The present invention relates to a method of carbon nanomaterial modification by plasma treatment, to composites which incorporate the plasma modified carbon nanomaterial, and to methods of producing those composites.
Carbon nanotubes (CNTs) have excellent mechanical properties, which make them attractive candidates for application in high performance composite materials. It is known that CNTs could have a Young's modulus of up to 1 TPa and a tensile strength approaching 60 GPa. The use of carbon nanotubes in many polymer/CNT composites, including poly(methyl methacrylate), epoxy resin, poly(vinyl alcohol) and polystyrene, has increased the mechanical properties of the composites, such as Young's modulus, hardness, and tensile strength.
In order to obtain good physical properties for CNT/polymeric composites, the surface of CNTs needs be modified to improve the nanotube dispersion and to enhance the interfacial strength. The existing technique for nanotube surface modification, oxidation of CNTs, has some disadvantages. The acid oxidation of CNTs will often destroy the structures of CNTs, add more defects on the surface of CNTs, and cut the CNTs into shorter fragments. The interfacial properties of the acid-oxidized carbon nanotubes can be improved by subsequent chemical modification through reactions characteristic of acid-oxidization-induced carboxylic groups. However, this requires complicated chemical processes which are often carried out in toxic solvents, and it increases both the cost for final products and the risk of environmental pollution.
Accordingly, there is a need for improved methods for surface modification of carbon nanomaterials, for improved composites, and for methods of making the composites.
The present invention meets this need by providing a polymer-carbon nanomaterial composite. The composite includes a polymer matrix; and plasma-modified carbon nanomaterials having surface functional groups attached thereto, wherein the carbon nanomaterial is selected from carbon nanotubes, carbon nanofibers, carbon nanoparticles, carbon black, nanodiamond, fullerenes, or combinations thereof.
Another aspect of the invention is a method of making a polymer-carbon nanomaterial composite. The method includes providing carbon nanomaterials selected from carbon nanotubes, carbon nanofibers, carbon nanparticles, carbon black, nanodiamond, fullerenes, or combinations thereof; exposing the carbon nanomaterial to a plasma in the presence of a monomer to form plasma-modified carbon nanomaterial having surface functional groups attached thereto; providing a polymer matrix; and blending the plasma-modified carbon nanomaterial with the polymer matrix.
Another aspect of the invention is a method of modifying carbon nanomaterials. The method includes providing carbon nanomaterials selected from carbon nanotubes, carbon nanofibers, carbon nanparticles, carbon black, nanodiamond, fullerenes, or combinations thereof; and exposing the carbon nanomaterial to a plasma in the presence of a monomer to form plasma-modified carbon nanomaterial having surface functional groups attached thereto.
Plasma treatments for surface modification of carbon nanotubes in a dry state were developed. Interactions between the plasma-modified carbon nanotubes and polymeric matrices depend on the nature of functional groups induced by the plasma treatment. In order to develop polymeric/CNT composites with enhanced mechanical properties, the plasma-induced functional groups need to have strong interactions with the polymeric chains and/or other fillers in the polymeric matrix.
Although the examples describe the use of carbon nanotubes, the invention is not so limited. Other carbon nanomaterials, including, but not limited to carbon nanofibers, carbon nanoparticles, carbon black, nanodiamond, and fullerenes, can also be used.
The surface modified CNTs do not have a harmful effect(s) on their morphological structures. The modified CNTs can be used as enforcement fillers in polymeric materials, especially rubbers. The plasma treated CNTs with surface functional groups can be dispersed well in the rubber matrices, such as hydrogenated nitrile butadiene rubber (HNBR) and fluoroelastomers, to form rubber/CNT composites by conventional melting blending methods, including extrusion, roll milling, and solvent method. The polymer/CNT composites possess improved mechanical properties.
The surfaces of CNTs are modified in a plasma reactor, in which functional groups from plasma monomer vapor(s) are grafted onto the nanotube surfaces. The plasma monomers used in this study include, but are not limited to, acetic acid, hexane, acetonitrile, acrylic acid, methacrylic acid, and acetaldehyde. Suitable functional groups include, but are not limited to, acetic acid groups, hexane groups, acetonitrile groups, acrylic acid groups, methacrylic acid groups, acetaldehyde groups, alkyl amine groups, alcohol groups, or combinations thereof.
Suitable materials for the polymer matrix include, but are not limited to, rubbers, elastomers, thermoplastics, thermosets, synthetic inorganic polymers, biopolymers, coordination polymers, or combinations thereof. Suitable rubber matrix materials include, but are not limited to, hydrogenated nitrile butadiene rubber, styrene butadiene rubber, acrylonitrile butadiene styrene or combinations thereof. Suitable elastomer matrix materials include, but are not limited to, fluoroelastomers, ethylene propylene rubber, or combinations thereof. Suitable thermoplastic matrix materials include, but are not limited to, poly(vinyl acetate), ethylene vinyl acetate, polyacrylonitrile, polyethylene, polypropylene, or combinations thereof. Suitable thermoset matrix materials include, but are not limited to, urea-formaldehyde, epoxy, melamine, or combinations thereof.
The plasma-modified carbon nanomaterials are generally present in an amount of less than about 8 wt %, alternatively less than 6 wt %, alternately less than about 4 wt %, alternatively less than about 2 wt %, or alternatively less than about 1 wt %.
The polymer-carbon nanomaterial composite generally has at least one improved mechanical property compared to the polymer matrix without the plasma-modified carbon nanomaterial. Improved mechanical properties include, but are not limited to, elongation, tensile strength, storage modulus, loss modulus, or stress.
The polymer-carbon nanomaterial composite can be used to make a wide variety of products. Examples include, but are not limited to, inflatable packers, mechanical packers, plugs, cup packers, electrical cables, conductive cables, wirelines, o-rings, bonded seals, seal backup rings, motors, casing/tubing patches, cementing plugs, bottom plugs, shock/impact absorbers, or pump protectors.
The polymer-carbon nanomaterial composite can be made by exposing carbon nanomaterial to a plasma in the presence of a monomer to form plasma-modified carbon nanomaterial having surface functional groups attached thereto. The plasma-modified carbon nanomaterial is blended with the polymer matrix.
Suitable methods for blending the plasma-modified carbon nanomaterial with the polymer matrix include, but are not limited to, by melt blending, or mechanical mixture. Suitable melt blending methods include, but are not limited to, extrusion, roll milling, solvent method, or combinations thereof.
The carbon nanomaterial is typically exposed to the plasma for a time in the range of about 10 sec to about 2 hr, alternatively about 10 sec to about 15 min. The pressure is typically in the range of about 10 to about 30 mTorr. The vapor pressure of the monomer is typically in a range from about 50 mTorr to about 1,000 mTorr, alternatively about 50 mTorr to about 500 mTorr.
The invention may be more readily understood from the following examples, which are intended to illustrate the invention, but not limit the scope thereof.
The plasma treatment of carbon nanotubes was carried out under different chemical vapors (alkyl amines, alcohols, aldehyde, acetonitrile, acetic acid, and hexane) with a base pressure of 20 mTorr and monomer vapor pressure ranging from 50 to 1000 mTorr for 10 sec to 2 h.
The XPS (X-ray photoelectron spectroscopy) survey spectrum of the raw CNTs (
After the plasma treatment with acetic acid vapor at 100 mTorr for 10 s, the XPS survey spectrum of the plasma-modified CNTs (
A desired amount of the acetic acid or hexane plasma-treated CNTs was mixed with a predetermined amount of raw HNBR sample in an extruder at 160° C. for 10 min before being extruded out. The crosslinking initiator was added to the HNBR/carbon nanotube mixture and blended at 100° C. for another 10 min. The mixture sample was pressed at 176° C. (350° F.) for 20 min to crosslink in a sheet form.
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A desired amount of the acetic acid plasma-treated carbon nanotubes was mixed with 10 wt % PVAc in N,N-dimethyl formamide with a ratio of 1:1 by weight, followed by sonication at room temperature for 1 h. The solvent was evaporated under vacuum at 80° C., and the solid residue was further dried in oven at 80° C. for 48 h to produce the PVAC/MWNT mixture. A desired amount of the PVAc/MWNT mixture was added into a HNBR sample in an extruder at 160° C. for 10 min and extruded out. Weight ratios of the MWNT/HNBR in the resulting samples are 1/100, 2/100 and 4/100 (designated as: HNBR/PVAc/CNT-x, where x is the weight ratio of MWNT to HNBR). The crosslinking initiator was added to the MWNT/HNBR mixture at 100° C. for 10 min. The samples were pressed at 176° C. (350° F.) for 20 min to crosslink in a sheet form. The control sample was the raw HNBR crosslinked using the above procedure with the crosslinking initiator, but without CNTs.
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The storage modulus and loss modulus of the HNBR/PVAc/CNT composite samples increased in comparison with pure HNBR at high temperature (
A desired amount of the acetic acid plasma-treated carbon nanotubes was mixed with 10 wt % EVA in an extruder at 180° C. for 10 min and extruded out. A predetermined amount of the EVA/MWNT mixture was added into the HNBR sample in an extruder at 180° C. for 10 min and extruded out. Weight ratios of MWNT/HBNR in the resulting samples are 1/100, 2/100 and 4/100 (designated as: HNBR/PVAc/CNT-x, where x is the weight ratio of MWNT to HNBR). The crosslinking initiator was added to the HNBR/PVAc/CNT mixture at 140° C. for 10 min. The resultant samples were pressed at 176° C. (350° F.) for 20 min to crosslink in a sheet form. The control sample was the raw HNBR sample crosslinked by above procedure with the crosslinking initiator, but without CNTs.
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A desired amount of the plasma-treated carbon nanotubes was mixed with fluoroelastomer (VTR 8655, or GBL 6005, available from Schlumberger) sample at 170° C. for 10 min. and then extruded out. Weight ratios of the plasma treated MWNT/fluoroelastomer are 1/100, 2/100, and 4/100 (designated as: fluoroelastomer/CNT-x, where x is the weight ratio of MWNT to HNBR). The crosslinking initiator (A178 Luperco and A317 Taic DLC, available from Schlumberger) was added to the MWNT/fluoroelastomer mixture at 110° C. for 10 min. The weight ratio of A178/A317/fluoroelastomer was 3.3/1.9/100. The resultant sample was pressed at 180° C. for 30 min to crosslink in a sheet form. The control sample was made using the above procedure, but without carbon nanotubes.
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For the hexane plasma-treated MWNTs, the VTR 8655 fluoroelastomer/MWNT composites also showed an enhanced strength at break, even with 1 wt % loading of the hexane plasma-treated MWNTs in the rubber matrix. Further increase in the loading of the hexane plasma-treated MWNTs (4 wt %) caused the strength at break to slightly decrease within the experimental error (
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For the GBL 6005/MWNT composites with the hexane plasma-treated MWNTs, only the composite sample with 4 wt % hexane plasma-treated MWNTs showed an enhanced strength at break of 2412 psi. With the load of the hexane plasma-treated MWNTs below 4 wt %, the strength at break of the composite samples was similar to the of the pure GBL 6005 sample.
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While certain representative embodiments and details have been shown for purposes of illustrating the invention, it will be apparent to those skilled in the art that various changes in the apparatus and methods disclosed herein may be made without departing from the scope of the invention.