Thermoplastic Polyurethanes Based on Poly ... - ACS Publications

18 Thermoplastic Polyurethanes Based on Poly(oxyethylene-oxypropylene) Glycols The Dependence of Properties and Injection Moldability on Molecular Structure F. X. O'SHEA Downloaded by UNIV LAVAL on April 29, 2016 | http://pubs.acs.org Publication Date: November 30, 1981 | doi: 10.1021/bk-1981-0172.ch018

Uniroyal, Inc., World Headquarters, Middlebury, CT 06749

Thermoplastic polyurethanes (TPU) are a versatile family of elastoplastic materials characterized by outstanding toughness and abrasion resistance. These materials are prepared from three principal reactants, a difunctional polyol, a difunctional chain extender and a diisocyanate in accordance with the following reaction:

They are linear block copolymers of the (AB) type in which A is the "soft" segment derived from the polyol and Β is the "hard" segment derived from the diisocyanate and chain extender. Theoretically, infinite molecular weight would be achieved at an isocyanate to total hydroxyl ratio of 1.0. Thus the number of moles of diisocyanate is equal to the sum of the number of moles of polyol and chain extender as shown. Hardness of the materials can be varied by altering the molar ratio of chain extender to polyol which in turn affects the weight ratio of hard segment to soft segment in the polymer. A principal obstacle to growth in TPU usage has been the high cost of these polymers because of the expensive raw materials required. Among commercially available diisocyanates only methylenebis(phenylisocyanate), pure MDI, produces x

0097-6156/81/0172-0243$05.00/0 © 1981 American Chemical Society Edwards et al.; Urethane Chemistry and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Downloaded by UNIV LAVAL on April 29, 2016 | http://pubs.acs.org Publication Date: November 30, 1981 | doi: 10.1021/bk-1981-0172.ch018

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acceptable i n j e c t i o n moldable polymers and then only w i t h a l i m i t e d number o f c h a i n extenders such as 1,4-butanediol and the b i s ( h y d r o x y e t h y l e t h e r ) o f hydroquinone. The s t r u c t u r e s of these compounds are shown i n F i g u r e 1. U n t i l r e c e n t l y , the p o l y o l component was r e s t r i c t e d t o p o l y ( o x y t e t r a m e t h y l e n e ) g l y c o l (PTMG), p o l y c a p r o l a c t o n e d i o l and to a d i p a t e e s t e r d i o l s such as p o l y ( e t h y l e n e a d i p a t e ) d i o l . The s t r u c t u r e s o f these p o l y o l s a r e shown i n F i g u r e 2. This paper d e s c r i b e s work which we have c a r r i e d out on TPU elastomers d e r i v e d from p o l y ( o x y p r o p y l e n e ) g l y c o l s (PPG p o l y o l s ) and from p o l y ( o x y e t h y l e n e - o x y p r o p y l e n e ) g l y c o l s as the p o l y o l components of the elastomers. Our i n t e r e s t i n these m a t e r i a l s i s based on the lower cost o f these p o l y o l s , e s p e c i a l l y s i n c e the p o l y o l comprises about 45 t o 65 percent by weight of the raw m a t e r i a l s used i n p r e p a r i n g most TPU elastomers. F i g u r e 3 shows the s t r u c t u r e o f these p o l y o l s . The p o l y Coxy e thy lene-oxypropy lene) g l y c o l s are o f t e n r e f e r r e d to as " t i p p e d " PPG p o l y o l s s i n c e they are commonly prepared by p o s t r e a c t i n g a PPG p o l y o l w i t h ethylene o x i d e . The r e a c t i o n i n c r e a s e s the p o l y o l r e a c t i v i t y by c o n v e r t i n g secondary h y d r o x y l groups t o primary h y d r o x y l groups although the k i n e t i c s o f the r e a c t i o n precludes 100% conversion t o primary h y d r o x y l terminated p o l y o l . Therefore, i n the s t r u c t u r e shown i n F i g u r e 3, ζ equals zero on some p o l y o l molecules. The p r i n c i p a l focus o f t h i s paper w i l l be on the e f f e c t s o f p o l y o l molecular weight, p o l y o l oxyethylene group content, NCO/OH r a t i o and c h a i n extender s t r u c t u r e on low temperature and e l e v a t e d temperature p r o p e r t i e s , thermal s t a b i l i t y d u r i n g p r o c e s s i n g , and p r o c e s s a b i l i t y , i n p a r t i c u l a r i n j e c t i o n molding behavior. Experimental M a t e r i a l s . 1,4-Butanediol was vacuum d r i e d (60°C., 2mm Hg) f o r s i x hours and then s t o r e d over molecular s i e v e s . The PPG p o l y o l s used were PPG 1025 and PPG 2025 from Union Carbide C o r p o r a t i o n . Tipped PPG p o l y o l s were s u p p l i e d through the courtesy o f F. J . P r e s t o n o f the O l i n C o r p o r a t i o n Research Center, New Haven, Connecticut. P o l y o l s were vacuum d r i e d (100°C., 2 mm. Hg) f o r one hour immediately p r i o r t o use. The MDI, Isonate 125 MF from Upjohn Company, was s t o r e d a t 50°C. and decanted p r i o r t o use so t h a t only water w h i t e , c l e a r m a t e r i a l was used. The BHEHQ was from Eastman Chemical Products. P r e p a r a t i o n o f Elastomers. Two methods f o r p r e p a r i n g TPU elastomers are commonly used. I n the prepolymer method, the p o l y o l i s prereacted w i t h a l l o f the MDI and the r e s u l t a n t p r e polymer, o f t e n c o n t a i n i n g f r e e MDI, i s subsequently r e a c t e d w i t h the c h a i n extender. I n the masterbatch method, the p o l y o l and c h a i n extender a r e premixed and then combined w i t h the MDI,

Edwards et al.; Urethane Chemistry and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

O'SHEA

Poly(oxyethylene-oxypropylene) Glycol Polyurethanes

DIISOCYANATE 0CN

CH2

NC0

^C^"

MDI


CHAIN EXTENDERS HOCH2CH2CH2CH2OH

1,4-Butanediol BHEHQ

HOCH2CH2O - ^ ^ - O C H 2 C H 2 0 H

Figure 1. Hard segment chemical components.

POLYOLS HO [CH2CH2CH2CH2O ] H

PTMG

X

Ο Ο Η [0(CH ) C]x ORO [C(CH ) 0]yH 2

2

5

Polycaprolactone Diol

5

J

Ο Ο » » HOCH2CH2O [ C(CH )4COCH CH 0 ] H 2

2

2

Polyethylenep , a d i

X

a t e ) D l o

Figure 2. Typical polyols.

PPG CH3

CH3

1

1

H [OCHCH2] ORO [CH2CHO] H x

y

"TIPPED" PPG CH3

CH3

H [ OCH2CH2lw IOCHCH2 ]χ ORO [ CH2CHO ] [ CH2CH2O ] H y

Figure 3.

z

PPG-type polyols.


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random p o l y m e r i z a t i o n being allowed to take p l a c e . We used both methods i n our s t u d i e s . Slabs c a s t from the r e a c t i o n s were g r a n u l a t e d , extruded, p e l l e t i z e d , and then i n j e c t i o n molded i n t o t e s t plaques. The prepolymer method was used p r i m a r i l y w i t h the untipped or l i g h t l y t i p p e d PPG p o l y o l s to a v o i d p o l y m e r i z a t i o n d i f f i c u l t i e s a r i s i n g from the d i f f e r e n c e i n r e a c t i v i t y between the secondary h y d r o x y l group of the p o l y o l and the primary h y d r o x y l group of the c h a i n extender. S t r u c t u r e - p r o p e r t y r e l a t i o n s h i p comparisons were always made between polymers p r e pared by the same method. Test Methods. The standard p h y s i c a l - m e c h a n i c a l p r o p e r t i e s of the elastomers were measured a t ambient temperature w i t h an INSTRON T e n s i l e T e s t e r . Cold impact was measured by the f a l l i n g d a r t t e s t i n which a 3" χ 6" χ 0.125" sample i s p o s i t i o n e d i n an i n v e r t e d U shape, c o n d i t i o n e d four hours at -30°C, and impacted a t the h i g h e s t p o i n t of the specimen by a weighted two-inch diameter rod-shaped plunger rounded on the end to a one-inch r a d i u s . The plunger i s dropped from a predetermined h e i g h t to g i v e 5 mph impact velocity. Heat sag was determined i n a t e s t i n which a f o u r - i n c h e x t e n s i o n of a 2" χ 6" χ 0.125" plaque i s heated f o r 30 minutes at 121°C. and i t s d e f l e c t i o n from h o r i z o n t a l i s measured. The DTA measurements were made on a DuPont Model 900 Thermal Analyzer u s i n g a heat-up r a t e of 20°C./min. The DSC c o o l i n g curves were obtained using a Perkin-Elmer DSC-1B a t a c o o l i n g r a t e of 20°C./min. Molecular weight determinations were made i n THF a t room temperature u s i n g a Waters Model 200 g e l permeation chromatograph. The hardness b u i l d - u p t e s t was c a r r i e d out on a one-half ounce l a b o r a t o r y i n j e c t i o n molding machine. Using a 2.5" χ 2.5" χ 0.125" plaque mold h e l d a t a constant temperature, polymer melt was i n j e c t e d i n t o the mold. The mold was opened at v a r i o u s times a f t e r i n j e c t i o n , the p i e c e removed, and i t s Shore A hardness measured e x a c t l y f i v e seconds a f t e r opening the mold. The development of hardness could be followed as a f u n c t i o n of quench time s t a r t i n g w i t h quench times as s h o r t as 10 seconds. R e s u l t s and D i s c u s s i o n When t h i s work began, an important a p p l i c a t i o n f o r TPU elastomers was i n automobile e x t e r i o r body components a s s o c i a t e d w i t h impact absorbing bumper systems. C o l d impact s t r e n g t h , r e s i l i e n c e , r e s i s t a n c e to heat d i s t o r t i o n a t p a i n t oven temper a t u r e s and r a p i d i n j e c t i o n molding c y c l e s were a l l r e q u i r e d . Polymers prepared from untipped PPG p o l y o l s have some i n herent d e f i c i e n c i e s . Table 1 shows data on i n j e c t i o n molded TPU elastomers based on PTMG and PPG p o l y o l s .


18.

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Table 1.

247

TPU Elastomers From PPG & PTMG P o l y o l s , MDI and 1,4-Butanediol. 1-04 NCO/OH, 52% by Weight P o l y o l . A


Polyol P o l y o l M o l e c u l a r Weight -30°C. Drop Impact Tg, DTA Heat Sag, 4" E x t e n s i o n , 121°C. Bashore Rebound

Β

C

PTMG 1000 Pass -54°C

PPG 1000 Fail -3Q°C

PPG 2000 Pass -53°C

2.5'I I 38

3.5 I I 21

1.5If 34

Comparisons were made between polymers c o n t a i n i n g an equal weight percent of s o f t segment so that the e f f e c t s o f segment l e n g t h could be evaluated independent of segment content. Thus the polymer based on 2000 molecular weight PPG p o l y o l i n Table 1 has both a s o f t segment l e n g t h and an average hard segment l e n g t h twice that of the other polymers even though the weight percent of s o f t and hard segments i s the same f o r a l l t h r e e polymers. The data show that the TPU based on 1000 molecular weight PPG has poorer low temperature p r o p e r t i e s than the PTMG based polymer as i n d i c a t e d by i t s l a c k of c o l d impact r e s i s t a n c e and i t s h i g h e r s o f t segment g l a s s t r a n s i t i o n temperature. I t a l s o d i s p l a y s poorer heat sag r e s i s t a n c e and i s l e s s r e s i l i e n t than the PTMG based polymer. I n a d d i t i o n , the polymer from PPG 1000 does not i n j e c t i o n mold w e l l because o f slow " f r e e z e - o f f " and high shrinkage. The comparison t o PTMG i s much more f a v o r a b l e when 2000 molecular weight PPG i s used i n s t e a d o f 1000 molecular weight. In a d d i t i o n t o improved c o l d impact r e s i s t a n c e , heat sag and r e s i l i e n c e , t h i s polymer freezes o f f r a p i d l y on i n j e c t i o n mold i n g and does not s h r i n k e x c e s s i v e l y . The d i f f e r e n c e s between the two PPG based polymers can be 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 polymer morphology. I t has been w e l l e s t a b l i s h e d through work by Cooper (1 ), by S e e f r i e d (2) , by Wilkes (3) and others t h a t the p r o p e r t i e s of TPU elastomers r e s u l t from thermodynamic i n c o m p a t i b i l i t y o f the s o f t and hard segments which leads t o microphase s e p a r a t i o n , domain f o r m a t i o n and the development o f long-range order i n the hard segment domains. The data i n Table 1 suggest that the PPG s o f t segment i s more compatible w i t h the MDl/butanediol hard segment than i s PTMG, r e s u l t i n g i n l e s s complete phase s e p a r a t i o n . The g l a s s t r a n s i t i o n of the PPG s o f t segment, t h e o r e t i c a l l y -78°C. a t i n f i n i t e l e n g t h , i s s h i f t e d t o a h i g h e r temperature because o f r e s t r a i n t s imposed by segment m i x i n g . I n the same way, the s o f t e n i n g temperature of the hard segment i s s h i f t e d downward l e a d i n g t o poorer r e s i s t a n c e t o heat d i s t o r t i o n . I n c r e a s i n g the segment l e n g t h s through the use of 2000 molecular weight p o l y o l g i v e s more

American Chemical Society Library 1155 16th St. N. W. Edwards et al.; Urethane Chemistry and Applications Washington, 0. C. 20036 ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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complete phase s e p a r a t i o n and l e s s segmental mixing r e s u l t i n g i n improved c o l d impact r e s i s t a n c e and b e t t e r r e s i s t a n c e to heat sag. The e f f e c t i v e n e s s of i n c r e a s e d s o f t segment molecular weight i n promoting phase s e g r e g a t i o n has been demonstrated by Schneider and P a i k Sung (4) f o r p o l y e s t e r - T D I polyurethanes. P r o c e s s i n g S t a b i l i t y . D e s p i t e i t s improved p r o p e r t i e s , the polymer based on PPG 2000 was found to be i m p r a c t i c a l because of i n s t a b i l i t y a t p r o c e s s i n g temperatures. R e t e n t i o n of the polymer melt a t 204°C. i n the b a r r e l of an i n j e c t i o n molding machine f o r times as s h o r t as 10 minutes gave molded p a r t s w i t h reduced t e n s i l e s t r e n g t h and l o s s of impact r e s i s t a n c e . I n e f f e c t i v e n e s s of a n t i o x i d a n t s and n i t r o g e n b l a n k e t s i n p r e v e n t i n g t h i s break down i n d i c a t e s t h a t i t i s thermal and not o x i d a t i v e . Work by B e a c h e l l (5) and by Dyer (6) has e s t a b l i s h e d that the polyurethane bond can d i s s o c i a t e a t these temperatures to g i v e polymer fragments terminated by i s o c y a n a t e and h y d r o x y l f u n c t i o n a l i t y as shown i n F i g u r e 4. Although t h i s r e a c t i o n i s r e v e r s i b l e i t could be d r i v e n to the r i g h t i f the d i s s o c i a t e d fragments undergo subsequent s i d e r e a c t i o n s or i n some way become l e s s a v a i l a b l e to one another f o r recombination. Morphology could p l a y a r o l e s i n c e the polymer prepared from PPG 1000 i s s t a b l e a t 204°C. even though i t has twice the number of p o l y o l to i s o c y a n a t e urethane bonds a v a i l a b l e for dissociation. E f f e c t of Qxyethylene Content on S t a b i l i t y . The p r o c e s s i n g i n s t a b i l i t y of TPU elastomers based on PPG 2000 can be overcome by the use of ethylene oxide " t i p p e d " PPG p o l y o l s , e s p e c i a l l y those c o n t a i n i n g h i g h l e v e l s of oxyethylene groups, e.g., 30% to 45% by weight. The improved s t a b i l i t y of the polymers based on such p o l y o l s i s demonstrated i n Table 2. Table 2.

Wt. % EO 10 30 45

E f f e c t of Oxyethylene Group Content of 2000 M.W. P o l y o l on S t a b i l i t y of TPU Elastomer.

Min, @ 204°C*

T e n s i l e , MPa

2 20

22.1 5.5

1.78 χ 10 0.88 χ 1 0

5

2 20

24.8 15.9

1.86 χ 1 0 1.16 χ 1 0

5

2 20

24.8 23.4

2.05 χ 10 1.47 χ 1 0

5

5

5 5

* R e t e n t i o n time i n b a r r e l of i n j e c t i o n molding machine b e f o r e i n j e c t i o n i n t o mold.



18.

O'SHEA


Increased oxyethylene group content i n the p o l y o l r e s u l t s i n improved s t a b i l i t y as r e f l e c t e d i n b e t t e r r e t e n t i o n of t e n s i l e s t r e n g t h w i t h thermal exposure. I n t u r n , t h i s c o i n c i d e s w i t h b e t t e r r e t e n t i o n of the weight average molecular weight of the polymers as determined by GPC measurements. There i s s t r o n g evidence t h a t thermal s t a b i l i t y i s dependent p r i m a r i l y on oxyethylene group content r a t h e r than primary h y d r o x y l content. Table 3 shows t h a t polymer prepared from a 2000 molecular weight p o l y o l w i t h 45% by weight oxyethylene group content but o n l y 46% primary h y d r o x y l content i s more s t a b l e than one prepared from a p o l y o l w i t h 30% by weight oxyethylene groups but w i t h 82% primary h y d r o x y l . Table 3.

Dependence of S t a b i l i t y on Oxyethylene Group Content of Polyol.

P o l y o l M o l e c u l a r Weight Weight % EO 1° -OH Content % Tensile Retention*

A

Β

C

2000 45 93 94

2000 45 46 85

2000 30 82 64

*Melt r e t a i n e d 20 minutes i n the b a r r e l a t 204°C. b e f o r e i n j e c t i o n i n t o mold compared w i t h melt h e l d f o r two minutes. These data suggest t h a t the thermal i n s t a b i l i t y of the polymer based on PPG 2000 a r i s e s from d i s s o c i a t i o n of the urethane bonds i n the melt but w i t h slow recombination because of segmental i n c o m p a t i b i l i t y which g i v e s r i s e to fragment s e p a r a t i o n making the r e a c t i v e ends u n a v a i l a b l e f o r recombination. High oxyethylene group content appears to improve c o m p a t i b i l i t y of the s o f t and hard segments s u f f i c i e n t l y to a l l o w recombination, thus r e s u l t i n g i n a more s t a b l e polymer. By t h i s r e a s o n i n g , the polymer based on untipped PPG 1000 i s t h e r m a l l y s t a b l e because of g r e a t e r c o m p a t i b i l i t y between the s o f t and hard segments i n the melt as a r e s u l t of the s h o r t e r segment l e n g t h s . The e f f e c t s of p o l y o l molecular weight and oxyethylene group content on the thermal s t a b i l i t y of these polymers have been r e p o r t e d independently by Bonk and Shah (7) · I n j e c t i o n M o l d a b i l i t y . The i n j e c t i o n molding behavior of these m a t e r i a l s i s another response which can be i n f l u e n c e d s i g n i f i c a n t l y by s t r u c t u r a l v a r i a t i o n s . For example, commercial s c a l e molding t r i a l s on a 45 Shore D automotive polymer based on a 2000 molecular weight p o l y ( o x y e t h y l e n e - o x y p r o p y l e n e ) g l y c o l which contained 45% by weight oxyethylene groups showed t h a t a c h i e v a b l e c y c l e times were s t r o n g l y i n f l u e n c e d by the NCO/OH


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r a t i o o f the f o r m u l a t i o n used t o produce the polymer. Although an NCO/OH r a t i o o f 1.0 t h e o r e t i c a l l y would produce i n f i n i t e molecular weight polymer, i n p r a c t i c e a s m a l l excess o f d i i s o c y a n a t e i s o f t e n used i n order t o account f o r i m p u r i t i e s and minor s i d e r e a c t i o n s which would reduce molecular weight. The excess i s o c y a n a t e ensures h i g h molecular weight through the branching mechanisms shown i n F i g u r e 5. With the automotive grade polymer an NCO/OH r a t i o of 1.05 or higher gave polymers which were d i f f i c u l t t o demold i n reasonable c y c l e times and tended t o s t i c k i n the mold. When made a t an NCO/OH r a t i o o f 1.01, the polymer demolded very r e a d i l y but tended t o show s t r e s s marks a f t e r p a i n t i n g and d r y i n g o f the demolded p a r t s and, i n some t o o l s , d i d not f i l l out the p a r t adequately. These d i f f e r e n c e s could not be r e c o n c i l e d on the b a s i s o f melt v i s c o s i t y but appeared i n s t e a d t o r e f l e c t d i f f e r ences i n the " f r e e z i n g o f f " o f the polymer d u r i n g molding. I t was p o s s i b l e to measure these d i f f e r e n c e s by the hardness b u i l d up t e s t d e s c r i b e d i n the experimental s e c t i o n . E f f e c t o f NCO/OH R a t i o on Hardness B u i l d - u p . F i g u r e 6 shows the dramatic e f f e c t o f NCO/OH r a t i o on the development of hardness w i t h quench time f o r the 45 Shore D automotive polymer. A r a t i o o f 1.01 g i v e s a polymer which develops hardness v e r y r a p i d l y w h i l e i n c r e a s i n g the NCO/OH r a t i o leads t o slower and slower hardness development, p a r t i c u l a r l y i n the c r i t i c a l e a r l y seconds a f t e r i n j e c t i o n . T h i s slow hardness b u i l d - u p can account f o r the long demolding time and tendency f o r mold s t i c k ing observed w i t h m a t e r i a l s made a t an NCO/OH r a t i o o f 1.05 o r h i g h e r . On the other hand, the very r a p i d f r e e z e - o f f o f the polymer made a t the 1.01 r a t i o could account f o r molded-in s t r e s s e s because o f premature hardening o f the polymer over the long d i s t a n c e s t r a v e l e d by the c o o l i n g melt i n a commercial t o o l . Polymer made a t an NCO/OH r a t i o o f 1.03 was found t o have an optimum r a t e o f hardness b u i l d - u p . I t s hardness b u i l d - u p curve, shown i n F i g u r e 6, was e s s e n t i a l l y superimposable w i t h that o f a commercially a c c e p t a b l e PTMG-based polymer and i n f i e l d t r i a l s i t molded a t the same c y c l e time w i t h o u t d i f f i c u l t y . Thus molding behavior i n l a r g e automotive molds c o u l d be r e l a t e d t o a simple t e s t which can be c a r r i e d out on a r e l a t i v e l y s m a l l sample. C o n t r o l o f NCO/OH r a t i o was demonstrated t o be c r i t i c a l i n these m a t e r i a l s t o o b t a i n polymer which can be molded a c c e p t a b l y and reproducibly. Another way t o r e p r e s e n t hardness development i s t o s e l e c t two quench times r e p r e s e n t i n g c r i t i c a l p o i n t s i n a hardness b u i l d up curve f o r the polymer type o f i n t e r e s t and adding together the two readings t o o b t a i n a hardness b u i l d - u p index. Table 4 shows index v a l u e s obtained from the a d d i t i o n o f the 10 second and 15 second readings f o r the polymers d e s c r i b e d i n F i g u r e 6. These d i f f e r e n c e s i n hardness b u i l d - u p (HBU) a r e b e l i e v e d t o r e s u l t from d i f f e r e n c e s i n the k i n e t i c s o f domain f o r m a t i o n t a k i n g


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