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Tensile properties of clear coating exposed to accelerated and natural
weathering
NATIONAL RESEARCH COUNCIL OF CANADA DIVISION OF BUILDING RESEARCH
DBR INTERNAL REPORT N0.434
T E N S I L E P R O P E R T I E S O F C L E A R C O A T I N G EXPOSED T O ACCELERATED AND NATURAL WEATHERING
by H . E . A s h t o n and M. Y a s e e n
Checked by: p
.
J.
s
Approved by: L.
W.
G . Date: M a r c h 1977PREFACE The p r o p e r t i e s o f t e n s i l e s t r e n g t h and e l o n g a t i o n g r e a t l y i n f l u e n c e t h e b e h a v i o r o f c o a t i n g s d u r i n g t h e i r s e r v i c e l i f e . Pigmented c o a t i n g s t h a t e x h i b i t a r a p i d i n c r e a s e f o l l o w e d by a marked d e c r e a s e i n t e n s i l e s t r e n g t h d u r i n g e x t e r i o r c x p o s u r e a r e n o t l i k e l y t o p e r f o r m w e l l . T h i s s t u d y was u n d e r t a k e n t o d e t e r m i n e i f changes i n t h e t e n s i l e p r o p e r t i e s o f c l e a r c o a t i n g s a r e c l o s e l y r e l a t e d t o t h e i r d u r a b i l i t y on e x t e r i o r wood s i d i - n g . A c c e l e r a t e d w e a t h e r i n g i s a common method f o r r e d u c i n g t h e time r e q u i r e d t o o b t a i n r e s u l t s from n a t u r a l e x p o s u r e . U s u a l l y , however, o n l y v i s u a l a s s e s s m e n t o r measurement o f a p p e a r a n c e f a c t o r s i s used t o f o l l o w t h e e f f e c t o f w e a t h e r i n g . Another o l ? j e c t i v e o f t h e work was t o s e e w h e t h e r mechanical p r o p e r t i e s would b e more i n f o r m a t i v e and g i v e a b e t t e r comparison between n a t u r a l and a r t i f i c i a l w e a t h e r i n g . The r e p o r t d e s c r i b e s t h e t e n s i l e s t u d i e s made on t h r e e t y p e s o f c l e a r f i n i s h e s and r e l a t e s t h e d i f f e r e n c e s i n b e h a v i o r t o c o m p o s i t i o n . The r e l e v a n c e o f s t r e s s / s t r a i n t e s t s t o e x t e r i o r performance i s d i s c u s s e d . Ottawa March 1977 C . B . Crawford, D i r e c t o r , DBR/NRC
TENSILE PROPERTIES OF CLEAR COATING EXPOSED TO ACCELERATED AND NATURAL WEAlMERING
b Y
H . E . Ashton and M. Yaseen
The a t t r a c t i v e f o r c e developed between a c o a t i n g and t h e s u b s t r a t e d u r i n g t h e p r o c e s s o f a p p l i c a t i o n i s mainly r e s p o n s i b l e f o r a d h e s i o n ( l l
.
I n t e r n a l s t r e s s e s developed w i t h i n t h e c o a t i n g i n t h e c o u r s e o f d r y i n g , however, g r e a t l y i n f l u e n c e a d h e s i o n and o t h e r mechanicalproper tie^(^,^^^).
The magnitude o f b o t h t h e f o r c e o f i n t e r a c t i o n and t h e i n t e r n a l s t r e s s e s depend mostly on t h e type o f b i n d e r , t h e n a t u r e o f pigments o r f i l l e r , t h e k i n d o f s u r f a c e p r e p a r a t i o n , t h e mode o f a p p l i c a t i o n and t h e r e l a t i v e humidity o f t h e environment.The performance o f a n o r g a n i c c o a t i n g i n s e r v i c e i s r e l a t e d t o i t s
physico-chemical p r o p e r t i e s and i t s a b i l i t y t o w i t h s t a n d mechanical s t r a i n s ( 5 1
.
The chemical changes t a k i n g p l a c e i n t h e c o a t i n g i n t h e c o u r s e o f w e a t h e r i n g t e n d t o make i t h a r d and b r i t t l e and u l t i m a t e l y i t f a i l s by c r a c k i n g and f l a k i n g p r o b a b l y when t h e magnitude o f i n t e r n a l s t r e s s e s exceeds t h e a d h e s i v e f o r c e s .The c o a t i n g a l s o e x p e r i e n c e s some s t r a i n s because o f t h e dimensional changes i n t h e s u b s t r a t e w i t h changes i n t h e w e a t h e r . Unlike m e t a l s , o r g a n i c c o a t i n g s deform under r e l a t i v e l y low l o a d s . I f t h e c o a t i n g h a s s u f f i c i e n t f l e x i b i l i t y t o w i t h s t a n d t h e s t r a i n s i t may r e g a i n i t s
o r i g i n a l s t a t e a f t e r t h e s t r e s s i s r e l e a s e d . With i n c r e a s i n g time o f exposure t h e c o a t i n g l o s e s f l e x i b i l i t y and u l t i m a t e l y t h e s t r a i n s exceed
i t s a b i l i t y t o respond, r e s u l t i n g i n f a i l u r e .
C o a t i n g s t h a t l a s t f o r a r e a s o n a b l y l o n g p e r i o d have been developed u s i n g h i g h l y improved r e s i n s and pigments. The g r e a t l y i n c r e a s e d s e r v i c e l i v e s o f c o a t i n g s h a s c o n s e q u e n t l y e x t e n d e d t h e time r e q u i r e d f o r a s s e s s i n g t h e i r d u r a b i l i t y t o n a t u r a l w e a t h e r i n g . The need t o a s s e s s t h e p r o p e r t i e s o f a c o a t i n g w i t h i n a s h o r t p e r i o d has l e d t o t h e development o f a number o f a c c e l e r a t e d w e a t h e r i n g d e v i c e s and t e s t s . I n c o n v e n t i o n a l methods f o r r a p i d e v a l u a t i o n o f d u r a b i l i t y , c o a t i n g s a r e exposed t o c o n d i t i o n s presumed t o b e r e l a t e d t o w e a t h e r i n g and v i s u a l a s s e s s m e n t s o f t h e changes t a k i n g p l a c e a r e made. U s u a l l y g l o s s and c o l o r r e t e n t i o n a r e measured and o c c a s i o n a l l y changes i n
f l e x i b i l i t y a r e determined. When t h e a c c e l e r a t e d w e a t h e r i n g t e s t r e s u l t s a r e compared w i t h t h o s e o b t a i n e d from n a t u r a l w e a t h e r i n g , however,
s a t i s f a c t o r y c o r r e l a t i o n s have n o t u s u a l l y been f o u n d ( b to 3,
.
One c a u s e i s p r o b a b l y t h a t i n an a c c e l e r a t e d w e a t h e r i n g machine n o t o n l y does t h e s o u r c e e m i t r e l a t i v e l y h i g h r a d i a t i o n b u t a l s o t h e d i s t r i b u t i o n o f t h eenergy i n t h e u l t r a v i o l e t r e g i o n i s n o t always comparable t o t h e d i s t r i b u t i o n o f energy i n s u n l i g h t . B a t t e l l e Memorial ~ n s t i t u t e ( l 4,
copducted a survey on p r e d i c t i n g t h e s e r v i c e l i f e o f o r g a n i c c o a t i n g s and concluded t h a t t h e poor p r e d i c t a b i l i t y o f most a c c e l e r a t e d t e s t s i s due t o t h e d i s p r o p o r t i o n a l a c c e l e r a t i o n o f one o f t h e f a c t o r s with r e s p e c t t o t h e o t h e r s . N e v e r t h e l e s s , because o f t h e time f a c t o r , a c c e l e r a t e d w e a t h e r i n g t e s t s a r e s t i l l used t o o b t a i n p r e l i m i n a r y
r e s u l t s which s h o u l d s u b s e q u e n t l y be confirmed by n a t u r a l exposure t e s t s . Another approach has been t o measure t h e mechanical p r o p e r t i e s o f f i l m s t h a t have been exposed t o n a t u r a l weathering. S i n c e t e n s i l e s t r e n t h and e l o n g a t i o n a r e more i n f o r m a t i v e than s i m p l e f l e x i b i l i t y t e s t s p l O ) , i t was thought t h a t t r e n d s i n e x t e r i o r performance c o u l d be d e t e c t e d a t a r e l a t i v e l y e a r l y s t a g e of exposure. Tne rocedure c a n ,
g
o f c o u r s e , a l s o b e u s e d with a r t i f i c i a l w e a t h e r i n g ( 1 5 , 1 ) on t h i s b a s i s . T h i s s t u d y t h e n was undertaken t o determine t h e changes t a k i n g p l a c e i n t h e t e n s i l e p r o p e r t i e s o f c l e a r c o a t i n g s exposed t o a c c e l e r a t e d and n a t u r a l w e a t h e r i n g c o n d i t i o n s and t o compare t h e r e s u l t s o b t a i n e d from t h e two t y p e s o f exposure.
EXPERI MENTAL
M a t e r i a l s and A p p l i c a t i o n
The same p h e n o l i c s , a l k y d s and modified c o a t i n g s used i n t h e p r e v i o u s s t u d i e s were exposed t o a c c e l e r a t e d and n a t u r a l weathering. T h e i r compositions a r e summarized i n Tables 1 t o 3 . The p r e p a r a t i o n o f f i l m s used t o measure mechanical p r o p e r t i e s has a l s o been d e s c r i b e d ( 1 7 ) . A c c e l e r a t e d Weatherine
A i r - d r i e d c o a t i n g s on t i n f o i l were mounted c a r e f u l l y on a rack s o t h a t no f o l d i n t h e c o a t i n g could develop d u r i n g h a n d l i n g and weathering. To determine t h e e f f e c t o f a c c e l e r a t e d w e a t h e r i n g on t e n s i l e s t r e n g t h and f l e x i b i l i t y , t h e mounted c o a t i n g s were exposed i n a twin c a r b o n - a r c Weather-Ometer
@
o p e r a t i n g on a c y c l e o f 12 hours l i g h t w i t h o u t w a t e r and 12 hours very high r e l a t i v e humidity without l i g h t . The l a t t e r was s u b s t i t u t e d f o r t h e 12-hour w a t e r s p r a y i n t h e c c l e c u s t o m a r i l y used i nl
li,
t h i s l a b o r a t o r y t o t e s t c l e a r f i n i s h e s on wood
.
The s u b s t i t u t i o n was n e c e s s a r y because t h e f o r c e o f t h e w a t e r s p r a y might damage c o a t i n g sexposed f o r any l e n g t h o f time. I n subsequent t e s t s on c l e a r f i n i s h e s on wooden p a n e l s i t was found t h a t t h e l i g h t - h u m i d i t y c y c l e took twice a s long t o o b t a i n t h e same r e s u l t s a s t h e l i g h t - w a t e r s p r a y . The r a t i o i n t h i s c a s e may b e c l o s e r t o one because f r e e f i l m s would absorb w a t e r more r a p i d l y t h a n a t h r e e - c o a t system a p p l i e d t o wood.
N a t u r a l Weathering
To s t u d y t h e changes t a k i n g p l a c e i n t e n s i l e p r o p e r t i e s , c o a t i n g s a p p l i e d on t i n f o i l were exposed o u t d o o r s . Thick, 8- by 1 2 - i n . g l a s s
p l a t e s were u s e d a s s u p p o r t s d u r i n g e x p o s u r e . A s s u g g e s t e d i n
ASTM
D2370 a t h i n l a y e r o f c a s t o r o i l was a p p l i e d t o one s i d c o f t h e g l a s s p l a t e t o h o l d t h e c o a t e d f o i l I n c l o s e c o n t a c t . Extreme c a r e i s needed i n d e c i d i n g t h e amount o f c a s t o r o i l t o b e a p p l i e d . Thc f o i l b e a r i n g t h e c o a t i n g was t h e n p l a c e d on t h e g l a s s p l a t e and t h e s u r f a c e c o m p l e t e l y smoothed w i t h a t i s s u e . Excess f o i l was f o l d e d o v e r a l l f o u r s i d e s o f t h e p l a t e and t h e c o a t i n g t a p e d t o t h e p l a t e w i t h w a t e r p r o o f p l a s t i c - c o a t e d t a p e l e a v i n g a s u f f i c i e n t l y l a r g e a r e a f o r e x p o s u r e .The three-week a i r - d r i e d c l e a r f i n i s h e s were exposed on r a c k s f o u r f e e t above t h e ground f a c i n g s o u t h a t an a n g l e o f 45" a t t h e e x p o s u r e s i t e on t h e N a t i o n a l Research Council p r o p e r t y i n Ottawa. Exposure o f 20 s e t s o f t h e 12 p a r a p h e n y l p h e n o l i c v a r n i s h e s began d u r i n g J u l y . The same n u d e r o f s e t s o f 1 3 a l k y d c o a t i n g s and o f t h e 12 m o d i f i e d formula- t i o n s were exposed i n I i u g u s t and O c t o b e r 1968 r e s p e c t i v e l y . Twenty s e t s a l l o w e d a t o t a l exposure time o f a b o u t 16 months because o f more f r e q u e n t t e s t i n g d u r i n g t h e f i r s t few n ~ n t h s o f e x p o s u r e . A t t?,,, end o f e a c h s e l e c t e d p e r i o d o f n a t u r a l o r a c c e l e r a t e d w e a t h e r i n g one s e t o f each t y p e o f c o a t i n g was removed from t h e r a c k s f o r t e s t i n g . The o t h e r s e t s were l e f t u n d i s t u r b e d f o r f u r t h e r e x p o s u r e . The f o i l b e a r i n g t h e c o a t i n g was removed from t h e s u p p o r t and examined f o r any damage t h a t might have o c c u r r e d d u r i n g e x p o s u r e . I t was t h e n p l a c e d between two waxed p a p e r s h e e t s and c u t i n t o o n e - i n . ( 2 5 mm) s t r i p s u s i n g a Thwing-Albert
p r e c i s i o n sample c u t t e r . The c o a t i n g s i d e o f t h e s t r i p s was t a p e d a t b o t h ends and t h e s t r i p s p l a c e d on a mercury b a t h . When t h e c o a t i n g s f l o a t e d f r e e l y a f t e r amalgamation o f t h e t i n , t h e y were p i c k e d up w i t h t w e e z e r s a t a t a p e d e n d . Each f i l m was s p r e a d o n waxed p a p e r and t h e o r i g i n a l u n d e r s i d e c a r e f u l l y c l e a n e d w i t h a camel h a i r b r u s h . To o b t a i n a r e l a t i v e l y c l e a n s u r f a c e and p e r m i t e a s y h a n d l i n g o f t h e f r e e f i l m on mercury, i t was n e c e s s a z y t o u s e a l a r g e amount o f mercury i n a s p a c i o u s amal gamation b a t h .
T e n s i l e T e s t s
The o n e - i n . - w i d e s t r i p o f t h e u n s u p p o r t e d c o a t i n g was t a p e d a t e a c h end t o p r o v i d e a t h r e e - i n . (76-mm)gauge l e n g t h . The w a t e r p r o o f p l a s t i c - c o a t e d p r e s s u r e - s e n s i t i v e c l o t h t a p e a l s o a c t e d a s a g r i p f o r t h e
t e n s i l e machine jaws. The t e s t specimens were p u l l e d on a T i n i u s - O l s e n U - C e l t r o n i c
@
t e n s i l e t e s t e r e q u i p p e d w i t h a 12,000-gram c e l l . T h i s machine a l l o w s t h e l o a d t o b e d e c r e a s e d i n s t e p s down t o 120 g f o r f u l l - s c a l e d e f l e c t i o n . I t was u s u a l l y o p e r a t e d i n t h e 600-, 1,200- o r 2,400-g r a n g e s depending upon t h e s t r e n g t h o f t h e c o a t i n g . I n most c a s e s t h e c o a t i n g was p u l l e d a t a s t r a i n r a t e o f 0 . 1 2 i n . ( 3 mm) p e r minute which i s f o u r p e r c e n t o f t h e gauge l e n g t h p e r minute.The maximum l o a d i n grams a p p l i e d t o t h e 1 - i n . - w i d e s t r i p o f t h e c o a t i n g e i t h e r a t b r e a k o r a t d e f o r m a t i o n ( y i e l d p o i n t ) was d e t e r m i n e d d i r e c t l y from t h e r e c o r d i n g c h a r t on t h e machine. The t e n s i l e s t r e n g t h was c a l c u l a t e d i n t e r n s o f pounds p e r s q u a r e i n c h and e x p r e s s e d a s t h e maximum l o a d a t t h e p o i n t o f b r e a k o r y i e l d .
The maximum e x t e n s i o n i n t h e l e n g t h o f t h e t e s t specimen a t t h e p o i n t o f b r e a k was t a k e n a s t h e measure of f l e x i b i l i t y o f t h e c o a t i n g and r e p o r t e d i n terms o f p e r c e n t e l o n g a t i o n . The e x t e n s i o n i n t h e l e n g t h was determined from t h e r a t e o f s t r a i n and t h e time r e q u i r e d f o r b r e a k i n g t o o c c u r . E l o n g a t i o n was c a l c u l a t e d a s t h e p e r c e n t i n c r e a s e i n t h e gauge l e n g t h o f t h e c o a t i n g .
Because o f t h e v a r i a b i l i t y e n c o u n t e r e d i n t e n s i l e t e s t s conducted on t h i n f i l m s o f o r g a n i c c o a t i n g s , i t had been p l a n n e d t o u s e a t l e a s t t e n specimens o f each m a t e r i a l f o r one t e s t . However, i n t h e c a s e o f some c o a t i n g s exposed t o a c c e l e r a t e d o r n a t u r a l w e a t h e r i n g f o r f a i r l y long p e r i o d s t h e number o f t e s t specimens was reduced b e c a u s e t h o s e t h a t developed d e f e c t s were d i s c a r d e d . For example, most p h e n o l i c c o a t i n g s exposed i n t h e Weather-Ometer f o r more t h a n 1 2 c y c l e s and o u t d o o r s f o r more t h a n one y e a r became s o b r i t t l e t h a t t h e y b r o k e e i t h e r i n t h e p r o c e s s of removing them from t h e amalgamation b a t h o r w h i l e p r e p a r i n g
them f o r t h e t e n s i l e t e s t . Because o f t h i s t h e r e s u l t s f o r c o a t i n g s o f l o n g e r e x p o s u r e times were o b t a i n e d from a l i m i t e d number o f specimens o r c o u l d n o t b e o b t a i n e d a t a l l .
When r e s u l t s from a n i n d i v i d u a l specimen were low b o t h f o r e l o n g a - t i o n and t e n s i l e s t r e n g t h compared w i t h o t h e r specimens o f t h e same m a t e r i a l s u b j e c t e d t o t h e same w e a t h e r i n g p e r i o d , t h e y were r e j e c t e d . The p r o c e d u r e - d e s c r i b e d i n ASTM D2370 s u g g e s t s t h a t t h e l o w e s t f i v e r e s u l t s b e d i s c a r d e d a u t o m a t i c a l l y r e g a r d l e s s o f how t h e y d i f f e r from t h e o t h e r s . Some d i f f i c u l t y was e x p e r i e n c e d i n s e l e c t i n g r e s u l t s from specimens w e a t h e r e d f o r f a i r l y l o n g p e r i o d s . In most t e s t s where t h e specimen broke i n t h e approximate c e n t r e o f t h e gauge l e n g t h , t h e
r e s u l t s were t a k e n as r e 1 i a b l e
.
However, specimens o f c o a t i n g s w e a t h e r e d f o r a l o n g p e r i o d f r e q u e n t l y s h a t t e r e d a t t h e t i m e o f b r e a k b e c a u s e of poor f l e x i b i l i t y . I n such c a s e s t h e v a l i d i t y o f t h e r e s u l t s wasc o n s i d e r e d q u e s t i o n a b l e s o were n o t r e p o r t e d . Cases can be s e e n i n t h e Tables where t h e t e s t d a t a do n o t a g r e e w i t h t h o s e from t h e p r e c e e d i n g o r f o l l o w i n g e x p o s u r e . They a r e r e p o r t e d h e r e t o i l l u s t r a t e t h e
v a r i a b i l i t y e n c o u n t e r e d i n d e t e r m i n i n g t h e t e n s i l e s t r e n g t h and f l e x i - b i l i t y of c l e a r c o a t i n g s .
RESULTS AND DISCUSSION
Because t e n s i l e s t r e n g t h and e l o n g a t i o n a r e i n v e r s e l y r e l a t e d f o r most o r g a n i c c o a t i n g s , i t i s e s s e n t i a l t o c o n s i d e r both p r o p e r t i e s a t t h e same time t o o b t a i n a more complete p i c t u r e o f t h e mechanical p r o p e r t i e s o f t h e s e m a t e r i a l s . For example, h i g h t e n s i l e s t r e n g t h i s
o n l y b e n e f i c i a l i f t h e r e i s a l s o some e l o n g a t i o n , o t h e r w i s e t h e m a t e r i a l i s b r i t t l e . Conversely, h i g h e l o n g a t i o n r e q u i r e s a r e a s o n a b l e l e v e l of t e n s i l e s t r e n g t h o r t h e c o a t i n g w i l l b e weak. I f a m a t e r i a l has b o t h h i g h t e n s i l e s t r e n g t h and h i g h e l o n g a t i o n i t i s tough, b u t t h i s combina-
t i o n i s t h e e x c e p t i o n w i t h o r g a n i c c o a t i n g s .
Because o f t h e e f f e c t o f one mechanical p r o p e r t y upon t h e o t h e r , i t
sometimes done by i n t e g r a t i n g t h e a r e a under t h e s t r e s s - s t r a i n c u r v e and c o n s i d e r i n g t h e r e s u l t a s a measure o f t o u g h n e s s . T h e o r e t i c a l l y , t h e b e s t way t o t r e a t t h e t e n s i l e p r o p e r t i e s o f v i s c o e l a s t i c m a t e r i a l s i s by means o f f a i l u r e e n v e l o p e s ( 18)
.
T h i s , however, i s n o t normal p r a c t i c eb e c a u s e o f t h e l a r g e number o f t e s t s r e q u i r e d t o d e f i n e t h e f a i l u r e envelope c o m p l e t e l y . Consequently, t h e r e s u l t s o b t a i n e d i n t h i s s t u d y w i l l b e p r e s e n t e d i n t h e u s u a l terms o f t e n s i l e s t r e n g t h and e l o n g a t i o n . E f f e c t o f A c c e l e r a t e d Weathering (AW) on T e n s i l e P r o p e r t i e s P h e n o l i c s The t e n s i l e s t r e n g t h and e l o n g a t i o n o f p h e n o l i c v a r n i s h e s s u b j e c t e d t o
AW
a r e l i s t e d i n T a b l e s 4 and 5 . The manner i n which t h e s e p r o p e r t i e s change i s i l l u s t r a t e d i n F i g u r e s 1 t o 4 .With t h e 20-gal v a r n i s h e s shown i n F i g u r e s 1 and 2 , t h e one made w i t h t u n g o i l had t h e h i g h e s t i n i t i a l t e n s i l e s t r e n g t h b u t t h e l o w e s t e l o n g a t i o n . A f t e r o n l y one day o f AW, however, v a r n i s h e s made w i t h t h e t h r e e o t h e r o i l s d e c r e a s e d i n e l o n g a t i o n t o t h e same low l e v e l . The l i n s e e d v a r n i s h approached t h e t e n s i l e s t r e n g t h o f t h e t u n g v a r n i s h w h i l e t h e soya and d e h y d r a t e d c a s t o r were somewhat lower.
The i n i t i a l h i g h s t r e n g t h and low e l o n g a t i o n o f t h e t u n g v a r n i s h i n comparison w i t h t h e o t h e r s a r e a t t r i b u t e d t o t h e h i g h d e g r e e o f c o n j u - g a t e d u n s a t u r a t i o n p r e s e n t i n t u n g o i l . T h i s c o n t r i b u t e s t o r a p i d d r y i n g s o t h a t t h e f i l m i s c l o s e r t o i t s f i n a l a i r - d r i e d p r o p e r t i e s than a r e t h o s e c o n t a i n i n g t h e s l o w e r d r y i n g o i l s . I n t h e Weather-Ometer, t h e r a d i a n t e n e r g y a c c e l e r a t e s t h e d r y i n g p r o c e s s . C o n s e q u e n t l y , a 1 1 f o u r v a r n i s h e s d e c r e a s e t o t h e same l e v e l o f e l o n g a t i o n . The d i f f e r e n c e i n t e n s i l e s t r e n g t h i l l u s t r a t e s t h e d i f f e r e n c e i n r e a c t i v i t y between t h e o i l s d u r i n g v a r n i s h c o o k i n g . Tung and l i n s e e d r e a c t more w i t h t h e r e s i n , r e s u l t i n g i n h i g h e r t e n s i l e s t r e n g t h s a t a l l t i m e s d u r i n g t h e e x p o s u r e . The peak i n s t r e n g t h r e a c h e d a f t e r t h r e e t o f o u r days i s most l i k e l y r e l a t e d t o t h e p r o c e s s e s of c r o s s l i n k i n g and c h a i n s c i s s i o n which a r e c a u s e d by u l t r a v i o l e t l i g h t ( 19)
.
C r o s s l i n k i n g i s a c o n t i n u a t i o n o f t h e d r y i n g p r o c e s s s o a t f i r s t t h e t e n s i l e s t r e n g t h i n c r e a s e s . A f t e r many o f t h e a v a i l a b l e s i t e s a r e used up, c h a i ns c i s s i o n becomes predominant l e a d i n g t o d e c r e a s e s i n m o l e c u l a r s i z e and a c o r r e s p o n d i n g l y lower s t r e n g t h .
The e f f e c t o f o i l c o n t e n t o n changes i n mechanical p r o p e r t i e s d u r i n g
AW i s shown i n F i g u r e s 3 and 4 . Again a l l 4 v a r n i s h e s had low l e v e l s o f e l o n g a t i o n a f t e r o n e day o f e x p o s u r e . The i n i t i a l e l o n g a t i o n o f t h e 4 0 - g a l v a r n i s h i s u n e x p e c t e d l y low compared n o t o n l y w i t h t h a t of t h e 30-gal v a r n i s h b u t a l s o t h e 40-gal l i n s e e d v a r n i s h . The t e n s i l e s t r e n g t h a s w e l l a s t h e e l o n g a t i o n o f t h e two s h o r t t u n g o i l v a r n i s h e s a r e t h e same, w i t h i n e x p e r i m e n t a l e r r o r , t h r o u g h o u t t h e e x p o s u r e . E v i d e n t l y t h e a d d i t i o n a l r e s i n d i d n o t c a u s e t h e 1 5 - g a l v a r n i s h t o become more b r i t t l e t h a n t h e 20-gal o n e . With t h e v a r n i s h e s c o n t a i n i n g more t u n g o i l t h e peak i n t e n s i l e s t r e n g t h i s lower and was reached a few days l a t e r .
Based on i t s h i g h e r f i n a l t e n s i l e s t r e n g t h t o g e t h e r w i t h a f l e x i b i l i t y comparable t o t h e o t h e r s , one might e x p e c t t h e 30-gal v a r n i s h t o I)c t l l c
most d u r a b l e . When exposed on wood p a n e l s i t a c t u a l l y i s l e s s d u r a b l e t h a n t h e 15- and 20-gal t u n g p h e n o l i c s .
S i m i l a r l y , t h e e l o n g a t i o n r e s u l t s i n Table 5 might l e a d one t o conclude t h a t t h e 4 0 - g a l soya and DH c a s t o r s h o u l d p e r f o r m b e t t e r t h a n a l l t h e o t h e r p h e n o l i c s . In f a c t , t h e i r performance on exposed wood i s r e l a t i v e l y p o o r . One f a c t o r a s s o c i a t e d w i t h t h i s might b e t h e i r low t e n s i l e s t r e n g t h s .
A 1 kyds
The t e n s i l e s t r e n g t h s and e l o n g a t i o n r e s u l t s from c l e a r a l k y d f i l m s exposed t o AW a r e given i n T a b l e s 6 and 7 . Curves f o r most o f t h e s e f i n i s h e s a r e p l o t t e d i n F i g u r e s 5 t o 8 . When compared w i t h p h e n o l i c v a r n i - s h e s , a l k y d s have much lower t e n s i l e s t r e n g t h s which i n c r e a s e d u r i n g e x p o s u r e more g r a d u a l l y . The l a t t e r i s p r o b a b l y r e l a t e d t o t h e g r e a t e r t r a n s p a r e n c y o f a l k y d s t o W . As might be expected from t h e i r c o m p o s i t i o n , a l k y d s a r e more f l e x i b l e t h a n p h e n o l i c v a r n i s h e s and r e t a i n t h i s f l e x i b i l i t y o v e r a c o n s i d e r a b l y l o n g e r p e r i o d . I n s p i t e o f t h e i r b e t t e r mechanical p r o p e r t i e s , c l e a r a l k y d s a r e l e s s s a t i s f a c t o r y t h a n p h e n o l i c s on e x t e r i o r wood because t h e y o f f e r l e s s p r o t e c t i o n t o t h e s u b s t r a t e . F i g u r e s 5 and 6 i l l u s t r a t e t h e e f f e c t o f o i l c o n t e n t on changes i n t e n s i l e p r o p e r t i e s when t h e t y p e o f o i l and d i b a s i c a c i d i n t h e a l k y d a r e c o n s t a n t . Although a l l t h e r e s i n s were o b t a i n e d from t h e same m a n u f a c t u r e r , t h e r e a r e o b v i o u s anomalies when t h e r e s u l t s a r e compared w i t h c o m p o s i t i o n . The t e n s i l e s t r e n g t h s o f t h e a l k y d s w i t h t h e h i g h e s t
and t h e lowest o i l c o n t e n t s a r e i n t h e e x p e c t e d o r d e r b u t t h e t h r e e w i t h i n t e r m e d i a t e o i l c o n t e n t s a r e i n t h e r e v e r s e . The i n i t i a l e l o n g a - t i o n s o f t h e extremes i n o i l c o n t e n t a r e a l s o t h e o p p o s i t e o f what would b e e x p e c t e d from t h e i r t e n s i l e s t r e n g t h s . The a l k y d w i t h 6 2 . 5 p e r c e n t soya o i l (NRP 913) which has very l i t t l e s t r e n g t h a l s o h a s t h e
l o w e s t f l e x i b i l i t y f o r t h e f i r s t 6 days w h i l e t h e s h o r t o i l a l k y d (916) w i t h t h e h i g h e s t t e n s i l e s t r e n g t h a l s o h a s t h e g r e a t e s t e l o n g a t i o n b e f o r e AW. The l a t t e r , however, e v e n t u a l l y drops t o t h e l o w e s t , i n k e e p i n g w i t h i t s i n c r e a s e i n s t r e n g t h .
The poor mechanical p r o p e r t i e s o f t h e l o n g o i l alkyd a r e c o n s i s t e n t w i t h t h e b e l i e f t h a t i n such a l k y d s some o f t h e o i l i s p r e s e n t a s a m i x t u r e o n l y and i s n o t c h e m i c a l l y bonded t o t h e alkyd m o l e c u l e . I t i s i n t e r e s t i n g t h a t w h i l e t h e 6 2 . 5 p e r c e n t soya alkyd i s c l o s e i n composi- t i o n t o t h e 59 p e r c e n t a l k y d , i t s t e n s i l e p r o p e r t i e s a r e r a t h e r i n f e r i o r which was r e f l e c t e d i n t h e i r r e l a t i v e d u r a b i l i t i e s on e x t e r i o r w o o d ( 2 0 )
.
NRP 912 and 914 w i t h s i m i l a r t e n s i l e p r o p e r t i e s a r e q u i t e s i m i l a r i n e x t e r i o r performance, b e i n g a b o u t t h e b e s t i n t h e group o f a l k y d s t e s t e d a t t h a t t i m e . I t i s d i f f i c u l t t o draw c o n c l u s i o n s about t h e e f f e c t o f d i b a s i c a c i d o r o f o i l t y p e from t h e c u r v e s shown i n F i g u r e s 7 and 8. NRP 1055behaved s i m i l a r l y t o 916 i n t h a t i t had h i g h i n i t i a l e l o n g a t i o n which d e c r e a s e d r a p i d l y and f a i r t e n s i l e s t r e n g t h which i n c r e a s e d m o d e r a t e l y . f i e c o r r e s p o n d i n g soya a l k y d (914) showed t h e same t y p e o f b e h a v i o r a 1 though w i t h lower s t r e n g t h and h i g h e r f l e x i b i l i t y . Both t h e l o n g e s t o r t h o - and i s o - p h t h a l i c a l k y d s had v e r y l i t t l e s t r e n g t h b u t t h e l a t t e r c o n t a i n s 15 p e r c e n t more o i l . The o t h e r t h r e e i s o a l k y d s had somewhat h i g h e r s t r e n g t h s which s u g g e s t s t h a t i n l o n g o i l i s o a l k y d s t h e o i l r e a c t s more t o become a p a r t o f t h e molecule. I t i s known t h a t o r t h o p h t h a l i c a n h y d r i d e t e n d s t o form i n t e r n a l r i n g compounds t h u s r e t a r d i n g f o r m a t i o n o f polymers w i t h h i g h m o l e c u l a r w e i g h t s . The r e a s o n f o r t h e e x t r e m e l y h i g h f l e x i b i l i t y o f i s o a l k y d E 6 i s unknown. P e r h a p s 67 p e r c e n t o i l c o n t e n t i s optimum f o r f l e x i b i l i t y i n i s o p h t h a l i c a l k y d s w h i l e t h e l e v e l a p p e a r s t o b e c o n s i d e r a b l y lower w i t h o r t h o a l k y d s . The i n c r e a s e i n f l e x i b i l i t y a t t h e s t a r t o f AW o f many o f t h e a l k y d s i s a t t r i b u t e d t o t h e f i l m s becoming toughened by c r o s s l i n k i n g . I n i t i a l l y t h e y were s o s o f t and weak t h a t t h e y b r o k e a t low l o a d s and e x t e n s i o n s .
Modified C o a t i n g s
The t e n s i l e s t r e n g t h s o f t h e m o d i f i e d o i l s , p h e n o l i c s and a l k y d s a r e g i v e n i n T a b l e s 8 and 9. R e p r e s e n t a t i v e c u r v e s a r e shown i n F i g u r e s 9 and 10 f o r a m o d i f i e d o i l and p h e n o l i c s and i n F i g u r e s 11 and 1 2 f o r a l k y d s . The v a l u e s f o r t h e u n m d i f i e d c o a t i n g s a r e a l s o p l o t t e d f o r e a s e o f comparison.
A s might be e x p e c t e d from t h e i r condensed chemical s t r u c t u r e , m o d i f i c a t i o n w i t h CNSL-hexamine o r Cardanol-hexamine p r o d u c t s t e n d e d t o i n c r e a s e t e n s i l e s t r e n g t h s w h i l e markedly d e c r e a s i n g t h e f l e x i b i l i t y o f b o t h l o n g and s h o r t p h e n o l i c s b a s e d on slow o r s e m i - d r y i n g o i l s . A s t h e s h o r t v a r n i s h e s a r e a l r e a d y b r i t t l e t h e s e changes c a n n o t b e c o n s i d e r e d advantageous u n l e s s o t h e r d e s i r a b l e p r o p e r t i e s , such a s r e d u c e d w a t e r t r a n s m i s s i o n o r a b s o r p t i o n , a r e c o n f e r r e d . The CNSL m o d i f i c a t i o n i n c r e a s e d t h e t e n s i l e s t r e n g t h o f t h e 20-gal v a r n i s h e s more t h a n
Cardanol b u t b o t h r e d u c e d f l e x i b i l i t y t o t h e same low l e v e l . Modifica- t i o n o f t h e 40-gal v a r n i s h e s a f f e c t e d e l o n g a t i o n s i m i l a r l y w h i l e t e n s i l e s t r e n g t h was i n c r e a s e d r e l a t i v e l y more a l t h o u g h n o t t o t h e same a b s o l u t e l e v e l a s t h e 20- g a l v a r n i s h e s .
With a l k y d s t h e same t r e n d s can b e s e e n i n F i g u r e s 11 and 1 2 . The o r i g i n a l low s t r e n g t h o f t h e l o n g and v e r y l o n g a l k y d s was i n c r e a s e d w h i l e f l e x i b i l i t y was d e c r e a s e d . The l a r g e d i f f e r e n c e s i n e l o n g a t i o n between t h e o r i g i n a l a l k y d s were r e d u c e d t o minor v a r i a t i o n s i n t h e f i r s t f e w days o f e x p o s u r e and t h e n d i s a p p e a r e d . M o d i f i c a t i o n had t h e l e a s t e f f e c t on t h e t e n s i l e s t r e n g t h o f t h e v e r y l o n g a l k y d , E2, f o l l o w e d by t h e s h o r t o i l 916, ( T a b l e 8 ) .
The two o i l s were t h e o n l y modified m a t e r i a l s t h a t r e t a i n e d a f a i r d e g r e e o f e l o n g a t i o n p r o b a b l y b e c a u s e no o t h e r r e s i n was p r e s e n t . T h e i r t e n s i l e s t r e n g t h , however, was low. 'Ihe i n c r e a s e i n s t r e n g t h and t h e r e m a r k a b l e d e c r e a s e i n f l e x i b i l i t y upon AiV o f most o f t h e m o d i f i e d
c o a t i n g s s u g g e s t t h a t r a d i a n t energy c a u s e s r a p i d chemical r e a c t i o n s between t h e m o d i f i e r and t h e o i l s i n t h e p h e n o l i c and a l k y d f i n i s h e s .
E f f e c t o f N a t u r a l Weathering (NW) on T e n s i l e P r o p e r t i e s P h e n o l i c s The t e n s i l e s t r e n g t h and e l o n g a t i o n o f p h e n o l i c v a r n i s h e s s u b j e c t e d t o NW a r e l i s t e d i n T a b l e s 10 and 11. S e l e c t e d m a t e r i a l s a r e p l o t t e d i n F i g u r e s 1 3 t o 1 6 . A s i n AW t h e s h o r t o i l v a r n i s h e s i n c r e a s e d i n t e n s i l e s t r e n g t h when f i r s t exposed and t h e n d e c r e a s e d a f t e r two t o f o u r months' e x p o s u r e . The i n c r e a s e i n s t r e n g t h was more g r a d u a l and t h e peak s t r e n g t h c o n s i d e r - a b l y lower i n NW. Tung and l i n s e e d were a g a i n c l o s e i n s t r e n g t h e x c e p t f o r t h e i n i t i a l and f i n a l v a l u e s w h i l e soya and DH C a s t o r e x h i b i t e d s i m i l a r s t r e n g t h s . V a r n i s h e s c o n t a i n i n g 6 7 p e r c e n t o f t h e two slow d r y i n g o i l s soon l o s t t h e i r i n i t i a l e x t e n s i b i l i t y , a s i n AW, and a f t e r 15 days t h e f l e x i b i l i t y o f a l l f o u r v a r n i s h e s was p r a c t i c a l l y t h e same. Even w i t h v a r n i s h e s c o n t a i n i n g 80 p e r c e n t o i l , t h e r e was no s i g n i f i c a n t d i f f e r e n c e i n e l o n g a t i o n between t h e o i l s a f t e r 45 d a y s 1 NW.
With r e g a r d t o t h e e f f e c t o f t h e p r o p o r t i o n o f t h e same o i l , i t can be s e e n i n F i g u r e 15 t h a t t h e two s h o r t e r t u n g v a r n i s h e s a g a i n had
h i g h e r t e n s i l e s t r e n g t h s t h a n t h o s e c o n t a i n i n g more o i l . The peak v a l u e was a l s o lower t h a n t h a t a t t a i n e d i n AW. I n t h e f i r s t month t h e 4 0 - g a l
tung e x h i b i t e d a maximum i n e x t e n s i o n t h a t was h i g h e r t h a n i n AW b u t i t
t h e n d e c r e a s e d t o t h e same low l e v e l a s t h e o t h e r s . These changes i n t h e mechanical p r o p e r t i e s o f t h e p h e n o l i c v a r n i s h e s a r e b e l i e v e d t o b e c a u s e d by t h e r e l a t i v e l y h i g h i n t e n s i t y o f s o l a r r a d i a t i o n d u r i n g t h e months o f J u l y and August.
A 1 kyds
The mechanical p r o p e r t i e s o f a l k y d s exposed t o
NW
a r e g i v e n i n Tables 12 and 1 3 w i t h r e p r e s e n t a t i v e m a t e r i a l s shown i n F i g u r e s 17 t o 20.I n c o n t r a s t t o t h e p h e n o l i c s , t h e maximum t e n s i l e s t r e n g t h w a s much h i g h e r i n NW t h a n i n Ah' and was n o t r e a c h e d u n t i l a f t e r a y e a r ' s
e x p o s u r e . I n f a c t , most o f t h e l o n g e r o i l a l k y d s were s t i l l i n c r e a s i n g i n t e n s i l e s t r e n g t h a t t h e end o f t h e t e s t . F l e x i b i l i t y a f t e r one y e a r was c o r r e s p o n d i n g l y lower i n d i c a t i n g t h a t t h e AW was n o t l o n g enough t o
g i v e t h e e q u i v a l e n t o f n a t u r a l e x p o s u r e f o r a y e a r .
The o r d e r o f s t r e n g t h o f t h e o r t h o a l k y d s shown i n F i g u r e 1 7 approximates t h a t i n AW ( F i g u r e 5) although i n t h e l a t t e r t h e one c o n t a i n i n g t h e l e a s t o i l became b r i t t l e b e f o r e t h e end of t h e t e s t . Hence i t c o u l d n o t be determined whether i t d u p l i c a t e d i t s b e h a v i o r t o
NW where i t f i n a l l y became weaker t h a n samples 912 and 914. Except f o r t h e weakest one, t h e s e a l k y d s d i d n o t e x h i b i t t h e peak i n f l e x i b i l i t y
t h a t o c c u r r e d i n AN. Again a l k y d s 912 and 914 showed s i n i i l n r changes i n mechanical p r o p e r t i e s . Comparison o f t h e t e n s i l e s t r e n g t h s i n F i g u r e s 7 and 19 r e i n f o r c e s t h e i m p r e s s i o n t h a t t h e a c c e l e r a t e d t e s t was t e r m i n a t e d e a r l y w i t h r e s p e c t t o e x t e r i o r e x p o s u r e . The l o n g e s t o r t h o a l k y d a n d a l l t h e i s o a l k y d s e x h i b i t a peak i n e x t e n s i b i l i t y a f t e r a b o u t 45 d a y s ' e x p o s u r e . Long i s o a l k y d s E4 a n d E6 have a b o u t t h e same t e n s i l e s t r e n g t h a s o r t h o a l k y d s c o n t a i n i n g l e s s o i l w h i l e t h e i r f l e x i b i l i t y i s somewhat s u p e r i o r t o t h a t o f t h e o r t h o a l k y d o f e q u i v a l e n t o i l c o n t e n t , E l . Thus t h e s e two a l k y d s have t h e b e s t combination o f mechanical p r o p e r t i e s . The l a b o r a t o r y - p r e p a r e d l i n s e e d a l k y d a t t a i n e d maximum t e n s i l e s t r e n g t h i n s i x months and r e t a i n e d i t f o r a n o t h e r s i x months b u t u n f o r t u n a t e l y , due t o ' t h e f a s t d r y i n g p r o p e r t i e s o f t h e o i l , i t l o s t a l l f l e x i b i l i t y i n t h r e e months. Soya o r t h o a l k y d w i t h t h e same amount o f o i l i n c r e a s e d i n t e n s i l e s t r e n g t h and d e c r e a s e d i n f l e x i b i l i t y a t a s l o w e r r a t e t h a n t h e l i n s e e d formula. Modified C o a t i n g s The v a l u e s f o r t h e s e m a t e r i a l s a p p e a r i n T a b l e s 14 and 15 w i t h c u r v e s i l l u s t r a t i n g t h e i r b e h a v i o r i n F i g u r e s 21 t o 24. M o d i f i c a t i o n w i t h Cardanol- o r CNSL-hexamine c o n d e n s a t e i n c r e a s e d t h e t e n s i l e s t r e n g t h o f s o y a and DH C a s t o r p h e n o l i c v a r n i s h e s exposed t o n a t u r a l w e a t h e r i n g , w i t h CNSL h a v i n g t h e g r e a t e r e f f e c t . A s w i t h t h e o r i g i n a l v a r n i s h e s t h e maximum s t r e n g t h was c o n s i d e r a b l y lower i n N1V t h a n i n AW b u t t h e d e c r e a s e i n f l e x i b i l i t y was r a p i d i n b o t h e x p o s u r e s . The two m o d i f i e d o i l s r e t a i n e d e x t e n s i b i l i t y f o r a b o u t s i x months b u t t h e n l o s t i t f a i r l y q u i c k l y . T h i s t o g e t h e r w i t h t h e i r low t e n s i l e s t r e n g t h t h r o u g h o u t t h e t e s t i n d i c a t e s t h a t films o f t h e s e m o d i f i e d o i l s a r e r a t h e r weak.
Modified a l k y d s behave more l i k e p h e n o l i c s t h a n l i k e t h e o r i g i n a l a l k y d s i n t h a t t h e y l o s e f l e x i b i l i t y r a p i d l y and t h e peak i n t e n s i l e s t r e n g t h was much lower i n NW t h a n i n AW. E6 a n d i t s m o d i f i c a t i o n , 1065, show t h e g r e a t e s t d i f f e r e n c e i n t h i s t y p e o f b e h a v i o r which i s p r o b a b l y r e l a t e d t o t h e f a c t t h a t Cardanol and CNSL a r e long c h a i n s u b s t i t u t e d p h e n o l s . Of t h e m o d i f i e d a l k y d s , 1065 l o s t f l e x i b i l i t y most r a p i d l y i n NW w h i l e i n AW it was 1067. However, 1065 r e a c h e d a h i g h e r i n i t i a l p l a t e a u i n t e n s i l e s t r e n g t h a n d took l o n g e r t o r e a c h i t s peak b e f o r e d e c l i n i n g t h a n d i d 1066 a n d 1067 ( F i g u r e 23). The m o d i f i e d s a f f l o w e r i s o a l k y d r e t a i n e d i t s e l o n g a t i o n r e l a t i v e l y w e l l compared t o t h e unmodified v e r s i o n ( F i g u r e 2 4 ) .
Comparison o f N a t u r a l and A c c e l e r a t e d Weathering
I n g e n e r a l , t h e time dependent change i n f l e x i b i l i t y o f t h e
d i f f e r e n t c o a t i n g s was f a i r l y comparable i n b o t h e x p o s u r e s . D i f f e r e n c e s i n t h e a p p a r e n t b e h a v i o r o f a l k y d s can be a t t r i b u t e d i n p a r t t o t h e s h o r t
l e n g t h o f t h e a r t i f i c i a l t e s t compared w i t h t h a t o f t h e p e r i o d o f n a t u r a l w e a t h e r i n g . For example, E6 would have a l m o s t t h e same c u r v e i n F i g u r e
20 a s i n F i g u r e 8 i f t h e n a t u r a l e x p o s u r e were t e r m i n a t e d a t 250 d a y s . The t e n s i l e s t r e n g t h s a r e i n t h e same r e l a t i v e o r d e r w i t h i n a group b u t p h e n o l i c and p h e n o l i c - m o d i f i e d c o a t i n g s have h i g h e r v a l u e s i n AW
w h i l e a l k y d s a r e h i g h e r i n NW. ( I n making comparisons i t s h o u l d b e n o t e d t h a t t h e p s i s c a l e i n F i g u r e s 5 a n d 7 i s t w i c e a s l a r g e a s i n t h e o t h e r s . ) The d i f f e r e n c e i n b e h a v i o r may b e c a u s e d by p h e n o l i c s a b s o r b i n g more UV
r a d i a t i o n i n t h e a c c e l e r a t e d t e s t t h a n t h e y do on e x t e r i o r e x p o s u r e . I t i s known t h a t t h e c a r b o n a r c p r o d u c e s two l a r g e p e a k s i n t h e n e a r UV t h a t a r e n o t p r e s e n t i n s u n l i g h t a n d t h a t p h e n o l i c s a b s o r b more towards t h e v i s i b l e r e g i o n t h a n do a l k y d s . Hence, t h e a c c e l e r a t e d t e s t may be t o o s e v e r e f o r t h e p r o p e r e v a l u a t i o n o f p h e n o l i c s . There does n o t a p p e a r t o b e a l o g i c a l e x p l a n a t i o n o f why a l k y d s r e a c h a l o w e r t e n s i l e s t r e n g t h i n AV where t h e i n t e n s i t y o f r a d i a t i o n i s g r e a t e r . CONCLUSIONS I n a c c e l e r a t e d and n a t u r a l w e a t h e r i n g , p - p h e n y l p h e n o l i c v a r n i s h e s b a s e d on t u n g o i l e x h i b i t e d t e n s i l e s t r e n g t h s s l i g h t l y h i g h e r t h a n l i n - s e e d a n d m o d e r a t e l y h i g h e r t h a n s o y a and d e h y d r a t e d c a s t o r v a r n i s h e s . R e g a r d l e s s o f f i l m s t r e n g t h , a l l v a r n i s h e s l o s t f l e x i b i l i t y soon a f t e r e x p o s u r e . I n c r e a s i n g t h e o i l c o n t e n t h a d l i t t l e e f f e c t on f l e x i b i l i t y o f e x p o s e d f i l m s . The d i f f e r e n c e s i n s t r e n g t h between t h e o i l s a r e a t t r i b u t e d t o d i f f e r e n c e s i n r e a c t i v i t y w h i l e c o o k i n g t h e v a r n i s h e s whereas t h e changes d u r i n g e x p o s u r e a r e t h o u g h t t o b e c a u s e d by r a d i a t i o n - i n d u c e d c r o s s - l i n k i n g f o l l o w e d by s c i s s i o n . The h i g h e r t e n s i l e s t r e n g t h o b s e r v e d i n p h e n o l i c s s u b j e c t e d t o AW i s b e l i e v e d t o b e t h e r e s u l t o f t h e c o n s i d e r - a b l y h i g h e r i n t e n s i t y o f some p o r t i o n s o f t h e UV r a d i a t i o n g e n e r a t e d by c a r b o n a r c s . P h e n o l i c s a b s o r b more i n t h i s r e g i o n t h a n a l k y d s s o a r e more a f f e c t e d by AW. Alkyds h a d l o w e r t e n s i l e s t r e n g t h t h e n p h e n o l i c s i n AW b u t i n NW t h e maximum s t r e n g t h o f a l l b u t l o n g o r t h o p h t h a l i c and v e r y l o n g i s o p h t h a l i c a l k y d s was h i g h e r t h a n p h e n o l i c s . I n b o t h t e s t s t h e i n c r e a s e i n s t r e n g t h o f a l k y d s was more g r a d u a l t h a n w i t h p h e n o l i c s . Alkyds were a l s o more f l e x i b l e a n d r e t a i n e d t h i s p r o p e r t y l o n g e r i n b o t h
t e s t s . I s o p h t h a l i c a l k y d s h a d a b e t t e r c o m b i n a t i o n o f t e n s i l e p r o p e r t i e s t h a n o r t h o a l k y d s o f e q u i v a l e n t o i l l e n g t h w i t h t h e i r optimum o i l c o n t e n t a p p e a r i n g t o b e a b o u t 6 7 p e r c e n t . O r t h o a l k y d s o f h i g h o i l c o n t e n t and i s o a l k y d s o f v e r y h i g h o i l c o n t e n t h a d low t e n s i l e s t r e n g t h s w i t h o u t a c o r r e s p o n d i n g i n c r e a s e i n f l e x i b i l i t y . T h i s i s c o n s i d e r e d t o b e c o n f i r - mation t h a t i n s u c h a l k y d s some o f t h e o i l i s p r e s e n t a s a m i x t u r e o n l y . The h i g h o i l c o n t e n t c o a t i n g s were s o f t a f t e r a i r d r y i n g f o r s i x weeks o r more. C o n s e q u e n t l y , when f i r s t s u b j e c t e d t o t h e t e n s i l e t e s t
During n a t u r a l and a c c e l e r a t e d w e a t h e r i n g t h e y became somewhat t o u g h e r a n d were a b l e t o e x t e n d more b e f o r e b r e a k i n g . Hence t h e y e x h i b i t e d a n i n c r e a s e i n f l e x i b i l i t y i n e a r l y e x p o s u r e p e r i o d s .
On t h e b a s i s o f mechanical p r o p e r t i e s , 40 c y c l e s o f a c c e l e r a t e d w e a t h e r i n g , u s i n g t h e 12-12 c y c l e b u t s u b s t i t u t i n g h i g h h u m i d i t y f o r w a t e r s p r a y , were n o t l o n g enough t o produce t h e changes t h a t o c c u r r e d i n a l k y d s i n one y e a r o f n a t u r a l w e a t h e r i n g . However, t h e a c c e l e r a t e d t e s t a p p e a r s t o have had a g r e a t e r e f f e c t on t h e t e n s i l e s t r e n g t h o f p h e n o l i c s . E v i d e n t l y an e x p o s u r e p e r i o d a p p l i c a b l e t o one t y p e o f c l e a r f i n i s h i s n o t n e c e s s a r i l y a p p r o p r i a t e t o a n o t h e r , a t l e a s t i f f r e e f i l m s a r e b e i n g exposed. W i t h i n t h e o r t h o a l k y d group e x t e r i o r e x p o s u r e r e s u l t s c o r r e l a t e d b e t t e r w i t h mechanical p r o p e r t i e s t h a n w i t h c o m p o s i t i o n . F l e x i b i l i t y i s g e n e r a l l y c o n s i d e r e d t o b e a n i m p o r t a n t p r o p e r t y o f c o a t i n g s , p a r t i c u l a r l y t h o s e d e s i g n e d f o r u s e on s u b s t r a t e s s u b j e c t t o r e l a t i v e l y l a r g e dimensional changes r e s u l t i n g from t e m p e r a t u r e o r h u m i d i t y v a r i a t i o n s . R a d i a n t e n e r g y i n a c c e l e r a t e d and n a t u r a l w e a t h e r i n g c a u s e s marked changes i n t h e mechanical p r o p e r t i e s of
p h e n o l i c s which r a p i d l y a t t a i n maximum s t r e n g t h w i t h a c o n c o m i t a n t l o s s i n f l e x i b i l i t y . On t h e o t h e r hand, t h e t e n s i l e p r o p e r t i e s o f most
a l k y d s change a t a s l o w e r r a t e and t h e y r e t a i n t h e i r f l e x i b i l i t y f o r a f a i r l y l o n g p e r i o d . T h i s b a s i c p r o p e r t y o f a l k y d s i s one r e a s o n f o r t h e good e x t e r i o r performance o f pigmented a l k y d s . C l e a r a l k y d s , however, when exposed on wood t o n a t u r a l o r a c c e l e r a t e d w e a t h e r i n g do n o t p e r f o r m n e a r l y s o w e l l a s t h e s h o r t o i l p h e n o l i c s which have v e r y l i t t l e f l e x i b i l i t y . T h i s p r o p e r t y , t h e r e f o r e , i s n o t s o i m p o r t a n t a s w a t e r a b s o r p t i o n , w a t e r v a p o r p e r m e a t i o n o r t r a n s p a r e n c y t o W l i g h t i n r e l a t i o n t o t h e performance o f c l e a r f i n i s h e s even on a d i m e n s i o n a l l y u n s t a b l e s u b s t r a t e such a s wood. ACKNOWLEDGEMENTS The a u t h o r s acknowledge w i t h t h a n k s t h e p r e p a r a t i o n o f t h e v a r n i s h e s a n d films by G . A . O I D o h e r t y , L . R . Dubois and R . C . S e e l e y . M r . S e e l e y a l s o measured t h e mechanical p r o p e r t i e s o f t h e f i l m s .
REFERENCES
1 . Gusman, S . S t u d i e s o f t h e Adhesion o f O r g a n i c C o a t i n g s . O f f i c i a l D i g e s t 34, 451, p . 886, ( 1 9 6 2 ) .
2. Zubov, P . I . and L . A . L e p e l k i n a . I n t e r n a l S t r e s s e s i n P o l y e s t e r C o a t i n g s . K o l l o i d Zh 24, 1 , p . 30, ( 1 9 6 2 ) .
3. Zubov, P . I . , L . A . S a k h a r e v a and Yu, P . Smirnova. I n f l u e n c e o f I n t e r n a l S t r e s s e s on t h e S e r v i c e Length o f P o l y m e r i c C o a t i n g s . Dokl. Akad. Nauk SSR, 150, 2 , p . 359, ( 1 9 6 3 ) .
4 . Dannenberg, H . D e t e r m i n a t i o n o f S t r e s s e s i n Cured Epoxy R e s i n s . SPE J o u r n a l , 21, 7, p
.
669, (1965).
5 . T a l e n , H.W. P h y s i c a l Examination o f P a i n t s i n R e l a t i o n t o t h e i r P r a c t i c a l P e r f o r m a n c e . Chemistry and I n d u s t r y , No. 4 9 , p . 1 5 6 4 ,
( 1 9 6 5 ) .
6 . C-D-I-C S o c i e t y f o r P a i n t Technology. An E v a l u a t i o n o f Various Weatherometers f o r Determining t h e S e r v i c e L i f e o f O r g a n i c C o a t i n g s . O f f i c i a l D i g e s t , 34, 454, p . 1191, (1962) and 37, 490, p . 1371, ( 1 9 6 5 ) . 7 . B u l l e t t , T.R. D u r a b i l i t y o f P a i n t . J . O . C . C . A . , 51, 10, p . 894, ( 1 9 6 8 ) . 8 . P a p i l l o , P
.
J . A c c e l e r a t e d Outdoor W e a t h e r a b i l i t y T e s t i n g o f Pigments i n P a i n t . J . P a i n t Tech. 40, 524, p . 359, ( 1 9 6 8 ) . 9 . S a t t e l m e y e r , R . and K . R . R e i c h e r t . D e g r a d a t i v e A c t i o n o f L i g h t on P a i n t Films d u r i n g N a t u r a l and A r t i f i c i a l W e a t h e r i n g . Deut. Farben Z e i t . , 22, 4 , p . 137, (1968).1 0 . Ashton, H . E . A c c e l e r a t e d T e s t i n g and O r g a n i c C o a t i n g s . P a i n t and V a r n i s h P r o d u c t i o n , 59, 9 , p
.
34, (1969).11. N o r t h w e s t e r n S o c i e t y f o r P a i n t Technology. A C o n t i n u e d S t u d y o f t h e Dew-Cycle Method o f A c c e l e r a t e d T e s t i n g . J . P a i n t Tech. 42, 540, p . 6 0 , ( 1 9 7 0 ) .
1 2 . Hoffmann, E . W e a t h e r i n g o f P a i n t Films and Development o f A c c e l e r a t e d T e s t s . J . P a i n t Tech. 43, 563, p . 9 7 , (1971)
.
1 3 . M i t t o n , P.B. a n d D.P. R i c h a r d s . A Model f o r O b j e c t i v e Development o f A c c e l e r a t e d W e a t h e r i n g T e s t s . J . P a i n t Tech. 43, 563, p . 107, (1971).
1 4 . Brand, B . G . , L . J . Nowacki, W . M i r i c k a n d E . R . M u e l l e r . P r e d i c t i n g S e r v i c e L i f e o f O r g a n i c C o a t i n g s , - A S t a t e - o f - t h e - A r t Review. J . P a i n t Tech. 40, 524, p . 396, ( 1 9 6 8 ) .1 5 . S c h u r r , G . G . , T.K. Hay and M. Van Loo. P o s s i b i l i t y o f P r e d i c t i n g E x t e r i o r D u r a b i l i t y by S t r e s s j S t r a i n Measurements. J . P a i n t Tech. 38, 501, p . 591, (1966).
1 6 . Rao, R . V . , M. Yaseen and J . S . Aggarwal. Mechanical P r o p e r t i e s o f Alkyd F i l m s and t h e E f f e c t o f D i f f e r e n t Weathering C o n d i t i o n s
.
J . O . C . C . A . , 51, 4 , p . 324, (1968).1 7 . Yaseen, M. And H . E . Ashton. E f f e c t o f Method o f Free Film P r e p a r a t i o n on P h y s i c a l P r o p e r t i e s o f O r g a n i c C o a t i n g s . DBR I n t e r n a l Report 427, Ottawa, (1976).
18. Z o r l l ,
U.
F a i l u r e Envelopes o f P a i n t Films a s R e l a t e d t oP i g m e n t a t i o n and S w e l l i n g . X I t h FATIPEC Congress, F l o r e n c e , 1972,
NRC T e c h n i c a l T r a n s l a t i o n 1787, Ottawa, (1975)
.
1 9 . Ashton, H . E . The Weathering o f O r g a n i c B u i l d i n g M a t e r i a l s . I n P r o g r e s s i n C o n s t r u c t i o n S c i e n c e and Technology No. 1, e d . by- P . J . Horrobin and J . W . Simpson, Medical & T e c h n i c a l P u b l i s h i n g Co., L a n c a s t e r , England, (1971)
.
20. Ashton, H . E . C l e a r F i n i s h e s f o r E x t e r i o r Wood; F i e l d Exposure T e s t s . J. P a i n t Tech. 39, 507, p . 212, (1967).
TABLE 1
COYPOSITION OF PARAPHENYLPHEWOLIC VARNISHES
*
I m p . G a l . p e r 1 0 0 l b . R e s i n NRP F o r m u l a N o . 1 0 2 0 89 3 8 9 4 9 0 1 1 0 2 1 9 0 2 9 0 3 9 0 5 1 0 2 2 1 0 2 3 1 0 2 4 1 0 2 5 S o l u t i o n C h a r a c t e r i s t i c s P e r c e n t S o l i d s 5 1 5 0 5 0 5 0 4 9 . 5 5 1 4 9 . 5 5 0 5 0 5 0 5 0 49 G - H V i s c o s i t y A - B C D B - C D B - C D C - D B C - D C E O i l C o n t e n t % V o l a t i l e C o n t e n t TY p e T u n g T u n g T u n g T u n g L i n s e e d L i n s e e d L i n s e e d L i n s e e d S o y a S o y a D e h y d r a t e d C a s t o r D e h y d r a t e d C a s t o r A r o m a t i c S o l v e n t s 8 5 . 3 3 3 . 3 1 0 . 0-
4 9 . 4 3 0 . 6 2 0 . 2 1 0 2 8 . 6 3 4 . 7 3 3 . 3 2 . 6 M i n e r a l S p i r i t s 1 4 . 7 6 6 . 7 9 0 . 0 1 0 0 . 0 5 0 . 6 6 9 . 4 7 9 . 8 9 0 7 1 . 4 6 5 . 3 6 6 . 7 9 7 . 4 A p p r o x . l e n g t h * 1 5 2 0 30 40 1 5 2 0 30 40 2 0 40 20 40 P e r c e n t o f S o l i d s 5 8 . 3 6 6 . 7 7 5 8 0 5 8 . 3 6 6 . 7 7 5 8 0 6 6 . 7 8 0 6 6 . 7 8 0TABLE 2 CO:.lPOSITION OF ALKYDS
*
P e r o i l c o n t e n t o f a l k y d s m a r k e d w i t h*
c a l c u l a t e d f r o m r e p o r t e d f a t t y a c i d c o n t e n t . O t h e r c o m m e r c i a l a l k y d s a r e r e p o r t e da s
o i l c o n t e n t . NRP F o r m u l a No. 9 1 2 9 1 3 9 1 4 9 1 5 9 1 6 1 0 5 5 * * 1 0 5 6 * * E l E2 E3 E4 E5 E6* *
A l k y d s p r e p a r e d i n t h e l a b o r a t o r y . O i l C o n t e n t TY3e S o y a S o y a S o y a S o y a S o y a L i n s e e d S o y a S a f f l o w e r S a f f l o w e r L i n s e e d - S o y a S o y a S o y a S o y a P e r c e n t o f S o l i d s 5 9 . 4 " 6 2 . 5 * 5 6*
4 8*
3 9 . 6 5 4 5 6 . 5 7 0 8 3 8 5 7 5 7 2 6 7 % P h t h a l i c C o n t e n t S o l u t i o n P e r c e n t S o l i d s5
0 5 0 5 0 42 40 ' 5 0 5 0 60 60 60 5 0 5 0 5 0 I s o m e r O r t h o O r t h o O r t h o O r t h o O r t h o O r t h o O r t h o O r t h o Is0 Is0 Is0 Is0 Is0 C h a r a c t e r i s t i c s G - H V i s c o s i t y A - B A - B D - F E G - H A - B A - B Al A1 A l C - D C - D C - D P e r c e n t o f S o l i d s 25 24 30 35
39 3 5 3 1 1 7 9 1 3L
8 20 28T A B L E 3
TABLE 4
TEXSILE STRENGTH OF PHENOLICS DURING ACCELERATED WEATHERING Maximum L o a d
a t
B r e a k o r Y i e l d P o i n t-
PSI
Days
Ex- p o s e d 0 0 . 5 0 1 2 4 6 8 1 0 1 2 1 5 - G a l l o n 2 0 - G a l l o n 1 0 2 0 T u n g 2 8 4 3 5 1 0 8 5 4 4 4 5 5 9 6 5 8 1 5 5 0 5 0 4 5 4 8-
2 8 8 8 8 9 3 T u n g 3 0 6 9 4 6 1 6 5 2 7 3 5 4 4 7 6 1 1 5-
3 7 6 6 3 3 5 2 2 3 7 0 1 0 2 1 L i n s e e d 2 4 7 7 5 2 5 7 5 7 6 2 5 8 1 0 5 0 1 4 4 4 9 4 2 3 7 9-
2 1 0 2 30-G a l l o n
9 0 5 T u n g 1 8 7 8 230'6 2 6 6 2 2 9 7 1 4 4 5 4 4 4 2 3 4 6 3 8 4 4 8 0 4 0 2 6 i 4 0 - G a l l o n 9 0 2 L i n s e e d 1 2 6 0 3 9 4 6 4 9 7 0 5 3 3 3 5 7 3 1 4 1 9 1-
2 6 6 6-
9 0 3 L i n s e e d 9 1 0 1 6 9 0 2 0 7 9 2 8 8 8 4 4 7 4 4 0 5 3 3 8 5 5 4 5 3 9-
9 0 1 T u n g 1 1 9 6 1 7 8 1 2 3 2 4 2 8 5 4 3 0 6 9 3 5 0 1 3 1 9 5 3 0 9 0-
1 0 2 2 S o y a 5 5 6 1 7 1 8 2 9 4 8 2 8 7 7 4 2 4 1 3 0 0 4 2 7 3 2 2 4 7 0-
1 0 2 4 DH C a s t o r 9 8 2 2 0 0 4 3 4 5 3 4 2 0 0 4 3 9 1 2 9 6 4 3 2 8 2 2 5 4 2-
1 0 2 5 CH C a s t o r 3 3 4 5 1 3 6 8 6 5 9 4 1 2 3 0 1 2 2 8 1 1 8 7 1 2 1 9 1 6 8 4 I 9 0 5 L i n s e e d 7 1 2 1 1 5 4 1 5 7 2 1 7 4 1 2 8 5 7 3 5 4 1 3 2 4 4 3 2 6 5-
1 0 2 3 S o y a 6 7 2 7 4 3 4 1 3 8 4 8 2 2 8 1 7 8 5 6 8 5 4-
TABLE 5
FLEXIBILITY PROPERTIES OF PHENOLICS D U R I N G ACCELERATED WEATHERING
Maximum E l o n g a t i o n
a t
B r e a k-
P e r c e n tI
D a y s Ex- p o s e d 0 0 . 5 1 2 4 6 8 1 0 1 2 1 5 - G a l l o n 1 0 2 0 T u n g 9 . 1 8 . 6 9 . 4 8 . 5 6 . 7 9 . 6 5 . 9 5 . 1 5 . 8 1 0 2 1 L i n s e e d 6 . 7 6 . 8 5 . 8 7 . 5 5 . 2 5 . 7 6 . 4 4 . 0 2 . 9 2 0 - G a l l o n 1 0 2 4 DH C a s t o r 8 9 3 T u n g 30- G a l l o n - 4 0 - G a l l o n 9 0 5 T u n g 3 9 . 7 2 5 . 5 1 3 . 8 8 . 8 8 . 2 9 . 7 9 . 1 6 . 9 6 . 0 8 . 2 1 1 . 2 9 . 9 1 0 . 1 1 0 . 6 5 . 2 3 . 7 5 . 1 4 . 4 9 0 1 T u n g 1 2 . 3 2 9 . 1 9 . 2 9 . 5 8 . 6 9 . 1 7 . 8 8 . 9-
9 0 2 L i n s e e d 9 0 3 L i n s e e d 7 6 . 4 2 7 . 8 2 1 . 9 8 . 4 8 . 7 1 0 . 1 7 . 0 4 . 9 7 . 9 1 0 2 2 S o y a 2 1 . 7 9 . 5 1 0 . 1 7 . 9 1 0 . 5 6 . 2-
4 . 2-
9 0 5 L i n s e e d 6 3 . 8 3 8 . 9 3 5 . 9 2 4 . 4 8 . 8 8 . 7 8 . 9 8 . 5-
1 0 2 3 S o y a 2 6 . 1 4 7 . 6 5 6 . 6 5 1 . 6 3 2 . 9 3 8 . 4 2 3 . 9 1 5 . 5-
7 5 . 4 2 7 . 3 93.11
1 5 . 9 1 0 2 5 DH C a s t o r 5 3 . 8 5 4 . 5 6 4 . 5 5 3 . 4 2 9 . 5 3 4 . 5 2 5 . 0 2 2 . 5 2 1 . 6 8 . 3 8 . 3 8 . 8 4 . 6 5 . 1 3 . 9-
9 . 9 8 . 8 8 . 5 4 . 5 5 . 7 5 . 2-
TABLE 6
TENSILE STRENGTH OF ALKYDS DURING ACCELERATED WEATHERING
Maximum L0a.d at Break or Yield Point
-
PSI
Days
Ex-
posed
0
6
12
18
30
40
4 . 1Iso-phthalic Commercial.Alkyds
Ortho-phthalic Alkyds
Lab.Prepd.
912
Soya
710
679
884
-
1119
1051
1055
.Lin-
seed
1105
1123
1166
1676
2411
2254
E5
Soya
200
205
-
228
547
676
914
Soya
411
431
-
823
1040
985
913
Soya
76
55
46
-
78
-
1056
Soya
121
168
-
75
273
-
E4
Soya
139
163
181
267
342
722
E2
Saf-
flower
60
47
110
-
-
549
E6
Soya
156
375
-
421
564
519
&E3
Lin-
seed
Soya
6
6
44
62
-
52
-
Commercial
915
Soya
316
618
653
526
849
719
916
Soya
759
767
830
914
1673
-
El
Saf-
flower
122
97
86
-
114
-
TABLE 7
FLEXIBILITY PROPERTIES OF ALKYDS DURING ACCELERATED WEATHERING
Maximum Elongation at Break
-
Per cent
Days
Ex-
posed
0
6
12
18
30
40
Iso-phthalic Commercial Alkyds
Ortho-phthalic Alkyds
E6
Soya
84.1
137.2
111.0
93.1
100.9
93.5
Commercial
E2
Saf-
flower
42.8
52.5
49.3
-
55.4
17.4
912
Soya
100.1
100.0
81.2
55.6
57.0
44.3
Lab.Prepd.
E4
Soya
51.0
67.5
70.4
57.8
72.3
32.9
E3
Lin-
seed
Soya
40.2
56.3
60.9
66.0
55.7
-
1055
Lin-
seed
91.0
86.2
72.0
42.6
7.8
8.0
E5
Soya
55.8
57.7
58.3
48.7
58.6
46.0
913
Soya
71.0
90.8
79.3
69.0
40.7
-
1056
Soya
88.2
123.8
-
80.0
41.9
-
914
Soya
106.8
101.9
110.2
77.5
32.0
50.0
9
16
Soya
164.2
137.2
106.6
79.7
17.4
-
915
Soya
111.9
136
133.5
74.3
58.8
87.5
El
Saf-
flower
55.5
62.6
61.0
-
71.2
-
TABLE 8
TENSILE STRENGTH OF MODIFIED PHENOLICS, O I L S AND ALKYDS DURING ACCELERATED WEATHERING Maximum L o a d a t B r e a k
