GB2066099A - Pneumatic radial tires - Google Patents

Abstract

A pneumatic radial tire (1) comprises a tread portion (2) a pair of side portions, a pair of bead portions and a carcass (5) of a radial structure reinforced with a belt; the carcass consisting of at least one rubberized cord layer produced by embedding cords of a polyethylene terephthalate polyester having a low degree of polymerization from an intrinsic viscosity of from 0.45 to 0.8 in a coating rubber containing not more than 80% by weight of natural rubber and having a swelling network chain concentration of from 25x10<-5>to 60x10<-5> mole/g.rubber, measured according to the conventional method.

Description

SPECIFICATION Pneumatic radial tires This invention relates to pneumatic radial tires, and more particularly to pneumatic radial tires having a good ride feeling, a low rolling resistance and an improved durability.

Radial tires have good wear resistance, cornering performance and the like, but have poor ride feeling as compared with bias tires because cords having a high modulus of elasticity, such as steel cords or the like, are used as a belt reinforcement.

Various attempts have been made to solve the above-mentioned drawbacks of radial tires. Among them, there are known the use of high modulus cords such as rayon cords as a carcass for accelerating the decay of vibration, the selection delivery based on the irregular degree of tire uniformity and the like.

In the former case, however, it is attempted to enhance the modulus of the rayon cord according to the hydrogen bond of its molecular structure, so that the energy loss is large during rotation of the tire and the rolling resistance tends to increase. In the latter case, the selection operation becomes complicated in the factory and also the scrap rate increases, so there there is a problem from the point of view of resource-saving.

Recently, it has been proposed to employ polyesters having a relatively low degree of polymerization and a highly increased modulus (i.e. having an intrinsic viscosity reduced to about 0.6-0.8 without losing the inherent characteristics of polyester or the small energy loss,) as a carcass material for radial tires or a ply cord as disclosed in Japanese Laid Open Patent No.4,902/78. In such tires, however, there are often observed rubber fracture at the turnup end of the carcass ply and separation between the ply cord and the rubber. The reasons for this are believed to be as follows. That is, the difference in the modulus between the ply cord and the surrounding rubber increases at the turnup end of the carcass ply due to the increase of the modulus of the cord, which causes strain concentration in the turnup end.As a result, cracks are produced and grow in the rubber during rotation of the tire and finally the continued use of the tire becomes impossible. Further, the ply cords and the ply coating rubber are subjected to an internal pressure loading after inflation of the tire and a repeated loading during rotation of the tire and the influence of such loads is particularly conspicuous in the side and shoulder portions of the tire, so that the ply cords and ply coating rubber creep into a direction of expanding the tire width. If the creep behaviour of the ply coating rubber is considerably large as compared with that of the ply cord, however, strain is produced between the ply cord and the ply coating rubber owing to the creeping and increases with lapse of running time and finally separation between the ply cord and the ply coating rubber is caused.

Furthermore, the reduction of the intrinsic viscosity of the polyester brings about a reduction in the fatigue resistance of the polyester, so that the strength of the polyester cord after running of the tire is considerably reduced.

It has now been found, in accordance with the present invention, that the above-mentioned drawbacks may be mitigated by improving the modulus and creep behaviour of the ply coating rubber or by increasing the crosslinking density or so-called swelling network chain concentration v8 of the rubber.

According to the invention there is provided a pneumatic radial tire comprising a tread portion, a pair of side portions extending from said tread portion, a pair of bead portions extending from said side portions, and a carcass of a radial structure reinforced with a belt arranged inside said tread portion, in which said carcass consists of at least one rubberized cord layer produced by embedding cords, each composed of a polyester consisting essentially of polyethylene terephthalate and having an intrinsic viscosity (n) of from 0.45 to 0.85 (as measured at 25"C using orthochlorophenol as solvent), in a coating rubber composed of a rubber composition containing not more than 80% by weight of natural rubber based on the total weight of rubber ingredients and having a swelling network chain concentration (v5) of from 25x10-5 to 60x105 mole/g.

rubber, according to the conventional manner.

In the following description reference will be made to the accompanying drawings, wherein: Figure 1 is a graph showing the relationship between the swelling network chain concentration and the breaking strength of the coating rubber; Figure 2 is a graph showing the relationship between the swelling network chain concentration and the elongation at rupture of the coating rubber; and Figure 3 is a schematic radial half section of an embodiment of pneumatic radial tire according to the invention.

The polyester cords to be used in the invention have a lower degree of polymerization as compared with the conventionally used polyester cords and are defined by an intrinsic viscosity (n) of 0.45 to 0.8, as measured at 25"C using orthochlorophenol as solvent. (The intrinsic viscosity is the viscosity obtained when a value of relative viscosity measured at a predetermined concentration is extrapolated to zero concentration). The higher is the value of the intrinsic viscosity, the higher is the degree of polymerization.

According to the invention, the coating rubber to be used for the formation of the rubberized cord layer embedding the above polyester cords according to the conventional manner must contain not more than 80% by weight, from preferably 30 to 75% by weight, from preferably 30 to 75% by weight, of natural rubber based on the total weight of rubber ingredients because it is necessary to prevent reduction of the cord strength after running of the tire, which is produced by the reduction of the fatigue resistance of the polyester due to the reduction of its intrinsic viscosity. If the content of natural rubber exceeds 85% by weight, the reduction of cord strength becomes marked.As rubbers to be blended with natural rubber, mention may be made of synthetic rubbers such as polybutadiene rubbers, styrene-butadiene copolymer rubbers, synthetic polyisoprene rubbers and the like; the use of polybutadiene rubbers and styrenebutadiene copolymer rubbers being preferred.

Furthermore, the coating rubber must have a swelling network chain concentration, v5, of from 25x1 Od to 60x10-5 mole/grubber, preferably from 30x10-5 to 50x10-5 mole/grubber. If v5 is less than 25x10-5 moleig-rubber, the modulus and creep behaviour of the coating rubber are insufficient to achieve the object of the invention whilst if v5 exceeds 60x105 moleig-rubber, the breaking strength and elongation at rupture of the coating rubber tend to be reduced as shown in Figures 1 and 2, though the strain between the ply cord and the rubber reduces with the increase of v5, and as a result the coating rubber tends to be broken.Thus, it has been confirmed from experimental results that V5 of more than 60x10-5 mole/g-rubber cannot achieve the objects of the invention.

v5 is measured as follows: Firstly, a sample of vulcanized rubber composition is weighed prior to the swelling. Then, the sample is swollen by immersing it in benzene at 350C for 48 hours. after the swelling, the sample is gently wiped with filter paper and placed in a weighing bottle. After the bottle has been closed, the swelled sample is weighed to calculate the volume fraction (vr) of rubber in the swelled sample. The calculation of vr in the swelled sample is carried out by dividing the rubber volume prior to the swelling, which is obtained by subtracting a volume of fillers (carbon black, silica and the like) in the sample from the volume of the sample prior to swelling, by the sum of the benzene volume in the swelled sample and the rubber volume prior to swelling.

Then, v5 is determined according to the following Flory - Huggins' equation: v5(mole/g-rubber)

wherein V is a molar volume of benzene and 11 is a constant of interaction, for example, 0.43 for natural rubber or synthetic polyisoprene rubber, 0.41 for polybutadiene rubber and 0.39 for styrene-butadiene copolymer rubber.

The pneumatic radial tires of the above-mentioned construction according to the invention can suffer less premature failures due to the rubber cracking in the turnup end of the carcass ply and the separation between the ply cord and the ply coating rubber, which are drawbacks in conventional tire using polyester cords of low degree of polymerization as a carcass, to considerably improve the durability, and provide a good ride feeling and a low rolling resistance, so that they are very advantageous in practical use.

In order that the invention may be well understood, the following Example is given by way of illustration only.

Example Pneumatic radial tires 1 having the construction shown in Figure 3, (tire size: 165 SR 13) were prepared by using a polyester cord layer, which was produced by embedding low polymerization polyester cords (1500 d/2) each having an intrinsic viscosity (N) of 0.50 or 0.75 in various coating rubbers having the compositions as shown in Table 1, as a carcass 5 for the tire 1 and using two steel cord layers as a belt 3. In Figure 3, numeral 2 represents the tread rubber, numeral 4 represents an inner liner, numeral 6 a bead filler, numeral 7 a bead wire and numeral 8 a turnup end of the carcass ply.Each of the tires 1 was then tested with respect to ride feeling, rolling resistance, cracking resistance at the turnup end of the carcass ply, retention of peel strength and retention of cord strength. Byway of comparison, the same tests were made with respectto a tire containing conventional polyester cords having an intrinsic viscosity of 0.90 or the rayon cords as a carcass. The results obtained are shown in Table 1.

The tests were performed as follows: 1) Ridefeeling The test tire was contacted with an iron drum of 3 m diameter provided with a protrusion having a width of 5 cm and a height of 1 cm, which was rotated by a driving motor. The radial vibration of the tire when riding over the protrusion was measured by an accelometer as a force acting on the tire shaft. Then, the amplitude of vibration in the first period was obtained from the recorded wave form as a hit index, which is expressed by a reciprocal number on the basis of the tire No. 1. Furthermore, the decay of vibration was obtained as a damping factor from the recorded wave form and defined by an index on the basis of the tire No. 1. The higher the value, the better the ride feeling.

2) Rolling resistance The test tire was contacted with an iron drum of 2 m diameter, which was rotated by a driving motor and then the rotational speed of the drum was raised to a predetermined value. Thereafter, the drive motor was stopped to allow the drum to run by inertia, during which the rolling resistance of the tire was measured on the basis of the deceleration speed of the drum and defined by an index on the basis that the tire No. 1 is 100.

The higher is the value, the lower is the rolling resistance.

3) Cracking resistance at turnup endofcarcassply After the tire had been run on public roads over a distance of 60,000 km, the turnup end of the carcass ply was cut out with a knife to measure the size of cracks at that end, which was defined by an index on the basis that the tire No. 1 is 100. The smaller is the value, the better is the cracking resistance.

4) Retention of peel strength After the tire had been run on public roads over a distance of 60,000 km, the tire was dissected to measure the peel strength between the belt and the carcass, which was compared with the peel strength before running to measure the retention (%) of peel strength.

5) Retention ofcordstrength After the tire had been run on public roads over a distance of 60,000 km, the cords of the carcass ply were taken out from the tire to measure the cord strength, which was compared with the cord strength before the running to determine the retention (%) of cord strength.

TABLE: Tire No. 1 2 3 4 5 6 7 8 9 10 Ply cord Kind of liber poly- poly- poly- poly- poly- poly- poly- poly- poly- rayon of carcass ester ester ester ester ester ester ester ester ester Intrinsic viscosity 0.90 0.90 0.50 0.50 0.50 0.50 0.50 0.75 0.50 Natural rubber (wt,%) 100 80 100 80 80 80 80 80 75 100 Polybutadiene rubber (wt.%) 20 20 20 20 Coating Styrene-butadiene rubber copolymer rubber (wt.%) 20 20 Synthetic polyisorprene 25 rubber (wt.%) Swelling network chain concentration vs 30 30 55 20 50 65 50 38 56 65 Ride Hitindex 100 100 100 103 100 105 105 100 100 105 feeling Decay index 100 100 140 135 140 140 140 120 140 140 Rolling resistence 100 100 100 102 100 100 100 102 102 90 Cracking resistance Results at turnup end of 100 100 70 120 70 130 75 90 70 100 carcass ply Retention of peel Strength 75 76 73 50 73 60 75 76 70 70 Retention of cord strength

Claims (3)

1. A pneumatic radial tire comprising a tread portion, a pair of side portions extending from said tread portion, a pair of bead portions extending from said side portions, and a carcass of a radial structure reinforced with a belt arranged inside said tread portion; in which said carcass consists of at least one rubberised cord layer produced by embedding cords, each composed of a polyester consisting essentially of polyethylene terephthalate and having an intrinsic viscosity (n) of from 0.45 to .85 (as measured at 25"C using orthochlorophenol as solvent), in a coating rubber composed of a rubber composition containing not more than 80% by weight of natural rubber based on the total weight of rubber ingredients and having a swelling network chain concentration (v ) of from 25 x 10-5 to 60 x 10-5 mole/g.rubber according to the conventional manner.

2. A pneumatic radial tire as claimed in Claim 1, wherein said coating rubber has a swelling network chain concentration (v ) of from 30 x 10-5 to 50 x 10-5 mole/g-rubber.

3. A pneumatic radial tire as claimed in Claim 1 substantially as hereinbefore described with reference to the Example herein.