The Chemical Nature of Chalking in the Presence of Titanium Dioxide

12 The Chemical N a t u r e of C h a l k i n g i n the Presence

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of T i t a n i u m D i o x i d e Pigments HANS G. VÖLZ, GUENTHER KAEMPF, HANS GEORG FITZKY, and ALOYS KLAEREN BAYER AG, D-4150 Krefeld 11 and D-5090 Leverkusen, West Germany Pigmented paint systems exposed to weathering are liable to undergo an oxidative destruction in the course of time. At an advanced stage of destruction, the pigment particles - originally completely surrounded by binder - are laid bare and can easily be removed from the surface of the paint film. As the effect continues, the pigment particles l a i d bare in the upper paint layers are progressively washed off by atmospheric precipitation. In the case of white or light pigments one speaks of the phenomenon of "chalking" to express that the pigment particles - like chalk - stick on being touched once they are l a i d bare by the destruction of the paint matrix. Anatase pigments show a far higher degree of chalking than r u t i l e pigments and are therefore unsuitable for the production of e. g. outdoor paints. 1. Chalking process. 1.1 Chalking. The destruction of the binder which we c a l l chalking has been known and feared for a very long time. The definition according to ASTM D 659 i s as follows: "Chalking i s that phenomenon manifested in paint films by the presence of loose removable powder, evolved from the film i t s e l f , at or just beneath the surface. Chalking may be detected by rubbing the film with the fingertip or other means." Chalking thus involves chemical changes in the binder due to the effect of atmospheric and meteoro logical influences. Rainfall washes the remaining non-gaseous decomposition products away u n t i l the pig ment i s finally exposed. On being subjected to fur ther weathering, the pigment exposed in the upper lay ers of paint i s finally carried away by the r a i n f a l l . 0097-6156/81/0151-0163$05.00/0 © 1981 American Chemical Society In Photodegradation and Photostabilization of Coatings; Pappas, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


164

PHOTODEGRADATION

A N D PHOTOSTABILIZATION

O F COATINGS

S i m p l e as t h e word " c h a l k i n g " may sound, t h e process behind i t i s extremely complicated. There are i n f a c t two main p r o c e s s e s w h i c h a r e r e s p o n s i b l e f o r t h e d e s t r u c t i o n o f t h e b i n d e r as o u t l i n e d above: - UV d e g r a d a t i o n (UVD) - t h e p h o t o c a t a l y t i c o x i d a t i o n c y c l e (POC) UV d e g r a d a t i o n i s a c h e m i c a l d e s t r u c t i o n p r o c e s s w h i c h l e a d s , under short-wave l i g h t , t o t h e d i r e c t o x i d a t i o n o f t h e b i n d e r by means o f a t m o s p h e r i c oxygen. The r e a c t i o n t a k e s p l a c e m a i n l y v i a p h o t o a c t i v a t e d s t a t e s o f the binder macromolecules. E x c i t e d s t a t e s o f t h e 02 m o l e c u l e s p l a y a minor r o l e . UVD o c c u r s w i t h o u t t h e pigment b e i n g i n v o l v e d and i s t h e r e f o r e o f no f u r t h e r i n t e r e s t i n t h i s connection. I n t h e p h o t o c a t a l y t i c o x i d a t i o n c y c l e , on t h e o t h e r hand, t h e pigment p l a y s a p a r t a s a c a t a l y s t . The p r o c e s s o f p h o t o c a t a l y t i c o x i d a t i o n c a u s e d by T i 0 2 p i g m e n t s i s f u l l y u n d e r s t o o d now (1, 2, 3, 4 ) . I t i s t h e s u b j e c t o f t h e p r e s e n t p a p e r , w h i c h comp r i s e s a d e s c r i p t i o n of t h i s oxidation cycle, the r e a s o n s f o r i t as shown by e x p e r i m e n t s and t h e p r a c t i c a l conclusions. 1.2 P h o t o c a t a l y t i c o x i d a t i o n c y c l e . The POC i s e x p l a i n e d below w i t h t h e a i d o f F i g . 1 ( 4 ) . The s t a r t i n g p o i n t i s a boundary i n t e r f a c e between t h e T i O ? p a r t i c l e s and t h e b i n d e r : on t h e Ti0 s u r f a c e , t h e p r e s e n c e o f water has l e d t o t h e f o r m a t i o n o f s u r f a c e h y d r o x y l g r o u p s £ri •OH J . The f i r s t s t e p c o n s i s t s i n t h e a b s o r p t i o n o f a quantum h v o f s h o r t w a v e l e n g t h and t h e f o r m a t i o n o f an e x c i t o n ( e l e c t r o n / h o l e p a i r ) : 2

( 1 )

h

^ h i * * . OKI,

e

+

p

The e x c i t o n r e a c t s f u r t h e r i n s t a n t a n e o u s l y w i t h a s u r f a c e h y d r o x i d e i o n and a T i i o n of the l a t t i c e , f o r m i n g a h y d r o x y l r a d i c a l and a (formal) T i ion. 4

+

J

4 +

(2)

p + [Ti '0H~]

(3)

e + [Ti

4

+

]

>[Ti

4 +

>fTi

3 +

+

] + 'OH J.

The n e x t s t e p i s t h e a d d i t i o n o f an a t m o s p h e r i c oxygen m o l e c u l e , w h i c h t a k e s o v e r t h e e l e c t r o n f r o m the T i and t u r n s i n t o ° 2 ( d s ) 3

+

:

a

(4)

[Ti

3 +

]

+ 0

2

» [Ti

4 +

.0"

( a d s )

]

In Photodegradation and Photostabilization of Coatings; Pappas, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

12.

VOLZ E T AL.

Chemical

Nature

of

165

Chalking

T h i s i s f o l l o w e d by t h e i m p o r t a n t r e a c t i o n i n w h i c h w a t e r i s consumed. The w a t e r r e a c t s i n t h e f o r m o f i t s d i s s o c i a t i o n products: ( 5 )


(6)

+

°2(ads) [Ti

4 +

+

H —>H0-

] + OH"

>[Ti

4 +

.OH"].

As a r e s u l t , we g e t a p e r h y d r o x y l r a d i c a l . A t t h e same t i m e the T i 0 s u r f a c e has r e t u r n e d t o i t s i n i t i a l s t a t e and t h e c y c l e i s c o m p l e t e d . The r e a c t i o n as a whole l o o k s l i k e t h i s : 2

Ti0 (7)

H0 2

+ 0

2

9

. hv

*

» *0H +

HO*.

The h i g h r e a c t i v i t y o f t h e r a d i c a l s "OH and HO" i s w e l l known; i t b r i n g s about t h e o x i d a t i v e d e s t r u c t i o n o f t h e b i n d e r , w h i c h we c a l l t h e " p h o t o c a t a l y t i c o x i d a t i o n c y c l e " (POC). I t i s a c y c l e i n w h i c h w a t e r and oxygen a r e c o n s t a n t l y b e i n g consumed t o d e s t r o y the b i n d e r . 2. E x p e r i m e n t a l

arguments.

2.1 I n f l u e n c e o f w a t e r . The e x p l a n a t i o n o f t h e POC i s t h e r e s u l t o f s t u d i e s l a s t i n g more t h a n t e n y e a r s (4). The most i m p o r t a n t s t a g e s and f a c t s a r e p r e s e n t e d below. When we s t a r t e d our work, o t h e r a u t h o r s were o n l y d i s c u s s i n g r e a c t i o n s f o r t h e POC i n w h i c h w a t e r was n o t p r e s e n t . (What i s meant h e r e i s H^O as a c h e m i c a l r e a c t i o n p a r t n e r , n o t t h e "wasEing-off l i q u i d " ) . T h i s i s s u r p r i s i n g , because a number o f a u t h o r s had r e p e a t e d l y o b s e r v e d t h a t no d e s t r u c t i o n by POC can t a k e p l a c e w i t h o u t w a t e r . We t h e r e f o r e began o u r own s t u d i e s by d e v o t i n g p a r t i c u l a r a t t e n t i o n t o r e a c t i o n s w i t h w a t e r . We c a r r i e d o u t c o m p a r a t i v e measurements o f t h e w a t e r v a p o r and oxygen p e r m e a b i l i t y o f b i n d e r f i l m s . The r e s u l t we o b t a i n e d was t h a t t h e p e r m e a b i l i t y o f w a t e r t h r o u g h p i g m e n t e d b i n d e r s under p r a c t i c a l c o n d i t i o n s i s on a v e r a g e 10 g r e a t e r ( c a l c u l a t e d on t h e number o f mols) t h a n t h a t o f O2. (Tab. I) T h i s showed good agreement w i t h d a t a g i v e n i n l i t e r a t u r e on unpigmented b i n d e r s and p o l y m e r s . In a s p e c i a l w e a t h e r i n g d e v i c e we c o n s t r u c t e d o u r s e l v e s (4), we l a t e r c a r r i e d o u t c h a l k i n g t e s t s i n w h i c h g r e a t c a r e was t a k e n t o e x c l u d e w a t e r w h i l e a l l o w i n g a i r t o e n t e r u n h i n d e r e d . T h i s and t h e 3



PHOTODEGRADATION


O F COATINGS

Organic Coatings and Plastics Chemistry Figure

1.

Schematic

of the photocatalytic

oxidation

cycle (POC)

(12)

Table I. Permeation of H 0 and O, Through Paint Films Under Atmospheric Conditions (2) 2

Molar permeation Molar permeation coefficient coefficient of H 0 of 0 2

2

Desmodur/Desmophen paint, unpigmented

3,1 10-11

2,0 10-7

Desmodur/Desmophen paint, 8,5% p.v.c, rutile untreated 3,6 • 10-11

1,0 10"

Nitrosynthetic lacquer, unpigmented

4,5 • 10-8

8,0 • 10-11

Nitrosynthetic lacquer, 8,5% p.v.c, rutile untreated Alkyd enamel, unpigmented Alkyd enamel, 8,5% p.v.c, rutile untreated

4,3 • 10-8

Q

9,3 • 10-11

10,0 •

7

10-11

4,5 • 10-8 4,0 10-8

Farbe & Lack



12.

VOLZ E T AL.

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Nature

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167

subsequent t e s t s were c a r r i e d o u t w i t h a i r - d r y i n g b i n d e r s b a s e d on a l k y d r e s i n s and a l s o w i t h p l a s t i c s (j>) . I n o r d e r t o p r e v e n t UVD a s f a r a s p o s s i b l e , s u i t a b l e c u t - o f f f i l t e r s were u s e d . To r e c o r d t h e l e v e l of d e s t r u c t i o n , the weight l o s s of the s p e c i m e n s was m e a s u r e d i n r e l a t i o n t o t h e w e a t h e r i n g time ( g r a v i m e t r i c t e s t ) . A s e r i e s of p a i n t specimens w i t h graduated pigment volume c o n c e n t r a t i o n s (p.v.c.) was p l a c e d i n t h e w e a t h e r i n g d e v i c e . T h e r e s u l t was t h a t , d e s p i t e the p r e s e n c e of atmospheric oxygen, t h e POC a c t u a l l y c o m e s t o a c o m p l e t e s t a n d s t i l l w h e n H20 i s a b s e n t ( F i g . 2). I t i s t h e r e a c t i o n s (5) and (6) w h i c h c a n n o t t a k e p l a c e - t h e c y c l e i s i n t e r r u p ted after equation (4). A s h a s b e e n s h o w n , t h e POC s t a r t s f r o m s u r f a c e h y d r o x y l groups which a r e c o n s t a n t l y b e i n g newly f o r m e d i n t h e c y c l e b y H2O. The e x i s t e n c e o f t h e s e h y d r o x y l g r o u p s was i n s u r e d w h e n we b e g a n o u r i n v e s t i g a t i o n s ; i n Germany, i t h a d become known p a r t i c u l a r l y as a r e s u l t of the s t u d i e s c a r r i e d out b y BOEHM a t a l . ( 6 ) , w h i c h w e r e b a s e d o n t h e r e s e a r c h r e s u l t s of other authors. 2.2 I n f l u e n c e of atmospheric oxygen. At the t i m e we s t a r t e d o u r s t u d i e s , n o n e o f t h e o t h e r a u t h o r s had s a t i s f a c t o r i l y d e a l t w i t h the q u e s t i o n of whether oxygen as w e l l as water t a k e s p a r t i n the POC. O u r own t e s t s o n t h i s q u e s t i o n w e r e c a r r i e d o u t i n the p r e v i o u s l y mentioned s p e c i a l weathering device (4). T h i s d e v i c e o f f e r s t h e p o s s i b i l i t y o f p r o d u c i n g w e a t h e r i n g c o n d i t i o n s w i t h an e x a c t l y pre-determined gas p h a s e . P a i n t specimens w i t h v a r y i n g p . v . c . o f Ti02 p i g m e n t ( a n a t a s e a n d r u t i l e ) w e r e w e a t h e r e d w i t h i r r i g a t i o n , b u t m a k i n g s u r e t o e x c l u d e any o x y g e n . H e r e , t o o , t h e POC came t o a c o m p l e t e stands t i l l ( F i g . 2). Weathering under these c o n d i t i o n s l e d t o a marked g r a y i n g of the specimens, which d e c l i n e d s l o w l y when t h e y w e r e p l a c e d i n a i r a g a i n . I f t h e g r a y e d samples were p u t i n t o h o t water, t h e g r a y i n g r e c e d e d w i t h i n a few seconds ( F i g . 3). T h e explanat i o n i s t h a t t h e POC i s s t o p p e d b e f o r e t h e r e a c t i o n (4); t h e g r a y s h a d e i s t h e v i s i b l e s i g n o f t h e presence of T i . O n l y a m a s s i v e s u p p l y o f H2O will e n s u r e t h a t t h e r e a c t i o n s o f e q u a t i o n s (4) a n d (5) continue unhindered. These t e s t s were a l s o r e p e a t e d w i t h t h e s i m u l t a n e o u s e x c l u s i o n o f H2O a n d 02- A s expected, t h e r e i s no p h o t o c a t a l y t i c o x i d a t i o n a n d t h e s p e c i m e n s d o n o t t u r n g r a y . We n a t u r a l l y a l s o c a r r i e d 3

+



168

PHOTODEGRADATION

AND

PVK

(%)

PHOTOSTABILIZATION

OF

COATINGS

• Farbe & Lack

Figure 2. Rate of degradation = f(PVC); anatase/alkyd resin: (1) weathering under exclusion of H 0; (2) weathering under exclusion of 0 ; (3) weathering in the presence of H 0 and 0 (4) 2

2

2

2

Figure 3. Regeneration of the graying by water: (A) array covered against irradiation; (B) irradiated array; (C) regenerated array by dipping in water (4)



12.

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Chemical

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169

Chalking

o u t t e s t s i n t h e p r e s e n c e o f H2O and O2 p a r a l l e l t o a l l o f t h e s e r i e s and always o b s e r v e d d e s t r u c t i o n t h r o u g h t h e POC ( F i g . 2 ) . These t e s t s a r e an i m p r e s s i v e d e m o n s t r a t i o n o f t h e i n t e r p l a y o f H2O and O2 i n the p h o t o c a t a l y t i c o x i d a t i o n c y c l e . 2.3 Dependence on t h e w a v e l e n g t h o f t h e i n c i d e n t light. Knowing t h e t h r e s h o l d w a v e l e n g t h Xq, above w h i c h no f u r t h e r POC d e s t r u c t i o n t a k e s p l a c e , i s o f i n t e r e s t f o r s e v e r a l r e a s o n s . On t h e one hand, i t was e x p e c t e d t h a t t h e v a l u e o f A g i s r e l a t e d t o t h e e n e r g e t i c d a t a o f t h e r u t i l e and a n a t a s e l a t t i c e . S e c o n d l y , i t i s p o s s i b l e on knowing Ag t o s e p a r a t e t h e p a r t o f t h e UVD i n t h e c h a l k i n g b e i n g u n d e s i r a b l e f o r t e s t i n g t h e p h o t o c h e m i c a l pigment a c t i v i t y f r o m t h e POC p a r t . We made use i n o u r t e s t s o f UV c u t - o f f f i l t e r s , which c u t o f f the i r r a d i a t i o n spectrum below A = 300, 335, 375, 395, 435 nm ( 7 ) . The most i m p o r t a n t r e s u l t we o b t a i n e d was t h a t t h e h i g h e s t r e l a t i v e d e s t r u c t i o n r a t e i s o b t a i n e d on c o a t i n g s p i g m e n t e d w i t h T i 0 a n a t a s e u s i n g t h e 375 nm c u t - o f f f i l t e r ( r e l a t e d i n each c a s e t o t h e UVD o f t h e unp i g m e n t e d f i l m ) . S h i f t i n g t h e a b s o r p t i o n edge i n t h e d i r e c t i o n of h i g h e r wavelengths, the i n f l u e n c e of t h e pigment d e c l i n e s i n l e a p s and bounds and t h e dependence on t h e PVC d i s a p p e a r s ( F i g . 4 ) . T h i s p r o v e s t h a t Tig must be between 375 and 395 nm f o r a n a t a s e . Below 375 nm, t h e UVD becomes i n c r e a s i n g l y r e s p o n s i b l e . T h i s f i n d i n g harmonizes very w e l l w i t h t h e c o n c e p t t h a t t h e p r i m a r y s t e p i n t h e POC i s t h e f o r m a t i o n o f an e x c i t o n . T i 0 2 i s an e x t r i n s i c semic o n d u c t o r o f t h e n - t y p e and t h e d i s t a n c e between t h e v a l e n c e and t h e c o n d u c t i o n band i s : - f o r r u t i l e , 3.05 eV, i . e . o p t i c a l a b s o r p t i o n edge 415 nm - f o r a n a t a s e , 3 . 2 9 eV, " " 385 nm. W i t h t h e a b s o r p t i o n o f a quantum w i t h an energy o f more t h a n 3.05 eV r e s p . 3.29 eV, an e l e c t r o n i s l i f t e d o u t o f t h e v a l e n c e band and i n t o t h e conduct i o n band, t h e r e b y f o r m i n g an e x c i t o n ( F i g . 5 ) . T h i s i n t e r p r e t a t i o n i s a l s o s u p p o r t e d by t h e m o l e c u l a r o r b i t a l t h e o r y and t h e c r y s t a l f i e l d t h e o r y r e g a r d i n g the bonding c o n d i t i o n s i n the T i 0 lattice. 2.4 ESR d e t e r m i n a t i o n o f r a d i c a l s p e c i e s . 2.4.1 OH and HO? r a d i c a l s . R a d i c a l s t a t e s a r e g e n e r a l l y connected w i t h the presence of unpaired e l e c t r o n s , w h i c h makes t h e ESR method s u i t a b l e f o r d e t e r m i n i n g them. ESR d e t e r m i n a t i o n o f t h e OH r a d i c a l i n t h e POC was p u b l i s h e d by us i n 1970 ( 1 ) . W i t h t h e a i d o f t e s t s p e c t r a , two l i n e s g^ = 2.0118 and 2

2


PHOTODEGRADATION

170

A N D PHOTOSTABILIZATION O FCOATINGS

rel weight loss


600-

% 375 nm

500400300-

•

2001000-

395 nm

ll

limn

0 2.5 5 10 15 20 25 %

0 2,5 5 10 15 20 25 %

p.v.C.

N.V.V.T. Figure 4.

Total weight loss dependence of anatase pigmented length and on p.v.c. (1)

paint films on wave-

hv ( X < £ 1 5 n m )

conduction band (empty)

_

3 d ° ( Ti M 4

Ti * (3d ) 3

1

E= 3,05eV valence band (filled)

™"^(p)^ -

I

2p (0 ") 6

2

Farbe & Lack

Figure 5.

Schematic

of energy band model of the rutile form of Ti0

2

(4)


12.

VOLZ E T AL.

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Nature

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111

g2 = 2.0134 were o b s e r v e d between -60 °C and -175 ° C . A c c o r d i n g t o t h e s t a t e o f t h e knowledge a t t h a t t i m e , g i was a t t r i b u t e d t o t h e f r e e r a d i c a l and g2 t o a complex compound. Nowadays, t h e v i e w i s t h a t b o t h l i n e s b e l o n g t o complex compounds f r o m T i ^ *0H, HO2 and H2O2. A d e c i s i o n a s t o w h i c h l i n e s h o u l d be a t t r i b u t e d t o t h e 'OH o r t h e HO2 complex i s so d i f f i c u l t because both s p e c i e s a r e i n c o r r e l a t i o n t o each o t h e r a c c o r d i n g t o Downloaded by NORTH CAROLINA STATE UNIV on December 26, 2012 | http://pubs.acs.org Publication Date: April 8, 1981 | doi: 10.1021/bk-1981-0151.ch012

+

(8)

HO* + H 0

*0H + H 0

2

2

2

Many a u t h o r s have c a r r i e d o u t a g r e a t d e a l o f work on p r o v i n g t h e e x i s t e n c e o f H2O2. F o r t h e f i r s t t i m e PAPPAS and FISCHER suceeded i n d e t e r m i n i n g i t w i t h t h e a i d o f an e x t r e m e l y s e n s i t i v e a n a l y s i s method w i t h p e r o x i d a s e a s u s e d i n b i o c h e m i s t r y ( 8 ) . The c o n c e n t r a t i o n s d e t e r m i n e d i n t h e POC a r e e x t r e m e l y low and a r e a " b y - p r o d u c t " o f t h e POC, b u t a r e o f no f u r t h e r i m p o r t a n c e w i t h i n t h e POC i t s e l f . I f H2O2 were t h e o x i d i z i n g agent i n t h e POC, t h e n p h o t o i n a c t i v e s u b s t a n c e s l i k e ZnO o r CdS ought t o show a v e r y h i g h d e s t r u c t i o n by POC s i n c e i n aqueous systems t h e y p r o v i d e H 0 c o n c e n t r a t i o n s w h i c h a r e s e v e r a l o r d e r s o f magnitude h i g h e r . I t was f o u n d t i m e a n d a g a i n i n t h e s e s t u d i e s t h a t a n a t a s e , r u t i l e and t r e a t e d r u t i l e p i g m e n t s d i f f e r e d i n t h e q u a n t i t y o f r a d i c a l s they produced ( F i g . 6 - 8 ) . These d i f f e r e n c e s c o r r e s p o n d w i t h t h e f a m i l i a r stages i n the photochemical a c t i v i t y o f these pigments. 2.4.2 T i 3 + c e n t e r s . I t was a l s o p o s s i b l e t o demonstrate t h e f o r m a t i o n o f paramagnetic Ti centers during the weathering o f T i 0 with the a i d o f o u r ESR measurements a t 1.3 K. Here, t o o , a n a t a s e and r u t i l e showed q u a n t i t a t i v e d i f f e r e n c e s i n conformity with the d i f f e r e n c e s i n the p h o t o a c t i v i t y o f b o t h modif i c a t i o n s ^ _ (1) . 2.4.3_Adsorbed 0^. S e v e r a l a u t h o r s have o b s e r ved t h a t O ^ ^ j g ) i s formed on t h e T i 0 s u r f a c e under c h a l k i n g c o n d i t i o n s . We were a b l e t o p r o v e t h i s i n our ESR measurements ( 4 ) . A t g = 2.009, two ESR l i n e s i n d i c a t e t h e t r a n s f e r o f t h e e l e c t r o n t o t h e added 0 molecule. 2.5 S i m u l a t i o n o f d e s t r u c t i o n by POC. The d e s c r i p t i o n o f t h e POC g i v e n h e r e l e a d s u s t o t h e idea of "simulating" thecycle, i . e . s p e c i f i c a l l y p r o d u c i n g OH o r HO2 r a d i c a l s and a l l o w i n g them t o r e a c t w i t h t h e p a i n t f i l m . To p r o d u c e t h e r a d i c a l s , we made u s e o f t h e d e c o m p o s i t i o n o f water m o l e c u l e s 2

2

3

2

2

2


+

PHOTODEGRADATION


172

g 2.024 2.012 2.000 1.988 1.976 1.964

Figure

6.

ESR measurements


O F COATINGS

g 2.024 2012 2.000 1.988 1.976 1.964

on anatase/water: (Mt) not exposed to light; exposed to light

(right)

'OH

Figure 7. ESR measurements on untreated rutile Ti0 in water: (top) not exposed to light; ( b o t t o m j exposed to light 2

Figure 8. ESR measurements on treated rutile/water: (top) not exposed to light; ( " b o t t o m ) exposed

to light

« ' 9 2024

' 2.012 2.000 1

' 1.988 1.976 1

««H

1

1

1

1

g 2.024

2.012

2.000

1.988

1.976


1

1964

•— 1.964


12.

VOLZ E T AL.

Chemical

Nature

of

173

Chalking

by h i g h - f r e q u e n c y d i s c h a r g e ( F i g . 9) w i t h i n a s p e c i a l l y d e v e l o p e d a p p a r a t u s . The t e s t s were c a r r i e d out w i t h p a i n t specimens p i g m e n t e d w i t h T i 0 2 ( a n a t a s e and r u t i l e ) , and p a r a l l e l t o t h i s , t h e same specimens were s u b j e c t e d t o n o r m a l w e a t h e r i n g i n a c o n v e n t i o n a l d e v i c e . When d i s t i n c t s i g n s o f c h a l k i n g a p p e a r e d , t e s t s were c a r r i e d o u t on b o t h s e r i e s o f specimens u s i n g t h e KEMPF t e s t . We f o u n d t h a t t h e r e p l i c a s o f t h e specimens f r o m t h e h i g h - f r e q u e n c y a p p a r a t u s d i d not d i f f e r f r o m t h o s e f r o m t h e w e a t h e r i n g d e v i c e ( 1 ) . These r e s u l t s a r e i n an i m p r e s s i v e agreement w i t h t h e f a c t t h a t h y d r o x y l and p e r h y d r o x y l r a d i c a l s a r e r e s p o n s i b l e f o r t h e phenomena w h i c h we o b s e r v e d i n t h e POC and w h i c h c o n t r i b u t e t o c h a l k i n g . 3. C o n c l u s i o n s ,

practical

consequences.

3.1 P h e n o m e n o l o q i c a l c o n s e q u e n c e s . 3.1.1 P h o t o a c t i v i t y o f T i O ^ m o d i f i c a t i o n s . We s h o u l d be a b l e t o e x p e c t o f a r e a c t i o n s y s t e m drawn up by e x p e r i m e n t t h a t i t r e f l e c t s o r e x p l a i n s t h e o b s e r v e d phenomena c o r r e c t l y . I n p a r t 2, a few s u c h f a c t s were d e a l t w i t h i n t h e l i g h t o f t h e POC r e a c t i o n s y s t e m . H e r e a r e a few more: - the d i f f e r i n g photoactivity of the T i 0 modifications - t h e " i n t e r n a l d e s t r u c t i o n " t h r o u g h t h e POC - t h e p r o t e c t i v e e f f e c t o f t h e Ti02 pigments a g a i n s t UV degradation. Of t h e m o d i f i c a t i o n s a n a t a s e and r u t i l e , t h e a n a t a s e ought t o have t h e h i g h e s t p h o t o s t a b i l i t y i f we l o o k o n l y a t t h e p o s i t i o n o f t h e a b s o r p t i o n edge. But s i n c e t h e e n e r g y c o n t e n t o f t h e e x c i t o n i s h i g h e r with the anatase than i t i s with the r u t i l e , the p r o b a b i l i t y of r e a c t i o n with the T i i o ni s greater d e s p i t e t h e s m a l l e r number o f e x c i t o n s . The f a c t t h a t r u t i l e i s more s t a b l e i s a l s o e x p l a i n e d by t h e more r i g i d bond o f i t s s u r f a c e h y d r o x y l g r o u p s . BOEHM (9) has shown t h i s by c o n s i d e r i n g t h e bond s t r e n g t h o f t h e OH" on T i 0 f r a c t u r e s u r f a c e s . A c c o r d i n g l y , a l t h o u g h t h e r u t i l e more r e a d i l y forms e x c i t o n s , J b h e p o s i t i v e h o l e r e a c t s slower w i t h t h e 0H~. 3.1.2 I n t e r n a l d e s t r u c t i o n and volume s h r i n k a g e . S i n c e d e s t r u c t i o n by t h e POC t a k e s p l a c e a t t h e i n t e r f a c e between T i 0 and b i n d e r , t h e r e f o r e b i n d e r i s n o t o n l y decomposed a t t h e s u r f a c e o f t h e p a i n t f i l m . T h e r e i s a l s o a c e r t a i n amount o f d e s t r u c t i o n i n s i d e and, w i t h s u f f i c i e n t f l e x i b i l i t y o f t h e b i n d e r and s u f f i c i e n t a d h e s i o n between t h e pigment and t h e b i n d e r , t h i s must l e a d t o a s h r i n k a g e i n volume. 2

4

+

2

2


PHOTODEGRADATION

174

[Skt]

A N D PHOTOSTABILIZATION O F COATINGS

l =

f(>0

10

\


i 5 -

i! i!

u 290

/7'\ 300 310

|! 320

330

340

350

360

. [nm] Farbe & Lack

Figure 9. UV emission spectra of various molecules in the high-frequency discharge (2). I(\) = intensity, measured on radiation receiver, A = wavelength. ( ) OH(H>0); (• • •; 0 ; (—' — ') N , tenfold reduced 2

2

200-| mg/(100cm2 h) 200 nm

120

35 nm

120

200 hours N.V.V.T.

Figure 10. Dependence of degradation of pigmented PE sheets (anatase, 0.2% p.v.c.) of different thickness on weathering time, by gravimetric measurement (1)



12.

VOLZ E T AL.

Chemical

Nature

of

Chalking

175

We were a b l e t o p r o v e t h i s by s e v e r a l methods i n d e p e n d e n t o f one a n o t h e r : - D e s t r u c t i o n by p h o t o c a t a l y t i c o x i d a t i o n i n t h e c a s e of v a r y i n g t h i c k n e s s e s of the p a i n t f i l m . O n l y when a c e r t a i n f i l m t h i c k n e s s i s r e a c h e d , do we f i n d a c o n s t a n t v a l u e f o r t h e w e i g h t l o s s . I f t h e f i l m t h i c k n e s s i s t o o low, t h e r a d i a t i o n i s n o t u s e d e n t i r e l y f o r t h e POC, some o f i t p a s s e s unused t h r o u g h the p a i n t f i l m ( F i g . 1 0 ) . I f t h e d e s t r u c t i o n would o n l y t a k e p l a c e on t h e s u r f a c e o f t h e f i l m , t h e n t h i s e f f e c t would n o t o c c u r . - D i f f e r e n c e between t h e measured d e c r e a s e i n f i l m t h i c k n e s s and t h a t c a l c u l a t e d f r o m t h e w e i g h t l o s s . Since with i n t e r n a l d e s t r u c t i o n i t i s only " l i g h t " b i n d e r m a t r i x w h i c h d i s a p p e a r s and n o t "heavy" pigment, t h e r e must be a d i f f e r e n c e between t h e two measurements. T h i s i s a l s o a c t u a l l y f o u n d ( F i g . 11). - Increase i n the p.v.c. I f there i s a shrinkage i n t h e volume, t h e n t h i s s h o u l d become e v i d e n t by an i n c r e a s e o f t h e p . v . c . D e t e r m i n a t i o n s have shown t h a t t h e p . v . c . i n c r e a s e s by s e v e r a l % ( F i g . 1 2 ) . 3.1.3 P r o t e c t i v e p r o p e r t i e s of r u t i l e pigments. A f u r t h e r consequence o f our p h o t o c a t a l y t i c o x i d a t i o n c y c l e i s the p r o t e c t i v e f u n c t i o n which r u t i l e p i g ments e x e r t a g a i n s t UV d e g r a d a t i o n . T h i s e f f e c t can be shown by s e v e r a l methods: - M o r p h o l o g i c a l i n v e s t i g a t i o n s . W i t h the a i d o f scanning e l e c t r o n micrographs, i t i s p o s s i b l e to d e m o n s t r a t e t h i s e f f e c t v i s i b l y ( 1 0 ) . The t e s t r e s u l t s can be r o u g h l y o u t l i n e d as f o l l o w s : w i t h a n a t a s e p i g m e n t s , d e s t r u c t i o n by p h o t o c a t a l y t i c o x i d a t i o n t a k e s p l a c e more r a p i d l y t h a n by UV d e g r a d a t i o n and t h e r e f o r e g o v e r n s t h e o v e r a l l p i c t u r e of c h a l k i n g ( F i g . 13). A f t e r weathering, t h e a n a t a s e p a r t i c l e s remain on t h e p a i n t s u r f a c e i n what might be d e s c r i b e d as " p i t s " ( F i g . 1 4 ) . In t h e c a s e o f t r e a t e d o r c o a t e d r u t i l e p i g m e n t s , on t h e o t h e r hand, d e s t r u c t i o n t h r o u g h t h e POC i s s l o w e r t h a n by UV d e g r a d a t i o n , w h i c h means t h a t UV d e g r a d a t i o n p l a y s t h e major r o l e ( F i g . 1 3 ) . R u t i l e pigment p a r t i c l e s t h e r e f o r e s t a n d on " p e d e s t a l s " , w h i c h have r e m a i n e d b e h i n d b e c a u s e t h e pigment p a r t i c l e s c a s t a shadow and p r e v e n t e d t h e UV r a d i a t i o n from r e a c h i n g the p a i n t at these r e g i o n s ( F i g . 15). - S p l i t t i n g up t h e t o t a l amount o f d e g r a d e d p a i n t m a t e r i a l i n t o a p e r c e n t a g e o f pigment and a p e r centage of b i n d e r . G r a v i m e t r i c a n a l y s i s of the d e s t r u c t i o n p r o c e s s e n a b l e s t h e mass l o s s t o be


PHOTODEGRADATION

176


O F COATINGS

H (cal) 2018161412. 10stabilized and treated

8-


64-

Figure 11. Comparison between film thickness values H, obtained by experiment and by gravimetric measurement (1)

2H (exp.)

0 6

8

10 12 14 16 18 20 um

N.V.V.T.

A

p.v.c.

N.V.V.T.

Figure

12. Dependence of increase in p.v.c. of PE sheets on weathering (anatase, 15% p.v.c; 240 pm thickness (1)

Ti0 rutile

time

Ti0 anatase

2

2

a) un weathered

Figure 13. Schematic of the degradation processes during the weathering of binders pigmented with Ti0 (2)

weathering time 240-260 h

weathering time 20-40 h

2

Farbe & Lack



12.

VOLZ E T AL.

Chemical

Nature

of

Chalking

Figure coating,

111

14. Ti0 anatase pigmented chalk rating 4, general view (2) 2

Figure 15. TiO, rutile pigmented ing, chalk rating 4, general view


coat(2)


178

PHOTODEGRADATION


OF

COATINGS

d i v i d e d i n t o pigment and b i n d e r p a r t s (7_)• When we l o o k a t t h e p r o p o r t i o n o f b i n d e r t h a t h a s become d e s t r o y e d , we f i n d t h a t , w i t h i n c r e a s i n g p . v . c , - i n t h e case o f anatase i t a l s o i n c r e a s e s ( F i g . 16), - i n t h e c a s e o f u n t r e a t e d r u t i l e i t r e m a i n s about t h e same ( F i g . 1 7 ) , - i n the case of t r e a t e d r u t i l e i t decreases (Fig.18). The l a t t e r o b s e r v a t i o n i s p r o o f o f t h e p r o t e c t i v e e f f e c t of s t a b i l i z e d r u t i l e pigments. 3.2 Means o f r e d u c i n g c h a l k i n g . 3.2.1 C h o o s i n g t h e most s t a b l e T i 0 2 m o d i f i c a t i o n . I t h a s been shown t h a t c h a l k i n g i s t h e r e s u l t o f two main p r o c e s s e s : UV d e g r a d a t i o n and t h e p h o t o c a t a l y t i c o x i d a t i o n c y c l e . What c a n be done t o s t a b i l i z e T i 0 2 p i g m e n t s i n s u c h a way t h a t t h e y c a u s e a s l i t t l e c h a l k i n g as p o s s i b l e ? A few p o s s i b i l i t i e s have a l r e a d y been m e n t i o n e d : - p r o t e c t i o n f r o m UV r a d i a t i o n , - e x c l u s i o n o f water, - e x c l u s i o n o f oxygen. I t i s o b v i o u s t h a t t h e s e p o s s i b i l i t i e s a r e unr e a l i s t i c i n p r a c t i c e as a means o f p r e v e n t i n g c h a l k i n g . Outdoor c o a t i n g s c a n n e i t h e r be p r o t e c t e d f r o m r a d i a t i o n , n o r f r o m water, n o r f r o m oxygen. T h e r e a r e , however, o t h e r p o s s i b i l i t i e s , s u c h a s s e l e c t i n g the Ti02 modifications with the highest p h o t o s t a b i l i t y . T h i s h a s a l r e a d y been d e a l t w i t h . R u t i l e p i g m e n t s have a h i g h e r s t a b i l i t y t h a n a n a t a s e p i g m e n t s , b u t i t c a n be improved even f u r t h e r by i n c o r p o r a t i n g f o r e i g n i o n s and by s u i t a b l e a f t e r treatment : 3.2.2 R e d u c i n g t h e number o f s u r f a c e Ti-OH groups. Improved s t a b i l i t y i s p o s s i b l e by i n c o r p o r a t i n g f o r e i g n ions instead of T i * into layers near t o t h e s u r f a c e o f t h e pigment p a r t i c l e . Pigment p r o d u c e r s have been d o i n g t h i s f o r many y e a r s . A l t h o u g h t h e i n c o r p o r a t i o n o f Zn o r A13+ i o n s i n t o the Ti02 a l s o leads t o the formation o f s u r f a c e h y d r o x y l i o n s , t h e y a r e u n a b l e t o f o r m OH r a d i c a l s , s i n c e t h e r e i s no s u i t a b l e e n e r g y s t a g e a v a i l a b l e as with T i / T i . 3.2.3 D e s t r o y i n g g e n e r a t e d OH and HO2 r a d i c a l s . The t r e a t e d pigment p a r t i c l e i s s u r r o u n d e d by a " c a t a l y t i c a l l y a c t i v e w a l l " i n the form o f substances w i t h a v e r y h i g h s u r f a c e a r e a . The g e n e r a t e d OH r a d i c a l s then r e a c t t o a l a r g e extent according t o +

4

(9) and

+

3

+

2 -OH — > destroy

H 0 2

each o t h e r

+

f

0

2

by r e c o m b i n a t i o n . Use h a s a l s o


12.

VOLZ E T AL.

Chemical

Nature

of

179

Chalking

1,00-


0,60-

0,20-

2,5 5 10 15 20 25 %

2,5 5 10 15 20 25 %

p.V.C.

N.V.V.T Figure 16. Dependence of total degradation and degradation of binder on p.v.c; sample: alkyd resin pigmented with Ti0 anatase ((weight loss rate: mg/(100 cm • 2

2

h))(l)

0,25-

0,20-

0,15-

2,5 5 10 15 20 25 %

2,5 5 10 15 20 25 %

p.V.C.

N.V.V.T Figure 17. Dependence of total degradation and degradation of binder on p.v.c; sample: alkyd resin pigmented with Ti0 rutile (untreated) (weight loss rate: mg/ (100 cm • h)) (1) 2

2



180

PHOTODEGRADATION

AND

PHOTOSTABILIZATION

OF

COATINGS

0,15-

0,05-

2,5 5 10 15 20 2 5 %

2,5 5 10 15 20 25 %

p.v.C.

N.V.V.T

Figure 18. Dependence sample: alkyd resin, Ti0

2

of total degradation and degradation of binder on p.v.c. rutile (coated) (weight loss rate: mg/(100 cm • h)) (1) 2

Figure 19. Ti0 rutile pigment treated with AI and Si aquates (\\) 2


12.

VOLZ E T AL.

Chemical

Nature

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Chalking

181

been made o f t h i s mechanism f o r a l o n g t i m e : The h i g h l y s t a b i l i z e d Ti02 r u t i l e pigments a r e " a f t e r t r e a t e d " o r " c o a t e d " , u s u a l l y w i t h one o r s e v e r a l l a y e r s o f S i and A l o x i d e aqueous compounds. The f a c t t h a t t h e s e compounds have a s t r u c t u r e w i t h h i g h s u r f a c e a r e a ( F i g . 19) h a s a l r e a d y been shown ( 1 1 ) . I t ensures a f a s t recombination o f the majority of the radicals.


4.

Conclusions.

The c h a l k i n g o f p a i n t s i s b r o u g h t about by two e l e m e n t a r y p r o c e s s e s : U V - d e g r a d a t i o n (UVD) and t h e p h o t o c a t a l y t i c o x i d a t i o n c y c l e (POC). O n l y w i t h t h e POC do p i g m e n t s a c t a s a c a t a l y s t . F o r T i 0 2 p i g m e n t s i n binders, the photocatalytic oxidation cycle i s c l e a r . One w a t e r m o l e c u l e and one oxygen m o l e c u l e a r e c o n v e r t e d i n t o one h y d r o x y l and one p e r h y d r o x y l r a d i c a l . The r a d i c a l s a r e t h e c a u s e o f b i n d e r d e s t r u c t i o n . Some o f t h e e x p e r i m e n t a l p r i n c i p l e s f o r t h e c h e m i c a l n a t u r e o f t h e POC a r e d e a l t w i t h h e r e : t h e p a r t p l a y e d by t h e water, t h e p a r t p l a y e d by t h e oxygen, t h e dependence on w a v e l e n g t h , ESR d e t e r m i n a t i o n o f t h e r a d i c a l s p e c i e s and t h e s i m u l a t i o n o f d e g r a d a t i o n due t o t h e POC. The p r a c t i c a l c o n s e q u e n c e s f r o m t h e r e a c t i o n s y s t e m o f t h e POC p r o v i d e e x p l a nations of the d i f f e r i n g p h o t o a c t i v i t y of the Ti02 m o d i f i c a t i o n s , an i n t e r p r e t a t i o n o f t h e i n t e r n a l d e s t r u c t i o n combined w i t h volume s h r i n k a g e o f t h e p a i n t f i l m and e x p l a n a t i o n s o f t h e p r o t e c t i o n a f f o r ded by r u t i l e p i g m e n t s a g a i n s t U V - d e g r a d a t i o n . Measures f o r r e d u c i n g c h a l k i n g f o l l o w f r o m t h e r e a c t i o n s y s t e m : t h e c h o i c e o f t h e most s t a b l e T i 0 2 m o d i f i c a t i o n ( r u t i l e ) , t h e r e d u c t i o n o f s u r f a c e Ti-OH through the i n c o r p o r a t i o n o f f o r e i g n ions i n t o the Ti0 l a t t i c e and t h e d e s t r u c t i o n o f formed OH r a d i c a l s by s u i t a b l e a f t e r t r e a t m e n t o f t h e T i 0 2 p i g m e n t . 2


182

PHOTODEGRADATION AND PHOTOSTABILIZATION OF COATINGS

Abstract. The photocatalytic oxidation cycle (POC) i s that process in chalking in which the pigment participates. This paper has shown the chemical reaction scheme for the course of this process as well as the experimental results confirming this scheme. The practical consequences of the reaction course are to be considered under two different aspects: on the one hand interpretation of the phenomenological ob servations (like differing photoactivities of the TiO modifications, "internal" destruction on chal king, protective function of the r u t i l e pigment against UV degradation), on the other hand the measures that can be taken to reduce the photoactivity of the TiO2 pigments (selection of the modification of the highest s t a b i l i t y , incorporation of foreign ions in the TiO crystal lattice, surrounding of pig ment particles by aftertreatment). Acknowledgment.We would like to thank Prof. Dr. H. P. BOEHM, Munich University, for contributingjnany fruitful discussions and interesting suggestions.


2

2

Literature Cited 1.

Völz, H.G., Kämpf, G., Fitzky, H.G., "X. FATIPEC -Congress, Congressbook", Vlg. Chemie, Weinheim, 1970, p. 107 2. Völz, H.G., Kämpf, G., Fitzky, H.G., farbe + lack, 1972, 78, p. 1037 3. Völz, H.G., Kämpf, G., Fitzky, H.G., Progr. Org. Coat., 1973, 1, p.1 4. Völz, H.G., Kämpf, G., Klaeren, A . , farbe + lack, 1976, 82, p. 805 5. Kämpf, G., J. Coat. Technol., 1979, 51, p. 51 6. Boehm, H.P., Herrmann, M., Kaluza, U . , Z. Anorg. Allgem. Chemie, 1970, 372, p. 308 7. Völz, H.G., Kämpf, G., Klaeren, A . , "XV. FATIPEC -Congress, Congressbook", N.V.V.T., Amsterdam, 1980, p. III-41 8. Pappas, S.P., Fischer, R.N., J. Paint Technol., 1974, 46, p. 65 9. Boehm, H.P., Z. Anorg. Allgem. Chemie, 1967, 352, p. 156 10. Kämpf, G., Papenroth, W., Holm, R., J. Paint Technol., 1974, 46, p. 56 11. Kämpf, G., Völz, H.G., farbe + lack, 1968, 74, p. 37 RECEIVED

October 20, 1980.


The Chemical Nature of Chalking in the Presence of Titanium Dioxide

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