Chapter 18
Polymers with Improved Flammability Characteristics Downloaded via IOWA STATE UNIV on October 1, 2018 at 00:25:14 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
W. T. Whang and Ε. M. Pearce Polymer Research Institute, Polytechnic University, Brooklyn, NY 11201
The flame resistance of polymeric materials was enhanced with the modification of chemical structure and the incorporation of additives. The polymers with more fused heterocyclic structures showed higher thermal stability and more char yield, thus - poly benzoxazole > poly(2,4-difluoro-1,5-phenylene t r i mellitic amide-imide) > poly(2,4-difluoro-1,5-phenyl ene isophthalamide). The poly(amide-imide) showed good solubility in Ν,Ν-dimethyl acetamide and Ν,N -dimethyl formamide with better processability than the polyamide. Among the investigated additives zinc chloride was the best additive to improve the flame resistance of nonsubstituted poly(1,3-phenylene iso phthalmide) (PMI). The material system increase 40% of the char yield and 5 units of the oxygen index when compared with a pure PMI.
U s u a l l y t h e thermal s t a b i l i t y o f çolymeric m a t e r i a l s i s r e l a t e d t o t h e c h e m i c a l bond s t r e n g t h . ^ T h e r m a l s t a b i l i t y c a n be enhanced w i t h s t r o n g e r bond s t r e n g t h which can be a c h i e v e d u s i n g resonance s t a b i l i z a t i o n i n condensed r i n g s t r u c t u r e s . Although many p u b l i c a t i o n s have d i s c u s s e d s t r u c t u r e r e l a t i o n s h i p s and t h e r m a l s t a b i l i t y , f e w o f them have p r e s e n t e d s y s t e m a t i c correlations. We were i n t e r e s t e d i n f u r t h e r c o r r e l a t i n g s t r u c t u r e , t h e r m a l s t a b i l i t y , and flame r e s i s t a n c e o f a s e r i e s polymers, which e x h i b i t e d a s i m i l a r i t y i n t h e mechanism o f thermal degradation. When p o l y ( 2 , 4 - d i f l u o r o - 1 , 5 - p h e n y l e n e t r i m e l l i t i c araide-imide) and p o l y ( 2 , 4 - d i f l u o r o - 1 , 5 - i s o p h t h a l a m i d e ) were p y r o l y z e d a t 400 - 500°C, i t was f o u n d t h a t benzoxazole groups were generated on t h e backbone o f these two polymers.^3 I t i s a l s o v e r y i n t e r e s t i n g t h a t the onset o f t h e second-step d e c o m p o s i t i o n o f these two polymers was c l o s e t o t h a t of p o l y b e n z o x a z o l e . 0097-6156/90Λ)425-0266$06.00Λ) © 1990 American Chemical Society
Nelson; Fire and Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
18.
WHANG & PEARCE
Polymers with Improved Flammability Characteristics 267
Experimental Monomer P r e p a r a t i o n and S o l v e n t P r e p a r a t i o n 1,3-phenylene diamine 1.3- p h e n y l e n e d i a m i n e was p u r i f i e d by vacuum e s s e n t i a l l y white s o l i d s . (m.p 62.5-63°C). 2.4- d i f l u o r o - 1 , 5 - p h e n y l e n e
sublimation to
diamine
T h i s diamine was prepared from 2 , 4 - d i n i t r o - l , 5 - d i f l u o r o benzene by a c a t a l y t i c r e d u c t i o n over SnCl^/HCl a t 50-60oC f o r 3 h o u r s . ^ The diamine was r e c r y s t a l l i z e d from benzene t o y i e l d a w h i t e product (mp. 112.5-113 C). U
2.4-diamino-l,5-benzenediol d H y d r o c h l o r i d e ^ »
1
m-Phenylene d i a c e t a t e was d i s s o l v e d s l o w l y i n 4 t o 5 t i m e s i t s volume of fuming n i t r i c a c i d w h i l e c o o l i n g w i t h an i c e water b a t h f o r 2 or 3 m i n u t e s . The s o l u t i o n was poured onto c r a c k e d i c e , filtered I m m e d i a t e l y , then washed w i t h b o i l i n g a l c o h o l . The y e l l o w s o l i d was s a p o n i f i e d by b o i l i n g i t i n 30% h y d r o c h l o r i c a c i d s o l u t i o n f o r 30 m i n u t e s . R e c r y s t a l l i z a t i o n from water gave y e l l o w n e e d l e s of 2 , 4 - d i n i t r o - l , 5 - b e n z e n e d i o l w i t h mp. 212-213°C. 2 , 4 - D i n i t r o - 1 , 5 - b e n z e n e d i o l was t h e n r e d u c e d w i t h s t a n n o u s c h l o r i d e i n c o n c e n t r a t e d h y d r o c h l o r i c a c i d a t 50-60°C. At the b e g i n n i n g , the s o l u t i o n was c l e a r , t h e n some w h i t e c r y s t a l s p r e c i p i t a t e d form the r e a c t i o n s o l u t i o n . A f t e r c o o l i n g , the r e s u l t a n t p r e c i p i t a t e was c o l l e c t e d by f i l t r a t i o n and washed w i t h c o l d water to g i v e 2 , 4 - d i a o : i n o - l , 5 - b e n z e n e d i o l d i h y d r o c h l o r i d e - n o mp. b e f o r e i t decomposed above 2uC°C. Acid Chlorides I s o p h t h a l o y l c h l o r i d e was r e c r y s t a l l i z e d from n-hexane, which was d i s t i l l e d over sodium w i r e . T r i m e l l i t i c a n h y d r i d e a c i d c h l o r i d e was p u r i f i e d by reduced pressure d i s t i l l a t i o n . N,N-Dimethylacetamide C o m m e r c i a l l y a v a i l a b l e Ν, N-dimethylacetamide was p u r i f i e d by vacuum d i s t i l l a t i o n over c a l c i u m h y d r i d e to c l e a r , colorless l i q u i d s and then kept dry i n a d e s i c c a t o r . P r e p a r a t i o n of P o l y ( p h o s p h o r i c a c i d ) P o l y ( p h o s p h o r i c a c i d ) was prepared by adding a 1.52/1 w e i g h t r a t i o of phosphorus p e n t o x i d e to 85% p h o s p h o r i c a c i d i n i c e b a t h and t h e n h e a t i n g a t 150°C f o r 6 h o u r s , w i t h s t i r r i n g under n i t r o g e n atmosphere.
Nelson; Fire and Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
268
FIRE A N D POLYMERS
Polymerization P o l y ( 2 , 4 - d i f l u o r o - 1 . 5-phenylene phenylene i s o p h t h a l a m i d e
i s o p h t h a l a m i d e ) and
poly(1.3-
The diamine was d i s s o l v e d i n Ν,N-dimethyl-acetamide by s t i r r i n g under a n i t r o g e n stream. A f t e r t h e d i a m i n e was c o m p l e t e l y d i s s o l v e d , e q u a l moles of i s o p h t h a l o y l c h l o r i d e was added a l l a t once. S t i r r i n g was c o n t i n u e d f o r about t h r e e hours a f t e r which the r e a c t i o n m i x t u r e was poured i n t o hot water and m a g n e t i c a l l y stirred. The polymer was washed w i t h hot water a t l e a s t t h r e e times and then e x t r a c t e d w i t h acetone t o remove low m o l e c u l a r weight s p e c i e s . The polymer was d r i e d i n vacuum oven a t about 70°C o v e r n i g h t . Poly(2,4-difluoro-1,5-phenylene t r i m e l l i t i c
amide-imide
P o l y ( 2 , 4 - d i f l u o r o - 1 , 5 , p h e n y l e n e t r i m e l l i t i c a m i d e - i m i d e ) was p r e p a r e d by a two-step procedure · *2 At the f i r s t s t e p , the polyamic a c i d was prepared by r e a c t i n g 2 , 4 - d i f l u o r o - 1 , 5 - p h e n y l e n e diamine w i t h t r i m e l l i t i c a n h y d r i d e a c i d c h l o r i d e ( w i t h the mole r a t i o of one t o one) i n anhydrous Ν, N-dime thy l a c e t amide a t room t e m p e r a t u r e under n i t r o g e n . A f t e r r e a c t i o n , the polymer was poured i n t o water and p r e c i p i t a t e d . A f t e r f i l t r a t i o n , the w h i t e s o l i d was washed w i t h d i s t i l l e d water and d r i e d i n a vacuum oven. The p o l y ( a m i d e - i m i d e ) was o b t a i n e d from h e a t i n g the polyamic a c i d at 220°C f o r 3 h o u r s . The polyamic a c i d was d i s s o l v e d i n N,Nd i m e t h y l acetamide o r Ν,Ν-dimethyl f o r m a m i d e , c a s t on g l a s s p l a t e s , and the s o l v e n t evaporated i n a vacuum oven t o form a polyamic a c i d f i l m b e f o r e h e a t i n g a t 220°C. P o l y [ b e n z o ( 1 , 2 - d : 5,4-d']-l,3-phenylene Equimolar q u a n t i t i e s of 2 , 4 - d i a m i n o - l , 5 - b e n z e n e d i o l d i h y d r o c h l o r i d e and i s o p h t h a l i c a c i d were mixed i n f r e s h p o l y ( p h o s p h o r i c a c i d ) u s i n g a h i g h - s h e a r s t i r r e r under a slow stream of n i t r o g e n gas. The system was heated a t 40°C f o r 6 h o u r s , a t 60°C f o r 18 h o u r s , a t 12D°C f o r 6 h o u r s , a t 160°C f o r 8 h o u r s , and a t 220°C f o r 24 hours. The r e s u l t a n t m i x t u r e was d a r k brown. The polymer was p r e c i p i t a t e d from w a t e r . A f t e r f i l t r a t i o n and w a s h i n g w i t h w a t e r and methanol, the s o l i d product was then d i s s o l v e d i n m e t h a n e - s u l f o n i c a c i d , f i l t e r e d and p r e c i p i t a t e d by the a d d i t i o n of methanol. The s o l i d was washed w i t h c o n c e n t r a t e d ammonium h y d r o x i d e , w a t e r , methanol, methanol/benzene m i x t u r e s ( w i t h a volume r a t i o of 1/1), and f i n a l l y benzene. The f i n a l product was dark brown. Characterization The polymers were c h a r a c t e r i z e d by IR s p e c t r a i n KBr p e l l e t s . A D i g i l a b FTS-IMX I n f r a r e d Spectrometer was used f o r t h i s purpose. The oxygen i n d i c e s of the polymers were measured by u s i n g t h e G e n e r a l E l e c t r i c oxygen index equipment. The oxygen i n d e x (01)
Nelson; Fire and Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
18.
WHANG & PEARCE
Polymers with improved Flammability Characteristics 269
i s d e f i n e d as che minimum c o n c e n t r a t i o n of oxygen i n an oxygenn i t r o g e n atmosphere t h a t i s n e c e s s a r y t o support a flame. volume of O2 01 =
x 100 volume of 0^
+
volutie of N2
A DuPont 910 d i f f e r e n t i a l s c a n n i n g c a l o r i m e t e r (DSC) and a DuPont 951 t h e r m o g r a v i m e t r i c a n a l y z e r (TGA) connected t o a DuPont 1090 thermal a n a l y z e r tfftre used t o study the t r a n s i t i o n d a t a , t h e r m a l s t a b i l i t y , and char y i e l d , r e s p e c t i v e l y , f o r a l l the polymers. The DSC was run under a n i t r o g e n stream a t a f l o w r a t e of 80 c.c./min. and a t a h e a t i n g r a t e of 20°C/min.. R e s u l t s and D i s c u s s i o n s The c h e m i c a l s t r u c t u r e s of these polymers were c h a r a c t e r i z e d u s i n g FT-IR. Poly(1,3-phenylene i s o p h t h a l a m i d e ) (PMI) and p o l y ( 2 , 4 - d i f l u o r o - l , 5 - p h e n y l e n e i s o p h t h a l a m i d e ) (2,4-DIF-PMI) showed N-H s t r e t c h i n g bands a t 3400-3200 cm" and C==0 s t r e t c h i n g bands(amide I ) a t 1630-1650 cm" . P o l y ( 2 , 4 - d i f l u o r o - 1 , 3 - p h e n y l ene t r i m e l l i t i c amide-imide) (2,4-DIF-PMTAI) showed a d d i t i o n a l bands a t 1740 and 1796 cm" c o r r e s p o n d i n g t o imide C==0 s t r e t c h i n g band a t 1625 cm" and C-O-C s t r e t c h i n g bands a t 1255 and 1050 cm" . As shown i n F i g u r e 1, the onset of d e c o m p o s i t i o n temperature was i n t h i s o r d e r : p o l y b e n z o x a z o l e > 2,4-DIF-PMTAI> 2,4-DIF-PMI. The char y i e l d of these polymers f o l l o w e d the same sequence. I t showed t h a t the i n t r o d u c t i o n of more f u s e d h e t e r o c y c l i c r i n g s i n t o the backbone of the polymers enhanced the t h e r m a l s t a b i l i t y and flame r e s i s t a n c e of p o l y m e r i c m a t e r i a l s . I t was i n t e r e s t i n g t h a t 2,4-DIF-PMTAI and 2,4-DIF-PMI showed a two-step d e c o m p o s i t i o n . The onset of d e c o m p o s i t i o n a t t h e second s t e p of these two polymers were c l o s e t o each o t h e r . More i n t e r e s t i n g , they were a l s o c l o s e t o the onset of d e c o m p o s i t i o n of p o l y b e n z o x a z o l e . We have c o n f i r m e d t h a t the d e c o m p o s i t i o n of t h e s e two polymers a t 400-550°C r e s u l t e d i n the f o r m a t i o n of benzoxazole u n i t s on the b a c k b o n e . ^ 1
1
1
1
1
1
The zole the high
h i g h t h e r m a l s t a b i l i t y and flame r e s i s t a n c e of p o l y b e n z o x a were due t o the resonance s t a b i l i z a t i o n of the a r o m a t i c and h e t e r o c y c l i c s t r u c t u r e s , were c o n t r i b u t e d s u b s t a n t i a l l y t o bond s t r e n g t h . The s o l u b i l i t i e s o f 2,4-DIF-PMI and 2,4-DIF-PMTAI were f a i r l y good i n Ν,Ν-dimethyl acetamide and Ν,Ν-dimethyl f o r a m i d e . The p o l y b e n z o x a z o l e c o u l d not be d i s s o l v e d i n any o r g a n i c s o l v e n t and c o n c e n t r a t e d s u l f u r i c a c i d c o u l d d i s s o l v e i t . Four Lewis a c i d s and a r a d i c a l t r a p p e r were used as a d d i t i v e s i n PMI i n o r d e r t o a l t e r n a t e the d e g r a d a t i o n mechanisms t o
Nelson; Fire and Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
270
FIRE A N D
POLYMERS
F i g u r e 1. TGA of ( a ) p o l y b e n z o x a z o l e (— — . — . ) , ( b ) 2,4-DIFPMTAI ( ) , and ( c ) 2,4-DIF-PMI( ) at a heating r a t e of 20 C/min under a n i t r o g e n stream a t 200 c.c./min. u
Nelson; Fire and Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
18.
Polymers with improved Flammabitity Characteristics 271
WHANG & PEARCE
f a v o r the t r a n s f o r m a t i o n of the polymer to c h a r . The d a t a on the t h e r m a l s t a b i l i t y and flame r e s i s t a n c e of PMI w i t h and w i t h o u t 10% a d d i t i v e s were shown i n Table 1. The dynamic TGA thermograms of PMI w i t h and w i t h o u t a d d i t i v e s were shown i n F i g u r e 2.
T a b l e 1. E f f e c t of A d d i t i v e s i n PMI on Thermal S t a b i l i t y and F l a m m a b i l i t y Residue of the pure A d d i t i v e s IDT* a d d i t i v e a t 10% 900°C, (%) (°C) _ Clay Charcoal A1 0 ZnCl NiC12 2
3
2
* IDT * The
440 440 440 439 416 422
_ 100 100 100 5 16
Char Y i e l d % 900°C a f t e r Oxygen 700° 800° 900° C o r r e c t i o n * * Index 51.1 61.2 60.5 62.9 63.4 52.6
45.2 57.3 55.1 59.9 59.2 48.2
47.5 58.3 57.1 60.0 60.8 50.0
45.5 52.6 50.1 54.4 65.2 52.4
83 41 40 39 43 39
I n i t i a l d e c o m p o s i t i o n temperature. char y i e l d a f t e r c o r r e c t i o n f o r the i n o r g a n i c a d d i t i v e .
The t h e r m a l s t a b i l i t y of PMI w i t h a d d i t i v e s i s not changed by the i n t r o d u c t i o n of c l a y , c h a r c o a l , AI2O3. These a d d i t i v e s were q u i t e s t a b l e over the temperature range under s t u d y (up 900°C). The lower t h e r m a l s t a b i l i t y observed f o r the c a s e s of Z n C l and NiC12 as a d d i t i v e s may have r e s u l t e d from a change i n the d e g r a d a t i o n mechanism. The c h a r y i e l d of 900°C, a f t e r c o r r e c t i n g f o r the a d d i t i v e s to g i v e a c o r r e c t e d char y i e l d , was c a l c u l a t e d on the b a s i s of the f o l l o w i n g e q u a t i o n : 2
Char Y i e l d of PMI with Additives
Char Y i e l d C o n t r i b u t e d from A d d i t i v e s
Char Y i e l d = ( a f t e r c o r r e c t i o n ) Weight F r a c t i o n of PMI
before
Degradation
The char y i e l d c o n t r i b u t e d by the a d d i t i v e s a l o n e was evaluated by measuring the r e s i d u e o b t a i n e d from the pure a d d i t i v e s by TGA. Based on the char y i e l d a f t e r c o r r e c t i o n , we found t h a t a l l the a d d i t i v e s improved the char y i e l d of PMI. ZnCl was 40% h i g h e r than t h a t of pure PMI. The Z n C l p r o b a b l y c a t a l y z e s the r e a c t i o n of cyano groups w i t h i t s e l f or w i t h a r o m a t i c amide r i n g s , but we have not e x p e r i m e n t a l l y s t u d i e d t h i s . The o t h e r f o u r a d d i t i v e s gave 5-9% enhancement o f c h a r y i e l d s . C l a y , N i C l but were l e s s e f f i c i e n t . The oxygen i n d i c e s of PMI w i t h these a d d i t i v e s were h i g h e r than t h a t of pure PMI, but d i f f e r e n c e s were not s i g n i f i c a n t . 2
2
2
Nelson; Fire and Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
272
FIRE A N D POLYMERS
Nelson; Fire and Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
18.
WHANG & PEARCE
Polymers with Improved Flammability Characteristics 273
Acknowledgment The a u t h o r s w i s h t o e x p r e s s t h e i r t h a n k s t o t h e U.S. Army Research O f f i c e , Research T r i a n g l e P a r k , N.C. f o r s p o n s o r i n g t h i s work under the g r a n t No. DAAG-29-79-C-0124. Literature Cited 1.
P. Ε. Cassidy, "Thermally Stable Polymers", Marcel Decker, N.Y. (1984.
2.
W. T. Whang, M. Kapuscinska, and Ε. M. Pearce, J . Polym. S c i . , Polym. Sym., 74, 109 (1986).
3.
A. C. Karydas, W. T. Whang, and Ε. M. Pearce, J . Polym. S c i . , Polym. Chem. Ed., 22, 847 (1984).
4.
Y. P. Krasnov, V. I. Logunova, and S. B. Sokolov, Polym. S c i . , USSR, 8, 2176 (1966).
5.
G.F.L. Ehlers, K. P. Fisch, S c i . , A-1, 8, 3511 (1970).
6.
H. L. Friedman, M. Goldstein, and G. A. G r i f f i t h , Anal., Proc. Int. Conf., 2nd, 1, 405 (1968).
7.
D. A. Chatfield, I. N. Einhorn, R. W. Mickelson, and J . H. F u t r e l l , J . Polym. S c i . , Chem. Ed., 17, 1353 (1979).
8.
D. A. Chatfield, I. N. Einhorn, R. W. Mickelson, and J . H. F u t r e l l , J . Polym. S c i . , Chem. Ed., 17, 1367 (1979).
9.
Β. H. Nicolet (1927).
and W. R. Powell, J . Polym.
Thermal
and W. J . Ray, J . Am. Chem. Soc., 49, 1801
10.
P. G. W. Typke, Ber. Dtsch. Chem. Ges., 16, 552 (1883).
11.
J . F. Wolfe (1981).
12.
P. F. F r i g e r i o , L. H. Tagle, and F. R. Diaz, Polymer, 22, 1571 (1981).
and F. E. A r n o l d , Macromolecules,
14, 909
RECEIVED November 20, 1989
Nelson; Fire and Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
