HK1021613B - Conductive polymer composition - Google Patents

Description

This invention relates to electrical heaters containing PTC conductive polymer compositions.

Conductive polymer compositions which exhibit PTC (positive temperature coefficient of resistance) behavior are well-known and are particularly useful for self-regulating strip heaters and circuit protection devices. Reference may be made for example, to U.S. patent Nos. 3,793,716, 3, 823,217, 3,858,144, 3,861,029, 3,914,363, 4,017,715, 4,177,376, 4,188,276, 4,237,441, 4,242,573, 4,246,468, 4,286,376, 4,304,987, 4,318,881, 4,330,703, 4,334,148, 4,334,351, 4,388,607, 4,400,614, 4,425,497, 4,426,339, 4,435,639, 4,459,473, 4,514,620, 4,520,417, 4,529,866, 4,534,889, 4,543,474, 4,545,926, 4,547,659, 4,560,498, 4,571,481, 4,574,188, 4,582,983, 4,631,392, 4,638,150, 4,654,511, 4,658,121, 4,659,913, 4,661,687, 4,667,194, 4,673,801, 4,698,583, 4,719,335, 4,722,758, and 4,761,541, European Patent Publication Nos. 38,718 (Fouts et al., published October 28, 1981), 158,410 (Batliwalla et al., published October 16, 1985), and 231,068 (Barma et al, published August 5, 1989). Conductive polymer compositions generally contain a crystalline polymer matrix and a carbon black dispersed in the matrix. Carbon blacks vary in particle size, surface area, structure, and surface chemistry, all of which influence the properties, e.g. flexibility and conductivity, of conductive polymers containing them. The surface chemistry of a carbon black can be altered by heat or chemical treatment, either during the production of the carbon black or in a post-production process, e.g. by oxidation. Oxidized carbon blacks frequently have a pH less than 5.0, and a high resistivity.

Carbon blacks having low resistivity are generally used to make PTC conductive polymers -- see for example U.S. Patent Nos. 4,237,441 (van Konynenburg et al.) and 4,388,607 (Toy et al.). However, U.S. Patent No. 4,277,673 (Kelly) discloses self-regulating heaters in which the PTC conductive polymer comprises a highly resistive carbon black. Kelly uses a process to make self-regulating heaters, in which a PTC composition which comprises an organic fluoropolymer matrix which has a crystallinity of at least 5%, and at least 4% by weight of the composition of a carbon black having a pH of less than 4 is melt extruded as a strip around a pair of wire electrodes, and the extrudate is then annealed (i.e. held at a temperature above the melting point of the polymer) for an extended time. The conductive polymer, as initially extruded, has a very high resistivity, and the annealing is necessary in order to reduce the resistivity to a useable level. According to Kelly, conductive polymers containing highly resistive carbon blacks anneal more rapidly than conductive polymers in which only conductive carbon blacks are present.

Conductive polymers are usually shaped by melt extrusion. Thin layers of conductive polymers can also be produced by solvent-based processes, and the resulting products have relatively low resistivity but suffer from poor stability.

We have discovered that laminar conductive polymer heaters having good stability can be made through the use of thick film inks containing a crystalline fluoropolymer and a carbon black having a pH of less than 4.0. Accordingly, this invention provides an electrical heater which

• (I) comprises (A) a PTC element which is composed of a conductive polymer composition which (i) exhibits PTC behavior, (ii) has a resistivity of 10 to 10,000 ohm-cm, and (iii) comprises (a) an organic fluoropolymer matrix which has a crystallinity of at least 5% and a melting point Tm, and(b) at least 4% by weight of the composition of a carbon black which has a pH of less than 4.0; and(B) two electrodes which can be connected to a source of electrical power to pass current through the PTC element; and
• (II) is such that if it is maintained at Tm for 50 hours, and is then cooled to 20°C, it has a resistance at 20°C from 0.25 Ri to 1.75 Ri; wherein Ri is the initial resistance at 20°C;

characterized in that

• (III) the PTC element is a laminar element which has been prepared by applying a polymer thick film ink which comprises the organic fluoropolymer, the carbon black and a solvent; and
• (IV) the electrodes are metal sheet or conductive paint electrodes attached to the PTC element.

The conductive polymer composition in the heaters of the invention exhibits PTC behavior in the operating temperature range of the heater, i.e. it has an R₁₄ value of at least 2.5 or an R₁₀₀ value of at least 10 (and preferably both), and preferably also has an R₃₀ value of at least 6, where R₁₄ is the ratio of the resistivities at the end and the beginning of a 14°C range, R₁₀₀ is the ratio of the resistivities at the end and the beginning of a 100°C range, and R₃₀ is the ratio of the resistivities at the end and the beginning of a 30°C range. By contrast, a composition having "ZTC" character increases in resistivity by less than 6 times, preferably less than 2 times, in any 30°C temperature range within the operating range of the heater.

The carbon blacks used in this invention have a pH less than 4.0, preferably less than 3.0, measured prior to mixing with the polymer. Such a carbon black generally has a relatively high volatile content, i.e. a high amount of oxygen chemisorbed on its surface. The amount of oxygen can be increased by oxidation in a post-production process. The resulting carbon black will have a higher surface activity. Particularly preferred are carbon blacks having a ratio of resistivity (in ohm-cm) to particle size (in nanometers) less than or equal to 0.1, preferably less than or equal to 0.09, particularly less than or equal to 0.08. The resistivity is determined by the procedure described in Columbian Chemicals Company bulletin "The Dry Resistivity of Carbon Blacks" (AD1078). In this test, 3 grams of carbon black in powder form are placed inside a glass tube between two brass plungers. A 5 kg weight is used to compact the carbon black. The height of the compacted carbon black and the resistance in ohms between the brass plunger electrodes are noted, and the resistivity is calculated.

Commercially available low pH carbon blacks can be used. Alternatively, nonoxidized carbon blacks may be treated, e.g. by heat or appropriate oxidizing agents, to produce carbon blacks with appropriate surface chemistry.

Other conductive fillers may be used in combination with the low pH carbon black. These fillers may comprise nonoxidized carbon black (i.e. a carbon black with a pH of at least 5.0), graphite, metal, metal oxide, or any combination of these. Preferably the low pH carbon black is present in amount at least 5%, preferably at least 10%, particularly at least 20%, e.g. 25 to 100%, by weight of the total conductive filler, and/or in amount at least 5%, preferably at least 6%, particularly at least 8%, by weight of the solid components of the total composition.

The fluoropolymers used in this invention have a crystallinity of at least 5%, preferably at least 10%, particularly at least 15%, e.g. 20 to 30%. Suitable polymers include polyvinylidene fluoride, ethylene/tetrafluoroethylene copolymers, and terpolymers of vinylidene fluoride, hexafluoropropylene, and tetrafluoroethylene; and blends of two or more such polymers. The term "fluoropolymer" is used herein to denote a polymer which contains at least 10%, preferably at least 25%, by weight of fluorine, or a mixture of two or more such polymers. In order to achieve specific physical or thermal properties for some applications, it may be desirable to blend one crystalline polymer with another polymer, either crystalline or amorphous. When there are two or more polymers in the composition, the blend must have a crystallinity or at least 5%. The crystallinity, as well as the melting point T_m, are determined from a DSC (differential scanning calorimeter) trace on the conductive polymer composition. The T_m is defined as the temperature at the peak of the melting curve. If the composition comprises a blend of two or more polymers, T_m is defined as the lowest melting point measured for the composition (often corresponding to the melting point of the lowest melting component).

The composition may comprise additional components, e.g. inert fillers, antioxidants, flame retardants, prorads, stabilizers, dispersing agents. Mixing is preferably conducted by solvent-blending. The composition may be crosslinked by irradiation or chemical means.

The resistivity of the conductive polymer composition depends on the function of the heater, the dimensions of the PTC element, and the power source to be used. The resistivity may be, for example, 10 to 1000 ohm-cm for heaters powered at 6 to 60 volts, or 1000 to 10,000 ohm-cm or higher for heaters powered at voltages of at least 110 volts. The laminar PTC element may be of any shape to meet the requirements of the heater, and may be screenprinted or applied in any suitable configuration. The electrodes, are in the form of metal sheet or conductive (e.g. metal- or carbon-filled) paint.

The heaters of the invention show improved stability under thermal aging and electrical stress. When a heater is maintained at a temperature equal to Tm for a period of 50 hours, the resistance at 20°C measured after aging, i.e. R_f50, is from 0.25 R_i to 1.75 R_i, preferably from 0.40 R_i to 1.60 R_i, particularly from 0.50 R_i to 1.50 R_i, where R_i is the initial resistance at 20°C. If a similar test is conducted for 300 hours, the resistance at 20°C after 300 hours, R_f300, is generally from 0.50 R_i to 1.50 R_i, preferably from 0.60 R_i to 1.40 R_i, particularly from 0.70 R_i to 1.30 R_i. It is to be understood that if a heater meets the resistance requirement when tested at a temperature greater than T_m, it will also meet the requirement when tested at T_m. Similar results will be observed when the heater is actively powered by the application of voltage. The change in resistance may reflect an increase or decrease in heater resistance. In some cases, the resistance will first decrease and then increase during the test, possibly reflecting a relaxation of mechanically-induced stresses followed by oxidation of the polymer. Particularly preferred compositions may exhibit stability which is better than a 30% change in resistance.

The invention is illustrated by the following examples, some of which are comparative examples, as indicated by an asterisk *.

Examples 1 to 10

For each example, an ink was prepared by blending the designated percent by weight (of solids) of the appropriate carbon black with dimethyl formamide in a high shear mixer. The solution was the filtered and powdered Kynar™ 9301 (a terpolymer of vinylidene fluoride, hexafluoropropylene, and tetrafluoroethylene with a melting point of about 88°C, available from Pennwalt) in amount equal to (100 - % carbon black) was added to the filtrate and allowed to dissolve over a period of 24 to 72 hours. (Approximately 60% solvent and 40% solids was used in making the ink). Silver-based ink electrodes (Electrodag™ 461SS, available from Acheson Colloids) were printed onto ethylene-tetrafluoroethylene substrates and samples of each ink were applied. Samples of each ink were aged in ovens at temperatures of 65, 85, 107 and 149°C. Periodically, the samples were removed from the oven and the resistance at room temperature (nominally 20°C), R_t, was measured. Normalized resistance, R_n, was determined by dividing R_t by the initial room temperature resistance, R_i. The extent of instability was determined by the difference between R_n and 1.00. Those inks which comprised carbon blacks with a pH of less than 4 were generally more stable than the inks comprising higher pH blacks. TABLE I

Stability of Conductive Inks After Aging at Elevated Temperature for 300 Hours (Resistance Measured at Room Temperature)
Carbon Example/Black	pH	Wt% CB	Rn @ 65°C	Rn @ 85°C	Rn @ 107°C	Rn @ 149°C
*1 Conductex™ SC	7.0	3.0	1.22	1.75	5.61	6.39
*2 Raven™ 1500	6.0	3.0	1.01	1.92	11.88	20.0
*3 Raven™ 890	6.5	6.0	1.27	1.77	2.92	6.07
*4 Raven™ 850	7.0	4.0	1.32	2.05	4.08	8.48
*5 Raven™ 1000	6.0	4.0	1.18	1.43	1.94	4.40
*6 Raven™ 16	7.0	5.6	1.11	1.89	-	-
7 Raven™ 5750	2.1	8.1	0.87	0.92	0.97	0.56
8 Raven™ 1040	2.8	9.1	0.96	1.15	1.47	1.34
9 Raven™ 1255	2.5	6.0	1.04	1.26	1.12	0.65
10 Raven™ 14	3.0	7.0	0.82	1.00	-	-
Notes to Table I: (1) Conductex and Raven are trademarks for carbon blacks available from Columbian Chemicals. (2) Wt% CB indicates the percent by weight of carbon black used in each ink. (3) Carbon blacks in Examples 1, 2 and 3 produced inks with ZTC character.

Measurements on two samples at 93°C (i.e. T_m + 5°C) showed that after 50 hours Example 6 (pH = 7.0) had an R_n of 2.53 and Example 10 (pH 3.0) had an R_n of 1.48.

The R_n values for Examples 1 to 6 and Example 7 to 10 were averaged for each time interval at the test temperatures. The results, shown in Table II, indicate that the carbon blacks with high pH values were significantly less stable than those with low pH values.

Additional tests were conducted on samples from Examples 6 and 10 in order to determine the stability of the compositions under applied voltage. After measuring the initial room temperature resistance, the samples were placed in environmental chambers maintained at either 20 or 65°C and appropriate voltage was applied to each sample in order to produce comparable watt densities. Periodically, the voltage was disconnected and the resistance of each sample measured. R_n was calculated as previously described. It is apparent from the results in Table III that the samples containing the oxidized carbon black were more stable than those with nonoxidized carbon black.

Examples 11 to 14

Following the procedure of Examples 1 to 10, inks were prepared using Kynar™ 9301 as a binder and incorporating the carbon blacks listed in Table IV. The resistance vs. Temperature characteristics were measured by exposing samples of each ink to a temperature cycle from 20°C to 82°C. The height of the PTC anomaly was determined by dividing the resistance at 82°C (R₈₂) by the resistance at 20°C (R₂₀). It was apparent that at comparable resistivity values, the PTC anomaly was higher for the low pH carbon blacks than for the high pH carbon blacks. TABLE IV

Carbon Example/Black	pH	D (nm)		DBP (cc/100g)			Wt%	Rho (ohm-cm)	PTC Height
*11 Raven™ 1000	6.0	28	95	63	2.46	0.088	4.0	750	3.1x
12 Raven™ 1040	2.8	28	90	99	19.20	0.695	9.1	720	13.0x
*13 Raven™ 450	8.0	62	33	67	1.36	0.021	5.0	150	23x
14 Raven™ 14	3.0	59	45	111	4.36	0.074	12.0	100	42x

Claims (7)

1. An electrical heater which

   (I) comprises

   (A) a PTC element which is composed of a conductive polymer composition which (i) exhibits PTC behavior, (ii) has a resistivity of 10 to 10,000 ohm-cm, and (iii) comprises

   (a) an organic fluoropolymer matrix which has a crystallinity of at least 5% and a melting point T_m, and

   (b) at least 4% by weight of the composition of a carbon black which has a pH of less than 4.0; and

   (B) two electrodes which can be connected to a source of electrical power to pass current through the PTC element;

   (II) is such that if it is maintained at T_m for 50 hours, and is then cooled to 20°C, it has a resistance at 20°C from 0.25 R_i to 1.75 R_i; wherein R_i is the initial resistance at 20°C;

   characterized in that

   (III) the PTC element is a laminar element which has been prepared by applying a polymer thick film ink which comprises the organic fluoropolymer, the carbon black and a solvent; and

   (IV) the electrodes are metal sheet or conductive paint electrodes attached to the PTC element.
2. A heater according to claim 1 characterized in that the PTC element has been prepared by screen printing the polymer thick film ink.
3. A heater according to claim 1 or 2, characterized in that the carbon black having a pH of less than 4.0 is the sole conductive filler in the conductive polymer composition.
4. A heater according to any one of the preceding claims characterized in that the carbon black has a pH of less than 3.0.
5. A heater according to any one of the preceding claims, characterized in that the composition further comprises (i) carbon black which has a pH which is at least 5.0 or (ii) graphite.
6. A heater according to any one of the preceding claims characterized in that the carbon black has a particle size of D nanometers and a dry resistivity R_CB such that (R_CB/D) is less than or equal to 0.1.
7. A heater according to any one of the preceding claims characterized in that the conductive polymer has been crosslinked.