34 The Electrochemical Society Interface • Winter 2003

The last several years havebeen difficult for producers ofU.S. chlor-alkali chemicals.Low prices for chlorine andcaustic and escalating fuel

costs have resulted in several plant clo-sures and have squeezed the profitabil-ity of those that have remained. Forthose plants operating withdiaphragm cell technology, which stillcomprises a major portion of domesticproduction, conversion to membranecells is not a viable option.Replacement of diaphragm cells withmembrane cells entails abandonmentof existing highly efficient causticevaporator trains, provision for solidsalt to replace solution-mined salt, andsignificant brine treatment upgrades,along with writing off the originalinvestment in diaphragm cells.

In addition to the direct cost issues,there are indirect costs associated withdiaphragm cell production of chlorine.These issues center on the environ-mental health and safety concernsover asbestos, which is currently usedin diaphragm cells as a separator tokeep products from mixing. Althoughthe chlorine industry has an excellenttrack record of using asbestos safely,many businesses have been forced intobankruptcy due to high damageawards resulting from the past sale ofasbestos-containing products. A greatdeal of pressure exists to find alterna-tive diaphragm cell separators, if onlyto assure continuity of supply of rawmaterials.

PPG Industries has responded tothis difficult period by developing anew separator for its diaphragm cellsoperating at its plants at Natrium,West Virginia, and Lake Charles,Louisiana. The new separator isasbestos-free, energy efficient, anddurable. The new separator, which wehave named Tephram, is capable ofretrofit to existing diaphragm cellfacilities with minimal changes toexisting equipment and procedures.Full circuit operation has been under-

Responding to a HarshBusiness Environment:

A New Diaphragm for the Chlor-Alkali Industry

by Sajjad Ahmed and Peter Foller

way at a small circuit at our Natriumlocation for over 10 years with theTephram diaphragm. Just this year, PPGcompleted conversion of its largestdiaphragm cell area at Lake Charlesfrom asbestos to Tephram diaphragms.Currently, conversion of the Natriumplant’s largest circuit is underway. The

success seen so far indicates that thediaphragm will be adaptable to a vari-ety of cell designs. The cells to whichthe Tephram diaphragm has been suc-cessfully applied at PPG locationsinclude the Columbia N6, the GlanorV-1244, and the Oxytech MDC-55.

FIG. 1. Manufacture of pTFE Microfibrils.

FIG. 2. Typical pTFE Microfibrils.

T

The Electrochemical Society Interface • Winter 2003 35

The requirements for any new chlor-alkali diaphragm are very demanding.Here is a partial list:

• Resistance to chlorine and caustic• Separation of chlorine and hydrogen• Permeability sufficient to match the

desired brine feed• High conductivity• Resistance to brine upsets • Rapid recovery from outages• Minimal impact on established plant

procedures and existing capital• Long life

What is a TephramDiaphragm?

The Tephram diaphragm is a fluo-ropolymer-based separator comprised

of pTFE (polytetrafluoroethylene)microfibrils and a secondary, longer,pTFE fiber. The longer fiber, DuPont’sTeflon Floc pTFE fiber, offers good rein-forcement of the diaphragm but is toocoarse to be used as the primary fiber.The microfibrils are prepared by a PPG-patented process and comprise the bulkof the diaphragm material. Themicrofibrils are made by forcing anaqueous suspension of pTFE powderthrough an orifice at high pressure. Theprocess is depicted in Fig. 1, and theresulting microfibril product is shownin Fig. 2.

The microfibrils prepared by thisprocess become highly entangled in thedeposition process so that there is noneed for high temperature sintering toassure diaphragm integrity. The fact

that the diaphragm is an entangledmat instead of sintered is thought toplay an important part in avoidinghydrogen transfer from the catholyteto anolyte along Teflon pathways dur-ing cell operation, a problem com-monly associated with perfluorocar-bon based diaphragms. To assure thatthe diaphragm is wettable, DuPont’sNafion solution is used to coat thepTFE so that the normally hydropho-bic fibers are permanently wetted.

The pTFE microfibrils and TeflonFloc fiber components are mixed withother deposition aids in a water-basedslurry containing 2-4 wt.% suspendedsolids and vacuum deposited directlyonto the cell cathode. Both wovenscreen and perforated plate cathodescan be used. The diaphragm matis then dried at low temperature(< 100 °C) to remove water. The factthat drying steps are done at low tem-perature puts less stress on cell compo-nents. A topcoat is next applied byvacuum depositing inert inorganicfillers into the diaphragm to adjustpermeability as desired. The fillers typ-ically used are Attapulgite clay and zir-conium oxide. Once the topcoat isapplied, the diaphragm is dried and isready for assembly into a cell.

The steps required for Tephramdiaphragm manufacture are:

• High shear manufacture of pTFEmicrofibrils

• Mixing of aqueous slurry (filteraids, surfactants, viscosity modifi-er, etc.)

• Vacuum deposition of diaphragmonto cathode and drying

• Topcoat application (zirconiumoxide & Attapulgite clay)

• Drying• Cell assembly

Except for the microfibril prepara-tion and some mixing equipment, thediaphragm manufacture uses the sameequipment as asbestos diaphragms.This allows for easy transition fromasbestos to Tephram diaphragms. Thefinal Tephram diaphragm is typicallyabout 0.10 inch thick and has an areadensity of about 0.4 lb/ft2. A photo ofa completed Tephram diaphragm onone of 12 cathode elements of a bipo-lar Glanor V-1244 electrolyzer isshown as Fig. 3.

A fully assembled 25 Ton Cl2/dayGlanor V-1244 Electrolyzer ready tomove into place is shown as Fig. 4.Sixty four such electrolyzers were con-verted to Tephram diaphragms in theconversion of Lake Charles’ Plant C.

FIG. 3. Tephram Diaphragm on a Glanor V-1244 Electrolyzer Cathode.

FIG. 4. Assembled Glanor V-1144 Electrolyzer at Lake Charles “Plant C.”

36 The Electrochemical Society Interface • Winter 2003

Start-up andOperating Maintenance

of TephramDiaphragm Cells

Tephram diaphragms are started upand operated much like their asbestosdiaphragm counterparts. Thediaphragm is flushed with water toremove surfactants and to wet thediaphragm. This gives a low start-upvoltage and avoids excessive foamingfrom surfactant residue. Following thewater flush, the cell is filled with hotbrine and started at full operatingload. The cell is monitored closely dur-ing the first several hours of operationto judge diaphragm permeability. TheTephram diaphragm is initially morepermeable than required for highstrength caustic operation. This allowsfor the addition of materials to thebrine feed to adjust the permeability ofthe diaphragm to match circuitrequirements. The higher permeabilityalso gives a built in cushion for theoccasional brine upset, which typicallyadds an additional burden of magne-sium and calcium.

Materials used for permeabilitytrimming include primarilyAttapulgite clay and magnesiumhydroxide. The clay particles act by fil-tration and they swell within thediaphragm matrix. The magnesiumforms a gelatinous hydroxide precipi-tate upon contact with hydroxide dif-fusion or leakage from the catholyte.Careful attention to amounts of mate-rials added, flow rates, and pH allowfor permeability control with goodefficiency. Overdosing with magne-sium hydroxide can result in excessplugging and lower efficiency, just aswith asbestos. However, the magne-sium will generally be dissolved fromTephram diaphragms with continuedoperation, and the occasional circuitoutage, so that the efficiency loss isnot irreversible. Uneven magnesiumhydroxide precipitation is, neverthe-less, the most common cause of longerterm decline in efficiency withTephram diaphragms.

Operating Experiencewith Tephram

Diaphragm Cells

Operating results in the smallerColumbia N6 circuit at Natrium havebeen described earlier.1-4 Typically inthis smaller (160 T Cl2/day) circuit, thepower requirements were slightly bet-ter or equal to those obtained withasbestos diaphragms. The smaller cir-

cuit demonstrated the ruggedness ofthe Tephram diaphragm. Generallyspeaking, cell renewal replacementswere due to cell mechanical problems,such as mat leaks or gasket failures,before the diaphragm failed. Operatinglives as long as four years wereobtained. The ruggedness of thediaphragm has been further demon-strated by empirical evidence: the cir-cuit was mothballed for several monthswith no special precautions beingtaken, such as addition of reducingagents or cathodic protection, andrestarted with no unusual problems.

At Lake Charles’ 1,800 Ton Cl2/dayGlanor V-1244 Plant C circuit, Tephramdiaphragms have demonstrated both alonger life and better power efficiencythan asbestos diaphragms. Relativecomparisons of voltage and efficiencywith asbestos are shown in Figs. 5 and6. Absolute values for efficiency and

voltage are not shown in order to pro-tect company-confidential informa-tion. The data shown in the figures arefor a period where both types ofdiaphragms were still in operation sothat a same-time, same-circuit compari-son could be made.

The chlorine current efficiency val-ues were calculated using the standardOxy”6” equation which estimates effi-ciency from caustic concentration, oxy-gen in chlorine, and sodium chlorate incell liquor (see Eq. 1 above).

The results for the Lake Charles V-1244 electrolyzers show that the volt-age is slightly better and efficiency gen-erally about the same as for the asbestoscells. This gives a power consumptionadvantage to the Tephram diaphragmcells. It should be noted that the com-parison is conservative with respect tothe power consumption advantage.This is due to preferential removal of

FIG. 5. Glanor V-1244 Cell Voltage Comparison.

FIG. 6. Glanor V-1244 Cell Efficiency Comparison.

Eq. 1. The Calculation of Oxy “6” Current Efficiency for Chlorine.

The Electrochemical Society Interface • Winter 2003 37

the worst-performing shorter livedasbestos diaphragm cells while allTephram diaphragms were maintainedin operation as the conversion pro-ceeded. Also noteworthy is the sub-stantially longer life seen with theTephram diaphragm cells. Theasbestos cells have a life shorter thantwo years due to the degradation ofasbestos from load swings and brineupsets. These factors cause asbestosdiaphragms to become less permeableto brine flow. The Tephramdiaphragms are much more forgiving,because the fluorocarbon matrix is notchemically attacked as is asbestos.With load swings becoming morecommon as power costs and chlorinedemand fluctuate, the operating stabil-ity of Tephram diaphragms becomesan important advantage.

The average caustic strength andanodic byproduct performance of theLake Charles Plant C diaphragms islisted in Table I. The averages are for64 electrolyzers, each of which con-tains 12 bipolar cell units. The datawere taken shortly after the plant wasfully converted from asbestos toTephram diaphragms in 2003.

Conversion of the MDC-55 cell cir-cuit at Natrium, WV, has been started.The incentive for this location hasbeen the Tephram diaphragm’s dura-bility and recovery from load swingsand brine upsets. Of the circuit ofmonopolar MDC-55 cells, at this writ-ing approximately 15% have beenconverted to Tephram diaphragms.Because there is a large population ofasbestos diaphragm cells still in opera-tion, direct side-by-side comparisonsare possible. A plot of recent Tephramdiaphragm cell voltages is comparedwith the average voltage for asbestosdiaphragms during the same period inFig. 7.

A similar plot is given in Fig. 8 forOxy6 chlorine efficiency. The plotcompares efficiency for the two typesof diaphragms during the same operat-ing period.

With the MDC-55 cells, voltage andefficiency appear to be generally stableover the test period for the Tephramdiaphragms. Again, the asbestos time-line does not extend as far as theTephram diaphragm time-line due tothe shorter life of the asbestosdiaphragms.

The data in Table II are for a recentmeasurement of caustic strength andanodic by-product performance. Notethat hydrogen in chlorine is not anissue, being generally less for Tephram

Table I. Plant C Operating Data with TephramDiaphragms.

V-1244 Diaphragm NaOH, H2, O2,Electrolyzers Age, days gpl vol.% vol.%

Average..................764 ......................137 ....................0.12 ....................2.24

Table II. Natrium MDC-55 Cell Operating Data.

Diaphragm NaOH, H2, O2,Age, days gpl vol.% vol.%

TephramDiaphragms ............398 ....................126 ..................0.10....................1.87

AsbestosDiaphragms ............299 ....................141 ..................0.15....................2.15

FIG. 7. Natrium MDC-55 Cell Comparison of Voltage.

FIG. 8. Comparison of Asbestos and Tephram Diaphragms in Natrium MDC-55 Cells.

38 The Electrochemical Society Interface • Winter 2003

Table III. Comparison of Asbestos and TephramDiaphragms.

Life Asbestos Tephram

Life ......................................8-15 months ................................4 years

Ruggedness ..........................Easily plugged with ....................Recovers fromimpurities upsets

Responses to ........................Cycling destroys asbestos ............No long-termswings may go high level effect

Safety............................................Carcinogen, caustic used ....................No asbestos, non-in diaphragm preparation hazardous water-

base slurry

Materials Cost......................Low cost but uncertain supply ....Higher thanasbestos, securesupply

Electrical Power Use ..................................................................Lower orequivalent toasbestos

diaphragm cells than for the asbestosdiaphragm cells.

The power consumption for thecells with the two types of diaphragmare roughly equivalent, because volt-ages and efficiency values are veryclose (power is proportional to volt-age/efficiency). Even without a clearpower savings, operating advantagessuch as resistance to load swings andbrine upsets and longer diaphragm lifehave led to the decision to fully con-vert to the Tephram diaphragm inPPG’s MDC-55 circuit.

The current status of the conver-sion of PPG’s diaphragm circuits toTephram diaphragms is shown in Fig.9. At this time, over half of PPG’sdiaphragm cell chlorine capacity uti-lizes the Tephram diaphragm.

A summary comparison of PPG’sexperience with the Tephramdiaphragm is given as Table III. Theseconclusions were developed throughmore than a decade of R&D. Table IV,the U.S. patents for the Tephramdiaphragm, shows the program’s ori-gins in the 1980s.

PPG is well on the way to becomingasbestos-free at its diaphragm plantfacilities. This is being achieved undervery difficult business conditions forthe chlor-alkali industry. The move tothe non-asbestos diaphragm is beingdriven by two factors, power savingsand the operating flexibility to tolerateload swings to match productionneeds. This does not imply that loadswings have no impact on theTephram diaphragm, only that it willnot be destroyed, as will asbestos, and

FIG. 9. PPG Tephram Diaphragm Chlorine Production by Cell Type.

Patent # Inventor Title Year

6,299,939..............DuBois, et al. ................Method of Preparing a Diaphragm for an Electrolytic Cell ..................................2001

6,296,745..............DuBois, et al. ................Method of Operating Chlor-alkali Electrolytic Cells ............................................2001

6,059,944..............DuBois, et al. ................Diaphragm for Electrolytic Cell ..........................................................................2000

5,683,749..............DuBois, et al. ................Method of Preparing Asbestos-free Chlor-alkali Diaphragm ................................1997

5,630,930..............Maloney ......................Method of Starting a Chlor-alkali Diaphragm Cell..............................................1997

5,612,089..............Dilmore, et al. ..............Method of Preparing Diaphragm for use in Chlor-alkali Cells ............................1997

5,567,298..............DuBois, et al. ................Method of Operating Chlor-alkali Cells ..............................................................1996

5,192,401..............DuBois, et al. ................Diaphragm for Use in Chlor-alkali Cells ............................................................1993

5,188,712..............Dilmore, et al. ..............Diaphragm for Use in Chlor-alkali Cells ............................................................1993

5,030,403..............Pickens, et al.................Method of Making Polymeric Fibrils....................................................................1991

4,720,334..............DuBois, et al.. ..............Diaphragm for Electrolytic Cell ..........................................................................1988

4,680,101..............Darlington, et al.. ........Electrolyte Permeable Diaphragm including a Polymeric Metal Oxide ................1987

4,666,573..............DuBois, et al.. ..............Synthetic Diaphragm and Process of Use Thereof ................................................1987

Table IV. Tephram Diaphram Technology is protected by the following U.S.Patents.

The Electrochemical Society Interface • Winter 2003 39

that eventual recovery is possible. Thelonger life of the Tephram diaphragmallows for savings in cell renewal mate-rials and labor, since fewer cell turn-arounds are required. These savingsmore than offset the higher costs thatare unavoidably associated with fluoro-carbon materials. �

Acknowledgments

Teflon, Teflon Floc, and Nafion areregistered trademarks of DuPont.Tephram is a registered trademark ofPPG Industries, Inc.

The authors would like to acknowl-edge the contributions of many PPG co-workers: at the Chemicals GroupTechnical Center: C. Dilmore (ret.); D.DuBois (ret.); A. Maloney (ret.); J.Kinney (ret.); S. Pickens; J. Snodgrass

(ret.); M. Schmidt; and H. Schussler; atNatrium: D. Bush; J. Maxwell; C. Hill;and R. Toumala; at Lake Charles: J.Cimini; T. Jeffery (ret.); S. Richardson; J.Hutchins (ret.); and others.

References

1. C. Dilmore and D. DuBois, PPG’s TephramSynthetic Diaphragm Circuit Conversion;Modern Chlor-Alkali Technology, 6, 133(1995).

2. P. Foller and D. DuBois, PPG’s Non-AsbestosDiaphragm for the Chlor-Alkali Industry; TheChlorine Institute, Inc., 72nd AnnualMeeting, Washington, DC, March 27,1996.

3. P. Foller and D. DuBois, PPG TechnologyJournal, 3(1), 49 (1995).

4. P. Foller, D. DuBois, and J. Hutchins, PPG’sNon-Asbestos Diaphragm, SCI Symposium,London, June 1997.

About the Authors

Sajjad Ahmed is an Associate Director of R&D at PPG Industries and has ledR&D on behalf of PPG’s Chlor-Alkali and Derivatives business unit since 2000. Heholds two masters degrees, in chemical engineering from the University of Waterlooand in industrial engineering from the University of Pittsburgh. He came to PPGfrom Bechtel and Shell (SADAF) and has a wealth of major project experience. Mr.Ahmed may be reached by e-mail at sahmed@ppg.com.

Peter Foller is an Associate Director of R&D at PPG Industries and has led R&Don behalf of PPG’s Chlor-Alkali and Derivatives business unit and, since 2000, hehas led R&D on behalf of Transitions Optical, PPG’s joint venture in photochromicophthalmic lenses with Essilor International of France. He has been an active mem-ber of ECS since his graduate school days with the late Prof. C. W. Tobias at theUniversity of California at Berkeley. Dr. Foller may be reached by e-mail atfoller@ppg.com.