Electromagnetic radiation shielding composites and method of production thereof

Preparation of electromagnetic radiation shielding composite exhibiting high conductivity and low resistance is shown by applying an electrical potential difference across a composite comprising an electrically conductive material dispersed within a foamed matrix material and applying at least one additional electrical potential difference being greater than the potential difference which precedes it.

BACKGROUND OF THE INVENTION 
The present invention relates to electromagnetic radiation shielding 
composites which exhibit high conductivity. 
As technology progresses and produces more sophisticated electronic 
equipment, the environment becomes more polluted with electromagnetic (EM) 
radiation. This radiation has recently become recognized as a significant 
hazard to the health of individuals as well as to the operation of 
electronic equipment, and in particular to digital equipment that is 
generally more easily adversely affected by spurious radiation. There is 
therefore a need to control such random and undirected radiated 
electromagnetic energy and also shield the environment (including both 
biological life and equipment) from its effects. 
The shielding of both emitted radiation and incoming radiation has 
traditionally been done by metallic enclosures. Metal serves as a shield 
as a result of its high conductivity since the penetration of EM radiation 
is dependent upon the conductivity exhibited by the shield. In fact, 
continuous metal enclosures having a thickness in the range of 1/32 inch 
to 1/10 inch serve as effective shields for EM radiation over the 
radio-frequency band from the kilohertz to gigahertz range. The most 
common metallic enclosures are comprised of steel, aluminum and copper. 
However, such metallic shielding enclosures are unfortunately both heavy 
and costly. 
As an alternative, equipment has been enclosed in plastic structures 
consisting of polymeric moulding compounds which include conductive 
materials such as metallic fibers, carbon fibers, etc. Composites have 
also been modified to produce higher conductivity by the use of surface 
treatments such as conductive paints, spray plating, flame spraying, or 
vacuum metallizing. However, these techniques often either prove to be too 
costly or do not sufficiently increase the conductivity of the structure 
to provide the desired amount of shielding. 
Due to the flexibility of design provided by utilizing molded plastic parts 
as shielding composites, it is desirable to provide adequate 
electromagnetic shielding by incorporating metallic or other conductive 
materials (e.g., in flake or fiber form) into the polymeric matrix to 
eliminate the use of secondary coating operations. A further advantage of 
integrally incorporated particles is that control of the passage of 
electromagnetic waves is achieved by absorption as well as by reflection 
because of the bulk conductivity created. 
However, there is presently a need for an efficient and economical means of 
providing shielding which exhibits minimal resistance and high 
conductivity. Due to the fact that only limited amounts of conductive 
materials can be incorporated into such shielding composites without 
imparing the ability of the matrix material from which the composite is 
made to be molded, it is desired to provide a method to increase the 
conductivity of the composite without also having to also unnecessarily 
increase the amount of conductive material employed. 
In addition, the use of shielding composites comprised of structural foams 
is advantageous in certain instances where a lower weight composite is 
required. The maximum amount of conductive material which the foamed 
composite can contain may be less than the maximum amount which an 
unfoamed composite can contain since less effective volume may be 
available within which to disperse the material. 
Therefore, it is desirable to provide a method by which the conductivity of 
such shielding composites can be increased over and above that exhibited 
by the composite subsequent to incorporation of the conductive material. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide an efficient 
method of providing a shielding composite possessing a high conductivity. 
It is also an object of the present invention to provide an efficient 
method of providing a shielding composite which exhibits low resistance. 
It is also an object of the present invention to provide a method of 
increasing the conductivity of a foamed shielding composite. 
It is also an object of the present invention to obviate the problems of 
the prior art as discussed above. 
In one aspect of the present invention there is provided a method of 
preparing an electromagnetic shielding composite which exhibits high 
conductivity and low resistance comprising: 
providing a composite comprising an electrically conductive material 
dispersed within a matrix material; and 
applying an electrical potential difference across said composite of 
sufficient magnitude to increase the conductivity of the composite to the 
extent required to enable the composite to serve as an effective shield 
for electromagnetic radiation. 
In another aspect of the present invention there is provide a method of 
preparing an electromagnetic radiation shielding composite which exhibits 
high conductivity and low resistance comprising: 
providing a composite comprising an electrically conductive material 
dispersed within a matrix material; and 
applying an electrical potential difference across said composite such that 
a first and at least one additional electrical potential difference are 
applied across said composite, each said electrical potential difference 
being greater than the electrical potential difference which precedes it 
and each said additional electrical potential difference which is applied 
being of such a value so as to increase the conductivity of said composite 
in comparison to that which said composite exhibited prior to applying 
said additional electrical potential difference, said additional 
electrical potential difference being applied and the value thereof being 
increased until the conductivity of the composite is increased to the 
extent required to enable the composite to serve as an effective shield 
for electromagnetic radiation.

DETAILED DESCRIPTION OF THE INVENTION 
The electromagnetic radiation shielding composite of the present invention 
may take many forms. The composite is generally employed as a shield for 
electromagnetic radiation in the form of a three-dimensional composite 
which is shaped to provide the proper shielding effect. That is, the 
composite may consist of a sheet-like portion which may be used in 
conjunction with other sheet-like portions to form a multi-sided 
structure. Such a structure can be used to enclose equipment which is 
sensitive to the effects of electromagnetic radiation. Alternatively, the 
composite may take the form of a structure which is molded in a specific 
shape (e.g., a hemisphere) or a shape which conforms to the configuration 
of the equipment to be shielded. 
The composite comprises a matrix of non-conductive material within which is 
dispersed an electrically conductive material. The matrix material serves 
as a binder or matrix for the electrically conductive material and 
provides structural integrity for the shielding composite. The matrix 
material preferably comprises a thermosetting or thermoplastic resinous 
material within which the conductive material can be dispersed. Suitable 
materials include but are not limited to addition polymers of aliphatic 
monoolefins, ethylenically unsaturated carboxylic acids and the esters 
thereof, conjugated dienes and ethylenically unsaturated nitriles, amines, 
ethers and ketones. Such materials also include polyamides, polyesters, 
polycarbonates, polyurethane and phenolic resins. Thermoplastic materials 
such as polyethylene and polycarbonate are most preferred. Various types 
of ceramic materials may also be used as the matrix material. One skilled 
in the art can readily determine which materials are suitable for use as a 
matrix material as such materials are conventional and well known in the 
art. 
The method of the present invention finds particular applicability to 
shielding composites comprising a foamed matrix material within which is 
dispersed the electrically conductive material. Such a foamed matrix must 
be sufficiently dense to provide the desired structural integrity for the 
composite. Generally, the density of the foamed matrix will be between 
about 70 to 100 percent of the density exhibited by the unfoamed matrix 
material. The methods of manufacture of such a structural foams is known 
and includes the entrainment of air and the use of blowing agents to 
provide the foamed structure. Suitable structural foams which are 
commercially available include a polycarbonate structural foam denoted as 
General Electric FL 900 MOBAY FS600. 
Several types of electrically conductive filler materials may be used. For 
example, the conductive filler material may consist of carbon fibers or 
finely divided metallic solids comprised of copper, zinc, aluminum, 
silver, iron, nickel, gold, tungsten, niobium, carbon steel and stainless 
steel as well as alloys or physical admixtures thereof. Aluminum is the 
preferred metal for use in the present invention. This listing of suitable 
conductive materials is not all-inclusive and other conventional types of 
electrically conductive materials may be used in the present invention. 
The conductive material may also consist of non-conductive solids such as 
resinous or glass particles which have been coated with a conductive 
material (e.g., by conventional metallizing techniques) to render the 
solids conductive. Such non-conductive solids are preferably metallized 
such that the added metal forms a continuous layer on the surface of the 
non-conductive solid. 
The conductive material may be in any suitable form or shape such as 
flakes, fibers, spherical or non-spherical particles or powders. The 
conductive material may be of any suitable size but should be of such a 
size that sufficient quantity of the solids may be dispersed within the 
matrix material to permit the desired conductivity to be achieved. 
Exemplary sizes for metallic flakes may range from about 1 mil in thickness 
to about 50 by 40 mils in dimension. Exemplary fibers can range up to 
about 250 mils in length or greater and have a diameter of about 3 mils or 
greater. Preferably the length of such fibers is about 125 mils. 
The quantity of conductive material which is dispersed within the matrix 
material will generally range from 5 percent by weight to about 60 percent 
by weight based on the weight of the composite. Amounts greater than that 
can be used advantageously. However, difficulty may then arise when 
attempting to admix the conductive material with the matrix material if 
the amounts of the conductive material which are used are too great. 
Preferably, the conductive material will be employed in an amount ranging 
from about 5 to 40 percent by weight to achieve adequate workability of 
the admixture as well as sufficient shielding effectiveness. A most 
preferred range is about 15 to 25 percent by weight. Generally, however, 
it has been found that a higher loading volume percent of the electrically 
conductive material will enable higher conductivity to be achieved both 
before and after treatment of the composite by the method of the present 
invention. 
The composite which contains the electrically conductive material dispersed 
throughout a matrix material will exhibit a conductivity value in excess 
of that which is normally exhibited by the composite in the absence of the 
electrically conductive material (and, conversely, a lower resistivity). 
However, in order to both increase the conductivity and lower the 
resistivity exhibited by the composite, it is advantageous to subject the 
composite to the method of the present invention. 
Specifically, the method of the present invention involves providing an 
electrical potential difference across the shielding composite of 
sufficient magnitude to increase the conductivity of the composite to the 
extent required to enable the composite to serve as an effective shield 
for electromagnetic radiation. The conductivity of the composite can be 
increased by several magnitudes as a result of using the method of the 
invention. Both the dc resistivity and the RF transmission of the 
composite are similarly changed radically. 
The value for the potential difference which is provided to obtain the 
desired increase in conductivity will vary depending upon the type of 
matrix material employed as well as the type and amount of electrically 
conductive material dispersed therein. However, it has been found that 
providing a potential difference of between about 500 to 15,000 volts will 
be sufficient to achieve the desired results. 
Generally, the conductivity of a composite comprised of an unfoamed matrix 
will be increased sufficiently as a result of providing an electrical 
potential difference which is lower than that which may required to 
provide similar results in a composite comprised of the same foamed matrix 
material due to the greater inherent resistance of the foamed composite. 
For example, it is possible that the conductivity of a shielding composite 
comprised of an unfoamed matrix material may be sufficiently increased by 
providing an electrical potential difference of from about 500 to 1000 
volts. A similar shielding composite comprised of a foamed matrix material 
may require an electrical potential of from 6000 to 12,000 volts to 
provide the desired increase in conductivity. However, it is not possible 
to generalize about the magnitude of the electrical potential difference 
which will be required since it is dependent upon the physical 
characteristics of the shielding composite. 
It is possible, however, that the potential difference which is necessary 
to sufficiently increase the conductivity of the shielding composite may 
be of such a magnitude that it will physically degrade the composite upon 
application. For example, the resistance of the composite may be so high 
that the ability of the composite to act as a conductor is exceeded. In 
such a case, arcing may occur and the composite may become burned or 
charred. 
Such a consequence may be avoided by modifying the method by which the 
electrical potential difference is provided. That is, an electrical 
potential difference may be provided across the composite such that a 
first and at least one additional electrical potential difference are 
sequentially provided across the composite. Each potential difference 
which is provided in sequence is greater than the potential difference 
which was previously provided across the composite. As each additional 
potential difference is provided the conductivity of the composite will 
increase subject to the limitations discussed below. 
The electrical potential differences which are provided across the 
composite are increased in magnitude a sufficient number of times and to 
such a final value so that the desired increase of conductivity and 
reduction in resistivity in the composite are provided. The number of 
potential differences which are provided is not critical as long as the 
desired results are achieved. Accordingly, the number of potential 
differences provided may be as few as two or, in the alternative, be much 
greater than two, such as, for example, ten or more. Satisfactory results 
can generally be obtained by providing from three to six potential 
differences across the composite of differing but increasing magnitudes. 
As will be discussed hereafter, the number of potential differences of 
differing magnitude which are required to produce the desired results 
depends to a great extent upon the individual magnitude of the potential 
differences provided. 
The increase in magnitude of the electrical potential difference across the 
composite may be conducted in either a substantially continuous manner or 
in a step-wise manner (i.e., the value is increased intermittently). 
However, electrical potential differences are advantageously provided 
which are increased over a sufficiently large range of values. 
The initial potential difference which is provided preferably is about 500 
to 1000 volts. Advantageously, the value of each additional potential 
difference may be from 25 to 300 percent greater than the value of the 
potential difference which preceded it. An exemplary and non-limiting 
progression of values for the potential differences which may be provided 
may be 1000, 3000, 6000, 9000, 12,000 and 15,000 volts. 
It will be observed that increasing the potential difference across the 
composite will increase the conductivity thereof up to a certain point. 
That is, no further improvement will generally be observed for the 
conductivity after a certain threshold value for the potential difference 
is exceeded. However, the threshold value generally varies with each 
matrix material and the amount and type of conductive material dispersed 
therein, so it is not possible to quantify such a value without some 
experimentation. However, the threshold value will generally be 
significantly less for a composite comprised of a matrix material which is 
not foamed as opposed to a matrix material which is foamed. 
The electrical potential difference may be provided by many suitable 
methods, and is preferably provided in a direction which is perpendicular 
to the plane of the shield. 
For example, one method of applying the electric field to the composite 2 
involves the use of electrodes 4 which are applied to the surface of the 
composite in the form of phosphor-bronze finger stock such as are used for 
making contacts around the periphery of shielded room doors. These 
electrodes can be applied to both sides of a flat sheet-like composite and 
high-voltage applied to them. The composite is then passed between the 
electrodes at a suitable speed in the direction indicated. The multiple 
flexible fingers of metal conform to the surface of the composite and 
provide the electrical potential difference in a one-dimensional pattern; 
that is, in line contact. FIG. 1 illustrates such a method. 
The electric potential difference applied to the electrodes can be from 
various sources and frequencies. For example, a 60-hertz source has been 
found to be satisfactory. However, a potential difference of any frequency 
from dc to microwaves will similarly affect the composite, as the 
mechanism apparently depends upon a breakdown between particles within the 
composite caused by the electric field. 
Another suitable method of providing the electrical potential difference 
may be advantageous when the composite consists of a three-dimensional 
object such as a molded box or housing. An electrode is applied to the 
inner surface by metal foil which is easily conformed to the surface in 
any shape and the electric field than applied between this electrode and a 
flexible outer electrode which could scan the surface. The scanning may be 
done either automatically or by hand in the case of a very intricate 
shape. The inner electrode may also consist of a conducting liquid (e.g., 
salt water) if the continuity of the part under treatment is leakproof. 
A further method of providing the electrical potential difference is by 
exposure to a microwave field (e.g., in an oven) or by means of an 
antenna. This method has the significant advantage of being a non-contact 
application. For example, a flat composite sheet may be passed through a 
microwave field. 
The effect produced by the application of the electric field depends upon 
the power capabilities of the source. That is, the effect is dependent 
upon the internal impedance which limits the current output. If an 
electrode is used which is too long, the current per linear length of 
electrode may be lower that that which is needed to produce the necessary 
effect and little change in conductivity may be observed. However, with a 
larger capacity power source (e.g., a voltage with sufficient volt-ampere 
capacity), there is no inherent limitation on the length of electrode that 
may be used for treating a composite. 
The means by which dc conductivity and RF transmission are changed by use 
of the method of the present invention is not completely understood. It is 
believed, however, that the applied electrical potential difference causes 
a breakdown of the insulating film of non-conductive matrix material 
between the electrically conductive material. A network of conducting 
paths are believed to be formed in the composite, thus providing a bulk 
property change from low conductivity to high conductivity. Attempts have 
been made to observe such conducting paths by both visible microscopy and 
scanning electron microscopy without success. The hypothesis is, however, 
strengthened by the fact that the current output of the power source (a 
high voltage, high internal resistance transformer in most experiments) is 
continuously varying and has a waveform with high frequency components 
which suggests the presence of minute arcing within the composite. Surface 
arcing is also sometimes visible when a composite is passed between 
electrodes. This surface arcing varies with the amount of incorporated 
electrically conductive material that is near or at the surface of the 
shielding composite. 
The invention is additionally illustrated in connection with the following 
Examples which are to be considered as illustrative of the present 
invention. It should be understood, however, that the invention is not 
limited to the specific details of the Examples. 
EXAMPLE 1 
A polycarbonate structional foam composite comprised of a matrix denoted as 
General Electric FL 900 containing 20 percent by weight of a mixture of 
aluminum flakes and fibers is subjected to a series of treatments wherein 
an electrical potential difference of increasing magnitude is provided 
across the composite. The composite measures about 2.times.6.times.1/4 
inch and is in the form of a sheet. The composite sheet is moved through 
electrodes as depicted in FIG. 1 which are 13/4 inches wide at a speed of 
1 inch every 5 seconds. The magnitude of the various potential differences 
employed and the change in resistance of the composite across its smallest 
dimension are set forth in Table I below: 
TABLE I 
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Run Potential Difference.sup.a (volts) 
Resistance (ohms) 
______________________________________ 
Control Untreated Infinite 
1 1,000 34,000 
2 3,000 3,100 
3 6,000 60.5 
4 9,000 26.0 
5 12,000 43.0 
6 15,000 43.0 
______________________________________ 
.sup.a V.sub.1 -V.sub.2 in FIG. 1. 
EXAMPLE 2 
Three samples of a structural foam composite sheet comprised of a foamed 
matrix containing 30 percent by weight of aluminum flakes is subjected to 
a treatment wherein an electrical potential difference of 1000 volts is 
applied across the sheets. The attenuation in dB of electromagnetic 
radiation of differing frequencies is measured both before and after the 
voltage treatment. The results are depicted below in Table II: 
TABLE II 
______________________________________ 
Attenuation in dB of Electromagnetic 
Radiation 
Frequency 
(MHz) 0.5 1.5 5 15 50 250 500 960 
______________________________________ 
Sample A 
Untreated 6 6 6 2 2 4 5 8 
Treated 27 27 27 26 25 18 14 14 
Sample B 
Untreated 4 4 4 4 4 3 4 7 
Treated 38 38 38 38 37 32 30 28 
Sample C 
Untreated 1 1 1 0 0 2 3 8 
Treated 35 35 35 35 34 29 26 21 
______________________________________ 
The principles, preferred embodiments and modes of operation of the present 
invention have been described in the foregoing specification. The 
invention which is intended to be protected herein, however, is not to be 
construed as limited to the particular forms disclosed, since these are to 
be regarded as illustrative rather than restrictive. Variations and 
changes may be made by those skilled in the art without departing from the 
spirit of the invention.