CONCLUSIONS
ACKNOWLEDGMENT
The purpose of this paper was to present a convenient approach to calibration of luminescence equipment and to show how modern computers can be used for correction of the spectra used in the calculation of quantum efficiencies. Calibration data reported in this paper should not be applied to other instruments. Even though two instruments may he of the same manufacturer and model, the components (source lamp, phototube, electronics, etc.) may not function identically. For this reason, each separate instrument should he calibrated and if a component is changed it should be recalihrated.
The authors thank Esso Research Laboratories, Humble Oil and Refining Co., for permission to publish this material. Helpful discussions v i t h P. Debye and D. P. Shoemaker are gratefully acknowledged. LITERATURE CITED
(1) Benford, F., Lloyd, G. P., Schwarz, S., J . Opt. SOC. Am. 38, 445, 964 (19481. (2) Bo&, E. J., Proc. Roy. SOC.(London) 154A,349 (1936). (3) Bowen, E. J., Trans. Furaday SOC.50, 97 (1954). (4) Melhuish, W. H., J . Phys. Chem. 65, 229 (1961).
( 5 ) Ibid., 64,i62 (1960). (6) Melhuish, W.H., ,V.2.J . S a . Tech., B , 37, 142 (1955). (7) Parker, C. A,, A N ~ LCHEXI. . 34, 502 (1960). (8) Parker, C. A , , Rees, Il-. T., Analyst 85, 587 (1960). (9) Ibid., 87, 83 (1962). (10) Teale, F. W. J., Xeber. G., Biochem. J . 65, 476 (195i). (11) Tratny, F., Kokalas, J. J., J . A p p l i e d Spect. 16, 176 (1962). (12) Weber, G., Teale, F. IT-. J., Trans. Faraday SOC.53, 646 (195i). (13) White, C. E., Ho, AI., Weimer, E. Q.,ANAL. CHEU.32, 438 (1960).
RECEIVEDfor review June 6 , 1963. -4ccepted August 12, 1963. Division of Physical Chemistry, 145th Meeting, ACS, New York, September 1963.
Determination of Propylene in Ethylene-Propylene Copolymers by Infrared Spectrometry JAMES E. BROWN, MAX TRYON, and JOHN MANDEL National Bureau o f Standards, Washington 25, D.
b Pyrolysis of ethylene-propylene copolymers produces derivatives that are rich in unsaturated carbon-carbon functional groups, This unsaturation exhibits strong absorption in the region of infrared light. The ratio of the absorption of the vinyl groups to that of the vinylidene groups varies with the mole fraction of propylene in ethylene-propylene copolymers. M a king use of this ratio, an analytical method has been developed for determining propylene in both raw and vulcanized ethylene-propylene copolymers.
P
PREPARED by random copol>-nierization of ethylene and propylene nith Zeigler or S a t t a catalyats have a wide range of commercial and practical applications il , 7 ) . To achieve the elaqtic quality of rubber, the propylene content must he controlled within certain limit3 ( I , 13, 14. 18). The a o r k pre-ented in this paper deals n i t h method3 for determining the mole fraction of prnpplene in both ran and vulcanized copolymers. Several methods have been reported for determining propylene in raw ethylene-propylene copolymer;:. The basis for calibration of many of these methods is the work published by Natta (lh’), which involves measuring the infrared absorption of the polymer in CC14 solutions. The absorption a t 7.25 micron.., presumably due to methyl vibrations, is related to the propylene concentration in the copolymer. I n OLYMERS
2172
ANALYTICAL CHEMISTRY
C.
some cases the dissolution of copolymers m-ith lorn propylene content or some particular structures is difficult (5, 6 ) . Moreover, Satta’s solution method was calibrated against his radiochemical method (15), for which the precision of the method was not stated; a considerable amount of scatter is evident in the data presented. Typical methods that have used Xatta’s solution procedure (15) for calibration are described in publications by Wei (21) and Gossl ( 8 ) . These infrared methods avoid the solution problems by employing intensity measurements made on pressed films. The ratio of the absorption a t 13.9 microns to that a t 8.7 microns is related to the propylene content of the copolymer. Some objections (3, 16) to the use of solid films have been raised because of the effect of crystallinity on the absorption spectra in copolymers n-ith low propylene content. These film methods are reliable only over the range of 30 to 50 mole 7*propylene. Bau and Manaresi ( 2 ) also calibrated a mass spectrometric method by Xatta’s solution method (16). The propylene concentration was reported t o be related to a certain ratio of mass peaks measured in the mass spectra of the pyrolysis products of the copolymers. An approximate calibration could he established with homopolymer mixtures of polyethylene and polypropylene. Drushel and Iddings (6, 6) reported a method calibrated against C14-labeled copolymers. They found that the ratio of absorbances of two bands in the
C-H stretching region or two hand> in the C-H wagging region could be used to measure the propylene content of pressed films. I n 1953 Harms (9) and Kruse and Wallace (10) pointed out that most polymers will depolymerize in a rather characteristic fashion on heating, yielding product5 representative of the polymer. This technique has been used repeatedly a i a means for qualitative analyqis by infrared spectrometry. The formation of characteristic pyrolyzates served as the ha& for the method presented here. Cross (4) indicated that thermal degradation of linear aliphatic polymers produces terminal unsaturated functional groups of the type RCH=CH2 and that branched polymers give products consisting mainly of RR’C=CH,. I n addition, he ,showed that different t>pes of carboncarbon double bonds have specific absorption in the infrared region from 1000 cm.-’ to that juqt beyond 700 cm.-’ Copolymers of ethylene and propglene are very nearly qaturated (I?‘). Hoir ever, on pprolysis they yield volatile products that are rich in two specific types of uniaturation, namely, vinyl RCH=CH2 and T inylidene RR’C =CH, (20). The vinyl group absorbs a t about 909 cni.-’ and the vinylidene at about 889 em.-’ (4. 17). Thermal degradation of diolefin polymers also gives unsaturated products. For example, the absorption measured in a quantitative method for natural rubber determination in natural rubher and SB-R
Pyrolysis furnace (20)
Figure 1 . A. Transite sheet
6. Furnace from combustion train C. Stainless steel tube D. Steel plug with hole for thermocouple E. Thermocouple F. Heater leads G. Test tubes H. Heater coils S. Sample Thermocouple and heater Icads connect to an indicator-control device
misturer (18) is probably due to the vinyl groups derived from the butadiene part of the molecule ani1 the vinylidene groups from the methyl branchr; of the isoprene units. EXPERIMENTAL
Samples. Five copolymers were used for calibration purposes; they had nominal propylene concentrations of 10, 20, 31, 40, and 50 mole % assigned by t h e s u p p l e r from a niaterials balance based on the monomers used. Linear polyethylene and isotactic polypropylene (air. mol. wt. = 213,000) served, respPc,tlr-ely, as 0 a n d 100 mole % propylene samples for calibration purposes. The copolymers and the pol) propylene were vulcanized according to the recipe given in Table I. Apparatus. The a[)paratus used for the pyrolysis is sliann in Figure 1
(about 3 to 5 minutes), a portion of the pyrolyzate was placed between two NaCl windows having a 0.025-mm. lead spacer t o control cell thickness. The cell was then placed in the infrared beam and scanned through the frequency of 950 cm.-l to 850 em.-' (10.5 microns to 11.8 micons). The instrument controls were set for normal operation with NaCl optics except t h a t the recorder chart speed was increased by a factor of two on the Model 221. These expanded spectra facilitated locating the absorption maxima. The absorbances corrected for background were measured. The ratio, €2, of the absorbance near 909 cm.-' to that near 889 em.-' was R = Agog cm.-1/ calculated-i.e., A889 cm.-' RESULTS
Figure 2 shons some typical spectra obtained on the pyrolyzates by a Model 221 spectrophotometer in the region of 950 em.-' to 850 em.-' The nominal mole % of propylene is indicated above each spectrum. The absorbances of the t n o bands were corrected for background by drawing a straight line through the points of minimum absorbances-Le., 950 em.-' and 850 cm.-' The specimens from each sample were analyzed in a random sequence so that each set contained one specimen from each sample. The values of the ratio, R, range from 9.977 to 0.0289, respectively, for 0 to 100 mole yo propylene for the raw samples and from 5.440 to 0.0431, respectively, for 10 t o 100 mole % propylene for the vulcanized samplei. The common logarithm of the ratio, R, can be represented by a linear function of the mole % of propylene in the copolymer. The R values mere
0
Table 1.
Vulcanization Recipe. Parts by
Constituent weight Polymer 100 Phil black 0 25 Dicumyl peroxide 2 0.2 Sulfur Total compound 127.2 Compounds cured 45 minutes a t 150"
-
a
C.
multiplied by 100 to avoid negative number,. Tables I1 and I11 list the results, expressed as common logarithms of 100 R, for the raw and the vulcanized A separate samples, respectively. analysis 1% as made for each of these two sets of data, followed by a study of the relationship between the results for raw and vulcanized samples. ANALYSIS OF RESULTS
A. Precision. For t h e study of precision i t is best t o analyze first t h e d a t a for internal structure, without reference t o t h e propylene values assigned t o t h e samples by their manufacturer. Such a n analysis is designed t o reveal the reproducibility of t h e analytical techniques. both within sets a n d among sets of measurements made either a t different times or on different instruments. The statistical procedure is a n extension of the ordinary analysis of variance and involves linear regression techniques (11, 1 2 ) . The data of Tables I1 and I11 represent two-way classification-i.e., data arranged according to tm-o criteria
s,
17-
7
(19).
The specstra were s c a r m d using three infrared spectrophotometers, PerkinElmer, Models 221, 21, and 137. =I simple demountable sodium chloride cell with a 0.025-mm. lead spacer served to contain the liquid pyrolyzate. Procedure. A sample wighing about 0.25 gram was placcd in the pyrolysis vessel, as shown in Figure 1. T h e mouth of a smaller te-t tube (10 X 75 mni.) was adjuqted to \\ithin about 5 cm. of the mouth of the larger test tube (16 X 150 inni.) t o aid in directing the distillate along the hot wall of the larger tube and thereby assure maximum contact of the initial pyrolyzate with the hot pyrolysis tube. This technique yielded reproducible pyrolyzates. The assembly wa- placed in the furnace previously heated to about 450" C. The pyrolyzate condensing a t the open end of tfle test tube was allowed to run into a .hird test tube. When the pyrolysis was complete
Figure 2. Typical infrared spectra of pyrolyzates obtained from raw ethylene-propylene copolymers The position of the absorption peaks from left to right are near 9 0 9 cm-1 and 8 8 9 cm.-', respectively, for each pair and the nominal compositions are indicated in mole % propylene
VOL 35, NO. 13, DECEMBER 1963
2173
- -- - - - - - - - - - - - - - - - - - - - - - - - 0
1.95
0
I94
0
0
+2P
p
I93
I89
0
I88
C
O
L
-
-
10
1-4
2.556 2.612 2.649 2,662 2.736 2 689 2.613 2.655 2,626 2.651 2.644 2.657 2.6458 I
2,334 2.363 2.358 2.363 2.364 2.375 2.409 2,428 2.406 2.411 2.455 2.480 2.3953
2.125 2.136 2.168 2.170 2.144 2.143 2.153 2.140 2.120 2.130 2.114 2.136 2.1399
1,958 2.002 1.958 2.038 2.006 2.010 I . 992 1.990 2.023 2.016 2.012 2.000 2.0002
1.655 1.646 1.674 1.724 1.692 1.698 1.689 1.691 1.644 1.692 1.667 1.668 1.6783
2174
ANALYTICAL CHEMISTRY
0
0
fI
f2,Y
0
STANDARD DEVIATION OF S C A T T E R 0
0.659 0.705 0,742 0.744 0.733 0.652 0.635 0.729 0.780 0,779 0.678 0.697 0.7111
a The sets marked A are measurements made on a Model 221 Spectrophotometer; the seta marked B are the results obtained on the same samples using a Model 137 Spectro-
photometer.
0
0 0
Logl,(1O0 R ) a t various propylene concentrations, mole % 20 31 40 50 100
Seta
0
O
HEIGHT
Loglo( 1OOR) for Polymer Pyrolyzates (Vulcanized Samples)
Table 111.
1B 2-4 2B 3A 3B 4h 4B 5A 5B 68 6B Av.
-
0
1.97
SLOPE
- - - - - - - - - - - - - - - - - - - -- -- - ---- - - - - - -
carried out at-another time, the line corresponding to the first set will be located above that corresponding to the second set. The same situation might occur even more frequently for sets of measurements made on different instruments. The lines corresponding to all the sets would then form a bundle of approximately straight lines, most of which would not pass through the origin. Thus, the lines may have nonzero intercepts. &o. the lines corresponding to different sets vary OCcasionally in slope, as well as in intercept. Apart from such systematic differences between sets there is, of course, also a random component of variability, shown by the fact that for any given set
relation between mole yc propylene and log (100 R). We will see that this relation is approximately a straight line of slope 0.022. Thus, the standard deviation of scatter, in per cent propylene units, is
the points corresponding to the various samples do not fall exactly on a straight line, but scatter more or less randomly about such a line. The statistical anaiysis consistent with this viewpoint yields, for each set, three parameters: the height of the line, the slope of the line, and the measure of scatter about the line. The height of the line is its ordinate a t a point representing the €,rand average of all the values for all samples. The variability of these height values among sets is identical 7%-iththe familiar main effect of sets in the analysis of variance. The measure of scatter %boutthe line is the familiar standard error of estimate, which is essentially the root-meansquare deviation from the line. The values of the three parameters are plotted in control-ch,trt fashion in Figures 3 and 4. The (harts for height and for slope are proJided with t a o sigma limits. The interpretation of these lines is based on the fact that if no systematic differences exist among sets, the chance for any one point to fall outside the control limits is only 5%. Thuq, if appreciably mlxe than 57c of the points fall outside the control limits, there is evidence that systematic differences exist among sets. The practical importance of this is that in such a case the precision of the method will be poorer on a set-to-set basis than m hen it is measured within sets. This, in turn, decreases thc reliability of determinations involving the use of a calibration line prepared a t another time. The following inferences can be drawn from Figures 3 and 4:
c
c
- 0.02140 Ci where CS represents mole % propylene Y V = 2.824
(the values assigned by the manufacturer) and Y R and YV represent the measurement log (100 R) for the raw and the vulcanized samples, respectively. The standard deviations measuring the scatter in these two plots, in units of log (100 R), are: For the raw samples:
Std. dev. =
O x 8
=
0.022
1.35 mole % propylene
For the conditions under which the measurements were made, the precision corresponding to this value is the best that can be obtained by this method. 2. For the raw samples, several points lie outside the control limits for the slope, indicating slight systematic differences among sets. Further calculations show that the contribution of these systematic effects to the over-all variability of the method would be noticeable only a t extreme concentrations (close to 0 or 100% propylene). 3. For the vulcamzed samples, all but one point lie inside the control limits. The single outlying point is due to Run 1A, in which the measured values tended to be slightly low. Thus, on the whole, the measurements made on the vulcanized samples show little evidence of systematic differences among sets.
s =
s =
i
1
~
c
0.0480
Converted to mole values are
o.O4o7 = 0.022
% propylene, these
1.85 mole-% and
0.0480 = 0.022 2.18 mole-% ~
S o w , each point in the calibration curve is the average of 10 determinations for the raw samples and the average of 12 determinations for the vulcanized samples. From the measure of internal precision, std. dev. = 1.35 mole yo propylene, one would expect a scatter in the calibration curves measured by:
Std. dev.
=
1.35 r0 0.43 mole 74 propylene =
for the raiT- samples and Std. dev. = 1.35
z/~0.39 mole % propylene =
for the vulcanized samples. Actually me obtain 1.85 instead of 0.43 and 2.18 instead of 0.39 as the standard
Y E = 2.759 - 0.02197 CS
I-
0.0407
For the vulcanized samples :
B. Calibration. I n Figures 5 and 6 the measured values for the raw and the vulcanized samples have been plotted against the values of mole yc propylene assigned by the manufacturer. I n each case, the measurements are the averages of all the sets-Le., they are the values shown in the last row of Tables I1 and 111. For both plots straight lines have been fitted to the data by the method of least squares. The equations of these lines are : For the raw samples :
1. The standard deviation of scatter is the same for the raw and for the vulcanized samples. It; average value is 0.0298, in the scale of log (100 R). I n order to convert thi!s value to mole yGpropylene we must fi *stestablish the
3
For the vulcanized samples:
1
h
-i
J
\
'.,
c j
1 13
,
~ 20
,
1 30
/ 40
1
, 53
1
1
60
~ 7C
,
I
80
90
IO0
"C
1
0
MOL E % P R O P Y L E N E
Figure 5.
Calibra,tion curve for raw samples
The mole % propylene values are those assigned nominally to samples b y the supplier. Each point is the average of 10 determinations. The least square equation for the solid line is: YE = log lo(iC!C R j = 2.759 0.02197 CS where Ca i s mole 7"propylene
-
20
1
30
4C
50
1
60
73
50
1 I
1
93
30
M O L E % PQOPYLENE
Figure 6.
Calibration curve for vulcanized samples
The mole % propylene values a r e those assigned nominally to the samples b y the suppliers. Each point is the average of 1 2 determinations. The least square equation for the solid line is: YV = logl,JlOO R ) = 2 . 8 2 4 0.021 4 0 Co where CS is mole % propylene
-
VOL. 35, NO. 13, DECEMBER 1963
2175
deviation of scatter for the calibration curves. The explanation may be found, in part a t least, in the observation that points corresponding to the same sample in Figures 5 and 6 show the same pattern of deviation from their respective calibration lines. This suggests the possibility that the assigned values may themselves be in error. C. Relation between Results for Raw and Vulcanized Samples. Figure 7 is a plot of t h e measurements on t h e vulcanized samples us. t h e corresponding measurements on t h e raw samples. T h e straight line shown in this figure was obtained by assuming equal precision for t h e abscissa a n d t h e ordinate. The equation of t h e line is
Yv
=
0.135
s = 0.0066
This value, converted to p a cent propylene units, yields: Std. dev. = 0.022
-
0.30 mole
% propylene
This estimate is in agreement with the ’ provalues 0.43 and 0.39 mole % pylene derived from the precision of the data. This result lends quantitative support to the assumption that the assigned values for the propylene concentrations may be in error. DISCUSSION
The pyrolysis products collected from the copolymers were mainly liquids and amounted to about 40y0 of the polymer content of the original sample. However, the polyethylene homopolymer gave a wax solid. The crystalline nature of this pyrolyzate undoubtedly is a major reason for the deviation of R for this sample from the curve obtained using the copolyr ars and polypropylene. Consequently, the polyethylene R value was not included in calculating the line shown in Figure 5. The vulcanized samples were pyrolyzed without extractions. Preliminary studies showed no detectable differences between extracted and nonextracted vulcanized samples used in this study. However, in the procedure recommended by Harms (9), extraction is suggested for finished commercial samples that may contain volatile plasticizers and/or organic fillers which could yield interfering pyrolysis products. The random sequence of the analysis within each set ensures that possible drift in the instruments or other causes of gradual or periodic shifts will not introduce systematic errors in the calibration of the procedure. The
2176
The values plotted are the ordinates of Figures 5 and 6. The least-square equation for the solid line is:
Vv = 0.135
4- 0.9747 YE
+ 0.9747 Y R
The standard deviation of scatter about the line is
o’oo66 --
Figure 7. Comparison of results for vulcanized and raw copolymers
ANALYTICAL CHEMISTRY
AVG. OF LOG,, I O O R ( R A W )
agreement of the two calibration curves
ACKNOWLEDGMENT
is demonstrated in Figure 7 where the average values of loglo(100 R ) for the
The author< express their appreciation to 11.Ifan Steel, for carrling out the statistical computations. to Kathryn Wharton for obtaining the IR 21 spectra. and to the Hercules Powder Company for supplying the ethylenepropylene copolymers.
raw polymers are plotted against thore for the vulcanized polymers. The lower point on the left represents the polymer containing 100 mole % propylene. and the highest point on the right represents the polymer containing 10 mole yG propylene. Each of the points falls ver\nearly on the least-squares straight line because now the assigned concentration uncertainty is avoided. That this line is less than 45’ (approximately 44’) is probably due to the vulcanization effectively reducing the availability of propylene units to form vinylidene products. Since the cros-links ( I , I S ) on vulcanization are presumed to occur principally a t the tertiary hydrogens in the propylene units, the available propylene concentration would be reduced to a small extent. The deviation is greater a t the end repreienting 100yO propylene. This method is applicable for determining propylene concentration in copolymers that vary over an extremely broad range. To our best knonledge it is the first analytical method that has been suggested for determining propylene concentrations in vulcanized copolymers. The method should be of special usefulness in the analpi. of finished material.. Although it is recommended that one bhould firat establish a calibration curve, the equation> and method presented here could be used to give fairly accurate edimates of propylene concentrations IT ith doublebeam infrared instruments. The formula for calculating the mole % propylene i. %C, = K
- a lOglo(100 R )
where K = 125.6 and a = 45.52 for the raw samples, and K = 132.0 and a = 46.73 for the vulcanized samples cured by a recipe similar to the one given in Table I.
LITERATURE CITED
(1) Amberg, L. O., Robinson, A. E., Rubber Plastzcs Age 42,8175 (1961). ( 2 ) Bau, E., AIanaresi, P., ANAL. CHEY. 31, 2022 (19.59). ‘3) Corish, P. J., Ibzd., 33, 1798 (1961). (4) Cross, L. H., Richards, R. B., Willis, H. A., Dascussions Faraday SOC.9, 235 (1950). (5) Drushel, H. I-.,Iddings, F. il., ANAL. CHEY.3 5 , B (1963). (6) Drushel, H. V., Iddings, F. A., Division of Polymer Chemistry, 142nd
Meeting. ACS, Atlantic City, September
1962. (7) Ehy, L. T., Fusco, J. V., Rubber Age 91, 949 (1962). 181 Gossl. T., Illakromol. Chem. 42. 1
p. 1156. (11) IIandel, J., J . . l m Statistzcnl dssn. 56,878 (1961). 112) Mandel. J.. Lashof. T. IT..-4SY.U Bull. s o . 239,53 (1959). (13) Satta, G., Crespi, G., Diguilio, E., Ballini, G., Bruzzone, RI., Rubber P2astic.s Age 42, 53 (1961). 114) Katta, G., Crespi, G., Massanti, G., 1 dvassori, -4., Satori, G., Scaglione, P., Rubber -4ge 89, 636 (1961). (15) S a t t a , G., Mamanti, G., T’alvassori, A., Pajaro, G., Chim. Ind. ( J f z l a n ) 39, 733 (1957). (16) Small, ’E. B. bf., ANAL. CHEM.33, 1798 (1961). (17) Smith, D. C., Ind. Eng. Chenz. 48, 1161 (1956). (18) Smith, W.E., Stoffer, R. L., Hannan, R. B., J . Polymer Sei. 61,39 (1962). (19) Tryon, M., Horowitz, E., Mandel J., J . Res. .Vutl. BUT.Std. 5 5 , 219 (1955). (20) Van Schooten, J., Duck, E. IT.> Berkenbosh, R., Polymer 2, 357 (1961). (21) Wei. P. E.. ANAL. CHEM.33. 215 (1961).’ ~
RECEIVEDfor review April 17, 1963. hccepted July 26, 1963.
