10 Water Permeation Through Elastomers The Effects of Hostile Environments on Coatings and Plastics Downloaded from pubs.acs.org by UNIV OF TENNESSEE KNOXVILLE on 01/22/16. For personal use only.
PATRICK E. CASSIDY Texas Research Institute, Inc., Austin, TX 78746, and Southwest Texas State University, San Marcos, TX 78666
Long term monitoring of deionized (DI) and salt water through Neoprenes G and WRT reveal an increasing permeation rate with age for DI water. Several explanations for this behavior are offered, with supporting analytical data. The effects on permeation of elevated temperatures and/or static and dynamic mechanical stresses are examined. Temperatures of 60-70°C cause dramatic increases in permeation rate for both DI and salt water, with relative values depending on the rubber composition. Small static stresses were ineffectual. The dynamic stress applied—high frequency acoustic agitation—increased permeability through heating. P l a s t i c s and elastomers are g e n e r a l l y thought to be r e s i s tant to o r unaffected by mild environments such as water immers i o n . Commonly, an organic coating polymer or s e a l i s accepted as p r o v i d i n g a water-tigjht b a r r i e r even when the device i s immersed i n water. But water w i l l permeate the b a r r i e r , i f only slowly. The phenomenon of water permeation i s reviewed i n (1,2). There a r e three f a c t o r s that can adversely a f f e c t a materi a l employed as a b a r r i e r : time, temperature, and mechanical s t r e s s . While the l a s t two are u s u a l l y considered when designi n g a piece of hardware, the f i r s t i s not always taken i n t o account. P a r t of the reason f o r d i s r e g a r d i n g time i s the l a c k of r e l i a b l e aging i n f o r m a t i o n . In order to evaluate a m a t e r i a l ' s l i f e t i m e expectancy i n a short l a b o r a t o r y t e s t the temperature i s increased and a temperature-time r e l a t i o n s h i p i s developed f o r some property o f the m a t e r i a l . Of course, there i s a l i m i t to how high the temperature can be (e.g., 80-90°C f o r Neoprenes) without inducing extraneous and u n r e a l i s t i c chemical r e a c t i o n s . More d i f f i c u l t sometimes i s the d e c i s i o n c r i t e r i o n : when the decay o f the 0097-6156/83/0229-0153S06.00/0 © 1983 American Chemical Society
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m a t e r i a l property (e.g., t e n s i l e strength) i s such that the m a t e r i a l i s considered f a i l e d f o r a s p e c i f i c a p p l i c a t i o n . T h i s paper considers Neoprene elastomers and t h e i r use as b a r r i e r s t o water p e n e t r a t i o n . Long time e f f e c t s on permeab i l i t y and composition w i l l be discussed as w i l l the e f f e c t s on p e r m e a b i l i t y of temperature and high-frequency mechanical (acoustic) agitation. Discussion E f f e c t s of Long-Term Water Exposure. The usual procedure f o r e v a l u a t i n g the p e r m e a b i l i t y of an elastomer i s to monitor water t r a n s m i s s i o n r a t e through the rubber u n t i l a steady s t a t e i s a t t a i n e d (ASTM D 1653), u s u a l l y a period of a few days. T h i s measured permeation r a t e i s then used to p r o j e c t t o t a l water transmission over the expected l i f e t i m e of a device, which may be s e v e r a l years ( 3 ) . T h i s i s v a l i d , of course, only i f no c r i t i c a l changes (e.g., degradation, leaching of components, hardening, e t c . ) take place i n the elastomer as i t ages. However, s e v e r a l aging r e a c t i o n s are known. The most severe i s perhaps the e x t r a c t i o n of components of the rubber i n t o the l i q u i d permeant. Another i s the continued s w e l l i n g of the e l a s tomer due to increased d i s s o l u t i o n of the permeant i n t o the membrane. In any case, long-time e x t r a p o l a t i o n s of p e r m e a b i l i t y r a t e s determined over short durations may not be v a l i d (4)· Indeed, long-term measurements of d e i o n i z e d (DI) water permeation through samples of Neoprene G (316 days at 60°C) and Neoprene WRT (75 days at 25°C) gave n o n l i n e a r permeation curves. F i g u r e 1 i l l u s t r a t e s t h i s f o r Neoprene G. Shown are p l o t s of weight l o s s versus time i n permeation c e l l s f i l l e d with DI water and with 3.5% s a l t water. The l a t t e r y i e l d e d a s t r a i g h t - l i n e p l o t , i . e . , constant permeation r a t e , which v e r i f i e s the technique and i s explained by the f a c t that s a l t water causes o n l y a very small change i n the composition of the rubber. The r a t e of DI water permeation was 139% g r e a t er at the end of the t e s t period than at the beginning. A Neoprene WRT sample showed a g a i n to 450% of i t s o r i g i n a l value, and t h i s under even milder c o n d i t i o n s than the Neoprene G t e s t ( 5 ) . The Neoprene G r e s u l t s are tabulated i n Table I .
Figure I. Water permeation through Neoprene G at 60° C. Key to permeant: Π, deionized water; and o, 3.5% NaCl solution.
Time - days
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156 Table I.
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DI SALT
Long-Term Permeation
Total Time (days)
T o t a l Water Permeated
316 316
16.53 11.51
Results
C o n d u c t i v i t y of Face Wash# (ohm" cm""*) 1
is!
8.6 X 7.6 Χ 1
10~ 10"
6
6
Permeation Ratet (mg/cm^/day) 1.69/2.35 1.43
1
# Pure DI water i s 5 χ 10"*> ohm" cm" t For DI water, i n i t i a l and f i n a l r a t e s are given There are s e v e r a l p o s s i b l e explanations f o r t h i s n o n e q u i l ibrium behavior. The composition of the Neoprene may be changed by the l e a c h i n g of components i n t o the l i q u i d permeant, thereby opening pores to increased c a p i l l a r y flow of water ( v i s - a - v i s d i f f u s i o n ) . Changes i n composition of the elastomer and the r e s i d u a l permeant are discussed i n l a t e r paragraphs. Another p o s s i b l e explanation may be i n c r e a s i n g s o l u b i l i t y of DI water i n these Neoprene formulations, due, of course, to the a d d i t i v e s . Other studies c a r r i e d out at 5, 25 and 50°C over 24-40 day terms have shown that fresh water continues to be ab sorbed by Neoprene WRT without reaching e q u i l i b r i u m ( v i d e i n f r a ) . Because p e r m e a b i l i t y i s dependent on s o l u b i l i t y , a c o n t i n u i n g i n c r e a s e i n the l a t t e r w i l l a f f e c t the former s i m i l a r l y . A t h i r d p o s s i b l e explanation i s that absorption of water i n elastomers which c o n t a i n water s o l u b l e f i l l e r s has been shown to create pockets of s o l u t i o n which continue to grow u n t i l the os motic pressure i s balanced by the containment pressure of the rubber. This introduces a very complex p i c t u r e f o r water perme a t i o n : there are pockets of permeant, which cause mechanical s t r e s s e s i n the rubber matrix, c r e a t i n g thereby channels f o r water d i f f u s i o n . In the Neoprene G t e s t s summarized i n F i g u r e 1 and Table I the outer (dry) faces of the permeated rubber samples were washed w i t h DI water to determine i f s o l u b l e s a l t s were present. The c o n d u c t i v i t y values l i s t e d i n Table I show o n l y a very s l i g h t d i f f e r e n c e , which i s probably of l i t t l e import. The i n t e r n a l water permeant was, however, changed by the l o n g term aging. The water was pale yellow. A n a l y s i s by emission spectroscopy showed notable changes i n the c o n c e n t r a t i o n s of 4 of the 16 met a l s which were monitored. These r e s u l t s are l i s t e d i n Table I I .
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Table I I . Metal Concentrations (%) i n Residual Permeants
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Metal Al Mg Na Fe
Concentration (%) Deionized Salt Water Water 7 χ 10~ 0.18 0.01 7 χ ΙΟ"
5
4
5 χ 10~ 0.05 1.9* 3 χ ΙΟ"
2
3
C o r r e s p o n d s to 4.8% NaCl s o l u t i o n . Aluminum appeared i n the r e s i d u a l s a l t water permeant at a con c e n t r a t i o n 700X that i n the DI water. Inspection of the permea t i o n c e l l s revealed much more i n t e r n a l c o r r o s i o n a s s o c i a t e d with the s a l t water permeant than with the fresh water; hence, the source of aluminum. Sodium, of course, o r i g i n a t e d from the Neo prene, and s a l t water removed much l e s s than d i d f r e s h water. The high concentration of sodium i o n i n the r e s i d u a l s a l t water permeant (4.8% v i s - a - v i s the i n i t i a l 3.5%) r e s u l t e d from the l o s s of water by permeation while r e t a i n i n g s a l t . The source f o r magnesium and i r o n was also the Neoprene. The permeated rubber was a l s o analyzed f o r metals by emis s i o n spectroscopy. Of the 16 metals present only sodium showed any a p p r e c i a b l e change i n c o n c e n t r a t i o n . S a l t water reduced the sodium content of the rubber by 17% (from 0.12% to 0.10%) whereas DI water reduced i t by 92% ( t o 0.01%). Two samples of Neoprene G were s l i c e d i n t o 40 ym s e c t i o n s w i t h a cryogenic microtome to examine i o n m i g r a t i o n . The sam ples were from an unused piece of m a t e r i a l and one which had been subjected to s a l t water permeation at 60°C f o r 316 days. Several s l i c e s of each sample were analyzed by ESCA ( a l s o c a l l e d XPS) f o r c a t i o n content. F i g u r e 2 shows the r e l a t i v e abundance of s e v e r a l elements versus depth i n the rubber. (Note that the l o g a r i t h m i c s c a l e deemphasizes the d i f f e r e n c e s along the y a x i s ) . A p o s s i b l e movement of species i n the d i r e c t i o n of flow ( r i g h t to l e f t ) i s apparent i n t h i s graph. Or, species may be removed by leaching by the permeant, which would r e s u l t i n low values on the wet s i d e . The l a t t e r argument i s weakened by the f a c t that a t s l i c e No. 16, 87% of the way through the rubber, the abundance of most of the species exceeds t h e i r o r i g i n a l value. E f f e c t s of E l e v a t e d Temperature Most r a t e processes increase w i t h i n c r e a s i n g temperature, for example, the s o l u b i l i t i e s of s o l i d s i n l i q u i d s . Of course, there are exceptions, e.g., the s o l u b i l i t y of water i n polyimides (6); and with p l a s t i c s and elastomers there are upper temperature
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5000rControl Values
30001
2438
367
/
ο 127
/ ο
^^ro
•
J
Δ
56
— —
29
•
Direction of Flow 0 JL
0.4
0.8
1.2
± from Dry Side - mm Depth J I L 20 30 10 Slice Number
1.6
2.0
40
J 50
N.D.
oxygen; A, Figure 2. Element abundance vs. depth in sample. Key: φ, carbon: aluminum: o, silicon: sodium: and Δ, sulfur.
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l i m i t s above which the processes themselves may be a l t e r e d . Solu b i l i t i e s and p e r m e a b i l i t i e s have been evaluated f o r Neoprenes (3^7_), polyisoprenes and n a t u r a l rubbers (8) f o r temperatures to 60 °C. F i g u r e 3 i l l u s t r a t e s the change of s o l u b i l i t y of water i n WRT Neoprene with temperature. The s o l u b i l i t i e s of both deioni z e d and 3.5% s a l t water were measured at 5, 25 and 50°C. The f i g u r e shows that DI water i s more soluble than s a l t water i n WRT Neoprene and that i n c r e a s i n g temperature increases s o l u b i l i t y . The s o l u b i l i t y of DI water a t 50°C does not approach equil i b r i u m even a f t e r 25 days. From these data a t o t a l weight gain of 300% was p r e d i c t e d . Assuming that d i f f u s i v i t y behaves s i m i l a r l y , p e r m e a b i l i t y would be expected to i n c r e a s e with temperature. Several elastomers were evaluated f o r p e r m e a b i l i t y to s a l t and d e i o n i z e d water a t s e v e r a l temperatures. A WRT Neoprene (Burke Rubber Co. Type 5109) was tested at 23, 40 and 70°C using both s a l t water (3.5%) and DI water as permeants. The r e s u l t s f o r s a l t water are l i s t e d i n Table I I I and f o r DI water i n Table IV. S a l t water permeability i n c r e a s e d 24 times with a 47°C temperature r i s e w h i l e the DI water increase was 33 times. The d i f f e r e n c e may be due to t h e much greater s o l u b i l i t y of DI water r e l a t i v e to s a l t water. Table I I I . Temp. (°C) 23 40 60 70
P e r m e a b i l i t y of Burke 5109 Neoprene to S a l t Water
Permeant
Permeability (g-cm/cm^-day)xl0^
3.5 wt-% NaCl Solution
33.3 55.6 190 792
Table IV. Temp. (°C) 23 40 60 70
Permeation Constant (ng-cm/cm^-hr-torr) 66.4 42.3 53.7 142.7
P e r m e a b i l i t y o f Burke 5109 Neoprene to DI Water
Permeant
DI Water
Permeability (g-cm/cm^-dayjxlO^ 13.3 50.0 255 433
Permeation Constant (ng-cm/cm^-hr-torr) 26.3 37.6 71.2 77.2
Energies of a c t i v a t i o n determined from an Arrhenius plot of these data y i e l d e d values of 12.8 kcal/mole f o r the NaCl water permeant samples and 15.3 kcal/mole f o r the DI water permeant samples.
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160 EFFECTS OF HOSTILE ENVIRONMENTS
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