Materials Degradation Caused by Acid Rain - ACS Publications

22 Effects of Acid Rain on Painted Wood Surfaces: Importance of the Substrate

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 6, 2016 | http://pubs.acs.org Publication Date: September 25, 1986 | doi: 10.1021/bk-1986-0318.ch022

R. Sam Williams U.S. Department of Agriculture, Forest Products Laboratory, One Gifford Pinchot Drive, Madison, WI 53705-2398 The effects of acid rain on painted materials can be seen in least two phenomena, degradation of the coating and degradation of the substrate. Most research on acid degradation and painted materials has focused on degradation of coatings caused by gaseous pollutants such as sulfur dioxide and nitrogen dioxide--known precursors of acid rain. This work showed that the type of pigment and extenders used in the paint formulation had a direct bearing on paint performance in an acid environment. The degradation of the substrate also has a direct bearing on coating performance. This substrate degradation may involve different failure mechanisms; therefore, future acid rain research should include the reaction to acid rain of the substrate-coating interface. Preliminary work at the Forest Products Laboratory has shown minor increases in weathering rate of uncoated wood specimens that were dipped in dilute acid periodically during xenon-arc accelerated weathering. The effect of this wood degradation on subsequent coating performance is unknown but is the topic of continuing research.

The mere mention o f the term " a c i d r a i n " o r a c i d d e p o s i t i o n ( o r a c i d p r e c i p i t a t i o n ) can u s u a l l y generate l i v e l y d i s c u s s i o n , c o n t r o v e r s y and even c o n f r o n t a t i o n among f r i e n d s , s c i e n t i s t s , and n a t i o n s . One a s p e c t o f a c i d r a i n now b e i n g d i s c u s s e d i s i t s r o l e i n the degradat i o n o f p a i n t s and p a i n t e d s u r f a c e s , even though u l t r a v i o l e t l i g h t i s c e r t a i n l y a much more severe f a c t o r i n d e g r a d i n g many p o l y m e r s , i n c l u d i n g the polymer ( b i n d e r ) i n p a i n t s . The term " a c i d r a i n " i t s e l f i s n o t e a s i l y d e f i n e d ; however f o r the purpose o f t h i s d i s c u s s i o n , l e t us i n c l u d e under a c i d d e p o s i t i o n those s o l i d s , l i q u i d s , g a s e s , o r a e r o s o l s o f man-made o r i g i n . The degree o f c o n t a m i n a t i o n , pH l e v e l s , t y p e o f a n i o n s , o r e c o l o g i c a l s i g n i f i c a n c e a r e n o t d e f i n e d , a l t h o u g h c o n t i n u e d emphasis i n t h e s e areas o f r e s e a r c h i s i m p o r t a n t . T h i s r e p o r t w i l l focus on t h e e f f e c t This chapter not subject to U.S. copyright. Published 1986, American Chemical Society

Baboian; Materials Degradation Caused by Acid Rain ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

22.

WILLIAMS

Effects of Acid Rain on Painted Wood Surfaces

311

of a c i d d e p o s i t i o n on wood and p a i n t e d ( f i n i s h e d ) wood. F i n i s h i n g problems and d e g r a d a t i o n o f f i n i s h e s depend on t h e s u b s t r a t e and, for t h i s reason, c o n s i d e r a b l e d i s c u s s i o n of the m a t e r i a l p r o p e r t i e s of wood a r e i n c l u d e d .


Normal Weathering

o f Wood

B e f o r e d i s c u s s i n g a c i d d e p o s i t i o n e f f e c t s on wood, a b r i e f r e v i e w o f normal wood w e a t h e r i n g may be h e l p f u l . (A g l o s s a r y o f w o o d - r e l a t e d terms i s i n c l u d e d a t t h e end o f t h i s paper.) As mentioned p r e v i o u s l y , UV d e g r a d a t i o n can be a severe problem w i t h many polymers. From a polymer c h e m i s t r y v i e w p o i n t , wood i s s i m i l a r t o o t h e r polymer composites. And as i s t r u e o f many s y n t h e t i c polymer c o m p o s i t e s , b o t h t h e m a t r i x and f i b e r a r e o r g a n i c polymers. I n wood, t h e f i b e r s are composed o f c e l l u l o s e c r y s t a l l i t e s surrounded by amorphous c e l l u l o s e and h e m i c e l l u l o s e s . The m a t r i x polymer i s a h i g h l y c r o s s l i n k e d a r o m a t i c polymer c a l l e d l i g n i n . L i g n i n , w i t h i t s abundance of p h e n o l i c , methoxy p h e n o l i c , and ketone groups, has s u f f i c i e n t chromophores t o be an e x c e p t i o n a l l y good UV l i g h t a b s o r b e r (.1). The a b s o r p t i o n o f t h i s energy by t h e l i g n i n i s t h e main cause o f UV d e g r a d a t i o n o f wood. T h i s UV d e g r a d a t i o n i s m a n i f e s t i n an i n i t i a l c o l o r change f o l l o w e d by t h e g r a d u a l e r o s i o n o f t h e wood s u r f a c e . T h i s e r o s i o n o r w e a t h e r i n g i s n o t t o be confused w i t h decay. Decay i s caused by f u n g i and can l e a d t o r a p i d d e t e r i o r a t i o n throughout t h e volume o f t h e wood. Weathering, on t h e o t h e r hand, i s a s u r f a c e d e t e r i o r a t i o n ; and, a l t h o u g h t h e i n i t i a l c o l o r changes can be seen w i t h i n days o r even h o u r s , t h e s u r f a c e e r o s i o n proceeds v e r y s l o w l y . The e r o s i o n r a t e f o r s o l i d wood i n temperate zones i s i n t h e o r d e r o f 1/8 t o 1/2 i n c h p e r c e n t u r y and depends m a i n l y on amount o f UV expos u r e and t h e wood s p e c i e s O , 2 ) . Other d e g r a d i n g f a c t o r s i n c l u d e m o i s t u r e , m e c h a n i c a l a b r a s i o n , temperature, and p o l l u t i o n ( 3 ) . Wood P r o p e r t i e s That A f f e c t C o a t i n g Performance Wood and wood-based m a t e r i a l s such as plywood, f i b e r b o a r d , papero v e r l a y e d p a n e l s , and f l a k e b o a r d , have s p e c i f i c p r o p e r t i e s which must be t a k e n i n t o account when f o r m u l a t i n g f i n i s h e s f o r them. Wood changes dimension as t h e m o i s t u r e c o n t e n t v a r i e s between a p p r o x i m a t e l y 30% ( f i b e r s a t u r a t i o n ) and 0% ( o v e n d r y ) . M o i s t u r e c o n t e n t s above f i b e r s a t u r a t i o n o c c u r as water i n t h e c e l l lumen and do n o t produce f u r t h e r d i m e n s i o n a l change. W i t h i n t h e range from f i b e r s a t u r a t i o n t o ovendry t h e amount o f d i m e n s i o n a l change depends on s p e c i e s (Table I ) , i n d i v i d u a l t r e e s ( F i g u r e 1 ) , and t h e type o f cut ( T a b l e I ) . Because t h e d i m e n s i o n a l changes d i f f e r i n t h e r a d i a l and t a n g e n t i a l d i r e c t i o n s o f wood, v a r i o u s c u t s d i s t o r t d i f f e r e n t l y ( F i g u r e 2 ) . Note t h e u n i f o r m s h r i n k a g e i n t h e r a d i a l ( v e r t i c a l g r a i n e d ) c u t p i e c e ( F i g u r e 2 ) . The p a i n t i n g c h a r a c t e r i s t i c s o f v a r i o u s s p e c i e s a r e l i s t e d (Table I I ) . V e r t i c a l - g r a i n e d c u t w e s t e r n r e d c e d a r and redwood l e a d as t h e most p a i n t a b l e s u b s t r a t e s . Comp a r i s o n o f t h e r a d i a l s h r i n k a g e v a l u e s f o r these s p e c i e s show them to be among t h e most d i m e n s i o n a l l y s t a b l e . I n a d d i t i o n t o t h e s e g r o s s f e a t u r e s , t h e m i c r o s t r u c t u r e a t t h e wood-paint i n t e r f a c e i s e x t r e m e l y i m p o r t a n t . Because m o i s t u r e e f f e c t s d i m e n s i o n a l changes i n wood, m o i s t u r e i s t h e b i g g e s t enemy o f p a i n t e d wood. How a c i d



2

Baldcypress Cedar: AlaskaA t l a n t i c whiteEastern redcedar IncenseNorthern whitePort-OrfordWestern redcedar Douglas-fir: Coast Interior north I n t e r i o r west

Species

7..2 10,.1 6 .8

4..9 6..9 5..0

2.,2

4..6

2..4 12 .4 10 .7 11 .8

7..8 7..7

4.,7 5.,2

3. 1 3. 3

7..6 6 .9 7 .5

9.,2 8.,8

6. 0 5. 4

2. 8 2..9

4 .8 3 .8 4 .8

10..5

6. 2

3. 8

Percent

1

Hemlock ( c o n . ) : Western Larch, western Pine : Eastern white Jack Loblolly Lodgepole Longleaf Pitch Pond Ponderosa Red Shortleaf Slash Sugar

SOFTWOODS

Q • bpecies

7. 8 9. 1 6..1 6.,6 7..4 6..7 7..5 7,.1 7,.1 6 .2 7 .2 7 .7 7 .6 5 .6

4. 2 4. 5 2. 1 3. 7 4.,8 4..3 5.,1 4..0 5..1 3..9 3,.8 4,.6 5..4 2 .9

Percent

Tangential

8..2 10..3 12..3 11,.1 12,.2 10 .9 11 .2 9 .7 11 .3 12 .3 12 .1 7 .9

12.,4 14.,0

_ Volumetric

S h r i n k a g e from green t o ovendry m o i s t u r e c o n t e n t

(30)

__ Radial

S h r i n k a g e V a l u e s o f Domestic Softwoods

S h r i n k a g e from green t o ovendry m o i s t u r e c o n t e n t _ Radial Tangential Volumetric

Table I .


Baboian; Materials Degradation Caused by Acid Rain ACS Symposium Series; American Chemical Society: Washington, DC, 1986. 6,.9 7..9 7..5 8,.3 9,.2 7,.4 7,.0 6 .8 7 .1

3,.0 4,.4

Percent

Tangential

2..9 4..5 3.,4 4..3 4..4 2..6 3..3

Radial

Species

9,.7 11,.1

11..2 11..4 11..0 12..4 13..0 9..4 9..8

Virginia Western w h i t e Redwood: Old-growth Young-growth Spruce: Black Engelmann Red Sitka Tamarack

SOFTWOODS—con.

Volumetric

1

11..9 11..8 6..8 7..0 11..3 11..0 11..8 11..5 13..6

4..4 4..9 6..8 7..1 7..8 7..5 7..4

2..6 2.,2 4..1 3..8 3..8 4..3 3..7

Volumetric

7..2 7..4

Percent

Tangential

1

4.,2 4.,1

Radial


2

e x p r e s s e d as a p e r c e n t a g e o f t h e green d i m e n s i o n . C o a s t D o u g l a s - f i r i s d e f i n e d as D o u g l a s - f i r growing i n t h e S t a t e s o f Oregon and Washington west of t h e summit o f t h e Cascade Mountains. I n t e r i o r West i n c l u d e s t h e S t a t e o f C a l i f o r n i a and a l l c o u n t i e s i n Oregon and Washington e a s t o f b u t a d j a c e n t t o t h e Cascade summit. I n t e r i o r N o r t h i n c l u d e s t h e remainder o f Oregon and Washington and t h e S t a t e s o f Idaho, Montana, and Wyoming.

r: Balsam C a l i f o r n i a red Grand Noble Pacific silver Subalpine White ;mlock: Eastern Mountain

Species


T a b l e I . S h r i n k a g e V a l u e s o f Domestic Softwoods ( 3 0 ) - - c o n .



314

M A T E R I A L S D E G R A D A T I O N C A U S E D BY A C I D R A I N

I 0

1

1

4

1

1

8 M o i s t u r e

1

1

I

12 c o n t e n t

I

I

16

t

«

2 0

( p e r c e n t )

F i g u r e 1.—An i l l u s t r a t i o n o f v a r i a t i o n i n i n d i v i d u a l t a n g e n t i a l s h r i n k a g e v a l u e s o f s e v e r a l boards o f D o u g l a s - f i r from one l o c a l i t y , d r i e d from green c o n d i t i o n ( 3 0 ) . (ML85 5194)

F i g u r e 2 . - - C h a r a c t e r i s t i c s h r i n k a g e and d i s t o r t i o n o f f l a t s , s q u a r e s , and rounds as a f f e c t e d by the d i r e c t i o n o f t h e a n n u a l r i n g s . T a n g e n t i a l s h r i n k a g e i s about t w i c e as g r e a t as r a d i a l (30). (ZM 12494F)



2

Cedar: AlaskaC a l i f o r n i a incensePort-OrfordWestern r e d c e d a r WhiteCypress Redwood Products overlaid with resint r e a t e d paper Pine: Eastern white Sugar Western w h i t e Ponderosa F i r , commercial w h i t e Hemlock Spruce D o u g l a s - f i r (lumber and plywood) Larch

Wood

1 2 2 2 2 2 2 2 2 2

2 2 2 2 2 2 2 2 2

II II II III III III III IV IV

1

-1

--

1 1

1 1

1

--

1

Conspicuousness of c h e c k i n g ; 1— least, 2- -most

--1

Resistance to cupping; l--best 4--worst

Weathering

I

I I I I I I I

1

SOFTWOODS

Ease o f k e e p i n g well painted; I--easiest, V—most exacting

Faint Faint Faint Distinct Faint Faint Faint Strong Strong

--

Faint Faint Faint Distinct Distinct Strong Distinct

Degree o f f i g u r e on flat-grained surface

C o n t i n u e d on n e x t page

Cream Cream Cream Cream White P a l e brown White Pale red Brown

—

Yellow Brown Cream Brown L i g h t brown L i g h t brown Dark brown

Color of heartwood (sapwood i s always light)

Appearance

T a b l e I I . C h a r a c t e r i s t i c s o f Woods F o r P a i n t i n g and F i n i s h i n g ( o m i s s i o n s i n t h e t a b l e i n d i c a t e i n a d e q u a t e date f o r c l a s s i f i c a t i o n ) (30)



Alder Aspen Basswood Cottonwood Magnolia Yellow-poplar Beech Birch Cherry Gum

1

a i n t e d ;

p

p

i

n

g

;

0

III III III III III III IV IV IV IV

1 2 2

2 4

—

1 2 2

2 2 4 2 2 4 4

P a l e brown P a l e brown Cream White P a l e brown P a l e brown P a l e brown L i g h t brown Brown Brown

L i g h t brown L i g h t brown Brown

2 2 2

IV IV IV

HARDWOODS

Brown

Faint Faint Faint Faint Faint Faint Faint Faint Faint Faint

Distinct Strong Strong

Faint

Conspicuousness Color of Degree o f f checking; heartwood f i g u r e on 1-least, (sapwood flat-grained 2--most i s always surface light)

SOFTWOODS--con.

i-best 4--worst

u

Appearance

2

1

c

Resistance t o

Weathering

IV

P I--easiest, V-most exacting

^l

Ease o f keeping

C h a r a c t e r i s t i c s o f Woods F o r P a i n t i n g and F i n i s h i n g ( o m i s s i o n s i n t h e t a b l e i n d i c a t e inadequate date f o r c l a s s i f i c a t i o n ) ( 3 0 ) — c o n .

Lauan (plywood) Pine : Norway Southern (lumber and plywood) Tamarack

Wood

Table I I .


WILLIAMS


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d e p o s i t i o n i n t e r a c t s w i t h m o i s t u r e and a f f e c t s p a i n t e d wood i s unknown a t t h i s t i m e . D i m e n s i o n a l changes are a f u n c t i o n o f d e n s i t y ; t h e r e f o r e the h i g h e r d e n s i t y latewood changes more t h a n the lower d e n s i t y e a r l y wood. Thus the l a r g e areas o f latewood i n f l a t - g r a i n e d lumber h o l d p a i n t p o o r l y ( F i g u r e 3 ) . T h i s i s p a r t i c u l a r l y t r u e o f the l a r g e latewood bands o f t e n found i n plywood. M i n i u t t i ( 4 , 5) r e p o r t e d d i f f e r e n t s w e l l i n g o f earlywood and latewood i n v e r t i c a l - g r a i n lumber. He a l s o showed t h a t the d i f f e r e n t i a l s w e l l i n g o f p o o r l y machined f l a t g r a i n s i d i n g developed severe s t r a i n s i n the c o a t i n g over t h e s e areas ( F i g u r e 4 ) . T h i s r a i s e d g r a i n developed c r a c k s over the latewood p o r t i o n s o f the s w e l l e d wood ( F i g u r e 5 ) . A c i d D e p o s i t i o n E f f e c t s on C o a t i n g s A comprehensive r e v i e w o f the e f f e c t s o f p o l l u t i o n on c o a t i n g s a p p l i e d t o wood and many o t h e r m a t e r i a l s was p u b l i s h e d i n 1979 (6) and i t i s not my i n t e n t t o d u p l i c a t e t h i s e f f o r t . I w i l l however, r e v i e w some o f the work t h a t impacts d i r e c t l y on wood and wood finishing. Of f i n i s h e s used o u t s i d e , the polymers t h a t can o f f e r the b e s t p r o t e c t i o n a g a i n s t a c i d d e p o s i t i o n are those not c o n t a i n i n g a c i d s e n s i t i v e groups such as e s t e r s . I n c l u s i o n of a c i d r e s i s t a n t p a i n t b i n d e r s such as v i n y l s , u r e t h a n e , and e p o x i e s would produce a c i d r e s i s t a n c e o n l y i f the o t h e r components are a l s o a c i d r e s i s t a n t . S a p o n i f i c a t i o n o f e s t e r s c a t a l y z e d by h y d r o x y l s formed d u r i n g c o r r o s i o n o f s t e e l s u b s t r a t e s may a l s o degrade p o l y e s t e r s ( 7 ) . V i r t u a l l y a l l o f the r e s e a r c h on a c i d d e p o s i t i o n e f f e c t s on f i n i s h e s has d e a l t w i t h c o a t i n g d e g r a d a t i o n and d i d not i n c l u d e the e f f e c t on the s u b s t r a t e . I n e a r l y l a b o r a t o r y experiments o f SO^ e f f e c t s on p a i n t , Holbrow (8) showed t h a t some o i l - b a s e d p a i n t s d r i e d more s l o w l y i n a SO^ contaminated atmosphere. The e f f e c t was dependent upon SO^ c o n c e n t r a t i o n , type o f o i l , and pigment.

Paints

based on l i n s e e d o i l , b o d i e d dehydrated c a s t o r o i l s , and tung o i l , w i t h t i t a n i u m d i o x i d e p i g m e n t s , were more s u s c e p t i b l e t o d r y i n g r e t a r d a t i o n t h a n unbodied dehydrated c a s t o r o i l and b a s i c pigments such as w h i t e l e a d o r z i n c o x i d e . The g r e a t e s t e f f e c t o c c u r r e d w i t h i n the f i r s t day o r two o f cure and the e f f e c t was more pronounced under m o i s t u r e condensing c o n d i t i o n s . The SO^ exposure d u r i n g the e a r l y s t a g e s o f o i l cure rendered the f i l m s m o i s t u r e s e n s i t i v e and t h e y w r i n k l e d under f u r t h e r exposure t o m o i s t c o n d i t i o n s . The d r y i n g o f l a t e x p a i n t s has n o t been e v a l u a t e d . The s o i l i n g o f p a i n t s by v a r i o u s p a r t i c u l a t e s has been documented. These p a r t i c u l a t e s i n c l u d e s u l f a t e s and c h l o r i d e s o f i r o n , c a l c i u m , and z i n c (8) as w e l l as dust from a l k a l i n e mortar ( 9 ) . Spence and Haynie (10) d i s c u s s e d two s u r v e y s by M i c h e l s o n and T o u r i n , and Booz e t a l . i n which an attempt was made t o c o r r e l a t e c o n c e n t r a t i o n o f a t m o s p h e r i c p a r t i c u l a t e m a t t e r w i t h the frequency of r e p a i n t i n g . A l t h o u g h t h e s e s u r v e y s showed a c o r r e l a t i o n , t h e r e are many unanswered q u e s t i o n s . The e f f e c t s o f o t h e r p o l l u t a n t s , the t y p e o f p a i n t u s e d , and the s o c i a l and economic f a c t o r s a f f e c t i n g p a i n t i n g frequency were not t a k e n i n t o a c c o u n t .



22.

WILLIAMS


F i g u r e 3 . - - P a i n t f a i l u r e over latewood o f f l a t - g r a i n e d (30). (M147 211-12)

319

siding



320


F i g u r e 4 . — E n l a r g e d view o f t h e end g r a i n o f p a i n t e d wood b e f o r e ( t o p ) and a f t e r (bottom) s w e l l i n g w i t h water. The p a i n t i s one t h i c k c o a t o f s o l v e n t - b o r n e p r i m e r . (Upper): Dry. (Lower): Wet (4). (M121 550)

F i g u r e 5 . - - E n l a r g e d views o f t h e p a i n t e d f a c e ( t o p ) and end g r a i n (bottom) o f a p i e c e o f wood s i m i l a r t o t h a t i n F i g u r e 4. The c r a c k i n t h e t h i c k c o a t o f s o l v e n t - b o r n e p r i m e r i s over t h e earlywoodlatewood j u n c t i o n and developed a f t e r water was p l a c e d on t h e end g r a i n t o s w e l l t h e crushed springwood c e l l s beneath t h e band o f latewood ( 4 ) . (M121 551)


22.

WILLIAMS


321

G u t f r e u n d ( H ) used exposure o f p a i n t f i l m s t o 0^ as a means o f


p r e d i c t i n g p a i n t performance. H i g h e r grades o f p a i n t e m b r i t t l e d l e s s d u r i n g exposure t o t h i s p o l l u t a n t . I n the e a r l y 1970's a s e r i e s o f experiments were begun by Campbell, Spence, S c h u r r , and o t h e r c o l l a b o r a t o r s t o a s s e s s t h e e f f e c t s o f a c i d d e p o s i t i o n on p a i n t f i l m s . The work was a l o g i c a l c o n t i n u a t i o n o f p r e v i o u s work by Holbrow, T i c e , and G u t f r e u n d . I n the f i r s t o f t h e s e e x p e r i m e n t s , u s i n g s t a i n l e s s s t e e l s u b s t r a t e s , the a c c e l e r a t e d e r o s i o n o f t h e p a i n t s u r f a c e by SO^ and 0^ was e v a l uated by a t t e n u a t e d t o t a l r e f l e c t a n c e i n f r a r e d s p e c t r o s c o p y (ATR I R ) , 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), and by measuring p a i n t e r o s i o n or l o s s g r a v i m e t r i c a l l y . I n subsequent work, t h e p a n e l s were e v a l uated o n l y g r a v i m e t r i c a l l y . A 1974 s t u d y by Campbell e t a l . (12) i n v o l v e d b o t h l a b o r a t o r y and f i e l d exposure o f f i v e c o a t i n g systems on s t a i n l e s s s t e e l panels: o i l - b a s e d house p a i n t ; l a t e x house p a i n t ; i n d u s t r i a l maintenance p a i n t ; c o i l - c o a t i n g p a i n t ; and n i t r o c e l l u l o s e / a c r y l i c a u t o motive p a i n t . The f o u r f i e l d exposure s i t e s were n o r t h c e n t r a l N o r t h Dakota; Los A n g e l e s , C a l i f o r n i a ; C h i c a g o , I l l i n o i s ; and V a l p a r a i s o , Indiana. The r e s e a r c h f o c u s e d on a c c e l e r a t e d e f f e c t s o f p o l l u t i o n on e r o s i o n , t h e normal d e g r a d a t i v e mechanism f o r a good p a i n t system, and d i d n o t i n c l u d e c a t a s t r o p h i c f a i l u r e s caused by l o s s o f p a i n t a d h e s i o n . The w e i g h t l o s s o f specimens a t low S0^ c o n c e n t r a t i o n s seemed t o c o r r e l a t e w e l l w i t h t h e p a i n t s h a v i n g CaCO^ e x t e n d e r p^g_ ments (Table

I I I ) . A t h i g h e r l e v e l s o f SO^ (1.0 ppm), t h e e r o s i o n

r a t e s were s u b s t a n t i a l l y h i g h e r . The g r e a t e s t i n c r e a s e i n e r o s i o n r a t e o c c u r r e d w i t h o i l - b a s e d house p a i n t h a v i n g CaCO^ e x t e n d e r pigments. ATR IR s p e c t r o s c o p y e v a l u a t i o n o f t h e s u r f a c e showed more r a p i d breakdown o f t h e b i n d e r i n t h e o i l / a l k y d p a i n t s t h a n i n t h e a c r y l i c - b a s e d l a t e x p a i n t s . Shaded specimens showed s i m i l a r t r e n d s but l e s s d e g r a d a t i o n . S u r f a c e e v a l u a t i o n u s i n g SEM s u b s t a n t i a t e d the r e s u l t s from t h e weight l o s s measurement. ATR IR s p e c t r o s c o p y showed t h a t o i l - b a s e d house p a i n t s have t h e g r e a t e s t s u r f a c e degradat i o n . L a t e x house p a i n t s a l s o showed severe s u r f a c e d e g r a d a t i o n . The r e s u l t s w i t h 0^ were l e s s c l e a r a l t h o u g h t h e o i l - b a s e d p a i n t was more s e v e r e l y a f f e c t e d than t h e l a t e x p a i n t . R e s u l t s o f l a b o r a t o r y exposure t o S0^ c o r r e l a t e d w e l l w i t h t h e outdoor exposure; t h e highest erosion rate occurred f o r coatings having a c i d - s e n s i t i v e e x t e n d e r pigments ( m a i n l y CaCO^) i n a r e a s o f h i g h p o l l u t i o n . The l a b o r a t o r y exposure v a l u e s o b t a i n e d f o r unshaded specimens were g e n e r a l l y h i g h e r than f o r t h e low c o n c e n t r a t i o n S0^ (0.1 ppm) and almost as h i g h as t h e h i g h c o n c e n t r a t i o n

S0^ (1.0 ppm).

based house p a i n t c o n t a i n i n g CaCO^ was an e x c e p t i o n , h a v i n g

Oiltwice

as h i g h an e r o s i o n r a t e f o r 1.0 ppm SO^ exposure (shaded) as f o r t h e 0.1 ppm S0^ (unshaded).

I n most cases t h e e f f e c t o f l i g h t and SO^

appeared a d d i t i v e e x c e p t f o r t h e o i l - b a s e d house p a i n t . Here t h e e r o s i o n was more t h a n t w i c e as f a s t as t h e sum o f S 0 and l i g h t 9


Baboian; Materials Degradation Caused by Acid Rain ACS Symposium Series; American Chemical Society: Washington, DC, 1986. 1 ± 2.2

2 ± 2.2

1 ± 1.3

1 ± 1.8

3 ± 5.4

9 ± 4.3

3 ± 1.9

0 ± 6.4

Oil

11 ±

Coil

I n d u s t r i a l maintenance

± 9.4

refinish

0

ppm

47 ± 10.6 99%

19 ± 10.5 99%

13 ± 2.9 50%

8 ± 2 99%

3 ± 1.4 95%

SHADED

1.0

concentration

0.1 ppm

SO^

ppm

Controls

4.6

Latex

Automotive

Coating

21 ± 15 87% 4 ± 4.2 55%

6 ± 8.*

6.1

6.8

1 ±

±

11 ± 3 30%

2 ± 1.6 95%

3 ± 1.4 96%

ppm

2 ± 4.4

1 ± 2.3

1.0

concentration

0.1 ppm

0^

T a b l e I I I . S l o p e o f E r o s i o n Data ( m i l s l o s s χ 1θ" / h r w i t h 95% c o n f i d e n c e l i m i t s ) Accompanied by a T - t e s t P r o b a b i l i t y (%) t h a t a S t a t i s t i c a l D i f f e r e n c e E x i s t s Between the R e s p e c t i v e S l o p e f o r a G i v e n P o l l u t a n t Type and L e v e l v s . the Zero P o l l u t a n t L e v e l , Based on A c c e l e r a t e d Weathering Data C o l l e c t e d a t 400, 700, and 1,000 Hours Only (used, w i t h p e r m i s s i o n , from Campbell e t a l . (12))


Baboian; Materials Degradation Caused by Acid Rain ACS Symposium Series; American Chemical Society: Washington, DC, 1986. 22 ± 2

20 = 7.2

Oil

141 ± 19 99%

34 ± 4.7 99%

22 ± 7 66%

12 ± 3.3

9 ± 1.7

11 ± 1 99%

3 ± 13.4

12 ± 2 . 3

3 ± 2.6 75%

4 ± 5

Coil

1.5

4 ±

5.1

OA

2 ±

ppm

UNSHADED

1.0

concentration

0.1 ppm

SO^

19 ±

refinish

0 ppm

Controls

I n d u s t r i a l maintenance

Latex

Automotive

Coating

28 ± 14 85% 15 ± 2.5 94% 45 ± 10.5 99%

10 ± 14.1

6 ± 3.3

22 ± 17.2

2 ±

± 5.< 93%

5 ± 1.3 99%

ppm

0.3

2 ± 1.7

1.0

concentration

0.1 ppm

0^

T a b l e I I I . S l o p e o f E r o s i o n Data ( m i l s l o s s χ ΙΟ" / h r w i t h 95% c o n f i d e n c e l i m i t s ) Accompanied by a T - t e s t P r o b a b i l i t y (%) t h a t a S t a t i s t i c a l D i f f e r e n c e E x i s t s Between the R e s p e c t i v e S l o p e f o r a G i v e n P o l l u t a n t Type and L e v e l v s . the Zero P o l l u t a n t L e v e l , Based on A c c e l e r a t e d Weathering Data C o l l e c t e d a t 400, 700, and 1,000 Hours Only (used, w i t h p e r m i s s i o n , from Campbell e t a l . ( 1 2 ) )


>

ο SX.

S*

S' ο

«s

in


324

exposure. A l t h o u g h t h i s r e s u l t might suggest some s y n e r g i s m between t h e s e two e f f e c t s w i t h t h e o i l - b a s e d house p a i n t , t h e data do n o t s u p p o r t e x t e n d i n g t h i s t o o t h e r p a i n t s . I t i s l i k e l y an i s o l a t e d e f f e c t caused by combining a h i g h l y U V - s e n s i t i v e b i n d e r w i t h CaCO^


e x t e n d e r pigments. The r e s u l t s from t h e s e experiments should be viewed w i t h some c a u t i o n because o f p o s s i b l e c o m p l i c a t i o n s i n measu r i n g w e i g h t l o s s . A d d i t i o n o r c o n d e n s a t i o n type r e a c t i o n s o f p o l l u t a n t s w i t h p a i n t components, d i f f u s i o n o f p o l l u t a n t s i n t o t h e f i l m , o u t g a s s i n g o f t h e p a i n t , and a c c u m u l a t i o n o f d i r t ( p a r t i c u l a r l y i n t h e f i e l d e x p o s u r e s ) can a l l b i a s t h e w e i g h t - l o s s measurements. I n a l a t e r s t u d y (1^3) , a c o n t r o l l e d environment chamber was used to i d e n t i f y d i r e c t and p o s s i b l e s y n e r g i s t i c e f f e c t s o f SO^, NO^, and 0^ on an o i l - b a s e d house p a i n t , an a c r y l i c l a t e x house p a i n t , a v i n y l c o i l c o a t i n g , and an a c r y l i c c o i l c o a t i n g , a l l on aluminum s u b s t r a t e s . V a r i a b l e s i n chamber c o n d i t i o n s i n c l u d e d t e m p e r a t u r e , r e l a t i v e humidi t y (RH), p o l l u t a n t s ( S 0 , N 0 , and 0 ) , dew, and l i g h t . The chamber 2

2

3

was c o n s t r u c t e d such t h a t t h e p a i n t s u r f a c e s were s u b j e c t e d t o m o i s t u r e - c o n d e n s i n g c o n d i t i o n s . The r e s u l t o f t h e s t u d y , based o n l y on w e i g h t l o s s , i n d i c a t e d t h a t o i l - b a s e d house p a i n t s h a v i n g s i l i c a t e e x t e n d e r s were a f f e c t e d by SO^ and RH. D e g r a d a t i o n t o t h e a c r y l i c and v i n y l c o i l c o a t i n g s was v e r y s l i g h t . I t i s s i g n i f i c a n t that the l a t e x house p a i n t f a i l e d because o f c o r r o s i o n o f t h e aluminum substrate. I n t h e case o f porous f i l m s such as l a t e x p a i n t s , t h e d i f f u s i o n o f SO^ t h r o u g h t h e f i l m was s u f f i c i e n t t o b r i n g about catastrophic paint f a i l u r e . I n a more r e c e n t 30-month outdoor exposure s t u d y i n v o l v i n g n i n e s i t e s i n t h e S t . L o u i s , M i s s o u r i , a r e a , t h e performance o f goodq u a l i t y o i l - and l a t e x - b a s e d house p a i n t s on s t a i n l e s s s t e e l were e v a l u a t e d by w e i g h t - l o s s measurements (14). The p a i n t s d i d n o t c o n t a i n CaCO^ e x t e n d e r pigments. Spencer and Haynie r e p o r t e d no e f f e c t s caused by SO^, b u t e r o s i o n r a t e s were a f u n c t i o n o f time o f w e t n e s s , t e m p e r a t u r e , and s u n l i g h t . No e x p l a n a t i o n was g i v e n f o r change i n SO^ e f f e c t s i n comparison w i t h t h e e a r l i e r i n v e s t i g a t i o n s . Because t h e p a i n t s t h a t performed p o o r l y i n t h e e a r l i e r s t u d i e s contained a c i d s e n s i t i v e extenders, the higher r e s i s t a n c e of the p a i n t s used i n t h e l a t e r s t u d y may be a t t r i b u t e d t o t h e l a c k o f t h e s e e x t e n d e r s . Spence and Haynie r e p o r t e d s i g n i f i c a n t l y lower e r o s i o n r a t e s f o r o i l - v e r s u s l a t e x - b a s e d p a i n t s . However, e r o s i o n r a t e s a l o n e may n o t be a good c r i t e r i a f o r e v a l u a t i n g l a t e x - v e r s u s o i l based p a i n t s . The major cause o f p a i n t f a i l u r e on wood i s n o t e r o s i o n b u t c a t a s t r o p h i c f a i l u r e o f t h e wood-paint i n t e r f a c e and subsequent p e e l i n g o f t h e p a i n t . R e d u c t i o n s i n e r o s i o n r a t e s t h r o u g h f o r m u l a t i n g h i g h l y c r o s s l i n k e d p a i n t s may be c o u n t e r p r o d u c t i v e . A c i d D e p o s i t i o n E f f e c t s on Wood and C e l l u l o s i c M a t e r i a l s The e f f e c t s on wood and o t h e r c e l l u l o s i c m a t e r i a l s o f a c i d r a i n and the o x i d e s o f n i t r o g e n and s u l f u r have been r e p o r t e d . Raczkowski (3) found t h a t e x p o s i n g s t r i p s o f microtomed spruce ( P i c e a a b i e s K a r s t . ) t o s u n l i g h t , wind, and r a i n r e s u l t e d i n de-


22.

WILLIAMS


325


creased t e n s i l e s t r e n g t h compared w i t h unexposed c o n t r o l s . Twelve s e t s o f specimens were exposed f o r 1-month p e r i o d s over 1 y e a r . The l o s s i n t e n s i l e s t r e n g t h a l o n g the g r a i n was g e n e r a l l y d i r e c t l y r e l a t e d t o the amount of s u n l i g h t d u r i n g the summer and f a l l ( F i g u r e 6 ) ; however, the l o s s i n s t r e n g t h f o r w i n t e r and s p r i n g seemed h i g h e r t h a n c o u l d be accounted f o r on the b a s i s of s u n l i g h t alone. The a c i d i t y of the r a i n and SO^ l e v e l s were h i g h e r d u r i n g the w i n t e r and s p r i n g , and t h i s h i g h e r l o s s was a t t r i b u t e d t o t h e s e higher a c i d i t y l e v e l s . I n i t i a l i n s p e c t i o n of the data i n d i c a t e d t h a t the main e f f e c t was caused by s u n l i g h t and t h a t t h e r e may be an a c i d e f f e c t p a r t i c u l a r l y d u r i n g the w i n t e r months. However the summer a c i d l e v e l s seem r a t h e r low. The pHs ranged from 6 t o 7.5 ( F i g u r e 7 ) . The pH of r a i n caused o n l y by atmospheric CO^ s h o u l d be 5.6. I f the study had i n c l u d e d another c o n t r o l t h a t was exposed t o s u n l i g h t but not t o a c i d c o n d i t i o n s , i t might have been p o s s i b l e t o s e p a r a t e t h e s e two e f f e c t s . I n a s e r i e s of t h r e e p a p e r s , A r n d t and Gross (15-17) r e p o r t e d on c o l o r and weight changes caused by outdoor exposure and a c c e l e r a t e d w e a t h e r i n g of wood. I n one of the p a p e r s , p a i n t e d s t e e l was a l s o evaluated. I n b o t h the wood and p a i n t e d s t e e l , o n l y c o l o r changes c o u l d be observed f o l l o w i n g outdoor and a c c e l e r a t e d w e a t h e r i n g . C o n c l u s i o n s based on c o l o r change must be viewed w i t h some c a u t i o n because, i n the case of p a i n t , minor pigment i n s t a b i l i t y c o u l d show c o l o r change w i t h o u t s e r i o u s l y a f f e c t i n g the b i n d e r i n t e g r i t y . With most wood, c o l o r changes are r e l a t e d t o the i n s t a b i l i t y of e x t r a c t i v e s , which have l i t t l e t o do w i t h the i n t e g r i t y of the wood. I n a d d i t i o n , the g r a v i m e t r i c measure of w e a t h e r i n g was c o m p l i c a t e d by s e v e r a l c o v a r i a b l e s ; the changes i n SO^ l e v e l s i n d i f f e r e n t l o c a t i o n s a l s o had d i f f e r e n t m o i s t u r e exposure, s o l a r exposure, and dry d e p o s i tion rates. The d e g r a d a t i o n of wood s u r f a c e s by ozone (0^) was a l s o s t u d i e d ( 1 8 ) . Exposure of wood a t two m o i s t u r e c o n t e n t s ( 6 % and s a t u r a t e d ) t o 0^ c o n c e n t r a t i o n s of 0.5, 1.0, and 1.5% r e s u l t e d i n a weight l o s s o f wood. However, the specimens showed no l o s s o f c r y s t a l l i n i t y or s t r e n g t h p a r a l l e l t o the g r a i n . I t appeared t h a t the l o s s i n w e i g h t was caused by d e g r a d a t i o n of the e a s i l y a c c e s s i b l e h e m i c e l l u l o s e s . The c r y s t a l l i n e c e l l u l o s e was not degraded and t h e r e f o r e the s t r e n g t h was u n a l t e r e d . Evans and Banks (19) exposed microtomed s e c t i o n s of Lime ( T i l i a v u l g a r i s ) and C o r s i c a n p i n e ( P i n u s n i g r a ) t o d i l u t e s u l f u r i c , s u l f u r o u s , n i t r i c , a c e t i c , and f o r m i c a c i d s , pH 2.0 t o 6.0, f o r up t o 12 months a t 40°C. C o n t r o l s were soaked i n water a t the same tempera t u r e . A t a pH of 2.0, a l l a c i d t r e a t m e n t s caused g r e a t e r l o s s of s t r e n g t h and toughness compared w i t h the c o n t r o l s . The amount of s t r e n g t h l o s s was pH dependent. The most i n t e r e s t i n g r e s u l t , however, was t h a t the s t r e n g t h l o s s caused by s u l f u r o u s a c i d was g r e a t e r than a l l o t h e r a c i d s a t s i m i l a r pHs. They noted t h a t the s t r e n g t h l o s s e s may be caused by a c o m b i n a t i o n of b o t h h y d r o l y t i c d e g r a d a t i o n of the h e m i c e l l u l o s e s and a s u l f o n a t i o n r e a c t i o n of the lignin. Scanning e l e c t r o n m i c r o s c o p y (SEM) data showing f a i l u r e a t the l i g n i n - r i c h m i d d l e l a m e l l a f u r t h e r s u p p o r t e d the s p e c u l a t i o n t h a t l i g n i n d e g r a d a t i o n i s p a r t i a l l y r e s p o n s i b l e f o r the decrease i n s t r e n g t h and toughness.




80

A

M

J

J A 1977

S

O

N

D

J

F

M 1978

F i g u r e 6.—The l o s s o f t e n s i l e s t r e n g t h o f spruce m i c r o s e c t i o n s , t h e mean monthly i n s o l a t i o n , and t h e a c i d i t y o f t h e r a i n w a t e r ( 3 ) . (ML85 5193) 10

Γ

A

M

J

J

A S 1977

O

N

D

J

F

M 1978

F i g u r e 7.—Mean monthly pH o f r a i n w a t e r and s u l p h u r d i o x i d e c o n c e n t r a t i o n i n t h e a i r ( 3 ) . (ML85 5192)



22.

WILLIAMS


327

F e i s t (20) and W i l l i a m s (21) have used xenon a r c a c c e l e r a t e d w e a t h e r i n g t o determine the e f f i c a c y of s u r f a c e t r e a t m e n t s o f wood. The d e g r a d a t i o n o f the s u r f a c e i s m a n i f e s t as e r o s i o n and can be measured m i c r o s c o p i c a l l y ( 2 ) . I n r e c e n t work a t the F o r e s t P r o d u c t s L a b o r a t o r y ( 2 2 ) , these t e c h n i q u e s were used t o determine the e f f e c t o f a c i d treatment on the e r o s i o n r a t e of w e s t e r n redcedar (Thuja p l i c a t a Donn ex D. Don). F i v e r e p l i c a t e specimens o f w e s t e r n redcedar and t h e i r l o n g i t u d i n a l l y end-matched c o n t r o l s were a r t i f i c i a l l y weathered (xenon a r c Weather-Ometer, a p p r o x i m a t e l y 24-h l i g h t and 4-h d e i o n i z e d water spray d a i l y ) . The Weather-Ometer was shut down f o r about 1 h Monday t h r o u g h F r i d a y i n o r d e r t o soak the specimens i n d i l u t e a c i d f o r 15 min. S i x types of a c i d soaks were used: n i t r i c and s u l f u r i c a c i d s a t pHs o f 3.0, 3.5, and 4.0. I found s i m i l a r r e s u l t s w i t h b o t h a c i d s . Compared w i t h the unsoaked c o n t r o l s , the 3.0 pH a c i d caused a 10% i n c r e a s e i n e r o s i o n r a t e . At a pH o f 3.5 the e f f e c t was a 4% i n c r e a s e due t o the a c i d , and no e f f e c t was found a t a pH o f 4.0. A l t h o u g h o n l y a l i m i t e d amount of work has been done on a i r p o l l u t i o n e f f e c t s on wood, the e f f e c t s on o t h e r c e l l u l o s i c m a t e r i a l , such as c o t t o n y a r n and f a b r i c , have been s t u d i e d by s e v e r a l groups (23-28). With minor d i f f e r e n c e s as t o the importance o f n i t r o g e n o x i d e s , the g e n e r a l r e s u l t s of these s t u d i e s showed t h a t t h e r e was an a c c e l e r a t e d d e g r a d a t i o n e f f e c t caused by p o l l u t i o n . These s t u d i e s compared the s t r e n g t h of f a b r i c f o l l o w i n g v a r i o u s exposures o f c o t t o n c l o t h t o outdoor environments h a v i n g v a r y i n g amounts o f p o l l u t i o n . The r e s u l t s i n d i c a t e d a s t r o n g c o r r e l a t i o n between s t r e n g t h l o s s and SO^ c o n c e n t r a t i o n , w i t h s o i l i n g a l s o h a v i n g an impact. A l s o as p o i n t e d out by these r e s e a r c h e r s , the s o l a r e f f e c t s cannot be negl e c t e d , p a r t i c u l a r l y d u r i n g the summer months. As f o r the mechanism o f d e g r a d a t i o n o r the i n t e r a c t i o n s of the v a r i o u s f a c t o r s , (SO^, s o i l i n g , and l i g h t ) , these a s p e c t s o f the problem have not been a d d r e s s e d . I n a d d i t i o n , the e f f e c t o f wet d e p o s i t i o n v e r s u s d r y d e p o s i t i o n has not been a d e q u a t e l y a d d r e s s e d . The work showing the e f f e c t o f s o i l i n g covers o n l y one a s p e c t o f the dry d e p o s i t i o n problem. The r e s u l t s o f the work w i t h c e l l u l o s e c l e a r l y demonstrate an e f f e c t o f a c i d d e p o s i t i o n . F u r t h e r work i s needed t o d e l i n e a t e the v a r i o u s f a c t o r s and the r e l a t i o n s h i p of the v a r i o u s f a c t o r s w i t h one another. F i n a l Comments There are s e v e r a l avenues f o r f u t u r e r e s e a r c h i n the r e s i s t a n c e o f wood and wood c o a t i n g s t o d e g r a d a t i o n by a c i d r a i n . P r e v i o u s r e s e a r c h on a c i d d e p o s i t i o n e f f e c t s on wood c o a t i n g s d i d not t a k e i n t o account the e f f e c t of the s u b s t r a t e but l i n k e d poor performance i n a c i d environments t o the extenders i n the p a i n t . I t s h o u l d be r e l a t i v e l y easy t o f o r m u l a t e p a i n t s t o overcome t h i s e r o s i o n problem. However, i n wood i t i s c a t a s t r o p h i c f a i l u r e t h a t causes most p o o r - p a i n t performance. Thus, the e f f e c t of a c i d s on t h i s i n t e r f a c e f a i l u r e s h o u l d be addressed i n f u t u r e work. I t i s i m p o r t a n t t o t e s t c o a t i n g s on the s u b s t r a t e f o r w h i c h they are designed. R e s e a r c h on p a i n t e d s t e e l w i l l not l i k e l y be a p p l i c a b l e t o p a i n t e d wood because the p r o p e r t i e s o f the p a i n t must be d i f f e r e n t i n o r d e r t o accommodate the v a s t l y d i f f e r e n t p r o p e r t i e s




328

of s t e e l and wood. A h i g h l y c r o s s - l i n k e d m o i s t u r e b a r r i e r a p p l i c a b l e t o s t e e l w i l l a c t u a l l y t r a p m o i s t u r e a t the wood-paint i n t e r f a c e and c o u l d cause p e e l i n g . These c o a t i n g s are b r i t t l e and are prone to c r a c k i n g , which l e a d s to f u r t h e r m o i s t u r e p e n e t r a t i o n . Paints for wood p r o d u c t s s h o u l d be more f l e x i b l e and porous. I n the r e s e a r c h done so f a r , specimens have not u s u a l l y been exposed t o the s p e c i f i c l o c a t i o n s where a c i d d e p o s i t i o n i s most l i k e l y t o o c c u r on a p a i n t e d s t r u c t u r e - - p r o t e c t e d areas such as under eaves and s o f f i t s . I n f a c t , i n t e r c o a t p e e l i n g of z i n c - c o n t a i n i n g p a i n t s under eaves has been l i n k e d to the f o r m a t i o n of m o i s t u r e s e n s i t i v e s a l t s t h r o u g h the a c t i o n of p o l l u t a n t s (j^2). The l i n k i s r a t h e r c i r c u m s t a n t i a l and f u r t h e r r e s e a r c h i s needed. The d i s c o l o r a t i o n or " f r o s t i n g " of l a t e x p a i n t s i s r e p o r t e d to be caused by the c o n c e n t r a t i o n of p o l l u t a n t s such as SO^, NO^, or H^S

i n protected

areas such as under eaves and

soffits

(12,

29)•

When t h e s e p o l l u t a n t s are absorbed by p a i n t f i l m s under m o i s t cond e n s i n g c o n d i t i o n s , the a c i d s t h a t form may p e n e t r a t e the f i l m and r e a c t w i t h some pigments. The s a l t s formed by t h i s r e a c t i o n l e a v e a g r a y i s h - w h i t e d e p o s i t on the p a i n t a f t e r the water evapo r a t e s . The problem i s more n o t i c e a b l e w i t h d a r k e r c o l o r s . I t i s c o n c e i v a b l e t h a t a c i d or SO^ would d i f f u s e through the f i l m over an extended p e r i o d of time and thus have an impact on the wood-paint i n t e r f a c e . I t s h o u l d be k e p t i n mind, however, t h a t the pH o f most wood s p e c i e s i s a l r e a d y i n the range of w e t - a c i d d e p o s i t i o n . Under dry d e p o s i t i o n c o n d i t i o n s , h i g h e r b u i l d u p o f a c i d (SO^) may o c c u r a t t h i s i n t e r f a c e . I t would be w o r t h w h i l e t o d e s i g n an experiment t o measure the a c i d c o n d i t i o n s under the f i l m . F u t u r e r e s e a r c h on a c i d d e g r a d a t i o n s h o u l d determine which components o f wood are most a f f e c t e d by a c i d and what e f f e c t t h i s d e g r a d a t i o n has on p a i n t performance. Assuming t h e r e i s an a c i d environment a t the wood-paint i n t e r f a c e , some h y d r o l y s i s of the h e m i c e l l u l o s e s i s p r o b a b l e . I t i s a l s o l i k e l y t h a t amorphous c e l l u l o s e would be h y d r o l y z e d . Recent work by Banks (19) l i n k e d l i g n i n d e g r a d a t i o n t o s u l f u r o u s a c i d . C r y s t a l l i n e c e l l u l o s e s h o u l d remain u n a l t e r e d ; but i f h y d r o l y s i s of the h e m i c e l l u l o s e s does o c c u r , i t i s not n e c e s s a r i l y a s u f f i c i e n t l y s e r i o u s c o n d i t i o n t o weaken the p a i n t a d h e s i o n . We know from r e c e n t work a t the F o r e s t P r o d u c t s L a b o r a t o r y t h a t b r i e f exposure o f wood t o u l t r a v i o l e t l i g h t ( n a t u r a l s u n l i g h t ) p r i o r to p a i n t i n g d r a s t i c a l l y decreases p a i n t adhesion. T h i s exposure a f f e c t s m a i n l y the l i g n i n . Thus, i t seems t h a t of the t h r e e main c o n s t i t u e n t s o f wood ( c e l l u l o s e , h e m i c e l l u l o s e , and l i g n i n ) , l i g n i n i s the most c r i t i c a l f o r p a i n t a d h e s i o n . Evaluating l i g n i n s u s c e p t i b i l i t y to d e g r a d a t i o n by a c i d d e p o s i t i o n may be the key t o u n d e r s t a n d i n g the e f f e c t s o f a c i d d e p o s i t i o n on wood and f i n i s h e d wood. Disclaimer

Notice

The use of t r a d e , f i r m , or c o r p o r a t i o n names i n t h i s p u b l i c a t i o n i s f o r the i n f o r m a t i o n and convenience o f the r e a d e r . Such use does not c o n s t i t u t e an o f f i c i a l endorsement o r a p p r o v a l by the U.S. Department o f A g r i c u l t u r e o f any p r o d u c t or s e r v i c e t o the e x c l u s i o n o f o t h e r s t h a t may be s u i t a b l e .


22.

WILLIAMS

Literature 1.

2. 3.


4. 5. 6. 7.

8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

20.

21. 22. 23. 24. 25. 26. 27. 28.


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Feist, W. C.; Hon, D. N.-S. In "Chemistry of Solid Wood;" Rowell, R. M., Ed.; ADVANCES IN CHEMISTRY SERIES No. 207, American Chemical Society: Washington, DC, 1984; pp 401-51. Feist, W. C.; Mraz, E. A. Forest Prod. J. 1978, 28(3), 38-42. Raczkowski, J. Holz als Roh-und Werkstoff 1980, 38(6), 231-34. Miniutti, V. P. Offical Digest 1963, 35(460), 451-71. Miniutti, V. P. Forest Prod. J. 1964, 571-76. Gross, H. Effects of air pollution on surface finishes and similar coatings," Erich Schmidt Verlag, Berlin, 1979. Wicks, Z. W., Jr. In "Corrosion Control by Coatings," Leidheiser, Η., Jr., Ed.; Science Press: Princeton, 1979; p. 29-34. Holbrow, G. L. J. Oil Colour Chemists' Assoc. 1962, 45(10), 701-18. Tice, E. A. J. Air Pollution Control Assoc. 1962, 12, 533. Spence, J. W.; Haynie, F. H. J. Paint Technol. 1972, 44(574), 70-74. Gutfreund, K. J. Paint Technol. 1966, 38(503), 732-39. Campbell, G. G.; Schurr, G. G.; Slawikowski, D. E.; Spence, J. W. J. Paint Tech. 1974, 46(593), 59-71. Spence, J. W.; Haynie, F.; Upham, J. B. J. Paint Technol. 1975, 47(609), 57-63. Haynie, F. H.; Spence, J. W. J. Air Pollution Control Assoc. 1984, 34, 941-44. Arndt, U.; Gross, U. Staub Reinhaltung der Luft. 1974, 34(6), 225-27. Arndt, U.; Gross, U. Staub Reinhaltung der Luft. 1976, 36(10), 405-10. Arndt, U.; Gross, U. Staub Reinhaltung der Luft. 1977, 37(2), 53-55. Dick, J. L.; Murphey, W. K. Research Briefs. 1972, 6(2), 4-6. Evans, P. D.; Banks, W. B. The International Research Group on Wood Preservation, Working Group III, Preservatives and Methods of Treatment, Sixteenth Annu. Meet., Brazil, 12-17 May 1985. Document No: IRG/WP/3326, 21 Feb. 1985. Feist, W. C. "Protection of wood surfaces with chromium trioxide;" Res. Pap. FPL 339, Madison, WI, U.S. Department of Agriculture, Forest Service, Forest Products Laboratory; 1979. Williams, R. S. J. Appl. Polym. Sci. 1983, 28, 2093-103. Williams, R. S. Unpublished. Race, E. J. Soc. Dryers Colourists 1949, 65, 55-63. Bogaty, H.; Campbell, K. S.; Appel, W. D. Textile Res. J. 1952, 22, 81-83. Morris, Μ. Α.; Young, Μ. Α.; Molig, T. A. Textile Res. J. 1964, 34, 563-64. Brysson, R. J.; Trask, B. J.; Upham, J. B.; Booras, S. G. J. Air Pollution Control Assoc. 1967, 17(5), 294-98. Brysson, R. J.; Trask, B. J.; Cooper, A. S., Jr. Am. Dyestuff Rep. 1968, 57, 512-17. Brysson, R. J., Walker, A. M.; Cooper, A. S., Jr. Textile Res. J. 1975, 45, 154-59.


330

MATERIALS

DEGRADATION

CAUSED BY ACID RAIN

29. Olympic Stain, A division of Clorox Company. Olympic Technical Information Bulletin No. 14, Bellevue, WA; 1977. 30. Forest Products Laboratory. Wood Handbook: Wood as an engineering material; Agric. Handb. 72, rev., Washington, DC; U.S. Department of Agriculture; 1974. 31. Feist, W. C.; Oviatt, A. E. "Wood siding--installing, finishing, maintaining," Home and Garden Bull. 203, Washington, DC; U.S. Department of Agriculture; 1983. 23 p.


G l o s s a r y ( E x t r a c t e d from the F o r e s t P r o d u c t s Wood Handbook, 1974.)

Laboratory,

ANNUAL GROWTH RING. The l a y e r o f wood growth put on a t r e e d u r i n g a s i n g l e growing season. I n the temperature zone the annual growth r i n g s o f many s p e c i e s ( e . g . , oaks and p i n e s ) a r e r e a d i l y d i s t i n g u i s h e d because o f d i f f e r e n c e s i n the c e l l s formed d u r i n g the e a r l y and l a t e p a r t s o f the season. I n some temperate zone s p e c i e s ( b l a c k gum and sweetgum) and many t r o p i c a l s p e c i e s , annual growth r i n g s are n o t e a s i l y r e c o g n i z e d . CELLULOSE. The c a r b o h y d r a t e t h a t i s the p r i n c i p a l c o n s t i t u e n t o f wood and forms t h e framework o f the wood c e l l s . DECAY. The d e c o m p o s i t i o n o f wood substance by f u n g i . Advanced ( o r t y p i c a l ) decay. The o l d e r stage o f decay i n w h i c h the d e s t r u c t i o n i s r e a d i l y r e c o g n i z e d because the wood has become punky, s o f t and spongy, s t r i n g y , r i n g s h a k e d , p i t t e d , or crumbly. Decided d i s c o l o r a t i o n o r b l e a c h i n g o f the r o t t e d wood i s o f t e n apparent. I n c i p i e n t decay. The e a r l y stage o f decay t h a t has not p r o ceeded f a r enough t o s o f t e n o r o t h e r w i s e p e r c e p t i b l y i m p a i r t h e hardness o f the wood. I t i s u s u a l l y accompanied by a s l i g h t d i s c o l o r a t i o n o r b l e a c h i n g o f the wood. DIMENSIONAL STABILIZATION. S p e c i a l t r e a t m e n t o f wood t o reduce t h e s w e l l i n g and s h r i n k i n g t h a t i s caused by changes i n i t s m o i s t u r e c o n t e n t w i t h changes i n r e l a t i v e h u m i d i t y . EARLYWOOD. The p o r t i o n o f the annual growth r i n g t h a t i s formed d u r i n g the the e a r l y p a r t o f the growing season. I t i s u s u a l l y l e s s dense and weaker m e c h a n i c a l l y t h a n latewood. EDGE-GRAIN LUMBER. Another term f o r v e r t i c a l - g r a i n lumber. EXTRACTIVE. Substances i n wood, n o t an i n t e g r a l p a r t o f the c e l l u l a r s t r u c t u r e , t h a t can be removed by s o l u t i on i n hot o r c o l d w a t e r , e t h e r , benzene, o r o t h e r s o l v e n t s t h a t do not r e a c t c h e m i c a l l y w i t h wood components. FIBER SATURATION POINT. The s t a g e i n the d r y i n g o r w e t t i n g o f wood a t which the c e l l w a l l s a r e s a t u r a t e d and the c e l l c a v i t i e s f r e e from water. I t a p p l i e s t o an i n d i v i d u a l c e l l o r group o f c e l l s , not t o whole b o a r d s . I t i s u s u a l l y t a k e n as a p p r o x i m a t e l y 30 p e r c e n t m o i s t u r e c o n t e n t , based on ovendry w e i g h t . FINISH (FINISHING). Wood p r o d u c t s such as d o o r s , s t a i r s , and o t h e r f i n e work r e q u i r e d t o complete a b u i l d i n g , e s p e c i a l l y t h e i n t e r i o r . A l s o , c o a t i n g s o f p a i n t , v a r n i s h , l a c q u e r , wax, e t c . , a p p l i e d t o wood s u r f a c e s t o p r o t e c t and enhance t h e i r d u r a b i l i t y or appearance. FLAT-GRAINED WOOD. Lumber t h a t has been sawed p a r a l l e l t o the p i t h and a p p r o x i m a t e l y tangent t o the growth r i n g s . Lumber i s



22.

WILLIAMS


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c o n s i d e r e d f l a t g r a i n e d when the a n n u a l growth r i n g s make an a n g l e o f l e s s t h a n 45° w i t h the s u r f a c e o f the p i e c e . HEARTWOOD. The wood e x t e n d i n g from the p i t h t o the sapwood, the c e l l s o f which no l o n g e r p a r t i c i p a t e i n the l i f e p r o c e s s e s o f the t r e e . Heartwood may c o n t a i n p h e n o l i c compounds, gums, r e s i n s , and o t h e r m a t e r i a l s t h a t u s u a l l y make i t d a r k e r and more decay r e s i s t a n t t h a n sapwood. HEMICELLULOSE. A c e l l u l o s e l i k e m a t e r i a l ( i n wood) t h a t i s e a s i l y decomposable as by d i l u t e a c i d , y i e l d i n g s e v e r a l d i f f e r e n t simple sugars. LATEWOOD. The p o r t i o n o f the annual growth r i n g t h a t i s formed a f t e r the earlywood f o r m a t i o n has ceased. I t i s u s u a l l y denser and s t r o n g e r m e c h a n i c a l l y t h a n earlywood. LIGNIN. The second most abundant c o n s t i t u e n t o f wood, l o c a t e d p r i n c i p a l l y i n the secondary w a l l and t h e m i d d l e l a m e l l a , which i s the t h i n cementing l a y e r between wood c e l l s . Chemicallyi t i s an i r r e g u l a r polymer o f s u b s t i t u t e d p r o p y l p h e n o l groups, and thus no s i m p l e c h e m i c a l f o r m u l a can be w r i t t e n f o r i t . LUMEN. I n wood anatomy, the c e l l c a v i t y . MOISTURE CONTENT. The amount o f water c o n t a i n e d i n the wood, u s u a l l y e x p r e s s e d as a p e r c e n t a g e of the w e i g h t o f the ovendry wood. RADIAL. C o i n c i d e n t w i t h a r a d i u s from the a x i s o f the t r e e o r l o g t o the c i r c u m f e r e n c e . A r a d i a l s e c t i o n i s a l e n g t h w i s e s e c t i o n i n a p l a n e t h a t passes t h r o u g h the c e n t e r l i n e o f t h e t r e e t r u n k . QUARTERSAWED LUMBER. Another term f o r v e r t i c a l - g r a i n e d lumber. RELATIVE HUMIDITY. R a t i o o f the amount o f water vapor p r e s e n t i n the a i r t o t h a t which the a i r would h o l d a t s a t u r a t i o n a t the same t e m p e r a t u r e . I t i s u s u a l l y c o n s i d e r e d on the b a s i s o f the w e i g h t o f the vapor b u t , f o r a c c u r a c y , s h o u l d be c o n s i d e r e d on the b a s i s o f vapor p r e s s u r e s . SAPWOOD. The wood of p a l e c o l o r near the o u t s i d e o f the l o g . Under most c o n d i t i o n s the sapwood i s more s u s c e p t i b l e t o decay t h a n heartwood. SPRINGWOOD. (See EARLYWOOD.) SUMMERWOOD. (See LATEWOOD.) TANGENTIAL. S t r i c t l y , c o i n c i d e n t w i t h a t a n g e n t a t the c i r c u m f e r e n c e o f a t r e e o r l o g , or p a r a l l e l t o such a t a n g e n t . I n p r a c t i c e , however, i t o f t e n means r o u g h l y c o i n c i d e n t w i t h a growth r i n g . A t a n g e n t i a l s e c t i o n i s a l o n g i t u d i n a l s e c t i o n through a tree or l i m b p e r p e n d i c u l a r t o a r a d i u s . F l a t - g r a i n e d lumber i s sawed tangentially. VERTICAL-GRAINED LUMBER. Lumber t h a t has been sawed so t h a t the wide s u r f a c e s extend a p p r o x i m a t e l y a t r i g h t a n g l e s t o the annual growth r i n g s . Lumber i s c o n s i d e r e d v e r t i c a l - g r a i n e d when the r i n g s form an a n g l e o f 45° t o 90°with the wide s u r f a c e o f the piece. WEATHERING. The m e c h a n i c a l o r c h e m i c a l d i s i n t e g r a t i o n and d i s c o l o r a t i o n o f the s u r f a c e o f wood caused by exposure t o l i g h t , the a c t i o n o f d u s t and sand c a r r i e d by w i n d s , and the a l t e r n a t e s h r i n k i n g and s w e l l i n g . RECEIVED

January 2, 1986


Materials Degradation Caused by Acid Rain - ACS Publications

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