Patent Application
20140339075
This invention relates to compositions containing conductive particles, articles comprising such compositions, and methods using such compositions and articles.
Graphite is a well-known material which comprises planar layers of carbon atoms. In each layer, the carbon atoms are arranged in a honeycomb lattice and are separated by a distance of about 0.142 nm, and the distance between the layers is about 0.335 nm. Graphite can be treated to increase the distance between adjacent layers (a process known as exfoliation, which is alternatively referred to as separation or expansion), and optionally to modify the graphite in other ways, for example by oxidizing or otherwise functionalizing carbon atoms at the edges of the layers. The final stage of exfoliation results in the single layer material known as graphene, which is “an atomically thick two-dimensional sheet composed of sp2 carbon atoms arranged in a honeycomb structure”. However, the term “graphene” is sometimes used in a way which is, strictly speaking, incorrect, to refer to exfoliated graphites which do not contain any completely separate carbonaceous sheets.
Reference may be made, for example, to Journal of Materials Science 22 (1987) 4196-4198 (Chung), Nature Nanotechnology 3 (2008) 563-568 (Hernandez et al.), U.S. Pat. No. 7,550,529, U.S. Pat. No. 6,811,917, Macromolecules 2010, 43, 6515-6530 (Kim et al.) and ACS Nano 5 (3), 2268-2270 (2011) (Li et al.). Each of those documents is incorporated herein by reference for all purposes.
Corrosion of metals is an important problem. Corrosion affects ships, submarines, offshore and submerged structures and facilities, oil and gas pipelines, infrastructure including bridges and highways and myriad other industries, and costs the US an estimated $276 billion annually. For example, the direct costs of corrosion to the US marine shipping industry are estimated at $2.7 billion annually.
A number of methods for mitigating corrosion are known. In some systems, which are referred to as impressed current cathodic protection systems, a material is made anodic by connecting it to the positive side of a battery or power supply, and the metal to be protected is made cathodic by connecting it to the negative side of the battery or power supply. The anode can be an extended anode or a point anode. The anode and the power supply must be such that the current density at all points on the substrate is high enough to control corrosion, but not so high as to cause problems such as damage to the substrate, for example embrittlement of the substrate or disbonding of a protective coating on the substrate. In addition, if the system is to have an adequate life, the anode itself must not be corroded at a rate which necessitates its replacement at frequent intervals.
In one type of impressed current system, which is described in U.S. Pat. No. 4,502,929, the entire disclosure of which is incorporated herein by reference for all purposes, the anode makes use of a composition comprising an organic polymer and carbon black or graphite dispersed in the organic polymer. The anode can for example be a nickel-coated multi-strand copper wire buss with the conductive polymer extruded over it. However, at the current densities preferably used, the conductive polymer becomes brittle and cracks, thus exposing the core, which rapidly becomes corroded. This disadvantage can be mitigated by surrounding the conductive polymer anode with a sacrificial material such as coke breeze, on which most of the electrochemistry occurs. However, this expedient involves additional costs and can only be used when the anode is placed in soil, not when the anode is placed in water. This type of anode has been used for terrestrial applications (pipe lines, tank bottoms etc.), but has not been used for marine applications, which generally require higher currents and current densities than can be used with this type of anode (typically 16 to 100 mA per linear foot and about 0.2 mA/cm2.
We have discovered, in accordance with the present invention, that improved results can be obtained in systems in which a chemical reaction takes place at the surface of an anode, for example in corrosion protection systems, dewatering systems, e.g. for stabilizing soil, and systems to reduce biofouling, through the use of an anode which comprises a polymer and, mixed with the polymer, a conductive filler which comprises an exfoliated graphite. The term “exfoliated graphite” is used in this specification to include any product obtained by treating graphite to increase the separation between some or all of the carbonaceous layers of graphite. The term also includes equivalent products obtained by so-called bottom-up processes which have been used to synthesize graphene, e.g. chemical vapor deposition, arc discharge, chemical conversion, reduction of carbon monoxide, and unzipping carbon nanotubes. Thus, the term “exfoliated graphite” is used in this specification to include the products which are commonly referred to as exfoliated graphite (including partially exfoliated graphite), expanded graphite, surface-enhanced graphite, graphite nanoplatelets, few-layer graphene and a single sheet graphene itself, and mixtures of two or more of these. For example, the term includes an exfoliated graphite produced by one of the methods described in the Macromolecules document incorporated by reference.
In a first aspect, this invention provides a method in which an electrochemical reaction, generally oxidation, of a material is carried out on the surface of an electrode, generally an anode, comprising a conductive polymer which comprises a polymer and, mixed with the polymer, exfoliated graphite. The material which is oxidized is preferably a material which occurs naturally in the environment, for example water (which is converted into oxygen and hydrogen ions) or sodium chloride (which is converted into chlorine and sodium ions. In one preferred method, the corrosion of a metallic substrate is inhibited by establishing a potential difference between the substrate as a cathode and an anode comprising a conductive polymer composition which comprises exfoliated graphite. In another preferred method, water is removed from a water-containing material, e.g. from soil, which lies between a metallic substrate and an electrode comprising a conductive polymer composition which comprises exfoliated graphite, by establishing a potential difference between the metallic substrate and the electrode. In a second aspect, this invention provides a metallic article which has a coating thereon of a conductive polymer composition which comprises a polymer and, mixed with the polymer, exfoliated graphite. Such articles are useful in the methods of the first aspect of the invention.
In one embodiment of the second aspect of the invention, the metallic article is an elongate article which has one dimension which is much larger than, for example at least 20 times, either of the other dimensions, and which comprises
In another embodiment of the second aspect of the invention, the metallic article comprises
The metallic article can for example be part of a ship which, in use, is below the water line.
In another embodiment of the second aspect of the invention, the metallic article is suitable for use as a point anode in a corrosion protection system, for example as one of a number of point anodes spaced away from a metallic substrate which is subject to corrosion. Such a point anode can for example have dimensions which do not differ from each other by more than a factor of three.
In a third aspect, this invention provides a conductive polymer composition which comprises a polymer, and, mixed with the polymer, exfoliated graphite, the composition having a resistivity in the range of 0.05-1000, e.g. 0.1-100 or 0.1-10, ohm·cm.
In a fourth aspect, this invention provides a conductive polymer composition which comprises a polymer and, mixed with the polymer, exfoliated graphite, the composition having a current density of at least 2 mA/cm2 when it is subjected to a test which makes use of an electrochemical cell which (i) consists of (a) an anode having the composition as its exterior surface, (b) a graphite cathode, (c) a saturated calomel electrode (SCE), and (d) an electrolyte of 3% salt water, the anode, the cathode, and the calomel electrode being located at the corners of an equilateral triangle having a side of about 1 inch (25 mm) and (ii) the anode is operated at a voltage of 1.6 volts versus the SCE. Preferably the composition has a current density of at least 2 mA/cm2 when the anode is operated at a voltage of 1.55 volts versus the SCE. It is particularly preferred that the composition has a current density of at least 2 mA/cm2 when the anode is operated at a voltage of 1.50 volts.
The invention is illustrated in the accompanying drawings, in which
In the Summary of the Invention above and in the Detailed Description of the Invention below, reference is made to particular features of the invention. It is to be understood that the disclosure of the invention in this specification includes all possible combinations of such particular features. For example, where a particular feature is disclosed in the context of a particular aspect or a particular embodiment, that feature can also be used in combination with other particular aspects and embodiments, and in the invention generally, except where the context excludes that possibility. The invention disclosed herein include embodiments not specifically described herein and can for example make use of features which are not specifically described herein, but which provide functions which are the same, equivalent or similar to, features specifically disclosed herein.
The term “comprises” and grammatical equivalents thereof are used herein to mean that, in addition to the features specifically identified, other features are optionally present. For example, a composition or device “comprising” (or “which comprises”) components A, B and C can contain only components A, B and C, or can contain not only components A, B and C but also one or more other components. The terms “consisting essentially of” and grammatical equivalents thereof are used herein to mean that, in addition to the features specifically identified, other features may be present which do not materially alter the claimed invention. The term “at least” followed by a number is used herein to denote the start of a range beginning with that number (which may be a range having an upper limit or no upper limit, depending on the variable being defined). The term “at most” followed by a number is used herein to denote the end of a range ending with that number (which may be a range having 1 or 0 as its lower limit or a range having no lower limit, depending upon the variable being defined). When a range is given as “(a first number) to (a second number)” or “(a first number)-(a second number)”, this means a range whose lower limit is the first number and whose upper limit is the second number. The terms “plural”, “multiple”, “plurality” and “multiplicity” are used herein to denote two or more than two features.
Where reference is made herein to “a” or “an” feature, this includes the possibility that there are two or more such features (except where the context excludes that possibility). Where reference is made herein to two or more features, this includes the possibility that the two or more features are replaced by a lesser number or greater number of features providing the same function, except where the context excludes that possibility. The numbers given herein should be construed with the latitude appropriate to their context and expression; for example, each number is subject to variation which depends on the accuracy with which it can be measured by methods conventionally used by those skilled in the art.
This specification incorporates by reference all documents referred to herein and all documents filed concurrently with this specification or filed previously in connection with this application, including but not limited to such documents which are open to public inspection with this specification.
Parts, percentages and ratios given in this specification are by weight unless otherwise noted.
Except where otherwise noted, the testing of the anodes described below was carried out in an electrochemical cell which consists essentially of (a) an anode having the composition as its exterior surface, (b) a graphite cathode, (c) a saturated calomel electrode (SCE), and (d) an electrolyte of 3% salt water, the anode, the cathode, and the calomel electrode being located at the corners of an equilateral triangle having a side of about 1 inch (25 mm).
The conductive filler can, but does not necessarily, consist essentially of an exfoliated graphite. The exfoliated graphite can be the product of a single process, or mixture of exfoliated graphites produced by different processes. The conductive filler can be an exfoliated graphite which contains substantially no single layer graphene, or can be a mixture of single layer graphene and byproducts of a process used to partially convert graphite into single layer graphene. Examples of exfoliated graphites which can be used include milled graphite, expanded graphite, and graphite nanoplatelets. The exfoliated graphite preferably has a surface area greater than about 10, preferably greater than 15, m2/gram for graphite that has not been extensively exfoliated, and can be much greater, e.g. 100-5000, or 500-2000, m2/gram for more completely exfoliated graphites.
The filler can also contain additional particulate conductive materials, for example a carbon black, e.g. a highly conductive carbon black such as Vulcan XC 72 (available from Cabot Corporation) or another carbon black having a resistivity 0.5-2 times the resistivity of Vulcan XC-72 and/or another carbon black and/or acetylene black, and/or graphite. These additional conductive materials can be used to reduce the resistivity of the conductive polymer composition to a desired level. The carbon black can for example have a specific gravity of about 1.8. The amount of the carbon black, based on the weight of the exfoliated graphite, can be, for example, 5-20%, preferably 8-15%.
The conductive polymer compositions used in this invention often contain an amount of the exfoliated graphite, and optionally one or more additional particulate conductive fillers, such that the resistivity of the composition is less than 100, preferably less than 10, particularly less than 5 ohm·cm, e.g. 0.1-100, or 0.1-10, or 0.1-5 ohm·cm.
When the composition comprises a thermoplastic or thermoset polymer with the conductive filler uniformly or non-uniformly dispersed therein, or such a composition which has been cross-linked, the amount of the particulate conductive filler can for example be 2.5-65%, preferably 5-60%, e.g. 20-55%. Compositions which are based on a thermoplastic polymer are particularly useful for the preparation of flexible strip anodes and flexible sheet anodes. When the conductive polymer is dispersed in a sintered polymer in the form of a multiplicity of particles which are melt bonded to each other, but which retain some particulate identity, or in ultrahigh molecular weight polyethylene, much higher amounts of filler can be used, e.g. 65-95%, preferably 70-90%. Such compositions are particularly useful for the preparation of anodes which are not flexible, e.g. point anodes, and anodes which have limited flexibility and comprise relatively inflexible sections containing the conductive polymer and relatively flexible sections in which the metallic substrate is covered by a flexible non-conductive polymer or a flexible conductive polymer.
The polymer with which the conductive filler is mixed can optionally have any one of the following characteristics, or any possible combination of one or more of the following characteristics.
Graphene and some modified graphites have a very low bulk density, which makes it difficult to mix them uniformly with a polymer. In one method, the conductive polymer composition is prepared by a three-step process which comprises (1) a first step in which a polymer in the form of a very fine powder (particle size preferably in the range of 1-100 microns) is mixed with the conductive filler, the ratio by weight of the polymer to the conductive filler preferably being in the range of 0.7:1-10:1, e.g. 1:1-10:1; (2) a second step in which the mixture from the first step is warm or cold pressed into porous sheets, pellets or granules; and (3) a third step in which the product of the second step is mixed with the remainder of the polymer (if any) in a Brabender mixer, Banbury-type internal mixer, compounding extruder, or the like.
In another method, the composition is prepared by a two-step process which comprises (1) warm or cold pressing the conductive filler into porous sheets, pellets or granules, and (2) mixing the porous sheets, pellets or granules with additional molten polymer in a Brabender mixer, Banbury-type internal mixer, or compounding extruder (e.g. a twin screw extruder or a Buss Kneader).
In another method, the composition is prepared by blending the conductive filler with a particulate polymer to obtain a homogeneous mixture, and then sintering the mixture to cause the polymer particles to bond to each other and to the conductive filler.
In another method, the composition is prepared by metering the conductive filler and the polymer into an extruder, or by metering the conductive filler and a monomer mixture into a melt reactor.
In another method, the composition is prepared by mixing the conductive filler with the polymer dissolved in a solvent, or with the polymer dispersed in an aqueous system, followed by casting the mixture, and removing the solvent or water.
The articles of the second aspect of the invention can optionally have any one of the following characteristics, or any possible combination of one or more of the following characteristics.
The articles of the invention can be prepared by methods known to those skilled in the art. For example, the ingredients can be dry blended in a Henschel™ or other high-speed mixer, and the resulting blend can be shaped. When the polymer is a thermoplastic, the shaping can be by extrusion or injection molding or by sintering. When sintering is used, preferably the blend is subjected to heat and/or light under moderate pressure such that individual polymer particles bond to the conductive filler and to each other to form a unitary structure, but not such that the polymer particles lose their separate identity. When the polymer is a thermoset, the shaping can be by heating in a mold.
The methods of the first aspect of the invention can optionally have any one of the following characteristics, or any possible combination of one or more of the following characteristics.
A surprising feature of the invention is that, when the conductive polymer comprises an exfoliated graphite, it is possible to make use of conductive polymer compositions having higher resistivity values, e.g. greater than 10 ohm·cm, than the resistivities of the conductive polymer compositions previously used in corrosion protection systems, which are will typically about 0.5-5 ohm·cm. A high resistivity value is advantageous because it helps to mitigate the non-uniform current distribution resulting from the resistance of the central conductor. The greater the distance from the power supply, the greater the resistance of the central conductor. The higher the resistivity of the conductive polymer composition, the less the variation in the total resistance with the distance from the power supply, and the more uniform the current distribution along the length of the anode.
The invention is illustrated in the following examples. Where reference is made in the examples to “Graphene”, the product in question is a product which was supplied by ACS Corporation, Oakland, Calif. as “Graphene”, but which is believed to be a partially exfoliated graphite produced from graphite. Similarly, where reference is made to graphene nanoplatelets, the product in question is a product which was supplied by XG Sciences Inc., Lansing, Mich. as graphene nanoplatelets, but which is believed to be a partially exfoliated graphite produced from graphite.
The following experiments were carried out to compare the performance of anodes in which the conductive polymer comprises (a) acetylene carbon black (typically used in commercially available conductive polymer anodes), or (b) Asbury 99 graphite (a synthetic high bulk density graphite with an average particle size of 15μ) combined with Vulcan XC72 carbon black (Cabot), or (c) “Graphene”. The acetylene carbon black, Vulcan XC72 and the graphite were commercially available products. In these tests, the anodes were operated in 3% saltwater at a current density of 2 mA/cm2, unless otherwise noted.
The anodes were prepared using the following steps.
The stability of the anode samples was determined by setting up an electrochemical cell with a graphite rod as the cathode, the conductive polymer sample as the anode, a standard calomel reference electrode, and with 3% sodium chloride solution as the electrolyte. The cell was powered with about 10.5-11.0 V DC power supply, and the cell current was adjusted by means of a resistor so that the initial current was 2 mA/cm2. This setting was left unchanged for the duration of the experiment. The potential between the anode and the cathode was measured at least once a day. Initially, the voltage is relatively constant, but at the point where the majority of the conductive filler has been consumed, the voltage rises quickly, indicating the end of the useful life of the anode. The anode to reference electrode potential was significantly lower for the exfoliated graphite-containing anodes than for the carbon-containing and the graphite-containing examples.
The useful life of the acetylene black samples was quite short; the useful life of the graphite/Vulcan 72 samples was about twice the useful life of the acetylene black samples; and the useful life of the exfoliated graphite-containing samples was about 5 times the useful life of the acetylene black samples.
Additional conductive polymer samples were prepared and tested using the conductive carbon blacks, synthetic graphites, natural flake graphites, and exfoliated graphite containing materials which are identified in Table 1 below. The description given in the Material Type column of Table 1 for the exfoliated graphite-containing materials in samples 5-13 is the description given to the material by the supplier of the material.
Test samples 1-13 were prepared using one or more of the carbonaceous materials identified in Table 1, linear low density polyethylene (LLDPE), Irganox (an antioxidant available from CIBA Specialty Chemicals) and Sunpar 2280 (a process aid available from Sunoco) in the quantities shown in Table 2 below. Samples 3-13 contain 5.6% of XC-72 (a highly conductive carbon black) in order to decrease the resistivity of the composition. Samples 1-5 were prepared by mixing the ingredients together at 200° C. for 5 minutes in a 60 cc Brabender mixer. Samples 6-13 were prepared in the same way except that the ingredients were formed into porous sheets by cold pressing them in a plastic bag, and then feeding the porous sheets into the Brabender mixer.
The composition produced in the mixer was then molded around a 0.078 inch (2 mm) diameter titanium wire to an outer diameter of about 0.305 inch (about 8 mm). After the molded composition had cooled, heat-shrinkable tubing was shrunk around the top and bottom sections of the molded composition, leaving an exposed central section of the composition having a defined surface area. The resulting samples were tested by setting up an electrochemical cell with the sample as the anode and a graphite rod as the cathode, and with 3% sodium chloride solution as the electrolyte. The cell was powered with a 10.5-11.0 V DC power supply, and the current was adjusted by means of a resistor so that the initial current was 2 mA/cm2 as in Example 1, and then evaluated to determine the amount of materials consumed.
The amount of materials consumed and the time of operation at 2 mA/cm2 can be used to calculate an anode capacity in either Amp hours/cc or Amp hours per gram. The anode to cathode voltage and the anode to reference voltage were monitored and can be used to determine the catalytic activity of the materials. Table 2 below shows the specific compositions prepared and the capacity of the anode in amp hours per gram. The anodes in which the conductive filler was a carbon black or a synthetic graphite had capacities between 2 and 5 A Hr/g. The anodes in which the conductive filler was an exfoliated graphite had capacities between 7.5 and 57 A Hr/g.
The procedure of Example 2 was used to prepare and test anodes containing conductive polymer compositions containing the ingredients set out in Table 3 below. In Table 3, Sint HDPE is an abbreviation for sintered high density polyethylene, LLDPE is an abbreviation for linear low density polyethylene, LLDPE g MalAnh is an abbreviation for maleic anhydride grafted LLDPE, HDPE-MalAnh is an abbreviation for maleic anhydride grafted high density polyethylene, PVDF is an abbreviation for polyvinylidene fluoride, PDVF Blend is an abbreviation for a blend of polyvinylidene fluorides, and UHMWPE-Oil is an abbreviation for a composite prepared by combining ultra high molecular weight polyethylene (UHMWPE), process oil and graphene followed by subsequent extraction of the process oil. The Anodeflex referred to in Table 3 is the commercially available product which embodies the disclosure of U.S. Pat. No. 4,502,929. The conductive fillers identified in Table 3 as A99, A60, A3243, Graphene, A3775, A3806, A3725, A4827, M15, C500, AcB and XC are further identified in Table 1 as Asbury DQA99, Asbury A60, Asbury 3243, ACS Corp. Graphene, Asbury 3775, Asbury 3806, Asbury 3725, Asbury 4827, XGSciences xGnP-M15 and, XGSciences xGnP-0500, Acetylene Black and Vulcan XC-72. The percentages of conductive filler in Table 3 are based on the total weight of the composition.
The results of testing these anodes are shown graphically in
Sintered anodes were prepared as follows.
1. The ingredients, and the amounts thereof, set out in Table 4 below are used to prepare sintered sheets A-F by the following steps.
a) Weighed amounts of the conductive filler and the powdered polymer are blended together, for example in a Keyence Hybrid mixer, model HM-501.
b) The resulting blend is spread evenly within a metal frame window, sandwiched between two Teflon release sheets, and compacted into a sheet under high pressure at room temperature, e.g. 4000 psi (280 kg per square centimeter) for two minutes.
c) The resulting sheet is sintered by heating in a hydraulic press at about 300° F. (150° C.) for samples A, B, C and D, and at about 350° F. (175° C.) for samples E and F, for a total of about five minutes, under 5-10 tons pressure, followed by cooling in a cold press for two minutes under pressure of 3000 psi (210 kg per square centimeter).
2. The sheets prepared in step 1 are laminated around a titanium wire as in Example 2.
The conductive fillers identified in Table 4 as Graphene and xGnP M15 are further identified in Table 1 as ACS Graphene and XGSciences xGnP-M15.
Sintered anodes were prepared by laminating a sintered sheet having one of the compositions in Table 4, over a titanium wire as described in Example 2 which had been previously coated with a composition containing linear low density polyethylene and a conductive filler. The thickness of the sintered sheet was 0.085 inch for samples A and C, 0.010 inch for sample D, 0.0815 inch for sample E, and 0.011 inch for sample F. Compositions A, C and D were laminated over a wire which had been coated with a composition containing LLDPE and 50.2% of acetylene black, Composition E was laminated over a wire which had been coated with a composition containing LLDPE and about 50% of xGnP-M15, and Composition F was laminated over a wire which had been coated with a composition containing LLDPE and 50% of a 44/6 blend of acetylene black and XC-72. The resulting anodes were tested as in Example 2 and the results are shown in Table 5 below.
The results of the testing are shown graphically in
Another method for processing highly filled conductive composites based on UHMWPE involves a first step of softening the polymer in oil, mixing the exfoliated graphite with the softened polymer, pressing the mixture into a sheet, and then extracting the oil. This method was used to produce an anode in which the conductive polymer composition contained 76.1 wt % “Graphene” in UHMWPE, as further described below.
The anode was prepared by laminating the resulting sheet around a titanium wire, as in Example 2, to prepare an anode having a diameter of 0.305 inch. The anode was tested as in Example 2 and had a lifetime of 5010 hours and an anode capacity of 41.61 A·hr/g
A conductive polymer composition was prepared by milling together 64 g expanded graphite, 30 g cis-1-4 polybutadiene, 4 g triallylisocyanurate and 1 g t-butyl per benzoate. The resultant material was warm pressed into a cylindrical mold around a titanium wire as in Example 2, and heat cured at 125° C. for 1 hr.
The ingredients of Sample 12 in Table 2, plus 1% TAIC and 0.1% Irganox 1010 were compounded on a Buss Kneader extruder and pelletized. The pellets were extruded around a nickel plated stranded copper 8 gauge copper conductor to form a ½″ (12.5 mm) diameter anode. Three sections of the anode were subjected to 5 Mrad ionizing radiation from an electron beam and subsequently heated to 125° C. for 10 minutes.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US2013/020423 | 1/5/2013 | WO | 00 | 7/3/2014 |
| Number | Date | Country | |
|---|---|---|---|
| 61631546 | Jan 2012 | US |
