NTC heating wire and preparation method thereof

Abstract

The application provides an NTC heating wire and a preparation method thereof. The NTC heating wire comprises, from inside to outside, a central tensile core, a heating core wire layer, a first insulation isolation layer, a main sensing layer, a second insulation isolation layer, a sentinel sensing layer and an outer insulation protection layer. The heating core wire layer is a metal resistance wire spirally wound on the outer surface of the central tensile core. The main sensing layer and the sentinel sensing layer each comprise a thermoplastic elastomer matrix and conductive fillers dispersed therein, and each is internally provided with at least two signal electrodes. The aging rate of the sentinel sensing layer material is greater than that of the main sensing layer material. The NTC heating wire realizes self-monitoring of the aging state through an innovative double-channel sensing structure and material design. The core lies in that two sensing layers with similar characteristics but different aging rates are arranged: the main sensing layer is used for conventional temperature monitoring, and the sentinel sensing layer is designed as a “sacrificial layer” whose aging process is significantly faster than that of the main sensing layer.

Description

Technical Field

[0001] This application relates to the field of electric heating element technology, specifically to an NTC heating wire and its preparation method. Background Technology

[0002] In existing technologies, flexible electric heating products such as electric blankets and electric clothing typically use heating wires with built-in NTC (negative temperature coefficient) sensors for temperature monitoring and control. The resistance of the NTC sensor decreases systematically as the temperature rises, thus achieving closed-loop control of the heating temperature and ensuring safe use. These heating wires usually consist of a metal resistance wire for heating and an NTC material layer for temperature measurement, with a tendency towards integration and composite structures to improve product flexibility and reliability. With technological advancements, multi-layer co-extruded composite structures have emerged, integrating heating, sensing, and insulation layers into a single unit, effectively reducing the wire's outer diameter and improving production efficiency and user experience.

[0003] However, existing NTC heating wires have a potential safety hazard: the NTC sensing material, the core of temperature sensing, undergoes irreversible chemical aging over time. Under prolonged heat and oxygen conditions, the polymer matrix material gradually degrades, causing a drift in its resistance-temperature characteristics, specifically manifested as changes in the B constant and an abnormal increase in resistance. This drift can lead to temperature measurement errors, potentially causing the control system to misjudge the actual temperature, resulting in excessively high or low heating temperatures. When the deviation accumulates to a dangerous level, the product already poses a safety risk, but users and the control system remain unaware that the sensing function has failed, lacking an effective failure warning mechanism. Summary of the Invention

[0004] To address the technical problem that aging of the sensing material in existing NTC heating wires leads to the failure of temperature monitoring and the inability to provide early warnings, this application provides an NTC heating wire and its preparation method.

[0005] The first aspect of this application provides an NTC heating wire, comprising a central tensile core, a heating core wire layer, a first insulating layer, a main sensing layer, a second insulating layer, a sentinel sensing layer, and an outer insulating protective layer arranged coaxially from the inside to the outside; the heating core wire layer is a metal resistance wire spirally wound on the outer surface of the central tensile core; both the main sensing layer and the sentinel sensing layer contain a thermoplastic elastomer matrix and conductive fillers dispersed therein, and each has at least two built-in signal electrodes; the aging rate of the sentinel sensing layer material is greater than the aging rate of the main sensing layer material.

[0006] This NTC heating wire achieves self-monitoring of aging status through an innovative dual-channel sensing structure and material design. Its core lies in the use of two sensing layers with similar characteristics but different aging rates: a main sensing layer for routine temperature monitoring, and a sentinel sensing layer designed as a "sacrificial layer" whose aging process is significantly faster than the main sensing layer. During operation, the control system simultaneously monitors the resistance values ​​R_main and R_sentinel of both sensing layers and calculates their ratio K. Since both sensing layers operate under the same temperature and mechanical environment, the K value remains relatively stable during normal operation. However, over time, the sentinel sensing layer, due to its faster aging rate, experiences a much greater increase in resistance than the main sensing layer, resulting in a continuous and significant unidirectional change in the K value. By setting a warning threshold for K value changes, the system can detect material aging trends and issue warnings before the temperature measurement accuracy of the main sensing layer exceeds the allowable range, indicating that the product's lifespan is nearing its end and maintenance or replacement is required, thus effectively preventing safety accidents caused by sensor failure. The coaxial structure ensures that the two sensing layers sense the same average temperature, eliminating measurement interference caused by uneven heating or asymmetrical heat dissipation conditions, and guaranteeing the accuracy and reliability of aging monitoring.

[0007] Furthermore, the aging rate of the sentinel sensing layer material is greater than that of the main sensing layer material, which is achieved through at least one of the following methods: the molecular weight of the matrix of the sentinel sensing layer material is lower than that of the matrix of the main sensing layer material; the mass fraction of the antioxidant in the sentinel sensing layer material is lower than that of the antioxidant in the main sensing layer material; and the sentinel sensing layer material also contains an aging-promoting agent. These methods precisely control the aging kinetics of the sentinel sensing layer by altering the stability of the polymer chain, its free radical scavenging ability, and its catalytic oxidation pathway, making it a reliable "aging indicator." Specifically, the molecular weight of the matrix of the sentinel sensing layer material can be more than 20% lower than that of the matrix of the main sensing layer material; the main sensing layer material may contain an antioxidant, and the content of the antioxidant is 0.5-2 wt%, while the sentinel sensing layer material does not contain an antioxidant; the aging-promoting agent may be a metal stearate, such as cobalt stearate, and its content is 0.01-0.1 wt%.

[0008] Furthermore, the B-constant (25 / 50) of the main sensing layer is 2000-9000K; the B-constant (25 / 50) of the sentinel sensing layer is 2000-9000K, and the absolute value of the difference between the B-constant (25 / 50) of the main sensing layer and the sentinel sensing layer is no greater than 200K. Designing the B-constants of the two sensing layers to be within a similar range (200K) ensures that the K-value fluctuation caused by temperature changes is very small within the operating temperature range, thereby highlighting the K-value change caused by aging, improving monitoring sensitivity and accuracy, and avoiding false alarms.

[0009] Furthermore, the thermoplastic elastomer matrix is ​​a thermoplastic polyurethane, such as polyester-type TPU or polyether-type TPU, which has good elasticity, wear resistance, and processability, making it suitable as the sensing layer matrix of the flexible heating wire. The conductive filler is a thermosensitive organic-inorganic composite powder, the raw materials of which include, by weight percentage: 0-10% metal powder, 40-90% metal oxide powder, 0.1-20% conductive polymer powder, 0.5-10% coupling agent, and 0.2-10% additives. Using this composite powder as a conductive filler, the coupling agent and conductive polymer can greatly improve the interfacial compatibility between inorganic particles and the TPU matrix, reduce the risk of material embrittlement caused by high filler content, make the resistance of the heating wire more stable when bent, and at the same time give the material excellent NTC (negative temperature coefficient) thermosensitive characteristics. Specifically, based on the total mass of the main sensing layer and the sentinel sensing layer, the content of the thermosensitive organic-inorganic composite powder can be selected from any value among 25%, 28%, 30%, 32%, and 35%.

[0010] Furthermore, the central tensile core is made of aramid fiber or polyester fiber, providing the heating wire with excellent mechanical strength and tensile properties, preventing breakage due to external forces during use. The metal resistance wire is made of iron-chromium-aluminum alloy wire or nickel-chromium alloy wire, possessing stable resistivity and excellent high-temperature resistance and oxidation resistance, ensuring long-term stability of the heating function.

[0011] The second aspect of this application provides a method for preparing an NTC heating wire, comprising the following steps: preparing the main sensing layer material and the sentinel sensing layer material; using a multi-layer coaxial co-extrusion process, a pre-prepared inner core assembly consisting of a central tensile core and a metal resistance wire spirally wound on its outer surface is used as the wire feeding substrate, and simultaneously fed into a co-extrusion die with the first insulating layer material, the main sensing layer material and its corresponding main sensing layer signal electrode, the second insulating layer material, the sentinel sensing layer material and its corresponding sentinel sensing layer signal electrode, and the outer insulating protective layer material for co-extrusion molding to obtain the NTC heating wire. This preparation method, through multi-layer coaxial co-extrusion technology, can construct a precise coaxial multi-layer structure in one molding process, resulting in high production efficiency, good product consistency, strong interlayer bonding, and stable achievement of the technical effects of this invention.

[0012] The present invention has the following beneficial effects:

[0013] This invention transforms the unmeasurable performance drift of the sensing material into a significant change in the measurable resistance ratio K by setting a main sensing layer and a sentinel sensing layer with differentiated aging rates. This enables self-monitoring and early warning of the aging state of the heating wire, greatly improving the long-term safety of electric heating products.

[0014] This invention employs a coaxial multilayer structure, which tightly couples the two sensing layers in space, allowing them to sense the same ambient temperature and mechanical stress. This effectively eliminates the influence of thermal asymmetry and mechanical stress interference on aging judgment, ensuring the accuracy and reliability of monitoring results.

[0015] This invention optimizes the type and content of conductive fillers, so that the sensing layer meets electrical performance requirements while exhibiting a weak piezoresistive effect. This ensures the stability of the resistance ratio K under complex working conditions such as stretching and bending, and further improves the robustness of aging monitoring.

[0016] The preparation method of the present invention adopts a multi-layer coaxial co-extrusion process, which realizes the integrated molding of complex structures. The process is simple, the production efficiency is high, it is easy to scale up production, and the product has uniform thickness of each layer, good concentricity, and stable performance. Detailed Implementation

[0017] To facilitate understanding of this application, a more complete description will be provided below. This application can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of the disclosure of this application.

[0018] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of the application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified. In the description of this application, "several" means at least one, such as one, two, etc., unless otherwise explicitly specified.

[0019] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0020] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0021] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0022] Unless otherwise specified, the percentage content mentioned in this application refers to mass percentage for solid-liquid mixtures and solid-phase-solid mixtures, and volume percentage for liquid-phase-liquid mixtures.

[0023] Unless otherwise specified, all percentage concentrations mentioned in this application refer to the final concentration. The final concentration refers to the proportion of the added component in the system after the addition of that component.

[0024] Unless otherwise specified, the temperature parameters in this application may be either constant temperature processing or processing within a certain temperature range. The constant temperature processing allows for temperature fluctuations within the precision range controlled by the instrument.

[0025] The term "particle" as used in this application, or a substance with a defined particle size distribution, is not necessarily spherical in shape; it may be irregular and can be either primary or secondary particles. The particle size of irregular particles is calculated as the average of their maximum and minimum diameters.

[0026] In some preferred embodiments, the central tensile core may be selected from any one of aramid fiber, high-strength polyester fiber, glass fiber or ultra-high molecular weight polyethylene fiber; the metal resistance wire may be selected from any one of iron-chromium-aluminum alloy wire, nickel-chromium alloy wire or constantan wire.

[0027] In some preferred embodiments, both the main sensing layer and the sentinel sensing layer contain two signal electrodes (e.g., tin-plated copper wires) extending parallel to the axial direction of the NTC heating wire. On the cross-section of the heating wire, the two signal electrodes within the same sensing layer are symmetrically distributed at 180 degrees about the axis of the heating wire, and are completely encapsulated within their respective sensing layer materials. This parallel symmetrical arrangement ensures a constant and uniform spacing between the two signal electrodes, thereby guaranteeing the stability and consistency of the resistance values ​​(R_main and R_sentinel) measured along the length of the sensing layer, and avoiding temperature misjudgments or aging false alarms caused by fluctuations in electrode spacing.

[0028] In some preferred embodiments, the thermoplastic elastomer matrix may be selected from any one of polyester-type thermoplastic polyurethane, polyether-type thermoplastic polyurethane, styrene-based thermoplastic elastomer, or polyolefin-based thermoplastic elastomer.

[0029] In some preferred embodiments, the aging-promoting agent can be selected from any one of organometallic salts such as cobalt stearate, cobalt naphthenate, or cobalt isooctanoate, and its content can be arbitrarily selected between 0.01, 0.03, 0.05, 0.08, or 0.1 wt%, as long as it can significantly accelerate the aging of the sentinel sensor layer.

[0030] In some preferred embodiments, the antioxidant may be selected from one or more of hindered phenolic antioxidants (such as 1010, 1076) and phosphite antioxidants (such as 168), and its total content in the main sensing layer may be arbitrarily selected between 0.5, 1.0, 1.5 or 2.0 wt%.

[0031] In some preferred embodiments, the traction speed of coaxial co-extrusion molding can be arbitrarily selected between 3, 5, 8, 10 or 15 m / min, and the die temperature can be arbitrarily selected between 185, 190, 195, 200 or 205°C, as long as good melt flow and composite of each layer of material can be guaranteed.

[0032] Example 1: This example provides an NTC heating wire and its preparation method.

[0033] The preparation method of the thermosensitive organic-inorganic composite powder is as follows: In a reactor, add 0.2 parts of silver powder, 80 parts of copper oxide, 30 parts of zinc oxide, and 20 parts of vanadium oxide, stir and heat to 50°C, add 7 parts of vinyltriethoxysilane and 0.5 parts of p-nonylphenoxypropylsulfonic acid triethanolamine, continue to heat to 80°C and ball mill and disperse for 1 hour, then add 8 parts of polypyrrole and disperse at low speed for 30 minutes to obtain the thermosensitive organic-inorganic composite powder for later use.

[0034] The main sensing layer material, by mass, includes: 62.5 parts of thermoplastic polyurethane A (polyester type, Shore hardness 90A, number average molecular weight approximately 85,000); 30 parts of thermosensitive organic-inorganic composite powder; 0.5 parts of antioxidant 1010; 0.4 parts of antioxidant 168; 0.3 parts of zinc stearate; 0.3 parts of ethylene bis-stearamide; 6 parts of maleic anhydride grafted polymer compatibilizer; and a B constant (25 / 50) of 7421.

[0035] The sentinel sensor layer material, by mass parts, includes: 45 parts of thermoplastic polyurethane B (polyester type, Shore hardness 85A, number average molecular weight approximately 48,000); 17.5 parts of thermoplastic polyurethane C (polyether type, Shore hardness 80A, number average molecular weight approximately 55,000); 30 parts of thermosensitive organic-inorganic composite powder; 0.05 parts of cobalt stearate (aging aid); 0.25 parts of zinc stearate; 0.2 parts of ethylene bis-stearamide; 7 parts of maleic anhydride grafted polymer compatibilizer, B constant (25 / 50) 7267; the first, second, and outer insulation layers are all made of thermoplastic polyurethane (polyester type, Shore hardness 85A); the signal electrode is tin-plated copper wire (copper core diameter 0.08mm).

[0036] The preparation method of NTC heating wire is as follows: Weigh the raw materials for the main sensing layer and the sentinel sensing layer according to the above formula, and premix them separately in a high-speed mixer for 10 minutes. Then, melt-blend, extrude, cool, and pelletize the premixed materials in a twin-screw extruder to obtain granules for the main sensing layer and the sentinel sensing layer. The extruder temperature for the main sensing layer is set to 155-192℃, and the extruder temperature for the sentinel sensing layer is set to 150-187℃. Using a five-layer coaxial co-extrusion production line, the pre-prepared inner core assembly, consisting of a central tensile core and iron-chromium-aluminum alloy wire spirally wound on its outer surface, is used as the wire feeding substrate. This is combined with four signal electrodes and five types of granules (first insulation layer, main sensing layer, second insulation layer, sentinel sensing layer, outer insulation layer, and outer insulation layer). The layers are fed into the corresponding flow channels of the co-extrusion die. Through the design of the die flow channels, two signal electrodes are guided to be extruded parallel to each other along the axial direction within the main sensing layer and are symmetrically distributed at a 180-degree center on the cross-section. Simultaneously, two other signal electrodes are guided to be extruded parallel to each other along the axial direction within the sentinel sensing layer and are also symmetrically distributed at a 180-degree center on the cross-section. The temperatures of the five extruders are set at 150-195℃, the die temperature at 190℃, the core wire tension is controlled at 2.5N, and the signal electrode tension at 0.12N. The screw speed of each extruder is adjusted to ensure that the thickness of each layer reaches the design value, and the traction speed is controlled at 8m / min. After the extrudate is cooled and shaped in a cooling water tank, it is wound up by a take-up machine to obtain the finished NTC heating wire.

[0037] Example 2: It is basically the same as Example 1, except that the aging-promoting agent cobalt stearate is not added to the sentinel sensing layer material, and the amount of thermoplastic polyurethane B is increased to 45.05 parts. The aging is accelerated only by the low molecular weight of the matrix and the absence of antioxidants.

[0038] Example 3: Basically the same as Example 1, except that the sentinel sensing layer matrix is ​​a single polyester TPU (number average molecular weight of about 50,000), the amount of which is 62.5 parts, and it is not blended with polyether TPU.

[0039] Example 4: Basically the same as Example 1, except that the total antioxidant content in the main sensing layer is 1.5wt%, antioxidant 1010 is 0.8 parts, antioxidant 168 is 0.7 parts, and the amount of thermoplastic polyurethane A is reduced to 61.9 parts.

[0040] Example 5: Basically the same as Example 1, except that the central tensile core is high-strength polyester fiber (200dtex).

[0041] Example 6: Basically the same as Example 1, except that the metal resistance wire is a nickel-chromium alloy wire (0.08mm in diameter).

[0042] Comparative Example 1: This comparative example does not include the sentinel sensing layer.

[0043] This comparative example provides a single-sensor layer heating wire with a structure similar to that of Example 1, but it only includes a central tensile core, a heating core wire layer, a first insulating isolation layer, a main sensing layer (with the same formulation as Example 1), and an outer insulating protective layer. Its preparation method is also correspondingly simplified, using only a three-layer co-extrusion process.

[0044] Comparative Example 2: In this comparative example, the aging rates of the sentinel sensing layer and the main sensing layer are basically the same.

[0045] This comparative example is basically the same as Example 1, except that the formulation of the sentinel sensing layer material is exactly the same as that of the main sensing layer material, that is, the molecular weight of the matrix, the type and content of antioxidants, and the type and content of conductive fillers are all the same. Therefore, its aging rate is basically the same as that of the main sensing layer.

[0046] Comparative Example 3: In this comparative example, the aging rate of the sentinel sensing layer is slower than that of the main sensing layer.

[0047] This comparative example is basically the same as Example 1, except that the same antioxidant as the main sensing layer is added to the sentinel sensing layer material, and no aging-promoting agent is added. At the same time, the amount of thermoplastic polyurethane B is reduced to 44.15 parts.

[0048] Accelerated aging test: The heating wire samples (10 meters long) of each embodiment and comparative example were placed in an 85°C constant temperature oven for accelerated aging. Every 72 hours, they were taken out and kept at 25°C for 1 hour to measure the resistance of the main sensing layer R_main and the resistance of the sentinel layer R_sentinel (comparative example 1 had no R_sentinel). The resistance ratio K = R_sentinel / R_main was calculated.

[0049] Initial performance test: Test the initial resistance and initial K value of each sample at 25℃, and perform mechanical bending test (K value change rate after 10,000 bends).

[0050] Table 1 Test results of the examples and comparative examples

[0051]

[0052] As shown in Table 1, Examples 1-6 all exhibited significant changes in the K value. After aging for 432 hours, the relative change rate of the K value exceeded 30%, reaching a maximum of 55.1%, far exceeding the settable warning threshold (e.g., 20%). This indicates that the present invention, by setting a fast-aging sentinel sensing layer, can effectively and reliably monitor the aging state. Simultaneously, after 10,000 bends, the K value change rate of all examples was less than 1.2%, indicating that the heating wire of the present invention has good mechanical stability, and mechanical stress has minimal interference with aging monitoring. This is because the NTC heating wire achieves self-monitoring of the aging state through an innovative dual-channel sensing structure and material design. Its core lies in setting two sensing layers with similar characteristics but different aging rates: the main sensing layer is used for conventional temperature monitoring, while the sentinel sensing layer is designed as a "sacrificial layer," whose aging process is significantly faster than that of the main sensing layer. When the heating wire is working, the control system simultaneously monitors the resistance values ​​R_main and R_sentinel of the two sensing layers and calculates their ratio K. Because both sensing layers operate under the same temperature and mechanical environment, the K-value remains relatively stable during normal operation. However, over time, the sentinel sensing layer ages faster, resulting in a much greater increase in resistance than the main sensing layer, causing a continuous and significant unidirectional change in the K-value. By setting a warning threshold for K-value changes, the system can detect material aging trends and issue an early warning before the temperature measurement accuracy of the main sensing layer exceeds the allowable range, indicating that the product's lifespan is nearing its end and maintenance or replacement is necessary. This effectively prevents safety accidents caused by sensor failure. The coaxial structure ensures that both sensing layers sense the same average temperature, eliminating measurement interference caused by uneven heating or asymmetrical heat dissipation, and guaranteeing the accuracy and reliability of aging monitoring.

[0053] Comparing Example 1 and Example 2, Example 1, due to the addition of an aging-promoting agent, had a higher K-value change rate (46.9%) than Example 2 (31.1%), which relied solely on low molecular weight and no antioxidants to accelerate aging. This indicates that the aging-promoting agent is an effective means to further amplify aging signals and improve early warning sensitivity.

[0054] Comparing Examples 1 and 3, Example 1, which uses a polyester / polyether blend matrix, showed a higher rate of change in K value (46.9%) than Example 3, which uses only a single polyester matrix (30.3%). This indicates that by introducing a more easily degradable polyether component, the aging of the sentinel layer can be accelerated more effectively.

[0055] Comparing Examples 1 and 4, the rate of change of K value (55.1%) in Example 4 further increased after increasing the antioxidant content of the main sensing layer. This indicates that by enhancing the stability of the main sensing layer and widening the "aging gap" between it and the sentinel layer, the change of K value can be more significant, thereby providing a wider dynamic monitoring range and higher early warning accuracy.

[0056] Comparative Example 1, lacking a sentinel sensing layer, cannot monitor the K-value. Although the resistance of its main sensing layer changes after bending and aging, it cannot distinguish whether the change is due to temperature, mechanical stress, or aging, thus lacking aging self-monitoring capability. Comparative Example 2's sentinel layer and main layer age at similar rates, resulting in a slow change in the K-value (17.6%). If a warning threshold of 20% is set, it may not trigger a timely warning, or the warning time may be too late, negating the purpose of early warning. Comparative Example 3, by adding sufficient antioxidants to the sentinel layer and removing aging-promoting agents, significantly slows its aging rate, bringing it close to that of the main layer, resulting in an extremely weak change in the K-value (only 7.6%). If a warning threshold of 20% is set, it will completely fail to provide early warning.

[0057] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0058] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. An NTC heating wire, characterized in that, It includes a central tensile core, a heating core wire layer, a first insulating isolation layer, a main sensing layer, a second insulating isolation layer, a sentinel sensing layer, and an outer insulating protective layer, arranged coaxially from the inside out; The heating core wire layer is a metal resistance wire spirally wound on the outer surface of the central tensile core; Both the main sensing layer and the sentinel sensing layer contain a thermoplastic elastomer matrix and conductive fillers dispersed therein, and each has at least two built-in signal electrodes, which are used to measure the resistance of the sensing layer. The resistance of the main sensing layer is R_main, and the resistance of the sentinel sensing layer is R_sentinel. The resistance ratio K is calculated as K = R_sentinel / R_main. The aging rate of the sentinel sensing layer material is greater than that of the main sensing layer material. The increase in resistance of the sentinel sensing layer material is much greater than that of the main sensing layer material, resulting in a continuous and significant unidirectional change in the value of K.

2. The NTC heating wire according to claim 1, characterized in that, The aging rate of the sentinel sensing layer material is greater than that of the main sensing layer material, which is achieved through at least one of the following methods: The molecular weight of the matrix of the sentinel sensing layer material is lower than that of the matrix of the main sensing layer material; The mass fraction of antioxidant in the sentinel sensing layer material is lower than the mass fraction of antioxidant in the main sensing layer material; The sentinel sensing layer material also contains aging-promoting agents.

3. The NTC heating wire according to claim 2, characterized in that, The molecular weight of the base material of the sentinel sensing layer is more than 20% lower than that of the base material of the main sensing layer.

4. The NTC heating wire according to claim 2, characterized in that, The main sensing layer material contains an antioxidant, and the content of the antioxidant is 0.5-2 wt%; the sentinel sensing layer material does not contain an antioxidant.

5. The NTC heating wire according to claim 2, characterized in that, The aging aid is a metal stearate, with a content of 0.01-0.1 wt%.

6. The NTC heating wire according to claim 1, characterized in that, The B constant (25 / 50) of the main sensing layer is 2000-9000K; the B constant (25 / 50) of the sentinel sensing layer is 2000-9000K, and the absolute value of the difference between the B constant (25 / 50) of the main sensing layer and the sentinel sensing layer is no greater than 200K.

7. The NTC heating wire according to claim 1, characterized in that, The thermoplastic elastomer matrix is ​​thermoplastic polyurethane, and the conductive filler is a thermosensitive organic-inorganic composite powder, the raw materials of which include, by weight percentage: 0-10% metal powder, 40-90% metal oxide powder, 0.1-20% conductive polymer powder, 0.5-10% coupling agent, and 0.2-10% additives.

8. The NTC heating wire according to claim 7, characterized in that, The content of the thermosensitive organic-inorganic composite powder is 25-35 wt% based on the total mass of the main sensing layer; the content of the thermosensitive organic-inorganic composite powder is 25-35 wt% based on the total mass of the sentinel sensing layer.

9. The NTC heating wire according to claim 1, characterized in that, The central tensile core is made of aramid fiber or polyester fiber, and the metal resistance wire is made of iron-chromium-aluminum alloy wire or nickel-chromium alloy wire.

10. A method for preparing an NTC heating wire as described in any one of claims 1-9, characterized in that, The process includes the following steps: preparing the main sensing layer material and the sentinel sensing layer material; using a multi-layer coaxial co-extrusion process, a pre-prepared inner core assembly consisting of a central tensile core and a metal resistance wire spirally wound on its outer surface is used as the wire feeding substrate, and simultaneously fed into a co-extrusion die with the first insulating isolation layer material, the main sensing layer material and the corresponding main sensing layer signal electrode, the second insulating isolation layer material, the sentinel sensing layer material and the corresponding sentinel sensing layer signal electrode, and the outer insulating protective layer material for co-extrusion molding to obtain the NTC heating wire.

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