Epoxy Resin Chemistry II - American Chemical Society

10 Effects of Impurities on Hydrolytic Stability and Curing Behavior

Downloaded by GEORGE MASON UNIV on December 25, 2014 | http://pubs.acs.org Publication Date: June 8, 1983 | doi: 10.1021/bk-1983-0221.ch010

GARY L. HAGNAUER and PETER J. PEARCE Army Materials and Mechanics Research Center, Polymer Research Division, Watertown, MA 02172 Preparative l i q u i d chromatography procedures are described for isolating highly pure N,N'-tetraglycidyl methylene d i a n i l i n e (TGMDA) from commercial TGMDA resins. Synthesis by-products or impurities are found to have a significant effect on the physical properties and curing behavior of the commercial resins. For example, the viscosity at 50°C of purified TGMDA is about 1/10th that of the commercial resins and impurities in the commercial resins accelerate the rate of hydrolysis of TGMDA. The presence of impurities lowers the temperature for the thermal polymerization of TGMDA but raises the glass transition temperature of the thermal polymerization products. However, impuri t i e s lower both the curing temperature for the reaction of TGMDA-diaminodiphenyl sulfone mixtures and the glass transition temperature of the reaction products. The heats of reaction are inversely proportional to the epoxy equivalent weights of the TGMDA resins. Epoxy r e s i n s c o n t a i n i n g Ν,Ν'-tetraglycidyl m e t h y l e n e d i a n i l i n e (TGMDA) a r e w i d e l y used i n t h e m a n u f a c t u r e o f f i b e r r e i n f o r c e d s t r u c t u r a l composites f o r aerospace a p p l i c a t i o n s . TGMDA i s t h e p r i n c i p l e component ( c a . 60-80%) i n C i b a - G e i g y C o r p o r a t i o n ' s A r a l d i t e MY720 and i n F I C C o r p o r a t i o n ' s G l y a m i n e G-120. O t h e r components i n t h e s e r e s i n s may i n c l u d e c h l o r o h y d r i n s , g l y c o l s , d i m e r s , t r i m e r s and h i g h e r o l i g o m e r s . A d d i t i o n a l l y , w a t e r , o r g a n i c s o l v e n t s and i n o r g a n i c s a l t s may be present as i m p u r i t i e s . I t i s g e n e r a l l y b e l i e v e d that h y d r o l y z a b l e c h l o r i d e s , f r e e h y d r o x y l s , and t r a c e i m p u r i t i e s have a s i g n i f i c a n t e f f e c t on t h e r e a c t i v i t y o f t h e r e s i n and may a d v e r s e l y i n f l u e n c e t h e r e l i a b i l i t y and p e r f o r m a n c e o f t h e c o m p o s i t e m a t e r i a l . However, t h e e f f e c t s o f i m p u r i t i e s on t h e p r o p e r t i e s and c u r i n g b e h a v i o r o f TGMDA r e s i n s have n o t been i n v e s t i g a t e d s y s t e m a t i c a l l y and t h e p r o p e r t i e s o f " p u r e " TGMDA a r e unknown. This chapter not subject to U.S. copyright. Published 1983, American Chemical Society

In Epoxy Resin Chemistry II; Bauer, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

194

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In t h i s paper, p r e p a r a t i v e l i q u i d chromatography p r o c e d u r e s a r e d e s c r i b e d t o i s o l a t e h i g h l y p u r e TGMDA f r o m t h e c o m m e r c i a l r e s i n s , and t h e p r o p e r t i e s o f p u r i f i e d TGMDA a r e compared w i t h t h o s e o f r e c e n t l y m a n u f a c t u r e d MY720 and G-120 r e s i n s . High performance l i q u i d chromatography (HPLC), d i f f e r e n t i a l scanning c a l o r i m e t r y (DSC), and DC r e s i s t a n c e t e c h n i q u e s a r e a p p l i e d t o e v a l u a t e t h e h y d r o l y t i c s t a b i l i t y and c u r i n g b e h a v i o r o f t h e r e s i n s and r e s i n f o r m u l a t i o n s c o n t a i n i n g d i a m i n o d i p h e n y l s u l f o n e (DDS). Experimental The TGMDA r e s i n s A r a l d i t e MY720 ( b a t c h No. 5093) and G l y a m i n e G-120 ( L o t No. 1003) were o b t a i n e d r e c e n t l y f r o m C i b a G e i g y C o r p . a n d F I C C o r p . , r e s p e c t i v e l y , and were s t o r e d i n c l o s e d c o n t a i n e r s a t -13°C. The p u r i f i c a t i o n was c o n d u c t e d i n two s t a g e s u s i n g a W a t e r s A s s o c i a t e s P r e p LC/System 500 w i t h P r e p P A K - 5 0 0 / S i l i c a columns. S t a g e 1. S e p a r a t i o n f r o m c o m m e r c i a l r e s i n . M o b i l e Phase: Methylene C h l o r i d e F l o w R a t e : 60 ml/min Concentration: 2 0 % w/v Inject: 600 m l Run Time: 40 m i n S t a g e 2. F u r t h e r P u r i f i c a t i o n o f S t a g e 1 M a t e r i a l . M o b i l e P h a s e : 6 0 % E t h y l A c e t a t e / 4 0 % Hexane v / v F l o w R a t e : 200 ml/min Concentration: 2 0 % w/v Inject: 5 ml Run Time: 15 m i n A W a t e r s A s s o c i a t e s ALC/GPC-244 i n s t r u m e n t w i t h M6000A s o l v e n t d e l i v e r y s y s t e m s , M720 s y s t e m c o n t r o l l e r , 710B WISP a u t o i n j e c t i o n s y s t e m , M440 UV d e t e c t o r , and M730 d a t a module was used f o r t h e HPLC a n a l y s e s . The f o l l o w i n g c o n d i t i o n s were used f o r r e v e r s e phase HPLC: Column: R a d i a l PAK C w i t h RCM100 module Concentration: 0.5% w/v I n j e c t Volume: 10 VL F l o w R a t e : 3 ml/min Detector: UV 254 nm Mobile Phase: ( 6 5 % H 0/35% CH CN) t o ( 5 0 % H 0/50% CH C N ) , 20 m i n f g r a d 6. 1 8

3

( 5 0 % H 0/50% CH CN) t o ( 1 0 % H 0/90% 10 m i n f g r a d 6. 5

5

CH C N ) , 5

( 1 0 % H 0/90% CH CN) t o ( 1 0 % H 0/20% THF/70% CH C N ) f 10 m i n , g r a d 7. and f o r s i z e e x c l u s i o n HPLC: Column: μ S t y r a g e l ( 1 0 0 0 , 2 x 5 0 0 , 3x100 A ) Concentration: 0.5% w/v I n j e c t Volume: 10P1 F l o w R a t e : 2 ml/min M o b i l e P h a s e : THF Detector: UV 254 nm 3



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Reagent grade water was prepared from d i s t i l l e d water using a M i l l i p o r e M i l l i - Q 2 water p u r i f i c a t i o n system. Distilled-ing l a s s , UV grade tetrahydrofuran (THF) and a c e t o n i t r i l e (CH-CN) was used as received from Burdick & Jackson Labs. S o l u t i o n s were prepared i n volumetric f l a s k s and were f i l t e r e d through 0.2VIM M i l l i p o r e membrane f i l t e r s . In a d d i t i o n to HPLC, elemental a n a l y s i s , vapor phase osmometry* Fourier Transform Infrared Spectroscopy (FTIR), GC/MS, H and C NMR techniques were applied to v e r i f y the p u r i t y o f the r e s i n samples. Epoxy e q u i v a l e n t weights (EEW) were determined by the standard nonaqueous t i t r a t i o n method (J_) using chloroform as the s o l v e n t . A c o r r e c t i o n f o r t e r t i a r y amine was obtained by conducting the t i t r a t i o n i n the a b s e n c e o f the quaternary h a l i d e . V i s c o s i t y measurements were made at 50 C on a Rheometrics Mechanical Spectrometer Model RMS-7200 i n steady shear mode with the cone and p l a t e geometry. To evaluate the h y d r o l y t i c s t a b i l i t y o f the r e s i n samples, t h i n l a y e r s o f the l i q u i d r e s i n s were aged at 60°C and 96% r e l a t i v e humidity. The r e s i n s were sampled p e r i o d i c a l l y and HPLC was used t o monitor the formation of h y d r o l y s i s products as a f u n c t i o n of aging time. The HPLC operating parameters were s i m i l a r to those described p r e v i o u s l y f o r reverse phase a n a l y s i s except f o r the mobile phase c o n d i t i o n s which are shown below: (70% H 0/30% THF), 25 min. (70% H^O/30% THF) to (10% H 0/30% THF/60% CH CN), 20 min, grad. 7 (10% Η^0/30% THF/60% CH^N)? 5 min. 5

Curing s t u d i e s were performed using a DuPont 990 Thermal Analyzer f o r d i f f e r e n t i a l scanning c a l o r i m e t r y (DSC) and a s p e c i a l l y designed apparatus f o r DC r e s i s t a n c e measurements. Glass t r a n s i t i o n temperatures Τ o f the polymerization products were determined by DSC a n a l y s i s ! P u r i s s grade 4,4'diaminodiphenyl sulfone (DDS) was used as received from KochL i g h t Labs. Results and D i s c u s s i o n The s i z e exclusion and reverse phase HPLC chromatograms o f MY720 and G-120 are shown i n F i g u r e s 1 and 2. UV-absorbing components are c h a r a c t e r i z e d by having peak r e t e n t i o n times which depend upon t h e i r molecular s t r u c t u r e and s o l u b i l i t y p r o p e r t i e s . The s i z e or amplitude o f the peaks are d i r e c t l y p r o p o r t i o n a l to the concentrations o f the components being monitored; and more than one component may have the same r e t e n t i o n time, e s p e c i a l l y i n the case o f s i z e e x c l u s i o n HPLC. In Figure 1, higher oligomers, t r i m e r s and dimers o f TGMDA have shorter r e t e n t i o n times than TGMDA (2163 seconds), which i s shown to be the major component i n both samples. Better r e s o l u t i o n i s obtained by reverse phase HPLC. In Figure 2, high molecular weight,



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MY720

^76%

G-120

Figure

1.

S i z e E x c l u s i o n HPLC o f C o m m e r c i a l TGMDA

Resins.


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F i g u r e 2.

R e v e r s e Phase HPLC o f C o m m e r c i a l TGMDA

Resins.


197


198

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oligomeric components have r e t e n t i o n times higher than TGMDA; whereas more polar components with hydrolyzable c h l o r i d e s and f r e e hydroxyls tend t o have shorter r e t e n t i o n times. I t i s noted that although both samples appear t o have the same types o f components, the r e l a t i v e amounts o f components i n each sample are quite d i f f e r e n t . The e f f e c t s o f such components on the p r o p e r t i e s o f TGMDA w i l l be discussed i n t h i s s e c t i o n . The EEW and % epoxy values determined by t i t r a t i o n , % TGMDA c a l c u l a t e d from the HPLC analyses, and v i s c o s i t i e s o f s e v e r a l TGMDA samples are compared i n Table I . Samples H, G, D and Ε were obtained by preparative l i q u i d chromatography. Sample D r e s u l t e d from Stage 1 o f a marginally s u c c e s s f u l separation where the column was overloaded. Samples G and H were i s o l a t e d from Stages 1 and 2, r e s p e c t i v e l y , o f the p u r i f i c a t i o n procedure. Sample Ε c o n s i s t e d p r i m a r i l y o f higher oligomers and was the m a t e r i a l which remained on the column i n Stage 1 and was s t r i p p e d from the column with THF. The % epoxy values are based upon the t h e o r e t i c a l EEW value 105.5 g/eq f o r the TGMDA monomer. The r e s i n G-120 has a l a r g e r TGMDA concentration and a higher % epoxy than MY720. The v i s c o s i t y o f the samples increases with EEW and depends on the concentration o f the higher molecular weight components. Sample H i s water c l e a r , G i s pale yellow, D i s l i g h t amber, and the other samples are dark amber i n c o l o r . Table I

SAMPLE EEW (g/eq) % EPOXY H 107 99 G 108 98 D 111 95 G-120 118 89 MY720 127 83 Ε 168 63

% TGMDA (HPLC) VISCOSITY SIZE EXCLUSION/REVERSE PHASE (CP) 50°C 99.0 98.9 98.5 97.7 1270-1590 87.7 88.0 76.2 77.7 5000-1500* 61.5 69.2 17600 9.0 50400-54700

•Manufacturer's s p e c i f i c a t i o n s S i z e exclusion and reverse phase HPLC chromatograms f o r some of the samples are compared i n Figures 3 and 4. I t i s noted that samples Η and G have a s i n g l e major component; i . e . , the TGMDA monomer. Preparative LC concentrates the minor components and leaves only about 9.4J o f the TGMDA monomer i n sample E. Appreciable d i f f e r e n c e s are also observed upon comparing the FTIR spectra o f samples H, G-120 and Ε (Figure 5 ) . The r e l a t i v e l y sharp IR absorption bands, the large epoxy absorbance at 905cm and the absence o f an hydroxyl band are noted i n the spectrum o f sample H. Sample Ε has a large hydroxyl absorption band and an epoxy absorbance nearly h a l f the s i z e o f that observed f o r sample H. In comparison with sample H, the


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199


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absorption bands for sample Ε are broader and poorer resolved which suggest that sample Ε i s heterogeneous and more complex structurally. The p u r i f i e d TGMDA samples are found t o be considerably more h y d r o l y t i c a l l y stable than the commercial r e s i n s . In the e a r l y stages o f h y d r o l y s i s , the major product i s i d e n t i f i e d as p a r t i a l l y hydrolyzed TGMDA with three epoxy groups unreacted and one epoxy group ring-opened t o form an α-glycol. This product i s noted as a peak (11.1 minutes) i n the HPLC chromatograms (Figure 6 ) . As the samples age at 60 C and 96% RH, the product forms and the peak grows u n t i l the r e s i n s s t a r t c r o s s l i n k i n g and become i n s o l u b l e . The weight percentage o f t h i s product i s p l o t t e d as a f u n c t i o n o f exposure time i n Figure 7. The h y d r o l y t i c s t a b i l i t i e s o f the fresh MY720 and G-120 samples are quite s i m i l a r ; however, an older sample o f MY720 (obtained i n 1978) was found t o be s i g n i f i c a n t l y l e s s s t a b l e than the recent batch eventhough they i n i t i a l l y had nearly i d e n t i c a l % TGMDA. Over an aging period o f one week, the % TGMDA i n sample G changes very s l i g h t l y compared t o the changes i n the commercial r e s i n s (Table ID. Table I I H y d r o l y t i c S t a b i l i t y o f TGMDA Resins 60°C, 96% RH

Sample

Time t o Form 2% H y d r o l y s i s Product

0

15 days 1 2.1 0.4

98 76 66 66

G (96% TGMDA) G-120 MY720 (new) MY720 (old)

% TGMDA 7 days 96 59 47 44

1

(day" ) 0.084 0.35 0.37 0.35

The r a t e o f formation o f the h y d r o l y s i s product has a f i r s t order dependence on the concentration o f the h y d r o l y s i s product (Figure 8 ) . d

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204

EPOXY RESIN CHEMISTRY

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HAGNAUER

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forming. The rate constants o f the commercial r e s i n s are n e a r l y i d e n t i c a l (Table I I ) . The smaller constant f o r the p u r i f i e d r e s i n suggests that the h y d r o l y s i s mechanism i s complex and i s a f f e c t e d by the presence o f minor components, i n a d d i t i o n to the h y d r o l y s i s product analyzed, behaving as c a t a l y s t s . E f f e c t s o f r e s i n p u r i t y are observed i n the thermal polymerization o f the TGMDA samples and i n the curing behavior o f TGMDA/DDS mixtures. C h a r a c t e r i s t i c exotherm curves are generated using DSC (5°C/min). T y p i c a l l y , the thermal polymerization r e a c t i o n s occur over a narrow temperature range and have sharp exotherm peaks. In comparison, the shape o f the DSC exotherm curve and temperature range o f the TGMDA/DDS (20 wt%) c u r i n g r e a c t i o n i s quite s e n s i t i v e t o the presence o f i m p u r i t i e s (Figure 9 ) . For both r e a c t i o n s the exotherm temperature maximum T and the enthalpy -ΔΗ decrease as the concentration o f i m p u r i t i e s or o f components other than TGMDA increase (Table I I I ) . The Δ Η value i s d i r e c t l y r e l a t e d to the % epoxy and the decrease i n Τ i s e v i d e n t l y caused by i m p u r i t i e s behaving as c a t a l y s t s . e x Q

β χ ο

Table I I I DSC A n a l y s i s (5°C/min, Ν )

Sample

Thermal Polymerization o f TGMDA Samples ^AH(cal/g) Τ Tg(°C) T

c

C

1 0.92

0.89 0.83 0.84 0.62

X

Q

245 238 233 237 232 228

244 240 231 212 202 181

Impurities have a s i g n i f i c a n t e f f e c t on the g l a s s t r a n s i t i o n temperatures Tg o f the polymerization products (Table I I I ) . With i n c r e a s i n g amounts o f i m p u r i t i e s , the Tg o f the thermal polymerization products i n c r e a s e ; whereas the Tg o f the DDS curing r e a c t i o n products decrease. I t i s noted that the Tg o f the thermal polymerization product o f sample H i s 110 C lower than the value o f the curing r e a c t i o n product. The e f f e c t o f i m p u r i t i e s on the Tg o f the DDS curing r e a c t i o n product can be understood as a consequence o f i m p u r i t i e s decreasing the average f u n c t i o n a l i t y o f r e a c t i v e species while tending t o c a t a l y z e epoxy homopolymerization. The e f f e c t o f i m p u r i t i e s on the Tg o f the thermal polymerization products i s more d i f f i c u l t to understand. Conceivably, homopolymerization i n v o l v i n g the intramolecular r e a c t i o n o f d i g l y c i d y l groups may occur and thereby lower the c r o s s l i n k d e n s i t y and hence the Tg o f the thermal polymerization product t o a greater extent as the TGMDA r e s i n becomes more pure. Impurities, such as those c o n t a i n i n g ouglycol groups, may enhance


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E.0 DC RESISTANCE, 177°C (350°F) TGMDA+DDS (20%) •

H.0+ •

Alog & 10

1

G-120

^

2.0 .

0.0

^ G (96% TGMDA)

2000

H000

t (seconds) F i g u r e 10. DC R e s i s t a n c e Reaction.

Measurements o f TGMDA/DDS C u r i n g


6000

10.

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the average f u n c t i o n a l i t y by i n i t i a t i n g polymerization which would tend t o r a i s e the c r o s s l i n k d e n s i t y and,therefore the Tg of the products o f the r e s i n s which are l e s s pure. Since a l l the thermal polymerization samples were prepared using the same heating c y c l e , i t i s u n l i k e l y that degradation would have occurred t o the extent t o account for the lower Tg o f the product obtained from h i g h l y pure TGMDA. FTIR s t u d i e s (2) o f the thermal polymerization may help e x p l a i n the e f f e c t o f i m p u r i t i e s on Tg. DC r e s i s t a n c e studies o f the TGMDA/DDS c u r i n g r e a c t i o n i n d i c a t e that i m p u r i t i e s a f f e c t p r o c e s s a b i l i t y . Changes i n e l e c t r i c a l resistance£l(ohms) are r e l a t e d t o the m o b i l i t y o f the charge c a r r i e r s and hence t o the v i s c o s i t y o f the r e a c t i o n medium. As the r e a c t i o n mixture approaches the r e a c t i o n temperature, the DC r e s i s t a n c e measured between two e l e c t r o d e s achieves a minimum value d . . Onset o f the c u r i n g r e a c t i o n generates an exotherm which upsets thermal e q u i l i b r i u m ; however, as c u r i n g proceeds the r e a c t i o n mixture becomes i n c r e a s i n g l y more viscous, e s p e c i a l l y with the onset o f g e l a t i o n , and the resistance increases. i s p l o t t e d versus c u r i n g time t for commercial'and p u r i f i e d ~ TGMDA/DDS (20 wt%) mixtures at 177°C. The commercial sample s t a r t s r e a c t i n g e a r l i e r and shows a much greater increase i n with r e a c t i o n time than the purer sample. The p l o t for sample G has a p e c u l i a r s t e p l i k e c h a r a c t e r i s t i c which may be r e l a t e d t o a d i f f e r e n c e i n polymerization mechanism or perhaps t o an experimental anomaly. This study has shown that synthesis by-products or i m p u r i t i e s i n TGMDA r e s i n s have a s i g n i f i c a n t e f f e c t on r e s i n p r o p e r t i e s and curing behavior. Future work w i l l involve i d e n t i f y i n g and evaluating the e f f e c t s o f s p e c i f i c i m p u r i t i e s . Preparative l i q u i d chromatography w i l l be used t o p u r i f y l a r g e q u a n t i t i e s o f TGMDA t o determine the e f f e c t s o f i m p u r i t i e s on rheology and on the mechanical p r o p e r t i e s o f the cured r e s i n .


t

Acknowledgment The authors are g r a t e f u l t o Mr. Walter X. Zukas f o r the v i s c o s i t y measurements, t o Mr. Bernard R. LaLiberte and Mrs. Emily McHugh f o r running the DSC a n a l y s i s and t o Dr. Richard J . Shuford for providing the DC r e s i s t a n c e measurements. Literature Cited 1. Jay, R. R., Anal. Chem., 1964, 36, 667. 2. Mones, E. T.; Morgan, R. J . , Polym. Prep., Am. Chem. Soc., Div. Polym. Chem., 1981, 22(2), 249-250.. RECEIVED

December 16, 1982


Epoxy Resin Chemistry II - American Chemical Society

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