Size Exclusion Chromatography Analysis of Epoxy Resin Cure Kinetics

22 Size Exclusion Chromatography Analysis of Epoxy Resin Cure Kinetics 1

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on June 9, 2016 | http://pubs.acs.org Publication Date: March 30, 1984 | doi: 10.1021/bk-1984-0245.ch022

GARY L. HAGNAUER and PETER J. PEARCE

Polymer Research Division, Army Materials & Mechanics Research Center, Watertown, MA 02172 Liquid size exclusion chromatography (SEC) is ap plied to investigate the isothermal cure kinetics of the reaction between pure N,N'-tetraglycidyl methy lene dianiline (TGMDA) and 4,4'-diaminodiphenyl sul fone (DDS) monomers over the temperature range 121° to 187°C. Intermediate reaction products are isola ted by preparative SEC, identified and used as standards for SEC calibration. Monomer and soluble reaction product concentrations, molecular weight averages, and gel content are monitored as functions of reaction time by analytical SEC. A 3rd order rate expression describing the early stages of cure is established and Arrhenius relationships describ ing the temperature dependence of the rate constant and the onset of gelation are determined. Reaction mechanisms are discussed and the effects of varia tions in stoichiometry of TGMDA/DDS resins on the network structure and properties of the cured resin are considered. Epoxy r e s i n s c o n t a i n i n g Ν,Ν'-tetraglycidyl methylene d i a n i l i n e (TGMDA) and 4,4-diaminodipheny1 sulfone (DDS) are widely used i n the manufacture 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 a i r c r a f t . However, the commercial r e s i n formulations are g e n e r a l l y q u i t e complex and may include s e v e r a l d i f f e r e n t types o f epoxy r e s i n s , a d d i t i o n a l c u r i n g agents, c a t a l y s t s , organic s o l v e n t s , and a d d i t i v e s t o f a c i l i t a t e processing or modify p r o p e r t i e s o f the cured r e s i n . An accurate assessment o f the cure k i n e t i c s i s v i r t u a l l y impossible since the r e s i n s are o f t e n p a r t i a l l y reacted or "staged" d u r i n g t h e i r f o r m u l a t i o n and "prepregging" which generates a host of i l l - d e f i n e d , intermediate r e a c t i o n products and because a v a r i e t y of r e a c t i o n s which proceed at d i f f e r e n t r a t e s and by d i f f e r e n t mechanisms may occur during cure. Indeed i m p u r i t i e s 'Current address: Materials Research Laboratories, Ascot Vale, Victoria, Australia T h i s c h a p t e r not subject t o U . S . c o p y r i g h t . P u b l i s h e d 1984, A m e r i c a n C h e m i c a l S o c i e t y

Provder; Size Exclusion Chromatography ACS Symposium Series; American Chemical Society: Washington, DC, 1984.


334

SIZE EXCLUSION CHROMATOGRAPHY

and s y n t h e s i s by-products present i n commercial TDMDA are found t o have a s i g n i f i c a n t e f f e c t on c u r i n g behavior when DDS alone i s added as the c u r i n g agent (_1) . To begin to understand the c u r i n g behavior of the commerc i a l r e s i n s i t i s e s s e n t i a l f i r s t t o invest igate and understand the c u r i n g chemistry of simpler model systems. P r e l i m inary s t u d i e s have shown that i t i s p o s s i b l e to a c c u r a t e l y monitor cure k i n e t i c s and, at l e a s t d u r i n g the e a r l y stages of cure, to eluc idate the c u r i n g chemistry i f pure TGMDA and DDS monomers are used (_2). In t h i s paper 1iquid s i z e e x c l u s i o n chromatography (SEC) i s a p p l i e d to invest igate the isothermal cure k i n e t i c s o f the react i o n between pure TGMDA and DDS monomers over the temperature range 121° to 187°C. The o b j e c t i v e i s t o gain a b e t t e r understanding of the epoxy r e s i n c u r i n g chemistry and t o evaluate the temperature dependence o f the c u r i n g react ion. Intermediate react ion products are i s o l a t e d by préparâtive SEC, i d e n t i f i e d and used as standards for SEC c a l i b r a t i o n . Monomer and s o l u b l e r e a c t i o n product concen trât i o n s , molecular weight averages and g e l content are monitored as func t i o n s o f react ion time by SEC. From s t o i c h i o m e t r i c s t u d i e s a r a t e e x p r e s s i o n d e s c r i b i n g the e a r l y stages o f cure i s developed and an Arrhenius r e l a t i o n s h i p i s determined from the temperature dependence o f the r a t e constant. Experimental Préparât ive 1 i q u i d chromatography techniques were a p p l i e d to p u r i f y the TGMDA monomer (_1 ). The monomer used f o r t h i s study i s a pale y e l l o w 1 i q u i d w i t h a v i s c o s i t y of approximately 1300 c e n t i p o i s e at 50°C and an epoxy e q u i v a l e n t weight (EEW) o f 108g/eq. The t h e o r e t i c a l EEW o f the TGMDA monomer i s 105. 5g/eq. The c u r i n g agent DDS i s a white, c r y s t a l l i n e (mp, 162°C) powder and was h i gh1y pure (approx., 99%) as r e c e i v e d from A l d r i c h Chemical Co. TGMDA/DDS r e s i n formulât ions were prepared by h e a t i n g the weighed components to 90°C and then mixing to form homogeneous s o l u t i o n s (approx. , 30g). Except d u r i n g sampling the r e s i n formulât ions were stored i n sealed c o n t a i n e r s a t -13°C. Chromatographic and s p e c t r o s c o p i c analyses showed that no r e a c t i o n occurred d u r i n g mixing and that upon storage the formulât ions remained unreacted f o r at l e a s t 6 months. A Perkin-Elmer DSC IB instrument was used to study the isothermal cure (polymerizat ion) behav i o r o f the r e s i n formulât ions. Samples (5-10mg) were weighed i n aluminum DSC sample pans on a microbalance and t r a n s f e r r e d to the DSC heat i n g stage. The temperature o f the heat i n g stage was preset at the c u r i n g temperature and the cures were conducted i n a n i t r o g e n atmosphere. About 10-20 samples per r e s i n f o r m u l a t i o n were p a r t i a l l y cured over a range of time i n t e r v a l s . The react ions were terminated by r a p i d l y lowering the temperature and t r a n s f e r r i n g the sample pans t o 25 mL v o l u m e t r i c f l a s k s and adding


22.

H A G N AU Ε R A N D P E A R C E

Epoxy Resin Cure Kinetics

335

t e t r a h y d r o f u r a n (THF). To f a c i l i t a t e d i s s o l u t i o n t h e f l a s k s were a g i t a t e d and t h e s a m p l e s a l l o w e d t o s o a k f o r 1-k d a y s . Except for t h r o u g h a 0 . 2 μΜ M i l l i p o r e membrane f i l t e r i n p r e p a r a t i o n f o r SEC analysis. O n l y s o l u b l e components were a n a l y z e d by SEC.


A Waters A s s o c i a t e s ALC/GPC-244 instrument w i t h M6000A solvent d e l i v e r y system, M720 system c o n t r o l l e r , M730 data module, 710B WISP a u t o - i n j e c t o r and M440 UV d e t e c t o r was used f o r the SEC analyses and operated under the f o l l o w i n g c o n d i t i o n s :

Column Set : uStyragel (2 χ 50θΧ, 3 χ ΙΟοΧ) Sample Concentration: 0.2-0.5 μ g/PL I n j e c t i o n Volume : 20-60 μ L Mobile Phase : THF (UV grade, Burdick & Jackson Labs) Flow Rate : 1 mL/min Detector: UV 254nm Run Time : 45 min A Waters A s s o c i a t e s Prep LC System/500 was used f o r p r e p a r a t i v e SEC. Samples were i n j e c t e d u s i n g a 12 mL loop v a l v e and the column set c o n s i s t e d of two 2.5-in diameter χ 4 - f t length columns w i t h 80ÎOOS and 700/2000iP S t y r a g e l packing. Operating c o n d i t i o n s are shown below : Sample Concentration : 20g/100mL Mobile Phase : THF (UV grade, Burdick & Jackson Labs) Flow Rate : 40mL/min Detector : d i f f e r e n t i a l r e f r a c t ive index (RI) Run Time : 94 min Cure Mechanism Εpoxy-amine c u r i n g r e a c t i o n s are known to be exceedingly complex. More than one r e a c t i o n can occur and the temperature dependence of each r e a c t i o n may be q u i t e d i f f e r e n t . For the TGMDA-DDS system, moisture and r e s i n i m p u r i t i e s can not only behave as c a t a l y s t s but a l s o may a f f e c t the network s t r u c t u r e and p r o p e r t i e s of the cured res i n (2^). Because of the t e t r a - f une t i o n a l nature of the monomers, s t e r i c e f f e c t s may lead to a l t e r n a t i v e r e a c t i o n s and h i g h l y c r o s s 1 inked network s t r u c t u r e s may occur r e l a t i v e l y e a r l y i n the c u r i n g reaction. Indeed as p o l y m e r i z a t i o n proceeds, v i s c o s i t y increases and the glass t r a n s i t i o n temperature of the r e a c t i o n mixture g r a d u a l l y approaches and may exceed the c u r i n g temperature. As a r e s u l t , r e a c t i v e species become d i f f u s i o n 1imited and e v e n t u a l l y may e i t h e r seek other react ion pathways or stop r e a c t i n g e n t i r e l y .


336

SIZE E X C L U S I O N

CHROMATOGRAPHY

Denoting the s t r u c t u r e s

0

0

CH VcH2 2

χ

CH -CH-CH

/

x

2

^

2

N--NH 2

2

H N-R'~

s

2

0 DDS

the most l i k e l y r e a c t i o n s and r e a c t i o n product s t r u c t u r e s are shown below (1)

epoxy-primary amine addit ion OH ~R-CH -CH-CH -NH-R~ }

f

2

(2)

2

2

epoxy-secondary amine a d d i t i o n ί OH ^R-CH -CH-CH -^2N-R^ 2

(3)

ν CH OH ~ R-CH -CH-NH-R^< 2

or

2

and isomers

epoxy-hydroxyl a d d i t i o n

j 0-CH -CH-CH -R~ ~R-CH -CH-CH -NH-R~ * ^ 2

2

2

2

plus isomers and other products from the r e a c t i o n of species from (1) and (2)

(4)

epoxy-epoxy homopolymerization -£-ÇH-CH -0-4 ÇH ~R~ 2

n

2

I


22.

HAGNAUER A N D PEARCE

Epoxy Resin Cure

Kinetics

337


P r e p a r a t i v e SEC Samples f o r p r e p a r a t i v e SEC were prepared by p a r t i a l l y p o l y m e r i z i n g about 10g of a TGMDA/DDS mixture. To optimize the formation of the i n i t i a l , r e l a t i v e l y simple react ion products, a TGMDA r e s i n form ulât ion c o n s i s t i n g o f 25% by weight DDS was cured f o r 23 minutes at 145°C i n vacuo. The RI d e t e c t o r trace from the p r e p a r a t i v e SEC of t h i s r e a c t i o n mixture i s i l l u s t r a t e d i n Figure 1. Four i n j e c t ions were made s u c c e s s i v e l y at 60 minute i n t e r v a l s and f r a c t i o n s 1:1 and 2:1 were c o l l e c t e d as i n d i c a t e d . F o l l o w i n g the a p p l i c a t i o n of a vacuum to remove s o l v e n t , approximately 0.3g f r a c t i o n 1:1 and 0. l g f r a c t i o n 2:1 were r e a l i z e d . Analyt i c a l SEC and reverse phase high performance 1 i q u i d chromatography (HPLC) showed that f r a c t i o n 1:1 was 97% pure. Using F o u r i e r transform i n f r a r e d ( F T I R ) and H V C ^ NMR "spectroscopy, f r a c t i o n 1:1 was i d e n t i f i e d as the TGMDA-DDS epoxy-primary amine a d d i t i o n product shown below. F r a c t i o n 1:1 Ο

OH

H2N-^S^P>^H-CH CH-CH ?

Ο CrfeCH-CH,

2

/ N - ^ C H ^ ^ N ;

6

CH^CH-CHo

CHpCH-ChU

^

2

V

Although a s i n g l e peak was observed u s i n g SEC, f r a c t i o n 2:1 was found t o c o n t a i n three components by HPLC a n a l y s i s . The SEC a n a l y s i s i n d i c a t e s that the components of f r a c t i o n 2:1 have q u i t e s i m i l a r molar volumes. To a s c e r t a i n the r a t i o of monomers i n each component and o b t a i n more informat ion about the r e a c t i o n mechanism, FTIR s p e c t r a of the 2:1 components were run and compared w i t h the s p e c t r a of TGMDA, DDS, and the 1-1 product. Absorbance bands f o r hydroxy (3500 cm""*) and secondary amine (3410 cm *) groups were apparent i n the s p e c t r a o f a l l three components. There was no evidence suggest i n g the presence of a l i p h a t i c ether 1inkages of the type -CH -0-CH - or ^CH-0-CH -. From the r e s u l t s o f the chromato graphic and FTIR analyses, the f o l l o w i n g s t r u c t u r e s are p o s t u l a t e d f o r the components found i n f r a c t ion 2:1. -

2

2

2

F r a c t i o n 2:1 Ο

OH

Ο

OH

ο

CH^WH,

CH^CH-CH^HN-^S^P^H-CH^CH-C^

CH^CH-Chi,

CH^CH-CHj Ο

CH2ÇH-CH2 Ο

CHjCH-CH,

CHgCH-CHj? Ο


V


SIZE E X C L U S I O N

DDS

—I

j

60

j

1

j

1

j

,

j

1

70

1

j

80

!

CHROMATOGRAPHY

TGMDA

1

1

1

1

90

t (min) F i g u r e 1 . P r e p a r a t i v e SEC o f TGMDA/DDS (25%) r e s i n r e a c t e d 2 3 min at 1 U 5 °C.


r

22.

HAGNAUER AND

Epoxy Resin Cure Kinetics

PEARCE

O

OH

Ο

OH

^^1~{^"

Size Exclusion Chromatography Analysis of Epoxy Resin Cure Kinetics

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