Triboelectric property modification and selection of fabrics for filtration applications

A method is provided for specifying and altering the electrical properties of fibers and fabrics. This allows the prescription of filter media prepared from these constituents for consistently optimum or near optimum performance in the collection of particulate matter. The method may include the steps of determining the triboelectric properties of fabrics having other desirable filter media characteristics, modifying the triboelectric properties of these fabrics as needed to preferentially selected properties and utilizing, selectively, modified fabrics as the filter medium for optimally attracting gas entrained electrically charged particles to the surface of the filter. The modification of the triboelectric properties may be realized chemically or by dyeing the fibers and fabric. The selected fabric has triboelectric characteristics that provide maximum attraction for the dust to be filtered so that when possible, as is most common, agglomeration of the particles on the surface of the filter is promoted, and the density of the particulate on the surface of the filter is increased. Additionally, determination and modification techniques are proposed for fabrics utilized as filter media and the blending of the included fibers, filaments or yarns of selected fibers so modified. These are then combined into a medium for use in filtering particulate matter having particles of various electric charges.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the modification and manipulation of the 
triboelectric properties of filter material to provide fabrics having 
predictable triboelectric properties for use as filter media. More 
particularly, the invention relates to the modification or adjustment of 
filter fabric media according to predictably calibrated triboelectric 
properties for use in dust collection operations so as to optimize the 
performance of the filters in relation to the particulate matter to be 
filtered. 
2. Description of the Prior Art 
The U.S. Environmental Protection Agency (EPA), in 1970, originally set 
forth a National Ambient Quality standard for particulate matter. Since 
that time, U.S. industry has commenced practices which reduced the mass of 
particulates on an average by 20% by the early 1980's, despite increased 
industrial developments. In view of further industrial expansion and 
especially in view of the projected greater use of coal to generate 
electrical power, particulate pollutants will have a tendency to increase 
unless appropriate control measures are taken. As regulations became more 
stringent, especially in regard to particle size, control devices for 
removing particulate matter became more restricted. The most common 
methods in increasing order of apparent potential acceptability based on 
performance are: mechanical collectors, particulate wet scrubbers, 
electrostatic precipitators, and fabric filters. Fabric filtration is a 
process not greatly different in principle of operation from that of the 
common home vacuum cleaner. Particulate matter is removed from a dirty air 
(gas) stream by virtue of separation processes that occur at or near the 
fabric surface. Five such mechanisms are identified in the separation 
process: inertial deposition, brownian movement, direct interception, 
gravitational sedimentation and electrostatics. Except for the 
electrostatic involvement feature, each of these has been well described 
in the prior art. Because filtration is among the most reliable, efficient 
and economical methods for removing particulate matter from gases, 
baghouses are being applied more universally for controlling emissions. As 
an example, baghouses may be applied to coal-fired utility boilers, and 
are one of the few air pollution control techniques easily capable of 
meeting the more stringent anticipated emission standards. 
Although fabric filters are well known as being capable of collecting very 
small particulates, a high level of removal from industrial process gases 
is not routinely achieved. One reason for this is that not all fibers used 
in constructing the filter perform in the same manner, even where the 
chemical composition of the fibers is presumably identical in a favorably 
constructed fabric. Additionally, it appears that natural electrical 
forces clearly influence the filtration process. In fact, it appears that 
substantially all industrial processes produce particulate matter with 
charges, positive and negative. Although considerable information on the 
mechanics of the filtration process for uncharged particles is available, 
very little has been provided with regard to natural electrical effects in 
fabric filtration. This is despite the fact that particulate matter 
reaching the fabric filter is rarely uncharged and the medium itself is 
rarely devoid of an electric field. Accordingly, particles entering 
conventional collectors are mostly charged, sometimes far more extensively 
than at other times, but usually of mixed polarity. The type of generating 
process determines the magnitude of the charge, with grinding and other 
energy intensive operations producing particulate matter with extremely 
high levels of charge. 
It is generally accepted that electrostatic attraction draws particles from 
the gas stream to fibers if the two are oppositely charged. Even if only 
one of the particles or filtering fabric is charged, a naturally induced 
charge will be created on the other. This results in a polarizing force 
that causes attraction and particle movement from the gas stream to the 
oppositely charged fiber. However, although as stated above, the 
particulate matter and/or the fabric filter may have electrostatic charges 
thereon, the polarity, magnitude and durability of the triboelectrically 
induced charge depends upon the inherent properties of the materials, 
including their chemical make-up and the electrical resistivity. 
Electrical augmentation, the practice of electrically charging the 
gas-entrained particles and/or applying an electric field to the 
collecting medium, can provide excellent filtration features. These 
artificial charging conditions are, however, applicable only to 
non-combustible, electrically chargeable particles. Another limitation is 
that they require special processing and collection facilities, 
electrodes, electrical circuits, and the like. The most commonly proposed 
electrical augmentation techniques utilize a corona discharge to impress a 
charge on the particulate matter and/or a high D.C. voltage on wire 
electrodes appropriately located on or near the surface of the collecting 
fabric. One of the more serious limitations of electrode systems proposed 
for such augmentation is the short life of the circuitry. 
A very significant portion of the improved filtration performance gained by 
electrical augmentation or artificial charging may be achievable simply by 
balancing the natural charging properties of the fabric with those 
provided by the particulate matter. By utilizing natural charges, that is 
by using a fabric filter medium of appropriate inherent triboelectric 
properties relative to those of the particulate matter being collected, it 
is possible to deposit a low air-flow resistant cake without electrical 
augmentation. By suitably balancing the natural triboelectric properties 
of the medium in relation to those of the particles being collected, 
conditions are realized for approaching the ultimate level of filtration 
performance now attained only by electrical augmentation. 
Practically all of the commercial fibers used for filtration fabrics 
respond to contact electrification and because of molecular variations, 
gain or lose electrons differently. Different fibers, therefore, become 
charged at different polarities. When listed in a downward order from 
electropositive to electronegative, a series may be developed, referred to 
as the triboelectric series (TE), and any material fabric or dust, may be 
included according to its electrostatic polarity relative to others in the 
list. Triboelectrification is the frictional process by which substances 
such as fabrics, particles, and the like, when abraded or rubbed by other 
substances and separated, develop electrostatic charges. Polarity of the 
acquired charge on the rubbed material to that on the rubbing substance 
depends upon the inherent character of the rubbed substance. The magnitude 
of the acquired charge depends upon various qualities of both the rubbed 
and the rubbing materials including the differences in their spacing in 
the triboelectric series, the roughness of their surfaces, the environment 
to which they are exposed and other parameters. Natural charging refers to 
the charging process that occurs naturally in the course of handling 
materials of all types. Particulate matter acquires electrostatic charges 
by contact with or rubbing against other substances such as the walls of 
ducting or during formation/production as generated at high temperatures, 
grinding, and the like. The triboelectric properties of fabrics generally 
are either not significant or have not been recognized to be critical or 
useful in their normal service applications. 
In dry filtration, however, this characteristic of the medium appears to 
control the process and dictate its performance. An ideal balance between 
the electrostatic charges on the collecting filter medium provides optimal 
or near optimal filtration parameters in terms of pressure drop, 
efficiency, gas flow-through, fabric cleanability and other dependent 
variables. Although non-electrically augmented filtration operations 
presently are anticipated to be the most common collection methods, 
controlling the filter operation by balancing the electrostatic properties 
of the particles and of the filter medium has received little or no 
consideration. 
The opportunity to utilize natural electrostatic effects fully has been 
restricted to some extent by non-availability of appropriate media and 
most seriously by the triboelectric limitations of commercially available 
fabrics. The most serious problems have included triboelectric 
non-uniformity among even supposedly identical fabrics, and the 
limitations of the inherent triboelectric properties of an otherwise 
suitable fabric. The fabrics marketed for filter media use presently do 
not always permit the choice of the desired triboelectric properties, 
neither are the fabrics constructed from blends of fibers having a 
selectable preferred balance of electropositive and electronegative fibers 
for filtering gas entrained particles of both charges, to optimize the 
process. 
SUMMARY OF THE INVENTION 
The present techniques overcome the triboelectric limitations imposed on 
commercially available fabrics by preferentially and specifically 
adjusting or modifying the triboelectric properties of fabric filter media 
in order to realize optimal or near optimal filtration performance in the 
collection of precharged particulate matter. The selection of the 
preferred filter fabric may then be made on the basis of electrical 
properties as well as upon the need to meet the temperature, chemical, 
economic and other requirements. 
A simple modification process to provide consistent, desirable 
triboelectric properties is described. The process modifies useful fabrics 
such that they may be consistently and accurately categorized and assigned 
to a specific position in the triboelectric series. This position then 
corresponds to one which is most optimal for a particulate being filtered. 
Additionally, fibers may be modified triboelectrically and a fabric 
produced from a blend of fibers having the desirable properties for use as 
filter fabrics wherein the particulate being filtered has various 
electrical charges. For example, DACRON polyester fibers vary greatly in 
their triboelectric properties, and although ideal as filter medium for 
other reasons, may not be most favorably utilized unless appropriately 
modified in their electrical properties to provide consistent performance. 
Accordingly, the utilization of DACRON based upon its triboelectric 
properties is not always practical unless the fiber or the fabric made of 
the fiber is preferentially altered to provide the desired TE qualities. 
The described alteration may be made chemically by conventional chemical 
means, and more preferentially by dyeing. Dyeing provides the necessary 
chemical changes while further providing a simple and accurate 
identification method for the chemically altered fabrics. In either case, 
the alteration must be durable in order to withstand the conditions of 
service. Dyes are preferentially applied since in addition to achieving 
the needed electrical features routinely in commercially available 
facilities, the presence of a colorfast specific color denoting a position 
in the triboelectric series has considerable appeal. With prior knowledge 
of the subject particulates charge features, the filter fabric required to 
provide optimal collection parameters can be determined easily. For 
example, the advantage of having a highly electropositive red fabric and a 
very electronegative blue fabric or, perhaps, a blend of the fibers or 
yarns from the two base fibers in a fabric having a purple shade, would 
have specifically useful features. The first two would offer the needed 
triboelectric properties for optimally collecting negative triboelectric, 
commonly known as TE(-), or positive TE, known as TE(+) particles. The 
fabric containing the blend mixture of fibers/yarns would serve best in 
collecting dust in which half of the particles were positively charged and 
half were negatively charged. It would be apparent that the desirable 
blend will be governed by the actual charge distribution of particles in 
the particulate matter to be filtered. 
A method is disclosed, therefore, by which the TE properties of fibers in 
fabric filter media may be determined and changed predictably to meet 
requirements dictated by the TE properties of the particulate matter being 
collected and, thereby, to attain optimal or near optimal collection 
parameters. 
Any alteration of a fiber's TE property must be realized by a process that 
makes the change relatively permanent. Since charges are generated and 
carried on the surface of the fibers, an unbonded surface coating is 
useless for charge modification. But two fundamentally different types of 
treatment have been shown to be effective. One of these is by coordinate 
bonding through chemical addition or substitution with the substrate fiber 
to form a new, different and chemically bonded finish with suitable ionic 
qualities to influence the TE as desired. There are several processes of 
this type, two of which have been used and described below with the TE 
results indicated in Table 2 and another which is mentioned. 
Disperse dyes are prime examples of those agents that modify the surface by 
absorption to become a new and relatively permanent part of the fiber. 
Other dyes that become part of a fiber's surface and provide new and 
different ionic features influence the TE properties of these modified 
fibers differently. 
Chemical Reactions for TE Modification 
Any reagent capable of reacting with the fiber substrate, polymeric or 
monomeric, and providing a characteristic ionic group, will serve to 
modify the TE properties of that fiber. For modifying polyester as well as 
other polymeric fibers with active hydrogen atoms or those with bonded 
water molecules, it is possible to effect modification with such reagents 
as the isocyanates, silanes and Grignards which become a new part of the 
fiber and, therefore, may alter its TE properties. Evidence is provided in 
NATURE, 349: 683 (1991) that water forms hydrogen bonds to the aromatic pi 
electrons of certain organic compounds like the polyesters and RYTON, for 
example. With isocyanate on RYTON: 
##STR1## 
With dimethyldichlorosilane on RYTON to produce a silicone complex: 
##STR2## 
Isocyanate Reactions 
The ability of isocyanates to perform in this way has been demonstrated 
clearly by the treatment applied by Mobay Chemical Company to samples of 
the CS&S, bulk knitted polyester filament yarn filter fabric. The 
extremely high TE (+) properties imparted by some of the treatments as 
identified below is verification that new surfaces with cationic features 
have been achieved by direct reaction with the polymeric or monomeric 
reagents to form a durable, abrasion resistant, reacted finish. 
Isocyanate Reaction With Other Active H Atoms 
Examples are given utilizing polyester, or pe. 
{RN=C.dbd.O+R*OH(pe).fwdarw.RONHR*(pe)+CO.sub.2 } 
Reference should be made to Table 2 for the relative TE positions of 
treated fabrics. 
Mobay A-(pe)+Bayhydrol 123 (reaction with polymeric pe) 
Mobay B-(pe)+Desmodur E-21 (MDI=methylenediphenyldiisocyanate) (crosslinks 
through active H/moisture on pe) 
Mobay C-(pe)+Desmodur N-1 (HDI=hexamethylenediisocyanate) (crosslinks 
through active H/moisture on pe) 
Mobay D-(pe)+Impranil DLN polyurethane (15%) 
Mobay E-(pe)+Bayhydrol 123 (15%) 
Mobay F-(pe)+Desmodur N-75 and Desmophen (670A-80) (Desmophen N-75 
crosslinks with Desmophen (670A-80 on pe)) 
With reference to Table 2, the differences in TE contributions of those 
treatments which react with the polyester polymer and produce a strong 
cationic influence compared to those that provide a less strong cationic 
surface coating will be apparent from the locations of such treated CS&S 
knitted (pe) Dacron fabrics in the series. The more or less 
electropositive contributions of those reagents that become part of the 
fiber and provide an amine or amide (TE+) end group is apparent. 
Silane Reactions 
Silanes react with moisture, loosely or pi-bonded to a complex polymer, for 
example, HOH: 
##STR3## 
and with pi-bonded water of pe, or RYTON: 
##STR4## 
Grignard Reactions 
Among other reagents capable of changing the TE properties of fibers are 
those of the Grignard type. These are formed from alkyl halides with 
metallic magnesium, usually in the presence of anhydrous ethyl ether, 
other higher series ethers, tertiary amines and even hydrocarbons: 
R*MgX+(HOH, ROH, RHO, or RNH.sub.2).fwdarw.(HOMgX, ROMgX, RHMgX, or 
NHMgX)+R*H 
The reactions allow Grignard reagents to modify fibers with a variety of 
endgroups including active hydrogens, pi-bonded or otherwise attached 
water molecules, primary and secondary amines, acidic groups, alkyl 
halides and those containing acetylenic groups. The reactions may be 
considered as follows: 
with an active hydrogen RH+R'MgX.fwdarw.RMgX+R'H 
with water HOH+R'MgX.fwdarw.HOMgX+R'H 
with amines --NHH+R'MgX.fwdarw.--NHMgX+R'H 
with acids --C.dbd.O--OH+R'MgX.fwdarw.--C.dbd.O--OMgX+R'H 
with halide RX+R'MgX.fwdarw.RMgX.sub.2 +RR' 
with acetylinic groups --C.tbd.CH+R'MgX.fwdarw.--C.tbd.CMgX+R'H 
Dyeing Processes 
Dyes become part of the fiber surface by absorption. Any dye, therefore, 
capable of penetrating and changing the fiber's TE properties by 
introducing a different ionic group, offers a means for predictably 
modifying this property for achieving optimal or near optimal filtration 
performance, especially for collecting the agglomerating types of 
particulate matter. MAXILON RED GRL-BR(HC)200 from Ciba-Geigy Corporation 
is an Azo dye made from an aryl quaternary amine and sodium napthyl 
sulfonate producing a triazide nitrogen. The dye becomes part of the 
polyester fiber, absorbing into the polymer's surface, dissolving therein, 
but not chemically bonded. The dye is fast, i.e., it resists fading by 
washing, and the like, and can be expected to remain an effective 
component of the polyester fiber/fabric used in the normal filtration 
environments to which it is exposed. The presence of amine and/or amide 
groups account for this dye's ability to confer electropositive TE 
qualities. The surface solution or adsorption of the dye contributes to 
the fastness or durability of the modification. RITE REACTIVE YELLOW B-RLN 
as supplied by Rite Industries Inc. is a difunctional dye of vinyl sulfone 
and monochlorotriazine. The presence of chlorine in this dye is believed 
to contribute to the TE (-) quality that it confers to the CS&S polyester. 
Another example is RIT ALL PURPOSE CONCENTRATED TINT AND DYE, produced by 
Special Products, an Affiliate of CPC International Inc. While the 
compositions of these dyes have not been revealed, it will be obvious from 
the locations of the RIT-dyed polyester fabric in Table 2, the various 
dyes not only provide different colors but also impart markedly different 
TE properties. 
Firestone's Solution Dyed Polyester Fibers/Fabrics 
Fabrics with yellow solution dyed polyester filament yarns in the filling 
and different colored solution dyed polyester filament yarns in the warp 
as provided by Firestone Fibers and Textiles Company, Box 450, Hopewell, 
Va. 23860, were evaluated triboelectrically. Not only were the positions 
of the six different fabrics in the TE series found to be quite different 
but the samples also produced substantial variations in generated voltage 
when rubbed by the reference material. These effects are indicated as 
follows: 
______________________________________ 
color TE Total V* 
______________________________________ 
blue +4.1 14.0 
green +2.5 6.5 
orange +0.1 9.4 
red/orange +0.04 14.2 
yellow +2.5 6.5 
white +1.5 15.5 
black +1.3 15.2 
______________________________________ 
While the types of dye used on these polyester filament yarns are not 
identified except that they are of the solution type, it will be evident 
that each has a different influence on the TE properties of the same 
polyester fiber and, therefore, might be used to alter such features 
predictably. 
The process thus provides a method for selecting a fabric filter by 
preferentially and specifically adjusting the TE properties of known 
fabric filter media to conform to a specific charge polarity and magnitude 
calibrated for the collection of known charged particulate matter. 
Additionally, the process provides a grouping of filter fabrics modified 
through the preferential adjustment of the TE properties of available 
useful fabrics for selective adoption in filtering particulate matter 
having charged particles attracted optimally to materials only having 
certain TE properties. 
Modification of the TE properties of known fibers to specific locations in 
the TE series is disclosed, such that a fabric may be constructed having a 
blend of fibers of various selective TE properties for use as a filter 
medium in filtering particulate matter having particles of various charge 
distribution. 
A method is also provided to select a filter for a particular filtration 
application, the method including the steps of determining and changing 
the TE properties of a fabric to conform to a specific charged polarity 
and magnitude, constructing filters from materials calibrated to such 
properties, determining the charge polarity and magnitude of particulate 
matter to be filtered, and selecting filters calibrated to optimally 
attract such particles and cause aggregation of those that are subject to 
such transformation. Accordingly, the present invention includes the 
determination of the TE properties of fabrics having other desirable 
filter media characteristics, modifying the TE properties of these fabrics 
and utilizing, selectively, modified fabrics as the filter medium for 
optimally attracting gas entrained electrically charged particles to the 
surface of the filter. The invention also includes the determination and 
modification of the TE properties of fibers utilized in fabrics having 
other desirable filter media characteristics and blending yarns of 
selected fibers so modified into a fabric for use in filtering particulate 
matter comprising particles having various electrical charges, the yarn 
blend being such that certain of the fibers best attract others of the 
particles to the surface of the fabric. 
These and other particular features and advantages of the present invention 
will become more fully understood upon reference to the presently 
preferred embodiments thereof and the examples set forth. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Particle separation by filter media occurs-by more than the simple process 
of entrapment of the individual particles, since the voids in most fabrics 
are usually many times greater than the size of the collected individual 
particles. Ordinarily, the separation process is relatively poor until a 
suitable layer or particulate matter is collected and forms a bridging 
type accumulation across the openings. Once a particulate base or cake is 
formed over the fabric surface, the collection efficiency increases to a 
value approaching 100%, depending on the medium, the particulate and 
processing conditions. Particulate matter that agglomerate on the 
collecting surface can be removed while some cake may remain. For those 
particulates that do not agglomerate on the collecting surface, less 
effective cleaning must be accepted in order to achieve high collection 
efficiency. 
It is generally accepted that electrostatic attraction draws particles from 
the gas stream to fibers when the two are oppositely charged. When 
electrostatic forces of attraction are suitable, particle-to-particle 
contact and, thereby, agglomeration is enhanced. This is believed to be 
the primary basis for the porous type structure of the deposits collected 
under favorable conditions of electrostatic charging. In the process of 
separating gas entrained particulate matter by fabric filtration, 
efficiency of particle removal, the rate of gas flow through the fabric 
and the effectiveness of the cake removal operation are all maximized by 
formation of such a porous deposit of collected particulate matter. The 
ideal type of deposition is realized by optimizing the electrostatic 
balance between particles being collected and the collecting filter 
medium. Electrical augmentation by artificial charging can lead to ideal 
collection parameters, but for general use, the durability of the 
electrical circuitry and non-applicability to a combustible environment is 
a limitation of these processes. By balancing the particulate and fabric 
natural charges, a significant portion of improved contact is determined 
by the inherent properties of the fibers that make up the fabric, by the 
construction of the fabric, by the particulate itself, and by operating 
conditions. Therefore, the natural TE properties of the medium may be 
employed to deposit a low air flow resistant cake without electrical 
augmentation to approach the ultimate level of filtration performance now 
achieved by such augmentation. However, the opportunities to utilize 
natural electrostatic effects fully are limited to a large extent by the 
availability of appropriate media and more so by the variability of 
commercial fabrics. Accordingly, by predetermined modification of the TE 
properties of the medium, media may be selected to provide the best 
filtration performance for given particulates produced by a given process. 
Consequently, an electrical balance is required between the filter media 
and the particles being collected, and it is important to realize that the 
strong electrical charge imbalance between the filter media and the 
particles contributes not only to attract the particles, but to attract 
these particles so effectively as to cause aggregation of those types of 
particulate matter that can undergo such a transformation. It is this 
aggregation or change in the effective particle size on the surface and 
not the inside the fabric structure, that is the key to the process 
provided by electrostatic interactions. Such interactions may be provided 
by the utilization of naturally generated electrostatic charges on the 
particles and on the collecting fabric. The advantage of increasing the 
particles size by agglomeration is evident on the basis of size alone, but 
such change also leads an increase in density for many types of 
particulate matter. This added feature assists in the removal of the 
collected cake during the cleaning phase of the filtration cycle simply by 
reason of the gravitational effect; moreover, the greater density 
contributes to a reduction in particle reentrainment, i.e. the drawing of 
collected particles or dust back up into the filter bag of a baghouse. 
The triboelectric properties include the relative position, positive to 
negative, in the series, the magnitude of the charge generated and the 
rate of charge dissipation. The latter is significant since fabrics which 
discharge at low rates, although useful for agglomerating 
difficult-to-aggregate dust, are also more difficult to clean. The 
determination of the TE properties of fabrics may be accomplished by a 
controlled rubbing (or contact) and separation test as fully described in 
the American Dyestuff Reporter, Volume 57, No. 15, pages 31-33 dated Jul. 
15, 1968. The method is quite simple, in that a narrow strip of test 
fabric is positioned in a frame and rubbed by a strip of reference fabric 
mounted on a rotatable insulated disk. The reference fabric is engaged 
with the test fabric and rotated through a fixed number of revolutions or 
time period. The test fabric is then contacted by a probe connected to an 
electric meter which measures the generated electrostatic voltage with 
respect to both polarity and magnitude. Using reference fabrics of known, 
predetermined and established polarity which preferably have the same 
construction, e.g., woven in the same pattern of essentially the same 
fiber/yarn sizes, other fabrics may then be located in a series relative 
to the reference materials by their charges when rubbed with these 
reference materials. Examples of these reference materials are NYLON, 
which is electropositive and a dinitrile, DARLAN, which is 
electronegative. As the trials are repeated by rubbing and separating 
different samples of fabric, each may then be located in this same TE 
series. Through repetition of this process with other fabrics, a TE series 
may be prepared by locating the various test fabrics relative to the 
reference fabrics. Such a series is shown in Table 1, which is similar to 
the table in EPA Report 600/7-78-142b. It should be noted that the 10 volt 
scale is strictly arbitrary, selected for ease in calculating values. 1000 
or 5000 would be equally appropriate and possibly more realistic. 
It is critical, in carrying out these tests, that the reference and test 
sample fabrics be clean since most foreign substances will confer 
different surface features. Subsequent to the TE position testing, a 
charge dissipation rate of the fabric is determined in order to determine 
the voltage remaining on the test samples after a fixed period of time, 
which is arbitrarily set at two minutes. This charge dissipation rate is 
measured as a percentage of the voltage derived from the tests made to 
determine the TE series. Thus, it gives a value which is a percentage of 
the charge lost in two minutes and again is considered to be arbitrary. 
The scale for the values is arbitrarily established by setting values with 
respect to the known electropositive and electronegative reference fabrics 
and comparing the test fabrics therewith as more fully described in U.S. 
Pat. No. 3,487,396. Table 1 may thus establish TE positions of the various 
filter fabrics. The numbers included in brackets after the fabric name 
represent the relative TE discharge rate at 50% relative humidity. Most of 
the fabrics in Table 1 are wovens, while certain of the yarns are spun and 
others are continuous filament. 
TABLE 1 
______________________________________ 
A TRIBOELECTRIC SERIES 
(Estimated Triboelectric Positions of Some Filter Fabrics) 
(an arbitrary scale) 
______________________________________ 
(+) very electropositive (+) 
+8 protein (WOOL A) 20%! 
+7 protein (WOOL B) 80%! 
polyphenylene sulfide (RYTON, st.) 20%! 
+6 ---- polyamide (NYLON) {afc 800b- REF.} 
fiberglass 35%! 
+5 polyester (DACRON O) 50%! 
+4 
polyester (DACRON A) 90%! 
+3 
+2 
polyester (KODEL) 20%!, polyester (DACRON) 40%! 
+1 
aramid (NOMEX) 60%! 
acrylic {copolymer} (ORLON) 30%! 
-1 
acrylic homopolymer! (DRALON T) 30%! 
-2 
polyester (DACRON B) 30%!, polypropylene 50%! 
-3 
-4 ---- acrylic dinitrile! (DARLAN) {afc 5546-REF.} 
-5 
ptfe (TEFLON) 0%! 
-6 ectfe (HALAR) 20%! 
aramid (KEVLAR) 45%! 
(-) very electronegative (-) 
______________________________________ 
Note: All locations above +6 and below -4 are approximate. 
%! = apparent rate of charge dissipation. 
In filtration, because the charges that are acquired by particles in normal 
processing should be neutralized at the fabric surface to promote 
aggregation, this condition is best achieved by means of the fabric which 
offers the greatest attraction, i.e., most widely separated from the dust 
in the TE series. All materials, including particulate matter, can be 
located in the TE series. Fibers with consistent TE properties offer 
features for accurate prescription in the collection of certain dusts. 
Consider, for example, the opportunities provided for filtration of a 
given dust at elevated temperatures, e.g., 400.degree. F. For 
electropositive properties, fiberglass media is available to attract and 
agglomerate electronegative particles. Similarly, because of its 
electronegative properties, a TEFLON fabric offers those TE features well 
suited for attracting and aggregating electropositive particles. For a 
mixture of positive and negative particles, NOMEX may be expected to offer 
somewhat better conditions, or agglomerating features, than either of 
these other high temperature media. 
It would appear evident that in the filtration of a dust in which the 
particles carry both electronegative and electropositive charges, the more 
ideal medium will be that which can attract particles of both polarity. 
Ideally, this fiber blended yarn would consist of a negative TE polarity 
fiber concentration equivalent to 100% minus the percent concentration of 
negatively charged particles and a positive TE polarity of a concentration 
of 100% minus the percent concentration of the positively charged 
particles, assuming all particles carry charges. 
One clear indication of the variability in TE properties among polyester 
fabrics and how this influences the filtration process was indicated 
experimentally in a test comparing three polyester fabrics having 
substantially the same permeability, having relative TE positions of -2.5, 
+4.8 and +1.4. Electric furnace dust was utilized as the test material. 
The filter medium having the TE position of -2.5 short cycled and 
essentially failed after collecting a small amount of the dust; the filter 
medium having the TE location of +4.8 performed somewhat better but not 
well; and the filter fabric having the midrange TE position of +1.4 
performed very well and could have been used for a substantial period of 
time to collect considerable dust. 
Since DACRON fibers vary greatly in their TE properties, and are generally 
ideal as a filter medium for other reasons, they may not be used most 
favorably in every appropriate application without modification in 
electrical properties. Since inherent chemical properties determine the 
electrical characteristics that dictate the location of the material in 
the TE series, it is evident that the DACRON fiber types that appear in 
different positions in the TE series must possess different surface 
chemical features. Accordingly, the prescription of DACRON, based upon TE 
properties, is not always practical unless the fiber/fabric is 
preferentially altered, chemically, to provide the desired TE qualities. 
Chemical modification, whether by conventional chemical means or by 
dyeing, therefore, is the principle for adjusting the TE properties of the 
fabric (fibers) therein to meet known and preferred locations in the TE 
series as related to those of the collected particulate matter. 
In aqueous processes involving fabrics, chemical alteration is often 
accomplished by anionic or cationic reagents. The anionic treatment allows 
either retention or causes an enhancement of the electronegative 
properties of the processed fabric while the cationic finishes alter or 
enhance electropositive features. When these ionically active agents are 
applied to media for filtration applications, they produce similar changes 
and also provide either the same TE polarity or a reversal in the polarity 
of the original substrate. For example, the reaction of a polyester fabric 
having inherent TE properties that locate it in the mid position of the 
series, with an anionic treatment, causes the fabric to become far more 
electronegative. Similarly, when the same basic fabric substrate is 
reacted with a cationic reagent, it becomes more electropositive in the TE 
series. Anionic reagents are those that in a liquid subjected to an 
electric potential, collect at the anode. These reagents are represented 
by such chemicals as those containing hydroxide, carbonate and phosphate. 
Cationic reagents in a liquid subjected to electric potential collect at 
the cathode. These agents are represented by chemical makeup of such 
active positive ions as the amines and amides. It is thus evident that the 
reaction may be a simple chemical, an active modifying or resin-forming 
agent or a reactive dye. These reactions and the resulting change brought 
about by them in the TE position of the fabric has been verified by 
applying a cationic dye to the near mid-TE position, such as -0.5 DACRON 
(No. 107), to become very a electropositive medium at a position of +5. 
Referring to Table 2, an anionic dye, applied to the same near mid-TE 
position caused the fabric to change to a new TE position of -4.

EXAMPLES AND TESTING 
In testing using two household dyes, such as RIT manufactured by Special 
Products, an affiliate of CPC International, one that was labeled navy 
blue and another scarlet, although not recommended for polyester or 
acrylic fibers, were applied successfully to pre-cleaned, woven, napped 
polyester fabric. Canadian Wheelabrator Fry No. S350/154, piece No. 35900 
was utilized for this test and was cleaned using nonionic detergent, a 
140.degree. F. wash and a thorough rinse. This fabric in the cleaned 
condition had a TE position of -0.1. When dyed with the blue dye, the 
fabric's TE position was raised to the +0.3 location. The change in the 
scarlet dyed sample was found to be downward to a more negative polarity 
of -0.7 in the TE series. Not only are these noted polarity differences 
real and significant, but the magnitude of the charges generated by 
rubbing the three fabrics with NYLON in one instance and with DARLAN in 
another also differ greatly. The charge developed by rubbing against NYLON 
was -8.8 volts for the blue sample, -14.1 volts for the undyed fabric and 
-15.7 volts for the scarlet sample. Similarly for the test materials 
rubbed with DARLAN, the blue sample responded with -6.5 volts, the undyed 
with -9.2 volts and the scarlet dyed fabric with +7.9 volts. The TE data 
are shown graphically in Table 2. 
TABLE 2 
______________________________________ 
A TRIBOELECTRIC SERIES 
Showing Positions of Some Fabrics 
before and after Modification 
(an arbitrary scale) 
______________________________________ 
(+) very electropositive (+) 
+8 
+7 *CS&S+C.G.Max.RedGRL 
*W.F.154+C.G.Max.RedGRL 
*RYTON, st. *CS&S+MobayC 
+6 ---- polyamide (NYLON) {afc 800b- REF} 
*CS&S+MobayD 
*CS&S+MobayE 
+5 *DACRON107+cat.dye *CS&S+MpbayA 
+4 *FIR.ST.(b) 
+3 
*CS&S *FIR.ST.(gr), 
*FIR.ST.(wh) 
+2 
*CS&S+MobayF, 
*FIR.ST.(ye) 
+1 *FIR.ST.(bk) 
*W.F.#154+RIT(b) *FIR.ST.(ro) 
0 *W.F.#154 *FIR.ST (o) 
*W.F.#154+RIT(s) 
-1 
-2 
-3 
-4 ---- acrylic dinitrile! (DARLAN) {afc 5546-REF.} 
*DACRON#107+an.dye 
-5 *CS&S+RITE(y) 
*CS&S+dmdcsi *W.F.#154+dmdcsi 
-6 *RYTON, st.+dmdcsi 
(-) very electronegative (-) 
______________________________________ 
Note: All locations above +6 and below -4 are approximate. 
Legend: CS&S = BEANE, bulk knit, fil.; W.F. = WHEELABRARTOR FRY, st., 
woven napped; FIR.ST. = FIRESTONE solution dyed filling yarns; Mobay = 
MOBAY CHEMICAL CO. with A,B,C,D,E & F urethane finishes; st. = staple 
(short) fiber; fil. = filament fiber; b = blue; bk = black; o = orange; r 
= red; wh = white; ye = yellow; C.G.MAX.GRL = CIBA GEIGY MAXILON RED 
GRLBR; RITE = RITE REACTIVE (yellow) from RITE INDUSTRIES INC.; RIT = RI 
TINT AND DYE, Special Products of CPC INTERNATIONAL INC.; cat. = cationic 
an = anionic; dmdcsi = dimethyldichlorosilane. 
These test results indicate conclusively that polyester and other fiber 
based fabrics may be altered to become either electronegative or 
electropositive in the TE series with a preselected dye or chemical, which 
is selected specifically for its effect on the fibers. Other fibers and 
fabrics are also amenable to the changes by dyeing or chemical alteration. 
Accordingly, any fiber that can be modified chemically, whether directly 
or by means of a bridging coupler or by means of a pre-etch, should 
respond to appropriate treatment and provide predetermined TE properties. 
Although tests were directed toward conveying NYLON-like TE properties to 
polyesters, it should be relatively easy to make NYLON more 
electronegative by chemical modification or dyeing. Only the 
non-reactivity of the fiber limits the opportunities for TE adjustment. 
Thus, fibers of TEFLON and the olefins would be expected to present more 
difficulties in the modification process; even so, some modifications 
should be possible. 
Once the TE properties of the desired material have been adjusted as 
desired, that material may be utilized as the filter medium for dusts 
having particulates most attractive to its polarity and magnitude. Or 
stated in another manner, the position of a particulate in the TE series 
may be determined and the filter medium having the most attractive 
opposite polarity TE position can be selected from the modified fabrics. 
A variety of techniques are available for determining the TE properties of 
particles. For example, in EPA Report No. 600/7-78-142a, September, 1978, 
G. W. Penney described impingement methods for charging dust with sequent 
charge determination. One test utilized a tungsten carbide target on which 
the dust (silica) impinged at high velocity. The charge was read on an 
electric meter connected to the tungsten target. Penney later used a 
fabric filter as the target as supported on a metal screen which was 
connected directly to an electric meter. The current collected by the 
screen was measured by the meter and the rate of air flow through the 
filter was determined by means of a calibrated orifice. While an indirect 
approach for the determination of TE properties of the particles is 
described herein, the data obtained by appropriate particulate 
detection/measurement systems is preferred, at least for comparative data. 
Once the TE properties of the filter media are known, filtration tests may 
be conducted with such media selected for particular TE features. If, for 
example, different media of essentially the same construction, but made 
with fibers with TE properties ranging from those that are electropositive 
to those that are more electronegative are evaluated under the same 
controlled conditions, the influence of the TE position becomes evident. 
As pressure drop remains low and flow rates remain high without dust 
leakage, the more ideal media are found and the TE characteristics of 
these media specify those preferred for optimum performance. Once so 
located, the apparent TE features of the dust are indicated approximately 
and the most ideal fabric filter medium may be specified, especially as 
the filtration tests are extended to fine-tune the analysis. 
Accordingly, a method is provided for selecting a filter for filtration 
application, the method including the steps of determining the TE 
properties of a fabric, chemically changing the TE properties to conform 
to specific desired properties, and utilizing fabric selected with the 
desired properties for media to filter dust and other particulate matter 
having TE characteristics most attracted to the fabric for promoting 
agglomeration on the surface of the media and an increase in density for 
the particulate so filtered. 
Numerous alterations of the structure herein disclosed will suggest 
themselves to those skilled in the art. However, it is to be understood 
that the present disclosure relates to the preferred embodiment of the 
invention which is for purposes of illustration only and not to be 
construed as a limitation of the invention. All such modifications which 
do not depart from the spirit of the invention are intended to be included 
within the scope of the appended claims.