Chemical Reactions on Polymers - American Chemical Society

Chapter 24

Analytical Study of Photodegraded p-Aramid in an O Atmosphere 18

2

Madeline S. Toy and Roger S. Stringham

Downloaded by UNIV OF SYDNEY on September 24, 2015 | http://pubs.acs.org Publication Date: December 22, 1988 | doi: 10.1021/bk-1988-0364.ch024

Science Applications International Corporation, 5150 El Camino Real, Los Altos, CA 94022

18

The photodegradation of para-aramid in an O atmos phere allows the differentiation between the accelerated experimental photooxidative conditions from its usual daylight exposure effects. This study illustrated an estimation of the rates of photooxidation of a commer cial para-aramid product (i.e., DuPont's Kevlar-29 woven fabric) based on the oxygen-18-labelled carbon dioxide ( CO and CO ) decarboxylated from the sample. The oxygen-18-labelled atoms, which are insert ed in the macromolecules, were analyzed for the photo degradation processes. This technique also allows the radial O-distribution measurement from the fiber surface toward the fiber center. 2

46

48

2

2

18

T

!

The aramids i s a series of isomeric f u l l y aromatic polyamides, which can withstand s e r v i c e - l i f e stress at high temperature without deformation and dégradât ion ( 1^,^) . The ^ r a m i d s inherent flame resistance, high thermal and chemical s t a b i l i t y , and high modulus f u l f i l l a new source for engineering materials. Para-aramid shows one of the greatest high modulus fibers developed to date(3). The two members of the aramid family, which have achieved the commercial importance, are developed by DuPont: 1) Nomex, poly(m-phenylene isophthalamide), and 2) Kevlar, poly(p-benzamide) or poly(p-phenylene terephthalamide). Kevlar f i b e r s are supplied by DuPont as Kevlar-29 and 49. The former i s characterized by high t e n s i l e strength and the l a t t e r by high i n i t i a l modulus (_4). Some Kevlar-29 end uses are i n ropes and cables, which are as strong as steel at o n e - f i f t h the weight, and i n b a l l i s t i c vests. Some Kevlar-49 end uses are i n reinforcing resins and composites for aerospace structures, boat h u l l s , and sport equipments. The molecular structure of Kevlar i s a regular alternating pdirected benzene ring and amide group. The amide linkage shows a degree of double bond character and i s regarded as a r i g i d planar unit(5). The intermolecular hydrogen-bonding, which the amide l i n k f a c i l i t a t e s , imposes a very ordered arrangement on the polymer chains generating a laminar structure(6). The f i b e r s have a Tg (glass 1

0097-6156/88/0364-0326$06.00/0 © 1988 American Chemical Society

In Chemical Reactions on Polymers; Benham, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

24.

T O Y AND STRINGHAM

Photodegraded p-Aramid

327

t r a n s i t i o n temperature) above 300 C, and can be heated without decom position or melting to temperatures exceeding 500°C(7). Many applications of this para-aramid fabric result i n the sunlight exposure between 300 and 450 nm, where the l i g h t absorp tion of para-aramid and the solar spectrum overlap(8). In this region the sunlight deteriorates the aramids and has been reported to occur i n the absence of oxygen and i n air(9,10). The photodegraded para-aramid i n a i r generates carboxylic acid groups accom panied by a rapid loss of molecular weight and mechanical properties (10). This paper reports an a n a l y t i c a l study of the photodegraded Kelvar-29 fabric i n 0.2 atm 0 (99%). 1 8

2


Experimental Reagents. Oxygen-18-labelled gas (99%) was purchased from ICON, a gaseous mixture of 100 ppm ethane and helium from A l l t e c h , helium from Matheson, concentrated s u l f u r i c acid (Ultrex) from J . T. Baker, and chloroform and dimethylacetamide (DMAc) from A l d r i c h . The reagents were used as received unless otherwise s p e c i f i e d . Fabric Cleaning. The Kevlar-29 woven fabric was obtained through the courtesy of Naval Weapons Center. A special pair of serrated shears was purchased from Technology Associates for cutting the f a b r i c . The fabric (2.5 cm χ 18 cm) was placed i n a Soxhlet thimble and extracted by 100 ml of chloroform for 24 hours to remove i t s surface lubricants (about 3% by weight). The fabric was then removed from the thimble and agitated i n a 20 ml of hot d i s t i l l e d DMAc for 15 minutes, before i t was placed back into the thimble and extracted for another 8 hours using fresh chloroform solvent. The solvent-cleaned f a b r i c was dried i n a vacuo at room temperature. Photolysis Chamber. A high pressure ozone-free mercury-xenon arc lamp (200 watts) was purchased from UVP (San Gabriel, CA). Its irradiance was estimated at 125 times of the s u n ( l l ) . The photolysis chamber consisted of a Pyrex conical pipe outside and a quartz sleeve inside. The top and bottom aluminum end plates with the Pyrex (7.6 cm id) and the quartz (4.5 cm od) tubes enclosed an annular chamber of 21.5 cm height with gas i n l e t and outlet valves attached. The photolysis chamber s ozone free Hg-Xe arc lamp, which was v e r t i c a l l y suspended at the center of the quartz sleeve, using two Lucite high voltage connectors, passed i t s irradiance through the quartz wall onto the fabric i n a confinded 99% oxygen-18-labelled atmosphere at a pressure of 0.2 atm (the p a r t i a l pressure of 0 i n the atmosphere) and at a specified temperature and time. 1

2

Photolysis Procedure. The solvent-cleaned Kevlar-29 f a b r i c swatch (2.5 cm χ 18 cm) was placed around the outside quartz tube inside the photolysis chamber, which was subsequently evacuated, before 1^02 (99%) was introduced to 0.2 atm. The photolysis chamber was preheated to the specified photooxidation temperature, before the Hg-Xe lamp was turned on. The temperature, which was held constant i n the chamber by adjusting the a i r flow around the lamp, was moni tored by a thermocouple placed next to the fabric sample inside the chamber. After the photooxidation had continued for the specified


328

CHEMICAL REACTIONS ON POLYMERS


time p e r i o d , t h e lamp was t u r n e d o f f and t h e p h o t o l y s i s chamber was evacuated and c o o l e d t o ambient t e m p e r a t u r e . Room a i r was l e t i n and t h e p h o t o l y s i s chamber was opened t o remove t h e o x i d a t i v e l y photodegraded f a b r i c swatch. T h i s p r o c e d u r e was r e p e a t e d on d i f f e r e n t swatches a t s e v e r a l temperatures v e r s u s p h o t o l y s i s t i m e s . P r e p a r a t i o n o f S o l u b l e Sample f o r D e c a r b o x y l a t i o n . The p h o t o l y z e d f a b r i c swatch was p l a c e d i n 20 ml o f d i s t i l l e d DMAc a t 100°C f o r 5 m i n u t e s . The DMAc s o l u t i o n was f i l t e r e d t o remove any i n s o l u b l e f i b r o u s m a t e r i a l from t h e d e c a r b o x y l a t i o n f l a s k (a round 30 ml twoneck P y r e x f l a s k w i t h a p r o t r u d e d bottom w e l l ) and d r i e d i n vacuo w i t h t h e temperature kept below 50°C l e a v i n g a r e s i d u a l f i l m on one side of the f l a s k . The p u r i f i e d s u l f u r i c a c i d ( i . e . , p r e h e a t e d t h e commercial u l t r a p u r e grade H2SO4 a t 200°C f o r 24 h r under vacuo t o remove i t s d e c a r b o x y l a t i n g contaminants and s u b s e q u e n t l y c o o l e d t o ambient t e m p e r a t u r e ) , 0.5 m l , was added t o t h e bottom w e l l o f t h e decarboxy l a t i o n f l a s k and s u b s e q u e n t l y p l a c e d under vacuo. A known amount of a GC-standard (640 mm o f 100 ppm C2^6 ^ ) i °duced i n t o the f l a s k f o r t h e t o t a l CO2 d e t e r m i n a t i o n . The a c i d was then a l l o w ed t o d i s s o l v e t h e photodegraded r e s i d u a l f i l m on t h e s i d e o f t h e f l a s k by t i l t i n g . The a c i d s o l u t i o n was l e f t s t a n d i n g f o r 20 min u t e s a t room temperature. nH e

w

a

s

ntr

A n a l y t i c a l I n s t r u m e n t s f o r Carbon D i o x i d e A n a l y s e s . A gas chroma tography ( C a r l e S e r i e s 100) and a GC-mass s p e c t r o m e t e r (LKB9000) were used w i t h i d e n t i c a l GC-columns (2 mm i d and 6 m l e n g t h ) , w h i c h were purchased from A l l t e c h and packed w i t h 80% Porapak Q 80/100 and 20% Porapak Ν 80/100. The s i x - p o r t m i n i - s w i t c h i n g v a l v e was purchased from Hach and used t o t r a p t h e t o t a l carbon d i o x i d e e v o l v e d and t h e C2H5 s t a n d a r d from t h e d e c a r b o x y l a t i o n f l a s k i n t o i t s c o l l e c t i o n / i n j e c t i o n l o o p a t l i q u i d n i t r o g e n temperature. T h i s c o l d l o o p was warmed t o ambient temperature and i t s t r a p p e d c o n t e n t s were v a p o r i z e d . The c o l l e c t e d vapor m i x u t r e i n t h e l o o p was swept i n t o the GC's h e l i u m stream (2 atm p r e s s u r e ) and i n t o t h e column and then i n t o t h e t h e r m i s t o r d e t e c t o r , when t h e v a l v e was t u r n e d t o i n j e c t i o n mode. The sample was a n a l y z e d f o r t h e t o t a l CO2 c o n t e n t and t h e C2H6 s t a n d a r d . A f t e r t h e C0 had passed through t h e G ^ s d e t e c t o r , the vapor m i x t u r e was then c o l l e c t e d i n a p o r t a b l e gas c e l l , w h i c h c o n s i s t e d o f a s i x - p o r t m i n i - s w i t c h i n g v a l v e and a c o l l e c t i o n / i n j e c t i o n l o o p , again c h i l l e d t o l i q u i d n i t r o g e n temperature. The c o l l e c t e d sample a t -196°C was i s o l a t e d from t h e f l o w o f the GC's h e l i u m gas stream and then t h e l o o p was warmed t o ambient temperature f o r GC-mass s p e c t r o s c o p i c a n a l y s e s . The gas c e l l , which c o n t a i n e d t h e i s o t o p i c C0 and t h e C2H5 s t a n d a r d i n h e l i u m a t one atmosphere, was p l a c e d i n t h e i n j e c t i o n h e l i u m f l o w o f t h e GC-mass s p e c t r o m e t e r f o r t e n m i n u t e s , b e f o r e t h e m i n i - s w i t c h i n g v a l v e was t u r n e d t o i n j e c t t h e vapor c o n t e n t s i n t o t h e i n s t r u m e n t . A f t e r t h r e e m i n u t e s , t h e CO2 peak e l u t e d . The superimposed peaks were sampled t e n times d u r i n g t h e i r e l u t i o n and t h e r e l a t i v e i s o t o p i c q u a n t i t i e s o f C 0 , C 0 , C 0 and C 0 were determined. 2

2

4 5

4 6

2

4 7

2

4 8

2

2


24.

T O Y AND STRINGHAM

329

Photodegraded p-Aramid

R e s u l t s and D i s c u s s i o n The o x i d a t i o n under d a y l i g h t exposures i s a c o n t i n u o u s ongoing p r o c e s s , t h a t a l t e r s t h e p a r a - a r a m i d sample. The use o f 0 atmosphere a l l o w s t h e d i f f e r e n t i a t i o n between t h e a c c e l e r a t e d e x p e r i m e n t a l p h o t o o x i d a t i o n c o n d i t i o n s and i t s u s u a l d a y l i g h t exposure e f f e c t s . T h i s paper d e t e r m i n e s t h e r a t e s o f p h o t o o x i d a t i o n s o f Kevlar-29 f a b r i c based on t h e o x y g e n - 1 8 - l a b e l l e d c a r b o n d i o x i d e ( i . e . , ^^C0 and C 0 ) d e c a r b o x y l a t e d from t h e sample. In o t h e r words, t h e o x y g e n - 1 8 - l a b e l l e d atoms, which a r e i n s e r t e d i n the Kevlar macromolecules, a r e being analyzed to d e t e r mine t h e p h o t o d e g r a d a t i o n processes. At 25°C t h e K e v l a r macromolecule r e t a i n s i t s p o l y m e r i c s t r u c t u r e i n c o n c e n t r a t e d s u l f u r i c a c i d s o l v e n t . The r a p i d d e c a r b o x y l a t i o n observed a t 25°C i n T a b l e I , which was exposed f o r 7 min a t 100°C i n 0.2 atm "^0 i n t h e p h o t o l y s i s chamber, i s suggested t o o r i g i n a t e from t h e o x i d i z e d t e r m i n a l groups o f t h e macromolecules (10,12). I n c o n t r a s t , t h e a c i d d e c a r b o x y l a t i o n a t 196°C appears t o b r e a k down t h e K e v l a r macromolecules c o m p l e t e l y . A thermal d e g r a d a t i o n p a t t e r n has been r e c o g n i z e d f o r t h e d e c a r b o x y l a t i o n o f K e v l a r i n s u l f u r i c a c i d a t 196°C. T h i s thermal d e c o m p o s i t i o n p a t t e r n i s used as a d e c a r b o x y l a t i o n model, which c o n s t i t u t e s t h e same two types o f d e c a r b o x y l a t i o n r e a c t i o n s a t 196°C ( e . g . , R l and R2 i n T a b l e I) : one y i e l d s one mole o f C0 p e r {Cyl^NO)- i e t y and t h e o t h e r g i v e s two moles o f C0 p e r {CyH^NO} m o i e t y . The h a l f l i f e of t h e former (R2) i s 4 t o 12 h r and i s suggested t o o r i g i n a t e from t h e amide l i n k a g e s c a r b o n y l groups. The l a t t e r s h a l f l i f e (Rl) i s 660 h r and i s from t h e two o f t h e s i x carbons o f i t s a r o m a t i c rings. F i g u r e 1 shows t h e t y p e s o f d e c a r b o x y l a t i o n ( R l and R2) as t h e pseudo f i r s t o r d e r r e a c t i o n s . They a r e f i r s t o r d e r r e a c t i o n s , because t h e b i m o l e c u l a r second o r d e r p r o c e s s ( d e c a r b o x y l a t i o n o f K e v l a r i n c o n c e n t r a t e d H S0^) c o n t a i n s one o f t h e r e a c t a n t s ( c o n c e n t r a t e d H S0^) i n a g r e a t e x c e s s t o t h e o t h e r r e a c t a n t ( t h e s o l v a t ed s u r f a c e l a y e r ) . F i g u r e 1 a l s o i l l u s t r a t e s t h e changes i n c o n c e n t r a t i o n s w i t h times o f R l and R2. The R2 i n t e r c e p t i s 32 χ 10"^ moles, i . e . , t h e i n i t i a l c o n c e n t r a t i o n o f R2 a t about 11 χ 10"^ moles; whereas t h e i n i t i a l c o n c e n t r a t i o n o f R l i s about 21 χ 10"^ moles. The r a t i o o f R l t o R2 r e a c t i o n s i s about 2:1, which conforms w i t h a l l t h e d e c a r b o x y l a t i o n r e a c t i o n s ( e . g . , T a b l e I ) a t 196°C, whether t h e sample was o r was n o t exposed t o p h o t o l y s i s chamber conditions. There a r e c o n s i s t e n t l y t h r e e moles o f C0 e v o l v e d p e r •(CyH^NO)- moiety o f seven carbons e l i m i n a t i n g as one mole C0 a t t i ^ of 4 t o 12 h r (R2) and two moles C0 a t t i ^ o f 660 h r ( R l ) . T a b l e I a l s o shows t h a t t h e main component o f t h e t o t a l C0 e v o l v e d , which i s determined by GC, i s t h e ^ C 0 , and t h e t o t a l o f the o t h e r C0 i s o t o p e s ( e . g . , 46co and C 0 ) i s about 10" o f C0 . The low c o n c e n t r a t i o n s o f t h e i s o t o p i c ^ C 0 j C 0 are s t i l l , however, an e a s i l y measurable q u a n t i t y o f GC-mass s p e c t r o m e t e r . The -*C0 i s used as t h e s t a n d a r d , because t h e measurement o f ^ C 0 r e l a t e s d i r e c t l y t o the q u a n t i t y o f t h e abundant C 0 . I t s low c o n c e n t r a t i o n l e v e l s h e l p t o improve t h e measuring a c c u r a c y o f t h e o t h e r C0 i s o t o p e s , v'hich a r e a l s o i n low c o n c e n t r a t i o n . The moles 2

4 8


2

2

2

2

m o

2

1

1

2

2

2

2

2

2

2

2

4 8

2

4

2

2

2

4 8

2

a

n

(

2

4

4

2

2

4 4

2

2



330


Table 1, Decarboxylation Data of Photodegraded Kevlar

C0

2

Isotope

Decarboxylation Moles of Total CO Evolved 31.6 χ 10 6

ça C0

4 6

4 8

C0 C0

2

2

1.9 χ 10"

2

7.0 χ 1 0 "

9

1 1

Rate of C0 Evolution/ t

3

at

Moles of Total CO Evolved

Rate of C0 Evolution/ t

* 6.98 χ 10"^ 3.49 χ 10"'

Two types of reactions : (Rl) 660 hr (R2) 4 to 12 hr

1 to 10 min

4.0 χ 10"

42 h r

1 to 10 min

4.3 χ 10"

?

?

L

J / 2

J / ?

1 to 10 mi η

7

9

d

8 min

The total carbon available for decarboxylation from 0.415 g Kevlar fabric sample [expressed as ·Λ-0 ΗςΝ0-4- moieties in moles, i . e . , 3.49 χ 10"" moles] « 3 χ 3.49 χ 10 « 1.05 χ 10 moles, where 3 designates three carbon atoms from a — r - C H - M H — m o i e t y of 7 carbons eliminating as 7

5

7

b 2 Includes extrapolated data. There are consistently two types of decarboxylation reactions that occur at 196 C (Rl and R2) of a given Kevlar sample [expressed as «4-C HcN0-*moieties in moles], whether i t was exposed or not exposed to photolysis: one y i e l d s one mole of C0 per -4-C H N0-4-> moiety and the other gives two moles of C0 per -M^JfgNO-r- moiety ( e . g . , F i g . 1 ) . Two rates (a f a s t and a slow) may be involved. C 0

#

7

2

d

7

5

2



10

-5

10

i

R 2

40

120

J160

200

_1_

Decarboxylation Time, hr (O Rl and R2)

00

6

6

= 11 χ 10~ moles]

240

6

280

1 Intercept [the I n i t i a l concentration = 21 χ 10~ moles]

intercept [the i n i t i a l concentration = (32 - 21 ) χ 10"

Figure 1. A representative plot o f j ^ C C ^ concentration versus decarboxylation time for the two types of pseudo f i r s t order decarboxylation reactions at 196°C. The r a t i o R2 intercept over R l intercept i s 1:2.

[ ^-

—r

10

Decarboxylation Time, hr (• R2)


320

CO CO

3

ι

Ο-

•η

Α.

5 ο ο

χ > 2

Ο

H

Η Ο

332


of the t o t a l CO2 from the GC determination i s used for the tion of the moles of -^C02 as follows:

calcula

4

45 Moles of the t o t a l C0

2

χ 0.01108 = moles of

C0

2

where 0.01108 i s the fraction of -^C i n carbon at mass 12. The presence of ^ C 0 and C02 are measured from their respective GC/ms peak areas compared to that of The moles of are then corrected by subtracting the ^ C 0 and CU2 occurring naturally i n the CO2 sample. Some of the possible photooxidized s i t e s of the Kevlar macro molecules before decarboxylation are i l l u s t r a t e d below (Equations 1 and 2): 4

48

2

4

48


2

These 02~inserted macromolecules can be differentiated from the ongoing -^c^-oxidized macromolecules by GC-mass spectrometer. When a sample of the same Kevlar-29 fabric was i d e n t i c a l l y treated i n ^ 02-atmosphere except i n the absence of photolysis (in the dark) for 24 hours, the results of C0 evolved at 25° and 196°C from the GC-analysis were the same as the samples under photolysis (e.g., Table I ) . However, the gross difference was i n the analyses of the GC-mass spectroscopic data, where no oxygen-18-labelled C0 isotopes evolved ( i . e . , there was no ^ C 0 or ^ c o above the natural background l e v e l s ) . Figures 2 and 3 i l l u s t r a t e the pseudo f i r s t order decarboxyla tion reactions of C 0 (the slow reaction with t ^ = 42 hr) and C 0 (the fast reaction with ti^ = 8 min) . The photodegraded sample was exposed for 7 minutes at 100°C i n 0.2 atm ^ ^ 2 . The concentration of -^O-Kevlar i s expressed as -fCyH^NO)- i n moles and i s equal to the t o t a l ^ C 0 minus evolved ^C02 i n moles for Figure 2 and the t o t a l C 0 minus evolved C 0 i n moles for Figure 3. Figure 4 shows the r a d i a l d i s t r i b u t i o n from the f i b e r surface toward the f i b e r center (0.00065 cm). The area under the curves are the t o t a l mole ratios C0 /-(C H NO> at 1.15 χ 10~" for curve a and C0 /{C H NO> at 1.23 χ 10" for curve b. Most of the C0 (95%) was evolved during the f i r s t 30 minutes. Successive surface layers, which were then dissolved separately i n d i s t i l l e d DMAc solvent and decarboxylated i n concentrated s u l f u r i c acid, where each was analyzed for ^C02 and C02. Although the most severely photodegraded part of the f i b e r occurred at the l i g h t exposed sur face, i t appeared that the ^ ^ - d i s t r i b u t i o n s r e throughout the f i b e r . Table II summarizes the decarboxylation data of Kevlar and i t s r a d i a l d i s t r i b u t i o n of oxygen as a constituent i n C02» ^C02 and C O . The data were obtained from 0.415g Kevlar-29 f a b r i c , 8

2

2

4

4

2

2

4 6

4 8

2

2

4

4

2

4 8

4 8

2

2

46

4

2

48

7

5

5

2

7

5

4 8

2

4

48

w e

44

4

48

?




CO CO CO

Ό

ft.

Λ.

"1

c*

ο « "•» ο a.

>

Ο

Η

>



334




en

CO

to

&

-t

îL

Ό

C X .

*t

a.

Ο

ο

>

ο

ΜΗ

Η »

>

H Ο *

Chemical Reactions on Polymers - American Chemical Society

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