Conductive laminate and method of producing the same

A laminate of insulating material having fine metallic articles dispersed therein is rendered highly conductive by passing electrical energy through the laminate of sufficient strength to break down the dielectric resistance of the insulating material at the points of contact of the metallic articles.

BACKGROUND OF THE INVENTION 
Metallic or metal-coated articles, such as metallized glass fibers, are 
incorporated into plastic laminates to dissipate electrostatic charge and 
to provide shielding against electromagnetic radiation, among other 
benefits. These properties depend on the electrical conductivity of the 
metallic articles. The most common methods of making such laminates 
involve using metal-coated fibers which have been cut into relatively 
short lengths of about one-half to one inch or even shorter. The 
electrical properties of such laminates rely on effective metal-to-metal 
contact between the short lengths of metallized fiber. At high 
frequencies, capacitative or inductive effects between adjacent fibers may 
reduce the importance of metal-to-metal contact. 
In making the laminates, the short lengths of metal-coated fiber are 
combined in with fluid polymerizable material which becomes the plastic 
matrix. In the fabrication of the laminate, the many individual lengths of 
metallized fiber develop a thin film of the insulating plastic, perhaps as 
thin as a few hundred-thousandths of an inch, at the point of contact with 
adjacent metal-coated fibers. This thin insulating film reduces or 
destroys the electrical conductivity at the fiber junction points. Since 
the metal-coated fibers in the finished laminate take the form of a 
three-dimensional network, there is a myriad of potential parallel paths 
for the flow of electric current. The conductivity of the laminate is the 
combined effect of the conductivity of the many individual current paths. 
SUMMARY OF THE INVENTION 
The invention relates to a method and resultant product of improving the 
electrical conductivity of laminates of insulating material having fine 
metallic articles dispersed therein. I have discovered that by causing an 
electric current to pass through such a laminate under proper conditions 
the conductivity of the laminate will be significantly and permanently 
improved. These proper conditions require that the extremities of the 
laminate be connected briefly to a source of electrical energy which is of 
sufficiently high potential to break down the dielectric resistance of the 
insulating material in the junction points between the metallic articles, 
but which is not so high as to cause physical degradation of the laminate. 
The required potential and the maximum permissible energy dissipation 
depend on the nature of the laminate being treated. Energy dissipation in 
the laminate should not reach excessive levels at which the metallic 
articles or the insulating material would burn. 
The required conditions for applying electrical energy to the laminate may 
be obtained by various means, including but not necessarily limited to 
those described further herein. In a preferred method the secondary 
current of a high-voltage induction coil is passed through the laminate. 
Both the potential and energy dissipation are controlled by the parameters 
of the induction coil; typically the secondary delivers a very high 
potential but very little energy. 
Another means of practicing the invention involves passing the secondary 
current of a low-frequency transformer through the laminate. The applied 
potential is determined by the turns ratio of the transformer and the 
voltage supplied to the primary. The energy dissipated may be controlled 
by any one of a number of well-known means, such as current-limiting 
devices or circuits in the primary or secondary of the transformer or a 
high reactance transformer design. In a further variation, an electrical 
capacitor is discharged through the laminate. The applied potential is 
determined by the voltage to which the capacitor is charged, and the 
energy dissipated during the discharge is determined by the energy storage 
capacity of the capacitor. 
Practice of the invention, as described above or through similar means, 
results in a significant and permanent improvement in the electrical 
conductivity of the laminate. In a preferred laminate of plastic or resin 
material incorporating metal-coated fibers, it is believed that impressing 
a voltage in excess of the dielectric strength of the plastic, typically 
within the range of 100 to 500 volts per mil (0.001 inch), punctures or 
breaks down the plastic, the extent of which depends on the amount of 
current applied. Since the thickness of the plastic film separating 
contact points of the individual metal-coated fibers is generally much 
less than one mil, a relatively small potential is sufficient to cause 
dielectric breakdown. In theory, once these thin insulating layers between 
adjacent fibers have been broken down, there are many more parallel 
current paths set up and the conductivity of the laminate is greatly 
improved. 
The essential features and further advantages of the invention are set 
forth in detail below in conjunction with the following drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The invention is applicable to laminates of insulating material having fine 
metallic articles dispersed contiguously therein. The preferred laminate 
is made of plastic or resin material and incorporates metal-coated glass 
fibers or comparable materials. Such laminates may be fabricated under any 
suitable process including: hand lay-up, machine lay-up, spray-up, low 
pressure molding, matched die molding, injection molding, filament 
winding, and pultrusion. Besides metal-coated glass fibers, metallic 
articles such as fine strips of aluminum foil, wire, or flakes may also be 
used. 
The preferred laminates are made with commercial polyester resins and 
incorporate various weight percentages of metal-coated glass fibers cut to 
various lengths. The metallized fibers may be cut from fiber bundles or 
strands. Good results have been obtained for metallized fiber contents 
ranging from 5% to 30% by weight in cut lengths ranging from one-quarter 
inch to one inch. Comparable or better results could be obtained with 
higher percentage loadings of metallized fiber or longer cut lengths. The 
method of fabricating the laminate and the uniformity of distribution of 
the metallized fiber would also be significant variables. 
In practicing the invention with plastic laminates, it is necessary to make 
electrical connections to at least two points on the laminate. These 
points should be as widely separated as the geometry of the laminate 
allows, so that the current flows through the greatest possible volume. 
The surfaces of a plastic laminate are usually resin-rich, i.e. the 
dispersed metal-coated fibers are not exposed at the surfaces or edges but 
are usually covered by an insulating layer of resin. (Since the resin in 
fluid prior to curing, it tends to flow around and enclose the fibers.) 
Because of this resin-rich condition, measures must be taken to ensure a 
good electrical connection between the metal-coated fibers and the 
contacts applied to the surfaces of the laminate. 
The drawings illustrate some of the measures which may be used to make 
effective electrical connections to the metallized fibers within the 
laminate. In FIG. 1A opposite edges 11 of the laminate 10 are sanded to 
remove the surface layer of resin and expose the metallized fibers. These 
edges are then coated with a conductive paint, which usually consists of 
finely divided silver or copper in a suitable vehicle. After the paint 
dries, electrical connection may be made to the painted edges. In FIG. 1B 
small tabs 12 of thin aluminum foil are imbedded in the plastic matrix 
during fabrication of the laminate 10 so that they are in direct contact 
with the metal-coated fibers. One end of each foil tab extends beyond the 
resin-rich surface, and electrical connection can be made to it by means 
of clips, clamps, or screws. In FIG. 1C self-tapping screws 13 are driven 
in undersized holes drilled into the laminate 10 through the metallized 
fiber material. The helical screw threads cut through the matrix material 
and make contact with the metal coating on the fibers. Wires or terminal 
lugs can be attached under the screw heads. 
The edges of the laminate shown in FIG. 1D are sanded to expose the 
metallized fibers, and electrical connection is made by metal strips or 
bars 14 pressed or clamped against the sanded edges. When the high-voltage 
high-frequency induction coil is used, a simplified version of FIG. 1D may 
conveniently be employed. Without sanding the edges, a grounded metal bar 
is laid against one edge of the laminate and the high voltage terminal of 
the induction coil is brought close to the opposite edge and moved slowly 
along the entire edge. The high voltage spark penetrates the insulating 
layer of plastic to reach the metallized fibers beneath. In FIG. 1E holes 
are drilled in the laminate and taper-reamed, for example, to a standard 
taper of 0.25 inch per foot. Metal pins 15 of the same taper are driven 
into the holes. The metallized fiber is exposed in the wall of the hole, 
and the taper gives a wedging action which provides good contact. 
Electrical connections are made to the taper pins. There are of course 
many other methods that may be used to ensure that contact be made to the 
metal coating on the fibers and not merely to the insulating surface of 
the laminate. 
The specific parameters of applying electrical energy to the laminate vary 
widely depending on the dimensions and characteristics of the laminate 
being treated. According to the invention, the applied voltage must be 
high enough to cause dielectric breakdown of the thin insulating films of 
plastic between conductive fibers and to cause a flow of current through 
the laminate. The existence of this condition can readily be determined by 
use of electrical measuring instruments such as an ammeter or voltmeter 
or, in the case of the induction coil, by observing the passage of sparks 
from the high voltage terminal to the laminate. The power dissipated 
should be the minimum necessary to develop good electrical conductivity in 
the laminate. 
FIG. 2 shows a circuit of a typical induction coil for use in the 
invention. The power source may be either alternating or direct current at 
a voltage depending on the design of the transformer 16. The primary 
winding 17 of the transformer consists of a relatively small number of 
turns wound on a soft iron core. An interrupter 18 is located so that the 
magnetic field developed in the core by a flow of current in the primary 
pulls the armature away from a stationary contact. This causes the primary 
circuit to open and the magnetic field to collapse, thus allowing the 
armature to touch the stationary contact whereupon the sequence is 
repeated. The collapse of the magnetic field induces a current in the 
secondary winding 19, which consists of very many turns of fine wire. The 
frequency of this current is determined by the inductance of the primary 
winding and the capacitance of the capacitor 20, and the voltage is 
determined by the turns ratio. A device 21 for adjusting the spring 
tension on the interrupter armature is usually provided so that the time 
rate of change of the magnetic field can be maximized. In a typical 
induction coil used in the invention, the power source is 120 volts at 60 
Hertz, the output voltage 25,000 to 30,000 volts, the power output 5 
watts, and the frequency 500 kiloHertz. 
FIG. 3 shows a typical capacitor discharge circuit used in the invention. A 
transformer 22 and rectifier 23 constitute a source of rectified direct 
current at a peak voltage suitable for the working voltage rating of a 
capacitor 24. A resistor 25 is connected in series with the capacitor to 
limit the peak current flow during the charging or discharging. When the 
switch 26 is connected to the rectifier 23, capacitor 24 becomes charged 
to the peak voltage of the power supply. When switch 26 is connected to 
the output circuit 27, capacitor 24 discharges through the laminate. 
Typical values for the components for the capacitor discharge circuit are: 
a transformer secondary voltage of 300 volts RMS, 423 volts peak; a 
silicon diode rectifier rated at 20 amperes and 1000 peak inverse voltage; 
an electrolytic capacitor of 100 microfarads, 450 VDC working voltage; a 
100 ohm resistor rated at 2 watts; and an energy storage capacity of the 
circuit of approximately 18 watt-seconds. 
FIG. 4 illustrates a typical current-limited transformer circuit for use in 
the invention. The primary winding 28 of the transformer 29 is connected 
in series with an incandescent lamp bulb 30, which acts as a current 
limiting device. When switch 31 is closed a high voltage is developed 
across the secondary of the transformer which breaks down the insulating 
plastic between the metallized fibers in the laminate and causes a current 
to flow through the laminate. This causes the current in the primary to 
increase, which in turn increases the current through lamp 30 and causes 
the tungsten filament to heat. Because tungsten has a high temperature 
coefficient of resistivity, the resistance of the lamp 30 increases 
sufficiently to limit the primary current to a safe value. Series resistor 
32 limits the current through the laminate during the short time required 
for the lamp filament to heat. Typical values for the components of the 
circuit are: a transformer of 120 volts, 60 Hz primary, 480 volts RMS 
secondary; a 60 watt, 120 volt tungsten lamp bulb; a 100 ohm resistor 
rated at 2 watts; and a maximum current through the laminate of 
approximately 100 milliamperes. 
The above-described circuits for practicing the invention were tested on 
laminate samples made by hand-lay-up in the form of sheets either 6 inches 
square and 0.25 inches thick or 9 inches square and 0.125 inches thick. 
The resistance of the laminate samples before and after treatment 
according to this invention was measured with an ohmmeter having a 
sensitivity of 20,000 ohms per volt. The measurements were also confirmed 
by readings obtained by using an electronic ohmmeter and by measuring the 
voltage drop across the specimen during passage of a known alternating 
current. 
EXAMPLE I 
43 grams of a 30-filament strand of metallized glass fiber chopped to 1/2 
inch lengths were spread evenly in an aluminum mold 9 inches square and 
0.125 inch thick (inside dimensions). 170 grams of W. R. Grace Co. Hatco 
12184 polyester resin mixed with 1% by weight of Luperco ATC catalyst 
(containing 50% benzoyl peoxide in tricresyl phosphate) was poured into 
the mold and distributed evenly over and through the fiber material. The 
mold was placed between two sheets of Mylar film and cured in a laboratory 
hydraulic press with platens heated to 110.degree. C. for 10 minutes under 
a load of 6 tons. After cooling and removal of overflowed resin the 
finished laminate weighed 205 grams with a resulting fiber content of 21%. 
Three strips one inch wide and nine inches long were cut from this 
laminate. Electrical connections to the laminate were made as shown in 
FIG. 1C, by drilling 0.106 inch holes 1/2 inch from each end and driving 
#6.times.3/8 inch pan head tapping screws into the holes. The resistance 
between the two tapping screws on the first strip was measured with a 
volt-ohmmeter and found to be 35,000 ohms. One screw was then connected to 
a ground wire, and the high voltage terminal of an induction coil as shown 
in FIG. 2 was placed in contact with the other screw for a period of 30 
seconds. After this treatment the resistance was measured at 42 ohms. The 
resistance of the second strip was measured to be 26,000 ohms. A capacitor 
discharge apparatus as shown in FIG. 3 was discharged once through the 
laminate. After this treatment the resistance was measured at 43 ohms. The 
resistance of the third strip was measured to be 39,000 ohms. Power was 
applied to the laminate from a current-limited transformer as shown in 
FIG. 4 for 30 seconds. After this treatment the resistance was found to be 
10 ohms. 
EXAMPLE II 
36 grams of a 720-filament yarn of metallized glass fiber chopped to 1/2 
inch lengths were mixed with 150 grams of catalyzed Hatco 12184 polyester 
resin as described in Example I. The mixture was poured into an aluminum 
mold 6 inches square and 0.25 inch thick (inside dimensions) and rolled to 
obtain uniform distribution free from air bubbles. The laminate was cured 
by hot pressing under the conditions described in Example I. The finished 
laminate weighed 178 grams, giving a fiber content of 20%. Electrical 
connections were formed in the laminate using tapping screws as in Example 
I. 
The resistance between the two screws was measured with a volt-ohmmeter and 
found to be 1 megohm (1,000,000 ohms). The screws were then connected to 
the induction coil, as in Example I, for a period of 15 seconds. After 
this treatment the resistance between the screws was found to be 900 ohms. 
EXAMPLE III 
17 grams of a 660-filament metallized glass fiber roving chopped to 1/2 
inch lengths were spread evenly in an aluminum mold 6 inches square and 
0.25 inch thick (inside dimensions). 150 grams of W. R. Grace Co. Hatco 
GR-394 polyester resin mixed with 1% by weight of Lupersol DDM catalyst 
(containing 60% methyl ethyl ketone peroxide in dimethyl phthalate) was 
poured into the mold and distributed evenly over and through the fiber 
material. The laminate cured overnight at room temperature. The weight of 
the finished laminate was 163 grams, giving a fiber content of 10%. 
Two opposite edges of the laminate were sanded to expose the metallized 
fiber and painted with a coat of silver conductive paint, as shown in FIG. 
1A. The resistance between the painted edges were measured to be in excess 
of 50 megohms. A capacitor discharge apparatus as shown in FIG. 3 was 
discharged once through the laminate. After this treatment the resistance 
between the silvered edges was found to be 600 ohms, equivalent to a 
volume resistivity of the laminate of 381 ohm-centimeters. 
EXAMPLE IV 
A laminate sample containing 25% by weight of metal-coated fibers 1 inch 
long showed a resistance of 1,100 ohms. A potential of 12 volts at 60 Hz. 
was applied with a current-limited transformer as shown in FIG. 4. After 
this treatment, the resistance dropped to 600 ohms. Another sample 
containing 20% by weight of metal-coated fibers 1 inch long showed a 
resistance of 115 ohms; after treatment with the induction coil, the 
resistance was 36 ohms. 
A further sample containing 25% by weight of metal-coated fibers 1 inch 
long showed a resistance of 1050 ohms. After treatment with the capacitor 
discharge circuit, the resistance was 17 ohms. The laminate was then 
connected to a source of 60 Hz. alternating current and a potential of 33 
volts was applied, causing a steady current of 400 to 500 milliamperes to 
flow. The resistance of the laminate increased to 66 ohms after treatment 
of a few seconds under these conditions. This test demonstrated the 
adverse effect of excessive energy dissipation in the laminate. 
In the above tests, treatment of the laminate by the induction coil proved 
to be the preferred method. The high voltage generated by the induction 
coil is adequate for almost any type of plastic laminate. The visible 
sparks are direct evidence that the current is passing through the 
material, and the electrical connection for purposes of treatment is 
considerably simplified. The energy delivered is low enough to avoid 
damage to the flexural strength and desired properties of the laminate. 
The invention has application to laminates of other types of plastic or 
other insulating materials or having other forms of metallic bodies 
incorporated therein. Electrical treatment of the laminate can be 
accomplished by methods analagous to those described above in order to 
achieve the desired improvement in the conductivity of the laminate. It 
will be understood that the above described methods and resultant products 
are merely exemplary and that those skilled in the art may make many 
variations and modifications without departing from the spirit and scope 
of the invention. All such modifications and variations are intended to be 
within the scope of the invention as defined in the appended claims.