The Effects of Hostile Environments on Coatings and Plastics

11 Polyurethane Aging in Water and Methanol Environments DOUGLAS L. FAULKNER, MICHAEL G. WYZGOSKI, and MARK E. MYERS, JR.

Downloaded by CORNELL UNIV on May 30, 2017 | http://pubs.acs.org Publication Date: September 29, 1983 | doi: 10.1021/bk-1983-0229.ch011

General Motors Research Laboratories, Warren, MI 48090-9055

The tensile properties of polyester-based thermoplastic polyurethanes were studied as a function of the time of exposure of the plastics to water, methanol, methanol-water, methanol-isooctane, and methanol-water-isooctane. The resulting decrease in the tensile properties of the plastics was attributed to reaction of the plastics with water and methanol. As indicated by the decrease in properties, reaction with methanol is initially faster, but the reaction rate with water increases with time -- presumably because of the autocatalytic nature of the reaction. Nuclear magnetic resonance spectroscopy indicated that the reaction mechanisms with methanol and water were transesterification and hydrolysis, respectively. Gel permeation chromatography revealed that both reactions resulted in a marked reduction in the molecular weight distribution of the plastics, which correlated well with losses in tensile properties. Mechanisms f o r polyurethane degradation i n c l u d e h y d r o l y s i s and chemical exchange r e a c t i o n s . H y d r o l y s i s i n v o l v e s a r e a c t i o n o f the polymer with water which r e s u l t s i n chain cleavage. Chemical exchange i s a general term which includes such r e a c t i o n s as g l y c o l y s i s , a m i n o l y s i s , t r a n s e s t e r i f i c a t i o n and the l i k e . These r e a c t i o n s , l i k e h y d r o l y s i s , r e s u l t i n molecular weight r e d u c t i o n through random chain s c i s s i o n . U l t i m a t e l y a l l of these r e a c t i o n s w i l l cause an attendant l o s s of t e n s i l e p r o p e r t i e s . T h i s paper presents r e s u l t s from aging p o l y e s t e r based p o l y urethanes i n methanol and water environments and i n other mixtures of these media. The i n f l u e n c e of aging on mechanical p r o p e r t i e s was assessed by measuring t e n s i l e strengths and elongations a t

0097-6156/ 83/0229-0173S06.00/0 © 1983 American Chemical Society

Garner and Stahl; The Effects of Hostile Environments on Coatings and Plastics ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

174

EFFECTS OF HOSTILE ENVIRONMENTS

various exposure times. Chemical analyses were used to independently monitor the extent of degradation and to determine the aging mechanisms. Experimental M a t e r i a l s . I n j e c t i o n molded Texin 591AR t h e r m o p l a s t i c polyurethane, s u p p l i e d by Mobay Chemical Company, was one of the two polyurethanes used i n t h i s i n v e s t i g a t i o n . (Hereafter t h i s r e s i n i s r e f e r r e d to as polymer "A".) T h i s r e s i n i s a p o l y e s t e r based polyurethane and i s considered a medium grade i n terms of hardness or f l e x i b i l i t y . Nuclear magnetic resonance (NMR) char a c t e r i z a t i o n revealed that f o r polymer A the molar r a t i o of p o l y e s t e r to diphenylmethane-4,4 -diisocyanate (MDI) i s 2.07, and the e s t e r i s p o l y ( b u t y l e n e a d i p a t e ) . The m a t e r i a l was obtained i n p e l l e t form to prepare samples f o r aging experiments. P e l l e t h a n e CPR 2102 polyurethane, obtained from the Upjohn Company, was the other polyurethane s t u d i e d . (Hereafter t h i s m a t e r i a l i s r e f e r r e d to as polymer "B".) For polymer Β the molar r a t i o of p o l y e s t e r to ΜΌΙ i s 2.95, and the e s t e r i s p o l y ( c a p r o l a c t o n e ) . T h i s m a t e r i a l i s reported to have i n i t i a l p r o p e r t i e s s i m i l a r to polymer A when both m a t e r i a l s are post cured at 110°C f o r 16 hours. The polymer Β samples were p r e v i o u s l y post cured. In a d d i t i o n , polymer Β i s reported to provide s u p e r i o r r e s i s t a n c e to h y d r o l y t i c a t t a c k compared to other p o l y e s t e r based p o l y urethanes . D i s t i l l e d water, isooctane, and methanol were used as aging media. Isooctane and methanol were reagent grade and were used as r e c e i v e d without f u r t h e r p u r i f i c a t i o n . Isooctane was s e l e c t e d as a r e l a t i v e l y i n e r t f l u i d to study the e f f e c t of methanol concen tration.


1

T e n s i l e Sample P r e p a r a t i o n . Polymer A t e n s i l e bars were i n j e c t i o n molded according to the s u p p l i e r ' s recommendations. Dimensions were 3.2 by 3.8 mm with a length of 62.0 mm. Polymer Β was sup p l i e d i n 1.7 mm t h i c k sheets from which 3.2 mm wide t e n s i l e bars were c u t . Aging of T e n s i l e Bars. T e n s i l e bars were aged i n t e s t tubes c o n t a i n i n g the aging media. The t e s t tubes were heated i n an aluminum block bath and were equipped with water cooled condensers to minimize l i q u i d l o s s through evaporation. A l l samples were aged at 80°C except samples aged i n methanol, where the aging temperature was 60°C to prevent the b o i l i n g of methanol (b.p.=64.5°C). A l l aging l i q u i d s were m i s c i b l e when heated except f o r a mixture of water, methanol, and isooctane, which formed a two-phase mixture. I n t h i s case, the samples were suspended i n the i s o o c t a n e - r i c h phase. A f t e r aging, samples were d r i e d i n a vacuum oven at 80°C and r e e q u i l i b r a t e d at room temperature under


11.

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175

50% r e l a t i v e humidity before t e n s i l e t e s t i n g . At the termination of these experiments, the v a r i o u s s o l v e n t mixtures used to age the samples were evaporated to dryness, and the r e s u l t i n g residues were saved f o r NMR a n a l y s i s . T e n s i l e strengths and elongations were measured at room temperature using t e n s i l e specimens with a gauge length of 15.9 mm. An I n s t r o n was employed with a crosshead speed of 508 mm/min.


Chemical A n a l y s i s Nuclear Magnetic Resonance (NMR). Samples were d i s s o l v e d i n dimethyl s u l f o x i d e (

176



Results Aging of T e n s i l e Bars. Polyurethane t e n s i l e samples were aged i n methanol, water, an equal volume mixture of the two, and other mixtures c o n t a i n i n g isooctane. While only g r a p h i c a l r e s u l t s of polymer A t e n s i l e p r o p e r t i e s are presented i n t h i s s e c t i o n , s i m i l a r r e s u l t s were obtained with polymer B. T e n s i l e p r o p e r t i e s of both polymers A and Β are l i s t e d i n Table I . A p l o t d e p i c t i n g the e f f e c t of aging i n water on the t e n s i l e strength of polymer A i s shown i n F i g . 1. The p l o t i s c h a r a c t e r i z e d by an i n i t i a l slow decrease i n strength with time which a c c e l e r a t e s a f t e r 10 days. T h i s increase i n the r a t e of t e n s i l e strength l o s s i s c o n s i s t e n t with the a u t o c a t a l y t i c nature of the h y d r o l y s i s r e a c t i o n . The r e a c t i o n generates an a c i d which i n t u r n c a t a l y z e s f u r t h e r h y d r o l y s i s of the polyurethane ( 1 ) . As shown i n F i g . 2, t e n s i l e strength of polymer A decreases more r a p i d l y i n i t i a l l y i n methanol than i n water even though aging i n methanol i s conducted at 60°C (20°C lower than i n water). The rate of property l o s s i s observed not to i n c r e a s e with time. T h i s i s expected s i n c e the degradation r e a c t i o n i n methanol ( t r a n s e s t e r i f i c a t i o n ) i s not a u t o c a t a l y t i c . The r e d u c t i o n of t e n s i l e e l o n g a t i o n i n the two media shows a s i m i l a r behavior. T e n s i l e p r o p e r t i e s of the polyurethanes aged i n solvent mix tures c o n t a i n i n g v a r i o u s amounts of methanol were a l s o evaluated. The r e l a t i v e e f f e c t s of the composition of the mixtures on the t e n s i l e strength of polymer A are shown i n F i g . 3. The data appear to be systematic when expressed i n terms of mole f r a c t i o n of methanol ( i n d i c a t e d i n F i g . 3 i n p a r e n t h e s i s ) . A two-phase mixture r e s u l t e d upon the a d d i t i o n , on a volume b a s i s , of a 25% a l i q u o t of an equal volume methanol-water s o l u t i o n to isooctane ( i n an attempt to form a 75-12.5-12.5 i s oo c t ane-me thano1-wa ter s o l u t i o n ) . No volume change was observed i n the isooctane l a y e r upon the a d d i t i o n of the methanol-water s o l u t i o n . Given the p o l a r i t y d i s s i m i l a r i t i e s of water and isooctane, i t i s reasonable to expect that the methanol p a r t i t i o n i n g would favor much higher methanol concentrations i n water than i n i s o o c t a n e . Since the samples were exposed only to the upper isooctane l a y e r , which presumably contained l i t t l e methanol, l e s s degradation occurred here than i n other mixtures c o n t a i n i n g higher methanol contents. Chemical A n a l y s i s NMR. The NMR spectrum of an unexposed sample of polymer A, shown i n F i g . 4, i n d i c a t e s that the polymer contains MDI, ethylene g l y c o l , and poly(butylène a d i p a t e ) . The assignments are as shown (2), and the v a r i o u s sets of bands are given l e t t e r d e s i g n a t i o n s . Chemical s h i f t s are i n d i c a t e d as parts per m i l l i o n (ppm) from TMS.



(50-50)

e

20 30 10 20 30

20 30 10 20 30 10 20 30 10

-10

Aging Time (Days)

18.82 18.27 23.92 15.65 7.31

25.85 5.23 2.36 0.59 6.70 2.76 0.93 14.89 10.27 7.86 22.61 46.75 39.16 43.78 23.65 8.55

51.71 43.64 13.03 11.38 35.85 15.10 8.89 39.99 25.30 16.89 47.78

T e n s i l e S t r e n g t h (MPa) Polymer A Polymer Β

T e n s i l e P r o p e r t i e s of Polyurethanes

e

907 850 1090 928 299

1060 219 32 16 347 107 32 955 640 331 1060

1070 1030 1010 869 251

1020 1220 448 368 1070 592 261 1050 949 731 1030

X Elongation Polymer A Polymer Β

* Aging temperatures were 80 C except f o r methanol where the temperature was 6 0 C .

**

Η

Water

M

M

Isooctane-methanol-water (75-12.5-12.5)

*·

Methanol-water

**

Isooctane-methanol (75-25)

**

Control Methanol

Aging Medium*

Table I .





FAULKNER ET AL.

Polyurethane Aging


30 r

Time

(days) i n Methanol

Figure 2. Kinetics of tensile strength loss of polymer A aged in methanol at 60° C.


180



30 r

I

ι

ι

ι

0

10

20

30

Time (days) Figure 3. Kinetics of tensile strength loss of polymer A aged in methanol solvent mixtures at 80°C. XhteOH is the mole fraction of methanol



FAULKNER ET AL.

Polyurethane Aging

Figure 4. NMR spectrum and peak assignments of unexposed polymer A.


182


In methanol or water environments the polymer i s expected to cleave at the e s t e r linkage as shown below: O H

H O

-O-C-N

(Q}

o H

CH

2

(Q)

2

o II

CHJ-O-C-CHJ-CHJ-CHJ-CHJ-C-O


O H

N-C-0-CH -CH -0-C-N 2

H O (Q}

CH

y

ο M

4 I

(QS

2

N-C-O-CHJCHJ-CHJ"

ο H

CHJ-CHJ-CHJ-^-O-C-CHJ-CHJ-CHJ-CHJ-C-O-CHJ-CHJ .

I f methanol i s present, the methyl e s t e r should form at the cleavage points ( t r a n s e s t e r i f i c a t i o n ) and a c t as a " t r a c e r " i n the NMR spectrum. I f water alone causes the cleavage ( h y d r o l y s i s ) , no new bands w i l l appear i n the spectrum because the a c i d OH formed i s d i f f i c u l t to detect by proton NMR. In the present study the expected methyl e s t e r s were sometimes observed i n the polymer a f t e r aging i n methanol environments. Figure 5 shows the methyl e s t e r peak observed i n an NMR spectrum of polymer A aged 30 days i n methanol. I t i s p o s s i b l e to o b t a i n q u a n t i t a t i v e data from the NMR spectrum i n t e g r a l . R e f e r r i n g to F i g . 4, i f the i n t e g r a l of peaks designated b i s d i v i d e d by 8 (the number of protons i n the MDI c o n t r i b u t i n g to t h i s resonance), we have a number f o r the r e l a t i v e amount of MDI present, that i s , r e l a t i v e moles MDI » b/8 In a s i m i l a r manner, we can obtain a number f o r the r e l a t i v e amount of poly(butylene a d i p a t e ) present by d i v i d i n g the i n t e g r a l of peak g by 8 (the number of protons i n the poly(butylene adipate) c o n t r i b u t i n g to t h i s resonance), that i s , r e l a t i v e moles p o l y e s t e r = g/8 -, «. . . moles p o l y e s t e r . Thus, we can o b t a i n a value f o r the r a t i o r*-—i=r= > moles MDI moles p o l y e s t e r _ g/8 _ g moles MDI " b/8 ~ b c

Λ

F i g u r e 6 shows that the molar r a t i o of p o l y e s t e r to MDI decreases with aging time i n methanol. In methanol, the degraded p o l y e s t e r i s s u f f i c i e n t l y s o l u b l e i n the medium and i s subse quently extracted from the polyurethane substrate (as w i l l be shown l a t e r ) . The polyurethane i s a l s o h i g h l y swollen by methanol, thereby enhancing the rate of d i f f u s i o n of p o l y e s t e r chain segments out of the polyurethane matrix. In c o n t r a s t , when the m a t e r i a l i s exposed to water (see F i g . 7) the molar r a t i o i s p r a c t i c a l l y unchanged even though t e n s i l e p r o p e r t i e s have



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Aging

·*— Chemical S h i f t

(ppm from TMS)

Figure 5. NMR spectrum of polymer A tensile specimen aged 30 d in methanol at 60° C.

3.00

r

ο

ο I 0

ι

»

1

10

20

30

Time (days) i n Methanol Figure 6. Relative composition of polymer A aged in methanol at 60° C.




3.00 ρ

I 10

Time

I 20

(days)

i n Water

Figure 7. Relative composition of polymer A aged in water at 80° C.


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d e t e r i o r a t e d ( F i g . 1)· The degraded p o l y e s t e r i s not s o l u b l e i n water nor i s the polyurethane h i g h l y swollen i n water. Conse quently, NMR techniques are not as u s e f u l f o r monitoring p o l y urethane aging In water as i n methanol environments. In general, r e s u l t s from NMR a n a l y s i s i n a l l media i n d i c a t e that the p o l y e s t e r to MDI molar r a t i o remained roughly constant i n aging media which contained water since the degraded p o l y e s t e r i s not e x t r a c t e d . In a d d i t i o n , more change occurred In the molar r a t i o of p o l y e s t e r to MDI i n polymer A than i n polymer Β during aging. In Table I I , the amount of methyl e s t e r ( d e t e c t a b l e by NMR a n a l y s i s ) i s shown to increase with methanol content i n isooctane s o l u t i o n s . This provides evidence that the extent of chain s c i s s i o n i s p r o p o r t i o n a l to methanol content. Table I I . E f f e c t of Composition of Isooctane-Methano1 S o l u t i o n s on Methyl E s t e r Formation i n Polymer Β Aged 30 Days at 80°C 0 II

ν ι η * Methanol Content /TT ι

The Effects of Hostile Environments on Coatings and Plastics

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