Surface modified carbonaceous materials

Details Surface modified carbonaceous materials Surface modified carbonaceous materials

2001-04-06

2003-01-14

Nutter, Nathan M. (Department: 1711)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Electrode

C525S183000, C428S317100

Reexamination Certificate

active

06506517

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a new process for grafting polymers at the surface of carbonaceous materials and to the application of materials thus obtained as an electronically conductive additive, in particular in electrochemical generators, as electronically conductive deposit, or as an ink.
PRIOR ART
Carbonaceous materials play a major part in many fields, for example: structural elements (composite material based on carbon fibers, . . . ), pigments (ink, toner, paint, . . . ), electrodes (double layer capacitance, electrolysis of aluminum, . . . ), ion exchangers (removal of heavy metals, purification of water, . . . ), catalyst supports (combustible material battery, . . . ), mineral charges (rubber industry, tire, . . . ), . . .
This wide range of applications is inherent to the specific physico-chemical properties of carbonaceous materials: heat resistance, good electrical conductivity, wide range of specific surface (typicallly of the order of m
2
/g to thousands of m
2
/g), coloring power, . . .
Among all the physico-chemical parameters characterizing a carbonaceous material, one of the most important is the state of its surface which conditions, in final analysis, the interface between the carbonaceous material and the surrounding medium. It is thus well known that various functional groups may be present at the surface of a carbonaceous material or may be introduced later on during its synthesis through different processes: chemical (oxidation, sulfonation, . . . ), physico-chemical (electrolytic treatment, plasma, . . . ) or thermal.
It is thus possible to introduce various functions at the surface of carbonaceous materials: radicals, carbanions, carboxylic acids, sulfonic acids, phosphonic acids, hydroxyls, amines, pyridines, halogens, anhydrides, lactones, pyrones, quinones, etc. The concentration of these different groups at the carbon surface varies typically from 0.1 to 10 meq/g of carbon. It is therefore possible to control and finally modify the surface properties of these materials, for example their acid-base character or their hydrophilic-hydrophobic character. Reference will be made in this connection to the work of I. N. Ermolenko, L. P. Lyubliner & N. V. Gulko: Chemically Modified Carbon Fibers, VCH Publishers Inc., New York, 1990. Thus, carbon powders carrying carboxyl functions at their surface find an important outlet in the field of pigments for high capacity ink jet printers (see Technical Notice Degussa AG: “Pigmentru&bgr;e/Pigment Blacks”).
The possibilities of functionalizing a carbonaceous material are thus sufficiently varied to consider initiating numerous chemical reactions from its surface. Among all the opportunities to modify the interface properties, grafting of polymers is of particular interest. For example, the presence of various ultrafine particles (carbon, titanium oxide, silica, ferrite, . . . ) at the surface of a grafted polymer enables to improve the dispensability of the materials thus modified in organic solvents or in suitable polymer matrices. It is thus possible to obtain colloidal dispersions of a carbon whisker which are very stable in tetrahydrofurane by grafting at their surface≈1 g of polystyrene per gram of carbon (N. Tsubokawa, T. Yoshihara, Y. Sone,
J. Polym. Sci., Part A, Polym. Chem.,
Vol. 30, 561-567, 1992).
Different strategies of synthesis were used for grafting polymers at the surface of a carbonaceous material. Two large families may be mentioned: initiation of polymerization of a monomer from a functional group which is present at the carbon surface, and grafting of a prepolymer containing at least one chemical function capable of reaction with the surface of carbon.
In the first case, a number of works were concentrated on initiating the anionic, cationic or free radical polymerization of monomers, more often by modifying a functional group which is present on a carbonated material to make it active towards the type of polymerization under consideration. Let us mention for example the possibilities offered from a carboxylate function which is easily introduced by oxidation of a carbonated material: transformation into an alkaline metal salt (Li, Na, K. Cs, Rb, . . . ) for anionic polymerization, transformation into an acylium salt (acylium perchlorate, acylium antimony hexafluoride, . . . ) for cationic polymerization or the transformation into peroxyesters for free radical polymerization.
In the second case, other works dealt with the grafting of prepolymers containing a chemical function which is capable of reacting with a group which is present at the carbon surface, most of the time by modifying the group present on the carbon atom to make it reactive. Let us mention for example the possibilities offered by the transformation of carboxyl functions into isocyanate, epoxide, acid chloride, . . . thus enabling grafting of, for example, a polyethylene glycol, a polyvinyl alcohol, a polyethylene imine, a silicon diol, a silicon diamine, . . .
These different processes of synthesis are not completely satisfactory, because they necessitate the chemical modification of the group which is fixed on the carbon atom and/or the polymer which is intended to be grafted makes these synthesis complex and costly to operate on a large scale. Moreover, graftings initiated by polymerization cause a problem of chain endings whose posterior reaction may be unfavourable to the operation of systems integrating these carbonated materials. They also require separation of the grafted chains on the carbon from the polymer chains which are formed in solution.
SUMMARY OF THE INVENTION
In order to overcome these limitations, the inventors have studied this problem and propose, in the present invention, a new process for the preparation of carbonated materials which are grafted with polymers, which consists of fixing at the surface of carbonated materials, polymer segments via ester or amide bonds by a mechanism of dehydration which is easy to implement.
The invention is also concerned with the application of these materials as an additive of electronic conduction in particular in electrochemical generators, as an anti-corrosive deposit and/or an electronic conductor on different supports (plastic, aluminum, . . . ), and as inks, in particular offset inks.
DESCRIPTION OF THE INVENTION
The present invention proposes a new process enabling grafting of polymers bearing at least one carboxyl, hydroxyl or amine function, from carboxyl, amine or hydroxyl functions which are present at the surface of carbonaceous materials which are in the form of powders or fibers, the different possibilities being summarized in the reaction scheme given below. In this manner, a chemical bond is provided via an ester or amide bond.
The novelty of this process resides in the fact that the reaction is carried out in such a manner that the carbon which is intended to be modified by polymer segments is in suspension in a solution containing the polymer which is intended to be grafted, and in the fact that the solvent is selected so as to be able to carry out the reaction of dehydration between a carboxyl function and an amine and/or an hydroxyl function at a suitable temperature.
Among the solvents, dimethylformamide or dimethyalcetamide are well suited for this type of reaction. They permit to dissolve a number of polymers and it was possible to show that the reaction was carried out within a few hours at temperatures of the order of 100-150° C. An alternative is offered by the utilization of a solvent which enables to carry an azeotropic distillation, such as chloroform, carbon tetrachloride, benzene, toluene, chlorobenzene or tetrachloroethane. These solvents enable to achieve the reaction of dehydration in a few hours, at the most at the boiling temperature of the solvent, i.e. for these four solvents at temperatures between 61 and 147° C.
Among functionalized carbons, carbons carrying amine functions are obtained by treating carbon fibres or powders with nitrogen containing gases (ammonia, amines, . . . ) such as described for e

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