Note: Descriptions are shown in the official language in which they were submitted.
F. 1081
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STATIC DISSIPATIVE ~AT
Background of the Invention
The invention relates to a static-dissipative
web construction, suitable for example as a floor
mat to enable personnel-accumulated static electricity
to safely discharge from a pérson standing on the mat.
Various static-discharge mat constructions have
been proposed, ranging from such highly conductive
configurations as to permit the hazard of substantially
instantaneous discharge, to slow-leaking constructions
which exhibit undesirable dependence upon ambient
humidity. Between these extremes,~Patent No. 4,208,696
to Lindsay, et al. describes a multi-layer static-
dissipative web wherein an open-weave fabric in the
form of cotton scrim is rendered electrically conductive
~using carbon in a :Latex binder) and is interposed between
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upper and lower layers of relatively low conductivity,
to produce mat constructions having an overall volume
resistivity between 101 and 1011 ohm-cm and surface
resistance in the order of 108 ohms per square;
Lindsay et al. predicate their results on the foraminous
nature of their conductive open-weave fabric. While
the Lindsay, et al. product is in many respects satis-
fàctory, it is prone to delamination, and for many
applications an order of magnitude reduction in
surface resistance is desirable, i.e., to the order
of 107 ohms per square.
Brief Statement of the Invention
It is an object of the invention to provide an
improved static-dissipative web or mat of the character
indicated.
It is a specific object to produce such a web or
mat that is inherently not prone to delamination and
which exhibits a surface resistance in the order of
10 ohms per square, as measured pursuant to ASTM
Standard D257-76.
A general object is to provide such a web or mat
construction which is of relative simplicity, which
uses readily controllable and available component
materials, and which is inherently relatively insensitive
to environmental humidity.
The invention achieves the foregoing objects by
employing a thin continuous film of graphite-compounded
polyermic material such as a bonded conductive flexible
plate on the underside of the upper polymeric layer of
low conductivity, the thin film having a surface
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resistance in the order of 102 ohms per square, as
measured pursuant to said ASTM Standard D257-76.
By employing polymeric material of the same nature,
e.g., polyvinyl chloride in both these layers, as
well as in an expanded conductive cushioning bottom
layer, laminar bonding is optimized, and sensitivity
to varying humidity is minimized. Also, the continuous
nature of all layers, and the continuous nature of
their interface bonding, provides a continuous volume
within which electrostatic charge may dissipate and
distribute over the thin film of the conductive inter-
mediate layer, as distinguished from the discrete
paths in which charge dissipation must be channeled
in the foraminous-scrim network configuration of
Lindsay, et al.
Detailed Description
_ . _
The invention will be described in detail for a
preferred embodiment, in conjunction with the accompany-
ing drawings, in which:
Fig. 1 is an enlarged sectional view through a mat
construction of the invention;
Fig. 2 is an exploded view in perspective, to permit
better identification of components of the construction
of Fig. l;
Figs. 3a to 3f are views similar to Fig. 1, to
il-lustrate successive stages in fabrication of the
construction of Fig. l; and
- Fig. 4 is a schematic diagram to illustrate steps
in a continuous process for making the construction
of Fig. 1.
1198~7;~ (
In Figs. 1 and 2, the invention is shown in the
form of a static~dissipating web or mat comprising
three bonded layers 10-11-12 of differently conductive
polymeric material, such as polyvinyl chloride. The
top or upper layer 10 and the bottom or lower layer 12
may each be of the same solid-cast construction, but
as shown, the lower layer 12 is an expanded version of
the same polymeric material; and both layers 10-12
incorporate one or more conductive ingredients to enable
each of layers 10-12 to have a volume resistivity in
the range 107 to 112 ohm-cm. The inner or intermediate
layer 11 is a thin film of preferably the same polymeric
material containing an electrically conductive ingredient
such as carbon black and exhibiting a surface resistance
in the order of 10 ohms per s~uare, as measured pursuant
to ASTM Standard D257-76. A highly satisfactory conductive
vinyl film for use at conductive layer 11 is known as
Condulon, a trademark and product of Pervel Industries,
Inc., Plainfield, Connecticut.
In a specific illustrative example, the solid upper
and expanded lower layers 10-12 are both of polyvinyl
chloride, with added conductive plasticizer, which may
be co~llercially available products known as Markstat
AL-15 or di-octyl-adipate (DOA)*, or a combination of
the two. If mixed, it is preferred that the proportion
by weight of the AL-15 to the DOA be 2:1, their combina-
tion accounting for 12 percent of the total dry-ingredient
mix. In the illustrative example, the solid top layer 10
was 35 mils thick, the inner layer 11 was 2 mils thick,
and the expanded lower layer was 88 mils thick, for an
Markstat AL-15 is a product of Argus Division, ~itco
Chemical Company, Brooklyn, New York; and DOA is available
from various sources, including Monsanto, St. Louis, ~lissouri.
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overall thickness of 125 mils. The upper and lower
layers each exhibited a volume resistivity of about
ohm-cm and a surface resistance of 108 ohms per
square, and the surface resisitvity of the inter-
mediate layer was 300 ohms per square. Overallsurface resistance of the consolidated mat was
measured at the exposed surface of top layer 10 to
be`107 ohms per square, and the time for discharge
of a 5kV potential was 0.05 second.
Continuous manufacture of the described web will
be described in connection with Figs. 3 and 4, commencing
with a supply reel 15 of suitable casting web 16
continuously advancing from left to right, in the
sense of Fig. 4. The casting web 16 may be a release-
coated fabric or paper, and in the latter event the
casting surface thereof is preferably embossed (as in
Fig. 3a), for ultimate aesthetic purposes at the
exposed upper surface of layer 10.
A first casting of liquid-mixed polymeric-coat
ingredients is made at 17 and the same is cured or
fused at 18, thereby establishing the solid layer 10
atop the casting paper 16, as shown in Fig. 3b. A
separate supply of conductive film for inner layer 11
is available from a reel 19 and is guided for bonded
lamination to layer 10, under heat and pressure, at
heated squeeze rolls 20, to produce intermediate
product shown in Fig. 3c. A second casting of liquid-
mixed polymeric-coat ingredients (this time with an
expanding component) is then made at 21, so that the
developing product appears as in Fig. 3d, wherein the
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numeral 12' will be understood to designate the
as-yet unexpanded liquid coat applied at 17.
Passage through an expanding oven 22 enables
controlled uniform expansion of the coat 12' to
its ultimate thickness, as bottom layer 12, the
same being consolidated in a fusing oven 23, with
the appearance shown in Fig. 3e. The product is
now completed by stripped removal of the casting
paper and its separate accumulation at 24, leaving
finished product (Fig. 3f) available for reel
accumulation at 25.
Static-dissipating vinyl mat material, produced
as described is found to meet all stated objects.
Static-dissipating conductivity is an order of
magnitude (i.e., 10 times) better, that is, a surface
resistance of 10 ohms per square, as compared with
108 ohms per square of a vinyl product of the Lindsay,
et al. patent, and delamination is virtually impossible.
Ambient humidity and/or water immersion are found to
have no significant effect on electrical properties.
The inherent capability of Condulon to discharge a 5kV
charge in 0.02 second enables design modification to
increase the speed of electrostatic discharge, from the
0.05 second time observed for the described mat, either
by creating a less-thick top layer 10 or by increasing
the proportion of conductive plasticizer in the low-
conductivity layers 10-12. Generally, the thickness
range of layer 10 may be between 25 and 50 mils, the
thickness range of layer 11 may be between 1 and 5 mils,
and the thickness range of layer 12 may be between 25
and 125 mils, expansion being optional and dependent
upon ultimate use.
While the invention has been described in detail
for the preferred embodiment, it will be understood
that modifications may be made without departlng from
the scope of the invention.
