Physicll Properties of Foam for Protecting Plants Against Cold Weather

Harry J. Rraud and Jerry L. Chesness hlertam ASAE &fntsen ASAE

A spray-on layer of lightweight foam lu serve as a thermal insol;ltor for

plants 11% been su gested as an ideal mld protection metfod. The requisites of r loam for insulation are easy to l>re.define: low cost, easy generatinn, non-tnxir, sufficient stability to main- tain ;tt!qaste corer overnight in freez- ing tempernturcs, and self-destructinn when temperatures rise nbove the dam- aging level.

The success of foam as a f r eae pro- ; tnzioll ngnt depends on its thermal ; insulating capacity in a destructive en- 1 \im~~rnent. The most urgent problem I to bc solved before relinble pl;mt p m : tenion n n be achieved is to formulate fmm u.it11 stability to resist wind and

-d&erieriornting effects of freezing t m - peclmrer.

The tl~ickoess oE a foam layer re- uired for crop protection depends on

$c low tcmpcrnturc cxpectations, thc themill energy stored in the soil and the insulating ability of the foam layer. Tl~~nnal prulwrtirs of foam pertinent to heat flow have not been reported. Tl~e thermal conductivity, density and rl,?cil;c Leal are all pertinent quantities nccdc~l in heat flow c;llc~dations.

The rpcib oljectives set forth for this study were:

fa) To formulate foam solutions ul~klj produce Lighlweight foam with o\rmigl>t stability. A 12-hr foam life was set a a goal.

(h) To develop a foam generator . e.lpl)le of producing foam ei th the

desired expatision and in quantities st~ffiriel~t for laboratory analyses and field plot coverage. Foam propertics a:td method of generation are intimately rPI2tmI.

( c ) To determine the physical prop- erties of foam and their change witll tilne. Of interest are the propelties of dr~aity, yield, thermal cuntlnctivity. slwific heat and volume change with time. k u s e of the phenomenon of drai~r~ge .ad slnspected change in foam pmperties uith time, the per t inent properties would have to 1x evaluated

- . -.

NOMENCLATURE .~ -

~~ --

Symlrxl Unit. X""?. Dor.ri*iort

P I I h wt -8 It F~~nnl drnrlty WJLrr

%'I Ih Faun, wd&l Wri,qht of &-n m l ~ m e

v~ 11, Fnaton rlslilmnv Volumr. brzh19 iclmmted foam

WI lb WaIw wel~ht \%'eight. known vnluntc

w. Ih Solute airhr U'righl. mu- volume

v" Ih Solute ""ltmr xnn- *"I-

7 Ill Air rolumc unkmun *du-

!h p s lh Solute mnrmrmtion "Strollth of SoluIilm." W,/Wy E eu R p r lh Foam ~rpanninn Volumo/unit wt of SoluHm. = 1,

m R per Ib Foam 9cld volunr/"nit vtigbt Of nlvte PI

Yw

y. cu It ncr cu It Fmm yield \ 'ol~mrlimil rnlvnn. 01 rolvts Ih cr It SSdutr drnrily W/.V,

XI. q. x:, Solute i n p d i c n l s Foam .&. .lahiliren X I + Xp + X:, = 1

CI. C2. C3 $ a lb Cart. Cort p r llait anlpht v cu n w F~~~ va~amc ~ c ~ d . rmm hand

dr*h and width w In. Falm band width D ~ r n l b on plnntim! wrm d In. Foam band depth Varier with orotenion level required s n ROW spr ing

=,,. 8 p a F ~ n r m* prr nrrc 1) $ per 'mein. Poll" rmt P" nnc i.,. F,,r ruw oop rn""..C

Bhl pn lb dec F swcinr hrot 2 Btu m h r It dcg F 'rhnms1 mdsc , i r ( ty

C. t per Ih Solute mlt C, = X I C, + X2 C2 + Xi CI

continuously. A foam layer would suf- fer degradation in a hostile environ- ment.

The sequel to this study, field ex- periments with foam for protecting strawberry plants, is reported by Ches- nrss and nrsud (5).

THE NATURE OF FOAM Foams are inherently unstable sys-

tems. Freshly generated foam consists of Itluid inlerf:ccer surro~mding small pockets of gas. The viscous nature of a rich foam mix mmposed of small hub- blcs gives it appearance and properlies like a solid material even though its constituents arc in the liquid and gase- ous state. Shortly after generation, ex- L ~ S liquid in the habble wl l s begins to drain at n rate dependent on the constituents, tcmperatore and si7a of the buhbles.

Foam Drainage

For A t i o n s which are lnnsformed to a froth bv adtation. aeration. or

the solution and air which occurs i n the genemting device and is, therefore, un arbitrary measure unless related to foam viscobity and density.

Jacobi ct nl. (12) and Bikerman (2) note that drainage is caused by both loss of excess free liquid in the bubble walls ilud the effluent from hunting bubbles. Differentintion between the two is not a simple matter.

The rate at which the liquid drains can be measured easily using volu- metric devices. A method of char- acterizing foam drainage is based on time required for one fr~urth thp liqei<l volume to drain. The quarter drainage time is measured bv placing a pan of freshly generated f a ~ m on a slightly t i l t~d shelf and mllerting the liqui~l drainage.

Evaporation of liquid from bubble walls can cause loss of foam weight. Tts mngnihlde would be significant in a windy, dry atmosphere. For foam ex- posed to conditions excellent for mass transfer of moisture to the environ-

2 - ram x". en-%I3 vs prnmtrd at thr. W i n - other meam, the rate at which the froth metlt, foam loss due to mass loss might

IW : n r t i h ~ of the Amnicsn Secinv or ~rrieal- ,,,,,I s,,;.,,, in ~ \ i ~ ~ ~ . III;~~;., ~~~~b~ returns to liquid can lle expressed with he more destructive than weakening I!n*l. w n n m *rnnf .d h~ the S t r ~ ~ r t ~ ~ r * a decay, function. It is asnuned that duc to drainage. Drainage cnuses 11 and E".irn""lrnt Ui\<.i0,,. rn t l , , t h c , m - ~ ~ ~ ~ ~ J. BRAUD n n ~ JERRY the dramage from the froth will be loss of liquid and is acmmpanied hy ~,2;l~f~~;~;;";0;;;;~,i~2;p;;;;2;7 2;: complete and the solution will even- bursting of some bubbles but tbe loss in..l~ st.,,. uniser.ity mton ~n~~ ,c . . and ,,.. toi~lly return to liquid. Fonm dninngr: of bnhble stnlctu~re dws nnt ner~ssarily . 1 nyri;lllturrl nngi~~rcriuz Je- ~~.~tennmt. u n i n l 4 i l ~ 01 ~ m r ~ i ~ , nthen.. is affected by the degree of miring of occur as fast as drninage. Persistenrp of

lgiO . TRASSACTIONS nr m e ASAE

-- - -

foam will tlot go lt;~nd-in-lmnd a i t l ~ prior to miring with water and sulse- 170~oimi qualitv is greatly alT~rted 5 time of wtclltia-~ 01 liquicl it, the f c ~ ~ t 1 1 C I I I C ~ ~ i n ~ ~ r m r a t i o ~ ~ of xis. Let the cast eerrer;ttiorl mrthorl. Sevsrnl wries t .. a

&r unit &ight of the threc ;<gents l,c roam generators w e e 1,uilt and teW C,, t,, C,. The weight fractions of Hecause of tlle i11tim:de relations b rnch X,. X, and X., add to unity. .so t w e e ~ ~ ~ronerties of faan and the rnel .~ -~ ,

Foam Density and Expnnni,,,, ~ l ~ e the unit mst of the solute od of gen;ratiun, the study required. density of freslnlr g e ~ ~ + . n , ~ ~ ~ l lo.,,,, is C. - X, C, + xI C, + y, c;, concurrent effort to devise n methq e~sily me;aol-erl I y c i s tc l~i~l~ .t l i l r ~ c l of generating the foam and at the ~ilr

conbuine~ full i,nd i t l lc The utlit cust of fclam, C,, foulld by tilne to tire foam; p ~ l r ~ i weight of the knowr~ vnl~snne. Esp.tn- rli\.i<ling the unit cmt of solute C. hy pr,,pefiies. sion, E, is the reripror.~l of fu.tm t l~e yield YV is A program of foam testing was v sity it1 the mehic units mrl tlar I.PI.~~>I.O- c .- Cs up in an attempt to lormulate mia c:tl of specific gravity in Ernglirl~ nlcitr. I -

Y" \r.hich would: ( a ) Pre ldu~~ fmtm \ti(

1 1 :I re;lxnvablc ntnount of energy b E - - = -

*. - Thermal Conductivitv ~>arted bv witatin11 and ;or turbulo p, S.G.

Consider the volntne lnd tv~igllt n.1,~- tion in the foam genelutiotb l>rocrr.i. We stalt with s knnwn \alulne of solute, V,, which is added to a kn~o\\-n vol~~me of water. V,. An nnkvtown amount of air, V., is incorpo~~tetl into the soluti~rn to produce ;I r o ln~~ le of foam, V,.

v, = V" + v, + V,, Foam exptnsion. E = VriW,. the MI- ume of foam produced per unit \ vc i~ l~ l of solution depends nrl the mnount of air incorprntcd into the solutiotl is, the mixing process. If the foam-mnking equipment p~.oduces exmllcnt dispel- kion of the ;air into very small bttl~hlrs. a smnll sample of foam a n be weiglnrrl to reveal foitm densitv, 0,. The to;ml

The t11enn:tl ~orldt~ctirity of fotm nrrnpris~d of il semi-stahle huhble-like structure is not unchanging with time. Hi~te and degree of foam drainage aIol>g nith liquid removed by evaporation rvonld be the primary factors causing cllilt~gr in strucNre. These factors not- \\-ithstnnding it should be rclxtively r.m! to define the t h m a l conductivity i l l tcnns of a minimal rate and rnagni- tudr of change expected with time. 'I ltr simplest and most straightforward :11'proac11 to evaluating the thennal mn- cluctivity of itnr matmkil is to physi- I;JI!. n~odcl a one-dimensional stady- state hott mnduction sihlation. This was thv approach tsken in this study.

, ~, ;nixing: (b) Ilaw adequate rxl,ansii for ecanomy and ( c ) have sh\v drat age and stability to withst;rnd simuliie cold \ventlier exposure in a wind tulln test.

A specially constructed wind tuna wWi~$ used to produce temperature am wind conclitions similar to what wad I,e expected during cold weatl~er car ditions in the field. A heat Row trai! rlucer built into the wind tunnel flm was used ns the primary mearuring di rice to ascertsin the thelmal condu tivit)r of thc foam 1:lyer and its rha~!! wit11 time. Persistence af the foam I~I!? w;a measured lry recording the suln dcnce of the foam layer over :r y i i m of 1.2 hr evpusure to conlrolled envua mental conditions in the mnnel.

1 .~ . .

density by definition Persistence is the foam property of F~~~ ti^,,

w, p, - - Vf

Feum Yield It is of interest to kt~cnv the volume of fcam pnrdl~xd per unit weight of t h e solute. From defiai- tiuns above:

'+', _ w. + w, P f = - - - v, V' + v, + v.

Foam yield. Y,, depends on expansion and concentration of salute.

v 1 - y =-= 3;; -- , W" prW" Pr

+ - 1 -- - w, - (1 +-) = W" PI w,

1 1 1 - (1 + ) = E ( 1 + -) PC F F

Where F, the concmtration of the solute on a weight basis, is defined by the relatiooship F = W.;W,. Foam yield on volume biais. Y,., the foam volume per unit vo~urna uf solute.

r a

Since F is a dirnenrionlcss ratio, the units of yield Y, and Y, are the re- cipwal units used fur p, und p..

Foam Cast Foam solutions are made xvith ;I solute, either liquid or dissolved solid agents in ;\n aclrlmus solution. A~sumr that the agents itre premixed

2

prim~~l-v interesl and least reported in t ie i t t . Fomi persistence is the mrinsare of foam \,olume retention with time. There seems to be nn st;mrLud- iwd nletllud for measu~ing. The de- rrP:me in tl~ickeers of a foam layer with time cin~ l ~ e used as a measure. This rnetl~wl i, re;tlistic where a layer of foam ivauld l,e placed over low grow- ing plants or over bare soil and thus ex~osed to winrl cumnts.

Pmistence \dues are meaningful only when the physical conditions of the envilornnent <,iausing the destruc tion alr given. Mapr elements of de- struction ise wind forces. dust in at- mosphere. iind ellrrts of changes in tem- perature. F;tdol-s of destruction inher- cct 10 the mix are drainage. chemical chilagem wit11 time, and h;ological de- gratldtion.

IS\~ES.~II:ATIO~.

It wils initially nssomerl that mm- mercial fire-fiehtine faun @enerators

T l ~ e mechanical function of a fm generator is essentially to mix the sdi tiun with air it, such a manner tlil

large numbers of small bubbles arc pu. duced. In all. five foam generata were hnilt. The velucily nf air flm through the screens wils an importur variable of the gener~ttio~~ p r w m an m~ttrol of it was the cnitiral factor. 'Ih final design for velocity mntrol ;an prolmrtioning wi~s a mne-slri~pcd grt enttor.

The generator consists of hvo car with the mouths facing each other an mnultted with the axes in u vertk direction, s w Fig. I. A flexillle fair hose attaches to the outlet of the ul per cone tc direct the foam to the pir uf application. Compressed air sen! as the source of energy for meatit foam and provides the air for ilxn poration into b~thhles. Air at 40 p Hnws upward fur injertiun into :I rr turi section. The pressure drop in 11 venlttri section is sufficient to suck i

0 - ~~~~~

could be use; forvproduciug the dc- thc salntion. sired foam and tllat fire foam mncen- As the air iet rushes thmuell 11, trates ruuld he used for plant-inrulat- ing foam. Early exposure tests of foam samples subjected lu simulated mld weather ma~~litions in a wind t~nlnel showed that greater foam stability would be requirerl far uvcrnight ex- posure to ntmosl~heric conditioni t h ~ n is retluilpd for fire-fighting.

venturi section. violent agilation w?: IIIP foilm sr,lntion a~uscs vely smi bul~bles to form. As the air-liquid-fr~;n mixture moves upward into the m l r the \mi~llcr hohhlcs pass through th screen rnesh. Large liquid droplets N I x ~ k thl the walls of the loser cm al~rl drilin into the rutranee of tln

TRANSACTIONS OF T ~ E ASAE . 19

FIG. 1 Air supply lines delivers mm- prncd sir to venlari action where liquid Inam mlulion from container ir drrwlb into the sir meam. The sir-liqaid huhhle mir- brr imvo upward into Iowa hall of cone rand lltroudn refitline meenr. Foam zen-

o w where they encounter the high v?lcxity flnn strcilm.

Tw.0 synthetic detergentr, Orv~rs K liquid (Prodor and Camhle Cornpony) itnd Vuponol C (E. I. DuPoiit Co.) wm: wed ;as foaming agents. To IIC-

ccmodiatc stabilizing ingrarlients, the co~lcentration of swface agent wLS

usndly held at 1 percent or P percent. D~zinege sncl density data were ob-

tained for all samples xt room tempera- ture. I3ec;l<ae kmperckture had il very I > r ~ v ~ ~ t ~ n ~ e d effect on drainitge rate and persistence, mimy of thc foitm snm~ ples were evaluated in $1 57 F environ- ment of a walk-in refrigerator and in u near freezing temperature of 38 F. Pan samples of some foams were placed in- side it 15 F chamher to learn their re- action to freezing.

---r.-.... ~~ ~

Of all stabilizing ingredients tested. only three matcri~ls \r.erc able to re- tard quarter dninage time to more Ill;m .an Iruor, regardless of the colicen- tration med. Gelatin. citrus pectin ltnd starch phosphate ( a corn starch de- rivdtive), \\,hen used in cwncentrations of 2 p-rcent surface ngcnt arid 2 pcr- cent st;~l)ilizer, produed foam with quarter drainage time in excess of one hour. A comparison of dncinage time among fire foam, starch phosphste. pec- tin and gelatin stabilized foams is given in Fig. 2.

Short Life Foams Foam generated from commercisl fire-fighting solutions w;ts found to he ia~udequate for meet- ing the persistence requircmcnt (Fig. 2).

foam when exposed to cold wind. The tenacity of the skin l~olds the foam ta- gether. Loss of foum by m a s transfer of liquid into the wind is greatly rc- duced once the surface seals with the skin fnrrnntion.

Sodium plypectate (19). a dcriv~l- tive of cihl~s pectin. was found to havt* excellent skin-forming ahility whet1 conibined with calcium. Drainage time \r.ils retarded fmm 180 min with IIO

calcium to as long ns 50 hr with cal- cium. The rate of gel of the snlution~ could be controlled by the avail;~biliv of the calcium salt. Properties of thc foams derived from mixes which were liquid ut room tcmpe~.aturc are listed in T~l)le I. Good skin formation of foam was ol,served only with pectate con- tent above I mrcent.

Starch phGphate Starch phosplvate is an excellent hinding agent. It al- lowed copious fmtm prorhruliun~ either when wed alone or wit11 other stabiliz- ing agents.

Uccrease in temperinhrre of expmts-e iocresed drainage time from 50 min at 1 percent concentration to 180 mill at 35 1:. Starch phwph;tte wherl used alone produced no skin surface on the foan and was unahle to u.ithst;ltid wind. The starch filaln's stability when frozen was excellent, crpecinlly nt the lower densities.

Gelatin Foam Mixes with inherent gel forming prop~rties were far super- ior to those which would not gel. TIIF gel point of gelatin in I percent to 2 nrrcent men>hr~t ion is ne;lr 65 F. thlls 1 ~~

\lurr than 40 materials were ffrmo- The stabilizing agents dismimcl solutiotis Ivad to be held above this IrwI ii,to solutions m d the properties were s~lpollin, agar, gum ;~rallic, sev- temperature prior to foaming. d Ihdr fuams were evaluated. era1 starch types, wheat flour, bagi155e. I,, Table 1, it is shown thut gelatins,

\ ~ . ~ , ~ ~ s ~ l u t i o n r were fo~mulatml methyl cellulose, peanut flour and ap- especially the APM series, were ;dde tbrw i,tgredients: R foaming agent pectin. because of either dilficr~lty to form ~tahle foams with quarter drain- ( ~ ~ , , d ~ ~ t i c detergent), a film stabiliz- in foaming, fast drainage and sd)se- age time greater than 100 hr. The gela- ins ;,g~~lt if ~ ~ c c e s s a ~ , a moisture v e n t irrqs of stmctul-e, or freeze d?- t i , foa,n forms a very tough skin sur- hil~din:, agent or dra in age-retarding ~~'"ction. fa<= rvhen chilled. I3ecause of the ~CPIII . 111 all. this gave 46 klgredient Pectin Foam A11 important result of negligible drainage pml~elty, the gela- ~ail,inatians in v;~riuus concentrations the gelling aldlity of pectin is tbat &a tins were chosen its the superior stahiliz- fur r ~utbl d 209 mixes. tough skin forms on the surface of the ing agent fur field appliuatio~l (5).

TABLE I. PROPERTIES OF FOAM - - -. -

. .- snlulio.. ~- -~ ~ -. F-mt

Slrhiliro <~n,mlmticn Q,,..nn Bindine n m t -nmrmtralitm Exmn.tcm Yield Dr*inaKr ~ l m m c n * - -. -. wmnt PF.LT"L

S,di###t# ~ d l ~ a t ~ 1. . ' 16.8 7 7 180 ",in Yrry sb"n skin. nnllmt itlam ~..li,,.,, ,".l~~~.tnh , , CnHPO.. lii me wr nl 20.1 4 9 50 hr HW skin tamxtimrlion rich rianus m i x

S.!i,,m ,nc,%dia1r I li CnHPO.. 10 my wr gnu 15.8 465 511 hr Gd skin lc~marlnn

S..Ii<$m ms??~>cwtbxt. j.3 CIIIIPO.. 25 ml! wr pm 19 5.55 10 hr E-ll~nt ~tnl~ilits LXI liln ixdynuctote C~ttl'O,. ton ms ~ r r nn 30.8 39s 18 h. E ~ C . . I I ~ ~ inam hlr. rtrlnle skin-id-

,e*arr $IXICIIIII jobp.~talc I OIHPO.. SO mx per y m 111.5 13NO 40 hr ~ r c l skin frmrtin,. c~lerllmt B ~ n n

l l l l l i t ~ ( d # s # . ]ntats. I C~IIIPO,. 150 nla 8-r nl-l JIB 6R.5 140 nln E \ : ~ ~ I 'b 1n.m

1 : ~ h ~ ~ ~ 7 .4PM 1 Stnmh h!,sph,hntc I 49.5 I % l i >I00 hr \Vwk 1-m s t r u c l ~ ~ t ~ ~ clYmf 0.3

(;~:dbtm 7 .APhl 1 Starch hrph*tr . I IN.,? 1100 >l00 hr U'mk fmln sinlctmr c b w d 0.6

: A ! 7 P X I 1 Stttrrh ~rkrmhiltr I 5.5 1130 ,100 hr Emllmt rkin, rich hanl mix (:s:.#r,n 1lF L Starch phn<ph,hlr 1 41.1 1410 48 hr \\'~rl rkinx-stnu draina~e-larec

hlll lhl~ (.r?a!n.n 7 APH I .... 42. I420 >lo0 hr Wcek stin <;~:.bb.n : AP>f 1 .... .... 39.1 t l G O >1W hr X.:.lrllml r*in-CXcrllrnt htds I;v~A,," , *P>, 3 0 35.3 918 >I00 hr Vcrv sh.nnl: skin-crrrllCnt fmnn

... . r,,>t<:ily. - - - - - - -- 'All nkn nmde r i l h 2 perrmt Or\.w K Liqnld fa hxrmin~ tlml. ,. Fonm~ MI,I*.~IPS d~mninrd nl 38 F. Solotiuna mc held at nmm tm~~rrnrturc micar tu h-imlin~.

1971 THAKSACIIONS OF THE ASAE 3

& m -- - 5== -=== -- -- -- =-

I I ~ o n L O O ~ lo000

T i m . .inutc.

FIG. 9. Comparison of drainage rrte af foams. The effect af stabilizing agent an dr6noge rntc Is shown. Gelatin stabilized detergent foam to d v e desired foam life, F'loaed regions rep rercnt compo.ite data of dl drainage tests.

The slow drainage of gelatin fmams was apparently due to the gel forma- tion in the cold environment. Gel for- mation depends on the gel strength of the material, conuentmation, pH, tem- perature and additives. In this stud,^, the mixes were formulated withgelatin concentration from 0.5 perwnt to 3 pcrccnt.

Mixes with less tlian I percent gela- tin lrad little skin-forming ability. and thosc in excess of 2 percent would gel in solution at room teml=rature priur to funming.

Because of the high mmpatability of selatin and starch phuspll;~te, stitrch phosphate can he used as a gelatin ex- tender as s h a m by the slow drainage of starch-gelatin mixes shown in the I . 17te mst advantage of starch as a gelatin extender is definite because starch phosphate costs approximately 1/5 that of gelntin.

The gelatins with higher gel strength were superior in skin-forming qn:tlities than gelatins with low gel strength. Type 7 APM (Tahle 1) was the-su- perior one lrpcause of its lack of drain- age and ability to form a tough skin when exposed to cold temperatures.

Expansirm Limits for Stable Foam

Regardless of the ingredient used for foam formulation, definite upper limits to expansion were found for a long life foam. In no case was it pus- sible to pmclnce n foam ablc to with- stand wind when the expansion was greater than 100. Foams with expan- sion less than 20 were heavy, wet foams. Economy dictates that expan- sion he as high as passihle, while stn- hility requiments call for an expansion greater than 30.

Thermal Conductivity and Persistence

Method An insulated wind tunnel was used to provide a thermal envimn- ment in which the conductivity and

FIG. 3 Cut section through tunnel Row of test rreh'an. lac tiou of tlnermaoouplcs end heat llaw hanrductlr is 3hown.

persistence of the insulating fonm could be analyzed.

A l-ft square test area 1 in. deep (depth of flour insulation) was made in the center of the 18 in. by 6 ft in- sulated floor of the wind tunnel. This size and lomtiun insured sufficient in- sulation around its circumference to prohibit lateral heat transfer. A cross sectional view of the tcst section is shown in Fig. 3.

Under steady-state conditinns (no heat amunolation) the rate of heat flow through the insulating foam C ~ I

be expressed l y Fonrieis Law:

Where: q, is rntc of heat h.ansfer through the

foam, Btu per hr. k, is the thermal conductivity of the

foam, Htu per hr ft deg F A is the cross sectional area of foam

through which heat flows, sq ft IW,/ID( ip the tempetatllre gradient

within the foam, deg F per fl.

Sin= the heat flow is one dimen. sional, the rate of heat conduction thmigh the foam must equal the rnle of heat conduction through the ply- wood.

Simnltaneo~ur (or approximately so) rne;uuremcnts of the rate of heat con- duction th~vugh the foam and thlnu h the plywood and ihe temperatnrc grarfi- ent within the fonm must be made at short intervals of time, in order to quantize the thermal conductivity of the foam.

Heat flux was measured with a HY- Therm bidirectional heat sensor (Hy Cd Engineeritkg Co., Santa Fe Springs, Calif.) The heat sensor wns i~atallml in the e n t e r of the test area flush with the bottom of the plywood floor.

The temperatures at six hlcaliom, Fig. 3, in a vertical rofile in the test section of the tunnef were rnedsured

and recorded: (a) amhient air strm flowing under the lmttom of the t t section, (h) the surface of the h flux sensor exposed to ambient air Hn (c) the fo:lm plywood intcrfnce in b test cavity. and ( d ) 0.3. 0.7 and 4 in. (free airstream in the tunnel) a h the floor of the test men. A SIn~ll d mounted vertically in the test sectiu was used to indicate the foam depl' Change in depth with time was th measure of persistence.

Results Because of th. unst;thl nature of foam the thermal mnductir* ch:~ngcs with time. Temperature p d ents measured in the 1%. thick foai

layer were linear for fresh homogeneov foam, hut varied within the foam dept as foam structure altered with time.

The variability of thermal mndo: tivity within a foam layer (function r depth) was recognized as significant i itself. however, the heat flow throd the entile layer can be predicted if a etiective conductivity value for t1t

whole layer is known. Tljc tcchnirp employed enabled us to detennine a eRective thermal conductivity far . foam layer in a wld nir stream. cud tions similar to field exposure.

Fig. 4 gives thermal conductivity a a function of time. Thermal mndm tivity appmaches a mnskant value b less tlian nun hour. Conductivity is ont a weak function of foam density in th. range 1 to 2 lb per cu f t and was in dependent of foam ingredients. fi mean d u e , k = 0.061 t 0.006 & p ~ r hr f t deg F was found in eigl: long-te~m tests with stahle foams. Th change in instantaneous mnductivic with time can be attributed to tlr change in bubble size due to drainng

Persistence Subsidence of the fm layer with time served as criteria d persistence. Actually, drainage rate ur! nn exrellent indimtor of stability. Od those foams with little or no drainag h id good persistence in the wind tun nel exposure. A comparison of persist.