JPS5818722B2 - Self-regulating electrical article and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to self-regulating electrical articles and methods of making the same.

Conductive thermoplastic compositions have been obtained by adding conductive carbon black to a polymer matrix.

One class of such compositions takes advantage of the nonlinear positive temperature resistance coefficient exhibited by a particular material to create self-regulating or current-limiting semiconductor products. I was getting it.

U.S. Patent No. 3,243 by Kohler
, 753 discloses one such composition in which a polymer matrix containing 25
Compositions formed around carbon blacks having grades 75 to 75 are described.

Carbon black whose polymeric matrix becomes electrically conductive as an array of interconnected conductive paths as the temperature of such composition increases, either by increasing the ambient temperature or by resistive heating caused by the passage of electric current. It expands at a faster rate than the particles.

Reducing the number of conductive paths that carry current reduces the amount of heat generated by force heating according to I2R.

This self-limiting feature can be used, for example, in thermal tracing in chemical plants for pipe deicing.
Can be used in tubes to maintain the flow properties of viscous syrups.

For such applications, articles formed from conductive compositions ideally reach and maintain a temperature at which the energy lost by heat transfer to the surroundings is equal to the energy gained from the electrical current.

If the environmental temperature decreases, the resistance value decreases as the temperature of the product decreases, increasing the amount of heat generated and increasing heat transfer to the surroundings.

Immediately, heat transfer and heat generation become equal again.

Conversely, as the environmental temperature increases, the heat transfer from the conductive product decreases, and the increase in resistance generated from the temperature intensifier increases.
Reduce or stop heating according to 2R.

Of course, self-regulation
ng) The conductive compositions can also be used for applications other than resistive heating, such as heat sensing and circuit switching applications.

However, in any case, the high carbon black content, which is characteristic of most prior art compositions, is a disadvantage.

High carbon black content results in poor stretchability and stress cracking resistance, resulting in low temperature embrittlement.

Furthermore, high carbon black content appears to adversely affect the current regulating properties of the conductive composition.

If a semiconducting thermoplastic composition is externally heated and its resistance is plotted against temperature (abscissa), the curve obtained changes from the low room temperature value (Ri) to the ``peak resistance'' (R
Up to point p) it shows an increase in resistance with temperature, followed by a sharp drop in resistance with further increase in temperature due to the melt phase of the polymer matrix.

To avoid a decrease in resistance due to its concomitant irreversible change in resistance properties, cross-linking the polymer matrix will in turn cause the resistance to level off at the peak temperature and remain at a constant value even as the ambient temperature increases further. be.

Crosslinked semiconducting articles with high carbon black content exhibit undesirably low resistance when brought to peak temperature by exposure to very high or low ambient temperatures.

In such cases, due to poor heat transfer properties from the product, ■
This prevents the heat generated by 2RP from dissipating and causes burns.

with a substantially low carbon black content, inter alia:
Plasticity and other physical properties are improved and substantially R
It would be desirable to produce p/Ri boosted semiconducting self-regulating products.

However, it is not possible to achieve this objective with heavy bodies having a low carbon black content, since they exhibit a very high room temperature resistance.

Cabot Corp.
Pigment Black Technical Report (
Pigment B 1ackTechnical
Report) S-8's miscellaneous text titled ``Carbon black for conductive plastics'' describes the carbon content of various polymers containing Vulcan X C-72'' oil furnace black. −
It is shown that the resistance curve exhibits a resistance of 100,000 ohms or more and increases toward an asymptote at about 15% carbon black content.

Other publications have similarly reported high resistance at low carbon black contents.

Traditionally, self-limiting compositions have been used, such as Epstein (Ep
3,435,401, but when the carbon black content was lowered, the extrudate was essentially at the value of the polymer matrix itself, 107 ohms or less. It showed a room temperature resistance of

In fact, British Patent No. 1,201,166 recommends not using the thermal fusing process when significant electrical conductivity is desired with less than about 20% carbon black.

For the first time ever, self-limiting extrudates have been obtained which have the advantage of low carbon black content and yet exhibit room temperature (hereinafter 21°C (70°F)) resistances in the useful range of about 5 to about 100,000 ohms-α. .

In this case, the relationship between carbon black content and room temperature resistance satisfies the formula: % (where L is the weight factor of the carbon black in the extrusion composition).

After extrusion in a conventional manner, exposure to a time-temperature relationship that is considerably more severe than that previously used for strain relief or improving electrode wettability, such as 149°C (300°F) for a period on the order of 24 hours. The resistance can be significantly reduced by thermally altering the structure of the carbon black in the uncrosslinked product.

The resulting product is suitable for cryoprotection and other self-limiting applications, exhibits a high Rp/Ri, and has other advantages of a low carbon black content.

In particular, unlike extrudates with high carbon black content, the resistance-temperature properties are stable on storage, yet are not affected by repeated temperature cycling.

A polymer matrix in which conductive carbon black is dispersed in any proportion to obtain a self-limiting composition.
The non-linear coefficient of thermal expansion must be generally appropriately indicated,
For this reason, crystallinity is considered to be important.

Generally, polymers exhibiting a degree of crystallinity of at least about 20% as determined by X-ray diffraction are suitable for the practice of this invention.

Among the many polymers used in carrying out this invention are low, medium and high density polyethylene and polypropylene, polyolefins such as polybutene-1, poly(
dodecamethylene pyromellitimide), terpolymers of ethylene-propylene copolymers and nonconjugated dienes, polyvinylidene fluoride, and polyvinylidene fluoride-tetrafluoroethylene copolymers.

As will be recognized by those skilled in the art, limiting temperatures to suit the intended application (eg, antifreeze, isothermal, etc.) can be determined by appropriate selection of the polymeric matrix material.

For example, 37.80G (100°F), 54.4°
G (130°F), 65.6'C (150°F), 82
．． 2°C (180°F) and 121°C (250°P
Elements that are self-limiting at temperatures as low as ) can be made using wax-poly(ethylene-vinyl acetate) blends, low density polyethylene, high density polyethylene, polypropylene, and polyvinylidene fluoride, respectively.

Other criteria for selecting a polymer include, in particular cases, desirable elongation, environmental resistance, ease of extrusion, etc., as is well known.

A particularly preferred matrix material is carbon black mixed with the first formulation component to form a masterbatch;
It is now a multi-component formulation mixed with the main polymer component.

The first and second polymer blend components are selected such that their free energy of mixing with each other is positive.

The associated incompatibility apparently acts to segregate the carbon black contained within the polymer matrix into certain regions of the general environment, and such formulations are extremely stable to temperature cycling during use. It turns out that there is.

With single-component matrices, the result has sometimes been that progressively higher temperatures must be reached to deliver the same wattage at temperature cycles.

Of course, even in the case of single-component MAI-IJ wax,
Satisfactory stability against temperature cycling is achieved with the low carbon black content obtained according to the invention.

Typically, minor amounts of polymeric formulation components are selected to provide good compatibility with carbon black for the formulation components present in large amounts, but not for specific physical properties desired in the total extrudate. selects the major component.

The major formulation components are preferably present in a weight ratio of at least about 3:1 to the minor components with which the carbon black is initially mixed.

Presently, the most preferred formulations have polyethylene as the primary component, others include ethylene-vinyl ester copolymers such as ethylene-vinyl acetate or ethylene-ethyl acrylate copolymers.

A particularly preferred extrudate contains about 70:20 by weight polyethylene:ethylene-ethyl acetate copolymer.

The carbon blacks used are those conventionally used for conductive carbon blacks, such as furnace blacks and channel blacks.
It is of a high type (structure).

Other conventional additives such as antioxidants and the like may be used as long as their amount and properties do not defeat the purpose of the invention.

A particularly interesting class of advantageous color additives has been found to be materials such as waxes that are compatible with the main formulation components and melt at lower temperatures.

As a result, certain wattages can be obtained at lower temperatures because the presence of the wax increases the resistance of the composition near the melting point of the wax.

Compounding methods may be conventional and generally include Banbury processing, milling and pelletizing before pressure extruding the self-limiting elements from the melt.

In a preferred embodiment, the carbon black-containing matrix is extruded onto a pair of spaced elongated electrodes to form elements rod-shaped or, most preferably, dumbbell-shaped in cross-section, and the extruded thermoplastic covers the electrodes and Try to connect them.

Now, to provide one of the most common uses for self-limiting heaters, which is to protect pipes from freezing, at least about 13-26 watts (13-26 watts/7 m) from the heater to its surroundings must be used.
It is desirable for the heater to have a heat output capable of transferring heat of 13 W/m (4 watts/foot) at commonly used voltages ranging from 120 to 480 volts. should be about 6,000 to 100,000 ohms-α, but of course, to obtain values as large as 8 watts/foot, the resistance will be lower at a particular voltage.

However, after extrusion of formulations containing no more than about 15% carbon, the room temperature resistance is greater than about 107 ohms, but usually on the order of the resistance of the dielectric polymer matrix itself.

At such resistance values, the wattage available under voltage is essentially zero.

By subjecting the extrudate to a temperature above the melting point of the polymer for a substantially longer period of time than would previously be used to improve electrode wettability etc. if a self-limiting product were obtained by other methods. It has been found that the electrical conductivity of the extrudate can be greatly increased by doing so.
Resistance values in the range of 5 to about 100,000 ohms can be obtained with carbon contents of less than about 15%, and in fact even with carbon black contents of less than about 10%, resistances of 10,000 ohms can be obtained.
Lower room temperature resistance values than mine are usually obtained.

Although thermal texturing obviously involves microscopic movement of carbon particles of a type not normally associated with "annealing," that term is used here for convenience.

Annealing above about 121°C (250°F), preferably at least about 149°C (300°F)
and in any case at a temperature above the melting point or melting range of the polymer matrix in which the carbon black is dispersed.

It will be appreciated that the time for conducting the annealing will vary depending on the nature of the particular Mal-IJ wax and the amount of carbon black contained therein.

In all cases, annealing is performed so that the resistance of the annealed element is determined by the formula 2 L + 5 l og 10 R<
45, preferably <40, and the required time for a particular case can be readily determined experimentally.

Typically, annealing is performed for a period of time in excess of 15 hours, and usually at least about 24 hours of annealing is performed.

If the device is held at the annealing temperature continuously for the required period of time, cooling may be adjusted so that upon completion of the annealing, it takes at least about 11/2 hours to return to room temperature.

However, the total annealing residence time required is at least about 3
It has been found that if the annealing is divided into two equal steps, but the device is returned to room temperature between each annealing step, the cooling adjustment becomes less important.

Because the polymer matrix of the carbon black-containing extrudate is in a molten state during annealing, an extruded insulating coating of thermoplastic material that retains its shape when brought to the annealing temperature is applied to the extrudate before annealing. is preferred.

Once annealing has been completed and a further insulating coating, such as polyethylene, is optionally applied, the self-limiting element can be subjected to ionizing radiation at an intensity sufficient to desirably crosslink the carbon black-containing core. .

The dose is visually selected to achieve sufficient crosslinking to provide the thermal stability required for the particular intended application without unduly reducing the polymer Ma I-IJ wax crystallinity.

That is, the total crystallinity of the crosslinked carbon black-containing matrix should be avoided to be less than about 20%.

Along these lines, the irradiation dose may range from about 2 to 15 megarads or more in special cases, preferably about 1
It is 2 megarads.

The invention is further described in the following examples with respect to preferred embodiments thereof.

In the examples, all parts and references are by weight and all resistance values are measured at room temperature and on a Wheatstone bridge unless otherwise indicated.
Measured in e).

Example 1 34.5ky (76 bond) polyethylene (0.92
density of 9 g/cc) and 34% Palcan XC-72
and ethylene-ethyl acrylate copolymer (0,930g
/cc density, ratio 18 ethyl acrylate) mixture 1
4.5 kg (32 lb) was placed in a Banbury mixer along with 1 lb of antioxidant.

The ram was closed and mixing started.

The temperature is approximately 116°C (240°F) - 1218°C (25
When the temperature (0°F) was reached, the batch was opened, placed in a two-roll mill, cut into strips, and fed into a pelletizing extruder.

The pelletized formulation was then applied to two parallel thin tinned copper electrodes (each electrode was a wire consisting of 19 wires;
(diameter 0.09z) to form an extrudate with a generally dumbbell-shaped cross section.

The electrodes are separated (center to center) by 6.99 mrn (0.275 in.) and the interconnecting web is a semiconducting composition surrounding the electrodes, approximately 0.13 mm (15 mils) apart and at least 0.20 mm (8 mil) thickness.

Extrusion of the composition was carried out in a plasticizing extruder fitted with a crosshead die so that the composition was extruded around the electrodes.

The same extruder was then operated to produce 0.20 mm (8 mil) thick polyurethane (Texin).
591-A, Mobeso ml-poration (Mobay
An insulating coating (commercially available from Co., Ltd.) was extruded.

For any geometry, a conventional tube extrusion process may be used, in which a vacuum [e.g.
08 H20 (5-201 nH20)].

The coated product was then placed on an aluminum disk (660 mm2
6 diameter) and exposed to 149°G (300°F) for 24 hours in a circulating air oven.

After this thermal change in structure, the sample was cooled to room temperature for about 11/2 hours, and then the resistance of the sample was measured at various temperatures.

I got the following data. Examples 2-9 Further extrudates were made using various polymers and carbon black contents according to the method of Example 1 except for the conditions described below.

The polymer matrices for the various examples were as follows; (2) a 3=1 blend of low density polyethylene: ethylene-ethyl acrylate copolymer; (3) low density polyethylene: ethylene-acetic acid. Vinyl copolymer 5:
(4) polyvinylidene fluoride; (5) medium density polyethylene: 3:1 blend of ethylene-ethyl acrylate copolymer; (6) high density polyethylene: ethylene-
3:1 blend of ethyl acrylate copolymer; (7) ethylene/propylene copolymer (Polyallomer from Eastman Chemical Co.); (8)
Polybutene-1; and (9) polyvinylidene fluoride/tetrafluoroethylene copolymer [Pennwalt Chemical Co.]
``Kynar 5200''].

For each formulation, the carbon black was first mixed with a minor component of the polymer formulation, and the resulting masterbatch was mixed with the other polymer components.

Coated extrudates of each composition exhibited nonlinear positive temperature coefficients of resistance.

Table 2 shows the data obtained.

Example 10 The procedure of Example 1 was repeated to obtain identical polyurethane coated extrudates.

The extrudates were then exposed to 149° C. (below 300° C.) for nine periods with each period of 3 hours and cooling to room temperature between periods.

Thereafter, a final polyethylene insulation coating (0.30 mm (12 mils) thick) was also applied to the annealed product by a pre-extrusion process and 1-Me was applied at a total dose of 12 megarads.
The whole was crosslinked by exposure to an electron beam of v.

The produced strips exhibited the resistance values indicated therein at the temperatures indicated in Table 3.

Claims (1)

Claims: 1. A pair of parallel spaced elongated electrodes, electrically coupling the electrodes to a melt-extruded conductive polymer composition having a non-linear positive temperature coefficient of resistance. , and conductive carbon black and at least 2 as determined by X-ray diffraction.
a web of a composition comprising a crystalline thermoplastic polymer having a total crystallinity of 0%; A self-regulating electrical article comprising an insulating coating of a melt-extruded polymer that retains its shape at the crystalline melting point of the polymer, wherein the conductive polymer composition contains over 15 weight by weight of conductive carbon black. and have a room temperature resistivity that satisfies the formula % (where L is the weight factor of the carbon black in the composition and R is the ohm-temperature resistivity of the composition). A self-regulating electrical article characterized by: 2 a) Melt extrude a conductive polymer composition around a pair of parallel spaced elongated electrodes, the conductive polymer composition having a non-linear positive temperature coefficient of resistance, and consisting of conductive carbon black and a crystalline thermoplastic polymer having a total crystallinity of at least 20% as determined by X-ray diffraction; A method of making a self-regulating electrical article comprising melt extruding an insulative coating of a composite composition, comprising: (a) having a non-linear positive temperature coefficient of resistance;
(c) a crystalline thermoplastic polymer having a total crystallinity of at least 20% as determined by X-ray diffraction; and an amount of conductive carbon black not more than 15% by weight of the composition; Melt extruding a conductive polymer composition having a room temperature resistivity after extrusion of at least 10 Q, 000 ohm·I around a pair of parallel spaced elongated electrodes; An insulating composition is melt-extruded around the obtained extrudate to form an insulating coating that retains its shape in the next step C, and the crystallization heat in the C6 conductive polymer composition is The product of step B is annealed at a temperature above the melting point of the plastic polymer and at a temperature at which the insulating coating retains its shape; (where L is the weight factor of the carbon black in the composition and R is the ohm-temperature resistivity of the composition). A method of manufacturing a self-regulating electrical article.