5_stress cracking of nylon polymers in aqueous salt solutions

7/23/2019 5_Stress Cracking of Nylon Polymers in Aqueous Salt Solutions

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J OURNA L OF MAT E RI AL S SCIE NCE 22 1987) 1715-1723

t ress c rack ing o f ny lon po lymers in aqueous

sa l t so lut ions

Part N y l o n s a l t in t e ra c t i o n s

M I C H A E L G . W Y Z G O S K I , G L E N E. N O V A K

Polymers Department , Genera l Mo tors Research Laborator ies, Warren, M ich ig an 4 80 90 -90 55,

USA

A previous investigation J. Mater. ScL 1 987 ) 1707 ) has sho wn that nylon polymers are

suscep t ible to st ress cracking by aqueous calcium chlor ide at sl igh t ly elevated temperatures

50 C).. in the present study, the ny lon -sa l t interactions were investigated using thin f i lms of

both nylon 6 and nylo n 6,6. Chemical, thermal, infrared and dyn am ic mechanical ana lyses

we re performed along wi th tensi le prop erty determinations. W eigh t gain measurements indi-

cated that above 50C large increases in sorption occurred for f i lms immersed in either

aqueous c alcium , l ithium , or magne sium chlorides. Both salt ions and wate r we re absorbed at

the elevated temperature. In con trast to these results, the data for sodiu m c hlorid e indica ted

that only water was absorbed at al l temperatures. The sorpt ion process was shown to be

physical ly reversible since the absorbed salt could be removed by immersion in pure water.

Mo reover the sorpt ion of the sal t solut ion at 100 C did n ot p roduce chem ical degradation or

signi f icant crystal l izat ion of the nylons. At higher temperatures the absorbed sal t did modi fy

the recrystall izat ion beh aviou r and increas e the Tg of the n ylons. S ignif ica nt red uction s in ten -

si le modulus were observed for nylon f i lms equi l ibrated in the sal t solut ions and tensi le elon-

gations at rupture were increased. The effect of increasing temperature and salt type on the

sorp tion process paral lels the prev ious ly observed stress-cracking ac tivi ty of the aqueous salt

solut ions . I t is therefore su ggested that a plas t icizat io n-l ike mechan ism is responsible for the

sal t - induced crazing of the crystal l ine nylons.

1. n t r o d u c t i o n

Bo th o r g an ic l i qu id s an d in o r g an ic sa l t so lu tio n s can

cau se en v i r o n m en ta l s t r e s s c r ack in g o f n y lo n p o ly -

mers [1, 2] . Results discussed in a previous paper [3]

d em o n s t r a t ed th e r o l e o f p o ly m er - l i q u id so lu b il i ty i n

the s t ress c rack ing of ny lon by organ ic so lven ts .

Ra pid ly so rbed l iqu ids were observed to p las t ic ize the

sur face , a l lowing homogeneous s t ress re laxa t ion to

o ccu r i n co m p e t i t i o n w i th c r aze g ro wth . T o m in im ize

th is su r face p las t ic iza t ion ef fec t a su bsequ ent s tud y of

n y lo n c r ack in g in aq u eo u s sa l t so lu tio n s em p lo y ed a

f r ac tu r e m ech an ic s ap p r o ac h [ 4] . W i th t h i s t e ch n iq u e

th e f a i l u r e ch a r ac t e r i s t i c s o f r a zo r - cu t p r ec r ack ed

sp ec im en s o f n y lo n p o ly m er s we r e ex am in ed . P r e -

l im in a r y r e su lt s i n d i ca t ed th a t sa l t - in d u ced c r ack in g

o ccu r r ed o n ly a t e l ev a t ed t em p e r a tu r e s ( ab o v e 5 0 C).

Mo r eo v e r , t h e t o t a l f a i l u r e t im e was d e t e r m in ed

la r g e ly b y th e c r aze g r o wth k in e t i c s . Cr ack g r o wth

o ccu r r ed r ap id ly o n ly a f t e r a c r azed r eg io n h ad

p r o p ag a ted ac r o ss n ea r ly t h e en t i r e sam p le c r o ss -

sec t ion . In add i t ion , the p rev ious resu l t s showed tha t

n y lo n 6 was m o r e su scept ibl e t o c r ack in g th an n y lo n

6 ,6 , whi le ny lon 11 was essen t ia l ly immune to s t ress

crack ing . T hou gh d i f fe rences in suscep t ib i l i ty fo r the

v a r io u s p o ly m er s can b e a t t r i b u t ed to d i ff e r en ces i n

c r y s t a ll i n it y an d /o r ch em ica l s t r u c tu r e ( am id e g r o u p

co n cen t r a t i o n ) , t h e t em p e r a tu r e d ep en d en ce o f t h e

0022-2461/87 03.00 + .12 1987 Chapman and Hall Ltd

s t r e ss - c r ack in g p h en o m en o n i s n o t r e ad i ly ex p la in ed

by these fac to rs .

T h e p r e sen t s tu d y f u r th e r ad d r e sse s t h e m ech a n i sm

o f sa l t - in d u ced c r ack in g o f n y lo n p o ly m er s b y ex am in -

in g th e so r p t io n o f sa lt so lu t io n in n y lo n 6 co m p ar e d

wi th n y lo n 6 , 6 an d b y d e f in in g th e e f fec t o f ab so r b ed

sa l t o n th e s t r u c tu r e an d p r o p e r t i e s o f t h e se p o ly m er s .

2 . E x p e r i m e n t a l p r o c e d u r e

2 . 1 . M a t e r i a l s

E x t r u d ed f ilm s o f n y lo n 6 ,6 an d n y lo n 6 w e r e p r o v id ed

b y E . I . d u Po n t , W i lm in g to n , De lawar e , USA ( Z y te l

2 7 8 1 ) an d Al l i ed , Mo r r i s to wn , New Je r sey , USA

(Capron ER20) , respec t ive ly . The f i lm th icknesses

were 0 .0027 cm for ny lon 6 , 6 and 0 .0034 cm fo r ny lo n

6 . Sa tu r a t ed aq u eo u s so lu t io n s o f so d iu m , ca l c iu m ,

l i t h iu m , an d m ag n es iu m ch lo r id e s we r e p r ep a r ed a t

r o o m t em p e r a tu r e . T h e l a t t e r was i n t h e f o r m o f a

hyd ra te , MgC12 - 6H 20. An aly t ica l reagen t g rade sa l t s

we r e u sed. No a t t em p t was m a d e to co m p en sa t e fo r t h e

increased so lub i l i ty o f the sa l t a t h igher tem pera tures .

2 2

W e i g h t g a i n s

W eig h t g a in s we r e m easu r ed u s in g th e t h in f i lm s to

assess the percen tage and ra te o f absorp t ion in var ious

inorgan ic sa l t so lu t ions a t 50 , 75 and 100C. Fi lms

wer e d i e - cu t t o g iv e a 2 .5 c m x 2 .5 cm sam p le . Up o n

1715


2/9

r e m o v a l f r o m t h e s a lt s o l u t i o n s a m p l e s w e r e d i p p e d i n

w a t e r t o r e m o v e e x c e s s s a lt o n t h e f il m s u r f a c e , b l o t t e d

w i t h t i s s u e s , a n d w e i g h e d . P r e l i m i n a r y e x p e r i m e n t s

h a d s h o w n t h a t t h e b r i e f i m m e r s i o n i n w a t e r w a s

n e c e s s a r y t o g i v e r e p r o d u c i b l e r e s u l t s . I n a d d i t i o n ,

s e l ec t e d s a m p l e s w e r e i m m e r s e d i n w a t e r a t 1 0 0 C t o

d e s o r b t h e s a l t s o l u t i o n i n o r d e r t o a s s e s s t h e a m o u n t

o f e x t r a c t a b l e m a t e r i a l i n t h e f il m s . T h e f il m w e i g h ts

w e r e u s e d a s a n i n d i c a t i o n o f t h e c o m p l e t e n e s s o f t h e

d e s o r p t i o n p r o c e s s a n d t h e r e t a i n e d w a t e r w a s s u b s e -

q u e n t l y r e m o v e d b y d r y i n g a t 1 0 0 C i n a v a c u u m

o v e n .

2 . 3 . C h e m i c a l a n a l y s i s

N y l o n f i l m s a m p l e s w h i c h w e r e e q u i l i b r a t e d i n s a l t

s o l u t i o n s a t v a r i o u s t e m p e r a t u r e s w e r e a l s o a n a l y s e d

f o r t h e i r c a t i o n c o n t e n t . T h e f i l m s r e m a i n e d i n t h e

h e a t e d s a l t s o l u ti o n s u n t i l j u s t p r i o r t o t h e a n a l y s i s t o

p r e v e n t d e s o r p t i o n a t r o o m t e m p e r a t u r e . T h e t e c h-

n i q u e i n v o l v e d d i s s o l v i n g t h e s a m p l e i n h y d r o c h l o r i c

a c i d a n d s u b s e q u e n t l y d e t e r m i n i n g t h e c a l c i u m , l i t h -

i u m , s o d i u m , o r m a g n e s i u m c o n c e n t r a t i o n w i t h a

M o d e l 4 6 A t o m i c A b s o r p t i o n S p e c t r o p h o t o m e t e r

( P e r k i n -E l m e r , N o r w a l k , C o n n e c t i c u t , U S A ) . B a s e d

u p o n t h e m e a s u r e d i o n c o n t e n t a n d t h e t o t a l w e i g h t

g a i n , t h e p a r t i t i o n i n g o f t h e s a l t a n d w a t e r i n t h e f il m s

w a s c a l c u l a t e d .

2 4 Molecu lar we ight determinat ions

G e l p e r m e a t io n c h r o m a t o g r a p h y ( G P C ) c u r v e s o f th e

n y l o n s w e r e o b t a i n e d f r o m s a m p l e s d i s s o lv e d i n f r e sh l y

d i s ti l le d m - c r e s o l h e a t e d t o 6 0 C a t a p o l y m e r c o n -

c e n t r a t i o n o f 0 . 5 w t . T h e fi l te r e d s o l u t i o n s w e r e

i n je c te d i n t o a W a t e r s 1 5 0- C A L C / B P C C h r o m a t o -

g r a p h ( M i l l i p o r e I n c . , M i l f o r d , M a s s a c h u s e t t s ) . T h e

c o l u m n s c o n t a i n e d p o l y s t y r e n e - d i v i n y l b e n z e n e g el s

a n d w e r e p u r c h a s e d f r o m t h e S h o d e x C o r p o r a t i o n o f

J a p a n . T h e c o l u m n s w e r e h e a t e d t o 1 10 C . M o l e c u l a r

w e i g h ts o f t h e n y l o n s w e r e o b t a in e d f r o m t h e G P C

c u r v e s u s i n g a u n i v er s a l c a l i b r a ti o n c u r v e . T h e M a r k -

H o u w i n k c o e f f ic i e n ts ( K a n d a v a l u e s ) f o r t h e n y l o n s

w e r e c a lc u l a t e d u s i n g i n j e c t i o n - m o u l d e d n y l o n 6 sa m -

p l es o f k n o w n m o l e c u l a r w e i g h t s w h i c h w e r e s u p p l i e d

b y t h e A l l i e d C o r p o r a t i o n .

2 5 Infrared spectrosco py

A P e r k e n - E l m e r M o d e l 2 8 3 B i n f r a r e d s p e c t r o m e t e r

w a s u s e d t o o b t a i n t r a n s m i s s i o n i n f r a r e d s p e c t r a o f

t h i n f i lm s o f n y l o n . I n i t ia l e x p e r i m e n t s w i t h t h e

e x t r u d e d f i l m s i n d i c a t e d t h a t t h e y w e r e t o o t h i c k f o r

t r a n s m i s s i o n s p e c t r o s c o p y . T h e r e f o r e , t h i n n e r f i l m s

w e r e p r e p a r e d b y c a s t i n g f r o m f o r m i c a c i d s o l u t i o n s

( 1 0 w t ) . T h e f il m s w e r e d r i e d i n a v a c u u m o v e n a t

1 0 0 C f o r 2 h t o r e m o v e t h e s o l v e n t . C h a n g e s i n t h e

i n f r a r e d s p e c t r a w e r e r e c o r d e d f o r f i l m s b e f o r e a n d

a f t e r e q u i l i b r a t i o n i n s a t u r a t e d a q u e o u s c a l c i u m

c h l o r i d e a t 1 00 C .

2 . 6 . D i f f e r e n t i a l

scanning calor imetry

D S C )

D S C w a s e m p l o y e d t o d e t e r m i n e t h e e f f e ct o f t h e

a b s o r b e d c a l c i u m c h l o r i d e s o l u t i o n o n t h e T g a n d t h e

m e l t in g b e h a v i o u r o f t h e n y l o n . F o r t h is p u r p o s e s c an s

w e r e m a d e a t 2 0 C m i n i u s i n g a s a m p l e s i ze o f 4 t o

1716

5 m g . A f t e r t h e f i r s t h e a t i n g t o a b o v e t h e m e l t i n g

t e m p e r a t u r e , a r e p e a t e d s c a n w a s m a d e . S i n c e m o s t o f

t h e w a t e r i s d r iv e n o f f d u r i n g t h e f ir s t h e a ti n g , t h e

s e c o n d D S C t r a c e p r e s u m a b l y i n d i c a t e s th e e f fe c t o f

a b s o r b e d c a l c i u m c h l o r i d e a l o n e , w h e r e a s t h e f i r s t

s c a n i n d i ca t e s t h e c o m b i n e d e f f ec t o f a b s o r b e d s a l t

a n d w a t e r .

T h e D S C w a s a l so u s e d t o d e t e r m i n e t h e p e r c en t a g e

c r y s t a l li n i t y o f n y l o n 6 a n d n y l o n 6 ,6 . F o r t h e s e

m e a s u r e m e n t s 1 0 m g s a m p l e s w e r e u s e d w i t h t h e D S C

i n t h e t i m e - b a s e m o d e . M e l t i n g p e a k a r e a s w e r e

m e a s u r e d w i t h a p l a n i m e t e r a n d t h e c o r r e s p o n d i n g

p e r c e n t a g e c r y s t a l li n i t y w a s b a s e d u p o n h e a t s o f

f u s i o n o f 1 9 1 a n d 1 9 6 J g -1 f o r n y l o n 6 a n d n y l o n 6 , 6 ,

r e s p e c ti v e ly . T h e s e v a l u e s a r e t h e a v e r a g e s o f s e v e r a l

r e p o r t e d v a l u e s f o r e a c h p o l y m e r [ 5 ] .

2 .7 . D y n a m i c m e c h a n i c a l m e a s u r e m e n t s

T h e T g w a s d i f f ic u lt t o d e f i n e f r o m t h e D S C t r a ce s . T o

m o r e c l e a r l y d e fi n e t h e e f f e ct o f t h e a b s o r b e d s o l-

u t i o n o n t h e T g , d y n a m i c m e c h a n i c a l p r o p e r t i e s w e r e

m e a s u r e d u s i n g t h e R h e o v i b r o n D D V - I I - C ( T o y o

B a l d w i n C o . L t d . ) . E x t r u d e d f i lm s a m p l e s ( 0 .5 c m x

3 .0 c m ) w e r e r u n a t a h e a t i n g r a t e o f 1 C m i n - 1 a n d a

f ix e d f r e q u e n c y o f 11 H z .

2 8 T e n s i l e p r o p e r t y measurements

D i e - c u t s p ec i m e n s ( A S T M T y p e C ) w e r e e q u i li b r a te d

a t 1 0 0 C i n s a t u r a t e d a q u e o u s c a l c i u m c h l o r i d e a n d

m a i n t a i n e d a t t h is t e m p e r a t u r e i n s o l u t i o n u n t i l j u s t

p r i o r t o t e st in g . U p o n r e m o v a l f r o m t h e s o l u t i o n s , th e

f il m s w e r e d i p p e d i n w a t e r t o r e m o v e e x c e s s s a lt o n t h e

s u r f a c e , b l o t t e d w i t h t i s s u e s a n d m o u n t e d i n t h e

I n s t r o n t e s te r . D u r i n g t h e f e w m i n u t e s r e q u i r e d f o r

t h i s p r o c e d u r e , t h e f i l m t e m p e r a t u r e h a d d e c r e a s e d t o

r o o m t e m p e r a t u r e . E l o n g a t i o n w a s c a l c u l a t e d b a s e d

u p o n t h e i n i t i a l g r i p s e p a r a t i o n . T h e c r o s s h e a d s p e e d

w a s 5 m m m i n - 1 .

3 . R e s u l t s

3 1 W eight gains

F i g s 1 a n d 2 s h o w t h e e f f e c t o f t e m p e r a t u r e a n d t i m e

o n t h e w e i g h t g a i n s f o r n y l o n 6 a n d n y l o n 6 , 6 t h i n

f il m s i m m e r s e d i n s a t u r a t e d a q u e o u s c a l c i u m c h l o r i d e

100 r

8

7O

t /

O ~ C ~ , | , | ~ ,

0 5 1 0 1 5 2 0 2 5 3 0

T i m e l j 2 h 1 / 2 )

0

|

3 5 4 0

Figure

Effect of temperature on the w eight gains of nylon 6 thin

films in saturated aqueou s calcium chloride: (zx) 50 C, (O ) 75 C,

(o) 100 C . F or com parison the equilibrium level for nylon 6 in

water at 100C is shown by the dashed line.


3/9

3 5

3 t ~ 0 0 n u

2 5

w

c

~ 2 0

{ 5

1 0 0

5

t _ _

I I

0 5 1 0 1 5 2 0 2 5 3 0

T i m e 1 / 2 h 1 1 2 )

Figure

Effectof temperature on the weightgains of nylon 6,6 thin

films in saturated aqueo us calcium chloride: (A) 50 C, (O) 75 C,

(o) 100 C. F or co mparison the equilibrium evel for ny lon 6,6 in

wa ter at 100 C is show n by the dash ed line.

s o l u t io n s . C o m p a r i n g t h e r e su l t s fo r t h e t w o p o l y m e r s

i t i s n o t e d t h a t r e l a t i v e ly l o w w e ig h t g a in s a r e o b se r v e d

a t 5 0 C , t h o u g h t h e n y l o n 6 a b s o r b s m o r e t h a n t h e

n y lo n 6 , 6 . A t h ig h e r t e m p e r a tu r e s s i g n i f i c a n t i n c r e ase s

a r e o b s e r v e d w i t h e q u i l i b r i u m v a l u e s b e i n g g r e a t e r

t h a n t h e w e i g h t g a i n s o b s e rv e d i n p u r e w a t e r ( d a s h e d

l in e s i n F ig s 1 a n d 2 ). F o r n y lo n 6 , 6 t h e e q u i l i b r iu m

w e i g h t g a i n i n c re a s e s s y s t e m a t i c a l l y a s t e m p e r a t u r e

in c r e ase s . H o w e v e r , i n t h e c a se o f n y lo n 6 a g r e a t e r

w e i g h t g a i n o c c u r s a t 7 5 C t h a n a t 1 00 C .

R e p e a t e d m e a s u r e m e n t s u s i n g m u l t i p l e s a m p l e s

d e m o n s t r a t e d t h a t t h e r e p r o d u c i b i l i t y o f t h e e q u i l i -

b r i u m w e i g h t g a i n s w a s p o o r . F o r e x a m p l e , a t 7 5 C

n y l o n 6 f i l m s g a v e m a x i m u m w e i g h t g a i n s r a n g i n g

f r o m 6 5 t o 1 1 0 % . T h i s s c a t t e r i s a t t r i b u t e d t o t h e

d i s t o r t i o n o f t h e fi lm s w h e n h i g h l y s w o l le n a n d t o t h e

a s so c i a t e d d i f f ic u l t ie s i n r i n s in g a n d b lo t t i n g t h e su r -

f a c e s i n a r e p r o d u c ib l e ma n n e r . G e n e r a l l y l e s s s c a t t e r

w a s n o t e d ( a n d l e ss d i s t o r ti o n ) f o r n y l o n 6 , 6. D u e t o

th e i n a c c u r a c y o f t h e se d a t a i t i s n o t p o s s ib l e t o

c h a r a c t e r i z e t h e so r p t i o n k in e t i c s ( F i c k i a n a s a g a in s t

C a se I I ) o r t o a c c u r a t e ly a s se s s d i ff e r e n ce s i n r a t e f o r

t h e n y l o n 6 c o m p a r e d w i t h n y l o n 6 , 6 . H o w e v e r , i n

sp i t e o f t h e i n a c c u r a c y i t is p o s s ib l e t o c o n c lu d e f r o m

t h e s e r e s u lt s t h a t n y l o n 6 a b s o r b s g r e a t e r a m o u n t s o f

t h e c a l c i u m c h l o r i d e s o l u t i o n a t a l l t e m p e r a t u r e s c o m -

p a r e d t o t h e n y lo n 6 ,6 . Mo r e o v e r , t h e e f f e c t o f t h e

in c r e a s in g t e mp e r a tu r e o n so r p t i o n p a r a l l e l s t h e e f f e c t

o f i n c re a s i n g t e m p e r a t u r e o n s tr es s c r a c k i n g o f b o t h

p o ly me r s i n t h e s a l t so lu t i o n [ 4 ] .

A

0 3

9

8

7

6

4

3

2

1

0 1 2 3 4

H 2 0

Na CI (50 s a t u r a t i o n )

N a C I s a t u r a t e d )

I I I I I I I I I

5 6 7 8 9 10 11 12 13

T i m e 1 /a h l / 2 )

Figure

Weight gains for nylon 6 6 thin films in water or in aque ous

sodium chloride solutions at 100 C.

TABLE I Equilibr iumweight gains for nyl on 6,6

A q u e o u s C o n c e n t r a t i o n T e m p e r a t u r e W e i g h t ain

solution (M) ( C) (%)

NaC1 6.1" 100 4.6

CaC12 6.7* 100 28.5

LiC1 15.0" 100 25.9

MgC12 5.7* 100 23.8

CaC12 5.7 100 23.0

LiC1 5.7 100 8.7

CaCI2 3.3 100 12.4

CaC12 6.7* 85 17.4

LiC1 15.0* 85 14.3

MgC12 5.7* 85 22.6

*Saturated solutions.

T h e w e i g h t g a i n s f o r n y l o n 6 , 6 f i l m s i m m e r s e d i n

w a t e r o r a q u e o u s s o d i u m c h l o r i d e s o l u ti o n s a re s h o w n

in F ig . 3 . A s t h e c o n c e n t r a t i o n o f s a l t i n c r e a se s t h e

m a x i m u m w e i g h t g a i n d e c r e a s e s . T h u s t h e s o d i u m

c h lo r id e a c t s a s a d i l u e n t l o w e r in g t h e c o n c e n t r a t i o n

( a n d t h u s t h e a c t i v i t y ) o f t h e w a te r w i th a r e su l t a n t

d e c r e a se i n t h e e q u i l i b r iu m so r p t i o n l e v el o f w a te r .

T h i s b e h a v io u r d i f fe r s m a r k e d ly f r o m th a t o b se r v e d f b r

n y l o n 6 , 6 f i lms i n t h e c a l c iu m c h lo r id e ( F ig . 2 ) .

R e s u l t s f o r n y l o n 6 , 6 i n l i t h i u m a n d m a g n e s i u m

c h lo r id e so lu t i o n s w e r e s imi l a r t o t h a t i n t h e c a l c iu m

c h l o r i d e a s s h o w n i n T a b l e I . F o r t h e s e s a t u r a t e d

s o l u t i o n s a t 1 0 0 C t h e a m o u n t o f a b s o r b e d s a l t a n d

w a t e r w a s g r e a t e r t h a n 2 0 % i n a ll ca se s . T h i s d e m o n -

s t r a t e s t h a t a l l t h r e e o f t h e se s a l t so lu t i o n s i n t e r a c t

s t r o n g l y w i t h t h e n y l o n i n c o n t r a s t t o t h e r e su l t s f o r

s o d i u m c h l o ri d e .

S o l u t i o n s h a v i n g e q u i m o l a r c o n c e n t r a t i o n s w e r e

a l so i n v e s t i g a t e d b y d i l u t i n g t h e c a lc i u m a n d l i t h iu m

c h lo r id e t o g iv e 5 .7 M so lu t i o n s , t h e s a m e a s t h e s a tu -

r a t e d m a g n e s i u m c h l o ri d e . W e i g h t g a i n s f o r n y l o n 6 ,6

e q u i l i b r a t e d i n t h e se so lu t i o n s a r e a l so sh o w n in

T a b le I . T h e r e i s a s i g n i f i c a n t r e d u c t io n i n t h e w e ig h t

g a i n i n t h e d i l u t e d l it h i u m c h l o r i d e c o m p a r e d t o t h e

s a t u r a t e d s o l u t i o n .

D a t a f o r n y l o n 6 , 6 i n a c a l c i u m c h l o r i d e s o l u t i o n

d i lu t e d t o 5 0 % o f t h e s a tu r a t i o n ( 3.3 M) is a l so sh o w n

in T a b le I . T h e r e su l t s su g g e s t t h a t t h e e q u i l i b r iu m

w e i g h t g a in i s d i r e ct l y p r o p o r t i o n a l t o t h e a m o u n t o f

c a l c iu m c h lo r id e i n t h e so lu t i o n . F o r e x a mp le , i n 6 . 7 M

c a lc iu m c h lo r id e a w e ig h t g a in o f 2 8 . 5 % w a s o b se r v e d ,

w h i l e in t h e 3 . 3 M so lu t i o n t h e w e ig h t g a in w a s 1 2 . 4 % .

F o r d i r e c t c o m p a r i s o n w i t h c r a z e g r o w t h k i n e t i c s

d a t a ( t o b e r e p o r t e d [ 6 ] ) , w e ig h t g a in s w e r e a so

m e a s u r e d a t 8 5 C . C o m p a r e d t o t h e h i g h e r t e m -

p e r a tu r e , t h e e q u i l i b r iu m w e ig h t g a in s d e c r e a se d , w i th

m a g n e s i u m c h l o r i d e b e i n g t h e m o s t s o l u b l e f o l l o w e d

b y c a l c iu m a n d l i t h iu m c h lo r id e s . T h e p r e c i s io n i n t h e

d a t a i s i n su f f i ci e n t t o d e t e r m in e w h e th e r t h e o r d e r o f

so lu b i l i t y a t 1 0 0 C i s re v e r se d c o m p a r e d t o 8 5 C .

D a ta f o r n y lo n 6 f i lms a r e l i s t e d i n T a b l e I I . T h e

r e su l t s i n d i c a t e t h a t i n t h e 5 0 % sa tu r a t e d ( 3 .3 M) c al -

c i u m c h l o r i d e t h e n y l o n 6 s h o w e d a s i m i l a r w e i g h t

g a i n t o t h e n y l o n 6 ,6 ( 12 . 9 % c o m p a r e d w i t h 1 2 . 4 % ,

r e sp e c ti v e ly ). A l so i n t h e e q u imo la r ( 5 .7 M) s o lu t i o n s

c o n s i d e r a b l y le ss a b s o r p t i o n o c c u r r e d i n t h e l i t h i u m

c h lo r id e . A t

55

C , f o r t h e s a tu r a t e d so lu t i o n s , a s i g -

n i f i ca n t l y h i g h e r s o r p t i o n o f a q u e o u s m a g n e s i u m

7 7


4/9

T A B L E I I E q u i li b ri u m w ei g ht g a in s f o r n y l on 6

A q u e o u s C o n c e n t r a t i o n T e m p e r a t u r e W e i g h t g a i n

so lu t ion (M) ( C ) (%)

CaC12 6.7* i00 44.8

CaC12 3.3 100 12.9

CaC12 5.7 100 25.0

LiC1 5.7 100 8.5

MgC12 5.7* 100 26.6

CaC l 2 6.7* 55 1.6

LiC l 15.0 55 1.3

M g C l2 5.7* 55 13.6

*Sa tu ra ted so lu t ions .

c h l o r i d e w a s o b s e r v e d . R e s u l t s a t 5 5 C f o r t h e o t h e r

t w o s a l t s o l u ti o n s w e r e l o w e r t h a n e x p e c t e d . Re p e a t e d

me a s u r e me n t s i n d i c a t e d c o n s i d e r a b l e s c a t t e r i n r e s u lt s

f o r c a l c i u m a n d l i t h i u m c h l o r i d e s t h o u g h ma x i mu m

w e i g h t g a i n s w e r e al w a y s b e l o w 1 0 . I n ma g n e s i u m

chlo r ide the weigh t ga ins were cons i s t en t ly abo ve 10

t h o u g h t h e r a t e o f s o r p t i o n w a s n o t n e c e s s a ri l y mo r e

rap id than fo r the o ther sa l t s . Th i s wi l l be d i scussed

fu r ther in con junc t ion wi th c raze g rowth k ine t i cs [6 ] .

3 2 C h e m i c a l a n a l y s is

T o m o r e c l e a r l y de f in e th e c o m p o s i t i o n o f th e s o r b e d

so lu t ion , t he eqm fib ra t ed f i lms were ana lys ed fo r the i r

s o d i u m, c a l c i u m, l i t h i u m, o r ma g n e s i u m c o n t e n t .

W e i g h t g a i ns w e r e r e c o r d e d f o r e a c h f i lm s a mp l e u s e d

i n t h e a n a l y s i s w i t h i mme r s i o n t i me s b a s e d o n t h e

r e s u lt s in F i g s 1 a n d 2 . F r o m t h e c a t i o n d e t e r mi n a t i o n

the to t a l s a l t con ten t (as suming e l ec t r i ca l neu t ra l i ty )

a n d t h e w a t e r c o n t e n t w e r e c a l c u la t e d . T a b l e s I I I a n d

I V l i st t h e me a s u r e d r e s u lt s a l o n g w i th t h e c a l c u l a te d

va lues . At 50 and 100C in ca l c ium ch lo r ide , rep l i -

c a t e e x p e r i me n t s w e r e p e r l o r me d , a n d i n t h e s e c a s e s

t h e v a l u e s g i v e n i n T a b l e I I I a r e a v e r a g e s . A s n o t e d

p r e v i o u s l y , c o n s i d e r a b l e s c a t t e r o c c u r r e d f o r n y l o n 6

equ i l ib ra t ed a t 75 o r 100 C, b u t no t fo r n y lon 6 ,6 .

Fo r t h e c a l c i u m c h l o r i d e ( T a b l e I I I ) t h e a n a l y s i s

c o n f i r ms t h a t b o t h s a l t a n d w a t e r w e r e a b s o r b e d i n t o

the ny lon a t a l l t empera tu res . A s ign i f i can t increase in

t h e a mo u n t o f t h e a b s o r b e d c a l c iu m c h l o r i d e o c c u r r e d

abo ve 50 C, para l l e l ing the increase in overa l l weigh t

g a i n f o r b o t h m a t e r i a l s . A n a p p a r e n t m a x i m u m

weigh t ga in was n o ted a t 75 C fo r the ny lo n 6 (F ig . 1 ).

O n t h e o t h e r h a n d , t h e p a r t it i o n i n g o f t h e s a l t s o l u t i o n

( Ca C1 2 / H 2 0 m o l a r r a t i o s ) i n e a c h o f th e n y l o n p o l y -

me rs ind ica tes tha t t he e f fec tive conc en t ra t ion o f sa l t

so lu t ion in the po lymer f i lms increases sys t emat i ca l ly

w i t h t e mp e r a t u r e . A m u c h g r e a t e r te mp e r a t u r e d e p e n -

d e n c e i s i n d i c a t e d f o r t h e n y l o n 6 , 6 e v e n t h o u g h t h e

weigh t ga ins a re genera l ly lower . In a l l cases , t he

concen t ra t ion in the f i lms i s l es s than the concen-

t ra t ion o f sa l t i n the sa tu ra t ed l iqu id . Fo r exam ple ,

sa tu ra t e d aqu eou s ca lc ium ch lo r ide i s 6 .7 M, which i s

s ign if i can tly abo ve the sa l t /water co ncen t ra t ions in the

nylon , even at 100 C.

Fo r n y l o n 6 t h e i n cr e a s e in t e mp e r a t u r e f r o m 5 0 t o

75 C w as seen to a f fec t t he to t a l w eigh t ga in in a m uch

mo r e d r a ma t i c ma n n e r t h a n i t a f f e c t e d t h e r e l a t i v e

a m o u n t s o f c a l c iu m c h l o r i d e a n d w a t e r . T h e r e a s o n

w h y t h e t o t a l w e i g h t g a i n d e c r e a s e d a t 1 0 0 C c o m-

p a r e d t o 7 5 C i s n o t k n o w n ; h o w e v e r , t h e d i s t o r t i o n

of the f i lms and l ack o f p rec i s ion sugges t fu r ther

s tud ies a re needed to conf i rm th i s . Compar ing these

resu l t s wi th the p re v ious s t res s - rup tu re da ta [4 ] , i t is

no ted tha t t he to t a l weigh t ga in co r re l a t es wel l wi th

s t res s -c rack ing ac t iv i ty ra ther than the sa l t concen-

t ra t ion in the ny lon .

Fo r l i th i u m a n d ma g n e s i u m c h l o r id e s o l u t io n s d a t a

w e r e t a k e n o n l y a t 1 0 0 C f o r n y l o n 6 , 6 . Re s u l t s a r e

g iven in Tab le IV . In bo th cases the ana lys i s conf i rms

tha t t he increased weigh t ga in , com pare d to pure wa ter ,

is d u e t o t h e s o r p t i o n o f b o t h s a l t a n d a d d i t i o n a l

water .

The weigh t ga in and chemica l ana lys i s resu l t s

d e m o n s t r a t e t h a t a p a r t ia l s o l v a t i o n o f th e n y l o n

mat r ix occurs a t t hese s l igh t ly e l eva ted t empera tu res

in aqueous sa l t so lu t ions . Whether the so lva t ion i s

c o n f i n e d t o t h e a mo r p h o u s ( r a t h e r t h a n c r y s t a l l i n e )

r e g io n s , a n d w h e t h e r i t in d u c e s c h e mi c a l d e g r a d a t i o n

or a t rue p las t i c i za t ion o f the ny lon (as water does ) ,

a re d i scussed in the fo l lowing sec t ions .

3 3

Molecular weight

d e t e r m i n a t i o n

T h e n u m b e r a v e r a g e ( . ~ t ) , w e i g h t a v e ra g e ( 31 w ) a n d

z - a v e r a g e ( ~ t ) mo l e c u l a r w e i g h t s o f th e e x t r u d e d

ny lon f i lms a re g iven in Tab le V . T he e f fec t o f ca l c ium

c h l o r id e e x p o s u r e o n t h e mo l e c u l a r w e i g h t w a s e x a m-

i n e d to a s s e s s w h e t h e r d e g r a d a t i o n o f th e p o l y m e r

occur red in the sa l t so lu t ion . S ince some uncer t a in ty

ex i s t ed in the in i t i a l molecu lar weigh t averages o f the

ny lon 6 f i lm, s tud ies were conducted us ing the ny lon

6 ,6 f i lm. The resu l t s i n Tab le V ind ica te tha t no

d e g r a d a t i o n o c c u r r e d a f t e r f o u r d a y s o f i mme r s i o n a t

100C fo r the ny lon 6 ,6 . Cons ider ing the re l a t ive ly

r a p i d t ime s c a le o f t h e e n v i r o n me n t a l c r a z in g a n d

crack ing [4 ] , t h i s resu l t sugges t s tha t c hem ica l a t t ack

o f t h e n y l o n b y t h e s a lt s o l u t i o n i s n o t r e s p o n s i b l e f o r

t h e s a l t - in d u c e d c r a c k in g . A f t e r 3 0 d a y s o f i mme r s i o n ,

there i s some ind ica t ion tha t t he ny lon i s degrad ing

s ince the mo lecu lar weigh t i s decreas ing . In v i ew of the

T A B L E I I I A n a l y s i s o f n y l o n fi lm s e q ui l i b ra t e d i n sa t u r a t e d a q u e o u s c al c i u m c h l o r id e

S a m p l e T e m p e r a t u r e W e i g h t

( C ) ga in

( )

C o m p o s i t i o n ( w t % )

Ca CaC12 H20

C o n c e n t r a t i o n

in f i lm

(M)

N ylo n 6 50 7.8 0.6 1.8 5.5 3.0

75 78.2 4.5 12.3 31.6 3.5

100 42.0 3.1 8.5 20.4 3.9

Ny lon 6,6 50 3.0 0.2 0.6 2.4 2.4

75 22.0 2.3 6.3 11.7 4.9

100 28.5 3.1 8.5 13.8 5.6

7 8


5/9

6

0

t -

O

. . . . . / r y D n yl o n ----.

f

7

I [ E q u i l i b r a t e d i n

,~ aqueous C a C I

D r y n y l o n

I

I i

51 8 I I

4 0 0 0 3 5 0 0 3 0 0 0 O 0 1 6 0 0 1 4 0 0

W a v e n u m b e r o r e 1 }

Figure Infrared spectra of cast nylon 6 film before and after

equilibration in saturated aqueous calcium chlor ide at 100 C.

magnitude of the decrease, even after 30 days, it is

reasonable to presume that the previously observed

effect of the salt solution on stress-rupture behaviour

in short time exposures is primarily physical in nature

rather than an irreversible chemical degradation. Of

course, at the lower temperatures, such as 50 or 75 C,

degradation is even less likely to occur.

3.4. Infrared spectroscopy

Since nylon 6 generally absorbed larger amounts of

the salt solution, infrared studies were conducted

using this polymer. The infrared spectra of nylon 6

both before and after equilibration in saturated

aqueous calcium chloride at 100C are shown in

Fig. 4. Changes in the shape and/or location of the

absorption bands are observed at 3300, 1640 and

1550 cm- 1. These bands are at tributed to vibrations of

-N -H , -C =O , and -C -N molecules in the nylon

chain, respectively [7]. The slight shift in frequency, or

broadening, for each of these bands is in the direction

expected for a change in envi ronment of the molecules

from an unassociated state to a more associated state

[8]. Similar band-shifts are not observed for nylon 6

saturated with water, and non-polar diluents induce an

opposite shift. Since an extensive intermolecular

hydrogen bonding network is known to exist in nylon

polymers [9], the infrared results suggest that the cal-

cium chloride solution does not simply disrupt this

network, but in fact replaces it with some form of

association of the hydrated ions and amide groups in

the nylon.

The infrared spectra of the salt-equilibrated film

also shows a broad band at 3400cm -1 as a shoulder

on the sharper - N- H absorption band at 3300 cm -j .

In a previous study of a cast nylon film which con-

tained only calcium chloride, i.e. no water) an intense

broad absorption was also observed at this frequency

[10]. This was attributed to free, or non-hydrogen-

bonded, N- H groups in accordance with the results of

Trifan and Terenzi [9] for salt-free nylon. However, the

3400 cm-1 absorption reported here may also be attri-

buted to the O-H groups of the absorbed water. A

similar, though less intense, absorption was noted in

this study in the infrared spectra of a nylon 6 film

which was saturated with water. Also, vacuum drying

at 100C decreased the intensity of this band. Assum-

ing that this band at least in part is due to free N-H

groups, the existence o f such groups seemingly contra-

dicts the extensive broadening of the 3300 cm -1 band

on the lower-frequency side. That is, the absorbed

calcium chloride would appear to be causing an

increase in both free N - H and more associated N- H.

To avoid this contradiction it is tentatively suggested

tha t the 3400 cm-I band is entirely due to the large

amount of absorbed water in the nylon films used in

this study.

3 5 D i f fe ren t ia l scann ing ca lo r imet ry

The DSC trace of a dry nylon 6 film is shown in

Fig. 5. A slight endotherm occurred above 100 C,

perhaps due to release of residual moisture or other

impurity. The major transition for the polymer is the

crystalline melting endothe rm at 220 C with an indi-

cation that some recrystallization occurred above

160C exotherm just prior to melting). A repeated

run of the dry nylon after melting and quenching gave

a similar result to the initial trace.

The DSC analysis of nylon 6 after equilibration in

saturated calcium chloride at 100C is also shown in

TA B LE IV Analysis of nylon 6,6 films equilibrated in saturated aqueous salt solutions at 100C

Solution Concentration Weight Composition ( ) Concentration

(M) gain in film

( ) Cation Salt Water

M)

CaClz 6.7 28.5 3.1 8.5 13.8 5.6

LiC1 15.0 25.9 0.6 3.4 17.1 4.7

MgCI2 5.7 23.8 1.1 4.4 t4.8 3.1

NaC1 6.1 4.6 0 0 4.6 -

H20 - 8.5 0 0 8.5 -

TA BL E V Molecular weights of nylon films

Sample Descript ion M, (x 104) 2~rw (x 104) mz ( 105)

Capron ER20 Extrude d film nylon 6 4.72 ~ 60* ~ 800*

Zytel 2781 Extrude d film nylon 6,6 4.30 14.4 4.78

Zytel 2781 4 days at 100C in sat. CaC1z 4.32 14.5 4.67

Zytel 2781 30 days at 100C in sat. CaCI2 3.96 12.3 3.99

*Sample had a high molecular weight tail giving uncertain Mw and ~14 .

1719


6/9

e

0

W

D r y n y l o n6

S NtYuI:at: q U t ~ ~ b~ a s t ~ ~ ~ -

I I I I I I I

0 50 100 150 200 250 300 3 5 0

T e m p e r a t u r e

C)

Figure D i f f e re n t i a l s c a n n i n g c a l o r i m e t r y t r a c e s o f n y l o n 6 f il m

s h o w i n g t h e e f fe c t o f a b s o r b e d a q u e o u s c a l c i u m c h l o ri d e o n t h e

m e l t i n g b e h a v i o u r .

Fig. 5. The initial trace is complex, particularly above

150 C, where multiple melting peaks are observed.

After melting and quenching, the repeated run indi-

cates that the nylon 6 film was completely amorphous

with some indication of a broad Tg above 150 C. The

latter is 100C above the normal 50C Tg of nylon 6.

The effect of absorbed salt in the absence of water) in

preventing crystallization of nylon 6 has been

previously reported [11]. Presumably much of the

water is evaporated during the first DSC scan leaving

primarily a nylon- sal t mixture. The complex melting

behaviour observed in the initial DSC heating is

believed to be an indication that recrystallization

occurs more readily for the nylon 6 containing both

salt and water, in contrast to the effect of salt alone.

Films equilibrated at 50 and 75C also exhibit the

multiple melting peaks, and also could be quenched to

an amorphous or near-amorphous condition. How-

ever, if the equilibrated films were dried in vacuum to

partially remove water prior to DSC analysis, the

resultant DSC traces show much less intense pre-

melting phenomena.

Similar DSC results were observed for nylon 6,6

films. Fig. 6 shows the DSC scans for the control and

the salt solution-equilibra ted film. Although the nylon

6,6 melts at a higher temperature than nylon 6 265

compared wi th 220 C), the effect of absorbed salt and

.u_

0

e .

U J

D r y n y lo n 6

S e c o n d - - h e a t i n g~ ~ ~ ~ ~~ t

I I I I I

0 50 100 150 200 2 5 0

T e m p e r a t u r e

C)

f

' ~ - - - - u

I I

300 3 5 0

Figure 6

D i f f e re n t i a l s c a n n i n g c a l o r i m e t r y t r a c es o f n y l o n 6 , 6 fi l m

s h o w i n g t h e ef fe c t o f a b s o r b e d a q u e o u s c a l c i u m c h l o r i d e o n t h e

m e l t i n g b e h a v i o u r .

water is the same; namely, a complex melting and

recrystallization was noted which is absent in the dry

nylon 6,6. Again the reheat showed that the melted

and quenched sample was amorphous or melts above

325 C) with indications of a Tg near 125 C. As with

the nylon 6, it is suggested that the initial effect of the

salt and water is to promote recrystallization of the

nylon 6,6 at lower temperatures above 135 C); how-

ever, once the water has evaporated, the salt alone

inhibits the recrystallization process allowing a

quenched sample to be totally amorphous.

The DSC results do not establish whether the nylon

films are less crystalline or more crystalline after

equilibration in the saturated aqueous calcium

chloride at 100 C. To address this question, crystal-

linity measurements were made for films which were

soaked in water after prior equilibration in salt solu-

tions. The water soaking removed the absorbed cal-

cium chloride as verified by weight losses. To remove

the water, the films were dried to constant weight in

vacuum at 100 C. In the subsequent DSC measure-

ments, these films gave a single melting endotherm,

similar to unmodified dry nylon, with no evidence of

lower melting forms and with relatively small recrys-

tallization exotherms.

Crystallinity determinations are given in Table VI

along with the amount of extractable material. The

T A B L E V I P e r c e n t a g e c r y s t a l l in i t y a n d e x t r a c t a b l e s f o r e q u i l i b r a t e d n y l o n f i l m s

S a m p l e T r e a t m e n t C r y s t a l l i n it y ( ) E x t r a c t a b l e s ( )

N y l o n 6 ( fi lm ) N o n e 2 6 -

W a t e r , 5 0 C 2 8 0

W a t e r , 1 0 0 C 3 0 1 . 4

C a C l z , 7 5 C 2 8 0 . 7 , 0 . 7 *

Ca C I ~ , 100 C 33 3 . 2 , 5 . 6*

N y l o n 6 ( i n j e c t i o n - m o u l d e d ) N o n e 3 1 -

N y l o n 6 , 6 ( fi lm ) N o n e 2 9 -

W a t e r , 5 0 C 3 4 0

W a t e r , 1 0 0 C 3 3 1 . 2

C a C l z , 1 0 0 C 3 2 0 , 3 . 2 *

N y l o n 6 , 6 ( i n j e c t i o n - m o u l d e d ) N o n e 3 7 -

* R e p e a t e d m e a s u r e m e n t .

1 720


7/9

latter refers to the amount of material extracted from

the nylon films by water or by the aqueous calcium

chloride during the weight gain experiment.

o r

example, the nylon 6 film in water at 100 C evidenced

a 1.4 loss in weight (determined after vacuum dry-

ing). Only small increases in crystall inity are observed

due to immersion in the salt solution. Similar increases

a r e

noted after immersion in water. The uncertainty in

these measurements is at least _+ 2 based upon

repeated runs of the same sample. Also, if the extracted

material is assumed to contribute to the amorphous

content initially, then extracted samples should

evidence an apparent increase in crystallinity. Based

upon these considerations, it is clear tha t only minor

changes, if any, occur in crystallini ty as a result o f the

equilibration of either type of nylon in calcium

chloride solutions, For comparison, a sample of nylon

6 annealed at 200 C in air for 24 h was determined to

have a crystallinity of 48 . Thus, a significant

increase is attainable for the nylon, but apparently the

rate of recrystallization is not significant at 100C

even for the highly swollen films.

Based upon these results, the complex melting

phenomenon in the DSC scans is attributed to

processes which occur at higher temperatures during

the heating of the samples in the DSC. At 100 C (and

below) the sorption of salt solution by the nylon is

considered to be a physically reversible process with

no large-scale changes in crystall inity resulting from

the penetration of the solution into the amorphous

regions of the nylon. It is also apparent from these

measurements that the difference in crystallinity

between nylon 6 and nylon 6,6 is not significant

enough to explain the weight-gain differences or the

corresponding susceptibility to stress cracking noted

previously [4].

3.6. Dynamic mechanical m e a s u r e m e n t s

The DSC results could not be used to clearly define the

Tg of the nylon and the effect on Tg of the absorbed salt

solution. The results did suggest that the salt alone

increases the Tg, presumably due to the stiffening

effect of the amide-salt complex formation reported

by others [11]. Water, on the other hand, is well known

to lower the nylon Tg, in the usual manner that

diluents plasticize amorphous polymers [12]. The

combined effect of absorbed salt and water has not

been reported. Rheovibron measurements were there-

fore employed to reveal the effect of the absorbed salt

solution.

Fig. 7 shows the change in loss factor, tan& with

temperature for nylon 6,6 which was previously

equilibrated at 100C in saturated aqueous calcium

chloride. Also shown is the effect of vacuum drying to

remove water from the film, thereby indicating the

influence of the calcium chloride alone. Following the

usual convention the various transitions in nylon are

labelled e, fl and 7 with the c~ transi tion being identified

with the glass transition of the nylon [13].

For the dry nylon the glass transit ion can be seen to

initiate above 50 C with the peak in tan5 at 83 C. The

absorbed calcium chloride solution dramatically shifts

the Tg peak to higher temperatures (peak in tan5 at

- - [ { a } I i I

re~ 130C

v

. . . I

100 50 0 100 150 2 0 0

Temperature C)

Figure Dynamic mechanical loss factor (tanS) against tem-

perature showing he effect of absorbed calciumchloride solution

on the transitions of the nylon6,6. (a) equilibrated n CaC12, then

vacuum-dried to remove water; (b) equilibrated at 100C in sat-

urated aqueous CaCI~; (c) dry sample.

130 C) and also causes a significant broadening of the

peak. Such a shift in T~ suggests a restricting effect of

the absorbed salt-wate r mixture on molecular motion

in the amorphous phase of the nylon. However, due to

the possible loss of water from the film during the

Rheovibron test the exact location of the Tg of the

equilibrated film is uncertain. Also the onset of Tg may

not be shifted as much as the tan5 peak.

After water is removed from the swollen nylon 6,6

film by vacuum drying, the Tg tan5 peak is shifted to

much higher temperatures with the maximum in

tan5 near 190 C. This shows the strong effect of the

salt alone in complexing with the nylon chains to

completely suppress the segmental chain motions

which normal ly would have initiated above 50 C. The

Tg increase agrees with the DSC results reported here

and by others [11]. These results suggest that the salt

solution does not act as a conventional plasticizer for

nylon; in other words, the Tg decrease expected from

the large amount of absorbed solution is offset by the

strong interactions with the salt ions.

The beta and gamma transitions are also affected by

the absorbed salt solution with the beta loss peak

especially being enhanced. Absorbed water has been

reported to have a similar effect on this transition

which is associated with carbonyl group motion [13].

Fig. 7 demonstrates that the removal of water causes

a noticeable sharpening of the beta tan5 peak while the

gamma tan5 peak is unchanged. In view of the latter

being associated with local oscillations of non-polar

methylene groups [13], this observation is not surpris-

ing and in fact confirms the infrared results in suggest-

ing an association of the salt ions with the polar

portions of the nylon.

3 . 7 . T e n si Le p r o p e r ty m e a s u r e m e n t s

Table VII lists the properties of nylon 6 and nylon 6,6

thin films before and after equilibration in saturated

calcium chloride at 100C. Modulus and strength

values were calculated using the dimensions of the d ry

films. A large decrease in modulus was observed for

both nylons as a result of the absorption of aqueous

calcium chloride. Lower strengths were also observed,

1721


8/9

T A B L E V I I Te ns i l e prope r t i e s of ny lon f i lm s e qui l ibra te d in s a tura te d a que o us c a lc ium c hlo r ide a t 100 C*

S a m p l e T r e a t m e n t M o d u l u s ( G P a ) T e n s i le s t r e n g t h

( M P a )

E l o n g a t i o n a t

b r e a k ( )

N y l o n 6 V a c u u m - d r i e d c o n t r o l 1 .0 6 4 2 . 4

Ca C12, 100C for 1 .5h 0 .32 24 .8

Ny lon 6 ,6 Va c u um -d r ie d c on t ro l 2 . 03 108.1

Ca CI2 , 100C for 24 h 0 .83 101.9

8.0

18.5

125

178

* M o d u l u s a n d s t r e n g t h w e r e c a l c u l a te d u s i n g d i m e n s i o n s o f d r y f il m s,

especially for the nylon 6, and elongations at break

were also increased for both polymers.

The tensile property measurements suggest that the

effect of the diffusion of salt solution into the amor-

phous regions is to weaken the nylon matrix. Since

these measurements were performed at room tem-

perature, it must also be expected that at elevated

tempera ture the effects reported here would be further

enhanced. Thus even lower moduli and loss of load-

bearing capability would be anticipated due to the

absorption of salt solution at 50 to 100 C.

The reproducibility of the tensile property measure-

ments was demonstrated by replicate experiments

with both nylon 6 and nylon 6,6. In view of the infra-

red and dynamic mechanical measurements, some stif-

fening modulus increase) had been anticipated rather

than the observed softening. A direct comparison of

these results is complicated by the uncertainty in the

amount o f water which evaporated from the samples.

Since the elapsed time period from removal of

samples from the salt solutions to tensile testing) was

considerably less than in the dynamic mechanical

measurement of Tg, the tensile properties are thought

to more nearly represent the equilibrated film con-

ditions. The relat ion of the observed changes to the Tg

of the equilibrated films will require further study.

However, the tensile property changes reported here

are representative of a plasticization-like action by the

salt solution.

4 D i s c u s s i o n

The results of this investigation have shown that those

aqueous salt solutions which were previously shown

to cause crazing of nylon are also partially soluble in

the material at elevated temperatures. This includes

calcium, lithium, and magnesium chlorides. Sodium

chloride, which showed no tendency to induce crazing,

was also not absorbed into the nylon structure. The

chemical analysis confirmed that both salt ions and

water are absorbed into the nylon with the partitioning

of the salt and water being dependent on temperature

as well as on nylon type.

The crystallinity measurements of films previously

equilibrated in salt solutions suggested that the salt is

sorbed into the amorphous regions of nylon without

inducing significant changes in crystallinity. Similarly

no degradation in molecular weight was detected as a

result of salt-solution absorption. Thus the evidence

indicates that the sorption process is physically revers-

ible and appears similar in this regard to the swelling

of amorphous polymers by organic solvents.

However, the ionic nature of the salt solution

coupled with the intermolecular hydrogen bonding o f

722

not swol le n f i lm d im e ns ions .

the nylon lead to a much more complicated polymer -

liquid interaction than the van der Waals type of

bonding in organic solvents. For example, the infrared

spectroscopy results indicated that the absorbed cal-

cium chloride formed an association with the amide

groups in the nylon. This was observed in spite of the

large amount of water in the equilibrated films.

Evidence that the association of the amide groups

with the hydrated salt ions stiffens the nylon chains

was provided by the dynamic mechanical measure-

ments which showed a shift of the glass transition to

higher temperatures. Such a shift is indicative of a

restricted mobility for the nylon polymer chains in the

amorphous phase in the presence of the aqueous salt

solution.

That absorbed salt alone restricts the amorphous

phase mobility is shown by the very high Tg for the

vacuum-dried film. DSC results also indicate an

increased Tg for the second heating of equilibrated

films. These films are also thought to contain very

little water. This confirms the observations previously

reported by others for ny lon -sa lt systems [11]. For the

film equilibrated in the aqueous solution, the loss of

water during the dynamic test leaves some doubt as to

the precise location of T~. Since absorbed water

decreases Tg while salt alone increases it, one expects

an intermediate effect of the salt solution. The fact

that salt solution induced crazing occurs at tempera-

tures just above the Tg of the dry nylon makes the

question of the Tg location of even greater interest.

For example, if absorbed salt raises the Tg, the sorp-

tion process should in turn be retarded. On the other

hand, a lowering of Tg is more compatible with sol-

vent crazing models developed for amorphous glassy

polymers. Unfortunately the available evidence does

not allow a resolution of the question o f the Tg of the

salt solution equilibrated nylon films.

The tensile property measurements, on the other

hand, indicate that the modulus is decreased as a

result of the sorption process. The inference is that the

Tg may also be reduced or at least substantially

broadened. The reduction in modulus is considered

very significant in terms o f proposing a mechanism for

salt-solution-induced crazing. It establishes a parallel

between absorbed salt solution in nylon and absorbed

organic solvents in glassy polymers, since the latter

can also reduce modulus and increase tensile elong-

ation. Thus relaxation controlled craze growth models

developed for solvent crazing of glassy polymers

[14, 15] may be equally applicable to salt solution

induced craze growth in crystalline nylon. This will be

examined in detail in the next paper in this series [6].

The results of the present study are compatible with a


9/9

p l a s t ic i z a t io n l i k e m e c h a n i s m f o r sa l t in d u c e d c r a z i n g

s i n c e i t h a s b e e n c l e a r l y d e m o n s t r a t e d t h a t t h e s a l t

s o l u t i o n p e n e t r a t e s t h e n y l o n s t r u c t u r e w i t h o u t

d e s t r o y i n g t h e c r y s ta l li n e s t r u c tu r e a n d w i t h o u t c a u s

i n g e x t e n s i v e r e c r y s t a ll iz a t io n o r m o l e c u l a r w e i g h t

d e g r a d a t i o n .

e f e r e n c e s

I . C . D . W E I S K E , Kuns to f f e 54 (1964) 626.

2 . P . D U N N a n d G . F . S A N S O M , J Appl Poly m Sc i 13

(1969) 1641.

3 . M . G . W Y Z G O S K I , i n M a c r o m o l e c u l a r S o l u t i o n s: S o l -

v e n t P r o p e r t y R e l a t i o n s h i p s i n P o l y m e r s e d i t e d b y R . B .

S e y m o u r a n d G . A . S t a h l ( P e r g a m o n , N e w Y o r k , 1 98 2) p . 4 1 .

4 . M . G . W Y Z G O S K I a n d G . E . N O V A K ,

J Mate r Sc i

22 (1987) 1707.

5 . J . B R A N D U P a n d E. H . I M M E R G U T ( ed s) . P o l y m e r

1 4 H a n d b o o k ( 2 n d E d n ) (W i l e y , N e w Y o r k , 1 9 7 5 ) p . 1 1 1- 5.

6 . M . G . W Y Z G O S K I a n d G . E . N O V A K , i n p r e p a ra t i o n .

7 . D . O . H U M M E L , I n f r a r e d S p e c t r a o f P o l y m e r s ( I n t e r-

s c ie nc e , Ne w Y ork , 1966) p . 61.

8 . C . G . C A N N O N , Spe c t roc him Ac ta 16 (1960) 302.

9 . D . S . T R I F A N a n d J. F . T E R E N Z I ,

J Poly m Sc i 28

(1958) 443.

1 0. P . D U N N a n d G . F . S A N S O M ,

ibid

13 (1969) 1657.

1 1. H . K I M a n d P . J . H A R G E T ,

J Appl Phy s

56 (1979)

6072.

1 2. M . I . K O H A N , N y l o n P l a s t i c s ( W i l e y , N e w Y o r k , 1 9 73 )

p. 329.

1 3. N . G . M c C R U M , B . E . R E A D a n d G . W I L L I A M S ,

A n e l a s t i c a n d D i e l e c t r i c E f f e c t s i n P o l y m e r i c S o l i d s

( W i l e y , N e w Y o r k , 1 9 67 ) p . 4 8 0.

1 4. J . G . W I L L I A M S a n d G . P . M A R S H A L L , Proc R Soc

A342 (1975) 55 .

1 5. E . J . K R A M E R a n d R . A . B U B E C K , J Poly m Sc i 16

(1978) 1195.

Received 2 June

and accepted 18 August 1986

723

5_stress cracking of nylon polymers in aqueous salt solutions

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