PPTC material using a single-stage high-temperature application process

Abstract

This invention provides a method for producing polymer positive temperature coefficient (PPTC) materials that are suitable for flexible applications and have a low environmental impact. [Solution] A method for preparing a PPTC compound and conformally applying the compound to a substrate, comprising the steps of: preparing and mixing raw materials to form a PPTC mixture; processing the PPTC mixture in a polymer compounding apparatus; heating the PPTC mixture and performing a beam irradiation step on the polymer compound to be extruded as a PPTC compound to crosslink the polymer chains within the PPTC compound; and applying the PPTC compound to the substrate using a high-temperature application step. Here, the PPTC compound is heated and conformally applied to the surface of the substrate.

Description

Technical Field

[0001] The embodiments relate to the field of circuit protection devices including fuse elements.

Background Art

[0002] Polymer positive temperature coefficient (PPTC) devices may be used as overcurrent protection devices, overheat protection devices, current sensors, or temperature sensors in various applications. In overcurrent protection or overheat protection applications, a PPTC device may be regarded as a resettable fuse and may be designed to exhibit low resistance when operating under certain conditions such as low current. The resistance of a PPTC device may change due to direct heating caused by a temperature rise in the environment of the PPTC device or through resistive heating generated by the current flowing through the PPTC device. For example, a PPTC device may include a composite PPTC material formed of a polymer material and a conductive filler, and this PPTC material may transition from a low-resistance state to a high-resistance state due to a thermally induced change in the polymer material, such as a melting transition or a glass transition. At a transition temperature, sometimes called the "trip temperature" (the trip temperature can range from room temperature to a temperature well above room temperature), the polymer material expands and the conductive network of conductive filler particles within the PPTC material is disrupted, which can cause a significant decrease in the conductivity of the PPTC material. Such resistance changes give the PPTC material fuse-like characteristics, and this resistance may be reversible when the PPTC material cools back to room temperature.

[0003] PPTC materials are typically formed by compounding polymer materials and conductive fillers, and then extruding or die-forming the compounded mixture into chips or sheets of the desired shape. PPTC materials produced by this type of process are generally rigid and unsuitable for conformal application to flexible substrates, which may be desired for some applications (e.g., flexible heaters or temperature sensors). PPTC materials suitable for flexible applications can be created by a process that involves dissolving the polymer material in a solvent and mixing the resulting solution with conductive fillers to form a "PPTC ink." This PPTC ink can then be printed onto the desired surface or otherwise conformally applied, after which the ink is dried (i.e., the solvent is evaporated). This type of process has several drawbacks. For example, the steps of dissolving the polymer material when preparing the PPTC ink, and removing the solvent after applying the ink, can be time-consuming. Furthermore, the solvents required to dissolve the polymer material have a significant environmental impact.

[0004] This disclosure provides further information regarding these and other considerations. [Overview of the Initiative]

[0005] This summary is provided to introduce various concepts that will be further described in detail later in a simplified form. This summary is not intended to clarify any important or essential features of the claimed subject matter, nor is it intended to assist in determining the scope of the claimed subject matter.

[0006] A method for preparing a polymer positive temperature coefficient (PPTC) compound and conformally applying such a compound to a substrate according to one embodiment of the present disclosure may include the steps of: preparing and mixing raw materials to form a PPTC mixture; processing the PPTC mixture in a polymer compounding apparatus, where the PPTC mixture is heated and extruded as a PPTC compound; subjecting the polymer compound to a beam irradiation step to crosslink the polymer chains within the PPTC compound; and applying the PPTC compound to a substrate using a high-temperature application step, where the PPTC compound is heated and conformally applied to the surface of the substrate. [Brief explanation of the drawing]

[0007] Hereafter, various embodiments of the disclosed method will be described as examples, with reference to the attached drawings.

[0008] [Figure 1] This flowchart illustrates a method for preparing a polymer positive temperature coefficient compound and conformally applying such a compound to a substrate, according to embodiments of the present disclosure. [Modes for carrying out the invention]

[0009] The embodiments will be described more fully below with reference to the accompanying drawings illustrating several exemplary embodiments. The subject matter of this disclosure may be embodied in many different forms and should not be construed as being limited to the embodiments described herein. These embodiments are provided so as to make this disclosure complete and sufficient and so as to convey the scope of the subject matter to those skilled in the art. In the drawings, similar numbers throughout refer to similar elements.

[0010] In this specification, elements or operations listed in the singular and beginning with the word "a" or "an" are understood to include multiple elements or operations unless otherwise indicated. Furthermore, various embodiments in this specification are described in the context of one or more elements or components. An element or component may comprise any structure arranged to perform a particular operation. While an embodiment may be described, for example, with a limited number of elements in a particular topology, that embodiment may include more or fewer elements in alternative topologies required for a given implementation example. Note that any reference to "one embodiment" or "an embodiment" means that the particular features, structures, or characteristics described in relation to that embodiment are included in at least one embodiment. The phrases "in one embodiment," "in some embodiments," and "in various embodiments" appear in various places in this specification, but not all of them necessarily refer to the same embodiment.

[0011] An improved method for preparing polymer positive temperature coefficient (PPTC) compounds and conformally applying such compounds to a substrate (e.g., a substrate in a PPTC device) is described herein in accordance with this disclosure. A flowchart illustrating an example of such a method is shown in Figure 1.

[0012] Referring to block 100 of the example method shown in Figure 1, the raw materials for forming the PPTC compound may be gathered and prepared (e.g., weighed or measured). These raw materials may include polymers, conductive fillers, and optionally one or more additives. The polymers included in the PPTC compound may be selected from semicrystalline polymers, such as polyethylene copolymers (ethylene vinyl acetate, ethylene and acrylic acid copolymer, ethylene butyl acrylate copolymer, polyolefin elastomer, polyethylene oxide, etc.), fluororesins (polyvinyl fluoride, polytetrafluoroethylene, ethylene tetrafluoroethylene copolymer, etc.), polyesters (polycaprolactone, etc.), polyethers (polyethylene glycol, polytetrahydrofuran, etc.), polyurethanes, polyamides or copolymers thereof, and diene elastomers or copolymers thereof. The disclosure is not limited thereto. The amount of polymer in the PPTC compound may be in the range of 60 to 95% by volume.

[0013] The conductive filler contained in the PPTC compound may be selected from carbon black, carbon fiber, carbon nanotubes, graphite, graphene, or particles of metal or metal carbide (e.g., copper, nickel, tungsten carbide, titanium carbide, etc.). This disclosure is not limited in this respect. The amount of conductive filler in the PPTC compound may be in the range of 1 to 40% by volume.

[0014] Selective additives in PPTC compounds may include, but are not limited to, inorganic fillers, flame retardants, antioxidants, coupling agents, arc suppressors, crosslinking agents, and pigments (e.g., those that visually represent PPTC properties such as trip temperature).

[0015] The above-mentioned raw materials, including polymers, conductive fillers, and selective additives, may be weighed, and appropriate amounts of each raw material may be mixed together to form a PPTC mixture.

[0016] Referring to block 110 of the example method shown in Figure 1, the PPTC mixture may be processed in a conventional polymer compounding apparatus. Here, the PPTC mixture may be heated and extruded. In such a process, the PPTC mixture may be heated to a temperature range of 100–350°C, depending on the type of polymer used. The heated PPTC mixture may be extruded in the form of wires, sheets, or pellets, depending on the type of polymer compounding apparatus used. This disclosure is not limited in this respect. The extruded product is hereafter referred to as the “PPTC compound”.

[0017] Referring to block 120 of the example method shown in Figure 1, the PPTC compound may undergo a beam irradiation process that crosslinks the polymer chains within the PPTC compound. This process can form a network of polymer chains within the PPTC compound. This greatly contributes to the "resettable" property of the PPTC compound (i.e., the ability of the PPTC compound to decrease in electrical resistance when cooled to a temperature below its trip temperature). Since the beam irradiation is performed before the PPTC compound is mounted in the device, the beam irradiation process may be called a "pre-beam irradiation" process. The pre-beam irradiation process may be performed using any conventional beam irradiation method and associated equipment, and this process includes, but is not limited to, electron beam irradiation and gamma ray irradiation. When electron beam irradiation is used, the beam irradiation dose may be in the range of 1 to 100 Mrad (10 to 1000 kGy). When gamma ray irradiation is used, the beam irradiation dose may be in the range of 1 to 100 kGy. This disclosure is not limited in this respect. In various embodiments, the pre-beam irradiation step may be omitted, and instead, the beam irradiation step may be performed after the PPTC compound has been mounted on the device.

[0018] Referring to block 130 of the example method shown in Figure 1, the PPTC compound may be applied to the substrate using a heating step (hereinafter referred to as the "high-temperature step"). In this case, the PPTC compound can be melted or softened and applied to the substrate in a manner that adheres along the surface of the substrate. The high-temperature step may include high-temperature pressurization, high-temperature jetting, molding, etc. In various embodiments, the substrate may be a flexible polymer insulating film, and may be formed from, for example, polyimide, polyester, polyamide, fluorinated polymer, polyolefin, silicone, epoxy, etc. The disclosure is not limited in this respect. The substrate may form a base for a flexible PPTC element (i.e., a PPTC element that can be conformally applied to another structure / surface), and may include a pair of metal electrodes bonded to the substrate for connecting the PPTC element in a circuit. When the PPTC compound is applied to the substrate using the high-temperature step, it can bridge and connect the metal electrodes and create a conductive path between them.

[0019] In various embodiments, the high-temperature step in block 130 of the method example may be a high-temperature pressurizing step. This may include cutting the PPTC compound to a desired size and shape to form “PPTC elements” suitable for application to a substrate (for example, suitable for bridging the metal electrodes of a PPTC element). The PPTC elements may then be placed on a substrate, which may be placed in a high-temperature pressurizing machine. The high-temperature pressurizing machine can apply heat and pressure to laminate the PPTC elements onto the substrate. In various examples, the high-temperature pressurizing may be carried out in a temperature range of 80 to 350°C (depending on the melting point of the polymer), a pressure range of 20 to 10,000 psi (0.14 to 69 MPa) (depending on the type of polymer and the high-temperature pressurizing temperature), and a time range of 1 to 6,000 seconds. The disclosure is not limited thereto.

[0020] Referring to block 140 of the example method shown in Figure 1, the PPTC element may undergo various processes after the PPTC element has been applied to the substrate. Such processes may be called “post-application” processes. Post-application processes may include, but are not limited to, beam irradiation (in addition to or instead of the pre-beam irradiation process described above), insulating coating, heat treatment, packaging, etc. This disclosure is not limited in this respect.

[0021] Those skilled in the art will understand the numerous advantages provided by the method disclosed herein. For example, the method disclosed herein allows for the easy application of PPTC elements to flexible substrates in a conformal manner requiring only one high-temperature step. Furthermore, the method disclosed herein eliminates the need for any environmentally harmful solvents conventionally used when applying PPTC compounds to flexible substrates.

[0022] Although this embodiment is disclosed with reference to a specific embodiment, numerous modifications, alterations, and changes are possible to the embodiment described without departing from the scope and realm of this disclosure, as set forth in the appended claims. Therefore, this embodiment should not be limited to the embodiment described, but may encompass the entire scope as defined by the following claims and their equivalents.

Claims

1. A method for preparing polymer positive temperature coefficient (PPTC) compounds and conformally applying such compounds to a substrate, The step of preparing and mixing raw materials to form a PPTC mixture; In the step of processing the PPTC mixture in a polymer compounding apparatus, the PPTC mixture is heated and extruded as a PPTC compound; A step of performing a beam irradiation process on the PPTC compound to crosslink the polymer chains within the PPTC compound; and In the step of applying the PPTC compound to the substrate using a high-temperature application process, the PPTC compound is heated and conformally applied to the surface of the substrate. A method for providing this.

2. The method according to claim 1, wherein the PPTC mixture comprises a polymer and a conductive filler.

3. The method according to claim 2, wherein the polymer includes at least one of ethylene vinyl acetate, ethylene and acrylic acid copolymer, ethylene butyl acrylate copolymer, polyolefin elastomer, polyethylene oxide, polyvinyl fluoride, polytetrafluoroethylene, ethylene tetrafluoroethylene copolymer, polycaprolactone, polyethylene glycol, and polytetrahydrofuran.

4. The method according to claim 2, wherein the conductive filler includes at least one of carbon black, carbon fiber, carbon nanotube, graphite, graphene, copper, nickel, tungsten carbide, and titanium carbide.

5. The method according to claim 2, wherein the PPTC mixture further comprises an additive.

6. The method according to claim 5, wherein the additive includes at least one of inorganic fillers, flame retardants, antioxidants, coupling agents, arc suppressants, crosslinking agents, and pigments.

7. The method according to claim 1, wherein the PPTC mixture is heated to a temperature range of 100 to 350°C before extrusion molding.

8. The method according to claim 1, wherein the beam irradiation step is an electron beam irradiation step.

9. The method according to claim 1, wherein the beam irradiation step is a gamma-ray irradiation step.

10. The method according to claim 1, wherein the beam irradiation step is a pre-beam irradiation step performed before applying the PPTC compound to the substrate.

11. The method according to claim 1, wherein the beam irradiation step is a first beam irradiation step performed before applying the PPTC compound to the substrate, and the method further comprises a second beam irradiation step performed after applying the PPTC compound to the substrate.

12. The method according to any one of claims 1 to 11, wherein the high-temperature application step includes one of high-temperature pressurization, high-temperature jetting, and molding.

13. The method according to any one of claims 1 to 11, wherein the high-temperature application step is a high-temperature pressurization step, the PPTC compound is cut to a desired size and shape to form PPTC elements, and the PPTC elements are laminated onto the substrate using a high-temperature pressurizer.

14. The method according to claim 13, wherein the high-temperature pressurization step is performed in a temperature range of 80 to 350°C and a pressure range of 20 to 10,000 psi (0.14 to 69 MPa).

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