Tuesday, June 29, 2010

Graphene-Rubber Nanocomposites Possess Qualities Like Carbon Nanotube Composites But Are Much Cheaper to Make

The Trustees of Princeton University (Princeton, NJ) received U.S. Patent 7,745,528 for functional graphene-rubber nanocomposites. The graphene can be produced at a much lower cost than can carbon nanotubes but offers similar characteristics to carbon nanotube composites.  The functional graphene-polymer nanocomposite is made by either solution or melt processing of polymer matrix and functionalized graphene sheets (FGS). 

Professor of Chemical and Biological Engineering, Director, Program in Engineering Biology Robert Prud'Homme, Bulent Ozbas, Ilhan Aksay, Richard Register and Douglas Adamson invented a polymer composition, containing a polymer matrix which contains an elastomer; and a functional graphene which displays no signature of graphite and/or graphite oxide, as determined by X-ray diffraction.  The nanocomposite exhibits excellent strength, toughness, modulus, thermal stability and electrical conductivity.

Over the past five decades, industrial scale `composite materials` have been produced by adding numerous minerals and metals to thermosetting, thermoplastic and elastomeric polymers. As compared to the bulk polymers, these composites have shown moderate mechanical performance improvements in properties such as Young's modulus, tensile strength, abrasion resistance and storage modulus. However, recent advances in nanoscale particle synthesis have dramatically accelerated the growth of the composite industry. The capacity to synthesize and characterize atomic-level particles has produced a new generation of high-performance fillers.

The incorporation of these sub-micron fillers in polymers, known as `nanocomposites`, has produced unparalleled performance improvements, easily outpacing earlier attempts. Commercial demand for nanocomposite materials has exploded. The possible applications for such materials cover a wide range of industries including food packaging, gasketing, automotive applications, portable electronic devices, etc.

The inventors of the graphene nanocomposites investigated the mechanical, thermal, electrical, and gas barrier properties of graphene filled polymer nanocomposites. Single graphene sheets with high surface area (>600 m2/g) are obtained from oxidation process that is followed by rapid exfoliation of graphite.

As a result of this process the graphene sheets are decorated mainly with epoxide, carboxylic acid and hydroxyl groups and, thus, unlike carbon nanotubes no surface functionalization is necessary to improve the dispersion of graphene fillers in the polymer matrix. In addition to vulcanized natural (NR) and styrene-butadiene (SBR) rubbers, polystyrene-polyisoprene-polystyrene (PS-PI-PS) and PDMS have been used as a matrix to study the dispersion of functional graphene (FG) and the resultant physical properties.

Exfoliated graphite offers the mechanical performance of CNTs at the price of clay fillers. Exfoliated graphite has the potential to generate a new class of affordable, high-performance nanocomposites.

Graphite may be exfoliated using chemical or thermal methods. In these methods, the weakly bound graphene sheets break apart. The backbone of each method centers on the acid treatment of graphite 

The functional graphene-polymer nanocomposite can be employed in all the areas where polymers (plastics, rubber, elastomers, etc.) are used for gas barrier applications including tires and packaging. The functional graphene-polymer nanocomposite can be used in many areas where both traditional and nano-composites are used including tires, bladder, packaging material, and in other areas where good gas permeation, mechanical and electrical properties are required.

The material is relatively easy to manufacture compared to other nano-sized filler that are used in the composites and it is naturally available (graphite). Unlike other available fillers in the industry (carbon nanotubes, clay, carbon black), properties such as electrical conductivity, low gas permeation, high strength and high modulus can be all accessed in the same material due to graphene's plate like structure. 

Graphite is an excellent lubricant especially in high temperature applications due easy sliding of graphene sheets over each other. The Princeton inventors expect functionalized graphene shteets (FGS) 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 FGS make it an attractive additive to coatings that must maintain stability exposed to sunlight. Coatings include preferably black coatings. FGS can be used as an additive for roofing sealers, caulks, elastomeric roofing membranes, and adhesives.

FGS 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 etc.

FGS can be added to a ceramic matrix to improve the electrical conductivity and the fracture toughness of the material. The partially oxidized surface of FGS offers stronger interaction with the ceramic matrix, especially with metal oxides and silicon oxides in particular. For example, FGS can be mixed with a silicon alkoxide material and then the silicon alkoxide can be condensed to form an amorphous material silicon oxide material containing well-dispersed FGS nano-platelets.

The hydroxyl and epoxide groups on the FGS surface can condense with the silicon alkoxide to form strong covalent bonds between the matrix and the FGS filler. Low loadings of FGS in such materials impart improved fracture strength and conductivity. FGS-glass and FGS-ceramic composites can also be applied as thermoelectric materials. Similar techniques can also be used to create tinted and UV-protective grades of glass. FGS can also be used to reinforce cement and in other building material applications.

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