The Effects of Hostile Environments on Coatings ... - ACS Publications

17 Environmental Corrosion of Polymers Causes and Main Reactions B. RÅNBY and J. F. RABEK

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 22, 2016 | http://pubs.acs.org Publication Date: September 29, 1983 | doi: 10.1021/bk-1983-0229.ch017

The Royal Institute of Technology, Department of Polymer Technology, Stockholm, Sweden

Air pollutants such as particulate matter, sulfur oxides, nitrogen oxides, hydrocarbons and photochemical oxidants such as ozone, atomic oxygen, singlet oxygen and peroxyacetylnitrate may cause serious corrosion of polymeric materials: rubber,plastics and fibres. The paper reviews the problem of environmental corrosion of polymers considering the effects and mechanisms of corrosion by the particular air pollutants. Protection and stabilization of rubber and plastics against these degradation processes is of great importance from an industrial point of view. It also affects the saving of raw materials and energy. Polymers more resistant to environmental corrosion mean longer lifetime which decreases the necessary production. A i r p o l l u t i o n i s u s u a l l y defined as the presence i n the outdoor atmosphere, o f substances put there d i r e c t l y or i n d i r e c t l y by an a c t o f man, i n amounts which are detrimental to h e a l t h and s a f e t y o r i n t e r f e r e with the fulJ._use o f m a t e r i a l s o r p r o p e r t y , e.g. made from polymeric m a t e r i a l s . Man i s not the only agent able to p o l l u t e the atmosphere. There are many n a t u r a l processes that do so such as p o l l i n a t i o n o f p l a n t s , v o l c a n i c emptions, dust stormes and f o r e s t f i r e s . There are a l s o secondary p o l l u t a n t s i n the atmosphere, formed i n the a i r from primary p o l l u t a n t s , due to acts by man, e.g. smoke, i n d u s t r i a l p o l l u t a n t s and photochemical smog formed i n the a i r from substances emitted from automobile exhaust and other sources. The major a i r p o l l u t a n t s a r e P a r t i c u l a t e matter. There i s a l a r g e v a r i e t y o f such m a t e r i a l s i n the a i r , p r i m a r i l y s o l i d matter r a t h e r than l i q u i d . The p a r t i c l e s come from a v a r i e t y o f sources, they a r e o f d i f f e r e n t s i z e , shape, c o l o r , texture, and chemical composition. They can remain suspen-

0097-6156/ 83/ 0229-0291 $06.00/ 0 © 1983 American Chemical Society

In The Effects of Hostile Environments on Coatings and Plastics; Garner, David P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

292

EFFECTS OF HOSTILE ENVIRONMENTS

ded i n the a i r f o r p e r i o d s ranging from a few seconds to a few years. S u l f u r o x i d e s , i n the form o f s u l f u r d i o x i d e ( S 0 ) s u l f u r t r i o x i d e (S0«), s u l f u r i c a c i d (H^SO^) and s u l f a t e s (SO^). Host of these p o l l u t a n t s are emitted to the atmosphere as s u l f u r d i o x i d e , which i s c h e m i c a l l y converted by o x i d a t i o n to S0~ and s u l f a t e s i n the a i r . Both p a r t i c u l a t e matter from smoke ana s u l f u r oxides are known to be components i n the smogs of London. They are mainly the r e s u l t of combustion of c o a l . Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 22, 2016 | http://pubs.acs.org Publication Date: September 29, 1983 | doi: 10.1021/bk-1983-0229.ch017

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Carbon monoxide (CO) i s produced p r i m a r i l y by the combustion of g a s o l i n e i n automobile engines. Nitrogen o x i d e s , i n the form o f n i t r i c oxide (NO) and n i t r o g e n d i o x i d e ( î ^ ) . Both gases are b y p r o d u c t s from combustion o f f u e l , at h i g h temperatures causing chemical combinations o f n i t r o g e n and oxygen. The products are i n g r e d i e n t s i n the subsequent product i o n of photochemical smog. Hydrocarbons, which are a l a r g e c l a s s o f chemical compounds mainly formed by incomplete burning o f f u e l and i n v o l v e d i n the smog p r o d u c t i o n . Photochemical o x i d a n t s , which are c h e m i c a l l y o r photochemically formed i n the a i r . The s t a b l e oxidants are ozone ( 0 ^ ) , and peroxya c e t y l n i t r a t e (PAN), the unstable are atomic oxygen (0) and s i n g l e t oxygen ( 0^). Photochemical oxidants are mainly produced d u r i n g power gener a t i o n , i n i n d u s t r i a l operations and are a l s o from n a t u r a l sources. The s i n g l e t oxygen (3-6) i s to a l a r g e e x t e n t r e l a t e d to automobile exhaust, which i s the main cause of the photochemical smog i n the Los Angeles a r e a . T h i s c l a s s i f i c a t i o n (1-6) o f a i r p o l l u t a n t s has been l e g a l l y e s t a b l i s h e d by the Environmental P r o t e c t i o n Agency (USA) as the N a t i o n a l Ambient A i r Q u a l i t y Standards under the terms of the Clean A i r A c t . A i r p o l l u t a n t s (1-6) are mainly the r e s u l t of burning o f v a r i o u s types of f u e l s : c o a l , o i l , and gas f o r h e a t i n g of b u i l d i n g , generation o f e l e c t r i c i t y and g a s o l i n e f o r t r a n s p o r t e t c . The use o f g a s o l i n e and d i e s e l f u e l produces some of the p a r t i c u l a t e matter, much o f the hydrocarbons and n i t r o g e n o x i d e s , and hence much of the photochemical smog, and almost a l l the carbon monoxide. Coal and o i l combustion i n i n d u s t r y and l o c a l h e a t i n g produces most o f s u l f u r oxides and p a r t i c u l a t e s . N a t u r a l gas i s the " c l e a n e s t " f u e l . Combustion o f t h i s gas produces some n i t r o g e n o x i d e s , but not as much as the burning o f g a s o l i n e i n automobile engines. I n d u s t r i a l processes produce o n l y h a l f as much p a r t i c u l a t e matter as f u e l combustion, one f o u r t h o f the SO^, and s m a l l e r port i o n s o f the other a i r p o l l u t a n t s . The p o l l u t i o n caused by i n d u s t -


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293

Environmental Corrosion

ry i s l e s s s i g n i f i c a n t than p o l l u t i o n from f u e l combustion i n automobiles. Having d e f i n e d the v a r i o u s a i r p o l l u t a n t s , we now c o n s i d e r b r i e f l y the types of e f f e c t s which are p o s s i b l e i n environmental c o r r o s i o n o f polymers.


Mechanical c o r r o s i o n o f polymers by p a r t i c u l a t e matter The p a r t i c l e s i n the a i r over an urban area a r e of a v a r i e t y of s i z e s , shapes and chemical composition, ranging from t i n y , s p h e r i c a l metal p a r t i c l e s from m e t a l l u r g i c a l fumes to huge, porous conglomerates of sooty carbon, s o i l p a r t i c l e s , f l y ash, and f l y dust of a l l types. The s i z e and shape of the p a r t i c l e s w i l l a l most t o t a l l y determine the s u r f a c e c o r r o s i o n o f polymers, p l a s t i c s , the polymer i n p a i n t s and l a c q u e r s . The important q u e s t i o n i s how these p a r t i c l e s behave i n the a i r , how f a r and how f a s t the wind c a r r y them, and what e f f e c t s they can have on polymeric materials. P a r t i c l e s l a r g e r than about ten microns are emitted i n t o the a i r p r i m a r i l y by p h y s i c a l processes, such as g r i n d i n g and abras i o n , and f l a s h from f u e l combustion. The p a r t i c l e s tend to s e t t l e out onto the ground (or on the s u r f a c e of polymeric m a t e r i a l s ) r a t h e r r a p i d l y . These are the p a r t i c l e s gathered as d u s t f a l l . I n environmental p o l l u t i o n c o n t r o l the d u s t f a l l m a t e r i a l i s weighed and reported as tons per square m i l e per month o r , i n m e t r i c u n i t s , as grams per square meter per day. D u s t f a l l p l a y s a fundamental r o l e i n the a b r a s i v e c o r r o s i o n of polymer s u r f a c e s but i t a l s o decreases the photochemical processes (degradation, c r o s s l i n k i n g and o x i d a t i o n processes) by s c a t t e r i n g and a b s o r p t i o n o f harmful sun i r r a d i a t i o n . U n l i k e d u s t f a l l , p a r t i c l e s smaller than about ten microns i n s i z e remain suspended i n the a i r f o r long periods o f time. Turbul e n t motion i n the a i r i s able to keep them a l o f t more o r l e s s permanently. These p a r t i c l e s may a l s o cause a slow and very f i n e s u r f a c e c o r r o s i o n o f the polymeric m a t e r i a l s . They a l s o absorb and s c a t t e r sun i r r a d i a t i o n , but they do not p r o t e c t polymer s u r f a c e s a g a i n s t harmful exposure to sun i r r a d i a t i o n l i k e l a r g e p a r t i c l e s . The f i n e s t p a r t i c l e s , those below 0.1 micron i n s i z e , are so small that they have no v i s i b l e e f f e c t on the c o r r o s i o n o f a polymer s u r f a c e . The c l e a n e s t a i r almost always contains a t l e a s t s e v e r a l hundred such p a r t i c l e s per c u b i c centimeter, and i n p o l l u ted urban a i r t h i s may reach 100,000 per c u b i c centimeter. Even at these l a r g e numbers, the small p a r t i c l e s d o n t have c o r r o s i o n e f f e c t and they can be ignored. There i s a s p e c i a l k i n d o f p a r t i c u l a t e matter so c a l l e d "black smoke" that cover polymer surfaces very t i g h t l y and i s d i f f i c u l t to remove. Black smoke may c o n t a i n s t a b l e unpaired e l e c trons ( f r e e r a d i c a l s ) which may p l a y a s p e c i f i c r o l e i n photostab i l i z a t i o n o f polymeric m a t e r i a l s by scavenging f r e e r a d i c a l s formed from p h o t o l y s i s of polymers. f


294


The amount of p a r t i c u l a t e matter emitted i n t o the atmosphere amounts to m i l l i o n s o f tons each year as reported f o r USA (Table I ) . Corresponding data f o r Sweden are not a v a i l a b l e . Table I .

Annual emissions of p a r t i c u l a t e matter (estimates f o r 1970 i n USA)


6

Emissions, m i l l i o n s Source category o f tons per year Fuel combustion Transportât i o n Gasoline motor v e h i c l e s 0.3 Diesel a i r c r a f t , trains, vessels 0.3 0.1 Off-highway v e h i c l e s Total transportation 0.7 S t a t i o n a r y sources Coal 5.6 Fuel O i l 0.4 N a t u r a l gas 0.2 Wood 0.6 T o t a l s t a t i o n a r y source 6.8 T o t a l f u e l combustion I n d u s t r i a l processes A g r i c u l t u r a l burning S o l i d waste d i s p o s a l Miscellaneous Total

7.5 13.3 2.4 1.4 1.5 26.1

Percent of t o t a l

1.1 1.2 0.4 2.7 21.5 1.5 0.8 2.3 26.1 28.8 51.0 9.2 5.3 5.7 100.0

I t i s apparent that the b u l k o f t h i s p a r t i c u l a t e matter comes from f u e l combustion sources, p r i m a r i l y e l e c t r i c - p o w e r p l a n t s and heating b o i l e r s , and from i n d u s t r i a l processes. T h i s f a c t should be taken i n t o c o n s i d e r a t i o n o f s u r f a c e c o r r o s i o n and/or covering by the d u s t f a l l l e v e l s i n the o l d i n d u s t r i a l i z e d c i t i e s , whereas values approaching 2000 tons/mi month are found i n heavy i n d u s t r i a l r e g i o n s . In the outermost suburbs o f a b i g c i t y the d u s t f a l l would be 5 tons/mi^ month. At r e l a t i v e l y low v e l o c i t i e s , s m a l l p a r t i c l e s h i t t i n g a p l a s t i c m a t e r i a l with impact w i l l abrade the s u r f a c e and a l t e r i t s p r o p e r t i e s . At some c r i t i c a l v e l o c i t y , e.g. o f a v e h i c l e o r blowing a i r the target s u r f a c e i s penetrated and the p a r t i c l e s are deformed o r break up as they pass i n t o the m a t e r i a l . Penetrat i o n of the p l a s t i c surfaces by the p a r t i c u l a t e matter i s u s u a l l y accompanied by the formation o f a deep c y l i n d r i c a l c a v i t y o f the target p l a s t i c . At h y p e r v e l o c i t i e s (e.g. i n hypersonic a i r c r a f t o r r o c k e t s ) , the impact o f p a r t i c l e matter on a p l a s t i c s u r f a c e t r a n s f e r s a tremendous amount o f k i n e t i c energy w i t h i n a small r e g i o n . T h i s energy i s p a r t i t i o n e d i n t o heat, l i g h t , e x p l o s i v e mass e j e c t i o n , and shock wave generation^. The temperatures are of s u f f i c i e n t



17.

RAN BY AND RABEK


295

magnitude to produce l o c a l i z e d m e l t i n g thermal degradation and even complete p y r o l y s i s of the polymer. The magnitude and modes of damage to p l a s t i c s by p a r t i c u l a t e matter impact i s dependent upon a number of parameters, such as mass, s i z e , shape, v e l o c i t y , p h y s i c a l s t a t e and t r a j e c t o r y of the p a r t i c l e . The d e n s i t y , hardness, s t r e n g t h , sonic v e l o c i t y , c r y s t a l s t r u c t u r e , and other p r o p e r t i e s of the m a t e r i a l s are a l s o s i g n i f i c a n t . Soft and rubbery p l a s t i c s have higher s p a l l a t i o n t h r e s h o l d and l e s s c r a t e r damage than r i g i d p l a s t i c s . Greater p e n e t r a t i o n frequency i s a l s o obtained with the impact of small p a r t i c l e s on s o f t p l a s t i c s or a b l a t i n g p l a s t i c s c o n t a i n i n g a molten surface l a y e r . P l a s t i c composites c o n t a i n i n g many i n t e r faces are b e t t e r able to withstand impulsive s t r e s s of s h o r t magn i t u d e , because they d i s t r i b u t e energy over a wide area. Punctures, c r a t e r s , cracks and other types of mechanical c o r r o s i o n which are noted i n p a r t i c l e impact on p l a s t i c s may cause s e r i o u s degradation and even r e s u l t i n c a t a s t r o p h i c f a i l u r e of the m a t e r i a l . C o r r o s i o n of polymers by s u l f u r oxides S u l f u r oxides (SO ) i n c l u d e not only S 0 but a l s o s u l f u r t r i o x i d e (SO^). S u l f u r i c a c i d mist and s u l f a t e s may a l s o be d e r i v e d from s u l f u r oxides, but they are commonly not d e f i n e d as p a r t of SO . Most of the s u l f u r contamination i s emitted i n the form of s u f f u r d i o x i d e with about 1 to 3 percent of s u l f u r t r i o x i d e mixed in. In the a i r , S0~ r e a c t s with oxygen, ammonia, and other compounds, i n c l u d i n g tne water vapor present i n a i r , to form s u l f u r i c a c i d mist, l i q u i d drops of concentrated a c i d , as w e l l as v a r i o u s other s u l f a t e s , The a c i d a e r o s o l e s a t t a c k polymer s u r f a c e s and cause l u s t r e l e s s n e s s of the s u r f a c e s . The main r e a c t i o n o| S0« with hydrocarbons i s formation of sulphonic a c i d s (RSO^H) . un the other hand S0« shows photochemical r e a c t i v i t y under sun i r r a d i a t i o n (up to 388 ran). The e x c i t e d form of SO2 r e a c t s with s e v e r a l polymejg_|ueh as p o l y o l e f i n s , p o l y s t y r e n e , rubbers, nylons and a c r y l a t e s . The main r e a c t i o n s are p h o t o s u l f o n a t i o n accompanied by f r e e r a d i c a l o x i d a t i o n and chain s c i s s i o n and/or c r o s s l i n k i n g r e a c t i o n s . I t has a l s o been reported that the mixture SO2-O2 absorbs l i g h t e f f i c i e n t l y below 300 nm and a substant i a l amount of s i n g l e t oxygen 0 i s produced with a quantum y i e l d of 0 > . 2

ό

PH + · Ν 0

o

2

b

Ë*.P* + HN0

2

(1)

The polymer a l k y l r a d i c a l (P*) formed can f u r t h e r r e a c t with molecular oxygen and produce polymer peroxy r a d i c a l s ( c f . Sect i o n 7) . N i t r o x y r a d i c a l s may a l s o reae|ywith unsaturated bonds i n polymers according to the r e a c t i o n s :



298 *NO

(2)

*>m + 0

2

-CH = CH- + NO + 0 -

-CH-CH-

(3)

I i Ν

0

0 CH-CH- (4)

-CH = CH- + N 0 — • -CH-CHo


2

i i

ι · N0

Ν

o

0

0 -CH-CH-

+

-CH = CH-

I i Ν

*-CH-CH-

/

\

0 = Ν·

0

\

I! '

(5) 0

/

-CH-CH-

0

Formation of n i t r o x i d e and iminoxy r a d i c a l s i s enhanced by u l t r a v i o l e t r a d i a t i o n . This i n d i c a t e s that the photochemical d i s s o c i a t i o n of NO2 i n t o n i t r i c oxide and oxygen atoms plays an im portant r o l e i n the r e a c t i o n with double bonds. Photochemical smog The photochemical smog i n the atmosphere i s a r e s u l t o f che m i c a l and photochemical r e a c t i o n s ^ n ^ g h e a i r between oxides of n i t r o g e n , oxygen and hydrocarbons . The photochemistry of smog i n v o l v e s very complicated r e a c t i o n s which are i n t e n s i v e l y i n v e s t i g a t e d by s e v e r a l l a b o r a t o r i e s round the world. In the absence o f hydrocarbons, the c y c l e i n i t i a t e d by N0 c o n s i s t s of three r e a c t i o n s : 1. N i t r o g e n oxide has an unpaired e l e c t r o n and i s a brown gas. I t absorbs s o l a r energy and i s d i s s o c i a t e d i n t o one molecule of NO and one f r e e oxygen atom, 2

•N0

2

+ hv

*Ν0·+ 0

(6)

2. The f r e e oxygen atom r e a c t s with an oxygen molecule ( 0 ) to form ozone ( 0 ^ ) , 2

0 + 0

2

*0

(7)

3

Ozone absorbs s o l a r energy and i s e a s i l y photolysed i n t o a t o mic oxygen (0), s i n g l e t oxygen ( 0 ) and molecular oxygen ( 0 ) : 9

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17. RAN BY AND RABEK

299

3

3

0

3

+ hv (11,800 A)

* 0 ( Ig) + 0

0

3

+ hv (6200 Â)

* 0 ( Δ ) + 0

0

3

+ hv(4600 Â)

• 0 ( Σ~) + 0

1

3

0

1

1

1

(10)

2

+ 0

(3

0 V

+ hv (4000 A)

»

+ hv(2600 Â)

* 0

2

l

1

+

(Σ) ζ g

j Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 22, 2016 | http://pubs.acs.org Publication Date: September 29, 1983 | doi: 10.1021/bk-1983-0229.ch017

(9)

2

3

0

(8)

2

+ 0

(12)

3 i s molecular oxygen i n the ground s t a t e *Δ

a n c

* \g

(triplet).

are two forms of e x c i t e d states

(singlets).

3. An ozone molecule may a l s o r e a c t with a NO molecule produced i n r e a c t i o n ( 6 ) , which forms n i t r o g e n dioxide again: 0

+ NO

3

*0

2

+ N0

(13)

2

In the presence of hydrocarbons these r e a c t i o n s become more complex: Ν0

χ

( W

+ hv

2

cycle { 0 + 0

* N0- + 0

(14)

*0

(15)

+ M

2

+ M

3

0

3

+ NO-

*N0

0

3

+ NO^

*N0^ + 0Λ produces (17) L n i t r i c acid

N 0

3

+

N 0

2""F0

+ 0

2

2 H N 0

(16)

2

(

3J

N0-+ N0:-T7-^ 2HN0 z " ^ ^

1

8

)

n i t r o u s a c i d (19)

o

2

HN0 + hv

*Ν0·+·0Η

2

CO +-0H

^C0

H0

*N0£ + Ό Η

2

+ NO

HC- + 0

2

HC- + 0

3

2

+ HO^

(20) CO e f f e c t (21) produces an OH chain (22)

*R0 -

(23)

2

^R0 - + RCHO aldehyde (24) and peroxy radicals

HC* + OH + 0

2

2

^ R 0 * + RCHO 2

(25)



300 HC- + R 0

*-R0 ' + RCHO

2

(26)

2

H0 * + NO

*N0« + Ό Η

2

o x i d a t i o n o f NO to (27) N0 2

R0

2

+ N0

»PAN

2

production of peroxy-/0 \2S) acetylnitrate / || I RC00N0


2

S e v e r a l oxygen j p e c i e s such as atomic oxygen ( 0 ) , ozone (0^) and s i n g l e t oxygen ( 0 ) which are formed i n a photochemical smog are very r e a c t i v e with polymers ( c f . S e c t i o n on o x i d a t i o n ) . 2

P h o t o s e n s i t i z e d r e a c t i o n by hydrocarbons The amounts o f hydrocarbons emitted i n t o the a i r are m i l l i o n s of tons each year as estimated f o r USA. (Table I V ) . Corresponding data f o r Sweden are not a v a i l a b l e .

Table IV. Annual emissions o f hydrocarbons (estimates f o r 1970 i n USA) Emissions, m i l l i o n s Percent Source category of tons per year of t o t a l F u e l combustion Transportation Gasoline motor v e h i c l e s Diesel, a i r c r a f t , trains, vessels Off-highway v e h i c l e s Total transportation S t a t i o n a r y sources Coal Fuel O i l N a t u r a l gas Wood T o t a l s t a t i o n a r y source T o t a l f u e l combustion I n d u s t r i a l processes A g r i c u l t u r a l burning S o l i d waste d i s p o s a l Miscellaneous Total

16.6 0.9 2.0 19.5

47.6 2.6 5.7 55.9

0.2 0.1 0.3

0.6 0.3 0.8

0.6

-

-1.7

20.1 9.5 2.8 2.0 0.5 34.9

57.6 27.2 8.0 5.7 1.5 100.0

Organic f r e e r a d i c a l s formed from hydrocarbons i n a photo chemical smog (prev. S e c t i o n ) may i n i t i a t e f r e e r a d i c a l o x i d a t i o n of polymers ( c f . next S e c t i o n ) .


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301


Some hydrocarbons emitted are more a c t i v e than o t h e r s , espe c i a l l y p o l y c y c l i c aromatic hydrocarbons, which even i n small amounts may p h o t o s e n s i t i z e formation o f s i n g l e t oxygen ( 0^) by an energy t r a n s f e r mechanism: Hydrocarbons + hv

»(hydrocarbons)


*) means e x c i t e d s i n g l e t and/or t r i p l e t molecules. * (hydrocarbons) + 0

2

(29)

s t a t e o f the hydrocarbon

+ hydrocarbons +

1 0

2

(30)

The p o l y c y c l i c aromatic hydrocarbons a r i s e from f o s s i l f u e l combustion, but t h e i r annual average c o n c e n t r a t i o n i s r a t h e r low, e.g.anthracene 0.05-10.0 nanograms (10 ) per cubic meter. I t has been reported that aromatic p o l y n u c l e a r hydrocarbons are s e n s i t i z e r s and a c e ^ l e ^ a t e p h o t o o x i ^ t i o n o f d i f f e r e n t p o l y mers, e.g. p o l y o l e f i n s and rubbers Free r a d i c a l o x i d a t i o n processes Organic f r e e r a d i c a l s (R.) which are formed i n photochemical smog r e a c t w i t h oxygen and produce peroxy r a d i c a l s (R0 *) which are r a t h e r l o n g l i v e d and may s e l e c t i v e l y a b s t r a c t hydrogen from polymers (PH) and produce polymer r a d i c a l s (Ρ·)· 2

R* + 0

2

R0 - + PH 2

*R0 2

*R00H + P-

(31) (32)

Polymer a l k y l r a d i c a l s (Ρ·) add molecular oxygen to produce polymer peroxy r a d i c a l s (Ρ00·) which may f u r t h e r a b s t r a c t hydrogen from the same and/or neighbouring polymer molecule. In t h i s way a c y c l e o f f r e e r a ^ c ^ J o x i d a t i o n s occurs and i s known as "autox o d i a t i o n process ' : 11

p. + o

2

POO* + PH

*>Ρ00· ^POOH + P-

(33) (34)

Oxygen r e a c t s f a s t w i t h a l l types o f a l k y l and a l l y l polymer r a d i c a l s formed. The o x i d a t i o n may cause the polymeric m a t e r i a l s to c r o s s l i n k which causes hardening or shrinkage and may show up as s t r e s s c r a c k i n g . The a u t o x i d a t i o n may degrade the macromolec u l e s to lower molecular weight and v o l a t i l e products, causing volume decrease o r s t r e s s c r a c k i n g o f the polymer. I t may a l s o lead to a c i d i c products which w i l l d i s c o l o u r the m a t e r i a l or b r i n g about f u r t h e r degradations.



302


O x i d a t i o n processes with atomic oxygen, ozone and s i n g l e t oxygen The photochemical smog contains s e v e r a l oxygen s o e c i e s such as atomic oxygen (0), ozone (0^) and s i n g l e t oxygen ( 0^) which are h i g h l y r e a c t i v e with s e v e r a l polymers. The c o n c e n t r a t i o n o f 0 and 0j i s low and t h e i r e f f e c t i s considered to be l e s s important i n the t o t a l c o r r o s i o n o f polymers than the e f f e c t s o f ozone, n i t r i c and s u l f u r i c a c i d s . Atomic oxygen (0) r e a c t s r a p i d l y with s e v e r a l polymers (PH) by a b s t r a c t i o n o f hydrogen and formation o f polymer o x y r a d i c a l s (P0-) ' ' PH + 0 p. + o

** P- + -OH 2

(35)

— * P0£

(36)

H i g h l y branched polymers such as polypropylene and polymers with other l i n k s , e.g. poly(oxymethylene), are most r e a d i l y attacked by atomic oxygen. P e r f l u o r i n a t e d polymers, rubber v u l c a n i z e d with s u l f u r , and h i g h l y aromatic polymers are the most r e s i s t a n t . Ozone (O3) p l a y s a s i g n i f i c a n t r o l e i n o x i d a t i o n and degrad a t i o n o f s e v e r a l p^|ym|r^s^çh^gs: p o l y o l e f i n s , rubber, p o l y s t y rene and polyamides . Ozone may a b s t r a c t hydrogen and form polymer p e r o x y r a d i c a l s (POO*): 9

PH + 0

9

9

1» POO* + -OH

3

(37)

or add to double bonds with formation o f ozonides, which are uns t a b l e and e a s i l y decomposed i n t o aldehydes and a c i d s : 0

-CH=CH- + 0

/ 0

\ 0

I

I

0

\

* - C - C-

o

H

-C

H r

e

a

HO

I +

H c

t

C-

(38)

0

s

S i n g l e t oxygen (^ί^) mainly with unsaturated polymers by "enetype" r e a c t i o n s with |^rm|t|gn o f hydroperoxy groups and s h i f t i n g o f the double bond ' : 9

UUH. l

-CH -CH=CH- + 0 2

2

*

-CH=CH-CH-

(39)

As a r e s u l t o f these o x i d a t i o n r e a c t i o n s polymeric m a t e r i a l s g r a d u a l l y lose t h e i r u s e f u l mechanical p r o p e r t i e s . During the o x i d a t i o n degradation (mainly by b e t a - s c i s s i o n processes) and c r o s s l i n k i n g r e a c t i o n s occur.


17.

RÂNBY AND RABEK


Non-oxidative


303

processes

In p o l l u t e d a i r other degradative r e a c t i o n s i n the absence of oxygen may a l s o occur. The presence o f water, e s p e c i a l l y a c i d i c water, may b r i n g about h y d r o l y t i c r e a c t i o n s and, i f oxygen a l s o i s present, may increase the o x i d a t i o n r e a c t i o n s . A l l these r e a c t i o n s may be going on simultaneously and i n the most complicated manner. The r e s u l t i n g lower molecular weight u n i t s can cause changes i n flow, shape, strength and other mechanical p r o p e r t i e s . P h y s i c a l changes may r e s u l t from d i s t o r t i o n o f the polymeric m a t e r i a l by s u n l i g h t due to photodegradation and p h o t o c r o s s l i n k i n g and can be p h o t o s e n s i t i z e d by the presence o f i m p u r i t i e s on the surface which e f f i c i e n t l y absorb l i g h t . Energy t r a n s f e r processes may p l a y a considerable r o l e . D i f f u s i o n and m i g r a t i o n o f a d d i t i v e s from the polymer matrix i n t o a surface may have both a c c e l e r a t i n g or s t a b i l i z i n g e f f e c t s . The s i d e e f f e c t may be a hardening o f the m a t e r i a l from the l o s s o f p l a s t i c i z e r s or from the l o s s o f other low molecular a d d i t i v e s or even water, which can act as p l a s t i c i z e r s . D i f f e r e n t i a l expansion of organic and i n o r g a n i c a d d i t i v e s can r e s u l t i n c r a c k i n g o f the polymeric m a t e r i a l s . These r e a c t i o n s may add to the r e a c t i o n s caused by p o l l u t e d atmosphere. They are p a r t i c u l a r l y p o s s i b l e a t temperatures near those where changes or property t r a n s i t i o n s occur ( f o r example at s o f t e n i n g and m e l t i n g p o i n t s o r b r i t t l e p o i n t s o f the m a t e r i a l s ) . S e r v i c e L i f e o f Polymeric

M a t e r i a l s i n P o l l u t e d Atmosphere

The d e t e r i o r a t i o n o f p l a s t i c s and other polymeric ma^egj.a^ i n p o l l u t e d atmosphere takes place i n a number o f stages ' : 1. 2. 3. 4. 5. 6.

Loss o f g l o s s Minute c r a z i n g Severe c r a z i n g and c r a c k i n g Leaching Loss of reinforcement from the s t r u c t u r e Gradual breakup

The timescale f o r these processes to occur i s dependent on the type of polymer, the content of added m a t e r i a l s , p l a s t i c i z e r s and a d d i t i v e s such as a n t i o x i d a n t s , p h o t o s t a b i l i z e r s , thermostab i l i z e r s , e t c ) , f i l l i n g and r e i n f o r c i n g m a t e r i a l s such as p a r t i c les and f i b e r s , e t c . The f i r s t observed e f f e c t i s the l o s s o f surface g l o s s which i s an obvious s i g n o f d e t e r i o r a t i o n o f a polymeric m a t e r i a l . The gloss can be l o s t by a b r a s i o n from p a r t i c u l a t e matter (sometimes i n r a i n ) , surface l e a c h i n g and minute and severe c r a z i n g . A f t e r extended exposure times the polymeric m a t e r i a l s may l o s e t h e i r mechanical p r o p e r t i e s f o r p r a c t i c a l use.



304

D i s c o l o r a t i o n of polymeric m a t e r i a l s i s u s u a l l y the r e s u l t of complicated photochemical r e a c t i o n s and the e f | e c t of surface r e a c t i o n with S0 , N0 , * s i n g l e t oxygen ( 0 >. The s e r v i c e l i f e of p l a s t i c s and other polymeric m a t e r i a l s i s a p a r t i c u l a r problem i n Los Angeles and Tokyo because of the spe c i a l c l i m a t i c c o n d i t i o n s f o r formation of photochemical smog. T h i s s i t u a t i o n i s by no means confined to the USA and Japan. Photoche m i c a l smog and environmental c o r r o s i o n of polymers i s commonly observed i n some European c i t i e s , notably Madrid, Athens and London. In Stockholm where a i r p o l l u t i o n i s at a low l e v e l these problems are p r e s e n t l y not seen so s e r i o u s . 3 η α


2

2

2

Conclusions The c o n c e n t r a t i o n of a i r p o l l u t a n t s i s very important f o r the longterm p r o p e r t i e s of polymeric m a t e r i a l s . One would expect that the problem of environmental c o r r o s i o n of polymers i s n e g l i g i b l e i n comparison with the p h y s i c a l , t h e r m a l and photochemical ageing of polymers. The t a b l e s I-IV show that annual emission of p o l l u t a n t s i n t o the atmosphere i s tremendous, and the e f f e c t s on polymers of i n d i v i d u a l p o l l u t a n t s ( p a r t i c l e s , s u l f u r oxides, n i t r o g e n oxides, hydrocarbons, oxygen compounds, a c i d r a i n , etc) should be taken i n t o s e r i o u s c o n s i d e r a t i o n and be the subject of more d e t a i l e d research. The cost of environmental c o r r o s i o n must i n c l u d e the d e t e r i o r a t i o n of such d i v e r s e o b j e c t s as polymeric b u i l d i n g m a t e r i a l s , outdoor e l e c t r i c a l l i n e s with polymeric i n s u l a t i o n , rubber t i r e s , painted s u r f a c e s , e t c . The c o s t can only be estimated and no d e f i n i t e f i g u r e s are a v a i l a b l e . A d d i t i o n a l costs i n c l u d e the work f o r replacement and s u b s t i t u t i o n of corroded polymeric m a t e r i a l s , e.g. i n machinery and b u i l d i n g c o n s t r u c t i o n s . Observations of polymeric m a t e r i a l s , e.g. house p a i n t , used i n heavy p o l l u t e d a r e a s demonstrate that the e f f e c t s of environmental c o r r o s i o n are c l e a r l y v i s i b l e . T h i s paper does not cover a l l aspects of t h i s subject. The references c i t e d here are s e l e c t e d from hundreds of p u b l i s h e d r e p o r t s , and they are given as examples to i l l u s t r a t e the impor tance of the problem. The need f o r more research i s amply v e r i f i e d , both on the e f f e c t s and t h e i r prevention. Since 1971 our department i s i n v o l v e d i n study of photodegradat i o n , s e n s i t i z e d photooxidation and p h o t o s t a b i l i z a t i o n of polymers. A b i g e f f o r t i s made i n the study of s i n g l e t oxygen o x i d a t i o n of s e v e r a l important polymers, such as polydienes, p o l y ( v i n y l c h l o r i d e ) , p o l y s t y r e n e and p o l y e s t e r s , r e a c t i o n s of atomic oxygen and ozone with polydienes and p h o t o s t a b i l i z a t i o n of these polymers. As a r e s u l t of t h i s work, a USA-Swedish Workshop on "Photodegrad a t i o n and P h o t o g g a b i l i z a t i o n of Polymers" was organized i n Stockholm i n 1981 . At t h i s meeting s e v e r a l problems i n c l u d e d i n t h i s paper were d i s c u s s e d . The workshop concluded that a broad i n t e r n a t i o n a l cooperation should be explored and current research on t h i s problem expanded.



17.

RAN BY AND RABEK


305

Areas r e q u i r i n g s p e c i a l a t t e n t i o n i n the f u t u r e i n c l u d e improved s t a b i l i z a t i o n processes f o r e x i s t i n g polymers and the synthesis of new polymeric m a t e r i a l s which are more r e s i s t a n t against environmental c o r r o s i o n than those p r e s e n t l y used. The i m p l i c a t i o n s of the s t a b i l i t y o f n a t u r a l polymers f o r the p r e d i c t i o n o f s e r v i c e l i f e o f s y n t h e t i c polymers,the general e f f e c t s o f h y d r o l y s i s , the l i g h t - i n d u c e d h y d r o l y t i c degradation are r e l e v a n t research t o p i c s . This paper includes r e s u l t s from our research programs on photooxidation and p h o t o - s t a b i l i z a t i o n o f polymers supported by the Swedish N a t i o n a l Board f o r T e c h n i c a l Development (STU) and a p r o j e c t i n the NORDFORSK program on s i n g l e t oxygen r e a c t i o n s .

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