GB2041945A

GB2041945A - Rubber Compositions

Info

Publication number: GB2041945A
Application number: GB7848938A
Authority: GB
Original assignee: Kuraray Co Ltd
Current assignee: Kuraray Co Ltd
Priority date: 1978-12-18
Filing date: 1978-12-18
Publication date: 1980-09-17
Also published as: GB2041945B

Abstract

Rubber compositions with improved processability and capable of affording a vulcanizate with improved properties, including adhesion to metal surfaces, comprise a natural rubber and a modified liquid polyisoprene rubber that is an adduct of maleic anhydride or a derivative thereof with a liquid polyisoprene rubber with a cis-1,4 content of not less than 70% and a molecular weight of 8,000 to 100,000, the amount of maleic anhydride or its derivative being 0.1 to 15 mol % per isoprene unit in the liquid polyisoprene rubber.

Description

SPECIFICATION Rubber Composition This invention relates to rubber compositions.

It is one of the recent trends in the rubber industry in general to use low-molecular-weight plasticizers such as process oil, lanolin and dioctyl phthalate so as to decrease the degree of plasticity of unvulcanized rubber compounds, thereby improving their processability and saving labour and energy. Whereas the use of low-molecular-weight plasticizers certainly causes a decrease in the viscosity of unvulcanized rubber compounds and an increases in their flowability, and thus contributes greatly to savings of labour and energy in the manufacture of rubber goods, it also brings about a decrease in strength-related properties, for example, the green strength of rubber compounds in the unvulcanized state, which decrease produces trouble in the moulding step.At the same time deteriorations in the properties of vulcanizates, such as hardness and modulus, are inevitable, and therefore they are not always satisfactory for the manufacture of such rubber goods as belts, hoses, tires, rolls, vibration absorbers and rubber shoes, where the performance of the products necessitates severe and strict requirements. Moreover, when metal reinforcements are used, such as in the case of belts, hoses, tires and vibration absorbers, the use of lower-molecular-weight plasticizers such as those mentioned above is undesirable for reasons which concern adhesion to the metal.

In order to solve this rather contradictory problem, namely simultaneously increasing flowability in the unvulcanized state and workability, typically green strength, a rubber composition has been proposed in which a low-molecular-weight polyisoprene rubber with an intrinsic viscosity in the range of 0.1 to 1 is incorporated as low molecular weight plasticizer into a solid rubber, see for example, British Patent No. 990,439 and U.S. Patent No. 3,335,202. Further, British Patent No. 1,111,978 discloses polyisoprene rubbers having various intrinsic viscosity distributions between 0 and 20 dl/g. In these methods, however, adhesion to metal reinforcements is not sufficient.

Further, it is known that introduction of a polar group, for example maleic anhydride, into a solid cis-1,4 polyisoprene rubber improves the green strength, see for example, U.S. Patents Nos.

3,567,691,3,507,838, 3,502,736,3,664,248, 3,887,527,3,909,473 and 3,898,193 and British Patents Nos. 1,404,291, 1,406,935 and 1 ,2 11,795. However, these are also not fully satisfactory in respect to flowability and workability.

The present invention is based on the discovery that a new rubber composition containing natural rubber and a particular polyisoprene rubber has improved properties in the vulcanized and unvulcanized state. The present invention provides a rubber composition comprising a natural rubber and a modified liquid rubber that is an adduct of maleic anhydride or a derivative thereof with a liquid polyisoprene rubber with a cis-1,4 content (as herein defined) of not less than 70% and a molecular weight (as herein defined) of 8,000 to 100,000, the maleic an hydride or its derivative being present in an amount of 0.1 to 15 mol% per isoprene unit in the liquid polyisoprene rubber.

The natural rubber to be used in the present invention, is, for example, SMR-5L, SMR-20, SMR50, RSS No. 1, RSS No. 2, RSS No. 3, ADS or pale crape. In order to produce a sufficient effect in relation to green strength, it is preferred that the Mooney viscosity (ML,+4(100 C)) of the natural rubber is not less than 35 and is not more than 130, preferably 90. These natural rubbers may be used either alone or as a mixture of two or more of them. A small amount of other synthetic solid polydiene rubbers may be incorporated.Such solid polydiene rubbers are synthetic rubbers with a molecular weight of at least 300,000, such as cis-1,4-polyisoprene rubber (IR), polybutadiene rubber (BR), styrene-butadiene copolymer rubber (SBR), styrene-isoprene copolymer rubber (SIR), acrylonitrilebutadiene copolymer rubber (NBR), acrylonitrile-isoprene copolymer (NIR), isobutylene-isoprene copolymer (IIR), and ethylene-propylene-diene terpolymer (EPDM). Adjustment of the Mooney viscosity of a value within the above range can be made by mastication of the natural rubber on an open roll mill or Banbury mixer.

The molecular weight of the liquid polyisoprene rubber before modification, its cis-1,4-content and the kind and amount of the added functional group are very important factors.

The unmodified liquid polyisoprene rubber which is in the base of said liquid rubber has a molecular weight in the range of 8,000 to 100,000, preferably in the range of 1 5,000 to 55,000.

When the molecular weight is much lower than 8,000, not only is the green strength improving effect small, but also there is a decrease in the efficiency of cross-linking of the rubber composition when it is combined with a polydiene rubber, which results in a deterioration in the physical properties of the vulcanizate. When the molecular weight is much higher than 100,000, the viscosity becomes too high to enable the processability to be improved. The term "molecular weight" used herein means the viscosity-average molecular weight (abbreviated to "M"), which is calculated from the following equation: [?1]= 1.21 x 10-4M077 where [X1] is intrinsic viscosity as determined with toluene solutions at 300C.

Moreover, it is necessary that the cis-1,4 content of the unmodified liquid polyisoprene rubber be not less than 70%, preferably not less than 80%. If the cis-1,4 content is too low, the liquid polyisoprene rubber itself is already too rigid, and, in addition, not only is processability poor, but also the efficiency of vulcanization of the rubber composition is low and the properties of the vulcanizate are insufficient. The term "cis-1,4 content" used herein is that measured by infrared absorption spectrophotometry.

The unmodified liquid polyisoprene rubber with the desired specific molecular weight and microstructure can be prepared, for example, by anionic polymerization, radical polymerization or coordination anionic polymerization of isoprene. It can also be prepared by thermal decomposition at a high temperature (e.g. 1 800 to 3O00C) of a natural rubber or a solid synthetic cis-1 ,4-polyisoprene rubber prepared by Ziegler polymerization or anionic polymerization. However, the liquid polyisoprene rubber prepared by thermal decomposition has a very strong smell and is highly coloured, due to byproducts of the thermal decomposition, and production of a product of constant quality is difficult, hence preparation by thermal decomposition is not desirable.Those liquid polyisoprene rubbers that are produced by anionic polymerization using a lithium catalyst are preferred, because they do not contain a gel portion and show a ratio of (weight-average molecular weight):(number-average molecular weight) of not more than 3:1 and a narrow range of molecular-weight distribution.

Such anionic polymerization will now be described in more detail. Isoprene monomer is polymerized by the use of metallic lithium or an organolithium such as methyl lithium, propyl lithium, butyl lithium or distyrenyl lithium as catalyst, in the presence or absence of a solvent. As is well known, the molecular weight of the polymer can easily be regulated by adjusting the ratio of the amount of isoprene monomer to that of the catalyst used. The use of a solvent facilitates the control of the polymerization and is therefore appropriate.

The derivatives of maleic anhydride to be used in practising the invention include maleic acid, monoesters and diesters of maleic acid, maleamide and maleimide.

The addition reaction of maleic anhydride or a derivative thereof to the liquid polyisoprene rubber can easily be carried out, for example by adding a maleic anhydride or a derivative thereof to a liquid polyisoprene rubber with a molecular weight falling within the above specified range and heating the mixture in the presence or absence of a solvent and in the presence or absence of a radical initiator.

The solvent is generally a hydrocarbon or a halogenated hydrocarbon, preferably an inert solvent such as n-butane, n-hexane, n-heptane, cyclohexane, benzene, toluene or xylene.

Besides those adducts of maleic an hydride or its derivatives that are obtained by reaction of a liquid rubber prepared in the manner mentioned above with maleic anhydride and/or a derivative thereof such as maleic acid, a maleate, maleamine or maleimide, products derived from the adduct of a liquid polyisoprene rubber and maleic anhydride by esterifying, amidating or imidating one or both of the carboxyl groups originating from the maleic anhydride with an alcohol, such as methanol, ethanol or n-propanol, ammonia or an amine such as n-propylamine or n-butylamine, in the presence or absence of a catalyst, such as p-toluenesulfonic acid, according to circumstances, are also generally used.In consideration of the viscosity stability in the case where the modified liquid rubber is stored for a long period of time, alcohol derivatives or amine derivatives of the modified liquid polyisoprene rubber are preferred to the maleic-anhydride-modified liquid polyisoprene rubber itself. However, a neutralized carboxylated liquid polyisoprene rubber obtained from the adduct of a liquid polyisoprene rubber and maleic anhydride by neutralizing the carboxyl groups originating from the maleic acid with the hydroxide of metal such as potassium, sodium, barium, calcium or aluminium cannot be used in the compositions of the present invention because the said derivative is itself inclined to become gelled or completely crosslinked and does not contribute to the improvement of flowability and processability of the composition.

Since the amount of maleic anhydride or its derivative added to the liquid polyisprene rubber influences the properties of the finally obtained vulcanizate, it is important that that amount be in the range 0.1 to 1 5 mol%, preferably 0.5 to 10 mol%, per isoprene monomer unit. With an excessively small amount of maleic anhydride or a derivative thereof, the desired simultaneous improvement in processability, vulcanizate hardness and adhesion to metals cannot be produced. On the other hand, with an excessively large amount, deterioration in processability and in properties of the vulcanizate is observed, supposedly due to the formation of nonuniform system at the time of mixture with a solid polydiene rubber.

According to the invention, the preferred amount of the liquid rubber is 5 to 45% by weight, particularly 10 to 30% by weight, of the total amount of the natural rubber and the liquid rubber.

Excessively small or large amounts of the liquid rubber produce less effect on the green strength specified hereinafter. Moreover, when the amount of the liquid rubber is too great, remarkable deterioration in properties of the vulcanizate results in addition to the problems mentioned above.

The rubber composition of the present invention may contain, besides the above-mentioned natural rubber and the liquid rubber, a variety of rubber additives. Typical examples of such additives are vulcanizing or curing agents such as sulphur and peroxides, vulcanization accelerators, activators such as zinc oxide, magnesium oxide and stearic acid, fillers such as carbon black, clay, calcium carbonate, surface-treated calcium carbonate, silica, hydrated silicates, pulverized mica, talc and pulverized asbestos, plasticizers such as process oil and other liquid rubbers, antioxidants, and ultraviolet-degradation inhibitors. In accordance with the invention, a single natural rubber or a mixture of two or more natural rubbers may be used. Furthermore, the natural rubber may be replaced partly by some other rubber or plastics material, so long as the desired result is achieved.Further, metallic materials such as steel, brass, brass-plated steel, zinc, zinc-plated steel, chrome-plated steel, copper and aluminium may be used in the form of plates, cords or powders as reinforcements.

The rubber composition of the present invention is produced on an apparatus conventional in the rubber industry in general, such as a roll mill, Banbury mixer, kneader or some other internal mixer.

The rubber composition of the invention is especially suitable for the manufacture of steel reinforced radial tires, steel-reinforced conveyor belts, steel-reinforced rubber hoses and other products that have to meet especially severe requirements set out with respect to their properties. The rubber composition of the invention with a white filler added, such as clay, silica or calcium carbonate, is suitable for the manufacture of rubber shoes and other rubber goods for industrial use, for instance, because such a rubber composition, when vulcanized, produces a remarkably improved reinforcing effect in respect of hardness and modulus of the vulcanizate.

The following examples further illustrate the invention.

Example 1 and Comparaive Example 1 A liquid polyisoprene rubber with a molecular weight of 35,000 and a cis-1,4 content, as measured by infrared absorption spectrophotometry, of 85% was prepared by polymerization of monomeric isoprene in n-heptane in the presence of sec-butyl lithium as catalyst. The liquid polyisoprene rubber (100 parts by weight) was dissolved in toluene, and 5 parts by weight of maleic anhydride was added, and the mixture was stirred with heating at 1 500C and then poured into acetone. The resulting precipitate was dried, giving a modified liquid polyisoprene rubber (MAn-LIR[A]) which contained 0.7 mole of maleic anhydride per 100 isoprene units. Using this modified liquid rubber Man-LIR[A] or a process oil and according to the recipe in Table 1, three rubber compounds were prepared in a roll mill.The Mooney viscosity and green strength of each compound and the hardness of each vulcanizate and bond strength to a brass plate were as shown in Table 1.

Table 1 1-A 1-B* 1-C Natural rubber RSS No. 3 100 80 80 MAn-LIR[A] 20 20 Process oil1' 20 Carbon black GPF2 45 45 45 Zinc oxide 5 5 5 Stearic acid 1 1 1 Sulphur 0.8 0.8 0.8 Accelerator NS3' 2.2 2.2 2.2 Antioxidant NS-64 1 1 1 Mooney viscosity of compound ML1+4(1000C) 61 32 30 Green strength5' (kg/cm2) 2.9 4.1 1.9 Hardness of vulcanizate6' 62 65 57 Bond strength7 of vulcanizate to a brass plate (g/cm) 327 1,310 457 Notes: * Compound in accordance with the invention 1 ) Sunthene 450 (Sun Oil Company) 2) Product of Mitsubishi Chemical Industries Ltd.

3) Nocceler NS: N-tert-butyl-2-benzothiazolyl sulphenamide (Ouchi-Shinko Chemical Industrial Co. Ltd.) 4) Nocrac NS-6: 2,2-methylenebis(4-methyl-6-tert-butylphenol) (Ouchi-Shinko Chemical Industrial Co. Ltd.) 5) Tensile strength shown by a specimen 2 mm thick made of an unvulcanized compound subjected to tension at rate of drawing of 2 cm/min.

6) Hardness on JIS K 6301 A scale of the vulcanizate (1 450C, 25 minutes) 7) Peel strength shown in a peeling test at a separation angle of 900 according to JIS K 6301 by a specimen prepared by placing an unvulcanized compound between two JIS Class 2 brass plates and subjecting the assembly to press vulcanization at 1 450C for 25 minutes.

As is clear from Table 1, compound I-B according to the invention, where a natural rubber was combined with the modified liquid polyisoprene rubber, showed a decreased Mooney viscosity, an improved flowability and a high green strength and at the same time brought about a high bond strength of the vulcanizate to a brass plate. In other words, the performance of the composition of the invention was sufficient in respect of flowability and processability in the unvulcanized state and in properties of the vulcanizate, including bond strength of the vulcanizate to the metal. On the contrary, compound I-C where a process oil was used showed a much decreased green strength, although it showed a decreased Mooney viscosity and an improved flowability; decrease in bond strength to the brass plate also resulted.

Example 2 ancfComparative Example 2 Three polyisoprene rubbers with molecular weights of 3,000, 35,000 and 230,000 respectively, each with a cis-1,4 content as measured by infrared absorption spectrophotometry of about 85%, were prepared by polymerization of isoprene monomer in n-neptane as solvent with the amount of sec-butyl lithium varied. Each of these polyisoprene rubbers was dissolved in toluene, maleic anhydride was added in an amount of 5 parts by weight per 100 parts of the polyisoprene rubber, the mixture was heated at 1 500C with stirring and then poured into acetone, and the reaction product that precipitated was collected and dried to give a modified polyisoprene rubber with a bound maleic anhydride content of about 0.7 mol%.Using these modified polyisoprene rubbers, three rubber compounds were prepared according to recipe I-B in Example 1. The Mooney viscosity and green strength of each compound and the hardness of each vulcanizate and its bond strength to a brass plate were as shown in Table 2.

Table 2 Compound ll-A Il-B* Il-C Molecular weight of polyisoprene before modification 3,000 35,000 230,000 Mooney viscosity of compound ML1+4(1000C) 32 32 70 Green strength (kg/cm2) 1.9 4.1 4.5 Hardness of vulcanizate (JIS A) 57 65 63 Bond strength of vulcanizate to a brass plate (g/cm) 711 1,310 631 * Compound in accordance with the invention As can be seen in Table 2, compound ll-B, where the modified liquid polyisoprene rubber having a molecular weight falling within the range specified by the invention was used, showed a markedly decreased Mooney viscosity in the unvulcanized state and a high green strength, and produced a vulcanizate with a sufficient hardness and a high bond strength to a brass plate.On the other hand, compound Il-A, which contained a modified liquid polyisoprene rubber with a molecular weight below the specific range showed a good flowability but a low green strength and produced a low bond strength of the vulcanizate to a brass plate. Compound Il-C, which contained a modified polyisoprene rubber with a molecular weight exceeding the said range, did not reveal desirable improvement in flowability or in vulcanizate-metal bond strength.

Example 3 and Comparative Example 3 A liquid polyisoprene rubber with a molecular weight of 35,000 and a cis-1,4 content of 85% was prepared in the same manner as in Example 1. This liquid polyisoprene rubber was dissolved in toluene, and maleic anhydride was added in amounts of 0.5, 5 and 30 parts by weight per 100 parts by weight of the liquid polyisoprene rubber, respectively. After heating each mixture at 1 500C with stirring, the mixture was poured into acetone. The reaction product that precipitated was collected and dried. Modified liquid polyisoprene rubbers with contents of bound maleic anhydride of 0.05, 0.7 and 19.0 mol%, respectively, were obtained. Three rubber compounds were prepared on a roll mill, according to the recipe of compound i-B in Example 1, but using the three modified liquid polyisoprene rubbers as plasticizers. The Mooney viscosity and green strength of each compound and the hardness of the vulcanizate and its bond strength to a brass plate were as shown in Table 3.

Table 3 Compound 111-A 111-B* 111-C Bound maleic anhydride content in modified liquid polyisoprene rubber (mol %) 0.05 0.7 10.0 Mooney viscosity of compound ML1+4(1000C) 31 32 69 Green strength (kg/cm2) 2.0 4.1 4.2 Hardness of vulcanizate (JIS A) 57 65 59 Bond strength of vulcanizate to a brass plate (g/cm) 478 1,310 298 *Compound according to the invention It is noted from Table 3 that compound Ill-B, which contained a modified liquid polyisoprene rubber with a bound maleic anhydride content falling within the range specified by the invention, successfully achieved the object of the invention.On the other hand, compound Ill-A, which contained a modified liquid polyisoprene rubber whose bound maleic anhydride content was smaller than the lower limit of the said range, caused a low green strength and a low bond strength of the vulcanizate to a brass plate, whereas compound Ill-C, which contains a modified liquid polyisoprene rubber with a bound maleic anhydride content greater than the upper limit of the said range showed an insufficient decrease in Mooney viscosity and produced a low bond strength of the vulcanizate to a brass plate.

Example 4 and Comparative Example 4 A liquid polyisoprene rubber (LIR[B]) with a molecular weight of 51,000 and a cis-1,4 content as measured by infrared absorption spectrophotometry of 82% was prepared by polymerizing monomeric isoprene in the presence of sec-butyl lithium, with n-heptane used as solvent. Separately, a liquid polyisoprene rubber (LIR[C]) with a molecular weight of 57,000 and a cis-1,4 content of 42% was prepared by polymerization in the presence of diethyl ether and butyl lithium.Both the liquid polyisoprene rubbers were dissolved in xylene, maleic acid was added to the solution in an amount of 7 parts by weight per 100 parts by weight of the liquid polyisoprene rubber, the mixture was heated at 1 600C with stirring, ethanol was then added, the whole mixture was poured into acetone, and the precipitate was collected and dried. There were obtained modified liquid polyisoprene rubbers, designated, respectively, as E-MAn-LIR[B] and E-MAn-LIR[C], each with an ethyl maleate content of 3.8 mol%. In accordance with the recipes shown in Table 4 rubber compounds were prepared on a kneader using the modified liquid polyisoprene rubbers.The Mooney viscosity and green strength of each compound, the hardness of the vulcanizate and the bond strength of the vulcanizate to a brass plate were as shown in Table 4.

Table 4 IV-A lV-B* /V-C* lV-D Natural rubber SMR-5L 100 90 80 90 E-MAn-LIR[B] 10 20 E-MAn-LIR[C] - - 20 Process oil1 3 3 3 3 Carbon black FEF2 45 45 45 45 Zinc oxide 6 6 6 6 Stearic acid 1 1 1 1 Sulphur 2 2 2 2 Accelerator MSM 1 1 1 1 Antioxidant RD4 1 1 1 1 Cobalt naphthenate 3 3 3 3 Mooney viscosity of compound ML1+4 (1 000C) 65 50 53 72 Green strength (kg/cm2) 8.9 12.7 13.9 Hardness of vulcanizate51 (JIS-A) 64 66 68 59 Bond strength of the vulcanizate to a brass plate5 Interface Break in Break in peeling rubber rubber between layer layer rubber and metal Notes: * Compounds according to the invention 1) Sunthene 450 (Sun Oil Company) 2) Manufactured by Mitsubishi Chemical Industries Ltd.

3) Nocceler MSA: N,N-oxydiethylene-2-benzothiazolyl sulphenamide (Ouchi-Shinko Chemical Industrial Co. Ltd.) 4) Antigen RD: polymer of 2,2,4-trimethyl-1 ,2-dihydroquinoline (Sumitomo Chemical Co. Ltd.) 5) Vulcanization conditions: 1450C, 25 minutes.

It can be seen from Table 4 that, even if the base rubbers for the preparation of modified liquid polyisoprene rubbers have approximately the same molecular weight, the Mooney viscosity and green strength of the rubber composition comprising a natural rubber and a modified liquid polyisoprene rubber, the hardness of the vulcanizate and the bond strength of the vulcanizate to a brass plate are greatly influenced by difference in cis-1,4 content among the base rubbers.Thus, compounds IV-B and IV-C, where modified liquid rubber E-MAn-LIR[B] based on a liquid polyisoprene rubber with a cis-1,4 content of 82% was incorporated, each had a Mooney viscosity lowered sufficiently for processing as well as an improved green strength, and at the same time the vulcanizate had an improved hardness and the bond between vulcanizate and brass plate did not show interface peeling but showed good adhesion. On the other hand, compound IV-D, which contained modified liquid rubber E-MAn-LIR[C] based on a liquid polyisoprene rubber with a cis-1,4 content of 42%, showed a high Mooney viscosity and bad processability, and the vulcanizate had a decreased hardness.

Example 5 and Comparative Example 5 Using the same modified liquid polyisprene rubber as that used in Example 1, rubber compounds were prepared on a roll mill according to the recipes shown in Table 5. The compounds were evaluated in terms of Mooney viscosity and green strength, in hardness and stress at 300% elongation of the vulcanizate, and in the bond strength between the vulcanizate and chrome-plated steel (Table 5). Table 5 shows that the use of the modified liquid polyisoprene rubber improved simultaneously the workability and processability of the green rubber, the hardness of the vulcanizate and the vulcanizatemetal bond strength.

V-A V-B* V-C* Natural rubber RSS No. 1 90 70 55 Modified liquid polyisoprene - 20 20 SBR-1 500i 10 10 25 Clay 130 130 130 Zinc oxide 5 5 5 Stearic acid 3 3 3 Sulphur 3 3 3 Accelerator DM2 1.1 1.1 1.1 Accelerator DT3 0.4 0.4 0.4 Dicyclohexylamine 1.1 1.1 1.1 Antioxidant NS-6 1 1 1 Mooney viscosity of compound ML,+4 (100 C) 37 25 31 Green strength (kg/cm2) 0.7 1.3 1.2 Hardness of vulcanizate (JIS A) 64 73 70 Stress at 300% elongation41 (kg/cm2) 78 112 98 Bond strength between the vulcanizate and chrome-plated steel Interface Break in Break in peeling rubber rubber between layer layer rubber and metal Notes: * Compound according to the invention 1) Styrene-butadiene copolymer rubber with a styrene content of 23.5% (Mooney viscosity: 52) (Japan Synthetic Rubber Co. Ltd.) 2) Nocceler DM: dibenzothiazyl disulphide (Ouchi-Shinko Chemical Industrial Co. Ltd.) 3) Nocceler DT: di-o-tolylguanidine (Ouchi-Shinko Chemical Industrial Co. Ltd.) 4) Stress at 300% elongation of the vulcanizate according to JIS K 6301.

Example 6 and Comparative Example 6 A liquid polyisoprene rubber with a molecular weight of 27,000 and cis-1 ,4 content of 81% was prepared in the same manner as in Example 1. This liquid polyisoprene rubber was dissolved in toluene, 10 parts by weight of maleic anhydride per 100 parts of the above rubber was added, the mixture was heated at 1 900C with stirring, ammonia was then passed through the reactor, and the reaction mixture was poured into acetone. The precipitate was dried, and there was obtained a modified liquid polyisoprene rubber with a bound maleamide content of 4.1 mol%. Using this modified liquid polyisoprene rubber, the unmodified liquid polyisoprene rubber, the unmodified liquid polyisoprene rubber or a process oil, rubber compounds as shown in Table 6 were prepared on a roll mill.

As is clear from Table 6, the use of the modified liquid rubber improved the processability and the hardness of the vulcanizate and further the vulcanizate-metal bond strength in comparison with the case where the process oil was used. Moreover, when the unmodified liquid polyisoprene rubber was used, decrease in the hardness resulted, and the improvement in vulcanizate-metal adhesion, though appreciable, was not so significant as in the case where the modified liquid polyisoprene rubber was used.

Table 6 Vl-A Vl-B Vl-C* Natural rubber RSS No. 31) 85 85 85 Process oil 15 - - Unmodified liquid polyisoprene rubber 15 Modified polyisoprene rubber - 15 Carbon black GPF 30 30 30 Carbon black FEF 20 20 20 Zinc oxide 5 5 5 Stearic acid 3 3 3 Sulphur 2.8 2.8 2.8 Accelerator NS 1 1 1 Cobalt naphthenate 3 3 3 Antioxidant NS-6 1 1 1 Mooney viscosity of compound ML+4 (1 000C) 54 55 58 Green strength (kg/cm2) 6.0 9.2 1 5.8 Hardness of vulcanizate2 (JIS A) 58 62 70 Adhesion of vulcanizate with zinc-plated steel plate, peel strength2) (kg/cm) 0.8 1.4 3.5 Note: : * Compound according to the invention 1) Mooney viscosity ML+4 (1 000C)=71 2) Vulcanization conditions: 1450C, 30 minutes.

Claims

1. A rubber composition comprising a natural rubber and a modified liquid rubber that is an adduct of maleic anhydride or a derivative thereof with a liquid polyisoprene rubber with a cis-1,4 content (as herein defined) of not less than 70% and a molecular weight (as herein defined) of 8,000 to 100,000, the maleic anhydride or its derivative being present in an amount of 0.1 to 1 5 mol% per isoprene unit in the liquid polyisoprene rubber.

2. A composition as claimed in Claim 1 comprising 95 to 55% by weight of the natural rubber and, correspondingly, 5 to 45% by weight of the modified liquid rubber.

3. A composition as claimed in Claim 1 or 2 in which the liquid polyisoprene rubber has a cis-1,4 content of not less than 80% and a molecular weight of 1 5,000 to 55,000.

4. A composition as claimed in any preceding claim in which the modified liquid rubber is an adduct of a liquid polyisoprene rubber with maleic anhydride or a derivative thereof in an amount of 0.5 to 10 mol% per isoprene unit in the liquid polyisoprene rubber.

5. A composition as claimed in Claim 1 substantially as hereinbefore described in any one of Examples 1 to

6.