JPH0559557B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to electrical means in which an electrode is in contact with a conductive polymer composition, and to a method of manufacturing. Conductive polymer compositions are well known. They consist of organic polymers containing dispersed finely divided conductive fillers, such as carbon black or particulate metals. Some of these compositions are so-called PTC (Positive Temperature Coefficient;
positive temperature coefficient) behavior. The terminology traditionally used to describe PTC behavior is varied and inaccurate. As used herein, the terms "composition exhibiting PTC behavior" and "PTC composition" refer to
At least one temperature range within the range -100°C to approximately 250°C (hereinafter "critical range")
range”), the composition having a
At the beginning of this temperature range the composition has a resistivity of about ¹⁰⁵ ohm-cm or less; and within this temperature range the composition has an _R14 value of at least 2.5 or an _R100 value of at least 10 (preferably and desirably have an R ₃₀ value of at least 6. Here, R ₁₄ is the ratio of the resistivity at the end of the 14°C range and the beginning, R ₁₀₀ is the ratio of the resistivity at the end and the beginning of the 100°C range, and R ₃₀ is the ratio of the resistivity at the end of the 30°C range. It is the ratio of the resistivity at the starting point and the starting point. The term "PTC element" refers to an element consisting of the PTC composition described above. When plotting the logarithm of the resistance of a PTC composition measured between two electrodes in contact with the element versus temperature, it is often
However, it is not always the case that a sharp change in slope is observed over a portion of the critical temperature range, in which case it is possible to extrapolate a substantially linear section on either side of the section showing the sudden change in slope. The term "switching temperature" (usually abbreviated as Ts) is used to refer to the temperature at the point where the two intersect. PTC compositions in such PTC elements are described herein as having a "useful Ts." Ts from 0℃
Preferably it is between 175°C, for example between 50°C and 120°C. Conductive polymer compositions, particularly PTC compositions, are useful in electrical means where the composition contacts an electrode, usually a metal electrode. Devices of this type are usually manufactured by a process consisting of extruding or casting a molten polymer composition around or onto one or more electrodes. In known methods, the electrode is not heated prior to contact with the polymeric composition, or is heated to a localized extent, e.g., to a temperature well below the melting point of the composition, e.g., as in conventional wire coating techniques. It can only be heated to temperatures below 65°C.
[Temperatures throughout this specification are expressed in degrees Celsius. ] Well-known examples of such means include a generally ribbon-shaped core of conductive polymer composition, a pair of longitudinally extending electrodes embedded near the ends of the core, generally stranded electrical wires. , and an outer layer of protective and insulating composition. Particularly useful heaters are those whose composition is
It exhibits PTC behavior and is therefore self-regulating. In the manufacture of such heaters, whose composition contains less than 15% carbon black, in order to obtain a sufficiently low resistivity,
In the prior art, it was necessary to anneal the heater for a long period of time to obtain 2L + 5log ₁₀ R45 (where L is the weight percent of carbon black and R is the resistivity in ohms and centimeters at room temperature). taught by. A disadvantage of devices consisting of electrodes and conductive polymer compositions in contact with them, particularly with strip heaters, is that the longer they are used, the higher their resistance and the lower their power output. This is especially true when it is subjected to thermal cycling. Means-to-means differences in contact resistance between electrodes and carbon black-filled rubber are important for comparison of the electrical properties of such means and for comparisons of the electrical properties of such rubbers, especially at high resistivities and low voltages. It is known to be an obstacle to accurate measurements of resistivity; and it has been suggested that the same is true for other conductive polymer compositions. Various methods have been proposed to reduce the contact resistance between a rubber filled with carbon black and a test electrode placed in contact with it. A preferred method is to vulcanize the rubber while it is in contact with a brass electrode. Another method is
These include copper-plating, vacuum coating with gold, and using a colloidal solution of graphite between the electrode and the specimen. For more information, visit Applied Science Publishers.
“Conductive Rubber and plastics” by R.H. Norman, published by Science Publishers.
(1970), Chapter 2. from now,
It will be clear that the factors governing the magnitude of such contact resistance have not been fully elucidated. The inventors have found that the lower the initial contact resistance between the electrode and the conductive polymer composition, the lower the increase in total resistance over time. The inventors have also discovered that by placing or maintaining the electrode and polymer together in contact with each other at a temperature above the melting point of the composition, the contact resistance between them is reduced. The term "melting point of a composition" is used herein to refer to the temperature at which a composition begins to melt. The amount of time that the electrode and the composition, each at a temperature above the melting point of the composition, must be in contact with each other to achieve the desired outcome is:
Extremely short. A time of more than 5 minutes does not further significantly reduce the contact resistance, and times of less than 1 minute are often sufficient and are therefore recommended. The processing times are thus orders of magnitude different compared to the times required by known annealing treatments for reducing the resistivity of compositions, such as those described in U.S. Pat. Nos. 3,823,217 and 3,914,363. . Accordingly, in accordance with one aspect of the invention, the invention provides an electrical heating circuit comprising: (A) a power source; and (B) a self-regulating strip heater; The heater (1) (a) exhibits PTC behavior, (b) has a resistivity at 21°C of less than 50,000 ohm-cm, and (c) contains carbon black dispersed in a crystalline polymer. a melt-extruded core of a conductive polymer composition comprising: (2) (a) embedded parallel to each other in the conductive polymer composition; and (b) longitudinal ends connected to the power source. (i) a carbon black content (L) in the conductive polymer composition; , weight %) is (a) not more than 15 weight %, and (b) has a relationship to the resistivity (R, ohm cm) of the composition at 21°C as follows: 2L + 5log ₁₀ > 45 , (ii) the average linearity ratio between the electrodes does not exceed 1.2;
and (iii) the power source has a voltage of at least 100 volts. The self-regulating strip heater of the present invention comprises an electrode and a conductive polymer composition in contact therewith, and is suitable for use in an electrical circuit in which current flows through the conductive polymer to the electrode. A method of making a suitable means, the method comprising melt extruding a molten thermoplastic conductive polymer composition onto an electrode, the method comprising: (1) contacting the electrode with the conductive polymer for the first time; Time,
The electrode is at a temperature above 65°C, and (2) the electrode and the conductive polymer in contact with it are
Tm (Tm is the temperature at which the conductive polymer composition begins to melt) for a time sufficient to reduce the contact resistance between the conductive polymer and the electrode. It can be manufactured by a method. Both Tp and Te are preferably at least 20°C higher than Tm, especially at least 55°C.
Tp and Te both represent the ring and ball structure of the polymer composition.
-and-Ball) above the softening temperature is often preferred. It is desirable to subject the conductive polymer composition to pressure to help intimately integrate the conductive polymer composition with the electrode. The pressure is generally at least 14
Kg/cm ² , preferably at least 21 Kg/cm ² , for example 21
and 200 Kg/cm ² , especially at least 35 Kg/cm ² , for example 35 to 70 Kg/cm ² . The inventors have also discovered that contact resistance can be correlated to the force required to peel the electrode from the polymer composition. According to this, the means consisting of an electrode in contact with a conductive polymer composition, in particular a stranded wire electrode embedded in the conductive polymer composition, has a force (P) for peeling the electrode from the means. is preferably at least 1.4 times Po. Here Po consists of the same electrode in contact with the same conductive polymer composition, and that it is in contact with the conductive polymer composition melting the electrode at a temperature not higher than 24°C and the polymer composition It is the force required to peel off the same electrode from a means manufactured by a method consisting of cooling the object while in contact with the electrode. Peeling force P
and Po at 21°C as detailed below.
Measure with. Cut a 5.1 cm long linear sample of a heater strip (or other means) containing a 5.1 cm long electrode. Peel the polymer from the electrode over 2.5 cm at one end of the sample. The exposed electrode is lowered through a small hole slightly larger than the electrode in a rigid metal plate held in a horizontal plane. The bare end of the electrode is tightly clamped by a movable clamp below the metal plate, and the other end of the sample is held loosely above the metal plate to keep the electrode vertical. Next, move the movable fasteners.
Move vertically downward at 5.1 cm/min and measure the maximum force required to peel the conductor from the sample. We also found that for strip heaters, currently the most widely used means by which electrical current is passed through conductive polymer compositions, contact resistance is a quantity that can be easily measured as described below. It has also been found that it can be correlated with the linearity ratio. In the present invention, the linearity ratio of a strip heater is defined as the resistance value at 30 mV/resistance value at 100 V, and the resistance value is determined by contacting with a probe penetrating the outer coating and conductive polymer core of the strip heater. Measured at 21°C between two electrodes. Since the contact resistance can be ignored at 100V, the closer the linearity ratio is to 1, the smaller the contact resistance. The invention is useful with any type of electrode, such as a metal plate, strip or wire, but also with electrodes having irregular surfaces, such as twisted wire electrodes such as those commonly used in strip heaters. This is particularly the case for woven wire electrodes (such as those described in DE-OS 26 35 000-5) and expandable electrodes such as those described in DE-OS 2 655 543-1. Preferred stranded wires are silver-coated and nickel-coated copper wires, which are less susceptible to problems such as melting or oxidation than tin-coated or uncoated copper wires. Of course, the latter type of copper wire can also be used without difficulty as long as the operating temperature is not too high. The conductive polymer composition used in the present invention generally includes:
Contains carbon black as a filler in an amount of up to 15% by weight. Often the composition is PTC
It is preferable to show behavior. The resistivity of the composition is generally lower than 50,000 ohm cm at 21°C, e.g.
100 to 50,000 ohm cm. For strip heaters designed to be powered by 115 volts or more alternating current, the composition typically has a resistivity of 2,000 to 50,000 ohm-cm, such as 2,000 to 40,000 ohm-cm. Preferably, the composition is thermoplastic. However, it may be lightly crosslinked, or in the process of crosslinking, as long as it is sufficiently fluid under the contact conditions to become intimately integrated with the electrode. Preferably, the polymer is a crystalline polymer. The strip heater of the electrical heating circuit of the present invention generally has two electrodes separated by a distance of 0.15 to 1 cm, although larger separations, such as 2.5 cm or more, may be employed. can. The conductive polymer core may be of conventional shape, provided that it is not larger than its minimum dimension, in particular three times the circular cross-section, in particular
Preferably it has a cross-section of no more than 1.5 times, for example no more than 1.1 times. In one preferred embodiment of manufacturing the self-regulating strip heater of the electrical heating circuit of the present invention, the electrode and polymer composition are heated separately before being contacted. In this embodiment, the composition is preferably melt extruded onto the electrodes, such as by extrusion around a pair of spaced wire electrodes using a cross head die. The electrode is preheated to a temperature (Te) which may be above or below the melting temperature (Tp) of the polymeric composition, but generally above (Tp-55) and preferably above (Tp-30). Tp
will normally be significantly higher than the melting point of the composition, for example 30°C to 80°C higher. Of course, neither the electrode nor the composition should be heated to temperatures at which significant oxidation or other degradation occurs. In another embodiment of the manufacture of the self-regulating strip heater of the electrical heating circuit of the present invention, the composition is formed in contact with the electrodes (without preheating the electrodes) and then these are brought into contact with each other. While heating, the composition is heated to a temperature above the melting point of the composition. Care must be taken to ensure effective reduction of contact resistance by this method. Optimal conditions will depend on the electrode and composition, but increasing time, temperature and pressure will facilitate achieving the desired results. Pressure can be applied, for example, in a press or by nip rollers. This aspect of the manufacture of the self-regulating strip heater of the electrical heating circuit of the invention is limited, for example, so that at the end of the annealing, the carbon black content L and the resistivity R are 2L+5log ₁₀ R>45. This is particularly useful when only a typical annealing process is performed. One method of heating the electrode and the surrounding composition is to pass a high current through the electrode, thereby generating the desired heat by resistive heating of the electrode. In yet another embodiment of the manufacture of the self-regulating strip heater of the electrical heating circuit of the present invention,
The conductive polymer composition is initially in the form of a shaped article, such as one or more spherules or pellets, and is not shaped into contact with an electrode.
The electrode and composition are heated together under pressure, for example in a compression mold. Especially when the conductive polymer composition exhibits PTC behavior, it is often preferred that the composition be crosslinked in the final product. Crosslinking can be carried out as a separate step after the treatment to reduce the contact resistance, in which case crosslinking by radiation is preferred. Alternatively, crosslinking can be carried out simultaneously with the above treatment, in which case chemical crosslinking with the aid of crosslinking initiators such as peroxides is preferred. The invention is illustrated by the following examples. Some of these are comparative examples. The strip heaters in each example were manufactured as described below. The conductive polymer composition is
Medium density polyethylene containing antioxidants is blended with carbon black masterbatch containing ethylene/ethyl acrylate copolymer to achieve the indicated weight percentages.
This was obtained by providing a composition containing carbon black. The melting point of the composition (ie, the temperature at which the composition begins to melt) was about 100°C, and the temperature at which the composition completed melting was about 115°C. diameter
The composition was melt extruded at a melt temperature of about 180 DEG C. onto a pair of silver-coated stranded copper wires through a cross-headed die having a 0.36 cm circular orifice. Each wire had a diameter of 0.08 cm, consisted of 19 strands, and the axis of the wire was on the diameter of the orifice and at a point 0.2 cm away from it. cross
Before reaching the head die, the wire was preheated by passing it through a 60 cm long oven at 800°C. The temperature of the wire entering the die was 82°C in Comparative Examples 1, 4 and 6, where the speed of the wire passing through the oven and die was 21 m/min, and 165°C in Examples 2 and 7; Example 3
and 5, the temperature was 193°C. An insulating skin for the extrudate was then provided by melt extruding a 0.051 cm thick layer of chlorinated polyethylene or ethylene/tetrafluoroethylene copolymer around the extrudate.
The coated extrudate was then irradiated to crosslink the conductive polymer composition. Examples 1-3 These examples, of which Example 1 is a comparative example, illustrate the effect of linearity ratio (LR) on power output when subjecting a heater to temperature changes. In each example, measure the linearity ratio of the heater, then
Connect to a 120 volt AC power source and reduce the ambient temperature to 3.
Continuously varied in minute cycles. That is, the temperature was increased from -37℃ to 65℃ over 90 seconds, and then
The temperature was lowered to -37°C over 90 seconds. The maximum power output of the heater in each cycle was measured initially and from time to time and expressed as a ratio of the initial maximum power output (P _N ). The polymer composition used in Example 1 contained approximately 26% carbon black. The polymer compositions used in Examples 2 and 3 contained approximately 22% carbon black. The results obtained are shown in Table 1 below.

Table * Comparative Examples Examples 4-7 These examples, summarized in Table 2 below, illustrate the effect of preheating the electrode on the linearity ratio and peel strength of the product.

[Table] * Comparative Example The peel strength of the heater strips of Examples 7 and 6 was 1.45.

Claims (1)

[Scope of Claims] 1. An electrical heating circuit comprising (A) a power source and (B) a self-regulating strip heater, the self-regulating strip heater comprising: (1) (a) melting of a conductive polymer composition exhibiting PTC behavior, (b) having a resistivity at 21°C of less than 50,000 ohm-cm, and (c) comprising carbon black dispersed in a crystalline polymer. an extruded core; (2) two longitudinally extending electrodes (a) embedded parallel to each other in the conductive polymer composition; and (b) connected to the power source; 3) a layer of a protective and insulating composition surrounding the conductive polymer; (i) the carbon black content (L, weight %) of the conductive polymer composition is: (a) 15% by weight; % or less, and (b) the resistivity (R, ohm cm) of the composition at 21°C has a relationship of 2L + 51og ₁₀ R>45, and (ii) the average linearity between the electrodes. The ratio does not exceed 1.2,
and (iii) an electrical heating circuit characterized in that the power supply is at a voltage of at least 100 volts. 2. The electrical heating circuit according to item 1 above, wherein the average linearity ratio between the electrodes does not exceed 1.10.