degradation mechanism of pu mdi based

7/26/2019 Degradation Mechanism of PU Mdi Based

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Prog. Polym. Sci. Vo l . 15 , 735-7 50 , 1990 007 9-67 00/9 0 $0 .00 + .50

Printed in Gre at B rita in . A l l r ights reserved. 1 99 0 P e r g a m o n P r es s p ie

A D V A N C E S I N P H O T O D E G R A D A T I O N A N D

S T A B I L I Z A T I O N O F P O L Y U R E T H A N E S

B. P. THAPLIYAL an d R. CI~NDRA *

D epa r tmen t o f Ch em i s tr y D el h i Co l l e ge o f En g i neer i n g D el h i 110006 I nd i a

C O N T E N T S

I.

I n t r o d u c t i o n

2 . M e c h a n i s m o f p h o t o d e g r a d a t i o n

3 . E f fe c t o f p h y s ic a l s t a t e o n p h o t o d e g r a d a t io n

4 . P h o t o s t a b i l i z a t io n o f p o l y u r e t h a n e s

5.

C o n c l u s i o n

R e f e r e n c e s

735

735

740

743

748

748

I . I N T R O D U C T I O N

A l l s y n t h e t i c a n d n a t u r a l o r g a n i c p o l y m e r s a r e p r o n e t o p h o t o d e g r a d a t i o n .

H o w e v e r t h e ra te o f d e t e r io r a t i o n v a r ie s e n o r m o u s l y w i t h t h e p o l y m e r s tr u c -

t ur e. A l t h o u g h t h e p h o t o d e g r a d a t i o n a n d s t ab i li za t io n o f p o l y u r e t h a n e s h a s

b e e n s t u d i e d s i n c e t h e ir c o m m e r c i a l i n t r o d u c t i o n i t i s o n l y r e c en t l y t h a t t h e k e y

p h o t o d e g r a d a t i v e s te p s h a v e b e e n c o n fi rm e d a n d s o m e p r o g r es s h a s b e en m a d e

i n th e s y s t e m a t i c u n d e r s t a n d i n g o f th e m o d e s o f a c ti o n o f e f fe c ti v e co m m e r c i a l

s t a b i l i z a t i o n . T h e s y s t e m a t i c d e v e l o p m e n t o f U V s t a b i l i z e r s f o r a p o l y m e r i c

s y s t e m r e q u i re s a d e t ai le d k n o w l e d g e o f th e n a t u r e a n d i m p o r t a n c e o f p h o t o -

i n it ia t i o n r e a c t i o n s a n d t h e r e a c t io n o f k e y i n te r m e d i a t e s i n t h e p h o t o d e g r a -

d a t i o n . I n t h e p r e s e n t r e v i e w w e w i l l c r i t i c a l l y d i s c u s s p h o t o d e g r a d a t i o n

m e c h a n i s m s a n d t h e ef fe c t o f ch e m i c a l a n d p h y s i c a l s tr u c t u r e o n d e g r a d a t i o n

b e f o r e w e c o n s i d e r p o t e n t i a l p h o t o s t a b i l i z a t i o n m e c h a n i s m s .

2 . M E C H A N I S M O F P H O T O D E G R A D A T I O N

I n it ia l s t u d i e s ~-3 o n t h e p h o t o d e g r a d a t i o n o f p o l y u r e t h a n e s w e r e m a d e in t h e

e a r ly 1 9 60 s b u t a d e f in i te m e c h a n i s m c o u l d n o t b e e s ta b l i sh e d d u e t o t h e

d i ff ic u lt ie s i n i d e n t if ic a t io n o f t h e p r i n c i p a l p h o t o d e g r a d a t i o n p r o d u c t s o f t h e

p o l y m e r i c s y s t e m s . M o s t o f th e s tu d i e s w e r e c e n t er e d a r o u n d t h e c o m m e r c i a ll y

a v a i l a b l e p o l y u r e t h a n e s d e r i v e d f r o m a r o m a t i c d i i s o c y a n a t e s s u c h a s t o l u e n e

T o w h o m a l l c o r r e s p o n d e n c e s h o u l d b e a d d r e s s e d .

735


2/16

736 B.P. THAPLIYAL and R. CHAND RA

d i i s o c y a n a t e ( T D I ) a n d d i p h e n y l m e t h a n e d i i s o c y a n a t e ( M D I ) w h i c h t u r n e d

y e l lo w o n s t o ra g e o r i n a c t u a l u s ag e . A l i p h a t ic i s o c y a n a t e - b a s e d p o l y u r e t h a n e s

s how rem arkab le pho to res i s tanc e bu t the i r pho tode g rad a t ion s tud ies a re l imi ted.

S o m e i n v e st ig a t o rs , 4~n s t u d y i n g t h e p h o t o d e g r a d a t i o n o f a r o m a t i c p o l y -

u r e t h a n e s, p o s t u l a t e d t h e f o r m a t i o n o f t h e fo l lo w i n g m o n o - a n d d i q u i n o n e

imide p roduc ts .

. . . . . O 2 C ~ N H = = - - ~ - - C H 2 - = = = = = ~ N H ~ C O 2. . ..

t

. . . . . O a C ~ N ~ : : : ~ C H ~ N H ~ C O 2 .. . . .

t

. . . . . O 2 C ~ N @ C @ N ~ C O 2 . . . .

L o s s o f s o m e4-n us e fu l mec han ic a l p roper t i e s on expos ure to s un l igh t was

f o u n d t o r e su l t f r o m t h e U V i n it ia t e d a u t o - o x i d a t i o n o f th e u r e t h a n e l in k a g e t o

y ie ld qu inon e- im ide typ e st ructu res. P o lyure thanes f rom M D I w ere m os t a ffec ted

by l igh t. P o lym ers bas e d on 4 -4 ' -me thy lene b i s (cyc lohexy l i s oc yana te ) had

go od s pec t r a l and c o lour s tab i l i ty and exc e l l en t s t r e s s - s t r a in p rop er ty r e ten t ion .

T h e U V a n d v is ib l e a b s o r p t i o n s p e c t r a o f p o l y m e r i c fi lm s a f te r p h o t o i r r a d i -

a t ion s how ed a p rog res s ive s h i f t tow ard the v i s ib le r eg ion , r e s u lt ing in a ye l low

fi lm. Osawa

eta/

3 s u g ge s te d t h a t p h o t o d e g r a d a t i o n o f p o l y u r e t h a n e s p r o c e e d e d

v ia exc ited t r ip l e t s t a te s , fo rm ing an in t r ap o lym er exc im er in the in i ti a l s t age and

e x ci p le x e s b e t w e e n t h e p o l y m e r a n d t h e d e g r a d a t i o n p r o d u c t s o r i n t e rm e d i a t e s

a f te r i rr ad ia t ion . P o ly ure tha nes we re exc i ted by i r r ad ia t ion a t 290 and 346 nm

and emi t ted l igh t a t 310 and 420nm.

Al len a nd M c Ke l le r 14 r epo r ted an e l ec t ron ic s pec t ros c o py an d l a se r f l a sh

p h o t o l y s i s s t u d y o n M D I a n d p o l y e s t e r d i o l - b a s e d p o l y u r e t h a n e e l a st o m e r s .

F l u o r e s c e n c e a n d p h o s p h o r e s c e n c e e m i s s i o n s w e r e o b s e r v e d . A l t h o u g h t h e

s ourc e o f f luo res c enc e c ou ld no t be iden ti f ied , the phos p hores c e nc e w as r epo r ted

t o o r i g i n a t e f r o m a b e n z o p h e n o n e - t y p e o x i d a t i o n i m p u r i t y . I t w a s f o u n d t h a t

a f te r a bo u t 25 h r o f i r r ad ia t ion , v i r tua l ly a l l the f luo res c enc e emis s ion h ad

d is app ea red whi l e re l a t ive ly l it tl e c han ge ha d oc c ur red in the phos pho res c enc e .

W i t h l o n g e r p e r i o d s o f ir r a d ia t i o n , a g r a d u a l d e c r e a se i n th e p h o s p h o r e s c e n c e

in tens ity was ob s e rved , bu t th i s c hange be c am e d if f ic u l t to m on i to r s a t i s fac to r i ly

a s t h e y e l lo w - b r o w n d i s c o l o r a t io n o f t h e film d e v e l o p e d . N o p h o s p h o r e s c e n c e

was o bs e rv ed fo r the f ilm i r r ad ia ted fo r 400 h r .

Beac he ll and Ch ang 15 inves t iga ted the s ys tema t ic pho tode g rad a t ion o f

u r e th a n e m o d e l c o m p o u n d s . T h e p h o t o o x i d a t i o n p r o d u c t s o f et hy l N - p h e ny l


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PHOTOD EGRADA TION AND STABILIZATION OF POLYURETHANES 737

c a r b a m a t e w e r e i d en t if ie d , T h e p h o t o d e g r a d a t i o n o f t h e p o l y u r e t h a n e b a s e d o n

T D I a n d e t h y l e n e g l y c o l , c o n t a i n i n g a h y d r o g e n a t o m ~ t o t h e c a r b a m a t e

n i t r o g e n a t o m , w a s p o s t u l a t e d t o p r o c e e d b y t h e f o l l o w i n g p h o t o - F r i e s

m e c h a n i s m .

. . . . .

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Y a m a g a t a a n d c o w o r k e r s 6 c a r r ie d o u t s tu d i e s o n t h e i n fl u e nc e o f U V l ig h t o n

u r e th a n e c o m p o u n d s a n d n o t e d t h e b e h a v i o r o f t h e rm o p l a s t ic p o l y u r e th a n e s

i n t h e p r e se n c e o f U V l ig h t. F l u o r e s c e n c e , U V , I R s p e c t r o s c o p y a n d E S R d a t a

ind ic a ted tha t in the in i ti a l s t ages o f UV i r r ad ia t ion the u re than e l inkage wa s

bro ken v ia an e l ec t roph i l i c mec han is m, the re fo re dec reas ing dens i ty in the in it ia l

s t ages. C ros s l ink ing oc c ure d in the la te r s t ages by a r ad ic a l me c han is m and wa s

a c c o m p a n i e d b y t h e f o r m a t i o n o f c o l o r e d m a t e r i a l. N o a c k a n d S c h w e t li c k ~7

s u g g e s te d th a t t h e y e ll o w i n g o f u r e t h a n e p o l y m e r s u n d e r U V i r r a d i a ti o n w a s

d u e t o o x i d a t i o n o f f r e e - N H 2 g r o u p s f o r m e d b y t h e p h o t o l y s i s o f t h e u r e t h a n e

g r o u p s . U V s p e c tr a l a n d E S R s t u d ie s i n d ic a t e d t8 t h a t p h o t o d e g r a d a t i o n w a s

d e p e n d e n t o n t h e k i n d o f p o l y o l u s e d t o p r e p a r e t h e p o l y u r e th a n e .

B y ta k i n g i n t o a c c o u n t t h e e f fe c t o f e x c it a ti o n w a v e l e n g th , G a r d e t t e a n d

L e m a i r e '9 p o s t u l a t e d t h a t t h e p h o t o l y s i s o f M D I - b a s e d p o l y u r e t h a n e s p r o c e e d s

b y t w o e n t ir e ly u n r e l a t e d p r o c e ss s . F r o m G a r d e t t e a n d L e m a i r e 's w o r k 2 i t c a n

b e c o n c l u d e d t h a t t h e d e g r a d a t i o n o f a r o m a t i c p o l y u r e t h a n e s b y i rr a d i a t io n

m a y b e a c o u n t e d f o r b y t w o m e c h a n i sm s b a s e d o n t h e f o r m a t i o n o f d if fe r en t


4/16

738

B. P. THAPL IYA L and R. CHAN DRA

prod uc ts . Th e f i r st is obs e rv ed a t s ho r t w ave leng ths ( < 340 nm ) an d p roc ee ds

t h r o u g h a p h o t o - F r i e s t y p e r e a c t i o n . T h e s e c o n d m e c h a n i s m i s o b s e r v e d a t

l o n g e r w a v e l e n g t h s a n d i n v o lv e s fo r m a t i o n o f c o l o r e d s p e c ie s d e r iv e d f r o m

p r i m a r y h y d r o p e r o x i d e s . T h e s e t w o p a t h s w e r e d i f f e r e n t i a t e d b o t h b y i n c o r -

p o r a t i o n o f T i n u v in 7 70 i n to t h e p o l y m e r a n d b y s e l ec ti o n o f v a r io u s i r r a d ia t i o n

w a v e l e n g t h s # Q u i n o n e - d i i m i d e p r o d u c t s a r e o b t a i n e d w h e n w a v e l e n g t h s o f

l ig h t a b o v e 3 4 0 n m a r e u se d . H o w e v e r , f o r m a t i o n o f p h o t o - F r i e s r e a r r a n g e m e n t

t y p e p r o d u c t s a n d u n s u b s t i t u t e d a r y l a m i n e c le a v a g e p r o d u c t s o c c u r w h e n l ig h t

belo w 340 nm is used for pho to lys is . 2

h v > 3 4 n m

hv ~ 34Ohm

..... O 2 C ~ N H ~ H = - - - - ~ N H ----CO~ ....

+

T h e p h o t o o x i d a t i v e b e h a v i o r o f th e r m o p l a st ic p o l y u r e th a n e s b a s e d o n

a r o m a t i c a n d a l ip h a t ic d i i s o c y a n a t e s a n d p o l y e t h e r a n d p o l y e s t e r p o l y o l s w a s

a l s o i n v e s ti g a te d b y t h e s a m e g r o u p . 2~ U p o n p h o t o o x i d a t i o n , h y d r o p e r o x i d e

f o r m a t i o n t a k e s p l ac e in t h e h y d r o c a r b o n m o i e t ie s o f t h e p o l y u r e t h a n e s a s

s h o w n b e l o w :

R R R

)

~ C H ~ *' ~ C ~ *' ~ C ~ *, ~

_ _ o _ _ .

F o u r t y p e s o f h y d r o p e r o x i d e s w e re f o rm e d , d e p e n d i n g u p o n t h e s tr u c tu r e o f t he

p o l y u r e t h a n e e l a st o m e r :

0

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. . . . - C H~NH ~C ~O . . . . .

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(c) (D)


5/16

P H O T O D E G R A D A T I O N A N D S T A B IL I Z A T IO N O F P O L Y U R E T H A N E S 7 3 9

T h e r e sp e c t iv e h y d r o p e r o x i d e s w e r e f o r m e d f r o m t h e C H 2 g r o u p ~t t o t h e

p h e n y i g r o u p o f d i is o c y a n a t e ( A ) , th e C H 2 g r o u p a

to

the e s te r g roup (B) , the

C H 2 g r o u p ~t t o t h e e t h e r g r o u p ( C ) a n d t h e C H 2 g r o u p a t o t h e c a r b a m a t e

n i t r o g e n a t o m ( D ) .

T h e p r i m a r y p h o t o c h e m i c a l r e a c t i o n s i n p o l y u r e t h a n e s l e a d i n g t o a c c u m u -

la t ion o f f r ee r ad ic a l s , and fu r the r r eac t ions o f f r ee r ad ic a l s by UV pho to -

ly sis o f p o l y u r e t h a n e s b a s e d o n i , 4 - b u t a n e d i o I - H D I , 1 , 6 - h e x a n e d i o l -H D l

c o p o l y m e r , m e t h y l N - e t h y l c a r b a m a t e a n d d i i s o p r o p y l h e x a m e t h y l e n e b i s c a r -

b a m a t e w e r e s t u d i e d . 22 T h e p r i m a r y s t e p i n p h o t o l y s i s w a s c l e a v ag e o f t h e C - O

b o n d o f t h e u r e th a n e g r o u p . S u b s e q u e n t d e g r a d a t i o n f o l lo w e d a r a d ic a l c h a in

m e c h a n i s m .

B r a u m a n e t a l 23 f o u n d t h a t t h e s u b s t i tu t i o n o f t h e a l ip h a t i c d i i so c y a n a t e

m e t h y l e n e b i s (4 - c y c lo h e x y l i s o c y a n a t e ) f o r M D I g a v e a p o l y m e r r e si s ta n t t o

l i g h t - i n d u c e d d i s c o l o r a t i o n a n d d e g r a d a t i o n . A s d i s c u s s e d p r e v i o u s l y , p h o t o -

d e c o m p o s i t i o n o f a r o m a t i c p o l y u r e t h a n e s i s s u p p o s e d t o p r o c e e d v i a c l e a v ag e

o f N - C a n d C - O b o n d s in th e u re t h a n e g r o u p . P h o t o c h em i c a l d e g ra d a t i o n o f

a l i p h a t i c u r e t h a n e g r o u p s c o n t a i n i n g a h y d r o g e n a t o m o n t h e c a r b o n a t o t h e

c a r b a m a t e n i t ro g e n , w a s p o s t u l a t e d t o r e s e m b l e t h a t o f a n a m i d e : a n i n it ia l

N - C , a n d p o s s ib l y C - O , b o n d c le a v a ge in t he u r e t h an e g r o u p w a s f o ll o w e d b y

h y d r o g e n a b s t r a c t i o n . S u b s e q u e n t o x i d a t iv e d e g r a d a t i o n o f t hi s s it e a p p a r e n t l y

o c c u r s v ia a h y d r o p e r o x i d e i n te r m e d i a t e. T h i s p r o c e s s d o e s n o t o c c u r r e ad i ly i f

t h e h y d r o g e n s a r e n o t a c t iv a t e d , a s e v i d e n c e d b y t h e p h o t o l y t i c s t a b il i ty o f t h e se

p o l y u r e t h a n e s .

W i les 24 obs e rved a us e fu l wa y o f s tudy ing the in te r ac t ion o f pho to ns wi th

po lymers , i . e . to iden t i fy the s ign i f ic an t c hromophores in eac h s ys tem and to

d e t e r m i n e th e im p o r t a n t p r i m a r y p r o c e s s e s f o l lo w i n g p h o t o n a b s o r p t i o n . F o r

example , s uc h an ana lys i s has been us e fu l in a s s ign ing the c o r r ec t r e l a t ive

i m p o r t a n c e t o N o r r i s h t y p e I a n d I I p h o t o r e a c t i o n s f o r p o l y m e r s c o n t a i n i n g

c a r b o n y l g r o u p s . T h e i r r a d i a ti o n o f p o l y u r e t h a n e s d e r i v e d f r o m d i m e t h y l g ly -

o x i m e a n d h e x a m e t h y l e n e d i i s o c y a n a t e 25 i n v o l v e d t w o i n it ia l p r o c e s se s o f N - O

a n d C - O b o n d s ci ss io n . T h e r a t io o f N - O a n d C - O c le a v ag e s d e p e n d s o n t h e

w a v e l e n g t h o f t h e li g h t u se d . T h e C - O b o n d c l e a v a g e re s u lt e d in p o l y m e r

d e g r a d a t i o n , w h i le t h e s p ec ie s f o r m e d b y N - -O b o n d c l e a v a g e w e r e t h o u g h t t o

u n d e r g o r e c o m b i n a t i o n p r o c es s e s. U V i r ra d i a t i o n o f s o l u t i o n s a n d film s o f

p o l y p r o p y l e n e g l y c o l - T D I c o p o l y m e r s d e c re a s e d th e u r e t h a n e g r o u p a b s o r p -

tio n 26"27b y b re a k i n g N - -C a n d C - O b o n d s . S u c h w a s m o n i t o r e d b y U V , I R a n d

N M R s p e c t r o s c o p y . T h e i n f lu e n c e o f t h e p h y s i ca l s ta t e o f t h e p o l y m e r a n d

i r r ad ia t ion dos e on the r eac t ion were e s tab l i s hed .

3 . E F F E C T O F P H Y S I C A L S T A T E O N P H O T O D E G R A D A T I O N

T h e t e r m " p o l y u r e t h a n e " i s c o n v e n i e n t l y u s e d t o d e s c r i b e a w i d e v a r i e t y o f

po lymer ic ma te r ia l s inc lud ing p las t i c s , e l a s tomers , f ibe r s , foams ( f l ex ib le and


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7 4 0

B . P . T H A P L I Y A L a n d R . C H A N D R A

rigid), surface coatings and adhesives. With the rapid expansion of this class of

materials over the past decades, polyurethanes now include those polymers

which contain a significant number of urethane groups together with a variety

of other structurally important groups, e.g. ester, ether and urea linkages. The

relationship between their chemical and physical forms is crucially important

from the viewpoint of their photostability.

Most commercial polyurethane elastomers derive their high level of strength

and flexibility from the segmented structure. A number o f investigators28-33 have

shown that polyurethane elastomers consist of alternating hard and soft seg-

ments. The soft segments are formed from linear polyether or polyester chains

whose glass transition temperature is sufficiently below the desired service

temperature of the polyurethane. Soft segments which crystallize on chain

extension are particularly desirable from the viewpoint of ultimate tensile

strength. The hard segments originate from a diisocyanate and a chain extender

or a crosslinking agent (e.g. diol or diamine). These hard segments have a T8 or

Tm well above the anticipated service temperature of the polyurethane and

hold the rubber together by physical crosslinking (which can involve interchain

hydrogen bonding). These hard segments prevent the material from flowing so

that the elasticity is maintained. These polyurethanes behave as rubbers when

the hard segment content is lower than 40% and as glassy materials when the

hard segment content is greater than 40%.

The photodegradation of polyurethanes34 based on aromatic diisocyanates

can be dependent on the physical state of the polymer. Hoyle

e ta[ .

35-39 have

studied aromatic diisocyanate-based aromatic systems extensively to find out

the effect of the physical state of the polymer on the photostability. In one

investigation39 they have shown that for simple polyurethanes based on MDI

and a single diol, films with low crystallinity and high flexibility degraded

rapidly upon exposure to UV radiation. In the case o f segmented polyurethanes

based on MDI and more than one diol, the extent and mechanism of photolytic

decomposition was dependent on the hard segment content in the polymer. As

the content of hard segments increased, both the percent crosslinked gel and

discoloration generated upon photolysis decreased dramatically. The formation

of photo-Fries rearrangement products was found to increase with an increase

in the molecular weight of the poly(ethylene oxide) soft segment.

Fluorescence spectroscopy40 was used to determine the effect of flexibility and

crystallinity of the polyurethanes on the degradation pathway. I t was observed

that amorphous films yielded an ortho photo Fries rearrangement product,

whereas the semicrystalline film did not. Photodegradation of MDI and TDI 4~ -

based polyurethanes depended on the chain flexibility and the matrix crystallinity.

Both absorbance change and gel formation decreased as the stiffness and

crystalline content of the film increases.

There are a wide variety of compositional variables which can affect the

degree of phase segregation and hard segment organization and, accordingly,


7/16

PHOTODEGRADATION AND STABILIZATION OF POLYURETHANES 741

the s am ple p roper t i e s. I n the TD I -ba s ed po lyure thanes , morph o log ic a l va r ia t ions

h a v e b e e n o b s e r v e d in p o l y m e r s fr o m t h e c o m m o n l y u s e d i s o m e r s, 2 ,4 - T D l a n d

2 , 6 -T D I . 4z I n 2 , 4 - T D I p o l y m e r s , t h e a s y m e t r i c p l a c e m e n t o f t h e i s o c y a n a t e

res idues wi th r e s pec t to the me thy l g roup c an r e s u l t in s ome head to t a i l

i s omer iz a t ion in the ha rd s egmen t . In 2 ,6 -TDI po lymers , th i s p rob lem i s abs en t

s in c e th e m o l e c u l e is s y m m e t r i ca l . T h e p h o t o s t a b i l i t y o f p o l y u r e t h a n e s b a s e d o n

t h e s e is o m e r s c a n b e e x p l a in e d o n t h e b a s is o f s y m m e t r y .

T h e d e g r a d a t i o n r a t e o f p o l y u r e t h a n e s 43 p r e p a r e d f r o m v a r i o u s d i i s o c y a n a t e s

by r eac t ing wi th r e f ined c as to r o il inc reas ed in the o rde r M D I < T D I < 1 ,5 -

N D I < H D I u p o n e x p o s u re t o a t m o s p h e r e s o f 8 0 % a n d 9 5 % r el at iv e h u m i d it y .

T h e d e g r a d a t i o n w a s a n a u t o c a t a l y t i c p r o c e ss a n d w a s a cc e l e ra t e d b y th e c u r in g

c a ta lys t s , i . e . t r i e thy lene d iamine and d ibu ty l t in d i l au ra te . The s tab i l i ty o f

p o l y u r e th a n e s b a s e d o n H D I - B D is f o u n d t o b e g r e a te r t h an t h a t o f t h e o t h e r

p o l y u r e t h a n e s . "

P e t e r a n d c o w o r k e r s45 s t u d i e d t h e e f fe c t o f c o m p o s i t i o n a n d m o r p h o l o g y o n

t h e p h o t o - a n d t h e r m o c r o s s li n k i n g o f p o l y ( e th e r u r e t h a n e u r e a s) c o n ta i n i n g

d iac e ty lene l inkages . I t was s een tha t c o po ly m ers wi th phas e s ep a ra t ion c om -

p o s i t i o n w e r e m o r e p h o t o c h e m i c a l ly a n d t h e rm a l l y r e a ct iv e t o w a r d s c r o s sl in k -

i n g t h a n h o m o g e n o u s p o l y u r e t h a n e s .

The c hanges in c ha in l eng th and the i r d i s t ribu t ions a s we ll a s the c hanges in

c h e m ic a l s tr u c tu r e w e r e f o ll o w e d b y G P C , N M R a n d I R 46 af ter subje c t ing the

p o l y u r e t h a n e p r e p o l y m e r s t o p h o t o o x i d a t iv e d e g r a d a ti o n . T h e c h r o m a t o g r a p h i c

r e s u l t s s h o w e d t h a t t h e p h o t o o x i d a t i v e d e c o m p o s i t i o n w a s f o l l o w e d b y a

dec reas e in mo lec u la r mas s toge the r wi th an inc reas e in po lyd is pe r s i ty . Th is

ind ica ted an inhom ogen ous d eg rada t ion whic h was a c ons equenc e o f the s peci fic

c o u r s e a n d o f t h e in t e n si ty o f p h o t o o x i d a t i v e d e g r a d a t i o n .

In o rde r to de te rm ine the e ffec t o f po lyo l s t ruc tu re , the phys ic o -mec han ic a l

p r o p e r t i e s o f p o l y u r e t h a n e s p r e p a r e d f r o m M D I w i th 1 , 4 -b u t a n ed i o l a n d

p o l y e t h e r a n d p o l y e s t e r p o l y o l s w e r e e x a m i n e d a f t e r p r o l o n g e d e x p o s u r e t o

wea the r ing . ~ Th e low er re s i stanc e to wea the r ing foun d fo r po ly e the r u re thanes ,

c om pa red to tha t o f po lye s te r u re thanes , r e f lec ted the ir lower re s i stanc e to

t e m p e r a t u r e c h a n g e a n d U V r a d i a t i o n .

S t ruc tu ra l c hanges in cros s linked po lyure thanes r e su l ting f rom pho too x ida t ive

d e g r a d a t i o n a r e d e t e r m i n e d b y t h e n a t u r e o f th e o l i g o e st e rs u s e d f o r t h e ir

p repa ra t ion . 47 The k ine t ic s o f oxyg en ab s o rp t ion and s t ruc tu ra l c ha nges w ere

e x a m i n e d i n th e p h o t o c h e m i c a l o x i d a t i o n o f p o l y u r e t h a n e s p r e p a r e d f r o m

a li p h at ic p o l y is o c y a n a te s . T h e a m o u n t o f a b s o r b e d o x y g e n d e p e n d e d o n t h e

c r o ss l in k d e n s i ty o f t h e p o l y u r e t h a n e s a n d o n t h e n a t u r e o f t h e o l ig o e s te r b l o ck s .

T h e l ig h t s t a b i li t y o f p o l y u r e t h a n e s a n d p o l y ( u r e t h a n e u r e a s) ~ d e r iv e d f r o m

po ly(e thy lene ad ipa te ) d io l , po ly (p en ty len e ad ipa te ) d io l , po lyc a pro la c ton e d io l ,

p o l y ( p r o p y l e n e o x i d e ) gl y co l , p o l y o x y p r o p y l e n e d ia m i n e a n d 1 ,4 b u t a n e d i o l

w e r e s t u d i e d b y a X e n o t e s t 1 2 0 0 a p p a r a t u s a n d a c o r r e l a t i o n w a s e s t a b l i s h e d

be tw een the c hemic a l s t ruc tu re and the l igh t s t ab i li ty o f the po lymers .


8/16

742 B.P. THAPLIYAL and R. CHANDRA

T h e l ig h t r e s i st a n ce a n d p h o t o d e g r a d a t i o n 49 o f a r o m a t i c p o l y e s te r s , a r o m a t i c

po lyc a rbon a tes and po lyure thanes bas ed on a l ipha tic d i is oc yana tes and po lyes te r

a l ip h a t i c e s te r a n d a r o m a t i c e s t e r p o l y o l s a n d o f a r o m a t i c d i i s o c y a n a t e s a n d

p o l y e t h e r p o l y o i s h a v e a l s o b e e n i n v e st ig a t ed . T h e a p p e a r a n c e o f p o l y u r e t h a n e s

f r o m a l i p h a t i c d i i s o c y a n a t e s a n d p o l y e s t e r p o l y o l s r e m a i n e d u n c h a n g e d a f t e r

7 2 h r i n a X e n o t e s t a p p a r a t u s , w h i le p o l y u r e t h a n e s f r o m a r o m a t i c d i i s o c y a n a t e s

a n d p o l y e t h e r p o l y o l s s h o w e d s o m e c h a n g e i n a p p e a r a n c e ( b u t l e s s t h a n s o m e

o t h e r t y p e s o f p o l y u r e t h a n e s ) .

I n g e n e ra l , m a j o r p h o t o p r o c e s s e s o b s e r v e d in s o l id p o l y m e r s a r e d e p e n d e n t

upon tempera tu re . Above the g la s s t r ans i t ion t empera tu re (Tg) r e l a t ive ly l a rge

s e g m e n t s o f th e m o l e c u l e c a n u n d e r g o e x t e n s iv e m o t i o n . A b o v e Tg p o l y m e r

c ha in mobi l i ty i s s u f f ic ien t to fo rm c yc l ic in te rmed ia te s , jus t a s in s o lu t ion .

P h o t o a c o u s t i c s p e c t r o s co p i c s t u d ie s ~ h a v e s h o w n t h a t t h e d e g r a d a t i o n o f

p o l y u r e t h a n e s c a n b e i n i t i a t e d a t a r e l a t i v e l y l o w t e m p e r a t u r e a n d l o w U V

d o s a g e a b o v e T g. I t w a s s e e n th a t e f f ec t o f h e a t is m o r e p r o n o u n c e d t h a n t h e

effect o f l ight.

N a p h t h y l c a r b a m a t e s h a v e b e e n s h o w n t o f o r m e x c i m e r s t h a t a r e s t a b i l i z e d

by h ydro gen bon d ing inhe ren t in the pa r t i c ipa t ing s pecies. 52 S ub s t i tu t ion o f a

m e t h y l g r o u p o n t h e ce n t r a l n i tr o g e n a t o m o f th e c a r b a m a t e m o i e t y s e v e re l y

l i m i t s e x c i m e r f o r m a t i o n . A p o l y u r e t h a n e b a s e d o n n a p t h y l e n e d i i s o c y a n a t e

( N D I ) f o r m s in t r a m o l e c u l a r e x c im e r s i n d i lu t e s o lu t i o n s in b o t h g o o d a n d p o o r

s o lven ts . However , the exc imer fo rmat ion i s s ign i f ic an t ly enhanc ed in poor

s o lv e n ts , w h e r e i n t r a m o l e c u l a r c o n t a c t b e t w e e n n a p t h y l e n e c a r b a m a t e g r o u p s i s

f a v o u r e d . T h e p h o t o p h y s i c s o f e x c i m e r f o r m a t i o n i n t h e p o l y u r e t h a n e i s i n te r -

p r e te d o n t h e b a si s o f a n " i s o la t e d m o n o m e r " s c h e m e ?

P o l y u r e t h a n e s c o n t a i n i n g d i a n t h r a c e n e u n i t s i n t h e m a i n c h a i n w e r e p h o t o -

chemical ly d issoc ia ted by i r rad ia t ion a t 300

nl' D. 53

Th e efficiency o f photoch em ical

r e a s so c i a ti o n g a v e a n i n d i ca t i o n o f t h e m o b i l i ty o f t h e t e r m i n a l p o l y m e r s e g-

m e n t s c o n ta i n i n g a n t h r a c e n e g r o u p s . D u r i n g p h o t o l y s i s t h e te r m i n a l a n t h r a c e n e

g r o u p s p r o d u c e d s a n d w i c h d i m e r s w h i c h g e n e r al ly d i ss o c i a te d u p o n w a r m i n g :

t h e p o l y u r e t h a n e s h o w e d r e t e n t io n o f th e s a n d w i c h d i m e r s t ru c t u r e a t r o o m

t e m p e r a t u r e .

T h e r e p o r t e d p h o t o d e c o m p o s i t i o n o f a p o l y e s te r d i o l - M D I p o l y u r e t h a n e a n d

i t s m o d e l c o m p o u n d s i n s o l u t i o n w a s n o t i n f l u e n c e d b y p i p e r y l e n e o r t r i p l e t

q u e n c h e rs , i n d ic a ti n g t h a t p h o t o d e c o m p o s i t i o n d o e s n o t s t a rt f r o m t h e e x te r n a l

t r ip le t s t a te bu t f rom the exc i ted s ing le t s t a te J 4 Th e en ergy levels and l i fe t ime of

p o l y u r e t h a n e p h o t o d e c o m p o s i t i o n w e r e 9 8 .6 K c a l / m o l a n d 3 .2 n s e c r e sp e c ti v e ly

in the exc i ted s ingle t s ta te and 2 .93 nsec in the exc i ted t r ip le t s ta te .

4. P H O T O S T A B I L I Z A T I O N O F P O L Y U R E T H A N E S

A l t h o u g h a l a rg e n u m b e r o f d i ff e re n t ty p e s o f c o m p o u n d s a r e k n o w n t o b e

e f fec t ive UV s tab i l i z e r s , on ly a f ew c la s s es o f thes e c ompounds a r e us e fu l in


9/16


p r e v e n t in g t h e p h o t o d e g r a d a t i o n o f p o l y u r e t h a n e s . I t is w e l l k n o w n t h a t m a n y

ino rgan ic p igmen ts s uc h as i ron ox ides (F e203 and F e304) , c h romic ox ide

(Cr203) , r ed l ead ox ide (P b304) , z inc ox ide (ZnO) and t i t an ium d iox ide (T iO2)

h a v e b e e n u s e d a s s t ab i li z er s f o r p o l y u r e th a n e s . Z n O is re p o r t e d t o b e o n e o f th e

m os t eff ic ien t and eco no m ical wh i te inorga nic pho tos tab i l izers , 55 espec ia l ly in

t h e U V r e g io n f r o m 2 4 0 t o 3 8 0 n m . C h a s e

e t a l 56

prepa red m e ta l ox ide pa rt i cl e s ,

e .g . Zn O , S b203 , WO3 and M oO3 o f ave rag e d iam ete r fo r s t ab il i zing po ly -

ure thane f ilms. C rek ov and h is c o l l eagues 57 m ade e f fo r ts to im prov e the m ec h-

an ic a l , the rma l , hyd ro ly t ic and l igh t s t ab i l i ty o f po lyure thane f i lms by f i l l ing

wi th CuO , M OO3, B iz O3 , Cr20~ , T iO2 and W O3. W ate r abs o rp t ion dec reas ed

when po lyure thanes were f i l l ed wi th me ta l ox ides . The f i l l ed po lymer was more

res is tan t to ac ids than to a lkal is .

G a r d e t t e a n d L e m a i r e 58 s h o w e d t h a t T iO 2 i n h ib i te d t h e p h o t o o x i d a t i o n o f

H D I - p o l y e t h e r a n d p o l y e s t e r p o l y u r e t h a n e s m o r e e ff e ct iv e ly t h a n a r o m a t i c

d i i s oc ya na te -b as ed p o lyure thane s . T he e f fec t o f "/-Fe203 on the the rm al and

p h o t o o x i d a t i o n o f p o l y u r e t h a n e c o a t i n g s w a s r e m a r k a b l e . 59

M a n y o r g a n i c p i g m e n t s s u c h a s a z o , a n t h r a q u i n o n e a n d t h i o i n d i g o c o m -

pou nds , quina cridone s, isoindolinones, perylenes, dioxazines a nd phtha locya nines

hav e wid e ap pl ica t ion in th e plas t ic in dus try . 6'6~ Fe do see v e t a l 6z f o u n d t h a t

v a r i o u s i n d o p h e n o l s a r e s t a b il iz e r s f o r p o l y u r e t h a n e s . S o m e a z o c o m p o u n d s 6~

s u c h a s p - h y d r o x y a z o b e n z e n e a n d p - a m i n o a z o b e n z e n e w e r e ef fe c ti v e r e t a r d e rs

i n c o m p a r i s o n t o t h e U V a b s o r b e r s T i n u v i n 3 2 6 , 2 , 4 - d i h y d r o x y b e n z o p h e n o n e

a n d d i b u t y l d i t h i o c a r b a m a t e s .

P h o t o s t a b l e c o l o r e d a r o m a t i c p o l y u r e t h a n e s w e r e p r e p a r e d f r o m p o l y o l s

s t r u c t u r a l l y d y e d w i t h a n t h r a q u i n o n e d e r i v a t i v e s b y L i t v i n e n k o a n d h i s c o l -

l ea g u e s. 64 T h e c o l o r o f t h e d y e d p o l y o l d e p e n d e d u p o n t h e a n t h r a q u i n o n e

c o m o n o m e r u s e d . F l e x ib l e p o l y u r e t h a n e c o a t i n g s w e r e p r e p a r e d b y th e r e a c t io n

o f th e p o l y o l s w i th T D I . T h e i n c r e a se i n p h o t o s t a b i l i ty o f p o l y u r e t h a n e s b a s e d

o n s t r u c t u r a l l y d y e d p o l y o l s w a s d u e t o t h e o - h y d r o x y k e t o n e p h o t o i s o m e r i z -

a t ion in the po lyo l mo lec u le .

F luo res c en t pho tos tab i l i z e r s abs o rb l igh t and immed ia te ly r eemi t the l igh t a t

a longer wave leng th in the fo rm o f f luo res c en t l igh t. N um ero us o p t ic a l b r igh ten -

ing agen ts abs o rb UV rad ia t ion and r eemi t the ene rgy as v i s ib le l igh t . S uc h

" b r i g h te n i n g " c o m p o u n d s a r e c o m m o n l y u s e d to c o m p e n s a t e f o r th e y e ll o w in g

dur ing the p roc es s ing o f po lym er ic ma te r ia l s and to inc rease b r il l ianc e and

w h i t en e s s. C o u m a r i n s a n d s t il b e n e s a r e e x a m p l e s o f th e c o m m o n o p t i ca l b r i g h t-

e n in g c o m p o u n d s u s e d i n p o l y m e r s . T h e e f fe c t o f c o u m a r i n s , s ti lb e n e d e r iv a t iv e s

a n d o t h e r c o m m o n b r ig h te n in g a g e n ts o n t h e p h o t o d e g r a d a t i o n o f p o ly e s t er

po lyure thanes was s tud ied by inves t iga t ing the c hanges in r e s idua l b reak ing

s t r eng th . 65 S om e o f the c ou m ar in and s t ilbene de r iva t ives were fo und to be

e f fe c ti v e r e t a rd e r s o f t h e p h o t o d e g r a d a t i o n o f p o l y m e r s a n d t h e ir e f fe c ti v en e s s

was c om pa rab le to tha t o f c o m m on s tab il i ze r s , e .g. tr i az o le de r iva t ives . Th e

r e t a r d a t i o n m e c h a n i s m o f t h e re p r e s e n t a ti v e c o m p o u n d s w a s d i s cu s s e d o n t h e


10/16

744 B.P. THAPLIYAL nd R. CHANDRA

b a s is o f th e d e c a y o f e t h y l p h e n y l c a r b a m a t e d u r i n g p h o t o i r r a d i a t i o n a n d o n t h e

b a s is o f f l u o re s c e nc e em i s s io n o f t h e p o l y m e r w i t h a n d w i t h o u t t h e a d d i t i v e .

R3N H ~ O

COUMARINS

SO~R

NAPI-r~OTRIAZOLYL TILBENF~

F l u o r e s c e n t c o m p o u n d s a r e v e r y e ff e ct iv e p h o t o s t a b il i ze r s b u t u n f o r t u n a t e l y

the po or l igh t and the rmal s t ab i l ity o f m os t o f thes e fluo res c en t ma te r ia ls r e s u lt s

in a gradual decrease in the ir e f f ic iency. (Only one l ight-s table f luorescent

c o m p o u n d , 6 , 1 3 -d i c hl o ro - 3 ,1 0 d i p h e n y l t r ip h e n o d i o x a z i n e , w a s f o u n d t o b e a n

excel len t U V s tabi l izer for ce l lu lose es ter p las t ics . 66)

c t

P henyl e s te r s o f benz o ic ac id were the f ir st ph o tos tab i l i z e r s to be us ed t ec hn i-

c a lly and they a re s til l ava i l ab le bec aus e o f the i r low pr ic e . U nfo r tun a te ly m os t

o f them tu rn ye l low on expo s ure to U V l igh t. Th is p rob lem l imi t s the i r us e a s

U V a b s o r b e r s f o r co l o r le s s a n d t r a n s p a r e n t p o l y m e r s . I t is w e l l k n o w n t h a t t h e

y e l lo w i n g o f t he s e c o m p o u n d s is d u e t o a p h o t o c h e m i c a l r e a r r a n g e m e n t o f th e

F r ie s type , g iv ing de r iva t ives o f d ih ydro xyb enz o phe non es . 67-7 A m on g the

h y d r o x y b e n z o p h e n o n e s , t h e o - h y d r o x y b e n z o p h e n o n e s a r e g o o d s t a b i l i z e r s

whi le p -h ydr oxy ben z op hen on es ac t a s ph o to deg rada t ion s ensit iz er s. Th e 1 ,4 -

d i h y d r o x y b e n z o p h e n o n e i s o m e r i s , h o w e v e r , a v e r y g o o d p h o t o s t a b i l i z e r f o r

p o l y u r e t h a n e s f '

o o " - o

* ( n , A * )

i t ~ t l ( - , , ) l ~ ( n . ;~

0 ~ ) H ~ O ( ~ O H 0


11/16


Many workers have found that UV absorbers, e.g. Tinuvin 326 and 2,4

dihydroxybenzophenone, are effective for retardation of the photodegradation

of polyurethanes.63'7~ In addition, benzotriazole derivatives have been used as

stabilizers for polyurethanes.72 As in the case of o-hydroxybenzophenones, the

(o-hydroxyphenyl)benzotriazoles may form internal hydrogen bonds. Their

photostabilization mechanism is considered to be a rapid tautomerism in the

excited state: ~'73'74

Merril and Bennett75 found that (o-amidophenyl)benzotriazoles may form an

intramolecular hydrogen bond:

R R

N--H N H

The distribution of a benzotriazole stabilizer (Tinuvin 328) in crystalline poly-

propylene has been investigated by UV microscopy.76 The results show that the

stabilizer accumulates in the non-crystalline region and between polypropylene

spherulites, and is rejected by crystalline regions. These observations are very

important for understanding the photostabilization mechanism of polymers.

The excellent mechanical properties of benzotriazole-polyurethane systems offer

scope for interesting studies which involve varying the crystallinity and flexi-

bility of the polyurethanes.

The hydroxphenyl-s-triazines are a new class of stabilizers which have recently

been investigated by Heller73 and used in polyurethanes.77

Ra

The mechanism by which the photostabilization of aromatic polyurethanes is

achieved by o-hydroxyphenyl-s-triazines is similar to that of o-hydroxyben-

zophenones and o-hydroxyphenylbenzotriazoles. The light fastness and protec-

tive powers of hydroxyphenyl-s-triazines are influenced by:

(1) The number of o-hydroxy groups. Compounds with a higher number of

o-hydroxyphenyl groups show better protective power and stronger absorp-

tion of UV light of long wavelengths.


12/16

746

B. P. THAPLIYAL and R. CHAND RA

(2) T he b as ic i ty o f the t ri az ine r ing is r educ ed by the s ubs t i tuen t s R , an d R : an d

the l igh t f a stnes s o f the r e s u l t ing c om po un d inc reas es.

In t r am olec u la r bo nd s a r e obv io us ly no t a p re requ is i t e fo r a s t ab il iz ing mec h-

an i s m. Tr iaz o les 73 wh ic h a re pho toc hem ic a i ly ac t ive have , h owe ver , no pho to -

s tab il iz ing e f fec t and the c om po un ds m ay ac t a s pho tos ens it i z e rs .

i

~ " R 2 ~ R 2

R3 ~ R3

Rt R:

4H - 1,2,4 - triazoles 2H - 1,2,4 - triazoies

Thes e c om po un ds s ensi ti ze the d i s c o lo ra t ion o f po lye s te r r es in s. H ow ever ,

they c an b e us e fu l in ex tend ing the s he l f l if e o f unc ured po ly ure th ane mix tu res .

A h e a t - c u r a b l e p o l y u r e t h a n e w i th g o o d s h e l f life w a s p r e p a r e d b y a d d i n g a s m a l l

a m o u n t o f t ri a zo l e t o th e p o l y m e r f l T h u s a p r e p o l y m e r p r e p a r e d f r o m a p o l y -

( t e tr a m e t h y l e n e a d i p a t e ) p o l y o l a n d T D I w a s m e l t b le n d e d w i th 1 , 4 -b u t a n ed i o l,

1 ,2,6-hexanetriol, 4H -1,2,4-tr iazole and tr iphen yllead ac etate at 60C. I t ha rde ne d

a t r o o m t e m p e r a t u r e a n d w a s g r a n u la t e d . T h e g r a n u l e s g el le d a t 5 0 C in 8 m i n

a f t e r o n e o r t w o w e e k s s t o r a g e b u t p o l y m e r w i t h o u t t r ia z o le h a d a g e l t im e o f

5 min a f te r one w eek and wa s in fus ib le a f te r tw o w eeks s to rage .

Am ines and pheno l s ac t a s typ ic a l an t iox ida n ts and the i r e ffec tiveness i s

h i g h l y d e p e n d e n t u p o n t h e n a t u r e o f t h e p o l y m e r t o w h i c h t h e y a r e a d d e d .

A r o m a t i c p o l y u r e t h a n e s h a v e b e e n s ta b i li ze d 79 b y m i x i ng w i t h p r i m a r y m o n o -

f u n c t io n a l p h e n o l s a n d p r i m a r y o r s e c o n d a r y a m i n e s ( 0 . 2 - 2 g m o l / g m o l o f fr e e

N C O ) i m m e d i a t e ly a f t e r sy n t he s is . S o l u t i o n s o f a m i n o a c i d h y d r a z id e c h a i n -

ex tended po ly ure th ane s w ere s tab il i zed by add i t ion o f amines , s The s o lu t ion

wi th t r i e thy lamine s how ed no c hange in v i s c os ity o r c o lo r o n s tand ing overn igh t .

T h e f lu o r e sc e n c e o f T D I - b a s e d p o l y u r e t h a n e s w a s q u e n c h e d b y s a l ic y li c a c i d

e s te r s, a m i d e s a n d h y d r a z id e s , s' Q u e n c h i n g a p p r o x i m a t e l y o b e y e d t h e F o e r t e r

and P e r r in e qua t ion ac c om pan ied b y a s ens i ti z a tion o f the f luo res c enc e o f the

s a l ic y l ic ac id de r iva t ives . The pho toye l lowing o f the po lyure thanes was a l s o

inhibi ted by the sa l icyl ic ac id der ivat ives .

Nic ke l c he la te s s uc h as n ic ke l d ibu ty ld i th ioc a rbamate have indus t r i a l app l i -

c a t ions a s typ ic a l quen c her s fo r po lyure than es , s2

CH3(CH2)3. ~S ]2N i

\~c//

CH3(CH:)3~ ~ S -

Y a m a g a t a a n d h is a s s o ci a te s 83 o b s e r v e d t h e e f fe c t o f U V l ig h t o n u r e t h a n e

c o m p o u n d s a n d t h e e ff e ct o f ri n gl e t o x y g e n a n d s t ab i li z er o n t h e p h o t o d e g r a -

da t io n o f po ly ure th ane r es in s. The e f fec t o f adde d s tab i li z e rs , e .g . the


13/16

P H O T O D E G R A D A T I O N A N D S T A B I L IZ A T I O N O F P O L Y U R E T H A N E S 7 47

dibutyldithiocarbamates of Ni or Zn, Irganox 1010, BHT and Sanol on the

photodegradat ion of a polyurethane was examined in the presence of ringlet and

triplet oxygen. The higher the quenching capacity of the stabilizer for singlet

oxygen, the smaller the photodegradation rate o f the polyurethane. The presence

of singlet oxygen was more important in photodegradation o f polyurethanes in

the initial stages compared to that of triplet oxygen. No difference was observed

between the ringlet and triplet oxygen after prolonged irradiation.

Propylene glycol-xylene diisocyanate polymers having improved thermal and

weathering stability were prepared by the addition of nickel dialkyldithiocarba-

mate u at any stage before completion of the urethane polymer format ion.

MDI-poly(oxypropylene)-glycerol ether copolymer and an acrylic polyol-

MDI copolymer were photostabilized by antioxidants8s (e.g. nickel dibutyl-

dithiocarbamate and Irganox) having large quenching indexes for singlet oxy-

gen. A linear relation was observed between the yellowing index and color index.

An aromatic polyurethane having a tertiary hydrogen in the polymer chain was

susceptible to photodegradation but was stabilized by the addition of an anti-

oxidant. The gloss index was decreased with increasing yellowing and coloration.

Nickel dodecylthiophosphate 6 is also described for quenching the excited

state of polyurethanes. Zinc dibutyldithiocarbamates63 were also found to be

effective as UV absorbers and for the retardation o f photodegradation, as were

some azo compounds.

Some patents recommending various chemical compounds as efficient photo-

stabilizers have been published in the last decade. The majority of these com-

pounds have not, however, found application on an industrial scale. Piperidine,

stilbene and coumarin derivatives have been used as stabilizers for polyurethanes.

Morimura e t a l s 7 prepared and used 2,2,6,6-tetramethylpiperidine derivatives as

stabilizers for polyurethanes.

Ishada e t a l ~ 8 increased the light and heat resistance of polyurethanes by the

addition of cytidine, uridine or a similar pyrimidine base to the polymer.

Synergism in the stabilization effect was also observed. Polymer containing

0.5% cytidine lost only 1.0% o f its weight during I hr at 220C and was

unchanged in color after 192hr in a Fadeometer. Similarly 100 parts poly-

urethane~9 melt-blended with 0.25 part (4-organo oxy-2,2,6,6-tetramethyl

piperidines) and bis(2,2,6,6-tetramethyl piperidyl esters) was molded into

0.5 mm thick plates and tested for light stability in a Fadeometer at 45C.

Murayama e t a l 9 prepared 4-alkoxy-2,2,6,6-tetramethyl-l,2,5,6-tetrahydro-

pyridines and melt-blended these stabilizers with polyurethanes. Molded sheets,

when exposed to UV radiation, become embrittled only after 660 hr, whereas a

control sample containing no stabilizer became embrittled within 60 hr.

Keberle and coworkers9~ improved the light stability of aromatic polyurethanes

by adding sodium 2-(fl-aminopropionamido)-2-methylpropanesulphonate as

additive. The additive was added to anionic-emulsifier-free polyurethane disper-

sions which were used to prepare colorless films or coatings with good light

resistance.


14/16

748

B. P. THAPLIYAL and R. CHANDRA

The m etho ds o f pos s ib le d i r ec t de te rm ina t ion o f ligh t s t ab i li z e r and an t iox i -

dan t c on ten t o f po ly ure th ane s a r e i l lus t r a ted by M od y . 92 Di f fe ren t ana ly t ica l

t e c h n iq u e s w e r e u s e d f o r d e te r m i n i n g a d d i t i v e c o n c e n t r a ti o n s , e .g . G C , H P L C

a n d G P C . A l s o , a g e n e r a l m e t h o d f o r s c r e e n i n g t h e a d d i t i v e s i n t h e p o l y m e r

u s in g t h i n -l a y e r c h r o m a t o g r a p h y w a s d e v e l o p e d . T h e s e te c h n i q u e s w e r e u s e d t o

a s s u r e t h e a d d i t io n o f th e p r o p e r a m o u n t a n d c o n c e n t r a t i o n o f t h e a d d i ti v e t o

t h e p o l y m e r b e f o r e a n d a f t e r p ro c e s s in g a n d d u r i n g t h e a g i n g o f p o l y u r e t h a n e s .

5. CONCLUSION

A b e t t e r u n d e rs t a n d in g o f t he m e c h a n is m s o f p h o t o d e g r a d a t i o n a n d p h o t o -

s t a b i l i z a t i o n h a s p r o d u c e d p o l y u r e t h a n e s w i t h i n c r e a s e d p h o t o r e s i s t a n c e

th roug h the us e o f m ore e f fec tive s tab il iz e r s. H ow eve r , m ore know ledge i s

n e e d e d a b o u t t h e p r i m a r y a n d s e c o n d a r y r e a c t i o n s i n v o l v e d i n o x i d a t i v e

p h o t o d e g r a d a t i o n .

U r e t h a n e m o d e l s y s t e m s h a v e a lr e a d y r e c e iv e d in c r e as e d a t t e n t i o n a n d t h e ir

us e has f ac i l i ta ted the s epa ra t ion o f the f ac to r s invo lved in the c omplex pho to -

d e g r a d a t i o n p r o c e ss . H o w e v e r , g e n e r a li z a ti o n s p r o j e c t e d f r o m s m a l l -m o l e c u l e

m o d e l c o m p o u n d s t ru c t u r e s f o r s tu d i e s o f a s i ng le p o l y u r e t h a n e i n a p a r t ic u l a r

phys ic a l s t a te c an no t b e o f in te r es t fo r a ll po lyure than es . T here fo re , s tud ies o f

p h o t o d e g r a d a t i o n a n d s t a b il iz a t io n o f p o l y u r e t h a n e s h a v i n g d i f fe r e n t p h y s ic a l

s t ru c t u r e s m a y p r o v i d e b e t t e r u n d e r s t a n d i n g o f t h e u r e t h a n e s y s te m s . I n a r e c e n t

s t u d y 93 i t has b een ob s e rved th a t m orph o log ic a l c han ges t ak e p lac e a s a r e s u lt

o f p h o t o o x i d a t i v e d e g r a d a t i o n a n d a r e t h e p r i m a r y c a u s e o f t h e f a il u re o f

a rom at ic u re thane s ys tems . The r e s ea rc h f ie ld o f po lyu re thane pho to deg rad a t ion ,

t h e r e f o r e , o f f e r s a m p l e o p p o r t u n i t y f o r t h e p o l y m e r c h e m i s t a n d p h y s i c i s t t o

c o l l ab o ra te in wo rk tha t is no t on ly in te ll ec tua lly s t imu la t ing b u t o f p rac t ic a l

impor tanc e a s we l l .

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15/16

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