Polymer Wear and Its Control - American Chemical Society

23 D e t e r i o r a t i o n P r o c e s s e s of Polymeric M a t e r i a l s and T h e i r I n f l u e n c e on t h e Durability of R e i n f o r c e d

Concrete

Polymer Wear and Its Control Downloaded from pubs.acs.org by UNIV OF TEXAS AT EL PASO on 10/31/18. For personal use only.

Toshio Fukushima and Kenji Motohashi Building Research Institute, Ministry of Construction, The Japanese Government, 1-Tatehara, Oho-machi, Tukuba-gun, Ibaraki Prefecture 305, Japan

From the viewpoint of prediction of service lives, the photochemical deterioration processes of polymers used as paints and finishes are theoretically analyzed based upon unsteady state dynamics. Theoretical results are compared with experimental data under natural and accelerated exposure. Infrared spectra and scanning micrographs show that the deterioration proceeds continuously inwards from the surface, but differently with the exposure conditions. Parabolic (√t ) law was derived approximately for the increase in the depth of the deteriorated layer of polymers with time. Paying attention to the influence of the deterioration of polymeric finishes, the parabolic law involving a constant term was also derived for the progress of carbonation of concrete. These parabolic laws well predict the progress of deterioration and explain the protective function of finishes on reinforced concrete. The development o f t h e p e t r o c h e m i c a l i n d u s t r y has e n a b l e d t h e mass s u p p l y o f v a r i o u s t y p e s o f polymers w i t h a t t r a c t i v e p r o p e r t i e s . A l though many polymers a r e w i d e l y used i n v a r i o u s f i e l d s ( e . g . as p a i n t s , a d h e s i v e s , c o a t i n g s and f i n i s h e s i n h o u s i n g ) , t h e y a r e sub j e c t t o g r a d u a l d e t e r i o r a t i o n under n a t u r a l exposure. W i t h t h e enlargement o f r e c e n t s o c i a l needs f o r t h e c o n s e r v a t i o n o f n a t u r a l r e s o u r c e s and energy, and f o r t h e h i g h f u n c t i o n s o f i n s e r v i c e m a t e r i a l s , t h e importance o f t h e concept o f d u r a b i l i t y a s performance w i t h t i m e and t h e p r e d i c t i o n o f s e r v i c e l i v e s o f m a t e r i a l s has been i n c r e a s i n g l y a p p r e c i a t e d among many r e s e a r c h e r s and e n gineers i n various f i e l d s . I n pursuit o f r a t i o n a l c r i t e r i a f o r e v a l u a t i n g t h e p r o g r e s s o f d e t e r i o r a t i o n and f o r i d e n t i f y i n g d e t e r i o r a t i o n mechanisms o f m a t e r i a l s , a l o t o f p h e n o m e n o l o g i c a l d a t a have been accumulated f o r t h e d e t e r i o r a t i o n p r o c e s s e s . However t h e r e s t i l l remain major d i f f i c u l t i e s t o be s o l v e d f o r a n a l y t i c a l methods o f t h e deterioration processes. On t h e o t h e r hand, i n h o u s i n g under o r d i n a r y a t m o s p h e r i c e n v i r 0097-6156/85/0287-0347$06.00/0 © 1985 American Chemical Society

American Chemical Society Library 115516th St, H.% Washington, O.C. 20036

POLYMER WEAR AND ITS CONTROL

348

onment, t h e c a r b o n a t i o n o f c o n c r e t e i s v e r y i m p o r t a n t from t h e p o i n t o f v i e w o f t h e d u r a b i l i t y o f r e i n f o r c e d c o n c r e t e component, because i t causes t h e n e u t r a l i z a t i o n o f c o n c r e t e , r e s u l t i n g i n t h e r e d u c t i o n o f c o r r o s i o n p r o t e c t i v e f u n c t i o n a g a i n s t r e i n f o r c i n g b a r s . Many e f f o r t s have been done t o d e l a y o r s u r p r e s s t h e p r o g r e s s o f n e u t r a l i z a t i o n o f c o n c r e t e e f f e c t i v e l y , e.g. b y u s i n g p a i n t s and f i n i s h e s . T h i s r e p o r t d e a l s w i t h dynamic p r o c e s s e s o f t h e d e t e r i o r a t i o n o f polymers o f t e n used as p a i n t s and f i n i s h e s i n h o u s i n g , and a l s o r e f e r s t o t h e i r i n f l u e n c e as t h e r e d u c t i o n i n p r o t e c t i v e performance on t h e d u r a b i l i t y o f r e i n f o r c e d c o n c r e t e . The d e t e r i o r a t i o n p r o c e s s e s o f polymers by t h e s i m u l t a n e o u s a c t i o n o f u l t r a v i o l e t (UV) l i g h t and d i f f u s i v e oxygen i s e x p l a i n e d t h e o r e t i c a l l y based upon u n s t e a d y s t a t e dynamics. The p a r a b o l i c l a w (/Γ law) i s d e r i v e d f o r a t y p i c a l p a t h f o r t h e p r o g r e s s o f t h e d e t e r i o r a t i o n o f polymers inwards from t h e s u r f a c e ( l ) , and compared w i t h some e x p e r i m e n t a l d a t a . The same p a r a b o l i c l a w i n v o l v i n g a c o n s t a n t t e r m was a l s o d e r i v e d f o r t h e carbonation o f concrete, which w e l l explains t h e r e t a r d a t i o n e f f e c t s o f f i n i s h e s on t h e c a r b o n a t i o n (2_). Dynamic A n a l y s i s o f D e t e r i o r a t i o n P r o c e s s e s o f P o l y m e r i c

Materials

O u t l i n e o f t h e T h e o r e y i c a l Model. The main assumptions f o r t h e un s t e a d y s t a t e dynamics a r e as f o l l o w s * l ) O n l y polymer m o l e c u l e s w h i c h a r e r a i s e d i n t o e x c i t e d s t a t e b y a b s o r b i n g UV l i g h t (photon f l u x , n o ; w a v e l e n g t h , λ) near t h e a b s o r p t i o n band c h a r a s t e r i s t i c o f polymers can p a r t i c i p a t e i n p h o t o c h e m i c a l r e a c t i o n s ( e f f i c i e n c y , η ; molar c o n c e n t r a t i o n , C * ) . (2) P h o t o c h e m i c a l r e a c t i o n s a r e : i ) d e p o l y m e r i z a t i o n o f a c t i v a t e d polymer m o l e c u l e s ( f i r s t o r d e r r e a c t i o n , R = ] q C * J , i i ) phot ο-oxidation by t h e c o l l i s i o n o f d i f f u s i v e oxygen ( i n i t i a l and time-dependent molar c o n c e n t r a t i o n , C^Q ^ ^A.; ^ ^ t i v e d i f f u s i o n c o e f f i c i e n t , D A ) and a c t i v a t e d polymer m o l e c u l e s ( s e c ond order- r e a c t i o n , R 2 = k 2 C * C A ) · We can c o n s i d e r t-his r e a c t i o n t o be q u a s i - f i r s t o r d e r r e a c t i o n ( R = k i 'C^) ( 3 ) D e t e r i o r a t e d p o l y mer m o l e c u l e s (molar c o n c e n t r a t i o n , C ) do n o t d i f f u s e and r e m a i n i n t h e o r i g i n a l p o s i t i o n s , (h) The i n f l u e n c e o f t e m p e r a t u r e on photo chemical reactions i s included i n material property constants (lq , k ) and d i f f u s i v e c o e f f i c i e n t ( D A ) a c c o r d i n g t o t h e A r r h e n i u s l a w . (5) D e t e r i o r a t i o n p r o c e e d s inwards f r o m t h e s u r f a c e and t h e degree o f d e t e r i o r a t i o n v a r i e s w i t h exposure t i m e (t) and t h e depth f r o m t h e s u r f a c e (x) . As t h e d e t e r i o r a t i o n a d v a n c e s , d e t e r i o r a t e d polymer m o l e c u l e s h a v i n g i n f r a r e d (IR) a c t i v e f u n c t i o n a l groups as c a r b o n y l (-CO) a r e c r e a t e d , and t h e degree o f d e t e r i o r a t i o n i s r e f l e c t e d i n IR s p e c t r a and i n t h e m o r p h o l o g i c a l change o f t h e s u r f a c e . The deteriorated surface l a y e r r e s u l t s i n the reduction i n the f l e x u a l s t r e n g t h and i n p r o t e c t i v e performance o f p o l y m e r s . (6) Compared w i t h t h e t h i c k n e s s o f t h e p l a t e o f polymer, t h e d e t e r i o r a t e d depth (6) c a n be c o n s i d e r e d t o be s m a l l enough f o r t h e t h i c k n e s s o f t h e p l a t e t o be r e g a r d e d as i n f i n i t e l y l a r g e . B a s i c d i f f e r e n t i a l e q u a t i o n s f o r t h e dynamic a n a l y s i s , t o g e t h e r w i t h t h e i n i t i a l and boundary c o n d i t i o n s , a r e summarized as f o l l o w s : x

a n (

e

e c -

2

B

2

dC /dt A

=

D 8 CA/3X 2

a

3C*/8t = ( n n / N 0

2

- kx'C

)C e

0

0

,9C /at = kxC* + k - C B

x

A

x

(1)

A

e x p ( - C e x ) - k *C* - k C 0

x

x

A

(2) (3)

23.

Deterioration of Polymeric Materials

FUKUSHIMA AND MOTOHASHI

349

Here, NQ i s Avogadro's c o n s t a n t ; C Q , i n i t i a l molar c o n c e n t r a t i o n o f u n d e t e r i o r a t e d polymer m o l e c u l e s ; ε , molar a b s o r p t i o n c o e f f i c i e n t . χ

[ i n i t i a l Conditions];

t £ 0: C* = C

[Boundary C o n d i t i o n s ] ; [ i ]

= Cg = 0

A

t > 0 ,x = 0 : C = C o A

[II]

t > 0 , x +

[III]

t

> 0,

χ

A

œ : C

= 0

A

dC /dx

= 6:

=

B

0

R e s u l t s o f Dynamic A n a l y s i s . S o l v i n g t h e simultaneous d i f f e r e n t i a l e q u a t i o n s 1.-3. under t h e g i v e n i n i t i a l and boundary c o n d i t i o n s b y t h e L a p l a c e t r a n s f o r m a t i o n method, a n a l y t i c a l s o l u t u o n s U.-8. a r e obtained i n the form o f dimensionless concentrations: ΦΑ

ξ

C (x,t)/C A

A

= (l/2)[exp(- x/k!7D + exp(x/kx7D

A

A

) e r f c ( x / 2 / D ~ t - /kf^t)

)erfc(x/2v^t" + /kpF")]

(h)

Φ* - C * ( x , t ) / C f l = [1 - exp(-kjt)] e x p ( - 0 ε χ ) - ( C / C * ) k i ' φ 0

φ

Ξ C ( x , t ) / C * = {kit

Β

B

χ

A0

0

A

H e r e , C* = C n n e / N 0

0

x

5

- [1 - exp(kit)]}[exp(- C e x/ki)]

+ (k ' - k! ) ( C o / C * ) / f o d t + [1 - e x p ( - k t ) ] x

()

Α

A

x

x

(C /C*) A0

(6)

A

0

On t h e o t h e r hand, u s i n g t h e boundary c o n d i t i o n [ i l l ] , we can o b t a i n a p p r o x i m a t e l y t h e p a r a b o l i c l a w f o r t h e p r o g r e s s o f t h e depth o f t h e d e t e r i o r a t e d l a y e r as f o l l o w s : δ = /et β = D (5k A

(7) x

+ 2kr)(Tkx - 2 k i ' ) / U i - 2 k i ' ) ( 3 k ] . - 2k! Ο

Dynamic A n a l y s i s o f t h e P r o g r e s s o f C a r b o n a t i o n

(8)

o f Concrete

The n e u t r a l i z a t i o n o f c o n c r e t e l e a d s t o r e d u c t i o n o f t h e c o r r o s i o n p r o t e c t i v e f u n c t i o n o f c o n c r e t e a g a i n s t r e i n f o r c i n g s t e e l , and has an i m p o r t a n t i n f l u e n c e on t h e d u r a b i l i t y o f r e i n f o r c e d c o n c r e t e s t r u c t u r e s . The n e u t r a l i z a t i o n i s i n f l u e n c e d by v a r i o u s f a c t o r s ( c o n c e n t r a t i o n o f C 0 gas, t y p e o f c o n c r e t e , water-to-cement r a t i o ( W / C ) , water c o n t e n t , t y p e o f f i n i s h e s and t h e i r t h i c k n e s s and p e r m e a b i l i t y , t e m p e r a t u r e and h u m i d i t y c o n d i t i o n s , e t c . ) . From t h e p h y s i c o chemical point o f view, t h i s process can be c o n s i d e r e d t o be t h e d i f f u s i o n o f C 0 inwards i n t o c o n c r e t e f r o m t h e s u r f a c e , accompanied by t h e c o n v e r s i o n o f C a ( 0 H ) i n t o CaC03. I n t h i s c o n t e x t , unsteady s t a t e dynamics has been done f o r t h e p r o g r e s s o f n e u t r a l i s a t i o n o f c o n c r e t e i n o r d e r t o r a t i o n a l l y u n d e r s t a n d t h e p r o c e s s and t h e i n f l u e n c e o f f i n i s h e s on t h e p r o c e s s (_3, k). The main assumptions a r e as f o l l o w s : ( l ) I n t h e i n i t i a l s t a t e , C a ( 0 H ) e x i s t s homogeneously i n s e m i - i n f i n i t e c o n c r e t e ( i n i t i a l molar c o n c e n t r a t i o n , C^q), and t h e c a r b o n a t i o n p r o c e s s b e g i n s when CO2 i n gas phase ( i n i t i a l molar c o n c e n t r a t i o n , C ) comes i n t o c o n t a c t w i t h t h e s u r f a c e o f c o n c r e t e ( t a k e χ = 0 a t t h e s u r f a c e ) . (2) D i f f u s i v e C 0 (molar c o n c e n t r a t i o n , C a ; d i f f u s i o n c o e f f i c i e n t , D a i n c o n c r e t e ) f e e l s t h e mass t r a n s f e r r e s i s t a n c e a t t h e s u r f a c e o f c o n c r e t e ( s u r f a c e mass t r a n s f e r c o e f f i c i e n t , K g ) , unless the e q u i l i b r i u m C = mC i s s a t i s f i e d ( k. +°°). The r e t a r d a t i o n e f f e c t s o f f i n i s h e s on t h e d i f f u s i o n o f C 0 ( t h i c k n e s s , L; d i f f u s i o n c o e f f i c i e n t , D i n a f i n i s h ) a r e i n c o r p o r a t e d i n t o t h e 2

2

2

2

a g

2

2

2

a

a 2

al

ag

2


350

t o t a l mass t r a n s f e r c o e f f i c i e n t ( K O d e f i n d as f o l l o w s : 1/W = ! / a g + L/D (9) (3) Ca(0H)2 d i f f u s e s s l o w l y backwards (molar c o n c e n t r a t i o n , C-^2 5 diffusion coefficient, i n concrete) from the inner l a y e r of c o n c r e t e , and c o v e r t s i n s t a n t a n e o u s l y i n t o CaCU3 w i t h CO2 w h i c h d i f f u s e s forwards from the surface. Based upon t h e above a s s u m p t i o n s , fundamental d i f f e r e n t i a l equa t i o n s a r e o b t a i n e d . L a p l a c e t r a n s f o r m a t i o n method was a l s o used t o s o l v e t h e s i m u l t a n e o u s d i f f e r e n t i a l e q u a t i o n s under t h e g i v e n i n i t i a l and boundary c o n d i t i o n s . A n a l y t i c a l s o l u t i o n s a r e o b t a i n e d i n t h e form o f d i m e n s i o n l e s s c o n c e n t r a t i o n s w h i c h i n v o l v e e r r o r f u n c t i o n s c o n c e r n i n g t i m e and t h e depth from t h e s u r f a c e . F o r t h e p r o g r e s s o f n e u t r a l i z a t i o n , t h e p a r a b o l i c l a w i n v o l v i n g c o n s t a n t terms was d e r i v e d as f o l l o w s ( X , n e u t r a l i z a t i o n d e p t h o f c o n c r e t e ) : a g

k

(10)

X = k /F - I e

k

e

= m /D /D a i

* = W^ag'

a 2

{(C /2C ) - D / a g

b 0

b2

D i} a

(ll) (12)

Experimental SEM O b s e r v a t i o n o f Masonry C o a t i n g M a t e r i a l s . F o r samples o f t h e masonry c o a t i n g m a t e r i a l s l i s t e d i n T a b l e I . , t h e s u r f a c e morpholog i c a l changes' due t o o u t d o o r exposure and s u n s h i n e carbon a r c i r r a d i a t i o n were i n v e s t i g a t e d by s c a n n i n g e l e c t r o n m i c r o s c o p y (SEM) by H i t a c h i Model S-l+50 a f t e r m e t a l l i z i n g w i t h p l a t i n u m by s p u t t e r c o a t i n g . The c r o s s s e c t i o n s o f t h e specimens were exposed by f r a c t u r e after freezing i n l i q u i d nitrogen. Masonry c o a t i n g m a t e r i a l s , which a r e t h e m i x t u r e o f o r g a n i c p o l y m e r , f i l l e r and f i n e a g g r e g a t e s , o f t e n a p p l i e d some 0 . 3 t o 15mm t h i c k by s p r a y i n g o r r o l l i n g on t h e v a r i o u s t y p e s o f e x t e r n a l s u r f a c e such as c o n c r e t e , cement m o r t a r , a s b e s t o s cement s h e e t s and o t h e r boards, are objects i n t h i s i n v e s t i g a t i o n . The c o a t e d l a y e r s o f masonry c o a t i n g m a t e r i a l s ( e x c l u d i n g sub s t r a t e s ) were exposed on t h e a c r y l i c r e s i n sheet mounted on the expos u r e r a c k s a t an a n g l e o f 30° f a c i n g s o u t h i n t h e B u i l d i n g Research I n s t i t u t e f o r one y e a r . Sunshine carbon a r c a c c e l e r a t i o n t e s t based on J I S A l U l 5 ( b l a c k p a n e l t e m p e r a t u r e 63±3°C, ion-exchanged water s p r a y 18 minutes i n e v e r y two h o u r s ) was conducted u s i n g specimens of t h e same samples used i n t h e outdoor exposure t e s t above up t o 1000 hours ( 5 ) . R e s u l t s . Examples o f t h e SEM micrographs o b t a i n e d a r e shown i n F i g u r e s 1 t o 8 . I t can be found t h a t t h e s u r f a c e morphology o f t h e outdoor exposed specimens i s e v i d e n t l y d i f f e r e n t from t h a t o f s p e c i mens exposed t o s u n s h i n e carbon a r c i r r a d i a t i o n i n t h e l a b o r a t o r y . s u r f a c e o f t h e specimens exposed outdoor i s u n i f o r m l y weathered ( F i g u r e l ) . On t h e c o n t r a r y , t h e s u r f a c e o f t h e specimens exposed t o the a c c e l e r a t e d a g i n g t e s t i s not u n i f o r m l y d e t e r i o r a t e d depending upon t h e s u r f a c e t e x t u r e s , but shows f u r t h e r t h e d e t e r i o r a t i o n i n concave p a r t s p r o b a b l y due t o l o n g e r r e m a i n i n g o f s p r a y e d water (Figure 2). The d i f f e r e n c e i n t h e s u r f a c e morphology o f mansory c o a t i n g ma-

A Β C D Ε

under single single under under

Materials

average t h i c k n e s s {im) type of specimen 1.5 coat ( a c r y l i c rubber emulsion) + top c o a t 1.4 l a y e r ( a c r y l i c r e s i n emulsion) 1.5 l a y e r ( a c r y l i c r e s i n emulsion) 1.6 coat ( a c r y l i c r e s i n emulsion) + top coat 1.6 coat ( a c r y l i c r e s i n emulsion) + top coat

Table I . Description of Masonry Coating

352


F i g u r e 1. M i c r o g r a p h o f A i n T a b l e I . outdoor e x p o s u r e ; 1 y e a r , m a g n i f i c a t i o n b a r ; 500μ .

Figure 2. Micrograph o f A i n Table I . a c c e l e r a t i o n t e s t ; 1,000 h r s . , m a g n i f i c a t i o n b a r ; 500u.

23.

FUKUSHIM A AND MOTOH ASHI


353

t e r i a l s i s more c l e a r l y o b s e r v e d i n m i c r o g r a p h s a t r a t h e r h i g h mag n i f i c a t i o n . The s u r f a c e o r g a n i c p o l y m e r e x p o s e d o u t d o o r s m i g h t b e g r a d u a l l y d e g r a d e d a n d i n o r g a n i c f i l l e r i n t h e c o a t e d l a y e r s became e x p l i c i t l y o b s e r v a b l e ( F i g u r e s 3 t o 5 ) . The d e g r e e o f t h i s d e t e r i o r a t i o n , namely c h a l k i n g b e h a v i o u r , d i f f e r s depending upon t y p e s o f o r g a n i c p o l y m e r , and r e a s o n a b l y corresponds t o t h e r e s u l t o f p r a c t i c a l c h a l k i n g t e s t b a s e d o n J I S Κ 5 5 l 6 . On t h e o t h e r h a n d , m i c r o c r a c k s ( F i g u r e s 6 a n d 7) a n d m i c r o f l a k i n g ( F i g u r e 8) c a n be o b s e r v e d on t h e s u r f a c e o f e v e r y specimen a f t e r t h e a c c e l e r a t i o n t e s t . Discussion Degradation of Polymers. From t h e r e s u l t s o f dynamic a n a l y s i s , t h e degree o f t h e d e t e r i o r a t i o n o f p o l y m e r s v a r i e s w i t h t i m e and t h e depth f r o m t h e s u r f a c e , and i s i n f l u e n c e d by t h e p h o t o c h e m i c a l r e a c t i o n c o n s t a n t s (kx , k ) a n d d i f f u s i o n c o e f f i c i e n t (D.). Figure 9 shows t h e t h e o r e t i c a l r e s u l t o f t h e i n f l u e n c e o f p h o x o - o x i d a t i o n on t h e d i s t r i b u t i o n o f oxygen i n p o l y m e r s . D e r i v i n g t h e t i m e dependence o f s u r f a c e d e t e r i o r a t i o n , we o b t a i n e q u a t i o n 1 3 . 2

φ (0,ΐ) Β

Ξ C (x,t)/C* B

2 ki*t(c

A 0

/C*)

= k]C

A O

t

(13)

T h i s shows t h a t t h e s u r f a c e d e t e r i o r a t i o n a d v a n c e s a p p r o x i m a t e l y i n p r o p o r t i o n t o exposure t i m e , and t h a t t h e p r o g r e s s o f d e t e r i o r a t i o n depends upon t h e s u r f a c e o x y g e n c o n c e n t r a t i o n a n d t h e r a t e c o n s t a n t o f p h o t o - o x i d a t i o n , though i t depends,of c o u r s e , l i n e a r l y on t h e i n t e n s i t y o f i n c i d e n t l i g h t . The s u r f a c e d e t e r i o r a t i o n c a n be o b s e r v e d i n IR a b s o r p t i o n s p e c t r a a n d i n t h e m o r p h o l o g i c a l change i n SEM, and f u r t h e r i n t h e change o f m a t e r i a l p r o p e r t i e s . W a t a n a b e e t a l . {k) have measured IR a b s o r p t i o n s p e c t r a o f s l i c e d samples o f o u t d o o r e x p o s e d p l a t e s o f p o l y s t y r e n e (PS) and o t h e r p o l y m e r s . T h e i r e x p e r i m e n t a l d a t a a r e shown i n F i g u r e 1 0 . T h e a b s o r p t i o n n e a r 1730 c m " , due t o t h e c a r b o n y l f u n c t i o n a l g r o u p , i s s e e n t o i n c r e a s e i n i n t e n s i t y w i t h t i m e . T h i s may show t h a t t h e photο-oxidation proceeds r a t h e r i n the surface l a y e r s of polymers. F i g u r e s 1 t o 8 show t h e m o r p h o l o g i c a l change o f m a s o n r y c o a t i n g m a t e r i a l s under outdoor and a c c e l e r a t e d a g i n g exposure. For o u t d o o r - e x p o s e d s a m p l e s , i t was o b s e r v e d t h a t a c r y l i c r u b b e r e m u l s i o n t y p e p o l y m e r i s g r a d u a l l y l o s t and t h a t f i l l e r becomes e x p l i c i t l y d i s c l o s e d a:fter one-year exposure ( c h a l k i n g phenomena)(Figures 1, k a n d 5 ) . F o r a c c e l e r a t e d - a g e d s a m p l e s , h o w e v e r , t h e f l a k i n g was o b s e r v e d . The d i f f e r e n c e i n t h e s u r f a c e m o r p h o l o g y b e t w e e n t h e o u t d o o r e x p o s u r e t e s t a n d l a b a r a t a r y t e s t c a n b e d u e t o many c o m p l e x f a c t o r s w h i c h have not been c o m p l e t e l y r e v e a l e d . On t h e b a s i s o f u n s t e a d y s t a t e d y n a m i c s , t h e d e g r e e o f t h e s u r f a c e d e t e r i o r a t i o n o f p o l y m e r s becomes h e a v i e r i n l i n e a r p r o p o r t i o n as t h e i r r a d a t i o n l e v e l o f i n c i d e n t l i g h t becomes h i g h e r , b u t t h e p r o g r e s s o f d e t e r i o r a t i o n inwards f r o m t h e s u r f a c e depends m a i n l y upon t h e d i f f u s i o n o f oxygen ( o r w a t e r ). I n t h e c a s e o f a c c e l e r a t e d - a g e d samples under r a t h e r s t r o n g and s h o r t - t e r m i r r a d i a t i o n by sunshine carbon arc i n the l a b a r a t o r y , the surface of polymers becomes s t r o n g l y d e t e r i o r a t e d as t h e r e s u l t o f r a p i d p h o t o c h e m i c a l r e a c t i o n s . However, t h e d e t e r i o r a t i o n o f polymers i s c o n s i d e r e d n o t t o e x t e n d t o t h e d e e p i n n e r l a y e r s . On t h e o t h e r h a n d , f o r out door-exposed samples under r a t h e r m i l d and l o n g - t e r m i r r a d i a t i o n 1

354


F i g u r e 3. M i c r o g r a p h o f A i n T a b l e I . i n i t i a l specimen, m a g n i f i c a t i o n b a r ; 5y.

F i g u r e k.

M i c r o g r a p h o f A i n T a b l e I . outdoor e x p o s u r e ; 1 y e a r , m a g n i f i c a t i o n b a r ; 5u .

23.


F i g u r e 5.

F i g u r e 6.


M i c r o g r a p h o f Ε i n T a b l e I . outdoor e x p o s u r e ; l y e a r , m a g n i f i c a t i o n b a r ; 5μ .

M i c r o g r a p h o f Β i n T a b l e I . a c c e l e r a t i o n t e s t ; TOO h r s . , m a g n i f i c a t i o n b a r ; 5u .

355


356

F i g u r e 7.

M i c r o g r a p h o f D i n T a b l e I . a c c e l e r a t i o n t e s t ; 700 h r s . , m a g n i f i c a t i o n b a r ; 5y .

Figure 8. Micrograph o f Ε i n Table I . a c c e l e r a t i o n t e s t ; h r s . , m a g n i f i c a t i o n b a r ; 5μ .

1000

23.


FUKUSHIM A AND MOTOHASHI

Figure 9

357

I n f l u e n c e o f phot ο-oxidation on t h e d i s t r i b u t i o n o f oxygen i n p o l y m e r i c m a t e r i a l s ( r e s u l t o f t h e o r e t i c a l calculation).

6

J

1

2000

1900

1

1800

WAVE NUMBER

I

1

L

1700 1600 1500 -1

(cm )

F i g u r e 1 0 . Change o f i n f r a r e d a b s o r p t i o n w i t h d i s t a n c e below t h e s u r f a c e o f p o l y s t y r e n (.PS) polymer exposed o u t d o o r s (at C h o s i ) f o r t h r e e y e a r s (Reproduced w i t h p e r m i s s i o n from Réf. XI C o p y r i g h t 1 9 7 9 , Japan I n d . Tech. A s s o c . ) .


358

"by n a t u r a l s u n - l i g h t , t h e s u r f a c e d e t e r i o r a t i o n i s c o n s i d e r e d t o be not so s t r o n g , and t h e d e t e r i o r a t i o n i s m a i n l y r u l e d by t h e d i f f u s i o n . The d e t e r i o r a t i o n i s c o n s i d e r e d t o proceed s l o w l y b u t deeper i n t o t h e i n n e r l a y e r s , t h i s c o n c e p t u a l diagram i s i l l u s t r a t e d i n F i g u r e 1 1 , w h i c h seems t o be one o f t h e reasons why t h e d i f f e r e n t morphology was caused on t h e s u r f a c e l a y e r s . The depth o f t h e d e t e r i o r a t e d l a y e r was t o be a few microns a t most f o r b o t h specimens exposed outdoors f o r one y e a r and t h o s e e x posed t o sunshine carbon a r c i r r a d a t i o n f o r 1000 hours by o b s e r v i n g t h e i r c r o s s s e c t i o n s . Of c o u r s e , t h e depth i s o n l y based on t h e mor p h o l o g i c a l changes; t h e r e f o r e , m o l e c u l a r s t r u c t u r e changes, such as formation o f carbonyl f u n c t i o n a l groups, c r o s s - l i n k i n g r e a c t i o n , r u p t u r e o f m o l e c i l a r c h a i n s e t c . a r e supposed t o be d i r e c t l y i n v i s i b l e . The d e t e r i o r a t i o n p r o g r e s s e s c o n t i n u o u s l y f r o m t h e s u r f a c e , and t h e depth o f t h e d e t e r i o r a t e d l a y e r i n c r e a s e s i n p r o p o r t i o n t o t h e square r o o t o f exposure t i m e as shown i n e q u a t i o n s 7. and 8. T h i s p a r a b o l i c (/t~ ) l a w was o b t a i n e d as t h e n a t u r a l d e r i v a t i o n based upon unsteady s t a t e dynamics, assuming t h e s i m u l t a n e o u s a c t i o n o f UVl i g h t and d i f f u s i v e oxygen. F i g u r e 12 i s t h e l e a s t - s q u a r e p l o t based on e x p e r i m e n t a l d a t a f r o m Kubota e t a l . (5)· I t can be seen t h a t many polymers show t h e d e t e r i o r a t i o n by t h e power l a w o f exposure t i m e ( t ; η = 0.5 - 1.0). The d i f f e r e n c e between t h e o r y and experiment i s c o n s i d e r e d t o be due t o t h e complex mechanisms n o t e x p l i c t l y t r e a t ed i n t h i s t h e o r y . n

R e t a r d a t i o n E f f e c t s o f P o l y m e r i c F i n i s h e s on C a r b o n a t i o n . The r e t a r d a t i o n e f f e c t s o f p o l y m e r i c f i n i s h e s can be w e l l e x p l a i n e d , u s i n g e q u a t i o n s 9. and 12., where Ζ i s t h e c o n s t a n t t e r m w h i c h i n c l u d e s t o t a l s u r f a c e mass t r a n s f e r c o n s t a n t , and d e s c r i b e s t h e retardation o f carbonation or induction period before the carbonation b e g i n s t o p r o c e e d . From e q u a t i o n 9. i t can be seen t h a t t h e e f f e c t s depend on t h e t h i c k n e s s and d i f f u s i o n c o e f f i c i e n t s o f f i n i s h e s (Figure 13). I f the f u n c t i o n a l thickness o f f i n i s h decreases, or the d i f f u s i o n - r e s i s t a n c e o f f i n i s h reduces by t h e d e t e r i o r a t i o n p r o c e s s e s , i t i s considered that the p r o t e c t i v e function o f polymeric f i n i s h e s a g a i n s t t h e c a r b o n a t i o n o f c o n c r e t e g r a d u a l l y degrades. A f t e r a g i v e n l o n g t i m e t h e n e u t r a l i z a t i o n depth i s seen t o be reduced i n p r o p o r t i o n t o t h e t h i c k n e s s o f a g i v e n f i n i s h i n g m a t e r i a l . Though t h e s e r e t a r d a t i o n e f f e c t s have n o t b e e n , so f a r , w e l l e s t a b l i s h d , F i g u r e 14 shows t h e r e t a r d a t i o n e f f e c t s o f i n o r g a n i c f i n i s h e s i n t h e f i e l d r e s e a r c h f o r i n d o o r v e r t i c a l w a l l s i n an e x i s t i n g r e i n f o r c e d b u i l d i n g aged 17 (Xo i n d i c a t e s t h e n e u t r a l i z a t i o n depth o f c o n c r e t e w i t h o u t f i n i s h e s ) . The r e s u l t s r e p r e s e n t t h e e v i d e n c e f o r t h e c o n s i d e r a t i o n s d e s c r i b e d above, and t h e importance o f t h e t h i c k n e s s as w e l l as t h e permeability o f f i n i s h e s i n the retardation o f carbonation. Conclusions The p h o t o c h e m i c a l d e t e r i o r a t i o n p r o c e s s e s o f p o l y m e r i c m a t e r i a l s o f t e n used as p a i n t s and f i n i s h e s a r e s t u d i e d b o t h t h e o r e t i c a l l y and e x p e r i m e n t a l l y . T h e o r e t i c a l models f o r t h e d e t e r i o r a t i o n p r o c e s s e s a r e developed based upon u n s t e a d y s t a t e dynamics. The r e s u l t s were compared w i t h e x p e r i m e n t a l d a t a under outdoor and a c c e l e r a t e d a g i n g

23.


359


F i g u r e 1 1 . Schematic model o f absorbed UV energy d i s t r i b u t i o n i n t h i c k n e s s f o r t h e outdoor exposure t e s t and t h e sun s h i n e carbon a r c s t y p e a c c e l e r a t i o n t e s t .

PS. PVC PMMA ABS POM PE

• + Δ 0 0 X

exposure s i t e : C h o s h i ..·· 1; PS 2 ; ΡΕ

Y = Y =

95.21 100.93

3 ; POM

Y =

.0.63 1^3.58X

_ 4 - - U ; PVC 5; ABS

Y = Y =

îoo.oox ·

6; PMMA

Y =

-J^ —

1

2

3

EXPOSURE TIME

4—

.09 .03

69.9^1 ?

1+.22Χ.0.39

χ

h (year)

F i g u r e 12. L e a s t - s q u a r e s p l o t u s i n g t h e power l a w f o r t h e d e p t h o f t h e d e t e r i o r a t e d l a y e r o f v a r i o u s polymers a f t e r v a r i o u s p e r i o d s o f outdoor exposure ( a t C h o s i ) . E x p e r i m e n t a l d a t a a r e f r o m Kubota e t a l . Réf. 12.


360 C 0 ; 5Ϊ T; 20°C R.H.; 60$ X =/ce/t-Z 2

D j = 3.83E+7 mm /month 2

a

D 2= 3.89E+U mn /month 2

a

1^2= 1296 nm /month 2

ce r-t ce

u Ο

t Ρ

1* Time

6

8

10

Τ(month)

1 2 9 5 mm/month

li

=30.0

(2):

=1813

lz

=21.5

(3):

=2590

I3 =15.0

(10 :

=25900

Ik =1.5

(1):

k

Ag

=

F i g u r e 1 3 . I n f l u e n c e o f t h e s u r f a c e mass t r a n s f e r r e s i s t a n c e on t h e p r o g r e s s o f c a r b o n a t i o n under a c c e l e r a t e d condition.

T o t a l Thickness o f F i n i s h i n g Materials L(mm) F i g u r e ik. R e l a t i o n s h i p between t h e c a r b o n a t i o n ( n e u t r a l i z a t i o n ) depth o f i n d o o r c o n c r e t e and t h e t o t a l t h i c k n e s s o f f i n i s h e s (Result o f f i e l d Research f o r e x t e r n a l v e r t i c a l w a l l s i n an e x i s t i n g r e i n f o r c e d c o n c r e t e aged 17).

23.



361

e x p o s u r e s . I n f r a r e d s p e c t r a and s c a n n i n g e l e c t r o n m i c r o g r a p h s show t h a t t h e d e t e r i o r a t i o n proceeds c o n t i n u o u s l y inwards f r o m t h e s u r f a c e o f p o l y m e r i c m a t e r i a l s , h u t t h e "behaviour o f d e t e r i o r a t i o n a r e d i f f e r e n t w i t h t h e exposed c o n d i t i o n s . P a r a b o l i c (/Γ ) l a w was d e r i v e d a p p r o x i m a t e l y f o r t h e i n c r e a s e i n t h e depth o f t h e d e t e r i o r a t e d l a y yer w i t h time. Paying a t t e n t i o n t o t h e influence o f the d e t e r i o r a t i o n o f poly m e r i c f i n i s h e s , t h e p a r a b o l i c l a w i n v o l v i n g a c o n s t a n t term was a l s o d e r i v e d f o r t h e p r o g r e s s o f c a r b o n a t i o n o f c o n c r e t e . These p a r a b o l i c laws combined p r e d i c t t h e p r o g r e s s o f t h e d e t e r i o r a t i o n under n a t u r a l w e a t h e r i n g and e x p l a i n w e l l t h e p r o t e c t i v e performance o f f i n i s h i n g m a t e r i a l s on r e i n f o r c e d c o n c r e t e . Acknowledgments The a u t h o r s would l i k e t o e x p r e s s t h a n k s t o Mr. T. N i r e k i , Head o f t h e D u r a b i l i t y D i v i s i o n , and D r . F. Tomosawa, Head o f t h e I n o r g a n i c M a t e r i a l s D i v i s i o n , M a t e r i a l s Department o f t h e B u i l d i n g R e s e a r c h I n s t i t u t e f o r t h e i r k i n d d i s c u s s i o n s . Thanks a r e e x p r e s s e d t o D r . H. Kubota and t h e i r co-workers f o r t h e i r s u p p l y i n g v a l u a b l e e x p e r i m e n t a l d a t a i n c o n s i d e r i n g t h e d e t e r i o r a t i o n from t h e s u r f a c e o f p o l y m e r i c m a t e r i a l s . They would l i k e t o e x p r e s s t h a n k s a l s o t o e d i t o r s o f t h e l i t e r a t u r e c i t e d here f o r t h e i r k i n d p e r m i s s i o n s f o r t h e c o p y r i g h t t r a n s f e r i n o r d e r f o r them t o use t h e p a r t o f t h e f i g u r e s and a r t i c l e s t o c o n s t i t u t e t h i s paper. Literature Cited 1. Fukushima, T. D u r a b i l i t y of Building Materials 1983, 1. 327-343 2. Fukushima, T.; Kawase, K.; Tomosawa, F.; Akashi, H. Proc. Annual Symp. A r c h i t . Inst. Japan, Tokyo, 1982, p. 253 ( i n Japanese). 3. Fukushima, T. Proc. Annual Symp. A r c h i t . Inst. Japan, Hokuriku, 1983, p. 199 ( i n Japanese). 4. Fukushima, T. Proc. 27th Japan Cong. Materials Res., Kyoto, Japan Sept. 1983, p.225 5. Motohashi, K.; N i r e k i , T. Proc. 3rd Internat. Conf. Durability of Building Materials and Components, Espoo, Finland, Aug. 1984 6. Watanabe, Y.; Kitajima, F.; H a t t o r i , S. Proc. 15th Symp. Polymer Res. Works Japan Ind. Tech. Assoc., Tokyo, 1979, p. 177 ( i n Jap anese) . 7. Kubota, H.; Suzuki, S.; Nishihara, O.: Yoshikawa, K; Shirota, T. i n b i d , p. 147 ( i n Japanese). R E C E I V E D January 23, 1985

Polymer Wear and Its Control - American Chemical Society

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