JPS63270827A - Yarn-shaped heat generator and its production - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an electrically heat-generating filamentous heating element that is highly flexible and durable for long-term use, and a method for manufacturing the same.

[Prior Art] Flexible heat-generating wires made of thin metal wires have traditionally been used to insulate or heat carriers, but electric blankets, electric blankets, etc.
It has become widely used in consumer products such as electric carpets, and due to its convenience, there is a trend that it will be increasingly diversified into products in the future.

Conventionally, resistors made of thin metal wires such as stainless steel wires and nichrome wires have been used for these heating elements, but when each of the above products is required to be flexible, flexible Used materials include those in which ultra-thin resistance wire is wound in a spiral shape around a polyester core thread, and those in which carbon is fixed to a fabric using a resin binder.

However, none of these can meet the required performance in terms of bending resistance, abrasion resistance, etc., and also lacks flexibility, so improvements are required.

One of the attempts to solve this problem was
There is a publication No. 470. This increases the flexibility of the heating element by providing a rubber or plastic layer containing conductive fine particles around the core yarn. Although this invention has improved the level of flexibility compared to fine metal wire, it is used in the clothing field such as rider suits, diaper suits, inner suits, gloves, shoes, etc., and electric blankets that require a high degree of bending durability. In this field, the level of flexibility is extremely low, and it has not yet been put into practical use.

Furthermore, this manufacturing method is similar to that of electric wire manufacturing, and requires large-scale equipment consisting of a conductive rubber or plastic heating and melting section, a heat-insulating piping section, and a melt-coating section. The melt-coated part has a diameter of 1 mm or less for introducing and extracting the core yarn.
It has two pores and is kept at a high enough temperature to make the rubber or plastic fluid. therefore,
The operation of passing the core thread through the two pores is very difficult to operate.

In addition, in Utility Model Publication No. 39-37687@, a solution of resin containing conductive fine particles is applied to the core thread, and then the solvent is removed.
A method of fixing is disclosed. The application means,
This is a method in which the core yarn is immersed in a solution and then the excess solution is removed using a fine-hole file slit or the like, and the equipment is simplified and the operability is improved somewhat compared to the above-mentioned melt coating method.

However, in the heating element obtained by this method, since the solution cannot be applied uniformly, temperature unevenness occurs when electricity is applied, and there is even a risk of causing burns or fire. Therefore, this method is not recognized as an appropriate method for producing filamentous heating elements.

In addition, in an attempt to obtain a highly flexible thread-like heating element,
There is Japanese Unexamined Patent Publication No. 109321/1983. In this method, a nylon conjugate filament is softened by heating or swollen by a swelling agent, and carbon particles are fixed to the surface layer of the filament to form a filamentous heating element. This heating element has too high a resistance value per length, making it unsuitable for use as a heating element. Further, it is difficult to uniformly fix carbon particles, and therefore the resistance value varies widely, making it impossible to stably supply the required resistance value industrially.

[Problems to be Solved by the Invention] The present invention improves these conventional problems, and provides a fabric with small variations in resistance value, high flexibility, good adhesion between the conductive layer and the core yarn, and bendable, The present invention provides a filamentous heating element that is difficult to peel off due to wear and the like and can be used stably for a long period of time, and a method for manufacturing the same.

[Means for Solving the Problems] In order to solve the above problems, the filamentous heating element of the present invention has the following configuration. That is, a filamentous heating element consisting of a core yarn, a conductive layer of a synthetic resin containing dispersed conductive fine particles covering the core yarn, and air bubbles, and whose weight variation CV (%) in the yarn axis direction satisfies the following formula. be.

C ≦ 6.0 (CV (%): CV (%) when the weight of 40 yarns of 25 cm each in the yarn axis direction is continuously measured) Among the above configurations, the core yarn is a flexible filament heating element. It provides durability against repeated folding and abrasion. At this time, it is important that air bubbles exist in the conductive synthetic resin layer covering the core thread.

The presence of air bubbles can dramatically improve the flexibility and repeat durability.

Furthermore, by setting the weight variation C2 in the yarn axis direction to 6.0% or less, the heat generation performance of the product satisfies the same level as products made of conventional thin metal wires. In particular, the weight variation C■ is preferably 4.0% or less. In other words, this is a necessary structural requirement to reduce resistance value variation to a practical level. Continuous measurement at 25 cm intervals is approximately equal to the distance between electrodes in a normal product, and is therefore an effective measurement method that takes into account the size of the product.

Further, a preferred method for manufacturing the filamentous heating element of the present invention has the following configuration. That is, a synthetic resin solution in which conductive fine particles are suspended is attached to the core yarn while being measured using a metering device, and then dried and fixed to form a conductive layer of synthetic resin containing dispersed conductive fine particles on the core yarn. This is a method for producing a filamentous heating element, characterized by forming a filamentous heating element.

As the material for the core thread used in the present invention, synthetic fiber or natural fiber thread is used. This material maintains stable performance for a long period of time at a temperature normally used as a heating element, that is, in the range of 20 to 100° C., and has good adhesion to the conductive layer. Thermoplastic synthetic fibers such as polyamide, polyester, and polyolefin have good non-hygroscopicity and chemical resistance, and are less susceptible to thermal deterioration in the above temperature range. This is preferable because it has a fuse function of blowing. In addition, aromatic polyamide, polybenzimidazole, polyphenylene triazole, polyoxadiazole, polyimide,
It is preferable to use heat-resistant fibers such as thermosetting resin fibers because they have advantages such as increasing the usable temperature range and significantly extending the product life.

The form of the core yarn used in the present invention is preferably one that has good adhesion to the conductive layer and is difficult to peel off. For example, spun yarn and double-structured yarn with short fibers on the surface layer have fuzz on the fiber surface, so they absorb synthetic resin well, and after becoming a thread-like heating element, the conductive layer is attached to the surface of the core yarn and the fuzz. Good tangle adhesion. Moreover, for this purpose, it is more preferable to use substantially non-bundling fibers as the core yarn. In this case, the synthetic resin solution adhering to the core yarn penetrates into the interior of the core yarn due to the non-binding nature of the core yarn, and is dried and fixed as it is. When looking at the cross section of the obtained filamentous heating element, the core yarn is coated with a synthetic resin conductive layer dispersed therein containing conductive fine particles. Because the bulky processed yarn has crimps, the contact area with the resin solution is large, and since there is little fluff on the fiber surface, there are no thick parts or protrusions due to fluff in the yarn axis direction after drying and fixation.
It is particularly preferred also from the viewpoint of good high-order workability. It is more preferable that the single fiber cross section of the fiber used for the core yarn is irregularly shaped.

The core yarn used in the filamentous heating element can be restored to its original form by immersing the filamentous heating element in a synthetic resin solution solvent and treating it with ultrasonic waves for an appropriate period of time.
This method can be used to confirm the core thread.

In addition, the above-mentioned non-focusing fiber is C measured by the following method.
Refers to fibers with an F value of 10 or less.

Using an automatic entanglement degree measurement device manufactured by Rotsiel (Switzerland),
The average moving distance Ω of the fibers was determined, and from this value, the CF value was calculated as 100/Ω, and this CF value was taken as a characteristic value indicating non-focusing property.

It is also preferable to treat the core yarn in advance with a substance that has high affinity for both the conductive fine particle-containing synthetic resin and the core yarn.

The synthetic resin containing dispersed conductive fine particles used in the present invention is not particularly limited as long as it maintains stable performance against temperature and has excellent adhesiveness, bending resistance, abrasion resistance, etc. Examples of resins that can be suitably used include polyurethane resins, acrylic resins, butyral resins, etc.
Particularly flexible ones are preferably selected.

As the conductive fine particles used in the present invention, for example,
Representative examples include carbon particles and metal particles. For example, various types of carbon black can usually be used as carbon particles, and the particle size is as follows:
It is usually 1 to 500 mμ, and preferably 10 to 200 mμ.

Also, graphite can be used as carbon particles,
As graphite, natural graphite, ie, phosphorous graphite, flaky graphite, earthy graphite, or artificial graphite with a size of 1 to 100 μm is preferably used, but phosphorous graphite or flaky graphite with a size of 5 to 50 μm is preferably used. It is preferable to use one with a size of . Furthermore, it is also preferable to use a mixture of carbon black and graphite as the carbon particles. The amount of carbon particles used can be changed as appropriate depending on the desired resistance value. For example, in order to obtain an appropriate resistance value for a heating element, carbon particles are usually used in an amount of 5 to 25% by weight, preferably 7 to 15% by weight in the resin solution.

When a synthetic resin solution in which such a large amount of conductive particles is suspended exhibits structural viscosity, its viscosity increases several times when it is left standing. For this reason, the efficiency of the weighing device described below deteriorates,
Uneven adhesion of the synthetic resin solution occurs. Therefore, as a remedy for this problem, it is preferable to stir the stored synthetic resin solution before measurement so that the synthetic resin solution always remains in a fluid state.

The resistance value of the filamentous heating element of the present invention can be appropriately set depending on the content of carbon particles dispersed in the synthetic resin, the thickness of the laminated layers, etc. For example, in the case of the above formulation, a resistance value of 1 to 100.07 m can be obtained.

The volume resistivity of the resin at this time is approximately 0.01 to 10 Ω·Cll1, depending on the thickness of the thread.

The conductive fine particles and the synthetic resin used in combination with the conductive fine particles can be selected depending on the purpose. It is also possible to reduce the resistance value by further twisting a plurality of filamentous heating elements to make them thicker.

As described above, the present invention has a heat generating layer containing bubbles, and the method thereof includes, for example, a method of mixing a blowing agent such as azobisisobutyronitrile (AIBN) into the suspension, for example, a method of mixing a blowing agent such as azobisisobutyronitrile (AIBN) into the suspension, This can be achieved by coating with a low-boiling point solvent such as and then drying at an ambient temperature higher than the temperature at which the solvent is normally evaporated (drying), so that the solvent vapor is surrounded in a heat-generating layer. . In addition, when foaming is performed using the above-mentioned AIBN, for example, AIBN is mixed in advance with a resin dissolved in dimethylformamide, and then 1
When dried at 50°C, it can be foamed by thermal decomposition.

There is no limit to the amount of air bubbles mixed in; if the amount is small, the flexibility of the filament-like heating element obtained will be at the level of the material, and if it is large, the flexibility will increase, but if it is too large, the mechanical strength and surface smoothness will decrease. . Therefore, it is preferable to determine it empirically or experimentally from the physical properties of each material constituting the filamentous heating element.

The heating layer of the filamentous heating element of the present invention may be a single layer, but may be laminated in multiple layers for the purpose of adjusting the electrical resistance value, smoothing the surface, etc. The number of layers to be laminated is not particularly limited, and the purpose can usually be achieved by laminating about 1 to 3 times. At this time, the concentration of carbon particles dispersed in each heating layer can be changed, for example, in order to improve the smoothness of the surface of the filamentous heating element. For example, the content may be 12% by weight, 10% by weight from the innermost layer, 5% by weight from the outermost layer, etc.

However, it is possible to make the metering deposit in one time, and it is more preferable to make it deposit in one time in terms of equipment.

An example of the filamentous heating element of the present invention and its manufacturing method will be shown below.

Preparation process> Preparation of core thread: Prepare thread without knots.

Preparation of resin suspension of conductive fine particles: A synthetic resin solution is prepared by dissolving and suspending the resin and conductive fine particles in an appropriate solvent. At this time, it is preferable to adjust the viscosity of the synthetic resin solution to 1 to 1000 poise, from the viewpoint of uniform adhesion and the discharge effect of the metering device, and more preferably to 2 to 200 poise. In addition, the solid content of the synthetic resin solution, that is, the weight ratio of components other than the solvent, can be adjusted to 10 to 50% by weight.
It is preferable from the viewpoint of energy consumption in the drying process after adhesion and workability in the suspension/dissolution process. The synthetic resin solution is sealed in a closed container to prevent evaporation of the solvent.

Measuring and Adhesion Step> The synthetic resin solution in which conductive fine particles are suspended is transferred from the sealed container to a measuring device through piping while being stirred. As the metering device, a known metering pump, particularly a gear pump, is suitably used. One of the purposes of the present invention is to reduce the variation in the resistance value of the strands, but by measuring the synthetic resin solution using a measuring device and making it adhere to the core yarn, the amount of adhesion in the axial direction of the strands can be reduced. It is possible to make the variation extremely small.

The measured synthetic resin solution is further transferred through piping to an adhesion device, and is adhered to the core yarn. The adhering device may be a known adhering device, but in order to continuously adhere a constant amount to the core yarn, it is preferable that the distance from the discharge hole of the synthetic resin solution to the point where the core yarn leaves the adhering device is short. For this purpose, it is preferable to use a core thread guide having discharge holes. In addition, since the thread path of the core yarn does not move within the attachment device, there is a thread guide groove such as a 7-shaped groove, a U-shaped groove, or a flat groove, and a discharge hole is bored in the recessed part of the thread guide groove. Particular preference is given to using a thread guide as an attachment device. FIG. 1 shows an example of a top view of the thread guide. The part indicated by the broken line is the transfer path for the measured synthetic resin solution. (a) is 7
CG') is a diagram of a thread guide groove, where (b) is a U-shaped groove, and CG' is a flat groove. As another embodiment of the yarn path guide, one in which a plurality of discharge holes are provided in the yarn path is preferably used. This is shown in FIG. 2, and this guide is advantageous in that the synthetic resin solution can be uniformly applied to the core yarn.

In order to reduce variations in the amount of adhesion, it is preferable to feed the core yarn to the adhesion device at a constant speed. For this purpose, a method is preferably used in which the yarn is fed and taken up by rollers before the adhering device and after the drying step described below. In order to attach the yarn with appropriate tension, it is also preferable to tension the yarn using front and rear rollers.

Drying process> The core yarn drawn out after the measuring and attaching process is transferred to the next drying process. For drying, normal ventilation drying may be used, but in order to improve productivity, various methods commonly used to accelerate drying may be used, such as heating the drying air or heating with an infrared lamp. .

The thus obtained filamentous heating element of the present invention has extremely low variation in the amount of resin deposited in the direction of the filament axis because the synthetic resin solution is uniformly adhered thereto.Also, since the filamentous heating element employs a dry fixation method, the filamentous heating element It has air bubbles inside and is highly flexible.
It has excellent mechanical properties such as bending resistance and abrasion resistance, and has a uniform resistance value per unit length of heating wire, so it can be advantageously used as a heating material for various heating element products.

[Example] Hereinafter, the present invention will be specifically explained with reference to Examples.

In addition, the viscosity, CF value, and durability in the present invention were measured by the following methods.

(Viscosity) A sample was collected in a 500mΩ cylindrical container, and the temperature was 30℃±1.
The viscosity immediately after production is measured using a BM type rotational viscometer (manufactured by Tokyo Keiki) at ℃.

In addition, prior to measurement, the sample is sufficiently stirred using a propeller mixer or a homomixer.

(CF value) Depends on the equipment and method described above, but the details are as follows.N
The number was set to 20.

FIG. 4 is a diagram schematically showing the apparatus. The measurement procedure is (1) Hang the yarn 9 on the movable pulley 10.

(2) The fixed needle 4 is inserted between the fibers constituting the yarn 9 substantially at right angles to the yarn axis direction.

<3> Fix the fixed needle 4 in the state of (2) to the wall.

(4) Attach loads 5 and 6 to both ends of yarn 9.

(5) Attach auxiliary load 7 to either side of load 5 or load 6.

(6) The needle 8 after the thread 9 moves against the fixed needle 4 due to the auxiliary load and the rotation of the movable pulley 10 stops.
The position of is read on a scale on a scale plate 11 fixed to the wall.

(7) Next, switch the auxiliary load 7 to one of the loads (8) As in <6>, read the position of the pointer 8 on the scale on the dial 11 after the movement and rotation have stopped. (Find the value of the total travel distance Ω (Cm) from the scale read at step 6 and complete the measurement.

Here, the loads 5 and 6 are 15 g, and the auxiliary load 7 is (denier number/filament number) 9.

(Number of Cutting and Folding) Measurements were made using an MIT double folding membrane testing machine with an additional load of 0.5 kg and a measurement sample length of 7 cm.

(Number of Cutting Wear) Measurements were made using a yarn-based abrasion tester with a sample length of 45 cm.

Example 1 Polyester type polyurethane resin (Dainichiseika Kagyo Co., Ltd.)
After uniformly dissolving methyl ethyl ketone and dimethyl formamide in a mixed solution (weight ratio 80:20) to a concentration of 16% by weight, carbon black with an average particle size of 40 mμ and graphite with a diameter of 10 μm were added to the carbon suspension solution. The synthetic resin solutions were adjusted to have a concentration of 7% and 5% by weight, respectively. The viscosity of this solution is 25 poise, and the solid content is 26
% by weight. The core yarn is polyester spun at a speed of 300
After spinning at 0 m/min, stretching and twisting was performed by a conventional method, and then the torque was reduced using a hot plate at 190°C.
A tentative twisted yarn with a trilobal cross section of 50 denier 72 filament was obtained. Two of these threads were combined to form a core thread. The CF value was 5.5.

The synthetic resin solution was sealed in a sealed container, stirred, and rotated once at zero.
．． A gear pump with a capacity of 0170C was used to transfer the synthetic resin solution from a closed container to the deposition device. The deposition device is the second
Using a flat groove ceramic yarn guide with each discharge hole diameter of 0.5 m and groove width of 0.81 rm as shown in the figure, the discharge amount was 0.273.
It was attached to the core thread at cc/min. The circumferential speed of the core yarn is 1m/
It was sent to the deposition device at a constant speed by a minute roller. The thread tension just before the applicator was 503.

The solvent was evaporated using a 1.25 m long heating cylinder located 15 cIn after the deposition device. At this time, by setting the temperature of the inner wall of the heating cylinder to 200° C., the evaporation of the solvent was accelerated to promote foaming, and the solidification of the synthetic resin solution was accelerated so that air bubbles were taken into the heat generating layer. In this way, the synthetic resin containing conductive fine particles dispersed therein is fixed to the core yarn, and the circumferential speed is 1m/1.
It was picked up by a roller.

The weight of the obtained yarn was measured continuously in the direction of the yarn axis of 40 yarns of 25 ctn each, and the average value was 0.026 g/
25 cm, standard deviation 0.00070S The CV value obtained by dividing the standard deviation by the average value was 2.7%. Also, when the resistance value of the same thread was measured, the average value was 3.44Ω/25 c
m, standard deviation was 0.123, and CV value was 3°6%.

From this, it was found that the method of the present invention allowed the synthetic resin solution to adhere uniformly in the yarn axis direction, and thus the variation in the resistance value of the filamentous heating element became extremely small. Further, a cross section of this filamentous heating element is shown in FIG. It was confirmed that the synthetic resin 2 had penetrated into the inside of the core yarn 3, that the core yarn 3 was dispersed in the synthetic resin 2, and that the adhesiveness was good, and that the presence of air bubbles 1 was observed.

Table 1 shows the results of measuring the number of cutting breaks and the number of cutting wear using this filamentous heating element and a nichrome wire and a commercially available cord heater for comparison.

(IX bottom margin) Table 1 Table 1 shows that the filamentous heating element of the present invention has outstanding durability compared to conventional metal heater wires.

A fabric-like heating element obtained using the thread-like heating element will be explained with reference to FIG. The fabric-like heating element 15 shown in the figure uses an electrode wire 16 plated with copper wire and a polyester thread 17 for the warp, and the thread-like heating element 14 and a polyester thread 18 for adjusting the amount of heat generated for the weft. A fabric-like heating element was made using an ordinary loom. Both sides of the fabric were coated with polyethylene melt for the purpose of providing a rough insulation coating.

Further, a lead wire 19 through which current is passed was connected to the electrode wire 16 by soldering 20. When electricity was supplied from a Ni-Cd battery to this fabric-like heating element sewn onto the lining of a vest, it was found to be extremely flexible with no local temperature unevenness, and was well received by those who tried it on.

In addition, when this filamentous heating element was immersed in dimethylformamide and subjected to ultrasonic treatment for 30 minutes, the synthetic resin was dissolved.
Although the core thread remained, it was confirmed that the shape of the core thread did not change from before attachment.

Example 2 A filamentous heating element was obtained in the same manner as in Example 1, except that the synthetic resin and the amount of carbon particles were changed, and the viscosity and solid content of the synthetic resin solution were changed. The results are summarized in Table 2. In addition,
The filamentous heating element of Nα3 was re-dried.

Table 2 As is clear from Table 2, even when the viscosity and solid content were changed, good weight uniformity was exhibited.

Example 3 A filamentous heating element was obtained in the same manner as in Example 1 except that the type of core yarn was changed. The results are summarized in Table 3. The core thread denier was approximately 300 denier in each case.

As is clear from Table 3, even when the core yarn type and CF value were changed, weight variations were small and good uniformity was exhibited. Furthermore, in the durability tests, all of the heaters were better than the commercially available cord heaters mentioned above. Of these, CF value 10
Particularly good results were obtained using the following non-bundling fibers as core threads.

Comparative Example 1 Instead of using the metered liquid supply method, the same core yarn as in Example 1 was immersed in a tank in which a synthetic resin solution with the same composition as in Example 1 was stored.
．． A filamentous heating element was obtained by drawing it out from a nozzle having a diameter of 9 m. The weight variation C■ is 6.3%, and the resistance value variation C■ is 10.5%.If the liquid is not metered and dispensed, the weight variation C■ is high, and the uniformity of the resistance value is correspondingly poor, causing filamentous heat generation. It was something that was inappropriate for my body. Furthermore, when a fabric-like heating element was manufactured in the same manner as in Example 1 and a current was applied to the lining of a vest, local temperature unevenness occurred, which was unpopular with those who tried it on.

[Effects of the Invention] In the present invention, the variation in the amount of deposited synthetic resin solution is small and the variation in resistance value is small, so that when it is made into a product, temperature unevenness can be kept so small that it is not felt at all. In addition, it is a thread-like heating element that is highly flexible, has good adhesion between the heating element layer and the core yarn, and is difficult to peel off due to bending, abrasion, etc., and can be used stably for a long period of time, making it possible to knit and weave. With this, we have been able to provide a heating element that can be applied to various uses such as the clothing field, the embedded field, agriculture, fisheries, and civil engineering fields. It can also be applied to all types of vehicles, including cars, trains, aircraft, ships, and space rockets.

[Brief explanation of the drawing]

1 and 2 show an example of a deposition device used in the present invention. FIG. 3 shows an example of the cross section of the filamentous heating element of the present invention. In the figure, 1: air bubbles, 2: synthetic resin, 3: core yarn. FIG. 4 is a diagram schematically showing the CF value measuring device. FIG. 5 is an explanatory diagram of a fabric-like heating element obtained by weaving a thread-like heating element. Patent applicant Toshi Co., Ltd. (2L)
(b) (C)
Figure 1 Figure 2 Engraving Figure 6 Engraving of the drawing Figure 4 Figure 5 Procedural amendment writing method) % formula % (1. Indication of the case 1988 Patent Application No. 100419 2. Name of the invention.

Claims (1)

[Scope of Claims] (1) Consists of a core yarn, a conductive layer of a synthetic resin containing dispersed conductive fine particles covering the core yarn, and air bubbles, and the weight variation CV (%) in the yarn axis direction is expressed by the following formula: A filamentous heating element that satisfies the following. CV≦6.0 (CV (%): CV (%) when the weight of 40 yarns of 25 cm each in the yarn axis direction is continuously measured) (2) The core yarn is a substantially non-bundling fiber. A filamentous heating element according to claim (1). (3) The filamentous heating element according to claim (1), wherein the core yarn is a bulky textured yarn. (4) A synthetic resin solution in which conductive fine particles are suspended is attached to the core yarn while being measured using a measuring device, and then dried and fixed to form a conductive layer of synthetic resin containing dispersed conductive fine particles on the core yarn. 1. A method for producing a filamentous heating element, characterized by forming a filamentous heating element. (5) The method for producing a filamentous heating element according to claim (4), wherein the synthetic resin solution has a viscosity of 1 to 1000 poise and a solid content of 10 to 50% by weight. (6) The method for manufacturing a filamentous heating element according to claim (4), characterized in that the synthetic resin solution is applied to the core yarn through a discharge hole bored in a yarn path guide for the core yarn. (7) The thread guide is characterized by having a thread guide groove such as a V-shaped groove, a U-shaped groove, or a flat groove, and in which a discharge hole is bored in the concave portion of the thread guide groove. A method for producing a filamentous heating element according to claim (6). (8) A method for producing a filamentous heating element according to claim (4), characterized in that substantially non-bundling fibers are used as the core yarn. (9) A method for producing a filamentous heating element according to claim (4), characterized in that a bulky textured yarn is used as the core yarn.