Quality Control of Curing Polyurethane Propellants by infrared Spectrometry EUGENE A. BURNS’ Propulsion Sciences Division, Stanford Research Institute, Menlo Park, Calif
b Infrared spectrometry has been utilized for quality control of polyurethane solid propellants. By monitoring the isocyanate N=C=O stretching frequency the hydroxyl 0-H stretching frequency, and the urethane C-0 stretching frequency, i t i s possible to determine the fraction of free isocyanate groups which remain after a specified time. This term, (NCO)i/ (NCO),, serves as an index to the repeatability of a particular propellant batch; it i s indicative o f batch temperature and ingredient composition. The ratio of isocyanate to hydroxyl initially i s evaluated present, (NCO),/(OH),, to predict the rate of polymerization reaction and mechanical properties of the finished propellant. A correlation of (NCO)i/(NCO), and the castability of the propellant i s presented.
F
of assuring quality control, rapid, quantitative methads are required for monitoring polyurethane propellant mixes before they are cast into rocket motors. Polyurethane propellants consist of a solid oxidizer dispersed homogenously in a polyurethane binder; the binder is prepared by reaction of long-chained diols, triols. or tetrols with a diisocyanate. Infrared spectrometry affords the opportunity to follow the consumption of starting materials and appearance of urethane groups as polymerization proceeds, The fraction of free isocyanate groups remaining after a specified time, (KCO),/(KCO)o, serves as an index to the repeatability of a particular propellant batch. This fraction is affected by two main factors: temperature and ingredient composition. Operator weighing errors in the amount of curing catalyst, burning rate catalyst, diol, triol, oxidizer, and diisocyanate will be reflected in the value of (NC0)J (NCO), to various extents. To eliminate the possibility that these errors might cancel one another, i t is necessary to repeat the determination after a specified interval. The rate of the polymerization reaction as well as the mechanical properties of the cured propelOR THE PCRPOSE
Present address, Propulsion Research Department, Space Technology Laboratories, Inc., Redondo Beach, Calif. 1270
ANALYTICAL CHEMISTRY
lant are dependent upon the ratio of isocyanate to hydroxyl initially present, and hence this determination will also be made. Three methods for quality control of polyurethane propellant mixes are presented: a quantitative, rapid, go-no go method for the ratio (NCO),/(NCO). that can be completed in 5 to 8 minutes; a quantitative method for determination of the ratio of isocyanate to hydroxyl initially present, (NCO)o/(OH)a>which can be completed in 15 to 20 minutes; and a rapid semiquantitative method for the determination of the isocyanateto-hydroxyl ratio at a specified time t,, (NCO),/(OH), which can be completed in 5 minutes. These methods employ a carbon tetrachloride extraction of the propellant mix; the difference in the time required for the methods depends upon whether the extraction is quantitative or not. A straight line relationship has been observed between the castability (flow rate) and the ratio (XCOJ (NCO).; however, this correlation depends on particle size of solid ingredients and temperature of the mix. GO-NO GO METHOD
The measurement of the fraction of free isocyanate remaining a specified time after the addition of toluenc diisocyanate serves as a rapid “go-no go” test for the repeatability of a propellant batch. If this fraction. (SCO) J(KCO)o fails to fall within empirically determined control limits for a certain propel-
Table 1. Summary of Results Obtained on the “Go-No Go” Method for Determination of the Fraction of Free Isocyanate in Batch G 38/59” ANCO ”lco ~ _ _ EXCO
mm.-
Reaction meq./ time, ml.
cco
mm.meq./ ml. X lo3
(XC0)i __
(NCOS 0.361 0.342 0.340 1.03 10.32 0.343 1.02 Av. 0.347 Std. dev. +0.0097 110’ F. reaction temperature and 0.11% w./w. catalyst in binder. hr. 1.00 1.02
X
lo3
3.05 4.75 3.41 5.39
5.40 9.24 6.60
lant batch, it does not necessarily meail the propellant will be inferior, but rather, that the curing rate is significantly different from the curing rate control. Procedure. Transfer as quickly as possible about 1 gram of propellant into a conical glass-stoppered centrifuge tube which contains about 10 ml. of reagent-grade carbon tetrachloride. Insert the stopper in the tube and shake vigorously until the solid portion of t h e sample is free-flowing. Centrifuge for 1 to 2 minutes and transfer a portion of the supernatant liquid into a 0.2-mm.. fixed-thicknesi sodium chloride, infrared cell and ohtain the infrared spectrum of the solution in the range 2450 to 2100 cm.-’ (4.1 to 4.7 microns) and 1800 to 1650 cm.-l (5.5 to 6.0 microns) with a spectrometer set a t a scanning rate of about 150 em.-’ per minute. Determine the absorbance of isocyanate ( A v c o ) and carbonyl (&o) stretching frequencies by the base-line method ( 3 ) and calculate the ratio ( S C O ) ,’(SCO), from the equation
\\here E ~ C and O ero are the niolar absorptivities of the isocyanate and carbonyl groups, reqpectirely. Discussion. If two optical cells are used to determine the absorbance of carbonyl and isocyanate, corrections in the path length, b , of the cell used for the determination of the group designated by the subscript must be applied
The path length is determined by measurement of interference fringes (4). This method is predicated on the fact that the molar concentration of isocyanate initially present is equal to the sum of the isocyanate and the carbonyl concentrations at any time t , ( 1 ) . hnother requirement, which has been verified experimentally, is that a uniform ratio of long-chained to short-chained molecules present is obtained in this partial extraction. Results typical of this method are shown in Table I. The relative standard deviation of this method is 2.80%.
If the value of (SCO),/(h-CO). falls outside of the empirically determined control limits, then the ratio of (NCO), (OH), must be determined to ascertain whether the propellent will have satisfactory n-echanical propero specities. If ( ~ C O ) o / ( O € ~is) within fication. then analysi: of the curing rate ibatalyst and total solids must be acinomplished to determine whether the propellant n ill cure with satisfactory mechanical properties.
Table II.
Batcha G 40161 (;41/62",c
G 55/78 DETERMINATICN OF THE ISOCYANATE-HYDROXYL RATIO
G 56/79
13y rnult,iple extrrictions, the total binder present in the polyurethane propellant can be examined. I n this manner i t is possible to ascertain whether the amount of isocyanate initially present was correct. Use of the :tbsorption band neai 3500 em.-' (2.86 microns) for quantitative determinations of hydroxyl is not fl2asible because of serious overlapping of the N-H stretching mode near 3300 cm-' (3.03 microns). The method for the determination of the ( S C O ) , , ,(OH), ratio is limited then, bemuse knowledge of (OH), is required. -. 1his is not a serious limitation, because the long-chained diol: and triols can be weighed accurately without fear of reaction with water in the air, which is a problem in handling the diisocyanat diluted to a volume. T7, with carbon tetrachloride. the conce~itiation of isocyaiiatv ires sent in the solution is
Siniilarly, the concentration of urethane linkages, equal to the carbonyl concentration is
Combining Equation> -1 and 3. r is calculated as follows : r =
~7
r .!sco
nuCscr) Lbscc,u:
.+
sG9 cco b c o u
(,-, '
Procedure. Add 7 to 10 mi. of reagent-grade carbon tetrachloride to :i 15-nil. glass-stoppered conical centri-
Summary of Determination of Isocyanate to Hydroxyl Ratio Obtained from Isocyanate and Carbonyl Infrared Absorption Bands
G 57/80 G 47/6gb
Reacfion time, hr. 2 0 2 0 1.0
1.0 1.13 1.17 0.92 0.97 1.00 1.17 1.21 1.37 1.25 1.25 4.00
nJV. meq./nil. 0 0487 0 0498 0.0461 0.0501 0.0522 0.0791 0.0634 0.0541 0.0767 0.0717 0.0485 0.0789 0.0758
0.0605 0.0465
b~coasco
-4co bcoaco
meq./nil. meq./ml. 0 0168 0 0176 0.0180 0.0188 0.0246 0.0349 0.0270 0.0233 0.0328 0.0306 0.0211 0.0329 0.0188 0.0154 0.0092
0 0311 0 0299 0.0295 0.0318 0.0292 0,0451 0.0398 0.0332 0.0482 0.0419 0.0284 0.0482 0,0556 0,0450 0,0389
--T __ _. .
~
Found,
Value 0 Y84 0 954 1.030 1.011 1.031
1.011' 1.047 1.044 1.056 1.011 1.021 1.028 0,981 0.998 1.013
av
Taken
0 Y6Y
1 000
1,021
1.000
1.02'2
1.030
1.049
1.030
1.0'20
1.030
0.997
1.030
The reaction temperature was 80" F., and the catalyst concentration was 0.2Oc; w./w. of the binder, unless otherwise designated. Reaction temperature 110' F. Catalyst concentration 0.11% w./w. of binder. 'l
r_
fuge tube, stopper, und weigh the dilution in carbon tetrachloride. T h c ~ tube to t h e nearest 5 nig. Maintain data, although the agreement is good. the tube upright throughout the could be improvcd with on-the-spot n-eighing and subsequent operations determination-Le., within 20 minutes. so t h a t carbon tetrachloride cannot The variability between the time the be lost b y seepage through the groundspectra were taken and the time the exglass stopper. > i ta specified reaction traction was complete is most likely rest h e , quickly open the tube, add about ponsible for the variability between the 3 to 4 grams of propellant mix, reresults. The variability within replistopper, and reweigh t h e tube. Shake cates, in general, i i quite good. vigorously until the solid material is free-floning. Centrifuge for 3 minutes) RAPID METHOD FOR DETERMINATION OF and then quantitatively decant t'he ISOCYANATE TO HYDROXYL RATIO supernatant liquid into a 50-ml. ~ o l u metric flask. Rinse the residual liquid .I rapid control determination of the and solid from the stopper area ryith a stream of carbon tetrachloride (from a isocyanate to hydroxyl ratio which is polyethylene squeeze bottle) into the independent of knowledge of the initial centrifuge tube. Add carbon tetrahydroxyl concentration can be obtained chloride to bring the total volume in the -:1 use of the propllant extract prepared tube to about 12 ml.. stopper, shake. in the "go-no go" method. centrifuge, unstopper, and decant into The method *tcniG from the detcrthe same volumetric fla.sk. Repeat mination of the hydrox>-land i-oc).ana,te these operations until the extraction is absorption. Infrarecl cell- of t-iiffcring complete (usually 4 estractions). Fill the volumetric flask to the mark with optical path must be ernployed Iiwau+eof carbon tetrachloride, mis well, and then the large differences of molar ah-orptransfer a portion of the s - h t i o n to a tivitie. of the liylrosyl :inti iwqxiiate 0.5-mm. fixed-thickness ~odiiiiiichloride bards. The value of the hydroxyl coninfrared cell. Obtain the infrared speccentration is not accurate because of contrum of the soliition for the ranges 2450 tribution from the overlapping imido to 2100 c m - l (4.1 to 4.7 microns) and stretching band. Hon-ever, with suit1800to 1650cni.-' ( 5 . 5 to 6.0 microns) at able care the apparent, hydroxyl concena scanning rate that does not exceed 150 tration determination is reproducible. em.-' per minute. Determine the cell The method measures the ratio ( S C O ) , / thickness by the intcrference fringe method. Determine the absorbance of (OH) at' time t , , which may not be equal the isocyanate and cwbonyl bands bjto the initial rat,io, ( S C O ) . , (OH),., bethe base-line method and calculate the cause the extent of side reactions may value of r using Equation 6. vary and because of the amounts origiResults. Results typical of the nally taken. Although measurement of isocyanate to hydrosyl ratio deterthe hydroxyl peak is inaccurate, this ratio mined by this method are shon-n in can be used as a control method after Table 11. I n t'his study t h e data were sufficient data have been obtained oii obtained approxiniatcly 2 hours after the urethanation ~vns quenched liy repetive batches to aasure reliability. VOL. 35,
NO. 9, AUGUST 1963
1271
0 46
Table 111.
Typical Results of the Rapid Determination of Isocyanate to Hydroxyl Ratio
BatchfL (;
3S/.iD
G 40/6l C; 41/62
Reaction t enip er:%tiire,O 110
S0
110
b?;cO, m n .
( S_ C -O ) , _
bo^, mm.
.I?;co
.1 O H
0.503
0.356
0.087
0 . si1
0,503
0.553 0.397 0.6% 0.665 0.581 0.608
0.126 0.101 0.149
0.96 0.S6 0.92 0 . 86 0 s5 0.92
0,0942 0 ,0942
0.094%
0.501
0.168
0.148 0 . 143
0 42
[OH),
s ? -
038
129/154
z
128/153 l26/15 I
1
,
.ill batrhes contained 0.11 :i w./w. ferric acetylwetonate catalyst in the binder, :md :ill sampling occurred about 1 hour after toluene diisocyamte addition.
0 34
Replicate Values of (NCO)./(NCO), Obtained as a Function of Time on Propellant Extracts Compared with Castability Data
0 30
'1
Table IV.
(SCO),
Batch 1) 126/151
D 127/152
L) 1"/153 11 129/154
I ) 130/133
Reaction time, 1ir.
1. 3 3 2.45 3.40 4.50 1.30
2.27
3.30 4.45 2.30 3.37 4.50 1.2s 2.33 i3.40 4 . 4,5 1.2s
3.38
3.32
4.65 1.20 2.22 3 . ;33 I ) 132/137 1 .:30 2.28 3.27 4.62 1.25 I ) 1:33/15s 3.43 3 , 23 4.37 Grand pooled stand. dev 1 ) 131/136
1
-
(SCO), Pooled stand.
\-a1ue 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
482
345 319 315 416 372
349 339 360 326
324
408 364
354
327 444 403 380
372 509 449 444 464
429
387 385 474 438 408
390
Stand. dev.
dev. for bstch
1 0 ,0268 10.0080
10.0119
1 0.0010 10.0028 10,0021
0 0169
f0.00 17 3Z0. 0113 1 0 .0066
0 0037
50,0061
f.0.0028
0 0074
3Z0.0076 1 0 ,0033 10,0067 10.0055
0 0060
10.0092 f.0.0057 10.0057 10.000 1 0 . 0120 =k0.0170 1 0 .0044 10.0066 1 0 .0036 10.0016 1 0 ,0041
0 0071
0 010')
0 004.3
10.0064 10.0090 10.0072 10.0030
0 0C68 0 00S8
Value interpolated to satisfy reaction tinie.
Procedure. Sample and extract the uncured propellant described in the "go-no go" method. Measurr the absorbance of the hydroxyl band b e h e m 4000 to 3200 cm.-l (2.5 to 3.1 microns) using a 1.0-mm. fixed-thickness .odium chloride infrared cell at a winning rate not to exceed 400 em.-] per minute. The base line determination (3) is very critical in this instance; hence, care must be exercised when the hase line is drawn. Calculate the ratio
1272
ANALYTICAL CHEMISTRY
130/t55
0
of isocyanate t o hj-droxj-1 conwntrations in the hinder a t the time, t i , a.; follom:
Results. Typical results of thiq method are liqted in Table 111. The values are some 10yc low In absolute magnitude; howevw, they are reproducible to about 37,, and therefore can serve as a rough index for the (XCO), '(OH), ratio.
20 40 CASTABILITY
60
80
100
- poundr/hour 68
Figure 1 . Fraction of isocyanate remaining as a function of castability of propellant and ammonium perchlorate particle size blend N H 4 C I 0 4 particle size: Ground = 1 1 microns Unground = Nominal 2 0 0 microns Ground, Unaround Batch % % 80.0 20.0 D 126/151 2 7 . 4 7 2.6 D 12811 5 3 40.3 59.7 D 12911 5 4 D 130/155 51.6 48.4
CORRELATION OF ( N C O ) i I ( N C O ) , RATIO WITH CASTABILITY
C'on,-idci,il)lc work has been carried ow a correlation Dety of a particular propellant hatch and the fraction of isnc y n a t e wniaining in the niis after a ..pccific,d timc t , . In Table Is' thc ~-aluc> of ( S C ' O ) , ,(XCO),, :ire listed for 12 batchcs as a function of time with the interl)olatcd castahilitj- t'ogether with ralculated standard deviations for each tletrmiinntion. The grand pooled .tandaid dcviation for the value of ( S C O ) , ( S C O ) , i.- +0.008S or + 1.7 to 2.8yc Iclatil-e, n-hich is the error to hc m analysis by infrarc d I t follon-a, therefow. of .hort-chaincd inolecul(!q to long-chained molecules which are ext,ractrd i.; wproducible in an extraction at a particular timr interval. Plota of (XCO), (XCO),,a3 a function of ca.tability (flow rate) shon- the exllrctcd dirwtional correlation. -1s the propellant cures, the castability dccreases, a. does (XCO)i/(SCO),. Com~ ~ a r i - oofn the ( S C O ) , / ( S C O ) ,z's. casta,bility curw among batches shows that several Imanieters affect the curvc. l'aranic~tcrs which affect the curing rate of thr. propellant, (catalyst, temperature, animonium ion, dio1,'triol concentrations, etc.) and parameters which p r i m a d y affect the castability (part'icle size of the solid ingredients, etc.) can
cause major shifts in thi! slope or shape of the curve, or they can compensate for one another. For example, if the data of Table IT' are examined closely for the T-arious batch parameters, it is seen that for Batches D 126,'151 t>hroughD 133,158 a good straight line is obtained for thc plot of ( X C O ) L ~ ' ( S C O )2s. o castability n-ith different slopes when different cabalysts are used; when the pai,ticle size, distribution of the solid ingrrdient~is varied, the slope remains nearly the s m e ; however, different intercrpts arc observed. In Figure 1 (lata deinonst,rating this latter effcct are -h(iivii. For the same degree of polymerizatioii, the castability increased in the following oydcr of mnionium perchloyate dist'ribution 51.6%, 59.7 and 72.6Yc ungroiind, and has the ma mum ra5tability whe:i the oxidizer 1)article size distribution n-as that which gave thv maximum hulh density.
time, t, the sum of the isocyanate concentration ( S C O ) , and carbonyl COILcentration (CO),, equals the initial concentration of isocyanate (KCO),,
(SCO),
=
(SCO),
+ (CO),
(6)
Substituting Lambert-Beer-Bouger law definitions and experimental parameters in Equation 8 the follon-ing results
ACKNOWLEDGMENT
where 771 is the initial concentration of isoeyanat,e, meq. per gram; the rrst of the terms have been defined earlier. From laboratory experiments or on actual quantitative extractions of the propellant mix for a series of different extraction times, st'raight line curves fitting the expression of Equation 9 result when -Irco, zr.bsco is plottrd as a function of -Icn zcbco. The molar absorptivities of the isocyanate and carbonyl gIoups are calculated from the slope, s, and intercept, I , of the straightline ciir1.e as follows: I
DETERMINATION OF MOLAR ABSORPTIVITIES
For the d[+rminatio i of the various ratios described abow, an accurate i ~ i l u cfor the molar a bsorpbivities of isocyanate, carbonyl, and hydroxyl are required-the values rqresentative of the grouping in the exact medium. Throughorit this work a necesary as3iiniption \\-hose validity has been pro\.eii clscwhere ( I ) is t,hat a t any
The molar absorptivity of the hydroxyl group is obtained from knonn diluted concentrations of polypropylenc glycol in a manner similar to that ieported previously ( 2 ) . The valueb of €\LO, ~ C O , and used in Tables I, 11, and 111 (sodium chloride optic-) ale 1165, 376, and 47.6 ml.:meq.-cm., I C spectirely. The xalues of E \ C O and ecO used in Table IV (prism-grating interchange) are 1289 and 450 nil. meq -em.
and e[,, =
- esco -~
1\11)
The author acknowledges the efforts of E. ;i.Lawler and -1.Longwell who asaided 17-ith some of the determination-. LITERATURE CITED
(1) Burns, E;. A . , Lawler, E. A , , J . ( ' h e u i .
Ena. Datn. in mess. ( 2 ) B"urns, k,d., Muraca, R. F., A%\ CHEM.31, 397 (1959).
%I,.
(3) Heigl, J. J., Bell, 11.F., White, J. I-., Ibzd., 19, 293 (194i). ( 4 ) Smith, D. C . , Miller, F. C J . Opt. S O C . -4m.34, 130 (1944).
RECEIXED for review December 31, 1962. Accepted ?.lay 10, 1963. Presented in part at the Division of Analytical Cheniistry, 144th Meeting, ACS, Los hgeles, Calif., April 1963. Work supported by TJnited Technology Corporation.
Gravimet r'ic Deter minutio n of Germanium in Si Iico n-Germa nium AI I oys K. L. CHENG and B. L. GOYDISH RCA laboratories, Princeton, N. 1. b When silicon-germanium alloys are dissolved in a mixture o f nitric and hydrofluoric acids, the silicon i s completely volatilized as its fluoride. nf o r t una t e I y , a pp ro;< ima t e 1y 5 0 of the germanium present may also be lost due to the formatilm of a volatile g e r ma nium fluor ide (:ompI ex. Citric acid can effectively eliminate this loss, thus making it possible to determine germanium gravimetrically as GeOt. This has resulted in an accurate method for determining the composition of silicon-gernianium alloys, wherein, after dissoluticn of the sample and volatilization o f tlie silicon as its fluoride, the citric acid i s removed b y ignition, leaving a reijidue of determinable GeOl. Complexants other than citric acid have been studied; however, only citric ticid has been found to be completely effective.
u
70
I
DLTAILLD study ( 2 >6 ) of the intrinsic optical absorption y e c t r a and the electronic propertie- of the germanium-silicon alloy -y'tem, it became necessary to analyze these alloys quaiititatively. -illthough qpectrographic and polarographic methods for analyzing the germanium-ilicon alloys have heen reported, their accuracy does not meet the requirement for the work currently carried out in our laboratories. The ition of the alloy, must t h an accuracy of better than i1% relative error over the range of 20 to 100% gernianium. The standard deviation of the spectrographic method (4) is only 3=8 and the accuracy of the polarographic methcd is not better than 1 2 to 3% relative error. The accuracy of the polarographic method to better than +lye relative error was probably mistakenly claimed (3, 5 ) .
s- A
The j ~ r o p o m lnicthod involve3 separation of silicon as silicon fluoride a i d deterniination of germanium as Ge02, citric acid being uied t o prevent the loss of germanium as a volatile fluoride. Silicon is determined by difference. Ah x-ray fluoresence method for analyzing the silicon-germanium alloys has been developed ( 1 ) . EXPERIMENTAL
K i t h gentle hrating, disqolre thc .ample containing 10 t o 100 mg. of germanium in a covered, tared 50-nil. platinum crucible, using 2 nil. of concentrated nitric acid, 1 nil. of 4b7G hydrofluoric acid, and 10 ml. of 1M citric acid. *ifter diisolving the cample, wash t h e cover with a minimum amount of water and evaporate on a hot plate a t approsimately 300" C. to dryness. Then ignite Procedure.
VOL. 35, NO. 9 , AUGUST 1963
1273
