CONDUCTIVE COATING FIELD OF INVENTION

This invention generally relates to coatings. More particularly the invention relates to methods of producing a compound for use as a corrosion inhibiting conductive coating and the conductive coating produced from the compound.

BACKGROUND ART

Corrosion inhibiting of metals using conductive polymers was first suggested since about two decades ago. Among all known conductive polymers, polyaniline has received considerable attention in the field of corrosion science. This is due to its ease of preparation, excellent environmental stability and features of redox properties associated with the nitrogen chain.

Polyaniline containing organic coatings had been known to offer corrosion protection of steel in acid and saline media. In most of these applications, polyaniline was doped with either hydrochloric acid, sulphonic acid, phosphonic acid, diocytyl phosphate or camphor sulfonic acid and the doped polyaniline was used in the manufacture of the coating.

The usage of conductive polymers for inhibiting corrosion of various metals has been widely described in numerous patent fillings, including US patent nos. 5,853,621 and 5,532,025 (doped polyaniline with polymeric binder), US patent no. 6,015,613 (metal and conductive polymer laminate), US patent no. 6,060, 116 (doped polyaniline and thermoplastic composition), US patent no. 6,500,544 (epoxy resin and non-conducting polyaniline), US patent no. 7, 179,604 (ligno-sulfonic acid doped polyaniline with acrylic or formaldehyde resin), US publication no. 2013/0130056 Al (wax and doped polyaniline composition with liquid paraffin), US patent nos. 5,645,890, 5,928,795 and 6, 117,558 (raw polyaniline, either in doped or un-doped forms).

There are several reasons that limit the application of raw polyaniline in terms of corrosion inhibition, i.e. the high brittleness and poor adhesion to its substrate, particularly in a corrosive environment. Troch-Nagels et al [J. Appl. Electrochem., 1992, Volume 22, pg. 756-764] found that polyaniline film electro-polymerised in a nitric acid solution could not offer any protection from corrosion and it is also pointed out that the adhesion of polyaniline film to the substrate was poor and brittle. Lu et al [Synthetic Metals, 1995, Volume 71, pg. 2163-2166] reported that the coatings of polyaniline deposited from an aqueous solution exhibited rather poor adhesion to the steel substrate. Wessling [Synthetic Metals, 1998, Volume 93, pg. 143-154] drew a conclusion that the proper coatings on metals with polyaniline from dispersion could lead to a significant shift of the corrosion potential in the direction of noble metals and to the formation of a passive metal oxide layer on the surface of the metal. Besides this, the author also emphasized that the polyaniline coating prepared by any suitable method must adhere well to the metal surface, especially under the corrosion conditions.

US patent no. 7,033,639 teaches a coating composition comprising a polyaniline particle and a binder which comprises a rubber component. Aside from the two components, several other components were added to the composition such as an oxidant and also a cross-linking agent. Although polyaniline is the main molecule used in the coating composition, the coating of the US patent is optimised for corrosion inhibition instead of increasing the conductivity properties of the coating. This is due to the low amounts of polyaniline used in the composition i.e. ~3 wt% which is insufficient to provide conductivity to the rubber compound. Further, the main disadvantage of conductive polyaniline is its limited thermal processability. It is observed that the US patent has process temperatures of up to 120°C. This further contributed to reduced conductivity of the compound.

CN publication no. 101033301 A teaches another method to produce a coating that mainly comprises pre-doped polyaniline and a rubber component. The doping agent for polyaniline was not identified in this publication. Secondary components were added to the composition such as dodecane sulfonic acid as a dispersant and m-cresol as a secondary doping agent. The coating produced through the method utilizes a complex method that includes mixing, heating, drying, exposing to IR light, vacuum desiccating and soaking and distilled water. It is observed that the CN publication has reaction temperatures of up to 80°C which has resulted in the poiyanline coating possessing only average values of conductivity. There is a need to formulate and prepare a novel electrically conductive coating that exhibits high level of corrosion inhibiting properties as well as improved conductivity values and at the same time, possesses good physical properties (e.g. tensile strength and modulus) for application on various metal surfaces using a simple method of preparation while maintaining the thermal processability of polyaniline. This invention thus aims to alleviate some or all of the problems of the prior art.

SUMMARY OF THE INVENTION

In an aspect of the invention, there is provided a method of producing a compound mixture for use as a corrosion inhibiting conductive coating comprising the following steps:

i) providing a rubber masterbatch solution;

ii) separately providing an electrically conductive filler masterbatch solution; and

iii) mixing the rubber masterbatch solution and the electrically conductive filler masterbatch solution until homogeneous to produce a compound mixture; wherein the compound mixture has an electrical volume resistance of at most 10 ⁹ ohm.

In an embodiment, the rubber masterbatch solution may comprise an appropriate amount of an organic solvent and an appropriate amount of a rubber component wherein the organic solvent and the rubber component are mixed until homogeneous. The rubber component may comprise natural rubber or synthetic rubber. The natural rubber may comprise epoxidized natural rubber (ENR) and the synthetic rubber may comprise poly(butadiene-co-acrylonitrile) rubber. The ENR may be of a grade containing about 20.0 to about 75.0 mole% of epoxide content and preferably about 25.0 to about 50.0 mole% of epoxide content.

The poly(butadiene-co-acrylonitrile) rubber may be of a grade containing about 17.0 to about 55.0 weight% of acrylonitrile content and preferably about 30.0 to about 48.0 weight% of acrylonitrile content.

The rubber component may be dissolved in the organic solvent at concentrations of about 10.0 to about 100.0 mg/ml and preferably about 20.0 to about 50.0 mg/ml. In an embodiment, the electrically conductive filler masterbatch solution may comprise an appropriate amount of an organic solvent and an appropriate amount of an electrically conductive filler component wherein the organic solvent and the electrically conductive filler component are mixed until homogeneous. The electrically conductive filler component may comprise doped polyaniline. The doped polyaniline may comprise dodecylbenzenesulfonate polyaniline. The dodecylbenzenesulfonate polyaniline may be of a protonation level of about 40.0 to about 100.0% and preferably about 48.0 to about 60.0%.

The electrically conductive filler component may be dissolved in the organic solvent at concentrations of about 10.0 to about 50.0 mg/ml and preferably about 15.0 to about 25.0 mg/ml.

In a separate embodiment, the organic solvent may comprise chloroform or toluene. In an embodiment, the ratio of the rubber masterbatch solution to said electrically conductive filler masterbatch solution in step (iii) may be about 50.0 to about 99.0 weight% and preferably about 70.0 to about 90.0 weight%.

In a further embodiment, the mixing in step (iii) may be conducted at temperatures of about 10°C to about 100°C and preferably about 23°C to about 60°C.

In a second aspect of the present invention, there is provided a corrosion inhibiting conductive coating comprising the compound mixture of this invention wherein the coating is formed by:

i) providing a thin layer of the compound mixture on a surface; and

ii) drying the compound mixture to form the corrosion inhibiting conductive coating.

In an embodiment, the thin layer of the compound mixture of step (i) is provided by casting or coating of the mixture on a surface.

It is an aim of the present invention to provide a compound for use as a corrosion inhibiting conductive coating that is suitable especially, though not exclusively, for use on metal surfaces. The compound exhibits high level of corrosion inhibiting properties as well as improved conductivity values and at the same time, possesses good physical properties (e.g. tensile strength and modulus) for application on various metal surfaces using a simple method of preparation which enables maintenance of the thermal processability of polyaniline. The compound and corrosion inhibiting conductive coating of this invention provides for various advantages which will be further elaborated in the following pages.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated, although not limited, by the following description of embodiments made with reference to the accompanying drawings in which:

FIG. 1 illustrates the basic chemical structure of the smallest repeat unit of an epoxidized natural rubber molecule;

FIG. 2 illustrates a preferable molecular structure of poly(butadiene-co-acrylonitrile);

FIG. 3 illustrates the basic chemical structure of the smallest repeat unit of a dodecylbenzenesulfonate polyaniline molecule;

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present invention is directed to methods of producing a compound mixture for use as a corrosion inhibiting conductive coating and a conductive coating produced from the compound. Compound Mixture

The compound mixture mainly comprises a rubber component and an electrically conductive filler component. Any suitable rubber component may be used. Natural or synthetic rubbers may be used as the rubber component. Preferably, the natural rubber used is epoxidized natural rubber (ENR). The synthetic rubber is preferably poly(butadiene-co-acrylonitrile).

ENR is a type of chemically modified natural rubber harvested from the Hevea Braziliensis tree which is manufactured by reacting the harvested natural rubber with peroxy formic acid. ENR conveys several desirable properties such as good dispersion level of fillers, good tensile properties and improved oxidation resistance. The smallest repeat unit of an epoxidized natural rubber molecule is shown in Figure 1. Any suitable form of ENR may be used as the rubber component. ENR of any grade between 20.0 to 75.0 mole% of epoxide content is preferred. ENR of 25.0 to 50.0 mole% of epoxide content is particularly preferred. An important mechanical feature of poly(butadiene-co-acrylonitrile) is resistance to oils, acids and aliphatic hydrocarbons, low temperature flexibility and is resilient at a range of temperatures. These properties are desirable especially for use as corrosion inhibiting coatings. Any suitable form of poly(butadiene-co-acrylonitrile) rubber may be used as the rubber component. Poly(butadiene-co-acrylonitrile) of any grade between 17.0 to 55.0 mole% of acrylonitrile content is preferred. Poly(butadiene-co-acrylonitrile) of 30.0 to 48.0 mole% of acrylonitrile content is particularly preferred. A preferable molecular structure of poly(butadiene-co-acrylonitrile) is illustrated in Figure 2.

In order to increase the conductivity properties of the compound, a sufficient amount of a suitable electrically conductive filler component is added. Any suitable electrically conductive filler may be used. Preferably, the electrically conductive filler component used is doped polyaniline.

Doped polyaniline is selected due to its low toxicity, good thermal stability, high electrical conductivity properties. The compound also possesses good compatibility with natural or synthetic rubbers. The doped polyaniline preferably is dodecylbenzenesulfonate polyaniline. The smallest repeat unit of a dodecylbenzenesulfonate polyaniline molecule is shown in Figure 3. Dodecylbenzenesulfonate polyaniline of any level of protonation between 40.0 to 100.0% is preferred. Dodecylbenzenesulfonate polyaniline with a protonation level of 48.0 to 60.0% is particularly preferred.

It was found that the use of poly(butadiene-co-acrylonitrile) and dodecylbenzenesulfonate polyaniline had a synergistic effect where low levels electrical conductivity percolation thresholds and higher conductivities were observed in the resulting compound. Method of Producing the Compound Mixture

The method for producing the compound mixture of the present invention mainly comprises the following simple steps:

i) providing a rubber masterbatch solution;

ii) separately providing an electrically conductive filler masterbatch solution; and

iii) mixing the rubber masterbatch solution and the electrically conductive filler masterbatch solution until homogeneous to produce a compound mixture.

The rubber masterbatch solution mainly comprises the rubber component as described and an organic solvent. The preferred amount of rubber component dissolved in the organic solvent in the rubber masterbatch solution is about 10.0 to 100.0 mg/ml of organic solvent. 20.0 to 50.0 mg/ml of rubber component to organic solvent is particularly preferred. The rubber component and organic solvent are mixed until homogenous to produce the rubber masterbatch solution.

The electrically conductive filler masterbatch solution mainly comprises the electrically conductive filler component as described and an organic solvent. The preferred amount of electrically conductive filler component dissolved in the organic solvent in the rubber masterbatch solution is about 10.0 to 50.0 mg/ml of organic solvent. 15.0 to 25.0 mg/ml of electrically conductive filler component to organic solvent is particularly preferred. The electrically conductive filler component and organic solvent are mixed until homogenous to produce the electrically conductive filler masterbatch solution.

Any suitable organic solvent may be used for producing the rubber masterbatch solution and the electrically conductive filler masterbatch solution. For example, chloroform or toluene may be used as the preferred organic solvent. The rubber masterbatch solution and electrically conductive filler masterbatch solution are then combined and mixed until homogenous to produce a compound mixture.

In step (iii), the composition of rubber masterbatch solution to electrically conductive filler masterbatch solution is preferably 50.0 to 99.0 weight%. A composition of 70.0 to 90.0 weight% is particularly preferred. The mixing of step (iii) may be conducted at any suitable temperature ranging from 10°C to 100°C. The mixing of step (iii) may be conducted without any heating. Temperature of 23°C to 60°C is particularly preferred, where necessary, heat is applied by any known technique, preferably by way of an electrical heater. It is noted that prior art methods uses some form of heating at various steps of the process and may also involve re-heating which consumes large amounts of energy.

Also, as the mixing of step (iii) may be conducted at room temperature as the operating temperature of the process, the polyaniline's processability is increased as polyaniline has limited thermal processability. If processing temperatures exceed the thermal processability of polyaniline, the conductivity of the resulting polyaniline compound decreases.

Any suitable mixing device may be used. A mechanical stirrer such as a magnetic stirrer may be used to facilitate mixing.

Corrosion Inhibiting Conductive Coating

The method for producing the corrosion inhibiting conductive coating of the present invention mainly comprises the following steps:

i) providing a thin layer of the compound mixture on a surface; and

ii) drying the compound mixture to form the corrosion inhibiting conductive coating. The corrosion inhibiting conductive coating may be formed by casting where the compound mixture is dispensed into a mould and allowed to dry to form the corrosion inhibiting conductive coating. Alternatively, the corrosion inhibiting conductive coating may be formed by dipping a metal surface into the compound mixture and allowing the compound mixture to dry, thus forming the corrosion inhibiting conductive coating on the metal surface.

Any suitable drying method may be used. For example, the compound mixture is allowed to air dry on the metal surface, forming the corrosion inhibiting conductive coating. Another example of drying is using an oven dryer. The corrosion inhibiting conductive coating produced from the rubber compound of the present invention has been observed to exhibit very low electrical volume resistance of 10 ¹ to 10 ⁹ ohms and good non-aged physical properties i.e. tensile strengths of up to 18. MPa, elongation of up to about 790.0%, with a modulus at 300% elongation and 15.1 MPa and of course excellent corrosion inhibiting properties as described in the following Examples.

EXAMPLE

The following Examples illustrate the various aspects, methods and steps of the process of this invention. These Examples do not limit the invention, the scope of which is set out in the appended claims.

Example 1

Preparation of Rubber Coating Rubber based coatings with various compositions of doped polyaniline were produced in order to determine their electrical resistance values and corrosion inhibiting behaviour.

Natural rubber Epoxidized natural rubber based coatings are prepared as follows:

The rubber masterbatch solution was prepared by using epoxidized natural rubber with 48.0±3.0 mole % epoxidation level. Chloroform was used as the organic solvent. The concentration of rubber masterbatch solution is 20.0 mg rubber/mL solvent. The electrically conductive filler masterbatch solution was prepared by using dodecylbenzenesulfonate polyaniline with 42.0% protonation level. Chloroform was used as the organic solvent. The concentration of filler masterbatch solution is 16.5 mg doped polyaniline/ml organic solvent. Rubber masterbatch solution was added to the electrically conductive filler masterbatch solution in appropriate amounts in order to obtain the following compositions (wt% rubber : wt% doped polyaniline), 50.0:50.0, 60.0:40.0, 70.0:30.0, 80.0:20.0, 90.0: 10.0, 95.0:5.0, 97.5:2.5 and 99.0: 1.0 respectively. Each of the above compound mixtures were magnetically stirred for 24 hours at room temperature prior to either casting or coating process.

Synthetic rubber

Poly(butadiene-co-acrylonitrile) based coatings are prepared as follows: Rubber masterbatch solution was prepared by using poly(butadiene-co-acrylonitrile) with 48.2±1.0 weight% acrylonitrile content. Toluene was used as the organic solvent. The concentration of rubber masterbatch solution is 50.0 mg rubber/ml organic solvent. The electrically conductive filler masterbatch solution was prepared by using dodecylbenzenesulfonate polyaniline with 42.0% protonation level. Toluene was used as the organic solvent. The concentration of electrically conductive filler masterbatch solution is 16.5 mg doped polyaniline/ml organic solvent. Rubber masterbatch solution was added to the electrically conductive filler masterbatch solution in appropriate ratios in order to obtain the following compositions (wt% rubber : wt% doped polyaniline), 50.0:50.0, 60.0:40.0, 70.0:30.0, 80.0:20.0, 90.0:10.0, 95.0:5.0, 97.5:2.5 and 99.0:1.0 respectively. Each of the above compound mixtures were magnetically stirred for 24 hours at room temperature prior to either casting or coating process.

Example 2

Measurement of Electrical Resistance Values for Rubber Coatings A pressed pellet of doped polyaniline (diameter 13 mm, thickness 1.5 mm) was placed in a holder with two spring-loaded metallic contacts in order to measure its electrical resistance value. Samples of pure rubber and rubber-doped polyaniline blend were cast respectively from their blend solution onto square microscope slides (625 mm ² ), and the solvent was allowed to evaporate for 24 hours. All electrical resistance values were measured by using the guarded 2-probe techniques. The equipment employed was a Keithley 6517A Electrometer with built in-voltage source up to lkV. Results from the measurement are shown in Table 1. Table 1: Magnitude Orders of Electrical Volume Resistance Values (Ohms) for Test Samples of

Rubber Coatings prepared accordingly to Example 1

All pure rubbers are good electrical insulators, with volume resistance values in the region of xlO ¹⁴ to xlO ¹⁶ ohms. The dodecylbenzenesulfonate polyaniline showed a low electrical resistance value in the region of xlO ¹ ohms. In general, the electrical resistance values of rubber coatings decreased with the increasing content of doped polyaniline. Example 3

Determination of Corrosion Inhibiting Behavior of Rubber Coatings

Total immersion technique was used to determine the corrosion inhibiting behavior of all rubber coatings. All compound mixtures as prepared according to Example 1 were kept in an air tight jar to avoid any solvent evaporation. Carbon steel samples of circular shape were cut from a steel metal sheet in order to minimise the exposed end grain according to ASTM G31 standard. The samples were polished by fine size emery paper (600 grit), cleaned, degreased with acetone (GPR, Sigma-Aldrich), dried and weighed with an electronic balance (Mettler Toledo AL204, accuracy of ± 0.1 mg). Sample shape of being circular in shape and sizes of diameter between 37mm and 39mm and thickness of between 2.5mm and 3.5mm were made convenient to laboratory corrosion tests according to ASTM G31 standards. Three sets of test samples with respective diameters of about 38 mm and thickness of approximately 3 mm, with a hole of about 8 mm in diameter were prepared for each compound mixture. First, each test sample was dip-coated for 1 min in the compound mixtures and then left for 1 hour to dry; the second dip was for another 1 min then dried in an oven for 48 hours at 40°C. A carbon steel sample without rubber coating was also prepared as the control for this test.

After the end of immersion test period (60 days) in 5.0 wt % sodium chloride aqueous solution, the test samples were taken out and the coating removed and cleaned according to ASTM Gl standards. The performance of the coating was examined through calculation of the sample's mass loss (in mean value) and the results are shown in Table 2. The greatest corrosion inhibiting behavior in 5.0 wt % sodium chloride aqueous solution was observed for all carbon steel samples coated with 10.0-30.0 weight % of doped polyaniline.

Table 2: Mean Values of Total Mass Loss for Carbon Steel Samples with Rubber Coatings prepared according to Example 1

Example 4

Physical Strengths of Rubber Coatings

Test samples of all rubber coatings prepared according to Example 1 showed some tensile property values (measured accordingly to the British Standard, BS ISO 37) as summarized in Tables 3-5. All rubber coatings prepared according to Example 1 showed good physical strength, i.e. tensile strengths up to about 18.7 MPa, elongations at break % up to about 790.3% and modulus at 300% elongation up to about 15.1 MPa. Table 3: Tensile Strength Values of Rubber Coatings prepared according to Example 1

Table 4: Elongation at Break Percentage [EB%] Values of Rubber Coatings prepared according to Example 1

Table 5: Modulus at 300% Elongation [M300] Values of Rubber Coatings

prepared according to Example 1

Conclusion

It is observed that the sample rubber coating with 70.0 to 90.0 weight% of rubber to doped polyaniline possesses the desired qualities required in a corrosion inhibiting conductive coating. This is based on the results of Table 2 to 5 where the maximum physical properties can be obtained without compromising on the conductivity of the coating as shown in Table 1. It is observed that the above rubber samples have the least amount of corrosion i.e. total mass loss, highest tensile strength and modulus at 300% elongation with optimal values of electrical volume resistance and elongation values where the composition of rubber to doped polyaniline is 70.0 to 90.0 weight%.

As will be readily apparent to those skilled in the art, the present invention may easily be produced in other specific forms without departing from its scope.