|Title of Invention||
A METHOD OF RENDERING A POROUS GRAPHITE PLATE IMPERVIOUS TO FLUIDS
|Abstract||A method of rendering a porous graphite plate impervious to fluids, comprising the steps of placing at least one such plate in a chamber capable of withstanding vacuum(10"2 Torr) and positive pressures up to 10 Kgcm-2; degassing the plate, under vacuum created in the chamber, for removal of any gas entrained in the plate; submerging the plate, in the chamber, in a solventless thermosetting resin or in a thermosetting resin solution(10-40% by weight of resin in the solution), such as epoxy resins, phenolic resins, polyester resins, polyimide resins, unsaturated polyester resin, and forcing the resin into the pores of the plate by introducing, into the chamber, inert pressurised, such as, air, nitrogen; evacuating the said pressurised gas, thereafter, to create a vacuum in the chamber; repeating the cycle of successively subjecting the plate to pressurised gas and vacuum, until the said plate is impregnated to a predetermined loading of the resin; removing the plate from the chamber and allowing the solvent to evaporate, leaving the resin in the pores of plate to polymerize therein.|
This invention relates to a method of rendering a porous graphite plate impervious to fluids. In recent years considerable attention has been directed to the development of Polymer Electrolyte(PEM) fuel cell stacks for transportation applications. Further interest in such fuel cells has been intensified by the demand for nonpolluting power sources. A polymer electrolyte fuel cell is a type of electrochemical fuel cell which employs a membrane electrode assembly(MEA). The electrochemical cell comprises of an anode, a cathode, and a polymer electrolyte membrane. The impervious graphite plates are used as gas separators and reactant distributor-cum-current collectors in the PEM fuel cell stack. Since Graphite plates are used as gas separators, these plates have to be impervious to reactant gases at operating conditions of fuel-cell. Commercially available graphite plates are fine grained and have porosity ranges from 10 to 20 %(volume ). This porosity generally admits mixing of oxygen and fuel, in particular, hydrogen and resulting in the uncontrollable combustion. Thus, impregnation of the plates with resin to make them impervious is essential for the use of these plates in PEM fuel cell. Impregnation of porous graphite materials with resin and pyrolysis is frequently desired in order to make them denser and also to increase structural strength. AH the process of densification by impregnation by pyrolysation of those liquid impregnants though reduces the number of interconnected pores opening onto the surface of the body, it does not afford sufficiently impervious plates which are suitable as separator in the fuel cells. The conventional methods would not appear to be capable of giving sufficiently high impervious graphite plates at necessarily lower costs and may thus have practical limitations. In addition, the process itself is tedious because graphite plates undergo deformation when it is subjected to high temperature treatment and also it will not be cost effective. For the PEM fuel ceils to become a commercially viable system for the applications that are envisaged it is still necessary to develop cost effective method of impregnation to obtain graphite plates of stability and reasonable good electrical conductivity. A method is provided, herein, for making impervious graphite plates by vacuum pressure resin impregnation which may be used in the manufacture of PEM fuel cells. Such impervious graphite plates can find other applications also, apart from use in PEM fuel cells, so much so that the use of these plates is not confined to PEM fuel cells.
The present invention relates generally to graphite plates, which are used in the PEM fuel cell, more particularly, though not exclusively, reactant distributor cum current collector.
Impregnation of porous graphite materials with resin and pyrolysis is frequently desired in order to make them denser and also to increase structural strength. Fine grain, molded graphite parts have residual
porosity, which detracts from the use in some applications such as fuel cells those in which they are used as separators. This porosity generally admits mixing of reactants leading to uncontrollable combustion which will result in the generation of enormous amount of heat. The presence of pores/voids also weakens the material and results in a general loss of strength.
In order to overcome these difficulties, a wide variety of impregnation processes have been practiced in the graphite industry. For example decomposition of a gas phase to close interconnected pores and prevent unwanted penetration of a graphitic material has been suggested. The use of a solution of furfural and acetone with two catalytic additives to impregnate graphite bodies in order to reduce porosity and improve properties obtained after the body is cured in an acid bath and subsequently carbonized is also known. Unfortunately, both catalyst and acetone are required to achieve the desired results. Pitch admixed with a polymerizable liquid has also been used for impregnation. A typical impregnant admixture for impregnating carbonaceous electrodes in the manufacture of chlorine includes a liquid solution of furfuraldehyde, coal tar pitch and a diethylsulfate catalyst.
The densification and improved surface finish of graphite materials by impregnating with a liquid formulation containing furfural, para toluene sulfonic acid and tetraethylene glycol into fine grained, isotropic graphite bodies and then polymerization and pyrolyzation is also known. All the processes of densification by impregnation by pyrolyzation of those liquid impregnants, though reduce the number of interconnected pores opening onto the surface of the body, they do not afford sufficiently impervious plates which are ideally suitable as separators in fuel cells. The processes so far reported would not appear to be capable of giving sufficiently high impervious graphite plates necessarily of lower costs and may thus have practical limitations. In addition, the process itself is tedious because graphite plates undergo deformation when they are subjected to high temperature treatment and also it will not be cost effective. For PEM fuel cells to become a commercially viable system for the •applications that are envisaged, it is still necessary to develop cost effective methods of impregnation of graphite plates to obtain reasonable stability and good electrical conductivity.
Impregnation of solid porous graphite plates with another thermoset resin is frequently desired in order to make the graphite plate impervious to gases, or to increase structural strength. Impregnation is accomplished by capillary or other adsorptive action upon contact immersion of the graphite piece. It has been found that a process for the production of a resin impregnated impervious graphite plates for use in the PEM fuel cell comprises:
• Mixing epoxy resin with a suitable solvent at room temperature;
• Placing the graphite plates arranging them in the impregnation tank such that there is sufficient space provided for the free flow of solution while impregnation
• Evacuating the impregnation tank to vacuum by vacuum pump
• Submerging the graphite plate to be impregnated in the resulting impregnant;
• Pressurising the solution with nitrogen or air at 7 kg/cm2 pressure
• Draining and evacuation of the impregnation tank
• Repetition of the evacuation and pressurising till the required level resin loading
• Evaporation of the solvent and polymerisation of the resin in liquid impregnant in the pores of the graphite plates by holding it at room temperature for an appropriate time;
The impregnation tank made of stainless steel/mild steel container with top portion large enough to accommodate the plates to be impregnated. For convenience, a transparent glass seal over the top of the container must also be provided which gives sufficient visibility Inlet and outlet means for liquids and means for maintaining vacuum and gaseous pressures. The free volume of the chamber should be several times smaller than the volume of submerged graphite plates to avoid solvent loss and consequent change of concentration. Nitrogen/air should be provided at a pressure. For evacuating the setup, a simple mechanical vacuum pump is sufficient. The graphite plates are placed in the container, preferably in a rack, but without other support. In the preferred embodiment, the graphite plates are evacuated prior to immersion to remove entrained gas in the graphite plates. No pre-treatment of the graphite is necessary. Total available void volume or "available porosity," must be known to calculate the impregnant concentration necessary to impregnate the graphite with enough resin to produce impervious plates. Ease of penetration is partially a function of the pore size.
The thermosetting resin applied to the graphite plates may generally be selected from those thermosetting resins utilized in the production It is, of course, necessary that a thermosetting resin be selected which is either inherently liquid at the coating temperature or which may be modified to possess flowable properties at the coating temperature by the addition of a reactive modifier or diluent, or by dissolution in a solvent for the same. Suitable solvents which are commonly utilized in such solvent systems include acetone, methyl ethyl ketone, dimethyl ketone, perchloroethylene, methylene chloride, ethylene dichloride, dimethyl formamide, etc. The thermosetting resin dissolved in the solvent may be either uncured or partially cured (i.e., advanced). Illustrative examples of suitable thermosetting resins for use in the present process for the production of resin impregnated graphite plates include epoxy resins, phenolic resins, polyester resins, polyimide resins, unsaturated polyester resin etc.
A solution is next made up of the material with which it is desired to impregnate the graphite. The process is not limited by the nature of the solution, so long as it is relatively non-viscous and capable of wetting graphite. After preparation of the impregnant solution, the graphite plates are placed in the tank in which the impregnation is to take place, the lid is fastened, and the plates are evacuated then submerged into the impregnant solution. Two or three impregnations of the 20 percent resin solution are required for 30 gms loading of typical resin per plate.
Upon removal from the solution of thermoset resin, the impregnated graphite plate is kept at room temperature at which the solvent evaporates slowly and leaves the resin into the pores which polymerises into the voids of the graphite plates. The impregnation is easily carried out to full depth of plates provided the impregnant solution is relatively dilute and non-viscous. The impregnation tends to be rather non-uniform and partially dependent upon graphite pore size. The evaporation of solvent is slow. Attempts to increase the rate of evaporation of solvent by raising the temperature of the plates by heating the plates by placing them in an oven which was maintained at 70°C, resulted in bubbling, exudation and migration of the thermoset resin due to faster evaporation of the solvent which forced the resin out of the plates. The liquid impregnants employed are also very low in viscosity and completely wet the graphite plates allowing even the finest of the pores in the original graphite body to be filled with the impregnant. An initial evacuation is preferably carried out before the graphite plates are submerged with the impregnant solution. This resulted in better penetration of resin into voids of the graphite plates. ■Subsequent step comprises of degassing and impregnation of graphite plates in a series of vacuum and pressure cycles. In actual impregnation, the pressure, as well as duration, is determined empirically for each type of graphite plates and for each liquid concentration or viscosity, taking into consideration certain limiting factors such as porosity of the plates.
The gain in weight after impregnation was measured. This is indicative of the extent of impregnation. These impregnated plates were placed inside the jig, which is specially designed for the purpose of testing the porosity of graphite plates. The nitrogen/air at 3kg/cm2 is applied on one side of the plate. The other side was connected to the bubbler, which shows bubbling if there was any leak. EXAMPLE
Three graphite plates obtained from the open market with 10-20 percentage voids were selected for processing to determine the penetration characteristics of graphite of dimension 20 cm x 30cm. The plates were arranged in parallel by putting them in a rack in which graphite plates can be arranged such that the distance between two graphite plates was 0.5 cm. The plates were then evacuated under vacuum 10"2mm and the resin was allowed to enter into the impregnation chamber. The volume of the solution of resin was adjusted such that the top plate was submerged in the solution of the resin. The solution had a density of 0.86 g/cm3 and a viscosity of 20 centipoises. Three vacuum-pressure cycles
•were then applied and the impregnated plates were removed from the rack and the surface plates were cleaned with acetone to remove the resin at the surface and the solvent inside the plate was allowed to evaporate slowly at room temperature and the curing was carried out at room temperature. The objective of the present investigation is to produce cheaper impervious graphite plates with reasonably good electrical conductivity to suit the needs of PEM fuel cell applications among other applications, from commercially available graphite plates.
An aspect of the invention is to provide impregnants for use in the process identified above which are capable of wetting isotropic graphites, and consequentially, capable of filling even the finest pores of the structures into which they are impregnated. We claim:
1 A method of rendering a porous graphite plate impervious to fluids, comprising the steps of placing at least one such plate in a chamber capable of withstanding vacuum(10*2 Torr) and positive pressures up to 10 Kgcrrf2; degassing the plate, under vacuum created in the chamber, for removal
from solvents, such as, acetone, methyl ethyl ketone, methanol, ethanol, methylenechloride, ethylenechloride, dimethylformamide.
4 A method as claimed in any of the preceding Claims wherein the said resin is uncured
5 A method as claimed in any one of the preceding Claims 1 to 3 wherein the said resin is partially
.6 A method of rendering a porous graphite plate impervious to fluids substantially as herein described
and as illustrated by example 7 A porous graphite plate whenever rendered impervious to fluids by a method as claimed in any one
of the preceding Claims
|Indian Patent Application Number||326/MAS/2001|
|PG Journal Number||17/2010|
|Date of Filing||20-Apr-2001|
|Name of Patentee||SPIC SCIENCE FOUNDATION|
|Applicant Address||"MOUNT VIEW" 111 MOUNT ROAD , GUINDY CHENNAI 600 032.|
|PCT International Classification Number||G01N1/28|
|PCT International Application Number||N/A|
|PCT International Filing date|