Chemorheology of Thermosetting Polymers - American Chemical

1 Process Automation A Rheological and Chemical Overview of Thermoset Curing C. A . M A Y , M . R. DUSI, J. S. FRITZEN, D. K . HADAD, M . G. MAXIMOVICH, K . G. THRASHER, and A . WERETA, JR.

Downloaded by 80.82.77.83 on December 28, 2017 | http://pubs.acs.org Publication Date: August 29, 1982 | doi: 10.1021/bk-1982-0227.ch001

Lockheed Missiles and Space Company, Inc., Sunnyvale, CA 94086

As little as ten years ago the processing o f thermosetting polymers was more a skilled craft than a scientifically sound procedure. Whereas scientists i n laboratories throughout the world had synthesized and formulated many remarkably useful thermoset products, the processing was, f o r the most part, dependent on the judgement o f the operator. Beginning in the early 70's, instrumentation started becoming a v a i l a b l e which permitted accurate analysis o f complex formulations and various means o f monitoring and studying the chemistry and rheology o f their cure. Thus, a methodology was evolving whereby thermoset curing could be c o n t r o l l e d in a manner s i m i l a r t o other chemical processes precise raw material control and monitoring process chemistry to the proper end point. In the chapters which follow t h i s introductory dissertation, you will find many examples o f how contemporary instrumentation and methodology can be used to understand and control the product i o n o f useful artifacts from thermoset materials. Obviously, the various materials themselves cannot be overlooked. However, the reader should think beyond how a particular chemical structure is studied or analyzed to the techniques and instrumentation involved and how they may be used or modified to solve a problem of his or her i n t e r e s t . Herein will be found a heavy emphasis on chemical k i n e t i c s and rheology the i n t e r a c t i o n between the two goals achieved by a n a l y t i c a l techniques, a p p l i c a t i o n o f chemorheo l o g i c a l techniques to surface coatings, often the f i n a l step i n manufacturing, and finally the use of mathematics and computers i n the processing o f thermoset polymers. I t is the purpose o f t h i s chapter t o offer an introduction t o the instrumental methods of thermoset material and process control as they evolved from adhesive and composite studies i n the Lockheed Laboratories. The results have brought us to the threshold o f scientifically automated processing based on quantitative understanding. The development o f a s c i e n t i f i c a l l y c o n t r o l l e d hardware fabr i c a t i o n process i s not an easy task. A m u l t i - d i s c i p l i n e d scient i f i c approach i s required. The study team members must be s k i l l e d i n a n a l y t i c a l , physical and polymer chemistry, spectros0097-6156/83/0227-0001$07.00/0 © 1983 American Chemical Society May; Chemorheology of Thermosetting Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


2

CHEMORHEOLOGY OF THERMOSETTING POLYMERS

copy, c h e m i c a l , e l e c t r i c a l and manufacturing e n g i n e e r i n g , and computer s c i e n c e s . A t o t a l understanding o f the chemical and r h e o l o g i c a l events i s p r e r e q u i s i t e . By m o n i t o r i n g these events i t i s p o s s i b l e t o use the i n f o r m a t i o n t o c o n t r o l the process heat h i s t o r y and the t i m i n g o f o t h e r s p e c i a l events such as a p p l i c a t i o n o f pressure d u r i n g a l a m i n a t i o n o r bonding process. Phys i c a l r e s t r i c t i o n s , such as presses o r a u t o c l a v e s , g e n e r a l l y pre c l u d e r a p i d and d i r e c t measurement o f these events d u r i n g hard ware f a b r i c a t i o n . However, i n our work (1-5) and t h a t o f o t h e r s (6-12) i t has been shown t h a t changes i n the d i e l e c t r i c proper t i e s o f a c u r i n g r e s i n can be r e l a t e d t o the chemical and rheo l o g i c a l events o f the process. T h i s can be i l l u s t r a t e d by the data i n F i g u r e 1, a p l o t o f the changes i n v e c t o r v o l t a g e (V) and phase angle (ψ), i n an a l t e r n a t i n g c u r r e n t e l e c t r i c a l f i e l d o f 1000 Hz, as a f u n c t i o n o f a simple cure c y c l e ( T ) . As the cure proceeds, two peaks separated by a v a l l e y are observed. The f i r s t peak i s a s s o c i a t e d w i t h the m e l t i n g o f the m a t r i x m a t e r i a l , d e b u l k i n g of the hardware and the w e t t i n g o f f i b e r s , f i l l e r s , o r s u b s t r a t e s as i n the case o f adhesive bonding. The v a l l e y i n the d i e l e c t r i c curves i s a r e g i o n o f high d i p o l e m o b i l i t y when the m a t r i x v i s c o s i t y i s low. I t i s the p a r t o f the process where c o n s o l i d a t i o n p r e s s u r e i s normally a p p l i e d d u r i n g bonding o r l a m i n a t i o n . The f i n a l peak r e p r e s e n t s a r a p i d v i s c o s i t y i n c r e a s e as the r e s i n g e l s and then hardens and the d i p o l e movement becomes more r e s t r i c t e d . In shop p r a c t i c e d i e l e c t r i c p r o p e r t i e s are o b t a i n e d by b u i l d i n g a small c a p a c i t o r i n t o the hardware which has a minimal e f f e c t on hardware mechanical pro perties. The work d e s c r i b e d h e r e i n r e l a t e s p r i m a r i l y t o l a m i n a t i o n and bonding processes. However, the techniques are g e n e r i c t o most forms o f thermoset r e s i n p r o c e s s i n g . In the d i s c u s s i o n which f o l l o w s many o f the r e s i n systems c o n t a i n g l y c i d y l amines. The bulk o f the epoxy f o r m u l a t i o n s used i n the aerospace indus t r y today are based on t e t r a g l y c i d y l m e t h y l e n e d i a n i l i n e , I (T6MDA) and w i t h d i a m i n o d i p h e n y l s u l f o n e , I I (DDS). Systems based on

H,C-CHCH

.CHoCH-CHo

>QCH HQN( HC-CHCH2' CH2CH-CH2 A

2

2

2

0

X

^

9

^

H NO§-ONH 2

2

0

o

N

Tetraglycidylmethylenedianiline, I

Diaminodiphenylsulfone,

II

these products cure by two mechanisms, the c o n v e n t i o n a l amine/ epoxide a d d i t i o n (A) and homopolymerization o f the epoxide ( B ) .

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


1.

MAY

ET AL.

Process Automation

F i g u r e 1. I n f l u e n c e of P h y s i c a l Changes on D i e l e c t r i c P r o p e r t i e s .


3


Η Ο

ι

Η

Ο

/\ \ R-C-C + Ν R

A) Epoxy/Ami ne

R-C-C-N-R' I

Η

Η Η

Ο

/ \

B) Homopolymerization

x

ο

η R-C-C

N

oj o c c I


R

v

Η

ι R

Exact d e f i n i t i o n o f t h e cure chemistry i s d i f f i c u l t s i n c e formu l a t i o n s o f t h i s type may a l s o c o n t a i n a u x i l i a r y c a t a l y s t s (Lewis A c i d s ) which enhance t h e homopolymerization r e a c t i o n . We have demonstrated t h a t f o r m u l a t i v e changes, even minor ones,(13) can measurably a l t e r t h e cure r a t e and t h e subsequent d i e l e c t r i c response. Shown i n Table 1 a r e t h r e e epoxy prepreg Table 1 Prepreg Formulations Formula No. 1 2 3 TGMDA DDS BF -400 RC-2 3

100

100

100

32

32

32

-

1

-

1.43

f o r m u l a t i o n s . Each c o n t a i n s t h e same TGMDA/DDS r e s i n system. However, one r e s i n has no a c c e l e r a t o r (1) another (2) has a w i d e l y used BF~ monoethylamine s a l t and t h e t h i r d (3) c o n t a i n s a p r o p r i e t a r y Lewis A c i d w i t h t h e e l e c t r o n acceptor c o n c e n t r a t i o n equal t o t h a t o f t h e BF -400 system. Shown i n F i g u r e s 2 and 3 a r e d i f f e r e n t i a l c a l o r i m e t r y (DSC) and d i e l e c t r i c scans obtained from a simple 177°C (350°F) cure c y c l e . The DSC data shows t h a t t h e uncatalyzed system (1) d e f i n i t e l y r e a c t s a t a slower r a t e than the o t h e r two f o r m u l a t i o n s . Note a l s o t h a t t h i s system d i s p l a y s o n l y one peak i n t h e DSC w h i l e t h e a c c e l e r a t e d combinations show two. The l e f t hand peaks i n these l a t t e r two f o r m u l a t i o n s a r e caused by t h e a c c e l e r a t o r s , a p o i n t which w i l l be d i s c u s s e d mom entarily. 3

The d i e l e c t r i c scans ( F i g u r e 3) a l s o show d i f f e r e n c e s i n the r a t e s o f r e a c t i o n i n d i c a t i n g s e n s i t i v i t y t o chemical change. From these data i t can a l s o be concluded t h a t t h e p r o p r i e t a r y a c c e l e r a t o r causes t h e most r a p i d cure.



1. MAY ET AL.

Process Automation

5

400r

TEMPERATURE

FORMULA 1 FORMULA 2 FORMULA 3

•ο

Ζ
— δ δ — Λ — M TIME IN MINUTES

F i g u r e 15. D i e l e c t r i c Response from P r o d u c t i o n Mold.


Ho


F i g u r e 16. Normal Cure C y c l e vs P r e l i m i n a r y Closed-Loop Cure C y c l e .


22

CHEMORHEOLOGY OF THERMOSETTING

POLYMERS


ing. S i n c e both o f these changes are measurable by d i e l e c t r i c response, t o t a l automation o f the thermoset r e s i n process should be a c h i e v a b l e i n the near f u t u r e . I t i s e v i d e n t t h a t d i f f e r e n t f o r m u l a t i o n s w i l l r e q u i r e d i f f e r e n t a l g o r i t h m s , however, once one m a t e r i a l i s c o n t r o l l e d , c o n t r o l o f the o t h e r s should be r e a d i l y simple. Using chemical and p h y s i c a l c h a r a c t e r i z a t i o n methods t o assure c o n s i s t e n t s t a r t i n g m a t e r i a l s and d i e l e c t r i c measurements to c o n t r o l and v e r i f y the proper c u r e , a t o t a l l y c o n t r o l l e d process concept i s p o s s i b l e . A l l d e c i s i o n p o i n t s , f r o m prepreg acceptance through p a r t layup and c u r e c o n t r o l t o n o n - d e s t r u c t i v e hardware e v a l u a t i o n (NDE), a r e f r e e d from t h e e r r o r s p o s s i b l e by human d e c i s i o n . One concept o f t h i s type i s i l l u s t r a t e d i n F i g u r e 17 f o r the f a b r i c a t i o n o f f i b e r r e i n f o r c e d composite hardware. The coded incoming m a t e r i a l i s f i r s t s u b j e c t e d t o a s e r i e s o f l a b o r a t o r y (chemical c h a r a c t e r i z a t i o n ) t e s t s . I f these meet s p e c i f i c a t i o n s , mechanical c h a r a c t e r i z a t i o n s are run which determine accept a b l e f i b e r and i n t e r f a c i a l bonding s t r e n g t h s . S i n c e the l a t t e r i s the more expensive acceptance procedure, i t should be run a f t e r the s t a r t i n g prepreg meets the chemical and p h y s i c a l s t a n dards. S i m u l t a n e o u s l y , w i t h r e c e i p t o f the p r e p r e g , computer documentation o f s t o r a g e l i f e should b e g i n . T h i s data becomes a t r a c e a b l e e n t i t y f o r each hardware item made from the m a t e r i a l by l i n k i n g i t through a c e n t r a l computer a c t i v e data f i l e . As the next step the f a b r i c a t o r i s i d e n t i f i e d and h i s a c t i o n s are c o n t r o l l e d by means such as CRT v i d e o d i s p l a y s , v o i c e data e n t r y , e t c . The hardware i s then cured u s i n g a c l o s e d loop m i c r o p r o c e s s o r c o n t r o l l e d c y c l e i n v o l v i n g d i e l e c t r i c s i g n a l s which a l s o v e r i f y t h a t the hardware was p r o p e r l y processed. I f n o n - d e s t r u c t i v e e v a l u a t i o n shows the p a r t i s below an a c c e p t a b l e v o i d l e v e l and thus m e c h a n i c a l l y sound, hardware q u a l i t y i s assured w i t h a high degree o f c o n f i d e n c e . F u r t h e r , should a f a i l ure o c c u r a t some p o i n t d u r i n g i t s u s e f u l l i f e , every step o f the f a b r i c a t i o n i s t r a c e a b l e . Through a data a n a l y s i s each p a r t w i t h the same p r o d u c t i o n h i s t o r y can be l o c a t e d and c o r r e c t i v e a c t i o n can be taken. In a l l p r o b a b i l i t y w i t h b e t t e r s c i e n t i f i c unders t a n d i n g o f m a t e r i a l s and processes t h i s w i l l not happen. However, w i t h human l i v e s dependent on thermoset products i n the f u t u r e , t h i s type o f c o n t r o l i s worthwhile i n s u r a n c e . C u r r e n t hardware acceptance procedures are based on e i t h e r mechanical t e s t i n g o f t a g ends o r d e s t r u c t t e s t i n g o f a c t u a l p a r t s based on s t a t i s t i c a l s e l e c t i o n . Procedures o f t h i s type o n l y v e r i f y the q u a l i t y o f the t e s t specimen. They say n o t h i n g about the q u a l i t y o f the hardware being used. However, by u s i n g f a b r i c a t i o n processes based on sound s c i e n t i f i c p r i n c i p l e s and documenting t h a t these events have o c c u r r e d , we can have a high degree o f conf i d e n c e t h a t r e l i a b l e thermoset products have been produced. On


MAY ET AL.

Process Automation

RECEIVING BAR CODE ASSIGNED TO MATERIAL Downloaded by 80.82.77.83 on December 28, 2017 | http://pubs.acs.org Publication Date: August 29, 1982 | doi: 10.1021/bk-1982-0227.ch001

I

CHEMICAL CHARACTERIZATION\ MECHANICAL CHARACTERIZATION V\ ι : — STORAGE LIFE HISTORY CENTRAL PART FABRICATION COMPUTER BAR CODE ON EACH PART ACTIVE DATA CLOSED-LOOP CURE CYCIf H

I

REWORK REJECT

TREND ANALYSIS

NDE ACCEPTANCE DECISION ACCEPT TRACEABLE PART

CENTRAL COMPUTER HISTORICAL DATA

F i g u r e 17. Composite Hardware C o n t r o l Concept.



24

the b a s i s o f what has been d i s c u s s e d h e r e i n and i n t h e c h a p t e r s which f o l l o w , i t i s e v i d e n t t h a t t h i s time i s c l o s e a t hand.

Literature Cited

1) May, C.A., Fritzen, J.S. and Brown, G.R., International Con ference on Composite Materials, Proceedings, 264-84, Boston/ Geneva (1975). 2) May, C.A., SAMPE Series 20; 108-16, San Diego (April 1975). 3) Wereta, Α.,Jr.,and May, C.A., Org. Coatings and Plastics Chem., ACS Preprints 38, 679-84, Anaheim (March 1978). Downloaded by 80.82.77.83 on December 28, 2017 | http://pubs.acs.org Publication Date: August 29, 1982 | doi: 10.1021/bk-1982-0227.ch001

4) Wereta, Α.,Jr.,and May, C.A., J. Adhesion 12, 317-331 (1981). 5) Fritzen, J.S., Wereta, Α.,Jr.,and Arvay, E.A., SAMPE Series 22, 430-34, San Diego (April 1977). 6) von Hippel, Α., Dielectric Properties of Materials, MIT Press, Cambridge, MA. (1954) 7) Turner, A.Jr.,Mechanical Behavior of High Polymers, Interscience Publishers, New York (1948). 8) Smyth, C.P., Dielectric Behavior and Structure, McGraw-Hill, New York (1955). 9) McCrum, N.G., Reed, B.E. and Williams, G., Anelastic and Di electric Effects in Polymeric Solids, John Wiley and Sons, New York (1967). 10) Wrasidlo, W.J., Motions in Poly-Pyromellitimide, Boeing Scien tific Research Laboratories, Rpt. No. Dl-82-1061. 11) Yaloff, S.A. and Wrasidlo, W.J., J. Appl. Polymer Sci 16, 2159, (1972). 12) Hedvig, P., Dielectric Spectroscopy of Polymers, John Wiley and Sons, New York (1975). 13) May, C.A., Whearty, D.K. and Fritzen, J.S., SAMPE Series 21, 803-18, Los Angeles (April 1976). 14) St. Clair, T.L. and Jewell, R. Α., National SAMPE Tech. Conf. Series, 8, 82-93 Seattle (Oct. 1976). 15) Cheng, D.K., Analysis of Linear Systems, Addison-Wesly Pub lishing Co., Inc., Reading, MA. (1961). 16) May, C.A., Wereta, Α.,Jr., Fritzen, J.S. and Keck, F.L., AFWAL-TR-80-4171 (Nov. 1980). RECEIVED March 31, 1983 May; Chemorheology of Thermosetting Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

Chemorheology of Thermosetting Polymers - American Chemical

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