JP7757407B2 - Polyamide fiber for tire cord and its manufacturing method - Google Patents

Description

The present invention relates to polyamide fibers for tire cords and their manufacturing method, as well as twisted cords using the fibers and woven cord fabrics using the cords.

Polyamide fibers have excellent toughness, adhesiveness, and fatigue resistance, and are widely used as rubber reinforcing fibers such as tire cords for trucks, buses, construction, and aircraft.

As described in Patent Document 1 below, radial tires for aircraft are generally used under high internal pressure and heavy load conditions, which places strong demands on load-bearing capacity and suppression of tire radial growth. Furthermore, given the growing demand for cost and weight reduction, it is important for the reinforcing fibers used in radial tires for aircraft to reduce the amount of fiber without compromising its functionality. To meet these demands, there is a need to develop fibers with higher strength and rigidity.

As described in Patent Document 2, a known method for manufacturing tire reinforcement cords involves twisting polyamide fibers into a twisted cord, then applying an adhesive treatment (hereinafter also referred to as "dipping") to the twisted cord, followed by a tension heat treatment under tension. To produce reinforcement cords with higher strength and rigidity, twisted cords have come to be tension heat treated under higher tension. This increases the strength after dipping, contributing to further weight reduction in tires and enabling them to be used in tires for heavier load applications.

On the other hand, when polyamide fibers are used as rubber reinforcing fibers, twisted cords (hereinafter also referred to as "raw cords") are generally used, which are made by twisting a multifilament, which is an assembly of multiple monofilaments, and twisting these together in pairs or triplicates. However, when manufacturing raw cords, there may be variations in length, and depending on the application, extremely long raw cords may be required. In such cases, it may be necessary to connect the raw cords together at their ends.

A commonly known method for joining raw cord is the air splicing method, which involves entangling the fibers. This joining method offers the advantages of easy joining operations, excellent workability, and reduced swelling at the joining point (hereinafter also referred to as "air knot").

However, tire cords or tire cord cord fabrics with air knots joined using the air splicing method face the problem of being prone to cord breakage during the dipping process described above, when the twisted cords are unable to withstand the tension exerted during the high temperature and high tension heat treatment, resulting in broken cords. Such broken cords or dipped fabrics are treated as defective products and distinguished from normal products, so the current situation is that tire cord and tire cord cord fabric manufacturing is plagued by cord breakage issues.

Special table 2019-508595 publication Japanese Patent Application Publication No. 1-174628

In view of the problems of the prior art described above, the problem that the present invention aims to solve is to provide polyamide fibers for tire cords that suppress cord breakage during the manufacturing process of tire cords and tire cord woven fabrics.

As a result of extensive research and experimentation to solve the above-mentioned problems, the inventors unexpectedly discovered that air knot strength is important in preventing cord breakage under high temperature and tension during the manufacturing process of tire cords and tire cord woven fabrics, leading to the completion of this invention.

That is, the present invention is as follows.
[1] A polyamide fiber for tire cords having an air knot strength of 3.5 cN/dtex or more and 6.0 cN/dtex or less in an environment at a temperature of 180°C.
[2] The polyamide fiber for tire cord according to [1], wherein the number of inter-trip entanglements is 13 or more/m and 30 or less/m.
[3] The polyamide fiber for tire cords according to [1] or [2] above, having an air knot strength at 25°C of 6.4 cN/dtex or more and 9.0 cN/dtex or less.
[4] The polyamide fiber for tire cord according to any one of [1] to [3], having a knot strength at 25°C of 4.5 cN/dtex or more and 6.5 cN/dtex or less.
[5] The polyamide fiber for tire cord according to any one of [1] to [4] above, having a tensile strength of 8.7 cN/dtex or more and 11.0 cN/dtex or less.
[6] The polyamide fiber for tire cord according to any one of [1] to [5] above, having a dimensional stability of 12% or more and 20% or less.
[7] The polyamide fiber for tire cord according to any one of [1] to [6], wherein an ionic surfactant is applied to the surface in an amount of 0.01% by weight or more and 0.1% by weight or less per weight of the fiber.
[8] The following physical properties (1) to (5):
(1) The total fineness is 900 dtex or more and 2500 dtex or less;
(2) The elongation is 15% or more and 25% or less;
(3) The boiling water shrinkage rate is 4% or more and 8% or less;
(4) The finishing agent adhesion rate is 0.5% by weight or more and 1.5% by weight or less; and (5) The number of single yarns is 100 or more and 400 or less;
The polyamide fiber for tire cord according to any one of [1] to [7], which is a multifilament yarn having the following structure:
[9] The polyamide fiber for tire cord according to any one of [1] to [8] above, which has an air knot portion.
[10] A twisted cord using the polyamide fiber for tire cords described in [9] above.
[11] A woven blind fabric using the twisted yarn cord described in [10] above.
[12] A method for producing a polyamide fiber for a tire cord, in which a polyamide fiber spun by melt spinning is passed through one or more finishing agent application devices and a multistage drawing roller and wound up, wherein the drawing fixation temperature of the multistage drawing roller is 205°C or higher and 240°C or lower. The method for producing a polyamide fiber for a tire cord according to any one of [1] to [9] above.

The polyamide fiber for tire cords of the present invention reduces cord breakage under high temperature and tension during the manufacturing process of tire cords and tire cord woven fabrics.

FIG. 1 is a diagram illustrating an example of an apparatus for producing polyamide fibers for tire cords according to the present invention.

Hereinafter, embodiments of the present invention will be described in detail.
One embodiment of the present invention is a polyamide fiber for tire cords, which has an air knot strength of 3.5 cN/dtex or more and 6.0 cN/dtex or less in an environment at a temperature of 180°C.
The air knot strength of the polyamide fiber for tire cords of this embodiment in an environment at a temperature of 180°C is 3.5 cN/dtex or more and 6.0 cN/dtex or less, preferably 4.0 cN/dtex or more and 5.0 cN/dtex or less, and more preferably 4.2 cN/dtex or more and 4.5 cN/dtex or less. The air knot strength can be measured by the method described in the Examples below. An air knot strength of 3.5 cN/dtex or more in an environment at a temperature of 180°C can suppress twisted cord breakage during tension heat treatment. On the other hand, an air knot strength of 6.0 cN/dtex or less in an environment at a temperature of 180°C can prevent the single yarns in the air knot portion from becoming too tightly entangled, making it less likely that unevenness in the thickness of the cord fabric will occur due to an increase in the gauge at the seam. The air knot strength is a characteristic of the fiber, measured by performing a specific air knot operation on the fiber and measuring its yarn joining properties.

The air knot strength of the tire cord polyamide fiber of this embodiment at 25°C (room temperature) is preferably 6.4 cN/dtex or more and 9.0 cN/dtex or less, more preferably 6.8 cN/dtex or more and 8.0 cN/dtex or less, and even more preferably 7.0 cN/dtex or more and 7.5 cN/dtex or less. A room temperature air knot strength of 6.4 cN/dtex or more can suppress cord breakage even under tension heat treatment. On the other hand, a room temperature air knot strength of 9.0 cN/dtex or less prevents the single yarns in the air knot from becoming too tightly entangled, making it less likely that unevenness in the thickness of the cord fabric will occur due to a thicker gauge at the seam.

The room temperature (25°C) knot strength of the polyamide fiber for tire cords of this embodiment is preferably 4.5 cN/dtex or more and 6.5 cN/dtex or less, more preferably 5.0 cN/dtex or more and 6.0 cN/dtex or less, and even more preferably 5.2 cN/dtex or more and 5.8 cN/dtex or less. A 25°C knot strength of 4.5 cN/dtex or more can suppress cord breakage during tension heat treatment. On the other hand, a 25°C knot strength of 6.5 cN/dtex or less can be produced without a significant decrease in process stability due to increased fuzz or yarn breakage.

The tensile strength of the polyamide fiber for tire cords of this embodiment is preferably 8.7 cN/dtex or more and 11.0 cN/dtex or less, more preferably 9.0 cN/dtex or more and 10.5 cN/dtex or less, and even more preferably 9.4 cN/dtex or more and 10.2 cN/dtex or less. A tensile strength of 8.7 cN/dtex or more provides excellent tensile strength, contributing to the development of the air knot strength and knot strength. On the other hand, a tensile strength of 11.0 cN/dtex or less allows production without a significant decrease in process stability due to increased fuzz or yarn breakage.

The dimensional stability of the polyamide fiber for tire cords of this embodiment is preferably 12% or more and 20% or less, more preferably 15% or more and 19% or less, and even more preferably 16% or more and 18% or less. Dimensional stability can be calculated as the sum of the fiber's boiling water shrinkage and intermediate elongation (elongation at a load equal to 1/2 the breaking strength). Dimensional stability of 12% or more improves toughness in rubber reinforcement applications such as tire cords. On the other hand, dimensional stability of 20% or less contributes to strength retention at high temperatures, excellent tensile strength at high temperatures, and contributes to the expression of the air knot strength in a 180°C environment and knot strength at 25°C.

The intertrip entanglement number of the polyamide fiber for tire cord of this embodiment is preferably 13 or more and 30 or less, more preferably 13 or more and 25 or less, and even more preferably 17 or more and 23 or less. The intertrip entanglement number refers to the degree of entanglement measured by an automatic entanglement tester, which satisfies 60% strength relative to the trip level. If the intertrip entanglement number is 13 or more, entanglement of single yarns during air knot formation is promoted, resulting in a fiber with excellent air knot strength in an environment at a temperature of 180°C. On the other hand, if the intertrip entanglement number is 30 or less, single yarn unraveling during the twisting process and the resulting single yarn fuzzing are suppressed, preventing a decrease in twisting efficiency and a loss of woven quality.
In addition, from the viewpoint of making the shape of the air knots uniform and suppressing hanging slack during the production of the cord fabric, it is also preferable that the number of inter-trip entanglements is 25 or more/m and 30 or less/m.

The polyamide fiber for tire cords of this embodiment preferably has an ionic surfactant attached at a concentration of 0.01% by weight to 0.1% by weight per fiber, more preferably 0.012% by weight to 0.08% by weight, and even more preferably 0.02% by weight to 0.06% by weight. The inclusion of this amount of ionic surfactant enhances the effects of other finishing agent components, improving the extreme pressure properties of the yarn, facilitating the dispersion of stress applied to air knots during tension, and improving knot strength. The high strength of the polyamide fiber, combined with the presence of the ionic surfactant on the fiber surface, improves knot strength. An ionic surfactant content of 0.01% by weight or more improves knot strength and contributes to improved air knot strength. On the other hand, an ionic surfactant content of 0.1% by weight or less reduces roll surface contamination during the drawing process, leading to increased roll cleaning frequency. The ionic surfactant can be applied to the polyamide fiber by adding a finishing agent during the spinning process.

The polyamide fiber for tire cord of the present embodiment has the following physical properties (1) to (5):
(1) The total fineness is 900 dtex or more and 2500 dtex or less;
(2) The elongation is 15% or more and 25% or less;
(3) The boiling water shrinkage rate is 4% or more and 8% or less;
(4) The finishing agent adhesion rate is 0.5% by weight or more and 1.5% by weight or less; and (5) The number of single yarns is 100 or more and 400 or less;
Multifilament yarns having a total fineness of 900 dtex or more have sufficient mechanical properties when used for tire cords and tire cord woven fabrics. On the other hand, from the viewpoint of weight reduction, those having a total fineness of 2500 dtex or less are preferred.

The elongation is preferably 15 to 25%. An elongation of 15% or more provides sufficient toughness for rubber reinforcement applications such as tire cords. Furthermore, there is a trade-off between elongation and strength, and to achieve a balance with strength, it is preferable that the elongation be 25% or less.

The boiling water shrinkage rate is preferably in the range of 4 to 8%. If the boiling water shrinkage rate is 4.0% or higher, the heat shrinkage stress will be high, suppressing stretching during the dipping process and making it possible to obtain a high-strength dipped cord. On the other hand, if the boiling water shrinkage rate is 8.0% or lower, it will have excellent dimensional stability and contribute to the development of air knot strength at high temperatures.

In addition, the deposition rate of the finishing agent applied to the yarn during the spinning process is preferably in the range of 0.5% by weight to 1.5% by weight. Polyamide fibers for rubber reinforcement applications such as tire cords are drawn at high tension against a drawing roll. Therefore, the finishing agent, which reduces the frictional resistance between the individual fibers that make up the fiber, must be uniformly deposited on the fiber surface in order to provide excellent extreme-pressure properties and reduced frictional resistance with metal surfaces (smoothness). This prevents a decrease in strength of the vulcanized cord and improves fatigue resistance. If the deposition amount of the finishing agent is less than 0.5% by weight, the effect of reducing extreme-pressure properties and frictional resistance with metal surfaces (smoothness) is insufficient, making it difficult to consistently produce high-strength polyamide fibers. Furthermore, if the deposition amount of the finishing agent exceeds 1.5% by weight, the penetration of the dipping solution into the polyamide fiber during dipping is hindered, significantly reducing adhesion to rubber.

It is also preferable that the number of single threads is 100 or more and 400 or less. A multifilament with 100 or more single threads increases the specific surface area and allows for flexible response to external stress, improving rubber adhesion and fatigue resistance. On the other hand, a single thread count of 400 or less prevents the single threads from fusing together during melt spinning.

The polyamide fiber for tire cords in this embodiment may be at least one fiber selected from the group consisting of polyamide 6, polyamide 6.6, polyamide 11, polyamide 12, polyamide 6.10, polyamide 6.12, polyamide 4.6, their copolymers, and mixtures. Among these, polyamide 6.6 fiber primarily made of polyhexamethylene adipamide fiber is preferred. Polyhexamethylene adipamide refers to a polyamide fiber composed of 100% hexamethylenediamine and adipic acid and having a melting point of 250°C or higher. Polyamide 6.6 fiber may also be a fiber made of a polymer copolymerized or blended with polyamide 8, polyamide 6.1, polyamide 10, polyamide 6.T, etc., as long as the melting point is not less than 250°C.

Another embodiment of the present invention is a method for producing a polyamide fiber for a tire cord, in which a polyamide fiber spun by melt spinning is passed through one or more finishing agent application devices and a multistage drawing roller and wound up, wherein the drawing fixation temperature of the multistage drawing roller is 205°C or higher and 240°C or lower.
An example of the method for producing polyamide fibers for tire cords according to the present embodiment of the present invention will now be described.
FIG. 1 is an explanatory diagram showing an example of equipment for producing a tire cord polyamide fiber according to this embodiment. First, a molten polymer is uniformly heated by a part of a spinning machine called a spin head 1 and spun out of a spinneret 2. The spun polymer passes through a heating cylinder 3 located directly below the spinneret and is solidified by cold air from a cooling chamber 4 to form a yarn. The yarn gathered at each end is then treated with a finishing agent in a finishing agent application device 5 and then proceeds to a drawing process using a group of rollers consisting of a take-up roller 6, a first roller 7, a second roller 8, a third roller 9, and a fourth roller 10. Specifically, the yarn is taken up at a predetermined speed by roller 6, then guided under slight tension to a first-stage roller 7, from which it is drawn by multiple heated and drawn rollers 8, 9, and 10. The yarn is then fed to an entanglement device 11 and wound up by a winder 12.

The finishing agent applicator 5 is not particularly limited, but is generally a roll or nozzle type. The amount of finishing agent applied by the finishing agent applicator 5 is preferably 0.5% by weight or more and 1.5% by weight or less per fiber weight. A finishing agent of 0.5% by weight or more allows for stable processing from twisting to dip processing, eliminating single yarn breakage and improving quality. A finishing agent of 1.5% by weight or less prevents problems such as pulling out at the knot.

When applying a finishing agent to fibers, an ionic surfactant can be applied to the fibers. The ionic surfactant includes at least one selected from the group consisting of anionic surfactants containing a phosphorus atom and anionic surfactants containing a sulfur atom. Examples of anionic surfactants containing a phosphorus atom include, but are not limited to, metal or amine salts of alkyl phosphate esters (hereinafter abbreviated as phosphate), and metal or amine salts of polyoxyethylene alkyl phosphate. More specific examples include potassium lauryl phosphate, sodium lauryl phosphate, potassium octyl phosphate, and sodium octyl phosphate. Examples of anionic surfactants containing a sulfur atom include, but are not limited to, alkanesulfonates.

In addition to the ionic surfactants mentioned above, it is preferable to use a component with excellent smoothness and heat resistance as the finishing agent component to be oiled, so that the yarn can be smoothly drawn during the spinning process, from the perspective of yarn quality and industrial material applications. The component used as the smoothing agent is preferably an ester compound. It is preferable to use at least one ester compound selected from the group consisting of ester compounds with three or more ester bonds in the molecule and ester compounds with a sulfur element in the molecule.

Examples of ester compounds having a sulfur element in the molecule include (1) ester compounds of dicarboxylic acids and monohydric alcohols, such as dialkylthiodipropionates, and (2) ester compounds of monocarboxylic acids and monohydric alcohols, such as alkylmercaptopropionates.

Examples of ester compounds having three or more ester bonds in the molecule include (3) ester compounds of polyhydric alcohols and monocarboxylic acids, such as trimethylolpropane trialchelate, glycerin trifatty acid ester, pentaerythritol tetrafatty acid ester, and trimethylolpropane fatty acid ester; (4) ester compounds of polycarboxylic acids and monohydric alcohols, such as trialkyl trimellitate and triethyl citrate; and (5) natural fats and oils, such as castor oil, palm oil, and refined rapeseed oil. These components may be used alone or in combination of two or more.

Nonionic surfactants can be used as agents for modulating emulsification and friction. Examples include: (1) polyoxyalkylene polyhydric alcohol fatty acid ester-type nonionic surfactants, such as ether ester compounds obtained by condensing at least one compound selected from the group consisting of polyethylene glycol dialkylate, polyoxyethylene sorbitan monoalkylate, polyoxybutylene sorbitan trialchelate, polyoxypropylene castor oil, polyoxyethylene hydrogenated castor oil, polyoxyethylene propylene hydrogenated castor oil trialchelate, polyoxyethylene hydrogenated castor oil trialchelate, ethylene oxide (EO) adducts of castor oil, and EO adducts of hydrogenated castor oil with monocarboxylic acids and dicarboxylic acids; and (2) at least one compound selected from the group consisting of organic acids, organic alcohols, organic amines, and organic amides. Compounds having an alkylene oxide having 2 to 4 carbon atoms added to the species, more specifically, for example, ether-type nonionic surfactants such as polyoxyethylene fatty acid esters, polyoxyethylene fatty acid ester methyl ethers, polyoxyethylene alkyl ethers, polyoxyethylene polyoxypropylene alkyl ethers, polyoxyethylene polyoxypropylene nonylphenyl ethers, polyoxyethylene alkylamino ethers, and polyoxyethylene fatty acid amide ethers, (3) polyhydric alcohol partial ester-type nonionic surfactants such as sorbitan mono-fatty acid esters, sorbitan tri-fatty acid esters, and glycerin mono-fatty acid esters, and (4) alkylamide-type nonionic surfactants such as diethanolamine mono-fatty acid amides, etc. These components may be used alone or in combination of two or more.

The finishing agent can be applied in any form, including diluted with mineral oil or as a water-based emulsion, but it is preferable to apply it as an emulsion when considering compatibility with water in subsequent processes.

The multi-stage heated stretching rollers 10 apply a stretching temperature of 205°C or higher and 240°C or lower. The stretching temperature is preferably 210°C or higher and 230°C or lower, and more preferably 215°C or higher and 220°C or lower. A stretching temperature of 205°C or higher provides excellent dimensional stability at high temperatures during cord processing. On the other hand, if the stretching temperature is 240°C or higher, it is difficult to stably produce polyamide fiber with a boiling water shrinkage of 4% or higher.

The entanglement device 11 can be a known device that sprays compressed fluid onto the yarn using an entanglement nozzle. However, it is preferable to supply the compressed fluid to the yarn with an energy of 1.0 to 1.5 kW. The compressed fluid supply energy (air supply energy) can be calculated by multiplying the supply pressure (MPa) by the flow rate (Nm3/hr). The above supply energy range can be satisfied by arbitrarily selecting the supply pressure and the fluid inlet diameter of the entanglement nozzle. It is even more preferable to supply an energy of 1.3 kW or more and 1.5 kW or less. Satisfying the above supply energy range allows for an appropriate inter-trip entanglement number to be obtained without compromising process stability during twisting, promotes entanglement of single yarns during air knot creation, and produces fibers with excellent air knot strength.

The polyamide fiber of this embodiment can be suitably used for twisted cords (tire cords) connected with air knots, and for woven blinds made therefrom. Use of the polyamide fiber of this embodiment can reduce cord breakage during the manufacturing process of twisted cords and woven blinds.

EXAMPLES Next, the present invention will be specifically explained with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples.
First, the definitions and measurement methods of the physical properties used in the examples and comparative examples will be explained.

(1) Air knot strength and tenacity A Joint Air 116 manufactured by Machinetex Corporation was used as the connecting yarn, and the chamber and chamber cover of the air splicer were appropriately selected and attached depending on the fineness, and the polyamide fibers were spliced using the air splicer in the specified procedure under the following conditions.
Supply air pressure: 0.5 MPa
Air injection time: 1 second Thread length: 20 mm
For a 150 mm fiber sample (binder yarn) having one air knot, the tensile breaking strength was measured at test temperatures of 25°C (room temperature) and 180°C and a pulling rate of 300 mm/min in accordance with JIS L 1013 8.5, and the air knot strength at each temperature was recorded. Measurements were made twice per level, and the average value was recorded as the air knot strength.

(2) Knot strength and strength: According to JIS L 1013 8.6, a 150 mm fiber sample was measured at a pulling rate of 300 mm/min at 25° C. The measurement was carried out twice for each level, and the average value was taken as the knot strength.

(3) Intertrip entanglement number The number of entanglements at 60% strength of the trip level (intertrip entanglement number) was measured using a RAPID 600V manufactured by Lenzing Instruments under the conditions shown in Table 1 in accordance with ASTM D 4724. The measurement was carried out 10 times per level, and the average value was taken as the intertrip entanglement number.

(4) Fineness The fineness was measured in accordance with JIS L 1013 8.3.1 Method A, except that the rewinding speed was set to 60 times/min.

(5) Tensile strength, tensile strength, elongation, intermediate elongation According to JIS L 1013 8.5, a 150 mm fiber sample was measured at a pulling speed of 300 mm/min. Measurements were performed three times for each level, and the average value was taken as the tensile strength. However, the intermediate elongation was the elongation (%) at a load of 1/2 the breaking strength.

(6) Finishing agent adhesion rate: Measured in accordance with JIS L 1013 8.27. Cyclohexane was used as the solvent.

(7) Boiling Water Shrinkage: Measured in accordance with JIS L 1013 8.18.

(8) Dimensional stability: Dimensional stability was determined as the sum of the boiling water shrinkage and intermediate elongation (elongation at a load of 1/2 the breaking strength).

(9) Twisted Yarn Filament Disorder The raw yarn was twisted in two with a first twist of 26.5 turns/100 mm and a second twist of 26.5 turns/100 mm. The raw yarn package was joined using an air knotter in the same manner as in the air knot strength measurement described above to form a continuous twisted yarn. This twisted yarn cord was used to weave a blind, and the supplied twisted yarn cord was counted for filament disorder that required removal measures for weaving using a Daikosha KH-700 infrared sensor and a Kasuga Electric SF2-A photocell sensor. Filament disorder includes loosened or frayed single yarns in the twisted yarn cord, which may cause thickness unevenness when made into a blind. Twisted yarn filament disorder was evaluated per 1,000 twisted yarn cords, 1,000 meters, according to the following evaluation criteria.
(Evaluation criteria)
⊚: The number of disordered filaments is less than 1.0;
Good: The number of disordered filaments is 1.0 or more and less than 5.0;
△: The number of disordered filaments is 5.0 or more and less than 10.0;
×: The number of disordered filaments is 10.0 or more.

(10) Flatness In the weaving of the blinds obtained in (9) above, the blinds were visually inspected after weaving, and the hanging slack of the twisting cords was counted. Hanging slack includes cases where the twisting cords are taut or loose, causing the blind to be out of alignment with other cords, resulting in uneven streaks. If the twisting cords are taut, they appear glossy, and if they are loose, they appear blackish. Hanging slack was evaluated per 2,000 twisting cords and 1,000 m, according to the following evaluation criteria.
(Evaluation criteria)
◎: The number of hanging slacks is less than 1.0;
○: The number of hanging slacks is 1.0 or more, but it can be corrected by adjusting the tension of the roll of fabric;
x: The number of loosened cords is 1.0 or more, and the cord needs to be replaced.

(11) Cord Breakage Using the twisted yarn cord obtained in (9) above, a 3200 m long cord fabric was produced with a width of 1580 mm, a pick count of 2.0/50 mm, a warp count of 1091, and a warp density of 34.5/inch. The fabric was then subjected to a dipping treatment using an RFL solution (resorcinol-formalin-latex solution) at a processing speed of 86 m/min, a processing temperature of 140 to 240°C, and a processing tension of 300 to 5500 kgf. This weaving and processing treatment was repeated 10 times. The processing passability at this time was evaluated. Cord breakage was evaluated according to the following evaluation criteria.
(Evaluation criteria)
○: The cord breakage occurred less than 0.1 times, and there was no problem with process passability;
△: Cord breakage occurred 0.1 times or more but less than 2 times;
x: Cord breakage occurred two or more times, and there was a problem with process passability.

[Examples 1 to 4, Comparative Example 1]
A polyhexamethylene adipamide polymer with a relative formic acid viscosity of 82 was obtained by a known melt polymerization method using a vacuum at the final stage of the melt polymerization. This polymer, in its molten state, was introduced into a melt spinning machine and drawing machine as shown in Figure 1 to obtain a high-tenacity polyhexamethylene adipamide fiber with a 1440 dtex/210 f. Specifically, the polymer was melted at 300°C, uniformly heated to 305°C using a spin head 1, and extruded through a spinneret 2 with 210 holes. The extruded fiber was passed through a 70 mm-long heating cylinder 3 set at 300°C under a heated atmosphere, and then cooled and solidified in a cold air chamber 4 to form a filament. The filament then passed sequentially through a finishing agent applicator 5, a take-up roller 6, a first roller 7 to a third roller 9, and a fourth roller 10 set at 220°C. The filament was entangled in an entanglement applicator 11 and wound on a winder 12. Between the first roller 7 and the fourth roller 10, the draw ratio was changed and the yarn was wound so that the resulting raw yarn tensile strength would be the value (8.46 cN to 9.99 cN) shown in Table 2 below. The wound fibers were twisted to prepare raw cord and woven fabrics, which were then dipped in RFL liquid.

The finishing agent applicator 5 applied a finishing agent (containing 1.5% by weight of an ionic surfactant) with the following composition to a deposition rate of 1.1% by weight. Compressed air was applied to the entanglement applicator 11 at 0.45 MPa, with an air supply energy of 1.3 kW. The resulting polyamide 66 fibers were evaluated for tensile strength, air knot strength, knot strength, entanglement strength, dimensional stability, and other characteristics. Additionally, process stability, twisting filament disorder, and cord breakage in the twisting and dipping processes were evaluated. The results are shown in Table 2 below.
(Finishing agent composition)
(a1) Trimethylolpropane trilaurate: 10 parts by weight (a2) Dioleyl thiodipropionate: 40 parts by weight (b) POEO stearyl polyether: 20 parts by weight (c1) Hydrogenated castor oil EO25 triisostearate: 15 parts by weight (c2) Hydrogenated castor oil EO25 maleic acid stearate: 15 parts by weight (d) Ionic surfactant (d1) Alkyl (C10-16) sulfate sodium salt: 0.5 parts by weight (d2) Alkyl (C12-18) acid phosphate, alkyl (C12-14) amine salt: 1 part by weight

[Example 5]
The same procedure as in Example 1 was carried out except that the air supply energy in the entanglement step was set to 1.0 kW. The results are shown in Table 2 below.

[Example 6]
The same procedure as in Example 1 was carried out except that in the stretching step, the stretching and fixing temperature was set to 210° C. The results are shown in Table 2 below.

[Example 7]
The same procedure as in Example 1 was repeated, except that a finishing agent containing 4.5% by weight of an ionic surfactant of the following composition was applied in the oiling step. The results are shown in Table 2 below.
(ionic surfactants)
(d1) Alkyl (C10-16) sulfate sodium salt: 1.5 parts by weight (d2) Alkyl (C12-18) acid phosphate, alkyl (C12-14) amine salt: 3 parts by weight

[Comparative Example 2]
The same procedure as in Example 1 was carried out except that the air supply energy in the entanglement step was set to 1.6 kW. The results are shown in Table 2 below.

[Comparative Example 3]
The same procedure as in Example 1 was carried out except that the air supply energy in the entanglement step was set to 1.8 kW. The results are shown in Table 2 below.

[Comparative Example 4]
The same procedure as in Example 1 was carried out except that in the stretching step, the stretching and fixing temperature was set to 200° C. The results are shown in Table 2 below.

[Comparative Example 5]
The same procedure as in Example 1 was carried out, except that in the oiling step, a finish containing no ionic surfactant was applied so that the adhesion rate was 1.1% by weight. The results are shown in Table 2 below.

[Comparative Example 6]
The same procedure as in Example 1 was repeated, except that a finishing agent containing 0.7% by weight of an ionic surfactant of the following composition was applied in the oiling step. The results are shown in Table 2 below.
(ionic surfactants)
(d1) Alkyl (C10-16) sulfate sodium salt: 0.2 parts by weight (d2) Alkyl (C12-18) acid phosphate, alkyl (C12-14) amine salt: 0.5 parts by weight

[Comparative Example 7]
The same procedure as in Example 1 was repeated except that a finish containing no ionic surfactant was applied in the oiling step to a deposition rate of 1.1 wt % and the fiber tensile strength was 8.49 cN/stex. The results are shown in Table 2 below.

[Comparative Example 8]
The same procedure as in Example 1 was carried out except that the air supply energy in the entanglement step was set to 0.8 kW. The results are shown in Table 2 below.

The polyamide fiber for tire cords of the present invention reduces cord breakage under high temperature and tension during the manufacturing process of tire cords and tire cord woven fabrics.

REFERENCE SIGNS LIST 1 Spin head 2 Spinneret 3 Heating barrel 4 Cold air chamber 5 Finishing agent applying device 6 Take-up roller 7 First roller 8 Second roller 9 Third roller 10 Fourth roller 11 Entanglement applying device 12 Winder

Claims (11)

A polyamide fiber yarn for a tire cord having an air knot portion , wherein the air knot strength of the air knot portion in an environment at a temperature of 180°C is 3.5 cN/dtex or more and 6.0 cN/dtex or less. 2. The polyamide fiber yarn for tire cord according to claim 1, wherein the number of intertrip entanglements is 13 or more/m and 30 or less/m. 3. The polyamide fiber yarn for a tire cord according to claim 1 or 2, wherein the air knot strength at 25°C of the air knot portion is 6.4 cN/dtex or more and 9.0 cN/dtex or less. 3. The polyamide fiber yarn for tire cord according to claim 1 or 2, having a knot strength at 25°C of 4.5 cN/dtex or more and 6.5 cN/dtex or less. 3. The polyamide fiber yarn for tire cord according to claim 1 or 2, having a tensile strength of 8.7 cN/dtex or more and 11.0 cN/dtex or less. 3. The polyamide fiber yarn for tire cord according to claim 1 or 2, which has a dimensional stability of 12% or more and 20% or less. 3. The polyamide fiber yarn for tire cord according to claim 1, wherein an ionic surfactant is applied to the surface in an amount of 0.01% by weight or more and 0.1% by weight or less per weight of the fiber. The following physical properties (1) to (5):
(1) The total fineness is 900 dtex or more and 2500 dtex or less;
(2) The elongation is 15% or more and 25% or less;
(3) The boiling water shrinkage rate is 4% or more and 8% or less;
(4) The finishing agent adhesion rate is 0.5% by weight or more and 1.5% by weight or less; and (5) The number of single yarns is 100 or more and 400 or less;
The polyamide fiber yarn for tire cord according to claim 1 or 2, which is a multifilament yarn having the following structure: A twisted cord using the polyamide fiber yarn for tire cords according to claim 1 or 2. A blind woven fabric using the twisted yarn cord described in claim 9. 3. A method for producing a polyamide fiber yarn for a tire cord, comprising the steps of: spinning a polyamide fiber by melt spinning; passing the polyamide fiber through one or more finishing agent application devices and a multistage drawing roller; and winding the yarn ; wherein the drawing fixing temperature of the multistage drawing roller is 205°C or higher and 240°C or lower.