name a few.

[0032] The rigidity booster may be added to the rubber compositions disclosed herein as mixtures of the fatty acid salts.

[0033] The rigidity booster may be provided in amounts necessary to obtain the desired rigidity boost. In particular embodiments, the rubber compositions may include between 1 phr and 15 phr of the rigidity booster or alternatively between 1 phr and 10 phr, between 3 phr and 8 phr, between 3 phr and 10 phr, between 3 phr and 15 phr or between 15 phr and 20 phr of the rigidity booster. In particular embodiments the amount added is an amount that increases the MA10 in an identical rubber composition without the rigidity booster by at least 10 % or alternatively by at least 30 %, at least 50 %, at least 100 % or at least 150 %. In other embodiments the amount of rigidity booster is an amount that maintains an identical MA 10 compared to an identical rubber composition that lacks the rigidity booster but has more reinforcement filler, such as carbon black or silica. The benefit of reducing the filler is that it provides lower hysteresis and therefore improved rolling resistance in a tire. [0034] In addition to the rubber component and the rigidity booster as described above, the rubber compositions disclosed herein further includes a methylene acceptor/methylene donor reinforcing resin system. As was noted above, such reinforcing resin systems increase the rigidity of a rubber composition to levels greater than those that may typically be achieved with just a reinforcing filler. Such resin systems include both a methylene acceptor and a methylene donor that together cross-link to provide a three- dimensional reinforcing network within the rubber composition.

[0035] Examples of useful methylene acceptors include phenols, the generic name for hydroxylated derivatives of benzene and equivalent compounds. This definition covers in particular monophenols, for example phenol or hydroxybenzene, bisphenols, polyphenols (polyhydroxyarenes), substituted phenols such as alkylphenols or aralkylphenols, for example bisphenols, diphenylolpropane, diphenylolmethane, naphthols, cresol, t-butylphenol, octylphenol, nonylphenol, xylenol, resorcinol or analogous products and 3-hydroxydiphenyl- amine (3-HDPA) and/or 4-hydroxydiphenylamine (4-HDPA).

[0036] Useful methylene acceptors for particular embodiments are the novolac resins. These resins are phenol- aldehyde pre-condensates resulting from the condensation of phenolic compounds and aldehydes, for example formaldehyde. Novolac resins (also referred to as "two-step resins") require the use of a methylene donor as a curing agent to crosslink the novolac resins in the rubber composition just as do the other methylene acceptors. Such crosslinking thereby creates the three dimensional resin networks. Such curing normally takes place above 100 °C. An example of a suitable novolac resin is available from the SI Group with offices in Schenectady, NY under the product name HRJ- 12952. This novolac resin has a density of 1.25 g/cm and a melting point of 100 °C with less than 1% of unreacted phenol.

[0037] Particular embodiments of the rubber compositions disclosed herein include a methylene acceptor that is selected from a novolac resins, diphenylolmethane, diphenylolethane, diphenylolpropane, diphenylolbutane, a naphthol, a cresol or combinations thereof. Any of the methylene acceptors disclosed herein or that are otherwise known to those skilled in the art to be suitable for the purpose may be used in particular embodiments of the rubber compositions either singularly or in combination. Particular embodiments of useful rubber compositions may be limited to novolac resins as the useful methylene acceptor.

[0038] Suitable methylene donors may be selected from, for example, hexamethylenetetramine (HMT); hexamethoxymethylmelamine (HMMM); formaldehyde; paraformaldehyde; trioxane; 2-methyl-2-nitro-l-propanal; substituted melamine resins such as N-substituted oxymethylmelamine resins; glycoluril compounds such as tetramethoxymethyl glycoluril; urea- formaldehyde resins such as butylated urea- formaldehyde resins; or mixtures thereof. Hexamethylenetetramine (HMT), hexamethoxymethylmelamine (HMMM) or mixtures thereof are preferred methylene donors in particular embodiments.

[0039] The amount of methylene acceptor added to the rubber compositions disclosed herein may range, for example, from between 2 phr and 30 phr or alternatively between 2 phr and 25 phr or between 2 phr and 20 phr, between 2 phr and 10 phr, 5 phr and 25 phr, between 5 phr and 20 phr or between 10 phr and 20 phr of the methylene acceptor.

[0040] The methylene donor is added to the rubber composition as needed to provide the desired cross-linking with the methylene acceptors in an amount, for example, of between 8 wt% and 80 wt% of the total weight of the methylene acceptors in the rubber composition or alternatively between 10 wt% and 60 wt%, between 10 wt% and 40 wt% or between 15 wt% and 35 wt%. In particular embodiments, the methylene donor may be added to the rubber composition in an amount of between 0.5 phr and 15 phr or alternatively between 1 phr and 10 phr or between 1 phr and 5 phr.

[0041] In particular embodiments, a ratio of the total amount of methylene acceptors to the methylene donors may range between 1: 1 and 10: 1 by weight or alternatively between 3: 1 and 7: 1 or between 2: 1 and 8: 1. The ranges disclosed herein for particular embodiments may be combined with any of the ranges for the methylene acceptor disclosed herein.

[0042] For particular embodiments of the rubber compositions disclosed herein, the quantity of the rigidity booster may be within a set ratio with the quantity of methylene acceptor. Therefore, for example, it may be useful for the rigidity booster to be included in an amount that results in a ratio of the rigidity booster to the methylene acceptor to be in the range of between 1: 10 and 3: 1 by weight or alternatively between 1:8 and 2: 1, between 1:5 and 2: 1 or between 1:3 and 1: 1 by weight. [0043] In addition to the rubber component, the rigidity booster and the methylene acceptor/methylene donor reinforcing resin system as described above, the rubber compositions disclosed herein further include a reinforcing filler. Reinforcing fillers are well known in the art and include, for example, carbon blacks and silicas. Any reinforcing filler known to those skilled in the art may be used in the rubber composition either by themselves or in combination with other reinforcing fillers. In particular embodiments of the rubber composition disclosed herein, the reinforcing filler is selected from silicas, carbon blacks or combinations thereof. In other embodiments, the reinforcement filler is limited to essentially carbon black or alternatively to just carbon black and no more than 20 phr of silica or no more than 10 phr of silica or 0 phr of silica.

[0044] Carbon black, which is an organic filler, is a well-known reinforcement filler. Any carbon black known in the art and suitable for the given purpose may be used in the rubber compositions disclosed herein. Suitable carbon blacks of the type HAF, ISAF and SAF, for example, are conventionally used in tire treads. Non-limitative examples of carbon blacks include, for example, the N115, N134, N234, N299, N326, N330, N339, N343, N347, N375 and the 600 series of carbon blacks, including, but not limited to N630, N650 and N660 carbon blacks.

[0045] As noted above, silica may also be useful as reinforcement filler. The silica may be any reinforcing silica known to one having ordinary skill in the art including, for example, any precipitated or pyrogenic silica having a BET surface area and a specific CTAB surface area both of which are less than 450 m 2 /g or alternatively, between 30 and 400 m 2 /g may be suitable for particular embodiments based on the desired properties of the cured rubber composition. Particular embodiments of rubber compositions disclosed herein may include a silica having a CTAB of between 80 and 200 m 2 /g, between 100 and 190 m 2 /g, between 120 and 190 m 2 /g or between 140 and 180 m 2 /g. The CTAB specific surface area is the external surface area determined in accordance with Standard AFNOR-NFT-45007 of November 1987.

[0046] Highly dispersible precipitated silicas (referred to as "HDS") may be useful in particular embodiments of such rubber compositions disclosed herein, wherein "highly dispersible silica" is understood to mean any silica having a substantial ability to disagglomerate and to disperse in an elastomeric matrix. Such determinations may be observed in known manner by electron or optical microscopy on thin sections. Examples of known highly dispersible silicas include, for example, Perkasil KS 430 from Akzo, the silica BV3380 from Degussa, the silicas Zeosil 1165 MP and 1115 MP from Rhodia, the silica Hi- Sil 2000 from PPG and the silicas Zeopol 8741 or 8745 from Huber.

[0047] The total amount of reinforcement filler that may be added to the rubber compositions disclosed herein may range, for example, between 40 phr and 150 phr or alternatively between 40 phr and 100 phr or between 45 phr and 85 phr. These examples are not meant to limit the invention though particular embodiments of the rubber compositions disclosed herein may fall within these ranges of the total amount of reinforcement filler.

[0048] When silica is added to the rubber composition, a proportional amount of a silane coupling agent is also added to the rubber composition. The silane coupling agent is a sulfur-containing organosilicon compound that reacts with the silanol groups of the silica during mixing and with the elastomers during vulcanization to provide improved properties of the cured rubber composition. A suitable coupling agent is one that is capable of establishing a sufficient chemical and/or physical bond between the inorganic filler and the diene elastomer; which is at least bifunctional, having, for example, the simplified general formula "Y-T-X", in which: Y represents a functional group ("Y" function) which is capable of bonding physically and/or chemically with the inorganic filler, such a bond being able to be established, for example, between a silicon atom of the coupling agent and the surface hydroxyl (OH) groups of the inorganic filler (for example, surface silanols in the case of silica); X represents a functional group ("X" function) which is capable of bonding physically and/or chemically with the diene elastomer, for example by means of a sulfur atom; T represents a divalent organic group making it possible to link Y and X.

[0049] Any of the organosilicon compounds that contain sulfur and are known to one having ordinary skill in the art are useful for practicing embodiments of the present invention. Examples of suitable silane coupling agents having two atoms of silicon in the silane molecule include 3,3'-bis(triethoxysilylpropyl) disulfide and 3,3'-bis(triethoxy- silylpropyl) tetrasulfide (known as Si69). Both of these are available commercially from Degussa as X75-S and X50-S respectively, though not in pure form. Degussa reports the molecular weight of the X50-S to be 532 g/mole and the X75-S to be 486 g/mole. Both of these commercially available products include the active component mixed 50-50 by weight with a N330 carbon black. Other examples of suitable silane coupling agents having two atoms of silicon in the silane molecule include 2,2'-bis(triethoxysilylethyl) tetrasulfide, 3,3'- bis(tri-t-butoxy-silylpropyl) disulfide and 3,3'-bis(di t-butylmethoxysilylpropyl) tetrasulfide. Examples of silane coupling agents having just one silicon atom in the silane molecule include, for example, 3,3'(triethoxysilylpropyl) disulfide and 3,3' (triethoxy-silylpropyl) tetrasulfide. The amount of silane coupling agent can vary over a suitable range as known to one having ordinary skill in the art. Typically the amount added is between 7 wt. % and 15 wt. % or alternatively between 8 wt. % and 12 wt. % or between 9 wt. % and 11 wt. % of the total weight of silica added to the rubber composition.

[0050] Particular embodiments of the rubber compositions disclosed herein may include no processing oil or very little, such as no more than 5 phr. Processing oils are well known to one having ordinary skill in the art, are generally extracted from petroleum and are classified as being paraffinic, aromatic or naphthenic type processing oil, including MES and TDAE oils. Processing oils are also known to include, inter alia, plant-based oils, such as sunflower oil, rapeseed oil and vegetable oil. Some of the rubber compositions disclosed herein may include an elastomer, such as a styrene-butadiene rubber, that has been extended with one or more such processing oils but such oil is limited in the rubber composition of particular embodiments as being no more than 10 phr of the total elastomer content of the rubber composition.

[0051] The rubber compositions disclosed herein may further include, in addition to the compounds already described, all or part of the components often used in diene rubber compositions intended for the manufacture of tires, such as plasticizers, pigments, protective agents of the type that include antioxidants and/or antiozonants, vulcanization retarders, a vulcanization system based, for example, on sulfur or on a peroxide, vulcanization accelerators, vulcanization activators, extender oils and so forth. There may also be added, if desired, one or more conventional non-reinforcing fillers such as clays, bentonite, talc, chalk or kaolin. [0052] The vulcanization system is preferably, for particular embodiments, one based on sulfur and on an accelerator but other vulcanization agents known to one skilled in the art may be useful as well. Vulcanization agents as used herein are those materials that cause the cross-linkage of the rubber and therefore may be added only to the productive mix so that premature curing does not occur, such agents including, for example, sulfur and peroxides. Use may be made of any compound capable of acting as an accelerator of the vulcanization of elastomers in the presence of sulfur, in particular those chosen from the group consisting of 2-mercaptobenzothiazyl disulfide (abbreviated to "MBTS"), N- cyclohexyl-2-benzothiazolesulphenamide (abbreviated to "CBS"), N,N-dicyclohexyl-2- benzothiazolesulphenamide (abbreviated to "DCBS"), N-tert-butyl-2- benzothiazolesulphenamide (abbreviated to "TBBS"), N-tert-butyl-2-benzothiazole- sulphenimide (abbreviated to "TBSI") and the mixtures of these compounds. Preferably, a primary accelerator of the sulfenamide type is used.

[0053] The vulcanization system may further include various known secondary accelerators or vulcanization activators, such as zinc oxide, stearic acid and guanidine derivatives (in particular diphenylguanidine).

[0054] The rubber compositions that are embodiments of the present invention may be produced in suitable mixers in a manner known to those having ordinary skill in the art. Typically the mixing may occur using two successive preparation phases, a first phase of thermo-mechanical working at high temperature followed by a second phase of mechanical working at a lower temperature.

[0055] The first phase, sometimes referred to as a "non-productive" phase, includes thoroughly mixing, for example by kneading in a Banbury type mixer, the various ingredients of the composition but excluding the vulcanization system and the methylene donor. It is carried out in a suitable kneading device, such as an internal mixer, until under the action of the mechanical working and the high shearing imposed on the mixture, a maximum temperature of generally between 120 °C and 190 °C is reached.

[0056] After cooling the mixture a second phase of mechanical working is implemented at a lower temperature. Sometimes referred to a "productive" phase, this finishing phase consists of incorporating the vulcanization system and the methylene donor into the rubber composition using a suitable device, such as an open mill. It is performed for an appropriate time (typically, for example, between 1 and 30 minutes or between 2 and 10 minutes), and at a sufficiently low temperature, i.e., lower than the vulcanization temperature of the mixture and lower than the cross-linking temperature of the methylene donor/acceptor system, so as to protect against premature vulcanization or resinification cross-linking.

[0057] The invention is further illustrated by the following examples, which are to be regarded only as illustrations and not delimitative of the invention in any way. The properties of the compositions disclosed in the examples were evaluated as described below.

[0058] Mooney Plasticity (ML 1+4) was measured in accordance with ASTM Standard D1646. In general, the composition in an uncured state is molded in a cylindrical enclosure and heated to 100 °C. After 1 minute of preheating, the rotor turns within the test sample at 2 rpm, and the torque used for maintaining this movement is measured after 4 minutes of rotation. The Mooney Plasticity is expressed in "Mooney units" (MU, with 1 MU = 0.83 Newton-meter).

[0059] Moduli of elongation (MPa) were measured at 10% (MA10) at a temperature of 23 °C based on ASTM Standard D412 on dumb bell test pieces. The measurements were taken in the second elongation; i.e., after an accommodation cycle. These measurements are secant moduli in MPa, based on the original cross section of the test piece.

[0060] Hysteresis losses (HL) were measured in percent by rebound at 60 °C at the sixth impact in accordance with the following equation:

HL (%) = 100 (Wo - Wi)/Wi,

where Wo is the energy supplied and Wi is the energy restored.

[0061] The elongation property was measured as elongation at break (%) and the corresponding elongation stress (MPa), which is measured at 23 °C in accordance with ASTM Standard D412 on ASTM C test pieces.

Example 1

[0062] This example illustrates the effectiveness of fatty metallic soaps as a rigidity booster in 100% natural rubber compositions containing formalphenolic resins. It also shows the effect of the loadings of the rigidity booster in the rubber compositions containing various amounts of the formalphenolic resins.

[0063] Rubber formulations were prepared with the component amounts shown in Table 1. The carbon black was N330. The formalphenolic resin, or the methylene acceptor, was the Novolac resin HRJ- 12952 available from SI Group with offices in Schenectady, NY. The methylene donor was hexamethylenetetramine (HMT). The vulcanization package included sulfur, accelerators, ZnO, and stearic acid. The antidegradant included standard antidegradant additives such as 6PPD. The fatty Zn soap was Struktol CY48 provided by the Struktol Company, with melting point of about 96°C.

[0064] For each of the formulations described in Table 1, the natural rubber and all the other materials except for the sulfur, accelerators and the hexamethylenetetramine (HMT) were added to a Banbury mixer and processed until well incorporated. The mixture was then dropped from the mixer and transferred to a mill where it was cooled.

Table 1 - Formulations and Physical Properties

[0065] The sulfur, the accelerator, and the hexamethylenetetramine were added into the cooled mixture on the mill and the productive mix was milled for a time until the components were well mixed. The product was then tested for its properties in accordance with the testing procedures described above. For the cured properties, the product was cured for 25 minutes at 150 °C. The results of the physical properties are shown in Table 1.

[0066] As can be seen from the results in Table 1, the addition of the fatty soap to the rubber compositions having the reinforcing resins greatly increased the MA10 when compared to the witness formulation having only the reinforcing resins. One advantage of this is that the amount of carbon black in the rubber composition can be reduced to give the same level of rigidity as the sample without the fatty soap, thereby expectedly providing benefits that can be obtained from having less carbon black such as improved processability and improved rolling resistance for tire applications.

Example 2

[0067] This example demonstrates the effect of various fatty Zn soaps on the rigidity of the rubber compositions containing 50/50 NR/SBR blend elastomers and formalphenolic resins. It also provides examples of materials that are not rigidity booster.

[0068] Rubber formulations were prepared with the component amounts shown in Table 2. The fatty soap CY was Struktol CY48, and the fatty soap ZB was Struktol ZB49, and both were provided by the Struktol Company, with melting points of 96°C and 119°C, respectively. The fatty alcohol/ester was AFLUX 42 provided by Rhein Chemie with offices in Pittsburgh, PA. The formulations were mixed and cured as described in Example 1.

Table 2 - Formulations

Table 3 - Physical Properties

[0069] As can be seen in the results of Table 3, both the fatty Zn soaps of different melting points provided increases in rigidity of the inventive formulations F2-1 and F2-2. However, the comparative formulation C2, which included a fatty alcohol/ester, did not provide a rigidity boost.

[0070] The terms "comprising," "including," and "having," as used in the claims and specification herein, shall be considered as indicating an open group that may include other elements not specified. The term "consisting essentially of," as used in the claims and specification herein, shall be considered as indicating a partially open group that may include other elements not specified, so long as those other elements do not materially alter the basic and novel characteristics of the claimed invention. The terms "a," "an," and the singular forms of words shall be taken to include the plural form of the same words, such that the terms mean that one or more of something is provided. The terms "at least one" and "one or more" are used interchangeably. The term "one" or "single" shall be used to indicate that one and only one of something is intended. Similarly, other specific integer values, such as "two," are used when a specific number of things is intended. The terms "preferably," "preferred," "prefer," "optionally," "may," and similar terms are used to indicate that an item, condition or step being referred to is an optional (not required) feature of the invention. Ranges that are described as being "between a and b" are inclusive of the values for "a" and "b."

[0071] It should be understood from the foregoing description that various modifications and changes may be made to the embodiments of the present invention without departing from its true spirit. The foregoing description is provided for the purpose of illustration only and should not be construed in a limiting sense. Only the language of the following claims should limit the scope of this invention.