JPH0256887A - 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 and making no need of annealing treatment. 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 L (wt.%) of carbon black in the conductive polymer composition is not higher than 15% and the resistance R (ohm.centimeter) of the composition is so controlled as to satisfy 2L+5log10 R >45 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. They consist of organic polymers containing finely divided conductive fillers, such as carbon black or particulate metals, dispersed therein. Some such compositions are so-called PTC (Posit
ive Temperature Coer ric
1ent; positive temperature coefficient) behavior. The terminology traditionally used to describe PTC behavior is varied and inaccurate. As used herein, the terms "composition exhibiting rpTc behavior" and "rp'rc composition" refer to -10
At least one temperature range within the range of 0°C to approximately 250°C (hereinafter referred to as the "critical range")
ge"), 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 or an R14 value of at least 10.

(preferably both) and preferably has an R8° value of at least 6. Here, [14 is the ratio of resistivity at the end of the 14°C range and the beginning, R10° is the ratio of the resistivity at the end and the beginning of the 100°C range, and R8° is the ratio of resistivity at 30°C. It is the ratio of the resistivities at the end and the beginning of the range.The term "rPTC element" means an element made of the PTC composition described above.The ratio of the resistivity of the PTC composition measured between two electrodes in contact with the element. When plotting logarithmic values against temperature, 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 such cases, a substantially linear portion on either side of the portion showing the sudden change in slope may be extrapolated. The term “switching temperature” refers to the temperature at the point where things intersect.
We use the term "g temperature" (usually abbreviated as Ts).
C compositions are described herein as having a useful TsJ, where Ts is between 0°C and 175°C, e.g.
Preferably it is between 120°C and 120°C.

“Conductive polymer compositions, particularly PTC compositions, are useful in electrical means in which the composition contacts an electrode, usually a metal electrode. It is usually manufactured by a process consisting of extruding or casting a molten polymer composition onto or onto the polymer composition. In known methods, the electrode is not heated prior to contact with the polymer composition, or 65 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 ℃.

[Temperatures throughout this specification are expressed in degrees Celsius. ] Well-known examples of such means include a generally ribbon-shaped core of a conductive polymeric composition, a pair of longitudinally extending electrodes embedded near the ends of the core, typically stranded electrical wires. ,
and a flexible strip heater consisting of an outer layer of protective and insulating composition. 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+ 51008+
The prior art has shown that it is necessary to anneal the heater for a long period of time such that on < 45 (where L is the weight percent of carbon black and R is the resistivity in ohm-cm at room temperature). taught.

? ! One disadvantage of devices consisting of electrodes and conductive polymer compositions in contact with them, especially 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.

of the contact resistance between the electrode and the carbon black filled rubber,
``Differences between tools pose an obstacle to the comparison of the electrical properties of such tools and to the accurate measurement of the resistivity of such rubbers, especially at high resistivities and low voltages. I don't know.

It has been suggested that the same holds 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. A preferred method is to vulcanize the rubber while avoiding contact with the brass electrodes. 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.
"ConducLive Rubber and Plastics" by R.H. Norman, published by CE Publishers.
and P 1assies” (1970).
Please refer to chapter. 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 discussed, 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
(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. I will provide a. Both Tp and Te are preferably at least 20°C higher than Tm, especially at least 55°C.

TI) and Te both represent the ring and ball formula (Ring) of the polymer composition.
-and-B at l) 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', preferably at least 2 I kg/cm', such as from 21 to 200 kg/cm', especially at least 35 kg/cm',
For example, 35 to 70 kg/cm".

The inventors have also found that contact resistance scales with the force required to peel the electrode from the polymeric 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 21C 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 in the case of strip heaters, where the current amount of current passed through a conductive polymer composition is a widely used means, contact resistance is a readily measurable quantity as described below. A certain linearity ratio
It was also found that there is a correlation 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; and (3) a layer of a protective and insulating composition surrounding the conductive polymer, wherein (i) the carbon black content (L, weight %) is (a
N5% or (b) less than 15%, the resistivity (It) of the composition.

ohm cm) 2L + 510g+. R〉45 and (ii) The average linearity ratio between any pair of electrodes is l.

il self-regulating strip heater characterized by not exceeding 2! 17? do. The linearity ratio of a strip heater is defined as the resistance at 30 MV toov, 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 the moon, the smaller the contact resistance becomes.

The invention is useful for electrodes of any type, such as metal plates, strips or wires, or electrodes with irregular surfaces, such as twisted wire electrodes, braided wire electrodes, such as those commonly used in strip heaters. electrodes (such as those described in German Patent Application No. 2,635.000-5) and German Patent Application No. 2,655,54.
Expander pull (expa pull) as described in No. 3-1
This is especially true for ndable) electrodes. Preferred stranded wires are silver-coated and nickel-coated copper wires, which are less susceptible to melting or oxidation than tin-coated or uncoated steel wires. However, the latter type of copper wire can 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.
Senna, for example 100 to 50,000 ohm-cm. For strip heaters designed to be powered by +15 volts or more alternating current, the composition is generally between 2.000 and 50.0 volts.
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 and the strip heaters to which this invention relates generally have two electrodes separated by a distance of 0.15 to 1 cm, although a larger separation, e.g. 2°, is possible. 5 cm or more can be used. The 6 cores of the conductive polymer may be of conventional shape, but they are of minimal dimension, especially 3 cores of circular cross section.
It is preferred to have a cross section of no more than a factor of 1.5, 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 while being brought into contact. In this embodiment, the composition is, for example, 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 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. 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. do. 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 applicable, 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, or At the end of annealing, carbon black content (L) and resistivity (R
) is particularly useful if only a limited annealing process is performed such that 2 r, + 5108+oR>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 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.

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 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 had a diameter of 0.08 cm, consisted of 19 strands, and the axis of the wire was at a point on and 0.2 cm away from the orifice diameter. Upon reaching the cross-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; In Examples 3 and 5 it was 193°C.

An insulating skin for the extrudate was then provided by melt extruding a 0.051 centimeter thick layer of chlorinated polyethylene or ethylene/tetrafluoroethylene copolymer around the extrudate. The 11-overturned extrudate was then irradiated to crosslink the conductive polymer composition.

Examples 1-3 These examples, of which Example I 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. The effect is illustrated by increasing the temperature from -37°C to 65°C over 90 seconds and then decreasing the temperature 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 (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.

PN LRPN LRPN No LR 1
1.3+ 1.11 1500 0.5
1.6 1.3 - 1 11100'
0.3 2.1 1.2 - 11700
- - 1.1 1.1 1* Comparative Examples Examples 4-7 These examples are summarized in Table 2 below. Effect of electrode preheating on product linearity ratio and peel strength Table 2 Carbon black Percentage Linearity ratio *4
22 1.6*6
23 1.357
23 1.1* Comparative Example The peel strength of the heater strips of Examples 7 and 6 was 1.
It was 45.

Particularly preferred embodiments of the present invention are listed below: 1, (1)
Melting a conductive polymer composition exhibiting P'rC# behavior and having a resistivity of 50,000 ohm-cm at 21"C1,:m and comprising carbon black dispersed in a crystalline polymer. an extruded core; (2) at least two longitudinally extending electrodes embedded parallel to each other in the conductive polymer composition; and (3) a protective and insulating composition surrounding the conductive polymer composition. (I) The content of carbon black in the conductive polymer is 1
5% by weight or less, and (ii) the conductive polymer composition has 5o at 21°C;
o o O o o o It is possible to melt-extrude the extruded composition under conditions such that it is not necessary to anneal the extruded composition at a temperature above the crystalline melting point of the conductive polymer composition in order to not reduce the resistivity below ohm-cm. A self-regulating strip heater 2 characterized in that the average linearity ratio between any pair of electrodes does not exceed 12. The average linearity ratio between any pair of electrodes does not exceed 1°lO. Machine strip heater 3 The carbon black content ff1 (L, weight %) in the composition is correlated to the resistivity (R1 ohm cm) of the composition so that 2 L + 5 log, oR > 45. The strip heater according to 1 or 2 above, which is attached

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 1
5% by weight or less, and (ii) the conductive polymer composition has a
can be melt extruded 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 less than 1,000 ohm-cm; and (iii) the average linearity ratio between any pair of electrodes is 1.2.
A self-regulating strip heater characterized by not exceeding . 2. The strip heater according to item 1 above, wherein the average linearity ratio between any pair of electrodes does not exceed 1.10. 3. Content of carbon black in the composition (L, weight %
) is correlated to the resistivity (R, ohm cm) of the composition such that 2L+5log_1_0R>45.
Strip heater as described in section.