Epoxide Equivalent Weight Determination by Carbon-13 Nuclear

7 Epoxide Equivalent Weight Determination by Carbon-13

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on September 21, 2015 | http://pubs.acs.org Publication Date: December 3, 1979 | doi: 10.1021/bk-1979-0114.ch007

Nuclear Magnetic Resonance W. Β. MONIZ and C. F. PORANSKI, JR. Chemistry Division, Naval Research Laboratory, Washington, D C 20375

We have been involved f o r some time i n the development and a p p l i c a t i o n of nuclear magnetic resonance (nmr) techniques f o r the c h a r a c t e r i z a t i o n of thermosetting polymers. The e f f o r t has been d i r e c t e d toward a n a l y s i s of both prepolymers and cured r e s i n systems. We have p r e v i o u s l y described how carbon-13 nmr can a i d the a n a l y s t d i s t i n g u i s h v a r i o u s types of epoxy r e s i n s and i d e n t i f y r e a c t i v e d i l u e n t s which may be present (1), and have published a comprehensive c a t a l o g of proton and carbon-13 nmr spectra of epoxy r e s i n s and curing agents ( 2 ) . We have a p p l i e d the r e l a t i v e l y new technique of proton-enhanced carbon-13 nmr to the a n a l y s i s of s o l i d , cured epoxy systems ( 3 ) . I t appears that t h i s technique w i l l be u s e f u l not only i n c h e m i c a l l y charac t e r i z i n g cured systems, but a l s o i n probing t h e i r molecular dynamics. In t h i s paper we d i s c u s s the use of carbon-13 nmr to measure epoxide equivalent weights of epoxy r e s i n s based on the d i g l y c i d y l ether of bisphenol A (DGEBA). The r e s u l t s obtained to date i n d i cate that the carbon-13 technique could be an a t t r a c t i v e a l t e r n a t i v e to current methods. Epoxy r e s i n s based on DGEBA are represented by the general formula shown below. Commercial r e s i n s are mixtures of such s t r u c t u r e s (oligomers) with v a r i o u s values of n. The epoxide equivalent weight (EEW) measures the number of epoxide groups a v a i l a b l e f o r r e a c t i o n during cure. In a r e s i n composed of o l i g o mers, the EEW i s an average f o r the mixture. The r e s i n system processing parameters which are of p r a c t i c a l importance i n c l u d e EEW, v i s c o s i t y , and i n some cases, the r e l a t i v e

\

r

? "

-τ

J n This chapter not subject to U.S. copyright. Published 1979 American Chemical Society

In Epoxy Resin Chemistry; Bauer, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

J*

84

EPOXY RESIN CHEMISTRY

proportions of the v a r i o u s n-oligomers. Zucconi (A)> f ° example, has reported that problems i n manufacture and performance of epoxy p r i n t e d - c i r c u i t boards were traced to d i f f e r e n c e s i n the oligomer d i s t r i b u t i o n of epoxy r e s i n batches which had met v i s c o s i t y and EEW s p e c i f i c a t i o n s . He has developed a q u a l i t y c o n t r o l procedure based on l i q u i d chromatography f o r monitoring oligomer d i s t r i b u t i o n . The carbon-13 nmr technique described here measures EEW, but not oligomer d i s t r i b u t i o n . Dorsey, et a l . , have d e s c r i b e d a proton nmr method f o r EEW determination which uses a measured amount of sym-tetrachlorοeth ane as an i n t e r n a l standard ( 5 ) . They r e p o r t r e s u l t s on f i v e r e s ins with EEW's below 300. Hammerich and Willeboordse have analyzed the p r e c i s i o n of proton nmr EEW determinations (6). Some of the problems i n accurate area measurement they p o i n t out, such as c a r bon-13 s a t e l l i t e s and i n s u f f i c i e n t s e p a r a t i o n of main peaks, are not of concern i n the carbon-13 nmr EEW determination. While proton nmr i s s a t i s f a c t o r y f o r determinations on low and medium EEW epoxy r e s i n s , the method l o s e s accuracy as the EEW i n c r e a s e s . As shown i n F i g u r e 1, the proton nmr spectrum of DER 332LC (EEW=175) has sharp m u l t i p l e t s and i n t e g r a t i o n of the r e l a t i v e areas i s s t r a i g h t f o r w a r d . But, as the spectrum of EPON 1004 (EEW^950) shows, when the EEW i n c r e a s e s , the m u l t i p l e t s broaden due to overlap with the l i n e s of the a l i p h a t i c protons i n the b r i d g i n g groups of the oligomers. For carbon-13 nmr, the l a r g e chemical s h i f t d i s p e r s i o n and the use of proton decoupling o b v i a t e such problems. F i g u r e 2 shows carbon-13 nmr s p e c t r a of the r e g i o n of i n t e r e s t f o r two epoxy r e s i n systems. The three l i n e s at 44, 50, and 70 ppm form the b a s i s f o r the carbon-13 nmr method of EEW determination. As η i n the general oligomer s t r u c t u r e i n c r e a s e s , the number of b r i d g i n g carbons i n creases, but the number of t e r m i n a l epoxide groups remains the same, two per oligomer molecule. For the i d e a l i z e d n-oligomer, the r a t i o of t e r m i n a l g l y c i d y l ether carbons of types c,d, or e, to b r i d g i n g carbons (type b) i s 2/3n. Now, the i n t e n s i t y of the carbon-13 l i n e at 70 ppm i s the sum of the i n t e n s i t i e s of the l i n e due to the t e r m i n a l ether methylene carbon, I , and those due to the b r i d g i n g carbons, 1^. Although Ι cannot §e measured d i r e c t l y ,


r

b

OH

Δ

c

λ

d I

Y

e

i t s value can be obtained by measuring the intensity of either the line at 44 ppm (I ) or 50 ppm (1^), or their average value, I'. Thus the ratio, 2?3n, can be expressed by - I') and η = 2(I - I')/3I\ b


MONiz AND PORANSKI

Epoxide Equivalent Weight Determination

85


7.

Figure 1. Proton NMR spectra (100 MHz) of DER 332LC (upperj and Epon 1004 (lowerj.




EPON 828 EEW ~ 190

EPON 1002 EEW ~ 650

70

40 ppm

Figure 2.

The 40-70 ppm region of the carbon-13 Ν MR spectra (15 MHz) of two epoxy resins


7.

MONiz AND PORANSKI


87


T h i s a n a l y s i s assumes that a p a r t i c u l a r epoxy r e s i n c o n s i s t s of only one n-oligomer, which of course i s not the t r u e p i c t u r e . In order to i l l u s t r a t e that the η d e r i v e d by the above equation y i e l d s the average η that would be found by chemical a n a l y s i s , we generated a set of h y p o t h e t i c a l oligomer mixtures and c a l c u l a t e d t h e i r EEW's based on carbon-13 nmr and chemical a n a l y s i s . As shown i n Table I, the two methods g i v e e q u i v a l e n t r e s u l t s .

Table I. Epoxide E q u i v a l e n t Weights C a l c u l a t e d f o r Four H y p o t h e t i c a l Epoxy Formulations . n-oligomer Λ

mole percent of n-oligomer , a b c

0 3 5 8 10 15

EEW

fchemical (carbon-13

i n formulation d

5 8 6 10 15 56

2 8 72 10

1766

1532

568

1012

1760

1533

568

1008

3 5

3 2 75 9 5 6

63 10 8 5 8 6

Q u a n t i t a t i v e aspects Carbon-13 nmr became an important experimental technique only w i t h the development of F o u r i e r transform (FT) nmr. But the FT nmr experiment r e q u i r e s a t t e n t i o n to a number of f a c t o r s i n order f o r the measurements to be q u a n t i t a t i v e l y meaningful (]_,8^9). These may be d i v i d e d i n t o two c a t a g o r i e s : i n s t r u m e n t a l , which a f f e c t FT nmr of any nucleus; and those r e l a t e d to s p i n dynamics, which f o r some n u c l e i , i n c l u d i n g carbon-13, demand greater a t t e n t i o n . Among the instrumental f a c t o r s i s the power envelope of the r f i n t e r r o g a t i n g p u l s e . I t must cover the s p e c t r a l r e gion of i n t e r e s t u n i f o r m l y so that a l l n u c l e i being studied are a f f e c t e d to the same degree. The f i l t e r used i n the d e t e c t i o n system must a l s o have uniform response across the spectrum. Other instrumental c o n s i d e r a t i o n s are computer r e l a t e d and i n clude f a c t o r s such as memory s i z e , word l e n g t h and ADC r e s o l u t i o n ( 9 ) . Most modern FT nmr spectrometer systems are designed to s a t i s f y the above requirements, but one must always v e r i f y proper o p e r a t i n g parameters f o r the a n a l y s i s being run. With regard to s p i n dynamics, two areas of concern i n c a r bon-13 nmr are the s p i n - l a t t i c e r e l a x a t i o n time, T , and the n


88


nuclear Overhauser effect,NOE. The r e l a x a t i o n time, Τ-, i s a mea sure of the recovery of nuclear magnetization to e q u i l i b r i u m a f t e r the r f p u l s e . In a given molecule, T- s of the v a r i o u s c a r bons may vary widely. Since the accumulation of adequate s i g n a l to n o i s e r a t i o i n carbon-13 FT nmr i s obtained by r e p e t i t i v e l y p u l s i n g the nuclear system, one must assure that a l l n u c l e i have returned to e q u i l i b r i u m between p u l s e s . The NOE i s a r e s u l t of the proton decoupling used to e l i m i n a t e proton-carbon s p i n c o u p l i n g . The carbon nmr s i g n a l s a r e enhanced v i a the proton system, but the enhancement may not be the same f o r a l l carbons i n a molecule. Both r e l a x a t i o n e f f e c t s and NOE can be d e a l t w i t h i n sev e r a l ways f o r q u a n t i t a t i v e carbon-13 measurements (7,8). The approach we used i n v o l v e d gated decoupling to surpress NOE, and a s u f f i c i e n t l y long delay time between pulses to assure complete recovery of carbon magnetization. The delay time was determined by measuring the T , s of the carbons of i n t e r e s t i n s e v e r a l of the epoxy r e s i n systems. The r e s u l t s , given i n Table I I , show that a delay time between pulses of 16sec (5 times the longest T- (8)) would be adequate f o r these m a t e r i a l s . In our EEW determinations, we used a pulse delay of 35sec.


f

f

Table I I . T

1

Values f o r Carbons of I n t e r e s t Τ (sec)

Resin

EPON 828 EPON 1001 EPON 1002

χ

44ppm

50ppm

70ppm

1.2 1.2 1.0

3.2 2.9 2.7

1.3 0.4 0.3

Results Table I I I g i v e s the EEW's we obtained using t h i s carbon-13 approach f o r a s e r i e s of commercial epoxy r e s i n s . I t a l s o g i v e s the v a l u e s obtained by chemical a n a l y s i s and the v a l u e s from the l i t e r a t u r e (10). The carbon-13 r e s u l t s f a l l i n t o two groups. For the l i q u i d r e s i n s carbon-13 nmr g i v e s r e s u l t s g e n e r a l l y low*er than the ranges given i n the l i t e r a t u r e . For the s o l i d r e s i n s of medium EEW, the carbon-13 r e s u l t s a r e g e n e r a l l y w i t h i n the l i t e r a t u r e ranges. Except f o r two cases, the chemical r e s u l t s are 5-10% higher than the carbon-13 r e s u l t s , and a r e c o n s i s t e n t l y higher than the l i t e r a t u r e ranges. T h i s carbon-13 nmr method f o r determining EEW has a number of f e a t u r e s which make i t worth c o n s i d e r a t i o n as an a l t e r n a t i v e


7.

MONiz AND PORANSKI


89


to other methods. The amount of sample required i s 0.4g or less. Neither precision weighings nor standardized solutions are need ed. The analyst i s free for other tasks while the FT nmr spec trometer automatically accumulates the data. In an analytical laboratory or quality control environment where EEW measurements are made on a routine basis, calibration curves and standard data acquisition procedures should result i n measurement times of 20 minutes or less. T

Table III. EEW s Found by Carbon-13 nmr and Chemical Analysis Resin DER 332LC EPON 826 EPON 828 EPON 828° ΕΡΙΚ0ΤΕ 828 ARALDITE 7071 EPON 1001 EPON 1002 EPON 1004

Carbon-13 178 172 176 181 171 522 568 686 971

Chemical 178 190 194 196 193 529 600 762 1086

Literature 170-175 180-188 185-192 185-192 185-192 450-530 450-530 6QQ-700 875-1025

^Reference 10 Manufactured i n the United States ^Manufactured i n Canada Assumed to be the same as the value for EPON 828

Experimental The samples for the carbon-13 nmr measurements were CDCl^ solutions (O.2g/ml solvent) of commercial samples of the epoxy resins i n 10mm o.d. sample tubes. The measurements were made at 15MHz with a JEOL FX60Q spectrometer system, A 9Q°ClOMSec) pulse was used with gated decoupling to surpress the NOE Q). The data collection block was 2048 points for a spectral width of 2200Hz. The observation f i l t e r was set at 2000Hz, substantially i n excess of the 1100Hz used for best signal to noise i n the quadrature detection mode. This assured negligible attenuation of the signals of interest due to the Butterworth f i l t e r characteris t i c s . The data block was expanded by zero f i l l i n g to 16,384 points before Fourier transformation. Exponential multiplica tion equivalent to a line broadening of 1.6Hz was applied. The number of accumulations was 512. The pulse repetition rate was 35sec, longer .than 10 times the longest T^ of the carbons of interest. Relaxation times, T^, were measured by the inversion/ recovery method (11) on the solutions used for the quantitative measurements. Chemical analysis (HC1 t i t r a t i o n , single determi nation) of epoxide equivalent weights were performed by Schwarz kopf Microanalytical Laboratory, Woodside Ν. Y,



90


Literature cited 1. Poranski, J r . , C.F., and Moniz, W.B., J . Coat. Tech., 1977, 49, (632), 57. 2. Poranski, J r . , C.F., Moniz, W.B., Birkle, D.L., Kopfle, J . T . , and Sojka, S.A., "Carbon-13 and Proton NMR Spectra for Char acterizing Thermosetting Polymer Systems: I. Epoxy Resins and Curing Agents", Naval Research Laboratory Report 8092, Washington, D.C., 1977. 3. Garroway, A.N., Moniz, W.B., and Resing, H.A., "Carbon-13 NMR in Solving Macromolecular Problems", ACS Symposium Series, in press. 4. Zucconi, T.D., "Proceedings of the Fifth Conference on Com posite Materials: Testing and Design", ASTM Special Publica tion, Philadelphia, Pa., 1979, in press. 5. Dorsey, J . G . , Dorsey, C . F . , Rutenberg, A.C., and Green, L . A . , Anal. Chem., 1977, 49, 1144. 6. Hammerich, A.D., and Willeboordse, F . G . , Anal. Chem., 1973, 45, 1696. 7. Werhli, F.W., and Wirthlin, T., "Interpretation of Carbon-13 NMR Spectra", Heyden and Son, Ltd., New York, 1976, p 264. 8. Shoolery, J . N . , Prog. NMR Spectrosc., 1977, 11, 79. 9. Randall, J . C . , "Polymer Sequence Determination; Carbon-13 NMR Method", Academic Press, New York, 1977, Chap. 5. 10. Tanaka, Y., Okada, Α., and Tomizuka, I., "Epoxy Resins", C. A. May and Y. Tanaka, Eds., Marcel Dekker, Inc., New York, 1973, Chap. 2. 11. Mullen, Κ., and Pregosin, P.S., "Fourier Transform NMR Techniques: A Practical Approach", Academic Press, New York, 1976, p 65. RECEIVED May 21, 1979.


Epoxide Equivalent Weight Determination by Carbon-13 Nuclear

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