U.S. patent number 3,801,418 [Application Number 05/235,179] was granted by the patent office on 1974-04-02 for transparent anti-static device.
This patent grant is currently assigned to The United States of America as represented by the United States Atomic. Invention is credited to Clarence C. Cornelis, Harry A. Hanna.
United States Patent |
3,801,418 |
Cornelis , et al. |
April 2, 1974 |
TRANSPARENT ANTI-STATIC DEVICE
Abstract
A transparent anti-static device having a generally transparent
electrically conductive metal film on a substrate with a silicon
monoxide film superimposed on the conductive film to eliminate
buildup of static electricity. The coating can be deposited on a
transparent substrate such as glass or plastic on, as an example, a
window or safety shield, thereby permitting observation through the
substrate of a tool or explosive being processed.
Inventors: |
Cornelis; Clarence C. (Fort
Madison, IA), Hanna; Harry A. (Burlington, IA) |
Assignee: |
The United States of America as
represented by the United States Atomic (Washington,
DC)
|
Family
ID: |
22884435 |
Appl.
No.: |
05/235,179 |
Filed: |
March 16, 1972 |
Current U.S.
Class: |
428/38; 359/894;
427/164; 428/332; 427/109; 427/166; 428/922; 52/171.1 |
Current CPC
Class: |
C03C
17/3649 (20130101); C09K 3/16 (20130101); C03C
17/3655 (20130101); C03C 17/3681 (20130101); H05F
3/02 (20130101); C03C 17/3605 (20130101); C03C
17/36 (20130101); F41H 13/00 (20130101); Y10S
428/922 (20130101); Y10T 428/26 (20150115) |
Current International
Class: |
F41H
13/00 (20060101); C09K 3/16 (20060101); C03C
17/36 (20060101); H05F 3/02 (20060101); B32b
001/04 (); H05b 033/28 () |
Field of
Search: |
;117/211,217,71R ;52/171
;102/105 ;161/44,45,41,43 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Dallin et al., "The Development of Electrical Conducting
Transparent Coatings for Acrylic Plastic Sheets," Wade Tech. Rep.
53-378 (1-1954) pp. 9-15..
|
Primary Examiner: Leavitt; Alfred L.
Assistant Examiner: Esposito; M. F.
Attorney, Agent or Firm: Horan; John A. Resendez;
Ignacio
Claims
What is claimed is:
1. A generally transparent, anti-static window shield for explosive
handling apparatus comprising an electrically conductive annular
support frame; a planar optically transparent electrically
insulative substrate encircled and supported by said support frame;
an annular electrically conductive gasket intermediate said frame
and substrate; a permanent, optically transparent, electrically
conductive film of uniform thickness between about 30 and about 40
angstroms and of about 80% nickel-20% chromium disposed directly on
and coextensive with said substrate and intermediate said substrate
and said gasket; a permanent, optically transparent, abrasion
resistant silicon monoxide film being of uniform thickness of from
about 500 angstroms to about 600 angstroms superimposed directly on
and coextensive with said electrically conductive thin film
intermediate said first mentioned film and said gasket, to protect
said electrically conductive film from wear and to provide a visual
indication of film state and anti-static properties; and a
grounding connector electrically connected with said gasket and
with said electrically conductive film for removing electrical
charges therefrom.
2. The window shield of claim 1 wherein said films together have
between about 75% and about 90% optical transmission, a surface
resistivity of about 100,000 ohms per square and an electrostatic
field attenuation better than about 99.99%.
Description
BACKGROUND OF INVENTION
A cause of many accidental detonations of primary explosives has
been proven to be high sensitivity to static electricity. A source
of static hazard is the highly nonconductive plastic and otherwise
transparent materials used in operational shields in explosive
handling processes. These materials, such as copolymers of
polymethacrylate and ethylacrylate, i.e., arcylic plastics, and
also polycarbonate plastics, can accumulate up to 15,000 volts on
their surface and can easily supply enough energy to cause
detonations of explosive agents such as lead azide or lead
styphnate. Surface conductivity of present transparent anti-static
coatings generally range between a few million ohms to several
hundred million ohms per square. Although some of these anti-static
materials are presently in use, the desired requirements of optimum
safety in a hazardous area, coupled with minimum maintenance,
minimum design change requirements and maximum protection are not
entirely met and therefore these anti-static materials are not
satisfactory. The greatest disadvantage of commercially available
transparent anti-static coatings is that they are removed by light
wear and abrasion to the treated surface such that the user is
never entirely sure if they remain in effect or not, i.e., their
use is unreliable unless frequent testing is conducted. Required
are permanent, reliable generally transparent anti-static means
which permit application to transparent, electrically nonconductive
surfaces of windows, shields, or other devices.
SUMMARY OF INVENTION
In view of the above requirements, it is an object of this
invention to provide a generally transparent, electrically
conductive, permanent coating which may be deposited on an
electrically nonconductive material.
It is a further object of this invention to provide a permanent
electrically conductive coating which may be applied to devices
such as windows, safety or operational shields and still retain
transparency for viewing the product being processed.
It is a further object of this invention to provide a permanent
anti-static coating which may be applied to various substrates and
which will provide an electrostatic field attenuation of better
than 99 percent.
It is a further object of this invention to provide a permanent
anti-static coating which provides an attenuation of alternating
electrostatic fields or electrostatic fields changing in magnitude
or polarity with time which exceeds 99 percent up to an equivalent
frequency of at least 60 cycles per second.
It is a further object of this invention to provide a coating with
an outer protective layer of good scratch or abrasion resistance,
and which coating provides a visual indication of coating
effectiveness based upon the color of the coating.
Various other objects and advantages will appear from the following
description of the invention, and the most novel features will be
particularly pointed out hereinafter in connection with the
appended claims. It will be understood that various changes in the
details, materials and arrangement of the parts which are herein
described and illustrated in order to explain the nature of the
invention may be made by those skilled in the art without departing
from the principles and scope of this invention.
The invention comprises an electrically conductive, transparent,
permanent coating of conductive and silicon monoxide layers which
may be applied to nonconductors for anti-static electrical
protection.
DESCRIPTION OF DRAWING
The present invention is illustrated in the accompanying drawings
wherein;
FIG. 1 is a perspective view of an embodiment of this
invention.
FIG. 2 is a cross sectional view along line AA of FIG. 1.
FIG. 3 is an exploded view of an area B of FIG. 2 showing details
of this invention.
FIG. 4 illustrates the optical transmission properties of the
anti-static coating.
FIG. 5 illustrates the percent shielding efficiency of this
invention as a function of the frequency of the applied
electrostatic field.
FIG. 6 illustrates the voltage reduction efficiency of a DC or
steady static electrostatic field.
DETAILED DESCRIPTION
FIG. 1 illustrates one embodiment of this invention. In FIG. 1, a
window, shield, or other appropriate device 10 is shown which may
include a suitable mounting or supporting frame front and rear
portions 11, an electrically conductive gasket 12, an electrically
nonconductive, and preferably optically transparent sheet or
substrate 13 which may commonly be of plastic or glass as described
below. One of the transmissive surfaces of substrate 13, such as
surface 14 shown in FIG. 2, may have disposed thereon a generally
transparent coating 15 formed in accordance with this invention.
The coating 15 and its placement is better shown in FIG. 2 which
also better describes or displays the method of assembling the
window or other appropriate device 10. FIG. 3 is an amplified or
enlarged view of area B in FIG. 2. FIG. 3 shows, in exaggerated
proportions for the sake of illustration, the surface 14 of the
substrate 13 and the positioning of the coating 15 which comprises
an electrically conductive metal film 16 and a protective coating
17 which may be a silicon monoxide film thereagainst to provide the
desired electrical shielding of substrate 13.
It has been found that the conductive metal film 16 may be formed
from a material taken from the group consisting of nickel and
nickel-chromium alloys such as 80% Ni-20% Cr and other alloys
thereof to provide the desired conductivity levels to achieve
highly effective electrostatic shielding with high levels of
optical transmission for the normally nonconductive substrate 13.
The protective thin film 17 may be made of a material which
provides an outer protective coating of good durability resistant
to abrasion and wear and chemical attack such as to impart
permanent characteristics, to the coating 15 and which is
substantially optically transparent and of good optical quality. A
protective thin film material which exhibits these desired
properties has been found to be silicon monoxide at a thickness of
from about 500 to about 600 angstroms. Gasket 12 may be made of any
suitable conductive rubber material, such as polyethylene plastic
filled with carbon black or any type of flexible plastic filled
with carbon black or other suitable conductive particles, such as
silver particles, which can make electrical contact between film 16
and frame portion 11, if frame portion 11 is conductive, or
directly with grounding means 20.
Gasket 12 may be eliminated if frame 11 is machined flat to achieve
maximum contact with the applied coating 15. This may be
impractical and consequently a soft, compressible and conductive
gasket may be used to establish an intimate contact area. Frame
portion 11 may also be a conductor or be formed from conductive
portions and, since in contact with gasket 12, may provide the
means for charge dissipation to some suitable grounding means 20. A
typical embodiment which has been successfully used with this
invention has device 11 being an explosive or operational shield
wherein means 20 is electrically connected with said electrical
thin film for removing electrical charges therefrom. The substrate
13 may be a suitable plastic such as a copolymer of
polymethacrylate and ethylacrylate, i.e., acrylic plastics or also
polycarbonate plastics, or a suitable glass. The method of
depositing coating 15 on the substrate 13 may vary but vacuum
deposition may be used with success. Process parameters are
determined by the skill of the operators and the type of equipment
available. A typical process for vacuum deposition of the
conductive film 16 involves cleaning the substrate 13 used as a
target with a suitable alcohol, such as isopropyl alcohol, placing
the dried sheet in an appropriate size chamber, evacuating the
chamber to a suitable low pressure such as about 5 .times.
10.sup..sup.-4 microns, and vaporizing the material to be deposited
(using a tungsten filament or the like) to a thin film thickness of
from about 30 to about 40 angstroms on the target substrate 13.
Preferably the thin film coating thickness and feed rate may be
selected such as by typical automatic or the like controls, to
achieve the desired coating properties of electrical conductivity,
optical transmission, strength and reliability without being
severely affected by outgassing of substrate 13 during application
of the coatings.
The protective thin film 17, such as silicon monoxide film, may
then be vacuum deposited using granular silicon monoxide suitably
disposed in the deposition chamber. Preferably, the thin film
thickness 17 may be deposited over electrically conductive thin
film 16, such as by automatically controlling the process in a
manner known in the art, to a thickness between about 500 and about
600 angstroms of silicon monoxide. Total coating 15 thickness may
therefore typically vary between 530 and 640 angstroms. It has been
found that 35 angstroms of vaporized 80% Nickel--20% Chromium
material produces a film which will attenuate a steady state
electrostatic field by at least 99.99 percent while transmitting
between 75 and 90 percent light in the optical spectrum (400 to 800
millimicrons). This typical film exhibits a surface resistivity of
about 100,000 ohms per square. Surface resistivity is a constant
for all systems of measurement of area where the width and length
of the area being measured are equal, i.e., a square. Consequently,
a unit of area is not specified since surface resistance will be
identical for a given material regardless of the designation as
ohms/per square inch, per square foot, etc.
The surface resistivity figure of 100,000 ohms per square applies
to the finished coating which includes an overlay of silicon
monoxide which itself is nonconductive. The actual metal film may
have a surface resistivity much lower than 100,000 ohms per square.
If a heavy layer of silicon monoxide were used, the surface
resistivity value may be well above the 100,000 ohms per square.
The 500 to 600 angstrom thick silicon monoxide film provides a
highly transparent overlay for protection of the metallic film and
possesses excellent optical properties with no detectable
aberration. FIG. 4 illustrates the optical transmission properties
of a typical coating of these materials and thicknesses for which
there was an optical transmission average of approximately 87%.
The normal outgassing of the nonconductive plastic substrate
(passage of gas molecules from the plastic to air) does not
interfere with the coating which is of sufficient thinness not to
inhibit the slow leakage of gas therethrough and, consequently,
virtually preventing formation of gas bubbles or disbonds at the
coating substrate interface. Any desired substrate materials, as
enumerated above and others, may be used with this invention except
for those materials having such a high outgassing rate which may
raise problems in achieving the required vacuum to vacuum deposit
the desired layers and with subsequent degradation of deposited
layers.
The use of protective coating 17 imparts permanent characteristics
to the coating 15 such as to resist abrasion, wear, chemical
attack, etc. Further, the protective coating generally has a tinge
or hue such that cursory visual examination quickly reveals if the
coating has been damaged. This is especially critical when working
with explosives and reduces prior extensive inspection
requirements. If the coating has been damaged, the color or hue of
the thin film 17 will be different and be visually evident.
Coating thickness may affect the final electrical conductivity of
the deposited metal film and the protective effects of the silicon
overlay. Nickel, or an 80% Nickel 20% Chromium alloy, may be
selected as a suitable metal because its function of thickness
versus conductivity is more linear than many other metals having
practical boiling points. The temperature of the target during and
after the vacuum deposition process may also be important as
affecting electrical conductivity properties. Since high
temperatures may tend to agglomerate the metal grains, thereby
reducing the conductivity but not significantly altering the
optical properties, all possible steps to minimize temperature
elevation should be taken.
A major advantage of this invention is the Faraday Cage action
provided by the coating around the solid substrate to which it is
applied. Faraday action is the shielding effect which attenuates or
reduces the strength of an electrostatic or electromagnetic field
through the material. Since the coating conductive, excellent
anti-static properties are present also. FIG. 5 shows that the
frequency response of a safety shield using this coating is high
enough to provide adequate protection against 60 cycle radiation
and also against changes in field strength arising from mechanical
movement.
The electrostatic shielding efficiency is measured by placing the
coated plastic or coated glass between a source of static
electricity, such as a metal plate charged to the indicated
voltage, and the probe of an electrostatic voltmeter. The drop in
potential, which is the result of the invention herein described
being placed in front of the metal plate, is recorded on a chart
and the efficiency is determined. As an example, metal screen with
14 wires per inch will typically attenuate an electrostatic field
by 99.6%. Further testing using this invention, at equal and higher
voltages showed the electrostatic shielding efficiency to be in
excess of 99.99%. The results of this testing are shown in FIG. 5.
This means that a source of static electricity representing 1,000
volts on the operator side of the shield will be reduced to less
than 0.1 volt on the other side of the shield. FIG. 6 illustrates
the drop in steady static field strength when the invention is
used, or shielding efficiency in regard to nonchanging fields.
The coating may be readily checked visually by noting the
uniformity of the light brown tint of coating 15, and also
electrically with a volt-ohmeter. If the coating should be removed
on some areas of the operational shield due to excessive friction,
abrasion, etc., it will show up as a light patch which can be
confirmed by a volt-ohmeter. The coated side may be placed facing
the explosive or other material being protected from static
electricity. Friction on the uncoated side will produce static
electricity but this cannot penetrate the coating due to its
Faraday effect.
Although the primary description throughout the specification has
been of a transparent substrate, with a transparent anti-static
coating, this invention is not limited to transparent substrates
and may be used wherever an anti-static coating is required and to
which this invention may be suitable. As such, this invention could
also be used on opaque materials as well as transparent substrates.
Although a coating may be used on both sides of the substrate, the
optical qualities would be reduced, i.e., less visibility, greater
light reflections, etc. Further a coating on both sides would form
an excellent capacitor, i.e., two conductive surfaces separated by
an insulator. If the ground connection on one side were to be
accidentally disrupted, a large quantity of energy could be stored
in the "capacitor" producing a great hazard. A one sided coating
cannot produce this effect.
A particularly successful application of this invention has been
its usage on operational shields in explosive handling processes.
This use has effectively eliminated premature detonation of
explosive agents such as lead azide or lead styphnate due to static
electricity discharges resulting from charge buildup on
nonconductive surfaces.
* * * * *