Pptc compositions and devices having low switching temperature and sharp crystallization behavior

Abstract

The present invention provides PPTC compositions and devices having low switching temperatures and sharp crystallization behavior. The PPTC device can include a first electrode and a second electrode disposed opposite the first electrode. The PPTC device can include a PPTC layer disposed between the first electrode and the second electrode, the PPTC layer including a polymer matrix formed of a thermoplastic polyurethane (TPU) material.

Description

[0001] This application is a divisional application of application No. 202010040506.0, filed on January 15, 2020, entitled "PPTC Composition and Device with Low Switching Temperature and Sharp Crystallization Behavior". Technical Field

[0002] The implementation plan covers the field of circuit protection equipment, including fuse equipment.

[0003] Discussions in related fields

[0004] In various applications, polymer positive temperature coefficient (PPTC) devices can be used as overcurrent or overtemperature protection devices, as well as current or temperature sensors. For example, a PPTC device may comprise a polymer material and a conductive filler, which provides a mixture that transitions from a low-resistance state to a high-resistance state due to changes in the polymer material, such as melt transition or glass transition. At such transition temperatures (sometimes called trip temperatures), which can often be in the range of room temperature or higher, the polymer matrix can expand and disrupt the conductive network, making the composite material less conductive. This change in resistance gives PPTC materials similar melting-off characteristics, and the resistance can be reversible when the PPTC material cools back to room temperature.

[0005] For many applications, a sharp change in conductivity as a function of temperature is useful. This sharp electrical change, in turn, depends on a sharp change in the state of the polymer matrix forming the PPTC device. For high-temperature switching applications with trip temperatures above 100°C, many different polymer compositions are known, including polyethylene (PE), polyvinylidene fluoride (PVDF), ethylene tetrafluoroethylene (ETFE)-based polymers, polyaramids, etc. Each of these polymer systems is characterized by being a semi-crystalline or crystalline polymer, exhibiting a relatively well-defined melting point to impart sharp switching behavior. Furthermore, the crystallization behavior of PPTC switches upon cooling is relatively well-defined, allowing for the reuse of this material across multiple switching events. However, the development of low-temperature PPTC devices is hampered by the lack of polymers with well-defined melting and crystallization behaviors below 100°C. Summary of the Invention

[0006] In one embodiment, a PPTC device material is provided. The PPTC device may include a first electrode and a second electrode disposed opposite to the first electrode. The PPTC device may include a PPTC layer disposed between the first electrode and the second electrode, the PPTC layer comprising a polymer matrix formed of a thermoplastic polyurethane (TPU) material.

[0007] In another embodiment, a method of forming a PPTC device may include forming a polymer matrix material based on a polycaprolactone material or a thermoplastic polyurethane (TPU) material. The method may include: dissolving the polymer matrix material in a solvent to form a polymer mixture; mixing a conductive filler into the polymer mixture to form an ink; and applying the ink to a substrate.

[0008] In yet another embodiment, the polymer positive temperature coefficient (PPTC) device may include a strip, a first electrode disposed on the strip, and a second electrode disposed on the strip. The PPTC device may also include a PPTC film disposed on the strip between the first and second electrodes, the PPTC film comprising a polymer matrix material formed of thermoplastic polyurethane (TPU). Attached Figure Description

[0009] Figure 1 A side view of a PPTC device according to an embodiment of the present disclosure is depicted;

[0010] Figure 2 A side cross-sectional view of another PPTC device according to an embodiment of the present disclosure is depicted;

[0011] Figure 3A A side cross-sectional view of another PPTC device according to an embodiment of the present disclosure is depicted;

[0012] Figure 3B Depicting Figure 3A Top view of some variations of PPTC devices;

[0013] Figure 4 A graph depicting the resistance of a reference material and another set of PPTC materials arranged according to an embodiment of the present disclosure versus temperature is shown.

[0014] Figure 5A A graph depicting the resistance of an additional PPTC material arranged according to an embodiment of the present disclosure during heating is shown.

[0015] Figure 5B Depicting Figure 5A The resistance of PPTC material during cooling is plotted against temperature; and

[0016] Figure 6 The process flow according to other embodiments of this disclosure is shown. Detailed Implementation

[0017] Embodiments of the invention will now be described more fully below with reference to the accompanying drawings, in which exemplary embodiments are illustrated. These embodiments should not be construed as limited to those described herein. Rather, they are provided so that this disclosure will be thorough and complete, and will fully convey their scope to those skilled in the art. Throughout the drawings, similar figures refer to similar elements.

[0018] In the following detailed description and / or claims, the terms “on,” “overlapping,” “set on,” and “above” are used. “On,” “overlapping,” “set on,” and “above” can be used to indicate that two or more elements are in direct physical contact with each other. Additionally, the terms “on,” “overlapping,” “set on,” and “above” can indicate that two or more elements are not in direct contact with each other. For example, “above” can mean that one element is on top of another element without contacting each other, and there may be another element or elements between these two elements. Furthermore, the term “and / or” can mean “and,” it can mean “or,” it can mean “exclusive or,” it can mean “one,” it can mean “some, but not all,” it can mean “neither,” and / or it can mean “both,” although the scope of the subject matter protected by the claims is not limited in this respect.

[0019] In various embodiments, novel methods and compositions for forming PPTC devices are provided, including, for example, cryogenic sensors, fuses, and other devices for protecting batteries, circuit boards, FETs, and diodes. According to some embodiments, devices having switching temperatures in the range of 50°C to 70°C are provided. Furthermore, in some embodiments, compositions that produce reset or crystallization temperatures below the switching temperature are provided, including fast-reset materials, as discussed below. According to various embodiments, PPTC materials are provided that exhibit acceptable solubility in a range of solvents, enabling their use in inks and thus facilitating low-cost manufacturing processes, and are compatible with a variety of substrates (flexible, thermosensitive, or rigid) and conductors. Therefore, this embodiment provides the possibility of using materials sensitive to high temperatures or oxygen, as only low-temperature baking can be used to remove solvents and reflow polymers.

[0020] In various implementations, PPTC devices are fabricated based on adhesive materials, which can often be sticky and difficult to mix, and may exhibit partial crystallinity, such as relatively low crystallinity. Therefore, these materials have not previously been considered for PPTC applications.

[0021] According to various embodiments of this disclosure, PPTC devices based on polycaprolactone (PCL) or PCL are provided. PCL exhibits a melting point of approximately 60°C, which enables the fabrication of cryogenic sensors or switches. In some embodiments, PPTC materials and devices based on copolymers derived from PCL are provided. In some embodiments, PCL-based polyurethanes are provided, while in other embodiments, polyurethanes based on other materials such as polyesters are provided.

[0022] In specific embodiments, PPTC materials are formed from thermoplastic polyurethane (TPU), as detailed below. Typically, TPU is a block copolymer based on polyols or long-chain diols, chain extenders or short-chain diols, and diisocyanates. The soft blocks of the copolymer consist of polyols and isocyanates, while the hard blocks consist of chain extenders and isocyanates. This combination imparts both elastomer and toughness properties. Generally, three types of TPU are known: polyester-based, polyether-based, and polycaprolactone-based. In TPU, the hard block portions can represent crystalline regions that undergo a reversible melt transition during which the individual hard block components on different polymer chains dissociate from each other during heating above the melt temperature and recrystallize back into blocks upon cooling below the melt temperature.

[0023] As shown below, in some embodiments, the PPTC material is formed from a polymer comprising structural units such as caprolactone, polycaprolactone, urethane, polyurethane, polyol, glycol, and other materials. This polymer can be a copolymer having caprolactone, urethane, polyol, and other units incorporated into the polymer chain. In specific embodiments, an appropriate level of crosslinking can be imparted to the PPTC polymer material to provide mechanical stability during use.

[0024] By incorporating appropriate amounts of conductive or semiconductive particles into caprolactone-based polymers and / or TPU-based polymers, low-resistance to high-resistance (PPTC anomalous effect) switches can be produced, wherein the switching behavior is related to the melt transition in the polymer, as detailed in the embodiments below.

[0025] Turning Figure 1The image shows a side view of a PPTC device 100 according to an embodiment of the present disclosure. The PPTC device 100 may include a substrate 102, a PPTC device layer 104, and opposing electrodes, shown as electrodes 106. In some embodiments, the substrate 102 may be a flexible substrate, such as a strip. The opposing electrodes may be arranged such that both electrodes are disposed on the substrate 102, and the PPTC device layer 104 is also disposed on the substrate 102. Thus, at a given switching temperature, the PPTC device 100 can be used for current flowing generally parallel to the plane of the substrate 102, such as current flowing along the x-axis, resulting in an increase in resistance. According to an embodiment of the present disclosure, the PPTC device layer 104 may include a caprolactone-based polymer and a conductive filler, or a TPU-based polymer and a conductive filler, and may exhibit a melting temperature in the range of 50°C to 70°C. Thus, the PPTC device 100 can be used in a variety of applications where the switching temperature is useful in the range of 50°C to 70°C, including as a sensor in a battery device.

[0026] Figure 2 Another PPTC device is presented, shown as PPTC device 110. In this case, a pair of electrodes, shown as electrodes 116, are disposed on opposite sides of a PPTC device component 114 with a suitable conductive filler (such as a caprolactone-based or TPU-based polymer material). Thus, PPTC device 110 can act as a cryogenic sensor or switch to increase the resistance of a current traveling along the z-axis.

[0027] Go to Figure 3A The image shows a side view of another PPTC device 120 according to an embodiment of the present disclosure. Figure 3B A top plan view depicting some variations of the PPTC device 120 shows a linear array of PPTC components 124 arranged on a strip substrate.

[0028] The PPTC device 120 may include a strip 122 that acts as a substrate, to which multiple PPTC components are attached. As an example, typically... Figure 1The plurality of PPTC components are arranged in electrical series and attached to strip 122. The PPTC components can be electrically connected to each other using conductors 126 arranged along the surface of strip 122. The switching temperature of PPTC component 124 is in the range of 50°C to 70°C, therefore PPTC device 120 can be used in applications requiring current limitation above approximately 50°C to 70°C. In certain embodiments, PPTC device 120 can be applied to another component or device to be protected, such as a battery or battery pack, where operation will be limited to a design temperature such as 60°C. In some instances, strip 122 can be attached to various different locations on the battery or battery pack to monitor the temperature at each location using one of the PPTC components 124, thereby monitoring the temperature at various different locations.

[0029] Now go to Figure 4 The figure illustrates resistance measurements of a series of PPTC devices arranged according to embodiments of the present disclosure. In this example, the PPTC device is formed from a PPTC component based on polycaprolactone PPTC material, having a switching temperature of approximately 55°C, as shown. In these samples, the autothermal height (ATH), i.e., the logarithm of the resistance ratio between the low-temperature and high-temperature states, is approximately 2.4. Therefore, when the PPTC component undergoes a melt transition at approximately 55°C within a temperature range of approximately 10°C, the resistance increases by more than two orders of magnitude. This resistance change may be sufficient for certain applications such as sensor applications. It is noteworthy that a given degree of crosslinking of the polycaprolactone material may be required in applications where switching is necessary to maintain the mechanical stability and integrity of the PPTC device. In these cases, crosslinking can be achieved, as a method, by exposing the PPTC material to radiation. In this case, these embodiments are not limited.

[0030] In specific embodiments of this disclosure, a crosslinking promoter may be added to caprolactone to promote crosslinking upon exposure to radiation. For example, triallyl cyanurate (TAC) or triallyl isocyanurate (TAIC) may be added in small amounts to caprolactone to promote crosslinking. It is well known that TAIC, such as 1% by weight, can promote crosslinking of polycaprolactone at absorbed doses as low as 20 kGY, where up to 80% crosslinking can be observed. Without TAIC, caprolactone may remain essentially uncrosslinked (0%) at such absorbed doses.

[0031] It is worth noting that in some embodiments, the switching temperature, approaching 60°C, can be adjusted to some extent by using different crosslinking methods (such as catalysts or exposure to radiation beams such as electron beams). In some instances, for example, such as... Figure 4As shown, when the temperature is raised to well above the melting temperature, PPTC materials exhibit a small negative temperature coefficient (NTC effect), but this effect can be eliminated by radiation crosslinking (electron beam or gamma-ray radiation).

[0032] Furthermore, to improve crystallization behavior, polycaprolactone can be synthesized with relatively low molecular weights, such as below 20,000, and in some cases around 14,000. Additionally, polycaprolactone can be synthesized with a narrow molecular weight distribution, such as below 1.5 (characterized by the ratio of weight-average molecular weight to number-average molecular weight), to increase the abruptness of the switching transition.

[0033] While crosslinking of caprolactone can be promoted by TAIC through exposure to radiation doses, crosslinking can also generally reduce the ATH value of pure caprolactone-based PPTC materials. Therefore, according to another embodiment of this disclosure, for applications requiring crosslinking to stabilize caprolactone-based PPTC devices, additional components may be added to caprolactone.

[0034] According to various embodiments of this disclosure, PPTC materials may comprise copolymers based on caprolactone and known thermoplastic urethane monomers. Known thermoplastic urethanes (TPUs) containing PCL are used in many applications such as adhesives. In some embodiments, the copolymer may also comprise polyol-based materials.

[0035] Figure 5A and Figure 5B The resistive behavior of a PPTC material arranged according to an additional embodiment of the present disclosure is shown. In this example, the PPTC material is a low-temperature TPU copolymer comprising a combination of diisocyanate, bio-based polyol (such as castor oil or soybean oil), and chain extender diol (such as 1,4-butanediol).

[0036] More specifically, the PPTC material is formed from a mixture of 30% conductive filler and 70% polymer matrix, wherein the polymer matrix is ​​a commercially available polymer—Pearlbond—manufactured by Lubrizol Advanced Materials, Inc. TM ECO 590. (For example...) Figure 5A As shown, the switching temperature is approximately 55°C, and the ATH is approximately 7, meaning that the resistance increases by approximately seven orders of magnitude when switching to the high-temperature state. Furthermore, as... Figure 5B As shown, the reset to the low resistance state during cooling does not occur until approximately 48°C. The difference between the temperature at which the transition to the high resistance state is completed during heating (61°C) and the temperature at which the transition to the low resistance state begins during cooling (48°C) is greater than 10°C.

[0037] Depending on whether the sample is heated or cooled, this switching hysteresis between high and low resistance provides an additional safety measure, as the high-resistance state is maintained well below the switching temperature, preventing the protected device from returning to "conduction" until crystallinity is re-established at a temperature 12°C below the switching temperature. In other words, in circuits where current is limited by a PPTC device in a high-temperature, high-resistance state, as the PPTC device cools, the high-resistance state remains below the switching temperature, thus limiting the current through the protected device or circuit until the temperature is well below the switching temperature.

[0038] like Figure 5B Furthermore, recrystallization behavior is very pronounced at the recrystallization temperature. Without being limited by any specific theory, it is believed that the combination of the aromatic polyurethane and the bio-based polyol portion of the copolymer matrix can lead to extremely rapid recrystallization of the PPTC material, thereby allowing PPTC devices to respond quickly as they cycle from low temperature to high temperature and back.

[0039] In other embodiments, PCL-based polymeric materials can be used for low-temperature PTC materials, including known adhesive materials such as Pearlbond. TM 100. Pearlbond TM 103. Pearlbond TM 220. Pearlbond TM 223 and Pearlbond TM DIPP 119.

[0040] According to different embodiments of this disclosure, caprolactone-based PPTC materials or TPU-based PPTC materials can be prepared using known preparation methods. In various embodiments, to prepare the PPTC material, the polycaprolactone or TPU material is first dissolved in a variety of solvents, particularly aromatic solvents such as toluene, xylene, mesitylene, anisole, or mixtures thereof. This solution can be mixed with conductive materials such as carbon-based conductors such as carbon black, graphite nanotubes or graphene, tungsten carbide, titanium carbide, nickel, copper, or other metals or semiconductors (doped with ZnO, MnOx, FeOx, NiOx, tin oxide, other sulfides, tellurides, etc.) to form a conductive composite material for use as a PPTC material after drying.

[0041] In some methods according to this embodiment, an appropriate amount of caprolactone-based or TPU-based polymer, used as the polymer matrix, is mixed with an appropriate amount of carbon black to produce a final volume percentage of polymer to carbon black in a 70 / 30 ratio. In a specific example, Pearlbond... TM ECO 590 is first dissolved in xylene to form a polymer solution, while carbon black is added to the polymer solution to produce the final 70 / 30 volume ratio after drying.

[0042] The advantage of these types of polymer materials is that compounding can be easily performed using standard methods (e.g., Brabender, twin-screw extruders, or other methods) to produce polymer sheets that can be used directly (stamped and melted onto interdigitated electrodes) or laminated between metal foils. These materials can be pressed and laminated at relatively low temperatures, such as 120°C, which helps to maintain the properties of the materials present in the formulation with minimal property changes. Furthermore, due to the low melting point of the materials, they can also be mixed or compounded and dispensed via a heated syringe. Additionally, PPTC material sheets can be cut or stamped into any desired shape factor and interconnected to devices using standard methods.

[0043] After formulation, these materials can be used in both PTC and NTC materials and devices. Because the switching temperatures of these devices are typically in the 60°C range, it is possible to process PPTC devices containing materials that may oxidize or react with oxygen or water at high temperatures, thus avoiding the high temperatures required for conventional compounding.

[0044] This low-temperature processing capability further facilitates the use of polymer solutions or inks to be manufactured, which can be applied using any standard technique, such as direct liquid dispensing, screen printing, stencil printing, inkjet printing, etc. Low-viscosity solutions can be prepared using relatively high solvent levels, while thicker materials can be prepared using higher polymer solids levels (i.e., solutions with less solvent). The compatibility of the materials (granules are soluble) has been demonstrated using up to 50% by weight levels of xylene or anisole. This compatibility allows for use in a variety of low-cost applications.

[0045] Furthermore, these polymeric materials of this embodiment are prone to precipitation upon exposure to other common solvents such as isopropanol or ethanol. Therefore, in some embodiments, a solution of caprolactone-based or TPU-based polymers, conductive additives, and solvents can be prepared as a bulk material or dispensed onto a suitable substrate, device, or support, followed by exposure to an alcohol additive such as isopropanol vapor. In this way, the final PPTC material can be rapidly deposited into a bulk material or deposited in situ onto a substrate without requiring a baking or drying process after dispensing the PPTC material.

[0046] Furthermore, in implementations requiring mechanical integrity and reusability of PPTC switches, as described above, TPU copolymers exhibit several advantages over pure polycaprolactone PPTC devices. The copolymers described above form high-quality polymer sheets, while homopolymer PCL sheets are relatively brittle and prone to breakage during pressing, making them difficult to process using stamping or pressing techniques. Moreover, copolymers formed from diisocyanates, polyols, and glycols differ from PCL homopolymers in that they exhibit superior performance when compounded without Prorad additives (materials that promote radiation crosslinking).

[0047] In other embodiments of this disclosure, any suitable ester-containing monomer may be introduced to provide functions other than polycaprolactone or polyester, as well as other urethane functions, to customize the properties of these materials.

[0048] Turn now Figure 6 An exemplary process flow 300 is illustrated. At block 302, a polymer material is formed based on caprolactone or TPU. In some instances, the polymer material may be a homopolymer of polycaprolactone. In other examples, the polymer material may be a copolymer, including a low-temperature thermoplastic urethane forming material. In a particular embodiment, the polyol may be a bio-based polyol, including castor oil or soybean oil.

[0049] In box 304, the polymer material is dissolved in a solvent. Suitable solvents include aromatic solvents such as toluene, xylene, mesitylene, anisole, or mixtures thereof. In one method, the polymer material is provided in powder form or as granules, which is mixed with the solvent using a rapid mixer, by rolling, or by ultrasonic treatment. In some variations, mixing is carried out without heating, while in other variations, the mixture is heated to temperatures up to 80°C to accelerate the dissolution rate of the polymer material.

[0050] In box 306, the conductive filler is mixed with the polymer mixture to form the ink. In various embodiments, the conductive filler may include carbon materials, metals, conductive ceramic materials, conductive oxides, semiconductors, etc. The amount of conductive filler can be selected to produce a total volume fraction of 5% to 80% conductive filler after solvent removal from the ink. Typically, the polymer material is first dissolved in a solvent to ensure homogeneity before box 306, although in some embodiments, the conductive filler may be added to the polymer before box 304.

[0051] In some variations, the mixing of the conductive filler is accomplished through known processes such as rapid mixing, ultrasonic treatment, rolling, and heating. Rapid mixing using ceramic blocks facilitates rapid sample preparation and provides significant agitation. Ultrasonic treatment offers an excellent option for extruding air bubbles from the polymer / conductive filler solution, ensuring the final PPTC material is bubble-free, as bubbles would otherwise degrade electrical properties.

[0052] In block 308, ink is applied to a substrate. In some embodiments, ink may be dispensed onto a substrate comprising circuitry disposed on a strip or other substrate, the circuitry including, for example, opposing electrodes. Thus, ink may be dispensed onto areas on the substrate containing opposing electrodes, such as interdigitated electrodes filling gaps between electrodes.

[0053] In box 310, the ink is dried after being dispensed onto the substrate. In some embodiments, as the ink is deposited as a layer, it is heated in an oven for a duration of 10-30 minutes to dry into a solid film. In some variations, ink drying can be achieved by first allowing the solvent within the ink to evaporate at ambient temperature before performing the second stage of heating. This two-stage method helps to avoid bubble formation. In another variation, ink drying can be performed, for example, by performing a low-temperature baking at, for example, 60°C before a final baking at 120°C.

[0054] While embodiments of the invention have been disclosed with reference to certain embodiments, numerous modifications, alterations, and variations of said embodiments are possible without departing from the spirit and scope of this disclosure as defined in the appended claims. Therefore, embodiments of the invention are not limited to the described embodiments and may have the full scope defined by the language of the following claims and their equivalents.

Claims

1. A method for forming a PPTC device, comprising: Polymer matrix materials are formed based on polycaprolactone or thermoplastic polyurethane (TPU) materials; The polymer matrix material is dissolved in a solvent to form a polymer mixture; A conductive filler is mixed into the polymer mixture to form an ink; Apply the ink to the substrate; The substrate is kept at room temperature for at least 30 minutes to allow a first portion of the solvent to evaporate. as well as The substrate is heated at a temperature of 60°C to 150°C, thereby causing a second portion of the solvent to evaporate.

2. The method according to claim 1, wherein the application comprises: Perform direct liquid dispensing, screen printing, stencil printing, or inkjet printing.

3. The method according to claim 1, wherein, The solvent includes up to 80% by weight xylene, mesitylene, or anisole.

4. The method of claim 1, further comprising exposing the ink to an alcohol additive.

5. The method according to claim 1, further comprising: Prior to mixing the conductive filler, use a rapid mixer, rolling mixer, or ultrasonic tool at room temperature and 80°C. ο The polymer matrix material is mixed with the solvent at a mixing temperature between C and [other temperatures].

6. The method of claim 1, further comprising exposing the ink to a radiation beam, wherein, A portion of the polymer matrix material is cross-linked.

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