PNEUMATIC TYRES

Abstract: Pneumatic tyres having an improved steering response have a tread portion which comprises a blend of a base elastomer and, as a steering response substance, a polymer of an unsaturated compound selected from methacrylic acid, acrylic acid and lower alkyl esters thereof. The steering response substance is preferably a graft copolymer of isoprene and methylmethacrylate. A preferred amount of the steering response substance is in the range from 5 to 60% by weight of the total of the base elastomer and the steering response substance.

Description: This invention relates to pneumatic tyres having improved performance on the road; in particular it concerns means of improving steering response or frictional grip on wet road surfaces (referred to herein as "wet grip"). In a preferred form of the invention both steering response and wet grip are improved. Steering response can be defined as the rate of deviation from the "straight ahead" vehicle position during driving by the application of a small steering input (about 15° of steering wheel revolution or about 0.5° slip angle). It is a very important factor in motor racing. Attempts made in the past to increase steering response (which is closely related to a tyre's cornering stiffness) have been by increasing the stiffness of the tyre compound in general and the tread in particular. However, in this past work it has been found that increasing the compound stiffness results in a corresponding reduction in the wet grip of the resulting tyres for they then have a lower energy absorption potential (loss compliance). Chemical Abstract Vol.79, No. 4,1973, 20012q (a paper by A.V. Shilkin et al , Proizvod Shin. Rezinothekh Asbetotekh Izdelii Ref. SB, No. 1, 6-8) discloses the use of liquid butadiene-styrene-methacrylic acid copolymers as softening agents for styrene-butadiene rubber (SBR) and natural rubber. A paper by M.A. Wheelans in Ind. Gomma 22(4), SI, 53-4 discloses the use of a graft copolymer of natural rubber and methyl methacrylate as a reinforcing resin for natural rubber to provide materials having a hardness intermediate between that of normal, traditional resilient rubbers and harder materials such as ebonites. The materials thus produced are said to be suitable for shaping by injection moulding. It has now surprisingly been found that by inclusion of graft copolymers of a polyisoprene (for example natural rubber) and an unsaturated compound selected from methacrylic acid, acrylic acid and lower alkyl esters thereof (referred to herein as "steering response substances") the steering response of tyres can be increased substantially without any substantial detrimental affect on wet grip. Accordingly, the present invention provides a pneumatic tyre whose tread portion is characterised in that it comprises a blend of a base elastomer and, as a steering response substance, a graft copolymer of polyisoprene and an unsaturated compound selected from methacrylic acid, acrylic acid and lower alkyl esters thereof; and the steering response substance is present in the blend in an amount of from 5 to 60% by weight of the total of said base elastomer and the steering response substance. The lower alkyl ester of methacrylic acid or acrylic acid is conveniently one in which the lower alkyl group has 1 to 6 carbon atoms, for example it can be methyl, ethyl, propyl ( n - or iso -) or butyl ( n -, iso - sec - or t -). Said ester is conveniently a lower alkyl ester of methacrylic acid, for example it can be methyl methacrylate, and it is with particular reference to the use of methyl methacrylate as the ester that the following description relates. The steering response substance is conveniently a polyisoprene to which has been grafted a lower alkyl ester of methacrylic acid or acrylic acid. By way of example, where the base elastomer (also referred to below as the "base polymer") is a natural rubber, the steering response substance can be a product obtained by graft copolymerisation of natural rubber with said ester in monomeric form, the graft copolymerisation being effected, for instance, between an emulsion or dispersion of the base elastomer and the ester, the content of the lower alkyl ester in the steering response substance being, for instance, in the range from 5% to 60% (for instance 15, 20, 30, 40, 45 or other proportion in the range 15 to 50%) by weight of the steering response substance. As stated above, the steering response substance is present in an amount in the range from 5% to 60% by weight of the total of said base elastomer and the steering response substance. In producing the tyres of the present invention the steering response substance is used in any convenient proportion in said range such that it will, on the one hand, provide a useful increase in steering response of pneumatic tyres whose tread portion comprises the elastomer composition after vulcanisation and, on the other hand, is not sufficiently high as to derogate from the other desirable properties of the composition. Suitable amounts of the steering response substance depend, to some extent at least, on its content of the lower alkyl ester. Where the base elastomer is an SBR rubber the amount of steering response substance is preferably in the range from 10 to 50 parts, especially 15 to 30 parts per 100 parts of said blend; where the base elastomer is a natural rubber the amount of steering response substance is preferably in the range 10 to 60, especially 15 to 40 or 50 parts per 100 parts of said blend. As can be seen from the Examples below, good results have been obtained using the steering response substances referred to as MG30 and MG49 in an amount of 20 parts per 100 parts of blend. In a preferred form of the invention the blend comprises: (a) an SBR elastomer or a cis -1,4-polyisoprene as the base elastomer, and (b) a graft copolymer of polyisoprene and methyl methacrylate as the steering response substance, said graft copolymer constituting from 10 to 40% by weight of the total of said base elastomer and the steering response substance. If desired, the amount of steering response substance can be assessed in terms of the amount of methyl methacrylate (or other lower alkyl ester) in the tyre tread composition. For example, the amount of poly (methyl methacrylate) in said blend can be in the range from 3 to 20% by weight of the total of said base elastomer and the steering response substance. Thus, where as in Example 2 below the steering response substance MG30 is used in an amount of 20 parts per 100 parts of blend and contains 30% by weight of methyl methacrylate, the content of methyl methacrylate in the tread composition is about 6%; the corresponding figure for the use in Example 3 of MG 49 at the rate of 20 parts per 100 of blend is about 9.8% methyl methacrylate content. The steering response substance can be incorporated in the elastomer composition in any convenient way; however, in general it is incorporated in the composition concurrently with incorporation of the vulcanisation agents and in like manner. Instead of, or as well as, the lower ester of methacrylic acid or acrylic acid there can be used in this invention methacrylic acid and/or acrylic acid. Thus, for example the steering response substance can be a graft copolymer of a polyisoprene and methacrylic acid and/or acrylic acid. The invention is illustrated by the following Examples of which Examples 1 and 4 are control or comparative examples containing no added steering response substance and Examples 2, 3, 5 and 6 are examples of the invention containing a steering response substance. The base elastomeric polymers used in Examples 1, 2 and 3 and 4, 5 and 6, respectively, were an emulsion SBR and a natural rubber; and the compounding ingredients where chosen so that the compositions of Examples 1 - 3 were suitable for the tread of motor car tyres and the compositions of Examples 4 - 6 for truck tyres. Examples 7 - 12 relate to vulcanised compositions obtained by vulcanising the compositions of Examples 1 - 6 respectively. In the Examples all "parts" are by weight unless stated to the contrary and the abbreviations used have the following meanings: MG 30 and MG 49 are each a graft copolymer obtained by graft polymerising methyl methacrylate in natural rubber latex and containing, respectively, nominal proportions of 30 and 49% by weight of poly(methyl methacrylate). They have been obtained from the Rubber Research Institute of Malaysia, PO Box 150, Kuala Lumpar, Malaysia. SBR (23%S) - a styrene-butadiene copolymer having a styrene content of 23% by weight. SMR 20 - a natural rubber N 375 black - a carbon black Enerflex 94 - an aromatic extending oil sold by British Petroleum Company 6PPD - an antidegradant TMQ - an antidegradant CBS - cyclohexylbenzthiazyl sulphenamide vulcanisation accelerator The stearic acid referred to in the Examples was a blend of fatty acids sold for use in the rubber industry and containing fatty acids having nine to 21 carbon atoms in the hydrocarbon chain. The rubber compositions of Examples 1 to 6 were obtained by blending together in an internal mixer the ingredients as shown in Table 1, and the resulting compositions were moulded into test pieces, the cure being under the conditions referred to below. The rubber compositions of Examples 2 and 3 have the same constitution as that of the control Example 1 except that they contain 20 parts of MG30 and MG 49, respectively in place of 20 parts of the base polymer. Similarly, the rubber compositions of Examples 5 and 6 have the same constitution as that of Example 4 except that they contain 20 parts of MG 30 and MG 49, respectively in place of 20 parts of the base polymer. EXAMPLES 7 to 12 Part of each of the six rubber compositions of Examples 1 to 6 was moulded into test samples and subjected to vulcanisation conditions comprising 15 minutes while the temperature was raised to 165°C followed by 20 minutes at that temperature. The resulting vulcanised test samples were tested using the dynamic response apparatus described in the paper by J.E. Smith and E.C. Sumner, published in "Proceedings of The International Rubber Conference 1972". The test conditions and the values obtained for complex modulus and loss factor are shown in Table 2 below. A second part of each of the six rubber compositions of Table 1 was tested to assess its wet grip on a road surface. Each of those compositions was used as the tread compound of model tyres of size 2.25- 8 inches (57 - 203 mm). The tyres were moulded under the following vulcanisation conditions: 60 minutes at 155°C. These model tyres were subjected to two tests as follows: Grip on a wet Delugrip road surface (Delugrip is a Registered Trade Mark) was measured using the internal drum machine described in a paper by R.J. Blythe published in "Proceedings of The International Rubber COnference", 1986, Gothenburg, Sweden. Measurements of the wet grip were made for peak and locked wheel sliding friction over a range of speeds from 8.9 to 22.4 m/second. The test was repeated using a Bridport pebble surface instead of the Delugrip surface. The results obtained were normalised relative to the control. The results (together with mean normalised values for the two surfaces) are shown in Table 3. From Tables 2 and 3 it will be seen that use of the MG30 and MG49 has resulted in an increase in stiffness and in wet grip, especially where the base polymer natural rubber. The rheometry of each of the rubber compositions of Examples 7 to 12 is shown in Table 4 below. The meanings and means of calculation of the parameter symbols shown in Table 4 are given in Table 5 below.

Claims: A pneumatic tyre whose tread portion is characterised in that it comprises a blend of a base elastomer and, as a steering response substance, a graft copolymer of polyisoprene and an unsaturated compound selected from methacrylic acid, acrylic acid and lower alkyl esters thereof; and the steering response substance is present. in the blend in an amount in the range from 5 to 60% by weight of the total of said base elastomer and the steering response substance. A pneumatic tyre according to Claim 1, in which the polyisoprene is natural rubber latex. A pneumatic tyre according to Claim 1 or 2, in which said unsaturated compound is methyl methacrylate. A pneumatic tyre according to Claim 1, in which said amount is in the range from 15 to 50% by weight. A pneumatic tyre according to Claim 1, in which said blend comprises: (a) an SBR elastomer or a cis -1,4-polyisoprene as the base elastomer, and (b) a graft copolymer of polyisoprene and methyl methacrylate as the steering response substance, said graft copolymer constituting from 10 to 40% by weight of the total of said base elastomer and the steering response substance. A pneumatic tyre according to Claim 3, in which the amount of poly(methyl methacrylate) in said blend is in the range from 3 to 20% by weight of the total of said base elastomer and the steering response substance.

Inventors: BLYTHE, ROBERT JOHN, POWELL, BRIAN DAVID WILLIAM
Applicants: SUMITOMO RUBBER INDUSTRIES LIMITED
Application year: 1989
Main classification: B60C1
IPC codes: C08L33, C08L21, C08L9, B60C1, C08L51