CA1176452A - Conductive polymer compositions containing fillers - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
The invention relates to electrical devices embodying conductive polymer compositions. The compositions have improved electrical stability and comprise a crystalline polymer component and a filler component which has a total surface area of at least 1800 m2 per 100 cc. of composition and which comprises carbon black and a non-conductive particulate filler, each present in amount of at least 4% by volume of the composition. The compositions may have PTC, ZTC or NTC resistivity/temperature characteristics. Useful electrical devices according to the invention include heaters, circuit control devices and temperature sensors.

Description

This invention relates to electrical devices embodying conductive polymer compositions.
Conductive and semi-conductive compositions comprising carbon black dispersed in a polymer are known. They may have room temperature resistivities ranging from less -than 1 ohm.cm to 108 ohm.cm or more, and may exhibit positive tempera-ture coefficient (PTC) behavior, zero temperature coefficient (ZTC or constant wattage) behavior or negative temperature coefficient (NTC) behavior. Reference may be made, for example, to United States Patent Nos. 2,978,665, 3,351,882/ 3,823,217, 3,861,029, 3,950,604, 4,017,715, 4,177/376 and 4,246,468, and to German OLS Nos.

2,413,475, 2,746,602, 2,755l076 and 2,821,570. Recent advances in this field are described in German OLS Nos. 2,948,350, 2,948,281, 2,949,173 and 3,002,721, in our Canadian applications 352,413, 352,414, 358,274 and 363,205, in our Canadian applications filed contemporaneously with this application, namely 375,795, 375,877, 375,879 and 375,886, and in our United States Patent Nos. A,314,231, 4,317,027 and 4,352,083.

-3-5~"''h ~ ~ 7~ 4 ~ ~ 157/113 MP0714 The resistivity of a conductive polymeL at a particular temperature can be changed by subjecting it to thermal cycling, i.e by exposing it to elevated temp~
erature followed by cooling. The change is often particularly marked when the composition is first exposed to elevated temperature after it has been shaped. When the polymer is an elastomer, although initial exposure to elevated temperature often produces the greatest change, the room temperature resistivity remains very unstable, i.e. it is changed very substantially by further exposure to heat. When the polymer is crystalline, a composition having high resistivity can often be annealed above the polymer melting point to reduce its room temperature resistivity to a value which is in the range 1~2 to 106 ohm.cm and which is relatively stable, i.e. it is changed, but only relatively slowly, by Further exposure to heat. However, 0ven this relatively slow change in resistivity is undesirable. We have discovered that in certain compositions, as defined below, these continued changes in room temperature resistivity on therrnal cycling (aFter an initial heat treatment) can be reduced or even substantially eliminated by inclusion oF a suitable amount of a non-conductive filler.

In one aspect, the inve~tion provides electrical devices comprising a conductive polymer element and at least two electrodes which can be connected to a power source so that current flows through the element, the conductive polymer element being formed from a conductive polymer composition comprising a crystalline polymer component and a particulate filler component which has been dispersed in the polymer component and which comprises carbon black and a non-conductive particulate filler, said composition having a resistivity Of 102 to 106 ohm.cm at 23C after having been subjected to a thermal cycle which consists of heating the composition from 23C to Tt st and cooling the composition from TteSt to 23 C, where TteSt is 20C above the crystalline melting point of the polymer, wherein said particulate filler component has a total surface area of at least 1800 m~ per 100 cc. of composition and comprises at least 4% by volume of the composition of carbon black and at least 4% by volume of the composition of at least one non-conductive particulate filler.
Where reference is made herein to heating or cooling a composition, the heating or cooling is effected in such a way "' `' ~:~7~52 that the temperature of the composition changes at the rate of 2 C
per minute.
The stability of the compositions to thermal cycling can be expressed as the ratio of the resistivity of the composition after the thermal cycle defined above to its resistivity after a second thermal cycle as defined. The higher the resistivity of the composition, the more the value of this ratio differs from the ideal value of l. However, we have founcl that the difference between 1 and the said ratio is consistently less than it is for comparable compositions which do not contain a non-conductive filler as defined.
The electrical devices of the invention may, for instance, take ~he form of heaters, sensors or circuit control devices.
The polymer component used in this invention preferably has a crystallinity of at least 1%, ~enerally a-t least 2%, preferably at least 5%, particularly at least 10~, especially at least 20%, as measured by x-ray diffraction.

In one preferred class of compositions, the crystalline polymer component is cross-linked, the gel fraction oF the polymer (as calculated from the measured gel Fraction of the composition) preFerably being at lzast 0.6, especially at least 0.75.

When the polymer component is crystalline, it generally comprises at least 40~, preferably at least 60,~o~
especially substantially 100~, by weight of une or more polymers having a carbon-containing backbone. When the polymer component comprises more than one polymer, it is often preferred that each of the polymers should be crystal-line. 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 on~ olefin, preferably ethylene, ano units derived From at least one olefinically unsaturated comonomer containing a polar group, preferably vinyl ace-tate, an acrylate ester, eg. methyl or ethyl acrylate, or acrylic or methacrylic acid, said comonomer-derived units preferably constituting at least 10~ and generally not more than 30O by weight of the copolymer; and crystalline poly-mers which comprise 50 to 100~, preferably 8û to 100$, by by weight of -CH2CHCI- or -CH2CF2- units, eg. poly-vinylidene fluoride or a copolymer of vinylidene fluoride eg. with tetrafluorethylene. Other suitable crystalline ~7~452 157/113 MP0714 polymers are disclosed in the patents and apDlications referred to aboYe.

The polymer component, when crystalline, generally provides up to 60o~ preFerably 40 to 60~o ~ by weight of the composition, the remainder being provided by the filler component and other non-particulate ingredients such as antioxidants.

The polymer component can also be non-crystal-line, but in this case, the filler component must be such that the composition has an elongation (as measured by ASTM D638) oF at most 1C~ot preferably at most 2~, at 23C
and preFerably of at most 50Z at lOO~C. The non-crystalline polymer component preferably has an elongation of at least 25~ at 23~C. Typically such polymers are elastomers or thermoplastic elastomers prod~ced by cross-linking a composition obtained by dispersing the filler component in an elastomeric gum, ie a non-crystalline polymer which exhibits elastomeric properties when cross-linked and which preferably has a glass transition temperature below 23C. The non-crystalline polymers can have a carbon-containing bacl<bone, e.g. non-crystalline chlorinated polyethylene and fluorinated elastomer~ such as Vitnn.

The compositions generally contain a single carbon black, but mixtures of carbon blacks can be used.
The carbon black will preFerably be the sole conductive ~ -r~rJ~rJ~ J~ -8-~7~

material in the composition. We have surprisingly found that, for a composition containing a particular polymer and a given volume percent of a particular carbon black based on the polyme., the presence of the non-conductive filler does not cause any great chanye in the reslstivity of the composition at 23aC or the basic nature of its resistivity/temperature relationship (ie.
PTC, ZTC or NTC). It is, therefore, possible to produoe compositions having desired properties by making use of the information given in the patents and applications referred to above in order to select suitable combinations of carbon black and polymer. Thus in one preferred class of compositions, which generally exhibit consta~t wattage behavior? the carbon black has a particle size (D) in millimicrons and a surface area (5) in m2/g such that 5/D is at least 10, preferably at least 12, especially at least 18, with D preferably being less than 27, especially less than 18, particularly less than 15, millimicrons. In another preferred class of compositions, which generally exhibit PTC behavior, the carbon black has a particle size (D) of 20 to 150 millimicrons, eg. 20 to 75 millimicrons, and a surface area in m2/gram (5) such that 5/D is not more than 10; preferably the value of the quantity S x volume of filler component D ~ p~onent i9 less than 1.

7~

The volume of carbon black is at least 4 eg. 4 to 20~o~ by volume of the composition, with 10 to 45~ generally being used for PTC compositions, 4 to 20,o for ZTC compositions and 4 to 45u for NTC compo-sitions.

The non-conductive particulate fillers have resistivities of greater than 106, preferably greater than 10a, and often greater than 101, ohm.cm.
The particles may be solid or, in suitable cases, hollow.
Excellent results have been Gbtained using glass beads.
Other inorganic fillers which can be used include titanium dioxide, silica and antimony trioxide. Organic particulate fillers can also be used1 for sxample 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 may have a particle size which is smaller or greater than the carbon black, for example From 0.1 to 100 microns, preferably 1 to 70 microns.

It is usually convenient to use a single non-conductive filler1 but mixtures of fillers can be used.
Thc filler may have a Rurface coating of a wetting or coupling agent to render it more readily dispersible in the polymer component.

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The volume of non~conductive filler is at least 4%, eg. 4 to 80%, preferably 6 to 30%, by volume of khe composition.
In PTC compositions, the volume of non-conductive filler is preferably less than, eg. 0.3 to 0.8 times, the volume of carbon black. In ZTC compositions the volume of non-conductive filler is preferably greater than, eg. 2 to 12 times, preferably 3 to 10 times, the volume of the carbon black.
The quantities and types of carbon black and non-conductive filler used should be such that they have a total surface area of at least 1800 m2 per 100 cc. of composit:Lon, preferably at least 2000 m2/100 cc., especially at least 3000 m2/100 cc., particularly at least 4000 m2/100 cc., with higher values, eg. 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. The particulate filler component generally provides at least 10%, preferably at least 20%, particularly at least 25%, by volume of the composition.
Certain NTC compositions containing carbon black and a non-conductive filler are among those described and claimed in our Canadian application filed contemporaneously herewith, No. 375l886.

Certain compositions containing carbon black and an arc-controlling additive such as alumina trihydrate are among those described and claimed in our Canadian application filed contemporaneously herewith, No. 375,879.
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 of another melt-shaping technique into an element of the desired shape, any cross-linking thereof being carxied out after such shaping.
The invention is illustrated by the following Examples, in which Examples 1, 6 and 7 are comparative examples. The ingredients used in the Examples and the amounts thereof are set out in Table I below. The resistivities of the compositions as - prepared, 20 ~ ~ after a first thermal cycle as defined, 21 ~, and after a second thermal cycle as defined, 22 ~ ~ are shown in Table 2.
In the accompanying drawings, both Figures 1 and 2 are graphs showing the resistivity of a composition according to the invention plotted against the temperature at which the resistance measurement was made.

EXAMPLE_I (Comparative) The ingredients were introduced into a Banoury mixer with water-cooled rotors turning at high g~ar and were mixed at high gear For 4.5 minutes and at low gear for 1.5 minutes. The mixture was dumped, coo1ed and granulated. The granules were dried under vacuum at 70C
for 16 hours. A portion oF the dried granules was compres-sion molded into a slab 0.05 cm. thick. Rectangular samples 2.5 x 3.8 cm. were cut from the slab and electrodes were placed on the samples by painting ~.6 x 2.5 cm.
strips of a silvzr-Viton~composition (Electrodag~504) on both surFaces at each end of the sample. The ~amples wsre thermally conditioned by maintaining them at 160C For 15 minutes by external heating and then cooling to room temperature. The resistivity of the composition was calculated from sample resistance measurements taken at 3C intervals as the samples were subjected to two thermal cycles, in the first of which the sample was externally heated from 23 to 160C and then cooled at 23C and in the second of which the sample was externally heated from 23 to 180 and then cooled to 23C

Figure 1 shQws the resi~tivity of the compo-sition a~ a function of temperature during these cycles, the first cycle being shown as a broken line and the second as a solid line.

fY/~ Pt ~~ K ~ 13 -5;~

The procedure of Example I was Followed except that the ingredients were as shown in Table I; the rotor was steam-heated until the torque increased considerably, when the steam was turned off and water was turned on; and the ingredients were mixed at high gear for 4 minutes.
Figure 2 shows the resistivity of the composition during the heating and cooling cycles, the first cycle being shown as broken line and the second as a solid line.

This example shows the production of a planar heater comprising planar mesh electrodes having between them a PTC layer and a contig~ous zrc layer composed of a composition according to the invention.

Preparation of_ZTC Sheet Material Master Batch 1 was prepared From the ingredients shown in Table I. The ingredients were introduced into a Banbury mixer whose rotor had been preheated by steam and wa~ turning at Fourth gear. When the torque had increased considerably, the iteam to the rotor was turned oFF and water was passed through the rotor to cool it. Mixing was continued at Fourth gear For 2.5 mins. after the water had :~7~5;~

been turned on and for a further 2 mins. at third gear.
The mixture was dumped, held on a steam-heated mill, extruded into a water bath through a a.s cm. extruder fitted with a pelletizing die, and chopped into pellets.
The pellets were dried under vacuum at 60C for at least 18 hours.

Master Batch 2 was prepared From the ingredients shown in Table I. The ingredients were introduced into a Banbury mixer whose rotor was water-cooled and was turning at fourth gear; mixing was carried out at Fourth gear for 2 mins. and at third gear for 1.75 mins. The mixture was dumped, cooled and granulatedO The granules were dried under vacuum at 60C for at least 18 hours.

The Final mix, containing the ingredients shown in Table I, was prepared by introducing 11,523 9. of Master Batch 1, 3,127 9~ of Master Batch 2, 3,48û 9. of high density polyethylene (Marlex 6003) and 77.7 g. of anti-oxidant into a Banbury ~ixer whose rotor was water-cooled and was turning at high gear; mixing was carried out at high gear for 4 mins. and at low gear for 1 min. The mixture was dumped, held on a steam-heated miLl, extruded into a water bath through a 8.9 cm. extruder fitted with a pelletizing die, and chopped into pellets. The pellets were dried under vacuum at 70C for 24 hours, and then r /7~ ~ D /~: ~r~

s~

extruded into sheet 30 cm. wide and 0.053 cm~ thick, , ~ using a Davis-Standard~Extruder fitted with a 15 inch sheet die and operating at 20 RPM with a throushput of 122 cm/minute, The sheet was stored under argon.

Preparation of PTC Sheet Material The ingredients shown in Table I for the PTC
material were introduced into a Banbury mixer. The mixture was dumped from the Banbury and converted into sheet by the same procedure as the Final Mix. The sheet was stored under argon.

Preparation of Heater Rectangles 22.2 x 22.9 cm. were cut from the ZTC
sheet material and from the PTC sheet material, and dried under vacuum at 60C For 9 hours. Two rectangles 20.3 x lS Z2.9 cm, were cut from a sheet of fully annealed nickel mesh that had been thoroughly cleaned. The rectangles were sprayed until the nickel was completely covered, but the mesh apertures were not Filled, with a composition containing 60 parts by wcight of methylethyl ketone and 40 parts of Electrodag 502. The coated mesh rectangles were dried under vacuum for 2 hours at 100C.
r~ ~, b~ f r~ , 5 The PTC, ZTC and mesh rectangles were laminated to each other by layering a fluoroglass sheet (a release shest of a glass-fiber reinforced Fluorinated polymer), a mesh electrode, a PTC layer, a ZTC layer, another mesh electrode, and another Fluoroglass sheet in a mold and pressing with a 30.5 cm. press with plate temperatures of (224C) (top) and 215C (bottom) for 3.5 minutes at l2.5 tonnes ram pressure. The mold was then cooled in an 45 cm. cold press with air cooling at ram pressure for 5 minutes.

The resulting heater blank was masked, leaving 3.8 cm. at each end unmasked. A razor was used to scrape away PTC or ZTC material (which had been pressed through the coated mèsh) from the mesh on opposite sides of the heatar in the unmasked area. The scraped area on each side of the heater blank was then further abraded with a grit blaster.

Strips 2.5 x 25.4 cm. were cut from flat and fully annealed Cu mesh which had besn thoroughly cleaned.
One aide of the strips was coated with a silver/silicone contact elastomer and strips were then dried in vacuum at room temperature for a minimum of 4 hours. One end of 1~764S2 157/113 MP0714 each of the strips were then bent back at a 45 angle and 0.02 x 0.5 x 15.2 cm. flat copper wire was soldered onto the bent end. One of these strips was applied to each oF
the abraded areas of the heater blank with the silver side down and then each side of the heater blank was covered with a 22.2 x 3.~ x 0.03 cm. polyethylene sheet. The assembly was placed between two 1.3 cm. aluminum plates and compression molded at 200~C for 3 min. at 2,300 kg.
pressure, and then placed in the cold press for 10 minutes at 2,30U kg. pressure.

2191 9. of Master Batch 1 as used in Example 3, 535 9. of Master Batch 2 as used in Example 3, 709 9. of Alathon 7D50 and 16 9. of the antioxidant were mixed in a Banbury mixer t3.3 minutes at high gear, 1 minute at low gear), to give a final mix having the composition shown in Table 1. The mixture was dumped, cooled and granulated.
The granules were dried under vacuum at 60C For at least 18 hours and then extruded into a tape using a 3/4 inch Brabender~extruder and a 7.6, 0.05 cm. tap0 die at 40 RPM.
The extruded tape was taken off on a roll stack with the top roll at 115C and the bottom roll at 95C.

f~O~~f~/'S -lB-~1764~2 157/113 MP0714 The ingredients listed in Table I were introduced into a steam preheated ~anbury mixer turning at high gear and were mixed for approximately one minute in high gear. At this point the torque increased considerably, the cooling water was turned on, and the mixing continued at high gear for one minute, then low gear for one minute.
The material was dumped, cooled, and granulated, and the granules were dried at 60C in vacuum for at least 18 hours. The granules were extruded into tape as in Example 4.

EXAMPLE 6 (Com arative) 642 9. of Master Batch 2 as used in Example 3, ~ 1836 9. of Marlex~6003 and 41 9. of antioxidant were added to a 5 lb. Banbury mixer turning at high gear. After 4.75 mins., the torque increased considerably and cooling water was turned on; mixing was continued for 1.5 minutes, and the mixture was then dumped, cooled and granulated. The granules, whose composition is shown in Table I, wece dried under vacuum for at least 18 hours at 60C. The granules were extruded into tape a9 in Example 4.

1~L76452 EXAMPLE_7 (Comparative) The ingredients listed in Table I were introduced into a Banbury mixer turning at high gear. After mixlng for 5.6 minut:es at high gear and 1.4 minutes in low S gear, the mixture was dumpedl cooled and granulated. The granules were dried~at 60C in vacuum at le~st 1a hours.
:~ The granules were ~extruded into tape as in Example 4.

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Claims (8)

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

1. Electrical devices comprising a conductive polymer element and at least two electrodes which can be connected to a power source so that current flows through the element, the conductive polymer element being formed from a conductive polymer composition compris-ing a crystalline polymer component and a particulate filler component which has been dispersed in the polymer component and which comprises carbon black and a non-conductive particulate filler, said composition having a resistivity of 102 to 106 ohm.cm at 23°C after having been subjected to a thermal cycle which consists of heating the composition from 23°C to Ttest and cooling the composition from Ttest to 23°C, where Ttest is 20°C above the crystalline melting point of the polymer, wherein said particulate filler component has a total surface area of at least 1800 m2 per 100 cc. of composition and comprises at least 4% by volume of the composition of carbon black and at least 4% by volume of the composition of at least one non-conductive particulate filler.

2. Electrical devices according to claim 1 wherein the polymer component has a crystallinity of at least 10% and constitutes up to 60% by weight of the composition; the carbon black has a particle size (D) of 20 to 150 millimicrons and a surface area (S) in m2/gram such that S/D is not more than 10;
and the volume of non-conductive filler is 0.3 to 0.8 times the volume of carbon black.

3. Electrical devices according to claim 2 wherein the amount of carbon black is 10 to 45% by volume of the composition.

4. Electrical devices according to claim 2 or 3 wherein the quantity is less than 1.

5. Electrical devices according to claim l wherein the polymer component has a crystallinity of at least 10% and constitutes up to 60% by weight of the composition; the carbon black has a particle size (D) which is less than 27 millimicrons and a surface area (S) such that S/D is at least 12; and the volume of non-conductive filler is 2 to 12 times the volume of carbon black.

6. Electrical devices according to claim 5 wherein the amount of carbon black is 4 to 20% by volume of the composition.

7. Electrical devices according to claim 2, 3 or 5 wherein the conductive polymer composition has an elongation at 23°C of at most 10%.

8. Electrical devices according to claim 1, 2 or 5 wherein the particulate filler component has a total surface area of at least 8,000 m2 per 100 cc. of composition and provides at least 20%
by volume of the composition.