JPH059281B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an improvement in a pneumatic tire using foamed rubber for the tread rubber layer, which is used for running during the winter season or from the winter season through the summer season.

Conventional Technology Winter tires that are designed for driving on icy roads in the winter and all-season tires that are used for driving on icy roads in the winter as well as normal paved roads in the summer have a tread pattern that is a plurality of grooves (usually 3 to 5 equally spaced) extending therein and a number of lateral grooves extending across these circumferential grooves from one end of the tread to the other; consisting of a land area defined by
A so-called block type tread is common.

This type of block-type tread is sometimes applied to summer tires, but in the case of winter tires and all-season tires, it is used to improve the snow gripping characteristics (road holding on snowy roads) when driving on snowy roads. Compared to summer tires, the circumferential and lateral grooves of the tread are made wider and deeper, while winter tires in particular have studs (or spikes) on the land area in consideration of slip resistance on icy roads. ) have been used for a long time. Such studded tires reliably exhibit driving, braking, and steering performance when driving on icy roads, which are particularly problematic in terms of driving safety. However, even though they are winter tires, they do not only drive on icy and snowy roads; they are often used for a certain period of time on roads without snow or even after the ice has disappeared. It is widely known that road damage caused by studs driven into tires on the paved road surface and pollution caused by the dust generated at this time have become social problems.

In view of the above problems, so-called studless tires, which replace studded tires, are made using ice compounds (rubber) that are designed to maintain flexibility even when the temperature drops, especially for on-ice performance. Types that use a treaded rubber layer have mainly been developed, but unlike this type of improvement of the properties of the rubber itself based on rubber compounding, we have developed foamed rubber, which is not normally used as a component of tires. A type of material to be used as a tread rubber layer was filed in a patent application filed in 1983 by the applicant.
It was developed and proposed in No. 77081 (Japanese Unexamined Patent Publication No. 62-283001). This proposal focuses on the generation of friction that stops slipping on icy or snowy roads, and the water film (water and pseudo-liquid layer formed by melting ice and snow) that exists between the tire and the road surface, which is a major factor in slipping. By using foamed rubber containing a certain amount of air bubbles in the tread rubber layer, the rubber itself does not lose its flexibility even at low temperatures, and the countless edges create an edge effect and a normal A major feature is that it can be expected to have a water-repellent effect that cannot be obtained with rubber.
According to this, it was confirmed that compared to tires using ice compounds as studless tires, a much higher level of performance on ice, represented by driving, braking, and maneuverability on icy roads, can be achieved. This excellent on-ice performance is due to the foamed rubber's flexibility, edge effect, and water removal properties as described above.

Problems to be Solved by the Invention A tire using foamed rubber for the tread rubber layer has the following problems:
As mentioned above, it has excellent on-ice performance, but its on-ice performance hardly changes even when the tread rubber layer is worn, so it can be used not only in winter but also for long periods from winter to summer. It has been confirmed through numerous tests that this is possible. However, it has also been found that some problems arise with such long-term use. One of the problems
The first is cracks that occur at the bottom of the tread groove. This phenomenon is rarely seen in tires using ice compounds, but once a crack occurs, it can grow and, in extreme cases, cause a block in the tread.
This develops into peeling or separation from the belt layer, causing serious problems in terms of durability.

Therefore, the present inventors have conducted various studies in view of the above-mentioned problem of crack generation, and according to the research,
As mentioned above, this type of tire has relatively wide and deep tread grooves, and is susceptible to circumferential force (due to driving and braking) and lateral force (due to cornering) when running. It was found that dynamic strain occurs repeatedly in the trench bottoms that connect the land regions, and as a result, the burden on the trench bottoms is higher than expected and exposed to extremely severe conditions. Ta. When we examine the foamed rubber located at the bottom of the groove in detail, we find that the foaming rate in the center of the thickness is higher than that at both ends of the thickness, due to the large number of bubbles contained in this foamed rubber. Because the partition wall is formed thinner, the strength of this part is weaker than that of the upper and lower parts of the thickness, and when dynamic strain occurs repeatedly at the groove bottom of the tread due to running, the foam rubber at the groove bottom position Stress concentrates in the center of the weak thickness and a crack nucleus occurs, which then spreads throughout the thickness.Furthermore, the foaming rate in the center is related to the thickness at the bottom of the groove, and if the thickness is thin, It was found that the bubble diameter and the number of bubbles at the upper and lower ends of the thickness as well as at the center area decreased, the foaming rate decreased, and the strength per unit area improved. Based on the above findings, and as a result of further research, we formed the tread rubber layer into a multi-layer structure consisting of the above foamed rubber layer and a substantially non-foamed base rubber layer, and performed normal vulcanization molding using a mold. When the foamed rubber layer is supported by the base rubber layer at the groove bottom position, it is formed as a sufficiently thin constriction with a small foaming rate, thereby effectively suppressing the occurrence of cracks at the groove bottom. The present invention was achieved based on the discovery that

The object of the present invention is to effectively suppress cracks occurring at the groove bottom of the tread in the above-mentioned pneumatic tire for winter or all-season use in which the tread rubber layer is made of foamed rubber having closed cells.

Means for Solving the Problems This invention uses foamed rubber having closed cells for the rubber layer of the tread, and the tread has a plurality of circumferential grooves and a number of lateral grooves extending across the circumferential grooves and the ends of the tread. In a pneumatic tire comprising directional grooves and a land portion defined by the groove group, the tread rubber layer has a foamed rubber layer made of the foamed rubber located on the radially outer side and a foamed rubber layer made of the foamed rubber located on the radially inner side. a base rubber layer made of substantially non-foamed rubber located thereon, and the foamed rubber layer forms a thin constricted portion on the base rubber layer at the groove bottom position of the circumferential groove and the lateral groove. and is interconnected with the foamed rubber layer located in the land portion, and the foaming rate in the constricted portion is smaller than the foaming rate of the foamed rubber layer in the land portion.

Effect As described above, in the present invention, the tread rubber layer is configured as a multilayer structure, and the foamed rubber layer at the bottom of the groove is supported by the base rubber layer, which is substantially non-foamed rubber, and is thin and has a small foaming rate. By being configured as a constricted portion, the strength of the foamed rubber layer at this portion is high, and the occurrence of cracks at the groove bottom is effectively suppressed.

EXAMPLES Hereinafter, the present invention will be described in detail based on examples and drawings.

1 to 4 are diagrams showing preferred embodiments of the pneumatic tire of the present invention.

In Figures 1, 2, and 3, pneumatic tire 1 is a radial tire for passenger cars (tire size
165SR13), with a customary cylindrical tread rubber layer 3 on the tread 2 at the radially outer end.
Also, a pair of bead portions 5 are provided at the radially inner end.
between the bead portions 5, a carcass 6 consisting of rubberized cords arranged approximately radially in this embodiment, and two layers extending circumferentially of the tire on the outside of the crown portion 2 of the carcass 6. It is reinforced by arranging a non-stretchable belt 7. This belt 7 is arranged radially inside the tread rubber layer 3. Also, Torezdo 2
A side wall 8 is provided at both ends of the shoulder 4 , that is, between the shoulder 4 and both bead portions 5 .

The tread rubber layer 3 uses a foamed rubber layer 3A having closed cells on the radially outer side that engages with the road surface, and a base rubber layer 3B made of ordinary non-foamed rubber on the radially inner side of the foamed rubber layer 3A.
is located. Here, non-foamed rubber is suitable for the inner base rubber layer, but foamed rubber with a significantly small foaming rate can also be used if necessary. Further, a third rubber layer having a foaming rate intermediate between the foaming rates of both rubber layers 3A and 3B is provided between the foamed rubber layer 3A and the base rubber layer 3B, for example, in order to reduce the difference in rigidity at the boundary between these layers. It is also possible to arrange.

Here, regarding the hardness of the rubber layer, generally speaking, the inner base rubber layer 3A is smaller than the outer foam rubber layer 3A.
It is preferable that B has higher hardness, and Shore A
When measured as hardness, the foamed rubber layer 3A ranges from 35 degrees to 53 degrees, and the base rubber layer 3B ranges from 54 degrees to 80 degrees.
A range of degrees is recommended for practical purposes.

The foamed rubber layer 3A is considered as a proportion of the total area of the treaded rubber layer (this proportion is based on the land area 12
(approximately corresponds to the proportion of the total thickness of the foamed rubber layer 3A), and should be at least 10% from the viewpoint of wear life related to performance on ice and snow. That is, if this ratio is too small, the base rubber layer 3B will be exposed in the middle of the wear life of the tire 1, and the performance on ice and snow will be lost, so that the tire must be used as a normal summer tire thereafter. However, the percentage will be touched on later,
From the above-mentioned purpose of arranging the base rubber layer 3B,
It is preferably 80% or less.

In addition, the foaming ratio V of the foamed rubber layer 3A is generally preferably in the range of 5% to 50%, from the viewpoint of the above-mentioned performance on ice and abrasion resistance on drying roads.
A range of 30% is more preferred. Note that the foaming rate V is calculated by the following formula.

V=(ρ ₀ /ρ ₁ -1)×100(%) In this, ρ ₀ is the density of the rubber solid phase part of the foamed rubber (g/cm ³ ), and ρ ₁ is the density of the foamed rubber (g/cm ^{3 )} . ). Furthermore, the average cell diameter of the closed cells of the foam rubber is preferably in the range of 5 μm to 150 μm, and is preferably in the range of 10 μm to 150 μm.
A range of 100 μm is more preferred. If the bubble diameter is too small and the average bubble diameter is less than 5 μm, the effect of ice and snow performance will generally be insufficient. On the other hand, if the bubble diameter is too large and the average bubble diameter exceeds 150 μm, the wear resistance and land area There is a risk that the restoring force, settling, cut resistance, etc. may decrease.

Further, the foamed rubber is molded by adding a foaming agent to a normal rubber compound and heating and pressurizing the mixture according to a normal tire manufacturing method. As the blowing agent, for example, dinitroso pentamethylene-tetraamine, benzenesulfonyl hydrazide, etc. are used.

3D is an auxiliary rubber layer made of ordinary non-foamed rubber and has physical properties similar to the outer cover rubber of the sidewall 8. As shown in FIGS. 2 and 3, this auxiliary rubber layer 3D covers and protects a part of the groove bottom of the lateral groove 10 from the joint part of the foamed rubber layer 3A and base rubber layer 3B at the side part of the tread rubber layer 3. There is. The function of this auxiliary rubber layer 3D is to protect the shoulder 4, which has a relatively large strain and is susceptible to cut damage, and to suppress separation at the joint end between the foam rubber layer 3A and the base rubber layer 3B. be.

Further, in both side areas 3a and the central area 3b of the tread 2, a large number of lateral grooves 10 extending in the transverse direction from the shoulder 4 (tread end) are arranged on the circumference of the tire. Further, two linear circumferential grooves 11A are provided in the central area 3b, and deformed zigzag circumferential grooves 11A are provided in both side areas 3a.
B is provided. These lateral grooves 10 intersect with the circumferential grooves 11A and 11B to define a block-shaped land portion 12. Here, foam rubber layer 3
A forms thin constrictions 14 at the groove bottoms 10a of the lateral grooves 10, circumferential grooves 11A, and circumferential grooves 11B, and these constrictions 14 cover the foamed rubber layer 3A located in the land portions 12. They are interconnected on the radially outer side of the base rubber layer 3B. The foaming rate of this constricted portion 14 is sufficiently small.
The foaming rate is at least smaller than that of the land portion 12, and therefore the strength per unit area of the constricted portion 14 is high, and the constricted portion 14 is substantially formed of the non-foamed base rubber layer 3B. This effectively suppresses the occurrence of cracks at the bottom of the groove. Note that 15 is a sipe, which is provided on the land portion 12 in the lateral direction of the tire.

To manufacture a tire with such a multi-layered tread rubber layer, first, the inner rubber layer (which becomes the non-foamed base rubber layer 3B) and the rubber compounded with the above-mentioned foaming agent are added to the inner rubber layer (which becomes the non-foamed base rubber layer 3B). layer (later foamed to become foamed rubber layer 3A),
If necessary, rubber layers (which will become non-foamed auxiliary rubber layers 3D) are placed on both sides of these rubber layers to form a treaded rubber layer having a substantially trapezoidal cross section. In this case, the outer and inner rubber layers are somewhat thicker at both ends, but can have a substantially uniform thickness over the entire width. A green tire is formed by bonding the tread rubber layer thus obtained to the crown portion of a carcass layer formed in a separate process together with the belt layer. Next, this green tire is vulcanized in a mold, and in the mold, vulcanization is carried out from the inside of the green tire using steam pressure via a bladder in a conventional manner. At this time, the inner surface of the vulcanization mold for molding the tread rubber layer has a large number of protruding ridges corresponding to the circumferential and lateral grooves, so these ridges form the shape of the green tire during vulcanization. Apply strong pressing force to the flat outer surface of the tread rubber layer. Here, when a pressing force is applied to the tread rubber layer in this way, the tread rubber layer between the heated ridges and the highly rigid belt layer softens and flows into the recesses between the ridges. At this time, because the pressure is high in this pressed area, the gas generated by the foaming reaction moves from the groove bottom area to the land area and diffuses, and some of the gas is transferred to the inner rubber layer. It also migrates and spreads. Therefore, at the end of vulcanization, the foamed rubber layers 3A in the land portion 12 mutually interact in the circumferential direction and width direction of the tire 1 via the thin constricted portions 14 with a small foaming rate of the foamed rubber layers 3A located at the groove bottom. It becomes a form connected to. At this time, the base rubber layer 3B also has the remaining thickness to form a land portion and a constricted portion. In this way, the foaming rate of the constricted portion 14 of the foamed rubber layer 3A is small;
In addition, since the base rubber layer 3B is formed to be thin, the stiffness of the constricted portion 14 is increased.
This effectively suppresses the occurrence of groove bottom cracks.

In this case, the thickness D3A of the foamed rubber layer 3A at the width direction center E of the groove, that is, the constriction part 14 is
For the reasons mentioned above, the total thickness D3 from the groove bottom surface 10b to the cord reinforcing layer, here the belt 7, is
It should be 70% or less, more preferably 50% or less. FIG. 5 shows the results of a test conducted separately on this point, and shows the foaming rate along the thickness direction of the foamed rubber layer 3A at the groove bottom 10a, that is, the constriction 14, when the thickness is 5 mm. Thing A,
For the sake of convenience, the three types, B, which is 4 mm, and C, which is 2 mm (the total thickness of the groove including the base rubber layer is 7 mm), are calculated as equivalent to the number N of foamed cells per unit area. . In addition, in the tires used in this test, the thickness of the foam rubber layer and base rubber layer in the case of a green tire was 13.5 mm and 2.5 mm for A, 12 mm and 4 mm for B, respectively.
mm and C were 7 mm and 9 mm. As shown in the figure, the number N of foams on the groove center line E decreases as the thickness D3A of the foamed rubber layer 3A decreases sequentially from A to B to C. For this reason, the constriction 14
is strengthened by this reduction in foaming rate, and furthermore, by reducing the thickness of the constricted portion 14, the base rubber layer 3 is strengthened.
The thickness of B increases accordingly (D3 in Figure 4).
-D3A), the reduction in the thickness of the constricted portion 14 works more advantageously in terms of reinforcing the constricted portion 14 at the groove bottom position.

Next, in order to confirm the effects described later, a tread rubber layer of a tire was molded using the rubber shown in Attached Table 1 to prepare a test tire. Here, the ratio of the thickness of the outer rubber layer (later foamed to form the foamed rubber layer 3A) in the green tire before vulcanization to the total thickness of the tread rubber layer was 40%. In addition, the entire tread rubber layer of this test tire was composed of the foamed rubber of Composition 1 shown in Attached Table 1, and a comparative tire was prepared. In Attached Table 1, various characteristics or physical properties of Composition 1 (foamed rubber) are shown at point P on land area 12 of tire 1 (see Figures 2 and 4).
is the value at .

As a result of measuring the foaming rate V of the foamed rubber layer 3A in the tread rubber layer 3 according to the method described later, it was found to be 22% at point P of the land portion 12 (see Figures 2 and 4) as shown in Attached Table 1. Neck part 14
It was 5 to 10%, especially 10% at point Q (see Figure 4), which indicates the center of the thickness. In this case, the ratio of the thickness D3A to the total thickness D3 is 30
%, and the ratio of the thickness of the foamed rubber layer 3A to the total thickness of the treaded rubber layer at the center of the land portion 12 was 45%.

Next, using each of these tires, we tested handling performance, braking performance on ice, pitching performance on snow, ride comfort performance,
Wear resistance and groove bottom crack resistance tests were conducted.
Here, all tires used in the test were of different sizes.
165SR13, and the same tread pattern as shown in Figure 1 was used. The results are shown in Attached Table 2 with the comparative tires set as an index of 100.

Hereinafter, methods for measuring the various properties of the tread rubber layer and the various properties of the tire will be briefly explained.

Average cell diameter and foaming ratio V The average cell diameter of foamed rubber is determined by cutting out a block-shaped sample from the foamed rubber of the test tire, taking a photograph of the cross section of this sample with an optical microscope at a magnification of 100 to 400, and then measuring the average cell diameter of foamed rubber by cutting out a block-shaped sample from the foamed rubber of the test tire, and taking a photograph of the cross section of this sample with an optical microscope at a magnification of 100 to 400. The bubble diameter of the bubbles was measured and expressed as the arithmetic mean value. In addition, the foaming rate V of foamed rubber is determined by cutting the sample into a 2 μm thin piece,
Density after being left for one week to stabilize after vulcanization: ρ ₁
(g/cm ³ ), and on the other hand, the density ρ ₀ (g/cm ³ ) was measured based on a similar sample cut from non-foamed rubber (solid phase rubber) and calculated using the above formula. .

Cell diameter and number of closed cells A block-shaped sample was cut out from the foamed rubber of the test tire, and a photograph of the cross section of this sample was taken at a magnification of 100~
400 optical microscope to determine the diameter of the closed cells. Next, the diameter of the closed cells is 5 μm.
The number of cells mentioned above was measured over a total area of 4 mm ² or more, and the number of cells per unit area of closed cells 1 mm ² was calculated.

Dynamic modulus of elasticity of foam rubber A rectangular sample (width
4.6 mm long, 30 mm long, 2 mm thick) and measured at 30°C using a dynamic elastic modulus meter (manufactured by Iwamoto Seisakusho Co., Ltd.).
Measured at ℃, frequency of 60Hz, and amplitude distortion of 1%.

Maneuverability The test tire was installed in a normal maneuverability tester indoors, and the cornering power was measured at a load of 395 kg, and the performance of the comparison tire was set as 100 and expressed as an index. Here, the larger the value, the better the maneuverability.

Braking performance on ice After each of the four test tires was installed on a passenger car with a displacement of 1800 c.c., the braking distance was measured on ice at an outside temperature of -5°C, and the performance of the comparative tire was set as 100 and expressed as an index. Here, the larger the value, the better the braking performance.

Snow climbing performance: After installing each of the four test tires on a passenger car with a displacement of 1800cc.
The climbing time on a snowy road of m was measured and expressed as an index with the performance of the comparison tire set as 100. here,
The larger the value, the better the hill climbing performance.

Ride comfort performance The test tire was attached to the fixed shaft of a normal bump-over vibration testing machine indoors, and the shaft load fluctuation when riding over a bump was measured at a load of 395 kg, and the performance of the comparison tire was set as 100 and expressed as an index. Here, the larger the value, the better the ride comfort performance.

Wear resistance performance After installing four test tires on a passenger car with a displacement of 1800c.c., the tires were driven on public roads for 10,000km to measure the amount of change in tread depth, and the performance of the comparison tires was compared to 100%.
It is expressed as an index. Here, the larger the value, the better the wear resistance performance.

Groove bottom crack resistance performance Four test tires of each type were mounted on a passenger car with a displacement of 1800 c.c. and driven for 20,000 km on a public road, and the occurrence of cracks in the groove bottom was observed.

Effects of the Invention From Table 2, there is almost no difference in braking performance on ice, hill climbing performance on snow, and riding comfort between the comparison tire and the test tire because the effects are due to the use of the foamed rubber layer as the tread rubber layer. Regarding handling performance and abrasion resistance, it was shown that the effect of using a composite rubber layer of a base rubber layer and a foamed rubber layer as a tread rubber layer is remarkable, and furthermore, it shows that the effect of using a composite rubber layer of a base rubber layer and a foamed rubber layer is remarkable, and furthermore, it shows that the effect of using a composite rubber layer of a base rubber layer and a foamed rubber layer is significant. It can be seen that the performance has been effectively improved. As described above, the present invention improves and maintains various tire performances in winter and summer, while taking advantage of the advantages of foam rubber, which particularly improves ice and snow performance, while addressing its disadvantages.
In particular, it was possible to effectively suppress the occurrence of cracks in the bottom of the groove on both sides of the tread, and the durability was significantly improved.

[Brief explanation of drawings]

Fig. 1 is a partial plan view of a pneumatic tire showing an embodiment of the present invention, Fig. 2 is a sectional view taken along the - arrow in Fig. 1, Fig. 3 is a sectional view of the main part thereof, and Fig. 4 is also a sectional view thereof. FIG. 5, which is a cross-sectional view of the main part, is a graph showing the relationship between the thickness of the foamed rubber layer at the groove bottom and the number of foamed rubber layers. DESCRIPTION OF SYMBOLS 1...Pneumatic tire, 3...Treaded rubber layer, 3A...Foamed rubber layer, 3B...Base rubber layer, 3D...Auxiliary rubber layer, 7...Belt layer, 10
...Transverse groove, 11A, B...Circumferential groove, 14...
...neck part.

[Table] *: Dinitropentamethylenetetramine

【table】

【table】

Claims (1)

[Claims] 1. Foamed rubber having closed cells is used for the rubber layer of the tread, and the tread has a plurality of circumferential grooves, a large number of lateral grooves extending across these circumferential grooves and the tread ends, and a group of grooves. In the pneumatic tire, the tread rubber layer includes a foamed rubber layer made of the foamed rubber located on the outside in the radial direction, and a substantially non-foamed rubber located on the inside in the radial direction. a base rubber layer, and the foamed rubber layer forms a thin constricted portion on the base rubber layer at the bottom position of the circumferential groove and the lateral groove, and the foamed rubber layer is located on the land portion. 1. A pneumatic tire, wherein the foamed rubber layer is interconnected with the foamed rubber layer, and the foaming rate in the constricted portion is smaller than the foaming rate of the foamed rubber layer in the land portion.