Conductive releasable adhesive and method of making same

A conductive releasable adhesive and method of making same for electrostatic discharge controlled surfaces in which a multiplicity of interlacing electrically conductive fibers is suspended in a releasable adhesional substance forming a conductive matrix throughout the volume of the adhesional substance for facilitating dissipation of electrical charges.

FIELD OF INVENTION 
This invention relates to a conductive releasable adhesion for affixing 
electro-static discharge controlled surfaces to flooring and other 
surfaces and a method of making it. 
BACKGROUND OF INVENTION 
Static electricity causes problems ranging from a mere nuisance to 
degradation and catastrophic failures of electrical devices when 
tribo-electric charges build up on a person and are then discharged into 
electronic equipment upon contact. The advent of large scale integration 
of electrical components has resulted in even greater sensitivity to 
electro-static discharge ("ESD"). One estimate of the cost of ESD-caused 
information loss, disruption, and damaged equipment is as high as $10 
billion per year. Consequently, many methods have been employed to combat 
ESD in manufacturing and other facilities where sensitive electronic 
components are exposed. 
With regard to ESD controlled surfaces such as floors, it has become 
apparent that a major component of the total electrical continuity sought 
to be achieved in preventing ESD is the adhesive used to secure the ESD 
floor covering. This is because in most work environments, the floor is 
one of the largest single surfaces and the primary source of static-charge 
buildup. To combat this problem, conductive or static dissipative flooring 
is often applied, usually in the form of tiles, modules, or roll goods. 
The adhesive or cement used to secure the tiles to the existing flooring 
must itself provide for and facilitate continuous electrical continuity 
from tile to tile, and from tile to ground. Therefore, various conductive 
adhesives have been developed. The most generally used type is a 
conductive epoxy. Epoxies, by their nature, however, are nonreleasable and 
therefore not desirable for modular floors which otherwise allow for easy 
reconfiguration. Additionally, epoxy does not facilitate easy replacement 
of individual tiles which have become worn or damaged. There are other 
reasons epoxies are not desirable for many applications. Some epoxies 
require a long set-up time which disrupts operations and increases the 
overall cost associated with installing the ESD flooring. Because of its 
consistency, conductive epoxy is difficult to apply, further adding to the 
cost of installation. Additionally, epoxies involve a mixing of resin and 
hardener before application, and often require special handling and 
ventilation because of the strong odors generated by the epoxies. 
Generally, epoxies also have a very short mixed or pot life. Finally, 
epoxies are often difficult to correctly apply to achieve the desired 
conductivity and also difficult to clean off tools, furniture, and 
equipment. Most importantly, however, epoxy applied to ESD flooring does 
not allow the flooring to be easily reconfigured or releasable and does 
not allow easy replacement of worn or damaged individual tiles. 
Modular ESD flooring in the form of tiles is desirable for many 
applications since a given area can be easily reconfigured and maintained. 
Furthermore, this type of ESD flooring allows easy access to wiring 
running in the floor. Releasable adhesives allow such reconfiguration and 
maintenance, but there is currently no effective releasable conductive 
adhesive for ESD flooring. Using releasable type adhesives, some attempts 
were made to develop a conductive adhesive for ESD flooring. For example, 
carbon black powders have been added to currently available releasable 
adhesives. The result, however, was that the carbon diluted the tackiness 
and strength of the dry adhesive rendering it unusable. 
SUMMARY OF INVENTION 
It is therefore a primary object of this invention to provide a releasable 
conductive adhesive for electrostatic discharge controlled surfaces. 
It is a further object of this invention to provide such a releasable 
conductive adhesive which is pressure sensitive. 
It is a further object of this invention to provide a releasable conductive 
adhesive which has an ESD-safe surface electrical resistance. 
It is a further object of this invention to provide a releasable conductive 
adhesive in which the conductive properties of the adhesive do not affect 
the tackiness and strength of the releasable adhesive. 
This invention results from the realization that a conductive releasable 
adhesive for ESD controlled surfaces can be accomplished by suspending a 
multiplicity of conductive fibers interlaced in a releasable adhesional 
substance to form a conductive matrix capable of facilitating dissipation 
of electrical charges. 
This invention features a conductive releasable adhesive for affixing ESD 
controlled surfaces to flooring or other surfaces. Conductive fibers are 
suspended in a releasable adhesional substance to form an interlaced 
conductive matrix that facilitates dissipation of electrical charges. 
In a preferred embodiment, the adhesional substance may be a pressure 
sensitive acrylic polymer aqueous emulsion such as ABOND.TM. M-277, and 
the fibers may be acrylic fibers conductively coated with copper sulfide 
such as "Thunderon" synthetic conductive fibers and may have a Denier 
value of between 0.5 and 25. The fibers may have a length of between 0.5 
and 15 mm, a diameter of 0.0005 to 0.0025 inch, and comprise from between 
0.1 to 10 percent of the adhesive by weight thereby achieving an adhesive 
in which the fibers are inert with respect to the adhesional substance not 
affecting its tackiness or adhesive strength. These parameters achieve a 
adhesive which when applied to a non-conductive surface achieves a maximum 
surface electrical resistance of 2.times.10.sup.7 ohms for ESD flooring. 
The invention also features a method of making a conductive releasable 
adhesive for electrostatic discharge controlled surfaces including the 
steps of providing a releasable adhesional substance and mixing the 
adhesional substance with conductive fibers to suspend the fibers in the 
adhesional substance to form a conductive matrix throughout the volume of 
the adhesional substance. 
In a preferred embodiment, a ribbon blender is used to mix the fibers with 
the adhesional substance. 
DISCLOSURE OF PREFERRED EMBODIMENT 
Other objects, features and advantages will occur to those skilled in the 
art from the following description of a preferred embodiment and the 
accompanying drawings, in which: 
FIG. 1 is a schematic representation of a cross section of a quantity of 
the conductive releasable adhesive according to this invention; 
FIG. 1A is a magnified view of the material according to this invention. 
FIG. 2 is a block diagram showing a method of making the conductive 
releasable adhesive according to this invention; 
FIG. 3 is a schematic diagram showing the mixing of conductive fibers with 
releasable adhesive according to the method of making the conductive 
releasable adhesive of this invention; and 
FIG. 4 is a cross-sectional schematic view of the conductive releasable 
adhesive according to this invention employed to secure an electrostatic 
discharge controlled surface to existing flooring.

This invention may be accomplished with acrylic fibers conductively coated 
with copper sulfide such as "Thunderon" synthetic conductive fibers 
suspended in a pressure sensitive releasable adhesional substance such as 
ABOND.TM. M-277, forming an interlaced conductive matrix with an 
optimal surface electrical resistance for facilitating dissipation of 
electrical charges. 
The fibers may be mixed with the adhesive in a blender and the resultant 
adhesive may be applied to an existing flooring and allowed to dry for a 
time before the electrostatic discharge controlled surface is applied. In 
this way, modular ESD flooring may be applied and since the adhesive is 
releasable, the flooring may be easily reconfigured and/or individual 
tiles can be easily replaced without disrupting operations taking place in 
the location where the ESD flooring is applied. 
There is shown in FIG. 1 the conductive releasable adhesive 10 for ESD 
controlled surfaces according to this invention. Conductive fibers 12 are 
suspended in releasable adhesional substance 16 to form an interlaced 
conductive matrix 18. In a preferred embodiment, the releasable adhesional 
substance may be a pressure sensitive acrylic polymer aqueous emulsion 
such as ABOND.TM. M-277. Also, the fibers may be acrylic fibers 
conductively coated with copper sulfide such as "Thunderon" synthetic 
conductive fibers having a Denier value of between 0.5 and 25, and a 
diameter of between 0.0005 to 0.0025 inch. Preferably, the strands have a 
length of between 0.5 and 15 mm and comprise from 0.1 to 10 percent of the 
adhesive by weight. Conductive fibers other than acrylic fibers coated 
with copper sulfide could be used as long as they form a conductive matrix 
with sufficient conductive properties. For example, carbon coated acrylic 
or propylene type conductive fibers could be used. 
In a preferred embodiment, "Thunderon" fibers are used having a Denier 
value of 2, a diameter of about 0.0007 inch, and a length of about 5 mm, 
and comprising approximately 1% of the total wet weight of the adhesive. 
Experiments involving these parameters resulted in a conductive releasable 
adhesive which, before application of the ESD flooring, had a maximum 
surface electrical resistance of 2.times.10.sup.7 ohms using the general 
testing methodology of ASTM F-150 at a potential of 100 volts. It has been 
determined that this is ideal for ESD protective floor coverings. Also, 
these levels have been achieved with fibers which are inert with respect 
to the releasable adhesional substance--that is, they do not adversely 
affect any of the inherent properties of the releasable adhesive. 
Furthermore, it has been found that these parameters result in an adhesive 
which is easy to apply. Using a 1/16 inch semi-circular notched trowel, 
the fibers are kept adequately suspended and are spread evenly forming an 
interlaced conductive matrix having the properties delineated above. 
Fibers too long get caught in the teeth of the trowel used to apply the 
adhesive or get caught on the roller used to spread the adhesive. Fibers 
too short do not provide the required conductive matrix unless cost 
prohibitive quantities of short fibers are added. 
It has been established that the use of such a conductive releasable 
adhesive is cost effective over generally available conductive epoxies 
since the modular floor tiles are not destroyed upon removal and 
replacement during reconfiguration of a given space. Furthermore, the 
set-up time is often much shorter than that of common epoxies. 
Accordingly, the installation period is shorter, further lessening the 
costs associated with installation. 
The fibers 12 are mixed at step 22 in FIG. 2 with the releasable adhesional 
substance, from step 20. This method results in the conductive releasable 
adhesive at step 24 with the above described properties. 
Special ribbon blender 28, FIG. 3, may be used to mix the fibers 30 from 
step 22 of FIG. 2 with the adhesional substance 32 supplied at step 20, 
FIG. 2. In a preferred embodiment, after the releasable adhesional 
substance 16 is placed in special ribbon blender 28, small quantities of 
fibers 12 are slowly dispersed by hand into blender 28 near its perimeter 
while the blender operates at slow speed. As the quantity of fibers 12 is 
increased, blender speed is increased somewhat until the correct 
percentage of fibers 12 to adhesional substance 16 is achieved, and then 
the blender 28 is operated at full capacity for a time to assure proper 
dispersion of fibers 12 in adhesional substance 16. The fibers 12 tend to 
adhere to blades of the ribbon blender 28 if the blender speed is too fast 
and/or quantities of fibers 12 are added too quickly. Therefore, for best 
dispersion of fibers 12 in adhesional substance 16 to achieve the desired 
conductive matrix 18, the above gradual progression of increasing the 
quantity of fibers 12 at step 22, FIG. 2, as the speed of ribbon mixer 28, 
FIG. 3, increases is suggested. 
As applied to secure an electrostatic discharge controlled surface 34 to 
existing flooring 38, FIG. 4, the conductive releasable adhesive 10 
according to this invention allows the surface 34 to be modular since the 
adhesive 10 is releasable and yet it securely adheres the surface 34 to 
the existing flooring 38 since the fibers do not affect the releasable 
properties of the adhesive. 
In application, flooring 38 is properly prepared and thoroughly cleaned. 
Using a 1/16 inch semicircular notched trowel, the conductive releasable 
adhesive is fully spread on the floor 38 to achieve coverage of 
approximately 30 yards per gallon. The adhesive 10 is allowed to dry for 
an average of two hours. As it dries, adhesive 10 turns from a cloudy to 
clear appearance if ABOND.TM. M-277 is used as the releasable 
adhesional substance 16, FIG. 1, supplied at step 20, FIG. 2. Surface 34, 
for example ESD carpet tiles, is then applied. Adhesive 10 is easily 
removed from tools, etc., with soapy water followed by a rag dampened in 
chlorinated solvent. This ease of clean-up is a further advantage over 
epoxies. 
Although specific features of the invention are shown in some drawings and 
not others, this is for convenience only as each feature may be combined 
with any or all of the other features in accordance with the invention. 
Other embodiments will occur to those skilled in the art and are within the 
following claims: