Resins for Aerospace - ACS Publications - American Chemical Society

24

Resins for Aerospace CLAYTON A. MAY

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 6, 2015 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch024

Arroyo Research and Consulting Corporation, 2661 Beach Road, H-67, Watsonville, CA 95076

Epoxy Resins Phenolic Resins Polyurethanes High-Temperature Resins Adhesives Sealants Thermoset Processing Science and Control

In 1979 at the American Chemical Society/Chemical Society of Japan Chemical Congress in Honolulu, the Division of Organic Coatings and Plastics Chemistry sponsored a symposium entitled "Resins for Aerospace." This was an acknowledgment and tribute to the fact that the aerospace industry is a heavy contributor to the use and understanding of thermoset resin systems. The papers from this symposium later appeared as a 35-chapter book (1). Topics ranged from the design and testing of urethane launch tube seals to a search for readily removable coatings for electronic applications. In the aerospace industry, resinous polymers encompass a wide variety of hardware applications for aircraft, missiles, and space structures. In aircraft, resins are used as a matrix material for primary (flight-dependent) and secondary fiber-reinforced composite (FRC) structures, adhesives for the bonding of metal and composite hardware components, electronic circuit board materials, sealants, and radomes. Missile applications include equipment sections, motor cases, nose cones, carbon-carbon composites for engine nozzles, adhesive bonding, and electronics. As the exploration of outer space intensifies, applications will become even more exotic. FRC w i l l be used to construct telescopes, antennas, satellites, and eventually housing and other platform structures where special properties such as weight, stiffness, and dimensional stability are important. There is l i t t l e question that FRC w i l l be the largest user of resins in aerospace. As illustrated by Figure 1, a l l composite aircraft w i l l soon be available on a commercial scale. Approxi0097 6156/85/0285-0557S07.00/0 © 1985 American Chemical Society

In Applied Polymer Science; Tess, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


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APPLIED POLYMER SCIENCE

Figure 1.

The Lear Fan " a l l composite" a i r c r a f t Lear Fan Corp.).

(photo courtesy of



24.

MAY

Resins for Aerospace

559

mately 95% of the a i r f r a m e s t r u c t u r e of the Lear Fan i s made from composite m a t e r i a l s , p r i m a r i l y g r a p h i t e / e p o x y . T h i s a i r c r a f t i s c u r r e n t l y undergoing F e d e r a l A v i a t i o n A d m i n i s t r a t i o n (FAA) c e r t i f i c a t i o n t e s t s and should obtain approval i n the near future. Beech A i r c r a f t has r e c e n t l y announced p l a n s to b u i l d an " a l l graphite" aircraft. Although the consumption of these materials i n aerospace i s but a s m a l l f r a c t i o n of the r e s i n o u s polymers used i n i n d u s t r y , the impact of the needs and a p p l i c a t i o n s i s large. V i r t u a l l y a l l modern day s t r u c t u r a l a d h e s i v e s have aerospace o r i g i n s . The need f o r e l e v a t e d temperature performance r e s u l t e d i n polymers useful for such d i v e r s e a p p l i c a t i o n s as e l e c t r i c a l i n s u l a t i o n and brake l i n i n g s . The search f o r c o a t i n g s to r e s i s t r a i n e r r o s i o n and UV l i g h t contributed h e a v i l y to the technology of the polyurethanes. M i l l i o n s of d o l l a r s i n hardware and human l i v e s w i l l soon be dependent on aerospace s t r u c t u r e s f a b r i c a t e d from s t r u c t u r a l a d h e s i v e s and f i b e r - r e i n f o r c e d composites. T h i s s i t u a t i o n has d i c t a t e d a more s c i e n t i f i c understanding of s t r u c t u r a l polymer c h a r a c t e r i z a t i o n and p r o c e s s i n g dynamics. Chemical v a r i a t i o n s i n s t a r t i n g raw material formulations can no longer be t o l e r a t e d for fear of unforeseen l o n g - t e r m degradation effects. Cure chemistry must be understood and documented as proof of p r o p e r l y processed a s s e m b l i e s . These needs have l e d to major advances i n chromatography, thermal a n a l y s i s , and spectroscopy of thermoset r e s i n systems and have opened the door toward a b e t t e r understanding of the s c i e n t i f i c s i g n i f i c a n c e of cure k i n e t i c s and l i q u i d and s o l i d state rheology. I t i s the purpose of t h i s chapter to discuss the types and uses of resins for aerospace and a l s o to document aerospace contributions t o t h e s c i e n c e and u n d e r s t a n d i n g o f s t r u c t u r a l p o l y m e r s . Thermoplastics w i l l not be a part of t h i s discussion. They do have aerospace a p p l i c a t i o n s , most notably, i n the i n t e r i o r furnishings of commercial a i r c r a f t . However, i t i s the thermoset resins that have been the major contributor to aerospace hardware technology. This chapter w i l l deal with the chemistry and a p p l i c a t i o n s of epoxies, phenolics, urethanes, and a v a r i e t y of current vogue h i g h temperature polymers. A p p l i c a t i o n s i n f i b e r - r e i n f o r c e d p l a s t i c s w i l l be d i s c u s s e d i n the i n d i v i d u a l s e c t i o n s on r e s i n c h e m i s t r y where appropriate. Separate sections w i l l deal with adhesives and sealants. Adhesives are most important because, as e a r l y h i s t o r y demonstrates, they l e d the way to the a p p l i c a t i o n of r e s i n s i n aerospace. A section i s a l s o included on s i l i c o n e and p o l y s u l f i d e sealants. A l t h o u g h these m a t e r i a l s are e l a s t o m e r s r a t h e r than r e s i n s , no d i s c u s s i o n of aerospace polymers would be complete without some mention. Some major thermosetting polymers have been o m i t t e d from t h i s r e v i e w . Among t h e s e are the unsaturated polyesters, melamines, ureas, and the v i n y l esters. Although these products do find t h e i r way i n t o aerospace a p p l i c a t i o n s , the uses are so s m a l l that a d e t a i l e d discussion i s not warranted. Epoxy Resins Of a l l the resins used by the aerospace industry, epoxy resins have, by far, gained the widest acceptance. They offer a v e r s a t i l i t y that i s u n a t t a i n a b l e by any of the other m a t e r i a l s t h a t w i l l be



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d i s c u s s e d . As raw m a t e r i a l s , they range i n v i s c o s i t y from w a t e r t h i n l i q u i d s to h i g h m o l e c u l a r weight s o l i d s . As such they are a d a p t a b l e to a wide v a r i e t y of manufacturing processes. For example, i n the manufacture of f i b e r - r e i n f o r c e d p l a s t i c (FRP) s t r u c t u r e s , low v i s c o s i t y would be r e q u i r e d f o r a process such as f i l a m e n t winding or p u l t r u s i o n , whereas a s o l i d form of the r e s i n would be prerequisite for the dry lay-up processing of laminates for electronic applications. C r o s s - l i n k i n g of these polymers i s accomplished by combinations of resins and curing agents. The broad range of materials a v a i l a b l e for these purposes affords a wide v a r i e t y of curing conditions and s t r u c t u r a l properties. Cures range from very short periods of time at room temperature to much longer periods at elevated temperatures for resin-curing agent combinations that are capable of e l e v a t e d temperature performance. When cured, polymer properties range from s o f t , f l e x i b l e m a t e r i a l s to hard, tough, c h e m i c a l - r e s i s t a n t , and e l e v a t e d t e m p e r a t u r e - r e s i s t a n t products. C h a r a c t e r i s t i c a l l y , the epoxy r e s i n s s h r i n k l e s s d u r i n g cure than most other thermoset resins, and no v o l a t i l e by-products are generated during the cure. Epoxies a l s o provide e x c e l l e n t e l e c t r i c a l i n s u l a t i n g c h a r a c t e r i s t i c s , outstanding chemical and s o l v e n t resistance, and, above a l l , e x c e l l e n t a d h e s i o n . The most common a e r o s p a c e a p p l i c a t i o n s of epoxy resins are i n adhesives and FRP; however, they a l s o f i n d use i n s u r f a c e c o a t i n g s and a v a r i e t y of e l e c t r i c a l and electronic applications. The most commonly used epoxy r e s i n i s the d i g l y c i d y l e t h e r of b i s p h e n o l A and i t s h i g h e r homologs. The pure d i g l y c i d y l e t h e r (n = o) i s a low-melting c r y s t a l l i n e s o l i d . However, the commercial grades s t a r t as a l i q u i d with a v i s c o s i t y of approximately 40 P and range upward to v a l u e s of n e q u a l to 18 or more f o r some of the higher molecular weight coating grades.

There are three general chemical reactions basic to the curing of epoxy r e s i n s : the r e a c t i o n of epoxides w i t h amines, t h e i r r e a c t i o n s w i t h c a r b o x y l i c a c i d s or a n h y d r i d e s , and c a t a l y t i c homopolymerizations. The most common curing agents are the amines wherein each of the amino hydrogens r e a c t w i t h an o x i r a n e r i n g . Depending on the number of amino hydrogens per m o l e c u l e and the s u p p o r t i n g c h e m i c a l s t r u c t u r e , a wide v a r i e t y of p r o p e r t i e s and cures are a v a i l a b l e . A l i p h a t i c amines afford rapid cures at low to modest temperatures, but o n l y moderate elevated-temperature performance to 125 °C maximum. The aromatic amines are u s e f u l to around 175 °C and have e x c e l l e n t chemical and s o l v e n t resistance but r e q u i r e h i g h e r cure temperatures. Common p r a c t i c e i s to cure the r e s i n system at the d e s i r e d use temperature. The c y c l o a l i p h a t i c amines offer an i n t e r e s t i n g compromise between the other two amines. They have u s e f u l p r o p e r t i e s i n the 150 °C r e g i o n but can be cured under r e l a t i v e l y m i l d conditions, around 60 °C.


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H

0 ? ? CH>-R' , 2- - ' RNH + CH -CH-R' —— RNHCH, - CH -R' — * -»RN CH,-CH-R' 0

2

H

C H

C H

1

X

c


R

2

I

OH Carboxylic acid and anhydride curing agents are used to a lesser e x t e n t i n aerospace a p p l i c a t i o n s as compared to the amines which have mechanical properties and cure conditions that can be t a i l o r e d to a wider v a r i e t y of s p e c i f i c a p p l i c a t i o n s . Anhydrides tend to be somewhat b r i t t l e but o f f e r u s e f u l s e r v i c e as h i g h as 250 °C w i t h novolac-type epoxy resins. In addition, the a l i p h a t i c d i c a r b o x y l i c acid anhydrides give tough and sometimes f l e x i b l e properties which are useful i n encapsulation a p p l i c a t i o n s . The curing reactions are h i g h l y complex, and the mechanisms have been the subject of numerous a r t i c l e s i n the t e c h n i c a l l i t e r a t u r e . The e a r l i e s t mechanism, as proposed by F i s c h and Hofmann (.2), i n v o l v e d the r e a c t i o n of the anhydride w i t h an a l c o h o l , e i t h e r h y d r o x y l f u n c t i o n a l i t y of the r e s i n i t s e l f or some h y d r o x y 1 c o n t a i n i n g i m p u r i t y such as m o i s t u r e . The c a r b o x y l group of the h a l f - a c i d ester then reacts with an epoxide to form a second ester l i n k a g e and generates a new h y d r o x y l group as the r e a c t i o n continues. When t h i s mechanism p r e v a i l s , optimum properties r e s u l t at an anhydride:epoxide molar r a t i o of 0.85:1. This r e s u l t i n d i c a t e s t h a t a secondary r e a c t i o n occurs d u r i n g the cure. The most probable i s an acid-catalyzed epoxide homopolymerization (see below). 0 C R

x

n

0 C~0R'

,,

0 + R*0H-~ R'

V

N

0 0 C"0R' + R -CH-CH — R' 7

L

C-0H

R (C0)~ 0

etc.

"c-O-CrL-CH R"

6 6 6 OH I t was l a t e r found that Lewis acids and bases were c a t a l y s t s for the epoxide-anhydride r e a c t i o n . The p r e f e r r e d s t o i c h i o m e t r y w i t h respect to anhydride and epoxy proved to be 1:1. This r e l a t i o n s h i p led Fischer (3) to propose a mechanism wherein the f i r s t step was a r e a c t i o n between the anhydride and the a c c e l e r a t o r to form a carboxyl anion. This anion i n turn reacts with an epoxide, and the reaction repeats i t s e l f . 0 0 0 +

A

+

/CNR. 0 CNR. R' 3 . -C—R' 3 R (COLO etc. C C-0C-0-C-C-R' ^ 0 0 0 0" The t h i r d c u r i n g r e a c t i o n of importance to the aerospace i n d u s t r y i s epoxide homopolymerization. The most p r e v a l e n t i s c a t i o n i c p o l y m e r i z a t i o n i n d u c e d by L e w i s a c i d s and may be i l l u s t r a t e d as f o l l o w s (4): n

R

A

0 + RN:

+ R

C

N


562


0 X Y~+R +

cft-CH-

+nRCH-CH

x

Y"..X0 /

\

2

+

X(0CH CHR) 2


R CH-CH

n

0

+

....Y"

R CH-CH

2

A l t h o u g h t h i s r e a c t i o n does n o t l e a d t o a h i g h d e g r e e o f p o l y m e r i z a t i o n w i t h monoepoxides, the p o l y f u n c t i o n a l i t y of most epoxy r e s i n s l e a d s to h i g h l y c r o s s - l i n k e d s t r u c t u r e s and good thermal p r o p e r t i e s . The most important Lewis a c i d i s boron t r i f l u o r i d e , most commonly used i n the form of an amine s a l t which f a c i l i t a t e s handling and c o n t r o l s the exothermic heat of reaction. Other compounds i n t h i s category i n c l u d e the h a l i d e s of t i n , aluminum, z i n c , boron, s i l i c o n , i r o n , t i t a n i u m , magnesium, and antimony. The fluoroborates of these metals have a l s o been reported as c a t a l y s t s . Lewis a c i d s are a l s o used to c a t a l y z e the amineepoxide reaction as described i n the f o l l o w i n g discussion. The c l a s s of epoxy r e s i n s most commonly used i n aerospace a p p l i c a t i o n s today are the g l y c i d y l amines. P o s s e s s i n g good adhesive c h a r a c t e r i s t i c s and a h i g h degree of i n t e r l a m i n a r shear, they find widespread use i n FRP. The most popular i s t e t r a g l y c i d y l methylenedianiline (TGMDA) cured with diaminodiphenyl sulfone (DDS). The mechanisms of cure are h i g h l y complex. Because Lewis a c i d c a t a l y s t s are many times included i n the r e s i n formulations, both amine-epoxide and h o m o p o l y m e r i z a t i o n r e a c t i o n s are important c o n s i d e r a t i o n s when s t u d y i n g the cure c h e m i s t r y . As many as nine d i f f e r e n t c h e m i c a l r e a c t i o n s can be w r i t t e n to d e s c r i b e the c u r e . Morgan et a l . (5) have studied t h i s system e x t e n s i v e l y and concluded that the presence of a BF3 amine complex enhances the consumption of the epoxide group by homopolymerization during cure even when l e s s than the s t o i c h i o m e t r i c amount of DDS i s present i n the r e s i n system. 0

A

TETRAGLYCI DYLMETHYLENEDIANI LINE 0 0 Dl AMI NODI PHENYLSULFONE Epoxy r e s i n s are the most v e r s a t i l e product used i n aerospace composite hardware. They are employed both alone and i n combination w i t h a v a r i e t y of other r e s i n s to form a broad range of p r o d u c t s . The only l i m i t a t i o n to the number of a v a i l a b l e products rests i n the a b i l i t y of the forraulator to meet the ever-increasing demands for improved s t r u c t u r a l properties. Greater d e t a i l on the use of epoxy resins for bonding can be found i n the section on adhesives. In addition to FRP and adhesives, a t h i r d important use of epoxy resins i n aerospace i s i n surface coatings. Epoxy resin-polyamide combinations are used as primers under urethane top coats. The most



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general formulations i n v o l v e the lower molecular weight s o l i d grades of epoxy r e s i n s where n e q u a l s 2-4. The polyamide c u r i n g agents a r e t h e r e a c t i o n p r o d u c t o f an a l i p h a t i c amine s u c h as diethylenetriamine with C13 dimerized or trimerized fatty acids. Epoxy r e s i n s a l s o f i n d e x t e n s i v e use i n the f a b r i c a t i o n o f printed c i r c u i t board laminates. These are of great importance to the aerospace industry. Most laminations are made by using dry l a y up techniques because f l a t surfaces are i n v o l v e d . Here again the lower weight, s o l i d grades of r e s i n are used. However, because long ambient temperature s t o r i n g of the prepreg i s d e s i r a b l e , l a t e n t c u r i n g agents such as dicyandiaraide are most commonly employed. Epoxy novolacs are added to these formulations to improve e l e v a t e d temperature performance, and f i r e r e t a r d a n c e i s a c h i e v e d by incorporating resins based on tetrabromobisphenol A. Before l e a v i n g the subject of epoxy resins, toughening w i l l be d i s c u s s e d b r i e f l y because i t i s an area of c u r r e n t r e s e a r c h and development a c t i v i t y . Even though epoxy formulations are tougher than most h i g h g l a s s t r a n s i t i o n temperature (Tg) polymers, the aerospace community demands more. Normally, to toughen a material one adds a p l a s t i c i z e r . This a d d i t i o n , however, reduces the crossl i n k d e n s i t y of the system and l o w e r s the Tg. T h e r e f o r e , c u r r e n t attention has turned to the use of dispersions of s m a l l amounts of rubber or other elastomeric p a r t i c l e s i n the r e s i n that a r r e s t crack growth i n a b r i t t l e m a t r i x , thereby i n c r e a s i n g the f r a c t u r e toughness. In c u r r e n t t e c h n o l o g y t h i s r e s u l t i s a c c o m p l i s h e d by incorporating r e l a t i v e l y low molecular r e a c t i v e l i q u i d rubbers i n t o the epoxy f o r m u l a t i o n (j6, 7_). Dynamic mechanical s t u d i e s by Manzione and G i l l h a m (8) have shown that by c o n t r o l of the rubberepoxy c o m p a t i b i l i t y and cure c o n d i t i o n s a wide range o f rubber p a r t i c l e morphologies can be obtained. These morphologies r e s u l t i n different stress-response mechanisms. The elastomeric materials most commonly used for t h i s purpose are c a r b o x y l - t e r m i n a t e d a e r y l o n i t r i l e - b u t a d i e n e copolymers. The c a r b o x y l groups r e a c t w i t h the epoxy group t o produce an epoxyterrainated rubber t h a t promotes i n t e r f a c i a l bonding i n two-phase systems (9). By c o n t r o l l i n g the concentrations of M, X\ X 2 , Z, and U, a broad range of c o m p a t i b i l i t i e s can be a c h i e v e d . Impact r e s i s t a n c e and f r a c t u r e toughness are i n c r e a s e d w i t h minimal s a c r i f i c e i n molulus and elevated-temperature performance. 9

0 HO-C-R+(CH -CrKH^

0

2

1

CH CH

2

2

C*N

C-0 M OH

Phenolic Resins Phenolics are the oldest resins employed by the aerospace industry. Baekeland began h i s work on these materials around 1905, two years after the Wright brothers made t h e i r f i r s t a i r p l a n e f l i g h t . By 1910 phenolic resins were being used commercially i n laminates, moldings, and i n s u l a t i n g varnishes. As discussed i n the section on adhesive bonding, phenolic resins played an important e a r l y r o l e as a r e s i n for aerospace a p p l i c a t i o n s . They continue to be an important r e s i n for adhesives, a b l a t i v e s , and carbon-carbon composites. Their high


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char formation c h a r a c t e r i s t i c s c o u l d w e l l l e a d to a renewed importance i n aerospace a p p l i c a t i o n s . F i r e retardant resins, p a r t i c u l a r l y i n commercial a i r c r a f t , c o u l d w e l l be a demanded prerequisite i n the near future. Both r e s o l e and n o v o l a c types of p h e n o l i c r e s i n are used i n aerospace a p p l i c a t i o n s . The r e s o l e s are g e n e r a l l y made by u s i n g a l k a l i n e c a t a l y s t s and an excess of formaldehyde. The r e s o l e structure i s h i g h l y complex and i n v o l v e s both methylene and dimethyl ether bridges between the phenolic moieties. During cure both water and formaldehyde are e v o l v e d .

The novolac resins are prepared by using a c i d i c c a t a l y s t s and a d e f i c i e n c y of formaldehyde. Because t h i s type of r e s i n i s l e s s r e a c t i v e , c r o s s - l i n k i n g i s accomplished by the addition of a curing agent or c a t a l y s t . The most common i s hexamethylenetetramine or "hexa." The curing agent serves as a l a t e n t source of formaldehyde. As i n the case of the r e s o l e s , v o l a t i l e s are emitted d u r i n g the c u r e . The chemistry of the p h e n o l i c r e s i n i s o l d but complex and w e l l documented i n the l i t e r a t u r e (10).

n As indicated e a r l i e r , i n s p i t e of a long h i s t o r y of association with the aerospace industry, the products find only l i m i t e d use i n s p e c i f i c a p p l i c a t i o n s . The e v o l u t i o n of v o l a t i l e s during the cure c o m p l i c a t e s the f o r m a t i o n of i n t r i c a t e shapes. When c u r e d , the resins are h i g h l y c r o s s - l i n k e d and expectedly b r i t t l e . They a l s o oxidize r e a d i l y i n a i r at elevated temperatures. The main areas of aerospace a p p l i c a t i o n as i n d i c a t e d are a d h e s i v e s , a b l a t i v e s , and carbon-carbon composites. In a d h e s i v e s , because of t h e i r b r i t t l e n a t u r e , p h e n o l i c s are g e n e r a l l y formulated i n add mixture with other polymers. They are used as the primary c r o s s - l i n k i n g agents f o r n i t r i l e rubbers, p o l y ( v i n y l formal) and b u t y r a l resins, and epoxy resins. The other main aerospace a p p l i c a t i o n s a c t u a l l y take advantage of the poor o x i d a t i o n r e s i s t a n c e . During the space race of the l a t e 1950s and e a r l y 1960s, c o n s i d e r a b l e r e s e a r c h was conducted on the a b l a t i v e c h a r a c t e r i s t i c s of thermoset resins (11). Phenolics proved to be i d e a l materials providing one of the highest char y i e l d s and good short-terra, e l e v a t e d - t e m p e r a t u r e s t r e n g t h . High-density graphite fiber-reinforced laminates have proven to be an e x c e l l e n t a b l a t i v e heat s h i e l d m a t e r i a l (12). I f the char formation i s c a r r i e d out at high pressures under c a r e f u l l y c o n t r o l l e d conditions, e x c e l l e n t carbon-carbon composites r e s u l t . These materials are used i n reentry nose cones and rocket nozzles.


24. MAY

565



Polyurethanes The polyurethanes, although not used i n large volumes, have s p e c i f i c a p p l i c a t i o n s of v a l u e f o r aerospace. Accordingly, a brief d i s c u s s i o n of the c h e m i s t r y and the m a t e r i a l a p p l i c a t i o n s i s i n o r d e r . There are t h r e e c h e m i c a l r e a c t i o n s of importance: the r e a c t i o n of h y d r o x y l groups w i t h i s o c y a n a t e s to form urethanes (Reaction 1), the reaction of isocyanates with water to form amines ( R e a c t i o n 2), and the r e a c t i o n of amines w i t h i s o c y a n a t e s to form ureas (Reaction 3). H 0 R-N-C-O+^OH

— R-N-C-OR Urethane

R-N =00 + H 0

R - NH +C0

2

2

(2)

2

Amine H 0 H R-N = C=0+R N H — R - N - C - N - R 2

1

(3)

Urea A l t h o u g h t h e p o l y u r e t h a n e s form u s e f u l a d h e s i v e bonds, p a r t i c u l a r l y between m e t a l s and e l a s t o m e r s , t h e i r use i n the aerospace i n d u s t r y f o r bonding purposes i s l i m i t e d . Because polyurethanes tend to depolymerize above 120 °C and are subject to h y d r o l y s i s , and because aromatic urethanes autoxidize when exposed to thermal or UV l i g h t (13), e p o x i e s a r e the p r e f e r r e d bonding m a t e r i a l . Recently they were studied as launch s e a l s for both land and sea m i s s i l e launch tubes and were found to be superior to s e a l s based on neoprene rubber (14). The c h e m i c a l r e a c t i o n f o r t h i s a p p l i c a t i o n i s proposed to be t h a t between i s o c y a n a t e s and amines ( R e a c t i o n 3). The major aerospace use of i s o c y a n a t e c h e m i s t r y i s i n s u r f a c e c o a t i n g s . T h i s use r e s u l t e d from the d i s c o v e r y t h a t a l i p h a t i c i s o c y a n a t e s were v a s t l y s u p e r i o r to the c o n v e n t i o n a l a r o m a t i c isocyanates i n resistance to UV l i g h t . The preferred c r o s s - l i n k i n g agent i s hexamethylene d i i s o c y a n a t e . Because t h i s component i s h i g h l y t o x i c , the f o r m u l a t i o n s i n v o l v e i s o c y a n a t e - p o l y o l p r e c o n d e n s a t e s a p p l i e d o v e r e p o x y - p o l y a m i d e p r i m e r s . The precondensates g e n e r a l l y employ an excess of the d i i s o c y a n a t e . Thus, because a i r c r a f t u s u a l l y r e l y on ambient-temperature cures, R e a c t i o n s 2 and 3 a r e o f p r i m a r y i m p o r t a n c e f o r c o a t i n g applications. High-Temperature Resins No t r e a t i s e on r e s i n s f o r aerospace would be complete w i t h o u t discussing high-temperature polymers. There has been a continuing major r e s e a r c h e f f o r t i n t h i s a r e a . I n i t i a l work was devoted t o e l e v a t e d - t e m p e r a t u r e performance and s t a b i l i t y . L i t t l e or no



566


a t t e n t i o n was aimed at hardware f a b r i c a t i o n . The r e s u l t was a p l e t h o r a of unique, almost i m p o s s i b l e to h a n d l e , i n t r a c t a b l e polymers. Fortunately, i t was q u i c k l y recognized that polymer cost, a v a i l a b i l i t y , and p r o c e s s i b i l i t y were a l s o i m p o r t a n t , and the necessary compromises began. Hergenorther and Johnston have recently published an e x c e l l e n t s t a t u s r e p o r t on high-temperature polymers (15). The i n i t i a l high-temperature r e s i n s of s i g n i f i c a n c e were the l i n e a r polyimides formed by the reaction of an aromatic dianhydride such as S ^ ' j A j V - b e n z o p h e n o n e t e t r a c a r b o x y l i c d i a n h y d r i d e w i t h bis(aminophenylene)diamines. These products processed p o o r l y but d i d f i n d a l i m i t e d m a r k e t p l a c e i n the form of c o a t i n g s , m o l d i n g m a t e r i a l s , and a d h e s i v e s . Other t h e r m o p l a s t i c high-temperature polymers of i n t e r e s t were the p o l y b e n z i r a i d a z o l e s made by the condensation of aromatic b i s ( o - d i a r a i n e s ) w i t h d i p h e n y l aromatic d i c a r b o x y l a t e s and the polyphenylquinoxalines from bis(o-diaraines) and aromatic d i b e n z y l s . The development of more p r o c e s s i b l e thermoset high-temperature r e s i n s began when p o l y i m i d e s of lower molecular weight and improved flow were synthesized by end capping the polymer c h a i n w i t h monofunctional d i c a r b o x y l i c anhydrides capable of providing c r o s s - l i n k i n g v i a v i n y l polymerization. The most p o p u l a r end-cap compounds were the anhydrides of m a l e i c and Nadic a c i d s . 0 0

Typical Polyimide Resin The low molecular weight, improved p r o c e s s i b i l i t y approach was d e v e l o p e d f u r t h e r by the NASA-Lewis p o l y m e r i z a t i o n of monomeric reactants (PMR) concept (16). I t i s based on the fact that mixtures of p , p - m e t h y l e n e d i a n i l i n e w i t h a selected concentration blend of t h e m e t h y l or e t h y l h a l f e s t e r s o f N a d i c and benzophenone d i a n h y d r i d e s form a v i s c o u s mass y i e l d i n g composite prepregs and adhesives with d e s i r a b l e tack and drape. Drape i s a p a r t i c u l a r l y important shop c h a r a c t e r i s t i c because i t permits conformal movement of the prepregs on t o o l s u r f a c e s d u r i n g the f a b r i c a t i o n of the complex shapes r e q u i r e d f o r aerospace a p p l i c a t i o n s . The main problem associated with t h i s concept i s that v o l a t i l e s are e v o l v e d d u r i n g the e a r l y p r o c e s s i n g stages because of the i m i d i z a t i o n reactions. f



24.

MAY


567

NASA-Langley provided a further improvement i n p r o c e s s i b i l i t y by replacing the methylenedianiline with a commercial aromatic diamine m i x t u r e t h a t afforded even b e t t e r h a n d l i n g c h a r a c t e r i s t i c s (17). This type of product i s formed by the reaction of formaldehyde with a n i l i n e as a p r e c u r s o r i n i s o c y a n a t e manufacture. Fluorinecontaining thermoplastic polyimides and polyimides end capped with acetylene groups for c r o s s - l i n k i n g are a l s o a v a i l a b l e as p o t e n t i a l commercial high-temperature resins. A current version of the low molecular weight concept i s the use of bismaleimides made from aromatic diamines and/or mixtures thereof and m a l e i c anhydride i n admixture w i t h other v i n y l monomers. Several commercial formulations of t h i s type are now a v a i l a b l e as the neat resins or as FRP prepregs. There i s considerable current r e s e a r c h i n t h i s area because of the f a v o r a b l e p r o c e s s i b i l i t y . However, to date, FRP products of t h i s type are outperformed by the PMR-type polyimides by about 50 °C at elevated temperatures. The f a b r i c a t i o n of h i g h temperature performance composites i s the area of most current i n t e r e s t . The f a v o r i t e approaches are the PMR polyimides and b i s m a l e i m i d e / v i n y l monomer combinations as matrix resins. One t h e r m o p l a s t i c p o l y i m i d e f o r a d h e s i v e bonding i s commercially a v a i l a b l e , and research on the use of high-temperature r e s i n s f o r t h i s purpose c o n t i n u e s . P o l y i m i d e f i l m s and prepregs a l s o are c u r r e n t l y r e c e i v i n g c o n s i d e r a b l e a t t e n t i o n f o r the f a b r i c a t i o n of p r i n t e d c i r c u i t boards. Research i n the area of high-temperature r e s i n systems c o n t i n u e s to be p o p u l a r . The main d e t e r r e n t s to success remain: h i g h - p r i c e d s t a r t i n g m a t e r i a l s , l i m i t e d a v a i l a b i l i t y , and d i f f i c u l t p r o c e s s i b i l i t y . Adhesives I f there i s one area that can be s i n g l e d out as the c a t a l y s t for the aerospace a p p l i c a t i o n of r e s i n s , i t i s a d h e s i v e s . They have been used i n the f a b r i c a t i o n of aerospace hardware f o r over 60 y e a r s . Shortly after World War I , the Loughead brothers b u i l t an a i r c r a f t i n G o l e t a , C a l i f o r n i a , c a l l e d the S p o r t . I t was a bonded plywood veneer s t r u c t u r e t h a t employed a c a s e i n g l u e . The molds used to form the aerodynamic shape were made from concrete. This structure was f o l l o w e d s h o r t l y by a l i n e of commercial a i r c r a f t c a l l e d the Vega. This l i n e was produced by the same process and served notice to the then f l e d g l i n g a i r c r a f t industry that resins were to play an important r o l e i n the aerospace industry. Figure 2 shows t h i s e a r l y "assembly l i n e . " During the 1920s the a d h e s i v e was changed to a phenolic r e s o l e that was cured by induction heating. This type of construction was s t i l l i n use as l a t e as 1938. The next major advance i n bonding t e c h n o l o g y was the v i n y l phenolic "redux" adhesive developed by de Bruyne (18) during World War I I . I t was used for the assembly of the De H a v i l l a n d a i r c r a f t . Bonds were made by spreading a s o l u t i o n of a phenolic r e s o l e on the plywood parts to be bonded, s p r i n k l i n g t h i s coating with a powdered p o l y ( v i n y l f o r m a l ) r e s i n , and b l o w i n g o f f the excess powder. The s o l v e n t was a l l o w e d to e v a p o r a t e , and the bonding process was completed by the a p p l i c a t i o n of heat and p r e s s u r e . A l t h o u g h t h i s technique sounds crude, consistent high-performance bonds r e s u l t e d . The use of a h i g h m o l e c u l a r weight t h e r m o p l a s t i c to toughen a b r i t t l e thermoset was probably the f i r s t example of the commercial




568

Figure 2.

The Lockheed Vega production l i n e , e a r l y 1920s. Note the s i m i l a r i t y of these bonded wood s t r u c t u r e s to a i r c r a f t c u r r e n t l y being made from aluminum (photo c o u r t e s y of Lockheed - C a l i f o r n i a Company).



24. MAY

569


a p p l i c a t i o n of a polymer a l l o y and the s t a r t of s t r u c t u r a l adhesive formulation technology. During World War I I , research i n Germany l e d to the discovery of the urethanes. A d h e s i v e s based on h y d r o x y l - c o n t a i n i n g polymers, p r i m a r i l y synthetic hydroxyl-terminated polyesters, i n combination w i t h i s o c y a n a t e s , were e x c e l l e n t f o r bonding rubber to metal substrates. They were used to bond rubber tank treads to the metal cogs of the d r i v e chains. Simultaneously, epoxy resins were being developed both i n the United States and abroad (Switzerland). Thus, d u r i n g the l a t e 1940s, a number of o u t s t a n d i n g c a n d i d a t e s f o r s t r u c t u r a l bonding became a v a i l a b l e , and they are responsible for many of the products i n today's marketplace. From these e a r l y e f f o r t s , epoxy r e s i n s emerged as the l e a d i n g candidate. They were tougher than the phenolics, and no v o l a t i l e s were evolved during the cure. Compared to the isocyanates they had s u p e r i o r thermal s t a b i l i t y and moisture r e s i s t a n c e . The uses of epoxy resins for adhesive bonding i n aerospace a p p l i c a t i o n s are so w i d e s p r e a d t h a t t h e y w o u l d be i m p o s s i b l e t o d e s c r i b e i n a d i s s e r t a t i o n of t h i s l e n g t h . The r e s i n - c u r i n g agent combinations range i n t o the hundreds. In a d d i t i o n , epoxies are used i n combination w i t h other r e s i n s . Phenolic resins improve e l e v a t e d temperature bond performance. The use of nylon adds peel strength to the bond l i n e and g i v e s very h i g h - s t r e n g t h bonds at ambient temperatures. V i n y l resins are added to paste adhesives to improve the green s t r e n g t h d u r i n g bonding. N i t r i l e e l a s t o m e r s p r o v i d e toughness. The o n l y l i m i t a t i o n to the number of epoxy-based adhesives a v a i l a b l e for the aerospace industry r e s t s i n the a b i l i t y of the formulator to meet the ever-increasing demands for improved structural properties. Furthermore, the wide range of m a t e r i a l forms a v a i l a b l e to the f o r m u l a t o r permits a p p l i c a t i o n of the a d h e s i v e s i n the form of p a s t e s , powders, and h i g h l y tacky to dry tapes. As i l l u s t r a t e d by the data i n Table I , the choice of curing agent alone can lead to uses covering a broad range of temperatures (19). Another noteworthy aerospace adhesive was developed during the mid-1950s. I t was a combination of a phenolic resole and an epoxy r e s i n t h a t had a use temperature ranging up to 260 °C (500 °F) as shown by the l a s t e n t r y i n T a b l e I . The f o r m u l a t i o n c o n s i s t e d of the f o l l o w i n g (parts by weight (pbw)): S o l i d d i g l y c i d y l ether of bisphenol A (n=2-3) Phenolic resole (A-stage) Aluminum powder Dicyandiamide Cu 8-hydroxyquinoline

33 67 100 6 1

The adhesive i s manufactured i n tape form by a hot-melt process. I t i s a tacky s o l i d at room temperature. The i n t e g r i t y i s maintained by u s i n g a f i n e l y woven g l a s s f a b r i c s c r i m as the c a r r i e r . T h i s process i s an e x c e l l e n t example of the compromises required i n the technology of formulation. Some of the high-temperature performance t h a t i s expected from the p h e n o l i c r e s o l e i s s a c r i f i c e d f o r the improved bond strength and toughness afforded from the epoxy r e s i n . The f i l l e r i s added to make the thermal c o e f f i c i e n t of expansion of the cured a d h e s i v e more m e t a l l i c i n n a t u r e . Dicyandiamide i s the



resole

Phenolic

acid anhydrides

Carboxylic

330-350 (439-450)

200-300 (366-422)

(10.3)

300-350 (422-450)

l a t e n t (24.1) 1500 3000 (20.7) amines

Insoluble

3500 (24.1)

Aromatic 275-325 (408-436) amines (27.6) 2000 (13.8)

2500 (17.2)

2000 (13.8)

2500 (17.2)

3000 (20.7)

3000 (20.7)

175-210 (353-372)

tic tertiary amines

Mixed a l i p h a -

3000 (20.7)

2000 (13.8) 3000 (20.7)

(297)

75 (297) 175-210 (353-372)

75

3

2500 (17.2)

2500 (17.2)

3500 (24.1)

3500 (24.1)

3500 (24.1)

3000 (20.7) 3500 (24.1)

4000 (27.6)

2000 (13.8)

3000 (20.7)

3500 (24.1)

4000 (27.6)

3500 (24.1)

2500 (17.2) 2500 (17.2)

1000 ( 6.9)

2000 (13.8)

2500 (17.2)

3000 (20.7)

3500 (24.1)

1000 ( 6.9)

500 ( 3.4)

2000 (13.8)

1500 (10.3)

1500 (10.3)

2000 (13.8)

1500 (10.3)

Room -70 °F(216 K) Temp.(297 K) 180 °F(355 K) 250 °F(394 K) 300 °F(394 K) 500 °F(533 K)

Aliphatic amines

polysulf ides

Polyamides,

2

Approximate t e n s i l e shear strength to expect on aluminum, lb/in. (kPa x 1CT )

Bond Strengths Expected From Major Curing Agent Types

Typical Type of Curing Cure Agent Temp. °F (K)

Table I .


24. MAY


571


c u r i n g agent. I t promotes c r o s s - l i n k i n g of the epoxy r e s i n and c o r e a c t i o n w i t h the r e s o l e . The copper compound was added to increase the elevated-temperature, bond-performance l i f e of cured adhesive. The adhesive curing i s accomplished by heating the bonded assembly f o r 1 h at 177 °C (350 ° F ) . I f tougher adhesive bonds are required than are a t t a i n a b l e from conventional epoxy resins, they can be modified. A t y p i c a l example i n v o l v e s the carboxy1-terminated elastomer modifications discussed e a r l i e r . A modified base r e s i n i s f i r s t formed as f o l l o w s (pbw): Liquid d i g l y c i d y l ether of bisphenol A Carboxyl-terminated n i t r i l e rubber

100 10

The rubber and r e s i n are coreacted by h e a t i n g the mixture at around 100 °C i n the presence of a Lewis base. Most commonly t e r t i a r y amines are used f o r t h i s purpose. The morphology of the rubber dispersion i s important to bond performance and i s governed by the coreaction procedure. This material may be formulated i n t o a u s e f u l aerospace paste a d h e s i v e by the f o l l o w i n g combination of m a t e r i a l s (pbw): Rubber-modified r e s i n (above) Melamine r e s i n Fumed s i l i c a Dicyandiamide 3-(3,4-dichlorophenyl)-l,1-dimethylurea Aluminum powder

100 2 5 8 1.5 50

Melamine acts as a f l o w - c o n t r o l agent for the r e s i n . The fumed s i l i c a adds t h i x o t r o p h y to the mixture which r e t a r d s f l o w on v e r t i c a l s u r f a c e s . Dicyandiamide i n t h i s case i s a l a t e n t c u r i n g agent. Thus, the c a t a l y z e d a d h e s i v e e x h i b i t s l o n g pot l i f e on s t o r a g e at ambient temperatures. The urea d e r i v a t i v e i s an a c c e l e r a t o r f o r the cure. T h i s f o r m u l a t i o n can be cured i n the range of 110-120 °C whereas 177 °C i s the normal curing temperature for dicyandiamide-based formulations. Epoxies are not the o n l y r e s i n s used i n aerospace bonding. A common formulation i s a n i t r i l e / p h e n o l i c f i l m adhesive. A t y p i c a l formulation may consist of the f o l l o w i n g (pbw): N i t r i l e rubber Phenolic r e s i n Sulphur Zinc oxide Benzothiazyl d i s u l f i d e Carbon black

100 100 2 5 1.5 20

Here again i s a f o r m u l a t i o n compromise between the b r i t t l e , h i g h - t e m p e r a t u r e c h a r a c t e r i s t i c s of the p h e n o l i c r e s i n and the rubbery, low-temperature performance of the e l a s t o m e r . Note a l s o that the other ingredients are the type of material combination that i s normally associated with n i t r i l e elastomer v u l c a n i z a t i o n . R e p r e s e n t a t i v e of an u l t r a - h i g h - t e m p e r a t u r e a d h e s i v e i s the f o l l o w i n g polyimide-based formulation (pbw):


572


100 80 5 2


Linear condensation polyimide Aluminum powder Fumed s i l i c a Arsenic thioarsenate (AsAsS4)

Adhesives of t h i s type are d i f f i c u l t to process because of t h e i r h i g h m e l t i n g and poor f l o w c h a r a c t e r i s t i c s . They are n o r m a l l y applied from a s o l v e n t s o l u t i o n , i n t h i s case iV-methyl p y r r o l idone. The s o l v e n t must be removed during the bonding operation to obtain maximum elevated-temperature performance. A technique commonly used i s evaporation of the s o l v e n t by heating an open assembly p r i o r to the f i n a l bonding o p e r a t i o n . The f i n a l a s s e m b l y i s t h e n accomplished through conventional heat and p r e s s u r e a p p l i c a t i o n s . The a r s e n i c t h i o a r s e n a t e i s added as a s t a b i l i z e r to r e t a r d elevated-temperature aging. From t h i s b r i e f discussion i t can be seen that the formulator of aerospace adhesives has an almost i n f i n i t e arsenal of materials and combinations t h e r e o f by which u s e f u l products can be d e v i s e d . A good summary of the many types of materials a v a i l a b l e i s presented i n F i g u r e 3. As the data show, u s e f u l bond s t r e n g t h s can be obtained over a broad temperature range. Each d i f f e r e n t type of product d i s p l a y s i t s maximum s t r e n g t h i n a d i f f e r e n t temperature r e g i o n . T h i s r e f l e c t s the f a c t t h a t , i n a d d i t i o n to the i n h e r e n t s t r e n g t h c h a r a c t e r i s t i c s of a f o r m u l a t i o n , bond s t r e n g t h a l s o depends on the g l a s s t r a n s i t i o n temperature (Tg) of the basic r e s i n system. Normally, an adhesive formulation gives optimum strength i n the region of the Tg. I t i s i n t h i s region where thermal s t r a i n s on a bond l i n e are at a minimum (19). Sealants P o l y m e r i c s e a l a n t s are important m a t e r i a l s i n the aerospace industry, " i r t u a l l y every a i r c r a f t or space craft employs them i n one form or another. The most important r e s i n s used f o r t h i s purpose are the p o l y s u l f i d e s and the polydimethyl siloxanes. Of the two m a t e r i a l s , s e a l a n t s based on the p o l y s u l f i d e s are the most widely used. The f i r s t step i n the s y n t h e s i s of the l i q u i d p o l y s u l f i d e s i n v o l v e s the reaction of b i s c h l o r o e t h y l formal (Structure IV) and sodium p o l y s u l f i d e (Structure V) to form a d i - and t r i p o l y s u l f i d e mixed polymer (Structure VI). The r e s u l t i n g polymer i s then nClC H40CH 0C H4 c i + n N a S 2

2

IV

2

2

V

2#25

—>(-C H40CH 0C H4S 2

2

2

2#25

) + 2 n NaCl n

VI

reacted with a s p l i t t i n g s a l t mixture of sodium s u l f i d e and sodium acid s u l f i t e to form a lower molecular weight p o l y s u l f i d e polymer (Structure V I I ) . The concentration of the s p l i t t i n g s a l t s i s used to c o n t r o l the molecular weight of the l i q u i d p o l y s u l f i d e polymer that i s then formulated i n t o the desired sealant.


24. MAY

573

Resins for Aerospace HS 4G H4-0-CH2-O -C2H4-SS^-C2H4-0-CH2-0-C H4SH n VII


2

n

2

S e a l a n t s are used by the a i r c r a f t i n d u s t r y to form f l e x i b l e adherent b a r r i e r s that w i l l w i t h s t a n d c h e m i c a l a t t a c k . The major uses are seals around r i v e t e d j o i n t s , f i l e t i n g on overlapping bonded surfaces, and s e a l i n g f u e l tanks. They a l s o find some a p p l i c a t i o n w i t h e l e c t r i c a l components f o r p r o t e c t i o n from moisture and vibration. P o l y s u l f i d e sealants are supplied as both one-package and twopackage systems. A two-package system that uses lead dioxide as the curing agent i s the most common o v e r a l l . However, current a i r c r a f t systems are l a r g e l y one-package systems where the preferred curing agents are zinc or calcium peroxide. Because of the complexity of s e a l a n t adhesion problems, a v a r i e t y of primer f o r m u l a t i o n s have been developed for various surfaces. Most are based on c h l o r i n a t e d rubbers and s i l a n e s or admixtures t h e r e o f (20). A l t h o u g h epoxy r e s i n s a r e a l s o used as c u r i n g a g e n t s f o r t h e p o l y s u l f i d e e l a s t o m e r s , use i n a e r o s p a c e i s l i m i t e d . This p a r t i c u l a r combination i s used p r i m a r i l y by the b u i l d i n g trades. The s i l i c o n e s (mainly polydimethyl siloxanes) are considerably more e x p e n s i v e than the p o l y s u l f i d e s ; t h e r e f o r e , s i l i c o n e use i s r e s t r i c t e d to more s p e c i a l i z e d a p p l i c a t i o n s where s p e c i f i c a t t r i b u t e s of the materials are required. For example, v i r t u a l l y e v e r y U.S. m i s s i l e or s a t e l l i t e c o n t a i n s some form of s i l i c o n e sealant because of i t s long use l i f e , low v o l a t i l e content, and heat resistance. They are a l s o used around a i r c r a f t windows and doors. High-temperature f u e l tank sealants based on fluorocarbon-containing polysiloxanes have a l s o been the subject of considerable governmentsponsored research (21). Siloxane-based sealants cure either by condensation or a d d i t i o n reactions (22). One-package systems are a v a i l a b l e that cure i n the presence of atmospheric m o i s t u r e . The c r o s s - l i n k i n g r e a c t i o n r e s u l t s i n the e l i m i n a t i o n of e i t h e r a c e t i c a c i d or methyl e t h y l ketone depending on the r e s i n system. The more c o n v e n t i o n a l twopackage systems use m e t a l l i c soaps such as d i b u t y l t i n d i l a u r a t e to c a t a l y z e the c r o s s - l i n k i n g with the subsequent e l i m i n a t i o n of e t h y l alcohol. A d d i t i o n cures can a l s o be accomplished w i t h v i n y l containing polysiloxanes and a t r a n s i t i o n metal c a t a l y s t . Thermoset Processing Science and Control A recent and important aerospace contribution to the thermoset r e s i n f i e l d has evolved from efforts to develop a s c i e n t i f i c a l l y sound basis for thermoset r e s i n processing and c o n t r o l . I t r e s u l t e d from the exacting mechanical strength requirements placed on hardware by the i n d u s t r y . Over the past 10 years t h e r e has been c o n s i d e r a b l e a c t i v i t y i n the area of p r o c e s s i n g s c i e n c e . T h i s s i t u a t i o n i s p a r t i c u l a r l y t r u e i n the areas of a d h e s i v e bonding and FRP production. Because of the e f f o r t s of aerospace s c i e n t i s t s , augmented by the u n i v e r s i t i e s , s c i e n t i f i c techniques f o r s o l v i n g m a t e r i a l - and p r o c e s s - r e l a t e d problems are r a p i d l y d e v e l o p i n g . P r i o r to t h i s time bonding and l a m i n a t i o n were a l m o s t e n t i r e l y dependent on o p e r a t o r s k i l l s . Key changes made d u r i n g processes



574


were based on human decision and i n a d v e r t e n t l y l e d to f a b r i c a t i o n errors and material misuse. Fabrication of hardware based on the types of resins discussed h e r e i n i s o b v i o u s l y a c h e m i c a l p r o c e s s . To c o n t r o l a c h e m i c a l process the s t a r t i n g raw m a t e r i a l s must be p r e c i s e l y d e f i n e d , and the course of the c h e m i c a l process must be f o l l o w e d . In the aerospace industry t h i s means the s t a r t i n g r e s i n formulations must be consistent and p r e c i s e l y defined, and the chemical and p h y s i c a l changes that take place as a thermoset r e s i n cures must be monitored and c o n t r o l l e d . Studies i n the area of chemical d e f i n i t i o n of s t a r t i n g materials began more than 10 years ago (23). In a d d i t i o n to wet c h e m i s t r y , the l a b o r a t o r y measurements c u r r e n t l y used f o r t h i s purpose are 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), high-performance l i q u i d chromatography (HPLC), g e l permeation chromatography (GPC), infrared spectroscopy (IR), and rheology. Shown i n F i g u r e 4 i s a thermogram of a c a t a l y z e d TGMDA/DDS prepreg formulation. I t r e v e a l s considerable information about the m a t e r i a l and how i t c u r e s . The Tg of the prepreg i s a measure of the degree of B-staging. The presence of r e s i d u a l s o l v e n t s can be detected as shown. The onset of the cure, the i n f l u e n c e of accelerators, and the peak exotherm are other useful features of the thermogram. The area under the c u r v e i s the heat of r e a c t i o n , a processing parameter. Although the method has been c r i t i c i z e d for lack of p r e c i s i o n , common errors i n the prepreg composition and i t s processing can be observed by t h i s technique. I t i s a l s o a v a l u a b l e aid i n developing cure c y c l e s for new r e s i n systems. One of the most powerful t o o l s for q u a l i t a t i v e and q u a n t i t a t i v e examination of thermoset r e s i n f o r m u l a t i o n s i s chromatography. Shown i n Figure 5 i s an HPLC t y p i c a l of the TGMDA/DDS prepreg r e s i n system. The l o c a t i o n of a peak on the e l u t i o n p l o t i n d i c a t e s the presence of the d e s i r e d components of the f o r m u l a t i o n . The area under a g i v e n peak i s p r o p o r t i o n a l to i t s c o n c e n t r a t i o n when standardized. Note a l s o t h a t a r e a c t i o n peak appears on the chromatograph. T h i s peak can be used f o r an a c c u r a t e e s t i m a t e of the extent of B-stage d u r i n g prepreg f a b r i c a t i o n and subsequent storage. The area under the curing agent peak i s not an accurate measure of i t s c o n c e n t r a t i o n i n the s t a r t i n g f o r m u l a t i o n due to r e a c t i o n w i t h the r e s i n d u r i n g manufacture. The s t a r t i n g f o r m u l a t i o n c o n c e n t r a t i o n , however, can be c a l c u l a t e d by u s i n g the r e a c t i o n peak. With t h i s p a r t i c u l a r f o r m u l a t i o n , the s t a r t i n g ( t o t a l ) concentration of the curing agent can a l s o be measured p r e c i s e l y by IR a n a l y s i s by u s i n g a peak t h a t corresponds to the s u l f o n e group (24) . S u b t r a c t i v e a n a l y s i s w i t h F o u r i e r transform IR (FTIR) has a l s o been suggested as a future chemical characterization procedure (25) . A combination of FTIR w i t h HPLC appears to be another promising technique. Rheology i s a most recent a d d i t i o n to aerospace m a t e r i a l and process t e c h n o l o g y . I t can be used as an incoming m a t e r i a l inspection t o o l to predetermine the p r o c e s s i b i l i t y c h a r a c t e r i s t i c s of a given thermoset r e s i n system. Although l i q u i d state rheology i s c u r r e n t l y the most widely used method, s o l i d state measurements are u s e f u l i n d e t e r m i n i n g the extent of cure because they can be used to measure the T of a finished hardware item. Shown i n Figure g


24.

MAY

575


6000 (41.34) 5000 ( 34.45) 4000 (27.55)


3000 (20.67) 2000 (13.78)

1000 (6.89) Q_

Resins for Aerospace - ACS Publications - American Chemical Society

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