18 Influence of Sulfur Dioxide on Organic Coatings W. FUNKE and H. HAAGEN Institut für Technische Chemie, Universität Stuttgart und Forschungsinstitut für Pigmente und Lacke, E.V., Stuttgart, Federal Republic of Germany
As far as organic coatings are concerned, SO is one of the most important deteriorating factors of the environment. SO may react directly with metal surfaces to form sulfates, either before or after the coating is applied. In case of steel such sulfates stimulate atmospheric corrosion. Reactions are also possible with vehicles and pigments, giving rise to degradation and discoloration. Such reactions are sometimes assisted by oxygen and sunlight. Special emphasize will be given to the permeability of organic coatings to SO . Though literature and data are still scanty, experimental evidence exists that S0-permeability is quite remarkable. Problems and methods to neutralize or capture permeating SO in organic coatings by special film components will be discussed. 2
2
2
2
2
The i n c r e a s i n g emission o f SO2 i n t o the atmosphere (Table I) r a i s e s many s e r i o u s problems, one o f which i s the i n f l u e n c e of t h i s p o l l u t a n t on organic coatings and on t h e i r p r o t e c t i v e function!*"^ There a r e v a r i o u s important aspects o f t h i s problem which s h a l l be subsequently reviewed and d i s c u s s e d . Corrosion P r o t e c t i o n It i s w e l l known that SO^ anions s t i m u l a t e c o r r o s i o n o f s t e e l surfaces by preventing an i n s i t u formation of i r o n oxides, which may impede the d i f f u s i o n processes i n v o l v e d i n the c o r r o s i o n r e a c t i o n s ^ . These s u l f a t e anions a r e e i t h e r formed i n the a t mosphere by o x i d a t i o n o f S 0 o r by d i r e c t r e a c t i o n w i t h the s t e e l surface i n the presence o f water to form s o - c a l l e d s u l f a t e nests^. The l a t t e r transformation may take p l a c e a t the unprot e c t e d metal surface o r p o s s i b l y , at l e a s t i n p r i n c i p a l , a f t e r SO2 has permeated the organic f i l m and a r r i v e d at the metal supp o r t . The d i f f u s i o n of SO^ anions through organic coatings seems Z
0097-6156/ 83/0229-0309S06.00/0 © 1983 American Chemical Society
EFFECTS OF HOSTILE ENVIRONMENTS
310
Table I . C h a r a c t e r i s t i c SO2 Concentration L e v e l s Condition
ppm (v/v)
Emission Value Longtime Exp. (TA L u f t ) Emission Value Short Time Exp. (TA L u f t ) MEK-Value, Exp. Time 30 min. 24 h r s . 1 yr. P e r c e p t i o n o f Smell MAC-Value Smog Alarm M o r t a l Danger K e s t e r n i c h Test 0.2 1 SO /300 1 2.0 1 SO /300 1 2
2
0.05 0.15 0.37 0.11 0.04 0.5 2.0 0.8 400 c a . 600 c a . 6000
Large Towns ( 2x10° inhab.) max. v a l u e Medium Towns (0.5-2x10° inhab.) Smaller Towns ( 0.5x10° inhab.) Rural S t a t i o n Urban S t a t i o n —
Industrial Station I n d u s t r i a l Area (Ruhr, West Germ.) S0 -conc. S t u t t g a r t Summer Winter
mg/m 3
0.15 0.33 0.05 0.04 0.00010.0075 0.00750.015 0.03
0.134 0.4 1.0 0.3 0.1 1-34 5 2.14 1068 c a . 1600 c a . 16000
—
Lit,
1 1 1 1 1 1 1 1 1 2 2
0.4 0.9 0.14 0.13 0.0010.02 0.020.04 0.08
3 4 3 3 5 5 5
max. 1.28
max. 3.4
4
max. 0.015 max. 0.05
max. 0.04 max. 0.14
1 1
2
1 ppm=l cnrvm3=2.67 mg/nP MEK=Maximal Emission Concentration MAC=Maximal Allowable Concentration - Threshold L i m i t
Value
18.
FUNKE AND HAAGEN
311
Influence of Sulfur Dioxide
not to p l a y an important r o l e . Though l i t t l e i n f o r m a t i o n i s a v a i l a b l e on SO^ p e r m e a b i l i t y , i t may be assumed t h a t i t i s lower than that o f C l ~ . 8
SO9
Permeability
A l i t e r a t u r e search r e v e a l s d i s a p p o i n t i n g l y l i t t l e i n f o r m a t i o n on SO2 p e r m e a b i l i t i e s o f o r g a n i c c o a t i n g s . SO2 p e r m e a b i l i t i e s o f alkyd r e s i n s a r e t r e a t e d i n a s e r i e s o f papers^*"!!; however the p u b l i s h e d data are d i f f i c u l t to compare with other data because they a r e not presented i n conventional dimensions. Only s l i g h t l y more data a r e a v a i l a b l e on polymer f i l m s i n g e n e r a l , part o f which have been compiled (Table I I ) . Because o f d i f f e r i n g meas u r i n g c o n d i t i o n s and l a c k i n g informations on these c o n d i t i o n s as w e l l as on the m a t e r i a l s used, even the l i t t l e data on polymer f i l m s given by v a r i o u s authors are hard to comparel^-18 Theref o r e , the only conclusions that can be drawn from t h i s l i s t a r e that SO2 has low p e r m e a b i l i t y i n g l a s s y polymers, very high p e r m e a b i l i t y i n r u b b e r - l i k e polymers, and that there i s a s i g n i f i cant decrease i n SO2 p e r m e a b i l i t y with i n c r e a s i n g polymer c r y s tallinity. m
Pressure Dependency o f SO? P e r m e a b i l i t y As may be seen from Table I , concentrations o f SO2 i n the atmosphere a r e subjected t o c o n s i d e r a b l e v a r i a t i o n s . Moreover, SO2 concentrations used i n c o r r o s i o n t e s t s a r e l a r g e r by s e v e r a l orders o f magnitude. Therefore, i t becomes h i g h l y questionable whether data obtained a t high SO2 c o n c e n t r a t i o n l e v e l s may be a l s o a p p l i e d t o p r a c t i c a l exposure c o n d i t i o n s . SO2 p e r m e a b i l i t i e s o f a s e r i e s o f polymer f i l m s l i k e p o l y e t h y l e n e , polycarbonate. polyamide-*- as w e l l as p o l y a c r y l a t e and c e l l u l o s e t r i a c e tate-*-" have been shown to depend on SO2 pressure, e s p e c i a l l y at higher SO2 c o n c e n t r a t i o n l e v e l s . In the case o f o r g a n i c coatings such data are not a v a i l a b l e . I t i s t h e r e f o r e recommended t o study SO2 p e r m e a b i l i t y a t low SO2 concentrations comparable to p r a c t i c a l exposure c o n d i t i o n s . 2
Comparison o f SO2 P e r m e a b i l i t y w i t h P e r m e a b i l i t i e s o f Other Gases Contrary to other atmospheric gases l i k e N2, 0 o r CO2, the perm e a b i l i t y o f polymer f i l m s f o r SO2 i s very high (Table I I I ) . Obv i o u s l y the molecular s i z e o f SO2 i s not the dominating f a c t o r for i t s permeation r a t e . As the p e r m e a b i l i t y c o e f f i c i e n t P^ i s defined by the product o f the d i f f u s i o n c o e f f i c i e n t , D, and t h e s o l u b i l i t y , S, the S 0 s o l u b i l i t y o f the polymer f i l m s plays an important r o l e . In f a c t t h e few data published on s o l u b i l i t y o f SO2 i n polymers corroborate t h i s e x p e c t a t i o n . E q u i l i b r i u m s o l u b i l i t y o f SO2 i n p o l y a c r y l a t e was found t o be as h i g h as 21.5% by weight at 760 mmHgl^. j case o f a b i s p h e n o l A polycarbonate, 2
2
n
312
EFFECTS OF HOSTILE ENVIRONMENTS
Table I I . SO2 P e r m e a b i l i t i e s o f Polymer Films Polymer Film
Temp. °C
Polyethylene (low density) Polyethylene (high d e n s i t y ) Polyamide (Nylon 11) a) b) Polycarbonate (Lexon) a) b) Vinylidene ChlorideV i n y l c h l o r i d e Copolymer Polyethylene Terephthalate a) b) Polymethylmethacrylate Polyvinylchloride (rigid) Teflon Polystyrene C e l l u l o s e (cellophane) Cellulose Nitrate Vulcan Rubber S i l i c o n e Rubber
Comparative SO2, 0 , N 2
2
2
2
61.4 41 131 25
145 38
2
20.9 5.68 6.58 2.16 22.4 21,0
Lit.
12 13 12 13 13 12 14 13 14 13 15 13 16 17 14 14
:
2
2
4
0,201 5.27 0,201 0.132 0.116 4,2 22 52.7 176 1450 11800
13
PSO /PO
Polyethylene (230 m) Polyamide (61 m) Polycarbonate (131 m) Vinylidene Chloride V i n y l C h l o r i d e Copolymer P o l y a c r y l a t e (22.6 m) P o l y a c r y l a t e (22.6 m)
PS0 /PN PS0 /PC0
25 22 25 22 25 25 25 22 25 22 22
230
PSO
Table I I I . and co 2 P e r m e a b i l i t i e s of Polymer Films
Polymer F i l m
1. 2.
25 25 25 25 25 25
Thickness m
2
Lit.
6.8 47 15
12 12 12
65 515 19.2
12 19 19
1
2
18.
FUNKE AND HAAGEN
313
Influence of Sulfur Dioxide 0
13.5% by weight was o b s e r v e d ^ and explained by electron-donoracceptor complexes. Data o f weight i n c r e a s e o f non-pigmented organic coatings on 24 h r s exposure to SO2 at 760 Hg support these r e s u l t s (Table I V ) . Quite o b v i o u s l y many polymer f i l m s have a high a f f i n i t y to SO2. S 0 a b s o r p t i o n by polybutylmethac r y l a t e f i l m s was s t r o n g l y i n f l u e n c e d by the s o l v e n t used i n f i l m formation (Table V ) . Probably i n t h i s case the t o t a l absorption i s composed o f two concurrent processes: c l u s t e r formation o r f i l l i n g o f h o l e s which obeys a Langmuir isotherm and o r d i n a r y d i s s o l u t i o n which obeys the Henry Law. As has been shown e a r l i e r , f i l m formation may be accompanied by phase s e p a r a t i o n l e a d i n g to incoherent microscopic or submicroscopic solvent i n c l u s i o n s , which may provide the s i t e s f o r the Langmuir a b s o r p t i o n process. Again the questions remain whether both processes a l s o occur at p r a c t i c a l SO2 c o n c e n t r a t i o n l e v e l s and how much SO2 i s absorbed t h e r e . Moreover, a gradual r e l e a s e of r e t a i n e d s o l v e n t and s t r e s s - r e l a x a t i o n a s s i s t e d by SO2 a b s o r p t i o n may give r i s e to time dependency i n a b s o r p t i o n measurements. 2
2 1
Table IV. Weight Increase o f Polymer Films A f t e r 24 Hours Exposure to Pure SO2 at Atmospheric P r e s s u r e ^ 0
Polymer
Weight Increase %
Cellulose Nitrate PVC-Copolymer Alkyd/Melamine-Resin Epoxy-Resin Polyurethane/Acrylate
3 5 14 11 6
Table V. Influence o f Solvents Used i n F i l m Formation on S 0 Absorption A f t e r 24 Hours o f E x p o s u r e 2
26
PBMA Prepared
from S o l u t i o n With
Mineral S p i r i t s Toluene MIBK Ethylacetate Isopropanol
Weight Increase %
15 16 11 6 12
314
EFFECTS OF HOSTILE ENVIRONMENTS
E f f e c t of Humidity and Pigmentation on SO? P e r m e a b i l i t y I n c r e a s i n g humidity a l s o i n c r e a s e s S 0 p e r m e a b i l i t y . However, no s i g n i f i c a n t e f f e c t s can be observed i f water a b s o r p t i o n of p o l y mer f i l m s i s low. As most o r g a n i c c o a t i n g s absorb only s m a l l a mounts o f water, normal v a r i a t i o n s i n humidity should not s i g n i f i c a n t l y a f f e c t S 0 permeation. As w i t h other f i l l e r s , pigment a t i o n s i g n i f i c a n t l y reduces S 0 p e r m e a b i l i t y 2 2 by the same reason as does polymer c r y s t a l l i n i t y . 2
2
2
Reaction of the F i l m Forming Polymers and Pigments w i t h SO? As a r e s u l t of exposure to S 0 , embrittlement and d i s c o l o r a t i o n of polymer f i l m s have been observed. For chemical r e a c t i o n s to take p l a c e , u s u a l l y the presence o f water, oxygen and sometimes U V - r a d i â t i o n i s r e q u i r e d . With hydrocarbons and U V - r a d i â t i o n , s u l f i n i c a c i d s are formed ^. No r e a c t i o n takes p l a c e w i t h p o l y ethylene i n absence of l i g h t . P o l y e t h y l e n e and polypropylene c r o s s l i n k on exposure to l i g h t and S 0 . Besides a d i r e c t r e a c t i o n w i t h the polymer, a photochemical r e a c t i o n of a c t i v a t e d S 0 w i t h oxygen may lead to formation of ozone which then may a t t a c k polymer f i l m s by o x i d a t i v e d e g r a d a t i o n . A s e r i e s of i n o r g a n i c pigments l i k e chrome y e l l o w , chrome-molybdate and ZnO are a t tacked by S 0 w i t h formation of Cr ( I I I ) s u l f a t e , l e a d s u l f a t e , and ZnS0£ , r e s p e c t i v e l y . Heavy degradation and d i s c o l o r a t i o n were observed on exposure of coated aluminum w a l l panels to S 0 and U V - l i g h t ^ . A c t i v e c o r r o s i o n p r o t e c t i v e pigments l i k e red l e a d may n e u t r a l i z e the c o r r o s i o n - s t i m u l a t i n g a c t i o n of S 0 by formation of s l i g h t l y s o l u b l e PbSO^. As these pigments are i n c r e a s i n g l y p r o h i b i t e d because of t o x i c e f f e c t s and environmental p r o t e c t i o n , i t i s imperative to i n c o r p o r a t e other SO9 scavenging components i n organic coatings to i n c r e a s e t h e i r p r o t e c t i o n e f ficiency. 2
2
2
2
2
2
2
2
Literature Cited
1) J. Baumuller and U. Hoffmann, Bauphysik, 1, 22 (1982). 2) German Standardization DIN 50 018. 3) H. C. Wohlers andG.B. Bell, "Literature review of metropolitan air pollutant concentrations", Stanford Res. Inst. Report, 1958. 4) J. Ruf, "Korrosion", Schutz durch Lacke und Pigmente, Verlag W. A. Colomb, Stuttgart, 1972. 5) L. G. Johanson and N. G. Vannerberg, Werkstoffe und Korrosion, 32, 265 (1981). 6) K. Barton, S. Bartonova and E. Beranek, Werkstoffe und Korrosion, 25, 659 (1974). 7) H. Schwarz, Werkstoffe und Korrosion, 16, 93 (1965). 8) A. L. Glass and J. Smith, J. Paint Technology, 39, 490 (1967).
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9) M. Svoboda, H. Klicova and B. Knapek, FarbeundLack, 74, 659 (1968). 10) M. Svoboda, H. Klicova and B. Knapek, J. Oil Chem. Assoc., 52, 677 (1969). 11) M. Svoboda, H. Klicova and B. Knapek, Farbe und Lack, 75, 121 (1969). 12) E. G. Davis and M. L. Rooney, Kolloid Ztschr., Ztschr. f. Polymere 249, 1043 (1971). 13) Ε. G. Davis, M. L. Rooney and P. L. Larkins, J. Appl. Polym. Sci., 19, 1829 (1975). 14) M. Benarie and Bui-the-Chuong, Atmos. Environ., 3, 574 (1969). 15) Β. E. Saltzman, C. R. Feldmann and A. E. O'Keefe, Environ. Sci.Technol.,5, 1121 (1974). 16) J. Hanonsek and L. Herynk, Chem. Listy, 56, 376 (1962). 17) P. Y. Hsieh, J. Appl. Polym. Sci., 7, 1743 (1963). 18) R. M. Felder, R. D. Spence and J. K. Ferrell, J. Chem. Eng. Data, 20, 235 (1975). 19) D. L. Kuehne and S. K. Friedlander, Ind. Eng. Chem. Process Dev., 19, 609 (1980). 20) Ε. G. Davis and M. L. Rooney, J. Polym. Sci., 10, 2325 (1972). 21) W. Funke, J. Oil Chem. Assoc., 59, 398 (1976). 22) M. Svoboda, G. Klicova and B. Knapek, Zashchita Metallov, 9, 301 (1973). 23) H. H. G. Jellinek, F. Flajsman and F. J. Kryman, J. Appl. Polym. Sci., 13, 107 (1969). 24) S. Torlaski, L. Caggiati and G. Zorzella, "Fatipec Con ference Florence 1972", Conference book p. 207, 1972. 25) H. Haagen - Unpublished results. 26) H. Haagen -.Farbe und Lack, in print. RECEIVED April4,1983
