Photoozonization of Polypropylene - American Chemical Society

at 255 (strong, quantum yield of photolysis 0 =1.0), 305 (weak) and. 340 nm ... Experimental. Isotactic polypropylene (PP) was supplied by Polyscience...
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Chapter 14

Photoozonization of Polypropylene Oxidative and J.

Reactions

Caused by Ozone

Atomic Oxygen on Polymer Surfaces 1

1

1

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F. Rabek , J . Lucki , B. Rånby , Y . Watanabe , and B. J . Q u

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1

Department of Polymer Technology, The Royal Institute of Technology, S-100 44 Stockholm, Sweden Research Institute for Polymers and Textiles, Tsukubo, Japan University of Science and Technology of China, Hefei, Anhui, China 2

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Polypropylene films were oxidized by treatment with ozone and ozone- UV- light. The main products of photolysis of ozone are atomic oxygen, singlet oxygen and oxygen. All of these oxygen species such as ozone, atomic oxygen and singlet oxygen are very reactive with polymer surface. The changes that resulted in polypropylene surface oxidation were followed by IR(ATR), fluorescence, ESR and ESCA spectroscopy and the contact angle measurements. Polypropylene surface is oxidized more rapidly in the presence of atomic oxygen formed from photolysis of ozone than that of ozone,however, separation of both reactive oxygen species in this method is imposible. The surface oxidation products are mainly carbonyl and/or carboxyl groups, with lower level of hydro­ peroxide groups. The mechanisms of oxidation by ozone and atomic oxygen have been proposed. Ozone formed i n atmosphere i s a by-product of atmospheric photoc h e m i c a l r e a c t i o n s , mainly i n photochemical smog i n c i d e n t s and i t s c o n c e n t r a t i o n s may a t t a i n as h i g h as 50 ppm ( 1 ) . O r d i n a r i l y , ozone i n the lower atmosphere i s p r e s e n t o n l y i n very low c o n c e n t r a t i o n s i n the range of l e s s than 1 t o 2 or 3 ppm.High c o n c e n t r a t i o n of ozone i s formed c l o s e l y to h i g h - v o l t a g e i n s t a l a t i o n s and d u r i n g h i g h - v o l t a g e d i s c h a r g e s i n stormy weather. The low c o n c e n t r a t i o n s of ozone n o r m a l l y p r e s e n t i n the atmosphere are s u f f i c i e n t t o cause severe o x i d a t i o n and c r a c k i n g i n p o l y o l e f i n s (2-13) and many o t h e r polymers such as p o l y s t y r e n e (6,11,12,14,15), p o l y ( v i n y l c h l o r i d e ) (11,12,16) and r u b b e r s (11,12,17-20). Where the ozone c o n c e n t r a t i o n i s i n c r e a s e d by a i r p o l l u t i o n , h i g h e r a l t i t u d e s or the p r e s e n t of e l e c t r i c a l machinery, the r a t e of degrad a t i o n c o n s i d e r a b l y i n c r e a s e s . T h e p r e v e n t i o n of such d e g r a d a t i o n r e p r e s e n t s a matter of c o n s i d e r a b l e economic i n t e r e s t s i n c e i t can g r e a t l y improve s e r v i c e l i f e of polymers and p l a s t i c s . An a d d i t i o n a l a p p l i c a t i o n of t h i s r e s e a r c h i s to a p p r i s e the s u i t a b i l i t y of p o l y mers f o r upper atmosphere a p p l i c a t i o n (Space S h u t l e f l i g h t s ) where a p l e n t i t u d e of ozone and atomic oxygen p r e v a i l s . 0097-6156/88/0364-0187$06.00/0 © 1988 American Chemical Society

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C H E M I C A L REACTIONS ON

POLYMERS

I n s p i t e of e x t e n s i v e and s i g n i f i c a n t s t u d i e s on the r e a c t i o n of ozone w i t h polymers, n o t h i n g has been p u b l i s h e d on the r e a c t i o n of ozone p h o t o l y s i s p r o d u c t s w i t h p o l y o l e f i n s . Ozone has UV a b s o r p t i o n s a t 255 ( s t r o n g , quantum y i e l d of p h o t o l y s i s 0 =1.0), 305 (weak) and 340 nm (weak). Ozone absorbs a l s o a t l o n g e r wavelength (510,615,670 and 760 nm ) ( 2 1 , 2 2 ) . The main p r o d u c t s of p h o t o l y s i s of ozone a r e atomic oxygen, s i n g l e t oxygen and oxygen ( 2 2 ) . A l l of t h e s e t h r e e oxygen s p e c i e s , ozone, atomic oxygen and s i n g l e t oxygen, a r e v e r y r e a c t i v e w i t h most of a l l polymers ( 2 3 , 2 4 ) . U V - i r r a d i a t i o n a c c e l e r a ­ t e s the o z o n i z a t i o n of low m o l e c u l a r weight o r g a n i c compounds ( 1 8 ) . I n our study we have found t h a t U V - l i g h t g r e a t l y a c c e l e r a t e s the r a t e of ozone a t t a c k a t the p o l y p r o p y l e n e s u r f a c e . P r e s e n t e d r e s u l t s were undertaken t o determine mechanism of the p h o t o - o x i d a t i o n of p o l y p r o ­ pylene s u r f a c e upon U V - i r r a d i a t i o n of polymer f i l m s i n ozone. Experimental I s o t a c t i c p o l y p r o p y l e n e (PP) was s u p p l i e d by P o l y s c i e n c e I n c . W a r r i n g t o n (USA) and p u r i f i e d by a common method. Ozone was generated by Ozon G e n e r a t o r Model 502 ( F i s c h e r Labor und V e r f a h r e n t e c h n i k , Germany) equipped w i t h ozone c o n c e n t r a t i o n measuring d e v i c e . A m i x t u ­ r e o f ozone_«|nd oxygen a t t h e f l o w r a t e 500 ml/min and ozone c o n c e n t ­ r a t i o n 4x10 g/1 passed t h r o u g h a q u a r t z c e l l , c o n t a i n e d PP f i l m s (50-70 μπι), which c o u l d be i n a d d i t i o n i r r a d i a t e d w i t h UV or UV/VIS l i g h t from lamps: low p r e s s u r e mercury lamp, type HPK 125 W ( L I ) o r h i g h p r e s s u r e mercury lamp, type HPK 15, 1000 W ( L 2 ) , both P h i l i p s , H o l l a n d , from the d i s t a n c e 30 cm. Atomic oxygen was generated by p h o t o l y s i s o f ozone which passed through a q u a r t z c a p i l l a r y tube (0 2 mm and l e h g t h 60 mm), u s i n g the 253.7 nm l i n e from a l o w - p r e s s u r e mercury lamp ( L I ) . F l u o r e s c e n c e e m i s s i o n s p e c t r a were r e c o r d e d w i t h P e r k i n - E l m e r LS Luminescence S p e c t r o m e t e r , whereas IR ( t r a n s m i s s i o n and ATR s p e c t r a ) w i t h P e r k i n - E l m e r 1710 IR F o u r i e r Transform S p e c t r o m e t e r . ESR s p e c t r a were determined w i t h a B r u k e r ER-420 ESR Spectrometer u s i n g a c c e s s o r i e s f o r s o l i d samples, u l t r a v i o l e t i r r a d i a t i o n i n a room t e m p e r a t u r e . ESCA c o r e - l e v e l s p e c t r a f o r and 0^ were r e c o r d e d w i t h a Leybold-Heraeus Spectrometer u s i n g A I K Q ^ ^ e x c i t a t i o n r a d i a t i o n . T y p i c a l o p e r a t i n g c o n d i t i o n s f o r the X-rây gun were 13 kV and 14 mA and a p r e s s u r e of 3x10 mbar i n the sample chamber. The c o n t a c t a n g l e has been measured by a common method. The h y d r o p e r o x i d e (Ρ00Η) c o n c e n t r a t i o n was determined i o d o m e t r i c a l l y a f t e r d e c o m p o s i t i o n of the o z o n i d e s w i t h e x c e s s of a l c o h o l i c sodium h y d r o x i d e . g

R e s u l t s and D i s c u s s i o n B e i n g an endothermic a l l o t r o p e of oxygen, ozone may s e r v e as a p r e c u r s o r f o r r e a c t i v e oxygen s p e c i e s such as atomic oxygen and s i n g l e t oxygen. The a b s o p t i o n of l i g h t by ozone c o n s i s t s of t h r e e bands: 200-320 nm ( H a r t l e y band), 300-360 nm (Huggins band) and 440-850 nm (Chappuis band). The p r i m a r y p h o t o c h e m i c a l p r o c e s s e s d i f f e r c o n s i d e r a b l y i n each of t h e s e bands. The quantum y i e l d o f

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ozone p h o t o l y s i s a t 254 nm ( L I ) i s n e a r l y u n i t y ( 0 . 9 * 0.2^. The main p h o t o p r o d u c t s a r e atomic oxygen 0 and s i n g l e t oxygen 0 ( Δ ) according t o the reactions (22): 2

o

+

h

+

v

3

o( h)

X

0( D) + 0 l

0

2

0

(Α

(l) (2)

2

• O + 20

3

)

λ

2

• 20 + 0

3

( A ) +

l

l

+o

(3)

2

The quantum y i e l d o f ozone d e c o m p o s i t i o n a t 334 nm (L2) i s 4, i n d i c a ­ t i n g t h a t one o f t h e p r o d u c t s must be an e x c i t e d s p e c i e s c a p a b l e o f decomposing 0^ f u r t h e r . The p r i m a r y p r o c e s s o f t h e 0^ p h o t o l y s i s a t 334 nm o c c u r s a c c o r d i n g t o t h e r e a c t i o n s : 0

+

h

3

3

» 0( P) + \

3

0( P) + 0 ^ ( ^ o r

λ

3

Δ)

• + 0

0

3

2

λ

(£or Δ

+ 0

)

(5)

2

• O^P)

W

+ 20

2

(6)

From the mechanism the o v e r a l l quantum y i e l d o f 0 d e c o m p o s i t i o n i s 4. The r e s u l t s show t h a t a l l r e a c t i o n s (1-6) must be more c o m p l i c a t e d by t h e f a c t t h a t t h e ozone consumption o c c u r s i n two s t a g e s , a v e r y f a s t p r o c e s s ( l e s s than 50 jLtsec) f o l l o w e d by a slow p r o c e s s l a s t i n g many m i l l i s e c o n d s . Upon t h e exposure o f polymer samples t o ozone and light,four a c t i v e oxygen s p e c i e s such as ozone i t s e l f , atomic oxygen , s i n g l e t oxygen and oxygen may s i m u l t a n e o u s l y r e a c t w i t h a polymer. I n t h e case o f PP f i l m s , s i n g l e t oxygen does n o t r e a c t a t a l l w i t h a polymer s u r f a c e ( 2 5 - 2 7 ) . However, ESCA measurements (9,28) support r e a c t i o n of ( Δ ) with polyolefin surfaces. The PP sample exposed t o ozone shows f o r a r a p i d f o r m a t i o n o f polymer peroxy r a d i c a l s . A s i n g l e t unsymmetric l i n e ESR spectrum ( F i g . l ) i s v e r y c h a r a c t e r i s t i c f o r oxygenated polymer r a d i c a l s ( 2 9 ) . 3

F i g u r e 1. ESR s p e c t r a o f P00" r a d i c a l s formed d u r i n g exposure of p o l y p r o p y l e n e f i l m s t o : ( ) ozone and ( ) ozone and UV light (LI).

190

CHEMICAL REACTIONS ON POLYMERS

These ESR s p e c t r a a r e i n good agreement w i t h ESR s p e c t r a o f o z o n i z e d PP p u b l i s h e d p r e v i o u s l y ( 3 0 ) . The r a p i d f o r m a t i o n o f peroxy r a d i c a l s i n d i c a t e s t h a t ozone r e a c t s w i t h PP w i t h o u t i n d u c t i o n p e r i o d . I n t h e i n i t i a l stage o f r e a c t i o n t h e h y d r o p e r o x i d e groups (POOH) c o n c e n t r a ­ t i o n i n c r e a s e s and t h e r a t e o f POOH f o r m a t i o n i s l i n e a r l y dependent on t h e ozone c o n c e n t r a t i o n ( F i g . 2 ) . A f t e r prolonged o z o n i z a t i o n t h e c o n c e n t r a t i o n o f POOH remains almost c o n s t a n t .

0.0· 1x10 *

2x10~

4x10 '

[o ](g/D 3

F i g u r e 2. K i n e t i c curve o f POOH groups f o r m a t i o n i n p o l y ­ propylene f i l m a f t e r 10 hours exposure t o ozone. The f o r m a t i o n o f POOH d u r i n g s i m u l t a n e o u s exposure o f PP f i l m s t o ozone and l i g h t ( L I o r L 2 ) can n o t be o b t a i n e d k i n e t i c a l l y . The e x p e r i m e n t a l r e s u l t s show f o r r a p i d f o r m a t i o n o f h y d r o p e r o x i d e groups which a r e p a r t i a l l y decomposed under U V - i r r a d i a t i o n . There i s no l i n e a r dependence on t h e ozone c o n c e n t r a t i o n . The a n a l y s i s o f IR s p e c t r a o f PP samples exposed t o ozone o r _^ ozone-UV i r r a d i a t i o n show f o r t h e f o r m a t i o n o f a band a t 1714 cm ( F i g . 3 ) a t t r i b u t e d t o c a r b o n y l groups. The c a r b o n y l index o b t a i n e d from t r a n s m i s s i o n and ATR s p e c t r a o f PP f i l m s U V - i r r a d i a t e d (L2) i n ozone shows t h a t t h e c o n c e n t r a t i o n o f c a r b o n y l groups a t t h e s u r f a c e i s 8 times h i g h e r than i n a b u l k . ATR s p e c t r a show t h a t oxygen groups a r e l o c a t e d a t t h e s u r f a c e t o t h e depth o f 0.6 μπι. K i n e t i c c u r v e s o f c a r b o n y l groups f o r m a t i o n i n d i f f e r e n t e x p e r i m e n t a l c o n d i t i o n s a r e shown i n F i g . 4 . The h i g h e s t c a r b o n y l groups c o n t e n t i s o b t a i n e d d u r i n g U V - i r r a d i a ­ t i o n o f PP f i l m s i n ozone. The c o n c e n t r a t i o n o f c a r b o n y l groups i s 2 t i m e s h i g h e r than i n t h e case where samples were exposed o n l y i n ozone. I t i s g e n e r a l l y w e l l known f a c t t h a t t h e c o n c e n t r a t i o n o f c a r b o n y l and hydroperoxy groups i n s t r o n g l y o x i d i z e d polymer d e c r e a ­ ses r a p i d l y w i t h d i s t a n c e from the s u r f a c e ( 3 1 ) . The ESCA method a l l o w s f o r t h e p e n e t r a t i o n o f a very t h i n l a y e r o f t h e o r d e r 1-3 nm. ESCA s p e c t r a o f PP f i l m s t r e a t e d w i t h ozone and U V - i r r a d i a t e d i n ozone a r e shown i n F i g . 5 . S u r f a c e o x i d a t i o n o f PP f i l m s i s i n d i c a t e d i n t h e f o r m a t i o n o f the 0^ peak and t h e component i n the C, band due t o o x y g e n - c o n t a i n i n g groups. These r e s u l t s a r e s i m i l a r t o t h a t r e p o r t e d p r e v i o u s l y f o r s u r f a c e o z o n i z a t i o n o f p o l y e t h y l e n e films (9). g

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100

Figure 3. Formation of carbonyl group absorption at 1714 cm polypropylene samples exposed to ozone and UV l i g h t (L2). 1

0.6-,

0

in

2

4

6

8

Time(hr )

Figure 4. Kinetics of carbonyl group formation at 1714 cm"*" i n polypropylene samples: (0) and (t) ozone and UV l i g h t (L2); (Δ) and (A) ozone only; (0) and (·) ATR spectra; (Δ) and (A) transmission spectra.

192

CHEMICAL REACTIONS ON

τ

535

1

1

1

1

Γ

530 eV

«

ι

1

290

'

1

1

285

1

1

1

1

POLYMERS

Γ

280 eV

F i g u r e 5. ESCA s p e c t r a of p o l y p r o p y l e n e f i l m s : a. pure sample; b. exposed t o atomic oxygen from p h o t o l y s i s of ozone ( L I ) ; c. exposed t o ozone o n l y ; d. exposed t o ozone and UV l i g h t ( L I ) . The 0^ band ( l O x ) f o r samples b,c,d i s p r e s e n t e d a f t e r s u b s t r a c t i o n of the 0 ^ band of the sample a. g

The main d i f f e r e n c e i s t h a t the C. /0 i n t e n s i t y r a t i o f o r samples U V - i r r a d i a t e d i n ozone i s lower than tËat observed i n samples exposed i n ozone o n l y , what i n d i c a t e h i g h e r c o n t e n t of oxygen f u n c t i o n a l groups. The t a i l s on the h i g h bonding energy s i d e s of the C^ peaks show t h a t oxygen i n c o r p o r a t e d i n t o polymer s u r f a c e i s i n the form of d i f f e r e n t f u n c t i o n a l groups. By analogy w i t h s u r f a c e o x i d a t i o n of p o l y e t h y l e n e f i l m s ( 9 ) , these t a i l s can be a t t r i b u t e d t o component peaks a t b i n d i n g energy of 286.5, 288.5 eV, c o r r e s p o n d i n g t o carbon i n C-0, C=0 and C(0)0H groups. The ozone o x i d i z e d p r o d u c t s of p o l y e t h y l e n e i n c l u d e ketone and/or e t h e r s , e s t e r s and c a r b o x y l i c a c i d s (5,10,13,18). The shape of C^ envelope i n the case of PP irradiat e d i n ozone ( F i g . 5 d ) i s s i m i f a r t o t h a t observed i n o t h e r samples exposed t o ozone o n l y ( F i g . 5 c ) or t o a t o m i c oxygen ( F i g . 5 b ) . Ί

g

The most o b v i o u s change i n an o x i d a t i o n of the PP s u r f a c e r e s u l t s i n an i n c r e a s e i n the w e t t a b i l i t y of the polymer. The PP shows a g r e a t decrease i n c o n t a c t a n g l e w i t h water or w a t e r / a l c o h o l (3:1) a f t e r treatment w i t h ozone or ozone-UV i r r a d i a t i o n or atomic oxygen ( T a b l e 1 ) .

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T a b l e 1. Change i n the w e t t a b i l i t y o f p o l y p r o p y l e n e f i l m a f t e r exposure t o ozone and ozone-UV i r r a d i a t i o n

Experimental conditions

Contact angle (degrees) H 0 2

H 0 - C H OH ( 3 : 1 ) 2

2

5

Unexposed f i l m

85.5

51.8

A f t e r ozone t r e a t ment, 8 h r s

54.4

26.7

A f t e r ozone - UV irradiation, 8 hrs

18.2

11.3

Ozone d i f f u s e s r e a d i l y i n t o amorphous r e g i o n o f t h e p o l y e t h y l e n e (32) and o x i d a t i o n p r o b a b l y o c c u r s much deeper i n t h e s o l i d sample. Ozone a l s o a t t a c k s the c r y a t a l l i n e p a r t o f p o l y e t h y l e n e b u t i t has a slow i n i t i a t i o n s t a g e f o l l o w e d by more r a p i d o x i d a t i o n ( 1 3 ) . Because ozone does n o t d i f f u s e i n t o t h e c r y s t a l l i n e r e g i o n s (13,32), o x i d a t i o n i s r e s t r i c t e d t o t h e s u r f a c e . The r e s u l t i n g o x i d i z e d f u n c t i o n a l groups on t h e c r y s t a l l i n e r e g i o n s w i l l remain a t t h e s u r f a c e , whereas those formed i n t h e amorphous r e g i o n can d i f f u s e i n t o the bulk. Our r e s u l t s show t h a t n o t o n l y ozone but atomic oxygen formed o f p h o t o l y s i s o f ozone a r e r e s p o n s i b l e f o r o x i d a t i o n o f PP f i l m s u r f a c e s . P r e f e r e n t i a l a t t a c k o c c u r s i n t h e case o f ozone on double bonds i n PP, but s i n g l e bonds a l s o y i e l d oxygen c o n t a i n i n g groups. I t i s known t h a t p o l y o l e f i n s always c o n t a i n s m a l l amount o f o l e f i n i c u n s a t u r a t i o n s o f which v i n y l i d e n e i s more i m p o r t a n t than v i n y l i n p o l y p r o p y l e n e ( 3 3 ) . These groups can be r e s p o n s i b l e f o r a f l u o r e s c e n c e observed~Tn PP (34) ( F i g . 6 ) . I t i s g e n e r a l l y a c c e p t e d t h a t t h e s e groups a r e i n v o l v e d i n p h o t o - o x i d a t i v e r e a c t i o n s . V i n y l i d e n e decrease i s a s s o c i a t e d w i t h t h e r a p i d f o r m a t i o n and decay o f h y d r o p e r o x i d e groups. The f o r m a t i o n o f c o n j u g a t e d c a r b o n y l groups i n t h e l a t t e r stage of o x i d a t i o n i n place of v i n y l i d e n e i n d i c a t e s that the v i n y l i dene decay i s a s s o c i a t e d w i t h t h e o x i d a t i o n o f the a l l y l i c groups. F l u o r e s c e n c e observed i n PP f i l m s d i s a p p e a r s c o n t i n u o u s l y under t r e a t m e n t w i t h ozone ( F i g . 6 a ) and U V - i r r a d i a t i o n i n ozone ( F i g . 6 b ) or oxygen ( F i g . 6 c ) . These r e s u l t s show t h a t almost a l l o f o l e f i n i c unsaturations are oxidized. K i n e t i c c u r v e s o f f l u o r e s c e n c e d i s a p p e a r i n g ( F i g . 7) a l l o w f o r comparison o f t h e o x i d a t i o n e f f e c t , which i s t h e h i g h e s t i n t h e case o f U V - i r r a d i a t i o n i n ozone.

w

Ο

Ο Ζ

Ο ζ

>

M

>

5

w

η Χ

14.

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Photoozonization of Polypropylene

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1.0-

QO-)

0

1

1

1

1

1

r

10

20

30

AO

50

60

Time ( min )

Figure 7. Kinetics of fluorescence disappearing at 340 nm of polypropylene films: ( O ) treated with ozone only; ( φ ) ozone and UV l i g h t (L2); ( Δ ) UV irradiated (L2) i n oxygen. Excitation wavelwngth 240 nm.

CHEMICAL REACTIONS ON POLYMERS

196

There have been numerous t h e o r e t i c a l and e x p e r i m e n t a l e f f o r t s t o e x p l a i n t h e mechanism by which ozone r e a c t s w i t h double bonds o f u n s a t u r a t e d s u b s t a n c e s Q l , 1 2 , 3 5 ) . Perhaps t h e more w i d e l y a c c e p t e d r e a c t i o n i s t h e C r i e g e e mechanism which produces t h e two groups A and Β ( a s shown below) ( 36-42):

o-o-cr

0 /V R C=CR + 0 2

2

3

I - R C-CR

R2C—CR2

2

2

•R C-00

+ R C=0

2

2

(7)

P r o d u c t A, t h e s t r u c t u r e o f which C r i e g e e l e a v e s i n doubt, i s e x t r e m e l y u n s t a b l e and q u i c k l y r e v e r t s i n t o p r o d u c t s C and D through the i n t e r m e d i a t e B. The a n a l y s i s o f p u b l i s h e d d a t a on r e a c t i o n s o f ozone w i t h low m o l e c u l a r hydrocarbons shows t h a t double bonds r e a c t w i t h ozone more q u i c k l y than s a t u r a t e d bonds ( 1 2 ) . Ozone r e a c t s w i t h s a t u r a t e d h y d r o ­ carbons i n r e a c t i o n s i n which hydrogen a b s t ^ a c t i o n 2 i s f o l l o w e d by r e - h y d r i d i z a t i o n o f t h e carbon atom form sp t o sp s t a t e (43,44):

C-H

HO*

+

products

0,

(8)

I t hag been shown t h a t t r a n s i t i o n o f a backbone carbon from t h e s p ^ t o sp s t a t e i s promoted by t e n s i l e s t r e s s e s and i n h i b i t e d by compressive s t r a i n s (10,44). The a c c e l e r a t i o n o f t h e p r o c e s s o f ozone o x i d a t i o n o f t h e polymers under l o a d i s n o t a s s o c i a t e d w i t h t h e changes i n s u p r a m o l e c u l a r s t r u c t u r e o r segmental m o b i l i t y o f t h e c h a i n . The p r o b a b l y r e a s o n o f t h i s e f f e c t i s a d e c r e a s i n g o f t h e a c t i v a t i o n energy f o r hydrogen a b s t r a c t i o n ( 4 4 ) . The mechanism o f i n i t i a l s t a g e s o f t h e r e a c t i o n o f ozone w i t h PP can be r e p r e s e n t e d as:

o +

-CH-CHI CH

ΌΗ

(rapid)

(9)

2

n

-CH-CH- + 0 2 1 3 CH 3

-CH -C9

+

o

2

+

CH -C- + 'OH ( r a p i d ) 1 0 z

CH. (POO")

(10)

14.

RABEKETAL.

197

Photoozonization of Polypropylene

The polymer peroxy r a d i c a l s (POO*) a r e d e t e c t e d by ESR s p e c t r o s c o p y ( F i g . l ) . T h i s i s p a r t i c u l a r l y i n t e r e s t i n g s i n c e n e i t h e r o f t h e proposed p r o d u c t s o f ozone c h a i n s c i s s i o n d e s c r i b e d p r e v i o u s l y i n t h e C r i e g e e mechanism a r e s u s c e o t i b l e t o d e t e c t i o n by ESR. I t can be, t h e r e f o r e , c o n c l u d e d t h a t subsequent o r a d d i t i o n a l r e a c t i o n s immediately occur a f t e r chain s c i s s i o n o r the decomposition of the z w i t t e r i o n t h r o u g h p r o t o n m i g r a t i o n t a k e p l a c e . The z w i t t e r i o n s ( r e a c t i o n 7) can be v e r y r e a c t i v e and r e a c t w i t h themselves o r o t h e r p a r t o f t h e c h a i n ( 4 2 ) . Another p o s s i b l e r e a c t i o n which c o u l d be c o n s i d e r e d i s t h a t t h e z w i t t e r i o n may s t r i p a hydrogen atom from another c h a i n . I f t h e f r e e r a d i c a l s a r e formed by a d i r e c t hydrogen a b s t r a c t i o n o r by secondary r e a c t i o n s o f these t y p e s , ESR does n o t p r o v i d e a d i r e c t measure o f bond r u p t u r e i n PP.Formation o f o x i d i z e d groups such as C=0, 00H o r COOH i s a r e s u l t o f secondary r e a c t i o n s i n which f o r m a t i o n o f polymer hydroperoxy r a d i c a l (POO*) seems t o be essential:

POO*

I

-CH-CH -

(slow)

(11)

(slow)

(12)

(slow)

(13)

2

A l l o f t h e c h e m i c a l changes t h a t r e s u l t from o z o n i z a t i o n a r e o x i d a t i v e r e a c t i o n s i n c l u d i n g t h e b r e a k i n g o f t h e polymer c h a i n . A PP sample a f t e r o z o n i z a t i o n i n t h e presence o f U V - i r r a d i a t i o n becomes b r i t t l e a f t e r 8 h r s o f e x p o s u r e , whereas t h e same e f f e c t i n ozone i s n o t i c e a b l e a f t e r 50-60 h o u r s . D e g r a d a t i o n o f polymer c h a i n o c c u r s as a r e s u l t o f d e c o m p o s i t i o n o f peroxy r a d i c a l s . The o x i d a t i o n r a p i d l y reaches s a t u r a t i o n , suggesting the surface nature o f ozone and atomic oxygen a g a i n s t o f PP as a consequence o f l i m i t e d d i f f u s i o n o f both oxygen s p e c i e s i n t o t h e polymer. Ozone r e a c t s w i t h PP m a i n l y on t h e s u r f a c e s i n c e t h e r e a c t i o n r a t e and t h e c o n c e n t r a t i o n o f i n t e r m e d i a t e peroxy r a d i c a l s a r e p r o p o r t i o n a l t o t h e s u r f a c e a r e a and n o t the weight o f t h e polymer. I t has been found t h a t p o l y e t h y l e n e i s a t t a c k e d o n l y t o a depth o f 5-7 m i c r o n s ( 4 5 ) . Atomic oxygen o x i d a t i o n o f polymers has been r e p o r t e d by a few a u t h o r s ( 4 6 , 5 0 ) . Experiments were l i m i t e d t o the measurements o f w e i g h t - l o s s d a t a and changing o f the w e t t e a b i l i t y ( 4 6 - 4 8 ) , and o n l y two papers were devoted t o t h e study mechanism o f atomic oxygen o x i d a t i o n of polydienes (49,50). The PP samples exposed t o atomic oxygen show f o r t h e f o r m a t i o n o f polymer peroxy r a d i c a l s (POO*), which g i v e almost i d e n t i c a l ESR spectrum as i n t h e case o f ozone r e a c t i o n ( F i g . l ) . The ESCA s p e c t r a ( F i g . 5 ) i n d i c a t e t h a t atomic oxygen o x i d a t i o n i s more e f f e c t i v e than o z o n i z a t i o n . These r e s u l t s s u p p o r t our assumption t h a t o z o n i z a t i o n

198

CHEMICAL REACTIONS O N POLYMERS

of PP f i l m i n the presence o f U V - i r r a d i a t i o n i s probably a r e s u l t o f s i m u l t a n e o u s a t t a c k o f atomic oxygen, m o l e c u l a r oxygen (which i s p r e ­ sent i n e x c e s s i n ozone-oxygen m i x t u r e ) and ozone (which was n o t c o m p l e t e l y p h o t o l y s e d ) . Oxygen atoms were g e n e r a t e d by p h o t o l y s i s o f ozone, u s i n g t h e 253,7 nm l i n e from L I lamp. The i n i t i a l l y formed e x c i t e d oxygen atoms 0( D) a r e r a p i d l y d e a c t i v a t e d t o t h e ground s t a t e atomic oxygen 0, by c o l l i s i o n s w i t h t h e e x c e s s o f ozone mole­ c u l e s p r e s e n t . I n t h i s method i t i s i m p o s s i b l e t o s e p a r a t e both s p e c i e s , a t o m i c oxygen and ozone. mer

Mechanism o f t h e r e a c t i o n s which a r e b e l i v e d t o o c c u r when a p o l y ­ such as PP i s exposed t o atomic oxygen a r e f o l l o w i n g : H I -CH -Cζ ι CH^

+

0

— • -CH -C- + Zj CH^

"OH

(rapid)

(14)

(rapid)

(15)

(rapid)

(16)

(rapid)

(17)

OH -CH -C2

CH

-CH -C2

CH

+

*0H — - C H

2

- Ç CH

3

+

"OH — •

3

-CH=CCH

3

+ H 0 2

3

00"

I -CH -Ç-

+

2

CH

0

^

2

-CH -C2

CH

3

3

(POO*)- d e t e c t e d

-CH=C-

+

0

3

— •

products according reaction 7

POO"

+ PH

— • POOH

POOH

— •

P0"

+

+

"OH

P"

by ESR

spectroscopy

(rapid)

(18)

(slow)

(19)

(slow)

(20)

Slow r e a c t i o n which also occur during photo-oxidation and/or thermal o x i d a t i o n can take p l a c e d u r i n g o x i d a t i o n w i t h atomic oxygen, but t h e s e slow r e a c t i o n s a r e o f l i t t l e importance because o f t h e r a p i d o x i d a t i o n which u s u a l l y o c c u r s . More r e s u l t s which e x p l a i n atomic oxygen o x i d a t i o n mechanism o f PP, w i l l be p u b l i s h e d s e p a r a t e l y . Acknowledgnent. These investigations are part of a research program on the environmental degradation of polymers supported by the Swedish National Board for Technical Developments (SIU), which we gratefully acknowledge.

14.

RABEK ET AL.

Photoozonization of Polypropylene

199

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