The present invention relates to a polymeric tubular article comprising a hybrid reinforcing yarn, wherein the reinforcing yarn comprises a high strength core and at least one wrap of a sheath yarn wound about the core for better fatigue resistance at high temperature.

Prior Art

In recent years, the tendency to downsize automobile engines to reduce weight and improve fuel economy has been strengthened. Downsizing, and the resultant increase in specific power output cause many engine components to face new challenges. Among these, low-pressure fluid transfer hoses used under the hood become more complicated in shape on one hand, and on the other hand they have to withstand higher temperatures and pressures with lesser wall thickness. Therefore, dynamic fatigue resistance of such hoses at high temperatures becomes more important.

Low-pressure hoses are generally reinforced with textile yarns to withstand required internal pressures without compromising flexibility. Braid, spiral, wrap, and knit reinforcements are well known types of textile yarn reinforcements. But for hoses with complicated shapes, knit reinforcement is often preferred due to its flexibility. Knit reinforcement is accomplished by applying reinforcing yarns over the tube to form loops interlocking with each other. This chain-like structure results in yarns passing over other yarns. However, continuous radial expansion and contraction of the hose because of internal pressure changes cause these yarns to move back and forth and cut each other at knot points. Once a yarn is broken, the knit reinforcement starts to unravel at that point resulting in a local hole in the fabric. As a result, hose may excessively expand in that region and rupture. Thus, abrasion resistance of a reinforcing yarn determines its long-term dynamic fatigue performance, which in turn determines the operating life of the knitted hose.

l Yarns such as PEN, PBO, para-aramid, para-aramid copolymer, carbon and liquid crystal polyester are known to exhibit high tensile strength. However long-term dynamic fatigue performance of these yarns at high temperatures is not satisfactory because of yarn on yarn abrasion. In order to improve dynamic fatigue performance of low-pressure hoses, it is known to use composite cords composed of two or more yarns.

A composite or hybrid cord refers to a single yarn comprising strands of two or more kinds with different properties, wherein individual strands are core spun, twisted, commingled, air covered, wrapped etc. to obtain the final cord. The proportion and the orientation of each kind are adjusted according the desired properties of the final cord.

US Patent Application 20080202618 describes a hose reinforced with a hybrid cord comprising multiple monofilaments and short fibers entangled between them to increase adhesion of the cord to the rubber layers.

US Patent Application 20100224298 discloses a tire reinforced with a hybrid cord produced by commingling process, which includes two different multifilament yarns having different initial tangent moduli.

EP2066947 relates to a flexible hose reinforced with a hybrid cord wherein two yarns having different fatigue properties are twisted together in order to increase the fatigue performance. The disclosed hybrid yarn comprises a first yarn of co-para-aramid fibers and a second yarn of meta-aramid fibers which are twisted, plied, folded, or commingled together to form a single hybrid reinforcing yarn for the hose. However, in this method, the hybrid yarn may have a surface comprising both co-para-aramid fibers and meta-aramid fibers so that the pressure-carrying yarn is not fully isolated and it is still open to yarn-on-yarn abrasion and self-cutting at contact points. Furthermore, twisted co-para-aramid and meta-aramid yarns are expensive especially for coolant hoses.

Summary of the Invention

The object of the invention is to provide an inexpensive polymeric tubular article exhibiting improved fatigue resistance at high temperatures. This object is achieved by a tubular product which comprises an inner tube, a reinforcement, and an outer tube; wherein said reinforcement comprises a hybrid cord comprising a pressure carrying high strength core and at least one, preferably two wraps wound around the core in opposite directions. The term "yarn" within the context of the present invention is to be interpreted broadly to include a continuous strand of textile fibers, filaments or material in a form suitable for knitting, weaving or otherwise intertwining to form a textile fabric. "Fiber" refers to an elongate, individual, monolithic unit of matter that forms the basic element of a fabric and the "filament" refers to a continuous fiber of extremely long length. As used herein, the term "strand" refers to an ordered assemblage of textile fibers having a high ratio of length to diameter. For the purpose of this application, it shall be understood to mean a single fiber, filament, monofilament or yarn.

The term "cord" as used herein refers to a twisted or formed structure composed of one or more single or plied filaments, strands, or yarns of organic polymer or inorganic materials and "greige" is a descriptive term for yarns that have not received any bleaching, dyeing or finishing treatment after being produced.

According to the invention, the entire tensional load is carried by the longitudinally extending core. Wraps only function to protect and therefore increase the yarn-on-yarn abrasion resistance of the core to improve long term fatigue performance. In order to achieve this, the core is completely covered with the wrappings in such a way that the surface of the hybrid cord comprises only the wrapped sheath yarns. In this way, load carrying core yarns are isolated from each other so that possibility of self-cutting is eliminated. This structure also protects the core against yarn to metal abrasion which occurs during knitting, braiding and spiraling operation. The inventive tubular article can be used as a fluid transfer hose as well as a bellow, expansion joint or the like. Of particular importance is low-pressure fluid transfer hoses used in automotive applications like turbocharger, radiator, heater or fuel hoses.

FIG. l is a perspective view of the polymeric tubular article in accordance with the invention. FIG. 2 shows a typical knit fabric. FIG. 3 is a diagrammatic drawing of a composite cord constructed in accordance with the invention.

FIG. 4 is a graph showing the burst pressure values of samples before and after the fatigue life test. FIG. 5 is a graph showing a single pressure cycle of hose fatigue life test. Detailed Description

Figure l shows an arrangement of the inventive tubular article (l). Article comprises an inner tube (io) onto which a reinforcement (n) is knitted and an outer tube (12) covering the knitting. Figure 2 illustrates the details of a typical knit fabric. There are different types of knit fabrics called plain stitch, lock stitch etc, but in common, they all have the cord (111) forming loops (110) interlocking with each other and creating contact points (110').

Figure 3 illustrates a preferred embodiment of a hybrid cord (111) used in the reinforcement (11) of the tubular article (1) in accordance with the invention. In the preferred embodiment, the hybrid cord (111) comprises an elongated core (115) with at least one strand of a high strength yarn, a first wrap (116) of a sheath yarn wound around the core in one direction, and a second wrap (118) of a sheath yarn wound in opposite direction to the first wrap.

According to the invention, the load carrying core (115) comprises at least one strand of a high tensile strength material selected from the group consisting of polyester, liquid crystal polyester, polyamide, para-aramid, para-aramid copolymer, polyimide, polyphenylene-2,6-benzobisoxazole (PBO), polyketone (PK), polyetheretherketone (PEEK), glass, carbon, basalt, steel and blends thereof. Various forms of such para- aramid fibers are available from E.I. duPont de Nemours and Company under trademark Kevlar®, from Teijin Ltd. under trademark Twaron®, from Kolon Industries, Inc. under trademark Heracron® and from Hyosung Corporation under trademark Alkex®. Para-aramid copolymer fibers known under trademark Technora® are available from Teijin Ltd., and liquid crystal polyester, also known as aromatic polyester are sold by Kuraray Co. Ltd. under trademark Vectran®. Poly(p- phenylene-2,6-benzobisoxazole) (PBO) known as liquid crystalline polyoxazole fibers are commercially available from Toyobo Co Ltd under trademark Zylon®, and polyetheretherketone fibers made of polyether are sold by Zyex Ltd. under trademark Zyex®. Wraps (116, 118) of sheath yarns protect the core (115) to improve its yarn-on-yarn and yarn-to-metal abrasion resistance. Conventional fibers exhibiting improved fatigue resistance, like aliphatic polyamide, polyester, rayon, (PPS) polyphenylene sulfide, lyocell, acrylic, PTFE, meta-aramid or (POD) polyoxydiazole, polyacrylate are preferred for the yarns used in the first and second wraps (116, 118). Wraps can be constructed from the same material or different materials can be employed in each one.

In the preferred embodiment of the invention, the core (115) has a total linear density in the range of about 400 to 4000 dtex and sheath yarns of each wrap (116, 118) has about 40 to 400 dtex. Preferably linear density of the yarn of the second wrap (118) is equal to the first wrap (116).

According to the invention, the core (115) is completely covered with the wraps (116, 118) in such a way that the surface of the hybrid cord (111) comprises only the yarns of wrappings. In this way, in the knit fabric pressure carrying core yarns are isolated from each other so that possibility of self-cutting is eliminated. In the illustrated embodiment, the first and second wraps (116,118) are wound at the rate of 200 - 4000 turns per meter depending on the linear density and twist level of the core (115) and wrap (116,118) yarns.

In the preferred arrangement of the invention, two wraps in opposite directions are employed in such a way that a first wrap (116) is wound in one direction about the core (115) and a second wrap (118) is wound in an opposite direction to the first wrap. In this way, the possibility of unraveling of the wrappings at high stress contact points is eliminated. However, as long as the core (115) is completely covered, it is also possible to use a single wrap (116) wound in either S or Z direction. Or in alternative embodiments, more than two wraps can also be employed. In the latter case, each subsequent wrap is wound in a direction opposite to the previous wrap. In the preferred arrangement of the invention, the core (115) comprises a single strand, either composed of a single filament or filaments plied, textured, commingled, twisted or air covered together. However in alternative embodiments, two or more separate strands extended substantially parallel may compose the core, and the wraps (116, 118) may be wound around this bundle.

In the preferred arrangement, greige yarn is used in the core (115) and the wraps (116, 118). However in alternative embodiments the core or sheath yarns or the final hybrid cord can be treated with adhesives or bonding elastomers or may comprise plastic resin layers. Furthermore, in alternative embodiments, wet lubricants such as synthetic oil or grease, glycerol, polybutane, polymer ester, polyolefines, polyglycols, silicon, soap, natural or synthetic waxes, resins and tars with additives of organic and/or inorganic thickeners, such as, for example, organic polymers, polycarbamide, metal soap, silicates, metal oxides, silicic acid, organophilic betonite or dry lubricant such as Talcum, graphite powder, molybdenum disulfide, polytetrafluorethylene (PTFE), lead (Pb), gold (Au), silver (Ag), boron trioxide (BO3), lead oxide (PbO), zinc oxide (ZnO), copper oxide (Q12O), molybdenum trioxide (M0O3) and titanium dioxide (T1O2) can be applied to the core or sheath yarns prior to wrapping process. Alternatively, the final hybrid cord (111) can be dipped in lubricants stated above. During production, 1-10 cN/tex pre tension is applied to the core (115) before the wraps (116,118) are wound. First, the first wrap (116) is wound around the core (115) with closed coiling, right after, the second wrap (118) is wound around the first one again with closed coiling. Both the first and second wraps must be coiled in precision without any gap to cover the core (115) completely. The inventive tubular article (1) can be used as a fluid transfer hose, bellow, expansion joint or the like. Of particular importance is low-pressure fluid transfer hoses used in automotive applications like turbocharger, radiator, heater or fuel hoses.

The polymeric material used in inner (10) and/or outer tube (12) of the tubular article (1) can be selected among elastomers, termoplastic elastomers or flexible plastics. Of particular importance is vulcanized rubber compositions comprising any one of ethylene acrylic elastomer, ethylene propylene diene copolymer, ethylene propylene copolymer, chloroprene rubber, chlorinated polyethylene, chlorosulfonated polyethylene, polyepichlorohydrin, nitrile rubber, hydrogenated nitrile rubber, fluoro rubber, natural rubber, styrene-butadiene rubber, isoprene rubber, butyl rubber, bromobutyl rubber, chlorobutyl rubber, butadiene rubber, silicone rubber, acrylate rubber, ethylene-vinyl acetate rubber, and blends thereof.

The hybrid cord reinforcement (11) may reside between the inner and the outer tube (10, 12) and/or may be embedded in a layer (10, 12) of the product. Also, it should be noted that the invention is not limited to the knit reinforcement. As one skilled in the art will appreciate, disclosed hybrid cord reinforcement (11) may also be provided in the form of braid, spiral, wound or any other type of known textile yarn reinforcements.

A further object of the invention is to provide a method for producing a polymeric tubular article (1) exhibiting improved fatigue resistance at high temperatures. This object is achieved by a method comprising the steps of - forming a hybrid cord (111) by winding a first wrap (116) of a sheath yarn around a core (115) in a first direction, then winding a second wrap (118) around the first wrap in an opposite direction;

- forming the inner tube (10) from a polymeric material,

- forming a reinforcing layer (11) over the inner tube using the hybrid cord (111), - forming a polymeric outer tube (12) over the reinforcing layer.

The reinforcement (11) is preferably formed by knitting with plain stitch, lock stitch or any other stitch pattern. However it may also be formed by braiding or spiral winding or wrapping the hybrid cord over the inner tube.

The method may also include a final step of curing the polymeric tubular article (1) on a linear or curved mandrel. Curing may be accomplished by steam or air vulcanization.

In the following examples, performance of the inventive tubular product is illustrated in comparison with alternative designs. In examples 1 through 6, fatigue tests are carried out on an L-shaped radiator hose by cycling hot antifreeze fluid at 100% concentration and at 115 °C ± 2 °C according to the pressure cycle shown in Figure 5. During the test, 30 mm peak-to-peak oscillation at a frequency of 0.33 Hz is applied and the test is carried out in chamber at 85 °C ± 2 °C. Burst strength of each sample is measured before and after the fatigue life test by filling the hose with antifreeze agent and increasing the internal pressure at a rate of 980 to i960 kPa/min until the failure occurs.

For comparative purposes, each yarn construction is also individually tested to measure tensile strength and yarn-on-yarn abrasion resistance.

Tensile tests are performed according to ASTM D7269, "Standard Test Methods for Tensile Testing of Aramid Yarns". Yarns are pre-conditioned at 45 ± 5°C (113 ± 40°F) for 3 to 6 h. Distance between clamps are adjusted as 500 mm. Velocity of grip is set as 250 mm/sn.

To measure abrasion resistance of greige and hybrid cords, "dry method" of ASTM 6611 "Standard Test Method for Wet and Dry Yarn-on-Yarn Abrasion Resistance", is applied. In order to compare different embodiments, the same tensional load is used for each yarn type. Also to simulate hose reinforcement knot points, one turn yarn interwrap is applied.

Each of the six sample hoses has a 16 mm inner diameter and 3,5 mm wall thickness. EPDM composition is used for both inner and outer layers and the reinforcements are knit with a plain stitch on a 1 3/16" knitter head with 8 needles, 4 bobbins, at 3 mm per stitch for all samples. Each sample is vulcanized on a L-shaped curved metal mandrel in steam autoclave. Example 1

Sample hose 1 utilizes an example embodiment of the inventive hybrid cord where a 1100 dtex Kevlar continuous filament core yarn having a twist level of 90 tpm is first wrapped by a 167 dtex PES yarn with a twist level of 30 tpm at a rate of 1200 turns per meter. Then a second wrap of PES yarn having same linear density and twist level as the first wrap is wound around the first one again at a rate of 1200 turns per meter. Example 2

Example 2 has the same construction as that of the first embodiment of the reinforced hose except that the wrapping yarns on the Kevlar® 1100 core yarn are 220 dtex PES. This polyester yarn has higher linear density than the first embodiment. Example 3

Example 3 has the same construction as that of the second embodiment of the reinforced hose except that the wrapping yarns on the core Kevlar® 1100 yarn are 220 dtex PPS .

Example 4 Example 4 has the same construction as that of the second embodiment of the reinforced hose except that the wrapping yarns on the Kevlar® 1100 core yarn are 220 dtex Nomex® .

Example 5 Convential Hose

Example 5 has the same construction as that of the first embodiment of the reinforced hose except that reinforcing material is Kevlar® 1100 greige yarn.

Example 6 Comparative Hose

Example 6 has same construction as that of the first embodiment of the reinforced hose except that the reinforcing material is Kevlar® 1100 /PET 1100 twisted hybrid yarn.

Results are illustrated in Table 1 below.

Table 1

The results in Table 1 show that the inventive double wrapped hybrid cord has almost the same tensile strength when compared with conventional cord and the twisted hybrid cord, however it exhibits two to three times more resistance to yarn-on-yarn abrasion. Burst tests conducted on virgin samples show that the inventive double wrapped hybrid cord, conventional cord and the twisted hybrid cord all have comparable performances. On the other hand, the same tests conducted after 30,000 PVT cycles show the dramatic decrease in the burst pressure value of the samples constructed with conventional cord and the twisted hybrid cord whereas 80% to 93% of the burst retention capability can be preserved with the inventive hybrid cord. Results are also graphically shown in Figure 4.