Electrically conductive PTC screen printable ink composition with low inrush current and high NTC onset temperature

Abstract

An electrically conductive screen-printable PTC ink composition with low inrush current and high NTC onset temperature, consisting of at least two different polymers, polymer-1 and polymer-2; wherein the melting temperature difference between polymer-1 and polymer-2 must be greater than 50° C., and the mechanical strength of polymer-1 as expressed by Young's modulus must be greater than 200 MPa.

Claims

1. A positive temperature coefficient (PTC) composition comprising: 5-35 wt % of a first polymer; 1-20 wt % of a second polymer; 5-50 wt % of a conductive particulate agent; and 30-80 wt % of an organic solvent; wherein: the first polymer has a melting temperature that is at least 50° C. higher than a melting temperature of the second polymer; the first polymer has a Young's modulus between 550 MPa and 900 MPa; the first polymer is a polyvinylidene difluoride polymer; the second polymer is a polycaprolactone polymer or a polyacrylate polymer; and the organic solvent dissolves both the first and second polymers.

2. The composition of claim 1, wherein the organic solvent is at least one of a dibasic ester (DBE), methyl ethyl ketone (MEK), N-methyl-2-pyrrolidone (NMP), toluene, and xylene.

3. The composition of claim 1, wherein the second polymer has a melting temperature between 40° C. and 70° C.

4. The composition of claim 1, wherein the melting temperature of the first polymer is above 120° C.

5. The composition of claim 1, wherein the conductive particulate agent is selected from the group consisting of metallic powder, metal oxide, carbon black and graphite.

6. The composition of claim 1, wherein the conductive particulate agent is carbon black or low-structured carbon black.

7. The composition of claim 1, wherein a difference between a negative temperature coefficient (NTC) onset temperature of the composition and a switching temperature of the composition is between 56° C. and 98° C.

8. The composition of claim 1, wherein the composition has an R.sub.Ts/R.sub.−20 ratio of between 1.6 and 1.9, with R.sub.Ts equal to a resistance of the composition at a switching temperature of the composition, and R.sub.−20 equal to the resistance of the composition at −20° C.

9. The composition of claim 1, wherein a weight ratio of the first polymer to the second polymer is between 0.5 and 1.

10. A film comprising the composition of claim 1, wherein: a weight ratio of the first polymer to the second polymer is between 0.5 and 1; a difference between an NTC onset temperature of the composition and a switching temperature of the composition is between 56° C. and 98° C.; and the composition has an R.sub.Ts/R.sub.−20 ratio of between 1.6 and 1.9, with R.sub.Ts equal to a resistance of the composition at a switching temperature of the composition, and R.sub.−20 equal to the resistance of the composition at −20° C.

11. The film of claim 10, wherein the second polymer has a melting temperature between 40° C. and 70° C.

12. The film of claim 10, wherein the melting temperature of the first polymer is above 120° C.

13. The film of claim 10, wherein the conductive particulate agent is selected from the group consisting of metallic powder, metal oxide, carbon black and graphite.

14. An electric heater comprising the composition of claim 1, wherein: a weight ratio of the first polymer to the second polymer is between 0.5 and 1; a difference between an NTC onset temperature of the composition and a switching temperature of the composition is between 56° C. and 98° C.; and the composition has an R.sub.Ts/R.sub.−20 ratio of between 1.6 and 1.9, with R.sub.Ts equal to a resistance of the composition at a switching temperature of the composition, and R.sub.−20 equal to the resistance of the composition at −20° C.

15. The heater of claim 14, wherein the second polymer has a melting temperature between 40° C. and 70° C.

16. The heater of claim 14, wherein the melting temperature of the first polymer is above 120° C.

17. The heater of claim 14, wherein the conductive particulate agent is selected from the group consisting of metallic powder, metal oxide, carbon black and graphite.

Description

BRIEF DESCRIPTION OF THE 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 illustrates a typical PTC profile (TCR vs temperature);

(3) FIG. 2 illustrates a current vs temperature profile of a typical PTC heater.

(4) FIG. 3 illustrates a temperature vs time profile of a typical PTC heater.

(5) FIG. 4 illustrates a PTC profile of an embodiment.

(6) FIG. 5 illustrates a PTC profile of an embodiment.

(7) FIG. 6 illustrates a PTC profile of an embodiment.

(8) FIG. 7 illustrates a PTC profile of an embodiment.

(9) FIG. 8 illustrates a PTC profile of an embodiment.

(10) FIG. 9 illustrates a PTC profile of an embodiment.

(11) 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

(12) The present disclosure provides an electrically conductive PTC screen-printable ink composition with low inrush current and high NTC onset temperature and a method of making the same. The electrically conductive PTC ink composition presents both an efficient PTC effect at the switch temperature and maintains PTC effect beyond the switching temperature for a wide temperature range. When the present PTC ink composition is applied in a self-regulating heating element, it offers a low inrush current, a desirable regulated temperature, and a wider safety operation temperature range.

(13) In an embodiment, the PTC ink composition comprises at least two polymers, referred to as a first polymer (polymer-1) and a second polymer (polymer-2). The selection of the two polymers meet two criteria at the same time. The first criterion is that the difference in melting temperatures between the two polymers is greater than 50° C. The second criterion is that the Young's modulus of at least one of the two polymers is greater than 200 MPa.

(14) For convenience, we refer the polymer with higher melting temperature as polymer-1 herein, and the polymer with lower melting temperature as polymer-2. Accordingly, the switch temperature is mainly determined by the melting temperature of polymer-2, which may be selected from crystalline or semi-crystalline polymers with a melting temperature below 90° C., or below 80° C., or between 40-70° C.

(15) Accordingly, the structural strength of the PTC composition is provided by polymer-1. Therefore, the selected polymer-1 has a higher melting temperature (than polymer-2) and higher mechanical strength as expressed by a higher Young's modulus. To reduce or even eliminate the NTC effect, the melting temperature of polymer-1 can be at least 50° C. above the melting temperature of the selected polymer-2, thus, the melting temperature of polymer-1 can be above 120° C., or above 140° C. To reduce the inrush current and hold the PTC composition in place over the whole operating temperature range, the Young's modulus of polymer-1 can be greater than 200 MPa, or greater than 500 MPa.

(16) The selection of polymer-2 is mainly according to the requirement of the equilibrium temperature. The melting temperature of polymer-2 can be close to the equilibrium temperature required. And polymer-2 can have good adaptability with polymer-1. There is no requirement for polymer-2's Young's modulus.

(17) The selection of the solvent is based on its proper boiling point and the solubility of polymer resins used. All the polymers are completely dissolved in the organic vehicle prior to blending with other components. Any organic, inert liquid may be used as the solvent for the medium (vehicle) so long as the polymers are fully solubilized.

(18) Solvents may be selected from DBE-9, MEK, N-methyl-2-pyrrolidone (NMP), toluene, xylene, and the like. In one embodiment, the solvent used is the mixture of DBE-9 and NMP, which has a greater viscosity stability than each of them alone.

(19) Some conductive particles may be added to this PTC ink composition to provide electrically conductive function. The conductive particles can be one or a mixture of more than one of metallic powder, metal oxide, carbon black and graphite. The conductive particles may be carbon black, or a low-structured carbon black.

(20) In an embodiment, a positive temperature coefficient (PTC) composition comprises two different polymers: polymer-1 and polymer-2; where the melting temperature of polymer-1 is at least 50° C. higher than that of polymer-2, and the Young's modulus of polymer-1 is larger than 200 MPa. The composition comprises:

(21) a) 5-35 wt % polymer-1;

(22) b) 1-20 wt % polymer-2;

(23) c) 5-50 wt % carbon black;

(24) d) 30-80 wt % organic solvent for dissolving both polymer-land polymer-2.

(25) The PTC ink composition may be prepared according to the procedure consisting of the following steps: 1) the preparation of 5-55 wt. % polymer solution by dissolving the selected polymer-1 and polymer-2 in the selected solvent; 2) the preparation of an ink base by adding pre-calculated carbon black into the polymer solution; and 3) the preparation of final PTC ink composition by adding a proper amount of polymer solution into the ink base to adjust the ratio between carbon black and polymer so as to fine-tune electric resistance of the final PTC composition.

(26) The resulting PTC ink composition may be applied to substrates such as polyester films, e.g. DuPont® Teijin films, by a screen-printing process. After printing the PTC ink on a polyester film, it is cured in an oven at 120° C. for 10 minutes. Subsequently, a conductive paste suitable for use on polyester substrates such as DuPont® 5025 silver paste is printed over edges of the PTC ink and cured at 120° C. for 5 minutes. The cured film is then tested for resistance change with temperature. The resistance of the cured PTC film is measured as a function of temperature, so the PTC characteristics are determined.

(27) The composition will now be described in more detail with reference to the following examples. However, these examples are given for illustration only and are not intended to limit the scope of the present invention.

EXAMPLES

(28) In all these examples, all parts and percentages are by weight unless otherwise noted. Of these examples, polyvinylidene fluoride (PVDF) resins are purchased from Arkema USA, Poly(ε-caprolactone) diester with neopentyl glycol resins (CAPA) are purchased from Perstorp Sweden; Polyacrylate based intelimer polymer resins (IPA) are purchased from Air Products USA, Carbon Black is purchased from Cabot USA, and all other chemicals are purchased from Sigma-Aldrich Company USA unless otherwise specified.

(29) TABLE-US-00001 TABLE 1 Physical properties of selected polymers Melting Young's Commercial Chemical temperature modulus grade family (° C.) (MPa) Flex PVDF 100 80 KY2800 PVDF 140 550-900 PL1000 CAPA 35 NA* PL6100 CAPA 59 NA* IPA13 Intelimer 65 NA* NA* Not Available

(30) General procedure to prepare PTC inks: PTC inks exampled hereafter are prepared according to a general procedure as described below. First, a polymer solution is prepared by dissolving the selected polymer-1 and polymer-2 in a solvent mixture containing 50% of DBE-9 and 50% of NMP, where the ratios between the two polymers are given in each individual example. The overall concentration of polymer solution is controlled at 25%, and the dissolution of polymers is completed by stirring at 80° C. for 2 hours. Second, an ink master base is prepared by adding the conductive carbon black into the polymer solution and mixing for 30 minutes and then passing through a three-roll mill three times until all the carbon black particles in the mixture are less than 10 μm. Finally, the final ink is completely letdown by mixing a certain amount of the polymer solution with the master ink base to adjust the viscosity and sheet resistance of the final PTC ink.

(31) General procedure to evaluate a PTC ink: To evaluate the PTC inks prepared in these examples, film strips with silver contacts on both ends of the film strips are first prepared as detailed hereafter. DuPont® 5025 silver conductive ink is first printed on a polyester substrate, e g MELINEX ST505, commercially available from TEKRA USA, to create two silver lines, and then dried at 120° C. for 5 minutes in a box oven to create silver contacts. The PTC film strips are then constructed between these silver contacts by over-printing the exampled PTC ink and dried at 120° C. for 10 minutes in a box oven. Both silver paste and PTC ink are printed by a 280-mesh plastic screen. Thickness of silver contacts is about 20 μm while the thickness of the PTC film strip is about 7 μm, as measured by a calibrated micrometer screw gauge. The sheet resistances at different temperatures in a temperature controlled oven is precisely measured by an HP multimeter. With measured resistances at different temperatures, a PTC profile as illustrated in FIG. 1 is plotted. From this plotted PTC profile, typical points such as the switch temperature, T.sub.s, and the maximum temperature, T.sub.max, are found.

(32) Table 2 summarizes the polymers selected in preparing samples exampled including the ratio between two polymers and associated physical properties.

(33) TABLE-US-00002 TABLE 2 Exampled sample summary Melting P1 temperature Young's difference Example Polymer -1 Polymer-2 Ratio modulus (P1 − P2) No. (P1) (P2) (P1/P2) (MPa) (° C.) 1 Flex PL1000 2 80 65 2 Flex PL6100 2 80 41 3 Flex PL6100 1 80 41 4 KY2800 IPA13 1 550-900 75 5 KY2800 IPA13 0.5 550-900 75 6 KY2800 NA NA 550-900 NA

(34) In order to better demonstrate the inrush impact in a cool environment, a reference temperature of minus 20 degree Celsius (−20° C.) was selected to plot the TCR vs temperature for the examples listed in Table 2. So the inrush impact can be expressed by R.sub.Ts/R.sub.−20° C.. FIGS. 4-9 presents PTC profiles of these examples 1-6 respectively.

(35) TABLE-US-00003 TABLE 3 PTC Performance of listed examples T.sub.s T.sub.max T.sub.max − T.sub.s Example (° C.) (° C.) (° C.) R.sub.Ts/R.sub.−20°C. 1 70 97 27 27.0 2 40 100 60 10.1 3 48 70 22 110 4 57 155 98 1.6 5 57 113 56 1.9 6 105 137 32 8.7

(36) From FIGS. 4-9 and Table 3, it is clearly seen that Example 1 has a very low T.sub.max−T.sub.s (° C.) implying an NTC onset is close to the switching temperature, and a high R.sub.Ts/R.sub.−20° C. implying a high inrush impact, which failed due to the low Young's modulus of polymer-1. Example 2 exhibits a high T.sub.max−T.sub.s (° C.) implying its NTC onset is significantly postponed beyond the switching temperature but has a high R.sub.Ts/R.sub.−20° C. implying a high inrush impact, which also failed due to the low Young's modulus of polymer-1. Like Example 1, Example 3 has a very low T.sub.max−T.sub.s (° C.) implying an NTC onset is close to the switching temperature, and a high R.sub.Ts/R.sub.−20° C. implying a high inrush impact, which failed due to the low Young's modulus of polymer-1 and the small melting temperature difference between the polymer 1 and polymer-2.

(37) Comparing Examples 1-3, Examples 4-5 deliver the desired performance, including both the low inrush impact as expressed by a low value of R.sub.Ts/R.sub.−20° C. and the wide safety temperature range beyond the switching temperature as expressed by a value of T.sub.max−T.sub.s (° C.). In both examples, polymer-1 has a Young's Modulus greater than 500 MPa, and the melting temperature difference between polymer-1 and polymer-2 is greater than 50° C.

(38) In example 6, there is only polymer-1, no polymer-2. FIG. 9 shows both characteristics are not good: T.sub.max−T.sub.s is too small (32° C.), R.sub.Ts/R.sub.−20° C. is too large (8.7). This example shows the importance of the selection of polymer-2.

(39) 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.