Additive for masticating natural rubber, mastication method, composition obtained thereof and pneumatic tire

Abstract: An additive for masticating natural rubber comprising at least one compound selected from the group consisting of benzoic acids, naphthoic acids, dicarboxylic acids and compounds represented by Formula (IV): wherein m and n each represent an integer of 1 to 3; p represents an integer of 1 to 4; m + n + p = 6; X is O, NR 8 or (R 9 O) q ; R 6 is a C 1 ~ C 24 alkyl, a C 2 ~ C 24 alkenyl, or a C 6 ~ C 24 aryl group; and R 7 is H, OH, an alkyl, alkenyl, or aryl group; R 8 is H or C 1 ~ C 24 alkyl group; R 9 is a C 1 ~ C 4 alkyl group; q is an integer of 1 to 5, a mastication method and a natural rubber composition each of which uses the additive, and a pneumatic tire using the natural rubber composition.

Description: The present invention relates to a mastication method of natural rubber using an additive capable of reducing the amount of gel without lowering the molecular weight and capable of improving the processability as well, a pneumatic tire using the above natural rubber composition, and the use of the additive for masticating natural rubber. In rubber products such as tires, belts and hoses, there are blended various compounding agents such as vulcanizing agents and reinforcing agents. In order to facilitate this blending work and molding work of the rubber products, a mastication step for crude rubber (natural rubber) which is a raw material is essential. In general, the significance of mastication lies in providing crude rubber with satisfactory plasticity by giving shearing force to it. This mastication facilitates blending and milling work (work for milling homogeneously various compounding agents into crude rubber) like dispersing of various compounding agents such as vulcanizing agents and fillers. Further, the mastication stabilizes the dimensions, makes the skin fine and makes scorching difficult in after-steps of rolling and extruding, that is it serves the purpose for making it easy to carry out these operations. In this mastication, the molecular weight is also reduced by breaking the molecules of crude rubber to have the rubber plasticized by mere mechanical mastication using a roll machine, an internal mixer and the like. However, such mechanical mastication requires electricity, labor and time in very large quantities. Accordingly, it is generally known well that a peptizer is used in a mastication step of crude rubber. Thiophenols having substituents on an aromatic ring and disulfides have so far been used as a peptizer. However, there are such problems that the use of these peptizers allows the plasticization to go on to reduce the viscosity of the rubber and that at the same time the molecular chains of the rubber are broken and vulcanized rubber obtained by vulcanizing the rubber becomes inferior in breaking characteristics. On the other hand, in natural rubber there exist gels formed by intertwined molecular chains and by the reaction between functional groups contained in the isoprene chains of the natural rubber or the reaction of such functional groups with non-rubber components contained in the natural rubber. This gel component causes an increase in viscosity of crude rubber (storage hardening) or poor dispersion of the rubber compounding agents. Ordinary mastication reduces this gel component but breaks the molecular chains at the same time. Accordingly, the same problems as described above take place. Thus, so far there have been no techniques for selectively reducing the amount of gel without lowering the molecular weight of natural rubber. Attention is also drawn to the disclosures of DD-A-150473, US-A-3297607, US-A-4410656, and EP-A-0551818. An object of the present invention is to provide a mastication method of natural rubber using an additive capable of reducing the amount of gel of natural rubber without lowering the molecular weight thereof and capable of improving the processability as well, a natural rubber composition obtained by the above mastication method, and a pneumatic tire using the above natural rubber composition. Intensive investigations made by the present inventors in order to solve the conventional problems described above have resulted in finding that specific compounds have such effects that the amount of gel of natural rubber can be reduced without lowering the molecular weight thereof and the processability can be improved as well. Further researches have resulted in successfully obtaining an intended additive for masticating natural rubber, a mastication method of natural rubber using the above additive, a natural rubber composition obtained by the above mastication method, and a pneumatic tire using the above natural rubber composition. The present invention provides a mastication method of natural rubber, comprising a step of carrying out mastication at a start temperature of 60-85°C after adding 0.5 to 20 parts by weight of at least one additive for masticating natural rubber relative to 100 parts by weight of natural rubber, wherein said additive comprises at least one compound selected from the group consisting of benzoic acids represented by the following Formula (I), naphthoic acids represented by the following Formula (II), dicarboxylic acids represented by the following Formula (III) and aromatic polycarboxylic acid derivatives represented by the following Formula (IV): wherein R 1 and R 2 each represent -H, -CH 3 , -CH 2 CH 3 , - C(CH 3 ) 3 , -NH 2 , -OH, -SH, -NO 2 , -COPh, -Br, -Cl, -CH 2 CH 2 CH 3 , -CH(CH 3 ) 2 , -OCH 3 or -OC 2 H 5 and may be same or different, wherein R 3 and R 4 each represent -H, -OH or -NH 2 , HOOC―R 5 ―COOH wherein R 5 represents ―(CH 2 ) ℓ ― (ℓ is an integer of 1 to 14), and wherein m and n each represent an integer of 1 to 3; p represents an integer of 1 to 4; m + n + p = 6; when m≧2, a part or all of the carboxyl groups may be dehydrated to an anhydride; X is any one of O, NR 8 (R 8 is H or an alkyl group having 1 to 24 carbon atoms) and (R 9 O) q (R 9 is an alkyl group having 1 to 4 carbon atoms, and q is an integer of 1 to 5); R 6 is any one of an alkyl group having 1 to 24 carbon atoms, an alkenyl group having 2 to 24 carbon atoms and an aryl group having 6 to 24 carbon atoms; R 7 is any one of H, OH, an alkyl group, an alkenyl group and an aryl group; and a part or all of hydrogens in both R 6 and R 7 may be substituted with halogens. The benzoic acids represented by Formula (I) described above are preferably p-toluic acid and p-t-butylbenzoic acid. The dicarboxylic acids represented by Formula (III) described above are preferably terephthalic acid, isophthalic acid, and adipic acid. The aromatic polycarboxylic acid derivatives represented by Formula (IV) described above are preferably derivatives of any one of phthalic acid, trimellitic acid, pyromellitic acid and anhydrides thereof. A peptizer is preferably used in combination with the additive for natural rubber, the blending amount of said peptizer being 10% by weight or less based on the blending amount of the said additive. The invention in a further aspect provides a pneumatic tire characterized by using a natural rubber composition obtainable by the mastication method of natural rubber as described in the above. According to the present invention, there is provided a mastication method of natural rubber using an additive capable of reducing the amount of gel of natural rubber without lowering the molecular weight thereof and capable of improving the processability as well. The use of this additive provides a natural rubber composition in which the rubber compounding agents do not cause poor dispersion and vulcanized rubber obtained by vulcanizing the rubber composition does not become inferior in breaking properties. A pneumatic tire obtained by using the above rubber composition has excellent heat-generating properties and abrasion resistance. The embodiments of the present invention are explained in detail below. The additive for masticating natural rubber used in the method of the present invention comprises at least one compound selected from the group consisting of the benzoic acids represented by Formula (I) described above, the naphthoic acids represented by Formula (II) described above, the dicarboxylic acids represented by Formula (III) described above and the aromatic polycarboxylic acid derivatives represented by Formula (IV) described above. The additive for natural rubber used in the method of the present invention can reduce the amount of gel of the natural rubber without lowering the molecular weight thereof by adding at a mastication step of natural rubber. In particular, the processability can be improved as well by using the aromatic polycarboxylic acid derivatives represented by Formula (IV) described above. Examples of the benzoic acids represented by Formula (I) described above include benzoic acid, p-toluic acid, p-t-butylbenzoic acid, anthranilic acid, p-aminobenzoic acid, p-nitrobenzoic acid, o-benzoylbenzoic acid, salicylic acid and thiosalicylic acid. Among them, p-toluic acid and p-t-butylbenzoic acid are preferable in terms of the effects of the present invention, low cost and no sublimating property. Examples of the naphthoic acids represented by Formula (II) described above included-naphthoic acid, β-naphthoic acid and β-oxynaphthoic acid. Examples of the dicarboxylic acids represented by Formula (III) described above include terephthalic acid, isophthalic acid, adipic acid, citraconic acid, sebacic acid and dodecanoic diacid. Further, among them, terephthalic acid, isophthalic acid, and adipic acid are preferable in terms of the great effects of the present invention and low cost. Preferred examples of the aromatic polycarboxylic acid derivatives represented by Formula (IV) described above are derivatives of any one of phthalic acid, trimellitic acid, pyromellitic acid and anhydrides thereof. In the aromatic polycarboxylic acid derivatives represented by Formula (IV) described above, R 6 is preferably an alkyl group having 1 to 22 carbon atoms, and X is preferably O or (R 9 O) q . In particular, X is preferably O. Further, the aromatic polycarboxylic acid derivatives represented by Formula (IV) described above are preferably derivatives of any one of phthalic acid, trimellitic acid, pyromellitic acid and anhydrides thereof in which R 6 in Formula (IV) described above is an alkyl group having 1 to 22 carbon atoms and X is O or (R 9 O) q . Particularly preferable are such derivatives wherein X is O. More specifically, suitable such derivatives include monostearyl phthalate, monodecyl phthalate, N-octyl phthalamic acid, monodecyl trimellitate, monostearyl trimellitate, monostearyl pyromellitate and distearyl pyromellitate. Among them, monoalkyl phthalates are preferable in terms of the effect of reducing the amount of gel, plasticizing effect and low cost, and monostearyl phthalate is particularly preferable. The mastication method of natural rubber of the present invention is characterized in that the amount of gel of the natural rubber can be reduced without lowering the molecular weight thereof by masticating 100 parts by weight of natural rubber blended with 0.05 to 20 parts by weight of at least one compound selected from the benzoic acids represented by Formula (I) described above, the naphthoic acids represented by Formula (II) described above, the dicarboxylic acids represented by Formula (III) described above and the aromatic polycarboxylic acid derivatives represented by Formula (IV) described above. The blending amount of the additives represented by Formulas (I) to (IV) described above has been set to 0.05 to 20 parts by weight per 100 parts by weight of the natural rubber component because a blending amount of less than 0.05 part by weight can not provide the desired effects of the present invention. On the other hand, a blending amount exceeding 20 parts by weight exerts an adverse effect on the breaking characteristics of the vulcanized rubber to bring about undesired effects. The blending amount of the additives represented by Formulas (I) to (IV) varies within the range described above depending on the kind of natural rubber such as RSS (Ribbed Smoked sheets) and TSR (Technical Standard Rubber) and the grade, and is preferably within the range from 1 to 5 parts by weight from the viewpoint of cost. In the mastication method of the present invention, the additives described above may be used alone or in combination of two or more kinds thereof. The additives according to the present invention may be used in combination with thiophenols and disulfides which are usually used as peptizers by substituting for a part of the additives of the present invention. The blending amount of the peptizers has to be 10 % by weight or less based on the blending amount of the additives of the present invention. If the blending amount exceeds 10 % by weight, an adverse effect of breaking the molecular chains is increased, and the breaking properties of the vulcanized rubber obtained by vulcanization become inferior. Further, in the mastication method of the present invention, with respect to the masticating machine used, mastication time, rubber temperature and the like, the optimum machine and ranges shall be selected according to uses of the rubber. The masticating machine used may be, for example, a roll machine, an internal mixer or a Gordon plasticator. When, for example, a roll machine is used, the roll interval, roll revolution ratio, roll revolution number and the like are selected in the optimum ranges. A start temperature of mastication, though varied depending on uses of the rubber, is 60 to 85°C, preferably 70 to 80°C. Mastication time is 60 to 120 seconds, preferably 80 to 100 seconds. The mastication method of natural rubber according to the present invention can reduce the amount of gel of natural rubber without lowering the molecular weight thereof and improve the processability as well (these points shall be explained in further detail in the examples described later). Gels formed by intertwined molecular chains and the reaction between functional groups contained in the isoprene chains of natural rubber or the reaction of such functional groups with non-rubber components contained in the natural rubber are absent from the natural rubber composition obtained by the preceding mastication method of natural rubber. Thus, the amount of gel of the natural rubber can selectively be reduced without lowering the molecular weight thereof. Accordingly, the use (plastic processing) of this natural rubber composition makes it easy to disperse various compounding agents such as a vulcanizing agent and a filler and considerably facilitates operations at a mixing step, that is operations for milling homogeneously various compounding agents in non-productive mixing in which fillers such as carbon black, oils and an anti-oxidant are blended into crude rubber and milled and then in productive mixing in which sulfur and a vulcanization accelerator are blended and milled. Further, the use of the natural rubber composition stabilizes the dimensions, makes the skin fine and makes scorching difficult in rolling and extruding which are after-steps, that is makes it possible to carry out these operations very easily and efficiently and does not cause the breaking characteristics of the vulcanized rubber obtained by vulcanization to become inferior. The resulting natural rubber composition can suitably be used for rubber products such as conveyor belts, belts and hoses in addition to pneumatic tires. The pneumatic tire of the present invention is characterized by using the natural rubber composition obtained by the mastication method described above. In general, a pneumatic tire has a structure that comprises a tread part, a pair of side parts connected at both sides of said tread part and a pair of bead parts each of which is formed in the inner circumferences of said side parts and that is reinforced with a carcass ply formed by disposing carcass ply cords in a tire radial direction and a belt layer surrounding said carcass ply and buried inside of the tread part. The pneumatic tire of the present invention uses the natural rubber composition obtained by the mastication method described above and is blended, if necessary, with a suitable amount of synthetic rubber depending on the tire component members forming the tire structure described above. That is, if the tire component members are, for example, tread rubber, belt coating rubber, carcass ply rubber, bead filler rubber, rubber chafer and side wall rubber, the natural rubber composition obtained by the mastication method described above is blended, if necessary, with a suitable amount of synthetic rubber according to uses of these tire rubber component members. The synthetic rubber to be blended includes; for example, styrene-butadiene rubber (SBR), butadiene rubber (BR), isoprene rubber (IR), butyl rubber (IIR), halogenated butyl rubber (X-IIR), ethylene-propylene rubber (EPDM) and mixtures thereof but is not restricted thereto. In the production of the pneumatic tire of the present invention, first a rubber component comprising the natural rubber composition obtained by the mastication method described above and blended of necessary with synthetic rubber are blended with a filler such as carbon black, oils and an anti-oxidant depending on the preceding various tire component members to be applied to carry out non-productive mixing and then blended with sulfur and a vulcanization accelerator to carry out productive mixing. Then, the resulting rubber composition is applied to the respective tire component members. These component members are molded into a green tire on a drum, and then the green tire is vulcanized by means of a vulcanizing machine, whereby an intended pneumatic tire is obtained. In the production of the pneumatic tire of the present invention, with respect to the mixing machine, the vulcanizing machine, mixing time (non-productive mixing time and productive mixing time), mixing temperature (non-productive mixing temperature and productive mixing temperature), vulcanizing time and vulcanizing temperature, the optimum machines and ranges are selected depending on uses of the pneumatic tire. The pneumatic tire of the present invention thus constituted has excellent heat-generating property and abrasion resistance since reduction in the molecular weights of the natural rubber compositions used for the various tire component members described above is less than in conventional natural rubber compositions. The present invention will be further described with reference to the following non-limiting Examples. In the following Examples and Comparative Examples, the amounts of gel, the molecular weights and the Mooney viscosities were determined by the following methods. . Measurement of the amount of gel of masticated rubber In order to measure the amount of gel of masticated rubber, 0.2 g of the rubber was immersed in 60 ml of toluene and left standing at room temperature for 24 hours. Then, the gel component was separated by means of a centrifugal separator, and the solvent was evaporated in a vacuum drier. Then, the weight of the gel was measured. . Measurement of molecular weight of masticated rubber The molecular weight of masticated rubber was determined by means of GPC (gel permeation chromatography) manufactured by Tosoh Corp. after dissolving 0.03 g of the rubber in 30 ml of THF (tetrahydrofuran) and leaving standing at room temperature for 24 hours. ·Measurement of Mooney viscosity (ML 1+4 , processability) of masticated rubber The Mooney viscosity of the masticated rubber was measured for processability at 100°C by means of a Mooney viscometer SMV201 manufactured by Shimadzu Corp., and the value obtained after 5 minutes was employed. Examples 1 to 9 and Comparative Examples 1 to 4 Natural rubber was blended with the additives according to the recipes (parts by weight) shown in Table 1, and then was masticated by means of a banbury mixer for 90 seconds at a starting temperature of 80°C. The resulting masticated rubber was measured for amount of gel and molecular weight. These results are shown in the following Table 1. RSS No. 3 was used as natural rubber. Examples 10 to 17 and Comparative Examples 5 to 7 Natural rubber was blended with the additives according to the recipes (parts by weight) shown in Table 2, and then was masticated by means of a banbury mixer for 90 seconds at starting temperature of 80°C. The resulting masticated rubber was measured for amount of gel, molecular weight and Mooney viscosity. These results are shown in the following Table 2. RSS No. 3 was used as natural rubber. Considerations on Table 1 and Table 2 It has been confirmed from the general point of view that in Examples 1 to 9 of the present invention, the amounts of gel can be reduced without lowering the molecular weights as compared with Comparative Examples 1 to 4 falling outside the scope of the present invention, and that in Examples 10 to 17, the processabilities can be improved as well. From the individual cases, it can be seen that in Comparative Example 1 in which the natural rubber has not been masticated, the amount of gel is increased. In Comparative Example 2 in which the natural rubber has been masticated as usual, the amount of gel is also increased and the molecular weight is lowered as well. Further, it can be confirmed that in Comparative Examples 3 to 6 in which conventional peptizers have been added to masticate the natural rubber, the amounts of gel are reduced but the molecular weights are notably lowered as well. It can be seen that in Comparative Example 7 in which an insufficient amount of an aromatic polycarboxylic acid derivative (which is an additive which may be used in the method according to the present invention) is blended, the amount of gel becomes large. In contrast with this, it has been confirmed that in Examples 1 to 6 and Examples 9 to 17 in which sufficient amounts of suitable additives have been used to masticate the natural rubber, the amounts of gel can be markedly reduced without lowering the molecular weights, and further, the processabilities can be improved as well. Further, it has been confirmed that in Examples 7 and 8 in which a peptizer has been substituted for a part of the additive, the amounts of gel can be reduced without lowering the molecular weights so much. Examples 18 to 25 and Comparative Examples 8 to 12 The rubber compositions obtained by using the natural rubber compositions obtained in Examples 10 to 17 and in Comparative Examples 1, 2 and 5 to 7 and mixing them according to the recipes (parts by weight) shown in the following Table 3 by means of a banbury mixer under the following conditions were used for tread parts to prepare pneumatic tires (size 11R 22-5): Mixing conditions: non-productive mixing: start temperature 80°C, revolution number 50 RPM, time 4 minutes productive mixing: start temperature 80°C, revolution number 50 RPM, time 1 minute Vulcanizing conditions: 150°C, 25 minutes The respective pneumatic tires obtained in Examples 18 to 25 and Comparative Examples 8 to 12 were measured for heat-generating property and abrasion resistance by the following methods. These results are shown in the following Table 3. . Measurement of heat-generating property: A test tire was charged with air having a maximum air pressure prescribed in JATMA and subjected to a running test by means of a drum test machine at a speed of 65 km/hr with a load of 150 % of the maximum load applied to determine the maximum temperature in the inside of the tread. A reciprocal number of the temperature was calculated to show a heat-generating property by index, wherein that of Comparative Example was set at 100 (control). The larger the value, the better the heat-generating property. ·Measurement of abrasion resistance: A commercial car equipped with test tires was driven 5000 km on roads, and then the traveling distance per abrasion by 1 mm was calculated from the remaining grooves. The value of Comparative Example 10 was set at 100 (control), and the larger the value, the better the abrasion resistance of the tire.

Claims: A mastication method of natural rubber, comprising a step of carrying out mastication at a start temperature of 60 ∼ 85°C after adding 0.05 to 20 parts by weight of at least one additive for masticating natural rubber relative to 100 parts by weight of natural rubber, wherein said additive comprises at least one compound selected from the group consisting of benzoic acids represented by the following Formula (I), naphthoic acids represented by the following Formula (II), dicarboxyic acids represented by the following Formula (III) and aromatic polycarboxylic acid derivatives represented by the following Formula (IV): wherein R 1 and R 2 each represent -H, -CH 3 , -CH 2 CH 3 , -C(CH 3 ) 3 , - MH 2 , -OH, -SH, -NO 2 , -COPh, -Br, -Cl, -CH 2 CH 2 CH 3 , - CH(CH 3 ) 2 , - OCH 3 , or -OC 2 H 5 and may be same or different, wherein R 3 and R 4 each represent -H, -OH or -NH 2 , HOOC-R 5 -COOH wherein R 5 represents ―(CH 2 ) ℓ ― (ℓ is an integer of 1 to 14), and wherein m and n each represent an integer of 1 to 3; p represents an integer of 1 to 4; m + n + p = 6; when m≧2, a part or all of the carboxyl groups may be dehydrated to an anhydride; X is any one of O, NR 5 (R 6 is H or an alkyl group having 1 to 24 carbon atoms) and (R 9 O) q (R 9 is an alkyl group having 1 to 4 carbon atoms, and q is an integer of 1 to 5); R 6 is any one of an alkyl group having 1 to 24 carbon atoms, an alkenyl group having 2 to 24 carbon atoms and an aryl group having 6 to 24 carbon atoms; R 7 is any one of H, OH, an alkyl group, an alkenyl group and an aryl group; and a part or all of hydrogens in both R 6 and R 7 may be substituted with halogens. A method as claimed in claim 1, wherein the benzoic acids represented by Formula (I) are p-toluic acid, p-t-butylbenzoic acid, anthranilic acid, p-aminobenzoic acid, p-nitrobenzoic acid, o-benzoylbenzoic acid, salicylic acid and thiosalicylic acid. A method as claimed in claim 2, wherein the benzoic acids represented by Formula (I) are p-toluic acid and p-t-butybenzoic acid. A method as claimed in claim 1, wherein the dicarboxylic acids represented by Formula (III) are terephthalic acid, isophthalic acid and adipic acid. A method as claimed in claim 1, wherein the aromatic polycarboxylic acid derivatives represented by Formula (IV) are derivatives of any one of phthalic acid. trimellitic acid, pyromellitic acid and anhydrides thereof. A method as claimed in any of claims 1 to 5, wherein a peptizer is used in combination with the additive for natural rubber, the blending amount of said peptizer being 10 % by weight or less based on the blending amount of the said additive. A pneumatic tire characterized by using a natural rubber composition obtainable by the mastication method of natural rubber as claimed in any of claims 1 to 6. Use of an additive comprising at least one compound selected from aromatic polycarboxylic acid derivatives represented by the following Formula (IV) for masticating natural rubber: wherein m and n each represent an integer of 1 to 3; p represents an integer of 1 to 4; m + n + p = 6; when m≧2, a part or all of the carboxyl groups may be dehydrated to an anhydride; X is any one of O, NR 8 (R 8 is H or an alkyl group having 1 to 24 carbon atoms) and (R 9 O) q (R 9 is an alkyl group having 1 to 4 carbon atoms, and q is an integer of 1 to 5); R 6 is any one of an alkyl group having 1 to 24 carbon atoms, an alkenyl group having 2 to 24 carbon atoms and an aryl group having 6 to 24 carbon atoms; R 7 is any one of H, OH, an alkyl group, an alkenyl group and an aryl group; and a part or all of hydrogens in both R 6 and R 7 may be substituted with halogens.

Inventors: NISHI ISAO, NOHARA DAISUKE, TSUCHIHASHI MASAAKI, NISHI, ISAO, NOHARA, DAISUKE, TSUCHIHASHI, MASAAKI, NISHI, ISAO C/O KAO CORP., TSUCHIHASHI, MASAAKI C/O KAO CORP.
Applicants: BRIDGESTONE CORP, KAO CORP, BRIDGESTONE CORPORATION, KAO CORPORATION
Application year: 1998
Main classification: C08K5
IPC codes: C08K5, C08C19