Rubber composition for tyre

Abstract: A rubber composition for a tyre characteristically comprising a diene rubber having an epoxy group, either a monoamine compound or a diamine compound having a structural unit represented by the formula (1): (where R¹, R², R³, and R⁴ are each a hydrogen atom or an alkyl group having one to three carbon atoms, and X represents any structural formula) and carbon black, wherein the rubber composition has a dissipation factor at -10°C (tanδ(-10°C)) and a dissipation factor at 70°C (tanδ(70°C)) which satisfy the following expressions: 0. 03 ≦ tanδ(70°C) ≦ 0. 35 W.I.R = {tanδ(-10°C) - tanδc(-10°C) } / tanδc(-10°C) R.I.R = {tanδc(70°C) - tanδ(70°C) } / tanδc(70°C) T.I.R = W.I.R + 2 x (R.I.R) ≧ 0. 3 where tanδc(-10°C) and tanδc(70°C) are tanδ at -10°C and 70°C of a control rubber composition of the same formulation as the rubber composition except that the control rubber composition includes a diene rubber having no epoxy group and is free of the diamine compound having a structural unit represented by the formula (1).

Description: The present invention relates to rubber compositions for tyres, excellent in grip performance on a wet road (hereinafter referred to as "wet grip") and fuel saving. Recently fuel saving performance of automobiles has been increasingly desired, and since the fuel saving performance depends significantly on tyre characteristics, rubber compositions for tyres which provide for excellent fuel saving performance have been desired to be developed. On the other hand, the wet grip performance of tyres has been of a great interest from the viewpoint of driving safety. Generally, a tyre is required to have a low hysteresis loss so as to exhibit satisfactory fuel saving. However, a low hysteresis loss giving a low rolling resistance also leads to reduced grip performance. On the other hand, a tyre of higher wet grip performance naturally has a higher hysteresis loss which results in a higher rolling resistance and hence is poor in fuel saving performance. Thus, the wet grip performance and fuel saving performance of a tyre are not compatible with each other, and it is difficult to obtain a tyre satisfactorily exhibiting both the two performances. Therefore, intensive study has been made to develop a rubber composition which provides for in a tyre both wet grip performance and fuel saving performance. For example, it has been conventionally known that the wet grip performance is improved by using epoxidised natural rubber for a tyre tread. However, the fuel saving performance is adversely affected, because the rubber cannot escape from the non-compatible relationship between the two characteristics. In addition, such a rubber shows a strong rolling adhesion and hence tends to offer poor rolling workability. To compensate the adverse effect, use of silica instead of carbon black has been proposed (RUBBER WORLD, by C S L Baker, et al, August 1987, p.27 and ELASTOMERICS, BY Roland Newell, July 1992, P.22). However, this technique still involves disadvantages in workability, wear resistance and the like. Alternatively, there has been proposed a rubber composition in which a compound having at least two primary amino groups and a sulfur vulcaniser are mixed with synthetic cis-1,4-polyisoprene rubber having an epoxy group in order to improve the tensile stress and strength of vulcanised synthetic cis-1,4-polyisoprene (Japanese Examined Patent Publication No Hei 1(1989)-24166 and EP-A-068 468). Other tread compounds comprising epoxidised CIS-1,4 butadiene rubber and diaminodiphenyl ethers are disclosed by JP-A-58-019341. With these rubber compositions, however, improvements in both the wet grip performance and the fuel saving performance cannot be rendered compatible. This invention has been accomplished in view of the above-mentioned technical background. It is, therefore, an object of the invention to provide a rubber composition which provides for a tyre satisfactorily exhibiting both fuel saving and wet grip performances and improved rolling workability. According to the present invention, there is provided a rubber composition for a tyre comprising a diene rubber having an epoxy group, either a monoamine compound or a diamine compound having a structural unit represented by the formula (1): (where R 1 , R 2 , R 3 , and R 4 are each a hydrogen atom or an alkyl group having one to three carbon atoms, and X represents any structural formula) and carbon black, wherein the rubber composition has a dissipation factor at -10°C (tanδ(-10°C)) and a dissipation factor at 70°C (tanδ(70°C)) which satisfy the following expressions: 0. 03 ≤ tanδ(70°C) ≤ 0. 35 W.I.R = {tanδ(-10°C) - tanδc(-10°C) } / tanδc(-10°C) R.I.R = {tanδc(70°C) - tanδ(70°C) } / tanδc(70°C) T.I.R = W.I.R + 2 x (R.I.R) ≥ 0. 3 where tanδc(-10°C) and tanδc(70°C) are tanδ at -10°C and 70°C of a control rubber composition of the same formulation as the rubber composition except that the control rubber composition includes a diene rubber having no epoxy group and is free of the diamine compound having a structural unit represented by the formula (1), characterised by further comprising a carboxylic acid having two or more carboxyl groups in a molecule thereof. The invention will now be described by way of example only, in conjunction with the attached diagrammatic drawings in which:- Figure 1 is a graph showing the relationship between rubber compositions and respective tanδ thereof; and Figure 2 is a graph showing the relationship between rubber compositions and respective tanδ thereof. In the present invention, a diene rubber having an epoxy group in a molecule thereof (hereinafter referred to as "epoxidised diene rubber") is obtained by introducing an epoxy group in such a manner as to epoxidise any diene rubber such as natural rubber, polyisoprene rubber, polybutadiene rubber, solution-polymerised styrene-butadiene rubber) hereinafter referred to as "solution-polymerised SBR") or emulsion-polymerised styrene-butadiene rubber (hereinafter referred to as "emulsion-polymerised SBR") at double bonds thereof using an organic peroxide such as peracetic acid, to copolymerise a diene rubber monomer and a vinyl monomer having an epoxy group such as glycidyl methacrylate, or to graft-polymerise a diened rubber and a monomer having an epoxy group. Examples of specific diene rubbers having an epoxy group in a molecule thereof include epoxidised natural rubber, epoxidised polyisoprene rubber, epoxidised polybutadiene rubber, epoxidised styrene-butadiene rubber, an isoprene-glycidyl methacrylate copolymer, a butadiene-glycidyl methacrylate copolymer, and a stryene-butadiene-glycidyl methacrylate terpolymer. Among these, epoxidised natural rubber and epoxidised SBR are preferably used. In particular, epoxidised solution-polymerised SBR is preferable in terms of fuel saving. The content of epoxy group in the epoxidised diene rubber is not particularly limited, but is preferably about 0.1 to 60 groups to 100 monomer units of the rubber, more preferably 0.5 to 40 epoxy groups, most preferably 2 to 30 epoxy groups. If the content is less than 0.1, little improvement is found, whereas if it is more than 60, the resulting rubber has a excessively high glass transition temperature (Tg) which adversely affects convenience for use, workability, and compatibility with other rubbers for blending. The rubber component of the rubber composition according to the present invention may be a single epoxidised diene rubber, a blend of two or more kinds of epoxidised diene rubbers or a blend of an epoxidised diene rubber and a diene rubber having no epoxy group. When there is used a blend of an epoxidised diene rubber and a diene rubber having no epoxy group, the proportion of the epoxidised diene rubber in the rubber component is generally 40% by weight or more, though depending on the number of epoxy groups in the epoxidised diene rubber. Nevertheless, the overall content of the rubber component in the rubber composition is preferably comprised of the epoxidised diene rubber. Although the type of diene rubber having no epoxy group used for blending is not particularly limited, natural rubber and SBR are preferable. The amine compound for use in the present invention is either a monoamine compound having one amino group in a molecule or a diamine compound having two amino groups in a molecule. Although the monoamine compound is not particularly limited, examples thereof include primary amines such as N-hexylamine and cyclohexylamine, and secondary amines such as 4,4-methylene biscyclohexylamine. The diamine compound used in the present invention is a compound having a structural unit represented by the formula (1); In the formula (1), R 1 , R 2 R 3 and R 4 are each a hydrogen atom or an alkyl group having one to three carbon atoms. X represents any structural formula, for example, a straight or branched aliphatic alkyl group, an alicyclic hydrocarbon, an aromatic hydrocarbon including a polycyclic compound such as benzene ring or naphthalene, and a carbonyl group. Examples of diamine compounds satisfying the above requirements include aliphatic primary amines such as hexamethylene-diamine; aliphatic secondary amines such as N,N'-dimethyl-1,6-diaminohexane, tertiary amines such as N,N,N',N'-tetramethyl-1,3-propanediamine; aromatic primary amines such as p-phenylenediamine and tolulene-2,4-diamine; aromatic secondary amines such as N,N'-dimethyl-1,4-phenylenediamine; aromatic tertiary amines such as N,N,N',N'-tetramethyl-1,4-phenylenediamine; and urea derivatives such as tetramethyl urea. Among these, the aliphatic primary amines are particularly preferable. Of the aliphatic primary amines, hexamethylenediamine is preferably used in terms of its availability. The content of such a monoamine or diamine compound is appropriately selected depending on the type and content of epoxidised diene rubber in the rubber composition. To satisfy the tanδ requirements, the monoamine or diamine compound is used preferably in an amount of 0.1 mmol to 100 mmol per 100 parts by weight of rubber component, more preferably 0.5 mmol to 60 mmol, most preferably 1 to 40 mmol. For the carbon black used in the present invention, a carbon black having a nitrogen adsorption specific surface of 50 to 250m 2 /g and a DBP oil absorption of 60 to 150ml/g is preferably used. A carbon black having a nitrogen adsorption specific surface of less than 50m 2 /g or a DBP oil absorption of less than 60ml/g results in a rubber composition of poor wear resistance. On the other hand, a carbon black having a nitrogen adsorption specific surface exceeding 250m 2 /g results in poor dispersion, and a carbon black having a DBP oil absorption exceeding 150ml/g results in a rubber composition of increased viscosity and poor workability. The content of carbon black is preferably 20 to 150 parts by weight relative to 100 parts by weight of the rubber component. A content of less than 20 parts by weight results in insufficient reinforcing effect and poor wear resistance. On the other hand, a content of more than 150 parts by weight results in a rubber composition of poor workability and degraded physical properties. The rubber composition of the present invention is preferably incorporated with polycarboxylic acid having two or more carboxyl groups. The addition of polycarboxylic acid allows the resulting rubber composition to have an improved rolling workability and facilitates the preparation of the rubber composition satisfying the specific requirements of tanδ at given temperatures. Examples of polycarboxylic acids are aliphatic dicarboxylic acids such as suberic acid; aromatic dicarboxylic acids such as phthalic acid; and tricarboxylic acids such as tricarballic acid. Amongst these, the aliphatic dicarboxylic acids such as suberic acid are preferable. The amount of the polycarboxylic acid to be blends is appropriately selected depending on the kind and content of the epoxidised diene rubber used in the rubber composition. To satisfy the tanδ requirements, however, the polycarboxylic acid is used in such an amount as to give preferably 0.2 to 200 mmol of carboxylic group relative to 100 parts by weight of the rubber component, more preferably 1.0 to 120 mmol, most preferably 2 to 80 mmol. Preferably, the blending ratio of the polycarboxylic acid to the amine compound satisfies the following relationship. That is, the ratio of mol number of contained carboxyl group (M C ) to mol number of contained amino group (M A ), or M C /M A ranges from 0.2 to 1.8 (0.2 ≤ M C /M A ≤ 1.8). If the ratio is less than 0.2, the rolling workability of the resulting rubber composition is not sufficiently improved. If it is not less than 1.8, the T.I.R. characteristic of the resulting rubber composition is insufficiently improved. In the rubber composition of the present invention, various additives usually used in rubber compounding such as a vulcanising agent, a vulcanisation accelerator, a reinforcing agent, a filler, a softener, a processing aid and an antioxidant may be added as required in addition to the foregoing components. Although the rubber composition for tyres of the invention is of the formulation described above, the dissipation factor (tanδ) thereof is variable depending on the type and content of the epoxidised diene rubber, the type and content of the diamine compound, the type and content of other additives, the method of kneading and the method of vulcanisation. This invention is directed to rubber compositions for tyres, tanδ of which satisfies specific requirements. The general relationship between tanδ and temperature is explained referring to Figure 1. In Figure 1, the solid line a indicates the tanδ of a rubber composition incorporated with a diene rubber having no epoxy group, and dotted line b indicates a rubber composition incorporated with an epoxidised diene rubber. As can be understood from Figure 1, when an epoxy group is introduced by epoxidising double bonds of a diene rubber or by copolymerisation of a diene rubber with a vinyl compound having an epoxy group, generally the peek temperature of tanδ rises, whereby tanδ at temperatures ranging from -10°C to 70°C is generally increased. On the other hand, the two dot chained line d shows the tanδ of a rubber composition containing a diene rubber having no epoxy group and incorporated with a primary amino compound. Comparing with the case indicated by the line a, the peak temperatures of tanδ (T∝) scarcely changes, and tanδ in the low temperature region (for example, at -10°C) scarcely changes, but tanδ in the high temperature region (for example, at 70°C) becomes greater. In Figure 1, a one dot chained line c indicates the tanδ of a rubber composition containing an epoxidised diene rubber and a primary amino compound. Although T∝ of the rubber composition indicated by line c rises due to mixing of the epoxidised diene rubber, the degree of rise is smaller than that of the rubber composition indicated by the line b, and tanδ in the high temperature region is lower than that of the rubber composition indicated by the line d. It should be noted that the rubber compositions indicated by the lines a to d are of the same recipe (free of carbon black) except for the type of rubber component and the presence of the primary amino compound. Figure 2 shows the relationship between temperature and tanδ of rubber compositions containing carbon black. In the Figure, the solid line g indicates the tanδ of a rubber composition containing a diene rubber having no epoxy group and free of any primary amino compound, the dotted line e indicates the tanδ of a rubber composition containing an epoxidised diene rubber and free of any primary amino compound, and the one dot chained line f indicates the tanδ of a rubber composition comprising an epoxidised diene rubber and a primary amino compound. The rubber compositions indicated by the lines e to g are of the same recipe except for the type of rubber component and the presence of the primary amino compound. As can be understood from a comparison of e with g, the introduction of an epoxy group to a diene rubber raises T∝ and hence generally increases tanδ over a wide temperature range. On the other hand, from a comparison of e with f, the mixing of a primary amino compound is found to decrease tanδ in the low temperature region. These facts suggest that some reactions or interactions between amino groups and epoxy groups, and amino groups and various functional groups on the surface of carbon black such as carboxyl groups, hydroxly groups and quinone groups improve aggregation within each molecular chain or of molecular chains. Also, the reactions or interactions presumably reduce a high order structure of carbon black formed in rubber and improve the dispersibility of carbon black. It is known that the wet grip performance is closely related to tanδ in the low temperature region (for example, from -30 to 15°C), and the rolling resistance is closely related to tanδ in the high temperature region (for example, from 50 to 65°C). A rubber with larger tanδ in the low temperature region is know to exhibit better wet grip performance, while a rubber with small tanδ in the high temperature region is know to exhibit a more excellent fuel saving performance (refer to Y Sazito, "New Polymer Development for Low Rolling Resistance Tires", Kautschuk und Gummi Kunststoffe, 1986, p.39). In the present invention, the dissipation factor at -10°C (tanδ (-10°C)) is employed as an index of the wet grip performance and the dissipation factor at 70°C (tanδ (70°C)) is employed as an index of the rolling resistance. The tanδ (-10°C) and tanδ (70°C) of the rubber composition for tyres of the present invention satisfy the following expressions: 0. 03 ≤ tanδ(70°C) ≤ 0. 35 W.I.R = {tanδ(-10°C) - tanδc(-10°C) } / tanδc(-10°C) R.I.R = {tanδc(70°C) - tanδ(70°C) } / tanδc(70°C) T.I.R = W.I.R + 2 x (R.I.R) ≥ 0. 3 where tanδc (-10°C) and tanδc (70°C) are tanδ at -10°C and 70°C of a control rubber composition of a reference formulation, which is the same formulation as the rubber composition of the present invention except that the control rubber composition includes a diene rubber having no epoxy group as the rubber component thereof and is free of any amine compound and any carboxylic acid. T.I.R. is preferably 1.0 or more, especially 1.5 or more. In general, tanδ is greatly dependent on the types and contents of carbon black, softener and vulcanising agent. Conventionally, these are optimised taking consideration of the wet grip and fuel saving performances and, in addition, other various characteristics required such as wear resistance, workability and the like. For this reason, to examine the effects of improvements by the rubber component and diamine compound, it is required to compare the rubber composition of the present invention with the control rubber composition of the reference formulation. R.I.R. is doubled in the expression of T.I.R. to take merits and difficulties resulting from improvements into consideration. Further, tanδ (70°C) is restricted within the above range because the wear resistance is extremely degraded when tanδ (70°C) is less than 0.03, and the fuel saving performance is extremely degraded when it exceeds 0.35. The present invention will be more fully described by way of examples. It should be noted that such examples are only illustrative of the invention and not limitative thereof. Described first are methods of evaluating the characteristics of rubber compositions. Wet grip performance A µ-S test car provided with tyres made of each rubber composition was driven on a wet road at a speed of 40km/hr and then was braked. The friction coefficient (µ) was measured from the point where the wheels were locked to the point where the car stopped to determine the maximum friction coefficient between the points. The maximum friction coefficient of a tyre made from each rubber composition was indexed with the index of the maximum friction coefficient of a tyre made from rubber composition No. 2 (refer to Table 3) having a rubber component comprised of a mixture of SBR and natural rubber and free of any amine compound being assumed 100. The greater the index, the more excellent the wet grip performance. Fuel saving A tyre was rotated at a speed of 80 km/hr under a load of 350kgf to measure the rolling friction coefficient thereof. The maximum friction coefficient of a tyre made from each rubber composition was calculated as an index with the index of the maximum friction coefficient of a tyre made from rubber composition No. 1 (refer to Table 3) having a rubber component comprised of natural rubber and free of any amine compound being set as 100. The smaller the index, the smaller the rolling resistance, hence the more excellent the fuel saving performance. Rolling workability The rolling workability of each rubber composition was evaluated by kneading with two rolls of 8 inch diameter. The evaluation was based on the three ratings: O: no problem, Δ: practically no problem though slightly adhering to rolls, and X: significantly adhering to rolls to cause problem of workability. Diamine compound In Examples of the present invention 10 types (Nos. 1-10) of diamine compounds were used. Compounds Nos. 9 and 10 were primary amines, compound No. 3 was a secondary amine, and the others were tertiary amines. Diamine Compound No. Compound No.1 N,N,N',N'-tetramethyle-1,6-hexadiamine No.2 N,N,N',N'-tetramethyl-1,3-propanediamine No.3 N,N'-dimethyl-1,6-diaminohexane No.4 1,8-bis(dimethylamino)naphthalene No.5 Tetramethyl urea No.6 N,N,N',N'-tetramethyle-1,4-phenylenediamine No.7 p,p'-tetramethyl-diaminodiphenylnaphthalene No.8 4,4'-bis(dimethylamino)benzophenone No.9 Hexamethylenediamine No.10 Tolulene-2,4-diamine Rubber compositions Nos. 1-15 Rubber compositions Nos. 1 to 15 were compounded by mixing the compounding ingredients A listed in Table 2 with different amine compounds and rubber components shown in Table 3. Used as the epoxidised diene rubber was epoxidised natural rubber 25 having about 25 epoxy groups relative to 100 monomer units. Used as the carbon black was HAF with a nitrogen adsorption of 100ml/g. In Table 2 SUNNOS S is a wax, and NOCRACK 6C is an antioxidant. It is to be noted that amino group content (mmol) and formula (1) unit content (mmol) are respectively calculated from the amount of the amine compound relative to 100 parts by weight of the rubber component (hereinafter the same). Compounding ingredients A Amount (wt. parts) Rubber component 100 Carbon black (HAF) 50 Aromatic oil 5 Chinese white 5 Stearic acid 1 SUNNOX S 1 NOCRACK 6C 2 Sulfur 1.75 Vulcanisation accelerator varied Using the rubber compositions Nos. 1 to 15, strip rubber specimens with a width of 4mm and thickness of 2mm were prepared. Using a viscoelasticity meter (VBS-F3 manufactured by Iwamoto Manufacturing K.K.), tanδ at -10°C and 70°C were measured under such measurement conditions as a distance between clamps of 30mm, an initial extension of 3.3%, a dynamic amplitude of ±0.25% and a frequency of 10Hz, and then the characteristics W.I.R., R.I.R. and T.I.R. were calculated. For the calculation of W.I.R., R.I.R. and T.I.R., the rubber composition No. 1 was employed as a control rubber compositions. Formulations of rubber compositions Nos. 1 to 15, tanδ, W.I.R., R.I.R. and T.I.R. were as listed in Table 3. As can be seen from Table 3, the rubber compositions Nos. 1 and 2 were comparative examples containing no epoxidised diene rubber, and the rubber composition No. 3 was a comparative example containing an epoxidised diene rubber but free of any primary amino compound. On the other hand, the rubber compositions Nos. 4-15 were examples of the present invention which had a T.I.R. value of not less than 0.3. Wet grip and rolling friction were evaluated on the above-mentioned rubber compositions Nos. 1 to 15 by measuring methods described. The results were as shown in Table 4. Rubber composition No Wet grip Fuel saving Comp. Ex. No.1 95 100 No.2 100 111 No.3 111 117 Ex. No.4 116 105 No.5 113 89 No.6 116 90 No.7 114 93 No.8 114 97 No.9 114 95 No.10 112 101 No.11 107 100 No.12 105 102 No.13 112 96 No.14 113 94 No.15 114 122 As seen from Table 4, the rubber compositions according to examples of the present invention were more excellent in wet grip performance than the rubber compositions according to the comparative examples. Further, the greater the value of T.I.R. grew, the more excellent the fuel saving performance became. In particular, the rubber compositions with T.I.R. values of 1.0 or more were excellent in fuel saving performance relative to the comparative rubber compositions. The rubber compositions with T.I.R. values of 1.5 or more (Nos. 5-9, 13 and 14), in particular, were highly excellent in fuel saving performance. In the rubber compositions in accordance with the present invention, blending of hexamethylenediamine resulted in rubber compositions (Nos. 5 to 14) with T.I.R. value higher than that of the rubber composition resulting from blending of N-hesylamine or a monoamine (the rubber composition No. 4), and led to more improved wet grip and fuel saving performances. The use of an epoxidised diene rubber solely (the rubber compositions Nos. 4 to 8) as the rubber component was found to give a higher T.I.R. value and more improved wet grip and fuel saving performances than the use of a blend of an epoxidised diene rubber and a rubber having no epoxy group (the rubber compositions Nos. 9-14). Rubber Compositions Nos. 16-25 Rubber compositions Nos. 16 to 25 were prepared by mixing the compounding ingredients B listed in Table 5 with different types and contents of rubber components and diamine compounds (Nos. 1-8 in Table 1) shown in Table 6. The content of each diamine compound was such that the formula (1) unit content relative to 100 parts by weight of the rubber component was 8.6 mmol. The epoxidised diene rubber and carbon black (HAF) were the same as used in the rubber compositions No. 1-15. Compounding ingredients B Amount (wt parts) Rubber component 100 Chinese white 5 Stearic acid 1 Sulfur 1.75 Vulcanisation accelerator (Acc. NS) 1 Diamine compound varied Carbon black (HAF) 45 In the same manner as with the rubber composition No.1, tanδ at -10°C and 70°C of each of rubber compositions 16-25 were measured and then the characteristics W.I.R., R.I.R. and T.I.R. were calculated. For the calculation of W.I.R., R.I.R. and T.I.R., the rubber composition No. 16 was employed as a control rubber composition. As can be seen from Table 6, the rubber composition No. 16 was a comparative example containing no epoxidised diene rubber, and the rubber composition No. 17 was a comparative example containing an epoxidised diene rubber but free of any diamine compound. Further, the rubber compositions Nos. 23-25 was comparative examples which had T.I.R. values of less than 0.3. Wet grip and fuel saving performances and rolling workability were evaluated on the thus prepared rubber compositions Nos. 16 to 25 by the aforementioned evaluation method. The results are shown in Table 7. Rubber composition No Wet grip Fuel saving Rolling workability Comp. Ex. No. 16 92 89 O No. 17 110 109 X Ex. No. 18 113 95 ▴ No. 19 116 101 ▴ No. 20 115 87 ▴ No. 21 112 105 ▴ No. 22 110 104 ▴ Comp Ex. No. 23 109 113 ▴ No. 24 109 109 ▴ No. 25 112 110 ▴ As seen from Table 7, rubber compositions containing the epoxidised rubber and amine compound and exhibiting larger T.I.R. values tended to show excellent wet grip and fuel saving performances though they showed slight variations. Rubber compositions exhibiting T.I.R. values of 1.5 or more, in particular, showed excellent fuel saving performance (refer to compositions Nos. 18-20). Further it was found that the use of the epoxidised rubber resulted in a rubber compositions of degraded rolling workability (composition No. 17), but the combination of the amine compound with the epoxidised rubber recovered the rolling workability to such an extent as to cause no practical problem (compositions Nos. 18-25. Compounding of rubber compositions containing epoxidised natural rubber, diamine compound and carboxylic acid. Rubber compositions Nos. 26-33 Rubber compositions Nos. 26 to 33 were prepared by mixing the compounding ingredients C listed in Table 8 with different tyres and contents of rubber components, diamine compounds and carboxylic acids shown in Table 9. Compounding ingredient C Amount (wt parts) Rubber component 100 Zinc oxide 5 Stearic acid 1 Sulfur 1.75 Vulcanisation accelerator (Acc. NS) 1 Aromatic oil 16 SUNNOX S 1.5 NOCRACK 6C 1.5 Diamine compound varied Carboxylic acid varied Carbon black (N351) 55 Carbon black (N351) in Table 8 has a nitrogen adsorption specific surface area of 81m 2 /g and DBP oil adsorption of 100ml/g. As in the same manner as with the rubber composition No. 1, tanδ at -10°C and 70°C of each of the rubber compositions 26-33 were measured and then the characteristics W.I.R., R.I.R. and T.I.R. were calculated. For the calculation of W.I.R, R.I.R and T.I.R., the rubber composition No. 26 was employed as a control rubber composition. Table 9 shows compounding ingredients of rubber compositions Nos. 26-33, contents of amino group and carboxyl group, and values of tanδ, W.I.R., R.I.R. and T.I.R. As can be seen from Table 9, rubber composition No. 26 was a comparative example containing no epoxidised diene rubber, and rubber composition No. 27 was a comparative example containing an epoxidised diene rubber but free of any diamine compound. Further, rubber composition No. 28 was an example not containing any carboxylic acid, and rubber composition No. 33 was an example containing a mono-carboxylic acid (hexoic acid) instead of a dicarboxylic acid (suberic acid). All of these are precluded from rubber compositions according to claim 2. Rubber compositions Nos. 29 to 32 were examples incorporated with both the diamine compound and dicarboxylic acid which were included in rubber compositions according to claim 2. Wet grip and fuel saving performances and rolling workability were evaluated on the thus prepared rubber compositions Nos. 26 to 33 by the aforementioned evaluation method. The results were as shown in Table 10. Rubber Composition No. Wet grip Fuel saving Rolling workability Comp. Ex. No.26 98 102 O No. 27 112 114 X Ex. No. 28 112 97 ▴ No. 29 110 101 O No. 30 114 97 O No. 31 113 103 O No. 32 111 111 O No. 33 109 104 ▴ As seen from Table 10, examples containing the diamine compound exhibited satisfactory wet grip performance. Although the rubber compositions No.s 31-33 showed fuel saving performance inferior to that of the control rubber composition No. 26, such inferiority was within the allowable range. When taking the wet grip and fuel saving performances as a whole, the rubber compositions of examples were more excellent than the control rubber composition. The rubber compositions having T.I.R. values of 1.0 or more, especially 1.5 or more (Nos. 28 and 30) were more excellent in the fuel saving performance than the control rubber composition. Further, rubber compositions Nos. 28 and 33 containing the diamine compound but free of the dicarboxylic acid both exhibited slightly inferior rolling workability. Effect of epoxidised SBR. Rubber compositions Nos. 34-37 Rubber compositions Nos. 34 to 37 were prepared by mixing the compounding ingredients shown in Table 11. As the diamine compound hexamethylenediamine was used, and as the epoxidised rubber epoxidised natural rubber and epoxidised SBR-1 resulting from epoxidisation of solution-polymerised SBR-1 were used. The solution-polymerised SBR-1 was a product of Nippon Gosei Rubber Kabushiki Kaisha and contained 15% by weight of styrene and 57% by weight of a vinyl component. The epoxidised solution-polymerised SBR-1 was obtained in the following manner. A separable flask of 2 litres provided with a cooler, agitator, thermometer and funnel was charged with 1.5 litres of toluene, to which were added 81g of solution-polymerised SBR-1 and 1.62g of 2,6-di-tert-butyl-4-methylphenol. The mixture was subjected to complete dissolution by warming to 40°C. In turn, this solution was added with 0.085 mol of formic acid, and when the solution became homogeneous, an aqueous solution of hydrogen peroxide was added dropwise to the solution in an amount equivalent to 0.0245 mol of hydrogen peroxide. The mixture thus obtained was subjected to reaction at 40°C under stirring for 5 hours. The reaction solution was allowed to cool, and then washed with a diluted solution of sodium hydroxide, then with a washing water to remove unreacted formic acid and hydrogen peroxide. Finally, the resulting polymer was precipitated in excessive methanol and dried to give the epoxidised solution-polymerised SBR-1. As in the same manner as with the rubber composition No. 1, tanδ at -10°C and 70°C of each of the rubber compositions 34-37 were measured and then the characteristics W.I.R., R.I.R. and T.I.R. were calculated. For the calculation of W.I.R., R.I.R. and T.I.R., the rubber composition No. 34 was employed as a control rubber composition. Table 11 shows compounding ingredients of rubber compositions Nos. 34-37, contents of amino group and formula (1) unit, and values of tanδ, W.I.R., R.I.R. and T.I.R. Wet grip and fuel saving performances were evaluated on the thus prepared rubber compositions Nos. 34 to 37 by the aforementioned evaluation method. The results are shown in Table 11. Rubber Composition No Comparative Ex Example 34 35 36 37 Rubber component Natural rubber 60 - - - Epoxidised natural rubber 25 - 60 60 60 Solution polymerised SBR-1 40 40 40 - Epoxidised solution-polymerised SBR-1 - - - 40 Hexamethylenediamine - - 1 1 Amino group content (mmol) - - 17.2 17.2 Formula (1) unit content (mmol) - - 8.6 8.6 Carbon black 45 45 45 45 Chinese white 5 5 5 5 Stearic acid 1 1 1 1 Sulfur 1.75 1.75 1.75 1.75 Vulcanisation accelerator (Acc NS) 1 1 1 1 Characteristics tanδ(-10°C) 0.212 0.382 0.441 0.554 tanδ(70°C) 0.076 0.120 0.077 0.067 W.I.R. 0 0.80 1.08 1.61 R.I.R. 0 -0.58 -0.01 0.13 T.I.R. 0 -0.36 1.05 1.88 Evaluation Wet grip 97 108 110 117 Fuel saving 92 106 92 90 As seen from Table 11, the rubber composition No. 35 containing the epoxidised natural rubber was inferior in fuel saving performance though superior in wet grip performance to the rubber composition No. 34 which was free of the epoxidised natural rubber. The use of a combination of the epoxidised natural rubber and hexamethylenediamine permitted rubber compositions (Nos. 36 and 37) to exhibit improvements in both the wet grip and fuel saving performances. It can be reaffirmed from the comparison between the rubber compositions Nos. 36 and 37 that by using a rubber component entirely comprised of the epoxidised rubber, the wet grip performance is significantly improved (refer to the results shown in Table 4). Further it can be seen that by replacing a part of the epoxidised rubber with the epoxidised solution-polymerised SBR-1, the wet grip performance is improved (compare the rubber compositions Nos. 5 and 6 to No. 37). Rubber compositions Nos. 38-40 Rubber compositions Nos. 38 to 40 were prepared by mixing the compounding ingredients shown in Table 12. As the diamine compound hexamethylenediamine was used, and as the epoxidised rubber epoxidised SBR 1502 resulting from epoxidisation of SBR 1502 (emulsion-polymerised SBR) was used. The epoxidised emulsion-polymerised SBR 1502 was obtained in the following manner. A separable flask of 2 litres provided with a cooler, agitator, thermometer and funnel was charged with 1.5 litres of toluene, to which added were 81g of SBR 1502 and 1.62g of 2,6-di-tert-butyl-4-methylphenol. The mixture was subjected to complete dissolution by warming to 40°C. In turn, this solution was added with 0.17 mol of formic acid, and when the solution became homogeneous, an aqueous solution of hydrogen peroxide was added dropwise to the solution in an amount equivalent to 0.49 mol of hydrogen peroxide. The mixture thus obtained was subjected to reaction at 40°C under stirring for 5 hours. The reaction solution was allowed to cool, and then washed with a diluted solution of sodium hydroxide, then with a washing water to remove unreacted formic acid and hydrogen peroxide. Finally, the resulting polymer was precipitated in excessive methanol and dried to give the epoxidised SBR 1502. This reaction caused the glass transition temperature by Dsc measurement to vary from -55°C to 35°C, and a peak which was assumed to be attributed to epoxy group was observed near 890cm -1 by FT-IR. In the same manner as with the rubber composition No. 1, tanδ at -10°C and 70°C of each of the rubber compositions 38-40 were measured and then the characteristics W.I.R., R.I.R. and T.I.R. were calculated. For the calculation of W.I.R., R.I.R. and T.I.R., the rubber composition No. 38 was employed as a control rubber composition. Table 12 shows compounding ingredients of rubber compositions Nos. 38-40, contents of amino group and carboxyl group, and values of tanδ, W.I.R., R.I.R. and T.I.R. Wet grip and fuel saving performances were evaluated on the thus prepared rubber compositions Nos. 38 to 40 by the aforementioned evaluation method. The results are shown in Table 12. Rubber Composition No Comparative Ex Example 38 39 40 Rubber component SBR 1502 100 100 - Epoxidised SBR 1502 - - 100 Hexamethylenediamine - 1 1 Amino group content (mmol) - 17.2 17.2 Formula (1) unit content (mmol) - 8.6 8.6 Carbon black 45 45 45 Chinese white 5 5 5 Stearic acid 1 1 1 Sulfur 1.75 1.75 1.75 Vulcanisation accelerator (Acc NS) 1 1 1 Characteristics tanδ(-10°C) 0.217 0.192 0.797 tanδ(70°C) 0.126 0.137 0.091 W.I.R. 0 -1.12 2.67 R.I.R. 0 -0.09 -0.28 T.I.R. 0 -0.29 3.23 Evaluation Wet grip 96 95 126 Fuel saving 105 109 97 As seen from Table 12, the use of hexamethylenediamine solely without combination with the epoxidised natural rubber was found to contribute to no change in wet grip and fuel saving performances from the comparison between the rubber compositions Nos. 38 and 39. The combination of the epoxidised natural rubber with hexamethylenediamine caused both the wet grip and fuel saving performances to be improved (refer to the rubber composition No.40). Further, where the rubber component was entirely comprised of the epoxidised rubber, the use of the epoxidised SBR rather than the epoxidised natural rubber significantly improved the wet grip performance (compare the rubber composition No. 37 to No. 40). Rubber compositions Nos. 41-42 Rubber compositions Nos. 41 and 42 were prepared by mixing the compounding ingredients shown in Table 13. As the diamine compound hexamethylenediamine was used, and as the epoxidised rubber epoxidised solution-polymerised SBR-2 resulting from epoxidisation of solution-polymerised SBR-2 was used. The solution-polymerised SBR-2 was a product of SUMITOMO KAGAKU Co and contained 25% by weight of styrene and 40% by weight of a vinyl component. This epoxidisation caused the glass transition temperature to vary from -40°C to 26°C by DSC measurement. In the same manner as with the rubber composition No. 1, tanδ at -10°C and 70°C of each of the rubber compositions 41 and 42 were measured and then the characteristics W.I.R., R.I.R. and T.I.R. were calculated. For the calculation of W.I.R., R.I.R. and T.I.R., the rubber composition No. 41 was employed as a control rubber composition. Table 13 shows compounding ingredients of rubber compositions Nos. 41 and 42, content of amino group, and values of tanδ, W.I.R., R.I.R. and T.I.R. Wet grip and fuel saving performances were evaluated on the thus prepared rubber compositions Nos. 41 to 42 by the aforementioned evaluation method. The results are also shown in Table 13. Rubber Composition No. Comp Ex Example 41 42 Rubber component Solution-polymerised SBR-2 100 - Epoxidised solution-polymerised SBR-2 - 100 Hexamethylenediamine 0.6 Amino group content (mmol) - 10.3 Formula (1) unit content (mmol) - 5.2 Carbon black 45 45 Chinese white 5 5 Stearic acid 1 1 Sulfur 1 1 Vulcanisation accelerator (Acc NS) 0.6 0.6 Characteristics tanδ(-10°C) 0.450 1.146 tanδ(70°C) 0.140 0.104 W.I.R. 0 1.55 R.I.R. 0 0.26 T.I.R. 0 2.06 Evaluation Wet grip 111 130 Fuel saving 111 102 From the comparison of the rubber compositions Nos. 41 and 42 with the rubber compositions Nos. 39 and 40, it is seen that the use of the rubber component comprised of the solution-polymerised SBR rather than the emulsion-polymerised SBR tended to be responsible for lowered fuel saving performance though contributing to improved wet grip performance. Accordingly, when stress is laid on the wet grip performance with the fuel saving performance being kept at conventional degree, a rubber composition preferably comprises a combination of the rubber component entirely formed of the epoxidised solution-polymerised SBR, and the amine compound. As seen from Tables 11 to 13, the rubber compositions Nos. 37, 40 and 42 containing the epoxidised SBR as the epoxidised rubber exhibited higher T.I.R. values, and the combination of the amine compound with the epoxidised SBR contributed to superior wet grip and fuel saving performance. As has been described, the rubber composition in accordance with the present invention contains an amine compound and satisfies specific requirements of tanδ in the high temperature region (70°VC) and tanδ in the low temperature region (-10°C). The rubber composition exhibits significantly improved wet grip performance with the fuel saving performance little affected. Further, by varying the type and content of the amine compound, not only the wet grip performance but also the fuel saving performance can be improved. Therefore, the use of the rubber composition for a tyre of the present invention makes it possible to provide tyres assuring safety and satisfactory fuel saving performance. A rubber composition containing an amine compound and a carboxylic acid having two or more carboxyl groups according to another aspect of the present invention exhibits improved rolling workability and easily satisfies the requirements of tanδ. The use of such rubber composition will readily provide tyres assuring safety and satisfactory fuel saving performance.

Claims: A rubber composition for a tyre comprising a diene rubber having an epoxy group, either a monoamine compound or a diamine compound having a structural unit represented by the formula (1): (where R 1 , R 2 , R 3 , and R 4 are each a hydrogen atom or an alkyl group having one to three carbon atoms, and X represents any structural formula) and carbon black, wherein the rubber composition has a dissipation factor at -10°C (tanδ(-10°C)) and a dissipation factor at 70°C (tanδ(70°C)) which satisfy the following expressions: 0. 03 ≤ tanδ(70°C) ≤ 0. 35 W.I.R = {tanδ(-10°C) - tanδc(-10°C) } / tanδc(-10°C) R.I.R = {tanδc(70°C) - tanδ(70°C) } / tanδc(70°C) T.I.R = W.I.R + 2 x (R.I.R) ≥ 0. 3 where tanδc(-10°C) and tanδc(70°C) are tanδ at -10°C and 70°C of a control rubber composition of the same formulation as the rubber composition except that the control rubber composition includes a diene rubber having no epoxy group and is free of the diamine compound having a structural unit represented by the formula (1), characterised by further comprising a carboxylic acid having two or more carboxyl groups in a molecule thereof.

Inventors: KIKUCHI NAOHIKO, MATSUO TOSHIROU, MURAOKI KIYOSHIGE, TERAKAWA KATSUMI, UEDA MINORU, KIKUCHI, NAOHIKO, MATSUO, TOSHIROU, MURAOKI, KIYOSHIGE, TERAKAWA, KATSUMI, UEDA, MINORU
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LIMITED
Application year: 1995
Main classification: C08K5
IPC codes: C08L9, C08L15, C08L19, C08G59, C08L21, C08L63, C08K5, C08K3, C08K13, B60C1