CA1116063A - Monitoring device - Google Patents

Abstract

ABSTRACT
An improved device for measuring the amount of a selected component of a fluid mixture wherein the particulate sorbont material which collects the selected component is immobilized in a matrix of polytetrafluoro ethylene.

Description

3 91ll~103 IMPROVED MONITORIN~ ~EVIC~.

This lnvention relates to an improved device for analyzing the a~ount o~ a selected component in a fluid mixture, More particularl~ it relates to an lmproved personal monltor for determining the level o~ human exposure to harmful vapors and gases in ambi.ent air.
Personal monitors o~ the type descrlbed hereln are known and disclose~ ln U.S. Patents 3,924,219 and 3,950,980. ~hese monitors ma~ be attached to clothing in close proxlmity to the breathlng zone of the wearer.
~enerally, these monitors comprise body members ~orming a shallow chamber hav.lng an open end. Inside the chamber is dlsposed a collectin~ la~er which collects the pre-selected component fro~ the air. Acrosa the open end o~
the chamber and spaced rrom the collecting layer are one or more porous attenuating la~ers to prevent implngement o~ movln~ alr on the collecting layer and allow the selected components to reach the collecting la!yer by di~uslon.
; ~he collectlng substances use~ul in these monitors include materials whlch adsorb 3 absorb, react with or otherwise generally combine with the selected components belng measure~. In some cases the collecting substance i9 particulate ln nature such as actlvated charcoal, alumina or sllica. When particulate collecting materials are used, certain pro~lems have been encountered.

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Firstly, particulate material tends to shift position inside the monitor as the monitor is moved about. If the exposed surface of the collecting material is not smooth, uncertain concentration gradients may develop and the accuracy of subsequent measurements may be compromised. Secondly, when it is necessary to transfer the collecting material to a separate vial for analysis of the collected gases, some of the particulate material may be lost or incompletely removed from the monitor thereby introducing additional error into the measurements.
Thus, the need existed for a convenient means oE immobilizing or entrapping the particulate collecting substance ln a manner which would not impair its ability to combine with the selected components being monitored and which would not interfere with the subsequent analysis of the collected components.
The present invention effectively fulfills this need by providing a monitor wherein the particulate collecting substance is immobilized in a smooth flexible sheet formed by blending the collecting substance with an aqueous dispersion of polytetrafluoroethylene (PTFE) and subsequent formation of the mixture into a composite sheet.
Composite sheets of PTFE and various filler materials are known. U.S.
Patent ~o. 4,153,661, assigned to the same assignee as the present application, discloses high tensile strength porous PTFE composite sheets containing

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various fillers including carbon, sllica or alumlna, and a process ~or their preparation. The sheet 1~
described as beln~ uniformly porous making it suitable for use as a filtering material, for electrol~tic cells, as a gas diffusinn membrane and other uses known for PTF~ sheets.
Composite sheets ~ormed from PTFE and various flller materials are also disclosed, for example7 in the following U.S. Patents: 2,997,448, 3,010~536, 3,315~020,

3,ll07,2ll9 and 3,407,096. Although composite sheets com-prising PT~E and various ~iller materlals were prevlously known, lt was heretofore unrecognized that certain flller materlals contained therein could be utilized to collect selected gaseous components from a gaseous mixture for subsequent analysis. ~he surprislng discovery that particulate collectin~ materlal, e.~. activated charcoal, could be immo~llized into a PTFE sheet and retaln vlrtually all of its ability to combine wlth selected gaseous materials and the discovery that sorbed com-ponents could be readily desorbed from the sheet formkhe basis o~ the present lnvention.
According to the present invention there is provlded an improved devlce for measuring the amount of at least one selected component in a ~luid mixture. The device comprises body members ~orming a shallow chamber havlng an open end, a collectin~ layer disposed withln the chamber f'or combining wlth the selected component and one or more porous attenuating layers spaced apart from the collectlng layer and disposed across the open :. ~ ;. :,: : .:, :
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-` 3 end o~ the chamber. ~he improvement Or the pre~ent lnvent,ion resides ln the use Or a compo~lte sheet of polytetrafluoroeth~lene and a partlculate collecting material as the collecting la,ver. The P~FR composite sheet provides a collectin~ layer which is hiehly ef~icient in collecting the selected component. It presents a smooth, even collectin~ sur~ace which does not shi~t~ stratify, channel or otherwise chan~e po~-Ltion when the device is moved, Furthermore, the PTFE is chemlcally inert and does not lnterfere wlth the removal of the selected com-ponent from the sheet for subsequent analysis. The particulate collecting material is lmmobiliz~ed in the sheet so that none is lost from the device dur1ng use or during transfer from the device for analysis. ;~
Another embodiment of the invention resides in the use o~ the P~FE composlte sheet as an attenuatlng layer spaced from the detecting layer. In this embodi-ment, the sheet, whlch is highly porous, may be used to attenuate the ~low o~ the ~luid into the chamber and additionally serves as a selective barrier to remove selected components ~rom the fluid whlch may adversely a~fect the analysis. In this embodiment, the collectin~
layer may be a PTFE composite sheet or lt may be another type o~ collec~ing layer such as a gold film ~or mon- ~;
itoring mercury vapor.
~ he term collec~ing substance as used herein refers broadly to partlcula~e ma~erial wh~ch adsorbs, absorbs~ reacts wlth or otherw~se comblnes with a . ,/ , , ,, , . ~ . . . .
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,-,': :~ ' ';', , -, selected component o~ a ~luid mixture to remove the com-ponent from the mixture. For purposes o~ further dis-closure the terms "sorbent" and "sorbent material" will be used lnterchangea~ly wlth the term "collectln~ material."
The composite sheet materlal utillzed in the monitors of the invention is prepared by dlluting a P~FE
latex with water an~ adding thereto the particulake sor~
bent material. The resulting mixture is kneaded to ~orm a homogeneous paste. The pastè is subseauently calendered to ~orm a thin sheet material. The sheet is then dried at moderate temperatures, preferably under vacuum, to ~orm a composite whlch can be utillzed as a collectlng layer containing negl~glble contaminants.
The resultin~ composlte sheet material is smooth, ~lexible and porous, and possesses su~iclent tenslle strength 50 that lt may be cut, rubbed or handled without significant loss of the sorbent material.
In spite of the fact that the sorbent is intricately lncorporated into the composlte~ the sorbent character-istlcs thereof are not ~lp,niflcantly lmpaired.
The PTFE latex utilized in preparing the sheet materlal is an aqueous disperslon of negatively charged hydrophoblc collold containin~ particle~ in the general size ran~e o~ 0.05 to 0.5 mlcrometer~. A preferred material is Te~lo ~ 42 containlng 32-35% by welght solids of P~FE resln, which is avallable ~rom the E. I. DuPont Co~ Teflo ~ 42 ia partlcularly pre~erred as a commercial raw materlal slnce 1~ contalns no wetting :

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g'~ 13 agent. Teflo~ 30 or 30B~ slmllar products from E. I.
DuPont, may also be used, but the composlte sheet rormed therefrom require heat treatment at higher temperatures~
e.g. 300C, to remove the wetting agents.
The PTFE may be considered a~ a blnder in the ~orbent material and ~enerally comprises from about 5 to 50 percent by welght of the composite sheet, and pre-ferably about 15 to 25 percent by weight. The remainder Or the sheet is essentially the sorbent material in the pre~erred amount of 5t) to 95 percent by weight, and most preferably 75 to 85 percent by weight. However, in come instances, a portion of non-sorbent particulate flller may be substltuted for sorbent material. For example, up to about 50~ of the particulate material may be non-sorbent ~iller i~ the monitor is to be llmited to a short exposure period and/or ~or low concentratlons of monitored vapors.
The sorbent materlal is generally a particulate material of relatively ~ine parkicle slze and high surface area. For example9 the preferred actlvated charcoal utlllzed typlcally contains particles less than about 45 mlcrometers in diameter. ~About 75% less than 25 micrometers, 50~ less than about 12 micrometers and 5% less than 5 micrometers). ,enerally~ the partlcles wlll pass 400 mesh U.S~ Screen.
The par~iculate material utilized for the sorbent material in this composite are well known for their ability to sorb vapors of environmental consequence ~:, . : . . . , :
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;¢:~ 3 rrom an ambient akmosphere. hey must be es~entially insoluble in water. ~ctivated charcoal, alumlna or silica are typical example~ o~ ~uikable sorbent ma~erial.
In additlon, organic chromato~raphic substances such as Chromosorb, a co~olymer of styrene and divinyl benzene in the form of beads~ available ~rom Johns~Manville, may also be used in the composit;e.
As indicated earlier, a ~iller may be sub-stltuted ~or some of the sorbent material. The filler may consist o~ clay, talc, tltanium dloxlde3 calcium carbon-ate~ etc. and the partlcle sl~e is preferably les~ than about 1l5 mlcrometers.
The PTFE disperslon (generallv containin~ about 35 percent ~ollds) is mixed with sufflcient additional water and the particulate materlal to form a homo-geneous paste. The lar~er the partlcle slze o~ the sorbent material used, the less water ls required to form the paste. ~-The paste or putty-llke mass ls then trans-~erred to a calenderin~ device where it is calenderedbetween rolls preferably malntalned at about 50C to about 100C to form the sheet materlal. Preferably, the calendering rolls are made Or a rigld materlal such as ~teel A useful calenderlng device has a pair o~
rotatable opposed calendering rolls each o~ whlch may be heated and one o~ whlch may be ad~usted to~ard the other to reduce the ga~ or nip between the tWD. Typically, the 8aP ls adJusted to a setting of about 10 to 20 , , l,i ' ' . . .

milllmeters for the lnitial pass of the mass and, a~
calendering operations pro~ress, the gap is re~uced to produce a sheet of porouæl ~lexible, easily handled ~heet materlal. The sheet material f'ormed by passing through the calender rolls ma~J be ~olfled and rotated 90 to improve the strength Or the final sheet material. In a preferred mode of calenderlng, the folding and rotating Or the mass is per~ormed a~ter each pass. The thickne~s of the completed porous composite sorbent material is about 0.1 to 2.0 milllmeters~ with a pre~erred thlckness of about 0.3 to o.8 mm.
The resulting calendered sheet is baked, pre-~erably under vacuum, ln order to provide a porous, sorbent sheet materlal wlth minimal impurities which interfere with subse~uent anal~tical procedure. The heating step ls easily carried out in a vacuum oven at 150 to 3005 ~or from 0.5 to 5.0 hours, at atmospherlc pressure to a vacuum of about 100 Torr. It will be apparent to those skllled ln the art that baklng at a hlgher temperature under vacuum will require less time ~o eliminate contamlnants, whereas baking at lower temperatures atmospheric temperature will require longer time. Baking at higher temperatures, e.g. 3Q0C, for longer than about twenty mlnu~es may cause the sheet to become bri~tle. It i8 pre~erred to use a vacuum with an absolute pressure of about 50-125 Torr at a temperature o~ 150C for about 3.0 hours.
In spite of the ~act that the sorbent material .

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is bonded so well that the sheet materlal has appreclable tensile strength, and can be cut, ru~bed or ~lexed with-out loss o~ the sorbent, there 1s relatlvely little or no loss of the sorbent characterlskic. The surface area of the sorbent remains substantlally unchanged despite its lncorporatlon into the PT~E matrix. For example~ a sample of carbon having a surface area o~ 917 m2 per gram incorporated into Te~lo ~ 42 dlsperslon on an 80 percent carbon basis had a surface area o~ 734 m~/g of sheet material, The sur~ace area was determirled by nltro~en ~dsorption.
.Samples of ~eflo ~ dispersion-carbon sheet materlal were exposed to various organic vapors and the vapors were extracted with CS2 and anal~æed by gas chromatography. The data in ~able I indlcates the recover~J of the organic vapor.

TABLE I
Or~anic Vapor Heptane 103 Toluene 97 Chlorobenzene 94 Benzene 98 Methylethylketone 94 Monitors using these sorbent sheets as collect-ing layers can be used to monitor methyl ethyl ketone~
benzene, ethanol, heptane, toluene~ chlorobenzene, tri-chloroethylene~ sulfur dio~ide, chlorine~ ammonia, hydro~en sul~lde and hydrogen chlorlde and other materlals of environmental consequence. The monitor i5 exposed to , : . ., . ~:
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the environment rOr a predeter~ined length of ti~e, and ls generally po~itioned near the breathing zone of the wearer when use~ as a personal monitorO Following the exposure perio~ the collected vapors are desorbed from the collecting layer with an elutant such as carbon disul~ide. An allquot o~ the elutant containing, collected material is analyzed using conventional techniques such as gas chromatography.
Alternatively the sorbent sheets may be spaced from the collecting layer a,nA used to attenuate gas f'low lnto the chamber of the monitor. In thls position~ the sheet may serve as selective barrier to remove certaln selected vapors which may inter~ere wlth the analysls and allow the vapors to be analyzed to pass throu~h to the collecting layer. For example, a mercury vapor monitor such aY that described in ll.S. Patent No. 3,924,219 utllizes a gold film as the collecting (detecting) layer for the mercury vapor. The presence Or chlorine in a sampled environment may interrere with accurate measure-ment Or the amount of mercury vapor. Accordingly, it isdesirable to filter out chlorlne before it reaches the gold rllm collectlng layer. Thls can be accompli3hed by providing a ~ilter comprising a porous P~FE-Zeollte ~-composite ~heet in whlch the ~eollte has been reacted with an excess of lO percent N-phenyldi-lsopropanolamine in ethanol ~ollowing formation o~ the sheet. The sheet selectively rilters chlorine while allowlng passage of mercury vapor.

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, r~q'.~~ 3 The improved monitors o~ the inventlon are ~urther illustrated by the following nonllmiting examples.

This example illustrate~ the preparation Or PTFE - carbon composite sheets ~or use in or~anic vapor monitors. The effect o~ varying the amount o~ carbon in the sheet on various physlcal propertie~ of the sheet was determined.
The ~ollowing sheets were prepar~ed:
Weight of Wt. o~ PTFEVolume o~
Sample Carbon Disper3ionDeionized Water l 90 g 29 g180 ml 3 50 29 lO0

4 40 29 80 7 lO 29 20 The carbon used was Wltco Activated Charcoal, Type 965, which had been ~reviously ~round and cla~æi~ied to remo~e resldual flne particles and then heat treated at 600C under nltrogen ~or one hour or remove possible trace organlc contamlnant~. The ~inal pa ~icle slze was determined to be predominantly between 7 and 31 micro-meters. The PTFE u~ed was obtalned ~rom the Eo I. DuPontCompany under the Tradename Te~lo ~ 42 aqueous Te~lo di~perslon~ 34.4% solids. In each ca~e, the delonized water was added to the Te~lo ~ 42 dispersion. The dlluted Te~lo ~ 42 waq then added to the carbon. The mixture was stirred and then kneaded by hand to form a 3i~

homo~eneous paste.
The carbon-Te~lonR-water paste was then con-verted ~o a selr-supporting s~leet by milling on a two-roll rubber mlll having metal rollers controlled at a temperature of 80C. The millirlg procedure was as follow~:
1. ~he gap between the two metal rollers was ad~usted to approxlrnately 1/4 lnch (o.6 cm). The rollers were rotatlng ln opposite dlrections and arranged such that materlal lntroduced into the gap tended to be drawn lnto the gap and calendered under shear ~orces.
2. The homo~eneous paste was forced between the rollers 20 times. Between each pass the material was ~olded once and rotated at a 90 angle to the directlon of the previous pass.
3. The roll gap was decreased to approximately 1~ inch tO.3 cm). The material was ~orced between the rollers, ~olded, rotated 90 as be~ore ror ten passes.
4~ ~he roller gap was then reduced ln small steps without rurther ~oldin~ or rotatlng the sheet until the final sheet thickness was achieved.

5. The films were then dried in a vacuum , , .
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oven at 150C for 3 hour3 at a vacuum with an absolute pressure of 50 Torr. The sheets were all approximately 0.015 inches (O,3 mm) in thickness.
The sheet samples were then cut into circular wa~ers using a 1-3/16 inch (3.0 cm) cuttlng die. The wafers were used to conduct per~ormance tests, except ror poroslty and tensile strength wllich were determlned from the sheet samples themselves. 'rhe per~ormanc~ te~ts conducted are described as follows:
(1.) Volume and ~ensity - Th~ wafers were weighed on an analytlcal balance and the thlckness measured wlth a micrometer. The wa~er volume and density were calculated. The weight of carbon in each wa~er was cal-culated by multiplying the wafer weight by the percent carbon dry weight in the form ulat ion .
t2.) Adsorptlon Capacity - The adsorpkion capacity was tested by placing the wa~ers in a saturaked carbon tetrachloride (CC14) vapor chamber at room temperature ~or one hour and deter~inlng weight gainO The CC14 activlty wa~ calculated by divialng weight galn by -~
warer thickness~ ~he we~gh~ gain in grams was also dlvided by the welpht in grams o~
carbon ln each waf'er.

-(3.) Porosity - The poroslty was mea~ured wikh a W.& L.E. /urley Densometer, Model 4200.
The recor~ed value wa~ the time (in seconds) required to pass 10 cm3 o~ alr through a one square lnch (6.45 c~2) area of sheet at a di~rerential ore~sure o~
0.03 standard atmospheres.
(4.) Elutrlation E~ficlency - The wafer~ were placed ln indivldual fla~ containers with covers. A syringe was used ~o add 2.0 mlcrollter~ o~ methyl ethyl ketone (MEK) to each wafer, the cover snapped on and the sealed containers were allowed to ~tand overnl~ht, The nex~ day, 1.5 mlllillters Or carbon dlSUl~ld0 (CS2) added to the contalners. A~ter 30 mlnute~, 2 micro- :
liter samples of the liquld were with- :
drawn and in~ected into a Hewlett Packard Model 5840A gas chromatograph. Samples of a standard solution Or 2.0 ml¢roliter~ ;
MEK in 1.5 milliliter~ Or CS2 were al80 in,~ected (2 microliters). The elutrlation e~icienc~,r (percent MEK recovery) wa~
determined by comparislon of MEK p~ak area o~ ~he unknowns an~ the s~andards.
(5,) Ten~le Strength - Sheet tensile ~tr~ngth wa~ mea~ured on a ~M model Instron u~ing - ;. :, .

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one inch stri.ps~ one lnch Jaw separation and one inch/rninute crosshead s~eed. TJnits were in ~oun~s per square lnch.
The results ohtained are set forth in the ~ollo~lin~ Table I.

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TABLE I
Wafer Wafer Wafer Sample Carbon ThicknessWeight Density No. Wt, % (mils)(mm) (g) (g/cm3) 1 90.0 15.0 (0.38) 0.196 0.721 2 87.5 15.0 (0.38) 0~201 0.738 3 83.3 15.5 (0.39) 0.216 0.767 80.0 13.1 (0.33) 0.192 0.80~
75.0 15.0 (0.38) 0.229 0.842

6 66.6 1~.5 (0.37) 0.2~ 0.926

7 50.0 15.0 (0.38) ~.315 1.157 .~ : . ,: . : : :
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TABLE I CONTINUED

Sam- CCl CC1~ Gurley MFK Tensile ple Abso~bed Ac-tivlty Porosity Recovery Strength2 No. _ (mg) (mg/mm) (sec) (~) (psi)(Kg/cm ) 1101.5 267 1.197.5 68 (4~7~) 2106.0 279 2.3100.5 134 (9.42) 3107.6 276 3.4101.1 192 (13.5) 489.6 272 4.598.2 270 (19.0) 5100.2 264 5.896.4 245 (17.2) 692.9 251 11.2101.4 390 (~.4) 796.1 253 61.1103.4 9~2 (66.2) These data indicate that higher carbon loading gave higher porosity, lower sheet density and lower tensile strength. Sheet sorption properties remained essentially constant, and all sheets in this experiment were suitable for use in organic vapor monitors. The formulation can be tailored to individual applications.

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This example illustrates the effect of varying the thickness of -the PTFE-Sorbent composite sheet.
A homogeneous paste was formed from 40 g Witco Type 965 Activated Charcoal, 29 g Teflon~ 42 aqueous Teflo ~ dlspersion and 80 milliliters of deionized water.
Self-supporting sheets were prepared exactly as described in Example 1, except the final rubber mill roll gap was varied to achieve a range of sheet thicknesses. The samples were tested using procedures described in Example 1.

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- ~o -The above ~ata indicate that only the vapor capacity (CClll Adsorbed) and Gurley Porosit!~ are sub~
stantially a~ected by film thickness. Accordln~ly, sheet thlc]cness can be used to re~ulate these properties as desired for the snecl~lc lntencled applicatlon.
~ xampleæ ~-7 aré intended to demonstrate that a variety of sorbent materials can be used in the com posite sheets, and that use~ul sorbent activity is re-tained in the sheet.

EXAMPLE ~
A sample of Witco mype 965 Activated Charcoal was ~round an(~ sieved through 4~0 mesh screen ko give a ~article si7e predominantly between 5-25 microns. The charcoal was then heated at 60oo~ ~or one hour. A
homogeneous paste of 80 grams carbon, 60 grams aqueous Teflon~ 42 aqueous dispersion t3ll.4~ solids) and 160 grams deionized water was prepared and formed into 0.015 lnch (0.3~ mm) sheet by repeated passes through a two-roll steel roll rubber mill at ~C as described in Example 1.
The sheet was then dried in an oven at 150C ~or three hours under water asplrator vacuum (about 50 Torr absolute pressure). A circular~ 1-3/1~ inch (3.0 cm) diameter wafer was cut from the sheet and welghed. ~he weight o~ carbon ln the wafer was calculated. An approximately equi~alent amount of the ground, sieved and ~lred parent carbon powder was also wel~hed. ~he carbon wafer and carbon powder samples were both placed in a saturated benæene vapor chamber in metal weiÆhing dishes.

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A~ter three days, the weight gain was recorded for each ~am~le. The sorbent activity was c~lculated by computing the ~reight ~ain per gram o~ carbon. The activlty obtained for th~ carbon wa~er was ~.298 ~, benzene/g carbon com-pared ~Iith a value o~ 0.2~3 ~ benzene/~ carbon ~or theparent carbon ~a~ple. Accor~in~ly, the sorbent activit~
for the PTF~-carbon sheet ~ras virtuallv i~entical to that of the ~ree carbon.

~XA~IPLE 1~
A sample of .Tohns-Manvllle Chromosor ~ 102 (a porou3 stvrene divinylbenzene copolym~r having a surface area ln the ran~e of 30n-400 m2/g) was ground usinp, a mortar and pestle and t~len sieved through 140 mesh screen. A homogeneous paste was prepared from 5 g Chromoæor ~ 102, 3.75 g ~e~lo ~ 42, 7 m~ delonlzed water and 2 ml methanol. The methanol was necessary to pro-mote wettlng o~ the Chromosor ~. The paste was then calendered as descrlbed in Example 1 to give a self-supportlng~heet approximately 0.015 inch tO.3~ mm) thlck 20 and dried at 150C ~or 3 hours in the vacuum oven at an ~;
absolute pressure Or about 50 Torr. The sorbent acti~lty wa~ determined by comparlng weight of benzene adsorbed by ~he ~heet to the parent sorben~ ma~erial a~ ln Example 3. The absorptive activlty of ~he Chromosor 25 102/Teflo~42 ~hee~ wa3 0.769 g benzene/g Chromosor 102 compared ~o an activity of 0.750 g benzene/g Chromosorb~ 102 ~or parsn~ ma~erial.
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~xArlP3: E 5 A sample o~ ~sher ~ilica ~el rlrade 42 wa~
ground with mortar and pestle, siev~d through 140 mesh screen and converted lnto a PTFE composite sheet. ~he ~ormulation contained 2n g sleved sillca gel and 15 g Terlo ~ 42. No addltional water was necessary. The composlte sheet was formed a6 described in Example 1.
A~ter drying at 150C ~or three hours in the vacuum oven, the sorbent activlt~ of the ~ilm was compared to the parent sorbent material in a saturated ethanol vapor chamber at room temperature. ~he ac~ivity of the Te~lo ~-bonded ~llm was 0.204 g ethanol/g silica gel, compared to 0.265 g ethanol/g sllica gel ~or the parent material.

A sample o~ Ma~heson, Coleman and Bell Actlvated Alumina was ground and sleved as described ln Example 5. A homogeneous pa~te was ~ormed ~rom 20 g activated a~umina, 15 g Te~lon~ 42 disper~ion and 7 milliliters Or deionlzed water. The paste was calendered a~ descrlb~d ln Example lg bu~ ~ewer calendering passes were required to achieve tensile strength ~uitable for handllng. The sheet was dried at 150~C for 3 hour~ ln a vacuum oven, The ~orbent activity of the sheet was compared to that of the parent sorbent ln the sat-urated ethanol vapor chamberO The re~ult~ were com-plicated by the ~act that both fllm and parent sorbent re~dlly ad~orb moi~ture from the air durlng handling. ~be sorbent activlty of the sheet was ~ound to be 0.117 g " ~:

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- 23 _ ethanol/g activated alumlna, compared to 0.071 g ethanol/g activated alumina ror the parent sorbent.
Surprlsingly, the sorbent activity of the sheet was greater than that Or the parent material.

EXAr~ LR 7 Thls example illustrates the use o~ sorbent particles of larger particle size to ~orm the PTF~ com-poslte sheets.
R~ght g Or Tena ~ GC (60 x 80 mesh), a 2,6-diphenyl-p~phenylene oxide porous po:Lymer developed by Akzo Research Laboratorle~, was mixed wlth 5.9 gm of Te~lo ~ 30. (34% suspension o~ Te~lo ~ micro-particles in water stabllized with 4~ Triton~ X 100), and enough extra water to ~orm a homogeneous paste. The paste was then formed lnto a sheet 0.50 mm thick by repeated pa~ses through a two-roll steel roll rubber mill set at 50C. The ma~erial was calendered ~rom a s~arting thlckness o~ 0.5 cm to a rinal thickness o~ 0.5 mm three times. The ~llm was ~olded after the ~lrst two times and inserted lnto the roller~ at a 90 angle. A ~el~-supportlng sheet was obtained.

~his example illustrates a PTFE-carbon corn-posite sheet containing an inert ~iller.
Sixty g o~ carbon (partlcle siæe 37 to 15 micrometers) were mlxed with 20 g o~ kaolln, an inorgan1c clay ~iller, and 20 g of ~e~lo ~ 30 solids.

Enough water was added to ~orm the solids into a . ~ , : , ~ -:: ~ . .
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- 2LI _ homogeneous paste. The paste was then forme~ into a sheet 0.5 mm thick by repeated passes through a steel roll rubber mlll se~ at 50C. ~he material was rolled ~rom a starting thickness o~ 0.5 cm to a ~inal thlckness o~
0.5 mm three times. ~he sheet was folded after the first two times and ~nser~ed into the rollers at a 90 angle. The resulting sheet was dried in a muf~le ~urnace under a nitrogen purge at 3~0C ~or twenty mlnutes to remove the water and Trito ~X 100 contained in the Te~lo ~ 30. The resultin~ sheet was pliable, strong, and ~ree Or any shedding.

EXAMPLE_9 This example illustrates a PTFE composlte sheet ror use as a selectlve barrier ln a mercury monltor to filter out inter~erring chlorine.
A PTFE-Zeollte composite sheet was prepared by uniformly mixing 40 grams Linde Mo~ 541 molecular sieveg 16.5 grams Teflon~ 30 (59.r~ so~lds) an~ 60 grams delonlzed water. The resulting homogeneous paste was calendered at 80C a~ in E~ample 1 to a 0.017 lnch (0.~3 mm) sheet, and the sheet wa~ air dired at room temperature, In view of the lntended use as a selective barrler, and the corresponding need ~or good porosity character~stlcs, the sheet porosity was measured uslng the Model 4200 Gurley H P S tester. A$ a dif~erential test pressure o~ 0.03 standard atmospheresg 16.5 seconds were required to p~ss 10 cublc centimeters of air through loO s~uare inch t6.45 cm23 o~ ~he sample. This .. ~ .

favorably compares with 35 Gurley seconds for Celanese Celgar ~ 2500 and lndicates that the porosity o~ khe PTFE-Zeolite composite sheet is acceptable.
Mext the sheet was saturated with excess sol-ution of 10~ N-phenyldi-isopropanolamine (Isonol~ ln ethanol and the non-entrained excess solutlon was strlpped away.
Arter drying, the cornbined Isonol~ Zeolite, ,B Teflo ~sheet was tested as a chlorine gas ~ilter as rollows. Ten part~ per million Or chlorine ga~ was clrculated ln a chamber on one slde of the test rllm.
A gold fllm detecting layer as described in U.S. Patent No. 3,950,980 was u~ed to de~ermlne the tlme to chlorlne breakthrough. The breakthrough time was determined by measuring the ¢hange in electrical resistance of the gold film~ and was found to be in exces3 of 90 minutes. This lndicates over 15 hours prokection at the chlorlne Threshold Limit Value o~ 1 part per million.
Consi~tent with the need ~or ~iltering chlorine while passing mercury vapor, the sheet was next mounted on the outer ~urface o~ the 3M Brand Mercury Vapor Monitor No. 3600 ~ust below and adJacent to the standard Celgar ~ arrier rllm used in the productO The monl~or wa~ exposed to 0,1 mg Hg/cubic meter air ~or 8 hours and the change in resistance of the gold ~ilm wa~ 7.3% as compared to 8~9~ ~or standar~ controls without the selective barrier. Thi~ shows the utillty o~ the ~ilm as a quantitative chlorine barrler which passes mercury vapor, ~,, . , . ~ , ~

:: ~ - :, ~ - : , ~.~ a ~ t~
_ 26 -This example illustrates a Melamine-Terlon composlte sheet for a selective barrier for flltering chlorine gas and passin~ mercury vapor.
An Aldrich 99% purity Melamlne aheet wa~
prepared by uni~ormly mixing 30 ~rams Te~lo ~ 2 (34.4~
sollds) with 40 gram~ particulate Melamlne and 5 grams deionlzed water. The materlal was processed as in ~xample 8 to a 0.017 inch (t).4~ mm) sheet.
The aurle~ ~orosity te~t of Example 1 showed the rllm poroslty to be an acce~table 75 ~urley seconds.
The chlorlne reslstance test o~ ~xample 8 showed the sheet blocks chlorine for over 295 mlnutes. The mercury vapor transmisslon was shown ~o be 26% o~ the standard by the procedure used in Example 8.

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Claims (7)

914,103 The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a device for measuring the amount of at least one selected component in a fluid mixture com-prising body members forming a shallow chamber having an open end, a collecting layer disposed within said chamber for collecting said selected component and at least one porous attenuating layer spaced apart from said collecting layer and disposed across the open end of said chamber, the improvement wherein said collecting layer comprises a porous polytetrafluoroethylene sheet containing an effect-ive amount of a particulate sorbent for said selected component.

2. The device according to claim 1 wherein said polytetrafluoroethylene sheet comprises 5 to 50 percent by weight polytetrafluoroethylene.

3. The device according to claim 2 wherein said polytetrafluoroethylene sheet comprises 50 to 95 percent by weight of said particulate sorbent.

4. The device according to claim 3 wherein said polytetrafluoroethylene sheet comprises 75 to 85 percent by weight of said particulate sorbent.

5. The device according to claim 1 wherein the average particle size of said sorbent is less than 45 micrometers.

6. The device according to claim 1 wherein said sorbent is selected from the group consisting of carbon, alumina and silica,

7. The device according to claim 1 wherein said polytetrafluoroethylene sheet further comprises up to about 50 percent by weight of a non-sorbent filler.