Graphite foam material and method of making same

Details Graphite foam material and method of making same Graphite foam material and method of making same

1999-03-15

2001-04-17

Derrington, James (Department: 1731)

Plastic and nonmetallic article shaping or treating: processes

Carbonizing to form article

C264S029500, C264S109000, C264S028000, C264S328140, C264S328180, C423S448000

Reexamination Certificate

active

06217800

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to graphite foam material, of the type used for high temperature insulation and the like, and to a method of making the same. The invention also relates to a graphite material that may be used and added to provide protection from electrostatic discharge (ESD) or shielding from electromagnetic and radio interference (EMI/RFI).
2. Art Background
In the prior art, various forms of graphite material have been used as insulating materials in high temperature applications including industrial ovens and furnaces, vacuum furnaces and controlled atmosphere heating apparatus and the like.
One of the first such graphite insulating materials was powdered carbon black which had an appropriate amount of insulating capacity, but was very difficult to handle, relatively heavy, and extremely time consuming to replace. As a result with the advent of foam and resin chemistry, a number of newer materials were developed which were lighter and easier to handle, but which had the requisite insulating capacity. One such material is a carbon fiber insulating material made of a carbon fiber held in a matrix by a phenolic resin material, and formed into a board or block.
Static electricity and electrostatic discharge (ESD) are naturally occurring phenomena. Simply stated, static electricity is electrical energy at rest on a surface. It is generally created by the rubbing together and separating of two materials, one of which is usually non-conductive. Typically, one material gives up electrons and becomes positively charged; the other takes on the electrons and becomes negatively charged. ESD may be defined as the sudden discharge of an electrostatic potential from one body to another. A good example may be the shock one receives when touching a metal door knob after walking across a carpeted floor.
In many environments, ESD may damage or destroy sensitive electronic components, erase or alter magnetic media, or set off explosions or fires in flammable environments. These discharges may be caused by a variety of sources, most commonly there is a direct discharge from a person or equipment into a sensitive object.
One way of preventing ESD is to reduce the generation of charges in the first place. A second way of preventing ESD is to provide a ground path for the safe dissipation of accumulated charges to ground. A third method is to provide shielding or protection of devices and equipment from discharge through packaging. ESD may also be controlled with materials, such as conductive plastics, that do not generate high levels of charge, that dissipate charges before they accumulate to dangerous levels, or that provide electrostatic shielding to prevent charges from reaching the sensitive product.
Electromagnetic Interference (EMI) is electrical energy, either electromagnetic or in the radio frequency (RF) range in the case of radio frequency interference (RFI) that is radiated by specific sources. Some of these sources include computer circuits, radio transmitters, fluorescent lamps, color TV oscillators, electric motors, automotive ignition coils, overhead power lines, lightning, TV games, and many other resources. EMI/RFI may interfere with the operation of simple household appliances such as causing the unwanted operation of garage door openers. On another level, EMI/RFI may corrupt data in large scale computer systems, cause inaccurate readings and output in aircraft guidance systems, and interrupt the functioning of medical devices, such as pacemakers.
Proper shielding may prevent products from emitting electromagnetic or radio frequency energy to other susceptible equipment. Shielding may also protect susceptible equipment from the effects of externally radiated EMI/RFI as the shielding absorbs the energy emitted, converting it to thermal energy.
EMI thermoplastic composites are used primarily for shielding against emission or reception of EMI and RFI. Traditionally, shielding has been accomplished by encasing sensitive electronic parts in metal housings or by using metallic coatings on the inside of plastic housings. Thermoplastic compounds with appropriate shielding additives are cost effective alternatives in many applications due to their ability to take on complex shapes and maintain tight tolerances.
It is desirable to provide, at relatively low cost, a compound/s that may dissipate charges before they accumulate to dangerous levels, that provide electrostatic shielding to prevent charges from reaching the sensitive product. Moreover, it is desirable to provide, at relatively low cost, a material for shielding against emission or re-emission of electrostatic.
SUMMARY OF THE INVENTION
The present invention is a composition of matter, and specifically, a material comprising cryogenically treated graphite or carbon particles which are then expanded by thermal shock/gas expansion. The expanded particles are then combined with a phenolic resin, or the like, and then thermoset under pressure at an elevated temperature to form a hardened sheet or plate. The carbon or graphite particles can be obtained from previously expanded graphite which has been made into flexible graphite foil, and therefore, the present invention permits the recycling of graphite foil which is not otherwise commercially distributed. The method of making said material is also described and claimed.
The material has generally the same insulating and other physical characteristic as the prior art carbon fiber insulation materials, and it is less expensive than prior art materials.
Another advantage of the present invention is that it can utilize, without any drawbacks, recycled flexible graphite material, as a starting material. Such recycled flexible graphite material is currently typically being landfilled. Thus, the present invention is particularly advantageous as a benefit to the environment. Additionally, the advantageous method of making the material and the quality of the material made in accordance with the present invention provide additional benefits.
Another advantage is the reduced weight loss due to oxidation, resulting in longer furnace life between successive rebuilding of the furnace.
According to the present invention, one may use finished low density blocks, boards, billets, etc. to make higher density parts by cutting (i.e., using, for example, a cork bore or saw) to shape or mold the material into a desired shape and pressing using different pressures to obtain the desired density. For example, die formed rings can be made using the present invention. The compressive strength of the die formed rings was greater than or equal to the strength of some monolithic graphites of the prior art.
The present invention also includes a graphite material and a method for making the graphite material by using a thermoplastic material mixed with re-expanded graphite. The compound of thermo plastic material and re-expanded graphite is fed into an injection molding system at a relatively high temperature and injected into a mold where a plastic material is formed. The plastic material is then removed from the mold when the material is still very hot but hard set.
The present invention further includes in one embodiment thereof a method of making a graphite material. Flexible graphite is ground into a powder having a particle size in the approximate range of 25 to 80 mesh. The graphite powder is mixed in an amount ranging between approximately 10%-90% graphite powder by weight, with a resin, in an amount ranging between approximately 10%-90% by weight. The graphite powder, mixed with the resin is hot pressed.
The present invention also provides in another embodiment thereof a method of making a graphite material. A flexible graphite is ground into a powder having a particle size in an approximate range of 25-80 mesh. The graphite powder is soaked in a cryogenic liquid. The soaked graphite powder is expanded. The resulting graphite powder is mixed in an amount ranging between approximately 10%-90% by weight with a resin, in an amount ranging between ap

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