JPH0256886A - Self-controlling strip heater - Google Patents

Abstract

PURPOSE: To carry out melt extrusion on an electrode by specifying a linear ratio of carbon black as a filling agent for a melting thermoplastic conductive polymer composition and a strip heater. CONSTITUTION: A means suitable to be used for an electric circuit in which electric current flows to electrodes through a conductive polymer is constituted of the electrodes and the conductive polymer composition brought into contact with the electrodes. The content of carbon black in the conductive polymer composition is higher than 15% and the average linear ratio between any couple of electrodes of the strip heater does not exceed 1.2. Consequently, the electrodes and the polymer composition are separately heated before they are brought into contact with each other and melting extrusion is carried out on an electrode by carrying out extrusion molding on the surrounding of an electric wire electrode.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electrical means in which an electrode is in contact with a conductive polymer composition, and a method for making the same.

Conductive polymer compositions are well known. It consists of a finely divided conductive polymer 1, for example carbon black or an organic polymer containing particulate metals dispersed therein. Some such compositions are so-called PTC (Posi
tive TemperatureCoefTicie
nt; positive temperature coefficient) behavior. The terms conventionally used to describe r"rc behavior are varied and inaccurate. As used herein, the terms "composition exhibiting rPTC behavior" and "rPTC composition" refer to -100°C
to approximately 250°C (hereinafter referred to as the "critical range").
”), wherein at the beginning of this temperature range the composition has a resistivity of less than about 105 ohm-cm; and within this temperature range the composition has a resistivity of at least 2.5 Refers to a composition having an R14 value or an R10G value (preferably both) of at least 10, and preferably an R3° value of at least 6, where R14 is 14
It is the ratio of resistivity at the end and beginning of the °C range, R+
oo is the ratio of the resistivity at the end of the 100°C range and the beginning, and Rso is the ratio of the resistivity at the end and the beginning of the 30°C range. The term "rPTC element" refers to the element comprising the PTC composition described above. When the logarithm of the resistance of a PTC composition measured between two electrodes in contact with the element is plotted against temperature, a sharp change in slope is often, but not always, observed over a portion of the critical temperature range. In such cases, the term "switching temperature" is used to refer to the temperature at the intersection of the extrapolated substantially linear sections on either side of the section exhibiting the abrupt change in slope.
(commonly abbreviated as Ts). PTC compositions in such PTC elements are herein described as "useful Tsj", where Ts is between 0°C and 175°C, e.g. 1 from 50℃
Preferably it is between 20°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. 65℃
It will only heat up to temperatures below.

[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 a flexible strip heater consisting of an outer layer of protective and insulating fabric. Particularly useful heaters are those whose composition exhibits PTC behavior and are therefore self-regulating. In the manufacture of such heaters, the composition of which contains less than 15% carbon black,
To obtain a sufficiently low resistivity, 2L+51oog+o
The prior art teaches that it is necessary to anneal the heater for an extended period of time such that R<45, where % is the weight percent of the carbon black and R is the resistivity in ohms and centimeters at room temperature. has been done.

Disadvantages particularly associated with strip heaters in devices consisting of an electrode and a conductive polymer composition in contact therewith are:
The longer it is used, the higher its resistance and the lower its power output, especially when it is subjected to thermal cycling.

of the contact resistance between the electrode and the carbon black filled rubber,
Differences between instruments can be an obstacle to the comparison of the electrical properties of such instruments and to the accurate measurement of the resistivity of such rubbers, especially at high resistivities and low voltages. known; 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 rubber filled with carbon black and a test electrode placed in contact with it. The preferred method is
This involves vulcanizing the rubber while bringing it into contact with a brass electrode. Other methods include copper-plating, vacuum coating with gold, and using a colloidal solution of graphite between the electrode and the specimen. For more information, please visit Applied Science Publishers (A ppl
1edS science publishers)
Conductive Rubber and Plastics by R.H. Norman, published by
ve Rubber and P 1astics”)
(1970), Chapter 2. It will become clear from this 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, need to be in contact with each other to achieve the desired outcome is very short. A time of more than 5 minutes does not result in a further significant reduction in contact resistance, and times of less than 1 minute are often sufficient and are therefore recommended. Processing times are thus similar to those described, for example, in U.S. Pat.
The times required by known annealing treatments to reduce the resistivity of compositions, such as those described in No. 363, are of a completely different order of magnitude.

Accordingly, in one aspect, the present invention produces a means suitable for use in an electrical circuit comprising an electrode and a conductive polymer composition in contact therewith, in which electrical current is passed through the conductive polymer to the electrode. 1. A method comprising melt extruding a molten thermoplastic conductive polymer composition onto an electrode, the method comprising: (1) when the electrode is first contacted with the conductive polymer, the electrode is heated to 65°C; (2) The electrode and the conductive polymer in contact with it are at a temperature exceeding Tm
(Tn+ is the temperature at which the conductive polymer composition begins to melt)
The method of manufacture is characterized in that the conductive polymer is maintained at a temperature exceeding 100 mL for a time sufficient to reduce the contact resistance between the conductive polymer and the electrode. It is preferred that both 'rp and Te be at least 20° C., especially at least 55° C. higher than Tm. Tp and Te are the ring and ball type (Ri) of the polymer composition.
ng-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 614 kg/c+n', preferably at least 21 kg/am"1, such as from 21 to 200 kg/cm", especially at least 35 kg/cm".
", for example 35-70 kg/am"ζ.

The inventors have also found that contact resistance correlates with the force required to peel the electrode from the polymer composition. The invention therefore further provides means comprising an electrode, in particular a stranded wire electrode embedded in the conductive polymer composition, in contact with the conductive polymer composition, comprising means for stripping the electrode from the means. Provide a force (P) that is at least 1.4 times Po. where 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
is measured at 21'C as detailed below.

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. The movable clamp is then moved vertically downward at 5.1 cm/min and the maximum force required to peel the conductor from the sample is measured.

The inventors also found that for strip heaters, which are currently the most widely used means by which electrical current is passed through conductive polymer compositions, contact resistance is a readily measurable quantity as described below. linearity ratio
It was also found that it can be correlated with the ratio. Therefore, the present invention further provides (1) exhibiting PTC behavior;
a melt-extruded core of a conductive polymer composition comprising carbon black dispersed in a crystalline polymer; (2) embedded in the conductive polymer composition parallel to each other in a longitudinal direction; (3) a layer of a protective and insulating composition surrounding the conductive polymer; (i) the content (by weight) of carbon black is (a);
) more than 15% or less than (b month 5%) the resistivity (R) of the composition.

(ohm cm) satisfies 2 L + 5 log+oR > 45, and (ii) the average linearity ratio between any pair of electrodes does not exceed 1°2. The linearity ratio of a strip heater is defined as the resistance at 30 MV and the resistance at 100 mV, which is the resistance between two electrodes contacted by a probe penetrating the outer coating and conductive polymer core of the strip heater. measured at 21'C.

Since the contact resistance can be ignored at 100V, the closer the linearity ratio is to l, the smaller the contact resistance becomes.

The invention is useful with any type of electrode, such as a metal plate, strip or wire, but with irregular surfaces, such as twisted wire electrodes, braided wire electrodes, etc., such as those commonly used in strip heaters. electrodes (e.g. those described in West German Patent Application No. 2,635.000-5) and West German Patent No. 2,655,5
Expander pull (exp) as described in No. 43-1
andable)? This is especially true for the It pole. Preferred stranded wires are silver-coated and nickel-coated copper wires, which are less susceptible to melting or oxidation than tin-coated or uncoated copper wires. However, 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 contains carbon black as a filler in an amount of about 15% by weight, e.g.
It is contained in an amount of more than 20% by weight.

In many cases it is preferred that the composition exhibit PTC behavior.

The resistivity of the composition is typically 50,000 ohms at 21°C.
centimeters, for example lOO to 50,000 ohm centimeters. For strip heaters designed to be powered by +15 volts or more alternating current, the composition is typically between 2,000 and 50.
00 ohm cm, e.g. 2.000 to 40.0
It has a resistivity of 0.00 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 integrate tightly with the electrode. Preferably, the polymer is a crystalline polymer.

The strip heaters to which the present invention relates generally have 0.1
The two electrodes are separated by a distance of 5 to 1 cm, but larger separations, such as 2.5 cm or more, can also be used. Conductive polymer 6
The nucleus may be of conventional shape, provided that it has a minimum dimension, in particular not more than three times the circular cross-section, in particular 1.5
Preferably, it has a cross-section of no more than a factor of 1.1, for example no more than a factor of 1.1.

In one preferred embodiment of the invention, the electrode and polymer composition are heated separately before being contacted. In the folded embodiment, the composition is, for example, cut into a cross-head die (c
by extrusion molding around a pair of spaced wire electrodes using a ross head die).
Preferably, it is melt extruded onto the electrode. The electrode is preheated to a temperature (Te) which can 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 considerably 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 invention, the composition is brought into contact with the electrode (without preheating the electrode) and then heated while in contact 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 invention is useful, for example, when there is no need or preference for an annealing treatment when the composition contains more than 15% by weight of carbon black, such as more than 17% or 20% by weight; At the end of the process, carbon black content (L) and resistivity (R
) is particularly useful if only limited annealing is performed such that 2 L + 51ogloR>45.

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 another embodiment of the invention, the conductive polymer composition is initially in the form of a shaped article, such as one or more globules or pellets, and is not shaped into contact with the 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 forces with the aid of crosslinking initiators such as peroxides are preferred.

The invention is illustrated by the following examples. Some of these are comparative examples.

Strip heaters on each side were manufactured as described below. The conductive polymer composition is prepared by blending medium density polyethylene containing an antioxidant with a carbon black masterbatch containing an ethylene/ethyl acrylate copolymer to provide a composition containing the indicated weight percent carbon black. Obtained. The melting point (ie, the temperature at which the composition begins to melt) of the composition was about 100°C, and the temperature at which the composition completed melting was about 115°C. Diameter 0
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-head die having a ., 36 cm circular orifice. Each wire was 0.08 cm in diameter, consisted of 19 strands, and the axis of the wire was at a point on and 0.2 cm away from the orifice diameter. Before reaching the Cross-Head Rad 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 is
The speed of the wire passing through the oven and die is 21 m/
In Comparative Examples 1.4 and 6, the temperature was 82°C,
In Examples 2 and 7, the temperature was 165°C, and in Example 3
and 5, it 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. On each side, the linearity ratio of the heater was measured, then the heater was connected to a 120 volt AC power source and the ambient temperature was varied continuously in 3 minute cycles. That is, the temperature was increased from -37°C to 65°C over 90 seconds, and then decreased to -37°C over the next 90 seconds.

The maximum power output of the heater in each cycle is measured initially and from time to time and the ratio of the initial maximum power output (PN
).

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.

PN LRPN LRPN No LR l
1.31 1.11 1500 0.51
．． 61.3 - 1 11100 0.3
2.1 1.2 - 1 11700
- -1.1 1.1 1 1* 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 2% 11! Carbon black percentage *4 22 *6 23 * Comparative example linearity ratio! , 6 1.0 1.35 1.1 The peel strength of the heater strips of Examples 7 and 6 is 1.
It was 45.

Particularly preferred embodiments of the present invention are listed below; 1, (1)
It exhibits PTC behavior and has a resistivity of 50.0 at 2+'C.
(2) a melt-extruded core of a conductive polymer composition having a diameter of 0.00 ohm-cm and comprising carbon black dispersed in a crystalline polymer; at least two buried in parallel and extending longitudinally;
(3) a layer of a protective and insulating composition surrounding the electrically conductive polymer; (i) the content of carbon black in the electrically conductive polymer is at least 15% by weight; and (ii) the average linearity ratio between any pair of electrodes does not exceed 12, and the average linearity ratio between any pair of electrodes does not exceed 110. Strip heater 3, # The conductive polymer composition is a composition extruded at a temperature above the melting point of the conductive polymer composition to reduce the resistivity to below 50,000 ohm cm at 21°C. The strip heater according to item 1 or 2 above, which can be melt-extruded under conditions that do not require annealing.

Claims (1)

[Claims] 1. (1) A melt-extruded core of a conductive polymer composition exhibiting PTC behavior, having a resistivity of 50,000 ohm-cm at 21° C., and comprising carbon black dispersed in a crystalline polymer. (2) at least two longitudinally extending electrodes embedded parallel to each other in the conductive polymer composition; and (3) a layer of a protective and insulating composition surrounding the conductive polymer composition. (i) the content of carbon black in the conductive polymer is at least 15% by weight; and (ii) the average linearity ratio between any pair of electrodes does not exceed 1.2. A self-regulating strip heater. 2. 2. The strip heater of claim 1, wherein the average linearity ratio between any pair of electrodes does not exceed 1.10. 3. The conductive polymer composition has a temperature of 50,00 at 21°C.
under conditions such that there is no need to anneal the extruded composition at a temperature above the crystalline melting point of the conductive polymer composition to reduce the resistivity to below 0 ohm-cm;
The strip heater according to item 1 or 2 above, which can be melt-extruded.