High temperature shear stable nanographite dispersion lubricants with enhanced thermal conductivity and method for making

Abstract

A process for producing a nanographite dispersion in a fluid wherein the thermal conductivity of the dispersion is enhanced from the base fluid by more than 10% for a 1% graphite dispersion. A high purity graphite with high crystallinity and reduced surface damage and oxidation is selected as the starting material. The starting material is subjected to a process of wet media milling in the presence of dispersant and solvent fluid. The mill temperature is controlled to control and reduce surface damage to yield a nanographite with flake shape and controlled aspect ratio until a particle size average of 300 nm diameter and 50 nm is obtained. The process recycles a portion of the milled material to increase the ratio of small particle distribution to large particles in an intermediate product with small and large particle bi-modal distribution. The large particle distribution is removed by a separation process such as centrifugation or filtration.

1 . A product made from a process for producing a nanographite dispersion in a fluid having the thermal conductivity of the dispersion enhanced from the base fluid by more than 10% for a 1% graphite dispersion, comprising the steps of:
 selecting a high purity graphite with high crystallinity, reduced surface damage, and reduced oxidation as the starting material;   wet media milling said starting material in the presence of a dispersant and a solvent;   controlling a mill temperature and atmosphere to control and reduce surface damage to said starting material;   milling until a desired nanographite with flake shape and controlled aspect ratio is achieved;   milling until a particle size average of 300 nm diameter and 50 nm thick is or smaller is obtained;   recycling a portion of the milled material to increase the ration of small particle distribution to large particle in an intermediate product with small and large particle bi-modal distribution;   removing the large particle distribution from the finished product by centrifugation or filtration, and   adding said intermediate product to a lubricant.   
     
     
         2 . The product by process of  claim 1 , wherein said mill is selected from a dry ball mill, a wet ball mill, and a jet mill. 
     
     
         3 . The product by process of  claim 1 , wherein the dispersant is an ashless dispersant. 
     
     
         4 . The product by process of  claim 1 , wherein the dispersant is
 selected from the group consisting of a lipophilic hydrocarbon group and a polar functional hydrophilic group wherein the polar functional group comprises a carboxylate, ester, amine, amide, imine, imide, hydroxyl, ether, epoxide, phosphorus, ester carboxyl, anhydride, and nitrile, and the lipophilic group comprises an oligomeric or polymeric compound from 70 to 200 carbon atoms to ensure oil solubility and hydrocarbon polymers treated with various reagents to introduce polar functions including products prepared by treating polyolefins such as polyisobutene first with maleic anhydride, or phosphorus sulfide or chloride, or by thermal treatment, and then with reagents such as polyamine, amine, and ethylene oxide.   
     
     
         5 . The process of  claim 1 , wherein the dispersant is selected from the group consisting of a N-substituted polyisobutenyl succinimides and succinates, alkyl methacrylate-vinyl pyrrolidinone copolymers, alkyl methacrylate-dialkylaminoethyl methacrylate copolymers, alkyl methacrylate-polyethylene glycol methacrylate copolymers, and polystearamides. Preferred oil-based dispersants that are include dispersants from the chemical classes of alkylsuccinimide, succinate esters, high molecular weight amines, MANNICH base and phosphoric acid derivatives. Some specific examples are polyisobutenyl succinimide-polyethylenepolyamine, polyisobutenyl succinic ester, polyisobutenyl hydroxybenzyl-polyethylenepolyamine, bis-hydroxypropyl phosphorate, LUBRIZOL 890 (an ashless PIB succinimide), LUBRIZOL 6420 (a high molecular weight PIB succinimide), ETHYL HITEC 646 (a non-boronated PIB succinimide), a PIB Succinimide, and a dispersant VI improver ETHYL 5777. 
     
     
         6 . The product by process of  claim 1  wherein said starting material comprises a pasty liquid of particles with mean size less than 500 nanometers in diameter and having a range of from 100 to 500 nm in diameter and from 20 to 80 nm in thickness.