Method for improving rubbers

This invention relates to a method for improving a rubber having --C(CH₃)═C--type unsaturated bonds. More particularly, the present invention relates to a method for improving a rubber having --C(CH₃)═C--type unsaturated bonds, in green strength of carbon-containing unvulcanized blend of said rubber as well as in rebound resiliency of carbon-containing vulcanized blend of said rubber.

In order to obtain a rubber composition having an improved green strength, a method has conventionally been known (Japanese Patent Publication No. 10635/1973) wherein a rubber is reacted with a compound having a carboxyl group, such as maleic anhydride or a hydroxamylchloridic acid (e.g. terephthalohydroxamylchloridic acid).

Also, in order to obtain a rubber composition having an improved rebound resiliency, a method is known (Japanese Patent Laid-Open Publication No. 81301/1984) wherein a polyisoprene rubber is treated with a Lewis acid such as SnCl₄.

However, any of the above methods has been unable to attain both of improved green strength and improved rebound resiliency.

Therefore, the object of the present invention resides in providing a rubber material having an improved green strength and an improved rebound resiliency.

The present inventors found that a rubber, when subjected to both of a modification reaction by Lewis acid and a carboxylation reaction, shows such significant improvements in both green strength and rebound resiliency as can not be expected in conventional methods of subjecting a rubber only to a modification reaction by Lewis acid or to a carboxylation reaction.

Thus, according to the present invention there is provided a method for improving a rubber having --C(CH₃)═C--type unsaturated bonds, characterized by subjecting said rubber to two reactions consisting of a modification reaction by Lewis acid and a carboxylation reaction in any desired order.

As the rubber having --C(CH₃)═C--type unsaturated bonds, used in the present invention (hereinafter this rubber is referred, in some cases, to simply as a or the rubber), there can be mentioned a homopolymer rubber of a conjugated diene (e.g. isoprene, 2,3-dimethylbutadiene), a copolymer rubber of at least two of said conjugated dienes, a copolymer rubber of at least one of said conjugated dienes and at least one other monomer, a ring-opening polymer rubber of a cycloolefin (e.g. cyclopentene, norbornene), a polymer rubber of a cyclic diene (e.g. ethylidenenorbornene, cyclopentadiene), a polyolefin rubber such as a copolymer rubber between at least one of said cyclic dienes and at least one olefin, etc. Specific examples of these rubbers include a polyisoprene rubber, a butyl rubber, a polydimethylbutadiene rubber, a butadiene-isoprene copolymer rubber, an isoprene-styrene copolymer rubber, a butadiene-isoprene-styrene copolymer rubber, a butadiene-isoprene-acrylonitrile copolymer rubber and a styrene-isoprene-styrene block copolymer rubber.

In the modification reaction by Lewis acid of the present invention, a rubber is treated with a Lewis acid in a solution state wherein the rubber is dissolved in an organic solvent, or in the state of a rubber cement right after polymerization. The solvent can be selected freely from aromatic hydrocarbon solvents such as benzene, toluene and the like; paraffinic hydrocarbon solvents such as butane, hexane and the like; cycloparaffinic hydrocarbon solvents such as cyclohexane and the like; and halogenated hydrocarbon solvents such as chloroform, ethylene dichloride and the like. They can be used singly or in combination. Those solvents having an activity toward Lewis acids, such as alcohols, ketones, ethers and the like are restricted in their use amounts. The rubber concentration in the solution is decided appropriately between 1 and 30% by weight. The water content in the solution desirably is 400 ppm or below ordinarily.

The Lewis acid used in the present invention can be those ordinarily known. Typical examples include metal halides and semimetal halides (e.g. halides of such elements as Be, B, Al, Si, P, S, Ti, V, Fe, Zn, Ga, Ge, As, Se, Zr, Nb, Mo, Cd, Sn, Sb, Te, Ta, W, Hg, Bi and U, as well as halides of such oxides as PO, SeO, SO, SO₂ and VO), organic halides and complexes thereof. Specifically there are mentioned BF₃, (CH₃)₂ BF, AlCl₃, AlBr₃, (C₂ H₅)AlCl₂, POCl₃, TiCl₄, VCl₄, MoCl₆, (CH₃)SnCl₃, TeBr₄, etc. Particularly preferable are BF₃ (C₂ H₅)₂, SnCl₄, SbCl₅, WCl₆, SnBr₄, BCl₃, TeCl₄, etc.

The amount of the Lewis acid required for the modification is greatly affected by the kind of the solvent in the rubber solution or by water and other impurities present in the rubber solution and hence can not be specified generally. However, the amount ordinarily is 0.1 to 5 parts by weight, preferably 0.5 to 3 parts by weight per 100 parts by weight of the rubber. It is possible to substantially reduce the amount of the Lewis acid by together using an appropriate amount (e.g. 0.1 to 25 parts by weight, preferably 0.5 to 10 parts by weight per 100 parts by weight of the rubber) of a compound known as a cationic polymerization catalyst (e.g. a compound having active halogen such as trichloroacetic acid, tribromoacetic acid, water plus methyl alcohol, 2,4-dinitrophenol, p-nitrophenol or the like, or an organic compound having active halogen such as t-butyl chloride, triphenylmethane chloride, benzyl chloride or the like). The use of such a co-catalyst is also effective for the suppression of gel generation. The reaction temperature used in the modification reaction by Lewis acid is not particularly restricted but ordinarily is -20° to 100° C., preferably 10° to 60° C. The reaction time is not particularly restricted, either, but ordinarily is selected appropriately between 2 minutes and 10 hours.

The carboxylation reaction used in the present invention is not particularly restricted. It includes all reactions capable of introducing, using a modifier, carboxyl groups into a rubber in a solution state or in a rubber kneader without using any solvent. Specific example of these reactions are a maleic anhydride addition reaction in the presence of heat or a peroxide (CA '79 (2) 6443 Z); a method of reacting a rubber with a hydroxyamylchloridic acid such as terephthalohydroxyamylchloridic acid, oxyiminochloroacetic acid or the like (Japanese Patent Laid-Open Publication No. 10635/1973); a 3-carboxyphenylmaleimide addition reaction (Kauch. Rezina 1974 8-10 Chavichich, et al); a method of reacting a rubber with carbon monoxide in the presence of a metal carbonyl compound to introduce carboxyl groups into the rubber (CA'77 (12) 762989); and a carboxylation reaction using 4-carboxylphenyl-1,2,4-triazoline-3,5-dione (Japanese Patent Laid-Open Publication No. 86477/1974).

The solvent and reaction conditions (temperature, pressure and time) used in the carboxylation reaction can be selected from those used in the modification reaction by Lewis acid. Specifically, the conditions mentioned in the literatures and patents cited previously can appropriately be used depending upon the conditions of the carboxylation reaction adopted.

The use amount of the above mentioned modifier in the carboxylation reaction is not particularly restricted but usually is 0.01 to 20 parts by weight, preferably 0.1 to 10 parts by weight, per 100 parts by weight of the rubber.

In the present invention method for improving a rubber, a rubber is subjected to two reactions consisting of a modification reaction by Lewis acid and a carboxylation reaction in any desired order. There can be mentioned the following four typical methods.

(1) A method wherein (a) a rubber is subjected firstly to a modification reaction by Lewis acid in a solution state [industrially in the state of a rubber cement right after polymerization (this applies also to (2) to (4))] and then to a carboxylation reaction, or, (b) a rubber is subjected firstly to a carboxylation reaction and then to a modification reaction by Lewis acid.

(2) A method wherein a rubber is subjected to a modification reaction by Lewis acid in a solution state, a small amount (1 to 20 moles per 1 mole of the Lewis acid used) of an alcohol or acetone is added to stop the reaction, and then the rubber is subjected to a carboxylation reaction.

(3) A method wherein a rubber is subjected to a modification reaction by Lewis acid, a large amount of an alcohol, hot water or the like is added to stop the reaction and simultaneously to coagulate the reacted rubber, and then the coagulated rubber is subjected to a carboxylation reaction as it is or in a solution state.

(4) A method wherein a rubber is subjected to a carboxylation reaction as it is or in a solution state and then the reacted rubber is subjected to a modification rubber by Lewis acid in the state of the reaction mixture per se or in the state of a newly prepared solution.

The reaction mixture obtained in the above reaction is mixed with a large amount of an alcohol, hot water or the like to stop the reaction and simultaneously to coagulate the reacted rubber, after which the coagulated rubber is washed and dried to obtain an improved rubber.

The conditions of improvement reactions (modification reaction plus carboxylation reaction) used in the present invention are desirably selected so that the glass transition temperature and Wallace plasticity of the improved rubber become higher by 0.3° to 2.7° C. and 2 to 35 points, respectively, than those of the unreacted rubber. When the increase of the glass transition temperature or Wallace plasticity of the treated rubber is smaller than the above range, the treated rubber is not sufficiently improved in rebound resiliency and heat built-up. When the increase is larger than the above range, the treated rubber has decreased rebound resilience and increased heat built-up, thus having deteriorated characteristics. It is also preferable that the conditions of the modification reaction by Lewis acid be selected so that the amount of the gel generated is as little as possible. When the amount of the gel generated is very large (e.g. 10% or more) compared with that of the untreated rubber, the processability of the treated rubber is reduced.

When the improved rubber is mixed with ordinary compounding agents for rubbers (e.g. a vulcanizing agent, a vulcanizing accelerator, a vulcanizing aid, a reinforcing agent and a softener), and if necessary, also with ordinary rubbers, the resulting unvulcanized blend has an excellent green strength and consequently very good moldability. The blend, particularly, the blend containing carbon black, when vulcanized, is excellent in strength characteristics, rebound resiliency, etc. and accordingly is used not only in general applications but also particularly preferably in those applications requiring said properties, such as tire carcass and tread, rubber vibration insulators and the like. The improved rubber obtained from the present invention method can also be used in ordinary latex applications, in the form of a latex.

Next, the present invention will be explained specifically by way of Examples. In each Example, the analysis of improved rubber, the preparation of unvulcanized and vulcanized rubber blends of improved rubber and the testing of physical properties of these blends were conducted as follows.

Glass Transition Temperature

Measurement by a high sensitivity differential scanning colorimeter (DSC) (Model SSC-560 manufactured by Daini Seikosha) was conducted to obtain a curve. The inflection point of the curve was used as a glass transition temperature of the tested rubber sample.

Wallace Plasticity

It was measured at 100° C. using a Wallace rapid plastometer.

Amount of Carboxyl Group Introduced Into Rubber

It was measured by a neutralization titration method after the low molecular components present in the treated rubber have been removed by purification.

Preparation of Unvulcanized Rubber Blend

The materials shown in the following compounding formulation, other than sulfur and the vulcanization accelerator, were kneaded in a small Banbury mixer. To the resulting mixture were added sulfur and the vulcanization accelerator on a small roll, and they were kneaded to obtain an unvulcanized rubber blend.

______________________________________Compounding formulation(Examples 1 and 2)Rubber (treated)      100    parts by weightHAF carbon            50Aromatic oil          5Zinc oxide            5Stearic acid          2Sulfur                2.5N--oxydiethylene-2-   0.8benzothiazylsulfeneamide(vulcanization accelerator)N--isopropyl-N'--phenyl-p-                 1.0phenylenediamineCompounding formulation (Example 3)Butyl rubber (Exxon Butyl 268)                 100    parts by weight(treated)HAF carbon            50Sulfur                1.5Tetramethylthiuram disulfide                 1.0    parts by weight(vulcanization accelerator)2-Mercaptobenzothiazole                 0.5(vulcanization accelerator)______________________________________

Green Strength

An unvulcanized rubber blend was press-molded at 100° C. for 5 minutes to prepare an unvulcanized rubber sheet of 2 mm in thickness. Dumbbell-shaped specimens meeting JIS K 6301 No. 3 were punched from the rubber sheet and subjected to a tensile test at 25° C. at a pulling rate of 500 mm/min to measure a tensile stress at 500% elongation.

Tensile Test

An unvulcanized rubber blend was subjected to press vulcanization at 145° C. (160° C. for an unvulcanized butyl rubber blend) for a predetermined time to obtain a 2 mm thick sheet. Dumbbell-shaped specimens meeting JIS K 6301 No. 3 were punched from the sheet and subjected to a tensile test at 25° C. at a pulling rate of 500 mm/min.

Rebound Resiliency

It was measured at 25° C. using a Dunlop tripsometer on a vulcanized rubber blend obtained by heating an unvulcanized rubber blend at 145° C. (160° C. for an unvulcanized butyl rubber blend) for a predetermined time.

EXAMPLE 1

160 g of a polyisoprene rubber (cis--1,4 bond═98%) was dissolved in 4 liters of dehydrated benzene and placed in a sealed glass vessel (a separable flask). Thereto were added the Lewis acid and the co-catalyst both specified in Table 1, each as a benzene solution, and the whole mixture was subjected to a reaction with stirring, at 25° C. in a nitrogen atmosphere under the conditions specified in Table 1. 100 ml of methanol was added to stop the reaction. Subsequently, the acid compound specified in Table 2 was added as a methanol solution (100 ml), followed by addition of triethylamine. The resulting mixture was subjected to a reaction at 60° C. for 3 hours. The reaction mixture was poured into 8 liters of methanol containing 20 ml of 36% hydrochloric acid to coagulate the treated rubber. The coagulated rubber were made into fine pieces and washed. Then, this treated rubber in fine pieces was immersed in 3 liters of methanol containing 2 g of an oxidation inhibitor (2,6-ditertiarybutyl-4-methylphenol), washed and dried in a vacuum drier for 24 hours, whereby the samples A, B, C and D according to the present invention were obtained.

Comparative samples E and F were obtained in the same manner as above except that either the carboxylation reaction or the modification reaction by Lewis acid was not conducted.

                                  TABLE 1__________________________________________________________________________      Amount of    Amount of                         Reaction                                Reaction    Type of      Lewis acid            Type of                   co-catalyst                         temperature                                timeSample    Lewis acid      (mM)  co-catalyst                   (mM)  (°C.)                                (min)__________________________________________________________________________A   SnCl4       8    CCl3 COOH                    8    25     26B   BF3 O(C2 H5)2      10    MeOH   10    25     60C   SnCl4      15    t-BuCl 15    25     45D   SnCl4      30    --     --    25     30E   SnCl4       8    CCl3 COOH                    8    25     25F   --     --    --     --    --     --__________________________________________________________________________

              TABLE 2______________________________________                         Amount of                         triethylamineSample Type and amount (g) of acid compound                         (g)______________________________________A     Terephthalohydroxamylchloridic acid                       2.4   2.4B     Same as above         1.6   1.6C     Oxyiminochloroacetic acid                       0.8   0.8D     Same as above         3.2   3.2E     --                    --    --F     Terephthalohydroxamylchloridic acid                       2.4   2.4______________________________________

Unvulcanized and vulcanized blends of each sample and the untreated polyisoprene rubber were measured for physical properties. The results are shown in Table 3.

As is appreciated from Table 3, the samples A, B, C and D according to the present invention, as compared with the comparative samples E and F and the untreated polyisoprene rubber, are markedly improved in green strength and rebound resiliency.

                                  TABLE 3__________________________________________________________________________               Amount of               carboxyl                     Physical properties of                                       Physical properties of        Glass  group in-                     unvulcanized blend                                       vulcanized blend   Rubber        transition               troduced    Green       300% Tensile                                                 Elon-                                                     Rebound   wallace        temperature               (mole/100                     Wallace                           strength                                Vulcanization                                       stress                                            strength                                                 gation                                                     resiliencySample  plasticity        (°C.)               g rubber)                     plasticity                           (kg/cm2)                                time (min)                                       (kg/cm2)                                            (kg/cm2)                                                 (%) (%)__________________________________________________________________________This inventionA       64   -61.3  0.005 43    71   26     140  280  550 74B       58   -62.0  0.003 44    55   26     140  290  550 72C       56   -62.2  0.002 45    33   25     135  280  540 72D       60   -61.7  0.007 42    108  26     155  275  480 77ComparisonE       64   -61.3  --    43    4.4  25     130  280  500 68F       48   -62.8  0.005 42    29   24     140  280  530 66Untreated   46   -62.8  --    43    2.1  22     130  280  570 63polyisoprenerubber__________________________________________________________________________

EXAMPLE 2

160 g of a polyisoprene rubber (cis--1,4 bond═98%) was dissolved in 4 liters of dehydrated toluene and placed in a sealed glass vessel (a separable flask). Thereto were added the Lewis acid and the co-catalyst both specified in Table 4, each as a toluene solution, and the whole mixture was subjected to a reaction with stirring, at 25° C. in a nitrogen atmosphere under the conditions specified in Table 4. 100 ml of ethanol was added to stop the reaction. Subsequently, the compound(s) specified in Table 5 was (were) added and the resulting mixture was subjected to a reaction under the conditions specified in Table 5. The reaction mixture was poured into 8 liters of methanol to coagulate the treated rubber. The coagulated rubber was made into fine pieces and washed. This rubber in fine pieces was immersed in 3 liters of methanol containing 2 g of an oxidation inhibitor (2,6-ditertiarybutyl-4-methylphenol), washed and dried in a vacuum drier for 24 hours, whereby the samples G and H shown in Table 6 were obtained.

Comparative samples I and J were obtained in the same manner as above except that either the carboxylation reaction or the modification reaction by Lewis acid was not conducted.

These samples were subjected to the same measurements as in Example 1. As shown in Table 6, the same results as in Example 1 were obtained.

                                  TABLE 4__________________________________________________________________________     Amount of    Amount of                        Reaction                               Reaction    Type of     Lewis acid           Type of                  co-catalyst                        temperature                               timeSample    Lewis acid     (mM)  co-catalyst                  (mM)  (°C.)                               (min)__________________________________________________________________________G   SnCl4     6.8   CCl3 COOH                  6.8   25     60H   SnCl4     6.0   CBr3 COOH                  6.0   25     45I   SnCl4     6.8   CCl3 COOH                  6.8   25     60J   --    --    --     --    --     --__________________________________________________________________________

              TABLE 5______________________________________ Amount of  Amount of  Reaction Reaction maleic     benzoyl    temperature                                timeSample anhydride (g)            peroxide (g)                       (°C.)                                (hours)______________________________________G     8          --         100      5H     2          0.02        60      1I     --         --         --       --J     8          --         100      5______________________________________

                                  TABLE 6__________________________________________________________________________               Amount of               carboxyl                     Physical properties of                                       Physical properties of        Glass  group in-                     unvulcanized blend                                       vulcanized blend   Rubber        transition               troduced    Green       300% Tensile                                                 Elon-                                                     Rebound   wallace        temperature               (mole/100                     Wallace                           strength                                Vulcanization                                       stress                                            strength                                                 gation                                                     resiliencySample  plasticity        (°C.)               g rubber)                     plasticity                           (kg/cm2)                                time (min)                                       (kg/cm2)                                            (kg/cm2)                                                 (%) (%)__________________________________________________________________________This inventionG       61   -61.7  0.002 44    38   26     145  280  520 72H       59   -61.9  0.0015                     46    28   24     143  280  520 70ComparisonI       61   -61.7  --    46    4.0  25     135  280  540 67J       48   -62.8  0.002 48    16   26     150  280  490 64Untreated   46   -62.8  --    43    2.1  22     130  280  570 63polyisoprenerubber__________________________________________________________________________

EXAMPLE 3

100 g of a butyl rubber (Exxon Butyl 268 having an unsaturation degree of 1.5 to 2.0%, manufactured by Exxon Chemical) was dissolved in 4 liters of dehydrated benzene and placed in a sealed glass vessel (a separable flask). Thereto were added the compounds specified in Table 7, each as a benzene solution, and the whole mixture was subjected to a reaction with stirring, at 25° C. in a nitrogen atmosphere under the conditions specified in Table 7. 100 ml of methanol was added to stop the reaction. Subsequently, the compounds specified in Table 8 were added and the resulting mixture was subjected to a reaction at 60° C. for 3 hours. The reaction mixture was poured into 8 liters of methanol containing 20 ml of 36% hydrochloric acid to coagulate the treated rubber. The coagulated rubber was made into fine pieces and washed. This treated rubber in fine pieces was immersed in 3 liters of methanol containing 2 g of an oxidation inhibitor (2,6-ditertiarybutyl-4-methylphenol), washed and dried in a vacuum drier for 24 hours, whereby a sample K was obtained.

Comparative samples L and M were obtained in the same manner as above except that either the carboxylation reaction or the modification reaction by Lewis acid was not conducted.

These samples were subjected to the same measurements as in Example 1.

As shown in Table 9, the same results as in Example 1 were obtained.

              TABLE 7______________________________________ Amount of Amount of           Reaction SnCl4           trichloroacetic                       Reaction                               temperatureSample (mM)      (mM)        (min)   (°C.)______________________________________K     8         8           60      30L     8         8           60      30M     --        --          --      --______________________________________

              TABLE 8______________________________________    Amount of terephthalo-                      Amount ofSample   hydroxamylchloridic acid (g)                      triethylamine (g)______________________________________K        1.0               1.0L        --                --M        1.0               1.0______________________________________

                                  TABLE 9__________________________________________________________________________                 Amount of                 carboxyl  Rubber  Glass  group in-                       Physical properties of                                         Physical properties of  Mooney  transition                 troduced                       unvulcanized blend                                         vulcanized blend  viscosity          temperature                 (mole/100                       Green      Vulcanization                                         ReboundSample M1+8 (125° C.)          (°C.)                 g/rubber)                       strength (kg/cm2)                                  time (min)                                         resiliency__________________________________________________________________________                                         (%)*K     55      -60.5  0.002 68         22     38ComparisonL      55      -60.5  --    52         22     35M      50      -61.0  0.002 60         20     35Untreated  50      -61.0  --    50         20     33butyl rubber__________________________________________________________________________ *This invention