GB2106920A

GB2106920A - Conductive polymer compositions based on polyvinylidene fluoride

Info

Publication number: GB2106920A
Application number: GB08225680A
Authority: GB
Inventors: Andrew Au; Peter Van Konynenburg
Original assignee: Raychem Corp
Current assignee: Raychem Corp
Priority date: 1981-09-09
Filing date: 1982-09-09
Publication date: 1983-04-20
Also published as: JPH0334498B2; JPS5853939A; EP0074281A1; ATE35745T1; DE3278775D1; CA1236246A; GB2106920B; JPH0395248A; EP0074281B1

Abstract

Conductive polymer compositions based on polyvinylidene fluoride have improved properties when the polyvinylidene fluoride has a very regular structure characterized by a low head-to-head content in the repeating units. The improved properties include improved electrical stability when contacted by organic fluids and/or when maintained at elevated temperatures in air. Such compositions are particularly useful in the form of self-limiting heaters, e.g. for heating diesel fuel.

Description

1

SPECIFICATION

Conductive polymer composition This invention relates to conductive polymer com positions and devices comprising them.

Conductive polymer compositions, and devices comprising them, are known or are described in copending patent applications. Reference may be made for example to U.S. Patents Nos. 2,978,665, 3,243,753,3,351,882,3,571,777,3,793,716, 3,823,217, 3,861,029, 4,017,715, 4,177,376,4,188,276,4,237,441, 4,238,812, 4,242,573, 4,246,468, 4,255,698, 4,272,471 and 4,276,466; U. K. Patent No. 1,534,715; J. Applied Polymer Science 19, 813-815 (1975), Klason and Kubat; Polymer Engineering and Science 18, 649-653 (1978) Narkis et al; and German OILS Nos. 2,634, 999, 2,755,077, 2,746,602, 2,755,076, 2,821,799, 2,949,173 and 3,030,799; European Published Patent Applica- tions Nos. 0, 026,571, 0,028,142, 0,030,479, 0,038,713, 0,038,714, 0,038, 715, 0,038,716, 0,038,717 and 0,038,718; and the copending applications corresponding to U.S. Serial Nos. 150,909, and 254,352 and U.K. Applications Nos, 2,075,992A, 2,080,834A, 8209923,8217000,8216999 and 8216997. The disclosure of each of the patents, publications and applications referred to above is incorporated herein be reference.

Electrical devices containing conductive polymers generally (though not invaribly) comprise an outer jacket, usually of insulating material, to protectthe conductive polymer from damage by the surrounding environment. However, if no protective jacket is used, or if the jacket is permeable to harmful species in the environment, or if the conditions of use are such that the jacket may become damages, it is necessary or desirable to select a conductive polymer which is not damage (or which deteriorates at an acceptably low rate) when exposed to the sur- rounding environment. Exposure of conductive polymers to organic fluids generally results in an increase in resistivity; exposure to air, especially at elevated temperatures between room temperature and 350C below the melting point generally results in a decrease in resistivity both at the elevated temperature and at room temperature (a phenonenon known in the art as "resistance relaxation").

We have discovered that conductive polymer compositions which are based on polyvinylidene fluoride exhibit substantially improved stability if the polyvinylidene fluoride has a very regular structure which can be characterized by a low head-to-head content in the repeating units. Polyvinylidene fluoride is made up of repeating units of formula -CHCF,7, which can be arranged heat-to-tail (i.e. -CH2CFz CH2CFz-) or head-to-head (i.e. -CH2CFz CF2CHT-), and we have found that the lower the head-to-head content, the greater the stability of the resistivity of the composition when exposed to organic fluids and/or when exposed to air at elevated temperature. Previously known conductive polymer compositions based on polyvinylidene fluoride have GB 2 106 920 A 1 made use of polyvinylidene fluroide of relatively high head-to-head contents, namely at least 5.2% and generally higher, which are easier to process than the polymers used in the present invention.

In its first aspect, the present invention provides a conductive polymer composition which comprises (a) polyvinylidene fluoride having a head-tohead content of less than 5.0%, preferably less than 4.5%, particularly less than 4.0%, and (b) a particu late conductive filler, especially carbon black, dispersed in the polyvinylidene fluoride. The composition preferably exhibits PTC behavior.

In its second aspect, the invention provides an electrical device which comprises a conductive polymer element composed of a conductive polymer composition as defined above and at least one electrode in electrical contact with said element, for example, at least two electrodes which can be connected to a sou rce of electrical power and which when so connected cause current to flow through the conductive polymer element. Preferred devices are self- limiting heaters, e.g. flexible strip heaters, in which the conductive polymer composition exhibits PTC behavior. Such heaters are particularly useful for heating liquids in which the heater is immersed, especially diesel fuel (see U.K. applications Nos, 8216999 and 8216997.

In a third aspect, the invention provides a method of heating a liquid, particularly an organic liquid, especially diesel fuel, which comprises passing current through a self-limiting heater as defined above which is immersed in the liquid.

In a fourth aspect of the invention provides a fuel feedthrough and heating assembly which can be positioned and connected between a fuel filter and a fuel tank of a fuel supply system to provide means for heating fuel which is being pumped through a fuel line from the fuel tankto the fuel filter, said feedthrough and heating assembly comprising (A) a feedth rough comprising (i) a fuel conduit having at one end thereof a fuel line connector for connecting the feedthrough to a fuel line and at the other end thereof a fuel filter connector for connecting the feedthrough to a fuel filter; and (ii) a neck portion which protrudesfrom the fuel conduit between the ends thereof and which comprises a chamber; (B) a flexible self-limiting strip heater as defined above which preferably comprises a fuel-resistant insulating jacket, one end of the strip heater being within the chamber of the neck portion, and the strip heater passing through the fuel line connector and protruding from the fuel conduit; (C) insulated electrical leads connected to the electrodes of the heater, the connections lying within the chamber of the neck portion; (D) a fuel-resistant, water-resistant and insulat- ing composition which encapsulates (i) the connections between the electrodes and the leads, (ii) the insulation at the ends of the connected electrical leads and (iii) the insulating jacket at the end of the connected heater; and The drawings originally filed were informal and the print here reproduced is taken from a later filed formal copy.

2 GB 2 106 920 A 2 (E) a fuel-resistant gasket which prevents fuel which is being pumped through the fuel conduit from exiting through the neck portion.

Polyvinylidene fluorides suitable for use in this invention are commercially available. The head-tohead content of a polyvinylidene fluoride can be measured by those skilled in the art. We have found that the measured head-to-head contents of different samples of a polymer sold under a particular trade name can differ substantially. In general, the presently available polyvinylidene fluroides made by suspension polymerization (ratherthan emulsion polymerization) have lower head-to-head contents. The number average molecular weight of the polymer is generally at least 5,000, eg. 7,000 to 15,000.

The polyvinylidene fluoride is preferably a homopolymer of vinylidene fluoride, but the presence of small quantities of comonomers, (preferably less than 15%, particularly less than 5% by weight), eg. tetrafluoroethylene, hexafluoropropylene and ethylene, is not excluded. The polyvinylidene fluoride is preferably the sole crystalline polymer in the composition, but other crystalline polymers, eg.

other crystalline fluoropolymers, may also be present. The composition may contain relatively small amounts (preferably less than 35%, especially less than 20%, particularly less than 10%, by volume) of one or more elastomeric polymers, particularly solvent-resistant fluroine-containing elastomers and acrylic elastomers, which are usually added primarily to improve the flexibility and elongation of the composition.

The particulate conductive filler preferably com- prises carbon black, and often consists essentially of carbon black. Choice of the carbon black will influence the resistivity/temperature characteristics of the composition. Compositions exhibiting PTC behavior are preferred for many devices of the invention, especially self-limiting heaters, and for these a carbon black having a ratio of surface area (r&lg) to particle size (mu) of 0.03 to 6. 0 is preferred. For other uses, compositions exhibiting ZTC or NTC behavior may be preferred. The amount of conduc- tive filler used will depend upon the desired resistivity of the composition. For flexible strip heaters which are to be used for heating diesel fuel and powered by a 12 volt battery, we prefer a PTC composition whose resistivity at 25'C is less than 200 Kynar460 Furnex N765 Viton AHV Omya-BSH TAIC Kynar 460 is polyvinylidene fluoride available from Pennwalt and having a head-to-head content of about 5.5%.

Furnex N765 is a carbon black available from Columbia Chemical having a particular size of about 60 millimicrons, a surface area of about 32 m21g and a DBP value of about 112 cm3/100 g.

ohm.cm eg. about 10 to about 100 ohm.cm. In such compositions the amount of carbon black may for example be 16 to 25% by weight.

In addition to one or more conductive fillers, the compositions may also comprise other conventional additives, such as non-conductive fillers (including flame retardants), antioxidants and crosslinking agents (or residues thereof if the composition has been cross-linked).

The compositions of the invention are preferably cross-linked (particularly by irradiation), since this has been found to enhance their resistance to organic solvents.

Preparation of the compositions of the invention can be carried out in conventional fashion. Often it will be convenient to melt-extrude the composition directly into a water bath (which may be heated), and using this technique subsequent annealing is often not required.

The invention is illustrated by the following Exam- pies, in which Examples 1, 2,3,7,12 and 13 are Comparative Examples not in accordance with the invention.

EXAMPLE 1

The ingredients listed for Composition A in Table 1 below were mixed in a Banbury mixer. The mixture was dumped, placed on a steam-heated mill and extruded into a water bath through a 3.5 inch (8.9 cm) extruderfitted with a pelletizing die. The extrudate was chopped into pellets which were dried for 16 hours at WC.

The ingredients listed for Composition B in Table 1 were mixed and pelletized in the same way as for Composition A.

83% by weight of the Composition A pellets and 17% by weight of the Composition B pellets were tumble blended and dried at 11 ODC. The composition of the resulting Final Blend is shown in Table 1. Using a 1.5 inch (3.8 cm) diameter extruder fitted with a crosshead die having an orifice 0.4 inch (1.0 cm) x 0.1 inch (0.3 cm), the blend was melt-extruded over a pair of pre- heated 14 AWG (1.85 mm diameter) '19127 nickel-coated copper wires with a centerto-center separation of 0.25 inch (0.64 cm).m. The extrudate was passed immediately through a bath of water at room temperature, air-dried, and then irradiated to a dosage of 10 Mrad. The conductive polymer had a resistivity of about 50 ohm.cm at 2TC.

TABLE 1 Composition 8 CompositionA FinalBlend Wt(g) Wt-I.Vol% Wt(g) Wt-I. Vol% Wit% V01% 16,798 72 72.6 16,339 70 70.6 71.7 72.3 4,433 19 18.7 4,901 21 20.7 19.3 19.0 1,400 6 5.9 1,400 6 5.9 6.0 5.9 467 2 1.3 467 2 1.3 2.0 1.3 233 1 1.5 233 1 1.5 1.0 1.5 Viton AHV is a copolymer of hexafluoropropylene and polyvinyiidene fluoride manufactured by du Pont.

Omya-BSH is calcium carbonate available from 110 Omya Inc.

TAIC is triallyl isocyanurate, a radiation crosslinking agent.

1 3 GB 2 106 920 A 3 EXAMPLES2-6 The ingredients listed for Examples 2 to 6 in Table 2 below were mixed in a Banbury mixer. The mixture was dumped, granulated and dried for 72 hours at 75C under vacuum. Using a 0.75 inch (1.9 cm) single screw extruder fitted with a cross-head die having an orifice 0.3 inch (0.76 cm X 0.1 inch (0.3 cm), the blend was melt-extruded over a pair of pre-heated 18 AWG (1.2 mm diameter) 19127 nickel-coated copper wires 10 with a center-to-center separation of 0.25 inch (0.64) cm). The extrudate was passed immediately through a bath of water at room temperature, air-dried, and then irradiated to a dosage of 10 Mrad. EXAMPLES 7- 15 The ingredients shown for Examples 7-15 in Table 2 were mixed in a Banbury mixer, dumped and then granulated. The granulated materials were molded into slabs of thicknesses of 0.030" (0.076 cm) to O.OW' (.091 cm) by compression molding at 20WC for three minutes.

Table 2

Ex. No.

Ingredients 2C 3C 4 5 6 7C 8 9 10 11 12C 13C 14 15 Kynar450 77 90 88 Kynar460 77 89 Solef 1010 74 88.5 88 KF1100 74 89.5 88.5 KF1000 77 Dyflor 2000M Statex G 21 21 24 24 21 89.5 88.5 Vulcan XC72 8 9.5 10 8.5 8.5 10 9 9.5 9.5 Omya BSH 2 2 2 2 2 2 2 2 2 2 2 2 2 2 Resistivity 3.1 x 1 W 1.6x 101 1800 1850 2000 288 298 200 134 (ohm-cm) at 2TC Kynar 450 is polyvinylidene fluoride available from Perinwalt and having a heat-to-head content in the range 5.5 to 6.3.

Solef 1010 is a polyvinylidene fluoride available 25 from Solvay et cie of Belgium, and having a head-to head content of 4.1 %.

KF1000 and I(F1 100 are polyvinylidene fluorides available from Kureha Chemical Industry Co. of Japan, and having a head-to-head content of 3.5 to 30 3.8%.

Statex G is a carbon black available from Cities Services Co., Columbian Division having a particle size of about 60 millimicrons, a surface are of about 32 rr?lg and a DBP value of about 90 cm1A00 g. 35 Dyflor 2000 M is a polyvinylidene fluoride available from Kay-Fries, Inc., member of Dynamit Nobel Chemikalien of Federal Republic of Germany and having a head-to-head content of about 4.44.9.

Vulcan XC-72 is a carbon black available from 40 Cabot Co., having a particle size of about 30 millimicrons, a surface area of about 224 m21g and a DBP value of about 178 cm11100 g. TESTS FOR STABILITY IN ORGANIC SOL VENTS The extrudates obtained in Examples 1 and 4 were 45 compared by the following tests. Samples 2 inch (5.1 cm) long were cut from the extrudates. The samples were immersed in various solvents at 25C and the resistance of the samples was measured at intervals. The solvents used, and their solubility parameters, 50 were Solvent Solubility Parameter (ca 1/CM3)0..5 8.9 9.3 9.9 10.0 10.3 10.7 10.8 12.0 12.1 12.7 Toluene Methylethyl ketone (MEK) Acetone o-dichlorobenzene Acetic Anhydride Pyridine Dimethylacetamide (DMAC) Dimethylsulphoxide (DMSO) Dimethylformarnide (DIVIF) Ethanol The results for Examples 1 and 4are shown in figures 1 and 2 respectively of the accompanying drawings, where the ratio of the resistance at a given time (Rf) to the initial resistance (Rj) is plotted against time. The greater stability of the composition of the invention (Example 4, shown in Figure 2) is apparent.

The extrudates obtained in Examples 1 to 6 were compared in the following way. Samples 2 inch (5.1 cm) long were cut from the extrudates and were immersed in various test liquids maintained at 16(f F (71'C). The test liquids are listed below and include diesel fuel and various commercially available additives for diesel fuel alone and mixed with diesel fuel. At intervals, the samples were removed, cooled to WC and dried, and their resistance measured. Table 3 shows the value of the ratio Rf/Ri forthe different samples at various times. The additives tested, and their main ingredients, were as follows:

4 GB 2 106 920 A 4 812 Toluene, methanol, acetone, naphthalenic mineral oil and ethylene glycol monobutylether.

Fire Prep 100 Naphthalenic oil and partly oxidised 15 Conditioner Naphthalenic mineral oilland isop ropanol Gumout Naphthalenic mineral oil, non aromatic ester and aliphatic acid.

aliphatic hydrocarbon WVnn's Anti- Sta-Lube Naphthalenic mineral oil Knock Naphthalenic mineral oil, non Redline Naphthalenic mineral oil, barium aromatic ester, aliphatic amide, and Catalyst carbonate and aliphatic acid.

other inorganic carbonates, and 20 FPPF Ethyl cellulose, ethylene glycol mono sulfur-containing material butylether, and oxidised hydrocar Wynn's bons.

Example No.

Ri (ohms) Rf/Ri after 19 hours in B12.

Fire Prep 1000 Sta-Lube Red-line Catalyst Wynn's Conditioner Gumout Wynn's Anti Rf/Ri after hours in Diesel Fuel Rf/Ri after 69 hours in Diesel Fuel + 7% B12 Diesel Fuel + 7% WF Diesel Fuel + 10% gasoline Rf/Ri after 275 hours in Diesel Fuel RfIR; after 157 hours in Diesel fuel + 7% B12 Diesel Fuel + 7% FPPF Diesel Fuel + 10%gasoline Table 3

IC(C) 2 (C) 3 (C) 9.3 8.8 2.3 13X 104 28X 104 43X 104 3.3x 104 1.02 1.04 0.96 0.91 1.09 1.04 1.11 0.94 1.22 1.06 1.33 1.00 1.39 1.18 1.19 1.13 1.14 1.10 1.22 1.01 1.12 1.04 1.18 0.99 1.03 0.97 4 5 14.1 19.7 133 0.94 0.95 0.97 1.08 1.01 1.00 1.07 0.93 1.00 10.4 339 0.92 0.91 1.05 1.15 1.08 1.09 0.92 1.26 1.10 1.67 1.15 1.05 1.12 1.32 1.12 1.20 1.08 1.05 1.12 1.17 1.05 1.15 1.01 0.99 1.07 1.09 1.01 1.12 0.95 0.93 1.04 1.66 1.17 1.78 1.30 1.33 1.10 RESIS TA NCE RELAXA TION TES TS The compositions of Examples 7-15 were tested by the following tests. Samples 1 inch (2.54 cm) by 1.5 inch (3.8 cm) were cut from the molded slabs. Electrodes were formed on each sample by painting a strip 0.25 inch (0.62 cm) wide at each end with a suspension of silver particles (Electrodag 504avail- able from Acheson Colloids). The samples were annealed for 5 minutes at 2000C, and then cooled. The samples were then placed in an oven at 100'C and their resistances measured at intervals. It was found at the lower the head-to-head content of the

Claims

polymer, the less its change in resistance. CLAIMS

1. A conductive polymer composition which comprises a particulate conductive filler dispersed in polyvinylidene fluoride which has a headto-head content of less than 5.0%.

A k

2.97 1.37 1.08 1.35 1.47 1.17 1.14 1.27 1.28 1.06 1.01 1.16 2. A composition according to Claim 1 wherein the polyvinylidene fluoride has a head-to-head content of less than 4.5%.

3. A composition according to Claim 2 wherein the polyvinylidene fluoride has a heat-to-head content of less than 4.0%.

4. A composition according to anyone of Claims 1 to 3 wherein the conductive filler is carbon black.

5. A composition according to anyone of claims 1 to 4 which exhibits PTC behavior.

6. A composition according to Claim 1 substan tial ly as hereinbefore described.

7. An electrical device which comprises a conductive polymer element composed of a composition as claimed in any one of Claims 1 to 6; and (ii) at least one electrode which is in electrical contact vvith the conductive polymer element.

C i

8. A device according to Claim 7 which is a selfregulating heater in which the conductive polymer composition exhibits PTC behavior.

9. A device according to Claim 7 substantially as 5 hereinbefore described.

10. A method of heating diesel fuel which comprises passing current through a self-regulating heater as claimed in Claim 8.

11. A fuel feedth rough and heating assembly which can be positioned and connected between a fuel filter and a fuel tank of a fuel supply system to provide means for heating fuel which is being pumped through a fuel line from the fuel tank to the fuel filter, said feedthrough and heating assembly comprising (A) a feedth rough comprising (i) a fuel conduit having at one end thereof a fuel line connector for connecting the feedthrough to a fuel line and at the other end thereof a fuel filter connectorfor connecting the feedthrough to a fuel filter; and a neck portion which protrudes from the fuel conduit between the ends thereof and which comprises a chamber; (B) a flexible self-limiting strip heater as claimed in Claim 8 which comprises a fuel-resistant insulating jacket, one end of the strip heater being within the chamber of the neck portion, and the strip heater passing through the fuel line connector and protrud- ing from the fuel conduit; (C) insulated electrical leads connected to the electrodes of the heater, the connections lying within the chamber of the neck portion; and (D) a fuel- resistant, water-resistant and i nsu lating composition which encapsulates (i) the connections between the electrodes and the leads and (ii) the insulation at the ends of the connected electrical leads and (iii) the insulating jacket at the end of the heater; and (E) a fuel-resistant gasket which prevents fuel which is being pumped through the fuel conduit from exiting through the neck portion.

Printed for Her Majesty's Stationery Office by The Tweeddale Press Ltd., Berwick-upon-Tweed, 1983. Published at the Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.

GB 2 106 920 A 5