KR960000505B1 - Rubber compositions for tire treads - Google Patents

Abstract

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Description

Rubber composition for use in tire tread

1 is a graph showing the shear modulus of conventional rubber compositions illustrated for fracture temperature,

2a and 2b show a view similar to that of FIG. 1, but showing a temperature-to-shear modulus relationship and a temperature-to-loss factor relationship for rubber compositions and comparative compositions embodying the present invention, respectively.

The present invention relates to a rubber composition suitable for use in tire treads.

Automotive tires generally have to satisfy driving safety, riding comfort and cost reduction. As a result of the development of highway networks, there is an urgent need for improved devices that can provide reliable steering, cornering and braking properties during high speed driving.

In order to obtain improved tire characteristics, frictional performance is required, which can be defined as the friction between the tread of the tire and the road or pavement surface. Tread rubbers for this purpose must be made to be very sensitive to slight deformations on the road and hysteresis losses resulting from cyclic deformation of the tread on its surface during high speed, frictional contact. The more the tread wastes energy at the contact site through hysteresis loss, the greater the friction. According to the Williams-Landel-Ferriton-time superposition principle, such tread deformation is known to depend on the magnitude of the hysteresis loss determined at lower temperatures than the tire is used. Indeed, the loss factor (tanσ), often taken as a measure of hysteresis loss, correlates significantly with the coefficient of friction obtained by the tire, especially when measured at about 0 ° C.

Improved friction performance and reduced heat generation are considered difficult to achieve at the same time because large hysteretic losses cause unsatisfactory heat buildup and inadequate rolling. Periodic deformation of the tread is less frequent during conventional clouds, where instantaneous energy waste is known to be substantially the same as the tan σ value measured at relatively high temperatures in the range of 40-60 ° C., although it depends on the low temperature tan σ value during high speed sliding. Thus, the tan σ value can easily be dependent on temperature, ie high at about 0 ° C. and low at about 40-60 ° C., whereby both properties can be obtained.

Attempts have been made to improve hysteresis losses using styrene-butadiene copolymer rubbers having high styrene content and high glass-transition temperatures. However, conventional rubbers of the styrene-butadiene class tend to result in insufficient wear resistance with increasing styrene content and also inadequate low temperature resistance at elevated glass-transition temperatures. These rubbers provide too high modulus to follow the asperate road surface when exposed to reduced ambient temperatures. In other words, the energy waste is small even if the tan σ value is kept high, resulting in a reduction in the braking force. Rubber has a brittle structure at much lower ambient temperatures, which is dangerous.

By combining styrene-butadiene copolymer rubber, natural rubber or isoprene rubber having a glass-transition temperature of -50 ° C or higher and, if desired, butadiene rubber having a glass-transition temperature of -70 ° C or lower together with a selected kind of carbon black and a softening agent. It has now been found that excellent tires can be obtained using the resulting rubber composition.

It is therefore a primary object of the present invention to provide a new and improved rubber composition for use in a tire tread that is excellent in wear resistance, friction performance and low temperature resistance and which makes the tire suitable for four seasons of operation.

The above and other objects and features of the present invention will be better understood from the following detailed description.

According to one important aspect of the present invention, at least one styrene-butadiene copolymer rubber having a glass-transition temperature of at least -50 ° C and a natural or synthetic isoprene rubber and, if desired, butadiene rubber having a glass-transition temperature of at most -70 ° C Raw material rubber (the total amount is 100 weight part) comprised; 80-130 parts by weight of carbon black having a specific surface area of 110 m ² / g or more (this amount is based on the total amount of raw rubber); And 20-90 parts by weight of a softening agent having a viscosity-gravity constant between 0.90-0.98 (the amount is based on the total amount of raw rubber), which has a storage shear modulus of less than 500 MPa at −30 ° C. A rubber composition for use in the composition is provided.

A first preferred embodiment of the invention includes the use of three-component raw rubbers consisting of one or more styrene-butadiene copolymer rubbers (SBR), natural rubber (NR) or synthetic isoprene rubber (IR) and butadiene rubber (BR). do.

SBRs suitable for the purposes of the first embodiment should have a derived-transition temperature (Tg) of at least -50 ° C. Tg lower than −50 ° C. does not yield better results than conventional styrene-butadiene rubbers. SBR can be prepared by solution or emulsion polymerization, but preferably should have a styrene content of at least 25% by weight when emulsion-polymerized using an organic peroxide catalyst.

As used herein, BR should have a Tg of −70 ° C. or less. Tg above -70 ° C will not be effective in improving wear resistance and low temperature resistance. BR can be obtained by solution or emulsion polymerization in the presence of a Ziegler catalyst or a lithium catalyst. Preference is given to polybutadienes containing at least 90% by weight cis 1-4 bonds.

The amount of SBR, NR / IR and BR to be added should be in the range of 90-20, 5-50 and 5-30 parts by weight, respectively, in order. More than 90 parts by weight of SBR or less than 5 parts by weight of NR / IR and BR will not obtain sufficient wear resistance and adequate low temperature resistance. Less than 20 parts by weight of SBR, more than 50 parts by weight of NR / IR or more than 30 parts by weight of BR results in reduced frictional performance.

Carbon blacks useful as reinforcing fillers in the present invention include, for example, ISAF, SAF and ultra SAF carbon black. This drug should have a specific surface area (N ₂ SA) of at least 110 m ² / g as measured by nitrogen adsorption. N ₂ SA below 110 m ² / g results in insufficient friction performance and low wear resistance.

The amount of carbon black added should be in the range of 80-130 parts by weight based on 100 parts by weight of the raw rubber. An amount less than 80 parts by weight cannot provide sufficient friction performance and satisfactory wear resistance. An amount greater than 130 parts by weight results in a decrease in dispersion, which mechanically weakens the resulting composition.

Suitable softening agents are of petroleum type, specific examples of which include aromatic process oils. The softening agent should have a viscosity-gravity constant (VGC) of 0.90-0.98. VGC values dictate the Tg value of the composition, and VGCs less than 0.90 result in reduced frictional performance.

The amount of softening agent added may vary depending on the amount of reinforcing filler, but should be in the range of 20-90 parts by weight per 100 parts by weight of raw rubber. An amount less than 20 parts by weight may make the composition less stretchable and therefore less chipping-resistant. Amounts greater than 90 parts by weight can lead to a decrease in mechanical strength.

According to a second preferred embodiment of the invention, two-component raw rubbers are used which consist of at least one SBR and NR / IR.

SBR should have a Tg between -30 ° C and -50 ° C. Tg above −30 ° C. results in too high a breakdown temperature and cannot be used for commercial applications. Tg below −50 ° C. will be ineffective as in conventional styrene-butadiene rubbers. The SBR used in the second embodiment can be prepared as in the first embodiment but preferably has a styrene content of 25-50% by weight.

The amount of SBR and NR / IR added should be in the range of 90-40 and 10-60 parts by weight, respectively. SBR of more than 90 parts by weight or NR / IR of less than 10 parts by weight will not improve the wear resistance and low temperature resistance. Less than 40 parts by weight of SBR or more than 60 parts by weight of NR / IR can result in reduced frictional performance.

Suitable carbon blacks and softening agents for the second embodiment should be strictly observed as to provide a rubber composition of the desired properties which is the same as in the first embodiment with respect to the particular type and amount and expected by the present invention.

The rubber compositions according to the first and second embodiments should have a storage shear modulus (G ′) of less than 500 MPa at −30 ° C. The reason for this temperature limit is that tire use is rare below -30 ° C.

In general, rubbery materials suddenly increase in modulus at reduced ambient temperatures. This is known as the glass-transfer phenomenon, where rubber is converted into a glass state in which its thermal molecular motion is fixed. The rubber is thus stiff and prone to brittle structures only under significant deformation. The temperature at which such a brittle structure occurs is usually called the fracture temperature, which is closely related to the modulus of elasticity. As is evident from FIG. 1, a typical rubber composition has a G 'value of at least 500 MPa at the fracture temperature.

Conventional additives such as vulcanizing agents, vulcanization accelerators, antioxidants, processing aids and the like can be used.

The present invention will now be described by the following examples, which should not be considered as limiting the present invention.

[Example 1-15 and Comparative Example 1-13 (SBR-NR-BR raw material rubber)]

Numerous rubber compositions and controls according to the invention were prepared and vulcanized as shown in Tables 1-3. Viscoelasticity and wear were measured under the conditions given below and the results are given in these tables.

Loss factor (tanσ)

A viscoelastic spectrometer (Iwamoto Sesakusho Co., Ltd.) was used with a frequency of 20 Hz, an initial strain of 10%, and a dynamic strain of ± 2.0%. 0.70 or more are preferable.

Wear

Audit using a hardened pico type wear machine. The case of Comparative Example 1 is taken as the standard index. The larger the index, the greater the wear resistance.

Storage shear modulus (G ')

A dynamic mechanical analyzer (Leometric) was used, with frequency 10 Hz, shear strain 0.5% and unit MPa.

Glass-Transition Temperature (Tg)

A thermal analyzer (Dupont) was used and the temperature increase was 10 ° C / min and Tg is the intersection between the tangent line of the transition point and the extrapolated baseline (extrapolation-starting temperature).

Viscosity-Gravity Constant (VGC)

It was measured according to ASTM D 2140-81.

TABLE 1

Formulation: parts by weight

(1) emulsion-polymerized SBR, styrene content 23.5%, Tg -54 ° C NIPOL 1712 (Nippon Xeon Co.)

(2) emulsion-polymerized SBR, styrene content 35%, Tg -43 ° C NIPOL 9520 (Nippon Xeon Co.)

(3) emulsion-polymerized SBR, styrene content 45%, Tg -33 ° C

(4) solution-polymerized SBR, styrene content 31% Tg -46 ° C (Asahi Chemical Co.)

(5) BR, cis 1-4 bond 98%, Tg -105 ° C, NIPOL 1441 (Nippon Xeon Co.)

(6) NR, RSS No.3

(7) HAF Carbon Black N 339, N ₂ SA 80m ² / g

(8) ISAF carbon black N 220, N ₂ SA 115 m ² / g

(9) SAF Carbon Black N 110, N ₂ SA 200m ² / g

(10) Ultra SAF Carbon Black, N ₂ SA 200m / g

(11) Aromatic oil, VGC 0.95 (Fuji Kosan Co.)

(12) Paraffin oil, VGC 0.84 (Fuji Kosan Co.)

(13) N- (1.3-dimethylbutyl) -N'-phenyl-P-phenylenediamine

(14) N-cyclohexyl-benzothiazyl-sulfenamide

(1)-(5): Oil-in-oil rubber which contains 37.5 weight part of aromatic oils, respectively: The quantity shown in the table is the said oil.

TABLE 2

Formulation: parts by weight

(2)-(6), (9), (11), (13), (14): See footnotes in Table 1.

TABLE 3

Formulation: parts by weight

(2), (5)-(14): see footnotes in Table 1

Example 4 and Comparative Examples 1, 2 and 9 relate to the use of SBRs with varying styrene content. Styrene-rich SBR is inadequate to wear resistance and storage shear modulus (G ') given a sufficient loss factor (tanσ). Example 1-4 showing the present invention is very satisfactory for both tan sigma value and G 'value, and therefore also very satisfactory for low temperature resistance. As clearly shown in Figs. 2 (a) and 2 (b), Examples 2 and 4 are more gentle than Comparative Example 2 when considering the temperature-dependent curves of G 'and tan σ values.

Next, looking at the use of NR or IR mixed in different proportions with 20 parts by weight of BR, Examples 5-7 for Comparative Examples 3 and 4 show sufficient abrasion resistance and moderate G 'value, with the Does not decrease significantly.

Examples 8 and 9 are effective for improving abrasion resistance and G 'value when compared to Comparative Example 9 in which only SBR is used. Comparative Example 10 compared to Examples 10-12 using HAF carbon black with an N ₂ SA of 110 m ² / g or less did not provide a tan σ value increase.

Paraffin oil softeners having a VGC of 0.90 or less, Comparative Example 11 showed satisfactory G 'values but insufficient tanσ values.

Carbon blacks and oils outside of the ranges specified above would face reduced wear which was evident from Comparative Examples 12 and 13 over Examples 13-15.

Comparative Examples 6 and 8 are examples of the use of SBR, NR / IR and BR mixed in various ratios. Excess NR / IR or BR results in a sharply reduced tan σ value even if satisfactory in wear resistance.

[Example 16-20 and Comparative Example 14-18 (SBR-NR raw material rubber)]

Inspection rubber compositions corresponding to Tables 4 and 5 were prepared and vulcanized. Viscoelasticity and wear were measured and were the same as for SBR-NR-BR raw rubbers and the results were plotted.

TABLE 4

Formulation: parts by weight

(1)-(4), (6), (8), (11), (13), and (14): see footnotes in Table 1

(1)-(4): Oilfield rubbers containing 37.5 parts by weight of aromatic oil, respectively:

Chart the amount excluding such oils.

TABLE 5

Formulation: parts by weight

(2), (3), (6)-(9), (11)-(14): see footnotes in Table 1.

Examples 16 and 17 and Comparative Examples 14-16 relate to the use of SBR of various styrene contents. The styrene-rich SBR was not satisfactory in wear resistance and G 'value even though the tan σ value was sufficient. Examples 16 and 17 are excellent in tanσ value and G ', and thus excellent in low temperature resistance.

Comparative Example 17 compared to Example 19, using HAF carbon black with N ² SA of 110 m ² / g or less, showed insufficient tanσ values.

Softening agents with satisfactory G values of VGCs of 0.90 or less have no effect on tan σ value improvement and are incorporated into Comparative Example 18.

As is evident from Comparative Example 19 for Example 20, carbon blacks and oils outside the specified range result in reduced wear resistance.

Excess NR, Comparative Example 20 provides improved wear resistance but reduced tanσ value.

In this way, the present invention has been described above, and it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the scope of the appended claims.

Claims (8)

Rubber compositions for use in tire treads, comprising: (a) at least one styrene-butadiene copolymer rubber having a glass-transition temperature of -50 ° C. or higher and natural rubber or isoprene rubber and butadiene having a glass-transition temperature of -70 ° C. or lower; Raw rubber composed of rubber (total amount is 100 parts by weight); (b) 80-130 parts by weight of carbon black having a specific surface area of 110 m ² / g or more (the amount is based on the total amount of the raw rubber); And (c) 20-90 parts by weight of a softening agent having a viscosity-gravity constant between 0.90 and 0.98, the amount being based on the total amount of the raw rubber, thereby having a storage shear modulus of 500 MPa or less at -30 ° C. Rubber composition The rubber composition according to claim 1, wherein the raw rubber is composed of 90-20 parts by weight of the styrene-butadiene copolymer rubber, 5-50 parts by weight of the natural rubber or isoprene rubber, and 5-30 parts by weight of the butadiene rubber. 3. The rubber composition of claim 2, wherein the styrene-butadiene copolymer rubber has a styrene content of at least 25% by weight. The rubber composition according to claim 1, wherein the raw rubber is composed of 40-90 parts by weight of the styrene-butadiene copolymer rubber and 60-10 parts by weight of the natural rubber or isoprene rubber. The rubber composition of claim 4, wherein the styrene-butadiene copolymer rubber has a styrene content of 25-50% by weight. The rubber composition of claim 1, wherein the butadiene rubber is cis-polybutadiene containing 90% by weight or more of 1-4 bonds. The rubber composition according to claim 1, wherein the carbon black is furnace carbon black. The rubber composition of claim 1, wherein the softening agent is an aromatic process oil.

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