EP0074281B1

EP0074281B1 - Heating diesel fuel

Info

Publication number: EP0074281B1
Application number: EP82304744A
Authority: EP
Inventors: Peter Van Konynenburg; Andrew Au
Original assignee: Raychem Corp
Current assignee: Raychem Corp
Priority date: 1981-09-09
Filing date: 1982-09-09
Publication date: 1988-07-13
Also published as: GB2106920A; JPH0334498B2; JPS5853939A; EP0074281A1; ATE35745T1; DE3278775D1; CA1236246A; GB2106920B; JPH0395248A

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

This invention relates to a method of heating diesel fuel using a conductive polymer composition.
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, 3,351,777, 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 OLS 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 Applications 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, 0,038,718, 0,040,537, and 0,045,630.
Electrical devices containing conductive polymers generally (though not invariably) comprise an outer jacket, usually of insulating material, to protect the 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 damaged, it is necessary or desirable to select a conductive polymer which is not damaged (or which deteriorates at an acceptably low rate) when exposed to the surrounding 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 35°C below the melting point generally results in a decrease in resistivity both at the elevated temperature and at room temperature (a phenomenon known in the art as "resistance relaxation").
We have discovered that conductive polymer compositions which are based on polyvinylidene fluoride exhibit substantially improved stability of 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 -CH₂CF₂-, which can be arranged head-to-tail (i.e. ―CH₂CF₂―CH₂CF₂―), or head-to-head (i.e. -CH₂CF₂-CF₂CH₂-), 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 made use of polyvinylidene fluoride of relatively high head-to-head content, namely at least 5.2% and generally higher, which are easier to process than the polymers used in the method of the present invention.
In accordance with the present invention, there is provided a method of heating diesel fuel which comprises passing current through a self-regulating heater that has no outer protective jacket, which heater

(i) is immersed in diesel fuel, and
(ii) is composed of a conductive polymer composition which
- (a) comprises a particulate conductive filler dispersed in polyvinylidene fluoride which has a head-to-head content of less than 5%,
- (b) exhibits PTC behaviour, and
- (c) is in direct contact with the diesel fuel.

Polyvinylidene fluorides suitable for use in this invention are commercially available. The head-to-head 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 fluorides made by suspension polymerization (rather than emulsion polymerization) have lower head-to-head contents. The number average molecular weight of the polymer is generally at least 5,000, e.g. 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), e.g. tetrafluoroethylene, hexafluoropropylene and ethylene, is not excluded. The polyvinylidene -fluoride is preferably the sole crystalline polymer in the composition, but other crystalline polymers, e.g. 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 fluorine-containing elastomers and acrylic elastomers, which are usually added primarily to improve the flexibility and elongation of the composition.
The particulate conductive filler preferably comprises carbon black, and often consists essentially of carbon black. Choice of the carbon black will influence the resistivity/temperature characteristics of the composition, and a carbon black having a ratio of surface area (m²/g) to particle size (nanometers) of 0.03 to 6.0 is preferred. The amount of conductive filler used will depend upon the desired resistivity of the composition. For flexible strip heaters which are to be powered by a 12 volt battery for heating diesel fuel, we prefer a PTC composition whose resistivity at 25°C is less than 200 ohm - cm e.g. 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 used in the method of the invention 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 used in the method 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 used in the method of the invention can be carried out in a 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 Examples, in which Examples 1, 2, 3, 7, 12 and 13 are compositions of Comparative Examples not used in the method of 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) extruder fitted with a pelletizing die. The extrudate was chopped into pellets which were dried for 16 hours at 80°C.
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 110°C. 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)×0.1 inch (0.3 cm), the blend was melt-extruded over a pair of pre-heated 14 AWG (1.85 mm diameter) 19/27 nickel-coated copper wires with a center-to-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 25°C.

Examples 2-6

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 75°C 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)×0.1 inch (0.3 cm), the blend was melt-extruded over a pair of pre-heated 18 AWG (1.2 mm diameter) 19/27 nickel-coated copper wires 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 0.036" (.091 cm) by compression molding at 200°C for three minutes.

Tests for stability in organic solvents

The extrudates obtained in Examples 1 and 4 were 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 25°C and the resistance of the samples was measured at intervals. The solvents used, and their solubility parameters, were
The results for Examples 1 and 4 are shown in Figures 1 and 2 respectively of the accompanying drawings, where the ratio of the resistance at a given time (R,) to the initial resistance (R,) 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 160°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 25°C and dried, and their resistance measured. Table 3 shows the value of the ratio R_f/R_i for the different samples at various times. The additives tested, and their main ingredients, were as follows:

Resistance relaxation tests

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 504 available from Acheson Colloids). The samples were annealed for 5 minutes at 200°C, 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 polymer, the less its change in resistance.

Claims

1. A method of heating diesel fuel which comprises passing current through a self-regulating heater that has no outer protective jacket, which heater

(i) is immersed in diesel fuel, and

(ii) is composed of a conductive polymer composition which

(a) comprises a particulate conductive filler dispersed in polyvinylidene fluoride which has a head-to-head content of less than 5%,

(b) exhibits PTC behaviour, and

(c) is in direct contact with the diesel fuel.

2. A method according to claim 1, wherein the polyvinylidene fluoride has a head-to-head content of less than 4%.

3. A method according to claim 1 or 2, wherein the conductive filler is carbon black.

4. A method according to claim 3, wherein the carbon black has a ratio of surface area (meter²/gram) to particle size (nanometer) in the range from 0.03 to 6.0.