Determination of Hydroxyl Numbers by Near-Infrared Absorption C.
L. HILTON
Research Center,
U. S.
Rubber Co., Wayne, N. J.
b Duplicate analyses by the acetylation procedure for hydroxyl number require 1.5 man-hours and 3 hours of elapsed time. A faster method of analysis was desired. Duplicate determinations of hydroxyl numbers of certain polyesters and polyethers by near-infrared absorption analysis in t;.e region from 2.0 to 3.2 microns can be accomplished in 0.5 hour using a Beckman Model DK-2 spectrophotometer. This represents a saving of 1 man-hour and 2.5 elapsed hours per duplicate determination. Results with samples thus far analyzed s'-ow an average difference of less than 1 .O% relative between the chemical and the near-infrared methods.
T
HE usual method (IS) for determining hydroxyl numbers is by acetylation with acetic anhydride in pyridine. This requires a reflux period, hydrolysis of the excess acetic anhydride, and titration of the acetic acid formed by this hydrolysis. Duplicate analyses on a single sample require about 1.5 man-hours and 3 hours of elapsed time. The present paper discusses the determination of hydroxyl numbers of certain polyesters and polyethers by near-infrared absorption analysis in:the region from 2.6 to 3.2 microns. Duplicate determinations can be accomplished in 0.5 hour using a Beckman Model DK-2 spectrophotometer-a saving of 1man-hour and 2.5 elapsed hours per duplicate determination. The excellent reviews of the applications of near-infrared spectroscopy by Kaye (IO,11) and the work of Flett (4) led to the utilization of the absorption bands in the region from 2.7 to 2.9 microns for the quantitative determination of the hydroxyl contents of polyesters and polyethers. Barrow has discussed intensities of infrared hydroxyl bands (I) and Cannon hydrogen bonding (3). Mitchell, Bockman, and Lee (IS) used the range from 1.3 to 1.6 microns for the determination of acetyl content of cellulose acetate. Goddu (6-7) has studied phenolic hydroxyl, acids, alcohols, hydroperoxides, and oximes using a Beckman DK-2 instrument. Kaye (9) surveys the lib
1610
ANALYTICAL CHEMISTRY
erature from 1955 through 1958. Two additional articles utilizing the wavelength regions around 3 microns have just appeared (8, 8). Inasmuch as near-infrared absorption bands, with few exceptions, are due to overtones and overtone combinations involving hydrogen stretching vibrations, the only solvents which are optically transparent in this region are those containing no hydrogen atonis. Thus carbon tetrachloride and carbon disulfide are the only common solvents that meet this requirement. Because carbon' tetrachloride has lower volatility and a less disagreeable odor, it is preferred. However, most of the polyesters and polyethers involved in the present investigat,ion were insoluble or only partially soluble in carbon tetrachloride alone. From the optical standpoint, 10% (by volume) solutions of chloroform, tetrachloroethane, dioxane, iso-octane, cyclohexane, or petroleum ether were found to be satisfactory in the region from 2.6 to 3.2 microns. From the standpoint of solubility of the samples, chloroform was the best solvent. Consequently, the samples were dissolved in chloroform and diluted with carboh tetrachloride to give a final concentration of 10% chloroform by volume. The 0.50% ethyl alcohol contained in the analytical reagent grade chloroform as a preservative presented no problems so long as the same lot number was used in the sample and reference cells of the double-beam instrument. APPARATUS A N D MATERIALS
The instrument used was a Beckman Model DK-2 recording spectrophotometer with a temperature-regulated cell holder. The instrument settings were: scanning time 5; scale expanded 2X; time constant 0.2; sensitivity 0.50. At this sensitivity the nominal slit widths obtained were 0.36 mm. a t 2.90 microns; 0.21 mm. a t 2.85 microns; 0.19 mm. at 2.80 microns; and 0.35 mm. at 2.75 microns. Speciallymatched (within d=O.OlO absorbance unit) 1.00cm. silica cells were obtained from Beckman Instruments, Inc. These cells were satisfactory from 1.0to 3.2 microns. Reagent grade chemicals were used throughout.
PROCEDURE
The sample is weighed into a tared 1Wml. volumetric flask and dissolved in 10.0 ml. of analyzed reagent grade chloroform delivered from a buret. The sample size ranges from 1.0 to 2.0 grams, depending on the amount of hydroxyl group present. The solution is diluted to exactly 100 ml. with reagent grade carbon tetrachloride, and 1gram of anhydrous sodium sulfate is added to remove any water present. The solution is shaken for about 1 minute and the sodium sulfate is allowed to settle. The near-infrared spectrum from 3.2 to 2.0 microns is determined using a Beckman h,.Iodel DK-2 recording spectrophotometer. Although the useful range of the spectrum is from2.G to 3.2microns, the complete spectrum is obtained. In this way the near-infrared spectra of various types of materials can be compared from a series of standard spectra. The hydroxyl number is calculated using the proper equation. Where a p plicable, the acid number (milligram of potassium hydroxide per gram of sample) is determined and converted to per cent of free carboxyl. THEORY
The relative absorption of energy by the hydroxyl group compared with such groups as ester carbonyl, 4 H - ,
I
-C-H,
I
and -CHs
is sufficiently high
that changes in the proportions of these functional groups will have little effect on the total absorption. Free carboxylic acid groups, however, interfere. Consequently, a separate ,titration to determine the amount of free acid is necessary. If it is assumed that the absorption due to each functional group present in a compound is proportional to the amount of that group present, we obtain :
+
a,(compound) = a, group 1 ( X ) a. group 2 (Y) a, group 3 (1-X-Y) (1)
+
where X, Y, irnd 1-X-Y are the fractions of groups 1, 2, and 3,respectively, and a. is the absorptivity for the material concerned, absorptivity equsls A,/bc. A. is the absorbance of the solution, b is the optical path in centimeters, and c is the concentration in grams per liter. For the work with the polyesters,
n
FIGURE
NEAR-INFRARED SPECTRUM OF POLYESTER OF ETHYLENE GLYCOL AND ADIPIC ACID
700
.7 00
FIGURE 2
NEAR-INFRARED SPECTRUM OF POLYETHER OF GLYCEROL AND PROPYLENE OXIDE
A B
,600
A
sB
S 0 R
B A
n
400
-
,300
-
.PO0
-
B NA
N
E
400
4 \
C
i\
,500
0 R
300
WAVE LENGTH (MICRONS) 2.3
grouli 1 is taken as thv lij-clrosyl group, group 2 is the carboxyl, and group 3 represents the remainder of the compound. Equation 1 then becomes: 0 a, polyester = a. OH
(Y)
(x) + a,-/
+ a. remainder (1-X-Y)
\H (2)
For polyethers, there is no carboxyl present, so the equation can be further modified: a, polyether = a, OH ( X ) a. remainder (1-X) (3)
+
For this work peak absorptivities were used without attempting to correct for background. RESULTS
Because the position of the hydroxyl band and the relative ahorptivities of various groups vary from compound to compound, each new polyester or polyether must be calibrated using the results obtained by the chemical method. After the original calibration, the nearinfrared procedure may be used. The preliminary determinations were made using the ordinary cell holder supplied with the instrument. Failure to obtain duplication of absorptivities on repeat runs led to an investigation into the effect of tcrnperature on the determination. The temperature within the cell compartment was found to rise from 24.5' to 37.0' C. over a &hour period. The absorbance (log 1/T) of one particular solution was found to drop 0.010 for each degree rise in temperature. This corresponded to a change in absorbance over the temperature range involved. This decrease in absorptivity with increasing temperature indicates intramolecular bonding such as that reported by Burns and Muraca (9) for poly(propy1ene glycols).
wo
24
Beckman Instruments, Inc. tested their instruments at their Californiaplant .and found that this temperature rise is normal. Because such a variability in absorbance could not be tolerated, a ternperature-regulated cell holder was purchased. This permits the regulation of the temperature within the cell compartment to *0.1" C. With this a p paratus, excellent reproducibility was obtained. Table I shows the absorptivity values obtained for a series of polyesters of ethylene-propylene glycol and adipic acid. Figure 1 shows the absorption spectrum of such a polyester. The hydroxyl number of this polyester was 62.4, the acid number was 0.5, and the concentration was 9.08 grams per liter. The values for per cent hydroxyl and per cent carboxyl were obtained by titration procedures. By placing the experimental data in Equation 2, we obtain a series of equations which can be readily solved for absorptivity of the various groups. The absorptivities for the carboxyl and the remainder of the compound were found to be 0.400 snd 0.02918, respectively. W h e n the experimental data are inserted in Equation 2, we obtain:
% hydroxyl
= 40.5 a, 2.m
-
0.15%~Srbo~yl - 1.18 (4)
As shown in Table 11, the average per cent Merence between the chemical method and the near-idrared procedure is less than o.5y0 relative. The precision is of the same order of magnitude. The hydroxyl number is defined as the number of milligrams of potassium hydroxide equivalent to the acid conaumed in esterifying the hydroxyl groups present in 1 gram of sample. The per cent hydroxyl can thus be converted to hydroxyl number by multiplying by 561/17.0 or 33.0. The acid number . . is defined as the number of mdhgmms of potassium hydroxide required to neutralize 1 gram of sample.
e5
2 6
2,7
21)
2 9
3 0
31
Table 1. Absorptivity Values for Ethylene-Propylene Adipate Sample yo yo a, at 2.83 Microns No. OH COOH Polyester Hydroxyl 1 2.68 0.07 0.09530 2.497 0.09579 2.516 0.09570 2.513 2 1.89 0.04 0.07553 2.489 2.509 0.07589 0.07588 2.508 3 1.82 0.05 0.07342 2.453 0.07405 2.488 0.07108 2.499 4 1.69 0.17 0.07120 2.506 0.07116 2.504 0,07108 2.499 5 1.23 0.09 0.06003 2.528 0.05993 2.520 0.05986 2.514 Av. 2.502 Table 11. Comparison of Results by Chemical Method and Near-Infrared
% Hydroxyl Sample NearNo. Chemical infrared 1 2 3 4 5
2.675 1.887 1.820 1.683 1.230
2.682 1.882 1.802 1.676 1.234
% Difference
(Relative)
0.26 0.26 0.99 0.42 0.33 Av. 0.45
If the equation is expressed in terma of hydroxyl number and acid number, we obtain: Hydroxyl NO. = 1337 a. I.= - 0.396 acid No. - 38.94 (5) When a series of polyethers of glycerol and propylene oxide were analyzed by near-infrared, the absorptivities for the hydroxyl group and the remainder of the polyether were found by inserting the data given in Table 111 in Equation 3. The absorptivity for the hydroxyl group was found to be 2.581, while that for the remainder was 0.0149. Figure VOL. 31, NO. 10, OCTOBER 1959
1611
Table 111.
If the hydroxyl number is desired, Equation 6 is multiplied by 33.0 to give :
Hydroxyl Content of Polyethers
% % HYdroxyl a. 2.78 HY(Chem- PolyHy- drox 1 Sample ical) ether droxyl Near-?R LG42
1.45 0.05154 2.561 1 . 4 3 0.05210 2.600 0.05158 2.564 r,~t37 2.28 0.0741ii 2 . ~ 2 5 2,s 0.07431 2.616 0.07392 2.599 LG112 3.76 0.11107 2.577 3.75 0.11098 2.574 0.11088 2.572 LB240 7.11 0.19650 2.569 7 . 0 8 0.19504 2.549 7.13 0.19587 2.561 Av. 2.581
1.44 1.46 1.44 2.34 2.33 2.32 3.77 3.77 3.76 7.12 7.06
7.09
2 shows the absorption spectrum of a polyether with a hydroxyl number of 47.5. The concentration was 13.0 grams per liter. The equation devised for the per cent hydroxyl in the case of this polyether was:
% hydroxyl
= 39.2 a,
2.87
- 0.58
(6)
Hydroxyl No. = 1294 a . : . ~- 19.1 (7) The near-infrared procedure has been applied to polyesters of several M e r e n t glycols with various dibasic acids with excellent results. Although the work with polyethers has been more l i i t e d than that with the polyesters, results thus far obtained have been satisfactory. INTERFERENCES
Acids, alcohols, hydroperoxides, and all other hydroxyl-containing compounds absorb in the region from 2.6 to 3.2 microns. Amides, amines, and oximes also interfere. As the comrosition of each polyester or polyether will be known qualitatively, equations can be readily developed to correct for any interferences present. LITERATURE CITED
( I ) Barrow, G. M., J . Phys. Chern. 59, 1129-32 (1955).
(2) Burns, E. A., Muraca, R. F., ANAL.
CAEM.31, 397 (1959). (3) Cannon, C. G., Speetrochim. -4ch 10, 429 (1958). (4) Flett, M. St. C., ZW.,10, 21 (1957). (5) Goddu, R. F., ANAL. CHEM.30,
2009 (1958). (6) Goddu, R. F., Pittsburgh Conference
on Analytical Chemistry and Applied Spectroscopy, March 1958. (7) Goddu, R. F., Delker, D. A., ANAL. CHEM. 30,2013 (1958). (8) Kabssskalian, Peter, Townley, E. R., Yudis, M. D., Zbid., 31,375 (1959). (9) Kaye, W., Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1959. (10) &ye, W., Spectroehirn. Acta 6 , 257 (1954). (11) Zbid., 7, 181 (1955). (12) Mitchell, J. A., Bockman, C. D., Jr., Lee, A. V.,ANAL.CHEM.29,499 (1957). (13) Ogg, C. L., Porter, W. L., Willits,
C. O., IND.ENG.CHEM.,ANAL.ED. 17, 394 (1945).
RECEIVED for review March 27, 1959. Accepted June 15, 1959. Pittsburgh Conference on Analytical Chemuirtry and Aplied Spectroscopy, Symposium on dar-Infrsred Spectroscopy, March 1959. Contribution 181.
Terpolymer Rubbers Standardization of Infrared Analysis Radiotracer Methods
by Chemical and
GEORGE B. STERLING, JOHN G. COBLER, DUNCAN S. ERLEY, and FRED A. BLANCHARD The Dow Chemical Co., Midland, Mid. ,A procedure, involving the use of radioactive tracer techniques combined with standard chemical methods, has been developed for standardizing infrared spectroscopic measuhments of a methyl isopropenyl ketonebutadiene-acrylonitrile terpolymer. The technique is generally applicable where a component of the polymer can b e synthesized from radioactive materials. The infrared analysis, standardized by this technique, has been applied to the accurate product control of this terpolymer.
P
materials are tailored for specific end uses by copolymerising two or more monomers in definite proportions. Because the physical properties exhibited by the synthetic polymers depend largely on exact polymer composition, which generally ditrers from the monomer charge ratio, effort has been devoted to methods for their analysis. Infrared spectroscopy has OLYMERIC
1612
ANALYTICAL CHEMISTRY
proved particularly useful, as it is rapid, above determinations. Furthermore, specific, and accurate; but aamplea of the polymer cannot be redissolved in a known composition are required as solvent after being freed of water by any standards for quantitative work. Thia of the standard techniques such as a m paper describee a novel approach to the tropic distillation of the water, free% development of an infrared method for coagulation, or acid coagulation. The determining the composition of one of insolubility of the isolated polymer ie these tailored producta-a terpolymer thus a deterrent to most methods of of methyl isopropenyl ketone (MIE), chemical analysis. The calculation of butadiene, and acrylonitrile 0. acrylonitrile from a Kjeldahl nitrogen The method could be readily used on determination has been generally acmany products of this type. Cepted. Pepe, Kniel, and Cmba (7) have deCaldulation of the concentrationof the scribed an ultraviolet spectrophob monomer units from the elemental metric method for determining methyl analysis (carbon, hydrogen, nitrogen, isopropenyl ketone in polymers. The and oxygen), however, did not agree standard procedure for determining buwith the charged monomer ratios. The tadiene is to determinethe amount of unacrylonitrile concentration calculated saturation by iodine monochloride ad& from the Kjeldahl nitrogen value was tion (3,6, 6). These methods are a p about 10% lower than the charged ratio. plicable only if the polymer can be disDetermination of the nitrogen by #e solved in a suitable solvent. This Dumas method gave d t a which were terpolymer, however, is prepared by about 10% higher than the Kjeldahl emulsion polymerization and the h a l method, and agreed reasonably well product is obtained in latex form. T h e with the charged monomer ratio. This latex emulsion is not suitable for the revised nitrogen value, however, failed
