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Desiccant Selection For Maximizing Argon Concentration In IG Units by David C. Darwin, Ph.D., and Gregory R. Schoofs USGOPENINGS FEATURE Introduction T he drive toward lower U- values has created rapid growth in the production of argon-filled insulating glass (IG) units. Desiccant choice and usage can have a substantial impact on the argon concentration and the life of argon-filled IG units. To fab- ricate high quality argon-filled IG units, there are compelling reasons why we believe a blend of 3A molecular sieve/silica gel should be used in as many sides as practical. Desiccants which contain large pore molecular sieves (com- monly referred to as 4A and 13X), either alone or in blends, are detri- mental and should be avoided. The desiccant used in argon-filled IG units should have the following characteristics: "Desiccant choice and usage'can have a substantial impact on the argon concen- tration and the life of argon- filled 10units. If 1. The desiccant must adsorb water, thereby protecting the IG unit against moisture fogging; 2. The desiccant must adsorb hydrocarbons (and other volatile species), thereby protecting the IG unit against chemical fogging and agai~st staining coated glass; 3. The desiccant must not adsorb and desorb argon or nitrogen, thereby minimizing glass deflec- tion and seal stress; and 4. The desiccant should not contain pre-sorbed (i.e., previously adsorbed and held) nitrogen, thereby eliminating the outgassing of nitrogen into the sealed IG unit which, in turn, dilutes the argon concentration. Historically, silica gel, 3A molecular sieve, 4A molecular sieve, 13X molecu- lar sieve, and their blends have been used for desiccation of IG units. Let's take a look at these desiccants with respect to the four characteristics. Water Adsorption Capacity Of rGDesiccants It is generally recognized that all of the aforementioned molecular sieves strongly adsorb water. Of these desic- cants, 4A molecular sieve is the least expensive to manufacture; silica gel is the most expensive. Hydrocarbon Adsorption CapacityOf IG Desiccants Hydrocarbons (and other volatile species) are a common impurity in IG units. Sources of hydrocarbons include: 1. Painted metal muntins or spacers'; 2. Vinyl or plastic internal muntins, spacers, or keys which outgas hydrocarbons when exposed to heat or ultraviolet light (UV); 3. Chemicals used to clean or wipe down muntins, spacers, keys, etc.; 4. Cutting and machine lubricants; 5. "Touch up" paint used to repair marked or scratched muntins or spacers; and 6. Sealant systems containing volatiles. Types 3A and 4A molecular sieves have no capacity for hydrocarbons because their pore sizes are too small.2 This leaves 13X and silica gel to be considered for hydrocarbon removal. continued on page 2 @1999 USG/ass magazine. All rights reserved from Key Communications, Inc., GarrisonvilIe, VA. Reprinted with permission.
Transcript

DesiccantSelectionFor Maximizing ArgonConcentration InIG Unitsby David C. Darwin, Ph.D.,and Gregory R. Schoofs

USGOPENINGS FEATURE

Introduction

The drive toward lower U-values has created rapidgrowth in the productionof argon-filled insulatingglass (IG) units. Desiccant

choice and usage can have a substantialimpact on the argon concentration andthe life of argon-filled IG units. To fab-ricate high quality argon-filled IG units,there are compelling reasons why webelieve a blend of 3A molecularsieve/silica gel should be used in asmany sides as practical. Desiccants

whichcontainlarge poremolecularsieves(com-monlyreferred toas 4A and1 3 X ) ,eitheralone orin blends,are detri-mentalandshould beavoided.

The desiccant used in argon-filled IGunits should have the followingcharacteristics:

"Desiccantchoice and

usage'can havea substantial

impact on theargon concen-tration and thelife of argon-

filled 10units. If

1. The desiccantmust adsorbwater, therebyprotecting theIG unit againstmoisture fogging;

2. The desiccant must adsorbhydrocarbons (and other volatilespecies), thereby protecting the IGunit against chemical fogging andagai~st staining coated glass;

3. The desiccant must not adsorb

and desorb argon or nitrogen,thereby minimizing glass deflec-tion and seal stress; and

4. The desiccant should not contain

pre-sorbed (i.e., previouslyadsorbed and held) nitrogen,thereby eliminating the outgassingof nitrogen into the sealed IG unitwhich, in turn, dilutes the argonconcentration.

Historically, silica gel, 3A molecularsieve, 4A molecular sieve, 13X molecu-lar sieve, and their blends have beenused for desiccation of IG units. Let'stake a look at these desiccants withrespect to the four characteristics.

Water AdsorptionCapacityOf rGDesiccants

It is generally recognized that all ofthe aforementioned molecular sievesstrongly adsorb water. Of these desic-cants, 4A molecular sieve is the least

expensive to manufacture; silica gel isthe most expensive.

HydrocarbonAdsorptionCapacityOf IG Desiccants

Hydrocarbons (and other volatilespecies) are a common impurity in IGunits. Sources of hydrocarbons include:1. Painted metal muntins or

spacers';2. Vinyl or plastic internal muntins,

spacers, or keys which outgashydrocarbons when exposed toheat or ultraviolet light (UV);

3. Chemicals used to clean or wipedown muntins, spacers, keys, etc.;

4. Cutting and machine lubricants;5. "Touch up" paint used to repair

marked or scratched muntins orspacers; and

6. Sealant systems containingvolatiles.

Types 3A and 4A molecular sieveshave no capacity for hydrocarbonsbecause their pore sizes are too small.2This leaves 13X and silica gel to beconsidered for hydrocarbon removal.

continued on page 2

@1999 USG/ass magazine. All rights reserved from Key Communications, Inc., GarrisonvilIe, VA. Reprinted with permission.

DesiccantSelection

"Molecularsieveshaveanextremelyhighadsorptive

attractionforwater.II(

continued from page 1

It has long been recognized that thehydrocarbon capacity of type 13X mol-ecular sieve is dramatically reduced ifwater is present. Although the pores oropenings of 13X are large enough toadmit hydrocarbons, the preferentialadsorption of water severely restrictsthe ability of 13X to adsorb and holdhydrocarbons. As correctly stated by amajor desiccant manufacturer, "molec-ular sieves have an extremely highadsorptive attraction for water. Thisaffinity is so strong that water will nor-mally displace any other material that isalready adsorbed on the molecularsieves.,,3

W~

zenwo::8:30::::1301-0-wz.JIL.O0==m)(WM:1:....

~ ~ 20OW-0IX3Z::1::10000::~~ 1000Zw~1X3mO::omo«

One might hope that blends of 3Aand 13X would reduce the affinity of13X for water, but this is not the case.Data from molecular sieve suppliersclearly show that the attraction forwater on 3A, 4A, and 13X is essentiallythe same. Thus, if any water is presentin an IG unit containing a blend of 3Aand 13X, the water will be essentiallyequally adsorbed and distributed onboth molecular sieves. As the 13X mol-ecular sieve adsorbs water, both itsattraction and its capacity for hydrocar-bons decrease. As water continues to beadsorbed on 13X molecular sieve, pre-viously adsorbed hydrocarbons will be

outgassed into the"airspace."

Hydrocarbon andmoisture moleculesare, literally, like oiland water. Becausethey are fundamen-tally different mole-cules, a blend offundamentally dif-ferent adsorbentswith totally differ-ent properties is bestsuited for removingthe hydrocarbonsand moisture from

the "airspace." Asdescribed in U.S.Patent No.4,144,196 ("Abso-rbent for Use inDouble GlazedWindows"), a blendof 3A molecularsieve and silica gelis ideal. This blendis non-separatingbecause the densi-

40

ADSORPTION OF NITROGEN ON13X MOLECULAR SIEVE BEADS

ISOBAR: AIR AT SEA LEVEL

o.20 o 20 40 60 80 100 120

TEMPERATUREIN DEGREESF

FIGURE 1: Adsorption of nitrogen on fully activated 13X mole-cular sieve beads; the isobar is for air at sea level. Data arereplotted from those originally reported In reference four.

ties of both components areessentially the same. The 3A molecularsieve selectively and preferentiallyadsorbs water, leaving the silica geltotally active and free to adsorb hydro-carbons without any interference fromwater.

Need For LowDeflection Desiccants

Day to night temperature changescause an IG unit to behave like anaccordion. In accord with the "IdealGas Law," the pressure within a sealedIG unit increases whenever the temper-ature increases. This is like the pressureincreasing inside an automobile tirewhenever it heats up. The Ideal GasLaw also mandates that the pressurewithin an automobile tire or a sealed IG

unit decreases whenever the tempera-ture decreases.

Nitrogen and argon adsorption anddesorption from 13X and 4A molecularsieves as the temperature changesamplify the accordion effect. As shownin figure one, 13X molecular sieves, anddesiccant blends which contain 13X,adsorb nitrogen whenever the tempera-ture decreases and desorb ("outgas")nitrogen whenever the temperatureincreases. Virtually identical behavioroccurs with 4A molecular sieve.4Similar, though less pronounced,behavior also occurs on 13X and 4A ifargon is present instead of nitrogen.5Desorption, which occurs as the tem-perature increases, means that nitrogenor argon molecules are outgassed fromthe desiccant; this increases the numberof molecules in the "airspace," which,in turn, causes the pressure to increase.Similarly, adsorption of argon or nitro-gen at low temperatures removes

moleculesfrom the "air-

space,"therebydecreasing thepressure.

The pressurechanges result-ing from theadsorption/des-

orption of argon ornitrogen amplify the

pressure changes above and beyondthose mandated by the Ideal Gas Law.Glass deflections which occur unavoid-ably with any use of 13X or 4A molec-ular sieves:1. Distort reflected images;2. Harm the V-value of the IG unit;3. Stress or destroy the seal; and4. Reduce the life of argon-filled IG

units.A blend of 3A molecular sieve and

silica gel, which neither adsorbs noroutgasses nitrogen, oxygen, or argon, isthe obvious solution to this problem.6

FIGURE2: Maximum argon concentration In an IG unit with dimensions of two-foot bythree-foot by fl- inch whose "airspace" is argon-filled to an Initial level of 100 percent,the best case scenario. The muntin grid Is two long and two short spans. Blends ofvarious molecular sieves and silica gel are shown as 80 percent of the first listed com-ponent and 20 percent of the second, where SG denotes "silica gel."

Maximizing ArgonConcentration TnIGUnits

To achieve lower U-values by utiliz-ing argon gas, manufacturers want toreplace as much of the ambient air in IGunits as possible with argon gas. Toattain an argon fill level of 100 percent,all air, which is 79 percent nitrogen,must be eliminated from the "airspace."In the common practice of "lance fill-ing" argon through one or two holes inthe conventional IG spacer, the follow-ing factors limit the argon concentrationto less than 100 percent:1. Pre-sorbed nitrogen brought into

an IG unit on 13X or 4A molecularsieves;

2. Air in the free space between des-iccant particles and in the macrop-ores of the particles;

3. Air inside emptyspacers; and

4. Air inside hollowmuntins.

Large pore molec-ular sieves, such as13X, 4A, and desic-cant blends contain-ing 13X or 4A, readily

pre-sorb nitrogen from ambient air priorto fabricating and sealing an IG unit.4At sea level and 65F, every ounce offully-activated 13X has pre-sorbedapproximately 12.5 cubic inches ofnitrogen gas (see figure one). Type 4Aholds a nearly equal amount of nitro-gen.4 This translates to 5.5 spacer vol-umes of nitrogen for each spacer vol-ume filled with 13X or 4A molecularsieve. Additionally, the volume of freeair between the particles and in themacropores of the particles correspondsto approximately 0.54 spacer volumes,for a total of 6.04 volumes of nitrogenplus air for each spacer volume filledwith 13X or 4A.

A similar, though somewhat less pro-

nounced, effect occurs with any blendcontaining 4A or 13X molecular sieves.For example, a molecular sieve blendcontaining 20 percent 13X has pre-sorbed approximately 1.1 spacer vol-umes of nitrogen for each spacer vol-ume filled with a 20 percent 13Xmolecular sieve blend. Add to this thevolume of free air between the particles,and the total becomes approximately1.64 spacer volumes of nitrogen plus airfor each spacer volume filled with a 20percent 13X molecular sieve blend.

Type 13X, Type 4A, and molecularsieve blends containing 13X or 4A, willoutgas some of the pre-sorbed nitrogeninto the "airspace" as they equilibratewith the argon fill.5 Additional pre-sorbed nitrogen will be outgassed as the13X or 4A preferentially picks up mois-

ture or hydrocarbons from the interiorof argon-filled IG units and as thedesiccant increases in temperatureduring normal daytime heatcycles.3,4,5 Nearly all of the pre-sorbed nitrogen will eventually

outgas and accumulate in the

continued on page 4

DesiccantSelectioncontinued from page 3

"airspace." Nitrogen outgassed from13X and 4A molecular sieves:1. Dilutes the argon concentration;

and

2. Can create an over-pressure condi-tion in IG units, which causes glassdeflection and the associatedproblems.In contrast, there is virtually no pre-

sorbed nitrogen introduced into an IGunit in a 3A sieve due to the pore sizewhich prevents nitrogen adsorption; norin a silica gel which does not have anaffinity for the adsorption of nitrogen orargon.

Argon concentration can be dilutedfurther by air initially present in spacersand hollow muntins. Argon displace-ment of air from spacers, whetherempty or desiccant-filled, and hollowmuntins is an inefficient processbecause it takes a longer time for the airto be displaced and diffuse out of thespacers and muntins than the time typi-cally allocated to argon fill the "air-

---

space."As an example of

the dilution ofargon by air fromdesiccant, spacers,and muntins, weconsidered a two-

foot by three-foot by1/2-inch IG unit whose

"airspace" is argon-filled to an initial levelof 100 percent, thebest case scenario.Figure two comparesthe actual argon concen-tration following nitrogenand air dilution from the fourfactors outlined above. A minimum ini-tial argon fill (Le., concentration) of 90percent has been instituted as a certifi-cation standard by at least one NorthAmerican certification program.7Figure two shows that it is difficult tomeet this certification standard if the IGunit contains 4A or 13X molecularsieves, either alone or in blends.

Filling as many sides as practicalwith a 3A molecular sieve/silica gelblend provides a simple and inexpen-sive way to minimize argon dilution inIG units because:1. There is essentially no pre-sorbed

nitrogen to release into the "air-space;" and

2. The blend itself occupies volumein the spacers that would other-wise contain air not readily dis-placed during argon filling.

ConclusionBetter performance and

longer life of argon-filled IGunits can easily be obtainedwith proper desiccantselection and usage. Webelieve a blend of 3Amolecular sieve and silicagel to be unmatched with

its combination of highmoisture and hydrocarbon

capacity, minimum argonadsorption/ desorption,negligible amount ofpre-sorbed nitrogen,and minimum glassdeflection. USG

"Better performance

and longer life of

argon-filled 10units

can easily be obtained

with proper desiccant

selectionand usa~

References1. Juergen H. Braun and Daryl P.

Cobranchi, Journal of CoatingsTechnology, December 1995,Volume 67, Number 851, pages 55-62.

2. Donald W. Breck, "ZeoliteMolecular Sieves", John Wiley &Sons, 1973.

3. Union Carbide Corporation techni-calliterature, "DRY GAS?".

4. Donald Peterson, Zeolites, July1981, Volume 1, pages 105-112.

5. George W. Miller, AIChESymposium Series, 1987, Volume83, Number 259, pages 28-39.

6. U.S. Patent No. 4,144,196.7. IGMAC Certification Criteria.

David C. Darwin, Ph.D. is technicalservice manager-adsorbents forGrace Davison in Baltimore, MD.Gregory R. Schoofs is president ofSchoofs Inc., in Morage, CA.


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