Sunday, May 2, 2010

Graphene 'Worms' and Nano Accordions for Oil Spill Remediation Developed by Princeton Chemists

A method of producing nanofillers of thermally exfoliated graphite oxides (TEGO), a type of graphene that resembles worms or accordions on the nano-scale, may be used to produce composite materials for oil spill remediation according to its inventors Princeton University Professor of Chemical Engineering and  Director, Program in Engineering Biology Robert K. Prud'Homme, Professor of Chemical Engineering Ilhan A Aksay and researcher Douglas Adamson. 

The process of making expanded graphite materials with an accordion or "worm-like" structure has many applications, including electromagnetic interference shielding, oil spill remediation, and sorption of biomedical liquids. 

Thermally exfoliated graphite oxides (TEGO) may be used in polymer composites, particularly in conductive polymer composites. They may also be used as an  additive in elastomeric materials, in elastomer diffusion barriers, as a hydrogen storage medium, as material for supercapacitors, in flexible electrodes, as adsorbent material, as dispersant, as lubricant, in coatings, particularly in coatings that require UV stability. Further TEGO can be used in glass or ceramic composites, in thermoelectric composite materials, as pigments in inks, or as UV protective filler in composites. TEGO can also be used for electromagnetic shielding.   

The result of partial oxidation of graphite produces graphite oxide (GO). Many models have been proposed to describe the structure of graphite oxide. However, the precise structure of GO is still an area of active research.  Princeton University  (Princeton, NJ) received U.S. Patent 7,659,350 on February 9, 2010 for its method to produce TEGO.

The majority of these partially exfoliated graphite materials are made by intercalation of graphite with sulfuric acid in the presence of fuming nitric acid to yield expanded graphitic material. These expanded materials are then heated to yield an increase in the c-axis direction. While these materials are sometimes referred to as "expanded graphite" or "exfoliated graphite," they are distinct from the TEGO developed at Princeton.

For the "worm-like" expanded graphite oxide materials, the individual graphite or GO sheets have been only partially separated to form the "accordion" structures. Although the heating results in an expansion in the c-axis dimension, the typical surface area of such materials is in the order of 10-60 m2/g. Both the surface area below 200 m2/g and the presence of the 0002 peak of the pristine graphite corresponding to a d-spacing of 0.34 nm are indicative of the lack of complete separation or exfoliation of the graphene sheets.

While the term "graphene" is used to denote the individual layers of a graphite stack, and graphite oxide denotes a highly oxidized form of graphite wherein the individual graphene sheets have been oxidized, graphene is used to denote the layered sheet structure that may be in a partially oxidized state between that of native graphene and graphite oxide.

Material based on modified graphite is appropriate, for example, as a nanofiller for polymer composites, a conductive filler for composites, an electrode material for batteries and ultracapacitors, as a filler to improve diffusion barrier properties of polymers, and as a hydrogen storage material. The graphite nanoplatelet (GNP) material is distinct from previous graphitic materials, which lack one or more of the attributes required for a successful nanofiller.

Also, the Princeton invention relates to a material based on modified graphite that is electrically conductive and can confer electrical conductivity when formulated with a polymer matrix. The material based on modified graphite has a high aspect ratio so that it can perform as a barrier to diffusion when incorporated in a polymer composite

FIG. 12 shows in (A) and (B) Scanning electron microscope (SEM) images of TEGO-poly(methyl methacrylate) fracture surface. By using a high acceleration voltage (20 kV), the sub-surface morphology of TEGO nanoplates can be observed. The persistent wrinkled nature of the TEGO nanoplates within the composite provides for better interaction with the host polymer matrix.

 Currently expanded graphite is used as an absorbent for oil spill remediation and for the cleanup of other hazardous organic liquid spills. The hydrophobic surfaces are wetted by oil and thereby bind and hold oil. Other compounds used for spill remediation are clays, but these must be surface treated to may them hydrophobic enough to bind organic liquids. The high surface area of TEGO and its hydrocarbon surfaces make it an excellent absorbent material for oil and organic liquids. The TEGO can be contained in large porous sacks made from polypropylene or polyethylene fabric or porous film.

The low bulk density of TEGO make it attractive in that the amount of liquid that can be imbibed on a weight basis can be high. Liquid loadings between 100 to 10,000 wt:wt oil to TEGO can be achieved. In another embodiment the TEGO is co-processed with a polymeric binder in the form of a foam sheet. These open cell structure of the foam allow contact between the oil and the TEGO surfaces. The advantage of this system is that the absorbent system can be rolled for storage. 

Graphite is an excellent lubricant especially in high temperature applications due easy sliding of graphene sheets over each other. The Princeton inventors expect TEGO to display superior lubricating properties since the interactions between the graphene sheets are significantly weakened in comparison to graphite.

The UV light absorption capabilities of TEGO make it an attractive additive to coatings that must maintain stability exposed to sunlight. Coatings include preferably black coatings. TEGO can be used as an additive for roofing sealers, caulks, elastomeric roofing membranes, and adhesives.

TEGO absorbs UV radiation and can therefore be used to impart UV protection and to improve the lifetime of plastic components in outdoor use, such as hoses, wire coatings, plastic pipe and tubing. 

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