Electrically conductive composition

A substantially water and salt free electrically conductive composition used to assure good electrical conductivity between two points. The composition remains moist (i.e., it does not dry or harden) over an extended period of time.

BACKGROUND OF THE INVENTION 
1. Technical Field 
The invention relates to an electrically conductive composition. 
2. Background Art 
Electrically conductive compositions, such as gels, creams or jellies, have 
found utility in providing electrical connection between various locals. 
For example, it is customary for persons involved in the manufacture of 
integrated circuits and other sensitive electronic components to wear 
wrist straps that are electrically connected to ground to guard against 
the accumulation of electrostatic charges. U.S. Pat. No. 4,398,277 
discloses one such wrist strap which comprises a fabric wrist band which 
holds an electrically conductive stainless steel plate against the 
wearer's wrist. A wire conductor provides an electrical path from the 
metal wrist plate to ground. Additionally, the fabric wrist band is 
electrically conductive due to the presence of silver-plated filaments in 
the fabric. These filaments provide a continuous electrical path from the 
wearer's wrist to ground even if the stainless steel plate accidentally 
moves out of contact with the wearer's wrist. 
The use of a moisturizer, such as a skin lotion, is recommended to ensure 
good electrical contact between the wearer's body and the wrist strap, 
particularly in dry (e.g., low humidity) conditions. The use of 
moisturizers has, however, proven to be unreliable for this purpose as 
they tend to lose their skin-moistening effect upon evaporation of the 
water present in them. Additionally, both perspiration and moisturizers 
contain materials such as salts that can be corrosive to metals and which 
may, over a long period of time, gradually attack both the face of a 
stainless steel wrist plate and the silver plating of the filaments 
thereby reducing the electrical conductivity of the strap. Even a small 
reduction in electrical conductivity of the wrist strap is a major concern 
since the loss of conductivity could allow a sudden electrostatic 
discharge from the wearer to an electronic component being handled. Such a 
discharge could damage the component thereby either rendering it 
inoperative immediately or causing it to fail prematurely. 
Electrically conductive compositions, such as gels, are in widespread 
medical use for ensuring good electrical contact between the skin and 
electrodes such as are used in EKG tests. These compositions generally 
comprise humectants, bactericides, thickeners, water and electrolytic 
salts which enhance electrical conductivity. The use of such salts is 
detrimental to the long term efficacy of the wrist strap due to the 
corrosive effect they would have on the metals in the strap. Moreover, the 
water present in such compositions tends to evaporate within a small 
fraction of a typical wearing time encountered in the manufacture of 
integrated circuits. This would require frequent reapplication of such 
compositions thereby magnifying the problems caused by the presence of the 
salts, and adding to the inconvenience in using the strap. 
SUMMARY OF THE INVENTION 
The present invention overcomes these disadvantages. It provides an 
electrically conductive composition which is substantially free from water 
and conductivity enhancers such as electrolytic salts. Consequently, the 
composition can be used to provide good electrical conductivity between a 
wrist strap and the wrist of a wearer. This is particularly surprising 
since the composition is substantially free from water and electrolytic 
salts. Furthermore, since it is substantially free from water it remains 
effective for extended periods of time. 
In accordance with the present invention there is provided an electrically 
conductive composition which is substantially free from water and salts 
and which comprises a mixture of at least one polyhydric alcohol and at 
least one thickener. Substantially free from water means that the 
composition contains less than 10% by weight water. Substantially free 
from salts means that the composition contains less than 0.001% by weight 
of a salt which possess electrolytic capabilities (such as sodium or 
potassium chloride). 
The present invention is not limited to use with wrist straps of the type 
described above, but may be employed wherever it is desired to establish a 
conductive path between two points via a conductive medium. For example, 
the composition of the invention may be used in the device disclosed in 
copending application Ser. No. 07/029,340, now U.S. Pat. No. 4,762,497 
filed 3-23-87. The device of that application has two plastic parts, one 
of which is formed with a reservoir for an electrically conductive 
composition such as a gel. When the two parts are interconnected to entrap 
a fabric such as the sleeve of a cleanroom garment, the gel is squeezed 
through the fabric to complete an electrical path between an electrode in 
each of the parts, thus completing an electrical connection between a 
wrist strap and ground. In such use, the gel remains viable as long as the 
connector entraps the fabric. Upon removing the connector, the gel is 
easily cleaned out of the reservoir, permitting the connector to be 
reused. The gel may also be easily cleaned from the garment leaving it 
uncontaminated for the next use.

DETAILED DESCRIPTION 
The principal components of the present invention, that is the polyhydric 
alcohol and thickener, may be combined in a wide variety of concentration 
ratios. Preferably the thickener is present at a level of at least 1 part 
by weight per 100 parts by weight polyhydric alcohol. While the upper 
level of thickener employed is not critical, it preferably comprises no 
more than about 10 parts by weight per 100 parts by weight of the alcohol. 
At substantially higher levels, the composition becomes too viscous to 
conveniently handle. More preferably the thickener comprises from about 3 
to 7 parts by weight per 100 parts by weight polyhydric alcohol. 
The composition of the invention has an electrical volume resistivity of 
less than 5.times.10.sup.6 ohm-cm (preferably less than 1.times.10.sup.5 
ohm-cm). It is understood that as resistivity decreases, conductivity 
increases. 
Volume resistivity of the composition may be determined by immersing two 
circular electrodes (each 0.45 cm.sup.2 in area) in the composition such 
that they are about 0.5 cm apart from center point to center point. It is 
not necessary that the spacing between the electrodes be exactly 0.5 cm. 
However, it is necessary that the exact spacing be known. While immersed, 
a 5 volt potential is applied across the electrodes and the electrical 
resistance is measured. The volume resistivity (R.sub.v) is then 
determined from the equation 
EQU R.sub.v =(A/T)xR.sub.m 
where A is the area of one of the electrodes, T is the distance between the 
electrodes, and R.sub.m is the measured resistance. 
The composition of the invention preferably has a pH of 7 or less. While 
compositions of higher pH are within the scope of the invention it has 
been found that a bacteriacide and/or mold inhibitor is often required to 
prevent the growth of unwanted organisms. Typically from 0.01 to 0.1 part 
by weight of the bacteriacide and/or mold inhibitor per 100 parts by 
weight of the polyhydric alcohol is sufficient to prevent unwanted growth. 
While the lower pH limit is not critical, it is preferred that it not be 
below 3. Below 3 the acidity of the composition may have a negative effect 
on long term use. Additionally, the composition is less viscous (i.e., 
more fluid) at lower pH values. This may cause minor problems during use. 
However, it is noted that the pH of the composition may be increased 
through the addition of a minor amount of a basic material such as 
triethanol amine thereby causing the composition to become less fluid. 
Preferably the composition has a pH of from 4 to 7 and more preferably a 
pH of from 4 to 6. 
Polyhydric alcohols useful in the present invention are those compounds or 
polymers having more than one hydroxyl group. Preferably the polyhydric 
alcohol is water soluble and normally liquid at room temperature (e.g., 
20.degree. C.). When polyhydric alcohols which are not normally liquid at 
room temperature are employed in the composition of the invention, they 
should be mixed with one or more liquid polhydric alcohols. Examples of 
useful polyhydric alcohols are ethylene glycol, propylene glycol, 1,2,4 
butane triol, polyethyleneoxide (e.g., Carbowax.TM. 400) and the like. 
Glycerol may also be used as a polyhydric alcohol in the present 
invention, although a second polyhydric alcohol may have to be added in 
order to achieve the desired electrical conductivity. Those skilled in the 
art will, of course, recognize that ethylene glycol may cause some dermal 
reactions. 
Thickeners useful in the present invention are materials which cause the 
polyhydric alcohol to increase in viscosity. Examples of useful thickeners 
include polymers containing acid anhydride units. These polymers are 
molecules built up by the repetition of a sufficient number of small 
chemical units wherein it is preferred that at least 2 mole % of the units 
contain an acid anhydride group. 
A particularly preferred acid anhydride-containing polymer comprises the 
polymerization product of an addition-polymerizable acid anhydride (such 
as acetic anhydride, maleic anhydride and the like) with an olefinic, 
addition-polymerizable monomer free from the anhydride functionality such 
as acrylate esters (e.g., methyl acrylate, methyl methacrylate, butyl 
acrylate, etc.); vinyl ethers (e.g., methyl vinyl ether, 2-ethyl hexyl 
vinyl ether, decyl vinyl ether); vinyl acrylates (e.g., vinyl acetate, 
vinyl butyrate, etc.); olefins (e.g., ethylene, propylene, styrene, 
.alpha.-methyl styrene, isobutylene, etc.); olefinic polycarboxylic acid 
esters (e.g., dimethyl maleate, dimethyl fumurate, etc.; and vinyl halides 
(e.g., vinyl chloride, vinylidene dichloride). 
Other ingredients may be used in the composition of the invention to 
accomplish a desired result. For example, as noted previously, 
bacteriacides and/or mold inhibitors may be employed. Ingredients to 
adjust the pH of the final product may be employed. Typically these other 
ingredients are employed at minor levels, that is, up to 0.1 parts by 
weight per 100 parts by weight polyhydric alcohol. 
The composition of the invention may be easily prepared by combining the 
polyhydric alcohol, thickener and any optional ingredients in a suitable 
vessel and mixing in the presence of heat (e.g., 80.degree. C. to 
100.degree. C.) until the composition begins to thicken, typically from 1 
to 4 hours. The pH of the initial combination of ingredients may 
preferably be adjusted to about 7 by the addition of a minor amount of 
triethanol amine or the like. Once the composition has thickened its pH 
generally goes to a value of 5 or less. After thickening, the composition 
is cooled to room temperature. 
EXAMPLE 
A mixture of 51 grams of ethylene glycol, 16 grams of glycerol, 2 grams of 
a medium molecular weight copolymer of approximately equal parts of methyl 
vinyl ether and maleic anhydride (Gantrez.TM. 139 from GAF, Inc., weight 
average molecular weight of 138), and six drops (approximately 0.4 gm) of 
triethanol amine was heated to 85.degree. C. with stirring until gelling 
had started. This temperature was maintained for 3 hours to provide a gel 
which then was allowed to cool to room temperature. The pH of the gel was 
4.5 and its electrical resistivity was 2.4.times.10.sup.5 ohm-cm, 
surprisingly good in view of it being water-free and salt-free. 
This electrically conductive gel was spread onto the wrists of a number of 
persons, who then wore a wrist strap of the type disclosed in U.S. Pat. 
No. 4,398,277. Testing indicated that in each case, the electrical 
conductivity between the body of the wearer and the wrist strap was 
adequate, both immediately upon applying the wrist strap and after the 
wrist strap had been worn continuously for about eight hours. 
Using a syringe, this electrically conductive gel was used in a connector 
having two plastic parts each having an electrode in the base. The gel was 
squirted into the cavity of a tube of elastic, polymeric, electrically 
conductive, open-cell foam which was formed with a cavity that was open at 
both ends. The cavity was 0.32 cm in diameter and 0.96 cm in length. The 
gel-filled tube was inserted into one of the plastic parts. The other 
plastic part was interconnected to the first to entrap a piece of a 
cleanroom garment fabric (polyester) which had a thickness of about 0.125 
mm, compress the foam tube and transmit a squeezing force against the 
electrically conductive gel to force it to flow into the interstices of 
the fabric and into contact with the flat electrode of the other plastic 
part. The electrical resistance between the electrodes (each 1.0 cm in 
diameter) was measured to be 0.2 megohm. In order to bleed off static 
charges, the electrical resistance should be about one megohm or less.