Determination of the Epoxide Equivalent Weight of Glycidyl Ethers by Proton Magnetic Resonance Spectrometry J. G.. Dorsey,” G. F. Dorsey, A. C. Rutenberg, and L. A. Green Oak Ridge Y- 12 Plant, P.O. Box Y, Oak Ridge, Tennessee 37830 A proton nuclear magnetic resonance method to determine the epoxlde equivalent welght (EEW) of several commercial epoxy reslns has been developed. The preclsion and accuracy of the proton NMR method have been calculated for commerclal reslns as well as phenyl glycidyl ether. The data compare well with that of the conventlonal ASTM method of reactlng the epoxy with hydrogen bromide in glaclal acetic acid. The EEW of phenyl glycldyl ether at the 95% confidence interval determlned by the proton NMR method is 150.2 f 1.1 and by the ASTM method is 151.6 f 0.6 as compared to a theoretical value of 150.17. The proton NMR method is rapid, free of interferlng chemlcal reactlons, requlres a small sample, and simultaneously provides a flngerprlnt of the materlal Investigated.
The epoxide equivalent weight (EEW) is the weight of resin in grams which contains one gram-equivalent of epoxy. In general, if any epoxy group terminates each end of a resin, then the EEW is one-half the average molecular weight of a diepoxy resin and one-third the average molecular weight of triepoxy resin ( I ) . The EEW has been determined by many methods. The most frequently reported involves the addition of hydrogen halide to the epoxy group. 0 HO
\ / \ / /
C-C
\
\
t HX
-+
-C--CI
/
\
(1) X
Normally, H X (Equation 1) is hydrogen bromide dissolved in glacial acetic acid (2). The difference between the amount of acid added and the amount of acid unconsumed determined by titration with standard base determines the epoxy content. In addition to the hydrogen bromide-acetic acid method, other methods include the pyridinium chloride-pyridine method, the hydrochloric acid-potassium iodide method, potentiometric titration with hydrochloric acid, infrared, and nearinfrared spectroscopy ( I ) . The applicability of each method depends on the resin to be investigated, the extent to which undesirable side reactions occur, and the presence of impurities or additives in commercial resins or reagents used which interfere with the desired reaction. Reaction times vary from 15 min to 4 h for the individual methods (3). The proton NMR method was developed as an alternative to wet chemical and spectrophotometric procedures because it is rapid, free of interfering chemical reactions, requires a small sample, and simultaneously provides a fingerprint of the material investigated. EXPERIMENTAL Apparatus. Proton NMR spectra were obtained with a Bruker HFX 90 MHz spectrometer at a probe temperature of 328 K. The samples were heated to shift the labile hydrogen of the resin upfield from the methylene protons of the epoxide group. Reagents. All chemicals used including the internal standard, 1,1,2,2-tetrachloroethane,were reagent grade and used as supplied with the exception of PGE which was freshly distilled. The resins were manufactured by the following companies: PGE, Epons 826, 828, and 834 Shell Chemical Company; Epi-Rez: Jones-Dabney 1144
ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
CH3
CQCHCHiOQ B
C
D
E
c
0 QOCH,C/H~CH,
CH3 A
’
__
E
PPm
Figure 1. 90 MHz proton NMR spectrum of Epi-Rez 508 with 1,1,2,2-tetrachIoroethane added as internal standard
Company; and Araldite 6010: Ciba-Geigy Corporation. Procedure. Into a 1-mL volumetric flask with a ground glass stopper weigh within 0.5 mg 200 mg of resin and 200 mg of TCE. Dilute to 1 mL with carbon tetrachloride. Stopper and shake for 1 min. Pipet into a 5-mm NMR tube and add 2 or 3 drops of tetramethylsilane lock material. Obtain six integrals and calculate the EEW of the material. RESULTS AND DISCUSSION Commercial resins which include Araldite 6010, Epi-Rez 508, Epon 826, Epon 828, and Epon 834, as well as a freshly distilled monomer, phenyl glycidyl ether (PGE), were analyzed by both the proton NMR and HBr-acetic acid procedures. All resins investigated are diglycidyl ethers of bisphenol A (DGEBA) type resins. The EEW of the resins analyzed depends on n (Formula 2)
(2)
and varies from n = 0.2 to n = 0.72. The proton NMR spectrum of Epi-Rez 508 is shown in Figure 1 and consists of five distinct areas as denoted by the letters at the bottom of the spectrum. The singlet at F (5.89 ppm) is the standard, TCE. When n is assumed to be zero, the assignment of the five peaks (A, B, C, D, and E) is straightforward and is depicted on the spectrum. All integrals were calculated from an expanded spectrum (10 Hz/cm). The multiplet (B) at 2.67 ppm in Figure 1 is the absorption of two methylene protons of the epoxide group. The EEW is calculated by comparing the integral (IF)of the two protons of TCE with the integral (IB) of the two methylene protons of the epoxide group.
of Resin
)
As n increases I B decreases; as n increases the integral at D (Figure 1)will increase since a larger proportion of protons in the molecule will be alkoxy rather than epoxy in character. Thus, the monomer of DGEBA where n = 0 should exhibit
a spectrum similar t o Figure 1 while a polymer with n g 1 should exhibit a spectrum similar to that of Epon 834 shown in Figure 2. The ratio of the integrals, A:B:C:D:E, in Figure 1 should be 6:4:2:4:8 if n = 0 while the ratio of the integrals of a DGEBA resin where n = 1 should be 12:4:2:8:16. The hydroxyl proton and the proton attached to the carbon bearing the hydroxyl group are not included in the ratio. The hydroxyl group location is dependent on concentration and temperature. A value of 0.72 is estimated for n from the EEW of Epon 834. In Table I, the data determined by the proton NMR method for Araldite 6010, Epon 826, Epon 828, Epon 834, and Epi-Rez 508 are compared with literature values and data determined by the HBr-acetic acid method. Five or six aliquots were analyzed by the proton NMR method. Duplicate analyses were made by the HBr-acetic acid method. The spectrum of PGE is shown in Figure 3.
Figure 2. 90 MHz proton NMR spectrum of Epon 834
f-J- A
OCH p c t i - C HZ
(4)
It consists of four multiplets instead of the five in DGEBA. The integral at 2.62 ppm is used to calculate the EEW of the material. The EEW of PGE at the 95% confidence interval determined by the proton NMR method is 150.2 1.1 ( n = 6) and by the ASTM method is 151.6 f 0.6 (n = 6). The theoretical value is 150.17.
*
ACKNOWLEDGMENT The authors gratefully acknowledge the able assistance of M. H. Eager and E. H. Tomlinson who obtained the proton NMR spectra.
1 7
6
5
4
2
3
1
Figure 3. 90 MHz proton NMR spectrum of phenyl glycidyl ether
Table I. Comparison of the Epoxide Equivalent Weight Obtained by Proton Magnetic Resonance and HBr- Acetic Acid Methods with Literature Values
a
Resin
Literature Value
Araldite 6010 Epi-Rez 508 Epon 826 Epon 828 Epon 834
185-196(4) 171-177(5) 180-188(6) 185-192(6) 230-280(6)
Proton NMRa 189.5 * 173.3 i 185.6 i 190.7 i 271.5 i-
Values at 95% confidence interval.
HBr-acetic acid
1.7
188.4
3.1 1.0 3.0 2.8
175.2
182.6 194.4 263.8
LITERATURE CITED (1) H. Lee and K. Neville, “Handbook of Epoxy Resins”, McGraw-Hill Book Company, New York, N.Y., 1967, pp 4-14. (2) “Annual Book of ASTM Standards”. Voi. 20, American Society for Testing and Materials, Philadelphia, Pa, 1972, p 778. (3) J. L. Jungnickei, “Organic Analysis”, Vol. 1, J. Mitchell, Jr., et al., Eds., Interscience Publishers, Inc., New York, N.Y., 1953 p 127. (4) Ref. 1, pp 4-59. (5) Ref. 1, pp 4-63. (6) Ref. 1, pp 4-66
RECEIVED for review March 14,1977. Accepted April 27,1977. The Oak Ridge Y-12 Plant is operated by Union Carbide Corporation’s Nuclear Division for the US. Energy Research and Development Administration under U S . Government Contract W-7405-eng-26.
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