Chemorheology of Thermosetting Polymers - American Chemical

2 Cure Kinetics and Mechanical Properties of a Resin Matrix

Downloaded by UNIV OF CINCINNATI on February 18, 2015 | http://pubs.acs.org Publication Date: August 29, 1982 | doi: 10.1021/bk-1982-0227.ch002

Effects of Impurities and Stoichiometry G A R Y L . H A G N A U E R , P E T E R J . P E A R C E , B E R N A R D R. L A L I B E R T E , and M A R G A R E T E . R O Y L A N C E

Army Materials and Mechanics Research Center, Organic Materials Laboratory, Watertown, M A 02172

The effects of impurities, or synthesis by -products, and stoichiometry on the cure kinetics of Ν,Ν'-tetraglycidyl methylene dianiline (TGMDA) resins formulated with diaminodiphenyl sulfone (DDS) are evaluated using gel permeation chromato graphy and d i f f e r e n t i a l scanning calorimetry. During the early stages of cure (177°C), the predominant reaction is amine-epoxy addition represented by a third-order rate expression. Impurities increase the rate of reaction and lead to higher extents of reaction. Increasing the epoxy/amine equivalent r a t i o decreases the rate of reaction and leads to higher extents of cross l i n k i n g and reaction. The mechanical properties of a series of resin specimens cured from TGMDA/DDS formulations with varying stoichiometry and different impurity levels are evaluated using standard ASTM test methods. Impurities i n commercial TGMDA resins cured with DDS and changes in the epoxy/amine r a t i o (1.46-1.95) are found to have no significant effects on the mechanical properties of the cured resin matrix. Post-curing improves the tensile and compressive strength and decreases the stiffness of the resin matrix. 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) 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 aerospace a p p l i c a t i o n s . The monomer TGMDA i s the p r i n c i p l e component (ca. 60-80%) i n CibaGeigy Corporation's A r a l d i t e MY720 and i n FIC Corporation's Glyamine G-120. Synthesis by-products o c c u r r i n g i n these r e s i n s 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 , dimers, t r i m e r s and higher

This chapter not subject to U.S. copyright. Published 1983, American Chemical Society In Chemorheology of Thermosetting Polymers; May, C.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


26

CHEMORHEOLOGY OF THERMOSETTING POLYMERS

oligomers* A d d i t i o n a l l y , water, organic 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 has been shown t h a t t h e s y n t h e s i s by-products and 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 the p h y s i c a l p r o p e r t i e s , h y d r o l y t i c s t a b i l i t y and r e a c t i v i t y o f TGMDA OK At 50 C t h e v i s c o s i t y o f r e l a t i v e l y pure (96%) TGMDA i s about one-tenth t h a t o f the commercial r e s i n s . I m p u r i t i e s ( s y n t h e s i s by-products) a c c e l e r a t e the r a t e o f h y d r o l y s i s o f TGMDA and lower the c u r i n g temperature f o r the r e a c t i o n o f TGMDA with diaminodiphenyl sulfone (DDS). In t h i s paper the e f f e c t s o f i m p u r i t i e s and s t o i c h i o m e t r y on the TGMDA-DDS p o l y m e r i z a t i o n r e a c t i o n are i n v e s t i g a t e d . P r e l i m i n a r y data showing the e f f e c t s o f i m p u r i t i e s and post-cure treatment on the mechanical p r o p e r t i e s and moisture up-take o f the TGMDA/DDS r e s i n matrix are presented. The p u r i f i e d TGMDA monomer, t h e commercial r e s i n s G-120 and MY720, and a sample c o n s i s t i n g p r i m a r i l y o f TGMDA oligomers are used f o r t h i s study. Gel permeation chromatography (GPC) and 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) techniques are a p p l i e d t o evaluate t h e TGMDA-DDS cure k i n e t i c s a t 177°C Mechanical t e s t specimens are prepared and conditioned f o r t e n s i o n , compression, f l e x u r e , Izod impact and t o r s i o n pendulum analyses. Conclusions are made regarding the e f f e c t s o f i m p u r i t i e s , s t o i c h i o m e t r y and post-cure treatment on the cure behavior, s t r u c t u r e , s t r e n g t h , s t i f f n e s s , impact energy, g l a s s - t r a n s i t i o n temperature and moisture absorption p r o p e r t i e s o f the TGMDA/DDS r e s i n m a t r i x . Experimental Samples. The TGMDA r e s i n s A r a l d i t e MY720 (batch No. 5093) and Glyamine G-120 (Lot No. 1003) were obtained from Ciba Geigy Corp. and FIC Corg., r e s p e c t i v e l y , and were stored i n c l o s e d c o n t a i n e r s a t -13 C. The samples designated TGMDA and ••Residue" were i s o l a t e d from the G-120 r e s i n u s i n g p r e p a r a t i v e l i q u i d chromatography OK Sample TGMDA i s e s s e n t i a l l y pure monomer; whereas the Residue c o n s i s t s p r i m a r i l y o f higher molecular weight oligomers and r e l a t i v e l y p o l a r r e s i n components. Data c h a r a c t e r i z i n g the TGMDA r e s i n s are shown i n Table I . Epoxy e q u i v a l e n t weights (EEW) were obtained by the standard nonaqueous t i t r a t i o n method (2) u s i n g chloroform as the s o l v e n t . 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 GPC and high performance l i q u i d chromatography (HPLC) techniques f o r determining weight-% TGMDA monomer have already been described ( Ό . V i s c o s i t y measurements were made a t 50°C u s i n g a Rheometrics Mechanical Spectrometer Model RMS-7200 i n steady shear mode with cone and p l a t e geometry. To prepare formulations o f the TGMDA samples with DDS f o r GPC and DSC s t u d i e s , the components were f i r s t weighed ( c a . 30g t o t a l ) , then heated t o about 90°C and mixed u n t i l a homogeneous s o l u t i o n was formed a t 80-90°C HPLC and GPC analyses o f the formulations showed t h a t no apparent r e a c t i o n occurred during mixing. The r e s i n f o r m u l a t i o n s were stored i n sealed c o n t a i n e r s M

W

In Chemorheology of Thermosetting Polymers; May, C.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

Impurities and Stoichiometry of Resin Matrix

HAGNAUER ET AL.

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at -13 C and removed only f o r sampling. The c u r i n g agent DDS was h i g h l y pure ( c a . 99%) as r e c e i v e d from A l d r i c h Chemical Co. The compositions o f the f o r m u l a t i o n s are i n d i c a t e d i n p a r e n t h e s i s as weight percentage o f DDS. GPC Study. GPC i s used t o analyze changes i n the chemical compositions o f the TGMDA/DDS f o r m u l a t i o n s cured f o r d i f f e r i n g periods o f time a t 177°C. Molecular weights and t h e weight percentages o f TGMDA monomer. DDS. r e a c t i o n products and g e l a r e determined as a f u n c t i o n o f cure time. Samples (5-1Omg) were weighed on a microbalance and p a r t i a l l y cured on the h e a t i n g stage o f a Perkin-Elmer DSC 1B instrument i n a n i t r o g e n atmosphere. Cures were terminated by t r a n s f e r r i n g the sample pans t o 25ml v o l u m e t r i c f l a s k s and adding 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 the f l a s k s were a g g i t a t e d and the samples allowed t o soak f o r 1-4 days. Except f o r h i g h l y c r o s s - l i n k e d g e l . a l l the components were s o l u b l e . The sample s o l u t i o n s were d i l u t e d t o 25ml and f i l t e r e d through a 0.2PM M i l l i p o r e membrane f i l t e r p r i o r t o GPC a n a l y s i s . Only s o l u b l e components were analyzed by GPC. A Waters A s s o c i a t e s ALC/GPC-244 instrument w i t h M6000A s o l v e n t d e l i v e r y system. M720 system c o n t r o l l e r , M730 data module, 71 OB 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 GPC 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: PStyragel (2x500Â, 3x100À) Concentrâtion : 0.2-0.5 Pg/Pi I n j e c t i o n Volume: 20-60Pi Mobile Phase: THF (UV grade, Burdick & Jackson, Labs). Flow Rate: 1 ml/min Detector: UV 254nm Run Time: 45 min. T y p i c a l GPC chromatograms are shown i n F i g u r e 1. The chromatograms are d i s p l a c e d along the o r d i n a t e t o i l l u s t r a t e compositional changes. Reaction products e l u t e w i t h r e t e n t i o n times between 28 and 34 minutes. The i n d i c a t e d products were i s o l a t e d by p r e p a r a t i v e GPC f o r purposes o f i d e n t i f i c a t i o n and c a l i b r a t i o n . Areas under the GPC peaks and peak segments a r e d i r e c t l y p r o p o r t i o n a l t o the c o n c e n t r a t i o n s o f the components. Therefore, the weight-% o f each component or s e t o f components designated ^ may be c a l c u l a t e d ,

c

V

A

.100*

(

1

)

m · V ( m

iV s V

i s the c a l i b r a t i o n constant, A. i s the peak V s area of component i and m and V represent the sample where κ. =

(


2.

HAGNAUER ET AL.


c o n c e n t r a t i o n (yg/μΐ) and i n j e c t i o n volume ( P i ) . The s u b s c r i p t s denotes the r e s p e c t i v e parameters f o r the c a l i b r a t i o n s t a n d a r d ( s ) . I t i s noted t h a t the c a l i b r a t i o n constants f o r TGMDA and DDS are q u i t e s i m i l a r and t h a t the 1:1 product has a d i f f e r e n t constant which i s e s s e n t i a l l y i d e n t i c a l t o those of the higher molecular weight products. The c a l i b r a t i o n curve f o r determining molecular weights (MW) i s shown i n Figure 2. D i s c r e t e MWs obtained by averaging the MWs n


n


29


CHEMORHEOLOGY OF THERMOSETTING


POLYMERS

2.

HAGNAUER ET A L .

31 Impurities and Stoichiometry of Resin Matrix

of components e l u t i n g a t the same times are i n d i c a t e d as data p o i n t s and e x t r a p o l a t e d t o account f o r higher MW components. Standard equations are a p p l i e d t o c a l c u l a t e number-.weight- and z-average MWs -

M

L

i

(2)

N= M

I V

i


ΣΑ M

Σνν

Mζ

*- i

i

( M )

where A. i s the peak area f o r component(s) i ^ o f average molecular weight and the area segment f o r component(s) e l u t i n g i n the e x t r a p o l a t e d r e g i o n a t time t . over the i n t e r v a l t . - t . w i t h M. defined by 1

1

log Gel

M

t

1

1

= 6.697-0.1172-^

(5)

content i s determined i n d i r e c t l y according t o the equation m« %gel

r

_ x

1

x

1

κ ·· 100%

(6)

m-V DSC Study. DSC i s used t o determine the extent α and r a t e à of r e a c t i o n d u r i n g the i s o t h e r m a l cure o f the TGMDA/DDS f o r m u l a t i o n s . These parameters are then r e l a t e d t o c o m p o s i t i o n a l changes determined by GPC. Measurements were made using'a DuPont 990 thermal analyzer with 920 DSC module. Procedures d e s c r i b e d i n the instrument o p e r a t i n g manual were used f o r temperature and c a l o r i m e t r i c c a l i b r a t i o n . Samples were weighed ( c a . 20-30mg) i n open aluminum pans and run i n a n i t r o g e n atmosphere. An i n i t i a l b a s e l i n e was e s t a b l i s h e d by e q u i l i b r a t i n g the instrument a t the c u r i n g temperature w i t h empty sample pans and a run was i n i t i a t e d by i n s e r t i n g a f i l l e d sample pan. Thermal e q u i l i b r i u m was regained w i t h i n 2 minutes a f t e r sample i n s e r t i o n and t h e exothermic r e a c t i o n was considered complete when the recorder s i g n a l l e v e l l e d o f f t o a b a s e l i n e . The f i n a l b a s e l i n e was e x t r a p o l a t e d t o determine the t o t a l area under the exotherm curve and hence the i s o t h e r m a l heat o f cure ΔΗ . Each i s o t h e r m a l l y cured samgle was then analyzed u s i n g dynamic DSC a t a h e a t i n g r a t e o f 2 C/min t o o b t a i n the r e s i d u a l heat o f r e a c t i o n Η · Sample weight l o s s e s were n e g l i g i b l e . Data from the i s o t h e r m a l analyses were d i g i t i z e d and evaluated u s i n g a Hewlett Packard 9830 computer. Δ

Γ β 3



32

The t o t a l heat o f r e a c t i o n ΔΗ. . was c a l c u l a t e d from the sum of the i s o t h e r m a l and r e s i d u a l heal§. **^re * f o r e a c i l sample. For i s o t h e r m a l cures the extent o? r e a c t i o n o. a t time t. i s defined 1 a

(7)

i jSV^tot =

where^2j^Hj i s the cumulative heat generated or area i n t e g r a t e d


under the DSC exotherm curve over the time i n t e r v a l t ^ r O t o t . and AHj. i s the t o t a l heat or the sum o f the t o t a l i n t e g r a t e d areas. The extent o f r e a c t i o n i s assumed t o be p r o p o r t i o n a l t o the heat evolved d u r i n g cure. The r a t e o f r e a c t i o n i s d e f i n e d

at

or t ^ .

Mechanical T e s t i n g . A s e r i e s o f r e s i n p l a t e s ( c a . 0.1 i n c h t h i c k n e s s ) and c y l i n d r i c a l specimens (0.5 inch diameter) were prepared from TGMDA/DDS f o r m u l a t i o n s with v a r y i n g s t o i c h i o m e t r y and d i f f e r e n t i m p u r i t y l e v e l s f o r mechanical t e s t i n g . The TGMDA r e s i n , c u r i n g agent DDS. mixing implements and molds were preheated t o 120°C and the appropriate amount o f DDS was mixed i n t o the r e s i n u n t i l a l l v i s i b l e t r a c e s o f DDS disappeared. M a i n t a i n i n g the temperature a t 120°C the r e s i n f o r m u l a t i o n was degassed i n vacuo u n t i l a marked decrease i n the outgassing r a t e was noted ( c a . 10 min) and then c a r e f u l l y poured i n t o the preheated v e r t i c a l T e f l o n molds t o prevent entrapment o f a i r . The cure c y c l e c o n s i s t e d o f h e a t i n g the samples from 120 C t o 135 C (1 C/min). h o l d i n g a t 135°C f o r 4 hours, h e a t i n g t o 177 C (1 C/min) h o l d i n g a t 177°C f o r 2 hours and c o o l i n g s l o w l y overnight t o room temperature. Using t h i s technique e s s e n t i a l l y void f r e e c a s t i n g s were achieved. ASTM standard specimens and procedures were used f o r f l e x u r e (D-690). compression (D-695), Izod impact and t o r s i o n a l pendulum a n a l y s i s (TPA). For t e n s i o n . D1822 t e n s i l e impact specimens were s u b s t i t u t e d f o r D638 specimens t o conserve m a t e r i a l . Test specimens were machined from the p l a t e s and c y l i n d e r s u s i n g a water cooled diamond wheel. A l l the specimens were d r i e d i n vacuo at 100°C f o r three weeks before t e s t i n g or subsequent postcure treatment. Half the specimens were post-cured f o r 2 hours a t 225°C i n vacuo before t e s t i n g . S e l e c t e d specimens were immersed i n d i s t i l l e d water a t 80°C f o r 6 weeks f o r moisture uptake determinations. t

Q

t


2.


HAGNAUER ET A L .


Results and D i s c u s s i o n GPC S t u d i e s . F i g u r e 3(a) and (b) i l l u s t r a t e s how the composition o f the p u r i f i e d TGMDA r e s i n formulated w i t h DDS (25%) changes during cure a t 177°C. The formation o f g e l corresponds to a marked decrease i n the c o n c e n t r a t i o n o f the higher MW products o c c u r r i n g between 30 and 40 minutes ( a l s o see Figure 1 ) . Focusing on the e a r l y stage o f cure p r i o r t o the onset o f g e l a t i o n , i s o l a t i o n and i d e n t i f i c a t i o n o f the r e a c t i o n products shows t h a t simple amine-to-epoxy a d d i t i o n i s the predominant r e a c t i o n . Indeed F o u r i e r transform i n f r a r e d s p e c t r o s c o p i c a n a l y s i s o f the r e a c t i o n mixture a t v a r i o u s stages o f cure supports the c o n c l u s i o n t h a t there i s a one-to-one c o r r e l a t i o n between epoxide c o n c e n t r a t i o n and TGMDA c o n c e n t r a t i o n and t h a t no s i g n i f i c a n t s i d e r e a c t i o n s occur during the e a r l y stages o f cure. Hence. GPC may be a p p l i e d t o i n v e s t i g a t e the k i n e t i c s o f the TGMDA/DDS r e a c t i o n . Experimental data are p l o t t e d i n Figure 4 and show t h a t t h e concentrations C (mol/kg) o f TGMDA and DDS decrease i n p a r a l l e l as the r e a c t i o n time i n c r e a s e s ; i . e . , - d[TGMDA] dt

=

-d[DDS] dt

(9)

over n e a r l y 20% o f the t o t a l t h e o r e t i c a l extent o f r e a c t i o n . S t o i c h i o m e t r i c s t u d i e s show t h a t the r e a c t i o n i s f i r s t - o r d e r with respect t o the c o n c e n t r a t i o n o f TGMDA and second-order with respect t o DDS - d[TGMDA] _ H - [TGMDA] pTrun*1 [DDS] rnnci dt

2

3

( 1 Q )

The r a t e constants k- i n Table I I are c a l c u l a t e d from both the r a t e o f r e a c t i o n o f TGMDA and o f DDS. Co represents the i n i t i a l reactant concentrations. Table I I . GPC Rate Constants f o r the TGMDA/DDS Reaction a t 177°C

Sample TGMDA/DDS (15%) TGMDA/DDS (25%) TGMDA/DDS (37%)

Co(mol/kg) TGMDA DDS 2.01 0.605 1.78 1.01 1.49 1.49

2 -2 -1 k-(kg mol min ) TGMDA DDS 0.0179 0.0179 0.0184 0.0182 0.0224 0.0224

Molecular weight parameters f o r the TGMDA/DDS (25%) r e a c t i o n at 177 C are p l o t t e d i n Figure 5. The r a t e o f change and values of the parameters provide i n f o r m a t i o n r e l a t i n g t o the formation o f the g e l network. Parameters M and M are most s e n s i t i v e t o the formation o f high MW products and approach i n f i n i t y as the r e a c t i o n nears the onset o f g e l a t i o n . The f i n i t e values o f these W


34

CHEMORHEOLOGY OF THERMOSETTING POLYMERS 1

1

1

Γ


801

Time (min) Figure 3.

Changes i n the TGMDA/DDS (25*) (a) Reactant and (b) Reaction Product Concentrations (Weight t) with Cure Time ( t ) a t 177°C




HAGNAUER ET AL.


CHEMORHEOLOGY OF THERMOSETTING


36


POLYMERS

2.

HAGNAUER ET A L .


parameters beyond the onset o f g e l a t i o n and the downward curvature o f the p l o t s are a consequence o f the f a c t that only s o l u b l e components can be analyzed by GPC and that the highest MW products tend t o be incorporated i n t o the g e l network f i r s t . Most l i k e l y , Μ curves downward e a r l y because t h e GPC c a l i b r a t i o n i s no longer a p p l i c a b l e a t r e l a t i v e l y high extents o f r e a c t i o n where a v a r i e t y o f h i g h l y branched products are formed. Only M can be i n t e r p r e t e d beyond the onset o f g e l a t i o n ( s o l i d l i n e ) by i n c l u d i n g the weight f r a c t i o n o f g e l i n the equation χ


M

(11)

where C^ i s the weight f r a c t i o n o f components d e f i n e by Eqs. 1 and 6. DSC Study. From the a n a l y s i s o f the t o t a l heats o f r e a c t i o n of the commercial and f r a c t i o n a t e d TGMDA resin/DDS formulations over a broad s t o i c h i o m e t r i c range, i t i s concluded that the t o t a l heat o f r e a c t i o n d i v i d e d by the epoxy concentration has a constant value tot (eq. epoxy/100g)

. . " ( r — constant

1

2

)

and that the exotherm f o r the epoxy-amine r e a c t i o n i s e s s e n t i a l l y i d e n t i c a l t o that o f the epoxy-epoxy polymerization r e a c t i o n » φ

χ

c

a Φ

Φ

ι-Ι .Ο CO Η

mi


ο

•

•

00 CvJ eu ο

CO

•

• • CO

σ

Φ

σ

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%* CvJ

c*— CO

C0

CO

Chemorheology of Thermosetting Polymers - American Chemical

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