JP2007168711A - Pneumatic radial tire for heavy load - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the durability of a belt, and to reduce weight in a flat system heavy load pneumatic radial tire, especially in a flat pneumatic radial tire for a small-sized light truck. <P>SOLUTION: The pneumatic radial tire for a heavy load comprises a carcass 2, a belt layer 3 made from two or more steel belt plys, and at least one belt reinforcing layer 4. The cord of the belt reinforcing layer 4 is made from polyketone fibers, and an interval between the outermost layer steel cord of the belt layer 3 and a polyketone fiber cord of the belt reinforcing layer 4 is not less than 0.4 mm. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a heavy-duty pneumatic radial tire that achieves both improvement in belt durability and weight reduction, mainly a flat heavy-duty pneumatic radial tire, and more particularly to a flat pneumatic radial tire for a small light truck.

  Conventionally, on the outer periphery of the belt layer of a pneumatic radial tire for passenger cars, nylon (polyamide), aramid (Kevlar (registered trademark)), A belt reinforcing layer formed by winding a twisted cord of an organic fiber such as polyethylene-2,6-naphthalate (PEN) substantially spirally in the tire circumferential direction is provided. In recent years, it is also known to apply a polyketone (PK) fiber cord as a cord having a high initial tensile resistance to such a belt reinforcing layer (for example, Patent Documents 1 and 2).

  However, flat heavy-duty radial tires, especially flat pneumatic radial tires for small light trucks, have the following strict requirements, so the selection of belt structure and belt reinforcement is limited. In other words, no polyketone (PK) fiber cord has been applied to the belt reinforcing layer.

  That is, in general, flat heavy-duty radial tires, especially small light truck flat radial tires, have a higher internal pressure than passenger car tires, so the growth distortion at the end of the belt increases. As a result, strict characteristics are required against belt separation failure.

  Further, since the volume of the tread rubber is larger than that of the tire for passenger cars (the groove depth is deep), the growth strain at the belt end tends to increase due to the high heat generation of the tread. From this viewpoint, the belt separation failure is further increased. Strict characteristics are required.

In order to satisfy the severe requirements as described above and prevent belt separation failure of flat heavy load pneumatic radial tire, it is necessary to suppress the growth distortion of the belt end, but as one of its effective means, In general, in a flat-type heavy-duty pneumatic radial tire, a structure in which three steel belt layers are provided and two nylon reinforcing layers are arranged at the end of the belt layer has been applied.
JP 2000-14205 A (Claims etc.) JP 2001-334811 A (Claims etc.)

  However, due to the recent increase in safety activities and safety awareness of automobile manufacturers and tire manufacturers, there are occasional users who use air pressures higher than necessary. It has become necessary to set the required level for belt durability higher than before. On the other hand, with the increasing environmental awareness of automobile manufacturers and society, there is an increasing demand for reducing tire rolling resistance, and further weight reduction is required.

  Realizing both improved belt durability and weight reduction in this way has become an issue in recent flat-type heavy-duty radial tires, in particular flat-type radial tires for small light trucks, and there is a need to solve them. Yes.

  Accordingly, an object of the present invention is mainly to realize both improvement in belt durability and weight reduction in a flat heavy-duty pneumatic radial tire, particularly in a flat pneumatic radial tire for a small light truck.

  As a result of intensive studies to solve the above problems, the present inventors have found that the above object can be achieved by using a polyketone fiber cord as an organic fiber cord of the belt reinforcing layer under predetermined conditions. The invention has been completed.

That is, the pneumatic radial tire for heavy loads of the present invention is arranged in the radial direction of the carcass consisting of at least one carcass ply extending in a toroidal shape between a pair of bead cores and in the radial direction of the crown portion of the carcass. In a heavy-duty pneumatic radial tire comprising a belt layer made of a steel belt ply and at least one belt reinforcing layer disposed on the outer side in the tire radial direction of the belt layer,
The cord of the belt reinforcing layer is made of polyketone fiber cord, and the distance between the steel cord of the outermost steel belt ply of the belt layer and the polyketone fiber cord of the belt reinforcing layer is 0.4 mm or more, To do.

The distance between the steel cord of the outermost steel belt ply of the belt layer and the polyketone fiber cord of the belt reinforcing layer is preferably 0.5 to 0.6 mm. A rubber sheet having an unvulcanized thickness of 0.5 mm or more can be suitably disposed between the belt layer and the belt reinforcing layer. Further, an unvulcanized rubber in which an end rubber is disposed between the end of the belt layer and the end of the belt reinforcing layer adjacent thereto, or the belt end is molded by thick contour extrusion. May be attached as steel belt ply rubber. Furthermore, it is preferable that the belt reinforcing layer is composed of two layers at the end of the belt layer. Furthermore, the polyketone fiber cord has the following formulas (I) and (II),
σ ≧ −0.01E + 1.2 (I)
σ ≧ 0.02 (II)
(Where E is the elastic modulus (cN / dtex) at 49 N load at 25 ° C., and σ is the heat shrinkage stress (cN / dtex) at 177 ° C.). preferable. Furthermore, the present invention is particularly suitable for a flat radial tire for a small light truck.

  According to the present invention, it is possible to achieve both improvement in belt durability and weight reduction mainly in a flat heavy-duty pneumatic radial tire, particularly a flat pneumatic radial tire for a small light truck.

Hereinafter, preferred embodiments of the present invention will be described in detail.
FIG. 1 is a schematic cross-sectional view of a heavy duty pneumatic radial tire of the present invention. As shown in the figure, the radial tire of the present invention includes a carcass 2 composed of at least one carcass ply extending in a toroidal shape between a pair of bead cores 1 and two or more steels arranged in the crown portion radial direction. A belt layer 3 made of a belt ply and at least one belt reinforcing layer 4 which is disposed on the outer side in the tire radial direction and in which an organic fiber cord is wound substantially in the tire circumferential direction. Here, the belt reinforcing layer 4 of the preferred example shown in the drawing is arranged so that one of the two belt reinforcing layers 4 on the inner side in the tire radial direction covers the entire width of the belt layer 3, and the other on the outer side in the tire radial direction. Although the sheet is disposed only in the shoulder portion, the present invention is not limited to such an arrangement, and as shown in the following examples, the number and the arrangement position can be appropriately selected. The coating rubber for the belt reinforcing layer 4 is not particularly limited, and various compounded rubbers conventionally used for the belt reinforcing layer can be used.

  In the present invention, it is important that the cord of the belt reinforcing layer 4 is made of polyketone fiber cord. Hereinafter, the reason will be described in detail.

  Important performances in recent flat radial tires for small light trucks include belt durability, particularly belt durability at high speeds, and weight reduction. To improve belt durability, increase the number of belts (for example, from 2 to 3), and place a belt reinforcement layer at the end of the belt (from none to 1 or from 1 to 2). Is a useful tool. However, in this case, it becomes a heavy structure and weight reduction cannot be achieved. Conversely, weight reduction can be achieved by reducing the number of belts (for example, from 3 to 2) and reducing the belt reinforcement layer at the belt end (for example, from 2 to 1 or from 1 to none). However, in this case, the belt durability performance is deteriorated. Thus, belt durability performance and weight reduction are inherently in a trade-off relationship, and it has been quite difficult to achieve both. However, in the present invention, the PK fiber cord is used for the belt reinforcing layer under predetermined conditions, thereby overcoming this difficulty and realizing both of them.

  That is, in the present invention, first, the heat shrinkage stress characteristic (particularly at high temperature) of the PK fiber cord is utilized, and the cord is used for the belt reinforcing layer, thereby suppressing the belt end growth strain during high speed running or high load. And improved belt durability.

  In addition, PK fiber cords at about 120 ° C have about twice the heat absorption stress at high temperatures compared to conventional nylon fiber cords, so the number of belts can be reduced from the conventional structure or belt reinforcement at the belt end. It has been found that the number of layers can be reduced, and the weight can be reduced at the same time.

  In the present invention, as described above, in utilizing the heat shrinkage stress characteristic of the PK fiber cord, the distance between the steel cord of the outermost steel belt ply of the belt layer 3 and the polyketone fiber cord of the belt reinforcing layer 4 is set to 0. It is also important that the thickness is 4 mm or more, preferably 0.5 mm or more. Thereby, the failure resulting from the contact between the steel cord of the outermost steel belt ply of the belt layer 3 and the polyketone fiber cord of the belt reinforcing layer 4 due to the heat shrinkage of the PK fiber cord can be avoided. However, from the point of weight reduction, 0.6 mm or less is preferable.

  In order to provide such an interval, a rubber sheet having an unvulcanized thickness of 0.5 mm or more is preferably disposed between the belt layer 3 and the belt reinforcing layer 4. In addition, as shown in FIG. 2, when the belt reinforcing layer 4 is disposed only at the end of the belt layer 3, it is between the end of the belt layer 3 and the end of the belt reinforcing layer 4 adjacent thereto. It is preferable to dispose the end rubber 5. Alternatively, although not shown, unvulcanized rubber molded by contour extrusion with a thick belt end may be attached as a steel belt ply rubber.

  Specifically, the PK fiber cord used in the present invention is preferably a PK fiber cord described in detail below. That is, it is a multifilament twisted PK fiber having a total decitex per cord of 1000 to 6000 decitex. If the total decitex per cord is in the range of 1000 to 6000 decitex, it is possible to achieve high rigidity and light weight compared to the steel cord, which is a merit of the organic fiber. If the total decitex is less than 1000 decitex, the highly rigid belt reinforcing layer 4 cannot be constituted by one sheet. On the other hand, if it exceeds 6000 decitex, the gauge of the belt reinforcing layer becomes thick and tire weight increases.

Moreover, it is preferable that the PK fiber cord used in the present invention satisfies the relationship represented by the following formulas (I) and (II) in order to obtain the desired effect.
σ ≧ −0.01E + 1.2 (I)
σ ≧ 0.02 (II)
However, if σ is larger than 1.5, the shrinkage force at the time of vulcanization becomes too large, and as a result, the cords inside the tire and the rubber arrangement may be disturbed, leading to deterioration in durability and uniformity. , As the upper limit,
1.5 ≧ σ
It is preferable to satisfy the relationship represented by these. Here, the heat shrinkage stress σ is obtained by heating a 25 cm long fixed sample of the PK fiber cord before vulcanization subjected to a general dip treatment at a heating rate of 5 ° C./min at 177 ° C. It is the stress (unit: cN / dtex) generated in the cord, and the elastic modulus E is the elastic modulus at 49 N load at 25 ° C. of the same PK fiber cord, and the SS curve of the JIS cord tensile test Elastic modulus of unit cN / dtex calculated from tangent at 49N.

In addition, the PK fiber cord further includes the following formula (III),
α = T × D ^1/2 (III)
(Where T is the number of twists (times / 100 mm) and D is the total fineness (dtex) of the cord), and the twist coefficient α is preferably in the range of 850 to 4000. If the twist coefficient α of the PK fiber cord is less than 850, the heat shrinkage stress cannot be secured sufficiently. On the other hand, if it exceeds 4000, the elastic modulus cannot be secured sufficiently and the belt reinforcing ability is reduced.

  Furthermore, the PK fiber cord is preferably formed by twisting two filament bundles made of polyketone having a fineness of 500 to 3000 dtex. If the fineness of the filament bundle used for the PK fiber cord is less than 500 dtex, both the elastic modulus and the heat shrinkage stress are insufficient. On the other hand, if it exceeds 3000 dtex, the cord diameter becomes thick and the driving cannot be made dense.

  Furthermore, it is preferable that the PK fiber cord has a reversibility that shrinks at a high temperature and expands when returned to room temperature. As a result, the PK fiber cord in the belt reinforcement layer contracts at high temperatures, that is, during high-speed running, and can sufficiently suppress the tread from protruding by exerting a sufficient tagging effect. When the vehicle is traveling at a low speed, that is, at a low speed, the PK fiber cord in the belt reinforcing layer is stretched, and a sufficient contact area of the tire can be secured.

（式中、Ａは不飽和結合によって重合された不飽和化合物由来の部分であり、各繰り返し単位において同一であっても異なっていてもよい）で表される繰り返し単位から実質的になるものが好適であり、その中でも、繰り返し単位の９７モル％以上が１−オキソトリメチレン［−ＣＨ₂−ＣＨ₂−ＣＯ−］であるポリケトンが好ましく、９９モル％以上が１−オキソトリメチレンであるポリケトンが更に好ましく、１００モル％が１−オキソトリメチレンであるポリケトンが最も好ましい。 As a polyketone as a raw material of the PK fiber cord, the following general formula (IV),

(Wherein A is a portion derived from an unsaturated compound polymerized by an unsaturated bond, and may be the same or different in each repeating unit). Among them, a polyketone in which 97 mol% or more of the repeating units is 1-oxotrimethylene [—CH ₂ —CH ₂ —CO—] is preferable, and a polyketone in which 99 mol% or more is 1-oxotrimethylene. Is more preferable, and a polyketone in which 100 mol% is 1-oxotrimethylene is most preferable.

  In such polyketones, the ketone groups may be partially bonded to each other and the portions derived from the unsaturated compound may be bonded to each other, but the proportion of the portions in which the unsaturated compound-derived portions and the ketone groups are alternately arranged is 90 mass. % Or more, preferably 97% by mass or more, and most preferably 100% by mass.

  In the above formula (IV), the unsaturated compound forming A is most preferably ethylene, but propylene, butene, pentene, cyclopentene, hexene, cyclohexene, heptene, octene, nonene, decene, dodecene, styrene, acetylene. , Unsaturated hydrocarbons other than ethylene such as allene, and methyl acrylate, methyl methacrylate, vinyl acetate, acrylamide, hydroxyethyl methacrylate, undecenoic acid, undecenol, 6-chlorohexene, N-vinylpyrrolidone, diethyl ester of sulphonylphosphonic acid Further, it may be a compound containing an unsaturated bond such as sodium styrene sulfonate, sodium allyl sulfonate, vinyl pyrrolidone and vinyl chloride.

（上記式中、ｔおよびＴは、純度９８％以上のヘキサフルオロイソプロパノールおよび該ヘキサフルオロイソプロパノールに溶解したポリケトンの希釈溶液の２５℃での粘度管の流過時間であり、Ｃは、上記希釈溶液１００ｍＬ中の溶質の質量（ｇ）である）で定義される極限粘度［η］が、１〜２０ｄＬ／ｇの範囲内にあることが好ましく、３〜８ｄＬ／ｇの範囲内にあることがより一層好ましい。極限粘度が１ｄＬ／ｇ未満では、分子量が小さ過ぎて、高強度のポリケトン繊維コードを得ることが難しくなる上、紡糸時、乾燥時および延伸時に毛羽や糸切れ等の工程上のトラブルが多発することがあり、一方、極限粘度が２０ｄＬ／ｇを超えると、ポリマーの合成に時間およびコストがかかる上、ポリマーを均一に溶解させることが難しくなり、紡糸性および物性に悪影響が出ることがある。 Furthermore, as a polymerization degree of the said polyketone, following formula (V),

(In the above formula, t and T are the flow time of a viscosity tube at 25 ° C. of a diluted solution of hexafluoroisopropanol having a purity of 98% or more and a polyketone dissolved in the hexafluoroisopropanol, and C is the diluted solution. The intrinsic viscosity [η] defined by the mass (g) of solute in 100 mL) is preferably in the range of 1 to 20 dL / g, more preferably in the range of 3 to 8 dL / g. Even more preferred. If the intrinsic viscosity is less than 1 dL / g, the molecular weight is too small to obtain a high-strength polyketone fiber cord, and troubles such as fluff and yarn breakage occur frequently during spinning, drying and stretching. On the other hand, if the intrinsic viscosity exceeds 20 dL / g, it takes time and cost to synthesize the polymer, and it becomes difficult to uniformly dissolve the polymer, which may adversely affect the spinnability and physical properties.

  Furthermore, the PK fiber preferably has a crystal structure with a crystallinity of 50 to 90% and a crystal orientation of 95% or more. If the degree of crystallinity is less than 50%, the structure of the fiber is not sufficiently formed and sufficient strength cannot be obtained, and the shrinkage characteristics and dimensional stability during heating may be unstable. For this reason, the crystallinity is preferably 50 to 90%, more preferably 60 to 85%.

  The polyketone fiberization method includes (1) a method in which unstretched yarn is spun and then subjected to multistage hot stretching and stretched at a specific temperature and magnification in the final stretching step of the multistage hot stretching, (2 ) A method in which after the undrawn yarn is spun, hot drawing is performed, and the fiber after completion of the hot drawing is rapidly cooled with high tension applied. A desired filament suitable for the production of the polyketone fiber cord can be obtained by fiberizing the polyketone by the method (1) or (2).

  Here, the spinning method of the unstretched yarn of the polyketone is not particularly limited, and a conventionally known method can be employed. Specifically, JP-A-2-112413, JP-A-4-228613, Wet spinning method using an organic solvent such as hexafluoroisopropanol and m-cresol as described in JP-A-4-505344, WO99 / 18143, WO00 / 09611, JP2001 -164422, JP-A-2004-218189, JP-A-2004-285221, and the wet spinning method using an aqueous solution of zinc salt, calcium salt, thiocyanate, iron salt, etc. Among these, the wet spinning method using an aqueous solution of the above salt is preferable.

  For example, in a wet spinning method using an organic solvent, a polyketone polymer is dissolved in hexafluoroisopropanol, m-cresol, or the like at a concentration of 0.25 to 20% by mass, extruded from a spinning nozzle to be fiberized, and then toluene, ethanol, isopropanol The unstretched yarn of polyketone can be obtained by removing and washing the solvent in a non-solvent bath such as n-hexane, isooctane, acetone or methyl ethyl ketone.

  On the other hand, in the wet spinning method using an aqueous solution, for example, a polyketone polymer is dissolved in an aqueous solution of zinc salt, calcium salt, thiocyanate, iron salt or the like at a concentration of 2 to 30% by mass, and a spinning nozzle is formed at 50 to 130 ° C. Then, it is extruded into a coagulation bath and subjected to gel spinning, and further desalting and drying can be performed to obtain unstretched polyketone. Here, in the aqueous solution in which the polyketone polymer is dissolved, it is preferable to use a mixture of zinc halide and a halogenated alkali metal salt or a halogenated alkaline earth metal salt. An organic solvent such as an aqueous solution, acetone, or methanol can be used.

  Further, as a drawing method of the obtained undrawn yarn, a hot drawing method in which the undrawn yarn is heated and drawn to a temperature higher than the glass transition temperature of the undrawn yarn is preferable. The method (2) may be carried out in one stage, but it is preferably carried out in multiple stages. There is no restriction | limiting in particular as a method of heat drawing, For example, the method etc. which run a thread | yarn on a heating roll or a heating plate are employable. Here, the heat stretching temperature is preferably in the range of 110 ° C. to (melting point of polyketone), and the total stretching ratio is suitably 10 times or more.

  When polyketone fiberization is carried out by the method of (1) above, the temperature in the final stretching step of the multistage hot stretching is in the range of 110 ° C. to (the stretching temperature of the stretching step one step before the final stretching step). Moreover, the draw ratio in the final drawing step of multistage hot drawing is preferably in the range of 1.01 to 1.5 times. On the other hand, when polyketone fiberization is carried out by the method of (2) above, the tension applied to the fiber after completion of the hot drawing is preferably in the range of 0.5 to 4 cN / dtex, and the cooling rate in rapid cooling is 30 The cooling end temperature in the rapid cooling is preferably 50 ° C. or less. There is no restriction | limiting in particular as a rapid cooling method of the heat-stretched polyketone fiber, A conventionally well-known method can be employ | adopted, Specifically, the cooling method using a roll is preferable. In addition, since the polyketone fiber obtained in this way has a large residual elastic strain, it is usually preferable to perform relaxation heat treatment so that the fiber length is shorter than the fiber length after hot drawing. Here, the temperature of the relaxation heat treatment is preferably in the range of 50 to 100 ° C., and the relaxation ratio is preferably in the range of 0.980 to 0.999 times.

  The PK fiber cord is made of a multi-filament twisted PK fiber formed by twisting a plurality of the polyketone filaments. For example, the filament bundle made of the polyketone is subjected to a lower twist, and then two or three of them are combined. The twisted cord can be obtained by applying an upper twist in the opposite direction.

  In order to make the most effective use of the high heat shrinkage characteristics of PK fiber cords, the processing temperature during processing and the temperature of the molded product during use are close to the temperature indicating the maximum heat shrinkage stress (maximum heat shrinkage temperature). It is desirable to be temperature. Specifically, the processing temperature such as the RFL processing temperature and the vulcanization temperature in the adhesive processing performed as necessary is 100 to 250 ° C., and when the tire material generates heat due to repeated use or high speed rotation. Since the temperature can be as high as 100 to 200 ° C, the maximum heat shrink temperature is preferably in the range of 100 to 250 ° C, more preferably in the range of 150 to 240 ° C.

  In the present invention, in order to obtain the desired effect, the polyketone fiber cord is wound with a single wire or two wires, and the winding density is preferably 35 to 50/50 mm.

  In the pneumatic tire of the present invention, points other than the belt reinforcing layer 4 are not particularly limited. For example, although not shown, an inner liner is usually disposed in the innermost layer of the tire, and a tread pattern is appropriately formed on the tread surface. Further, in the pneumatic tire of the present invention, as the gas filled in the tire, normal or air having a changed oxygen partial pressure, or an inert gas such as nitrogen can be used.

Hereinafter, the present invention will be described in more detail with reference to examples.
Pneumatic radial tires in which the belt layer 3 and the belt reinforcing layer 4 are disposed as shown in FIGS. 3A to 3D were respectively manufactured according to the conventional method according to the following Table 1 and the conditions shown below.
(Tire size): 205 / 65R16
(Carcass ply cord): PET 1670 dtex / 2, twist number 40 × 40/10 cm, driving number 100 pieces / 10 cm, angle 0 °
(Belt ply cord): (1 + 6) × 0.37 mm steel cord, number of driven 40/10 cm, angle 70 °

(Belt reinforcement layer cord): Number of driving 40 / 50mm
(Elastic modulus E of PK fiber cord): 162 (analyzed after each test tire is taken out and calculated from the tangent at 49N of SS curve by JIS cord tension test, where E is the elastic modulus at 25 ° C and 49N load) (Unit: cN / dtex)
(PK fiber cord heat shrinkage stress σ): 2.6 (analyze and take out each test tire, σ is the heat shrinkage stress at 177 ° C., σ is a temperature increase of 5 ° C./min of a 25 cm long fixed sample) Heated at a speed, the stress generated at 177 ° C. was measured and calculated from the generated force (unit: cN / dtex)

Each obtained tire was evaluated according to the following evaluation method. These results are also shown in Table 1 below.
(Belt durability)
The test tire is mounted on a 16 × 5.5J rim, which is a JATMA standard rim, at an internal pressure of 600 kPa, and is run on the drum at 100 km / h at 1030 kg, which is the maximum load condition of JATMA. The distance was displayed as an index with the tire of Conventional Example 1 as 100. The higher the number, the better the result.
(Belt end running distortion)
The ratio of the tire circumference at the belt end before running the belt endurance test and the tire circumference at the belt end after running for 24 hours was defined as belt end running strain, and the tire of Conventional Example 1 was displayed as an index as 100. The higher the number, the better the result.
(Tire mass)
The reciprocal of the mass of the tire of Conventional Example 1 is taken as 100 and indicated as an index. The higher the number, the better the result.

1 is a schematic cross-sectional view showing a pneumatic tire according to an embodiment of the present invention. It is a typical sectional view showing the state where end part rubber was arranged between a belt layer and a belt reinforcement layer. It is typical sectional drawing which shows the arrangement | positioning state of the belt reinforcement layer in an Example.

Explanation of symbols

1 Bead core 2 Carcass 3 Belt layer 4 Belt reinforcing layer 5 End rubber

Claims (8)

A carcass made of at least one carcass ply extending in a toroidal shape between a pair of bead cores, a belt layer made of two or more steel belt plies arranged in the radial direction of the crown of the carcass, In a heavy-duty pneumatic radial tire comprising at least one belt reinforcing layer disposed on the outer side in the tire radial direction,
The cord of the belt reinforcing layer is made of polyketone fiber cord, and the distance between the steel cord of the outermost steel belt ply of the belt layer and the polyketone fiber cord of the belt reinforcing layer is 0.4 mm or more, Pneumatic radial tire for heavy loads.   The heavy-duty pneumatic radial tire according to claim 1, wherein a distance between a steel cord of the outermost steel belt ply of the belt layer and a polyketone fiber cord of the belt reinforcing layer is 0.5 to 0.6 mm.   The heavy-duty pneumatic radial tire according to claim 1 or 2, wherein a rubber sheet having an unvulcanized thickness of 0.5 mm or more is disposed between the belt layer and the belt reinforcing layer.   The heavy-duty pneumatic radial tire according to any one of claims 1 to 3, wherein an end rubber is disposed between an end of the belt layer and an end of the belt reinforcing layer adjacent thereto.   The heavy-duty pneumatic radial tire according to any one of claims 1 to 3, wherein unvulcanized rubber formed by contour extrusion with a thick belt end is attached as a steel belt ply rubber.   The heavy-duty pneumatic radial tire according to any one of claims 1 to 4, wherein the belt reinforcing layer includes two layers at an end of the belt layer. The polyketone fiber cord has the following formulas (I) and (II),
σ ≧ −0.01E + 1.2 (I)
σ ≧ 0.02 (II)
(Wherein E is an elastic modulus (cN / dtex) at a load of 49 N at 25 ° C., and σ is a heat shrinkage stress (cN / dtex) at 177 ° C.) The pneumatic radial tire for heavy loads as described in any one of 1-6.   It is a flat pneumatic radial tire for small light trucks, The heavy-duty pneumatic radial tire as described in any one of Claims 1-7.

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