Urethane Chemistry and Applications - American Chemical Society

29 Thermoplastic Polyurethane Elastomer Melt Polymerization Study

Downloaded by PENNSYLVANIA STATE UNIV on September 9, 2012 | http://pubs.acs.org Publication Date: November 30, 1981 | doi: 10.1021/bk-1981-0172.ch029

C. S. SCHOLLENBERGER, K. DINBERGS, and F. D. STEWART BF Goodrich Research & Development Center, Brecksville, OH 44141

Due to the relatively low volatility, compatibility, high reactivity, and liquid or low melting nature of their reactant components - the diisocyanate, the macroglycol, and the small chain-extender glycol - polyurethanes can be produced readily in the melt by the polyaddition process, even at moderate tempera tures and atmospheric pressure. Such polymerizability is attrac tive in that it enables the polymers to be formed quickly, efficiently, and in a relatively small space. There are no large solvent storage tanks to accomodate, no solvents nor reactants to recover and repurify for recycle, no contaminated water to clean before discharge, and no atmospheric pollution by gases or particulates to deal with. In an era of high capital costs, keen commercial competition, and worldwide concern for a clean environment a manufacturing process such as polyurethane elastomer melt polymerization obviously has much to recommend it. Polymer Formation. In the formation of thermoplastic polyurethane elastomers the reactants - diisocyanate, macrogly col, and small chain extender glycol - interact and join to gether end-to-end to produce essentially linear polymer chains. The chemical bond produced i n the c h a i n - b u i l d i n g l i n k a g e s i s the urethane group which i s covalent i n nature and reasonably s t r o n g , although the s t r e n g t h v a r i e s w i t h s t r u c t u r e . Urethane formation i s shown i n Equation 1.

0097-6156/81/0172-0433$09.25/0 © 1981 American Chemical Society

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

434

URETHANE CHEMISTRY AND APPLICATIONS

Equation 1.


•

Urethane Formation

'—NCO + (isocyanate group)

H0-~^(hydroxyl group)

j

>

NH-CO-0(urethane group)

+ Δ

Heat of r e a c t i o n i s reported to be -52 kcal/mole i n the case of the hexamethylene d i i s o c y a n a t e and 1,4-butanediol reac tion. The same d i i s o c y a n a t e , r e a c t i n g w i t h p o l y ( e t h y l e n e adipate) g l y c o l at 100°C was found to show a r e a c t i o n v e l o c i t y constant, k, of 0.00083 1 mol sec , w i t h an a c t i v a t i o n energy, E, of 11.0 k c a l / m o l e . ' At the same temperature diphenylmethane-4,4'-diisocyanate (MDI) r e p o r t e d l y reacted w i t h p o l y ( d i ethylene adinate) g l y c o l i n mono-chlorobenzene w i t h k = 0.00091 1 mole sec , and Ε = 10.5 k c a l / m o l e . ' As Equation 1 i n d i c a t e s , urethane formation i s an e q u i l i brium r e a c t i o n . The degree of d i s s o c i a t i o n of urethane product i n t o reactant isocyanate and hydroxyl groups i s dependent upon the ambient temperature and i s a constant at given temperature (Equation 2). 1

1

2

1

1

Equation 2.

1

4

3

4

Urethane E q u i l i b r i u m Constant

[NHCOO] = Κ [NCO][OH] We w i l l l a t e r note m a n i f e s t a t i o n s of the chemical e q u i l i b r i u m i n urethane p o l y m e r i z a t i o n s . Polymer S t r u c t u r e . In t h e r m o p l a s t i c polyurethane elastomer formation the p o l y m e r i z a t i o n produces polymer primary chains which c o n s i s t of a l t e r n a t i n g urethane-sparse s o f t segments ( d i i s o c y a n a t e - m a c r o g l y c o l l i n e a r r e a c t i o n product) and uret h a n e - r i c h hard segments ( d i i s o c y a n a t e - c h a i n extender l i n e a r r e a c t i o n p r o d u c t s ) . These primary chains tend to " v i r t u a l l y c r o s s l i n k " and " v i r t u a l l y chain extend" w i t h one another, p r i n c i p a l l y through the a s s o c i a t i o n of t h e i r hard segments, due to the hydrogen bonding of t h e i r urethane groups, the a s s o c i a t i o n of aromatic 7T e l e c t r o n s , e t c . , producing a g i a n t " v i r t u a l net work" of polymer chains, and thus e l a s t i c i t y i n the product. These v i r t u a l l i n k a g e s are r e l a t i v e l y l a b i l e and are r e v e r s i b l e w i t h heat and s o l v a t i o n , p e r m i t t i n g t h e r m o p l a s t i c and s o l u t i o n p r o c e s s i n g of the polymers. The foregoing i s depicted schema t i c a l l y i n F i g u r e 1.



SCHOLLENBERGER

ET AL.

Elastomer Melt Polymerization Study

VIRTUALLY CROSSLINKED/EXTENDED NETWORK of POLYMER PRIMARY CHAINS

ti POLYMER Figure 1.

Thermoplastic

Δ or SOLVENT

PRIMARY

CHAINS

methane elastomer

chain

organization.



436

URETHANE CHEMISTRY AND APPLICATIONS

I t i s thus seen that thermoplastic polyurethane elastomers have two types of thermal l a b i l i t y . That depicted i n Figure 1 i s benign and the b a s i s of the v e r s a t i l e p r o c e s s a b i l i t y of t h i s polymer c l a s s . That depicted i n Equation 1 becomes important at high temperatures and cannot be considered to be t o t a l l y benign i n that i t i n v o l v e s the r e v e r s i b l e d i s s o c i a t i o n of polymer primary chain bonds, which separations can be i r r e v e r s i b l e depending upon the polymer environment. I t would seem that such d i s s o c i a t i o n must predominate i n the areas of highest urethane group concentration, namely the polymer hard segments and t h i s suggests the p o s s i b i l i t y of some "scrambling" of polymer segmented s t r u c t u r e , e.g., during polymer melt p r o c e s s i n g , rheology s t u d i e s , e t c . and other problems which we do not address i n t h i s paper. I t i s the polymer changes r e l a t e d to Equation 1 that are the primary concern of the present paper. Although there i s considerable published l i t e r a t u r e on the r e l a t i o n of polyurethane p r o p e r t i e s to t h e i r chemical and p h y s i c a l s t r u c t u r e s , considerably l e s s has appeared concerning the d e t a i l e d nature of urethane p o l y m e r i z a t i o n . The thermoset nature of the c l a s s i c a l polyurethane systems, which has provided fewer o p p o r t u n i t i e s and thus l e s s s c i e n t i f i c i n t e r e s t f o r the study of the intermediate stages and course of urethane polymeri z a t i o n , l i k e l y helps account f o r t h i s . Published information on urethane p o l y m e r i z a t i o n d e t a i l l a r g e l y concerns thermoset urethane elastomers systems. In p a r t i c u l a r , the work of Macosko e t . a l . i s c a l l e d to a t t e n t i o n . The present paper supplements t h i s l i t e r a t u r e with information on the f u l l course of l i n e a r thermoplastic urethane elastomer formation conducted under random melt p o l y m e r i z a t i o n c o n d i t i o n s i n a s l i g h t l y modified Brabender P l a s t i C o r d e r r e a c t o r . V i s c o s i t y and temperature v a r i a t i o n s with time were continuously recorded and the e f f e c t s of s e v e r a l r e l e v a n t p o l y m e r i z a t i o n v a r i ables - temperature, composition, c a t a l y s t , s t a b i l i z e r , macrog l y c o l a c i d number, shortstop - are reported. The paper w i l l a l s o be seen to provide a d d i t i o n a l i n s i g h t i n t o the nature and behavior of thermoplastic polyurethane elastomers. 4

Experimental

1 3

Part

M a t e r i a l s . The reactants used i n t h i s study and designations are l i s t e d i n Table I.

their


29.

SCHOLLENBERGER ET AL.

Elastomer Melt Polymerization Study


Table I Reactants Used i n the Present Study

Name PTAd PTAd PTAd PTAd PTAd PTAd MDlJ MDlï* MDI 1,4-BD0 1,4-BD0 EDO Stannous Octate 1-Propanol l-Decanol TPU Irganox 10 1

Urethane Chemistry and Applications - American Chemical Society

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