Quantitative Determination of Some Inhibitors in Polymers by Ultraviolet Light Absorption F. W. BANES AND L. T. EBY, Standard Oil Development Company, A rapid and accurate method of analysis for inhibitors in polymeric materials employing ultraviolet light absorption has been developed which is especially desirable for inhibitors present in small concentrations. Correction for linear "background" absorption is made without need for direct measurement.
0
RGANIC polymeric substances are commonly subject to attack by atmospheric oxygen, which will cause changes in the physical properties of the polymers. Inhibitors are usually incorporated to prevent or retard polymer degradation caused by atmospheric and imposed oxidizing conditions. The use of stabilizers is, therefore, very widespread and is especially important in synthetic rubbers. Inhibitor concentration is an important factor in estimating the potential stability of a polymer. The Concentration requirements for stabilizing polymers of different types will vary with the type of polymer. The use of small concentrations of inhibitor has create4 an urgent need for a more precise method of inhibitor analysis. Production control requires an easy and rapid method of analysis as well as accurate results, even where higher concentrations of inhibitor are used. N-Phenyl-2-naphthylamine(commonly called phenyl-0-naphthylamine) has been used very widely in the stabilization of synthetic elastomers and, therefore, an effort was made to develop the best analytical method for this specific inhibitor. A method of analysis of primary aromatic amines involving titration R ith standardized nitrous acid (6) has been applied to its determination; however, when this method was used to determine the inhibitor present in a polymer in small concentrations, it was found unreliable in the absence of very careful contiol and expert manipulation. Craig ( 2 ) estimated the concentration of N-phenyl-2-naphthylamine in rubber by isolation of the hydrochloride. An adaptation of this method to Butyl rubber by a turbidimetric measurement of the amine hydrochloride has been used for production analysis but was unsatisfactory unless very careful control was maintained. An oxidative method usjng ceric sulfate has also been used with somewhat more reliable results. Investigation of rapid physical methods of analysis revealed that N-phenyl-2-naphthylamine has a very strong absorption in the near-ultraviolet region (4, 6, 7 ) which is very desirable for spectral analysis. Qualitative spectrographic determination of various rubber ingredients, including S-phenyl2-naphthylamine, has been described ( 3 ) . The use of spectrophotometry now appears to be the best method of analysis for N-phenyl-2-naphthylamine in polymers. Since polymers and various blending ingredients are not entirely transparent to ultraviolet light, it is necessary to subtract the "background" absorption (absorption due to materials other than S-phenyl-2-naphthylamine) to obtain a true estimation of inhibitor concentration in the polymer. A mathematical adaptation of the method which Wright (8) used in infrared spectroscopy in correcting for the background absorption, without actual measurement of the background absorption, was applied to this determination with considerable success. The method can be satisfactorily used with any polymer which can be dissolved in a solvent, or from which the inhibitor can be extracted by a solvent, where the background absorption is linear through the spectral points or wave lengths at which the inhibitor exhibits a maximum and two adjacent minimum absorptions. The correction for linear background absorption by optical density measurements a t three wave lengths is outlined below. Although certain forms of this mathematical approach are in use, to the authors'
Elizabeth,
N. J.
knowledge it has not been previously reported. (Derivation and first application of this equation to polymer analysis was carried out in this laboratory by T. S. Chambers.) TKOassumptions are made: (1) Beer's law is obeyed and (2) the background absorption is linear with wave length over the portion of the spectrum involved. The former assumption has been found valid for the concentrations employed. Knowledge of the system used in the analysis will determine whether the latter is true or is a rlose approximation which would introduce but little error. Figure 1 is a typical illustration of the absorp,tion of a solution of polymer containing an inhibitor.
.
SPECIFIC fXTlNCTlON COEFFICIENT
Khi" -
I
A""' WAVELENQTH
Figure 1.
Typical Absorption of Solution Containing an Inhibitor
of Polymer
K'min, Kmax, and K m i n are observed specific extinction coefficients of the polymer sample being analyzed for inhibitor content. Kimih,KFaX,and Ki:'" are t,he specific extinction coefficients of the background and K L m i n , K Y , and K P n are the specific extinction coefficients for the inhibitor. Let x = fraction of inhibitor in polymer; and 1 - r = fraction of polymer or background. Then I('min
r~K,'rnin + (1
- 2) KLmin
+ (1 - 2) Kp"x
Km,*x
zK,mnx
Kinin
xKF" + ( 1 -
2) K$'n
(1)
(2) (3)
The K's and Ki's are experimentally determined, leaving four unknowns. The vnlue of x can be determined without knowing KL'Sif one of the Kb's is known in terms of the others. From Figure 1 : (1 - ~)Ki:l""
535
(1 - 5)KF'"
+
536
INDUSTRIAL AND ENGINEERING CHEMISTRY
Substitute values of KLminand KY" from Equations 1 and 3 in Equation 5 : Kmax = (1 - n)(Kmin - .&min) + n(K'min - ~K'min) (7) 1--2
Table
I.
Wave Length, mr 282
Substitute value of Kpaxfrom Equation 7 in Equation 2:
284
Since K =
508 309 326 332
D LC
where D = optical density = log
L
I solvent I solution ~
= cell length = 1 em. C = concentration of polymer in solution in grams per liter I solvent = intensity of light through pure solvent I solution = intensity of light through polymer solution
Vol. 18, No. 9
Specific Extinction Coefficients of N-Phenyl-%naphthylamine and Uninhibited Polymers ~
Specific Extinition Coefficients, K (1. g.-1 CIII. -1) GR-5 in Perbunan in P B N in Butyl rubber P B N in CzHaCIz CzTI4Cla CpH4Ck iso-octane In iso-octane 49.7 0.01-0.06 54.0 ,.. 52.0 .. ., 51.5 0.05 0'03 85.6 01-0.04 85.0 ... ... 83.5 87.0 0.04 0.02 13 2 0.01'" 17.0 ... ... ..... 18.0 0.01 0.01 15.0
and their use permits simplification of Equation 9 to the following expressions:
yoPBN in Butyl rubber
=
1.73
(D308
- 0.60326 - 0.'40282) C
(10)
then
% inhibitor in polymer
% ' PBN in Perbunan and GR-S =
=
- (1 - n ) D d n - nD'dn] C [ K y x - (1 - n)Kmin - n K : m l n ] (9) 100[Dma=
EXPERIMENTAL
The choice of solvent should be such that it will dissolve both polymer and inhibitor and it must be easily ,obtained in a pufe state and preferably show low and linear absorption in the spectral region employed. Ethylene dichloride and chloroform were satisfactory solvents for GR-S and Perbunan. These solvents as well as carbon tetrachloride and iso-octane were suitable solvents for Butyl rubber. A Beckman quartz prism spectrophotometer (1) with a hydrogen discharge tube serving as the light source was used for all optical density measurements. Quartz cells with a light path of 1 em. held the liquid samples. Extinction coefficients of the inhbitors, in the solvents to be used for the analyses, were determined by optical measurements a t the wave lengths of the maximum and two adjacent minimum absorptions best suited for analytical purposes. Several concentrations were employed and an average extinction coefficient was calculated for the inhibitor a t each of the three wave lengths. It is important that measurements of extinction coefficients of inhibitors be made for each instrument, since the exact dispersion, resolving power, actual slit width, photocell spectral sensitivity, and amount of reflected and scattered light of the instrument are factors which affect the optical density measurements. Since only small samples of polymer were employed, care was exercised to get truly representative results. This was accomplished by milling and refining a sample of rubber from which portions were taken for analyses. AIilling was also beneficial in destroying gel in polymers which would introduce error from light reflection. Excessive milling and heating in contact with air were avoided. An accurately weighed portion of polymer was placed in a glass-stoppered Erlenmeyer flask and a known volume of solvent added from a pipet. Dissolution occurred on standing 1 or 2 days with occasional shaking, or i t was accelerated by stirring or heating. A clear sample was removed from any settled insoluble material (centrifuging may be used) and transferred to the quartz cell for optical density measurements. A matched quartz cell containing a solvent control was used as the reference zero optical density. The concentration of inhibitor in the polymer was calculated by means of Equation 9. Using this procedure, only a few minutes are required to determine the inhibitor content of a sample of dissolved polymer. DISCUSSION
The specific extinction cpefficients of A--phenyl-%naphthylamine showed slight variation in numerical value and spectral position with the solvent employed. The evaluation of the coefficients used in calculations in this paper are given in Table I
(D3'9
1.83 - 0.520332- 0.480*84)C
(11)
where the solvent for Butyl rubber is iso-octane and that for Perbunan and GR-S is ethylene dichloride. Solutions containing about 0.01 gram of N-phenyl-2-naphthylamine per liter (or about 0.3 gram of Butyl rubber or 0.05 gram of Perbunan or GR-S per 100 ml.) were most satisfactory for optical density measurements with the Beckman instrument. These solutions have opticat densities in the range of 0.1 to 1.0 for the wave lengths prescribed. The advantage of the ultraviolet absorption method for t h e determination of N-phenyl-2-naphthylamine in Butyl rubber a s compared u ith the turbidimetric determination by precipitated hydrochloride is evident from Table 11. An oxidation method involving ceric sulfate oxidation of the inhibitor gave results in good agreement with those obtained by the ultraviolet method.
Table II.
Comparison of Methods of Analysis of Butyl Rubber for N-Phenyl-%naphthylamine % N-Phenyl-%naphthylamine in Polymer
Polymer Composition Pure Butyl rubber Pure Butyl rubber 0.25% P B N a Pure Butyl rubber 2.5% ainc stearateo Pure Butyl rubber 2.5% zincstearate f 0.25% P B N a Butyl rubber (light colored)
+ + +
0
Chemical lrethods Hydrochloride Ceric turbidimetric sulfate 0.00 .
0.40
.. ... ... ...
0.63
0.38
0.22
0.00
Ultraviolet Method Corrected Uncorrected for backfor background ground 0.00 0.01 0.34
0.26
0.00
0.03
0.23
0.26
0.35
0.41
Added on mill.
The calculation of inhibitor content by using only the wave length of maximum absorption gives values listed in Table I1 as uncorrected for background. This calculation represents the maximum amount of inhibitor that can be in the polymer and is made by Equation 12: Maximum possible % PBN in polymer =
The uncorrected value does not always agree with that corrected for background absorption but tends to be higher where the background absorption is greater. The dark-colored Butyl rubber sample in Table I1 showed no inhibitor present by corrected ultraviolet analysis, while the un-
ANALYTICAL EDITION
September, 1946
537
Even fresh polymers may, however, become contaminated with impurities by milling or improper washing, so that the magType Polymers for N-Phenyl-2-naphthylamine nitude of the background absorption is increased, thereby making 5 S-Phenyl-2-naphthylamine in Polymer a background correction necessary if qccurate and reproducible Ultraviolet X e t h o d ~ k , ~method, ~ ~ i ~ ~ i rewlts are required. In Table I11 data are presented illustrating Corrected Uncorrected nitrous acid for backf o r backtitration of the application of ultraviolet absorption and Equations 11 and Polymer Compositions ground ground H O d c extract 13 to the determination of S-phenyl-2-naphthylamine in Buna0 00 Unstabilized Perbunan 0.00 0.02 type polymers, In certain cases the antioxidant was extracted Perbunan + 1.9% P B N added ... on mill (A) 1.91 1.99 from the polymers with glacial acetic acid and the extracts tiPerbunan (-4) milled 10 min... Utes a t 20' C. 1,88 1.96 trated with nitrous acid to starch-iodide end points. In these Perbunan (A) milled 10 mincases special precautions xere taken to obtain complete extrac... 1.80 1.96 Utes a t looo C. Perbunan (-4) aged 4 days, air tion and avoid losses which generally accompany this type of oven 85" C. 1.79 1.87 ... Perbun&n + 2.0% P B N added analysis. 1.91 on mill (B) 1 98 2 05 Perbunan commercial product Data are also presented in Table I11 t o show the effect of nat1.65 (C) 1 64 1.69 ural and imposed oxidizing conditions on the antioxidant contents Perbunan (C) aged 46 days, ... air oven, 60' C. 1 32 1.42 of polymers. Ultraviolet analyses of such polymers show not Perbunan, dark colored stock I 85 2.26 Unstabilieed GR-S 0 00 0.05 ,.. only decreased amounts of antioxidant present in the aged polyGR-S + 0.5% P B N added on mers, but also a variation in the tTvo methods of calculation 0.58 mill (A) 0 51 0.54 GR-S + 1.96% P B N added (Equations 11 and 13) which can be accounted for only by 1 . 9 8 on mill (B) 1.96 1.99 GR-S (B) aged 46 days, air assuming that oxidation products of the inhibitor and polymer oven, 60' C. 0.74 1.24 ... are contributing to the background absorption. a Parenthetical letter refers t o speoifio polymer-inhibitor compositions. Inspection of the absorption spectra of highly oxidized Perbunan and G R S polymers Table IV; Effect of A g i n g of Buna-Type Polymers on Ultraviolet Absorption and Inhibitor Analyses reveal that the characteristic peak absorption of the N %violet Method Chemical method, Yted Tincorrected nitrous acid phenyl-2-naphthylamine a t 309 -~~~~~~ okf o r backtitration of mp has been a t least partially ground HOAc extract destroyed, and the absorption Perbunan Original 1.64 1.69 ... 0 447 0.732 0.136 0 500 50 days, air has become more linear with 0 206 0.500 0.27 1.05 ... 0,313 0.076 oven 82' C. wave length than is the case 1.81 1.85 1.82 GR-S OriginzJ 0 485 0.805 0.148 0.500 50 days, air for the absorption of the 0.36 0.77 0.43 oven, 82' C . 0 360 0.333 0.117 0,500 GR-S Original solupure inhibitor, Under such ... 1.96 2.02 tion 0 I58 0,545 0.500 0.878 circumstances, application of Same soluaion aged 10 days a background correction rein diffused sunlight a t sults in much more acroom temcurate evaluations of the inperature 0 202 0.716 0.642 0,500 , 1.11 1.65 hibitor contents of the polymers than either measurement of the optical density of the polymer solution a t only one wave length, or application of corrected analysis indicated that inhibitor was still present. analytical procedures which depend on the reaction of a Storage stability of this polymer was very poor, indicating that inhibitor was absent. functional group of the inhibitor. The nitrous acid titration From a large number of inhibitor determinations on Butyl procedure, for example, fails to distinguish oxidation prodrubber containing about 0.25% N-phenyl-2-naphthylamine, the ucts of N-phenyl-2-naphthylamine from the pure inhibitor if following probable errors were obtained when corrected for backamine groups are present in the oxidation products and if ground absorption: these groups will form nitroso derivatives. The work of Rehner, Banes, and Robison ( 7 ) has demonstrated that amine Mean deviation from theoretical 10.01% PBN compounds may occur in oxidation products of K-phenyl-2Maximum deviation from theoretical * 0.04% PBN naphthylamine. Mean reproducibility *0.01% PBN The changes in ultraviolet absorption and inhibitor content Maximum deviation from mean *0.04% PBN of Perbunan and GR-S polymers due to oxidation promoted by heat and light are further illustrated in Table IV. The more The ultraviolet absorption of GR-S and Perbunan polymers which contain small amounts of such impurities as hydroquinone, soap, fatty acid, persulfate, and mercaptan is very low as compared to that of N-phenyl-2-naphthylamine. Since larger Table V. Wave Lengths Applicable for Ultraviolet Determination amounts of antioxidant are used in the Buna-type polymers of Inhibitors (about' 2.0y0) than in Butyl rubber (about 0.25%). the backWave Lengths in mp for ground absorption of unaged Buna-type polymers is of less sigAnalyses Solvent h'min Amax Xmin Inhibitor nificance in determining the inhibitor contents of the polymers Aminox" Ethylene dichloride 246 291 355 by ultraviolet absorption. I t is then usually necessary to measAgerite Stalitea Ethylene dichioride 251 286 353 ure the absorption of a polymer solution only a t the wave length Inhibitor 8567a Iso-octane 246 277 300 N-Phenyl-2-naphIso-octane 282 308 326 of maximum absorption (309 m,u) and calculate the inhibitor conthylamine N-Phenyl-2-naphEthylene dichloride 284 309 332 centration by Equation 13. Table
111. Comparison of Different Methods of Analysis of Buna-
I . .
~~
Maximum possible
70 PBX in
Perbunan or GR-S =
thylamine N-Phenyl-2-naphthylamine
Chloroform
284
309
332
a Aminox, Amine, ketone condensation product. Agerite Stalite, alkylated aromatic amine. Inhihitor 8567, phenolic type.
INDUSTRIAL AND ENGINEERING CHEMISTRY
538
probably correct analyses are those obtained where background corrections have been made. This method of analysis has been extended to other inhibitors. It is most reliable where $rge values for maximum extinction coefficients are encountered, The wave lengths which were used for analyses for certain of these inhibitors in polymers are shown in Table V. CONCLUSION
A spectrophotometric method of analysis for inhibitors in polymers has been developed. This method corrects for background absorption and has been found to be rapid and more reliable than chemical methods of analyses which were investigated.
Vol. 18, No. 9
LITERATURE CITED
(1) Brogden, C. E., IXD. ENG.CHEM., AXAL.ED.,13,696 (1941). (2) Craig, D., Ibid.,9, 56-9 (1937). (3) Dufraise, C., and Houpillart, J.,Reo. ghn. caoz~tchouc,16, No. 2.
44-50 (1939).
(4) International Critical Tables, Vol. V, p. 364, New York, Mc-
Gram-Hill Book Co., 1929. (5) Lunge, G., "Teclinical Methods of Chemical Analysis", English ed., tr. by C. -4.Keane, Vol. I, Part 11, pp. 877, 927-8, New York, D. Van Sostrand Co., 1911. (6) Purvis, J. E., Proc. Cambridge Phil. Soc., 24, 421-5 (1928). (7) Rehner, J., Jr., Banes, F. W., and Robison, S. B., J . Am. Chem. SOC.,67, 605-9 (1945). (8) Wright, N., IND.ENG.CHEM.,AXALED., 13, 1-8 (1941). P R E S E X T Ebefore O the Division of Rubber Chemistry a t the 109th Xleetinp of the AXERICAS CHEsi1c.a S O C I E T Y Atlantir . City, S . J.
Analysis of Binary Mixtures of Normal Aliphatic Dibasic Acids and Esters' Use of Composition-Melting Point Relations of the Acids DAVID F. HOUSTON AND WALTER A. VAN SANDT2 Western Regional Research Laboratory Albany, Calif. Bureau of Agricultural and Industrial Chemistry, Agricultural Research S. Department of Agriculture Administration,
d.
A n empirical method is presented for determining the melting ranges of fused and quenched samples of dibasic acid mixtures. Application to binary mixtures of alternate and adjacent acids containing six to twelve carbon atoms reveals that compositions may b e determined within 1 to 570, depending on the composition of the sample. Approximate data on eutectic temperatures of the systems are included.
M
IXTURES of similar dibasic acids often occur in oxidation
. products from unsaturated fatty acids, and existing methods for their separation and analysis are somewhat unsatisfactory. Crystallizations yield, wit'h difficulty, only small proportions of pure components. The precision of saponification or neutralization equivalent determinations is limited by the 7unit difference in equivalent weights of adjacent dibasic homologs. 1-acuum fractionation of esters gives perhaps the most effective separation, but boiling points of adjacent esters are rather close, and intermediate fractions inevitably remain. Additional composition-property data are desirable for corroborative purposes. The results of Gantter and Hell ( I ) for melting points of binary mixtures of suberic and azelaic acids indicated that this property might prove useful. The data, though irregular, showed that a marked melt,ing point depression occurred in the system, and hence a considerable range of values was available. The lack of further published information for similar systems appears traceable to the fact that powdered binary mixtures of dibasic acids frequently melt over wide temperature ranges without distinct relation to composition. An empirical method has now been developed for taking melting points which provides results closely related to composition and affords its determination within a few per cent. This method has been applied to the systems comprising adjacent and alternate pairs of acids containing six to twelve carbon atoms.
mg., bringing the mixtures t o complete fusion, stirring them during cooling, and powdering the solidified material in a mortar. Portions were introduced into capillary tubes 1.0 t o 1.5 mm. in diameter and liquefied by holding the tubes in a stirred and rapidly heated oil bath. Air bubbles, which frequently developed, were dislodged by momentarily removing the tubes and shaking them sharply as with a clinical thermometer. The melted samples were quenched by quickly transferring the capillary tubes t0.a stream of tap water. Quenched samples were 3 t o 5 mm. in height. Approximate melting ranges were observed during the fusion process and allow rapid temperature adjustment in taking the final measurements.
Table
1
2
Second article on thio subject appears on page 541. Present address, F. E. Booth Co.. Emeryville, Calif.
Dibasic Acids Used for Binary Mixtures Capillary
"h?
Carbon Atoms 6 7 8 9
Adipic Pimelic Suheric Azelaic
152.0 108,8-104,3 141.4-141,9 106,6-107.0
10
[Sebacic
132.8-133.1
11 12
Acid
1 11-Undecanedioic l:12-Dodecanedioic
110.3-1 10.8 128.7-129.0
Source Eastman product as received Prepared from castor oil acids by oxidation with nitric acid Eastman product, recrystallized From 12-hydrosystearic acid Reid b y method (2) of Hall and
z
a
!w I 90 O O k Q /
..
PREPARATION OF SAMPLES
-4cids used are presented in Table I. One-gram samples of mixtures were prepared by weighing the component acids t o 0.2
1.
s o . of
_. ...
SO
60
70
80
90
0 100
WT.% OF H I JHER G H E R MOL. W T . A C l D Figure 1.
Composition-Melting Point Curves for Binary Mixtures of Odd-Carbon Dibasic Acids
Temperalure scale to be shilled as indicated'lor individual curves
