Thermal substrate with high-resistance magnification and positive temperature coefficient

Abstract

A printed circuit that comprises a substrate, electrical interconnects and a double-resin ink having a positive temperature coefficient (PTC), wherein the double-resin ink has a resistance magnification of at least 20 in a temperature range of at least 20 degrees Celsius above a switching temperature of the double-resin ink, the resistance magnification being defined as a ratio between a resistance of the double-resin ink at a temperature ‘T’ and a resistance of the double-resin ink at 25 degrees Celsius. The substrate is a fabric or mesh, while the double-resin ink and the electrical interconnects are deposited onto the substrate.

Claims

1. A printed circuit comprising: a substrate; a double-resin ink having a positive temperature coefficient (PTC); and electrical interconnects; wherein: the substrate is a fabric or mesh; the double-resin ink and the electrical interconnects are deposited onto the substrate; the double-resin ink comprises: a first resin comprising a crystalline or a semi-crystalline polymer; and a second resin comprising a non-crystalline polymer; and the double-resin ink has a resistance magnification of at least 20 in a temperature range of at least 20 degrees Celsius above a switching temperature of the double-resin ink, the resistance magnification being defined as a ratio between a resistance of the double-resin ink at a temperature ‘T’ and a resistance of the double-resin ink at 25 degrees Celsius.

2. The printed circuit of claim 1, wherein the resistance magnification of the double-resin ink is at least 50.

3. The printed circuit of claim 2, wherein the temperature range is 30 degrees Celsius above the switching temperature.

4. The printed circuit of claim 1, wherein the switching temperature is between 0 and 160 degrees Celsius.

5. The printed circuit of claim 1, wherein the first resin provides a first PTC effect in a first temperature range and the second resin provides a second PTC effect in a second temperature range, the second temperature range being higher than the first temperature range.

6. The printed circuit of claim 5, wherein the double-resin ink comprises: a) 5-15 wt % of a thermally active first polymer resin having a melting point of 30° C.-70° C. and providing the first PTC effect in the first temperature range below 70° C., b) 5-15 wt % of a thermally active second polymer resin having a melting point of 70° C.-1400° C. and providing the second PTC effect in the first temperature range above 70° C., c) 10-30 wt % of conductive particles, d) 40-80 wt % of an organic solvent having a boiling point higher than 100° C., the organic solvent being capable of dissolving both the first resin and the second resin, and e) 0-5 wt % of additives comprising a dispersive additive, a wetting additive and a rheological additive, the additives having enhanced dispersing, wetting and rheological properties.

7. The printed circuit of claim 1, wherein the double-resin ink is deposited on to the substrate by screen printing, a thick film dispenser or a 3-D printer.

8. The printed circuit of claim 1, wherein the electrical interconnects comprise at least one of a conductive paste, a metal foil, a wire, a metal alloy, aluminum, copper, nickel; a copper-nickel alloy, and a high-conductivity electronic polymer.

9. The printed circuit of claim 1, wherein deposition of the double-resin ink provides resistors arranged in parallel in one or more columns.

10. The printed circuit of claim 9, wherein the resistors have a length from about 0.2 cm to about 10 cm.

11. The printed circuit of claim 9, wherein a temperature at each resistor is regulated independently.

12. The printed circuit of claim 9, wherein there are N columns, each column having n.sub.col resistors in parallel; each resistor having a resistance of R, an applied voltage V, and a total power dissipated, P, is:
P=n.sub.colNV.sup.2/R; with N≥1 and n.sub.co1≥2.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The foregoing and other advantages of the disclosure will become apparent upon reading the following detailed description and upon reference to the drawings.

(2) FIG. 1 shows the relationship of the PTC ratio R.sub.T/R.sub.25 versus temperature T of a typical PTC composition.

(3) FIG. 2 shows the relative resistance magnification of a high-resistance magnification (HRM) PTC ink that exhibits an upper resistance magnification greater than 200 above a temperature of 70 Celsius.

(4) FIG. 3 illustrates an embodiment of a printed circuit that comprises multiple resistors that

(5) regulate temperatures independently over each small area covered by each individual resistor.

(6) FIG. 4A illustrates a cross-section of an embodiment of a PTC coating on an insulating yarn or filament.

(7) FIG. 4B is a schematic of an embodiment of a mesh or fabric that incorporates an insulating yarn or filament constructed as shown in FIG. 4A.

(8) FIG. 4C illustrates a cross-section of an embodiment of a PTC coating on a conductive yarn or filament.

(9) FIG. 4D is a schematic of an embodiment of a mesh or fabric that incorporates a conductive yarn or filament constructed as shown in FIG. 4C.

(10) FIGS. 5A and 5B are each a schematic cross section of a PTC ink and interconnects deposited on a film (FIG. 5A) and subsequently laminated (FIG. 5B).

(11) FIGS. 5C-5F are each a schematic cross section of a laminated HRM PTC thermal substrate positioned in thermal communication with a fabric or mesh.

(12) While the present disclosure is susceptible to various modifications and alternative forms, specific embodiments or implementations have been shown by way of example in the drawings and will be described in detail herein. It should be understood, however, that the disclosure is not intended to be limited to the particular forms disclosed. Rather, the disclosure is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of an invention as defined by the appended claims.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(13) FIG. 1 shows the relationship of the PTC ratio R.sub.T/R.sub.25 versus temperature T of a typical PTC ink composition. When the temperature T is in excess of 70° C., the PTC ratio R.sub.T/R.sub.25 (also known as a “resistance magnification” or “resistivity magnification”) begins to decline. The PTC ratio R.sub.T/R.sub.25 is defined as the ratio between a resistance R.sub.T at temperature T and the resistance R.sub.25 at temperature 25° C. The lower resistance at the onset of NTC leads to excessive current flow and the heating element is overpowered. Therefore, the NTC temperature region is a potential safety-risk temperature region.

(14) FIG. 2 shows the relative resistance magnification of a high-resistance magnification (HRM) PTC ink suitable for applications in the present disclosure. The HRM PTC ink exhibits the following properties: its switching temperature is at about 40° C.; its resistance magnification (or PTC ratio R.sub.T/R.sub.25) rises to about 200 within 30° C. of the switching temperature; and it exhibits only PTC behavior 30° C. beyond the switching temperature. That is, within a span of 30° C. above the switching temperature, the current flowing through the HRM PTC ink is reduced by a factor of 200, thereby providing a self-regulating mechanism by which the power consumption is reduced by 200. In fact, further investigation reveals that the HRM PTC ink has an upper resistance magnification greater than 200 above a temperature of 70° C. Furthermore, no NTC effect is observable 30° C. above the switching temperature, implying use of the HRM PTC ink is safe within this temperature range.

(15) For example, common commercially available PTC inks typically manifest a resistance multiplication of 10-15, as shown in FIG. 1. On the other hand, the resistance magnification of an HRM PTC ink reaches 200 as shown in FIG. 2.

(16) Moreover, the NTC effect is well above the switching temperature as compared to the NTC effect depicted in FIG. 1. In addition, hysteresis effects are absent, transition regions are short and resistance recovery times are short. Finally, the slope of the curve for the HRM PTC ink in FIG. 2 is steeper than that depicted in FIG. 1, which allows more tightly controlled temperature regulation.

(17) These properties imply that the HRM PTC ink of FIG. 2 may be used in applications where a PTC effect, along with a high resistance magnification (i.e. power consumption reduction) are required between about 40° C. and 70° C. The high resistance magnification can be greater than 50, or greater than 100, or greater than 150, in the temperature range of interest.

(18) It follows that applications that require a PTC effect and high magnification resistance in a different temperature range can use an HRM PTC ink with a) a switching temperature; b) PTC effect; and c) high resistance magnification in the required temperature range.

(19) An example of an HRM PTC ink is disclosed in US Pat. Pub. No. 20170327707, incorporated herein by reference. However, as discussed above, any HRM PTC ink that exhibits substantially similar characteristics may be used. The requisite HRM PTC ink exhibits a switching temperature, high resistance magnification and a PTC effect over the temperature range required for the application.

(20) For example, the HRM PTC ink may comprise a first resin that provides a first PTC effect in a first temperature range and a second resin that provides a second PTC effect in a second temperature range, wherein the second temperature range is higher than the first temperature range.

(21) As an example, the double-resin HRM PTC ink may comprise about 10-30 wt % conductive particles; about 5-15 wt % of a first polymer resin; about 5-15 wt % of a second polymer resin; about 40-80 wt % of an organic solvent; and about 0-5 wt % other additives. The conductive particles can be one of, or a mixture of: a metallic powder, a metal oxide, carbon black and graphite. The first polymer resin may be a kind of crystalline or semi-crystalline polymer, such as polyurethane, nylon, and polyester. The second polymer resin may be a kind of non-crystalline polymer, such as acrylic resin. The selection of the solvent is based on its proper boiling point and the solubility of polymer resins, since the polymer resins are completely dissolved in the organic solvent prior to blending with other components. Any organic, inert liquid may be used as the solvent for the so long as the polymer is fully solubilized. As examples, the solvent may be selected from MEK, N-methyl pyrolidone (NMP), toluene, xylene, and the like. The other additives include a dispersing/wetting additive and a rheology additive.

(22) As an example of the double resin composition, the HRM PTC ink may comprise about 5-15 wt % of a thermally active polymer resin-1 having a melting point of 30-70° C. and providing a first temperature coefficient characteristic in the first temperature range below 70° C.; about 5-15 wt % thermally active polymer resin-2 having a melting point of 70-140° C. and providing a second positive temperature coefficient characteristic in the second temperature range above 70° C.; about 10-30 wt % conductive particles; about 40-80 wt % organic solvent having a boing point higher than 100° C., the organic solvent being capable of dissolve both the polymer resin-1 and polymer resin-2, and about 0-5 wt % additives. The additives may comprise dispersing additives, wetting additives and rheological additives, with the additives having enhanced dispersing/wetting and rheology properties. The first polymer resin may be a kind of crystalline or semi-crystalline polymer, such as polyurethane, nylon, and polyester. The second polymer resin may be a kind of non-crystalline polymer, such as acrylic resin.

Thermal Substrate

(23) In a thermal substrate comprising a substrate and an HRM PTC ink, the HRM PTC ink can provide the elimination of NTC and therefore avoid catastrophic failure. Moreover, the high resistivity of the HRM PTC ink in a temperature window of 20° C. to 40° C. above the switch temperature can provide precise temperature self-regulation with rapid time-to-temperature.

(24) Non-limiting examples of a substrate include a fabric, a mesh and a film.

(25) The present disclosure describes applications of the HRM PTC ink described above that extends or eliminates the onset of the NTC effect, offers magnification factors greater than 15, 25, 50, 100, 150 or 200, and switches in the range of 0°−160° C. Such applications are therefore safer, more reliable and dissipate minimal power at the switch temperature. Moreover, the wide switch temperature range of HRM PTC inks offers greater design flexibility and the steep temperature-resistance transition enables tighter temperature control.

(26) Thermal substrates that use HRM PTC ink may be created using various materials —depending on the specific application. For example, for outdoor wear, a nylon fabric may be used. For an industrial application that requires a higher operating temperature, a woven glass fiber mesh may be used. Similarly, various substrates may be used depending on the parameters of the application. For example, for clothing where low-weight and flexibility are required, thermoplastic polyurethane (TPU), polyester or a natural fabric such as cotton or a cotton blend is appropriate.

(27) In general, all-natural fibers, many polymer films and, in the case of meshes, metal wires are amenable to heating with a HRM PTC ink.

First Embodiment of a Thermal Substrate

(28) In some embodiments, a thermal substrate may be made by deposition of a HRM PTC ink onto a fabric or mesh.

(29) The HRM PTC ink may be deposited on the substrate by various techniques. For example, screen printing onto a substrate may be used successfully because of the favorable dispersion of the HRM PTC ink. Other suitable techniques include gravure or rotogravure (e.g. “doctor blade”) methods. The HRM PTC ink may also be dispensed over simple or complex surfaces using nozzles mounted on programmable robots or embedded in components by 3-D printing. Other methods of depositing a HRM PTC ink with substantial accuracy are known in the art.

(30) Once the HRM PTC ink has been deposited on a fabric or mesh, silver or other conductive paste may be deposited on the substrate to create electrical interconnects (e.g. contacts and bus lines) for use in heater applications. In some embodiments, other metals (e.g. metal foils or wires), metal alloys or electrically conductive materials such as, but not limited to, aluminum, copper, nickel and alloys thereof, or highly conductivity electronic polymers may be deposited on the substrate as a paste or ink to create interconnects In all cases, interconnects can be designed for minimal length to lessen the possibility of cracking.

(31) An HRM PTC ink is methodically deposited on a substrate such that the resulting circuit pattern provides for optimum power delivered by the resulting thermal substrate. For any given heating application, whereas the switch temperature is determined by the composition of the HRM PTC ink, the power delivered by the thermal substrate is determined by the HRM PTC ink's circuit pattern.

(32) FIG. 3 depicts a printed circuit that comprises multiple deposits of printed HRM PTC ink. Each deposit of ink 380 acts as a resistor that regulates temperatures independently over a small area covered by the individual resistor 380. Each resistor 380 in a column is powered by line voltage busses 390 and interconnects 391. The printed circuit arises from deposition of the HRM PTC ink and conductive interconnects onto the substrate, as described above.

(33) Since the HRM PTC resistor 380 material typically has high sheet resistance, the power, P, is determined by arranging the printed resistors in parallel in a column on the substrate, as shown in FIG. 3. The power dissipated by a column, P.sub.col, is:
P.sub.col=V.sup.2/R.sub.col
where V is the applied voltage and R.sub.col is the total resistance of the column. If the number of resistors in parallel within a column is n.sub.col and the resistance of individual resistor 380 is R, the resistance of the column, R.sub.col, is:
R.sub.col=R/n.sub.col.
For a total of N columns (as depicted in FIG. 3), the total power dissipated by the sheet, P, is
P=n.sub.colNV.sup.2/R.
The resistors may have a length of from about 0.2 cm to about 10 cm. The temperature at each resistor is independently regulated. This circuit pattern allows independent temperature control of small areas, controlled power delivery and temperature uniformity, or non-uniformity if desired, over the surface of the substrate regardless of the local thermal load. The gap between discrete resistors may also be reduced to zero to form a contiguous line of resistors with identical behavior of the heater, i.e. local self-regulation in response to local thermal load conditions. In all cases, the resistivity of the HRM PTC ink may be adjusted appropriately.

Second Embodiment of a Thermal Substrate

(34) In other embodiments, the HRM PTC ink may be deposited on a thread, yarn or mesh element for weaving into a fabric or mesh to create a thermal fabric or mesh.

(35) FIG. 4A illustrates a cross-section of an HRM PTC coating 420 on an insulating thread, yarn or filament 410. The HRM PTC ink is deposited as a coating 420 on the insulating thread, yarn or filament 410 and preferably sealed for electrical isolation on the outer surface with an insulating polymer 430. Coating and sealing may be done by dipping, extrusion or vapor deposition. Conduction of current is therefore along the length of the thread, yarn or filament. In this configuration, the HRM PTC ink is formulated for low resistivity, while multiple coated threads, yarns or filaments may be connected in parallel.

(36) In another embodiment shown in FIG. 4B, the HRM PTC ink 472 can be deposited on a conductive thread, yarn or filament 471 (e.g. such as a copper wire) and coated with another electrically conductive layer 473 such as copper or silver. As above, there can be an optional insulating layer 474 around the conductive layer 473. In this case, the HRM PTC ink may be formulated for high resistivity and electrical current flows radially inward from the outer conductive layer 473 to the conductive thread 471.

(37) FIG. 4C illustrates an example of how the coated threads 460 may be woven into the weft of a fabric or mesh to form a thermal substrate using an insulator as a thread, yarn or filament. Wires 450 and 451 carry supply voltage and are woven into the warp. Contact with the heater threads can be made by coating with pressure-sensitive adhesive and subsequently simultaneously applying heat and pressure, pulse welding, swaging, sealing with an overcoat or other means known in the art. The remaining threads 470 in the warp are standard fabric or mesh materials, e.g. polyester.

(38) FIG. 4D is a schematic of a mesh or fabric that incorporates a conductive yarn or filament constructed as shown in FIG. 4C. One wire carrying supply voltage 480 makes contact with the outer conductor layer of the heater threads 490, which constitute the weft; the other wire 481 makes contact with the inner conductor of the coated threads. The remaining threads 495 are the customary fabric material. The inner conductors are exposed by stripping the HRM PTC ink from the threads with solvent or mechanical means.

Third Embodiment of a Thermal Substrate

(39) In yet another embodiment a thermal substrate 505 may be made by depositing the HRM PTC ink 510 and conductive interconnects 511 onto a polymer film 520, as shown in FIG. 5A. The printed elements may be subsequently encapsulated by optionally laminating a second film 530 of the same composition (as that of film 520), resulting in a laminated thermal substrate 535, as shown in FIG. 5B. Lamination may be achieved by using a pressure and temperature adhesive. Suitable substrate and encapsulation materials include, but are not limited to: polyester, polyimide, polypropylene, rubber, silicone, thermoplastic polyurethane, laminates, ethylene-vinyl acetate (EVA) adhesive film, acrylate adhesive film and silicon adhesive film, fabric, silicone, and polyethylene terephthalate (PET). Additionally, the fabric or mesh heated by the thermal substrate 505 may have other layers of materials bonded to it such as, but not limited to: adhesive films, thermal barriers, reflective films, high or low emissivity films, absorptive films, alkaline resistant films, ground planes or EMI/RFI protective layers.

(40) If the primary heat transfer mechanism is conduction, the laminated thermal substrate 535 can be positioned in thermal communication with a fabric or mesh 560 in order to heat the fabric or mesh 560, as shown in FIG. 5C. In the embodiment shown, the laminated thermal substrate 535 may be merely positioned close to the fabric or mesh 560 to be in contact therewith. While laminated thermal substrate 535 (of FIG. 5B) is shown, thermal substrate 505 (of FIG. 5A) may be used in place of the laminated thermal substrate 535.

(41) Alternatively, the laminated thermal substrate 535 may be attached to the fabric or mesh 560 by a fastener 570 (such as, but not limited to: a rivet, snap, clasp or stud), as shown in FIG. 5D, or by various other means such as adhesives, sewing or removable clips. While laminated thermal substrate 535 (of FIG. 5B) is shown in in FIG. 5C, the thermal substrate 505 (of FIG. 5A) may be used in place of the laminated thermal substrate 535.

(42) If the primary mode of heat transfer is infrared radiation or convection, the laminated thermal substrate 535 may not need to be proximate to the fabric or mesh 560. Then, an air gap 580 may be configured, as shown in FIG. 5E, between the laminated thermal substrate 535 and the fabric or mesh 560. While laminated thermal substrate 535 (of FIG. 5B) is shown, the thermal substrate 505 (of FIG. 5A) may be used in place of the laminated thermal substrate 535.

(43) Whether the laminated thermal substrate 535 (or thermal substrate 505) is adjacent to the mesh or fabric 560 (as in FIG. 5C), fastened to the mesh or fabric 560 (as in FIG. 5D), or separated from the mesh or fabric 560 by an air gap 580 (as in FIG. 5E), an optional layer of material may be used for thermal insulation, water proofing, etc. FIG. 5F illustrated a configuration where a laminated thermal substrate 535 is fastened to a mesh or fabric 560, and placed near a layer of material 590, with an air gap 580. For example, in an embodiment, a heated jacket can have a fabric lining that has an HRM PTC film attached thereto, along with a waterproof fabric that forms an outer layer of the jacket.

(44) In various applications, the fabric or mesh heated by the thermal film may have a sensor positioned proximate to it or laminated in it, Furthermore, it may use a feedback loop to adjust its temperature based on the sensor. In other applications, the HRM PTC ink itself may be used as its own temperature sensor since it manifests such a strong and repeatable relationship between resistance and temperature. In such an application, an auxiliary circuit may be configured to measure real-time heater resistance for an accurate temperature integrated over the entire thermal film.

(45) While particular implementations and applications of the present disclosure have been illustrated and described, it is to be understood that the present disclosure is not limited to the precise construction disclosed herein and that various modifications, changes, and variations can be apparent from the foregoing descriptions without departing from the spirit and scope of an invention as defined in the appended claims.