Polyamide yarn with improved fatigue resistance and the use thereof for the manufacture of tire cord

Abstract

A drawn polyamide (preferably PA66) high tenacity yarn having a relative viscosity (RV) of about 90 to about 200 comprising PA66 characterized by a molecular weight distribution (Mz / Mw) of ≥ 1.40 and ≤ 1.50 ±0.03. The yarn finds utility in a variety of end uses including tire cord. The yarn provides improved fatigue resistance and / or cured-in rubber strength.

Description

POLYAMIDE YARN WITH IMPROVED FATIGUE RESISTANCE AND THE USE THEREOF FOR THE MANUFACTURE OF TIRE CORDCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present disclosure claims priority to and the benefit of U.S. Provisional Patent Application Serial No. 63 / 529,154, filed July 27, 2023, U.S. Provisional Patent Application Serial No. 63 / 529,161 filed July 27, 2023, U.S. Provisional Patent Application Serial No. 63 / 661,603 filed June 19, 2024, U.S. Provisional Patent Application Serial No. 63 / 661,600 filed June 19, 2024 and U.S. Provisional Patent Application Serial No. 63 / 661,599 filed June 19, 2024; the contents of which are hereby incorporated by reference herein in their entireties.FIELD

[0002] The present application relates to polyamide yarn suitable for tire cord fabric. In particular, the application relates to polyamide yarn having improved properties such as fatigue resistance and cured-in-rubber strength suitable for tire cord fabric and a method of making the polyamide yarn.BACKGROUND

[0003] Tire cord is a fabric element that is used to provide dimensional stability to the tire, and to help support the weight of a vehicle. Tire cord acts as a reinforcing material and can enhance durability of the tires. Tire cord can be made from a variety of materials including steel, rayon and polyester. Another common group of fibers that are being used for tire cord production are made of polyamide (or nylon polymer) which are considered as a high strength option when enhanced performance is required. Tire cord can be obtained by twisting or plying polyamide fibers. An adhesion treatment agent may be applied to the surface. The twisted yarns may also go through a process of heat setting before being embedded in rubber and eventually becoming one of the many parts of a tire.

[0004] During use, tires go through many cycles of loading and unloading during their lifetime which causes the yarns used therein, as reinforcing components, to lose their mechanical strength. As the number of loading cycles increases this can contribute to lowering the strength oftires and ultimately to tire deformation and failure. The loss of mechanical strength is referred to as “fatigue” and retention of mechanical strength is referred to as fatigue resistance.

[0005] One of the most important properties of fibers used in tire cords is fatigue resistance. Different methods have been previously used to enable improved fatigue resistance of tire cords made from polyamide. Examples of such methods are as follows:• A hybrid tire cord as described in patent publication CN113260747 B can be made of multifilament aromatic polyamide, aramid spun yarn, nylon, or polyester multifilament.• A sea-island composite industrial filaments (Sea-island fiber, also known as superconjugate fiber is formed by a core component dispersed in another polymer in a longitudinally continuous way. Patent publication W02007112665 describes a process for producing a sea-island composite industrial filaments including polyester as an island component and making polyamide the sea component. The sea-island composites produced by this method provide improved fatigue resistance amongst other features.)• Using specific treatment liquids comprised of emulsion-based and oil-based liquids are described in patent publications JP2566993 B2 and JP2782510 B2. These treatments are applied in the spinning process of the yarns to reduce the fiber-to-fiber friction.• A spinning process condition that is described in patent publication JP3281115 B2 through which certain dynamic mechanical properties are met.

[0006] In reference JP2015034361 A, it is reported that for making high strength polyamide fibers it is preferred to obtain a sulfuric acid relative viscosity (SARV) in the range of 3.5 - 3.8 by the solid-phase polymerization to run the liquid-phase and solid-phase polymerization equipment efficiently. Additionally, benzenesulfonate compound (5 - 300 ppm) is added as an oxidation stabilizer to the water used for cooling the cast polymer (exiting polymerization autoclaves) to reduce thermal oxidation and degradation of polymer during the solid-phase polymerization and melt spinning. The reference EP3674458 reports that the use of a PA66 fiber with SARV of more than 3.5 causes poor cost efficiency and challenges in fiber drawing, leading to problems achieving the requisite fiber strength. (A SARV of 3.5 translates to a formic acid RV of 95.) According to these references, the use of a fiber of more than 3.5 SARV, without using the benzenesulfonate compound, would result in frequent filament breakage and thus would be technically and economically unsuitable for producing continuous polyamide multifilament yarns.

[0007] Continuous demand for better performance of tires, for example for higher mileages, or for electric vehicles (EVs) with higher torque and vehicle weight applied to the tires, etc., shows that there is still a need for new material for tire cord fabrics and particularly for polyamide with improved fatigue resistance and cured-in-rubber strength.

[0008] It is an object of this invention to address the aforementioned problems and in particular to provide a polyamide yarn for use in tire cord fabric.SUMMARY

[0009] In accordance with one aspect of the invention there is provided a polyamide material having improved fatigue resistance and / or cured-in-rubber strength, preferably wherein the polyamide is polyhexamethylene adipamide (also known as polyamide 66, PA66 or nylon 66). The improvement in one or both of these properties is exhibited relative to a polyamide material not having the characteristics recited herein, and in particular polyamide material not having the combination of formic acid relative viscosity (RV) and molecular weight distribution (Mz / Mw) characteristics, and particularly not having the combination of RV, Mz / Mw and tenacity characteristics.

[0010] In particular, there is provided a drawn polyamide (preferably PA66) yarn having a formic acid relative viscosity (RV) of about 90 to about 200 and comprising polyamide (preferably PA66) characterized by a molecular weight distribution (Mz Mw) in the range of > 1.40 and < 1.50 ±0.03, preferably > 1.42 and < 1.49 ±0.03 and preferably > 1.45 and <1.48 ±0.03, and preferably wherein the yarn exhibits a tenacity of >8.5 cN / dtex.

[0011] In particular, the inventors have found that such a yarn provides a fatigue resistance and / or cured-in-rubber strength which is higher than that provided by a drawn polyamide (preferably PA66) yarn having a lower formic acid relative viscosity and an Mz / Mwvalue outside of the stated range, and higher than that provided by a drawn polyamide (preferably PA66) yarn having a lower formic acid relative viscosity and lower tenacity and an Mz / Mwvalue outside of the stated range.

[0012] In another aspect there is provided a tire cord made from or comprising the drawn polyamide (preferably PA66) yarn according to the application.

[0013] The tire cord preferably has a cured-in-rubber strength of > 50 N, preferably > 70 N, preferably > 100 N, preferably > 130 N, preferably > 150 N, preferably > 180 N, preferably > 210 N, preferably > 220 N.

[0014] In a particularly preferred embodiment, the tire cord has a cured-in-rubber strength of > 150 N, preferably > 180 N, preferably > 210 N, preferably > 220 N. It will be appreciated that the higher cured-in rubber strength ranges of this particularly preferred embodiment are achievable using drawn yarns of higher linear density.

[0015] In a further aspect there is provided the use of the drawn polyamide (preferably PA66) yarn defined herein in a tire cord for the purpose of improving the fatigue resistance and / or cured-in-rubber strength of a tire cord comprising said yarn. The use of the yarn for such purpose(s) is of particular utility in respect of yarns which exhibit a tenacity of >8.0 cN / dtex (preferably >8.5 cN / dtex), and preferably also an energy to break of at least 2.5 J and / or an elongation at maximum force of about 10 to about 30 percent (particularly at least 15.0 percent), and / or an elongation at 45N of about 7 to about 12 % (particularly about 8.5 to about 12%), preferably with the preferred thresholds and ranges for these parameters as defined herein.

[0016] In a further aspect there is provided the use of a tire cord comprising the drawn polyamide (preferably PA66) yarn defined herein in a tire for the purpose of enhancing the durability of said tire.

[0017] In a further aspect of the invention there is provided a tire comprising the tire cord as defined herein.

[0018] In a further aspect there is provided a method of preparing a drawn polyamide (preferably PA66) yarn according to the application comprising the steps of: a) shear heating the polyamide (preferably PA66) flakes until the polymer reaches a molten state; b) extruding the molten polymer through spinnerets; c) cooling the molten polymer that exits the spinnerets to solidify the filaments; d) applying a lubricating oil to the filaments; e) drawing the filaments by passing the filaments through one or more heated rollers to stretch and align the molecules; f) relaxing the drawn yarns by passing through another set of one or more rollers to achieve the required shrinkage on the final yarn andg) winding the finished filaments onto bobbins.

[0019] In a further aspect of the invention there is provided a method of preparing tire cord from the drawn polyamide yarn described herein comprising the steps of: a) twisting single yarns in Z or S direction wherein said single yarns are the drawn polyamide yarns described herein; and b) plying two or more single yarns together and twisting them in a direction which is opposite to the direction of twist in the single yarns, preferably with a similar number of twists per unit length as each single yarn.

[0020] For the intended application of tire manufacture, the tire cord produced in this way is normally then further processed by weaving the cords into fabric (“tire cord fabric”), which is then used in tire fabrication. The tire cord of the present invention is suitable as at least the warp component of the fabric. Typically 1000-1500 of such tire cords are woven into the fabric as the warp component. The drawn polyamide yarn of the present invention may be used, alternatively or additionally, as the weft component (also referred to as “fill” or “pick” yarn) of the fabric. Where drawn polyamide yarn of the present invention is present only as the warp component, any conventional weft component may be used. The weft component of tire cord fabric is normally present in the fabric at a relatively lower density than the warp component. The primary function of the weft component is to maintain a uniform warp spacing during subsequent processing, such as transportation, dipping, heat-treatment, calendering and tire fabrication etc. The fabric is then dipped in a cord adhesive solution, as is well-known in the art. Adhesives suitable for bonding the fabric to rubber compounds include resorcinol formaldehyde latex (RFL) mixtures. The fabric is then dried under controlled temperature and tension and for a controlled amount of time. The drying temperature should be below the melting point of the polyamide. For instance, tire cord made from drawn PA66 yarn is typically heat-treated under tension at a temperature of from about 150 to about 250°C preferably for a period in the range of from 10 seconds to 2 minutes, preferably under a 5 to 20% stretch.

[0021] In a further aspect of the invention, there is provided a tire cord fabric comprising the tire cord made from or comprising the drawn polyamide (preferably PA66) yarn according to the invention.DETAILED DESCRIPTION OF THE INVENTION

[0022] It is an object of this invention to provide industrial yarn having improved fatigue resistance and cured-in-rubber strength properties.

[0023] As a rule of thumb, an increase in the molecular weight of polymer (PA66 in particular) would be expected to improve the mechanical properties of products made with said polymer. The longer chains enable higher numbers of physical entanglements, and in some cases chemical crosslinking will also contribute to the changes in the behavior of material against mechanical forces. However, such changes can have contrasting impacts and so it is difficult to predict the exact balance of properties influenced by macromolecular structure changes. For example, increasing the molecular weight theoretically makes the material stronger due to higher entanglement, but can also result in a more brittle or degraded polymer, which means the polymer will break at smaller deformations. Therefore, making a higher molecular weight polymer is not always advantageous for the requirements of the end-use application.

[0024] PA66 is one of the commonly used engineered thermoplastics. PA66 is synthesized by polycondensation of hexamethylenediamine and adipic acid. PA66 is also referred to as nylon 66 and these terms are used interchangeably herein. Polyamides and particularly PA66 are often used as the base polymer for tire cord fabric due to their high mechanical strength, thermal resistance, and energy absorption characteristics.

[0025] The tire cord yarns of the present disclosure are composed of fibers which are in the form of continuous multi-filaments. In one embodiment, such filaments are formed by extrusion of molten polymer through spinnerets at high temperatures and pressures and subsequently quenched in air, coated with spin finish lubricant, drawn between pairs of rolls, entangled along the length of the fiber to make a coherent yarn and then wound up on carboard tubes as bobbins.

[0026] The polyamide may be manufactured by conventional means known in the art. The polyamides may be manufactured from intermediates produced via a biosynthetic pathway or via conventional petrochemical route. The relative viscosity of the polyamide may be increased by increasing the degree of polymerization, i.e. the molecular weight, of the polyamide as is known in the art. For instance, the molecular weight and relative viscosity may be increased by a solid- state polymerization (SSP) step, typically conducted under dry nitrogen at elevated temperature (for instance about 180°C).

[0027] An increase in molecular weight of aliphatic polyamides (such as PA66) is expected to increase the strength. However, the melt viscosity also increases with higher molecular weight, which makes the polymer processing more difficult. A common solution to this problem is to increase the processing temperature to reduce the polymer melt viscosity. This, however, comes at the cost of polyamide degradation which negatively affects the quality of polymer. As a result, when the molten polymer is spun, it cannot be drawn enough and therefore the achievable mechanical properties such as tenacity, elongation at maximum force and energy to break will be limited. Overall, therefore, simply increasing molecular weight is a difficult way to improve the properties of polymeric materials because striking the correct balance in properties is a significant challenge. However, the present inventors have found that making polyamide (preferably PA66) yarns with higher RV and modulating the molecular weight distribution of the polymers provides yarns with improved fatigue resistance.

[0028] It is known in the art that more severe conditions during polymerization (including temperature and residence time) can promote the formation of undesired side products. Certain of these undesired side products are referred to in the industry as “gel.” Gel formation in polyamides (and particularly PA66) refers to the process by which the polymer chains in the material crosslink and form a three-dimensional network, resulting in a gel-like structure. As gel formation increases, the fraction of linear polymer chains in the polymer decreases and therefore the molecular weight distribution diverges from the most probable Flory molecular weight distribution of linear condensation polymers (Flory, P. J., Molecular size distribution in linear condensation polymers, Journal of the American Chemical Society, 1936, 58(10), 1877-1885). Thus, the measured molecular weight distribution (measured as Mz / Mw) of step-growth polymers like polyamide 66 is considered an indicator of the polymer’s linearity or lack thereof. Gelling is typically initiated by exposure to excessive heat, radiation, or other chemical agents that cause the polymer chains to become crosslinked. The degree of crosslinking can affect the mechanical and physical properties of the polyamide, such as its strength, stiffness, and water absorption. Gel formation can also alter the processing behavior of the polyamide, requiring adjustments to be made in the manufacturing process to ensure consistent quality and performance. It is known in the art that micro- or nanoparticles of gelled polymer in the yarn create stress concentration points that hinder the drawing process of the yarns and limit the achievable tenacity and elongation at break.

[0029] The present inventors have found that controlling the value of the Mz / Mwparameter results in unexpected improvements in fiber properties. In particular, the present inventors observed that, at a drawn yarn RV range of 90-200 (and particularly in the range of 90-150, more particularly 90-130, and more particularly 90-120), if the polymer constituting the yarn has a molecular weight distribution (Mz / Mw) higher than 1.50, it fails in the spinning process due to frequent filament and process breakages which makes the process uneconomical.

[0030] The Mz / Mw parameter is measured by Size Exclusion Chromatography (SEC), as described hereinbelow. It will be appreciated that the values of the Mz / Mwparameter referred to herein are qualified with an error bar of ±0.03 in order to capture the uncertainty of the measurement method. It will be appreciated that the molecular weight distribution (Mz / Mw) parameter described herein is that of the polyamide constituting the drawn yarn.

[0031] The inventors have observed that the Mz / Mwparameter provides a precise measure of the molecular weight distribution (particularly its breadth) which allows accurate comparison between different samples.

[0032] It will be appreciated that the Mz / Mwparameter can be modulated using techniques which are conventional in the art and known to the skilled person. In particular, by controlling the process conditions during polymerization, the molecular weight distribution and gel formation, and hence the Mz / Mwparameter, can be controlled. For example, pre-heating the polyamide flakes before an SSP step is known to allow a shorter residence time in SSP which results in narrower molecular weight distribution of the resulting polymer (TWI651341-B). Another method is to limit the content of bishexamethylenetriamine within the polymer through directly subjecting an equimolar salt of adipic acid and hexamethylenediamine (HMD) to an SSP step while replenishing the escaped amount of HMD followed by melt polymerization and another SSP step (JP-3005692- B). Other known strategies to control the molecular weight distribution and the Mz / Mwparameter of polyamides include addition of polymerization catalysts, such as phosphoric acid or sodium hypophosphite, and modulation of the heating and decompression of the polymerization medium (EP-2871201-A). Other conventional methods for adjusting the molecular weight distribution and the parameter Mz / Mwinclude blending resins which have different molecular weights (EP- 3127945-A).

[0033] The polyamide of particular utility in the present invention is or comprises PA66, and preferably the polyamide is PA66. However, in alternative embodiments the polyamide maybe or may comprise at least one polyamide selected from nylon 6 (PA-6), nylon 7 (PA-7), nylon 4,6 (PA-4,6), nylon 4,10 (PA-4,10), nylon 5,6 (PA-5,6), nylon 5,10 (PA-5,10), nylon 6,10 (PA- 6,10), nylon 12 (PA- 12) and nylon 6,12 (PA-6,12). The preferred PA66 polyamide may be present in the yarns disclosed herein in combination with one or more of any of said polyamides of said alternative embodiments.

[0034] The yarn of the present disclosure preferably has a linear density in the range from about 200 to about 3000 decitex, preferably from about 300 to about 2500 decitex, preferably from about 400 to about 2500 decitex, preferably from about 600 to about 2300 decitex.

[0035] The number of filaments in the yarn is preferably about 30 to about 450, preferably about 60 to about 300, preferably about 100 to about 250.

[0036] The tenacity of the yarn is preferably > 8.0 cN / dtex, preferably > 8.5 cN / dtex, preferably > 9.0 cN / dtex. Of particular utility in the present invention is a yarn having a formic acid relative viscosity (RV) of about 90 to about 200 and a tenacity of >8.5 cN / dtex (preferably > 9.0 cN / dtex), and comprising polyamide (preferably PA66) characterized by a molecular weight distribution (Mz / Mw) in the range of > 1.40 and < 1.50 ±0.03, preferably > 1.42 and < 1.49 ±0.03 and preferably > 1.45 and <1.48 ±0.03.

[0037] The yarn preferably exhibits an elongation at maximum force of about 10.0 to about 30.0 percent, preferably about 15.0 to about 30.0 percent, preferably about 15.0 to about 27.0 percent, preferably about 15.0 to about 25.0 percent, preferably about 16.0 to about 25.0 percent, preferably about 17.0 to about 25.0 percent, preferably about 18.0 to about 25.0 percent of the original dimensions.

[0038] The elongation at 45 N of the yarn is preferably about 7% to about 12%, preferably about 8% to about 12%, preferably 8.5% to about 12%, preferably about 9% to about 12% relative to the initial length of the yarn before testing.

[0039] The energy to break of the yarn is preferably at least 0.6 J, preferably at least 1.0 J, preferably at least 1.5 J, preferably at least 2.0J, preferably at least 2.5 J, preferably at least 2.8 J. It will be appreciated that yarns with a higher linear density exhibit a higher energy to break. In a preferred embodiment, the energy to break of the yarn is preferably at least 2.5 J, preferably at least 2.8 J.

[0040] The drawn yarn formic acid relative viscosity (RV) is about 90 to about 200, preferably about 90 to about 150, preferably about 90 to about 130, preferably about 90 to about120. Preferably, the yarn has an RV of at least 95. Thus, preferably the RV is about 95 to about 200, preferably about 95 to about 150, preferably about 95 to about 130, preferably about 95 to about 120.

[0041] The formic acid relative viscosity (RV) is measured according to ASTM D789-19 (2019) using a 90% formic acid solution, as reported hereinbelow. It will be appreciated that the formic acid relative viscosity values defined herein are those of the polyamide constituting the drawn yarn.

[0042] There is a correlation between viscosity and molecular weight. Higher molecular weight polymers generally result in higher RV values. The molecular weight and RV affects the mechanical and physical properties of the polyamide in different applications, as well as consistency and quality in the manufacturing process.

[0043] The process of producing high RV polyamide fiber involves carefully controlling the polymerization reaction to achieve a high degree of polymerization (or chain length) in the resulting polymer. As is known in the art, this can be accomplished by adjusting reaction conditions such as temperature, pressure, and catalyst concentration, to optimize the polymerization kinetics and control the molecular weight distribution.

[0044] It will be appreciated that the yarns described herein are drawn yarns.

[0045] The percent of dry heat shrinkage of the yarn is preferably about 5 to about 10 percent, preferably about 6 to about 8 percent, about 6 to about 7 percent relative to the as-drawn fibers before the test.

[0046] The fatigue resistance is measured herein as the breaking strength retention relative to the strength before being fatigued, and is preferably >35%, preferably >50%, preferably >65%, preferably >70%. In a preferred embodiment the fatigue resistance is from about 70 % to about 90%.

[0047] The fatigue resistance and cured-in-rubber strength are measured on a tire cord made from or comprising the polyamide (preferably PA66) yarn of the invention.

[0048] It is a further aspect of the present methods and products that they preferably do not include using benzene sulfonate compounds, as the inclusion of such compounds may not be suitable for all applications. Additionally or alternatively, the methods and products disclosed herein do not comprise phosphorous-containing compounds. Phosphorous-containing compounds may or may not be included in the polymer as catalyzer or an agent to further improve polymerstability against degradation at higher temperatures. According to references EP2188421 Bl and US4966949 A, examples of suitable compounds include X(CH2)n PO3 R2, wherein X is selected from 2-pyridyl, -NH2, NHR', and N(R')2, n=2 to 5, R and R' independently are H or alkyl; 2- aminoethylphosphonic acid, potassium tolylphosphinate, or phenylphosphinic acid. Preferred compounds include 2-(2'-pyridyl) ethyl phosphonic acid, and metal hypophosphite salts including sodium and manganous hypophosphite. A base such as an alkali metal bicarbonate can be added to the catalyst to minimize thermal degradation, as described in US 5,116,919.

[0049] Preferably, the polyamide used in the present invention does not contain a phenolic antioxidant stabilizer, in particular a hindered phenolic stabilizer. Such compounds include alkylsubstituted and / or aryl-substituted phenols, particularly such hindered phenols compounds.

[0050] In another aspect of the invention there is provided a method of making a polyamide PA66 yarn, as described herein. In an embodiment the method includes shear heating polyamide 66 flakes until the polymer reaches a molten state. The heating is followed by extrusion of the molten polyamide through spinnerets with multiple holes and rapidly cooling the molten polymer that exits the spinnerets to solidify the filaments. Cooling can be done by a stream of air. A lubricant is then applied to filaments. The filaments are then passed through a series of one or more heated rollers to stretch and align the molecules. This process is called drawing. Drawing can occur in multiple stages with rollers rotating at different speeds and at different temperatures. The drawn yarns are then relaxed by passing through another set of one or more rollers to achieve the required shrinkage on the final yarn. The finished filaments are wound onto bobbins and optionally further processed.

[0051] In a further aspect of the invention there is provided a method of preparing tire cord from the drawn polyamide (preferably PA66) yarn as described herein. The tire cord may be made using any conventional method known in the art. Preferably, the method comprises the steps of: a) twisting single yarns in Z or S direction wherein said single yarns are the drawn polyamide yarns described herein; and b) plying two or more single yarns together and twisting them in a direction which is opposite to the direction of twist in the single yarns, preferably with a similar number of twists per unit length as each single yarn.

[0052] The resulting tire cord may then be further processed as described hereinabove.Test Methods:

[0053] The features of the polyamide yarn are evaluated using the following test methods.Linear Density of Yarn

[0054] Linear density of yarns is measured according to ASTM DI 907 - Option 6 with zero twist added.Dry Heat Shrinkage

[0055] Dry heat shrinkage of yarns is measured in hot air according to the method described in ASTM D4974 at a temperature of 177 °C and under 0.045 gf / dtex tension for a duration of 2 minutes.Formic Acid Relative Viscosity ( RV) (Drawn Fiber RV)

[0056] The relative viscosity (RV) is measured on the fiber according to ASTM D789-19 (2019) using a 90% formic acid solution. One 20 g fiber sample is required for each replicate of this analysis. The drawn fiber RV specifically applies to measurement of RV on fibers samples collected after the spinning process. Prior to RV measurement, each sample is treated to remove any remaining fiber lubricant oil, also known as spin finish. To remove the lubricant, each sample of fiber is soaked in enough methylene chloride to fully cover the sample. The sample is allowed to soak in a covered extraction funnel for twenty minutes with stirring. This procedure is then repeated. Once the second methylene chloride rinse is complete, the fiber is soaked in enough 1 : 1 methanol: methylene chloride to fully cover the sample. The sample is allowed to soak in a covered extraction funnel for twenty minutes with stirring. This procedure is repeated twice more. Once all five soak steps are complete, remaining solvent is blown out of the fiber sample with clean pressurized air. The fiber is then allowed to air dry completely in an exhaust hood. Once dry, ASTM D789-19 is followed to measure the formic acid relative viscosity of the drawn fiber sample.Fiber tensile testing

[0057] Maximum force (N) and elongation at maximum force (%) is assessed according to standard ASTM D3822 / D3822M-14 (2020) but with the amendments as listed below.

[0058] The initial gauge (clamp) length set on the Instron tensile tester is 254 mm. The Instron crosshead speed is set at 300 mm / min. Fiber specimens are cut initially to size 350 mm and while one end is fixed within the upper (moving) grip, the fiber is twisted 3 tpi (twist per inch). The other end is then fixed within the lower grip of the Instron. Tensile testing is done on 10specimens cut from the same bobbin. The reported result for maximum force (also known as breaking force or breaking load) is the average of the maximum force results of the 10 specimens which are tested in Newtons (N). The reported result for elongation at maximum force (also known as elongation at break, percentage elongation or percentage extension) is the mean average of the elongation at maximum force results of the 10 specimens which are tested (%). Tenacity is measured by the following equation.

[0059] Tenacity (cN / dtex) = Maximum Force (cN) / Linear Density (dTex)

[0060] Energy to break (J) is a measure of work required to break a fiber sample and is calculated based on the fiber tensile results (obtained as detailed above) as the area under the stressstrain curve.

[0061] The elongation at 45N is also assessed using the methodology described above.Mz / Mwvalue by Size Exclusion Chromatography (SEC)

[0062] The absolute molar mass distributions of the yarn samples are measured using size exclusion chromatography (SEC) with refractive index (RI) and multi-angle light scattering (MALS) detection. Two 300 x 7.5 mm PL hexafluoroisopropanol (HFIP) gel columns are used with guards together with Wyatt Technology HELEOS multi-angle light scattering and Wyatt Technology Optilab T-rEX refractive index detectors. The temperature of columns and the RI detectors is kept at 40 and 30°C, respectively. The samples are prepared at a nominal concentration of 2 mg / mL in mobile phase consisting of 0.01 M tetraethylammonium nitrate (TEAN) in hexafluoroisopropanol (HFIP). After dissolving for 4 hours, the sample solutions are filtered through 0.45 pm polytetrafluoroethylene (PTFE) syringe filters and a volume of 200 pL is injected in the mobile phase flow for each measurement. Flow rate of solvent is 0.8 ml / min and the run time is 40 minutes for each measurement. The molar mass averages for each sample are calculated by averaging the results from two injections of the sample and the molecular weight distribution is calculated as follows: z-average molecular weightMolecular weight distribution (Mz / Mw) = weight average molecular weight

[0063] As is conventional in the art, Mz and Mw are defined as follows:where n = 1 gives M = Mw, and n = 2 gives M = MzFatigue Resistance, as measured by Disk Fatigue testing

[0064] Tire cords are made by twisting two separate yarns at Z direction at a specified number of twists per unit length (8.5 tpi was used in this study) and then twisting them together at S direction at a specified number of twists per unit length (8.5 tpi was used in this study). The cords are then passed through the adhesive bath containing resorcinol formaldehyde latex (RFL) mixture and then dried at 150 °C for ca. 1 min under tension.

[0065] The cords are then embedded in rubber and the rubber is cured to make the test specimens. The breaking strength loss of cords made of yarns are measured according to ASTM D6588 / D6588M-11(2016). The test is done under 15% compression and 10% tension for 11.5 hours. Cured-in-rubber cords are removed from the embedding rubber after the specimens are fatigued and the breaking strength loss is measured using the following equation:Breaking Strength Loss (%)Breaking Strength before fatigue cycles (IV) — Breaking strength after fatigue cycles (IV) Breaking Strength before fatigue cycles (IV)Cured-in-rubber strength

[0066] Tire cords are made using the method described in the previous section. Cured-in- rubber strength is measured by wrapping adhesive (RFL) treated cord around a 7.3 cm by 27.6 cm clean, flat steel plate with approximately 0.6 mm spacing between adjacent wraps of cord. When the desired number of wraps (generally 5) have been made, the two ends of the cord are tied together at the back of the plate using a double square knot to firmly secure the sample to the plate. A 7.3 cm by 27.6 cm piece of rubber of appropriate composition (in this case a typical passenger tire carcass stock formulation), 0.76 mm thick, is placed on top of the cords wrapped around the plate. The sample is then cured in a hydraulic press for 20 minutes at 177±2 °C under 3.3 (3000 kgm) tons pressure. At the end of the curing cycle the sample is removed from the press and the exposed cords on the back side of the plate are immediately cut. After cooling to room temperature, the cords are pulled from the rubber and then allowed to condition at 24°C / 55% RH for at least 48hours. Cured-in-rubber breaking strength is then determined using a 15.2 cm gauge length and a strain rate of 120% / min.Processability

[0067] During the spinning trials, the condition of multifilament is monitored between spinneret and winders and the processability is rated with a number between 1 and 4, where 1 is the best condition and the lowest number of breaks is seen and 4 the worst condition (comparatively) and it means the process goes through many breaks and has to be started over every few minutes.Examples

[0068] A series of example yarns are selected to evaluate the features of the yarns having varying relative viscosity and Mz / Mwvalues. The polymerization conditions of the PA66 of Examples 1 and 2 are adjusted using the techniques described hereinabove to provide a combination of high RV and Mz / Mwvalues which fall within claim 1. Comparative Examples 1 to4 exhibit either RV or Mz / Mw values which fall outside claim 1. Comparative Examples 3 and 4 correspond to Comparative Examples 1 and 2 but produced using lower draw ratios. The yarns are evaluated according to the measurement methods disclosed herein, and the results are presented in Table 1. For fatigue resistance and cured-in-rubber strength, tire cords are made by twisting two separate yarns per example (1400 dtex each) at Z direction (8.5 tpi) then twisting them together at5 direction (8.5 tpi).Table 1

[0069] The RV of a yarn generally increases with increasing molecular weight and as such can be used as a representative measurement for the molecular weight of a sample. Examples 1 and 2 are provided to exemplify yarns having an RV in the preferred range of 90- 130. Comparative Examples 1 and 3 are selected to have a lower molecular weight and consequently a lower RV, below the preferred range. Comparative Examples 2 and 4 were intended to provide examples with a higher molecular weight and consequently a higher RV of about 140, but were instead foundto have an RV value of 102. While not wishing to be bound by theory, it is speculated that the lower-than-expected RV for Comparative Examples 2 and 4 is the result of polymer degradation which occurs more frequently in processing of higher molecular weight polyamides.

[0070] As can be seen from the results in table 1, Comparative Example 2 exhibits an elongation at maximum force of only 14.6% and an energy to break of only 2.3 J, which is lower than the other samples. Thus, while the fatigue resistance (strength retention) of Comparative Example 2 is comparable with that of Example 2, the strength was already at a low level. Additionally, the processability is worse for Comparative Example 2 than for the other samples. These results suggest that increased degradation is occurring in this sample.

[0071] Table 1 shows fatigue resistance improves as RV increases for Example 1, Example 2 and Comparative Example 1. Comparative Example 2, which is expected to have the highest molecular weight, does not have a higher RV but rather, has comparable fatigue resistance to Example 2. As noted above, it is thought that the RV for the Comparative Example 2 is lower than expected due to degradation occurring during the processing. Taking this into account, the data shows a correlation between increasing RV (as a representation of increased molecular weight) and improved fatigue resistance.

[0072] The samples of Table 1 are analyzed for molecular weight distribution in the fully drawn fiber. It is observed that when the polymerization process is controlled well enough to avoid degradation and branching, and to achieve the desired molecular weight distribution (Mz / Mw) of the present invention, an increase in RV results in both higher fatigue resistance (strength retention) and cured-in-rubber strength. In other words, the improvement of both fatigue resistance and cured-in-rubber strength associated with higher molecular weight (RV) is only observed when the Mz / Mw of the polymer constituting the drawn yarn is within the ranges of the present invention. Comparative example 2 does not follow the same trend as other examples and it can be seen that it has Mz / Mw outside of the claimed range and accordingly, processability is the worst amongst other examples and both the fatigue and cured-in- rubber strength is not improved.

[0073] Although the invention has been described with reference to certain specific embodiments, various modifications thereof will be apparent to those skilled in the art without departing from the purpose and scope of the invention as outlined in the claims appended hereto. Any examples provided herein are included solely for the purpose of illustrating the invention andare not intended to limit the invention in any way. The disclosures of all prior art recited herein are incorporated herein by reference in their entirety.

Claims

CLAIMS:

1. A drawn polyamide yarn having a formic acid relative viscosity (RV) of about 90 to about 200 comprising PA66 characterized by a molecular weight distribution (Mz / Mw) in a range of > 1.40 and < 1.50 ±0.03.

2. The polyamide yarn according to claim 1 wherein the molecular weight distribution (Mz / Mw) is in a range of > 1.42 and < 1.49 ±0.03, preferably > 1.45 and < 1.48 ±0.03.

3. The polyamide yarn according to claim 1 or 2 wherein the RV is about 90 to about 150, preferably about 90 to about 130, preferably about 90 to about 120.

4. The polyamide yarn according to any preceding claim wherein the yarn has an RV > 90.

5. The polyamide yarn according to any preceding claim wherein the linear density of the yarn is 200-3000 decitex.

6. The polyamide yarn according to any preceding claim wherein the number of filaments of the yarn is 30-450.

7. The polyamide yarn according to any preceding claims wherein the tenacity of the yarn is >8.0 cN / dtex, preferably >8.5 cN / dtex, preferably >9.0 cN / dtex.

8. The polyamide yarn according to any preceding claim wherein the energy to break of the yarn is at least 0.6J, preferably at least 1.0 J, preferably at least 1.5 J, preferably at least 2.0J, least 2.5 J, preferably at least 2.8 J.

9. The polyamide yarn according to any preceding claim wherein the yarn exhibits an elongation at maximum force of about 10.0 to about 30.0 percent, preferably about 15.0 to about 30.0 percent, preferably about 15.0 to about 27.0 percent, preferably about 15.0 to about 25.0 percent, preferably about 16.0 to about 25.0 percent, preferably about 17.0 to about 25.0 percent, preferably about 18.0 to about 25.0 percent of the original dimensions.

10. The polyamide yarn according to preceding claim wherein the dry heat shrinkage is 5- 10%.

11. The polyamide yarn according to any preceding claim wherein the elongation at 45N is about 7 to about 12 %.

12. The polyamide yarn of any preceding claim which does not contain benzenesulfonate compound or contains benzenesulfonate in amounts of less than 5 ppm.

13. The polyamide yarn according to any preceding claim wherein the yarn further comprises a phosphorous- containing compound as a thermal stabilizer, preferably wherein the phosphorous- containing compound is selected from: compounds defined by the formula X-(CH2)nPO3R2, wherein X is selected from 2-pyridyl, -NHz, NHR', and N(R')z, n=2 to 5, R and R' independently are H or alkyl; 2-aminoethylphosphonic acid, potassium tolylphosphinate, or phenylphosphinic acid; 2-(2'-pyridyl) ethyl phosphonic acid; and metal hypophosphite salts including sodium and manganous hypophosphite.

14. The polyamide yarn according to any of claims 1 to 12 which does not comprise a phosphorous-containing compound.

15. The polyamide yarn according to any preceding claim which does not contain a phenolic antioxidant stabilizer, in particular a hindered phenolic stabilizer, in particular alkyl-substituted and / or aryl-substituted phenols.

16. The polyamide yarn according to any preceding claim wherein the yarn exhibits a tenacity of >8.5 cN / dtex (preferably >9.0 cN / dtex) and an elongation at maximum force of at least 15.0 percent, and preferably also an energy to break of at least 2.5 J and / or an elongation at 45N of about 8.5 to about 12%.

17. The polyamide yarn according to any preceding claim which provides a fatigue resistance and / or cured-in rubber strength which is higher than that provided by a polyamide yarn having a lower relative viscosity and an Mz / Mwvalue outside of said range.

18. The polyamide yarn according to any preceding claim wherein the polyamide is or comprises at least one polyamide selected from polyamide 6,6 (PA66), nylon 6 (PA-6), nylon 7 (PA-7), nylon 4,6 (PA-4,6), nylon 4,10 (PA-4,10), nylon 5,6 (PA-5,6), nylon 5,10 (PA-5,10), nylon 6,10 (PA-6,10), nylon 12 (PA- 12) and nylon 6,12 (PA-6,12), and preferably wherein the polyamide is or comprises PA66, and preferably wherein the polyamide is PA66.

19. A tire cord made from or comprising the yarn of any one of claims 1-18.

20. The tire cord of claim 19 wherein the fatigue resistance is more than 35%, preferably more than 50%, preferably more than 65%, preferably wherein the fatigue resistance is from about 70 % to about 90%.

21. The tire cord of claim 19 or 20 wherein the cured-in-rubber strength of the tire cord is > 50 N, preferably > 70 N, preferably > 100 N, preferably > 130 N, preferably > 150 N, preferably > 180 N, preferably > 210 N, preferably > 220 N.

22. A method of preparing a drawn yarn according to any one of claims 1-18 comprising the steps of: a) shear heating polyamide flakes until the polymer reaches a molten state; b) extruding the molten polymer through spinnerets; c) cooling the molten polymer that exits the spinnerets to solidify the filaments; d) applying a lubricating oil to the filaments; e) drawing the filaments by passing the filaments through one or more heated rollers to stretch and align the molecules; f) relaxing the drawn yarns by passing through another set of one or more rollers to achieve the required shrinkage on the final yarn and g) winding the finished filaments onto bobbins.

23. A method according to claim 22 wherein the drawing in step e) occurs in multiple stages with rolls rotating at different speeds and at different temperatures.

24. A method of preparing tire cord according to any one of claims 19-21 comprising: a) twisting single yarns in Z or S direction wherein said single yarns are the drawn polyamide yarns described herein; and b) plying two or more single yarns together and twisting them in a direction which is opposite to the direction of twist in the single yarns, preferably with a similar number of twists per unit length as each single yarn.

25. A tire comprising the tire cord of any of claims 19 to 21.

26. The use of a drawn polyamide yarn as defined in any of claims 1-18 in a tire cord for the purpose of improving the fatigue resistance and / or cured-in-rubber strength of a tire cordcomprising said yarn, wherein said yarn has a formic acid relative viscosity (RV) of about 90 to about 200 and comprises polyamide characterized by molecular weight distribution (Mz / Mw) in a range of > 1.40 and < 1.50 ±0.03.

27. The use of a tire cord according to any of claims 19-21 in a tire for the purpose of enhancing the durability of said tire, wherein said drawn polyamide yarn has a formic acid relative viscosity (RV) of about 90 to about 200 and comprises polyamide characterized by molecular weight distribution (Mz / Mw) in a range of > 1.40 and < 1.50 ±0.03.

28. A tire cord fabric made from or comprising the tire cord of any of claims 19-21. 1

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