CA1168432A

CA1168432A - Conductive polymer compositions

Info

Publication number: CA1168432A
Application number: CA000375886A
Authority: CA
Inventors: Alan J. Gotcher; Robert W. Fouts, Jr.; David W. Whitney
Original assignee: Raychem Corp
Current assignee: Raychem Corp
Priority date: 1980-04-21
Filing date: 1981-04-21
Publication date: 1984-06-05
Also published as: JPS5730758A

Abstract

ABRIDGEMENT

The invention relates to conductive polymer compositions which comprise carbon black dispersed in a crystalline polymer and which exhibit PTC behavior below the melting point and very pronounced NTC behavior above the melting point. The invention recognises the value of such NTC behavior and discloses the selection of the polymer, carbon black and other components to obtain such NTC
behavior. The compositions are useful in temperature-sensing devices and other electrical devices. Figure 1 shows the resistivity/temperature curve For a typical composition.

Description

This invention relates to conductive polymer compositions.

PTC compositions comprising a crystalline polymer and carbon black are well known~ Such compo-sitions increase in resistivity as the temperature isincreased, reaching a peak around the crystalline melting point of the polymer and, unless they are cross linked, then show NTC behavior, (see for example ~. Applied Polymer Science, 19, 813-815 (1975), Klason and Kubat, and Polymer Engineering and Science, lS, 649-653 (197~), Narkis et al). This NTC behavior has been widely regarded as a disadvantage and For practical applications such compositions have, therefore, usually been cross-linked.
However, we have now realized that if the resistivity of such a composition falls sharply enough and far enough after reaching a peak, then the composition is of value in temperature-sensing devices and other electrical devices, especially such devices which comprise an element composed of such an NTC composition and at least one other element composed of another conductive polymer composition.
Furthermore, we have discovered how to select the components of the composition in order to provide such valuable NTC
behavior.

In one aspect, the present invention provides a conductive polymer composition which .~

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- ~6~3432 comprises carbon black dispersed in a crystalline polymer which has a crystalline mel-ting point T and which is substantially free from cross-linking, said composition having a resistivity at 23C, e of 102 to 108 ohm.cm, said composition, when heated, from 23C to an elevated temperature above Tm, having a resistivity which reaches a maximum at a temperature Tpeak and then decreases rapidly in the temperature range Tpeak to (Tpeak ~ 25) C to a valume, e min' which is less than 105 ohm.cm and less than K x 23 ~, where K is 0.35.
In the polymer composition, carbon black, as a particulate filler, is preferably present in an amount of at least 4~ by volume of the composition.
According to a preferred embodiment of the invention the K value mentioned above is about 0.2, especially 0.1 and more particularly O.OS.
Throughout this specification, when reference is made to heating or cooling the conductive polymer composition, the heating or cooling is carried out so that the temperature of the composition changes at 2C per minute.

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It is desirable that the composition should show a very sharp rise in resistivity as it is cooled from temperatures above Tm. Accordingly, it is preferred that the composition, when cooled from (Tpeak + ~5)C
to 23C~ has a resistivity, as it is cooled, which increases to a maximum at a temperature Tpeak B' and then falls to a value 21 e at 23~C~ and which has a minimum value in the temperature range Tpeak B to (Tpeak B + 25)C which is less than K1 x 213 ~ where Kl is 0~35~ preferably 0.2 especially 0.1, particularly 0~05~

The polymer component used in this invention preferably has a crystallinity of at least 5o~ especially at least 10o~ particularly at least 20o~ as measured by X-ray diffraction. When the polymer component comprises more than one polymer, it is often preferred that each of the polymers should be crystalline. Suitable crystalline polymers include polyolefins, especially polyethylene (high density and low density) and polypropylene;
halogenated polyolefins such as chlorinated polyethylene;
copolymers which consist essentially of units derived from at least one olefin, preferably ethylene, and units derived from at least one olefinically unsaturated comonomer containing a polar group, preferably vinyl acetate, an acrylate ester, eg. methyl or ethyl acrylate, _~_ ~;

or acrylic or methacrylic acid, said comonomer-derived units preferably constituting at least 10,o and generally not more than 30O by weight of the copolymer; and crystalline polymers which comprise 50 to 100~, preferably 80 to 1ûO~ by weight of -CH2CHCl- or -CH2CF2- units, eg. polyvinylidene fluoride or a copolymer of vinylidene fluoride eg. with tetrafluorethylene. Other suitable crystalline polymers are disclosed in German Offenle-gungschriften 2,821,799, 2,937,708 and 2,948,350 (Case Nos.
MP0253, MP0275 and MP0280).

The polymer component may for example provide up to 90O~ usually 40 to 90O~ by weight of the composition, the remainder being provided by the filler component and other non-particulate ingredients ~uch as antioxidants.

The compositions generally contain a single carbon black, but mixtures of carbon blacks can be used.
The carbon black is preferably the sole conductive material in the composition.

In one preferred class of compos tionsJ the carbon black has a particle size (D) in millimicrons and a surface area (S) in m2/g such that S/D is at least 10, preferably at least 12, especiaIly at least 18, with D preferably being less than 27, especially less than 18, particularly less than 15, millimicrons. The volume of carbon black 43~

used is generally at least 4O~ eg. 4 to 20~, especially 4 to 5O~ by volume oF the composition.

The carbon black may be the sole particulate filler. However, especially when the carbon black has an S/D ratio of less than 10, it is preferred that the particulate filler component also contains at least 4~0, by volume of the composition, oF a non-conductive particulate filler, i.e. a filler having a resistivity greater than 106 ohm.cm, preferably greater than 108 ohm.cm and often greater than 101 ohm.cm. The particles may be solid or, in suitabls cases, hollow. Excellent results have been obtained using glass beads. Other inorganic fillers which can be used include titanium dioxide, silica and antimony trioxide. ûrganic particulate fillers can also be used, for example particles which are composed of an organic polymer having a softening point such that the particles remain as a discrete particulate phase during use of the composition~ The non-conductive filler will usually have a particle size greater than the carbon black, for example from 0.1 to 100 microns, preferably 0.5 to 70 microns.

It is usually convenient to use a single non-conductive filler, but mixtures of fillers can be used. The filler may have a surface coating of a wetting or coupling 3~

agent to render it more readily dispersible in the polymer component.
The volume of non-conductive filler (if used) is preferably at least 4%, eg. 4 to 80~, preferably 6 to 30~, by volume of the composition and is preferably greater than, e.g.

2 to 12 times, preferably 3 to 10 times, the volume of the carbon black.
The quantities and -types of carbon black and any non-conductive filler used should preferably be such that the fil].er component has a total surface area of at least 1800 m2 per 100 cc. of composition, preferably at least 2000 m2/100 cc, especially at least 3000 m2/100 cc., particularly at least 4000 m2/100 cc, with yet higher values, e.g. at least 8,000 m2/100 cc, at least 10,000 m2/100 cc and at least 12,000 m2/100 cc being particularly preferred. In this connection, reference may be made to our Canadian Application Serial No. 375,856.
The filler component can be dispersed in the polymer component in any suitable way. It is often convenient to use a master batch technique, ie. to disperse the carbon black in a part of the polymer and the non-conductive filler in another part of the polymer, and then to mix the two master batches and the remainder of the polymer. The dispersion can be shaped by molding or extrusion or another melt-shaping technique into an element of the desired shape.

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~i -8-The invention is illustrated by the following Examples. The ingredients used in the Examples and the amounts thereof are set out in the Table below. After mixing the ingredients and compression molding the mixture into a slab as described in the Examples, rectangular samples 2.54 x 1.8 cm were cut From the slabs and silver electrodes were provided on the samples by painting 0.63 x 2.54 cm. strips of a silver-Viton~
composition (Electrodag~504) on both surFaces at each end of the sample. Resistance measurements were taken at 3nC
intervals as the sample was subjected to two thermal cycles. In the first cycle the sample was externally heated from 23 to 160C, maintained at 160C For 15 minutes, and then cooled to 23C. In the second cycle the 15 sample was externally heated from 23 to 180C, maintained at 180C for 15 minutes and then cooled to 23C. Figures l to 4 show the resistivities of the compositions of Examples l to 4 respectively during these cycles, the first cycle being shown as a broken line and the second as a solid line.

EXAMPLE l The ingredients were mixed for 5 minutes in a Banbury mixer with water-cooled rotors turning at high gear. The mixture was dumped, cooled and granulated. The granules were dried under vacuum at 70C for 16 hours. A
portion of the dried granules was compression molded into a slab 0.076 cm thick.

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3~2 The procedure of Example 1 was followed except that the rotor was steam-heated for the first 1.4 min. of mixing at high gear, after which the steam was turned off, and water was passed through the rotor. After an addi-tional 1.5 min. mixing, the mixture began to flux, and mixing was continued for an additional 1.7 min.

The ingredients shown under Master Batch 1 in the Table were introduced into a Banbury mixer with a steam-heated rotor turning at high gear. When the torque had inc~eased considerably, the steam to the rotor was turned off and water was passed through the rotor to cool it. Mixing was continued for 4 mins. after the water had been turned on. The mixture was dumped, cooled and granu-lated. The granule~ were dried under vacuum at 60C for 18 hours.

The ingredients shown under Master Batch 2 in the Table were introduced into a 8anbury mixer with a water-cooled rotor turning at high gear, and were mixed at high gear for 3.25 min. and at low gear for 2.5 min. The mixture was dumped, cooled and granulated. The granules were dried under vacuum at 60C for 18 hours.

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The final mix 7 containing the ingredients shown in the Table, was prepared by introducing 2,154 9. of Master Batch 1, 439 9. oF Master Batch 2, 829 9. of high density polyethylene (Marlex 6003) and 18.5 9. of anti-oxidant into a Banbury mixer whose rotor was water-cooled and was turning at high gear; mixing was carried out at high gear for 3.3 min. and at low gear for 1 min. The mixture was dumped, cooled and granulated. The granules were dried under vacuum at 70C for 16 hours, and then compression molded into a slab 0.053 cm thick.

The procedure of Example 3 was followed except that the final mix was prepared by mixing 2,1a6.5 9. of Master batch 1, 461 9. of Master Batch 2, 780 9. of high density polyethylene (Marlex 6003) and 1704 9. oF anti-oxidant, and the samples were heat-treated, before the electrodes were proYided thereon, by heating them to 20ûC
and then cooling them to room tempsrature.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A conductive polymer composition which comprises carbon black dispersed in a crystalline polymer which has a crystalline melting point T and which is substantially free from cross-linking, said composition having a resistivity at 23°C, 23°C , of 102 to 108 ohm.cm, said composition, when heated from 23°C to an elevated temperature above Tm, having a resistivity which reaches a maximum at a temperature Tpeak and then decreases rapidly in the temperature range Tpeak to (Tpeak+ 25)°C to a value,C min , which is less than 105 ohm.cm and less than K x 23°C where K is 0.35.

2. A composition according to Claim 1 wherein said carbon black (a) is present in an amount of 4 to 20% by volume of the composition, (b) has a particle size, D, in millimicrons and a surface area, S, in m2/gram such that S/D is greater than 10 and (c) is dispersed in a polymer component having a crystallinity of at least 10%.

3. A composition according to Claim 2 wherein S/D is at least 12 and D is less than 18 millimicrons.

4. A composition according to Claim 3 wherein S/D is at least 18.

5. A composition according to Claim 3 or 4 wherein D is less than 15 millimicrons.

6. A composition according to Claim 1 wherein the particulate filler component comprises (l) carbon black in amount at least 4% by volume of the composition, and (2) a non-conductive particulate filler in amount at least 4% by volume of the composition, said filler component having a total surface area of at least 1,600 m2 per 100 cc. of composition.

7. A composition according to Claim 6 wherein said filler component has a total surface area of at least 4,000 m2 per 100 cc.
of composition.

8. A composition according to Claim 6 or 7 wherein the volume of the non-conductive filler is from 2 to 12 times the volume of the carbon black.

9. A composition according to Claim 6 or 7 wherein said carbon black (a) is present in amount 4 to 20% by volume of the composition, (b) has a particle size, D, in millimicrons and a surface area, S, in m2/gram such that S/D is greater than 10 and (c) is dispersed in a polymer component having a crystallinity of at least 10%.

10. A composition according to Claim 6 or 7 wherein said carbon black (a) is present in amount 4 to 20% by volume of the composition, (b) has a particle size, D, which is less than 18 millimicrons and a surface area, S, in m2/gram such that S/D is greater than 12 and (c) is dispersed in a polymer component having a crystallinity of at least 10%.

11. An electrical device which comprises a conductive element composed of a conductive polymer composition and at least two electrodes for passing electrical current through the conductive element, wherein the conductive polymer composition is a compo-sition as claimed in Claim 1; 2 or 6.